Page 248 - Adsorption Technology & Design, Elsevier (1998)

Selected adsorption processes 225

Many industrial countercurrent fractionation processes for the separation
of components operate on the principle of either the three and four section
cascade or the simulated moving bed. A summary of simulated moving bed
and countercurrent fractionation processes is given in Table 5.1. With the
exception of the Hypersorption process all are presently operated com-
mercially.

7.7.5 Sorbex processes
A family of similar processes have been developed by UOP for a variety of
difficult industrial separations. These have been reviewed by Broughton et
al. (1970), de Rosset et al. (1981) and Broughton and Gembicki (1984). Each
of these operates on the same principle as the simulated moving bed system
previously described (see Figure 7.16). The generic title of the industrial
processes referred to is Sorbex. The configuration of the separation unit is,
however, based upon a single packed column rather than three or four
interconnected beds. Figure 5.13 shows how the single column is divided up
into sections into which fluid may be introduced or from which fluid may be
withdrawn by means of specially designed flow distributors. A pump sited
external to the column enables circulation of fluid from bottom to top of the
packed bed. Only four of twelve connections to the column are utilized at
any given time. Flows are switched by means of a rotary valve so that the
desorbent, extract, feed and raffinate connections are simultaneously
advanced by one bed section in the direction of fluid flow. Because of the
switching of connection points between rotary valve and column, flow varies
through the circulating pump, which must be capable of handling a steady
controlled flow at four different flow rates.
Choice of adsorbent material and desorbent fluid is crucial to the
economic viability of any particular Sorbex process. It is important that the
extract fluid consisting of the more strongly adsorbed component and
desorbent (A + D) and the raffinate product consisting of the least strongly
adsorbed component and desorbent (B + D) are capable of being separated
by downstream distillation. Ideally the separation factor for A and D, aAD,
should be equal to that for B and D, aao. However, such is rarely the case
although when D is adsorbed more strongly than the raffinate product but
less strongly than the extract product, specified product purities can
normally be achieved.
The Parex process for the separation of isomers of xylene is based on the
Sorbex configuration as is the Ebex process for the recovery of ethyl
benzene. Both these processes utilize cationic forms of X and Y zeolites with
toluene or p-diethylbenzene as desorbent in the Parex process and toluene
as desorbent for the Ebex process. The Molex process (also based on the