The Preliminary Design of Heat-Integrated Multicomponent Distillation Sequences Through Generation of Flowsheet Superstructures by Duncan Leeson Centre for Process Systems Department of Chemical Engineering Imperial College London September 2018 Submitted in part fulfilment of the requirements for the degree of Doctor of Philosophy of Imperial College London First Supervision: Prof. Nilay Shah Second Supervision: Prof. Paul Fennell Dr. Niall Mac Dowell Financial Support: Shell Global Solutions BV Engineering and Physical Sciences Research Council Examiners: Dr. Claire S. Adjiman Dr. Vassilios S. Vassiliadis Declaration of Originality I hereby certify that I am the sole author of this work, and that all material in this dissertation that is not my own work has been properly acknowledged. List of Publications D. Leeson et al., "Simultaneous Design of separation sequences and whole process energy integration" Chemical Enginering Research and Design 125 (2017) pp. 166-180 D. Leeson et al., "A Techno-economic analysis and systematic review of carbon capture and storage (CCS) applied to the iron and steel, cement, oil refining and pulp and paper industries, as well as other high purity sources" International Journal of Greenhouse Gas Control 61 (2017) pp. 71-84. The copyright of this thesis rests with the author and is made available under the Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work. 1 Abstract Given that in 2012 distillation was estimated to consume roughly 3% of energy globally, managing the heating and cooling duties of these processes is essential [52]. Currently, many process design strategies involve designing the separation system before energy integration is considered, leading to suboptimal solutions. In this thesis, a model for the preliminary design of a distillation sequence is presented as a MILP, using only basic thermodynamic data from feed components. The model has been developed based on a reduced superstructure with temperatures calculated via a discrete grid. Process streams from elsewhere in the plant are also considered concurrently, and consideration of these often changes the optimal sequence. The examples suggest cost reductions of over 30% when ancillary streams are not considered when compared to a basic heuristical approach, and up to 50% when a small number of additional process streams are included. The model was then further developed to include multieffect distillation, where separators are considered as a system of two parallel columns. Including the opportunity for multieffect distillation led to changes in the optimal sequence, with associated costs reduced by up to a further 30% compared to the previous iteration of the model. The model has been demonstrated on two industrial case studies; crude oil refining and platformate separation. Both examples demonstrate the flexibility of the model to deal with complex industrial problems, with the crude example showing optimise under an uncertain feedstock, while the platformate demonstrates the importance of including as much information about the process as possible to find the optimal result. As a preliminary design tool, the model should be used as initialisation for detailed process design. This has been investigated, with the distribution between multieffect columns made continuous. This led to further cost reductions with a short solution time, due to the initialisation offered by the linear models. 2 Acknowledgements I am eternally grateful to my supervisor, Prof. Nilay Shah, for his guidance and support throughout the course of my PhD, and for the encouragement to persevere when things were difficult. Thank-you for your positivity and the freedom to choose the areas that were most interesting for me, and the subtle guidance you would give to ensure that I stayed on the right track. I hugely appreciate your encouragement to branch out and get involved in projects which were so different from my own, meaning that I have got to the end of this PhD with a broader outlook on professional life as an engineer. From the four years of undergraduate where you were my tutor, and over the course of this PhD, it has been a pleasure working alongside you. I would also like to thank both Prof. Paul Fennell and Dr. Niall Mac Dowell, for their encouragement and assistance throughout the project, and the fresh approach they took to tackling any problems from a different perspective. From working together on industrial carbon capture and storage, I have also been fortunate enough to work with both of you across a range of different subjects, and I thank you both for the opportunity to work alongside you on these. It has been a pleasure to work alongside you both, and I have thoroughly enjoyed it. I am not sure I could have managed to complete this PhD without the close group of friends that I have here at Imperial, and I would like to thank all of you for your company and for your support over the last four years. A special mention has to be made for the Four Bees, without which the last few years would have been much duller, and with much less laughter. To all of you who have made the last few years much more entertaining, you have my thanks. Above all, I would like to thank Mum, Dad and Mhairi for your support and patience offered throughout the course of not just this PhD, but my entire schooling and academic career. From the first stages of my education, you've supported me and encouraged me to go further, and I am proud to show you just what I have achieved. Finally, thank you Konzi for your support and being there for me, and for making everything seem so much sunnier. 3 Contents 1 Introduction 14 1.1 Background . 14 1.2 Distillation Processes . 15 1.3 Crude Oil Refining . 16 1.4 Aim & Problem Statement . 18 2 Literature Review 21 2.1 Distillation Modelling . 21 2.1.1 Historical Methods of Distillation Calculation . 22 2.1.2 Non-equilibrium stage models . 25 2.2 Pinch analysis and Heat Exchange Network Design . 28 2.3 Separation Process Synthesis Modelling . 33 2.3.1 Distillation Sequence Classification . 33 2.3.2 Heuristical Process Design . 36 2.3.3 Non-Superstructure Approach to Process Synthesis . 37 2.3.4 Superstructure Generation with Sharp Splits . 40 2.3.5 Superstructure Generation with Non-sharp Splits . 41 2.3.6 Superstructure Generation with Energy Integration . 45 2.4 Summary of Literature . 52 3 Methodology 53 3.1 Separation Superstructure Model . 53 4 3.1.1 Background . 53 3.1.2 Mathematical model construction and assumptions . 54 3.2 Heat Exchange Network . 60 3.2.1 Background . 60 3.2.2 Differences with standard Pinch Analysis . 60 3.2.3 Mathematical Model Description . 62 3.3 Summary of Methodology . 64 4 Preliminary Heat-Integrated Design of Distillation Sequences 66 4.1 Background . 66 4.2 Mathematical model for construction of optimal heat-integrated multicomponent distillation sequence . 68 4.2.1 Separation system . 68 4.2.2 Heat Integration . 73 4.2.3 Overall System Heat Duties . 77 4.3 Scalability of model . 79 4.4 Case Studies . 81 4.4.1 Data and Problem construction . 81 4.4.2 Baseline Scenario . 83 4.4.3 Separation Sequence Integration . 85 4.4.4 Plant-wide Integration . 87 4.5 Conclusions . 90 5 Preliminary Design of Distillation Sequences with Column Pressurisation and Capital Costs 93 5.1 Introduction . 93 5.2 Mathematical model of optimal heat-integrated multicomponent distillation sequence 96 5.2.1 Separation System Model . 96 5.2.2 Heat Integration . 101 5.2.3 System heat duties & Utility Cost function . 106 5 5.2.4 Pressure Calculations . 107 5.2.5 Pumping Cost Calculation . 108 5.2.6 Capital Costing . 109 5.2.7 Objective Function . 113 5.3 Case Studies . 113 5.3.1 Data and Problem construction . 113 5.3.2 Results . 116 5.4 Conclusions . 121 6 Application Of Distillation Sequencing Model to Industrial Processes 124 6.1 Crude Oil Refining . 124 6.1.1 Background to Oil Refining . 124 6.1.2 Classification of Crude Oils . 125 6.1.3 Database Population and Assay Comparison . 127 6.1.4 Model Construction . 128 6.1.5 Results . 134 6.2 Separation of Aromatics (BTX) from Platformate . 139 6.2.1 Methodology . 139 6.2.2 Results . 142 6.3 Conclusions . 145 7 Non-linear Distillation Sequence Design 147 7.1 Introduction . 147 7.2 Separation System Model . 148 7.3 Case Study . 157 7.3.1 Data and Problem Construction . 157 7.3.2 Example 1 . 159 7.3.3 Example 2 . 161 7.4 Conclusion . 163 6 8 Conclusions and Recommendations for Future Work 166 A Nomenclature 172 A.1 Dimensions/Indices . 172 A.2 Parameters . 172 A.3 Variables . 174 A.4 Binary Variables . 176 B Detailed Case Study Solution Data for Chapter 4 178 B.1 Baseline Scenario . 179 B.2 Optimised Separation Sequence . 180 B.3 Integrated Separation Sequence . 181 B.4 Case Study Composite Curves . 182 C Detailed Solution Data 184 D Crude Oil Assay Data 186 E GAMS Models 191 E.1 Chapter 4 - Preliminary heat integrated separation sequencing model.