Fp. Mccandless, A COMPARISON OF MEMBRANE CASCADES, SOME ONE-COMPRESSOR RECYCLE PERMEATORS, AND DISTILLATION, Journal of membrane science, 89(1-2), 1994, pp. 51-72
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.