SEPARATION OF SUPERCRITICAL CO2 AND ISOOCTANE MIXTURES WITH AN ASYMMETRIC POLYIMIDE MEMBRANE

Asymmetric polyimide Kapton(R) membranes were made by casting a solution of 18 wt. % polyamic acid and 5 wt.% phenanthrene in dimethylacetamide at 80-100 degrees C, with a four-minute evaporation time, followed by a cyclization process of thermal treatment in a bath of dioctyl sebacate under N-2, in three steps: 1 hr at 373 K, 1 hr at 473 K and 1 hr at 573 K. With a membrane cast at 80 degrees C, the best value of 12.8 was obtained for the separation factor, alpha(i.oct./CO2), in experiments conducted at 423 K and 8-12 MPa for supercritical fluid feed mixtures with a CO2/iso-octane molar concentration ratio of 25:1. The CO2 fluxes of both the pure components and the mixture are directly proportional to the trans-membrane pressure difference up to 12 MPa and can be plotted on a straight line which does not pass through the origin but intercepts at the horizontal axis. The flux of iso-octane in the mixture is enhanced compared with that of the pure iso-octane by 1.4 times from 4.7X10(-9) mol/(m(2)-sec-Pa) to 6.4X10(-9) mol/(m(2) sec-Pa).