Carbon molecular sieve dense film membranes derived from Matrimid® for ethylene/ethane separation

Development of dense film carbon molecular sieve (CMS) membranes for ethylene/ethane (C2H4/C2H6) separation is reported. A commercial polyimide, Matrimid (R), was pyrolyzed under vacuum and inert argon atmosphere, and the resultant CMS films were characterized using pure C2H4 and C2H6 permeation at 35 degrees C, 50 psia feed pressure. The effects on C2H4/C2H6 separation caused by different final vacuum pyrolysis temperatures from 500 to 800 degrees 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 inverted perpendicular C where an optimum combination of C2H4 permeability similar to 14-15 Barrer with C2H4/C2H6 selectivity similar to 12 was observed. A modified heating rate protocol for 675 degrees 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. (C) 2011 Elsevier Ltd. All rights reserved.