Page 235 - Adsorption Technology & Design, Elsevier (1998)

214 Selected adsorption processes

vaporized organic solvents from air, the removal of organic compounds
from waste water streams, ion exchange processes and sugar decolouriza-
tion. Although the principle inherent in all of these processes is the
adsorption, and hence removal, of an undesirable component from a
process fluid which itself is not adsorbed, the mode of operation varies from
one process to another. For example, the regeneration of the adsorbent may
be by means of heat application or by a purge at a temperature close to that
at which the adsorption occurs. In solvent recovery processes, steam
stripping is a common procedure for regeneration. The mode of solids
conveying also varies from one process to another. The Union Carbide
Purasiv process for the recovery of organic solvents from gaseous streams
(Keller 1983) is illustrated in Figure 5.9.
The operation of simple countercurrent flow adsorption systems can be
represented by a McCabe-Thiele diagram. Figure 7.13 shows the operating
lines for adsorption and desorption and a single straight equilibrium line
(assuming adsorption and desorption occur at the same temperature). The
flow diagram for the process corresponding to the McCabe-Thiele diagram
is juxtaposed. The operating lines are defined by mass balances over the
adsorption and desorption columns. For adsorption
(7.2)
S (q - qF) = F (c - CF)
and for desorption

(7.3)
S (q- qD) = D (c-CD)
S, F and D represent the flowrates per unit cross section of the adsorbed phase,
feed and desorbent, respectively, cr and co are the concentrations of the single
component adsorbate in the feed and desorbing fluid, respectively, and qF and
qD are the corresponding adsorbed phase concentrations. The net flow of the
adsorbate must be downwards in the adsorption section and upwards in the
desorption section in order to achieve removal of the undesired component by
the downward flowing adsorbent and regeneration of the contaminated solid
by the fluid used for desorption. This implies F < SK and D > SK, where K is
the adsorptive equilibrium constant. The desorbent flow rate is therefore
greater than that of the circulating adsorbent. In order that mass transfer is in
the directions required during adsorption and desorption then CE < CF and co
< CR where subscripts E and R refer to extract and raffinate. Such operations
are, in general, uneconomic, and only suitable when there is a plentiful supply
of cheap purge fluid.
If regeneration of the adsorbent is achieved at a higher temperature than
used for adsorption and without the introduction of an inert purge, the
equilibrium line corresponding to desorption will lie below the equilibrium
line for adsorption and the requirement that the flow of desorbent be greater