Vapor permeation is a membrane-based process for separating binary or multi-component mixtures of miscellaneous organic fluids for example the dehydration of vaporous solvent stream.
The membrane acts as a selective barrier between the two phases, the vapor phase of the feed to be separated and the vaporous phase of the permeate. Separation of components of the feed is based on a difference in transport rates of individual components of the feed mixture through the membrane, not on thermodynamic equilibrium or differences in volatilities. That means that mixtures which cannot be separated easily by distillation due to the formation of azeotropes, can easily be separated by pervaporation or vapour permeation.
Driving force for the transport of a component through the membrane is a gradient in the chemical potential, maintained by a difference in the partial pressures of the components on the two sides of the membrane. The components at the upstream side (feed) of the membrane are at their respective saturated partial pressure. At the downstream side (permeate) the partial pressure is lowered; therefore the component passing through the membrane evaporates and leaves the membrane as a vapor. The low partial pressure at the permeate side is maintained by condensing the vaporous permeate at a sufficiently low temperature. All non-condensable components have to be removed from the permeate side by means of a vacuum pump, in order to allow for an unhindered convective transport of the vapor to the condenser and an effective condensation, which is essential to the vapor permeation process.
The advantages of the vapour permeation process is the requirement for a smaller membrane area and the reduced aggressiveness of the solvents compared to pervaporation process.