Modelling and Control of Reactive Distillation Processes Nicholas C. T. Biller A thesis submitted for the degree of Doctor of Philosophy of the University of London Department of Chemical Engineering University College London London WCIE 7JE September 2003 ProQuest Number: 10014376 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest. ProQuest 10014376 Published by ProQuest LLC(2016). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 A bstract Reactive distillation has been applied successfully in industry where large capital and energy savings have been made through the integration of reaction and distillation into one system. Operating in batch mode, in either tray or packed columns, offers the flexibihty required by pharmaceutical and hne chemical industries for producing low volume/high value products with varying specifications. However, regular packed or tray columns may not be suitable for high vacuum operations due to the pressure drop across the column section and short path distillation may be more applicable. The objective of this thesis is to investigate the control of reactive distillation in batch columns, tray and packed, and in short path columns. In order to study control fully, it is necessary to develop rigorous dynamic models that accurately capture the process behaviour. The higher the degree of rigour, the more accurately the process conditions and dynamics are captured. However, more rigorous models are more computationally expensive to implement and can be prone to numerical errors, introduced for instance during linearisation. Therefore, in this thesis, the degree of modelling rigour required for both simulation and control purposes is explored in detail for tray and packed batch columns and short-path columns. For batch tray columns, it is demonstrated that to accurately capture the change in process conditions, it is necessary to model pressure dynamics and employ a dynamic energy balance. For packed columns, distributed rate based modelling is compared to lumped equilibrium modelling and it is found that due to the varying conditions within the packing, the efficiency changes, resulting in mismatch between the two methods. The short-path distillation column which has hitherto only been modelled at steady-state, is modelled using a dynamic rate based model, essential for investigating control. Having developed the dynamic models, the control and controllability of these reactive distillation processes are examined. General control properties of reactive batch distillation are discussed and methods are presented for applying linear controllability tools to these non-linear process models. The linear models are then employed to demonstrate the implications for control when adopting one of the three processes. Acknowledgements I would like to thank my supervisor, Dr. Eva Sprensen, for her guidance and encouragement througout the course of this work. I would also like to thank the students and staff, past and present, of the Computer Aided Process Engineering group for the lively and useful discussions and the occasional, but necessary, excursions to Huntley St. The financial support from the Engineering and Physical Sciences Research Council and the Centre for Process Systems Engineering is gratefully acknowledged. Finally, I would like to thank Anna for her constant love and support. C ontents Abstract 2 Acknowledgements 3 List of figures 10 List of tables 14 1 Introduction 15 1.1 M otivation ....................................................................................................................... 15 1.2 Reactive distillation ...................................................................................................... 16 1.3 Batch column operation ................................................................................................ 17 1.3.1 Control of reactive batch distillation ............................................................ 18 1.4 Short-path column operation ..................................................................................... 19 1.4.1 Falling film e v a p o ra to rs.................................................................................... 19 1.4.2 Wiped him evaporators ........................................................................................20 1.4.3 Short-path distillation ........................................................................................... 20 1.5 Objectives of this w o rk....................................................................................................... 21 1.6 Outline of this t h e s i s .......................................................................................................... 22 1.7 Main contributions ..............................................................................................................23 4 CONTENTS 5 2 Literature review 25 2.1 Modelling of reactive batch distillation ........................................................................ 25 2.1.1 Introduction .......................................................................................................... 25 2.1.2 Reactive batch distillation literature .................................................................26 2.1.3 Reactive batch distillation conclusions ............................................................. 40 2.2 Modelling of short-path distillation ...............................................................................41 2.2.1 Introduction .......................................................................................................... 41 2.2.2 Short-path literature .............................................................................................41 2.2.3 Short-path conclusions ......................................................................................... 47 2.3 Conclusions ...........................................................................................................................48 3 Modelling of RBD in tray columns 49 3.1 Tray column modelling ......................................................................................................49 3.1.1 Rigorous model .......................................................................................................50 3.1.2 Assumptions ..........................................................................................................51 3.1.3 Initial conditions ................................................................................................... 51 3.1.4 Integration ............................................................................................................. 52 3.1.5 Simplified m odel ................................................................................................... 53 3.2 Comparison between simphfied and rigorous models ............................................ 53 3.2.1 Ethyl acetate case study ......................................................................................53 3.2.2 Case study results ................................................................................................56 3.3 Conclusions .......................................................................................................................... 60 CONTENTS 6 4 Modelling of RBD in packed columns 64 4.1 Rate-based modelling of packed columns .....................................................................65 4.1.1 Modelling of mass and energy transfer ............................................................. 65 4.1.2 Modelling of hydrodynamics ................................................................................66 4.1.3 Modelling of chemical reactions .........................................................................67 4.1.4 Packed column m odelling ....................................................................................68 4.2 Reactive batch distillation case study ...........................................................................69 4.2.1 Column design .......................................................................................................69 4.2.2 Column operation ................................................................................................70 4.2.3 Effect of discretisation ......................................................................................... 71 4.2.4 Determination of H EX ? ................................................................................. 72 4.3 Comparison between rate based and equilibrium m o d e ls ......................................80 4.4 Conclusion .......................................................................................................................... 86 5 Control of reactive batch columns 88 5.1 In tro d u c tio n .......................................................................................................................... 89 5.1.1 Control of batch distillation columns ..............................................................91 5.2