Increasing H2/N2 separation selectivity in CHA zeolite membranes by adding a third gas

• Propane, n-butane, and ethane decreased H2 and N2 permeances in SAPO-34 membranes.
• Propane increased the H2/N2 separation selectivity from 4 to 9 over two days.
• Propane's effect on H2/N2 selectivity was less at higher temperatures.
• n-Butane also increased the H2/N2 separation selectivity but ethane did not.
• The change in selectivities is attributed to decreasing N2 diffusivity, rather than due to competitive adsorption.

Adding propane or n-butane in low concentrations to a H2/N2 feed increases the H2/N2 separation selectivity in SAPO-34 membranes by decreasing the N2 permeance more than the H2 permeance. More than two days were required to reach steady state because propane and n-butane diffuse slowly into the zeolite pores, but they adsorb strongly and thus decrease H2 and N2 loadings. Although the loading of the more-weakly adsorbed H2 is expected to decrease more, the N2 permeance decreased more than the H2 permeance apparently because the diffusivity of the larger N2 molecule decreased more. Propane more than doubled the H2/N2 selectivity at 298 K, but its effect on selectivity decreased as the temperature increased because the propane loading in the SAPO-34 pores decreased. Ethane also decreased H2 and N2 permeances, but it did not change H2/N2 selectivity. As with propane, ethane decreased H2 loadings more than N2 loadings, and decreased N2 diffusivity more than H2 diffusivity. However, the smaller ethane had less effect on the N2 diffusivity than did propane, so the net effect was that the selectivity did not change. The effects of alkane adsorption in SAPO-34 membranes were reversed when the alkanes were removed from the feed.