Separation of supercritical fluid mixtrues of CO 2 and petroleum components with an asymmetric polyimide membrane

Abstract

Asymmetric polyimide Kapton® membranes were made by casting a solution of 18 wt% polyamic acid and 5 wt% phenanthrene in dimethylacetamide at 343–363 K, 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 h at 373 K, 1 h at 473 K and 1 h at 573 K. The CO 2 fluxes of both pure CO 2 and mixtures of CO 2 and 2,2,4-trimethylpentane are directly proprotional to the transmembrane pressure difference up to 12 MPa and for the mixture are 0.80–0.95 times lower than those for pure CO 2. The lines do not pass through the origin but intercept at the horizontal axis. The fluxes of 2,2,4-trimethylpentane in the mixture are enhanced by a factor 1.2–1.3 as compared to that of pure 2,2,4-trimethylpentane. Separation experiments of supercritical fluid feed mixtures with CO 2 and several pertroleum components for a molar feed ratio of 15:1 were carried out at 323, 373 and 423 K and 8–12 MPa. The measured separation factors for the aliphatic hydrocarbon solutes 2,2-dimethylbutane, n-hexane, 2,2,4-trimethylpentane and n-octane were 4.4-3.3, 4.8-3.5, 6.0-4.0 and 6.3-4.4, respectively, and for the aromatic hydrocarbon solutes benzene, toluene, p-xylene and o-xylene 5.3-3.7, 6.1-4.3, 7.0-5.1 and 7.3-5.2, respectively. The measured separation factors for aliphatic hydrocarbon solutes were always higher than those calculated from the flux data for pure CO 2 and pure hydrocarbon while the separation factors for aromatic hydrocarbon solutes were always lower than the calculated ones.