Effects of impurities on CO2/CH4 separations through SAPO-34 membranes

The effects of H2O, N2, C2H4, C3H8, and n-C4H10 impurities on CO2/CH4 separation were studied for SAPO-34 membranes on stainless steel supports. The CO2 permeance decreased by 12%, but the CO2/CH4 selectivity was stable after 12 days of exposure to 170 ppm. Permeances and selectivity were not significantly affected when 3% N2 was in the feed. Adding 1% hydrocarbons (C2-C4) to the feed decreased both permeance and selectivity; C4 hydrocarbon caused the largest decrease because heavier hydrocarbons have higher heats of adsorption and thus inhibit CO2 permeation more significantly. Higher hydrocarbon concentrations decreased the permeance and selectivity more. When four impurities were in the feed, a SAPO-34 membrane separated CO2 from CH4 with selectivity as high as 55 and a CO2 permeance of 4.9 × 10−8 mol/(m2 s Pa) at 297 K and a feed pressure of 2.2 MPa. The percent impurity in the feed at low pressure was a good indication of the separation behavior at higher pressures. Higher impurity partial pressures did not result in more loss in permeance or selectivity when the CO2 and CH4 partial pressures were corresponding increased. The permeances and selectivities were not permanently reduced by impurities, but were restored by removing the impurity from the feed or by calcining.