Development of Membrane Separation of Supercritical Fluid

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Thin porous silica membranes were prepared on cylindrical porous alumina substrate by the sol-gel technique. The controlled particle size of silica sol gave a relatively sharp pore distribution of the membrane at less than a few nanometers. This thin porous silica membranes were used to measure the permeation rate of the supercritical carbon dioxide (SCCO_2) and the rejection of the solutes (polyethylene glycols and triolein). The permeation rate, which is division of the mole permeation flux with the transmembrane pressure, increased with increase in the pressure to reach the maximum near the critical pressure and decreased to approach the almost constant value at the higher pressure. The appearance of the maximum, however, was not significant when the average pore diameter of the membrane became smaller or the temperature became higher. This behavior of the permeation rate could be probably produced by the effect of pressure on the kinematic viscosity of SCCO_2 and the condensation of SCCO_2 in the membrane pores. When the membrane with the average pore size of 1.5nm was used, the rejection of PEG400 changed from a slightly negative value to a positive value with increase in the transmembrane pressure, while the rejection of triolein did not change. The membrane with the average pore size of 0.6nm retained well PEG400 the rejection of which reached 0.8, and the further improvement of the thin porous silica membranes could be useful for the membrane processing of the supercritical fluid mixture.