Membrane and Desalination Technologies - TCE Moodle Website

50 J. Ren and R. Wang
Nonporous dense
membrane
Composite membranes
Isotropic microporous membranes have a rigid, interconnected pore, and voided structure
distributed randomly; its pore diameter is in the order of 0.01–10 mm. The particles larger than
the largest pores are completely rejected and the particles smaller than the largest pores, will
be partially rejected by deep filtration and screen filtration according to the hydrodynamic and
hindered transport theory. The separation process is controlled by the pore size distribution of
microporous membranes and the hydrodynamic conditions. The microporous membranes are
prepared by phase separation, tracked etch, stretching, or leaching, etc. The phase separation
is the most important method for the isotropic microporous membrane preparation.
Electrically charged membranes are referred to either anion-exchange membranes or
cation-exchange membranes, which possess dense/microporous structures carrying fixed
positive or negatively charged ions. The charge and the concentration of the ions in solution
are the key factors to control the separation process.
2.2. Anisotropic Membranes
Isotropic microporous
membrane
Integrated asymmetric
membranes
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Charged membrane
Supported liquid
membranes
Fig. 2.2 Schematic diagram of different membrane morphologies [adapted from ref. (2)].
Anisotropic membranes are layer structures, changing the porosity and pore size over the
whole membrane wall. The anisotropic membranes usually have a very thin surface layer
supported on a thick microporous substrate. The thin skin layer is the selective layer to
perform separation, while the microporous substrate mainly provides the mechanical
strength. Because of the very thin selective layer, the membrane fluxes are very high.
Integrally asymmetric membranes, composite membranes and supported liquid membranes
are in the category of anisotropic membranes.
Integrally asymmetric membranes were developed by Loeb and Sourirajan in 1960s (3),
which were prepared by the phase inversion method using a single membrane material. The
membrane porosity and pore size change in different layers. A selective thin layer exists on
the top surface followed by more and more loose layers. The thin skin layer and porous
substrate are coupled with each other as they are formed simultaneously in the phase
inversion processphase inversion process.
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