Carbon molecular sieve membranes for natural gas purification: Role of surface flow

Carbon molecular sieve membranes (CMSM) were prepared from the pyrolysis of polyimide films within a temperature range of 600°C‐800°C under nitrogen stream. The membrane samples were characterized and tested for the permeation of He, CH 4 , CO 2 , and N 2 at different pressures and temperatures, respectively. The CMSM700 membrane (pyrolyzed at 700°C) showed an ideal selectivity of ~ 11 for N 2 /CH 4 with a permeability of 2.18 × 10 −15 mol · m/m 2 · s · Pa for N 2 . The separation mechanism for the N 2 /CH 4 pair was shown to be largely molecular sieving rather than surface flow. The membrane showed an ideal selectivity of ~ 500 for the CO 2 /CH 4 pair with a CO 2 permeability of 9.72 × 10 −14 mol · m/m 2 · s · Pa. The permeability of He was lower than that of CO 2 , suggesting that the surface flow played a significant role in the CO 2 permeation. The updated permeability‐selectivity tradeoff curves show that this CMSM membrane compared favourably with other membrane materials reported in the literature for the removal of N 2 and CO 2 from CH 4 for natural gas upgrading.