Page 238 - Adsorption Technology & Design, Elsevier (1998)

216 Selected adsorption processes

than that of the feed is no longer necessary. A fraction of raffinate (the fluid
from which the undesired component has been removed) can also be used
for regeneration because now CF < CE. The flow diagram and appropriate
McCabe-Thiele diagram for such a system is sketched in Figure 7.14 from
which it may be seen that the raffinate has been stripped of the contaminant
in the feed. Introducing the feed at the desorption section, however, results
in the extract (the fluid containing the undesired component emerging from
the desorption section) becoming enriched with the adsorbate.

7.7.2 Adsorptive fractionation
The principles outlined in Section 7.7.1 may be applied to the separa-
tion of components from a mixed feed of A (the more strongly adsorbed
component) and B (the least strongly adsorbed component). By dividing
the column, through which the solid adsorbent flows downward counter-
current to the upward flowing fluid, into a cascade of operating sections
and relying on displacement desorption by a third inert fluid D (the
desorbent), two products (E the extract and R the raffinate) are gen-
erated. The extract will contain a preponderance of A, the more strongly
adsorbed component, while the raffinate will consist mainly of B, the
least strongly adsorbed component. There is a formal similarity between
adsorption fractionation and distillation: introducing a binary component
feed F near the middle of the column, a raffinate product enriched in B
may be obtained from the uppermost section of the column and an ex-
tract product enriched in A from the lower section of the column. The
principle of operation may be gleaned from Figure 7.15. Two McCabe
-Thiele diagrams are necessary to represent the system, one with the
equilibrium line for component A and the other for component B.
Flowrates in each of the four sections into which the column is divided
are adjusted so that desorption of A and B occurs in the column sections
1 and 2 while adsorption of A and B occurs in sections 3 and 4. The net
flow of A should be downwards in sections 1, 2 and 3 and upward in
section 4 so that A is moving continuously to the extract port. Similarly
the net flow of B should be upwards in sections 2, 3 and 4 and downward
in section 1 so that B moves continuously toward the raffinate port. Feed
F is introduced between sections 2 and 3 and the desorbent D and solids S
circulated in opposite directions, D upwards and S downwards. The oper-
ating lines on the McCabe-Thiele diagrams corresponding to each section
are shown in Figure 7.15. Note that the operating lines may cross the
equilibrium line only at the feed point and effluent (extract and raffinate
products) points. The resultant operating lines thus form a hexagon.
Slopes of each operating line are determined by relative flowrates.