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Separation processes play a critical role in industry, including the removal
of impurities from raw materials, purification of products, separation of
recycle streams, and removal of contaminants from air and
effluents (Figure 1). In most of the process industries, the separation process accounts for more than 40-70%
of capital and operating cost, and proper use of selective separation process can
reduce costs and increase profits.
The separation of fluid mixtures in various processes are classified into
two categories: (1) Separation by equilibrium distribution; and (2) separation
by differences in transport rates. Majority of the processes are equilibrium
distribution (evaporation, distillation, extraction, adsorption, absorption, etc.).
Thermodynamic equilibrium is established between the two phases and
both phases show the same temperature and all components have the same
chemical potential in both phases. Component can be highly enriched in one phase
and depleted in the other. The two phases are separated by appropriate means,
the enriched component can be recovered by establishing a new equilibrium at
a different temperature or pressure. Repetition of this at the end will lead to
a phase in which one of the components is present at the required
concentration. The separation by difference in transport rates can be achieved by an
additional means. In some cases a membrane is used to separate the two phases from
each other. A driving force, i.e., a gradient in pressure, concentration, temperature
or electrical field is applied over the membrane. Under the influence of the
driving force, components from the mixture to be separated, held at a higher
chemical potential, transport through the membrane to the side of the lower potential. |
