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Membrane separations by partial vapor pressure gradient

Pervaporation (PV)

Pervaporation (PV) is the separation of liquid mixtures by partial vaporization through a dense permselective membrane. Unlike the other membrane processes, a phase change occurs when the permeate changes from liquid to vapor during its transport through the membrane. PV is an enrichment technique similar to distillation; however, unlike distillation, PV is not limited by the vapor-liquid equilibrium. As a matter of fact, PV has been commercially applied to the separation of azeotropic mixtures (dehydration of alcohol). The heart of the PV is a nonporous membrane, which either exhibits a high permeation rate for water but does not permeate organics, or vice versa. A gradient in the chemical potential of the substances on the feed side, and the permeate side is the driving force for the process, which can be represented by partial vapor pressures on both sides of the membrane. The driving force is kept at a maximum by applying low pressure (vacuum or sweep gas) to the permeate side of the membrane, combined with immediate condensation of permeated vapors.

Pervaporation processes have found use in the chemical industry to break azeotropic water/alcohol mixtures and to perform separations that are highly energy-intensive when distillation is used. Over the decades, a growing amount of attention has been paid to the application of pervapo-ration to environmental problem.

The performance of pervaporation is commonly evaluated by two experimental parameters: the permeation flux and the selectivity. The performance of a pervaporation process is assessed by the flux of the permeating species and the selectivity of the species (Equation 3.31):

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