Separation can also be based on kinetic selectivity, i.e. on a difference in migration velocity of the relevant components. This principle is used in a membrane separation operation as represented schematically in Figure 1.5 for a binary mixture of two components A and B.
Separation is based on the difference in diffusion velocity of components A and B through the selective membrane. Assuming that the transfer of component B through the selective membrane is faster, the mixture that goes through the membrane is enriched in component B, while the residual mixture is enriched in component A.
Membrane separation does not involve a change in phase and the two existing phases can be either gaseous (gaseous permeation) or liquid (ultrafiltration, reverse osmosis). However, a phase change along with a selective transfer through a membrane can in some cases enhance separation (e.g. in pervaporation).
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