A COMPARISON OF MEMBRANE CASCADES, SOME ONE-COMPRESSOR RECYCLE PERMEATORS, AND DISTILLATION

For more than a decade the continuous membrane column (CMC) has been t outed as being a revolutionary separation device since it is capable o f producing a highly enriched product without cascading. Most studies have ignored membrane cascades in favor of the one-compressor permeato r (OCP) designs, implying that a CMC or one of the other OCPs is alway s more efficient than multi-compressor cascades. Moreover, it has been claimed that a CMC is completely analogous to a packed distillation c olumn, and the design and performance of CMCs have been analyzed in te rms of concepts developed for equilibrium processes. This paper shows how basic separation science and cascade theory applies both to membra ne permeation and distillation, and shows that a CMC cannot be analogo us to distillation because the recycle flow pattern in distillation is characteristic of that in a countercurrent recycle cascade (CRC), whi le it is not in a CMC. On the other hand, a number of membrane permeat ors can be connected in series to form the recycle flow pattern of a C RC. A theoretical comparison of the performance of ideal CMCs with mem brane CRCs provides an insight as to why some of the one-compressor pe rmeator designs require much larger membrane areas and compressor duti es than that required by CRCs to obtain the same separation. Although the enrichment that occurs in a CMC is increased with increasing recyc le rate similar to that in a CRC, the mechanism is entirely different in the two designs. It is suggested that a CMC and some of the other O CPs are single permeation stages with enrichment that depends both on recycle rate and cut. The consequences of some of the thermodynamic di stinctions between the rate-governed permeation and equilibrium proces ses such as distillation are briefly explored.