5. Membrane materials for gas permeation

A wide variety of membrane structures are available for gas permeation. However, most of the materials used industrially comprise a thin, dense layer of selective polymer on a porous support of the same type (so-called asymmetrical membranes) or of a different type (composite membranes). The deposition of a "caulking" layer made of a highly permeable but non-selective polymer (such as silicone) on the active layer, proposed in 1980 for Air Products membranes, is very often applied, in order to limit the drop in performance due to defects in the dense matrix.

Membrane materials can also be flat (for use in flat or spiral modules (Figure 5 )) or cylindrical, in which case they are referred to as hollow fibers (Figure