Page 58 - Adsorbents - fundamentals and applications

SIMPLE CRITERIA FOR SORBENT SELECTION 43

or in its expanded form (using the Langmuir isotherm):
b
q 1 q m 1 1
S = · (3.63)
b
q 2 q m 2 2
The above parameter can thus be used to compare the performance of two sor-
bents. The better sorbent will have a correspondingly higher value for S.
Returning to the AFM discussed before (Eq. 3.57), its similarity to the param-
eter S (Eq. 3.63) now becomes obvious. If the equilibrium adsorption data seem
to give a good fit to the Langmuir equation (as is usually the case for commonly
occurring gas separation applications), then the selectivity α 1,2 would not differ
significantly under adsorption and desorption conditions. As a result, α 2 /α des
ads
would reduce to simply α 1,2 , which is indeed the case with the parameter S
defined above. Also the N 2 (or q 1 ) factor is common to both the parameters,
the only difference is the addition of the parameter q 2 in the denominator. The
latter parameter serves to increase the sensitivity of the parameter and, more
importantly, makes it a non-dimensional value.

Test of Sorbent Selection Parameter (S). To demonstrate the usefulness
of the S parameter in comparing adsorbent performances, examples from pub-
lished PSA simulation results were used. Both of these examples deal with the
production of O 2 by separating it from N 2 present in atmospheric air. The two
pairs of sorbents to be considered for comparison are as follows: (1) LiX or
+
LiLSX (Si/Al = 1.0, 100% Li − exchange) and NaX (or 13X), and (2) LiX
+
(Si/Al = 1.0, 100% Li − exchange) and LiAgX (Si/Al = 1.0, 1.1 Ag per unit
+
cell and ∼94 Li per unit cell). These two pairs have been previously analyzed
+
for their performance for air separation by PSA using computer simulations of a
proven model by Rege and Yang (1997) and Hutson et al. (1999).
A five-step PSA cycle (i.e., a commercially used cycle) was used for both
groups of sorbents in this test. The steps involved in each cycle are as fol-
lows: (1) pressurization with the feed gas, namely 22% O 2 (mixture of O 2 with
Ar impurity included) and 78% N 2 ; (2) high-pressure adsorption with the feed
gas or feed step; (3) co-current depressurization; (4) countercurrent blowdown;
(5) countercurrent low-pressure purge with the product of the feed step (oxygen).
All the steps above involved equal duration (30 s). Thus the time required for
the completion of each PSA cycle was 2.5 min. The model assumed only two
adsorbable components, namely O 2 and N 2 . The less-strongly adsorbed species,
like Ar, were grouped with O 2 with the assumption that all contaminants in air
like CO 2 and water vapor were removed completely prior to feeding by using
pretreatment beds. The product of each cycle comprised a volumetric mixture of
the output stream of the feed step and the co-current depressurization step. This
product stream was partly used to purge the bed countercurrently in step (5). The
gas was fed to the PSA beds at 298 K.
The model used assumed an adiabatic bed, negligible pressure drop in bed
and axial dispersion. Further details about the simulation model used as well as