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High pressure pure- and mixed-gas separation of CO₂/CH₄ by thermally-rearranged and carbon molecular sieve membranes derived from a polyimide of intrinsic microporosity

Swaidan, Raja, Ma, Xiaohua, Litwiller, Eric, Pinnau, Ingo
Journal of membrane science 2013 v.447 pp. 387-394
artificial membranes, carbon, carbon dioxide, heat treatment, methane, natural gas, permeability, polymers
Natural gas sweetening, one of the most promising venues for the growth of the membrane gas separation industry, is dominated by polymeric materials with relatively low permeabilities and moderate selectivities. One strategy towards improving the gas transport properties of a polymer is enhancement of microporosity either by design of polymers of intrinsic microporosity (PIMs) or by thermal treatment of polymeric precursors. For the first time, the mixed-gas CO₂/CH₄ transport properties are investigated for a complete series of thermally-rearranged (TR) (440°C) and carbon molecular sieve (CMS) membranes (600, 630 and 800°C) derived from a polyimide of intrinsic microporosity (PIM-6FDA-OH). The pressure dependence of permeability and selectivity is reported up to 30bar for 1:1, CO₂:CH₄ mixed-gas feeds at 35°C. The TR membrane exhibited ~15% higher CO₂/CH₄ selectivity relative to pure-gas feeds due to reductions in mixed-gas CH₄ permeability reaching 27% at 30bar. This is attributed to increased hindrance of CH₄ transport by co-permeation of CO₂. Interestingly, unusual increases in mixed-gas CH₄ permeabilities relative to pure-gas values were observed for the CMS membranes, resulting in up to 50% losses in mixed-gas selectivity over the applied pressure range.