Dense film polyimide membranes for aggressive sour gas feed separations

Dense film membranes of the copolyimide 6FDA-DAM:DABA (3:2) are studied for simultaneous removal of CO₂ and H₂S from sour natural gas streams. Pure and mixed gas permeation as well as pure gas sorption data are reported at 35°C and pressures up to 62bar. The H₂S partial pressures used are representative of highly aggressive field operations. Penetrant-induced plasticization effects are evident at feed pressures below 1bar in pure H₂S feeds; sub-T_g thermal annealing is used to effectively mitigate this effect, and these annealed films are used throughout the study. Surprisingly, H₂S/CH₄ selectivity nearly doubles for mixed gas testing in comparison to the pure component ideal selectivity values and approaches the level of a state-of-the-art glassy polymer, cellulose acetate (CA), at H₂S partial pressures above 2bar. Furthermore, permeation experiments using a 9.95% H₂S, 19.9% CO₂, 70.15% CH₄ mixture at low feed pressures give CO₂/CH₄ selectivity of up to 49-over 30% greater than the pure component selectivity for 6FDA-DAM:DABA (3:2). The overall sour gas separation performance of this polyimide is comparable to high-performance rubbery polymer membranes, which have been reported for only moderate H₂S partial pressure feeds, and is superior to that for CA based on a practical combined acid gas separation efficiency metric that we introduce. Finally, methods for continued development of the current polyimide membrane material for aggressive sour gas separations are presented.