Pervaporation

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

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. Driving force for the separation here is the difference in the partial pressures of the components on the two sides and not 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 its 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 Raoult's law for a liquid and by Dalton's 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. A relatively new membrane in the field hydrophylic membranes is the ceramic membranes with the actual separation layer being made of amorphous silica. This is in fact a membrane which is porous, with pores ranging around 4 Ångstrøm, large enough to let water molecules pass through and retain any other solvents that have a larger molecular size such as ethanol. Recent novell hydrophylic ceramic membranes can also be based on titania or zirconia.

Hydrophobic membranes are often PDMS based where the actual separation mechanism is based on the solution-diffusion model described above.

Pervaporations is a very mild process and hence very effective for separation of those mixtures which can not survive the harsh conditions of distillation.

Applications

 * Solvent Dehydration: dehydrating the ethanol/water and isopropanol/water azeotropes
 * Continuous water removal from condensation reactions such as esterifications to enhance  conversion and rate of the reaction.
 * Membrane introduction mass spectrometry
 * Removing organic solvents from industrial waste waters.
 * Combination of distillation and pervaporation/vapour permeation