Page 16 - Adsorbents fundamentals and applications

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INTRODUCTORY REMARKS

Separation may be defined as a process that transforms a mixture of substances
into two or more products that differ from each other in composition (King, 1980).
The process is difficult to achieve because it is the opposite of mixing, a process
favored by the second law of thermodynamics. Consequently, the separation steps
often account for the major production costs in chemical, petrochemical, and phar-
maceutical industries. For many separation processes, the separation is caused by
a mass separating agent (King, 1980). The mass separating agent for adsorption is
adsorbent, or sorbent. Consequently, the performance of any adsorptive separation
or purification process is directly determined by the quality of the sorbent.
Due to the progress made in sorbent and cyclic process developments, adsorp-
tion has already become a key separations tool that is used pervasively in
industry. Adsorption is usually performed in columns packed with sorbent parti-
cles, or fixed-bed adsorbers. The high separating power of chromatography that
is achieved in a column is a unique advantage of adsorption as compared with
other separation processes. The high separating power is caused by the continuous
contact and equilibration between the fluid and sorbent phases. Under conditions
free of diffusion limitation, each contact is equivalent to an equilibrium stage or
theoretical plate. Usually several hundred to several thousand such equilibrium
stages can be achieved within a short column. Thus, adsorption is ideally suited
for purification applications as well as difficult separations. Partly because of this
unique advantage, adsorption is well-positioned to play a key role in the devel-
opment of many future energy and environmental technologies. The simulated
moving-bed technology is a good example of using adsorption to perform dif-
ficult separations, where satisfactory separations are achieved by using sorbents
with separation factors as low as 2.
There are only a handful of generic sorbents that are commercially avail-
able. These are the sorbents being used in the current adsorption processes.

Adsorbents: Fundamentals and Applications, Edited By Ralph T. Yang
ISBN 0-471-29741-0 Copyright  2003 John Wiley & Sons, Inc.

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