Pervaporation with a "reverse-selective" (hydrophobic) membrane is a promising technology for the energy-efficient separation of alcohols from dilute alcohol-water streams, such as those formed in the production of biofuels. Pervaporation depends on the selectivity and throughput of the membrane, which in turn is highly dependent on the membrane material. A nanocomposite approach to membrane design is desirable in order to combine the advantages and eliminate the individual limitations of previously-reported polymeric and zeolitic membranes. In this work, a hollow-fiber membrane composed of a thin layer of polymer/zeolite nanocomposite material on a porous polymeric hollow fiber support is developed. The hollow fiber geometry offers considerable advantages in membrane surface area per unit volume, allowing for easier scaling and higher throughput than flat-film membranes. Poly(dimethyl siloxane) (PDMS) and pure-silica MFI zeolite (silicalite-1) were investigated for these membranes. Iso-octane was used to dilute the dope solution to provide thinner coatings. Previously-spun non-selective Torlon hollow fibers were used as the support layer for the nanocomposite coatings. To determine an acceptable method for coating fibers with uniform, defect-free coatings, flat-film membranes (0 to 60 wt% MFI on a solvent-free basis) and hollow-fiber membranes (0 and 20 wt% MFI) were fabricated using different procedures. Pervaporation experiments were run for all membranes at 65C with a 5 wt% ethanol feed. The effects of membrane thickness, fiber pretreatment, coating method, zeolite loading, and zeolite surface treatment on membrane pervaporation performance were investigated.

A new innovative polymer pyrolysis method was proposed for creation of attractive carbon molecular sieve (CMS) membranes. Oxygen exposure at ppm levels during pyrolysis was hypothesized and demonstrated to make slit-like CMS structures more selective and less permeable, which I contrary to ones expectation. Indeed prior to this work, any exposure to oxygen was expected to result in removal of carbon mass and increase in permeability. The results of this study indicated that the separation performance and CMS structure may be optimized for various gas separations by careful tuning of the oxygen level. This finding represents a breakthrough in the field of CMS membranes. Simple replacement of pyrolysis atmospheres from vacuum to inert can enable scale-up. The deviation in CMS membrane performance was significantly reduced once oxygen levels were carefully monitored and controlled. The method was shown to be effective and repeatable not only with dense films but also with asymmetric hollow fiber membranes. As a result, this work led the development of the "inert" pyrolysis method which has overcome the challenges faced with previously studied pyrolysis method to prepare attractive CMS membranes. The effect of oxygen exposure during inert pyrolysis was evaluated by a series of well-controlled experiments using homogeneous CMS dense films. Results indicated that the oxygen "doping" process on selective pores is likely governed by equilibrium limited reaction rather than (i) an external or (ii) internal transport or (iii) kinetically limited reaction. This significant finding was validated with two polyimide precursors: synthesized 6FDA/BPDA-DAM and commercial Matrimid®, which implies a possibility of the "inert" pyrolysis method application extending towards various precursors. The investigation was further extended to prepare CMS fibers. Despite the challenge of two different morphologies between homogeneous films and asymmetric hollow fibers, the "inert" pyrolysis method was successfully adapted and shown that separation performance can be tuned by changing oxygen level in inert pyrolysis atmosphere. Moreover, resulting CMS fibers were shown to be industrially viable. Under the operating condition of ~80 atm high pressure 50/50 CO2/CH4 mixed gas feed, the high separation performance of CMS fibers was shown to be maintained. In addition, elevated permeate pressures of ~20 atm did effect the theoretically predicted separation factor. While high humidity exposures (80%RH) resulted in reduced permeance, high selectivity was sustained in the fibers. Recommendations to overcome such negative effects as well as future investigations to help CMS membranes to be commercialized are provided.

The goal of this project was to understand and model the performance of hybrid inorganic-organic membranes under realistic operating conditions for hydrocarbon gas/vapor separation, using butane isomers as the model vapors and a hybrid membrane of 6FDA-DAM-5A as an advanced separation system. To achieve the set goal, three objectives were laid out. The first objective was to determine the factors affecting separation performance in dense neat polymer. One main concern was plasticization. High temperature annealing has been reported as an effect means of suppressing plasticization. A study on the effect of annealing temperature was performed by analyzing data acquired via sorption and permeation measurements. Based on the findings from this study, a suitable annealing temperature was determined. Another factor studied was the effect of operating temperature. In deciding a suitable operating temperature, factors such as its possible effect on plasticization as well as reducing heating/cooling cost in industrial application were considered. Based on the knowledge that industrial applications of this membrane would involve mixture separation, the second objective was to understand and model the complexity of a mixed gas system. This was investigated via permeation measurements using three feed compositions. An interesting transport behavior was observed in the mixed gas system, which to the best of our knowledge, has not been observed in other mixed gas systems involving smaller penetrants. This mixed gas transport behavior presented a challenge in predictability using well-established transport models. Two hypotheses were made to explain the observed transport behavior, which led to the development of a new model termed the HHF model and the introduction of a fitting parameter termed the CAUFFV fit. Both the HHF model and CAUFFV fit showed better agreement with experimental data than the well-established mixed gas transport model. The final objective was to explore the use of mixed matrix membranes as a means of improving the separation performance of this system. A major challenge with the fabrication of good mixed matrix membranes was the adhesion of the zeolite particle with the polymer. This was addressed via sieve surface modification through a Grignard treatment process. Although a Grignard treatment procedure existed, there was a challenge of reproducibility of the treatment. This challenge was addressed by exploring the relationship between the sieves and the solvent used in the treatment, and taking advantage of this relationship in the Grignard treatment process. This study helped identify a suitable solvent, which allowed for successful and reproducible treatment of commercial LTA sieves; however, treatment of lab-made sieves continues to prove challenging. Based on improved understanding of the Grignard treatment reaction mechanism, modifications were made to the existing Grignard treatment procedure, resulting in the introduction of a "simplified" Grignard treatment procedure. The new procedure requires less control over the reaction process, thus making it more attractive for industrial application. Permeation measurements were made using mixed matrix membranes in both single and mixed gas systems. Selectivity enhancements were observed under both single and mixed gas systems using sieve loadings of 25 and 30wt%. The Maxwell model was used to make predictions of mixed matrix membrane performance. Although the experimental results were not in exact agreement with Maxwell predictions, the observed selectivity enhancement was very encouraging and shows potential for future application. Recommendations were made for future study of this system.

Distillation and absorption columns offer significant energy demands for future development in the petrochemical and fine chemical industries. Membranes and adsorbents are attractive alternatives to these classical separation units due to lower operating cost and easy device fabrication; however, membranes possess an upper limit in separation performance that results in a trade-off between selectivity (purity) and permeability (productivity) for the target gas product, and adsorbents require the need to be water-resistant to natural gas streams in order to withstand typical gas compositions. Composite membranes, or mixed-matrix membranes, are an appealing alternative to pure polymeric membrane materials by use of a molecular sieve “filler” phase which has higher separation performance than the pure polymer. In this thesis, the structure-property-processing relationships for a new class of molecular sieves known as zeolitic imidazolate frameworks (ZIFs) are investigated for their use as the filler phase in composite membranes or as adsorbents. These materials show robust chemical and thermal stability and are a promising class of molecular sieves for acid gas (CO₂/CH₄) separations. The synthesis of mixed-linker ZIFs is first investigated. It is shown that the organic linker composition in these materials is controllable without changing the crystal structure or significantly altering the thermal decomposition properties. There are observable changes in the adsorption properties, determined by nitrogen physisorption, that depend on the overall linker composition. The results suggest the proposed synthesis route facilitates a tunable process to control either the adsorption or diffusion properties depending on the linker composition. The structure-property-processing relationship for a specific ZIF, ZIF-8, is then investigated to determine the proper processing conditions necessary for fabricating defect-free composite membranes. The effect of ultrasonication shows an unexpected coarsening of ZIF-8 nanoparticles that grow with increased sonication time, but the structural integrity is shown to be maintained after sonication by using X-ray diffraction, Pair Distribution Function analysis, and nitrogen physisorption. The permeation properties of composite membranes revealed that intense ultrasonication is necessary to fabricate defect-free membranes for CO₂/CH₄ gas separations. Finally, the separation properties of mixed-linker ZIFs is investigated by using adsorption studies of CO₂ and CH₄ and using composite membranes with differing linker compositions. Adsorption properties of mixed-linker ZIFs reveal that these materials possess tunable surface properties, and a selectivity enhancement of six fold over ZIF-8 is observed with mixed-linker ZIFs without changing the crystal structure. Gas permeation studies of composite membranes reveal that the separation properties of mixed-linker ZIFs are different from their parent frameworks. By proper selection of mixed-linker ZIFs, there is an overall improvement of separation properties in the composite membranes when compared to ZIF-8.

Mixed matrix membranes (MMMs) consisting of a polymer bulk phase and an inorganic dispersed phase have the potential to provide a more selective membrane because they incorporate the selectivity of a zeolite dispersed phase while maintaining the ease of use of a polymer membrane. A critical problem in MMM applications is control over the polymer-zeolite interface adhesion during fabrication which can detrimentally impact membrane performance. In this work, MgOxHy (1≤x≤2, 0≤y≤2) nanostructures have been grown on pure-silica MFI and aluminosilicate LTA zeolites through four surface deposition techniques: Grignard decomposition reactions, solvothermal and modified solvothermal depositions, and ion-exchange induced surface crystallization. The structural properties of the surface nanostructures produced by each of the four methods were thoroughly characterized for their morphology, crystallinity, porosity, surface area, elemental composition, and these properties were used to predict the method’s suitability for use in composite membranes. The nanostructured zeolites were used in mixed matrix membranes (MMMs) at two MMMs weight loadings. The dispersion, mechanical properties, and CO₂/CH₄ gas separation properties were measured MMMs made with each method of functionalized LTA. All functionalization methods improve adhesion with the polymer observable by microscopy, the dispersion of particles, and the elastic modulus and hardness of the membrane. Gas permeation measurements prove the quality and effectiveness of the Ion Exchange membrane for CO₂/CH₄ separation by its significant increase in selectivity over the pure polymer. Lastly, the interface between the two materials was studied by probing the interfacial polymer mobility using NMR spin-spin relaxation measurements and mechanical mapping of membrane cross sections. It was shown that the nanostructures have both steric and chemical interactions with the polymer. Mapping of the elastic modulus indicated that functionalization methods that resulted in poorer zeolite coverage also disrupted the mechanical properties of the membrane at the interface of the materials. The investigations in this thesis provide detailed structure-property relationships of surface-modified molecular sieves and nanocomposite membranes fabricated using these materials, allowing a rational approach to the design of such materials and membranes.

The overall focus of this thesis is on the development and understanding of nanoporous layered silicates and membranes, particularly for potential applications in gas separations. Nanoporous layered materials are a rapidly growing area of interest, and include materials such as layered zeolites, porous layered oxides, layered aluminophosphates, and porous graphenes. They possess unique transport properties that may be advantageous for membrane and thin film applications. These materials also have very different chemistry from 3-D porous materials due to the existence of a large, chemically active, external surface area. This feature also necessitates the development of innovative strategies to process these materials into membranes and thin films with high performance.

The global market for membrane separation technologies is forecast to reach $16 billion by the year 2017 due to wide adoption of the membrane technology across various end-use markets. With the growth in demand for high quality products, stringent regulations, environmental concerns, and exhausting natural resources, membrane separation technologies are forecast to witness significant growth over the long term (Global Industry Analysts Inc., 2011). The future of membrane technology promises to be equally exciting as new membrane materials, processes and innovations make their way to the marketplace. The current trend in membrane gas separation industry is, however, to develop robust membranes, which exhibit superior separation performance, and are reliable and durable for particular applications. Process simulation allows the investigation of operating and design variables in the process, and in new process configurations. An optimal operating condition and/or process configuration could possibly yield a better separation performance as well as cost savings. Moreover, with the development of new process concepts, new membrane applications will emerge. The thesis addresses developing models that can be used to help in the design and operation of CO2 capture processes. A mathematical model for the dynamic performance of gas separation with high flux, asymmetric hollow fiber membranes was developed considering the permeate pressure build-up inside the fiber bore and cross flow pattern with respect to the membrane skin. The solution technique is advantageous since it requires minimal computational effort and provides improved solution stability. The model predictions and the robustness of the numerical technique were validated with experimental data for several membrane systems with different flow configurations. The model and solution technique were applied to investigate the performance of several membrane module configurations for air separation and methane recovery from biogas (landfill gas or digester gas). Recycle ratio plays a crucial role, and optimum recycle ratios vital for the retentate recycle to permeate and permeate recycle to feed operation were found. From the concept of two recycle operations, complexities involved in the design and operation of continuous membrane column were simplified. Membrane permselectivity required for a targeted separation to produce pipeline quality natural gas by methane-selective or nitrogen-selective membranes was calculated. The study demonstrates that the new solution technique can conveniently handle the high-flux hollow fiber membrane problems with different module configurations. A section of the study was aimed at rectifying some commonly believed perceptions about pressure build-up in hollow fiber membranes. It is a general intuition that operating at higher pressures permeates more gases, and therefore sometimes the membrane module is tested or characterized at lower pressures to save gas consumption. It is also perceived that higher pressure build-up occurs at higher feed pressures, and membrane performance deteriorates at higher feed pressures. The apparent and intrinsic permeances of H2 and N2 for asymmetric cellulose acetate-based hollow fiber membranes were evaluated from pure gas permeation experiments and numerical analysis, respectively. It was shown that though the pressure build-up increases as feed pressure increases, the effect of pressure build-up on membrane performance is actually minimized at higher feed pressures. Membrane performs close to its actual separation properties if it is operated at high feed pressures, under which conditions the effect of pressure build-up on the membrane performance is minimized. The pressure build-up effect was further investigated by calculating the average loss and percentage loss in the driving force due to pressure build-up, and it was found that percentage loss in driving force is less at high feed pressures than that at low feed pressures. It is true that unsteady state cyclic permeation process can potentially compete with the most selective polymers available to date, both in terms selectivity and productivity. A novel process mode of gas separation by means of cyclic pressure-vacuum swings for feed pressurization and permeate evacuation using a single pump was evaluated for CO2 separation from flue gas. Unlike transient permeation processes reported in the literature which were based on the differences in sorption uptake rates or desorption falloff rates, this process was based on the selective permeability of the membrane for separations. The process was analyzed to elucidate the working principle, and a parametric study was carried out to evaluate the effects of design and operating parameters on the separation performance. It was shown that improved separation efficiency (i.e., product purity and throughput) better than that of conventional steady-state permeation could be obtained by means of pressure-vacuum swing permeation. The effectiveness of membrane processes and feasibility of hybrid processes combining membrane permeation and conventional amine absorption process were investigated for post-combustion CO2 capture. Traditional MEA process uses a substantial amount of energy at the stripper reboiler when CO2 concentration increases. Several single stage and multi-stage membrane process configurations were simulated for a target design specification aiming at possible application in enhanced oil recovery. It was shown that membrane processes offer the lowest energy penalty for post-combustion CO2 capture and likely to expand as more and more CO2 selective membranes are developed. Membrane processes can save up to 20~45% energy compared to the stand-alone MEA capture processes. A comparison of energy perspective for the CO2 capture processes studied was drawn, and it was shown that the energy requirements of the hybrid processes are less than conventional MEA processes. The total energy penalty of the hybrid processes decreases as more and more CO2 is removed by the membranes.

The global market for membrane separation technologies is forecast to reach $16 billion by the year 2017 due to wide adoption of the membrane technology across various end-use markets. With the growth in demand for high quality products, stringent regulations, environmental concerns, and exhausting natural resources, membrane separation technologies are forecast to witness significant growth over the long term (Global Industry Analysts Inc., 2011). The future of membrane technology promises to be equally exciting as new membrane materials, processes and innovations make their way to the marketplace. The current trend in membrane gas separation industry is, however, to develop robust membranes, which exhibit superior separation performance, and are reliable and durable for particular applications. Process simulation allows the investigation of operating and design variables in the process, and in new process configurations. An optimal operating condition and/or process configuration could possibly yield a better separation performance as well as cost savings. Moreover, with the development of new process concepts, new membrane applications will emerge. The thesis addresses developing models that can be used to help in the design and operation of CO2 capture processes. A mathematical model for the dynamic performance of gas separation with high flux, asymmetric hollow fiber membranes was developed considering the permeate pressure build-up inside the fiber bore and cross flow pattern with respect to the membrane skin. The solution technique is advantageous since it requires minimal computational effort and provides improved solution stability. The model predictions and the robustness of the numerical technique were validated with experimental data for several membrane systems with different flow configurations. The model and solution technique were applied to investigate the performance of several membrane module configurations for air separation and methane recovery from biogas (landfill gas or digester gas). Recycle ratio plays a crucial role, and optimum recycle ratios vital for the retentate recycle to permeate and permeate recycle to feed operation were found. From the concept of two recycle operations, complexities involved in the design and operation of continuous membrane column were simplified. Membrane permselectivity required for a targeted separation to produce pipeline quality natural gas by methane-selective or nitrogen-selective membranes was calculated. The study demonstrates that the new solution technique can conveniently handle the high-flux hollow fiber membrane problems with different module configurations. A section of the study was aimed at rectifying some commonly believed perceptions about pressure build-up in hollow fiber membranes. It is a general intuition that operating at higher pressures permeates more gases, and therefore sometimes the membrane module is tested or characterized at lower pressures to save gas consumption. It is also perceived that higher pressure build-up occurs at higher feed pressures, and membrane performance deteriorates at higher feed pressures. The apparent and intrinsic permeances of H2 and N2 for asymmetric cellulose acetate-based hollow fiber membranes were evaluated from pure gas permeation experiments and numerical analysis, respectively. It was shown that though the pressure build-up increases as feed pressure increases, the effect of pressure build-up on membrane performance is actually minimized at higher feed pressures. Membrane performs close to its actual separation properties if it is operated at high feed pressures, under which conditions the effect of pressure build-up on the membrane performance is minimized. The pressure build-up effect was further investigated by calculating the average loss and percentage loss in the driving force due to pressure build-up, and it was found that percentage loss in driving force is less at high feed pressures than that at low feed pressures. It is true that unsteady state cyclic permeation process can potentially compete with the most selective polymers available to date, both in terms selectivity and productivity. A novel process mode of gas separation by means of cyclic pressure-vacuum swings for feed pressurization and permeate evacuation using a single pump was evaluated for CO2 separation from flue gas. Unlike transient permeation processes reported in the literature which were based on the differences in sorption uptake rates or desorption falloff rates, this process was based on the selective permeability of the membrane for separations. The process was analyzed to elucidate the working principle, and a parametric study was carried out to evaluate the effects of design and operating parameters on the separation performance. It was shown that improved separation efficiency (i.e., product purity and throughput) better than that of conventional steady-state permeation could be obtained by means of pressure-vacuum swing permeation. The effectiveness of membrane processes and feasibility of hybrid processes combining membrane permeation and conventional amine absorption process were investigated for post-combustion CO2 capture. Traditional MEA process uses a substantial amount of energy at the stripper reboiler when CO2 concentration increases. Several single stage and multi-stage membrane process configurations were simulated for a target design specification aiming at possible application in enhanced oil recovery. It was shown that membrane processes offer the lowest energy penalty for post-combustion CO2 capture and likely to expand as more and more CO2 selective membranes are developed. Membrane processes can save up to 20~45% energy compared to the stand-alone MEA capture processes. A comparison of energy perspective for the CO2 capture processes studied was drawn, and it was shown that the energy requirements of the hybrid processes are less than conventional MEA processes. The total energy penalty of the hybrid processes decreases as more and more CO2 is removed by the membranes.

Natural gas is one of the fastest growing primary energy sources in the world today. The increasing world demand for energy requires increased production of high quality natural gas. For the natural gas to be fed into the mainline gas transportation system, it must meet the pipe-line quality standards. Natural gas produced at the wellhead is usually "sub-quality" and contains various impurities such as CO2, H2S, and higher hydrocarbons, which must be removed to meet specifications. Carbon dioxide is usually the most abundant impurity in natural gas feeds and high CO2 partial pressures in the feed can lead to plasticization, which causes loss of some methane product and may ultimately render the membrane ineffective. Moreover, the presence of highly sorbing higher hydrocarbons in the feed can further reduce membrane performance. Covalent crosslinking has been shown to increase plasticization resistance in dense films by suppressing the degree of swelling and segmental chain mobility in the polymer, thereby preserving the selectivity of the membrane. This research focuses on extending the dense film success to asymmetric hollow fibers. In this work, the effect of high pressure CO2 (up to 400 psia CO2 partial pressure) on CO2/CH4 mixed gas separation performance was investigated on defect-free the hollow fiber membrane at different degrees of crosslinking. All the crosslinked fibers were shown to exhibit good resistance to selectivity losses from CO2 induced plasticization, significantly more than the uncrosslinked fibers. Robust resistance of the hollow fiber membranes in the presence of toluene (a highly sorbing contaminant) was also demonstrated as the membranes showed no plasticization. Antiplasticization was found to occur in the presence of toluene feeds with the crosslinkable fibers used in this work.

Membrane separations are rapidly growing alternatives to traditionally expensive gas separation processes. For natural gas purification, membranes are used to remove carbon dioxide to prevent pipeline corrosion and increase the heating value of the natural gas. The robust chemical and physical properties of polyimide membranes make them ideal for the numerous components and high pressures associated with natural gas production. Typically, the performance of membranes changes over time as a result of physical aging of the polymer. Previous work shows that the thin selective layer of an asymmetric hollow fiber membrane, the morphology of choice for gas separations, ages differently than a thick dense film of the same material. Also, carbon dioxide, which is highly soluble in most polymers, can actively swell and plasticize polymer membranes at higher pressures. In this work, free acid groups present in the model polyimide are covalently crosslinked to stabilize the matrix against plasticization. Physical aging of two different crosslinked derivatives are compared to the free acid polyimide through gas permeation, gas sorption, and refractive index measurements. Thick (~50 m) and thin (~650 nm) films are examined to determine the effects of sample dimension on physical aging. The crosslinking mechanism employs diol substituents to form ester linkages through the free acid group. However, the annealing treatment, above the glass transition temperature, used to "reset" the thermal history of the films is found to form a new crosslinked polymer. Characterization of this new crosslinking mechanism reveals a high-temperature decarboxylation of the free acid creates free-radical phenyl groups which form covalent crosslinks through other portions of the polymer structure. Since ester crosslinks may be vulnerable to hydrolysis in aggressive gas feed streams, this new mechanism of crosslinking may create a more robust membrane for aggressive separations. In addition to the physical aging study, supercritical carbon dioxide conditioning of the two glycol crosslinked polyimides is compared to the free acid polymer. In this case, the free acid polymer is not crosslinked since the esterification crosslinking reaction occurs at much lower temperature than the decarboxylation mechanism. The free acid polymer displays an atypical permeation response under supercritical carbon dioxide conditions which suggests a structural reorganization of the polymer occurs. The crosslinked polymers do not exhibit this type of response. Mixed gas permeation confirms a substantial decrease in the productivity of the free acid polyimide and reveals the enhanced stability of the crosslinked polyimides following the supercritical carbon dioxide conditioning. Finally, examination of structurally similar fluorine-containing polyimides following approximately 18 years of aging allows the study of polymer structure on physical aging. A 6FDA-based polyimide is compared to a BPDA-based polyimide to understand the effects of bulky, CF3 groups on physical aging, and polyimides with diamine isomers reveal the effects of structural symmetry on physical aging.