Process Simulation and Control Using Aspen

astern

PROCESS SIMULATION AND CONTROL USING

METHANOl

BUTENES

RDCOLUMN

CCS

AMIYA K. JANA

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.

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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

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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

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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

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 .

8

35 42

.

Prnhlpms

_

28

4 Computation of Dew Point Temperature 1 3 5 T-xy and P-xy Diagrams of a Binary Mixture .

,

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

.

.

.

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

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

4

189

.

4

.

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 .

5

.

229-284

Tnt.roHiirt.ion Dynamics and Control of a Continuous Stirred

229

Tank Reactor (CSTR)

230

3 Dynamics and Control of a Binary Distillation Column

255

Summary and Conclusions

279

Prnhlpms ,

,

References 6

Dynamics and Control of Pressure-driven Processes il

Tnt.rndnrtinn

6 2

Dynamics and Control of a Reactive Distillation (RD) Column

f

.

.

,..

279

284

285-313 285

286

Summary and Conclusions

310

Problems References

31J 313

Index

315-317

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Part I

Steady State Simulation and Optimization using Aspen Plus

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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

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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

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FIGURE 1.1

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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.

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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

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Copyrighted material

INTRODUCTION AND STKPWISK ASPEN PI.US

SIMULATION

7

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To develop a lowsheet, irst choose a unit operation available in the Model Library.

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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 .

.

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In the Model Library, there are ive built-in separators. A brief description of these models is given below.

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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.

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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

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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

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FLASH

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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).

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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

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Then click OK. Again, hit OK when the Aspen Plus engine window pops up and

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subsequently, proceed to create the lowsheet. Creating flowsheet

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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. '

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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.

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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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INTRODUCTION AND STFPWISK ASPEN PLUS

SIMULATION

11

Clicking on Material STREAMS, move the mouse pointer over the red arrow at the

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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. ..

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PROCESS SIMULATION AND CONTROL USING ASPEN

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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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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

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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.

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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.

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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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0233

H-C6H14

0 400

0 463

0 138

-

Mok Trie .

.

.

Mm/Spitlan Sflprntms { Heat Eicchangeit { Cdum | Reactori | PrMtue Chmgeii j Mmpdalai | Soldi j Use. Models j -

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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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i

1 1

raliH

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

following process flow diagram (see Figure 1.34) in the Flowsheet Window by connecting the input and output streams with the flash 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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I

gl *w

.

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FIGURE 1.34

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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-

gag i

3 abi

3

3 »l alai

ij ,

u m it »

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FIGURE 1.35

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INTKODUCTION AND STHPWISE ASFKN PLUSIM SIMULVTION

31

'

In the next, the Aspen Plus accounting information are given (see Figure 1.36). _

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FIGURE 1.36

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

r«

mm

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FIGURE 1.37

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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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=1 3

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bl

-

-

8

i

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F-3

.

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Q-S-o-'g-'ii D

FIGURE 1.38

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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INTRODUCTION AND STEPWISR ASI-KNJ>LU

sim

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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Specifying block information Hit Afort 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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FIGURE 1.44

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

"

-

,

.

.

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 As we start Aspen Plus from 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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f

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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FIGURE 1.64

f

It should be noted that if we move the T-xy plot slightly or close it, we ind Figure 1.65 having a databank. Some of these values have been used to make the plot (Figure 1.64). n3K|fc!»|-qM!!H 3i -

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FIGURE 2.12

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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Viewing input summary

For input information, press Ctrl + Alt + I on the keyboard or select Input Summary from the View pulldown menu (see Figure 2.16). CBSES Fie

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Press Afexf button or click on Reactions and get the window (as shown in Figure 2.26).

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FIGURE 2.26

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 finally click on OK Subsequently, select POWERLAW in the Enter Type list and hit OK to get the screen .

as shown in Figure 2.27. Ffc

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Viewing results In the next

,

select Solver Settings, choose figsuto Summary/Sf ms in the list on the

left and finally get the results shown in Figure 2.35 in a tabulated form.

78

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dmbl

Melm mbhjsM«!] 21 g

r m«i »

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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). <

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FIGURE 2.49

Hit A exf knob and obtain two stoichiometric relations as shown in Figure 2 50. .

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FIGURE 2.50

In the simulation of the present problem we use partial pressure basis (applicable for vapour only) and therefore, the Power law expression has the following form: ,

,

( f > r = k

exp

R

1

To

(2.5) ,

P represents the partial pressure (N/m2). If fo is not specified, the above equation

where 18 replaced by: ,

E ri

n

88

PROCESS SIMULATION AND CONTROL USING ASPEN r= kTn exp

RT,

mPif1

2 6) .

For the prescribed reactions, values of the pre-exponential factor and activation energy 2 are provided in the two forms, shown in Figures 51(a) and (b). To apply the Arrhenius law, we put zero for temperature exponent n and left the box, allotted for datum .

temperature T0, empty. i»f.i

I r

mi r»

! .isi; I - IB!

as

±1 "

[i) cfwe-. sciwio.

1

3

d

E

§

ill

*

ai F

* a ?

Si Bacfa i

PR

0 R-1

StflEfiMS

RSI

ffrteM

REqui

Rtjfcto

RCSTFI

RPVJ5

RBVch

FIGURE 2.51(a) ». Ea »«, 0«, r i, a .

.

-

i

' .

r u>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 STROIMS

111 *

RS'jc

HTot)

Qg

RGtfc,

ftCSIR

ftFy

'

"

.

'

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

An

IPE a-wm ftcpwl*



'

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 ,

process flow 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 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).

PetroFrac is commonly employed to fractionate a petroleum feed. This rigorous model simulates the refinery columns, such as ADU, VDU, fluidized-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 flow rate of 200 Ibmol/hr at 750F and 15 psia This feed is required to ,

.

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| \ 0 >Nh| *i lacal -

*bs

atrsitrro rkx sot iabli

taw

EH

-

HUM

fit -. I

FIGURE 3.21

ASPEN PLUS SIMU1.ATI0N OF DISTILLATION MgggUj

121

Hitting Next followed by OK, we have the Run Status screen (see Figure 3.22).

i r -i _

rr

.i.ipi

.

HMtf mutt

WIWW

(M

i

ibi

f

fMF(«

iMMO

*fik

SOik

i*rfi«

f

_

HH-'Ifci

'*.

.

FIGURE 3.22

Viewing results

f

In the next, select Blocks/DSTWU/Results rom the Data Browser. In the following (Figure 3.23), we get the answers as: Minimum reflux ratio = 7.724 Actual reflux ratio = 8.751

Minimum number of stages = 33.943 Actual number of stages = 67.887 Feed location = 40.417

f

f

Save the work by choosing File I Save As /... in the menu list on the top. We can name the ile whatever we like. Remember that a backup ile (*.bkp) takes much less space than f

a normal Aspen Plus documents ile (*.apw). Viewing input summary

If we wish to have the input information, press Ctrl + Alt + I on the keyboard or select Input Summary from the View pulldown menu (see Figure 3.24).

Copyrtghtod material

122

PROCESS SIMULATION AND CONTROL USING ASPEN dim -

I

-i .iai,

r..|-M= IT

I

i 'j-i .

ibi

i a SH

r -

1S?497652 »3*J12.;3 -

Br.il.

ifM IM|NMM

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sti| s

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lalxt '

i

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irpuc SuwMry creic«d by Aspen Plus Bel. 11

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.

at 10:15:40 Tho

Jol

12,

Directory c:\Pr09ran f nes'.Aspenrechvworklng foIi working FoldersVupen plus 11.1 .

[TITLE

2007

Fllenw c :\users\4kjana\AppMt«\Local\Te«p- ap6336. trt ~

'SinulatiorL of 3 Shortcut Cist Illation column'

I-UNITS

EPXC

Ikf-STREjWS COMVEW ALL

bescfiiPTiON Central simulation with Eoallih units : F. psl, Ib/hr Ibool/hr, Btu/hr. coft/hr. ,

property Method: nort

Flow basis for Input: Mole Strea* report composition: HoU flow

PSOP-SDUSCES PUHEll C0KPOMEKTS

ETHANE C2H5 / ElKfLEKE C2H>

PROPERTIES Pk-SOAVE

PROP-OATA RirSKD-l IH-W.ITS ENC PROP-LIST BKSKI3

BPVAL ETHANE ETHYLENE . OlOOMOfriJO BPVAL ETMYLENE ETHANE iTREW

.0100000000

F

S085TRE»t fIXEO TEKP"' 5. PRES-1S t«0LE-FLOW-200 M016-FRAC ETHANE 0.6 - ETWlENt 0 4 .

.

,

-

»±

| - w.

-

11

- I

ii

-

'

MIUU

Da

oM

t

-

I

IM*a

«0

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US YOU UKE

Sttwrnf

v a *

Flo»*e«bftg Opbon* MoM

ft-1 a

,1-! Took

4 Gfl

2Jil

i

STREAMS

DSTWU

Dim

Rrft«c

E- ad

Muffnc

SCFfac

PaBcfiae

Ratftac

Batetfrac

farHaip.pMn

| .si) Chapter 3 ttowJWcri | 4]Q>«pMf2-lto«rfi Wtrt H

A xn Pkj. - SM«i

. fi'"

FIGURE 3.27(b)

In the Setup/Report Options / Stream sheet select basis as shown in Figure 3.28. ,

:

.

fib b«

on To*

r Uhv 0 Becwrt Opdcro

i

n« u»v rnvhw Hdp

R

1 >IB3|- . { lal

UMbl'1

-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 *

SIRCMC

tWSrtten | S

Mtatt | HMEwhangm



T

FV«

itun

Wr*s-

r-i-i-i

i .w

-

-

i -igi *m

3a !« Mn* ;lrM*N£ .

cats

IHYUNC

>

O

o

,.

si

-

Pttrfnc

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

itfi>t|

-

3EfSiF -q

l-li Fi-Hid QUIH

3

|m*

dr it

(200

,

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.

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.

134

PROCESS SIMULATION AND CONTROL USING ASPEN n* E#

M«m

Dal* Toch ft* Utrvy WMdaw M*.

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>

ASPEN PLUS

SIMULATION OF DISTILLATION MODELS

141

Specifying stream information

In the next, use the Data Browser menu tree to navigate to the Streams /F/Input / Specifications sheet. Entering the values of all state variables and component mole fractions, we get this picture (see Figure 3.46). .

I rH-H-F

" -IB I'll IW Ml

I2S

II- P=!J

f» -

k

& I

'U*w

'

5?

3

~

I-

-P--

....

3

3 -

,

3 3

!?-

Mftg

«!Ei

M=

-*.

-~

»

'O--------

FIGURE 3.46

Specifying block information

f

Open the Configuration sheet choosing Blocks /RADFRAC in the list on the left. In the problem statement, the information on number of stages, condenser type, vapour distillate low rate and reflux rate are given (see Figure 3.47).

r£Z3_

3=

'

:

.

3

mam

t

ea .

rauv

s-.rv,

Ma

Cm

mm

«.

FIGURE 3.47

.*>>

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. fl. Ml

D«i

T«* ft* PW Uom* VMcm N*

I

CJ

31 P«p 3«.

.

Q flow

i rj dv 20

Um4

S .

t . . 9

,

' RADFRAC

0-

Hi j j flndxra *

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

to*

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. F

r

oawnd

*j 1

3

9TIJ3K?SI

T- t c I

i

20

i

20

j

Tiit tasoU 1

"

--

B

to

171 385017

f

195 3*a»6

'

aj

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

In the next, select Plot Wizard from 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

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1

15" CH

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tufi

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.

Q?1!*

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,

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_

-

J

T.w Sot,

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j

J

i

51 OK

J

s- j

wFodo FkMntln CCCOTHI CQCQS H) Hv4m >

BM

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Join*

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60 - .

JP"" Pi" CH

-

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m&

tS»

so..

w«

fafc.-A»»H.MI

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 PetroFrac column and report the flow 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).

I

I

I

I

I

I

It

Cioata a Utm SmMan Using A+

f

A+

Blank Smiiabon

Temptale

"

OpendnEwiingSimulatior

e*r\ADU BOOK apw

DAeoolAChaptesVAOU OWN xm D:V0ook\Chap(«sViDU t*{> .

0«

|

I

Adobe Ao-dia P,d«»« I Chace.s

W \ 4]Q*i** 2

HmcHW ||

Av

Pka

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 blank process flowsheet. 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.

rinF. I- I-h HI

I .Ml

I

lal

iN

J?

it Jb

[1 ir .

tr ffc* | turn* t

Jt

#

lb

J

1

ir Op

ft .

IT

if

& ir

4

if"

i> Bp # I .

c

a

.

-

W 11 1

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. '

t.

r

k

-

»l ~l

I f

i i I fV

i I "I -I *\

I J

.|:-liS|-. I .:M -

1 >hl

-3idPy!!il

"

3 ficwtw

|Bfni;tFf.

I

3

~

3

< i-j :

1 aj c v-lu

STRUMS

'

KTVM

OM

fikfiK

PtftaPut

RatoFiv

8«=rfi«

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. .

D|tfiB|

_

iJ e|jgJ njKjfti i-g -i ».i Pn

zi

{ |h| Miigj .}

bJiiif

_

j

-

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n

| n b iei

i

0.1

cs

nm

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no

w,

1

ED-**

rv -

D

-

""!««'

wwi

»

.,

m,,,

s

«

,

t«,«

FIGURE 3.70

i

i*

6

ASPEN PLUS

SIMULATION OF DISTILLATION MODELS

155

As we hit Next icon, an input form for Stream STEAM opens up. After filling out, it

looks like Figure 3.71

.

t*

*

V««

D*»

T«w

fVn PW

Marwt VMem

M-i rv -

-

[ft

3

HuTorCoew

| EOOptan |

31 1*0 .

F/traBy

£

Mol(Qior Stioam

i:

FtranKn

|r

ict

d

ia 1

d

H 11500

g ST

:

E S S

LGHTS

3 SE>1 3 O SlOS a

*

Total NlSOO

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'

itiputCMeM*

fV

MMd iAw. } SwoWort 1 HutEttharpei' Columw j Rwawt j h w O* '

STREAMS

DSIWU ntnjl

r. .1

Raifitc

EntlKl

MJtf

t)_-.r... Pt fwFrac

SCfiac

.

| M*v niE | S | U«Modes | njj Bat efiae -

._

Bwctjj.. fiac

b

;

-

S . * AwnWM

NUVI

cj/Ki

i OTote

t'

FIGURE 3.71

In Figures 3,72(a) (b) and (c), three filled input forms are shown for STM1, STM2 ,

and STM3 streams

fitf

'

csk

Wew

,

Dm

Tooli

H i

PW

Ibtw,

JMI mi "

9

dSfb

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: _

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| Ma:-. Flow

N . _

.

|f

d

y t- tr-, Minodi j

_

ISO

[p..

I

dl*'

d

C1

d

-O

d

-

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J 0*3

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_

-

ICI

cs

1

NC5

1

J

H20

fEEL

tniii

S kM Cans,..

i fM-I-8-If'(S'#'C'4

-

SWUM

MTMI

'

MM* (renFI

FIGURE 3.72(a)

" '

156

PROCKSS SIMULATION AND CONTROL USING ASPEN

O

tn

J9J»J

Ua

1

S3 2J

ApecMicotioo

I rMMOM«M I MI I M* I IMOMa |

i .r.

£>*

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M/w

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IOib

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lau«

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PfUcfrac

Raiefiae

j 4] Aapen_ Mo«by Mct | - Cha r3 Hritirft

Botk

Batctfrac

j jUwtdTwro Hwcao )[

Aapen PVj« . Soul

«

j

FIGURE 3.87

Configuring settings

In the subsequent step, hit Next symbol and fill up the three setup input forms as shown in Figures 3.88(a), (b) and (c). Re

E(k Mm

D*«

Tooii

Pur.

Pid

Lbra>y Wfida-*

H >

n| |H| J JMg|jgl nsMfeKNM H j i-i-I- PT L r _

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r |cOW>/HH [

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DM C

STREAMS

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QaU

RxF.ac

E«kI

M F.oc

P»tof.ac

R fi

t

£J

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T-r1

FIGURE 3.91(b)

ouiul .1 ! ami 01 al-rtfcloKlyl » ! 3

I MilBj} 3 «i -fe!

SfEf P

-

3

-

Li

j\

,

r

ir

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). -

He &k Vfe* CW* Tec* Fin fa u

, Wi-do- Up

Mai -I..[ **m *?! oMfel

M *'l :2l_J_liiJjdElj*J.£iJ

i r 1.14- nr

-i

i

iai 1 I

PtoimiI >/!:cnjcnni| Com*

sws. WOfiBEH 5

© _

HMttr,

}

J MfMffd

Urn*

1W

6*jb

Flow

:.

Fto-iatm

U*.

| .

SfBEAMS

:

Djrf

f&F-K

E-iKa

W-if.s:

SCF

P

ff-t

.

fistf.ic

B«c»f >

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. .

.

,

E«

rtca a(s

"m«

Rji

fta

Jay,

7fr»e«

-

fop *

E

7]

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fl (t*Sf EEC

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1

ftiufnt

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| tjHA.MTMa We {j

FIGURE 3.94

J

<

n

-S

' to. **** tr j -

Mil

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.

-

E«

V*-

OA

TMh

4*

ci»1H| *U M*} J T-l l-l' F* _

-

raMai

H'"I n . I M .| i?i m £ 'I .IBI

-

I

laai «M

mjihTiiiiTiii liiriSij

StPEAMi

[ .11VU

C««

RadF.-a-

Mjfffac

SCFr«c

Prttofra-

RatsF

Bt&f'*

.

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. fit

SX

.Vn

t« 1

It.-.

a»>

P«

UrVf

loHal am Lr_ia_ J rr

_

.

_

i

1 *LiJ «)W

la

.

~

3 aaad '

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a

aria

ei

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d

.

d

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ra-1 *l)4s

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ozn

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0152

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*

uam

mmm

M*

sfgl..

j»[di

> nn

I :mii

In Tm. t trv I

Mil

t Mai

lil/TU '» «»« K ua

MtM i

'

MIMU

Om

IWw i«

IOai

DMTw

FIGURE 3.101

taXw

\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).

5 -

I »

-

- "

.

-

I rijo- - 11;

.--

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.

J T -

I-1 l- iv

i-g

I

i»< siW

I --. I - l - I

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. '

38 2i£B_uaas

'

3

-'-i ~ _u-j.ii2a

~--

5=.

-

o _

AH

.>

Mfc.

lw

Mto

W.

W«

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

;Ti=E4Mi

DSTVU

Dag

R»Jf>:

Erftaci

M frac

SCF»c

PihoF-JC

flyrf.K

fcj'ctfix

FIGURE 3.106

Finally (see Figure 3 107), select the Vary tab and enter the following information: .

>

4. Lfc

_

Ar-

Cm« to-Ji Hpi Ptoi

Ubr,

AWtow

»H>

MHj_U jff) nkiaiaKiH n.| P >|'l"l imi i ibi j r iii nr _

' .

'

_

I ~l

1 fil

i i

X

U

-

i

/-,

6 r

J

-

Q '

3 "

-

0 0

r

r

! M6

>

r-s. _

i

>

o

I -'

-

Tan,

I

'

> fi-

-

| HmMH I r>MMf'CM«M | Mr iW I SOW: | UwMaW- |

»gj '

I

a

FIGURE 3.107

?

:

1

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. &k 'Aw U«j

Tw, fu.

w**«.

el w| Mii*MaJi£)!id r

I I "I

-

j

Slock:

ISSOAfilS

USFUC

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1

t

1

I

1

1

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-

! OSTWU

ail * J "

Dan

.

Ir.

C.1K31

sicwr-

- fl-.j.J.

Hbi Ik/TcI

ML

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ttt/To)

i

»

c.aui*

i

3TH£*«S

Ici

XL

Lhtt i.e. i

Brfrac

311

BOT

O

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

Temperature (8F)

Pressure (psia)

Flow rate Ob/hr)

STEAM

Main tower

350

50

12000

STEM!

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.

Part II Chemical Plant Simulation

using Aspen Plus

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 n-C

pi

5

.

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).

Copyrighled malarial

\SI'KN PWB

SIMULATION OP CHKMK \l PLANTS

hmbj_lj__bJ'ii mmLii-jd .3 J.i.'isim

i.

191

:

si

In

ro..-

AM n i.tu.

1

I

1 1" -

.'

I

1 «

FIGURE 4.2

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

.

1»

1-

.

l-.

i -"Pi

g :.r.:

I

iai

Fopui C2 CJ C*

IU cs .

*

!S

*

_

j wy tm 1: -

.

.

J O pOa**

_

i i r

.

id

5

-.

| > M

i-w

'

-

I'M

-

3 "

3

j. a.

-j],. ,.

j

tti»» .rtl.m '-if

2j twi- m*.

'

:

".

ww.

r w

sit.

FIGURE 4.14

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) .

Click on Afert 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

FU

Jtnry

Wndov.

I Ms

HA J2J

L TJ i i -

"

3

r Preci.ie "iciMie r Pt«tiu>«(Mo

I

"

3

I

) 38

0 j

EWSK Rtcnc

H jsn

l -iai

/Specifcaliooij CflfcOalKr. OsHjw ] flaihOphor.t (? Ptmp

"

3 "

3

P

Result

J

_

Al

Ptnp.

ro-jcertrterma'eincxJ To no'rr roi ed»i ijncd » Mea ' c fx :- Hm - * Osu .>: -- (Mnu.

I W ci -

ai c;

j a o .

ai «

.

a pi

EOCorwOpten

2

-

Q

-

iM .

FIGURE 4.18

9 -m *

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

.

.

-

B a B O B w Z

'

93

us states; ~. Jt

n

tjxll huz - jjtkt:

a = c» n

lice*

fi"

mu

z.

=i3»i

arc:

a

Bsmi:

csrta

-

"

.

v. ex.

-vOS

FIGURE 4.19

Viewing results

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 file 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

3 >bJ.'

V<

tat,

-

AWl-

-*k

'

1 - M|

.

11 >id illF-Z]»J iJ _

OKI to U rciM r. ffMaitcV

P Hbi

P Ir-imi .«! no >» j tKW,

I

FIGURE 4 27

.1

208

PROCESS SIMULATION AND CONTROL USING ASPEN

Specifying components

The components that are involved in the monomer manufacturing process are ethylene (C2H4), chlorine (CI2), 1,2-dichloroethane (C2H4C12), vinyl chloride (C2H:iCl} and hydrogen chloride (HC1). In order to get a blank component input form, choose Components/ Speciftcatiojis in the left pane of the Data Browser window. Defining all species in the Selection sheet, we have Figure 4.28. 1 Data

ma

(ComponaO Spooftoiioni . Dal a Bnnrasr)

Tods

fkn

Plot

Library

Window

Hdp

510*101

_

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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. Dltfiui j | fricl gj n\'(\%\**\! |h| ] v| gj

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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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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, first 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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In the subsequent step, run the program with Initialization run mode. As it is finished, go back to Dynamic mode. Then, open the plot sheets for both the controllers. The regulatory behaviour is illustrated in Figure 5.40 giving changes in feed temperature

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 %

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45

toluene

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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)

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horizontal

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5

25

5

25

Sump

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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 fluid and coolant In the simulation. assume constant reboiler heat duty and apply the UNIFAC base property method. ,

.

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 flow 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.

(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

the flowsheet 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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PROCESS SIMULATION AND CONTKOI, IISINC ASPEN

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choose Properties/Specifications and gel the property Inpul form. a property method originall} Includes several models for calculating the physical properties For the distillation example, set the UNIPAC base method b> .

in Aspen simulation

,

scrolling down (see Figure 5.47). Specifying stream Information ,"

next, (.pen Streama IFEED IInput ISpecifications sheel Entering the given

Values lor all State variables and teed eompo Figure 6 'IH

ion Ihe slream mpnl lorm looks like .

Specifying block information ,"

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) ,

260

.

PROCESS SIMULATION AND CONTROL USING ASPEN >

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Starting Aspen Dynamics

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 file 'Ch5 5 3 RadFrac.dynf. As a result, the Aspen Dynamics window appears (see Figure 5.57) accompanying with the closed-loop process flow .

_

_

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)) Controlled variable: liquid level in the reflux drum Manipulated variable: distillate flow rate Controller action: direct

DYNAMICS AND CONTROL OF FLOW-DRIVEN PROCESSES

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Again hold the ControlSignal icon, drag it onto the process flowsheet and drop it on

the blue outgoing arrow marked OutputSignal from the CCT block. As Select the

Control Variable dialog box appears (see Figure 5.63), choose 'CCT.OP' by name and press OK.

Immediately, a solid black arrow representing the controller output signal is automatically generated. Move the mouse pointer to reflux stream and make a connection to InputSignal2 port. To use the reflux flow rate as control variable, select BLOCKSC'RADFRAC). Reflux.FmR' in the dialog box and click OK (see Figure 5.64). '

Now the binary distillation column is coupled with four control schemes, LCI, PC2, LC3 and CCT, and the closed-loop process looks like Figure 5.65. The subsequent discussion includes the modification of different tuning properties of the CCT controller.

DYNAMICS AND CONTROL OK FLOW DRIVKn PROCESSES

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First we wish to see the default tuning properties. So double-click on the CCT block and then hit Configure symbol in the faceplate to open the Configure dialog box (see Figure 5.66). ,

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DYNAMICS AND CONTROL OF FLOW-DRIVEN PROCESSES

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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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Modifying ranges for process variable and controller output we hit the Ranges tab, the Configure dialog box (see Figure 5.68) shows the ranges imposed

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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 He

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Clearly, the CCB controller shows satisfactory set point tracking performance against a pulse input change It is observed from Figure 5.74 that owing to strong .

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

Component

Flow rate

(kmol/hr) toluene methane

hydrogen

j cold-out hoi Temperature = HOOT

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Pressure = 498 psia

200 Dead time

2300

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1000

pi

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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.

(a) Simulate the heat exchanger model using the shortcut method, countercurrent flow 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 flow rates for both the disturbance or wild stream (Fw) and

5

.

.

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 flow rate accordingly. m

Input 2

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PI

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Input 1

FIGURE 5.77

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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")

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STREAMS("W ) Fcn("WATER")

150.0

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FIGURE 6.3

When the Connect to Engine window appears, use the default Server type (Local PC). Creating flowsheet

The process flow diagram includes a feed pump a feed compressor, a distillation column and three control valves. The complete process flowsheet 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 ,

.

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

At the beginning of data entry, fill up Global sheet followed by Accounting sheet under Specifications of Setup folder. Moreover, select 'Mole' fraction along with 'Std.liq.volume' flow basis in Stream sheet under Report Options [see Figures 6.5(a)

,

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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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In Figures 6.9(a) to (d), first 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 fill 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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Choose Blocks/RDCOLUMN/Convergence and fix up the maximum iterations to 200 (see Figure 6.16). : F*s

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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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299

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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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SeiBa> | StoWOf | H** ttfhwigsn [ CtWn j RwCCi- Pi«b.b OianiiBn | Mjr xWa! | Soldi 1 UaaMod-h

o

-

"

tetF

Con*

f

SI REAMS

MCon*

Vrt.t

Po? C\ jFcWffiVAifuriFVji 11.1

sun I

y-

FIGURE 6.19(a)

i

r -i-i -i- nr

\

lai

RWMtt rt.

r.-., .

MICE

J

ft

1

UMfl

HHi t*e»< I bWltoi I " f Kl«ro». | DAim | PmOo.

Pimm* OunoMi | UitvJ**, | ioW, ] u«UuM<

FIGURE 6.19(b)

HUM

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). .

l r LL i

_

_

_

-

3

-

fV

! .mi

_

1

mi

:

us 5"'

o«flto.M1[iB-.p»*ii 1

mi

-

-

.

FIGURE 6.20(a)

:

*

V*m

Dm*

Twt

Rw

Ptai

Ltm-

Wndn*

|jLi sial -J ,

I l l

I r I M- fj "

-

3

I en It U»M«

UAod

a

jo win

.

5!

-

FIGURE 6.20(b)

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

302

PROCESS SIMULATION AND CONTROL USINd ASI'KN I y I'M i hi- Mojiin i) fie

-

Edi

Vww

Data

ToWl« .

I[lr53

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.

Notice that by the similar way, we can design the flow controller for butene feed of the RD column.

Top section

In addition to the LCI and PC2 control structures, the distillate composition can be

controlled by manipulating the reflux flow rate. In an alternative approach, along withf the pressure control (PC2), we can control the reflux drum level by the manipulation o the reflux rate and the distillate flow 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 and controlled by the manipulation of the fresh methanol feed rate. The butene flow rate is flow-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).

c 0 a *q -

M

-

m

5

JC»r.Lt»*v

TV)-HM*

m m .

r«uv(.-,

m

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

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, first highlight a block or stream, then right-click to point Forms and finally select the item that we want to access.

DYNAMICS AND CONTROL OF PRESSURE-DRIVEN PROCESSES

309

EBB

A

-

-

1

1 r»5fl

ill 1

A

jf5

i

4--

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). ,

Ot-

[ZtK-

o5 3

F

4 -

x-i

-

mf

Lii 13 iJ

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 .

,

,

,

.

Here, we have used the proportional gain of 1 %/% and integral time of 20 min

for

all composition controllers. The effects of disturbance in butene feed temperature have been depicted in Figure 6.28. mm Fte

View

Took

Q b: B SQi SfnUahon

Wtxjow

Heb

IS

.iV IDynamic

r tt Tf Gi«i|oo5

3

j i; a* v»

_

h «

'

IB » fl

b? t!

Cl B

K

I phut !-:-»&-

-

oV>*

i-r*

| jjDlWS-MiCCToll.. I

x

se

r-

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 flowsheet responds satisfactorily under load variable

change and measurement lag.

SUMMARY AND CONCLUSIONS | In Chapter 5, we have studied the dynamics and control of the flow-driven chemical processes. Here, a case study has been conducted on a MTBE catalytic distillation column using the pressure-driven dynamics. The complete process flow 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 bottoms stream, several control structures have been configured with the flowsheet 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, (a) simulate the flowsheet 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 ,

(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

.

f

6

,

-

312

PROCESS SIMULATION AND CONTROL USING ASPECT Pure ethylbenzene

Temperature = 260oC Pressure = 1 bar

Flow rate = 100 kmol/hr

"

M !

[pptI

-

o

cv

|feed|-1

-

COMPRESS

FIGURE 6.30 6

.

RSTOIC

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 ft3 (75% liquid). For the liquid-phase reaction the inlet streams Fl and F2, are specified in Table 6.2. ,

,

,

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 flowsheet to compute the product compositions ibi configure the control schemes for maintaining the liquid level pressure and ,

,

temperature in the CSTR. and

(c) investigate the closed-loop process response under any disturbance input 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

.

6

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 .

tot-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 "

.

,

,

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.

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 ArrhrniUH Inw, Bf*. 70 ASPEN. :J

(iontrol modali icon, 2(18 ( outI'ol Mi mil icon, 2(18

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 diatillation column

,

Binary mixture

,

\2

'Mth

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 LHHW, 54, 93

Rename block, 11, 193

LMTD, 256

Rename stream, 193

MTBE column, 286

Measurement lags, 309, 310

Report file, 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

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

Stepwise, 7 Stoichiometric coefficients, 237

UNIFAC. 287

User Models, 7

Stream information. 18. 33 Stream table, 22

Styrene, 55

Vapour fraction 210

SULZER, 287 SYSOPO*. 18

Variable number 180

,

.

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