Page 206 - Adsorption Technology & Design, Elsevier (1998)

192 Selected adsorption processes

concentration of the most strongly adsorbed component in the bed. This is
achieved by removing the gas contained in the adsorbent voids which,
following the initial two steps, will have entrapped gas at the same
composition as the feed. The pressurization and feed steps, during which
adsorption occurs, are shortened in duration so that the cocurrent depres-
surization step can be initiated before breakthrough of components from the
bed. The bed is subsequently desorbed by blowdown and purge steps in the
cycle. The net benefit of cocurrent depressurization is increased purity of the
most strongly adsorbed component in the product which, in consequence,
enhances the recovery of the least strongly adsorbed component.

7.2.3 Pressure equalization

To help conserve expenditure of mechanical energy during a PSA cycle it
was suggested by Marsh et al. (1964) that two columns could be interconnec-
ted at a particular stage of the cycle so that the pressure energy contained by
the gas in a bed at high pressure could be shared with a bed which has been
subject to blowdown (and thus at a lower pressure) and which, as a result,
becomes partially pressurized in readiness for repressurization. Pressure
equalization steps enable gas separations to be realized economically on a
large scale. It is now common to include pressure equalization in a cycle
when four or more beds in series comprise a PSA unit (Berlin 1966, Wagner
1969). Benefits of pressure equalization include increased product recovery
and steadier continuous flow of the most strongly adsorbed component from
the unit.

7.3 COMMERCIAL PSA PROCESSES

A brief description of the more common PSA processes for drying,
purification and separation of gaseous components clarifies how cocurrent
depressurization and pressure equalization steps are incorporated into the
basic PSA cycle. A variety of different sequences of operation of adsorber
beds exists and the manner in which these steps are introduced depends on
particular circumstances such as plant utilities, throughput required and cost
of equipment and operation.

7,3.1 Drying of air (Skarstrom 1975)
The drying of air and other gaseous streams may be accomplished by
thermal swing processes. Pressure swing adsorption operation can, how-
ever, achieve even lower dewpoints than thermal swing operation and has