Practical Distillation Control

Practical Distillation Control

William L. Luyben, Editor

InmiI VAN NOSTRAND REINHOLD ~ ____ New York

Copyright © 1992 by Van Nostrand Reinhold Softcover reprint of the hardcover 1st edition 1992 Library of Congress Catalog Card Number: 92-10642 ISBN 978-1-4757-0279-8 ISBN 978-1-4757-0277-4 (eBook) DOl 10.1007/978-1-4757-0277-4 All rights reserved. No part of this work covered by the copyright hereon may be reproduced or used in any form or by any means-graphic, electronic, or mechanical, including photocopying, recording, taping, or information storage and retrieval systems-without written permission of the publisher.

Manufactured in the United States of America.

Published by Van Nostrand Reinhold 115 Fifth Avenue New York, New York 10003

Chapman and Hall 2-6 Boundary Row London, SE 1 8HN, England

Thomas Nelson Australia 102 Dodds Street South Melbourne 3205 Victoria, Australia

Nelson Canada 1120 Birchmount Road Scarborough, Ontario MIK 5G4, Canada

16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

Ubrary 01 Congress Catalotling-in-Publication Data Practical distillation control/edited by William L. Luyben

p. Ctn.

Includes bibliographical references and index. ISBN 978-1-4757-0279-8 1. Distillation apparatus.

I. Luyben, William L. TP159.D5P73 1992 66O'.28425-dc20

2. Chemical process control.

92-10642 CIP

John E. Anderson Hoechst Celanese Corpus Christi, TX 78469

(Chapter 19)

Page S. Buckley Consultant Newark Delaware 19713

(Chapter 2)

James J. Downs Advanced Controls Technology Eastman Chemical Company Kingsport, TN 37662

(Chapter 20)

James G. Gerstle Amoco Corporation Naperville, Illinois 60566

(Chapter 12)

Vincent G. Grassi II Air Products and Chemicals, Inc. Allentown, PA 18195-1501

(Chapters 3 and 18)

Kurt E. Haggblom Process Control Laboratory Abo Akademi 20500 Abo, Finland

(Chapter 10)

David A. Hokanson Exxon Chemicals H. R. Rotterdam No. 231768 The Netherlands

(Chapter 12)

Henk Leegwater DSM 6190 AA Beek The Netherlands

(Chapter 16)

Contributors

William L. Luyben Department of Chemical Engineering Lehigh University Bethlehem, PA 18015

(Chapters 1, 11, 22, 24, and 25)

Randy C. McFarlane Amoco Corporation Amoco Research Center Naperville, IL 60566

(Chapter 7)

Charles Moore Department of Chemical Engineering University of Tennessee Knoxville, TN 37996

(Chapter 8)

Cristian A. Muhrer Air Products and Chemicals, Inc. Allentown, PA 18195-1501

(Chapters 23 and 25)

Antonis Papadourakis Rohm and Haas Co. Bristol, P A 19007

(Chapter 4)

Ferdinand F. Rhiel Corporate Division of Research & Development Bayer AG D-5090 Leverkusen, Germany

(Chapter 21)

John E. Rijnsdorp University of Twente 7500AE Enschede Netherlands

(Chapter 4)

Daniel E. Rivera Department of Chemical Engineering Arizona State University Tempe, Arizona 85287

(Chapter 7)

v

vi Contributors

F. Greg Shinskey The Foxboro Co. Foxboro, MA 02035

(Chapter 13)

Sigurd Skogestad Chemical Engineering University of Trondheim, NTH N-7034 Trondheim, Norway

(Chapter 14)

Terry L. Tolliver Monsanto Co. St. Louis, MO 63167

(Chapter 17)

Bjorn D. Tyreus Engineering Department E. I. DuPont de Nemours & Co. Wilmington, DE 19898

(Chapters 5 and 9)

Ernest F. Vogel Advanced Control Technology Tennessee Eastman Co. Kingsport, TN 37662

(Chapter 6)

Kurt Waller Process Control Laboratory Abo Akademi 20500 Abo, Finland

(Chapters 10 and 15)

This book is dedicated to Bea, Gus, and Joanne Luyben and Bill Nichol, four of the most avid bridge players I have ever known. Run 'em out!

Preface

Distillation column control has been the subject of many, many papers over the last half century. Several books have been de­voted to various aspects of the subject. The technology is quite extensive and diffuse. There are also many conflicting opinions about some of the important questions.

We hope that the collection under one cover of contributions from many of the leading authorities in the field of distillation control will help to consolidate, unify, and clarify some of this vast technology. The contributing authors of this book represent both industrial and academic perspectives, and their cumulative experience in the area of distillation control adds up to over 400 years! The collection of this wealth of expe­rience under one cover must be unique in the field. We hope the readers find it effec­tive and useful.

Most of the authors have participated at one time or another in the Distillation Con­trol Short Course that has been given every two years at Lehigh since 1968. Much of the material in the book has been subjected to

the "Lehigh inquisition" and survived! So it has been tested by the fire of both actual plant experience and review by a hard-nosed group of practically oriented skeptics.

In selecting the authors and the topics, the emphasis has been on keeping the ma­terial practical and useful, so some subjects that are currently of mathematical and the­oretical interest, but have not been demon­strated to have practical importance, have not been included.

The book is divided about half and half between methodology and specific applica­tion examples. Chapters 3 through 14 dis­cuss techniques and methods that have proven themselves to be useful tools in at­tacking distillation control problems. These methods include dynamic modelling, simu­lation, experimental identification, singular value decomposition, analysis of robustness, and the application of multivariable meth­ods. Chapters 15 through 25 illustrate how these and how other methods can be ap­plied to specific columns or important classes of columns.

ix

The use in this book of trademarks, trade names, general descriptive names, and so forth, even if they are not specifically identified, should not be taken as indication that such names, as understood by the Trade Marks and Merchandise Act, may be freely used by anyone.

Contents

Preface ix

Part 1 Techniques and Methods 1

1 Introduction, William L. Luyben 3

1-1 Importance of Distillation in Industry 3 1-2 Basic Control 3

1-2-1 Degrees of Freedom 3 1-2-2 Fundamental Variables for Composition Control 5 1-2-3 Pressure Control 6 1-2-4 Level Control 8

1-3 Uniqueness of Distillation Columns 9 1-4 Interaction between Design and Control 10

1-4-1 Increasing Column Size 10 1-4-2 Holdups in Column Base and Reflux Drum 11 1-4-3 Effects of Contacting Devices 11 1-4-4 Sensors 11

1-5 Special Problems 11 1-5-1 High-Purity Products 11 1-5-2 Small Temperature Differences 12 1-5-3 Large Temperature Differences 12 1-5-4 Gravity-Flow Reflux 12 1-5-5 Dephlegmators 13

1-6 Conclusion 13 References 13

2 Historical Perspective, Page S. Buckley 14

2-1 Introduction 14 2-2 What is Control? 14 2-3 Column Design Methods 15 2-4 Column Tray and Auxiliary Design 15

2-4-1 Tray Design 15 2-4-2 Reboiler Design 16 2-4-3 Flooded Reboilers 16 2-4-4 Column Base Designs 16 2-4-5 Overhead Design 16

2-5 Instrumentation 17 2-6 Control System Design Methods 19 2-7 Process Control Techniques 21 2-8 Influence on Distillation Control 22 2-9 Conclusion 23

xiii

XlV Contents

3

4

2-10 Comments on Reference Texts References

Part 2 Methods

Rigorous Modelling and Conventional Simulation, Vincent G. Grassi II

3-1 Overview 3-1-1 Conventional Simulation

3-2 Distillation Process Fundamentals 3-2-1 Continuity Equations 3-2-2 Vapor-Liquid Equilibrium 3-2-3 Murphree Vapor Phase Stage Efficiency 3-2-4 Enthalpy 3-2-5 Liquid and Froth Density

3-3 Computer Simulation 3-3-1 Algebraic Convergence Methods 3-3-2 Equilibrium Bubble Point Calculation 3-3-3 Equilibrium Dew Point Calculation 3-3-4 Distillation Stage Dynamic Model 3-3-5 Bottom Sump 3-3-6 Condenser 3-3-7 Reflux Accumulator 3-3-8 Feedback Controllers

3-4 Writing a Dynamic Distillation Simulator 3-5 Plant-Model Verification 3-6 Computational Performance 3-7 Conclusions 3-8 Nomenclature

References

Approximate and Simplified Models, Antonis Papadourakis and John E. Rijnsdorp

4-1 Introduction 4-2 Classification of Simple Models 4-3 Simple Steady-State Models 4-4 Partitioning of the Overall Dynamic Model

4-4-1 Introduction 4-4-2 Assumptions 4-4-3 Propagation of Vapor Flow and Pressure Responses 4-4-4 Propagation of Liquid Flow and Liquid Holdup Variations 4-4-5 Propagation of Vapor and Liquid Concentrations

4-5 Linear Models 4-5-1 Linear Models in the Time Domain 4-5-2 Linear Models in the Laplace Domain 4-5-3 Linear Models in the Frequency Domain

4-6 Nonlinear Models 4-6-1 Simplifying Assumptions 4-6-2 Number of Components 4-6-3 Number of Stages-Orthogonal Collocation References

24 24

27

29

29 30 32 32 34 35 36 36 37 37 38 39 39 41 41 42 43 43 44 45 45 46 46

48

48 48 49 50 50 52 53 53 54 55 55 55 59 61 61 62 62 69

Contents xv

5 Object..()riented Simulation, B. D. Tyreus 72

5-1 Introduction 72 5-2 Ideal Dynamic Simulator 73 5-3 Object-Oriented Programming 74

5-3-1 Classes and Objects 75 5-3-2 Modelling a Column Tray 75 5-3-3 Inheritance and Polymorphism 79

5-4 Distillation Column Simulation with Object-Oriented Programming 81 5-4-1 Structured Models 81 5-4-2 Structured Models and Object-Oriented Programming 82

5-5 Experience in Using Object-Oriented Simulation for Distillation 83 5-6 Conclusion 84

References 84

6 Plantwide Process Control Simulation, Ernest F. Vogel 86

6-1 Introduction 86 6-2 Applications of a Plantwide Process Control Simulator 86

6-2-1 Process Control 88 6-2-2 Process Design 89 6-2-3 Process Safety 89 6-2-4 Example 90

6-3 Benefit from Plantwide Process Control Simulation 91 6-4 Defining the Scope of a Plantwide Process Simulation 91 6-5 Building a Plantwide Process Simulation 92

6-5-1 Programming Environment 92 6-5-2 Equation Solving Environment 92 6-5-3 Steady-State-Dynamic Flowsheet Simulation Environment 93 6-5-4 Practical Considerations 93

6-6 Features of a Plantwide Process Simulator for Control Strategy Design 94 References 95

7 Identification of DistiUation Systems, R. C. McFarlane and D. E. Rivera 96

7-1 Introduction 96 7-1-1 Discrete Transfer Function Models for Distillation Systems 97 7-1-2 Iterative Methodology of System Identification 98

7-2 Perturbation Signal Design 99 7-2-1 Discussion 99 7-2-2 Pseudo-random Binary Sequence Signals 100

7-3 Model Structure Selection and Parameter Estimation 103 7-3-1 Bias-Variance Trade-Offs in System Identification 103 7-3-2 Nonparametric Methods 105 7-3-3 Parametric Models 106 7-3-4 Identification for Control System Design 109 7-3-5 Identifiability Conditions for Closed-Loop Systems 115 7-3-6 Treatment of Nonlinearity 117

7-4 Model Validation 119 7-4-1 Classical Techniques 119 7-4-2 Control-Relevant Techniques 120

7-5 Practical Considerations 122 7-6 Example 123 7-7 Nomenclature 136

References 138

xvi Contents

8 Selection of Controlled and Manipulated Variables, Charles F. Moore 140

8-1 Introduction 140 8-2 Sensor and Valve Issues 140

8-2-1 Inventory Control Concerns 140 8-2-2 Separation Control Concerns 142 8-2-3 Loop Sensitivity Issues 142

8-3 Location of Temperature Sensors 145 8-3-1 Determining Temperature Sensitivities 145 8-3-2 Selecting a Temperature Sensor for Single-Ended Control 147 8-3-3 Selecting Temperature Location for Dual-Ended Control 148

8-4 Selecting Sensor Type: Temperature versus Composition 157 8-4-1 Limitation of Temperature Sensors 157 8-4-2 Operational Concerns with Using Process Analyzer 159 8-4-3 Schemes for Using Analyzers in Distillation Control 160 8-4-4 Analyzer Resolution Requirements versus Location 161 8-4-5 Determining Composition Sensitivities 162 8-4-6 Selecting an Analyzer Location for Single-Ended Control 162 8-4-7 Selecting Analyzer Locations and Focus for Dual-Ended Control 165

8-5 Other Roles for Column Analyzers 169 8-5-1 Feedforward Control 169 8-5-2 Recycle Inventory Control 170 8-5-3 Measuring and Documenting Variation 170

8-6 Selecting Manipulated Variables 170 8-6-1 Steady-State Considerations 171 8-6-2 Dynamic Considerations 173 8-6-3 Plantwide Considerations 174

8-7 Summary and Conclusions 176 References 177

9 Selection of Controller Structure, B. D. Tyreus 178

9-1 Introduction 178 9-1-1 Control Design Principles 179

9-2 Manipulative Variables 179 9-2-1 Manipulative Variables and Degrees of Freedom 181

9-3 A Methodology for Selection of Controller Structure 183 9-3-1 Level and Pressure Controls 184 9-3-2 Composition Controls 184 9-3-3 Optimizing Controls 185

9-4 Examples 185 9-4-1 A Column with a Stripping Section Sidestream 185 9-4-2 A Column with a Rectifying Section Sidestream 188

9-5 Conclusion 191 References 191

10 Control Structures, Consistency, and Transformations, Kurt E. Hiiggblom and Kurt V. Waller 192

10-1 Introduction 192 10-2 Some Basic Properties of Distillation Control Structures 194

10-2-1 Energy Balance Structure (L, V) 194 10-2-2 Material Balance Structures (D, V) and (L, B) 196

Contents xvii

10-3 Consistency Relations 198 10-3-1 (L, V) Structure 198 10-3-2 (D, V) Structure 199 10-3-3 (L, B) Structure 200

10-4 Transformations between Control Structures 201 10-4-1 Transformation from (L, V) to (D, V) 201 10-4-2 Transformation from (L, V) to (L, B) 203

10-5 Control Structure Modelling-the General Case 204 10-5-1 Compact Description of Control Structures 204 10-5-2 Consistency Relations 205 10-5-3 Transformations between Arbitrary Structures 206 10-5-4 Complex Distillation Columns 207

10-6 Application 1: Numerical Examples of Control Structure Transformations 208 10-6-1 (D, V) Structure 209 10-6-2 (LID, V) Structure 209 10-6-3 (LID, VIF) Structure 210 10-6-4 (LID, VIB) Structure 210

10-7 Application 2: Use of Consistency Relations in Transformations 211 10-8 Application 3: Process Dynamics 212 10-9 Application 4: Identification of Consistent Models 214

10-9-1 Reconciliation of Control Structure Models 215 10-9-2 Numerical Example 216

10-10 Application 5: Relative Gain Analysis 219 10-10-1 Some Analytical Relations between Relative Gains 220 10-10-2 Numerical Example 221

10-11 Application 6: Synthesis of Decoupled Control Structures by Transformations of Output Variables 221 10-11-1 Derivative of Output Transformations 222 10-11-2 Numerical Examples 223 10-11-3 Discussion of Output Decoupling Structures Suggested in the Literature 224

10-12 Application 7: A Control Structure for Disturbance Rejection and Decoupling 225 Acknowledgment 226 References 227

11 Diagonal Controller Tuning, William L. Luyben 229

11-11 Introduction 229 11-1-1 The Problem 230 11-1-2 Alternatives 230 11-1-3 LACEY Procedure 233 11-1-4 Nomenclature 233

11-2 Selection of Controlled Variables 235 11-3 Selection of Manipulated Variables 235

11-3-1 Morari Resiliency Index 235 11-3-2 Condition Number 237

11-4 Tuning Diagonal Controllers in a Multivariable Environment 237 11-4-1 Review of Nyquist Stability Criterion for SISO Systems 237 11-4-2 Extension to MIMO Systems 238 11-4-3 BLT Tuning Procedure 239 11-4-4 Examples 239

11-5 Pairing 243 11-5-1 Elimination of Unworkable Pairings 244 11-5-2 Tyreus Load Rejection Criterion 246

11-6 Conclusion 247

xviii Contents

12 Dynamic Matrix Control Multivariable Controllers, David A. Hokanson and James G. Gerstle

12-1 Introduction 12-2 Basics of DMC Mathematics

12-2-1 Convolution Models 12-2-2 Prediction Errors 12-2-3 Control Solution 12-2-4 Move Suppression

12-3 Review of Model Identification 12-3-1 DMC Model Identification Background 12-3-2 Integrating Process Model Identification 12-3-3 Multivariable Model Identification 12-3-4 Nonlinear Transformations

12-4 Design Aspects of a Multivariable DMC Controller 12-4-1 Weights 12-4-2 Constraints

12-5 Implementation Steps for a DMC Controller 12-5-1 Initial Design 12-5-2 Pretest 12-5-3 Plant Test 12-5-4 Model Identification 12-5-5 Controller Building and Simulation 12-5-6 Controller and Operator Interface Installation 12-5-7 Controller Commissioning 12-5-8 Measuring Results

12-6 DMC Applications on Industrial Towers 12-6-1 Hydrocracker C3 - C4 Splitter 12-6-2 Hydrocracker Preftash Column 12-6-3 Benzene and Toluene Towers 12-6-4 Olefins Plant Demethanizer 12-6-5 Olefins Plant C2 Splitter

12-7 Summary References

13 DistiUation Expert System, F. G. Shinskey

13-1 Introduction 13-1-1 On-line versus Off-line Systems 13-1-2 Expertise in a Knowledge Domain 13-1-3 Logical Rule Base 13-1-4 First Principles and Mathematical Modelling

13-2 Configuring Distillation Control Systems 13-2-1 Nonlinear Multivariable System 13-2-2 Relative Gain Analysis 13-2-3 Establishing a Performance Index 13-2-4 Applications and Objectives

13-3 Rule Base for Simple Columns 13-3-1 Economic Objective 13-3-2 Maximizing Recovery 13-3-3 Controlling Both Product Compositions 13-3-4 Floating Pressure Control

248

248 249 249 253 253 254 255 255 256 257 258 260 260 261 261 262 263 263 265 266 267 267 267 268 268 268 268 269 270 270 271

272

272 272 273 273 274 275 275 277 279 283 284 284 285 286 287

Contents xix

13-4 Rule Base for Sidestream Columns 288 13-4-1 Classifications 288 13-4-2 Configuration Rules 288

13-5 Nomenclature 290 References 290

14 Robust Control, Sigurd Skogestad 291

14-1 Robustness and Uncertainty 291 14-2 Traditional Methods for Dealing with Model Uncertainty 292

14-2-1 Single-Input-Single-Output Systems 292 14-2-2 Multi-Input-Multi-Output Systems 293

14-3 A Multivariable Simulation Example 294 14-3-1 Analysis of the Model 294 14-3-2 Use of Decoupler 296 14-3-3 Use of Decoupler When There is Model Uncertainty 297 14-3-4 Alternative Controllers: Single-Loop PIO 298 14-3-5 Alternative Configurations: DV Control 299 14-3-6 Limitations with the Example: Real Columns 301

14-4 RGA as a Simple Tool to Detect Robustness Problems 301 14-4-1 RGA and Input Uncertainty 301 14-4-2 RGA and Element Uncertainty/Identification 302

14-5 Advanced Tools for Robust Control: IL Analysis 303 14-5-1 Uncertainty Descriptions 304 14-5-2 Conditions for Robust Stability 305 14-5-3 Definition of Performance 307 14-5-4 Conditions for Robust Performance 307

14-6 Nomenclature 308 References 309

Part 3 Case Studies 311

15 Experimental Comparison of Control Structures, Kurt V. Waller 313

15-1 Introduction 313 15-2 Manipulator Choice for Decentralized Control 314 15-3 Experimental Apparatus 315 15-4 Mixture Distilled 316 15-5 Control Structures Studied 316 15-6 General Comments on the Experiments 318 15-7 (D, V), (V, D), and (L, B) Structures 320 15-8 (D/(L + D), V) Structure 321 15-9 (D /(L + D), V /B) Structure 322 15-10 Comparison of Four Conventional Control Structures 324

15-10-1 One-Point Control 324 15-10-2 Two-Point Control 325

15-11 Structure for Disturbance Rejection and Decoupling 326 15-12 Controller Tuning for Robustness against Nonlinearities 328 15-13 Summary and Conclusions 328

Acknowledgment 329 References 329

xx Contents

16 Industrial Experience with Double Quality Control, Henk Leegwater 331

16-1 Introduction 331 16-2 Quality Control 331 16-3 Single Quality Control 331 16-4 Why Double Quality Control? 332

16-4-1 Energy Consumption Versus Degradation of Valuable Product 332 16-4-2 Optimizing Throughput 333

16-5 Quality Measurements 334 16-6 Why Multivariable Control for Double Quality Control? 336 16-7 Heat and Material Balance in Relation to Separation 337 16-8 Net Heat Input 338 16-9 Separation Indicators 339

16-9-1 Separation Performance Indicator 339 16-9-2 Separation Accent Indicator 339 16-9-3 Interpretation 339

16-10 Development of the Control Scheme for the C2 Splitters 340 16-10-1 Characterization of the Column Operation 340 16-10-2 Column Simulation 340

16-11 Introduction of the New Control Schemes into Industrial Practice 341 16-12 C2 Splitter with Heat Integration 341

16-12-1 Basic Controls 342 16-12-2 Quality Measurements 342 16-12-3 Previous Control Scheme 342 16-12-4 Improved Control Scheme 342 16-12-5 Condenser-Reboiler Level Control 343 16-12-6 Experiences 346

16-13 C2 Splitter without Heat Integration 347 16-13-1 Basic Controls 347 16-13-2 Quality Measurements 347 16-13-3 Previous Control Scheme 347 16-13-4 Interaction 347 16-13-5 Improved Control Scheme Using Qnet/F and the Separation Factors 348 16-13-6 Experiences 349

16-14 Conclusion 350

17 Control of Distillation Columns via Distributed Control Systems, T. L. Tolliver 351

17-1 Introduction 17-1-1 Historical Background on Distributed Control Systems 17-1-2 Advantages over Analog Instrumentation 17-1-3 Future Trends

17-2 Case I 17-2-1 Process Background-Debottlenecking 17-2-2 Control Scheme Design 17-2-3 Implementation Details 17-2-4 Results

17-3 Case II 17-3-1 Process Background-Reduced Capital 17-3-2 Control Scheme Design 17-3-3 Implementation Details 17-3-4 Results

17-4 Case III 17-4-1 Process Background-Energy Conservation 17-4-2 Control Scheme Design

351 351 352 353 353 353 355 357 359 359 359 359 361 363 364 364 364

Contents xxi

17-4-3 Implementation Details 366 17-4-4 Results 367

17-5 Summary 368 References 368

18 Process Design and Control of Extractive Distillation, Vmcent G. Grassi II 370 18-1 Overview 370

18-1-1 Extractive and Azeotropic Distillation 371 18-1-2 History 373 18-1-3 Process Description 374

18-2 Phase Equilibria 375 18-2-1 Relative Volatility 375 18-2-2 Residue Curves 377

18-3 Process Design 380 18-3-1 Degrees of Freedom 380 18-3-2 Process Design Procedure 382 18-3-3 Total Cost Relations 386

18-4 Process Control 387 18-4-1 Control System Economics 388 18-4-2 Measured Variables 392 18-4-3 Extraction Tower Nonlinearities 394 18-4-4 Extraction Tower Open Loop Dynamics 398 18-4-5 Control Schemes 398 18-5 Conclusions 403 References 403

19 Control by Tray Temperature of Extractive Disdlladon, John E. Anderson 405

19-1 Situation 19-2 Analysis 19-3 Solution 19-4 Conclusions

405 405 408 412

20 Disdlladon Control in a Plantwlde Control Environment, James J. Downs 413

20-1 Introduction 20-2 Plantwide Component Inventory Control

20-2-1 Component Inventory Control for a Tank 20-2-2 Component Inventory Control for a Process

20-3 Acetaldehyde Oxidation Process Case Study 20-3-1 Description of the Problem 20-3-2 Low Boiler Column Analysis 20-3-3 Component Inventory Control Analysis 20-3-4 Control of the Acetaldehyde Oxidation Process

20-4 Conclusions

21 Model-Based Control, F. F. Rhiel

21-1 Introduction 21-2 Design of the Control Concept

21-2-1 Process Description

413 414 414 417 423 423 427 432 438 439

440

440 440 440

xxii Contents

21-2-2 Design of the Observer Model 442 21-2-3 Control Concept 444

21-3 Results of Model-Based Control 444 21-3-1 Observer and Controller Behaviour 444 21-3-2 Introducing the New Control Concept in the Production Plant 445

21-4 Comparison between Conventional and New Control Concept 446 21-5 Conclusions 449 21-6 Appendix 449

References 450

22 Superfractionator Control, William L. Luyben 451

22-1 Occurrence and Importance 451 22-2 Features 451

22-2-1 Many Trays 452 22-2-2 High Reflux Ratios 452 22-2-3 Flat Temperature Profile 452 22-2-4 Slow Dynamics 452

22-3 Alternative Control Structures 453 22-4 Industrial Example 455

22-4-1 Process 455 22-4-2 Plant Dynamic Tests 455 22-4-3 Simulation 455 22-4-4 Results 455

22-5 Tuning the D-B Structure 457 22-5-1 Transformations 457 22-5-2 Example 458 22-5-3 Fragility of D-B Structure 459

22-6 Pitfalls with Ratio Schemes 459 22-7 Superfractionator with Sidestream Example 461

22-7-1 Process 461 22-7-2 Plant Dynamic Tests 462 22-7-3 Steady-State Analysis 462 22-7-4 Simulation Results 466

22-8 Conclusion 467 References 467

23 Control of Vapor Recompression Distillation Columns, Cristian A. Muhrer 468

23-1 Introduction 468 23-2 Design 469 23-3 Dynamics and Control 470 23-4 Alternative Compressor Control Systems 471

23-4-1 Compressor Performance Curves 472 23-4-2 Plant Characteristic Curve 474 23-4-3 Description of Alternative Compressor Controls 475 23-4-4 Dynamic Performance 477

23-5 Case Studies 478 23-5-1 Steady-State Design 478 23-5-2 Dynamic Models 481 23-5-3 Control System Design 482 23-5-4 Results for Specific Systems 488

23-6 Conclusion 23-7 Nomenclature

References

24 Heat-Integrated Columns, Wdliam L. Luyben

24-1 Introduction 24-2 Types of Systems

24-2-1 Energy Integration Only 24-2-2 Energy and Process Integration

24-3 Economic Incentives 24-4 Limitations 24-5 Control Problem 24-6 Total Heat-Input Control 24-7 Incentives for Composition Control of All Products 24-8 Conclusion

References

25 Batch Distillation, Cristian A. Muhrer and Wdliam L. Luyben

25-1 Introduction 25-2 Basic Operations

25-2-1 Process 25-2-2 Composition Profiles 25-2-3 Slop Cuts

25-3 Assessment of Performance: Capacity Factor 25-4 Models

25-4-1 Differential Distillation 25-4-2 Pseudo-Steady-State Models 25-4-3 Rigorous Dynamic Models 25-4-4 Fitting Models to Experimental Batch Distillation Data

25-5 Comparison with Continuous Distillation 25-6 Reflux Ratio Trajectories 25-7 Pressure Trajectories

25-7-1 Constant Pressure 25-7-2 Constant Reflux-Drum Temperature

25-8 Column Design 25-9 Slop Cut Processing

25-9-1 Alternatives 25-9-2 Results

25-10 Inferential Control of Batch Distillation 25-10-1 Problem 25-10-2 Basic Insight 25-10-3 Quasidynamic Model 25-10-4 Extended Luenberger Observer

25-11 Conclusion References

Index

Contents xxiii

489 490 490

492

492 492 492 498 499 501 502 502 504 507 507

50S

508 508 508 509 510 510 511 511 512 512 513 514 514 515 515 515 515 517 517 518 519 519 519 521 523 525 528

529