Dense film polyimide membranes for aggressive sour gas feed separations

Abstract Dense film membranes of the copolyimide 6FDA-DAM:DABA (3:2) are studied for simultaneous removal of CO 2 and H 2 S 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 62 bar. The H 2 S partial pressures used are representative of highly aggressive field operations. Penetrant-induced plasticization effects are evident at feed pressures below 1 bar in pure H 2 S feeds; sub- T g thermal annealing is used to effectively mitigate this effect, and these annealed films are used throughout the study. Surprisingly, H 2 S/CH 4 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 H 2 S partial pressures above 2 bar. Furthermore, permeation experiments using a 9.95% H 2 S, 19.9% CO 2 , 70.15% CH 4 mixture at low feed pressures give CO 2 /CH 4 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 H 2 S 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.