A membrane separation process used to separate volatile substances from dilute solutions in which the membrane provides a selective barrier. Being independent of vapour–liquid equilibria, pervaporation operates by permeation of a substance (p. 279) through the membrane followed by its evaporation into the vapour phase. The separation is based on a difference in transport rates of individual components in the liquid retentate and vapour permeate on either side of the membrane. The upstream side of the membrane is typically at ambient pressure and the downstream side is under vacuum to allow the evaporation of the selective component after permeation through the membrane. The driving force for the separation is the difference in the partial pressures of the components on either side of the membrane.Pervaporation is used by many industries for purification and separation and is popular due to its simplicity, low energy consumption, and low temperature and pressure operation. It is effective for separating diluting solutions containing small amounts of a component to be removed. Hydrophilic membranes are used for dehydration of alcohols containing small amounts of water, while hydrophobic membranes are used for the removal of organic substances from aqueous solutions. It is a less aggressive separation process than distillation and is therefore used for the separation of ethanol from yeast fermentations, removal of water from esterification reactions, organic solvents from industrial wastewater, and for separating hydrophobic flavour compounds from aqueous solutions.