Page 99 - Adsorption Technology & Design, Elsevier (1998)

5

Processes and cycles

Adsorbent particles have a finite capacity for fluid phase molecules and
therefore extended contact with a feedstock will ultimately lead to the
creation of a thermodynamic equilibrium between the solid and fluid phases.
At this equilibrium condition the rates of adsorption and desorption are
equal and the net loading on the solid cannot increase further. It now
becomes necessary either to regenerate the adsorbent or to dispose of it. For
those applications in which it is economically favourable to regenerate the
adsorbent it is necessary to devise processes in which the regeneration
method can be incorporated.

5.1 FIXED AND MOVING BED PROCESSES

Vessels and columns which hold the adsorbent in a fixed position appear
initially to provide distinct advantages over their counterparts in which the
adsorbent is allowed to move. First, such equipment is simple and relatively
inexpensive to fabricate. Secondly, minimal attrition of adsorbent occurs
when it remains fixed in position, although it should be noted that attrition in
fixed bed processes which are subject to frequent changes of pressure and
flow direction still remains a practical industrial problem. However, despite
their simplicity, fixed beds have many disadvantages:

(1) As fluid is passed through a fixed bed of adsorbent the transfer of
adsorbate molecules from the feed to the solid initially occurs at the bed