Improved SAPO-34 Membranes for CO_2/CH_4 Separations

摘要

SAPO-34 membranes were synthesized by in situ crystallization on a porous, tubular stainless-steel support when the Si/Al gel ratios were 0.3, 0.2, and 0.15. For a Si/Al gel ratio of 0.1, a mixture of SAPO-34 and SAPO-5 formed. The SAPO-34 membranes effectively separated CO_2 from CH_4 for conditions at or near industrial requirements. At 295 K and a pressure drop of 140 kPa, the CO_2/CH_4 selectivity was 170 for a membrane with a Si/Al gel ratio of 0.15. The CO_2 permeance was 1.2 x 10~(-7) mol m~(-2) s~(-1) Pa~(-1). The selectivity increased as the temperature decreased, and at 252 K the CO_2/CH_4 selectivity was 560. SAPO-34 membranes with Si/Al gel ratios of 0.15 were also prepared by seeding the support surface with SAPO-34 crystals. These membranes had lower CO_2/CH_4 separation selectivity (115) but higher CO_2 permeance (4.0 x 10~(-7) mol m~(-2) s~(-1) Pa~(-1)) at 295 K and a pressure drop of 140 kPa. At 7 MPa, a membrane had a CO_2/CH_4 selectivity of 100 for a 50/50 feed at 295 K. The permeances and the separation selectivity were constant for a week at 295 K for a 75% CO_2 feed at 5.7 MPa.