A comparison of some recycle permeators for gas separations

Abstract

The performances of several different recycle permeators are compared by calculating the enrichment, extend of separation, and area requirements for different overall stage cuts and recycle ratios, using the separation of air as an example. Membrane modules investigated included a two-unit series type permeator with permeate recycle, a continuous membrane column with high-pressure feed, and two single-unit countercurrent permeators with permeate recycle (both high-pressure and low-pressure feed). The use of Rony's extent of separation in the analysis greatly aids in the visualization of limiting permeator performance at different recycle ratios. The composition of O 2 in the high-pressure product (reject) stream becomes zero at specific values of the stage cut which depend on recycle ratio for all but the low-pressure feed module. This would require an infinite permeator area, hence stage cuts greater than this limiting value are not possible. For the case of the low-pressure feed unit, the reject stream composition approaches a minimum finite value that depends on pressure ratio in the permeator. This limiting minimum composition would also be reached at specific values of stage cut that depend on recycle ratio and hence a stage cut greater than this limiting value cannot be obtained. Other permeator types (co-current, perfect mixed models) do not behave in this manner. It is concluded that the two-unit series type permeator gives the best separation and requires smaller membrane areas at high recycle ratios. Performance of the four permeators is not greatly different at lower recycle ratios.