Page 311 - Adsorbents fundamentals and applications

296 SORBENTS FOR APPLICATIONS

Introducing only 1 Ag per unit cell can significantly improve the PSA sepa-
ration. Cost estimates have been made that showed that a significant gain can be
obtained by the use of Ag-containing Li-LSX zeolite.

10.1.5. Oxygen-Selective Sorbents

Since the N 2 /O 2 ratio in air is approximately 4, much less work is needed to
separate air by using an O 2 -selective sorbent. This is indeed practiced in industry
by kinetic separation (in a PSA cycle) using carbon molecular sieve (CMS)
(Yang, 1987; Coe, 1995). CMS has been widely used for nitrogen generation by
PSA (not VSA). 4A zeolite has also been used for nitrogen generation, mainly
for small-volume fuel tank blanketing. Oxygen-binding cobalt complexes have
been of long-standing interest and will be discussed here.

Carbon Molecular Sieves. Excellent carbon molecular sieve materials have
been developed in industry. On these sorbents, the O 2 ,N 2 , and Ar isotherms
are approximately equal because they all adsorb by van der Waals interactions,
and their polarizabilities are approximately the same. But the diffusivity ratio
for O 2 (Ar)/N 2 is approximately 30 due to differences in molecular size (Chen
et al., 1994).
Details of the CMS preparation and its adsorption equilibrium and kinetics for
air separation are given in Chapter 5 (5.7.2). PSA with CMS is widely used for N 2
production from air. The simple Skarstrom cycle (see Chapter 3) has been used,
and the typical feed pressure is 8 atm, whereas the desorption pressure is near
ambient. The maximum N 2 purity is 99.5% (Notaro et al., 1999). The adsorbent
inventory is well below 500 lb per ton/day of N 2 capacity, and the largest existing
single PSA unit is 100 tons/day for N 2 purity of 95% (Notaro et al., 1999). The
power consumption is around 300 kWh per ton N 2 . The low bed inventory as
well as the low energy requirement while using the rather inefficient PSA cycle
reflects the fact that air contains only 21% O 2 , which is the adsorbed component.

4A Zeolite. NaA (4A) zeolite has also been used in small volume, enriched
N 2 generation by PSA, for fuel tank blanketing of military aircrafts (particularly
helicopters). A high-pressure air is readily available from the pressurized engine
bleed or shaft-driven compressor air. The feed pressure is regulated to about
25 psig before feeding to the PSA unit. Fast PSA cycles, typically 2.5 s/cycle,
are used. These on-board units generate a gas containing less than 12% O 2 ,than
that which is the required limit for fuel tank blanketing. The N 2 enrichment is also
accomplished by kinetic separation due to faster diffusion of O 2 in 4A zeolite.

Oxygen-Binding Cobalt Complexes. Since the 1940’s, considerable work
has been devoted to the study of oxygen-binding transition metal complexes,
mostly in attempts to mimic important biological oxygen carriers, such as hemo-
globin and myoglobin. Because of their similarity to the natural heme proteins,
O 2 -binding iron complexes have received the most attention. However, it is