Page 52 - Adsorbents - fundamentals and applications

TEMPERATURE SWING ADSORPTION AND PRESSURE SWING ADSORPTION 37

to achieve an enrichment ratio of 4000 (from 0.1 ppm in the feed air to 400 ppm
in the desorption product). The two basic ideas were reflux (or rinse) and parallel
pressure equalization (as discussed above). The first PSA was super-atmospheric,
and the second one was sub-atmospheric, both containing 13X zeolite.
As the environmental applications are becoming increasingly more important
for PSA, more studies are needed for the enrichment and recovery of sorbate. As
already seen, the fundamental as well as operational issues for enrichment are
different from those involved in purification and bulk separation and have not
been well understood.

Radial-Bed PSA. The development of radial-bed PSA is aimed at oxygen pro-
duction from air (Rota and Wankat, 1991; Poteau and Eteve, 1993; Chiang and
Hong, 1995a and 1995b; Smolarek et al., 1998). Due to the relatively linear
isotherms of nitrogen and oxygen on zeolites, O 2 -PSA has adopted the vacuum
swing mode since the 1980’s, that is, adsorption at approximately 1 atm with
desorption at 0.2–0.4 atm, depending on the sorbent (Rege and Yang, 1997).
With the invention/improvements of the sorbent (notably the invention of LiLSX
zeolite by Chao, 1989), the trend for O 2 -PSA has been to use shorter cycle times
and higher flow rates. Eventually the bed pressure drops and, to a lesser degree,
the pellet diffusion rates have become the limitation. As a consequence, shorter
beds are used. The development of annular beds, or radial flow beds, is a further
improvement of the shallow beds. In these beds, the sorbent is contained in an
annular layer. The feed air flows radially from the outside toward the inner part.
The reason for the feed from outside in the radial bed is to reduce the velocity
variation. As the nitrogen is adsorbed, the gas velocity reduces and the cross
sectional area in the radial bed is also reduced. In addition, it is possible that,
due to the increased interstitial flow velocity toward the center, the concentration
wavefront is sharpened, as the isotherm is of the favorable type (i.e., concave
shape) (Yang, 1987). This further helps the separation. Chiang’s invention was
unique in that very small particles (3 µm crystals) were used in thin annular beds
where pressure drop was exploited for the separation (Chiang and Hong, 1995a
and 1995b).
The radial-bed development will further improve O 2 -PSA. Significant (and
likely larger) improvements can also be achieved from the invention of new
sorbents. With the availability of better sorbents, nitrogen production from air
with zeolites could be competitive with molecular sieve carbon.

PSA Reactor. The idea of a PSA reactor was first suggested by Vaporciyan
and Kadlec (1987; 1989). The basic idea is to combine sorption and catalytic
reaction in order to shift the thermodynamic equilibrium of the reaction. The
sorbent selectively adsorbs one of the products and is regenerated during the low
pressure half-cycle. By doing so, the conversion is increased and simultaneous
separation is also accomplished. Vaporciyan and Kadlec (1989) demonstrated the
idea for CO oxidation reaction by mixing a 5A zeolite sorbent and a Pt/alumina
catalyst in the adsorber. This idea was pursued by Sircar and co-workers (Carvill