DISTILLATION CONTROL DISTILLATION CONTROL An Engineering Perspective CECIL L. SMITH A JOHN WILEY & SONS, INC., PUBLICATION Copyright © 2012 by John Wiley & Sons, Inc. All rights reserved Published by John Wiley & Sons, Inc., Hoboken, New Jersey Published simultaneously in Canada 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, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. 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TP156.D5D586 2012 660'.28425–dc23 2011041437 Printed in the United States of America ISBN: 9780470381946 10 9 8 7 6 5 4 3 2 1 v CONTENTS Preface ix 1 Principles 1 1.1. Separation Processes 2 1.2. Total Material Balance 9 1.3. Refl ux and Boilup Ratios 13 1.4. Total Material Balance around Condenser 18 1.5. Total Material Balance around Reboiler 21 1.6. Component Material Balances 24 1.7. Energy and the Separation Factor 28 1.8. Multicomponent Distillation 35 1.9. Stage-by-Stage Separation Model 38 1.10. Formulation of the Control Problem 47 1.11. Tower Internals 50 1.12. Flooding 55 1.13. Tray Hydraulics 59 1.14. Inverse Response in Bottoms Level 62 1.15. Composition Dynamics 65 References 69 2 Composition Control 70 2.1. Product Specifi cations 71 2.2. Columns in Series 75 2.3. Composition Analyzers 78 vi CONTENTS 2.4. Temperature 83 2.5. Distillate Composition Control: Constant Boilup 91 2.6. Distillate Composition Control: Constant Bottoms Flow 96 2.7. Operating Lines 100 2.8. Temperature Profi les 106 2.9. Feed Composition Disturbances 111 2.10. Bottoms Composition Control 116 2.11. Propagation of Variance in Level Control Confi gurations 122 2.12. Level Control in Direct Material Balance Confi gurations 126 3 Pressure Control and Condensers 136 3.1. Pressure Control 137 3.2. Once-Through Heat Transfer Processes 142 3.3. Water-Cooled Condensers 147 3.4. Flooded Condensers 151 3.5. Air-Cooled Condensers 159 3.6. Partial Condensers 162 3.7. Atmospheric Towers 167 3.8. Vacuum Towers 169 3.9. Floating Pressure/Pressure Minimization 173 Reference 179 4 Reboilers and Feed Preheaters 180 4.1. Types of Reboilers 181 4.2. Steam-Heated Reboilers 185 4.3. Hot Oil 195 4.4. Fired Heaters 198 4.5. Feed Preheater 200 4.6. Economizer 204 References 208 5 Applying Feedforward 209 5.1. Feed Flow and Composition 210 5.2. Internal Refl ux Control 220 5.3. Extreme Feedforward 226 5.4. Feedforward for Bottoms Level 229 5.5. Feedforward for Column Pressure 234 5.6. Product Compositions 238 Reference 242 6 Unit Optimization 243 6.1. Energy and Separation 244 6.2. Optimization of a Column 250 CONTENTS vii 6.3. Constraints in Distillation Columns 255 6.4. Control Confi gurations for Single Constraint 258 6.5. Control Confi gurations for Multiple Constraints 266 References 272 7 Double-End Composition Control 273 7.1. Defi ning the Problem 273 7.2. Options for Composition Control 275 7.3. Relative Gain 283 7.4. Relative Gains from Open Loop Sensitivities 290 7.5. Relative Gains for Other Confi gurations 294 7.6. Ratios for Manipulated Variables 296 7.7. Effect of Operating Objectives 300 7.8. MPC 303 8 Complex Towers 306 8.1. Heat Integration 307 8.2. Side Heater/Side Cooler 311 8.3. Sidestreams 316 8.4. Withdrawing a Liquid Sidestream 319 8.5. Withdrawing a Vapor Sidestream 322 8.6. Composition Control in Sidestream Towers 324 Index 329 ix PREFACE Two observations constitute the basis for this book: 1. Despite its thirst for energy, distillation continues to be widely used for separations. Effi ciently operating these columns requires a high degree of automatic control. 2. Virtually all column designs are based on a steady - state separation model. Especially for columns separating nonideal materials, there is no alternative. The perspective of this book is that the steady - state separation model should also be the basis for developing the control confi guration for the column. Yes, a steady - state model! Although the technology to do so is widely available, extending to a dynamic model is not necessary for developing the column control confi guration. The most crucial component of every process control application is devel- oping the piping and instrumentation (P & I) diagram that defi nes the control confi guration for the process and for each unit operation, such as distillation, within that process. If the P & I diagram is correct, the loops can be successfully commissioned and tuned to deliver the required performance. But where the confi guration is defi cient, the usual consequence is tuning diffi culties. Until the defi ciencies in the P & I diagram are corrected, neither automatic tuning, tuning techniques, nor experienced tuning professionals can succeed. For something so crucial to success in process control, one would think rigorous procedures would be available to derive the P & I diagram from the process characteristics, operating objectives, and so on. Instead, the usual x PREFACE practice is basically copying — the control confi guration from a sister plant with the same or similar process is used as the starting point for the P & I diagram. This works reasonably well in power generation, pulp and paper, oil refi ning, and other industries where the same basic process technology is being repli- cated, but with different production rates, different feedstocks, and so forth. How many outright mistakes have been copied? How many times has a poorly performing confi guration been copied when a better performing confi guration could be implemented? Despite an occasional “ war story, ” the answers to such questions are largely opinions. One should expect better, specifi cally, a rigorous procedure for translating the characteristics of the process (as expressed by models) and the operating objectives into a P & I diagram. This would also be useful when choosing between design alternatives, thus promoting the integration of process design and process control. Steady - state models are now available for all unit operations, and such models are the basis for most modern plant designs. Especially for continuous processes, the process fl ow sheet is developed using these models. Such models should also provide the basis for developing the P & I diagram. For too long, the primary focus of process control has been the linear systems theory. Rarely is such technology useful in developing a P & I diagram. This perspective is the basis of another misconception, specifi cally, that the dynamic behavior of the process dictates the appropriate control confi gura- tion. This seems to translate to “ control every variable with the nearest valve ” as the guiding principle for developing a P & I diagram. Is this done con- sciously? Not usually, but if you examine enough P & I diagrams, it seems to turn out that way. However, if process dynamics receive the primary cons- ideration in developing the control confi guration, this would often translate to “ control every variable with the nearest valve. ” The steady - state characteristics of the process largely determine the appro- priate control confi guration. What is the direct and long - term infl uence of a fi nal control element on one or more controlled variables? When developing a P & I diagram, the customary practice is to rely on a qualitative assessment. While this is often suffi cient, processes can be subtle and occasionally behave very differently from what is expected. When this occurs, the resulting P & I diagram is defi cient. This prospect increases with the complexity of the process, with the haste with which the P & I diagram must be developed, and with the inexperience of the developer of the P & I diagram. Process characteristics are best expressed in the form of a model for the process. Given the current availability of such models, it is time to begin relying on a quantitative assessment of process characteristics. This is short of the ultimate goal, namely to derive the P & I diagram from such models. However, this is a step in the right direction, and distillation is a good unit operation to use as the starting point. Operating variables such as product fl ows, refl ux, and boilup affect the composition of all product streams, but not to the same degree. The selection of the control confi guration is preferably based on a PREFACE xi quantitative assessment of their effect. For this, the steady - state separation model suffi ces. Single - end composition control is rather forgiving. Double - end composi- tion control is not. The same can be said for sidestream towers for which two product compositions must be controlled. For columns separating well - behaved materials, statements can be developed to guide the choice of the control confi guration. However, these statements must be used cautiously for columns separating nonideal materials. In either case, the preferable approach is to base the choice of the control confi guration on a quantitative assessment of column behavior computed from the steady - state separation model used for column design. C ecil L. S mithHouston, Texas November 28, 2011 1 PRINCIPLES 1 Distillation Control: An Engineering Perspective, First Edition. Cecil L. Smith. © 2012 John Wiley & Sons, Inc. Published 2012 by John Wiley & Sons, Inc. A distillation column obtains separation through energy. Consequently, it seems intuitive that a product composition must be controlled by manipulating a term relating to energy. When the composition of both product streams from a two - product tower must be controlled, this suggests the following approach: • Control the distillate composition by adjusting the refl ux. • Control the bottoms composition by adjusting the boilup. For most columns, this control confi guration exhibits a substantial degree of interaction, which translates to operational problems in the fi eld. An alternate approach is as follows: • Control the composition of one of the products (distillate or bottoms) by adjusting an energy term (refl ux or boilup). • Control the composition of the other product by adjusting the respective product draw. For most applications, the degree of interaction is much lower. With this approach, one of the compositions is being controlled by directly adjusting a term in the column material balance. Consequently, this presenta- tion begins with various material balances (entire tower, condenser only, [...]... (D or L) 2 Control reflux drum level with the other flow However, level cannot be controlled by manipulating a very small flow If L/D > 1, drum level cannot be controlled by manipulating D Figure 1.8 presents the two possible control configurations, which are designated direct material balance control and indirect material balance control The... control 2 Control bottoms level with the other flow However, level cannot be controlled by manipulating a very small flow If V/B > 1, drum level cannot be controlled by manipulating B Figure 1.10 presents the two possible control configurations, which are designated direct material balance control and indirect material balance control The... the level controller to satisfy the steady-state material balance for the reboiler: 24 PRINCIPLES TABLE 1.2 Control Configurations for Bottoms Composition Direct Material Balance Control Control configuration Manipulated variable for composition Manipulated variable for bottoms level Solution of reboiler material balance Preferred for level control if Impractical if Indirect Material Balance Control Figure... towers This chapter reviews the general principles of distillation that are relevant to process control, including • material balances, energy, and separation; • composition control, through either energy terms or product flows; • the stage-by-stage separation models for multicomponent distillation and their utility in control analyses; • tray towers and packed towers; • column dynamics 1.1 SEPARATION... close If the feed flow is constant, then Cooling Media Condenser Reflux Drum LT Distillate, D Feed, F LT Reboiler Heating Media Bottoms, B Figure 1.4 Holdups in a column 10 PRINCIPLES 1 any long-term change in the distillate flow must be offset by an equal and opposite change in the bottoms flow; 2 any long-term change in the bottoms flow must be offset by an equal and opposite change in the distillate flow... of every level controller is to close some material balance To assure that the column material balance closes, every column control configuration must contain one of the following: 13 REFLUX AND BOILUP RATIOS 1 The reflux drum level is controlled by manipulating the distillate flow 2 The bottoms level is controlled by manipulating the bottoms flow Providing both is also an option 1.3 REFLUX AND BOILUP RATIOS... material balance control and indirect material balance control pertain to how the value of the distillate flow is determined Table 1.1 summarizes the attributes of the two configurations The configuration in Figure 1.8a is the direct material balance control configuration Values for D and L are determined as follows: TOTAL MATERIAL BALANCE AROUND REBOILER 21 D—specified by the distillate composition controller;... equations is referred to as an integrating process An alternate term is ramp process (the response to any upset is a ramp in the holdup or level) or non-self-regulated process (the process will not seek an equilibrium unless control actions are taken) 1.2.6 Level Control An integrating process does not seek its own equilibrium If there is an imbalance in the total material balance, the result is one of... balance for the column: F = D + B The terms direct material balance control and indirect material balance control pertain to how the value of the bottoms flow is obtained Table 1.2 summarizes the attributes of the two configurations The configuration in Figure 1.10a is the direct material balance control configuration Values for B and V are determined as follows: B—specified by the bottoms composition controller;... balance control (b) Indirect material balance control TABLE 1.1 Control Configurations for Distillate Composition Direct Material Balance Control Control configuration Manipulated variable for composition Manipulated variable for drum level Solution of condenser material balance Preferred for level control if Impractical if Indirect Material Balance Control Figure 1.8a Distillate D Reflux L L = VC − D L>D L/D . DISTILLATION CONTROL DISTILLATION CONTROL An Engineering Perspective CECIL L. SMITH A JOHN WILEY &. PRINCIPLES 1. any long - term change in the distillate fl ow must be offset by an equal and opposite change in the bottoms fl ow; 2. any long - term change in