Molecular Geometry:

There are a number of methods based on molecular geometry. They require a permeable barrier to provide separation. The techniques are based on the following:

• Molecular sieves
• Gel filtration

The permeable barrier separation process of practical importance utilizes semi- permeable membranes as the restrictive surface. The membranes permit the passage of certain chemical species completely while preventing or strongly retarding the permeation of others. It might be mentioned here in which the transport rate of permeation has to be high enough to achieve reasonably rapid separation. A good mechanical and chemical stability of these membranes is important. There are different types of membranes available which are as follows:

• Microporous membranes,
• Homogenous membranes,
• Charged membranes, and
• Thin membranes.