Cascades, membranes

As a footnote, considerable simplification in the mathematical separation representations could result by assuming that the respective molar flow rates remain constant throughout the membrane unit. Such is the practice in distillation calculations, where there is mass transfer in both directions. The assumption is similarly made in absorber or stripper calculations, where only one key component is involved. This condition, called constant molal overflow in distillation and absorber and stripper derivations and calculations, may also be accommodated in the case of multistage or cascade membrane calculations, as derived and utilized... 

It is first assumed that perfect mixing can be attained for single-stage and multistage or cascade membrane operations and that a condition of constant molal (or molar) overflow or underflow can be approximated during multistage operations. 

Figure 7 is a schematic representation of a section of a cascade. The feed stream to a stage consists of the depleted stream from the stage above and the enriched stream from the stage below. This mixture is first compressed and then cooled so that it enters the diffusion chamber at some predetermined optimum temperature and pressure. In the case of uranium isotope separation the process gas is uranium hexafluoride [7783-81-5] UF. Within the diffusion chamber the gas flows along a porous membrane or diffusion barrier. Approximately one-half of the gas passes through the barrier into a region... 

FIG. 22-64 Cascade arrangement for membrane processing to maintain cross-flow velocity as permeate is removed. 

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