High performance carbon molecular sieve membranes for ethylene/ethane separation derived from an intrinsically microporous polyimide
Abstract An intrinsically microporous polymer with hydroxyl functionalities, PIM 6FDA OH, was used as a precursor for various types of carbon molecular sieve (CMS) membranes for ethylene/ethane separation. The pristine polyimide films were heated under controlled N 2 atmosphere at different stages from 500 to 800 degC. All CMS samples carbonized above 600 degC surpassed the polymeric ethylene/ethane upper bound. Pure gas selectivity reached 17.5 for the CMS carbonized at 800 degC with an ethylene permeability of about 10 Barrer at 2 bar and 35 degC, becoming the most selective CMS for ethylene/ethane separation reported to date. As expected, gravimetric sorption experiments showed that all CMS membranes had ethylene/ethane solubility selectivities close to one. The permselectivity increased with increasing pyrolysis temperature due to densification of the micropores in the CMS membranes, leading to enhanced diffusivity selectivity. Mixed gas tests with a binary 50:50 v/v ethylene/ethane feed showed a decrease in selectivity from 14 to 8.3 as the total feed pressure was increased from 4 to 20 bar. The selectivity drop under mixed gas conditions was attributed to non ideal effects: (i) competitive sorption that reduced the permeability of ethylene and (ii) dilation of the CMS that resulted in an increase in the ethane permeability.