dwm OF DISTILLATION COLUMN CONTROL SYSTEMS PAGE S. BUCKLEY WILLIAM L. LUYBEN JOSEPH P. SHUNTA PRINCIPAL CONSULTANT. ENGINEERING DEPARTMENT E.I. DU PONT DE NEMOURS & CO. PROFESSOR OF CHEMICAL ENGINEERING & CONSULTANT LEHIGH UNIVERSITY SENIOR CONSULTANT, ENGINEERING DEPARTMENT E.I. DU PONT DE NEMOURS & CO. Edward Arnold Design of Distillation Column Control Systems 8 Instrument Society of America 1985 All rights reserved Printed in the United States of America In preparing this work, the author and publisher have not investigated or considered patents which may apply to the subject matter hereof. It is the responsibility of the readers and users of the subject matter to protect themselves against liability for infringement of patents. The information contained herein is of a general educational nature. Accordingly, the author and publisher assume no responsibility and disclaim all liability of any kind, however arising, as a result of using the subject matter of this work. The equipment referenced in this work has been selected by the author as examples of the technology. No endorsement of any product is intended by the author or publisher. In all instances, the manufacturer's procedures should prevad regarding the use of specific equipment. No representation, expressed or implied, is made with regard to the availability of any equipment, process, formula, or other procedures contained herein. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher: Instrument Society of America 67 Alexander Drive P. 0. Box 12277 Research Triangle Park North Carolina 27709 United States of America ISBN 0-7131-3551-4 Library of Congress Cataloging in Publication Data Buckley, Page S. Design of distillation column control systems. Includes indexes. 1. Distillation apparatus. 2. Chemical process control. 1. Luyben, William L. 11. Shunta, Joseph P. 111. Title. TP159D5B83 1985 660.2'8425 84-27813 ISBN 0-7131-3551-4 Book design by Raymond Solomon Production by Publishers Creative Services Inc., New York Preface t his is a book about the design of disullation column control systems. It is written primarily fiom the standbint of an engineering design organization, and is based on years of experience with large design projects as well as on personal plant experience. Most new investment dollars go into new or modemized facilities, and it is in the design phase of projects for these facilities that the most opportunities occur and flexibility exists to influence process control. Consequently this book is aimed primarily at design personnel. It is our hope, however, that it will also be usell to those who have to operate or troubleshoot existing plants. Part I is an introduction, including a perspective on control and a brief review of fundamentals of &stillation, with emphasis on topics that will be of interest to the control engneer rather than to the column design engineer. The distillation review, it is hoped, will be particularly usell to nonchemical enpeers. Part I1 of the book, on concepts and configurations, discusses some practical aspects of distillation control. Once the requirements for a particular column in a particular process are understood, design engineers must make at least a preliminary choice of equipment arrangements and control system configuration. In this section we have mostly avoided the use of mathematics and control theory. It is our hope that our discussions of equipment and control system arrangements will be usell to process engineers, production supervisors, main- tenance engineers, and instrument engineers seeking guidelines, alternatives, and perspectives. Part I11 focuses on the quantitative design of distillation control systems. It is aimed at professional control engineers and any others concerned with the numerical definition and specification of control system performance. Probably the most important development in process control system design since about 1950 was the evolution of a substantial body of theory and mathematics, plus a large catalog of control system studies. Together, these permit quantitative design of most process control systems with a considerable degree of multivariable control. It is the purpose of this book to indicate the range of this technology, which has been developed for distillation control, to the point where it can be economically and reliably used for design. The ultimate economic advantages include lower plant investment (particularly in tankage), lower operating costs, and closer control of product quality. For the most part, we have stayed with the modest theory of single-input, single-output (SISO) systems presented in previous books: Techniques of Process Control by P. S. Buckley (Wiley, 1964) and Process Mohling, Simulation, and Control fm Chemical Engineers by W. L. Luyben (McGraw-Hill, 1973). This kind of theory and mathematics not only is adequate for noninteracting systems and for simple interacting systems, but it has the advantages of requiring minimum formal training and of permitting low design costs. “Modernyy or “optimal” control techniques are mentioned only briefly here because their use on real, industrial-scale distillation columns has been quite limited to date. These techniques are still being actively researched by a number of workers, and it is hoped that they eventually will be developed into practical design methods. As of the date of the writing of this book, however, these mathematically elegant methods are little used in industry because of their complexity, high engineering cost, and limitation to relatively low- order systems. Simulation techniques also are not covered since there are several texts that treat this topic extensively. In the past five years, we have witnessed the introduction and proliferation of microprocessor-based digital controls of various sorts that are intended to replace analog controls. In fact, most of the newly installed control systems are of this type. In addition, we are seeing more control being implemented in process control computers. Sampled-data control theory has taken on new importance because of these developments and so we have included a chapter on previous work we have done in this area as it relates specifically to distillation columns. The concepts we present are quite basic as opposed to the recent advances in adaptative, multivariable, and predictive control, but we hope they will benefit those interested in synthesizing single-loop sampled-data controllers. Many thanks are due our associates in the Du Pont Company, particularly R. K. Cox, and throughout the industrial and academic communities for helpful comments and suggestions. Many of the concepts presented in this book have been vigorously debated (over untold cans of beer) during the Distillation Control Short Courses held at Lehgh University every other spring since 1968. We also wish to thank Leigh Kelleher for major assistance in formatting and editing, Arlene Little and Elaine Camper for typing, and Ned Beard and his Art Group for preparing the illustrations. Pade S. Buckley William L. Luyben Joseph P. Shunta Nomenclature i n this work an effort has been made: (1) to use symbols and units commonly employed by chemical engineers, (2) to define each symbol in a chapter when the need for that symbol arises, and (3) to keep symbols and units as consistent as possible from chapter to chapter. A few symbols, however, have different meanings in different parts of the text. The list that follows contains the major symbols and their usual meanings: a A B C Cll D E F Bc BL h H i K 1 Lo LR M Mw P P C f transportation lag or dead time, usually seconds or minutes area, ft2 bottom-product flow, mols/min specific heat, pcu/lbm "C acoustic capacitance, fi5/lbf control-valve flow coefficient, gallons per minute of water flow when valve pressure drop is 1 psi diameter, feet, or top-product flow rate fi-om condenser or condensate receiver, mols/min Murphree tray efficiency cycles/minute or cps feed rate to column, mols/min ft Ibmass mass-force conversion factor, 32.2 - - sec' Ib force local acceleration due to gravity, ft/sec2 heat-transfer film c&cient, head of liquid or liquid level, feet fl (has different meaning when used as subscript) static gain distance, feet external reflux, mols/min liquid downflow in column, mols/min liquid holdup, mols molecular weight pressure, psi pressure, lbf/fi?, or atmosphere, or mm Hg pcu/sec "C fi? 523 524 Pcu 5 Q 4 R 5 t T U V VI- W Y ZF Z Y 29 X z a Ml- 5 6, 60 h Am CL P E 7 + 0 Nomenclature pound centigrade units (heat required to heat one pound of water 1°C) vapor pressure of pure component, speciesj heat flow, pcu/sec, or fraction of feed that is liquid (molar basis) flow rate, ft3/sec or ft3/min reflux ratio, LJD Laplace transform variable time, seconds or minutes temperature, degrees Celsius or Kelvin, or sampling time interval in sampled-data control systems pcu/sec f? "C overall heat-transfer coefficient, ~ vapor flow, mols/min, or volume, ft3 volume in tank corresponding to level transmitter span, AHT weight rate of flow, usually Ibm/sec weight, lbm mol fraction more volatile component in a liquid mol fiaction more volatile component in a vapor z-transform variable, or mol fraction more volatile component in feed acoustic or hydraulic impedance, Ibf sec/fi5 relative volatility specific heat ratio, or activity coefficient liquid-level transmitter span, feet, corresponding to full-scale output difkrence between set-point signal and signal from measurement device damping ratio in a quadratic expression arbitrary input signal arbitrary output signal latent heat of vaporization, pcu/lbm molar latent heat of vaporization, pcu/mol viscosity, lbmlft sec = centipoise/ 1488 density, lbm/ft3 time constant, usually seconds or minutes enthalpy, pcu/lbm fi-equency, radians/unit time Subscripts Q quadratic B bottom of tower R reset, or reflux 525 L H f f i i SP 0 S st C D OL light component or key heavy component or key feed feedforward inlet arbitrary tray location or component outlet stripping section set point steam controller distillate (top product) open loop (used outside of brackets) Symbols on Illustrations CC or xc FC LC PC TC HS LS HL LL cw composition control flow control liquid level control pressure control temperature control high signal selector low signal selector high signal limiter low signal limiter cooling water Individual barred terms (e.g., V, P) indicate average values. Combined barred terms [e.g., HG(z)] have special meaning in sampled-data control systems (see Chapter 21). K,G,(s) measurement transfer function K,G,(s) controller transfer function K,,G,(s) control valve transfer function KpGp(s) process transfer function Contents Preface Part I INTRODUCTION Chapter 1 Strategy for Distillation-Column Control 1.1 Distillation Control Objectives 1.2 Arrangements for Maw-id-Balance Control 1.3 Fundamentals of Composition Control 1.4 Compensation for Various Disturbances 1.5 Startup and Shutdown 1.6 Control System Design Philosophy 1.7 Procedure for Overall Control System Design 1.8 Column Design Philosophy and Control System Design 1.9 Existing Columns-Typical Practices and Troubleshooting 1.10 Conventions Followed in This Book 1.11 Literature Chapter 2 Fundamentals of Distillation 2.1 Introduction 2.2 Tray Hydraulics 2.3 Vapor-Liquid Equilibrium Fundamentals 2.4 Graphical Solution Techniques 2.5 Effects of Variables Part II CONCEPTS AND CONFIGURATIONS Chapter 3 Overhead System Arrangements 3.1 Introduction 3.2 Types of Condensers 3.3 Atmospheric Columns 3.4 3.5 Pressure Columns-Vapor Product 3.6 Miscellaneous Pressure-Control Techniques 3.7 3.8 Vacuum and Pressure Columns-Liquid Product Gravity-Return Reflux Versus Pumped-Back Reflux Control Techniques with Air-Cooled Condensers pwe mtt 1 3 3 6 11 12 13 14 19 19 20 21 22 25 25 28 30 49 65 67 69 69 70 72 80 84 86 90 99 V vi contents 3.9 ‘Tempered” Versus Once-Through Coolant 3.10 Level Control of Condensate Receiver and Required Holdup Chapter 4 Column-Base and Reboiler Arrangements 4.1 Introduction 4.2 Vertical Thermosyphon Reboilers 4.3 Flooded Thermosyphon (Steam-Side) Reboilers 4.4 Forced-Circulation Reboilers 4.5 Flooded-Bundle Kettle Reboilers 4.6 Internal Reboilers 4.7 Steam Supply and Condensate Removal 4.8 Required Holdup for Level Control 4.9 Miscellaneous Column-Base Designs 4.10 Miscellaneous Reboiler Designs Chapter 5 Feed System Arrangements 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 General Comments Feed Flow Control Feed Temperature Control Feed Enthalpy Control Feed Tray Location Feed Tank Sizing Feed System for Double-Column Systems Feeds with Makeup/Purge to Tankage Feed Systems in Sequences of Columns With and Without Recycles Chapter 6 Level Control and Feedforward Options 6.1 Introduction 6.2 6.3 6.4 Unfavorable Control Schemes 6.5 Unreasonable Control Schemes Material-Balance Control in Direction Opposite to Flow Material-Balance Control in Direction of Flow Chapter 7 Control of Sidestream Drawoff Columns Side-Draw Columns with Large Sidestreams 7.1 Introduction 7.2 100 100 109 109 110 114 116 117 119 122 126 130 133 137 137 137 140 141 143 144 145 149 151 153 153 154 157 166 166 169 169 169 contents 7.3 7.4 7.5 7.6 7.7 Chapter 8 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 Chapter 9 9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 Side-Draw Columns with Small Sidestreams Composition Control of Side-Draw Columns An Improved Approach to Composition Control of Side- Prefiactionator Plus Sidestream Drawoff Column Other Schemes Draw Columns Minimizing Energy Requirements Introduction Cons e rv a ti o n Design Considerations in Heat-Recovery Schemes Multiple Loads Supplied by a Single Source Single Source, Single Load Split Feed Columns Combined Sensible and Latent Heat Recovery Energy Recovery by Vapor Recompression Application of Protective Controls to Distillation Columns Introduction Overrides and Interlocks Implementation of Overrides Controllers in Override Circuits Anti Reset-Windup Feedforward Compensation with Overrides Overrides for Column Overhead System Overrides for Column-Base System Automatic Stamp and Shutdown “Idle” or Total Reflux Miscellaneous Overrides Design Considerations Overrides for Side-Draw Columns Chapter IO Indirect Composition Measurements 10.1 Introduction 10.2 Single-Tray Temperature 10.3 Differential Temperature 10.4 Differential Vapor Pressure vii 170 170 174 176 180 181 181 181 182 183 186 189 189 189 193 193 194 195 199 200 202 205 208 21 1 21 3 214 21 7 220 229 229 229 230 231 [...]... 249 255 256 273 279 279 288 289 Part 111 QUANTITATIVE DESIGN OF DISTILLATION CONTROL SYSTEMS Approaches to Quantitative Design Chapter 12 12.1 12.2 12.3 12.4 12.5 Ways of Designing Control Systems Kinds of Information Available Functional Layout of Control Loops Adjustment of Controller Parameters (Controller Tuning) Enhanced Control of Distillation Columns via On-Line Models 293 295 295 297 299 303 305... Control 16.1 Introduction 16.2 Level Control of Simple Vessels 16.3 Level Control of Overhead Condenser Receiver Via TopProduct Withdrawal 16.4 Level Control of Overhead Condenser Receiver Via Reflux Manipulation 16.5 Column- Base Level Control Via Bottom-Product Manipulation 16.6 Column- Base Level Control Via Feed Flow Manipulation 16.7 Column- Base Level Control Cascaded to Steam FlowControl 16.8 Column- Base... 21.10 Conventional control of X2 with set-point disturbance 516 2 1.1 Tracking sampled-data control of X2 with set-point disturbance 1 517 21.12 Conventional control of X2 with feed composition disturbance 518 2 1.13 Tracking sampled-data control of X2 with feed composition disturbance 519 21.14 Comparison of conventional and tracking PL control 520 Strategy for Distillation- Column Control i n chemical... relatively minor items Proper original design is by far the best way to guarantee satisfactory operation and control Therefore, in this book we will approach the design of integrated distillationcolumn control systems as a systems problem in process design The application of feedforward, feedback, and protective controls wdl be coordinated with the sizing and proper location of process holdups to achieve both... level control via bottom product 162 Material balance control in direction of flow, reflux drum level control via reflux, base level control via bottom product 163 Material balance control in direction of flow, reflux drum level control via dstillate, base level control via boilup 164 6.9 Like Figure 6.8 but with reflux ratioed to dstdlate 165 7.1 Basic control scheme for column with sidestream drawoff... column with sidestream drawoff 171 7.2 7.3 7.4 Controls for liquid sidestream drawoff column 172 Alternate control scheme for column with sidestream drawoff 173 Scheme for control of sidestream composition 175 7.5 Control of terminal composition 176 7.6 (A) In the control system finally chosen, the toluene impurity content in the dlstillate producer is controlled by the reflux ratio (B) The five alternative... noninteracting control of column product compositions 1 i DISTILLATION CONTROL OBJECTIVES The starring point of any design project is a definition of objectives For distillation there are many possible approaches, but the one chosen here is one the authors have found broadly useful in virtually all kinds of processes.’ It has three main facets: (1)material-balance control, (2) product quality control, and... Material-Balance Control C 0 L Q) 3 C 3 0 0 E d 8 s 2 m 8 2 CI z E 3 3 c e 3 - m d E 8 8 c m d - -?$ ' zE= 3 e,o 3 iz&% U Q 9 10 Strategy for Dljtillatwn -Column Control FIGURE 1.5 Distillation column with material balance control in direction of f o lw 1.3 Funahmentnls of Composition Control 11 1.3 FUNDAMENTALS OF COMPOSITION CONTROL Let us consider briefly what must be done to a column to keep terminal... signal flow diagram for column pressure control via manipulation of inert gas and vent valves 412 17.8 Reduction of signal flow diagram of Figure 17.7 412 17.9 Signal flow diagram for column pressure control via manipulation of inert gas and vent valves when reboiler steam is flow or flow ratio controlled 413 17.10 Reduced form of Figure 17.9 414 17.11 Column pressure control via flooded condenser drainnegligible... product 140 5.4 Column feed temperature control with economizer and preheater 141 5.5 Column feed enthalpy control with economizer and preheater 142 5.6 Column with multiple feed trays 144 5.7 Feed system for a split column 146 5.8 Feed system for split vacuum columns 147 5.9 Feed systems for column in parallel 148 5.10 Makeuptpurge feed systems 150 6.1 Bottom product demand, overhead level control via . DISTILLATION CONTROL SYSTEMS Chapter 12 Approaches to Quantitative Design Ways of Designing Control Systems Functional Layout of Control Loops Adjustment of Controller Parameters (Controller. 7.4 7.5 Control of terminal composition 7.6 Basic control scheme for column with sidestream drawoff Controls for liquid sidestream drawoff column Alternate control scheme for column with. and Shutdown 1.6 Control System Design Philosophy 1.7 Procedure for Overall Control System Design 1.8 Column Design Philosophy and Control System Design 1.9 Existing Columns-Typical Practices