Page 296 - Adsorbents fundamentals and applications

AIR SEPARATION 281

the finding of N 2 /O 2 selectivity by zeolites, no effort was made to separate air
by adsorption until the 1960’s, after the invention of synthetic zeolites types A
and X, and the pressure-swing adsorption (PSA) cycles.
The invention of types A and X zeolites by Milton (1959) made it possible
for these zeolites to be available at controlled quality and guaranteed supply. The
inventions of PSA cycles by Skarstrom (1960) and by Guerin de Montgareuil and
Domine (1964) have been discussed in detail by Yang (1987). Inspired by these
inventions, dreamers in industry began contemplating the possibility of separating
air at ambient temperature (as opposed to 77 K for cryogenic processes) by
adsorption. 5A (CaA) and 13X (NaX) zeolites were used (and are still being used
in some instances) as the sorbents. The PSA technology development encountered
some challenges that were unexpected from small-diameter laboratory column
experiments, for example, the “cold spot” problem (large temperature depression
near the feed end, Collins, 1977; Yang, 1987) and the “creeping death” of beds
(due to accumulation and freezing of water). The history of the PSA technology
development may be reflected by the decline in the cost of O 2 from PSA as
shown in Figure 10.1.
Prior to ca. 1980, PSA systems were used with both adsorption and desorption
pressures well above atmospheric. These systems were low in capital (due to
simplicity) but high in power consumption (since both N 2 and O 2 in the feed
are compressed compared with vacuum swing adsorption (VSA) where only the
waste gas is evacuated). The availability of improved sorbents and lower cost
vacuum equipment led to the development of VSA, which is typically operated
with adsorption pressure slightly above atmospheric and desorption pressure of
typically 0.2 atm. A further breakthrough occurred in 1989, with the invention
of LiLSX zeolite (low silica X, with Si/Al = 1) (Chao, 1989). The LiLSX is
currently the best commercial sorbent for air separation and will be discussed

100

80
Product cost % of ’72 60 Introduction of VPSA

(LiLSX zeolite)
40

20

0
1972 1976 1980 1984 1988 1992 1996 2000
Year
Figure 10.1. PSA oxygen product cost (in constant dollars). 5A and 13× zeolites were used
before 1990, and LiLSX zeolite is used after 1990 (courtesy of J. P. Kingsley of Praxair, Inc.).