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Abstract

Pressure-dependent anisotropic behavior is provided by a size selective separation membrane system having two successive porous layers. The first layer is a soft, elastic, effectively porous material, and the second layer is a substantially rigid porous material. When positive pressure presses the first layer against the second layer, the second layer serves as a rigid support, and the first layer is compressed to reduce its effective pore size. The first layer material is characterized by sufficient compressibility, that, when a pressure differential of 100 psi is applied across the thickness of the system, the effective flow rate is reduced by a factor of at least 20 compared to the flow rate when 100 psi is exerted in the opposite direction. A substance can be separated from a liquid mixture using the membrane system by: positioning the second layer of the membrane system in contact with the mixture; exerting pressure on the mixture to force liquid with the substance first through the pores of the rigid layer and the soft layer of the system; and reversing the flow through the membrane system by exerting pressure on the resulting filtrate, in a manner to compress the soft material sufficiently to reduce the pore size to prevent passage of the substance. The membrane system is manufactured by coagulating the layers from polymers in solution.

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