Gas separation by permeators with high‐flux asymmetric membranes

Abstract

The permeation behavior of the high-flux asymmetric membrane differs from that of the conventional symmetric membrane. A calculation method for predicting the gas separation performance of a permeator with asymmetric membrane is presented. The model takes into account the permeate pressure drop and is applicable to both hollow-fiber and spiral-wound modules. The effect of permeate-feed flow pattern on module performance is analyzed. It is shown that for the high-flux asymmetric membrane, the countercurrent flow pattern is not necessarily always the preferred operating mode. The mathematical model is verified by large-scale field pilot-plant experiments for helium recovery from natural gas using large hollow-fiber modules (220 m2/unit). C. Y. PANAlberta Research Council Edmonton, Alberta, Canada SCOPEThe conventional symmetric membrane has a homogeneous structure with uniform permeation properties across its thickness. This type of membrane has not been widely used for gas separation mainly due to low rates of permeation imposed by the membrane thickness required for maintaining membrane integrity and strength. Recent development of asymmetric membranes, however, has made membrane permeation an important unit operation for gas separation. The membrane consists of an ultrathin skin and a porous supporting layer with negligible resistance to gas flow. The skin, which acts as the separation barrier, is highly permeable due to its thinness. This permits the use of highly selective polymers with inherently poor permeability for specific gas separation. The presence of the porous supporting layer, howeve:, renders the permeatiop behavior of the asymmetric membrane somewhat different from that of the familiar symmetric membrane. A calculation method for predicting the performance of a permeator with the high-flux asymmetric membrane is presented. Both hollow-fiber and spiral-wound modules are considered. The effect of flow pattern on the performance of the asymmetric membrane is found to be significantly different from that of the symmetric membrane. Laboratory and pilot-plant data are presented to substantiate the mathematical model. CONCLUSIONS AND SIGNIFICANCEThe porous supporting layer of the asymmetric membrane prevents the mixing of permeate fluxes of varying compositions on the membrane skin surface. Consequently, the asymmetric membrane always gives rise to cross-flow type of permeation regardless of the flow pattern and direction of the bulk permeate stream flowing outside the porous layer. It is this characteristic that sets the permeation behavior of the asymmetric membrane apart from the conventional symmetric membrane.It is shown that, for the asymmetric membrane with narrow permeate flow path such as hollow fibers, the permeate pressure build-up is strongly dependent on the feed-permeate (the bulk stream) flow pattern. The countercurrent mode has the lowest permeate pressure build-up but the feed flow is in the undesirable direction in relation to the permeate pressure build-up. The cocurrent pattern, on th...

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