Membrane Modeling, Simulation and Optimization for Propylene/Propane Separation Dissertation by Ali K. Alshehri In Partial Fulfillment of the Requirements For the Degree of Doctor of Philosophy King Abdullah University of Science and Technology Thuwal, Kingdom of Saudi Arabia © June 2015 Ali K. Alshehri All Rights Reserved 2 The dissertation of Ali K. Alshehri is approved by the examination committee. Committee Chairperson: Prof. Dr. Zhiping Lai Committee Members: Prof. Dr. Zhiping Lai Prof. Dr. Ingo Pinnau Prof. Dr. Shuyu Sun Prof. Dr. Ali Abbas 3 Abstract Membrane Modeling, Simulation and Optimization for Propylene/Propane Separation Ali Alshehri Energy efficiency is critical for sustainable industrial growth and the reduction of environmental impacts. Energy consumption by the industrial sector accounts for more than half of the total global energy usage and, therefore, greater attention is focused on enhancing this sector’s energy efficiency. It is predicted that by 2020, more than 20% of today’s energy consumption can be avoided in countries that have effectively implemented an action plan towards efficient energy utilization. Breakthroughs in material synthesis of high selective membranes have enabled the technology to be more energy efficient. Hence, high selective membranes are increasingly replacing conventional energy intensive separation processes, such as distillation and adsorption units. Moreover, the technology offers more special features (which are essential for special applications) and its small footprint makes membrane technology suitable for platform operations (e.g., nitrogen enrichment for oil and gas offshore sites). In addition, its low maintenance characteristics allow the technology to be applied to remote operations. For these reasons, amongst other, the membrane technology market is forecast to reach $16 billion by 2017. 4 This thesis is concerned with the engineering aspects of membrane technology and covers modeling, simulation and optimization of membranes as a stand-alone process or as a unit operation within a hybrid system. Incorporating the membrane model into a process modeling software simplifies the simulation and optimization of the different membrane processes and hybrid configurations, since all other unit operations are pre- configured. Various parametric analyses demonstrated that only the membrane selectivity and transmembrane pressure ratio parameters define a membrane’s ability to accomplish a certain separation task. Moreover, it was found that both membrane selectivity and pressure ratio exhibit a minimum value that is only defined by the feed composition, product purity and the recovery ratio. These findings were utilized to develop simple and accurate empirical correlations to predict the attainability behavior in real membranes, which showed good agreement with experimental and simulation results for various applications. Furthermore, the attainability of the most promising two and three-stage membrane systems are discussed by considering the complete well mixed assumption. The same behaviors that describe single-stage attainability are also recognized for multiple-stages. This discussion leads to a major discovery regarding the nature of the relationship between the attainability parameters in a multiple-stage membrane system with that of a single-stage system. Study of the economics of the multiple-stage membrane process for propylene/propane separation identifies the technology as a potential alternative to the conventional distillation process, even at the existing membrane performance, but conditionally at low to moderate membrane cost and sufficient durability. 5 To study the energy efficiency of membrane retrofitting to an existing distillation process, a shortcut method was developed to calculate the minimum practical separation energy (MPSE) of the membrane and distillation processes. It was discovered that the MPSE of the hybrid system is only determined by the membrane selectivity and the applied transmembrane pressure ratio in three stages. At the first stage, when selectivity is low, the membrane process is not competitive to the distillation process. At the second medium selectivity stage, the membrane/distillation hybrid system can help to reduce the energy consumption; the higher the membrane selectivity the lower the energy requirement. The energy conservation is further improved as the pressure ratio increases. At the third stage, when both the selectivity and pressure ratio are high, the hybrid system will change to a single-stage membrane unit, resulting in a significant reduction in energy consumption. The energy at this stage continues to slowly decrease with selectivity but increases slightly with pressure ratio. Overall, the higher the membrane selectivity, the more energy that is saved. These results should be very useful in guiding membrane research and their applications. Finally, an economic study is conducted concerning hypothetical membranes and the necessity for low cost and more durable membranes rises as the key for a viable hybrid process. 6 Acknowledgements I would like to begin by expressing my deepest and sincere gratitude to the sole of King Abdullah who was inspired to inspire all scholars in Saudi Arabia and the world. My deep thanks to my advisor Dr. Zhiping Lai for the continuous support, guidance and mentorship over the last four years. Learning his systematic way of conducting scientific analysis will be of great importance to my future as it was during my research. My appreciation is extended to the committee members Prof. Ingo Pinnau, Prof. Shuyu Sun and Prof. Ali Abbas. They have directed me during the research period and provided helpful comments and suggestions. Sincere thanks are due to my colleagues in the Advanced Membrane and Porous Material Center for their encouragement and support. I am really indebted to Saudi Aramco Oil Company and its management who believed in my ability and provided me with the chance to continue study and achieve another milestone in my life. Special Thanks goes to my Aramco Advisors Mr. Ahmed Mohamed and Mr. Abdullah Al-Gahtani for their continuous support. Finally, I would like to extend my gratitude to my family. They have always encouraged me and assisted me during the research. Foremost among these is my mother, wife, sisters and brothers without whose constant support and encouragement over the long months of writing, this work would not have been finished. At the end I want to thank my son Zaid and the three angles Wasan, Layan and Wajd for taking care of me during the rough times I passed through. I wish the acknowledgement page was large enough to name each and every one who has touched my life in so many meaningful ways. 7 List of Symbols A membrane area (m2) C gas specific heat J/(molK) P d hollow fiber diameter (m) D distillate flow (mol/s) E separation energy (J/mol) F feed flow rate (mol/s) f normalized feed flow rate (𝑓 = 𝐹/𝐹 ) 0 G permeate flow rate (mol/s) g normalized permeate flow rate (𝑔 = 𝐺/𝐹 ) 0 m number of feed streams n number of components P feed initial pressure (Pa) 0 𝑃 feed side pressure (Pa) ℎ 𝑃 permeate side pressure (Pa) 𝑙 P compressor outlet pressure (Pa) out Q heat exchanger duty (J/s) 8 q mixture liquid fraction R retentate flow rate (mol/s) or ideal gas constant r molar fraction of retentate stream Re Reynolds number R minimum reflux ratio min S selectivity that is defined as the permeability ratio of the most permeable j component relative to component j T feed temperature (K) T compressor inlet temperature (K) in T compressor outlet temperature (K) out V vapor flow rate, mol/s W machinery shaft work, J/s x molar fraction of feed stream y molar fraction of permeate stream z number of side streams  relative volatility of component i i 9  permeability that is defined as the flux normalized for transmembrane pressure difference and membrane thickness (mol/msPa)  permeability of the most permeable component (mol/msPa) m  feed to permeate pressure ratio (𝑃 /𝑃) ℎ 𝑙  target component enrichment (y /x ) A A  stage-cut defined as the flow rate ratio of permeate stream to the stage feed λ latent heat/enthalpy of vaporization (J/mol)  underwood polynomial root  recovery ratio of the target component (𝐺𝑦 /𝐹𝑥 ) 𝐴 𝐴  ratio between recycle stream flow rate to the raw feed flow rate (Q/F) ϕ separation coefficient (yA(1−xA)) xA(1−yA) 10 Table of Contents 1. Introduction _____________________________________________________ 24 1.1 Background...............................................................................................24 1.2 Motivation ................................................................................................28 1.3 Research objectives ..................................................................................29 1.4 Thesis outline............................................................................................31 1.5 References ................................................................................................31 2. Membrane Background & Literature Review _________________________ 36 2.1 Histrory of membrane technology ............................................................36 2.2 Membrane separation background ...........................................................39 2.3 Membrane module designs .......................................................................43 2.3.1 Plate-and-Frame design ................................................................... 43 2.3.2 Spiral wound design ........................................................................ 44 2.3.3 Hollow fiber design ......................................................................... 45 2.4 Module flow patterns ................................................................................48 2.5 Membrane systems for gas separations ....................................................49 2.5.1 Single-stage configuration ............................................................... 51 2.5.2 Multi-stage configuration ................................................................ 53 2.5.3 Membrane/distillation hybrid configurations .................................. 57 2.5.3.1 Pre-distillation hybrid configuration ............................................ 59 2.5.3.2 Post-distillation hybrid configuration ........................................... 60