Surprising plasticization benefits in natural gas upgrading using polyimide membranes

Hydrogen sulfide (H2S) and carbon dioxide (CO2) are acid gases that must be removed from natural gas prior to transmission and use. Here, we report the H2S/CH4 and CO2/CH4 separation performance of two polyimide membranes, i.e. 6FDA-DAM and 6FDA-DAM/DABA (3:2) for various realistic gas compositions and conditions. So-called plasticization effects of the polyimides are generally viewed as negative features when using such membranes, but we report important applications where H2S is present in the feed and the polyimide membrane plasticization is actually a tool for performance optimization. In fact, we identify cases where polyimide plasticization can provide large performance benefits for H2S/CH4 separations. We further analyze the transport mechanisms in terms of sorption and diffusion factors to understand this surprising discovery for various important feeds and conditions. The 6FDA-DAM membrane showed H2S permeability of 495 barrer and H2S/CH4 selectivity of similar to 31 with CO2 permeability of 301 barrer and CO2/CH4 selectivity of similar to 19 for a 20% H2S, 20% CO2 and 60% CH4 feed at 35 degrees C and 46 bar. Such CO2/CH4 performance and higher H2S/CH4 separation performance for aggressive high pressure feeds exceeds that of rubbery polymers, making the glassy materials ideal for processing natural gas feeds containing H2S and CO2.