Page 50 - Adsorbents fundamentals and applications

TEMPERATURE SWING ADSORPTION AND PRESSURE SWING ADSORPTION 35

Rudelstorfer, 1976; Yang, 1987). In the Polybed PSA, typically 9 to 10 beds are
used. This has been practiced for hydrogen purification, where product recovery
is important. For air separation, however, oxygen recovery is not as important
(versus increased costs from more beds), and no polybed PSA is used. In some
cases, the EQ step is accomplished by connecting the CD effluent to the mid-point
of another bed that is to be re-pressurized. This can also alleviate the problem
with the “reversed” concentration profile.
Still another clever method to alleviate the “wrong” concentration profile prob-
lem of the EQ step is the use of “parallel equalization,” (PEQ) by Hirose and
co-workers (Yoshida et al., 1998; 2000). This was done for the enrichment of Xe
from air, in the second-stage PSA, where five parallel connections were made
between the two beds. By doing so, the Xe is more enriched in the discharge
end of the bed and consequently a more enriched Xe can be produced.

Pretreatment Beds. Water vapor and carbon dioxide, both very strongly
adsorbed on zeolites, created a major problem in the separations of air as well as
other mixtures when using zeolites. These gases are not easily desorbed and hence
tend to accumulate in the bed, leading to a stoppage in operation. The problem
can be solved by using a separate pretreatment bed outside the PSA system,
which must be regenerated separately from the PSA. For process simplicity, it
is desirable to include the pretreatment bed(s) within the PSA system so the
integrated system can be regenerated as a whole.
The idea of integrating the pretreatment beds into the PSA system was first
suggested by Heinze (1962), and improved by Tamura (1967), mainly by allowing
a flow in the adsorption step. The sorbent in the pretreatment bed should be
different from that in the main bed since a relatively inefficient sorbent is used
in the former, unlike the efficient sorbent used in the main bed. Although both of
the aforementioned processes were commercialized for oxygen production (Lee
and Stahl, 1973), a more efficient process was later developed by Sircar and
Zondlo (1977). In the latter process, a desiccant such as silica gel or alumina is
placed in the pretreatment bed, while the main bed contains a zeolite. By this
arrangement, water vapor and carbon dioxide are adsorbed in the pretreatment
beds, which are easily regenerated along with the main beds by pressure swing.
An additional advantage of using small pretreatment beds is that the temperature
fluctuations in the smaller beds are less severe.
The idea of integrating the pretreatment beds in PSA has also been used in
hydrogen purification, in which an inefficient sorbent in the pretreatment beds
adsorbs pentane and heavier compounds, whereas the main beds adsorb methane,
ethane, and propane (Alexis, 1967). This idea should be useful in many multi-
component separations.
The pretreatment bed can be combined with the main bed; that is, two sor-
bents are packed in the same bed with the desiccant (alumina or silica gel)
placed near the feed end followed by the zeolite bed. This arrangement is widely
used in the production of oxygen from air (Armond, 1979), as well as other
separations.