Page 8 - Adsorption Technology & Design, Elsevier (1998)

The development of adsorption technology 5

of molecular sieve carbons, which are less hydrophilic than zeolites, and can
therefore separate wet gaseous streams effectively.
Although the development of a whole range of laboratory synthetic zeolites,
stimulated by the researches of Barter, precipitated a rapid growth in
commercial pressure swing adsorption (PSA) processes (a selection of which
are described in Chapter 7), as a historical note it should be stated that the first
patents filed for such processes were due to Finlayson and Sharp (1932) and
Hasche and Dargan (1931). More than two decades elapsed before two
commercial processes for the separation of air, patented by Guerin de
Montgareuil and Domine (1964) and Skarstrom (1958), became the foundation
for pressure swing adsorption separation techniques on a commercial scale.
The essential difference between the earlier thermal swing processes (TSA),
and the pressure swing process (PSA) is in the method by which the adsorbent
is regenerated following adsorption of the most strongly adsorbed component
of a gaseous or liquid mixture. Increase in temperature of the adsorbent bed is
the driving force for desorption in TSA processes whereas reduction in total
pressure enables desorption in PSA processes. The rapid growth in the number
of patents for PSA processes shown in Figure 1.1 is testimony to the successful
commercialization of these processes. Their prominence is due principally to
the much shorter cycle times required for the PSA technique than TSA
methods. Thermal swing processes require cycle times of the order of hours on
account of the large thermal capacities of beds of adsorbent. Reduction in
pressure to achieve desorption may, on the other hand, be accomplished in
minutes rather than hours. Not all TSA processes can, however, be simply
transposed into PSA processes solely because of the difference in adsorbent
bed regeneration times. TSA processes are often a good choice when
components of a mixture are strongly adsorbed, and when a relatively small
change in temperature produces a large extent of desorption of the strongly
adsorbed species. PSA processes are more often adopted when a weakly
adsorbed component is required at high purity: furthermore, cycle times are
much shorter than in TSA processes and therefore greater throughputs are
possible utilizing PSA techniques.
TSA and PSA processes are, by virtue of the distinct adsorption and
regeneration components of the cycle, not continuous processes, although a
continuous flow of product may be achieved by careful design and bed
utilization. Moving bed and simulated moving bed processes are, however,
by their very nature truly continuous. Examples of these are given in
Chapter 7, but here it suffices to say that a number of continuous commercial
processes for the separation of aromatic mixtures, the separation of
n-paraffins from branched and cycloalkanes, the production of olefins
from olefin and paraffin mixtures and the isolation of fructose from corn
syrup, have been in operation since the early 1980s.