Process Simulation and Control Using Aspen
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PROCESS SIMULATION AND CONTROL USING
METHANOl
BUTENES
RDCOLUMN
CCS
AMIYA K. JANA
This Thispage pageintentionally intentionallyleft leftblank blank
Rs. 295.00
PROCESS SIMULATION AND CONTROL USING ASPEN
Amiya K. Jana
@ 2009 by PHI Learning Pnvate Limited, New Delhi. All rights reserved. No part of this book may be reproduced In any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-978-81-203-3659-9
The export rights of this book are vested solely with the publisher.
Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Jay Print Pack Private Limited, New Delhi-110015.
Rs. 295.00
PROCESS SIMULATION AND CONTROL USING ASPEN
Amiya K. Jana
@ 2009 by PHI Learning Pnvate Limited, New Delhi. All rights reserved. No part of this book may be reproduced In any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-978-81-203-3659-9
The export rights of this book are vested solely with the publisher.
Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Jay Print Pack Private Limited, New Delhi-110015.
This Thispage pageintentionally intentionallyleft leftblank blank
Rs. 295.00
PROCESS SIMULATION AND CONTROL USING ASPEN
Amiya K. Jana
@ 2009 by PHI Learning Pnvate Limited, New Delhi. All rights reserved. No part of this book may be reproduced In any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-978-81-203-3659-9
The export rights of this book are vested solely with the publisher.
Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Jay Print Pack Private Limited, New Delhi-110015.
This Thispage pageintentionally intentionallyleft leftblank blank
Rs. 295.00
PROCESS SIMULATION AND CONTROL USING ASPEN
Amiya K. Jana
@ 2009 by PHI Learning Pnvate Limited, New Delhi. All rights reserved. No part of this book may be reproduced In any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-978-81-203-3659-9
The export rights of this book are vested solely with the publisher.
Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Jay Print Pack Private Limited, New Delhi-110015.
Contents
Preface Acknowledgements
Part I
vii ix
Steady State Simulation and Optimization
using Aspen Plus 1
.
Introduction and Stepwise Aspen Plus
Simulation:
Flash Drum Examples 1 1 .
3-53
Aspen: An Introduction
3
2 Getting Started with Aspen Plus Simulation 1 3 Stepwise Aspen Plus Simulation of Flash Drums 1
4 7
.
.
13 1
Built-in Flash Drum Models
13 2
Simulation nf a Flash nmm
.
.
7 ,
1 33 .
.
1 3
.
,
Computation of Bubble Point Temperature
.
Summary and Conclusions
50
,
,
,
,
Reference 2
,
Aspen Plus 2 1 .
50
53
Simulation of Reactor Models
Built-in Rpartor Models
54-106 54
2 Aspen Plus Simulation of a RStoic Model 2 3 Aspen Plus Simulation of a RCSTR Model 2 4 Aspen Plus Simulation of a RPlug Model 2
8
35 42
.
Prnhlpms
_
28
4 Computation of Dew Point Temperature 1 3 5 T-xy and P-xy Diagrams of a Binary Mixture .
,
.
.
.
25
Aspen Plus Simulation of a RPlug Model using LHHW Kinetics Summary and Conclusions .
55 65 78 93 104
Prohlpms
704
Reference
106 v
Copyrighted maierlal
VI
3
.
CONTENTS
Aspen Plus
Sinmlation of Distillation Models
107-185
3 1
Rnilt-in nistillntinn Mndols
107
32
Aspen Plus Simulation of the Binary Distillation Columns
108
.
3
.
3 2 1
Simulation of a DSTWTT Mnripl
IQfl
3 9. 9
Simulation of a RaHFrnr MoHpI
122
3 Aspen Plus Simulation of the Multicomponcnt Distillation Columns Simnlnt.ion of a RaHFrar MoHpI
13fi
332
Simulation of a PetroFrac Model
148
.
.
3
.
3
.
.
4 Simulation and Analysis of an Absorption Column
164
5 Optimization using Aspen Plus
178
Part II .
Chemical Plant Simulation using Aspen Plus
Aspen Plus 4 1
4
.
4
.
181 l2
f
Summary and Conclusions Problems
4
136
3 3 1
Simulation of Chemical Plants
189-226
TntrnHnrtion
2 Aspen Plus Simulation of a Distillation Train
189
3 Aspen Plus Simulation of a Vinyl Chloride Monomer (VCM) Production Unit
203
Summary and Conclusions
220
Prnhlpms
;
,
220
-
References
Part III 5
.
226
Dynamics and Control using Aspen Dynamics
Dynamics and Control of Flow-driven Processes 5J 52 .
229-284
Tnt.roHiirt.ion Dynamics and Control of a Continuous Stirred
229
Tank Reactor (CSTR) 5
.
230
3 Dynamics and Control of a Binary Distillation Column
255
Summary and Conclusions
279
Prnhlpms ,
,
References .
Dynamics and Control of Pressure-driven Processes il
Tnt.rndnrtinn
6 2
Dynamics and Control of a Reactive Distillation (RD) Column
f
6
.
,..
279
284
285-313 285
286
Summary and Conclusions
310
Problems References
31J 313
Index
315-317
Copyrlghled maierlal
r
Preface
"
The future success of the chemical process industries mostly depends on the ability to design and operate complex, highly interconnected plants that are profitable and that meet quality, safety, environmental and other standards To achieve this goal, the software "
.
tools for process simulation and optimization are increasingly being used in industry.
By developing a computer program, it may be manageable to solve a model structure of a chemical process with a small number of equations. But as the complexity of a plant integrated with several process units increases, the solution becomes a challenge. Under this circumstance, in recent years, we motivate to use the process flowsheet simulator to
solve the problems faster and more reliably. In this book, the Aspen
software package
has been used for steady state simulation, process optimization, dynamics and closedloop control. To improve the design, operability, safety, and productivity of a chemical process
with minimizing capital and operating costs, the engineers concerned must have a solid knowledge of the process behaviour. The process dynamics can be predicted by solving the mathematical model equations. Within a short time period, this can be achieved
f
f
quite accurately and eficiently by using Aspen lowsheet simulator. This software tool is not only useful for plant simulation but can also automatically generate several control structures, suitable for the used process flow diagram. In addition, the control parameters, including the constraints imposed on the controlled as well as manipulated variables. are also provided by Aspen to start the simulation run. However, we have the option to modify or even replace them.
This well organized book is divided into three parts. Part I (Steady State Simulation
and Optimization using Aspen Plus
) includes three chapters. Chapter 1 presents the f
introductory concepts with solving the lash chambers. The computation of bubble point and dew point temperatures is also focused. Chapters 2 and 3 are devoted to simulation of several reactor models and separating column models, respectively.
Part II (Chemical Plant Simulation using Aspen Plus
) consists of only one chapter
(Chapter 4). It addresses the steady state simulation of large chemical plants. Several
individual processes are interconnected to form the chemical plants. The Aspen Plus simulator is used in both Part I and Part II. vii
Copyrighted maierlal
viii
PREFACE
The Aspen Dynamics package is employed in Part III (Dynamics and Control using Aspen Dynamics ) that comprises Chapters 5 and 6. Chapter 5 is concerned with the f
dynamics and control of low-driven chemical processes. In the closed-loop control study
,
the servo as well as regulatory tests have been conducted. Dynamics and control of pressure-driven processes have been discussed in Chapter 6. The target readers for this book are undergraduate and postgraduate students of chemical engineering. It will be also helpful to research scientists and practising engineers. Amiya K. -Jana
Copyrighted maierlal
Acknowledgements
It is a great pleasure to acknowledge the valuable contributions provided by many of my well-wishers. 1 wish to express my heartfelt gratitude and indebtedness to Prof. A.N.
Samanta, Prof. S. Ganguly and Prof. S. Ray, Department of Chemical Engineering, IIT Kharagpur. I am also grateful to Prof. D. Mukherjee, Head, Department of Chemical Engineering, IIT Kharagpur. My special thanks go to all of my colleagues for having
created a stimulating atmosphere of academic excellence. The chemical engineering students at IIT Kharagpur also provided valuable suggestions that helped to improve the presentations of this material.
I am greatly indebted to the editorial staff of PHI Learning Private Limited, for their constant encouragement and unstinted efforts in bringing the book in its present form.
No list would be complete without expressing my thanks to two most important people in my life-my mother and my wife. I have received their consistent encouragement and support throughout the development of this manuscript.
Any further comments and suggestions for improvement of the book would be gratefully acknowledged.
rial
Part I
Steady State Simulation and Optimization using Aspen Plus
Copyrigf
This Thispage pageintentionally intentionallyleft leftblank blank
Rs. 295.00
PROCESS SIMULATION AND CONTROL USING ASPEN
Amiya K. Jana
@ 2009 by PHI Learning Pnvate Limited, New Delhi. All rights reserved. No part of this book may be reproduced In any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-978-81-203-3659-9
The export rights of this book are vested solely with the publisher.
Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Jay Print Pack Private Limited, New Delhi-110015.
CHAPTER
i
Introduction and Stepwise Aspen Plus Simulation: Flash Drum Examples
11 .
ASPEN: AN INTRODUCTION
By developing a computer program, it may be manageable to solve a model structure of
a chemical process with a small number of equations. However, as the complexity of a plant integrated with several process units increases, solving a large equation set f
becomes a challenge. In this situation, we usually use the process lowsheet simulator,
such as Aspen Plus
and PRO/II
(AspenTech). ChemCad
(Chemstations), HYSYS
(Hyprotech)
(SimSci-Esscor). In 2002, Hyprotech was acquired by AspenTech.
However, most widely used commercial process simulation software is the Aspen software.
During the 1970s, the researchers have developed a novel technology at the Massachusetts Institute of Technology (MIT) with United States Department of Energy funding. The undertaking, known as the Advanced System for Process Engineering (ASPEN) Project, was originally intended to design nonlinear simulation software that could aid in the development of synthetic fuels. In 1981, AspenTech, a publicly traded company, was founded to commercialize the simulation software package.
AspenTech went public in October 1994 and has acquired 19 industry-leading companies as part of its mission to offer a complete, integrated solution to the process industries (http://www.aspentech.eom/corporate/careers/faqs.cfm#whenAT).
The sophisticated Aspen software tool can simulate large processes with a high degree of accuracy. It has a model library that includes mixers, splitters, phase separators, heat exchangers, distillation columns, reactors, pressure changers, manipulators, etc. By interconnecting several unit operations, we are able to develop a
f
process low diagram (PFD) for a complete plant. To solve the model structure of either a
Copynghled material
4
PROCESS SIMULATION AND CONTROL USING ASPEN
a single unit or a chemical plant, required Fortran codes are built-in in the Aspen simulator. Additionally, we can also use our own subroutine in the Aspen package. The Aspen simulation package has a large experimental databank for thermodynamic and physical parameters. Therefore, we need to give limited input data for solving even a process plant having a large number of units with avoiding human errors and spending a minimum time.
Aspen simulator has been developed for the simulation of a wide variety of processes, such as chemical and petrochemical, petroleum refining, polymer, and coalbased processes. Previously, this flowsheet simulator was used with limited
applications. Nowadays, different Aspen packages are available for simulations with promising performance. Briefly, some of them are presented below. Aspen Plus-This process simulation tool is mainly used for steady state simulation of
chemicals, petrochemicals and petroleum industries. It is also used for performance monitoring, design, optimization and business planning. Aspen Dynamics-This powerful tool is extensively used for dynamics study and closed-
loop control of several process industries. Remember that Aspen Dynamics is integrated with Aspen Plus.
Aspen BatchCAD-This simulator is typically used for batch processing, reactions and distillations. It allows us to derive reaction and kinetic information from experimental data to create a process simulation. Aspen Chromatography-This is a dynamic simulation software package used for both batch chromatography and chromatographic simulated moving bed processes. Aspen Properties-It is useful for thermophysical properties calculation. Aspen Polymers Plus-It is a modelling tool for steady state and dynamic simulation, and optimization of polymer processes. This package is available within Aspen Plus or Aspen Properties rather than via an external menu.
Aspen HYSYS-This process modelling package is typically used for steady state simulation, performance monitoring, design, optimization and business planning for petroleum refining, and oil and gas industries.
It is clear that Aspen simulates the performance of the designed process. A solid understanding of the underlying chemical engineering principles is needed to supply reasonable values of input parameters and to analyze the results obtained. For example, a user must have good idea of the distillation column behaviour before attempting to use
Aspen for simulating that column. In addition to the process flow diagram, required input information to simulate a process are: setup, components properties, streams and blocks. ,
12 .
GETTING STARTED WITH ASPEN PLUS SIMULATION
Aspen Plus is a user-friendly steady state process flowsheet simulator. It is extensively used both in the educational arena and industry to predict the behaviour of a process by using material balance equations, equilibrium relationships, reaction kinetics, etc.
Using Aspen Plus, which is a part of Aspen software package, we will mainly perform in this book the steady state simulation and optimization. For process dynamics and
INTRODUCTION AND STEPWISE ASPEN PLUS
SIMULATION
5
f
closed-loop control, Aspen Dynamics (formerly DynaPLUS) will be used in several subsequent chapters. The standard Aspen notation is used throughout this book. For example, distillation column stages are counted from the top of the column: the condenser is Stage 1 and the reboiler is the last stage. As we start Aspen Plus rom the Start menu or by double-clicking the Aspen Plus icon on our desktop, the Aspen Plus Startup dialog appears. There are three choices and we can create our work from scratch using a Blank Simulation, start from a Template or Open an Existing Simulation. Let us select the Blank Simulation option and click OK (see Figure 1.1). MM
MM
'Ml
I
I-
FIGURE 1.1
f
The simulation engine of Aspen Plus is independent rom its Graphical User Interface (GUI). We can create our simulations using the GUI at one computer and run them connecting to the simulation engine at another computer. Here, we will use the simulation engine at Local PC'. Default values are OK. Hit OK in the Connect to Engine dialog (Figure 1.2). Notice that this step is specific '
to the installation.
f
f
The next screen shows a blank Process Flowsheet Window. The irst step in developing a simulation is to create the process lowsheet. Process flowsheet is simply defined as a blueprint of a plant or part of it. It includes all input streams, unit operations, streams that interconnect the unit operations and the output streams. Several process units are listed by category at the bottom of the main window in a toolbar known as the Model Library. If we want to know about a model, we can use the Help menu from the menu bar. In the following, different useful items are highlighted briefly (Figure 1.3). Copyrighted material
6
PROCESS SIMULATION AND CONTROL USING ASPEN
Connect to Engine Serve« type
Local PC
Liter Into Node name:
Uset name Password
Working dfedory:
Q Save as Default Cormeciion OK
Exit
Help
FIGURE 1.2
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Next button
Data Browser button
Solver Settings button
Material STREAMS icon Status bar
H
/ lfcMM/5«iln«t | Sipiram | H«rfEKtwgvt | Calm | Rmovi | PmtutO*no*i | MrauMeti | Sat* | UmtUoM j Model Library toolbar s
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FIGURE 1.3
Copyrighted material
INTRODUCTION AND STKPWISK ASPEN PI.US
SIMULATION
7
f
f
To develop a lowsheet, irst choose a unit operation available in the Model Library.
f
Proprietary models can also be included in the lowsheet window using User Models option. Excel workbook or Fortran subroutine is required to define the user model. In the subsequent step, using Material STREAMS icon, connect the inlet and outlet streams
with the process. A process is called as a block in Aspen terminology. Notice that clicking f
on Material STREAMS, when we move the cursor into the lowsheet area red and blue
arrows appear around the model block. These arrows indicate places to attach streams f
to the block. Red arrows indicate required streams and blue arrows are optional. When the lowsheet is completed, the status message changes from Flowsheet Not
Complete to Required Input Incomplete. After providing all required input data using input forms, the status bar shows Required Input Complete and then only the simulation results are obtained. In the Data Browsery we have to enter information at locations where there are red semicircles. When one has finished a section, a blue checkmark
appears. In subsection 1.3.2. a simple problem has been solved, presenting a detailed stepwise simulation procedure in Aspen Plus. In addition, three more problems have
also been discussed with their solution approaches subsequently. 13
STEPWISE ASPEN PLUS SIMULATION OF FLASH DRUMS
.
1 3 1 Built-in Flash Drum Models .
.
f
In the Model Library, there are ive built-in separators. A brief description of these models is given below.
f
Flash 2: It is used for equilibrium calculations of two-phase (vapour-liquid) and threephase (vapour-liquid-liquid) systems. In addition to inlet stream(s), this separator can include three product streams: one liquid stream, one vapour stream and an optional water decant stream. It can be used to model evaporators, lash chambers and other single-stage separation columns.
Flash 3: It is used for equilibrium calculations of a three-phase (vapour-liquid-liquid) system. This separator can handle maximum three outlet streams: two liquid streams and one vapour stream. It can be used to model single-stage separation columns. f
Decanter: It is typically used for liquid-liquid distribution coeficient calculations of a two-phase (liquid-liquid) system. This separator includes two outlet liquid streams along
with inlet stream(s). It can be used as the separation columns. If there is any tendency of vapour formation with two liquid phases, it is recommended to use Flash3 instead of Decanter.
f
Sep 1: It is a multi-outlet component separator since two or more outlet streams can be produced rom this process unit. It can be used as the component separation columns. Sep 2: It is a two-outlet component separator since two outlet streams can be withdrawn from this process unit. It is also used as the component separation columns.
At this point it is important to mention that for additional information regarding a built-in model, select that model icon in the Model Library toolbar and then press Fl on the keyboard.
8
PROCESS SIMULATION AND CONTROL USING ASPEN
132 .
.
Simulation of a Flash Drum
Problem statement
r
A 100 kmol/hr feed consisting of 10, 20, 30, and 40 mole% of propane, c-butane, n-pentane and n-hexane, respectively, enters a lash chamber at 15 psia and 50oF. The lash drum (Flash2) is shown in Figure 1.4 and it operates at 100 psia and 200oF. Applying the SYSOP0 property method, compute the composition of the exit streams. f
f
,
3
-
FLASH
f
A lowsheet of a lash drum. f
FIGURE 1.4
Simulation approach
From the desktop, select Start button followed by Programs, AspenTech, Aspen Engineering Suite, Aspen Plus Version and Aspen Plus User Interface. Then choose Template option in the Aspen Plus Startup dialog (Figure 1.5).
I 1- l-MHM*
FIGURE 1.5
As the next window appears after hitting OK in the above screen, select General with English Units (Figure 1.6). Copyrighted material
INTRODUCTION AND STEPVV1SE ASPEN PLUSIM SIMULATION -Hi
9
1
1
'
.
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;1
I
i.-
L
-
.
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i
-
FIGURE 1.6
Then click OK. Again, hit OK when the Aspen Plus engine window pops up and
f
subsequently, proceed to create the lowsheet. Creating flowsheet
f
f
Select the Separators tab from the Model Library toolbar. As discussed earlier, there are ive built-in models. Among them, select Flash2 and place this model in the window. Now the Process Flowsheet Window includes the lash drum as shown in Figure 1.7. By
default, the separator is named as Bl. '
nia*lHl mU JM ??1
a-i-m * -ai-o "d 3
r
-
I l-l
SI Hi bl' 3
0
-
0 9 «=>. 8 - C .
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1 FIGURE 1.7
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10
PROCESS SIMULATION AND CONTROL USING ASPEN1
To add the input and output streams with the block, click on Streams section (lower left-hand comer). There are three different stream categories (Material, Heat and Work), as shown in Figure 1.8.
3
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-
l
,
1 Ma I
,
J--
XQ.o Q lr -
-
FIGURE 1.8
Block Bl includes three red arrows and one blue arrow as we approach the block
after selecting the Material STREAMS icon. Now we need to connect the streams with f
the lash chamber using red arrows and the blue arrow is optional. The connection procedure is presented in Figure 1.9.
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-
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-
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FIGURE 1.9
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INTRODUCTION AND STFPWISK ASPEN PLUS
SIMULATION
11
Clicking on Material STREAMS, move the mouse pointer over the red arrow at the
f
inlet of the lash chamber. Click once when the arrow is highlighted and move the cursor so that the stream is in the position we want. Then click once more. We should see a stream labelled 1 entering the drum as a feed stream. Next, click the red arrow
coming out at the bottom of the unit and drag the stream away and click. This stream is marked as 2. The same approach has been followed to add the product stream at the f
top as Stream 3. Now the lowsheet looks like Figure 1.10. Note that in the present
case, only the red arrows have been utilized. ..
-
.
,
0
a
.
>
1 -
Of.
.
F
OQ-o-O-itFIGURE 1.14
14
PROCESS SIMUIvVTION AND CONTROL USING ASPEN
f
Although optional, it is a good practice to ill up the above form for our project giving the Title (Flash Calculations) and keeping the other items unchanged (Figure 1.15). .
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3 ri-i
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-
*
(0-eo.o-1
-
FIGURE 1.15
In the next step (Figure 1.16), we may provide the Aspen Plus accounting information (required at some installations). In this regard, a sample copy is given with the followings: User name: AKJANA
Account number: 1
Project ID: ANYTHING Project name: AS YOU WISH
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.
si
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FIGURE 1.16
Copyrighted material
INTRODUCTION AND STEPWISE ASPEN PLUS
SIMULATION
15
f
We may wish to have streams results summarized with mole ractions or some other basis
that is not set by default. For this, we can use the Report Options under Setup folder. In the f
subsequent step, select Stream sheet and then choose Mole raction basis, ...
.
.
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As illed out, the form shown in Figure 1.17, inal results related to all inlet and product streams will be shown additionally in terms of mole raction. Remember that all values in the inal results sheet should be given in the British unit as chosen it previously. Specifying components
Clicking on Next button or double-clicking on Components in the column at the left side and then selecting Specifications, we get the following opening screen (Figure 1.18).
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FIGURE 1.18
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16
PROCESS SIMULATION AND CONTROL USING ASPEN1"
Next, we need to fill up the table as suggested in Figure 1 18. A Component ID is essentially an alias for a component It is enough to enter the formulas or names of the components as their IDs Based on these component IDs, Aspen Plus fills out the Type .
.
.
,
Component name and Formula columns But sometimes Aspen Plus does not find an exact match in its library. Like in the present simulation, we have the following screen (Figure 1.19) after inserting chemical formulas of the components in the Component ID .
,
column. 3513 I
Toolt
Run
Plot
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PROCESS SIMULATION AND CONTROL USING ASPEN
f
Aspen Plus suggests a number of possibilities. Among them, select a suitable component name (N-BUTANE) and then click on Add. Automatically, the Component name and Formula for Component ID N-C4H10 enter into their respective columns. For last two components, we follow the same approach. When all the components are completely defined, the illed component input form looks like Figure 1.22. m
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FIGURE 1.22
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The Type is a specification of how Aspen calculates the thermodynamic properties. For luid processing of organic chemicals, it is usually suitable to use 'Conventional* option. Notice that if we make a mistake adding a component, right click on the row and then hit Delete Row or Clear.
Specifying property method
Press Next button or choose Properties I Specifications from the Data Browser. Then if we click on the down arrow under Base method option, a list of choices appears. Set the SYSOPO' method as shown in Figure 1.23. A Property method defines the methods and models used to describe pure component and mixture behaviour. The chemical plant simulation requires property data. A wide variety of methods are available in Aspen Plus package for computing the properties. Each Process type has a list of recommended property methods. Therefore, the Process type narrows down the choices for base property methods. If there is any confusion, we may select All' option as Process type. '
Specifying stream information In the list on the left, double click on Streams folder or simply use Next button. Inside that folder, there are three subfolders, one for each stream. Click on inlet stream F, and
f
enter the temperature, pressure, low rate and mole fractions. No need to provide any data for product streams L and V because those data are asked to compute in the present problem (see Figure 1.24). This property method assumes ideal behaviour for vapour as well as liquid phase.
C
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INTRODUCTION AND STEI'WISK ASPEN PLUS
SIMULATION
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Specifying block information
Hitting Next button or selecting Blocks/FLASH in the column at the left side, we get the block input form. After inserting the operating temperature and pressure, one obtains Figure 1.25.
20
PROCESS SIMULATION AND CONTROL USING ASPRN U3SE Toob
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FIGURE 1.25
Now the Status message (Required Input Complete) implies that all necessary information have been inserted adequately. Moreover, all the icons on the left are blue. It reveals that all the menus are completely filled out. If any menu is still red, carefully enter the required information to make it blue. Running the simulation
Click on Next button and get the following screen (see Figure 1 26). To run the simulation, press OK on the message. We can also perform the simulation selecting Run from the Run pulldown menu or using shortcut key F5 .
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The Control Panel, as shown in Figure 1.27, shows the progress of the simulation. It presents all warnings, errors, and status messages.
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FIGURE 1.29
If we click on Stream Table button, the results table takes a place in the Process Flowsheet Window, as shown in Figure 1.30. Fie Edt
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FIGURE 1.32
When the next window pops up (see Figure 1.33)
,
select General with Metric Units
and then hit OK.
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FIGURE 1.33
In the next
,
press OK in the Connect to Engine dialog. Once Aspen Plus connects to
the simulation engine, we are ready to begin entering the process system.
30
PROCESS SIMULATION AND CONTROL USING ASPEN
Creating flowsheet
Using the Flash2 separator available in the equipment Model Library, develop the
f
following process low diagram (see Figure 1.34) in the Flowsheet Window by connecting f
the input and output streams with the lash drum. Recall that red arrows are required ports and blue arrows are optional ports. To continue the simulation, we need to click either on Next button or Solver Settings as discussed earlier. Note that whenever we have doubts on what to do next, the simplest way is to click the Next button.
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Configuring settings
From the Data Browser, choose Setup I Specifications. The Title of the present problem is given as 'Bubble Point Calculations'. Other items in the following sheet remain untouched (see Figure 1.35). However, we can also change those items (e.g., Units of measurement. Input mode, etc).
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INTKODUCTION AND STHPWISE ASFKN PLUSIM SIMULVTION
31
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In the next, the Aspen Plus accounting information are given (see Figure 1.36). _
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Specifying components
Click on TVex button or choose Components /Specifications in the list on the left. Then define all components and obtain the following window (see Figure 1.37). rfc
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32
PROCESS SIMULATION AND CONTROL USING ASPEN
Specifying property method
Hit Next button or select Properties / Specifications in the column at the left side. In Property method, scroll down to get RK-Soave. This equation of state model is chosen for thermodynamic property predictions for the hydrocarbon mixture (see Figure 1.38).
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Hitting ATex/ button twice, we have the following picture (see Figure 1.39). The binary parameters are tabulated below. When we close this window or cbck OK on the message. it implies that we approve the parameter values. However, we have the opportunity to
edit or enter the parameter values in the table. In blank spaces of the table, zeros are there. It does not reveal that the ideal mixture assumption is used because many
thermodynamic models predict non-ideal behaviour with parameter values of zero.
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33
Specifying stream information
Click OK. Alternatively, use the Data Browser menu tree to navigate to the Streams/1/ Input/Specifications sheet. Then insert all specifications for Stream 1 as shown in Figure 1 40 J
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Hit Afot or select Blocks/BUBBLE from the Data Browser. After getting the blank input
form, enter the required inputs (Pressure = 18 bar and Vapour fraction = 0) for block BUBBLE (see Figure 1.41). -
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INTKODUCTION AND STKPWISK ASI'KN PLUS
SIMULATION
35
Choosing Blocks/BUBBLE/Results in the column at the left side, we get the
following results summary for the present problem (see Figure 1.44).
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From the results sheet, we obtain the bubble point temperature = 42.75411960C. 13 .
4
.
Computation of Dew Point Temperature
Problem statement
Compute the dew point temperature at 1.5 bar of the hydrocarbon mixture, shown in Table 1.2, using the RK-Soavc property method. TABLE 1.2
Component
Ci C2 Ca
Mole fraction 0 05 .
0 1 .
0 15 .
0 15
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.
.
f
Assume the mixture inlet temperature of 250C, pressure of 5 bar and low rate of 120 kmol/hr.
36
PROCESS SIMULATION AND CONTROL USING ASPEN
Simulation approach f
As we start Aspen Plus rom the Start menu or by double-clicking the Aspen Plus icon on our desktop theAspe?i Plus Startup dialog appears (see Figure 1.45). Select Template option ,
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As Aspen Plus presents the window after clicking OK as shown Figure 1.45, choose General with Metric Units. Then press OK (see Figure 1.46).
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INTRODUCTION AND STEPWISE ASPEN PLUS
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37
Subsequently, dick OK when the Aspen Plus engine window pops up. Creating flowsheet
f
In the next, we obtain a blank Process Flowsheet Window. Then we start to develop the process lowsheet by adding the Flash2 separator from the Model Library toolbar and joining the inlet and product streams by the help of Material STREAMS (Figure 1.47).
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Now the process low diagram is complete. The Status bar in the bottom right of the above window (see Figure 1.47) reveals Required Input Incomplete indicating that input data are required to continue the simulation. Configuring settings
Hitting Next button and then clicking OK, we get the setup input form. The present problem is titled as Dew Point Calculations' (see Figure 1.48). In Figure 1.49, the Aspen Plus accounting information are provided. '
Specifying components
f
Here we have to enter all the components we are using in the simulation. In the list on the left, choose Components /Specifications and ill up the table following the procedure explained earlier (see Figure 1.50).
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Viewing results
As we click OK on the above message the Control Panel appears showing the progress of the simulation. After the simulation is run and converged we notice that the Results Summary tab on the Data Browser window has a blue checkmark Clicking on that tab ,
,
.
will open up the Run Status. If the simulation has converged it should state ,
"
Calculations were completed normally" (see Figure 2 13). Pressing Next button and then OK, we get the Run Status screen In the subsequent .
.
step, select Results Summary /Streams in the list on the left and obtain the final results (see Figure 2.14). Save the work done by choosing File/Save As/...in the menu list on the top.
If we click on Stream Table knob just above the results table, the results are recorded
in the Process Flowsheet Window, as shown in Figure 2.15
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For input information, press Ctrl + Alt + I on the keyboard or select Input Summary rom the View pulldown menu (see Figure 2.16). CBSES Fie
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Right click on Available reaction sets, hit New button, then either accept default name R-l or give a name as we want for the reaction set and inally click on OK Subsequently, select POWERLAW in the Enter Type list and hit OK to get the screen
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as shown in Figure 2.27. Ffc
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select Solver Settings, choose igsuto Summary/Sf ms in the list on the f
In the next
left and finally get the results shown in Figure 2.35 in a tabulated form.
78
PROCESS SIMULATION AND CONTROL USING ASPEN1 B» Ebl V«- D*. TMi. Hun fW ijt
I f
J4J«J
MiiM m
I I i PT
! .leal I
i
"
I
I - Ml tM
i
i
I
I "
3 '-"
l
il
il
i
-
nil 1
am
0541
1000
0«5
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0 001
0J30
nooo
tso 601
-
mmmi
1
sm
'
ITTre
0«J
MPPM
DOM
0 98)
| HuiE«*w> ! C>i liai g f
3Sif*r-~3 *m si ISrolWoneilheHPVjgMocW
Vdd|*MMC
o-> '
HSteic
tVM
i s u -= u
myt
RG|te. t
-
SIKAMS
|
RC?tR
Rptq
RBtuh
FIGURE 2.40(A) UaTSil
>
nt
Mm OKk TMIp An W L±>»v WWo* H*p
arsi aiobdj-/Deicnmn >/Acciwnlina| 0>agr>o«(«ci {
MMi- pdim I Smmnc I HulE«chv4«i | Cot-mi. lo«'«" | Pimm«C»W f
[T
hB- 1 -1 - 8 Q l5ia
f
SIRLWi
__
R.'*
RfrMl
RGfaU;
W Iff
O Hf''-.
FIGURE 2.40(b)
gMdiM-AiMf But " I
"
.
'
U
.
82
PROCESS SIMULATION AND CONTROL USING ASPEN
dmbl
Melm mbhjsM«!] 21 g
r m«i »
K C
r SM
twwrH »4i , »« flow W 'IK
»O-S-0 y FIGURE 2.40(c)
Specifying components
From the Data Browser, select Specifications under the Components folder. As we provide the chemical formula of the components in the Component ID column, the other columns of the table are automatically filled up (see Figure 2.41). f
Fit E* Htw D«i Tat* tji RsT Ihwf fntotr Hife f
<
IMM
FvmU,
cia Modab |
a §is q-u '
RStot
Rrail
lE
f
SIR£>M$
D*«
f
'
i)
*"
FIGURE 2.46
Specifying reaction information
Hitting Afof button and clicking on New we have the following forms (see Figures 2.47(a) and (b)) for reaction number 1 (2C6H6 -> C12H10 + H2). Since the reaction rate constants are defined with respect to benzene we convert the stoichiometric coefficient of benzene r
,
,
to unity for both the reactions Obviously the reactions are second-order. .
,
Jala_jj iiei wj nHM'.teM ».| m .| |h| .| pi 1
r..l..|,.l it
1 .ibi-
.
®|
1 / ial
Rmmm
I1-I'
A
v
R 1
ii
-
o
-
0 S 0
1J -
FIGURE 2.47(a)
a .a
#
1
86
PROCESS SIMULATION AND CONTROL USING ASPEN HI »l
D|tf|y|
I I
i-«i» rr
'i-.joii
"
HMfcl""
»|-»l «l|Ii
(31 50*10. 5m;
a i
.
3 >>l Dj J n.|
C6M6
KiMtel«daNUT/T>>|"*'(Ewto
8 i 0
Rt
fjfl
I Mill
a
,
"
11
FIGURE 2 51(b) .
Lin
i
ASPKN PLUS
SIMULATION OF REACTOR MODELS
89
Running the simulation
r
Hitting Afet button and running the simulation, we obtain the Control Panel (Figure 2.52) showing the progress of the present simulation.
i r-i I ! f»
-i-igi
'
_
_
1
w aisd
(0 9 S 8 O = U M
t
mm
"tj~
mm
m>
mm
FIGURE 2.57
Again click on Next and get the form, shown in Figure 2.58.
J
92
PROCESS SIMULATION AND CONTROL USING ASPEN1
I mim 1?! r3l-Mod> |
si u=u 111 *
HTot)
Qg
RGtfc,
tCSIR
tFy
f
RS'jc
f
STROIMS '
"
.
'
8M,
FIGURE 2.59
Note that the plot window can be edited by right clicking on that window and selecting Properties In the properties window .
,
the user can modify the title, axis scale,
font and colour of the plot Alternatively, double-click on the different elements of the .
plot and modify them as we like to improve the presentation and clarity.
ASPEN PLUS
2
.
5
SIMULATION OK KKACTOR MOOEI
93
ASPEN PLUS SIMULATION OF A RPlug MODEL USING LHHW KINETICS
Problem statement
In acetic anhydride manufacturing, the cracking of acetone produces ketene and methane according to the following irreversible vapour-phase reaction:
CH3COCH3 -> CH2CO + CH4 acetone
ketene
methane
f
f
This reaction is irst-order with respect to acetone. Pure acetone feed with a low rate of 130 kmol/hr enters a PFR at 7250C and 1.5 atm. The kinetic data for the Aspen Plus simulation are given below. k = 1.1 s"1
E = 28.5 x 107 J/kmol n=0
T0 = 980 K The unit of pre-exponential factor clearly indicates the |C 1 basis. To use the LangmuirHinshelwood-Hougen-Watson (LHHW) kinetic model, set zero for all coeficients under Term 1 and that for all coeficients except A under Term 2. Take a very large negative value for coeficient A. The sample adiabatic PFR is 3 m in length and 0.6 m in diameter. Applying the SYSOP0 base method, compute the component mole fraction in the product stream. f
f
f
,
Simulation approach
As we select Aspen Plus User Interface, first the Aspen Plus Startup window appears, as shown in Figure 2.60. Choose Template option and press OK.
f
2I=flHJ-J-Lag Pl-W i-H=J Tl
I I I 'IW *l
1
1
-I
I
**mmm*mH
MM
FIGURE 2.60
94
PROCESS SIMULATION AND CONTROL USING ASPEN
In the next, select General with Metric Units and again hit OK button (see Figure 2.61)
.
pea
M
IPE a-wm tcpwl* f
An
'
Penmen
1
"
'
11
'C*
'
FIGURE 2.61
As the Connect to Engine dialog pops up
,
click OK.
Creating flowsheet
From the Model Library toolbar we have selected RPlug reactor and developed the ,
f
process low diagram as displayed in Figure 2.62 He &
3an Tocfc fir FW mI Jy»r, WnSe* Htfc
Qi lHI aiai
|a| yj nl-i-iaKKi i w.| 3
rlttF-I l- l PT
s,flt M
.
I
Mi
_
I
i ii
T
To)
(2.8)
If Tq is ignored, Eq. (2.3) replaces the above expression.
Note that all the notations
n
K = k
E exp
R
k
used in Eq. (2.8) have been defined earlier. The driving force is expressed by:
f N
A
n c?
and the adsorption is modelled as: M
N
nc"J Li=i where,
In (Ki) =Ai + Bi/T + Ci IniT) + D.T
(2.9)
Here, m is the adsorption expression exponent M the number of terms in the adsorption expression, N the number of components a the concentration exponent, K2, K, the equilibrium constants [Eq (2.9)], A,, it, Q, the coefficients and I Notice that the ,
,
f
.
concentration term C used in the above discussion is dependent on the [CJ basis Say when [CJ basis is selected as molarity the concentration term represents the component molar concentration (kmol/m3); similarly when [CJ basis is partial .
for example
,
,
pressure, the concentration term represents the component partial pressure (N/m2). Providing required data we have the illed kinetic sheet shown in Figure 2.71. Click on Driving Force to obtain a blank form as shown in Figure 2.72. f
,
,
Select 'Term 1' and then 'Molarity' as [CJ basis Under Concentration exponents for set acetone exponent to 1. Similarly for products set ketene and methane exponents to 0 Also enter zero for all four driving force constants as mentioned in the .
reactants,
,
.
problem statement (see Figure 2 73). In the subsequent step (see Figure 2.74), select Term 2' rom the pulldown Enter f
.
term menu Since the given reaction is irst-order with respect to acetone no second term enter zero for all exponents and coefficients Owing to
and there is the method Aspen Plus uses to specify a reaction, we should insert a very large negative value for f
.
,
.
coetticient A (say
on Next icon
.
,
-106)
to make Term 2 essentially zero [see Eq (2 9)1 Finally click n-
.
,
ASPEN PLUS
0m
f
t*
-
L T
Om
Ta«i
.av*
VMw
I I 'i r-i
-
SIMULATION OF REACTOR MODELS
-I -lei
I
!«!
3 -~
.
j
.
3
,
ii w
j9 O -
:
'm CH3COOC2H5 + H20 The inlet stream, consisting of 50 mole% acetic acid, 45 mole% ethanol and 5 mole% water, is fed to a REquil model with a flow rate of 400 kmol/hr at 750C and 1.1 atm. The reactor operates at 80oC and 1 atm. Using the NRTL property method, simulate the reactor model and report the compositions of the product streams. .
7 Ethylene is produced by cracking of ethane in a plug low reactor. The irreversible elementary vapour-phase reaction is given as: f
2
C2H6 - C2H4 + Hg ethane ethylene hydrogen f
Pure ethane feed is introduced with a low rate of 750 kmol/hr at 800CC and
5 atm. The reactor is operated isothermally at inlet temperature. The kinetic data for the LHHW model are given below (Fogler, 2005). 5
.
k = 0.072 s"1 £ = 82 x 103 cal/mol
Tq = 1000 K
|C,] basis = Molarity The reactor length is 3 m and diameter is 0.8 m. Using the SYSOP0 thermodynamic model, simulate the reactor. 2 8 Repeat the above problem replacing the PFR by a stoichiometric reactor with 80% conversion of ethane. If require, make the necessary assumptions. 2 9 In acetic anhydride manufacturing, the cracking of acetone occurs and produces ketene and methane according to the following irreversible vapour-phase reaction: .
.
CH3COCH3 i CHoCO + CH3
1
106
PROCESS SIMULATION AND CONTROL USING ASPEN
In the CSTR model, ketene is decomposed producing carbon monoxide and ethylene gas. K
'
CH2CO-> CO + 0.5 C2H4 where, 15
,
rk = K
.
'
-
K=
26586
exp 22.8-
K' = exp 19.62-
mol/lit s . atm15
T 25589
mol/lit . s
[C,] basis = Partial pressure
Here, -rA is the rate of disappearance of acetone (A), -rk the rate of disappearance of ketene ik), PA the partial pressure of A, and K and K the reaction rate '
constants. Pure acetone feed with a flow rate of 130 kmol/hr enters the reactor at 7250C and 1.5 atm. The reactor with a volume of 1
.
4 m3 operates at 700oC
and 1.5 atm. Applying the SYSOPO base method compute the component mole fractions in the product stream ,
.
REFERENCE | Fogler
,
H. Scott (2005), Elements of Chemical Reaction Engineering
3rd ed.. New Delhi
.
,
Prentice-Hall of India
CHAPTER
Aspen Plus Simulation of Distillation Models
31 .
BUILT-IN DISTILLATION MODELS
An Aspen simulation package has nine built-in unit operation models for the separating column. In the Aspen terminology, these packages are named as DSTWU, Distl, RadFrac. Extract. MultiFrac, SCFrac, PetroFrac, RateFrac and BatchFrac. Under these categories,
several model configurations are available. Note that Extract model is used for liquidliquid extraction. Among the built-in column models, DSTWU, Distl and SCFrac
r
represent the shortcut distillation and the rest of the distillation models perform igorous calculations.
DSTWU model uses Winn-Underwood-Gilliland method for a single-feed two-product fractionating column having either a partial or total condenser. It estimates minimum number of stages using Winn method and minimum reflux ratio using Underwood method. Moreover, it determines the actual reflux ratio for the specified number of
stages or the actual number of stages for the specified reflux ratio, depending on which is entered using Gilliland correlation. It also calculates the optimal feed tray and reboiler as well as condenser duty. Remember that this model assumes constant molar overflow and relative volatilities.
Distl model includes a single feed and two products, and assumes constant molar
overflow and relative volatilities. It uses Edmister approach to calculate product composition. We need to specify a number of stages, e.g. feed location, reflux ratio,
pressure profile and distillate to feed iD/F) ratio. Actually, when all the data are provided, we can use this column model to verify the product results. RadFrac is a rigorous fractionating column model that can handle any number of feeds as well as side draws. It has a wide variety of appUcations, such as absorption,
stripping, ordinary distillation, extractive and azeotropic distillation, reactive distillation, etc. MultiFrac is usually employed for any number of fractionating columns and any number of connections between the columns or within the columns. It has the ability to simulate the distillation columns integrated with flash towers, feed furnaces, side 107
Copyrighted material
108
PROCESS SIMUKATION AND CONTROL USING ASPEN
strippers, pumparrounds, etc. This rigorous column model can be used as an alternative of PetroFrac, especially when the configuration is beyond the capabilities of PetroFrac As mentioned earlier, SCFrac is a shortcut column model. It simulates a distillation .
unit connected with a single feed, multiple products and one optional stripping steam
.
It is used to model refinery columns, such as atmospheric distillation unit (ADU) and vacuum distillation unit (VDU).
f
PetroFrac is commonly employed to fractionate a petroleum feed. This rigorous model simulates the refinery columns, such as ADU, VDU, luidized-bed catalytic cracking (FCC) fractionator, etc., equipped with a feed furnace, side strippers, pumparounds and so on. RateFrac is a rate-based nonequilibrium column model employed to simulate all
types of vapour-liquid separation operations, such as absorption, desorption and distillation. It simulates single and interlinked columns with tray type as well as packed type arrangement.
BatchFrac is a rigorous model used for simulating the batch distillation columns. It also includes the reactions occurred in any stage of the separator. BatchFrac model does not consider column hydraulics, and there is negligible vapour holdup and constant liquid holdup. It is worthy to mention that for detailed information regarding any built-in Aspen
Plus model, select that model icon in the Model Library toolbar and press Fl. In this chapter, we will simulate different distillation models, including a petroleum refining column, using the Aspen Plus software. Moreover, an absorption column will be analyzed. In addition to the steady state simulation the process optimization will ,
also be covered in the present study. 3
2
.
ASPEN PLUS SIMULATION OF THE BINARY DISTILLATION COLUMNS
32 1 .
.
Simulation of a DSTWU Model
Problem statement
A feed stream, consisting of 60 mole% ethane and 40 mole% ethylene enters a DSTWU column having a low rate of 200 Ibmol/hr at 750F and 15 psia This feed is required to ,
f
.
fractionate in a distillation column capable of recovering at least 99 6% of the light key .
component in the distillate and 99 9% of the heavy key component in the bottoms. The sample process operates at 300 psia with zero tray-to-tray pressure drop The pressure .
.
in the reboiler as well as condenser is also 300 psia In the simulation, use total .
30 theoretical stages (including condenser and reboiler) and a total condenser Applying the RK-Soave property method simulate the column and calculate the minimum reflux ratio, actual reflux ratio minimum number of stages actual number of stages, and .
,
,
,
feed location.
Simulation approach
From the desktop select Start button and then click on Programs, AspenTech, Aspen ,
Engineering Suite
,
Aspen Plus Version and Aspen Plus User Interface. Then choose Template option in the Aspen Plus Startup dialog and hit OK (see Figure 3 1). ,
.
ASPEN PLUS
Q\a\m -I -I
SIMULATION OF DISTILLATION MODKUS
|r| IB3|- . { lal
UMbl'1
0 Becwrt Opdcro
i
-I
-3>>JialajNil
G«mi4 I Ftatt««t j Btaek Vsi»m} Proper j ADA | H«MtebancUWinsD««nMpwl
;
Ffaubwit
FiMjonbMa
j f? Mote
P Moio
r mm
' P S»S
'
T SUHovcAjw
i jjj Bock.
ShWDfamM
Iff Si«ndsd(BOoctant
r SUi viAm ;
_
K ConpoAMvAweftMoftKlnn
-
; tff- [geTTe r WUaPBcoUw)
1 P SoK-rewatfwwKw*
Wo«tWu.iartwiw* MOM
tWSrten | S Mtatt | HMEwhangm t
*
1>rf
l
f
' DSTMl DSTMJ
f
SIRCMC
.ac
FIGURE 3.28
e
IK
ole' as well as 'Mass' fraction
ASPEN PLUS
SIMULATION OF DISTILLATION MODELS
131
Specifying components In the list on the left, choose Components /Specifications to define the components. f
Using the component names, ethane and ethylene, as their IDs, we obtain the illed table as shown in Figure 3.29. =W
54
*»
:«>
T
FV«
itun
Wr*s-
r-i-i-i
i .w
-
-
i -igi *m
3a !« Mn* ;lrM*N£ .
cats
IHYUNC
>
O
o
,.
si
-
r
Pttfnc
R«rfW::
B«ct#.*;
FIGURE 3.29
Specifying property method From the Data Browser select Specifications under Properties folder and then set RK,
Soave base method to compute the physical properties (see Figure 3 30). .
Fit
'
CM
0|I>'.|--|T .
»MBJ|v
mrai
B«i«»a(tw6 .
OwoCTylO
J
fflr.SOAVt - r
«J|a.
3 1
J
j
1
IJ
.
It 5T««)J
' OSTWU
FIGURE 3.30
132
PROCESS SIMULATION AND CONTROL USING ASPEN
Specifying stream information
Use the Data Browser menu tree to navigate to the Streams /F/Input / Specifications
sheet. Inserting the given values for the feed stream, Figure 3.31 is obtained. r
3EfSiF -q
l-li Fi-Hid QUIH
3
|m*
dr it
(200
itfi>t|
-
,
jbrt*.
1j
1 Hm(t jr r:
Ejiki
CMlma | FtMcicn | RMMfOungett | Htrie-Mm \ SiteJ UtftKoM |
Mulfia:
SCFlK
FmfiK
PmfiiK
9*0*1*:
FIGURE 3.31
Specifying block information
In the left pane of the Data Browser window select Blocks/RADFRAC/Setup. Fill up the Configuration sheet as shown in Figure 3 32. ,
.
Sa To* Rn Pa tfea/ WMw Help
I r.-.|-.i-l fT
Nv i
11] isN
3 g BO VMbki
a «*
H*o4erHcuv*i
O Dwt
7]F
1 ?98
(trrotA,
i
Ha
-
FIGURE 3.32
ASPEN PLUS
SIMULATION OF DISTILLATION MODELS
133
Under Setup subfolder, the filled Streams sheet looks like Figure 3.33.
i 'to
.
1
«
«!«.;
>
|
| MB I M»» |
it r
-
FIGURE 3.33
In the next, simply input 300 psi under Stage 1/Condenser pressure. Aspen simulator assumes that the column operates isobarically if no additional pressure information is provided (see Figure 3.34).
IB I' W tl*)
i:.,ir.ir.ii.0.ii'.fi..#.s .j-. FIGURE 3.34
Running the simulation
To run the simulation, hit Next and then OK to observe the progress of the simulation in the Control Panel window, shown in Figure 3.35.
PROCESS SIMULATION AND CONTROL USING ASPEN n* E#
M«m
Dal* Toch t* Utrvy WMdaw M*. f
134
I ~
_.
iJ
-
l i-.i T -
*
l"l "li-dail 4;|
ai Jfilp
Hao«Miafl input «p«cl tmi jiowshmt;
-
>C«lcaJ.«ttQn» t»«in . . .
Block: aAcrajkC
«ra»l
STREAMS
PSTWU
Dirf
Radfw , lAaci
MUtfxic;
SCFuw
fahoFrac
Ratrffac
Batetfrac
__
_
FIGURE 3.35
Viewing results
Click on Solver Settings followed by Results Summary and Streams, we have the table, shown in Figure 3.36, accompanying the results of all individual streams. Save the f
work in a folder as a ile.
j He Ed) Urn Curt TooU Ron
Plot Ifinry WndoM l-!. 'nrari
(oiMitilax: Mil flo"
Miiauas main
unimi
yxio-
ikkmiu
4
tiHu« ei-» -
rmkM C!"
W'OITl Ultll'l
HWii ill wii:i
CfWuM EIMK .oio»Xo>
142
4 PROCESS SIMULATION AND CONTROL USING ASPEN
In the subsequent step, specify the feed tray location in the Streams sheet as shown in Figure 3.48. D«i
T«* t* PW Uom* VMcm N* f
fl. Ml
I
CJ
31 P«p 3«.
.
Q flow
i rj dv 20
Um4
S .
t . . 9
,
' RADFRAC
0-
f
Hi j j lndxra Cj
T3 '..
FIGURE 3.54
f
It is a good habit to save the work done at least at this moment. If we wish to see the tabulated results with the process low diagram in a single sheet, simply hit Stream Table button just above the results table (see Figure 3.55).
JMMI .til _!.) a nH-|»l%l
2-MET-01
"
t QwpW M I JO
o*
t
BPVAL
IW l lhAJT M- [
wnfta 5 } > AtWwAort* ||
-APF7EAJ
>
1 17a>
FIGURE 3.56
(b) First, choose Blocks /RADFRAC /Profiles in the column at the left side
Accordingly, we have the stage-wise data as shown in Figure 3 57. .
mm 3fl *«-
J >>j am w
rmi I c
1
Vapofto.
*j 1
3
9TIJ3K?SI
T- t c I
i
20
i
20
j
Tiit asoU 1
"
B
--
o
171 385017
f
195 3*a»6
'
aj
t
r
oawnd
t
F
20
i
3ra
20
i
201354309
ia
i
;oo
anwsssi
9
liJnuiiJlii
i
}»
lit n
121)
;:
llJ
fir
STntAMS
||
.
.
- i
:iT:;nr-si
I HME
| Mh | M.UoM |
jgwc
tW-«
H«rf.>i
*mtr»C
FIGURE 3.57
J
ASPEN PLUS"" SIMULATION OF DISTILLATION MODELS
147
f
In the next, select Plot Wizard rom the Plot dropdown menu or press Ctrl + Alt + W
on the keyboard to get Figure 3.58.
I ... '. 1H|_U*1«*) nKW*haMH n
I |h| .131.31
71
_
j
.1
. «Cj owlWelcome to Aspen P!b» net Wlzwdl
1 am
r
|
a 'mm ijte>
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1
15" -
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tufi
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.
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,
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naa
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-
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s- j
wFodo FkMntln CCCOTHI CQCQS H) Hv4m >
BM
seisa
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60 - .
JP"" Pi" CH
m&
tS»
so..
w«
ac.-A»»H.MI f
'
f
-
NU«
iQVjre 1740
FIGURE 3.59
,
148
PROCESS SIMULATION AND CONTROL USING ASPEN
Select the plot type under the heading of Temp and press Finish button to obtain a '
plot of Temperature (0F) vs. 'Stage' (see Figure 3.60).
rinwii-ii
I
I
J
s
<
i
io
u
£
15
i«
:
k
w
tr
tt
is-a
it
FIGURE 3.60
r
Recall that the above plot window can be edited by ight clicking on that window and selecting Properties. Then the user can easily modify the title, axis scale, font and colour of the plot. 332 .
.
Simulation of a PetroFrac Model
Problem statement
An artificial petroleum refining column (PRC), shown in Figure 3.61, consists of a feed f
furnace and a distillation tower. The tower has two pumparound circuits, a partial condenser and three side strippers. The furnace (single stage lash type) operates at
25 psia and provides a fractional overflash of 40% (StdVol basis) in the tower. The outlet stream of the furnace goes to the tower on Stage 22. The tower has 26 stages f
f
with a Murphree stage eficiency equal to 90%. A steam stream, STEAM, is introduced at the bottom of the ractionator (26th stage with on-stage convention). There are another three steam streams, STM1, STM2 and STM3, used in the side strippers. The condenser
runs at 15.7 psia with a pressure drop of 5 psi. The tower pressure drop is equal to 4 psi. The distillate rate is 10000 bbl/day and the distillate vapour fraction in the condenser is 0.2 (StdVol basis).
Copyrighted malarial
ASPEN PLUS
SIMULATION OF EHSTILLATIQM MODELS
149
<
LIGHTS
WATER
STMl sir,STM2
BOT
A lowsheet of a petroleum refining column. f
FIGURE 3.61
f
A hydrocarbon mixture with the following component-wise low rates enters the furnace at 1170F and 44.7 psia (see Table 3.3). TABLE 3.3
Flow rate (bbl/day)
Component
Ci c2 C3
3 65 575
i-C4
1820
«-c4
7500
i-C5
30000
n-C5
42000
H2O
250
In Table 3.4, two pumparound circuits and three side strippers are specified. TABLE 3.4
Loeatum
Pumparound (drawoff type)
Draw stage
1 (partial) 2 (partial)
Specifications
Return stage
Flow rate
Heat duty
(bbl/day)
(MMBtu/hr)
8
6
49000
1
12
1000
40 (for cooling) 17 (for cooling)
-
-
Location
Stripper
No. of
Stripper
Draw
Return
Stripping
stages
product
stage
stage
steam
Bottom product flow rate (bbl/day)
1
5
SID1
6
5
STMl
11000
2
4
SID2
12
11
STM2
15000
3
3
SID3
19
18
STM3
8000
1
150
PROCESS SIMULATION AND CONTROL USING ASPEN
Four steam streams used in the column model are described in Table 3.5. TABLE 3.5
Specifications
1
Steam stream
Location
Temperature (0F)
Pressure (psia)
Flow rate (lb/hr)
STEAM
Main tower
350
50
11500
STM1
SID1 stripper
350
50
4000
STM2
SID2 stripper
350
50
1500
STM3
SID3 stripper
350
50
1000
Considering the 'BK10' base method under 'REFINERT process type, simulate the f
PetroFrac column and report the low rates (bbl/day) of all product streams. Simulation approach
Select Aspen Plus User Interface. When the Aspen Plus Startup dialog appears choose Template and click on OK (see Figure 3.62).
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FIGURE 3.62
As the next window pops up (see Figure 3.63), select Petroleum with English Units
and press OK knob
.
,
ASPEN PLUS SIMULATION OF DISTILLATION MODELS
mm
151
'Mm. 1
linn
v-
.
..
L
EJ J-J
-
.
.
..
FIGURE 3.63
Click OK when the Connect to Engine dialog appears. The next screen presents a f
blank process lowsheet. Creating flowsheet
Select the Columns tab from the Model Library toolbar. As we expand the PetroFrac block icon, a variety of models is displayed as shown in Figure 3.64. Select a model icon and press Fl to know more about that.
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FIGURE 3.64
-
152
PROCKSS SIMULATION AND CONTROL USING ASPEN
As the distillation tower described in the problem statement, it is appropriate to choose CDUIOF PetroFrac model. Then place it in the flowsheet window. Adding all
incoming and outgoing streams and renaming the streams as well as block, the process f
low diagram takes the shape as shown in Figure 3.65.
L'Mi-
111" A
KM*
1W
mm m
31*
1Mb
-jM«.
FIGURE 3.65
Configuring settings Click Next to continue the simulation (see Figure 3.66). In the Title field, enter Simulation of a Petroleum Refining Column'. Open the Accounting sheet keeping untouched the other global defaults set by Aspen Plus. '
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FIGURE 3.69
Specifying stream information
Next the Streams /FEED /Input/Specifications sheet appears with the Data Browser menu tree in the left pane Entering the feed data, Figure 3.70 is obtained. .
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ASPEN PLUS
SIMULATION OF DISTILLATION MODELS
155
f
As we hit Next icon, an input form for Stream STEAM opens up. After illing out, it
looks like Figure 3.71
.
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FIGURE 3.71
In Figures 3,72(a) (b) and (c), three illed input forms are shown for STM1, STM2
f
,
and STM3 streams
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FIGURE 3.72(a)
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156
PROCKSS SIMULATION AND CONTROL USING ASPEN
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tj
The three windows shown in Figure 3.79(a), (b) and (c), specify the side strippers d on the given input data. ,
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FIGURE 3.79(c)
Although the Status bar says Required Input Complete, we have to specify the two pumparound circuits connected with the main fractionator. Select Blocks/PRC/ Pumparounds in the list on the left. Click on New as the object manager appears. We may accept the default Pumparound ID T-l Then specify the first pumparound circuit (see Figure 3.80). '
.
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162
PROCESS SIMULATION AND CONTHOL USING ASI'KN
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Select again Blocks/PRC/Pumparounds to reopen the pumparounds object manager. By the same way, ill out the form for second pumparound circuit, shown in Figure 3.81. |»'|H|
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Running the simulation Hit Next icon and click OiTto run the simulation
.
The Control Panel window is presented
in Figure 3.82.
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ASPEN PLUS
SIMULATION OF DISTILLATION MODELS
163
Viewing results
f
From the Data Browser, choose Results Summary /Streams and obtain the table, shown in Figure 3.83, that includes the low rates (bbl/day) of all product streams. Save the work done. l.;,ft-i
-
o-
FIGURE 3.83
To obtain the input information (see Figure 3.84), select Input Summary from the View pulldown menu.
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Copyrighted material
164 34 .
PROCESS SIMULATION AND CONTROL USING
ASPEN
SIMULATION AND ANALYSIS OF AN ABSORPTION COLUMN
Problem statement
A hydrocarbon vapour enters an absorption column below the bottom stage and the absorbent enters above the top stage. The column operates at 75 psia with no pressure
drop and it has four equilibrium stages. The absorber is specified in Figure 3.85. GAS-PDT
Absorbent
Pure n-C10 Temperature = 90oF Pressure = 75 psia Flow rate = 1000 Ibmol/hr ABSORBENT
oAo-rttu
Gas Feed
Temperature = 90oF
Pressure = 75 psia Component
Flow rate
(Ibmol/hr)
LIQ-PDT
280
c2
150
C3
240
n-C
170
4
n-C
150
5
FIGURE 3.85 A Tlowsheet of an absorption column.
Apply the Peng-Robinson equation of state
model in the simulation.
(a) Simulate the absorber model (ABSBR2 under RadFrac) and compute the product compositions.
f
(b) Perform the sensitivity analysis by examining the effect of absorbent low rate on the exiting C3 concentration in the top product (c) Compute the absorbent low rate to keep 15 mole% of C, in the gas product (GAS-PDT). 3
f
.
Simulation approach
(a) Double-click Aspen Plus User Interface icon on the desktop. When Aspen Plus window pops up, select General with English Units Template as shown m Figures 3.86(a) and (b).
ASPEN PLUS
;
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,
SIMULATION OF DISTILLATION MODELS
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FIGURE 3.92
169
170
PROCESS SIMULATION AND CONTROL USING ASPEN
Select the Streams tab to specify stream location. Under Convention, there are two
feeding options: On-Stage and Above-Stage. In the present problem, the top stage is the first stage and the bottom stage is the fourth one. Therefore the absorbent is fed above Stage 1 and the gas feed is introduced above Stage 5 (see Figure 3.93). -
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FIGURE 3.93
In the next step (see Figure 3 94), select Pressure tab to specify the pressure profile across the absorption column In this case, the column is operated isobarically at 75 psia. Under Top stage / Condenser pressure enter 75 psi. Aspen software assumes that the column operates isobarically if no additional information is provided. .
.
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ASPEN PLUS"' SIMULATION OF DISTILLATION
MODELS
171
Hit Next foUowed by OK to run the simulation. The Control Panel window is
shown in
Running the simulation
Figure 3.95. Ft.
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FIGURE 3.95
Viewing results
Choosing Results Summary /Streams in the left pane of the Data Browser window get the results as shown in Figure 3.96.
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FIGURE 3.101
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\SPEN PLUS
SIMULATION OF DISTIUJVTION MODELS
175
From the Data Browser, select Model Analysis Tools/Sensitivity/C3/Results to
display the tabulated data (see Figure 3.102).
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.
-
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.--
FIGURE 3.102
In order to represent the results graphically, highlight a column in the table and select X-Axis Variable (Ctrl + Alt + X) from the Plot pulldown menu. By the similar way. select Y Axis Variable (Ctrl + Alt + Y) for the next column. Then select Display Plot (Ctrl + Alt + P) from the Plot menu and obtain Figure 3.103.
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J FIGURE 3 103
176
PROCKSS SIMULATION AND CONTROL USING ASPEN
(c) In the left pane of the Data Browser window {see Figure 3.104), open Flowsheeting Options folder and then select Design Spec. We need to provide
this design spec a name in the same manner that we did for the sensitivity analysis. Press New, enter DSC3* and click on OK. '
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FIGURE 3.104
Select ihTettJ under Define tab. Then enter 'CS' as a variable name and press OK. In the next step (see Figure 3.105), the following information are required to input: Type: Mole-Frac Stream: GAS-PDT
Substream: MIXED
Component: C3
v . .
K-MJ-B-M-iM-fFIGURE 3.105
Copyrlghiod material
ASPEN PLUS1M SIMULATION OF DISTILLATION MODELS
177
In the subsequent step (see Figure 3.106), select the Spec tab. Design specification data are noted below:
Spec: C3 Target: 0.15 Tolerance: 0.001
DSTVU
Dag
R»Jf>:
Erftaci
M frac
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PihoF-JC
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FIGURE 3.106
Finally (see Figure 3 107), select the Vary tab and enter the following information: .
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FIGURE 3.107
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178
PROCESS SIMULATION AND CONTROL USING ASPKN"
Type: Mole-Flow Stream: ABSORBEN
Substream: MIXED
Component: NC10
Manipulated variable limits Lower: 500
Upper: 1500
As we run the simulation, we get the screen, shown in Figure 3.108. Tw, fu.
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FIGURE 3.114
SUMMARY AND CONCLUSIONS | At the beginning of this chapter a brief of all built-in column models of Aspen software has been presented Several separating columns, including a petroleum refining column and an absorber have been simulated using Aspen Plus. The process optimization has ,
.
,
also been discussed with an example. The present study covers both the binary as well as multicomponent systems Interested readers may try to simulate the models given .
in the exercise
.
182
PROCESS SIMULATION AND CONTROL USING ASPKN
PROBLEMS| 3
.
1 A feed mixture, consisting of 60 mole% ethanol and 40 niole% water, is to be
f
f
separated by using a DSTWU model having a low rate of 100 kmol/hr at 40oC and 1 atm so as to recover at least 85% of the light key component in the liquid distillate and 80% of the heavy key component in the bottoms. The column operates at 1 atm with no pressure drop throughout. In the simulation, consider the reflux ratio of 1.5 and a total condenser. Applying the Wilson property method, simulate the column and ind out the minimum number of stages, actual number of stages, and feed position. .
2 A feed stream, consisting of 50 mole% ethane and 50 mole% ethylene. enters a Distl column having a low rate of 200 Ibmol/hr at 750F and 15 psia. This separator f
3
runs at 300 psia with no tray-to-tray pressure drop. The pressure in the reboiler as well as condenser is also 300 psia. The feed enters the model at 6th stage and the column has total 15 theoretical stages (including condenser and reboiler) and a total condenser. If the reflux ratio is 7 and the distillate to feed ratio is 08
compute the mole fraction of ethane in both the product streams with applying the RK-Soave equation of state model. 3 3 A feed mixture specified in Figure 3.115 is to be distilled by a rigorous RadFrac model (FRACT2). The column consists of total 24 equilibrium stages (including condenser and reboiler) with a stage pressure drop of 2 kPa. Consider the .
.
.
condenser (total) pressure of 125 kPa and the top stage (Stage no. 2) pressure of f
130 kPa. The distillate low rate is 120 kmol/hr and the reflux ratio (mole basis)
is 2. A side product (vapour) is withdrawn from 14th stage. Applying the SoaveRedlich-Kwong (SRK) property method, simulate the column model and report the product compositions. Feed
Temperature = 110nF Pressure = 175 kPa
Dj
O
Sj
cC.
B\
$
Feed stage = 10 (above stage) Component benzene
Flow rate (Ibmol/hr) 250
toluene
80
diphenyl
10 FIGURE 3.115
.
4 A reboiled stripper is to be employed to remove mainly propane and lighter
components from a feed stream, shown in Figure 3.116. It has total 6 stages (including condenser and reboiler) and no condenser. The bottoms rate is 100 Ibmol/hr and the column pressure is 150 psia throughout. Using the PengRobinson thermodynamic method, simulate the RadFrac model (STRIP2) and ind out the product compositions.
f
3
A flowsheet of a distillation column.
Copyrighted material
ASPEN PLUS
SIMULATION OF DISTILLATION MODELS
183
Feed
D -
Temperature = 40oF Pressure = 300 psia
Feed stage = 1 (above stage) Component
Flow rate
(Ibmol/hr) c,
60
c2 c3
150
75
n-C
4
175
n-C5
60
n-C s
35
FIGURE 3.116 3
.
A flowsheet of a stripping column.
5 A feed mixture of cyclopentane and cyclohexane is to be separated employing a liquid-liquid extraction unit at 250C and 1 atm with the use of methanol as a
f
solvent. The schematic diagram of the process with feed specifications is given in Figure 3.17. The process unit, having toted ive stages, is operated adiabatically. Applying the UNIQUAC property method, simulate the extraction model (ICON1) and note down the product compositions. Feed
Temperature = 30oC Pressure = 1 atm
Feed stage =
1 Flow rate
Component
(Ibmol/hr) 250
cyclopentane cyclohexane
750
EXTRACT
FEED
Solvent
SOLVENT'
-
RAFFINAT
,
Temperature - 30DC Pressure = 1 atm
Feed stage = Component
5
Flow rate
(Ibmol/hr) 1000
FIGURE 3.117 .
6 A gas consisting of 40 mole% ammonia, 60 mole% air at 20CC, 25 psia, flowing at the rate 120 kmol/hr, is to be scrubbed counter-currently with water (pure) entering at 60oC and 30 psia at a rate 100 kmol/hr. The column operates at 1 atm throughout
and it has four stages. Using the UNIFAC thermodynamic model, (a) simulate the RadFrac absorber (ABSBR2) and determine the exiting ammonia concentration in
the gas product, (b) Perform the sensitivity analysis by examining the effect of absorbent low rate on the exiting ammonia concentration in the top product. f
3
A lowsheet of an extraction column. f
methanol
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184
7 An artificial petroleum refining column (PRC) shown in Figure 3.118 consists of a feed furnace and a fractionation tower. The tower includes one pumparound circuit, a partial condenser and one side stripper. The furnace (single stage flash type) operates at 20 psia and provides a fractional overflash of 50% (StdVol basis) in the tower. The outlet stream of the furnace enters the tower on stage 18
.
.
The column has total 20 stages. A steam stream, STEAM, is fed at the bottom of the fractionator (20th stage with on-stage convention). There is another steam stream, STEM1, used in the side stripper. The condenser runs at 15 psia with a
pressure drop of 5 psi. The tower pressure drop is equal to 5 psi. The distillate rate is 12000 bbl/day and the distillate vapour fraction in the condenser is 0.25 (StdVol basis). The liquid product, SID1, is withdrawn from 5th stage with a flow rate of 2000 bbl/day.
A hydrocarbon mixture with the given component-wise low rates (Table 3.6) f
3
PROCESS SIMULATION AND CONTROL USING ASPEN
enters the furnace at 120oF and 45 psia.
LIGHTS
WATER
IS
o
-
SID1
STEM1 -O FEED
STEM
FIGURE 3.118 A flowsheet of a petroleum refining column TABLE 3.6
Component
Flow rate (bbl/day)
c2
100
10
C
3
600 1800
n-C
4
7500 30000
1-0,
42000
nrCt
250
H 0
250
2
SID2
C>
BOT
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ASHEN PLUS
SIMULATION OF DISTILLATION MODELS
185
The pomp around circuit (for cooling) and the side stnpper are specified with the following information (see Table 3.7). TABLE 3.7
Location
Specifications i
Pumparound
Draw
Return
Flaw rate
Temperature
idrauoff type)
stage
stage
(bbl/day)
feF,
8
6
40000
20
I (partial)
Location
Stnpper
Stnpper product
Draw
Return
Stripping
stages
stage
stage
steam
5
SID1
12
10
STEM1
No. of
1
Bottom product flow rate (bbl/day; 15000
Two steam streams, used in the column model, are described in Table 3.8. TABLE 3
Specifications Steam stream
Location
STEAM
STEM!
Temperature (8F)
Pressure (psia)
Flow rate Ob/hr)
Main tower
350
50
12000
Stnpper
350
50
5000
Selecting the PENG-ROB base method under RE FINE RV process type simulate the model using a PetroFrac column and report the flow rates (bbl/day > of all ,
product streams.
This Thispage pageintentionally intentionallyleft leftblank blank
Rs. 295.00
PROCESS SIMULATION AND CONTROL USING ASPEN
Amiya K. Jana
@ 2009 by PHI Learning Pnvate Limited, New Delhi. All rights reserved. No part of this book may be reproduced In any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-978-81-203-3659-9
The export rights of this book are vested solely with the publisher.
Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Jay Print Pack Private Limited, New Delhi-110015.
Part II Chemical Plant Simulation
using Aspen Plus
This Thispage pageintentionally intentionallyleft leftblank blank
Rs. 295.00
PROCESS SIMULATION AND CONTROL USING ASPEN
Amiya K. Jana
@ 2009 by PHI Learning Pnvate Limited, New Delhi. All rights reserved. No part of this book may be reproduced In any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-978-81-203-3659-9
The export rights of this book are vested solely with the publisher.
Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Jay Print Pack Private Limited, New Delhi-110015.
Aspen Plus Simulation of Chemical Plants
4 1 .
INTRODUCTION
In the last three chapters, we have studied in detail the simulation of individual processes, such as flash drum, dryer, chemical reactor, distillation column including petroleum refining process, absorber, stripper and liquid-liquid extraction unit, using
Aspen Plus
software. Here, by a 'chemical plant' we mean a chemical process
f
integrated with several single process units. The chemical process industries usually include flash chamber, mixer, splitter, heat exchanger, pump, compressor, reactor, fractionator, ilter and so on. It is easy to simulate even a large chemical plant by the use of Aspen software package. In the present chapter, the simulation of two chemical process flowsheets is discussed. They are a distillation train and a vinyl chloride monomer (VCM) manufacturing unit. After thoroughly reading this chapter and simulating the solved
examples in hand, we will be able to use Aspen Plus flowsheet simulator for solving a wide variety of chemical plants. To improve the flowsheet simulation skills, it is recommended to solve the problems given in the exercise. 4 2 .
ASPEN PLUS SIMULATION OF A DISTILLATION TRAIN
Problem statement
f
A hydrocarbon stream H is supplied at 50C and 2.5 atm. The pump Pi discharges the feed F at 10 atm. In Table 4.1 the component-wise low rates are tabulated for stream H.
The schematic representation of the complete process integrated with a pump and f
ive DSTWU column models (Cl, C2, C3, C4 and C5) is shown in Figure 4.1.
189
Copyrk
190
PROCESS SIMULATION AND CONTROL USING ASPEN TABLE 4.1 F/ouj rate (kmol/hr)
Component
10 35
50 130 200 180 200 5
n-C
.
pi
C1
C3
cs
C4
A lowsheet of a distillation train. f
FIGURE 4.1
C2
For Aspen Plus simulation of the distillation train, required information are given in Table 4.2. TABLE 4.2 Column
Condenser
Reboiler
(abbreviation)
pressure (aim)
pressure (atm)
Deethanizer (CD
9
9
Depropanizer (C2)
5
6
Deisobutanizer (03) Debutanizer (04)
4
4
3
3
Deisopentanizer (05)
2
2
All distillation models have total 20 theoretical stages (including condenser and reboiler) and a total condenser. For the light key (LK) and heavy key (HK), we expect 99.9% and 0.1% recovery, respectively, in the distillate of all columns. Using the PengRobinson property method, simulate the distillation train and report the compositions
of all distillation products. Simulation approach From the desktop, select Start button followed by Programs, AspenTech, Aspen Engineering Suite, Aspen Plus Version and finally Aspen Plus User Interface. Then
choose Template option in the Aspen Plus Startup dialog (see Figure 4.2).
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As wo hit OK button, the following window appears (sec Figure 13). Based on the units used in the problem statement we select General with Metric Uliits, ,
in-i
.
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i -"Pi
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Specifying block information As we hit Next button the block input form appears. The deethanizer column is specified ,
with the given data as shown in Figure 4.15. arn
HUN
,«*«rJKmvwa
u
FIGURE 4 16(d) .
r
Click on Afet and specify the pump (PI) outlet by providing the discharge pressure
of 10 atm (see Figure 4.17).
200
PROCESS SIMULATION AND CONTROL USING ASPEN
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ASPEN PLUS"' SIMULATION OF CHEMICAL PLANTS
201
Notice that if there are no red semicircles in the left it can be said that the data ,
entry for running the Aspen simulator is complete. Running the simulation
As we approve the simulation run, the Control Panel, displayed in Figure 4 19, .
shows
the progress of the flowsheet simulation in addition to a message o[ Results Available
.
.
1
.
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Viewing results
f
Choose Results Summary /Streams in the column at the left side and obtain the compositions of all distillation products as shown in Figure 4.20. We may save the work by choosing File/Save As/...using the menu list on the top. W< tan give a name of the ile whatever we like. Note that if we click on Stream Table,
the results summary table is incorporated in the Process Flowsheet Window, as shown in Figure 4 21. .
Viewing input summary
If we wish to have the systematic input information, press Ctrl + Alt + I on the keyboard or select Input Summary from the View pulldown menu (see Figure 4*22). In order to create a report file (*.rep) for the present problem, we may follow the approach presented in Chapter 1 It is worthy to mention that the report file contains all necessary information on the solved Aspen Plus problem including given process .
,
data and computed results
202
PROCESS SIMULATION AND CONTROL USING ASPEN
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PROCESS SIMULATION AND CONTROL USING ASPEN 1 M
Running the simulation
As we press Next button, Aspen Plus displays a message as shown in Figure 4.32. Since the data entry is fully complete, the simulator seeks user permission to run the program. I >! |h| ] v| gj
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In the subsequent step (see Figure 5.8), select Stream sheet with opening the Report Options form under Setup folder and include Mole fraction item. E*
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The forward reaction as well as the backward reaction is represented with their stoichiometric coeficients and exponents in two sheets shown in Figures 5.14(a) and (b). f
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246
PROCESS SIMULATION AND CONTROL USING ASPEN"1
f
Alternatively, to obtain the Configure dialog box, irst double-click on the controller block LCI and then click on Configure symbol (yellow colour) in the faceplate as shown in Figure 5.26. I
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By the similar way, we obtain the tuning data sheet, shown in Figure 5.27, for the temperature controller TC2.
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Dynamics window
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4- PROCESS SIMULATION AND CONTROL USING ASPEN
First make sure that all the items in the Configure dialog box and faceplate are correct. In order to execute the dynamic closed-loop simulation click on Run button in ,
the toolbar. During the simulation run, give a step change in the set point value
of
reactor liquid level from 0.914029 to 1.1 metre at time = 1 5 hours. Typing the new set .
point value in the faceplate, press Enter button on the keyboard so that the Operator set point value in the Configure dialog box also changes automatically to 1 1 .
meter
Note that the new set point must be within the specified ranges of PV In Figure 5 36 the servo performance of the level controller is depicted for 5 hours as selected earlier Obviously, the plot also includes the manipulated input profile ,
.
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De 70oC at time = 3 hours). .
254
PROCESS SIMULATION AND CONTROL USING AS PEN
Clearly, the proportional integral controller with default tuning parameters values shows a high-quality temperature tracking performance. As stated if the performance of any controller is not satisfactory, we have the option to tune the parameters simply by trial-and-error method. If we introduce a set point change in the reactor temperature the TC2 controller ,
,
takes necessary action with adjusting the heat duty to compensate for the changes
.
But interestingly, the liquid level remains undisturbed. Figure 5.38 confirms this fact
.
At this point we can conclude that loop 1 affects loop 2, but loop 2 does not affect loop 1 Actually here the interaction is in a single direction. (d) Viewing regulatory performance of LCI and TC2:
.
To perform the
regulatory study, we need to introduce at least a single change in the input disturbance. However, here we consider two subsequent step changes in the feed temperature. Initially, the feed temperature changes from 75 to 80oC at time = 2 hours and then the temperature (80oC) returns to 750C after 1.2 hours To change the feed temperature twice as prescribed above, irst we need to open the feed data sheet by double-clicking on the FEED block in the process flowsheet (see Figure 5.39).
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255
DYNAMICS AND CONTROL OF FLOW-DRIVEN PROCESSES in the feed data sheet. For brevity, the faceplate and configure dialog box included in the Aspen Dynamics window, shown in Figure 5.40. 2
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It is obvious that the reactor liquid level remains unchanged with a change in feed temperature since there is no interaction involved On the other hand, the reactor temperature is disturbed However the TC2 controller provides satisfactory disturbance .
.
,
rejection performance under this situation. So far we have studied mainly the closed-loop behaviour of a reactor system coupled
with Aspen-generated control schemes. We did not include any additional controller with the CSTR model In Section 5.3 we consider a distillation example to elaborate this point. .
,
5 3 DYNAMICS AND CONTROL OF A BINARY DISTILLATION COLUMN Problem statement
A partially vaporized binary mixture of benzene and toluene enters a RadFrac distillation model as displayed in Figure 5 41. .
he column has total 25 theoretical stages (including condenser and reboiler) and
operates at a pressure in the reflux drum of 18 psia and reboiler of 21 psia. The ow rate is 285 Ibmol/hr and reflux ratio is 2 2 (mole basis) .
.
256
PROCESS SIMULATION AND CONTROL USING ASPEN Feed Specifications o
TOP
Flow rate = 600 Ibmol/hr
Temperature = 225° F
Pressure = 21 psia Feed stage = 13 (above stage)
FEED
Mole %
Component benzene
45
toluene
55
FIGURE 5.41
BOTTOM
A flowsheet of a distillation column
.
In Table 5.1, the reflux drum and the base of the column (the 'sump' in Aspen terminology) are specified. It is fair to use an aspect ratio (length to diameter ratio) of 2 (Luyben, 2004). TABLE 5.1 Item
Vessel type
Head type
Height / Length (ft)
Reflux drum
horizontal
elliptical elliptical
5
25
5
25
Sump
-
Diameter (ft) .
.
The column diameter is 5 ft. Use default values for other tray hydraulic parameters (e.g., tray spacing, weir height and weir length to column diameter ratio). Consider logmean temperature difference (LMTD) assumptions for the total condenser. Actually the LMTD is calculated using the temperatures of process luid and coolant In the simulation. assume constant reboiler heat duty and apply the UNIFAC base property method. ,
f
.
Simulate the column model to obtain the products mole fractions. Keeping the default level and pressure control algorithms unaltered, inspect the servo as well as regulatory performance of a proportional integral (PI1 controller that is required to insert to control the benzene composition in the distillate by manipulating the reflux low rate. (0 Devising an another PI control scheme to maintain the benzene composition in the bottom product with the adjustment of heat input to the reboiler, observe the interaction effect between the top and bottom composition loops. f
(a) (b)
Simulation approach
(a) Select Aspen Plus User Interface and when the Aspen Plus window pops up. choose Template and press OK. In the subsequent step, select General with
f
English Units and hit OK button. To open the process lowsheet window, click OK when the Aspen Plus engine window appears.
Creating flowsheet
From the Model Library toolbar, select the Columns tab. Place the RadFrac model on f
the lowsheet window and add the feed as well as two product streams. Renaming all the streams along with distillation block, we have Figure 5.42.
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259
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.
,
scrolling down (see Figure 5.47). Specifying stream Information ,"
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lefl pane ol the Data Browser window select Blocks IRADFRACI Setup to open Configuration sheet and then insorl the required datn (see Figure 5.49) ,
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Open a blank dynamic simulation window for the example column, following a similar procedure as previously shown for the CSTR problem. In the next, simply open the flow-driven dynamic ile 'Ch5 5 3 RadFrac.dynf. As a result, the Aspen Dynamics window appears (see Figure 5.57) accompanying with the closed-loop process low f
.
_
f
_
diagram. The flowsheet actually includes the three default control schemes LCI, PC2 and LC3 to monitor the reflux drum liquid level, top stage pressure and column base liquid level, respectively.
In the present discussion, we do not want to change anything of the three automatically inserted control strategies. All data, including timing parameters, ranges,
bias values and controller actions, remain untouched. A little detail of these control structures is given below. Loop 1 Controller: LCI
Type of controUer: P-only (since integral time is very large (60000 minutes)) f
Controlled variable: liquid level in the reflux drum Manipulated variable: distillate low rate Controller action: direct
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DYNAMICS AND CONTROL OF FLOW-DRIVEN PROCESSES
273
Obviously, some of the default values set by Aspen Dynamics are not acceptable. For example, the operator set point value of process variable (benzene composition in distillate) should not be greater than L Secondly, the CCT controller action must be
Reverse'. In addition, the value of control variable (reflux flow rate) at steady state is
'
usually used as
bias value.
We have two options in our hand to correct the default values. Either manually we can do it or Aspen Dynamics can automatically initialize the values of set point process variable, control variable, bias and ranges. Note that the controller action is ,
changed only manually. It is wise to initialize the values by the help of Aspen Dynamics For this, press Initialize Values button in the Configure dialog box and use 'Reverse' .
controller action. It is obvious in the window, shown in Figure 5.67, that the values of SP, PV and OP in the faceplate change automatically to their steady state values. If
this approach fails to initialize the simulation of controller model with the steady state data, check and replace, if necessary, the values of PV and OP with their steady state values by double clicking on signal transmission lines (input to the controller and output from the controller).
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PROCESS SIMULATION AND CONTROL USING ASPEN
In addition, the used constraints are reported below: Process variable
Range minimum: 0.0 Range maximum: 0.1 Output
Range minimum: 6000000 Btu/hr Range maximum: 18000000 Btu/hr Viewing interaction effect between two composition loops
To observe the effect of interaction between two composition loops, the set point value of bottoms composition of benzene has been changed twice. The simulation result is depicted in Figure 5.74 for a step increase (0.0033 -> 0.0045 at time = 1.5 hours) followed by a step decrease (0.0045 -> 0.0025 at time = 3 hours). EC
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Clearly, the CCB controller shows satisfactory set point tracking performance against a pulse input change It is observed rom Figure 5.74 that owing to strong f
.
interaction between the two composition loops of the distillation column the set point changes in bottom loop affect the top product composition Similarly, when any set point change is introduced in the top composition loop, the bottom product composition ,
.
will also be affected.
DYNAMICS IND CONTROL OF FLOW-DRIN KN I'HOrKSSKS
279
SUMMARY AND CONCLUSIONS | This chapter has investigated the closed-loop process dynamic characteristics using
Aspen Dynamic- package. To observ e the controller performance in terms of set point tracking and disturbance rejection, a CSTR in addition to a distillation column have been illustrated The default control strategies have been tested for the reactor example, whereas the two additional composition control loops have been included along with the default control laws for the distillation example. Several simulation experiments have been executed for both the processes under flow-driven dynamic simulation. Note
that Chapter 6 presents the dynamic simulation and control of more rigorous pressuredriven dynamic process.
PROBLEMS| 5 1 A feed mixture of benzene and toluene is fed to a flash drum (Flash2). The .
separator operates at 1.2 atm and 100oC For dynamic simulation, required feed specifications are provided in Figure 5.75. .
Feed
Temperature = 25°C Pressure = 3 bar FLASH
Flow rate = 100 kmol/hr
Component
>o PI
Mole fraction
benzene
06
toluene
04
.
.
FIGURE 5.75 '
A flowsheet of a flash drum.
a) Use the SYSOP0 property method to compute the amounts of liquid and vapour products and their compositions
.
.
b) As shown in Figure 5 75, employ a PI control scheme to monitor the .
temperature in the flash drum by manipulating the heat duty
.
(c) Show the closed-loop servo performance with +10% and then -10% step changes in the flash temperature
.
(d) Report the tuning parameters obtained by trial-and-error method, controller action and ranges imposed 2 A vapour mixture of toluene, methane and hydrogen is heated using a shell and tube heat exchanger (HeatX) The superheated steam is used as a heating medium. Complete specifications required for closed-loop dynamic simulation are shown in .
5
.
.
Figure 5 76. .
280
PROCRSS SIMULATION AND CONTROL USING ASPEN Hot Stream Out
Pressure = 14 psia
Cold Stream In
I
Temperature = 2780F Pressure = 500 psia
HOT-OUT
Cold Stream Out Flow rate
Component
(kmol/hr)
j cold-out hoi Temperature = HOOT
ICOLD-INf
Pressure = 498 psia
200
toluene
Dead time
2300
methane
HOT-INK
1000
hydrogen
pi
>AT
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ii Hot Stream In
Temperature = 1160oF Pressure = 14.7 psia Flow rate = 5110 kmol/hr
FIGURE 5.76
A flowsheet of a heat exchanger.
f
(a) Simulate the heat exchanger model using the shortcut method, countercurrent low direction and NRTL-RK property method. (b) Include a PI control structure to observe the set point (cold stream outlet temperature) tracking performance and the manipulated input (steam inflow rate) profile. In the closed-loop simulation experiment, assume that the temperature sensor takes 1 minute time (dead time) to measure the controlled variable. Report the used tuning properties. (c) Examine the regulatory performance by introducing + 10% and subsequently 10% step changes in the inlet temperature of the cold stream. -
3 Device a cascade control scheme for the above heat exchanger and investigate the controller performance. 5 4 A liquid mixer model with a typical ratio controller (Seborg et al. 2003) is shown in Figure 5.77. The low rates for both the disturbance or wild stream (Fw) and
5
.
f
.
the manipulated stream (FE) are measured, and the measured ratio, R = FE/Fw is calculated. The output of the ratio element is sent to a ratio controller (PI) that compares the calculated ratio Rm to the desired ratio Rd (set point) and adjusts the manipulated low rate accordingly.
f
m
Input 2
>o Ratio
PI
>
Input 1
FIGURE 5.77
A flowsheet of a mixer
| POT >
,
DYNAMICS AND CONTROL OF KLOW-DRIVKN PROCESSES
281
The input data are shown in Table 5.2 for simulation. TABLE 5.2 Stream
Temperature CO
Pressure (aim)
E
50
1
W
60
1
Flow rate (kmol/hr)
Pe
Composition
= 100
pure ethanol
= 150
pure water
Process variable at steady state = 0.667 (FE/FW = 100/150) Controller output at steady state = 100 kmol/hr Proportional gain = 4 %/% Integral time = 20 minutes Controller action = reverse
(a) Appljang the SYSOPO base property method, simulate the mixer model operated at 1 atm. (b) Using the given controller properties and default ranges, report the ratio controller performance with two consecutive set point step changes (0.667 -> 0.72
Double-click on Input 1 transmission line and ill up Tables 5.3(a) and (b). f
Hint:
0.65) in the ratio.
TABLE 5.3(a) Value *,
Spec
>STREAMS("E ) Fcn("ETHANOL")
100.0
Free
STREAMS("W ) Fcn("WATER")
150.0
Free
o
5
60
8
35
FIGURE 5.78
A flowsheet of a stripping column
.
(a) Using the Peng-Robinson thermodynamic method simulate the RadFrac (STRIP2) model and compute the product compositions. (b) Keeping the default controllers (PCI and LC2) unaltered configure a composition control scheme (PI) coupling with a 'Dead time' and 'Noise elements to maintain the propane mole fraction in the distillate by manipulating the reboiler heat duty as shown in Figure 5 79. Use the given ,
,
'
_
.
closed-loop data and execute the dynamic simulations to test the developed composition controller performance
.
5
.
6 Ethylene is produced by cracking of ethane in a stoichiometric reactor. The irreversible elementary vapour-phase reaction is given as .
C2H6 -i C2H4 + H2 ethane
ethylene
hydrogen
shown in Figure 5.79, with a low 5 atm. The reactor operates at inlet
Pure ethane feed enters the reactor model rate of 750 kmol/hr at 800oC and 5
,
f
5
PROCESS SIMULATION AND CONTROL USING ASI'RN""
.
temperature and pressure with 80% conversion of ethane
.
DYNAMICS AND CONTROL OF FTOW-DRIVKN PROCESSES
283
pi
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A lowsheet of a reactor f
FIGURE 5.79
f
U) Using the SYSOPO thermodynamic method, simulate the reactor model. (b) Develop a control loop as configured in the low diagram to maintain the desired reactor temperature by the adjustment of heat duty. Considering the measurement lag of 1 minute, inspect the servo as well as regulatory control performance. Report the tuning properties used to achieve a satisfactory closed-loop performance. 7 A binary feed mixture consisting of methylcyclohexane fMCH) and toluene is introduced above tray number 14 of a RadFrac distillation model, shown in Figure 5.80.
O
1 phenol [
O
1 FEED h
FIGURE 5.80
A lowsheet of a distillation column f
.
It is dificult to separate this close-boiling system (MCH-toluene) by simple binary distillation Therefore, phenol is used as an extractant and introduced above tray number 7 of the column The two input streams have the following f
5
.
.
specifications shown in Table 5.5. ,
TABLE 5.5
Stream
Temperature (*C)
PHENOL FEED
Pressure 'bar)
Flow rate
Mole fraction
105
14
100 nrVhr
10
105
14
181.44 kmol/hr
0 5/0.5
.
.
.
.
(MCH/toluene)
The column has 22 theoretical stages (including condenser and reboiler) with a total condenser The distillate rate and reflux ratio are given as 90.72 kmol/hr and 8 (mole basisrespectively. The pressure profile is defined with Stage 1 .
pressure of 1 10316 bar and Stage 22 pressure of 1 39274 .
bar. Use LMTD
assumptions for the condenser The reboiler heat duty is assumed constant. The reflux drum and sump are specified in Table 5.6.
284
PROCESS SIMULATION AND CONTROL USING ASPEN TABLE 5.6 Item
Reflux drum
Sump
Head type
Vessel type horizontal -
Height /Length (m)
Diameter (m)
elliptical
15
0 75
elliptical
15
0 75
.
.
.
.
The column diameter and tray spacing are given as 2 m and 0.6 m respectively, ,
(a) Simulate the distillation column using the UNIFAC property method to compute the composition of MCH in the distillate and that of phenol in the bottom product. (b) In addition to the default level and pressure controllers, insert a PID structure to control the MCH composition in the top product by manipulating the flow rate of PHENOL stream.
(c) Produce the plots to show the closed-loop control responses, and report the tuning parameters, control actions and operating ranges for controlled as well as manipulated variables used.
REFERENCES| Luyben, W.L., (2004), "Use of Dynamic Simulation to Converge Complex Process Flowsheets", Chemical Engineering Education, pp. 142-149. Seborg, D.E., T.F. Edgar and D.A. Mellichamp, (2003), Process Dynamics and Control, 2nd ed., John Wiley & Sons, Inc.
CHAPTER
6
Dynamics and Control of Pressure-driven Processes
61 .
INTRODUCTION
To know the transient characteristics of a complicated chemical plant, we need a dynamic
process simulator. It is well-recognized that Aspen Dynamics is such an efficient flowsheet simulator used for dynamic process simulation. As we have seen in Chapter 5,
Aspen Dynamics simulator can be employed to design a process as well as its associated control strategies.
Aspen Dynamics extends an Aspen Plus
steady-state model into a dynamic process
model. If the steady state Aspen Plus simulation is exported to Aspen Dynamics, there is a necessity to choose either flow-driven dynamic simulation or pressure-driven dynamic simulation In a rigorous pressure-driven simulation, pumps and compressors are inserted where needed, to provide the required pressure drop for material low. Control valves are installed where needed, and their pressure drops selected. For good .
f
,
,
control
,
the pressure drop across a control valve should be greater than 0.1 bar. The
f
f
luid that lows through a valve should normally be liquid-only or vapour-only because the two-phase low through a control valve is unusual f
.
It should be pointed out that for a pressure-driven case, we must not insert a valve in the suction of a pump or at the discharge of a compressor (compressor speed or its equivalent compressor work is manipulated). The control valves are positioned on the
f
luid streams such that the controllers can manipulate the valve positions.
f
The simple low-driven dynamic simulations have been discussed in detail in the
previous chapter. Therefore simulation
.
,
here we are intended to study the pressure-driven
A reactive or catalytic distillation column is exampled for the rigorous
pressure-driven Aspen Dynamics simulation as well as control.
285
286 6
.
2
PROCESS SIMULATION AND CONTROL USING ASPEN
DYNAMICS AND CONTROL OF A REACTIVE DISTILLATION (RD) COLUMN
Problem statement
The methyl tertiary butyl ether (MTBE) column configuration (Jacobs and Krishna, 1993) chosen for the simulation is shown in Figure 6.1. Pure methanol (MeOH) feed (liquid) Temperature = 320 K Pressure = 1 aim Flow rate = 711.30 kmol/hr
Feed stage = 10 (above-stage) 0| METHANOL
Jy
Ft
tCl-fpisT-Q CV2
PUMP
Butenes feed (vapour)
CH butenes]-1 "
Temperature = 350 K
1-H l-IbotI- '
CV3
Pressure = 1 aim Flow rale = 1965.18 kmol/hr
RDCOLUUN
Feed stage = 11 (above-stage) COMPRESS
Mol fracl
Component /so-butene (IB)
0 36
-butene (NB)
0 64
.
.
FIGURE 6.1
A lowsheet for the production of MTBE. f
n
The RD column (RadFrac) consists of 17 theoretical stages, including a total condenser and a partial reboiler. Reactive stages are located in the middle of the column, Stage 4 down to and including Stage 11. In Aspen terminology, the numbering of the stages is top downward; the condenser is Stage 1 and the reboiler is the last stage. MTBE is produced by reaction of IB and MeOH: (CH3)2C = CHa + CH3OH «-»(CHgk COCH3 IB
MeOH
MTBE
The liquid-phase reaction is catalyzed by a strong acidic macroreticular ion exchange
resin, for example Amberlyst 15. and n-butene does not take part in the reaction (inert). The forward and backward rate laws (Seader and Henley, 1998; Rehfinger and Hoffmann, 1990) are derived in terms of mole fractions, instead of activities (products of activity coefficient and mole fraction): '
Forward rate: rf= 3 67 x 1012 exp
9244(M
-
.
IB
.
Backward rate: r,, = 2 67 .
x 1017 exp
RT -
134454> RT
xMeOH , VMTB!'
DYNAMICS AND CONTROL OF PRESSURE-DRIVEN PROCESSES
287
Here, z represents the liquid-phase mole fraction. The pre-exponential factors, including the activation energy (kJ/kmol), are given in SI units. The catalyst is provided only for reactive stages (8 stages total), with 204.1 kg of catalyst per stage (Seader and Henley. 1998). The used catalyst is a strong-acid ion-exchange resin with 4.9 equivalents of acid groups per kg of catalyst. So, the equivalents per stage are 1000 or 8000 for the
8 stages. In some references, the equivalents per stage are directly given.
f
The column, starting from Stages 2 to 16, is packed with 'MELLAPAK' (vendor: SULZER) having a size of 250Y. Use 'Simple packing' hydraulics and the height equivalent to a theoretical plate (HETP) may be considered as 1 m. The distillation column diameter is 6 m. Stage 1 (condenser) pressure is 11 atm with a column pressure drop of 0.5 atm. The reflux ratio is set to 7 (mole basis) and the bottoms low rate is 640.8 kmol/hr. In the MTBE synthesis process, it is desirable to obtain a bottom product
containing high-purity MTBE and a distillate containing high-purity NB. In Table 6.1 the reflux drum and the sump (the next-to-last stage in the column) are specified. TABLE 6.1 Item Reflux drum
Sump
Vessel type
Head type
Height/Length (m)
Diameter (m)
horizontal
elliptical
2
1
elliptical
22
-
.
1 1 .
The pump delivers the liquid stream POUT at 11.7 atm. The compressor (isentropic) has discharged the vapour feed FV at 11.5 atm. The three control valves (adiabatic
f
lash) CV1, CV2 and CV3 have the outlet pressures of 11.5 atm, 10.8 atm and 11.3 atm respectively. Using the UNIFAC base property method, f
(a) simulate the process lowsheet to obtain the distillation product summary, and
(b) develop the control configurations to achieve the desired product purity under disturbance input. Simulation approach
(a) Start the Aspen program by double-clicking the Aspen Plus User Interface icon
on the desktop. Then select Template option and press Oif (see Figure 6.2).
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FIGURE 6.2
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PROCESS SIMULATION AND CONTROL USING ASPEN"
We choose General with Metric Units option and hit OK button (see Figure 6.3). 016*11 l
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When the Connect to Engine window appears, use the default Server type (Local PC). Creating flowsheet
The process low diagram includes a feed pump a feed compressor, a distillation column and three control valves. The complete process lowsheet drawn in an Aspen window should somewhat resemble the one shown in Figure 6 4 Recall that Aspen has a tool in the toolbar that automatically takes the user through the required data input in a
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.
stepwise fashion. The blue Next button does this.
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FIGURE 6.4
.
DYNAMICS AND CONTROL OF PRKSSIIRE-DRI\T N PROCESSES ,
289
Configuring settings
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At the beginning of data entry, ill up Global sheet followed by Accounting sheet under Specifications of Setup folder. Moreover, select 'Mole' fraction along with 'Std.liq.volume' low basis in Stream sheet under Report Options [see Figures 6.5(a)
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Specifying components
The components involved in the example system are MeOH (CH40) IB (C4H8-5), NB (C4H8-1) and MTBE (C5H120-D2). Within the parentheses, the chemical formulas used in Aspen terminology are mentioned (see Figxire 6.6). ,
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DYNAMICS AND CONTROL OF PRESSURE-DRIVEN PROCESSES
291
Specifying property method
The user input under the Properties tab is probably the most critical input required to run a successful simulation. This has been discussed in much greater detail in the previous chapters. This key input is the Base method found in Global sheet under Specifications option. Set UNIFAC for the present project (see Figure 6 7). .
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Under the Streams tab, we have used Specifications sheets to input the data for both the feed streams, BUTENES and METHANOL [see Figures 6.8(a) and (b)]. «k
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In Figures 6.9(a) to (d), irst the feed compressor details are giver.. Subsequently, the three control valves, CVl, CV2 and CV3, are specified.
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DYNAMICS AND CONTROL OF PRESSURE-DRIVEN PROCESSES
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In the list on the left, choose Blocks I RDCOLUMN I Setup to ill up Configuration sheet (see Figure 6.11).
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The pressure profile of the sample RD column is described in window shown in Figure 6 13. .
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In the next, dick Dynamic tab wader Blocks/RDCOLUMN The design specifications of the reflux drum and sump are reported in Figures 6 17(a) and (b). .
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PROCESS SIMULATION AND CONTROL USINd ASRKN1
Hit Next icon to open the Reactions folder. For the forward reaction (Reaction No 1) and the backward reaction (Reaction No. 2), the stoichiometric coefficients and exponents are defined under Kinetic' Reaction type in the two sheets as shown in .
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Figures 6.19(a) and (b).
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DYNAMICS AND CONTROL OF PRESSURE DRIVEN PROCESSES -
301
The Power law kinetdc data for both the reactions are provided in Figures 6 20(a) and (b). .
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Running steady state simulation and viewing results As we hit Next knob followed by OK, Control Panel window pops up. Under Summary/Streams the results are displayed in Figure 6.21. ,
Results
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FIGURE 6.24
The controller CC4 is tuned by trial-and-error approach and the parameter values have been chosen as:
Proportional gain = 5 %/% Integral time = 5 min Use default values for other items including bias signal, ranges, etc.
f
Notice that by the similar way, we can design the low controller for butene feed of the RD column.
Top section
In addition to the LCI and PC2 control structures, the distillate composition can be
f
controlled by manipulating the reflux low rate. In an alternative approach, along withf the pressure control (PC2), we can control the reflux drum level by the manipulation o f
the reflux rate and the distillate low rate can be adjusted by a ratio control law to give a constant reflux ratio. In the present case, the former control scheme has been incorporated for performance study.
DYNAMICS AND CONTROL OF PRESSURE-DRIVKN PROCESSES
307
Bottom section
In the bottom section of a distillation column, it is a common practice that either the bottom product purity or the tray temperature near the bottom of the column, which has a strong correlation with the product purity, is controlled at its desired value by
the manipulation of the reboiler heat duty. For the sample process, we have implemented a composition control structure for product quality control. As the CC4 control block has been connected, similarly we can incorporate the other control structures discussed above with the distillation flowsheet. The window,
shown in Figure 6.25, includes a closed-loop scheme in which the MTBE purity is controlled in the bottoms by adjusting the reboiler heat input and the methanol impurity in the top is controlled by manipulating the reflux flow rate. As stated earlier, the concentration of methanol on the reactive stage it is being fed to (Stage 10) is measured f
and controlled by the manipulation of the fresh methanol feed rate. The butene flow rate is low-controlled. The liquid levels in the reflux drum and the base of the column
are maintained by the distillate flow rate and the bottoms flow rate, respectively. The condenser heat removal is manipulated to control the column pressure. All of the structures are single-input/single-output (SISO) structures with PI controllers (P-only on levels).
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m
5
JC»r.Lt»*v
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.
uMflr*ti»f nm ?i li 460C at time = 3.9 hours) have been
introduced to examine the performance of the closed-loop RD process. A change in feed temperature affects the internal composition in the reactive zone. This, in turn, may deteriorate the product quality. The system responses to temperature disturbance are illustrated in Figure 6.26. It is obvious that the proposed structure is able to maintain the MTBE purity in the bottoms under the influence of disturbance variable. It can
f
f
also prevent excessive losses of both methanol and iso-butene in the products. Each Aspen Dynamics model includes different plots and tables from which we can easily access the simulation inputs as well as results. For this, irst highlight a block or stream, then right-click to point Forms and inally select the item that we want to access.
DYNAMICS AND CONTROL OF PRESSURE-DRIVEN PROCESSES
309
EBB
A
-
-
1
1 r»5fl
ill 1
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FIGURE 6.26
Performance of the closed-loop RD process with Measurement lags Aspen Dynamics screen, shown in Figure 6.27 includes three dead time blocks (DTI, ,
DT2 and DT3) connected with three composition controllers (CC4 CC6 and CC7). ,
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4 x-i
-
-
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FIGURE 6.27
310
PROCESS SIMULATION AND CONTROL USING ASPEN
The measurement lag of 15 sec (0 25 min) is used in all composition loops. To incorporate a dead time for a measured variable say methanol mole fraction on Stage 10, highlight DTI block right-click on the block, point to Forms and then select Configure to open the configure table. In the Value cell enter 0.25 min as a sensor dead time. Follow the same approach for other two dead time blocks .
,
,
,
.
or
f
Here, we have used the proportional gain of 1 %/% and integral time of 20 min
all composition controllers. The effects of disturbance in butene feed temperature have been depicted in Figure 6.28. mm View
Took
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A«»»Pte-t..W. | y 650C at time = 8 hours) and subsequently a step increase (65 -> 760C at time = 15 hours) have been considered in the simulation study. The developed closed-loop process lowsheet responds satisfactorily under load variable
change and measurement lag.
SUMMARY AND CONCLUSIONS |
f
f
In Chapter 5, we have studied the dynamics and control of the low-driven chemical processes. Here, a case study has been conducted on a MTBE catalytic distillation column using the pressure-driven dynamics. The complete process low diagram includes a distillation column, a feed compressor, a feed pump and three control valves. In the
MTBE synthesis process, a bottom product containing high-purity MTBE and a top product enriched with n-butene are obtained. To maintain the MTBE purity in the f
bottoms stream, several control structures have been configured with the lowsheet in
DYNAMICS AND CONTROL OF PKKSSURK-DRIVKN PROCKSSKS
.*J 1 1
Aspen Dynamics. All of the structures are SISO schemes with PI controllers (P-only on levels). The controllers have been tuned by simply using heuristics. The proposed closed-loop process provides satisfactory results under disturbance input and measurement lag.
PROBLEMS 6
.
1 A binary mixture of ethanol and l-propanol enters a flash drum (Flash2) The
f
feed specifications are shown in Figure 6.29 with the process low diagram
.
Liquid mixture
(UQ-MIX)
CV2
Temperature = 90X Pressure = 1,4 bar Flow rate = 120 kmol/hr
cCH liq-mix f»B-[fgiE CV1
Mol fract
Component ethanol
06
1-propanol
04
(pF]->t'i-|pdt-uq1-o
-
.
CV3
FIGURE 6.29
A flowsheet of a flash drum
f
The lash chamber operates at 90oC and 1.2 bar. The vertically placed separator with a length of 2 m and diameter of 1 m has elliptical head type. All the control valves have a pressure drop of 0.2 bar. Applying the RK-Soave thermodynamic model as a base property method, f
(a) simulate the lowsheet to obtain the product compositions, (b) design the two control schemes to maintain the pressure and liquid level in f
the lash chamber, and
(c) examine the performance of the designed controllers.
2 Styrene is produced by dehydrogenation of ethylbenzene Here we consider an
.
.
irreversible reaction: -
C2H5 -> CgHs - CH = CH2 + H2
ethylbenzene
styrene
hydrogen
The process low diagram that consists of a reactor (RSTOIC) a feed compressor ,
f
(COMPRESS) and a control valve (CV) is shown in Figure 6 30 .
An isentropic compressor discharges the FEED stream that enters the RStoic reactor at 2 bar The reactor runs at 260oC and 2 bar The control valve involves .
a pressure drop of 0 2 bar Use the fractional conversion of ethylbenzene equals .
08 .
.
Applying the Peng-Robinson thermodynamic method.
(a) simulate the lowsheet and ' b) observe the closed loop process response employing the flow controllers. f
6
,
-
312
PROCESS SIMULATION AND CONTROL USING ASPECT Pure ethylbenzene
Temperature = 260oC Pressure = 1 bar
Flow rate = 100 kmol/hr
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-
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FIGURE 6.30 .
A flowsheet for the production of styrene.
3 The hydrogenation of aniline produces cyclohexylamine in a CSTR according to the following reaction:
C6H5NH2 + 3H2 -> CeHnNHa aniline hydrogen cyclohexylamine
The complete process flowsheet is provided in Figure 6.31. It includes a pump having a discharge pressure of 41.2 bar, an isentropic compressor having a discharge pressure of 41 bar, an elliptical head-type vertically placed reactor having a length of 1 m and three control valves with a pressure drop of 0.2 bar in each.
FEED
FL
F1
P1
CV1
-
u
CV2
PUMP
>
<
F2
>ff J
1 PDT-LIQ \-0
CV3
COMPRESS
RCSTR
FIGURE 6.31
A flowsheet for aniline hydrogenation
The reactor operates at 41 bar and 120oC and its volume is 1200 t3 (75% liquid). For the liquid-phase reaction the inlet streams Fl and F2, are specified in Table 6.2. ,
,
f
6
RSTOIC
,
TABLE 6.2
Reactant
Pure aniline (Fl)
Pure hydrogen (F2)
Temperature (°C) 40 -
12
Pressure (bar)
Flow rate (kmol/hr)
7
45
7
160
DYNAMICS AND CONTROL OF PKKSSURE DRIVEN PROCESSES
313
Data for the Arrhemus law:
Pre-exponentiaJ factor = 5 x lO8 m3/kmol s Activation energ>' = 20 000 .
Btu/lbmol
ICJ basis = Molanty Use the SYSOP0 base property method in the simulation. The reaction is firstorder in aniline and hydrogen, and the reaction rate constant is defined with respect to aniline.
(a) Simulate the lowsheet to compute the product compositions ibi configure the control schemes for maintaining the liquid level pressure and f
,
,
temperature in the CSTR. and
(c) investigate the closed-loop process response under any disturbance input 6
.
6
4 Repeat the above problem with adding a time lag of 0.2 min in temperature measurement and carry out the closed-loop process simulation to report the disturbance rejection performance of the developed scheme 5 In addition to the level, pressure and temperature controllers, include the flow controllers with the flowsheet, shown in Problem 6.3. and inspect the closed-loop
.
process response.
REFERENCES | Al-Arfaj. M A. and W L Luyben (2000) "Comparison of Alternative Control Structures for an Ideal Two-product Reactive Distillation Column Ind. Eng. Chem. Res., 39, .
"
,
pp 3298-3307.
Al-Arfaj. M A and W L. Luyben (2002) "Control Study of Ethyl fert Butyl Ether Reactive Di-tillation." Ind. Eng Chem Res., 41, pp. 3784 -3796. ,
.
Jacobs. R. and R Krishna (1993) "Multiple Solutions in Reactive Distillation for Methyl .
ot-Butyl Ether Synthesis Ind. Eng. Chem. Res., 32. pp 1706-1709. Kaymak D B and W L. Luyben (2005) "Comparison of Two Types of Two-temperature "
t
.
,
,
Control Structures for Reactive Distillation Columns pp 4625-4640.
"
,
Luyben
Ind. Eng. Chem. Res , 44,
W L. i2004i "Use of Dynamic Simulation to Converge Complex Process Chemical Engineering Education pp. 142-149
,
Flowsheets
"
.
,
Rehfinger A and U Hoffmann (1990) .
,
"Kinetics
of Methyl Tertiary Butyl Ether Liquid
Phase Synthesis Catalyzed by Ion Exchange Resin-I Intrinsic Rate Expression in Liquid Phase Activities Chem Eng. Set.. 45. pp. 1605-1617. .
"
.
Seader J D and E J Henley Sons In< . New York .
11998)
.
"Separation
'
Process Principles, John Wiley &
.
W Bng, S J I) s H WonK and E K Lee (2003) "Control of a Reactive Distillation Column m the Kinetic Regime for the Synthesis of n Butvl Acetate Ind Eng. Chem Re* . ,
.
"
.
42
.
pp B182-5194.
This Thispage pageintentionally intentionallyleft leftblank blank
Rs. 295.00
PROCESS SIMULATION AND CONTROL USING ASPEN
Amiya K. Jana
@ 2009 by PHI Learning Pnvate Limited, New Delhi. All rights reserved. No part of this book may be reproduced In any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN-978-81-203-3659-9
The export rights of this book are vested solely with the publisher.
Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-110001 and Printed by Jay Print Pack Private Limited, New Delhi-110015.
Index
ABSBR2. 164
('hmmnil
AbNorplittn cnliunn, UM AnounlinK mformnhon. I I. 'M. 58
Compoaonl ))>, I Ml
Acetone, 93
('onfiguro dialog bosp li'io
Activation energy, (>r>
Control pnncl, 20
Adsorption, 100
Control vnlvi'M. 22!)
Aniline, M
(iontrol modali icon, 2(18 ( outI'ol Mi mil icon, 2(18
ArrhrniUH Inw, Bf*. 70 ASPEN. :J
Aopen Aapen Aapen Ahpimi Aapen Aapen Aapen
(
'
phtnt, 180
omponpnt tijuiii( lianisuir column, n>7
,
DcHi n ipac, I7(i Di'w point, 35
Direcl acting control, 243 Diaplay plot, I7i>
.
Anpcn prnpcrl ich I
Diatillation, l()7 Diatillation train, 180, 100 Diatl, 107
Hhmc method )H Bati hKrac I0H ,
,
Binary mixture
,
\2
Mth
'
Binary diatillation column
,
I )nvirin tor i r, 100 I )i mn modela, 7
Dryer, r>2
BK10 tr>i Block 7
D8TWII. 107. 108
,
Block inftirmation 33 ,
''
'-Me
"
'
point
28
,
I dynamic mode, 253 i kynumicN library, 2(i7 Dvnn I'M IS. r>
'
t'tnlvtir dialillation 28fi ,
T
OIJIOK 152 .
flbemCad
,
.
1
Mi hyll'onMtne, 56 rixpOlll'lltN, 2il7
316
INDKX
Flash 2, 3, 7
Peng-Robinson 60 PetroFrac 108 PetroFrac model 1 48 ,
Flow-driven, 229
Flow-driven simulation, 229
,
,
Formula, 116
Plot wizard 48. 90, 147 POLYSRK 204 Power law 54, 87 ,
Fraction basis, 195
,
FSpht, 204
,
Pre-exponential factor 65 ,
Pressure-driven simulations 229, 285 ,
Geometrv data, 237
PRO/1ITM 3 ,
Process flowsheet window 9 ,
Process variable 249 ,
HETP, 287
Hvdraulics sheet, 299
Property method 18, 32. 39 Pulse input 253 Pumparound circuits 149 ,
,
HYSYSTM, 3
,
Initialization mode, 253
Initialize values button, 273
Input summary, 23, 64
RadFrac, 107
RadFrac model, 127
Ranges tab, 247 RateFrac, 108
Ketene, 93
RBatch, 54
Kinetic, 74
RCSTR, 54
Kinetic factor, 100
RCSTR model, 230
Kinetic reaction type, 300
Reconnect destination, 192
Kinetic sheets, 238
Reconnect source, 193 REFINERY, 154
Regulatory performance, 254, 275 Rename block, 11, 193
LMTD, 256
Rename stream, 193
MTBE column, 286
Measurement lags, 309, 310
Report ile, 23, 122 Report options, 15 Requil, 54 Results plot dialog box, 251 Reverse acting control, 243
MELLAPAK, 287
RGibbs, 54
Methane, 93
RK-Soave, 28. 32
Model library, 5 Molarity, 76 Multi-input/multi-output, 243
RPlug. 54, 78 RStoic, 54, 55 Run status, 62
MultiFrac, 107
RYield, 54
NRTL, 52
SCFrac, 108
f
LHHW, 54, 93
Material STREAMS, 7
Pause at time, 251
Sensitivity analysis 172 Sep 1, 2, 7 Separators, 42 Servo performance 275 Setup, 15 Side strippers 149 Single-inputysingle-output, 243 Solver settings, 13
PENG-ROB, 140
SRK, 52
,
Object manager, 179
Operator set point, 247 Optimization, 178
,
,
INDEX
f
Stepwise, 7 Stoichiometric coeficients, 237
UNIFAC. 287
User Models, 7
Stream information. 18. 33 Stream table, 22
Styrene, 55
Vapour raction 210
SULZER, 287 SYSOPO*. 18
Variable number 180
f
,
.
Vinyl chloride monomer 189, 203 ,
Temperature approach, 262
Wilson model 43
Template. 5
Winn-Underwood-Gilliland method 107
,
,
317
PROCESS SIMULATION AND CONTROL USING
ASPEN
AMIYA K. JANA
As Ihe complexilv of a plant integrated with several process units increases solving Ihe model structure with a large equation ,
set becomes a challenging task. To overcome this situation, various process flowsheet simulators are used. This book describes the simulation, optimisation, dynamics and closed-loop control of a wide variety of chemical processes using the most popular commercial flowsheet simulator Aspen '"
.
The book presents the Aspen simulation of a large variety of chemical units, including flash drum, continuous stirred tank reactor (CSTR), plug flow reactor (PFR), petroleum refining column, heat exchanger, absorption lower, reactive dislittation, disiillation
train, and monomer production unit. It also discusses the dynamics and control of flow-driven as well as pressure-driven chemical processes using Ihe Aspen Dynamics package. KEY FEATURES
Acquaints Ihe students with the simulation of large chemical plants with several single process units.
Includes a large number of worked out examples ittustrated in step ay-step format for easy understanding of the concepts. f
*
Provides chaptered problems lor extensive practice.
This book is suitable for the undergraduate and postgraduate students of chemical engineering. It will also be helpful to research scientists and practising engineers. THE AUTHOR
Amiya K. Jana received his B.E. degree in chemical engineering in 1998 from Jadavpur University, M.Tech. in chemical engineering
in 2000 from IIT Kharagpur, and Ph.D. in chemical engineering in 2004 from IIT Kharagpur. Presently. Or. Jana is Assistant Professor at IIT Kharagpur. His areas of research include control system process intensification, ,
and modelling and simulation. He is also the author of ChemiesJ Process Mode/ting and Computer Simukuon published by PHI learning.
You may also be interested in Process Control: Concepts. Dynamics and Applications, S.K. Singh Heat Transfer: Principles and Applications Binay K. Dutta ,
Principles of Mass Transfer and Separation Processes, Binay K. Dutta A Textbook of Chemical Engineering Thermodynamics, K.V. Narayanan lSBN:')7fl-flWD3-3l.S1-,1
Rs. 295.00 www.phlndia.com
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