Pervaporation: Most Suitable Membrane Process

1.4 Pervaporation Principle

Pervaporation implying permeation and 'vaporization' is the most suitable membrane process for liquid mixtures. It is a separation process in which a binary or multi-component liquid mixture is separated by partial vaporization through a suitable dense non-porous membrane as shown in Figure. While one side of membrane layer is in contact with mixture, other side is kept under very low total or partial pressure with the help of a vacuum or a sweeping gas respectively. Molecules selectively passing through the boundary layer due to their polarity, higher affinity and faster diffusivity are vaporized and removed from the surface immediately. Therefore, desired species are enriched at permeate side during remaining ones

In other word, the membrane is defined as a structure having lateral dimensions greater than its thickness, through which mass transfer occurs under a various driving forces.

The word membrane originates from the latin word 'membrana' which means a skin. Membranes keep things separated in living world. They pass material selectively. The membrane may be a selective or contacting barrier. In the former case, it controls the exchange between the two regions next to it in a specific manner, whereas in the latter case, its function is mainly to contact the two regions between which the transport occurs. A membrane can be homogeneous or heterogeneous, with symmetric or asymmetric structure, in solid or liquid state, it can carry a positive or negative charge, or can be neutral or bipolar. Transport through a membrane is affected by convection, by diffusion or individual molecules induced by an electric field or concentration, pressure or temperature gradient. Membrane can also physically or chemically modify the permeating species, prevent permeation or regulate the rate of permeation. Thus membranes may be either passive or active, depending upon its ability to change the chemical nature of the permeating species. The membrane thickness may vary from as small as 100 micron to several millimetres. Another characteristic

The commercially and scientifically most relevant processes are pressure driven processes such as reverse osmosis, ultra and microfiltration, pervaporation or gas separation; concentration gradient driven processes such as: dialysis; and electrical potential driven process such as