Applied Science, Faculty of
Engineering, School of (Okanagan)
DSpace
UBCO
Alwai Alcheikhhamdon, Yousif
Doctor of Philosophy - PhD
University of British Columbia
The work presented in this thesis investigates the use of membrane technology in lieu of the energy-intensive distillation process for propylene - propane separation. The polymeric membranes lack the selectivity necessary for efficient separation. A recent breakthrough occurred when the metal-organic framework filler “ZIF-8” was mixed with Polymers of Intrinsic Microporosity (PIMs) to synthesize mixed-matrix films with high propylene selectivity. At high ZIF-8 loadings, the compatibility between the organic PIM and the semi-organic ZIF-8 deteriorates. This work was the first to propose functionalizing the PIM with a “carboxyl” (-COOH) group to enhance the compatibility with ZIF-8. Due to the carboxyl’s higher electronegativity, stronger H-bonding is induced between the mixed-matrix components compared to the hydroxyl (-OH) functionalization proposed in previous studies; the -COOH functionalized polymer “6FDA-TrMCA” was selected for this purpose. In this work, material development activities were limited to the pristine “6FDA-TrMCA” films preparation. The performance of the mixed-matrix membrane was approximated using the Maxwell model, meeting the targeted measures. Since membrane simulation software tools are exclusive to the commercial membrane suppliers, the “quantitative” impact of the selectivity boost on the process performance measures (e.g., purity, recovery, recycle) is not measurable. As part of this work, a novel HYSYS extension “MemCal” was matured in collaboration with a partner to support the simulation of membranes in Aspen HYSYS. The pure polymeric and the mixed-matrix membranes were simulated for their performance in single- and in two-stage designs meant for propylene’s commercial grades production; factors such as feed the propylene content and trans-membrane pressure ratio were accounted for in the analyses. The polymeric membranes were found incapable of producing neither the chemical nor the polymer grades of propylene, even in a two-stage process. Alternatively, the mixed-matrix membranes with the recommended selectivity of 35 (i.e., feasible using 6FDA-TrMCA and ZIF-8) were found capable of producing the chemical and the polymer grades at practical recovery when utilized in the single- and two-stage designs, respectively. The future research should grant more attention to the mixed-matrix high-pressure operation subject, as the latter is a prerequisite for technology’s success.
PROPYLENE - PROPANE SEPARATION USING MIXED MATRIX FILMS MADE OF FUNCTIONALIZED POLYMERS AND ZEOLITIC-IMIDAZOLATE FRAMEWORKS by Yousif Alwai Alcheikhhamdon B.Sc., American University of Sharjah, 2009 M.Sc., King Abdullah University of Science and Technology, 2010 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY in THE COLLEGE OF GRADUATE STUDIES (Mechanical Engineering) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) April 2021 © Yousif Alwai Alcheikhhamdon, 2021 ii The undersigned certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis entitled: Propylene - Propane Separation using Mixed Matrix Films Made of Functionalized Polymers and Zeolitic-Imidazolate Frameworks Submitted by Yousif Alwai Alcheikhhamdon in partial fulfillment of the requirements for the degree Doctor of Philosophy in Mechanical Engineering Dr. Mina Hoorfar, School of Engineering, UBC Supervisor Dr. Ingo Pinnau, Advanced Membranes & Porous Materials Center, KAUST Co-Supervisor Dr. Homayoun Najjaran, School of Engineering, UBC Supervisory Committee Member Dr. Kasun Hewage, School of Engineering, UBC University Examiner Dr. Boguslaw Kruczek, University of Ottawa External Examiner Additional Committee Members include: Dr. Keekyoung Kim, School of Engineering, UBC Supervisory Committee Member iii Abstract The work presented in this thesis investigates the use of membrane technology in lieu of the energy-intensive distillation process for propylene - propane separation. The polymeric membranes lack the selectivity necessary for efficient separation. A recent breakthrough occurred when the metal-organic framework filler “ZIF-8” was mixed with Polymers of Intrinsic Microporosity (PIMs) to synthesize mixed-matrix films with high propylene selectivity. At high ZIF-8 loadings, the compatibility between the organic PIM and the semi-organic ZIF-8 deteriorates. This work was the first to propose functionalizing the PIM with a “carboxyl” (-COOH) group to enhance the compatibility with ZIF-8. Due to the carboxyl’s higher electronegativity, stronger H-bonding is induced between the mixed-matrix components compared to the hydroxyl (-OH) functionalization proposed in previous studies; the -COOH functionalized polymer “6FDA-TrMCA” was selected for this purpose. In this work, material development activities were limited to the pristine “6FDA-TrMCA” films preparation. The performance of the mixed-matrix membrane was approximated using the Maxwell model, meeting the targeted measures. Since membrane simulation software tools are exclusive to the commercial membrane suppliers, the “quantitative” impact of the selectivity boost on the process performance measures (e.g., purity, recovery, recycle) is not measurable. As part of this work, a novel HYSYS extension “MemCal” was matured in collaboration with a partner to support the simulation of membranes in Aspen HYSYS. The pure polymeric and the mixed-matrix membranes were simulated for their performance in single- and in two-stage designs meant for propylene’s commercial grades production; factors such as feed the propylene content and trans-membrane pressure ratio were accounted for in the analyses. The polymeric iv membranes were found incapable of producing neither the chemical nor the polymer grades of propylene, even in a two-stage process. Alternatively, the mixed-matrix membranes with the recommended selectivity of 35 (i.e., feasible using 6FDA-TrMCA and ZIF-8) were found capable of producing the chemical and the polymer grades at practical recovery when utilized in the single- and two-stage designs, respectively. The future research should grant more attention to the mixed-matrix high-pressure operation subject, as the latter is a prerequisite for technology’s success. v Lay Summary Propylene is an important feedstock for the production of various petrochemicals, most importantly, the thermo-plastic polymer “polypropylene”. Before its utilization, propylene must be separated off its associated propane in one of the most energy-intensive and complex industrial distillations. The work presented in dissertation investigates the use of membrane technology in lieu of conventional distillations for separating propylene from propane. For completeness, the work combines the aspects of membrane material development together with membrane unit’s design and modeling to demonstrate technology’s prospects in this field. The work here demonstrates that with proper membrane material selection, appropriate unit process design, and under certain operational envelope, membranes can be made qualified for the propylene-propane separation application. The work recommends a combination of compatible materials to form a mixed-matrix material that is selective towards propylene. Furthermore, the work demonstrates that the multi-stage design can moderate the performance offsets caused by the low membrane selectivity. vi Preface The work presented in this thesis is the outcome of an original research conducted by the author under the supervision of Professor Mina Hoorfar from the School of Engineering at UBC Okanagan, and Professor Ingo Pinnau from the Advanced Membranes and Porous Materials Center at KAUST. The work includes reviewing the literature, identifying knowledge gaps, setting research targets, conducting the experiments, performing simulations, analyzing the results, and writing the thesis. The results presented in this thesis were published in peer-reviewed journals. Author’s supervisor, Dr. Hoorfar is the corresponding author in most of the relevant publications, and author’s co-supervisor, Dr. Pinnau is a co-author as applicable. Author’s contribution in each publication is explained under the respective listing. a. Article published or submitted in refereed journals • Y. Alcheikhhamdon and M. Hoorfar, “Natural gas quality enhancement: A review of the conventional treatment processes, and the industrial challenges facing emerging technologies,” Journal of Natural Gas Science and Engineering, vol. 34, pp. 689–701, 2016. (Lead author: conducting a detailed review of existing natural gas treatment technologies, comparing them, and writing the paper) • Y. Alcheikhhamdon and M. Hoorfar, “Natural gas purification from acid gases using membranes: A review of the history, features, techno-commercial challenges, and process intensification of commercial membranes,” Chemical Engineering and Processing - Process Intensification, vol. 120, pp. 105–113, 2017. vii (Lead author: conducting a detailed review about the status of membrane technology in natural gas treatment industry, analyzing the technology market competency, and writing the paper) • Y. Alcheikhhamdon, M. Hoorfar, and B. Chen, “A novel tool for the modeling, simulation and costing of membrane-based gas separation processes using Aspen HYSYS: Optimization of the CO2/CH4 separation process,” Computers & Chemical Engineering, vol. 117, pp. 11–24, 2018. (Lead author: presenting the idea, performing analyses using the software, and writing the paper) • Y. Alcheikhhamdon, I. Pinnau, M. Hoorfar and B. Chen, "Propylene -propane separation using Zeolitic-Imidazolate Framework (ZIF-8) membrane: process techno-commercial evaluation", Journal of Membrane Science, vol. 591, p. 117252, 2019. (Lead author: presenting the idea, performing analyses using the software, providing recommendations for future work, and writing the paper) • B. Chen, Z. Meng, H. Ge, Y. Alcheikhhamdon, M. Hoorfar, L. Liu, T. Yang, X. Fang, "Optimization of residual oil hydrocrackers: integration of pump-free ebullated-bed process with membrane-aided gas recovery system", Energy Fuels, p. 2584-2597, 2019. (co-author: simulating the process units, providing recommendations, and reviewing the relevant sections in the manuscript) • A. M. Gaffney, N. Duprez, K. Louthan, B. Borders, J. Gasque, A. Siegfried, T. Stanford, K. Roberts, Y. Alcheikhhamdon, M. Hoorfar, S. Majumdar, H. Murnen, B. Chen "Ethylene production using oxidative dehydrogenation: viii effects of membrane-based separation technology on process safety & economics", Catalysis Today, 2020. (co-author: simulating the process units, providing recommendations, and reviewing the relevant sections in the manuscript) b. Other refereed contributions • Y. Alcheikhhamdon and M. Hoorfar (2016). Membrane Gas Absorption for Offshore Natural Gas CO2 Removal. CSME. Kelowna. June 26, 2016. (Lead author: presenting the idea, conducting a review about the concept technology status, and writing the paper) ix Table of Contents ABSTRACT ......................................................................................................... III LAY SUMMARY ................................................................................................... V PREFACE ........................................................................................................... VI LIST OF TABLES ............................................................................................... XII LIST OF FIGURES ........................................................................................... XIV ACKNOWLEDGEMENTS .............................................................................. XVIII DEDICATION ................................................................................................... XIX CHAPTER 1: INTRODUCTION ............................................................................ 1 1.1 Importance of C3s Separation .......................................................... 1 1.2 Cracking Process Feedstock from Natural Gas .............................. 3 1.3 Paraffins Cracking Process .............................................................. 5 1.4 Conventional Olefin-Paraffin Separation ......................................... 7 1.5 Motivation ......................................................................................... 11 1.6 Objectives ......................................................................................... 11 1.7 Scope and Plan ................................................................................ 11 1.8 Assumptions and Limitations ......................................................... 14 1.9 Thesis Organization ......................................................................... 15 CHAPTER 2: C3H6 / C3H8 SEPARATION MEMBRANES (PREVIOUS WORK) 17 2.1 Separation Principle ........................................................................ 17 2.2 Membranes Classification and Chemistry ..................................... 21 2.3 Polymeric Membranes Tested for C3s Separation ........................ 25 2.4 Alternative Membrane Materials and Transport Mechanisms ...... 29 2.5 MOFs for Propylene-Propane Separation ...................................... 30 2.5.1 ZIF-8 Chemistry and Crystalline Structure .................................. 30 x 2.5.2 Properties of Pristine ZIF-8 Membranes ..................................... 33 2.5.3 Process Conditions Impact on Pure ZIF-8 Membrane Performance ................................................................................................................. 35 2.6 Mixed-Matrix Membranes for Propylene-Propane Separation ..... 35 2.6.1 Impact of Interfacial Defects on MMMs Performance.................. 37 2.6.2 Functionalization of Polymers used in MMMs (Previous Work) .. 39 2.7 Summary ........................................................................................... 47 CHAPTER 3: MATERIAL DEVELOPMENT - EXPERIMENTAL WORK ........... 49 3.1 Polymer of Intrinsic Microporosity (PIM) Selection ..................... 49 3.2 Film Components ............................................................................ 51 3.2.1 Synthesis of "Carboxyl" Functionalized PIM ............................... 52 3.2.2 ZIF-8 Filler .................................................................................. 53 3.3 Film Preparation .............................................................................. 53 3.3.1 Preparation of "Carboxyl" Functionalized PIM Film ..................... 53 3.3.2 Preparation of Mixed-Matrix Films............................................... 55 3.4 Permeability and Selectivity Test Results and Analyses ............. 57 3.5 Testing and Characterization Methods .......................................... 60 3.5.1 Pure-Gas Permeation Tests ........................................................ 62 3.5.2 Mixed-Gas Permeation Tests ...................................................... 64 3.5.3 Film Density / Thickness Calculations ......................................... 66 3.5.4 Thermogravimetric Analysis (TGA) ............................................. 67 3.5.5 Sample Imaging .......................................................................... 68 3.6 Summary ........................................................................................... 69 CHAPTER 4: PROCESS DESIGN, MODELING, AND SIMULATION ............... 70 4.1 Process Design ................................................................................ 70 xi 4.2 Process Modeling ............................................................................ 72 4.2.1 Logarithmic Method ..................................................................... 74 4.2.2 Series Solution Method ............................................................... 76 4.3 Coding and Integration of "MemCal" ............................................. 80 4.4 Summary ........................................................................................... 83 CHAPTER 5: MEMBRANE UNITS PERFORMANCE ASSESSMENT .............. 84 5.1 Single-Stage Process Assessment ................................................ 84 5.1.1 Single-Stage: Chemical Grade Production .................................. 87 5.1.2 Single-Stage: Polymer Grade Production ................................... 89 5.1.3 Single-Stage: From the Chemical to the Polymer Grade ............ 90 5.1.4 Single-Stage: Further Analyses .................................................. 91 5.2 Two-Stage: Process Capability Assessment ................................. 92 5.2.1 Two-Stage: Chemical Grade Production ..................................... 96 5.2.2 Two-Stage: Polymer Grade Production ....................................... 98 5.3 Summary ......................................................................................... 100 CHAPTER 6: CONCLUSION, CONTRIBUTION, AND FUTURE WORK ........ 102 6.1 Conclusion ..................................................................................... 102 6.2 Contributions ................................................................................. 103 6.2.1 Membrane Materials ................................................................. 103 6.2.2 Membrane Unit Modeling and Simulation ................................. 103 6.2.3 Membrane Unit Process Design ............................................... 104 6.3 Future / Planned Work – Recommendations ............................... 104 BIBLIOGRAPHY .............................................................................................. 107 xii List of Tables Table 2-1: Propylene and propane physico-chemical properties ........................ 25 Table 2-2: Experimental data for the permeability and selectivity of C3s in pristine polyimide-based PIMs ........................................................................................ 27 Table 2-3: Experimental data for C3s permeability in pristine ZIF-8 films ........... 34 Table 2-4: Carboxyl-functionalized polymers reported in previous studies ......... 45 Table 2-5: C3s separation properties of previously-developed ZIF-8 based MMM films .................................................................................................................... 46 Table 3-1: Classification of 6FDA-based polyimides based on diamine functional groups ................................................................................................................. 50 Table 3-2: Preparation of Pristine Polymeric Film – calculation details .............. 54 Table 3-3: Permeability / selectivity of methyl- / carboxyl-functionalized PIMs ... 58 Table 3-4: Effect of carboxyl / methyl groups on FFV / BET surface area of PIMs............................................................................................................................ 59 Table 3-5: Imagine techniques used in mixed-matrix films characterization ....... 68 Table 4-1: Analogy between the transfers in heat and mass exchangers ........... 75 Table 5-1: Basis for single-stage unit simulation envelope ................................. 87 Table 5-2: Best possible propylene recovery in a single-stage unit producing the “chemical grade” ................................................................................................. 87 Table 5-3: Best possible propylene recovery in a single-stage unit producing the “polymer grade” .................................................................................................. 89 Table 5-4: Propylene recovery in a single-stage unit producing the “polymer grade” from the chemical grade ..................................................................................... 91 Table 5-5: Basis for two-stage units’ simulation envelope .................................. 95 xiii Table 5-6: Two-stage: recycle ratio required for producing the chemical grade at 90% recovery ...................................................................................................... 96 Table 5-7: Two-stage: recycle ratio required for producing the chemical grade at 99% recovery ...................................................................................................... 96 Table 5-8: Two-stage: recycle ratio required for producing the polymer grade at 90% recovery ...................................................................................................... 98 Table 5-9: Two-stage: recycle ratio required for producing the polymer grade at 99% recovery ...................................................................................................... 99 xiv List of Figures Figure 1-1: Main processes for polypropylene production using a feedstock sourced from natural gas ...................................................................................... 1 Figure 1-2: Cracking process feedstock recovery from natural gas using refrigeration and gas expansion ........................................................................... 5 Figure 1-3: Preliminary fractionation and olefin / paraffin separation conducted downstream of hydrocracker for recovering ethylene and propylene using energy-intensive distillations ............................................................................................. 8 Figure 1-4: Simplified diagram for a conventional C3s distillation tower (splitter) involving an energy-intensive reflux system in column’s overhead ....................... 9 Figure 1-5: Pressure-dependent boiling points of propylene and propane estimated using Peng-Robinson EOS ................................................................................. 10 Figure 1-6: Research areas contributing toward the development of C3s separation membranes ......................................................................................................... 12 Figure 1-7: Research execution plan - workflow diagram ................................... 14 Figure 2-1: Dual-mode sorption effect encountered in glassy polymeric membranes ......................................................................................................... 20 Figure 2-2: Classification of membranes based on their chemistry, structure, porosity, and modularization ............................................................................... 22 Figure 2-3: Examples of common membrane polymers: cellulose acetate: CTA, polyimides: Matrimid®, and perfluorinated polymers: Cytop™ ............................ 24 Figure 2-4: Upper bound of traditional polymers and PIMs for propylene-propane separation (log-scale) ......................................................................................... 27 Figure 2-5: ZIF-8 salt made of zinc ion (metal) and 2-methylimidazole (organic linkers) ................................................................................................................ 31 xv Figure 2-6: 3D representation of ZIF-8 crystalline structure, reproduced from [62] with permission from The American Chemical Society ....................................... 32 Figure 2-7: Dihydroxyl functionalized polymer "6FDA-SBIDA" used as the continuous phase in a mixed-matrix film containing ZIF-8 as a selective filler for C3s’ separation ................................................................................................... 42 Figure 2-8: Experimental data (pure- and mixed-gas) versus Maxwell model predictions of permeability and selectivity of MMMs made of 6FDA-SBIDA polymer and ZIF-8 filler for C3s’ separation...................................................................... 43 Figure 2-9: Plasticization resistance of pure ZIF-8, pure 6FDA-SBIDA, and MMMs made of both when tested at various feed pressures ......................................... 45 Figure 3-1: Synthesis reaction of "6FDA-TrMCA" polymer from corresponding dianhydride and diamine ..................................................................................... 52 Figure 3-2: Pristine 50μm thick polymeric film made of "6FDA-TrMCA" casted in 70mm dia. petri dish undergoing solvent evaporation in triple-covered containers in the presence of a solid desiccant .................................................................... 54 Figure 3-3: Selectivity of 6FDA-TrMCA / ZIF-8 mixed-matrix films as a function of ZIF-8 loading as predicted by the Maxwell model ............................................... 58 Figure 3-4: Sketch of the permeation box used for measuring film’s permeability in pure- / mixed-gas tests ....................................................................................... 60 Figure 3-5: Pure-gas permeation setup used for measuring film’s permeability based on the continuously recorded pressure rise in the constant-volume permeate side of the film, test conducted at fixed feed pressure and temperature ............. 61 Figure 3-6: Pressure rise at film’s permeate side with time-lag and steady-state slope demonstrated ............................................................................................ 63 xvi Figure 3-7: Mitutoyo thickness gauge and a synthesized film ready for digital scanning ............................................................................................................. 67 Figure 3-8: Thermogravimetric analyzer Q5000 and TGA curve for a mixed-matrix film contaminated with solvent and water ........................................................... 68 Figure 4-1: Single- / two-stage (permeate concentration-based) membrane units for C3s separation .............................................................................................. 71 Figure 4-2: Component mass balance on a differential membrane segment operated in counter- and cross-flow modes ........................................................ 73 Figure 4-3: Series solution method used for solving segmental mass balances in counter-current flow modes ................................................................................ 78 Figure 4-4: MemCal representation of a membrane stage in Aspen HYSYS .... 81 Figure 4-5: MemCal user interface in Aspen HYSYS with multiple selections subject to user’s preferences .............................................................................. 81 Figure 4-6: Reproduction of Coker et.al results for a ternary separation case using MemCal .............................................................................................................. 82 Figure 4-7: MemCal used in the simulation of a two-stage membrane unit plus recycle in Aspen HYSYS .................................................................................... 83 Figure 5-1: Single-stage process for propylene-propane separation: design scheme and simulation ....................................................................................... 84 Figure 5-2: Dew-point pressure of 70 wt% propylene C3-mixture as a function of temperature ........................................................................................................ 86 Figure 5-3: Single-stage: minimum selectivity required for producing the “chemical grade” at 90% recovery ...................................................................................... 88 Figure 5-4: Single-stage: minimum selectivity required for producing the “polymer grade” at 90% recovery ...................................................................................... 90 xvii Figure 5-5: Trade-off between propylene’s purity and recovery in single-stage designs ............................................................................................................... 92 Figure 5-6: Two-stage unit (permeate gas concentration based) modeled in HYSYS for C3s’ separation ................................................................................ 94 Figure 5-7: Propylene recovery versus recycle ratio in polymeric membrane based two-stage unit operated at various PRs (A vs. B) for chemical grade production 97 Figure 5-8: Propylene recovery versus recycle ratio in MMM-based two-stage unit operated at various PRs (A vs. B) for chemical grade production ...................... 98 Figure 5-9: Propylene recovery versus recycle ratio in MMM-based two-stage unit operated at various PRs (A vs. B) for polymer grade production ...................... 100 xviii Acknowledgements First and foremost, I would like to sincerely thank my supervisor, Dr. Mina Hoorfar for her unlimited support, guidance, and encouragement throughout the program. Her dedication, enthusiasm, and professionalism will continue to inspire me in my upcoming career. I strongly believe that I was fortunate to have such as a patient, flexible, and understanding supervisor. I would also like to thank my co-supervisor, Dr. Ingo Pinnau for his guidance throughout my research. His valuable directions made the research more focused, structured, and up to date with the ongoing research. With the continuous productive technical discussions, Dr. Ingo and his team demonstrated the value of collaborative research. Also, I would like to thank all his team members, particularly Dr. Giuseppe Genduso and Dr. Bader Ghanem for their support while at KAUST Advanced Membranes & Porous Materials Center. I would also like to extend my sincere thanks to Dr. Bo Chen from the Dalian Research Institute of Petroleum and Petrochemicals for his unlimited support, particularly in the field of modeling and simulation. The appreciation is extended to the research committee members, Drs. Peter Englezos, Homayoun Najjaran, and Keekyoung Kim, who enriched the work with their high-level guidance in terms of research direction and progress. Last but not the least, I wish to thank my support system: my parents, lovely wife and siblings for their unconditional and unlimited support. xix Dedication To my beloved parents Parts of this chapter have been published as i) review paper (Alcheikhhamdon et al., Journal of Natural Gas Science and Engineering (2016), Vol. 34, pg. 689-701), and ii) research article (Alcheikhhamdon et al., Journal of Membrane Science (2019), Vol. 589). 1 Chapter 1: Introduction 1.1 Importance of C3s Separation Propylene (C3H6) is an unsaturated hydrocarbon that is highly reactive, causing it being an important feedstock for various industries. In fact, propylene is the second most important feedstock in the petrochemical industry, behind ethylene [1], with a production of ~75 million tons in year 2014 [2]. By 2025, the global production of propylene is expected to exceed 132 million tons, two-thirds of which is utilized in the production of the well-known thermo-plastic polymer “poly-propylene” [3]. Propylene’s use is extended to the manufacture of iso-propyl alcohol, propylene oxides, acrylic acid, and many other vital products [1], [4]. Propylene is not a naturally occurring chemical but is rather produced through a series of operations including a chemical reaction. In a thermal cracking process known for its intensive energy use, the saturated hydrocarbons (paraffins) are converted to unsaturated ones (olefins, e.g. propylene). Propylene production is mostly sourced from the light alkanes present in the natural gas. The processes demonstrated in Figure 1-1 up to the olefin (alkene)-paraffin (alkane) separation are elaborated in the following sections of this chapter. Figure 1-1: Main processes for polypropylene production using a feedstock sourced from natural gas 2 Unluckily, none of the production technologies yield pure propylene (C3H6); the chemical reactions associated with the hydrocracking processes produce a hydrocarbon mixture that must undergo series of separation processes for propylene’s purification. Among the various separation processes, the separation of propylene (C3H6) from its associated propane (C3H8) is one of the most energy- intensive operations. Despite the insignificant boiling point difference, distillation remains the only technique adopted industrially for their separation. C3s distillation process is continuously criticized for its complex design, intensive energy demand, and adverse environmental impact. The criticism intensifies as energy price increases, and as the environmental regulations become more stringent. Membrane-based process form an appealing alternative due to their compactness, low-energy demand, and their friendly environmental impact [5]. Since the mid- of 1980s, membrane materials have encountered several improvements, contributing toward more reliable and separation capable membranes [5]. Between 2016 and 2024, the membrane-based gas separation market is expected to grow by 8.4% per annum [6]. Currently, several gas separation industries (e.g. O2/N2, CO2/CH4, etc.) are based on polymeric membranes. However, for the C3H6 / C3H8 separation application in specific, the developed membranes’ performance was either poor or unsustainable; the latter justifies the absence of industrial records for the use of membrane technology for C3s’ separation. Novel membrane materials are being developed and tested for C3H6 / C3H8 separation. The development of mixed-matrix membrane (MMM) films that utilize the novel Metal-Organic Frameworks (MOFs) as propylene selective filler is an example of these advancements. Improving the compatibility between the filler and the polymer in mixed-matrix films is one of the key objectives. Compared to membranes’ material development, 3 researches often grant less attention to other equally important subjects, namely process design. In fact, process design manipulation assists in overcoming shortages in membrane performance (e.g. low-selectivity). The research work presented in this dissertation combines the aspects of process design together with membrane material development to improve the competency of membrane technology in C3s’ separation field, against the conventional thermal-based distillation units. 1.2 Cracking Process Feedstock from Natural Gas Natural gas forms the primary input for miscellaneous petrochemical industries, including propylene production. Methane (CH4), which is the primary component of any natural gas, is often accompanied with high molecular weight hydrocarbons (e.g. C3H8, C4H10, and C5+). These high molecular weight hydrocarbons form the so-called "Natural Gas Liquids (NGLs)" when collected in liquid phase. The NGLs form an important feedstock for the downstream "hydrocracking" process. The hydrocracking process converts the NGLs (saturated hydrocarbons, paraffins) into more valuable unsaturated hydrocarbons (olefins, e.g. propylene). In fact, the NGL recovery process is not a must if the feedstock of the hydrocracking process is secured from an alternative source (e.g. LPG: mixture of C3H8, C4H10). The source of the hydrocracking process feedstock is dependent on the process economics and the business scope demarcations. The separation of NGLs from bulk natural gas is mostly achieved using "external" refrigeration, wherein the natural gas temperature is deliberately dropped to condense (liquefy) the heavy hydrocarbons. The cold residual gas discharged from the cryogenic unit is often utilized to pre-cool the cryogenic unit feed (internal refrigeration), and thus reduce the external refrigeration unit load. The mechanical 4 refrigeration unit design parameters e.g. refrigerant selection, cycle type, and process conditions are set based on the aimed cooling. Gas cooling by expansion is an alternative option, which avoids the expensive capital and operating costs of mechanical refrigeration units. In the expansion process, the natural gas pressure is sacrificed using a throttling valve; the latter makes use of the "Joule-Thompson" (JT) effect for dropping the temperature. Gas cooling can alternatively be achieved using expanders. The higher isentropic efficiency of the turbines compared to the JT valves causes them to result in lower gas temperature at an equal expansion pressure. Obviously, the turbines are more expensive than the JT valves due to their complex design. The design often involves both the expander and the JT valve in parallel, where the JT valve serves as a backup in case of expander’s outage. Whether performed using a JT valve or an expander, gas expansion is a main process concern as it results in process gas pressure loss. Thus, when using an expander, a compressor coupled to the turbine (using a common shaft) and utilizing its produced power can be installed to recover the pressure loss, forming the so called "Turbo-expander (TE)". Though the recovery is not 100% efficient, it is still beneficial in minimizing the pressure loss. If NGL recovery is extensive, i.e. if the recovery of low molecular weight hydrocarbons (e.g. ethane) is targeted in liquid form, a combination of both mechanical refrigeration and expansion (in series) is often considered, see Figure 1-2 [7], [8]. 5 Figure 1-2: Cracking process feedstock recovery from natural gas using refrigeration and gas expansion Other licensed technologies such as oil absorption is available for NGLs’ recovery. In this process, a synthetic oil with high affinity toward NGLs is used for selectively absorbing the heavy hydrocarbons from the bulk natural gas. Once extracted, the absorption oil undergoes a regeneration process to separate the extracted NGLs. The absorption process suffers from several drawbacks including the high energy demand and the undesired lights (e.g. methane) pick-up. Compared to mechanical refrigeration and gas expansion, the absorption technology is less encountered [7], [8]. 1.3 Paraffins Cracking Process Olefins (such as ethylene or propylene) are not naturally occurring chemicals, but are rather synthesized, mainly sourced from the steam or the catalytic cracking of paraffins; these processes account for 70% and 28% of the global propylene production, respectively. Other technologies such as propane dehydrogenation and methanol to propylene (MTP) conversion account for the residual production (2%) [9]. Traditionally, propylene is by-product to the ethylene produced from the thermal or the catalytic cracking of high molecular weight hydrocarbons (e.g. heavy oils, naphtha, etc.) [10]. With the increased natural gas availability, more cracking units are now being operated with high molecular weight hydrocarbons (e.g. NGLs) 6 sourced from natural gas. In the steam cracking units, the paraffins, (e.g. C2-C4 alkanes) are diluted with steam and heated using furnaces at high-pressure up to 600-900C before being converted into olefins [1]. In spite of the extensive heat integration, the heat consumed in the cracking units still account for a major portion of the energy consumed in the global petrochemical industry. This massive energy consumption is associated with significant carbon footprint; around 200 million tons of CO2 has been emitted to the atmosphere in year 2000 while producing the heat required for cracking processes [11]. For energy optimization, the cracked product heat is often utilized for preheating the feedstock, for generating HP steam, and possibly for providing the duty required in downstream separation units. Multiple olefins and byproducts result from the cracking unit; these products depend on the feedstock, the cracking temperature, the time spent in the furnace, and the steam to the feedstock ratio. Compared to the steam cracking, catalytic cracking is less energy intensive. The availability of a metal catalyst reduces the activation energy required for cracking the paraffins, allowing the reaction at reduced temperatures [1]. Despite its current less market share, the use of catalytic cracking is steadily increasing, noting that its less energy intensive. In fact, the share of catalytic cracking in the domestic US propylene production market has increased from 45% in 1998 to 58% in 2010. The selected catalyst influences the products of a cracker. The reaction kinetics can be controlled (e.g. by manipulating the temperature, altering the catalyst, etc.) for controlling the yield of a cracker, i.e. to maximize the production of a certain olefin (e.g. propylene) over another [12]. Recently, few "on-purpose" (licensed) technologies were introduced to the market. Unlike the conventional cracking processes which produce a mixture of products, 7 the “on-purpose” processes produce a specific olefin product, resulting in simplified downstream separation. It should be noted that the “on-purpose” process requires a specific feedstock that varies depending on the adopted technology. Methanol to propylene conversion (Lurgi MTP™ by Air Liquide) and propane dehydrogenation (Oleflex™ by UOP) are examples of these processes. By 2025, the "on-purpose" processes are expected to source 30% of the global propylene production [13]. 1.4 Conventional Olefin-Paraffin Separation The hydrocracking process effluent includes a mixture of olefins (CnH2n), unreacted paraffins (CnH2n+2), other hydrocarbons (formed by side reactions), carbon dioxide, carbon monoxide, and hydrogen gases [12]. These products must be separated and handled independently. Each of these products is valuable in its own way. However, this research work will emphasize on propylene and its separation from propane. C3s distillation is one of several distillations that must be conducted for propylene’s purification. The separation process which is conducted downstream of the cracking process constitutes of two main steps: a) preliminary fractionation, and b) olefin-paraffin separation, see Figure 1-3 [12]. 8 Figure 1-3: Preliminary fractionation and olefin / paraffin separation conducted downstream of hydrocracker for recovering ethylene and propylene using energy-intensive distillations Preliminary fractionation refers to the process wherein the products are separated based on their carbon number (C2, C3, etc.), i.e. irrespective of the hydrocarbons’ saturation status (olefins vs. paraffins). Based on the scope of the petrochemical plant, preliminary fractionations and olefin-paraffin separation processes might not be conducted in the same facility. In fact, the olefin (e.g. propylene) - paraffin (e.g. propane) separation is more challenging than preliminary fractionation due to the insignificant difference between the separated components boiling points. The C3H6 / C3H8 boiling point difference is limited to around 5 to 10 oC depending on the operating pressure (see Figure 1-5), causing the C3-splitter to record an exceptional distillation tower height (90-100 m), tray count (200+ trays), and reflux ratio (between 12-20) [14], [15]. The abnormal height imposes several difficulties in column’s fabrication, construction, and transportation. Despite the extensive heat integration, the C3s distillation is recognized as the US most energy-intensive distillation process [10]. In fact, it is estimated that the olefin / paraffin (e.g. ethylene 9 / ethane, and propylene / propane) separation processes account for ~0.3% of the global industrial power demand [16]. Figure 1-4 shows a simplified C3-splitter [3]. Figure 1-4: Simplified diagram for a conventional C3s distillation tower (splitter) involving an energy-intensive reflux system in column’s overhead The operating and the design parameters are critical for optimizing the C3-splitter cost. Factors such as pressure, temperature, reflux ratio and tray count all influence the separation cost. Figure 1-3 summarizes the operating conditions and the design parameters of a C3-splitter. The C3-splitter is intentionally operated at high pressure (16-26 atm) to gain the following: • Elevating splitter’s temperature profile, which in-turn allows the use of a low-cost cooling medium (e.g. water) in column’s reflux system [17]. • Increasing the difference between the C3s boiling points, causing a reduction in the splitter’s tray count. • Reducing the gas volumetric flowrate, and thus reducing splitter’s diameter. The 1st and the 2nd advantages are demonstrated in Figure 1-5 [3]. The savings associated with the above advantages overweigh the cost rise caused by column metal thickness increase. The operating and the design parameters of C3-splitters 10 have been optimized by the extensive studies conducted in this field, and the practical experience acquired from operating numerous splitters around the world. The proper allocation of the feed stream across the various column trays, and the recovery of propylene’s product heat for feed stream pre-heat are few examples of columns’ design optimizations [18]. Figure 1-5: Pressure-dependent boiling points of propylene and propane estimated using Peng-Robinson EOS Depending on the design and the operating conditions, a C3-splitter could produce the chemical (92-95 wt%) or the polymer (99.5-99.8 wt%) grades of propylene [17]. The drawbacks of the conventional C3-splitters, i) the intensive power demand, ii) the large unit footprint, and iii) the complex operation, caused the industry and the academia to investigate alternative technologies for propylene (C3H6) - propane (C3H8) separation. Adsorption by solid desiccants, absorption by liquid solvents, and membranes are among the explored processes [10]. The use of membranes for C3H6 / C3H8 has been under research for some time. However, up to date, there 11 is no record for a competent conventional membrane that is capable of separating propylene from propane. 1.5 Motivation The conventional thermal-driven C3s distillation towers are expensive to operate, energy-intensive, and unfriendly to the environment [3]. The slowdown in the global economics, the increase in the energy price, and the recent / upcoming stringent environmental regulations are all factors promoting the search for an alternative process, with lower energy demand and reduced carbon emissions. The fact that membranes are distinguished with these features motivates researchers to mature this technology further for the C3s’ separation application. 1.6 Objectives This work targets improving the competency of membrane technology in the C3s’ (propylene-propane) separation field, with the objective of reducing the separation process energy consumption and associated carbon emissions. 1.7 Scope and Plan Up to date, the membranes developed for propylene-propane separation service are not qualified to replace the conventional distillation towers. The disqualification is regarded to membrane’s unsatisfactory performance (e.g., low selectivity), the challenges associated with membranes’ large-scale manufacture, and the inability to assess the performance of membranes when placed in a process due to the lack of simulators. Nevertheless, it is still believed that membranes can ultimately share a portion of the C3s’ separation market once the limitations are moderated; this moderation demands the collaboration of membrane material chemists, membrane manufacturers, and process design / simulation engineers. Figure 1-6 12 demonstrates the areas of development that contribute toward the development of reliable membrane units for C3s’ separation. The work in this thesis targets all the areas in Figure 1-6, except for the membrane’s large-scale manufacture. Figure 1-6: Research areas contributing toward the development of C3s separation membranes The research objectives listed below, which are categorized by the area, all target toward improving the capability of membrane units for C3’s separation: a) Membrane Material Development This research targets the development of reliable and capable mixed-matrix films which utilize "ZIF-8" as a filler for selectivity enhancement. This research targets improving the adhesion between the selective filler and the hosting polymer by improving the interfacial compatibility. b) Process Modeling and Simulation It is essential for users to have access to a reliable, fast converging, and user-friendly simulator that is capable of modeling membrane units of complex designs (e.g., two-stage + recycle). The development of such a simulator will augment the research related to gas separation, noting that membrane researchers will be able to evaluate process needs, set research targets, and assess the performance of 13 newly-developed membranes in a “process”. Further, a simulator will allow users to optimize membrane units’ designs for cost-effective separation; the latter reflects positively on the competency of membrane technology in C3s separation market. In this work, a simulation tool will be utilized to evaluate the performance of the mixed-matrix films in single- and two-stages C3s separation processes. c) Membrane Unit Process Design Though the manipulation of process design is a way that can be adopted to mitigate membranes’ imperfection (low-selectivity), the literature often grants less attention to this subject compared to the membrane’s material development. In this research work, the single- and the two-stage processes will be assessed for their ability to produce the chemical and the polymer grades of propylene at different combinations of membrane selectivity and pressure ratios. In fact, overcoming the selectivity burden via process design manipulation will cause researchers to focus on other critical aspects, including but not limited to, membrane industrial-scale manufacture, tolerance to contaminants, performance sustainability, etc. Figure 1-7 summarizes the research work execution plan. 14 Figure 1-7: Research execution plan - workflow diagram 1.8 Assumptions and Limitations The limitations in each research area are addressed in the respective chapter as necessary. However, the following general limitations / assumption are to be noted: - No industrial record available for the manufacture of mixed-matrix membranes. The work presented in this thesis assumes that turning mixed-matrix materials into real membrane products is viable. The modularization aspect (includes thin layer synthesis) is to be investigated, and hence was proposed as future work. - No data is available for the commercial-scale cost of “ZIF-8” filler. In this work, it is assumed that ZIF-8 cost is reasonable, and thus would not discourage the use of ZIF-8 in synthesizing mixed-matrix materials. - The process design and simulation work assumed operating the mixed-matrix membranes with high pressure ratio. However, the mixed-matrix materials were not tested to for the entire simulated pressure ratio, and was therefore proposed as future work. 15 - The membrane pre-treatment requirements were not addressed as part of this work, due to the lack of information regarding feed’s potential contaminants. - The work does not elaborate on the propylene product status upon its discharge from the membrane unit. The study scope is limited to the propylene product purity and recovery, and does account for aspects such as propylene product pressure level or phase. 1.9 Thesis Organization The thesis is comprised of 6 chapters which are organized as follows: Chapter 1: Provides an overview of the conventional processes adopted in industry for producing high-purity propylene from natural gas. It also emphasizes on the drawbacks of the conventional propylene-propane distillation process, and the requirement for an alternative separation technology. Furthermore, the chapter explains the motivation and the objectives of this research. Chapter 2: Explains the principles of the membrane-based gas separation, with an emphasis on the propylene-propane separation application. The current status of conventional polymeric membranes, and their shortfalls in the C3s’ separation field are demonstrated. Furthermore, the chapter summarizes the previous work related to the development of Mixed- Matrix membranes (MMMs) made of Polymers of Intrinsic Microporosity (PIM) and Metal Organic Frameworks (MOF) for C3s’ separation purpose. The chapter elaborates on the improvements made in the aspects of transport properties and polymer-filler compatibility. Chapter 3: Details the experimental work related to the synthesis and testing of the polymeric and mixed-matrix membranes for propylene-propane separation. Furthermore, the chapter describes the apparatus and the experimental setups 16 used in the synthesis, testing, and characterization of the pure polymeric and the mixed-matrix films. Chapter 4: Explains the importance of process design, modeling, and simulation in optimizing the performance of membrane-based propylene-propane separation units. To facilitate this work, this chapter reveals a novel simulation tool that can be used to compute the performance of membranes in complex processes. Chapter 5: Presents the analyses performed to assess membranes’ capability in producing the commercial grades of propylene considering various combinations of design variables. The work examines the performance of the polymeric and the mixed-matrix membranes when installed in a process, and aims at illustrating the added-value of MMMs. Lastly, the work underlines the importance of process design manipulation in overcoming materials’ performance offsets. Chapter 6: Lists the main conclusions, outlines the contributions of this research, and provides a list of recommendations for the future research.17 Parts of this chapter have been published as i) review paper (Alcheikhhamdon et al., Chemical Engineering & Processing: Process Intensification (2017), Vol. 120, pg. 105-113), ii) research article (Alcheikhhamdon et al., Journal of Membrane Science (2019), Vol. 589). Chapter 2: C3H6 / C3H8 Separation Membranes (Previous Work) In this chapter, the principles of membrane-based gas separation are explained, and the shortfalls associated with the use of conventional polymeric membranes for C3s’ separation are demonstrated. Furthermore, the chapter summarizes the previous work related to the development of Mixed- Matrix membranes (MMMs) made of Polymers of Intrinsic Microporosity (PIM) and Metal Organic Frameworks (MOF) for C3s’ separation purpose. The chapter elaborates on the improvements made in the aspects of transport properties and polymer-filler compatibility. 2.1 Separation Principle The polymeric membrane constitutes of a thin layer of organic material, capable of separating gas mixtures when pressure difference is imposed across it. A selective membrane permeates one gas (or more) across it at a rate higher than that of the other gases; the separation occurring across the membrane is a result of the preferential permeation of a certain gas over the other gases. The membranes are pre-engineered for the preferential permeation of certain gases while rejecting the others. The permeating (fast) gases sacrifice their pressure while overcoming the hydrodynamic resistance imposed by membrane’s thickness, through which the permeation occurs from the feed to the permeate side. On the other hand, the non-permeating (slow) components experience insignificant pressure loss while flowing from the feed to the residual side of a membrane [19], [20]. Gas separation membranes are of the dense type; the pores of these membranes are so small to the extent that the membranes are often denoted as "non-porous". 18 In these membranes, gas’s mobility is defined by the solution-diffusion model. In its simplest form, this model is presented by the Fick’s first law of diffusion [21]: 𝐹𝑙𝑢𝑥𝑔𝑎𝑠,𝑖 = −𝐷𝑔𝑎𝑠,𝑖(𝛥𝐶𝑔𝑎𝑠,𝑖/𝑡) = −𝐷𝑔𝑎𝑠,𝑖 𝑆𝑔𝑎𝑠,𝑖(𝛥𝑃𝑔𝑎𝑠,𝑖/ 𝑡) [2-1] Where, 𝐷𝑔𝑎𝑠,𝑖 is the diffusivity of the gas “i” in the membrane material ΔC𝑔𝑎𝑠,𝑖 is the concentration difference of gas “i” across the membrane thickness ΔP𝑔𝑎𝑠,𝑖 is the partial pressure difference of gas “i” across the membrane 𝑆𝑔𝑎𝑠,𝑖 is the solubility of the gas “i” in the membrane material 𝑡 is the thickness of the membrane dense layer In a dense membrane, the permeability of a gas depends on two factors: diffusivity and solubility [21]. Diffusivity is a function of the gas kinetic diameter; the mobility of the gas occurs in the free volume formed between the polymeric membrane chains as these chains flex [21]. Obviously, gases with large kinetic diameter experience high flow resistance and therefore low diffusivity, and vice versa. The kinetic diameter is often associated with gas’s molecular weight (𝑀𝑊𝑔𝑎𝑠 ↑ → 𝐷𝑔𝑎𝑠 ↓). However, this correlation does not apply consistently. The solubility of a gas in membrane’s substrate is another permeability controlling factor; the solubility is often associated with gas’s condensability and molecular weight (𝑀𝑊𝑔𝑎𝑠 ↑ → 𝑆𝑔𝑎𝑠 ↑). Again, this correlation does not apply consistently. The diffusivity and solubility of components often run oppositely (𝐷𝑔𝑎𝑠 ↑ → 𝑆𝑔𝑎𝑠 ↓), i.e. components with high diffusivity often suffer from low solubility, and vice versa. The latter implies that the permeation rate of a gas is controlled by the balance of the diffusivity and solubility factors [21]. In terms of the mechanism, the solution-diffusion process starts by the adsorption of gas into membrane’s high-pressure surface, followed by the diffusion across membrane’s thickness, and finally the desorption from the other membrane 19 end (low-pressure permeate side) [22]. When pure permeation tests are conducted for propylene (C3H6) and propane (C3H8) gases under equal trans-pressure across the same membrane, Equation [2-1] simplifies to [2-2] as following: αC3H6/C3H8 = FluxC3H6/FluxC3H8 = (DC3H6/DC3H8) x (SC3H6/SC3H8) [2-2] Where, αC3H6/C3H8 is membrane’s propylene to propane selectivity The intrinsic selectivity (α) of a membrane is a measure of its performance, i.e. its preference to permeate a certain component over another while the effect of partial pressure is excluded. The diffusivity ratio ( DC3H6DC3H8 ), whether it is less or greater than unity, is often predicted by comparing the kinetic diameters of gases, whereas the solubility ratio ( SC3H6SC3H8 ) is best concluded from sorption tests. Comparing the gases physical / chemical properties only assist in anticipating the sorption test results. The molecular sieving ability of a membrane is generally dictated by its morphology, whereas its sorption potential is highly influenced by the material chemistry; it is worth highlighting that the morphology and the material chemistry of a membrane are inter-related. The interaction between the gas and the membrane material is best understood from analyzing the results of a sorption test; the latter assist in understanding the effect of solubility on membrane’s permeability and selectivity. A sorption test for a glassy polymeric film typically starts with a sharp non-linear rise in the total sorbed gas, followed by a relatively slower linear rise at higher-pressures, see Figure 2-1. 20 Figure 2-1: Dual-mode sorption effect encountered in glassy polymeric membranes Two sorption modes govern gas’s solubility in a membrane film, i) the Langmuir mode observed at low-pressure, wherein the gas occupies the vacant voids (sorption sites) within the structure, and ii) the Henry’s mode observed at the entire pressure range, wherein the gas saturates the dense film material [23]. The dual mode sorption is summarized in Equation [2-3]: 𝑐 = 𝑆. 𝑃 = 𝑐𝐷 + 𝑐𝐻 = 𝑘𝐷𝑃 + 𝑐𝐻′ 𝑏𝑃1+𝑏𝑃 [2-3] Where, 𝑐 is the concentration of a gas in membrane’s film 𝑃 is the pressure applied on film’s-high pressure side 𝑆 is the pressure-normalized total solubility of a gas in a membrane film 𝑐𝐷 is the gas concentration caused by Henry’s mode sorption 𝑐𝐻 is the gas concentration caused by Langmuir’s mode sorption 𝑘𝐷 is the pressure-normalized Henry’s solubility of a gas (Henry’s constant) 𝑐𝐻’ is the pressure-normalized Langmuir solubility of a gas (Langmuir constant) 21 𝑏 is the Langmuir affinity parameter (gas-membrane interaction factor) In general, the increase in feed pressure often boosts film’s permeability. However, this is to be distinguished from the permeability increase caused by plasticization; the latter is observed when dealing with condensable gases (e.g. C3s) [22]. The selectivity obtained from the ratio of pure-gas tests’ permeabilities does not represent the actual performance of a membrane. Membranes often demonstrate lower selectivity when tested for mixed-gases due to the “competitive-sorption” phenomena that can only be demonstrated in mixed-gas tests [23]. In brief, when multiple gases are fed to a membrane simultaneously, each of the gases will compete for the limited sorption sites within the membrane, thereby reducing the sorption sites available for the others. In C3s separation application, the less condensable gas (propylene - C3H6) loses sorption sites to the more condensable gas (propane - C3H8), causing a drop in the solubility ratio ( SC3H6SC3H8 )𝑚𝑖𝑥𝑒𝑑, and therefore a drop in the permeability of propylene compared to propane [24]. 2.2 Membranes Classification and Chemistry Membranes can be classified according to their porosity, material chemistry, structure, and modularization, see Figure 2-2. 22 Figure 2-2: Classification of membranes based on their chemistry, structure, porosity, and modularization From the chemistry perspective, membranes could be polymeric (organic), non-polymeric (inorganic), or a mixture of both (mixed matrix). Most of the membranes utilized in gas separation are polymeric, the knowledge of manufacturing polymeric membranes for gas separation applications is inherited from manufacturing similar membranes yet for reverse osmosis (water desalination) [25]. The non-polymeric membranes are rarely used in industry due to their material cost, brittleness, and the difficulties associated with their manufacture. The mixed-matrix membranes are anticipated to shape the next generation of gas separation membranes, since these membranes combine the advantages of the polymeric membranes with the outstanding separation performance of the inorganic fillers [26]. The introduction of the fillers assists in overcoming the permeability / selectivity trade-off experienced in standard polymeric membranes. The existing techniques adopted in the polymeric membranes’ industrial manufacture are not expected to be impacted much by the introduction of the fillers to synthesize the mixed-matrix [27]. 23 From the porosity perspective, the membranes are either porous or non-porous (dense). The former is not suitable for gas separation, whereas the latter is suitable given it small pore size. The non-porous membranes are made of either glassy or rubbery polymers; the two differ by their polymeric chains’ flexibility and mobility. Compared to the rubbery materials, the polymeric chains of the glassy materials experience less mobility (rigid chains); this offers the glassy materials the ability to sieve the gases based on their molecular size. However, this advantage occurs at the cost of reduced permeability, given the limited free volume. Most of the gas separation applications utilize membranes made of glassy polymers, primarily in applications where the separation is based on molecular size exclusion. From the structural perspective, the membranes constituting of single material are either homogeneous or asymmetric; these terms refer to the pore size distribution across membrane’s thickness. The membranes made of multiple polymer layers are denoted as "composite" membranes. In these membranes, a thin selective layer is deposited on the top of a thick mechanical support layer made of different material; both forms are commercialized. Lastly, the markets offer membranes in two module configurations, namely the spiral-wound and the hollow-fibers [28]. The spiral-wound are distinguished by their high tolerance to foulants, whereas the hollow fiber modules are distinguished by their compactness (high surface area to footprint ratio) [28]. The flat-sheet membranes are rarely found in large-scale industries due to their limited surface area; their usage is limited to the experimental setups. Most of the commercial membranes are derivatives of the cellulose acetate, the polyimides, and the perfluorinated polymers [29], an example of each is shown in Figure 2-3 [28]. Presently, ten polymers account for 90% of the commercialized 24 gas-separation membranes [30]. The satisfactory performance of these materials and the experience acquired from decades of manufacturing and operating such membranes for water desalination (reverse osmosis) justify their dominance. Figure 2-3: Examples of common membrane polymers: cellulose acetate: CTA, polyimides: Matrimid®, and perfluorinated polymers: Cytop™ Among the materials listed in Figure 2-3, the polyimides, which are formed as a result of an imidization reaction between a dianhydride and a diamine, are eminent with an exclusive permeability-selectivity combination, thereby reserving a unique spot on the Robeson curve (permeability-selectivity plot). Apart from the selectivity aspect, the polyimides are known for their excellent chemical / thermal resistance, as well as their good mechanical strength. However, the physical aging and the plasticization concerns persist [22]. In brief, the polyimides membranes are better candidates than the other polymers for C3s’ separation purpose. 25 2.3 Polymeric Membranes Tested for C3s Separation Compared to thermal distillation units, the membrane units are characterized by their reduced energy demand, reduced footprint, simplified operation, and expedited construction and installation schedules [31]. Moreover, the membrane units are distinguished by their low-turndown ratios, and their competency (from the economic perspective) even at low design throughputs. Several studies investigated the use of conventional polymeric membranes for propylene-propane separation, aiming at replacing the energy intensive distillation processes [32]. Several lab-scale membrane films were developed and tested for C3s’ separation purpose. The sorption tests recommend no significant difference between the solubility of propylene and propane in membrane’s material, causing the solubility ratio ( SC3H6SC3H8 ) being ~1. The selectivity of these films toward propylene is caused by the diffusivity advantage ( DC3H6DC3H8 ), noting the smaller size of propylene gas molecule compared to propane (see Table 2-1). Table 2-1: Propylene and propane physico-chemical properties Gas Chemical Structure Molecular Weight (g/mol) Van der Waals Diameter (pm) Lennard-Jones diameter (pm) Normal Dewpoint (°C) Propylene C3H6 42.08 403 [33] 468 [34] -47.6 Propane C3H8 44.10 416 [33] 512 [34] 506 [35] -42.0 Industrially, the poor separation performance and the durability concerns have so far prevented the use of conventional polymeric membranes for C3s separation [36]. In polymeric membranes, the selectivity enhancement (separation efficiency) occurs at the cost of the permeability (mass transport rate) and vice versa [27]. Robeson [37] periodically collects and publishes the separation data for the latest developed polymeric films for various gas separation applications. The so-called 26 “Robeson Upper Bound” maps the maximum recorded selectivity at a particular permeability value; the latter qualifies the upper bound to be a useful tool for rating the performance of newly developed materials [37]. Out of the various glassy polymeric developed for C3H6 / C3H8 separation, the polyimides involving 4,4′- (Hexafluoroisopropylidene) diphthalic anhydride, known as “6FDA”, capture most of the literature’s attention [9]. These polyimides, particularly the ones customized with pre-engineered diamines are denoted as polymers of intrinsic microporosity (PIMs). These polymers are designed to have a unique combination of micro- and ultra-micro pores, causing the membranes made of them to have an outstanding performance that qualifies them to define a new upper bound [9], [38]. Burns et al. [38] defined the upper bound for C3s separation membranes when operated at 35C (the 2003 blue line in Figure 2-4); Kookos [39] described the propylene-propane upper-bound by another correlation detailed elsewhere. Based on the PIMs improved performance, a new upper bound (the 2016 orange line in Figure 2-4) was defined; the 2016 fit is developed based on 2003 slope (due to the same gas pair) considering a visual fit connecting the best tested PIMs, i.e. the fit is over-optimistic [2]. When the PIMs in Table 2-2 data are mapped on the sketch, their actual selectivity values fall below the 2016 line, implying that the 2016 fit demonstrated in Figure 2-4 is exaggerated. Thus, the 2003 bound forms a more realistic benchmark to evaluate the newly developed PIMs. Additionally, Figure 2-4 illustrates the added-value of blending a selective filler along with PIMs to form the mixed-matrix material (elaborated later in Section 2.6). 27 Figure 2-4: Upper bound of traditional polymers and PIMs for propylene-propane separation (log-scale) Table 2-2 lists the performance of various 6FDA-based films, tested for pure-gas. Table 2-2: Experimental data for the permeability and selectivity of C3s in pristine polyimide-based PIMs Polymer P (atm.) T (oC) Pure- / Mixed- Gas Test C3H6 Permeability (Barrer) Selectivity C3H6 / C3H8 Ref. 6FDA-mPDA 3.8 35 Pure 0.13 10 [40] 6FDA-IPDA 3.8 35 Pure 0.58 15 [40] 6FDA-6FpDA 3.8 35 Pure 0.89 16 [40] 6FDA-DDBT 2.0 50 Pure 1.80 20 [41] 6FDA-TrMPD 2.0 50 Pure 30.0 11 [35] Matrimid® 2.0 35 Pure 0.10 10 [38] 6FDA-Durene 3.5 35 Pure 14.5 10.6 [42] 6FDA-Durene/ DABA (9/1) 3.5 35 Pure 13.2 11.6 [42] FDA-Durene/ DABA (7/3) 3.5 35 Pure 10.2 14.2 [42] 1 Barrer = 10-10 cm3 (STP).cm / cm2.sec.cmHg Most of the performance data quoted in literature (including these in Table 2-2) are based on pure-gas permeation tests [40]. The same membranes demonstrate lower selectivity values when tested for mixed gases (realistic feed). Compared to the pure-gas tests, Tanaka et al. [35] and Staudt-Bickel et al. [40] reported 40% and 50% drop in αC3H6/C3H8 under mixed-gas testing conditions, respectively. Furthermore, Staudt-Bickel et al. [40] observed that polyimide-based membranes 28 experience selectivity loss when tested at higher pressures due to the "plasticization" effect; the latter refers to the swelling of the membrane matrix due to the high concentrations of C3s in film’s material [43]. In terms of the mechanism, plasticization is a pressure dependent phenomenon that is caused by the dissolution of the gases in polymer’s material. At high-pressures, significant amount of the gas dissolves in the polymer material which facilitates the polymer inter-chain flexibility, causing an increase in the permeability and drop in the selectivity [42]. Most of the selectivity data quoted in literature pertains to low-pressure tests (1 to 2 atm). Few researchers attempted to suppress plasticization in 6FDA-based polymers [9], [42], [43]. Polymer cross-linking and post-treatment dense film annealing (thermal treatment at 200-400 oC) were both proven effective in increasing film’s resistance against plasticization. Das et al. [9] demonstrated that under pure-gas tests, 6FDA-6FpDA membranes hold their performance up to 5 atm when subject to film annealing. Askari et al. [42] reported similar work yet for other polymers and co-polymers, namely 6FDA-Durene, 6FDA-Durene/DABA (9/1), and 6FDA-Durene / DABA (7/3). The permeation data listed in literature (including these in Table 2-2) pertain to fresh films tested hours or days post their synthesis. Due to the “physical-aging” effect, the films do not maintain their performance when tested months later. In fresh films made of amorphous glassy polymers, the free volumes are in a “non-equilibrium” stage, and they often relax with time causing the membrane film to be denser, less permeable, but more selective. The aging kinetics (which often slow over with time) is dependent on many factors including membrane’s environment, material, and thickness. Polymeric films of high free volume (e.g. PIMs) and low thickness are prone to accelerated aging kinetics [24], [28], [44]. Low et al. [45] 29 summarized the several factors that influences the physical aging of PIMs in a detailed study. In terms of the targeted separation properties, the previous studies recommended a minimum selectivity αC3H6/C3H8 of 35 and a minimum propylene permeability of 1 Barrer to replace the conventional C3s’ distillation process [28], [46]. Up to date, there is no record for a conventional polyimide membrane film that meets these properties when tested for mixed C3s at high-pressure. 2.4 Alternative Membrane Materials and Transport Mechanisms The poor selectivity of pristine polymeric membranes triggered the search for alternative membrane materials and transport mechanisms. The development of the facilitated transport [47], [48] and the carbon molecular sieve (CMS) [49], [50] membranes are examples of the attempts. However, these developments were commercially unsuccessful due to concerns related to their long-term performance and large-scale manufacture. The facilitated transport membranes experience irreversible performance loss upon the poisoning of their selective ion carriers, whereas the CMS membranes experience manufacturing difficulties as a result of their brittleness [14]. Furthermore, the CMS membranes are prone to performance loss once exposed to air, causing their handling and preservation being a logistical challenge [51]. Extensive research has been conducted in the area of Mixed-Matrix Membranes (MMMs) which consist of a polymeric matrix and an inorganic porous selective filler (e.g. zeolites, Metal Organic Frameworks (MOFs), CMS, carbon nanotubes, metal-oxides, etc.). Filler’s selection is dependent on the separation objective, and filler’s compatibility with the nominated polymer. Compared to the other fillers, the MOFs demonstrate better compatibility with the organic polymeric matrix due to their 30 organic-linkers. Furthermore, the ability to tailor the size and the structure of MOF’s pore based on the application requirement (via altering the metal or the organic linker) marks the MOFs with an added-value that is not feasible using other fillers. The MOFs are also distinguished with an outstanding separation efficiency and an exceptional chemical and thermal stabilities [27]. The mixed-matrix films that utilize MOFs are capable of surpassing the permeability / selectivity lines encountered in pristine polymeric films. The studies conducted in this field demonstrated that the MOF-based mixed-matrix membranes (MMM) possess great potentials in separating gases of comparable molecular sizes, e.g. propylene-propane mixtures [14], [36], [52]. 2.5 MOFs for Propylene-Propane Separation 2.5.1 ZIF-8 Chemistry and Crystalline Structure "Zeolitic Imidazolate Frameworks” (ZIFs) represent a novel class of ordered porous crystalline materials that is distinguished by very precise molecular structure [53]. Upon their blending with a polymer (as filler), and subject to proper compatibility, the ZIFs can boost the overall selectivity of mixed matrix materials [53]. ZIFs are a development of the trending research area "reticular chemistry", and belong to the family of Metal-Organic Frameworks (MOFs) [54], [55]. The frameworks are hybrid materials made of an inorganic metal atom surrounded by organic linkers, which together form a cage-type crystalline structures [53]. ZIFs are distinguished by properties similar to these of zeolites, i.e. the exceptional inner surface area, the precise molecular structure, and the exceptional thermal stability. In fact, the term “zeolitic” is attributed to the “zeolite-like” properties of the framework. The metal ions (cause of tetrahedral structure) and the imidazolate (a bridging atom) in ZIFs resembles the aluminosilicate and the oxygen atoms in zeolites, respectively [56]. 31 However, compared to real zeolites, ZIFs are more porous, flexible and tunable [57]. In fact, their flexibility and tunability allow their blending with polymeric materials to form membrane films of outstanding separation performance. It should be noted that beside their intended use in mixed-matrix membranes’ synthesis, the ZIFs have already been utilized in catalysis, biological applications, gas storage applications, drug delivery, chemical sensors, and separation adsorbents [27], [58]. Herm et al. [59] elaborated the utilization of MOFs as adsorbents for various separation application, including propylene’s polymer grade production. It is worth indicating that around 20,000 different MOFs were reported in literature [60]. Yet, limited number of these MOFs persisted to separation applications. The Metal Organic Framework (MOF) "ZIF-8" has been elaborated in the literature repeatedly for its use in propylene / propane separation application [14], [52], [61]. Its suitability is attributed to its distinguished crystal aperture size, which allows the selective permeation of propylene while rejecting propane. The MOF “ZIF-8”, elsewhere described as Zn(MeIM2) [56], is formed when the transition divalent metal cation (Zn+2) coordinates with the de-protonated N atom in the anionic organic part (2-methyli-midazole) to form an expanded three-dimensional sodalite (SOD) structure of controlled-window size [36], [56], see Figures 2-5 and 2-6. Figure 2-5: ZIF-8 salt made of zinc ion (metal) and 2-methylimidazole (organic linkers) 32 Figure 2-6: 3D representation of ZIF-8 crystalline structure, reproduced from [62] with permission from The American Chemical Society The nano-porous aperture is bounded by six-membered ring; its non-expanded pore size is 3.4 Å, calculated from the crystallographic data obtained from X-ray diffraction (XRD) [33], [56], [57]. The ZIF-8 frameworks are somewhat flexible, given the presence of the organic imidazole linkers and their associated bonds rotation [63]. The aperture of the unflexed ZIF-8 framework is 3.4 Å, whereas its flexed pore size is between 4.0 to 4.2 Å [33], [57]. The degree of flexibility is just perfect, since it allows the permeation of propylene at a rate much higher than that of propane [33]. Additionally, the large internal cavity of ZIF-8 (11.6 Å), connected through small apertures, distinguishes this MOF with a remarkable diffusivity [42], [56]. As for the surface area of ZIF-8, Park et al. [56] estimates this to be 1,947 m2/g. The kinetic separation properties of a MOF (e.g. ZIF-8) are defined by the selected metal ion and organic linker, which together define the internal cavity of a MOF, and its structural flexibility [16]. The materials used in the synthesis of ZIF-8 are available, and the synthesis techniques have been addressed in the literature [56], [64]. The materials used in 33 the synthesis of ZIF-8 are less-costly when compared to the materials used in the manufacture of inorganic membranes and zeolites. ZIF-8 is a prototype for the MOF related research, noting the numerous separation applications where it can be utilized [65]. Other zinc and cobalt based ZIFs (ZIF-1 to -12) were reported elsewhere in the literature [56]. Researchers have recently studied the impact of metal ion (cation) substitution on the framework properties. The substitution of the zinc metal (in ZIF-8) with cobalt results in the formation of the so-called "ZIF-67" [66]. Though its framework structure was found similar to that of ZIF-8, the pore size of ZIF-67 was found smaller (3.3 Å) and the framework was found less flexible; The reduced framework flexibility is regarded to the stiff Co-N bond compared to the Zn-N bond in ZIF-8. The higher electronegativity of Co compared to Zn causes a stiffer ionic bond, which in-turn restricts the rotation of the organic linkers [66]. The ZIF-67 smaller pore size and stiffer structure cause its selectivity being double that of ZIF-8, but at the cost of reduced permeability [66]. In terms of its utilization as a filler in MMMs, researchers have reported an agglomeration concerns when utilizing ZIF-67 at loadings > 30% [66]. In view of these shortfalls, the research work presented in this thesis will continue to adopt the commercially available MOF prototype “ZIF-8”. 2.5.2 Properties of Pristine ZIF-8 Membranes The transport of gas in pure ZIF-8 membranes is governed by the solution-diffusion mechanism. In fact, the precise molecular structure of ZIF-8 reflects positively on its sieving ability, and therefore its selectivity. Zhang et al. [33] reported a selectivity αC3H6/C3H8 of 130 in pure-gas environments. In another work [67], Zhang et al. reported a selectivity of 122 and a permeability of 277 Barrer. In terms of the selectivity breakdown, the supporting information of [53] reported a solubility ratio 34 ( SC3H6SC3H8 ) of 1.06 and a kinetic ratio ( DC3H6DC3H8 ) of 94. Obviously, the outstanding selectivity is attributed to the kinetic factor (size-exclusion). As for the outstanding ZIF-8 permeance, the results presented in the supporting information of the same reference [53] suggest that the high permeance is attributed to the high-solubility of propylene gas in the ZIF-8 material. Though it is not the interest of this research, it is worth highlighting that ZIF-8 offers no sieving effect for C2s (ethylene / ethane), as the ZIF-8 framework aperture size is larger than the molecular size of both C2H4 and C2H6 [68]. Pan et al. [14] synthesized and tested five pristine ZIF-8 films, and tested them under identical conditions to demonstrate results’ reproducibility. Similarly, Kwon et al. [52] prepared and tested three films. Unlike these films which were tested at low-pressures, the films prepared by Ma et al. [61] were tested at high-pressures. Table 2-3 demonstrates the conditions and the consistent results of these tests. The selectivity figures are either equal or above the minimum selectivity "35" required for substituting distillation [28]. Table 2-3: Experimental data for C3s permeability in pristine ZIF-8 films Developer Propylene Permeance (GPU) Selectivity (C3H6/C3H8) Pure- / Mixed- Gas Test Feed P (atm) T (oC) Ref Pan et al. 83.0 ± 19.0 34.6 ± 6.4 Mixed Atm ~22 [14] Kwon et al. 79.8 ± 2.7 38 ± 1.1 Mixed Atm Room [52] Ma et al. 85 – 100 45 Mixed Up to 7 Room [61] 1 GPU = 10-6 cm3(STP) cm-2 s-1 cmHg-1 = 3.35 x 10-10 mol m-2 s-1 Pa-1 The defects formed during the synthesis process (cracks, gaps, etc.) have adverse impact on film’s selectivity (compared to ZIF-8 crystal). Using surface coating, Sheng et al. [69] were able to boost the ZIF-8 membrane selectivity to around 100, i.e. close to that of ZIF-8 crystal. The coating is proven effective in blocking the 35 surface defects. The coated membranes demonstrated sustainable performance at feed - permeate pressure difference of around 6 atm [69]. 2.5.3 Process Conditions Impact on Pure ZIF-8 Membrane Performance Limited studies investigated the effect of temperature variation on the performance of ZIF-8 membranes. The experiments performed by Pan et al. [14] and Hara et al. [36] both demonstrated that the increase of temperature results in a drop in ZIF-8 membrane’s selectivity. The tests performed by Pan et al. [14] demonstrated that between 0 to 60 oC, ~ 5% drop in selectivity was experienced for each 10 oC increase in operating temperature. The described trend implies that operating the membrane at lower temperature would be more effective. Furthermore, the thermal analyses suggest that ZIF-8 membranes are thermally stable; this is evidenced by results’ repeatability when the films are subjected to temperature cycles [14]. As for the feed gas (C3s) composition impact, Pan et al. [14] demonstrated that the selectivity of ZIF-8 membrane increases (up to 40) as the feed gas composition approaches pure propylene, and decreases (down to 25) as the gas composition approaches pure propane. The opposite was demonstrated in the supplementary material of [61] upon operating the permeate side under vacuum. 2.6 Mixed-Matrix Membranes for Propylene-Propane Separation Limited studies analyzed the feasibility of manufacturing ZIF-8 membranes on an industrial scale [65]. The experience gained from the manufacture of non-polymeric membranes recommend that the manufacture of pristine ZIF-8 membranes would be challenging, in view of material’s brittleness and synthesis difficulties [65]. The membranes elaborated in Table 2-3 (Section 2.5.2) were synthesized on a lab-scale, with limited cross-sectional area and thickness to avoid defects formation. Instead of being the only membrane constituent, researchers investigated the use 36 of “ZIF-8” as a filler that is dispersed in a continuous polymeric phase to form the so-called Mixed-Matrix Membranes (MMM) [70]. In fact, the rapid and the parallel developments experienced in the area of PIMs and MOFs accelerated the studies related to their combined use (in MMMs) for olefin-paraffin separation [22]. Ordonez et al. [71] was the first to prepare a MMM using ZIF-8 and the well-known Matrimid® polymer. Although it was not intended for C3s’ separation, the mixed-matrix performance data that was reported for other tested gases were promising. The continuous polymeric phase in the mixed matrix offers the membrane a degree of flexibility to support its industrial manufacture, noting that traditional polymers are solution-processable. Also, the polymeric materials are relatively cheaper than the filler, i.e. blending the polymer with the filler would drop the overall membrane cost. Filler’s superior selectivity (sieving ability) and higher permeability compared to the polymer both improve the overall mixed-matrix performance compared to the pure polymeric membrane, subject to the absence of interfacial defects [53]. The higher permeability is of the mixed-matrix is attributed to the intrinsic high-permeability of the filler (e.g. ZIF-8), the associated increase in polymeric chains’ distance, and the additional free volume formed because of ZIF-8 particles’ introduction [42]. Unlike the PIMs which suffer from physical aging effects, the MOFs are more stable given their somewhat stable structures [44]. The permeability and the selectivity of a non-defected MMM is often predicted by the Maxwell model presented in equation [2-4]. The model applies best for MMMs of low filler concentration (low-loading) [72]. The ideal selectivity can be calculated from the ratio of permeabilities when equation [2-4] is applied once for propylene, and another for propane. 𝑃𝑀𝑀𝑀 = 𝑃𝑝 [𝑃𝑍𝐼𝐹−8 + 2𝑃𝑝 − 2 𝜙𝑍𝐼𝐹−8 (𝑃𝑝−𝑃𝑍𝐼𝐹−8)𝑃𝑍𝐼𝐹−8 + 2𝑃𝑝 + 𝜙𝑍𝐼𝐹−8 (𝑃𝑝−𝑃𝑍𝐼𝐹−8) ] [2-4] 37 𝑃 Permeability of propylene or propane (mol m-2 Pa-1 s-1) 𝜙 Volume of fraction of the filler (ZIF-8) 𝑝 a subscript which denotes the polymer The permeability and the selectivity of MMMs are expected to be somewhere between these of the pure components (MOF and polymer); the values depend on the MOF’s (e.g. ZIF-8) volume fraction. The minimum permeability and selectivity combination is equivalent to that of the pure polymeric film, noting that its diffusivity and solubility, which their product defines film’s permeability, is a function of the polymer’s free volume and composition [27]. On the other hand, the best possible permeability and selectivity combination is defined by that of the pure filler, i.e. it is not possible to synthesize MMMs with a selectivity exceeding that of pure ZIF-8 (e.g. 100 to 150 for propylene / propane), while noting that filler’s properties are dependent on MOF’s pore geometry and surface chemistry [27]. Lower selectivity or higher permeability than these predicted by the Maxwell model at a particular loading are possible indications of interfacial defects. Other models such as Bruggeman model [73], Maxwell–Stefan model [74] [75] which account for further factors, e.g. the interfacial layer influence, filler particles' size distribution, and filler particles agglomeration impacts are elaborated elsewhere [72]. The weights of the filler (ZIF-8) and the polymer are recorded precisely during the film synthesis process. The volumes (and thus the volume fractions) of the MMM film components can be calculated using the densities of the pure components. 2.6.1 Impact of Interfacial Defects on MMMs Performance The physical and the chemical stability of ZIF-8 as well as the inherent hydrophobic nature of its organic ligands, all enhance the compatibility between ZIF-8 and the polymeric material in MMMs. Still, these advantageous properties are not sufficient 38 to eliminate inter-crystalline defects in mixed-matrix structures, particularly at high ZIF-8 loadings. After all, the MOF (e.g. ZIF-8) includes a non-organic part (e.g. Zn) which is not expected to have a perfect compatibility with the organic polymer. It is important to note that the poor adhesion between the mixed-matrix components is either regarded to the interfacial voids, or the rigidification of polymer around the filler particles [27]. The interfacial defects are more of a concern when using glassy polymers, and even more concerning when utilizing PIMs (often denoted as “super-glassy” polymers). Unlike the rubbery polymers which their chains adjust to fit the ZIF-8 particles, the chains of the glassy polymers are rigid [76]. Irrespective of their nature, the interfacial defects create non-selective channels that would reduce membrane’s selectivity compared to the ideal selectivity predicted by the Maxwell model [53]. Additionally, these defects weaken the mechanical integrity of the synthesized films. Lin et al. [27] summarized the state of the art strategies / methods that could be followed to minimize the formation of interfacial defects in MMMs, including but not limited to, a) Designing the interaction between the MOF / the polymer: the MOF is selected such that the chemical group present in its organic linker is identical to the group present in the polymeric chain, e.g. blending ZIF-8 with polyimides since both contain amine groups [76]. However, this approach is not always viable, since the selections of the filler and the polymer are influenced by other factors such as the components’ individual separation performance. Though not commonly used, computational methodologies (molecular simulation) can be used for the initial screening of the polymer / MOF pairs, instead of synthesizing and testing actual films made from all the possible combinations of MOF / polymer [77]. 39 b) Polymer functionalization: detailed in Sections 2.6.2 and 3.2.1. c) MOF surface modification: the modification is more demanding compared to the functionalization of the polymer, detailed in Section 2.6.2. d) Adding an interface agent (e.g. ionic liquids) to heal the defects formed between the filler and the polymer; this subject is outside the scope of this research. e) Film preparation techniques, e.g. the alteration of the MOF / polymer solutions mixing sequence [27], or the in-situ MOF growth (often referred as single-step preparation) method where there is no requirement for multiple solutions [76]. Experimentally, a significant difference between the measured and the estimated (theoretical) densities of the synthesized film might be an indication of interfacial defect [53]. The large macroscopic interfacial gaps formed within the membrane structure causes the defected membrane to be of a lower density compared to a non-defected one. The ideal (non-defected) membrane density can be calculated using equations [2-5] or [2-6] depending on the available data. 𝜌𝑀𝑀𝑀 = 𝜌𝑍𝐼𝐹−8 𝜙𝑍𝐼𝐹−8 + 𝜌𝑝 𝜙𝑝 [2-5] 𝜌𝑀𝑀𝑀 =1 𝑋𝑍𝐼𝐹−8𝜌𝑍𝐼𝐹−8 + 𝑋𝑝𝜌𝑝 [2-6] 𝜌 Density of the MMM, ZIF-8, or the polymer 𝜙 Volume of fraction of the filler (ZIF-8) or the polymer 𝑋 Mass fraction of the filler (ZIF-8) or the polymer 𝑝 a subscript which denotes the polymer The ZIF-8 particle density is specified in the literature at 0.95 g/cm3 [53]. 2.6.2 Functionalization of Polymers used in MMMs (Previous Work) The formation of interfacial defects between the MOF and the polymer exasperates at high filler loadings, with highly-glassy polymers, and under the condition of poor 40 polymer-filler compatibility [53], [76]. It is noticed that material scientists overrate the advantage of MOFs’ organic linkers in improving the adhesion between the MOFs and the organic polymeric matrix [27]. Despite its advantages, the organic part of the MOF is not sufficient to the extent that it would eliminate the formation of interfacial defects, particularly at high loadings of the filler, and when utilizing micron-size MOFs. It should be noted that replacing the hosting polymer for better adhesion with the selected filler is not always an option, as the utilized polymer must hold an acceptable performance (selectivity / permeability) toward propylene, such that the resultant mixed-matrix performance is reasonable. "Functionalization" denotes a “limited” modification that is applied to the polymer’s chemical structure using a pre-selected functional (chemical) group; the selection is dependent on the modification objective. In fact, Park et al. [78] was the first to introduce the concept of polymer "functionalization" yet for improvements related to membrane synthesis. Functionalization was also proven effective in improving the resilience of pristine polymeric films against plasticization [79], [80]. Recently, Ma et al. [53] reported an improvement in the compatibility between the filler and the polymer upon functionalizing the polymer with a hydroxyl group due to the H-bonding induced between the constituents. Similar observation was reported by Tien-Binh et al. [81] who blended hydroxyl supported co-polyimide with amine group (-NH2) containing MOF “MIL-53(Al)”. Compared to the non-supported co-polyimide, the polymer supported with the hydroxyl group (-OH) demonstrated better compatibility with the MOF due to H-bonding [81]. The deep-rooted organic and polymer chemistry fields are understood much better than the emerging reticular chemistry field. Therefore, it is somewhat easier to functionalize the PIMs compared to MOFs functionalization; the synthesis of a 41 functionalized polymer is feasible in a conventional lab, whereas MOFs’ surface modification is expected to demand further extensive facilities. Still, Ma et al. [82] demonstrated the feasibility of functionalizing MOF (UiO-66, zirconium-based) with hydroxyl group for its use as a filler in a mixed-matrix film meant for CO2 / CH4 separation. Similarly, Venna et al. [83] modified the surface of MOF “UiO-66-NH2” with a phenyl acetyl group before being used in preparing a mixed-matrix film using the well-known Matrimid® polymer for CO2 / N2 separation. In terms of the compatibility enhancement, the affinity between the functionalized polymer and the ZIF-8 (as a MOF) is attributed to the hydrogen bond (H-bonding) formed between the positively charged (𝛿+) "H" of the hydroxyl group, and the electronegative (𝛿−) "N" of the ZIF-8 organic ligand. Although H-bonding is not as strong as the ionic or the covalent bonds, its strength is enough for "adhesion" improvement. The work performed by Ma et al. [53] has proven the positive impact of utilizing a dihydroxyl functionalized polyimide, namely "6FDA-SBIDA" in the preparation of the ZIF-8 based MMMs. The preparation of the intrinsically microporous polyimide "6FDA-SBIDA" has been reported in a previous work [84] by the same group (see Figure 2-7) [53]. Apart from the polymer / filler adhesion improvement benefits, the hydroxyl functionalization was advantageous in boosting the selectivity of the polymer itself. Compared to the non-hydroxyl version of the same polymer, the hydroxyl-functionalized version showed double the selectivity (α= 30, against 15). Expectedly, this occurred at the cost of permeability which dropped to almost one third of the non-functionalized permeability [2]. The interaction between the functionalized polymer and ZIF-8 was proven by both Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS). 42 Figure 2-7: Dihydroxyl functionalized polymer "6FDA-SBIDA" used as the continuous phase in a mixed-matrix film containing ZIF-8 as a selective filler for C3s’ separation The MMM films synthesized from the functionalized polymer "6FDA-SBIDA" and ZIF-8 showed high tolerance toward high filler loadings (up to 65%) at excellent compatibility, and thus have demonstrated better performance [53]. Furthermore, the MMMs synthesized from functionalized polymers have demonstrated high resistance to plasticization at elevated feed pressures. Figure 2-8 illustrates the permeability and selectivity of the mixed matrix made of 6FDA-SBIDA polymer and ZIF-8 filler, when tested for pure- and mixed-gases at 2 bar and 35 oC [53]. The permeability and selectivity of the films, as predicted by the Maxwell model, are demonstrated on the same figure. The pure permeability and selectivity values used in the Maxwell model predictions are based on previous work by Swaidan et al. [34] (for 6FDA-SBIDA) and Sheng et al. [69] (for ZIF-8); these values are reported in Figure 2-8. For the ZIF-8 in specific, the selectivity used in the Maxwell 43 model (=100) is not far from that the value reported by Zhang et al. [67] in a previous work (=130). Figure 2-8: Experimental data (pure- and mixed-gas) versus Maxwell model predictions of permeability and selectivity of MMMs made of 6FDA-SBIDA polymer and ZIF-8 filler for C3s’ separation Figure 2-8 indicates that up to ~50% loadings, and in pure-gas environments, the experimental results match the predictions of the Maxwell model almost perfectly; this perfect fit implies minimum interfacial defects between the functionalized polymer “6FDA-SBIDA” and the ZIF-8 filler. The exceptional adhesion between the components is attributed to the functionalization effect. A deviation between the Maxwell prediction and the pure-gas tests was observed at loadings > 50%. The deviation is not caused by the interfacial gaps, but rather due to of ZIF-8 particles agglomeration, as proven by the TEM (Transmission Electron Microscopy) images. In terms of the films’ strength, the elasticity (Young’s modulus) and the tensile strength tests (conducted as part of the same work [53]) suggest that the interfacial 44 defects start at ~33 wt% loading. However, with no adverse impact on membranes’ performance though, since the defects are not severe. Similar to the trend observed in the pure polymeric membranes, and referenced to the pure-gas tests, lower selectivity values were recorded when testing the mixed-matrix films for mixed gases (50:50), see Figure 2-8. The mixed-gas data cannot be compared to the Maxwell model predictions, since the model does not account for the competitive sorption effect detailed in Section 2.1. “Plasticization” (swelling and selectivity loss) resistance at high feed pressures is a critical factor that defines the overall performance of 6FDA-SBIDA / ZIF-8 MMMs. The tests performed by Ma et al. [53] demonstrated that the films made of pure 6FDA-SBIDA or pure ZIF-8 are prone to severe plasticization (and thus selectivity loss) at high pressures. Alternatively, the MMMs made of 6FDA-SBIDA and ZIF-8 have demonstrated unforeseen plasticization resistance, see Figure 2-9. A similar behavior was observed by Shahid et al. [85] when testing mixed-matrix films made of ZIF-8 and Matrimid® for CO2 / CH4 separation. In terms of the polymer, the resistance to plasticization is explained by the presence of MOFs which restrict the polymeric chains’ mobility. As for the MOF filler, its resistance to plasticization is attributed to the confining effect imposed by the polymer, which preserves MOF’s inherent sieving capacity [2]. The resistance to plasticization is advantageous as it allows membranes to operate at high-pressures, causing a reduction in the membrane area. In another work by Abdulhamid et al. [80], another polar group, namely "carboxyl", was utilized to functionalize the polymer in an attempt to suppress plasticization. "Carboxyl" functionalization is also associated with slower physical aging kinetics [30]. Askari et al. [42] reported the use of “carboxyl” functionalized co-polymers, namely 6FDA-Durene / DABA (ratio of 9/1) and 6FDA- 45 Durene / DABA (ratio of 7/3), in the preparation of ZIF-8 based mixed-matrix films for C3s’ separation, see Table 2-4. Figure 2-9: Plasticization resistance of pure ZIF-8, pure 6FDA-SBIDA, and MMMs made of both when tested at various feed pressures Table 2-4: Carboxyl-functionalized polymers reported in previous studies Polymer Name Polymer / Co-polymer Structure Polar Functional Group Reference 6FDA-TrMPD (-DAM) No [42], [53], [66], [86] 6FDA-Durene No 46 Polymer Name Polymer / Co-polymer Structure Polar Functional Group Reference PIM-6FDA-OH (SBIDA) Yes (Hydroxyl) 6FDA-Durene / DABA (9/1) Yes (Carboxyl) 6FDA-Durene / DABA (7/3) Yes (Carboxyl) 6FDA: 4,4′-(hexafluoroisopropylidene)diphthalic anhydride TrMPD (-DAM): 2,4,6-trimethyl-m- phenylenediamine (-DAM) DABA: 3,5-diaminobenzoic acid Durene: 1,2,4,5-tetramethylbenzene SBIDA: 3,3,3′,3′-tetramethyl-1,1′-spirobisindane-5,5′-diamino-6,6′-diol In the work by Askari et al. [42], the objective of utilizing the carboxyl (-COOH) supported polymer, i.e. DABA (3,5-diaminobenzoic acid) was to facilitate the cross-linking (forming active sites) of the polymeric chains for plasticization suppression. The work by Askari et al. [42] has not explicitly recognized the advantage of using a “carboxyl” supported polymer in improving the adherence between the polymer and ZIF-8. However, this advantage can still be inferred from the consistency observed between the pure-gas test data and the Maxwell estimations even at ZIF-8 loadings as high as 40 wt%; see the corresponding data of 6FDA-Durene / DABA (9/1) in Table 2-5 [42], [53], [66]. Table 2-5: C3s separation properties of previously-developed ZIF-8 based MMM films Polymer Material ZIF-8 Loading (wt%) Maxwell Model Pure-Gas Test 𝑷C3H6 (Barrer) 𝜶C3H6/C3H8 𝑷C3H6 (Barrer) 𝜶C3H6/C3H8 6FDA-TrMPD (-DAM) Pristine polymer N/A N/A 16.6 11.3 20 wt% 27.1 19 37.7 20.7 47 Polymer Material ZIF-8 Loading (wt%) Maxwell Model Pure-Gas Test 𝑷C3H6 (Barrer) 𝜶C3H6/C3H8 𝑷C3H6 (Barrer) 𝜶C3H6/C3H8 6FDA-Durene* Pristine polymer N/A N/A 14.5 10.58 20 wt% 26.4** 16.9** 25.9 16.93 PIM-6FDA-OH (SBIDA) Pristine polymer N/A N/A 3.5 30 Variable Figure 4-4 Figure 4-4 Figure 4-4 Figure 4-4 6FDA-Durene / DABA (9/1)* Pristine polymer N/A N/A 13.2 11.68 20 wt% 24.3 17.87 20.5 18.08 30 wt% 32.5 21.90 29.7 23.18 40 wt% 42.7 26.55 47.3 27.38 6FDA-Durene / DABA (7/3)* Pristine polymer N/A N/A 10.2 14.17 20 wt% 18.6 20.03 19.1 25.47 * Annealed at 200 C for improving plasticization resistance. ** Maxwell data calculated from the pristine polymer and the pure ZIF-8 (P=277 Barrer and α=122) data. *** Data in this table is retrieved from the work by Askari et al. [42]. Additionally, the work by Askari et al. [42] demonstrated that the mixed-matrix films experience less selectivity loss compared to the pristine polymeric films when tested for mixed-C3s; similar observation was reported by Ma et al. in [53]. Finally, Askari et al. [42] demonstrated that the “carboxyl” groups are associated with selectivity enhancement at the permeability cost. A similar observation was noted when testing the PIMs listed in Section 3.4 of this thesis. 2.7 Summary The separation of propylene from propane using conventional membrane materials is a challenge, noting the very similar physico-chemical properties of these gases. The advancements made by materials scientists in developing highly permeable and selective materials, namely, “Polymers of Intrinsic Microporosity (PIMs)” are still not enough for qualifying pure polymeric material for C3s’ separation. Blending Metal-Organic Framework (MOF) with PIM has been proven effective in boosting 48 the permeability and the selectivity of the resultant mixed-matrix. However, the adhesion between the MOF and the PIM at high MOF loading remains a concern. PIMs’ functionalization with a “-OH” group was proposed by material scientists to induce H-bonding between the PIM and the MOF, thereby improving the adhesion. Still, these defects are observed in the mixed-matrix at MOF loading > 50wt%; such loading is required to achieve the targeted selectivity of ~35. 49 Chapter 3: Material Development - Experimental Work This chapter elaborates the experimental work related to the synthesis and testing of the polymeric and mixed-matrix membranes for propylene-propane separation. Furthermore, the chapter describes the apparatus and the experimental setups used in the synthesis, testing, and characterization of the pure polymeric and the mixed-matrix films. Due to the circumstances associated with the pandemic, the experimental work related to the mixed-matrix films’ preparation and testing was unfinished. The experimental work presented in this chapter was conducted at KAUST’s Advanced Membranes and Porous Material (AMPM) Center, KSA. 3.1 Polymer of Intrinsic Microporosity (PIM) Selection The performance of the mixed-matrix is highly-influenced by the choice of polymer. Beside the permeability and selectivity towards propylene gas, the polymer must demonstrate excellent compatibility with the filler (ZIF-8) for better adhesion. In this work, the functionalization of the polymer with a carboxyl (-COOH) group is proposed in lieu of the hydroxyl (-OH) functionalization addressed in previous work [53] (Section 2.6.2). Due to its higher electronegativity compared to the hydroxyl, the carboxyl is expected to induce stronger H-bond between the electronegative "N" in the ZIF-8 organic-linker side, and the positively charged “H” in the polymer carboxyl side. Also, the stronger H-bond is expected to add further strength to the film’s resistance against plasticization. In terms of the polymer choice, several films made of 6FDA-based polyimides have been recently reported, see Table 3-1. These contorted polymers containing macro-molecular backbones are referred to as “Polymers of Intrinsic Microporosity 50 (PIMs)”. These polymers are featured with a continuous network of intermolecular and interconnected microcavities, causing the associated films being remarkably permeable [6]. The PIMs are distinguished with a unique balance between micro- and ultra-micro pores, causing them to be permeable and selective [22]. Although the films synthesized from these polymers were not tested for C3s, their great performance in conventional separations (e.g., O2/N2, CO2/CH4, H2/CH4) suggest promising performance in regards to C3s’ separation. Table 3-1: Classification of 6FDA-based polyimides based on diamine functional groups Anhydride Diamine Polymer Structure Polar Functional Group Tri-Methyl Ref. 6FDA m-Phenylenediamine 6FDA-mPDA No No [87] DABA 6FDA-DABA Yes (Carboxyl) No [87] TrMPD 6FDA-TrMPD (-DAM) No Yes [67], [87] TrMCA 6FDA-TrMCA Yes (Carboxyl) Yes [80] 6FDA: 4,4′-(hexafluoroisopropylidene)diphthalic anhydride DABA: 3,5-diaminobenzoic acid TrMPD: 2,4,6-trimethyl-m- phenylenediamine TrMCA: 3,5-diamino-2,4,6-trimethylbenzoic acid The balance between polymer’s inter-chain rigidity and spacing defines polymeric film’s separation performance [88]. In fact, the engineering of polymer’s molecular structure could lead to remarkable improvements in permeability / selectivity [30]. 51 Budd’s and McKeown’s research groups studied this subject in depth [89]. Factors such as the functional groups’ polarity, bulkiness, and location (sides versus core) all influence the properties of a polymeric film, as demonstrated later in Table 3-4. Out of the polymers listed in Table 3-1, "6FDA-TrMCA" was selected for the mixed-matrix film preparation. The commercially available fluorinated dianhydride "6FDA" was selected as the polymer first building block, given the several recommendation for its use in the synthesis of PIMs [35], [40], [78], [79]. The bulky “-CF3” groups in 6FDA are responsible for restricting the adjacent phenyl rings torsional motion (limits polymeric chains packing), leading to an increase in the Fractional Free Volume (FFV), and thus a boost in the permeability [90]. As for the polymer second building block, the carboxyl containing diamine "TrMCA" was selected in order to yield a carboxyl functionalized polymer. The diamine is selected to contain three methyl groups as these result in bulky polymer chain sides, boosting material’s FFV and thus improving the permeability. It is worth highlighting that 6FDA-TrMCA is the carboxyl functionalized version of 6FDA-TrMPD, and also the Tri-Methyl customized version of 6FDA-DABA (see Table 3-1). To the best of our knowledge, this is the first research work that recommends the use of a "carboxyl" functionalized polymer for enhancing the adherence between the organic-polymer and the semi-organic ZIF-8 in the mixed-matrix films intended for propylene-propane separation. The carboxyl functionalized PIM (6FDA-TrMCA) proposed as part of this work is more permeable than the carboxyl-functionalized co-polymers reported in previous studies (see Tables 2-4 and 2-5). 3.2 Film Components The sections below detail the preparation and the sourcing of the mixed-matrix film components. 52 3.2.1 Synthesis of "Carboxyl" Functionalized PIM The synthesis of the polymer "6FDA-TrMCA" is not the main focus of this research. However, in brief, the reaction between 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (hereinafter denoted as 6FDA) and 3,5-diamino-2,4,6-trimethylbenzoic acid (hereinafter denoted as TrMCA) in equimolar quantities yields the polymer of interest when conducted at the conditions detailed below [80] (see Figure 3-1). Figure 3-1: Synthesis reaction of "6FDA-TrMCA" polymer from corresponding dianhydride and diamine In essence, the reaction is conducted at a high temperature (~200 oC) in the presence of a high-boiling point solvent (m-cresol) and a catalytic amount of isoquinoline. The water resulting from the polycondensation reaction is stripped-off using nitrogen. The precipitation of the polymer is achieved by cooling down the reaction and adding methanol. Lastly, the polymer is filtered and dried at 150 oC for 24 hours. In terms of the reactants, 6FDA is purchased from Sigma-Aldrich whereas TrMCA was synthesized from reacting 2,4,6-trimethylbenzoic acid (from Sigma-Aldrich) with sulfuric / nitric acids at room temperature. The precipitated, filtered, and then dried (24 hours at 100 oC) powder, namely 2,4,6-Trimethyl-3,5-dinitrobenzoic acid, was subsequently reacted with hydrazine monohydrate (N2H4·H2O) in the presence of Pd/C (palladium on carbon) catalyst to form the second polymer part “TrMCA”. 53 3.2.2 ZIF-8 Filler The MOF selected as the mixed-matrix film filler, i.e., ZIF-8 (2-Methylimidazole zinc salt), was not synthesized, but rather purchased from Sigma-Aldrich (Basolite® Z1200 by BASF). However, the ZIF-8 synthesis protocol is detailed by Demir et al. in [91]. Prior to its use in the mixed-matrix film preparation, the thermally-stable ZIF-8 powder was dried in an oven for 24 hours at ~200 oC to ensure that the material is free of contaminants (solvents, moisture, etc.). 3.3 Film Preparation 3.3.1 Preparation of "Carboxyl" Functionalized PIM Film The preparation of the pristine polymeric films (no ZIF-8) is essential for (i) setting a performance benchmark to demonstrate the added-value of ZIF-8, and (ii) obtaining 𝑃𝐶3𝐻6 / 𝑃𝐶3𝐻8 in pure polymeric film as input to the Maxwell model. For the preparation of the polymeric film solution, 250 mg of the 6FDA-TrMCA polymer powder was mixed with 9.2 mL of anhydrous tetrahydrofuran (THF) to form a 3 wt% solution; magnetic stirring was used for dissolving the polymer. The homogeneous solution was extruded through 0.45-µm hydrophobic PTFE syringe filter to remove the suspended particles and the cross-linked polymers (if any). The filtered solution was then casted in a petri dish of 70 mm diameter to dry the film off the solvent. The rapid evaporation of the volatile solvent THF results in internal stresses, which in-turn causes a non-uniform film surface, possibly with wrinkles and shrinkages (referenced to the casting area). To slowdown the evaporation, the solution is dried off in triple-covered containers as demonstrated in Figure 3-2. 54 Figure 3-2: Pristine 50μm thick polymeric film made of "6FDA-TrMCA" casted in 70mm dia. petri dish undergoing solvent evaporation in triple-covered containers in the presence of a solid desiccant The solution is dried off the solvent in the presence of a desiccant that adsorbs the water present in the atmosphere due to solvent’s hygroscopic nature. Once dried, the film was placed in an oven for 24 hours at 200 oC for complete solvent removal. The 70mm diameter / 50μm thick film was punched using a standard diameter cutter to form multiple films (discs) with a smaller yet identical diameter. The calculations related to film’s preparation are summarized in Table 3-2. Table 3-2: Preparation of Pristine Polymeric Film – calculation details Parameter Calculation Details Film diameter 50-90mm based on the available petri dish size. A large petri dish allows more films to be punched yet at the cost of more polymer requirement. Film thickness 50-100μm films are preferred due to less polymer consumption, and the less time required to reach steady-state condition during permeability tests. Also, the specified thickness guarantees the film being self-supported. Film volume Petri dish cross-sectional area x film thickness Film density 1.1 and 1.3 g/cc depending on the polymer, verified post film preparation. Film weight Film volume x assumed film density (Note A) Solvent weight Calculated based on the targeted polymer in solution (wt%) Solvent volume Calculated based on the required solvent weight and the solvent density Solution concentration 3 wt% polymeric solution is typically prepared, guarantees the full polymer dissolution in the solvent, and also the formation of reasonable film thickness. 55 Parameter Calculation Details A. Differences between the real and the assumed density of the polymeric film results in a deviation between the synthesized (measured by thickness gauge) and the targeted (assumed in calculations) thicknesses. However, this is not a concern if the synthesized film thickness is reasonable. The 50-100μm films synthesized for the experimental purposes are self-supported. However, this is not the case in commercial membranes wherein a porous support layer is required underneath the much thinner separation layer. Despite being made of the same polymer, thin membrane films might exhibit different separation performance compared to the thick ones, due to the differences in polymeric chains arrangements [6], [92]; this implies that the materials demonstrating promising performance cannot be qualified for separation unless being tested with thin films. In a conventional membrane laboratory, film’s thickness can be controlled either by manipulating polymer’s concentration in the later-dried polymeric solution, or by altering the casting area, i.e., the petri dish size. In addition to the separation properties, the thickness of a film influences its aging kinetics. Compared to the thick films prepared in conventional laboratories, the more realistic thin films age faster. As for the impact on the permeability tests, film’s thickness impacts the time required to reach saturation; thin films often reach the steady-state before the thick films, particularly when testing bulky / condensable gases. 3.3.2 Preparation of Mixed-Matrix Films Due to labs closure during the COVID-19 pandemic, the intended preparation and the testing of “6FDA-TrMCA + ZIF-8” mixed-matrix films were not completed, and were therefore proposed as future work. Nonetheless, the procedure for preparing “6FDA-TrMCA + ZIF-8” films is not different from “6FDA-SBIDA-OH + ZIF-8” films’ preparation, reported earlier by Ma et al. in [53]. The dried ZIF-8 powder is first dissolved in < 5 mL of THF using an ultrasonic horn. Then, the 6FDA-TrMCA 56 powder is added to the same solution until the targeted ZIF-8 loading is met, and placed on a roller mixer to form a homogenous solution. The solution is then stripped off with N2 to concentrate the polymer to ~ 5 wt%, before the solution undergoes another cycle of 24-hour mixing. Unlike the pristine polymeric film which is casted in a leveled petri-dish, the mixed-matrix solution is casted on a glass plate with a doctor blade to ensure a uniform distribution of the ZIF-8 filler over the casting area. The casted film is then placed in an oven that is operated at room temperature with nitrogen used for flushing THF. Next, the film undergoes another cycle of drying in the same oven at 250 oC for 24 hours, before it becomes ready for the permeability tests. During the synthesis process, the “priming” method elaborated in literature [27] can be applied for improving the filler “ZIF-8” / polymer adhesion. In this method, and prior to mixing the filler and the polymeric solutions, a small amount of the polymer is added to the filler’s solution to coat the filler’s particles. To some extent, the described coating minimizes the filler / polymer interfacial stresses, thereby reducing the formation of defects. It should be noted that the “ZIF-8” particles’ size has a direct influence on the ZIF-8 / polymer compatibility. In fact, the smaller diameter (nano-size) particles are preferred [76], since these particles demonstrate better dispersion and lesser interfacial voids across the mixed-matrix film when compared to the larger diameter (micron-size) MOFs. However, agglomeration of the nano-particles remains a concern [27]. The ability to synthesize “thin” (< 1 μm) defect-free mixed-matrix films is expected to be challenging, particularly as the difference between the targeted film thickness and the ZIF-8 particle size drops; this subject is to be addressed in a separate work that prioritize the aspects of film defects and mechanical integrity. 57 3.4 Permeability and Selectivity Test Results and Analyses Beside the intended separation gases (e.g., C3H6 / C3H8), the synthesized films are often tested for a set of standard gases (He, H2, O2, N2, CH4, CO2). The results of these tests allow better understanding of material’s characteristics, and also serves as a reference should the material be nominated for their separation. In terms of the sequence, switching between the non-condensable gases (He, H2, N2, O2,) was relatively quick compared to the condensable ones (CH4, CO2). In fact, the condensable gases require more time to desorb off film’s material, particularly when testing thick films. A film degassing time of at least two and eight hours was adopted post the permeability tests involving non-condensable and condensable gases, respectively. For the highly-condensable C3H6 and C3H8 gases, a degassing time equivalent to the testing time (at least) was planned after each test. Due to the circumstances associated with the pandemic, the 6FDA-TrMCA testing was limited to the standard gases. However, the results of the 6FDA-TrMPD serves as a useful reference to anticipate 6FDA-TrMCA (C3H6 / C3H8) selectivity range. 6FDA-TrMCA is the carboxyl-functionalized form of 6FDA-TrMPD (see Table 3-1). Table 3-3 results suggest that 6FDA-TrMCA offers higher O2/N2, CO2/CH4, and H2/CH4 selectivity compared to 6FDA-TrMPD. Accordingly, it is anticipated that 6FDA-TrMCA αC3H6/C3H8 would be higher than that of 6FDA-TrMPD, i.e., > 11. In this analysis, it is assumed that it would be 1.5x that of 6FDA-TrMPD. In terms of the permeability, the analysis of 6FDA-mPDA as well as 6FDA-TrMPD permeability tests’ data both suggest that the permeability of the polymer towards propylene (C3H6) gas would be in the same order of magnitude as that of methane, refer Table 3-3. Assuming that the same applies to 6FDA-TrMCA, the permeability towards propylene would be in the range of 3.2 Barrer. 58 The Maxwell model was then utilized to predict the ZIF-8 loading required to achieve a selectivity range between 25 to 35, considering the assumptions made above for the pristine 6FDA-TrMCA permeability and selectivity. According to the model, the required ZIF-8 loading would be in the range of 45 to 70 wt% (α = 25 → 35) as demonstrated in Figure 3-3. It is anticipated that such loading will not be associated with interfacial defects due to the carboxyl induced H-bonding. Figure 3-3: Selectivity of 6FDA-TrMCA / ZIF-8 mixed-matrix films as a function of ZIF-8 loading as predicted by the Maxwell model Table 3-3: Permeability / selectivity of methyl- / carboxyl-functionalized PIMs Film Material Permeability Data (Barrer) Selectivity Data Reference He H2 N2 O2 CH4 CO2 C3 H6 C3 H8 O2 / N2 CO2 / CH4 H2 / CH4 C3 H6 / C3 H8 6FDA-mPDA (a) - 46 0.62 3.6 0.2 14 0.13 0.013 5.8 70 230 10 [40], [80] 6FDA-DABA - 49 0.42 2.86 0.15 11.1 - - 6.8 74 327 - [80] 6FDA-TrMPD (b) (c) - 407 25.3 91.4 23 498 30.0 2.72 3.6 22 18 11 [35], [80] 6FDA-TrMCA (c) - 193 5.52 26.4 3.2 144 - - 4.8 45 61 - [80] 59 Film Material Permeability Data (Barrer) Selectivity Data Reference He H2 N2 O2 CH4 CO2 C3 H6 C3 H8 O2 / N2 CO2 / CH4 H2 / CH4 C3 H6 / C3 H8 6FDA-TrMCA (d) 156 160 3.4 17.9 1.76 87.0 - - 5.3 49.4 90.9 - Current work a) For C3s: 6FDA-mPD tested at P=3.8 atm / T=35 oC b) For C3s:6FDA-TrMPD tested at P=2.0 atm / T=50 oC c) 85-100 μm thick film d) 70 μm thick film - aged film The effects associated with the addition of the methyl and the carboxyl groups are understood best from analyzing the data listed in Tables 3-3 and 3-4. A comparison between 6FDA-mPDA / 6FDA-TrMPD data, or 6FDA-DABA / 6FDA-TrMCA data illustrates that the methyl groups' (3 x −CH3) addition reflects positively on the permeability, but adversely on selectivity; the first effect overweighs the second. When placed at the polymeric chain sides, the bulky groups cause the polymeric chains to interlock, which in-turn increases material’s FFV and drops its density. Table 3-4: Effect of carboxyl / methyl groups on FFV / BET surface area of PIMs Polymer Carboxyl Functionalization Tri-Methyl Fractional Free Volume (FFV) BET Surface Area (m2/g) Density (g/cm3) 6FDA-mPDA No No 0.165 [80] 0.156 [40] 66 [93] 1.47 [40] 6FDA-DABA Yes No No data No data No data 6FDA-TrMPD No Yes 0.251 [80] 0.239 [94] 450 [80] 1.24 [80] 1.23 [94] 6FDA-TrMCA Yes Yes 0.218 [80] 260 [80] 1.31 [80] A comparison between 6FDA-mPDA / 6FDA-DABA data, or 6FDA-TrMPD / 6FDA-TrMCA data illustrates that the carboxyl-functionalization (mainly intended for adhesion improvements) causes the structure to be denser, reflecting positively on the selectivity, but adversely on the permeability, i.e., opposite to the effects of the methyl group addition. Since the carboxyl-functionalization results in a drop in the permeability, its application is advised to be in conjunction with the methyl-functionalization; this is the case in 6FDA-TrMCA which applies both. 60 Compared to the basic 6FDA-mPDA that is neither supported by the tri-methyl nor by the carboxyl groups, 6FDA-TrMCA, selected for the mixed-matrix synthesis, offers much higher permeability at the cost of a minor selectivity loss. But most importantly, 6FDA-TrMCA is expected to offer superior compatibility with the filler, which the other PIMs do not offer, particularly at high ZIF-8 loadings. Though not tested as part of these experiments, the carboxyl-functionalized materials are expected to be more resistant to plasticization. 3.5 Testing and Characterization Methods The permeability of a thin film was measured using the assembly (permeation box) available at KAUST’s Advanced Membranes & Porous Materials Center, refer Figure 3-4. The test conditions (e.g., the feed pressure, and the temperature) can be manipulated to the specific test requirement. Figure 3-4: Sketch of the permeation box used for measuring film’s permeability in pure- / mixed-gas tests 61 The temperature-controlled box integrates multiple items (detailed in Figure 3-4) to form the experimental setup illustrated in Figures 3-5. Figure 3-5: Pure-gas permeation setup used for measuring film’s permeability based on the continuously recorded pressure rise in the constant-volume permeate side of the film, test conducted at fixed feed pressure and temperature The experimental setup consists of high- / low-pressure sections, separated by the tested film. In terms of the test mechanism, a constant high pressure is applied to film’s feed side, while the fixed volume at the low-pressure side experiences a pressure rise. The permeability of the tested film is calculated based on the rate of the pressure rise, as detailed later in this section. Obviously, a rapid pressure rise implies that the tested film is highly permeable, and vice versa. The tested film is sandwiched between two adhesive-backed aluminum tape donuts. Epoxy resin is applied to the interface between the film and the aluminum tape to guarantee proper sealing to force the gas flow solely through the tested film. The resin often dries in 24 hours, after which the permeation cell flanges can be tightened for starting the test. The data recorded during the tests are passed to Data Acquisition Board (DAQ), and subsequently to LabView® software for 62 recording / plotting data, and performing permeability calculation based on the equations detailed in the following sections. 3.5.1 Pure-Gas Permeation Tests By applying the time derivative of the ideal gas law to the permeate side of the setup, we will obtain 𝑅𝑇𝑑𝑛𝑑𝑡= 𝑉𝑝𝑑𝑝𝑝𝑑𝑡 [3-1] “Permeance” is defined as the flux (flow / area) normalized by the total trans-membrane pressure difference (see below): 𝑃𝐿 =1𝐴 (𝑝𝑓−𝑝𝑝) 𝑑𝑛𝑑𝑡 [3-2] In the setup demonstrated in Figure 3-4, the permeate pressure is often negligible compared to the feed. On this basis, and upon substituting the permeation rate 𝑑𝑛𝑑𝑡 in the equation [3-2] by [3-1], the permeance can be expressed as 𝑃 =𝑉𝑝𝑅𝑇 𝐿𝐴 1𝑝𝑓 𝑑𝑝𝑝𝑑𝑡= 𝐷. 𝑆 [3-3] where, 𝑃 is the tested film permeability for the flowing gas 𝑉𝑝 is the permeate side constant volume 𝑝𝑓 , 𝑝𝑝 are the feed and the permeate side pressures 𝐿 is the tested film thickness 𝐴 is the tested film cross-sectional area that is available for permeation 𝑅 is the universal gas constant 𝑇 is the permeate vessel-controlled temperature 𝐷 is the diffusivity of a gas through the tested film 𝑆 is the solubility of a gas in the tested film 63 𝑑𝑝𝑝𝑑𝑡 is the steady-state rise in permeate pressure The permeation test is often held for a minimum duration of 10 times the test time-lag (𝜃), and subject to the establishment of a steady-state pressure rise. The time-lag (𝜃) is the intercept of the pressure curve steady-state (SS) slope with the time-axis (𝑡𝑖), as demonstrated in Figure 3-6. The start time (𝑡0) must be subtracted from the intercept time (𝑡𝑖) if the former is not equal to zero; 𝑡0 is the time at which the film is exposed to a high-pressure, i.e., the time at which the feed gas valve was opened. Figure 3-6: Pressure rise at film’s permeate side with time-lag and steady-state slope demonstrated Slow gases (e.g., those with large molecular structures) are often associated with significant time-lag. Bulky gases require more time to diffuse through, saturate, and establish a steady-state concentration gradient across the film. The time-lag fundamentals are detailed elsewhere [95]. The pure-gas selectivity is calculated based on the permeability values obtained from testing the film for each gas (A & B) separately. 64 α AB (𝑝𝑢𝑟𝑒)=𝑃𝐴𝑃𝐵 [3-4] The pure-gas permeability and selectivity values are useful in benchmarking and comparing materials. However, these values might not reflect the real performance of membranes when tested in real (mixed-gas feed) conditions, noting the adverse impact of the “competitive-sorption” effect, as detailed in Section 2.1. Kruczek et al. [96] proposed an alternative mechanism for measuring the diffusivity and the permeance of a low-permeability films utilizing Roger’s short-time method [97]. The method is particularly useful when testing slow-gases (e.g., propylene), noting that the steady slope demonstrated in Figure 3-6 might require several days to be established, especially when testing low-permeance films. The method presented by Kruczek et al. is not utilized in this work. Instead, the conventional time-lag method was adopted. In another study, Wu et al. [98] studied the validity of the time-lag method for testing mixed-matrix films, and compared the result of this method to the numerical solution of the Fick’s law (applied in 3-D) when solved for the diffusion coefficient; the study did not object the use of the time-lag method for the mixed-matrix films. 3.5.2 Mixed-Gas Permeation Tests The pressure rise concept detailed in the previous section is still applicable for the permeability / selectivity measurement in mixed-gas environments, yet subject to few modifications. In a mixed-gas test, the rise in the component’s partial pressure is used in lieu of the rise in the total permeate pressure (used in a pure-gas test) [24]. When the time derivative of the ideal gas law is applied to the permeate side of the setup (yet for a particular gas) the following relation is obtained: 𝑅𝑇𝑑𝑛𝑖𝑑𝑡= 𝑉𝑝𝑑𝑝𝑝,𝑖𝑑𝑡 [3-5] 65 The rise in the component’s partial pressure is not measurable. Thus, Dalton’s law is applied to express the rise in terms of the total permeate pressure. 𝑅𝑇𝑑𝑛𝑖𝑑𝑡= 𝑉𝑝 𝑦𝑖𝑑𝑝𝑝𝑑𝑡 [3-6] In the above relation, the term “Permeance” is defined as the component flux (flow / area) normalized to the component trans-membrane partial pressure difference. 𝑃𝑖𝐿 =1𝐴 (𝑝𝑓,𝑖−𝑝𝑝,𝑖) 𝑑𝑛𝑖𝑑𝑡 [3-7] In the setup demonstrated in Figure 3-4, the permeate pressure is often negligible compared to the feed. Furthermore, the Dalton’s law is applied to eliminate the partial pressure expression: 𝑃𝑖𝐿 =1𝐴 𝑝𝑓 𝑥𝑖 𝑑𝑛𝑖𝑑𝑡 [3-8] Upon substituting the permeation rate 𝑑𝑛𝑖𝑑𝑡 in the equation [3-8] by [3-6], the mixed-gas permeance can be expressed as 𝑃𝑖 =𝑉𝑝𝑅𝑇 𝐿𝐴 1𝑝𝑓 𝑑𝑝𝑝𝑑𝑡 𝑦𝑖 𝑥𝑖 = 𝐷𝑖 𝑆𝑖 [3-9] where, 𝑖 subscript denotes a parameter specific to a certain gas 𝑦𝑖 is the mole (volume) fraction of a certain gas in the permeate vessel 𝑥𝑖 is the mole (volume) fraction of a certain gas in the feed which is known. All other parameters are as described in the previous section. Equation [3-9] is used to calculate the permeance of a gas through a film in mixed-gas environment. The mixed-gas selectivity is calculated based on equation [3-9] when used for two different gases (e.g., A and B). α AB (𝑚𝑖𝑥𝑒𝑑)=𝑃𝐴𝑃𝐵= 𝑦𝐴 𝑦𝐵⁄ 𝑥𝐴 𝑥𝐵⁄= [ 𝑦𝐴(1− 𝑦𝐴)⁄ 𝑥𝐴(1− 𝑥𝐴)⁄]𝑏𝑖𝑛𝑎𝑟𝑦 𝑚𝑖𝑥𝑡𝑢𝑟𝑒 𝑜𝑛𝑙𝑦 [3-10] 66 Compared to the pure-gas setup, the mixed-gas setup is supplemented with a gas chromatograph (GC) for analyzing the permeate gas composition (𝑦𝑖). The pressure at the permeate vessel might be insufficient to flow the gas toward the GC’s inlet. In such a case, helium can be used to induce the flow as demonstrated in Figure 3-4. The feed gas composition (𝑥𝑖) is often known to users, pre-ordered from the gas supplier based on the test requirements. 3.5.3 Film Density / Thickness Calculations The geometrical density of a film is calculated from its measured mass and volume. Film’s volume is calculated based on its thickness and cross-sectional area. The thickness of a film is measured using a digimatic thickness gauge (Mitutoyo 547-561S, 0-12mm range, 0.0005" resolution, +/-0.001 accuracy), see Figure 3-7. For an averaged thickness measurement, the same film is measured at five different locations, i.e., at the four edges and the center. The cross-sectional area is calculated using "ImageJ" software, post the films being digitally scanned. For digital calibration, an object with an exactly known dimension is scanned along with the films. The measurement of the cross-sectional area is performed post the dry-out of the epoxy resin to allow an accurate measurement of the area available for permeation, see Figure 3-7. 67 Figure 3-7: Mitutoyo thickness gauge and a synthesized film ready for digital scanning 3.5.4 Thermogravimetric Analysis (TGA) Controlling the quality of the synthesized film is essential, mainly in terms of the film’s thermal stability, impurities, and the filler’s content in the mixed matrix. The quality of the synthesized film is very dependent on chemists’ compliance with the synthesis procedures. A thermal-based method, namely Thermogravimetric Analysis (TGA) is used for verifying the quality of the synthesized films. The main downside of this method is that the film is sacrificed during the analysis. Thus, from the film prepared for the permeability test, a small portion is often reserved for TGA. During the analysis, the weight of the sample is continuously measured as its temperature increases. Foreign liquids (e.g., solvent, water, etc.) could be present in the synthesized film, due to either imperfect solvent evaporation, or the use of contaminated solvents. A drop in the sample weight at or around the boiling-point of a suspected liquid is a fair proof of its presence; the contamination level is proportional to the observed weight loss, see Figure 3-8. 68 Figure 3-8: Thermogravimetric analyzer Q5000 and TGA curve for a mixed-matrix film contaminated with solvent and water TGA is also utilized to confirm the mass fraction of a filler in a mixed-matrix film. The mass of ZIF-8 in a polymer-based mixed-matrix film is back-calculated from the weight of the ZnO residue found at ~800 oC (wherein no polymer remains). The calculation of ZIF-8 mass based on that of ZnO is subject to stoichiometry. Further, TGA provides an insight into the thermal-stability of a film, i.e., its resistance to decomposition at various temperatures. 3.5.5 Sample Imaging Several imaging techniques can be used for characterizing the mixed-matrix films, subject to the availability of the corresponding instruments in the synthesis lab. Table 3-5 summarises the unique data provided by each technique. Table 3-5: Imagine techniques used in mixed-matrix films characterization Technique Test Objective Transmission Electron Microscopy (TEM) Demonstrates the compatibility between ZIF-8 and the hosting polymer in mixed-matrix films. Furthermore, detects the presence of interfacial gaps and ZIF-8 particles agglomeration (if any). X-ray Diffraction (XRD) Verifies the preservation of the chemical identities for both the ZIF-8 and the polymer post the mixed-matrix film formation. Furthermore, it provides rough information about sample’s free volume. X-ray Photoelectron Spectroscopy (XPS) Proves the chemical interaction between ZIF-8 particles and the hosting polymer in mixed-matrix films. 69 Technique Test Objective Fourier Transform Infrared Spectroscopy (FT-IR) Proves the chemical interaction between ZIF-8 particles and the hosting polymer in mixed-matrix films. Dynamic Light Scattering (DLS) Determines ZIF-8 particles’ size distribution. Differential Scanning Calorimetry (DSC) Polymer glass transition temperature. 3.6 Summary The “carboxyl” functionalization of PIMs has been proposed as a mean to improve the adhesion with the ZIF-8 fillers, aiming at developing competent C3s’ separation membranes. Although not proven experimentally, the positive effect of carboxyl in terms of inducing H-bonding between the film constituents is expected to surpass that of hydroxyl-functionalization proposed in previous studies. Furthermore, the carboxyl-functionalized PIM “6FDA-TrMCA” is expected to exhibit fair separation properties that would allow its use in the formation of mixed-matrix material along with ZIF-8 for the purpose of C3s’ separation. The preparation procedures as well as the characterization and the testing methods for the pristine polymeric “6FDA-TrMCA” film as well as the mixed-matrix film were elaborated in this chapter. Parts of this chapter have been published as a research article (Alcheikhhamdon et al., Journal of Membrane Science (2019), Vol. 589). 70 Chapter 4: Process Design, Modeling, and Simulation This chapter illustrates the importance of process design, modeling, and simulation in optimizing the performance of membrane-based propylene-propane separation units. To facilitate this work, this chapter reveals a novel simulation tool that can be used to compute the performance of membranes in complex processes. "Aspen HYSYS" is utilized as the platform for running the tool. 4.1 Process Design The depropanizer column produces a C3 mixture that is often 50-70wt% propylene (referred to as refinery grade), balance being propane. The downstream C3-splitter is responsible for upgrading the propylene from 50-70wt% to the chemical (93wt%) or to the polymer (99.5wt%) grades, see Figure 1-3. Remarkable advancements have been made toward increasing the performance and the robustness of commercial membranes. However, for C3s’ in specific, the membranes’ performance figures are still insufficient for replacing the distillation-based C3-splitters. In a single-stage unit, the low-selectivity of a membrane causes a trade-off between the produced propylene purity and recovery, causing the units being incompetent as compared to distillation. The installation of a recovery stage (two-stage unit) is an effective mean to overcome the purity / recovery trade-off, thereby allowing the production of high-purity propylene at competent recoveries [99]. However, the recovery stages are often associated with additional capital and operational costs, noting the presence of inter-stage compressors. Process design selection is dependent on the feed quality, process conditions, membrane selectivity, and purification objectives. In fact, most of the knowledge associated with the membrane units’ process design is acquired from a similar 71 research work conducted for the CO2 / CH4 separation application [99], [100]. Figure 4-1 demonstrates the pressure and the concentration profiles in single- / two-stage processes that are intended for C3s’ separation. Figure 4-1: Single- / two-stage (permeate concentration-based) membrane units for C3s separation Single-stage units are distinguished by their limited footprint, simplified operation, and low cost. However, these advantages occur at the cost of poor propylene recovery. Compared to the single-stage designs, the two-stage designs moderate the recovery concern at the cost of increased process complexity, and relatively higher capital / operational expenses. The previous studies recommended a minimum selectivity αC3H6/C3H8 of 35 and a propylene permeability no less than 1 Barrer for replacing the well-established distillation process [28], [46]. Chapter 5 details the adequacy of these figures. Figure 4-1 does not demonstrate the pre-treatment facilities which are commonly installed upstream of membranes for preserving them against contaminants and foulants. The extent of the pre-treatment (outside the scope of this research) varies from simple to the complex designs depending on the feed gas quality, and is best advised by the membrane supplier [31]. 72 4.2 Process Modeling The determination of multi-stage membrane units’ performance is one of the challenges encountered in research and industry, noting the numerous equations that must be solved simultaneously for computing the overall performance. In fact, the optimization of the multi-stage units’ design is obstructed by the numerous material balances that are to be solved. The complexity of modeling, and therefore simulation, is proportional to a) unit’s stage count, b) number of gases, and c) the presence of recycle streams. The use of a simplified model results in a poor prediction of actual unit’s performance, which in-turn causes end-users to be doubtful about adopting membrane technology for any gas separation application, including C3s. The elaboration illustrates the importance of developing a powerful simulation tool that can support the simulation of unrestricted number of membrane stages, gases, and recycles. In this research, the well-known thermodynamic simulator “Aspen HYSYS” was selected as the platform for the simulation tool. The use of this software allows the utilization of its in-built computational algorithms and fluid packages in resolving the complex mass balances, and in predicting gases’ thermodynamic behaviors, respectively. Apart from these advantages, “Aspen HYSYS” is user-friendly and commonly used by the researchers in this field. Irrespective of the modelling technique, equations [4-1] to [4-4] are fundamental in solving any membrane model material balance. The mass transport in non-porous dense polymeric membranes is based on the solution-diffusion mechanism [21]: 𝐹i = 𝐽i (𝑃 𝑥local – 𝑝 𝑦local) 𝐴 [4-1] Where, 𝐹i Permeation rate of component 𝑖 (mol s-1) 73 𝐽i Permeance of component 𝑖 (mol m-2 Pa-1 s-1) 𝐴 Membrane area (m2) 𝑃 Feed-side pressure (Pa) 𝑝 Permeate-side pressure (Pa) 𝑥local Feed-side component 𝑖 molar fraction 𝑦local Permeate-side component 𝑖 molar fraction 𝑖 Component index In membrane module section, the mass balance is presented by the following [70]: 𝐹pi = 𝐹fi – 𝐹ri [4-2] Where, 𝐹pi Permeation rate of component 𝑖 (mol s-1) 𝐹fi Feed rate of component 𝑖 (mol s-1) 𝐹ri Residual rate of component 𝑖 (mol s-1) The composition of the gas on either side of the membrane is dynamic. Hence, Fick’s law [21] demonstrated in equation [4-1] must be applied on a "differential" basis, rather than across the entire membrane stage, see Figure 4-2 [101]. Figure 4-2: Component mass balance on a differential membrane segment operated in counter- and cross-flow modes Equation [4-3] demonstrates the differential form of equations [4-1] and [4-2]: 𝑑𝐹pi = 0 – 𝑑𝐹ri = + 𝐽i (𝑃 𝑥local – 𝑝 𝑦local) 𝑑𝐴 [4-3] 74 Equation [4-3] is applicable to all possible flow modes. The counter- and the cross- flow modes are encountered in the hollow-fiber and the spiral-wound modules, respectively [101]. The co-current mode is rarely used due to the rapid depletion of permeation potential across the membrane. The composition at the high- (𝑥local) / low- (𝑦local) pressure sides varies with the area (𝑑𝐴), causing the integration of equation [3-3] being challenging. The integration of equation [4-3] over the entire membrane area is crucial for computing the performance across a stage. On a high-level, the solution for equation [4-3] can be demonstrated by the following: ∫ 𝑑𝐹pi = ∫ 𝐽i (𝑃 𝑥local – 𝑝 𝑦local) 𝑑𝐴 = 𝐹pi = 𝐽i 𝛥𝑃𝑖,𝑎𝑣𝑔 𝐴 [4-4] Where, 𝛥𝑃𝑖,𝑎𝑣𝑔 is the averaged permeation potential which its representation is subject to the modeling method; several techniques have been proposed in the literature. The following sub-sections cover the fundamentals, advantages and the limitations of each method. 4.2.1 Logarithmic Method The logarithmic approach by Davis R. [101] is one the simplest modeling methods. The analytical solution, which is the signature of this method, allows its tabulation in a spreadsheet running in the simulator’s background. The simulator exchanges the information with the spreadsheet until the simulation converges at the specified tolerance. Unluckily, the logarithmic method is only valid for binary systems, i.e. the method is not suitable for multi-component gases. Mass transfer in membranes is analogous to heat transfer in shell and tube exchangers. The analogy details are described in Table 4-1: 75 Table 4-1: Analogy between the transfers in heat and mass exchangers Parameter Heat Transfer Mass Transfer Surface Area Provider Heat exchanger Membrane (mass exchanger) Transfer Mode Convective Diffusive and convective Equation 𝑄 = 𝑈 𝐴 𝛥𝑇lm [4-5] Where, 𝑄: Heat transfer rate 𝑈: Heat Transfer coefficient 𝐴: Heat exchanger area 𝛥𝑇lm: Logarithmic mean T difference 𝐹pi = 𝐽i 𝛥𝑃i,avg 𝐴 [4-6] Where, 𝐹pi: Mass transfer rate 𝐽i: Mass Transfer coefficient (Permeance) 𝐴: Membrane surface area 𝛥𝑃i,: Average permeation potential, its calculation is subject to the modelling technique Transfer Potential Local Temperature difference Localized component partial pressure difference Analogy By Comparison 𝛥𝑃i,avg is analogous to 𝛥𝑇lm → 𝛥𝑃i, avg ≃ 𝛥𝑃i,lm System Sketch Transfer Potential Equation 𝛥𝑇lm = ΔTa – ΔTb𝑙𝑛 (ΔTaΔTb) [4-7] 𝛥𝑇i,a: Temperature gradient at side “a” 𝛥𝑇i,b: Temperature gradient at side “b” 𝛥𝑃i,Avg = 𝛥𝑃i,lm = ΔPi,a – ΔPi,b𝑙𝑛 (ΔPi,aΔPi,b) [4-8] 𝛥𝑃i,a: Component’s partial pressure difference at side “a” 𝛥𝑃i,b: Component’s partial pressure difference at side “b” Permeation Potential Described elsewhere for all modes (co-current, counter-current and cross flow) a) In cross-flow mode (spiral-wound), the permeate side composition is somewhat uniform whereas the high-pressure side concentration is dynamic. The solution of the two-component (binary) system is as following: ∆𝑃i,avg = [𝑥i̅ P − 𝑦𝑃 p ] [4-9] 𝑥i̅ =𝑥𝐹,𝑖− 𝑥𝑅,𝑖ln(𝑥𝐹,𝑖𝑥𝑅,𝑖) [4-10] 𝛼 =𝐽𝑖𝐽𝑗 [4-11] 𝑃𝑟 =𝑝𝑃 [4-12] 𝐵 = 0.5 [1 + 1(𝛼−1) 𝑃𝑟+ 𝑥i̅𝑃𝑟 ] [4-13] 𝑦𝑃 = 𝐵 − [𝐵2 − 𝛼(𝛼−1) 𝑃𝑟𝑥i̅ ]0.5 [4-14] b) In countercurrent flow mode (hollow-fibers), the feed and permeate sides’ composition is dynamic. The solution for two-component (binary) system is as following: ∆𝑃i,avg =(𝑥𝐹,𝑖𝑃− 𝑦𝑎,𝑖 𝑝)−(𝑥𝑅,𝑖𝑃−𝑦𝑏,𝑖 𝑝)ln(𝑥𝐹,𝑖𝑃−𝑦𝑎,𝑖 𝑝𝑥𝑅,𝑖𝑃−𝑦𝑏,𝑖 𝑝) [4-15] 76 Parameter Heat Transfer Mass Transfer 𝑦a,i : permeate composition opposing feed 𝑦b,i : permeate composition opposing residue All the parameters indicated above are known to process designers as a design input, except for the composition of the permeate at the side opposing the residual gas, which can be determined by solving the material balance of the involved components simultaneously. The solution to 𝑦b,i is similar to that obtained in equation [3-14], except that 𝑥i̅ is exchanged by 𝑥𝑅,𝑖. Whether its cross flow or countercurrent, solving the above equations should be under the constrain demonstrated in equation [3-16], which implies that the summation of all n components (gases) molar fractions at any side of the membrane and at any point across the flow path should be equal to 1. ∑ 𝑥𝐹 =𝑛𝑖=1 ∑ 𝑥𝑅 = ∑ 𝑦𝑙𝑜𝑐𝑎𝑙 =𝑛𝑖=1𝑛𝑖=1 ∑ 𝑦𝑃 = 1𝑛𝑖=1 [4-16] Where the subscriptions 𝐹, 𝑅, 𝑃 denote feed, residual and permeate gas, respectively. The symbol 𝑛 denote the total number of gases in the stream. The logarithmic method suffers from significant solution errors, particularly at high separation-cuts (permeate to feed ratio). To overcome this concern, the total membrane area is often divided into smaller yet equal segments, to which the logarithmic approach can be applied successively. However, this solution can be somewhat complex, causing users to select other modelling techniques. 4.2.2 Series Solution Method The deficiency of the logarithmic approach, namely the inability to model multi-component membrane-based gas separation systems, has called for the development of an alternative modelling approaches. With the series solution method, Coker D. [102] was able to model endless number of gases under co-current, countercurrent and cross flow modes. Further, this method is distinguished 77 with unique capabilities such as modeling permeate purges, and tabulating gases’ permeance values as functions of pressure and temperature; both features result in more realistic modelling. These features reflect positively on the assurance and the speed of the convergence, which are crucial for simulators. This method is based on the principle of dividing the length of a membrane fiber (axial division) into 𝑁 segments installed in series, and conducting the material balance on each individually yet consecutively [102]. The mass balance equations must be solved simultaneously, since the input to one segment is the output from another (except for the boundaries). The equations are solved efficiently using the Thomas algorithm for solving tridiagonal matrices [103]. Below lists the assumption adopted in the series solution method [102]: • Membranes are operated in steady state. • Shell-side pressure loss is insignificant to be calculated. • Bore side pressure loss estimated using Hagen-Poiseuille equation [104]. • Gas composition varies across fiber’s length, but no axial mixing occurs. • Fibers are identical in material, morphology, diameter and thickness. • Fiber’s performance is scaled-up to compute the overall unit performance. Figure 4-3 summarizes the principle of this method when applied to a hollow fiber operating in "counter-current" flow mode [102]. For propylene-propane separation application, the parameters marked in red squares, namely feed gas 𝐿N+1 rate, feed gas composition 𝑋j,N+1, and the targeted product (propylene) composition 𝑦j,N are often set by the process requirements. On the other hand, the parameters marked in green squares, namely the membrane unit area (𝑁 ∗ ∆𝐴) and the permeate purge rate 𝑉o / composition 𝑦j,o are design variables that are to be adjusted to achieve the targeted product (propylene) quality. 78 Figure 4-3: Series solution method used for solving segmental mass balances in counter-current flow modes Equation [4-17] expresses the differential area around which the method is applied: ∆𝐴 =𝜋 𝐷𝑜𝐻 𝑇𝑓𝑖𝑏𝑒𝑟𝑁 [4-17] Where, ∆𝐴 Differential hollow-fiber area 𝐷o Fiber outer diameter 𝐻 Fiber effective separation length 𝑇fiber Fibers count 𝑁 Assumed number of segments In Figure 4-3, the subscript “k, k-1, k+1” refers to the segment from which a flow is discharged, whereas “j” refers to a gas component. The individual component rate (ℓj,k or ʋj,k) is obtained from multiplying the total stream rate (𝐿k or 𝑉k) by the individual component fraction (𝑥j,k or 𝑦j,k) at a consistent location. Hence, ℓj,k = 𝑥j,k * 𝐿k [4-18] ʋj,k = 𝑦j,k * 𝑉k [4-19] 𝐿k = ∑ ℓ𝑗,𝑘𝑅𝑗=1 : R is the total number of handled gases [4-20] 𝑉k = ∑ ʋ𝑗,𝑘𝑅𝑗=1 : R is the total number of handled gases [4-21] A mass balance for component "𝑗" around stage 𝑘 yields the following: (ℓj,k+1 - ℓj,k) + ʋj,k-1 - ʋj,k = 0 [4-22] Component 𝑗 permeation rate: 𝑚j,k = ℓj,k+1 - ℓj,k [4-23] 79 Component 𝑗 permeation rate: 𝑚j,k = 𝐽j (𝑃 𝑥j,k – 𝑝 𝑦j,k) ∆𝐴 [4-24] Equations [4-18] and [4-19] are used to eliminate the mole fractions from [4-24]. Furthermore, equation [4-23] is used to eliminate 𝑚j,k from [4-24] to yield: ʋj,k =−𝑉k 𝑝 ∆𝐴 𝐽j[ℓj,k+1 − (1 +𝑃 ∆𝐴 𝐽j𝐿k ) ℓj,k ] [4-25] Equation [4-25] is used to solve the consecutive mass balances. However, the model fails to converge when the membranes permeate is operated at full vacuum (p= 0 bara). To overcome this limitation, equation [4-25] is plugged into equation [4-22] to eliminate ʋj,k and ʋj,k-1 : 𝐵j,k ℓj,k-1 + 𝐶j,k ℓj,k + 𝐷j,k ℓj,k+1 = 0 [4-26] Where, Bj,k =−𝑉k-1 𝑝 ∆𝐴 𝐽j(1 +𝑃 ∆𝐴 𝐽j𝐿k-1 ), Cj,k = 1 +𝑉k-1 𝑝 ∆𝐴 𝐽j+𝑉k 𝑝 ∆𝐴 𝐽j(1 +𝑃 ∆𝐴 𝐽j𝐿k ) , Dj,k =−𝑉k𝑝 ∆𝐴 𝐽j− 1 The implementation of equation [4-26] for component 𝑗 on 𝑁 segments results in a set of equations. These equations are rearranged to a square matrix (𝑁 × 𝑁) for solving them simultaneously. [ Cj,1 Dj,1 0 0 0Bj,2 Cj,2 Dj,2 0 00 Bj,k Cj,k Dj,k 00 0 Bj,N-1 Cj,N-1 Dj,N-10 0 0 Bj,N Cj,N ] X [ ℓj,1ℓj,2ℓj,kℓj,N-1ℓj,N ] = [ -Bj,1000-Dj,N] [4-27] Once the residual-side flowrates ℓj,1 to ℓj,N are calculated, a simple component mass balance and a total mass balance around a segment can reveal the unknown compositions. In fact, the complex calculations are impractical unless coded into a software. The series solution method was implemented into a HYSYS extension called “MemCal” [105]. This extension was used for producing most of the simulation results in this research. 80 4.3 Coding and Integration of "MemCal" None of the thermodynamic simulators offer "membrane unit" among the standard default unit operations. The inability to solve the mass balance equations prevents the determination of membrane units’ performance. Further, the inability to solve the mass balance equations analytically implies that these equations are to be solved numerically [105]. Pan [106] proposed a differential algorithm to solve the associated equations using the ‘shooting method’. However, this method fails to converge at high stage-cuts, i.e. at high permeate to feed ratios. The described limitation prevents the adoption of Pan’s method in process simulators. The series solution method by Coker et al. [102] is efficient in solving Boundary Value Problems (BVPs), and in overcoming the convergence concerns experienced in Pan’s model. In the series solution model, the individual components’ differential equations are first discretized through the upwind finite difference method (FDM). Then, the Gaussian elimination algorithm [103] is used to solve the equations, and thus map the flowrates across the membrane. The adopted algorithm is robust, efficient, and converges at stage cuts as high as 99.9%. Bo Chen implemented this algorithm and integrated it with Aspen HYSYS to result in a power simulation tool denoted as "MemCal". The algorithm was first coded in Visual Basic (VB.NET) and then compiled into a Dynamic Link Library (DLL). The DLL file communicates with HYSYS through a Component Object Model (COM), which allows the exchange of data from and to HYSYS. Further, an External Definition File (EDF) was established to define the user-interface. The interface (see Figures 4-4 and 4-5) is user-friendly and enables easy and quick modeling of membrane processes [105]. 81 Figure 4-4: MemCal representation of a membrane stage in Aspen HYSYS Figure 4-5: MemCal user interface in Aspen HYSYS with multiple selections subject to user’s preferences The interface allows the manipulation of flow mode (co-current / counter-current), module’s configuration (hollow-fiber / spiral-wound), membrane area, permeance (and hence selectivity), and permeate pressure. The algorithms running in the background vary according to the user’s selections. In terms of model’s validation, MemCal’s output was compared with the results presented by Coker et al. [102] for the "ternary separation" case. Figure 4-6 demonstrates the results being 82 reproduced successfully [105]. The minor offset in the results is regarded to the calculations’ tolerances, which can be tuned at the cost of increased convergence time [105]. In addition to the described ternary separation verification, tool’s output was compared to the results of the binary systems short-cut methods. Since there are no analytical solutions to the multi-component (more than two gases) systems, MemCal results were cross-checked against the numerical solutions of previously solved systems; the results were found consistent. Figure 4-6: Reproduction of Coker et.al results for a ternary separation case using MemCal An input / output interface dashboard (spreadsheet) can be linked to the model to retrieve the critical inputs / outputs from the simulation. This interface is crucial as it allows users to compare and analyze the results, particularly when performing sensitivity / optimization studies. A costing model can also be linked to the model. Figure 4-7 demonstrates a two-stage membrane process simulated in HYSYS. 83 Figure 4-7: MemCal used in the simulation of a two-stage membrane unit plus recycle in Aspen HYSYS 4.4 Summary A novel, powerful, and user-friendly tool supporting the simulation of membranes within the thermodynamic simulators’ environment was demonstrated. Compared to the application-specific tools reported earlier in the literature, “MemCal” is more generic, and supports the simulation of membrane-based gas separation involving multiple gases (> 2) in multi-stage designs. Furthermore, “MemCal” is featured with its ability to run within the simulator environment, allowing the use of all simulator standard functions including (recycle, adjust, optimize, costing, etc.); such features were exclusive to the commercial membrane suppliers. Using “MemCal”, simple and complex process designs will be evaluated (Chapter 5) for assessing the ability of the various membrane types (polymeric, mixed-matrix, and ZIF-8) to upgrade the refinery grade of propylene to the commercial grades. It is planned to add further complexity to the “MemCal” coding to accommodate additional user-preferences, as detailed later in Section 6.3 (Future Work). 84 Chapter 5: Membrane Units Performance Assessment This chapter presents the analyses performed to assess membranes’ capability in producing the main commercial grades of propylene under various combination of design variables. The presented work examines the performance of the polymeric and the mixed-matrix membranes when installed in a process, and aims at illustrating the added-value of MMMs. Lastly, the work underlines the importance of process design manipulation in overcoming materials’ performance offsets. The single- and the two-stage processes are elaborated in Section 5.1 and 5.2, respectively. Each of the processes is simulated for the production of the chemical and the polymer grade considering pure-polymeric, mixed-matrix, and pure ZIF-8 membranes. 5.1 Single-Stage Process Assessment The fundamentals of the membrane unit process design are demonstrated best in the simplest design form, i.e. the single stage (see Figure 5-1). Figure 5-1: Single-stage process for propylene-propane separation: design scheme and simulation 85 Generally, the inability to achieve a targeted separation is regarded to limitations in membrane’s performance or operating conditions. In fact, both factors constrain the single-stage membrane system from producing the purified forms of propylene (chemical: 93.0 wt%, polymer: 99.5 wt%) at competent recovery values. A positive partial pressure difference must be maintained across the membrane to permeate the fast gas (e.g., propylene). Huang et al. [107] demonstrated that even if a perfect membrane (α = ∞) was utilized, the concentration of the fast gas at the permeate side cannot be boosted from the feed side concentration by a factor exceeding the pressure ratio (PR), see equation [5-1] and [5-2]. 𝐸𝑛𝑟𝑖𝑐ℎ𝑚𝑒𝑛𝑡 𝑙𝑖𝑚𝑖𝑡 = 𝑝𝑟𝑜𝑝𝑦𝑙𝑒𝑛𝑒 𝑖𝑛 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 (𝑣𝑜𝑙% )𝑝𝑟𝑜𝑝𝑦𝑙𝑒𝑛𝑒 𝑖𝑛 𝑓𝑒𝑒𝑑 (𝑣𝑜𝑙% )<𝑓𝑒𝑒𝑑 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 [5-1] 𝑀𝑎𝑥 𝑝𝑟𝑜𝑝𝑦𝑙𝑒𝑛𝑒 𝑖𝑛 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 (𝑣𝑜𝑙%) = 𝑝𝑟𝑜𝑝𝑦𝑙𝑒𝑛𝑒 𝑖𝑛 𝑓𝑒𝑒𝑑 (𝑣𝑜𝑙%) 𝑥 𝑃𝑅 [5-2] Assuming the use of a perfect membrane (α = ∞), the production of the chemical (93.0 wt%) and the polymer (99.5 wt%) grades from 50-70 wt% propylene feed is not PR limited, as the PR imposed across the membrane stage is expected to be between 5 and 15. In fact, the PR effect becomes more influential when using poor membranes (low selectivity). Figure 1-3 illustrates that the membrane unit, that is to replace the C3-splitter, will be fed from the depropanizer overhead at a pressure as high as 15 atm. Similar to previous relevant studies [61], [108], and based on the fact that the depropanizer downstream of the catalytic cracking produces this mixture, the membrane units are assumed to be fed with 70 wt% propylene (balance propane) gas mixtures. Membrane units are not expected to receive the feed gas above 15 atm, since the 70 wt% propylene feed that is typically sourced at 40oC would then liquefy, refer the condensation pressure in Figure 5-2. Expectedly, operating the permeate at a reduced pressure (e.g., vacuum) is advantageous in maximizing the permeation 86 potential. However, to avoid its downstream recompression, the permeate must be left with some positive pressure (2-3 atm) to support the downstream operations (e.g., propylene’s transfer and storage). Based on the elaboration made on the feed and the permeate sides’ operating pressures, a realistic trans-membrane PR would be less than 15. Figure 5-2: Dew-point pressure of 70 wt% propylene C3-mixture as a function of temperature Apart from feasibility of producing the targeted propylene purity, the technical competency of a process is also defined by its propylene product recovery: 𝑃𝑟𝑜𝑝𝑦𝑙𝑒𝑛𝑒 𝑅𝑒𝑐𝑜𝑣𝑒𝑟𝑦 (%) = 𝑝𝑟𝑜𝑝𝑦𝑙𝑒𝑛𝑒 𝑓𝑙𝑜𝑤 𝑖𝑛 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑝𝑟𝑜𝑝𝑦𝑙𝑒𝑛𝑒 𝑓𝑙𝑜𝑤 𝑖𝑛 𝑓𝑒𝑒𝑑 𝑥 100 % [5-3] Unluckily, the single-stage units suffer from an unavoidable tradeoff between the purity and the recovery of produced propylene (demonstrated later in Figure 5-5). The single-stage unit was simulated at all the possible combinations of variables in Table 5-1. The simulations in Section 5.1 aims at assessing the single-stage unit in terms of i) its ability to produce the desired propylene purity, and ii) its resultant 87 propylene recovery. The results are summarized in Tables 5-2 and 5-3 depending on the product grade (purity). Table 5-1: Basis for single-stage unit simulation envelope Parameter Min Value Max Value Basis Feed composition 50 wt% propylene 70 wt% propylene Min: Worst possible feedstock quality Max: C3 splitter column typical feed quality Membrane selectivity (𝛼) 10 35 100 Min: Least polyimide selectivity in Table 2-2 Mid: Mixed Matrix (polymer + ZIF-8) selectivity Max: Pure filler (ZIF-8) selectivity Pressure ratio (PR) 5 15 Min: Feed / permeate pressure: 15 atm / 3 atm Max: Feed / permeate pressure: 15 atm / 1 atm Product purity 93.0 wt% propylene 99.5 wt% propylene Min: Chemical grade composition Max: Polymer grade composition Flow mode Counter-current Most efficient flow mode 5.1.1 Single-Stage: Chemical Grade Production The simulation results suggest that polymeric membranes (α=10) are not qualified for producing the chemical grade (93.0 wt%) of propylene at an acceptable recovery value, even at the best combination of Table 5-1 design variables. As for MMMs (α=35), their success in producing the chemical grade is subject to the feed quality, most successful when upgrading the 70 wt% feed. Table 5-2: Best possible propylene recovery in a single-stage unit producing the “chemical grade” Chemical Grade Product Pressure Ratio Polymeric Membrane Selectivity (𝜶=10) Mixed-Matrix Membrane Selectivity (𝜶=35) Pure ZIF-8 Films Selectivity (𝜶=100) Feedstock: 50 wt% propylene PR = 5 Not viable Recovery = 55% Recovery = 84% PR = 15 Not viable Recovery = 80% Recovery = 96% Feedstock: 70 wt% propylene PR = 5 Recovery = 45% Recovery = 93% Recovery = 97% PR = 15 Recovery = 65% Recovery = 98% Recovery = 99% Lastly, the pure ZIF-8 films (α=100) were found capable of producing the chemical grade at an exceptional recovery. However, the manufacture of these membranes on an industrial scale is unlikely with the available technologies, and thus will not 88 be elaborated further. In view of the above results, and in order to assist material scientists in setting “selectivity” targets for the future developed membranes, the single-stage process was simulated considering various combinations of feedstock composition and pressure ratio; the simulations aim at computing the minimum membrane selectivity that would allow the production of the chemical grade at 90% propylene recovery. Since the recovery in conventional C3s distillation towers is ~99%, it is anticipated that 90% recovery would be the least recovery value acceptable to users in order to start considering the membrane technology as a legitimate replacement to distillation. When handling a feed equivalent to that of the C3s distillation (70 wt% propylene), a single-stage unit at the specified pressure ratio range would require a membrane selectivity of 17 to 27 (Figure 5-3B) for upgrading the feed to the chemical grade while achieving the 90% recovery; the selectivity range is feasible using ZIF-8 based MMMs. When handling a poor feedstock (50 wt% propylene), the minimum selectivity that would be required (α=50-320, Figure 5-3A) exceeds that feasible using ZIF-8 based MMMs. At low PR, the required selectivity even exceeds that of pure ZIF-8 (α=100-130). In fact, boosting the PR is as important as boosting the selectivity, if not more important (see the curve trends in Figure 5-3). Figure 5-3: Single-stage: minimum selectivity required for producing the “chemical grade” at 90% recovery 89 Based on the above results, and in view of the limited selectivity, it can be concluded that the single-stage units’ application window is very limited, and the use of multi-stage designs is a must for achieving high purity and recovery values. 5.1.2 Single-Stage: Polymer Grade Production None of the simulated membranes, including pure ZIF-8, was capable of producing the polymer grade (99.5 wt%) at competent recovery. The recovery values were poor even at the best possible combination of design variables, see Table 5-3. Table 5-3: Best possible propylene recovery in a single-stage unit producing the “polymer grade” Polymer Grade Product Pressure Ratio Polymeric Membrane Selectivity (𝜶=10) Mixed-Matrix Membrane Selectivity (𝜶=35) Pure ZIF-8 Films Selectivity (𝜶=100)) Feedstock: 50 wt% propylene PR = 5 Not viable Not viable Not viable PR = 15 Not viable Not viable Not viable Feedstock: 70 wt% propylene PR = 5 Not viable Not viable Recovery = ~1% PR = 15 Not viable Not viable Recovery = 12% Similar to the analyses conducted in the previous section, the minimum selectivity that is necessary for producing the polymer grade at 90% recovery was calculated. Considering a 70 wt% feed, the required selectivity (α=300-850) was found well above that achievable using MMMs, see Figure 5-4. The selectivity range required for upgrading a 50 wt% feed is even higher than the 70 wt% feed range, and thus was not simulated. 90 Figure 5-4: Single-stage: minimum selectivity required for producing the “polymer grade” at 90% recovery 5.1.3 Single-Stage: From the Chemical to the Polymer Grade Sections 5.1.1 and 5.1.2 demonstrated that due to membrane selectivity or process PR limitations, the single-stage processes are generally incapable of producing the chemical and the polymer grades when fed with 50-70 wt% propylene. To complete the investigation of single-stage units’ application window, the analyses presented in this section aim at assessing the possibility of using the single-stage process for a less intensive separation, namely upgrading the chemical grade quality (93.0 wt%) to the polymer grade (99.5 wt%). As illustrated in Table 5-4, both the polymeric and the MMMs failed in upgrading the chemical grade to the polymer grade quality at the desired 90% recovery. Despite the success of pure ZIF-8 membranes in this application, as indicated in Section 5.1.1, the manufacture of these membranes is unlikely with the available technologies; their corresponding results are reported for reference only. 91 Table 5-4: Propylene recovery in a single-stage unit producing the “polymer grade” from the chemical grade Polymer Grade Product Pressure Ratio Polymeric Membrane Selectivity (𝜶=10) Mixed-Matrix Membrane Selectivity (𝜶=35) Pure ZIF-8 Films Selectivity (𝜶=100)) Feedstock: Chemical Grade (93.0 wt% propylene) PR = 5 Not viable Recovery = 71% Recovery = 96% PR = 15 Not viable Recovery = 80% Recovery = 98% At the analyzed PR range (5 to 15), the selectivity that is necessary for producing the polymer grade at minimum recovery of 90% ranges from 47 to 58. These values are not achievable using polymeric membranes, and have not been achieved so far in any of the synthesized ZIF-8 based MMMs (although theoretically possible). The failure of the single-stage unit for this service adds up to the previous failures reported in Section 5.1.2, and the partial failures reported in Section 5.1.1. 5.1.4 Single-Stage: Further Analyses Material scientists are persistently enhancing the selectivity of ZIF-8 based MMMs. However, they often fail in assessing the advantage of selectivity enhancement on unit’s performance “quantitively”. For qualitative impact assessment, the trade-off between propylene’s purity and recovery was plotted at various PR and selectivity combinations (considering Table 5-1 basis), see Figure 5-5. 92 Figure 5-5: Trade-off between propylene’s purity and recovery in single-stage designs When aiming to produce the chemical grade (93.0 wt%) from a typical C3-splitter feed (70 wt% C3H6), boosting the selectivity from 35 to 100 (i.e. to that of pure ZIF-8), while PR is fixed, results in a negligible recovery gain. It should be noted that a membrane with selectivity of 35 already results in a recovery close to 100%. The same boost in selectivity, i.e., from 35 to 100, is still inadequate for producing the polymer grade at competent recovery. In view of these observation, the benefit of boosting the selectivity further (i.e., above 35) is questionable. In fact, increasing the pressure ratio across the membrane has been proven to be more effective than increasing the selectivity above 35. 5.2 Two-Stage: Process Capability Assessment A multi-stage process is capable of overcoming the single-stage process shortfalls, mainly the tradeoff between produced propylene’s purity and recovery. Unluckily, the recovery enhancement occurs at the cost of a complex process scheme, larger 93 footprint, and additional capital investment; all caused by the additional membrane stage and the accompanying inter-stage compressor. Nevertheless, the boost experienced in propylene’s purity and recovery is worth it, especially when used for the polymer grade production. Most of the literature related to multi-stage designs pertain to the membrane-based CO2 / CH4 separation process (natural gas industry), wherein process designers attempt to suppress the slip of CH4 along with the permeating CO2 [99], thereby minimizing product losses [99]. Process design customization is dependent on various factors including, but not limited to, separation objective, process conditions, membrane’s selectivity, and most importantly, product’s collection side (residual versus permeate). Due to their complex design, difficult operation, large footprint, and high cost, the three-stage (and more) units elaborated in literature are rarely encountered in industry [109], [110]. In fact, the two-stage units form the most practical and encountered form of multi-stage processes. Out of the several two-stage designs listed in relevant literature [3], [99], the one based on “permeate concentration” (Figure 5-6) is the most suitable for C3s separation. In this design, the semi-purified propylene produced from the first stage permeate is compressed and subsequently fed to the second membrane stage for further concentration. The propylene gas, at the desired grade, is collected from the second stage permeate at a low pressure. The concentrated propane is retrieved from the first-stage residue side at the high-pressure. The high-pressure residual gas from the second stage is recycled back to the feed for reprocessing, thereby minimizing propylene losses. The recycle streams in two-stage designs complicate the mass balance solution, and thus demands the use of a simulator. All the items in the two-stage design were simulated using HYSYS default functions, except for the membrane 94 which its simulation was supported by the extension “MemCal (detailed in Section 4.3). Figure 5-6: Two-stage unit (permeate gas concentration based) modeled in HYSYS for C3s’ separation The role of the second stage, from where the purified propylene is collected, is to fine-tune the purity. Thus, an automatic adjust block “ADJ-2” was imposed on the model to manipulate the area of the second stage such that the desired propylene grade purity is always met. Another block “ADJ-1” manipulates the area of the first stage such that the targeted recovery is met; the two adjust blocks work simultaneously. The two-stage design in Figure 5-6 was simulated at all the possible combinations of Table 5-5 variables. 95 Unlike single-stage units where the recovery is uncontrollable (purity-dependent), the two-stage units allows such control via recycle rate adjustment; the latter is doable via adjusting the first stage membrane area. The two-stage units were assessed for their ability to meet propylene recovery of 90% (minimum) and 99% (same as the C3 distillation tower) at the desired grade purity. Table 5-5: Basis for two-stage units’ simulation envelope Parameter Min Value Max Value Basis Feed composition 50 wt% propylene 70 wt% propylene Min: Worst possible feedstock quality Max: C3 splitter column typical feed quality Membrane selectivity (𝛼) 10 35 100 Min: Least polyimide selectivity in Table 2-2 Mid: Mixed Matrix (polymer + ZIF-8) selectivity Max: Pure filler (ZIF-8) selectivity Pressure ratio (PR) 5 15 Min: Feed / permeate pressure: 15 atm / 3 atm Max: Feed / permeate pressure: 15 atm / 1 atm Product purity 93.0 wt% propylene 99.5 wt% propylene Min: Chemical grade Max: Polymer grade Propylene Recovery 90% 99% Min: Least acceptable economically Max: Equivalent to C3s distillation tower Flow mode Counter-current Most efficient flow mode The use of a two-stage design does not guarantee the production of the desired purity at the targeted recovery. Thus, the two-stage units must be simulated at the various combinations of design variables for i) assessing the separation feasibility, and ii) estimating the required recycle ratio. In fact, the size, and thus the cost, of a two-stage unit are correlated with the recycle rate. Though increasing the recycle reflects positively on recovery, the latter occurs at the cost of an additional membrane area, a larger inter-stage compressor, and a higher power demand. The feasibility of producing the chemical and the polymer grades at the desired recovery values are assessed in Sections 5.2.1 and 5.2.2, respectively. 96 5.2.1 Two-Stage: Chemical Grade Production In this section, the two-stage unit is assessed for its ability to produce the chemical grade (93.0 wt%) of propylene at 90 and 99% recovery, and if found possible, the recycle rate (RR) is computed by the simulator, see Tables 5-6 and 5-7. Table 5-6: Two-stage: recycle ratio required for producing the chemical grade at 90% recovery Chemical Grade at 90% Recovery Pressure Ratio Polymeric Membrane Selectivity (𝜶=10) Mixed-Matrix Membrane Selectivity (𝜶=35) Pure ZIF-8 Films Selectivity (𝜶=100) Feedstock: 50 wt% propylene PR = 5 RR = 71% RR = 8% RR = 2% PR = 15 RR = 26% RR = 2% RR = 0%a Feedstock: 70 wt% propylene PR = 5 RR = 11% RR = 0%a RR = 0%a PR = 15 RR = 4% RR = 0% a RR = 0% a a. 0% recycle implies that process is achievable with a single-stage, and the second stage is not required. b. Excessive implies that the recycle ratio is > 100% (multiple of the feed rate), which is uneconomical due to a large membrane area, high compression capacity, and large unit footprint. c. Recycle rates are calculated as following: total recycle mass flow / total fresh feed mass flow x 100% Table 5-7: Two-stage: recycle ratio required for producing the chemical grade at 99% recovery Chemical Grade at 99% Recovery Pressure Ratio Polymeric Membrane Selectivity (𝜶=10) Mixed-Matrix Membrane Selectivity (𝜶=35) Pure ZIF-8 Films Selectivity (𝜶=100) Feedstock: 50 wt% propylene PR = 5 RR = Excessiveb RR = 42% RR = 23% PR = 15 RR = Excessiveb RR = 12% RR = 4% Feedstock: 70 wt% propylene PR = 5 RR = Excessiveb RR = 11% RR = 5% PR = 15 RR = 37% RR = 2% RR = 0% a a. 0% recycle implies that process is achievable with a single-stage, and the second stage is not required. b. Excessive implies that the recycle ratio is > 100% (multiple of feed rate), which is uneconomical due to a large membrane area, high compression capacity, and large unit footprint. c. Recycle rates are calculated as following: total recycle mass flow / total fresh feed mass flow x 100% The results in Table 5-6 demonstrate that achieving 90% recovery is feasible, even at the worst design variables, including the use of polymeric membranes (𝛼=10). 97 When ZIF-8 based MMMs (𝛼 ≥ 35) are utilized, the units recorded insignificant, and in some cases zero recycle demand; the latter implies single-stage sufficiency. Compared to the 90%, the 99% recovery cases are associated with higher recycle demand. However, the recycle demand could be excessive (multiples of the feed) to the extent that the process is regarded impractical and uneconomical, see Table 5-7. In most of the simulated cases, the polymeric membranes failed in achieving the 99% recovery. Oppositely, the MMMs recorded an impressive performance. For a cost-effective design, the dependency of the propylene recovery on unit’s recycle must be carefully analyzed. Figures 5-7 and 5-8 demonstrate that the rate of recovery improvement encounters a slowdown at high recycle ratios. Thus, the recycle rate must be optimized such that the revenues gained from the extra recovery overweigh the cost of the additional recycle. Figure 5-7: Propylene recovery versus recycle ratio in polymeric membrane based two-stage unit operated at various PRs (A vs. B) for chemical grade production 98 Figure 5-8: Propylene recovery versus recycle ratio in MMM-based two-stage unit operated at various PRs (A vs. B) for chemical grade production 5.2.2 Two-Stage: Polymer Grade Production The analyses in the previous section were repeated yet for the polymer grade. The production of the ultra-pure form (99.5 wt%) is more challenging than the chemical grade, and if feasible, it would be associated with higher recycle rates to achieve the same desired propylene recoveries (90% and 99%), see Tables 5-8 and 5-9. Table 5-8: Two-stage: recycle ratio required for producing the polymer grade at 90% recovery Polymer Grade at 90% Recovery Pressure Ratio Polymeric Membrane Selectivity (𝜶=10) Mixed-Matrix Membrane Selectivity (𝜶=35) Pure ZIF-8 Films Selectivity (𝜶=100) Feedstock: 50 wt% propylene PR = 5 RR = Excessiveb RR = 90% RR = 12% PR = 15 RR = Excessiveb RR = 28% RR = 6% Feedstock: 70 wt% propylene PR = 5 RR = Excessiveb RR = 21% RR = 8% PR = 15 RR = Excessiveb RR = 6% RR = 2%a a. 0% recycle implies that process is achievable with a single-stage, and the second stage is not required. b. Excessive implies that the recycle ratio is > 100% (multiple of feed rate), which is uneconomical due to a large membrane area, high compression capacity, and large unit footprint. c. Recycle rates are calculated as following: total recycle mass flow / total fresh feed mass flow x 100% 99 Table 5-9: Two-stage: recycle ratio required for producing the polymer grade at 99% recovery Polymer Grade at 99% Recovery Pressure Ratio Polymeric Membrane Selectivity (𝜶=10) Mixed-Matrix Membrane Selectivity (𝜶=35) Pure ZIF-8 Films Selectivity (𝜶=100) Feedstock: 50 wt% propylene PR = 5 RR = Excessiveb RR = Excessiveb RR = 81% PR = 15 RR = Excessiveb RR = 98% RR = 14% Feedstock: 70 wt% propylene PR = 5 RR = Excessiveb RR = Excessiveb RR = 27% PR = 15 RR = Excessiveb RR = 39% RR = 4% a. 0% recycle implies that process is achievable with a single-stage, and the second stage is not required. b. Excessive implies that the recycle ratio is > 100% (multiple of feed rate), which is uneconomical due to a large membrane area, high compression capacity, and large unit footprint. c. Recycle rates are calculated as following: total recycle mass flow / total fresh feed mass flow x 100% Tables 5-8 and 5-9 illustrate the failure of polymeric membranes in producing the polymer grade, even in a two-stage process. On the other hand, despite their high recycle ratios, the MMMs were found capable of producing the polymer grade at the desired recoveries (90% recovery: at all PRs, 99% recovery: at high PRs only). The results of the pure ZIF-8 films, which their industrial manufacture is unlikely, are provided for future studies’ reference only, and thus were not be elaborated. In Figure 5-9, the relation between the recovery and the recycle ratio is illustrated for the MMMs only, noting the failure of polymeric membranes in producing the polymer grade. The analyses are demonstrated in Figures 5-9A/B for the low- and the high-PRs, respectively. 100 Figure 5-9: Propylene recovery versus recycle ratio in MMM-based two-stage unit operated at various PRs (A vs. B) for polymer grade production Figure 5-9 suggests that to be as competent as the thermal distillation in terms of producing the polymer grade at 99% recovery, the ZIF-8 based MMMs must be able to stand a PR of ~15, without sacrificing its selectivity or mechanical integrity. It should be noted that the hydroxyl-functionalized ZIF-8 based mixed-matrix films prepared by Ma et al. [53] were tested at maximum PR of 7, i.e., less than 15. Films’ performance at PR of 15 is to be investigated. As for the lower recovery (90%), the production of the polymer grade using MMMs was found feasible, even at the least PR (=5) and with the worst feedstock quality (50 wt% propylene). 5.3 Summary The chapter assessed the capability of the polymeric, the mixed-matrix, and the pure ZIF-8 membranes in producing the two-main commercial grades of propylene, under various process design configurations. The results confirmed the finding of previous studies, namely the need for boosting the selectivity above that offered by the conventional polymeric membranes, noting their limited application window. 101 The mixed-matrix films, at the literature suggested selectivity of 35, were found capable of producing the chemical grade when utilized in a single-stage process, and both the grades (chemical and polymer) when utilized in a two-stage process. The simulations suggest that increasing the selectivity up to that of the pure ZIF-8 film (unlikely) is still not enough for producing the polymer grade in a single-stage process. In a two-stage process, and compared to the MMMs’ performance, the advantage of increasing the selectivity was found limited to a minor reduction in recycle. Although the latter results in unit cost cut, boosting MMMs’ selectivity above 35 would be challenging due to the defects concern detailed in Chapters 2 and 3. In view of the limited added-value of enhancing the selectivity above 35, and since the recovery and the purity shortages can be mitigated by process design, material chemists are advised to focus on the aspects other than selectivity enhancement, including but not limited to, MMMs’ manufacturability, mixed-gas performance, and high pressure-ratio tolerance. From the techno-economic viewpoint, it is important to acknowledge the product state of the conventional distillation and membrane technology. This subject is not addressed in the previous relevant work. As a byproduct to the distillation column reflux operation, the conventional distillation process yields the propylene in the liquefied form. In contrary, the membrane process produces it in the gaseous state, and therefore might demand liquefaction step, based on the following operations. Apart from the product phase, it is worth highlighting that the distillation units are only justified at high-capacities. Alternatively, the membranes can be utilized even for low-capacities, noting their somewhat linear capacity-cost relation. 102 Chapter 6: Conclusion, Contribution, and Future Work 6.1 Conclusion The separation of propylene from its associated propane downstream the cracking process is one of the most important yet challenging industrial processes. In this work, the use of membrane technology for C3s’ separation in lieu of the energy-intensive thermal-based distillation process was investigated. The work combined the aspects of membrane material development together with membrane unit process design, while aiming at reducing the separation process energy requirement, and thus cutting-down the associated carbon emissions. The performance of PIMs is still insufficient for qualifying membranes to complete with the robust distillation process. Blending MOF with PIM for developing a propylene selective mixed-matrix material was a novel development. However, the defects formed between the semi-organic MOFs and the organic PIMs caused a slowdown in the development of mixed-matrix materials. This work proposed the use of the polar group “carboxyl” for PIMs functionalization, in order to improve the adhesion between the functionalized PIM and the propylene selective MOF “ZIF-8”. To bridge the gap the between the “material development” and the “separation process” aspects, the novel tool “MemCal” was adopted to assess the performance of the previous and the newly developed materials when used in a process. Beside the propylene product purity aspect, the work granted equal importance to the propylene recovery aspect, as the latter influences the economic viability of the technology for this application. As for the factors influencing the performance, the work demonstrated that the pressure ratio effect is as important as membrane’s selectivity effect in defining technology’s performance score. 103 Finally, using simulation, the work demonstrated that the selectivity shortages can be somewhat moderated by adopting more complex process designs. Accordingly, the work defined the application window of the polymeric and the mixed-matrix membranes in the C3s’ separation field. 6.2 Contributions The following sections summarize the contributions of this work by the sub-subject. 6.2.1 Membrane Materials The use of “carboxyl” functionalized polymers was addressed in preceding studies for various objectives. However, this work was the first to propose their use along with ZIF-8 for synthesizing mixed-matrix membranes for C3s’ separation, making use of the H-bonding induced between the Carboxyl-functionalized PIM and ZIF-8. The positive effect of “carboxyl” functionalization is expected to exceed that of the “hydroxyl” proposed in earlier studies. Although the synthesis of the mixed-matrix film made of 6FDA-TrMCA and ZIF-8 was unfinished due to labs closure during the pandemic, the performance of the films made of the carboxyl supported PIM 6FDA-TrMCA and ZIF-8 is anticipated to be promising, particularly from the constituents’ compatibility perspective. Once functionalized, the PIMs can form the continuous phase of the mixed-matrix, and propylene selectivity between 25 to 35 can be accomplished with moderate to high ZIF-8 loadings. This feasible selectivity range is adequate for upgrading the refinery grade of propylene to the commercial grades, particularly in a multi-stage process. 6.2.2 Membrane Unit Modeling and Simulation A novel modeling tool “MemCal” that supports the modeling of membranes in common thermodynamic simulators was revealed in this work. Using this powerful tool, the polymeric / mixed- matrix membranes were simulated for analyzing their 104 capability in separating C3s’ when in single- and in two-stage designs; membranes’ performance was evaluated based on their ability to meet the targeted propylene product purity and recovery. The performance offsets, caused by insufficient membrane selectivity, were found somewhat manageable with process design, i.e., by adopting the two-stage design instead of the single-stage; the latter is a key finding as it allows membrane scientists to divert their focus to critical aspects other than the selectivity enhancement, including but not limited to MMMs’ performance sustainability, modularization, manufacturability, and other topics as detailed in the Future Work section. 6.2.3 Membrane Unit Process Design The work defined the application window for the polymeric (α~10) and the ZIF-8 based mixed-matrix membrane (α~35), and by the mean of simulation confirmed the conclusion of the preceding studies that the polymeric membranes are short of the selectivity required for C3s’ purification, particularly when targeting to produce the ultra-pure form of propylene. Using the two-stage design proposed in this work, the production of the chemical (using polymeric membrane) and the polymer (using mixed-matrix membrane) grades was found feasible at a recovery comparable to that of distillation, which in-turn demonstrates the ability to moderate the effect of poor membrane performance via process design manipulation. Also, the work demonstrated the added-value of operating the membrane with high PR, thereby providing a guidance for the future work. 6.3 Future / Planned Work – Recommendations Based on simulations’ outcomes, and upon reviewing the relevant literature, the following knowledge gaps still exist; these areas must be investigated and resolved before the MMM-based process can be regarded as a distillation alternative. 105 • Performance of ZIF-8 based MMMs at high PR The simulation results suggest that operating the MMMs at high PRs is a must for achieving a propylene recovery comparable to that of the thermal distillation (99%), particularly when targeting the production of the ultra-pure form (polymer grade). Up to date, the ZIF-8 based MMMs were tested at maximum PR of 7 [53], whereas the PR necessary for producing the polymer grade at 99% recovery is ~15 (two-stage process, 𝛼=35, 50-70 wt% propylene feed). Thus, should the future researchers fail in boosting the mixed-matrix selectivity (under mixed-gas condition) above 25-35, it would be then crucial to guarantee the performance of these materials at high PR to compensate the poor selectivity. In fact, the future researchers are not advised to boost the mixed-gas selectivity of a material above 25-35, as that would probably be at the cost of brittle mixed-matrix material structure (due to high ZIF-8 loading). Also, as proven by the performed simulations, boosting the selectivity above 35 would not qualify the single process for the polymer grade production, and would not also reduce the two-stage process recycle ratio significantly. Therefore, it is advisable not to pursue further research in boosting the selectivity above the indicated range, but rather to focus on other aspects such as membrane high PR operation (including plasticization resistance), constituent’s compatibility (e.g. via carboxyl functionalization), processability, and tolerance to contaminants. • Manufacturing and modularization of ZIF-8 based MMMs Up to date, no record is available for the industrial modularization of ZIF-8 based MMMs, i.e., for the delivery of these membranes in the hollow-fiber or spiral-wound forms. The mixed-matrix membrane manufacturing limitations hinder technology’s commercialization. All the literature data pertains to relatively thick flat-sheet films, 106 which are synthesized and tested in labs under ideal conditions; the separation performance data as well as the polymer / filler compatibility in thin (real) films might differ from these of thick ones. 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Thesis/Dissertation
eng
Mechanical Engineering
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Propylene - propane separation using mixed matrix films made of functionalized polymers and zeolitic-imidazolate frameworks
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