Page 202 - Adsorption Technology & Design, Elsevier (1998)

188 Selected adsorption processes

Most commercial processes for drying and separation of gases utilize two
or more packed adsorbent beds. The simplest of such arrangements is two
packed beds, one acting as an adsorbent bed while the other (having already
been exposed to the gas stream as an adsorbent bed) acts as a regenerator.
The role of each bed is then reversed, the adsorber being regenerated while
the freshly regenerated bed becomes the adsorber. The cycle is then
repeated at predetermined intervals. Although each bed is being operated
batchwise, a continuous flow of feed and product is achieved which reaches a
steady state following a number of cycles of operation. The length of a cycle
depends primarily on whether each adsorbent bed is regenerated by raising
its temperature (thermal swing) or lowering its pressure (pressure swing).
The choice between thermal and pressure swing modes of operation is
largely dictated by economics although other technical factors are of
importance. Because adsorption is an exothermic process and strongly
adsorbed species have relatively high heats of adsorption, a small increase in
temperature is capable of reducing the bed loading of strongly adsorbed
components by large amounts. This means that the desorbate can be
recovered at high concentration. Heat losses from beds of adsorbents
militate against high efficiency and the large thermal capacity of an
adsorbent bed translates into relatively long times for heating and cooling,
thus contributing to lengthy cycle times. The most convenient way of raising
the temperature of the bed to be regenerated is by purging the bed with a
preheated gaseous stream. Availability of low grade steam or waste heat at
an adjacent plant location would be one factor favouring the choice of
thermal swing operation. On the other hand pressure swing operation would
be preferred when a relatively weakly adsorbed component of an adsorbable
mixture is required as a high purity product. Furthermore, the adsorbent is
used efficiently in pressure swing operations and the cycle times are
considerably reduced below those needed for thermal swing operations. The
desorbed components of the initial mixture fed to a pressure swing unit are,
however, only recovered at relatively low purities. Figure 5.14 illustrates the
difference between thermal and pressure swing operations. It should be
noted that mechanical energy is expended during pressure swing operations
whereas thermal energy, being cheaper than mechanical energy, is utilized
during thermal swing operations.
Another method of adsorbent regeneration is known as purge stripping.
An inert gas purge removes adsorbate from the bed without change of
temperature or pressure. Inert purge stripping is uncommon in practice
because it is only applicable to rather weakly adsorbed components. A
combination of inert purge and thermal swing operations, however,
facilitates desorption of more strongly adsorbed components. If the increase
in bed temperature is relatively small when an inert purge is employed,