Page 32 - Adsorbents - fundamentals and applications

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SORBENT SELECTION:

EQUILIBRIUM ISOTHERMS,

DIFFUSION, CYCLIC
PROCESSES, AND

SORBENT SELECTION
CRITERIA

The selection of a proper sorbent for a given separation is a complex problem.
The predominant scientific basis for sorbent selection is the equilibrium isotherm.
Diffusion rate is generally secondary in importance. The equilibrium isotherms
of all constituents in the gas mixture, in the pressure and temperature range of
operation, must be considered. As a first and oversimplified approximation, the
pure-gas isotherms may be considered additive to yield the adsorption from a
mixture. Models and theories for calculating mixed gas adsorption (Yang, 1987)
should be used to provide better estimates for equilibrium adsorption. Based
on the isotherms, the following factors that are important to the design of the
separation process can be estimated:

1. Capacity of the sorbent, in the operating temperature and pressure range.
2. The method of sorbent regeneration — for example, temperature or pressure
swing — and the magnitude of the required swing.
3. The length of the unusable (or unused) bed (LUB).
4. The product purities.

The LUB is approximately one-half the span of the concentration wavefront,
or the mass transfer zone. The LUB is primarily determined by the equilibrium
isotherm (Yang, 1987). A sharp concentration front, or a short LUB, is desired
because it results in a high sorbent productivity as well as a high product purity.

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

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