Pervaporation

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Pervaporation is a method for the separation of mixtures of liquids by partial vaporization through a non-porous membrane.

[edit] Theory

The membrane acts as a selective barrier between the two phases, the liquid phase feed and the vapor phase permeate. It allows the desired component(s) of the liquid feed to transfer through it by vaporization. Typically, the upstream side of the membrane is at ambient pressure and the downstream side is under vacuum to allow the evaporation of the selective component after permeation through the membrane. The fractionation process is mainly due to polarity differences, and not to the volatility difference of the components in the feed. The word is derived from the two basic steps of the process, firstly the permeation through the membrane by the permeate, then it's evaporation into the vapor phase. This process is used by a number of industries for several different processes, including purification and analysis, due to its simplicity and in-line nature.

The driving force for transport of different components is provided by a chemical potential difference between the liquid feed/retentate and vapor permeate at each side of the membrane. The retentate is the remainder of the feed leaving the membrane feed chamber, which is not permeated through the membrane. The chemical potential can be expressed in terms of fugacity, given by Raoults law for a liquid and by Daltons law for (an ideal) gas. It should be noted that during operation, due to removal of the vapor-phase permeate, the actual fugacity of the vapor is lower than anticipated on basis of the collected (condensed) permeate. Separation of components (e.g. water and ethanol) is based on a difference in transport rate of individual components through the membrane. This transport mechanism can be described using the solution-diffusion model, based on the rate/ degree of dissolution of a component into the membrane and its velocity of transport (expressed in terms of diffusivity) through the membrane, which will be different for each component and membrane type leading to separation.

Pervaporation is effective for diluting solutions containing trace amounts of the component to be removed. Based on this, hydrophilic membranes are used for dehydration of alcohols containing small amounts of water and hydrophobic membranes are used for removal/recovery of trace amounts of organics from aqueuous solutions. Pervaporations is a very mild process and hence very effective for separation of those mixtures which can not survive the harsh conditions of distillation.

[edit] Applications

Solvent Dehydration: dehydrating the ethanol/water and isopropanol/water azeotropes
Continuous water removal from condensation reactions such as esterifications
Membrane introduction mass spectrometry