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Development of dense film carbon molecular sieve (CMS) membranes for ethylene/ethane (C2H4/C2H6) separation is reported. A commercial polyimide, Matrimid®, was pyrolyzed under vacuum and inert argon atmosphere, and the resultant CMS films were characterized using pure C2H4 and C2H6 permeation at 35 °C, 50 psia feed pressure. The effects on C2H4/C2H6 separation caused by different final vacuum pyrolysis temperatures from 500 to 800 °C are reported. For all pyrolysis temperatures separation surpassed the estimated ‘upper bound’ solution processable polymer line for C2H4 permeability vs. C2H4/C2H6 selectivity. C2H4 permeability decreased and selectivity increased with increasing pyrolysis temperature until 650–675 °C where an optimum combination of C2H4 permeability ∼14–15 Barrer with C2H4/C2H6 selectivity ∼12 was observed. A modified heating rate protocol for 675 °C showed further increase in permeability with no selectivity loss. CMS films produced from argon pyrolysis showed results comparable to vacuum pyrolysis. Further, mixed gas (63.2 mol% C2H4 + 36.8 mol% C2H6) permeation showed a slightly lower C2H4 permeability with C2H4/C2H6 selectivity increase rather than a decrease that is often seen with polymers. The high selectivity of these membranes was shown to arise from a high ‘entropic selection’ indicating that the ‘slimmer’ ethylene molecule has significant advantage over ethane in passing through the rigid ‘slit-shaped’ CMS pore structure. |
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