INTRODUCTION
Although adsorption has been used as a physical-chemical process for many years, it is
only over the last four decades that the process has developed to a stage where it is now
a major industrial separation technique. In adsorption, molecules distribute themselves
between two phases, one of which is a solid whilst the other may be a liquid or a gas.
The only exception is in adsorption on to foams, a topic which is not considered in this
chapter.
Unlike absorption, in which solute molecules diffuse from the bulk of a gas phase to
the bulk of a liquid phase, in adsorption, molecules diffuse from the bulk of the fluid to
the surface of the solid adsorbent forming a distinct adsorbed phase.
Typically, gas adsorbers are used for removing trace components from gas mixtures.
The commonest example is the drying of gases in order to prevent corrosion, condensation
or an unwanted side reaction. For items as diverse as electronic instruments and biscuits,
sachets of adsorbent may be included in the packaging in order to keep the relative
humidity low. In processes using volatile solvents, it is necessary to guard against the
incidental loss of solvent carried away with the ventilating air and recovery may be
effected by passing the air through a packed bed of adsorbent.
Adsorption may be equally effective in removing trace components from a liquid phase
and may be used either to recover the component or simply to remove a noxious substance
from an industrial effluent.
Any potential application of adsorption has to be considered along with alternatives
such as distillation, absorption and liquid extraction. Each separation process exploits
a difference between a property of the components to be separated. In distillation, it
is volatility. In absorption, it is solubility. In extraction, it is a distribution coefficient.
Separation by adsorption depends on one component being more readily adsorbed than
another. The selection of a suitable process may also depend on the ease with which the
separated components can be recovered. Separating n- and iso-paraffins by distillation
requires a large number of stages because of the low relative volatility of the components.
It may be economic, however, to use a selective adsorbent which separates on the basis
of slight differences in mean molecular diameters, where for example, n- and iso-pentane
have diameters of 0.489 and 0.558 nm respectively. When an adsorbent with pore size of
0.5 nm is exposed to a mixture of the gases, the smaller molecules diffuse to the adsorbent
surface and are retained whilst the larger molecules are excluded. In another stage of the
process, the retained molecules are desorbed by reducing the total pressure or increasing
the temperature.-------
MULTICOMPONENT ADSORPTION
The three isotherms discussed, BET, (H–J based on Gibbs equation) and Polanyi’s
potential theory involve fundamentally different approaches to the problem. All have
been developed for gas–solid systems and none is satisfactory in all cases. Many workers
have attempted to improve these and have succeeded for particular systems. Adsorption
from gas mixtures may often be represented by a modified form of the single adsorbate
equation. The Langmuir equation, for example, has been applied to a mixture of n__
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