Improved SAPO‐34 Membranes for CO2/CH4 Separations

CO2 separation from CH4 is important in natural-gas processing because CO2 reduces the energy content, and it is acidic and corrosive in the presence of water. Plants using CO2-selective polymeric membranes were installed in the 1980s, and currently the largest membrane facility processes 20 000 000 standard cubic meters per day. However, high CO2 pressures can plasticize polymeric membranes and decrease their separation ability, although a recent study by Lin et al. reported that CO2 pressures (up to at least 1.7 MPa) increased permeabilities and selectivities for highly branched, crosslinked poly(ethylene oxide) membranes. Zeolite membranes have potential for separating CO2 from CH4 owing to their superior thermal, mechanical, and chemical stability, good erosion resistance, and stability at high CO2 pressures. Zeolites are inorganic crystalline structures with uniform-sized pores of molecular dimensions. Because both CO2 (0.33 nm kinetic diameter) and CH4 (0.38 nm) are much smaller than the pores of largeand medium-pore zeolites, separation with these membranes is mainly based on competitive adsorption, and selectivities have been low. Lovallo et al. obtained a selectivity of about 10 for a high-silica MFI membrane at 393 K. In contrast, small-pore molecular sieves such as zeolite T (0.41 nm pore diameter), DDR (0.36 × 0.44 nm), and SAPO-34 (0.38 nm) have pores that are similar in size to CH4 but larger than CO2. These membranes have high CO2/CH4 selectivities due to a combination of differences in diffusivity and competitive adsorption. Cui et al., using a T-type zeolite membrane, obtained a CO2/CH4 composition selectivity of 400 and a CO2 permeance of 4.6 × 10 mol m s Pa at 308 K for a feed pressure of 0.1 MPa and vacuum on the permeate side. Tomita et al. obtained a CO2/CH4 selectivity of 220 and a CO2 permeance of 7 × 10 mol m s Pa at 301 K for a pressure drop of 0.5 MPa for DDR membranes. We reported separation selectivities at 295 K as high as 95 and permeances of 2.4 × 10 mol m s Pa for SAPO-34 membranes. These membranes also were selective and stable in the presence of H2O, N2, C2H4, C3H8, and n-C4H10 impurities. [8] The molecular sieve SAPO-34 has the composition (SixAlyPz)O2, where x = 0.01–0.98, y = 0.01–0.60, and z = 0.01–0.52. Adsorption isotherms showed that CO2 adsorbs more strongly than CH4 on SAPO-34 crystals. Most gas separations in zeolite membranes have used relatively low feed pressures, but feeds from natural gas wells may be 7 MPa or higher. Since the CO2 adsorption capacities in FAUand T-type membranes are close to saturation at 0.1 MPa, their CO2 fluxes are not expected to increase much for CO2 feed partial pressures above 0.1 MPa. Another challenge is that selectivity may decrease with increasing pressure because Knudsen (∝ P) and viscous flow (∝ P) through nonzeolite pores contribute more to the gas permeation as the feed pressure increases. For example, the CH4 flux for T-type membranes increased more than linearly with increasing pressure drop, indicating that viscous flow contributed to CH4 flux. As the feed pressure increased from 0.1 to 0.5 MPa at 308 K, the CO2/CH4 selectivity in T-type membranes decreased from 400 to 150. In the current study, SAPO-34 membranes with higher CO2 permeances and CO2/CH4 selectivities than previously reported were obtained by modifying the gel composition and synthesis procedure. The highest selectivity of 560 (at 252 K) was obtained for a membrane with a Si/Al ratio of 0.15. These membranes were effective for conditions at or near industrial requirements (feed pressures up to 7.2 MPa for extended times). SAPO-34 membranes with various Si/Al gel ratios were synthesized by in situ crystallization on porous stainless-steel tubes. As shown in Table 1, SAPO-34 membranes were obtained when the Si/Al gel ratios were 0.3, 0.2, and 0.15. When the Si/Al gel ratio was 0.1, a mixture of SAPO-34 (structure: chabazite, pore size: 0.38 nm) and SAPO-5 (structure: AFI, pore size: 0.74 nm) was formed. Note that the tetraethylammonium ion can be used as a template either for SAPO-5 or SAPO-34, and less SiO2 in the gel favors SAPO-5 formation. These membranes were used to separate a 50:50 CO2/CH4 mixture at 295 K at a feed pressure of 222 kPa and a permeate pressure of 84 kPa. As shown in Table 1, CO2/CH4 selectivities were higher than 100 for all pure SAPO-34 membranes. Membrane M3, with a Si/Al ratio of 0.15, had the highest CO2 permeance and the highest CO2/CH4 selectivity of 170. The presence of SAPO-5 in membrane M4 significantly increased permeances because both CO2 and CH4 are much smaller than the SAPO-5 pores, and they should have lower selectivity than SAPO-34 pores. C O M M U N IC A IO N S