Page 242 - Adsorption Technology & Design, Elsevier (1998)

Selected adsorption processes 219

Difficulties associated with circulating solids experienced with the discon-
tinued Hypersorption process (Berg 1946) for the recovery of ethylene from
a low molecular weight gas have been circumvented by using fluidized beds,
one as an adsorbing section and another as a desorbing section. However,
losses of efficiency due to the back mixing of solids may severely affect the
fractionation when selectivity is constrained by the feed components having
similar physical properties. Alternatives including pulsed beds, in which the
solid adsorbent is intermittently fluidized, have been reported (Calmon and
Gold 1979). Fluid flow is upward for fluidizing and solids transfer and
downward when the bed is stationary and contacted with the process fluid.
Siegell et al. (1986) and Bellows et al. (1986) reported the use of magnetically
stabilized fluidized beds in which back mixing of solids is substantially
reduced.

7.7.3 Simulated moving beds

All of the problems associated with moving solids can be avoided by
switching the duty of each of the four sections so that each section assumes
the role of its succeeding neighbour downstream (in the direction of fluid
flow) by the operation of pneumatic valves (Ruthven and Ching 1989). Each
of the four columns is connected in series as shown in the flow diagram of
Figure 7.16 but, instead of a countercurrent flow of solids, the solids in each
column remain as a stationary bed. Countercurrent operation is then
simulated by advancing the desorbent, extract, feed and raffinate ports by
one column at specified intervals.
To achieve separation of the components A and B in the feed, certain
conditions of flow must be observed. Defining ~'i as the product of the
equilibrium constant K~ for component i and the ratio of simulated solid
motion S to fluid flow L,

~ti-" Ki (S]L) = Ki ~T] ~-U]
(7.4)

where u~ and u are the simulated solids and interstitial fluid velocities, re-
spectively, and e is the bed voidage. The flow conditions which must be
observed are, for each bed section,

~'A < 1, ~'B < 1 for section 1
(7.5)
~'A > 1, ~'B < 1 for section 2
(7.6)
?'A > 1, ~'B < 1 for section 3
(7.7)
?'A > 1, ?'B > 1 for section 4
(7.8)