High Pressure CO₂/CH₄ Separation Using Carbon Molecular Sieve Hollow Fiber Membranes

Carbon molecular sieve (CMS) hollow fiber membranes have been investigated for CO 2 /CH 4 separation. High-pressure (up to 1000 psia), mixed-gas feeds of 10% CO 2 /90% CH 4 on the shell side were examined for three different temperatures (24, 35, and 50 °C). The mechanical, permeance, and selectivity stabilities of the CMS membranes under high pressure were encouraging and could be industrially relevant for many high-pressure applications, such as CO 2 removal from natural gas. Two asymmetric polyimide precursor fibers, 6FDA/BPDA-DAM and Matrimid 5218, were pyrolyzed under vacuum to form the CMS membrane fibers. When pyrolyzed under identical protocols, the two types of CMS fibers had different permeation properties and physical characteristics. Modifications of the pyrolysis protocol and conditions were explored. Increasing the final pyrolysis temperature was shown to dramatically increase the CO 2 /CH 4 selectivity (>600) of the CMS membranes but was detrimental to the CO 2 permeance. On the other hand, using a helium purge gas instead of a vacuum environment during pyrolysis did increase CO 2 permeance but resulted in a significant loss of CO 2 /CH 4 selectivity. Shortening the thermal soak time at the final pyrolysis temperature was the most effective approach to increasing the CO 2 permeance while maintaining the CO 2 /CH 4 selectivity.