Influence of propane on CO2/CH4 and N2/CH4 separations in CHA zeolite membranes

• SSZ-13 membranes can be prepared with very few defects.
• Propane decreases CO2, N2, and CH4 permeances through SAPO-34 and SSZ-13 membranes.
• Propane affects SSZ-13 membranes less than SAPO-34 membranes.
• SSZ-13 membranes separate N2/CH4 mixtures with selectivities of 9–13.

Two types of CHA zeolite membranes (SAPO-34, SSZ-13) were used for CO2/CH4, N2/CH4, and CO2/i-butane separations at both low (270 and 350 kPa) and high (1.73 MPa) pressures. The SSZ-13 membranes were more selective, with CO2/CH4 separation selectivities as high as 280 and N2/CH4 separation selectivities of 12 at 270 kPa feed pressure. For both types of membranes, selectivities and permeances decreased as the feed pressure increased. The CO2/i-butane separation selectivities were greater than 500,000 for SAPO-34 membranes, indicating extremely low densities of defects because i-butane is too large to enter the CHA pores. The CO2/i-butane selectivities were smaller for SSZ-13 membranes (2,800–20,000), in part because the SSZ-13 layer was on the outside of the porous mullite tubes and sealing the membrane on the zeolite surface was more difficult than for the SAPO-34 membranes that were grown on the inside of glazed alumina tubes. Propane, in feed concentrations from 1 to 9%, significantly influenced separations by decreasing permeances in most cases. The effect was larger for N2/CH4 than for CO2/CH4 mixtures, apparently because the more strongly-adsorbing CO2 competes better than N2 with propane for adsorption sites. Although propane caused permeances to decrease significantly over time, selectivities decreased much less. Propane decreased permeances more for SAPO-34 membranes than for SSZ-13 membranes at 350 kPa, and at high pressure propane even increased CO2 permeances and decreased CH4 permeances in SSZ-13 membranes, thus significantly increasing CO2/CH4 selectivities. Propane permeances reached steady state relatively quickly because its permeation was mostly through defects, but CO2, N2, and CH4 permeances did not stabilize in the presence of propane, even after seven days. The effects of propane were reversible when it was removed from the feed and the membranes were heated.