High pressure CO2/CH4 separation using carbon molecular sieve hollow fiber membranes

Carbon molecular sieve (CMS) hollow fiber membranes have been investigated for CO2/CH4 separation. High-pressure (up to 1000 psia), mixed-gas feeds of 10% CO2/90% CH4 on the shell side were examined for three different temperatures (24, 35, and 50 degreesC). 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 CO2 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 CO2/CH4 selectivity (>600) of the CMS membranes but was detrimental to the CO2 permeance. On the other hand, using a helium purge gas instead of a vacuum environment during pyrolysis did increase CO2 permeance but resulted in a significant loss of CO2/CH4 selectivity. Shortening the thermal soak time at the final pyrolysis temperature was the most effective approach to increasing the CO2 permeance while maintaining the CO2/CH4 selectivity.