Distillation Control E-Book

Cecil L. Smith

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 457

Veröffentlichungsjahr: 2012

Beschreibung

Learn to Design the Best Control Configuration for Any Distillation Column Today, distillation is by far the most common separation technique used in the chemical and petroleum industries. All distillation columns need to be carefully controlled in order to meet specified production and quality levels. Distillation Control enables readers to do this by approaching the subject from a process to develop, analyze, and troubleshoot all aspects of column controls. Readers are efficiency and effectiveness and minimizing coats. Distillation Control begins with a chapter dedicated to underlying principles, including separation processes, reflux and boilup ratios, and composition dynamics. Next, the author covers such critical topics as: * Composition control * Pressure control and condensers * Reboilers and feed preheaters * Application of feedforward * Unit optimization * Complex towers As readers progress through the text, they'll discover that the best control configuration for a distillation column is largely determined using steady-state process characteristics. The stage-by-stage separation models that the author sets forth for column design, therefore, provide information that is essential in developing the optimal control configuration. In addition to its clear explanations, Distillation Control is filled with clear diagrams and illustrations that clarify complex concepts and guide readers through multi-step procedures. Engineers as well as other professionals working in process facilities that use distillation to separate materials will fin that this book enables them to implement the latest tested and proven distillation control methods to meet their particular processing needs.

Leseprobe

Table of Contents

Cover

Title page

Copyright page

PREFACE

1 PRINCIPLES

1.1. SEPARATION PROCESSES

1.2. TOTAL MATERIAL BALANCE

1.3. REFLUX AND BOILUP RATIOS

1.4. TOTAL MATERIAL BALANCE AROUND CONDENSER

1.5. TOTAL MATERIAL BALANCE AROUND REBOILER

1.6. COMPONENT MATERIAL BALANCES

1.7. ENERGY AND THE SEPARATION FACTOR

1.8. MULTICOMPONENT DISTILLATION

1.9. STAGE-BY-STAGE SEPARATION MODEL

1.10. FORMULATION OF THE CONTROL PROBLEM

1.11. TOWER INTERNALS

1.12. FLOODING

1.13. TRAY HYDRAULICS

1.14. INVERSE RESPONSE IN BOTTOMS LEVEL

1.15. COMPOSITION DYNAMICS

2 COMPOSITION CONTROL

2.1. PRODUCT SPECIFICATIONS

2.2. COLUMNS IN SERIES

2.3. COMPOSITION ANALYZERS

2.4. TEMPERATURE

2.5. DISTILLATE COMPOSITION CONTROL: CONSTANT BOILUP

2.6. DISTILLATE COMPOSITION CONTROL: CONSTANT BOTTOMS FLOW

2.7. OPERATING LINES

2.8. TEMPERATURE PROFILES

2.9. FEED COMPOSITION DISTURBANCES

2.10. BOTTOMS COMPOSITION CONTROL

2.11. PROPAGATION OF VARIANCE IN LEVEL CONTROL CONFIGURATIONS

2.12. LEVEL CONTROL IN DIRECT MATERIAL BALANCE CONFIGURATIONS

3 PRESSURE CONTROL AND CONDENSERS

3.1. PRESSURE CONTROL

3.2. ONCE-THROUGH HEAT TRANSFER PROCESSES

3.3. WATER-COOLED CONDENSERS

3.4. FLOODED CONDENSERS

3.5. AIR-COOLED CONDENSERS

3.6. PARTIAL CONDENSERS

3.7. ATMOSPHERIC TOWERS

3.8. VACUUM TOWERS

3.9. FLOATING PRESSURE/PRESSURE MINIMIZATION

4 REBOILERS AND FEED PREHEATERS

4.1. TYPES OF REBOILERS

4.2. STEAM-HEATED REBOILERS

4.3. HOT OIL

4.4. FIRED HEATERS

4.5. FEED PREHEATER

4.6. ECONOMIZER

5 APPLYING FEEDFORWARD

5.1. FEED FLOW AND COMPOSITION

5.2. INTERNAL REFLUX CONTROL

5.3. EXTREME FEEDFORWARD

5.4. FEEDFORWARD FOR BOTTOMS LEVEL

5.5. FEEDFORWARD FOR COLUMN PRESSURE

5.6. PRODUCT COMPOSITIONS

6 UNIT OPTIMIZATION

6.1. ENERGY AND SEPARATION

6.2. OPTIMIZATION OF A COLUMN

6.3. CONSTRAINTS IN DISTILLATION COLUMNS

6.4. CONTROL CONFIGURATIONS FOR SINGLE CONSTRAINT

6.5. CONTROL CONFIGURATIONS FOR MULTIPLE CONSTRAINTS

7 DOUBLE-END COMPOSITION CONTROL

7.1. DEFINING THE PROBLEM

7.2. OPTIONS FOR COMPOSITION CONTROL

7.3. RELATIVE GAIN

7.4. RELATIVE GAINS FROM OPEN LOOP SENSITIVITIES

7.5. RELATIVE GAINS FOR OTHER CONFIGURATIONS

7.6. RATIOS FOR MANIPULATED VARIABLES

7.7. EFFECT OF OPERATING OBJECTIVES

7.8. MPC

8 COMPLEX TOWERS

8.1. HEAT INTEGRATION

8.2. SIDE HEATER/SIDE COOLER

8.3. SIDESTREAMS

8.4. WITHDRAWING A LIQUID SIDESTREAM

8.5. WITHDRAWING A VAPOR SIDESTREAM

8.6. COMPOSITION CONTROL IN SIDESTREAM TOWERS

Index

Copyright © 2012 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New Jersey

Published simultaneously in Canada

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Library of Congress Cataloging-in-Publication Data:

Smith, Cecil L.

 Distillation control : an engineering perspective / Cecil L. Smith.

p. cm.

 Includes bibliographical references and index.

 ISBN 978-0-470-38194-6

 ISBN 978-1-118-25968-9 (epdf)

 ISBN 978-1-118-25969-6 (epub)

 ISBN 978-1-118-26009-8 (mobi)

 1. Distillation. I. Title.

 TP156.D5D586 2012

 660'.28425–dc23

2011041437

ISBN: 9780470381946

PREFACE

Two observations constitute the basis for this book:

1. Despite its thirst for energy, distillation continues to be widely used for separations. Efficiently 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 configuration 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 configuration.

The most crucial component of every process control application is developing the piping and instrumentation (P&I) diagram that defines the control configuration 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 configuration is deficient, the usual consequence is tuning difficulties. Until the deficiencies 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 practice is basically copying—the control configuration 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 refining, and other industries where the same basic process technology is being replicated, but with different production rates, different feedstocks, and so forth. How many outright mistakes have been copied? How many times has a poorly performing configuration been copied when a better performing configuration could be implemented? Despite an occasional “war story,” the answers to such questions are largely opinions.

One should expect better, specifically, 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 flow 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, specifically, that the dynamic behavior of the process dictates the appropriate control configuration. 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 consciously? Not usually, but if you examine enough P&I diagrams, it seems to turn out that way. However, if process dynamics receive the primary consideration in developing the control configuration, this would often translate to “control every variable with the nearest valve.”

The steady-state characteristics of the process largely determine the appropriate control configuration. What is the direct and long-term influence of a final 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 sufficient, processes can be subtle and occasionally behave very differently from what is expected. When this occurs, the resulting P&I diagram is deficient. 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 flows, reflux, and boilup affect the composition of all product streams, but not to the same degree. The selection of the control configuration is preferably based on a quantitative assessment of their effect. For this, the steady-state separation model suffices.

Single-end composition control is rather forgiving. Double-end composition 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 configuration. 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 configuration on a quantitative assessment of column behavior computed from the steady-state separation model used for column design.

CECIL L. SMITH

Houston, Texas

November 28, 2011