Optimal operation of unconventional batch distillation columns

OPTIMAL OPERATION OF UNCONVENTIONAL BATCH DISTILLATION COLUMNS
by

Haydn I. Furlonge

January 2000

A thesis submitted for the degree of Doctor of Philosophy of the University of London and for the Diploma of Membership of the Imperial College

Department of Chemical Engineering and Chemical Technology Imperial College of Science, Technology and Medicine London SW7 2BY, United KingdomAbstract

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Unconventional column con gurations and operating policies o er opportunities for increasing the exibility and energy e ciency of batch distillation. The overall objective of this thesis is to investigate the optimal operation of batch distillation taking into account these di erent possibilities. The approach adopted is characterised by three key features. Firstly, relativelydetailed dynamic models of tray and packed columns are used for studying batch distillation operations. Secondly, rigorous dynamic optimisation techniques are employed for examining various operating policies for unconventional columns. Thirdly, a wide range of the available degrees of freedom is exploited. This approach is essential in determining the full potential of unconventional columns andin making valid comparisons between them. The separation of a multicomponent zeotropic mixture using a multivessel column comprising of trays is investigated. It is shown that optimising the initial distribution of the feed among the vessels results in a signi cant improvement in column performance. For some separations, withdrawing product from the vessels into accumulators is better than total reux operation in terms of energy consumption. Open-loop optimal operation is also compared to a recently proposed feedback control strategy. The energy consumption of a conventional column is found to be about twice that of a multivessel column having the same number of stages. The optimal operations of conventional and middle vessel packed columns are also examined. The separation of anazeotropic mixture using extractive batch distillation is considered. A distributed packed column model is used. In contrast to the results of simulation-based studies reported in the literature, we have found that the operating pro t of the conventional column is slightly higher than that of the middle vessel column. This highlights the need for applying rigorous optimisation before drawing rm conclusionson the superiority of one column con guration over another.

Acknowledgements
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I wish to thank both my supervisors, Dr. Eva S rensen and Professor Constantinos C. Pantelides, for their guidance throughout the course of this work. Their contribution to my academic progress is signi cant. My examiners, Dr. Nilay Shah and Professor XavierJoulia, deserves mention. An implicit but signi cant contribution to the solution of the numerical problems encountered in this thesis was the helpful and friendly Computer Aided Process Engineering group at University College London. Kian's assistance with the submission of the thesis, Alma's time-out tips (especially the compulsory visit to Portobello Market) and football with the lads will beremembered. The many sacri ces made by my family, and in particular by my wife, Sarrah, cannot be overemphasised. My mother's con dence in me and her insistence that I ful l my ability is primarily responsible for me leaving the warmth and sunshine of Trinidad to study in not-so warm and sunny London. My wife's support has seen this endeavour to its fruition, while memories of my brothers, sisters andniece have dissipated some of the gloom and have served as an incentive to nish. Financial support obtained from the British Council and the Engineering and Physical Sciences Research Council is gratefully acknowledged.

Dedication
To my mother for her encouragement and my wife for her support

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Table of Contents
Abstract Acknowledgements Dedication List of Figures List of Tables
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