TABLE OF CONTENTS In differential permeation, the point compositions of the phases are considered to vary linearly with position along the surface(s) of the membrane. A steady state is assumed, so that the compositions are independent of time.
One phase is called the permeate V, the other phase the reject L. The designators also refer to the (molar) flow rate of each of the phases, which varies with position. The reject is the continuation of the feed-stream. Its inlet rate and composition are those of the feed, its final or exit rate and composition depend on the permeation effected.
If the permeate is withdrawn at each point along the membrane surface, then the situation corresponds somewhat to that of bulk differential vaporization or condensation. [1], [2] However, in the case at hand, the reject is considered to be a flow system of changing composition rather than a bulk phase of uniform composition. Moreover, the change in composition of the reject phase is to be a function of linear position only and not of time.
The permeate so withdrawn may subsequently be accumulated, but this is independent of the permeation process per se. Furthermore, the accumulation may be withdrawn as if concurrent to the flow of the reject phase or as if countercurrent to the reject phase. The total accumulation and its composition are the same in each situation; it is "after the fact."
[1]Hoffman, E. J. Azeotropic and Extractive Distillation. New York: Wiley-Interscience, 1964; Huntington, NY:...
