Multiflash for Windows Manual

User Guide for

Multiflash for Windows Infochem/KBC Advanced Technologies plc

Version 4.4 February 2014

Infochem/KBC Advanced Technologies plc Unit 4, The Flag Store 23 Queen Elizabeth Street London SE1 2LP Tel: +44 [0]20 7357 0800 Fax: +44 [0]20 7407 3927 e-mail: [email protected]

This User Guide and the information contained within is the copyright of Infochem Computer Services Ltd.

Infochem/KBC Advanced Technologies plc Unit 4, The Flag Store 23 Queen Elizabeth Street London SE1 2LP, UK Tel:+44 [0]20 7357 0800 Fax:+44 [0]20 7407 3927 e-mail:[email protected]

Disclaimer While every effort has been made to ensure that the information contained in this document is correct and that the software and data to which it relates are free from errors, no guarantee is given or implied as to their correctness or accuracy. Neither Infochem Computer Services Ltd nor any of its employees, contractors or agents shall be liable for direct, indirect or consequential losses, damages, costs, expenses, claims or fee of any kind resulting from any deficiency, defect or error in this document, the software or the data.

Contents Overview

1

Introduction ............................................................................................................................... 1 The Multiflash GUI ..................................................................................................... 1 Multiflash Software System....................................................................................................... 1 Document Organisation ............................................................................................................. 2 New Features and Changes in Version 4.4 and 4.3 ..................................................... 2 New Features and Changes in Version 4.2 .................................................................. 2 Running Multiflash...................................................................................................... 2 HELP ........................................................................................................................... 3 Case studies ................................................................................................................. 3 Appendix - Multiflash Commands .............................................................................. 3 Installation ................................................................................................................................. 3

New Features and Changes in Version 4.4 and 4.3

5

Introduction ............................................................................................................................... 5 Models ....................................................................................................................................... 5 Huron-Vidal-Pedersen mixing rule.............................................................................. 5 Sutton Model for surface tension................................................................................. 5 LBC ............................................................................................................................. 5 Salt component ............................................................................................................ 5 High accuracy reference eos for water-ammonia binary system ................................. 6 New high accuracy reference eos ................................................................................ 6 Activity Coefficient models......................................................................................... 6 Performance enhancements ......................................................................................... 6 Phase key components................................................................................................. 6 Windows GUI............................................................................................................................ 6 PVTSim import tool .................................................................................................... 6 Models tab ................................................................................................................... 6 Inhibitor calculator ...................................................................................................... 6 Surface tension ............................................................................................................ 7 Petroleum Fraction Input Table................................................................................... 7 Tables......................................................................................................................................... 7 OLGA tables................................................................................................................ 7 Interfaces ................................................................................................................................... 7 Excel Interface............................................................................................................. 7 CAPE-OPEN ............................................................................................................... 7 Databanks .................................................................................................................................. 7 Infodata........................................................................................................................ 7 DIPPR.......................................................................................................................... 7

New Features and Changes in Version 4.2

9

Introduction ............................................................................................................................... 9 Models ....................................................................................................................................... 9 CSMA model............................................................................................................... 9 Mercury ....................................................................................................................... 9 Poynting correction ..................................................................................................... 9 Activity coefficient model for gas phase ..................................................................... 9 LBC viscosity model ................................................................................................. 10

User Guide for Multiflash for Windows

Contents  iii

Binary Interaction Parameters ................................................................................................. 10 Multiflash phase equilibrium algorithm................................................................................... 10 Flash calculations..................................................................................................................... 10 Databanks ................................................................................................................................ 10 Infodata...................................................................................................................... 10 Windows GUI.......................................................................................................................... 11 PVTSim import tool .................................................................................................. 11 Reid vapour pressure ................................................................................................. 11 Liquid dropout and Wax precipitation curve............................................................. 11 New icon for Asphaltene precipitation curve ............................................................ 11 Phase envelopes for solids......................................................................................... 11 Hydrate models.......................................................................................................... 11 Inhibitor calculator .................................................................................................... 11 PVT Analysis ............................................................................................................ 11 Retrograde Dew Point ............................................................................................... 11 Calculation options.................................................................................................... 11 Usability .................................................................................................................... 12 Tables....................................................................................................................................... 12 OLGA tables.............................................................................................................. 12 Multiflash Excel Interface ....................................................................................................... 12 Joule-Thompson coefficient ...................................................................................... 12

Getting Started

13

Starting Multiflash ................................................................................................................... 13 Multiflash Main Window......................................................................................................... 13 Input section .............................................................................................................. 14 Conditions ................................................................................................................. 14 Fluid identification .................................................................................................... 14 Compositions............................................................................................................. 14 Results window ......................................................................................................... 14 Menu options............................................................................................................. 14 The Toolbar ............................................................................................................... 18 Defining a problem in Multiflash ............................................................................................ 18 Loading an existing problem file ............................................................................................. 18 Loading a problem setup file ..................................................................................... 18 Calculations ............................................................................................................... 19 The results ................................................................................................................. 19 Additional calculations .............................................................................................. 20 Setting up a new problem ........................................................................................................ 21 Clearing previous problems....................................................................................... 21 Defining the components........................................................................................... 21 Defining the models .................................................................................................. 22 Set Input Conditions .................................................................................................. 23 Carrying out the flash calculation.............................................................................. 23 Other calculations...................................................................................................... 24 Phase envelope .......................................................................................................... 24 Saving the problem setup......................................................................................................... 25 Backup file ................................................................................................................ 26 Loading a existing MFL file .................................................................................................... 26 Warning option for matching and PVT form........................................................................... 26 Printing the output ................................................................................................................... 26 Saving the output ..................................................................................................................... 27 How to exit the program .......................................................................................................... 27 Technical support..................................................................................................................... 28

Models

29

Introduction ............................................................................................................................. 29 What is a model? ..................................................................................................................... 29 What types of model are available? ......................................................................................... 29

iv  Contents

User Guide for Multiflash for Windows

Equation of state models.......................................................................................................... 30 When to use equation of state methods ..................................................................... 30 Equations of state provided in Multiflash ................................................................................ 30 Ideal gas equation of state ......................................................................................... 30 Peng-Robinson equation of state ............................................................................... 30 Peng-Robinson 1978 (PR78) equation of state.......................................................... 31 Redlich-Kwong (RK) and Redlich-Kwong-Soave (RKS) equations......................... 31 Advanced Equation of state options .......................................................................... 31 When to use cubic equations of state......................................................................... 32 Cubic plus association (CPA) model......................................................................... 32 PSRK equation of state.............................................................................................. 32 ZJ (Zudkevitch-Joffe) model ..................................................................................... 33 PC-SAFT equation of state........................................................................................ 33 Lee-Kesler (LK) and Lee-Kesler-Plöcker (LKP) equations of state.......................... 34 Benedict-Webb-Rubin-Starling (BWRS) equation of state ....................................... 34 Multi-reference fluid corresponding states (CSMA) model ...................................... 35 IAPWS-95 ................................................................................................................. 35 Water-Ammonia ........................................................................................................ 35 Carbon Dioxide high-accuracy model ....................................................................... 36 GERG-2008............................................................................................................... 36 GERG-2008 (Infochem extension)............................................................................ 36 Activity coefficient methods.................................................................................................... 37 Activity coefficient equations in Multiflash .............................................................. 37 Gas phase models for activity coefficient methods ................................................... 38 When to use activity coefficient models.................................................................... 38 Models for solid phases ........................................................................................................... 39 Solid freeze-out model .............................................................................................. 39 Scaling and general freeze-out model........................................................................ 39 Modelling hydrate formation and inhibition.............................................................. 39 Modelling wax precipitation...................................................................................... 42 Modelling asphaltene flocculation............................................................................. 43 Other thermodynamic models ................................................................................... 43 Transport property models....................................................................................................... 44 Viscosity.................................................................................................................... 44 Thermal conductivity................................................................................................. 45 Surface Tension ......................................................................................................... 46 Diffusion coefficient.................................................................................................. 46 How to specify models in Multiflash....................................................................................... 47 Using the menu.......................................................................................................... 47 Loading a model from a .mfl file............................................................................... 48 Setting up preferred models in Multiflash ................................................................. 48 How to change a model ............................................................................................. 48 Models for solid phases ........................................................................................................... 48 Hydrates .................................................................................................................... 48 Pure solid phase......................................................................................................... 49 Waxes ........................................................................................................................ 50 Asphaltenes ............................................................................................................... 51 Combined Solids Model ............................................................................................ 52 Troubleshooting - models ........................................................................................................ 52 Model is not available ............................................................................................... 52 Groups not available for UNIFAC model ................................................................. 52 Binary interaction parameters .................................................................................................. 53 BIPs and models ........................................................................................................ 53 Temperature dependence of BIPs.............................................................................. 54 BIPs available in Multiflash ...................................................................................... 54 Viewing BIP values................................................................................................... 55 Units for BIPs............................................................................................................ 57 Supplementing or overwriting BIPs .......................................................................... 58 BIPs for CSMA and GERG mixing rule ................................................................... 59 Troubleshooting - BIPs............................................................................................................ 60

User Guide for Multiflash for Windows

Contents  v

Units .......................................................................................................................... 60 BIP databank ............................................................................................................. 60 Differences between the PR Model in Multiflash and Aspen Hysys ....................................... 60

Components

63

Introduction ............................................................................................................................. 63 Normal components................................................................................................... 63 Petroleum fractions.................................................................................................... 65 Defining a mixture ................................................................................................................... 65 Specifying the data source......................................................................................... 66 Selecting components................................................................................................ 66 Adding, inserting, replacing and deleting components.............................................. 69 Viewing and editing pure component data ................................................................ 70 User-defined components ........................................................................................................ 72 Adding a user-defined component............................................................................. 72 Specifying data for a user-defined component .......................................................... 73 Models and input requirements ................................................................................. 74 Stream types ............................................................................................................................ 76 Hydrate inhibitors .................................................................................................................... 79 Inhibitor calculator: alcohols/glycols ........................................................................ 79 Salt calculator ............................................................................................................ 81 Troubleshooting - components ................................................................................................ 84 Databank not found ................................................................................................... 84 Databank not licensed................................................................................................ 85 Component cannot be found...................................................................................... 86 Too many components in the mixture ....................................................................... 87

Petroleum fluids

89

Introduction ............................................................................................................................. 89 PVT Lab Analysis input .......................................................................................................... 89 Component list .......................................................................................................... 91 Petroleum fluid composition ..................................................................................... 94 Molecular weight and specific gravity ...................................................................... 95 Total amount of fluid................................................................................................. 96 Water cut ................................................................................................................... 97 Total Wax Content .................................................................................................... 97 SARA Analysis ......................................................................................................... 97 Pseudocomponents .................................................................................................... 98 Characterisation......................................................................................................... 99 User Defined Cuts ................................................................................................... 100 Saving a PVT Analysis............................................................................................ 101 Black Oil Analysis ................................................................................................................. 101 Input data................................................................................................................. 102 Distillation curves .................................................................................................................. 102 TBP distillation........................................................................................................ 102 ASTM D86 distillation ............................................................................................ 104 PVT Lab Analysis input with n-paraffin analysis.................................................................. 105 n-Paraffin distribution ............................................................................................. 105 Characterisation....................................................................................................... 107 Estimated n-paraffin distribution............................................................................. 108 Troubleshooting – PVT Analysis........................................................................................... 109 Sensitivity to characterisation.................................................................................. 109 Presence of water..................................................................................................... 109 Defining petroleum fractions ................................................................................................. 109 Basic characterisation properties ............................................................................. 109 Other properties....................................................................................................... 110 Entering petroleum fractions ................................................................................... 110 Editing petroleum fraction data ............................................................................... 112 Deleting petroleum fractions ................................................................................... 112

vi  Contents

User Guide for Multiflash for Windows

Problems defining a petroleum fraction .................................................................. 113 Delumping tool ...................................................................................................................... 113 How to use the delumping utility ............................................................................ 114 Matching using petroleum fraction properties ....................................................................... 116 Matching dew and bubble points............................................................................. 116 Matching Density/Volume ...................................................................................... 122 Matching wax data/WAT ........................................................................................ 123 Matching liquid viscosity ........................................................................................ 125 Matching vapour viscosity ...................................................................................... 126 Problems when matching ........................................................................................ 127 Petroleum Fluid Blending...................................................................................................... 127 Blending method ..................................................................................................... 128 Fluid file name......................................................................................................... 128 Fluid amounts .......................................................................................................... 129 Model definition ...................................................................................................... 129 Blending procedure ................................................................................................. 129 Example for blending .............................................................................................. 130 Example with waxy crudes...................................................................................... 132 Example with asphaltenic crudes ............................................................................ 133

Input conditions

137

Introduction ........................................................................................................................... 137 Specifying compositions........................................................................................................ 137 Specifying temperature, pressure and volume ....................................................................... 138 Specifying enthalpy, entropy and internal energy.................................................................. 139 Troubleshooting - input conditions........................................................................................ 139

Calculations

141

Introduction ........................................................................................................................... 141 The basis of a flash calculation.............................................................................................. 141 Flashes available in Multiflash .............................................................................................. 142 Isothermal (P,T) flash.............................................................................................. 142 Isenthalpic flashes ................................................................................................... 143 Isentropic flashes ..................................................................................................... 143 Isochoric flashes ...................................................................................................... 143 Bubble and dew point flashes.................................................................................. 143 Fixed phase fraction flashes .................................................................................... 144 Phase Envelopes ...................................................................................................... 148 Phase Envelopes for solids ...................................................................................... 157 Liquid dropout curve calculation............................................................................. 158 Hydrate calculations ................................................................................................ 159 Wax calculations ..................................................................................................... 159 Tolerance calculations ............................................................................................. 161 Reid Vapour Pressure .............................................................................................. 162 Property output in Multiflash................................................................................................. 164 Troubleshooting - flash calculations ...................................................................................... 165 Plot the phase envelope ........................................................................................... 166 Use the P,T flash ..................................................................................................... 166 Limit the number of phases ..................................................................................... 167 Consider all types of solution .................................................................................. 167 Provide a starting estimate....................................................................................... 167 Provide a key component ........................................................................................ 168

Units

169 Introduction ........................................................................................................................... 169 Default units .......................................................................................................................... 169 Changing units ....................................................................................................................... 170 Troubleshooting - units.......................................................................................................... 171

User Guide for Multiflash for Windows

Contents  vii

Output

173

Introduction ........................................................................................................................... 173 The results window................................................................................................................ 173 Font........................................................................................................................................ 174 Writing the results to a file .................................................................................................... 174 Printing the output ................................................................................................................. 175 Calculation output.................................................................................................................. 175 Manipulating the Output.......................................................................................... 177 Phase Labelling...................................................................................................................... 178 Aqueous phase labelling.......................................................................................... 178 Enthalpy definition ................................................................................................................ 178 Activity Models ....................................................................................................... 179 Entropy definition .................................................................................................................. 180 Activity Models ....................................................................................................... 180 Errors and warning messages ................................................................................................ 181 Displaying status for current settings..................................................................................... 181 Troubleshooting - output ....................................................................................................... 181 The output does not include everything expected ................................................... 181 Phase labelling......................................................................................................... 182 Fonts ........................................................................................................................ 182

Interfaces with other programs

183

Introduction ........................................................................................................................... 183 Pipesim PVT files .................................................................................................................. 183 OLGA .................................................................................................................................... 184 OLGA hydrate file................................................................................................... 186 OLGA wax file ........................................................................................................ 186 Prosper PVT files................................................................................................................... 187 To generate the file.................................................................................................. 187 CAPE-OPEN Interface .......................................................................................................... 188 PVTSim CHC file import tool ............................................................................................... 189

Help

191 Introduction ........................................................................................................................... 191 On-line help ........................................................................................................................... 191 Help Topics ............................................................................................................. 191 Multiflash Error Codes ............................................................................................ 193 Check for Updates ................................................................................................... 193 About Multiflash ..................................................................................................... 194 Technical support................................................................................................................... 194

Case studies - Pure component data

197

Introduction ........................................................................................................................... 197 Physical properties of a pure component ............................................................................... 197 Defining the problem in Multiflash ......................................................................... 197 Obtaining properties from the Pure Component Data option .................................. 200 Excel interface ......................................................................................................... 202

Case studies - Phase equilibria

205

Introduction ........................................................................................................................... 205 Oil and gas systems ............................................................................................................... 205 Calculating the bubble point curve .......................................................................... 206 Calculating the dew point curve .............................................................................. 207 Phase envelope ........................................................................................................ 208 Adding water to the system ..................................................................................... 209 Including a petroleum fraction ................................................................................ 210

viii  Contents

User Guide for Multiflash for Windows

Other flash calculations ........................................................................................... 212 PVT Analysis......................................................................................................................... 214 User defined carbon number cuts ............................................................................ 219 TBP curves .............................................................................................................. 221 Black Oil Analysis ................................................................................................................. 223 Delumping tool – Case study................................................................................................. 225 Refrigerant mixtures .............................................................................................................. 229 Polar systems ......................................................................................................................... 230 Modelling a polar mixture ....................................................................................... 230 Liquid-liquid equilibria ........................................................................................... 234 Vapour-liquid-liquid equilibria................................................................................ 235 Azeotropes............................................................................................................... 235 Eutectics .................................................................................................................. 236 Polymers ................................................................................................................................ 237 Data input ................................................................................................................ 237 Co-Polymers............................................................................................................ 239

Case studies - Hydrate dissociation, formation and inhibition

243

Introduction ........................................................................................................................... 243 Defining the hydrate models.................................................................................................. 243 Fluid phase model ................................................................................................... 244 Hydrate model ......................................................................................................... 244 Nucleation model .................................................................................................... 245 Ice model ................................................................................................................. 245 Scale model ............................................................................................................. 245 Phases ...................................................................................................................... 245 Hydrate calculations with Multiflash..................................................................................... 246 Will hydrates form at given P and T ?..................................................................... 246 Hydrate formation and dissociation temperature at given pressure......................... 247 Hydrate formation and dissociation pressure at given temperature ......................... 250 Hydrate phase boundary .......................................................................................... 251 Other flash calculations with hydrates..................................................................... 251 Maximum water content allowable before hydrate dissociation ............................. 251 Calculations with inhibitors ................................................................................................... 252 Can hydrates form at given P and T ?...................................................................... 252 Hydrate dissociation temperature at a given pressure ............................................. 254 Hydrate dissociation pressure at a given temperature ............................................. 254 Hydrate phase boundary .......................................................................................... 254 Amount of inhibitor required to suppress hydrates ................................................. 255 Salt inhibition .......................................................................................................... 256 Scale precipitation ................................................................................................................. 258

Case studies – Wax precipitation

260

Introduction ........................................................................................................................... 260 Defining the wax model......................................................................................................... 260 Calculating wax appearance temperature (WAT).................................................................. 261 Calculating wax precipitation ................................................................................................ 264

Case studies – Asphaltene flocculation

267

Introduction ........................................................................................................................... 267 Input data ............................................................................................................................... 267 Defining the asphaltene model .............................................................................................. 268 Asphaltene matching ............................................................................................... 270 Saturation P at reservoir T ....................................................................................... 272 Calculating asphaltene precipitation conditions .................................................................... 272 Sensitivity of calculations to variation in input data.............................................................. 276 Choice of Analysis method ..................................................................................... 276 Data Availability ..................................................................................................... 277

User Guide for Multiflash for Windows

Contents  ix

No reservoir or precipitation conditions .................................................................. 280 Gas injection .......................................................................................................................... 282 Titration ................................................................................................................................. 283

Case studies – Combined solids

287

Introduction ........................................................................................................................... 287 Asphaltene precipitation ........................................................................................................ 287 Wax and Asphaltene precipitation ......................................................................................... 288 Hydrates, Waxes and Asphaltenes......................................................................................... 289

Case studies – Excel spreadsheets

293

Introduction ........................................................................................................................... 293 UNFACFIT.xls ...................................................................................................................... 293 Notes........................................................................................................................ 294 UNIFAC .................................................................................................................. 294 Activity model worksheets ...................................................................................... 294 VLEFIT.xls............................................................................................................................ 295 Solids.xls................................................................................................................................ 296 PVT Analysis .......................................................................................................... 296 Match bubble point.................................................................................................. 297 Wax ......................................................................................................................... 298 Asphaltenes ............................................................................................................. 299 Asphaltene with gas injection.................................................................................. 300 Hydrates .................................................................................................................. 301

Case study – Mercury partitioning

303

Introduction ........................................................................................................................... 303 Defining the mercury model .................................................................................................. 303 Calculating mercury partitioning and dropout ....................................................................... 304 Other calculations .................................................................................................................. 308 Distribution of mercury species ............................................................................................. 308

Appendix - Multiflash Commands

311

Introduction ........................................................................................................................... 311 Commands ............................................................................................................................. 311 When you may need to use commands.................................................................................. 311 Defining models..................................................................................................................... 312 What the model definition means ............................................................................ 312 Supplying an external file of BIPs ......................................................................................... 313 Defining phase descriptors and key components ................................................................... 314

Index

x  Contents

317

User Guide for Multiflash for Windows

Overview

Introduction Multiflash is a powerful and versatile system for modelling physical properties and phase equilibria. It can be used as a stand-alone program or in conjunction with other software. This manual describes the features of the Multiflash Windows Graphical User Interface (GUI) and explains how it can be used to solve engineering problems.

The Multiflash GUI The Multiflash Windows GUI gives you access to the full capabilities of the program, including: 

All the thermodynamic and transport properties needed for engineering studies.



Comprehensive fluid characterisation and model tuning for petroleum fluids.



Flash calculations to determine the phases present at specified conditions and their type, composition and amounts.



Modelling solids formation, including pure solids, halide scales, hydrates, waxes and asphaltenes.

It is easy to set up all aspects of a study: components, models, units, type of calculation etc. via menu options or tool bar buttons. This configuration can then be saved for future use with the GUI or other applications. Virtually any flash calculation can be carried out irrespective of the number and type of phases present. Complete phase envelopes can be plotted showing phase boundaries and critical points.

Multiflash Software System In addition to the Windows GUI, it is possible to use the Multiflash calculation engine in a variety of ways. There is a Multiflash add-in for Microsoft Excel and an interface for use with Matlab. A CAPE-OPEN physical property package interface allows Multiflash to be used by any application that is CAPE-OPEN enabled. Multiflash may also be used with any software that can call a Windows DLL. We provide support for applications written in various programming languages including C++, Visual Basic and Fortran. Linux applications can also be supported.

User Guide for Multiflash for Windows

Overview  1

Separate documentation is available for each of these interfaces.

Document Organisation The rest of this document is divided into the following sections.

New Features and Changes in Version 4.4 and 4.3 New developments and additions are listed, although more details of how to use new features will be covered in the appropriate section. Information is also given on changes to models or data which may give rise to different results from those obtained with earlier versions of Multiflash. Version 4.3 was an interim internal release of Multiflash.

New Features and Changes in Version 4.2 The developments done in the previous version are listed for reference.

Running Multiflash Each section provides details on different aspects of the software.

Getting Started Describes the different parts of the Multiflash main window and shows how to use the program by running a simple example with step-by-step instructions.

Models This section describes the mixture models available in Multiflash and shows how to define a model. How and when to use models is reviewed, together with the availability and use of model interaction parameters. Detailed specification of the models can be found in a separate manual.

Components The types of components that are available from Multiflash are defined, together with a description of the physical property databanks available. This section also shows how to search for and select components.

Petroleum Fluids Covers a number of topics related to modelling petroleum fluids: how to use the information measured by a PVT laboratory to produce a compositional fluid model; how to define the properties of petroleum fractions (pseudocomponents); how to use experimental data to tune the petroleum fluid model.

Input Conditions The necessary conditions for carrying out different types of calculations are defined, together with how to enter and change these within the program.

Calculations The final step in running Multiflash, the specification of the calculations which can be carried out, and the circumstances where they might be most appropriate are outlined.

2  Overview

User Guide for Multiflash for Windows

Units This section defines the standard working units of the software, the range of options available for input and output units and how to change them.

Output This section reviews the different levels of output available, where output is reported and how it may be saved.

HELP The various types of help available and how to access them are reviewed.

Case studies Additional examples of how to tackle typical problems using Multiflash are provided.

Appendix - Multiflash Commands The Multiflash command language is used to store information about the problem setup (databanks, components, models etc.) in Multiflash .mfl files. Although users should not need to understand the command language it is fully described in the Multiflash Command Reference manual. In some circumstances it may be necessary to use Multiflash commands to configure the software. The Tools menu has a Command option which allows commands to be entered. Those commands that users of the Multiflash GUI may find useful are discussed in the appropriate sections of the User Guide or in the Appendix.

Installation Information on how to install Multiflash software is provided in the separate Installation Guide for Multiflash for Windows.

User Guide for Multiflash for Windows

Overview  3

New Features and Changes in Version 4.4 and 4.3

Introduction As usual the new version includes the results of general maintenance and improvements in numerical methods over the past year, as well as performance enhancements. Specific features are described below.

Models Descriptions and references detailing the models are provided in the User Guide for Models and Physical Properties.

Huron-Vidal-Pedersen mixing rule This mixing rule has been implemented for both the Peng-Robinson and the Redlich-Kwong-Soave equations of state. Binary interaction parameters are included for defined components.

Sutton Model for surface tension. This model has been implemented to allow for a computationally inexpensive way to calculating the surface tension of systems containing water.

LBC It is now possible to specify a critical volume specifically for the LBC model for each component, and to specify the LBC model parameters A1-A5.

Salt component The salt pseudo-component can now be used with the following models: PR, PRA, PRA-Infochem, PR78, PR78A, PR78A-Infochem, RKSA, and RKSAInfochem.

User Guide for Multiflash for Windows

New Features and Changes in Version 4.4 and 4.3  5

High accuracy reference eos for water-ammonia binary system The Tiller-Roth high accuracy corresponding equation of state model for waterammonia binary system is now available.

New high accuracy reference eos High accuracy reference eos for dodecane, DME, R161, R236EA and R236FA are implemented in Multiflash 4.4. For the details, refer to “Multi-reference fluid corresponding states (CSMA) model” on page 35.

Activity Coefficient models For very light components, like N2, O2, etc, the Poynting is disabled. This allows a more correct description of mixtures of these components with heavier ones such as water.

Performance enhancements The cubic equations of state and the CPA equations have been optimized in such a way that from Multiflash 4.3 onwards it is possible to perform flash calculations about 1.5 to 2 times faster than the previous versions of Multiflash.

Phase key components It is now possible to specify multiple key components for phases. It is useful for defining aqueous phases where the amount of water may be small.

Windows GUI PVTSim import tool It is now possible to import characterised fluids from PVTSim if they were exported to a CHC file. This tool accepts systems with aqueous components, and allows the user to define the desired number of phases. It is possible to select an option to define models where the density of the gas phase is calculated using the GERG-2008 model while the rest of the thermodynamic properties are calculated using cubic models.

Models tab The "Select Model Set" tab for Cubic Eos now also has an option where the GERG 2008 model can be used to estimate the density of the vapour phase. The Huron-Vidal-Pedersen mixing rule can be selected for PRA and RKSA. A second liquid hydrocarbon was added to the Mercury model tab.

Inhibitor calculator The salt component is again part of the inhibitor calculator.

6  New Features and Changes in Version 4.4 and 4.3

User Guide for Multiflash for Windows

Surface tension The user can select the MacLeod-Sugden 2 phase variant for the calculation of surface tension. This model can be selected under the MCSA (MCS-Advanced) tab.

Petroleum Fraction Input Table The user can now specify the critical volume to use in the LBC model in the pseudo component generation table. The property has the name VCLBC and is only used in the LBC model. The maximum dimension of the petroleum fraction input table is now extended from 25 to 100.

Tables OLGA tables The OLGA table generator was made more robust and more compliant with the file format accepted by OLGA. The maximum dimension of the temperature and pressure grid is now extended from 50 to 100 for the current version of Multiflash.

Interfaces Excel Interface The Excel-AddIn can now be used in 64 bit versions of Microsoft Office 2010 and later.

CAPE-OPEN Better support for multithreaded applications. Native support for 64bit applications.

Databanks Infodata Saturated liquid surface tension of MEG, TEG corrected.

DIPPR The DIPPR 2013 databank is now available on request.

User Guide for Multiflash for Windows

New Features and Changes in Version 4.4 and 4.3  7

New Features and Changes in Version 4.2

Introduction As usual the new version includes the results of general maintenance and improvements in numerical methods over the past year. Specific features are described below.

Models Descriptions and references detailing the models are provided in the User Guide for Models and Physical Properties.

CSMA model New high accuracy corresponding state models are implemented for the refrigerants: R1234YF and R1234ZE(E) in Multiflash 4.2.

Mercury The mercury model has been extended, so that it can now be used in connection with PR78A and CPA, as well as with RKSA.

Poynting correction The Poynting correction has been modified to give zero correction to the enthalpy and entropy at saturation pressure. The enthalpy, entropy and heat capacity calculated with activity coefficient models with Multiflash 4.2 are therefore different from the results with previous version, but the new pure component values are closer to correlations for saturated liquid Cp.

Activity coefficient model for gas phase A new model, ACG (activity coefficient model for gas phase), has been included in Multiflash to allow the user to calculate thermal properties for the gas phase based on correlations for saturated liquid heat capacity and heat of vaporisation.

User Guide for Multiflash for Windows

New Features and Changes in Version 4.2  9

LBC viscosity model The LBC viscosity model has been fixed to work properly with petroleum fraction with carbon number lower than C7.

Binary Interaction Parameters Several BIPs were added or corrected in Multiflash version 4.2.

CPA model CPA water/Hg, MEG/Hg, correct BIP for ethane/MEG, THF/water, O2+H2O, O2+n-octane

RKSA-Info, CPA and PR H2 + (methane, ethane, propane, n-butane, n-pentane, n-hexane, n-heptane, noctane, n-decane, n-dodecane, n-hexadecane, n-eicosane, n-octacosane, nhexatriacontane, carbon dioxide and water)

RKSA O2+H2O

PRA O2+n-octane

Multiflash phase equilibrium algorithm Make the Pressure-Enthalpy and Pressure-Entropy flashes more robust in particular in the single phase region.

Flash calculations In Multiflash 4.2, the Reid vapour pressure calculation is implemented. The Reid vapour pressure (RVP) is usually employed by refineries to quantify and modify the vaporization of gasolines and other volatile petroleum products. For the details, see the section on “Reid Vapour Pressure” on page 162. When the flash calculation involves solid phases, the errors about not being able to calculate viscosity of a phase were removed. It is not possible to calculate viscosity of solid phases. The Joule-Thompson coefficient has been added as an output property of the flash calculations.

Databanks Infodata New components, R365mfc, R1234YF and R-1234ZE(E) are added into the INFODATA databank. The ideal gas heat capacities of Na+, Cl-, Ca++ and Br- in the databank have been revised. The ions Mg++, Ba++, Sr++, H+, CO3--, HCO3-, OH- and SO4—have been added to the INFODATA databank.

10  New Features and Changes in Version 4.2

User Guide for Multiflash for Windows

Windows GUI PVTSim import tool It is now possible to import characterised fluids from PVTSim if they were exported to a CHC file.

Reid vapour pressure It is now possible to calculate the Reid Vapour pressure via the calculation

Liquid dropout and Wax precipitation curve The liquid dropout and wax precipitation curve tools have been improved to allow more control of the plotting.

New icon for Asphaltene precipitation curve A new tool was added to determine graphically the precipitation of asphaltenes at constant pressure.

Phase envelopes for solids The automatic calculation of the V/L phase envelope as well as solid phase boundaries has been made more robust. It is now possible to right click in area of this plot and to a PT flash.

Hydrate models The RKSA-Infochem model has been removed from the hydrate model selection form, so that it is now only possible to select CPA-Infochem, with or without the electrolyte model. It is possible to load an .mfl file with the RKSA-Infochem hydrate model defined and use it in Multiflash 4.2.

Inhibitor calculator The salt component tab has been removed from the inhibitor calculator.

PVT Analysis The PVT analysis tool has been updated to allow the user to specify density in API degrees.

Retrograde Dew Point A new button was added to the main window to calculate automatically the retrograde dew point at a fixed temperature with having to resort to the Fixed Phase Fraction tool.

Calculation options The calculations options have been simplified. Now only “Normal”, “Upper Retrograde” and “Unspecified” are show for the type of solution where that is necessary.

User Guide for Multiflash for Windows

New Features and Changes in Version 4.2  11

Usability In the forms where is necessary to input data, such as the Matching forms or the PVT Analysis input, the user is warned that those values are lost if the dialog box is closed without performing any operation with the data.

Tables OLGA tables The OLGA table generatas made more robust and more compliant with the file format accepted by OLGA.

Multiflash Excel Interface Joule-Thompson coefficient The Joule Thompson coefficient was added to the list of properties that is possible to get in a flash calculation.

12  New Features and Changes in Version 4.2

User Guide for Multiflash for Windows

Getting Started

Starting Multiflash Start Multiflash by clicking on the Multiflash 4.4 shortcut on the desktop.

Alternatively, from the Windows Start menu choose All Programs and then Multiflash 4. The Multiflash Main Window will be displayed.

Multiflash Main Window

User Guide for Multiflash for Windows

Getting Started  13

Input section The Input section of the Multiflash main window is located below the Toolbar. It comprises the Conditions and Fluid identification widgets.

Conditions The input conditions, such as temperature and pressure, used in Multiflash calculations are shown below the Toolbar. The current units are shown next to each value.

Fluid identification A text box labelled Fluid identification is located to the right of the Conditions section of the main window.

Use of the box is optional but it does allow you to add any comments or notes and, subsequently, to save these as part of the .mfl file. This can be useful for future reference, perhaps for identifying the study and the source of the fluid data, etc. When the file is loaded again any notes will be shown in the Fluid identification box.

Compositions The Compositions button is located below the Fluid identification box. It allows the fluid composition to be entered once components have been selected.

Results window All the phase equilibrium flash calculations, error/warning messages, echoes and results from Multiflash operations are displayed in the main window of Multiflash.

Menu options The menus allow you to control all aspects of running Multiflash. Options are grouped under the main menu headings of File, Edit, Select, Tools, Calculate, Table and Help.

File The File menu controls the loading, saving, clearing and printing of setup files as well as the saving and printing of results.

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User Guide for Multiflash for Windows

In menu items ending with triple dots you will need to define further items, such as the directory and file to be loaded. A dialogue box will be displayed to allow you to do this. The last nine recently-used setup files are listed in the File menu. To load a file from the list, double-click on the file name.

Edit This controls the normal windows editing functions of Cut, Copy and Paste, which can be used on text in the results window.

Select The Select menu option allows you to select the fluid-phase components, components that may appear as pure solids (Freeze-out Components), petroleum fluid characterisation (PVT Lab. Input) physical property models, level of property output, stream types, units of measurement and the use of starting values for calculations. All the menu options except Use Starting Values activate dialogue boxes which are described in later sections of this guide. Items marked with the right pointing triangle contain submenus.

User Guide for Multiflash for Windows

Getting Started  15

Tools The Tools menu groups together options for displaying information about the current problem, utilities for handling data and settings that control the calculations or other aspects of the way Multiflash works.

The Command option can be used to enter Multiflash commands (see the Appendix on page 311). This is not normally necessary but may sometimes be useful for setting options that are not, otherwise, accessible in the Multiflash GUI. Pure Component Data and BIPs options allow you to view and edit the properties of any component in the mixture and any binary interaction parameters being used. The Inhibitor calculator allows you to add water and hydrate inhibitors (alcohols, glycols, salts) in volume, mass or molar units. A salt analysis may be entered in a wider variety of units. The Matching function tunes the properties of petroleum fractions in the mixture to reproduce user-supplied measurements. The quantities for which data may be supplied are: dew points, bubble points/GOR, liquid viscosity and liquid density. The wax model may be tuned to match a wax appearance temperature or precipitation data and the asphaltene model parameters may be tuned to match flocculation or titration data. The Blend Fluids option allows the user to blend (mix) a number of fluids by mass, volume or molar amounts. The Preferences option allows the user to set the default behaviour when Multiflash is started. You can set the preferred units and the calculated properties, the locations of files used by Multiflash, the appearance of the results in the results window and the default models to be used for calculations. All

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User Guide for Multiflash for Windows

these preferences are stored in the Windows Registry. Any changed items will be used the next time that Multiflash is started. Settings such as models and units may be changed at any time during a Multiflash session using the Select menu but this has no effect on the Preferences. The Show option allows you to see the current problem status such as the whole problem description or the models, pure component data source or BIP bank in force.

Calculate The Calculate menu provides a choice of flash calculations. Different types of calculations are grouped together as: Standard Flashes; Bubble and Dew Point Flashes; Fixed Phase Fraction Flashes; (see “Fixed phase fraction flashes” on page 144), the tolerance calculation, (see “Tolerance calculations” on page 161), the phase envelope calculation , (see “Phase Envelopes” on page 148) , phase envelope for solids, special-purpose Hydrate and Wax calculations, liquid dropout, waxes, asphaltene precipitation curve calculations, and Reid vapour pressure calculation.

Table The Table menu is for creating input files for use with other applications, currently PIPESIM, OLGA and Prosper.

See “Interfaces with other programs” on page 183.

Help The HELP menu enables you to get help on a variety of topics, see “Help” on page 191.

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Getting Started  17

The Toolbar The Toolbar provides a quick way of accessing some of the most common operations. Holding the cursor over a toolbar button displays a tool-tip that describes its function. For example, the PT button for the flash at fixed P and T calculation.

Defining a problem in Multiflash This means selecting the components in the mixture and setting their compositions, choosing the models you wish to use to calculate properties and setting the input conditions (e.g. temperature and pressure). The steps are described in detail in the appropriate sections below. In this tutorial we will first use an existing problem file and then go through the steps required to set up a problem from scratch.

Loading an existing problem file Infochem supplies a series of sample problem setup files covering a variety of typical problem types. These can be used as examples when testing the program or can be used as a basis for defining your own setup files. By convention problem setup files for Multiflash have the extension .mfl. The file used for the simple tutorial is C4C5.mfl. The system is 0.4 moles of butane and 0.6 moles of pentane. The model used to describe the system is the Peng-Robinson equation of state with the pure component data taken from the INFODATA databank.

Loading a problem setup file The problem setup file for our tutorial is C4C5.mfl. To load the file: 1.

From the File menu choose “Load Problem Setup”

Or Click on the Load Problem Setup button

on the toolbar.

This will display the file selection dialogue

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which will show a list of available setup files (*.mfl) contained in the problem files directory. By default this is the directory where Multiflash was installed. This directory might be different if earlier versions of Multiflash were used on your computer. After the Multiflash is launched, the Multiflash working directory can be changed from the menu option, Tools/Preferences/General/Folders. 2.

Select C4C5.mfl and click on Open or double-click on the file name. The file will be read by Multiflash and the contents are echoed in the Results window. The file contains the complete definition of the problem including: 

Data sources



Models



Phases



Components



Compositions



Temperature and pressure

The input conditions section of the main window will look like this

Calculations You can now carry out a flash calculation at the specified temperature and pressure by clicking on the PT flash toolbar button

.

The results The results of any calculations are displayed in the Results section of the main Multiflash window.

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Getting Started  19

You may need to scroll the window to see all the results. Size of window can also be changed by clicking and dragging its borders and corners.

Additional calculations Any of the input conditions may be changed by entering new values to overwrite or supplement those shown in the Conditions section of the main window. Simply type the value for the input condition in the appropriate text box, ensure that all necessary input conditions are defined for the flash calculation you wish to carry out and then click on the appropriate toolbar button or select the calculation from the Calculate menu. Compositions for a mixture may be altered by clicking on the composition button and editing the right-hand column of the drop-down table where the amounts of each component are defined. To replace a component or to add new components see “Adding, inserting, replacing and deleting components” on page 69. Some simple changes are shown below:

Change the pressure 1.

Click in the Pressure box of the Conditions panel in the main window and change the pressure to 14e5 Pa.

2.

Click on P,T flash toolbar button or use the Calculate menu and choose Standard Flashes and then P.,T Flash.

3.

The calculation shows that the system is a one-phase liquid under these conditions.

Change the composition 1.

20  Getting Started

Click on the Compositions… button in the Fluid identification section of the main window. The component names are displayed in the first column and the number of moles of each component in the mixture is shown in the second column. Edit the “mole” column of the drop-down table so that there are 9 moles of butane and 1 mole of pentane.

User Guide for Multiflash for Windows

2.

Recalculate the P,T flash as above. The mixture is now two-phase.

Carry out an isenthalpic flash 1.

Enter a value of 1000 in the Enthalpy box.

2.

Click on the P,H flash toolbar button menu.

or use the Calculate

Setting up a new problem To set up a new problem you must enter the following information: 

Data sources



Models



Components



Compositions



Input conditions

Clearing previous problems You can restore all settings in Multiflash to the state when the program was started by selecting the “Clear Problem Setup” option from the File menu.

Defining the components Choose the components for any problem by clicking on Components in the Select menu. Alternatively you can click on the Select Components toolbar button,

. Either will display the Select Components Dialogue box.

The default data source is the Infochem fluids databank which is called Infodata. If you have licensed the DIPPR databank this may be selected from the dropdown list. Components may be selected in a variety of ways e.g. by name, by scrolling through a list or by searching for a formula or substring. The various methods are fully described in “Selecting components” on page 66.

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Getting Started  21

Choose the components needed for the problem, in this case butane and pentane as follows: 1.

Make sure the Name option button is selected.

2.

Click in the box next to “Enter name” and type butane and then press the key or click on the Add button. BUTANE is transferred to the Components selected list. Do the same for pentane.

3.

Click on the Close button, this will return you to the Main Window.

Defining the models To define the models and phases to be used for the calculations choose Model set from the Select menu.

Each tab of this window groups together similar types of model, e.g. cubic equations of state, activity coefficient models and so on. For general advice on which models to choose for a particular application and more information about each model, see “What types of model are available?” on page 29 or consult the “Models and Physical Properties” manual. We will use the Peng-Robinson equation of state: click on PR. You can also change the transport property models and the phases to consider but the default set will usually be appropriate. Click on the Define Model button. The following message should confirm that the models have been successfully defined.

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Click on OK and then Close the Select Model set dialogue to return to the main window.

Set Input Conditions Before a flash calculation can be carried out you must define all necessary input conditions, including the composition of the mixture. All input conditions are specified in the Condition section of the main window.

Compositions To enter the composition click on the Composition button. The drop-down table shows the components in the left-hand column. The amount of each component in the mixture is typed in the right-hand column. The default unit for amounts is moles. Note that the amounts do not have to sum to 1 or 100 or any other value. Click on the Compositions button and enter 0.4 for butane and 0.6 for pentane.

The composition table can be hidden by clicking on the up-arrow button

Pressure and temperature Other input conditions will depend on the type of flash calculation to be carried out, e.g. for an isothermal flash you must enter a pressure and a temperature in the appropriate text boxes and in the units shown next to them. The units may be changed as described in the section “Units” on page 169. Type 9e5 in the Pressure box and 375 in the temperature box.

Carrying out the flash calculation To carry out a flash calculation you either click on the appropriate flash button on the toolbar or select the required flash from the Calculate menu. The most commonly encountered flash options have been allocated toolbar buttons; these are the isothermal flash, dew and bubble points, isenthalpic and isentropic flashes at fixed pressure and fixed phase fraction flashes. Other flashes, such as isochoric flashes or isentropic and isenthalpic flashes at fixed temperature, are specified using the Calculate menu. To calculate the bubble point at the pressure of 9 bar click on To carry out an isothermal flash at 9 bar and 375K click on A short description of the function of each toolbar button is displayed when the cursor is placed over the button.

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Getting Started  23

Other calculations Any of the input conditions may be changed by entering new values to overwrite or supplement those shown in the Input section of the main window. Compositions for a mixture may be altered as described above. Ensure that all necessary input conditions are defined for the flash calculation you wish to carry out and then click on the appropriate toolbar button or select the required flash from the Calculate menu. To replace a component or to add new components see “Adding, inserting, replacing and deleting components” on page 69.

Phase envelope You can plot the complete phase envelope by clicking on the phase envelope button

or selecting Phase Envelope from the Calculate menu.

Click on the VLE AutopPlot button; the vapour-liquid phase boundary will be displayed in a separate window. Click No in response to the message "Maximum number of points reached …" if the phase envelope is completed. If more points along the phase envelope are required, click Yes to complete the envelope.

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User Guide for Multiflash for Windows

The legend can be changed by clicking Options button on the Phase Diagram form. The phase diagram may be edited or printed as described in, ”Customising the phase envelope plot” on page 155. Alternatively it can be exported to Excel (Excel 97 or later).

Saving the problem setup Once you have defined the components and models you can create a problem setup file containing this information for future use. If compositions and other input conditions are set these values will also be saved in the file. To save the setup either click on Save problem setup button,

, or

select “Save Problem Setup” or” Save problem Setup As” from the File menu. The dialogue box allows you to specify the name of the .mfl file and the directory where you want it stored.

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Getting Started  25

If the problem file to be saved is based on an existing file, the default file name given will be the same as the existing one and a warning message will pop up.

Otherwise Multiflash will provide a default file name which can be overwritten. Keep in mind that in order to write files into the default mfl directory you should have a right to do this. If your system administrator deprived you of such a right, it might be useful to copy the entire directory to somewhere in your working space and continue to work with Multiflash in there.

Backup file For any existing MFL files loaded to Multiflash, a backup file with a file extension .MFB will be created if the existing MFL file is overwritten with the changes.

Loading a existing MFL file A warning message is given when loading a saved MFL file to Multiflash to overwrite the current project.

Warning option for matching and PVT form A warning message is given when leaving the form without matching the data from all the matching forms and PVT forms.

Printing the output You can print the output from your calculations by selecting “Print Results” from the File menu or clicking on the Print Results button, This Print dialogue (see below) allows you to select the printer and its settings and print out all the output currently stored in the results window.

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If you only wish to print part of the output you should select the relevant section by highlighting it with the cursor. This time the print range in the print panel shows you the option of printing out only the selected text. Alternatively you can cut or copy the relevant sections and paste them to another application, such as Word.

Saving the output The output from Multiflash is described in detail elsewhere, see “Output” on page 173. All the output from any Multiflash session is automatically stored in a file called MFLASH.LOG in the Multiflash working directory. This file will be re-named the next time the program is started. The names are allocated sequentially as MFLASH_1.LOG, MFLASH_2.LOG, etc., up to MFLASH_9.LOG. If you wish to save the output to another file, select Save Results from the File menu. A dialogue box allows you to choose the file name and directory. The convention is that the extension for output files is .out, but you may alter this if you wish. Note that the amount of output produced by any calculation or problem setup file is limited to 300kb of text. If there is too much output only the last 300kb is displayed on the screen or in the log file.

How to exit the program To exit Multiflash select Exit from the File menu. If the current fluid definition has changed the user is asked if saving is necessary.

User Guide for Multiflash for Windows

Getting Started  27

Technical support For contact information see “Technical support” on page 194.

28  Getting Started

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Models

Introduction This section defines what a model is in terms of the Multiflash nomenclature, which models are available and when you might wish to use them, as well as how to specify and use them in Multiflash. For information on specifying models, see “How to specify models in Multiflash” on page 47. Detailed model descriptions may be found in our separate User Guide for Models and Physical Properties.

What is a model? Within the context of Multiflash, a model is a mathematical description of how one or more thermodynamic or transport properties of a fluid or solid depends on pressure, temperature and composition.

What types of model are available? The key calculation carried out in Multiflash is the determination of phase equilibrium. This is based on the fundamental relationship that at equilibrium the fugacity of a component is equal in all phases. For a simple vapour-liquid system

fiv  fil v

where f i is the fugacity of component i in the vapour phase and fugacity of component i in the liquid phase.

f i l is the

The models used in Multiflash to represent the fugacities in terms of temperature, pressure (and composition) fall into two groups: equation of state methods and activity coefficient methods. The basis of each of these methods is described below. With an equation of state (EOS) method all thermodynamic properties for any fluid phase can be derived from the equation of state. With an activity coefficient method the vapour phase properties are derived from an equation of state, whereas the liquid properties are determined from a combination of models which include a representation of the excess properties. Multiflash may also be used to calculate the phase equilibrium of systems containing solid phases, either mixed or pure. These may occur either when a normal fluid component freezes or may be a particular solid phase such as a hydrate, wax or asphaltene. Models used to represent these solids are discussed below.

User Guide for Multiflash for Windows

Models  29

The transport properties of a phase (viscosity, thermal conductivity and surface tension) are generally derived from semi-empirical models which will be discussed later.

Equation of state models An equation of state describes the pressure, volume and temperature (PVT) behaviour of pure components and mixtures. Most equations of state have different terms to represent the attractive and repulsive forces between molecules. Any thermodynamic property, such as fugacity coefficients and enthalpies, can be calculated from an equation of state relative to the ideal gas properties of the same mixture at the same conditions.

When to use equation of state methods Equations of state can be used over wide ranges of temperature and pressure, including the subcritical and supercritical regions. They are frequently used for ideal or slightly non-ideal systems such as those related to the oil and gas industry where modelling of hydrocarbon systems, perhaps containing light gases such as H2S, CO2 and N2, is the norm. Equation of state methods do not necessarily well-represent highly non-ideal chemical systems such as alcoholwater. For this type of system, at low pressure, an activity coefficient approach is preferable but at higher pressure you may need to use an equation of state with excess Gibbs energy mixing rules, such as RKSA(Infochem). All equations of state will describe any system more accurately when binary interaction parameters (BIPs) have been derived from the regression of experimental phase equilibrium data. BIPs are adjustable parameters that are used to alter the predictions from a model until these reproduce as closely as possible the experimental data. The use of interaction parameters in Multiflash is discussed separately; see “Binary interaction parameters” on page 53. The thermal properties of any fluid phase can be derived from an equation of state. However, one property which is often poorly represented by the simpler equations of state is the liquid density. Multiflash offers enhanced versions of both the Redlich-Kwong-Soave (RKS) and Peng-Robinson (PR) cubic equations of state where the equation of state parameters can be fitted to reproduce both the pure component saturated vapour pressure using a databank correlation and the saturated liquid density at 298K or Tr=0.7 (Peneloux method). These are referred to in Multiflash as the advanced version of the particular equation of state.

Equations of state provided in Multiflash The following equations of state are available in Multiflash.

Ideal gas equation of state This model is normally used in conjunction with an activity coefficient method when the latter is used to model the liquid phase. It could also be used to describe the behaviour of gases at low pressure.

Peng-Robinson equation of state The Peng-Robinson (PR) equation is a cubic equation of state.

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Peng-Robinson 1978 (PR78) equation of state The 1978 revised version of the Peng-Robinson equation has a different treatment for the parameter  . This model removes a defect in the original equation where heavy components with higher acentric factors become more volatile than components with somewhat lower acentric factors. For any mixture containing components with acentric factors greater than 0.49 the PR78 equation will give different results and must therefore be treated as a different model.

Redlich-Kwong (RK) and Redlich-Kwong-Soave (RKS) equations Like Peng-Robinson, the Redlich-Kwong and Redlich-Kwong-Soave equations and their variants are examples of simple cubic equations of state.

Advanced Equation of state options The advanced implementation of both the Peng-Robinson and the RedlichKwong-Soave equations of state (PRA, PR78A and RKSA models) contain additional non-standard features. These include the ability to match stored values for the liquid density and the saturated vapour pressure and a choice of mixing rule.

The Peneloux density correction This correlation is used to match the density calculated from the equation of state to that stored in the chosen physical property data system. For light gases, the density is matched at a reduced temperature of 0.7 and the volume correction is assumed constant. In Multiflash, for liquid components the volume shift is treated as a linear function of temperature; the density is matched at 290.7K and 315.7K so as to reproduce the density and thermal expansivity of liquids over a range of temperatures centred on ambient. However, a third term is available, see the User Guide for Models and Physical Properties, and the user may enter all three coefficients as pure component properties.

Fitting the vapour pressure curve For each component the equation of state a parameter is fitted by linear regression to the vapour pressure over a range of reduced temperatures corresponding to the stored data. Up to 5 coefficients may be used but fewer coefficients will be fitted if there are insufficient data or if the extrapolation to low temperatures is unrealistic. If there is no vapour pressure equation for a component, the standard expression for each equation of state is used.

Mixing Rules The standard mixing rule for the cubic equations of state is the, so-called, van der Walls 1-fluid mixing rule. This is a simple recipe for obtaining the properties of a mixture by combining the pure-component properties. It is a widely used and highly effective method for many non-polar mixtures encountered in the oil and gas industries. For highly non-ideal systems it is often useful to be able to use a Gibbs energy excess model (e.g. UNIQUAC or NRTL) as part of the mixing rule for the equation of state. The possibilities are outlined in the User Guide for Models and Physical Properties.

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Models  31

When to use cubic equations of state The simple cubic equations of state, PR and RKS, are widely used in engineering calculations. They require limited pure component data and are robust and efficient. Both PR and RKS are used in gas-processing, refinery and petrochemical applications. They will usually give broadly similar results, although if one model has been fitted to experimental data and there are no interaction parameters for the other then the optimised model is always preferable. There is some evidence that RKS gives better fugacities and PR better volumes (densities) but both can be improved if the Peneloux correction is used. For most applications we would recommend the use of the RKSA (or PRA, PR78A) model sets which use the Peneloux correction, fit the EOS parameters to match the vapour pressure and use the Van der Waals 1-fluid mixing rules. RKSA with the Infochem mixing rules can be used as part of the hydrate model and provides extra flexibility to represent the highly non-ideal aqueous system. It does, however, require suitable BIPs for such systems. The API variant of RKS is applicable to petroleum systems and mixtures containing hydrogen. The RK EOS may be used instead of the ideal gas model for the vapour phase of systems where the liquid phase is being modelled with an activity coefficient model. Finally, the GUI provides a checkbox that allows the user to use the GERG 2008 model to calculate the density of the vapour phase while estimating the rest of the thermodynamic properties (e.g., fugacity coefficients, vapour pressures, etc.) with the selected cubic Eos.

Cubic plus association (CPA) model The CPA model consists of the Redlich-Kwong-Soave equation plus an additional term based on Wertheim’s theory that represents the effect of chemical association. The CPA model also uses the Peneloux density correction to match the liquid density calculated from the equation of state to that stored in the chosen physical property data system. The volume shift is a linear function of temperature which is set to match the saturated liquid density at two different temperatures. For light gases, a constant volume shift is used that is fitted to the gas’s liquid density at a reduced temperature of 0.7.

When to use CPA. The CPA model is the recommended model for hydrate calculations, or other cases including water, methanol, ethanol, MEG, DEG, TEG and salts. For other (non-polar) components CPA reduces to the RKSA EOS.

PSRK equation of state This model consists of the RKSA equation of state with vapour pressure fitting, the Peneloux volume correction and the PSRK type mixing rules. The excess Gibbs energy is provided by the PSRK variant of the UNIFAC method. This is the same as the normal VLE UNIFAC model except that the group table has been extended to include a large number of common light gases.

When to use PSRK The PSRK model is an extension of the UNIFAC method. It is intended to predict the phase behaviour of a wide range of polar mixtures using the solution of groups concept as embodied in UNIFAC. The main benefit of PSRK is that it is able to handle mixtures containing gases much better than UNIFAC and unlike

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a normal equation of state it can handle polar liquids. This is because (a) it uses an equation of state with an excess Gibbs energy mixing rule thereby avoiding problems of how to handle supercritical components in an activity coefficient equation; (b) the UNIFAC group parameter table has been extended in PSRK to include 32 common light gases.

ZJ (Zudkevitch-Joffe) model The ZJ equation of state model is a variant of the original RK cubic eos. Unlike the original RK, the “a” and “b” parameters are expressed explicitly in terms of the critical temperature and pressure, the “a” and “b” parameters in ZJ eos are defined by simultaneously solving the equations of fugacity coefficients along the saturation line and the equation of pressure for both vapour and liquid phase.

When to use ZJ model The model provides a tool for giving better predictions on enthalpy departures of saturated and compressed liquids, both pure and liquid mixtures with suitable interaction parameters (BIPs). However the BIPs are required in order to use the model as the default BIPs in our databank are not regressed against any experimental data.

PC-SAFT equation of state The PC-SAFT equation is a development of the SAFT model that has been shown to give good results for a wide range of polar and non-polar substances including polymers. Polymers are one of the most important areas of application of PC-SAFT. The model appears to be one of the most accurate and realistic equations of state currently available for modelling polymer systems. PC-SAFT stands for the Perturbed Chain Statistical Associating Fluid Theory and it incorporates current ideas of how to model accurately the detailed thermodynamics of fluids within the framework of an equation of state. The mathematical structure is very complex and cannot be conveniently described in this guide. Further information and references are provided in the User Guide for Models and Physical Properties. The Multiflash version includes an implementation of the association term of PC-SAFT which follows the same general structure as the association term in the CPA model. We also include the dipolar and quadrupolar terms when the dipole moment and quadrupole moments are available. Polymers are not well defined chemical compounds but rather a distribution of chain molecules of varying molecular weight. In Multiflash, polymers must be represented by one or more pseudo components which must be set up as userdefined components. Using PC-SAFT, every pseudo component for a given polymer must be assigned the same values of the pure-compound parameters SAFTSIGMA (in metres, not Ångstrom units) and SAFTEK. In addition, the SAFTM parameter must be specified. This is normally quoted as a ratio to the molecular weight, so it has to be calculated for each polymer pseudo component knowing the molecular weight. For polystyrene, for example, Gross and Sadowski give the ratio as 0.019, so for a polystyrene pseudo component of molecular weight 100000, the SAFTM parameter should be set to 1000000.019=1900, etc. Additionally, the user can define association parameters if the polymer forms hydrogen bonds. These parameters are SAFTBETA which defines the volumetric or entropic parameter, and SAFTEPSILON, the energy or enthalpy parameter. Multiflash also provides an extension to the PC-SAFT definition: so that the user can also supply a heat capacity parameter SAFTGAMMA for the association term. For the association term to be non-zero, the user must also

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Models  33

define the parameter SAFTFF which denotes the number of donor bonding sites per segment of polymer. Values of PC-SAFT parameters for polymers can be found in Modelling Polymer Systems Using the Perturbed-Chain Statistical Associating Fluid Theory Equation of State by Gross and Sadowski in Industrial and Engineering Chemistry Research, 41, 1084, (2002) and in Modelling of polymer phase equilibria using Perturbed-Chain SAFT by Tumakaka, Gross and Sadowski in Fluid Phase Equilibria, 194-197, 541, (2002). Multiflash allows the user to define up to four polymer segments which can be used to define any number of homopolymers or copolymers following the method of Tumakaka, Gross and Sadowski described in the reference above. If the polymer is formed from only one type of segment, it is a homopolymer of that segment; if it is formed of two or more types of segment, it is a copolymer. Multiflash also has a version of PC-SAFT with simplified mixing rules as proposed by researchers at the Danish Technical University. The same pure component parameters can be used for this model variant but the model interaction parameters will be different.

Lee-Kesler (LK) and Lee-Kesler-Plöcker (LKP) equations of state The LK and LKP methods are 3 parameter corresponding states methods based on interpolating the reduced properties of a mixture between those of two reference substances.

When to use LK or LKP The methods predict fugacity coefficients, thermal properties and volumetric properties of a mixture. However, they are rather slow and complex compared to the cubic equations of state and are not particularly recommended for phase equilibrium calculations, although they can yield accurate predictions for density and enthalpy. They would normally be applied to non-polar or mildly polar mixtures such as hydrocarbons and light gases.

Benedict-Webb-Rubin-Starling (BWRS) equation of state The method is an 11 parameter non-cubic equation of state. For methane, ethane, ethylene, propane, propylene, iso-butane, n-butane, iso-pentane, n-pentane, hexane, heptane, octane, carbon dioxide, hydrogen sulphide and carbon dioxide, the pure component parameters are set to values recommended by Starling in his book ‘Fluid Thermodynamic Properties for Light Petroleum Systems’, Gulf Publishing Co., Houston, 1973. For other substances the pure component parameters are estimated using correlations developed by Starling and Han which are given in the same book.

When to use the BWRS equation The BWRS equation is included for the convenience of users that wish to reproduce calculations based on this method. It is not generally recommended. The BWRS equation can give accurate volumetric and thermal property predictions for light gases and hydrocarbons. Given suitable interaction parameters it should give reasonable vapour-liquid phase equilibrium predictions but we do not provide many BIPs in our databank. Owing to its complexity, it requires more computing time than the cubic equations of state.

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Multi-reference fluid corresponding states (CSMA) model The CSMA model is based on a collection of very accurate equations of state for a number of reference fluids. It will provide accurate values of thermodynamic properties for any of the reference fluids (see below for a list) and it uses a 1fluid corresponding states approach to estimate mixture properties. It is formulated so that mixture properties will reduce to the (accurate) pure component values as the mixture composition approaches each of the pure component limits.

Reference fluids The following substances are included in the model: acetone ammonia argon

fluorine helium heptane

benzene iso-butane n-butane 1-butene iso-butene cis-2-butene trans-2-butene carbon dioxide carbon monoxide carbonyl sulphide decane DME dodecane ethane ethanol ethylene

cyclohexane iso-hexane n-hexane hydrogen hydrogen sulphide krypton methane methanol neon nitrogen nitrogen trifluoride nonane octane oxygen iso-pentane neo-pentane

n-pentane propane propylene sulphur dioxide SF6 toluene water xenon R11 R113 R114 R115 R116 R12 R123 R1234YF R1234ZE R124 R125

R13 R134A R14 R141B R142B R143A R152A R161 R218 R22 R227EA R23 R236EA R236FA R245FA R32 R365MFC R41 RC318

The equations of state are taken from various sources and do not all have the same quality or range of applicability. Other hydrocarbons and petroleum fractions are included using a generalised equation of state.

IAPWS-95 The reference equation of state used for water is the IAPWS-95 scientific formulation. It is also available as a separate model option. For water the recommended equations for transport properties have also been implemented.

Water-Ammonia The high accuracy corresponding state model for water-ammonia binary system is based on the work developed by Beiner Tillner-Roth and Daniel G . Friend. The model not only has the high accuracy models for pure water and ammonia but also has terms for correcting the mixing behaviour. The model covers the thermodynamic space between the solid-liquid-vapour boundary and the critical locus, and is also valid in the vapour and liquid phases for pressures up to 40 MPa. It represents vapour-liquid equilibrium properties with an uncertainty of  0.01 in liquid and vapour mole fractions. Typical uncertainties in the singlephase regions are  0.3% for the density and  200 J / mol for enthalpies.

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Reference: Beiner Tillner-Roth and Daniel G . Friend, J. Phys. Chem. Ref. Data,Vol, 27, No. 1, 1998.

Carbon Dioxide high-accuracy model The reference EOS of Span & Wagner and high-accuracy models for the transport properties of carbon dioxide are available as a separate model option.

GERG-2008 The CSMA model also includes the GERG-2004/2008 natural gas model. This is an industry-standard high-accuracy model for mixtures of natural gas components: methane nitrogen CO2 ethane propane n-butane iso-butane

n-pentane iso-pentane hexane heptane octane nonane decane

argon oxygen hydrogen CO2 water helium H2S

The model includes appropriate BIPs for all components in the list. The model is fully described in the publication:, O. Kunz, R. Klimeck, W. Wagner, M. Jaeschke, The GERG-2004 wide-range equation of state for natural gases and other mixtures, GERG Technical Monograph 15 (2007).

Applicability Applications of the model include: acid gas injection; natural gas pipelines and processes; CO2 transport and carbon sequestration; water/steam systems; air; instrument calibration and multi-phase meters. The model performs best for mixtures that do not involve strong specific interactions and for any of the pure substances in the list above. For mixtures, appropriate binary interaction parameters are needed for good accuracy. BIPs are included for the following components: CO2, H2S, methane, ethane, propane, butane, pentane, and water. The mixture model is applicable to systems that do not contain free water. The GERG-2004/2008 model is a well-verified standard. It is probably the best model for natural gas mixtures containing the components listed above.

GERG-2008 (Infochem extension) GERG-2008 has been extended to provide a pseudo reference eos for petroleum fractions or components that the high accuracy eos are not available. With this extension, the model can be used for modelling the fluid phase behaviour of light condensates containing small amounts of residuals or a mixture with some components that the high accuracy eos are not available. For a component or a peseudo-component in a mixture with no high accuracy eos, a four-parameter corresponding-states principle (CSP) model proposed by Sun and Ely(2005) is used to generalize the universal technical Equation of State (EOS). This CSP model implemented in Multiflash is in the form of the Helmholtz free energy, and two non-spherical fluids of propane and octane are used as the reference fluids for nonpolar or weakly polar components. For the details of the method, refer to the publication given below.

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Reference: L. Sun, J.F. Ely, A corresponding states model for generalized engineering equations of state, International Journal of Thermophysics, Vol. 26, No. 3 (2005).

Activity coefficient methods Liquid activity methods are based on the following equation for the fugacity of component i in the mixture

f i l  xi i f i*, l In an ideal solution the liquid fugacity of each component is directly proportional to the mole fraction of the component, ie. the activity coefficient  i is equal to 1 The ideal solution assumes that all molecules interact with the same intermolecular potential. This assumption is reasonable for molecules of a similar size and similar type. However, most real mixtures deviate significantly from ideality and the activity coefficient is different from unity.

Activity coefficient equations in Multiflash A number of activity coefficient equations are available in Multiflash. Details of each model may be found in the User Guide for Models and Physical Properties.

Ideal solution model The ideal solution model may be used when the mixture is ideal, i.e. when there are no mixing effects. It can also be used for single components to calculate some pure component properties from the physical property databank.

Wilson E equation This model may be used for vapour-liquid equilibrium calculations but it is not capable of predicting liquid-liquid immiscibility. Binary interaction parameters are provided in our INFOBIPS bank for some component pairs. If no BIPs are included for your particular mixture then to obtain accurate predictions you must supply binary interaction parameters values in the correct units.

Wilson A equation This model, which is a simplified form of the Wilson E model, may be used for vapour-liquid equilibrium calculations but it is not capable of predicting liquidliquid immiscibility. To obtain accurate predictions you must supply binary interaction parameters (BIP) values, which are dimensionless.

NRTL equation The NRTL model may be used for vapour-liquid, liquid-liquid and vapourliquid-liquid calculations (the VLE option should be used for VLLE). Again if BIP values are not provided in the BIP databank, INFOBIPS, they must be supplied for accurate predictions. In cases where the user does not specify any value for the third adjustable parameter,  ij , it is automatically set to 0.3 if the VLE version of NRTL is specified or to 0.2 if the LLE version is specified. Note that  ij   ji so only  ij need be supplied.

UNIQUAC equation The UNIQUAC model may be used for vapour-liquid, liquid-liquid and vapourliquid-liquid calculations. There are UNIQUAC VLE and LLE variants as for the

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NRTL equation. Again BIP values must be supplied for accurate predictions if they are not included in INFOBIPS. For VLLE the variant chosen should be guided by the BIPs available.

Regular Solution theory Regular solution theory can be used for vapour-liquid calculations for mixtures of non-polar or slightly polar components. The theory is applicable to systems which exhibit negligible entropies and volumes of mixing. However, it has been largely superseded by equations of state.

Flory-Huggins The Multiflash implementation of Flory-Huggins theory includes a correction term. The Multiflash expression reduces to the standard Flory-Huggins theory if all interaction parameters are set to zero. However, to obtain reasonable results it is usually necessary to adjust the values of the interaction parameters to fit the data. Flory-Huggins theory is able to describe systems which include some long chain molecules. It has consequently been applied to model polymer systems but it has been to some extent superseded by other models such as PC-SAFT. However, Flory-Huggins theory still offers the advantages of speed and simplicity.

UNIFAC method The UNIFAC method is similar to UNIQUAC but the interaction parameters are predicted based on the molecular group structure of the components in the mixture. The model is completely predictive and does not require the user to supply BIPs.

Dortmund Modified UNIFAC method For Dortmund modified UNIFAC, the two binary parameters between components are treated as quadratic functions of temperature. Dortmund modified UNIFAC is better able to represent the simultaneous vapour-liquid equilibria, liquid-liquid equilibria and excess enthalpies of polar mixtures than the original UNIFAC method. Like original UNIFAC, however, it does not allow for the presence of light gases in the mixture.

Gas phase models for activity coefficient methods The normal choices for the gas phase model would be the perfect gas equation, the RK equation of state or a virial equation of state. They are described in detail in the "Models and Physical Properties Guide". A second virial coefficient model such as Hayden and O'Connell (HOC) can account for gas phase non-idealities up to pressures of about 5 to 10 bar. The implementation of the HOC model in Multiflash allows the vapour phase association of substances such as acetic acid to be represented.

When to use activity coefficient models Activity coefficient models are usually used to model any combination of polar and non-polar compounds including those exhibiting very strong non-ideality. If the gas phase model is ideal pressures should be limited to 3-5 bar. If RedlichKwong or another equation of state is used to model the gas phase then the pressure limit is higher, of the order of 10-20 bar. However, the mixture should be subcritical. If the mixture contains some components which are above their

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critical point, such as dissolved gases, then you should be aware that the properties of such components, e.g. vapour pressure, will be extrapolated. NRTL, UNIQUAC and UNIFAC may be used to model VLE, LLE and VLLE but both Wilson models are limited to VLE only. To obtain accurate predictions from any of the activity coefficient models, except UNIFAC, it is necessary to use interaction parameters. If these are not available in INFOBIPS for your system then you need to supply interaction parameters for the missing values which match the model specification given above and which are in the correct units. The parameters in Infobips are in standard SI units, Jmol-1, except for Wilson A where the BIPs are dimensionless. The user can supply BIPs in other units provided these units are specified correctly.

Models for solid phases Multiflash may also be used to calculate the phase equilibrium of systems containing solid phases, either mixed or pure. These may occur either when a normal fluid component freezes or may be a particular type of solid phase such as a hydrate.

Solid freeze-out model This model is used to calculate the thermodynamic properties of pure solid phases formed by freezing one or more of the components in the fluid mixture. It may be applied to any component where this may be a consideration. Solid freeze-out can be used to model the solidification of compounds such as water, carbon dioxide or methane, for example in natural gases. It can also be used to model eutectics.

Scaling and general freeze-out model In its general form, the freeze-out model can be applied to any solid phase of fixed composition, which must be defined. The model can for example be applied to hydrated salts such as monoethylene glycol (MEG) monohydrate or to crystalline mineral salts, i.e. scales. The model is available as part of the hydrates module where it may be applied to halide scales.

Modelling hydrate formation and inhibition Natural gas hydrates are solid ice-like compounds of water and the light components of natural gas. They form at temperatures above the ice point and are therefore a serious concern in oil and gas processing operations. The phase behaviour of systems involving hydrates can be very complex because up to seven phases must normally be considered. The behaviour is particularly complex if there is significant mutual solubility between phases, e.g. when inhibitors or CO2 are present. Multiflash offers a powerful set of thermodynamic models and calculation techniques for modelling gas hydrates.

Hydrate model The recommended hydrate model is based on the CPA equation of state for the fluid phases plus the van der Waals and Platteeuw model for the hydrate phases. The original Infochem model is also available. It is based on a modification of the RKS equation of state (RKSAINFO) for the fluid phases. The two models usually perform in a very similar way. The models also represent the inhibition

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effects and partitioning between phases of the common hydrate inhibitors: methanol, ethanol, MEG, DEG, TEG and salts. The main features of the models are: 

Our description of hydrate phase behaviour uses a thermodynamically consistent set of models for all phases present including hydrate structures I, II and H, ice, water, liquid and gas. The vapour pressures of pure water and sublimation pressures of ice are very accurately reproduced.



The following natural gas hydrate formers are included: methane, ethane, propane, isobutane, butane, nitrogen, CO2 and H2S.



Other hydrate formers that are not usually present in natural gas but which form structure I or II are also included. These compounds are: SF6 , ethylene, propylene, cyclopropane, oxygen, argon, krypton, xenon and THF.



Parameters are provided for the following compounds that form hydrate structure II in the presence of small ‘help-gases’ such as methane or nitrogen: cyclopentane, benzene and neopentane. These compounds and the structure H formers listed below may be present in condensate and oil systems.



Structure H hydrates form in the presence of small ‘help-gases’ such as methane or nitrogen but the formation temperatures are significantly higher (about 10 K) than pure methane or nitrogen hydrate. In practice it seems that structure II hydrates form before structure H but, if there is enough water, structure H may be formed too. The structure H model includes parameters for: isopentane neohexane 2,3-dimethylbutane 2,2,3-trimethylbutane 2,2-dimethylpentane 3,3-dimethylpentane methylcyclopentane methylcyclohexane cis-1,2-dimethylcyclohexane 2,3-dimethyl-1-butene 3,3-dimethyl-1-butene cycloheptene cis-cyclooctene adamantane ethylcyclopentane 1,1-dimethylcyclohexane ethylcyclohexane cyclohexane cycloheptane cyclooctane

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

The thermal properties (enthalpies and entropies) of the hydrates and ice are included allowing isenthalpic and isentropic flashes involving these phases.



Calculations can be made for any possible combination of phases including cases without free water. No modification of the phase models is required to do this.



The properties of the hydrates have been fixed by investigating data for natural gas components in both simple and mixed hydrates to

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obtain reliable predictions of structure I, structure II and structure H hydrates. 

The properties of the empty hydrate lattices have been investigated and the most reliable values have been adopted.



Proper allowance has been made for the solubility of the gases in water so that the model parameters are not distorted by this effect. This is particularly important for carbon dioxide and hydrogen sulphide which are relatively soluble in water.



Correct thermodynamic calculations of the most stable hydrate structure have been made.

The model is used to calculate the conditions (temperature or pressure) corresponding to the experimental determination of the hydrate dissociation point. This is equivalent to the conditions where the first very small quantity of hydrate appears after a sufficiently long time: the thermodynamic formation point. Before the thermodynamic formation point is reached hydrate cannot form - this point is also called the stability limit. Beyond the stability limit hydrate can form but may not do so for a long time. The model has been tested on a wide selection of open literature and proprietary experimental data. In most cases the hydrate dissociation temperature is predicted to within ±1K.

Hydrates in water sub-saturated systems Hydrates can form even in systems where there is no free water present. Our hydrate model is capable of predicting this phenomenon, although the data available for validating the results are very limited. What we have noticed is that for systems with very little water and at high pressures the predicted hydrate dissociation temperatures using RKSAINFO and CPA tend to diverge with increasing pressure, with CPA predicting lower hydrate dissociation temperatures than RKSAINFO. There are no data presently available to confirm which is correct. We can provide alternative model parameters if it required to change the behaviour of either model.

Nucleation model The nucleation model was developed in collaboration with BP as part of the EUCHARIS joint industry project. This model is an extension of the existing thermodynamic model for hydrates described above. In order to extend the nucleation model for use with Multiflash, the following enhancements to the nucleation model were made: 

The model was extended to cover the homogeneous nucleation of ice and fitted to available ice nucleation data.



The model was generalised to cover, in principle, nucleation from any liquid or gas phase.



A correction for heterogeneous nucleation was included that was matched to available hydrate nucleation data.



An improved expression was adopted for fluid diffusion rates.



More robust numerical methods were introduced into the program.

The nucleation model provides an estimate of the temperature or pressure at which hydrates can be realistically expected to form. The model is based on the statistical theory of nucleation in multicomponent systems. Although there are limitations and approximations involved in this approach it has the major benefit that a practical nucleation model can be incorporated within the framework of a traditional thermodynamic hydrate modelling package.

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Comparisons of model predictions with experimental data have been made where possible. In general measurements of hydrate nucleation result in an experimental error of  2ºC and predictions are usually within this error band. With the combination of the hydrate model and the nucleation model it is possible to predict the hydrate risk area which lies between the formation and dissociation boundaries.

Inhibitor modelling Thermodynamic hydrate inhibitors decrease the temperature or increase the pressure at which hydrates will form from a given gas mixture. The CPA-based hydrate model includes parameters for water with methanol, ethanol, MEG, DEG, TEG and salts. The RKSAINFO-based model also includes parameters for some less-common inhibitors: iso-propanol, propylene glycol and glycerol. The treatment of hydrate inhibition has the following features: 

The model can explicitly represent all the effects of inhibitors including the depression of the hydrate formation temperature, the depression of the freezing point of water, the reduction in the vapour pressure of water (i.e. the dehydrating effect) and the partitioning of water and inhibitor between the oil, gas and aqueous phases.



The model has been developed using all available data for mixtures of water with methanol, ethanol, MEG, DEG and TEG. This involves representing simultaneously hydrate dissociation temperatures, depression of freezing point data and vapour-liquid equilibrium data.



An electrolyte model is available in Multiflash, and a salinity calculator tool is provided (see “Salt calculator” on page 81) which allows the salt composition to be entered in a variety of ways. The Electrolyte model includes the ions: Na+, K+, Ca++, Cl- and Br-, and the salinity calculator can be used to convert a water analysis that includes other ions into an equivalent amount of Na+, K+, Ca++ , Cl- and Br-, or salt pseudo-component.



The solubility of hydrocarbons and light gases in water/inhibitor mixtures has also been represented.

Modelling wax precipitation Waxes are complex mixtures of solid hydrocarbons that freeze out of crude oils if the temperature is low enough. Under conditions of interest to the oil industry, waxes consist mainly of normal paraffins. Waxes are thought to consist of many crystals each of which is a solid solution of n-paraffins of a fairly narrow range in molecular weight. The wax model in Multiflash was developed by Coutinho. The main features of the model are: 

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It represents wax as a solid solution. There are two versions of the model, the Wilson and UNIQUAC variants. The version normally selected in Multiflash is the Wilson model which approximates the wax as a single solid solution. This approach is relatively simple to apply and gives a good representation of the data, so it is recommended for general engineering use. The more complex UNIQUAC variant models the tendency of waxes to split into several separate solid solution phases. The UNIQUAC variant can be activated by configuration files that can be

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supplied by Infochem for users who wish to simulate the detailed physical chemistry of wax precipitation. 

It gives good predictions of waxing behaviour, both wax appearance temperature and the amount of wax precipitated at different temperatures. The method is applicable to both live and dead oils.



It requires that the normal paraffins are explicitly present in the fluid model, as these are the wax forming components. It is essential to use the Infochem petroleum fluid characterisation procedure to enter the measured n-paraffin concentrations or else to estimate the n-paraffin distribution. The composition of the wax phase is determined by the known thermal properties (normal melting point, enthalpy of fusion, etc.) of the n-paraffins combined with their solution behaviour in both oil and wax phases.



In principle the wax model can be used in conjunction with any conventional cubic equations of state. The default option in the Multiflash implementation is RKSA.

Modelling asphaltene flocculation Asphaltenes are polar compounds that are stabilised in crude oil by the presence of resins. If the oil is diluted by light hydrocarbons, the concentration of resins goes down and a point may be reached where the asphaltene is no longer stabilised and it flocculates to form a solid deposit. Because the stabilising action of the resins works through the mechanism of polar interactions, their effect becomes weaker as the temperature rises, i.e. flocculation may occur as the temperature increases. However, as the temperature increases further the asphaltene becomes more soluble in the oil. Thus, depending on the temperature and the composition of the oil, it is possible to find cases where flocculation both increases and decreases with increasing temperature. The Multiflash asphaltene model is a variant of CPA, where the association term is used to describe the association of asphaltene molecules and their solvation by resin molecules. The interactions between asphaltenes and asphaltenes-resins are characterised by two temperature-dependent association constants: K AA and K AR . The remaining components are described by the van der Waals 1-fluid mixing rule with the usual binary interaction parameters kij so the asphaltene model is completely compatible with existing engineering approaches that work well for describing vapour-liquid equilibria. The model is a computationallyefficient way of incorporating complex chemical effects into a cubic equation of state. In the following publication the CPA based asphaltene model in Multiflash is compared to a PC-SAFT approach from the literature: Zhang, X., Pedrosa, N. and Moorwood, T., Modeling asphaltene phase behaviour: Comparison of methods for flow assurance studies, Energy Fuels, 26 (5), 2011.

Other thermodynamic models Multiflash also includes the COSTALD a corresponding states method for estimating the density of liquid mixtures. The COSTALD method can provide very accurate volumes for pure substances and simple mixtures, such as LNG. It is valid for liquids on the saturation line and compressed liquids up to a reduced temperature of 0.9. It is not necessarily accurate for heavy hydrocarbon mixtures with dissolved gases.

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Transport property models For each of the transport properties, viscosity, thermal conductivity and surface tension, Multiflash offers several approaches to obtaining values for mixtures. One route is to calculate the property for a mixture by combining the property values for the pure components of which it is composed; the mixing rule approach. The other is to use a predictive method suitable for the property in question.

Viscosity SuperTRAPP Model The SuperTRAPP method is a predictive extended corresponding states model that uses propane as a reference fluid. It can predict the viscosity of petroleum fluids and well-defined components over the entire phase range from the dilute gas to the dense fluid. The Infochem implementation of SuperTRAPP model includes modification to ensure that the viscosity of aqueous solutions of methanol, ethanol MEG, DEG and TEG or salts and ions are predicted reasonably well. Overall the SuperTRAPP method is the most versatile method for viscosity predictions and its performance is generally better than the other methods available in Multiflash. We would recommend this method for oil and gas application. It is the default viscosity model for use with equations of state. Reference: Huber, M. L. & Hanley, H.J.M. (1996) The corresponding-states principle: Dense Fluids. In J. Millat, J. H. Dymond & C. A. Nieto (Eds.), Transport properties of Fluids: Their correlation, Prediction and Estimation. Cambridge University Press.

Pedersen Model This is a predictive corresponding states model originally developed for oil and gas systems. It is based on accurate correlations for the viscosity and density of the reference substance which is methane. The model is applicable to both gas and liquid phases. The Infochem implementation of the Pedersen model includes modifications to ensure that the viscosity of liquid water, methanol, ethanol, MEG, DEG and TEG and aqueous solutions of these components or salt are predicted reasonably well. We would recommend this method for oil and gas applications. Reference: Pedersen, Fredenslund and Thomassen, Properties of Oils and Natural Gases, Gulf Publishing Co., (1989).

Twu Model This is a predictive model suitable for oils. It is based on a correlation of the API monograph for kinematic viscosity plus a mixing rule for blending oils. It is only applicable to liquids. Reference: Twu, Generalised method for predicting viscosities of petroleum fractions, AIChE Journal, 32, 2091, (1986).

Lohrenz-Bray-Clark method This model is a predictive model which relates gas and liquid densities to a fourth degree polynomial in reduced density. In Multiflash the fluid densities are derived from any chosen equation of state, rather than the correlations proposed by Lohrenz et al. This has the advantage that there is no discontinuity in the dense phase region when moving between liquid-like and gas-like regions.

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Multiflash also allows two variants of the LBC model. The first uses the original LBC method to estimate the critical volume of petroleum fractions and takes the critical volume of other components from the chosen data source. The second variant fits the critical volume of each component to reproduce the liquid viscosity at the boiling point. The method is mainly applicable to the types of components found in oil and gas processing but we would recommend that the SuperTRAPP method is normally used in these cases.

Liquid viscosity mixing rule This method obtains the liquid mixture viscosity by applying a simple mixing rule to the pure component saturated liquid viscosities generated from a databank. Each component in the mixture must have a liquid viscosity correlation stored in the databank.

Vapour viscosity mixing rule The viscosity of a gas mixture at low density is calculated from the databank correlations for the zero pressure gas viscosities of the pure components. Each component in the mixture must have a vapour viscosity correlation stored in the databank.

Thermal conductivity SuperTRAPP thermal conductivity method The SuperTRAPP method is an extended corresponding states model that uses propane as a reference fluid. It is applicable to both gas and liquid phases. The model can be used for petroleum fluids and well-defined components. The thermal conductivity is defined as the sum of internal and translational contributions. The latter are divided into three contributions: dilute gas, residual and critical enhancement. The Multiflash model does not include a critical enhancement term. For pure substances this can result in under-prediction of the thermal conductivity near the critical region. However for a mixture the critical enhancement is usually very small and negligible. The performance of the Super TRAPP method is generally better than the CLS method.

Chung-Lee-Starling thermal conductivity method This is a predictive method for both gas and liquid mixture thermal conductivities. It requires the critical properties, Tci , Vci and  ci for non-polar components. For polar and associating fluids the dipole moment and an association parameter are also required. Association parameters for water, acetic acid and the lower alcohols are provided. The fluid density is required as part of the calculation and this quantity may be obtained from any of the thermodynamic models in Multiflash. This method can be used for oil and gas processing and also for polar mixtures.

Liquid thermal conductivity mixing rule This method obtains the liquid mixture thermal conductivity by applying a simple mixing rule to the pure component saturated liquid thermal conductivities generated from a databank. Each component in the mixture must have a liquid thermal conductivity correlation stored in the databank.

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Vapour thermal conductivity mixing rule The thermal conductivity of a gas mixture at low density is calculated from the correlations for zero density gas thermal conductivity of the pure components at the same temperature.

Surface Tension Linear Gradient Theory model The Linear Gradient Theory model uses the properties of the phases in equilibrium to determine the interfacial tension. The key property is the density gradient that exists across the interface. With this model it is possible to estimate the interfacial tension between Liquid/Gas and Liquid/Liquid phases. It can be used in combination with the any EOS-based fluid model except: LKP, CSMA; the asphaltene model. Reference: Zuo, Y. X. and Stenby, E. H., A Linear Gradient Theory Model for Calculating Interfacial Tensions of Mixtures, Journal of Colloid & Interface Science, 182 p12, Elsevier (1996).

Macleod-Sugden method (MCS). This method predicts the surface tension (liquid-vapour) of a mixture based on the pure component parachors stored in a databank. It is mainly applicable to the types of component found in oil and gas processing. In this implementation the vapour phase is described by the ideal gas equation. Reference: Pedersen, Fredenslund and Thomassen, Properties of Oils and Natural Gases, Gulf Publishing Co., (1989).

Macleod-Sugden 2-phase variant (MCSA). This method predicts the surface tension (liquid-vapour) of a mixture based on the pure component parachors stored in a databank. It is mainly applicable to the types of component found in oil and gas processing. In this implementation the gas phase is described by the selected model for the gas phase (instead of the ideal gas equation) and therefore is more accurate than the 1-phase variant. Reference: Pedersen, Fredenslund and Thomassen, Properties of Oils and Natural Gases, Gulf Publishing Co., (1989).

Surface tension mixing rule The surface tension for a liquid mixture may be calculated from the correlations for the surface tension of the pure saturated liquids at the same temperature and pressure using a power law model.

Diffusion coefficient Fuller method The Fuller method calculates gas phase diffusion coefficients. It is an empirical modification of Chapman-Enskog theory.

Hayduk-Minhas method The Hayduk-Minhas method calculates liquid phase diffusion coefficients. It consists of a number of empirical correlations for different classes of mixture.

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User Guide for Multiflash for Windows

How to specify models in Multiflash Using the menu From the Select menu choose Model Set. The models selection dialogue is shown below.

Under each tab are listed similar types of models or models for different applications. Unless you have already set other default model preferences (see below) the Cubic EoS tab is displayed with RKSA as the default model. The default transport property models are shown. Other models can be selected or the transport property calculations can be turned off by selecting None. Diffusion coefficient models can be added if required. For most model choices up to four phases may be considered. These are: gas, liquid1, liquid2 and water. You can limit the number of phases to be considered. For example, you may know that your problem has only a gas and one possible liquid phase. In this case deselecting liquid2 and water may speed up calculations and make the phase equilibrium easier to solve. Once you are satisfied with your model selection click the Define Model button and you will see a message to confirming your choice.

Click OK and then Close to return to the main Multiflash window.

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Loading a model from a .mfl file When you load an .mfl file that includes model definitions you do not need to go through the Select Model Set process unless you wish to change details of the models or phases. The options shown in the Select Model Set window reflect as closely as possible the contents of the .mfl file loaded but it is possible to have options in the file that do not correspond to options that can be displayed.

Setting up preferred models in Multiflash If you wish to have Multiflash always start up with your preferred model selection you can do this by selecting Preferences/Models from the Tools menu. The options are as shown for Select/Model Set above. The preferred set of models and phases is stored in the Windows Registry and will be in force the next time you start Multiflash. You can still select any other models as required.

How to change a model You may wish to compare the predictions from one model with those from another. This is easily achieved. Selecting a new model set will remove all existing model-related information including: thermodynamic models; transport property model; phases and BIPs. For example, all you need to do to change from using RKSA to Peng-Robinson: open the Select Model Set dialogue from the Select menu and click on PR. This action will remove the previous model definition and load a new one and you are ready to repeat your earlier calculation with a new model or define a new problem. Similarly you may change the current methods for calculating the transport properties.

Models for solid phases Hydrates From the Select menu choose Model Set and then click on the Hydrates tab.

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The fluid model used in conjunction with the solid phase hydrate model is CPAInfochem. The model includes parameters to describe the effect of thermodynamic hydrate inhibitors on hydrate formation and the partitioning of the inhibitor between aqueous, gas and oil phases. The inhibitors include: methanol, ethanol, MEG, DEG and TEG. To model the effect of salts it is necessary to select the “CPA-Infochem + Electrolyte” model, which combines the electrolyte model in Multiflash with the CPA-Infochem model. The model is described in the section “Inhibitor modelling” on page 42. The default setting is to include models for hydrate structures: Hydrate 1 and Hydrate 2. In the majority of cases for natural gases, condensates or oils Hydrate 2 is the most stable hydrate form. However, there are some fluids, with certain compositions such as those with high concentration of methane or H2S, where the most stable hydrate structure can change from Hydrate 2 at low pressures to Hydrate 1 at high pressures. Hydrate structure H is unlikely to be present in most practical situations but may be selected if required. Calculation of the hydrate dissociation temperature and pressure will be quicker if you exclude the hydrate nucleation option. However, if you want to include the nucleation model select Phase Nucleation from the list of phases. With the electrolyte model it is possible to allow for the formation of Halide scales. If this option is selected then the appropriate phases will be added automatically, based on the component formula, e.g. NaCl, NaCl.2H2O, KCl, CaCL2, CaCl2.2H2O, CaCl2.4H2O, CaCl2.6H2O, NaBr, NaBr.2H2O, KBR or CaBr2.6H2O. The Halide Scales option will only be available if the Electrolyte model is selected and will slow down calculations so should only be used if solid formation is suspected.

Pure solid phase The freeze-out model can be applied to describe any pure solid component such as ice, CO2 etc. It may ne used in conjunction with any model set for the fluid phases but most commonly will be used with one of the cubic EoS. From the Select menu choose ‘Freeze-out Components’. This will produce a window displaying the components in the stream, e.g.

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Checking, or unchecking, the box next to each name allows you to add or remove the freeze-out model for that component. Each solid component may form a separate phase. There is a limit (20) on the total number of (fluid and solid) phases that may be considered. A message box will appear to confirm that the model has been defined (or removed) for the selected compound, e.g.

The name assigned to each pure solid phase is “SOLID” plus the compound name, e.g. ‘SOLIDEICOSANE’ (except in the case of water where it will be ICE).

Waxes The wax model in Multiflash is the Coutinho model, described in “Modelling wax ” on page 42. To define the wax model select the Waxes tab. Fluid phases are represented by the RKSA model. For more information on how to use the wax model see “Case studies – Wax precipitation” on page 260.

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The default phases include water. This may be deselected if no water is present. The transport property models apply to the fluid phases. Click on Define Model and then Close.

Asphaltenes To define the asphaltene model select the ‘Asphaltenes’ tab. Fluid phases are represented by the RKSA model. Water is not one of the default phases but may be added if required. Click on Define Model and then Close. For more information on how to make best use of the asphaltene model see “Case studies – Asphaltene flocculation” on page 267.

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Combined Solids Model The Combined Solids tab allows you to specify any combination of wax, hydrate and asphaltene solid phases. The fluid phase model is RKSA-Infochem and the effect of salts on hydrates is represented by the electrolyte model.

If you define only one type of solid you will be asked to use the individual solid model tab.

Troubleshooting - models Although each version of Multiflash is thoroughly tested it is always possible that you may come across problems. Please report any errors to us so that they can be investigated and corrected. There are other problems related to using Multiflash that can be resolved by the user. Some of those related to models are discussed below; others will be outlined in the relevant section.

Model is not available If a model on a Select Model Set tab is greyed out this usually means that it is an optional add-on and has not been licensed. You may not have licensed all the possible model options. The optional models include: hydrates; wax; asphaltenes; mercury; PC-SAFT; CSMA; IAPWS-95 and the CO2 High Accuracy model.

Groups not available for UNIFAC model The UNIFAC model generates the binary interaction parameters from group contributions. Although the majority of components in both the INFODATA and DIPPR databanks can be constructed from UNIFAC groups this is not possible for all components. Complex cyclic components are typical examples. If this is the case then you will see a warning message

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and in the main window *** WARNING

-13201 ***

Missing UNIFAC structure for one or more components However, if you define the model first this warning message will not be generated until you have defined components and tried to carry out a calculation. If you do encounter a situation where standard UNIFAC groups are missing, particularly for light gases, you should try loading the PSRK model. PSRK is a variant of UNIFAC with additional structural groups. If there are missing groups then you should note that because UNIFAC is the liquid phase model, if you try to carry out flashes involving the liquid phase, e.g. dew and bubble point calculations the flash will fail. However, the gas phase for the UNIFAC VLE model is defined with a separate gas phase model and isothermal flashes will apparently work with the stream being reported as all gas. It is possible to add user-defined UNIFAC groups, although there are no specific menu options. You would have to enter commands through the command box. Information on the commands can be found in the Multiflash Command Reference Manual.

Binary interaction parameters Binary interaction parameters (BIPs) are adjustable factors that are used to alter the predictions from a model until these reproduce as closely as possible the experimental data. BIPs are usually generated by fitting experimental VLE or LLE data to the model in question. For the UNIFAC and PSRK models, BIPs are predicted by group contribution methods and BIPs apply between pairs of components. However, the fitting procedure may be based on both binary and multi-component phase equilibria information, the former being the most common.

BIPs and models The more a BIP varies from its default value, the greater the adjustment required to make the underlying model fit measured data. For some models the BIPs have some physical significance but they are usually treated as empirical adjustment factors. Different models also require different numbers of BIPs. The cubic equations of state (RK, RKS, RKSA, PR, PR78, PRA. PR78A, ZJ) require only a single BIP that may be constant or linear or quadratic function of temperature. The default value is zero. A BIP will usually have a small positive or negative value and the magnitude is usually much less than 1. For LK, LKP and CSMA the default value of the interaction parameter is 1. When non-standard mixing rules are used, e.g. when using RKSA(Infochem), then the number of BIPs increases. For the Gibbs excess energy type mixing rules (MHV2-type and Huron-Vidal-type) the number of BIPs will be determined by the activity coefficient model used to describe the liquid phase.

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For the RKSA-Infochem mixing rule or the Huron-Vidal-Pedersen (HVP) 3 BIPs are needed. It is important to note that the HVP mixing rule can work with two different types of BIP sets. For information on how to indicate which BIP set is being used, please refer to the User Guide for Models and Physical Properties. PC-SAFT requires two symmetric BIPs, in addition to the pure component model parameters. The default value for both is zero. CPA uses the same interaction parameters as RKSA with the addition of three association parameters to describe cross association and self association where this occurs. Activity models generally reduce to the ideal mixing model when all BIPs are zero (the default value). The Wilson A and UNIQUAC models require two BIPs; Wilson E needs the same number of BIPs, but these are not interchangeable with Wilson A. NRTL needs two binary energy interaction parameters and a nonrandomness factor,  . With the exception of  , these BIPs may take a wide range of numerical values and more that one set may adequately represent the same experimental data. The BIPs for the activity models are asymmetric and it is important to define the binary pair of components i and j in the correct order to agree with the fitted or reported BIPs. The NRTL  parameter defaults to 0.3 for VLE calculations and 0.2 for LLE calculations. Values derived from fitting to experimental data will vary but are unlikely to be much greater than 0.6. The Regular Solution and Flory Huggins models both use a single symmetric BIP with a default value of zero.

Temperature dependence of BIPs In most of the open literature sources the reported binary interaction parameters will be temperature independent, i.e. constants. However, Multiflash allows for any BIP to be temperature dependent with either a linear or a quadratic dependence, although we do not recommend this for the NRTL  parameter.

BIPs available in Multiflash Our main BIP banks are applicable to oil and gas processing operations and called appropriately, OILANDGAS and INFOBIPS. INFOBIPS contains all the BIPs for the cubic equation of state models PR, PRA, PR78, PR78A, PRA-HVP, RKS, RKSA, RKSA-HVP, RKSA (Infochem) and CPA for hydrocarbons, water, methanol, glycols, H2S, CO2 and N2. It also includes BIPS for WilsonE and the VLE variants of NRTL and UNIQUAC, based on the data reported in the Dechema Chemistry Data Series. Other BIPs are also included for models such as CSMA, GERG-2008, BWRS and LKP. These values are generally for particular mixtures not covered in the standard correlations. OILANDGAS contains BIP correlations that can be used to estimate BIPs for the PR, PR78, RKS, CPA and LKP models for hydrocarbon and light gas mixtures that include petroleum fractions. A BIP bank, INFOLLBIPS, stores BIPs for use with the LLE variants of NRTL and UNIQUAC. For the UNIFAC model, BIPs are predicted from group contributions. The data records for pure components in the INFODATA databank contain information on UNIFAC groups, where applicable, enabling the BIPs to be generated. Groups for use with the PSRK and Dortmund Modified UNIFAC models are also stored as part of the pure component UNIFAC record. Where the groups

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vary, Multiflash applies the correct group structure to match the chosen model. We have created an Infochem version of the DIPPR databank that includes the same group information. You will see a warning message if the group contributions are missing for any chosen component. It is possible to have two BIP banks in force for any problem. When an equation of state model is defined Multiflash will first search INFOBIPS and then OILANDGAS. If any BIPs are still missing they will be set to default values. The VLE variants of the activity methods only access INFOBIPS, but the LLE variants access INFOLLBIPS, followed by INFOBIPS.

Viewing BIP values You can look at the values of any BIPs used in Multiflash calculations, including those from the INFOBIPS databank and any supplied by the user. To do this Define your model and mixture or load a problem setup file. From the Tools menu select BIPs. The Show BIP Values window will be displayed. The following example shows BIPs for decane and water for the RKS model.

A BIP dataset is assigned when the model is defined. The number of BIP datasets listed depends on the models that use BIPs and the number of BIPs for the model (see below). In the above example RKSBIP identifies the BIPs for the RKS EoS and LGSTBIP identifies the BIPs for the LG surface tension model. Select the RKSBIP dataset and click on Edit to view or change BIP values or the temperature dependence of the BIPs.

The Write to Output button displays the information in the results window: show bipset RKSBIP; BIPSET: RKSBIP COMPS ORDER VALUES 1 2 0 TEMPERATURE FUNCTION: EOS

0.5055762 UNITS: none

The output includes the name of the BIP set. The Infochem convention is to use the name of the model followed by BIP, e.g. RKSBIP, PRBIP, PR78BIP, PR78ABIP, RKSABIP, RKSABIP3 (for RKSA + Infochem mixing rule ), LKBIP, LKPBIP, WILSONBIP2, NRTLBIP3, UNIQUACBIP2 for VLE

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versions of the model, NRTLLLEBIP3, UNIQUACLLEBIP2 for the LLE implementations. If the BIP name is followed by a digit, this indicates the number of BIPs for the model or no digit means that there is only one BIP for the model. For the CPA model the bipset name is ASSOCBIP, while the PC-SAFT BIPs are designated by SAFTBIP and SAFTBIP-2. Regular Solution and Flory Huggins BIPs are named REGULARBIP. The components are referred to by the number they are assigned in Multiflash, i.e. the sequence in which they appear in the components list. The ORDER is the degree of the temperature dependence of the BIP. 0 means it is temperature independent, i.e. constant, 1 that it has linear T-dependence and 2 that it has quadratic T-dependence. The final line shows you the name of the temperature function. This will be EOS for equations of state or a dimensionless BIP, such as REGULARBIP for Flory Huggins or Regular Solution. Activity models are assigned an Activity temperature function, while the first set of CPA or PC-SAFT BIPs (ASSOCBIP, SAFTBIP) will be labelled EOS and the second (ASSOCBIP-2, SAFTBIP-2) will be labelled Association. Multiflash will check that the correct temperature function is used for the model selected. For equations of state (EOS) the function is,

kij  a0  a1T  a2T 2 For the association term, the first BIP adjusts the volume parameter, the second the energy parameter and the third the heat capacity parameter. The final element is the Unit for the BIP. See below for more information. A second example shows output for the methanol plus water binary and the RKSA (Infochem) model. The use of an NRTL type mixing rule means this model requires two asymmetric parameters and one symmetric parameter.

In this case the water methanol asymmetric parameters have also been fitted with a linear temperature dependence. In a third example we consider a mixture of water, methanol, methane, and ethane, and the RKS-HVP model. The HVP mixing rule can work with two different types of BIPs, 1) "VDW-like" parameters (i.e., kij), which are symmetrical, dimensionless, and are equal to those used in the classical mixing rule of the RKS equation. 2) "HVP-like" parameters, which are composed of symmetrical (ij) and asymmetrical (gij/R) BIPs, and where the asymmetrical parameters have units of K.

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We show below the output obtained for this mixture and model. In this case all pairs except one have “HVP-like” BIPs defined. Only the pair methane-ethane has “VDW-like” parameters, indicated by the absence of a symmetric BIP for this pair.

If the user has access to additional "HVP-like" parameters to those in Multiflash, these can be easily introduced by directly writing them in the output window. In the case of the asymmetrical parameters, care must be taken of introducing them in the correct order. For example, if the BIP found in the literature indicates that it corresponds to water-methanol it should be entered in the cell corresponding to the row "water" and column "methanol" (i.e., in the example above, the upper diagonal value of 276).

Units for BIPs The BIPs for most equation of state methods, Wilson A, Regular Solution and Flory Huggins are dimensionless. For other activity methods and the two CPA association parameters the BIPs have associated units. The default units in Multiflash are J/mole. If BIPs from external sources are used in Multiflash it is important that either the BIP units are changed to match the input values or the numeric values of the BIPs are changed to J/mole. The choice of units appears once the Units button is activated in the BIP display.

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J/mol is the Multiflash standard for the dimensioned activity model BIPs. cal/mol and K are the values used in the Dechema Data Series for activity model VLE and LLE BIPs. The “Aspen” format allows you to transfer the BIP values for the NRTL equation from Aspen Plus without further change. The actual input functions for the activity BIPs are as follows: In J/mol K

Aij=a + bT + cT2

In cal/mol K

Aij/4.184=a + bT + cT2

In K

Aij/R=a + bT + cT2

Dimensionless

Aij/RT=a + b/T + cT

Aspen format

Aij/RT=a + b/T + cT

For the NRTL equation, the  parameter is defined as follows: All formats except Aspen ij= a + bT + cT2 Aspen format

ij= a + b(T-273.15) + c(T-273.15)2

Supplementing or overwriting BIPs If you have interaction parameters available and wish to supplement or overwrite those stored in Multiflash you can do this using Tools/BIPs from the menu. However, you must make sure that the BIPs you supply conform to the model definition used in Multiflash and, for activity models, that you have specified and supplied BIPs in the correct units and in the correct order. Once the BIPset is displayed, as shown above, you can type your own BIP values into the appropriate cell to overwrite stored values. To take a simple example, load the C4C5.mfl file used for the simple tutorial Using Tools/ BIPs you can see that for the Peng-Robinson model the BIP for butane/pentane is set to zero. Given the nature of the mixture this is the expected value. However, you can enter a BIP and see the effect on the calculated results. If you enter a value of 0.1 you can see that the calculated bubble point temperature at 9 bar has changed from 371K to 359K.

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In the grid of BIPs you will notice that the symmetric BIP is displayed twice. You need only edit one of the cells; Multiflash will automatically enter this value in the other cell once you click on OK. If the model is changed the user-supplied BIPs values will be retained unless a model is selected that does not use the same BIP dataset. For example, if you simply want to change the number of phases or use a different transport property model, you don’t have to enter your user BIPs again. You can save your new BIPs for the current case permanently by saving the problem setup file. If, for certain binary pairs, you wish to overwrite the Infochem supplied BIPs every time you run Multiflash you may wish to store these in a separate input file. You should define the model and components and then use the BIPSet window to enter the values and then save the file. If the units of the BIPs you want to enter are different from the default units in Multiflash you can change the units as described above before entering the values. If you change the units after entering the BIP values or for existing BIPs in Multiflash, the values will automatically be updated. If you have a file of BIPs that contains values for components that are not in your current stream you will see a warning message for each missing pair: *** WARNING

-223 ***

One or more components in this BIP set are not currently defined If you know the reason for the warning it may be ignored because BIPs for components that are in the stream will be loaded and included in any subsequent calculations. You should note that if you change the model such that a different BIPset is required then the BIPs read from a file will be discarded and replaced by those from databanks or correlations included in the new definition.

BIPs for CSMA and GERG mixing rule For the CSMA with GERG mixing rule or GERG-2008 models, three BIPs are required for improving the critical temperature and volume of the mixture as well as the weighting factor for the binary high accuracy departure functions for modelling the mixing behaviour. The default names for the three BIPs with the GERG-2008 model are GERGBIP, GERGBIP-2 and GERGBIP-3. For GERGBIP and GERGBIP-2 each pair of components can potentially have three parameter coefficients. The first two are for the constant or linear (in temperature) interaction parameter. These two are symmetric. However the third value (0.9963365 and 1.003677 in the table for GERGBIP for methane and ethane as shown below) is actually a reciprocal of each other. So when entering this value, you need to be careful about the order of the BIP.

The third BIP, GERGBIP-3 is the weighting factor of the binary high accuracy departure functions. By default, it is set to zero. If the mixing rule has the departure function equation available for the binary system, a non-zero value will be displayed otherwise it will be zero.

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Troubleshooting - BIPs Units Reference: Dechema Chemistry Data Series (CDS) ISSN 0840-9645 Volume I to XVI Publisher: Dechema

We cannot stress often enough that to obtain correct results the BIPs entered must match the Multiflash definition and be in the correct units. A very good source of phase equilibrium data and BIPs is the Dechema Chemistry Data Series which is in several volumes. It is useful to note that their standard convention is to report activity model BIPs for VLE in cal/mol. Either these need to be converted to J/mol for use in Multiflash or the Units for BIPs must be changed to cal/mol. Similarly, the LLE BIPs for UNIQUAC and NRTL either need to be multiplied by the gas constant R (8.314 JK-1mol-1) for use in Multiflash or the BIP units must be set to K.

BIP databank The names of the main BIP databanks for equations of state and activity coefficient models are INFOBIPS and OILANDGAS, which are the names included automatically in all the relevant model sets. From Multiflash 4.2, the BIPs for all the relevant model sets are defined from INFOBIPS and BIPs correlations are obtained from OILANDGAS.

Differences between the PR Model in Multiflash and Aspen Hysys The Peng-Robinson (PR) and PR78 equation of state (EOS) models implemented in Multiflash is based on the original publications by Peng and Robinson (1976, 1978). By default the PR model is used for all phase properties: fugacities, density and thermal properties. Based on information in Aspen Hysys documentation the standard Hysys PR model works as follows. The fugacity coefficients of components in a mixture are obtained from the PR78 EOS. This means that if the components in Multiflash have the same critical properties, acentric factors and BIPs as in Hysys, the calculated phase equilibrium (phase amounts and compositions) at given pressure and temperature should be very similar. However, the volumetric properties (density) and thermal properties (enthalpy, entropy) may differ. The reason for this is that the Hysys model uses a version of the COSTALD model to obtain the liquid density at sub-critical conditions. Thermal properties are evaluated from the Lee-Kesler model. The recommended version of the PR model in Multiflash is PRA78A. Amongst other features, this model allows the use of a volume-shift adjustment that considerably improves density predictions compared to the original PR or PR78

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models. To obtain a match between Hysys PR liquid densities and PRA78A for systems including petroleum fractions it is recommended to use the volume matching tool (see section “Matching density/volume”) to adjust the volume shift parameters to match densities from Hysys. It is also possible to specify that the LK model should be used in Multiflash to calculate thermal properties. You should contact Infochem technical support for assistance if you wish to do this. Note that absolute values of enthalpy and entropy in Multiflash and Hysys will, most likely, differ irrespective of the model used. This is because the (arbitrary) enthalpy and entropy zero points are different. Enthalpy and entropy differences should, however, be comparable.

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Components

Introduction Multiflash recognises two types of component. Normal components are pure compounds such as hydrocarbons, petrochemicals and chemicals which may exist as gas, liquid or solid depending on conditions of temperature and pressure. A petroleum fraction is a pseudo component, usually a complex mixture of hydrocarbons, whose aggregate properties are characterised by standard tests, the results of which may be found in PVT laboratory reports. The physical properties for each type of component are stored or defined differently.

Normal components The physical properties of normal components are usually stored in databanks. Multiflash offers two, INFODATA and DIPPRTM. INFODATA is the Infochem fluids databank which contains data on several hundred compounds and is always supplied as part of Multiflash. DIPPR, produced under the auspices of AIChE, currently has data for around 2000 compounds, and is extended annually. DIPPR is offered as an optional module for Multiflash. For details of how to find a list of components or to search for a specific compound see “Selecting components” on page 66.

Properties of normal components Both INFODATA and DIPPR store data for each variable property (e.g. vapour pressure, liquid density) of a component as a function of temperature. Properties which are not temperature dependent are stored as constant values. A list of the properties available in DIPPR and INFODATA is shown below. Some properties may be missing for individual components. Constant properties Molecular Weight Critical Temperature Critical Pressure Critical Volume Critical Compressibility Factor Melting Point Triple Point Temperature Triple Point Pressure Normal Boiling Point (at 1 atm) Liquid Molar Volume at 298.15K Standard Ideal Gas Enthalpy of Formation at 298.15K

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Standard Ideal Gas Gibbs Energy of Formation at 298.15K Standard Ideal Gas Entropy at 298.15K Enthalpy of Fusion at Melting Point Entropy of Fusion at Melting Point Heat capacity change on fusion Volume change on fusion Standard Net Enthalpy of Combustion at 298.15K Acentric Factor Radius of Gyration Parachor Solubility Parameter at 298.15K Dipole Moment van der Waals Volume (UNIQUAC r) van der Waals Area (UNIQUAC q) Refractive Index Flash Point Lower Flammability Limit Upper Flammability Limit Autoignition Temperature plus some model specific parameters. In addition there is a group of properties that allow unique identification of the name and type of the component. These are TYPE CAS number FORMULA FAMILY code (Deprecated since DIPPR 2013) Normal databank components will be TYPE 1, petroleum fractions will usually be TYPE 12. Temperature Dependent Properties Solid Density Liquid density Vapour Pressure Enthalpy change on evaporation (latent heat) Solid Heat Capacity Liquid Heat Capacity Ideal Gas Heat Capacity Second Virial Coefficient Liquid Viscosity Vapour Viscosity Liquid Thermal Conductivity Vapour Thermal Conductivity Surface tension Both databanks, in addition to the properties above, also contain the UNIFAC or PSRK group structures, where applicable. The INFODATA databank also stores some additional properties which include: Constant properties Polarizability Quadrupole moment PC-SAFT model parameters Entropy of Formation Specific gravity Isobaric expansivity Temperature Dependent Properties Relative permittivity (dielectric constant)

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These properties are available for a small proportion of components in the databank. For temperature dependent properties both databanks contain information on the upper and lower temperature limits for the correlation used for each component. Extrapolation routines are included in Multiflash so that pure component properties from either bank behave reasonably beyond the temperature limits of the stored correlation. INFODATA contains only a limited range of components mainly suitable for oil and gas applications. Although every effort has been made to ensure that the data stored are correct we do not offer INFODATA as a quality assured databank. DIPPR is developed under the auspices of the American Institute of Chemical Engineers. The databank contains a broad range of components including hydrocarbons, petrochemicals, chemicals and some metals. The correlations used in Multiflash are the recommended set for each property and component. Questions concerning quality codes and sources of data for the DIPPR databank should be referred to Infochem.

Petroleum fractions Petroleum fractions are discussed in detail in the section “Petroleum Fluids”

Defining a mixture In this section we describe how to define a mixture containing normal components. To define a mixture including petroleum fractions or to set up fluid characterisation based on a PVT laboratory report see “Petroleum fluids” on page 89. The maximum number of components in any mixture in the current version of Multiflash is 200. From the Select menu choose Components or click on the Select Components toolbar button This will display the Select Components dialogue box

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Specifying the data source The Infochem fluids databank is the default data source. To choose any of the other data sources click on the button to the right of data source and select the required data source from the list.

Selecting components If the specified data source is a databank, e.g. INFODATA or DIPPR, then the components for any stream can be selected in a variety of ways depending on your knowledge of the contents of the databank and the name or synonym by which your chosen component is listed in the bank. If you are uncertain about any of these then there are various search strategies in place to help you find the components you need.

Select components by name This is a simple and rapid method once you know the components available. Click the Name option button.

You can enter the name of the component you wish to add to the mixture in the Enter Name box; it may be in upper- or lower-case or any combination. Press or click the Add button.. If you make a spelling error or if the component is not in the selected data bank you will get an error message

Select components by scrolling through a list Choose the All Components option button and a list of the standard names for the compounds stored in the chosen databank will appear. You can then scroll through the list until you see a component you require. This is quite convenient for INFODATA but may take a long time for a large databank like DIPPR. The component names are not sorted; they appear in the order of components in the databank selected.

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The component may then be selected for use in Multiflash either by doubleclicking on the name or by clicking on the name to highlight the component and then clicking the Add button. The name of the selected compound will then appear in the “Components selected in Multiflash” box. Further components may be selected in the same way.

Synonyms If you are not certain whether a particular name in the databank list represents the component of interest to you, or if are not sure that you have the correct name for a component, you may wish to check the alternative names (synonyms) stored for that compound. Click the Synonyms option button and type the name in the Enter name box. Press or click the Search button.

The list of synonyms stored for this component will be displayed. If this proves to be the component you wanted you can then choose the synonym to be used in the Multiflash output to identify the component by selecting it in the normal way (double clicking on the name or highlighting the name with a single click and using the Add button).

User Guide for Multiflash for Windows

Components  67

In the list of synonyms for ‘glycol’ shown above ‘ethylene glycol’ is the main name for the component in the INFODATA databank but any of the alternatives may be used. The last synonym in the list is 000107-21-1, which is the Chemical Abstracts registry number.

Select components by formula You may only know the formula of the component you wish to select. In this case select the Formula option button and type the formula in the Enter formula box. Compounds with matching formulas will be displayed and may be selected in the usual way.

It is important that the formula is defined in terms of standard chemical symbols, e.g. C6H10O, not c6h10o. Neither of the compounds corresponding to the chosen formula are in INFODATA, if you replace DIPPR with INFODATA as the data source and repeat the search the you will see the following warning. This also contains a reminder of the correct nomenclature for the formula in case this was the source of the problem.

You can also search the databanks using a partial formula. You m ay replace the number of any of the atoms in a component by a *, but you must name all the different atoms in the compound you are searching for. For example, C2Cl*H* will find all ethanes that contain chlorine.

Select components by substring You may wish to search the bank using only a portion of the name: perhaps you are interested in seeing which components contain a certain subgroup. Select the Substring button and type the portion of the name which identifies the group of interest in the text box. You can specify whether the string is to be at the start of a name, at the end of a name or anywhere in a name using the * character. For example methane* finds all names starting with methane; *methane finds all

68  Components

User Guide for Multiflash for Windows

names ending in methane and *methane* finds all names containing methane. Entering methane alone searches for an exact match.

Adding, inserting, replacing and deleting components You may add additional components to an existing mixture, replace components or delete them. You can add a component into an existing mixture at any given point in the selected list using the insert command.

Adding a component Components are added to the selected list for a mixture as described earlier, by: 

Double clicking on a component in a list.



By pressing the key after entering a name in the text box (with Name option button selected).



By selecting the compound from a list and clicking on the Add button.

If you try to add the same component twice you will be warned, for example,

and the action will not be allowed.

Inserting a component If you want to add a component to the selected list, but in a particular position, perhaps so that the compounds are in order of carbon number in the program output, then:

User Guide for Multiflash for Windows

Components  69

Select the new compound as usual by entering the name or highlighting it in a list Highlight (by clicking on it) the compound in the Compounds selected list above which you wish to insert the new component Click on the insert button.

Deleting components If you wish to remove one or more components from the selected list then, in the Compounds selected text box: Select the component(s), then Click on the Delete button The compound(s) will then be removed from the list.

Replacing a component If you wish to replace one component with another you can do this without first deleting it from the selected list. Select the component to be replaced Select the new component (either by typing in the name or using one of the other selection methods) Click on Replace. The new component replaces the highlighted component. Note that when you replace an existing component the amount of the new component in the mixture remains the same as the amount of the component it replaced unless you change the composition in the Composition drop down table.

Viewing and editing pure component data If you wish to obtain the values of any property for a pure component at specific conditions then you should consult the case study shown in “Case studies - Pure component data” on page 197 . If, however, you want to look at the stored data record for any component in the mixture then this can be done by using the Tools/Pure Component Data menu option. The components in your stream will be displayed in a subsidiary window together with a list of stored properties.

70  Components

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Highlight the component of interest and the property or group of properties required, click on Edit and the relevant section of the data record will be displayed. For constants the individual property values will be shown:

User Guide for Multiflash for Windows

Components  71

whilst for temperature dependent properties the coefficients of the correlation equation will be displayed. For details of the correlations used see the User Guide for Models and Physical Properties.

Any of these data may be overwritten by typing the new value in the appropriate cell in the correct units. The units for the constant properties may be changed using the Units button. Temperature-dependent property correlations always use Kelvin as the unit of temperature. You can also use this Edit facility to change the name of the component for the duration of any calculations. The data record can also be displayed in the main window using the Write to Output button.

User-defined components You may add user-defined pure components, for instance when the component you require is not available in our databanks. However, you should note that for this option you must supply all the data required for the models you use. This will include constants such as the critical properties and coefficients for the temperature dependent correlations such as perfect gas Cp etc. The minimum data requirements for different models are listed in “Models and input requirements” on page 74. This option is not recommended for petroleum fractions; see “Petroleum fluids” on page 89.

Adding a user-defined component To add a new user-defined component open the Select Components dialog box and select User-defined component from the Data source list. You should then enter the component name and press or click on Add. This operation creates a new component with a name but no data.

72  Components

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Specifying data for a user-defined component The physical properties for this component will be specified using the Tools/Pure component Data option in Multiflash main window. Return to the main window and activate this option and select your user-defined component.

Constants Click Edit to enter constant values.. The Value column will initially be empty. Enter the property values for your component, e.g.

You can change the units if necessary. Not all properties must be given a value; it depends on the models you use.

Temperature-dependent properties To enter a temperature dependent property, such as the perfect gas heat capacity, choose the property and click on Edit, or double click on the property description. A form will be displayed showing the component name and property but with the remaining text boxes blank.

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Components  73

As soon as you enter the Equation number the correct number of cells for the coefficients of this equation will be displayed and you enter the values of the coefficients. The temperature limits for your correlation should also be entered.

The equation types available for each property are set out in the User Guide for Models and Physical Properties, together with a description of each equation and the number and order of the coefficients. All equations are specified as a function of temperature in kelvin and you should only enter coefficients which have been fitted in kelvin. Click on OK to define the property.

Models and input requirements A basic minimum data set for any component should include the following: 

molecular weight,



critical temperature,



critical pressure,



acentric factor



perfect gas heat capacity.

The following table lists the data required in addition to the above for each model.

74  Components

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Model Thermodynamic RKS RKSAPI RKSA RKSAINFO RKS-HVP PR, PR78 PR-HVP PRA, PR78A PSRK LK, LKP BWRS CSMA or GERG-2008 CPA PC-SAFT

Ideal mixing NRTL Wilson E UNIQUAC UNIFAC Dortmund Modified UNIFAC Regular Solution Flory-Huggins Perfect gas RK Hayden O’Connell

Pure solid freeze-out

Hydrate Wax

Asphaltene Mercury partitioning model

User Guide for Multiflash for Windows

Additional data required None None vapour pressure and saturated liquid density As RKSA None None None As RKSA As RKSA plus UNIFAC subgroup structures None critical volume (VCRIT) None As RKSA plus association parameters (ASSBE, ASSEP, ASSGA) for associating components. PC-SAFT parameters (SAFTEK, SAFTSIGMA, SAFTM, SAFTKAPPA, SAFTEPSILON, SAFTFF Note (1) critical properties and acentric factor are used to generate starting values for flash calculations but do not affect the computed results from PC-SAFT. (2) SAFTKAPPA, SAFTEPSILON and SAFTFF are only needed for associating components. vapour pressure, saturated liquid density, enthalpy of evaporation As Ideal mixing As Ideal mixing As Ideal mixing plus UNIQUAC surface and volume parameters (UNIQQ, UNIQR) As Ideal mixing plus UNIFAC subgroup structures (UNIFAC) As Ideal mixing plus UNIFAC subgroup structures (UNIFAC) As Ideal mixing plus solubility parameter (SOLUPAR) and molar volume at 25°C (V25). As Ideal mixing plus solubility parameter (SOLUPAR) and molar volume at 25°C (V25). None None radius of gyration (RADGYR), dipole moment (DIPOLEMOMENT), Hayden-O’Connell association parameter (HOCASS) melting point (TMELT), enthalpy of fusion (HMELT), heat capacity change on fusion (CPMELT), volume of fusion (VMELT) potential parameters (HYD1, HYD2, HYD3), cavity occupation code (HYDOC) melting point (TMELT), enthalpy of fusion (HMELT), heat capacity change on fusion (CPMELT), volume of fusion (VMELT) vapour pressure, saturated liquid density As specified fluid model

Components  75

Transport properties Pedersen Twu LBC Lohrenz-Bray-Clarke CLS Chung-Lee-Starling SuperTRAPP models Macleod-Sugden (MCS and MCSA) Costald Liquid viscosity mixing rule Vapour viscosity mixing rule Liquid thermal conductivity mixing rule Vapour thermal conductivity mixing rule Surface tension mixing rule Linear Gradient Theory Diffusivity – Fuller's method Diffusivity - HaydukMinhas method

None boiling point (TBOIL), vapour pressure, saturated liquid density critical volume (VCRIT), or model specific VC (VCLBC), specific gravity (SG), dipole moment (DIPOLEMOMENT) critical volume (VCRIT), dipole moment (DIPOLEMOMENT) critical volume (VCRIT), dipole moment (DIPOLEMOMENT) parachor (PARACHOR) saturated liquid density liquid viscosity vapour viscosity liquid thermal conductivity vapour thermal conductivity surface tension Saturated liquid surface tension (STENSION) chemical formula (FORMULA), UNIFAC subgroup structures (UNIFAC). critical molar volume (VCRIT), normal boiling point (TBOIL), parachor (PARACHOR) dipole moment (DIPOLEMOMENT), saturated liquid density, chemical formula (FORMULA), UNIFAC subgroup structures (UNIFAC).

To save the pure component data you have entered you must save the data to a file, using File/Save Problem Setup.

Stream types It is possible to define a number of stream types in Multiflash. Each stream type consists of a subset of all the defined components and may be associated with its own set of models. The stream type concept is not particularly useful in the Multiflash GUI. It is primarily intended to support process simulation applications where different sets of components (with different models) may be present in different unit operations or sections of a flowsheet. A simple example, shown in the Multiflash Excel manual, is to describe a mainly hydrocarbon stream containing some water and glycol using a cubic equation of state for high and low pressure separator flashes but to change to an activity model to look at glycol regeneration from the recombined water streams. We will describe how to set up stream types in the Multiflash GUI. However, using the GUI, the composition of a sub-stream cannot be changed without altering the composition of the overall stream and it is difficult to show a realistic practical application in the GUI. Initially we have defined an input stream containing 4 hydrocarbons, a petroleum fraction, water and MEG and supplied a composition.

76  Components

User Guide for Multiflash for Windows

If you then activate the stream type selection using Select\Stream Types\Define you will be asked to define a model for your stream, in this case PRA. The format is exactly the same as the usual Select Model Set with the same options.

Once you have clicked on Define Model, a message box will confirm that the model has been successfully defined. Click on OK to activate the Define Stream Type text box.

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Components  77

We will first define a stream type called ‘Overall’ that contains all the components. Enter the name, make sure the ‘All components’ option button is selected and click Define Stream Type. Click Close to return to the Select Models dialog. A second stream can be defined the same way. This time we have selected the NRTL VLE model. Click the ‘Selected components’ option button, select water and MEG and enter the stream name ‘MEG’.

Click on Define Stream Type, then Close and Close again to return to the Multiflash main window. You can use the Tools/Show/Streamtypes menu item to display the list of defined stream types. show Sts; NO. OF STREAM TYPES 1 OVERALL 2 MEG

2

If you wish to assign BIPs for this stream you do this using Tools/BIPs, Click on the model/BIP name for the stream and enter the values in the BIP grid. If you try to define a further stream with the same name as a previous stream type you will be warned that the stream exists and asked if you wish to replace it.

However, you can define a further stream with the same components and model if you call it by another name. To Delete a stream type choose the Select/Stream Type/Delete option, highlight the name of the stream to be deleted and click on Delete Stream Type.

78  Components

User Guide for Multiflash for Windows

When you wish to carry out calculations relating to a particular stream, activate that stream using Select/Stream Types/Select Active, highlight the stream you wish to work with and Select Stream Type.

You can then carry out any Multiflash calculations using that stream with its selected components and the composition defined in the drop down Composition table.

Hydrate inhibitors Some of the pure components in INFODATA act as hydrate inhibitors, see “Inhibitor modelling” on page 42. The most common are methanol, ethanol, MEG, DEG, TEG and salts. Any of these can be included in the component list and their composition defined as shown in “Specifying compositions” on page 137. Indeed this is the way you should specify isopropanol, propylene glycol and glycerol. However, for methanol, ethanol, glycols and salt you can also use the Inhibitor Calculator.

Inhibitor calculator: alcohols/glycols The Inhibitor Calculator is included to simplify the addition of common inhibitors. It is designed to calculate the amount of inhibitor or inhibitors to be added to the amount of water present in the mixture in order to reach a userdefined inhibitor concentration. This concentration may be specified in mass, molar or volume units. The Inhibitor Calculator can also be used to add both water and the inhibitors to the components list. If they have not already been included they will automatically be added to the components list along with the calculated amounts. To calculate the amount of methanol, ethanol or glycols relative to the amount of pure water present in the mixture: From the Tools menu select Inhibitor Calculator and click on the Alcohols/Glycols tab.

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Components  79

Select a databank for the pure component properties; INFODATA is the default. If you have already defined the amount of water in the stream it will be shown in the dialog box. Otherwise, you must first enter the amount of water to which the inhibitor is to be added. The units for the amount of water are the currently selected units for component amounts and may be changed by clicking on the Units button. You can specify the amount of inhibitors as a mass %, mole % or volume % (at 1 atm and 60ºF). The concentration specified is the concentration of each inhibitor as a percentage of the total amount of water plus inhibitors. In the example shown below the amount of methanol added will be 10% by mass of the total amount of (water + methanol + MEG).

Click the Add button. The components will be added to the component list with the amounts as specified. The total percentages of all inhibitors must sum to be less than 100.

Click Close button to return to the main window.

80  Components

User Guide for Multiflash for Windows

The inhibitors are added in the selected input units for amounts even if the concentration is specified in other units. Note that specifying a given concentration of inhibitor (relative to water plus inhibitors) does not mean that this will be the concentration in the aqueous phase when the mixture is flashed. The inhibitor components and water will partition between all phases present. For example, methanol will typically be present to a significant extent in the oil and gas phases as well as in the aqueous phase. If you are carrying out fixed phase fraction flashes with glycol inhibitors at high concentrations (of the order of 75 wt% plus) you should allow for the possibility of the glycol forming a solid phase. The melting points of the pure glycols are relatively high. To do this you should set up a freeze-out model for the glycol using the Select/Freeze-out Components option, see “The Solid Freeze-out model”“Pure solid phase” on page 49.

Salt calculator The salt calculator provides a way of specifying the concentration of various salts in water using commonly-reported laboratory measurements. You can include the ions in your component list by selecting them from INFODATA. However, an easier approach is to use the “Salts / Ions” tab in the Inhibitor Calculator.

Electrolyte model The electrolyte model is a detailed model of the ionic species in mixed electrolyte solutions. It includes the following ions: Na+, K+, Ca++, Cl- and Br- . The effect of other ions is obtained by determining an equivalent amount of those listed. Alternatively, the RKSA-Infochem model can deal with ions by defining an equivalent amount of salt pseudo-component. The water content of your mixture can be defined from Select Components and the Composition box, or entered on the Alcohols/Glycols tab as described above. Click on the “Salt / Ions” tab. You can then define your salt from:

User Guide for Multiflash for Windows

Components  81

an Ion Analysis table

a Salt Analysis table

82  Components

User Guide for Multiflash for Windows

or the total amount of dissolved solids.

Clicking on the Add button initiates the calculation of the amounts of salt pseudo-component or Na+, K+, Ca++, Cl- and Br- ions to be added to the mixture in the input units set for amounts. In the cases of an ion analysis or a salt analysis you can choose to express the salt concentration in terms of equivalent amounts of salt pseudo-component, Na+, K+, Ca++, Cl- and Br- ions, or just in terms of Na+ and Cl- ions by clicking the corresponding option button. Clicking on the Add button will display a table of the salts added, e.g.

The corresponding ions or the salt pseudo-component will be added to the list of components in Multiflash and the amount of salt pseudo-component or of each ion will be entered in the Composition drop-down table. If you define only negative or positive ions in an Ion Analysis table an error message will be displayed.

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Components  83

If you try to add more salt than is physically realistic the Salt Calculator does not generate a warning but the amounts of the ions may be not realistic. If you enter values on more than one analysis option the amount of salt to be added will be taken from the table for which you activate the Add button. In the case of adding ions explicitly (i.e., not salt pseudo-component), the electrolyte model can only be selected as part of the hydrate model, i.e. in conjunction with the CPA fluid phase model. The model selection is made using the Hydrates tab in Select Model Set dialog. If you have added ions to the mixture and then select a model that does not include the electrolyte model a warning is displayed. For example

If you have selected a fluid phase model that does not include the electrolyte model and then use the salt calculator to add ions a warning will be displayed.

If you wish to add both inhibitors such as methanol and salts you should enter the required concentrations in the Alcohols/Glycols tab and then the salts in the Electrolyte Model tab. Clicking on Add will then add the correct level of chosen inhibitors and ions.

Troubleshooting - components Databank not found All licensed databanks will be placed in the installation directory. However, it is possible that the files may have been moved or overwritten. If a databank cannot be found then the following warning message will appear when you use the Select Component dialogue box.

84  Components

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If you attempt to load a Multiflash problem file that defines the databank and the databank cannot be found a message similar to the following will appear.

The message in the results window will be something like:: *** ERROR 12952 *** Cannot open databank files: check for correct file name and location. The path used by Multiflash to find databank files is set in the Preferences Window under Folders. Initially this will be the installation directory but you can change the path if you have moved the databank to another location.

Databank not licensed If you have not licensed DIPPR then warnings will appear.

User Guide for Multiflash for Windows

Components  85

If you have an earlier version of DIPPR and try to run Multiflash with the latest version, then following error message will appear.

The only corrective action is to substitute INFODATA as the data source or extend your licence.

Component cannot be found The component you need may not be stored in the selected databank. The warning message is self-explanatory

Before you accept this, however, it is worth checking 

That you have spelt the name correctly



That a formula or substring search cannot identify the component under another name



That formula searches are specified in standard chemical nomenclature

If a component is not present in the INFODATA databank you should try the DIPPR databank if you have a license. If you are planning a study and find that a component is missing please check with Infochem. We may be able to locate the required data. TIP

86  Components

If the component you cannot find in the databank is present only in small or trace amounts it may be possible to substitute a similar compound without significant error. However, this will clearly depend on the particular calculation and application.

User Guide for Multiflash for Windows

Too many components in the mixture The maximum number of components in a mixture in the current version of the software is 200. If you try to select more components you will be warned that the limit has been reached. TIP

If you have some components of similar type and size in your mixture, preferably present in small amounts, then it may be worth combining them to reduce the overall number of components. This is particularly useful when dealing with natural gases and gas condensates that have been analysed in great detail.

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Components  87

Petroleum fluids

Introduction Petroleum fluids are typically complex mixtures consisting of many thousands of hydrocarbon components. It is not practical to identify all these components by analytical methods. Even if it were possible to carry out an analysis it would not be feasible to model the physical properties of the fluid by including all the identified components. The practical approach adopted in the oil and gas industry is to base the model of a fluid on limited compositional analysis and other standard tests that are carried out by commercial PVT laboratories. In this section we describe how to use the information in a PVT laboratory report to construct a compositional fluid model in Multiflash; this is what is meant by the term characterisation. Characterising a petroleum fluid is an essential pre-requisite to studying the phase behaviour and other properties of the fluid. Many applications are discussed in the Case Studies that are included in this User Guide. The objectives of the Multiflash characterisation procedure can be summarised as: 

To make optimum use of measured data.



Construct a compositional model that is not restricted to a particular thermodynamic model.



Ensure that phase behaviour calculations not sensitive to the number of pseudocomponents.



Achieve high fidelity based on compositional information.



Allow model tuning to reproduce reliable experimental measurements.

PVT Lab Analysis input The primary information in a PVT lab report is a compositional analysis of the fluid. This analysis is normally carried out by gas chromatography. The gas and liquid from a separator test or bottom-hole sample are analysed separately and the results are usually recombined to give a reservoir fluid composition. The lighter hydrocarbons such as methane, ethane, propane etc. are individually identified along with some inorganic compounds such as nitrogen, CO2 and H2S. The analysis for hydrocarbons with more than 6 or 7 carbon atoms is generally reported as single carbon number fractions (SCNs) which are actually represent compounds in boiling point ranges. For example, a C9 SCN contains all hydrocarbons that boil between the normal boiling point of n--octane + 0.5ºC

User Guide for Multiflash for Windows

Petroleum fluids  89

and the normal boiling point of n-nonane + 0.5ºC. The analysis stops at a certain C-number which is reported as a plus fraction. The plus fraction amount contains all the material in the heavy end of the fluid and often represents a substantial proportion of the fluid. A typical example of a reservoir fluid analysis is shown below. It extends to C36+ Component

H2

Reservoir Fluid Mole % Weight %

Hydrogen

0.00

0.00

0.00

0.00

0.00

H 2S

Hydrogen sulphide

0.00

0.00

0.00

0.00

0.00

CO2

Carbon dioxide

0.00

0.00

3.32

1.95

0.88

N2

Nitrogen

0.00

0.00

1.88

1.11

0.32

C1

Methane

0.00

0.00

35.52

20.86

3.45

C2

Ethane

0.08

0.01

12.11

7.14

2.21

C3

Propane

0.82

0.20

18.66

11.30

5.13

iC4

i-Butane

0.51

0.17

4.29

2.73

1.64

nC4

n-Butane

1.84

0.60

9.62

6.41

3.83

C5

neo-Pentane

0.00

0.00

0.03

0.02

0.01

iC5

i-Pentane

1.48

0.60

3.14

2.45

1.82

nC5

n-Pentane

2.36

0.95

3.67

3.13

2.33

C6

Hexanes

5.10

2.46

3.23

4.00

3.55

Me-Cyclo-pentane Benzene Cyclo-hexane

1.60 0.24 2.12

0.75 0.11 1.00

0.60 0.09 0.91

1.01 0.16 1.41

0.88 0.12 1.22

C7

C8

C9

Heptanes

6.27

3.51

1.13

3.25

3.35

Me-Cyclo-hexane Toluene

3.76 1.54

2.07 0.79

0.61 0.22

1.91 0.76

1.93 0.72

Octanes

7.57

4.85

0.54

3.44

4.05

Ethyl-benzene Meta/Para-xylene Ortho-xylene

0.59 1.97 0.72

0.35 1.17 0.43

0.03 0.07 0.02

0.26 0.85 0.31

0.29 0.93 0.34

Nonanes

6.40

4.60

0.23

2.78

3.67

Tri-Me-benzene

0.72

0.48

0.00

0.30

0.37

C10

Decanes

6.99

5.57

0.07

2.93

4.29

C11

Undecanes

5.90

4.86

0.01

2.44

3.70

C12

Dodecanes

4.73

4.26

0.00

1.95

3.24

C13

Tridecanes

4.55

4.46

0.00

1.88

3.38

C14

Tetradecanes

3.71

3.95

0.00

1.53

3.00

C15

Pentadecanes

3.70

4.26

0.00

1.53

3.24

C16

Hexadecanes

2.79

3.46

0.00

1.15

2.63

C17

Heptadecanes

2.36

3.13

0.00

0.97

2.37

C18

Octadecanes

2.28

3.21

0.00

0.94

2.44

C19

Nonadecanes

1.97

2.89

0.00

0.81

2.20

C20

Eicosanes

1.56

2.41

0.00

0.65

1.83

C21

Heneicosanes

1.37

2.23

0.00

0.56

1.69

C22

Docosanes

1.21

2.06

0.00

0.50

1.57

C23

Tricosanes

1.07

1.91

0.00

0.44

1.45

C24

Tetracosanes

0.97

1.80

0.00

0.40

1.36

C25

Pentacosanes

0.87

1.69

0.00

0.36

1.28

C26

Hexacosanes

0.75

1.51

0.00

0.31

1.15

C27

Heptacosanes

0.69

1.45

0.00

0.29

1.10

C28

Octacosanes

0.63

1.36

0.00

0.26

1.04

C29

Nonacosanes

0.59

1.33

0.00

0.24

1.01

C30

Triacontanes

0.55

1.27

0.00

0.23

0.97

C31

Hentriacontanes

0.50

1.21

0.00

0.21

0.92

C32

Dotriacontanes

0.42

1.04

0.00

0.17

0.79

C33

Tritriacontanes

0.40

1.02

0.00

0.16

0.77

C34

Tetratriacontanes

0.35

0.93

0.00

0.15

0.71

C35

Pentatriacontanes

0.28

0.75

0.00

0.11

0.57

C36+

Hexatriacontanes plus

3.12 _____ 100.00

10.88 _____ 100.00

0.00 _____ 100.00

1.29 _____ 100.00

8.26 _____ 100.00

Totals: C36+ Molecular Weight (g mol-1) Density at 60°F (g cm-3)

90  Petroleum fluids

Flashed Liquid Flashed Gas Mole % Weight % Mole %

625 0.9925

User Guide for Multiflash for Windows

To enter the compositional data into Multiflash click on the Select/PVT Lab input menu item.

button or the

The following sections describe how to use this form.

Component list A list of possible components is provided based on what is typically reported by a PVT laboratory. The data for real components is taken from the databank specified at the top of the form. From C9 onwards the list contains SCNs. The real components in the list may be configured to your requirements. If there are components included that do not appear in your analysis they may be ignored or, if you prefer, they may be deleted by selecting them and clicking the Component Delete button. If you have additional components that you wish to add put the cursor in the component cell above which you wish to add a new compound and click on Component Insert button above the composition table. You will be able to select from a list of valid components:

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By default the “Databank Component” option is selected, but blank lines can be inserted in order to add more petroleum fractions, for example, C120, if necessary. Only hydrocarbons can be added to the component list in the PVT Analysis. If you try to characterise a fluid with a non-hydrocarbon component such as acetone, the analysis will fail with a warning message to indicate that the component cannot be added.

If you wish to add this component to carry out phase equilibria calculations then you should characterise your fluid without the component present, return to the main window and then add the component using Select Components. If a proposed new component is already in the list and the amount of the component is left blank, then the component will be ignored and the PVT characterisation will proceed as normal. If the component is already in the list with either a positive or zero amount then an error message will be generated when you try to do the characterisation.

When carrying out characterisation of a mixture with a misspelled component name or with the component which is not in the data source of your choice another error message will be generated.

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Only one data source can be active when carrying out PVT characterisation. This means that if the component you wish to add is only available in DIPPR (if you have licensed this) changing the data source from Infodata to DIPPR will also change the source of pure component properties for all the discrete components. If you wish the component list of your choice to be generated automatically you can do this by adding the PVT Analysis component list to your MFCONFIG.dat file. The location of the MFCONFIG.dat file is set under Tools/Preferences/General, and will be the same path as for the Problem Files (.mfl).

The mfconfig.dat file is an ASCII text file that needs to contain the following type of instructions.

The file must start with the keywords “pvtanalysis info components” in that order separated by blanks. Then the user types in the names of the components to be displayed. Single carbon number cuts are specified by the letter “C” followed by the carbon number. The ampersand is shorthand for creating a list of single carbon number cuts, so in this example “C6 & C100” creates a list of all the cuts from C6 to C100 inclusive. The ampersand can only be used once before the last name in the list of components. The file must be terminated with two semicolons.

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Petroleum fluid composition How you enter the petroleum fluid composition will depend on the information supplied by the PVT Laboratory. If you have a recombined reservoir fluid analysis (as shown above) you should use the ‘Single Fluid’ input. If possible, enter the compositional data in mass units rather than molar units. The reason for this is that the GC analysis measures compositions by mass rather than by moles and it is best to use values that are as close as possible to the actual measurements. If you only have the separator gas and separator liquid analysis you should use the Liquid+Gas tab on the form as shown below.

In this case it is necessary to enter the correct value of the recombination gas-oil ratio (GOR) as reported by the laboratory. Often there are several GORs reported that refer to different separators and it is essential to make sure that the appropriate value is used. The gas composition may be entered in either molar or mass units since all the gas phase components have a well-defined molecular weight. The liquid phase composition should be entered in mass units if possible. It is usually best to use the reservoir fluid composition provided by the laboratory because this avoids the complication of recombining the gas and liquid. Compositions can be entered in either mass or mole % by using the drop down menu at the top of the column and different units can be chosen for gas and liquid. However, changing from mass % to mole % once amounts have been entered will not lead to a unit conversion, the same values will be retained but in different units. As amounts are entered the Total % will be updated. If the Total % does not equal 100% you will be offered the option to normalise the percentages before the characterisation is carried out. Although the pseudo components will normally run sequentially in terms of single carbon number it is possible to have data from a non-laboratory source where the SCNs are not sequential. You can still enter these, leaving gaps as appropriate; if the PVT analysis is saved or

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recalled, zero amounts will be allocated for SCNs where no composition was provided. You can paste fluid compositions into the PVT Analysis form. For example, if you receive your report from the PVT laboratory as an Excel spreadsheet, then provided your PVT Analysis component list matches (or you have altered it to match) that from the laboratory, you can simply copy the reported composition from the spreadsheet and paste it into the appropriate column in the PVT Analysis form.

Molecular weight and specific gravity It is usually helpful to provide additional information on the molecular weight (MW) and specific gravity (SG) of the fluid. If you have entered a single fluid composition the options are:

For the Liquid+Gas input the ‘Single fluid’ field is replaced by a ‘Total liquid’ field. SG is the specific gravity relative to water at 60ºF and 1 atm. It is also possible to specify the density in API degrees. You can convert from API gravity to SG gravity using the following formula: SG = 141.5/(API + 131.5) The MW and SG of the Stock Tank Oil (STO) can be measured quite reliably and are the best values to enter. The properties of the heaviest SCN (plus fraction) are not normally measured and are obtained by calculation. The values should not be used unless no other information is available. If none of these values is supplied Multiflash will estimate the values based on the fluid distribution you have supplied. Our general advice is that if you have a lean gas or light condensate, i.e. where the C6+ fraction is only a minor proportion of the total fluid, you should allow the program to estimate the MW and SG. For heavier condensates with a detailed analysis to C20 or above it is also probably better not to specify a MW. For oils you should, preferably, enter the MW and SG of the STO as these are usually measured values. In order to provide some guidance when running Multiflash we have supplied some warning messages. If the molar fraction of C6+ is 0.5 and you fail to supply either molecular weight or specific gravity a similar warning message will ask you to check this.

Also, if you supply a molecular weight which does not appear compatible with the carbon number distribution you have entered you may see either of the following warning messages:

or

If the molecular weight appears to be too large this may still be correct you should just check the entry. If the molecular weight appears to be too small, it is probably because the molecular weight of the heaviest SCN is less than that allocated to the previous SCN in the distribution. This usually indicates a mistaken entry, but this is not always the case and the program will continue with the characterisation should you choose to ignore the warnings.

Total amount of fluid If you wish to specify the total amount of fluid that will be generated you can do this by entering a value in the ‘Total amount of fluid’ box. You can choose the units from the drop-down list. The value does not affect the composition; it is simply used to scale the amounts. If the amount is left blank the amount generated depends on the way the composition is entered. For example, if the composition is entered in mass units and the sum is 100 then 100 g of fluid will be generated. If a water cut is specified (see below) the total fluid amount will include the water.

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Water cut The Water Cut box allows you to add water to the fluid. The amount can be specified as a volume percent of the hydrocarbon liquid phase at standard conditions (60 ºF, 1 atm). You should note the following points 

The procedure used is approximate but does allow for loss of water to the vapour phase.



If you have already defined an aqueous phase by using the inhibitor calculator (or otherwise), entering a water cut value will alter the amount of water in the mixture but leave the composition of inhibitors unchanged, effectively altering the inhibitor concentration.



Entering 0% for the Water cut will leave water in the component list but overwrite any existing water amount by zero concentration.



Leaving water cut blank will leave any aqueous phase already defined unaffected.

Adding water will not affect the hydrocarbon fluid characterisation although it may affect the subsequent phase equilibria calculations. As an alternative you can specify the water cut as part of the petroleum fluid property matching: see “Matching dew and bubble points” on page 116.

Total Wax Content The Total Wax Content box provides the simplest means to enter information which can be used to model waxing behaviour. Modelling fluids including wax is discussed in the section “Case studies – Wax precipitation” on page 260. The Total Wax Content is the total amount (in mass) of C20+ n-paraffins ( relative to Stock Tank Oil) that is determined using the industrial standard UOP Method 46, where the wax is precipitated by addition of a polar solvent such as acetone. In Multiflash, the Total Wax Content is used to estimate the n-paraffin distribution in crude oils, that is required by the Coutinho wax model for modelling wax phase behaviour. If neither n-paraffin distribution nor the wax content is measured, an empirical correlation can be used to estimate the total wax content by ticking the Estimate Wax Content box, from which a n-paraffin distribution can be established. However this estimation method should only be valid for oils and only be used if no other information is available. If either measured wax data or the estimation method is specified, the characterisation procedure will generate both n-paraffin components and non-nparaffin pseudocomponents. However, to model the solid wax formation, the wax model must be selected from the Select Model Set dialog. The recommended analytical information for modelling wax is a measured nparaffin distribution and this input is described in the section “PVT Lab Analysis input with n-paraffin analysis” on page 105.

SARA Analysis A SARA(Saturates, Aromatics, Resins and Asphaltenes) analysis is the industry standard fractionation method for dividing crude oils into four components according to their polarizability and polarity. The analysis is usually expressed in mass % relative to the stabilised oil.

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The SARA analysis is used to characterise resin and asphaltene components that are used by the asphaltene model. The asphaltene modelling procedure is discussed in the section “Case studies – Asphaltene flocculation” on page 267. If the amount of asphaltene or the resin/asphaltene ratio is unknown, it can be estimated by the characterisation procedure by ticking the Estimate RA box. However, we recommend that you should use the measured values if at all possible. The SARA analysis values should only be entered if you intend to use the Multiflash asphaltene model. Other models may produce results with the asphaltene and resin components but will not give the correct phase behaviour.

Pseudocomponents The Pseudocomponents section of the form provides control on how the petroleum fraction pseudocomponents are generated.

The ‘Start pseudocomponents at’ box sets the lowest SCN at which to generate lumped pseudocomponents, i.e. pseudocomponents that represent a group of SCNs. The default setting is C6 but it may be changed using the scroll buttons. For example, if set to C10 this means that C6, C7, C8 and C9 will be represented by individual pseudocomponents and lumped pseudocomponents will start at C10. The smallest SCN from which you can start the pseudo component split is C6. The largest number depends on the carbon number distribution and on the SCN components defined in the component list. If you try to start from a larger SCN than the heaviest SCN in the component list you will get a warning:

If you wish to start the pseudocomponent split at a larger SCN than the heaviest SCN you will need to add additional SCN components to the component list. The ‘Number of pseudocomponents required’ box sets the number of lumped pseudocomponents that will be generated. The default value is 15 but in many cases a smaller number of lumped pseudocomponents will be quite adequate. The maximum number of components in Multiflash is 200. If you input a larger number of pseudo components than that Multiflash will change it to 200. If a wax content has been specified the corresponding boxes for n-paraffin pseudocomponents will be active. The default setting and recommended value for the n-paraffin pseudo-components is 15.

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Characterisation The characterisation procedure is started by clicking on the ‘Do Characterisation’ button. If the inputs provided are consistent with the requirement of the fluid characterisation in Multiflash, you will see a message to indicate that the characterisation has been successfully completed

The carbon number distribution entered and the function used to extrapolate the heavy end will be displayed.

The values can be exported to Excel by Clicking the “Write To Excel” button on the plot form. If you do not wish to see this plot then it can be disabled by unchecking the “Display SCN distribution” check box in the bottom left-hand corner of the PVT Lab Analysis form. Close the Carbon Number Distribution plot and the PVT Lab Analysis form. The components and amounts generated will be displayed in the main window. For example

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The numbers in the first column are simply sequence numbers for the components. Pseudocomponents have a P appended to the number. The second column contains the component name. Lumped pseudocomponents are allocated names that correspond to the carbon number range, eg. C6-19. The third column gives the amount of each component in the selected input units. The final column shows the lower- boundary for the SCNs included in each lumped pseudocomponent.

User Defined Cuts Multiflash will automatically adjust the boundaries between the lumped pseudocomponents to give the optimal approximation to the full SCN distribution for the number of pseudocomponents specified. However, you can also control the boundaries between the pseudocomponents by ticking ‘User Defined Cuts’ and pressing the ‘Define Cuts’ button. A drop-down box allows the user to define the required boundaries as carbon numbers. These carbon numbers represent the point in the distribution where one pseudo component finishes and the next one starts; they are the lower boundaries of each pseudo component, not the average carbon number of the pseudo component. The following example, showing part of the ‘PVT Analysis’ form, illustrates a case where the user wishes the distribution to split above C10. The boundaries are set at 10, 15, 20, 30 and 40 allow the user to create the following pseudo components: C10-15, C15-20, C20-30, C30-40 and C40+. Note that when you specify the carbon number boundaries, the first boundary determines where the pseudo components must begin, and it overrides the entry in the box ‘Start pseudocomponents at’

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If you leave all the entries blank in the carbon numbers drop-down box, Multiflash estimates the pseudo component boundaries automatically, just as if you had not specified user-defined cuts. If you only partially fill the carbon numbers box, generation of pseudo components stops when a blank entry is encountered. Multiflash also expects that the carbon numbers will be entered in a sequence of increasing values. If this is not done, some entries will be skipped, and a reduced number of pseudo components will be produced. The carbon numbers must all be greater than 5.5. Attempt to input a smaller the following warning will be triggered.

The facility allows you to define compatible pseudo components for several fluids with the same carbon number ranges.

Saving a PVT Analysis Before you can save your PVT fluid composition you must have carried out a successful characterisation. Once this has been done your input file can be saved as usual and when reloaded the original fluid composition will be displayed allowing you try different characterisations.

Black Oil Analysis In some circumstances a user may have very limited compositional data for a fluid. A compositional analysis may not have been measured or the data may have been generated from another application. The Black Oil analysis provides a way of constructing a compositional fluid model from limited data. The Black oil input is the third tab in the PVT Lab Fluid Analysis form.

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Input data The minimum data required are: 

Gas gravity. This is the MW of the gas divided by the MW of air (28.964).



Stock Tank Oil specific gravity SG (relative to water at 60ºF).



Solution GOR: the amount of gas relative to oil at standard conditions.

Additional data that may be entered if available but is optional: 

Watson K-factor for the oil. Kw = (Tb)1/3 / SG, where Tb is the boiling point in ºR



Gas Analysis. The gas analysis need not be complete; only the mole percentages of the components named on the form can to be entered, and they need not sum to 100%.

Other inputs are as for the compositional PVT Analysis.

Distillation curves Distillation curves are generated by standard experiments that report the amount of an oil that is distilled as a function of temperature. They are more common for refinery applications than in the upstream oil industry. The fourth tab on the PVTLab Fluid Analysis form may be used to enter either TBP data or D86 data.

TBP distillation True boiling point (TBP) distillation is the default distillation type on the ‘Distillation Curves’ tab.

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The TBP values are entered against the cumulative amount distilled as a volume percent. The temperature units can be selected using the drop-down list on the form. The molecular weights and specific gravities are optional and need only be entered if they have actually been measured. Note that they describe a TBP cut; for example the cut from 0% to 10% by volume has a molecular weight of 136 and a specific gravity of 0.782. However, the TBP of 165.6 °C corresponds to 0% distilled off and 176.7 °C corresponds to 10% distilled off, so the first cut is that which distils between 165.6 and 176.7 °C. By clicking the ‘Do Characterisation’ button, the characterisation proceeds in the same way as for a standard compositional analysis except that the result of the regression is conventionally expressed in terms of cumulative amounts as shown below for the same example.

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If the software cannot represent the data then a warning message will be shown to indicate that the characterisation procedure has failed. It may be that the data has been entered incorrectly or is inconsistent.

ASTM D86 distillation D86 is a standard analytical procedure that resembles TBP distillation but is simpler. The oil sample is placed in a single vessel and progressively heated to drive off gas. To enter a D86 analysis, the ‘Distillation Curves’ tab is used as above, but the D86 radio button is selected. The cumulative volume distilled is entered in the table together with the temperatures, but unlike TBP curves, there is no option for entering molecular weights or specific gravities as these cannot be measured by the D86 procedure. The example below illustrates a D86 analysis.

Multiflash converts D86 curves into an equivalent TBP curve using the method of Riazi and Daubert which is described in Analytical Correlations Interconvert Distillation Curve Types from Oil & Gas Journal, 84, 50, 25 August 1986. The resulting TBP curve is then characterised on pressing the ‘Do Characterisation’ button. The regression for this case is shown in the following plot.

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PVT Lab Analysis input with n-paraffin analysis For modelling phase behaviour including waxes the input information should ideally include a compositional analysis of the fluid plus an analysis of the nparaffin content. There is a separate PVTLab Fluid Analysis input form to handle this sort of data. In order to have a reliable and realistic description and, therefore, better performance of the characterisation procedure, the distribution of heavy paraffinic components well beyond nC20 would be desirable to give a realistic prediction by the characterisation procedure. To characterise a waxy crude with n-paraffin distribution, click on the toolbar button or use the Select/PVT Input for n-paraffins menu item. The form has four tabs which allow for different types of fluid and n-paraffin analysis.

n-Paraffin distribution In the first tab the total fluid composition is entered as for the standard PVT analysis in mole or mass percent, which would be expected to add to 100%. If it does not a warning message provides the opportunity to normalise it. The nparaffin distribution is that measured for the STO, again in mass or mole%. These units can be set independently of those used for the total fluid. The nparaffin distribution in the stock tank oil would normally be expected to sum to substantially less than 100%. We check that the total for the n-paraffins is less than 100%.

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The second tab allows for n-paraffin data which have been measured differently. In this case the total fluid composition is entered in the normal manner as mole or mass percent. However, the n-paraffin distribution is described as the fraction of each individual SCN (single carbon number cut) above C6 which is nparaffin. In this case the unit, mass or mole, for the n-paraffin distribution must match that of the Total fluid composition – and the unit heading will reflect this.

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The remaining two tabs operate the same way but allow for the overall fluid to be described in terms of gas plus liquid with a recombination GOR. In this case the fraction of n-paraffin will be that proportion of the liquid phase rather than the overall fluid. In any case if you try to enter an n-paraffin fraction or percentage for cuts below C6 a warning message will be issued.

Characterisation You can control the starting point for n-paraffin lumped pseudocomponents and the number of lumped n-paraffin pseudocomponents in a similar way to the procedure described on page 98. The default starting point for n-paraffin pseudocomponents is N6 (n-paraffin with 6 carbon atoms) and the default number of lumped n-paraffin pseudocomponents is 15. It may be possible to use fewer pseudocomponents but if the number is too small the wax phase will not be well-described and Clicking on the Do Characterisation button will analyse the data and start the characterisation procedure. The measured and fitted distributions are displayed for n-paraffins and non-n-paraffins if the compositions are given in a same unit. An example is shown below.

After closing this form and the PVTLab Fluid Analysis form the list of nparaffin and non-n-paraffin pseudocomponents is displayed in the main window.

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The n-paraffin pseudocomponents are denoted by an N prefix, eg. N6-8. The non-n-paraffin pseudocomponents are denoted by an I prefix, eg. I6-22. The number after the prefix indicates the carbon number range that the pseudocomponent is based on.

Estimated n-paraffin distribution If you do not a have measured n-paraffin distribution Multiflash can estimate the distribution and generate n-paraffin pseudocomponents. On the standard PVTLab Fluid Analysis form you can specify the wax content, or get Multiflash to estimate the wax content; see “Total Wax Content” on page 97.

The starting point and number of n-paraffin pseudocomponents should be specified as described above. Clicking on the Do Characterisation button displays a warning that the n-paraffin distribution will be estimated

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Troubleshooting – PVT Analysis This section describes some common problems that may be encountered.

Sensitivity to characterisation Not all the PVT analyses we have tested are well reproduced by the PVT characterisation fitting method. If this happens, it may be due to the inconsistency between the SCN distribution and the information for the molecular weight and/or specific gravity for the plus fraction or STO or total fluid. If you do see a warning related to the inconsistency, use the phase envelope tracer to see how the phase envelope is affected by characterising the fluid with and without the information for the molecular weight and/or specific gravity.

Presence of water Defining a water cut using the PVT analysis form will alter the amount of water in the overall stream but without changing the amounts of any components which are not hydrocarbons and are not in the list of discrete components. It can therefore affect the composition of an aqueous phase defined elsewhere in the program.

Defining petroleum fractions If the petroleum fractions in your mixture have been pre-assigned, their properties and amounts can be entered directly using the "Add/Remove Petroleum fractions" input form from the Select Components form. The most common reason for doing this is to re-use a characterisation from another application such as a process simulation package.

Basic characterisation properties The list of properties that may be used to support characterisation of petroleum fractions is: Component name Carbon number Molecular weight (g/mol) Specific gravity at 60ºF relative to water at 60ºF Normal boiling point Critical temperature Critical pressure Pitzer’s acentric factor However, not all of these are necessary. The minimum input sets are the component name together with the carbon number or molecular weight or specific gravity or boiling point or the information of critical temperature, critical pressure and acentric factor. Other properties that are not specified will be calculated by Multiflash.

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Other properties The properties that are estimated, if they have not been provided, are: Carbon number Molecular weight Normal boiling point Critical temperature Critical pressure Critical volume Acentric factor Parachor Dipole moment Enthalpy of formation Standard entropy Melting point Enthalpy change on melting Entropy change on melting Heat capacity change on melting Perfect gas Cp Saturated liquid density Saturated vapour pressure Enthalpy of evaporation Liquid Cp Liquid viscosity Liquid thermal conductivity Vapour viscosity Vapour thermal conductivity Surface tension

Entering petroleum fractions Click on the Select Components toolbar button or choose the Select/Components menu option. You will probably wish to define a number of pure components as already described in the section “Defining a mixture” on page 65.

To define a petroleum fraction click the “Add/Remove Petroleum fractions” button to launch the Petroleum Input form.

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If you have a pseudo component reported to be, say, C7 but have no physical properties for this cut you can simply type in the component name, e.g. C7, and the carbon number, in this case 7 then click on “Calculate” button. The remaining physical properties required to characterise the fraction will be calculated by Multiflash using the industry standard correlations of petroleum fraction recommended by Riazi and Al-Sahhaf.

On the form, the user-entered values are displayed in red and the values calculated by Multiflash are in black. By default the type of petroleum fraction, that is given in the form under the Extended properties Tab, is set to Normal. Click the OK button to close the form.

If not all the properties are known, the rest of the physical properties for the petroleum fractions will be estimated by Multiflash and displayed in black as shown below.

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The default display for the number of petroleum fractions on the form is set to 25 but the maximum number of petroleum fractions can be entered in this form is 100. After defining the petroleum fractions, click OK button or click the Calculate button to define the fractions in Multiflash before closing the form. If a set of characterised petroleum fractions are needed to be defined, this new Petroleum Input form allows you to import the physical properties by copying and pasteing from Excel spreadsheets. To do this, right-click the mouse to select the options to do the Copy and Paste on the form. Note that there is a Units button available in the Petroleum Input form. Check the units before entering data to define the fractions. The reference for the physical property correlations: Riazi, M.R. and Al-Sahhaf, T.A., Fluid Phase Equilibria 117 217 1996.

Editing petroleum fraction data Having defined your petroleum fractions and carried out some calculations it is possible you may wish to change the definition slightly. For instance you may wish see how a different data input set alters a phase envelope. This should be done by going to the Petroleum Fraction Input form. To edit and replace the petroleum fraction definition: Return to the Select Components Window by clicking on the Select Components button or using the menu option On the Select Components form, click the “Add/Remove Petroleum fractions” button to open the Petroleum Fraction Input form. On the form all the necessary physical properties of the defined petroleum fraction will be displayed. Delete the values for any data you wish to recalculate, e.g. any of the critical properties, then change or add values for the remaining input definition. Click on Calculate button or OK button to update the physical properties of the fractions in Multiflash so that the new or modified fraction definition will be loaded. Alternatively, you may wish only to change a single property of a petroleum fraction without recalculating any other properties which depend on it, perhaps to determine the sensitivity of a calculation to that property alone. This should be done using the Tools/Pure Component Data option and changing the stored property value. If you change the molecular weight of a petroleum fraction this way then the critical properties of the fraction will not be re-calculated.

Deleting petroleum fractions To delete one or group of petroleum fractions, highlight the row or rows on the Petroleum Fraction Input form and then right-click the mouse to select the Delete Selected Rows option.

To confirm the deleting click the Yes button on the following message dialog.

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Then click the calculate button to update the component properties or OK button to close the form. Please note that clicking Cancel button to close the form will not load the change onto Multiflash and the petroleum fractions will not be fully updated.

Problems defining a petroleum fraction The properties you enter to characterise a petroleum fraction on the Petroleum Fraction Input form will be checked when you click on the Calculate/OK button on the form. If they are physically unrealistic or cannot be processed by the petroleum fraction suite of correlations you will be warned and the warning or error message may be given in a message dialog box or displayed on the main window of Multiflash, and the fraction will not be accepted by Multiflash. The obvious problems will be entering a negative number for a quantity which must be positive, e.g. molecular weight, specific gravity, critical temperature and pressure or acentric factor. The warning message will pop-up after the Calculate or OK button is clicked.

The detailed message will be displayed on the main window.

It is possible for a temperature to be negative in the chosen units, but if it is also negative in absolute units a slightly different message will appear. The best advice we can offer is to make sure that you check that the units (displayed next to each of the input text boxes) match the numerical values you are putting in. A different section will discuss how to change units, see “Changing units” on page 170.

Delumping tool The PVT Analysis tool in Multiflash is used to characterise a fluid based on an existing analysis carried out in the PVT lab. As mentioned in the previous chapter, the objective of the characterization procedure is to produce a mixture of components that more closely describes the analysed fluid. The produced components can be lumped into pseudocomponents in order to get a simpler mixture while preserving the main characteristics of the fluid such as bubble points.

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If the fluid is characterized in Multiflash all the PVT analysis inputs are stored so it is easy to re-characterized the fluid with different setting as necessary. For example it may be needed to reduce or increase the number of pseudo components or add other inputs such as the molecular weight of the fluid. For any fluids characterized by a different software package, it is now possible to re-characterize the fluids using the Delump Tool in Multiflash.

How to use the delumping utility The delumping tool can be used with any previously characterized fluid. It can also handle fluids where asphaltenes and waxes are present. In the case with waxes the n-paraffin components will also be delumped. To launch the Delump tool click the Delump button: window will appear if there is a current defined fluid.

. The following

The Current Fluid table (1) displays the components and amounts of the current fluid. After delumping it will display the components and amounts of the delumped fluid. The Delumping status (2) shows the current status of the delumping tool. In this case it shows that the current fluid is not delumped. The Fluid summary (3) lists the number of components of each type:

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

Real components: methane, ethane, etc



Petroleum fractions: standard petroleum fractions, C6, C7, C7+, etc



Normal paraffin fractions: fractions n-paraffin components, N6, N7, N7+, etc



Iso-paraffin: petroleum fractions that are not part of the n-paraffin distribution, example I6, I7, etc

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

Resin fractions: petroleum fractions constituted by resins, example R6, R7, etc



Asphaltene: the amount of the asphaltene component in the current units.

The fluid is ready to be delumped. Click the “Delump” button. The resulting fluid is displayed in the Current Fluid table. NOTE:

Any Matched information such as bubble points, dew points, density will be discarded when the fluid is delumped.

The number of components of each type has increased and new options are now available for the delumped fluid. Clicking the “Revert” button puts the fluid in its original lumped state. It is now possible to export the delumped fluid to the PVT Lab Fluid Analysis tool by clicking the “Open PVT tool” button. It will open the standard PVT lab Fluid analysis tool if no n-paraffins are present and the PVT lab Fluid analysis with n-paraffin distribution tool otherwise. If the fluid has no n-paraffin analysis, one can be added by ticking the “Add n-Paraffin analysis” on the Delump Fluid form.

By ticking this option the “PVT lab Fluid analysis with n-paraffin distribution” tool will open instead of the standard one. After the PVT tool is opened the user has the option to change the number of pseudo-components, add molecular weight, specific gravity, SARA analysis and water cut if necessary and to re-characterise the fluid.

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Matching using petroleum fraction properties Sometimes the properties calculated for your fluid will not correspond to known or measured values for that property. Where the mixture includes petroleum fractions it is possible to adjust properties of the petroleum fractions to modify a property of the overall mixture. Typical mixture properties that can be adjusted by altering fraction properties are the dew and bubble points, the viscosity, the volume/density and, when the model is available, the wax appearance temperature. The asphaltene precipitation point is matched by adjusting the model parameters rather than the properties of any fractions and will be discussed in the relevant section. Matching of dew and bubble points, bubble point/GOR/Water cut or density/volume is only available with variants of RKS, PR or CPA model options, although the flexibility and method of fitting will vary.

Matching dew and bubble points Petroleum fraction can be re-defined by adjusting their properties to reproduce known experimental data, e.g. the dew point or bubble point of a mixture. The reflected changes of the properties of a petroleum fraction to re-produce the dew or bubble point experimental data depend on the equation of state models used when matching. If the RKSA, PRA, PR78A, or CPA models are used, the vapour pressures of each of the pseudo components are modified. The modifications are defined by two Mathias Copeman parameters. The keywords saved to the .mfl file are model-dependent. For example the keywords MCRKSA1 and MCRKS2 are for RKSA model, MCPRA1 and MCPRA2 for PRA model. If other equation of state models are used (such as the standard RKS) the acentric factor of the pseudo components are adjusted to match the dew point or bubble point experimental data. Matching is carried out using the Tools/Matching menu option. The matching function works with any number of fractions. Typically you may know an experimental upper retrograde dew point. You define your stream including one or more petroleum fractions and, if you wish, check the calculated dew point pressure or temperature using a fixed phase fraction flash, see “Fixed phase fraction flashes” on page 144. Our example has 9 petroleum fractions, the properties of the heaviest, C20+, are shown below.

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If the calculated dew point does not match the experimental values, activate the matching facility using Tools/Matching/Dew point menu option.

and enter the values for one or more experimental dew points and the type of dew point you are calculating (the Upper retrograde solution is the default setting for dew point). Click on the Match button and the vapour pressure of the fractions will be adjusted until the calculated and experimental values match. A plot will show a comparison of the matched data to the experimental data and the unmatched calculations.

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The two adjusted Mathias Copeman parameters will be reported in the main window together with the critical temperature and pressure. match table dewpoint upper retrograde temperatures 305.35 340.95 398.55 442.55;pressures 462.8 401.6 303.5 229.6;;; PETROLEUM FRACTIONS PHYSICAL PROPERTIES: TC/K PC/bar MC1 MC2 11P 515.614 31.2927 1.0045936 -0.12532595 12P 545.103 31.0298 1.0698024 -0.18301678 13P 570.756 29.346 1.142895 -0.23326444 14P 597.629 27.167 1.2325474 -0.28864297 15P 666.44 21.7559 1.5491369 -0.46237886 16P 734.346 17.2713 1.8226226 -0.62069818 17P 734.346 17.2713 1.8226226 -0.62069818 18P 734.346 17.2713 1.8226226 -0.62069818 19P 749.95 16.2654 1.9057277 -0.66510979 EXPERIMENTAL AND CALCULATED VALUES: DEWPOINT T(exp)/K P(exp)/bar P(calc)/bar 305.35 462.8 450.52454 340.95 401.6 413.32432 398.55 303.5 319.64004 442.55 229.6 207.69892

where C17+ is the 19th component. The adjusted Mathias Copeman parameters MCRKS1 and MCRKS2 for the RKSA model will be reflected in the pure component record.

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The new fraction properties can be saved using File/Save Problem Setup. The second Mathias Copeman Parameter MCRKS2 values may not be displayed if they do not result in a better fit to the dew point line or there is only one dewpoint specified in the matching form. Since Multiflash version 3.9 the matching facility for bubble points has been expanded to include GOR, liquid density and optionally the water phase fraction (Water cut). This is accessed through the Tools/Matching/Bubble Point/GOR option.

If only bubble point data are available then these are entered using the bubble point data table. If no bubble point data is available at all, it is still possible to match GOR. To fit the GOR you must enter the conditions at which the supplied GOR were measured and the GOR itself. The units may either be standard cubic feet per barrel of liquid at the T,P specified or the equivalent in standard m3/m3. The liquid density is optional but, if provided, must be at the same T,P conditions as the GOR. The experimental value for water phase fraction has to be expressed as volume percentage of the total liquid phase obtained at the separator condition: hydrocarbon and water. A choice of units is provided for liquid density including specific gravity in relation to water. The GOR and liquid density units in the bubble point matching form are not affected if the Units option is changed. The T,P units may be changed but in GOR conditions and bubble point units must be the same. The plot will show the phase envelope before and after matching with the information specified in the Match Bubble Point / GOR Data form. For the case with water cut specified in the matching, the phase envelopes for both the unmatched and matched case are the fluid without presence of water.

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The output in the results window will show the adjusted property values for each of the properties matched, and the comparative values to experimental data. For the bubble point matching the Mathias Copeman parameters are changed as described for the dew point matching. For fitting to liquid density the Peneloux shift parameters are altered as described below for density/volume matching.

The GOR and the water phase fraction are matched by changing the fluid composition: change non-aqueous components to match GOR and change water amount to match water cut. The output shows the original and adjusted composition for each fluid component and the ratio of the two. The initial amount of water will always be set to zero, even if it is not zero before using the matching facility.

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Although some warning messages are triggered, e.g. entering a GOR but without T,P conditions, it is not possible to trap inconsistencies in the entered data. The best way of spotting major inconsistencies is the comparison of the matched and unmatched phase envelopes. If, for instance, the GOR is entered in the wrong units then there may be a good match for bubble point, density and GOR but the phase envelope change is significantly different and merits closer inspection of the experimental data.

The ratio of adjusted composition to original data may also indicate possible inconsistencies. Similarly any GOR entered as part of the PVT Analysis and used to calculate the recombined fluid composition is ignored for the purposes of Bubble point/GOR matching and is not checked for consistency. Any amount of water added during PVT characterization, where a water cut was asked, will also be discarded.

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Major differences in the phase envelopes may indicate that it is worth checking and eliminating inconsistencies.

Matching Density/Volume The procedure for matching density/volume is similar to matching dew and bubble points. The matching is flexible and, as with the viscosity, the phase to be matched can be specified.

In this case the amended property values are the coefficients for the Peneloux volume shift parameter. match table volume OVERALL temperatures 381.15 381.15 381.15 381.15 350 350 350 350;pressures 360 350 340 330 100 80 60 40;Volume kg/m3 202.375 268.229 253.93 249.228 93 73 55 35;;; PETROLEUM FRACTIONS PHYSICAL PROPERTIES: PENELOUX VOLUME SHIFTS/(m3/mol) CONST. TERM TEMP. DEP. TERM 10P 0.00171839 -4.31382E-6 11P 0.00198798 -5.00294E-6 12P 0.00222093 -5.58758E-6 13P 0.00241593 -6.07539E-6

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14P 15P 16P 17P 18P 19P 20P 21P 22P 23P 24P EXPERIMENTAL AND CALCULATED VALUES: VOLUME T(exp)/K P(exp)/bar 381.15 360. 381.15 350. 381.15 340. 381.15 330. 350. 100. 350. 80. 350. 60. 350. 40.

0.00261675 0.00283127 0.00309332 0.00328365 0.00353665 0.00387479 0.0042509 0.0047303 0.00514017 0.00526666 0.00557851

Vol(exp)/kg/m3 262.375 258.229 253.93 249.228 93. 73. 55. 35.

-6.57719E-6 -7.11674E-6 -7.77269E-6 -8.24893E-6 -8.88117E-6 -9.72486E-6 -1.06627E-5 -1.18573E-5 -1.28798E-5 -1.31955E-5 -1.39722E-5

Vol(cal)/kg/m3 263.20947 258.47254 253.59631 248.57439 94.583391 73.892152 53.908961 34.831117

Matching wax data/WAT The method used for matching the wax precipitation curve or wax appearance temperature (WAT) to experimental data is to adjust the melting temperature of the petroleum fractions in the mixture. From Tools/Matching menu, Select Wax Phase to activate the Wax data dialog box.

The table can be used to match WAT or the amount of wax precipitated for a given pressure as the temperature falls. In the case of WAT you may wish to give the amount of wax as zero mass(or mole)%, in which case it does not matter of you specify the wax as a function of the oil plus wax phase or total fluid. However, it may be more realistic to give a positive amount of wax that reflects the nature of the measurement technique used. We recommend 0.045mass% for CPM or 0.3mass% for DSC. Once you have entered the data , click on the Match button and the melting temperature and the enthalpy of melting of petroleum fractions will be adjusted to match the information provided. The adjusted melting temperature or enthalpies will be displayed in the main window.

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After matching a plot will be generated but the type of plot depends on the data entered on the form. If it is a set of wax precipitation data at a given pressure or temperature, wax precipitation curves as a function of temperature/pressure are plotted.

If the data are for a fixed wax phase fraction, the wax phase boundary line at the fixed phase fraction is plotted.

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The new fraction properties can be saved using File/Save Problem Setup for future use. With the Coutinho wax model the adjustment is made only to the nparaffin pseudo-components as it is these which form the wax phase.

Matching liquid viscosity This facility allows you to match a known total hydrocarbon liquid viscosity or the stock tank oil viscosity. From the Tools/Matching menu select Viscosity.

Make sure LIQUID1 or LIQUID2 is selected from the list of possible phases and enter the temperature and pressure conditions as well as the liquid viscosity data to be matched. The matching procedure works by altering the reference viscosity of the petroleum fractions and these will be reported in the main window, together with a comparison of the experimental and fitted values of viscosity. For example

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Prior to this a comparison plot will be generated for any isotherm or isobar for which experimental data was supplied. If only a single viscosity point is matched the plot will be supplied for the isotherm.

The reference viscosities will also be reflected in the pure component data record for the petroleum fractions. The matching facility will attempt to match the data supplied whether or not it appears physically realistic. No warning will be issued if liquid viscosities increase with increasing temperature.

Matching vapour viscosity The same procedure given above can be applied to matching the viscosity of vapour phase. There is only one vapour phase, GAS in this case. With a defined viscosity model for both GAS and LIQUID, only one set of viscosity data of either liquid or gas phase can be matched at a time for a typical fluid.

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Problems when matching It is possible that matching to a particular property, such as the bubble point, may require a significant change to the properties of the petroleum fractions. If the petroleum fraction property that is adjusted is altered by more than 10% then you will see a warning message. For example: match bubblepoint; *** WARNING -13581 *** Adjustment to petroleum fraction properties is probably physically unrealistic

However, the adjusted property, e.g. acentric factor, may still be reasonable, in which case you should continue with your calculation. If you consider the adjusted properties to be physically unrealistic you need to check the compatibility of the characterisation and bubble point data you have entered. If Multiflash is unable to reconcile the petroleum fraction characterisation with the value to match the error message will be match bubblepoint; *** ERROR 20565 *** Quadratic extrapolation failed to improve solution *** ERROR 20404 *** The Matching procedure has failed *** ERROR 448 *** The matching calculation has not converged.

Significant differences in the matched and unmatched phase envelopes when matching multiple properties, such as bubble point, GOR and density, may indicate inconsistent experimental data but this may not generate a warning message.

Petroleum Fluid Blending The blending option allows mixing, or blending, of already characterised petroleum fluids to provide a new fluid characterisation, for example when two pipes intersect. The blending facility allows up to four separate fluids, each of which must be defined in a problem file (.mfl) or be the current fluid in use in Multiflash , to be blended together in relative amounts specified by the user to produce a new fluid described by its own set of pseudo-components. The properties and relative amounts of the blend’s pseudo-components are automatically calculated by the Multiflash blending procedure. To display the blending form, click on the input menu item.

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button or the Tools/Blend Fluids

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Blending method The blending method first picks out the discrete components such as methane, ethane, carbon dioxide, etc. from each fluid and adds them together in the correct proportions. The method then picks out the dominant pseudo-components from the mixture. It adds all the non-dominant pseudo-components to the most physically similar dominant pseudo-components and averages the physical properties of the resulting blended pseudo-components. The dominant pseudocomponents are those with the highest concentrations in the mixture for each range of molecular weight and also those that occupy the extreme positions of the molecular weight distribution. The method has the following advantages: It is automatic and requires no user intervention. It works for any type of fluid that can be represented in Multiflash and saved in problem files, although it does work best if the fluids and the pseudo-component distributions are similar. The properties of the blend change smoothly with changing blend ratios. The properties of the unblended fluids also change smoothly as small amounts of other fluids are added, i.e. the method shows correct limiting behaviour. The method of averaging the properties of the blended pseudo-components is exactly the same as that used in the PVT analysis procedure to create the pseudocomponents used to represent the properties of the original petroleum fluids. The method also handles waxy and asphaltenic crudes thereby predicting the likely wax or asphaltene formation from the fluid blend. For any volume blending, it is recommended that the fluid model is defined in the original MFL files so that the density can be calculated at the standard or the given condition if the density is not specified. Without knowing the density, the volume blending will fail.

Fluid file name The user can specify up to three different problem files describing different fluids that are to be blended. In addition, the fluid that is currently loaded in Multiflash can also be one of the fluids to be blended. The file names can be directly entered in the text boxes, or alternatively the user can click on Browse to search for each file in turn.

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Fluid amounts The relative amounts can be entered for each fluid in a variety of volume, mass and molar units which can be selected from the drop-down list. If an amount is left blank or set to zero, none of that fluid is added to the blend. So, if the fluid currently loaded in Multiflash is not required in the blend, its amount is omitted or set to zero. If a volume unit is selected, the program needs to calculate the density of the fluid as part of the blending procedure. The program uses the information provided in the window ‘User Specified Conditions for volume blending’ to find the fluid density. By default the ‘standard condition’ is ticked, in which case the program uses the model defined for that fluid to calculate its density at standard conditions (1 atm and 60degF). If no model is defined for that fluid, Wilson correlation is used to estimate the K values and then the fluid is flashed at the standard condition to find the fluid density. If ‘standard condition’ is not ticked, the user has two other options. If the density is entered, its value is used directly to calculate the blend ratio. Alternatively, if the density is not specified but the temperature and pressure are, the program uses the model for that fluid to calculate its density at the specified temperature and pressure. Note that the table for ‘User Specified Conditions for volume blending’ can only be filled in if volume units are selected for the fluids.

Model definition One of the fluids can be selected as providing the model definition for the fluid blend. The models and phases defined for the blended fluid are then the same as for the selected fluid. Likewise the information stored for all the associated utilities such as matching, PVT analysis, the inhibitor and salinity calculator, etc. will be taken from the same selected fluid. Alternatively, no fluid need be selected in which case the definition of blended fluid will include only the components and their properties with no associated model definitions or utility information. If the blended fluids are asphaltenic, the parameters of the asphaltene models for each of the constituent fluids will be averaged to give a prediction of possible asphaltene precipitation in the blend. However, to obtain this prediction the user must select a model definition for one fluid as, if no models are selected, the asphaltene model parameters will be lost.

Blending procedure The blending procedure is initiated by clicking OK. If there is a fluid currently defined in Multiflash, a warning will appear as follows:

On clicking Yes, the previous fluid definition is lost, and the blend becomes the currently defined fluid. If No is clicked, nothing happens giving the user the option to save the current fluid description, for example, before proceeding. In the example below, 5 kg of fluid described in file PetFluid.mfl is blended with 1 kg of fluid described in file Blackoil.mfl. The models and associated information for the blend are set the same as those in file Blackoil.mfl.

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Once the blend is defined as the current fluid, the properties of the blend can be modified just like any other Multiflash fluid, and the fluid definition can be saved at any time. If suitable information is available about the properties of the blend, the model can be modified by fitting to this information using the Multiflash match utilities.

Example for blending The case below uses two standard Multiflash examples of petroleum fluid models defined in Petfluid.mfl and Blackoil.mfl. The first is an oil for which a PVT analysis is provided and which is characterised by 5 pseudocomponents. The second is an oil for which no analysis exists, the properties of which have been estimated by the Multiflash black-oil input option. The second case is characterised by 15 pseudocomponents, so the two cases are sufficiently different to present a demanding test of the blending procedure. Start by loading file Petfluid.mfl using the menu option File/Load Problem Setup option. Click the phase envelope button and press VLE Autoplot. The phase envelope appears (after you select continue twice). By default Multiflash adds to a legend for each phase generated. The legend can be changed using Options in the bottom of the graph window, e.g. adding the mfl file name for the fluids before blending. Without clearing anything, go back to menu option File/Load Problem Setup to load file Blackoil.mfl. Repeat the VLE Autoplot again using Continue. Both phase envelopes are now visible on the same plot as shown:

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Now the blending procedure can be set up using the blend facility to blend the two fluids. Without deleting the phase envelopes, press the blend button in the main Multiflash window and select both fluids using the browse buttons if desired. Start by mixing 0.25 moles of Petfluid.mfl into 0.75 moles of Blackoil.mfl as illustrated:

The model definitions used in Blackoil.mfl are selected as the ones to be used for the blended fluid. On pressing OK, the blending procedure creates a model for the fluid blend in Multiflash. The phase envelope for the blend can then be added to the existing plots using VLE Autoplot. The procedure can then be repeated by going back to the Blend Fluids window and specifying ).5 moles of both oils, pressing OK and calculating the phase envelope. Finally the process can be repeated with 0.75 moles of Petfluid.mfl and 0.25 moles of Blackoil.mfl. The resulting diagram will appear as shown:

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As expected, the phase envelopes of the blends move between the two outlying envelopes of the original fluids depending on the blending proportions.

Example with waxy crudes The next case involves two standard examples of waxy crudes defined in waxycondensate.mfl and waxycrude.mfl. The first is a condensate crude with an n-paraffin distribution estimated from the total wax content; the second has a measured n-paraffin distribution. The second is much heavier and waxier as its n-paraffin number goes to much higher molecular weights termination at nC80+ as opposed to nC62+. First load waxycondensate.mfl and use the VLE Autoplot button on the phase envelope window to plot the fluid phase envelope. On the phase tab select the wax phase and press the Plot button to obtain the wax line. The result is as shown below. (The pressure axis is limited to 450 atm. This is done by selecting the Frame tab in the phase envelope window and entering 450 in the maximum pressure box for plotting.). Note that the phase envelope calculations are time consuming because these examples have a very large number of components.

Next the same calculations are repeated for waxycrude.mfl and the results to give the following phase diagram. The wax line occurs at a higher temperature for waxycrude.mfl because the fluid contains heavier n-paraffin components.

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Now the two crudes can be blended together using the blending procedure. To illustrate, the following plot shows a mixture of 0.25 mole for the waxycondensate.mfl and 0.75 mole for waxycrude.mfl with the final model set to that of waxycrude.mfl, although it does not matter which fluid definition is used as the models for both are the same.

As expected, the vapour-liquid phase envelope for the blended crude has mixed effects from both crudes. The waxy oil is much heavier and has much stronger effect on the phase envelope of the blended crude at the higher temperature end, whereas the waxy condensate has more contribution to lower temperature end. However, the blended wax line is very similar to that of waxycrude.mfl because the heavier n-paraffins in waxycrude.mfl continue to dominate the point of wax precipitation in the blended mixture.

Example with asphaltenic crudes In this example two asphaltenic crudes asphex2.mfl and asphex3.mfl are used. First, asphex2.mfl is loaded and the bubble point line is plotted by clicking the Plot button in the phase envelope form. The asphaltene envelope is added by selecting the asphaltene phase and plotting it from 8500 psia downwards.

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The result is characteristic of an asphaltenic oil where asphaltene precipitation occurs when the temperature falls below the plotted asphaltene boundary.

Next, the asphaltenic crude asphex3.mfl is loaded and the bubble point line is plotted. The asphaltene line is then selected and plotted from 150 bar upwards.

Finally 0.5 moles of both oils are blended together with the model definition obtained from the asphex2.mfl fluid. The bubblepoint line and asphaltene envelope of the blended mixture are plotted. The result is shown below. The bubble point line is intermediate between those of the two original oils. The properties of the resins and asphaltenes in the blend is calculated from those of the original oils using simple averaging rules; in this case the predicted asphaltene line of the blend is closer to that of asphex2.mfl. The predicted behaviour is highly speculative as there are no data in the public domain which can be used to support any model for the blending of asphaltenic crudes. So for engineering calculations the predictions must be treated with caution. For the details on how to plot the bubblepoint line and asphaltene envelopes, refer to the Asphaltenes case study chapter.

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Input conditions

Introduction Once you have chosen the model for your mixture and selected the components, the next step is to specify the input conditions for the problem. In Multiflash these may be: 

Component compositions



Temperature



Pressure



Volume



Enthalpy



Entropy



Internal energy

The component compositions must be specified but only a subset of the other conditions are needed depending on the calculation to be carried out. Input conditions may be specified in a wide range of units; for information on how to change the units see “Changing units” on page 170. The units currently selected are displayed next to each input field. As you may wish to change the input conditions frequently, the majority are grouped together in the Conditions section of the main window.

If input conditions are defined in a problem setup file they will be displayed when the file is loaded.

Specifying compositions Component compositions can be specified or modified in three places. Normally, they will be defined in the drop down table under Compositions. However, it is possible to enter them under the PVT Lab. Fluid Analysis Form (see ”PVT Lab Analysis” on page 89) or to enter the composition of an inhibitor through the Inhibitor Calculator (see “Inhibitor calculator” on page 79). To specify the amount of any component in a mixture using the drop down table under Compositions activate the table by clicking on the Compositions button.

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The table lists the selected components and the compositions should be entered in the right-hand column, in the units shown. If the composition has been defined in a problem setup file it will appear in the table. The compositions can be changed by overwriting any of the set values. In the drop down table all input units for compositions (amounts) are total amounts in either molar or mass units. There is no requirement that they sum to one or any other value. If you wish to enter mole fractions then the units should be set to mole, and you must ensure that the values sum to one. Otherwise they will be totalled in moles and the fractions scaled accordingly. To enter mass fractions select a mass unit for amounts, eg. g, and enter values that sum to one. When using the stream type option to allocate a selection of the components in the overall stream to a sub-stream the amounts of the components in that substream remain those designated in the Composition drop down table. If they are altered there this will be also be reflected in the overall composition of the stream. N.B. In the Inhibitor Calculator and the PVT Analysis Forms the compositions are specified as mole, mass and occasionally volume%. When specifying composition using the PVT Analysis Form you have the opportunity to normalise the compositions if they do not add up to 100%. Certain criteria apply to all three ways of specifying composition, although the warning messages may differ slightly. The messages shown here are generated from the Composition drop down table. You must define a non-zero amount for at least one component in a mixture for any calculation to proceed, although you may set amounts for some components in a mixture to zero in order to remove them temporarily from the mixture definition. If no composition is defined, i.e. all entries are set to 0, then you will be warned when activating a flash calculation,

If you enter an unacceptable value, e.g., a negative amount in the composition column you will be asked to re-enter the amount once you try to activate a flash calculation or enter another input condition, for example

Note that you can copy the composition of a phase from the results window and paste this into the Composition table to carry out further calculations.

Specifying temperature, pressure and volume For most of the standard flashes you must define either temperature or pressure plus one other input variable. For an isothermal flash, of course, you need both P and T.

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The numerical value for the temperature, pressure or volume should be entered in the appropriate text box, in the units shown. If the quantity has been set in a problem setup file it will be displayed. The values entered are checked when you move to another input condition or attempt to do a calculation or display another dialogue box. If the value is unacceptable, eg. a negative absolute temperature, you will be asked to re-enter the value. If you fail to enter temperature, pressure or volume and this is required for the flash calculation chosen then a warning will be given. For example, if you try to carry out an isothermal flash at fixed P,T specifying a pressure but not specifying an input temperature the following error will be reported Flash at fixed P and T: *** ERROR 259 *** Temperature not specified - Calculation not carried out

Specifying enthalpy, entropy and internal energy These input conditions are used when calculating isenthalpic and isentropic flashes and flashes at fixed internal energy. Enthalpy, entropy and internal energy are defined relative to an arbitrary zero point or datum (see below). Consequently they can take both positive and negative values and, therefore, there is no check on the value entered. If the value entered for any one of them is physically unrealistic you may fail to get a converged solution for a flash calculation and the error message will report this failure. The default datum points for enthalpy and entropy set the value of both to zero in the perfect gas state at 298.15K and 1 bar for each compound. Multiflash provides other choices for the datum points which can be set in the units tab for the property. The enthalpy datum may be set as compounds or elements and for entropy there is a further choice of standard. Compounds corresponds to the default described above. The other settings might be useful in the context of chemical reactions. More complete descriptions of how these properties are defined is given in the sections Enthalpy definition on page 178 and Entropy definition on page 180. As with temperature, pressure and volume if you fail to set an input value for any of these quantities and choose a flash calculation involving them you will see the related error message warning that they have not been specified and that the calculation has not been carried out.

Troubleshooting - input conditions It is always possible to make mistakes when entering numerical values. You should therefore check carefully, particularly if you feel the results appear unusual, that 

the values for the input conditions are correct



they are in the correct units and that



if they are fixed quantities for the flash calculation chosen, they appear correctly in the output

Another problem relating to units may occur if the input conditions are set in a problem setup file, but the input units are not specifically defined. In this case, when the file is loaded, it will be assumed that the values correspond to the units currently set in Multiflash. If this is not your intention it will clearly lead to an incorrect result in your terms, but may not be reported as an error in Multiflash.

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Calculations

Introduction The calculations available in Multiflash may be divided into two broad categories. Firstly there are simple flash calculations where two quantities such as pressure and temperature are specified and the amounts and compositions of all phases at equilibrium are calculated. Other calculation options such as the Phase Envelope or Reid Vapour Pressure depend on a sequence of flashes to follow a phase boundary or to simulate a laboratory procedure. Before carrying out any calculation you must specify the components, compositions and model(s). The other input conditions appropriate to the calculation must also be set. For example, the pressure and temperature for a PT flash. After you have performed a calculation any of the input conditions can be changed and further calculations carried out. Most of the calculations in Multiflash work with a single fluid composition and return a result for a single set of input conditions. If you need to carry out many calculations and generate tabular output this can be done easily using the Multiflash Excel interface.. See the separate document User Guide for Multiflash Excel Interface for details.

The basis of a flash calculation In a flash calculation the overall composition and any two of the following variables are fixed: 

Temperature (T)



Pressure (P)



Volume (V)



Enthalpy (H)



Entropy (S)



Internal energy (U)



Amount of a phase

The flash calculation allows you to determine, subject to the constraints imposed (the two fixed quantities), the number and type of phases present and the composition and properties of those phases. This is based on the thermodynamic principles that at equilibrium

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

The fugacities of each component in all phases are equal



The temperatures of all phases are equal

Calculations  141



The pressures of all phases are equal

Multiflash offers a comprehensive range of flash calculations, a list of which is given below. The most widely used of these are then discussed individually.

Flashes available in Multiflash The following flashes may be calculated in Multiflash. Standard flashes P,T flash

Isothermal flash

P,H flash

Isenthalpic flash at fixed pressure

T,H flash

Isenthalpic flash at fixed temperature

P,S flashIsentropic flash at fixed pressure T,S flash

Isentropic flash at fixed temperature

H,S flash

Flash at fixed enthalpy and entropy

P,V flash

Isochoric flash at fixed pressure

S,V flash

Isochoric flash at fixed entropy

T,V flash

Isochoric flash at fixed temperature

U,V flash

Flash at fixed internal energy and volume

P,U flash

Flash at fixed internal energy and pressure

T,U flash

Flash at fixed internal energy and temperature

Bubble and dew point flashes P, Dew point Dew point at fixed pressure T, Dew point

Dew point at fixed temperature

T, Retrograde dew point

Upper retrograde dew point at fixed temperature

P, Bubble point

Bubble point at fixed pressure

T, Bubble point

Bubble point at fixed temperature

Fixed phase fraction flash P, Fixed phase fraction Flash with a fixed amount of a specified phase and fixed pressure T, Fixed phase fraction

Flash with a fixed amount of a specified phase and fixed temperature

All the above flashes may be activated from the Calculate menu and selecting the flash type and, where appropriate, the particular flash. The flash conditions are taken from the Conditions section of the main window. The most widely used flashes can also be activated by clicking on the appropriate button in the tool bar.

Isothermal (P,T) flash A calculation of the equilibrium conditions of a given mixture at specified pressure and temperature is called an isothermal or P,T flash. It allows you to determine the number and type of phases present and the properties of those phases. It is the most widely applicable and the most reliable of the flash calculations. It also has a unique solution. To carry out an isothermal flash for a given mixture:

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Ensure you have chosen a model Enter the temperature and pressure in the Conditions section Click on the tool bar button , or, alternatively, choose P,T, Flash from the Standard Flashes list in the Calculate menu.

Isenthalpic flashes These are flashes at fixed enthalpy. The most useful is the P,H flash which is used when studying flow through valves or pipelines. A typical calculation sequence is: Determine the enthalpy of a stream at a given P,T (isothermal flash) and enter the calculated enthalpy in the Conditions section Drop the pressure by entering a lower pressure under Conditions Click on the

button or specify P,H flash from the menu bar.

The P,H flash also has a unique solution. You can also plot lines of constant enthalpy on a phase diagram. See the description on page 148.

Isentropic flashes The isentropic, fixed entropy, flashes are used when you are looking at adiabatic and reversible processes such as a turbo expander where you would use the P,S flash. Like the P,H flash the P,S flash has a unique solution. The H,S flash has applications in the turbine industry. As with the isenthalpic flash, enter the value for the entropy which you want to remain fixed in the Entropy box, and enter the value for the other variable (usually P or H). The P,S flash may be activated through either the tool bar button or the menu, but the latter is the only option for activating the H,S flash. You can also plot lines of constant entropy on a phase diagram. See the description on page 148.

Isochoric flashes The isochoric or fixed volume flashes are used when looking at closed systems, such as vessels. The two most used are the T,V flash where you know the temperature and the U,V flash where you know the internal energy. Both of these flash specifications have unique solutions. Neither of these flashes has a tool bar button assigned, both are activated only through Calculate in the Menu bar. You can also plot lines of constant volume on a phase diagram. See the description on page 148.

Bubble and dew point flashes In a simple two phase system a dew point is the first point at which liquid appears. This will be a temperature if the pressure is fixed and a pressure if the temperature is fixed. In a multiphase situation it is possible to have more than one dew point (you will probably have more than one liquid phase). The dew point calculation will return the primary normal dew point, ie. the temperature (at fixed pressure) at which the first liquid phase appears as the temperature is reduced or the lowest pressure at which a liquid phase appears (at fixed temperature).

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In most oil and gas systems there is also a retrograde dew point corresponding to retrograde condensation or condensation of liquid from a gas phase as the pressure is reduced. The retrograde dew point pressure (at fixed temperature) is the pressure at which a liquid phase appears as the pressure is reduced Dew point calculations can be activated from the Calculate menu or toolbar buttons:

and

for normal dew points and

for retrograde dew points.

Dew point calculations are particular applications of a fixed phase fraction flash (see below). Dew points corresponding to the appearance of each liquid phase can be calculated with the fixed phase fraction flash by specifying the name of the phase and setting the fixed amount of the phase to zero. A common application is to calculate both the hydrocarbon liquid dew point and water dew point in oil and gas systems. The bubble point is the first point at which gas appears as the pressure is reduced at fixed temperature or the temperature is increased at fixed pressure. Bubble point calculations can be activated from the Calculate menu or toolbar buttons:

and

.

Depending on the temperature or pressure specified and where this is in relation to the phase envelope and critical point it is possible that a dew point or bubble point calculation does not have a solution.

Fixed phase fraction flashes This is a flash where the temperature or pressure is fixed plus the fraction of one of the possible phases for a given mixture. The phase fraction may be defined in molar, mass or volume units. The fraction may take any value between 0 and 1. The Fixed Phase Fraction Flash (FPFF) is a generalisation of the dew and bubble point calculations described above.

Phases Multiflash is a multiphase phase equilibrium program that can handle up to twenty possible phases at any time with the current configuration. Any individual calculation will consider the possibility of all specified phases but the maximum which may exist together at equilibrium is limited to seven. The phase types that are included in the current version of Multiflash are 

Vapour



Liquid



Pure solid



Fixed composition solid



Hydrate



Wax



Asphaltene

It is clearly possible to have more than one of all these types except vapour. However, the software and models are structured such that it would not be sensible to define more than one wax or asphaltene. In order to identify each phase uniquely they are assigned names and, in some cases, key components, see “What the model definition means” on page 312. The standard set of names used for the different phases is listed below.

Phase names GAS

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LIQUID1

first liquid phase

LIQUID2

second liquid phase

WATER

liquid phase with key component water

ICE

solid phase with freeze-out model used for water

HYDRATE1

hydrate phase, structure I

HYDRATE2

hydrate phase, structure II

HYDRATEH

hydrate phase, structure H

WAX

wax phase

ASPHALTENE

Asphaltene phase with key component asphaltene

If you apply the freeze-out model to any phase the default name is generated by adding “solid” before the component name, e.g. SOLIDDECANE. Fixed composition phases are included for the halide scales: NaCl, NaCl.2H2O, KCl, CaCl2.2H2O, CaCl2.4H2O, CaCl2.6H2O, NaBr, NaBr.2H2O, KBr and CaBr2.6H2O. It is possible to create phases with any name, not just the standard list shown above. If you load a problem setup file that was not created in the Multiflash GUI you may see other names. The phase names are used to identify the different phases when carrying out fixed phase flashes and to identify the phases in the Multiflash output. A supercritical phases (often termed ‘dense phase’) represented by equation of state models cannot be assigned an unambiguous phase type. In other words there is no way to distinguish between a gas phase and a liquid phase. The rule used in Multiflash is that a supercritical phase is labelled as GAS if VT2 > VcTc2, where Vc is the pseudo critical volume and Tc is the pseudo-critical temperature.

Key components A key component helps to identify a particular phase. The rule used is that the key component should be present in the phase to the maximum amount relative to the total mixture composition. Alternatively a minimum key component can be specified; this means that the component should be present in the phase in the minimum relative concentration. Since version Multiflash 4.4, multiple key components can be specified for the phases. The phases set up by the Multiflash GUI identify one liquid phase (WATER) with aqueous components as key components as the key component and the other two liquid phases (LIQUID1 and LIQUID2) with aqueous components as a minimum key component. The aqueous key components are the mostly used polar components such water, methanol, ethanol, glycols, etc. Key components are only needed when a flash calculation must identify a phase uniquely (e.g. search for a particular phase fraction). Examples are the fixed phase fraction flash or a phase envelope calculation. If you request any of these calculations for a phase without a specified key component, an error message will be returned. For example, if you try to calculate the water dew point when there is no water in the mixture you will see the following message: *** ERROR 20148 *** Key components of fixed phase not present or in trace amounts. *** ERROR 21011 *** Illegal input specification

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The other two liquid phase descriptors are not distinguished in terms of key component as these are set in the model definitions prior to any components being defined. In most cases the fixed phase fraction flashes and phase envelope calculations will solve without any additional information. However, if the lack of unique identification may lead to problems in reaching the correct solution a warning will be issued. *** WARNING -20131 *** Key component not specified for multiple liquid/solid phases To prevent potential problems you should nominate at least one key component for one of two non-aqueous liquids. To do this it is necessary to use the Command window from the Tools menu. See ‘Defining phase descriptors and key components’ on page 314. See also the command reference manual. For example the following command will set heptane as the key component (present in highest concentration) for the phase liquid1. keys liquid1 heptane; Another possibility is to use the following specifications for selecting the lightest and heaviest liquid phases: keys liquid1 heaviest; keys liquid2 lightest; Whilst the latter may appear convenient you should remember the rule that the key component should be present in the phase to the maximum amount relative to the total mixture composition. If you have a mixture rich in methane then when the first liquid forms it may have more methane than the heaviest component and thus be labelled liquid2. For the same reason when specifying a liquid in terms of a specific component it is often more useful to choose one in the middle on the component range, e.g. heptane, rather than the heaviest hydrocarbon. Another occasion when you need to be particularly careful in your choice of key components is when using an Excel spreadsheet to carry out linked flashes or recycles where the composition of your streams can change significantly and hence the phase labelling can change even though there is no actual phase change.

Using the fixed phase flash To use the fixed phase flash either click on the fixed phase flash button for fixed pressure menu.

or fixed temperature

or select the option from the Calculate

The following dialogue box will be displayed.

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To see the list of possible phases click on the arrow to the right of the Select Phase list box. Select the phase you want to fix by clicking on the phase name; this will then appear as the selected phase. Then Select the basis for the fixed phase fraction flash. The options are mole fraction, mass fraction and volume fraction for general use and nucleation for use in hydrate calculations. Enter the fraction of the phase that you want to fix. This fraction must be between 0 and 1. To search for the temperature or pressure at which the phase first appears (the phase boundary) set the phase fraction to 0. However you can also look for the conditions at which there is a specified fixed amount (>0) of any phase. Finally click on Do flash. For systems exhibiting normal dew and bubble points the section to the left of the dialogue box labelled “type of solution” can remain with the Normal (default solution) option button selected. However, most oil and gas systems exhibit more complex phase behaviour. A typical phase diagram is shown below.

The critical point is where the gas and liquid phases become identical, having the same density and composition. It is also possible to have liquid-liquid critical points where two liquid phases become identical. The cricondentherm is the maximum temperature at which a two phase mixture can exist and the cricondenbar is the maximum pressure at which a two phase mixture can exist. If the pressure is reduced along an isotherm from the liquid or dense gas region (to the left of the critical point) it reaches its bubble point, where the light components no longer remain dissolved in the heavier liquid components and separate off as a gas. In gas condensates the gaseous components are in excess and the heavier liquid components in the minority. In this case, as the pressure is reduced (to the right of the critical point) the liquids drop out of the gas phase at the retrograde dew point and the amount of liquid increases as the pressure decreases. This is known as retrograde. condensation.. As the pressure is reduced further, the liquid components evaporate again and the liquid disappears at the normal dew point. For example, consider the phase envelope shown above, which has a large retrograde region. Taking a temperature of 300oF, for example, there are two dew points; one at 2.5 psia (the normal dew point) and one at 2977 psia (the retrograde dew point). The Type of Solution setting allows Multiflash to calculate either the normal or retrograde solution to a fixed phase fraction flash.

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In the special case of the dew point there are dedicated calculation options as described previously. The Unspecified setting for the type of solution is sometimes useful when dealing with complex phase behaviour that does may be quite different from the familiar Vapour + liquid situation discussed above. The Unspecified setting will cause Multiflash to search for the nearest solution to its starting point without any distinction between normal and retrograde behaviour. One example of where Unspecified may be needed is when calculating an asphaltene phase boundary above the bubble point. TIP

If you find a situation where you get failures for dew and bubble point calculations, or the solutions are not in the region you expect, then it is worth carrying out some isothermal (P,T) flashes or to plot the phase diagram.

Phase Envelopes The Phase Envelope utility will trace any phase boundary including gas, liquid and solid on pressure-temperature co-ordinates. The phase envelope utility will work with any Multiflash model but some models, such as activity coefficient models, are only valid at low pressures and will not produce closed phase boundaries. Other features include: 

the ability to trace selected phase boundaries on the basis of mass or volume fraction in addition to mole fraction and the nucleation boundary.



the flexibility to include the plotting of constant enthalpy, entropy, volume or free energy boundaries.



A generalisation of the plotting facility so that for any chosen phase boundary it is possible to plot any phase or property against another.

The use the Phase Envelope utility click on the Phase Envelope toolbar button, or select the option from the Calculate menu. A window similar to the following is displayed:

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The buttons along the bottom of this window perform calculations or other actions the three tabs that control what is calculated and plotted.

VLEAutoPlot The VLEAutoPlot button will generate a vapour-liquid phase boundary starting from the dew point line at low pressure. This is the usual phase envelope calculation. None of the settings on the Phase tab or Initial Values tab are used by this option. It is possible that the message box shown below will be displayed

The default for the maximum number of points to calculate is 100. This may not be sufficient to complete the phase boundary. Clicking on Yes will allow the calculations and plot to continue. If the calculations have reached any limit within the 100 points the message will not appear.

Phase tab The Type of Solution, Select Phase, Select Basis and Phase Fraction boxes are used when you click on the Plot button. The Select Phase box allows you to select the specific phase for which you wish to plot a boundary, for example WATER. The Phase Fraction box sets the value of fraction of the phase selected to be plotted. For example, to plot the water dew point line select the WATER phase and set the phase fraction to 0 and click on Plot. A phase fraction of zero should always be used to plot the boundary where a phase first appears (or disappears). Phase fractions greater than zero will plot lines where the selected phase is present with the fraction specified. As with the dew, bubble and fixed phase fraction calculations there may not be a solution for

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the specification made. For example, if the phase fraction of the water phase is set as 1.0 it is unlikely that there will be a solution unless the composition is almost pure water. The Type of Solution settings have the same meaning as discussed previously. Normal should usually be selected. The Select Basis box allows you to select the property to be plotted.

The mole fraction, mass fraction and volume fraction properties refer to the fraction of the specified phase. To calculate a phase appearance boundary (with a fraction of 0) it is recommended that you select the mole fraction basis. Selecting enthalpy, entropy, volume or internal energy will disable the Type of Solution and Select phase boxes because the property line plotted is the total for all the phases present. The value of the property to be plotted should be entered in the units shown. The units can be changed by clicking on the Units button. Click the Plot button to plot the constant property line.

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Initial Values Tab The settings on the Initial Values Tab are used when the Plot button is clicked. They are disregarded for the VLEAutoplot operation.

The Initial Values section allows you to set a pressure or temperature value for the start of the phase boundary or property plot.. The Start from setting selects either the pressure or temperature as the starting point and the Initial value to section sets the initial direction for the pressure or temperature. Pressure increasing from 1 bar is the default setting and is usually a good choice for many phase boundaries. However, you should be aware that occasionally there may not be a solution for a particular boundary at this pressure and it may be necessary to vary initial conditions to start the tracing of the phase boundary. When plotting phase boundaries for solids it is often a good strategy to start from a high pressure and change the direction to decrease.

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If the boundary still proves difficult to trace, starting values for both the temperature and pressure can be provided. Checking the Use starting value box will then provide Multiflash with an initial guess for the starting point and may make it possible to trace the phase boundary.

Frame Tab The Frame tab allows you to set P,T boundaries for the calculation and to set the properties you wish to see displayed on the x and y axes.

The section headed For Calculation allows setting of minimum and maximum values for temperature and pressure that will be used when calculating a phase boundary or property line. It is usually best not to enter any limits because the ranges for the calculation are usually unknown in advance. If the initial point on a line falls outside the specified range no further calculations will be done. The For Plotting section controls how the results of the calculation are displayed. Normally the x-axis displays temperature and the y-axis displays pressure. However these settings can be changed to enthalpy, entropy, volume, internal energy or the fraction of any phase. All the possibilities are shown in the drop-down list for each axis. The minimum and maximum values for plotting on each axis can also be set. If the T,P range for calculation is limited then the plotted values will also be limited irrespective of the plotting range set.

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A typical phase envelope plot might be a VLE plot for a hydrocarbon fluid. The axes will be pressure versus temperature.

By changing the plotting variables it is also possible to look at the liquid mole fraction as a function of temperature.

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Before the new plot is displayed you will be reminded that you have altered at least one of the axis properties and that boundaries plotted on the previous axes will be lost.

If you wish to return to the standard P,T axes to plot you must remember to alter the axes yourself in the Frame tab, they will not be reset automatically. The Frame tab also allows you to change the maximum number of points calculated at any one time. A limit is needed because some phase boundaries do not have a natural end point or form closed loops. The default value for the maximum is 100. This limit can be increased if desired.

Phase Envelope Output A typical phase envelope output is shown below. The boundary labelled V/L=0 is generated with the VLEAutoPlot function. The other phase boundaries are added one at a time with the Plot function.

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Any critical points are labelled with a C and discontinuities, where phase boundaries cross, are marked with a D. The numerical values for each point plotted are displayed in the main window. Critical points or discontinuities are marked with a C or D in the table. Phase envelope output: T/K P/bar 39 294.300 110.80 40D 294.300 110.80 41 294.304 110.95 42 294.306 111.03 The first column is the number of the point plotted.

Customising the phase envelope plot From the Phase Envelope window you can choose to keep or delete the most recently plotted phase boundary or to clear the whole plot. If you do not clear the plot any new phase boundaries will be added to the existing plot. This is a useful way to look at how multiple phases relate to each other or to plot quality lines (lines for differing amounts of a liquid phase) for any phase envelope.

Options The Options button allows you to delete or customise each line on the plot.

You can change the label, the colour and line style. Tool bar buttons on the plot window also give access to the controls of the graphics package. The Edit button provides detailed adjustments.

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Add Data The Add Data button allows you to add data points to the plot. These would typically be experimental points that you wish to compare with predictions. Clicking on Add Data displays a table.

Enter the data to be plotted (which can be pasted) and click on Save and Plot. Several data series can be added.

Write to Excel Although you can call all Multiflash functions directly from an Excel spreadsheet, including the Phase envelope function, you can also transfer the plot and phase boundary points generated in the Multiflash GUI to Excel. Once you are happy with a plot clicking on Write to Excel will generate a spreadsheet with a Chart corresponding to the tabular values reflected in the main window with the values themselves posted to a worksheet. The “Write to Excel” facility is supported for Excel 97 onwards.

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PT flash It is possible to do a PT flash at any point on the phase diagram plot. Moving the cursor across the plot displays the temperature and pressure as X, Y coordinates above the plot.

Right-clicking the mouse displays the PT Flash button. Click on this button to carry out a PT flash at the conditions shown. The output is displayed in the main window.

Phase Envelopes for solids The phase envelopes for solids button, is designed to make it easier to generate all the fluid and solid phase boundaries simultaneously by just clicking the button. The phase boundaries plotted depend on the models and phases currently defined. If there are no solid phases present, only the V/L phase envelope will be plotted. A PT flash can be carried out by right clicking with the mouse as described above. The following plot shows an example including hydrates, ice, water, liquid hydrocarbon and vapour phases.

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The next example shows a complex phase diagram including hydrate, ice, wax and asphaltene phases together with the usual vapour, hydrocarbon liquid and water phases.

Liquid dropout curve calculation Liquid dropout curves are routinely measured by PVT Laboratories as part of a constant mass expansion experiment on condensates. The liquid dropout is defined as the volume percentage of the hydrocarbon liquid phase at a given pressure relative to the total volume of the mixture at the upper retrograde dewpoint. The experiment is carried out at a fixed temperature. To calculate a liquid dropout curve choose the option from the Calculate menu or click on the toolbar button A typical example of liquid dropout curves at two different temperatures is shown below.

It is possible that an upper retrograde dewpoint is not found at the given temperature, ie. the condition is on the bubblepoint side of the critical point. In such cases the volume of the liquid phase is calculated relative to the total fluid volume at the saturation point (bubble point) at the given temperature. An example is shown below.

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Hydrate calculations All the flash calculations in Multiflash can include solid phases such as hydrates. The hydrate dissociation temperature or pressure can be calculated using the Fixed Phase Fraction Flash. However, since there is a possibility of up to three hydrate phases (with different structures), specific calculation options for hydrates are provided to simplify the task. To calculate the hydrate dissociation temperature at fixed pressure or hydrate dissociation pressure at fixed temperature select the Hydrates option from the Calculate menu or click on the appropriate toolbar button: or . The flash will return the temperature or pressure at which a hydrate phase is present in zero amount (the dissociation point). The hydrate structure and all the phase properties are also displayed. For more information on calculations with hydrate see the section Case studies Hydrate dissociation, formation and inhibition on page 243 .

Wax calculations As with other solids, a wax phase can be included in any flash calculation in Multiflash. In addition there are two dedicated calculations involving wax.

Wax Appearance Temperature The wax appearance temperature (WAT) is the temperature at which a small amount of wax phase appears as the mixture is cooled at a specified pressure. An experimental measurement of the WAT requires a detectable, non-zero, amount of wax to be formed. However, the calculation of the WAT is very sensitive to small amounts of heavy alkanes in the mixture so very different WAT values can be obtained depending on the criteria for detectability. The two most common experimental techniques are Cross Polar Microscopy (CPM) and Differential Scanning Calorimetry (DSC). The CPM technique is more sensitive than DSC and, in general, detects the presence of a smaller amount of wax. Hence the WAT measured by CPM will usually be higher than the measurement by DSC. After reviewing the WAT measurements in our database we have developed guidelines for detectability limits for the two techniques and these are included in the WAT calculator. To do a WAT calculation click on the toolbar button or select the Wax option from the Calculate menu. The following window is displayed.

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For CPM we suggest a criterion of 0.045 mass percent wax as the detectability limit and for DSC the suggested value is 0.3 mass percent. It must be emphasised that these figures are guidelines and may not apply in all cases. Clicking the Calculate WAT button will calculate the temperature corresponding to the wax percentage specified at the pressure set in the input conditions.

Wax Precipitation Curve The wax precipitation curve option calculates the amount of wax precipitated as a percentage of the wax plus oil phases as a function of temperature. Measurements of the amount of wax formed as a function of temperature are, in general, more reliable than WAT measurements which, by definition, are at the measurement limit. To calculate a wax precipitation curve click on the toolbar button or select the Wax option from the Calculate menu. A window similar to the following is displayed

Enter the pressure for the calculation in the pressure box and click Calculate. The curve is displayed in the plot window and a table of values is shown in the Multiflash output window Wax Precipitation Curve Pressure: 1. T (degC) 0. 4. 8. 12. 16. 20. 24. 28.

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bar

Wax mass percent in liquid(+wax) 5.12253 4.52423 3.89114 3.23714 2.57435 1.93908 1.40141 1.00565

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32. 36. 40. 44. 48. 52. 56. 60. 64. 66.40

0.725042 0.525915 0.377582 0.271559 0.195085 0.131349 0.0696203 0.0172402 0.00132507 0.

The starting temperature is 0°C, or the equivalent in other units, and the finishing temperature is the calculated WAT for zero % wax. The maximum number of points is twenty but the actual number of points will depend on the WAT, the units used and the step size.

Tolerance calculations Tolerance calculations are used to determine the amount of a component or mixture that must be added to the original stream to achieve a given phase split. A typical application is to determine how much inhibitor is required to suppress hydrate formation. Another example might be to determine the amount of water needed to saturate a gas. Tolerance calculations are carried out at fixed temperature and pressure and for a fixed phase fraction specified in moles. To carry out a tolerance calculation specify the model and the components in your stream. These should include the component or components for which you wish to determine the amount to be added. In the Composition table specify the composition for your mixture. The component/components for which the tolerance calculation is being carried out need not be there in zero amount if they are already present in the stream. However, it is more usual to set their composition to zero, for example zero amount of methanol when you wish to calculate the amount needed to inhibit hydrate formation. Using Calculate/Tolerance Calculation opens the following window

In the Phase Specified tab window set the phase and the fraction of that phase for the fixed phase element of the flash. In the Composition of Second Fluid tab enter the amount of the single component (usually 1.0) or the composition of the mixture to be added to the main stream to meet the phase fraction constraint set.

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Assuming you have already set the temperature and pressure click on the Calculate button. The results in the main window will show the amount of second fluid to be added to the original stream to meet the set constraints. In the example this is the amount of methanol to be added to the original inhibitor free stream to meet the condition of zero hydrate phase, ie. to prevent hydrate formation at the given conditions..

Reid Vapour Pressure The Reid vapour pressure (RVP) is usually employed by refineries to quantify and modify the vaporization of gasolines and other volatile petroleum products. RVP is a measure of the volatility of a fluid; it is reported as the gauge pressure of a unit volume of the petroleum fluid in equilibrium with 4 unit volumes of air at 100 °F (37.8 °C). The Reid vapour pressure of a fluid is different from its true vapour pressure, as it is calculated considering air as part of the fluid composition. Reid vapour pressures are obtained experimentally according to the ASTM-D323 test method. This standard is for volatile crude oils and volatile non-viscous petroleum liquids and it excludes liquefied petroleum gases. To calculate the RVP of a mixture select Reid Vapour Pressure from the Calculate menu. The calculation mimics the experimental procedure, as described in the ASTMD-323 method. This standard requires that the mixture is one phase liquid or liquid + vapour at the simulated experimental conditions. Water should not be present in the mixture. Other phase behaviour, such as multiple liquid or solid phases are not allowed by the experimental method and Multiflash will generate and an error message No liquid phase or multiple condensed phases were found at the flash conditions Another error message will be shown if the fluid is not sufficiently volatile to be considered a volatile petroleum product:

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Negative Reid Vapour pressure obtained. Mixture pressure is lower than 1 atm Reid vapour pressure calculations can be done with both real components and petroleum fractions using any appropriate thermodynamic model. However, special care is recommended when selecting the model for the calculation. While for most of the standard volatile fuels any cubic equation of state can be selected, when dealing with oxygenated fuels, such as mixtures with ethanol, the CPA Infochem model should be used to ensure that the correct phase behaviour is predicted. The following example uses a mixture of 93.57 mol % isopentane and 6.43 mol % ETBE (ethyl tert-butyl ether) (file ReidVP_ex1.mfl). The reported experimental Reid vapour pressure is 19.1 psi. Using the RKSA model s calculated value of 18.8 psi is obtained.

Note that this example assumes that you have a DIPPR databank license this is because ETBE is not available in Infodata databank and must be defined from the DIPPR databank. If you do not have a DIPPR license, the example ReidVP_ex2.mfl can be used instead. This is a mixture of different gasoline components (in the range C5-C7) and ethanol. As ethanol is part of the mixture, the CPA model should be used to ensure realistic predictions (the RKSA model will wrongly predict liquid-liquid phase splitting).

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The calculated value is now 10.8 psi. A low volatility gasoline will have a Reid vapour pressure around 6 psi, while a high volatility gasoline will be in the range of 15 psi. The example presented thus corresponds to a hypothetical gasoline of intermediate Reid vapour pressure

Property output in Multiflash Multiflash provides several levels of physical property output following a flash calculation. The default is to list the phases present , with their amounts, compositions, thermal and volumetric properties. It is possible to get more output such as heat capacities and compressibility and fugacity and activity coefficients for each component in each phase. You can also calculate and display diffusivity provided you have checked the “include diffusivity coefficient” box when defining the model set. The output can also be reduced to a minimum of phases and compositions. The level of output can be set using the Select/Property Output menu option. The following dialogue box will be displayed

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You can also trigger the same dialogue box using Tools/Preferences to set your preferred level of property output as the default. Activity coefficients may not be a valid property for all components in all phases. This will depend on the component and the model. If the property is not valid N/A will be displayed. The default setting is equivalent to the first two options being selected plus the transport properties. The later will be omitted if no transport models are defined To reduce the amount of output deselect any properties not of interest. If you choose Heat Capacity and Speed of Sound etc. the phase amounts and composition plus the volume and thermal property output are also automatically selected. Examples of the property output options are shown in “Calculation output” on page 175.

Troubleshooting - flash calculations This is the most difficult area for which to give general guidance. Multiflash is capable of handling complex mixtures which may exhibit multiphase phase equilibrium. Some flash calculations gave a unique solution. These include (P,T), (P,H), (P,S), (T,V), (U,V) and (S,V). In principle, these flash calculations should always have a solution. However, in practise, the range of conditions for which a solution can be found depends on the models used. For example, if the volume specified for a flash is smaller than the b parameter for a cubic equation of state it is not possible to solve any flash that includes the volume. Dew and bubble point calculations (and fixed phase fraction flashes in general) are common cases of flashes that may have no solution, a single solution or multiple solutions. When one of these calculations fails it will produce output similar to the following in the main output window Bubble point at fixed P: *** ERROR 20292 *** Cannot find converged point - max. iterations *** ERROR 20024 ***

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Cannot find starting point for calculation - there may be no solution. *** ERROR 344 *** The flash calculation has not converged The difficulty is to assess the reason for the error and how best to investigate the cause and take corrective action. An error may be reported when there is genuinely no solution to the problem posed. The error above, for example, resulted from asking for a bubble point at a pressure above the cricondenbar for the gas condensate discussed earlier. As you can see from the phase diagram presented there is no bubble point at this pressure as it is above the pressure at which the two phase mixture will exist. In general you will not be able to solve bubble or dew point problems at pressures above the cricondenbar or temperatures above the cricondentherm. Another type of phase envelope where problems can arise is shown below

As you can see the phase envelope turns up at low temperatures and high pressures. This is also typical of phase envelopes with a significant amount of hydrogen where you may not find a solution for the bubble point at low temperatures. What can you do if the method used to find a solution is not capable of solving the problem posed? There are various strategies which may help:

Plot the phase envelope The phase envelope plotter is a convenient way to determine the phase boundaries and what phases you might expect to be present under given temperature and pressure conditions. For information on plotting the phase envelope see “Phase Envelopes” on page 148. However, plots at high pressure based on activity models will not be meaningful.

Use the P,T flash The P,T flash is the most reliable of the flashes and the least likely to fail. It can be used where you are experiencing problems with other flashes to see where you are in terms of the phase envelope. Even the P,T flash may occasionally have problems. Again hydrogen containing mixtures can pose difficulties; it may not be possible to find a solution where the mixture is all liquid even using an isothermal flash.

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You can use a right click in a phase envelope plot to carry out a PT flash anywhere on the plot. See the description on page 157.

Limit the number of phases Multiflash is designed to consider the possibility of all the phases specified in the Model Set being formed and to establish the most stable solution. The model sets for equations of state are generally configured for four phases, GAS, LIQUID1, LIQUID2 and WATER. If you are having problems finding a solution and think you know there are fewer phases, possibly only GAS and LIQUID1, or LIQUID1 and LIQUID2, then it may be worth while reducing the number of phases to be considered. This can be done by switching off one of the phases in the Model Set dialog box.

Consider all types of solution As we have discussed above, for complex mixtures and phase diagrams you may not find the solution in the region you want by the “normal” solution route. Plot the phase envelope or carry out some P,T flashes to see what type of phase envelope you have and whether you should be seeking a retrograde solution.

Provide a starting estimate If you feel you have specified the flash, correct input conditions and correct type of solution but convergence errors are still being reported it may be worth providing a starting guess for temperature or pressure. This is useful for any flash other than the PT flash where both quantities are already specified. From the Select menu click on Use Starting Values.

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Carry out the flash calculation. The temperature or/and pressure (as appropriate for the flash calculation) set in the input conditions text box will be used as the starting value for the calculation. To turn off the use of starting values, use the Select menu again and click to remove the check mark by Use Starting Values . Unfortunately in the vicinity of the critical point the use of starting values may not be sufficient to give convergence as calculations in this region can be particularly sensitive to estimates of the composition.

Provide a key component As we have discussed earlier, “Key components” on page 145,. default key components are defined for the standard phase descriptors. If difficulties occur in uniquely identifying a phase then providing a more specific key component for a phase may help find the solution.

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Units

Introduction A significant proportion of the apparent errors and problems reported for Multiflash can be tracked down to a mismatch between the units the user thinks he/she is using and those currently set in the software. The importance of matching units cannot be stressed too much. Having said that, what units are available for you to select? Internally all Multiflash calculations are carried out in SI units. This cannot be changed. Unless altered by the user, the default input and output units are also SI and any numerical input values for any property will be assumed to be in SI. The input values of component amounts for any calculation are total amounts. The output defaults are that individual compositions of any phase are given as fractions but the amount of each phase is the total amount.

Default units In the absence of any other information the default input and output units are also SI. However, if you prefer to work in other units most of the time it is possible to set these as your preferred option. This is done through the Tools menu by selecting the Preferences/General option. Under the Default Units tab select your choice of units for any property. In the example below the default unit for pressure is set to bar rather than Pa.

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If you exit Multiflash and re-launch the software the pressure units will automatically be set to bar. However, you should be aware that if you load an .mfl file where you had included numeric values for pressures, but not specified the units, these will now be assumed to be in bar. This problem can only arise if you create your own .mlf files using a text editor rather than using the Multiflash GUI. The input amounts for any calculation are total amounts. They may be set as percentages in the PVT Analysis Form or the Inhibitor Calculator, but they will be converted to absolute amounts for calculation or display in the Composition drop down table. The output defaults are that individual compositions of any phase are given as fractions but the amount of the phase is the total amount. As with any other units these defaults can be changed.

Changing units You may change any of the input or output units at any time. This does not affect the default unit settings described above. From the Select menu choose Units or click on the Select input and output units toolbar button

170  Units

. This displays the Unit Selection window:

User Guide for Multiflash for Windows

To view the currently selected units for any property click on the corresponding tab. To change the input units for any property click on the option button next to your choice of unit in the Input unit column. Do the same in the Output unit column to change the output units for that property. The input and output units may be different. The units for output (calculated) composition may be changed from fractions to total amounts on the Amounts tab. It is not possible to change the input composition to fractions. The Unit Selection window can be also accessed by clicking the Units button from a dialog box such as Phase envelope, Tolerance calculation, defining Petroleum fractions, Pure Component Data, Matching options, Tables etc. Units can be also be altered as a "block" by choosing "All Metric" or "All British". Units for all properties will be reset to: All Metric; mole, DegC, bar,m3/mol, J/mol, J/mol/K, cP, W/m/K, N/m, m2/2 All British; lbmol, DegF, psi, ft3/lbmol, BTU/lbmol, BTU/lbmol/F, cP, BTU/hr/ft/F, dyne/cm, cm2/s If you save a problem setup the unit definition will also be saved as part of the problem setup file.

Troubleshooting - units The only problems we have encountered with units arise from a mismatch of those currently set in the software and those assumed by the user. It is always worth checking the units are what you expect if you run into problems.

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Output

Introduction The numerical output from Multiflash appears in the results window which occupies the bulk of the main window. The output is also written automatically to a log file called MFLASH.LOG. It is also possible to print results or to write them to another file of your choice. How you do this will be discussed in “Writing the results to a file” on page 174.

The results window As soon as Multiflash opens the banner appears in the results window.

The banner contains information on the serial number for your copy of Multiflash; the time and date of the current run for documentary purposes and information on the path of the Multiflash application files location. It also shows the contents of the MFCONFIG.DAT file if you have one. The path for the location of the Multiflash log file can be found from the “About Multiflash …” under the Help menu of the Multiflash main window. When a problem setup file (.mfl file) is loaded the contents of the files are also displayed in this window. Before the output from each calculation, the results window displays a separator followed by a comment line to identify the calculation. For example Dew point at fixed T: A confirmation message may be displayed be certain menu options. For example, the Clear Problem Setup option in the File menu shows the following message to confirm that the previous setup has been removed Clear current problem setup

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All the results from the Multiflash calculation will be sent to the results window, including comments, warning and error messages. This output will be considered in detail later. There is a limit of 300kb of text that can be stored in the results window. Earlier results will be overwritten once this limit is reached. Similarly if you choose to write the output to a file of your own choice only the contents of the results window will be stored. However, all results will be written to MFLASH.LOG unless the results from any set of calculations exceed 300kb. If some of the output is not visible in the results window then it can be viewed by scrolling vertically or horizontally. It is also possible to extend the size of the window by dragging.

Font The default font for displaying results in the output window is Lucida Console, 10 pt. This can be changed through the Tools menu by selecting the Preferences/General option and the Sheet Configuration tab and clicking on the Fonts button.

The possible fonts available will depend on the particular PC installation. When changing fonts we recommend that you always choose a mono-spaced font so that the column format is retained.

Writing the results to a file As well as being displayed in the results window the output is also sent to a log file which may be examined later. The log file is called MFLASH.LOG. Each time Multiflash runs the previous log files are renamed sequentially as MFLASH_1.LOG, MFLASH_2.LOG up to MFLASH_10.LOG. This allows you to refer back to earlier Multiflash sessions. The oldest file MFLASH_10,LOG is deleted when a new Multiflash session is started. If you wish to keep any of the log files you should rename them. You can also save all the output to a file at any time using the Save Results option from the File menu. A confirmation message is displayed

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If you have previously selected part of the output the following message is displayed.

A file save dialogue allows you to choose the file name and folder. The default extension for output files is .out, but you may choose any extension that is allowed by Windows.

Printing the output You can print the output from your calculations by selecting the Print Results option from the File menu or clicking on the toolbar button

.

The Print window allows you to configure your printer as in any Windows application. If you only wish to print part of the output you should select the relevant section by highlighting it with the cursor. You then select print results as above. Alternatively you can cut or copy the relevant sections and paste them to another application, such as Word using the Edit menu.

Calculation output Calculation output will vary slightly depending on the type of calculation and the level of physical property output specified but will be in the style

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The first line of output reports the temperature and pressure. These will be either the input conditions or the values calculated from the flash. This is followed by three messages: No. Phases reports the number of phases that have been found at the particular temperature and pressure. CONVERGED means that the calculation has been completed successfully. If a solution has not been found then error messages will be produced and any results would be labelled FAILED. If a warning has been generated then you may see the message ?UNCONVERGED. In the latter two cases the error or warning messages should be read carefully to assess the problem. Multiflash can only check for the stability of a solution with respect to the formation of another phase of a type already specified. For example, if you have only defined two liquid phases the possible formation of a vapour or solid cannot be checked

STABLE means that no further phases (out of the list provided) will form. Sometimes a solution may be labelled as UNSTABLE, meaning that further phases would form if more phases were allowed for in the problem setup. An example might be if only vapour and one liquid phase are specified but in fact the true solution is vapour-liquid-liquid. If you want to look for the extra phase then you must include an additional phase descriptor in your model definition. If, however, you are at very low temperatures you may be looking at a metastable solution and wish to retain only the number of phase descriptors already defined. The results of a hydrate nucleation calculation will always be labelled unstable because it is not an equilibrium point. It is also possible to see the message MARGINALLY STABLE. This means that, although the solution is stable, changing the conditions slightly might result in the formation or disappearance of a phase, for example near the critical point. The remaining output depends on the setting for the physical property output level. For each phase the phase name is given underneath to identify it uniquely. The previous example shows output from the default setting. The column labelled OVERALL gives the total mixture properties (where applicable). If molar output units are selected for amounts (default) the compositions shown are mole fractions, but the total amount of each phase is in molar units. If mass-based output units for amounts are chosen the compositions displayed are mass fractions and the totals are in the selected mass units. If you want the output in absolute amounts not fractions you set this on the Amounts tab of the Select Units option. Z is the compressibility factor calculated from the same model as used to calculate fugacity coefficients (K-values). The average molecular weight has units of g/mol. ‘Den/V’ is the molar or specific density or volume calculated from the volumetric model in the selected density/volume output units. U, H, S and G are the internal energy, enthalpy, entropy and Gibbs energy of each phase. If you have extended the physical property output then the additional properties will be listed below these, e.g.

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Cp (J/mol/K) heat capacity at fixed pressure Cv (J/mol/K ) heat capacity at fixed volume Sp.Sound (m/s) thermodynamic speed of sound Compress (1/bar) the compressibility Expansiv (1/K) the expansivity JT (K/bar) the Joule-Thomson coefficient Visc.(Pas ) viscosity Th.C.(W/m/K ) thermal conductivity

STen (N/m)

surface tension

The format for the surface tension output depends on the surface tension model selected. For the Macleod-Sugden model (MCS) there is an entry for each liquid phase. This is the liquid-gas interfacial tension. For the linear gradient theory model (LGST) the interfacial tension between each pair of phases is displayed in a grid. This will include liquid-liquid interfacial tension as well as the liquid-gas values. If the fugacity coefficients and activity coefficients have been selected the coefficients for each component in each phase are listed as follows: COMPONENT

BUTANE PENTANE

OVERALL

PHASE1 PHASE2 GAS LIQUID1 fug. coeff. fug. coeff. .86231 1.2403 .79165 .56033

Activity coefficients may not be a valid property for all components in all phases. This will depend on the component and the model. If the property is not valid N/A will be displayed. Finally, values for diffusion coefficients are displayed. If the model definition has been set to include diffusivity then the output will automatically include diffusion coefficients. If Diffusivity has been set for property output but not included in the Model Set definition then the property label will appear but not the numeric values. PHASE1 GAS Diffusion coefficient (m2/s ) ACETONE WATER ACETONE 8.00080E-06 1.86663E-05 WATER 1.86663E-05 4.29283E-05 PHASE2 LIQUID1 Diffusion coefficient (m2/s ) ACETONE WATER ACETONE 7.15035E-09 2.96215E-09 WATER 2.96215E-09 4.55884E-09

Manipulating the Output As described above the appearance of the output can be configured to your liking. Font type, size and colour can be altered as can the background of the results window. Gridlines can be added to clearly distinguish between columns. You can copy any proportion of a column and paste this directly into another grid. The composition of a liquid from a flash can be copied and directly pasted to the input composition table. The amount of a second phase to be added to the overall stream composition from a tolerance calculation can be copied and added to the Composition table.

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Phase Labelling The list of phases displayed after a flash calculation shows the phase label for each phase. At low pressures there is usually a clear distinction between a gas phase and a liquid phase. This is no longer the case in the ‘dense phase’ region above the vapour-liquid phase envelope where the phase present is supercritical. In such cases there is no physical distinction between a gas and a liquid. The phase label assigned by Multiflash is arbitrary. The criterion used to decide whether to call a supercritical phase gas or liquid is as follows. 

A phase is labelled as gas if

VT 2  Vc Tc2 where Tc and Vc are

the pseudo-critical temperature and volume. 

Otherwise the phase is labelled as liquid.



The pseudo-critical properties are model-dependent and do not correspond to the true critical properties for a mixture.

These conditions only apply to equation of state type models that can represent both gas and liquid phases. The phase properties such as density, enthalpy etc. are not affected by the label attached to the phase since, by definition, the phase is supercritical and there is only one solution for the volume of the equation of state model.

Aqueous phase labelling The aqueous phase labelling in the current version of Multiflash has changed. In previous versions only one key component was used, but now it is possible to define multiple key components. For version 4.4 onwards, the water phase is assigned a new special keyword “AQUEOUS”. This means that the following list of components will be used as water key components: 

water



methanol



ethanol



MEG



DEG



TEG



Glycol



Propylene glycol



Propylene glycol monomethyl ether



dipropylene glycol



dipropylene glycol monomethyl ether

The non-aqueous phases are assigned also a new keyword as key component: “*”.The “*” keyword is used to denote a phase which has negligible amounts of the positive key components of all the other phase descriptors. This is now the default behaviour when using the Graphical User Interface.

Enthalpy definition In Multiflash the enthalpy is calculated as

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H  H ref  H pg  H res Where:

H ref is the (arbitrary) enthalpy value in a reference state to be defined; H pg is the perfect gas contribution to the enthalpy which is calculated by integrating the equation for the perfect gas heat capacity;

H res is the residual enthalpy which is calculated from the thermodynamic model specified for thermal properties. The absolute value of enthalpy has no physical meaning but enthalpy differences are measurable quantities therefore H ref can be chosen at will. Multiflash has two possible reference states that are user selectable. By default we set to zero the enthalpy of each pure component in the perfect gas state at 298.15K and 1 atm. This reference state (or datum) is referred to as the ‘compound’ datum in the units selection window of Multiflash. See “Units” on page 169. An alternative datum in Multiflash (the ‘elements’ datum) sets the enthalpy of each element to zero in the perfect gas state at 298.15K and 1 atm. This datum produces enthalpy values that are much larger numerically than the ‘compound’ datum but enthalpy differences between two states are the same. When calculating chemical reaction equilibrium the ‘elements’ datum must be used because it is the elemental entities that are conserved rather than the molecular entities. If you observe differences in enthalpy values calculated with Multiflash and other software this is likely to be because of different choices of datum. However, enthalpy differences should not be very different.

Activity Models For activity methods the default route for calculating the enthalpy is as described above. However, there are two additional routes by which the enthalpy of each component can be calculated. Although the alternatives described below are available, it is recommended that they should only be used if the consequences and implications are well-understood.

Liquid enthalpy based on saturated liquid heat capacity The data stored for each pure component will normally include a correlation for the saturated liquid heat capacity (Cp) as a function of temperature. The enthalpy can be calculated based on the liquid Cp correlation instead of the perfect gas Cp correlation as usual. With this option for the liquid phase the pure component enthalpy at the reference state, 298.15K and 1 atm is calculated with the existing procedure described above. Then the total enthalpy of pure components at the system temperature is calculated by adding the integral of the liquid C p from the reference temperature to the given temperature plus the enthalpy change on evaporation at 298.15K, the Poynting correction and the excess enthalpy calculated from the activity coefficient models. The enthalpy for the gas phase is calculated by the default method. The consequence is that that the liquid heat capacity is based on the stored correlation for the liquid phase so it may be expected to be more accurate than the value calculated by the default method that is, ultimately, based on the perfect gas heat capacity. However the enthalpy difference between gas and liquid is no longer based on the databank correlation for enthalpy change on evaporation.

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Gas and liquid Enthalpy based on saturated liquid heat capacity With this option the liquid phase enthalpy is calculated as described above. The gas phase enthalpy is obtained by subtracting the enthalpy change on evaporation from the liquid phase value. This procedure provides liquid phase enthalpy and Cp based on the databank correlation for liquid Cp. The enthalpy difference between gas and liquid is based on the correlation for the enthalpy change on evaporation. However, the gas phase enthalpy is no longer based on the perfect gas heat capacity and would, therefore, be expected to be less accurate than in the default method.

Entropy definition In Multiflash the entropy is calculated as

S  S ref  S pg  S res Where:

S ref is the (arbitrary) entropy value in a reference state to be defined; S pg is the perfect gas contribution to the entropy which is calculated by integrating the equation for the perfect gas heat capacity;

S res is the residual entropy which is calculated from the thermodynamic model specified for thermal properties. Although the absolute entropy can be argued to have a physical interpretation, in practise, it is entropy differences that are experimentally accessible and S ref can be chosen at will. Multiflash has three possible reference states that are user selectable. By default we set to zero the entropy of each pure component in the perfect gas state at 298.15K and 1 atm. This datum is referred to as the ‘compound’ datum in the units selection window of Multiflash. See “Units” on page 169. The ‘elements’ datum sets the entropy of each element to zero in the perfect gas state at 298.15K and 1 atm. This datum produces values that are much larger numerically than the ‘compound’ datum but entropy differences between two states are the same. When calculating chemical reaction equilibrium the ‘elements’ datum must be used. The third possibility is the ‘standard’ datum which is sometimes called the ‘third-law or ‘absolute’ entropy. This means that the reference entropy is chosen so that the entropy of each component in the perfect gas state at 298.15K and 1 atm is equal to the standard entropy of that component. The standard entropy is relative to a zero value at absolute zero. The standard datum may also be used in chemical reaction analysis since the results are equivalent to the elements datum.

Activity Models The two alternative calculation routes for the enthalpy have corresponding routes for the entropy. They always apply to both properties.

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Errors and warning messages Errors are usually associated with Multiflash output indicating a failure to find a solution or that the results may be invalid. Error messages and warning messages are displayed in the results window. Each error and warning has a unique identification number. Errors are denoted by positive numbers and warnings by negative numbers. The error number is followed by a single line description of the problem. For example Dew point at fixed T: *** ERROR 20292 *** Cannot find converged point - max. iterations *** ERROR 20024 *** Cannot find starting point for calculation - there may be no solution. *** ERROR 344 *** The flash calculation has not converged

When several errors are reported, as above, it is the first error that is closest to the fundamental problem. For more information on each error code see Multiflash Error Codes in the Help menu. The information on the module in which the error occurred and the subprogram name is intended for use by Infochem technical support. Warning messages should be checked carefully. In many cases they may be ignored if the cause is understood.

Displaying status for current settings Show options in the Tools menu allow the user to check the status of some Multiflash options. The options are summarised in the following table. Show options BIP databank

The name of the active BIP bank for the problem e.g. oilandgas4

Models

List of the model set

Phase descriptors

Phase descriptor names for the phases to be included in calculation

Problem

Full definition of the current problem

Pure Component Databank

The pure component databank current at the time of request

Results

Re-display of the results from the last calculation

Stream Types

Displays number and names of active streams

The output from the Show Problem option is essentially identical to the information written to an .mfl file is the current problem setup is saved.

Troubleshooting - output Problems relating to the flash calculations, which may be reflected in the output, have been dealt with earlier. However, some problems directly related to the output may be worth mentioning.

The output does not include everything expected Remember that there is a limit of 300kb for the results window. If your output exceeds this then, as the last 300kb are retained, earlier results may be

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overwritten. You can of course cut and paste results to another document when you reach the limit or take the results from the log file. The output may not contain all the properties expected. This may be because they have not been selected, see “Property output in Multiflash” on page 164, or because a model for the property has not been defined. You should ensure that you have defined transport property models when creating PVT files for PIPESIM or TAB files for OLGA.

Phase labelling The phase names attached to each of the possible phases which may form enables you to keep track of the phase output. One case where some confusion may arise is for dense phases, where it is not possible to decide unambiguously whether the dense fluid is a liquid or a gas. In this case the phase name may change with slight alterations in conditions. A change in the phase label does not affect the correctness of the results or phase properties, it is only the label attached to the phase that is ambiguous.

Fonts Occasionally users have reported not being able to read the output in the results window. This appears to be the result of particular PC installations where the font type or size has been re-set to choices that are incompatible with the display. To change the font setting see “Font” on page 174.

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Interfaces with other programs

Introduction In addition to using Multiflash to do stand-alone calculations it is possible to create property data files for use by other application programs. In addition, Multiflash can also import files from other programs (e.g., PVTSim) to perform calculations. Multiflash can currently produce Pipesim PVT files, files for input to OLGA, property files for Prosper, and CAPE-OPEN property package files. Multiflash can also import CHC files from PVTSim.

Pipesim PVT files Pipesim is a general purpose simulator for modelling fluid flow in oil and gas wells, flow lines and pipeline systems. It is a product of Schlumberger Information Systems. Multiflash can produce a PVT data file for use by Pipesim. The file contains all the physical property information required by Pipesim. It consists of a series of flash calculations on a grid of pressure and temperature values. The information stored includes the stream composition and, for each grid point: liquid volume fraction, water cut volume fraction, liquid density, gas density, gas compressibility factor, gas molecular weight, liquid viscosity, total enthalpy, total entropy, liquid heat capacity, gas heat capacity and liquid surface tension. For a complete definition of PVT files see the Pipesim manual. To generate the file: Define the models (which must include models for surface tension and viscosity) and the mixture (components and composition) as usual Select Pipesim from the Table menu, then Fill in the dialogue box. with the pressure and temperature grid points and the name of the output file. The recommended file extension is .PVT

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Click on Calculate table An alternative to entering each pressure and temperature individually is to use the Equidistant Points Tab which allows you to enter start and end points for P and T and an increment or a number of points. Assuming there are no problems the output will go directly to the .PVT file, it will not be sent to the results window. In the results window you will see the message Pipesim table written to file: pipe1.pvt If there are problems calculating any of the entries in the table the following message is displayed

. and more details are reported in the output window. It should be noted that any BIPs specified by the user in Multiflash will not be stored in the .pvt file, although numerical values will have been calculated using them. However, in line with recent changes to PIPESIM .pvt files written with Multiflash will contain information on the version of the OILANDGAS BIP correlations and the viscosity model used.

OLGA OLGA is a general purpose transient simulator for modelling fluid flow in flow lines and pipeline systems. It is a product licensed by the SPT Group. Multiflash can produce a PVT data file for use in OLGA. The file contains all the physical property information required by OLGA. It consists of a series of flash calculations on a grid of pressure and temperature values. The information stored includes all the properties required by OLGA for either two-phase or three-phase problems as set out in SPT Technical Note No. 1. From Multiflash

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4.4, the maximum dimension of the temperature and pressure grid has been extended up to 100. To generate the file: Define the models (which must include models for surface tension, thermal conductivity and viscosity) and the mixture (components and composition) as usual Select OLGA from the Table menu, then

Fill in the dialogue box with the pressure and temperature grid points and the name of the output file. The recognised extension for OLGA tables is .tab. This will be allocated automatically if you use the Browse facility to identify a suitable folder and save the file there. Alternatively you can type in the .tab extension as part of the file name. If you want the file saved in a folder directory you must enter the full pathname, otherwise the file will be saved in the current default directory. If the file already exists, the fluid PVT table will be added to the end of the existing file if the append box is ticked, otherwise the entire file will be overwritten with the new PVT table. OLGA requires a File id (starting with a letter not a number). If you don't provide one, Multiflash will assign one automatically. Click on Calculate table If the file already exists and the append box is not ticked, a warning message will be displayed

Clicking Yes causes the file to be overwritten with the new table. Clicking No means nothing will happen.

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Assuming there are no problems with the calculations the output will go directly to the .named file, it will not be sent to the results window. In the results window you will see the message Olga

table written to file: oil.tab

Similarly to the Pipesim table option you have the choice of entering the P and T points for the table or specifying ranges and increments or the number of points. If any of the input is unacceptable, such as specifying negative pressures or absolute temperatures or if Multiflash cannot solve the flash calculation at any grid point then a warning message will appear. This may be an indication that you have incorrect input

or the calculation routines may fail with the message

OLGA hydrate file In addition to the PVT table, Multiflash can also write an OLGA hydrate file defining the boundary of the hydrate forming region. This is done by entering a file name in the hydrate file box or clicking the Browse button. The default extension for a hydrate file is .hyd. The hydrate file is written at the same time as the PVT table when the Calculate Table button is clicked. The same style of warning will appear if an existing hydrate file is about to be overwritten. If the hydrate file box is left blank, the hydrate calculations are omitted. If the hydrate file is required but the hydrate model and hydrate phases are not defined, the following warning is displayed. Choose the Model Set option from the Select menu to define the hydrate model. It should then be possible to generate the OLGA hydrate file.

OLGA wax file Multiflash can also write an OLGA wax file defining paraffin wax precipitation. This is done by entering a file name in the wax file box or clicking the Browse button. The default extension for a wax file is .wax. The wax file is written at the same time as the PVT table when the Calculate Table button is clicked. The

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same style of warning will appear if an existing wax file is about to be overwritten. If the wax file box is left blank, the wax calculations are omitted. If the wax file is required but the wax model is not defined, the following warning is displayed. Choose the Model Set option from the Select menu to define the wax model. It should then be possible to generate the OLGA wax file.

Prosper PVT files Prosper is a well performance, design and optimisation program for modelling well configurations for the oil and gas industry. It is a product of Petroleum Experts. Multiflash can produce a PVT data file for use by Prosper. The file contains all the physical property information required by Prosper. It consists of a series of flash calculations on a grid of temperature and pressure values. For a complete definition of PVT files please see the Prosper manual.

To generate the file Define the models (which must include model for viscosity) and the mixture (components and composition) as usual From the Table menu choose Prosper. The Prosper PVT Table window is displayed:

Enter the file name. The recommended file extension is .pvt The Fluid Id field is an optional identifier that will be written to the file. You can select the (input) units of temperature and pressure for the table from the drop-down boxes. Note that the units in the table file are fixed with the temperature in ºF and the pressure in psig. Enter the temperature and pressure points for the table. For each isotherm, up to 15 pressures can be specified on the form. The properties tabulated can be selected using the check-boxes in the Properties frame. The properties are defined as follows:

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Interfaces with other programs  187

Keyword

Description

Unit

GOR

Gas-Oil ratio

scf/sbl

OFV

Oil formation volume factor

RB/sbl

OVIS

Oil viscosity

cPoise

ODEN

Oil density

lb/ft3

OCOM

Oil compressibility

1/psi

GFVF

Gas formation volume factor

ft3/scf

GVIS

Gas viscosity

cPoise

ZFAC

Gas Z factor

GDEN

Gas density

lb/ft3

CGR

Reservoir condensate-gas ratio

bbl/MMscf

WVIS

Water viscosity

cPoise

WCOM

Water compressibility

1/psi

To generate the file click on Calculate table. Assuming there are no problems the output will go directly to the .PVT file, it will not be sent to the results window. In the results window you will see a message similar to PROSPER table written to file: C:\Program files\Infochem\Prosper1.pvt

If any of the input is unacceptable, such as specifying negative pressures or absolute temperatures or if Multiflash cannot solve the flash calculation at any grid point then a warning or error message will be displayed

and the errors are reported in the output window.

CAPE-OPEN Interface Infochem/KBC has been an active participant in developing and testing the CAPE-OPEN (CO) standards. The Multiflash CO modules implement versions 1.0 and 1.1 of the standard and support the PropertyPackageManager/ThermoSystem and PropertyPackage interfaces. The interface has been tested for interoperability with Petro-SIM, Aspen+, CoCo, ProII, gPROMS, Hysys and Simulis. To generate a Multiflash CO Property Package for use with the Multiflash CAPE-OPEN module simply set up the problem as usual. Instead of saving the Problem Set-up choose the Export CO Property Package option from the File menu. It is not necessary to specify which version of the CO interface you intend to use. For more information see the document the CAPE-OPEN Interface for Multiflash 4.4 document which is installed when the interface is selected during the installation of Multiflash.

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PVTSim CHC file import tool PVTSim is a general thermodynamic properties package of Calsep. The files generated by PVTSim (.CHC files) can be easily imported into Multiflash to perform thermodynamic calculations. To import a CHC file simply: 1) Select Import PVTSim CHC file from the File menu. The following window should appear:

2) Click the "Browse" button and search for the .CHC file that you desire to import. Then click on the "Open" button. 3) Choose the options for model import regarding models for viscosity, thermal conductivity, and surface tension. Otherwise the suggested default models will be used. 4) Click on the phases you want to define. Otherwise the suggested default phases will be defined. 5) Optional: if the original mixture as defined in the CHC file does not contain water but water is desired in the mixture, simply click on the "Add water" tick box. 6) Optional: use the GERG 2008 model to calculate the vapour phase density. 7) Click the "Import" button.

User Guide for Multiflash for Windows

Interfaces with other programs  189

Help

Introduction Help is provided in various ways: 

This document: User Guide for Multiflash for Windows



On-line Help



Website support



Technical support

On-line help The on-line help is accessed through the Help menu

Help Topics Help Topics provides access to the Contents, Index and Search facilities for Multiflash Help.

User Guide for Multiflash for Windows

Help  191

Selecting any active topic will bring up the related help text.

Related topics are further sub-headings and more help is displayed by selecting any of these. Some of the help text may be displayed in green. If this is underlined with a solid line, clicking on the text will allow you to jump to another help screen related to the text. If the text is underlined with a dotted line, clicking on it will result in a pop-up box containing a glossary definition or a margin note. Selecting the Search tab button in the on-line help window allows you to specify a particular topic you are interested in, either by typing in a description or selecting from the list displayed.

192  Help

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Clicking on the Display button or double clicking on the topic allows you to move to the related help text.

Multiflash Error Codes The error codes are the identification codes printed in the main window when an error occurs. The code numbers may be entered and a short explanation is given.

Check for Updates If your computer has an internet connection you can check for the latest update for the version of Multiflash that you are running. For example

User Guide for Multiflash for Windows

Help  193

About Multiflash The About option provides information about the Multiflash software and your license. It is particularly useful when reporting any problems to Infochem’s technical support team.

Version Info shows the version numbers of the Multiflash dll (the calculation engine) and the GUI. License Info includes your serial number and the expiry date of the license. Contact details gives information on how to contact Infochem, Location, Configuration and Log Files shows the path for the Multiflash application location and log files which are set by the Tools/Preferences/General/Folders option. You should check that these folders are set correctly if you experience problems in accessing data or messages. For further information see the Multiflash Installation Guide. The configuration file is an optional Multiflash command file called MFCONFIG.dat file that is run when Multiflash starts. If it is not found the file is not listed. The .log file is the file which automatically records the input and output information for any run.

Technical support To report bugs found in Multiflash, please email us. The support email address is: [email protected]. If you need further assistance contact us at:

194  Help

User Guide for Multiflash for Windows

Infochem/KBC Advanced Technologies plc. Unit 4 The Flag Store 23 Queen Elizabeth Street London SE1 2LP UK Telephone: +44 (0)20 7357 0800 Fax:+44 (0)20 7407 3927

User Guide for Multiflash for Windows

Help  195

Case studies - Pure component data

Introduction Although the primary purpose of Multiflash is to calculate the thermodynamic properties of mixtures there may be occasions when you simply want to know the properties of a pure component, particularly those from a particular data source. This is quite simple.

Physical properties of a pure component If you want the physical properties of a pure component, for example octane, over a range of temperatures you must either: 

Define the problem in Multiflash



Load an existing problem setup file containing the compound(s) into Multiflash



Load an existing problem setup file into Multiflash and edit the problem to add the compound of interest

If you want to know the stored values for the temperature independent properties or the correlation coefficients of a temperature dependent property choose Pure Component Data from the Tools menu bar as described previously, see “Viewing and editing pure component data” on page 70.

Defining the problem in Multiflash Choose a suitable model for the problem. If you wish to obtain the properties as set in the pure component databank choose the ideal gas and ideal solution models. This is the Ideal Mixing model set.. With this option all properties will be taken from the databank correlations except liquid Cp which is calculated from the vapour phase model and the enthalpy of vaporisation. (If any other model set is defined then only the pure component properties needed for that model set will be taken from the bank. Other properties will be calculated using the model set definitions which include models for the transport properties).

To specify the Ideal Mixing model set: From the Select menu choose Model Set and click on the Activity Models tab and select Ideal Mixing for the liquid phase and Perfect Gas for the gas phase. For transport properties choose the mixing rules models.

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Case studies - Pure component data  197

Click on the Define Model button and then on OK, For more information on models see “Models” on page 29.

Specify the pure component of interest From the Select menu choose Components or click on the toolbar button Specify the data source and component in the Select Components window

The default data source is the Infodata databank. To change this click on the down-arrow to the right of the data source box and choose another databank from the list. Type the component name in the Enter name box and press the enter key or click the Add button. Other ways of selecting components are described in the section “Defining a mixture” on page 65 .

Specifying the physical property output level If you are interested in pure component data you will probably want to output all available calculated properties. Choose the Property Output option from the Select menu. Check the boxes for the properties you want given as output, e.g. heat capacity and/or transport properties. Checking the volume and/or the heat capacity boxes will also check all previous boxes, because those properties are required for obtaining the requested property.

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Enter a composition for the stream In this case where we have a pure component the composition is not important provided it is a positive value. In the main window, click on the Compositions button, and enter a value of 1.0 in the right-hand column of the table next to octane. The input for this example is summarised in file octane.mfl.

Carry out a the flash calculation To obtain the properties of liquid octane on the saturation line you must carry out a bubble point calculation at a specified temperature. Enter a temperature, say 400K in the temperature box in the Conditions section. Click on the Bubble point at fixed temperature toolbar button the calculation

User Guide for Multiflash for Windows

to carry out

Case studies - Pure component data  199

The reported pressure is the saturated vapour pressure at 400K, the other properties are listed below the phase equilibrium output. As we are dealing with a pure component exactly the same property results would be obtained if we had specified a dew point flash at the same temperature. The next temperature can be entered in the temperature box in the Conditions section and the bubble point flash repeated at this temperature.

Obtaining properties from the Pure Component Data option The output from a flash calculation does not directly contain any of the pure component data values that are stored in a databank. To obtain this information choose the Pure Component Data option from the Tools menu. The following window is displayed

As we are dealing with a single component this will be the only choice available and should be highlighted. Click on a property in the property list and doubleclick or click on the Edit button to view or change the property. For constants the properties are displayed in the current units but these can be changed. For temperature-dependent properties such as the vapour pressure, the identifier of the correlating equation and the equation coefficients are displayed. It is usually not advisable to change correlation coefficients unless you are entering new values from some alternative data source. The equation identifiers are defined in the User Guide for Models and Physical Properties. You can also print the properties in the results window by clicking Write to Output. This output can then can be saved or copied into other files. show components "OCTANE" data ; 1 OCTANE MOLECULARWEIGHT TCRIT PCRIT VCRIT ACENTRICFACTOR TBOIL HFORMATION SSTANDARD

200  Case studies - Pure component data

114.231 569.32 2497000 2056.359 0.396 398.82 -208446.9 466.7252

g/mol K Pa mol/m3 K J/mol J/mol/K

User Guide for Multiflash for Windows

TMELT HMELT SMELT CPMELT VMELT RUNIQUAC QUNIQUAC THLWATER VHLWATER DIPOLEMOMENT POLARIZABILITY QUADRUPOLEMOMENT PARACHOR RADGYR HOCASS GFORMATION SFORMATION TTRIPLE PTRIPLE HCOMBUSTION V25 SOLUPAR SOLIDSOLUPAR ZCRIT REFRACTINDEX TFLASH TAUTO FLAMLOWER FLAMUPPER SPGRAVITY EXPANSIVITY OMARKS OMBRKS OMAPR OMBPR CNUMBER REFVISCOSITY REFVISST REFVISPD REFVISTW REFVISLB LJEVISC LJBVISC EOSC TYPE HDATUM SDATUM COMPREFNO MCRKS1 MCRKS2 MCRKS3 MCPR1 MCPR2 MCPR3 HYDOC HYD1 HYD2 HYD3 ASSBETA ASDBETA ASSEPSILON ASDEPSILON ASSGAMMA ASDGAMMA ASSDELTA ASDDELTA ASSFF ASDFF ASSAC ASSBC ASSKAPPA SAFTKAPPA SAFTEPSILON SAFTGAMMA SAFTFF SAFTEK

User Guide for Multiflash for Windows

216.37 20740. 50.00791 50814.48 5.8486 4.9360

0.

351.4 4.68040E-10 0. 16000. -752.7986 216.38 2.1083 -5074150 6120.925 15447.5 0.2587676 1.39505 286. 479. 0.8 6.5 0.7066211

K J/mol J/mol/K J/mol/K mol/m3

K m3/mol debye Å3 debye Å (dyn cm-1) 1/4 cm3/mol m J/mol J/mol/K K Pa J/mol mol/m3 (J/m3)1/2 (J/m3)1/2

K K vol % vol % 1/K

Pas Pas Pas Pas Pas J/K m 1. 1. 1. 93.

J/mol

1/K

J m3/(mol)2 m3/mol

K

242.78

K

Case studies - Pure component data  201

SAFTSIGMA SAFTLAMBDA SAFTM SAFTQ SAFTFRQ VSRKS1 VSRKS2 VSRKS3 VSPR1 VSPR2 VSPR3 CPIDEAL

3.8373E-10

m

3.8176

m3/mol m3/mol/K m3 K/mol m3/mol m3/mol/K m3 K/mol -3721.3925 -1.3945 0. 1.9471999 20. 1.38007 568.95 .3775 0. 5407.5898

1. -32384.514 76 290. -5.7709999 0. CPSOLID 5. -24. 1.34E-5 2.094E-8 PSAT 3. -7.9121099 -4.50132 260. HVAP 1. 54909.031 0. 0. LDENS 1. 2032.52 0. 568.381 LVISC 2. -20.462999 1497.4 0. 0. 216.38 VVISC 1. 3.1191E-08 0.92925 0. 216.38 1000. LTHCOND 5. 0.21867631 -0.00038267 0. 0. 216.38 VTHCOND 1. -8758. 0.8448 0. 339. 1000. STENSION 1. 0.05278999 1.2323 0. 0. 216.38 CPLIQUID 5. 224.83 -0.18663 0. 0. 216.38 SDENS 5. 8340.9004 -3.1515 0. 0. 133.15 VIRIALCOEFF 1. .00027389999 -5.65219E-04 -1.161662E-05 2.58796E-06 284.38 DIELECTRIC CARNUMBER 000111-65-9 FORMULA C8H18 FAMILYCODE AA UNIFAC CH3 2 CH2 6

4. 5.6339999 10000. -0.0085359998 216.37 -3.8043499 0. 568.381 0.375 1.3789999 398.83 55.092 2.1762921e-7 500 -2.71210E+10 0. 568.7 .00095890998 460. 0. 216.38 -3.63347E-04 1500.

The output includes the pure component constant properties and the coefficients for the temperature dependent property correlations. The definitions of the pure component correlations are given in the User Guide for Models and Physical Properties.

Excel interface If you wish to produce tabular output or for graphical output for properties other than phase boundaries the most convenient tool is the Multiflash Excel interface. The example file PURE.xls shows how to construct the following table of properties for octane. Liquid properties on the saturation line TEMP

PRESSURE

CP

ENTHALPY DENSITY VISCOSITY

THCOND

SURTEN

Pa

J/mol/K J/mol mol/m3 Pas W/m/K N/m 231.4 -47032 6255 0.0006945 0.1299 0.02341

275

427.8

300

2038.

247.7

-41043

6117

0.0004974

0.1235

0.02099

325

7232.

264.4

-34641

5970

0.0003783

0.1173

0.01861

350

20572.

281.4

-27819

5814

0.0003015

0.1114

0.01629

375

49414.

299.0

-20563

5646

0.0002493

0.1058

0.01404

400

104101.

317.4

-12856

5464

0.0002123

0.1004

0.01185

425

197831.

337.3

-4668

5265

0.0001839

0.09535

0.009729

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450

346415.

359.7

4045

5042

0.0001605

0.09055

0.007696

475

568184.

386.4

13359

4788

0.0001412

0.08601

0.005760

500

884337

422.1

23420

4488

0.0001250

0.08175

0.003941

For more information on the Excel interface see the document: User Guide for Multiflash Excel Interface.

User Guide for Multiflash for Windows

Case studies - Pure component data  203

Case studies - Phase equilibria

Introduction The main purpose of Multiflash is to determine the phase equilibria and thermodynamic properties of complex mixtures. The simple tutorial shown earlier, see “Getting Started” on page 13, was based on calculating the phase equilibria of a binary hydrocarbon system. Here we will look at a more complex hydrocarbon system and the phase equilibria of a polar mixture.

Oil and gas systems Initially we will look at the phase envelope of a system which contains six components: methane, ethane, propane, butane, hexane and decane. As with the pure component data system discussed earlier the case study can be set up interactively or by using a problem setup file. Only the former will be discussed in detail, but the setup file, hycbvle.mfl, is provided and may be used to load models and components or as an example for writing or editing your own files for handling similar cases. Specify the model Any of the equation of state models would be suitable for handling this problem. We have chosen the advanced version of RKS but very similar results would be expected using advanced PR, PR78 or RKS. Select Select on the menu, then Select Model set from the sub-menu, followed by Selecting Equations of state Tab from the Select Model set dialogue box and again, Selecting RKS (Advanced) with the default option for transport property models.

Finally, click on OK in the message box and Close in the Model set dialogue box.

User Guide for Multiflash for Windows

Case studies - Phase equilibria  205

Specify the components The INFODATA databank will be the default data source and this is acceptable for this case study, so we can move directly to specifying the components. You can activate the Select Components dialogue box by either Clicking on the select components button or Selecting Select from the menu bar and Selecting Components from the sub-menu

The various methods for selecting or searching for components have been shown before, see “Selecting components” on page 66. As our current system contains simple well known compounds they have been selected by Choosing the Name option button Typing the component name in the Enter name text box and pressing the enter key after the name to load it for Multiflash or Clicking on Add to load the component. Click on Close to load the components Define the composition Click on Compositions in the Conditions section, and Type in the compositions in the drop down table. For our example it is: Methane Ethane Propane Butane Hexane Decane

0.45 0.20 0.10 0.10 0.10 0.05

Calculating the bubble point curve You can calculate the bubble point curve either by doing a series of individual bubble/dew point calculations, or by generating the phase envelope. Individual calculations can either be at fixed temperature or pressure. Technically there is

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no particular reason to prefer one over the other; for this particular example we will fix the temperature and have our output pressure in bar. To change the pressure units, click on the Select input and output units button , then in the Tab control click on Pressure and click in the Output option button box against bar. (You can also change the input units for pressure if you wish.) Having specified the model, components, compositions and units, Enter the first temperature, 250K, in the Conditions section and Click on the T, Bubble button,

.

Repeat the last two steps, increasing the temperature by 25K each time. At 400K you still have a stable solution reported, but by 425K you will notice a failure message Bubble point at fixed T: *** WARNING -20323 *** Possible instability in solution (constrained flash) *** ERROR 20334 *** Constrained flash solution unstable *** ERROR 20028 *** Cannot solve flash problem - other unspecified error *** ERROR 344 *** The flash calculation has not converged. Investigating the bubble point curve using reduced temperature steps you will see that the solution is stable up to 404K and fails at 406K. The compositions of the liquid and gas phases are very similar, indicating that we are probably close to the critical region. You can either investigate this area of the phase envelope further using P,T flashes or move to calculating the dew point curve to formulate a view of the phase envelope.

Calculating the dew point curve As with the bubble point curve, the dew point curve can be calculated from a series of dew point calculations at either fixed temperature or pressure. Again we will use calculations at fixed temperature for this example As with the bubble point curve, start at 275K and calculate the dew point at 25K intervals using the T, dew point button,

.

This time the dew point calculation fails to converge at 475K. Returning to the last successful convergence at 450K, increase the temperature in 5K steps. This time the first failure to converge occurs at 470K. We know that it is probable that the critical point is around 400K so it is a reasonable assumption that this system has a significant retrograde region. Check this by repeating the dew point calculation at 450K but this time by using the fixed phase fraction route and looking for the upper retrograde solution. To do this: Click on the fixed phase flash at fixed temperature button,

’

In the dialogue box which is then activated, Click on the roll down arrow to the right of Select phase and from the drop down menu click on LIQUID1. Choose Mole Fraction under Select Basis and enter a phase fraction of 0.0 in the text box and click on the Upper retrograde type of solution. This fixed phase flash simulates a dew point calculation.

User Guide for Multiflash for Windows

Case studies - Phase equilibria  207

Click on Do flash. The calculated dew point pressure is now 115 bar for the retrograde region, whereas for the “normal” dew point the calculated pressure was 28.2 bar. This confirms that we have a retrograde region for this system. To calculate the full dew point curve you therefore need to increase the temperature at 1K intervals above 460K, using the normal T, dew point flash, until you meet the first convergence failure, at which point you are just beyond the cricondentherm. You should now switch to fixed phase fraction flashes at fixed temperature, set the options as described and reduce the temperature in small steps. In this way the retrograde dew point curve can be generated back to 403 K.

Phase envelope The same problem can be investigated more easily using the phase envelope calculator. Set up the problem as before, but instead of carrying out individual dew and bubble point calculations Select Calculate/Phase Envelope or click on the Phase Envelope button, Click on V/L Autoplot and Click on Yes when the message box appears asking if more points should be calculated. The resulting plot includes the dew and bubble point lines and the critical point is labelled.

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The output in the results window will allow you do identify the critical point explicitly 70C

402.731 147.11

Adding water to the system It is quite common in the oil and gas industry to find some water present with hydrocarbon systems, which will probably form a separate liquid phase. In this case study we will add water and look for the water dew point line. To add water to the input stream With the hydrocarbon case study defined, Click on the Select component button In the Select components dialogue box, enter water in the Enter name text box then press the enter key or click on Add Click on Close In the main window, click on composition and in the drop down table enter a composition for water, say 0.2.

Calculating the water dew point line If there are more than two liquid phases present then using Multiflash to calculate the dew point, defining either temperature or pressure, will result in finding the primary dew point, in this case the hydrocarbon liquid dew point. To find the dew point for the second liquid phase, in this case water, you must use the fixed phase fraction flash and look for the temperature or pressure where there is a zero amount of that phase. Therefore, for the water dew point, Enter 300K in the Conditions section of the main window. Click on the fixed phase fraction flash at fixed temperature button,

.

In the resulting dialogue box, click on the arrow under Select phase and select water. Under Select basis choose Mole Fraction and enter a phase fraction of 0.0. The “normal” type of solution should be satisfactory for the water dew point.

Click on Do flash Repeat the calculation at increasing temperatures to obtain the water dew point line. Alternatively, plot the water dew point line using the Phase Envelope calculator by selecting the water phase at 0.0 molar phase fraction.

User Guide for Multiflash for Windows

Case studies - Phase equilibria  209

Including a petroleum fraction The heaviest component in our hydrocarbon stream is decane, but often the heavier end of oil or gas condensate systems is defined as a petroleum fraction rather than as a single specified component. Each petroleum fraction will consist of a mixture of components and the fraction as a whole will be defined in terms of its molecular weight, density and possibly boiling point, although the first two properties are the most likely to be reported. Often the heavy end will be reported as a single fraction, e.g. C7+, although sometimes a more detailed analysis may be available breaking the heavy end down into several fractions. Multiflash includes petroleum fraction correlations which may be used to predict the thermodynamic and transport properties of the fraction based on the data available, see “Defining petroleum fractions” on page 109. For this case study we will remove decane and water from our stream and replace decane with a petroleum fraction of molecular weight 234 and specific gravity 0.838. To delete components Assuming the stream definition for the last case study is loaded Click on Select components button In the Select Components dialogue box, select water in the list of components selected for Multiflash, then click on Delete. This will remove water from the list. Repeat this for decane. You should now be left with methane, ethane, propane, butane and hexane. To add the petroleum fraction In the Select components dialogue box, click the “Add/Remove Petroleum Fractions” button to launch the Petroleum Fraction Input form. Enter C7+ in the Component name column, 234 for Molecular Weight and 0.838 for specific gravity on the form.

210  Case studies - Phase equilibria

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Click on Calculate or OK button to include the fraction in the defined stream. If Calculate button is clicked, the rest of the physical properties which will be calculated are displayed in black on the form. The user-entered values are in red. Click on Close In the main window click on Composition and enter 0.05 for the amount of C7+. The petroleum fraction is now included in the stream definition and the phase envelope calculation may be repeated with the new stream, although the cricondenbar, cricondentherm and retrograde regions will now be different. If you only know that the petroleum fraction is C7+ but do not have a reported MW or specific gravity you can simply fill in 7 as the Carbon number as follows and click Calculate button

and Multiflash will determine the properties from a set of standard tables. The calculated values by Multiflash are displayed in black and user-entered values are in red.

User Guide for Multiflash for Windows

Case studies - Phase equilibria  211

Other flash calculations Many engineering applications involve a wide range of flash calculations, not just those related to determining the phase envelope. For example, an isenthalpic flash at fixed pressure can be used to simulate the expansion of a stream through a valve Basing this case study on the simple hydrocarbon stream (and model) we first defined Methane Ethane Propane Butane Hexane Decane

0.45 0.20 0.10 0.10 0.10 0.05

we must initially carry out a P,T flash at the upstream conditions to determine the enthalpy and then a P,H flash at the exit pressure. Having loaded the model set and stream information Enter the upstream temperature, 300K and pressure, 50 bar Click on the P,T flash button The calculated total enthalpy is -10539.6 J/mol

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The stream is then throttled isenthalpically to 10 bar, by Entering the new pressure, 10 bar under Conditions Entering the calculated enthalpy under Conditions Clicking on the P,H flash button The calculated temperature at outlet has dropped to 273.134K.

You can also add the isenthalpic boundary for -10539.6 J/mol to your phase envelope.

User Guide for Multiflash for Windows

Case studies - Phase equilibria  213

PVT Analysis Many users will receive a PVT Analysis for the composition of an oil or gas from one of the PVT laboratories and wish to use this as input to Multiflash. These reports follow a fairly standard format and the PVT Lab Analysis form endeavours to reproduce this to make entering information as easy as possible. The facility to add or delete components from the generated list is also useful. The form is discussed in detail in section “PVT Lab Analysis” on page 89. The case study we are considering here is based on a problem setup file called pvt_anal2.mfl. To enter a PVT Analysis when you have no measured n-paraffin distribution either choose the Select/PVT Lab Input menu option or click on the The Lab Analysis form will then be displayed.

icon.

Initially we will consider a case where you only have a single fluid composition. First select the data source for your discrete (i.e. well-defined) pure components.

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This can be Infodata or DIPPR. In this example we use Infodata. Next at the top of the column headed Single fluid you can choose either mass or mol % as appropriate by clicking on the down arrow. If your PVT report offers a choice of mole or mass %, it is the mass % that is the experimentally measured data and should be given preference for separator oils. For this case the composition is provided as mass%. Next enter the compositions of the discrete components and the compositions of the petroleum cuts. In the form the pseudocomponents or single carbon number (SCN) cuts are labelled C6, C7 etc. In your PVT Laboratory report they may be referred to as hexanes, heptanes, etc., with the heaviest being labelled as a plus fraction such as C20+ or eicosanes+. In our example the heaviest SCN is C20. The overall percentage will be totalled as you enter the compositions. If the final total is not 100 you will be offered the opportunity to normalise the compositions when you characterise the fluid. You can enter further information to define the stream, such as the molecular weight of the Stock Tank Oil (STO), the total fluid or the heaviest SCN or the specific gravity of either the heaviest SCN or the STO. We have provided general advice on when such data should be supplied in “Petroleum fluids” on page 89. As the fluid in question has a heavy end (C6+) which comprises more than 50% of the stream we should supply this information if possible. We have therefore entered the molecular weight of the heaviest SCN but if you have the molecular weight of the total fluid available this may be preferable as this is again the measured quantity. You are now ready to define the basis of your characterisation by choosing where in your existing analysis you want to start redistributing the remaining fluid into new pseudocomponents and how many pseudocomponents you want to split this heavy end into. We’ve started with the simplest case where we have chosen to start the split at the heaviest SCN and only allocate one pseudocomponent. Effectively we are only allocating physical properties to the existing SCNs. Click on the Do Characterisation button and you will see a message box such as

followed by a screenshot of the experimental data and the fitted distribution

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Click on OK and Close to return to the main window where the new fluid composition will be reported

The output lists the components and their composition in the units requested. The additional column indicates the lower boundaries of the particular cut, for example C6 comprises the cut from C5.5 to C6.5. Properties of the individual pseudocomponents may be viewed using Tools/Pure Component Data as usual and further calculations can be carried out on the basis of this characterisation. At this point, having successfully characterised the fluid, you can also save the input as an .mfl file. A useful way of seeing how changing characterisations alter the results of phase calculations is to use the phase envelope generator. For instance, plot the phase envelope of this fluid.

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You can investigate various aspects of the characterisation and the sensitivity of the phase envelope to changing these. You can include an n-paraffin distribution by ticking the Estimate Wax Content box located in the PVT analysis form. Set the starting point for the n-paraffin to N6 with 15 n-paraffins. In this case the names and compositions of the fraction cuts will differ,

If you return to the PVT Lab Analysis form and instead of the heaviest SCN choose "Single fluid" and enter a MW of 68. Do the characterisation (without nparaffins) and plot the phase envelope. Then see what the effect is of extending the heaviest SCN to further fractions, by leaving C20 as the start of the pseudocomponents but choosing to split it into 5 pseudocomponents. Alternatively you can group the components by starting the pseudocomponent split at C8 and grouping the plus fraction into 15 pseudocomponents. You can see that this alters the cricondenbar but the major effect is on the

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cricondentherm. (The line for C8, 15PF is on top of the line for C20, 8PF).

Next, return to the original fluid definition and re-plot the phase envelope (first clear the previous plot), then in the PVT Analysis form enter a water cut. This is defined in terms of the volume percentage of the total fluid that is water. In this case choose 3 %. In the main window plot the new phase envelope and the water phase boundary.

Finally, return to the original fluid analysis again and this time add a separator gas. Here we will look at a simple problem where the gas is 100 % methane added at a GOR of 100 m3/m3. Move to the Liquid + Gas tab and enter 100 next to methane in the left hand column headed "Gas" and in the Recombined fluid section of the PVT form set the GOR units to m3/m3 and enter 100.

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Do the characterisation and return to the main window and plot the new phase envelope by clicking on VLE AutoPlot.

User defined carbon number cuts In the previous examples the carbon number cuts, SCN, have been defined by the software. If you wish you can over-ride these by choosing your own SCN. Starting with the pvt_anal2.mfl file go to the PVT Analysis form and change the starting point for the pseudocomponents to C6 with five pseudocomponents. Do the characterisation and return to the main window; you will see that the plus fraction has been defined with five SCN cuts

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Now, plot the phase envelope and then return to the PVT Analysis. This time check the box for user defined cuts. Clicking on "Define Cuts" button will show a drop down box that allows you to set the SCN cuts for the specified number of pseudo components (in this case 5).

After setting the SCN to 6, 10, 20, 30, and 40, and performing the characterization, the results window shows the amounts and starting points for the new SCN.

For these user defined SCNs the phase envelope is identical to the original.

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A more distorted distribution (e.g., with cuts at SCN of 6, 30, 35, 37, and 40), may affect the phase envelope more.

TBP curves If your PVT analysis data, instead of a detailed SCN/Composition report, is based on assay data, such as a True Boiling Point (TBP) curve or a D86 analysis, you can enter this and convert the data to fixed carbon number cuts. Go to the PVT Analysis and click on the tab marked Distillation Curves and enter your data. This case study is based on the TBP.mfl file, which has volume % and boiling point data but no Molecular weight or specific gravity.

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Once you have entered this, proceed as usual and do the characterisation If this is successful the plot will show the comparison of data and fitted distribution,

and the carbon number distribution will be reported in the results window.

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Once you have defined a fluid model you can carry out calculations or plot the phase boundary as for any fluid.

Black Oil Analysis The black oil analysis offers the user an opportunity to take a very limited input specification (known as Black Oil input) for a condensate or oil, and from this generate a compositional analysis. Our example is based on the blackoil.mfl file. To do a black oil analysis open the PVT form and select the “Balck Oil Analysis” tab. The minimum required input is the gas gravity (relative to air), the STO specific gravity (relative to water) at 60F and 14.7 psi and the solution GOR. The latter is the volume of gas produced at surface standard conditions divided by the volume of oil entering the stock tank at standard conditions. It is often referred to as Rs.

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The remainder of the form is the standard PVT, except that you do not provide molecular weight or specific gravity. You can choose the pseudocomponent distribution as normal, depending on the final application. In this case the chosen split is 15 fractions from C6+. Clicking on Do Characterisation generates the message that the characterisation has been successfully completed – in this case there is no compositional information to generate the compositional plot. The new composition is echoed in the main window and the phase envelope can be plotted as before.

Additional data can be added such as the Watson K-factor and/or the Gas analysis.

Plotting the phase envelopes shows the effect of including this data.

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Delumping tool – Case study A simple fluid with only one petroleum fraction is used as a starting point for this delumping example. The composition of the fluid is given in the following table.

Example fluid: Component

Amount (mol)

Methane

95

Ethane

3

Propane

2

Co2

0.1

N2

0.15

C6+

5

The C6+ fraction has the following properties: Tc (K)

Pc (Pa)

Acentric factor

Molecular weight

863

1.5e6

0.9

250

All the remaining properties will be calculated by Multiflash once this information is entered in the Petroleum Fraction Input form from the Select Components tool. Save the file as “original_fluid.mfl” After defining the fluid, click the delump button

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Click the “Delump” button.

From only one component in the plus fraction, 60 components were generated. Now, click “Close” to close the delump tool. When prompted to use the delumped fluid as it is click “yes”.

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The resulting fluid may be saved into an MFL file for future reference with the name “full_delumped.mfl”. Open the Delumping tool again by clicking the “Delump” button. Click the “Open PVT tool” button. The following window will appear:

Then, click “Do Characterisation” to characterise the fluid with the default settings. Note that the first petroleum fraction is C14, which states that the original C6+ was heavier than the name implied. After characterising the fluid, with the default settings the resulting components and amounts are displayed in Multiflash main window:

This fluid can now be saved in file named “delump_rechar.mfl”.

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A comparison of the different fluids can now be made, in terms of the shape of the phase envelope. As an example the default RKS (Advanced) model is used.

Note that this is a very simple example to illustrate how to use the delumping tool. The delumping procedure is more reliable if the original fluid contains more than 1 petroleum fraction. For this simple fluid the phase envelopes of the delumped and the re-characterised fluids differ from the original fluid, while better agreement is usually obtained when the original fluid contains more information (more petroleum fractions). An example with a more complex fluid and with wax is shown below. Open the file “wax.mfl” that is shipped with Multiflash. In this file the original fluid has 30 petroleum fractions. Use the same procedure as described above. The resulting phase envelopes are shown below:

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Refrigerant mixtures Several of our customers have used Multiflash to determine the properties of refrigerants. A number of pure refrigerants are included in the INFODATA databank and in the CSMA model. We have also fitted data for a large number of refrigerant mixtures and the BIPs have been stored in the INFOBIPS databank. For mixtures we have used the RKSA model to determine the thermodynamic properties of the mixture except for the liquid volume/density which is calculated using the ideal solution model. This model combination has been included in a model configuration file called refrig.mfl. To determine the properties of any refrigerant mixture, first load refrig.mfl using File/Load problem setup. The refrigerant mixture can then be defined as normal using Select Components and providing the composition. However, there are several well defined refrigerant mixtures which have been allocated refrigerant numbers e.g. R407A. This is a mixture of the pure refrigerants, R32, R125 and R134A, with a fixed composition (in mass percentages) of 20/40/40. To help our users we have set up .mfl files defining components/compositions for R401A R401B R401C R402A R402B R404A R405A R406A R407A R407B R407C R407D R407E R408A R409A R409B R410A R410B R411A R411B R414B R417A R500 R501 R502 R503 R504 R507A R508A R508B To determine the dew point properties of R407A Load refrig.mfl

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Load R407A.mfl Specify pressure, e.g. 25 bar or 25e5 Pa Click on the P, Dew point flash icon or menu item And the results will be displayed in the results window Dew point at fixed P:

Polar systems For polar systems, not related to oil and gas problems, the best model is often an activity model, such as Wilson-E, NRTL, UNIQUAC or UNIFAC, but binary interaction parameters are usually needed to obtain accurate results. For the first three models, Multiflash has BIPs available for many binary pairs but where these are missing you need to supply them. The UNIFAC BIPs are generated from group structures. Before carrying out phase equilibrium calculations for polar streams using an activity coefficient model we recommend that you check the availability of BIPs for your system and look up interaction parameters for the binary pairs where none are available from INFOBIPs. An alternative is to fit experimental data to a model used in Multiflash or generate data from UNIFAC and fit this to the model of your choice. We have provided sample spreadsheets which allow you to do both using the Excel interface. Reference: Dechema Chemistry Data Series Vols I to XIV, Dechema

A good source of experimental data and BIPs is the series of volumes in the “Chemistry Data Series”, published by Dechema. The UNIFAC model will provide estimates of vapour-liquid and liquid-liquid equilibria without the need for BIPs.

Modelling a polar mixture Using INFOBIPS As polar mixtures are usually non-ideal you may have some information on their phase behaviour and wish to know how best to reproduce this. A simple example is the acetone/water mixture. Described in ACETH2O.mfl. The Dechema data series referred to has several sets of data for this system. We have taken, at

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random, the data by Kojima et al, Kagaku Kogaku 32, 149 (1968) and based the example on one experimental point Pressure 760 mmHg Temperature

60.39 degC

x(acetone)

0.4000

y(acetone)

0.8426

We can use Multiflash to see how well different models and different sets of parameters represent this data. Depending on the relative importance to your application of accurate temperature or phase composition we can fix P and x and calculate T and y using a bubble point calculation at fixed P or fix P and T and calculate x and y with a P,T flash. We would usually suggest using activity coefficient models to predict phase behaviour for non-ideal mixtures. If you have BIPs available either from INFOBIPS or from any other source for any particular activity model then this is the model you should use. Specify the mixture by Clicking on the Select components button With the Infochem fluids databank as the default data source Type acetone in the Enter name text box and Click on the Add button. Select water in the same way Click on Close In the Composition drop-down table enter 0.4 mol of acetone and 0.6 mol of water. Set the units (from Select/Units from the menu option or the select units button) in the Tab control so that the input and output temperatures are in degC and the pressures in mmHg. Enter 760 mmHg as the pressure Select a suitable model Select in order Select/Model Set/Activity Models/WilsonE to choose the Wilson E model To check whether BIPs are stored in INFOBIPs for this mixture go to Tools/BIPs and when the Show BIP Value box appears click on Edit.

As you can see BIPs are available. Click on the P, Bubble point flash button and the model prediction is a temperature of 61.4 degC and a vapour phase fraction of acetone of 0.821. This is in reasonable agreement with the experimental results. It may be possible to improve prediction by over-writing the stored BIPs with those reported in

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Dechema for that particular experimental dataset, remembering that the BIPs must be in the correct units. You can repeat the calculation using the UNIFAC model where BIPs are calculated on the basis of group structure. The corresponding results are a bubble point temperature of 60.9 degC and a vapour phase fraction of acetone of 0.824.

Entering BIPs You may have a mixture where there are no stored BIPs. Take for example the system carbon tetrachloride/hexane. If you repeat the earlier steps to select components, units and models and again look at the BIPs using the Tools options you will see that for the Wilson-E model no BIPs are available.

Set the pressure to 1 bar, and specify the composition to 0.325 mole carbon tetrachloride and 0.675 mole hexane. Click on the P, Bubble point flash button to see the model prediction using the default, BIPs = 0.0. The predicted result is a bubble point temperature at 1 bar of 344.3K and a vapour phase fraction of carbon tetrachloride of .270. The experimental data is 342.8 K with y =0.286. However, for this data set we have fitted BIPs for the WilsonE model; 266.61 and 461.91 J/mol. You can enter these BIPs into Multiflash using the Tools /BIPs from the menu bar. Select Tools, then select BIPs In the Show BIP values box highlight WILSONBIP2 and click on Edit Remember to ensure that you have specified the correct units for the numeric values of the BIPs

WilsonBIP2 is the name recognised by Multiflash for BIPs relating to the Wilson E model. This model has two BIPs, which may be expressed as constant, linear or quadratic functions of temperature. The fitted BIPs like those from Dechema are temperature independent, i.e. constant. The BIPs are asymmetric and not interchangeable. It is therefore important to input the values in the correct order. If the BIPs are reported for pair A-B, the first value must go in the cell in row A, column B, and the second value must go in the cell in row B, column A. If your BIP units are not the default J/mol change the units by clicking on the Units button in the BIP box

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Enter the new BIPs in the selected units

Click on OK and then on Close in the Show BIP values box. Click again on the P, bubble point button. This time the predicted bubble point temperature is 342.9K and the gas phase composition is 0.288. These give a better prediction of the bubble point pressure and improvement in the prediction of the gas phase composition of acetone. The calculation can be repeated for each of the activity methods, including UNIFAC. To do this: Select the new model set, this will clear the previous model and BIPs, but retain the components, compositions and units. Calculate the P, bubble point without BIPs Remember NRTL has 3 constant coefficients, not 2: two asymmetric and one symmetric

Enter BIPs, as given in the table below (in J/mol), using the Tools/BIPs option. The bipset names are the model name followed by BIP, e.g. NRTLBIP3, UNIQUACBIP2. BIPs are not required for UNIFAC as they are generated from group contributions. Calculate the bubble point temperature again. with the BIPs. You should reproduce the following results when the BIPs are entered in J/mol. Temperature/K Wilson E

No BIPs

BIPs: 266.6, 461.9 UNIQUAC (VLE)

No BIPs

BIPs: 208.5, -12.84 NRTL (VLE)

344.3

0.270

342.9

0.288

344.2

0.271

342.9

0.288

344.0

0.273

BIPs: 276.8, 284.9, .3

342.7

0.287

UNIFAC (VLE)

343.4

0.279

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No BIPs

y1

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You can see from the results the importance of using interaction parameters for non-ideal systems. All the calculations were carried out with the ideal model for the vapour phase. For this system using the Virial (HOC) model, or RK for the vapour phase model does not give significant differences. However, for some components, such as acetic acid which may dimerise, using the Virial (HOC) model would be beneficial. You could also enter different BIPs for this system and examine the sensitivity of the results to these variations.

Liquid-liquid equilibria Many non-ideal polar systems exhibit liquid-liquid equilibria. To model these systems you must choose an activity model capable of predicting two liquid phases, the Wilson model cannot do this. Two suitable models are UNIQUAC LLE and NRTL LLE. The LLE version of NRTL usually has the  parameter set to 0.2 by default. Again to obtain realistic results you should enter BIPs and we have incorporated BIP data for over 300 systems into our INFOLLBIPs databank. If you take UNIQUAC or NRTL parameters taken from the Dechema Chemistry data Series they will be in K so you must choose the correct input units or convert them by multiplying by the gas constant (8.314 JK-1mol-1). A typical mixture which exhibits liquid/liquid behaviour is butanol/water Dechema Chemistry Data Series. Volume V, Part 1, page 236. If you have set up a problem with only one gas and one liquid phase defined you may see a warning message above a vapour-liquid or one liquid phase solution. Two different warnings will be given depending on the problem: Flash at fixed P and T: *** WARNING -20001 *** Unstable solution, more phases exist. T =298.25K P=1.00000E+05Pa ? CONVERGED ..UNSTABLE Flash at fixed P and T: *** WARNING -20006 *** Type of phases present do not agree with phase descriptors. These warnings indicates that you should consider looking for another liquid phase, and define a second phase descriptor for this. Use the Select Model Sets, or the model configuration files provided for activity methods, to ensure that two liquid phases are available. To carry out the case study in Multiflash Select the UNIQUAC LLE model Select butanol and water from INFODATA Set the butanol and water compositions to .5 mol each You can check that BIPs are available using the Tools/BIP option. Set the temperature to 298.15K and the pressure to 1e5 Pa. Carry out a P,T flash. The mixture will be a single liquid phase. If you increase the water concentration to 0.7 with 0.3 mole butanol and repeat the two phase flash you get a liquid-liquid solution.

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Vapour-liquid-liquid equilibria The situation is a little more complicated if you have VLLE. None of the activity models that can handle VLLE do this well without specific tuning of the BIPs. The VLE variants of UNIQUAC and NRTL are configured to be used with GAS and Liquid1 and the LLE variants with Liquid1 and Liquid2. To calculate VLLE with either you must first "Switch-on" the missing phase.

The only difference is the source of BIPs and we have increased the options by allowing two BIP databanks to be in force at any time. The LLE model options are configured to first search INFOLLBIPs, then to supplement any missing BIPs from INFOBIPs. The VLE variants are configured only to search INFOBIPs. For VLLE it may therefore be preferable to use the LLE model variant. However, it is difficult to make an absolute recommendation, trying both approaches and assessing the differences might be useful. You can also add INFOLLBIPS to a VLE variant using Tools/Command and entering bipdata INFOBIPS

INFOLLBIPS;

Azeotropes You can use Multiflash to identify azeotropes; the temperature and pressure where the composition of the liquid and gas phase are identical. They can be investigated by carrying out a series of flash calculations but this is timeconsuming. For binary azeotropes you might wish to consider using the Excel interface. Below is a plot of gas phase versus liquid phase composition for the propanol water system using Excel. The azeotrope is identified at a mole fraction of 0.46 propanol. The plot was generated using the Wilson-E model, and by making a series of bubble point calculations at 1 bar at varying composition of the liquid phase.

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You can vary the pressure or temperature and see the effect on the azeotropic point, or investigate the effect of adding a third component.

Eutectics Similarly, you can use Multiflash to determine a eutectic, although again this is most easily seen using an Excel spreadsheet. An example would be a mixture of benzene and naphthalene. In addition to defining a fluid phase model, for example RKSA, you also need to allocate the freeze-out model to each component. Using the Multiflash functions in Excel you then carry out a series of fixed phase fraction calculations to identify, for a given pressure, the temperature at which solid benzene and solid naphthalene form. A plot of the predicted temperature versus composition for both shows the eutectic point. For the RKSA fluid phase model and 1 bar this is predicted to be at 269.9 K and a mole fraction of benzene of .871.

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Polymers The PC-SAFT equation of state in Multiflash can be used to model the phase behaviour of polymers, solvents and gases. PC-SAFT is a complex equation and to be able to develop a robust implementation capable of predicting multiple phases is unusual. Owing to the high molecular weight, and correspondingly low mole fractions, of polymers in solution considerable effort has gone into improving the phase algorithms in Multiflash, but we are aware that further improvements may be necessary.

Data input The Infochem/KBC databanks do not contain any data for polymer components. Polymers should be defined using the Multiflash User Defined component route (Select/Components) and either saved as part of a complete .mfl file or a partial file containing the polymer data only. The required input data are: Critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), PC-SAFT parameters (SAFTEK, SAFTSIGMA, SAFTM, SAFTKAPPA, SAFTEPSILON, SAFTFF) Ideal gas Cp (CPIDEAL). Users should note that TCRIT, PCRIT and ACENTRICFACTOR are necessary to generate starting values for flash calculations. They are not required by the PC-SAFT model, and therefore do not affect the calculated results. Also SAFTKAPPA, SAFTEPSILON and SAFTFF are only needed for associating components. If your system is polydisperse, i.e. has the same polymer but with a range of molecular weight, then you can enter several polymers with varying properties, each called by a different name. This is analogous to setting up different petroleum fractions, although we do not yet have a facility to help the user set up the data for polymers. Using PC-SAFT, every pseudocomponent for a given polymer must be assigned the same values of the pure-compound parameters SAFTSIGMA (in metres, not Ångstrom units) and SAFTEK. In addition, the SAFTM parameter must be specified. This is normally quoted as a ratio to the molecular weight, so it has to be calculated for each polymer pseudocomponent knowing the molecular weight. For polystyrene, for example, Gross and Sadowski give the ratio as 0.019, so for a polystyrene pseudocomponent of molecular weight 100000, the SAFTM parameter should be set to 1000000.019=1900, etc. Our example polymer.mfl describes a simple binary of polystyrene plus butane. The required properties of polystyrene are included in the input file, as are the pure component SAFT parameters for butane. Load the input file in the normal way and carry out a P,T flash at the input conditions supplied. The results show a liquid-liquid split as expected.

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You can also carry other flashes, such as a bubble point calculation

PC-SAFT has two interaction parameters. No interaction parameters for PCSAFT are stored; both are set to zero by default. As with all Multiflash models it is possible for the user to enter BIPs through the Tools/BIP command or as part of the input file. As with CPA, in most cases it is the parameter, SAFTBIP that is adjusted. As the polymer is non-volatile, changing the BIP may affect the phase distribution and phase compositions more than the bubble point pressure or temperature. The result of altering the SAFTBIP from 0.0 to .05 for our sample system is shown below

A slightly more complex system, polymer2.mfl, shows the unusual ability of Multiflash to deal with complex systems. In this example pentane replaces Butane as a solvent and we introduce the styrene monomer and water in the mixture. In this case a bubble point calculation predicts the presence of four phases.

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The series of papers by Sadowski et al, detailed in the "Models and Physical Properties Guide”, are a useful source of input parameters for PC-SAFT for several polymers. N.B. Occasionally the phase labels, LIQUID1 and LIQUID2 may be interchanged. If this causes confusion they can be forced to stay the same by defining a key component for one of the liquid phases. You can use the Tools/Command menu option and enter the command Key liquid2 heaviest: Before PC-SAFT was implemented, Flory-Huggins was sometimes used for polymer calculations. This is still possible but not recommended. To apply the Flory Huggins model, in addition to the critical parameters required by the Multiflash algorithms the additional data needed are: vapour pressure (PSAT), saturated liquid density (LDENS), solubility parameter (SOLUPAR) and molar volume at 25°C (V25). Estimated properties have been included in the file polymer3.mfl for polystyrene and the predicted bubble point for the polystyrene-butane binary shown below.

Co-Polymers PC-SAFT can also be applied to co-polymers. Multiflash allows the user to define up to four polymer segments which can be used to define any number of homopolymers or copolymers. If the polymer is formed from only one type of segment, it is a homopolymer of that segment; if it is formed of two or more types of segment, it is a copolymer. We will look at an example where the constituent segments are ethylene and propylene (PE and PP). The fluid is set up in the copol.mfl file. The appropriate PC-SAFT parameters were taken from papers by Sadowski et al. as are the BIPs. A co-polymer structure (PEP1) has to be defined, as well as the physical properties of the co-polymer. The MW, T, P and acentric factor are required although the latter three are only used as starting values and arbitrary numbers may be assigned as long as Tc is high and Pc very low in line with the low

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volatility of polymers. Next a “template” has to be created to indicate the structure of the co-polymer. This is done by specifying "Bond Fractions", i.e., the total fraction of bonds between different type of segments. For example, in the case where there are regular alternating ethylene and propylene segments, the fraction of PP-PE bonds is equal to 1.0 (all bonds are of type PP-PE). Conversely, the fractions of direct PE-PE or PP-PP bonds will be equal to zero (no PE-PE or PP-PP bonds are present because the monomers alternate). To specify the bond structure, we select Tools/Pure Component Data/SAFT bond fractions. After clicking on Edit we get

The names of the constituent segments are entered as shown. The pattern of bond fractions shown is that for an alternating co-polymer as described above. In the case of a random co-polymer, the bond fraction pattern would be: Bond Fracs 0.25 0.5

0.25

Our input file, copol.mfl, has a co-polymer with a MW of 96400 g/mol. It is present at 15 wt% in a solvent, 1-butene (85 wt%). Calculation of the polymer cloud point is a difficult calculation. You can calculate this using a series of P,T flashes to see, for a given temperature, the pressure at which a second liquid phase appears or disappears. Another useful technique is to set the temperature of interest and calculate a bubble point. For this fluid the bubble point at 100 degC is shown below.

If you only have a gas and one liquid phase at the bubble point, then changing the pressure will not result in a liquid-liquid separation. If at the bubble point you have gas and two liquid phases then you can calculate the cloud point, the

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point at which a second liquid phase appears, using the fixed phase fraction flash at specified T, even though this is not completely straight forward. The solution type should be set to “unspecified” and it may sometimes be necessary to use starting values. Depending on the mixture you might need to look for liquid1 (rather than liquid2) to find the cloud point. That is the case for this fluid. (Starting value for pressure should be 82 bar).

A PT flash calculation for this mixture at 100 degC and 83 bar returns one phase, with label liquid1.

It might therefore be counterintuitive that to find the cloud point you need to look for liquid1 rather than liquid2, but a flash at 100 degC and 82 bar shows us that it really is the phase liquid1 which appears/disappears at the cloud point.

Labelling the phases for a system like this is not straight forward, which is the reason for these somewhat surprising results. When two liquid phases are present it is straight forward to keep track of which is which, in the sense that the solvent phase is labelled liquid1 and the polymer phase is labelled liquid2. The problem is when only one liquid phase is present. Because the amount of polymer is reasonably small the combined phase is given the label of the solvent phase. This is what causes the problem at the cloud point, where the solvent starts coming out of the combined phase, and forms an almost pure solvent phase, and the new phase really is liquid1, and the existing phase changes label to liquid2.

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A complex picture of the phase behaviour of co-polymers of the same type but differing molecular weight can be built up as shown in the following figure. PEP of varying MW and 1-Butene Wt% .15PEP:.85 Butene 300 M=.709kg/mol 250

M=5.9kg/mol M=26kg/mol

P/bar

200

M= 96.4kg/mol

150

100

50

0 0

50

100

150

200

250

T/C

To specify the co-polymers of differing MW you need only change the MW using Tools/Pure Component properties, all other SAFT properties remain the same. Provided the overall composition is specified in mass units it will remain the same even if the co-polymer MW is varied.

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Case studies - Hydrate dissociation, formation and inhibition

Introduction This section is only applicable if your copy of Multiflash includes the hydrates option. Natural gas hydrates are solid ice-like compounds of water and the light components of natural gas. Also, some heavier hydrocarbons found in gas condensates and oils are known to form hydrates if smaller molecules such as methane or nitrogen are present to stabilise the structure. Hydrates may form at temperatures above the ice point and are therefore a serious concern in oil and gas processing operations. The phase behaviour of systems involving hydrates can be very complex because up to seven phases must normally be considered, even without considering the possibility of scale formation. The behaviour is particularly complex if there is significant mutual solubility between phases, e.g. when inhibitors or CO2 are present. Multiflash offers a powerful set of thermodynamic models and calculation techniques for modelling hydrates. The models used in Multiflash for hydrates and hydrate inhibition have been briefly described, see “Hydrate model” on page 39, or our separate guide to models and physical properties. Components known to form hydrates are also listed.

Defining the hydrate models To ensure that reliable results are obtained it is particularly important that the correct set of models and phase descriptors are used. The Hydrate model sets contain a complete description of model and phase specifications. To define a hydrate model, select Model Set from Select option in the menu bar and click the Hydrates tab to activate the hydrates dialog box. The Hydrate model is then defined by selecting the relevant hydrate phases, i.e. Hydrate 1, Hydrate 2 or Hydrate H; the default is for Hydrate1 and Hydrate2 to be selected. The thermodynamic hydrate model will calculate the hydrate dissociation temperature or pressure, i.e. the point at which hydrates can form. To predict the temperature or pressure at which hydrates will definitely form you need to calculate hydrate nucleation. To do this you should also select Phase Nucleation. Phase Nucleation in the list of phase descriptors always works in conjunction with one of the solid phases such as any hydrate phase or the ice phase.

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Therefore the selection of Phase Nucleation does not increase the number of phases associated with the model used.

If you have a high concentration of salt then you may need to consider the possibility of salt precipitation. Multiflash can consider the formation of chloride and bromide scales. As this may not be a problem for many systems this option is not considered by default. If you think you may have a problem you should check the Halide Scales box. This will increase the number of phases that must be considered but the additional phases will be added automatically when the box is checked, the user does not have to do anything.

Fluid phase model To carry out the full range of hydrate calculations with all the available inhibitors the recommended fluid phase model is the CPA-Infochem. The CPA model is based on advanced RKS equation of state with additional association terms for describing the chemical association among the polar components such as water and methanol. Two sets of BIPs are required, one is required by the RKSA and another set is required by the association terms to describe the cross association between polar components. The required binary interaction parameters by CPA for hydrocarbons, light gases, water and inhibitors are available from the BIP databanks, INFOBIPS and the BIP correlations of OILANDGAS. For inhibition with methanol, ethanol, MEG, DEG or TEG, CPA is the recommended model as it reproduces the partitioning of methanol and MEG between water and hydrocarbon vapour and liquid phases more accurately than RKSA-Infochem with the Modified NRTL Infochem mixing rule.

Hydrate model The thermodynamic hydrate model consists of lattice parameters for the empty hydrate and parameters for the interaction of gas molecules with water in the hydrate. There are different parameter values for each hydrate structure, Hydrate 1, 2 and H. In addition the hydrate must be associated with a liquid phase model that is used to obtain the properties of water. It is important that this is the same model that is used for water as a fluid phase.

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Nucleation model This model can be used to predict the nucleation of any hydrate phases and provides an estimate of the temperature or pressure at which hydrates can be realistically expected to form. The nucleation model is based on the statistical theory of nucleation in multicomponent systems. With the Infochem/KBC hydrate model described above and the nucleation model, the hydrate formation and dissociation boundaries can be predicted. Between these two boundaries is the area of hydrate risk.

Ice model Ice is treated as a pure solid phase. The Infochem/KBC freeze-out model can be used to model the solidification of any component. As with the hydrate phase it is necessary to associate the solid phase model with a liquid phase model that is used to obtain the properties of water. It is important that this is the same liquid model that is associated with the hydrate phase. The nucleation model can also be used to predict the temperature or pressure at which ice starts to nucleate.

Scale model In its general form, the freeze-out model can be applied to any solid phase of fixed composition, which must be defined. The model can for example be applied to hydrated salts such as monoethylene glycol (MEG) monohydrate or to crystalline mineral salts, i.e. scales.

Phases In most cases six phase descriptors (PDs) are required: gas, hydrocarbon liquid, aqueous liquid, hydrate 1, hydrate 2 and ice. At high pressures and/or low temperatures the “gas” phase may become liquid-like and a second non-aqueous liquid PD is needed. This is also the case if there is a significant amount of CO2 or H2S present. When considering structure H hydrates an additional phase descriptor is needed for hydrate H. In most practical cases a natural gas contains propane and the stable hydrate structure will be hydrate 2, although for very lean gases at higher pressures hydrate 1 may be the most stable form. Key components are defined to distinguish between the hydrocarbon and aqueous liquid phases. The phase names used in the hydrate models are: GAS, LIQUID1, LIQUID2, Water, Ice, HYDRATE1, HYDRATE2 and HYDRATEH. You can apply Phase Nucleation to both hydrates and ice, defined by the hydrate model. If Phase Nucleation is selected, this means that the nucleation model is defined and can be used to predict the nucleation of any of the hydrate phases or ice. In contrast to the thermodynamic hydrate model which allows all possible phases to be present when carrying out calculations, the nucleation model considers only the nucleation of the specified phase. At low pressures this can lead to predictions that the hydrate nucleation temperature is higher than the dissociation temperature. However, this is not a real situation as ice is not being considered except for nucleation. If Halide scales are to be considered then further phase descriptors are required. These must represent the correct fixed composition of the scale, these are: NaCl, NaCl.2H2O, KCl, CaCl2.2H2O, CaCl2.4H2O, CaCl2.6H2O, NaBr, NaBr.2(H2O), KBr, CaBr2.6(H2O). The addition of these phase descriptors is done automatically by Multiflash when the "halide scales" option is selected.

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Hydrate calculations with Multiflash In principle, hydrate calculations with Multiflash are no different from flash calculations for fluid phases alone. Multiflash treats fluid and solid phases on the same basis and the full range of flashes can be carried out for streams with hydrates. An important point to note is that you must include water in the mixture explicitly if you wish to do hydrate calculations. Unlike some other programs Multiflash does not assume that water is present unless you specify it. The amount of water may influence the results of the calculations, particularly when inhibitors or water-soluble gases are present.

Will hydrates form at given P and T ? To find out whether a mixture will start to form hydrates at a given pressure and temperature it is simply necessary to define your mixture, specify a hydrate model set and do a P, T flash. If you wish to start from a problem setup file we have provided hydrate.mfl, which describes a gas condensate. From the Select menu choose Model Set and click on the Hydrates tab. In the dialog box, select the relevant phases required and initially specify CPA as the fluid phase model.

Click on OK once the hydrate model set has been successfully defined and loaded. Specifying the components and composition The fluid for this case study is defined in the following table: Component METHANE ETHANE PROPANE ISOBUTANE BUTANE PENTANE

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Moles 85.93 6.75 3.13 0.71 0.88 0.57

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CO2 N2 WATER

1.31 0.72 10

Define the normal components in the usual way; click on the Select components button, enter the component name in the Enter name text box and press the enter key or click on Add to select them for loading into Multiflash. Close to go back to the main window. Click on composition and enter the correct number of moles for each component. Alternatively. Load the hydrate.mfl input file. Enter the temperature, 270K and the pressure, 1 MPa (remember to change the standard pressure units from Pa to MPa). The input units are defined in moles but the output units for this example are in g. Click on the P,T flash button You will see the following results in the results window.

Hydrate2 is formed at the specified conditions, and you can see that this is in agreement with the phase diagram. Note that the output shows the amount of hydrate formed just as it does for other phases. Clicking on the Phase Envelope button , selecting Hydrate 2 phase and setting HYDRATE2 phase fraction equal to 0.0, the HYDRATE2 phase boundary can be generated by clicking the Plot button.

Hydrate formation and dissociation temperature at given pressure The hydrate formation or dissociation temperature calculation is an example of a fixed phase fraction flash. The dissociation temperature is the point below which hydrates can form (also known as the equilibrium hydrate formation curve). The

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formation temperature is the point at which the nucleation of hydrates occurs and hydrates will form. Between these two points is the area of hydrate risk where hydrates may or may not form depending on the time scale (see figure below) and the kinetic consideration.

P r e s s u r e

H yd r a t e z o n e

H yd r a t e fo r m a t io n cu r ve

H yd r a t e r is k

H yd r a t e fr ee

H yd r a t e d is s o cia t io n cu r ve

Tem pera ture

To calculate the hydrate dissociation temperature at given pressure Retain the pressure at 1 MPa. Either Click on the Hydrate dissociation T at fixed P button,

or the Fixed

Phase Fraction Flash at specified pressure button, In the first case Multiflash will determine the most stable hydrate structure and report the dissociation temperature for this. In the second case a dialogue box will be activated, click on the button next to Select phase and from the list select Hydrate2. Select Normal from the Type of solution and enter 0.0 for the molar phase fraction

Click on Do flash The results,

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show that the Hydrate 2 is the most stable form and first begins to form at 276.1K. It is important with the fixed phase fraction flash to specify the correct hydrate structure to search for. If Hydrate1 was specified in the above example the calculation would fail because there is no solution where Hydrate1 is more stable than Hydrate2. In most cases of practical interest hydrate2 is the structure formed, although hydrate1 may be more stable at high pressures for streams containing a high concentration of methane or H2S. If hydrate1 were to be more stable it would be present in non-zero amount in the list of phases formed. If in doubt you can check with the P,T flash option which reports which hydrate structures are stable at any T and P.

Hydrate formation temperature at given pressure To calculate the hydrate formation temperature at 1 MPa, make sure the nucleation model has been defined: go to Select/Model Set/Hydrates, check the "Phase Nucleation" box and click on "Define Model". The nucleation calculation will not work unless the model has been correctly defined. Then, go to the Fixed Phase Fraction Flash – at specified P dialog box and select Nucleation from “Select basis” . Set the phase fraction text box to zero as before and then click the "Do flash" button.

If you try to calculate the hydrate formation temperature without first defining the nucleation model, then the calculation will not converge and error messages will appear:

If this happens, define the nucleation model by selecting "Phase Nucleation" in the Hydrates model dialog box and repeat the calculation. The calculated results with the nucleation model are displayed in the main screen. Note that the hydrate formation temperature at 1 MPa is now 268K, about 8 Kelvin lower than the hydrate dissociation temperature, 276.1K. Note that the nucleation calculation is, in the thermodynamic sense, inherently unstable, as reported.

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Hydrate formation and dissociation pressure at given temperature The hydrate formation or dissociation pressure calculation is analogous to the formation or dissociation temperature calculation, but is carried out with the fixed phase fraction flash at specified T option (using the appropriate button or menu option). The following example finds the hydrate dissociation pressure for the above mixture at 270K.

The hydrate first forms at 0.598 MPa. Under these conditions the hydrate forms from the ice phase rather than the liquid water phase. The hydrate formation pressure at the same temperature is 1.26 MPa.

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Hydrate phase boundary You can also use the phase envelope calculator to plot the hydrate phase boundaries for formation and dissociation for this stream by using the thermodynamic hydrate model and nucleation model - and add experimental data if available.

Other flash calculations with hydrates Once the hydrate model has been specified it is possible to do the same flash calculations as for other fluid phases. For example, an isenthalpic flash calculation can be carried out in the same way as shown for the oil and gas system, see “Other flash calculations” on page 212.

Maximum water content allowable before hydrate dissociation Multiflash can determine the maximum amount of water that may be present in a mixture at a given pressure and temperature before hydrates can form. Multiflash can achieve this by means of a "tolerance calculation". The tolerance calculation combines two mixtures in different ratios until a specified condition is met. The following example finds the maximum water content for the above mixture at 270K and 1 MPa before hydrates will form. In order to calculate this, the following steps are required: Click on Composition and enter 0.0 for the amount of water. The overall compositions must be specified on a water-free basis, but water must remain in the components list. Then, select Calculate from the menu bar, and select Tolerance Calculation. Then, select the required phase from "Select phase" box by clicking the downward-arrow on the right side of the box. In this example, the phase should be HYDRATE2. Set the phase fraction to zero in "Enter phase fraction" box. Click the "Composition of Second Fluid" tab to obtain the second stream of the mixture, then set the composition of water to 1.0 mole (i.e., pure water) and leave the rest to zero. Click Calculate to carry out the tolerance calculation. Click Close back to the main window.

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In the results window you will see,

The first column shows the overall composition at the hydrate dissociation point. The amount of second fluid added is the number of grams of water specified by the tolerance calculation which must be mixed with the original water-free stream to meet the condition of zero hydrate phase at the chosen P and T.

Calculations with inhibitors There is no fundamental difference between calculations with and without inhibitors. To investigate the effect of an inhibitor you can either add it to the list of components in the mixture and specify the amount in the total mixture just as for any other component or you can use the Inhibitor Calculator (see “Inhibitor calculator” on page 79 ) to add the amount of inhibitor relative to water. However, the inhibitor will not, of course, remain solely in the water phase but will partition between the different fluid phases present at equilibrium and the amount in a particular phase will depend on the conditions and the amounts of other components. This is exactly what happens in reality. All the calculations described above can be carried out in the presence of inhibitors.

Can hydrates form at given P and T ? This is based on a P,T flash calculation. The following example illustrates the calculation for the gas defined previously in contact with a solution of water plus methanol at a methanol fraction of 20% by mass. Using Tools/Inhibitor Calculator bring up the Inhibitor calculator window and add 20 mass% methanol to the 10 mole of water in the system.

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After pressing the Add button, the following window will show up stating that 20 wt% methanol is approximately equivalent to adding 1.405 moles of methanol to 10 mole of water

With the temperature at 270K and a pressure of 1 MPa Click on the P,T flash button.

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The results show that the addition of this concentration of methanol is sufficient to prevent hydrate formation even though some has been lost to the gas phase.

Hydrate dissociation temperature at a given pressure With the same mixture, calculate the hydrate dissociation temperature using the Hydrate dissociation T at given P button or the fixed phase fraction flash at fixed P with the hydrate 2 phase at 0.0 phase fraction.

You can see that, compared to the earlier calculation in the absence of methanol, the addition of methanol has reduced the hydrate dissociation temperature from 276.1 K to 266.5 K. Virtually all the methanol is in the aqueous phase at these conditions.

Hydrate dissociation pressure at a given temperature Again this is analogous to the calculation above but you use the Hydrate dissociation at given T button, , or specify a fixed phase fraction flash at fixed T. The hydrate dissociation pressure increases from 0.598 MPa to 1.51 MPa. The anti-freeze effect of methanol means that the hydrate forms from liquid water rather than ice as previously.

Hydrate phase boundary You can compare the hydrate phase boundary with and without inhibitor by plotting the new phase boundary with methanol present.

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Amount of inhibitor required to suppress hydrates Multiflash can determine the amount of inhibitor that must be added to the system at a given pressure and temperature in order to suppress hydrates. This is another example of a tolerance calculation and is therefore specified using the Tolerance Calculation from the Calculate menu. The overall compositions must be specified on an inhibitor-free basis. The inhibitor is entered as a second stream using the tolerance calculation. The phase required to be fixed and phase fraction can be specified in the Select phase and Enter phase fraction boxes, zero phase fraction of hydrate2 in this case. The tolerance calculation combines the two mixtures in different ratios until the specified condition is met. The following example finds the amount of methanol that must be added to suppress hydrates for the above mixture at 270K and 1 MPa. Remove methanol from the main stream by clicking on Composition and entering 0.0 mol for methanol. Note that methanol must still be in the Composition list. Select Calculate, then Tolerance Calculation to activate the Tolerance Calculation dialogue box. Select the required phase (HYDRATE2) from Select phase box. Set this phase fraction to zero. Click the Composition of Second Fluid tab to specify the composition of methanol as 1.0 mole and leave the remainder zero. Click Calculate to carry out the tolerance calculation. Click Close to go back to the main window.

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The first column shows the overall composition at the hydrate dissociation point. The predicted methanol concentration required is 1.3% on a mass basis with respect to the total stream, approximately 13.6 mass % with respect to water in the feed. The amount of second fluid added is the number of grams of the mixture specified by the tolerance calculation (in this case pure methanol) which must be mixed with the original inhibitor-free stream to meet the condition of zero hydrate phases

Salt inhibition The Electrolyte model treats the salts either as being composed of only Na+ and Cl- ions or of Na+, K+, Ca++ , Cl- and Br- ions. Unfortunately, the information supplied for the amount of salt in brine, formation or production water is not usually specified in the input format required. To help you with the conversion we have provided a Salinity Calculator, see “Salt calculator” on page 81 that converts various analyses into the equivalent amount of sodium, potassium, calcium, chloride and bromide ions. Load the hydrate.mfl file: Change the Model set from CPA-Infochem to CPA-Infochem + Electrolyte (Remember to Define Model). Select the Inhibitor Calculator from the Tools menu and the tab “Salts / Ions / Salt Analysis” For this particular example there is information on the composition of the formation water. mass% NaCl 6.993 CaCl2 0.735 MgCl2 0.186 KCl 0.066 SrCl2 0.099 BaCl2 0.036 Enter this data into the Salt Calculator

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By clicking on Add the Salinity Calculator will determine the ion concentration that needs to be added to the 10 mole of water in the mixture.

and this amount will be automatically entered in the Composition drop down table. Specify the fixed phase flash at constant pressure, setting hydrate2 to 0.0, and click on Do flash The output shows that the hydrate dissociation temperature at 1 MPa for this stream is reduced from 276.1K to 272.77K.

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Scale precipitation This feature allows for precipitation of NaCl, NaCl.2(H2O), KCl, CaCl2.6(H2O), CaCl.4(H2O), CaCl.2)H2O), NaBr, NaBr.2(H2O), KBr and CaBr2.6(H2O). This is activated by ticking the Halide Scales box in the Hydrates Model Set but can only be defined with “CPA Infochem + Electrolyte” fluid phase model option. If you have not specified such an option a warning message is generated.

For our example the salt concentration is not high enough to trigger the precipitation of a scale for hydrate calculations at 1 MPa. In principle, you can use fixed phase fraction flashes to see when any of the scales will form. But the temperatures may well be below those of operational interest. For example selecting NaCl.2(H2O)

We and performing the fixed phase flash, we obtain

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A similar calculation looking for the formation of CaCl2.6(H2O) produces seven phases but at 240.6K.

A more likely scenario occurs if the salt concentration is higher, e.g. 30wt% equivalent of NaCl. A flash at temperatures higher than hydrate dissociation conditions will show NaCl forming

whereas at the lower temperatures where a hydrate phase is present you will see NaCl.2(H2O) being formed.

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Case studies – Wax precipitation

Introduction This section is only applicable if your copy of Multiflash includes the Wax option. Waxes are far more difficult to understand than pure solids because they are complex mixtures of solid hydrocarbons that freeze out of crude oils if the temperature is low enough. Waxes are mainly formed from normal paraffins, but iso-paraffins and naphthenes are also present. As with hydrates the formation of waxes is a serious concern in oil and gas processing. Before discussing the modelling of wax precipitation it is worth referring to a paper by Erickson et al. SPE 26604, (1993). which compares the results of measuring wax appearance temperatures (WAT) using three different experimental techniques. For twelve oils, where there were measurements made by at least two different techniques, there was only one case of complete agreement between two methods. Otherwise the minimum difference between techniques was 8 ºF, the maximum difference was 55 ºF, whilst the average difference was 24 ºF. It appears that the accuracy of WAT measurements has improved in recent years, but it is still difficult to measure; it is realistic when assessing results to assume that experimental error in WAT values may amount to several degrees. We recommend measurements made by Cross Polar Microscopy (CPM) if available. We also recommend that positive amounts of precipitated wax are used to identify the WAT, rather than the strict thermodynamic interpretation of zero percent, the onset of wax phase formation. The suggested default values are 0.045 wt% for reproducing CPM measurements and 0.3 wt% for DSC. The equivalent defaults for mol% are 0.015 mol% for CPM and 0.1 mol% for DSC but there is no automatic conversion between mass and mol%. The mass or mole% of wax is related to the liquid plus wax phases.

Defining the wax model The wax model in Multiflash is based on work by Coutinho. For more details refer to the “Modelling wax precipitation” section on page 42. The Coutinho wax model is a solid solution model which requires information on the normal paraffins in the fluid. Predictions from the model are largely governed by the n-paraffin distribution. If no experimental data are available for the distribution it can be estimated from the total wax content. If these data too are lacking then Multiflash has procedures to estimate an n-paraffin distribution. Note that the estimation method is only valid for oils, not waxy condensates. The n-paraffin distribution may be defined differently from that for the remaining fluid but we have found that starting the n-paraffin pseudocomponents from C6 and splitting the plus fraction into 15 pseudo-components is again a useful default.

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With a limited experimental data set it is not possible to make any definitive statements concerning the accuracy of the model in predicting the WAT. However it is clear that Coutinho's model provides a much improved prediction of the amount of wax precipitated as a function of temperature compared with other published thermodynamic models.

Calculating wax appearance temperature (WAT) The calculation of the wax appearance temperature (WAT), formerly known as the cloud point is an example of a fixed phase fraction flash. To define the wax model, select the Waxes tab in the Select/Model Set option. Click the Define Model button to define the wax model in the problem definition. Then Click on Close.

Go to the PVT Analysis form to characterise your fluid. The use of this is described in detail in “PVT Lab Analysis” on page 89. If you have an n-paraffin distribution then you should open the PVT Analysis with n-paraffins using the button. However, in this example (input file wax.mfl), there is no n-paraffin distribution, only a wax content. So, in this case you should use the normal button. Enter the fluid composition and set both the pseudocomponents and n-paraffins to be split from C6 (or N6) into fifteen fractions.

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If you fail to enter a wax content or to ask for this to be estimated then you will still be able to characterise the fluid, but when you try to calculate the WAT you will see a warning message box indicating that you don’t have an n-paraffin distribution.

Provided you have n-paraffins in your fluid characterisation, you can then calculate the WAT at any pressure, by using the WAT,

button.

The pressure will be taken from the Pressure text box in the main window. From a study of many waxy fluids we recommend using a small positive amount of wax to identify the WAT and suggest default values for the most common measurement techniques. The default for CPM is preset, but this can be altered to any value, including 0%. Click on Calculate WAT to initiate the calculation. Our example for this case study is based on a supplied problem set up file called wax.mfl. This particular fluid has a reported experimental WAT based on two different measurement techniques. At 1 bar, using CPM the reported WAT was

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53ºC and using DSC was 40 ºC. The predicted WAT for the CPM default is 49.5ºC and for the suggested DSC default is 37.7ºC. You may wish to vary the n-paraffin distribution for the Coutinho model and see the effect on the predicted WAT. One suggestion is to extend the heavy end as far as possible. You can do this by setting the start of the n-paraffin distribution to something like N90 and only splitting into 1 n-paraffin. You will be warned that the distribution has been extended as far as possible and the highest nparaffin will be lower than the N90 set. In this case the heaviest n-paraffin is n76+ and the WAT for the CPM default is 46.4 C. Extending the n-paraffin distribution does not necessarily increase the WAT, as there are competing effects from both the properties of the new heaviest n-paraffin and the solubility of the reduced amount of this fraction. If you have measured values for the WAT, then you can tune the pseudocomponent properties for the model to match these values. This is done using the Tools/Matching/Wax Phase option. The Matching option is described in “Matching wax data/WAT” on page 123. For this particular example we can take the WAT at 1 bar to be 53ºC from the CPM measurement and use the characterisation as before where the n-paraffin lumping starts after N90, leading to a single continuous SCN n-paraffin distribution. Then enter the value or values for the WAT temperatures and corresponding pressures and phase fractions. The fraction chosen can be zero but should probably reflect the suggested defaults for the technique used for the WAT measurement.

The matching facility will amend the values for the melting temperature and the change of enthalpy on melting of the n-paraffin fractions. The wax boundary can be plotted using the phase envelope button and choosing the WAX phase. For plotting a wax phase boundary with a finite amount of wax present to reflect the technique used in the wax measurements, the value such as 0.045wt% by CPM and 0.3 wt% by DSC has to be converted to the value related to the total fluid if the gas phase is present. In this case, where we are considering dead oil, 0.00045 mass fraction may be used to plot the wax phase boundary.

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For a live oil the amount of wax will be defined with respect to the total fluid. This will vary with pressure, so in this case it may be better to choose a zero mass fraction for the plot. The wax boundary for a live oil has a distinctly different shape. The D marks the point where the wax boundary crosses the bubble point line.

The full range of flashes is available for the Wax model.

Calculating wax precipitation As with other phases, the amount and composition of the wax phase are determined as part of any flash calculations. Given the uncertainty of the WAT from some experimental techniques and the sensitivity of WAT calculations to the characterisation of the heaviest fractions, a better picture of wax precipitation can be derived from a calculation of the wax precipitated as a function of temperature at a given pressure. Using the Windows version of Multiflash you can carry out a series of PT flashes to see how the wax builds up as the heavier components solidify with decreasing temperature. However, a simpler way of performing this analysis is to use the wax precipitation curve button, . Clicking on this will activate the wax precipitation curve form to produce a table of wax mass percentage as a function of temperature with respect to the liquid plus wax precipitated at a given pressure. The default starting temperature is 0°C, or the equivalent in other units, and the finishing temperature is the calculated WAT for zero percent wax.

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There is also an option to set the starting temperature to any other value. The maximum number of points is twenty but the actual number of points will depend on the WAT, the units used and a sensible step. The pressure will be taken as that specified in the pressure text box on the wax precipitation curve form. The wax precipitation curve below was generated using wax.mfl as supplied. The numeric values are also reported in the main window of Multiflash. A series of wax precipitation curves can be calculated within the form to see the effect of pressures on the amount of waxes precipitated.

If you require additional results below 0 ºC, you can also use a command entered in the Tools/Command box. The format of the command is: WAXPC value_pressure value_Tstart value_Tincrement; The wax precipitation curve calculated from the specified starting value is reported in the main window. Additionally, you can use the Add Data button to add the measured WAT if you wish. The calculated values and the experimental data can be exported to an Excel file from the form for further editing if necessary.

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Case studies – Asphaltene flocculation

Introduction One of the major problems for the oil industry is the precipitation of heavy organics during production, transportation and refining or processing of crude oil. Asphaltenes are polar compounds that are stabilised in crude oil by the presence of resins. If the oil is diluted by light hydrocarbons, the concentration of resins goes down and a point may be reached where the asphaltene is no longer stabilised and it flocculates to form a solid deposit. Because the stabilising action of the resins works through the mechanism of polar interactions, their effect becomes weaker as the temperature rises, i.e. flocculation may occur as the temperature increases. However, as the temperature increases further the asphaltene re-dissolves in the oil. Thus, depending on the temperature and the composition of the oil, it is possible to find cases where flocculation both increases and decreases with increasing temperature. The Infochem model for asphaltenes is based on a cubic equation of state but has additional terms to describe the association of asphaltene molecules and their solvation by resins. The parameters for the model were initially developed from a study of nearly thirty sets of experimental measurements of asphaltene precipitation which includes both proprietary and public domain data. The model is complex and to ensure reliable results we recommend that you follow the procedure we suggest until you are familiar with the model and the behaviour of your particular fluid. The asphaltene model in Multiflash is primarily intended for calculating asphaltene precipitation from live oils. We are aware that many users have only titration data for dead (STO) oils. We have investigated how to use this titration data to set the asphaltene model parameters and this is discussed later in the case study.

Input data The ideal input data for the model are:

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

A compositional analysis of the live oil



The amount of asphaltene in the oil and the ratio of resin to asphaltene, often determined from the SARA analysis of a stock tank oil.



One set of asphaltene precipitation conditions.

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

Bubble point (optional) to “tune” the petroleum fraction properties.

For some samples you may only know the absolute weight % of asphaltenes and resins in the total live oil but not the full SARA analysis. In these cases you can just use the weight % of resins and asphaltenes in the stock tank oil and ignore the text boxes for saturates and aromatics in the PVT Lab Fluid Analysis dialog box. For the asphaltene model, the saturates and aromatics part of SARA are only used to normalise the weight % of resins and asphaltenes. The % asphaltene is taken to be that precipitated by n-heptane. Some laboratories report the wt % asphaltene precipitated by n-pentane. It is difficult to give exact guidance on how to convert the values of asphaltenes between npentane and n-heptane precipitation as this can vary from oil to oil and laboratory to laboratory. In general we have found that the wt % asphaltene precipitated by n-pentane is approximately twice that precipitated by n-heptane. However, ratios may vary from 1.3 to 2.7. If you do not have the complete data set we have developed correlations to assign the required parameters. The minimum set of data in this case are: 

A compositional analysis of the live oil



Reservoir temperature



Bubble point (optional), to “tune” the petroleum fraction properties.

Obviously, the more data available the better the model predictions. The prediction of asphaltene precipitation is not as sensitive to the characterisation of the fluid as the wax model. The PVT characterisation method has been improved to characterise asphaltenes correctly regardless of the number of pseudo-components requested for the whole fluid. However, we suggest you consider using a common characterisation procedure for any comparison. By default, the PVT Analysis facility starts the pseudocomponent split at C6 and split the fractions into 15 components. Finally, to model asphaltene precipitation reliably, two model parameters should be determined by matching two asphaltene precipitation onset data at two different temperatures. If such experimental data are available, then the data should be used to optimize the model parameters to reproduce the experimental data. If you do not have asphaltene precipitation onset data, two additional data may be used for tuning. They are the STO asphaltene titration onset data using n-heptane and reservoir condition. Using reservoir condition, based on the screening procedure suggested by de Boer et al (SPE 24987, 1992), assumes that the asphaltene is nearly saturated at the reservoir condition.

Defining the asphaltene model The cubic equation of state, RKSA, is defined as part of the model and it is not possible to choose a different fluid phase model. To set up the asphaltene model use the Select/Model Set menu option. Select the Asphaltenes tab, Click on Define Model to select the asphaltene model and Click on Close to return to the main window.

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The default phases for the asphaltene model are gas, liquid1 and asphaltene. In principle the Water phase can be included but the asphaltene model is developed with the reservoir fluids that water is not considered. Therefore it is better not to include water phase. However if water is included in the fluid, the asphaltene model parameters should be produced for the fluid composition excluding water. In this case, more caution should be taken when plotting the asphaltene phase envelopes as the presence of water in the mixture makes the phase equilibrium calculation more complex. The next step is to characterise your fluid. Go to the PVT Analysis form, described in detail in “PVT Lab Analysis” on page 89. Enter the fluid analysis and set the Start pseudocomponents to C6 and the number of pseudocomponents to 15. Enter any data you have on the molecular weight and/or specific gravity. The final step in the characterisation is to enter any data you have on the weight of asphaltene in the oil and the ratio of resin to asphaltene. The example we are going to look at next is based on a supplied problem set up file, asphex.mfl. We have assumed in this example that a full SARA analysis is available, but later in the case study we will go back and look at the options if you don’t have so much data. If you load asphex.mfl the PVT Analysis form will look like this:

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Once you are happy the data is correct Click on Do Characterisation. In this case a message box will tell you the characterisation has been successfully completed and show a plot of the data and fitted distribution. You can then Click on OK and on Close to go back to the main window. If you have any warnings or errors associated with your own examples then the relevant message box will appear. Those related to fluid characterisation in general are described in “PVT Lab Analysis” on page 89. The asphaltene model in Multiflash was developed based on experimental data where the resin/asphaltene (R/A) ratio was always greater than 2.5. However, some of our users have reported values below this, possibly because the SARA analysis was based on a different experimental technique. Depending on the actual R/A entered you may find that model parameters cannot be generated. We have added a warning message if R/A is below 2. Initially, on characterisation you will be warned of a possible problem.

You can ignore this warning message and see if model parameters can still be produced. You can also increase the resin/asphaltene ratio manually or delete the Resin amount from the SARA Analysis and tick the Estimate RA box in which case the model will generate a default R/A ratio. After characterisation the asphaltene component is named as ASPHALTENE, and the resin component(s) by an “R” prefix, e.g. R65+.

Asphaltene matching The next stage is to use the matching facility to “tune” the pseudocomponent properties and optimize the asphaltene model parameter. Multiflash allows for different tuning methods, based on the available experimental data. The matching of the bubble points is recommended,

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especially for light oils, although not always necessary. In any case, the use of the matching procedure for the asphaltene phase is strongly recommended. Although the default asphaltene model parameters are available for the case where no information is available, it is usually not recommended to use the default setting. For any cases without any measured asphaltene data, it is recommended at lease that the reservoir condition should be used to tune the model parameters. For matching multiple bubble points, we suggest you to use the Tools/Matching Bubble point form first before matching any asphaltene data. On the other hand, if you have only one measured bubble point, the bubbelpoint can be specified on the asphaltene matching form together with the asphaltene data. Note that the bubble point is always matched first before matching the asphaltene data. For this example we have several bubble points. Enter these into the Tool/Matching/Bubble point table.

Click on Match to generate the comparison of predicted and matched data.

Use the Tools/Matching/Asphaltene Phase to display the dialogue box, and enter the available data to obtain the asphaltene model parameters. There are three types of asphaltene data that can be used for tuning the asphaltene model parameters, which are summarised as follows. 1.

Asphaltene upper onset data, ideally at two different temperatures.

2.

Asphaltene onset titration data

3.

Reservoir conditions (pressure and temperature)

In our example below, we have used the reservoir conditions (241 oF, 8500 psi).

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Click on Match and then on Close: the optimised model parameter values will be displayed in the main window. The asphaltene model has now been defined. PLEASE NOTE: If all the three asphaltene data sets mentioned above are available, the most adequate data would be the asphaltene upper onsets, followed by the titration data and finally the reservoir conditions. Only one set of data will be saved in the MFL file after matching.

Saturation P at reservoir T In the case where only one asphaltene upper onset is available, the reservoir saturation pressure could be treated as an another asphaltene upper onset pressure at reservoir temperature provided that no asphaltene precipitation is found experimentally at the reservoir condition. The reservoir saturation pressure at reservoir temperature is usually measured. If not, the calculated saturation pressure at the reservoir temperature can be used instead.

Calculating asphaltene precipitation conditions Once the asphaltene model has been defined and the parameters generated you can carry out asphaltene precipitation calculations. If you are starting from the position where you only know the reservoir conditions and have no information on specific precipitation conditions we suggest you should use the phase boundary tracer to get an overall picture. This is extremely useful, but for these complex calculations it can be difficult to find convergence or even starting values. We recommend you to follow the procedure below. First plot the bubble point line. Use the Phase Envelope facility and plot the boundary for zero gas phase, in this case with the initial values for pressure set to 1000 psi.

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Click on Plot to display the phase boundary. This will usually show a point, or points, of discontinuity at high pressure, labelled D. This is the point where the asphaltene precipitation envelope crosses the bubble point line.

These points can be very useful for setting an appropriate starting pressure for the asphaltene phase envelope or for providing starting values, if these are required. For this example go back to the Phase Envelope and this time set:

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

the phase to ASPHALTENE,



the fraction of the asphaltene phase to zero,



the solution type to unspecified



the Initial value for pressure to 3500 psi with pressure increasing.

After clicking on Plot button, Multiflash will ask if you want more points to be plotted. Click Yes, until the asphaltene boundary becomes complete.

For other examples you may have to set the pressure to decrease or to plot the upper and lower boundaries separately. The upper boundary uses the Unspecified solution or Upper retrograde type solution, while the lower boundary, select the Normal type of solution. Alternatively you can try specifying temperature rather than pressure and/or providing a starting value. We have found asphaltene boundaries most difficult to plot for very light oils. If you have a known set of conditions and want to see if, and how much, asphaltene is present, you can use a simple P,T flash. Enter the temperature and pressure, for example 200 F and 4000 psi, Click on the P,T icon, or Select the P,T flash from the Calculate\Standard flashes menu. The phases present, and the composition and amount of each phase, will be reported in the main window. Before doing this you may find it useful to set the units for amounts to mass as this usually reflects the units of measurement.

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If you want to know the pressure at which asphaltene will start to precipitate at any given temperature then you should use a flash at fixed phase fraction and temperature. Set the temperature, in this case 200 F. Click on the icon or select the calculation option and the dialogue box will be displayed.

Select the ASPHALTENE phase and set the molar phase fraction to zero. To calculate the pressure at which asphaltene will first appear for pressures above the bubble point, select Unspecified or Upper retrograde as the solution type and Click on Do Flash. Multiflash will calculate the pressure on the upper asphaltene phase boundary, in this case 7958.74 psi. To obtain the pressure for the lower asphaltene phase boundary, below the bubble point, follow the same procedure but set the Type of solution to Normal. In this case the reported pressure is 1865.08 psi. You can determine the amount of asphaltene precipitated at any set of P,T conditions using an PT flash as described earlier. However, a simpler way for understanding the asphaltene precipitation as a function of pressure is to use the asphaltene precipitation curve button, . Clicking on this will activate the asphaltene precipitation curve form to produce a table of the asphaltene mass percentage as a function of pressures with respect to the liquid plus asphaltene precipitated at a given temperature. The starting

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pressure depends on the asphaltene precipitation upper onset pressure and the finishing pressure is related to the lower boundary of the asphaltene precipitation onset. The temperature will be taken as that specified in the temperature text box on the asphaltene precipitation curve form. The asphaltene precipitation curve below was generated using asphex.mfl. Load this file, re-characterise the fluid, match the bubble points and then match the asphaltene phase using the reservoir conditions, as explained before. A series of asphaltene precipitation curves can then be calculated within the form to see the effect of temperature on the amount of asphaltenes precipitated. The maximum point in the precipitation curve corresponds to the bubblepoint, where the asphaltene precipitation reaches its maximum.

If the temperature specified is outside of the asphaltene phase boundary, a warning message will be generated as follows, indicating that there is no asphaltene precipitation at the specified temperature.

Sensitivity of calculations to variation in input data Choice of Analysis method The previous calculations were carried out by characterising the fluid without an explicit n-paraffin distribution and matching to reservoir conditions. The plots below show the effect of including the n-paraffin distribution. Note that any recharacterisation will reset all the matched model parameters to the default. Therefore the experimental data must be matched again after re-characterisation, in this case the bubble points and the asphaltene parameters. The explicit inclusion of the n-paraffins for this case shows a small difference to the predicted asphaltene phase envelope (APE) in the low temperature region. This is mainly related to the change of the fluid due to the presence of nparaffins and the reservoir conditions data that are used to adjust the model parameters.

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However, if two asphaltene upper onset data are available for adjusting the model parameters, the difference in the asphaltene envelope caused by the presence of n-paraffins will be reduced. The formation of a solid wax phase actually enhances the asphaltene solubility in the oil for this case and the asphaltene envelope in the low temperature region is lowered as a result of the presence of solid waxes. The new marked “D” along the asphaltene envelope are the points where the wax phase boundary cross the asphaltene phase envelope. For more details on simultaneously calculating asphaltene, wax and hydrate solid phases, refer to “Case studies – Combined solids” on page 287.

Data Availability This example of asphaltene precipitation was based on a data set which comprised the compositional fluid analysis, a SARA analysis, bubble points and the reservoir conditions. In this case, the bubble points were close to the unmatched predictions and so matching to bubble points might not be expected to make a major difference to the asphaltene predictions. However, even in this

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case you can see a noticeable effect from the matching.

There are other data that may be missing and have an effect on the predictions.

No information on the amount of asphaltene in the oil We have provided a procedure to estimate the weight % of asphaltene in the oil if this data are not available. In this case in the PVT Analysis box you should tick the box for estimate RA, and then Click on Do Characterisation and Close.

Rematch the bubble point and asphaltene phase at the reservoir conditions as before and plot the asphaltene precipitation envelope (APE). It is important to include this step; the matched properties are re-set to default values when the fluid is re-characterised. The default procedure estimates both the weight % of asphaltene and the resin/asphaltene ratio. For this particular example the predicted weight % of asphaltene is very close to the reported value with 0.7 wt% asphaltene predicted compared to the experimental data of 0.5 wt%.

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No resin – asphaltene ratio Even if you do not have a SARA analysis you may have the weight % asphaltene. In that case you only need to estimate the Resin/Asphaltene ratio. Proceed as before, enter the wt% asphaltene (0.5 %) in the correct text box in the PVT Analysis, but still tick the Estimate RA box. Repeat the matching of bubble point and reservoir conditions again and plot the APE.

Although the estimated R/A ratio is lower, at 13, than the original, at 22, the resultant APE is very close. This is the result of two factors: once you reach a certain level of R/A the effect of increasing the R/A is reduced and the difference is compensated by slightly different model parameters obtained from matching to a specific precipitation point or reservoir condition.

No reservoir pressure If you only have the reservoir temperature we have included a facility to estimate this. Simply enter the bubble point data and reservoir temperature as before and initiate the matching procedure.

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For this particular example the resultant APE is reasonably close to the APE calculated from the real reservoir conditions. As a corollary to this we have noticed that you usually generate very conservative APEs when you have a very over-pressured reservoir. If you have a bubble point measurement at the same temperature as the reservoir temperature and the reservoir pressure is more than 2.5 times the bubble point a warning message is generated.

You can continue to match to your reservoir conditions although it may also be beneficial to generate the model parameters with an estimated reservoir pressure to see the likely sensitivity. If the bubble point is matched at a different temperature from the reservoir temperature no warning is issued.

No reservoir or precipitation conditions If you do not have either reservoir or precipitation conditions, then there are two default options for generating the asphaltene model parameters. If you have entered a bubble point in the Asphaltene matching but nothing for Reservoir conditions or Asphaltene precipitation then Multiflash will assume that the reservoir temperature is the same as the bubble point temperature and proceed to estimate the reservoir pressure as above. If nothing is entered for bubble point, reservoir conditions or asphaltene precipitation, the model parameters are generated from correlations based on data held in our database. On the Asphaltene matching form, just click the Match button to generate the default model parameters. This will generate a warning message. Then click OK to close the form.

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The results from using this route are very variable, depending on the fluid analysis and we cannot fully recommend its use. In this case the result would be a much more conservative APE.

Matching to asphaltene deposition data The assumption in this case is that you have more data than our basic example, real asphaltene precipitation data either from field conditions or an asphaltene precipitation measurement. In this case we have two asphaltene precipitation onset points at 241 F and 6921 psi and 120 F and 9150 psi. Simply enter these in the Asphaltene matching box instead of the Reservoir conditions.

For comparison purposes we have matched to each point individually and then to both points.

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To add data to the plot, you can use the Add Data option in the phase envelope plotter to include the measured precipitation points and the reservoir conditions.

Gas injection It is known that as gas is injected into a reservoir the likelihood of asphaltene precipitation is increased. The asphaltene model predicts this trend correctly. Return to the original APE, calculated from the asphex.mfl input file with matched bubble points and reservoir conditions. You can mimic gas injection by increasing the amount of methane by adding more moles of methane in the drop down composition box. If you increase the amount of methane from 8.32g to 12g and re-plot the APE you will see that the fluid bubble point line is at higher pressures and the APE has expanded.

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When looking at the effect of gas injection you should, of course, not rematch the fluid bubble point or asphaltene precipitation data as doing this will alter the petroleum fraction properties and set the matched model parameters to the default values. Usually the model parameters are matched with the asphaltene precipitation data of the original reservoir fluid, and then the same model parameters are used for modelling the asphaltene phase behaviour of the original reservoir fluids with gas injection. You should not use the PVT Analysis GOR option to add the injection gas to the reservoir fluid. Any re-characterisation cancels the properties and parameters derived from earlier matching and, as you now have a different fluid, the values of bubble point and reservoir conditions used for matching are no longer valid. If you have a complex injection gas and want to study the effect of different gas injection rates then we suggest the use of an Excel spreadsheet or use the blending procedure in Multiflash to blend the injection gas stream and the reservoir fluid. Please note that the reservoir fluid with bubblepoints matched and asphaltene model tuned should be selected for the model definition in the blending form, so that the asphaltene model parameters are based on the original reservoir fluid. If the asphaltene precipitation data are available for the blended mixtures, the data should be matched after blending.

Titration The Infochem/KBC asphaltene model is intended for use in predicting the asphaltene phase behaviour of live oils and the generation of the model parameters is based on asphaltene studies of live fluids. However, live oil asphaltene studies can be expensive, particularly with the requirement to obtain and transport bottom hole samples. Some of our users have asked whether titration measurements on dead oils could be used to generate the model parameters. To date we have only been able to obtain limited samples of titration data and have traced only one oil, in the public domain, where there is information on both asphaltenes in the live oil and reported titration on the associated stock tank oil (STO), enabling us to compare results. However, we understand that some of our users have applied this approach successfully, and the procedure for using titration data has been automated. The studies have been limited to titration with n-heptane. Our example is based on the titration.mfl file provided. The file includes the live oil composition and the wt% of asphaltene and resins. The reported value of asphaltene was 1.9 wt%, that of resins was 16.1 wt% for the STO. Characterise

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the fluid composition as usual and return to the main menu. Then go to Tools/Matching/Asphaltene phase. The reported onset amount of heptane to just cause asphaltenes to precipitate from the STO at ambient conditions is 1.4 cm3 per g tank oil. This has been converted to .962 g n-heptane using the known density. Enter this value and the bubble point (54.4 ºC and 156.2 bar) and click on Match.

The asphaltene model parameters will be reported in the main window as usual and the APE can now be plotted. The resultant APE is compared below to those generated from matching to a known flocculation point of 54.4 ºC and 200 bar and to a combination of reservoir temperature (54.4º C) and bubble point (54.4 ºC and 156.2 bar)

The APE predicted from matching to titration of the STO is very close to the APE from asphaltene precipitation measurements and both are less conservative than using reservoir conditions to provide the model parameters. It is believed this has been the experience for other fluids.

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If your titration data does not include the amount of heptane required to initiate asphaltene precipitation and if this has to be deduced from other titration results, then the procedure for parameter generation is slightly more complicated and requires the use of an Excel spreadsheet. In the Multiflash GUI either characterise the STO, if this composition is provided, or flash the characterised live oil to STO conditions, using the RKSA model to ensure that no separate asphaltene phase is formed. If you have bubble point data it is important that you match these before flashing to STO conditions. Using the STO composition, change the model set to asphaltene, match the asphaltene precipitation to ambient conditions and save the problem using the File/Save Problem Setup option. You then need to create an Excel worksheet to read this .mfl file. Details of how to do this are described in the Excel manual, but we have provided an example spreadsheet, titration_sto.xls. For our example we have generated the file STO.mfl from the fluid used in our titration example. This is the file that should be used in the Excel spreadsheet. In the spreadsheet you then need to do two things: add a new component to the list, heptane, and add a command line describing the asphaltene parameters. This can be copied from the Multiflash GUI by using Tools/Show/Problem to display the commands. The command line can be copied and pasted to the spreadsheet but for fitting purposes it must be set up so that the RAP parameter appears in a single cell so that it can be optimised using the Excel Solver, e.g. include c:\work directory\sto.mfl"; model MREFASPHALTENE RAEQUIL DATA AAPREEXP 1.00000000 AAEXP 1.00000000 RAPREEXP .636986 RAEXP 0.96698 ; component heptane; The spreadsheet, titration_sto.xls, is set up to optimise the value of RAPREEXP (RAP) using the Excel Tools/Solver by comparing the calculated wt% of asphaltene deposited for given amounts of heptane to the experimental wt% deposited. For our particular example the data reported in the paper included a live oil and a STO composition, a wt% asphaltene for the STO, an asphaltene precipitation point and five points for the heptane titration. The reported titration data are plotted below wt% asphaltene component precipitated as Fn C7 solvent 2 1.8 1.6

wt% asphaltene

1.4 1.2 1 0.8

Exptl

0.6 0.4 0.2 0 0

5

10

15

20

25

30

35

40

g C7/g oil

Fitting to the onset precipitation point using the matching facility produced the following parameters

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RAP RAE

0.94176 0.96636

Whereas fitting to the other four points in Excel gave parameters RAP RAE

0.94999 0.96636

These parameters represent the STO titration data well, but the amount of heptane to just initiate precipitation is slightly higher. You can check the predicted value for the amount of n-heptane required for the onset of asphaltene precipitation using a tolerance calculation with heptane as the second fluid. The predicted amount is .997 g/g oil rather than .962. wt% asphaltene component precipitated as Fn C7 solvent 2.00 1.80

wt% asphaltene

1.60 1.40 1.20 1.00

RAP fit

0.80

Exptl

0.60

predicted onset

0.40 0.20 0.00 0.000

5.000

10.000

15.000

20.000

25.000

30.000

35.000

40.000

g C7/g oil

It is clearly preferable to generate the live oil APE from live oil data. Some predictions are possible from titration data but it is important that all data are compatible, particularly the compositions of the STO and the flashed liquid and the physical properties of pseudo components by matching to bubble point data in both cases if you have them. For this example we also had a measured precipitation point and you can see the APEs resulting from the different approaches.

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Case studies – Combined solids

Introduction The previous three case studies looked at solid formation for hydrates, waxes and asphaltenes as separate problems. However, for some fluids at certain conditions it is possible that any or all of these may form at the same time. The formation of any one will affect the overall composition of the fluid and may therefore affect the formation of the other solids. To examine this possibility we have included a Combined Solids model option. The particular model options for each solid have been chosen to provide the best Infochem can offer whilst ensuring compatibility. The common fluid phase model is RKSA. The hydrate models therefore use RKSAINFO as the fluid model, combined with the Electrolyte salt model. The wax model is the Coutinho model and the asphaltene model is the standard Infochem model. The Combined Solids option is only designed to look at solid formation, if you want to study more complex problems such as hydrate inhibition you should choose the dedicated Hydrates model set. If you only choose a single type of solid phase in the Combined Solids option you will be asked to use the model for that type of solid instead, e.g. the hydrate model if only hydrate phases are chosen.

Asphaltene precipitation To understand what happens when more than one solid form a useful starting point is to examine asphaltene precipitation alone. The example input file provided is combsolid.mfl. This includes an oil composition to C20+ which has a molecular weight of 81, wt% resin of 12.04 and wt% asphaltene of 0.7. The fluid is characterised from C6 with 15 fractions. The resins and asphaltenes are allocated as shown below:

The asphaltene model parameters are matched with a bubble point of 120F and 2650 psia and an asphaltene precipitation point of 120F and 8750 psia. The predicted APE is plotted below (use the same strategy for plotting as described in the asphaltene case study).

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Wax and Asphaltene precipitation To see the effect of simultaneous wax and asphaltene precipitation we first need to re-characterise the fluid with an n-paraffin distribution in order to apply the Coutinho model. In the PVT form tick the box to estimate wax content as none is known. The n-paraffin distribution is also set to C6 and 15 fractions. The separation of the n-paraffins from the remainder of the liquid also alters the distribution and properties of the resins and asphaltenes:

This in itself will alter the resin/asphaltene interaction. Allowing the wax to form will then remove some of the n-paraffins from the fluid again changing the proportion of resins in the remaining fluid. To see the effect choose the Combined Solids option from Select/Model set and specify wax and asphaltenes as the solid phases. Eliminate Hydrates, water and ice for the time being.

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As the model has been re-defined we will need to match the asphaltene parameters again, using the same input data. The new parameters will be slightly different because of the altered distribution. Now plot the gas and asphaltene boundaries as before then add the wax boundary.

As you can see changing the resin distribution and removing some of the nparaffins has the effect of stabilising the asphaltene slightly.

Hydrates, Waxes and Asphaltenes To study the effect of allowing hydrates to form we can retain the fluid characterisation used for wax and asphaltene but need to add water. Do this using Select/Components. Initially set the water composition to zero. Return to the Combined Solids selection and add the hydrates, water and ice to the list of potential phases.

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As we have chosen to re-define the model we also need to re-match the asphaltene parameters. This is best done in the absence of water, which is why the initial water composition was set to zero. Using the same input data the asphaltene parameters will be exactly the same as for the wax and asphaltene study. Now add the water composition, 10g. Adding too much water may cause difficulties when plotting the APE. With water present use the phase envelope plotter to generate all the phase boundaries. Starting point for the asphaltene boundary may have to be changed as it is affected by the presence of the other solids.

The wax boundary is not affected by the addition of water or the formation of hydrate, which occurs at lower temperatures. However, the effect on the lower APE is significant. As the hydrate is formed the light gas hydrate formers are removed from the fluid. This is in effect the reverse of gas injection and the asphaltene is stabilised with precipitation occurring at higher temperatures.

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Of course with water present there is also the possibility of a separate water phase.

If the fluid is flashed at 70F and 1750 psia Multiflash will predict the formation of 6 phases; gas, hydrocarbon liquid, water, hydrate2, asphaltene and wax. With only 10g of water present reducing the temperature slightly removes the water phase owing to the formation of additional hydrate. Of course, in practice the formation of so many phases will be affected by kinetic factors as well as the thermodynamic equilibrium.

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Case studies – Excel spreadsheets

Introduction There are some problems that are best approached using Excel. Some of these are discussed in the Multiflash Excel manual, such as generating tables of pure component data or using linked flashes or recycles for simple flowsheet calculations. Those presented here were set up following user requests and include generating binary interaction parameters for activity coefficient models and linking predictions of solid formation to a common fluid analysis. By default, the spreadsheets are stored in the same directory with the mfl files, i.e. under the tree \Infochem\MF44\MFL Files from the Multiflash 4.4 installation on your computer. Although we have not included them here, some users wish to fit their own experimental data for components not included in their version of Multiflash. To help users to do this we do have available a series of spreadsheets for the fitting of pure component data. They are not issued with the standard installation but we will supply them on request.

UNFACFIT.xls UNIFAC is a very useful model as the binary interaction parameters are generated from the group structures of the pure components and so reasonable predictions of phase equilibria can be obtained for polar systems without the need for stored BIPs. However, there may be times when you wish to use an alternative model such as NRTL. Although we are continually expanding our BIP databank there may be some binary pairs in your mixture for which we do not have stored NRTL parameters. If you do not have the time to search for experimental data for the missing pairs, or are unable to find any, then this spreadsheet allows you to generate the phase equilibria data from UNIFAC, providing group structures are available for your chosen components, and then fit this data using another activity model. The spreadsheet has several worksheets.

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Notes The first spreadsheet consists of notes on how to use UNFICAFIT.xls and how to enter the fitted BIPs in Multiflash.

UNIFAC This is the worksheet where you generate the phase equilibria data (liquid and gas phase compositions and temperature or pressure) that you are going to fit. You specify, by entering information in the appropriate cell: 

The databank to act as the source of pure component data, either Infodata or DIPPR if you have it



The names of the two components for your binary pair



Whether you wish to generate data for an isotherm or isobar and your chosen temperature or pressure

The required data is generated by the Multiflash functions when the calculation in the spreadsheet is updated. It includes both column headings and plots. The composition range is fixed and the units are SI. There is no need to change these although it is possible. An Error box reports the status of the data generation. This should be OK if the UNIFAC group structures are available for your chosen components. An error status of –13201 would indicate that the structures are missing for one or both compounds. Once the data has been generated you can move to the worksheet for the model you wish to use, WilsonE, UNIQUAC VLE or NRTL VLE.

Activity model worksheets All the model worksheets function in a similar manner. The component names, conditions, temperature or pressure are copied from the UNIFAC worksheet as are the phase equilibria data. The user enters initial guesses for the BIPs, a useful default is to start with 0.0 for both, and using the Excel Tools/Solver to start the fitting procedure. Once the best solution has been reached the new BIPs will be reflected in the cells used for starting guesses and in the cells reporting the fitted BIPs. One of the useful benefits of using Excel is that the results are plotted for comparison with the UNIFAC generated data allowing the user to decide easily whether the solution is acceptable. If the solution is not acceptable then you can try 

starting with a different initial estimate for the BIPs



using a different criterion for minimisation. The default setting is to minimise the sum of squares of the differences between given and predicted temperature or pressure. It is possible to minimise differences in gas composition by changing the target cell in the Solver.



Change the constraints on the values for BIPs when fitting. We have set limits on the values the BIPs can take as part of the Tools/Solver utility. To change this constraint use the Change button on the Solver text box.

Once acceptable BIPs have been generated they can be entered and stored in Multiflash as described in the spreadsheet notes or “Units for BIPs The BIPs for most equation of state methods, Wilson A, Regular Solution and Flory Huggins are dimensionless. For other activity methods and the two CPA association parameters the BIPs have associated units. The default units in

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Multiflash are J/mole. If BIPs from external sources are used in Multiflash it is important that either the BIP units are changed to match the input values or the numeric values of the BIPs are changed to J/mole. The choice of units appears once the Units button is activated in the BIP display.

J/mol is the Multiflash standard for the dimensioned activity model BIPs. cal/mol and K are the values used in the Dechema Data Series for activity model VLE and LLE BIPs. The “Aspen” format allows you to transfer the BIP values for the NRTL equation from Aspen Plus without further change. The actual input functions for the activity BIPs are as follows: In J/mol K

Aij=a + bT + cT2

In cal/mol K

Aij/4.184=a + bT + cT2

In K

Aij/R=a + bT + cT2

Dimensionless

Aij/RT=a + b/T + cT

Aspen format

Aij/RT=a + b/T + cT

For the NRTL equation, the  parameter is defined as follows: All formats except Aspen ij= a + bT + cT2 Aspen format

ij= a + b(T-273.15) + c(T-273.15)2

Supplementing or overwriting BIPs” on page 59. Although NRTL has three parameters we have chosen not to fit all three but to default the third parameter, alpha, to 0.3. You can over-ride this if you wish but we would suggest that values for alpha should never be negative and should rarely be larger than 0.6. Currently the fitting is limited to constant values for the BIPs although this could be expanded if necessary.

VLEFIT.xls This is substantially the same as UNIFACFIT.xls but the starting point is experimental data rather than data generated from the UNIFAC model. Instead of the first UNIFAC worksheet there is an Experimental worksheet to enter the data. Again you can choose the data source for your pure component data and indicate whether your chosen data is along an isotherm or isobar. In order to minimise effort this spreadsheet does allow you to choose the units for temperature and pressure to match those measured. The temperature or pressure for the isotherm or isobar should be entered as should the values for x, y and associated T or P. We have chosen a limited array for data entry. If you have more data and are familiar with Excel you can extend the range although you will need to remember to change the cell references in the dependent worksheets. Otherwise you should limit the data by choosing suitable points from the data available.

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If you have less data then you should enter #N/A in the cells which would otherwise be empty. This is necessary for the Excel Solver to operate correctly. For some data sets you may have P, x or T, x but no data for gas composition, y. In this case it is advised to enter #N/A for the y compositions. If you fail to do this the Solver will still function provided the minimisation criterion is based on difference in temperature or pressure, which is the default. However, the plots for x, y will not be relevant and should be ignored. As with UNIFACFIT.xls you can fit the experimental data to generate BIPs for WilsonE, UNIQUAC VLE or NRTL VLE by choosing the appropriate worksheet.

Solids.xls Several of the engineers using the Multiflash GUI have found it fairly complex to obtain results for solid formation particularly for asphaltenes. Improvements to allow users to fit bubble point and asphaltene model parameters at the same time go some way to alleviating this problem. However, we had set up a spreadsheet, which many engineers find useful and which we will still issue as part of the implementation. Of course you will only be able to use any worksheet if you have licensed the appropriate model. The Solids.xls includes the recommended PVT analysis routines and solids models; the CPA/Electrolyte model for Hydrates, the Coutinho wax model for waxes and the asphaltene model is standard. The spreadsheet consists of several worksheets and each worksheet has the relevant models built-in in hidden rows or columns. As the PVT Analysis procedure is complex, the PVT calculations are carried out using the Multiflash GUI, and the mfl file written and referenced in the first worksheet. The Excel calculations are set to manual using the Tools/Options or Formulas/Calculation Options facility depending on the Excel's version. This is to prevent automatic calculation of the whole spreadsheet as new compositions are entered. To update any individual worksheet use Shift F9, to update the whole spreadsheet use F9.

PVT Analysis The user characterises the PVT using the Multiflash GUI and enters the full directory path and mfl file name to provide the characterised fluid information. As all calculations in the spreadsheet have to be referenced to this fluid characterisation we recommend that the characterisation is based on splitting the fluid plus fraction from C6 into 15 pseudocomponents for both iso-paraffins and n-paraffins. This controls the number of components and negates the need for the user to modify the Excel functions to ensure that all components and compositions are included in any calculation. The units for calculation are also set in this worksheet. This first worksheet includes a bubble point calculation at a temperature or pressure set by the user. This allows you to decide whether to tune the petroleum fraction properties to match a known bubble point or if this is unnecessary. The list of possible components is based on the default component list used in our PVT Analysis utility. The user enters the compositions and any other information available such as the molecular weight and specific gravity. The SARA analysis can be entered; the resin and asphaltene amounts are needed for the asphaltene model although they can be estimated if required. The user can specify the starting point and the number of fractions for the characterisation although we would recommend staying with C6 and 15 fractions.

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Match bubble point If the predicted bubble point is sufficiently different from a known value then this worksheet allows the user to match a known bubble point. The fluid composition is taken from the characterised fluid in the PVT Analysis worksheet. The user specifies the bubble point to be matched and the bubble point is recalculated after the match to confirm that this has been carried out. A new problem input specification is written into this worksheet as a result of the match but it is in hidden rows or column.

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Wax The next worksheet is dedicated to prediction of wax precipitation, although the worksheets for wax, asphaltene and hydrates are not inter-dependent and can be used in any order. The user can choose to use the fluid characterisation direct from the PVT Analysis or with petroleum fraction properties tuned to match a bubble point. The first set of calculations predicts the wax appearance temperature (WAT) without any further tuning plus the amount of wax formed at a user specified T,P. The calculation is carried out based on the fixed wax phase fraction flash function and the amount of wax is set to be .00015 mole fraction of wax in the liquid+wax phases. There is a function called MF_PWAT, equivalent to the WAT button in the GUI, which calculates the WAT at a fixed mass or mole fraction of the wax phase. This can be relative to either the total fluid or to the liquid plus wax phases. For matching a measured WAT for a finite amount of wax phase, a command is used for specifying the measured WAT in the Solids.xls. For the detailed information about how to use the commands, refer to the section of Matching wax data in the Multiflash Command Reference manual. The WAT is plotted automatically as a function of pressure. The starting pressure and step can be set by the user to obtain the pressure range of choice. These calculations can be repeated based on matching to a known WAT, if one is available.

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Asphaltenes The asphaltene worksheet again offers the choice of using the PVT Analysis characterisation direct or the characterisation after matching to a known bubble point. For asphaltene modelling we would recommend the latter. The options for producing the asphaltene model parameters are not as flexible in Excel as in the Windows front end. You are limited to two options, the flocculation or reservoir conditions and you must supply both temperature and pressure. The matching is done by using a command with the temperature and pressure referred to either asphaltene ADE or reservoir condition. For detailed command information, refer to the section on Matching asphaltene deposition point in the Multiflash Command Reference manual. After matching the user can set either temperature or pressure and calculate the corresponding P or T for the upper and lower boundaries of the flocculation envelope. There is also an option for entering a set of T,P and calculating the amount of asphaltene formed at those conditions. Every effort has been made to plot the asphaltene precipitation envelope automatically with starting points derived from the precipitation or reservoir conditions. However, as the engineers using our Windows phase envelope facility will appreciate it is difficult to make this absolutely fail-safe.

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Asphaltene with gas injection The effect of gas injection on asphaltene flocculation is perhaps most easily calculated in Excel. The asphaltene model parameters for any fluid should be retained when studying the effect of added gas. The gas injection worksheet is therefore dependent on the asphaltene worksheet for the problem input specification and the matching function is disabled. It serves only to report the conditions chosen for matching. The composition of the gas is entered and the ratio of gas to oil is based on a simple molar ratio. You can still change conditions of temperature and/or pressure to determine the boundaries for flocculation and the amount of asphaltene flocculated. The boundaries for asphaltene flocculation, with and without gas injection are plotted but it cannot be guaranteed that this will be available for all fluids and gas injection rates.

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Hydrates The hydrate functionality is well served in our Windows software and easily used. We have only added a hydrate worksheet here for completeness and to allow the engineer to carry out quick checks for possible hydrate formation. It encapsulates the main features of our hydrate model but does not have the flexibility of the Windows program. The user needs to add an amount of water to the fluid and this can either be pure water or produced water with the salt content defined by ion analysis or total dissolved solid. The hydrate dissociation temperature can be calculated at a single pressure or plotted as a function of pressure. The hydrate is defined as Hydrate2 only, which is the usual hydrate formed as the spreadsheet is designed to work with oils rather than natural gases. There is also a section in the worksheet for looking at the use of inhibitors. These are limited to the two most common, methanol or MEG. The spreadsheet predicts inhibitor amounts or concentrations required for hydrate inhibition at set conditions.

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Users who want to investigate hydrate behaviour only, may find the hydrateinfo.xls and hydratecpa.xls spreadsheets useful. They have a restricted component list (gases and gas condensates) but offer a choice of fluid and salt model and a wider choice of calculations.

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Case study – Mercury partitioning Introduction This section is only applicable if your Multiflash license includes the mercury model. Infochem has developed a mercury model for prediction of the solubility of mercury in natural gases and condensates, and the distribution of mercury between gas, condensate and water phases. The mercury model is available in combination with RKSA, PR78A or CPA-Infochem fluid models. In condensates and petroleum liquids, organomercury compounds are significant and may be the predominant form of mercury, although it is generally agreed that mercury in natural gas is almost all in elemental form. Dimethylmercury and diphenylmercury are chosen to represent light and heavy organomercury compounds respectively. The mercury model in Multiflash is based on data for the solubility both in hydrocarbons and in water, including data which is not accessible in public domain. The mercury model was originally developed based on the RKSA equation of state. This is our recommendation for the fluid phase model. However PR78A and CPA may be chosen for compatibility with fluid characterisation base on these models.

Defining the mercury model The mercury model may be defined by selecting the mercury tab in the Select/Model Set box

The phases which can be defined for this model in the model selection form are gas, liquid1 (hydrocarbon liquid), liquid2 (hydrocarbon liquid), water, liquidHg (liquid mercury), solidHg (solid mercury), and ice. We recommend that you do

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not define a second hydrocarbon liquid phase (liquid2) unless you anticipate the possibility of multiple hydrocarbon liquid phases.

Calculating mercury partitioning and dropout Once the mercury model has been defined the fluid components are defined using Select/Components as usual and entering the fluid composition in the Composition drop down menu. The example file for this case study is supplied as Hg_Example.mfl, and uses RKSA as fluid model. In this particular example we have only specified mercury but the principle is the same if the components dimethyl- and diphenylmercury are present.

To demonstrate the partitioning we envision a simplified flowsheet where the feed gas goes through a warm separator, the gas phase exiting this separator then enters a cold separator and the exiting gas is compressed for export.

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If the mercury model is used in an Excel spreadsheet or a third party simulator, the streams can be set to be merged and recycled automatically but in the Multiflash GUI it is only possible to use one stream composition as input at a time for flash calculations. Once Hg_Example.mfl has been loaded, enter the 4 degC and 77 bar of the warm separator condition and carry out a PT flash.

As shown above, the mercury partitions among the fluid phases (Gas, Liquid1 and Water) . To simulate the phase separation at the cold separator condition, the gas phase composition can be highlighted, copied and then pasted into the Composition drop down menu to provide the feed for the PT flash for the cold separator. The conditions for the cold separator are set to -15 degC and 41 bar, and a new PT flash is performed. These conditions correspond to having a choke between the two separators, which reduces the pressure from 77 bar to 41 bar, resulting in the temperature dropping to -15 degC.

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At these conditions the separator is cold enough for a separate liquid mercury phase to form. If the stream is cooled down even further, for example to -52 degC, then solid mercury will form instead and slightly more mercury drops out.

As can be seen from above, the mercury model can calculate mercury dropping out either as a liquid or as a solid, and a flash calculation shows whether mercury drops out at the given conditions. In Multiflash the fixed phase fraction flash can be used to calculate the temperature or pressure at which a pure mercury phase will start to drop out. If the gas from the cold separator is compressed to 100 bar, such as for gas export, then a fixed phase fraction flash at that pressure can be used to determine the temperature at which mercury will start to drop out.

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If we do the calculation for the gas output from the cold separator at -15 degC at 100 bar we can see that liquid mercury will begin to dropout at -8.7 degC.

If we use the cold separator at -52 degC and 41 bar instead, then mercury will not dropout till the temperature is below -48 degC at 100 bar, at which point solid mercury begins to form. This is because of the lower amount of mercury in the gas stream from the cold separator at -52 degC compared to the gas stream from the cold separator at -15 degC.

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Other calculations The mercury model can be used for any flash calculation such as P,S or P,H. The model can also be used for mercury partitioning in a gas dehydrator. TEG has been included in the component list for the example provided, but was present at zero concentration. If the TEG is added, e.g. 0.3 moles, then the calculations can be repeated but in this case starting with a simple dehydrator flash at 10 degC and 110 bar. You can then see the mercury partitioning into the aqueous TEG phase and the consequent effects.

Distribution of mercury species As mentioned in the introduction, the mercury model in Multiflash can model elemental mercury, dimethylmercury and diphenylmercury, but the model is not able to predict how much of a measured total mercury content is elemental mercury, and how much is organomercury. For a given total mercury content in a gas stream it is reasonable to assume that all the mercury is elemental mercury, as elemental mercury is much more volatile than the two organomercury components. However, if the measured total mercury content is for a separator liquid stream, it is difficult to know exactly how much of each of the three mercury components that corresponds to. Assuming all mercury is elemental might lead to an over-prediction of the amount of mercury in the evolved gas phase when the separator liquid is further processed. One possible way of solving the problem is to assume some distribution between the three mercury species in the separator liquid. (An equimolar distribution of dimethylmercury and diphenylmercury can be assumed if no information is

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available). If a measurement of the mercury content for the evolved gas phase is available, the amount of elemental mercury in the assumed distribution for the separator liquid can be adjusted to match the measurement for the gas phase. This approach is not very accurate and is not based on observations, but it will give the best possible estimate of the distribution of mercury between the species based on the available information, and will agree with the mercury content in both the separator liquid and the evolved gas.

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Appendix - Multiflash Commands

Introduction The Multiflash command language is common to all Multiflash implementations - Excel, VB etc. A complete list of all commands and information on how to use them is contained in the Multiflash Command Reference manual. In the Multiflash GUI the use of commands has largely been replaced by the menu choices and toolbar buttons. However, there are some Multiflash facilities that are not available as menu/toolbar options so there are a few circumstances when it is necessary to .make direct use of Multiflash commands.

Commands From the Tools menu choose Command. A command window will be displayed. You can then enter a command and either press or click on the Send command button to send the command to Multiflash.

The command shown in this example will provide details of the specified model, including the name of the associated BIP dataset.

When you may need to use commands The menu options available in Multiflash will allow you to specify and solve most problems. However, some of the areas where commands let you supply or display additional information, supplement or amend menu options or carry out additional calculations are:

User Guide for Multiflash for Windows



Defining models



Supplying an external file of BIPs

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

Defining phase descriptors and key components

Defining models The standard thermodynamic and transport property models available in Multiflash are specified in the model sets and model configuration files supplied. However, you may wish to set up your own model definition, for example you may wish to use an activity style mixing rule with an equation of state model. You may also wish to group together different models to describe different properties, such as using an equation of state to describe the properties of a refrigerant mixture but defining the density in terms of ideal mixing. In general the MODELS command defines a thermodynamic or transport property model for mixtures. The command has the format: MODEL

model_id

MF_model_name

[Model_options]…

where 1.

model_id is a user-defined name that will be used to refer to the particular combination of the property model and options specified. 2.

MF_model_name is the Multiflash name for the basic model. The list of recognised models is given in “Models and input requirements” on page 74. Model_options are additional keywords that describe model variants, references to other, previously-defined, models or references to the source of binary interaction parameters.

Many .mfl files are provided and will provide examples of how to set up a model definition.

What the model definition means It is not necessary to understand the Multiflash model definition to carry out useful calculations. The explanation in this section is provided so that you can refer to the details when you need to. For example you may wish to change the key component for a phase. In the example below the model is Peng-Robinson. The definition of a model configuration file will appear in the results window and includes: The source of pure component data, binary interaction parameters and petroleum fraction correlations. The last line below defines the enthalpy and entropy datum point. If it is not included then the default (as shown below) will be taken. Puredata infodata; bipdata oilandgas infobips; chardata infochar tbsoereide; datum enthalpy compound entropy compound; A command to remove all previously loaded models and binary interaction parameters: remove models; remove bipsets; A command to allow for the possibility of user supplied BIPs which are identified by the name PRBIP bipset PRBIP 1 constant; The definition of the thermodynamic model; consists of the command “model” followed by a model name, MPR, and a keyword to identify the model, PR and finally the name for a user supplied BIP set, PRBIP.

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model MPR PR PRBIP; The definition of the default models used for the transport properties: model MPDVISC pdvisc lfit; model MCLSMPR cls MPR; model MMCSMPR mcs MPR; A phase descriptor giving an identifier and a phase type for each possible phase which may be considered in the calculation: remove pd; pd Gas gas MPR * * MPDVISC MCLSMPR; pd Liquid1 liquid MPR * * MPDVISC MCLSMPR MMCSMPR; pd Liquid2 liquid MPR * * MPDVISC MCLSMPR MMCSMPR; pd Water liquid MPR * * MPDVISC MCLSMPR MMCSMPR; and finally, specification of a key component for each liquid phase so that it can be identified when multiple liquid phases are found. key Water 007732-18-5; key Liquid1 not 007732-18-5; key Liquid2 not 007732-18-5; The CAS number is that for water. This format is general and unambiguous and allows, for instance, for cases where the user has chosen the alternative name H2O for water. However, Multiflash will still recognise the original format of Key Water water;

Supplying an external file of BIPs We supply binary interaction parameters, for light gases and hydrocarbons for use with the cubic equations of state from the OILANDGAS databank and for activity coefficient models from INFOBIPS or INFOLLBIPS. If you have interaction parameters available for other binary pairs and other models you may wish to supplement or overwrite those stored in Multiflash. The Tools/BIPs facility has been supplied to allow you to do this easily without the need for commands and to save any changes by saving the problem setup file. However, if you wish to store a preferred set of BIPs and to overwrite the stored BIPs every time you run Multiflash it may be easier to set up your own input file. You can then overwrite our BIPs by loading this file or including it as part of your problem setup. In this case it may be easier to construct the file using Multiflash commands. The command for setting up a bipset takes the form bipset, bipset_name, number of BIPs for model, degree of temperature dependence, temperature function, units, binary pair, values for BIPs; The default names for the model bipsets are the model name followed by BIP e.g. PRBIP For example, A typical file of BIP data might be: bipset PRBIP 1 constant eos none bipset PRBIP 1 constant eos none bipset PRBIP 1 constant eos none

butane pentane 0.01 ; pentane hexane 0.01 ; hexane heptane 0.01 ;

N.B. You must reload this BIP file every time you change the model as one of the commands when a model is loaded removes existing bipsets. If you define the bipset incorrectly, e.g. if for butane pentane the temperature function is defined as activity with units of J/mol, then a warning message will appear when you try to use this with the PR equation

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bipset PRBIP 1 constant activity J/mol butane pentane 0.01 ; *** ERROR

584 ***

Existing BIP set uses a different temperature function Some examples for other models are shown below: bipset WILSONBIP2 2 constant activity j/mol acetone water 6139 1450; bipset NRTLBIP3 3 linear activity J/mol methanol water .22 4.4e-3 .30 -6.6e-3 .003 0.0

Defining phase descriptors and key components Now that you can define the phases to be considered as part of the model definition the need to define or erase phase descriptors using Tools/Command should be reduced. However, it may still occur. A phase descriptor (PD) contains all the information required to identify a phase and to retrieve its thermodynamic properties. A PD must be specified for each possible phase that Multiflash is to consider. The PD command is used to define a phase descriptor (PD). To exclude the formation of a particular phase type, e.g. gas, the corresponding PD should be omitted or erased. The command has the format: PD pd_id phase_type

model_identifiers

or PD pd_id erase The following table gives the valid options and settings:

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command parameter

description

notes

pd_id

user-defined name that will be used to refer to the particular instance of phase type and associated models

any unique alphanumeric string, e.g. liquid1

phase_type

a keyword that defines the phase type, valid settings are: condensed gas hydrate liquid solidsolution vapour vapour

1. gas, vapour and vapour are synonyms. 2. condensed means a pure solid phase. 3. solidsolution means a mixed solid phase

model_identifiers

identifiers for up to six models that will be used to evaluate the thermodynamic and transport properties of the phase. Models for the following properties may be specified in the order given:

1. the model identifiers are the user-defined names associated with the models (see description of MODELS command) 2. at least one thermodynamic model must be defined (the same model is then used for all thermodynamic properties) 3. transport property models need only be defined if output of these properties is required

1. fugacity (K-values) 2. volume/density (optional) 3. enthalpy/entropy (optional) 4. viscosity (optional) 5. thermal conductivity (optional) 6. surface tension (optional) erase

erases (removes) the PD 1. the pd_id must have been from the list of PDs available previously defined for Multiflash 2. all information associated with the PD is lost

The standard model sets and model configuration files include four phase descriptors for GAS, LIQUID1, LIQUID2 and WATER. If you use the Select/Freeze-out components option to apply the freeze-out model, see “Solid freeze-out model” on page 39, a phase descriptor will be automatically generated from the component name. However, you can change this name using the phase descriptor command. You may wish to erase a phase descriptor to limit the number of phases considered when solving a flash calculation, see “Phase ” on page 178. The KEY command is used to define a key component for a PD. A key component helps to identify a particular phase when two or more PDs would otherwise be indistinguishable. It is not necessary to define a key component unless a flash calculation needs to identify phases uniquely (e.g. a search for a particular phase fraction). The command has the format: KEY

pd_id key_component_id

or: KEY

pd_id not key_component_id

pd_id is a previously-defined phase descriptor name. key_component_id is the name of the component which is used to identify the phase. The rule used is that the key component should be present in the phase to the maximum amount relative to the total mixture composition. If the

User Guide for Multiflash for Windows

Appendix - Multiflash Commands  315

component name is preceded by the keyword not , this means that the component should be present in the minimum relative concentration. The model sets and model configuration files supplied identify one liquid phase as having water as the key component and the other two liquid phases in terms of not being the water phase. There may be a case where you would wish to allocate a key component to one of these, e.g. Key liquid2 CO2; or Key liquid1 heaviest

316  Appendix - Multiflash Commands

User Guide for Multiflash for Windows

Index

A About Multiflash 194 Activity coefficient equations in Multiflash 37 Activity coefficient methods 37 Activity coefficient model for gas phase 9 Activity coefficient models 6 see Models 37 Activity coefficients 177 Activity model worksheets 294 Activity Models 179, 180 Adding a component 69 Adding a user-defined component 72 Adding water to the system 209 Adding, inserting, replacing and deleting components 69 Additional calculations 20 Advanced Equation of state options 31 Amount of inhibitor required to suppress hydrates 255 Appendix - Multiflash Commands 3 Applicability 36 Aqueous phase labelling 178 Asphaltene matching 270 Asphaltene precipitation 287 Asphaltene with gas injection 300 Asphaltenes 51, 267, 299 Case study 51, 267 Defining Asphaltene model 51 Model 43 ASTM D86 distillation 104 Azeotropes 235

B Backup file 26 Basic characterisation properties 109 Benedict-Webb-Rubin-Starling (BWRS) equation of state 34 Benedict-Webb-Rubin-Starling model 34 Binary interaction parameters 10, 53, 54, 55, 57, 58, 294 Displaying values 55 Supplementing and overwriting BIPs 58 Temperature dependence 54

User Guide for Multiflash for Windows

Units 57, 294 BIP databank 60 BIP set Names 55 BIPs see Binary interaction parameters 53 BIPs and models 53 BIPs available in Multiflash 54 BIPs for CSMA and GERG mixing rule 59 Black Oil Analysis 101, 223 Blending method 128 Blending procedure 129 Bubble and dew point flashes 143 Bubble point 143

C Calculate 17 Calculating asphaltene precipitation conditions 272 Calculating mercury partitioning and dropout 304 Calculating the bubble point curve 206 Calculating the dew point curve 207 Calculating the water dew point line 209 Calculating wax appearance temperature (WAT) 261 Calculating wax precipitation 264 Calculation options 11 Calculation output 175 Calculations 2, 19 see Flash calculations 141 Calculations with inhibitors 252 Can hydrates form at given P and T ? 252 CAPE-OPEN 7 CAPE-OPEN Interface 188 Carbon Dioxide high-accuracy model 36 Carry out a the flash calculation 199 Carry out an isenthalpic flash 21 Carrying out the flash calculation 23 Case studies 3, 197, 205, 243 Asphaltenes 51, 267 Combined solids 287 Delump 225 Getting Started 13 Hydrate formation and inhibition 243 Phase equilibria 205 Polymers 237 Pure component data 197 PVT Analysis 214 Refrigerants 229 Wax 50, 260 Change the composition 20 Change the pressure 20 Changing units 170 Characterisation 99, 107 Check for Updates 193 Choice of Analysis method 276 Chung-Lee-Starling model 45 Chung-Lee-Starling thermal conductivity method 45 Clearing previous problems 21 Combined solids 287 Combined Solids Model 52

Index  317

Commands 3, 311 Appendix 311 Tools/Command option 311 Component cannot be found 86 Component list 91 Components 2, 63, 65, 66, 67, 68, 69, 70, 84, 197 Adding a component to a stream 69 Databanks 63 Deleting components from a stream 70 DIPPR 63 Displaying pure component data 197 Inserting a component in a stream 69 Maximum number in stream 65 Normal components 63 Properties of normal components 63 Replacing a component in a stream 70 Selecting by formula 68 Selecting by name 66 Selecting by scrolling through list 66 Selecting by substring 68 Selecting components 66 Synonyms of components 67 Troubleshooting 84 User defined 72 Compositions 14, 23, 137, 171 Amounts and fractions 138, 170, 171, 176 Specifying compositions 137 Conditions 14, 137, 139 Conditions section of main window 137 Specifying compositions 137 Specifying enthalpy 139 Specifying entropy 139 Specifying internal energy 139 Specifying pressure 138 Specifying temperature 138 Specifying volume 138 Troubleshooting 139 Consider all types of solution 167 Constants 73 Co-Polymers 239 Corresponding states (CSMA) model 35 CPA model CPA 10 CSMA Reference fluids 35 CSMA model 9 Cubic plus association (CPA) model 32 Current settings Display 181 Customising the phase envelope plot 155

D Data Availability 277 Data input 237 Databank not found 84 Databank not licensed 85 Databanks 7, 10 DIPPR 63 INFODATA 63 Default units 169

318  Index

Defining a mixture 65 Defining a problem in Multiflash 18 Defining models 312 Defining petroleum fractions 109 Defining phase descriptors and key components 314 Defining the asphaltene model 268 Defining the components 21 Defining the hydrate models 243 Defining the mercury model 303 Defining the models 22 Defining the problem in Multiflash 197 Defining the wax model 260 Deleting components 70 Deleting petroleum fractions 112 Delump Case study 225 Delumping tool 113 Delumping tool - Case study 225 Dew point 143 Differences between the PR Model in Multiflash and Aspen Hysys 60 Diffusion coefficient 46 Diffusion coefficients 46 DIPPR 7, 63 Displaying status for current settings 181 Distillation curves 102 Distribution of mercury species 308 Document Organisation 2 Dortmund Modified UNIFAC method 38 Dortmund Modified UNIFAC model 38

E Edit 15 Editing petroleum fraction data 112 Electrolyte model 81 Enter a composition for the stream 199 Entering BIPs 232 Entering petroleum fractions 110 Enthalpy Specifying for isenthalpic flash 139 Enthalpy definition 178 Entropy Specifying for isentropic flash 139 Entropy definition 180 Equation of state models 30 Equations of State Models 30 Equations of state provided in Multiflash 30 Errors 176, 181 Convergence 176 Messages 181 Troubleshooting 52, 60, 84, 139, 165, 171, 181 Errors and warning messages 181 Estimated n-paraffin distribution 108 Eutectics 236 Example for blending 130 Example with asphaltenic crudes 133 Example with waxy crudes 132 Examples Case studies 50, 51, 197, 205, 243, 260, 267, 303

User Guide for Multiflash for Windows

Getting Started 13 Excel interface 7, 202 Exiting Multiflash 27

F File 14 Files Loading problem setup files 18 OLGA PVT file 184 Output file 173, 174 PIPESIM PVT file 183 Problem setup (mfl) files 18, 25, 26 Prosper PVT file 187 Saving a problem setup file 25 Saving results in an output file 27 Fitting the vapour pressure curve 31 Fixed phase fraction flash 144 Fixed phase fraction flashes 144 Flash calculations 10, 139, 141 Bubble points 143 Definition of a flash calculation 141 Dew point 143 Fixed phase fraction flash 144 Isenthalpic flash 143 Isentropic flash 143 Isochoric flash 143 Isothermal (P,T) flash 142 List of available flash calculations 142 Retrograde flash calculations 147 Troubleshooting 165 Type of solution 147 Flashes available in Multiflash 142 Flory Huggins model 38 Flory-Huggins 38 Fluid amounts 129 Fluid file name 128 Fluid Identification 14 Fluid phase model 244 Font 174 Fonts 182 Frame Tab 152 Fugacity coefficients 177 Fuller method 46 Fuller model 46

G Gas and liquid Enthalpy based on saturated liquid heat capacity 180 Gas injection 282 Gas phase models for activity coefficient methods 38 GERG model 36 GERG-2008 36 GERG-2008 (Infochem extension) 36 Getting Started 2, 13, 205 Groups not available for UNIFAC model 52

User Guide for Multiflash for Windows

H Hayduk Minhas method 46 Hayduk-Minhas method 46 Help 191 HELP 3, 17 Help Topics 191 High accuracy reference eos for water-ammonia binary system 6 How to change a model 48 How to Enter Multiflash Commands 311 How to exit the program 27 How to specify models in Multiflash 47 How to use the delumping utility 114 Huron-Vidal-Pedersen mixing rule 5 Hydrate Inhibitor Calculator 79 Nucleation model 41, 49, 245 Hydrate calculations 159 Hydrate calculations with Multiflash 246 Hydrate dissociation pressure at a given temperature 254 Hydrate dissociation temperature at a given pressure 254 Hydrate formation and dissociation pressure at given temperature 250 Hydrate formation and dissociation temperature at given pressure 247 Hydrate formation temperature at given pressure 249 Hydrate inhibitors 79 Hydrate model 39, 244 Hydrate models 11 Hydrate phase boundary 251, 254 Hydrates 39, 48, 243, 255, 301 Case study 243 Hydrate inhibition case study 255 Hydrate inhibitor model 42 Hydrate model 39 Phase boundary 251 Hydrates in water sub-saturated systems 41 Hydrates, Waxes and Asphaltenes 289

I IAPWS-95 35 Ice model 245 Ideal gas equation of state 30 Ideal solution model 37 Including a petroleum fraction 210 INFODATA 7, 10 Infochem fluids databank 63 Inhibition Hydrate inhibition case study 255 Model for hydrate inhibition 42 Inhibitor calculator alcohols/glycols 79 Inhibitor Calculator 6, 11, 79 Inhibitor modelling 42 Initial Values Tab 151 Input conditions see Conditions 137

Index  319

Input Conditions 2 Input data 102, 267 Input files Loading a problem setup file 18 Problem setup files 18, 25, 26 Saving a problem set up file 25 Input section 14 Inserting a component 69 Installation 3 Interface to other programs 141, 202, 235 Excel spreadsheet 141, 202, 235 Interfaces 7 Internal energy specifying as a flash condition 139 Introduction 1, 5, 9, 29, 63, 89, 137, 141, 169, 173, 183, 191, 197, 205, 243, 260, 267, 287, 293, 303, 311 ISAPWS-95 35 Isenthalpic flashes 143 Isentropic flashes 143 Isochoric flashes 143 Isothermal (P,T) flash 142

J Joule-Thompson coefficient 12

K Key components 145, 314

L LBC 5 LBC viscosity model 10 Lee-Kesler (LK) and Lee-Kesler-Plöcker (LKP) equations of state 34 Lee-Kesler-Plöcker (LKP) model 34 Limit the number of phases 167 Linear Gradient Theory 46 Linear Gradient Theory model 46 Liquid dropout and Wax precipitation curve 11 Liquid dropout curve calculation 158 Liquid enthalpy based on saturated liquid heat capacity 179 Liquid thermal conductivity mixing rule 45 Liquid viscosity mixing rule 45 Liquid-liquid equilibria 234 Loading a existing MFL file 26 Loading a model from a .mfl file 48 Loading a problem setup file 18 Loading an existing problem file 18 Loading files Loading problem setup files 18 Log file 27, 173, 174 Saving the log file 27 Lohrentz-Bray-Clark model 44 Lohrenz-Bray-Clark method 44

320  Index

M Macleod-Sugden 2-phase variant (MCSA). 46 Macleod-Sugden method (MCS). 46 Macleod-Sugden model 46 Manipulating the Output 177 Mass fraction flash 144 Match bubble point 297 Matching 116, 122, 123, 128 Asphaltene flocculation 281 Dew and bubble points 116, 122, 128 Liquid viscosity 125, 126 Wax Appearance Temperature 123 Matching Density/Volume 122 Matching dew and bubble points 116 Matching liquid viscosity 125 Matching to asphaltene deposition data 281 Matching using petroleum fraction properties 116 Matching vapour viscosity 126 Matching wax data/WAT 123 Maximum water content allowable before hydrate dissociation 251 Menu options 14 Mercury 9 mfl files 18, 25, 26 Mixing rules 31 Liquid thermal conductivity 45 Liquid viscosity 45 Surface tension 46 Vapour thermal conductivity 46 Vapour viscosity 45 Model definition 129 Model is not available 52 Modelling a polar mixture 230 Modelling asphaltene flocculation 43 Modelling hydrate formation and inhibition 39 Modelling wax precipitation 42 Models 2, 5, 9, 22, 29, 37, 48, 53 Advanced equations of state 31 Asphaltenes 43 Benedict-Webb-Rubin-Starling (BWRS) 34 Binary interaction parameters (BIPs) 53 Chung-Lee-Starling 45 Corresponding states (CSMA) 35 COSTALD model 43 Cubic plus association (CPA) 32 Diffusion coefficients 46 Dortmund Modified UNIFAC 38 Flory Huggins 38 Fuller model 46 Gas phase models 38 GERG model 36 Hayduk Minhas method 46 Hydrate model 39 Hydrate nucleation model 41, 49, 245 IAPWS-95 35 Ideal gas 30 Ideal solution 37 Lee-Kesler-Plöcker (LKP) 34 Linear Gradient Theory 46 Liquid density model 43

User Guide for Multiflash for Windows

Lohrentz-Bray-Clark 44 Macleod-Sugden 46 Mixing rules 31 Model definitions 29 NRTL 37 Pedersen 44 Peneloux density correction 31 Peng Robinson (PR) 30 PSRK 32 Redlich-Kwong (RK) 31 Redlich-Kwong-Soave (RKS) 31 Regular Solution 38 Selecting model set 22 Selecting new model 48 Solid models 39 SuperTRAPP 45 Surface tension 46 Surface Tension 46 Transport property 44 Troubleshooting 52 Twu 44 UNIFAC 38 UNIQUAC 37 Viscosity 44 Wax 42 When to use activity methods 38 When to use BWRS 34 When to use CPA 32 When to use CSMA 36 When to use cubic equations of state 32 When to use equation of state methods 30 When to use LKP 34 When to use PSRK 32 When to use ZJ model 33 Wilson E 37 ZJ (Zudkevitch-Joffe) model 33 Models and input requirements 74 Models for solid phases 39, 48 Models tab 6 Molecular weight and specific gravity 95 Multiflash Calculations 142 Case studies 197 Conditions 137 Exiting the program 27 Getting Started 13 Interfaces to other programs 183 Models 29 Output 173 Overview 1 Software system 1 Starting the program 13 Units 169 Multiflash Error Codes 193 Multiflash Excel Interface 12 Multiflash main window 13 Multiflash Main Window 13 Multiflash phase equilibrium algorithm 10 Multiflash Software System 1

User Guide for Multiflash for Windows

Multi-reference fluid corresponding states (CSMA) model 35

N New Features 5, 9 New Features and Changes in Version 4.2 2 New Features and Changes in Version 4.4 and 4.3 2 New high accuracy reference eos 6 New icon for Asphaltene precipitation curve 11 No information on the amount of asphaltene in the oil 278 No reservoir or precipitation conditions 280 No reservoir pressure 279 No resin - asphaltene ratio 279 Normal components 63 DIPPR 63 INFODATA 63 Properties of normal components 63 Notes 294 n-Paraffin distribution 105 NRTL equation 37 NRTL model 37 Nucleation 248 Nucleation model 41, 49, 245

O Obtaining properties from the Pure Component Data option 200 Oil and gas systems 205 OLGA 184 PVT file 184 OLGA hydrate file 186 OLGA tables 7, 12 OLGA wax file 186 On-line help 191, 194 Other calculations 24, 308 Other flash calculations 212 Other flash calculations with hydrates 251 Other properties 110 Other thermodynamic models 43 Output 3, 27, 173, 174 Enthalpy datum 178 Entropy datum 180 Level of physical property output 164 Phase Identification 178 Saving the output file 27 Troubleshooting 181 Writing the results to a file 173, 174

P PC-SAFT equation of state 33 Pedersen Model 44 Peng-Robinson (PR) model 30 Peng-Robinson 1978 (PR78) equation of state 31 Peng-Robinson equation of state 30 Performance enhancements 6 Petroleum Fluid Blending 127

Index  321

Petroleum fluid composition 94 Petroleum Fluids 2 Petroleum Fraction Input Table 7 Petroleum fractions 63, 109, 112, 113 Defining petroleum fractions 109 Deleting petroleum fractions 112 Editing petroleum fraction data 112 Importing characterised petroleum fractions 109, 210 Troubleshooting 113 Petroleum Fractions 65 Phase descriptors 313 Phase diagram 147 Phase envelope 24, 148, 166, 208 Customising 155 Phase envelope output 154 Phase Envelopes 148 Phase Envelopes for solids 11, 157 Phase key components 6 Phase labelling 178, 182 Phase names 144 Phase tab 149 Phases 39, 144, 147, 166, 182, 245, 313 Example of a phase diagram 147 Example of a phase envelope 166 Phase descriptors 313 Phase labels 182 Phase names 144, 182 Phase types 144 Selecting phases 47 Solid phases 39 Physical properties of a pure component 197 PIPESIM PVT file 183 Pipesim PVT files 183 Plot the phase envelope 166 Plots H, S, U, V phase boundaries 148 Phase envelope 148 Polar systems 230 Polymers 237 Case study 237 Poynting correction 9 PRA 10 Presence of water 109 Pressure Specifying as an input condition 138 Pressure and temperature 23 Printing Output 26 Printing the output 26, 175 Problem setup file 25 Loading a problem setup file 18 Saving a problem setup file 25 Problem setup files 18, 26 Problems defining a petroleum fraction 113 Problems when matching 127 Properties Displaying pure component properties 197 Level of physical property output 164

322  Index

Physical property databanks 63 Properties of normal components 63 Property output in Multiflash 164 Prosper PVT file 187 Prosper PVT files 187 Provide a key component 168 Provide a starting estimate 167 Pseudocomponents 98 PSRK equation of state 32 PSRK model 32 Pure solid phase 49 PVT Analysis 11, 89, 101, 214, 296 Case study 214 n-paraffins 105 Saving an analysis 101 Troubleshooting 109 PVT Lab Analysis input 89 PVT Lab Analysis input with n-paraffin analysis 105 PVTSim CHC file import tool 189 PVTSim import tool 6, 11

R Redlich-Kwong (RK) and Redlich-Kwong-Soave (RKS) equations 31 Redlich-Kwong (RK) model 31 Redlich-Kwong-Soave (RKS) model 31 Reference fluids 35 Refrigerant mixtures 229 Refrigerants Case study 229 Regular Solution model 38 Regular Solution theory 38 Reid vapour pressure 11, 162 Reid Vapour Pressure 162 Replacing a component 70 Results 14, 19, 26, 27, 173 Level of physical property output 164 Printing the results 14, 26 Results window 173 Saving the results in a file 14, 27 Results window 14 Retrograde 147 Retrograde condensation 147 Retrograde dew point 147 Retrograde Dew Point 11 RKSA 10 RKSA-Info, CPA and PR 10 Running Multiflash 2

S Salt calculator 81 Salt component 5 Salt inhibition 256 SARA Analysis 97 Saturation P at reservoir T 272 Saving a PVT Analysis 101 Saving files

User Guide for Multiflash for Windows

Saving a problem setup file 25 Saving results Writing the results to a file 173, 174 Saving the output 27 Saving the problem setup 25 Scale model 245 Scale precipitation 258 Scaling and general freeze-out model 39 Searching for components By formula 68 By name 66 By scrolling through list 66 By substring 68 Identifying synonyms 67 Select 15 Select components by formula 68 Select components by name 66 Select components by scrolling through a list 66 Select components by substring 68 Selecting components 66 Sensitivity of calculations to variation in input data 276 Sensitivity to characterisation 109 Set Input Conditions 23 Setting up a new problem 21 Setting up preferred models in Multiflash 48 Show functions 181 Soave-Redlich-Kwong (RKS) model 31 Solid freeze-out model 39 Solids.xls 296 Specific gravity conversion 95 Specify the pure component of interest 198 Specifying compositions 137 Specifying data for a user-defined component 73 Specifying enthalpy, entropy and internal energy 139 Specifying temperature, pressure and volume 138 Specifying the data source 66 Specifying the physical property output level 198 Stability analysis 176 Starting estimate for flash calculations 167 Starting Multiflash 13 Stream types 76 SuperTRAPP model 44, 45 SuperTRAPP thermal conductivity method 45 SuperTRAPP Viscosity Model 44 Supplementing or overwriting BIPs 58 Supplying an external file of BIPs 313 Surface tension 7, 46 Models 46 Surface tension mixing rule 46 Sutton Model for surface tension. 5 Synonyms 67 Component synonyms 67

PSF file for HTFS programs 183 PVT file for PIPESIM 183 TBP curves 221 TBP distillation 102 Technical support 28, 194 Temperature Specifying as an input condition 138 Temperature dependence of BIPs 54 Temperature-dependent properties 73 The basis of a flash calculation 141 The Multiflash GUI 1 The output does not include everything expected 181 The Peneloux density correction 31 The results 19 The results window 173 The Toolbar 18 Thermal conductivity 45 Models 45 Titration 283 To generate the file 187 To specify the Ideal Mixing model set: 197 Tolerance calculations 161 Too many components in the mixture 87 Tools 16 Total amount of fluid 96 Total Wax Content 97 Transport properties Displaying transport property values 164 Models 44 Surface tension 46 Thermal conductivity 45 Viscosity 44 Transport property models 44 Troubleshooting 52, 60, 84, 109, 139, 165, 171, 181 Binary interaction parameters 60 Calculations 165 Components 84 Models 52 Output 181 PVT Analysis 109 Units 171 Troubleshooting - BIPs 60 Troubleshooting - components 84 Troubleshooting - flash calculations 165 Troubleshooting - input conditions 139 Troubleshooting - models 52 Troubleshooting - output 181 Troubleshooting - PVT Analysis 109 Troubleshooting - units 171 Tutorial 13 Twu Model 44

T

UNFACFIT.xls 293 UNIFAC 294 UNIFAC method 38 UNIFAC model 38 UNIQUAC 37 UNIQUAC equation 37

Table 17 Tables 7, 12 Tabular output 141, 183 Excel interface 141

User Guide for Multiflash for Windows

U

Index  323

Units 3, 57, 60, 169, 170, 171, 294 Default units 169 Selecting units 170 Troubleshooting 171 Units for BIPs 57, 60, 294 Units for BIPs 57 Updates 5, 9 Usability 12 Use the P,T flash 166 User defined carbon number cuts 219 User defined components 72 User Defined Cuts 100 User-defined components 72 Using INFOBIPS 230 Using the fixed phase flash 146 Using the menu 47

When to use the BWRS equation 34 When to use ZJ model 33 When you may need to use commands 311 Will hydrates form at given P and T ? 246 Wilson A equation 37 Wilson E equation 37 Wilson E model 37 Windows GUI 6, 11 Writing the results to a file 174

Z ZJ (Zudkevitch-Joffe) model 33

V Vapour thermal conductivity mixing rule 46 Vapour viscosity mixing rule 45 Vapour-liquid-liquid equilibria 235 Viewing and editing pure component data 70 Viewing BIP values 55 Viscosity 44 Matching liquid viscosity 125, 126 Models 44 VLEAutoPlot 149 VLEFIT.xls 295 Volume Specifying as a flash condition 138 Volume fraction flash 144

W Warning option for matching and PVT form 26 Warnings 176, 181, 234 Additional phases 234 Convergence 176 Water cut 97 Water-Ammonia 35 Wax 260, 298 Case study 50, 260 Coutinho Model 42 Defining Wax model 50 Matching WAT 123 Wax and Asphaltene precipitation 288 Wax Appearance Temperature 159 Wax calculations 159 Wax Precipitation Curve 160 Waxes 50 What is a model? 29 What the model definition means 312 What types of model are available? 29 When to use activity coefficient models 38 When to use CPA. 32 When to use cubic equations of state 32 When to use equation of state methods 30 When to use LK or LKP 34 When to use PSRK 32

324  Index

User Guide for Multiflash for Windows