Page 132 - Adsorption Technology & Design, Elsevier (1998)

122 Processes and cycles

As a variable for changing thermodynamic potential, a change in temper-
ature is much more effective than a change in pressure. However, the
final choice of regeneration method(s) depends upon technical and
economic considerations. The most common methods are changes in
temperature (thermal swing adsorption) and changes in pressure
(pressure swing adsorption). The general advantages and disadvantages
of each method together with some process examples are shown in
Table 5.2.

5.7 REDUCTION IN PARTIAL PRESSURE

Figure 5.14(a) shows schematically the effect of partial pressure on
equilibrium loading for a Type I isotherm with a temperature, say 7'1.
Reducing the partial pressure from pl to p2 causes the equilibrium loading to
be reduced from ql to q2. There are two ways in which the partial pressure
can be lowered. First, the total pressure in the system can be reduced.
Secondly, an inert gas can be used to lower the partial pressure while
maintaining the total system pressure. In the majority of pressure swing
separations a combination of the two methods is employed. Use of a purge
fluid alone is unusual. Changes of pressure can be effected very much more
quickly than changes of temperature and thus cycle times of pressure swing
adsorption (PSA) processes are typically of the order of minutes, and for
rapid cycle systems of the order of seconds. The faster the cycle time the
lower is the size of the equipment and the inventory of adsorbent. Thus PSA
processes are attractive for bulk gas separations and purifications for those
situations in which the adsorptive forces are relatively weak. Some PSA
processes are based on kinetic effects rather than on equilibrium effects in
which case it can be important not to allow sufficient time for the gases in the
bed to reach thermodynamic equilibrium with the adsorbent.
PSA processes are often operated at low adsorbent loadings because
selectivity between gaseous components is often greatest in the Henry's Law
region. An example of this is the separation of air components on 5A zeolite.
It is desirable to operate PSA processes close to ambient temperature to
take advantage of the fact that for a given partial pressure the loading is
increased as the temperature is decreased. It is not usually worth attempting
to reduce the temperature to below ambient.
The basic PSA process uses two beds which operate 180 ~ out of phase with
each other in four steps as shown schematically in Figure 5.15. The steps are
(i) pressurization with feed, although the product could be used instead, (ii)
production at elevated pressure, (iii) countercurrent depressurization,
although this could be cocurrent, and (iv) purge with a fraction of the