SEPARATION OF SUPERCRITICAL-FLUID MIXTURES OF CO2 AND PETROLEUM COMPONENTS WITH AN ASYMMETRIC POLYIMIDE MEMBRANE

Asymmetric polyimide Kapton(R) membranes were made by casting a soluti on of 18 wt% polyamic acid and 5 wt% phenanthrene in dimethylacetamide at 343-363 K, with a four-minute evaporation time, followed by a cycl ization process of thermal treatment in a bath of dioctyl sebacate und er N2 in three steps: 1 h at 373 K, 1 h at 473 K and 1 h at 573 K. The CO2 fluxes of both pure CO2 and mixtures of CO2 and 2,2,4-trimethylpe ntane are directly proportional to the transmembrane pressure differen ce up to 12 MPa and for the mixture are 0.80-0.95 times lower than tho se for pure CO2. The lines do not pass through the origin but intercep t 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 CO2 and several petroleum components for a molar fe ed 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 wer e 4.4-3.3, 4.8-3.5, 6.0-4.0 and 6.3-4.4, respectively, and for the aro matic 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 separati on factors for aliphatic hydrocarbon solutes were always higher than t hose calculated from the flux data for pure CO2 and pure hydrocarbon w hile the separation factors for aromatic hydrocarbon solutes were alwa ys lower than the calculated ones.