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Membranes are employed in:
- recovery of hydrogen from product streams of ammonia plants
- recovery of hydrogen in oil refinery processes
- separation of methane from the other components of biogas
- enrichment of air by oxygen for medical or metallurgical purposes
- enrichment of ullage by nitrogen in Inerting systems designed to prevent fuel tank explosions
- removal of CO2 from natural gas
- removal of H2S from natural gas
- removal of volatile organic liquids (VOL) from air of exhaust streams
- desiccation or dehumidification of air.
Usually nonporous polymeric membranes are used. The vapours and gases are separated due to their different solubility and diffusivity in the polymers. The permeability or permeation coefficient,P, of such nonporous membranes can generally be expressed as the solubility,S, of the gas in the membrane polymer multiplied by the diffusivity, D, of the gas in the polymer, i.e., P = D • S; in such cases, permeation is said to occur by a "solution-diffusion" model. Polymers in the glassy state are generally more effective for separation, and predominantly differentiate gases based on their different diffusivities. Small molecules of penetrants move among polymer chains according to the formation of local gaps by thermal motion of polymer segments. The free volume of the polymer, its distribution and local changes in distribution are of the utmost importance. The diffusivity of a penetrant depends mainly on its molecular size.
Porous membranes can also be utilized for the gas separation. The pore diameter must be smaller than the mean free path of gas molecules. Under normal conditions (100 kPa, 300 K), this is about 50 nm. In this case, the gas flux through the pore is proportional to the molecule's velocity, i.e., inversely proportional to the square root of the molecule's mass. This is known as Knudsen diffusion. Gas flux through a porous membrane is much higher than through a nonporous one by 3 to 5 orders of magnitude. The separation efficiency is moderate; hydrogen diffuses 4 times faster than oxygen. Porous polymeric or ceramic membranes for ultrafiltration serve the purpose. Note that when the pores are larger than the limit then viscous flow occurs, and hence no separation.
In special cases other materials can be utilized; for example, palladium membranes permit transport solely of hydrogen.
- Vieth W.R., Diffusion in and through Polymers, Hanser Verlag, Munich