Chemical Process Structures and Information Flows PDF
Preview Chemical Process Structures and Information Flows
BUTTERWORTHS SERIES IN CHEMICAL ENGINEERING SERIES EDITOR ADVISORY EDITORS HOWARD BRENNER ANDREAS ACRIVOS Massachusetts Institute of Technology The aty CoUege of CUNY JAMES E. BAILEY California Institute of Technology MANFRED MORARI California Institute of Technology E. BRUCE NAUMAN Rensselaer Polytechnic Institute J. R. A. PEARSON Schlumberger Cambridge Research ROBERT K. PRUD'HOMME Princeton University SERIES TITLES Chemical Process Equipment: Selection and Design Stanley M. Walas Chemical Process Structures and Information Flows Richard S, H. Mah Computational Methods for Process Simulation W, Fred Ramirez Constitutive Equations for Polymer Melts and Solutions Ronald G. Larson Fundamental Process Control David M. Prett and Carlos E, Garcia Gas-Liquid-Solid Fluidization Engineering Liang-Shin Fan Gas Separation by Adsoφtion Processes Ralph T. Yang Granular Filtration of Aerosols and Hydrosols Chi Tien Heterogeneous Reactor Design Hong H. Lee Molecular Thermodynamics of Nonideal Fluids Lloyd L. Lee Phase Equilibria in Chemical Engineering Stanley M, Walas Physicochemical Hydrodynamics: An Introduction Ronald F. Probstein Transport Processes in Chemically Reacting How Systems Daniel E, Rosner Viscous Flow: The Practical Use of Theory Stuart W, Churchill Chemical Process Structures and Information Flows RICHARD S. Ç. ĚÁÇ Department of Chemical Engineering Northwestern University Butterworths Boston London Singapore Sydney Toronto Wellington Copyright © 1990 by Butterworth Publishers, a division of Reed Publishing (USA) Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electromc, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Library of Congress Cataloging-in-Publication Data Mah, Richard S.H. Chemical process structures and information flows / Richard S.H. Mah. p. cm. — (Butterworths series in chemical engineering) Includes bibhographical references. ISBN0-409-90175-X 1. Chemical processes. I. Title. II. Series. TP155.M287 1989 660'.2815-dc20 89-23984 CIP British Library Cataloguing in Publication Data Mah, Richard S.H. Chemical process structures and information flows. 1. Chemical engineering. Process analysis I. Title 660.2'8 ISBN 0-409-90175-X Butterworth Publishers 80 Montvale Avenue Stoneham, MA 02180 10987654321 Printed in the United States of America. No single thing abides, but all things flow. Fragment to fragment clings; The things thus grow Until we know and name them. By degrees They melt, and are no more the things we know. Luadius (c. 99-55 B.C.) PREFACE Our understanding of the systems aspects of chemical processes received a tremendous boost with the introduction of computers. Until the late 1950s when computers first began to become accessible to engineers and researchers, process systems could only be studied analytically. Physical experimentation involving many variables and their interactions were too difficult and too expensive to carry out. Practically speaking, systems research was limited only to areas amenable to analytical mathematics and open only to investigators skilled in mathematical analysis. With the advent of the computers came simulation and optimization, and an immensely expanded domain for systems studies and range of tech niques. Each new wave of hardware and software advances brought forth new engineering applications, a new influx of investigators and practitioners, and new understanding of the important role played by problem structures and information flows. This book represents a first attempt to delineate the subject and provide a common fi-amework with respect to process applica tions. Structures and information flows are keys to our understanding of processes and to efficient computation, and grasping the basics is a rewarding experience well within the reach of every chemical engineer. This book has grown out of my effort to introduce the systems viewpoint to chemical engineering students. Material in this book has been used in courses for graduate students and for senior undergraduate students at Nortνiwestem. The chapters may be read fairly independendy, but most readers would probably want to work through Chapters 1 and 2 first. The remaining chapters are grouped around three themes, and may be studied in any order. Chapters 3,4 and 5 deal with the design of continuously operated processes. Chapters 6 and 7 treat batch plant scheduling and design. The last two chapters provide an introduction to the monitoring and treatment of xm Preface XIV process data. The importance of worked examples and problems cannot be overemphasized. Many fine but significant points will only be appreciated after going dirough die problems, and many concepts requke reinforcement for clarification. Witii worked examples in each chapter and more dian 180 problems, this book could be used in classes or for self-study. Experience shows that the terminology in batch plants and basic concepts of probability and statistics are not always familiar to chemical engineers. Material on these two topics is appended, and may be read in conjunction with the appropriate chapters. With topics of such diversity it is quite impossible to unify the diverse notations. A consistent set of notation is used and listed in each chapter. But different notations for tiie same variables are sometimes used in different chapters. The selection of topics was heavily influenced by the research of my students and colleagues and by my own appreciation of tiie subject. It is a pleasure to acknowledge die stimulating interactions and die fruitful research collaborations which contributed directiy to this book. In particular, I would like to acknowledge, in chronological order, the contributions of my former graduate students T. D. Lin, Gregory M. Stanley, Wai-Biu Cheng, Iren Suhami, Kazuyuki Shimizu, Comeliu lordache, Shankar Narasimhan, Alexandros Kretsovalis, Joseph Rosenberg and Chen-Shan Kao, and of my colleague Professor Ajit C. Tamhane. To the many more students who took my courses 710-D55 and D59 over the years my sincere thanks for their patience, suggestions, and encouragement To Nortiiwestem University I am grateful for a six-month leave in the academic year 1988-89, which allowed me to complete my manuscript. Professor Mark A. Stadtherr of the University of Illinois and several current graduate students at Northwestern including Hong-Ming Ku and Naim Faqir read chapters of my manuscript. For almost nine months four of my current graduate students shared with me the task of preparing a camera-ready manuscript. But for the assistance of Chen-Shan Kao, Anil Patel, Karl Schnelle, and Shufeng Tan, we may never have triumphed over electronic desktop publishing. At least not so soon! Finally, to my beloved family - my parents, my wife and my son, who suffered my chronic absenteeism in loving silence, I dedicate this book. RICHARD S.H. MAH Evanston, Illinois August 1989 1 INTRODUCTION 1-1. PHENOMENON-ORIENTED AND SYSTEM-ORIENTED VIEW POINTS Broadly speaking, engineering problems may be examined ftom two different viewpoints. First, they may be studied finom the viewpoint of their phenomenological content. For instance, if the problem is the design of a reactor, we would want to learn about the reaction mechanisms and kinetics, the properties of products and byproducts, the heat and mass transfer under the prevailing process conditions, the materials of construction, and so on. In each study we would like to exclude as much extraneous influences as possible in order to focus on the phenomenon under examination. In fact, the ideal situation would be one in which the phenomenon is totally isolated from all extraneous factors. The second viewpoint is to examine the problem as a system. In this case we might be interested in the interaction of reaction conditions with yield and conversion, separation of products and byproducts, controllability and mechanical design of equipment. The ideal situation for such a study would be attained, if the behaviors of all components in the system are completely understood so that our attention may be focussed on the behavior of the system as a whole. Just as a disciplinary purist exalts in the elegance and simplicity of his experiment which may have been attained through elaborate measures and painstaking analysis, so the system analyst revels in the richness of structures derived from simple, innocuous components. The two viewpoints are complementary. Both are needed in tackling practical problems of any complexity. 2 Chemical Process Structures Chap. 1 This book is concerned with the structures and information flows associated with a chemical process. In order to treat the subject matter in hand we will, of course, have to assume or acquire certain background knowledge about the physical, chemical and engineering aspects of a chemical process. But we will do so with the primary purpose of developing an understanding and perception of the underlying structure of the whole process. The relationship between this study and the various components of a chemical process may be further clarified by an analogy. Before we attempt to write in English, we will need to assume or acquire certain vocabulary. We can, of course, study the pronunciation, etymology and orthography of words in a dictionary. But the words by themselves do not make literature, even though the Oxford Dictionary may contain all the words in Shakespeare. What makes the literature rich is the structure of words. Remove those structure and you will have a collection of lifeless words which do not allow differentiation between masterpieces and the writmgs of neophytes. On the other hand the foregoing statement should in no way be construed as demeaning the importance of a thorough grasp of vocabulary or the intellectual merit of etymology and orthography. 1 -2. THE WHOLE IS MORE THAN THE SUM OF ITS PARTS It is important to note that the system as a whole may exhibit charac teristics above and beyond those of its individual components. A digital computer is one such example. The basic information is stored in binary (two-state) devices. By interpreting some of these bit patterns as instructions and others as data, we can construct an enormously powerful information processing machine. In chemical engineering a well-known system is the coupling of an exothermic chemical reactor with a simple two-stream heat exchanger. The relationships between the inlet and the outiet temperatures for the individual units are shown in Fig. 1-1. Both units exhibit stable behaviors. Now suppose the heat exchanger is connected to üie reactor to preheat the feed and to quench the product. Quite a different behavior is observed for the system as a whole, as shown in Fig. 1-2. Of the three points of intersection which satisfy the constraints imposed by the subsystems, only the upper point (reaction ignited) and the lower point (reaction extinguished) are attainable without external control. The middle point is unstable in the sense that even an infinitesimal perturbation will cause the system to move away from that point. Rudd and Watson (1968) noted that whereas the reactor alone caimot be operated with cold feed, the system can be so operated. Chap. 1 Introduction Hot Stream Cold Heat 1 Stream Exchanger Product Reactor Feed Fig. 1-1 Behavior of two subsystems. 1 Feed Heat Reactor Exchanger Product Fig. 1-2 Behavior of an integrated system. LU CVJ LU X ü CO LU Χ , ü 150 O -5 C h e m ic Compress Ü ai -t>4- tX3 Refrigerant al Process Str o u r ctu CoyV res oa ler ter T T I I ω ί CO -Φ CO C h í^ . 1
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