Page 127 - Adsorption Technology & Design, Elsevier (1998)

Processes and cycles 119

fully loaded bed at the feed end of the process is taken off-line and switched
to the outlet end of a parallel regeneration train. In this way the adsorbent
effectively moves countercurrent to the flow of fluids in both the adsorption
and regeneration steps. The perfect analogue with the truly continuous
countercurrent process exists when there is a very large number of beds but
the process advantages are gained only at the expense of a large number of
small beds with a complex system of pipes, valves and a control system. Most
of the advantages of this flow arrangement can be gained, however, with
relatively few beds in each train (Ruthven and Ching 1989). Some examples
of commercial processes are shown in Table 5.1.
In the second approach a single bed is used and the feed and product line
positions are changed periodically by means of valve arrangements. The
Sorbex simulated moving bed, shown schematically in Figure 5.13 (and
described more fully in Section 7.7.5), uses a relatively high boiling point
displacement fluid to regenerate the adsorbent. The feed, product and
displacement fluid lines are moved as a function of time by a complex rotary
valve. A pump is used to circulate the displacement fluid continuously from
the bottom to the top of the bed. The displacement fluid is separated
from the extract and raffinate products by distillation in separate units.
Several commercial applications of the Sorbex process exist as shown in
Table 5.1.

5.6 DESORPTION AND REGENERATION OF ADSORBENTS
In certain applications it may be economic to discard the adsorbent after use in
which case it may be necessary to describe it as a waste. Clearly the nature and
concentration of the adsorbates will dictate the disposal route to be followed.
Disposal would be favoured when the adsorbent is of low cost, is very difficult
to regenerate (perhaps because adsorbates are held by chemical forces) and the
non-adsorbed products of the adsorptive separation are of very high value. In
the majority of process applications, disposal of the adsorbent as a waste is not
an economic option and therefore regeneration is carried out either in situ or
external to the adsorption vessel to a extent sufficient that the adsorbent can be
reused. Practical methods of desorption and regeneration include one, or more
usually a combination, of the following:

(1) increase in temperature;
(2) reduction in partial pressure;
(3) reduction in concentration;
(4) purging with an inert fluid;
(5) displacement with a more strongly adsorbing species;
(6) change of chemical condition such as pH.