Page 203 - Adsorption Technology & Design, Elsevier (1998)

Selected adsorption processes 189

then the disadvantages of purely thermal swing processes are cir-
cumvented. The paths of thermal swing and pressure swing operations are
illustrated in Figure 5.14. Inert gas purge stripping is also illustrated in
Figure 5.14, but during this operation the temperature and total pressure
remain constant.
Regeneration of the adsorbent following adsorption can also be accomp-
lished by displacing the adsorbed component with a purge gas or a liquid
which is as strongly adsorbed as the adsorbable component of the feed. The
displacement fluid is subsequently separated from the extract by distillation.
Separation of linear paraffins of intermediate molecular mass from
branched chain and cyclic isomers is an example of a displacement purge
cycle, ammonia being the strongly adsorbed purge in the Ensorb process of
Exxon Corporation (see Section 7.6).
All of the processes alluded to above are fixed bed cyclic batch processes
providing a continuous flow of raffinate (the least strongly adsorbed
component) and extract (the more strongly adsorbed component). An
alternative method of separation of components by adsorption is to employ
continuous countercurrent systems (see Section 7.7) in which either the
adsorbent is circulated through the flowing feed stream or, by appropriate
manipulation of the flowing fluids, to simulate adsorbent circulation. An
example of the former methodology is the Hypersorption Process of Union
Oil Co. while an example of the latter method of operating is the Sorbex
Process of UOP (see Section 7.7.5).

7.2 PRESSURE SWING ADSORPTION (PSA) PROCESSES

7.2.1 The two-bed Skarstrom cycle
The simplest two-bed continuous pressure swing adsorption (PSA) process
was invented by Skarstrom (1960). Each bed acts alternately and sequen-
tially as an adsorber and a regenerator to complete one cycle of events. The
plant layout and pipework connections between the two columns is
illustrated in Figure 7.1 and the cycle is described by Figure 5.15 which
shows how each column is utilized during a single cycle. To illustrate the
operation, we suppose that each bed in Figure 7.1 contains a molecular sieve
zeolite adsorbent whose capacity for adsorbing nitrogen from air is greater
than its capacity for adsorbing the oxygen component of air. For the first step
column 1 is pressurized to several atmospheres with air while isolated from
column 2. During the second step of the cycle columns 1 and 2 are connected
and oxygen (which is the least strongly adsorbed component of air) together
with some nitrogen remaining unadsorbed issues from both columns;