Seoul National University - Process Modeling Using Aspen Plus

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Process Modeling Using Aspen Plus (User Interface & Basic Inputs)  S   e   o  u  l   N  a   t   i   o  n  a   l   U  n  i   v  e   r  s   i   t   y 

Spring Semester, 2012

TA : Ikhyun Ikhyun Kim Instructo Instr uctorr : En Sup S up Yoon

Flows Flowshee heett Simul Simulat ation ion • What What is flow flowshe sheet et simu simulation lation? ? – Use of a comput computer er program program to quantit quantitat atively ively model model the the characteristic characteristic equations equations of a chemical chemical process

• Uses Uses unde underl rly ying ing phy physical relat relation ionship ships s – Mass an and en ener erg gy balan alance ce – E quili quilibr briu ium m re relat lation ionshi ships ps – Rate ate correlat correlations ions (react (reaction ion and and mass/hea ass/heatt transfer) ransfer)

• Pre Predicts – S tream flowrat flowrates, es, composit compositions, ions, and proper propertties – Operat Operatin ing g condi condittions ions Chemical Process Modelin Modeling g & Simulatio Simulation n

(2/39)

Seoul National University 

Good Good Flowsh Flowsheeti eeting ng Practi Practice ce • Build large large flow flowshee sheetts a few few block blocks s at a tim time e – This facilit facilitat ates es trou troubleshoot bleshooting ing if err error or occur

• Not necess necessari arily ly a one-t one-to-one o-one correspo correspond ndence ence betw between pieces of equipment in the plant and Aspen plus blocks • E nsure sure flo flow wsheet sheet inp inputs are are rea reason sonab able le • Check that result are consistent consistent and realistic realistic

Chemical Process Modelin Modeling g & Simulatio Simulation n

(3/39)

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Some Important Features of Aspen+ • Rigor igorou ous s elect electro roly lytte simu simulation lation • Soli Solid handing • P etr etroleu oleum m hand andlin ling • Dat Data rre egressio ssion • Data fit • Opt Optimiza imizattion ion • User User routine ines

Chemical Process Modelin Modeling g & Simulatio Simulation n

(4/39)

Seoul National University 

Good Good Flowsh Flowsheeti eeting ng Practi Practice ce • Build large large flow flowshee sheetts a few few block blocks s at a tim time e – This facilit facilitat ates es trou troubleshoot bleshooting ing if err error or occur

• Not necess necessari arily ly a one-t one-to-one o-one correspo correspond ndence ence betw between pieces of equipment in the plant and Aspen plus blocks • E nsure sure flo flow wsheet sheet inp inputs are are rea reason sonab able le • Check that result are consistent consistent and realistic realistic

Chemical Process Modelin Modeling g & Simulatio Simulation n

(3/39)

Seoul National University 

Some Important Features of Aspen+ • Rigor igorou ous s elect electro roly lytte simu simulation lation • Soli Solid handing • P etr etroleu oleum m hand andlin ling • Dat Data rre egressio ssion • Data fit • Opt Optimiza imizattion ion • User User routine ines

Chemical Process Modelin Modeling g & Simulatio Simulation n

(4/39)

Seoul National University 

The User Interface Interface (Flowsheet)

Next Next butto n

Detherm internet

NIST/TDE Pure property

Process Flow Diagram

Model Library Tabs

Select Mode Button

Model Library Status  Ar ea

Chemical Process Modelin Modeling g & Simulatio Simulation n

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The User Interface (Data browser) browser) Data browser  Sub-specification tab

Specification & Result data menu tree

Description

Status  Ar ea

Chemical Process Modelin Modeling g & Simulatio Simulation n

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The User Interface (Run control panel)

Run Control Panel

Description of  sequential calculation

Calculation Sequence Summary of errors

Status  Ar ea

Chemical Process Modeling & Simulation

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Graphic Flowsheet Operations - Blocks • To place a block on the flowsheet: 1. Click a model category tab in the Model Library 2. Select a unit operation model; click the drop-down arrow to select an icon for the model 3. Click the model and then click the flowsheet to place the block; you can also click the model icon and drag it onto the flowsheet 4. Click the right mouse button to stop placing blocks

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Graphic Flowsheet Operations - Streams • To place a stream on the flowsheet: 1. Click the STREAMS icon in the Model Library 2. If you want to select a different stream type (Material, Heat or Work), click the down arrow next to the icon and choose a different type 3. Click a highlighted port to make the connection 4. Repeat step 3 to connect the other end of the stream 5. To place one end of the stream as either a process flowsheet feed or product, click a blank part of the Process Flowsheet 6. Click the right mouse button to stop creating streams

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Automatic Naming of Streams and Blocks • Stream and block names can be assigned automatically by Aspen Plus or entered by the user when the object is created • To modify the naming options: 1. Select Options from the Tools menu 2. Click the Flowsheet tab 3. Check or uncheck the naming options desired

• Stream and block names can be displayed or hidden 1. Select object, right-click, and choose Hide from the menu

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Modifying Blocks and Streams • To display Input and Results forms in the Data Browser: 1. Double-click the object of interest, or Select the block or stream, right-click, and select Input… from the menu

• To change the appearance of a block or stream: 1. Select object by clicking it with the left mouse button 2. Click the right mouse button while the pointer is over the selected object icon to bring up the menu for that object 3. Choose appropriate menu item

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Breaking and Splicing Streams • To break a stream on the Process Flowsheet: 1. Select the stream on the flowsheet and right mouse click 2. Select “Break Stream” for the stream menu 3. If results exist, you will be asked if you want to reconcile the stream 4. Enter the name of the new product stream created

• To splice two streams: 1. Select the two streams to be spliced (using the Shift or Ctrl key) 2. Right mouse click on one of the streams, select “Splice Streams” 3. Combined stream will have the name of the former feed stream

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Inserting Blocks • To insert a block on the Process Flowsheet: 1. Select the stream where you want to insert the block 2. Right-click and select “Insert Block” 3. If results exist, you will be asked if you want to reconcile the stream 4. Select the new block ID and type 5. The old stream is connected to the first inlet and outlet port for the new block 6. Additional streams may need to be added to complete flowsheet connectivity depending on the model

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Using the Mouse Buttons • Left-click



Selects a block, stream, object ID, or annotation

• Right-click



Brings up menu for the selected stream, block, or flowsheet Cancels placement of streams or blocks on the flowsheet



• Double-left-click





Opens the Data Browser to the stream or block Input form, or Results form for intermediate streams Edits text

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Saving an Aspen Plus Simulation • To save a file: 1. Select Save As from the File menu 2. Choose a File name 3. Choose an appropriate Save As Type Fi le Ty pe

Ex ten si on

Fo rm at

Des cr ip ti on

Document

*.apw

Binary

File containing simulation input, results and intermediate convergence information

Backup

*.bkp

ASCII

Archive file containing simulation input and results

Compound

*.apwz

Binary

Compressed file which contains the model (the BKP or APW file) and external files referenced by the model. You can add additional files such as supporting documentation to the APWZ file.

See Maintaining Aspen Plus Simulations section for information on other file formats Chemical Process Modeling & Simulation

(15/39)

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Functionality of Forms • When you click the left mouse button to select a field on a form, the Description area gives you information about that field. Use this content to help with data entry • Click the drop-down arrow in a field to bring up a list of  possible input values for that field – Typing a letter will bring up the next selection on the list that begins with that letter

• The Tab key will take you to the next field on a form • In tables, Aspen Plus always adds a single row below the last entry

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Help • Help Topics – Select Help Topics from the Help menu to launch online help: • Contents : Browse through the documentation, including User Guides and Reference Manuals • Index : Search for help on a specific topic using the index entries • Search : Search for a help on a topic that includes any word or words

• “What’s This?” Help – Click the “What’s This?” toolbar button and then click any area to get help for that item

• F1 Help – With the cursor in the desired field, press the function key to bring up help for field and/or sheet

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Basic Input • The minimum required inputs to run a simulation are: – – – – –

Setup Components Properties Streams Blocks

• Enter data on the input forms in the above order by clicking the Next Button • Or, these input folders can be located quickly using the Data menu or the Data Browser toolbar buttons Chemical Process Modeling & Simulation

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Status Indicators • Colors and shapes are used to describe the current status of input and results: Symbol

Status Input for the form is incomplete Input for the form is complete No input for the form has been entered. It is optional. Results for the form exist Results for the form exist, but there were calculation errors. Results for the form exist, but there were calculation warnings. Results for the form exist, but input has changed since the results were generated.

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Cumene Flowsheet Definition • Which Aspen Plus block would you use for each unit? RECYCLE REACTOR COOL

FEED REAC-OUT

RStoic Model

Chemical Process Modeling & Simulation

COOL-OUT

Heater Model

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SEP

Flash2 Model

PRODUCT

Seoul National University 

Setup • Most of the commonly used Setup information is entered on the Setup Specif icatio ns Global sheet – – – – –

Flowsheet title to be used on reports Run type Input and output units Valid phases (i.e., vapor-liquid or vapor-liquid-liquid) Ambient pressure

• Stream report options are located on the Setup Report Options Stream sheet

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Setup Specification Form

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Setup Run Type Flowsheet Assay Data Analysis

Standard Aspen Plus flowsh eet run. Flowsheet runs can contain property estimation,

assay data analysis, and/or property analysis calculations  A stan dal on e Assay Data Anal ys is and ps eud oc om po nen t generat io n r un.

Use Assay Data Analysis to analyze assay data when you do not want to perform a flowsheet simulation in the same run  A stan dal on e Data Regres si on ru n. Use Data Regression to fit physical property

Data Regression

model parameters required by Aspen Plus to measure pure component, VLE, LLE, and other mixture data. Data Regression can contain property estimation and property analysis calculations. Aspen P lus cannot perform data regression in a flowsheet run Properties Plus setup run. Use Properties P lus to prepare a property package for use

Properties Plus

with Aspen Custom Modeler or Aspen P inch, with third-party commercial engineering programs, or with your company's in-house programs. You must be licensed to use Properties Plus  A stan dal on e Proper ty An aly si s r un. Use P roperty Analysis to generate property

Property Analysis Property Estimation

tables, P T-envelopes, residue curve maps, and other property reports when you do not want to perform a flowsheet simulation in the same run. Property Analysis can contain property estimation and assay data analysis calculat ions Standalone Property Constant Estimatio n run. Use Property Estimation to estimate

property parameters when you do not want to perform a flowsheet simulation in the same run

Chemical Process Modeling & Simulation

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Setup Units • Units in Aspen Plus can be defined at three different levels: – Global Level (“Input Data” and “Output Results” fields on the Setup Specifications Global sheet) – Object level (“Units” field in the tip of any input form of an object such as a block or stream) – Field level

• Users can create their own units sets using the Setup Units-Sets Object Manager . Units can be copied from an existing set and then modified.

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Components • Use the Components Specifications form to specify all the components required for the simulation • If available, physical property parameters for each component are retrieved from databanks • Pure component databanks contain parameters such as molecular weight, critical properties, etc. The databank search order is specified on the Databanks sheet • The Find button can be used to search for components • The Electrolyte Wizard can be used to set up an electrolyte simulation Chemical Process Modeling & Simulation

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Components

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Properties • Use the Properties Specifications form to specify the physical property methods to be used in the simulation • Property methods are a collection of models and methods used to describe pure component and mixture behavior • Choosing the correct physical properties is critical for obtaining reliable simulation results    L 1.0    O    N    A    H0.8    T    E    M   n0.6   o    i    t   c   a   r 0.4    F   e    l   o    M0.2   r   o   p   a    V0.0 0.0

Raoult’s Law

   L 1.0    O    N    A    H0.8    T    E    M   n0.6   o    i    t   c   a   r    F 0.4   e    l   o    M0.2   r   o   p   a

RK-Soave

   V0.0 0.2

0.4

0.6

0.8

Liquid Mole Fraction METHANOL

1.0

0.0

   L 1.0    O    N    A    H0.8    T    E    M   n0.6   o    i    t   c   a   r    F 0.4   e    l   o    M0.2   r   o   p   a    V0.0 0.0

0.2

0.4

0.6

0.8

1.0

NRTL 0.2

0.4

0.6

0.8

1.0

Liquid Mole Fraction METHANOL

Liquid Mole Fraction METHANOL

• Selecting a Process Type will narrow the number of methods available Chemical Process Modeling & Simulation

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Properties

Chemical Process Modeling & Simulation

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Streams • Use Stream Input forms to specify feed stream conditions, including two of the following: – Temperature – Pressure – Vapor Fraction

• Plus, for stream composition either: – Total stream flow and component fractions – Individual component flows

• Specifications for streams that are not feeds to the flowsheet are used as estimates Chemical Process Modeling & Simulation

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Streams Input Form

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Blocks • Each Block Input or Block Setup form specifies operating conditions and equipment specifications for the unit operation model • Some unit operation models require additional specification forms • All unit operation models have optional information forms (e.g., Block Options form)

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Blocks Form

Chemical Process Modeling & Simulation

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Starting the Run • Select Control Panel from the View menu or press the Next button to be prompted – Execute the simulation when all required forms are complete. If  you are unsure, use the Next button to take you to any incomplete forms

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Control Panel • The Control Panel consists of a: – Message window showing the progress of the simulation by displaying the most recent messages from the calculations – Status area showing the hierarchy and order of simulation blocks and convergence loops executed – Toolbar that you can use to control the simulation

Run

Start or continue calculations

Step

Step through the flowsheet one block at a time

Stop

Pause simulation calculations

Reinitialize

Purge simulation results

Results

Check simulation results

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Reviewing Results • History file or Control Panel Messages – Contains any generated errors or warnings (Select History or Control Panel on the View menu to display the History file or the Control Panel)

• Stream Results – Contains stream conditions and compositions (For all streams, click Results Summary/Streams; for individual streams, click the stream name in the Streams folder, then select the Results form)

• Block Results – Contains calculated block operating conditions (In the Blocks folder, click the block, then select the Result form)

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Cumene Production Conditions

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Exercise) Benzene Workshop • Objective : Add the process and feed stream conditions to a flowsheet

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Exercise) Benzene Workshop • Results – What is the heat duty of the COOLER block? ___________  – What is the temperature in the FL2 block? ___________ 

Chemical Process Modeling & Simulation

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(Exercise) Benzene Workshop Results FEED Temperature, F Pressure, psi Vapor Frac Mole Flow, lbmol/hr Mass Flow, lb/hr Volume Flow, cuft/hr Enthalpy, MMBtu/hr Mole Flow, lbmol/hr HYDROGEN METHANE BENZENE TOLUENE COOL heat duty FL2 outlet temperature

COOL

VAP1

LIQ1

VAP2

LIQ2

1000 550 1 600 10221.99 17271.52 7.361

200 550 0.869 600 10221.99 6905.633 0.17

100 500 1 501.724 2628.668 6098.248 -2.776

100 500 0 98.276 7593.324 143.354 2.015

99.8 14.7 1 2.762 71.786 1123.627 -0.023

99.8 14.7 0 95.514 7521.538 140.415 2.037

405 95 95 5

405 95 95 5

404.239 93.477 3.935 0.073

0.761 1.523 91.065 4.927

0.754 1.398 0.6 0.01

0.007 0.125 90.464 4.917

-7.19 MMBtu/hr 99.83ºF

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Process Modeling Using Aspen Plus (RadFrac Models)  S   e   o  u  l   N  a   t   i   o  n  a   l   U  n  i   v  e   r  s   i   t   y 

Spring Semester, 2012

TA : Ikhyun Kim Instructor : En Sup Yoon

Basic Distillation

Feed

 The feed containing the components to be separated enters around the middle of the column.  The feed can be in any state from a cold liquid to a superheated vapor.

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Basic Distillation distillation stage

Liquid and vapor are in countercurrent contact throughout the column as the liquid flows down and the vapor flows up the column. At each distillation stage some of the vapor moving up the column is condensed and this in turn evaporates some of the liquid moving down the column. If there are two components in the feed, then a greater amount of the less volatile component will condense at each stage and a greater amount of the more volatile component will evaporate.

Feed

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Basic Distillation

distillation stage

Feed

 The rectifying section is the name given to the stages above the feed point, where the concentration of the more volatile component increases in both the liquid and the vapor.

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Basic Distillation

distillation stage  The stripping section is the name given to the stages below the feed point, where the concentration of the more volatile component decreases in both the liquid and the vapor.

Feed

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Basic Distillation condenser

overhead vapor 

cooling water

distillation stage

Feed

 The overhead vapor containing the most volatile components from the feed, moves from the top of the column to the condenser. In this heat exchanger, cooling water is used to condense the vapor to a liquid.

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Basic Distillation condenser

overhead vapor

cooling water reflux overhead product distillation stage  The liquid from the condenser is split into two parts: (a)  The reflux is fed back to the column where it moves down the column in countercurrent flow with the vapor flowing up to the column. (b) The overhead product contains liquid with a composition specified in the design of the column.

Feed

 The ratio of the reflux flowrate to the overhead product flowrate is called the reflux ratio and is an important parameter in the design and operation of any distillation column.

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Basic Distillation condenser

overhead vapor

cooling water reflux overhead product

distillation stage

 The bottom liquid , containing the least volatile components in the feed, flows from the base of the column to the reboiler . In this heat exchanger steam is used to vaporize some of  the liquid which flows back to the column in countercurrent flow with the liquid moving down the column.  The amount of heat fed to the reboiler determines the vapor flow up to the column.

Feed

reboiler 

steam

bottom liquid Chemical Process Modeling & Simulation

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Basic Distillation condenser

overhead vapor

cooling water reflux overhead product

distillation stage

 The bottom product, has a specified composition, fixed during the design of the column and is the second product stream from a distillation column.

Feed

 This is the end of the section naming the parts of a distillation column.

reboiler

steam

bottom liquid Chemical Process Modeling & Simulation

bottoms product (9/28)

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RadFrac: Rigorous Multistage Separation • Vapor-Liquid or Vapor-Liquid-Liquid phase simulation of: – – – – –

Ordinary distillation Absorption, reboiled absorption Stripping, reboiled stripping Azeotropic distillation Reactive distillation

• Configuration options – – – – –

Any number of feeds Any number of side draws Total liquid draw off and pumparounds Any number of heaters Any number of decanters

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RadFrac Flowsheet Connectivity Vapor Distillate

Top-Stage o r  Condenser Heat Duty

1

Heat (optional) Liquid Distillate Water Distill ate (optional)

Feeds Reflux

Products (optional)

Heat (optio nal) Pumparound

Decanters

Heat (optio nal) Heat (optio nal)

Bottom Stage or  Reboiler Heat Duty

Boil-up

Product Return

Nstage

Heat (optio nal)

Bottoms

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Column Superstructure

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RadFrac Flowsheet Connectivity Component EB STYRENE TAR

Mass Fraction 0.5843 0.4150 0.0007

(model tar as n-heptadecane)

ETHBZ-PD

COLUMN

FEED Flowrate Temperature Pressure

27550 lb/hr   110 F 760 torr  

Number of Stages

53+ Condenser + reboiler  Feed Tray 25 Reflux Ratio 6 Distillate Rate 16700 lb/hr   Condenser Pressure 45 torr  To p Tray Pr es su re 50 to rr   Bottom Pressure 105 torr   Subcooled reflux 45 F

Use NRTL for p roperi tes STYR-PD

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Example: EB-Styrene Column: Flowsheet

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Example: EB-Styrene Column: Setup

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Example: EB-Styrene Column: Component

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Example: EB-Styrene Column: Property(1)

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Example: EB-Styrene Column: Property(2)

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Example: EB-Styrene Column: Stream

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Example: EB-Styrene Column: Block(1)

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Example: EB-Styrene Column: Block(2)

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Example: EB-Styrene Column: Block(3)

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Example: EB-Styrene Column: Block(4)

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Example: EB-Styrene Column • Use Plot Wizard to examine column profiles

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Equilibrium Stage Approach • Model column as a stack of equilibrium stages (theoretical plates) Liquid

Vapor Product

Feed

Heat inp ut

Liquid Product

Vapor 

• Key Assumptions – Perfect mixing – Thermodynamic Equilibrium – Violation of assumptions handled by “tray efficiency” Chemical Process Modeling & Simulation

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Feed Convention • You must specify a convention when defining a feed  Ab ov e-st age (default)

On-stage

Decanter*

n-1

n-1

n-1

Vapor  Feed n Liquid n

Feed

Feed Decanter 

n n-1

* Decanter convention i s valid only f or vapor-liquid-liqui d separation. Chemical Process Modeling & Simulation

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Choosing Feed Convention • Above-Stage feed convention is the default. • Using the Above-Stage convention, a vapor feed can be introduced to the bottom stage by specifying Stage =N+1 • Use the On-Stage convention when you know the feed is one phase – Saves flash calculations – Avoids flash problems with supercritical systems

• Use the Decanter feed convention to introduce a feed directly into the decanter in a vapor/liquid/liquid application.

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Basic Column Specification • Column Configuration: – Number of stages (specified on RadFrac Setup Congifuration sheet) – Condenser and Reboiler types (specified on RadFrac Setup Configuration sheet) – Locations of Feed and Product Streams (specified on RadFrac Setup Streams sheet)

• Two of the following operating specifications (specified on RadFrac Setup Configuration sheet): – – – –

Distillate or Bottoms rate Distillate to feed ratio or Bottoms to feed ratio Reflux or Boilup rate or Boilup ratio Condenser or Reboiler duty

• Column pressure profile (specified on RadFrac Setup Pressure sheet)

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Process Modeling Using Aspen Plus (Reactor Models)  S   e   o  u  l   N  a   t   i   o  n  a   l   U  n  i   v  e   r  s   i   t   y 

Spring Semester, 2012

TA : Ikhyun Kim Instructor : En Sup Yoon

Reactor Models

Reactor 

[ Balance Based ]

[ Equilibrium Based ]

[ Kinetics Based ]

Yield Shift Reactor 

Equilibrium Reactor 

PFR

Conversion Reactor 

Gibbs Reactor 

CSTR

Chemical Process Modeling & Simulation

(2/27)

Seoul National University 

Balanced Based Reactors • Yield Shift Reactor   – Requires a mass balance only, not an atom balance  – No reaction stoichiometry required  – Is used to simulate reactors in which inlets to the reactor are not completely known but outlets are known

• Conversion Reactor   – Performs mass balance calculations based on reaction stoichiometry(or conversion) and flashes the outlet stream  – Used when reactions kinetics are unknown or unimportant

Chemical Process Modeling & Simulation

(3/27)

Seoul National University 

Equilibrium Based Reactors • Equilibrium Reactor   – Computes combined chemical and phase equilibrium by solving reaction equilibrium equations  – Useful when there are many components, a few known reactions, and when relatively few components take part in the reactions

• Gibbs Reactor   – A Gibbs free energy minimization is done to determine the product composition at which the Gibbs free energy of the products is at a minimum  – Do not require reactions stoichiometry

Chemical Process Modeling & Simulation

(4/27)

Seoul National University 

Kinetics Based Reactors • CSTR  – Use when reaction kinetics are are known and and when the reactor  reactor  contents have same properties as outlet stream  – Can model model equilibrium equilibrium reactions reactions simultaneousl simultaneously y with rate-based rate-based reactions

• PFR  – Handles only only rate-based rate-based reactions reactions  – A cooling stream stream is allowed  – You must provide provide reactor reactor length length and diameter  diameter 

Chemical Process Modelin Modeling g & Simulatio Simulation n

(5/27)

Seoul National University 

Using a Reaction ID (1) • Reac Reactio tion n IDs are are setu setup p as obj objec ects, ts, sep separ arat ate e from from the the reactor, reactor, and then referenced within the reactor(s) • A single single Reacti Reaction on ID ID can can be referen referenced ced in any any numb number  er  of kinetic kinetic reactors reactors (RCSTR, (RCSTR, RPlug and RBatch) RBatch) • Multip Multiple le reac reactio tion n sets sets can be refere reference nced d in in the the react reactor  or  models • Each Each Reac Reactio tion n ID ID can can have have mult multipl iple e and/o and/orr comp competi eting ng reactions Chemical Process Modelin Modeling g & Simulatio Simulation n

(6/27)

Seoul National University 

Using a Reaction ID (2) • To set set up a Reacti Reaction on ID, ID, go to to the the React Reaction ions, s, Reac Reactio tions ns Object Manager   – Click on New New to create a new Reaction Reaction ID  – Enter ID name name and select the reaction type from the drop-down box  – Enter appropriate appropriate reaction data in the the forms

Chemical Process Modelin Modeling g & Simulatio Simulation n

(7/27)

Seoul National University 

Power Law Reaction ID (1) • The general general Power Law kinetic kinetic reaction reaction rate rate is: Reaction Rate



Kinetic Factor 

 

[Componenti]

Exponenti

i

 – [Componenti] : concentration of component i  – Exponenti : kinetic exponent of component i

• Within Within a Reac Reactio tion n ID you need need to spec specify ify::  – Stoichiometry sheet: stoichiometric coefficient and kinetic e

xponent for each component i  – Kinetic sheet: kinetic factor data Chemical Process Modelin Modeling g & Simulatio Simulation n

(8/27)

Seoul National University 

Power Law Reaction ID (2) • For a rever reversib sible le kine kinetic tic reacti reaction, on, both both the the forwa forward rd and and reverse reactions have to be specified separately • Example:

k 1

      

2 A  3 B

C   2 D     k 2

Forward reaction

2 A  3 B

Reverse reaction

C   2 D

k 1

 C  

k 2

2 D

 2 A  3 B

 Assuming 2 nd order in A

 Assuming 1 nd order in C and D (overall 2nd order)

 – k1 : Kinetic factor for forward reaction  – k2 : Kinetic factor for reverse reaction

Chemical Process Modelin Modeling g & Simulatio Simulation n

(9/27)

Seoul National University 

Power Law Reaction ID (3) • Stoichiometry Stoichiometry coefficien coefficients ts quantitativ quantitatively ely relate relate the amount of reactants and products in a balanced chemical reaction  – By convention convention - negative negative for reactants reactants and positive positive for products

Forward reaction coefficients:

A:

B:

C:

D:

Reverse reaction coefficients:

A:

B:

C:

D:

• Kinetic Kinetic exponents exponents show how the the concentrat concentration ion of of each component affects the rate of reaction  – Typically obtained from experimental data

Forward reaction exponents:

A:

B:

C:

D:

Reverse reaction exponents:

A:

B:

C:

D:

Chemical Process Modelin Modeling g & Simulatio Simulation n

(10/27)

Seoul National University 

Power Law Reaction ID (4) Forward reaction

Coefficients Forward reaction: A: -2 B: -3 C: 1 D: 2 Reverse reaction: A: 2 B: 3 C: -1 D: -2

Reverse reaction

Exponents Forward reaction: A: 2 B: 0 C: 0 D: 0 Reverse reaction: A: 0 B: 0 C: 1 D: 1 Chemical Process Modeling & Simulation

(11/27)

Seoul National University 

Power Law Reaction ID (5) Kinetic Factor 



kT 

n

  E   exp     RT 

• If reference temperature, T0, is specified, Kinetic Factor  is expressed as: n

Kinetic Factor 



  E  1 1     T      k   T   exp  R   T   T    0     0     

 – k : Pre-exponential factor   – n : Temperature exponent  – E : Activation energy  – T0 : Reference temperature Chemical Process Modeling & Simulation

(12/27)

Seoul National University 

Power Law Reaction ID (6)

Chemical Process Modeling & Simulation

(13/27)

Seoul National University 

Heat of Reaction • Heat of reaction need not be provided for reactions • Heat of reaction are typically calculated as the difference between inlet and outlet enthalpies for the reactor  • If you have a heat of reaction value that does not match the value calculated by simulator, you can adjust the heats of  formation of one or more components to make the heat of  reaction match • Heat of reaction can also be calculated or specified at a reference temperature and pressure in an Conversion Reactor  Chemical Process Modeling & Simulation

(14/27)

Seoul National University 

Reactor Workshop (1) • Objective: Compare the use of diff erent reactor ty pes to model a reacti on 70% conversion of ethanol P-STOIC

F-STOIC RSTOIC FEED Feed: Temp = 70°C DUPL Pres = 1 atm Water: 8.892 kmol/hr  Ethanol: 186.59 kmol/hr   Acetic Acid: 192.6 kmol/hr 

F-GIBBS

P-GIBBS

RGIBBS F-PLUG

P-PLUG RPLUG

F-CSTR

Diameter = 0.3 m P-CSTR

Use the NRTL-HOC property method RCSTR Chemical Process Modeling & Simulation

Length = 2 m

(15/27)

Volume = 0.14 m3

Seoul National University 

Reactor Workshop (2) •

Reactor Conditions: Temperature = 70 ℃, Pressure = 1 atm

•

Stoichiometry: Ethanol + Acetic Acid ↔ Ethyl Acetate + Water 

•

Kinetic Parameters:  – Reactors are first order with respect to each of the reactants in the reaction (second order overall)  – Forward Reaction: k=1.9X108, E=5.95X107 J/kmol  – Reverse Reaction: k=5.0X107, E=5.95X107 J/kmol  – Reactions occur in the liquid phase  – Composition basis is Molarity

•

Hint: Check that each reactor is considering both Vapor and Liquid as Valid phases

Chemical Process Modeling & Simulation

(16/27)

Seoul National University 

Reactor Workshop (3) • Results RStoic

RGibbs

RPlug

RCSTR

 Amount of Ethyl Acetate produced (kmol/hr) Mass fraction Ethyl  Acetate in product stream Heat duty (kcal/hr)

Chemical Process Modeling & Simulation

(17/27)

Seoul National University 

Equilibrium Based Reactors • Equilibrium Reactors  – REquil  – RGibbs

• Do not take reaction kinetics into account • Solve similar problems, but problem specifications are different • Individual reactions can be at a restricted equilibrium using a temperature approach to equilibrium or molar  extent of reaction Chemical Process Modeling & Simulation

(18/27)

Seoul National University 

REquil : Equilibrium Reactor  • Computes combined chemical and phase equilibrium by solving reaction equilibrium equations • Useful when there are many components, a few known reactions, and when relatively few components take part in the reactions

Chemical Process Modeling & Simulation

(19/27)

Seoul National University 

REquil : Specifications • Specified on the REquil Input Specification sheet the Reactor Conditions :  – Specify two of  • Temperature • Pressure • Vapor Fraction • Duty

 – Valid phases • Vapor-Liquid • Vapor-Only • Liquid-Only • Solid-Only • NOT Vapor-Liquid-Liquid

Chemical Process Modeling & Simulation

(20/27)

Seoul National University 

REquili : Equilibrium • Calculates equilibrium constants from Gibbs energy • Can restrict equilibrium by specifying one of   – Molar extent of the reaction  – A temperature approach to chemical equilibrium

• Temperature approach is the number of degrees above the reactor temperature at which chemical equilibrium is determined, Tequil = TR + ∆T • By default REquil assumes that reactions will reach equilibrium. (Temperature approach = 0) Chemical Process Modeling & Simulation

(21/27)

Seoul National University 

RGibbs : Equilibrium Reactor  • Handles simultaneous phase and chemical equilibrium by minimizing the Gibbs free energy with phase splitting • Does not require reactions stoichiometry

Chemical Process Modeling & Simulation

(22/27)

Seoul National University 

RGibbs : Specifications (1) • Specified on the Setup Specifications sheet the :  – Reactor Conditions • Pressure and either Duty or Temperature

 – Calculations options for phase, chemical, and restricted chemical equilibrium  – Maximum number of fluid phases to consider in the equilibrium calculations

Chemical Process Modeling & Simulation

(23/27)

Seoul National University 

RGibbs : Specifications (2)

Chemical Process Modeling & Simulation

(24/27)

Seoul National University 

RGibbs : Phase Equilibrium Only • Tries to distribute all species among the specified solution phases by default • Use Setup Products Sheet to assign different sets of  species to each solution phase • You can assign different thermodynamic property methods to each phase

Chemical Process Modeling & Simulation

(25/27)

Seoul National University 

RGibbs : Phase & Chemical Equilibrium •

By default, all components entered on the Components Specification Selection sheet are possible fluid phase or solid products

•

You can limit the number of possible products by using the Setup Products sheet

•

Tries to distribute all species among the specified solution phases by default

•

Use Setup Products sheet to assign different sets of species to each solution phase

•

You can assign different thermodynamic property methods to each phase

Chemical Process Modeling & Simulation

(26/27)

Seoul National University 

Rgibbs : Setup Product sheet

Chemical Process Modeling & Simulation

(27/27)

Seoul National University 

 C  h  e   m  i   c  a   l   P   r  o  c  e   s   s   M  o  d  e   l   i   n  g  &  S   i   m  u  l   a   t   i   o  n

Process Modeling Using Aspen Plus (Logical Operation Tools)  S   e   o  u  l   N  a   t   i   o  n  a   l   U  n  i   v  e   r  s   i   t   y 

Spring Semester, 2012

TA : Ikhyun Kim Instructor : En Sup Yoon

Lesson Objectives • Use a sensitivity analysis to study relationships between process variables • Introduce the use of design specifications to meet process design requirements • Introduce usage of Microsoft Excel and Fortran Calculator blocks

Chemical Process Modeling & Simulation

(2/41)

Seoul National University 

Sensitivity Analysis • Allows user to study the effect of changes in input variables on process outputs • Changes made to a flowsheet input quantity in a sensitivity block do not affect the simulation; the sensitivity study is run independently of  the base case simulation • Located under Data Browser | Model Analysis Tools | Sensitivity • Results can be viewed by looking at the Results form in the folder for the Sensitivity block • Plot results to easily visualize relationships between different variables

Chemical Process Modeling & Simulation

(3/41)

Seoul National University 

Uses of Sensitivity Analysis • Studying the effect of changes in input variables on process (model) outputs • Graphically representing the effects of input variables • Verifying that a solution to a design specification is feasible • Rudimentary optimization • Studying time varying variables using a quasi-steady-state approach • Doing case studies

Chemical Process Modeling & Simulation

(4/41)

Seoul National University 

Sensitivity Analysis Example RECYCLE REACTOR COOL

FEED REAC-OUT

COOL-OUT

SEP

Filename: CUMENE-S.BKP PRODUCT

• Determine the effect of cooler outlet temperature on the purity of the product stream – What is the manipulated (varied) variable?

» COOL outlet temperature – What is the measured (sampled) variable? » Purity (mole fraction) of cumene in PRODUCT stream Chemical Process Modeling & Simulation

(5/41)

Seoul National University 

Steps for Using Sensitivity Analysis 1. Specify measured (sampled) variable(s) –

These are quantities calculated during the simulation to be used in step 4 (Define sheet)

2. Specify manipulated (varied) variable(s) –

These are the flowsheet variables to be varied (Vary sheet)

3. Specify range(s) for manipulated (varied) variable(s) –

Variation for manipulated variable can be specified either as equidistant points within an interval or as a list of values for the variable (Vary sheet)  Tip: You can check the Disable variable box to temporarily not vary that variable

4. Specify quantities to calculate and tabulate –

Tabulated quantities can be any valid Fortran expression containing variables defined in step 1 (Tabulate sheet)  Tip: Click the Fill Variables button to automatically tabulate all of the define variables

Chemical Process Modeling & Simulation

(6/41)

Seoul National University 

Case Studies • Use the Cases option to set up a case study with any number of manipulated variables • Use the Cases sheet to enter the input data for each case • This makes it much easier to run multiple sets of data through a single model

Chemical Process Modeling & Simulation

(7/41)

Seoul National University 

Specifying Cases

Chemical Process Modeling & Simulation

(8/41)

Seoul National University 

Plotting 1. Select the column containing the X-axis variable and then select X-Axis Variable from the Plot menu 2. Select the column containing the Y-axis variable and then select Y-Axis Variable from the Plot menu 3. (Optional) Select the column containing the parametric variable and then select Parametric Variable from the Plot menu 4. Select Display Plot from the Plot menu Note: To select a column, click the heading of the column with the left mouse button Chemical Process Modeling & Simulation

(9/41)

Seoul National University 

Sensitivity Analysis Results • What is happening below 70 F and above 300 F? °

Chemical Process Modeling & Simulation

°

(10/41)

Seoul National University 

Notes 1. Only quantities that have been input to the flowsheet should be varied or manipulated 2. Multiple inputs can be varied 3. The simulation is run for every combination of  manipulated (varied) variables unless using Cases 4. Check the Cases box to specify the variable values for a list of individual cases

Chemical Process Modeling & Simulation

(11/41)

Seoul National University 

Sensitivity Analysis Workshop • Objective: Use a sensitivity analysi s to study the effect of the recycle flowrate on the reactor d uty

• Part A: Starting with the cyclohexane flowsheet (CYCLOHEX.BKP), plot the variation of REACT duty as the recycle split fraction in LFLOW is varied from 0.1 to 0.4 • Part B: In addition to the split fraction (Part A), vary the conversion of benzene in the reactor from 0.9 to 1.0; tabulate the reactor duty and construct a parametric plot showing the dependence of the reactor duty on recycle split fraction and the conversion of benzene Note: Both of these studies (Parts A and B) should be set up within the same sensitivity analysis block Chemical Process Modeling & Simulation

(12/41)

Seoul National University 

Cyclohexane Production Flowsheet PURGE  Total flow =330 kmol/hr  T =50 C P =25 bar Molefrac H2 =0.975 N2 =0.005 CH4 =0.02

92% flow to stream H2RCY

°

H2IN

VFLOW

H2RCY

VAP FEED-MIX

REACT HP-SEP

RXIN  T =150 C P =23 bar °

BZIN  T =40 C P =1 bar Benzene flow =100 kmol/hr

 T =50 C Pdrop =0.5 bar

RXOUT

LTENDS

°

 T =200 C Pdrop =1 bar Benzene conv =0.998 °

LIQ

°

CHRCY

COLFD LFLOW

 Theoretical Stages =12 Reflux ratio =1.2 Bottoms rate =99 kmol/hr Partial Condenser with vapor distillate only Column Pressure =15 bar Feed stage =8

30% flow to stream CHRCY

PRODUCT COLUMN Specify cyclohexane mole recovery in PRODUCT stream equal to 0.9999 by varying Bottoms rate from 97 to 101 kmol/hr

Use the RK-SOAVE property method

Chemical Process Modeling & Simulation

(13/41)

Seoul National University 

Design Specifications (1) • Similar to a feedback controller • Allows user to set the value of a calculated flowsheet quantity to a particular value • Objective is achieved by manipulating a specified input variable • Located under Data Browser | Flowsheeting Options | Design Specs • Design specifications change the base case, and so results are shown in the core simulation results Chemical Process Modeling & Simulation

(14/41)

Seoul National University 

Design Specifications (2) • Can be graphically represented on the flowsheet by selecting Display Design-Spec, Transfer and Calculator  connections under Tools |

Options | Flowsheet tab.

Chemical Process Modeling & Simulation

(15/41)

Seoul National University 

Design Specification Example RECYCLE REACTOR COOL

FEED REAC-OUT

COOL-OUT

SEP

Filename: CUMENE-D.BKP PRODUCT

• Determine the cooler outlet temperature to achieve a cumene product purity of 98 mole percent: –

What is the manipulated (varied) variable?

» COOL outlet temperature –

What is the measured (sampled) variable?

» Mole fraction of cumene in PRODUCT stream –

What is the specification (target) to be achieved?

» Mole fraction of cumene in PRODUCT stream =0.98 Chemical Process Modeling & Simulation

(16/41)

Seoul National University 

Steps for Using Design Specifications (1) 1. Identify measured (sampled) variables –

These are flowsheet quantities, usually calculated, to be included in the objective function (Define sheet)

2. Specify objective function (Spec) and goal (Target) –

This is the equation that the specification attempts to satisfy (Spec sheet)

3. Set tolerance for objective function –

The specification is converged when the objective function equation is satisfied to within this tolerance (Spec sheet)

4. Specify manipulated (varied) variable –

This is the variable whose value changes in order to satisfy the objective function equation (Vary sheet)

Chemical Process Modeling & Simulation

(17/41)

Seoul National University 

Steps for Using Design Specifications (2) 5. Specify range of manipulated (varied) variable – These are the lower and upper bounds of the interval within which Aspen Plus will vary the manipulated variable (Vary sheet)

• By default, the units of the variable(s) used in the objective function (step 2) and those for the manipulated variable (step 5) are the units for that variable type as specified by the Units Set declared for the design specification; you can change the units using the Object-level Units dropdown list in the Data Browser toolbar; however, if you do, it changes the units for all sheets in this form; for example, if you change the units to MetCBar in the Specs sheet, the units in the Vary form are also MetCBar

Chemical Process Modeling & Simulation

(18/41)

Seoul National University 

Notes (1) 1. Only quantities that have been input to the flowsheet should be manipulated 2. The calculations performed by a design specification are iterative; providing a good estimate for the manipulated variable will help the design specification converge in fewer iterations; this is especially important for large flowsheets with several interrelated design specifications 3. The results of a design specification can be found under Results Summary | Convergence or Convergence | Convergence and by choosing the Results form in the appropriate solver block; alternatively, the final values of the manipulated and/or sampled variables can be viewed directly on the appropriate Stream or Block Results forms

Chemical Process Modeling & Simulation

(19/41)

Seoul National University 

Notes (2) 4. If a design-spec does not converge: i. ii. iii. iv. v. vi. vii.

Check to see that the manipulated variable is not at its lower or upper bound Verify that a solution exists within the bounds specified for the manipulated variable, perhaps by performing a sensitivity analysis Ensure that the manipulated variable does indeed affect the value of  the sampled variables Provide a better estimate for the value of the manipulated variable Narrow the bounds of the manipulated variable or loosening the tolerance on the objective function to help convergence Make sure that the objective function does not have a flat region within the range of the manipulated variable Change the characteristics of the convergence block associated with the design-spec (step size, number iterations, algorithm, etc.)

Chemical Process Modeling & Simulation

(20/41)

Seoul National University 

Design Specification Workshop • Objective: Use a design specification in the cycl ohexane flowsh eet to fix the heat lo ad on the reactor  by varying t he recycl e flow rate – The cyclohexane production flowsheet (CYCLOHEX.BKP) is a model of an existing plant; the cooling system around the reactor can handle a maximum operating load of 4.7 Gcal/hr; determine the amount of cyclohexane recycle necessary to keep the cooling load on the reactor to this amount: ________  kmol/hr Note: The heat convention used in Aspen Plus is that heat input to a block is positive, and heat removed from a block is negative

Chemical Process Modeling & Simulation

(21/41)

Seoul National University 

Cyclohexane Production Flowsheet PURGE  Total flow =330 kmol/hr

92% flow to stream H2RCY

 T =50 C P =25 bar Molefrac H2 =0.975 N2 =0.005 CH4 =0.02 °

H2IN

VFLOW

H2RCY

VAP FEED-MIX

REACT HP-SEP

RXIN  T =150 C P =23 bar °

BZIN  T =40 C P =1 bar Benzene flow =100 kmol/hr

 T =50 C Pdrop =0.5 bar

RXOUT  T =200 C Pdrop =1 bar Benzene conv =0.998

LTENDS

°

°

LIQ

°

CHRCY

COLFD LFLOW

 Theoretical Stages =12 Reflux ratio =1.2 Bottoms rate =99 kmol/hr Partial Condenser with vapor distillate only Column Pressure =15 bar Feed stage =8

30% flow to stream CHRCY

PRODUCT COLUMN Specify cyclohexane mole recovery in PRODUCT stream equal to 0.9999 by varying Bottoms rate from 97 to 101 kmol/hr

Use the RK-SOAVE property method Chemical Process Modeling & Simulation

(22/41)

Seoul National University 

Calculator Blocks (1) • Allows the user to write equations in a Microsoft Excel spreadsheet or in Fortran syntax to be executed by Aspen Plus • Located under Data Browser | Flowsheeting Options | Calculator • Results can be viewed by looking at the Results form in the folder for the Calculator block • Also, since Calculator blocks change the base case, the core simulation results reflect the influence of the Calculator block

Chemical Process Modeling & Simulation

(23/41)

Seoul National University 

Calculator Blocks (2) • Can be graphically represented on the flowsheet by selecting Display Design-Spec, Transfer and Calculator  connections under Tools |

Options | Flowsheet tab.

Chemical Process Modeling & Simulation

(24/41)

Seoul National University 

Uses of Calculator Blocks • Feed-forward control (setting flowsheet inputs based on upstream calculated values) • Express a function in terms of flowsheet variables to calculate profit, for example • Call external subroutines • Transfer variables between flowsheet objects and/or external files • Write to an external file, Control Panel, etc. • Create custom input/output summary forms Chemical Process Modeling & Simulation

(25/41)

Seoul National University 

Calculator Block Example (1) • Use a Calculator to set the pressure drop across the COOL block: RECYCLE REACTOR COOL

FEED REAC-OUT

V

COOL-OUT

DELTA-P

SEP

PRODUCT

Calculator Block DELTA-P =-10-9 * V2

CUMENE-FORTRAN.BKP CUMENE-EXCEL.BKP

• Pressure drop across heater is proportional to square of  volumetric flow into heater Chemical Process Modeling & Simulation

(26/41)

Seoul National University 

Calculator Block Example (2) • Which flowsheet variables must be accessed? » Volumetric flow of stream REAC-OUT This can be accessed in two different ways:

1. Mass flow and mass density of stream REAC-OUT 2. A prop-set containing volumetric flow of a mixture

» Pressure drop across block COOL

• When should the Calculator block be executed? » Before block COOL

• Which variables are imported and which are exported? » Volumetric flow is imported » Pressure drop is exported Chemical Process Modeling & Simulation

(27/41)

Seoul National University 

Steps for Using Calculator Blocks 1. Access flowsheet variables to be used within Calculator – All flowsheet quantities that must be either read from or written to, must be identified (Define sheet)

2. Write Fortran or Excel – Fortran includes both executable Fortran (Calculate sheet) and nonexecutable (COMMON, EQUIVALENCE, etc.) Fortran (click the Fortran Declarations button) and to achieve desired result – Microsoft Excel spreadsheet presents all the capabilities of the running version of Excel (click Open Excel Spreadsheet button)

3. Specify location of Calculator block in execution sequence – Specify directly (Sequence sheet), or – Specify with import and export variables Chemical Process Modeling & Simulation

(28/41)

Seoul National University 

Fortran • Simple Fortran can be translated by Aspen Plus and does not need to be compiled • A Fortran compiler must be present on the machine where the Aspen Plus engine is running to compile more complex Fortran code • Standard Fortran syntax should be used F F Column 1

VFLOW=FLOW/DENS DP=-1E-9*VFLOW**2 Column 7

Note: “F” in Column 1 not required when entering code on Calculate sheet

Chemical Process Modeling & Simulation

(29/41)

Seoul National University 

Fortran Interpreter  • Aspen Plus will interpret inline Fortran if it is possible • The following Fortran can be interpreted: – – – – – – – – – – –

Arithmetic expressions and assignment statements IF statements GOTO statements, except assigned GOTO WRITE statements that do not have unformatted text in them FORMAT statements CONTINUE statements DO loops Calls to some built-in Fortran functions REAL or INTEGER statements* * Enter on the DOUBLE PRECISION statements* Declaration sheet DIMENSION statements*

Chemical Process Modeling & Simulation

(30/41)

Seoul National University 

Built-In Fortran Functions • Calls to some built-in Fortran functions: DABS DACOS DASIN DATAN DATAN2 DCOS DCOSH DCOTAN

DERF DEXP DFLOAT DGAMMA DLGAMA DLOG DLOG10 DMAX1IABS

DMIN1 DMOD DSIN DSINH DSQRT DTAN DTANH

IDINT MAX0 MIN0 MOD

• You also can use the equivalent single precision or generic function names; but, Aspen Plus always performs double-precision calculations Chemical Process Modeling & Simulation

(31/41)

Seoul National University 

Statements Requiring compilation • The following statements require compilation: CALL CHARACTER COMMON COMPLEX DATA ENTRY EQUIVALENCE IMPLICIT

LOGICAL PARAMETER PRINT RETURN READ TOP SUBROUTINE

Chemical Process Modeling & Simulation

(32/41)

Seoul National University 

Fortran Notes 1. The rules for writing In-Line Fortran are as follows: a. b. c. d.

The Fortran code must begin in column seven or beyond Comment lines must have the letter “C” or a “ ; ” in the first column Column two must be blank No entry beyond column 72

2. Variable names should not begin with lZ or ZZ 3. When using the Fortran WRITE statement, you can use the predefined unit number NTERM to write to the Control Panel; for example: 10

wr i t e( NTERM, 10) f l ow f or mat ( ‘ Feed f l owr at e =‘ , G12. 5)

Chemical Process Modeling & Simulation

(33/41)

Seoul National University 

Using Microsoft Excel (1) • The Microsoft Excel workbook is embedded into the simulation for each Calculator block – When saving as a backup (.bkp file), a .apmbd file is created.  This file needs to be in the working directory

• Full functionality of Excel is available including VBA and Macros • Cells that contain: – Import variables have a green border – Export variables have a blue border – Tear variables have an orange border – Incomplete variables have a red border Chemical Process Modeling & Simulation

(34/41)

Seoul National University 

Using Microsoft Excel (2)

Item Connect Cell Combo box Define button Unlink button Delete button Refresh button Changed button

Use to… Attach a Define variable to the current cell of the Microsoft Excel spreadsheet Create a new Define variable or edit an existing one Remove/break the link between a cell and a Define variable, without deleting the Define variable Remove link between a cell and a Define variable and delete the associated Define variable Refresh the list of Define variables in the Connect Cell Combo box Cause the Calculator to be re-executed the next time you run the simulation

Chemical Process Modeling & Simulation

(35/41)

Seoul National University 

Using Microsoft Excel (3) • Using the Aspen Plus toolbar in Microsoft Excel, set up the Worksheet as shown below: Import Variables

=FLOW/DENS

Export Variable

= (-1e-9)*C4^2

Chemical Process Modeling & Simulation

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Seoul National University 

Notes 1. Only quantities that have been input to the flowsheet should be overwritten 2. On the Calculator Input Sequence sheet, the preferred way to specify where the Calculator block should be executed is to list the imported and exported variables 3. In addition to the Calculator Results form, you can also increase the Calculator defined variables Diagnostics message level in Control Panel or History file through the Diagnostics button on the Sequence sheet; this will print the value of all input and result variables in the Control Panel

Chemical Process Modeling & Simulation

(37/41)

Seoul National University 

Calculator blocks in the Process Flowsheet Window •

Calculator blocks (and Design Spec/Transfer blocks) can now be placed on the PFD using icons on the Manipulators tab of the Model Library 1. Dashed connection lines will indicate the unit operation models affected by these blocks 2. Their display can be toggled on/off from the Tools | Options | Flowsheet tab

Chemical Process Modeling & Simulation

(38/41)

Seoul National University 

Increasing Diagnostics Increase Calculator defined variables Diagnostics message level in Control Panel or History file to 8.

Cal cul at or Bl ock F- 1 VALUES OF ACCESSED VARI ABLES VARI ABLE VALUE ======== ===== DP - 2. 032782930000 FLOW 5428. 501858128 DENS 0. 1204020367004

In the Control Panel or History File

RETURNED VALUES OF VARI ABLES VARI ABLE VALUE ======== ===== DP - 2. 032790410000

Chemical Process Modeling & Simulation

(39/41)

Seoul National University 

Calculator Workshop (1) • Objective: Use a Calcul ator bloc k t o maintain the methane:water ratio in t he feed to a reactor  CH4 + H2O

Methane  T =150 F P = 900 psia



Water

3 H2 + CO

Hydrogen

Carbon Monoxide

°

CH4

REFORMER

MIX RXIN

 T =70 F P = 15 psia °

H2O

RXOUT

 T =1100 F P = 850 psia °

 T =1450 F PDrop = 20 psi CH4 conversion = 0.995 °

Use the Peng-Robinson Property Method

Chemical Process Modeling & Simulation

(40/41)

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Calculator Workshop (2) • In a methane reformer, hydrogen gas is produced by reacting methane with water, generating carbon monoxide as a by-product • The feed to the reformer consists of pure methane and water streams; these are mixed and heated prior to being fed to the reformer; the conversion of methane is 99.5%, and the molar ratio of methane to water in the feed is 1:4 • Set up a Sensitivity block and plot a graph showing the variation of reactor duty as the methane flowrate in the feed is varied from 100 to 500 lbmol/hr Note: Use a Calculator block so that the methane:water ratio in the feed is maintained constant for each Sensitivity case Chemical Process Modeling & Simulation

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Seoul National University 

 C  h  e   m  i   c  a   l   P   r  o  c  e   s   s   M  o  d  e   l   i   n  g  &  S   i   m  u  l   a   t   i   o  n

Process Modeling Using Aspen Plus (Flowsheet Convergence)  S   e   o  u  l   N  a   t   i   o  n  a   l   U  n  i   v  e   r  s   i   t   y 

Spring Semester, 2012

TA : Ikhyun Kim Instructor : En Sup Yoon

Flowsheet Convergence • To evaluate flowsheet convergence, determine the: – – – – –

Calculation sequence Tear stream Number Iterations to solution Pattern of err/tol value Convergence method used

• Everything you need to know in evaluating the convergence status is written to the Control Panel

Chemical Process Modeling & Simulation

(2/13)

Seoul National University 

Convergence Blocks • Every design specification and tear stream has an associated convergence block • Convergence blocks determine how guesses for a tear stream or design specification manipulated variable are updated from iteration to iteration • Aspen Plus-defined convergence block names begin with the character “$” • User convergence blocks can be specified under Data | Convergence | Convergence... – User defined convergence block names must not begin with the character “$”

Chemical Process Modeling & Simulation

(3/13)

Seoul National University 

Flowsheet Sequence • To determine the flowsheet sequence calculated by Aspen Plus, look under the “Flowsheet Analysis” section in the Control Panel or on the left pane of the Control Panel window under “Calculation Sequence” • User-determined sequences can be specified on the Convergence Sequence form – User-specified sequences can be either full or partial

Chemical Process Modeling & Simulation

(4/13)

Seoul National University 

 What a Tear Stream? • A tear stream is one for which Aspen Plus makes an initial guess, and iteratively updates the guess until two consecutive guesses are within a specified tolerance • Tear streams are related to, but not the same as, recycle streams • To determine the tear streams chosen by Aspen Plus, look under the “Flowsheet Analysis” section in the Control Panel • User-determined tear streams can be specified on the Convergence Tear form

Chemical Process Modeling & Simulation

(5/13)

Seoul National University 

Tear Streams (1)

• Which are the recycle streams? 6 & 7 • Which are the possible tear streams? 6 & 7; 2 & 6; 4 & 7; 3 • Which is the best choice for the tear stream?  The best tear stream choice is stream 3; if this stream is used, you only need to converge on one tear stream instead of two

Chemical Process Modeling & Simulation

(6/13)

Seoul National University 

Tear Streams (2) • Providing estimates for tear streams can facilitate or speed up flowsheet convergence (highly recommended, otherwise the default is zero) • If you enter information for a stream that is in a “loop,” Aspen Plus will automatically try to choose that stream to be a tear stream – If you enter initial estimates for an internal process stream, Aspen Plus will preferentially choose that stream (if it can) over other possible tear streams with no initial estimates

Chemical Process Modeling & Simulation

(7/13)

Seoul National University 

Reconciling Streams • Simulation results for a stream can be copied onto its input form • Select a stream on the flowsheet, click the right mouse button and select “Reconcile” from the list to copy stream results to the input form – Two state variables must be selected for the stream flash calculation – Component flows, or component fractions and total flow can be copied – Mole, mass, or standard liquid volume basis can be selected

Chemical Process Modeling & Simulation

(8/13)

Seoul National University 

Convergence Block Algorithms • Aspen Plus uses different convergence block algorithms for different functions: –  To converge tear streams: • • • •

WEGSTEIN DIRECT BROYDEN NEWTON

–  To converge design specs

and tear streams: • •

BROYDEN NEWTON

– To converge design specs: • • •

SECANT BROYDEN NEWTON

– For optimization: • •

SQP COMPLEX

• Make changes to global convergence options on the Convergence | Conv Options | Defaults form Chemical Process Modeling & Simulation

(9/13)

Seoul National University 

Flowsheet Convergence References • Online Help – Troubleshooting Flowsheet Convergence – Glossary

Chemical Process Modeling & Simulation

(10/13)

Seoul National University 

Convergence Workshop (1) • Objective: Converge this flowsheet; FEED 165 F, 15 psia 100 lbmol/hr XH2O =0.4 XMeOH =0.3 XEtOH =0.3

GLYCOL

°

70 F, 35 psia 50 lbmol/hr Glycol °

PREHEATR BOT-COOL

VAPOR

Area =65 ft2

PREFLASH FEED-HT

DP =0 Q =0

DIST

COLUMN  Theoretical Stages =10 Reflux Ratio =5 Distillate to Feed Ratio =0.2 Column Pressure =1 atm Feed Stage = 5

LIQ BOT

Save as CONV-R.BKP

Use NRTL-RK Property Method

Note: There are several ways to converge this simulation; use the questions on the following pages to aid in your methodology Chemical Process Modeling & Simulation

(11/13)

Seoul National University 

Convergence Workshop (2) • Questions to ask yourself: – What messages are displayed in the control panel? – Why do some of the blocks show zero flow? – What is the Aspen Plus-generated execution sequence for the flowsheet? – Which stream does Aspen Plus choose as a tear stream? – What are other possible tear streams?

• Recommendation: – Give initial estimates for a tear stream and re-run flowsheet • Of the three possible tear streams you could choose, which do you know the most about? (Note: If you enter information for a stream that is in a “loop,” Aspen Plus will automatically choose it as the tear stream and set up a convergence block for it) Chemical Process Modeling & Simulation

(12/13)

Seoul National University