Actual air separation across multilayer composite membranes

Multilayer composite membranes are fabricated from six types of thin films as selective layers, an ethyl cellulose (EC) thin film as a flexible spacer , and poly(ether sulfone) (PES) with 15-45 nm pore size or 100-120 mu m thi ckness as a porous support layer. The effects of the thin-film type and its layer number, operating temperature, and transmembrane pressure difference , as well as the operational time on the actual air-separation properties t hrough the composite membranes, are investigated by a constant pressure-var iable volume method. The results show that a pure polystyrene thin-film com posite membrane exhibits poor actual air-separation performance due to its brittleness, although it has a higher ideal oxygen over nitrogen separation factor. The oxygen-enrichment air (OEA) flux through all of the composite membranes tested increases significantly with increasing operating temperat ure and pressure difference. The oxygen concentration in the OEA increases slightly with an increase in operating temperature, and the oxygen concentr ation through the polystyrene/cholesteryl oleyl carbonate blend, top layer composite membrane exhibits the maximal value. As the transmembrane pressur e difference increases, the oxygen concentration in the OEA also exhibits t he maximal value. The maximum oxygen concentration can reach 39.1%, which i s achieved by the multilayer composite membrane consisting of a polystyrene /cholesteryl oleyl carbonate (95/5) monolayer, an EC single flexible spacer , and a PES support at 35 degrees C and a transmembrane pressure difference of 550 kPa. (C) 2000 John Wiley & Sons, Inc.