Membrane cascade schemes for the separation of LPG olefins and paraffins

Membranes of commercial polymers, such as polyimides, are not very efficient in separating multicomponent olefin-paraffin mixtures in one stage. In order to improve the separation performance of such materials, the application of various recycle and/or multistage membrane schemes has been examined. These design schemes are: single-stage permeation with permeate recycle (SSPR), "in series" membrane cascade (ISMC), counter-current recycle cascade (CRC) and continuous membrane column (CMC). The key parameter in the design of these membrane configurations is the relation of the heads separation factor with stage cut, which was derived from experimental results for LPG separation on polyimide Matrimid((R)) 5218. Parametric analysis of the examined configurations shows that CRC cascades can improve dramatically the separation efficiency of commercial polymeric membranes. In the optimum case of a non-ideal CRC scheme with five stages, the LPG mixture can be separated on polyimide into an olefinic and a paraffinic stream of high purity (99-99.9 mol%) with ultrahigh yield (98-100%). This, however, is achieved at the expense of increased membrane area requirements, which may be offset by the achieved separation performance and the value of the recovered products. (C) 2004 Elsevier B.V. All rights reserved.