The scale and complexity of an adsorption unit varies from a laboratory chromatographic
column a few millimeters in diameter, as used for analysis, to a fluidised bed several
metres in diameter, used for the recovery of solvent vapours, from a simple container
in which an adsorbent and a liquid to be clarified are mixed, to a highly-automated
moving-bed of solids in plug-flow.
All such units have one feature in common in that in all cases the adsorbent becomes
saturated as the operation proceeds. For continuous operation, the spent adsorbent must
be removed and replaced periodically and, since it is usually an expensive commodity, it
must be regenerated, and restored as far as possible to its original condition.
In most systems, regeneration is carried out by heating the spent adsorbent in a suitable
atmosphere. For some applications, regeneration at a reduced pressure without increasing
the temperature is becoming increasingly common. The precise way in which adsorption
and regeneration are achieved depends on the phases involved and the type of fluid–solid
contacting employed. It is convenient to distinguish three types of contacting:
(a) Those in which the adsorbent and containing vessel are fixed whilst the inlet and
outlet positions for process and regenerating streams are moved when the adsorbent
becomes saturated. The fixed bed adsorber is an example of this arrangement. If
continuous operation is required, the unit must consist of at least two beds, one of
which is on-line whilst the other is being regenerated.
(b) Those in which the containing vessel is fixed, though the adsorbent moves with
respect to it. Fresh adsorbent is fed in and spent adsorbent removed for regeneration
at such a rate as to confine the adsorption within the vessel. This type of arrangement
includes fluidised beds and moving beds with solids in plug flow.
(c) Those in which the adsorbent is fixed relative to the containing vessel which moves
relative to fixed inlet and outlet positions for process and regenerating fluids. The
rotary-bed adsorber is an example of such a unit
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