In Membrane Gas Separation, the off-gas is led over a semipermeable membrane of a suitable material and module. Under a given hydrostatic head, it is split in two streams, one rich in the targeted component the other depleted.
Good performance dictates that the membrane be more permeable to the targeted component than it is to the air or inlet gas molecules.
The efficiency of operation depends on the membranes selectivity, the pressure in the space, the pressure in the targeted component rich stream, and the extend to which the targeted component has to be stripped from the air stream.
If the original concentration of the targeted component in the air stream is very low, a high degree of vacuum will be required on the targeted component-rich side of the membrane.
Usually the retentate concentration is higher than the regulation allows, so further treatment is necessary.
The three primary types of membrane gas separation unit are: