Gas separation membranes can also beclassified to four groups, based on materials they are produced:· Facilitated-transport membranes· Inorganic membranes· Polymeric membranes· Mixed matrix membranes.
The facilitated transport membranes theoryis to establish strong bonds between interested gas molecules and membrane tofacilitate the diffusion of interested gas. They possess some disadvantagessuch as mechanical instability, low diffusivity, and defect formation. Inorganic membranes may have goodseparation performance, chemical and thermal stability 10.
However, sincethey are quite expensive, hard to handle and process, they are not preferred tobe used in industrial applications. Membranes prepared by polymeric materialsare cost effective, easy to produce and operate. However, since they arechemically and thermally vulnerable, their usage is limited by low-temperatureand non-reactive gases. In numerous cases the polymeric materials are used forthe fabrication of the membrane mostly glassy amorphous materials characterizedby high glass transition temperatures, good mechanical strength and anacceptable combination of gas permeability and selectivity properties.Polymeric membrane prepared by using high performance materials such aspolyimide, Polysulfone and polyethersulfone, exhibit high selectivitycoefficient and acceptable permeability values 10.The hybrid or mixed matrix membranes aredeveloped by mixing inorganic and polymeric membranes to overcome these problems.
They have mechanical and thermal stability, cost-effectiveness, and easyoperation condition as well as excellent gas separation properties. Polymer mixing together to form blend isanother method to obtain an enhanced polymeric membrane by combining differentpolymers with different properties. It is time and cost effective method toderive a polymeric membrane with certain performance rather than synthesizing anew polymer. Polymeric membranes may suffer from CO2plasticization at relatively higher temperatures and pressures, which may berequired for natural gas purification. Blending of two or more polymers issuggested to overcome or increase the resistance against plasticization andalso to increase the mechanical and thermal properties. Membrane gas separation method is most advantageousseparating system, since it combines several beneficial features that make themattractive for industrial applications. The features are described brieflybelow:i.
Separation is on basis of molecularsize, which means that the separation process could be carry out at ambient ormodest temperature. Thermally sensitive solutes can be treated without damage. Otherenvironmental stresses, such as imposed by chemical additives and high shearcan also be avoided or minimized.
ii. Membrane separation process did notrequire any phase change and any latent heat. Thus, it could save lost ofenergy consumption. iii.
Membrane devices are almost alwayscompact and modular, especially if membrane is provided in a bundle of hollowfibers and spiral wound that occupies high area per unit volume. This factoralso leads towards weight and space efficiency, which is important intransportation or offshore platform applications.