Gas separation membranes can also be
classified to four groups, based on materials they are produced:
Mixed matrix membranes.
The facilitated transport membranes theory
is to establish strong bonds between interested gas molecules and membrane to
facilitate the diffusion of interested gas. They possess some disadvantages
such as mechanical instability, low diffusivity, and defect formation.
Inorganic membranes may have good
separation performance, chemical and thermal stability 10. However, since
they are quite expensive, hard to handle and process, they are not preferred to
be used in industrial applications.
Membranes prepared by polymeric materials
are cost effective, easy to produce and operate. However, since they are
chemically and thermally vulnerable, their usage is limited by low-temperature
and non-reactive gases. In numerous cases the polymeric materials are used for
the fabrication of the membrane mostly glassy amorphous materials characterized
by high glass transition temperatures, good mechanical strength and an
acceptable combination of gas permeability and selectivity properties.
Polymeric membrane prepared by using high performance materials such as
polyimide, Polysulfone and polyethersulfone, exhibit high selectivity
coefficient and acceptable permeability values 10.
The hybrid or mixed matrix membranes are
developed by mixing inorganic and polymeric membranes to overcome these problems.
They have mechanical and thermal stability, cost-effectiveness, and easy
operation condition as well as excellent gas separation properties.
Polymer mixing together to form blend is
another method to obtain an enhanced polymeric membrane by combining different
polymers with different properties. It is time and cost effective method to
derive a polymeric membrane with certain performance rather than synthesizing a
Polymeric membranes may suffer from CO2
plasticization at relatively higher temperatures and pressures, which may be
required for natural gas purification. Blending of two or more polymers is
suggested to overcome or increase the resistance against plasticization and
also to increase the mechanical and thermal properties.
Membrane gas separation method is most advantageous
separating system, since it combines several beneficial features that make them
attractive for industrial applications. The features are described briefly
i. Separation is on basis of molecular
size, which means that the separation process could be carry out at ambient or
modest temperature. Thermally sensitive solutes can be treated without damage. Other
environmental stresses, such as imposed by chemical additives and high shear
can also be avoided or minimized.
ii. Membrane separation process did not
require any phase change and any latent heat. Thus, it could save lost of
iii. Membrane devices are almost always
compact and modular, especially if membrane is provided in a bundle of hollow
fibers and spiral wound that occupies high area per unit volume. This factor
also leads towards weight and space efficiency, which is important in
transportation or offshore platform applications.