Pervaporation

Pervaporation is a membrane-based separation process used to separate liquid mixtures by partial vaporization through a dense, selective membrane. A liquid feed contacts the membrane on the feed side, while the permeate side is kept under vacuum or swept with an inert gas to continuously remove the vapor that forms. The separation relies on differences in solubility and diffusivity of the components within the membrane, described by the solution-diffusion mechanism.

In operation, components dissolve into the membrane, diffuse across it, and vaporize on the opposite side. The driving force is the partial pressure difference across the membrane, maintained by the vacuum or sweep on the permeate side. Permeate is typically a vapor that is condensed outside the module.

Pervaporation is versatile for separating azeotropes and heat-sensitive mixtures. Common applications include dehydration of organic solvents (notably ethanol and isopropanol), removal of water from organic feedstocks, solvent recovery, and purification of aromas or essential oil components. It is also used for removing volatile organic compounds from aqueous streams and for trace solvent cleanup.

Membranes used fall into polymeric and inorganic categories. Polymeric membranes, such as polyimides, are tailored for high selectivity toward water over organics. Inorganic and ceramic membranes (e.g., silica, zeolites) offer thermal and chemical resistance for aggressive feeds or higher-temperature operation. Composite membranes combine a selective top layer with a robust support.

Advantages include lower energy for certain separations compared with distillation, especially for breaking azeotropes and for heat-sensitive feeds. Limitations involve a trade-off between flux and selectivity, susceptibility to swelling and plasticization, fouling, and the costs of membranes and vacuum or sweep systems. Pervaporation is often used in hybrid configurations with distillation to achieve efficient separations.