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 ~31 with CO₂ permeability of 301 barrer and CO₂/CH₄ selectivity of ~19 for a 20% H₂S, 20% CO₂ and 60% CH₄ feed at 35 °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.