Multi Flash

User Guide for

Multiflash for Windows Infochem Computer Services Ltd

Version 3.6 11 May 2007

Infochem Computer Services Ltd 13 Swan Court 9 Tanner Street London SE1 3LE 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 Computer Services Ltd 13 Swan Court 9 Tanner Street London SE1 3LE, 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 Multiflash phase equilibrium utility......................................................................................1 Chemreact ..................................................................................................................................2 Multiflash Software System ...................................................................................................................2 Documentation..........................................................................................................................................2 Overview....................................................................................................................................2 New Features and Changes in Version 3.6 ..........................................................................3 Running Multiflash ..................................................................................................................3 HELP ..........................................................................................................................................4 Case studies ...............................................................................................................................4 Appendix - Multiflash Commands........................................................................................4 Installation .................................................................................................................................................4

New Features and Changes in Version 3.6

5

Introduction...............................................................................................................................................5 Models ........................................................................................................................................................5 Hydrates .....................................................................................................................................5 Asphaltenes................................................................................................................................5 UNIFAC.....................................................................................................................................6 BIPs.............................................................................................................................................................6 Fluid characterisation...............................................................................................................................6 Matching....................................................................................................................................................6 Wax calculations.......................................................................................................................................6 Phase labelling ..........................................................................................................................................7 Databanks...................................................................................................................................................7 DIPPR.........................................................................................................................................7 Infodata.......................................................................................................................................7 Front-end....................................................................................................................................................8 Icons............................................................................................................................................8 Graphics .....................................................................................................................................8 Inhibitor Calculator..................................................................................................................8 Preferences.................................................................................................................................8 Model Set Tabs.........................................................................................................................8 Interfaces....................................................................................................................................................8 Installation .................................................................................................................................................8

Simple Tutorial

9

Introduction...............................................................................................................................................9 Starting Multiflash....................................................................................................................................9 Defining a problem in Multiflash ........................................................................................................10 Problem setup file ...................................................................................................................10 Loading an existing problem file .........................................................................................................10 Loading a problem setup file ................................................................................................10 The results................................................................................................................................11 Additional calculations..........................................................................................................12 Setting up a problem interactively .......................................................................................................13 Clearing previous problems ..................................................................................................14

User Guide for Multiflash for Windows

Contents • iii

Defining the model.................................................................................................................14 Defining the components ......................................................................................................14 Define Input Conditions........................................................................................................15 Carrying out the flash calculation........................................................................................16 Additional calculations..........................................................................................................16 Phase envelope........................................................................................................................16 Saving a problem setup .........................................................................................................................17 Printing the output..................................................................................................................................18 Saving the output....................................................................................................................................19 Starting a new problem..........................................................................................................................19 How to exit the program........................................................................................................................19

Input

21 Introduction.............................................................................................................................................21 Input files .................................................................................................................................................21 Menu options...........................................................................................................................................22 File ............................................................................................................................................22 Edit ............................................................................................................................................22 Select ........................................................................................................................................23 Tools .........................................................................................................................................23 Calculate...................................................................................................................................24 Table .........................................................................................................................................24 Help ...........................................................................................................................................24 Toolbar buttons.......................................................................................................................................25 Dialogue boxes, text boxes, tab controls, drop-down tables and menus.......................................25 Commands...............................................................................................................................................25

Models

27

Introduction.............................................................................................................................................27 What is a model? ....................................................................................................................................27 What types of model are available? ....................................................................................................27 Equation of state method.......................................................................................................28 When to use equation of state methods..............................................................................28 Equations of state provided in Multiflash ..........................................................................28 Activity coefficient methods................................................................................................33 Activity coefficient equations in Multiflash ......................................................................33 Gas phase models for activity coefficient methods ..........................................................35 When to use activity coefficient models .............................................................................35 Models for solid phases .........................................................................................................35 Other thermodynamic models ..............................................................................................41 Transport property models ....................................................................................................41 How to specify models in Multiflash..................................................................................................43 How to load a model..............................................................................................................43 What the model definition means........................................................................................45 How to change a model.........................................................................................................46 Loading hydrate models ........................................................................................................46 The Freeze-out model............................................................................................................48 How to define a wax model..................................................................................................48 How to define the asphaltene model...................................................................................49 Combined Solids Model........................................................................................................49 Troubleshooting - models .....................................................................................................................50 Incorrect path...........................................................................................................................50 Model is not licensed.............................................................................................................51 Groups not available for UNIFAC model..........................................................................51 Binary interaction parameters ..............................................................................................................52 Number of BIPs related to any model.................................................................................52 Units for BIPs..........................................................................................................................52 Temperature dependence of BIPs ........................................................................................53 BIPs available in Multiflash .................................................................................................53

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User Guide for Multiflash for Windows

Viewing BIP values................................................................................................................54 Supplementing or overwriting BIPs ....................................................................................56 Troubleshooting - BIPs..........................................................................................................................59 Units..........................................................................................................................................59 Order of components..............................................................................................................59 Number of BIPs for the model.............................................................................................59 Naming the components ........................................................................................................59 BIP databank...........................................................................................................................60 BIPs not displayed..................................................................................................................60

Components

61

Introduction.............................................................................................................................................61 Normal components...............................................................................................................61 Condensed components .........................................................................................................63 Petroleum fractions ................................................................................................................63 Defining a mixture (stream) in Multiflash .........................................................................................64 Specifying the data source....................................................................................................64 Selecting components ............................................................................................................65 Adding, inserting, replacing and deleting components ....................................................68 Viewing and editing pure component data.........................................................................69 User-defined components......................................................................................................................71 Models and input requirements............................................................................................73 Stream types ............................................................................................................................................76 Hydrate inhibitors ...................................................................................................................................79 Inhibitor calculator.................................................................................................................79 Salt calculator..........................................................................................................................80 Troubleshooting - components.............................................................................................................82 Databank not found................................................................................................................83 Databank not licensed............................................................................................................83 Component cannot be found.................................................................................................84 Too many components in the mixture.................................................................................84

Petroleum fractions

85

Introduction.............................................................................................................................................85 Defining petroleum fractions................................................................................................................85 PVT Analysis ..........................................................................................................................................86 Analysis method.....................................................................................................................87 Component list........................................................................................................................88 Fluid composition...................................................................................................................90 Wax Content............................................................................................................................96 Water cut..................................................................................................................................96 Total amount of fluid .............................................................................................................96 SARA Analysis .......................................................................................................................96 Characterisation......................................................................................................................97 Saving a PVT Analysis ..........................................................................................................99 Troubleshooting – PVT Analysis ........................................................................................................99 Sensitivity to characterisation ..............................................................................................99 Presence of water..................................................................................................................100 Calculating petroleum fraction properties ........................................................................................100 Editing petroleum fraction data..........................................................................................101 Matching using petroleum fraction properties.................................................................................102 Matching dew and bubble points.......................................................................................102 Matching Density/Volume ..................................................................................................108 Matching wax appearance temp erature ............................................................................110 Matching liquid viscosity....................................................................................................111 Problems defining a petroleum fraction ...........................................................................112 Problems when matching ....................................................................................................113

User Guide for Multiflash for Windows

Contents • v

Input conditions

115

Introduction...........................................................................................................................................115 Specifying compositions.....................................................................................................................115 Specifying temperature, pressure and volume .................................................................................117 Specifying enthalpy, entropy and internal energy ..........................................................................117 Troubleshooting - input conditions...................................................................................................117

Calculations (flashes)

119

Introduction...........................................................................................................................................119 The basis of a flash calculation..........................................................................................................119 Flashes available in Multiflash...........................................................................................................120 Isothermal (P,T) flash..........................................................................................................121 Isenthalpic flashes ................................................................................................................121 Isentropic flashes..................................................................................................................121 Isochoric flashes ...................................................................................................................122 Bubble and dew point flashes.............................................................................................122 Fixed phase fraction flashes................................................................................................122 Hydrate calculations.............................................................................................................126 Scale calculations .................................................................................................................126 Wax calculations...................................................................................................................126 Tolerance calculations.........................................................................................................128 Provide a starting estimate ..................................................................................................129 Phase diagrams ......................................................................................................................130 Property output in Multiflash .............................................................................................................137 Troubleshooting - flash calculations.................................................................................................138 Plot the phase envelope.......................................................................................................139 Use the P,T flash...................................................................................................................139 Limit the number of phases ................................................................................................139 Consider all types of solution.............................................................................................140 Provide a starting estimate ..................................................................................................140 Provide a key component....................................................................................................140 Chemical reaction.................................................................................................................................140 Troubleshooting - chemical reaction.................................................................................................141

Units

143 Introduction...........................................................................................................................................143 Working units........................................................................................................................................143 Default units ..........................................................................................................................................143 Changing units ......................................................................................................................................144 Changing units interactively ...............................................................................................144 Changing units in a problem setup file .............................................................................145 Troubleshooting - units........................................................................................................................146

Output

147

Introduction...........................................................................................................................................147 The results window. .............................................................................................................................147 Font.........................................................................................................................................................148 Writing the results to a file ..................................................................................................................148 Printing the output................................................................................................................................149 Calculation output................................................................................................................................150 Manipulating the Output.....................................................................................................152 Phase envelope output.........................................................................................................................152 Errors and warning messages .............................................................................................................152 Displaying status for current settings................................................................................................153 Troubleshooting - output.....................................................................................................................154 The output does not include everything expected...........................................................154 Phase labelling ......................................................................................................................154

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User Guide for Multiflash for Windows

Phase envelope......................................................................................................................154 Fonts .......................................................................................................................................154

Interfaces with other programs

155

Introduction...........................................................................................................................................155 Pipesim PVT files.................................................................................................................................155 OLGA .....................................................................................................................................................156 CAPE-OPEN Interface ........................................................................................................................158

Help

159 Introduction...........................................................................................................................................159 On-line help ...........................................................................................................................................159 Website support ....................................................................................................................................162 Technical support .................................................................................................................................162

Case studies - Pure component data

163

Introduction...........................................................................................................................................163 Physical properties of a pure component..........................................................................................163 Defining the problem interactively ....................................................................................163 Producing a problem setup file...........................................................................................167 Obtaining properties from Pure component Data option...............................................167 Excel interface.......................................................................................................................169

Case studies - Phase equilibria

171

Introduction...........................................................................................................................................171 Oil and gas systems ..............................................................................................................................171 Calculating the bubble point curve....................................................................................172 Calculating the dew point curve.........................................................................................173 Phase envelope......................................................................................................................174 Adding water to the system ................................................................................................175 Including a petroleum fraction ...........................................................................................176 Other flash calculations.......................................................................................................177 PVT Analysis ........................................................................................................................................179 Black Oil Analysis ...............................................................................................................................185 Refrigerant mixtures ............................................................................................................................186 Polar systems .........................................................................................................................................188 Modelling a polar mixture. .................................................................................................188 Liquid-liquid equilibria .......................................................................................................192 Vapour-liquid-liquid equilibria ..........................................................................................192 Azeotropes.............................................................................................................................193 Eutectics .................................................................................................................................193 Polymers.................................................................................................................................................194 Data input...............................................................................................................................194 Co-Polymers ..........................................................................................................................197

Case studies - Hydrate dissociation, formation and inhibition

201

Introduction...........................................................................................................................................201 Defining the hydrate models ...............................................................................................................201 Fluid phase model ................................................................................................................202 Hydrate model.......................................................................................................................202 Nucleation model..................................................................................................................202 Ice model................................................................................................................................202 Scale model...........................................................................................................................203 Phases .....................................................................................................................................203 Hydrate calculations with Multiflash................................................................................................203 Will hydrates form at given P and T ? ..............................................................................203

User Guide for Multiflash for Windows

Contents • vii

Hydrate formation and dissociation temperature at given pressure.............................205 Hydrate formation and dissociation pressure at given temperature .............................207 Hydrate phase boundary......................................................................................................208 Other flash calculations with hydrates..............................................................................208 Maximum water content allowable before hydrate dissociation..................................209 Calculations with inhibitors ................................................................................................................210 Can hydrates form at given P and T ? ...............................................................................210 Hydrate dissociation temperature at a given pressure....................................................211 Hydrate dissociation pressure at a given temperature ....................................................212 Hydrate phase boundary......................................................................................................212 Amount of inhibitor required to suppress hydrates ........................................................213 Salt inhibition........................................................................................................................214 Scale precipitation ................................................................................................................................215 RKSA(Infochem) mo del .....................................................................................................................217

Case studies – Wax precipitation

219

Introduction...........................................................................................................................................219 Defining the wax model ......................................................................................................................219 Coutinho wax model............................................................................................................220 Calculating wax appearance temperature (WAT)...........................................................................220 Calculating wax precipitation.............................................................................................................223 Multisolid model...................................................................................................................226

Case studies – Asphaltene flocculation

227

Introduction...........................................................................................................................................227 Input data ...............................................................................................................................................227 Defining the asphaltene model...........................................................................................................228 Calculating asphaltene flocculation conditions...............................................................................232 Sensitivity of calculations to variation in input data......................................................................235 Choice of Analysis method.................................................................................................235 Data Availability...................................................................................................................235 No reservoir or flocculation conditions............................................................................238 Gas injection..........................................................................................................................................239 Titration..................................................................................................................................................240

Case studies – Combined solids

245

Introduction...........................................................................................................................................245 Asphaltene flocculation.......................................................................................................................245 Wax and Asphaltene precipitation.....................................................................................................246 Hydrates, Waxes and Asphaltenes.....................................................................................................247

Case studies – Excel spreadsheets

251

Introduction...........................................................................................................................................251 UNFACFIT.xls .....................................................................................................................................251 Notes.......................................................................................................................................251 UNIFAC.................................................................................................................................252 Activity model worksheets .................................................................................................252 VLEFIT.xls ............................................................................................................................................253 SolidsB.xls and SolidsA.xls ................................................................................................................253 PVT Analysis ........................................................................................................................254 Match bubble point...............................................................................................................255 Wax.........................................................................................................................................255 Asphaltenes............................................................................................................................256 Asphaltene with gas injection.............................................................................................257 Hydrates .................................................................................................................................258

Case study – Mercury partitioning viii • Contents

260

User Guide for Multiflash for Windows

Introduction...........................................................................................................................................260 Defining the mercury model...............................................................................................................260 Calculating mercury partitioning.......................................................................................................261 Calculating mercury dropout..............................................................................................................263 Other calculations.................................................................................................................................264

Case studies - chemical equilibria

265

Introduction...........................................................................................................................................265 Xylene isomerisation ...........................................................................................................................265 Steam cracking of ethane ....................................................................................................................266

Appendix - Multiflash Commands

269

Introduction...........................................................................................................................................269 When you may need to use commands.............................................................................................269 Defining models ....................................................................................................................................269 Supplying an external file of BIPs.....................................................................................................270 Defining phase descriptors and key components ............................................................................270

Index

User Guide for Multiflash for Windows

275

Contents • ix

Overview

Introduction Multiflash is an advanced software package for performing complex equilibrium calculations quickly and reliably. The main utility is a multiple phase equilibrium algorithm that is interfaced to Infochem’s package of thermodynamic models and a number of physical property data banks. The program also contains Infochem’s Chemreact utility for performing simultaneous phase and chemical equilibrium calculations.

Multiflash phase equilibrium utility Multiflash can perform multiphase equilibrium calculations between any numb er of phases of different types. Each property of each phase may be described by a different thermodynamic model if required. The phases that can be modelled include gases, liquids and solids, for example hydrates, waxes and asphaltenes, depending on which models are available under your licence. Multiflash incorporates a phase stability analysis procedure whereby it can establish automatically which of the possible phases are present at equilibrium. Phase equilibria can be calculated for the following conditions:

User Guide for Multiflash for Windows

•

fixed pressure and temperature

•

fixed fraction of any specified phase at fixed pressure (includes dew and bubble points)

•

fixed fraction of any specified phase at fixed temperature (includes dew and bubble points)

•

fixed pressure and enthalpy

•

fixed temperature and enthalpy

•

fixed pressure and entropy

•

fixed temperature and entropy

•

fixed pressure and internal energy

•

fixed temperature and internal energy

•

fixed pressure and volume

•

fixed temp erature and volume

•

fixed internal energy and volume

Overview • 1

•

fixed entropy and volume

•

fixed enthalpy and entropy

•

fixed pressure, temperature and fraction of any specified phase (variable composition)

A full range of thermodynamic and transport properties is available for any fluid phase.

Chemreact Chemreact is a utility for performing simultaneous phase and chemical equilibrium calculations. Currently Chemreact can handle equilibria involving combinations of one gas phase, one liquid phase and any number of pure solids. It is interfaced to the same package of thermodynamic models and physical property data banks as the phase equilibrium utility. As in the phase equilibrium utility, Chemreact incorporates a phase stability analysis procedure to establish automatically which phases are present at equilibrium. Chemreact can calculate equilibria for the following specifications: •

fixed pressure and temperature

•

dew point at fixed pressure

•

dew point at fixed temp erature

•

bubble point at fixed pressure

•

bubble point at fixed temperature

When using Chemreact the user does not need to specify any reaction mechanism but only list all the possible products and reactants.

Multiflash Software System In addition to the Microsoft Windows program, Multiflash is also available as an add-in for use with the Microsoft Excel spreadsheet program, for use with Matlab, as a DLL and as a suite of sub-routines for interfacing to other application packages. A CAPE-OPEN physical property package interface is also available. The package can be accessed from programs written in Fortran, Visual Basic, VBA and C. All versions share the capability to use the problem setup files which can store all aspects of a user’s problem, see “Setting up a problem interactively” on page 13. Separate documentation is available for these interfaces.

Documentation The supporting documentation is grouped into sections.

Overview This section describes what the Multiflash software is used for, what it contains and how it is made available.

2 • Overview

User Guide for Multiflash for Windows

New Features and Changes in Version 3.6 New developments and additions are listed, although the detailed review of how to use new features will be covered in the appropriate section. Information is also given on changes to code or data which may give rise to different results from those obtained with earlier versions of Multiflash.

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

Simple tutorial This section is based on starting the program and running a simple example with step by step instructions.

Input This section provides an overview of the various ways you can enter data and commands in Multiflash for Windows.

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 type 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, including how to define and edit data for petroleum fractions.

Petroleum Fractions How to carry out PVT characterisation and calculate petroleum fraction physical properties.

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.

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.

User Guide for Multiflash for Windows

Overview • 3

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 Nearly all the facilities available in the command processor version of Multiflash have been incorporated as menu options in the current version of Multiflash for Windows, and can be now readily accessed from either menu options or tool bar. An example would be the ability to edit or change model binary interaction parameters (BIP) in the BIP values window by selecting the BIPs option from the Tools menu. However a Tools Command option is still available which allows the user to enter any command and apply this in the Windows version. Those commands which users of the Windows version of Multiflash may find useful are discussed in the appropriate section of the User Guide or in the Appendix. Commands are used to create or edit problem setup files for use with Multiflash. A Contents list and an Index are also supplied to help you find your way around the User Guide.

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

4 • Overview

User Guide for Multiflash for Windows

New Features and Changes in Version 3.6

Introduction There are several improvements and extensions in Multiflash 3.6. The major developments for this version are the inclusion of a model for chloride scales and a review and update of asphaltene mo del parameters. A new graphics plotting package replaces the package used for previous versions of Multiflash. The inclusion of chloride scales has required significant extension to the Multiflash flash algorithms. We have carried out a comprehensive QA procedure and in some cases you may find that calculated results vary from previous versions. Where we are aware of significant differences we will point these out. All our hardware dongles are valid for previous licensed versions of Multiflash so you can reproduce earlier calculations if required.

Models Descriptions and references detailing the models are provided in our "Models and Physical Property Guide"

Hydrates The extension to prediction of chloride scales is part of our hydrate module and can be activated as part of the Hydrate Model Set, see “Loading hydrate models ” on page 46. The scales considered in this version of Multiflash are NaCl, NaCl.2H2O, KCl, CaCl2.6H2O, CaCl2.4H2O and CaCl6.H2O. The effect on salt solubility of other hydrate inhibitors, such as methanol and MEG, are also considered.

Asphaltenes Since the addition of an asphaltene model several versions ago, mo re data on asphaltene flocculation has been published and we have carried out further proprietary studies. As a result of this new information we have carried a out a review and upgrade of the asphaltene model parameters. The greatest changes in ADE predictions are likely to be for reservoir fluids with MW between 180-240, but at temperatures of operational interest these are not likely to be greater than experimental error. For lighter oils the improved temperature dependence of the resin-asphaltene parameter means that the ADE are more likely to close at lower

User Guide for Multiflash for Windows

New Features and Changes in Version 3.6 • 5

temperatures. However, we would also recommend, if possible, two measurements of asphaltene onset for light oils if possible. In order for you to be able to use two ADE measurements, another improvement in MF3.6 is the extension of the Matching Asphaltene Phase form to process both points. In the PVT Analysis the SARA values must be supplied for the STO rather than the total fluid and the asphaltene content that precipitated by heptane.

UNIFAC In MF3.6 users can now add their own user-defined UNIFAC groups if no existing group is suitable. There are no specific menu options as yet. You would have to enter commands through the command box – and information on the commands can be found in the Command Reference Manual.

BIPs There have been some minor modifications to the existing binary interaction parameters between ions and other components for the Electrolyte salt model and new parameters to support salt precipitation.

Fluid characterisation The PVT Analysis form for fluid characterisation has been modified in MF3.6. There are now two forms, one for analyses including n-paraffins and one for those without. This allows us to introduce mo re flexibility for entering nparaffin data as it is supplied by the laboratory, without making the PVT form for more routine analyses too complex. The n-paraffin data can now be supplied either as total amounts in the STO or as a fraction of the STO cuts, see “PVT Analysis ” on page 86. The method for estimating STO composition has been improved to better reproduce supplied MW and specific gravity. The n-paraffin distribution generated from wax content has been improved, which may effect WAT calculations.

Matching Matching to measured WAT or to amounts of wax precipitated has now been allocated a specific menu option under Tools/Matching. Amounts of wax can be provided relative to either the oil plus wax phases or relative to the total material.

Wax calculations There have been some modification to wax calculations. In MF3.6 the amount of wax specified for calculations is now relative to the liquid plus wax phases for the wax matching, the wax precipitation curve calculation and WAT calculated using the WAT button. For the fixed phase fraction flashes and the phase boundary plotting the specified amounts of wax are relative to the total fluid as in previous versions. After careful consideration of the data we have decided to recommend that calculations of WAT should be considered relative to positive amounts of wax precipitated, rather than zero, as this appears to be the case in experimental

6 • New Features and Changes in Version 3.6

User Guide for Multiflash for Windows

measurements. Different measurement techniques rely on different amounts of precipitated wax to determine the WAT. Accordingly the WAT button in MF3.6 now allows you to specify the mass or mole% of wax to be precipitated at the calculated WAT and recommends specific values for WAT measurements by CPM (Cross Polar Microscopy) and DSC (Differential Scanning Calorimetry). The values can still be set to zero percent wax if preferred. See “Wax calculations” on page 126 for more information. The Wax precipitation curve is now plotted in addition to the values being displayed in the Window.

Phase labelling The criterion used to decide whether a supercritical phase is to be labelled as gas or liquid has been changed. This change has the effect that more of the supercritical region is considered to be gas. It does not affect any property values. Specific details of the change can be found in the Programmer’s Guide.

Databanks DIPPR The latest version of DIPPR linked to Multiflash is now DIPPR 2006. This has increased the number of components by 48. A list of the additional components is available on request.

Infodata The number of components in Infodata has increased to 240. Those added for Multiflash 3.6 are: 1-hexene 1,3-cyclopentadiene, 1,4-pentadiene 1-heptene 1-octene 2-methyl,1-butene 2-methyl,2-butene 2-methyl,1-heptene 2-methyl,1-hexene 2-methyl,1-pentene cyclohexene cyclopentene n-propylcyclohexane n-propylbenzene n-butylbenzene 1,2,4-trimethylbenzene 1,3,5-trimethylbenzene

User Guide for Multiflash for Windows

New Features and Changes in Version 3.6 • 7

Front-end Icons There is now a new icon for PVT analysis including a n-paraffin analysis

.

Graphics We have introduced a new graphics package in MF3.6, TeeChart from Steema Software. We feel that this provides a better display and improved facilities for adding experimental or other data, editing legends and titles.

Inhibitor Calculator MF3.6 will now save the original salt analysis data supplied during a problem calculation as part of the .mfl file and this can be displayed when the file is loaded. Ethanol is now displayed on the Alcohols/Glycols tab is the Inhibitor tab in addition to methanol, MEG, DEG and TEG.

Preferences There have been two extensions to Tools/Preferences. You can now set the preferred level of property output to appear when Multiflash is loaded. The display is the same as that used for Select/Property Output. If you wish to change the property level during any run you should still use the Select menu option. The other extension is Folders, where you can specify the directory where the software can find the Databank and Help files, the Problem files and the CAPEOPEN property packages. This means you can set a default working directory where you prefer to save your .mfl files.

Model Set Tabs The arrangement of models in the Model Set form has been modified slightly. There are now separate tabs for the cubic and non-cubic equations of state and an additional tab has been added for the mercury model.

Interfaces The Matlab/Simulink interface is upgraded with each new Multiflash release. MF3.6 now supports CAPE-OPEN 1.1 as well as 1.0

Installation The installation procedure will copy any Preferences set in Mf3.5 to MF3.6 as part of installation.

8 • New Features and Changes in Version 3.6

User Guide for Multiflash for Windows

Simple Tutorial

Introduction This section concentrates on how to run Multiflash using a simple problem as an example. The assumption is that the user is already familiar with Windows terms and techniques.

Starting Multiflash You start Multiflash by clicking on the MFW3.6 shortcut.

The Multiflash Main Window should then appear.

If, in a previous run of Multiflash, an error has caused an abnormal termination, you may see a message box saying Multiflash DLL already loaded. In this case you may need to click the Start button and select “Restart the computer”.

User Guide for Multiflash for Windows

Simple Tutorial • 9

Defining a problem in Multiflash You need, first of all, to define the problem you want to solve using Multiflash. This means choosing the model you wish to use to describe your system, setting the source for the pure component data, the components in the system and their compositions, the input conditions (e.g. temperature and pressure) and the calculation you wish to carry out. This is described in detail in the appropriate sections. Initially a simple problem is defined and Multiflash is used to calculate the physical properties of the system. 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 and interaction parameters from the default BIP databank called OILANDGAS. First we are going to calculate the bubble point (the point at which the gas phase first appears) at fixed pressure followed by an isenthalpic flash (a flash calculation at fixed enthalpy and, in this case, pressure). There are two ways of setting up the problem. You can prepare a problem setup file using a text editor, either by typing in all the necessary commands or by copying and editing an existing file. However, it is more usual to set up the problem completely within Multiflash for Windows and save the problem file for future use. This is activated by “Save Problem setup” from the File menu “Save Problem setup” will save the current problem definition, excluding flash calculation options.

Problem setup file Infochem supplies a series of sample problem setup files covering typical problem types. These can be used as examples when testing the program or can be copied and edited to define your own problems. By convention problem setup files for Multiflash have the extension .mfl.

Loading an existing problem file Infochem has provided several problem set up files (extension .mfl) covering a variety of typical problems. Any of these may be used as an example when running Multiflash; the setup file used for the simple tutorial is C4C5.mfl. If the problem set up file has been edited to include a full definition of the problem, including input conditions and a specification of the flash calculation, then once the file is loaded the flash calculation will be carried out and the results shown. If it contains only a partial definition of the problem then the remaining specifications must be completed interactively.

Loading a problem setup file The problem setup file for our tutorial is C4C5.mfl. This is loaded by

10 • Simple Tutorial

User Guide for Multiflash for Windows

1.

Either selecting File from menu bar, then selecting “Load Problem Setup” from the sub-menu or Clicking on Load problem setup button

.

This will activate the “Load Multiflash File” Dialogue box,

which will show a list of available setup files (*.mfl) contained in the current directory. 2.

Scroll to C4C5.mfl, which will now be highlighted, and either double click on it or select it and click on OK. The Dialogue box will disappear and the file will be read in. This file contains the complete definition of the problem including: •

Data sources

•

Model

•

Phase descriptors

•

Compounds

•

Compositions

•

Input conditions

•

Calculation

The results The results of any calculations are displayed in the results section of the main Multiflash window and the last set of input conditions echoed in the input conditions section of the window. You can scroll up and down to look at earlier calculations. The input conditions section of the main window will now look like this

User Guide for Multiflash for Windows

Simple Tutorial • 11

displaying the set pressure and the enthalpy for the final calculation, which was an isenthalpic flash. The results of the isenthalpic flash calculation will be the last item displayed the results window

You can scroll up to look at the earlier bubble point calculation at a pressure of 10e5Pa.

Additional calculations You can pre -set all the calculations you wish to carry out for any one problem within the problem setup file. However, it is probable that once you have seen the results of one calculation you may wish to make other changes or perform 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. 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 flash icon or select the required flash from the menu options. 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.

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To replace a component or to add new components see Adding, inserting, replacing and deleting components . Some simple changes are shown below:

Change the pressure 1.

Place cursor in Pressure box of the Conditions panel in the main window and change the pressure to 20e5 Pa.

2.

Click on P,H flash button or use the drop down menu by selecting Calculate in the menu bar, then selecting Standard flashes in sub-menu and finally selecting P,H flash in sub-sub-menu.

Change the enthalpy 1.

Change the enthalpy for the isenthalpic flash by typing -1000 in the enthalpy box of the Conditions section

2.

Recalculate the isenthalpic flash by repeating step 2 as above.

Change the composition 1.

Click on the Composition button in the Conditions 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 “mol” column of the drop-down table so that there are 3 moles of butane and 7 moles of pentane.

2.

Recalculate the isenthalpic flash as above

Carry out an isothermal flash 1.

Enter a temperature in K in the temperature text box in the Conditions section, e.g. 300K

2.

Click on the P,T flash button or select the P,T flash option starting from Calculate in the Menu bar.

Setting up a problem interactively You can define all the necessary input for a problem calculation interactively. This input must include:

User Guide for Multiflash for Windows

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Data sources

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Model

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Compounds

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Compositions

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Input conditions

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Flash Calculation

Simple Tutorial • 13

Clearing previous problems Each time you start Multiflash from the Windows icon you start with a “clean sheet”, i.e. there are no models or components pre-loaded. However, the INFODATA databank, see “Normal components ” on page 61, will be set as the default data source. If you have already carried out a series of calculations for a previous problem and want to start a new one then Select “Clear Problem Setup” from the File menu. This will remove the current specification and reset the unit definitions to those in place when you started the program. INFODATA will still remain as the default data source for the pure component data.

Defining the model To define the model Select Select in the menu bar and from the drop-down menu select Model set. A tabbed dialogue box will offer a choice of different types of model, e.g. equation of state, activity coefficient method, hydrate, wax and asphaltene or a combination of solid models. Within each group a specific model along with a number of phases may be chosen by name and a combination of transport models chosen. For general advice on which models to choose for an application and a detailed definition of each model, see “What types of model are available?” on page 27 or consult the “Models and Physical Properties” manual. Once you have selected a model a message will be displayed to tell you that the model has been successfully loaded, click on OK and close the Select Model dialogue box to return to the main window. The model set will include a data source for binary interaction parameters. We will use the Peng-Robinson equation of state, see ”Peng-Robinson equation of state” on page 28, for this problem. Select Select in the menu bar and from the drop-down menu select Model set to open the Select Model Set tabbed dialogue box. In the Model Set window, select Equations of State, and from the following menu select PR. You can also select phases and transport property models in this window but the default set will already be selected. After the selection, click the Define Model button. You should see the following message.

Click on OK to return to the Select Models dialogue box. Click on Close to return to the main window.

Defining the components Choose the components for any problem by Selecting Select then selecting Components from the sub-menu. Alternatively you can click on the Select Components button, . Either will result in the display of the Select Components Dialogue box.

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Specify the data source and the individual components. Components may be selected in a variety of ways e.g. by name, by scrolling through a list or by searching for a formula. The various methods are fully described in “Selecting components ” on page 65. Choose the components needed for the problem, in this case butane and pentane, by: 1.

Selecting Select again, but this time selecting Components from the sub-menu. Alternatively you can click on the Select Components button, . Either will result in the display of the Select Components Dialogue box. At this stage we will ignore the search facilities and enter the components by the simplest route possible.

2.

INFODATA will be the default data source. Make sure the Name option button is selected.

3.

Type butane in the Enter Name box and either press Enter or click on the Add button to transfer butane to the Components selected list. Do the same for pentane.

4.

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

5.

To check the components are correctly loaded, click on the Composition button and check the drop-down table, which should look like this

Define 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 input condition section of the main window. The composition is entered by clicking on the Composition button. The dropdown table shows the chosen components in the left-hand column and the amount of each component in the mixture is typed in the right-hand column. The default unit for amounts is moles.

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Other input conditions will depend on the type of flash calculation to be carried out, e.g. for an isothermal flash you must type in a pressure and a temperature in the appropriate text boxes and in the units shown next to them. Enter the input conditions in the Conditions section of the main window

Compositions 1.

Type the number of moles of each component, in this case 0.4 for butane and 0.6 for pentane.

2.

Close by clicking on roll-up button if you wish. The table can remain down if you prefer.

Pressure Type the pressure in Pa., in this case 10e5, in the Pressure text box.

Carrying out the flash calculation To carry out a flash calculation you either click on the appropriate flash button or select the required flash from the menu. Only the most commonly encountered flash options have been allocated buttons; these are the isothermal flash, dew and bubble points, isenthalpic and isentropic flashes at fixed pressure and fixed phase flashes. To carry out the latter you also have to fill in supplementary information in a dialogue box. Other flashes, such as isochoric flashes or isentropic and isenthalpic flashes at fixed temperature, are specified using the options accessed under Calculate in the menu bar. To calculate the bubble point at fixed pressure, either Click on the bubble point at fixed pressure button, or 1.

Select Calculate from the menu bar, then

2.

Select Bubble or dew point flashes in sub-menu, then

3.

Select P, Bubble point flash in the next menu.

To carry out the isenthalpic flash, type -11078 in the enthalpy text box and click on the P,H button. The results will be identical to those shown earlier.

Additional 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. 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 flash icon or select the required flash from the menu options. 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 68.

Phase envelope You can also plot the complete phase envelope by

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1.

Clicking on the phase envelope button or clicking on the Calculate menu followed by clicking on Phase Envelope

2.

In the Phase Envelope tabbed dialogue box click on VLE Autoplot

The phase envelope will be displayed in a separate window.

The phase diagram may be edited or printed as described elsewhere, ”Customising the phase envelope plot” on page 135.. Alternatively it can be exported to Excel, provided you are running Excel 97 or later.

Saving a problem setup Once you have defined the model, components, compositions and conditions interactively you can create a problem setup file containing this information for future use. It does not matter whether these were entered through dialogue boxes, from an existing problem setup file or the Tools/Command option. To save the setup either Select File from the menu bar and then select “Save Problem Setup” or Click on Save problem setup button, The dialogue box that is activated allows you to specify the name of the .mfl file and the directory where you want it stored.

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Multiflash will provide a default file name which can be overwritten.

Printing the output You can print the output from your calculations by Selecting File from the menu bar, then selecting Print Results or Clicking on the Print results button, This leads you to a print panel (see below), so that you can change the printer‘s setting and its properties, 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 147 All the output from any run is automatically stored in a file called MFLASH.LOG. This file will be overwritten when the program is started unless you rename it. If you wish to save the output to another file directly from Multiflash, then Select File from the menu Select Save Results As with printing the results you will see a message box which allows you to write the entire output window to file. If you have previously highlighted a section of the output then the message box offers the option of writing the selected text to file. 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 32kb of text. Any output beyond this limit will not be dis played on the screen or in the log file.

Starting a new problem To load a new problem Select File from the menu, followed by Selecting Clear Problem Setup. The message “Clear current problem setup” will appear in the results window. Select a new problem setup file or enter a new problem interactively.

How to exit the program To exit Multiflash Select File from the menu bar then Select Exit

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Simple Tutorial • 19

Input

Introduction This section concentrates on the methods of providing input information for Multiflash, rather than the specification of that input information, which will be discussed in the sections relating to the various types of information which must be supplied. The information you need to supply will include •

Model specification

•

Data sources for pure component data

•

Compounds in the mixture.

•

Input conditions including composition

•

Calculation (flash) to be carried out

You may also wish to alter •

Units

•

Level of property output

You can supply all of the input specification interactively, all of the input specification in a problem setup file or use a mixture of both.

Input files An input file may contain a full specification of the problem or define only part of it. We have usually referred to the former as problem setup files (extension .mfl) and have supplied several to use as test examples or templates for producing your own setup files. As a minimum they should define the model, data sources for pure components and model interaction parameters (BIPs) and components for your mixture. Usually they will also contain a composition and possibly the flash calculation and the input conditions related to it. We recommend that any setup file that contains the numerical values for any input variables also defines the units for these to prevent any mismatch on loading. Typical of partial setup files are those which are supplied to define a model (extension .mfc). These contain a specification of the model and a list of phase descriptors, which give the possible phases that may be encountered in calculations. The .mfc files we supply normally define up to four phases: one

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Input • 21

vapour phase, two liquid phases and a water phase. More phases are needed for some applications, for example the hydrate I/II model configuration file contains seven phase descriptors, the hydrate I/II/H model configuration file has eight phase descriptors. Exceptions are the input files for PVT analysis. They do not need to define either a model or a source for the BIPs but must include a source for the pure component data.

Menu options Multiflash for Windows includes a menu bar and several drop-down menus. These allow you to control all aspects of running Multiflash for Windows, 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 and the saving and printing of results.

Where you need to define further items, such as the directory and file to be loaded, a dialogue box will be activated. If the setup files you want have been used recently, you may find them listed. To load a setup file from the list, move the pointer to the file and double click on it.

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

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User Guide for Multiflash for Windows

Select The Select menu option allows you to select the components, freeze-out model and related components, PVT Lab. Input, models, level of property output, stream types, units and the use of starting values. All the menu options except Use Starting Values activate dialogue boxes.

Tools The Tools menu allows you to use commands to access all options that are supported by Multiflash. This is most useful for options which have not yet been allocated a button or specific menu item in Multiflash for Windows. The Tools menu also allows you to view and edit the properties of any component in the stream and any binary interaction parameters being used. The Inhibitor calculator allows you to add water and inhibitors without defining them in the pure component list. It also allows you to define the amount of the inhibitor in the water phase in mass, mole or volume units and the calculator then determines the amount of inhibitor to be added to the overall composition in the units currently in use. The Salt Calculator is part of the Inhibitor Calculator dialog box. The Matching function amends the properties of petroleum fractions in the stream to reproduce user supplied liquid viscosity, wax appearance temperature, density/volume and dew or bubble points/GOR. For the asphaltene phase the matching function modifies the asphaltene model parameters. The Preferences option allows the user to configure the appearance of the results in the results window, e.g. Font size , and to set the default units and level of property output for Multiflash. 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.

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Input • 23

Calculate The Calculate menu option controls the choice of flash options, tolerance calculation, phase envelope and chemical equilibria. Sub-menus are activated for Standard Flashes, Bubble and Dew Point Flashes, Hydrate and Wax flashes and Chemical Equilibria. Dialogue boxes are activated for Fixed Phase Fraction Flashes, (see “Fixed phase fraction flashes” on page 122), tolerance calculation, (see “Tolerance calculations” on page 128), and phase envelope, (see “Phase ” on page 130) .

Table The Table menu is for the option of creating output files for use with other applications, currently PIPESIM and OLGA. See “Interfaces with other programs ” on page 155.

Help The HELP menu enables you to get help on a variety of topics, see “Help” on page 159.

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Toolbar buttons Some of the most common menu options have been allocated individual Toolbar buttons (located in the Tool bar section of the main window or in individual dialogue boxes).

Dialogue boxes, text boxes, tab controls, drop-down tables and menus Where additional information is needed for any menu option then you may be asked to supply this through dialogue boxes, text boxes, tab controls, drop-down tables or menus. These are described individually in the sections relating to the input variable being specified.

Commands There may be some Multiflash facilities that have not been allocated a menu option. These are still accessible by using the Tools Command menu option. Some of these commands will be discussed in other sections where they are used in examples or case studies. They can be specified in Multiflash either as part of a problem setup file or through the Tools/Command menu, options, e.g.

will provide more details of the chosen model, including the name of the associated BIP set.

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Input • 25

Models

Introduction This section defines what a model is in terms of the Multiflash nomenclature, what models are available and when you might wish to use them, as well as the practical means of specifying and using them in Multiflash. For information on specifying models, see “How to specify models in Multiflash” on page 43. Detailed model descriptions may be found in our separate "Models and Physical Properties Guide".

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 will depend on pressure, temperature or composition.

What types of model are available? The key thermodynamic property calculation carried out within 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

f i v = f il f i v is the fugacity of component i in the vapour phase and f i l is the fugacity of component i in the liquid phase. where

The models used in Multiflash to represent the fugacities from the phase equilibrium relationship in terms of measurable state variables (temperature, pressure, enthalpy, entropy, volume and internal energy) 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 method all thermal 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 the summation of the pure component properties to which a mixing term or an excess term has been added. 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

User Guide for Multiflash for Windows

Models • 27

hydrate, wax or asphaltene. Models used to represent these solids are discussed below. The transport properties of a phase (viscosity, thermal conductivity and surface tension) are derived from semi-empirical models which will be discussed later.

Equation of state method 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 represent highly non-ideal chemical systems, such as alcohol-water, well. 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 factors 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 52. 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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Redlich-Kwong (RK) and Redlich-Kwong-Soave (RKS) equations Like Peng-Robinson, the Redlich-Kwong and Redlich-Kwong-Soave equation and its 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 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 “Models and Physical Properties Guide”, and the user may enter all three coefficients as pure component properties.

Fitting the vapour pressure curve For each component the constants are fitted by linear regression to the vapour pressure over a range of reduced temperatures corresponding to the stored data. Fewer than 5 coefficients will be fitted if there are insufficient data or if the extrapolation to low temperatures is unrealistic. If the vapour pressure is undefined, the correlation reverts to the standard equation for that component.

Mixing Rules 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 separate "Models and Physical Properties Guide".

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) model sets which use the Peneloux correction, fit a to the vapour pressure and use the Van der Waals 1-fluid mixing rules. RKSA with the Infochem mixing rules is 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.

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Models • 29

The API variant of RKS is applicable to petroleum systems and mixtures containing hydrogen, while RK 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.

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. In MF3.6 the CPA model may be used for hydrate calculations with methanol, ethanol, MEG, DEG, TEG and salt inhibition, as these are the only cases for which parameters are currently provided. Parameters for additional substances may be added in future versions of Multiflash.

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 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.

LCVM equation of state This model consists of the PRA equation of state with vapour pressure fitting, the Peneloux volume correction and the LCVM type mixing rules. The excess Gibbs energy is provided by the LCVM 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 number of common light gases found in petroleum fluids.

When to use LCVM The LCVM model is an extension of the Unifac method. It is intended to predict the phase behaviour of petroleum fluids mixed with polar compounds using the solution of groups concept as embodied in Unifac. The main benefit of LCVM is that it is better able to handle asymmetric mixtures. This is because it uses an equation of state with an excess Gibbs energy mixing rule that was specifically

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User Guide for Multiflash for Windows

designed to work with Unifac for mixtures of light gases and heavy hydrocarbons. The models discussed are all examples of cubic equations of state. Multiflash also includes several other equations of state that are non-cubic.

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 is 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 a manual. However, references where this information may be found are given in our "Models and Physical Properties Guide". 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. 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 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 Modeling 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 Modeling 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.

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Models • 31

Lee-Kesler-Plöcker (LKP) equation of state The LKP method is a 3 parameter corresponding states method based on interpolating the reduced properties of a mixture between those of two reference substances

When to use LKP The method predicts fugacity coefficients, thermal properties and volumetric properties of a mixture. However, it is rather slow and complex compared to the cubic equations of state and is not recommended for phase equilibrium calculations, although it can yield accurate predictions for density and enthalpy. It 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 BWRS equation method is an 11 parameter non-cubic equation of state. For methane, ethane, ethylene, propane, propylene, isobutane, n-butane, isopentane, 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 gives much more 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 owing to its complexity, it requires more computing time than the cubic equations of state.

Multi-reference fluid corresponding states (CSM) model The CSM 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 current model implementation includes reference equations of state for the following substances: argon, iso-butane, n-butane, CO, CO2 , ethane, ethylene, fluorine, H2S, hydrogen, methane, nitrogen, octane, oxygen, n-pentane, propane, water (IAPSW 95), xenon, helium, hexane, heptane, octane, ammonia, neon, propylene, R123, R152a, R124, R125, R134a, R22, R32, R11, R113, R114, R115, R116, R12, R13, R14, R23, and RC318. Hydrocarbons between pentane and octane are modelled as combinations of these substances. The equations of state are taken from various sources and do not all have the same quality or range of applicability.

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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.

Reference fluids The equations of state are taken from various sources and do not all have the same quality or range of applicability.

Applicability. The model is very accurate for pure substances that are included in the above list of reference substances. It is also applicable to near-ideal mixtures such as air but for the best results it is necessary to fit values of the binary interaction parameters to match experimental data. The model should not be used for nonideal mixtures such as water + CO2 etc.

Activity coefficient methods In an ideal liquid solution the liquid fugacity of each component in the mixture is directly proportional to the mole fraction of the component

f i l = xi f i *,l The ideal solution assumes that all molecules in the liquid solution are identical in size and are randomly distributed. This assumption is valid for mixtures made up of molecules of similar size and type, but for mixtures of unlike molecules you must expect varying degrees of non-ideality. The activity coefficient represents the deviation of the mixture from ideality, as defined by the ideal solution.

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

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.

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Models • 33

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

α

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

α ij = α ji

so only

α 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 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.

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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 separate "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 then pressures should be limited to 3-5 bar. If Redlich-Kwong 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 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.

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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 original Infochem model uses a modification of the RKS equation of state for the fluid phases plus the van der Waals and Platteeuw model for the hydrate phases. An alternative model uses the CPA model for the fluid phases. The hydrate models have also been extended to include hydrate structure H in addition to structures I and II. The model can explicitly represent all the effects of the presence of a range of thermodynamic inhibitors, although parameters for the CPA model are only provided for methanol, ethanol, MEG, DEG, TEG and salt. 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 H2 S.

•

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

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3,3-dimethyl-1-butene cycloheptene cis -cyclooctene adamantane ethylcyclopentane 1,1-dimethylcyclohexane ethylcyclohexane cyclohexane cycloheptane cyclooctane •

The thermal properties (enthalpies and entropies) of the hydrates and ice are included permitting 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 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 solubilities 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 hydrate equilibrium formation temperature at a given pressure or pressure at a given temperature where the first very small quantity of hydrate appears after a sufficiently long time. This point corresponds to the thermodynamic formation point, also known as the hydrate dissociation 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, with both RKSAINFO and CPA used to model the fluid phase, is capable of modelling this, 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. If this causes any difficulty it is possible to reproduce the CPA predictions with RKSAINFO by using parameters which reduce the Infochem mixing rule to van der Waals. These parameters, for methane, ethane and propane with water, are stored in the file vdwbip.mfc. They can be used to overwrite the existing BIPs for these binaries by loading this file after you have defined the hydrate model based on RKSAINFO as the fluid model.

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When compared to the available data all three possible variants (CPA, RKSAINFO with standard and vdw BIP) give hydrate dissociation results within experimental error.

Nucleation model The nucleation model was developed in collaboration with BP as part of the EUCHARIS joint venture. This model is an extension of the existing thermodynamic model for hydrates described above. In order to extend the nucleation model for use with the Multiflash program, 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. 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 existing Infochem hydrate model and the nucleation model, the hydrate formation and dissociation boundaries can be predicted between which is the hydrate formation risk area.

Inhibitor modelling Thermodynamic hydrate inhibitors decrease the temperature or increase the pressure at which hydrates will form from a given gas mixture. The original Infochem hydrate model includes parameters for the commonly used inhibitors: methanol, salts, and the glycols MEG, DEG and TEG and for the less well-tested inhibitors ethanol, iso-propanol, propylene glycol and glycerol. A new mixing rule was developed for the SRK equation of state to model the effects of the inhibitors on the fluid phases. The hydrate model using CPA to model the fluid phases is limited in the current version of Multiflash to hydrate calculations with pure water or with methanol, ethanol, MEG, DEG, TEG and salt. Additional parameters to extend the CPA model to cover the full range of thermodynamic inhibitors listed above may be included in future versions. The treatment of hydrate inhibition has the following features: •

38 • Models

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.

User Guide for Multiflash for Windows

•

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.

•

Two salt inhibition models are available. The older model is based on a salt component. The new model is a (restricted) electrolyte model. A salinity calculator tool is provided ( see “Salt calculator” on page 80 ) which allows the salt composition to be entered in a variety of ways. The salt component model expresses the salt composition in terms of an equivalent “salt component” present in INFODATA with the properties of sodium chloride. The Electrolyte model in versions of Multiflash up to and including 3.4 calculated the equivalent amount of Na+ and Cl- in the mixture. From MF3.5 the Electrolyte model was been extended to include K+ and Ca++ ions. This does not affect the equivalence calculations if the salt composition is entered as TDS (Total Dissolved Solid). If an Ion Analysis or Salt Analysis is supplied then any values entered for K+ or Ca++ will be allocated to those ions in the Composition table and the equivalence for any ions other than Na+, K+, Ca++ or Cl-will be expressed in terms of these four ions depending on cation valency, e.g. divalent cations such as Mg++ will be allocated to Ca++ The equivalent composition is based on experimental data for the freezing point depression and hydrate inhibition effect of salts.

•

The solubilities of hydrocarbons and light gases in water/inhibitor mixtures have also been represented.

The original binary interaction parameters for the RKSA fluid model stored for alkanes with MEG correctly reproduce all the reported data for alkane solubility in MEG, for MEG solubility in heptane and the inhibition effect of MEG on hydrate dissociation temperatures. However, the measured data for MEG solubility in alkanes was limited to a single data set for the solubility of MEG in heptane. For mixtures containing hydrocarbons greater than C7 the parameters in versions prior to MF2.9 predicted an increasing MEG solubility with increasing carbon number. This may have led to over prediction of the amount of MEG required to inhibit hydrates for heavy crude oils. Later versions of Multiflash include new parameters for alkanes with MEG which stabilise the solubility of MEG in higher alkanes and correctly reproduce the MEG inhibition effect on hydrates. However, they fail to predict the correct solubility of alkanes in MEG. The new parameters were included in the BIP databank, oilandgas4, which is still the current version.. If, for any reason, you wish to use the original parameters you can still retrieve these from the BIP databank, oilandgas3. For information on how to do this see “Supplementing or overwriting BIPs” on page 56.

Salinity Model The original salt model operates only on a sodium chloride equivalent basis. The model represents the effect of sodium chloride in aqueous solution by a special equation of state component called “salt component” or “saltcomp”. This model is designed to operate with the Advanced RKS equation although it is also usable with the Advanced PR equation. The salt component model cannot be used with the CPA model or any other equation of state. The electrolyte salt model is designed to be added on to any equation of state. The models selection form allows it to be selected for use with the Advanced RKS equation and the CPA model. From Multiflash version 3.5 it represents the

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Models • 39

salt as a combination of sodium, potassium, calcium and chloride ions. Future versions of Multiflash may extend this to other ions.

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. Multiflash includes two wax models, the Coutinho model and the Multisolid model. The features of the Coutinho model are: •

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

•

The model 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.

•

The model requires that the normal paraffins are explicitly present in the fluid model, as these are the wax forming components. The user must therefore use the PVT Analysis either 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 nparaffins 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 options in the Multiflash implementation is RKSA.

The Multisolid wax model is still available as an alternative option. The features of this model are: It approximates waxing behaviour by representing the wax as a mixture of separate phases, each one of which corresponds to a pure pseudo component. The composition of the wax phase is determined by the assumed thermodynamic properties (normal melting point, enthalpy of fusion, etc.) for the petroleum fractions heavier than C6 but which are not asphaltenes or resins. The multisolid wax model should be used in conjunction with conventional cubic equations of state. The default option from the usual model set selection is RKSA but the multisolid model with PRA may be accessed from the waxpra.mfc file. The original Infochem oil characterisation method should be used in conjunction with the Multisolid wax model, as the model was set up to be compatible with this method. We recommend that the C6+ fraction of the oil should be represented by 15 pseudo components and the RKSA model should be selected.

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

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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 asphaltene model is based on the RKSA cubic equation of state with additional terms 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

K AA and K AR . The remaining components are described by the van der Waals 1-fluid mixing rule with the usual binary in teraction parameters k ij so constants:

the asphaltene model is completely compatible with existing engineering approaches that are adequate for describing vapour-liquid equilibria. The model is a computationally-efficient way of in corporating complex chemical effects into a cubic equation of state.

Other thermodynamic models Multiflash also incorporates a corresponding states method for estimating the density of liquid mixtures, the COSTALD model. 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.

Transport property models For each of the transport properties, viscosity, thermal conductivity and surface tension, Multiflash offers two 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

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. It is the default viscosity model for use with equations of state. 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 nomograph for kinematic viscosity plus a mixing rule for blending oils. It is only applicable to liquids.

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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 Lohrentz 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. 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 Pedersen 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

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 ω 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 mo dels 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

Macleod-Sugden method This method predicts the surface tension of a liquid 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. 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 diffusion coefficients. It is an empirical modification of Chapman-Enskog theory.

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

How to specify models in Multiflash There are three ways to specify models in Multiflash. The model can be defined as part of a complete problem setup file, extension .mfl. Loading a complete problem setup file was described earlier as part of the simple tutorial, “Loading a problem setup file” on page 21. The model can also be easily specified through the menu options (as a “Model set” or applying freeze-out components to model pure solid phases such as ice) or by loading a model configuration file, extension .mfc, which defines the model only, not the complete problem.

How to load a model Loading the model definition is usually the first step in setting up a problem. The model may be specified from the menu or by loading a model configuration file or it may be included in the problem setup file.

Defining the model from the menu To define the model from the menu bar first Select Select, then Select Model set and click on your chosen model. If this is the fist time it has been called in any Multiflash run the Select Model Set dialog box will appear

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Models • 43

with its default option set to Cubic Equations of State. The dialog box includes a list of the thermodynamic models, a list of possible phase descriptors, a choice of model sets for the transport properties and the ability to include diffusivity calculations.

Models for other options, such as •

Non-cubic EOS models

•

Activity Models

•

Hydrates

•

Waxes

•

Asphaltenes

•

Combined solids

•

Mercury

may by defined by clicking on the appropriate tab when similar dialog boxes will be activated. The Select Model Set will reflect, as far as possible, the model setting in the problem file loaded. The default for a model, other than the activity models, is to include four phase descriptors, gas, liquid1, liquid2 and water and use the Pedersen. CLS and Macleod-Sugden set of transport property models. You may select any of the other sets of transport models if you wish, or None if you don’t wish to calculate any transport properties. You can also 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 defining only gas and liquid1 may speed up calculation . Once you are satisfied with your model selection click the Define Model button. The model should load immediately and you will see a message to confirm this, for example:

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The message that the model has been successfully loaded will also be displayed in the results window and sent to the log file. Click OK to go back to the model definition dialog box, and then click ‘Close’ to return to the main Multiflash window. To redefine models, repeat the steps as described above. The previous model set will be overwritten automatically. Each model set contains a complete definition of the thermodynamic and transport property models and a list of phases, including key components for that phase.

Defining the model from a model configuration file If you prefer you can load a model definition from a model configuration file, extension .mfc. Model configuration files are supplied for all the model options available in Multiflash. Each one contains exactly the same information as the Model set, with the addition that the pure component data source is preset to be INFODATA. Like all Multiflash input files the model configuration file may also include comments. The content of the .mfc file is displayed in the results window. If the model is not loaded successfully for any reason a message box appears and detailed error messages are displayed in the results window.

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 comp onent 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. 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;

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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; With the model successfully loaded you can continue to s pecify other aspects of the problem such as the components and conditions. For more information on commands see the Appendix to the printed User Guide.

How to change a model You may wish to compare the predictions from one model with those from another. This is easily achieved. Each model set or model configuration file contains the commands to remove all currently defined models, phase descriptors and BIPs. For example, all you need to do to change from using Peng-Robinson to RKS is to Click on RKS in the Select Models dialogue box or Load the RKS.mfc file. Either of these actions 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 default method for calculating the transport properties.

Loading hydrate models If you define hydrate models using model configuration files such as hydrate.mfc and hydrateh.mfc, a complete set of models and phases descriptors will be specified. However, specifying hydrate models through the Select Model Set option allows selection of the different forms of hydrate individually. To define a hydrate model first Select Select/Model Set from menu bar. Select Hydrates tab to activate the Hydrates dialog box.

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The user can define the fluid phase model to be used. The RKSA(Infochem) fluid phase model has parameters defined for several inhibitors, including salt calculated using the original Salt Model, see “Modelling hydrate formation and inhibition” on page 36. The CPA-Infochem fluid phase model was introduced to improve predictions of inhibitor partitioning, with parameters provided for methanol, MEG, DEG and TEG. From MF3.5 model parameters have been added for ethanol, which have been tested using hydrate inhibition and such data for ethanol partitioning as are available. Two Salt models have been implemented. The latest Salt Model , based on an electrolyte approach, can be used with either RKSA(Infochem) or CPA as the fluid phase model and is accessed by selecting the “+ Electrolyte” options. Both Salt Models are described in “Modelling hydrate formation and inhibition” on page 36. The default setting is to include models for both 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 containing high proportions of methane or H2 S, where the most stable hydrate structure can change from Hydrate 2 at low pressures to Hydrate 1 at high pressures. If you also want to consider Hydrate H being formed you should select this as an additional option. Calculation of the hydrate dissociation temperature and pressure will be quicker if you exclude the hydrate nucleation option. However, If you also want to include the nucleation model select Phase Nucleation from the list of phase descriptors. A new option for MF3.6 is to be able to calculate the formation of chloride scales. This is only available with the Electrolyte model and the selection of Chloride scales will not be allowed if the RKSA(Infochem) or Association (CPA-Infochem) model is selected. If Chloride scales is selected then the appropriate phase descriptors will be added automatically, based on the component formula, e.g. NaCl, NaCl.2H2O, KCl, CaCL2, CaCl2.2H2O, CaCl2.4H2O or CaCl2.6H2O. As before Click the Define Model button and a message saying the model has been successfully defined will appear. Click OK for the model and phases specification to be reflected in the main window.

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Click the Close button to get back to the main screen. You can check which models are defined at any stage by using the Tools/Show/Models menu option. The models currently available will be listed in the results window.

The Freeze-out model The freeze-out model can be applied to any component. To apply the model to a component in a stream you need to define the component using the Select/Freeze-out component option. To do this Click on Freeze -out components from the Select menu This will produce a window displaying the components in the stream, e.g.

Checking the adjacent box allows you to add or remove the freeze-out model for any number of components within the overall limit on the total number of phases, which is twenty. A message box will appear to confirm that the model has been defined for the selected compound By default this sill be “solid” plus the compound name except in the case of water where it will be ice, e.g.

A similar message box appears if the compound is deselected.

How to define a wax model There is a choice of wax models since the introduction of the Coutinho model, described in “Modelling wax ” on page 40. Our recommendation would be to use the Coutinho model. However, the Multisolid model remains available for backward compatibility. In principle the Multisolid model can be based on any of the cubic equations of state to model the fluid phases by users writing their own model description but the default option uses RKSA as model parameters have been optimised with RKSA. However, for users who wish to use PRA as the fluid model we have retained the waxpra.mfc file. The wax model may be specified interactively or by loading the wax model configuration file, waxrksa.mfc (for the Multisolid model) or wa xcouth.mfc (for the Coutinho model). Either may be included in a problem setup file.

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To define the wax model in Windows interactive program: Click the Waxes tab to activate the waxes dialog box.

A message box will indicate that the model has been successfully defined.

How to define the asphaltene model The asphaltene model is defined in the same way as equations of state or the hydrate model; select the Asphaltenes tab in Select Model Set , click on Define Model and you will see a message box to say the model has been successfully defined. This will be reflected in the main window. For more information on how to make best use of the asphaltene model see “Case studies – Asphaltene flocculation” on page 227.

Combined Solids Model In earlier versions of Multiflash a user could append the multisolid wax model to a fluid phase or solid model, such as hydrates or asphaltenes. This allowed users to determine, for any flash condition, whether wax and hydrate (or wax and asphaltene) could form and how the presence of one solid might affect the formation of the other. In later versions we have a new tab, Combined Solids.

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The new tab allows you to specify any combination of wax, hydrate and asphaltene solid phases. To avoid over-complication specific models have been chosen for each of the phases, based on what we believe to be the best option. The fluid phase model is RKSA, as this is basis of tuned parameters for the other solid phases. The salt model is based on the Electrolyte approach. The wax model is Coutinho and there is currently only one choice of asphaltene model. If you define only one type of solid you will be asked to use the individual solid model tab.

Troubleshooting - models We have tested the Multiflash software as far as is possible and corrected any errors we have found, if you find any we haven’t please report them to us. However, there is also a category of problems, which we have run into ourselves, that can usually be resolved by the user. Some of those related to models are discussed below, others will be outlined in the relevant section.

Incorrect path All the supplied problem setup files and model configuration files will be copied to the installation directory. However, it is possible that you will wish to create a working directory containing your own problem files, which is different from the program installation directory. In this case you should copy the .mfc files into your working directory or always refer to any .mfc files using the full path name. For example a sample file may contain the command include pr.mfc which should read in the contents of the pr.mfc file. If the pr.mfc file is not in the same directory as C4C5a.mfl then you will see a message of the type

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where the pathname refers to your working directory, and the message in the main window will be: remove all; include pr.mfc *** ERROR 402 *** Cannot open this file. If the installation directory was c:\MF then the correct command would be, include "c:\MF\pr.mfc"; You should either change the .mfl file or copy the .mfc files to the designated directory to correct the problem.

Model is not licensed You may not have licensed all the possible model options. The standard Multiflash licence includes equations of state and activity models, but not the CSMA, IAPWS-95, PC-SAFT, hydrate, wax, asphaltene or mercury models, which are separate options. The models that are not available will be disabled.

Groups not available for UNIFAC model The UNIFAC model generates the binary interaction coefficients 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

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

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for the UNIFAC VLE model set is defined with a separate gas phase model and isothermal flashes will apparently work with the stream being reported as all gas. In MF3.6 users can now add their own user-defined UNIFAC groups, although there are no specific menu options. You would have to enter commands through the command box – and information on the commands can be found in the 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, with the exception of UNIFAC, LCVM and PSRK BIPs which are predicted by group contribution. 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.

Number of BIPs related to any model Different models require differing numbers of BIPs and these may or may not have physical significance. The cubic equations of state (RK, RKS, PR) require only a single BIP. The closer the binary system is to ideality the smaller the size of the BIP, which will be zero for ideal systems. It is unlikely that the value of the BIP will be greater than 1, although it is possible for it to be negative. For LKP 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. For the Infochem mixing rule 3 BIPs are needed. 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. 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 non-randomness factor, α . With the exception of α , 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 α 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 bipset; that is symmetrical, dimensionless and the default value is zero.

Units for BIPs The BIPs for equation of state methods are dimensionless, with the exception of two of the CPA association parameters. For some of the activity coefficient

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models they are dimensioned with the exception of Wilson A, Regular Solution and Flory Huggins. The dimensions used 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 selected 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 VLE and LLE BIPs. The “Aspen” format allows you to transfer the BIP values for the NRTL equation produced from Aspen Plus without further change.

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 α parameter.

BIPs available in Multiflash Our main BIP bank is applicable to oil and gas processing operations and called appropriately, OILANDGAS. This contains BIPs for the cubic equation of state models PR, PRA. RKS, RKSA and RKSA (Infochem) for hydrocarbons, water, methanol, glycols, H2 S, CO2 and N2, and for the CPA model. The correlations have been extended to include BIPs for the LKP model for hydrocarbon and light gas mixtures. Multiflash also provides methods for estimating BIPs for the PR, RKS and CPA equations for mixtures that include petroleum fractions. The INFOBIPS BIP bank includes BIPs for the WilsonE and the VLE variants of NRTL and UNIQUAC, based on the data reported in the Dechema Chemistry Data Series. Some BIPs are also available for equations of state such as BWRS, LKP and the cubics for systems not covered by the normal correlations, e.g. binaries including ammonia. Additional BIPs are added to INFOBIPS as time and data are available. 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 vary, Multiflash applies the correct group structure to match the chosen model. We have created an Infochem version of DIPPR that includes the same group information. You will see a warning message if the group contributions are missing for any chosen component.

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It is possible to have two BIP banks in force for any problem. All the .mfc model descriptions for the equations of state define both INFOBIPS and OILANDGAS. When the model is defined Multiflash will first search OILANGAS then INFOBIPS and 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. Between releases of Multiflash we may amend, or add to, the BIPs stored. For the OILANDGAS databank we will supply a copy of the previous banks in order to maintain backward compatibility. The current version will still be known as OILANDGAS4, and also the default, OILANDGAS. Previous versions such as OILANDGAS3 can be recalled if you wish, for details on how to do this see “Supplementing or overwriting BIPs” on page 56. For INFOBIPS we intend only to issue the latest version, if users wish to maintain access to earlier versions they should retain a copy of the relevant file.

Viewing BIP values You can look at the values of any BIPs used in Multiflash calculations, including those from the OILANDGAS databank and any supplied by the user. To do this Load a problem setup file or Define your model and mixture Select BIPs from Tools option in the menu bar. A dialog box for Show BIP Values will be generated. From there you can easily view or edit any BIP values and temperature dependence of BIPs. The BIPs values can be reported on the results screen by clicking on the Write to Output button. To take a particular example: Using Select/Component from the menu, select water and decane from INFODATA. Specify a model. Initially define the RKS model using the Select Model dialogue box. Select Tools Select BIPs and click on it The show BIP values dialog box is activated.

Select a BIP set in the dialog box. Click edit to view or change BIP values and temperature dependence of BIPs.

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Click on Write to Output button, the output will be shown in the results window: show bipset RKSBIP; BIPSET: RKSBIP COMPS ORDER VALUES 1 2 0 0.5055762 TEMPERATURE FUNCTION: EOS

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, RKSABIP, RKSABIP3 (for RKSA + Infochem mixing rule ), LKPBIP, WILSONBIP2, NRTLBIP3, UNIQUACBIP2 for VLE 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. If you define your own problem setup file you can name the BIP set as you wish. The BIP descriptions changed in Multiflash 3.4. The components are referred to by the number they are assigned in Multiflash, i.e. the sequence in which they appear in the comp onents 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 temperature dependence and 2 that it has quadratic temperature 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 BIPs (ASSOCBIP) will be labelled EOS and the second (ASSOCBIP-2) will be labelled Association. Multiflash will check that the correct temperature function is used for the model selected. The final element is the Unit for the BIP. For dimensionless BIPs this will be none, for stored VLE BIPs for activity models this will be J/mol, but if you have entered your own BIPs in different units it will reflect the units chosen. If you replace decane with methanol and re-define the model to be RKSA (Infochem) which has a non-standard mixing rule the BIP display will be more complicated. The use of an NRTL type mixing rule means this model requires two asymmetric parameters and one symmetric parameter.

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In this case the water methanol asymmetric parameters have also been fitted with linear temperature dependence. If you are using defined model sets or model configuration files then there will only be one set of BIPs for any problem. If you define a model set using two different models e.g. an equation of state in the vapour phase and an activity model in the liquid phase you may have two BIP datasets, one for each model.

In this case highlight the BIP dataset of interest.

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, you have specified and supplied BIPs in the correct units.. 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 not surprising. 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 10 bar has been reduced from 376.3K to 364.3K.

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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. The user supplied BIPs are not deleted if there is a model variation introduced by returning to the Select Model Set option, within certain limits. The BIP values will be retained unless a model is selected that does not use that BIP set. So, if you simply want to change the number of phase descriptors for a model 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 can create this file using the BIPset table or write the file with a text editor using standard commands. You should refer to the “Appendix - Multiflash Commands” on page 253. The default names for the model bipsets are the model name followed by BIP e.g. PRBIP, the number of BIPs, the temperature dependence, the temperature function, the units, components and BIP value. 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 ;

If you define the bipset incorrectly, e.g. 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 bipset PRBIP 1 constant activity J/mol butane pentane 0.01 ; *** ERROR

584 ***

Existing BIP set uses a different temperature function You might wish to create a file containing BIPs for a single binary pair but for various models. For example for our acetone water phase equilibria case study, see “Polar systems ” on page 188. This system acetone/hexane in this section provides a good example of BIP units. There are no BIPs stored in INFOBIPS but there are reported values in the Dechema Data Series. For the WilsonE model the values are 1067 and 326.8 cal/mol. To enter these go to the BIP display and click on Units. In the supplementary window select cal/mol and click on OK

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The Dechema values can then be entered directly.

If the Units are not changed then the BIP values must be entered as 4460 and 1366 J/mol. Changing Units when BIPs are already displayed will automatically change the units. If your file of BIPs contains a range of binary pairs, some of which contain components not in your current stream you will see a warning message for each such pair: *** WARNING

-223 ***

One or more components in this BIP set are not currently defined but where the components are in the stream the new BIPs will be loaded and included in any subsequent calculations. You should note that if you change the mo del such that a different BIPset is required then your input file of BIPs will be overwritten by the standard BIPdatabanks included in the model definition and you will need to reload your BIP file. If you wish to replace the current OILANDGAS databank with a previous version, for instance to reproduce earlier results, this is done using the Tools/Command function. You first delete the bipset for your model and then replace the databank, e.g. bipset RKSABIP3 erase; bipdata oilandgas3; will reinstate the BIP databank issued with versions 2.7 and 2.8. You can check which BIP banks are in force using the Tools/Show/BIP databank option.

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Troubleshooting - BIPs The methods of displaying and editing BIPs should limit the problems you are likely to encounter. There remain two possibilities:

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 Chemical 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-1 mol-1 ) for use in Multiflash or the BIP units must be set to K.

Order of components Equations of state with standard mixing rules usually have a single symmetric BIP. However, activity coefficient models have two asymmetric BIPs. It is important that you enter both asymmetric BIPs and in the correct cells so that the binary pair is consistent with BIPs supplied. If you do not use the Multiflash BIPset grid but use commands, either via an input file or Tools/Command then the following problems may occur.

Number of BIPs for the model When you are supplying BIPs it is important that you define and supply the correct number of BIPs. Wilson A, Wilson E and UNIQUAC require 2 BIPs. If you only specify 1 after the bipset name a warning message will appear; Model has the wrong no. of BIPs for the existing set A similar message will appear if you only specify 2 BIPs for NRTL which requires 3. The other possibility is that you specify the correct number of BIPs for the model, but fail to supply them all. The result will depend on the model and the default BIPs set. In some cases there is a generic default. If you specify NRTL with 3 BIPs and only supply 2 the default is to set the third parameter to 0.3, for NRTL VLE and to 0.2 for NRTL LLE. The next default level is to set the missing BIPs using internal databanks. Finally, the default is to set the BIP to an internal modeldependent default value. If you specify UNIQUACBIP to have 2 constant BIPs and supply only one BIP value before the end marker, the second BIP will be set to 0.0, and no warning message will be issued. We recommend that you always check your supplied BIPs using the Tools/BIPs option.

Naming the components If you forget to enter the component names for a binary pair before the BIP values you will upset the sequence of the command and the likely error message will be

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unrecognised keyword

BIP databank The name of the main BIP databank for equations of state is OILANDGAS and this is the name included in all relevant model sets and .mfc files. However, when you save a problem set-up the .mfl file will include the full name of the BIP bank used at the time. For Multiflash versions 2.4 and 2.5 this is OILANDGAS2, for version 2.6, 2.7 and 2.8 it is OILANDGAS3 and for 2.9, 3.0, 3.1, 3.2 3.3, 3.4, 3.5 and 3.6, OILANDGAS4. If you wish to update earlier input files you can edit them using a text editor or change the BIP bank as described above, “Supplementing or overwriting BIPs” on page 56. For WilsonE, NRTL, UNIQUAC and refrig.mfc the default databanks are INFOBIPS (VLE variants) or INFOLLBIPS (LLE variants).

BIPs not displayed This should no longer occur as we have increased the number of BIPs which can be displayed. If you do still encounter problems please let us know. In the unlikely eventuality that a particular BIP is important to you and is not displayed you can set up a new problem file containing only the pair/pairs of components of interest and display the BIPs for this smaller dataset.

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Components

Introduction Multiflash recognises three 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. Finally, a condensed component is a pure solid phase, in Multiflash these are used in chemical equilibrium calculations only. 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 about 242 compounds and is always supplied as part of Multiflash. DIPPR, produced under the auspices of AIChE, currently has data for over 1800 compounds, but 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 65.

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

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Standard Ideal Gas Enthalpy of Formation at 298.15K 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 Normal databank components will be TYPE 1, petroleum fractions will normally be TYPE 12.

Temperature Dependent Properties Solid Density Liquid density Vapour Pressure Enthalpy of vaporisation 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. For some components coefficients will also be stored for the second virial coefficient, relative permittivity and SAFT bond fractions used for model calculations. 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

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properties from either bank behave sensibly beyond the temperature limits of the stored correlation. INFODATA contains only a limited range of components, about 242, mainly suitable for oil and gas processing. 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.

Condensed components The INFOCOND databank includes about 140 solid compounds. The data are limited to molecular weight, normal melting point, normal boiling point, the enthalpy of formation in the condensed state at 298.15K, the standard entropy in the condensed state at 298.15K and the heat capacity in the condensed state as a function of temperature. It is intended for use with Infochem's chemical and phase equilibrium module, Chemreact.

Petroleum fractions Petroleum fractions are discussed in detail in “Petroleum fractions” on page 85. To define a single petroleum fraction the program requires certain characteristic properties and Multiflash will then estimate the other properties needed to support the range of calculations available in the program. The list of possible properties to support characterisation of the fraction are: Carbon number Molecular weight (g/mol) Specific gravity at 60o F relative to water at 60o F Normal boiling point Critical temperature Critical pressure Pitzer’s acentric factor However, not all of these are necessary. The minimum input sets are molecular weight, molecular weight and specific gravity; molecular weight and boiling point; boiling point and specific gravity; critical temperature, critical pressure and acentric factor. Alternatively the boiling point can be used instead of the acentric factor. This input data can be supplied by the user for each fraction or provided on the basis of a specified carbon number only. This is described in “Defining petroleum fractions” on page 85. The properties that are estimated, if they have not been provided, are: Carbon number Molecular weight Normal boiling point Critical temperature Critical pressure

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Components • 63

Critical volume Acentric factor Parachor Dipole moment Enthalpy of formation Standard entropy Perfect gas Cp Saturated liquid density Saturated vapour pressure Enthalpy of evaporation Liquid viscosity

Defining a mixture (stream) in Multiflash Multiflash calculates the properties of one stream (defined mixture) at a time. Most users of the interactive program will only be using a single stream throughout their calculations. However, a more recent option is the ability to define stream types, subsets of the original stream which contain only a defined sub-set of the original components. However, each stream type can be defined by a different model. For more information see “Stream types” on page 76. The original stream can be defined in two ways, either through the Select Components route or via the PVT Lab Fluid Analysis. Using Select Components the stream is defined by first choosing the data source; the databank which will be used to generate the pure component physical properties and then choosing which components in the mixture you wish to be taken from that source. More than one source can be used to define the stream. A common problem in oil and gas processing might require a choice of hydrocarbons from INFODATA and several petroleum fractions defined using the petroleum fraction correlations. Alternatively, some users may wish to define the stream using the fluid analysis report they have received from a PVT laboratory. Typically the fluid analysis will comprise a series of defined pure components (discrete components) and a series of pseudo components. The method of doing this is described in detail in “Petroleum fractions” on page 85. The maximum number of components in any mixture in the current version of Multiflash is 200. Defining the pure components in a stream will be discussed below.

Specifying the data source To define the data source first Select Select from the menu bar, then Select Components or click on the select components button,

.

Either will activate the Select Components dialogue box

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with the Infochem fluids databank as the default databank. To choose any of the other data sources click on the button to the right of data source. Select the chosen databank and this will appear in the data source text box.

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 scrolling through a list Choose the All Components option button and a list of the standard bank names for the compounds stored in the chosen data source 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.

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Components • 65

The component is then selected for use in Multiflash either by double clicking on the name or by selecting the name, which will then be highlighted and then clicking on the Add button. The name of the selected compound will then appear in the “Components selected in Multiflash” text box. Further components may be selected the same way.

Select components by name If you know the name or synonym under which a component is stored then firstly ensure the Name option button is selected, then type the name of the component in the text box. The name may be entered in upper or lower case.

The component will be transferred to the selected set by pressing the enter key after the name or by clicking on the Add button. 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 other names (synonyms) stored in the bank for that compound.

Synonyms To check the list of synonyms stored in a data source for any component type the name in the Enter name text box, make sure the Synonyms option button is selected and click on 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 select 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). In the list used to illustrate this facility we have chosen the DIPPR databank. One of the synonyms available is the DIPPR databank

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number for the component, in this case 1201. 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 text 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. In the current version of the program you may 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 names ending in methane and *methane* finds all names containing methane. Entering methane alone searches for an exact match.

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Components • 67

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 enter 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

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: Select the compound as usual by entering the name or highlighting it in a list Highlight (by clicking on it) the compound in the Multiflash list above which you wish to insert the new component

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Click on the insert button.

Deleting a component If you wish to remove a component from the selected list then, in the Compounds selected in Multiflash text box: Select the component, then Click on the Delete button The compound 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 163 in the printed User Guide. 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.

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Components • 69

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:

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whilst for temperature dependent properties the coefficients of the correlation equation will be displayed.

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. 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 We do allow you to add user-defined pure component data, 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 you need for your chosen model and properties, including the coefficients for the temperature dependent correlations. This option is not recommended for petroleum fractions, where you should choose the “Infochem petroleum fraction correlations” option, which allows you to enter minimum data and estimates the remaining properties. If you need to add a new pure component from the Data Source select Userdefined component from the drop-down list, then give the component name in the text box and press Enter.

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Components • 71

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, selecting your user-defined component and the Edit option, choosing to edit Constants initially. The Pure component data bank will appear with the Value column empty.

To define your component simply enter the value for the listed property, ensuring that you use the correct units, e.g.

To enter a temperature dependent property, such as the ideal heat capacity, choose the property keyword (in this case CPIDEAL) and click on Edit, or double click on CPIDEAL. A form will be displayed defining the component name and property but with the remaining text boxes blank.

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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 Multiflash Command Reference Manual, 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. The minimum data required to perform any phase equilibrium calculation with Multiflash using the basic cubic equations of state are critical temperature, critical pressure and acentric factor for each component. The advanced versions of the cubic EOS require the liquid density and vapour pressure correlations for the pure components. The data requirements are model dependent and models other than the cubic equations of state require additional data. Isenthalpic and isentropic calculations also require coefficients for the ideal-gas heat capacity for each component. To work in mass units rather than in molar units the molecular weight is needed.

Models and input requirements The minimum requirements for phase equilibrium and transport property calculations for the models available in Multiflash are given in the following Table.

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Components • 73

Model Thermodynamic RKS

RKSAPI

RKSA

RKSAINFO

PR

PRA

PSRK

LCVM

LKP

BWRS

CSM

CPA

PC-SAFT

Minimum input critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), ideal gas Cp (CPIDEAL) critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), ideal gas Cp (CPIDEAL) critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), vapour pressure (PSAT), saturated liquid density (LDENS), ideal gas Cp (CPIDEAL) critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), vapour pressure (PSAT), saturated liquid density (LDENS), ideal gas Cp (CPIDEAL) critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), ideal gas Cp (CPIDEAL) critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), vapour pressure (PSAT), saturated liquid density (LDENS), ideal gas Cp (CPIDEAL) critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), vapour pressure (PSAT), saturated liquid density (LDENS), ideal gas Cp (CPIDEAL) ), UNIFAC subgroup structures (UNIFAC) critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), vapour pressure (PSAT), saturated liquid density (LDENS), ideal gas Cp (CPIDEAL) ), UNIFAC subgroup structures (UNIFAC) critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), ideal gas Cp (CPIDEAL) critical temperature (TCRIT), critical pressure (PCRIT), critical volume (VCRIT) and acentric factor (ACENTRICFACTOR) for components not in model database, ideal gas Cp (CPIDEAL) critical temperature (TCRIT), critical pressure (PCRIT), critical volume (VCRIT), ideal gas Cp (CPIDEAL) critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), vapour pressure (PSAT), saturated liquid density (LDENS), association parameters (ASSBE, ASSEP, ASSGA), ideal gas Cp (CPIDEAL) critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), PC-SAFT parameters (SAFTEK, SAFTSIGMA, SAFTM, SAFTKAPPA, SAFTEPSILON, SAFTFF), ideal gas Cp (CPIDEAL). Note (1) TCRIT, PCRIT and ACENTRICFACTOR are necessary 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.

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Ideal solution NRTL Wilson E UNIQUAC

UNIFAC

Dortmund Modified UNIFAC Regular Solution

Flory-Huggins

Ideal gas RK Hayden O’Connell

Freeze-out

Hydrate Wax

Asphaltene

vapour pressure (PSAT), saturated liquid density (LDENS) vapour pressure (PSAT), saturated liquid density (LDENS) vapour pressure (PSAT), saturated liquid density (LDENS) vapour pressure (PSAT), saturated liquid density (LDENS), surface and volume parameters (UNIQQ, UNIQR) vapour pressure (PSAT), saturated liquid density (LDENS), UNIFAC subgroup structures (UNIFAC) vapour pressure (PSAT), saturated liquid density (LDENS), UNIFAC subgroup structures (UNIFAC) vapour pressure (PSAT), saturated liquid density (LDENS), solubility parameter (SOLUPAR) and molar volume at 25°C (V25). vapour pressure (PSAT), saturated liquid density (LDENS), solubility parameter (SOLUPAR) and molar volume at 25°C (V25). ideal gas Cp (CPIDEAL) critical temperature (TCRIT), critical pressure (PCRIT), ideal gas Cp (CPIDEAL) critical temperature (TCRIT), critical pressure (PCRIT), radius of gyration (RADGYR), dipole moment (DIPOLEMOMENT), HaydenO’Connell association parameter (HOCASS), ideal gas Cp (CPIDEAL) 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) critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), vapour pressure (PSAT), saturated liquid density (LDENS),

Transport properties Pedersen

Twu

LBC Lohrentz-Bray-Clarke

CLS Chung-Lee-Starling Macleod-Sugden

User Guide for Multiflash for Windows

critical temperature (TCRIT), critical pressure (PCRIT), molecular weight (MOLECULARWEIGHT) molecular weight (MOLECULARWEIGHT), boiling point (TBOIL), vapour pressure (PSAT), saturated liquid density (LDENS) critical temperature (TCRIT), critical pressure (PCRIT), critical volume (VCRIT), dipole moment (DIPOLEMOMENT), molecular weight (MOLECULARWEIGHT) critical temperature (TCRIT), critical volume (VCRIT), dipole moment (DIPOLEMOMENT), molecular weight (MOLECULARWEIGHT) parachor (PARACHOR)

Components • 75

Costald

Liquid viscosity mixing rule Vapour viscosity mixing rule Liquid thermal conductivity mixing rule Vapour thermal conductivity mixing rule Surface tension mixing rule Diffusivity – Fuller's method Diffusivity - HaydukMinhas method

critical temperature (TCRIT), critical pressure (PCRIT), acentric factor (ACENTRICFACTOR), saturated liquid density (LDENS) liquid viscosity (LVISC) vapour viscosity (VVISC) liquid thermal conductivity (LTHCOND) vapour thermal conductivity (VTHCOND) surface tension (STENSION) molecular weight (MOLECULARWEIGHT), chemical formula (FORMULA), UNIFAC subgroup structures (UNIFAC). critical temperature (TCRIT), critical molar volume (VCRIT), normal boiling point (TBOIL), parachor (PARACHOR) dipole moment (DIPOLEMOMENT), saturated liquid density (LDENS), 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 For certain applications you may wish to define one or more sub-streams formed from selected components of your overall input stream and describe these using different models. Typically, this will be when you are using Multiflash to define input data for a third party application such as a simulator where the composition of the fluid will be changing for various unit operations. As the composition changes different models may be more appropriate for different streams. 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 Multiflash for Windows. However, in the interactive version of Multiflash the composition of a substream cannot be changed without altering the composition of the overall stream and it is difficult to define a realistic practical application for the interactive version. Initially we have defined an input stream containing 4 hydrocarbons, a petroleum fraction, water and MEG and supplied a composition.

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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. If you have not defined a Model Set prior to activating the Select Stream Type option then you may wish to choose All components and call this your overall stream type.

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If you had already chosen a model using the Select/Model Set route before activating the Define Stream Type text box then creating an additional stream containing All components and using the same model may be superfluous. In addition to any new stream defined at this stage the program will also define an "Original" stream type containing this information. Click on Define Stream Type and on OK to define the stream. To check defined streams use the Tools/Show/Stream types option. show Sts; NO. OF STREAM TYPES 1

1

OVERALL

A second stream can be defined the same way. This time we have selected the NRTL VLE model and defined a stream called MEG containing only water and MEG

We can check this as before 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.

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When you wish to carry out calculations relating to a particular stream, activate that stream using Select/Stream Type/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 38. The most common are methanol, ethanol, MEG, DEG, TEG and salt. Any of these can be included in the component list and their composition defined as shown in “Specifying compositions” on page 115. 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 The Inhibitor Calculator was included to make the addition of inhibitors easier. The Inhibitor Calculator is designed to calculate the amount of inhibitor or inhibitors to be added to the amount of water present in the stream to reach a user defined inhibitor concentration. This concentration may be defined in mass, mole 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 compositions. To calculate the amount of methanol, ethanol or glycols relevant to the amount of pure water present in the stream first Select Inhibitor Calculator from Tools menu bar. The tabbed dialog box for water, alcohols and glycols will be activated.

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Components • 79

Select a databank from which the components are defined. If you have already defined the amount of water in the stream it will be reflected in the dialog box. Otherwise, you must first enter the amount of water to which the inhibitor is to be added (in the correct units) so that the boxes for methanol, MEG, DEG and TEG can be enabled. Then specify the required concentration in water of inhibitor or inhibitors in mass %, mole % or volume % at standard conditions. Standard conditions are normally considered to be 1 atmosphere and 60 DegF or 15 DegC.

Click Add button. Then the equivalent amount of inhibitor relevant to the amount of water will be added automatically to the list of compositions of the stream. Click Close button to return to the main window. Note that you can calculate the amounts of any combination of the four inhibitors to be added to water but the total percent of the inhibitors must be less than 100. 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 Freeze-out model” on page 48.

Salt calculator For the original salt model one of the components in the INFODATA databank is called “salt component”. This is used to represent a salt pseudo component, based on a sodium chloride equivalence, for use in calculating freezing point depression or hydrate inhibition, see “Calculations with inhibitors” on page 210 . For the new salt model, which is based on defining ions and not a salt pseudo component, the ions Na +, K+, Ca ++ and Cl- have been added to the databank. More ions may be added in future versions. To determine the inhibition effect of salt using either salt model you need to provide a description of the salt content of the aqueous stream but often the data for the salt, brine or formation/production water ion analysis will not be available to you in the appropriate units. To help generate the data in the form

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required by Multiflash we have an extended Tools/Inhibitor Calculator menu option. The water content of your stream can be defined from Select Components and the Composition box, or entered in the Water text box under the Water/Alcohols/Glycols tab as described above. Depending which salt model you wish to use you can either include the salt component or the Na +, K+ , Ca ++ and Cl- ions in your component list by selecting from INFODATA. However, an easier approach is to select either the “Salt Component Model” or the “Electrolyte Model” tab from the Inhibitor Calculator. With the appropriate tab selected you can define your salt from: an Ion Analysis table

a Salt Analysis table

or the total amount of dissolved solid.

Clicking on the Add Salt button then initiates the calculation of either the amount of salt component (Salt Component Model) or the amount of Na+, K+, Ca++ or Cl- ions (Electrolyte model) to be added to the stream in the units defining your composition

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Components • 81

The amount of either salt component or the ions will also be entered in the Composition drop-down table. If you try to add more salt than is physically realistic you may generate an error message such as

You will also generate error messages if you define only negative or positive ions in an Ion Analysis table.

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 Salt button. If you are carrying out fixed phase fraction flashes with a salt component present in your stream and a low ratio of water to salt you should allow for the possibility of a separate solid salt phase forming. To do this you should set up a freeze-out model for the salt component using the Select/Freeze-out Components option, see “Solid freeze -out model” on page 35. The "Salt Component " option can only be used with the RKSA(Infochem hydrate model. If a salt component is present in the components list and you redefine the model to CPA or RKSA(Infochem) + Electrolyte options the salt component will automatically be removed. Similarly, if the electrolyte salt model has been selected and Na +, K+, Ca ++ and Cl- are present in the component list they will be automatically removed on selection of the RKSA(Infochem) hydrate model. If you wish to add both methanol and salt you enter the required concentrations in the relevant Alcohols/Glycols and "Salt Component" or "Electrolyte" tab. Clicking on Add in any tab will add the correct level of chosen inhibitor.

Troubleshooting - components Despite all our efforts to test and debug the software there may be genuine errors which should be reported to us. However, there are a range of problems, commonly encountered, which can be rectified fairly easily. We hope the associated warning messages are useful but if not please let us know.

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Databank not found All licensed databanks will be loaded 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.

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 INFODATA format databank files. 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

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Components • 83

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 any of the available databanks, although this is less likely if you have DIPPR installed. The warning message is self-explanatory

Before you accept this, however, it is worth checking

TIP

•

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 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.

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 than this you will be warned that the limit has been reached. TIP

84 • Components

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.

User Guide for Multiflash for Windows

Petroleum fractions

Introduction In the oil and gas industry a fluid may not be entirely composed of well defined pure components. It may be composed of a combination of pure components plus one or more pseudo components or petroleum fractions. Such fractions are 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.

Defining petroleum fractions To define a single petroleum fraction the program requires certain characteristic properties and Multiflash will then estimate the other properties needed to support the range of calculations available in the program. The list of possible properties to support characterisation of the fraction are: Carbon number Molecular weight (g/mol) Specific gravity at 60o F relative to water at 60o F Normal boiling point Critical temperature Critical pressure Pitzer’s acentric factor However, not all of these are necessary. The minimum input sets are molecular weight, molecular weight and specific gravity; molecular weight and boiling point; boiling point and specific gravity; critical temperature, critical pressure and acentric factor. Alternatively the boiling point can be used instead of the acentric factor. The properties that are estimated, if they have not been provided, are: Molecular weight Normal boiling point Critical temperature Critical pressure Critical volume Acentric factor

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Petroleum fractions • 85

Parachor Dipole moment Enthalpy of formation Standard entropy Perfect gas Cp Saturated liquid density Saturated vapour pressure Enthalpy of evaporation Liquid viscosity Melting temperature Enthalpy of fusion Entropy of fusion Heat capacity change on fusion Volume change on fusion If you have the required input properties available, either from the PVT Analysis or from a previous characterisation (possibly from a simulation program), you can enter these and estimate the remaining properties as described in “Calculating petroleum fraction properties” on page 100. If you have pseudo components specified by carbon number and you wish to retain these but have no properties for them you can allocate these from the recommendations of Riazi and Al-Sahhaf, see ”Defining petroleum fractions” on page 85. However, you may also have available a compositional PVT analysis where you need to break down one or more pseudo components into further fractions or where you wish to regroup the fractions into fewer cuts. A typical application of extending the analysis of the “heavy end” of an oil would be when modelling waxes or asphaltenes. In this case you must first characterise the fluid and the PVT Analysis is designed to help you with this.

PVT Analysis The PVT Lab Analysis form was designed to allow you to enter the information supplied by PVT laboratories as quickly and easily as possible. As we actively improve and extend characterisation procedures the input form is modified, although the basic structure remains the same. At this point we have decided to establish two input forms rather than trying to extend the original. The “standard” PVT Analysis form will be used to enter analysis data for a total fluid, liquid plus gas or black oil. It can also be used to generate a n-paraffin distribution where no measured data are available. If a n-paraffin analysis has been measured then we have introduced a new PVT input with n-paraffins, which will usually apply only to use with the wax model. To initiate the standard PVT form click on the Lab input menu item.

86 • Petroleum fractions

button or the Select/PVT

User Guide for Multiflash for Windows

For analyses including n-paraffins click on the Input with n-paraffins menu item

button or use the Select/PVT

For this analysis there is no choice of analysis function, only Infoanal2 is used. If you have entered an analysis including n-paraffins but for any reason wish to use an analysis without n-paraffins you can generate this by opening the standard PVT form and the n-paraffins and other fluid components will be combined.

Analysis method The revised analysis method (Infoanal2) improves the way in which the fractions in a fluid are distributed, i.e. grouping and splitting fractions and the allocation of properties. For backward compatibility you can retain the original characterisation method (Infoanal1), based on the Whitson Gamma distribution. The new characterisation method has the following features: •

User Guide for Multiflash for Windows

The code is able accurately to regress the single carbon number analysis up to very high carbon numbers if given. The Whitson gamma function method which was used in the original method

Petroleum fractions • 87

was not able to fit actual experimentally measured distributions in some cases. •

The code will estimate the likely “tail” of the distribution corresponding to the plus fraction and make use of molecular weight data in estimating the tail. The method will function for extremes of a single C6+ plus fraction to cases where the analysis goes up to very high carbon numbers .

•

The code performs checks on user specified molecular weights and specific gravities to ensure they are consistent with the specified analysis.

•

As with the original characterisation method, the user can ensure that all inputs are genuine measured quantities, i.e. the liquid analysis can be specified on a mass basis, a separate gas analysis and recombination GOR can be given and the molecular weight and specific gravity can be specified for the stock-tank liquid.

•

The pseudo component lumping procedure uses a new method to ensure that the lumped equation of state model will behave as similarly as possible to the unlumped equation of state model.

Component list A list of possible components is already provided based on a typical list from a PVT laboratory. This consists partly of well-defined (discrete) compounds such as nitrogen or methane and partly of pseudo components such as C6, C7 etc. up to C65. These are often referred to as single carbon number (SCN) cuts. You can choose the data source for the discrete compounds, e.g. INFODATA or DIPPR. However, you may use a different laboratory which provides a different list of discrete components and you may therefore configure the list as you wish. To delete a component simply put the cursor in the component cell you wish to remove and click on the Component Delete button to remove it from the list. To add a component put the cursor in the component cell above which you wish to add a new compound and click on Component Insert. A blank row will be inserted. Simply choose the pure component data source from which you wish to take the data and type in the name of the new component to be added. For instance cyclopentane has been added to the list below.

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Only hydrocarbons can be added to the component list in the PVT Analysis. If you try to add a component such as acetone then you will see 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 comp onent 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.

If you mis -spell the component name or the component you wish to add is not in the data source of your choice another error message will be generated.

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

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Petroleum fractions • 89

And will be the same path as for the Problem Files (.mfl). The easiest way to set up the list as you want it is via Windows using the Tools \Show problem menu to reflect the changes in the main window.

Highlight the commands defining the pvtanalysis then copy and paste this to your MFCONFIG.dat file. The next time you run Multiflash and use the PVT Analysis the new component list will be generated automatically. Please delete the command “amounts” and the values following it in the MFCONFIG.dat file, otherwise the values will be automatically assigned to the components in the Fluid column of PVT analysis.

Fluid composition How you enter the fluid composition will depend on the information supplied by the PVT Laboratory: this may be supplied as information on the total reservoir fluid or as a separator fluid, separator gas and GAS to OIL ratio (GOR). If you only have a total reservoir fluid composition this should be entered using the Single Fluid option. If you have both gas and liquid you need to enter the fluid compositions using the Liquid-Gas option.

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The GOR is a value provided by the PVT laboratory carrying out the analysis to allow the user to recombine the gas and liquid analyses correctly in order to obtain the correct fluid composition for the recombined fluid. We have altered the description of GOR in the PVT form to reflect this. In order to carry out a characterisation for the Gas and Liquid analysis you must have a composition in both columns. 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 summed. 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 is saved or 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.

Additional fluid information You can provide additional information to define the fluid. This section of the PVT Analysis form was updated in MF3.5 If you have a single fluid composition then the options are:

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Petroleum fractions • 91

The additional properties can be either the molecular weight of the Stock Tank Oil (STO), the Single fluid or the heaviest SCN and/or specific gravity of the STO or the heaviest SCN. If none of these data are supplied the program will estimate the values based on the fluid distribution you have supplied. Prior to Multiflash 3.3 the layout of this section apparently enabled the user to enter the specific gravity of a total single fluid. However, as the specific gravity was taken to be at STO conditions this could be mis -leading as a total liquid specified might well include a significant amount of light components and would be twophase at STO conditions. If .mfl files from earlier versions of Multiflash, which included this specific gravity, are loaded the assumption will be that the value referred to the STO, which is likely to be correct in most cases. A warning message will be triggered.

If compositions are supplied for gas and liquid plus a GOR then the section for adding extra data has been modified slightly. In this case the molecular weight can be supplied for the STO, heaviest fraction or the liquid composition specified, which is not necessarily the STO.

Our general advice is that if you have a lean gas, i.e. where the C6+ fraction is only a minor proportion of the total fluid, you allow the program to estimate the molecular weight of the heaviest SCN. For condensates and heavy oils it is preferable to enter the molecular weight of the STO as this is usually an experimental value. The molecular weight for the heaviest SCN shown in analysis reports will probably have been assigned by the PVT laboratory using generalised tables for petroleum fraction properties. In order to give guidance when running Multiflash we have supplied some warning messages. If the mole fraction of C6+ is .5 and you fail to supply either molecular weight or specific gravity a similar warning message will ask you to check this.

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Also, if you supply a molecular weight which does not appear compatible with the carbon numb er 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 well 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. If you have a separate gas composition this should be entered in a similar way before doing any characterisation but you must supply the recombination GOR (gas to oil ratio) on a volume to volume basis. Again there is a drop down menu for the choice of units.

n-Paraffin distribution In MF3.6 entering a n-paraffin distribution has been substantially modified. The PVT analysis including n-paraffins is launched separately using the new button or the Select/PVT Input for n-paraffins menu item. The new form has four tabs. The first two allow the user to enter analyses for a single fluid, which may be a live or STO fluid, plus the n-paraffin distribution. The n-paraffin distribution may be described in two ways, either a percentage of the STO or as a fraction of each SCN, above C6, of the total fluid. In the first tab the total fluid composition is entered as for the standard PVT 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 n-paraffin 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 n -paraffin distribution in the stock tank oil would normally be expected to sum to substantially less than 100% but the only check we can currently make is that the total for the n-paraffins is less than 100%.

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Petroleum fractions • 93

The second tab allows for data which has 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 n-paraffin. 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.

The remaining two tabs operate the same way but allow for the overall fluid to be described in terms of gas plus liquid. 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 a n-paraffin fraction or percentage for cuts below C6 a warning message will be issued.

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A n-paraffin distribution can only be used with the new characterisation procedure, Infoanal2 so no option is offered for Infoanal1. Otherwise data entry is similar to any other PVT phase composition. If you have entered a n-paraffin dis tribution in previous versions of Multiflash For cases where the normal paraffin distribution has not been measured you should use the standard

form to estimate this.

Black Oil Analysis In some circumstances a user may have very limited compositional data for a fluid. Either this may not have been measured or the data may have been generated from other applications such as reservoir simulations. It may be that this limited data has to be converted to a more detailed compositional analysis in order to investigate other problems. The Black Oil Analysis can operate from the minimum input data of Gas gravity(relative to air), Stock Tank Oil specific gravity(relative to water) and Solution GOR. Additional data that may be available are the Watson K factor and a Gas Analysis and these may be entered as optional input.

The Black oil input is the last tab in the standard

option.

The characterisation proceeds as for any other PVT Analysis data.

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Petroleum fractions • 95

Wax Content The ability to define the wax content supports the use of the Coutinho wax model where you have no measured n-paraffin distribution. The value corresponds to the values given by the UOP total wax content test and is interpreted as giving the total C20+ n-paraffin content in the stabilised oil. It is used in the Coutinho wax model to calculate a n-paraffin distribution. If the wax content is unknown it too can be estimated by ticking the box for Estimate Wax Content. If a n-paraffin distribution is specified then the Wax Content and Estimate Wax Content boxes are both disabled.

Water cut In practice the hydrocarbon fluid may also contain water. If this is pure water it can be entered using the Water cut text box as a volume % of the total fluid. However, you should be aware of the consequences of using water cut if you have already defined an aqueous phase elsewhere, perhaps by entering an inhibitor solution, such as aqueous methanol. In this case entering a positive volume percent of water will alter the amount of water in the overall stream but leave the composition of methanol unaffected, effectively changing 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 and is the most appropriate choice in the circumstances. Adding water will not affect the hydrocarbon fluid characterisation although it may, of course, affect the subsequent phase equilibria calculations.

Total amount of fluid Your total fluid may comprise any combination of separator gas, separator liquid and water. These will be combined based on the individual gas and liquid compositions, the GOR and the water cut. The total amount of fluid text box allows you enter the further specification of the overall amount of fluid, which is then used to scale the compositions accordingly.

SARA Analysis PVT Laboratories can often supply additional information on the fluid composition in terms of a SARA analysis. This defines the relative amounts of components that are saturates, aromatics, resins and asphaltenes. The amo unts of saturates and aromatics are used in some characterisation schemes. Our current characterisation does not use this information but future extensions may do so. The SARA analysis is used by our asphaltene model. Currently, if you enter amounts for resin or asphaltene these components will be added to your mixture and will therefore affect your phase equilibria calculations. However, without an asphaltene model you should not infer that you can calculate asphaltene deposition. In general the SARA Analysis should only be used with the asphaltene model. 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 text box. However, we recommend that you should use the experimental data if you know them.

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Characterisation The characterisation procedure can be used to split or regroup the heavy end from C6 to the heaviest SCN and to allocate physical properties to the resulting pseudo components. There are two characterisation procedures, Infonanal1 (the original method) and Infoanal2 (the revised method which also allows a nparaffin distribution). If your problem has been stored in an .mfl file then the choice of characterisation procedure will also have been stored. Otherwise the first choice in the characterisation procedure is to choose the characterisation method; by default this will be set to Infoanal2. Perhaps the simplest case is to use the characterisation to allocate physical properties to the SCNs already defined by the PVT Analysis. To do this, in the Pseudo components section Scroll “Start pseudo components at” until you reach the heaviest SCN in your mixture Scroll “Number of pseudo components required” to 1 Click on “Do Characterisation” You should see a message to indicate that the characterisation has been successfully completed

If you are using Infoanal1 then the pseudo components and their compositions will be reflected in the main window on closing the PVT analysis form . In this case the amounts should match those entered. This will also be true for Infoanal2 but in this case there is an intermediate step which plots a comparison of the experimental analysis and the fitted distribution.

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. The physical properties related to any pseudo component may be viewed as usual using the Tools/Pure Component Data option.

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Petroleum fractions • 97

Splitting and regrouping the heavy end is very flexible. For example, to split the heaviest SCN into five further fractions simply Scroll “Start pseudo components at” until you reach the heaviest SCN in your mixture Scroll “Number of pseudo components required” to 5 Click on “Do Characterisation” Or to reduce the total number of SCNs from 10 (C6 to C15 for example) to 5 Scroll “Start pseudo components at” until you reach a suitable starting point e.g. C6 Scroll “Number of pseudo components required” to 5 Click on “Do Characterisation” You can start the pseudo component split anywhere from C6 to the heaviest SCN in your mixture. If you try to go beyond the heaviest SCN, you will be warned that you are exceeding this value

Similarly, you can split the heavy end into any positive number of pseudo components within the overall Multiflash limit of a maximum of 200 components in a mixture. If you try to go beyond this you will see errors reported, such as

If you ask for a large number of pseudo components, but less than the overall maximum, the characterisation will reproduce this as closely as possible but restrictions on the overall mass of the C6+ fraction may mean that the total number of pseudo components cannot be achieved. The names allocated to the pseudo component fractions will be generated by Multiflash and be of the form C11-15. If you choose to use the revised Infoanal2 characterisation without defining or estimating a n-paraffin distribution the output will be similar. If you wish to have a separate n-paraffin distribution you either enter the experimental compositions using the appropriate form or supply a wax content by either entering a known value or asking Multiflash to estimate this. You can then characterise the n-paraffin distribution in the same way as you do for the remaining fluid. In this case n-paraffin fractions are designated by the letter N, e.g. N11-15, and fractions in the remainder of the fluid by the letter I. If the nparaffin distribution is estimated a warning message to this effect will be displayed

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The n-paraffin distribution need not be identical to the remaining fluid distribution. For use with the Coutinho wax model our suggested default is C6 and 5 fractions for the main fluid and N6 and 15 fractions for the n-paraffin distribution. If an experimental n-paraffin distribution is supplied using the option then the experimental and fitted data for this distribution will be shown.

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.

Troubleshooting – PVT Analysis The PVT Analysis has been substantially improved since its first release. While we have made every effort to test it there may be improvements we need to make or problems we need to solve, so please get back to us if you spot any. Some difficulties we have encountered are:

Sensitivity to characterisation The distribution assumed in the original method is based on the Whitson Gamma distribution function. Not all the PVT analyses we have tested are well reproduced by this distribution and the revised method should provide a better alternative. If you do see a warning that the default distribution parameter has been used, then you should try the other distribution method or use the phase envelope tracer to see how splitting and re-grouping pseudo components may affect your phase calculations.

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Petroleum fractions • 99

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.

Calculating petroleum fraction properties If you know the input properties of the petroleum fraction or petroleum fractions you wish to include in your mixture, or your stream contains relatively few pseudo components. then you may choose to define the petroleum fraction properties directly using the Select Components option. First choose as your data source, the Infochem Petroleum fractions correlations from the Select Component dialog box. This will activate another dialogue box.

If you have a pseudo component reported to be C7 but have no physical properties for this cut you can simply type in the carbon number and then click on Add. The Pseudo component type should be left as Normal. The physical properties required to characterise the fraction will be allocated from the generalised table of petroleum fraction properties recommended by Riazi and AlSahhaf. Reference: Riazi, M.R. and Al-Sahhaf, T.A., Fluid Phase Equilibria 117 217 1996. Multiflash will also allocate a name to the petroleum fraction, or you can enter a name of your choice. You can look at the properties allocated to the fraction either by using the Edit function

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or through Tools/Pure Component Data in the main menu. If you wish to specify the basic physical properties of the fraction yourself you may do so. Activate the Petroleum fraction properties dialogue box and

fill in the name by which you wish to identify the fraction and any known properties in the chosen units. Select type of Pseudo component. The Normal option is the default for petroleum fractions which are not n-paraffins, asphaltenes or resins; the latter two categories are included for use in asphaltene modelling. Then click on the Add (or Insert or Replace) button to add the fraction to the selected list. Provided each fraction is identified by a different name then as many petroleum fractions as you wish may be added to the selection, provided the total number of components does not exceed 200, the current Multiflash limit. As discussed earlier the properties of any petroleum fraction are calculated from the input data supplied. To do this we use a number of standard industry correlations, which are listed in out “Models and Physical Properties Guide”. In Multiflash version 2.8 we changed the correlation for calculating the boiling point temperature. If you have problem files saved from previous versions (before 2.8) when you load these files the boiling temperature will be calculated using the new correlation (unless of course the boiling temperature is one of the input values). If for any reason you wish to calculate the boiling point from our original correlation you can do so by entering the following command using Tools/Command Chardata infochar tbapi; To ensure this is carried out correctly you first enter this command and then go to Select Components to edit the petroleum fraction record. This should be modified so that it only contains the original input specification, e.g. if you originally supplied only molecular weight and specific gravity you should remove the entries for boiling temperature, critical temperature and pressure and acentric factor. Using Replace will then initiate recalculation. You can check the results using Tools/Pure Component Data. Note that there is a Units button available in the Select Components window to change the units for entering data to define the fraction.

Editing petroleum fraction data Having defined your petroleum fraction 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 using the Edit option in the Select Components dialogue box.

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Petroleum fractions • 101

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 Highlight the petroleum fraction you wish to edit by clicking on it in the selected components section Click on the Edit button, which will activate the petroleum fraction properties dialogue box. This will show the current definition for the chosen fraction, showing all possible input data, including properties derived from a more limited input set. 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 Replace The new 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 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.

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 deposition point is matched by adjusting the model parameters rather than the properties of any fraction and will be discussed in the relevant section. Matching of dew and bubble points, bubble point/GOR 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 Your petroleum fraction can be re -defined by adjusting the properties of the petroleum fractions 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 point or bubble point experimental data depend on the equation of state models used when matching. If the RKSA, PRA 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 a 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. In earlier versions of Multiflash

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the user was limited to matching single dew or bubble points or a single viscosity value. In Multiflash 3.2 this was expanded to allow the option of entering a table of multiple values up to 10. From 3.4 this was increased to 20. 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 fixedphase fraction flash, see “Fixed phase fraction flashes” on page 122. Our example has 9 petroleum fractions, the properties of the heaviest, C20+, are shown below.

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

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Petroleum fractions • 103

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.

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 305.35 398.55 442.55;pressures 462.8 401.6 303.5 229.6;;;

104 • Petroleum fractions

340.95

User Guide for Multiflash for Windows

PETROLEUM FRACTIONS PHYSICAL PROPERTIES: TC/K

PC/bar

MC1

MC2

11P

515.61

31.293

0.83985

0.31249

12P

545.10

31.030

0.87122

0.33002

13P

570.76

29.346

0.90788

0.35784

14P

597.63

27.167

0.95284

0.39556

15P

617.81

25.482

0.98986

0.42828

16P

643.74

23.564

1.0377

0.47243

17P

687.08

20.486

1.1244

0.55600

18P

747.39

16.427

1.2626

0.69403

19P

811.89

13.044

1.4141

0.85153

EXPERIMENTAL AND CALCULATED VALUES: T(exp)/K

DEWPOINT

P(exp)/bar

P(calc)/bar

305.35

462.800000

455.265209

340.95

401.600000

407.966543

398.55

303.500000

315.039181

442.55

229.600000

215.530477

where C20+ is the 19th component. The adjusted Mathias Copeman parameters MCRKS1 and MCRKS2 for the RKSA model will be reflected in the pure component record.

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. Bubble points can be matched in a similar manner and similar plots generated, either for the whole phase envelope or over a selected temperature or pressure range.

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A recent upgrade was to expand the matching facility to match bubble points, GOR and a liquid density together. This is accessed through the Tools/Matching/Bubble point option.

If only bubble point data are available then these are entered using the bubble point data table. To simultaneously fit the GOR then 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. 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,

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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 is matched by changing the fluid composition. The output shows the original and adjusted composition for each fluid component and the ratio of the two. Component amounts adjusted to match GOR: original adjusted NITROGEN 0.35 0.358497 CO2 3.14 3.21454 METHANE 54.26 55.5711 ETHANE 8.57 8.77073 PROPANE 5.72 5.84139 ISOBUTANE 0.76 0.772932 N-BUTANE 2.45 2.48441 ISOPENTANE 0.75 0.748821 N-PENTANE 1.2 1.19059 C6 1.53 1.45224 C7 2.6 2.42592 C8 3.02 2.7916

ratio 0.9763 0.9768 0.9764 0.9771 0.9792 0.9833 0.9862 1.002 1.008 1.054 1.072 1.082

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.

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Petroleum fractions • 107

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. 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.

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In this case the amended property values are the coefficients for the Peneloux volume shift parameter. PETROLEUM FRACTIONS PHYSICAL PROPERTIES: PENELOUX VOLUME SHIFTS CONST. TERM

TEMP. DEP. TERM

10P

-0.297707E-3

5.70161E-7

11P

-0.339418E-3

6.6284E-7

12P

-0.37939E-3

7.41664E-7

13P

-0.408696E-3

7.98307E-7

14P

-0.448733E-3

8.74943E-7

15P

-0.492774E-3

9.61531E-7

16P

-0.550553E-3

1.07495E-6

17P

-0.616056E-3

1.20265E-6

18P

-0.710631E-3

1.38428E-6

19P

-0.84831E-3

1.64707E-6

20P

-0.9606E-3

1.8617E-6

21P

-0.103009E-2

1.99368E-6

22P

-0.103009E-2

1.99368E-6

23P

-0.103009E-2

1.99368E-6

24P

-0.109247E-2

2.11164E-6

EXPERIMENTAL AND CALCULATED VALUES: VOLUME

User Guide for Multiflash for Windows

T(exp)/K

P(exp)/bar

Vol(exp)/kg/m3

Vol(cal)/kg/m3

381.15

360.

262.37511

264.04855

381.15

350.

258.22911

259.28224

381.15

340.

253.9301

254.37638

381.15

330.

249.2281

249.32461

381.15

289.6

228.7901

226.28711

353.15

360.

291.25012

294.59372

353.15

340.

283.10012

284.80133

353.15

320.

273.96011

273.73597

353.15

300.

264.00011

261.87453

353.15

298.89

263.00011

261.19107

Petroleum fractions • 109

Matching wax appearance temperature The method used for matching the 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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No plot is generated when WAT are matched, but this may be incorporated in future versions. 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 hydrocarbon liquid viscosity. It cannot be used to match the viscosities of gas, water or solids. From the Tools/Matching menu Select Liquid Viscosity

Choose the correct phase from the list of possible phases and enter the temperature and pressure conditions and the liquid viscosity or viscosities to be matched. The matching procedure works by altering the reference viscosity of

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Petroleum fractions • 111

the fractions and these will be reported in the main window, together with a comparison of the experimental and fitted values of viscosity. For example PETROLEUM FRACTIONS PHYSICAL PROPERTIES: REFERENCE VISCOSITY/cP 11P

.24412

EXPERIMENTAL AND CALCULATED VALUES: VISCOSITY T(exp)/degC

P(exp)/bar

Visc(exp)/cP

Visc(cal)/cP

114.40

332.000000

.456000000

.446388733

114.50

290.300000

.430000000

.425686970

114.40

241.700000

.412000000

.401764279

114.40

219.300000

.393000000

.390433458

114.50

189.800000

.380000000

.375073303

114.40

187.800000

.382000000

.374320636

114.40

173.100000

.388000000

.397174169

114.90

157.400000

.408000000

.422478903

114.40

125.100000

.469000000

.486002624

114.90

65.7000000

.652000000

.647117892

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 is appears physically realistic. No warning will be issued if liquid viscosities increase with increasing temperature.

Problems defining a petroleum fraction The properties you enter to characterise a petroleum fraction will be checked when you click on the ADD button to select it. If they are physically unrealistic or cannot be processed by the petroleum fraction suite of correlations you will be warned and the fraction will not be accepted for selection. The obvious problems will be entering a negative number for a quantity which must be positive, e.g. molecular weight, specific gravity or acentric factor. The warning message will be self-explanatory

112 • Petroleum fractions

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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 144.

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.

User Guide for Multiflash for Windows

Petroleum fractions • 113

114 • Petroleum fractions

User Guide for Multiflash for Windows

Input conditions

Introduction Once you have chosen the model for your mixture and selected the constituent components, the next step is to specify the input conditions for the problem. In Multiflash these are •

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. All input conditions must be specified in the correct units. The units currently selected will be defined adjacent to the relevant text box or drop down table. For information on how to change the units see “Changing units ” on page 144. As you may wish to change the input conditions frequently, the majority are grouped together in the Conditions section of the main window.

Input conditions can be entered in the Conditions section or can be supplied in the problem setup file.

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 Analysis ” on page 86) or to enter the composition of an inhibitor through the Inhibitor Calculator (see “Inhibitor calculator” on page 79).

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Input conditions • 115

Note that in the Composition drop down table component names which contain a space have the space replaced by an underscore.

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. The table will list each of the selected components and the compositions should be entered in the right-hand column, in the units designated. If the composition has been defined in the 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 absolute units. If you wish to enter mole fractions then do this with the units set to mole, but you must check yourself that they add up to one. Otherwise they will be totalled in moles and the fractions scaled accordingly. 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 do 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 positive amount for at least one component in a mixture for any flash calculation to be successful, 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

The output format has been changed in recent versions so that you can copy and paste the values for a single column of output. For example you can copy the composition of the liquid phase from the results window and paste this into the Composition table to carry out further calculations.

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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. The numerical values for temperature, pressure and volume should be entered in the appropriate text box, in the correct units. If they have been set in the problem setup file they will be displayed in the text box. 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 input pressure or volume entered is unacceptable (perhaps a negative value) or the absolute temperature equivalent to any entry is unacceptable then you will be asked to reenter the input condition as with the composition. 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 activate 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. The default datum points for enthalpy and entropy set the value of both to zero for the perfect gas at 298.15K and 1 bar for compounds. Consequently enthalpy, entropy and internal energy can take both positive and negative values. There is therefore no check on the value entered for any of these three input conditions. 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. There are now two choices for the enthalpy datum point and three for entropy. These can be defined in the units tab for the property. Choosing “elements” is equivalent to the setting “Thermal properties relative to the elements” in Property Output in earlier versions. It is commonly used for chemical equilibria. The entropy can also be defined as the standard entropy. As with temperature, pressure and volume if you fail to set an input value for any of these variables 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

User Guide for Multiflash for Windows

•

the values for the input conditions are correct

•

they are in the correct units and that

•

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

Input conditions • 117

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 not necessarily to an error in Multiflash. TIP

118 • Input conditions

If you wish to include the input conditions in a problem setup file you should also define the input units for the conditions specified. The units specification, as well as the numerical value, will be displayed in the conditions window.

User Guide for Multiflash for Windows

Calculations (flashes)

Introduction All the calculations carried out in Multiflash are flash calculations. This is true even when you are only interested in obtaining the saturated liquid properties of a pure component: the initial calculation in this case is finding the bubble or dew point at a fixed temperature. Multiflash is designed to carry out a series of single flash calculations except for the phase envelope generator where a series of flashes are performed . Before activating a calculation you must specify the components, compositions, model and input conditions appropriate to the flash. After you have performed a calculation you then make any changes you want to any input variable and activate the flash again. When the phase envelope function is used to plot any phase line the results of the individual flashes carried out are shown in the main window. There is currently no other facility to automatically generate table style output. If you wish to generate tabular output frequently this can be done easily using our Excel spreadsheet interface. Of course, if you wish to calculate a series of flashes at changing conditions you can set them up in a problem setup file. Once this is loaded all the flashes will be calculated without further interaction on your part and you can scroll through the output in the results window.

The basis of a flash calculation. In a flash calculation any two of the following variables •

Temperature (T)

•

Pressure (P)

•

Volume (V)

•

Enthalpy (H)

•

Entropy (S)

•

Internal energy (U)

•

Amount of a phase

are fixed . Based on the thermodynamic principles that at equilibrium

User Guide for Multiflash for Windows

•

The fugacities of each component in all phases are equal

•

The temperatures of all phases are equal

•

The pressures of all phases are equal

Calculations (flashes) • 119

and the solution satisfies any set constraint (the two fixed variables remain the same) then the flash calculation allows you to determine, subject to the constraints imposed, the number and type of phases present and the composition and properties of those phases. 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

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

Tolerance calculation

Calculates the amount of a second stream which needs to be added to a stream in order to meet a fixed constraint e.g. amount of inhibitor to suppress hydrate formation

Phase envelope

A series of flashes to generate a phase diagram for any phase present

The fixed phase fraction can be specified in mole, mass or volume units. However, the particular example of tolerance calculations the fixed fraction is still restricted to molar units. Chemical Equilibrium Equivalent flashes to those described above but also taking into account simultaneous chemical equilibrium P,T Chemical Equilibrium

120 • Calculations (flashes)

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P, Dew Point Chemical Equilibrium T, Dew Point Chemical Equilibrium P, Bubble Point Chemical Equilibrium T, Bubble Point Chemical Equilibrium All the above flashes may be activated by clicking on Calculate in the menu bar, then clicking on the flash type, then clicking on the particular flash. A dialogue box for entering additional information is activated for the fixed phase fraction flashes. 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 activate an isothermal flash for a given mixture Ensure you have chosen a model Enter the temperature and pressure in Conditions Select Calculate in the menu bar Select Standard Flashes in the sub menu Select P,T flash. Alternatively click on the tool bar button,

in place of the last three steps.

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 is to Determine the enthalpy of a stream at a given P,T (isothermal flash) 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 plot the phase boundaries for given constant enthalpy values.

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. 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 text 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 plot the phase boundaries for given constant entropy values.

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Calculations (flashes) • 121

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. Neither of these flashes have tool bar buttons assigned to them, both are activated only through Calculate in the Menu bar. You can plot the phase boundaries for given constant volume values.

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. Similarly a bubble point denotes the first point at which gas appears. In a multiphase situation it is possible to have more than one dew point (you will probably have more than one liquid phase). The choice of the dew point flash will result in Multiflash calculating the first, or primary, dew point found at the given temperature or pressure. As dew and bubble points are particular applications of a fixedphase fraction flash, secondary dew points can be calculated by specifying the name of the phase, setting the fixed amount of the phase to zero for liquids, 1.0 for gas, and carrying out the fixedphase flash at a given pressure or temperature. Simple dew and bubble points can also be calculated this way and fixedphase flashes are discussed in more detail below. Dew and bubble point flashes for two phase systems (or the bubble point and primary dew point flash for a multiphase system) can be activated either from Calculate in the menu bar or tool bar buttons, and

for bubble point flashes

and

for dew point flashes.

Depending on the temperature or pressure specified and where this is in relation to the phase envelope it is possible that the problem posed will not have a solution.

Fixed phase fraction flashes This is the general application of 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, it does not have to be 0.0.

Phases Multiflash is a mult iphase 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

122 • Calculations (flashes)

•

Vapour

•

Liquid

•

Condensed (fixed composition)

•

Hydrate

•

Wax

•

Asphaltene

User Guide for Multiflash for Windows

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 45. In the Select Models option and in the standard model configuration files (.mfc) supplied we use a standard set of names for the different phases.

Phase names The standard phase names are: GAS

vapour phase

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. With the introduction of chloride scales as part of the hydrates model, further fixed composition phases are introduced for the possible scales: NaCl, NaCl.2H2O, KCl, CaCl2.2H2O, CaCl2.4H2O, CaCl2.6H2O. Using the command language you can of course allocate your own names to any of the phases. If you have created your own problem setup file you may have called these phases by different names. In which case these names will appear in the program in place of our standard nomenclature. 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. The allocation of phase labels to supercritical phases has altered slightly in MF3.6. Previously a supercritical phase was labelled as GAS if V>Vc, where Vc is the pseudo critical volume. The phase is now labelled GAS if VT 2 > VcTc2 .

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. If the 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. A key component is only needed when a flash calculation must identify a phase uniquely (e.g. search for a particular phase fraction). Examples are the fixedphase fraction flash or a phase envelope calculation.

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Calculations (flashes) • 123

If you request any of these calculations for a phase, the key component of which is missing, an error message will be returned. For example if you try to calculate a water dew point when there is no water in your stream the you will see the following message:

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 fixedphase 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 a key component for at least one of your two non-aqueous liquids. This can either be in terms of one of your specified components e.g. key liquid1 heptane; or we have allowed for two more general options, heaviest and lightest, e.g.: key liquid1 heaviest; key 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.

Defining the fixedphase flash To activate the fixedphase flash option you either click on the fixedphase flash button for fixed pressure or fixed temperature through Calculate in the menu bar.

or activate the option

Either will activate the following dialogue box. The number of possible phases for your specified problem will be shown. This example, which is for a hydrate I/II calculation, has seven phases. The equation of state model sets usually define four phases, (GAS, LIQUID1, LIQUID2 and WATER) whereas the activity coefficient models are usually defined with LIQUID1 and LIQUID2 or GAS and LIQUID1 with the option to add extra liquid phases.

124 • Calculations (flashes)

User Guide for Multiflash for Windows

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. Then enter the phase fraction you want to fix in the text box below. This fraction must be between 0 and 1. Usually it will be zero, i.e. you are looking for the pressure or temperature (depending whether you fixed T or P) at which this phase first appears. However you can also use this facility to 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 as it appears, with the Normal (default solution) option button selected. However, some systems, such as gas condensates can exhibit more complex phase diagrams, as shown below.

For “normal” systems if you reduce the pressure along an isotherm from the liquid or dense gas region you reach its bubble point, where the gaseous components no longer remain dissolved in the heavier liquid components and separate off as a gas.

User Guide for Multiflash for Windows

Calculations (flashes) • 125

In gas condensates the gaseous components are in excess and the heavier liquid components in the minority. In this case, as the pressure drops, the liquids drop out of the gas phase and eventually, as the pressure is reduced even more, the liquid components evaporate again. This is known as retrograde. condensation. For a “normal” fluid there is only one solution (calculated pressure) for the dew point at any given temperature. However, for systems exhibiting retrograde condensation there may be more than one solution. Consider the phase envelope above, which has a large retrograde region. Taking a temperature of 300o F, for example, there are two dew points; one at 2.5 psia (normal dew point) and one at 2977 psia (upper retrograde dew point). Multiflash, with the default setting of normal, will always search for the former when the gas phase fraction is set to 1.0 (to simulate the dew point). To search for the second solution the “Type of Solution” option button must be set to upper retrograde. With the addition of new models other “Types of Solution” become important. Experience with the asphaltene model has shown that unspecified should be set when solving for asphaltene flocculation above the bubble point. Otherwise, it is very difficult to specify exactly when different options for the “type of solution” setting will be appropriate. This depends on whether the pressure or temperature is being fixed and where the critical point for the mixture is on the phase envelope in relation to the cricondentherm (the maximum temperature at which a two phase mixture can exist) and the cricondenbar (the maximum pressure at which a two phase mixture can exist). The phase envelope above is labelled with the solution types used to calculate the phase envelope at fixed temperature. The maximum pressure at which the liquid (condensate) drops out of the gas is the retrograde dew point. The lowest pressure at which the condensate has all evaporated again is termed the normal dew 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.

Hydrate calculations The dissociation temperatures or pressures for hydrates can be calculated using the fixedphase pressure or temperature flashes and selecting the appropriate phase. For hydrates this meant you have to select the hydrate structure you are searching for. However, we have recently introduced two new flash buttons, and , which calculate the hydrate dissociation temperature at a given pressure and hydrate dissociation at a given temperature. Using the hydrate buttons means that you do not have to define the hydrate structure, the software will calculate the hydrate conditions for the most stable structure.

Scale calculations Salt precipitation conditions can be calculated using the fixed phase fraction flash once the Chloride Scale option has been specified as part of the hydrate model. The scale of particular interest must be chosen from the list in the dropdown menu and the onset conditions will be calculated. If other scales are stable at these condition the phase and amount of phase will be reported in the output.

Wax calculations In MF3.5 two new flash buttons were introduced for wax calculations. These were

126 • Calculations (flashes)

and

.

initiated the calculation of the WAT at zero fraction at

User Guide for Multiflash for Windows

the pressure set in the pressure text box. As discussed elsewhere in the manual calculation of WAT is very sensitive to small amounts of heavy end – and experimental measurements often require positive amounts of wax to be present. In MF3.6, after a detailed study of the experimental wax appearance temperatures we have in our database, we have modified the WAT calculations initiated by the button, at least for our recommended Coutinho wax model. Clicking on the button brings up a supplementary text box

We have suggested default amounts, in both mass% and mole%, which we have found correspond on average to measurements made by Cross Polar Microscopy (CPM) and Differential Scanning Calorimetry (DSC). The text box will display the recommended amounts for CPM in mass or mole%, whichever unit is chosen. The user may enter alternative amounts, including zero, if they wish. One other major difference is that the mass or mole% of wax is relative to the amount of liquid plus wax. If the Fixed Phase fraction flash is used then the amount of wax phase specified is relative to the total stream. Depending on whether you have a live or STO you may get different results if you choose to use this option A better picture of wax precipitation can often be obtained by looking at the amount of wax precipitated as a function of temperature. Prior to MF3.5 this had to be determined by a series of PT flashes or table generation in Excel. A table can now be generated using the

button.

Wax Precipitation Curve Pressure: T (degC)

1.

bar

Wax Weight fraction

0.

4.62303E-2

5.

3.63226E-2

10.

2.81384E-2

15.

2.10122E-2

20.

1.49992E-2

25.

1.01305E-2

30.

6.34561E-3

35.

3.51214E-3

40.

1.45667E-3

45.

0.

The starting temperature is 0°C, or the equivalent in other units, and the finishing temperature is the calculated WAT for zero % wa x. 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 or 1 bar if no pressure is specified. New for MF3.6 is a plot of the wax precipitation curve,

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If you require additional control of the table configuration then you can use a command entered in the Tools/Command box. The format of the command is WAXPC pressure tstart tincrement. A new table will be generated but to plot the data you will need to copy and paste to Excel.

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 fixedphase 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 open the subsidiary window

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In the Tolerance Calculation window set the phase and the fraction of that phase for the fixedphase element of the flash. Then select the Composition of Second Fluid tab. In the table enter the amount of the single component (usually 1.0) or the composition of the mixture to added to the main stream to meet the constraint set. 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. Tolerance Calculation:

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 done by selecting Use Starting Values from Select in the menu bar. The command line set startvalues will be shown in the results window. Activate the flash option. The temperature or pressure, whichever is appropriate for the flash option chosen, given in the input conditions text box will be used as the starting value for the calculation. To turn off the starting value, repeat the step above and the command line will be reported on the results window to show you that Use Starting Values option has been switched off. set nostartvalues 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 composition.

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Phase diagrams A major addition to Multiflash was the ability to calculate and plot phase envelopes. It is possible to trace any phase boundary including gas, liquid and solid on pressure-temperature co-ordinates. A VLE Autoplot function traces the primary dew and bubble point curves. You should note that the phase envelope should only be used for models based on an equation of state (or at low pressures for activity models). Since the first release of the phase boundary plotter we have added •

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.

•

improvements to handle cases where the line crosses a phase boundary at which there are no degrees of freedom according to the phase rule.

•

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

In MF3.6 we have changed the program used to generate the plots, giving more clarity and flexibility to the plotting functions. The Phase envelope is activated through the Calculate/Phase Envelope menu or using the phase envelope button,

.

The resulting window has three sections. The first

allows you to generate either the VLE phase envelope automatically or to set a specific phase boundary e.g. WATER in zero amount to define the water dew point line. You should also choose the appropriate “Type of Solution” as discussed previously, e.g. unspecified if you want to trace the asphaltene flocculation boundary above the bubble point pressure. If the VLE AutoPlot fails to trace the full phase envelope the liquid and gas phase boundary lines can be plotted individually.

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You may then select a variety of properties of that phase to be plotted, including phase fraction.

If you choose to plot any of these then you will be reminded of the current unit setting for the property when you enter the value.

The second section provides the initial conditions for any phase boundary and the conditions and direction for the boundary search. Experience has shown that using increasing pressure is a useful default. If the user does not set an initial value for the pressure a default value of 1 bar, or the equivalent in the units chosen, will be set. This value is only shown in the text box once the boundary has been plotted. While one bar is usually an appropriate default to start the plot you should also be aware that on some occasions 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. If the boundary still proves difficult to trace, starting values for the temperature or pressure as appropriate can be provided. For the phase envelope calculator the starting value is provided by entering a “guess” in the text box, i.e. a value for

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the temperature if you are defining pressure and you should check the “Use starting value box.

The final section 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 axis. For normal phase envelopes these will be temperature and pressure.

In the main results window T,P conditions, volume, enthalpy, entropy, internal energy and phase fractions of any other phases present as s pecified in the model specification for the problem are displayed. Any of these properties plus temperature and pressure can be plotted against any other. The choice is available from the drop down menu.

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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.

The grid may be deleted if you prefer. 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 allows you to change the maximum number of points calculated at any one time. The default is 100. If, when plotting any phase boundary, it is possible to calculate more points, then a displayed message will allow the phase envelope calculator to continue or to stop at any point.

The phase envelope will be shown at the same time as this message so that you can make an informed decision as to whether more points would be beneficial. The phase diagram window also remains open even when you return to the main window to carry out further calculations. When you want to close it click on the Close button. If the calculations have reached any limit within the 100 points the message will not appear. Any critical points are labelled with a C and discontinuities, where phase boundaries cross, are marked with a D.

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Similarly, the individual flash results which appear in the first three columns in the window are marked with C or D so that the numeric values at these points can be determined. 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. N.B. When plotting phase boundaries with a high level of salt component in the stream, to obtain sensible results you should allow a solid salt phase to form. If you are using the Salt Component model, set up a solid salt component using Select/Freeze-out Comp onent option, see “The Freeze-out model” on page 48. If you are using the Electrolyte salt component model then switch on the Chloride scale option.

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.

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The plot is always cleared if you Clear the problem setup and individual boundaries can be deleted by using Options, selecting the boundary and deleting. Individual plots can be customised. The Options button allows you to change the colour, type and legend for any boundary. Tool bar buttons within this window also access the graphics package with the facilities to edit the graph and axes’ titles etc. With the new graphics package it is easy to add data. Simply click on the Add Data button at the bottom of the plot

Paste the data to be plotted in the empty table that appears.

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Click on Save and Plot.

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 Multiflash for Windows 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.

Property output in Multiflash Multiflash provides several levels of physical property output. The default is to provide information on phases, compositions, thermal and volumetric properties. There are five higher levels providing more output such as fugacity and activity coefficients relative to each component in each phase. You can also calculate

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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 option. The following dialogue box will be displayed

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 output deselect any properties you don’t want. If you choose Heat Capacity and Speed of Sound the phase amounts and composition plus the volume and thermal property output are also automatically selected. Note that the thermal properties relative to elements option is no longer available as the enthalpy and entropy datum point can now be changed. Examples of the property output options are shown in “Calculation output” on page 158.

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. The reported errors are likely to be of the type Bub point at fixed P: *** 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 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

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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 This facility is an excellent way of determining 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 diagrams ” on page 130. 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.

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 and *.mfc files for the 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

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GAS and LIQUID1, or LIQUID1 and LIQUID2, then it may be worth while reducing the numb er 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 retrograde solutions. For retrograde solutions the terms upper and lower have no absolute meaning, they merely determine whether the search is for a lower or upper temperature or pressure.

Provide a starting estimate If you feel you have specified the correct input conditions but convergence errors are still being reported it may be worth providing a starting guess for temperature or pressure see “Provide a starting estimate” on page 129“.

Provide a key component As we have discussed earlier, “Key components” on page 123,. 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.

Chemical reaction Multiflash has a chemical equilibrium module which allows the simultaneous calculation of phase and chemical equilibrium. The phase equilibrium is currently limited to equilibria involving combinations of one gas phase, one liquid phase and any number of pure solid phases. The flash options available are •

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P,T Chemical Equilibrium

User Guide for Multiflash for Windows

•

Dew Point Chemical Equilibrium

•

Bubble Point Chemical Equilibrium

These are all activated from the menu route Calculate/Chemical equilibria followed by the specific flash definition. The P,T chemical equilibrium flash can also be activated by the

button.

Chemical equilibrium problems are treated in exactly the same way as any other phase equilibrium problem. There is no need to specify any reaction mechanism, only to include in the selected components list all the possible products and reactants you wish to consider. If you do not define both then no chemical reaction calculation will be possible.

Troubleshooting - chemical reaction Remember the standard model sets supplied allow for up to three liquid phases one of them water. As the chemical equilibrium model is limited to one liquid phase the software will only consider the first two phases, GAS and LIQUID1. You may therefore see a warning message Chemical equilibrium at fixed P and T: *** WARNING -30131 *** More than 2 fluid phases defined - first 2 used. This may be ignored provided that the first two phases (GAS and LIQUID1) are the ones you wish to use. If you are looking at a problem with pure solid phases the data for components in the pure solid phases should be taken from INFOCOND, rather than INFODATA. In this case the component will be assumed to be a solid and the phase types/names will not need to be adjusted to account for these solid phases. The components that appear in the mixed vapour and /or liquid phases should then be taken from one of the fluid component databanks as usual.

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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 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 the phase is the total amount.

Working units Internally all Multiflash calculations are carried out in SI units. This cannot be changed.

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 now possible to set these as your preferred option. This is done through the Tools/Preferences 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. 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.

Changing units In addition to setting your chosen default units you may change any of the input or output units on an individual problem basis. 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, ft 3 /lbmol, BTU/lbmol, BTU/lbmol/F, cP, BTU/hr/ft/F, dyne/cm, cm2 /s Of course, you can change any of the property units individually. Changing units can be done in a variety of ways

Changing units interactively The units may be changed for any property at any point. To change the units Select Select from the menu bar, then

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Select Units or Click on the Select input and output units button or activate the Units button in any appropriate dialogue box. Any of these actions will activate the Tab control for units selection:

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. This button will then be set. Do the same in the Output unit column to change the output units for that property. The input and output units do not have to match. The output composition may be changed from fractions to total amounts on the Amounts tab. It is not possible to change the input composition to fractions. This Tab control panel for units selection 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 and Tables.

Changing units in a problem setup file You can define your input and output units as part of a problem setup file. In fact we would recommend that you do define input units where you wish to include the numerical values for any of the input conditions in the file to avoid any possible mismatch with the units currently defined under Windows when the setup file is loaded. The command for defining units is of the form: inputunits property_name units_name; More than one set of input units may be defined at any time. You must finish the command with ; as an end marker. e.g. inputunits amounts kg temperature degF; Output units are defined in a similar manner after the command outputunits. If you use the command

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units both input and output units can be defined at the same time. If you save a problem set up in Windows 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 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 148.

The results window. As soon as Multiflash is activated 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. Problem setup files, loaded through the File/Load Problem Setup menu and the contents of model configuration 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 re flect the command. Examples would be Dew point at fixed T: to indicate that you have activated a dew point calculation at fixed temperature flash, or Clear current problem setup to confirm that you have cleared the previous problem using the File/Clear Problem Setup menu option.

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If you use the Tools/Command option to send a command this will also be displayed in full in the results window, after the comment “Command line”, e.g. Command line: show pvt; 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 32,000 characters 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 (defined in a problem setup file) exceeds 32,000 characters. If all 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 by the user from the Tools/Preferences menu option.

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 sent to the screen in the results window the output is also sent to a log file which may be examined later. The default log file is called MFLASH.LOG and this will be overwritten the next time you load Multiflash unless you rename it. To retain the log file for documentary purposes either rename MFLASH.LOG before running Multiflash again or send the contents of the results window to a specified file. This may be done by choosing the File/Save Results menu option. You will see a message box which allows you to write the entire output window to file.

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If you have previously highlighted a section of the output then the message box offers the option of writing the selected text to file.

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.

Printing the output You can print the output from your calculations by Selecting File from the menu bar, then Selecting Print Results or Clicking on the Print results 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.

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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

The first line of output reports the temperature and pressure. These may 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 would 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 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. 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). The default composition units are moles. The compositions shown are actually mole fractions, but the “Phase totals” are in moles. 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

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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. Cp (J/mol/K) Cv (J/mol/K ) Sp. Sound (m/s) Visc.(Pas ) Th.C.(W/m/K ) ST (N/m)

heat capacity at fixed pressure heat capacity at fixed volume thermodynamic speed of sound viscosity Thermal conductivity surface tension

If you have selected the thermal properties relative to the elements these are labelled as below: H S U G

(J/mol (J/mol/K (J/mol (J/mol

) ) ) )

enthalpy entropy internal energy Gibbs energy

If the fugacity coefficients and activity coefficients have been selected the coefficients for each component in each phase are listed as follows:

COMPONENT

OVERALL

BUTANE PENTANE

PHASE1 GAS fug. coeff. .86231 .79165

PHASE2 LIQUID1 fug. coeff. 1.2403 .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

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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.

Phase envelope output The phase envelope calculator produces plots of the chosen phase boundaries in the Phase Diagram window. The default is to plot each phase boundary with a solid line, but as a different colour, and the legends are the phase descriptor plus the fraction of that phase requested, with V/L=0 for the phase envelope generated by VLE Autoplot. The range of points calculated or plotted can be controlled by the Phase Envelope dialogue box and the plot customised, e.g. legends and line types changed as discussed in section “Customising the phase envelope plot” on page 135.. Phase envelope plots can be exported in several formats, including .wmf, .bmp and .jpeg. They can also be directly written to Excel, see “Write to Excel” on page 146.

Errors and warning messages It is not practical to provide a full list of potential error and warning messages. When they occur an error number will be reported in the results window, followed by a single line description of the error, e.g. *** ERROR 344 *** The flash calculation has not converged For more information you can go to the HELP/Multiflash Error codes. The information on the module in which the error occurred and the subprogram name is intended for use by Infochem technical support.

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Warning messages should be checked carefully. In many cases they may be ignored if the cause is understood. Errors are usually associated with Multiflash output indicating a failure to find a solution or that the results may be invalid.

Displaying status for current settings Show options in the Tools menu allow the user to check the status of most of the Multiflash options. The Show options are probably self-descriptive but they are summarised in the table.

Show options Amounts

Displays the amount of each component in the stream

BIP databank

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

Black Oil Analysis

Details of the input to the Black Oil Analysis PVT form

Characterisation method

The correlations used to predict physical properties of the petroleum fractions e.g. INFOCHAR TBSOEREIDE

Components

List of components in the input stream

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

PVT Analysis

Description of the settings in the PVT Lab Analysis form

Results

Repetition of the results from the last calculation

Stream Types

Displays number and names of active streams

For brevity much of the output from the show option will be in keyword form. For instance, under Characterisation method, TBSOEREIDE is the name of the correlation used for calculating the boiling point of a petroleum fraction. The alternative would be TBAPI. The output for Show Models will depend on whether the model has been set interactively or loaded as part of a problem file. In the latter case the thermodynamic model may only be referred to as MEOS. To obtain more information you would still have to use the Tools/Command option and type in Show model meos; If the model has been defined interactively more detail is given at the initial Show Models stage e.g. MPR is the Peng-Robinson model.

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Output • 153

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 32,000 characters for the results window. If your output exceeds this then, as the last 32,000 characters are retained, earlier results may be 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 137, or because a model for the property has not been defined. If you are not using a model configuration file or model set to define your model you must remember to include transport property models if you wish to obtain transport property output. This is particularly important 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, but in MF3.6 we have tried to improve the criteria to retain the gas phase label, even when the phase properties become more liquid like. 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.

Phase envelope This facility is unusual in its ability to plot multiple phase lines. The phase envelope output is designed so that you can build up complex phase diagrams by adding different phase boundaries. If you forget to clear the plot or clear the problem setup this may lead to the plot from a new problem being superimposed on that from an earlier plot. In this case either Delete the individual phase boundaries from the earlier plot using the Options facility or Clear the whole plot and start again.

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 our display. They can be set to acceptable choices, see “Font” on page 148.

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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. Multiflash can currently produce Pipesim PVT files and files for input to OLGA. The facility to produce HTFS Process Simulator Interface Files (PSF) has now been removed.

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 please 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 Table in the menu bar, then Select PIPESIM Fill in the subsequent 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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Interfaces with other programs • 155

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 the message Pipesim table written to file: pipe5.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 message will appear

. and the errors reported in the main 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 OILANDGAS 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 Scandpower A/S.

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Multiflash can produce a PVT data file for use by 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 Scandpower Technical Note No. 1. 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 Table in the menu bar, then Select OLGA Fill in the subsequent dialogue box. with the pressure and temperature grid points and the name of the output file. The recognised suffix for OLGA tables is .tab. This will be allocated automatically if you use the Browse facility to identify a suitable directory and save the file there. Alternatively you can type in the .tab suffix as part of the file name. If you want the file saved in a particular directory you must enter the full pathname, otherwise the file will be saved in the current default directory. 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 Assuming there are no problems 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

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

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Interfaces with other programs • 157

or the calculation routines may fail and the error may be

.

CAPE-OPEN Interface Infochem has been an active participant in developing and testing the CO standards. The Multiflash CO modules implement versions 1.0 and 1.1 of the standard and support the ThermoSystem and Property Package interfaces. The interface has been tested for interoperability with Aspen+, ProII, gPROMS and HYSYS. To generate a file from Multiflash that can be read by the Multiflash CAPEOPEN module simply set up the problem in Windows as usual. Instead of saving the Problem Set-up you use the file option for “Export CO Property Package”. You do not have to specify which version of the CO interface you intend to use. A separate manual for the CAPE-OPEN Interface is provided on the Multiflash CD-ROM.

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User Guide for Multiflash for Windows

Help

Introduction Help is provided in various ways: •

The printed User Guide

•

On-line Help

•

Website support

•

Technical support

On-line help The on-line help will be familiar to regular users of Windows applications. It is accessed through the HELP menu

Help Topics provides access to the Contents, Index and Search facilities for Multiflash Help. The Multiflash Error Codes detail the meaning of the Error and Warning codes. This will be a familiar facility for users of the Excel interface but new to Help in Multiflash for Windows.

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Help • 159

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 Search, either from the HELP menu or via the Search 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.

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Clicking on the Display button or double clicking on the topic allows you to move to the related help text. Methods of accessing on-line help are based on standard Windows techniques. However, the 'About Multiflash ....' section is particularly useful when trying to identify the causes of any problems which may arise when implementing or running the software.

Your serial number together with the version numbers for the .dll (here 3.6.26) and the interface (here 3.6.28) help us to uniquely identify the software you are using. If you are reporting problems to us, particularly if we are unable to reproduce them, we may ask you for this information.

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Help • 161

The lower text box shows the path for the Multiflash application location and log files which are set by two environment variables. The two environment variables are MF36APP and MF36USR. The MF36APP is the environment variable required by Multiflash which should be set during the Multiflash standard installation. You should check that the environment variable MF36APP is set correctly if you experience problems in accessing data or messages. For further information about the environment variable setting, see the Multiflash Installation Guide. The .log file is the file which automatically records the input and output information for any run. If you do not set up an MFCONFIG.dat file then the path for this will not be shown.

Website support We now have an additional level of help. Our website, http://www.infochemuk.comcan now be used to report bugs found in Multiflash and to see any bugs/problems we or other users have found. Where these have been fixed we will tell you which release version does or will contain the corrections. If we are still considering how to tackle the problem but know of a work around we will advise you of our suggestions.

Technical support If you need further assistance contact us at: Infochem Computer Services Ltd. 13 Swan Court 9 Tanner Street London SE1 3LE UK Telephone: +44 (0)20 7357 0800 Fax:+44 (0)20 7407 3927 e-mail: [email protected]

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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 into Multiflash

•

Edit an existing problem setup file and load it into Multiflash

•

Write a new problem setup file and load it into Multiflash

If you want to know the stored values for the temperature independent properties or the correlation coefficients of a temperature dependent property you must choose Pure Component Data from the Tools option from the menu bar described earlier, see “Viewing and editing pure component data.” on page 69.

Defining the problem interactively Having successfully loaded Multiflash, choose a suitable model for the problem. If you wish to obtain the properties from the data source correlations then you must use 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: Select Select from the menu, then Select Model set, followed by

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Case studies - Pure component data • 163

Selecting Ideal Mixing from the Activity Model selection. Choose the gas phase model as Perfect gas. The recommended and default model for transport properties for the Ideal model is Mixing rules. If you wish to calculate the diffusion coefficient remember to check the box for this to be included in the Model set. You will see a message to say that the Ideal Mixing model set has been successfully defined. Click on OK For more information on models see “Models ” on page 27. Specify the pure component of interest in this case octane Click on the Select Component button, or Select Select then Select Component from the menu bar. then specify the data source and component in the Select Components dialogue box

The data source is set by: Clicking on the arrow to the right of the data source text box and Clicking on the data source of interest, in this case INFODATA, the Infochem Fluids databank The component is specified by either: Selecting the Name option button, typing the component name in the enter name box, then pressing the enter key or clicking on Add Selecting the All components button, scrolling through the list of components which will appear in the dialogue box and either selecting octane, then clicking on Add or Double clicking on octane

164 • Case studies - Pure component data

User Guide for Multiflash for Windows

or Selecting the formula option button, typing C8H18 in the text box, clicking on Search, selecting octane in the “Components in databank” text box, then clicking on Add or double clicking on octane Specifying the physical property output level. If you are interested in pure component data you will probably want to output all available physical property data except thermal property data relative to elements. Select Select in the menu bar and then select Property output. In the resulting dialogue box, the first two are the default options for output of thermodynamic data, click Heat capacity/Speed of Sound and the set of transport property data and finally click on OK.

Enter the composition for the stream. In this case where we have a pure component the composition is not important provided it is a positive value. The input is summarised in file octane.mfl. In the main window, click on composition, then type 1.0 in the right-hand column of the table next to octane. To obtain the properties of liquid octane on the saturation line you must carry out a bubble point flash calculation at a specified temperature.

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Case studies - Pure component data • 165

Specify the temperature and the flash calculation Type the first temperature, say 250K, in the text box next to temperature in the input conditions section Click on the Bubble point at fixed temperature button , or select Calculate from the menu bar, then select Bubble and Dew point flashes and finally select T, Bubble point flash.

The reported pressure is the saturated vapour pressure at 250K, the other properties are listed below the phase equilibrium output. As we are dealing with a pure component exactly the same results would be obtained if we had specified a dew point flash at the same temperature. The next temperature should be entered in the text box in the Conditions section and the bubble point flash repeated at this temperature.

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Producing a problem setup file You can carry out the same calculation, or series of calculations using a problem setup file. We hope that the commands used are self explanatory. (Simply edit this file if you wish to obtain pure component data for another compound or properties at a different temperature). You can also overwrite the default choice of data source. To write a problem setup file yourself, use the file shown as an example or save the problem you have specified interactively

Obtaining properties from Pure component Data option Although the standard output from Multiflash does not contain any of the constant properties, or indeed the correlation coefficients for the temperature dependent properties, it is possible to obtain these. Select the components you are interested in, for example octane, as described above. Select Tools from the menu bar, followed by selecting Pure Component data, the following dialogue box will then be activated.

As we are dealing with a single component this will be the only choice available so making sure it is highlighted. Select a property in the list of Select property, click on Edit to view or change the property. 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

User Guide for Multiflash for Windows

114.231 569.32 2.49700E+06 2056.359 .39690

g/mol K Pa mol/m3

Case studies - Pure component data • 167

TBOIL HFORMATION SSTANDARD TMELT HMELT SMELT CPMELT VMELT RUNIQUAC QUNIQUAC THLWATER VHLWATER DIPOLEMOMENT PARACHOR RADGYR HOCASS GFORMATION TTRIPLE PTRIPLE HCOMBUSTION V25 SOLUPAR SOLIDSOLUPAR ZCRIT REFRACTINDEX TFLASH TAUTO FLAMLOWER FLAMUPPER SPGRAVITY EXPANSIVITY OMASCALE OMBSCALE CNUMBER REFVISCOSITY LJEVISC LJBVISC EOSC TYPE COMPREFNO MCRKS1 MCRKS2 MCRKS3 MCPR1 MCPR2 MCPR3 HYDOC HYD1 HYD2 HYD3 ASSBETA ASSEPSILON ASSGAMMA ASSDELTA ASSFF ASSAC ASSBC ASSKAPPA SAFTKAPPA SAFTEPSILON SAFTGAMMA SAFTFF SAFTEK SAFTSIGMA SAFTLAMBDA SAFTM VSHIFT1 VSHIFT2 VSPR1 VSPR2

398.82 -208446.9 466.7252 216.37 20740.

CPIDEAL

-32384.514 290.00 0.0000 -24. 2.094E-8 -7.9121099

CPSOLID PSAT

168 • Case studies - Pure component data

1.0000 76.000 -5.7709999 5. 1.34E-5 3.0000

50.00791 50814.48 5.8486 4.9360 .00000 351.40 0.468040E-9 .00000 16000. 216.38 2.1083 -5.07415E+06 6120.925 15448. .2587676 1.39505 286.00 479.00 .80000 6.5000 .7066211

K J/mol J/mol/K K J/mol J/mol/K J/mol/K m3/mol K m3/mol debye (dyn cm-1) 1/4 cm 3/mol m J/mol K Pa J/mol mol/m3 (J/m3)1/2 (J/m3)1/2 K K vol % vol % 1/K

Pas

1. 93.

J/m 1/K J m3/(mol)2 m3/mol K 242.78 3.83730E-10 3.8176

K m 1/mol

-3721.3925 4.0000 -1.3945 5.6330 0.0000 10000. 1.94719999 -0.008536 20. 216.37 1.38007 -3.8043499

User Guide for Multiflash for Windows

-4.5013199 1.0000 .00000 LDENS 1.0000 .00000 LVISC 2.0000 .00000 VVISC 1.0000 .00000 LTHCOND 5.0000 .00000 VTHCOND 1.0000 .00000 STENSION 1.0000 .00000 CPLIQUID 5.0000 .00000 SDENS 5.0000 .00000 CPSOLID 5.0000 1.34000E-05 VIRIALCOEFF 1.0000 -3.63347E-04 284.38 CARNUMBER 000111-65-9 FORMULA C8H18 FAMILYCODE AA UNIFAC CH3 2 HVAP

260.00 568.95001 54909.031 .37750 .00000 .00000 .00000 568.38098 2032.52 5407.5898 0.375 568.38098 -20.462999 1497.4 1.3789999 .00000 216.38 398.83 3.11910E-08 .92925 55.092 216.38 1000.0 .21560 -2.94830E-04 .00000 .00000 216.38 398.83 -8758.0 .84480 -2.71210E+10 339.00 1000.0 5.27890E-02 1.2323 .00000 .00000 216.38 568.70 224.83 -.1866 9.5891E-04 .00000 216.38 460.00 8340.9 -3.1515 .00000 .00000 133.15 216.38 -24.000 1.9472 -8.53600E-03 2.09400E-08 20.000 216.37 2.73900E-04 -5.65219E-04 -1.16166E-05 2.58796E-06 1500.0

CH2

6

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 Models and Physical Properties manual and the Multiflash Programmers Guide.

Excel interface The current version of Multiflash does not include options for tabular output or for producing graphical output for properties other than phase boundaries. If this is important to you then we would recommend our Excel interface. For example, the ideal.mfc file with the components set to octane was used in conjunction with this interface and Excel to produce the following output 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

275

427.762

231.4236

-47031.7

6254.853

0.000694

0.134522

0.023412

300

2038.06

247.738

-41042.5

6116.615

0.000497

0.127151

0.020986

325

7232.427

264.3592

-34641.4

5970.103

0.000378

0.11978

0.018612

350

20571.97

281.3794

-27819.2

5813.892

0.000302

0.112409

0.016294

375

49413.7

298.9716

-20563.4

5646.112

0.000249

0.105039

0.014036

400

104100.9

317.4437

-12855.7

5464.231

0.000212

0.09767

0.011846

425

197831.5

337.3402

-4668.02

5264.678

0.000184

0.091347

0.009729

450

346415.4

359.6594

4044.752

5042.146

0.000161

0.086478

0.007696

475

568184.1

386.3901

13359.5

4788.154

0.000141

0.082613

0.00576

500

884336.7

422.1065

23419.76

4487.608

0.000125

0.079471

0.003941

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Case studies - Pure component data • 169

Ideal gas properties at .001Pa TEMP

CP

ENTHALPY

DENSITY

VISCOSITY

THCOND

J/mol/K

J/mol

mol/m3

Pas

W/m/K

275

176.5271

-4228.471

4.37357E-07

4.80261E-06

0.010792

300

189.7323

350.1064

4.0091E -07

5.28049E-06

0.01224

325

202.7498

5256.578

3.70071E-07

5.75694E-06

0.013937

350

215.5304

10485.62

3.43637E-07

6.23188E-06

0.015937

375

228.0325

16030.77

3.20728E-07

6.70529E-06

0.0181

400

240.223

21884.65

3.00683E-07

7.17719E-06

0.020388

425

252.0771

28039.12

2.82996E-07

7.64758E-06

0.022801

450

263.576 9

34485.55

2.67274E-07

8.1165E -06

0.025336

475

274.711

41214.92

2.53207E-07

8.58399E-06

0.027994

500

285.4733

48218

2.40546E-07

9.05008E-06

0.030772

It is then easy to plot individual properties as a function of temperature or to compare property values from different data sources.

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User Guide for Multiflash for Windows

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 “Simple Tutorial” on page 9, 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, PR or RKS. Select Select, then Select Model set from the sub-menu, followed by Selecting Equations of state 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. Specify the components

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Case studies - Phase equilibria • 171

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 65. As our current system contains simple well known compounds they have been selected by Highlighting 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 they are: 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 using a series of flash calculations or by generating the phase envelope. Individual calculations may be a series of bubble point calculations at either fixed temperature or fixed pressure. Technically there is no particular reason to prefer one over the other; for this particular example we will fix temperature and have our output pressure in bar.

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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 375K you still have a stable solution reported but by 400K the solution is reported to be “marginally stable” but by 425K you will notice a failure message Bubble point at fixed T: *** WARNING -20331 *** Search converged to low accuracy *** WARNING -20023 *** Unspecified warning from lower level routines. *** ERROR 20034 *** Solution found is unstable. *** ERROR 344 *** The flash calculation has not converged Investigating the bubble point curve using reduced temperature steps you will see that athe solution is marginally stable up to 403K and fails at 404K. 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 dew point 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 fixedphase fraction route and looking for the upper retrograde solution. To do this: Click on the fixedphase 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 fixedphase flash simulates a dew point calculation.

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Case studies - Phase equilibria • 173

Click on Do flash. The calculated dew point pressure is now 114.527 bar for the retrograde region, whereas for the “normal” dew point the calculated pressure was 28.1796 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 cricondenbar. You should now switch to fixedphase fraction flashes at fixed temperature, set the options as described and reduce the temperature in small steps. This will define the retrograde dew point curve to 402K.

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.

The output in the results window will allow you do identify the critical point explicitly 53C

174 • Case studies - Phase equilibria

402.732 147.11

User Guide for Multiflash for Windows

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 can 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 the first temperature in the Conditions section of the main window. Click on the fixedphase 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.

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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 “Calculating petroleum fraction properties” on page 100. 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 on the arrow to the right of the Data source list box, then select Petroleum fractions correlations. In the dialogue box then activated: Enter C7+ in the Name box, enter 234 for Molecular Weight and enter 0.838 for specific gravity.

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Click on Add to include the fraction in the defined stream. Alternatively you could use the Replace option to replace decane with C7+. 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

and Multiflash will determine the properties from a set of standard tables.

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

User Guide for Multiflash for Windows

0.45 0.20 0.10 0.10

Case studies - Phase equilibria • 177

Hexane Decane

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 -10912.3 J/mol

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 276.468K.

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You can also add the isenthalpic boundary for -10912.3 J/mol to your phase envelope.

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 Analysis ” on page 86. The case study we are considering here is based on a problem setup file called pvt_anal2.mfl, which uses the Revised Analysis method.

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Case studies - Phase equilibria • 179

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 datasource for your discrete (i.e. well-defined) pure components. This can be Infodata or DIPPR and we have chosen Infodata. Next at the top of the column headed Single fluid 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. 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 “Fluid composition” on page 90. 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. We will use the default distribution method, Infoanal2. 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

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followed by a screenshot of the experimental data and the fitted distribution

Click on OK and Close to return to the main window where the new fluid composition will be reported

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.

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Case studies - Phase equilibria • 181

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.

You can investigate various aspects of the characterisation and the sensitivity of the phase envelope to changing these. For instance you can include a n-paraffin distribution by ticking the Estimate Wax Content box. Set the starting point for the n-paraffin to N6 with 15 n-paraffins. In the this case the names and compositions of the fraction cuts will differ,

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Change the distribution function to Infoanal1 and repeat the process. In neither case is the phase envelope significantly affected.

If you return to the PVT Lab Analysis form and instead of the heaviest SCN choose total liquid and enter a MW of 68. Do the characterisation 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

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Case studies - Phase equilibria • 183

fraction into 15 pseudocomponents. You can see that this alters the cricondenbar but the major effect is on the cricondentherm.

With Infoanal1 you cannot choose a starting point that is above the highest cut for which experimental data are entered. With Infoanal2 you can set the starting point to be any pseudocomponent cut provided this is lower than the highest cut in the Component column. The highest default cut is C100. Next, return to the original fluid definition and re-plot the phase envelope, then in the PVT Analysis form enter a watercut. 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 separator gas and in the Recombined fluid section of the PVT form set the GOR units to m3 / m3 and enter 100. Do the characterisation and return to the main window and plot the new phase envelope.

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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 normal compositional analysis. Our example is based on the blackoil.mfl file. 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 .

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 split is fifteen 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.

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Case studies - Phase equilibria • 185

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.

Refrigerant mixtures Several of our customers have used Multiflash to determine the properties of refrigerants. A number of pure refrigerants were added to our INFODATA databank several years ago and more recently in our CSM 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.mfc. To determine the properties of any refrigerant mixture, first load refrig.mfc 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

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R401B R401C R402A R402B R404A R405A R406A R407A R407B R407C R407D R407E R408A R409A R410A R411A R411B R414B R417A R500 R501 R502 R503 R504 R507A R508A R508B To determine the dew point properties of R407A Load refrig.mfc 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:

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Case studies - Phase equilibria • 187

Polar systems Multiflash is equally applicable to polar mixtures, although for systems of this type an activity model, such as Wilson-E, NRTL, UNIQUAC or UNIFAC, plus binary interaction parameters is 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. For UNIFAC BIPs are generated from group structure. 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 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

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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 61.41 degC and a vapour phase fraction of acetone of 0.82105. 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 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 60.88 degC and a vapour phase fraction of acetone of 0.82399.

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Case studies - Phase equilibria • 189

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 WilsonE model no BIPs are available.

Click on the P, Bubble point flash button to see the model prediction using the default, BIPs = 0.0. For a mixture of 0.325 mole carbon tetrachloride and 0.675 mole hexane 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 342.75K with y =0.286. However, for this data set we have fitted data for the WilsonE model of 266.61 and 461.91 J/mol. You enter 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, 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 that if the BIP values are reported for carbon tetrachloride as component 1, then the binary pair is entered with carbon tetrachloride as the first component. If you your BIP units are not the default J/mol change the units by clicking on the Units button in the BIP box

Enter the new BIPs in the selected units

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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 342.85K 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)

No BIPs

y1

344.3

.270

342.85

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

You can see fro m 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.

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Case studies - Phase equilibria • 191

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 α 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-1 mol-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. Flash at fixed P and T: *** WARNING -20001 *** Unstable solution, more phases exist. T =298.25K P=1.00000E+05Pa ? CONVERGED ..UNSTABLE This instability warning 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.

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

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INFOLLBIPS;

User Guide for Multiflash for Windows

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 time consuming. 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 clearly identified at a mole fraction of 0.42 propanol. Propanol - Water 1 0.9 0.8 y,propanol

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.2

0.4

0.6

0.8

1

x, propanol

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 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 296.3K and a mole fraction of benzene of .8626. The recently added Regular Solution model could also be used for this particular mixture. For this fluid phase model the predicted eutectic at 1 bar is 296.4K and .8638 mole fraction of benzene. As the plot below shows the results for the models are virtually indistinguishable.

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Case studies - Phase equilibria • 193

Benzene-Naphthalene eutectic 370.00

350.00

T/K

330.00

310.00

290.00

270.00

250.00 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

x Benzene

Polymers The inclusion of the PC-SAFT equation of state in Multiflash extended Multiflash applications to the calculation of 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 It will not be apparent to the user, but we have re-structured the pure component databanks to provide a more flexible structure. This will allow storage of the data required for new models, such as PC-SAFT. Although we made these changes in Multiflash 3.3 we have not yet included any data for polymer components. At present these need to be created 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 but do not affect the computed results from PC-SAFT. 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 molecularweight, 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. 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 pseudocomponents which must be set up as userdefined components.

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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.

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 involatile, changing the BIP may affect the phase distribution and phase compositions more than the bubble point prediction. The result of altering the SAFTBIP from 0.0 to .05 for our sample system is shown below

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Case studies - Phase equilibria • 195

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 as a fourth component. In this case a bubble point calculation predicts the presence of four phases.

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 components for one of the liquid phases. A useful options is to set Key liquid2 heaviest: Until the advent of PC-SAFT, 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.

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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 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. The physical properties of the co-polymer must also be defined. The MW, Tc, Pc 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 volatility of polymers.. Next a “template” has to be created to indicate the structure of the co-polymer. In this example there are regular alternating ethylene and propylene segments. The bond structure is generated through Tools/Pure component/SAFT bond fractions.

The names of the constituent segments are entered as shown. The pattern of bond fractions is that for an alternating co-polymer. A random co-polymer would have a bond fraction pattern

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Case studies - Phase equilibria • 197

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, at 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. If you only have a gas and one liquid phase 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 point at which a second liquid phase appears, using the fixedphase fraction flash at specified T. The solution type should be set to “unspecified” and it may sometimes be necessary to use starting values.

A complex picture of the phase behaviour of co-poly mers of the same type but differing molecular weight can be built up as shown in the following figure.

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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. In MF3.4 changing the co-polymer MW also changed the overall composition. From MF3.5, provided the overall composition is specified in mass, this 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 36, 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 is used. The Hydrate model sets contain a complete description of model and phase specifications (as do the relevant hydrate model configuration files). To define a hydrate model interactively, 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

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conjunction with one of the solid phases such as any hydrate phase or the ice phase. 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. In MF3.6 we have added the possibility of considering the formation of chloride 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 Chloride 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. The hydrate model sets and the hydrate model configuration files have the following definitions.

Fluid phase model To carry out the full range of hydrate calculations with all the available inhibitors the recommended fluid phase model is the advanced RKS equation of state with the a parameter fitted to the pure component vapour pressure, the Peneloux density correction and the Infochem mixing rule. The required binary interaction parameters for hydrocarbons, light gases, water and inhibitors are available from the OILANDGAS BIP dataset. However, for inhibition with methanol, ethanol, MEG, DEG or TEG, CPA may be preferable as it reproduces the partitioning of methanol and MEG between water and hydrocarbon vapour and liquid phases more accurately than RKSA plus the Infochem style mixing rule. As a result it will usually predict less conservative results for the amount of methanol required for a fixed inhibition. The differences between the model predictions will be most marked for systems with low water content and/or significant amounts of C6+.

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.

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 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 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.

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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 H2 S 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 or 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 temp erature. However, this is not a real situation as ice is not being considered except for nucleation. If chloride scales are to be considered then further phase descriptors are required. These must represent the correct fixed composition of the scale and in version MF3.6 are: NaCl, NaCl.2H2O, KCl, CaCl2.2H2O, CaCl2.4H2O, CaCl2.6H2O.

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. To define the case study interactively:

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Select Select, then select Model Set, followed by selecting Hydrates tab to activate the Hydrates dialog box. 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 CO2 N2 WATER

Moles 85.93 6.75 3.13 0.71 0.88 0.57 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 outputunits for this example are in g. Click on the P,T flash button You will see the following results in the results window.

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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.

Hydrate formation and dissociation temperature at given pressure

P r e s s u r e

Hydrate zone

Hydrate formation curve

Hydrate risk

Hydrate free

Hydrate dissociation curve

Temperature

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 formation temperature is the point at which the nucleation of hydrate occurs and hydrate will form. Between these two points is the area of hydrate risk where hydrates may or may not form depending on the time scale.

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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,

show that the hydrate2 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. There is no button for nucleation calculations so select Nucleation from “Select basis” in the Fixed Phase Fraction Flash – at

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specified P dialog box., Set the phase fraction text box to zero as before and then click Do flash button.

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.

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.

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.

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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.

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 177.

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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. This is an example of a Multiflash tolerance calculation. The overall compositions must be specified on a water-free basis. A second mixture composition is then specified using the Composition of Second Fluid tab in the Tolerance Calculation dialogue box.

For a water tolerance calculation this would be pure water. Under the Phase Specified tab the fixedphase and phase fraction can be specified using the Select phase and Enter phase fraction boxes, zero molar 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 maximum water content for the above mixture at 270K and 1 MPa before hydrates will form. As the overall composition must be specified on a water free basis, first remove water from the mixture by: Clicking on Composition and entering 0.0 for the amount of water. Water must remain in the components list. Select Calculate from the menu bar, then select Tolerance Calculation. Select the required phase from Select phase box by clicking the downwardarrow on the right side of the box, then set 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 and leave the rest to zero. Click Calculate to carry out the tolerance calculation. Click Close back to the main window. In the results window you will see,

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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 mi xture 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 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 with water plus 20% by mass of methanol relative to the water. 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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Alternatively 20 wt% methanol is approximately equivalent to adding 1.4 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.

The results show that the addition of this concentration of methanol is sufficient to prevent hydrate forming 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 fixedphase fraction flash at fixed P with the hydrate 2 phase at 0.0 phase fraction.

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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 a t a given temperature Again this is analogous to the calculation above but you use the Hydrate dissociation at given T button, , or specify a fixedphase fraction flash at fixed T. The hydrate dissociation pressure increases from 0.56 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. Select Calculate, then Tolerance Calculation to activate the Tolerance Calculation dialogue box. Select the required phase from Select phase box. Set 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 moles of the mixture specified by the tolerance calculation which must be mixed with the original inhibitor-free stream to meet the condition of zero hydrate phase.

Salt inhibition Multiflash models for hydrate inhibition include the inhibiting effect of saline water. The original salt model represents the salts as a single salt pseudocomponent which can be loaded from INFODATA. As sodium chloride is usually the dominant component, the model reduces other salt components to a sodium chloride equivalent basis and the databank stores the molecular weight of sodium chloride. The original Electrolyte salt model treats the salt as an electrolyte composed of Na + and Cl- ions only. The model extension in MF3.5 allowed for the salt to be described in terms of Na +, K+, Ca ++ and Cl- 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 80 that converts various analyses into either the equivalent amount of salt component or sodium, potassium, calcium and chloride ions. Load the hydrate.mfl file: Change the Model set from Association to Association + Electrolyte Select the Inhibitor Calculator from the Tools menu and the tab for the Electrolyte Model For this particular example there is information on the composition of the formation water.

NaCl CaCl2 MgCl2 KCl SrCl2 BaCl2

wt% 6.993 0.735 0.186 0.066 0.099 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 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 272.78K.

Scale precipitation The precipitation of Chloride Scales is a new feature of MF3.6. The new model allows for precipitation of NaCl, NaCl.2(H2O), KCl, CaCl2.6(H2O), CaCl.4(H2O) and CaCl.2)H2O). This is activated by ticking the Chloride Scales

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box in the Hydrates Model Set but can only be defined with an “Electrolyte” salt 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.

A similar calculation looking for the formation of CaCl2.6(H2O) produces seven phases but at 240K.

A more likely scenario is 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

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whereas at the lower temperatures where a hydrate phase is present you will see NaCl.2(H2O) being formed.

RKSA(Infochem) model You can repeat any of the above calculations using the RKSA(Infochem) model for the fluid phase. The chosen mixture is not one where we might expect to see significant differences between model predictions. The predictions of hydrate dissociation temperatures and pressures are virtually identical. The hydrate dissociation temperature at 1 MPa with RKSAINFO was 276.144K, with CPA it is 276.142K. Similarly the hydrate dissociation pressure at 270K was 0.598 MPa for both RKSAINFO and CPA. The partitioning results do show some differences between RKSAINFO and CPA. The amount of water required before hydrates form at 270K and 1 MPa changes from .800g (RKSAINFO) to .840g (CPA) for the composition specified and the amount of methanol required to inhibit hydrate formation at these conditions with 10 mole water present reduces from 30.45g (RKSAINFO) to 28.5g (CPA). Using the Electrolyte salt model with CPA or RKSAINFO gives results with trivial differences. To compare these to the results from the old salt model, you have to return to Select Model Set and this time define RKSA(Infochem) as the model. Defining this model will automatically remove the ions from the list of components. Now from the Inhibitor Calculator select the Salt Component Model tab and add the same salt composition to the water phase. This time .266 moles of Salt component are added. The predicted hydrate temperature at 1 MPa changes from 277.78K to 272.83K. Larger differences may occur at higher salt concentrations.

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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 isoparaffins 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 deposition it is worth referring to a paper by Erickson et al. SPE 26604, (1983). which comp ares 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. In MF3.6 we have also recommended that positive amounts of deposited wax are used to identify the WAT, rather than the strict thermodynamic interpretation of zero percent. The suggested default values are 0.045 wt% for reproducing CPM measurements and 0.3wt% 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%. This modification only applies if the Coutinho model is used. For the Multisolid model the WAT is calculated for zero mass or mol%. One other modification is that for the Coutinho model the mass or mole% of wax is related to the liquid plus wax phases rather than to the total fluid less any aqueous phase, which was the case in previous versions of Multiflash.

Defining the wax model There are two wax models in Multiflash. The wax model in earlier versions is a Multisolid model, whereas the more recent Coutinho model is a solid solution model. Both are described in more detail in “Modelling wax precipitation” on page 40. We recommend the use of the Coutinho model but retain the Multisolid model for backward compatibility.

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Coutinho wax model The Coutinho wax model is a solid solution model which requires more information than the Multisolid model. Predictions from Coutinho’s model are largely governed by the n-paraffin distribution. If no experimental data are available for this it can be estimated from the total wax content. If these data too are lacking then Multiflash has procedures to estimate them. The n-paraffin distribution may be defined differently from that for the remaining liquid phase but we have found that starting the n-paraffin pseudocomponents from C6 and splitting the plus fraction into 15 pseudocomponents is again a useful default. With a limited experimental data set it is not possible to make any definitive statements concerning the accuracy of the two models in predicting WAT, although it would appear that Coutinho's model will predict a WAT slightly higher than the Multisolid model. What is clear is that Coutinho's model provides a much improved prediction of the amount of wax precipitated as a function of temperature.

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. Two wax models are available in Multiflash..

Select the Coutinho model 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 Analysis ” on page 86. If you have an n-paraffin distribution then you should open the PVT Analysis using the new button. However, in this example, input file wax.mfl, there is no n-paraffin distribution only a wax content. In this case you should use the normal

button.

For the Coutinho model only the Infoanal2 distribution procedure is appropriate. As the input file includes the wax content, simply providing this value, or if it is not available ticking the “Estimate wax content box”, removes the option of choosing the Infoanal1 distribution. Enter the fluid composition and set both the

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pseudocomponents and n-paraffins to be split from C6 (or N6) into fifteen fractions.

If you fail to Enter a wax content or to ask for this to be estimated then you will 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.

Providing you have n-paraffins in your fluid characterisation you can calculate the WAT at any pressure, by using the WAT, button. With the Coutinho model clicking on the button initiates a further dialogue box

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 calculation. Our particular example is based on a supplied problem set up file called wax.mfl. This particular fluid has a reported experimental WAT based on three different measurement techniques. At 1 bar, using CPM the reported WAT was 53C, using NMR was 45C and using DSC was 40C. The predicted WAT for the CPM default is 51C and for the suggested DSC default is 39C. 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 n-

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paraffin will be lower than the N90 set. In this case the heaviest n-paraffin is n76+ and the WAT for the CPM default is 48.5 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 either model to match these values. This is done using the Tools/Matching/Wax Phase option. The Matching option is described in “Matching using petroleum fraction properties” on page 102. For this particular example we can take the WAT at 1 bar to be 53 DegC for CPM. Keep the characterisation for n-paraffin at N90 and 1 PF, then enter the value or values for the WAT temperatures and pressure and the phase fraction. The fraction chosen can be zero but should probably reflect the suggested defaults for the technique used for the WAT measurement.

For the Coutinho model 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. In this case it makes sense to plot the boundary for .00045 mass fraction as we have just matched to this value at 1 bar and we have a “dead” oil.

For a live oil the amount of wax will be with respect to the total fluid rather than the liquid. This will vary with pressure so in this case it may be better to choose zero mass fraction for the plot. The wax boundary for a live oil is a distinctly different shape. The D marks the point where the wax boundary crosses the bubble point line.

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The full range of flashes is available for the Wax models.

Calculating wax precipitation As with any other phase the amount and composition of the wax phase is determined as part of any flash calculation. 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 calculation of the wax precipitated as a function of temperature. 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 is to use the Wax precipitation curve button, . Clicking on this produces a table of the wax weight fraction as a with respect to the liquid plus gas precipitated as a function of reducing temperature. The starting temperature is 0°C, or the equivalent in other units, and the finishing temperature is the calculated WAT for zero percent wax. 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 or 1 bar if no pressure is specified. The wax precipitation curve below was generated using wax.mfl as supplied.

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Case studies – Wax precipitation • 223

If you require additional control of the table configuration then you can use a command entered in the Tools/Command box. The format of the command is WAXPC pressure tstart tincrement In MF3.6 the wax precipitation curve is also plotted

And you can use the Add Data button to add the measured WAT if you wish.

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You can also remove the n-paraffin estimation and switch to the Multisolid to compare the wax precipitation predicted by the two models, which is very different

However, if you add the experimental data for the amount of wax precipitated as measured by DSC you can see that the Coutinho model is more realistic.

If you have the Excel add-in, you can also do the calculations in a spreadsheet and compare models.

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Multisolid model If you want to try the Multisolid wax model for any of the calculations, then you need to re-characterise the fluid without n-paraffin, e.g. by deleting the wax content in the text box in the PVT analysis form and choosing the Infoanal1 distribution. Our recommended procedure for the Multisolid model is to start the pseudocomponents from C6 and to split the plus fraction into 15 pseudocomponents, regardless of the experimental analysis of the distribution. Then choose the Multisolid option from the Model Set.

When you activate the button the WAT is calculated immediately for zero percent wax at the pressure set in the text box. In this case the predicted WAT at 1 bar is 45.5C. You can use the fixedphase fraction flash to calculate the WAT for non-zero amounts of wax, but in this case the fraction of wax is related to the total fluid rather than the liquid plus wax phases.

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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 the 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 an additional term 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 deposition 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 flocculation from live oils. We are aware that many users have only titration data for dead (STO) oils. We have investigated using 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:

User Guide for Multiflash for Windows

•

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

•

One set of flocculation conditions for the asphaltene flocculation

Case studies – Asphaltene flocculation • 227

•

Bubble point (optional) to “tune” the petroleum fraction properties.

In MF3.6 we have modified Multiflash to take into account the fact that the SARA will have been measured for the stock tank oil rather than the total reservoir fluid. If you load files characterised in previous versions of Multiflash, where you have modified the SARA to represent the live oil, in MF36 the SARA will be taken to be for the STO. To correct this you will need to return to the original measurements or to carry out a P,T flash to re-calculate the STO value. 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 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 heptane. Some laboratories report the wt% asphaltene precipitated by pentane. It is difficult to give exact guidance on how to convert between pentane and 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 pentane is approximately twice that precipitated by heptane. However, ratios can vary from 1.3 to 2.7. If you do not have the complete data set we recommend 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 flocculation is not as sensitive to the characterisation of the fluid as the wax models. However, we suggest you consider using a common characterisation procedure. We recommend that in the PVT Analysis facility you start the pseudocomponent split at C6 and split the fraction into 15 components, regardless of the original experimental distribution. Finally, to model asphaltene flocculation successfully you need to adjust one model parameter to match actual flocculation data. If you have such experimental data available then you match to this. If you do not have any data there are two options. We have modified the screening procedure suggested by De Boer et al to adjust the parameter based on a knowledge of the reservoir conditions or you can use STO titration data using heptane if this is available,

Defining the asphaltene model The cubic equation of state, RKSA, is defined as part of the model, it is not possible to choose a different fluid phase model. This flexibility may be added at a later date. To set up the asphaltene model you can either load the asphalt.mfc file or use the Select/Model Set option. Select the asphaltene tab, Click on Define Model to select the asphaltene model and Click on Close to load the model.

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The default phases for the asphaltene model are gas, liquid1 and asphaltene. However, you can add a water phase if you wish. In Windows this can be done very simply by ticking the water check box. In the asphalt.mfc file we have commented out the lines defining the water phase and the key component for this phase. If you wish amend the .mfc file you must edit it to remove the # sign before the relevant lines. However, the asphaltene model parameters should be produced for the fluid composition excluding water. The next step is to characterise your fluid. Go to the PVT Analysis form, described in detail in “PVT Analysis ” on page 86. Set the Analysis method to the Infoanal2 method. 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 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 this 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 Analysis ” on page 86. Since the release of the asphaltene model we have had feedback from several users. In our original database of asphaltene measurements the resin/asphaltene 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 either ignore this warning message and see if model parameters can be produced, you can increase the resin/asphaltene ratio manually or delete the Resin amount from the SARA and tick the Estimate RA box and 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+. The next stage is to use the matching facility to “tune” the pseudocomponent properties and fix the one adjustable model parameter. We recommend that if you have bubble point data available you tune the petroleum fraction properties to match this. If you have multiple bubble points then we suggest you use the Tools/Matching Bubble point form. However, if you have only one measured bubble point we have incorporated this into the asphaltene matching form. In practice we have found that you do not always have to match to bubble point data to model the asphaltene data, although you almost always need such matching for light oils. However, we do recommend use of the matching procedure for the asphaltene phase. Although we have supplied a default for the case where you have no information to fix the adjustable model parameter we cannot recommend any of the subsequent results. But we do recognise that you may want to use the

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asphaltene model for screening purposes and we have developed a procedure to adjust this parameter based on a knowledge of the reservoir conditions or STO titration 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 values to obtain the asphaltene model parameters. Initially, we have used the reservoir conditions (241F, 8500 psi), although the dialogue box allows for two flocculation points (new in MF36) or titration data.

Click on Match and Close: the model parameter values will be displayed in the main window. The asphaltene model has now been defined.

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Calculating asphaltene flocculation conditions Once the asphaltene model has been defined and the parameters generated you can carry out flocculation calculations. If you are starting from the position where you only know the reservoir conditions and have no information on specific flocculation 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 follow this procedure. 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.

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 deposition envelope crosses the bubble point line.

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These points can be very useful for setting an appropriate starting pressure for the deposition envelope or providing starting values if these are required. For this example go back to the Phase Envelope and this time set the phase to asphaltene, the fraction to zero, the solution type to unspecified and the Initial value for pressure to 3500 psi with pressure decreasing. Ask for more points to be plotted until the asphaltene boundary is 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, the lower boundary the Lower retrograde 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 200F and 4000 psi, Click on the P,T icon, or Select the P,T flash from the Calculate\Standard flash menu. The phases present, and the composition and amount of each phase, will be reported. 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 deposit at any given temperature then you should use a flash at fixed phase fraction and temperature. Set the temperature, in this case 200F. Again Click on the icon or select the calculation option and the dialogue box will appear.

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 7948 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 Lower retrograde. In this case the reported pressure is 1874 psi. You can determine the amount of asphaltene flocculation at any set of P,T conditions using an isothermal flash as described earlier, You may like to get an idea of the amount of flocculation along any isotherm by carrying out a series of 4 or 5 calculations. The maximum flocculation will occur at the boiling point.

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Sensitivity of calculations to variation in input data Choice of Analysis method There is a choice of analysis method to characterise the fluid, the original method, Infoanal1 and the revised method, Infoanal2, which can also be used with and without a n-paraffin distribution. The asphaltene model has been modified in MF3.6 and Infoanal1 can longer be used with the asphaltene model. The initial calculations were carried out based on the default Infoanal2 analysis method and matching to reservoir conditions. The plots below show the effect of including the n-paraffin distribution. After any re -characterisation the data must be matched again, in this case the bubble points and the asphaltene parameters.

The inclusion of the n-paraffins for this case makes a small difference to the predicted ADE.

Data Availability This example of asphaltene flocculation 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 case you can see a noticeable effect from the matching.

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Case studies – Asphaltene flocculation • 235

There are other data that may be missing and have an effect on 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 is 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 deposition envelope (ADE). It is important to include this step; the matching procedure is cancelled when the fluid is recharacterised. The default procedure estimates both the weight % of asphaltene and the resin/asphaltene ratio. For this particular example the weight % of asphaltene with Infoanal2 is very close to the reported value at 0.7 wt% asphaltene 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 and 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 ADE.

Although the estimated R/A ratio is lower, at 13, than the original, at 22, the resultant ADE are 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 has a reduces plus matching to a specific flocculation point or reservoir condition compensates to some extent for changes in R/A.

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.

For this particular example the resultant ADE is reasonably close to the ADE calculated from the real reservoir conditions. As a corollary to this we have noticed that you usually generate very conservative ADE 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.

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Case studies – Asphaltene flocculation • 237

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 to the reservoir temperature no warning is issued.

No reservoir or flocculation conditions If you do not have either reservoir or flocculation 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 flocculation 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 flocculation the model parameters are generated from correlations based on data held in our database. The results from using this route are vary variable, depending on the fluid analysis and we cannot recommend its use. In this case the result would be a much more conservative ADE.

Matching to asphaltene deposition data The assumption in this case is that you have more data than our basic example, real deposition data either from field conditions or a asphaltene flocculation measurement. In this case we have two flocculation points at 241F and 6921 psi and 120F and 9150 psi. Simply enter these in the Asphaltene matching box instead of the Reservoir conditions.

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For comparison purposes we have matched to each point individually and then to both points.

With the new graphics package it is easy to add the actual data to the plot. Using the Add Data option in the phase envelope plotter you can add the measured flocculation points and the reservoir conditions.

Gas injection It is known that as gas is injected into a reservoir the likelihood of asphaltene flocculation is increased. The asphaltene model predicts this trend correctly. Return to the original ADE, calculated from the asphex.mfl input file with matched bubble points and reservoir conditions. You can mimic gas injection by

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Case studies – Asphaltene flocculation • 239

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 replot the ADE you will see that the fluid bubble point line is at higher pressures and the ADE has expanded.

When looking at the effect of gas injection you should, of course, not rematch the fluid bubble point or asphaltene phase deposition as doing this will alter the petroleum fraction properties and model parameters. 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.

Titration The Infochem asphaltene mo del was intended for use in predicting the asphaltene flocculation of live oils and the model parameter generation 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 can 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 any 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 heptane. Our example is based on the titration.mfl file provided. The file includes the live oil composition and the wt% asphaltene. The reported value was 1.9 wt% for the STO. Characterise the fluid composition as usual and the return to the main menu, Tools/Matching/Asphaltene phase. The reported amount of heptane to just cause asphaltenes to flocculate from the STO at ambient conditions is 1.4 cm3 per g tank oil. This has been converted to .962 g heptane using the known density. Enter this value and click on match.

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The asphaltene model parameters will be reported in the main window as usual and the ADE plotted. The resultant ADE is compared 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 ADE predicted from matching to titration of the STO is very close to the ADE from flocculation 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. If your titration data does not include the amount of heptane just to initiate flocculation and it has to be deduced from the other titration results then the procedure for parameter generation is slightly more complicated and requires the use of an Excel spreadsheet. In Multiflash for Windows either characterise the STO, if this composition is provided or flash the characterised live oil to STO conditions, using the RKSA model set to ensure that no separate asphaltene phase is formed. If you have bubble point data it is important that you match to this before flashing to STO conditions. Using the STO composition, change the model set to asphaltene, match the asphaltene flocculation to ambient conditions and save the problem using the File/Save Problem Setup option . You then need to create an Excel worksheet to read in 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 called from 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

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Case studies – Asphaltene flocculation • 241

asphaltene parameters. This can be copied from Multiflash for Windows 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\exampleoil.mfl"; model MREFASPHALTENE RAEQUIL DATA AAPREEXP 1.00000000 AAEXP 1.00000000 RAPREEXP .62542 RAEXP 0.98213 ; 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 flocculation 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 flocculation point using the matching facility produced the following parameters RAP

0.93567

RAE

0.96636

Whereas fitting to the other four points in Excel gave parameters RAP

0.95364

RAE

0.96636

These parameters represent the STO titration data well, but the amount of heptane to just initiate flocculation is slightly higher. You can check the predicted value for the amount of heptane required for the onset of asphaltene flocculation using a tolerance calculation with heptane as the second fluid. The predicted amount is .986 g/g oil rather than .962.

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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 ADE from live oil data. Some predictions are possible from titration data but 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 it. For this example we also had a measured flocculation point and you can see the ADEs 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 Coutinho and there is only one asphaltene model. The Combined Solids option is only designed to look at solid formation, if you want to study the more complex problems such as hydrate inhibition you should still choose the dedicated Hydrates model set. In fact, you will see messages to this effect if you only choose a single solid phase in the Combined Solids option.

Asphaltene flocculation To understand what happens when more than one solid forms a useful starting point is to examine asphaltene flocculation 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: R34-39

0.42815381

R39-45

3.0324184

R45-54

2.6016031

R54-66

2.1075435

R66-74

0.68416739

R74+

0.66073206

ASPHALTENE

0.55317548

The asphaltene model parameters are matched with a bubble point of 120F and 2650 psia and an asphaltene flocculation point of 120F and 8750 psia. The predicted ADE is plotted below.

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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 a 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: R34-38

0.28576627

R38-43

2.3119637

R43-48

2.0564111

R48-55

1.7829322

R55-64

1.5031864

R64-73

0.89704015

R73

0.016889776

R74+

0.66042869

ASPHALTENE

0.55317548

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 ADE. 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 upper ADE 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 flocculation occurring at lower temperatures. Of course with water present there is also the possibility of a separate water phase

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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 kinetics as well as thermodynamics.

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Case studies – Excel spreadsheets

Introduction Although only accessible if you have licensed the Excel add-in option 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. 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 NRT L. 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.

Notes The first spreadsheet consists of notes on how to use UNFICAFIT.xls and how to enter the fitted BIPs in Multiflash.

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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 then generated, including 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 criteria for minimisation. The default setting is to minimise on the sum of squares of the differences between given and predicted temperature or pressure. It is possible to minimise on 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. It is again possible to change this constraint using 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 “Supplementing or overwriting BIPs” on page 56. 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.

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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. 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 better to enter #N/A for the y compositions. If you fail to do this the Solver will still function provided the minimisation criteria is based on difference in temperature or pressure – 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.

SolidsB.xls and SolidsA.xls Several of the engineers using Multiflash for Windows 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 two spreadsheets, 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. There are two spreadsheets, SolidsB.xls and SolidsA.xls. SolidsB.xls includes the recommended PVT analysis routines and solids models; Anal2 for the PVT, the CPA/Electrolyte model for Hydrates, the Coutinho wax model for waxes and the asphaltene model is standard. For backward compatibility we have retained the SolidsA.xls. This spreadsheet takes a common PVT fluid analysis, which can be actively characterised in the first spreadsheet using the PVT Analysis 1 method. However, this analysis does not allocate any n-paraffin distribution and so the wax model is limited to Multisolid. The spreadsheets consists of several worksheets and each worksheet has the relevant models built-in in hidden rows or columns. In SolidsB.xls, as the revised PVT Analysis2 is more complex, in this spreadsheet the PVT must first be carried out using Multiflash for Windows, and the mfl file written and referenced in the first worksheet. The Excel calculations are set to manual using the Excel Tools/Options facility. 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.

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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 isoparaffins 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. This work sheet is fully active in SolidsA.xls and characterises the fluid according to the fluid composition provided. In common with our other spreadsheets input data are marked in red. 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.

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 predict 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 phase fraction flash function and the amount of wax is set to be .0002 mole fraction of the total fluid. In this version of the Mf3.6 XLL there is no function equivalent to the WAT button in the GUI which calculates the WAT at a fixed mass or mole percent of the liquid plus wax phases. The WAT is plotted automatically as a function of pressure. The starting pressure and step can 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. 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 flocculation envelope automatically with starting points derived from the flocculation or reservoir conditions. However, as the engineers using our Windows phase envelope facility will appreciate it is difficult to make this absolutely foolproof.

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Asphaltene with gas injection The effect of gas injection on asphaltene flocculation is most easily calculated in Excel. The asphaltene model parameters for any fluid should be retained when studying the effect of added gas and 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 especially 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. They can be added in fixed amount or the concentration required for hydrate inhibition at set conditions can be predicted.

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Users who want to investigate hydrate behaviour only may find the hydrateinfo.xls and hydratecpa.xls spreadsheets useful. These 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 copy of Multiflash includes the mercury model. Infochem has developed a mercury model in order to predict the solubility of mercury in natural gases and condensates, and the distribution of mercury between gas, condensate and water phases. An advanced form of the RedlichKwong-Soave equation of state (RKS), widely used in the oil and gas industry, has been used as the basis for the model. 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 ele mental form. To correctly predict the phase behaviour of both mercury and organomercury compounds, data was collected for solubility in hydrocarbons and water. Dimethylmercury and diphenylmercury are chosen to represent light and heavy organomercury compounds respectively. Additional measurements on mercury solubility, including that in TEG, provide access to data not in the public domain. The mercury model is supported by a special version of the Infodata databank and BIP correlations.

Defining the mercury model The mercury model my be defined by selecting the mercury tab in the Select/Model Set box

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or by loading the file mercury.mfc. The phases defined for this model are gas, liquid1 (hydrocarbon liquid), mercury (liquid mercury), water and solidmercury. If you do require a second hydrocarbon liquid phase then you must define an additional phase descriptor using Tools/Command as described in the Appendix – Multiflash commands in the User Guide.

Calculating mercury partitioning 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.

In this particular example we have only specified mercury but the principle is the same if components dimethyl and diphenyl mercury 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. If the mercury model is used in an Excel spreadsheet or a third party simulator the streams can be merged and recycled but the GUI only allows for one calculated stream composition to be used for the any flash. Once Hg_Example.mfl has been loaded enter the flash conditions 4 degC and 77 bar and carry out a P,T flash.

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The mercury partitions between the exiting streams from the warm separator. The gas phase compositions can be highlighted, copied and then pasted into the Composition drop down menu to provide the feed for the P,T flash for the cold separator. In the first case the conditions are set to -25 DegC and 41 bar.

At these conditions the separator is cold enough for a separate liquid mercury phase to form. If the temperature is even colder, -52 DegC, then mercury forms as a solid and slightly more mercury is formed.

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Calculating mercury dropout As can be seen from the examples above, the mercury model can calculate mercury dropping out either as a liquid or as a solid. The fixedphase fraction flash can also be used to calculate the temperature or pressure at which a pure mercury phase will drop out. If, for the example we are using, the gas from the cold separator at -25C and 41 bar is compressed to 100bar, such as for gas export, the PFRACF calculation can be used to determine the temperature at which liquid mercury will drop out.

If the gas from the cold separator at -52 DegC and 41 bar is used for this calculation the mercury concentration in the gas phase is much lower and a separate mercury phase will not form in the gas export line until solid mercury forms at -48.6 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.

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Case studies - chemical equilibria

Introduction Most thermodynamic simulation in process engineering centres on the need to calculate phase equilibria for separation processes, especially distillation. However chemical equilibria is another area where thermodynamic information can be exploited to make predictions about chemical processes. The chemical reaction module in Multiflash is a utility for performing simultaneous phase and chemical equilibrium calculations. It can handle equilibria involving combinations of one gas phase, one liquid phase and any number of pure solids. The chemical reaction module does not rely on reaction schemes. You do not need to specify any reaction mechanism but only list all the possible products and reactants. The applications are many and varied but a sample is discussed here

Xylene isomerisation To start with a very simple example, select ortho-, meta- and para-xylene from the INFODATA fluids bank. Select RKS using the route Select/Model set/Equations of State/RKS. The model set will be used to predict the phase properties and any of the cubic equation of state models would be acceptable for this case study. Enter a temperature of 575K and a pressure of 10 bar (remember to set the pressure and temperature units correctly). Click on the Chemical reaction button, The mixture will be in the gas phase. You may ignore the warning message. All model sets and model configuration files include phase descriptors for several phases to give the user greatest flexibility. However, the chemical reaction module is limited to one gas and one liquid phase, hence the warning message and the ?CONVERGED message. If you use the problem setup file, xylene.mfl, only one liquid phase is defined and the warning message will not appear. Also, the calculation will be reported as converged. Input any composition for the mixture provided there is a positive amount for each xylene.

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At the same conditions the ratios of the three xylenes will remain the same.

If you reduce the temperature to 300K and the pressure to 1 bar the mixture will be in the liquid phase. The equilibrium concentrations of the three xylenes are slightly, but not significantly, different and again are unaffected by the input compositions

Steam cracking of ethane This case study, defined in C2CRACK.mfl, to investigate the steam cracking of ethane is effectively a study of two competing reactions 2H2O + C2H6 = 2CO + 2H2O

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4H2O + C2H6 = 2CO2 + 7H2 You do not have to specify the reaction schemes only the products and reactants. Therefore, you should Select ethane, water, hydrogen, carbon monoxide and carbon dioxide from INFODATA In the drop down table under Comp osition enter 2 moles of water and 1 mole of ethane Select an equation of state model, e.g. RKS Set the temperature to 1000K and the pressure to 1bar. For this calculation it may also be useful to change the output from fractions to amounts, see “Changing units ” on page 144. Click on the chemical reaction button, Under these conditions 4.96 moles of hydrogen, 1.974 moles of carbon monoxide and 0.00668 moles of carbon dioxide are produced. Vary the ratio of water to ethane and note the increase in the amounts of carbon dioxide as the ratio increases. You can look at the enthalpy changes for the reactions by carrying out a simple P,T flash to obtain the enthalpy of the reactants and the chemical P,T reaction flash to see the enthalpy of the products.

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Appendix - Multiflash Commands

Introduction The Multiflash command language is common to all Multiflash implementations - DOS, 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 Windows version the use of commands has largely been replaced by menus and icons. However, not all the facilities available in the command processor version of Multiflash have been incorporated as menu options in the current version of Multiflash for Windows. An example might be reverting to a previous version of the Infochem BIP databank to maintain backward compatibility. To allow you full access to all Multiflash facilities a Tools command option is available, see “Tools ” on page 23, which allows you to enter a command and apply this in the Windows version. Commands are also used to specify problems in problem setup files. If the set up file is created by saving a problem specified interactively then the appropriate commands will be transferred automatically to the set up file.

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: •

Defining models

•

Supplying an external file of BIPs

•

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

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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 73. Model_options are additional keywords that describe model variants, references to other, previously-defined, models or references to the source of binary interaction parameters.

Any .mfc file will provide an example of how to set up a model definition.

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; e.g. bipset PRBIP 1 constant eos none methane butane 0.005; 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 Other examples are provided in “Supplementing or overwriting BIPs” on page 56. 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.

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.

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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 from the list of PDs available for Multiflash

1. the pd_id must have been previously defined 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 35, 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 “Troubleshooting - flash calculations” on page 138. 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

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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

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Bubble and dew point flashes 122 Bubble point 122

C

Index

A Activity coefficient equations in Multiflash 33 Activity coefficient methods 33 Activity coefficient models see Models 33 Activity coefficients 151 Activity model worksheets 252 Adding a component 68 Adding water to the system 174 Adding, inserting, replacing and deleting components 68 Additional calculations 12, 16 Additional fluid information 91 Advanced Equation of state options 29 Amount of inhibitor required to suppress hydrates 213 Analysis method 87 Appendix - Multiflash Commands 4 Asphaltene flocculation 245 Asphaltene with gas injection 257 Asphaltenes 5, 49, 227, 256 Case study 49, 227 Defining Asphaltene model 49 Model 40 Azeotropes 193

B Benedict-Webb-Rubin-Starling (BWRS) equation of state 32 Benedict-Webb-Rubin-Starling model 32 Binary interaction parameters 52, 53, 54, 56, 59 Displaying values 54 Supplementing and overwriting BIPs 56 Temperature dependence 53 Units 52 BIP databank 60 BIP set Names 55 BIPs 6 see Binary interaction parameters 52 BIPs available in Multiflash 53 BIPs not displayed 60 Black Oil Analysis 95, 185

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Calculate 24 Calculating asphaltene flocculation conditions 232 Calculating mercury dropout 263 Calculating mercury partitioning 261 Calculating petroleum fraction properties 100 Calculating the bubble point curve 172 Calculating the dew point curve 173 Calculating the water dew point line 175 Calculating wax appearance temperature (WAT) 220 Calculating wax precipitation 223 Calculation output 150 Calculations 3 seeFlash calculations 119 Calculations with inhibitors 210 Can hydrates form at given P and T ? 210 CAPE-OPEN Interface 158 Carry out an isothermal flash 13 Carrying out the flash calculation 16 Case studies 4, 163, 171, 201, 265 Asphaltenes 49, 227 Chemical equilibria 265 Combined solids 245 Hydrate formation and inhibition 201 Phase equilibria 171 Polymers 194 Pure component data 163 PVT Analysis 179 Refrigerants 186 Tutorial 9 Wax 219, 260 Change the composition 13 Change the enthalpy 13 Change the pressure 13 Changing units 144 Changing units in a problem setup file 145 Changing units interactively 144 Characterisation 97 Chemical equilibrium 2, 120, 140, 141, 265 Case study 265 Troubleshooting 141 Chemical reaction 140 Chemreact 2, 120, 140 Choice of Analysis method 235 Chung-Lee-Starling model 42 Clearing previous problems 14 Combined solids 245 Combined Solids Model 49 Commands 4, 25 Appendix 269 Tools/Command option 25 Component cannot be found 84 Component list 88 Components 3, 61, 63, 64, 65, 66, 67, 68, 69, 82, 83, 163 Adding a component to a stream 68

Index • 275

Condensed components 63 Databanks 61 Defining components in stream 64 Deleting a component from a stream 69 DIPPR 61 Displaying pure component data 163 INFOCOND 63 Inserting a component in a stream 68 Maximum number in stream 64 Normal components 61 Properties of normal components 61 Replacing a component in a stream 69 Selecting by formula 67 Selecting by name 66 Selecting by scrolling through list 65 Selecting by substring 67 Selecting components 65 Synonyms of components 66 Troubleshooting 82, 83 User defined 71 Compositions 16, 115, 145 Amounts and fractions 116, 144, 145, 150 Specifying compositions 115 Condensed components 63 INFOCOND 63 Conditions 115, 117 Conditions section of main window 115 Specifying compositions 115 Specifying enthalpy 117 Specifying entropy 117 Specifying internal energy 117 Specifying pressure 117 Specifying temperature 117 Specifying volume 117 Troubleshooting 117 Configuration files Model configuration files 22, 43, 45 Consider all types of solution 140 Co-Polymers 197 Corresponding states (CSM) model 32 Coutinho wax model 220 CSM Reference fluids 33 Cubic plus association (CPA) model 30 Current settings Display 153 Customising the phase envelope plot 135

D Data Availability 235 Data input 194 Databank not found 83 Databank not licensed 83 Databanks 7 DIPPR 61 INFOCOND 63 INFODATA 61 Default units 143 Define Input Conditions 15

276 • Index

Defining a mixture (stream) in Multiflash 64 Defining a problem in Multiflash 10 Defining models 269 Defining petroleum fractions 85 Defining phase descriptors and key components 270 Defining the asphaltene model 228 Defining the components 14 Defining the fixedphase flash 124 Defining the hydrate models 201 Defining the mercury model 260 Defining the model 14 Defining the model from a model configuration file 45 Defining the model from the menu 43 Defining the problem interactively 163 Defining the wax model 219 Deleting a component 69 Dew point 122 Dialogue boxes, text boxes, tab controls, drop-down tables and menus 25 Diffusion coefficient 43 Diffusion coefficients 43 DIPPR 7, 61 Displaying status for current settings 153 Documentation 2 Dortmund Modified UNIFAC method 34 Dortmund Modified UNIFAC model 34

E Edit 22 Editing petroleum fraction data 101 Entering BIPs 190 Enthalpy Specifying for isenthalpic flash 117 Entropy Specifying for isentropic flash 117 Equation of state method 28 Equations of State see Models 28 Equations of state provided in Multiflash 28 Errors 150, 152 Convergence 150 Messages 152 Troubleshooting 50, 59, 82, 117, 138, 141, 146, 154 Errors and warning messages 152 Eutectics 193 Examples Case studies 49, 163, 171, 201, 219, 227, 260, 265 Tutorial 9 Excel interface 169 Exiting Multiflash 19

F File 22 Files Loading a model configuration file 43, 45 Loading problem setup files 11 Model configuration (mfc) files 22, 43

User Guide for Multiflash for Windows

Output file 147, 148 PIPESIM PVT file 155 Problem setup (mfl) files 10, 17, 167 Saving a problem setup file 17 Saving results in an output file 19 Fixed phase fraction flash 122 Fixed phase fraction flashes 122 Flash calculations 117, 119 Bubble points 122 Chemical equilibrium 140 Definition of a flash calculation 119 Dew point 122 Fixed phase fraction flash 122 Isenthalpic flash 121 Isentropic flash 121 Isochoric flash 122 Isothermal (P,T) flash 121 List of available flash calculations 120 Retrograde flash calculations 125 Starting estimates 140 Troubleshooting 138 Type of solution 125 Flashes available in Multiflash 120 Flory Huggins model 34 Flory-Huggins 34 Fluid characterisation 6 Fluid composition 90 Fluid phase model 202 Font 148 Fonts 154 Front-end 8 Fugacity coefficients 151 Fuller model 43

G Gas injection 239 Gas phase models for activity coefficient methods 34 Graphics 8 Groups not available for UNIFAC model 51

H Hayduk Minhas method 43 Help 159 HELP 4, 24 How to change a model 46 How to define a wax model 48 How to define the asphaltene model 49 How to exit the program 19 How to load a model 43 How to specify models in Multiflash 43 Hydrate Inhibitor Calculator 79 Nucleation model 37, 47, 202 Hydrate calculations 126 Hydrate calculations with Multiflash 203 Hydrate dissociation pressure at a given temperature 212

User Guide for Multiflash for Windows

Hydrate dissociation temperature at a given pressure 211 Hydrate formation and dissociation pressure at given temperature 207 Hydrate formation and dissociation temperature at given pressure 205 Hydrate formation temperature at given pressure 206 Hydrate inhibitors 79 Hydrate model 202 Hydrate phase boundary 208, 212 Hydrates 5, 35, 201, 213, 214, 258 Case study 201 Hydrate inhibition case study 213 Hydrate inhibitor model 38 Hydrate model 35 Hydrate salt inhibition case study 214 Phase boundary 208 Hydrates, Waxes and Asphaltenes 247

I Ice model 202 Icons 8 Ideal gas equation of state 28 Ideal solution model 33 Including a petroleum fraction 176 Incorrect path 50 INFOCOND Condensed components 63 INFODATA 7 Infochem fluids databank 61 Inhibition Hydrate inhibition case study 213 Model for hydrate inhibition 38 Salt inhibition case study 214 Inhibitor Calculator 8, 79 Inhibitor modelling 38 Input 3 Input conditions seeConditions 115 Input Conditions 3 Input data 227 Input files 21 Loading a problem setup file 10 Problem setup files 10, 17, 167 Saving a problem set up file 17 Writing a problem setup file 167 Inserting a component 68 Installation 4, 8 Interface to other programs 119, 155, 169, 193 Excel spreadsheet 119, 169, 193 PIPESIM 155 Interfaces 8 Internal energy specifying as a flash condition 117 Introduction 1, 5, 9, 21, 27, 61, 85, 115, 119, 143, 147, 155, 159, 163, 171, 201, 219, 227, 245, 251, 260, 265, 269 Isenthalpic flashes 121 Isentropic flashes 121

Index • 277

Isochoric flashes 122 Isothermal (P,T) flash 121

K Key components 123, 270

L LCVM equation of state 30 LCVM model 30 Lee-Kesler-Plöcker (LKP) equation of state 31 Lee-Kesler-Plöcker (LKP) model 31 Limit the number of phases 139 Liquid-liquid equilibria 192 Loading a problem setup file 10 Loading an existing problem file 10 Loading files Loading a model configuration file 43, 45 Loading problem setup files 11 Loading hydrate models 46 Log file 19, 147, 148 Saving the log file 19 Lorentz-Bray-Clark model 41

M Macleod-Sugden model 42 Manipulating the Output 152 Mass fraction flash 122 Match bubble point 255 Matching 6, 102, 108, 110 Asphaltene flocculation 238 Dew and bubble points 102, 108 Liquid viscosity 111 Wax Appearance Temperature 110 Matching Density/Volume 108 Matching dew and bubble points 102 Matching liquid viscosity 111 Matching to asphaltene deposition data 238 Matching using petroleum fraction properties 102 Matching wax appearance temperature 110 Maximum water content allowable before hydrate dissociation 209 Menu options 22 mfc files 22, 43, 45 mfl files 10, 17, 167 Mixing rules 29 Liquid thermal conductivity 42 Liquid viscosity 42 Surface tension 42 Vapour thermal conductivity 42 Vapour viscosity 42 Model configuration files 22, 43, 45 Model is not licensed 51 Model set 43, 45 Selecting the model 43 Model Set Tabs 8 Modelling a polar mixture. 188 Modelling asphaltene flocculation 40

278 • Index

Modelling hydrate formation and inhibition 35 Modelling wax precipitation 39 Models 3, 5, 14, 27, 34, 46, 52 Advanced equations of state 29 Asphaltenes 40 Benedict-Webb-Rubin-Starling (BWRS) 32 Binary interaction parameters (BIPs) 52 Chung-Lee-Starling 42 Corresponding states (CSM) 32 Costald model 41 Cubic plus association (CPA) 30 Diffusion coefficients 43 Dortmund Modified UNIFAC 34 Flory Huggins 34 Fuller model 43 Gas phase models 34 Hayduk Minhas method 43 Hydrate model 35 Hydrate nucleation model 37, 47, 202 Ideal gas 28 Ideal solution 33 LCVM 30 Lee-Kesler-Plöcker (LKP) 31 Liquid density model 41 Lorentz-Bray-Clark 41 Macleod-Sugden 42 Mixing rules 29 Model configuration file 43 Model definitions 27 Model set 43 NRTL 33 Pedersen 41 Peneloux density correction 29 Peng Robinson (PR) 28 PSRK 30 Redlich-Kwong (RK) 28 Redlich-Kwong-Soave (RKS) 28 Regular Solution 34 Salinity model 39 Selecting model 43 Selecting model set 14 Selecting new model 46 Solid models 35 Surface tension 42 Transport property 41 Troubleshooting 50 Twu 41 UNIFAC 34 UNIQUAC 34 Viscosity 41 Wax 39 When to use activity methods 34 When to use BWRS 32 When to use CPA 30 When to use CSM 33 When to use cubic equations of state 29 When to use equation of state methods 28 When to use LCVM 30 When to use LKP 32 When to use PSRK 30

User Guide for Multiflash for Windows

Wilson A 33 Wilson E 33 Models and input requirements 73 Models for solid phases 35 Multiflash 1 Calculations 120 Case studies 163 Components 61 Conditions 115 Exiting the program 19 Input 21 Interfaces to other programs 155 Models 27 Output 147 Overview 1 Phase equilibrium 1 Simple tutorial 9 Software system 2 Starting the program 9 Units 143 Multiflash phase equilibrium utility 1 Multiflash Software System 2 Multi-reference fluid corresponding states (CSM) model 32 Multisolid model 226

N Naming the components 59 New Features 5 New Features and Changes in Version 3.6 3 No information on the amount of asphaltene in the oil 236 No reservoir or flocculation conditions 238 No reservoir pressure 237 No resin - asphaltene ratio 237 Normal components 61 DIPPR 61 INFODATA 61 Properties of normal components 61 Notes 251 n-Paraffin distribution 93 NRTL equation 33 NRTL model 33 Nucleation 205 Nucleation model 37, 47, 202 Number of BIPs for the model 59 Number of BIPs related to any model 52

O Obtaining properties from Pure component Data option 167 Oil and gas systems 171 OLGA 156 On-line help 159, 162 Order of components 59 Other calculations 264 Other flash calculations 177 Other flash calculations with hydrates 208

User Guide for Multiflash for Windows

Other thermodynamic models 41 Output 3, 19, 147, 148 Level of physical property output 137 Saving the output file 19 Troubleshooting 154 Writing the results to a file 147, 148 Overview 2

P PC-SAFT equation of state 31 Pedersen Model 41 Peng-Robinson (PR) model 28 Peng-Robinson equation of state 28 Petroleum fractions 61, 63, 85, 100, 101, 112 Defining a petroleum fraction 63, 85, 100 Editing petroleum fraction data 101 Splitting 98 Troubleshooting 112 Petroleum Fractions 3, 63 Phase descriptors 21, 45 Phase diagram 125 Phase diagrams 130 Phase envelope 16, 130, 139, 154, 174 Customising 135 Phase envelope output 152 Phase labelling 7, 154 Phase names 123 Phases 1, 21, 35, 45, 122, 123, 125, 139, 154, 203 Example of a phase diagram 125 Example of a phase envelope 139 Phase descriptors 21, 45 Phase labels 154 Phase names 123, 154 Phase types 122 Selecting phases 44 Solid phases 35 Types of phase 1 Physical properties of a pure component 163 PIPESIM PVT file 155 Pipesim PVT files 155 Plot the phase envelope 139 Plots H, S, U, V phase boundaries 130 Phase envelope 130 Polar systems 188 Polymers 194 Case study 194 Preferences 8 Presence of water 100 Pressure 16 Specifying as an input condition 117 Printing Output 18 Printing the output 18, 149 Problem setup file 10, 17, 167 Loading a problem setup file 10 Saving a problem setup file 17 Writing a problem setup file 167

Index • 279

Problem setup files 10 Problems defining a petroleum fraction 112 Problems when matching 113 Producing a problem setup file 167 Properties Displaying pure component properties 163 Level of physical property output 137 Physical property databanks 61 Properties of normal components 61 Property output in Multiflash 137 Provide a key component 140 Provide a starting estimate 129, 140 PSRK equation of state 30 PSRK model 30 PVT Analysis 86, 99, 179, 254 Case study 179 Saving an analysis 99 Troubleshooting 99

R Redlich-Kwong (RK) and Redlich-Kwong-Soave (RKS) equations 28 Redlich-Kwong (RK) model 28 Redlich-Kwong-Soave (RKS) model 28 Refrigerant mixtures 186 Refrigerants Case study 186 Regular Solution model 34 Regular Solution theory 34 Replacing a component 69 Results 11, 18, 19, 22, 147 Level of physical property output 137 Printing the results 18, 22 Results window 147 Saving the results in a file 19, 22 Retrograde 126 Retrograde condensation 126 Retrograde dew point 126 RKSA(Infochem) model 217 Running Multiflash 3

S Salinity model 39 Salt calculator 80 Salt inhibition 214 Hydrate salt inhibition case study 214 SARA Analysis 96 Saving a problem setup 17 Saving a PVT Analysis 99 Saving files Saving a problem setup file 17 Saving results Writing the results to a file 147, 148 Saving the output 19 Scale calculations 126 Scale model 203 Scale precipitation 215 Scaling and general freeze -out model 35

280 • Index

Searching for components By formula 67 By name 66 By scrolling through list 65 By substring 67 Identifying synonyms 66 Select 23 Select components by formula 67 Select components by name 66 Select components by scrolling through a list 65 Select comp onents by substring 67 Selecting components 65 Sensitivity of calculations to variation in input data 235 Sensitivity to characterisation 99 Setting up a problem interactively 13 Show functions 153 Simple tutorial 3 Soave-Redlich-Kwong (RKS) model 28 Solid freeze -out model 35 SolidsB.xls and SolidsA.xls 253 Specifying compositions 115 Specifying enthalpy, entropy and internal energy 117 Specifying temperature, pressure and volume 117 Specifying the data source 64 Splitting 98 Stability analysis 1, 150 Starting a new problem 19 Starting estimate for flash calculations 129, 140 Starting Multiflash 9 Steam cracking of ethane 266 Stream types 76 Supplementing or overwriting BIPs 56 Supplying an external file of BIPs 270 Surface tension 42 Models 42 Synonyms 66 Component synonyms 66

T Table 24 Tabular output 119, 155 Excel interface 119 PSF file for HTFS programs 155 PVT file for PIPESIM 155 Technical support 162 Temperature Specifying as an input condition 117 Temperature dependence of BIPs 53 The basis of a flash calculation. 119 The Freeze-out model 48 The output does not include everything expected 154 The results 11 The results window. 147 Thermal conductivity 42 Models 42 Titration 240 Tolerance calculations 128 Too many components in the mixture 84

User Guide for Multiflash for Windows

Toolbar buttons 25 Tools 23 Total amount of fluid 96 Transport properties Displaying transport property values 137 Models 41 Surface tension 42 Thermal conductivity 42 Viscosity 41 Transport property models 41 Troubleshooting 50, 59, 82, 99, 117, 138, 141, 146, 154 Binary interaction parameters 59 Calculations 138 Chemical reaction 141 Components 82 Models 50 Output 154 PVT Analysis 99 Units 146 Troubleshooting - BIPs 59 Troubleshooting - chemical reaction 141 Troubleshooting - components 82 Troubleshooting - flash calculations 138 Troubleshooting - input conditions 117 Troubleshooting - models 50 Troubleshooting - output 154 Troubleshooting - PVT Analysis 99 Troubleshooting - units 146 Tutorial 9 Twu Model 41

U UNFACFIT.xls 251 UNIFAC 6, 252 UNIFAC method 34 UNIFAC model 34 UNIQUAC 34 UNIQUAC equation 34 Units 3, 52, 59, 143, 144, 146 Default units 143 Selecting units 144 Troubleshooting 146 Units for BIPs 52, 59 Working units 143 Units for BIPs 52 Updates 5 Use the P,T flash 139 User defined components 71 User-defined components 71 Using INFOBIPS 188

Models 41 VLEFIT.xls 253 Volume Specifying as a flash condition 117 Volume fraction flash 122

W Warnings 150, 152, 192, 265 Additional phases 192 Chemical reaction 265 Convergence 150 Water cut 96 Wax 219, 255, 260 Case study 219, 260 Coutinho Model 39 Defining Wax model 48 Matching WAT 110 Multisolid Model 39 Wax and Asphaltene precipitation 246 Wax calculations 6, 126 Wax Content 96 Website support 162 What is a model? 27 What the model definition means 45 What types of model are available? 27 When to use activity coefficient models 34 When to use cubic equations of state 29 When to use equation of state methods 28 When you may need to use commands 269 Will hydrates form at given P and T ? 203 Wilson A equation, 33 Wilson A model 33 Wilson E equation 33 Wilson E model 33 Working units 143 Write to Excel 137 Writing the results to a file 148

X Xylene isomerisation 265

V Vapour-liquid-liquid equilibria 192 Viewing and editing pure component data. 69 Viewing BIP values 54 Viscosity 41 Matching liquid viscosity 111

User Guide for Multiflash for Windows

Index • 281