Aspen Plus Reference Manual, Unit Operation Models

Version

Aspen Plus

10

7

STE ADY STATE SIMUL ATION

Unit Operation Models REFERENCE MANUAL

AspenTech7

COPYRIGHT 1981—1998 Aspen Technology, Inc. ALL RIGHTS RESERVED The flowsheet graphics and plot components of ASPEN PLUS were developed by MY-Tech, Inc. ®   ® ® ADVENT , Aspen Custom Modeler , Aspen Dynamics , ASPEN PLUS , AspenTech , BioProcess Simulator (BPS), DynaPLUS, ModelManager, Plantelligence, the Plantelligence logo, POLYMERS PLUS®, PROPERTIES PLUS®, SPEEDUP®, and the aspen leaf logo are either registered trademarks, or trademarks of Aspen Technology, Inc., in the United States and/or other countries.

BATCHFRAC and RATEFRAC are trademarks of Koch Engineering Company, Inc. Activator is a trademark of Software Security, Inc. Rainbow SentinelSuperPro is a trademark of Rainbow Technologies, Inc. Élan License Manager is a trademark of Élan Computer Group, Inc., Mountain View, California, USA. Microsoft Windows, Windows NT, and Windows 95 are either registered trademarks or trademarks of Microsoft Corporation in the United States and/or other countries. All other brand and product names are trademarks or registered trademarks of their respective companies. The License Manager portion of this product is based on: Élan License Manager © 1989-1997 Élan Computer Group, Inc. All rights reserved

Use of ASPEN PLUS and This Manual This manual is intended as a guide to using ASPEN PLUS process modeling software. This documentation contains AspenTech proprietary and confidential information and may not be disclosed, used, or copied without the prior consent of AspenTech or as set forth in the applicable license agreement. Users are solely responsible for the proper use of ASPEN PLUS and the application of the results obtained. Although AspenTech has tested the software and reviewed the documentation, the sole warranty for ASPEN PLUS may be found in the applicable license agreement between AspenTech and the user. ASPENTECH MAKES NO WARRANTY OR REPRESENTATION, EITHER EXPRESS OR IMPLIED, WITH RESPECT TO THIS DOCUMENTATION, ITS QUALITY, PERFORMANCE, MERCHANTABILITY, OR FITNESS FOR A PARTICULAR PURPOSE.

Contents About the Unit Operation Models Reference Manual For More Information..............................................................................................................x Technical Support ..................................................................................................................xi 1 Mixers and Splitters Mixer .....................................................................................................................................1-2 Flowsheet Connectivity for Mixer....................................................................................1-2 Specifying Mixer...............................................................................................................1-3 FSplit.....................................................................................................................................1-5 Flowsheet Connectivity for FSplit...................................................................................1-5 Specifying FSplit ...............................................................................................................1-6 SSplit.....................................................................................................................................1-8 Flowsheet Connectivity for SSplit ....................................................................................1-8 Specifying SSplit ...............................................................................................................1-8 2 Separators Flash2....................................................................................................................................2-2 Flowsheet Connectivity for Flash2..................................................................................2-2 Specifying Flash2 .............................................................................................................2-3 Flash3....................................................................................................................................2-5 Flowsheet Connectivity for Flash3..................................................................................2-5 Specifying Flash3 .............................................................................................................2-6 Decanter................................................................................................................................2-8 Flowsheet Connectivity for Decanter ..............................................................................2-8 Specifying Decanter .........................................................................................................2-9 Sep.......................................................................................................................................2-12 Flowsheet Connectivity for Sep ......................................................................................2-12 Specifying Sep .................................................................................................................2-13 Sep2 .....................................................................................................................................2-14 Flowsheet Connectivity for Sep2 ....................................................................................2-14 Specifying Sep2...............................................................................................................2-15 3 Heat Exchangers Heater ...................................................................................................................................3-2 Flowsheet Connectivity for Heater..................................................................................3-2 Specifying Heater .............................................................................................................3-3 HeatX ....................................................................................................................................3-5 Flowsheet Connectivity for HeatX...................................................................................3-5 Specifying HeatX ..............................................................................................................3-6 References...........................................................................................................................3-18

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MHeatX .............................................................................................................................. 3-19 Flowsheet Connectivity for MHeatX ............................................................................ 3-19 Specifying MHeatX........................................................................................................ 3-20 Hetran ................................................................................................................................ 3-23 Flowsheet Connectivity for Hetran .............................................................................. 3-23 Specifying Hetran.......................................................................................................... 3-24 Aerotran ............................................................................................................................. 3-26 Flowsheet Connectivity for Aerotran ........................................................................... 3-26 Specifying Aerotran....................................................................................................... 3-27 4 Columns DSTWU ................................................................................................................................ 4-3 Flowsheet Connectivity for DSTWU ................................................................................ 4-3 Specifying DSTWU........................................................................................................... 4-4 Distl ...................................................................................................................................... 4-6 Flowsheet Connectivity for Distl...................................................................................... 4-6 Specifying Distl ................................................................................................................ 4-7 SCFrac.................................................................................................................................. 4-8 Flowsheet Connectivity for SCFrac ................................................................................. 4-8 Specifying SCFrac ............................................................................................................ 4-9 RadFrac.............................................................................................................................. 4-11 Flowsheet Connectivity for RadFrac ............................................................................ 4-12 Specifying RadFrac........................................................................................................ 4-13 Free-Water and Rigorous Three-Phase Calculations .................................................. 4-20 Efficiencies ..................................................................................................................... 4-20 Algorithms...................................................................................................................... 4-22 Rating Mode................................................................................................................... 4-23 Design Mode................................................................................................................... 4-24 Reactive Distillation ...................................................................................................... 4-25 Solution Strategies ........................................................................................................ 4-25 Physical Properties........................................................................................................ 4-28 Solids Handling ............................................................................................................. 4-28 MultiFrac ........................................................................................................................... 4-30 Flowsheet Connectivity for MultiFrac .......................................................................... 4-31 Specifying MultiFrac ..................................................................................................... 4-33 Efficiencies ..................................................................................................................... 4-41 Algorithms...................................................................................................................... 4-42 Rating Mode................................................................................................................... 4-42 Design Mode................................................................................................................... 4-42 Column Convergence..................................................................................................... 4-43 Physical Properties........................................................................................................ 4-46 Free Water Handling..................................................................................................... 4-46 Solids Handling ............................................................................................................. 4-46 Sizing and Rating of Trays and Packings .................................................................... 4-47 PetroFrac............................................................................................................................ 4-48 Flowsheet Connectivity for PetroFrac.......................................................................... 4-49 Specifying PetroFrac...................................................................................................... 4-51 Efficiencies ..................................................................................................................... 4-57

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Convergence....................................................................................................................4-58 Rating Mode....................................................................................................................4-59 Design Mode ...................................................................................................................4-59 Physical Properties.........................................................................................................4-60 Free Water Handling .....................................................................................................4-60 Solids Handling ..............................................................................................................4-61 Sizing and Rating of Trays and Packings .....................................................................4-61 RateFrac..............................................................................................................................4-62 Flowsheet Connectivity for RateFrac............................................................................4-63 The Rate-Based Modeling Concept................................................................................4-65 Specifying RateFrac .......................................................................................................4-66 Mass and Heat Transfer Correlations...........................................................................4-77 References...........................................................................................................................4-85 Extract ................................................................................................................................4-87 Flowsheet Connectivity for Extract...............................................................................4-87 Specifying Extract ..........................................................................................................4-88 5 Reactors RStoic ....................................................................................................................................5-2 Flowsheet Connectivity for RStoic ..................................................................................5-2 Specifying RStoic..............................................................................................................5-3 RYield....................................................................................................................................5-6 Flowsheet Connectivity for RYield..................................................................................5-6 Specifying RYield .............................................................................................................5-7 REquil ...................................................................................................................................5-8 Flowsheet Connectivity for REquil..................................................................................5-8 Specifying REquil .............................................................................................................5-9 RGibbs.................................................................................................................................5-10 Flowsheet Connectivity for RGibbs ...............................................................................5-10 Specifying RGibbs ..........................................................................................................5-11 References...........................................................................................................................5-15 RCSTR ................................................................................................................................5-16 Flowsheet Connectivity for RCSTR...............................................................................5-16 Specifying RCSTR ..........................................................................................................5-17 RPlug...................................................................................................................................5-21 Flowsheet Connectivity for RPlug.................................................................................5-21 Specifying RPlug ............................................................................................................5-22 RBatch ................................................................................................................................5-25 Flowsheet Connectivity for RBatch...............................................................................5-25 Specifying RBatch ..........................................................................................................5-26 6 Pressure Changers Pump .....................................................................................................................................6-2 Flowsheet Connectivity for Pump ...................................................................................6-2 Specifying Pump ...............................................................................................................6-3 Compr....................................................................................................................................6-9 Flowsheet Connectivity for Compr..................................................................................6-9 Specifying Compr ............................................................................................................6-10

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MCompr.............................................................................................................................. 6-13 Flowsheet Connectivity for MCompr ............................................................................. 6-13 Specifying MCompr........................................................................................................ 6-15 References .......................................................................................................................... 6-19 Valve................................................................................................................................... 6-20 Flowsheet Connectivity for Valve................................................................................. 6-20 Specifying Valve ............................................................................................................ 6-20 References .......................................................................................................................... 6-29 Pipe..................................................................................................................................... 6-30 Flowsheet Connectivity for Pipe ................................................................................... 6-30 Specifying Pipe .............................................................................................................. 6-31 Two-Phase Correlations ................................................................................................ 6-35 Closed-Form Methods.................................................................................................... 6-39 References .......................................................................................................................... 6-40 Pipeline .............................................................................................................................. 6-42 Flowsheet Connectivity for Pipeline............................................................................. 6-42 Specifying Pipeline ......................................................................................................... 6-43 Two-Phase Correlations ................................................................................................ 6-47 Closed-Form Methods.................................................................................................... 6-50 References .......................................................................................................................... 6-52 7 Manipulators Mult ...................................................................................................................................... 7-2 Flowsheet Connectivity for Mult...................................................................................... 7-2 Specifying Mult................................................................................................................ 7-3 Dupl ...................................................................................................................................... 7-4 Flowsheet Connectivity for Dupl...................................................................................... 7-4 Specifying Dupl................................................................................................................ 7-5 ClChng ................................................................................................................................. 7-6 Flowsheet Connectivity for ClChng................................................................................ 7-6 Specifying ClChng............................................................................................................ 7-6 8 Solids Crystallizer .......................................................................................................................... 8-3 Flowsheet Connectivity for Crystallizer .......................................................................... 8-3 Specifying Crystallizer ..................................................................................................... 8-4 References .......................................................................................................................... 8-11 Crusher............................................................................................................................... 8-13 Flowsheet Connectivity for Crusher............................................................................. 8-13 Specifying Crusher ........................................................................................................ 8-14 References .......................................................................................................................... 8-18 Screen ................................................................................................................................. 8-19 Flowsheet Connectivity for Screen ............................................................................... 8-19 Specifying Screen........................................................................................................... 8-19 References .......................................................................................................................... 8-22 FabFl .................................................................................................................................. 8-23 Flowsheet Connectivity for FabFl................................................................................. 8-23 Specifying FabFl............................................................................................................. 8-23

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References...........................................................................................................................8-26 Cyclone ................................................................................................................................8-27 Flowsheet Connectivity for Cyclone................................................................................8-27 Specifying Cyclone ..........................................................................................................8-28 References...........................................................................................................................8-35 VScrub.................................................................................................................................8-36 Flowsheet Connectivity for VScrub ................................................................................8-36 Specifying VScrub ...........................................................................................................8-37 References...........................................................................................................................8-39 ESP......................................................................................................................................8-40 Flowsheet Connectivity for ESP .....................................................................................8-40 Specifying ESP ................................................................................................................8-41 References...........................................................................................................................8-44 HyCyc ..................................................................................................................................8-45 Flowsheet Connectivity for HyCyc..................................................................................8-45 Specifying HyCyc ............................................................................................................8-46 References...........................................................................................................................8-51 CFuge ..................................................................................................................................8-52 Flowsheet Connectivity for CFuge ................................................................................8-52 Specifying CFuge............................................................................................................8-53 References...........................................................................................................................8-55 Filter ...................................................................................................................................8-56 Flowsheet Configuration for Filter................................................................................8-56 Specifying Filter .............................................................................................................8-56 References...........................................................................................................................8-59 SWash .................................................................................................................................8-61 Flowsheet Connectivity for SWash................................................................................8-61 Specifying SWash ...........................................................................................................8-62 CCD .....................................................................................................................................8-64 Flowsheet Connectivity for CCD ...................................................................................8-64 Specifying CCD...............................................................................................................8-65 9 User Models User .......................................................................................................................................9-2 Flowsheet Connectivity for User .....................................................................................9-2 Specifying User.................................................................................................................9-3 User2 .....................................................................................................................................9-4 Flowsheet Connectivity for User2 ...................................................................................9-4 Specifying User2...............................................................................................................9-5 10 Pressure Relief Pres-Relief...........................................................................................................................10-2 Specifying Pres-Relief ....................................................................................................10-2 Scenarios .........................................................................................................................10-3 Compliance with Codes ..................................................................................................10-6 Stream and Vessel Compositions and Conditions........................................................10-6 Rules to Size the Relief Valve Piping ............................................................................10-7 Reactions.........................................................................................................................10-9

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Relief System ............................................................................................................... 10-10 Data Tables for Pipes and Relief Devices................................................................... 10-12 Valve Cycling ............................................................................................................... 10-16 Vessel Types................................................................................................................. 10-16 Disengagement Models ............................................................................................... 10-18 Stop Criteria ................................................................................................................ 10-18 Solution Procedure for Dynamic Scenarios................................................................ 10-19 Flow Equations ............................................................................................................ 10-20 Calculation and Convergence Methods ...................................................................... 10-23 Vessel Insulation Credit Factor.................................................................................. 10-24 References ........................................................................................................................ 10-25 A Sizing and Rating for Trays and Packings Single-Pass and Multi-Pass Trays..................................................................................A-2 Modes of Operation for Trays .........................................................................................A-8 Flooding Calculations for Trays......................................................................................A-8 Bubble Cap Tray Layout .................................................................................................A-9 Pressure Drop Calculations for Trays ..........................................................................A-10 Foaming Calculations for Trays ...................................................................................A-11 Packed Columns ............................................................................................................A-12 Packing Types and Packing Factors.............................................................................A-12 Modes of Operation for Packing....................................................................................A-12 Maximum Capacity Calculations for Packing .............................................................A-13 Pressure Drop Calculations for Packing ......................................................................A-15 Liquid Holdup Calculations for Packing ......................................................................A-16 Pressure Profile Update ................................................................................................A-17 Physical Property Data Requirements.........................................................................A-17 References ..........................................................................................................................A-18 Index

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About the Unit Operation Models Reference Manual Volume 1 of the ASPEN PLUS Reference Manuals, Unit Operation Models, includes detailed technical reference information for all ASPEN PLUS unit operation models and the Pres-Relief model. The information in this manual is also available in online help and prompts. Models are grouped in chapters according to unit operation type. The reference information for each model includes a description of the model and its typical usage, a diagram of its flowsheet connectivity, a discussion of the specifications you must provide for the model, important equations and correlations, and other relevant information. An overview of all ASPEN PLUS unit operation models, and general information about the steps and procedures in using them is in the ASPEN PLUS User Guide as well as in the online help and prompts in ASPEN PLUS.

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For More Information Online Help ASPEN PLUS has a complete system of online help and context-sensitive prompts. The help system contains both context-sensitive help and reference information. For more information about using ASPEN PLUS help, see the ASPEN PLUS User Guide, Chapter 3. ASPEN PLUS Getting Started Building and Running a Process Model This tutorial includes several hands-on sessions to familiarize you with ASPEN PLUS. The guide takes you step-by-step to learn the full power and scope of ASPEN PLUS. ASPEN PLUS User Guide The three-volume ASPEN PLUS User Guide provides step-by-step procedures for developing and using an ASPEN PLUS process simulation model. The guide is task-oriented to help you accomplish the engineering work you need to do, using the powerful capabilities of ASPEN PLUS. ASPEN PLUS reference manual series ASPEN PLUS reference manuals provide detailed technical reference information. These manuals include background information about the unit operation models and the physical properties methods and models available in ASPEN PLUS, tables of ASPEN PLUS databank parameters, group contribution method functional groups, and a wide range of other reference information. The set comprises: • Unit Operation Models • Physical Property Methods and Models • Physical Property Data • User Models • System Management • Summary File Toolkit ASPEN PLUS application examples A suite of sample online ASPEN PLUS simulations illustrating specific processes is delivered with ASPEN PLUS. ASPEN PLUS Installation Guides These guides provide instructions on platform and network installation of ASPEN PLUS. The set comprises: • • •

ASPEN PLUS Installation Guide for Windows ASPEN PLUS Installation Guide for OpenVMS ASPEN PLUS Installation Guide for UNIX

The ASPEN PLUS manuals are delivered in Adobe portable document format (PDF) on the ASPEN PLUS Documentation CD. You can also order printed manuals from AspenTech.

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Unit Operation Models Version 10

Technical Support World Wide Web For additional information about AspenTech products and services, check the AspenTech World Wide Web home page on the Internet at: http://www.aspentech.com/ Technical resources To obtain in-depth technical support information on the Internet, visit the Technical Support homepage. Register at: http://www.aspentech.com/ts/ Approximately three days after registering, you will receive a confirmation e-mail and you will then be able to access this information. The most current Hotline contact information is listed. Other information includes: • • •

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Unit Operation Models Version 10

Chapter 1

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Mixers and Splitters This chapter describes the unit operation models for mixing and splitting streams. The models are:

Unit Operation Models Version 10

Model

Description

Purpose

Use For

Mixer

Stream mixer

Combines multiple streams into one stream

Mixing tees. Stream mixing operations. Adding heat streams. Adding work streams

FSplit

Stream splitter

Divides feed based on splits specified for outlet streams

Stream splitters. Bleed valves

SSplit

Substream splitter

Divides feed based on splits specified for each substream

Stream splitters. Perfect fluid-solid separators

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Mixers and Splitters

Mixer Stream Mixer Use Mixer to combine streams into one stream. Mixer models mixing tees or other types of mixing operations. Mixer combines material streams (or heat streams or work streams) into one stream. Select the Heat (Q) and Work (W) Mixer icons from the Model Library for heat and work streams respectively. A single Mixer block cannot mix streams of different types (material, heat, work).

Flowsheet Connectivity for Mixer

Material (2 or more)

Material Water (optional)

Flowsheet for Mixing Material Streams Material Streams Inlet

At least two material streams

Outlet One material stream

One water decant stream (optional)

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Chapter 1

Heat (2 or more)

Heat

Flowsheet for Adding Heat Streams Heat Streams Inlet

At least two heat streams

Outlet One heat stream

Work (2 or more)

Work

Flowsheet for Adding Work Streams Work Streams Inlet

At least two work streams

Outlet One work stream

Specifying Mixer Use the Mixer Input Flash Options sheet to specify operating conditions. When mixing heat or work streams, Mixer does not require any specifications.

Unit Operation Models Version 10

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Mixers and Splitters

When mixing material streams, you can specify either the outlet pressure or pressure drop. If you specify pressure drop, Mixer determines the minimum of the inlet stream pressures, and applies the pressure drop to the minimum inlet stream pressure to compute the outlet pressure. If you do not specify the outlet pressure or pressure drop, Mixer uses the minimum pressure from the inlet streams for the outlet pressure. You can select the following valid phases: Valid Phase

Solids?

Number of phases?

Free Water?

Phase?

Vapor-Only

Yes or no

1

No

V

Liquid-Only

Yes or no

1

No

L

Vapor-Liquid

Yes or no

2

No



Vapor-Liquid-Liquid

Yes or no

3

No



Yes or no

1

Yes



Yes or no

2

Yes



Yes

1

No

S

Liquid Free-Water

†

Vapor-Liquid Free-Water Solid-Only †

†

Check Use Free Water Calculations checkbox on the Setup Specifications Global sheet.

An optional water decant stream can be used when free-water calculations are performed. Mixer performs an adiabatic calculation on the product to determine the outlet temperature, unless Mass Balance Only Calculations is specified on the Mixer BlockOptions SimulationOptions sheet or the Setup SimulationOptions Calculations sheet. Use the following forms to enter specifications and view results for Mixer:

1-4

Use this form

To do this

Input

Enter operating conditions and flash convergence parameters

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View Mixer simulation results

Dynamic

Specify parameters for dynamic simulations

Unit Operation Models Version 10

Chapter 1

FSplit Stream Splitter FSplit combines streams of the same type (material, heat, or work streams) and divides the resulting stream into two or more streams of the same type. All outlet streams have the same composition and conditions as the mixed inlet. Select the Heat (Q) and Work (W) FSplit icons from the Model Library for heat and work streams respectively. Use FSplit to model flow splitters, such as bleed valves. FSplit cannot split a stream into different types. For example, FSplit cannot split a material stream into a heat stream and a material stream. To model a splitter where the amount of each substream sent to each outlet can differ, use an SSplit block. To model a splitter where the composition and properties of the output streams can differ, use a Sep block or a Sep2 block.

Flowsheet Connectivity for FSplit Material (any number)

Material (2 or more)

Flowsheet for Splitting Material Streams Material Streams Inlet

At least one material stream

Outlet At least two material streams

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Mixers and Splitters

Heat (any number)

Heat (2 or more)

Flowsheet for Splitting Heat Streams Heat Streams Inlet

At least one heat stream

Outlet At least two heat streams

Work (any number)

Work (2 or more)

Flowsheet for Splitting Work Streams Work Streams Inlet

At least one work stream

Outlet At least two work streams

Specifying FSplit To split material streams Give one of the following specifications for each outlet stream except one: • Fraction of the combined inlet flow • Mole flow rate • Mass flow rate • Standard liquid volume flow rate • Actual volume flow rate • Fraction of the residue remaining after all other specifications are satisfied FSplit puts any remaining flow in the unspecified outlet stream to satisfy material balance. You can specify mole, mass, or standard liquid volume flow rate for one of the following: • •

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The entire stream A subset of key components in the stream

Unit Operation Models Version 10

Chapter 1

To specify the flow rate of a component or group of components in an outlet stream, specify a group of key components and the total flow rate for the group (the sum of the component flow rates) on the Input Specifications sheet, and define the key components in the group on the Input KeyComponents sheet. Outlet streams have the same composition as the mixed inlet stream. For this reason, when you specify the flow rate of a key component, the total flow rate of the outlet stream is greater than the flow rate you specify. When FSplit has more than one inlet, you can do one of the following: • •

Enter the outlet pressure on the FSplit Input FlashOptions sheet Let the outlet pressure default to the minimum pressure of the inlet streams

To split heat streams or work streams Specify the fraction of the combined inlet heat or work for each outlet stream except one. FSplit puts any remaining heat or work in the unspecified outlet stream to satisfy energy balance. Use the following forms to enter specifications and view results for FSplit:

Unit Operation Models Version 10

Use this form

To do this

Input

Enter split specifications, flash conditions and calculation options, and key components associated with split specifications

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View split fractions for outlet streams, and material and energy balance results

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Mixers and Splitters

SSplit Substream Splitter SSplit combines material streams and divides the resulting stream into two or more streams. Use SSplit to model a splitter where the split of each substream among the outlet streams can differ. Substreams in the outlet streams have the same composition, temperature, and pressure as the corresponding substreams in the mixed inlet stream. Only the substream flow rates differ. To model a splitter in which the composition and properties of the substreams in the output streams can differ, use a Sep block or a Sep2 block.

Flowsheet Connectivity for SSplit Material (any number)

Material (2 or more)

Material Streams Inlet

At least one material stream

Outlet At least two material streams

Specifying SSplit For each substream, specify one of the following for all but one outlet stream: • • • •

Fraction of the inlet substream Mole flow rate Mass flow rate Standard liquid volume flow rate

SSplit puts any remaining flow for each substream in the unspecified stream. You cannot specify standard liquid volume flow rate when the substream is of type CISOLID, and mole and standard liquid volume flow rates when the substream is of type NC.

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Unit Operation Models Version 10

Chapter 1

You can specify mole or mass flow rate for one of the following: • The entire substream • A subset of components in the substream You can specify the flow rate of a component in a substream of an outlet stream. To do this, define a key component and specify the flow rate for the key component. Similarly, you can specify the flow rate for a group of components in a substream of an outlet stream. To do this, define a key group of components and specify the total flow rate for the group (the sum of the component flow rates). Substreams in outlet streams have the same composition as the corresponding substream in the mixed inlet stream. For this reason, when you specify the flow rate of a key, the total flow rate of the substream in the outlet stream is greater than the flow rate you specify. When SSplit has more than one inlet, you can do one of the following: • Enter the outlet pressure on the Input FlashOptions sheet. • Let the outlet pressure default to the minimum pressure of the inlet streams. The composition, temperature, pressure, and other substream variables for all outlet streams have the same values as the mixed inlet. Only the substream flow rates differ. Use the following forms to enter specifications and view results for SSplit: Use this form

To do this

Input

Enter split specifications, flash conditions, calculation options, and key components associated with split specifications

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View split fractions of each substream in each outlet stream, and material and energy balance results

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Mixers and Splitters

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Unit Operation Models Version 10

Chapter 2

2

Separators This chapter describes the unit operation models for component separators, flash drums, and liquid-liquid separators. The models are: Model

Description

Purpose

Use For

Flash2

Two-outlet flash

Separates feed into two outlet streams, using rigorous vaporliquid or vapor-liquid-liquid equilibrium

Flash drums, evaporators, knock-out drums, single stage separators

Flash3

Three-outlet flash

Separates feed into three outlet streams, using rigorous vapor-liquid-liquid equilibrium

Decanters, single-stage separators with two liquid phases

Decanter

Liquid-liquid decanter

Separates feed into two liquid outlet streams

Decanters, single-stage separators with two liquid phases and no vapor phase

Sep

Component separator

Separates inlet stream components into multiple outlet streams, based on specified flows or split frractions

Component separation operations, such as distillation and absorption, when the details of the separation are unknown or unimportant

Sep2

Two-outlet component separator

Separates inlet stream components into two outlet streams, based on specified flows, split fractions, or purities

Component separation operations, such as distillation and absorption, when the details of the separation are unknown or unimportant

You can generate heating or cooling curve tables for Flash2, Flash3, and Decanter models.

Unit Operation Models Version 10

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Separators

Flash2 Two-Outlet Flash Use Flash2 to model flashes, evaporators, knock-out drums, and other singlestage separators. Flash2 performs vapor-liquid or vapor-liquid-liquid equilibrium calculations. When you specify the outlet conditions, Flash2 determines the thermal and phase conditions of a mixture of one or more inlet streams.

Flowsheet Connectivity for Flash2 Vapor Heat (optional)

Material (any number)

Water (optional) Heat (optional)

Liquid

Material Streams Inlet

At least one material stream

Outlet One material stream for the vapor phase

One material stream for the liquid phase. (If three phases exist, the liquid outlet contains both liquid phases.) One water decant stream (optional) You can specify liquid and/or solid entrainment in the vapor stream.

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Unit Operation Models Version 10

Chapter 2

Heat Streams Inlet

Any number of heat streams (optional)

Outlet One heat stream (optional)

If you give only one specification (temperature or pressure) on the Input Specifications Sheet, Flash2 uses the sum of the inlet heat streams as a duty specification. Otherwise, Flash2 uses the inlet heat stream only to calculate the net heat duty. The net heat duty is the sum of the inlet heat streams minus the actual (calculated) heat duty. You can use an optional outlet heat stream for the net heat duty.

Specifying Flash2 Use the Input Specifications sheet for all required specifications and valid phases. For valid phases you can choose the following options: You can choose the following options

Solids?

Number of phases?

Free Water?

Vapor-Liquid

Yes or no

2

No

Vapor-Liquid-Liquid

Yes or no

3

No

Vapor-Liquid-FreeWater

Yes or no

2

Yes

Use the Input FlashOptions sheet to specify temperature and pressure estimates and flash convergence parameters. Use the Input Entrainment sheet to specify liquid and solid entrainment in the vapor phase. Use the Hcurves form to specify optional heating or cooling curves. Use the following forms to enter specifications and view results for Flash2:

Unit Operation Models Version 10

Use this form

To do this

Input

Enter flash specifications, flash convergence parameters, and entrainment specifications

Hcurves

Specify heating or cooling curve tables and view tabular results

Block Options

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View Flash2 simulation results

Dynamic

Specify parameters for dynamic simulations

2-3

Separators

Solids All phases are in thermal equilibrium. Solids leave at the same temperature as the fluid phases. Flash2 can simulate fluid phases with solids when the stream contains solid substreams or when you request electrolytes chemistry calculations. Solid Substreams Materials in solid substreams do not participate in phase equilibrium calculations. Electrolyte Chemistry Calculations You can request these on the Properties Specifications Global sheet or the BlockOptions Properties sheet. Solid salts participate in liquid-solid phase equilibrium and thermal equilibrium calculations. The salts are in the MIXED substream.

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Unit Operation Models Version 10

Chapter 2

Flash3 Three-Outlet Flash Use Flash3 to model flashes, evaporators, knock-out drums, decanters, and other single-stage separators in which two liquid outlet streams are produced. Flash3 performs vapor-liquid-liquid equilibrium calculations. When you specify outlet conditions, Flash3 determines the thermal and phase conditions of a mixture of one or more inlet streams.

Flowsheet Connectivity for Flash3 Vapor Heat (optional)

Material (any number)

1st Liquid Heat (optional)

2nd Liquid

Material Streams Inlet

At least one material stream

Outlet One material stream for the vapor phase

One material stream for the first liquid phase One material stream for the second liquid phase You can specify liquid entrainment of each liquid phase in the vapor stream. You can also specify entrainment for each solid substream in the vapor and first liquid phase.

Unit Operation Models Version 10

2-5

Separators

Heat Streams Inlet

Any number of heat streams (optional)

Outlet One heat stream (optional)

If you give only one specification on the Input Specifications Sheet (temperature or pressure), Flash3 uses the sum of the inlet heat streams as a duty specification. Otherwise, Flash3 uses the inlet heat stream only to calculate the net heat duty. The net heat duty is the sum of the inlet heat streams minus the actual (calculated) heat duty. You can use an optional outlet heat stream for the net heat duty.

Specifying Flash3 Use the Input Specifications sheet for all required specifications. Use the Input Entrainment sheet to specify solid entrainment. To specify optional heating or cooling curves, use the Hcurves form. Use the following forms to enter specifications and view results for Flash3: Use this form

To do this

Input

Enter flash specifications, key components, flash convergence parameters, and entrainment specifications

Hcurves

Specify heating or cooling curve tables and view tabular results

Block Options

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View Flash3 simulation results

Dynamic

Specify parameters for dynamic simulations

Solids All phases are in thermal equilibrium. Solids leave at the same temperature as the fluid phases. Flash3 can simulate fluid phases with solids when the stream contains solid substreams, or when you request electrolyte chemistry calculations. Solid Substreams Materials in solid substreams do not participate in phase equilibrium calculations.

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Unit Operation Models Version 10

Chapter 2

Electrolyte Chemistry Calculations You can request these on the Properties Specifications Global sheet or on the Input BlockOptions Properties sheet. Solid salts do participate in liquid-solid phase equilibrium and thermal equilibrium calculations. You can only specify apparent component calculations (Select Simulation Approach=Apparent Components on the BlockOptions Properties sheet). The salts will not appear in the MIXED substream.

Unit Operation Models Version 10

2-7

Separators

Decanter Liquid-Liquid Decanter Decanter simulates decanters and other single stage separators without a vapor phase. Decanter can perform: • Liquid-liquid equilibrium calculations • Liquid-free-water calculations Use Decanter to model knock-out drums, decanters, and other single-stage separators without a vapor phase. When you specify outlet conditions, Decanter determines the thermal and phase conditions of a mixture of one or more inlet streams. Decanter can calculate liquid-liquid distribution coefficients using: • An activity coefficient model • An equation of state capable of representing two liquid phases • A user-specified Fortran subroutine • A built-in correlation with user-specified coefficients You can enter component separation efficiencies, assuming equilibrium stage is present. Use Flash3 if you suspect any vapor phase formation.

Flowsheet Connectivity for Decanter Material (any number) Heat (optional)

1st Liquid 2nd Liquid Heat (optional)

Material Streams Inlet

At least one material stream

Outlet One material stream for the first liquid phase

One material stream for the second liquid phase You can specify entrainment for each solid substream in the first liquid phase.

2-8

Unit Operation Models Version 10

Chapter 2

Heat Streams Inlet

Any number of heat streams (optional)

Outlet One heat stream (optional)

If you specify only pressure on the Input Specifications sheet, Decanter uses the sum of the inlet heat streams as a duty specification. Otherwise, Decanter uses the inlet heat stream only to calculate the net heat duty. The net heat duty is the sum of the inlet heat streams minus the actual (calculated) heat duty. You can use an optional outlet heat stream for the net heat duty.

Specifying Decanter You can operate Decanter in one of the following ways: • Adiabatically • With specified duty • At a specified temperature Use the Input Specifications sheet to enter: • Pressure • Temperature or duty Use the following forms to enter specifications and view results for Decanter:

Unit Operation Models Version 10

Use this form

To do this

Input

Specify operating conditions, key components, calculation options, valid phases, efficiency, and entrainment

Properties

Specify and/or override property methods, KLL equation parameters, and/or user subroutine for phase split calculations

Hcurves

Specify heating or cooling curve tables and view tabular results

Block Options

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

Display simulation results

Dynamic

Specify parameters for dynamic simulations

2-9

Separators

Defining the Second Liquid Phase If two liquid phases are present at the decanter operating condition, Decanter treats the phase with higher density as the second phase, by default. When only one liquid phase exists and you want to avoid ambiguities, you can override the default by: • •

Specifying key components for identifying the second liquid phase on the Input Specifications sheet Optionally specifying the threshold key component mole fraction on the Input Specifications sheet

When

Decanter treats the

Two liquid phases are present

Phase with the higher mole fraction of key components as the second liquid phase

One liquid phase is present

Liquid phase as the first liquid phase, unless the mole fraction of key components exceeds the threshold value

Methods for Calculating the Liquid-Liquid Distribution Coefficients (KLL) When calculating liquid-liquid distribution coefficients (KLL), by default Decanter uses the physical property method specified for the block on the Properties PhaseProperty sheet or BlockOptions Properties sheet. On the Input CalculationOptions sheet, you can override the default by doing one of the following: • Specify separate property methods for the two liquid phases using the Properties PhaseProperty sheet • Use a built-in KLL correlation. Enter correlation coefficients on the Properties KLLCorrelation sheet. • Use a Fortran subroutine that you specify on the Properties KLLSubroutine sheet See ASPEN PLUS User Models for more information about writing Fortran subroutines.

Phase Splitting Decanter has two methods for solving liquid-liquid phase split calculations: • Equating fugacities of two liquid phases • Minimizing Gibbs free energy of the system You can select a method on the Input CalculationOptions sheet.

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Unit Operation Models Version 10

Chapter 2

If you select Minimizing Gibbs free energy of the system, the following must be thermodynamically consistent: • Physical property models • Block property method You cannot use the Minimizing Gibbs free energy of the system method when: You specify

On this sheet

Separate property methods for the two liquid phases

Properties PhaseProperty

The built-in correlation for liquid-liquid distribution coefficient ( KLL) calculations

Input CalculationOptions

A user subroutine for liquid-liquid distribution coefficient (KLL) calculations

Input Calculation Options

Equating fugacities of two liquid phases is not restricted by physical property specifications. However, Decanter can calculate solutions that do not minimize Gibbs free energy.

Efficiency Decanter outlet streams are normally at equilibrium. However, you can specify separation efficiencies on the Input Efficiency sheet to account for departure from equilibrium. If you select Liquid-FreeWater for Valid Phases on the Input CalculationOptions sheet, you cannot specify separation efficiencies.

Solids Entrainment If solids substreams are present, they do not participate in phase equilibrium calculations, but they do participate in enthalpy balance. You can use the Input Entrainment sheet to specify solids entrainment in the first liquid outlet stream. Decanter places any remaining solids in the second liquid outlet stream.

Unit Operation Models Version 10

2-11

Separators

Sep Component Separator Sep combines streams and separates the result into two or more streams according to splits specified for each component. When the details of the separation are unknown or unimportant, but the splits for each component are known, you can use Sep in place of a rigorous separation model to save computation time . If the composition and conditions of all outlet streams of the block you are modeling are identical, you can use an FSplit block instead of Sep.

Flowsheet Connectivity for Sep

Material (any number)

Material (2 or more) Heat (optional)

Material Streams Inlet

At least one material stream

Outlet At least two material streams

Heat Streams Inlet

No inlet heat streams

Outlet One stream for the enthalpy difference between inlet and outlet material

streams (optional)

2-12

Unit Operation Models Version 10

Chapter 2

Specifying Sep For each substream of each outlet stream except one, use the Sep Input Specifications sheet to specify one of the following for each component present: • Fraction of the component in the corresponding inlet substream • Mole flow rate of the component • Mass flow rate of the component • Standard liquid volume flow rate of the component Sep puts any remaining flow in the corresponding substream of the unspecified outlet stream. Use the following forms to enter specifications and view results for Sep: Use this form

To do this

Input

Enter split specifications, flash specifications, and convergence parameters for the mixed inlet and each outlet stream

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View Sep simulation results

Inlet Pressure Use the Sep Input Feed Flash sheet to specify either the pressure drop or the pressure at the inlet. This is useful when Sep has more than one inlet stream. The inlet pressure defaults to the minimum inlet stream pressure.

Outlet Stream Conditions Use the Sep Input Outlet Flash sheet to specify the conditions of the outlet streams. If you do not specify the conditions for a stream, Sep uses the inlet temperature and pressure.

Unit Operation Models Version 10

2-13

Separators

Sep2 Two-Outlet Component Separator Sep2 separates inlet stream components into two outlet streams. Sep2 is similar to Sep, but offers a wider variety of specifications. Sep2 allows purity (mole-fraction) specifications for components. You can use Sep2 in place of a rigorous separation model, such as distillation or absorption. Sep2 saves computation time when details of the separation are unknown or unimportant. If the composition and conditions of all outlet streams of the block you are modeling are identical, you can use FSplit instead of Sep2.

Flowsheet Connectivity for Sep2 Material Material (any number)

Material Heat (optional)

Material Streams Inlet

At least one material stream

Outlet Two material streams

Heat Streams Inlet

No inlet heat streams

Outlet One stream for the enthalpy difference between inlet and outlet material

streams (optional)

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Unit Operation Models Version 10

Chapter 2

Specifying Sep2 Use the Input Specifications sheet to specify stream and/or component fractions and flows. The number of specifications for each substream must equal the number of components in that substream. You can enter these stream specifications: • • • •

Fraction of the total inlet stream going to either outlet stream Total mass flow rate of an outlet stream Total molar flow rate of an outlet stream (for substreams of type MIXED or CISOLID) Total standard liquid volume flow rate of an outlet stream (for substreams of type MIXED)

You can enter these component specifications: • Fraction of a component in the feed going to either outlet stream • Mass flow rate of a component in an outlet stream • Molar flow rate of a component in an outlet stream (for substreams of type MIXED or CISOLID) • Standard liquid volume flow rate of a component in an outlet stream (for substreams of type MIXED) • Mass fraction of a component in an outlet stream • Mole fraction of a component in an outlet stream (for substreams of type MIXED or CISOLID) Sep2 treats each substream separately. Do not: • Specify the total flow of both outlet streams • Enter more than one flow or frac specification for each component • Enter both a mole-frac and a mass-frac specification for a component in a stream Use the following forms to enter specifications and view results for Sep2:

Unit Operation Models Version 10

Use this form

To do this

Input

Enter split specifications, flash specifications, and convergence parameters for the mixed inlet and each outlet stream

Block Options

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View Sep2 simulation results

2-15

Separators

Inlet Pressure Use the Input Feed Flash sheet to specify either the pressure drop or pressure at the inlet. This information is useful when Sep2 has more than one inlet stream. The inlet pressure defaults to the minimum of the inlet stream pressures.

Outlet Stream Conditions Use the Input Outlet Flash sheet to specify the conditions of the outlet streams. If you do not specify the conditions for a stream, Sep2 uses the inlet temperature and pressure.

❖

2-16

❖

❖

❖

Unit Operation Models Version 10

Chapter 3

3

Heat Exchangers This chapter describes the unit operation models for heat exchangers and heaters (and coolers), and for interfacing to the B-JAC heat exchanger programs. The models are:

Unit Operation Models Version 10

Model

Description

Purpose

Use For

Heater

Heater or cooler

Determines thermal and phase conditions of outlet stream

Heaters, coolers, condensers, and so on

HeatX

Two-stream heat exchanger

Exchanges heat between two streams

Two-stream heat exchangers. Rating shell and tube heat exchangers when geometry is known.

MHeatX

Multistream heat exchanger

Exchanges heat between any number of streams

Multiple hot and cold stream heat exchangers. Two-stream heat exchangers. LNG exchangers.

Hetran

Shell and tube heat exchanger

Provides interface to the B-JAC Hetran shell and tube heat exchanger program

Shell and tube heat exchangers, including kettle reboilers

Aerotran

Air-cooled heat exchanger

Provides interface to the B-JAC Aerotran air-cooled heat exchanger program

Crossflow heat exchangers, including air coolers

3-1

Heat Exchangers

Heater Heater/Cooler You can use Heater to represent: • Heaters • Coolers • Valves • Pumps (whenever work-related results are not needed) • Compressors (whenever work-related results are not needed) You also can use Heater to set the thermodynamic condition of a stream. When you specify the outlet conditions, Heater determines the thermal and phase conditions of a mixture with one or more inlet streams.

Flowsheet Connectivity for Heater Heat (optional)

Material (any number)

Heat (optional)

Material

Water (optional)

Material Streams Inlet

At least one material stream

Outlet One material stream

One water decant stream (optional)

Heat Streams Inlet

Any number of heat streams (optional)

Outlet One heat stream (optional)

3-2

Unit Operation Models Version 10

Chapter 3

If you give only one specification (temperature or pressure) on the Specifications sheet, Heater uses the sum of the inlet heat streams as a duty specification. Otherwise, Heater uses the inlet heat stream only to calculate the net heat duty. The net heat duty is the sum of the inlet heat streams minus the actual (calculated) heat duty. You can use an optional outlet heat stream for the net heat duty.

Specifying Heater Use the Heater Input Specifications sheet for all required specifications and valid phases. Dew point calculations are two- or three-phase flashes with a vapor fraction of unity. Bubble point calculations are two- or three-phase flashes with a vapor fraction of zero. Use the Heater Input FlashOptions sheet to specify temperature and pressure estimates and flash convergence parameters. Use the Hcurves form to specify optional heating or cooling curves. This model has no dynamic features. The pressure drop is fixed at the steady state value. The outlet flow is determined by the mass balance. Use the following forms to enter specifications and view results for Heater.

Unit Operation Models Version 10

Use this form

To do this

Input

Enter operating conditions and flash convergence parameters

Hcurves

Specify heating or cooling curve tables and view tabular results

Block Options

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View Heater results

3-3

Heat Exchangers

Solids Heater can simulate fluid phases with solids when the stream contains solid substreams or when you request electrolyte chemistry calculations. All phases are in thermal equilibrium. Solids leave at the same temperature as fluid phases. Solid Substreams Materials in solid substreams do not participate in phase equilibrium calculations. Electrolyte Chemistry Calculations You can request these on the Properties Specifications Global sheet or the Heater BlockOptions Properties sheet. Solid salts participate in liquid-solid phase equilibrium and thermal equilibrium calculations. The salts are in the MIXED substream.

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Unit Operation Models Version 10

Chapter 3

HeatX Two-Stream Heat Exchanger HeatX can model a wide variety of shell and tube heat exchanger types including: • Countercurrent and cocurrent • Segmental baffle TEMA E, F, G, H, J, and X shells • Rod baffle TEMA E and F shells • Bare and low-finned tubes HeatX can perform a full zone analysis with heat transfer coefficient and pressure drop estimation for single- and two-phase streams. For rigorous heat transfer and pressure drop calculations, you must supply the exchanger geometry. If exchanger geometry is unknown or unimportant, HeatX can perform simplified shortcut rating calculations. For example, you may want to perform only heat and material balance calculations. HeatX has correlations to estimate sensible heat, nucleate boiling, and condensation film coefficients. HeatX cannot: • • •

Perform design calculations Perform mechanical vibration analysis Estimate fouling factors

Flowsheet Connectivity for HeatX Cold Outlet Water (optional)

Hot Inlet

Hot Outlet Water (optional)

Cold Inlet

Unit Operation Models Version 10

3-5

Heat Exchangers

Material Streams Inlet

One hot inlet One cold inlet

Outlet One hot outlet

One cold outlet One water decant stream on the hot side (optional) One water decant stream on the cold side (optional)

Specifying HeatX Consider these questions when specifying HeatX: • Should rating calculations be simple (shortcut) or rigorous? • What specification should the block have? • How should the log-mean temperature difference correction factor be calculated? • How should the heat transfer coefficient be calculated? • How should the pressure drops be calculated? • What equipment specifications and geometry information are available? The answers to these questions determine the amount of information required to complete the block input. You must provide one of the following specifications: • • • • • • •

Heat exchanger area or geometry Exchanger heat duty Outlet temperature of the hot or cold stream Temperature approach at either end of the exchanger Degrees of superheating/subcooling for the hot or cold stream Vapor fraction of the hot or cold stream Temperature change of the hot or cold stream

Use the following forms to enter specifications and view results for HeatX: Use this form

To do this

Setup

Specify shortcut or detailed calculations, flow direction, exchanger pressure drops, heat transfer coefficient calculation methods, and film coefficients

Options

Specify different flash convergence parameters and valid phases for the hot and cold sides, HeatX convergence parameters, and block-specific report option

Geometry

Specify the shell and tube configuration and indicate any tube fins, baffles, or nozzles

UserSubroutines

Specify parameters for user-defined Fortran subroutines to calculate overall heat transfer coefficient, LMTD correction factor, tube-side liquid holdup, or tube-side pressure drop

Hot-Hcurves

Specify hot stream heating or cooling curve tables and view tabular results continued

3-6

Unit Operation Models Version 10

Chapter 3

Use this form

To do this

Cold-Hcurves

Specify cold stream heating or cooling curve tables and view tabular results

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View a summary of results, mass and energy balances, pressure drops, velocities, and zone analysis

Detailed Results

View detailed shell and tube results, and information about tube fins, baffles, and nozzles

Dynamic

Specify parameters for dynamic simulations

Shortcut Versus Rigorous Rating Calculations HeatX has two rating modes: shortcut and rigorous. Use the Calculation Type field on the Setup Specifications sheet to specify shortcut or rigorous rating calculations. In shortcut rating mode you can simulate a heat exchanger block with the minimum amount of required input. The shortcut calculation does not require exchanger configuration or geometry data. For rigorous rating mode, you can use exchanger geometry to estimate: • Film coefficients • Pressure drops • Log-mean temperature difference correction factor Rigorous rating mode provides more specification options for HeatX, but it also requires more input. Rigorous rating mode provides defaults for many options. You can change the defaults to gain complete control over the calculations. The following table lists these options with valid values. The values are described in the following sections.

Unit Operation Models Version 10

3-7

Heat Exchangers

Variable

Calculation Method

Available in Shortcut Mode

Available in Rigorous Mode

LMTD Correction Factor

Constant Geometry User subroutine

Default No No

Yes Default Yes

Heat Transfer Coefficient

Constant value Phase-specific values Power law expression Film coefficients Exchanger geometry User subroutine

Yes Default Yes No No No

Yes Yes Yes Yes Default Yes

Film Coefficient

Constant value Phase-specific values Power law expression Calculate from geometry

No No No No

Yes Yes Yes Default

Pressure Drop

Outlet pressure Calculate from geometry

Default No

Yes Default

Calculating the Log-Mean Temperature Difference Correction Factor The standard equation for a heat exchanger is:

Q = U ⋅ A ⋅ LMTD where LMTD is the log-mean temperature difference. This equation applies for exchangers with pure countercurrent flow. The more general equation is:

Q = U ⋅ A ⋅ F ⋅ LMTD where the LMTD correction factor, F, accounts for deviation from countercurrent flow. Use the LMTD Correction Factor field on the Setup Specifications sheet to enter the LMTD correction factor.

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Unit Operation Models Version 10

Chapter 3

In shortcut rating mode, the LMTD correction factor is constant. In rigorous rating mode, use the LMTD Correction Method field on the Setup Specifications sheet to specify how HeatX calculates the LMTD correction factor. You can choose from the following calculation options: If LMTD Correction Method is Then Constant

The LMTD correction factor you enter is constant.

Geometry

HeatX calculates the LMTD correction factor using the exchanger specification and stream properties

User subroutine

You supply a user subroutine to calculate the LMTD correction factor.

Calculating the Heat Transfer Coefficient To determine how the heat transfer coefficient is calculated, set the Calculation Method on the Setup U Methods sheet. You can use these options in shortcut or rigorous rating mode: If Calculation Method is

HeatX uses

And you specify

Constant value

A constant value for the heat transfer coefficient

The constant value

Phase-specific values

A different heat transfer coefficient for each heat transfer zone of the exchanger, indexed by the phase for the hot and cold streams

A constant value for each zone

Power law expression

A power law expression for the heat transfer coefficient as a function of one of the stream flow rates

Constants for the power law expression

In rigorous rating mode, three additional values are allowed:

Unit Operation Models Version 10

If Calculation Method is

Then

Exchanger geometry

HeatX calculates the heat transfer coefficient using exchanger geometry and stream properties to estimate film coefficients.

Film coefficients

HeatX calculates the heat transfer coefficients using the film coefficients. You can use any option on the Setup Film Coefficients sheet to calculate the film coefficients.

User subroutine

You supply a user subroutine to calculate the heat transfer coefficient.

3-9

Heat Exchangers

Film Coefficients HeatX does not calculate film coefficients in shortcut rating mode. In rigorous rating mode, if you use film coefficients or exchanger geometry for the heat transfer coefficient calculation method, HeatX calculates the heat transfer coefficient using:

1 1 1 = + U hc hh Where:

hc

=

Cold stream film coefficient

hh

=

Hot stream film coefficient

To choose an option for calculating film coefficients, set the Calculation Method on the Setup Film Coefficients sheet. The following are available: If Calculation Method is

HeatX uses

And you specify

Constant value

A constant value for the film coefficient

A constant value to be used throughout the exchanger

Phase-specific values

A different film coefficient for each heat transfer zone (phase) of the exchanger, indexed by the phase of the stream

A constant value for each phase

Power law expression

A power law expression for the film coefficient as a function of the stream flow rate

Constants for the power law expression

Calculate from geometry

The exchanger geometry and stream properties to calculate the film coefficient

The hot stream and cold stream film coefficient calculation methods are independent of each other. You can use any combination that is appropriate for your exchanger.

Pressure Drop Calculations To enter exchanger pressure or pressure drop for the hot and cold sides, use the Outlet Pressure fields on the Setup Pressure Drop sheet. In shortcut rating mode the pressure drop is constant.

3-10

Unit Operation Models Version 10

Chapter 3

In rigorous rating mode, you can choose how pressure drops are calculated by setting the pressure options on the Setup PressureDrop sheet. The following pressure drop options are available: If Pressure Option is

Then

Outlet Pressure

You must enter the outlet pressure or pressure drop for the stream.

Calculate from geometry

HeatX calculates the pressure drop using the exchanger geometry and stream properties

HeatX calls the Pipeline model to calculate tube-side pressure drop. You can set the correlations for pressure drop and liquid holdup that the Pipeline model uses on the Setup PressureDrop sheet.

Exchanger Configuration Exchanger configuration refers to the overall patterns of flow in the heat exchanger. If you choose Calculate From Geometry for any of the heat transfer coefficients, film coefficients, or pressure drop calculation methods, you may be required to enter some information about the exchanger configuration on the Geometry Shell sheet. This sheet includes fields for: • TEMA shell type (see the next figure, TEMA Shell Types) • Number of tube passes • Exchanger orientation • Tubes in baffle window • Number of sealing strips • Tube flow for vertical exchangers

Unit Operation Models Version 10

3-11

Heat Exchangers

E Shell One Pass Shell

F Shell Two Pass Shell with Longitudinal Baffle

G Shell

Split Flow

H Shell

Double Split Flow

J Shell

Divided Flow

X Shell

Cross Flow

TEMA Shell Types

3-12

Unit Operation Models Version 10

Chapter 3

The Geometry Shell sheet also contains two important dimensions for the shell: • Inside shell diameter • Shell to bundle clearance The next figure shows the shell dimensions.

Outer Tube Limit

Shell Diameter

Shell to Bundle Clearance

Shell Dimensions

Baffle Geometry Calculation of shell-side film coefficient and pressure drop require information about the baffle geometry within the shell. Enter baffle geometry on the Geometry Baffles sheet. HeatX can calculate shell-side values for both segmental baffle shells and rod baffle shells. Other required information depends on the baffle type. For segmental baffles, required information includes: • Baffle cut • Baffle spacing • Baffle clearances For rod baffles, required information includes: • Ring dimensions • Support rod geometry

Unit Operation Models Version 10

3-13

Heat Exchangers

The next two figures show the baffle dimensions. The Baffle Cut in the Dimensions for Segmental Baffles figure is a fraction of the shell diameter. All clearances are diametric.

Baffle Cut

Tube Hole

Shell to Baffle Clearance

Dimensions for Segmental Baffles Rod Diameter

Ring Outside Diameter

Ring Inside Diameter

Dimensions for Rod Baffles

Tube Geometry Calculation of the tube-side film coefficient and pressure drop require information about the geometry of the tubebank. HeatX also uses this information to calculate the heat transfer coefficient from the film coefficients. Enter tube geometry on the Geometry Tubes sheet.

3-14

Unit Operation Models Version 10

Chapter 3

You can select a heat exchanger with either bare or low-finned tubes. The sheet also includes fields for: • Total number of tubes • Tube length • Tube diameters • Tube layout • Tube material of construction The next two figures show tube layout patterns and fin dimensions. o

o

30

o

45

Tube Pitch

Tube Pitch Triangle

o

90

60

Rotated Square

Tube Pitch Rotated Triangle

Tube Pitch Square

Direction of Flow

Tube Layout Patterns

Fin Thickness Outside Diameter

Root Mean Diameter Fin Height

Fin Dimensions

Nozzle Geometry Calculations for pressure drop include the calculation of pressure drop in the exchanger nozzles. Enter nozzle geometry on the Geometry Nozzles sheet.

Model Correlations HeatX uses open literature correlations for calculating film coefficients and pressure drops. The next four tables list the model correlations.

Unit Operation Models Version 10

3-15

Heat Exchangers

Tube-side Heat Transfer Coefficient Correlations Mechanism

Flow Regime

Correlation

References

Single-phase

Laminar Turbulent

Schlunder Gnielinski

[1] [1]

Boiling - vertical tubes

Steiner/Taborek

[2]

Boiling - horizontal tubes

Shah

[3, 4]

Condensation - vertical tubes

Laminar Laminar wavy Turbulent Shear-dominated

Nusselt Kutateladze Labuntsov Rohsenow

[5] [6] [7] [8]

Condensation - horizontal tubes

Annular Stratifying

Rohsenow Jaster/Kosky method

[8] [9]

Shell-side Heat Transfer Coefficient Correlations Mechanism

Correlation

References

Single-phase segmental

Bell-Delaware

[10, 11]

Single-phase ROD

Gentry

[12]

Boiling

Jensen

[13]

Nusselt Kutateladze Labuntsov Rohsenow

[5] [6] [7] [8]

Kern

[9]

Condensation - vertical

Flow Regime

Laminar Laminar wavy Turbulent Shear-dominated

Condensation - horizontal

Tube-side Pressure Drop Correlations Mechanism

Correlation

Single-phase

Darcy’s Law

Two-phase

See Chapter 6

†

†

See Pipeline, Two-Phase Correlations, for the correlations available for two-phase pressure drop in a pipe.

Shell-side Pressure Drop Correlations

3-16

Mechanism

Correlation

References

Single-phase segmental

Bell-Delaware

[10, 11]

Single-phase ROD

Gentry

[12]

Two-phase segmental

Bell-Delaware method with Grant’s correction for twophase flow

[10, 11], [14]

Two-phase ROD

Gentry

[12]

Unit Operation Models Version 10

Chapter 3

Flash Specifications Use the Options Flash Options sheet to enter flash specifications. If you want to perform these calculations

Solids?

Set Valid Phases to

Vapor phase

Yes or no

Vapor-only

Liquid phase

Yes or no

Liquid-only

2-fluid flash phase

Yes or no

Vapor-Liquid

3-fluid flash phase

Yes or no

Vapor-Liquid-Liquid

3-fluid phase free-water flash

Yes or no

Vapor-Liquid-FreeWater

Solids only

Yes

Solid-only

Physical Properties To override global or flowsheet section property specifications, use the BlockOptions Properties sheet. You can use different physical property options for the hot side and cold side of the heat exchanger. If you supply only one set of property specifications, HeatX uses that set for both hot and cold side calculations.

Solids All phases are in thermal equilibrium. Solids leave at the same temperature as the fluid phases. HeatX can simulate fluid phases with solids when the stream contains solid substreams, or when you request electrolyte chemistry calculations. Solid Substreams Materials in solid substreams do not participate in phase equilibrium calculations. Electrolyte Chemistry Calculations You can request these on the Properties Specifications Global sheet or HeatX BlockOptions Properties sheet. Solid salts participate in liquid-solid phase equilibrium and thermal equilibrium calculations. The salts are in the MIXED substream.

Unit Operation Models Version 10

3-17

Heat Exchangers

References 1. Gnielinski, V., "Forced Convection in Ducts." In: Heat Exchanger Design Handbook. New York: Hemisphere Publishing Corporation, 1983. 2. Steiner, D. and Taborek, J., "Flow Boiling Heat Transfer in Vertical Tubes Correlated by an Asymptotic Model." In: Heat Transfer Engineering, 13(2):4369, 1992. 3. Shah, M.M., "A New Correlation for Heat Transfer During Boiling Flow Through Pipes." In: ASHRAE Transactions, 82(2):66-86, 1976. 4. Shah, M.M., "Chart Correlation for Saturated Boiling Heat Transfer: Equations and Further Study." In: ASHRAE Transactions, 87(1):185-196, 1981. 5. Nusselt, W., "Surface Condensation of Water Vapor." Z. Ver. Dtsch, Ing., 60(27):541-546, 1916. 6. Kutateladze, S.S., Fundamentals of Heat Transfer. New York: Academic Press, 1963. 7. Labuntsov, D.A., "Heat Transfer in Film Condensation of Pure Steam on Vertical Surfaces and Horizontal Tubes." In: Teploenergetika, 4(7):72-80, 1957. 8. Rohsenow, W.M., Webber, J.H., and Ling, A.T., "Effect of Vapor Velocity on Laminar and Turbulent Film Condensation." In: Transactions of the ASME, 78:1637-1643, 1956. 9. Jaster, H. and Kosky, P.G., "Condensation Heat Transfer in a Mixed Flow Regime." In: International Journal of Heat and Mass Transfer, 19:95-99, 1976. 10. Taborek, J., "Shell-and-Tube Heat Exchangers: Single Phase Flow." In: Heat Exchanger Design Handbook. New York: Hemisphere Publishing Corporation, 1983. 11. Bell, K.J., "Delaware Method for Shell Side Design." In: Kakac, S., Bergles, A.E., and Mayinger, F., editors, Heat Exchangers: Thermal-Hydraulic Fundamentals and Design. New York: Hemisphere Publishing Corp., 1981. 12. Gentry, C.C., "RODBaffle Heat Exchanger Technology." In: Chemical Engineering Progress 86(7):48-57, July 1990. 13. Jensen, M.K. and Hsu, J.T., "A Parametric Study of Boiling Heat Transfer in a Tube Bundle." In: 1987 ASME-JSME Thermal Engineering Joint Conference, pages 133-140, Honolulu, Hawaii, 1987. 14. Grant, I.D.R. and Chisholm, D., "Two-Phase Flow on the Shell Side of a Segmentally Baffled Shell-and-Tube Heat Exchanger." In: Journal of Heat Transfer, 101(1):38-42, 1979.

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Unit Operation Models Version 10

Chapter 3

MHeatX Multistream Heat Exchanger Use MHeatX to represent heat transfer between multiple hot and cold streams, such as in an LNG exchanger. You can also use MHeatX for two-stream heat exchangers. Free water can be decanted from any outlet stream. MHeatX ensures an overall energy balance but does not account for the exchanger geometry. MHeatX can perform a detailed, rigorous internal zone analysis to determine the internal pinch points and heating and cooling curves for all streams in the heat exchanger. MHeatX can also calculate the overall UA for the exchanger and model heat leak to or from an exchanger. MHeatX uses multiple Heater blocks and heat streams to enhance flowsheet convergence. ASPEN PLUS automatically sequences block and stream convergence unless you specify a sequence or tear stream.

Flowsheet Connectivity for MHeatX Cold Inlets (any number)

Hot Outlets Hot Inlets (any number)

Water (optional) Hot Outlets Water (optional) Cold Outlets

Water (optional)

Material Streams Inlet

At least one material stream on the hot side. At least one material stream on the cold side

Outlet One outlet stream for each inlet stream

One water decant stream for each outlet stream (optional) The inlet stream sides are non-contacting.

Unit Operation Models Version 10

3-19

Heat Exchangers

Specifying MHeatX You must give outlet specifications for each stream on one side of the heat exchanger. On the other side you can specify any of the outlet streams, but you must leave at least one unspecified stream. Different streams can have different types of specifications. MHeatX assumes that all unspecified streams have the same outlet temperature. An overall energy balance determines the temperature of any unspecified stream(s). You can use a different property method for each stream in MHeatX. Specify the property methods on the BlockOptions Properties sheet. Use the following forms to enter specifications and view results for MHeatX: Use this form

To do this

Input

Specify operating conditions, flash convergence parameters, parameters for zone analysis, flash table, MHeatX convergence parameters, and block-specific report options

Hcurves

Specify heating or cooling curve tables and view tabular results

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels and report options for this block

Results

View stream results, exchanger results, zone profiles, stream profiles, flash profiles, and material and energy balance results

Zone Analysis MHeatX can perform a detailed, rigorous internal zone analysis to determine: • Internal pinch points • UA and LMTD of each zone • Total UA of the exchanger • Overall average LMTD To obtain a zone analysis, specify Number of zones greater than 0 on the MHeatX Input Zone Analysis sheet. During zone analysis MHeatX can add: • • •

Stream entry points (if all feed streams are not at the same temperature) Stream exit points (if all product streams are not at the same temperature) Phase change points (if a phase change occurs internally)

MHeatX can also account for the nonlinearities of zone profiles by adding zones adaptively. MHeatX can perform zone analysis for both countercurrent and cocurrent heat exchangers.

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Unit Operation Models Version 10

Chapter 3

Using Flash Tables in Zone Analysis Use Flash Tables to estimate zone profiles and pinch points quickly. These tables are most useful for heat exchangers that have many streams, for which zone analysis calculations can take a long time. To use a Flash Table for a stream, specify the number of flash points for the stream on the MHeatX Input Flash Table sheet. When you specify a flash table for a stream, MHeatX generates a temperature-enthalpy profile of that stream before zone analysis, and interpolates that profile during zone analysis, rather than flashing the stream. You can also specify the fraction of total pressure drop in each phase region of a stream on the MHeatX Input Flash Table sheet. ASPEN PLUS uses these fractions to determine the pressure profile during Flash Table generation.

Computational Structure for MHeatX The computational structure of MHeatX may affect your specifications. Unlike other unit operation blocks, MHeatX is not simulated by a single computation module. Instead, ASPEN PLUS generates heaters and heat streams to represent the multistream heat exchanger. A Heater block represents streams with outlet specifications. A multistream heater block represents streams with no outlet specifications. The next figure shows the computational structure generated for a sample exchanger.

$LNGH02 S3

$LNGH03 S4

S5

HEATER

$LNGH04 S6

S7

S8 HEATER

HEATER $LNGQ03

$LNGQ04

$LNGQ02 $LNGHTR S1 LNGIN

S2 MHEATER LNGOUT

Example of MHeatX Computational Structure This computational sequence converges much more rapidly than simulation of MHeatX as a single block. Block results are given for the entire MHeatX sequence. In most cases, you do not need to know about the individual blocks generated in the sequence. The following paragraphs describe the exceptions.

Unit Operation Models Version 10

3-21

Heat Exchangers

Simulation history and control panel messages are given for the generated Heater blocks and heat streams. You can provide an estimate for duty of the internally generated heat stream. If the heat stream is a tear stream in the flowsheet, ASPEN PLUS uses this estimate as an initial value. You can give convergence specifications for the flowsheet resulting when MHeatX blocks are replaced by their generated networks. The generated Heater block and heat stream IDs must be used on the Convergence SequenceSpecifications and Convergence TearSpecifications sheets. Automatic flowsheet analysis is based on the flowsheet resulting when MHeatX blocks are replaced by generated Heater blocks. The generated Heater blocks, instead of the MHeatX block, appear in the calculation sequence. You can select generated heat streams as tear streams.

Solids MHeatX can simulate fluid phases with solids when the stream contains solid substreams, or when you request electrolyte chemistry calculations. All phases are in thermal equilibrium. Solids leave at the same temperature as the fluid phases. Solid Substreams Materials in solid substreams do not participate in phase equilibrium calculations. Electrolyte Chemistry Calculations You can request these on the Properties Specifications Global sheet or the MHeatX BlockOptions Properties sheet. Solid salts participate in liquid-solid phase equilibrium and thermal equilibrium calculations. The salts are in the MIXED substream.

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Unit Operation Models Version 10

Chapter 3

Hetran Interface to the B-JAC Hetran Program for Shell and Tube Heat Exchangers Hetran is the interface to the B-JAC Hetran program for designing and simulating shell and tube heat exchangers. Hetran can be used to simulate shell and tube heat exchangers with a wide variety of configurations. To use Hetran, place the block in the flowsheet, connect inlet and outlet streams, and specify a small number of block inputs, including the name of the B-JAC input file for that exchanger. You enter information related to the heat exchanger configuration and geometry through the Hetran standalone program interface. The exchanger specification is saved as a B-JAC input file. You do not have to enter information about the exchanger’s physical characteristics through the ASPEN PLUS user interface or through input language.

Flowsheet Connectivity for Hetran Cold Inlet Hot Inlet Hot Water (optional)

Hot Outlet Cold Outlet Cold Water (optional)

Material Streams Inlet

One hot inlet One cold inlet

Outlet One hot outlet

One cold outlet One water decant stream on the hot side (optional) One water decant stream on the cold side (optional)

Unit Operation Models Version 10

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Heat Exchangers

Specifying Hetran Enter the input for the shell and tube heat exchanger through the Hetran program’s graphical user interface. The input for Hetran in ASPEN PLUS is limited to: • The B-JAC input file name that contains the heat exchanger specification • A set of parameters to control how property curves are generated • A set of Hetran program inputs that you can change from within ASPEN PLUS (for example, fouling factors and film coefficients) Use the following forms to enter specifications and view results for Hetran: Use this form

To do this

Input

Specify the name of the B-JAC input file, parameters for calculating the property curves, optional Hetran program inputs, flash convergence parameters, and valid phases

BlockOption s

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View inlet and outlet stream conditions and material and energy balance results

Detailed Results

View overall results and detailed results for the shell side and tube side

Flash Specifications Use the FlashOptions sheet to enter flash specifications.

3-24

If you want to perform these calculations

Solids?

Set Valid Phases to

Vapor phase

Yes or no

Vapor-only

Liquid phase

Yes or no

Liquid-only

2-fluid flash phase

Yes or no

Vapor-Liquid

3-fluid flash phase

Yes or no

Vapor-Liquid-Liquid

3-fluid phase free-water flash

Yes or no

Vapor-Liquid-FreeWater

Solids only

Yes

Solid-only

Unit Operation Models Version 10

Chapter 3

Physical Properties To override global or flowsheet section property specifications, use the FlashOptions sheet. You can use different physical property methods for the hot side and cold side of the heat exchanger. If you supply only one set of property specifications, Hetran uses that set for both hot- and cold-side calculations.

Solids Hetran cannot currently handle streams with solids substreams.

Unit Operation Models Version 10

3-25

Heat Exchangers

Aerotran Interface to the B-JAC Aerotran Program for Air-cooled Heat Exchangers Aerotran is the interface to the B-JAC Aerotran program for designing and simulating air-cooled heat exchangers. Aerotran can be used to simulate aircooled heat exchangers with a wide variety of configurations. It can also be used to model economizers and the convection section of fired heaters. To use Aerotran, place the block in the flowsheet, connect inlet and outlet streams, and specify a small number of block inputs, including the name of the B-JAC input file for that exchanger. You enter information related to the air cooler configuration and geometry through the Aerotran standalone program interface. The air cooler specification is saved as a B-JAC input file. You do not have to enter information about the air cooler’s physical characteristics through the ASPEN PLUS user interface or through input language.

Flowsheet Connectivity for Aerotran Cold Water (optional) Hot Inlet

Cold (Air) Outlet

Hot Water (optional)

Hot Outlet

Cold (Air) Inlet

Material Streams Inlet

One hot inlet One cold (air) inlet

Outlet One hot outlet

One cold (air) outlet One water decant stream on the hot side (optional) One water decant stream on the cold side (optional)

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Unit Operation Models Version 10

Chapter 3

Specifying Aerotran Enter the input for the air-cooled heat exchanger through the Aerotran program’s graphical user interface. The input for Aerotran in ASPEN PLUS is limited to: • The B-JAC input file name that contains the heat exchanger specification • A set of parameters to control how property curves are generated • A set of Aerotran program inputs that you can change from within ASPEN PLUS (for example, fouling factors and film coefficients) Use the following forms to enter specifications and view results for Aerotran: Use this form

To do this

Input

Specify the name of the B-JAC input file, parameters for calculating the property curves, optional Aerotran program inputs, flash convergence parameters, and valid phases

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View inlet and outlet stream conditions and material and energy balance results

Detailed Results

View overall results, detailed results for the outside and tube side, and fan results

Flash Specifications Use the FlashOptions sheet to enter flash specifications.

Unit Operation Models Version 10

If you want to perform these calculations

Solids?

Set Valid Phases to

Vapor phase

Yes or no

Vapor-only

Liquid phase

Yes or no

Liquid-only

2-fluid flash phase

Yes or no

Vapor-Liquid

3-fluid flash phase

Yes or no

Vapor-Liquid-Liquid

3-fluid phase free-water flash

Yes or no

Vapor-Liquid-FreeWater

Solids only

Yes

Solid-only

3-27

Heat Exchangers

Physical Properties To override global or flowsheet section property specifications, use the FlashOptions sheet. You can use different physical property methods for the hot side and cold side of the air cooler. If you supply only one set of property specifications, Aerotran uses that set for both hot- and cold-side calculations.

Solids Aerotran blocks cannot currently handle streams with solids substreams.

❖

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❖

❖

❖

Unit Operation Models Version 10

Chapter 4

4

Columns This chapter describes the unit operation models for distillation columns using shortcut and rigorous calculations, and for liquid-liquid extraction. The models are: Model

Description

Purpose

Use For

DSTWU

Shortcut distillation design using the WinnUnderwood-Gilliland method

Determines minimum reflux ratio, minimum number of stages, and either actual reflux ratio or actual number of stages

Columns with one feed and two product streams

Distl

Shortcut distillation rating using the Edmister method

Determines separation based on reflux ratio, number of stages, and distillate-tofeed ratio

Columns with one feed and two product streams

SCFrac

Shortcut distillation for complex petroleum fractionation units

Determines product composition and flow, number of stages per section, and heat duty using fractionation indices

Complex columns, such as crude units and vacuum towers

RadFrac

Rigorous fractionation

Performs rigorous rating and design calculations for single columns

Ordinary distillation, absorbers, strippers, extractive and azeotropic distillation, three-phase distillation, reactive distillation

MultiFrac

Rigorous fractionation for complex columns

Performs rigorous rating and design calculations for multiple columns of any complexity

Heat integrated columns, air separation columns, absorber/stripper combinations ethylene plant primary fractionator quench tower combinations, petroleum refining applications continued

Unit Operation Models Version 10

4-1

Columns

Model

Description

Purpose

Use For

PetroFrac

Petroleum refining fractionation

Performs rigorous rating and design calculations for complex columns in petroleum refining applications

Preflash tower, atmospheric crude unit, vacuum unit, catalytic cracker main fractionator, delayed coker main fractionator, vacuum lube fractionator, ethylene plant primary fractionator and quench tower combinations

Rate-based distillation

Performs rigorous rating and design for single and multiple columns. Based on nonequilibrium calculations. Does not require efficiencies and HETPs.

Distillation columns, absorbers, strippers, reactive systems, heat integrated units, petroleum applications, such as crude and vacuum units, absorber-stripper combination

Rigorous liquid-liquid extraction

Models countercurrent extraction of a liquid stream using a solvent

Liquid-liquid extractors

RateFrac

Extract †

†

RateFrac requires a separate license and can be used only by customers who have purchased it through a specific license agreement with Aspen Technology, Inc.

This chapter is organized into the following sections:

4-2

Section

Models

Shortcut Distillation

DSTWU, Distl, SCFrac

Rigorous Distillation

RadFrac, MultiFrac, PetroFrac, RateFrac

Liquid-Liquid Extraction

Extract

Unit Operation Models Version 10

Chapter 4

DSTWU Shortcut Distillation Design DSTWU performs shortcut design calculations for single-feed, two-product distillation columns with a partial or total condenser. DSTWU assumes constant molal overflow and constant relative volatilities. DSTWU uses this method/correlation

To estimate

Winn

Minimum number of stages

Underwood

Minimum reflux ratio

Gilliland

Required reflux ratio for a specified number of stages or the required number of stages for a specified reflux ratio

For the specified recovery of light and heavy key components, DSTWU estimates: • Minimum reflux ratio • Minimum number of theoretical stages DSTWU then estimates one of the following: • Required reflux ratio for the specified number of theoretical stages • Required number of theoretical stages for the specified reflux ratio DSTWU also estimates the optimum feed stage location and the condenser and reboiler duties. DSTWU can produce tables and plots of reflux ratio versus number of stages.

Flowsheet Connectivity for DSTWU Heat (optional)

Heat (optional) Distillate Water (optional)

1 2

Feed N-1

N

Heat (optional)

Unit Operation Models Version 10

Bottoms Heat (optional)

4-3

Columns

Material Streams Inlet

One material feed stream

Outlet One distillate stream

One bottoms stream One water decant stream from condenser (optional)

Heat Streams Inlet

One stream for condenser cooling (optional) One stream for reboiler heating (optional)

Outlet One stream for condenser cooling (optional)

One stream for reboiler heating (optional) Each outlet heat stream contains the net heat duty for either the condenser or the reboiler. The net heat duty is the inlet heat stream minus the actual (calculated) heat duty. If you use heat streams for the reboiler, you must also use them for the condenser.

Specifying DSTWU Use the Input Specifications sheet to enter column specifications. The following table shows the specifications and what is calculated based on them: Specification

Result

Recovery of light and heavy key components

Minimum reflux ratio and minimum number of theoretical stages

Number of theoretical stages

Required reflux ratio

Reflux ratio

Required number of theoretical stages

DSTWU also estimates the optimum feed stage location, and the condenser and reboiler duties. DSTWU can generate an optional table of reflux ratio versus number of stages. Use the Input CalculationOptions sheet to enter specifications for the table.

4-4

Unit Operation Models Version 10

Chapter 4

Use the following forms to enter specifications and view results for DSTWU:

Unit Operation Models Version 10

Use this form

To do this

Input

Specify configuration and calculation options, block-specific report options, flash convergence parameters, valid phases, and DSTWU convergence parameters

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary results, material and energy balance results, and reflux ratio profile

4-5

Columns

Distl Shortcut Distillation Rating Distl simulates multistage multicomponent columns with a feed stream and two product streams. Distl performs shortcut distillation rating calculations for a single-feed, twoproduct distillation column. The column can have either a partial or total condenser. Distl calculates product composition using the Edmister approach. Distl assumes constant mole overflow and constant relative volatilities.

Flowsheet Connectivity for Distl Heat (optional)

Heat (optional) Distillate Water (optional)

1 2

Feed N-1

N

Heat (optional)

Bottoms Heat (optional)

Material Streams Inlet

One material feed stream

Outlet One distillate stream

One bottoms stream One water decant stream from condenser (optional)

Heat Streams Inlet

One stream for condenser cooling (optional) One stream for reboiler heating (optional)

Outlet One stream for condenser cooling (optional)

One stream for reboiler heating (optional)

4-6

Unit Operation Models Version 10

Chapter 4

Each outlet heat stream contains the net heat duty for either the condenser or the reboiler. The net heat duty is the inlet heat stream minus the actual (calculated) heat duty. If you use heat streams for the reboiler, you must also use them for the condenser.

Specifying Distl Use the Input Specifications sheet to enter the number of stages, reflux ratio, distillate to feed ratio, and other column specifications. Use the Input Convergence sheet to override default valid phases for condenser, convergence parameters for flash calculations, and model convergence parameters. Use the following forms to enter specifications and view results for Distl:

Unit Operation Models Version 10

Use this form

To do this

Input

Specify basic column configuration, operating conditions, Distl convergence parameters, and flash convergence parameters

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of column results and material and energy balance results

Dynamic

Specify parameters for dynamic simulation

4-7

Columns

SCFrac Shortcut Distillation for Complex Columns Use SCFrac to simulate complex distillation columns with a single feed, optional stripping steam, and any number of products. SCFrac also estimates the number of theoretical stages and the heating/cooling duty for each section. SCFrac can model complex columns, such as crude units and vacuum towers. SCFrac performs shortcut distillation calculations for columns with a single feed, one optional stripping steam stream, and any number of products. SCFrac divides a column with n products into n – 1 sections. These sections are numbered from the top down. SCFrac assumes: • Relative volatilities are constant for each section • The flow of liquid from section to section is negligible SCFrac does not handle solids. SCFrac can perform free-water calculations in the condenser.

Flowsheet Connectivity for SCFrac Distillate Side Products (any number)

Steam (optional)

Feed

Bottoms

Material Streams Inlet

One material feed stream One optional stripping steam stream (used for all sections)

Outlet One distillate stream

One bottoms stream At least one side product stream

4-8

Unit Operation Models Version 10

Chapter 4

Specifying SCFrac SCFrac divides an n–product column into n – 1 sections (see the next figure, SCFrac Multidraw Column). SCFrac numbers the column sections from the top down. For each section, you must specify: • Product pressure • Estimate of product flow or flow fraction based on feed flow You must specify the ratio of steam to product flow rate for all product streams except the distillate. You must also enter 2(n – 1) specifications from the following: • • • • •

Fractionation index (number of theoretical stages at total reflux) of a section Total flow, flow rate, or recovery of any group of components for a product stream Value of a property set property for a product stream (see ASPEN PLUS User Guide, Chapter 28) Difference of any pair of property set properties for one or a pair of product stream(s) Ratio of any pair of property set properties for one or a pair of product stream(s)

Because SCFrac performs steam calculations, water must always be present. All water flow leaves with the top product stream.

A Multidraw Column P1

P1

P2 Stream-1 P3 Stream-2 P4 Feed

Feed

Stream-3 P5

Stream-1 P2 Stream-2 P3 Stream-3 P4 Stream-4 P5

Stream-4

SCFrac Multidraw Column

Unit Operation Models Version 10

4-9

Columns

Use the following forms to enter specifications and view results for SCFrac:

4-10

Use this form

To do this

Input

Specify operating parameters, valid phases, SCFrac convergence parameters, and flash convergence parameters

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View condenser results, material and energy balance results, design specification results, section profiles, and product summary

Unit Operation Models Version 10

Chapter 4

RadFrac Rigorous Fractionation RadFrac is a rigorous model for simulating all types of multistage vapor-liquid fractionation operations. These operations include: • Ordinary distillation • Absorption • Reboiled absorption • Stripping • Reboiled stripping • Extractive and azeotropic distillation RadFrac is suitable for: • Two-phase systems • Three-phase systems • Narrow and wide-boiling systems • Systems exhibiting strong liquid phase nonideality RadFrac can detect and handle a free-water phase or other second liquid phase anywhere in the column. RadFrac can handle solids on every stage. RadFrac can handle pumparounds leaving any stage and returning to the same stage or to a different stage. RadFrac can model columns in which chemical reactions are occurring. Reactions can have fixed conversions, or they can be: • Equilibrium • Rate-controlled • Electrolytic RadFrac can also model columns in which two liquid phases and chemical reactions occur simultaneously, using different reaction kinetics for the two liquid phases. In addition, RadFrac can model salt precipitation. Although RadFrac assumes equilibrium stages, you can specify either Murphree or vaporization efficiencies. You can manipulate Murphree efficiencies to match plant performance. You can use RadFrac to size and rate columns consisting of trays and/or packings. RadFrac can model both random and structured packings.

Unit Operation Models Version 10

4-11

Columns

Flowsheet Connectivity for RadFrac Top Stage or Condenser Heat Duty

Feeds

Vapor Distillate 1

Heat (optional) Liquid Distillate Water Distillate (optional)

Reflux

Heat (optional) Heat (optional) Heat (optional) Bottom Stage or Reboiler Heat Duty

Products (optional) Decanters Return

Product

Boil-Up Nstage

Heat (optional) Bottoms

RadFrac can have any number of: • Stages • Interstage heaters/coolers • Decanters • Pumparounds

Material Streams Inlet

At least one inlet material stream

Outlet One vapor or liquid distillate product stream, or both

One water distillate product stream (optional) One bottoms liquid product stream Up to three side product streams per stage (optional) Any number of pseudo-product streams (optional) Each stage can have: • Any number of inlet streams • Up to three outlet streams (one vapor and two liquid) Outlet streams can be partial or total drawoffs of the stage flows. Decanter outlet streams can return to the stage immediately below. Or they can be split into any number of streams, each returning to a different user-specified stage. Pumparounds can go between any two stages, or to the same stage. Any number of pseudoproduct streams can represent column internal flows, pumparound flows, and thermosyphon reboiler flows. A pseudoproduct stream does not affect column results.

4-12

Unit Operation Models Version 10

Chapter 4

Heat Streams Inlet

One inlet heat stream per stage (optional) One heat stream per pumparound (optional)

Outlet One outlet heat stream per stage (optional)

One heat stream per pumparound (optional) RadFrac uses an inlet heat stream as a duty specification for all stages except the condenser, reboiler, and pumparounds. If you do not give two column operating specifications on the Setup Configuration sheet, RadFrac uses a heat stream as a specification for the condenser and reboiler. If you do not give two specifications on the Pumparounds Specifications sheet, RadFrac uses a heat stream as a specification for pumparounds. If you give two specifications on the Setup Configuration sheet or Pumparounds Specifications sheet, RadFrac does not use the inlet heat stream as a specification. The inlet heat stream supplies the required heating or cooling. Use optional outlet streams for the net heat duty of the condenser, reboiler, and pumparounds. The value of the outlet heat stream equals the value of the inlet heat stream (if any) minus the actual (calculated) heat duty.

Specifying RadFrac This section describes the following topics on RadFrac column configuration: • Stage Numbering • Feed Stream Conventions • Columns Without Condensers or Reboilers • Reboiler Handling • Heater and Cooler Specifications • Decanters • Pumparounds Use the following forms to enter specifications and view results for RadFrac: Use this form

To do this

Setup

Specify basic column configuration and operating conditions

DesignSpecs

Specify design specifications and view convergence results

Vary

Specify manipulated variables to satisfy design specifications and view final values

HeatersCoolers

Specify stage heating or cooling

Pumparounds

Specify pumparounds and view pumparound results continued

Unit Operation Models Version 10

4-13

Columns

Use this form

To do this

Pumparounds Hcurves

Specify pumparound heating or cooling curve tables and view tabular results

Decanters

Specify decanters and view decanter results

Efficiencies

Specify stage, component or sectional efficiencies

Reactions

Specify equilibrium, kinetic, and conversion reaction parameters

CondenserHcurves

Specify condenser heating or cooling curve tables and view tabular results

ReboilerHcurves

Specify reboiler heating or cooling curve tables and view tabular results

TraySizing

Specify sizing parameters for tray column sections and view results

TrayRating

Specify rating parameters for tray column sections and view results

PackSizing

Specify sizing parameters for packed column sections and view results

PackRating

Specify rating parameters for packed column sections and view results

Properties

Specify physical property parameters for column sections

Estimates

Specify initial estimates for stage temperatures, vapor and liquid flows, and compositions

Convergence

Specify convergence parameters for the column and feed flash calculations, and block-specific diagnostic message levels

Report

Specify block-specific report options and pseudostreams

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

UserSubroutines

Specify user subroutines for reaction kinetics, KLL calculations, tray sizing and rating, and packing sizing and rating

ResultsSummary

View key column results for the overall RadFrac column

Profiles

View and specify column profiles

Dynamic

Specify parameters for dynamic simulations

Stage Numbering RadFrac numbers stages from the top down, starting with the condenser (or starting with the top stage if there is no condenser).

Feed Stream Conventions Use the Setup Streams sheet to specify the feed and product stages. RadFrac provides three conventions for handling feed streams: • Above-Stage • On-Stage • Decanter (for three phase calculations only)

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Unit Operation Models Version 10

Chapter 4

(See the following figures, RadFrac Feed Convention Above-Stage and RadFrac Feed Convention On-Stage.) When the feed convention is Above-Stage, RadFrac introduces a material stream between adjacent stages. The liquid portion flows to the stage you specify. The vapor portion flows to the stage above. You can introduce a liquid feed to the top stage (or condenser) by specifying Stage=1. You can introduce a vapor feed to the bottom stage (or reboiler) by specifying Stage= the number of equilibrium stages + 1. Feed convention Decanter is used only in three-phase calculations (Valid Phases=Vapor-Liquid-Liquid on the Setup Configuration sheet) involving decanters. You can introduce a feed directly to a decanter attached to a stage using this convention.

n-1 Vapor Mixed feed to stage n Liquid n

RadFrac Feed Convention Above-Stage

n-1

Mixed feed to stage n

n

n+1

RadFrac Feed Convention On-Stage When the Feed Convention is On-Stage, both the liquid and vapor portions of a feed flow to the stage you specify.

Unit Operation Models Version 10

4-15

Columns

Columns Without Condensers or Reboilers You can specify the column configuration on the Setup Configuration sheet. If the column has no

Then specify

On sheet

Condenser

None for Condenser

Setup Configuration

Reboiler

None for Reboiler

Setup Configuration

Reboiler Handling RadFrac can model two reboiler types: • Kettle • Thermosyphon A kettle reboiler is modeled as the last stage in the column on the Setup Configuration sheet. Select Kettle for reboiler. By default, RadFrac uses a kettle reboiler. To specify the reboiler duty, enter Reboiler Duty as one of the operating specifications on the Setup Configuration sheet or leave it as a calculated value. A thermosyphon reboiler is modeled as a pumparound with a heater, from and to the bottom stage. Select Thermosyphon for Reboiler on the Setup Configuration sheet. Enter all other thermosyphon reboiler specifications on the Setup Reboiler sheet. The next figure shows the thermosyphon reboiler configuration. By default, RadFrac returns the reboiler outlet to the last stage using the On-Stage feed convention. You can also use the Reboiler Return Feed Convention on the Reboiler sheet to specify Above-Stage. This directs the vapor portion of the reboiler outlet to Stage= the number of equilibrium stages - 1.

Nstage - 1 Nstage

Reboiler Bottoms (B)

Thermosyphon Reboiler

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Chapter 4

The thermosyphon reboiler model has five related variables: • Pressure • Flow rate • Temperature • Temperature change • Vapor fraction You must specify one of the following: • • • • • • •

Temperature Temperature change Vapor fraction Flow rate Flow rate and temperature Flow rate and temperature change Flow rate and vapor fraction

If you choose an option consisting of two variables, you must specify the reboiler heat duty on the Setup Configuration sheet. RadFrac treats the value you enter for the reboiler heat duty as an initial estimate. The reboiler pressure is optional. If you do not enter a value, RadFrac uses the bottom stage pressure.

Heater and Cooler Specifications You can specify interstage heaters and coolers in one of two ways: • Specifying the duty directly on the HeatersCoolers SideDuties sheet • Requesting UA calculations on the HeatersCoolers UtilityExchangers sheet If you specify the duty directly on the HeatersCoolers SideDuties sheet, enter a positive duty for heating and a negative duty for cooling. If you request UA calculations on the HeatersCoolers UtilityExchangers sheet, RadFrac calculates the duty and outlet temperature of the heating/cooling fluid simultaneously with the column. The UA calculations: • Assume the stage temperature is constant • Use an arithmetic average temperature difference • Assume the heating or cooling fluid does not experience any phase change To request UA calculations, specify the: • • •

Unit Operation Models Version 10

UA Heating or cooling fluid component Flow and inlet temperature of the fluid

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Columns

You can specify the heat capacity of the fluid directly on the HeatersCoolers UtilityExchangers sheet or RadFrac can compute it from a property method. If RadFrac computes the heat capacity, you must also enter the pressure and phase of the heating or cooling fluid. By default, RadFrac calculates the heat capacity using the block property method. But you can also use a different property method. You can also specify the heat loss for sections of the column on the HeatersCoolers HeatLoss sheet.

Decanters For three-phase calculations (Valid Phases=Vapor-Liquid-Liquid on the Setup Configuration sheet), you can define any number of decanters. Enter decanter specifications on the Decanters form. For the decanter on the top stage, you must enter the return fraction of at least one of the two liquid phases (Fraction of 1st Liquid Returned, Fraction of 2nd Liquid Returned on the Decanters Specifications sheet). For decanters on other stages, you must always specify both Fraction of 1st Liquid Returned and Fraction of 2nd Liquid Returned. You can enter Temperature and Degrees Subcooling on the Decanters Options sheet to model subcooled decanters. If you do not specify Temperature and Degrees Subcooling, the decanter is operated at the temperature of the stage to which the decanter is attached. If side product streams are decanter products, you cannot specify their flow rates. RadFrac calculates their flow rates from the Fraction of 1st Liquid Returned and Fraction of 2nd Liquid Returned. By default RadFrac returns decanter streams to the stage immediately below. You can return the decanter streams to any other stage by entering a different Return Stage number on the Decanters Specifications sheet. You can split a return stream into any number of streams by giving a split fraction (Split Fraction of Total Return for the 1st Liquid and 2nd Liquid). Each resulting stream may go to a different return stage. When return streams do not go to the next stage, a feed or pumparound must go to the next stage. This prevents dry stages.

Pumparounds RadFrac can handle pumparounds from any stage to the same or any other stage. Use the Pumparounds form to enter all pumparound specifications. You must enter the source and destination stage locations for pumparounds. A pumparound can be either a partial or total drawoff of the: • Stage liquid • First liquid phase

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Chapter 4

• •

Second liquid phase Vapor phase

You can associate a heater or cooler with a pumparound. If the pumparound is a partial drawoff of the stage flow, you must enter two of the following specifications: • Flow rate • Temperature • Temperature change • Vapor fraction • Heat Duty If the pumparound is a total drawoff, you must enter one of the following specifications: • Temperature • Temperature change • Vapor fraction • Heat Duty Vapor fraction is allowed only when Valid Phases=Vapor-Liquid or Vapor-Liquid-Liquid. Use the Pumparounds Specifications sheet to enter these operating specifications. Pressure specification is optional. The default pumparound pressure is the same as the source stage pressure. RadFrac assumes that the pumparound at the heater/cooler outlet has the same phase condition as the pumparound at the inlet. You can override the phase condition using the Valid phases field on Pumparound Specifications sheet. RadFrac can return the pumparound to a stage using either the: • On-stage option • Above-stage option (returns the pumparound to the column between two stages) In three-phase columns, RadFrac can also return the pumparound to a decanter associated with a stage. You can select above-stage using the Return option field. RadFrac assumes the pumparound at the heater/cooler outlet has the same phase condition as the inlet. You can use Return-Phase on the Pumparounds Specifications sheet to assign a different phase at the heater/cooler outlet. Or you can specify Valid Phases=VaporLiquid or Vapor-Liquid-Liquid and let RadFrac determine the return phase condition from the heater/cooler specifications.

Unit Operation Models Version 10

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Columns

Free-Water and Rigorous Three-Phase Calculations RadFrac can perform both free-water and rigorous three-phase calculations. (See ASPEN PLUS Physical Property Methods and Models, Chapter 6.) These calculations are controlled by options you specify on the Setup Configuration sheet. You can select from three types of calculations: • • •

Free water in the condenser only Free water on any or all stages Rigorous three-phase calculations

When you choose free-water calculations in the condenser, only free water can be decanted from the condenser. You cannot use nonideal for the Overall Loop convergence method. Specify one of the following on the Setup Configuration sheet: Valid Phases=

On Sheet

For

Vapor-Liquid-FreeWaterCondenser

Setup Configuration

Free water in the condenser only

Vapor-Liquid-FreeWaterAnyStage

Setup Configuration

Free water on all stages

Vapor-Liquid-Liquid

Setup Configuration

Rigorous three-phase calculations

For RadFrac calculations, you must also specify which stages to test for two liquid phases on the Setup 3-Phase sheet. When you choose completely rigorous three-phase calculations on all stages selected, RadFrac makes no assumptions about the nature of the two liquid phases. You can associate a decanter with any stage. You cannot use Sum-Rates for the Overall Loop convergence method.

Efficiencies You can specify one of two types of efficiencies: • Vaporization • Murphree Vaporization efficiency is defined as: Effi v =

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yi , j K i, j x i , j

Unit Operation Models Version 10

Chapter 4

Murphree efficiency is defined as:

Eff i ,Mj =

y i , j − yi , j + 1 K k , j x i , j − y i , j +1

Where: K

=

Equilibrium K value

x

=

Liquid mole fraction

y

=

Vapor mole fraction

Eff

v

=

Vaporization efficiency

Eff

M

=

Murphree efficiency

i

=

Component index

j

=

Stage index

To specify vaporization or Murphree efficiencies, enter the number of actual stages on the Setup Configuration sheet. Then use the Efficiencies form to enter the efficiencies. For three-phase calculations, the vaporization and Murphree efficiencies you enter apply equally to the following equilibrium by default: • Vapor-liquid1 (VL1E) • Vapor-liquid2 (VL2E) You can use the Efficiencies form to enter separate efficiencies for VL1E and VL2E. You cannot enter separate efficiencies for VL1E and VL2E when you specify equilibrium reactions or when using Murphree efficiencies. You can use any of these efficiencies to account for departure from equilibrium. But you cannot convert from one efficiency to the other. Magnitudes of the efficiencies can be quite different. You should manipulate the Murphree efficiency to match the operating data when: • Efficiency is unknown • Actual column operating data are available When manipulating the Murphree efficiency, use design specifications on the DesignSpecs and Vary forms. Details on using and estimating efficiencies are described by Holland, Fundamentals of Multi-Component Distillation, McGrawHill Book Company, 1981.

Unit Operation Models Version 10

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Columns

Algorithms You can select an algorithm and/or initialization option for column simulation on the Convergence Basic sheet. The default standard algorithm and standard initialization option are appropriate for most applications. You can improve convergence behavior for the following applications using the guidelines described in this section: • Petroleum and Petrochemical Applications • Highly Nonideal Systems • Azeotropic Distillation • Absorbers and Strippers • Cryogenic Applications To change the algorithm and initialization option on the Convergence Basic sheet, you must first choose Custom as the option in the Convergence field on the Setup Configuration sheet.

Petroleum and Petrochemical Applications In petroleum and petrochemical applications involving extremely wide-boiling mixtures and/or many components and design specifications, you can improve the convergence efficiency and reliability by choosing Sum-Rates in the Algorithm field on the Convergence Basic sheet.

Highly Nonideal Systems When liquid phase nonidealities are exceptionally strong, choose Nonideal in the Algorithm field on the Convergence Basic sheet to improve the convergence behavior. Use this algorithm only when the number of outside loop iterations (using the standard algorithm) exceeds 25. You can also use the Newton algorithm for highly nonideal systems. Newton is better for columns with highly sensitive specifications. But it is usually slower, especially for columns with many stages and components.

Azeotropic Distillation For azeotropic distillation applications where an entraining agent separates an azeotropic mixture, specify the following on the Convergence Basic sheet: • •

Algorithm, Newton Initialization method, Azeotropic

A classic example of azeotropic distillation is ethanol dehydration using benzene.

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Chapter 4

Absorbers and Strippers To model absorbers and strippers, specify Condenser=None and Reboiler=None on the Setup Configuration sheet. The heat duty is zero for adiabatic operation. For extremely wide-boiling mixtures, specify one of the following: • Algorithm=Sum-Rates on the Convergence Basic sheet • Convergence=Standard on the Setup Configuration sheet and choose Absorber=Yes on the Convergence Basic sheet

Cryogenic Applications For cryogenic applications such as air separation, the standard algorithm is recommended. To invoke a special initialization procedure designed for cryogenic systems, specify Cryogenic for Initialization on the Convergence Basic sheet.

Rating Mode RadFrac allows the column to be operated in a rating mode or a design mode. Rating mode requires different column specifications for two- and three-phase calculations. For two-phase calculations, you must enter the following on the Setup Form: • Valid Phases=Vapor-Liquid or Vapor-Liquid-FreeWaterCondenser for handling free water in condenser • A Total, Subcooled, or Partial-Vapor condenser • Two additional column operating variables If the condenser or reflux is subcooled, you can also specify the degrees subcooling or the subcooled temperature. For three-phase calculations, you must specify Valid Phases= Vapor-LiquidLiquid or Vapor-Liquid-FreeWaterAnyStage (for free water calculations) on the Setup Configuration sheet. The required specifications depend on what you specify for the return fractions of the two liquid phases (Fraction of 1st Liquid Returned and Fraction of 2nd Liquid Returned) in the top stage decanter. The following table lists the three specification options:

Unit Operation Models Version 10

If you specified this on Decanters Specifications

Enter on Setup Configuration

Fraction of 1st Liquid Returned or Fraction of 2nd Liquid Returned, or no top decanter

A Total, Subcooled, or Partial-Vapor condenser and two operating specifications

Fraction of 1st Liquid Returned and Fraction of 2nd Liquid Returned

A Total, Subcooled, or Partial-Vapor condenser and one operating specification

Fraction of 1st Liquid Returned and Fraction of 2nd Liquid Returned

Two operating specifications, and an estimate for the amount of vapor in the distillate on the Estimates Vapor Composition sheet. RadFrac assumes a partial condenser with both vapor and liquid distillates.

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Columns

Design Mode RadFrac allows the column to be operated in rating mode or design mode. In design mode, use the DesignSpecs form to specify column performance parameters (such as purity or recovery). You must indicate which variables to manipulate to achieve these specifications. You can manipulate any variables that are allowed in rating mode, except: • Number of stages • Pressure profile • Vaporization efficiency • Subcooled reflux temperature • Degrees of subcooling • Decanter temperature and pressure • Locations of feeds, products, heaters, pumparounds, and decanters • Pressures of thermosyphon reboiler and pumparounds • UA specifications for heaters The flow rates of inlet material streams and the duties of inlet heat streams can also be manipulated variables. These are the design specifications: You can specify

For any

Purity

Stream including internal streams

Recovery of any components groups

Set of product streams, including sidestreams

Flow rate of any components groups

Internal stream or set of product streams

Temperature

Stage

Value of any Prop-Set property

Internal or product stream

Ratio or difference of any pair of Prop-Set properties

Single or paired internal or product streams

Flow ratio of any components groups to any other component groups

Internal streams to any other internal streams, or to any set of feed or product streams

†

†† †††

4-24

† ††

†††

Express the purity as the sum of mole, mass, or standard liquid volume fractions of any group of components relative to any other group of components . Express recovery as a fraction of the same components in any set of feed streams. See ASPEN PLUS User Guide.

Unit Operation Models Version 10

Chapter 4

Reactive Distillation RadFrac can handle chemical reactions. These reactions can occur in the liquid and/or vapor phase. The details about the reactions are entered on a generic Reactions form outside RadFrac. RadFrac allows two different reaction model types: REAC-DIST or USER. RadFrac can model the following types of reactions: • Equilibrium-controlled • Rate-controlled • Conversion • Electrolytic RadFrac can also model salt precipitation, especially in the case of electrolytic systems. You can request reaction calculations for the entire column, or you can restrict reactions to a certain column segment (for example, to model the presence of catalyst). For three-phase calculations, you can restrict reactions to one of the two liquid phases, or use separate reaction kinetics for the two liquid phases. To include reactions in RadFrac you must enter the following information on the Reactions Specifications sheet: • Reaction type and Reaction/Chemistry ID • Column section in which the reactions occur Depending on the reaction type, you must enter equilibrium constant, kinetic, or conversion parameters on the generic Reactions form outside RadFrac. For electrolytic reactions, you can also enter the reaction data on the Reactions Chemistry form outside RadFrac. To consider salt precipitation, enter the salt precipitation parameters on the Reactions Salt sheet or the Reactions Chemistry form outside RadFrac. To associate reactions and salt precipitation with a column segment, enter the corresponding Reactions ID (or Chemistry ID) on the Reactions Specifications sheet. For rate-controlled reactions, you must enter holdup or residence time data in the phase where the reactions occur. Use the Reactions Holdups or Residence Times sheets. For conversion reactions, use the Reactions Conversion sheet to override the conversion parameters specified on the Reactions Conversion form. RadFrac also supports User Reaction Subroutine. The name and other details of the reaction subroutine are entered on the UserSubroutines form.

Solution Strategies RadFrac uses two general approaches for column convergence: • •

Unit Operation Models Version 10

Inside-out Napthali-Sandholm

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Columns

The standard, sum-rates, and nonideal algorithms are variants of the inside-out approach. The MultiFrac, PetroFrac, and Extract models also use this approach. The Newton algorithm uses the classical Napthali-Sandholm approach. Use the Convergence form to select the algorithm and specify the associated parameters.

Inside-Out Algorithms The inside-out algorithms consist of two nested iteration loops. The K-value and enthalpy models you specify are evaluated only in the outside loop to determine parameters of simplified local models. When using nonideal, algorithm RadFrac introduces a composition dependence into the local models. The local model parameters are the outside loop iteration variables. The outside loop is converged when the changes of the outside loop iteration variables are sufficiently small from one iteration to the next. Convergence uses a combination of the bounded Wegstein method and the Broyden quasi-Newton method for selected variables. In the inside loop, the basic describing equations (component mass balances, total mass balance, enthalpy balance, and phase equilibrium) are expressed in terms of the local physical property models. RadFrac solves these equations to obtain updated temperature and composition profiles. Convergence uses one of the following methods: • Bounded Wegstein • Broyden quasi-Newton • Schubert quasi-Newton • Newton RadFrac adjusts the inside loop convergence tolerance with each outside loop iteration. The tolerance becomes tighter as the outside loop converges.

Newton Algorithm The Newton algorithm solves column-describing equations simultaneously, using Newton’s method. The convergence is stabilized using the dogleg strategy of Powell. Design specifications may be solved either simultaneously with the columndescribing equations or in an outer loop.

Design Mode Convergence RadFrac provides two methods for handling design specification convergence: • Nested convergence • Simultaneous convergence

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Chapter 4

Nested Design Spec Convergence (for all algorithms except SUM-RATES) The Nested Middle Loop convergence method attempts to satisfy the design specifications by determining the values of the manipulated variables (within their bounds) that minimize the weighted sum of squares function:

Φ=

∑ m

 ∧  G m − GM  Wm  Gm*    

2

Where: m

=

Design specification number

G

=

Calculated value

G

=

Desired value

G*

=

Scaling factor

w

=

Weighting factor

∧

The algorithm that manipulates the variables to minimize Φ does not depend on matching particular variables with corresponding design specifications. You should carefully select the manipulated variables and design specifications. Make sure that each manipulated variable has a significant effect on at least one design specification. The number of design specifications must be equal to or greater than the number of manipulated variables. If there are more design specifications than manipulated variables, assign weighting factors to reflect the relative importance of the specifications. The larger the weighting factor, the more nearly a specification will be satisfied. Scale factors normalize the errors, so that different specification types are compared on a consistent basis. When a value of a manipulated variable reaches a bound, that bound is active. If a problem has no active bounds and the same number of manipulated variables as design specifications, then Φ will approach zero (within some tolerance) when all specifications are satisfied. If there are active bounds or more design specifications than manipulated variables, RadFrac minimizes Φ . The weighting factors determine the relative degree to which the design specifications are satisfied.

Unit Operation Models Version 10

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Columns

Simultaneous Design Spec Convergence (for Algorithm=SUM-RATES, NEWTON) The Simultaneous Middle Loop convergence method algorithm solves the design specification functions simultaneously with the column-describing equations: ∧   G m − GM  Fm =   =0 Gm*    

Because the Simultaneous Middle Loop convergence method uses an equationsolving approach, there must be an equal number of design specifications and manipulated variables. In the nested method, no coupling is assumed between design specifications and manipulated variables. However, each design specification must be significantly affected by at least one manipulated variable. Bounds and weighting factors are not used. In general, the Simultaneous method gives better performance if all the specifications are feasible.

Physical Properties To override the global physical property method, use the Properties PropertySections sheet. You can specify different physical properties for different parts of the column. For three-phase calculations, you can specify separate calculation methods for Vapor-Liquid1 Equilibrium (VL1E) and Liquid1-Liquid2 Equilibrium (LLE). Use one of the following methods: • Associate separate property methods with VL1E and LLE using the Phase Equilibrium list box • Calculate VL1E using a property method. Specify LLE using liquid-liquid distribution (KLL) coefficients You can use the Properties KLLSections sheet to enter the KLL coefficients using a built-in temperature polynomial, and associate the coefficients with one or more column segments. Or you can use the Properties KLLCorrelations sheet to associate a user-KLL subroutine with one or more column segments.

Solids Handling RadFrac has two methods for handling inert solids: • Overall-balance • Stage-by-stage

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Chapter 4

Use the Solids handling option on the Convergence Basic sheet to select either an overall balance or stage-by-stage. The two methods differ in how they treat solids in the mass and energy balances. Neither method considers inert solids in the phase equilibrium calculations. However, salts formed by salt precipitation reactions (see Reactive Distillation) are considered in phase equilibrium calculations. The overall-balance method: • Temporarily removes all solids from inlet streams • Performs column calculations without solids • Adiabatically mixes solids removed from inlet streams with liquid product from the bottom stage The overall-balance method maintains an overall mass and energy balance around the column. But it does not satisfy individual stage balances. This is the default method. The stage-by-stage method treats solids rigorously in all stage mass and energy balances. The ratio of liquids to solids on a stage is maintained in the product streams withdrawn from that stage. The specified product flow is the total flow rate of the stream, including the solids. If a nonconventional (NC) solids substream is present in the column feeds, you must give all column flow and flow ratio specifications on a mass basis. When you specify a decanter, RadFrac can decant the solids partially or totally. By default, RadFrac decants the solids partially along with the second liquid phase. RadFrac uses the return fraction you specify for the second liquid phase (Fraction of 2nd Liquid Returned on the Decanters Specifications sheet) to decant the solids. If there is no second liquid phase in the decanter, RadFrac decants the solids partially along with the first liquid phase. RadFrac uses the return fraction you specify for the first liquid phase (Fraction of 2nd Liquid Returned on the Decanters Specifications sheet) in this case. You can request complete decanting of the solids by selecting Decant Solids Totally on the Decanters Options sheet.

Unit Operation Models Version 10

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Columns

MultiFrac Rigorous Fractionation MultiFrac is a rigorous model for simulating general systems of interlinked multistage fractionation units. MultiFrac models can handle a complex configuration consisting of: • Any number of columns, each with any number of stages • Any number of connections between columns or within each column • Arbitrary flow splitting and mixing of connecting streams MultiFrac can handle operations with: • Side strippers • Pumparounds • External heat exchangers • Single-stage flashes • Feed furnace Typical MultiFrac applications include: • Heat-interstaged columns, such as Petlyuk towers • Air separation column systems • Absorber/stripper combinations • Ethylene plant primary fractionator/quench tower combinations You can also use MultiFrac for petroleum refining fractionation units such as atmospheric crude units and vacuum units. But for these applications, PetroFrac is more convenient to use. Use MultiFrac only when the configuration is beyond the capabilities of PetroFrac. MultiFrac can detect a free-water phase in the condenser or anywhere in the column. It can decant the free-water phase on any stage. Although MultiFrac assumes equilibrium stage calculations, you can specify either Murphree or vaporization efficiencies. You can use MultiFrac for both sizing and rating trays and packings. MultiFrac can model both random and structured packings.

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Chapter 4

Flowsheet Connectivity for MultiFrac Top Stage or Condenser Heat Duty (optional)

Vapor Distillate 1 Reflux

Heat Liquid Distillate (optional) Water Distillate (optional)

Feeds

Side Products (optional)

Heat

Interconnecting Streams (Heater Optional)

Pumparounds and Bypasses (Heater Optional)

Interconnecting Streams (Heater Optional) Heat (optional)

Bottom Stage or Reboiler Heat Duty (optional)

Top Stage or Condenser Heat Duty (optional)

Nstage Nstage

Heat (optional) Bottoms (or Interconnecting Stream)

Vapor Distilate 1

Heat Liquid Distillate (optional) Water Distillate (optional)

Feeds

Side Products (optional)

Heat

Interconnecting Streams (Heater Optional)

Pumparounds and Bypasses (Heater Optional)

Interconnecting Streams (Heater Optional) Heat (optional)

Bottom Stage or Reboiler Heat Duty (optional)

Unit Operation Models Version 10

Nstage Heat (optional) Bottoms (or Interconnecting Stream)

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Columns

Material Streams Inlet

At least one inlet material stream

Outlet Any number of optional pseudo-product streams

Up to three optional outlet material streams per stage (one vapor, one liquid, and one free water) You can connect any number of columns by any number of connecting streams. For each column, any number of connecting streams can represent pumparounds and bypasses. These streams can flow between any two stages, or to the same stage. Each connecting stream can have an associated heater. Each column must have one liquid product or connecting stream leaving the bottom stage. The top stage of the main column (column 1) must have a product stream, which cannot be a connecting stream. The top stage of the other columns (column 2, 3, ...) must have a vapor product or a vapor connecting stream. The pseudoproduct streams represent column internal flows and connecting stream flows.

Heat Streams Inlet

One inlet heat stream per stage (optional) One inlet heat stream per connecting stream (optional)

Outlet One outlet heat stream per connecting stream (optional)

MultiFrac uses an inlet heat stream as a duty specification for all stages except the condenser, reboiler, and connecting streams. If you do not provide two column operating specifications on the Columns Setup Configuration sheet, MultiFrac uses a heat stream as a specification for the condenser and reboiler. If you do not provide two specifications on the ConnectStreams form, MultiFrac uses a heat stream as a specification for connecting streams. If you provide two specifications on the Columns Setup Configuration sheet or ConnectStreams form, MultiFrac does not use the inlet heat stream as a specification. The inlet heat stream supplies the required heating or cooling. You can use optional outlet heat streams for the net heat duty of the condenser, reboiler, and connecting streams. The value of the outlet heat stream equals the value of the inlet heat stream (if any), minus the actual (calculated) heat duty.

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Chapter 4

Specifying MultiFrac Individual columns are identified by column numbers. The numbering order does not affect algorithm performance. Column 1 has different specifications from the other columns. Within each column, the stages are numbered from the top down, starting with the condenser. Use the following forms to enter specifications and view results for MultiFrac:

Unit Operation Models Version 10

Use this form

To do this

Columns Setup

Specify basic column configuration and operating conditions

Columns HeatersCoolers

Specify interstage heaters/coolers

Columns FlowSpecs

Specify liquid and vapor flow specifications

Columns Efficiencies

Specify stage or component efficiencies

Columns Properties

Specify physical property parameters for column sections

Columns Estimates

Specify initial estimates for stage temperatures, and vapor and liquid flows and compositions

Columns Results

View column summary

Columns Profiles

View column profiles

InletsOutlets

Specify inlet and outlet material and heat stream locations

ConnectStreams

Specify sources and destinations of connecting material and heat streams, view connecting stream results

FlowRatios

Specify stream flow ratios

DesignSpecs

Specify design specifications, and view convergence results

Vary

Specify manipulated variables to satisfy design specifications and view final values

CondenserHcurves

Specify condenser heating or cooling curve tables and view tabular results

ReboilerHCurves

Specify reboiler heating or cooling curve tables and view tabular results

ConnectStreamHCurves

Specify connecting stream heating or cooling curve tables and view tabular results

TraySizing

Specify sizing parameters for tray column sections, and view results

TrayRating

Specify rating parameters for tray column sections, and view results

PackSizing

Specify sizing parameters for packed column sections, and view results

PackRating

Specify rating parameters for packed column sections, and view results

Convergence

Specify convergence parameters for column calculations, and block-specific diagnostic message levels

Report

Specify block-specific report options and pseudostream information

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

UserSubroutines

Specify user subroutine parameters for tray sizing and rating, and packing sizing and rating

ResultsSummary

View results of balances and splits

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Columns

Stream Definitions MultiFrac uses four types of streams: • External streams • Connecting streams • Internal streams • Pseudostreams External streams are standard MultiFrac inlet and outlet streams. They are identified by stream IDs. Connecting streams are within MultiFrac but external to individual columns. They can connect two columns, or stages of the same column (bypasses and pumparounds). You can associate a heater with any connecting stream. Connecting stream heaters are identified by connecting stream numbers. Internal streams are liquid or vapor flows between adjacent stages of the same column. An internal stream is identified by a source stage number and a column number. Pseudostreams store the results of internal and connecting streams. They are a subset of external outlet streams. Unlike normal outlet streams, pseudostreams do not participate in block mass balance calculations.

Required Specifications Follow these guidelines when entering specifications for column 1: • The number of stages must be greater than 1 • Two additional operating specifications are required • The distillate flow may not be a connecting stream You must specify: • Bottoms rate or distillate rate. The distillate rate includes both the vapor and liquid distillate flows • Either condenser duty, reboiler duty, reflux ratio or reflux rate • Distillate vapor fraction or condenser temperature If you specify the condenser stage temperature: • Both liquid and vapor distillate products must be present (distillate vapor fraction is greater than 0 or less than 1) • You must also specify an estimate for the distillate vapor fraction

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Chapter 4

Follow these guidelines when entering specifications for other columns: • The number of stages can be 1 (for example, to model a single-stage flash or feed furnace) • The distillate can be a connecting stream • MultiFrac calculates the distillate vapor fraction • The distillate rate includes only the vapor distillate flow and must be greater than zero. If a liquid distillate is present, specify flow on the InletsOutlets form. For columns with more than one stage, you may specify condenser duty, reboiler duty, bottoms rate, distillate rate, and reflux rate. For columns with one stage, you must specify either: • Bottoms rate • Distillate rate (includes only the vapor distillate) • Condenser duty

Feed Stream Conventions MultiFrac provides two conventions for handling feed streams (see MultiFrac Feed Convention Above-Stage and MultiFrac Feed Convention On-Stage in the following figures): • Above-Stage • On-Stage When Feed-Convention is Above-Stage, MultiFrac introduces a material stream between adjacent stages. The liquid portion flows to the stage (n) you specify. The vapor portion flows to the stage above (n – 1). You can introduce a liquid feed to the top stage (or condenser) by specifying Stage=1. You can introduce a vapor feed to the bottom stage (or reboiler) by specifying Stage=Number of stages + 1.

n-1

Vapor Mixed feed to stage n Liquid

MultiFrac Feed Convention Above-Stage

Unit Operation Models Version 10

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Columns

n-1

Mixed feed to stage n

n

n+1

MultiFrac Feed Convention On-Stage When Feed-Convention is On-Stage, both the liquid and vapor portions of a feed flow to the stage (n) you specify.

Connecting Streams MultiFrac allows any number of connecting streams. Any number of these streams can have the same: • Source column, stage, and phase • Destination column and stage MultiFrac introduces connecting streams on the destination stage regardless of their phase (that is, Feed Convention=On-Stage). All connecting streams can have a heater with heat duty, temperature, or temperature change specified. Use the ConnectStreams form to enter all specifications for connecting streams. Each terminal stream can be the source of a product stream and any number of connecting streams. If there is no product stream, at least one connecting stream must have an unspecified flow. For a connecting stream, required specifications depend on whether the stream: • • •

4-36

Has a flow rate that is fixed indirectly on the FlowRatios or Columns FlowSpecs form Is a terminal stream Is a pumparound to the top stage of column 1

Unit Operation Models Version 10

Chapter 4

For this type of connecting stream

You must specify †

One that does not satisfy the above conditions

Two of the following: flow, temperature (or temperature change), and duty

One whose flow is fixed indirectly on the FlowRatios or Columns FlowSpecs form

Either temperature (or temperature change), or duty

A terminal stream (vapor distillate or liquid bottoms)

Either temperature (or temperature change) or duty

†

†

†

Duty can default to 0 if necessary.

You can enter a second specification. If this specification is missing, MultiFrac uses the net flow from the stage excluding any other connecting stream with flow specifications. For a connecting stream that is the liquid pumparound to the top stage of column 1, enter two of the following: • Flow • Temperature (or temperature change) • Duty (specify 0 if there is no associated heater or cooler) If you enter only one of flow, temperature, or temperature change, MultiFrac uses the top stage duty for the missing requirement. When a stage is the destination of a connecting stream, MultiFrac uses the heat duty associated with the stage to determine the temperature of the connecting stream. When you enter the duty, temperature, or temperature change of the connecting stream, the stage duty does not affect the connecting stream temperature. Stage duty is properly accounted for in the stage enthalpy calculations. When a pumparound, bypass, or other connecting stream has a specified temperature change or outlet temperature, MultiFrac assumes that the specific value does not result in a phase change of any fraction of the stream. When you specify heat duty, a phase change may occur. Connecting streams can be either a total or partial drawoff of the stage flow. MultiFrac determines the drawoff type based on the number of specifications you give. If the drawoff type is

You enter

Partial

Two of the following: flow, temperature, temperature change, and heat duty

Total

One of the following: temperature, temperature change, and heat duty

† ††

Unit Operation Models Version 10

†

††

Enter zero for heat duty if heater is absent. Flow rate is taken as the net flow of the stage, excluding any product flow and any other connecting stream flow.

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Columns

MultiFrac allows total drawoff only for the top vapor stream and bottom liquid stream. For partial drawoffs you can specify the flow rate. Or MultiFrac can determine the flow rate based on one of the following: • Another flow specification (Columns FlowSpecs form) • A flow ratio specification (FlowRatios form) If you enter only one specification for pumparounds to the top stage of the main column, MultiFrac uses the top stage heat duty as the second specification. When a connecting stream has a specified temperature or temperature change, MultiFrac assumes the specified value does not result in a phase change of any fraction of the stream. When you specify the heat duty, a phase change can occur.

Heaters Use the Columns HeatersCoolers form to enter heater stage locations and duties. You can specify heaters indirectly by choosing a heater duty as the adjusted variable in one of the following forms: Form

Used to specify

Columns FlowSpecs

Stage liquid or vapor flow rate

FlowRatios

Vapor-to-liquid flow ratio

Flow Rate Specifications You can use the Columns FlowSpecs form to specify any stage liquid and vapor flow rates. The value you specify refers to the net flow of the stage liquid or vapor flow. This value excludes any portions withdrawn by side products and other connecting streams with flow specifications. This feature is typically used for specifying: • Internal reflux rate or total internal drawoff • Overflash in refining applications • Boilup rate For a terminal stream, flow specifications refer to the net flow of the stream excluding any portion withdrawn by connecting streams with flow specifications. Flow specifications include: • • •

Specifications provided on the ConnectStreams form Specifications fixed by the associated heater specifications Another FlowSpecs or FlowRatios specification

For an internal stream, flow specifications refer to the net flow of the stream excluding any portions withdrawn as products or connecting streams.

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Unit Operation Models Version 10

Chapter 4

When you enter a flow specification, MultiFrac adjusts the flow rate of a connecting stream or the duty of a heater. If the adjusted variable is

You enter the

A connecting stream flow

Connecting stream number in the IC-Stream field

A heater duty

Heater column and stage numbers

You can place the calculated heat duty in an outlet heat stream using the InletsOutlets form. Initial estimates for adjusted variables are not required. If a product or connecting stream of the same phase is leaving the stage, a specified value may be zero to model a total drawoff . MultiFrac will vary the heat duty associated with the heater of the same stage or another stage or the flow rate of an associated connecting stream to satisfy enthalpy and mass balances. If this will be varied

You must specify

Heat duty

Q-Column and Stage

Flow rate of a connecting stream

Stream number (IC-Stream)

Be cautious when selecting the: • Associated stage with varied heat duty • Connecting stream with varied flow rate An initial guess for the associated heat duty is not required.

Unit Operation Models Version 10

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Columns

Flow Ratio Specifications Use the FlowRatios form to specify the ratio of two flow rates. The flows can be of different phases, and come from any stage of any column. This feature is typically used for specifying the: • Internal reflux ratio • Overflash in refining applications • Boilup ratio For a terminal stream, the flows refer to the net flow of a stream, excluding any portion withdrawn by connecting streams with flow specifications. Flow specifications include those: • Specified on the ConnectStreams form • Fixed by either the associated heater specification, another Columns FlowSpecs sheet, or a FlowRatios Specifications sheet) For an internal stream, the flows refer to the net flow of the stream, excluding any portion withdrawn as products or connecting streams. When you specify a flow ratio, these will be varied to satisfy enthalpy and mass balances: • Heat duty of the same stage or another stage • Flow rate of an associated connecting stream When you enter a flow ratio specification, MultiFrac adjusts the flow rate of a connecting stream or the duty of a heater. If the adjusted variable is

You enter the

A connecting stream flow

Connecting stream number in the IC-Stream field

A heater duty

Heater column and stage numbers

You can place the calculated heat duty in an outlet heat stream using the InletsOutlets form. Initial estimates for these adjusted variables are not required. Be cautious when selecting the: • •

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Associated stage with varied heat duty Connecting stream with varied flow rate

Unit Operation Models Version 10

Chapter 4

Efficiencies You can specify one of two types of efficiencies: • Vaporization • Murphree Vaporization efficiency is defined as: Effi v =

yi , j Ki, j xi, j

Murphree efficiency is defined as: Effi ,Mj =

yi, j − yi , j +1 K i, j x i , j − yi , j +1

Where: K

=

Equilibrium K value

x

=

Liquid mole fraction

y

=

Vapor mole fraction

Eff v

=

Vaporization efficiency

Eff M

=

Murphree efficiency

i

=

Component index

j

=

Stage index

To specify vaporization or Murphree efficiencies, enter the number of actual stages on the Columns Setup Configuration sheet. Then use the Columns Efficiencies form to enter the efficiencies. You can use any of these efficiencies to account for departure from equilibrium. But you cannot convert from one efficiency to the other. Magnitudes of the efficiencies can be quite different. Details on using and estimating these efficiencies are described by Holland, Fundamentals of Multi-Component Distillation, McGraw-Hill Book Company, 1981.

Unit Operation Models Version 10

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Columns

Algorithms MultiFrac has three convergence algorithms. Use the Overall field on the Convergence Methods sheet to select the algorithm. The default standard algorithm is appropriate for most applications. Your choice of algorithm depends on the types of systems you are modeling: Application

Algorithm

Air separation

Standard

Close-boiling, e.g., C3 splitter

Standard

Wide-boiling, e.g., absorbers

Sum-Rates

Petroleum refining, e.g., crude unit

Sum-Rates

Ethylene plant primary fractionator

Sum-Rates

Highly-nonideal, e.g., azeotropic

Newton

Highly-coupled design specifications

Sum-rates or Newton

Rating Mode In rating mode, MultiFrac calculates column profiles and product compositions based on specified values of column parameters. Examples of column parameters are reflux ratio, reboiler duties, and feed flow rates.

Design Mode In design mode, use the DesignSpecs form to specify column performance parameters (such as purity or recovery). You must indicate which variables to manipulate to achieve these specifications using the Vary form. You can specify any variables that are allowed in rating mode, except: • Number of stages • Pressure profile • Efficiencies • Subcooled reflux temperature • Degrees of subcooling • Locations of feeds, products, heaters, and connecting streams

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Unit Operation Models Version 10

Chapter 4

The flow rates of inlet material streams and the duties of inlet heat streams can also be manipulated variables. You can specify

For any

Purity

Stream, including an internal stream

Recovery of any component groups

Set of product streams

Flow rate of any component groups

Internal stream or set of product streams

Temperature

Stage

Heat duty

Stage or connecting stream

Heat duty ratio

Stage or connecting stream to any other stage or connecting stream

Value of any Prop-Set property

Internal or product stream

Ratio or difference of any pair of properties in a Prop-Set

Single or paired internal or product stream

Flow ratio of any component groups to any other component groups

First group can be in any internal streams

†

†† ††† ††††

†

††

†††

††††

Express the purity as the sum of mole, mass, or standard liquid volume fractions of any group of components, relative to any other group of components. You can express recovery as a fraction of the same components in a subset of the feed stream. See ASPEN PLUS User Guide. The second group can be in any other internal streams, or set of feed or product streams.

Column Convergence MultiFrac uses the inside-out approach for column convergence. You can choose from two algorithm variants of this approach: • Standard • Sum-rates To select an algorithm, use the Overall field on the Convergence Methods sheet. The standard algorithm uses the standard inside-out formulation for the inside loop. It uses either the nested or simultaneous approach (specified as the Middle loop method on the Convergence Methods sheet) to converge the design specifications. This algorithm is appropriate for most systems. The sum-rates algorithm uses: • A sum-rates variant formulation for the inside loop • The simultaneous approach to converge the design specifications

Unit Operation Models Version 10

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Columns

Sum-rates is well suited for: • Wide-boiling systems • Columns with steep flow gradients MultiFrac also has the Newton algorithm, which uses a Napthali-Sandholm formulation. It solves the column-describing equations and design specifications simultaneously, using Newton’s method. This method can enhance convergence for highly-nonideal systems, such as azeotropic distillation. The Newton algorithm is generally slower than the other algorithms.

Design Specification Convergence MultiFrac provides two methods for handling design specification convergence: • Nested middle loop • Simult middle loop When you use the nested middle loop method, the algorithm attempts to satisfy the design specifications by determining the values of the manipulated variables (within their bounds) that minimize the weighted sum of squares function:

 G^ − G   Φ = ∑wm    G ** m  

2

Where:

m

=

Design specification number

G$

=

Calculated value

G

=

Desired value

G **

=

Scaling factor

w

=

Weighting factor

For purity and recovery, G$ and G are transformed by taking the logarithm, and G ** is taken as unity. When you use the simult middle loop method, the following algorithm solves the design specification functions simultaneously with the column describing equations:

(

)

Fm = G$ m − Gm / Gm** = 0

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Unit Operation Models Version 10

Chapter 4

The weighting factor is not available for this method. You can handle design specification convergence by using either scaling factors or weighting factors. The following algorithm attempts to satisfy design specifications by determining the values of the manipulated variables (within their bounds) that minimize the weighted sum of squares function:

 G$ − G  Φ = ∑wm  **   G  m

2

Where:

m

=

Design specification number

G$

=

Calculated value

G

=

Desired value

G **

=

Scaling factor

w

=

Weighting factor

Initialization Use Initialization Method on the Convergence Methods sheet to choose the initialization method. MultiFrac has two initialization procedures: • Standard • Crude Standard is appropriate for most systems. You must enter at least the top and bottom temperature estimates for each column. Crude invokes a special initialization procedure designed for petroleum refining and ethylene plant primary fractionator/quench tower applications. This procedure is designed for systems consisting of a main column connected to any number of sidestrippers. If you specify the following information on the Columns Setup and/or Columns FlowSpecs forms, you do not need to provide estimates: • •

All stripper bottoms flow rates Either the distillate or bottoms flow rate of the main column

Otherwise, you must enter at least the top and bottom temperature estimates for each column. You may enter profile estimates on the Columns Estimates form to enhance convergence. Temperature estimates are usually adequate. Highly nonideal systems may require composition estimates.

Unit Operation Models Version 10

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Columns

Physical Properties Use the BlockOptions form to override the global physical property method. You can specify a single property method on the BlockOptions form. MultiFrac uses this property method for all stages in all columns. Use the Columns Properties form to specify physical property methods when you use a separate property method for an individual column. You can also split a column into any number of segments, each using a different property methods.

Free Water Handling MultiFrac can perform free-water calculations. By default, MultiFrac performs free-water calculations for the main column condenser. The free-water phase, if present, is decanted. Use the Columns Properties form to request free-water calculations for additional stages in any column. You can define additional water decant product streams on the InletsOutlets form. You can use this capability to simulate the primary fractionator/quench tower combination of an ethylene plant.

Solids Handling MultiFrac handles solids by: • Temporarily removing all solids from inlet streams • Performing calculations without solids • Adiabatically mixing solids removed from inlet streams with main column liquid bottoms This calculation approach maintains an overall mass and energy balance around the MultiFrac block. But the bottom stage liquid product will not be in exact thermal or phase equilibrium with other bottom stage flows (for example, the bottom stage vapor flow).

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Unit Operation Models Version 10

Chapter 4

Sizing and Rating of Trays and Packings MultiFrac has extensive capability to size, rate, and perform pressure drop calculations for trayed and packed columns. Use the following forms to enter specifications: • TraySizing • TrayRating • PackSizing • PackRating See Appendix A for details on tray and packing types and correlations.

Unit Operation Models Version 10

4-47

Columns

PetroFrac Rigorous Fractionation PetroFrac is a rigorous model designed for simulating all types of complex vaporliquid fractionation operations in the petroleum refining industry. Typical operations include: • Preflash tower • Atmospheric crude unit • Vacuum unit • Catalytic cracker main fractionator • Delayed coker main fractionator • Vacuum lube fractionator You also can use PetroFrac to model the primary fractionator/quench tower combination in the quench section of an ethylene plant. PetroFrac can detect a free-water phase in the condenser or anywhere in the column. It can decant the free-water phase on any stage. Although PetroFrac assumes equilibrium stage calculations, you can specify either Murphree or vaporization efficiencies. You can use PetroFrac to size and rate columns consisting of trays and/or packings. PetroFrac can model both random and structured packings.

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Unit Operation Models Version 10

Chapter 4

Flowsheet Connectivity for PetroFrac

PetroFrac models column configurations consisting of a main column with any number of pumparounds and side strippers. You can specify a feed furnace. For single columns without pumparounds and side strippers, use RadFrac. For other multicolumn systems such as air separation systems, Petlyuk towers, and complex primary fractionators, use MultiFrac.

Material Streams Inlet

At least one inlet material stream One steam feed per stripper (optional)

Outlet One vapor or liquid distillate, or both

One free water distillate stream (optional) One bottoms product from the main column Any number of side products from main column (optional) Any number of water decant products from main column (optional) One bottoms product per side stripper Any number of pseudoproduct streams (optional)

Unit Operation Models Version 10

4-49

Columns

You can use any number of pseudoproduct streams to represent: • Column internal streams • Pumparound streams • Column connecting streams A pseudoproduct stream does not affect column results.

Heat Streams Inlet

One heat stream per stage for the main column (optional) One heat stream per pumparound heater/cooler (optional) One heat stream per stripper reboiler (optional) One heat stream per stripper bottom liquid return (optional)

Outlet One heat stream per stage for the main column (optional)

One heat stream per pumparound heaters/cooler (optional) One heat stream per stripper reboiler (optional) One heat stream per stripper bottom liquid return (optional) PetroFrac uses an inlet heat stream as a duty specification for all stages except the condenser, reboiler, pumparounds, and stripper bottom liquid return. If you do not give sufficient operating column specifications on the Setup Configuration sheet, PetroFrac uses a heat stream as a specification for the condenser and reboiler. If you do not give two specifications on the Pumparounds Specifications sheet, PetroFrac uses a heat stream as a specification for pumparounds. If you do not give two specifications for the bottom liquid return on the Strippers Setup LiquidReturn sheet, PetroFrac uses a heat stream as a specification. If you give two specifications on the Setup Configuration sheet or Pumparounds Specifications sheet, PetroFrac does not use the inlet heat stream as a specification. The heat stream supplies the required heating or cooling. Use optional outlet streams for the net heat duty of the condenser, reboiler, and pumparounds. The value of the outlet heat stream equals the value of the inlet heat stream (if any) minus the actual (calculated) heat duty.

Main Column The main column can have any number of inlet streams. It can also have up to three product streams per stage (one vapor, one hydrocarbon liquid, and one free water).

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Unit Operation Models Version 10

Chapter 4

Side Strippers The side strippers can have a steam feed. They must have a liquid bottoms product. You can use a heat stream as the heat source for the reboiler. If you do not specify the reboiler duty, bottoms flow rate, and steam feed, PetroFrac uses the heat stream as a duty specification. Optionally, the stripper liquid bottoms may be partially returned to the main column. To specify a bottom liquid return, you must enter two specifications on the Strippers Setup LiquidReturn sheet.

Feed Furnace You can specify a feed furnace. A feed furnace can have any number of feeds. The vapor and liquid streams from the furnace are fed to the stage where the furnace is attached.

Specifying PetroFrac Within each column or stripper, stages are numbered from the top down. If present, the main column condenser is stage 1. Use the following forms to enter specifications and view results of PetroFrac: Use this form

To do this

Setup

Specify basic column configuration and operating conditions

Pumparounds

Specify pumparound specifications and view results

Pumparounds Hcurves

Specify pumparound heating or cooling curve tables and view tabular results

Strippers Setup

Specify stripper operating specifications

Strippers Efficiencies

Specify stripper column or stage efficiencies

Strippers ReboilerHcurves

Specify stripper reboiler heating or cooling curve tables and view tabular results

Strippers TraySizing

Specify sizing calculation parameters for tray stripper sections, and view results

Strippers TrayRating

Specify rating calculation parameters for tray stripper sections, and view results

Strippers PackSizing

Specify sizing calculation parameters for packed stripper sections, and view results

Strippers PackRating

Specify rating calculation parameters for packed stripper sections, and view results

Strippers Properties

Specify physical property parameters for stripper sections continued

Unit Operation Models Version 10

4-51

Columns

4-52

Use this form

To do this

Strippers Estimates

Specify estimates for stripper temperatures and flows

Strippers Results

View stripper product stream and connecting stream results

Strippers Profiles

View stripper profiles

HeatersCoolers

Specify stage heating or cooling specifications

RunbackSpecs

Specify runback specification parameters

Efficiencies

Specify stage or component efficiencies

DesignSpecs

Specify design specifications, manipulated variables, and view results

CondenserHcurves

Specify condenser heating or cooling curve tables and view tabular results

ReboilerHcurves

Specify reboiler heating or cooling curve tables and view tabular results

TraySizing

Specify sizing calculation parameters for tray column sections, and view results

TrayRating

Specify rating calculation parameters for tray column sections, and view results

PackSizing

Specify sizing calculation parameters for packed column sections, and view results

PackRating

Specify rating calculation parameters for packed column sections, and view results

Properties

Specify physical property parameters for column sections

Estimates

Specify estimates for column temperatures and flows

Convergence

Specify convergence parameters

Report

Specify block-specific report options and pseudostreams

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

UserSubroutines

Specify user subroutines for tray and packing rating and sizing

Connectivity

View stream connectivity for the PetroFrac block

ResultsSummary

View key column results for the overall PetroFrac column

Profiles

View column profiles

Unit Operation Models Version 10

Chapter 4

Main Column You define the main column configuration using Condenser and Reboiler on the Setup Configuration sheet. PetroFrac allows six condenser types: • Subcooled • Total • Partial with vapor distillate product only • Partial with both vapor and liquid distillate products • No condenser, with pumparound to top stage • No condenser, with external feed to top stage You can specify one of three reboiler types: • Kettle reboiler • No reboiler, with pumparound to bottom stage • No reboiler, with external feed to bottom stage The types and number of required operating specifications depend on the column configuration. Normally, you must enter two column operating specifications. If either a condenser or a reboiler is absent, you must enter one specification. If both the condenser and reboiler are absent, do not enter any specification.

Feed Stream Handling Use the Setup Streams sheet to specify the feed and product stage locations. You may also identify a feed as the stripping steam, and override its flow by specifying a steam-to-product ratio. PetroFrac provides three conventions for handling feed streams (see PetroFrac Feed Convention Above-Stage and PetroFrac Feed Convention On-Stage in the following figures): • Above-Stage • On-Stage • Furnace When Feed-Convention is Above-Stage, PetroFrac introduces a material stream between adjacent stages. The liquid portion flows to the stage (n) you specify. The vapor portion flows to the stage above (n – 1). You can introduce a liquid feed to the top stage (or condenser) by specifying Stage=1. You can introduce a vapor feed to the bottom stage (or reboiler) by specifying Stage=Number of stages+1. When Feed-Convention is On-Stage, both the liquid and vapor portions of a feed flow to the stage (n) you specify.

Unit Operation Models Version 10

4-53

Columns

n-1

Vapor Mixed feed to stage n Liquid

PetroFrac Feed Convention Above-Stage

n-1

Mixed feed to stage n

n

n+1

PetroFrac Feed Convention On-Stage When Feed-Convention is Furnace, a furnace is attached to the stage (n) you specify. The feed enters the furnace before being introduced to the specified stage.

Feed Furnace PetroFrac can simulate a feed furnace simultaneously with the column/strippers. You can simulate the feed furnace as a simple heater or as a single stage flash with or without feed overflash bypass to the furnace. You can specify one of the following: • • •

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Heat Duty Temperature Fractional overflash

Unit Operation Models Version 10

Chapter 4

To do this

Use this sheet

Define a feed to the feed furnace

Setup Streams (Feed Convention)

Enter a furnace model type and associated specifications

Setup Furnace

You can select from three furnace model types, as shown in the next three figures. Main Column

Heat Feed

Furnace Modeled as a Stage Heat Duty Main Column

Feed

Furnace

Furnace Modeled as a Single Stage Flash

Unit Operation Models Version 10

4-55

Columns

Main Column

Feed Furnace

Furnace Modeled as a Single Stage Flash with Overflash Bypass

If Model=

PetroFrac models the furnace as

And calculates

Heater

Stage heat duty on the feed stage

—

Flash

Single-stage flash

Furnace temperature, degree of vaporization, vapor/liquid compositions

Flash-Bypass

Single-stage flash with the overflash bypassed back to the furnace

Furnace temperature, degree of vaporization, vapor/liquid compositions

Liquid Runbacks Use the RunbackSpecs form to specify the flow rate of liquid runback from any stage. When you enter a liquid runback specification, you must allow PetroFrac to adjust one of the following: • Flow rate of a pumparound • Duty of an interstage heater/cooler

Pumparounds Use the following sheets to enter specifications for pumparounds.

4-56

Use this sheet

To enter

Pumparounds Specifications

Pumparound connectivity and cooler/heater specifications

Report PseudoStreams

Pseudostream assignment for the pumparound

Hcurves Specifications

Heating/cooling curve specifications

Unit Operation Models Version 10

Chapter 4

Pumparounds are associated with the maincolumn. They can be total or partial drawoffs of the stage liquid flow. You must specify the draw and return stage locations for each pumparound. For partial drawoffs, you must specify two of the following: • Flow rate • Temperature • Temperature change • Heat Duty For total drawoffs, you must specify one of the following: • • •

Temperature Temperature change Heat Duty

Side Strippers Use the Stripper forms and sheets to enter specifications for side strippers. Side strippers may be either steam-stripped or reboiled. For steam strippers, you must enter a steam stream. You can override its flow rate by specifying a steamto-product ratio. For reboiled strippers, you must specify a reboiler duty. PetroFrac assumes: • A liquid draw goes from the main column to the top of the stripper. • The stripper overhead is returned to the main column. You must specify the draw and return stage locations. You can also: • Return a fraction of the stripper bottoms to the main column • Specify additional liquid draws from other stages of the main column as feeds to the strippers

Efficiencies You can specify one of two types of efficiencies: • Vaporization • Murphree Vaporization efficiency is defined as: Effi v =

yi , j K i, j x i , j

Murphree efficiency is defined as: Effi ,Mj =

Unit Operation Models Version 10

yi, j − yi , j +1 ki , j x i , j − yi , j +1

4-57

Columns

Where: K

=

Equilibrium K value

x

=

Liquid mole fraction

y

=

Vapor mole fraction

Eff v

=

Vaporization efficiency

Eff M

=

Murphree efficiency

i

=

Component index

j

=

Stage index

To specify vaporization or Murphree efficiencies, enter the number of actual stages on the Setup Configuration sheet and Strippers Setup Configuration sheet as Number of stages. Then use the Efficiencies and Strippers Efficiencies forms to enter the efficiencies. You can use any of these efficiencies to account for departure from equilibrium. But you cannot convert from one efficiency to the other. Magnitudes of the efficiencies can be quite different. Details on using and estimating these efficiencies are described by Holland, Fundamentals of Multi-Component Distillation, McGraw-Hill Book Company, 1981.

Convergence For convergence PetroFrac uses: • The sum-rates variant of the inside-out algorithm • A special initialization procedure designed for petroleum refining applications PetroFrac generally does not need initial estimates. For ethylene plant primary fractionator/quench tower combinations, you should provide temperature estimates. To enhance convergence, you may enter profile estimates on the following PetroFrac forms: • Estimates • Strippers Estimates Temperature estimates are usually adequate. You can increase convergence stability by selecting varying degrees of damping on the Convergence Basic sheet.

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Unit Operation Models Version 10

Chapter 4

Rating Mode In rating mode, PetroFrac calculates the column profiles and product compositions based on specified values of column parameters. Examples of column parameters are: • Reflux ratio • Reboiler duties • Feed flow rates • Furnace temperature • Pumparound loads

Design Mode In design mode you can manipulate subsets of the column parameters to achieve certain specifications on column performance. You can specify

For any

Purity

Stream, including internal streams

Recovery of any components group

Set of product streams

Flow rate of any components group

Internal stream or set of product streams

Flow rates of any components groups to any other component groups

Internal streams to any other internal streams, or set of feed or product streams

Temperature

Stage

Heat duty

Stage

Fractional overflash

Stage

TBP and D86 temperature gaps

Pair of product streams

TBP temperature

Product stream

D86 temperature

Product stream

D1160 temperature

Product stream

Vacuum distillation temperature

Product stream

API gravity

Product stream

Standard liquid density

Product stream

Specific gravity

Product stream

Flash point

Product stream

Pour point

Product stream

Refractive index

Product stream

†

††

continued

Unit Operation Models Version 10

4-59

Columns

You can specify

For any

Reid vapor pressure

Product stream

Value of any Prop-Set property

Internal or product stream

Difference of any pair of Prop-Set properties

Pair of product streams

Watson UOP K factor

Product stream

†

†† †††

†††

Express the purity as the sum of mole, mass, or standard liquid volume fraction of any group of components relative to any other group of components. Express recovery as a fraction of the same components in a subset of feed streams. See ASPEN PLUS User Guide, Chapter 28.

You can also specify overflash for a furnace feed stream.

Physical Properties Use the BlockOptions form to override the global physical property method. You can specify one method on this form, which PetroFrac uses for all stages in the main column and strippers. You can also split the main column or a stripper into any number of segments, each using a different property method. Use this sheet

When you use different properties for

Properties Property Sections

The main column

Strippers Properties Property Sections

A stripper

Free Water Handling PetroFrac can perform free-water calculations in the main column and side strippers. By default, PetroFrac performs free-water calculations for the main column condenser. The free-water phase, if present, is decanted.

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To do this

Use these sheets

Request free-water calculations for additional stages in the main columns and strippers

Properties Freewater Stages Strippers Properties Freewater Stages

Define additional water decant product streams for the main column

Setup Streams

Unit Operation Models Version 10

Chapter 4

Solids Handling PetroFrac handles solids by: • Temporarily removing all solids from inlet streams • Performing calculations without solids • Adiabatically mixing solids removed from inlet streams with main column liquid bottoms This calculation approach maintains an overall mass and energy balance around the PetroFrac block. But the bottom stage liquid product will not be in exact thermal or phase equilibrium with other bottom stage flows (for example, the bottom stage vapor flow).

Sizing and Rating of Trays and Packings PetroFrac has extensive capabilities to size, rate, and perform pressure drop calculations for trayed and packed columns. Use the following PetroFrac forms to enter specifications: • TraySizing, TrayRating, PackSizing, PackRating • Strippers TraySizing, Strippers TrayRating, Strippers PackSizing, Strippers PackRating See Appendix A for details on tray and packing types and correlations.

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RateFrac Rate-Based Distillation RateFrac is a rate-based nonequilibrium model for simulating all types of multistage vapor-liquid fractionation operations. RateFrac simulates actual tray and packed columns, rather than the idealized representation of equilibrium stages. RateFrac explicitly accounts for the underlying interphase mass and heat transfer processes to determine the degree of separation. RateFrac does not use empirical factors such as efficiencies and the Height Equivalent to a Theoretical Plate (HETP). RateFrac is applicable for: • Ordinary distillation • Absorption • Reboiled absorption • Stripping • Reboiled stripping • Extractive and azeotropic distillation RateFrac is suitable for: • • •

Two-phase systems Narrow and wide-boiling systems Systems exhibiting strong liquid phase nonideality

RateFrac can also detect and handle a free water phase in the condenser. RateFrac can model columns with chemical reactions. Reactions include: • Equilibrium • Rate-controlled • Electrolytic RateFrac models a complex configuration consisting of a single column or interlinked columns. The configuration may have: • • •

Any number of columns, each with any number of RateFrac Segments Any number of connections between columns or within each column Arbitrary flow splitting and mixing of connecting streams

RateFrac can handle operations with: • Side strippers • Pumparounds • Bypasses • External heat exchangers RateFrac can be used to • Rate existing columns • Design new columns

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You can define pseudoproduct streams to represent column internal flows or connecting streams in RateFrac. You can use Fortran Blocks, Sensitivity Analysis, and Case Study blocks to vary configuration parameters, such as feed location or number of segments. RateFrac can produce segmentwise column profile plots. RateFrac can be used with other ASPEN PLUS features and capabilities much in the same way as the equilibrium-based models, RadFrac, PetroFrac, and MultiFrac.

Flowsheet Connectivity for RateFrac Top Segment or Condenser Heat Duty (optional)

Feeds

Vapor Distillate or Interconnecting Stream 1

Reflux

Heat (optional)

Heat (optional) Liquid Distillate (optional) Water Distillate (optional) Side Products Interconnecting Streams (Heater optional)

Pumparounds and Bypasses (Heater optional)

Interconnecting Streams

(Heater optional) Heat (optional)

Bottom Segment or Reboiler Heat Duty (optional)

N

Heat (optional) Bottoms or Interconnecting Streams

RateFrac models single and interlinked columns. Any number of columns can be connected by any number of connecting streams. Each connecting stream can have an associated heater. Each column may have: • Any combination of packed and tray segments • Any number of connecting streams • Any number of side product streams

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Material Streams Inlet

At least one material stream

Outlet Up to two product streams (one vapor, one liquid) per segment

One water distillate product stream (optional) Any number of pseudoproduct streams (optional) Each column must have: • At least one vapor or liquid stream leaving the top segment • One liquid stream leaving the bottom segment When you model interlinked columns, the top and bottom streams can be connecting streams. However, the free-water stream from the condenser cannot be a connecting stream.

Heat Streams Inlet

One heat stream per segment (optional) One heat stream per connecting stream (optional)

Outlet One heat stream per connecting stream (optional)

RateFrac uses an inlet heat stream as a duty specification for all segments except the condenser, reboiler, and connecting streams. If you do not provide two column operating specifications on the Columns Setup Configuration sheet, RateFrac uses a heat stream as a specification for the condenser and reboiler. If you do not provide two specifications on the ConnectStreams Input sheet, RateFrac uses a heat stream as a specification for connecting streams. If you provide two specifications on the Columns Setup Configuration sheet or ConnectStreams Input sheet, RateFrac does not use the inlet heat stream as a specification. The inlet heat stream supplies the required heating or cooling. You can use optional outlet heat streams for the net heat duty of the condenser, reboiler, and connecting streams. The value of the outlet heat stream equals the value of the inlet heat stream (if any), minus the actual (calculated) heat duty.

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The Rate-Based Modeling Concept Most models available for simulating and designing multicomponent, multistage separation processes are based on the idealized concept of equilibrium or theoretical stages. This approach assumes that the liquid and vapor phases leaving any stage are in thermodynamic equilibrium with each other. The phase compositions, temperature, and vapor and liquid flow profiles are calculated by solving the governing material balances, energy balances, and equilibrium relations for each stage. In practice, columns rarely operate under thermodynamic equilibrium conditions. Vapor-liquid equilibrium prevails only at the interface separating vapor and liquid phases. The separation achieved in a multistage column depends on the interphase mass and heat transfer rate processes. Multicomponent mass transfer interactions can also have pronounced effects on the separation. When the equilibrium approach is used to model a tray column, a correction factor (referred to as an efficiency) attempts to account for the departure from equilibrium. Many definitions for efficiency exist, with wide variations in complexity and accuracy. In general, efficiencies depend on: • Physical characteristics of the equipment, such as column configuration • Hydrodynamics of the column • Fluid properties of the system Murphree vapor efficiencies are the most widely used. These efficiencies generally vary from stage to stage within a column, and from component to component. For multicomponent systems, there are no theoretical limitations on Murphree efficiencies. Experimental evidence shows that component efficiencies: • May vary strongly from component to component • Can take any value including negative values Methods used to calculate component efficiencies generally do not include the effect of the departure from thermal equilibrium. Packed columns are also designed using the equilibrium stage concept. However, HETP is commonly used in place of efficiencies. HETP varies with: • Type and size of the packing • Hydrodynamics of the column • Fluid properties of the system Like efficiencies, HETPs may vary strongly from point to point within a column and from system to system.

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RateFrac is based on a fundamental and rigorous approach. This approach avoids uncertainties that result when the equilibrium approach is used with estimated efficiencies or HETP. RateFrac directly includes mass and heat transfer rate processes in the system of equations representing the operation of separation process units. RateFrac: • Describes the simultaneous mass and heat transfer rate phenomena • Accounts for the multicomponent interactions between simultaneously diffusing species For nonreactive systems, RateFrac comprises: • • • •

Mass and heat balances around vapor and liquid phases Mass and heat transfer rate models to determine interphase transfer rates Vapor-liquid equilibrium relations applied at interfacial conditions Correlations to estimate mass and heat transfer coefficients and interfacial areas

For chemically reactive systems, RateFrac includes equations to account for the influence of chemical reactions on heat and mass transfer rate processes. For systems involving equilibrium reactions, RateFrac includes equations to represent the chemical equilibrium conditions. RateFrac completely avoids the need for efficiencies in tray columns or HETPs in packed columns. RateFrac has far greater predictive capabilities than the conventional equilibrium model.

Specifying RateFrac RateFrac numbers segments from the top down, starting with the condenser (or starting with the top segment if there is no condenser). Use the following forms to enter specifications and view results for RateFrac: Use this form

To do this

BlockParameters

Specify overall block parameters, convergence and initialization parameters, blockspecific diagnostic message levels, and feed flash convergence parameters

Columns Setup

Specify basic column configuration and operating conditions

Columns TraySpecs

Specify tray column section parameters

Columns PackSpecs

Specify packed column section parameters

Columns Reactions

Assign reactions to column sections, and specify vapor and liquid holdup data continued

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Use this form

To do this

Columns Estimates

Specify initial estimates for segment temperatures, and vapor and liquid flows and compositions

Columns EquilibriumSegments

Specify optional equilibrium segments and column efficiencies

Columns HeatersCoolers

Specify segment heating or cooling and utility exchangers

Columns FlowTempSpecs

Specify liquid, vapor, and temperature specifications

Columns Results

View column performance summary

Columns Profiles

View column profiles

Columns InterfaceProfiles

View column interface profiles

Columns EfficienciesFlooding

View tray and component efficiencies, packing HETPs, and flooding summary

Columns TransferCoefficients

View binary diffusion, binary mass, and heat transfer coefficients

InletsOutlets

Specify feed and product stream locations and conventions, inlet and outlet heat streams

ConnectStreams

Specify connecting stream sources and destinations and view results

DesignSpecs

Specify design specifications and view convergence results

Vary

Specify manipulated variables to satisfy design specifications and view final values

FlowRatios

Specify the flow ratio and view results

CondenserHcurves

Specify condenser heating or cooling curve tables and view tabular results

ReboilerHcurves

Specify reboiler heating or cooling curve tables and view tabular results

ConnectStreamHcurves

Specify connecting stream heating or cooling curve tables and view tabular results

Reports

Specify block-specific report options, and pseudostream information

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

UserSubroutines

Specify user subroutine parameters for mass and heat transfer coefficients, interfacial area, pressure drop, and kinetics

ResultsSummary

View material and energy balance results and overall split fractions

Column Numbering Individual columns are identified by a column number. The numbering order does not affect algorithm performance. Within each column, segments are numbered from top to bottom, starting with the condenser (when present).

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Stream Definition RateFrac uses four types of streams: • External streams • Connecting streams • Internal streams • Pseudostreams External streams are the standard RateFrac inlet and outlet streams. They are identified by stream IDs. Connecting streams are streams within RateFrac but external to individual columns. These streams are identified by connecting stream numbers. Connecting streams may connect two columns or segments of the same column (such as bypasses and pumparounds). You can associate a heater with any connecting stream. Heaters are identified by the connecting stream number. Internal streams are the liquid or vapor flows between adjacent segments of the same column. These streams are identified by a segment number and a column number. Pseudostreams store the results of internal and connecting streams. They are a subset of external outlet streams. Unlike normal outlet streams, pseudostreams do not participate in the block material balance calculations.

Material Feed Streams RateFrac uses two conventions for handling material feed streams (see RateFrac Feed Conventions in the following figures): • Above segment • On segment

Segment n-1

Mixed Feed to

Vapor Liquid

Segment n Segment n

RateFrac Feed Convention Above Segment

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Segment n-1

Liquid

Mixed Feed to Segment n Segment n Vapor

Segment n + 1

RateFrac Feed Convention On Segment When the feed convention is defined as Above segment, RateFrac introduces a material stream between adjacent segments. The liquid portion flows to segment n, specified as the feed segment. The vapor portion flows to the segment above (segment n-1 in the figure RateFrac Feed Convention Above segment). You can introduce a liquid to the top segment (or condenser) by specifying Segment=1. You can introduce a vapor feed to the bottom segment (or reboiler), by specifying the segment equal to the last segment in the column +1. When a two-phase feed stream is fed to segment 1, the vapor phase is combined directly with the vapor distillate. Similarly, when a two-phase feed stream is fed to the last segment of that column + 1, the liquid phase is combined directly with the liquid bottoms product. When the feed convention is defined as On segment, both the liquid and vapor portions of the feed flow to segment specified (segment n in the previous figure RateFrac Feed Convention On segment). RateFrac assumes that a vapor feed (or the vapor portion of a mixed feed) combines with the vapor phase in the segment it enters. RateFrac also assumes that a liquid feed (or the liquid portion of a mixed feed) combines with the liquid phase in the segment it enters.

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Column Configuration Specify the column configuration by indicating the following on the Columns Configuration sheet: • Number of segments • Presence or absence of condensers and reboilers • Equilibrium and nonequilibrium segments

Connecting Streams RateFrac allows any number of connecting streams. Any number of these streams can have the same: • Source column, segment, and phase • Destination column and segment RateFrac introduces connecting streams on the destination segment regardless of their phase (Convention = On Segment). All connecting streams can have a heater. Enter all specifications for connecting streams on the ConnectStreams Input sheet. RateFrac does not allow phase change for connecting streams. Connecting streams can be either a total or a partial drawoff of the segment flow. Enter the required specifications as follows: If the drawoff type is

You enter

Partial

Two of the following: flow, temperature or temperature change and heat duty

Total

One of the following: temperature or temperature change and heat duty

† ††

†

††

Enter zero for heat duty if heater is absent. Flow is taken as the net flow of the segment, excluding any product flow and any other connecting stream flow.

Required Specifications You must specify the total number of columns and connecting streams.

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Use this form

To enter

Such as

Columns TraySpecs

Tray specifications

Number of trays or Number of trays per segment Tray type Tray characteristics

Columns PackSpecs

Packing specifications

Total height of packing or Height of packing per segment Packing type Packing characteristics

Unit Operation Models Version 10

Chapter 4

You must also specify: • Inlet stream locations • Heat stream locations, heat duty, and phase • Pressure profile for each column • Condenser type • Two operating specifications for multisegment columns and one for singlesegment columns • Source and destination of any connecting stream and associated heater specifications • Outlet stream locations and phases. If the outlet stream is a side drawoff stream from a segment, you must specify its flow. A segment refers to one of the following: • A slice (or portion) of packing in a packed column (see the preceding figure, Nonequilibrium Segment in a Packed Column) • One (or more) tray(s) in a tray column (see the preceding figure, Nonequilibrium Segment in a Tray Column) A column consists of segments. To evaluate mass and heat transfer rates between contacting phases, RateFrac uses one of the following: • •

Height of packing in a packed segment Number of trays in a tray segment

Nonequilibrium Segment in a Packed Column

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Columns

Nonequilibrium Segment in a Tray Column

Equilibrium Stages RateFrac can model both equilibrium stages and nonequilibrium segments in the same column. Use the Columns EquilibriumSegments form to specify the location of equilibrium stages. When all stages are equilibrium, you can obtain the same results using RateFrac as you can using RadFrac, MultiFrac, or PetroFrac with ideal stages.

Reactive Systems RateFrac can handle kinetically controlled reactions and equilibrium reactions in both liquid and vapor phases. Chemical reactions can be of any type, including: • Simultaneous • Consecutive • Parallel • Forward • Reverse For kinetically controlled reactions, the kinetics can be defined by one of the following: • Built-in power law expressions • User-supplied Fortran subroutines

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For equilibrium reactions, the chemical reaction equilibrium constant can be defined either in terms of user-supplied coefficients for a temperature-dependent polynomial, or can be computed from the reference state free energies of participating components. RateFrac can model electrolyte systems using both the apparent and the true component approaches. Enter the following information on the Reactions form: • • •

Reaction stoichiometry Reaction type Phase in which reactions occur

Depending on the reaction type, you must enter either the equilibrium constant or kinetic parameters. For electrolytic reactions, you can also enter the reaction data on the Chemistry form. To associate reactions with a column segment, enter the corresponding Reactions ID (or Chemistry ID or User Reactions ID) on the Columns Reactions Specifications sheet. For rate-controlled reactions, you must enter holdup data for the phase where reactions occur. For these segments

Use this form to enter holdup information

Equilibrium

Columns Reactions

Tray

Columns TraySpecs

Packed

Columns PackSpecs

Heaters and Coolers Use the Columns HeatersCoolers Side Duties sheet to specify: • Heat duty for a segment • Heater segment location (column and segment) • Phase Use the Columns HeatersCoolers Utility Exchangers sheet to specify cooling (or heating) of any segment using a coolant (or heating fluid). You can use a heat stream to provide heat integration. Heat integration occurs when the duty recovered from another block is used as the heat source of heaters and coolers. Enter heat stream data on the InletsOutlets Heat Streams sheet.

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Columns

Physical Property Specifications Use the RateFrac BlockOptions form to override the global physical property property method. You can specify only one property method on the BlockOptions form. RateFrac uses this property method for the whole column. RateFrac does not allow multiple physical property methods.

Handling Free Water RateFrac can perform free-water calculations only in condensers.

Rating Mode In rating mode, RateFrac calculates temperatures, flows, and mole fraction profiles based on specified values of column parameters such as: • Reflux ratio • Product flows • Heat duties

Design Mode In design mode, use the DesignSpecs form to specify column performance parameters (such as purity or recovery). You must indicate which variables to manipulate to achieve these specifications using the Vary form. You can specify any variables that are allowed in rating mode, except: • Number of columns, segments, and connecting streams • Pressure profile • Locations of feeds, products, heaters, and connecting streams • Column configurations, including the number of trays, tray characteristics, height of packing, packing specifications

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The flows of inlet material streams and the duties of inlet heat streams can also be manipulated variables. You can specify

For any

Purity

Stream, including an internal stream

Recovery of any component groups

Set of product streams

Flow of any component groups

Internal stream or set of product streams

Component ratio

Internal stream and a second internal stream or feed streams and product streams

Temperature of vapor stream

Segment

Temperature of liquid stream

Segment

Heat duty

Condenser, reboiler, or a connecting stream

Value of any Prop-Set property

Internal or product stream

Ratio or difference of any pair of properties in a Prop-Set

Single or paired internal or product stream

†

†† †††

†

††

†††

Express the purity as the sum of mole, mass, or standard liquid volume fractions of any group of components, relative to any other group of components. You can express recovery as a fraction of the same components in a subset of the feed stream. See ASPEN PLUS User Guide, Chapter 28.

Calculating Efficiency and HETP From converged vapor and liquid composition profiles, RateFrac back-calculates the component Murphree vapor efficiencies. These efficiencies are defined for each component as the fractional approach to equilibrium of the vapor stream leaving any segment, with the liquid stream leaving the same segment. Eff ij =

y ij − y ij +1 K ij x ij − Yij +1

Where: Eff K x y i j

Unit Operation Models Version 10

= = = = = =

Murphree vapor efficiency Vapor-liquid equilibrium K value Liquid mole fraction Vapor mole fraction Component index Segment index

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Columns

For each segment of packed columns, RateFrac calculates the fractional approach to equilibrium using the same definition as used for Murphree vapor efficiency. RateFrac reports the height of packing required to achieve equilibrium as the HETP for that segment.

Convergence and Computing Time RateFrac must solve many more equations for a given column than an equilibrium model. Computing times for RateFrac are greater than they are for equilibrium models, particularly for problems containing many components. The solution algorithm RateFrac uses is an efficient, Newton-based simultaneous correction approach. RateFrac solution times increase with the square of the number of components. Solution times can be an order of magnitude greater than RadFrac, MultiFrac, or PetroFrac solution times for the same problems.

References for Built-In Correlations RateFrac uses well-known and accepted correlations to calculate: • Binary mass transfer coefficients for the vapor and liquid phase • Interfacial areas In general, these quantities depend on column diameter and operating parameters such as: • Vapor and liquid flow • Densities • Viscosities • Surface tension of liquid • Vapor and liquid phase binary diffusion coefficients Mass transfer coefficients and interfacial areas depend on: Packing characteristics

Tray characteristics

Type (random or structured)

Type (sieve, valve, or bubble-cap)

Size

Weir and flow path length

Specific surface area

Downcomer area

Material of construction

Weir height

The correlations involve well-defined dimensionless groups, such as the Reynolds, Froude, Weber, Schmidt, and Sherwood numbers. The correlations have been fitted to experimental measurements from laboratory and pilot plant absorption and distillation columns.

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The correlations RateFrac uses for mass transfer coefficients and interfacial areas are: Column type

Correlation used

Packed Columns (random packing)

Onda et al. (1968)

Packed Columns (structured)

Bravo et al. (1985, 1992)

†

Chan and Fair (1984)

Sieve Trays Valve Trays

Scheffe and Weiland (1987) †

Bubble-Cap Trays †

Grester et al. (1958)

These correlations do not provide the mass transfer coefficients and interfacial areas separately.

RateFrac allows you to write Fortran subroutines to calculate: • Binary mass transfer coefficients • Heat transfer coefficients • Interfacial areas The subroutines are described in the ASPEN PLUS User Models reference manual. By applying a rigorous multicomponent mass transfer theory (Krishna and Standart, 1976), RateFrac uses binary mass transfer coefficients to evaluate: • •

Multicomponent binary mass transfer coefficients Component mass transfer rates between vapor and liquid phases

RateFrac calculates the vapor phase and liquid phase heat transfer coefficients using the Chilton-Colburn analogy (King, 1980). This analogy relates: • Mass transfer coefficients • Heat transfer coefficients • Schmidt number • Prandtl number

Mass and Heat Transfer Correlations RateFrac uses several mass and heat transfer correlations for: • Packed columns. • Valve Tray columns • Bubble-Cap Tray columns • Sieve Tray columns

Packed Column RateFrac calculates the mass transfer coefficients and the interfacial area available for mass transfer using the correlations developed by Onda et al., 1968.

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Columns

The correlation for the liquid phase binary mass transfer coefficients is: 2/3  L  ρ  1/ 3   L  −1/ 2 L k in   ( ScinL ) a pd p   = 0.0051  aω µ L    gµ L  

(

)

0 .4

The correlation for the gas phase binary mass transfer coefficient is:

 g  RT g    G    = 5.23  k in   a p ug    a p Din  

0 .7

(Sc ) (a g 1/ 3 in

p

dp

)

−2

The interfacial area available for mass transfer is given by the correlation:

{

[

aω = a p 1 − exp − 145 . Re L

0.1

FrL −0.05We L

0 .2

(σ σ )

−0.75

c

]}

Where: 2

aρ L2 L L Re L = , FrL = 2 , We L = a p σρ L a pµ L gρ L and: L

=

Binary mass transfer coefficient for the binary pair i and n in the liquid phase (m/sec)

ρL

=

Density of liquid (kg/m 3 )

g

=

Acceleration due to gravity (m/sec 2 )

µL

=

Viscosity of liquid (Newton-sec/m 2 )

L

=

Liquid superficial mass velocity (kg/m 2 /sec)

aw

=

Wetted interfacial area (m 2 interfacial area/m 3 packing volume)

=

Schmidt number for the binary pair i and n in the liquid phase =

k in

L

Sc in

L

D in ap

µ L (ρ L DinL )

=

Binary Maxwell-Stefan diffusion coefficient for the binary pair i and n (m 2 /sec)

=

Specific surface area of the packing continued

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dp

=

Nominal diameter of packing or packing size (m)

g

=

Binary mass transfer coefficient for the binary pair i and n in the vapor phase (kg mole/atm/m 2 /sec)

=

Universal gas constant (m 3 atm/kg mole/K)

=

Gas phase temperature (K)

G

=

Gas superficial mass velocity (kg/m 2 /sec)

µg

=

Viscosity of gas mixture (Newton-sec/m 2 )

=

Gas phase Schmidt number for the binary pair i and n =

k in R

T

g

g

Sc in

ρg

(ρ D )

µg

g in

g

=

Density of gas mixture (kg/m 3 )

=

Gas-phase binary Maxwell-Stefan diffusion coefficient for the binary pair i and n (m 2 /sec)

σ

=

Surface tension (Newton/m)

σc

=

Critical surface tension of the packing material (Newton/m)

g

D in

Valve Tray Column RateFrac calculates the mass transfer coefficients and the interfacial area available for mass transfer using the correlations developed by Scheffe and Weiland, 1987. The correlation for the liquid phase binary mass transfer coefficient is:

( ) ( Re ) 0.68

ShinL = 125.4 Re g

0.09

L

(v )0.05 (ScinL )

0.5

The correlation for the gas phase binary mass transfer coefficients is:

( ) (Re )

Shing = 9.93 Re g

0.87

0.13

L

(ϖ) 0.39 (Scing )

0.5

The interfacial area available for mass transfer is given by the correlation:

( ) ( Re )

a = 0.27 Re g

Unit Operation Models Version 10

0.37

L

0.25

( ϖ) 0.52

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Columns

Where: L

Sh = L in

Re L =

k in ad ρ L D in L

g

, Sh = g in

k in ad ρ g D in g

, ScinL =

µL ρ L D in L

, Scing =

µg ρ g D in g

,

Gd Ld W , Re g = , ϖ= µg d µL

and: L

=

Liquid mass velocity (kg/m 2 /sec) (Velocity is based on tower active area.)

d

=

Geometric parameter of unit length (m)

µL

=

Viscosity of liquid mixture (Newton-sec/m 2 )

G

=

Gas mass velocity (kg/m 2 /sec) (Velocity is based on tower active area.)

µg

=

Viscosity of gas mixture (Newton-sec/m 2 )

L

=

Binary mass transfer coefficient for the binary pair i and n in the liquid phase (kg mole/m 2 /sec)

a

=

Interfacial area (m 2 interfacial area/m 2 tower active area)

ρL

=

Molar density of liquid (kg mole/m 3 )

=

Binary Maxwell-Stefan diffusion coefficient for the binary pair i and n (m 2 /sec)

=

Binary mass transfer coefficient for the binary pair i and n in the vapor phase (kg mole/m 2 /sec)

=

Molar density of gas mixture (kg mole/m 3 )

=

Gas-phase binary Maxwell-Stefan diffusion coefficient for the binary pair i and n (m 2 /sec)

ρL

=

Density of liquid mixture (kg/m 3 )

ρg

=

Density of gas mixture (kg/m 3 )

W

=

Weir height (m)

k in

L

D in

g

k in ρg g

D in

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Bubble-Cap Tray Column RateFrac calculates the product of the binary mass transfer coefficients and interfacial areas using the correlations developed by Grester et al., 1958. The product of liquid phase binary mass transfer coefficients and interfacial area is given by the correlation:

k in a = (4.127 × 108 DinL ) (0.21313F + 015 . ) Lt L 0.5

L

The product of gas phase binary mass transfer coefficient and interfacial area is given by the correlation:

k in a = g

(0.776 + 4.567h

w

− 0.2377 F + 104.85Q L )

(Sc )

g 0.5 in

G

Where: L

k in a L

D in

F

=

Binary mass transfer coefficient for the binary pair i and n in the liquid phase (kg mole/m 2 /sec)

=

Interfacial area (m 2 interfacial area/m 2 tower active area)

=

Binary Maxwell-Stefan diffusion coefficient for the binary pair i and n (m 2 /sec)

=

F-Factor =

µ g ρ1g/ 2  kg 1/2 / sec / m1/2  µg

=

Gas volumetric flow per unit active area (m 3 /sec/m 2 )

ρg

=

Density of gas mixture (kg/m 3 )

L

=

Liquid molar velocity (kg mole/m 2 /sec) (Velocity is based on active area.)

tL

=

Liquid residence time = 0.9998hL Z L / QL (sec)

hL

=

Liquid holdup = 0.04191 + 0.19hw + 2.4545QL − 0.0135 F ( m )

ZL

=

Liquid flow path length (m) continued

Unit Operation Models Version 10

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Columns

QL

=

Liquid flow per average path width (m 3 /sec/m)

hw

=

Outlet weir height (m)

g

=

Binary mass transfer coefficient for the binary pair i and n in the vapor phase (kg mole/m 2 /sec)

=

Gas molar velocity (kg mole/m 2 /sec) (Velocity is based on active area.)

=

Gas-phase Schmidt number for the binary pair i and n =

k in G

g

Sc in

µg g

D in

µg

(ρ D ) g

g in

=

Viscosity of gas mixture (Newton-sec/m 2 )

=

Gas-phase binary Maxwell-Stefan diffusion coefficient for the binary pair i and n (m 2 /sec)

Sieve Tray Column RateFrac calculates the product of mass transfer coefficients and interfacial areas using the correlations developed by Chan and Fair, 1984. The product of liquid phase binary mass transfer coefficient and interfacial area is given by the correlation: L k in a = (4.127 x108 DinL ) (0.21313F + 0.15) Lt L 0.5

The product of the gas phase binary mass transfer coefficient and interfacial area is given by the correlation:

k a= g in

( D ) (1030 F − 867 F ) g 0.5 in

2

h L 0.5

Where: L

k in a L

D in

=

Binary mass transfer coefficient for the binary pair i and n in the liquid phase (kg mole/m 2 /sec)

=

Interfacial area (m 2 interfacial area/m 2 tower active area)

=

Binary Maxwell-Stefan diffusion coefficient for the binary pair i and n (m 2 /sec) continued

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=

F

F-Factor =

µ gρg

( kg

1/ 2

/ sec / m1/ 2 )

µg

=

Gas volumetric flow per unit active area (m 3 /sec/m 2 )

ρg

=

Density of gas mixture (kg/m 3 )

L

=

Liquid molar velocity (kg mole/m 2 /sec) (Velocity is based on active area.)

tL

=

Liquid residence time = 0.9998hL Z L / QL (sec)

hL

=

Liquid holdup = 0.04191 + 0.19hw + 2.4545QL − 0.0135 F ( m )

ZL

=

Liquid flow path length (m)

QL

=

Liquid flow per average path width (m 3 /sec/m)

hw

=

Outlet weir height (m)

g

=

Binary mass transfer coefficient for the binary pair i and n in the vapor phase (m/sec)

=

Binary Maxwell-Stefan diffusion coefficient for the binary pair i and n (m 2 /sec)

F

=

Fractional approach to flooding gas velocity =

µgF

=

Gas velocity through active area at flooding (m/sec)

hL

=

Liquid height =

Γe

=

exp(− 12.55K s 0.91 )

B

=

0.0327 + 0.0286 exp(− 137.8hω )

Ks

=

µ g ρ g (ρ L − ρ g )

ρL

=

Density of liquid mixture (kg/m 3 )

k in g

D in

Unit Operation Models Version 10

1/ 2

µg / µgF

Γe hw + 1533Γe B(Q L / Γe ) ( m) 2/ 3

(

)

0.5

(m / sec)

4-83

Columns

Heat Transfer Coefficients RateFrac calculates the heat transfer coefficients, using the Chilton-Colburn analogy (King, 1980). The heat transfer coefficient is given by:

k av ( Sc)

2/3

=

htc Cpmix

Where:

4-84

k av

=

Average binary mass transfer coefficients (kg mole/sec)

Sc

=

Schmidt number

htc

=

Heat transfer coefficient (Watts/K)

Cpmix

=

Molar heat capacity (Joules/kg mole/K)

Pr

=

Prandtl number

Unit Operation Models Version 10

Chapter 4

References Bravo, J.L., Rocha, J.A., and Fair, J.R., "Mass Transfer in Gauze Packings," Hydrocarbon Processing, January, 91 (1985). Bravo, J.L., Rocha, J.A., and Fair, J.R., "A Comprehensive Model for the Performance of Columns Containing Structured Packings," ICHEME Symposium Series, 128, A439 (1992). Chan, H. and Fair, J.R., "Prediction of Point Efficiencies in Sieve Trays: 1. Binary Systems, 2. Multicomponent Systems," Ind. Eng. Chem. Process Des. Dev., 23, (1984) p. 814. Grester, J.A., Hill, A.B., Hochgraf, N.N., and Robinson, D.G., "Tray Efficiencies in Distillation Columns," AIChE Report, (1958). King, C.J., Separation Processes, Second Edition, McGraw-Hill Company, (1980). Krishna, R. and Standart, G.L., "A Multicomponent Film Model Incorporating a General Matrix Method of Solution to the Maxwell-Stefan Equations," AIChE J., 22, (1976) p. 383. Onda, K., Takeuchi, H., and Okumoto, Y., "Mass Transfer Coefficients between Gas and Liquid Phases in Packed Columns," J. Chem. Eng., Japan, 1, (1968) p. 56. Perry, R.H. and Chilton, C.H., "Chemical Engineers’ Handbook," Fifth Edition, McGraw-Hill Book Company, Section 18 (1973). Scheffe, R.D. and Weiland, R.H., "Mass Transfer Characteristics of Valve Trays," Ind. Eng. Chem. Res., 26, (1987) p. 228.

Unit Operation Models Version 10

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Columns

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Chapter 4

Extract Rigorous Extraction Extract is a rigorous model for simulating liquid-liquid extractors. It can have multiple feeds, heater/coolers, and side streams. Extract can calculate distribution coefficients using: • An activity coefficient model or equation of state capable of representing two liquid phases • A built-in temperature-dependent correlation (KLL Correlation sheet) • A Fortran subroutine (KLL Subroutine sheet) Although equilibrium stages are assumed, you can specify component or stage separation efficiencies. Extract can be used only for rating calculations. You can define pseudoproduct streams (Report PseudoStreams sheet) to represent extractor internal flows. You can use Fortran and sensitivity blocks to vary configuration parameters, such as feed location or number of stages.

Flowsheet Connectivity for Extract L2 Phase

L1 Phase Side feeds (any number)

1

Side products (any number)

Nstage L1 Phase

L2 Phase

Material Streams Inlet

One material stream to the first (top) stage, rich in the first liquid phase (L1) One material stream to the last (bottom) stage, rich in the second liquid phase (L2) One material stream per intermediate stage (optional)

Outlet One material stream for L1 from the last stage

One material stream for L2 from the first stage Up to two side product streams per stage, one for L1 and one for L2 (optional)

Unit Operation Models Version 10

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Columns

Specifying Extract Extract can operate in one of the following ways: • Adiabatically (default) • At a specified temperature • With specified stage heater or cooler duties You must specify: • Number of stages • Feed and product stream stage locations • Side product stream phase and mole flow rate • Pressure profile The first liquid phase (L1) flows from the first stage to the last stage. The second (L2) flows in the opposite direction. You must identify the key components in each phase using L1-Comps and L2-Comps on the Setup form. Extract can treat phase L1 as the solvent/extract phase or the feed/raffinate phase. Liquid-liquid distribution coefficients are required to represent the liquid-liquid equilibrium. Extract calculates these coefficients using one of the following methods: You can use

You enter

On sheet

Any physical property method that can represent two liquid phases

A global property method or an Opset name to override the global physical property method

BlockOptions Properties

A built-in temperature-dependent polynomial

Polynomial coefficients

Properties KLL Correlation

A Fortran subroutine

Subroutine name

Properties KLL Subroutine

Use the following forms to enter specifications and view results for Extract: Use this form

To do this

Setup

Specify basic column configuration and operating conditions

Efficiencies

Specify stage or component efficiencies

Properties

Specify parameters for KLL correlations and KLL subroutines

Estimates

Specify initial estimates for stage temperatures and compositions

Convergence

Specify convergence parameters and block-specific diagnostic message levels

Report

Specify block-specific report options and pseudostream information continued

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Unit Operation Models Version 10

Chapter 4

Use this form

To do this

Block Options

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View column performance summary, material and energy balance results, and split fractions

Profiles

View extractor profiles

Dynamic

Specify parameters for dynamic simulations

See ASPEN PLUS User Models for more information about Fortran subroutines.

❖

Unit Operation Models Version 10

❖

❖

❖

4-89

Columns

4-90

Unit Operation Models Version 10

Chapter 5

5

Reactors This chapter describes the unit operation models for reactors. The models are: Model

Description

Purpose

Use For

RStoic

Stoichiometric reactor

Models stoichiometric reactor with specified reaction extent or conversion

Reactors where reaction kinetics are unknown or unimportant but stoichiometry and extent of reaction are known

RYield

Yield reactor

Models reactor with specified yield

Reactors where stoichiometry and kinetics are unknown or unimportant but a yield distribution is known

REquil

Equilibrium reactor

Performs chemical and phase equilibrium by stoichiometric calculations

Reactors with simultaneous chemical equilibrium and phase equilibrium

RGibbs

Equilibrium reactor with Gibbs energy minimization

Performs chemical and phase equilibrium by Gibbs energy minimization

Reactors with phase equilibrium or simultaneous phase and chemical equilibrium. Calculating phase equilibrium for solid solutions and vapor-liquid-solid systems.

RCSTR

Continuous stirred tank reactor

Models continuous stirred tank reactor

One-, two, or three-phase stirred tank reactors with rate-controlled and equilibrium reactions in any phase based on known stoichiometry and kinetics

RPlug

Plug flow reactor

Models plug flow reactor

One-, two-, or three-phase plug flow reactors with rate-controlled reactions in any phase based on known stoichiometry and kinetics

RBatch

Batch reactor

Models batch or semi-batch reactor

One-, two-, or three-phase batch and semibatch reactors with rate-controlled reactions in any phase based on known stoichiometry and kinetics

RCSTR, RPlug, and RBatch are kinetic reactor models. Use the Reactions Reactions form to define the reaction stoichiometry and data for these models.

Unit Operation Models Version 10

5-1

Reactors

You do not need to specify heats of reaction, because ASPEN PLUS uses the elemental enthalpy reference state for the definition of the component heat of formation. Therefore, heats of reaction are accounted for in the mixture enthalpy calculations for the reactants versus the products.

RStoic Stoichiometric Reactor Use RStoic to model a reactor when: • Reaction kinetics are unknown or unimportant and • Stoichiometry and the molar extent or conversion is known for each reaction RStoic can model reactions occurring simultaneously or sequentially. In addition, RStoic can perform product selectivity and heat of reaction calculations.

Flowsheet Connectivity for RStoic Material (any number)

Heat (optional)

Heat (optional)

Water (optional) Material

Material Streams Inlet

At least one material stream

Outlet One product stream

One water decant stream (optional)

5-2

Unit Operation Models Version 10

Chapter 5

Heat Stream Inlet

Any number of heat streams (optional)

RStoic uses the sum of the inlet heat streams as the heat duty specification, if you do not specify an outlet heat stream. Outlet One heat stream (optional)

The value of the outlet heat stream is the net heat duty (sum of the inlet heat streams minus the calculated heat duty) for the reactor.

Specifying RStoic Use the Setup Specifications sheet to specify the reactor operating conditions and to select the phases to consider in flash calculations in the reactor. Use the Setup Reactions sheet to define the reactions occurring in the reactor. You must specify the stoichiometry for each reaction. In addition, you must specify either the molar extent or the fractional conversion for all reactions. When solids are created or changed by the reactions, you may specify the component attributes and the particle size distribution in the outlet stream using the Setup Component Attr. sheet and the Setup PSD sheet respectively. If you wish to calculate the heats of reaction, use the Setup Heat of Reaction sheet to specify the reference component for each reaction defined in the Setup Reactions sheet. You may also choose to specify the heats of reaction, and RStoic adjusts the calculated reactor duty, if needed. If you wish to calculate product selectivities use the Setup Selectivity sheet to specify the selected product component and the reference reactant component. Use the following forms to enter specifications and view results for RStoic:

Unit Operation Models Version 10

Use this form

To do this

Setup

Specify operating conditions, reactions, reference conditions for heat of reaction calculations, product and reactant components for selectivity calculations, particle size distribution, and component attributes

Convergence

Specify estimates and convergence parameters for flash calculations

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of operating results, mass and energy balances, heats of reaction, product selectivities, reaction extents, and phase equilibrium results for the outlet stream

Dynamic

Specify parameters for dynamic simulations

5-3

Reactors

Heat of Reaction RStoic calculates the heat of reaction from the heats of formation in the databanks when you select the Calculate Heat of Reaction option on the Setup Heat of Reaction sheet. The heats of reaction are calculated at the specified reference conditions based on consumption of a unit mole or mass of the reference reactant selected for each reaction. The following reference conditions are used by default: Specification

Default

Reference temperature

25 °C

Reference pressure

1 atm

Reference fluid phase

Vapor phase

You can also use the Setup Heat of Reaction sheet to specify the heats of reaction. The specified heat of reaction may differ from the heat of reaction that ASPEN PLUS computes from the heats of formation at reference conditions. If this occurs, RStoic adjusts the calculated reactor heat duty to reflect the differences. Under these circumstances, the calculated reactor heat duty will not be consistent with the inlet and outlet stream enthalpies.

Selectivity The selectivity of the selected component P to the reference component A is defined as:

S P, A =

 ∆P   ∆A 

Real

 ∆P   ∆A  Ideal

Where: ∆P

=

Change in number of moles of component P due to reaction

∆A

=

Change in number of moles of component A due to reaction

In the numerator, real represents changes that actually occur in the reactor. ASPEN PLUS obtains this value from the mass balance between the inlet and outlet.

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Unit Operation Models Version 10

Chapter 5

In the denominator, ideal represents changes according to an idealized reaction scheme. This scheme assumes that no reactions are present, except for the reaction that produces the selected component from the reference component. Therefore, the denominator indicates how many moles of P are produced per mole of A consumed in an ideal stoichiometric equation, or:

υ  ∆P  = P  ∆A  υ Ideal A where υ A and υ P are stoichiometric coefficients. This example shows how RStoic calculates selectivity: a1 A + b1 B → c1 C + d1 D c2 C + e2 E → p2 P a3 A + f3 F → q3 Q The selectivity of P to A is:

 Moles of P produced   c1∗ p2  S P, A =  /   Moles of A consumed   a1∗ c2  In most cases, selectivity ranges between 0 and 1. However, if the selected component is also produced from components other than the reference component, selectivity may be greater than 1. If the selected component is consumed in other reactions, selectivity may be less than 0.

Unit Operation Models Version 10

5-5

Reactors

RYield Yield Reactor Use RYield to model a reactor when: • Reaction stoichiometry is unknown or unimportant • Reaction kinetics are unknown or unimportant • Yield distribution is known You must specify the yields (per mass of total feed, excluding any inert components) for the products or calculate them in a user-supplied Fortran subroutine. RYield normalizes the yields to maintain a mass balance. RYield can model one-, two-, and three-phase reactors.

Flowsheet Connectivity for RYield Material (any number)

Heat (optional)

Heat (optional)

Water (optional) Material

Material Streams Inlet

At least one material stream

Outlet One product stream

One water decant stream (optional)

Heat Streams Inlet

Any number of heat streams (optional)

Outlet One heat stream (optional)

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Unit Operation Models Version 10

Chapter 5

If you give only one specification on the Setup Specifications sheet (temperature or pressure), RYield uses the sum of the inlet heat streams as a duty specification. Otherwise, RYield uses the inlet heat stream(s) only to calculate the net heat duty. The net heat duty is the sum of the inlet heat streams minus the actual (calculated) heat duty. You can use an outlet heat stream for the net heat duty.

Specifying RYield Use the Setup Specifications and Setup Yield sheets to specify the reactor conditions and the component yields. For each reaction product, specify the yield as either moles or mass of a component per unit mass of feed. If you specify inert components on the Setup Yield sheet, the yields will be based on unit mass of non-inert feed. Calculated yields are normalized to maintain an overall material balance. For this reason, yield specifications establish a yield distribution, rather than absolute yields. RYield does not maintain atom balances because you enter the fixed yield distribution. You can request one-, two-, or three-phase calculation. When solids are created or changed by the reactions, you can specify their component attributes and/or particle size distribution in the outlet stream using the Setup Component Attr. and Setup PSD sheets, respectively. Use the following forms to enter specifications and view results for RYield:

Unit Operation Models Version 10

Use this form

To do this

Setup

Specify reactor operating conditions, component yields, inert components, flash convergence parameters, and PSD and component attributes for the outlet stream

UserSubroutine

Specify subroutine name and parameters for the user-supplied yield subroutine

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of operating results, mass and energy balances for the reactor and phase equilibrium results for the outlet stream

Dynamic

Specify parameters for dynamic simulations

5-7

Reactors

REquil Equilibrium Reactor Use REquil to model a reactor when: • Reaction stoichiometry is known and • Some or all reactions reach chemical equilibrium REquil calculates simultaneous phase and chemical equilibrium. REquil allows restricted chemical equilibrium specifications for reactions that do not reach equilibrium. REquil can model one- and two-phase reactors.

Flowsheet Connectivity for REquil Material (any number)

Material (vapor phase) Material (liquid phase)

Heat (optional)

Heat (optional)

Material Streams Inlet

At least one material stream

Outlet One material stream for the vapor phase

One material stream for the liquid phase

Heat Streams Inlet

Any number of heat streams (optional)

Outlet One heat stream (optional)

If you give only one specification on the REquil Input Specifications sheet (temperature or pressure), REquil uses the sum of the inlet heat streams as a duty specification. Otherwise, REquil uses the inlet heat stream(s) only to calculate the net heat duty. The net heat duty is the sum of the inlet heat streams minus the actual (calculated) heat duty. You can use an outlet heat stream for the net heat duty.

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Unit Operation Models Version 10

Chapter 5

Specifying REquil You must specify the reaction stoichiometry and the reactor conditions. If no additional specifications are given, REquil assumes that the reactions will reach equilibrium. REquil calculates equilibrium constants from the Gibbs energy. You can restrict the equilibrium by specifying one of the following: • •

The molar extent for any reaction A temperature approach to chemical equilibrium (for any reaction)

If you specify temperature approach, ∆T, REquil evaluates the chemical equilibrium constant at T + ∆T, where T is the reactor temperature (specified or calculated). REquil performs single-phase property calculations or two-phase flash calculations nested inside a chemical equilibrium loop. REquil cannot perform three-phase calculations. Use the following forms to enter specifications and view results for REquil: Use this form

To do this

Input

Specify reactor operating conditions, valid phases, reactions, convergence parameters, and solid and liquid entrainment in the vapor stream

Block Options

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of operating results, mass and energy balances, and calculated chemical equilibrium constants

Solids Reactions can include conventional solids. REquil treats each participating solid component as a separate pure solid phase, not as a component in a solid solution. Any participating solids must have a free energy formation (DGSFRM) and enthalpy of formation (DHSFRM), or heat capacity parameters (CPSXP1). Solids not participating in reactions, including any nonconventional components, are treated as inert. These solids have no effect on the equilibrium calculations except on the energy balance.

Unit Operation Models Version 10

5-9

Reactors

RGibbs Equilibrium Reactor (Gibbs Free Energy Minimization) RGibbs uses Gibbs free energy minimization with phase splitting to calculate equilibrium. RGibbs does not require that you specify the reaction stoichiometry. Use RGibbs to model reactors with: • Single phase (vapor or liquid) chemical equilibrium • Phase equilibrium (an optional vapor and any number of liquid phases) with no chemical reactions • Phase and/or chemical equilibrium with solid solution phases • Simultaneous phase and chemical equilibrium RGibbs can also calculate the chemical equilibria between any number of conventional solid components and the fluid phases. RGibbs also allows restricted equilibrium specifications for systems that do not reach complete equilibrium.

Flowsheet Connectivity for RGibbs Material (any number)

Heat (optional)

Material (any number)

Heat (optional)

Material Streams Inlet

At least one material stream

Outlet At least one material stream

If you specify as many outlet streams as the number of phases that RGibbs calculates, RGibbs assigns each phase to an outlet stream. If you specify fewer outlet streams, RGibbs assigns the additional phases to the last outlet stream.

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Unit Operation Models Version 10

Chapter 5

Heat Streams Inlet

Any number of heat streams (optional)

Outlet One heat stream (optional)

If you specify only pressure on the Setup Specifications sheet, RGibbs uses the sum of the inlet heat streams as a duty specification. Otherwise, RGibbs uses the inlet heat stream(s) only to calculate the net heat duty. The net heat duty is the sum of the inlet heat streams minus the actual (calculated) heat duty. You can use an outlet heat stream for the net heat duty.

Specifying RGibbs This section describes how to specify: • Phase equilibrium only • Phase and chemical equilibrium • Restricted chemical equilibrium • Reactions • Solids Use the following forms to enter specifications and view results for RGibbs:

Unit Operation Models Version 10

Use this form

To do this

Setup

Specify reactor operating conditions and phases to consider in equilibrium calculations, identify possible products, assign phases to outlet streams, specify inert components and specify equilibrium restrictions.

Advanced

Specify atomic formula of components, estimates for temperature and component flows, and convergence parameters.

Block Options

Override global values for physical properties, simulation options, diagnostic message levels and report options for this block.

Results

View summary of operating results, mass and energy balances, molar compositions of fluid and solid phases present, the atomic formula of components, and calculated reaction equilibrium constants.

Dynamic

Specify parameters for dynamic simulations

5-11

Reactors

Phase Equilibrium Only To specify

Use this option

On

Phase equilibrium calculations only

Phase Equilibrium Only

Setup Specifications sheet

Maximum number of fluid phases that RGibbs should consider

Maximum Number of Fluid Phases

Setup Specifications sheet

Maximum number of solid solution phases

Maximum Number of Solid Solution Phases

Solid Phases dialog box from the Setup Specifications sheet

RGibbs distributes all species among all solution phases by default. You can use the Setup Products sheet to assign different sets of species to each solution phase. You can also assign different thermodynamic property methods to each phase. If there is a possibility that a solid solution phase may exist, use the Setup Products sheet to identify the species that will exist in that phase.

Phase Equilibrium and Chemical Equilibrium To specify

Use this option

On

Chemical equilibrium calculations (with or without phase equilibrium)

Phase Equilibrium and Chemical Equilibrium

Setup Specifications sheet

Maximum number of fluid phases that RGibbs should consider

Maximum Number of Fluid Phases

Setup Specifications sheet

Maximum number of solid solution phases

Maximum Number of Solid Solution Phases

Solid Phases dialog box from the Setup Specifications sheet

By default, RGibbs considers all components entered on the Components Specifications Selection sheet as possible fluid phase or solid products. You can specify an alternate list of products on the Setup Products sheet. RGibbs distributes all solution species among all solution phases by default. You can use the Setup Products sheet to assign different sets of species to each solution phase. You can also assign different thermodynamic property methods to each phase.

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Unit Operation Models Version 10

Chapter 5

RGibbs needs the molecular formula for each component that is present in a feed or product stream. RGibbs retrieves this information from the component databanks. For non-databank components, use the Properties Molec-Struct Formula sheet to enter: • Atom (the atom type) • Number of occurrences (the number of atoms of each type) Alternatively, you can enter the atom matrix on the Advanced Atom Matrix sheet. The atom matrix defines the number of each atom in each component. If you enter the atom matrix, you must enter it for all components and atoms, including databank components. If there is a possibility that a solid solution phase may exist, use the Setup Products sheet to identify the species which will exist in that phase.

Restricted Chemical Equilibrium To restrict chemical equilibrium: Specify

On

The molar extent of the reaction

Edit Reactions dialog box (from the Setup RestrictedEquilibrium sheet)

A temperature approach to equilibrium for individual reactions

Edit Reactions dialog box (from the Setup RestrictedEquilibrium sheet)

A temperature approach to chemical equilibrium for the entire system

Edit Reactions dialog box (from the Setup RestrictedEquilibrium sheet)

The outlet amount of any component as total mole flow or as a fraction of the feed of that component

Setup Inerts sheet

†

†

You can specify inert components by setting the fraction to 1.

For temperature approach specifications, RGibbs evaluates the chemical equilibrium constant at T + ∆T , where T is the actual reactor temperature (specified or calculated) and ∆T is the desired temperature approach. You can enter one of the following restricted equilibrium specifications for individual reactions: • The molar extent of a reaction • The temperature approach for an individual reaction Use the Setup RestrictedEquilibrium sheet to supply the reaction stoichiometry. If you enter one of the preceding specifications, you must also supply the stoichiometry for a set of linearly independent reactions involving all components in the system.

Unit Operation Models Version 10

5-13

Reactors

Reactions You can have RGibbs consider only a specific set of reactions. You can restrict the chemical equilibrium by specifying temperature approach or molar extent for the reactions. You must specify the stoichiometric coefficients for a complete set of linearly independent chemical reactions, even if only one reaction is restricted. The number of linearly independent reactions required equals the total number of products in the product list, including solids (see the Setup Products sheet), minus the number of atoms present in the system. The reactions must involve all participating components. A component is participating if it satisfies these criteria: • It is in the product list. • It is not inert. A component is inert if it consists entirely of atoms not present in any other product components. • It has not been dropped. A component listed on the Setup Products sheet is dropped if it contains an atom not present in the feed.

Solids RGibbs can calculate the chemical equilibria between any number of conventional solid components and the fluid phases. RGibbs detects whether the solid is present at equilibrium, and if so, calculates the amount. RGibbs treats each solid component as a pure solid phase, unless it is specified as a component in a solid solution. Any solid that RGibbs considers a product must have both: • Free energy of formation (DGSFRM or CPSXP1) • Heat of formation (DHSFRM or CPSXP1) Nonconventional solids are treated as inert and have no effect on equilibrium calculations. If chemical equilibrium is not considered, RGibbs treats all solids as inert. RGibbs cannot perform solids-phase-only calculations. RGibbs places all pure solids in the last outlet stream unless you specify otherwise on the Setup AssignStreams sheet. RGibbs can handle only a single CISOLID substream, which contains all conventional solids products defined as pure solid phases. RGibbs places the solid solution phases in the MIXED substream of the outlet stream(s). RGibbs cannot directly handle phase equilibrium between solids and fluid phases (for example, water-ice equilibrium). To work around this, you can list the same component twice on the Components Specifications Selection sheet, with different component IDs. If you want RGibbs to calculate the chemical equilibrium between these components: • Specify both component IDs on the Setup Products sheet. • Designate one ID as a solids phase component, the other as a fluid phase component.

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Unit Operation Models Version 10

Chapter 5

References Gautam, R. and Seider, W.D., "Computation of Phase and Chemical Equilibrium," Parts I, II, and III, AIChE J. 25, 6, November, 1979, pp. 991-1015. White, C.W. and Seider, W.D., "Computation of Phase and Chemical Equilibrium: Approach to Chemical Equilibrium," AIChE J., 27, 3, May, 1981, pp. 446-471. Schott, G. L., "Computation of Restricted Equilibria by General Methods," J. Chem. Phys., 40, 1964.

Unit Operation Models Version 10

5-15

Reactors

RCSTR Continuous Stirred Tank Reactor RCSTR rigorously models continuous stirred tank reactors. RCSTR can model one-, two-, or three-phase reactors. RCSTR assumes perfect mixing in the reactor, that is, the reactor contents have the same properties and composition as the outlet stream. RCSTR handles kinetic and equilibrium reactions as well as reactions involving solids. You can provide the reaction kinetics through the built-in Reactions models or through a user-defined Fortran subroutine.

Flowsheet Connectivity for RCSTR Heat (optional)

Material (any number)

Material

Heat (optional)

Material Streams Inlet

At least one material stream

Outlet One material stream

Heat Streams Inlet

Any number of heat streams (optional)

Outlet One heat stream (optional)

If you specify only pressure on the Setup Specifications sheet, RCSTR uses the sum of the inlet heat streams as a duty specification. Otherwise, RCSTR uses the inlet heat stream only to calculate the net heat duty. The net heat duty is the sum of the inlet heat streams minus the actual (calculated) heat duty. You can use an outlet heat stream for the net heat duty.

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Chapter 5

Specifying RCSTR You must specify the reactor operating conditions, which are pressure and either temperature or heat duty. You must also enter the reactor volume or residence time (overall or phase). Use the following forms to enter specifications and view results for RCSTR: Use this form

To do this

Setup

Specify reactor operating conditions and holdup, select the reaction sets to be included, and specify PSD and component attributes in the outlet stream

Convergence

Provide estimates for component flow rates, reactor temperature and volume, and specify flash convergence parameters, RCSTR convergence methods and parameters, and initialization options

UserSubroutine

Specify parameters for the user-supplied kinetics subroutine and block-specific report option for the kinetics subroutine

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of operating results and mass and energy balances for the block

Dynamic

Specify parameters for dynamic simulations

Reactions You must specify reaction kinetics on the Reactions Reactions forms and select the Reaction Set ID on the Setup Reactions sheet. You can specify one-, two-, or three-phase calculations. You can specify the phase for each reaction on the Reactions Reactions forms. RCSTR can handle kinetic and equilibrium type reactions.

Phase Volume In a multi-phase reactor, by default, ASPEN PLUS calculates the volume of each phase, using phase equilibrium results, as:

V Pi = VR

Vi f i ΣV j f j

Where:

Unit Operation Models Version 10

VPi

=

Volume of phase i

VR

=

Reactor volume

Vi

=

Molar volume of phase i

fi

=

Molar fraction of phase i

5-17

Reactors

You can override the default calculation by specifying the volume of a phase directly (Phase Volume) or as a fraction of the reactor volume (Phase Volume Frac) on the Setup Specifications sheet. Alternatively, when you specify the residence time of a phase in the reactor, ASPEN PLUS calculates the phase volume iteratively.

Residence Time ASPEN PLUS calculates the residence time (overall and phase) in the CSTR as: RT =

VR F * Σfi Vi

RTi =

V Pi F * f iVi

Where: RT

=

Overall residence time

RTi

=

Residence time of phase i

VR

=

Reactor volume

F

=

Total molar flow rate (outlet)

Vi

=

Molar volume of phase i

fi

=

Molar fraction of phase i

VPi

=

Volume of phase i

When the default calculation for phase volume, based on phase equilibrium results, is used, the phase residence time is equal for all phases. If you specify Phase Volume or Phase Volume Frac on the Setup Specifications sheet, the residence time for the phase specified in the Holdup Phase is calculated with the specified phase volume rather than the default phase volume.

Solids RCSTR can handle reactions involving solids. RCSTR assumes that solids are at the same temperature as the fluid phase. RCSTR cannot perform solids-phaseonly calculations.

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Unit Operation Models Version 10

Chapter 5

Scaling of Variables Four types of variables are predicted by RCSTR: component flow rates, stream enthalpy, component attributes and PSD (if present). RCSTR normalizes these variables, for faster convergence, by dividing each one by a scale factor. Two types of scaling are available in RCSTR: component-based scaling and substream-based scaling. Component-based scaling weighs each variable against its previous or estimated value. Substream-based scaling weighs each variable in a substream against the substream flow rate. For component-based scaling, minimum scale values are set by the Trace Scaling Factor in the Advanced Parameters dialog box (from the Convergence Parameters sheet). You may reduce the trace scaling threshold to increase the prediction accuracy of trace components. Component-based scaling generally provides more accuracy than substreambased scaling, especially for trace components. Use component-based scaling when: • The reaction network involves trace intermediates • The reaction rates are very sensitive to trace reactants (such as catalysts and initiators which participate in degradation reactions) The following tables summarize the scale factors used by each method.

Substream-based Scaling Method Variable Type

Variable

Initial Scale Factor

Component Flows

Component mole flow in outlet stream

Estimated outlet substream mole flow rate

Stream Enthalpy

Net enthalpy flow of outlet stream

Net enthalpy flow of inlet stream

Component Attributes (attr/kg)

Product of component mass flow (with attributes) and attribute value in outlet stream

Default attribute scale factor

PSD

Product of substream mass flow rate (with PSD) and PSD value in outlet stream

Default attribute scale factor

Note

Unit Operation Models Version 10

If any substream-based scaling factor is equal to zero, the default scaling factor is used instead (the default factor is 1.0 for component flow rates and 1.0E5 for stream enthalpy).

5-19

Reactors

Component-based Scaling Method Variable Type

Variable

Initial Scale Factor

Component Flows

Component mole flow in outlet stream

Larger of: - Estimated component mole flow in outlet stream - Product of Trace threshold and estimated outlet substream mole flow

Stream Enthalpy

Net enthalpy flow of outlet stream

Net enthalpy flow of inlet stream

Component Attributes (attr/kg)

Product of component mass flow with attributes and attribute value in outlet stream

Larger of: - Product of estimated attributed component mass flow and estimated attribute value in outlet stream - Product of Trace threshold and estimated outlet substream mole flow

PSD

Product of substream mass flow rate and PSD value in outlet stream

Larger of: - Product of estimated substream mass flow with PSDs and estimated PSD value in outlet stream - Product of Trace threshold and default attribute scale factor

5-20

Unit Operation Models Version 10

Chapter 5

RPlug Plug Flow Reactor RPlug is a rigorous model for plug flow reactors. RPlug assumes that perfect mixing occurs in the radial direction and that no mixing occurs in the axial direction. RPlug can model one-, two-, or three-phase reactors. You can also use RPlug to model reactors with coolant streams (co-current or counter-current). RPlug handles kinetic reactions, including reactions involving solids. You must know the reaction kinetics when you use RPlug to model a reactor. You can provide the reaction kinetics through the built-in Reactions models or through a user-defined Fortran subroutine.

Flowsheet Connectivity for RPlug Heat (optional)

Material

Material

Flowsheet Reactor without Coolant Stream

Material Coolant (optional)

Material

Material

Material Coolant (optional)

Flowsheet Reactor with Coolant Stream

Unit Operation Models Version 10

5-21

Reactors

Material Streams Inlet

One material feed stream One coolant stream (optional)

Outlet One material product stream

One coolant stream (optional)

Heat Streams Inlet

No inlet heat streams

Outlet One heat stream (optional) for the reactor heat duty. Use the heat outlet

stream only for reactors without a coolant stream.

Specifying RPlug Use the Setup Configuration sheet to specify reactor tube length and diameter. If the reactor consists of multiple tubes, you can also specify the number of tubes. You can specify the pressure drop across the reactor on the Setup Pressure sheet. Additional required input for RPlug depends on the reactor type. When you use this Reactor Type

And solid phase is

And fluid and solid phase temperatures are

Reactor with specified temperature

—

—

Reactor temperature, or temperature profile

Adiabatic reactor

Not present

—

No required specifications

Present

Same

No required specifications

Present

Different

U (fluid phase - solids phase)

Not present

—

Coolant temperature, and U (coolant - process stream)

Present

Same

Coolant temperature, and U (coolant - process stream)

Present

Different

Coolant temperature, U (coolant - fluid phase), U (coolant - solids phase), and U (fluid phase - solids phase)

Not present

—

U (coolant - process stream)

Present

Same

U (coolant - process stream)

Reactor with constant coolant temperature

Reactor with co-current coolant

Specify

continued

5-22

Unit Operation Models Version 10

Chapter 5

When you use this Reactor Type Reactor with co-current coolant

Reactor with counter-current coolant

And solid phase is

And fluid and solid phase temperatures are

Specify

Not present

—

U (coolant - process stream)

Present

Same

U (coolant - process stream)

Present

Different

U (coolant - fluid phase), U (coolant - solids phase), and U (fluid phase - solids phase)

Not present

—

Coolant outlet temperature or molar vapor fraction, and U (coolant - process stream)

Present

Same

Coolant outlet temperature or molar vapor fraction, and U (coolant - process stream)

Present

Different

Coolant outlet temperature or molar vapor fraction, U (coolant - fluid phase), U (coolant - solids phase), and U (fluid phase - solids phase)

For reactors with countercurrent external coolant, RPlug calculates the coolant inlet temperature. The result overrides your specified inlet coolant temperature. You can use a design specification that manipulates the coolant exit temperature or vapor fraction to achieve a specified coolant inlet temperature. For reactors with an external coolant stream, you can use different physical property methods and options (BlockOptions Properties sheet) for the process stream and the coolant stream. Use the following forms to enter specifications and view results for RPlug: Use this form

To do this

Setup

Specify operating conditions and reactor configuration, select reaction sets to be included, and specify pressure drops

Convergence

Specify flash convergence parameters, calculation options and parameters for the integrator

Report

Specify block-specific report options

UserSubroutine

Specify user subroutine parameters for kinetics, heat transfer, pressure drop, and list user variables to be included in the profile report

BlockOptions

Override global values for property methods, simulation options, diagnostic levels, and report options for this block continued

Unit Operation Models Version 10

5-23

Reactors

Use this form

To do this

Results

View summary of operating results and mass and energy balances for the block

Profiles

View profiles versus reactor length for process stream conditions, coolant stream conditions, properties, component and substream attributes, and user variables

Dynamic

Specify parameters for dynamic simulations

Reactions You must specify reaction kinetics on the Setup Reactions sheet, by referring to Reaction IDs that you select. You can specify one-, two-, or three-phase calculations. Specify the reaction phases on the Reactions Reactions forms. RPlug can handle only kinetic type reactions.

Solids Reactions can involve solids. Solids can be: • At the same temperature as the fluid phases • At a different temperature from the fluid phases (only for Reactor Types other than the reactor with specified temperature) In the latter case, you must specify the heat transfer coefficients on the Setup Specifications sheet.

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Unit Operation Models Version 10

Chapter 5

RBatch Batch Reactor RBatch is a rigorous model for batch or semi-batch reactors. Use RBatch when you know the kinetics of the reactions taking place. You can specify any number of continuous feed streams. A continuous vent is optional. The reaction runs until it reaches a stop criterion that you specify. Batch operations are unsteady-state processes. RBatch uses holding tanks and your specified cycle times to provide an interface between the discrete operations of the batch reactor and the continuous streams used by other models. RBatch can model one-, two-, or three-phase reactors.

Flowsheet Connectivity for RBatch Batch charge

Heat (optional) Vent (optional)

Continuous feed (any number)

Product

Material Streams Inlet

One batch charge stream (required) One or more continuous feed streams for semi-batch reactors (optional)

Outlet One product stream (required)

One vent stream for semi-batch reactors (optional)

Heat Streams Inlet

No inlet heat streams

Outlet One heat stream (optional)

Unit Operation Models Version 10

5-25

Reactors

Specifying RBatch Use the Setup Specifications sheet to specify the reactor conditions. Use the Setup Operations sheet to specify: • One or more stop criteria • Either a feed time or a batch cycle time Other required input for RBatch depends on reactor type. To establish the pressure of the vessel, enter one of the following specifications on the Setup Specifications sheet: • Constant pressure • Pressure profile • Reactor volume Use the Setup ContinuousFeeds sheet to enter mass flow rates for the continuous feeds at any number of points in time. You can thus simulate delayed feeds and step changes in feeds. For specified duty reactors, you can specify either a constant heat duty or a heat duty profile. For a reactor with constant duty, RBatch assumes adiabatic operation if you do not specify a heat duty. For reactors with specified coolant temperature, you must specify: • Coolant temperature • An over-all heat transfer coefficient • Total heat transfer area For constant temperature and specified temperature reactors, RBatch handles the temperature specification in one of the following ways: • By assuming perfect control • By interpreting the specified temperature(s) as the setpoint(s) of a PID controller Use the following forms to enter specifications and view results for RBatch: Use this form

To do this

Setup

Specify operating conditions, select reaction sets to be included, specify operation stop criteria, operation times, continuous feeds, and controller parameters

Convergence

Specify convergence parameters for flash calculations, integration, and pressure calculations

Report

Specify block-specific report options for profiles and reactor, vent, and vent accumulator property profiles

UserSubroutine

Specify parameters for the user kinetics subroutine, name and parameters for the user heat transfer subroutine, and user variables for the profile report. continued

5-26

Unit Operation Models Version 10

Chapter 5

Use this form

To do this

Block Options

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of block operating results and mass and energy balances

Profiles

View time profiles of reactor conditions, compositions, continuous feed stream flows, properties, component attributes, and user variables

Controller RBatch assumes perfect control when one of these conditions exists: • Pressure in the reactor is converged upon (that is, reactor volume is specified) • A single-phase batch reactor is used with no continuous feed streams If RBatch cannot assume perfect control, it interprets the specified temperature(s) as the setpoint(s) of a PID controller. This interpretation occurs when: • A two-phase reactor is used • RBatch performs pressure convergence calculations (that is, reactor volume is specified) • Continuous feeds are present during semi-batch operation Use the Setup Controllers sheet to specify the controller tuning parameters. The controller equation is: t  d (T − T s )  s s Q = M c  K (T − T ) + ( K / I ) ∫ (T − T )dt + KD  dt 0  

Where: Q

=

Reactor heat duty (J/sec)

Mc

=

Reactor charge (kg)

K

=

Proportional gain (J/kg/K)

T

=

Reactor temperature (K)

Ts

=

Temperature set point (K)

I

=

Integral time (sec)

D

=

Derivative time (sec)

t

=

Time (sec)

The gain factor is a specific gain per unit mass.

Unit Operation Models Version 10

5-27

Reactors

Reactions Reactions may or may not be present in RBatch. If they are, you must include the Reaction Set IDs on the Setup Reactions sheet. You can specify one-, two-, or three-phase calculations. You specify the reaction phases on the Reactions Reactions forms. RBatch can only handle kinetic type reactions.

Specifying Stop Criteria A reaction runs until one of your specified stop criteria reached. A stop criterion can be one of the following: • Reaction time • Reactor composition • Vent accumulator or continuous vent composition • Conversion of a component • Amount of material in the reactor or vent accumulator • Vent flow rate • Temperature in the reactor • Vapor fraction in the reactor • Any property specified on the Properties Prop-Sets Properties sheet As the stop criterion variable approaches its cut-off from above or below, you can specify whether or not RBatch should halt the reaction. If you specify more than one stop criterion, RBatch halts the reaction as soon as one of the criteria is reached. In addition, you must specify a halt time for the reaction. If the reaction does not reach the specified stop criteria by this time, RBatch halts the reaction.

Cycle Time You can specify a reactor cycle time. Or, you can let RBatch calculate it from your specified reaction and down times for draining, cleaning, and charging the reactor. If you do not specify reactor cycle time, then specify a feed cycle time. RBatch uses this time to determine the batch charge, because the reaction time is not known at the beginning of block execution. Note

If the reactor batch charge stream is in a recycle loop, you must specify the reactor cycle time.

Mass Balances Because RBatch uses different cycle times to calculate time-averaged flows, RBatch may not maintain a mass balance around the block. For example, suppose you specify a feed time of 30 minutes, but the down time plus the calculated value reaction time equals 45 minutes. The resulting net mass flow from the reactor is less than the charge flow by a factor of 45/30=1.5.

5-28

Unit Operation Models Version 10

Chapter 5

Remember that the mass balance pertains to the time-averaged inlet and outlet continuous streams. RBatch always satisfies a mass balance for its own internal batch computations. If there is no continuous feed stream, the mass balance around RBatch closes only if the cycle time is specified. This ensures that the same time is used for averaging the batch change and product streams. If there is a continuous feed stream, and it is not time-varying, the mass balance closes only if the cycle time is specified, and the specified value is equal to the calculated reaction time. In all other cases, the mass balance around RBatch does not close, although the compositions, temperature, and so on are correct.

Batch Operation RBatch can operate in a batch or in semi-batch mode. The reactor mode is determined by the streams you enter on the flowsheet. A semi-batch reactor can have a vent product stream, one or more continuous feed streams, or both. The vent product stream exits a vent accumulator. It does not exit the reactor itself. The vent accumulator is for the continuous (but time-varying) vapor vent leaving the reactor. The composition and temperature of each continuous feed stream remain constant throughout the reaction. The flow rate also remains constant, unless you specify a time profile for the flow rate of a continuous stream. Batch operations are unsteady-state processes. Variables like temperature, composition, and flow rate change with time, in contrast to steady-state processes. To interface RBatch with a steady-state flowsheet, it is necessary to use time-averaged streams. Four types of streams are associated with RBatch, as follows: Batch Charge The material transferred to the reactor at the start of the reactor cycle. The mass of the batch charge equals the flow rate of the batch charge stream, multiplied by the feed cycle time. The mass of the batch charge is equivalent to accumulating the batch charge stream in a holding tank during a reactor cycle. The contents of the holding tank are transferred to the reactor at the beginning of the next cycle . (See figure RBatch Reactor Configuration - No Vent Case.) To compute the amount of the batch charge, RBatch multiplies the flowsheet stream representing the batch charge by a cycle time you enter (either Cycle Time or Batch Feed Time). Batch Feed Time is not the time required to charge the reactor; it is a total cycle time used only to compute the amount of the charge. Batch Feed Time is required when Cycle Time is unknown. If Batch Feed Time differs from the actual computed cycle time, the RBatch flowsheet inlet and outlet streams are not in mass balance. However, all internal RBatch calculations and reports will be correct for the computed batch charge.

Unit Operation Models Version 10

5-29

Reactors

Continuous Feed A steady-state flowsheet stream fed continuously to the reactor during reaction. Its composition and temperature remain constant throughout the reaction. Its flow rate either remains constant or follows a specified time profile. Reactor Product The material left in the reactor at the end of the reactor cycle. The flow rate of the reactor product stream equals the total mass in the reactor, divided by the reactor cycle time. You can think of this process as analogous to transferring the reactor product to a product holding tank. This tank is drawn down during the next reactor cycle to feed the continuous blocks downstream (see figure RBatch Reactor Configuration - No Vent Case ). Vent Product The contents of the vent accumulator at the end of the reactor cycle. During the reactor cycle, the time-varying vent stream accumulates in the vent accumulator (see figure RBatch Reactor Configuration - Vent Case). The flow rate of the vent product stream is the total mass in the vent accumulator, divided by the reactor cycle time.

Feed Holding Tank Flowsheet Stream for Batch Charge

Batch charge transferred once each Reactor cycle

Product Holding Tank

Reactor product transferred once each cycle

Flowsheet Stream for Reactor Product

RBatch Reactor Configuration—No Vent Case

5-30

Unit Operation Models Version 10

Chapter 5

Feed Holding Tank

Vent Accumulator

Flowsheet Stream for Batch Charge Batch charge transferred once each Reactor cycle

Vent Holding Tank Vent Flowsheet Product Stream for transferred Vent once per Product cycle Product Holding Tank

Reactor product transferred once each cycle

Optional Flowsheet Stream for Continuous Feed

Flowsheet Stream for Reactor Product

RBatch Reactor Configuration—Vent Case

❖

Unit Operation Models Version 10

❖

❖

❖

5-31

Reactors

5-32

Unit Operation Models Version 10

Chapter 6

6

Pressure Changers This chapter describes the unit operation models for pumps and compressors, and models for calculating pressure change through pipes and valves. The models are: Model

Description

Purpose

Use For

Pump

Pump or hydraulic turbine

Changes stream pressure when the power requirement is needed or known

Pumps and hydraulic turbines

Compr

Compressor or turbine

Changes stream pressure when power requirement is needed or known

Polytropic compressors, polytropic positive displacement compressors, isentropic compressors, isentropic turbines

MCompr

Multistage compressor or turbine

Changes stream pressure across multiple stages with intercoolers. Allows for liquid knockout streams from intercoolers

Multistage polytropic compressors, polytropic positive displacement compressors, isentropic compressors, isentropic turbines

Valve

Valve pressure drop

Models pressure drop through a valve

Control valves and pressure changers

Pipe

Single segment pipe

Models pressure drop through a single segment of pipe

Pipe with constant diameter (may include fittings)

Pipeline

Multiple segment pipeline

Models pressure drop through a pipe or annular space

Pipeline with multiple lengths of different diameter or elevation

Use Pump, Compr, and MCompr models when energy-related information such as power requirement is needed or known.

Unit Operation Models Version 10

6-1

Pressure Changers

Pump Pump/Hydraulic Turbine Use Pump to model a pump or a hydraulic turbine. Pump is designed to handle a single liquid phase. For special cases, you can specify two- or three-phase calculations to determine the outlet stream conditions and to compute the fluid density used in the pump equations. The accuracy of the results depends on a number of factors, such as the relative amounts of the phases present, the compressibility of the fluid, and the efficiency specified. Use Pump to change pressure when the power requirement is needed or known. For pressure change only, you can use other models such as Heater. Pump can model pumps and hydraulic turbines. Use the Pump block to rate a pump or a turbine by specifying scalar parameters or by specifying the related performance curves. To use the performance curves, you can specify either: • •

Dimensional curves such as head versus flow or power versus flow Dimensionless curves such as head coefficient versus flow coefficient

Flowsheet Connectivity for Pump Work (optional)

Material Material (any number) Water (optional)

Work (optional)

Material Streams Inlet

At least one material stream

Outlet One material stream

One water decant stream (optional)

6-2

Unit Operation Models Version 10

Chapter 6

Work Streams Inlet

Any number of work streams (optional)

Outlet One work stream for the net work load (optional)

If you do not specify either power or pressure on the Setup Specifications sheet, Pump uses the sum of the inlet work streams as a power specification. Otherwise, Pump uses the inlet work stream(s) only to calculate the net work load. The net work load is the sum of the inlet work streams minus the actual (calculated) work load. You can use an optional outlet work stream for the net work load.

Specifying Pump Use the Setup Specifications sheet for Pump specifications. If you specify

Pump calculates

Discharge pressure

Power required or produced

Pressure increase (for a pump) or decrease (for a turbine)

Power required or produced

Pressure ratio (outlet pressure to inlet pressure)

Power required or produced

Power required (for a pump) or produced (for a turbine)

Discharge pressure

Curves of head, discharge pressure, pressure ratio, pressure change, or head coefficient

Power required or produced

Power curve

Discharge pressure

You can supply a Fortran subroutine to calculate performance curves in Pump. See ASPEN PLUS User Models for more information. Use the following forms to enter specifications and view results for Pump:

Unit Operation Models Version 10

Use this form

To do this

Setup

Specify operating conditions, efficiencies, net positive suction head parameters, specific speed parameters, valid phases, and flash convergence parameters

PerformanceCurves

Specify parameters and enter data for the performance curves

UserSubroutines

Specify name and parameters for the user performance curve subroutine

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of Pump results, material and energy balance results, and performance curve summary

6-3

Pressure Changers

NPSH Available The Net Positive Suction Head (NPSH) available for a pump is defined as:

NPSHA = Pin − Pvapor + H v + H s Where:

NPSHA

=

Net Positive Suction Head Available

Pin

=

Inlet pressure

Pvapor

=

Vapor pressure of the liquid at inlet conditions

Hv

=

Velocity head (= u 2 / 2 g , u is the velocity and g is gravitation constant)

Hs

=

Hydraulic static head corrected to the pump centerline

The NPSH available has to be greater than the NPSH required (NPSHR) to avoid cavitation. NPSH required is a function of pump design.

NPSH Required The Net Positive Suction Head (NPSH) required can be considered the suction pressure required by the pump for safe, reliable operation. The NPSHR can be specified using the performance curves on the PerformanceCurves NPSHR sheet, or calculated from the following empirical equation by specifying suction specific speed ( N ss ) on the Setup CalculationOptions sheet.

 N Q 0.5  NPSHR =    N ss 

4

3

Where:

6-4

NPSHR

=

Net Positive Suction Head Required

N

=

Pump shaft speed (rpm)

Q

=

Volumetric flow rate at the suction conditions

N ss

=

Suction specific speed

Unit Operation Models Version 10

Chapter 6

The units for Q and NPSHR are: US:

Q in gal/min and NPSHR in feet

Metric:

Q in cum/hr and NPSHR in meters

Specific Speed Specific speed and suction specific speed are two important parameters that define the suitability of a pump design for its intended conditions. The pump specific speed is defined as:

Ns =

N Q 0.5 Head 0.75

Where:

Head

= Head developed across the pump

Ns

= Specific speed

N

= Pump shaft speed (rpm)

Q

= Volumetric flow rate at the suction conditions

The units for Q and Head are:

Unit Operation Models Version 10

US:

Head in feet

Metric:

Head in meters

6-5

Pressure Changers

In general, pumps with a low specific speed are termed low capacity and those with a high specific speed are termed high capacity. For a turbine, the specific speed is defined as follows:

Ns =

N BHP 0.5 Head 1.25

Where:

Ns

=

Specific speed

BHP

=

Developed horsepower

Head

=

Total dynamic head across turbine

Suction Specific Speed Suction specific speed ( N ss ) is an index number for a centrifugal pump and is used to define its suction characteristic. It is defined as follows:

N ss =

N Q 0.5 NPSHR 0.75

Where:

NPSHR

=

Net positive suction head required for a pump or net positive discharge head required for a turbine

N ss

=

Suction specific speed

N

=

Pump shaft speed (rpm)

Q

=

Volumetric flow rate at the suction conditions

The units for Q and NPSHR are:

6-6

US:

Q in gal/min and NPSHR in feet

Metric:

Q in cum/hr and NPSHR in meters

Unit Operation Models Version 10

Chapter 6

Suction specific speed is a criterion of a pump’s performance with regard to cavitation. For a pump of normal design, values of N ss vary from 6,000 to 12,000 in US units. A typical value is 8,500.

Head Coefficient Head coefficient is defined as follows:

Headc =

Head g u2

Where:

Headc

=

Head coefficient

Head

=

Head developed across the pump

g

=

Gravitational constant

u

=

Impeller tip speed

Flow Coefficient Flow coefficient is the ratio of discharge throat velocity to impeller tip speed. It is defined as:

Flowc =

Q A1 u

A1 = π × d 12 / 4 Where:

Unit Operation Models Version 10

Flowc

=

Flow coefficient

Q

=

Volumetric flow rate

A1

=

Cross-sectional area of discharge throat

d1

=

Diameter of discharge throat

u

=

Impeller tip speed

6-7

Pressure Changers

The diameter of throat and diameter of impeller are related by the following empirical equation:

N s = 5500

d1 Diam

Where:

Ns

=

Specific speed at the best efficiency point

Diam

=

Diameter of impeller

You can specify Specific Speed ( N s ) on the Setup CalculationOptions sheet.

6-8

Unit Operation Models Version 10

Chapter 6

Compr Compressor/Turbine Use Compr to model: • A polytropic centrifugal compressor • A polytropic positive displacement compressor • An isentropic compressor • An isentropic turbine Use Compr to change stream pressure when energy-related information, such as power requirement, is needed or known. Compr can handle single-phase as well as two- and three-phase calculations. You can use Compr to rate a single stage of a compressor or a single wheel of a compressor, by specifying the related performance curves. Compr allows you to specify either: • Dimensional curves, such as head versus flow or power versus flow • Dimensionless curves, such as head coefficient versus flow coefficient Compr can also calculate compressor shaft speed. Compr cannot handle performance curves for a turbine.

Flowsheet Connectivity for Compr Material (any number)

Work (optional)

Work (optional)

Water (optional) Material

Material Streams Inlet

At least one material stream

Outlet One material stream

One water decant stream (optional)

Unit Operation Models Version 10

6-9

Pressure Changers

Work Streams Inlet

Any number of work streams (optional)

Outlet One work stream for net work load (optional)

If you do not specify either power or pressure on the Compr Setup Specifications sheet, Compr uses the sum of the inlet work streams as a power specification. Otherwise, Compr uses the inlet work stream(s) only to calculate the net work load. The net work load is the sum of the inlet work streams minus the actual (calculated) work load. You can use an optional outlet work stream for the net work load.

Specifying Compr If you specify

Compr calculates

Discharge pressure

Power required or produced

Power required (for a compressor) or produced (for a turbine)

Discharge pressure

Curves of head, power, discharge pressure, pressure ratio, pressure change, or head coefficient

Power required and discharge pressure

Discharge pressure and curves of head or power or head coefficient

Power required, discharge pressure, and shaft speed

Power required and curves of discharge pressure, pressure ratio, or pressure change

Discharge pressure, and shaft speed

When you use performance curves, you can specify either a scalar value of efficiency or efficiency curves. You can supply a Fortran subroutine to calculate performance curves in Compr. See ASPEN PLUS User Models for more information. Some required specifications depend on the compressor type. Specify the compressor type on the Setup Specifications sheet. You can model a polytropic compressor using either the GPSA or ASME method. You can model an isentropic compressor/turbine using either the GPSA, ASME, or Mollier-based methods. The GPSA method can be based on either: • Suction conditions • Average of suction and discharge conditions

6-10

Unit Operation Models Version 10

Chapter 6

The ASME method is more rigorous than the GPSA method for polytropic or isentropic compressor calculations. The Mollier method is the most rigorous for isentropic calculations. Use the following forms to enter specifications and view results for Compr: Use this form

To do this

Setup

Identify compressor specifications, calculation options, convergence parameters, and valid phases

Performance Curves

Specify parameters and enter data for the performance curves

User Subroutine

Enter performance curve subroutine parameters and name

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of Compr results, material and energy balance results, and performance curve summary

Dynamic

Specify parameters for dynamic simulations

Polytropic Efficiency The polytropic efficiency η p is used in the equation for the polytropic compression ratio:

n − 1  k − 1 =  η  k  p n The basic compressor relation is: n −1   Pin Vin  Pout  n ∆h =   − 1  n − 1  Pin   ηp     n  

Where: n k

Unit Operation Models Version 10

ηp

= = =

Polytropic coefficient Heat capacity ratio Cp/Cv Polytropic efficiency

∆h

=

Enthalpy change per mole

P V

= =

Pressure Molar volume

6-11

Pressure Changers

Isentropic Efficiency There are two equations for the isentropic efficiency ηs For compression:

ηs =

s hout − hin hout − hin

For expansion:

ηs =

hout − hin s hout − hin

Where : h s hout

= =

Molar enthalpy Outlet molar enthalpy assuming isentropic compression or expansion to the specified outlet pressure

Mechanical Efficiency Mechanical efficiency ηm is used to calculate the brake horsepower:

IHP = F∆h

BHP = IHP / ηm Where: IHP F

∆h BHP

ηm

6-12

= = =

Indicated horsepower Mole flow rate Enthalpy change per mole

= =

Brake horsepower Mechanical efficiency

Unit Operation Models Version 10

Chapter 6

MCompr Multistage Compressor/Turbine Use MCompr to model: • A multi-stage polytropic compressor • A multi-stage polytropic positive displacement compressor • A multi-stage isentropic compressor • A multi-stage isentropic turbine For polytropic compressors, MCompr can handle a single, compressible phase. For special cases you can specify two- or three-phase calculations. These calculations determine the outlet stream conditions and the properties used in the compressor equations. The accuracy of results depends primarily on the relative amounts of the phases present and the efficiency specified. The rigorous polytropic compressor uses real fluid properties calculated from the property method you specify. It does not assume ideal gas behavior. MCompr handles single-phase isentropic compressors and turbines. MCompr can also handle two- and three-phase mixtures. You can use MCompr to rate a multi-stage compressor, by using either: • Stage-by-stage dimensional performance curves, such as head versus flow or power versus flow • Wheel-by-wheel dimensionless performance curves, such as head coefficient versus flow coefficient MCompr can also calculate shaft speed. MCompr cannot handle performance curves for a turbine.

Flowsheet Connectivity for MCompr Work (any number) From Stage K-1

Feed to Heat (any number) Stage K+1 (any number)

Cooler Compressor

Stage K

To Stage K+1 Heat (optional)

Stage K Work (optional) Stage K

Unit Operation Models Version 10

Knockout Water (optional)

6-13

Pressure Changers

Material Streams Inlet

At least one material stream for the first compressor stage One or more material streams for stages after the first (optional). These streams enter the intercooler before the stages you specify.

Outlet One material stream leaving the last compressor stage

Either one optional knockout material stream for each intercooler for the liquid formed, or one optional global knockout for the liquid formed in all intercoolers Either one optional water decant stream for each intercooler, or one optional global water decant stream If you use liquid knockout outlet streams from one stage, you must use them for all stages. The last stage cannot have a liquid knockout material stream or a water decant stream.

Heat Streams Inlet

Any number of heat streams to each intercooler (optional)

Outlet Either one optional heat stream for the net heat load of each intercooler,

or one global heat outlet stream for the net heat duty for all intercoolers If you do not specify cooler conditions on the Setup Cooler sheet, MCompr adds the heat streams together and uses the total as a duty specification for the cooler. The net heat load equals the heat in the inlet heat streams minus the actual (calculated) heat duty. If you use a heat outlet from one stage, you must use one for all stages.

Work Streams Inlet

Any number of work streams to each compressor stage (optional)

Outlet Either one optional work stream for net work load, or one global work

stream for the net power for all compressor stages MCompr adds all work inlet streams together to provide the power requirement. If you do not specify power or pressure on the Setup Specs sheet, MCompr uses the total power as a power specification for the stage. The power in the outlet work stream equals the power in the inlet work streams minus the actual (calculated) power required. If you use a work outlet from one stage, you must use one for all stages.

6-14

Unit Operation Models Version 10

Chapter 6

Specifying MCompr If you specify

MCompr calculates

Discharge pressure

Power required or produced

Power required (for a compressor) or produced (for a turbine)

Discharge pressure

Curves of head, power, discharge pressure, pressure ratio, pressure change, or head coefficient

Power required and discharge pressure

Discharge pressure and curves of head or power or head coefficient

Power required and shaft speed

When you use performance curves, you can specify either a scalar value for efficiency or efficiency curves. You can supply a Fortran subroutine to calculate performance curves in MCompr. See ASPEN PLUS User Models for more information. MCompr can have an intercooler between each compression (or expansion) stage, and an aftercooler after the last stage. You can perform one-, two-, or three-phase flash calculations in the intercoolers. Each cooler can have a liquid knockout stream, except the cooler after the last stage. You can model a polytropic compressor using either the GPSA1 or ASME2 method. You can model an isentropic compressor/turbine using either the GPSA, ASME, or Mollier-based methods. The GPSA method can be based on either: • Suction conditions • Average of suction and discharge conditions The ASME method is more rigorous than the GPSA method for polytropic or isentropic compressor calculations. The Mollier method is the most rigorous for isentropic calculations. Use the following forms to enter specifications and view results for MCompr: Use this form

To do this

Setup

Identify multi-stage compressor specifications, stage specifications, cooler specifications, convergence parameters, and valid phases

Performance Curves

Specify parameters and enter data for the performance curves

User Subroutine

Specify performance curve user subroutine parameters and name

Hcurves

Specify heating or cooling curve tables and view tabular results continued

Unit Operation Models Version 10

6-15

Pressure Changers

Use this form

To do this

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of operating results, material and energy balance results, compressor and cooler profiles, and performance profiles

Dynamic

Specify parameters for dynamic simulations

Polytropic Efficiency The polytropic efficiency η p is used in the equation for the polytropic compression ratio:

n − 1  k − 1 =  η  k  p n The basic compressor relation is:

Pin Vin ∆h = n − 1 η p    n 

n −1   n   P  out  − 1  Pin    

Where: n

=

Polytropic coefficient

k

=

Heat capacity ratio Cp/Cv

ηp

=

Polytropic efficiency

∆h

=

Enthalpy change per mole

P

=

Pressure

V

=

Molar volume

Isentropic Efficiency There are two equations for the isentropic efficiency η s For compression:

ηs =

6-16

s hout − hin hout − hin

Unit Operation Models Version 10

Chapter 6

For expansion:

ηs =

hout − hin s hout − hin

Where : h

=

Molar enthalpy

s hout

=

Outlet molar enthalpy assuming isentropic compression or expansion to the specified outlet pressure

Mechanical Efficiency Mechanical efficiency ηm is used to calculate the brake horsepower:

IHP = F∆h

BHP = IHP / ηm Where: IHP

=

Indicated horsepower

F

=

Mole flow rate

∆h

=

Enthalpy change per mole

BHP

=

Brake horsepower

ηm

=

Mechanical efficiency

Parasitic Pressure Loss The parasitic pressure loss at the suction of a stage is calculated using the equation:

V2 ∆P = Kρ 2 Where:

Unit Operation Models Version 10

∆P

=

Parasitic pressure loss

K

=

Velocity head multiplier

ρ

=

Density

V

=

Linear velocity of process gas at suction conditions

6-17

Pressure Changers

Specific Speed The specific speed is defined as:

ShSpd (VflIn) 0.5 SpSpd = (Head) 0.75 Where: ShSpd

=

Shaft speed

VflIn

=

Suction volumetric flow rate

Head

=

Head developed

Specific Diameter The specific diameter is defined as:

ImpDiam (Head) 0.25 SpDiam = (VflIn) 0.5 Where: ImpDiam

=

Impeller diameter of compressor wheel

Head

=

Head developed

VflIn

=

Volumetric flow rate at suction conditions

Head Coefficient The head coefficient is defined as:

Hc =

Head g ( π ShSpd ImpDiam) 2

Where:

6-18

Head

=

Head developed

g

=

Gravitational constant

π

=

3.1416

ShSpd

=

Shaft speed

ImpDiam

=

Impeller diameter of compressor wheel

Unit Operation Models Version 10

Chapter 6

Flow Coefficient The flow coefficient is defined as:

Fc =

VflIn ShSpd (ImpDiam) 3

Where: VflIn

=

Volumetric flow rate at suction conditions

ShSpd

=

Shaft speed

ImpDiam

=

Impeller diameter of compressor wheel

References 1. GPSA Engineering Data Book, 1979, Chapter 4, pp. 5-6 to 5-10. 2. ASME Power Test Code 10, 1965, pp. 31-32.

Unit Operation Models Version 10

6-19

Pressure Changers

Valve Valve Pressure Drop Valve models control valves and pressure changers. Valve relates the pressure drop across a valve to the valve flow coefficient. Valve assumes the flow is adiabatic, and determines the thermal and phase condition of the stream at the valve outlet. Valve can perform one-, two-, or three-phase calculations.

Flowsheet Connectivity for Valve

Material

Material

Material Streams Inlet

One material stream

Outlet One material stream

Specifying Valve Use the Input Operation sheet to select the calculation type. If you select the Pressure changer option or the Design option for the calculation type, you must specify, on the same sheet, one of the following: • Outlet pressure • Pressure drop If you select the Pressure changer option, the specification is complete and Valve performs an adiabatic flash to calculate the thermal and phase condition of the outlet stream. If you select the Rating option for the calculation type, you must specify, on the same sheet, one of the following: • •

6-20

Flow coefficient at operating valve position Valve operating position (% Opening)

Unit Operation Models Version 10

Chapter 6

If you specify the valve operating position, you must also specify one of the following on the Input ValveParameters sheet: • Characteristic equation type and flow coefficient at maximum valve opening • Data for flow coefficient (Cv) versus valve opening in the Valve Parameters Table • A valve from the built-in library based on valve type, manufacturer, series/style, and size On the Input CalculationOptions sheet, you can specify that Valve: • Check for choked flow • Calculate cavitation index For vapor-containing streams, you must specify the pressure drop ratio factor (Xt) for the valve. For liquid-containing streams, if you specify that Valve check for choked flow, you must also specify the pressure recovery factor (Fl) for the valve. You can specify the pressure drop ratio factor and the pressure recovery factor for the valve in one of the following ways on the Input ValveParameters sheet: Specify Value at the operating valve position (Pres Drop Ratio Factor, Pres Recovery Factor) Data for pressure drop ratio factor (Xt) and for pressure recovery factor (Fl) versus valve opening (% Opening) in the Valve Parameters Table A valve from the built-in library based on Valve Type, Manufacturer, Series/Style, and Size

If you want to include the effect of head loss from pipe fittings on the valve flow capacity, you must specify the diameters of the valve and pipe fittings on the Input PipeFittings sheet. Valve uses the valve and pipe diameters, and estimates the piping geometry factor to account for the reduction in flow capacity. Use the following forms to enter specifications and view results for Valve:

Unit Operation Models Version 10

Use this form

To do this

Input

Specify valve operating conditions, flash convergence parameters, valid phases, valve parameters, sizes for pipe fittings, calculation options, and Valve convergence parameters

Block Options

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of operating results and mass and energy balances

6-21

Pressure Changers

Pressure Drop Ratio Factor The pressure drop ratio factor ( X t ) accounts for the effect of the internal geometry of the valve on the change in fluid density as it passes through the valve. The pressure drop ratio factor is the limiting value (under choked conditions) of the pressure drop ratio and is given by:

Xt =

1  dPch    Fk  Pin 

(1)

Where:

dPch

=

Pressure drop for choked vapor flow

Fk

=

Ratio of specific heats factor

Pin

=

Inlet pressure

You can specify the pressure drop ratio factor on the Input ValveParameters sheet in one of the following ways: • Choose a Library Valve • Enter data for Xt and % Opening in the Valve Parameters Table • Specify the value at the operating valve position in Valve Factors If you know the ratio of the gas sizing coefficient (C g ) to the liquid sizing coefficient (Cv ) , as defined in Fisher Controls Company Control Valve Handbook, you can calculate the pressure drop ratio factor (with the assumption Fk = 1) by either:

Cg  dPch   versus in equation (1) Cv  Pin 

•

Using valve manufacturer’s data for 

•

Using the expression

6.31 × 10 − 4  C g    Xt = Fk  Cv 

2

This relationship is based on equating the choked flow calculated (in US units of measure) with:

6-22

Universal Gas Sizing Equation

Wch = 106 . C g rPin

ISA Standard Valve Sizing Equation

Wch = N 6 Cv Y Fk X t rPin

Unit Operation Models Version 10

Chapter 6

Where:

Wch

=

Mass flow rate (choked flow)

r

=

Mass density of inlet stream

Y

=

Expansion factor (= 0.667 for choked flow)

N6

=

Numerical constant (= 63.3 for US units of measure)

If you specify the pressure drop ratio factor by choosing a valve from the built-in library or by entering data in the Valve Parameters Table on the Input ValveParameters sheet, Valve uses cubic splines to interpolate the value of the pressure drop ratio factor at the operating valve position. Valve uses the pressure drop ratio factor only when both of the following are true: • Vapor is present in the inlet stream • The Design or Rating option is selected for Calculation Type on the Input Operation sheet

Pressure Recovery Factor

( )

The pressure recovery factor Fl accounts for the effect of the internal geometry of the valve on its liquid flow capacity under choked conditions. The pressure recovery factor is defined as:

 dPch  Fl =    Pin − Pvc 

1/ 2

Where:

dPch

=

Pressure drop for choked liquid flow

Pin

=

Inlet pressure

Pvc

=

Pressure at the vena contracta in the valve

=

F f Pv

Pv

=

Vapor pressure of inlet liquid stream

Ff

=

Liquid critical pressure ratio factor

and

Pvc with

Unit Operation Models Version 10

6-23

Pressure Changers

You can specify the pressure recovery factor on the Input ValveParameters sheet in one of the following ways: • Choose a Library Valve • Enter data for Fl and % Opening in the Valve Parameters Table • Specify the value at the operating valve position in Valve Factors The pressure recovery factor is equivalent to the valve recovery coefficient K m , as defined in Fisher Controls Company Control Valve Handbook. You can use the valve recovery coefficient to calculate the pressure recovery factor as:

Fl =

Km

If you specify the pressure recovery factor by choosing a valve from the built-in library or by entering tabular data in the Valve Parameters Table on the Input ValveParameters sheet, Valve uses cubic splines to interpolate the value of the pressure recovery factor at the operating valve position. The pressure recovery factor is used in the Valve model calculations only when all of the following are true: • Liquid is present in the inlet stream • The Check for Choked Flow box is checked or the Set Equal to Choked Outlet Pressure option is selected on the Input CalculationOptions sheet • The Design or Rating option is selected for Calculation Type on the Input Operation sheet.

Valve Flow Coefficient The valve flow coefficient (Cv ) measures the flow capacity of the valve. The flow coefficient is defined as the number of US gallons per minute of water (at 60 °F) that will pass through the valve with a pressure drop of 1 psi. The valve flow coefficient relates the pressure drop across the valve to the flow rate as (Instrument Society of America, 1985) 1:

6-24

Liquid

W = N 6 Fp Cv r ( Pin − Pout )

Gas/Vapor

W = N 6 Fp Y r ( Pin − Pout )

with

Y = 1−

Pin − Pout 3 Fk X t Pin

Unit Operation Models Version 10

Chapter 6

Where:

W

=

Mass flow rate

N6

=

Numerical constant (based on the units of measure)

Fp

=

Piping geometry factor

Cv

=

Valve flow coefficient

Y

=

Expansion factor

Pin

=

Inlet pressure

Pout

=

Outlet pressure

r

=

Mass density of inlet stream

Fk

=

Ratio of specific heats factor

Xt

=

Pressure drop ratio factor

You can specify the flow coefficient in one of the following ways: • Use Flow Coef on the Input Operation sheet to specify the value at the operating valve position • Choose a Library Valve on the Input ValveParameters sheet • Enter data for Cv and % Opening in the Valve Parameters Table on the Input ValveParameters sheet • Specify Valve Characteristics in the Input ValveParameters sheet If you specify the flow coefficient by choosing a valve from the built-in library or by entering data in the Valve Parameters Table, Valve uses cubic splines to interpolate the value of the flow coefficient at the operating valve position.

Unit Operation Models Version 10

6-25

Pressure Changers

Characteristic Equation Type The characteristic equation for the valve relates the flow coefficient to the valve opening. Use the Input ValveParameters sheet to specify the characteristic equation type. The six built-in characteristic equations are: †

Type

Equation

Linear

V=P

Parabolic

V = 0.01P 2

Square Root

V = 10.0 P

Quick Opening

V =

Equal Percentage

V = Hyperbolic

†

V =

10.0 P

(10. + 9.9 × 10 −3 P 2 ) 0.01P 2 2.0 − 10 . × 10 −8 P 4 01 . P

(10. − 9.9 × 10 −5 P 2 )

Where: P = Valve opening as a percentage of maximum opening V = Flow coefficient as a percentage of flow coefficient at maximum opening

Piping Geometry Factor The piping geometry factor is defined as: Fp =

Cυp Cυ

Where: Cυp

=

Flow coefficient of the valve with attached fittings

Cυ

=

Flow coefficient of the valve installed in a straight pipe of the same size

The piping geometry factor accounts for the reduction in the flow capacity of a valve due to the head loss from the pipe fittings. The piping geometry factor has a default value of 1.0 if the valve and pipe fittings have the same diameter.

6-26

Unit Operation Models Version 10

Chapter 6

ASPEN PLUS calculates the piping geometry factor as (Instrument Society of America, 1985)1:

 ΣKC 2    υ Fp =  4 + 1   N 2d  

−0.5

with ΣK = K1 + K2 + K B1 − K B2 Where: 2

2

4

   d   d  d2  d2  K1 = 0.5 1 − 2  , K 2 = 10 .  1 − 2  , K B1 = 1 −   , K B2 = 1 −   D1  D2   D1   D2   

4

and: Fp

=

Piping geometry factor

Cυ

=

Valve flow coefficient

N2

=

Numerical constant (based on the units of measure)

d

=

Valve diameter

K1, K 2

=

Resistance coefficients of the inlet and outlet fittings

K B1 , K B2

=

Bernoulli coefficients for the inlet and outlet fittings

D1

=

Inlet pipe diameter

D2

=

Outlet pipe diameter

If the valve and pipe fittings diameters are different and you wish to include the effect of the additional head loss on the valve flow capacity, you must specify the valve and pipe diameters on the Input PipeFittings sheet.

Unit Operation Models Version 10

6-27

Pressure Changers

Choked Flow ASPEN PLUS calculates the limiting pressure drop for choked flow conditions using (Instrument Society of America, 1985)1:

(

Liquid

dPlc = F L Pin − F f Pυ

Vapor

dPυc = Fk X T Pin

with

 Pv  F f = 0.96 − 0.28   Pc 

2

) 0.5

Where: FL

=

Pressure recovery factor

Ff

=

Liquid critical pressure ratio factor

Fk

=

Ratio of specific heats factor

XT

=

Pressure drop ratio factor

Pin

=

Inlet pressure

Pυ

=

Vapor pressure at inlet

Pc

=

Critical pressure at inlet

dPlc

=

Limiting pressure drop, liquid phase

dPvc

=

Limiting pressure drop, vapor phase

For multi-phase streams, Valve takes the limiting pressure drop for choked flow to be the smaller of dPlc and dPvc . Flow in the valve is choked when the pressure drop exceeds this limiting pressure drop. Valve displays the choking status of the valve if you check the Check for Choking box on the Input CalculationOptions sheet.

6-28

Unit Operation Models Version 10

Chapter 6

Cavitation Index The likelihood of cavitation in a valve is measured by the cavitation index. ASPEN PLUS calculates the cavitation index as (Instrument Society of America, 1985)1:

 P − Pout   K c =  in  Pin − Pv  Where:

Kc

=

Cavitation index

Pin

=

Inlet pressure

Pout

=

Outlet pressure

Pv

=

Vapor pressure at inlet

The cavitation index definition is valid only for all-liquid streams. Valve calculates the cavitation index if you check the Calculate Cavitation Index box on the Input CalculationOptions sheet.

References 1. Flow Equations for Sizing Control Valves, ISA-S75.01-1985, Instrument Society of America, 1985.

Unit Operation Models Version 10

6-29

Pressure Changers

Pipe Pipe Pressure Drop Pipe calculates the pressure drop and heat transfer in a single segment pipe. You can also use Pipe to model the pressure drop due to fittings. Pipe handles a single inlet and outlet material stream. Pipe assumes the flow is one-dimensional, steady-state, and fully developed (that is, no entrance effects are modeled). Pipe can perform one-, two-, or three-phase calculations. Flow direction and elevation angle are arbitrary. To model multiple pipe segments of different diameters or elevations, use Pipeline instead of Pipe. If the inlet pressure is known, Pipe calculates the outlet pressure. If the outlet pressure is known, Pipe calculates the inlet pressure and updates the state variables of the inlet stream. Use Pipe to: • Calculate inlet or discharge conditions • Calculate pressure drops for one-, two-, or three-phase vapor and liquid flows

Flowsheet Connectivity for Pipe Material

Material

Material Streams Inlet

One material stream

Outlet One material stream

6-30

Unit Operation Models Version 10

Chapter 6

Specifying Pipe You must specify the following for Pipe: • Pipe length, diameter, roughness, and angle on the Setup PipeParameters sheet • Thermal specification type on the Setup ThermalSpecification sheet to determine whether Pipe operates with a temperature profile or temperature is calculated • Whether to integrate, assume constant dP/dL, or use a closed form equation on the Advanced Methods sheet • Frictional and holdup correlation when a closed form equation is not used on the Advanced Methods sheet • Pressure and temperature grid for fluid property calculations on the Advanced PropertyGrid sheet, if you request a pressure-temperature grid on the AdvancedCalculation Options sheet • Integration direction in which calculations proceed with respect to flow on the Advanced CalculationOptions sheet If the option selected is

Pipe needs the

And the integration direction is

Calculate pipe outlet pressure (default)

Inlet pressure

Downstream

Calculate pipe inlet pressure

Outlet pressure

Upstream

Pipe uses the inlet or outlet stream pressure to start the calculations. If the stream is an external feed to your flowsheet, or the outlet of a block that will execute after Pipe, use the Stream Specifications sheet to specify the stream pressure. If the integration direction is upstream, you can also specify the initial pressure for Pipe on the Advanced CalculationOptions sheet, by entering the outlet pressure. This pressure value will override the stream pressure entered on the Stream Specifications sheet. Select the flow calculation option on the Advanced CalculationOptions sheet to specify whether Pipe is to calculate the outlet or inlet stream flow and composition.

Unit Operation Models Version 10

If the option selected is

Pipe needs the

Reference inlet stream (default)

Inlet flow and composition

Use outlet stream flow

Outlet flow and composition

6-31

Pressure Changers

Use the following forms to enter specifications and view results for Pipe: Use this form

To do this

Setup

Specify pipe parameters, thermal specifications, fittings, flash convergence parameters and property profiles to be reported

Advanced

Specify calculation options, solution methods, property grid, integration parameters and Beggs and Brill coefficients

UserSubroutine

Specify pressure drop and/or holdup user subroutine name and parameters

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of Pipe results, inlet and outlet stream results, material and energy balance results, and profiles

Stream Specification You must initialize the inlet stream to Pipe whenever the option to reference inlet stream is selected, even if the inlet pressure is being calculated. Similarly, you must initialize the outlet stream whenever the option to use the outlet stream flow is selected. The initialized stream must be one of the following: • Entered on a Stream Specifications sheet • An outlet stream from part of the flowsheet executed (if option to use outlet stream flow is selected) • Transferred from another part of a flowsheet using a Transfer block

Physical Property Calculations You can specify that a rigorous flash is to be performed each time properties are calculated, by selecting the option to do Flash at Each Integration Step on the Advanced CalculationOptions sheet. If you select the option to Interpolate from Property Grid, Pipe will determine properties by interpolating in a table of property values at various temperatures and pressures. Specify one of the following if you use the Property Grid: • A range of temperatures and pressures on the Advanced Property Grid sheet. Pipe will calculate properties at these conditions and interpolate • The block ID of a Pipe block for which the option to interpolate from property grid was also selected, and which will be executed before the current block in the flowsheet

6-32

Unit Operation Models Version 10

Chapter 6

Pressure Drop Calculations Pipe can calculate pressure drop for either one-, two-, or three-phase vapor and liquid flows. If vapor-liquid flow exists, Pipe also calculates liquid holdup and flow regime (pattern). You may specify a flowing fluid temperature profile, or Pipe can determine it from heat transfer calculations. Pipe treats multiple liquid phases (for example, oil and water) as a single homogeneous liquid phase for pressure-drop and holdup calculations. Pipe automatically detects the special case of a single component fluid (for example, steam) and treats it appropriately.

Downstream and Upstream Integration For downstream and upstream integration, the combination of options selected for pressure and flow calculation on the Advanced CalculationOptions sheet determine which stream Pipe will update. The following table describes the available combinations. The next figure, Downstream and Upstream Integration, defines the inlet and outlet stream and pressure variables: If the pressure calculation option is

And the flow calculation option is

Then Pipe updates the

Calculate pipe outlet pressure

Reference inlet stream

Outlet stream only

Calculate pipe outlet pressure

Use outlet stream flow

Outlet stream thermodynamic conditions Inlet stream composition and flow

Calculate pipe inlet pressure

Use outlet stream flow

Inlet stream only

Calculate pipe inlet pressure

Reference inlet stream

Inlet stream thermodynamic conditions Outlet stream composition and flow

Inlet Stream

Inlet Pressure

Outlet Stream

Outlet Pressure

Downstream and Upstream Integration

Unit Operation Models Version 10

6-33

Pressure Changers

Design-Spec Convergence Loop Use caution when using Pipe inside a Design-Spec convergence loop. For example, you can manipulate the flow rate to a pipe to achieve a desired pipe outlet pressure. During the design specification convergence, the flow rate variables may become unreasonable in an intermediate iteration, causing Pipe to predict a negative pressure. Convergence difficulties occur as a result. You can avoid this situation by doing one of the following: • •

Keep the upper limit of the flow rate sufficiently low in Design-Spec Perform an upstream integration from the known outlet pressure. Select option to calculate pipe inlet pressure on the Advanced CalculationOptions sheet for this purpose. Define a Design-Spec to manipulate the flow rate to achieve the specified inlet pressure.

Erosional Velocity Erosional velocity is the velocity of the fluid in the pipe, above which the pipe material will start to break off. The fluid is traveling so fast that it starts to strip material from the walls of the pipe. In general use, the flow rate should be below this value. You can specify the erosional velocity coefficient on the Setup Pipe Parameters sheet. The erosional velocity is related to the erosional velocity coefficient by the following equation:

υc =

c ρ

Where:

υc = Erosional velocity in ft/second c = Erosional velocity coefficient (default=100)

ρ = Density in lbs/cubic ft

Methane Gas Systems Gas systems consisting mostly of methane occur frequently in the dense-phase region of wellbores and flowlines. In the dense-phase region, definable vapor and liquid phases do not exist. Equation-of-state methods classify the dense-phase material as either all vapor or all liquid. Significant differences in the predicted fluid transport properties may occur, depending on whether you choose the vapor or liquid state.

6-34

Unit Operation Models Version 10

Chapter 6

Experience has shown that gas system flow in the dense-phase region is best modeled by using vapor-phase properties. For systems consisting of mostly methane, where the pipe conditions lie above the cricondenbar of the phase envelope, specify vapor-only valid phase on the Setup FlashOptions sheet.

Modeling Valves and Fittings Pipe assumes that the pressure drop due to valves and fittings is distributed evenly along the specified length of the pipe. The total length Pipe uses in calculations corresponds to the specified pipe length, plus any equivalent pipe length due to valves, fittings, and miscellaneous L/D. If the pipe is not horizontal, Pipe adjusts the angle from the horizontal to achieve the same vertical rise or fall for the total length used in the calculations. This adjustment ensures the correct pressure drop due to elevation. If the order and position of the valves and fittings are important, you need to model each valve and fitting separately with a Pipe model, specifying zero length of pipe.

Two-Phase Correlations The following tables list the two-phase frictional pressure drop and holdup correlations available.

Two-Phase Friction Factor Correlations Pipe orientation

Inclination

Friction factor correlations

Horizontal

-2 deg to +2 deg

Beggs and Brill (BEGGS-BRILL) Dukler (DUKLER) Lockhart-Martinelli (LOCK-MART) User subroutine (USER-SUBR)

Vertical

+45 deg to +90 deg

Beggs and Brill (BEGGS-BRILL) Orkiszewski (ORK) Angel-Welchon-Ros (AWR) Hagedorn-Brown (H-BROWN) † User subroutine (USER-SUBR)

Downhill

-2 deg to -90 deg

Beggs and Brill (BEGGS-BRILL) Slack (SLACK) Darcy (DARCY) † User subroutine (USER-SUBR)

†

See ASPEN PLUS User Models.

continued

Unit Operation Models Version 10

6-35

Pressure Changers

Pipe orientation

Inclination

Friction factor correlations

Inclined

+2 deg to +45 deg

Beggs and Brill (BEGGS-BRILL) Dukler (DUKLER) Orkiszewski (ORKI) Angel-Welchon-Ros (AWR) Hagedorn-Brown (H-BROWN) Darcy (DARCY) † User subroutine (USER-SUBR)

†

See ASPEN PLUS User Models.

Two-Phase Liquid Holdup Correlations Pipe orientation

Inclination

Liquid holdup correlations

Horizontal

-2 deg to +2 deg

Beggs and Brill (BEGGS-BRILL) Eaton (EATON) Lockhart-Martinelli (LOCK-MART) Hoogendorn (HOOG) Hughmark (HUGH) † User subroutine (USER-SUBR)

Vertical

+45 deg to +90 deg

Beggs and Brill (BEGGS-BRILL) Orkiszewski(ORKI) Angel-Welchon-Ros (AWR) Hagedorn-Brown (H-BROWN) † User subroutine (USER-SUBR)

Downhill

-2 deg to -90 deg

Beggs and Brill (BEGGS-BRILL) Slack (SLACK) † User subroutine (USER-SUBR)

Inclined

+2 deg to +45 deg

Beggs and Brill (BEGGS-BRILL) Flanigan (FLANIGAN) Orkiszewski (ORKI) Angel-Welchon-Ros (AWR) Hagedorn-Brown (H-BROWN) † User subroutine (USER-SUBR)

†

See ASPEN PLUS User Models.

Note

6-36

Some of the related information for the two-phase friction factor and liquid holdup correlations was taken from "Two-Phase Flow in Pipes" by James P. Brill and H. Dale Beggs, Sixth Edition, Third Printing, January, 1991.

Unit Operation Models Version 10

Chapter 6

Beggs and Brill Correlation The Beggs and Brill correlation1 considers slip and flow regimes are considered with this method. Friction factor and holdup correlations depend on flow regime and pipe inclination. It is suitable for all inclinations, including vertical flow downward.

Dukler Correlation The Hughmark holdup method should be used with this pressure drop method. 2 The Dukler method was developed from field data using air-water mixtures in 1-inch pipes. It tends to overpredict frictional pressure drop. It is recommended in a design manual published jointly by the AGA and API.

Hagedorn-Brown Correlation The Hagedorn-Brown correlation3 considers slip between phases, but flow regime is not considered. It uses the same correlations for liquid holdup and friction factor for all flow regimes. It is an old method which works well for conventional oil wells. It is suitable for vertical upward flow, but not downward. It is generally recommended for gas wells, and is based on data obtained from U.S. Gulf Coast oil wells with 2-3/8 inch and 2-7/8 inch tubing.

Lockhart-Martinelli Correlation The Lockhart-Martinelli correlation4 is one of the oldest pressure drop correlations. It does not consider pressure drop due to acceleration. The method treats the vapor and liquid phases separately and uses a correction factor to find the 2-phase pressure gradient. Our implementation assumes turbulent gas and liquid phase flow.

Orkiszewski Correlation 5

Slip and flow regimes are considered in the Orkiszewski correlation . The friction factor and holdup correlation depend on the flow regime. It is suitable for vertical flow upward, but not downward. It is generally reliable for oil wells. It may exhibit problems for oil wells with high water cuts or high total gas to liquid ratios. It can significantly underpredict pressure drop for higher rate and higher 3 pressure wells (Beggs and Brill/1984) .

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Angel-Welchon-Ros Correlation The Angel-Welchon-Ros correlation method6, 7 was developed for low gas-to-liquid ratio water wells. It assumes no slip between the vapor and liquid phases when calculating liquid holdup.

Slack Correlation The Slack correlation method assumes a stratified flow regime, and should be used only for downhill flow.

Eaton Correlation The Eaton correlation8 holdup method was developed from data on 2- and 4-inch pipes with a gas-water-crude mixture, and a 17-inch pipe with a gas-oil mixture. It is often used with the Dukler frictional pressure drop correlation.

Flanigan Correlation The Flanigan correlation9 holdup methodwas developed from data taken in a 16-inch pipe. It calculates liquid holdup as a function of superficial gas velocity. It is suitable for inclined flow.

Beggs and Brill Correlation Parameters The following table lists the Beggs and Brill liquid holdup correlation parameters. Flow Regime

Name

Description

Segregated

BB1 BB2 BB3

Leading coefficient, A (default = 0.98) Liquid volume fraction exponent, alpha (default = 0.4846) Froude no. exp., beta (default = 0.0868)

Intermittent

BB4 BB5 BB6

Leading coefficient, A (default = 0.845) Liquid volume fraction exponent, alpha (default = 0.5351) Froude no. exp., beta (default = 0.0173)

Distributed

BB7 BB8 BB9

Leading coefficient, A (default = 1.065) Liquid volume fraction exponent, alpha (default = 0.5824) Froude no. exp., beta (default = 0.0609)

In addition, you can change the Beggs and Brill two-phase Friction Factor modifier, BB10 (default = 1.0).

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Chapter 6

Closed-Form Methods The following are closed-form methods: • Smith • Weymouth • AGA • Oliphant • Panhandle A • Panhandle B • Hazen-Williams

Smith The Smith method10 may be used for vertical dry gas flow. It should be considered for gas wells with condensate-gas ratios less than 50 bbls/mcf, water-gas ratios less than 3.5 bbls/mcf, and flow rates above the Turner predicted critical rate. Smith does not model gas well loadup, and will significantly under predict wellbore pressure drop if loadup is actually occurring. Smith results must be cross-checked against the Turner predicted critical rates to verify that the well is unloaded. Smith also does not model condensation of water vapor in the wellbore.

Weymouth 11

The Weymouth horizontal gas flow equation was first published in 1912. It is based on data taken on pipes with diameters from 0.8 inches to 11.8 inches. As a result, it is most accurate for smaller pipes having a diameter less than 12 inches.

AGA 12

The AGA method may be used for horizontal gas applications.

Oliphant The Oliphant method13 may be used for horizontal gas applications with pressures between vacuum and 100 PSI.

Unit Operation Models Version 10

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Pressure Changers

Panhandle A The Panhandle A method14 was developed by Panhandle Eastern for horizontal gas flow in large diameter cross country gas transmission lines. As a result, it is best used on lines having diameters larger than 12 inches. However, it does not account for gas compressibility (Z-factor), and assumes completely turbulent flow.

Panhandle B The Panhandle B method14 is a revised version of the Panhandle A method for horizontal gas flow and was developed by Panhandle Eastern. It is also called the "Panhandle Eastern Revised Equation". It accounts for the gas compressibility factor, and has revised exponents. This equation is not quite so Reynolds-Number dependent as the Panhandle A equation, although it, too, is best for pipe diameters of 12 inches or more.

Hazen-Williams The Hazen-Williams method14 was developed for the horizontal flow of water. When this method is used, the Hazen-Williams Coefficient must be specified in place of the Segment Efficiency on the Connectivity Edit dialog box.

References 1. Beggs, H.D. and Brill, J.P., "A Study of Two-Phase Flow in Inclined Pipes," Journal of Petroleum Technology, May 1973, pp. 607-617. 2. Dukler, A.E., Wicks, M., and Cleveland, R.G, "Frictional Pressure Drop in Two-Phase Flow: An Approach Through Similarity Analysis," AIChE Journal, Vol. 10, No. 1, January 1964, pp. 44-51. 3. Beggs, H.D. and Brill, J.P., "Two-Phase Flow in Pipes," University of Tulsa Short Course Notes, Third Printing, February 1984. 4. Lockhart, R.W. and Martinelli, R.C., "Proposed Correlation of Data for Isothermal Two-Phase, Two-Component Flow in Pipes," Chemical Engineering Progress, Vol. 45, 1949, pp. 39-48. 5. Orkiszewski, J., "Predicting Two-Phase Pressure Drops in Vertical Pipe," Journal of Petroleum Technology, June 1967, pp. 829-838.

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Chapter 6

6. Angel, R.R., and Welchon, J.K., "Low-Ratio Gas-Lift Correlation for CasingTubing Annuli and Large Diameter Tubing," API Drilling and Production Practice, 1964, pp. 100-114. 7. Ros, N.C.J., "Simultaneous Flow of Gas and Liquid as Encountered in Well Tubing," Journal of Petroleum Technology, October 1961, pp. 1037-1049. 8. Eaton, B.A. et al., "The Prediction of Flow Patterns, Liquid Holdup, and Pressure Losses Occurring During Continuous Two-Phase Flow in Horizontal Pipelines," Trans. AIME, June 1967, pp. 815-828. 9. Flanigan, Orin, "Effect of Uphill Flow on Pressure Drop in Design of TwoPhase Gathering Systems," Oil and Gas Journal, March 10, 1958, pp. 132141. 10. Smith, R. V., "Determining Friction Factors for Measuring Productivity of Gas Wells," AIME Petroleum Transactions, Volume 189, 1950, pp. 73-82. 11. Weymouth, T.R., Transactions of the American Society of Mechanical Engineers, Vol. 34, 1912. 12. "Steady Flow in Gas Pipes," American Gas Association, IGT Technical Report 10, Chicago, 1965. 13. Oliphant, F.N., "Production of Natural Gas," Report of USGS, 1902. 14. Engineering Data Book, Volume II, Gas Processors Suppliers Association, Tulsa, Oklahoma, Revised Tenth Edition, 1994.

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Pressure Changers

Pipeline Pipe Pressure Drop Use Pipeline to calculate the pressure drop in a straight pipe or annular space. Pipeline can: • Simulate a piping network with successive blocks, including wellbores and flowlines • Contain any number of segments within each block to describe pipe geometry • Calculate inlet or discharge conditions • Calculate pressure drops for one-, two-, or three-phase vapor and liquid flows. Pipeline treats multiple liquid phases (for example, oil and water) as a single homogeneous liquid phase for pressure-drop and holdup calculations. If vapor-liquid flow exists, Pipeline calculates liquid holdup and flow regime (pattern). You may specify a flowing fluid temperature profile, or Pipeline can calculate it from heat transfer calculations. Flow is assumed to be one-dimensional, steadystate, and fully developed (no entrance effects are modeled). Flow direction and elevation angle are arbitrary. To model a single pipe segment with constant diameter and elevation, you can also use Pipe.

Flowsheet Connectivity for Pipeline Material

Material

Pipeline Streams Material Streams Inlet

One material stream

Outlet One material stream

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Unit Operation Models Version 10

Chapter 6

Specifying Pipeline Use the Calculation Direction option on the Setup Configuration sheet to specify whether Pipeline is to calculate the outlet or inlet pressure. If Calculation Direction =

Pipeline will need the

And the integration direction is

Calculate outlet pressure (default)

Inlet pressure

Downstream

Calculate inlet pressure

Outlet pressure

Upstream

Pipeline uses the inlet or outlet stream pressure to start the calculations. If the stream is an external feed to your flowsheet, or the outlet of a block that will execute after Pipeline, use the Streams Specifications sheet to specify the stream pressure. You can also specify the initial pressure for Pipeline on the Setup Configuration sheet by entering the pressure value at the inlet or outlet. This pressure value overrides the stream pressure. Use the Pipeline flow basis option on the Setup Configuration sheet to specify whether Pipeline is to calculate the outlet or inlet stream flow and composition. If Pipeline flow basis=

Pipeline will need the

Use inlet stream flow (default)

Inlet flow and composition

Reference outlet stream flow

Outlet flow and composition

Use Thermal Options on the Setup Configuration sheet to specify whether or not the node temperatures are to be calculated by Pipeline using an energy balance. When you select the Specify Temperature Profile option, the temperature at each node can be specified. When you choose the Constant Temperature option, the temperature will be same at every node. You can define this temperature by specifying the inlet temperature (for downstream integrations) or the outlet temperature (for upstream integrations). If neither the inlet nor the outlet temperatures are specified, the temperature of the referenced stream will be used. When you choose the linear temperature profile option, you can specify the temperature at one or more nodes. Pipeline will do a linear interpolation between the temperatures specified to calculate the fluid temperature in each segment.

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Pressure Changers

Use the following forms to enter specifications and view results for Pipeline: Use this form

To do this

Setup

Specify pipeline configuration, segment connectivity and characteristics, calculation methods, property grid parameters, flash convergence parameters, valid phases, and block-specific diagnostic message level

Convergence

Override default values for integration parameters, downhill flow options, correlation parameters and Beggs and Brill coefficients (optional input)

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

UserSubroutines

Specify name and parameters for pressure drop and liquid holdup user subroutines

Results

View summary of Pipeline results, inlet and outlet stream results, profiles, and material and energy balance results

Stream Specification You must initialize the inlet stream to Pipeline whenever the Use Inlet Flow option is selected for Pipeline Flow Basis, even if the inlet pressure is being calculated. Similarly, you must initialize the outlet stream whenever you select the Reference Outlet Stream Flow option. The initialized stream must be one of the following: • On a stream form • An outlet stream from part of the flowsheet executed previously • Transferred from another part of a flowsheet using a Transfer block

Nodes and Segments Create at least one segment using the New button on the Pipeline Setup Connectivity sheet. Enter specifications for each segment on the Setup Connectivity Segment Data dialog box . For each segment, enter the inlet and outlet node names (maximum 4 characters). The required data depends on the options selected on the Setup Configuration sheet. If you select Do Energy Balance with Surroundings, you must specify a heat transfer coefficient (U-Value) and the ambient temperature. If you select the Linear Temperature Profile option, Pipeline uses the temperatures specified for the nodes to override the stream values. If specifications are not made for the nodes, then Pipeline uses the stream values. If you select Enter Node Coordinate, you must enter node coordinates (X, Y, and Elevation) for each segment node. You must enter Length and Angle for each segment if you select Enter Segment Length and Angle.

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Chapter 6

Physical Property Calculations You can specify a rigorous flash each time properties are calculated by selecting Do Flash at Each Step on the Setup Configuration sheet. If Interpolate from Property Grid is selected, Pipeline will determine properties by interpolating in a table of property values at various temperatures and pressures. Specify one of the following if you use the Property Grid: • A range of temperatures and pressures grid on the Setup PropertyGrid sheet. Pipeline calculates properties under these conditions and interpolates them. • The block ID of a Pipeline block for which you selected Interpolate from the Property Grid, and which will be executed before the current block in the flowsheet.

Pressure Drop Calculations Pipeline can calculate pressure drop for either one-, two-, or three-phase vapor and liquid flows. If vapor-liquid flow exists, Pipeline also calculates liquid holdup and flow regime (pattern). You may specify a flowing fluid temperature profile, or Pipeline can calculate it from heat transfer calculations. Pipeline treats multiple liquid phases (for example, oil and water) as a single homogeneous liquid phase for pressure-drop and holdup calculations. Pipeline automatically detects the special case of a single component fluid (for example, steam) and treats it appropriately.

Downstream and Upstream Integration For downstream and upstream integration, the combination of the selections made for Calculation Direction and Pipeline Flow Basis on the Setup Configuration sheet determine which stream Pipeline will update. The following table describes the available combinations. The next figure, Downstream and Upstream Integration, defines the inlet and outlet stream and pressure variables. If you specify Calculation Direction=

And Pipeline Flow Basis=

Then Pipeline updates the

Calculate Outlet Pressure

Reference inlet stream flow

Outlet stream only

Calculate Outlet Pressure

Use outlet stream flow

Outlet stream thermodynamic conditions Inlet stream composition and flow

Calculate Inlet Pressure

Reference Outlet Stream Flow

Inlet stream only

Calculate Inlet Pressure

Use Inlet Stream Flow

Inlet stream thermodynamic conditions Outlet stream composition and flow

Unit Operation Models Version 10

6-45

Pressure Changers

Inlet Stream

Outlet Stream

Inlet Pressure

Outlet Pressure

Downstream and Upstream Integration

Design Spec Convergence Loop Use caution when using Pipeline inside a Design-Spec convergence loop. For example, suppose you achieve a desired pipeline outlet pressure by varying the flow rate to the pipeline. In this case, the flow rate variable might cause Pipeline to predict negative pressures, resulting in convergence problems. You can avoid this situation by doing one of the following: • Keep the upper limit of the flow rate sufficiently low in the Design-Spec • Perform an upstream integration from the known outlet pressure. Use Calculate Inlet Pressure on the Setup Configuration sheet for this purpose. Your Design-Spec will then need to manipulate the flow rate to achieve the specified inlet pressure.

Erosional Velocity Erosional velocity is the velocity of the fluid in the pipe over which the pipe material will start to break off. The fluid is traveling so fast that it starts to strip material from the walls of the pipe. In general usage, the flow rate should be below this value. You can specify the erosional velocity coefficient in the C-Erosion field on the Segment Data dialog box on the Setup Connectivity sheet. The erosional velocity is related to the erosional velocity coefficient by the following equation:

vc =

c ρ

Where:

vc

=

Erosional velocity in ft/sec

c

= =

Erosional velocity coefficient (default=100) Density in lb/cubic ft

ρ

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Unit Operation Models Version 10

Chapter 6

Methane Gas Systems Gas systems consisting mostly of methane occur frequently in the dense-phase region of wellbores and flowlines. In the dense-phase region, definable vapor and liquid phases do not exist. Equation-of-state methods classify the dense-phase material as either all vapor or all liquid. Significant differences in the predicted fluid transport properties may occur, depending on whether you choose the vapor or liquid state. Experience has shown that gas system flow in the dense-phase region is best modeled by using vapor-phase properties. For systems consisting of mostly methane, where the pipeline conditions lie above the cricondenbar of the phase envelope, specify Valid Phases = Vapor only on the Setup FlashOptions sheet.

Two-Phase Correlations The following tables list the two-phase frictional pressure drop and holdup correlations available.

Two-Phase Friction Factor Correlations Pipe orientation

Inclination

Friction factor correlations

Horizontal

-2 deg to +2 deg

Beggs and Brill (BEGGS-BRILL) Dukler (DUKLER) Lockhart-Martinelli (LOCK-MART) Darcy (DARCY) † User subroutine (USER-SUBR)

Vertical

+45 deg to +90 deg

Beggs and Brill (BEGGS-BRILL) Orkiszewski (ORKI) Angel-Welchon-Ros (AWR) Hagedorn-Brown (H-BROWN) Darcy (DARCY) † User subroutine (USER-SUBR)

Downhill

-2 deg to -90 deg

Beggs and Brill (BEGGS-BRILL) Slack (SLACK) Darcy (DARCY) † User subroutine (USER-SUBR)

Inclined

+2 deg to +45 deg

Beggs and Brill (BEGGS-BRILL) Dukler (DUKLER) Orkiszewski (ORKI) Angel-Welchon-Ros (AWR) Hagedorn-Brown (H-BROWN) Darcy (DARCY) † User subroutine (USER-SUBR)

†

Unit Operation Models Version 10

See ASPEN PLUS User Models.

6-47

Pressure Changers

Two-Phase Liquid Holdup Correlations Pipe orientation

Inclination

Liquid holdup correlations

Horizontal

-2 deg to +2 deg

Beggs and Brill (BEGGS-BRILL) Eaton (EATON) Lockhart-Martinelli (LOCK-MART) Hoogendorn (HOOG) Hughmark (HUGH) † User subroutine (USER-SUBR)

Vertical

+45 deg to +90 deg

Beggs and Brill (BEGGS-BRILL) Orkiszewski (ORKI) Angel-Welchon-Ros (AWR) Hagedorn-Brown (H-BROWN) † User subroutine (USER-SUBR)

Downhill

-2 deg to -90 deg

Beggs and Brill (BEGGS-BRILL) Slack (SLACK) † User subroutine (USER-SUBR)

Inclined

+2 deg to +45 deg

Beggs and Brill (BEGGS-BRILL) Flanigan (FLANIGAN) Orkiszewski (ORKI) Angel-Welchon-Ros (AWR) Hagedorn-Brown (H-BROWN) † User subroutine (USER-SUBR)

†

See ASPEN PLUS User Models.

Note

Some of the related information for the two-phase friction factor and liquid holdup correlations was taken from "Two-Phase Flow in Pipes" by James P. Brill and H. Dale Beggs, Sixth Edition, Third Printing, January, 1991.

Beggs and Brill Correlation Slip and flow regimes are considered with this method. Friction factor and holdup correlations depend upon flow regime and pipe inclination. It is suitable 1 for all inclinations, including vertical flow downward.

Dukler Correlation The Hughmark holdup method should be used with this pressure drop method. The Dukler method was developed from field data using air-water mixtures in 2 1-inch pipes. It tends to over-predict frictional pressure drop. It is recommended in a design manual published jointly by the AGA and API.

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Chapter 6

Hagedorn-Brown Correlation The Hagedorn-Brown correlation3 considers slip between phases, but flow regime is not considered. It uses the same correlations for liquid holdup and friction factor for all flow regimes. It is an old method that works well for conventional oil wells. It is suitable for vertical upward flow, but not downward. It is generally recommended for gas wells, and is based on data obtained from U.S. Gulf Coast oil wells with 2-3/8 inch and 2-7/8 inch tubing.

Lockhart-Martinelli Correlation The Lockhart-Martinelli correlation4 is one of the oldest pressure drop correlations. It does not consider pressure drop due to acceleration. The method treats the vapor and liquid phases separately and uses a correction factor to find the 2-phase pressure gradient. Our implementation assumes turbulent gas and liquid phase flow.

Orkiszewski Correlation The Orkiszewsi correlation considers slip and flow regimes 5. The friction factor and holdup correlation depend on the flow regime. It is suitable for vertical flow upward, but not downward. It is generally reliable for oil wells. It may exhibit problems for oil wells with high water cuts or high total gas to liquid ratios. It can significantly underpredict pressure drop for higher rate and higher pressure 3 wells (Beggs and Brill/1984) .

Angel-Welchon-Ros Correlation This Angel-Welchon-Ros method6,7 was developed for low gas-to-liquid ratio water wells. It assumes no slip between the vapor and liquid phases when calculating liquid holdup.

Slack Correlation This method assumes a stratified flow regime, and should be used only for downhill flow.

Eaton Correlation The Eaton correlation8 holdup method was developed from data on 2- and 4-inch pipes with a gas-water-crude mixture, and a 17-inch pipe with a gas-oil mixture. It is often used with the Dukler frictional pressure drop correlation.

Unit Operation Models Version 10

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Pressure Changers

Flanigan Correlation The Flanigan correlation9 holdup method was developed from data taken in a 16-inch pipe. It calculates liquid holdup as a function of superficial gas velocity. It is suitable for inclined flow.

Beggs and Brill Correlation Parameters The following table lists the Beggs and Brill liquid holdup correlation parameters. Flow Regime

Name

Description

Segregated

BB1 BB2 BB3

Leading coefficient, A (default = 0.98) Liquid volume fraction exponent, alpha (default = 0.4846) Froude no. exp., beta (default = 0.0868)

Intermittent

BB4 BB5 BB6

Leading coefficient, A (default = 0.845) Liquid volume fraction exponent, alpha (default = 0.5351) Froude no. exp., beta (default = 0.0173)

Distributed

BB7 BB8 BB9

Leading coefficient, A (default = 1.065) Liquid volume fraction exponent, alpha (default = 0.5824) Froude no. exp., beta (default = 0.0609)

In addition, you can change the Beggs and Brill two-phase Friction Factor modifier, BB10 (default = 1.0).

Closed-Form Methods The following are closed-form methods: • • • • • • •

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Smith Weymouth AGA Oliphant Panhandle A Panhandle B Hazen-Williams

Unit Operation Models Version 10

Chapter 6

Smith The Smith method10 may be used for vertical dry gas flow. It should be considered for gas wells with condensate-gas ratios less than 50 bbls/mcf, water-gas ratios less than 3.5 bbls/mcf, and flow rates above the Turner predicted critical rate. Smith does not model gas well loadup, and will significantly underpredict wellbore pressure drop if loadup is actually occurring. Smith results must be cross-checked against the Turner predicted critical rates to verify that the well is unloaded. Smith also does not model condensation of water vapor in the wellbore.

Weymouth The Weymouth11 horizontal gas flow equation was first published in 1912. It is based on data taken on pipes with diameters from 0.8 inches to 11.8 inches. As a result, it is most accurate for smaller pipes having a diameter less than 12 inches.

AGA The AGA method12 may be used for horizontal gas applications.

Oliphant The Oliphant method13 may be used for horizontal gas applications with pressures between vacuum and 100 PSI.

Panhandle A The Panhandle A method14 was developed by Panhandle Eastern for horizontal gas flow in large diameter cross country gas transmission lines. As a result, it is best used on lines having diameters larger than 12 inches. However, it does not account for gas compressibility (Z-factor), and assumes completely turbulent flow.

Panhandle B The Panhandle B method14 is a revised version of the Panhandle A method for horizontal gas flow and was developed by Panhandle Eastern. It is also called the "Panhandle Eastern Revised Equation". It accounts for the gas compressibility factor, and has revised exponents. This equation is not quite so Reynolds-Number dependent as the Panhandle A equation, although it, too, is best for pipe diameters of 12 inches or more.

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Pressure Changers

Hazen-Williams The Hazen-Williams method14 was developed for the horizontal flow of water When this method is used, the Hazen-Williams Coefficient must be specified in place of the Segment Efficiency on the Connectivity Edit Dialog Box.

References 1. Beggs, H.D. and Brill, J.P., "A Study of Two-Phase Flow in Inclined Pipes," Journal of Petroleum Technology, May 1973, pp. 607-617. 2. Dukler, A.E., Wicks, M., and Cleveland, R.G, "Frictional Pressure Drop in Two-Phase Flow: An Approach Through Similarity Analysis," AIChE Journal, Vol. 10, No. 1, January 1964, pp. 44-51. 3. Beggs, H.D. and Brill, J.P., "Two-Phase Flow in Pipes," University of Tulsa Short Course Notes, Third Printing, February 1984. 4. Lockhart, R.W. and Martinelli, R.C., "Proposed Correlation of Data for Isothermal Two-Phase, Two-Component Flow in Pipes," Chemical Engineering Progress, Vol. 45, 1949, pp. 39-48. 5. Orkiszewski, J., "Predicting Two-Phase Pressure Drops in Vertical Pipe," Journal of Petroleum Technology, June 1967, pp. 829-838. 6. Angel, R.R. and Welchon, J.K., "Low-Ratio Gas-Lift Correlation for CasingTubing Annuli and Large Diameter Tubing," API Drilling and Production Practice, 1964, pp. 100-114. 7. Ros, N.C.J., "Simultaneous Flow of Gas and Liquid as Encountered in Well Tubing," Journal of Petroleum Technology, October 1961, pp. 1037-1049. 8. Eaton, B.A. et al., "The Prediction of Flow Patterns, Liquid Holdup, and Pressure Losses Occurring During Continuous Two-Phase Flow in Horizontal Pipelines," Trans. AIME, June 1967, pp. 815-828. 9. Flanigan, Orin, "Effect of Uphill Flow on Pressure Drop in Design of TwoPhase Gathering Systems," Oil and Gas Journal, March 10, 1958, pp. 132141. 10. Smith, R. V., "Determining Friction Factors for Measuring Productivity of Gas Wells," AIME Petroleum Transactions, Volume 189, 1950, pp. 73-82. 11. Weymouth, T.R., Transactions of the American Society of Mechanical Engineers, Vol. 34, 1912. 12. "Steady Flow in Gas Pipes," American Gas Association, IGT Technical Report 10, Chicago, 1965.

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13. Oliphant, F.N., "Production of Natural Gas," Report of USGS, 1902. 14. Engineering Data Book, Volume II, Gas Processors Suppliers Association, Tulsa, Oklahoma, Revised Tenth Edition, 1994.

❖

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❖

❖

❖

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Chapter 7

7

Manipulators This chapter describes the models for stream manipulators. The models are: Model

Description

Purpose

Use For

Mult

Stream multiplier

Multiplies component and total flow rates by a factor

Scaling streams by a factor

Dupl

Stream duplicator

Copies inlet stream into any number of duplicate outlet streams

Duplicating feed or internal streams

ClChng

Stream class changer

Changes stream class between blocks and flowsheet sections

Adding or deleting empty solid substreams between flowsheet sections

Use stream manipulators to modify stream variables for your convenience. They do not represent real unit operations.

Unit Operation Models Version 10

7-1

Manipulators

Mult Stream Multiplier Mult multiplies the component flow rates and the total flow rate of a material stream by a factor you supply on the Mult Input Specifications sheet. For heat or work streams, Mult multiplies the heat or work flow. Select the Heat (Q) and Work (W) Mult icons from the Model Library for heat and work streams respectively. Mult is useful when other conditions during the simulation determine the flow rate of the stream. Mult does not maintain heat or material balances. For material streams, the outlet stream has the same composition and intensive properties as the inlet stream.

Flowsheet Connectivity for Mult Material

Material

or

or

Heat

Heat

or

or

Work

Work

Material Streams Inlet

One material stream

Outlet One material stream

Heat Streams Inlet

One heat stream

Outlet One heat stream

Work Streams Inlet

One work stream

Outlet One work stream

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Chapter 7

The outlet stream must be the same type (material, heat, or work) as the inlet stream.

Specifying Mult The stream multiplication factor, specified on the Input Specifications sheet, is the only input required for Mult. This factor has to be positive for material streams. You can specify either a positive or negative factor for heat or work streams, thus allowing a change in direction for the heat or work flow. Use the Input Diagnostics sheet to override global values for the stream and simulation message levels specified on the Setup Specifications Diagnostics sheet. This model has no dynamic features. For material stream multipliers the pressure of each outlet stream is equal to the pressure of the inlet stream. The flow rate of each outlet stream is equal to the flow rate of the inlet stream multiplied by the factor as specified in the steady-state simulation.

Unit Operation Models Version 10

7-3

Manipulators

Dupl Stream Duplicator Dupl copies an inlet stream (material, heat, or work) to any number of duplicate outlet streams. It is useful for simultaneously processing a stream in different types of units. Select the Heat (Q) and Work (W) Dupl icons from the Model Library for heat and work streams respectively. Dupl does not maintain heat or material balances.

Flowsheet Connectivity for Dupl

Material

Material (any number)

Flowsheet for Duplicating Material Streams Material Streams Inlet

One material stream

Outlet At least one material stream, which is a copy of the inlet stream

Heat (any number)

Heat

Flowsheet for Duplicating Heat Streams Heat Streams Inlet

One heat stream

Outlet At least one heat stream, which is a copy of the inlet stream

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Chapter 7

Work (any number)

Work

Flowsheet for Duplicating Work Streams Work Streams Inlet

One work stream

Outlet At least one work stream, which is a copy of the inlet stream

Specifying Dupl Dupl requires no input parameters. Use the Input Diagnostics sheet to override global values for the stream and simulation message levels specified on the Setup Specifications Diagnostics sheet. This model has no dynamic features. For material stream duplicators the pressure of each outlet stream is equal to the pressure of the inlet stream. The flow rate of each outlet stream is equal to the flow rate of the inlet stream.

Unit Operation Models Version 10

7-5

Manipulators

ClChng Stream Class Changer ClChng changes the stream class between blocks and flowsheet sections. You can use ClChng to add or delete empty solid substreams between flowsheet sections. ClChng does not represent a real unit operation.

Flowsheet Connectivity for ClChng

Feed

Product

Material Streams Inlet

One material feed stream

Outlet One material product stream

Specifying ClChng ClChng does not require input. It copies substreams from the inlet stream to the corresponding substreams of the outlet stream. If a substream is

Then ClChng

In the outlet but not in the inlet

Initializes the substream to zero flow

In the inlet but not in the outlet

Drops the substream

ClChng does not maintain mass and energy balances if any dropped substream contains material flow or heat/work information.

❖

7-6

❖

❖

❖

Unit Operation Models Version 10

Chapter 8

8

Solids This chapter describes the unit operation models for solids processing such as crystallizers, solid crushers and separators, gas-solid separators, liquid-solid separators, and solids washers. The models are:

Unit Operation Models Version 10

Model

Description

Purpose

Use For

Crystallizer

Crystallizer

Produces crystals from solution based on solubility

Mixed suspension, mixed product removal (MSMPR) crystallizer

Crusher

Solids crusher

Breaks solid particles to reduce particle size

Wet and dry crushers, primary and secondary crushers

Screen

Solids separator

Separates solid particles based on particle size

Upper and lower dry and wet screens

FabFl

Fabric filter

Separates solids from gas using fabric filter baghouses

Rating and sizing baghouses

Cyclone

Cyclone separator

Separates solids from gas using gas vortex in a cyclone

Rating and sizing cyclones

VScrub

Venturi scrubber

Separates solids from gas by direct contact with an atomized liquid

Rating and sizing venturi scrubbers

ESP

Electrostatic precipitator

Separates solids from gas using an electric charge between two plates

Rating and sizing dry electrostatic precipitators

HyCyc

Hydrocyclone

Separates solids from liquid using liquid vortex in a hydrocyclone

Rating or sizing hydrocyclones

CFuge

Centrifuge filter

Separates solids from liquid using a rotating basket

Rating or sizing centrifuges

Filter

Rotary vacuum filter

Separates solids from liquid using a continuous rotary vacuum filter

Rating or sizing rotary vacuum filters

SWash

Single-stage solids washer

Models recovery of dissolved components from an entrained liquid of a solids stream using a washing liquid

Single -stage solids washer

CCD

Counter-current decanter

Models multi-stage recovery of dissolved components from an entrained liquid of a solids stream using a washing liquid

Multi-stage solids washers

8-1

Solids

This chapter is organized into the following sections:

8-2

Section

Models

Crystallizer

Crystallizer

Crushers and Screens

Crusher, Screen

Gas-Solid Separators

FabFl, Cyclone, VScrub, ESP

Liquid-Solid Separators

HyCyc, CFuge, Filter

Solids Washers

SWash, CCD

Unit Operation Models Version 10

Chapter 8

Crystallizer Mixed Suspension Mixed Product Removal Crystallizer Crystallizer models a mixed suspension, mixed product removal (MSMPR) crystallizer. It performs mass and energy balance calculations and optionally determines the crystal size distribution. Crystallizer assumes that the product magma leaves the crystallizer in equilibrium, so the mother liquor in the product magma is saturated. The feed to Crystallizer mixes with recirculated magma and passes through a heat exchanger before it enters the crystallizer. The product stream from Crystallizer contains liquids and solids. You can pass this stream through a hydrocyclone, filter, or other fluid-solid separator to separate the phases. Crystallizer can have an outlet vapor stream.

Flowsheet Connectivity for Crystallizer Vapor (optional) Material (any number)

Liquid and Solid Heat (optional)

Heat (optional)

Material Streams Inlet

At least one material stream

Outlet One material stream for liquid and solid

One optional vapor stream The outlet material stream should normally have at least one solid substream for the crystals formed. If you select Calculate PSD from Growth Kinetics or UserSpecified Values on the PSD PSD sheet, each substream must have a particle size distribution (PSD) attribute.

Unit Operation Models Version 10

8-3

Solids

If electrolyte salts are formed based on electrolyte chemistry calculations, a solid substream is not required when you select Copy from Inlet Stream on the PSD PSD sheet. If you do not use the vapor outlet stream, vapor products will be placed in the liquid/solid product stream.

Heat Streams Inlet

Any number of optional inlet heat streams

Outlet One optional outlet heat stream

If you give only one specification on the Setup Specifications sheet (temperature or pressure), Crystallizer uses the sum of the inlet heat streams as a duty specification. Otherwise, Crystallizer uses the inlet heat streams only to calculate the net heat duty. The net heat duty is the sum of the inlet heat streams minus the actual (calculated) heat duty. You can use an optional outlet heat stream for the net heat duty.

Specifying Crystallizer Crystallizer calculates crystal product flow rate and/or vapor flow, based on solubility data you supply. Or you can specify the chemistry for electrolyte systems instead of specifying solubility data. You must specify two of the following: • Crystallizer temperature • Pressure or pressure drop • Heat duty for the heat exchanger • Crystal product flow rate • Vapor flow

8-4

If you specify

Crystallizer calculates

Temperature and Pressure

Heat duty, crystal product flow rate, vapor flow rate

Pressure and Heat Duty

Temperature, crystal product flow rate, vapor flow rate

Temperature and Heat Duty

Pressure, crystal product flow rate, vapor flow rate

Pressure and Crystal Product Flow Rate

Temperature, heat duty, vapor flow rate

Temperature and Crystal Product Flow Rate

Pressure, heat duty, vapor flow rate

Pressure and Vapor Flow Rate

Temperature, heat duty, crystal product flow rate

Temperature and Vapor Flow Rate

Pressure, heat duty, crystal product flow rate

Unit Operation Models Version 10

Chapter 8

Use the following forms to enter specifications and view results for Crystallizer: Use this form

To do this

Setup

Specify operating parameters, crystal product and solubility parameters, recirculation options, and flash convergence parameters

PSD

Specify PSD and crystal growth calculation parameters

Advanced

Specify component attributes, convergence parameters, and name and parameters for user solubility subroutine

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of Crystallizer results, material and energy balance results, and crystal size distribution results

Recirculation Specifications You can model crystallizer with or without magma recirculation. To activate recirculation, specify one of the following on the Setup Recirculation sheet: • Recirculation fraction • Recirculation flow rate • Temperature change across heat exchanger If you want to model a different crystallization process flowsheet, you can use Crystallizer without recirculation, and use other blocks in the flowsheet to model the recirculation.

Solubility Crystallizer calculates the amount of crystal produced at its saturation (class II crystallization). You can provide solubility data in one of these ways: • Enter solubility data on the Setup Solubility sheet • Reference an electrolyte chemistry (defined in the Reactions Chemistry forms) in which the crystallizing component has been declared as a "salt" • Supply a subroutine to provide the saturation concentration or to calculate crystal product flow rate directly

Unit Operation Models Version 10

8-5

Solids

Saturation Calculation Method Choose the saturation calculation method from these options: • Solubility method: Identify the crystallizing component as solid product on the Setup Crystallization sheet. Enter solubility data on the Setup Solubility sheet. This data applies to the reactant species in the mixed substream. • Chemistry method: Create a new Chemistry on the Reactions Chemistry object manager. Enter the crystallization as a salt reaction on the Reactions Stoichiometry sheet. On the BlockOptions Properties sheet of the crystallizer, enter the Chemistry ID and select True Species for Simulation Approach. You must specify the crystallizing component as a Salt Component ID on the Setup Specifications sheet. • User Subroutine method: Identify the crystallizing component on the Setup Crystallization sheet and the solubility data basis and solvent ID on the Setup Solubility sheet. Specify a user subroutine to calculate saturation concentration or crystallizer yield on the Advanced UserSubroutine sheet. In general, when using the Solubility method, you should blank out the Chemistry ID field on the BlockOptions Properties sheet. If you specify chemistry when using the Solubility method, the chemistry must not contain the crystallizing component.

Supersaturation The degree of supersaturation is the driving force for crystallization processes. Supersaturation is defined as:

S = C − Cs Where: S

=

3 Supersaturation (kg of solute/m of solution)

C

=

Solute concentration

Cs

=

Solute saturation concentration

Because the crystallizer model assumes that the product magma is in phase equilibrium, this equation is not used. It is provided only for reference.

8-6

Unit Operation Models Version 10

Chapter 8

Crystal Growth Rate The crystal growth rate can be expressed as a function of the degree of supersaturation (S):

Go = kg S n Where:

Go

=

Growth rate dependence on supersaturation (m/s)

kg

=

Growth rate expression coefficient

n

=

Exponent

This expression is provided as background information only. In ASPEN PLUS, G o is calculated implicitly from the third moment of the population density. For a size-dependent growth rate, the growth rate is a function of crystal length (L):

G = G o (1 + γL )α

For 0 ≤ α ≤ 1

Where:

γ

=

Constant

α

=

Exponent

If the growth rate is independent of crystal size, then the values for γ and α are set to zero.

Unit Operation Models Version 10

8-7

Solids

Crystal Nucleation Rate The overall nucleation rate can be expressed as the sum of specific contributing factors (Bennett, 1984)1:

B o = kb G i MTj R k Where:

Bo

=

Overall nucleation rate

i, j, k

=

Exponents

kb

=

Overall nucleation rate expression coefficient

MT

=

Magma density = P/q (kg/m )

G

=

Crystal growth rate

R

=

Impeller rotation rate (revs/s)

P

=

Crystal mass flow rate (kg/s)

q

=

3 Volumetric flow rate of slurry in the discharge (m /s)

3

Population Balance If the feed stream contains no crystals, the population balance for a well-mixed continuous crystallizer can be written as (Randolph and Larson, 1988) 2:

d (nG ) qn + =0 dL V Where:

8-8

G

=

Crystal growth rate

n

=

3 Population density (no. /m /m)

L

=

Crystal length (m)

V

=

Crystallizer volume (m )

q

=

3 Volumetric flow rate of slurry in the discharge (m /s)

3

Unit Operation Models Version 10

Chapter 8

The boundary condition is n = n o at L = 0, where n o = B o / G is the population density of nuclei. For a constant crystal growth rate, the population density is:

−L n( L) = n o exp    Gτ  where τ = V / q is the crystal residence time.

PSD Statistics ASPEN PLUS calculates the crystal size distribution statistics once you select the Calculate PSD from Growth Kinetics option on the PSD PSD sheet. Properties of the distribution may be evaluated from the moment equations. The j-th moment of the particle size distribution is defined as: ∞

m j = ∫ Lj n( L) dL 0

The system reports several crystal size distribution statistics, measured on a volume or mass basis, including: • Mean size • Standard deviation • Skewness • The coefficient of variation (expressed as a percentage) The mean size is the mass-weighted average crystal size, as determined by the ratio of the fourth moment to the third moment, as follows:

L=

m4 m3

The skewness of a symmetric size distribution about the mean is zero. Negative values of skewness indicate the distribution is skewed toward the presence of small crystals. Positive values of skewness indicate the crystal distribution contains an excess of large crystals. Skewness is defined as

Unit Operation Models Version 10

∑ f ( x − mean) 3 (standard deviation) 3

8-9

Solids

The system uses the coefficient of variation to calculate variation related to the cumulative volume (or mass) distribution.

Coeff − Var(%) = 100

pd @ (.84) − pd @ (.16) 2 pd @ (.50)

where pd@ (x) is the particle diameter corresponding to fraction x of the cumulative volume (or mass) distribution. The fraction can be entered as the Fractional Coefficient on the PSD CrystalGrowth sheet; otherwise, it defaults to .16.

Calculating PSD The magma density, defined as total mass of crystals per unit volume of slurry, can be obtained from the third moment: ∞

M T = ρ c k v ∫ L3 n( L) dL 0

Where:

ρc

=

Density of crystal (kg/m )

kv

=

Volume shape factor of the crystal

3

Since:

−L n( L) = n o exp   ,  Gτ 

no =

Bo , Go

and B o = kb G i MTj R k these equations can be substituted into the third moment of population density, yielding:

M T = ρc k v ∫

∞

0

Gi − L L kb o M Tj R k exp  dL G  Gτ  3

where G = G o (1 + γL )α . Because L is made discrete by the increments of the particle size distribution, the equations can be solved for G o .

8-10

Unit Operation Models Version 10

Chapter 8

References 1. Bennett, R.C. "Crystallization from Solution," Perry’s Chemical Engineers’ Handbook, 6th Ed., pp. 19.24-19.40, McGraw-Hill, 1984. 2. Randolph, A.D. and Larson, M.A., Theory of Particulate Processes, 2nd Ed., Academic Press, 1988.

Unit Operation Models Version 10

8-11

Solids

8-12

Unit Operation Models Version 10

Chapter 8

Crusher Solids Crusher Use Crusher to simulate the breaking of solid particles. Crusher can model the wet or dry continuous operation of: • Gyratory/jaw crushers • Single-roll crushers • Multiple-roll crushers • Cage mill impact breakers Crusher assumes the feed is homogeneous. The breaking process creates fragments with the same composition as the feed. Crusher calculates the power required for crushing, and the particle size distribution of the outlet solids stream. Crusher does not account for the heat produced by the breaking process.

Flowsheet Connectivity for Crusher Feed

Crushed Solids

Work (optional)

Material Streams Inlet

One material stream with at least one solids substream

Outlet One material stream

Each solids substream must have a particle size distribution (PSD) attribute.

Unit Operation Models Version 10

8-13

Solids

Work Streams Inlet

No inlet work streams

Outlet One work stream containing the calculated power requirement

(optional)

Specifying Crusher Use the Input Specifications and Grindability sheets to specify operating conditions. You must enter the type of crusher and maximum particle diameter on the Input Specifications sheet. You must also specify the Bond work index or the Hardgrove grindability index for each solids substream on the Grindability sheet. The outlet flow rate of crushed product in the k-th size interval is:

Pk (β) = ∑ j

∑ i

Fij Si (β) Bik (β) + ∑ [1 − Sk (β)]Fkj j

Where:

Bik

=

Breakage function. Fraction of particles originally in size interval i that end up in size interval k

Fij

=

Flow rate of feed in the size interval i and particle size distribution j

Pk

=

Flow rate of solid in size interval k

Si

=

Selection function. Fraction of feed particles in size interval i to be crushed at the outlet diameter β

Sk

=

Selection function. Fraction of feed particles in size interval k to be crushed at the outlet diameter β

β

=

Crusher outlet diameter (Maximum Particle Diameter field)

i

=

Size interval counter within a PSD

j

=

PSD counter for multiple size distribution

If the inlet stream contains no liquid, then Crusher assumes dry crushing, and power requirements increase by 34%.

8-14

Unit Operation Models Version 10

Chapter 8

You can enter tabular values for the breakage ( Bik ) function on the Input BreakageFunction sheet and for the selection ( Si ) function on the Input SelectionFunction sheet, or let Crusher use the built-in tables (U.S. Bureau of Mines, 1977) (see the following two tables).

Breakage Function Correlations B ik (β) Feed size/solids outlet diameter 1.7 Ratio of product size to feed size

Multiple roll crusher

Gyratory/jaw crusher

Single roll crusher

Cage mill crusher

All crushers

1.0

1.0

1.0

1.0

1.0

1.0

0.8308

0.95

0.95

0.96

0.84

0.8972

0.5882

0.85

0.85

0.79

0.50

0.7035

0.4176

0.65

0.70

0.45

0.32

0.54

0.2065

0.35

0.35

0.20

0.15

0.2952

0.1041

0.22

0.20

0.10

0.052

0.1564

0.0522

0.14

0.19

0.05

0.019

0.0805

0.0368

0.11

0.17

0.03

0.011

0.0572

0.026

0.09

0.12

0.02

0.0066

0.0406

0.0131

0.03

0.08

0.0

0.002

0.0206

0.0

0.0

0.0

0.0

0.0

0.0

Selection Function Correlations, Si (β) Ratio of feed size to outlet diameter

Primary crusher

Secondary crusher

0.95

0.5695

0.7693

0.9

0.3817

0.6962

0.8

0.1716

0.5695

0.7

0.0771

0.4667

0.6

0.0347

0.3817

0.5

0.0156

0.3128

0.4

0.007

0.256

0.3

0.00315

0.2096

0.2

0.00145

0.1716 continued

Unit Operation Models Version 10

8-15

Solids

Ratio of feed size to outlet diameter

Primary crusher

Secondary crusher

0.1

0.0006

0.1405

0.05

0.00043

0.1271

0.001

0.00026

0.1153

0.0001

0.00026

0.1148

If the ratio of feed size to outlet diameter is greater than 1.0, then Si (β) = 0.85 . Use the following forms to enter specifications and view results for Crusher: Use this form

To do this

Input

Enter crusher operating parameters, the Bond work index or the Hardgrove grindability index, and user-specified selection and breakage functions

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of Crusher results and material and energy balances

Primary and Secondary Crushers Crushing operations are usually performed in stages. The reduction ratio is the ratio of the maximum diameter of feed particles to product particles. The reduction ratio in crushers ranges from 3 to 6 per stage. Feed particles are fed to the primary crushers. Outlet particles from the primary crushers are reduced further by the secondary crushers. Crusher uses different correlations for primary and secondary crushers. Use the Operating Mode field on the Input Specifications sheet to enter the type of crusher. To improve the efficiency of multistage crushers, use screens between stages.

Power Requirement The following equation determines the power requirement for Crusher:

POWER =

8-16

(

0.01

)

X F − X p × BWI × FLOWT XF × Xp

Unit Operation Models Version 10

Chapter 8

Where: POWER

=

Required power (Watt)

XF

=

Diameter larger than 80% of feed particle mass (m)

XP

=

Diameter larger than 80% of product particle mass (m)

BWI

=

Bond work index

FLOWT

=

Total solids mass flow rate (kg/s)

For dry crushing, power requirement increases by 34%. If X p is less than 70 micrometers, then the power required is further adjusted by:

 10.6 × 10 −6 + X p   POWER = POWER  Xp 1145 .  

Bond Work Index The Bond equation defines the work consumed in size reduction:

XF −

E = Ei

XF

XP

100 XP

Where: E

=

Work required to reduce a unit weight of feed with 80% passing a diameter X F microns to a product with 80% passing a diameter X p microns

Ei

=

Bond work index, that is, the work required to reduce a unit weight from a theoretical infinite size to 80% passing a diameter of 100 micrometers

The Bond work index is a semi-empirical parameter that depends on the properties of the material processed. The Bond work indices have been measured experimentally for a wide range of materials, and are available in Perry’s Chemical Engineers’ Handbook. Use experimental values with caution. The Bond work index is also a function of the: • •

Unit Operation Models Version 10

Particle size for non-homogeneous materials Efficiency of the size-reduction equipment

8-17

Solids

Hardgrove Grindability Index The Hardgrove grindability index indicates the difficulty of grinding coal based on physical properties such as hardness, fracture, and tensile strength. The Hardgrove grindability index can be approximated by:

BWI =

435 HGI 0.91

Where: BWI

=

Bond work index

HGI

=

Hardgrove grindability index

The HGI for some United States coals are available in Perry’s Chemical Engineers’ Handbook.

References 1. Computer Simulation of Coal Preparation Plants, U.S. Bureau of Mines, Grant No. GO-155030, Final Report August (1977). th 2. Perry’s Chemical Engineers’ Handbook, 6 Ed., McGraw Hill, 1984.

8-18

Unit Operation Models Version 10

Chapter 8

Screen Solids Separator Screen simulates the separation by screens of a mixture containing various sizes of solid particles into particles that have more uniform sizes than the original mixture. You can use Screen to model wet or dry operations and upper or lower level screens. Screen calculates the separation efficiency of the screen from the size of screen openings you specify.

Flowsheet Connectivity for Screen Overflow Feed Underflow

Material Streams Inlet

One material stream with at least one solids substream

Outlet One material stream for particles that do not pass through the

screen (overflow) One material stream for particles that pass through the screen (underflow) Each solids substream must have a particle size distribution attribute.

Specifying Screen Use the Input Specifications sheet to enter: • • • •

Unit Operation Models Version 10

Screen size opening Operating level (Upper or Lower) Operating mode (Wet or Dry) Entrainments

8-19

Solids

You can also use the Input SelectionFunction sheet to enter the following functions: • Selection function ( Si ) (optional) • Separation strength (optional) Use the following forms to enter specifications and view results for Screen: Use this form

To do this

Input

Specify screen parameters, operating conditions, and user-specified screen separation strength and selection functions

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of Screen results and material and energy balances

Upper and Lower Level Screens You can specify the operating level as Upper or Lower. The most efficient configuration is a multiple-deck screen with a series of Screen blocks. The inlet stream is fed over the upper level screen. The underflow from the upper level screens is fed over the lower level screens. Screen uses different correlations for upper and lower level screens. Screen calculates the flow rate of the screen overflow stream as:

Fo = ∑ Si ∑ Fij i

j

Where:

Si

=

Selection function. The fraction of feed particles in size range i that passes over the screen into the overflow product

Fij

=

Flow rate of feed in size range i and particle size distribution attribute j

Selection Function and Separation Strength Screen calculates the selection function for a certain size interval as:

Si =

exp A 1 − d p S o

Si = 1

8-20

[(

1

)]

for d p < S o

for d p ≥ S o

Unit Operation Models Version 10

Chapter 8

Where:

dp

=

Particle diameter

So

=

Size of screen opening

A

=

Separation strength

The default value of the screen separation strength, A, is a function of the size of the screen opening. Screen has four built-in functions (U.S. Bureau of Mines, 1977)1 for all possible combinations of screen types (see the table, Screen Separation Strength/Screen Size Correlation): • Upper level dry • Lower level dry • Upper level wet • Lower level wet You can enter your own separation strength value, separation strength correlation or selection function correlation on the Input SelectionFunction sheet. Screen then uses these selection function values for its mass balance calculation.

Screen Separation Strength/Screen Size Correlation Size of screen opening (m)

Dry, upper level

Dry, lower level

Wet, upper level

Wet, lower level

0.457

60

60

60

60

0.152

20

20

20

20

0.038

8

8

9

9

0.0095

8

6

8.5

6.6

0.00635

5

4

5.5

4.5

0.00236

3

2

3.5

2.3

0.00059

0.7

0.7

0.8

0.8

0.00042

0.6

0.6

0.7

0.7

0.000295

0.5

0.5

0.55

0.55

Separation Efficiency The separation efficiency of the screen is calculated as the ratio of the mass flow rate of the underflow to the fraction of the feed flow rate containing particles smaller than the screen openings.

Unit Operation Models Version 10

8-21

Solids

References Computer Simulation of Coal Preparation Plants, U.S. Bureau of Mines, Grant No. GO-155030, Final Report August (1977).

8-22

Unit Operation Models Version 10

Chapter 8

FabFl Fabric Filter FabFl is a gas-solids separator model used to separate an inlet gas stream containing solids into a solids stream and a gas stream carrying the residual solids. Use FabFl to simulate or design baghouse units in which solid particles are separated from the inlet gas stream. A baghouse consists of a number of cells in which vertically-mounted cylindrical fabric filter bags operate in parallel. You can use FabFl to rate or size baghouses.

Flowsheet Connectivity for FabFl Gas (overflow) Feed Solids (underflow)

Material Streams Inlet

One material stream with at least one solids substream

Outlet One overflow stream for the cleaned gas

One underflow stream for the solids particles Each solids substream must have a particle size distribution (PSD) attribute. Solids may be entrained in the overflow, based on the separation efficiency.

Specifying FabFl Use the Input Specifications sheet to specify operating conditions and baghouse characteristics.

Unit Operation Models Version 10

For these calculations

Set Mode=

And number of cells is

Rating

Simulation

Specified

Sizing

Design

Calculated

8-23

Solids

For sizing or rating calculations: If you enter

FabFl calculates

Maximum allowable pressure drop

Filtration time

Filtration time

Pressure drop

Use the following forms to enter specifications and view results for FabFl: Use this form

To do this

Input

Enter operating conditions, baghouse characteristics, and separation efficiency

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of FabFl results and material and energy balances

Operating Ranges FabFl uses empirical models because no theoretical models exist. Expect unreliable results when operating conditions exceed the ranges of the experimental data on which the models are based. Your data should fall within these ranges: • •

Diameter of solid particles between 10 −7 to 10 −4 m (0.1 to 100 micrometers) Maximum gas velocity through the cloth between 0.1 and 0.2 m/s (20 to 40 ft/min)

Filtering Time When rating fabric filters, FabFl calculates the filtering time t as: t=

∆Pf − ∆Pi CKVo2

Where:

8-24

∆Pf

=

Final pressure drop across collected dust and filter cloth

∆Pi

=

Pressure drop of the clean bag

C

=

Dust concentration

K

=

Dust resistance coefficient

Vo

=

Air to cloth ratio (gas velocity through the cloth)

Unit Operation Models Version 10

Chapter 8

The air to cloth ratio Vo is:

Vo =

Q ( N cell − N shake ) Abag N bag

Where: Q

=

Volumetric flow rate of the gas

N cell

=

Number of cells

N shake

=

Number of cells being cleaned

Abag

=

Total filter surface of all bags

N bag

=

Number of bags per cell

Resistance Coefficient The resistance coefficient K depends on the particle size and nature of solid particles. In an industrial-scale baghouse, the resistance coefficient also varies with time and bag position. If specific resistance coefficients are not available, the following values can be used as rough estimates 1: Dust particle diameter −6 m) ( 10

Resistance coefficients 2 [Pa/(kg/m ) (m/s)]

Less than 20

300,000

20 to 90

60,000

Greater than 90

15,000

These coefficients were determined from a small fabric filter. The filter has an air flow of 2 ft 3 / min through 0.2 ft 2 of cloth area (a filtering gas velocity of 10 ft/min). The pressure drop across the bag and dust was 8 inches of H 2 O . An approximation for the resistance coefficient 2 is: K=

1000 d p2

Where: dp

=

The average particle size in microns

The units for K are (inches of water)/(lbs dust/ft 2 of area)(ft/min velocity).

Unit Operation Models Version 10

8-25

Solids

Separation Efficiency The overall separation efficiency of the baghouse is:

∑∑Sη ij

ηo =

j

ij

i

Total inlet flow rate of solids

=

flow rate of solids removed from the inlet total inlet flow rate of solids

Where: Sij

=

Flow rate of solid j in size increment i

In FabFl, the separation efficiency is a function of the particle diameter of the solids. For large particles (greater than 10 micrometers in diameter), fractional collection efficiency ( ηi ) is 1.0. For particles smaller than 10 micrometers, efficiency decreases rapidly.

ηi

When

1.0

( d p ) av > 10 µm

0.0011 ( d p ) av + 0.989

1µm < (d p ) av < 10 µm

0.495 ( d p ) av + 0.495

( d p ) av < 1µm

You also can enter efficiency as a function of particle sizes on the Input Efficiency sheet to override the built-in correlations.

References 1. Air Pollution Engineering Manual, Public Health Service Publication No. 999AP-40, pp. 106-135, Washington D.C., DHEW (1967). 2. Billings, C.E. and Wilder, J., Handbook of Fabric Filter Technology, Vol. I, NIIS PB 200648.

8-26

Unit Operation Models Version 10

Chapter 8

Cyclone Cyclone Separator Cyclone separates an inlet gas stream containing solids into a solids stream and a gas stream carrying the residual solids. Use Cyclone to simulate cyclone separators in which solid particles are removed by the centrifugal force of a gas vortex. You can use Cyclone to size or rate cyclone separators. In simulation mode, Cyclone calculates the separation efficiency and pressure drop from a user-specified cyclone diameter. In design mode, the cyclone geometry is calculated to meet the user-specified separation efficiencies and maximum pressure drop. In both calculation modes, the particle size distributions of the outlet solids streams are determined.

Flowsheet Connectivity for Cyclone Gas Feed

Solids

Material Streams Inlet

One material stream with at least one solids substream

Outlet One stream for the cleaned gas

One stream for the solids Each solids substream must have a particle size distribution (PSD) attribute.

Unit Operation Models Version 10

8-27

Solids

Specifying Cyclone Use the Input Specifications sheet to specify the type of cyclone and operating conditions. Use the Input Dimensions sheet to enter cyclone dimensions, or use the Input Ratios sheet to enter ratios of cyclone dimensions. To perform these calculations

Specify

Cyclone calculates

Rating

Simulation mode Cyclone Diameter Number of Cyclones

Separation efficiency Pressure drop

Sizing

Design mode Separation Efficiency Maximum Pressure Drop (optional) Maximum Number of Cyclones (optional)

Cyclone diameter Number of cyclones

For rating calculations, if you specify Type=User-Specified or User-Specified Ratios, you can specify cyclone dimensions on the Input Dimensions or Input Ratios sheets. For design calculations, you must also enter the Maximum Number of Cyclones in parallel. If either of the following occurs, Cyclone calculates the number of cyclones in parallel: • The efficiency of a single cyclone is less than the required separation efficiency. • The calculated pressure drop exceeds the maximum pressure drop specified. Use the following forms to enter specifications and view results for Cyclone:

8-28

Use this form

To do this

Input

Enter cyclone specifications, dimensions, dimension ratios, separation efficiencies, and solids loading

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of Cyclone results and material and energy balances

Unit Operation Models Version 10

Chapter 8

Separation Efficiency The overall separation efficiency is: ηm =

flow rate of solids removed from the inlet total inlet flow rate of solids

ηm =

Co − Ci Qo Co − E E = = 1− Co Qo Co Qo Co

Where: Co

=

Concentration of solids in inlet gas

Ci

=

Concentration of solids in outlet cleaned gas

Qo

=

Inlet gas flow rate

E

=

Outlet emission rate of solids in the cleaned gas

Cyclone attains higher separation efficiencies with particles that are 5 to 10 microns or greater in diameter. For particles smaller than 5 microns, Cyclone efficiency decreases. Even with large particles, it is difficult to obtain a collection efficiency greater than 95%. If you enter a design efficiency higher than 95%, use either: • Multi-stage cyclones • Cyclone as a precleaner, followed by other collectors You can specify the Efficiency Correlation field on the Input Specifications sheet. If Efficiency Correlation=User-Specified, you can enter efficiency as a function of particle sizes on the Input Efficiency sheet.

Operating Ranges Cyclone uses correlations that are semi-empirical models. Do not expect satisfactory accuracy when the specified conditions exceed the ranges of experimental data from which the models were developed. In general, the pressure drop should be less than 2500 N / m 2 (10 inches of H 2 O ). The operating pressure should not exceed atmospheric pressure. The inlet gas velocity should be in the range of 15 to 27 m/s (50 to 90 ft/s). The Leith and Licht efficiency correlation is accurate for inlet velocities approximately 25 m/s (80 ft/s). The correlation overestimates the separation efficiency at high velocities.

Unit Operation Models Version 10

8-29

Solids

The Shepherd and Lapple correlation is accurate for particle sizes of 5 to 200 microns. This correlation tends to overestimate the efficiency of large particles (greater than 200 microns). The Shepherd and Lapple correlation also underestimates the efficiency of fine particles (smaller than 5 microns).

Pressure Drop Cyclone calculates the pressure drop (Shepherd and Lapple, 1939)1 as:

∆P = 0.0030 ρ f U t2 N h Where:

ρf

=

Density of the fluid

Ut

=

Inlet gas velocity

Nh

=

Inlet velocity speeds

Use the Input SolidsLoading sheet to enter values to correct for solids loading. The inlet velocity speed, N h , is:

Nk = K

ab De2

Where: K

=

Dimensionless ratio

a

=

Inlet height of the cyclone

b

=

Inlet width of the cyclone

De

=

Outlet diameter of the cyclone

The dimensionless ratio K is:

K=

8(Vs + Vnl / 2) abDc

Where:

8-30

Vs

=

Annular shaped volume above the exit duct to midlevel of the entrance duct

Vnl

=

Effective volume of the cyclone calculated by natural length l

Dc

=

Body diameter of the cyclone

Unit Operation Models Version 10

Chapter 8

The annular shaped volume Vs above the exit duct to midlevel of the entrance duct is: Vs =

π( s − a / 2 ) ( Dc2 − De2 ) 4

Cyclone Diameter Cyclone calculates the diameter of the body of the cyclone Dc as:  Qρ2f  (1 − b / Dc ) Dc = 0.0502  × 2.2   µ(ρ p − ρ f ) ( a / Dc ) (b / Dc ) 

0 . 454

Where: Q

=

Overflow gas flow rate

ρf

=

Density of the fluid

µ

=

Viscosity of gas fluid

ρp

=

Density of the particles

In this empirical equation, units are: Unit type

Unit

Length

Feet

Mass

Pounds

Time

Seconds

Dimension Ratios Use the Input Dimensions sheet to enter the dimensions of a cyclone when Mode=Simulation and Type=User-Specified. If you specify Type=User-Specified Ratios, you can use the Input Ratios sheet to enter dimension ratios (dimension / cyclone diameter) for a cyclone.

Unit Operation Models Version 10

8-31

Solids

The dimension ratios and some default values of the two built-in configurations are: Dimension ratio (dimension/cyclone diameter)

Type = High efficiency

Type = Medium efficiency

Cyclone diameter

1.0

1.0

Inlet height

0.5

0.75

Inlet width

0.2

0.375

Length of overflow

0.5

0.875

Diameter of overflow

0.5

0.75

Length of cone section

1.5

1.50

Overall length

4.0

4.0

Diameter of underflow

0.375

0.375

Number of gas turn in cyclone

7.0

4.0

Maximum diameter (m)

1.5

5.0

Minimum diameter (m)

0.1

0.1

Cyclone calculates the dimensions of the built-in cyclones using these ratios and the cyclone diameter you specify. The built-in configurations (Type=High or Medium) may not be the best designs. It is recommended that you enter dimensions or dimension ratios, if available.

Vane Constant Use the Vane Constant field on the Input Specifications sheet to specify the vane constant. The vane constant varies with the configuration of the inlet duct. In the common configuration, the inlet duct terminates at the wall of the cyclone. The vane constant is 16. To reduce friction loss, extend the duct into the interior of the cyclone. When the duct is in the middle of the cyclone separator, the vane constant is 7.5.

Cyclone Dimensions The next figure shows the Cyclone geometry. The table following the figure shows the Cyclone dimensions.

8-32

Unit Operation Models Version 10

Chapter 8

Dc b

De

s a

h

H

B

Cyclone Geometry The Cyclone design configurations are:

Unit Operation Models Version 10

Term

Description

High efficiency

High throughput

Dc

Body diameter

1.0

1.0

a

Inlet height

0.5

0.75

b

Inlet width

0.2

0.375

s

Outlet length

0.5

0.875

De

Outlet diameter

0.5

0.75

h

Cylinder height

1.5

1.50

H

Overall height

4.0

4.0

B

Dust outlet diameter

0.375

0.375

8-33

Solids

Solids Loading Correction The feed concentration of solids affects the separation efficiency. Concentration higher than 1.0 gm m 3 usually leads to higher efficiency. Smolik (1975)2, 3 presented the following relationship between the efficiency and solids concentration: 1 − ET*  c *  =  1 − ET  c 

a

Where: c*

=

"Low loading" solids concentration, 1.0 gm / m 3

c

=

Solids concentration

E *T

=

Total efficiency

ET

=

"Low loading" total efficiency

α

=

Exponent

Smolik gives values of α = 0.182. This form can only serve as a guide, because the effect of dust concentration depends on the nature of the solids, the humidity of the gas, and many other factors that do not figure in the existing correlations. The actual pressure drops with dust-laden gases are normally lower than those obtained with clean gas. Smolik gives an empirical correlation for the effect of feed concentration on pressure in the form: ∆p * = 1 − βc γ ∆p

Where: c

=

Solids concentration in the feed, g / m 3

∆p *

=

Pressure drop

∆p

=

Pressure drop with clean gas

β ,γ

=

Constants depending on the material

Smolik gives values of β = 0.02 and γ 0.6.

8-34

Unit Operation Models Version 10

Chapter 8

References 1. Shepherd, G.B. and Lapple, C.E., "Flow Pattern and Pressure Drop in Cyclone Dust Collectors," Industrial and Engineering Chemistry, 31, pp. 972984 (1939). 2. Smolik, J. et al., Air Pollution Abatement, Part I. Scriptum No. 401-2099 (in Czech). Technical University of Prague (1975). Quoted by Svarovsky, L., "Solid-Gas Separation," Handbook of Powder Technology, Williams, J.C. and Allen, T. (Eds.), Amsterdam: Elsevier, 1981. 3. Svarovsky, L., Solid-Gas Separation, Chapter 3, New York: Elsevier, 1981.

Unit Operation Models Version 10

8-35

Solids

VScrub Venturi Scrubber Use VScrub to simulate venturi scrubbers. Venturi scrubbers remove solid particles from a gas stream by direct contact with an atomized liquid stream. You can use VScrub to rate or size venturi scrubbers.

Flowsheet Connectivity for VScrub Liquid

Gas Feed Gas with Solids

Liquid and Solids

Material Streams Inlet

One stream for solids with at least one solids substream One stream for the atomized liquid

Outlet One stream for the cleaned gas

One stream for the liquid with solid particles

8-36

Unit Operation Models Version 10

Chapter 8

Specifying VScrub Use the VScrub Input Specifications sheet to specify operating conditions and parameters for sizing or rating calculations.

To perform these calculations

Set Mode =

Enter scrubber

VScrub calculates

Rating

Simulation

Throat Diameter Throat Length

Separation efficiency Pressure drop

Design

Separation efficiency

Liquid flow rate Throat diameter Throat length Pressure drop

†

Sizing

†

Because the required liquid flow rate is varied to meet the efficiency, the material balance is not satisfied if the calculated liquid flow rate is different from the rate you enter.

In both modes, VScrub also calculates the particle size distributions of the solids in the outlet streams. VScrub assumes that the liquid stream is introduced before or at the beginning of the scrubber throat. It also assumes the separation of the solid particles from the gas stream occurs only at the scrubber throat. Use the following forms to enter specifications and view results for VScrub:

Unit Operation Models Version 10

Use this form

To do this

Input

Specify operating parameters and throat operating conditions

Block Options

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of VScrub results and material and energy balances

8-37

Solids

Pressure Drop VScrub calculates the pressure drop (Yung, S. et al., 1977)1 ∆ p across the throat of the scrubber as: ∆p =

2ρl Vt 2 gc

(

 Ql    1 − x 2 + x 4 − x 2  Qg 

)

Where: ρl

=

Density of the liquid

Vt

=

Relative velocity of gas to liquid at the throat

gc

=

Conversion factor in Newton’s law of motion

Ql Qg

=

Liquid to gas volume flow rate

x

=

Dimensionless throat length defined by: 3lt C D ρ g x= +1 16 Dd ρl

lt

=

Throat length

CD

=

Drag coefficient, as a function of the Reynolds number (Dickinson and Marshall, 1968)2 N Re .

Where:

C D = .22 +

24 0.6 (1 + 0.15 N Re ) N Re

ρg

=

Density of the gas

ρl

=

Density of the liquid

Dd

=

Drop diameter (Sauter mean), defined by (Nukiyama, S., Tanasawa, Y. 1939)3: 585  σ l    Vt  ρl 

0.5

 µl  + 597    σ l ρ l 

0 . 45

1000Ql     Qg 

1.5

Where:

8-38

σl

=

Surface tension

µl

=

Viscosity of liquid

Unit Operation Models Version 10

Chapter 8

Separation Efficiency The separation efficiency (Yung, S., et al., 1978) 4 ηo is defined as: ηo =

=

Mass flow rate of particles in outlet liquid stream Mass flow rate of particles in inlet gas stream

∑ Sη i

i

Total inlet flow rate of solids

Where:

ηi

=

Efficiency for size increment i

Si

=

Mass flow rate of size increment i

References 1. Yung, S. et al., Journal of the Air Pollution Control Association, 27, 348 (1977). 2. Dickinson, D.R. and Marshall, W.R., AIChE Journal, 14, 541, (1968). 3. Nukiyama, S. and Tanasawa, Y., Transcripts of the Society of Mechanical Engineers (Japan), 5, 63 (1939). 4. Yung, S. et al., Environmental Science and Technology, 12, 456 (1978).

Unit Operation Models Version 10

8-39

Solids

ESP Electrostatic Precipitator Use ESP to simulate dry electrostatic precipitators. Dry electrostatic precipitators separate solids from a gaseous stream. Electrostatic precipitators have vertically mounted collecting plates with discharge wires. The wires are parallel and positioned midway between the plates. The corona discharge of the high-voltage wire electrodes first charges the solid particles in the inlet gas stream. Then the electrostatic field of the collecting plate electrodes removes the solids from the gas stream. You can use ESP to size or rate electrostatic precipitators.

Flowsheet Connectivity for ESP Gas Feed Solids

Material Streams Inlet

One material stream with at least one solids substream

Outlet One material stream for the cleaned gas

One material stream for the solids Each solids substream must have a particle size distribution (PSD) attribute.

8-40

Unit Operation Models Version 10

Chapter 8

Specifying ESP Use the Input Specifications sheet to specify parameters for sizing or rating calculations. To perform these calculations

Set Mode=

Enter

ESP calculates

Rating

Simulation

Number of plates Plate height Plate length

Separation efficiency Power required Corona voltage Pressure drop Precipitator width

Sizing

Design

Separation efficiency

Number of plates Precipitator dimensions Power required Pressure drop

You can specify maximum dimensions for sizing calculations on the Input Specifications sheet. Use the following forms to enter specifications and view results for ESP: Use this form

To do this

Input

Specify operating parameters and dielectric constants and precipitator dimensions

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of ESP results and material and energy balances

Operating Ranges The velocity of gas should be between 1 and 2.5 m/sec (for plate spacing 200 and 300 mm). If the gas velocity is larger than 3 m/s or less than 0.5 m/s, then the models for efficiency and pressure drop are not valid. This is because the transport of fine particles by turbulent diffusion may become more significant than transport by electrostatic force. ESP models wire-and-plate precipitators with relatively high dust concentration (≥ 1011 particles / m 3 or 0.1 kg / m 3 ). If the particle concentration is too low, ESP may overestimate the results. ESP is not suitable for a cylindrical electrostatic precipitator.

Unit Operation Models Version 10

8-41

Solids

Separation Efficiency The separation efficiency is defined as (Crawford, M. 1976)1: ηov =

Mass outlet flow rate of solids Total mass flow rate of the inlet solids substream

ηov = 1 −

 ( X s − L)q ps E c C  Cnvs exp   Cnvo 3πµdWV  

Where: Cnvs

=

Particle concentration at X s

Cnvo

=

Particle concentration at inlet

Xs

=

Point at which all particles have acquired a saturation charge

L

=

Plate length

q ps

=

Particle saturation charge

Ec

=

Collecting field strength ( = 0.25( Eo ))

C

=

Conningham correction factor

µ

=

Viscosity of the gas

d

=

Particle diameter

W

=

Distance between wires and plates

V

=

Actual gas velocity through the precipitator

The point at which all particles have acquired a saturation charge X s , is defined as:

Xs =

µdW 2 swV (Cnvo − Cnvs ) 0.332ε o E c C (0.8 E c Wsw − E 0 r0 )

Where:

8-42

sw

=

Distance between two wires

εo

=

Electric permissivity constant = 8.85 x 10 −12 c / vm

Eo

=

Corona field strength 2

ro

=

Corona radius

Unit Operation Models Version 10

Chapter 8

The collecting field strength Ec , is defined as:  T P To P     E c = 0.25  − VB f  o + 0.03 TPo ro    TPo 

Where: VB

=

Breakdown voltage

f

=

Roughness factor of wire

To

=

Atmospheric temperature

Po

=

Atmospheric pressure

T

=

Temperature

P

=

Pressure

The particle concentration at the point where the particles first have saturation charge, Cnvs is:

Cnvs =

0.212( k + 2) kd 2

0.8 E c

Ws w − E o ro

0.427 Ws w E c + 2 E o ro (0.533 Ws w − ro )

Where: k

= Dielectric constant ( = ε / ε o )

The particle saturation charge, q ps is: q ps =

3kπε o d 2 k+2

2 2.5 Eo ro  Ec + Wsw  3

 2 1.25 ro   −  Wsw  3

  

Pressure Drop ESP calculates the pressure drop across the precipitator as: ∆p = 45.5 ρ g Vg2

Where:

Unit Operation Models Version 10

ρg

=

Gas density

Vg

=

Gas velocity

8-43

Solids

Required Power The power required2 Pw to meet a specified separation efficiency is: Pw = 52.75 ln(1 − ηov ) Q

Where: Q

= Volumetric gas flow rate

Gas Velocity The models used in ESP are valid for inlet gas velocities ranging from 0.5 to 3 m/s. Outside this range, transport by turbulent diffusion becomes more significant than by electrostatic force and large errors should be expected.

Particle Diameter You can use ESP to model the separation of fine particles with diameters ranging from 0.01 to 10 microns. ESP is accurate when the inlet particle concentration is high (≥ 1011 particles / m 3 or 0.1 kg / m 3 ). If the concentration is too low, the model tends to overestimate the separation efficiency.

References 1. Crawford, M., Air Pollution Control Theory, Chapter 8: Electrostatic Precipitation, pp. 298-358. New York: McGraw-Hill, 1976. 2. White, H.J., Industrial Electrostatic Precipitation, 204, pp. 91-92 (1963).

8-44

Unit Operation Models Version 10

Chapter 8

HyCyc Hydrocyclone Solids Separator Use HyCyc to simulate hydrocyclones. Hydrocyclones separate solids from the inlet liquid stream by the centrifugal force of a liquid vortex. You can use HyCyc to rate or size hydrocyclones. In simulation mode (rating), HyCyc calculates the particle diameter with 50% separation efficiency from the user-specified hydrocyclone diameter. In design mode (sizing), HyCyc determines the hydrocyclone diameter required to achieve the user-specified separation efficiency of the solids with the desired particle size. In both calculation modes, pressure drop and the particle size distribution of the outlet solids streams are determined.

Flowsheet Connectivity for HyCyc Liquid

Feed

Solids

Material Streams Inlet

One liquid stream with at least one solids substream

Outlet One stream for the cleaned liquid with residual solids

One stream for solids Each inlet solids substream must have a particle size distribution (PSD) attribute.

Unit Operation Models Version 10

8-45

Solids

Specifying HyCyc Use the Input Specifications sheet to specify hydrocyclone operating conditions. To perform these calculations

Enter

HyCyc calculates

Rating

Simulation Mode Hydrocyclone Diameter

Separation efficiency Particle diameter with 50% separation efficiency Pressure drop, particle size distribution of outlet solids stream

Sizing

Design Mode Separation Efficiency

Hydrocyclone diameter Pressure drop, particle size distribution of outlet solids stream

To obtain practical dimensions when sizing hydrocyclones, enter the: • Maximum diameter of the hydrocyclone • Maximum pressure drop allowed across the hydrocyclone HyCyc designs multiple hydrocyclones in parallel if one of the following conditions exists: • The calculated diameter is greater than the maximum specified diameter. • The calculated pressure drop is greater than the maximum specified pressure drop. Use the following forms to enter specifications and view results for HyCyc: Use this form

To do this

Input

Specify simulation parameters, dimensions, tangential velocity correlation parameters, and separation efficiency

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of HyCyc results and material and energy balances

Operating Ranges HyCyc uses empirical and semi-empirical correlations. Expect unreliable results when operating conditions (Bradley, D., 1965)1 are outside the ranges of experimental data on which the models are based. In general, your data should fall within these ranges: • • • •

8-46

Particle diameter between and (5 to 200 micrometers) Hydrocyclone diameter between 0.01 and 0.6 m Pressure drop between 35 and 345 kPa Separation efficiency between 2% and 98%

Unit Operation Models Version 10

Chapter 8

The solids concentration should be less than 11% of the volume fraction, or less than 25% of the weight fraction.

Separation Efficiency Separation efficiency E is defined as:

E=

mass underflow rate of solids mass feedflow rate of solids

Reduced efficiency E ′ is defined as the fraction of feed solids that go to the underflow minus the fraction of the feed liquid that also goes to the underflow. E′ =

E − Rf 1 − Rf

Where R f is the volumetric ratio of underflow to feed flow (see Material Split , this chapter). The reduced efficiency is obtained from the following equation 2: 3    d      − 0.115   E ′ = 100 1 − exp − d     50   

Where: d

=

Diameter of the solid particles to be separated

d50

=

Particle diameter for which 50% of feed passes through underflow

In turn, d 50 is obtained from the following equation which includes operational and geometric parameters (Bradley, D., 1965)1:

d 50 Dc 3(0.38) n = α Di2

Unit Operation Models Version 10

0.5  µ Dc (1 − R f ) θ tan   2  Q(σ − ρ )

8-47

Solids

Where: Q

=

Volumetric flow rate at inlet

Dc

=

Chamber diameter

Di

=

Inlet diameter

n

=

Power of R in the tangential velocity distribution function

α

=

Inlet velocity loss coefficient

σ

=

Density of solid

Rf

=

Underflow rate/feed rate

θ

=

Cone angle

ρ

=

Density of liquid

µ

=

Viscosity of liquid

Material Split HyCyc splits the feed according to the following empirical correlation (Moder, J.M. and Dahlstrom, D.A., 1952)3: S=β(

Du ) Do

4. 4

Q −.44

Where:

8-48

S

=

Volume split = underflow rate/overflow rate

β

=

A constant, 6.13

Du

=

Diameter for underflow

Do

=

Diameter for overflow

Q

=

Inlet volumetric flow rate (gal/min)

Unit Operation Models Version 10

Chapter 8

The flow ratio R f (underflow rate/feed rate) is then obtained by: 1 − Rf =

1 1+ S

Tangential Velocity The following empirical correlation gives the tangential velocity V (Dahlstrom, D.A., 1954)4 in a hydrocyclone at a radius R: D  VR n = constant = α Vi  c   2 

n

Where: α

=

Inlet velocity loss coefficient

Vi

=

Inlet velocity

Dc

=

Diameter of the hydrocyclone

n

=

Exponent of radial dependence

R

=

Radius

For most cases, α and n are determined experimentally to be 0.45 and 0.8. These two variables are then used to determine d 50 .

Dimension Ratios Common hydrocyclones have the following ranges of dimension ratios (dimension/chamber diameter):

Unit Operation Models Version 10

Inlet diameter:

1/7

to

1/3

Length:

4

to

12

Overflow diameter:

1/8

to

1/2.3

Underflow diameter:

1/10

to

1/5

Cone angle:

9 deg.

to

20 deg.

8-49

Solids

Pressure Drop For the pressure drop correlation to be valid (overflow diameter/underflow diameter) should be 0.6 to 2.0. HyCyc uses the empirical pressure drop correlation (Dahlstrom, D.A., 1954)4: Q = 6.38 ( Do × Di ) 0.9 H 0.5

Where: Q

=

Volumetric flow rate (US gallons/minute) at the inlet

H

=

Height of fluid (feet) or length of vortex finder

Do

=

Overflow diameter

Di

=

Inlet diameter

Hydrocyclone Dimensions The next figure shows the HyCyc geometry. Inlet Di

Dc Do

L

θ

Du

Hydrocyclone Dimensions

8-50

Unit Operation Models Version 10

Chapter 8

The following table describes the HyCyc dimensions. Term

Description

Dc

Chamber diameter

Di

Inlet diameter

Do

Overflow diameter

Du

Underflow diameter

L

Length of hydrocyclone

θ

Cone angle

References st

1. Bradley, D., The Hydrocyclone, 1 edition., Pergamon Press, London (1965). 2. Yoshioka, H. and Hatta, Y., Kagaku Kagolar, Chemical Engineering, Japan, 19, 633 (1955). 3. Moder, J.M. and Dahlstrom, D.A., Chemical Engineering Progress, 48,75 (1952). 4. Dahlstrom, D.A., “Mineral Engineering Techniques,” Chemical Engineering Progress Symposium Series 50, No. 15, 41 (1954).

Unit Operation Models Version 10

8-51

Solids

CFuge Centrifuge Filter Use CFuge to simulate centrifuge filters. The centrifuge filters separate liquids and solids by the centrifugal force of a rotating basket. Use CFuge to rate or size centrifuge filters. CFuge assumes that the separation efficiency of the solids equals 1, so that the outlet filtrate stream contains no residual solids.

Flowsheet Connectivity for CFuge Liquid Feed Solids

Material Streams Inlet

One material stream with at least one solids substream

Outlet One material stream for the liquid

One material stream for the solids If you specify the particle size distribution (PSD), CFuge calculates the average particle size.

8-52

Unit Operation Models Version 10

Chapter 8

Specifying CFuge Use the Input Specifications sheet to specify operating conditions and the Input FilterCake sheet to specify filter cake properties. To perform these calculations

Enter

CFuge calculates

Rating

Diameter Rate of revolution Filter cake properties

Filtrate flow rate Filter cake moisture content Height of centrifuge basket

Sizing

List of centrifuge diameters and rates of revolution Filter cake moisture content (CFuge estimates if not entered)

Filtrate flow rate Filter cake moisture content Height of centrifuge basket

For sizing calculations, CFuge also calculates the liquid-handling capacities of all of the centrifuges you specify. CFuge selects the centrifuge with a liquid-handling capacity greater than or equal to the required filtrate flow rate. If more than one centrifuge satisfies this criterion, CFuge selects the one with the smallest capacity. If none of the centrifuges satisfies this criterion, CFuge selects the one with the highest filtrate flow rate. In both rating and sizing calculations, CFuge calculates the content and height of the centrifuge basket. Use the following forms to enter specifications and view results for CFuge: Use this form

To do this

Input

Specify centrifuge and filter cake parameters and centrifuge dimensions

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of CFuge results and material and energy balances

Filter Cake Characteristics Use the Input FilterCake sheet to specify: • • • • • •

Unit Operation Models Version 10

Cake resistance Moisture Content Sphericity Medium resistance Porosity The average diameter of the solid particles in the cake

8-53

Solids

The filter cake moisture content is the ratio of the mass flow rate of liquid to that of the solid in the outlet solids stream. The filter cake moisture content is an important design parameter. You should provide it if possible. If you do not enter it, CFuge calculates an estimate from the average particle diameter and cake parameters (Dombrowski, H.S., and Brownell, L.E., 1954) 1. If you enter the particle size distribution (PSD) of the inlet solid stream, CFuge calculates the average particle diameter, so you do not need to enter average diameter on the Input FilterCake sheet.

Filtrate Flow Rate CFuge calculates the filtrate volumetric flow rate from: Q=

1 ( F − WM ) ρl

Where: F

=

Feed liquid volumetric flow rate

M

=

Moisture content, mass of liquid/mass of dried solid (specified as Moisture Content on the FilterCake sheet or calculated by the model)

W

=

Dry solids feed rate

ρl

=

Liquid density

Pressure Drop CFuge calculates the pressure drop (Grace, H.P., 1953) 2 across the filter cake as: ∆p =

ρl ω 2 (r22 − r12 ) 2

Where:

8-54

ω

=

Rotational speed

r1

=

Radius of liquid surface

r2

=

Radius of inner wall of bowl

ρl

=

Liquid density

Unit Operation Models Version 10

Chapter 8

Separation Efficiency Separation efficiency, E, is defined as: E=

underflow rate of solids feedflow rate of solids

CFuge assumes that the separation efficiency of the solids equals 1, so that the outlet filtrate stream contains no residual solids.

References 1. Dombrowski, H.S., and Brownell, L.E., Industrial and Engineering Chemistry, 46, 6, 1207 (1954). 2. Grace, H.P., Chemical Engineering Progress, 49, 8, 427 (1953).

Unit Operation Models Version 10

8-55

Solids

Filter Rotary Vacuum Filter Use Filter to simulate continuous rotary vacuum filters. You can use Filter to rate or size rotary vacuum filters. Filter assumes the separation efficiency of the solids equals 1, so that the outlet filtrate stream contains no residual solids.

Flowsheet Configuration for Filter Filtrate Feed Solids

Material Streams Inlet

One material stream with at least one solids substream

Outlet One material stream for the liquid filtrate

One material stream for the solids

Specifying Filter Use the Input Specifications sheet to specify operating conditions and parameters.

8-56

Unit Operation Models Version 10

Chapter 8

To perform these calculations

Enter

Filter calculates

Rating

Simulation Diameter Width Rate of revolution Filter cake characteristics (optional)

Pressure drop across filter

Sizing

Design Maximum allowable pressure drop across the filter cake and medium Rate of revolution Filter cake characteristics (optional) Width to diameter ratio (optional)

Diameter Width

In both calculation modes, ASPEN PLUS determines the following: • Filtrate volumetric flow rate • Cake thickness • Mass fraction of solids in the solids filter cake Use the following forms to enter specifications and view results for Filter: Use this form

To do this

Input

Specify filter and filter cake parameters

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of Filter results and material and energy balances

Filter Cake Characteristics Filter assumes: • • •

The cake thickness is greater than 0.00635 m. The capillary number is greater than 1. The filter cake is incompressible or compacted uniformly throughout its thickness2

When the specific cake resistance α at the required pressure drop ∆P is not available, Filter can estimate it using the following empirical correlation:

α = α O (∆P)

k

Where:

Unit Operation Models Version 10

αO

=

Specific cake resistance at unit pressure drop

k

=

Cake compressibility

8-57

Solids

You can use this equation for interpolation and short-range extrapolation when some experimental data of α O and ∆P are available. α O is the intercept of the log-log plot of α versus ∆P. α and α O both have the units determined by the specified units set, and ∆P is always in Pascals. Use the Average Diameter field on the FilterCake sheet to specify the average diameter of solid particles in the filter cake. If you enter the particle size distribution (PSD) of the inlet solid stream, Filter calculates the average particle size.

Pressure Drop Filter calculates the pressure drop1 across the filter cake with:

 2 ∆ pωθ V  Q = ω RHV = RH    µα W 

1/ 2

Where: Q

=

Filtrate volume flow rate

ω

=

Angular velocity

R

=

Radius

H

=

Width

V

=

Filtrate volume per unit area

∆p

=

Pressure drop

θ

=

Wetting angle

µ

=

Viscosity

α

=

Filtration resistance

W

=

Solid mass per unit area

Separation Efficiency Separation efficiency, E, is defined as: E=

8-58

underflow rate of solids feedflow rate of solids

Unit Operation Models Version 10

Chapter 8

Filter assumes the separation efficiency of the solids equals 1, so that the outlet filtrate stream contains no residual solids.

References 1. Brownell, L.E. and Katz, D. I., Chemical Engineering Progress, 43, 11, 601 (1947). 2. Dombrowski, H.S. and Brownell, L.E., Industrial and Engineering Chemistry, 46, 6, 1207 (1954). Additional Reading: Brownell, L.E. and Katz, D. I., Chemical Engineering Progress, 43, 10, 537 (1947). Dahlstrom, D.A. and Silverblatt, C.E., Solid/Liquid Separation Equipment Scale Up, Chapter 2, Purchas, D.B., Ed., Uplands Press Ltd. (1977). Silverblatt, C.E., Risbud, H., and Tiller, F.M., Chemical Engineering, 127 (April 27, 1974).

Unit Operation Models Version 10

8-59

Solids

8-60

Unit Operation Models Version 10

Chapter 8

SWash Single-Stage Solids Washer Use SWash to simulate solids washers in which dissolved components in the entrained liquid of a solids stream are recovered by a washing liquid. SWash simulates a single-stage solids washer; it does not consider the presence of a vapor phase. SWash calculates the flow rates and compositions of the outlet solids and liquid streams from a user-specified liquid-to-solid mass ratio of the outlet solids stream and the mixing efficiency of the washer. For non-adiabatic operations, SWash determines the outlet temperature when outlet pressure and heat duty are given. Alternatively, SWash calculates the required heat duty when outlet temperature and pressure are specified.

Flowsheet Connectivity for SWash Liquid

Liquid

Solids

Solids

Heat (optional)

Heat (optional)

Material Streams Inlet

One stream for the solids particles with an entrained liquid One stream for the washing liquid

Outlet One stream for the washed solids particles

One stream for the washing liquid and entrained liquid from the inlet solids stream

Heat Streams Inlet

One stream for heat duty (optional)

Outlet One stream for net heat duty (optional)

Unit Operation Models Version 10

8-61

Solids

If you specify only pressure on the Input OutletFlash sheet, SWash uses the inlet heat stream as a duty specification. Otherwise, SWash only uses the inlet heat stream to calculate the net heat duty. The net heat duty is the inlet heat stream minus the actual (calculated) heat duty. You can use an outlet heat stream for the net heat duty.

Specifying SWash You must specify the mixing efficiency of the washer and the liquid-to-solid mass ratio of the outlet solids stream. For non-adiabatic operations, you must specify the pressure of the washer and one of the following: • The temperature of the washer • Heat duty (or an inlet heat stream without an outlet heat stream) Alternatively, SWash calculates the required heat duty when outlet temperature and pressure are specified. SWash assumes adiabatic operations if neither temperature nor heat duty is specified. Use the following forms to enter specifications and view results for SWash: Use this form

To do this

Input

Specify operating parameters, flash specifications, and convergence parameters

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of SWash results and material and energy balances

Mixing Efficiency The mixing efficiency of the washer, E, is defined as: S S x IN − xOUT E= S L x IN − xOUT

Where:

8-62

S x IN

=

Mass fraction of dissolved components in the entrained liquid of the inlet solids stream

S xOUT

=

Mass fraction of dissolved components in the entrained liquid of the outlet solids stream

L x OUT

=

Mass fraction of dissolved components in the outlet liquid stream

Unit Operation Models Version 10

Chapter 8

Bypass Fraction The bypass fraction is the fraction of liquid in the feed that bypasses the mixing, when mixing efficiency is less than 1. It is calculated as: Bypass fraction = (1 − mixing efficiency) ×

Unit Operation Models Version 10

liquid − to − solid ratio specified for SWash liquid − to − solid ratio in inlet solids stream

8-63

Solids

CCD Counter-Current Decanter CCD simulates a counter-current decanter or a multistage washer. CCD calculates the outlet flow rates and compositions from: • Mixing efficiency • Liquid-to-solid mass ratio of each stage CCD can calculate: • The heat duty profile from a specified temperature profile • The temperature profile from a specified heat duty profile CCD does not consider a vapor phase.

Flowsheet Connectivity for CCD Solids (Top feed)

Overflow

1

Feed To Underflow (optional) Product From Underflow (optional)

Product From Overflow (optional)

Feed To Overflow (optional)

Nstage

Underflow

Washing Liquid (Bottom feed)

Material Streams Inlet

One solids inlet material stream (top feed) One liquid inlet material stream (bottom feed) Any number of optional inlet material side streams per stage

Outlet One top product stream (overflow)

One bottom product stream (underflow) One optional stream per stage for the solids (underflow) One optional stream per stage for the liquid (overflow) Any number of pseudoproduct streams (optional)

8-64

Unit Operation Models Version 10

Chapter 8

Any number of pseudoproduct streams can represent internal underflows or overflows. A pseudoproduct stream does not affect the results of the simulation.

Specifying CCD Use the CCD Input Specifications sheet to enter the number of stages, pressure, mixing efficiency, and liquid-to-solid mass ratio. Use the CCD Input Streams to enter feed, product, and optional heat stream locations. On the CCD Input Temp-DutyProfiles sheet, note the following: If you enter

CCD calculates

Stage temperature

Stage heat duty

Stage heat duty

Stage temperature

Stage overall heat transfer coefficient

Stage temperature

You cannot enter both temperature profiles and heat duties or overall heat transfer coefficients. If you enter stage heat duty and/or an overall heat transfer coefficient, and you do not enter values for all stages, the system assumes unspecified values to be zero. Enter the medium temperature of each stage when you enter overall heat transfer coefficients. Use the Estimated Temperature field to enter estimated stage temperatures. Note

CCD interpolates unspecified values and, by default, assumes them to be the same as the ambient temperature.

Use the CCD Input PseudoStream sheet to transfer the internal overflow or underflow of a stage to a pseudostream. Use the following forms to enter specifications and view results for CCD:

Unit Operation Models Version 10

Use this form

To do this

Input

Specify operating parameters, temperature profile parameters, pseudostream information, and convergence parameters

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of CCD results, material and energy balances, and stage profiles

8-65

Solids

Component Attributes CCD does not consider the mixing of component attributes and PSDs. CCD assumes all outlet solids streams have the same attributes and PSD as the solids feed stream to stage one. CCD also assumes all outlet liquid streams have the same attributes and PSD as the liquid feed stream throughout the final stages.

Multistage Washer Profiles For any CCD profile, such as mixing efficiency, liquid-to-solid-ratio, temperature, duty, enter a value for every stage, as information becomes available. If you enter only some of the values for some stages, CCD generates the complete profile by linear interpolation of the given values. If you enter only one value, CCD assumes a constant profile of that value throughout the washer.

Mixing Efficiency The mixing efficiency of stage n is defined as: S S x IN − xOUT E= S L x IN − xOUT

Where:

8-66

S x IN

=

Mass fraction of dissolved components in the entrained liquid of the total inlet solids stream to stage n.

S x OUT

=

Mass fraction of dissolved components in the entrained liquid of the total outlet solids stream from stage n.

L x OUT

=

Mass fraction of dissolved components in the outlet liquid stream from stage n.

Unit Operation Models Version 10

Chapter 8

Medium Temperature The duty for each stage is calculated according to the following equations:

Qi = UAi (Tcalci − Tmed i ) Where:

Qi

=

Heat duty for stage i

UAi

=

Product of heat transfer coefficient and area for stage i

Tcalci

=

Calculated outlet temperature of stage i

Tmed i

=

Temperature of the heat transfer medium at stage i

❖

Unit Operation Models Version 10

❖

❖

❖

8-67

Solids

8-68

Unit Operation Models Version 10

Chapter 9

9

User Models This chapter describes the models that allow you to write your own unit operation models as Fortran subroutines. These subroutines must follow the guidelines described in the ASPEN PLUS User Models reference manual. The models are:

Unit Operation Models Version 10

Model

Description

Purpose

Use For

User

User-defined unit operation model

Model a unit operation using a user-supplied Fortran subroutine

Unit operations with four (or fewer) inlet and outlet streams

User2

User-defined unit operation model

Model a unit operation using a user-supplied Fortran subroutine.

Unit operations with no limit on number of streams

9-1

User Models

User User-Supplied Unit Operation Model User can model any unit operation model. You must write a Fortran subroutine to calculate the values of the outlet streams based on the inlet streams and parameters you specify. User and User2 differ only in the number of inlet and outlet streams allowed and the argument lists to the model subroutine. User is limited to a maximum of four material and one heat or work inlet stream and a maximum of four material and one heat or work outlet stream. User2 has no limits on the number of inlet and outlet streams.

Flowsheet Connectivity for User Material Heat (optional) Work (optional)

Heat (optional) Work (optional)

Material Streams Inlet

One to four inlet material streams

Outlet One to four outlet material streams

Heat Streams Inlet

One heat stream (optional)

Outlet One heat stream (optional)

Work Streams Inlet

One work stream (optional)

Outlet One work stream (optional)

9-2

Unit Operation Models Version 10

Chapter 9

Specifying User You must specify the name of the subroutine model on the Input Specifications sheet. You have the option of specifying: • A report subroutine name • Size of the integer and real arrays (INT and REAL) passed to the user model subroutine • Values of the integer and real arrays passed to the user model subroutine • Length of integer and real workspace vectors • Thermodynamic conditions of each outlet stream • Type of flash calculations (vapor, liquid, two-phase, three-phase) For information on writing Fortran subroutines for user models, see the ASPEN PLUS User Models reference manual. Use the following forms to enter specifications and view results for User:

Unit Operation Models Version 10

Use this form

To do this

Input

Specify name and parameters for user subroutine, calculation options, and outlet stream conditions and flash convergence parameters

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of User results and material and energy balances

9-3

User Models

User2 User-Supplied Unit Operation Model User2 can model any unit operation model. You must write a Fortran subroutine to calculate the values of the outlet streams based on the inlet streams and parameters you specify. User and User2 differ only in the number of inlet and outlet streams allowed and the argument lists to the model subroutine. User2 has no limits on the number of inlet and outlet streams. User is limited to a maximum of four material and one heat or work inlet stream, and a maximum of four material and one heat or work outlet stream.

Flowsheet Connectivity for User2 Material Heat (optional) Work (optional)

Heat (optional) Work (optional)

Material Streams Inlet

At least one inlet material stream

Outlet At least one outlet material stream

Heat Streams Inlet

Any number of heat streams (optional)

Outlet Any number of heat streams (optional)

Work Streams Inlet

Any number of work streams (optional)

Outlet Any number of work streams (optional)

9-4

Unit Operation Models Version 10

Chapter 9

Specifying User2 You must specify the name of the subroutine model on the User2 Input Specifications sheet. You have the option of specifying: • A report subroutine name • Size of the integer and real arrays (INT and REAL) passed to the user model subroutine • Values of the integer and real arrays passed to the user model subroutine • Length of integer and real workspace vectors • Thermodynamic conditions of each outlet stream • Type of flash calculations (vapor, liquid, two-phase, three-phase) For information on writing Fortran subroutines for user models, see ASPEN PLUS User Models reference manual. Use the following forms to enter specifications and view results for User2: Use this form

To do this

Input

Specify name and parameters for user subroutine, calculation options, and outlet stream conditions and flash convergence parameters

BlockOptions

Override global values for physical properties, simulation options, diagnostic message levels, and report options for this block

Results

View summary of User2 results and material and energy balances

❖

Unit Operation Models Version 10

❖

❖

❖

9-5

User Models

9-6

Unit Operation Models Version 10

Chapter 10

10

Pressure Relief This chapter contains detailed reference information on the ASPEN PLUS Pres-Relief model for pressure relief calculations. For information on using Pres-Relief, see the ASPEN PLUS User Guide, Chapter 33. This chapter describes the following topics: • Specifying Pres-Relief • Scenarios • Rules to size the relief valve piping • Compliance with codes • Stream and vessel compositions and conditions • Reactions • Relief system • Data tables for pipes and relief devices • Valve cycling • Vessel types • Disengagement models • Stop criteria • Solution procedure for dynamic scenarios • Flow equations • Calculation and convergence methods • Vessel insulation credit factor

Unit Operation Models Version 10

10-1

Pressure Relief

Pres-Relief Pressure Relief Model Use Pres-Relief to do either of the following: • Determine the steady-state flow rating of pressure relief systems • Dynamically model vessels undergoing pressure relief due to a fire or heat input specified by the user. You may specify that reactions occur in the vessel.

Specifying Pres-Relief Use Pres-Relief to do either of the following: • Determine the steady-state flow rating of pressure relief systems • Dynamically model vessels undergoing pressure relief due to a fire or heat input specified by the user. You may specify that reactions occur in the vessel Use the Setup form to specify the pressure relief scenario, general specifications such as the discharge pressure and the estimated flow rate, inlet stream conditions, initial vessel conditions, design rules, and any reactions (dynamic scenarios only) that occur. Use the Relief Device form to specify the relief system. You must select a relief device and specify its characteristics. You must also specify the vessel neck and the number of inlet and tail pipe sections to be used. Use the Dynamic Input form to specify the required parameters for dynamic scenarios. These include vessel specifications, disengagement models and details specific to the chosen scenario. For the fire scenario, you must specify the fire standard and the credits to be used. When the scenario is Dynamic run with specified heat flux, you must specify the heat input parameters. When the number of inlet and tail pipe sections exceeds 0, you must specify the details for each section in the Inlet Pipes and Tail Pipes forms. For dynamic scenarios, use the Operations form to specify one or more variables to be used as stop criteria. The simulation will stop when the value of any of these variables exceeds the user-specified limit.

10-2

Unit Operation Models Version 10

Chapter 10

Use the following forms to enter specifications and view results for Pres-Relief: Use this form

To do this

Setup

Specify pressure relief scenario, general specifications, initial stream conditions, design rules, and any reactions that occur (required input)

Relief Device

Specify the type of relief device and the characteristics of the device (required input)

Inlet Pipes

Specify piping, fittings, and valves immediately upstream of the relief device (optional input)

Tail Pipes

Specify piping, fittings, and valves immediately downstream of the relief device (optional input)

Dynamic Input

Specify parameters describing the dynamic event (required for dynamic scenarios)

Operations

Specify criteria that will terminate the dynamic simulation (required for dynamic scenarios)

Convergence

Override default methods and convergence parameters for the algorithms involved in the pressure relief simulation (optional input)

Block Options

Override default methods and options for property calculation, simulation, diagnostics, and reporting (optional input)

Results

Review calculated results and profiles for the steady-state scenarios

Dynamic Results

Review calculated results and profiles for the dynamic scenarios

Scenarios Scenarios are situations that cause venting through the pressure relief system to occur. Pres-Relief supports the following scenarios: • Dynamic run with vessel engulfed by fire • Dynamic run with specified heat flux into vessel • Steady state flow rating of relief system • Steady state flow rating of relief valve

Dynamic Run with Vessel Engulfed by Fire Use this scenario to model a vessel engulfed by fire. You must specify the vessel geometry and initial composition. ASPEN PLUS can compute the energy input for this scenario according to the following standards: • • •

Unit Operation Models Version 10

NFPA-30 API-520 API-2000

10-3

Pressure Relief

ASPEN PLUS assumes the calculated energy input is constant during the entire venting transient. ASPEN PLUS uses credit factors for drainage, water-spray, fire-fighting equipment, and insulation to reduce energy input, if appropriate for the chosen standard. You may specify a total credit factor instead of individual credit factors. You must specify the fire duration time. This is a dynamic scenario. The vessel contents and relief rate change as a function of time. The following tables describe how ASPEN PLUS calculates wetted area, energy input, and credit factors for each of the three standards.

Calculation of Wetted Area Vessel type

NFPA-30

API-2000

API-520

Horizontal

75% of total exposed area

75% of total area or area to a height of 30 ft. above grade, whichever is greater

Wetted area up to 25 ft. above grade (based on specified liquid level)

Vertical

Area up to 30 ft. above grade. Bottom plate is included if exposed

Area up to 30 ft. above grade. If on ground, bottom plate is not included.

Wetted area up to 25 ft above grade (based on specified liquid level). Bottom plate is included if exposed.

Sphere

55% of total exposed area

55% of surface area, or surface area to a height 30 ft. above grade, whichever is greater

Up to a maximum horizontal diameter or up to height of 25 ft. above grade, whichever is greater

Calculation of Q (Btu/hr), Based on Area (sq-ft) †

NFPA-30 and API-2000 Area range

Heat input

20 < area < 200

Q=20,000Area

200 < area < 1000

Q=199,300(Area

1000 < area < 2800

Q=963,400(Area

2800 < area †

Q=21000(Area

0.566

0.82

)

0.338

)

)

For NFPA-30 , QMAX=14,090,000 at 2800 square feet if operating pressure < 1 PSIG

API-520 Heat input Q=34,500(Area

10-4

0.82

)

Unit Operation Models Version 10

Chapter 10

Calculation of Credit Factors Type

NFPA-30

Insulation only

.3

API-2000

API-520 †

F=K(1660-TF)/21,000t

Same as API-2000

You must specify F Drainage only

.5

1.

Not defined

(Area > 200 sq. ft.) Water and drainage

.3

1.

Not defined

Water, insulation, and drainage

.15

NSUL

Not defined

Insulation and drainage

.15

Not defined

Not defined

(Area > 200 sq. ft.)

(Area > 200 sq. ft.) Drainage and prompt fire fighting effort

No credit

Not defined

0.6*INSUL

Portable

No credit factors allowed

Not defined

Not defined

†

See Vessel Insulation Credit Factor, this chapter.

Dynamic Run with Specified Heat Flux into Vessel This scenario is similar to the fire exposure scenario, except it can model any energy input. ASPEN PLUS can compute the energy input for this scenario in three ways depending on whether you specify: • A constant duty • A duty profile • An area for heat transfer, a heat transfer coefficient, and a source fluid temperature This scenario is a dynamic scenario and is typically used for electrical heaters and other energy sources.

Steady State Flow Rating of Relief System Use this scenario to find the flow rate through a specified relief system at the specified composition. For this scenario, you must enter your own: • Relief rate • Piping description • Feed stream composition • Feed stream condition

Unit Operation Models Version 10

10-5

Pressure Relief

Steady State Flow Rating of Relief Valve Use this scenario to find the flow rate through a valve, given the composition and condition at the entrance to the valve. This is the simplest scenario. It is similar to the steady state flow rating of relief system scenario, except no piping is allowed.

Compliance with Codes Pres-Relief allows two types of runs: • Code capacity • Actual capacity The primary purpose of the code capacity run is to ensure that the capacity of the relief system, rated as required by code, exceeds the maximum capacity dictated by the scenario. The maximum pressure reached during the relief event must be less than the code allowable accumulation. The Code Capacity run includes the: • ASME valve rating factor of .90 • Valve flow coefficient • A combination coefficient The combination coefficient is only included if a rupture disk/relief valve combination is being designed. Typical combination coefficients for NBBI certified combinations are close to 1.00. If the combination is not certified, the ASME code requires a combination coefficient of .90. The primary purpose of the actual capacity run is to provide the best estimate of the actual flow through the system. Design of downstream equipment (other than the tail pipe) is one example why you might need this information. The actual capacity run contains the valve flow coefficient, but not the ASME valve rating factor of .90 or the combination coefficient.

Stream and Vessel Compositions and Conditions For the steady-state scenarios, you must specify the composition and conditions (two of temperature, pressure, and vapor fraction) of the feed stream. You can do this on the Setup Streams sheet in two ways: • Reference an ASPEN PLUS stream • Give the composition and conditions of the stream as input to Pres-Relief

10-6

Unit Operation Models Version 10

Chapter 10

For the dynamic scenarios, you must specify the composition and the conditions in the vessel at the beginning of the pressure relief calculations. Do this by referencing an ASPEN PLUS stream, or by specifying the composition and two of temperature, pressure, and vapor fraction on the Setup Vessel Contents sheet. As with the steady-state scenarios, you may reference an ASPEN PLUS stream or give the composition and conditions as input to Pres-Relief. When vapor fraction is not specified, you may also specify: • •

Initial liquid fill fraction (fillage) of the vessel Pad-gas pressure and Component ID

Only two of temperature, pressure, and vapor fraction can be specified or referenced from a stream.

Rules to Size the Relief Valve Piping ASPEN PLUS uses several rules (3% rule, X% rule, and 97% rule) to size the inlet and outlet piping with PSVs. The rules use the following terminology: DSP CBP Psta Ptot IDP

= = = = =

BBP

=

Differential set pressure Constant back pressure Static pressure Static pressure + velocity pressure Inlet pressure drop Ptot (vessel) - Ptot (valve in) Built-up back pressure Psta (valve out) - CBP

These rules are applied for both actual and code capacity runs and are applied at the converged solution for the steady-state scenarios. For dynamic scenarios, the 3% Rule and X% Rule are applied once, at 10% overpressure. If all pressures are above 10% overpressure, the test is not performed and a warning is issued. If all pressures are below 10% overpressure, the highest pressure value is scaled up to 10% overpressure, and the scaled values are used in applying the rule. The 97% rule is applied when the pressure at the valve inlet is at or above 10% overpressure. None of the required standards mentions any of these rules except for the X% rule with X=10. The X% rule is mentioned in the non-mandatory appendix of the ASME code.

Unit Operation Models Version 10

10-7

Pressure Relief

3% Rule According to the 3% rule, the total pressure loss in the inlet must be less than 3% of the differential set pressure when the flow rate is equal to the code capacity of the valve at 10% overpressure.

IDP ≤ 0.03DSP For cases where the overpressure does not reach 10%, adjust the pressure drop rule by multiplying by the ratio of the maximum flowing pressure to 10% overpressure (psig).

IDP ≤ 0.03

RP 11 . SP

X% Rule According to the X% rule, the built-up back pressure must be less than X% of the differential set pressure when the flow rate is equal to the code capacity of the valve at 10% overpressure.

BBP ≤

X DSP 100

For cases where the overpressure does not reach 10% adjust the pressure drop rule by multiplying by the square of the ratio of the maximum flowing pressure to 10% overpressure (psig).

X  RP  BBP ≤   100  11 . PS 

2

97% Rule According to the 97% rule, 97% of the differential set pressure must be available across the valve anytime the over pressure is equal to or above 10% with a flow through the valve based on code capacity.

RP − CBP − IDP − BBP ≥ 0.97 DSP For cases where the overpressure does not reach 10%, apply the rule at peak overpressure.

10-8

Unit Operation Models Version 10

Chapter 10

Recommendations for Specific Valve Types For standard spring loaded valves or pop action pilot valves with unbalanced pilots vented to the discharge: The differential set pressure is the set pressure minus the constant back pressure.

DSP = SP − CBP Size the inlet piping using the 3% rule. Size the outlet piping using the 97% rule. -OrSize the outlet piping with the X% rule using X = 10. For balanced bellows spring loaded valves: The differential set pressure is the set pressure.

DSP = SP Size the inlet piping using the 3% rule. Size the outlet piping with the X% rule using X = 30. For modulating pilot operated valves with balanced pilots or pilots vented to atmosphere: The differential set pressure is the set pressure.

DSP = SP You can use the scenario required flow rather than the valve capacity for pressure drop calculations as an option. This can easily be simulated by changing the input orifice area until the overpressure reaches 10%. There is no inlet pressure drop rule. Size the outlet piping with the X% rule using X = 50.

Reactions If the protected vessel is a vertical, horizontal, API, spherical , or user-specified tank, you may model it with or without reactions. Specify the reactions by giving the Reactions ID on the Setup Reactions sheet.

Unit Operation Models Version 10

10-9

Pressure Relief

Relief System The venting system consists of: • A vessel neck • One or two sections of inlet pipe • The relief device itself • One or two sections of tail pipe In a simulation, the system being modeled may consist of an inlet pipe without a relief device, or a relief device connected to the vessel without an inlet pipe. The tail pipe is optional.

Relief Devices Pres-Relief can model the following types of relief devices: • Safety relief valves (PSVs; both liquid and gas/2-phase) • Rupture disks (PSDs) • Emergency relief valves (ERVs) • SRV/rupture disk combinations • Open vent pipes Internal tables (accessed from the ReliefDevice SafetyValve sheet) contain several standard commercially available valves, along with all the mechanical specifications and certified coefficients needed in the relief calculations. You may choose one valve from the tables, or enter your own valve specifications and coefficients. For liquid service valves, you must also specify the full-lift overpressure. This allows ASPEN PLUS to simulate some of the older style valves which do not achieve full lift until 25% overpressure is reached. For gas/2-phase service valves, you must also specify the average opening and closing factors. The valve does not open until the pressure drop across the valve reaches (opening factor * Dif-Setp). The valve closes when the pressure drop across it reaches (closing factor * Dif-Setp). In an actual capacity run, the rupture disk is modeled as a bit of resistance using the pipe model. The default value of L/D is 8 for a rupture disk with a diameter of 2 inches or less and 15 if the diameter is greater than 2 inches. You can override the default by specifying a value on the Relief Device Rupture Disk sheet. In the code capacity run, the rupture disk is modeled as an ideal nozzle with a certified discharge coefficient. If no certified discharge coefficient is available, a value of 0.62 is suggested. In a code capacity run in combination with a safety relief valve, the resistance of the rupture disk is modeled by the combination coefficient in the valve model.

10-10

Unit Operation Models Version 10

Chapter 10

The emergency relief vent is modeled as a nozzle. A de-rating factor of 0.9 is used in a code capacity run.

Piping System The inlet piping system can be made of one of the following: • One pipe section • Two sections of pipe plus a vessel neck, all with different diameters The tail pipe can be made of one section of pipe or of two sections of pipe with different diameters. For each pipe section, specify: • • • •

Pipe diameter Length Elevation Whether the pipes are screwed together or held together with flanges or welds

If pipes of different diameters are used, reducer and expander resistance coefficients ("K" factors) can be specified. ASPEN PLUS uses the equation K=4*fr*(L/D) to convert from resistance coefficients to equivalent L/D, where the term "fr" is the friction factor. Optional information for each section consists of the number of 90 degree elbows, straight tees, branched tees, gate valves, butterfly valves, transflo valves, and control valves. You can add other fittings not listed by specifying the L/D value. ASPEN PLUS calculates a total equivalent L/D before modeling the pipe section. You may also specify: • Ambient temperature at the inlet and outlet of the pipe • A heat transfer coefficient to exchange heat with the pipe contents While modeling the pipe section, ASPEN PLUS detects the choked condition in the pipe by keeping track of the Mach Number as integration down the pipe proceeds. If the Mach Number goes above 1.0, integration is stopped and a flag is returned to indicate that the pipe choked. Pipeline pressure drop modeling can work in two ways. You may specify one of the following: • Rigorous flashes are to be done at each step in the integration • A flash table is used during pipe integration

Unit Operation Models Version 10

10-11

Pressure Relief

If you request a table, specify the number of temperature and pressure points in the table. At each temperature-pressure pair, ASPEN PLUS performs a flash and calculates all necessary properties (density, viscosity, surface tension, and so on). As integration proceeds, ASPEN PLUS interpolates in this table to get the necessary properties. If properties outside the table are needed, a rigorous flash is performed at that point. In general, the pipe integration proceeds faster if the flash table is used. Several correlations are available, depending on the pipe inclination. The default method for all inclinations (holdup and frictional pressure loss) is Beggs and Brill. Other available options are: • Darcy • Lockhart-Martinelli • Dukler for frictional loss • Lockhart-Martinelli, Slack, and Flanigan for holdup

Data Tables for Pipes and Relief Devices Pres-Relief includes several customizable tables that list the available options for pipes, general purpose valves, safety relief valves, emergency relief vents, and rupture disks. You can modify the tables by changing data files. Then process the files through ModelManager Table Building System (MMTBS).

Pipes Pres-Relief includes a table of actual diameters for several steel pipe schedules. Use this table when choosing the piping for the inlet and tail pipes. You can modify this table by including more pipe materials and/or schedules. The following section shows the table organization.

10-12

Unit Operation Models Version 10

Chapter 10

first material of construction # of types first type # of diameters nominal diameter actual diameter nominal diameter actual diameter . . second type # of diameters nominal diameter actual diameter nominal diameter actual diameter . . second material of construction # of types first type # of diameters nominal diameter actual diameter nominal diameter actual diameter . . second type nominal diameter actual diameter nominal diameter actual diameter . .

General-Purpose Valves For general-purpose valves in the inlet or tail pipes, Pres-Relief includes a table of various manufacturers’ valves from 1 inch to 10 inches. The valves include: • • • • •

Unit Operation Models Version 10

Durco Plug Tufline Plug Jamesbury Ball AGCO Selector KTM Ball (L-Port and T-Port)

10-13

Pressure Relief

For each manufacturer, the table contains: • Valve type (for example., L-Port or T-Port) • Nominal diameter • Port area • Flow coefficient The table is organized as follows: first manufacturer # of types first type # of diameters nominal diameter nominal diameter . . second type # of diameters nominal diameter nominal diameter . .

port area port area

flow coeff flow coeff

port area port area

flow coeff flow coeff

Safety Relief Valves Pres-Relief includes a table of manufacturers’ safety relief valves. It contains valves for liquid and gas/2-phase service. For each valve, the table contains: • Service • Type • Manufacturer • Series, size (for example, 3L4) • Throat diameter • Inlet diameter • Outlet diameter • Discharge coefficient • Overpressure factor (for liquid service valves)

10-14

Unit Operation Models Version 10

Chapter 10

The table is organized as follows: Service (Liquid, Gas, or 2-phase) # of types first type # of manufacturers first manufacturer # of series first series # of sizes first size # of throat diameters throat diam inlet diam outlet diam throat diam inlet diam outlet diam . . throat diam inlet diam outlet diam throat diam inlet diam outlet diam

dischg coeff over pr factor dischg coeff over pr factor

dischg coeff over pr factor dischg coeff over pr factor

Emergency Relief Vents This table contains: • Nominal diameter • Effective diameter • Allowed setpoint for several Protectoseal and Groth emergency relief vents You must specify an over-pressure factor. The table is organized as follows: first manufacturer # of types first type # of nominal diameters nominal diameter effective diameter allowed setpoint nominal diameter effective diameter allowed setpoint . .

Rupture Disks This table contains manufacturers’ information on rupture disks. Each entry contains: • A manufacturer • Type • Nominal diameter • Actual diameter • Discharge coefficient

Unit Operation Models Version 10

10-15

Pressure Relief

The table is organized as follows: first manufacturer # of types first type # of nominal diameters first nominal diam actual diam discharge coeff second nominal diam actual diam discharge coeff . .

Valve Cycling If a relief valve is too large for a given application, valve cycling may occur. In this situation, the pressure in the vessel builds up to a point where the valve opens, but then closes almost immediately because enough material is released to lower the vessel pressure below the closing pressure. In some simulations, the valve may open and close several times per second. The simulation may run for a long time, just opening and closing the valve over and over. To stop such a simulation, you can specify whether or not to stop cycling, and how many openings and closings of the valve are allowed in a specified amount of time.

Vessel Types You must enter vessel geometry for the dynamic scenarios. You can choose one of the following vessel types: • Vertical Vessel • Horizontal Vessel • API Tank • Sphere • Heat exchanger shell • Vessel jacket • User-specified If you choose user-specified, you must specify surface area and volume. Surface area is also required for vessel jacket. Maximum Allowable Working Pressure (MAWP) with corresponding temperature is required for all vessel types. Some vessel types require diameter, length, and volume of internals.

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Unit Operation Models Version 10

Chapter 10

Vertical Vessel, Horizontal Vessel, and API Tank If you choose vertical vessel, horizontal vessel, or API tank, choose one of these head types: • Flanged and dished • Ellipsoidal • User-specified If you choose user-specified head type, you must specify the area and volume of a head.

Sphere If the protected vessel is a sphere, you must specify: • Diameter • MAWP with corresponding temperature • Volume of internals

Heat Exchanger Shell If the protected vessel is a heat exchanger shell, in addition to the items specified for a vertical vessel you must also specify whether the vessel is mounted vertically or horizontally.

Vessel Jacket If the protected vessel is a vessel jacket, you must specify: • MAWP with corresponding temperature • Volume of internals • Jacket volume

User-Specified If the protected vessel is user-specified, you must specify: • • • •

Unit Operation Models Version 10

Volume Area MAWP with corresponding temperature Volume of internals

10-17

Pressure Relief

Disengagement Models The following disengagement options are available: Option

Description

Homogeneous

Vapor fraction leaving vessel is the same as vapor fraction in vessel

All-vapor

All vapor leaving vessel

All-liquid

All liquid leaving vessel

Bubbly

DIERS bubbly model

Churn-turbulent

DIERS churn-turbulent model

User-specified

Homogeneous venting until vessel vapor fraction reaches the user-specified value, then all vapor venting

For the bubbly and churn-turbulent methods, ASPEN PLUS uses the DIERS “switch-point” calculations to compute the point at which total vapor-liquid disengagement occurs. Use the bubbly and churn-turbulent models only for vertical or API tanks.

Stop Criteria For dynamic scenarios, stop criteria need to be specified which will terminate the simulation. You must: • Select a specification type • Enter a value for the specification at which the simulation will stop • Select a component and substream for component-related specification types • Specify which approach direction (above or below) to use in stopping the simulation You may select from the following specification types: • Simulation time • Vapor fraction in the vessel • Mole fraction of a specified component • Mass fraction of a specified component • Conversion of a specified component • Total moles or moles of a specified component • Total mass or mass of a specified component • Vessel temperature • Vessel pressure • Vent mole flow rate or mole flow rate of a component • Vent mass flow rate or mass flow rate of a component

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Unit Operation Models Version 10

Chapter 10

You must also select the location of the stop criteria specification. You may select from the following locations: • Vessel • Relief vent system • Accumulator Certain restrictions apply depending on the location selected. When location = vessel, mole and mass flow rate are not allowed. When location = vent accumulator, only the following specifications are allowed: • • • •

Mass fraction of a specified component Mole fraction of a specified component Total moles of a specified component Total mass of a specified component

When location = vent, only the flowing specifications are allowed: • Mass fraction of a specified component • Mole fraction of a specified component • Vent molar flow rate • Vent mass flow rate

Solution Procedure for Dynamic Scenarios The problem to be solved is: Given the initial conditions in the vessel, a description of the pressure relief system, and the heat flow into the vessel, calculate the flow rate through the pressure relief system and determine if the pressure relief system meets code requirements. The problem is solved as outlined below. This algorithm is for the Heat-Input and Fire Scenarios. 1. Given the heat input to the vessel, solve the energy balance and flash equations along with the reaction equations for the vessel at the present time step. If any of the termination criteria are met, go to Step 6. The options for specifying termination criteria include: • • • •

Time for scenario exceeded Specified vapor fraction reached Vessel contents have reached specified value Pressure in the vessel is greater than the maximum allowed

2. If the pressure in the vessel is less than the device opening pressure, increment time and go to Step 1.

Unit Operation Models Version 10

10-19

Pressure Relief

3. Calculate the maximum flow rate possible through the pressure relief system. This value is calculated by finding the smallest diameter of any pipe or valve in the system, and calculating the sonic velocity through that diameter. 4. Calculate the pressure at the end of the vessel neck, after each section of the inlet pipe, after the pressure relief device, and after each section of the tail pipe based on the current flow estimate. If the pressure at the end of any section is less than the user-specified discharge pressure, it is not necessary to do the calculations for the next section. 5. If the pressure at the end of the pressure relief system is within tolerance of the user-specified discharge pressure, increment time and go to Step 1. Otherwise, calculate a new guess for the flow through the relief system and go to Step 4. 6. Given the flow at any time, check where the choke point is. If the choke point is not at the pressure relief valve, the system is unacceptable. Check if any applicable codes are violated. If so, the system is unacceptable.

Flow Equations Pipe Flow This is the general differential equation for flow through a constant diameter pipe:

  υ2   υ dp + G  υ dυ +  4 f  dL  + g sin ΦdL = 0 2D    2

(1)

Where:

υ

=

Specific volume of stream

p G f D L g

= = = = = = =

Static (flowing) pressure of stream Mass flow rate per unit area Friction factor Inside diameter of pipe Equivalent pipe length Acceleration due to gravity Vertical rise/equivalent pipe length

sin Φ

10-20

Unit Operation Models Version 10

Chapter 10

Φ represents the physical angle of the pipe with respect to the horizontal only if the equivalent pipe length is the same as the physical flow path length (that is, only pipe, no fittings or other resistances). The potential energy term in the equation assumes that the vertical elevation is distributed evenly along the entire equivalent length. For example, you have only a single 20 meter length of pipe that rises a total of six meters, then

sin Φ =

6 = 0.3 20

If the same system also includes a fitting resistance of 5 equivalent meters, then:

sin Φ =

6 = 0.24 20 + 5

Equation (1) applies to any flow system (all vapor, non-flashing liquid, flashing two-phase, non-flashing two-phase, etc.). All that is needed to solve the equation is the proper relationship between the pressure (p) and the stream specific volume ( υ ). This relationship is determined by the type of constraint chosen. For adiabatic flow, the defining equation is:

H + KE + PE = CONSTANT Where: H KE PE

= = =

Stream enthalpy Kinetic energy of stream Potential energy of stream

Between points 1 and 2:

H1 + KE1 + PE1 = H 2 + KE 2 + PE 2 Thus:

H 2 = H1 − ∆KE − ∆PE ASPEN PLUS flash routines can be used to calculate enthalpy at point 2.

Unit Operation Models Version 10

10-21

Pressure Relief

Nozzle Flow ASPEN PLUS calculates nozzle flow by treating the flow as adiabatic through a perfect nozzle which has no friction losses and is short enough so that any potential energy effects can be neglected. The actual flow is then calculated by applying a correction factor (the flow coefficient, Cd) to the flow calculated as if the nozzle behaved as perfect. Frictionless flow is described by:

udu + υ dp = 0

(2)

Where: u

υ

= =

Stream linear velocity Specific volume of stream

For adiabatic flow:

  u2 d  U + PV + + PE  = 0 2   Where: U PV

= =

Internal energy Pressure-volume product

Neglecting PE, and combining the definition of enthalpy (H = U + PV) into this equation gives:

dH + udu = 0

(3)

Combining (2) and (3) gives:

dH = υ dp

(4)

By definition:

dH =υ dp

(5)

(4) and (5) yield:

Tds = 0 or

ds = 0

Thus, adiabatic frictionless flow is isentropic.

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Unit Operation Models Version 10

Chapter 10

The flow equation (2) can be integrated to describe the flow through a perfect nozzle as follows: Let p0 = The upstream stagnation pressure where the velocity is zero (u0 = 0). Let p1 = The pressure in the nozzle throat at which the flow is accelerated to velocity u. Thus, the integrated form of (2) becomes: p

1 1 2 u = − ∫ υ dp 2 p01

which can be re-written (noting that u = G υ ): p1

G υ = −2 ∫ υ dp 2

2 1

(6)

p0

Equation (6) provides the means to calculate the flow rate through a perfect nozzle given the upstream stagnation pressure and the proper p-v relationship (which is isentropic). As one integrates (6) from p0 to p1, a maximum G indicates that the flow has become choked at the current value of p. (6) also serves as a method for converting between stagnation and static pressures at any point in the flow system (pipe or nozzle).

Calculation and Convergence Methods ASPEN PLUS uses the same equations used to model the safety relief valve as to model the conversion from stagnation to flowing pressure and back again. To be completely accurate, the valve should be modeled as in equation (6) in the Nozzle Flow section, this chapter. This model requires that constant entropy flashes be performed at each point in the integration of equation (6). This is a very time consuming calculation, so several options are provided to speed up the calculations. First, you can choose to do constant enthalpy flashes rather than constant entropy flashes through the nozzle. This speeds up the calculations by an order of magnitude, since the constant entropy flash is modeled by a series of constant enthalpy flashes converging on entropy. ASPEN PLUS also provides a shortcut method to calculate molar volume as a function of pressure during the nozzle integration. This method was developed by L. L. Simpson1 and gives very good results. Instead of doing a flash calculation to calculate the molar volume at each point in the integration, two flashes are done at the start and parameters are calculated which allow you to calculate the molar volume at other pressures without doing flashes.

Unit Operation Models Version 10

10-23

Pressure Relief

Vessel Insulation Credit Factor When Fire Standard API-520 or API-2000 is used, you may claim an insulation credit factor calculated from the formula:

F=

k (1660 − Tf ) 21000t

Where: k

=

Thermal conductivity of insulation, in British thermal units per hour per square foot per degree Fahrenheit per inch at mean temperature.

Tf

=

Temperature of vessel contents at relieving conditions, in degrees Fahrenheit.

t

=

Thickness of insulation, in inches.

Assuming a k value of 4.0, and Tf of 0.0, the following table, which was taken from API-2000, gives values of F for various values of insulation thickness:

10-24

Insulation thickness (t)

F Factor

6 inches (152 millimeters)

0.05

8 inches (203 millimeters)

0.037

10 inches (254 millimeters)

0.03

12 inches (305 millimeters) or more

0.025

Unit Operation Models Version 10

Chapter 10

References Simpson, L.L., "Estimate Two-Phase Flow in Safety Devices," Chemical Engineering, August, 1991, pp. 98-102. Additional Reading "Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries" Part I - Sizing and Selection, API Recommended Practice 520, American Petroleum Institute, 1220 L Street Northwest, Washington, D.C. 20005. "Venting Atmospheric and Low Pressure Storage Tanks," (Non-refrigerated and Refrigerated), API Standard 2000, American Petroleum Institute, 1220 L Street Northwest, Washington, D.C. 20005.

❖

Unit Operation Models Version 10

❖

❖

❖

10-25

Pressure Relief

10-26

Unit Operation Models Version 10

Appendix A

A

Sizing and Rating for Trays and Packings ASPEN PLUS has extensive capabilities to size, rate, and perform pressure drop calculations for trayed and packed columns. Use the following Tray/Packing forms to enter specifications: • TraySizing • TrayRating • PackSizing • PackRating These capabilities are available in the following column unit operation models: • • •

RadFrac MultiFrac PetroFrac

You can choose from the following five commonly-used tray types: • Bubble caps • Sieve • Glitsch Ballast® • Koch Flexitray® • Nutter Float Valve ASPEN PLUS can model a variety of random packings. You can also use any of the following types of structured packings: • • • • •

Unit Operation Models Version 10

®

Goodloe Glitsch Grid® Norton Intalox Structured Packing Sulzer BX, CY, Mellapak, and Kerapak Koch Flexipac, Flexeramic, Flexigrid

A-1

Sizing and Rating for Trays and Packings

For sizing and rating calculations, ASPEN PLUS divides a column into sections. Each section can have a different tray type, packing type, and diameter. The tray details can vary from section to section. A column can have an unlimited number of sections. In addition, you can size and rate the same section with different types of trays and packings. The calculations are based on vendor-recommended procedures whenever these are available. When vendor procedures are not available, well-established literature methods are used. ASPEN PLUS calculates sizing and performance parameters such as: • • • •

Column diameter Flooding approach or approach to maximum capacity Downcomer backup Pressure drop

These parameters are based on: • Column loadings • Transport properties • Tray geometry • Packing characteristics You can use the computed pressure drop to update the column pressure profile.

Single-Pass and Multi-Pass Trays You can use the column models in ASPEN PLUS to: • Size one- and two-pass trays • Rate trays with up to four passes Schematics of one-, two-, three-, and four-pass trays are shown in the next four figures. ASPEN PLUS performs and reports rating calculations for all panels. When specifying Weir heights, cap positioning, and number of valves:

A-2

For

Specify

One-pass tray

A single value

Two-pass tray

Up to two values, one for each panels A and B

Three-pass tray

Up to three values, one for each panel (A, B and C)

Four-pass tray

Up to four values, one for each panel (A, B, C and D)

Unit Operation Models Version 10

Appendix A

The values for the number of caps and number of valves applies for each panel. For example, two-pass trays have two A panels for tray AA, and two B panels for tray BB. Therefore, the number of caps per panel is the number of caps per tray divided by two. Similar consideration is necessary for three- and four-pass trays. If you specify only one value for multi-pass trays, that value applies to all panels. When specifying downcomer clearance and width:

Unit Operation Models Version 10

For

Specify

One-pass tray

A single value for the side downcomer

Two-pass tray

Up to two values, one for the side downcomer, one for the center downcomer

Three-pass tray

Up to two values, one for the side downcomer, one for the off-center downcomer

Four-pass tray

Up to three values: one for the side downcomer, one for the center downcomer, and one for the off-center downcomer

A-3

Sizing and Rating for Trays and Packings

Outlet Weir Length

Column Diameter

DC-WTOP WEIR-HT DCWBOT

DC-HT

DC-CLEAR

A One-Pass Tray

A-4

Unit Operation Models Version 10

Appendix A

Outlet Weir Length

Column Diameter

CTR. DC

CTR. DC

DC-WTOP

Below

~

~ WEIR-HT

DCWBOT

Panel A

DC-HT

DC-CLEAR

DCWTOP DCWBOT

Tray AA Side Downcomer

Panel B Tray BB Center DC-HT Downcomer

DC-CLEAR

~ ~

~~

A Two-Pass Tray

Unit Operation Models Version 10

A-5

Sizing and Rating for Trays and Packings

Outlet Weir Length

Column Diameter OFF-CTR.DC

OFF-CTR.DC DC-WTOP

DC-WTOP

WEIR-HT DC-HT

DCCLEAR

Panel A. B. C. DCOF DC-WBOT

DC-WTOP B

A

B

A

C

Panel C. B. A.

Panel A. B. C.

A Three-Pass Tray

A-6

Unit Operation Models Version 10

Appendix A

Outlet Weir Length

Column Diameter OFF-CTR.DC OFF-CTR.DC

SIDE DC

CTR.DC DC-WTOP

DC-WTOP

WEIR-HT DC-HT

Panel A. B. DC-WBOT

DC-WBOT DCCLEAR

D

D

Panel C. D.

C DCOF

A

B

B

A

Panel A. B.

A Four-Pass Tray

Unit Operation Models Version 10

A-7

Sizing and Rating for Trays and Packings

Modes of Operation for Trays ASPEN PLUS provides two modes of operation for trays: • Sizing • Rating In either mode, you can divide a column into any number of sections. Each section can have a different column diameter, tray type, and tray geometry. You can re-rate or re-design the same section with different tray types and/or packings. ASPEN PLUS performs the calculations one section at a time. In sizing mode, the column model determines tray diameter to satisfy the flooding approach you specified for each stage. The largest diameter is selected. In rating mode, you specify the column section diameter and other tray details. For each stage, the column model calculates tray performance and hydraulic information such as flooding approach, downcomer backup, and pressure drop.

Flooding Calculations for Trays For bubble caps and sieve trays, ASPEN PLUS provides two procedures for calculating the approach to flooding. The first procedure is based on the Fair 1 method. The second uses the Glitsch procedure 2 for ballast trays. This procedure de-rates the calculated flooding approach by 15% for bubble caps and by 5% for sieve trays. All other hydraulic calculations are based on the Fair and Bolles1,3 methods. For sizing calculations, you can also supply your own calculation procedure: = Specify

On form

Flooding calculation method = USER

TraySizing or PackSizing

Subroutine name

UserSubroutines

For valve trays (Glitsch Ballast, Koch Flexitray, and Nutter Float Valve trays), ASPEN PLUS uses procedures from vendor design bulletins.2,4,5

A-8

Unit Operation Models Version 10

Appendix A

Bubble Cap Tray Layout RadFrac uses cap diameter only for tray type CAPS. Valid entries are: Cap Diameter

Default Weir Height

Inches

Millimeters

Inches

Millimeters

3

76.2

2.75

69.85

4

101.6

3.00

76.20

6

152.4

3.25

82.55

Use the cap diameter to retrieve cap characteristics based on standard cap designs. For columns with diameter

The default is

Up to 48 in (1219.2 mm).

3 in (76.2 mm)

Greater than 48 in (1219.2 mm)

4 in (101.6 mm)

The following table lists standard cap designs: Materials

Stainless Steel

Nominal Size, in

3

4

6

U.S. Standard gauge

16

16

16

OD, in

2.999

3.999

5.999

ID, in

2.875

3.875

5.875

Height overall, in

2.500

3.000

3.750

Number of slots

20

26

39

Type of slots

Trapezoidal

Trapezoidal

Trapezoidal

Bottom

0.333

0.333

0.333

Top

0.167

0.167

0.167

Slot height, in

1.000

1.250

1.500

Height shroud ring, in

0.250

0.250

0.250

Cap

Slot width, in

continued

Unit Operation Models Version 10

A-9

Sizing and Rating for Trays and Packings

Materials Nominal size, in

Stainless Steel 3

4

6

U.S. Standard gauge

16

16

16

OD, in

1.999

2.624

3.999

ID, in

1.875

2.500

3.875

0.5-in skirt height

2.250

2.500

2.750

1.0-in skirt height

2.750

3.000

3.250

1.5-in skirt height

3.250

3.500

3.750

0.500

0.500

0.500

Riser

2.65

4.80

11.68

Reversal

4.18

7.55

17.80

Annular

3.35

6.38

14.55

Slot

5.00

8.12

14.64

Cap

7.07

12.60

28.30

Reversal/riser

1.58

1.57

1.52

Annular/riser

1.26

1.33

1.25

Slot/riser

1.89

1.69

1.25

Slot/cap

0.71

0.65

0.52

Riser

Standard heights, in

Riser-slot seal, in Cap areas, in

Area ratios

Pressure Drop Calculations for Trays Normally, RadFrac, MultiFrac, and PetroFrac treat the stages you enter as equilibrium stages. You must enter overall efficiency to: • Convert the calculated pressure drop per tray to pressure drop per equilibrium stage • Compute the column pressure drop If you do not enter overall efficiency, these models assume 100% efficiency. If you specify Murphree or vaporization efficiency, you should not enter overall efficiency. RadFrac, MultiFrac, and PetroFrac will treat the stages as actual trays.

A-10

Unit Operation Models Version 10

Appendix A

Foaming Calculations for Trays Suggested values for Ballast trays are: Service

System Foaming Factor

Non-foaming systems

1.00

Fluorine systems

0.90

Moderate foamers, such as oil absorbers, amine, and glycol regenerators

0.85

Heavy foamers, such as amine and glycol absorbers

0.73

Severe foamers, such as MEK units

0.60

Foam stable systems, such as caustic regenerators

0.30

Suggested values for Flexitrays are: Service

System Foaming Factor

Depropanizers

0.85-0.95

Absorbers

0.85

Vacuum towers

0.85

Amine regenerators

0.85

Amine contactors

0.70-0.80

High pressure deethanizers

0.75-0.80

Glycol contactors

0.70-0.75

Suggested values for Float valve trays are:

Unit Operation Models Version 10

Service

System Foaming Factor

Non foaming

1.00

Low foaming

0.90

Moderate foaming

0.75

High foaming

0.60

A-11

Sizing and Rating for Trays and Packings

Packed Columns The calculations for packings are based on the height equivalent of a theoretical plate (HETP). HETP=packed height/number of stages. The HETP is required. You can provide it using one of the following methods: • Enter it directly on the PackSizing or PackRating forms • Enter the packing height on the same form

Packing Types and Packing Factors ASPEN PLUS can handle a wide variety of packing types, including different sizes and materials from various vendors. For random packings, the calculations require packing factors. ASPEN PLUS stores packing factors for the various sizes, materials, and vendors allowed in a databank. If you provide the following information, ASPEN PLUS retrieves these packing factors automatically for calculations: • Packing type • Size • Material You may specify the vendor on the PackSizing or PackRating form. Is the vendor specified?

ASPEN PLUS uses

Yes

The packing factor published by the vendor

No

A value compiled from various literature sources

†

††

†,††

Fair, J.R., et al., "Liquid-Gas Systems," Perry’s Chemical Engineers’ Handbook, R.H. Perry and D. Green, ed., 6th ed. (New York: McGraw Hill, 1984). Tower Packings, Bulletin No. 15 (Tokyo: Tokyo Special Wire Netting Company).

You can enter the packing factor directly to override the built-in values. ASPEN PLUS uses the packing type to select the proper calculation procedure.

Modes of Operation for Packing The column models have two modes of operation for packing: • •

A-12

Sizing Rating

Unit Operation Models Version 10

Appendix A

In either mode, you can divide a column into any number of sections. Each section can have different packings. You can re-rate or re-design the same section with different packings and/or tray types. ASPEN PLUS performs the calculations one section at a time. In sizing mode, ASPEN PLUS determines the column section diameter from: • •

The approach to the maximum capacity A design capacity factor you specify

You can impose a maximum pressure drop per unit height (of packing or per section) as an additional constraint. Once ASPEN PLUS has determined the column section diameter, it re-rates the stages in the section with the calculated diameter. In rating mode, you specify the column diameter. ASPEN PLUS calculates the approach to maximum capacity and pressure drop.

Maximum Capacity Calculations for Packing ASPEN PLUS provides several methods for maximum capacity calculations. For random packings you can use: Method

For this type of packings †

Mass Transfer, Ltd. (MTL) ††

Norton

†††

MTL Norton IMTP

Koch

Koch

Eckert

All other random packings

† †† †††

Cascade Mini-Ring Design Manual (Tokyo: Dodwell & Company, Ltd., 1984). Intalox High-Performance Separation Systems, Bulletin IHP-1 (Akron: Norton Company, 1987). McNulty, K.J., "Hydraulic Model for Packed Tower Design." Paper presented at the American Institute of Chemical Engineers Spring Meeting in Houston, 1993.

For structured packings, ASPEN PLUS provides vendor procedures for each type. If you specify the maximum capacity factor, ASPEN PLUS bypasses the maximum capacity calculations. The definition of approach to maximum capacity depends on the type of packings.

Unit Operation Models Version 10

A-13

Sizing and Rating for Trays and Packings

For Norton IMTP and Intalox structured packings, approach to maximum capacity refers to the fractional approach to the maximum efficient capacity. Efficient capacity is the operating point at which efficiency of the packing deteriorates due to liquid entrainment. The efficient capacity is approximately 10 to 20% below the flood point. For Sulzer structured packings (BX, CY, Kerapak, and Mellapak), approach to maximum capacity refers to the fractional approach to maximum capacity. Maximum capacity is the operating point at which a pressure drop of 12 mbar/m (1.47 in-water/ft) of packing is obtained. At this condition, stable operation is possible, but the gas load is higher than that at which maximum separation efficiency is achieved. The gas load corresponding to the maximum capacity is 5 to 10% below the flood point. Sulzer recommends a usual design range between 0.5 and 0.8 for approach to flooding. For all other packings, approach to maximum capacity refers to the fractional approach to the flood point. Because there are different definitions for approach to maximum capacity, sizing results are not on the same basis for packings from different vendors, even when you use the same value for approach to maximum capacity. Direct performance comparison of packings from different vendors is not recommended. The capacity factor is:

CS = VS

ρV ρ L − ρV

Where:

A-14

CS

=

Capacity factor

VS

=

Superficial velocity of vapor to packing

ρV

=

Density of vapor to packing

ρL

=

Density of liquid from packing

Unit Operation Models Version 10

Appendix A

Pressure Drop Calculations for Packing For random packings, ASPEN PLUS provides several built-in methods to compute the pressure drop. Vendor

Pressure drop method

MTL

Vendor

Norton

Vendor procedure

Koch

Vendor procedure♦♦

Not specified

Eckert GPDC♦♦♦, Norton GPDC

† ††

††† ♦

♦♦

♦♦♦

§

§§

† ††, †††, ♦

††, †††, ♦

§

§§

, Prahl GPDC , Tsai GPDC

Cascade Mini-Ring Design Manual (Tokyo: Dodwell & Company, Ltd., 1984). Dolan, M.J. and Strigle, R.F., "Advances in Distillation Column Design," CEP, Vol.76, No.11 (November 1980), pp. 78-83. Intalox High-Performance Separation Systems, Bulletin IHP-1 (Akron: Norton Company, 1987). Intalox Metal Tower Packing, Bulletin IM82 (Akron: Norton Company, 1979). McNulty, K.J., "Hydraulic Model for Packed Tower Design." Paper presented at the American Institute of Chemical Engineers Spring Meeting in Houston, 1993. Fair, J.R., et al., "Liquid-Gas Systems," Perry’s Chemical Engineers’ Handbook, R.H. Perry and D. Green, ed., 6th ed. (New York: McGraw Hill, 1984), pp. 18-22. McNulty, K.J. and Hsieh, C.L., "Hydraulic Performance and Efficiency of Koch Flexipac Structured Packings." Paper presented at American Institute of Chemical Engineers Annual Meeting in Los Angeles, 1982. Tsai, T.C. "Packed Tower Program Has Special Features," Oil and Gas Journal, Vol. 83 No. 35 (September, 1985), p. 77.

If you specify the vendor, ASPEN PLUS uses the vendor procedure. If you do not specify the vendor, you can choose one of four different pressure drop methods. If you do not specify a method, ASPEN PLUS uses the Eckert generalized pressure drop correlation (GPDC).

Unit Operation Models Version 10

A-15

Sizing and Rating for Trays and Packings

For structured packings, vendor pressure drop correlations are available for all packings: Packing type

Pressure drop method

Goodloe

Vendor procedure

Glitsch Grid

Vendor procedure

Norton Intalox Structured Packings

Vendor procedure

Sulzer BX, CY, Mellapak, and Kerapak

Vendor procedure♦

Koch Flexipac, Flexeramic, and Flexigrid

Vendor procedure♦♦

† †† ††† ♦

♦♦

† †† †††

Goodloe, Bulletin 520A (Dallas: Glitsch, Inc., 1981). Glitsch Grid-Grid/Ring Combination Bed, Bulletin No. 7070 (Dallas: Glitsch, Inc., 1978). Norton Company, private communication, 1992. Spiegel, L. and Meier, W., "Correlations of the Performance Characteristics of the Various Mellapak Types." Paper presented at the 4th International Symposium of Distillation and Absorption, Brighton, England, 1987. McNulty, K.J., "Hydraulic Model for Packed Tower Design." Paper presented at the American Institute of Chemical Engineers Spring Meeting in Houston, 1993.

Liquid Holdup Calculations for Packing ASPEN PLUS performs liquid holdup calculations for both random and 6 structured packings. The calculations use the Stichlmair correlation. The Stichlmair correlation requires these parameters: • Packing void fraction and surface area • Three Stichlmair correlation constants ASPEN PLUS provides these parameters for a variety of packings in the built-in packing databank. If these parameters are missing for a particular packing, ASPEN PLUS will not perform liquid holdup calculations for that packing. You can also enter these parameters to provide missing values, or to override the databank values.

A-16

Unit Operation Models Version 10

Appendix A

Pressure Profile Update You can update the pressure profile using: • Computed pressure drops for the rating mode of both trays and packings • The sizing mode of packings If you choose to update the pressure profile, the column models solve the tray or packing calculation procedures simultaneously with the column-describing equations. For updating the pressure profile during calculations check Update Section Pressure Profile on the following forms: • • •

TrayRating PackSizing PackRating

Also, you can fix the pressure at the top or bottom of the column and you can specify this option on the above forms. The stage pressures become additional variables. ASPEN PLUS uses the pressure specifications given on the Pres-Profile form to: • Initialize the column pressure profile • Fix the pressure drop of stages for which the pressure profile is not updated

Physical Property Data Requirements Several physical properties that are not normally used for heat and material balance calculations are required for column sizing and rating. These properties are: • Liquid and vapor densities • Liquid surface tension • Liquid and vapor viscosities The physical property method that you specify for a unit operation model must be able to provide the required properties. In addition, the physical property parameters needed to calculate the required properties must be available for all components in the column. See the descriptions of properties in the ASPEN PLUS User Guide Volume 1, for details on specifying physical property methods and determining property parameter requirements.

Unit Operation Models Version 10

A-17

Sizing and Rating for Trays and Packings

References 1. Fair, J.R., et al., “Liquid-Gas Systems,” Perry’s Chemical Engineers' th Handbook, R.H. Perry and D. Green, ed. 6 ed., New York: McGraw Hill, 1984. rd 2. Ballast Tray Design Manual, Glitsch, Inc., Bulletin No. 4900, 3 ed., Dallas:1980.

3. Smith, B.D., “Tray Hydraulics: Bubble Cap Trays” and “Tray Hydraulics: Perforated Trays,” Design of Equilibrium Stage Processes, New York: McGraw Hill, 1963, pp. 474-569. 4. Koch Flexitray Design Manual, Koch Engineering Co., Inc. Bulletin No. 90, Wichita. 5. Nutter Float Valve Design Manual, Tulsa: Nutter Engineering Co., 1976. 6. Stichlmair, J., et al., "General Model for Prediction of Pressure Drop and Capacity of Countercurrent Gas/Liquid Packed Columns," Gas Separation and Purification, Vol. 3 (1989), p. 22.

❖

A-18

❖

❖

❖

Unit Operation Models Version 10

B

Index A Absorbers MultiFrac 4-30 RadFrac 4-23 RateFrac 4-62 Aerotran flash specifications 3-27 flowsheet connectivity 3-26 overview 3-26 physical properties 3-28 solids 3-28 specifying 3-27 AGA method Pipe model 6-39 Pipeline 6-51 Air separation MultiFrac 4-30 Air-cooled heat exchangers Aerotran 3-26 Algorithms convergence 4-22, 4-25, 4-27, 4-28, 4-42, 4-58 inside-out 4-26, 4-43 Newton 4-22, 4-26, 4-42, 4-44 nonideal 4-22, 4-26 standard 4-26, 4-42, 4-43 sum-rates 4-22, 4-26, 4-42, 4-43 Angel-Welchon-Ros correlation Pipe model 6-38 Pipeline 6-49 ASME method Compr 6-10 MCompr 6-15 Azeotropic distillation RadFrac 4-22

Unit Operation Models Version 10

Baffle geometry HeatX 3-13 Baghouses FabFl 8-23 resistance coefficients 8-25 separation efficiency 8-26 Ballast trays values A-11 Batch reactors RBatch 5-25 Beggs and Brill correlation Pipe model 6-37 Pipeline 6-48 Beggs and Brill correlation parameters Pipe model 6-38 Pipeline 6-50 B-JAC Aerotran interface 3-26 Hetran interface 3-23 Bolles method tray flooding calculations A-8 Bond work index (BWI) Crusher 8-14, 8-17 Brake horsepower Compr 6-12 MCompr 6-17 Bubble cap trays cap diameter A-9

C Cavitation index Valve model 6-29 CCD component attributes 8-66 flowsheet connectivity 8-64 medium temperature 8-67 mixing efficiency 8-66 overview 8-64 profiles 8-66 pseudostreams 8-65 specifying 8-65 Centrifuge filters CFuge 8-52 CFuge filter cake 8-53 filtrate flow rate 8-54 flowsheet connectivity 8-52 overview 8-52

Index-1

CFuge (continued) pressure drop 8-54 rating 8-53 separation efficiency 8-55 sizing 8-53 specifying 8-53 Chilton-Colburn analogy RateFrac 4-77, 4-84 ClChng flowsheet connectivity 7-6 overview 7-6 specifying 7-6 stream class change 7-6 Coal grinding 8-18 Column configuration RateFrac 4-70 Columns Distl 4-6 DSTWU 4-3 Extract 4-87 MultiFrac 4-30 packings A-12 PetroFrac 4-48 physical property requirements A-17 pressure drop calculations A-1 RadFrac 4-11, 4-16 RateFrac 4-62 rating A-1 SCFrac 4-8 sizing A-1 Component ratio RateFrac 4-75 Component separators Sep 2-12 Sep2 2-14 Compr ASME method 6-10 flowsheet connectivity 6-9 GPSA method 6-10 isentropic efficiency 6-12 mechanical efficiency 6-12 Mollier method 6-10 net work load 6-10 overview 6-9 performance curves 6-10 polytropic efficiency 6-11 specifying 6-10 steam pressure 6-9 Compressors Compr 6-9 Heater model 3-2

Index-2

Compressors (continued) MCompr 6-13 Condensers PetroFrac 4-51 RateFrac 4-71 Connecting streams RateFrac 4-70 Continuous stirred tank reactor RCSTR 5-16 Convergence algorithms 4-42, 4-43 RateFrac 4-76 Convergence algorithms PetroFrac 4-58 RadFrac 4-25 Coolers Heater model 3-2 RadFrac 4-17 RateFrac 4-73 Crude units SCFrac 4-8 Crusher Bond work index (BWI) 8-14, 8-17 breakage functions 8-14 flowsheet connectivity 8-13 Hardgrove grindability index (HGI) 8-14, 8-18 overview 8-13 power requirement 8-16 primary crusher 8-16 reduction ratios 8-16 secondary crusher 8-16 selection functions 8-14 specifying 8-14 Cryogenic applications RadFrac 4-23 Crystallizer crystal growth rate 8-7 crystal nucleation rate 8-8 flowsheet connectivity 8-3 magma recirculation 8-5 overview 8-3 particle size distribution (PSD) 8-9, 8-10 population balance 8-8 recirculation 8-5 saturation calculation 8-6 solubility 8-5 specifying 8-4 supersaturation 8-6 Cyclone design calculations 8-28 diameter calculation 8-31 dimension ratios 8-31

Unit Operation Models Version 10

Cyclone (continued) dimensions 8-28, 8-32 efficiency correlations 8-29 flowsheet connectivity 8-27 geometry 8-32 Leith and Licht correlation 8-29 operating ranges 8-29 overview 8-27 pressure drop 8-30 rating calculations 8-28 separation efficiency 8-29 Shepherd and Lapple correlation 8-29 solids loading correction 8-34 specifying 8-28 vane constant 8-32

D Darcy correlation Pres-Relief 10-12 Decanter model flowsheet connectivity 2-8 Gibbs free energy 2-10 KLL coefficients 2-10 liquid phases 2-10 liquid-liquid distribution coefficients 2-10 overview 2-8 phase-splitting methods 2-10 separation efficiencies 2-11 solids entrainment 2-11 specifying 2-9 Decanters CCD 8-64 Decanter model 2-8 Flash3 2-5 RadFrac 4-18, 4-29 Design mode RateFrac 4-74 Design mode convergence RadFrac 4-26 Design specification convergence MultiFrac 4-44 DIERS calculations Pres-Relief 10-18 Distillation Distl 4-6 DSTWU 4-3 MultiFrac 4-30 RateFrac 4-62 SCFrac 4-8

Unit Operation Models Version 10

Distl Edmister approach 4-6 flowsheet connectivity 4-6 overview 4-6 specifying 4-7 DSTWU flowsheet connectivity 4-4 Gilliland’s method 4-3 overview 4-3 reflux ratio 4-3 specifying 4-4 Underwood’s method 4-3 Winn’s method 4-3 Dukler correlation Pipe model 6-37 Pipeline 6-48 Pres-Relief 10-12 Dupl flowsheet connectivity 7-4 overview 7-4 specifying 7-5 Dynamic scenario algorithm Pres-Relief 10-19

E Eaton correlation Pipe model 6-38 Pipeline 6-49 Edmister approach Distl 4-6 Efficiencies Compr 6-12 MCompr 6-16, 6-17 RadFrac 4-20 Electrostatic precipitators ESP 8-40 Emergency relief vents (ERV) Pres-Relief 10-15 Equilibrium constants REquil 5-9 RGibbs 5-13 Equilibrium reactors REquil 5-8 RGibbs 5-10 ESP flowsheet connectivity 8-40 gas velocity 8-41, 8-44 operating range 8-41 overview 8-40 particle separation 8-42, 8-44

Index-3

ESP (continued) power requirement 8-44 pressure drop 8-43 separation efficiency 8-42 specifying 8-41 Ethylene plant primary fractionators MultiFrac 4-30 PetroFrac 4-48 Evaporators Flash2 2-2 Flash3 2-5 Exchanger configuration HeatX 3-11 Exchanger geometry HeatX 3-5 Extract flowsheet connectivity 4-87 overview 4-87 specifying 4-88

F FabFl calculation options 8-23 filtering time 8-24 flowsheet connectivity 8-23 operating ranges 8-24 overview 8-23 resistance coefficients 8-25 separation efficiency 8-26 specifying 8-23 Fabric filters FabFl 8-23 Fair method tray flooding calculations A-8 Feed furnaces PetroFrac 4-54 Feed stream conventions RateFrac 4-68 Feed streams PetroFrac 4-53 Film coefficients HeatX 3-10, 3-15 Filter model filter cake characteristics 8-57 flowsheet connectivity 8-56 overview 8-56 pressure drop 8-58 separation efficiency 8-58 specifying 8-56

Index-4

Filters CFuge 8-52 FabFl 8-23 Filter model 8-56 Flanigan correlation Pipe model 6-38 Pipeline 6-50 Pres-Relief 10-12 Flash tables zone analysis 3-21 Flash2 electrolytes 2-4 flowsheet connectivity 2-2 overview 2-2 solids 2-4 specifying 2-3 Flash3 electrolytes 2-6 flowsheet connectivity 2-5 overview 2-5 solids 2-6 specifying 2-6 streams 2-5 Flashes Flash2 2-2 Flash3 2-5 Flexitrays values A-11 Float valve trays values A-11 Fractionators PetroFrac 4-48 Free-water calculations MultiFrac 4-46 PetroFrac 4-60 RadFrac 4-20 RateFrac 4-74 FSplit flowsheet connectivity 1-5 overview 1-5 specifying 1-6

G Gas-solid separators Cyclone 8-27 ESP 8-40 FabFl 8-23 VScrub 8-36 General purpose valves Pres-Relief 10-13

Unit Operation Models Version 10

Gibbs free energy Decanter model 2-10 REquil 5-9 RGibbs 5-10 Gilliland’s correlation DSTWU 4-3 Glitsch Ballast method tray flooding calculations A-8 GPSA method Compr 6-10 MCompr 6-15

H Hagedorn-Brown correlation Pipe model 6-37 Pipeline 6-49 Hardgrove grindability index (HGI) Crusher 8-14, 8-18 Hazen-Williams method Pipe model 6-40 Pipeline 6-52 Heat exchangers Aerotran 3-26 computational structure 3-21 equations 3-8 Heater model 3-2 HeatX 3-5 Hetran 3-23 MHeatX 3-19 multistream 3-19 zone analysis 3-21 Heat transfer coefficient HeatX 3-9 Heater model electrolytes 3-4 flowsheet connectivity 3-3 overview 3-2 solids 3-4 specifying 3-3 Heaters Heater model 3-2 MultiFrac 4-38 RadFrac 4-17 RateFrac 4-73 Heat-interstaged columns MultiFrac 4-30 HeatX baffle geometry 3-13 electrolytes 3-17 exchanger configuration 3-11

Unit Operation Models Version 10

HeatX (continued) exchanger geometry 3-5 film coefficients 3-10, 3-15 flash specifications 3-17 flowsheet connectivity 3-6 heat transfer coefficient 3-9 log-mean temperature difference 3-8 model correlations 3-15 nozzle geometry 3-15 option sets 3-17 overview 3-5 physical properties 3-17 pressure drop 3-13, 3-14, 3-15 pressure drop calculations 3-10, 3-15 rating calculations 3-5, 3-6, 3-7, 3-8, 3-9 shell-side film coefficient 3-13 solids 3-17 specifying 3-6 streams 3-6 TEMA shells 3-11 tube geometry 3-14 tube-side film coefficient 3-14 zone analysis 3-5 HETP packings calculations A-12 RateFrac 4-75 Hetran flash specifications 3-24 flowsheet connectivity 3-23 overview 3-23 physical properties 3-25 solids 3-25 specifying 3-24 Hughmark method Pipe model 6-37 Pipeline 6-48 HyCyc dimension ratios 8-49 dimensions 8-50, 8-51 feed splitting 8-48 flowsheet connectivity 8-45 geometry 8-50 operating ranges 8-46 overview 8-45 particle velocity 8-49 pressure drop correlation 8-50 rating 8-46 separation efficiency 8-47 sizing 8-46 solids separation 8-45 specifying 8-46 velocity correlation 8-49

Index-5

Hydraulic turbines Pump model 6-2 Hydrocyclones HyCyc 8-45

I Inside-out algorithms MultiFrac 4-43 RadFrac 4-26 Isentropic compressors Compr 6-9, 6-12 MCompr 6-13 Isentropic turbines Compr 6-9 MCompr 6-13

K Kettle reboilers RadFrac 4-16 Knock-out drums Decanter model 2-8 Flash2 2-2 Flash3 2-5

L Leith and Licht correlation Cyclone 8-29 Liquid-liquid extraction Extract 4-87 Liquid-solid separators CFuge 8-52 Filter model 8-56 HyCyc 8-45 LNG exchanger MHeatX 3-19 Lockhart-Martinelli correlation Pipe model 6-37 Pipeline 6-49 Pres-Relief 10-12 Log-mean temperature HeatX 3-8

M Manipulators ClChng 7-6 Dupl 7-4

Index-6

Manipulators (continued) Mult 7-2 MCompr ASME method 6-15 brake horsepower 6-17 flow coefficient 6-19 flowsheet connectivity 6-14 GPSA method 6-15 head coefficient 6-18 isentropic efficiency 6-16 mechanical efficiency 6-17 Mollier method 6-15 overview 6-13 parasitic pressure loss 6-17 polytropic efficiency 6-16 specific diameter 6-18 specific speed 6-18 specifying 6-14, 6-15 MHeatX computational structure 3-21 electrolytes 3-22 flash tables 3-21 flowsheet connectivity 3-19 LNG exchanger 3-19 overview 3-19 solids 3-22 specifying 3-20 zone analysis 3-19, 3-20, 3-21 Mixer model flowsheet connectivity 1-2 overview 1-2 specifying 1-3 Mixers Heater model 3-2 Mixer model 1-2 Model correlations HeatX 3-15 Mollier method Compr 6-10 MCompr 6-15 Mult flowsheet connectivity 7-2 overview 7-2 specifying 7-3 MultiFrac algorithms 4-43 connecting streams 4-36 convergence algorithms 4-42, 4-43 design mode 4-42 design specification convergence 4-44 efficiencies 4-41 ethylene plant primary fractionator 4-30

Unit Operation Models Version 10

MultiFrac (continued) feed stream conventions 4-35 flow rate 4-38, 4-42 flow ratio 4-40 flowsheet connectivity 4-32 free-water calculations 4-46 heaters 4-38 initialization methods 4-45 Murphree efficiency 4-41 Newton algorithm 4-44 overview 4-30 packings 4-47 physical properties 4-46 property methods 4-46 rating mode 4-42 solids 4-46 specifying 4-33, 4-34 stream definitions 4-34 streams 4-32, 4-33, 4-35, 4-36, 4-42 sum-rates algorithm 4-43 trays 4-47 vaporization efficiency 4-41 Multistage fractionation units MultiFrac 4-30 Murphree efficiency MultiFrac 4-41 PetroFrac 4-57 RadFrac 4-21 RateFrac 4-65, 4-75

N Napthali-Sandholm algorithm RadFrac 4-26 Nested convergence RadFrac 4-27 Newton algorithm MultiFrac 4-44 RadFrac 4-22, 4-26 RateFrac 4-76 Nonequilibrium fractionation RateFrac 4-62 Nozzle geometry HeatX 3-15

O Oliphant method Pipe model 6-39 Pipeline 6-51

Unit Operation Models Version 10

Orkiszewski correlation Pipe model 6-37 Pipeline 6-49

P Packings calculations A-12 capacity calculations A-13 liquid holdup calculations A-16 MultiFrac 4-47 PetroFrac 4-61 pressure drop calculations A-15 pressure profile A-17 RateFrac 4-70 rating A-12 sizing A-12 specifying A-1 Stichlmair correlation A-16 types A-1, A-12, A-13 Panhandle methods Pipe model 6-40 Pipeline 6-51 Particle separation ESP 8-42, 8-44 PetroFrac condensers 4-51 convergence algorithms 4-58 design mode 4-59 efficiencies 4-57 ethylene plant primary fractionator 4-48 feed furnace 4-51, 4-54 feed streams 4-53 flowsheet connectivity 4-49 free-water calculations 4-60 liquid runback 4-56 main column 4-50, 4-51 Murphree efficiency 4-57 overview 4-48 packings 4-61 physical properties 4-60 property methods 4-60 pumparounds 4-56 rating mode 4-59 reboilers 4-51 side strippers 4-51, 4-57 solids 4-61 specifying 4-51 streams 4-49 trays 4-61 vaporization efficiency 4-57

Index-7

Petroleum refining fractionation MultiFrac 4-30 PetroFrac 4-48 Petroleum/petrochemical applications RadFrac 4-22 Physical properties columns A-17 HeatX 3-17 Physical property methods RateFrac 4-74 Pinch points estimating 3-21 Pipe model AGA method 6-39 Angel-Welchon-Ros correlation 6-38 Beggs and Brill correlation 6-37 Beggs and Brill correlation parameters 6-38 closed-form methods 6-39 Design-Spec convergence loop 6-34 downstream and upstream integration 6-33 Dukler correlation 6-37 Eaton correlation 6-38 erosional velocity 6-34 fittings modeling 6-35 Flanigan correlation 6-38 flash options 6-32 flowsheet connectivity 6-30 fraction factor correlations 6-35 Hagedorn-Brown correlation 6-37 Hazen-Williams method 6-40 holdup correlations 6-35 Hughmark method 6-37 integration direction 6-33 liquid holdup correlations 6-35 Lockhart-Martinelli correlation 6-37 methane gas systems 6-34 Oliphant method 6-39 Orkiszewski correlation 6-37 overview 6-30 Panhandle methods 6-40 physical property calculations 6-32 pressure drop calculations 6-33 pressure specification 6-31 Slack correlation 6-38 Smith method 6-39 specifying 6-31 stream specification 6-32 two-phase correlations 6-35 valve modeling 6-35 Weymouth method 6-39

Index-8

Pipeline AGA method 6-51 Angel-Welchon-Ros correlation 6-49 Beggs and Brill correlation 6-48 Beggs and Brill correlation parameters 6-50 closed-form methods 6-50 Design-Spec convergence loop 6-46 downstream and upstream integration 6-45 Dukler correlation 6-48 Eaton correlation 6-49 erosional velocity 6-46 Flanigan correlation 6-50 flowsheet connectivity 6-42 fraction factor correlations 6-47 Hagedorn-Brown correlation 6-49 Hazen-Williams method 6-52 holdup correlations 6-47 Hughmark method 6-48 integration direction 6-45 liquid holdup correlations 6-47 Lockhart-Martinelli correlation 6-49 methane gas systems 6-47 nodes and segments 6-44 Oliphant method 6-51 Orkiszewski correlation 6-49 overview 6-42 Panhandle methods 6-51 physical property calculations 6-45 pressure drop calculations 6-45 Slack correlation 6-49 Smith method 6-51 specifying 6-43 stream specification 6-44 two-phase correlations 6-47 Weymouth method 6-51 Pipes Pipe model 6-30 Pipeline 6-42 Piping system Pres-Relief 10-11 Plug flow reactors RPlug 5-21 Polytropic compressors Compr 6-9, 6-11 MCompr 6-13 Pres-Relief 3% rule 10-8 97% rule 10-8 Beggs and Brill correlation 10-12 calculation methods 10-23

Unit Operation Models Version 10

Pres-Relief (continued) capacity runs 10-6 code compliance 10-6 convergence methods 10-23 credit factors 10-4 Darcy correlation 10-12 data tables 10-12–10-16 DIERS calculations 10-18 disengagement options 10-18 Dukler correlation 10-12 dynamic scenarios 10-2, 10-7, 10-16, 10-18, 10-19 energy input calculations 10-4 fire scenario 10-3 flow equations 10-20 heat exchanger shell 10-17 heat flux scenario 10-5 insulation credit factor 10-24 Lockhart-Martinelli correlation 10-12 manufacturers' tables 10-12–10-16 nozzle flow equation 10-22 overview 10-2 pipe diameters 10-12 pipe flow equation 10-20 pipe specifications 10-11 reactions 10-9 relief system 10-10 relief system flow rating scenario 10-5 relief valve flow rating scenario 10-6 rupture disks 10-15 safety relief valves 10-14 sample solution 10-19 scenarios 10-3 sizing rules 10-7, 10-9 Slack correlation 10-12 specifying 10-2, 10-10, 10-11 spheres 10-17 steady-state scenarios 10-6 stop criteria 10-18 streams 10-7 user-specified vessel 10-17 valve cycling 10-16 valve types 10-10, 10-13 vents 10-15 vessel geometry 10-16 vessel head types 10-17 vessel jacket 10-17 wetted area calculations 10-4 X% rule 10-8 Pressure changers Compr 6-9 MCompr 6-13 Pipe model 6-30

Unit Operation Models Version 10

Pressure changers (continued) Pipeline 6-42 Pump model 6-2 Valve model 6-20 Pressure drop HeatX 3-13, 3-14, 3-15 Pressure drop calculations HeatX 3-10, 3-15 Pressure drop models Pipe model 6-30 Pipeline 6-42 Pressure relief systems Pres-Relief 10-2 Pump model flow coefficient 6-7 flowsheet connectivity 6-2 head coefficient 6-7 net positive suction head (NPSH) 6-4 overview 6-2 specific speed 6-5 specifying 6-3 suction specific speed 6-6 Pumparounds RadFrac 4-18 Pumps Heater model 3-2 Pump model 6-2

R RadFrac 4-23 absorbers 4-23 algorithms 4-22 azeotropic distillation 4-22 column configuration 4-13, 4-16 convergence algorithms 4-22, 4-25 convergence methods 4-26, 4-27, 4-28 coolers 4-17 decanters 4-18, 4-29 design mode 4-24 design mode convergence 4-26 design specifications 4-27 efficiencies 4-20 feed streams 4-14 flowsheet connectivity 4-12 free-water calculations 4-20 heaters 4-17 inside-out algorithms 4-26 kettle reboilers 4-16 Murphree efficiency 4-21 Napthali-Sandholm algorithm 4-26

Index-9

RadFrac (continued) Newton algorithm 4-22, 4-26 nonideal systems 4-22 overview 4-11 petroleum/petrochemical applications 4-22 physical properties 4-28 property methods 4-28 pumparounds 4-18 rating mode 4-23 reactive distillation 4-25 reboilers 4-16 salt precipitation 4-25 simultaneous convergence 4-28 solids handling 4-28 specifying 4-12 stage numbering 4-14 streams 4-12 strippers 4-23 thermosyphon reboilers 4-16 three-phase calculations 4-20, 4-23 two-phase calculations 4-23 UA calculations 4-17 vaporizaton efficiency 4-20 Rate-based modeling RateFrac 4-62, 4-65 RateFrac bubble-cap tray column 4-81 Chilton-Colburn analogy 4-77, 4-84 column configuration 4-70 column numbering 4-67 component ratio 4-75 connecting streams 4-70 convergence 4-76 coolers 4-73 correlations 4-76, 4-77 design mode 4-74 efficiencies 4-65, 4-75 equilibrium stages 4-72 feed stream conventions 4-68 flowsheet connectivity 4-63 Fortran subroutines 4-77 free-water calculations 4-74 heat transfer coefficients 4-84 heaters 4-73 HETP 4-65, 4-75 interfacial areas 4-76, 4-77, 4-79, 4-81, 4-82 mass transfer coefficients 4-76, 4-77, 4-79, 4-81, 4-82 Murphree efficiency 4-65 Newton algorithm 4-76 overview 4-62 packing specifications 4-70

Index-10

RateFrac (continued) physical property method 4-74 rate-based modeling 4-65 rating mode 4-74 reactions 4-72 reactive distillation 4-72 segments 4-71, 4-75 side duties 4-73 sieve tray column correlations 4-82 solution times 4-76 specifying 4-64, 4-66, 4-70 stream definitions 4-68 streams 4-63 tray column 4-79 tray column correlations 4-81, 4-82 tray specifications 4-70 utility exchangers 4-73 valve tray column 4-79 Rating mode RateFrac 4-74 RBatch batch operation 5-29 cycle time 5-28 flowsheet connectivity 5-25 mass balances 5-28 overview 5-25 reactions 5-28 specifying 5-26 stop criteria 5-28 temperature controller 5-27 RCSTR flowsheet connectivity 5-16 overview 5-16 phase volume 5-17 reaction kinetics 5-17 residence time 5-18 scaling methods 5-19 solids reactions 5-18 specifying 5-17 variable scaling 5-19 Reactions RateFrac 4-72 Reactive distillation RadFrac 4-25 Reactors RBatch 5-25 RCSTR 5-16 REquil 5-8 RGibbs 5-10 RPlug 5-21 RStoic 5-2 RYield 5-6

Unit Operation Models Version 10

Reboilers PetroFrac 4-51 RadFrac 4-16 Relief devices Pres-Relief 10-10 REquil equilibrium constants 5-9 flowsheet connectivity 5-8 Gibbs free energy 5-9 net heat duty 5-8 overview 5-8 solids 5-9 specifying 5-9 streams 5-8 RGibbs chemical equilibrium 5-12 flowsheet connectivity 5-11 overview 5-10 phase equilibrium 5-12, 5-13 reactions 5-14 restricted chemical equilibrium 5-13 solids 5-14 specifying 5-11 Rigorous distillation MultiFrac 4-30 PetroFrac 4-48 RadFrac 4-11 RateFrac 4-62 Rigorous extraction Extract 4-87 RPlug coolant 5-23 flowsheet connectivity 5-22 overview 5-21 reactions 5-24 solids 5-24 specifying 5-22 RStoic flowsheet connectivity 5-3 heat of reaction 5-3, 5-4 overview 5-2 product selectivity 5-3, 5-4 specifying 5-3 stream specifications 5-3 RYield calculation types 5-7 flowsheet connectivity 5-6 heat duty specification 5-7 overview 5-6 specifying 5-7 yield distribution 5-7

Unit Operation Models Version 10

S Salt precipitation RadFrac 4-25 SCFrac crude units 4-8 flowsheet connectivity 4-8 overview 4-8 specifying 4-9 vacuum towers 4-8 Screen flowsheet connectivity 8-19 operating levels 8-20 overview 8-19 screen size correlation 8-21 selection function 8-20 separation efficiency 8-21 separation strength 8-20 specifying 8-19 Sep flowsheet connectivity 2-12 inlet pressure 2-13 outlet stream conditions 2-13 overview 2-12 specifying 2-13 Sep2 flowsheet connectivity 2-14 inlet pressure 2-16 outlet stream conditions 2-16 overview 2-14 specifying 2-15 substreams 2-15 Separators Decanter model 2-8 Flash2 2-2 Flash3 2-5 Sep 2-12 Sep2 2-14 Shell heat exchangers Hetran 3-23 Shell-side film coefficient HeatX 3-13 Shepherd and Lapple correlation Cyclone 8-29 Shortcut distillation Distl 4-6 DSTWU 4-3 SCFrac 4-8 Simultaneous convergence RadFrac 4-28 Sizing recommendations Pres-Relief 10-9

Index-11

Slack correlation Pipe model 6-38 Pipeline 6-49 Pres-Relief 10-12 Smith method Pipe model 6-39 Pipeline 6-51 Solids Crystallizer 8-3 Flash2 2-4 Flash3 2-6 Heater model 3-4 MHeatX 3-22 RGibbs 5-14 Solids crushers Crusher 8-13 Solids separators CFuge 8-52 Crusher 8-13 Cyclone 8-27 ESP 8-40 FabFl 8-23 Filter model 8-56 HyCyc 8-45 Screen 8-19 VScrub 8-36 Solids washers CCD 8-64 SWash 8-61 Splitters FSplit 1-5 Sep 2-12 Sep2 2-14 SSplit 1-8 SSplit flowsheet connectivity 1-8 overview 1-8 specifying 1-8 Stichlmair correlation packings calculations A-16 Stoichiometric reactors RStoic 5-2 Stream classes changing 7-6 Stream definitions RateFrac 4-68 Stream manipulators ClChng 7-6 Dupl 7-4 Mult 7-2 Stream mixers Mixer model 1-2

Index-12

Stream multiplication Mult 7-2 Stream pressure changers Pump model 6-2 Stream splitters FSplit 1-5 SSplit 1-8 Streams combining 1-8 Flash3 2-5 splitting 2-12, 2-14 Strippers MultiFrac 4-30 RadFrac 4-23 RateFrac 4-62 Substream splitters SSplit 1-8 Sum-rates algorithm MultiFrac 4-43 SWash bypass fraction 8-63 flowsheet connectivity 8-61 mixing efficiency 8-62 overview 8-61 specifying 8-62

T TEMA shells HeatX 3-11 Thermosyphon reboilers RadFrac 4-16 Three-phase calculations RadFrac 4-20 Trays Bolles method A-8 bubble cap A-9 downcomer specifications A-3 Flexitrays A-11 float valve A-11 flooding calculations A-8 foaming calculations A-11 MultiFrac 4-47 PetroFrac 4-61 pressure drop calculations A-10 pressure profile A-17 RateFrac 4-70 rating A-2, A-8 sizing A-2, A-8 specifying A-1 types A-1

Unit Operation Models Version 10

Tube geometry HeatX 3-14 Tube heat exchangers Hetran 3-23 Tube-side film coefficient HeatX 3-14 Turbines Compr 6-9 MCompr 6-13 Pump model 6-2

U Underwood’s method DSTWU 4-3 Unit operation models user-supplied 9-2, 9-4 User model flowsheet connectivity 9-2 Fortran subroutines 9-3 overview 9-2 specifying 9-3 User2 flowsheet connectivity 9-4 Fortran subroutines 9-5 overview 9-4 specifying 9-5

V Vacuum filters Filter model 8-56 Vacuum towers SCFrac 4-8 Valve model calculation types 6-20 cavitation index 6-29 characteristic equation 6-26 choked flow 6-28 flow coefficient 6-24 flowsheet connectivity 6-20 overview 6-20 piping geometry factor 6-26 pressure drop calculation 6-20, 6-28 pressure drop ratio factor 6-22 pressure recovery factor 6-23 specifying 6-20 Valves cycling 10-16 Heater model 3-2 Pipe model 6-35

Unit Operation Models Version 10

Valves (continued) safety relief 10-14 types used in Pres-Relief 10-10, 10-13–10-16 Valve model 6-20 Vaporization efficiency MultiFrac 4-41 PetroFrac 4-57 RadFrac 4-20 Vents Pres-Relief 10-15 Venturi scrubbers VScrub 8-36 VScrub flowsheet connectivity 8-36 overview 8-36 pressure drop 8-38 rating 8-37 separation efficiency 8-39 sizing 8-37 specifying 8-37

W Weymouth method Pipe model 6-39 Pipeline 6-51 Winn's method DSTWU 4-3

Y Yield reactors RYield 5-6

Z Zone analysis HeatX 3-5 MHeatX 3-19, 3-20, 3-21

Index-13

Index-14

Unit Operation Models Version 10