Formation and characterization of asymmetric carbon molecular sieve and mixed-matrix membranes for natural gas purification

Membrane technology for gas separations continues to seek new, robust membrane materials with higher selectivities and productivities and greater tolerance to adverse environments. For the natural gas purification application (CO2 removal from CH4), this dissertation investigated: (1) asymmetric carbon molecular sieve (CMS) membranes and (2) mixed matrix membranes using CMS particles. In the study of pure CMS membranes, asymmetric polyimide hollow fibers were pyrolyzed under various pyrolysis conditions to form CMS hollow fibers. These CMS fibers were evaluated using high-pressure, mixed gas permeation experiments over a range of temperatures and feeds containing condensable hydrocarbon impurities. For pressures up to 1000 psia, it was demonstrated that CMS membranes exhibited attractive CO2/CH4 selectivities of 70 to 45 over the 24°C to 50°C temperature range and showed remarkable selectivity stability under condensable hydrocarbon exposure, which caused only a 10 to 20% reduction in productivity.; However, despite their attractive and robust membrane properties, costly and complex processing issues hinder the use of pure CMS membranes in industrial membrane units. The latter objective of this dissertation investigated the incorporation of these highly selective CMS materials as particles dispersed within a continuous, high-performance glassy polymer matrix. Modified casting techniques were developed to overcome initial challenges with poor polymer-sieve contact, enabling the formation of successful mixed matrix membrane films. High loadings (up to 38% by weight) of the CMS particles dispersed within polymer matrices were achieved from flat-sheet solution casting. Both pure gas and high-pressure, mixed gas permeation experiments on these mixed matrix films confirmed very impressive CO2/CH4 selectivity and CO2 productivity enhancements. Enhancements of as much as 45% in CO2/CH4 selectivity of the mixed matrix membrane (50 to 65) were observed over the intrinsic CO2/CH4 selectivities of the original polymer matrices. The technology of mixed matrix membranes using CMS particles offers great potential for future membrane materials development.