Improved AlPO-18 membranes for light gas separation

High-quality AlPO-18 membranes were synthesized using low-cost symmetric alumina supports by a single hydrothermal step, and showed to be useful for light gas separation. Single-gas permeances of CO₂, N₂, CH₄ and C₃H₈ decreased with increasing kinetic diameter. Single CO₂ and CH₄ permeance dependence of pressure drop was predicted by the Maxwell-Stefan diffusion model and was in agreement with experimental data. The best AlPO-18 membrane showed smaller-component permeances of 6.5, 6.3 and 1.0 × 10^-7 mol (m² s Pa)^-1 (equal to 1940, 1880 and 300 GPU) and mixture selectivities of 220, 45 and 22 for equimolar CO₂/CH₄, CO₂/N₂ and H₂/CH₄ binary mixtures at room temperature and 0.2 MPa pressure drop, respectively. The effects of temperature and pressure on the membrane's separation performance in the three binary mixtures were discussed.