Page 290 - Pressure Swing Adsorption

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266 PRESSURE SWING ADSORPTION
EXTENSIONS OF THE PSA CONCEPT 267
Driven H&allng so:
Platon Halt-Cycle
(73)
Each cycle therefore leads to a net upward movement of the solute. as
illustrated in Figure 7.l(b). Over a large.number of cycles a very high degree
of separatton can be achieved between the two reservoirs.
The main drawback of oararnetnc pumping 1s that 1t was originally
env1s1oned and reduced to practice as a batch process. In fact, published
accounts of experimental attempts indicate that perhaps 50 or more cycles
2
Heating are necessary to ::.tpproach cyclic steady state. Pigford and co-workers at-
and tempted to remedy that by suggesting a similar process, which they called
Cooling Cooling "cycling zone adsorption." Their concept was to admit feed to the first of
Jacket Half-Cycle several zones (fixed beds of adsorbent) connected m senes, rather than to
dnve it back and forth through a single fixed bed, as in parametnc pumping.
Dr1vlng1--~;z:1
Platon They were able to relate the number of zones for continuous operation to the
corresponding number of batchwise cycles. Nevertheiess, to achieve a good
(a)
separation in a contmuous system requiring several fixed beds has smce been
shown to be .1ust as 1moract1cal as a batchwise system that reqmred enormous
ttme (i.e., several days).
It 1s often said that PSA separations arc similar to paramctnc pumping
HOT COLO HOT COLO with pressure rather than temperature as the controlling thermodyn::.tm1c
variable. However, the relationship between parametric pumping and a

12
vl-----

13
(b)
Figure 7.1 Thermal· parametriC pump showing (a) the principle of operatmn and
(b) the zigzag progressfon of the concentration profile m successive cycles.

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easily understood from equilibrium theory. The wave velocity is given by
(Ea. 2.50):
V (7.1)
,I + ((! - <)/<) dq* /de 16
15
If adsorption ts exothermic:
da*) (dq*) (7.2) Figure 7.2 A pressure swing parametnc pump (the "molecular gate"). 1-6, motor
(Tc hot < -Tc cold and drive; 7-11, adjustable stroke pistons; 13, adsorbent bed; 12,12', product drawoff
oomts; 15,16, feed and ore-drier. (From Keller and Kuo,:: with perm1ss1on.)