Page 308 - Adsorbents fundamentals and applications

AIR SEPARATION 293

25
N adsorption at 25˚C (a)
2
B B
B J (b) J
B
Amount adsorbed (molec/uc) 15 B J H H J B H H H H H H H H
20
B
B
B
J
J
B
J
(c)
JJ J J J J J J JJ J J J
J
B
H
10
5
B B H F F (d) F
H F
F F F
0 F F F F
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Pressure (atm)
◦
Figure 10.11. N 2 adsorption isotherms (in molecules/unit cell) at 25 C, for (a) Ag-LSX-450
◦
◦
(treated at 450 C in vacuo), (b) Ag-LSX-350 (treated at 350 C in vacuo), (c) Ag-X, and
◦
(d) Ag-LSX treated at 100 C (Hutson and Yang, 2000a; with permission).
cell is limited. As seen from Table 10.3, with 2 Ag per unit cell, 0.9 Ag was at
site II*, whereas only 1.7 Ag were at this site in the sample with 41.8 Ag per
unit cell.
Figure 10.11 shows the N 2 isotherms on Ag-LSX heat-treated at 350 and
◦
450 C. A significant increase in N 2 adsorption was seen as the heat-treatment
◦
temperature was raised from 350 to 450 C. This increase in N 2 adsorption was
caused by the Ag at site II*, which interacted more strongly with N 2 .
+
The N 2 isotherms on mixed Li x Ag y -LSX zeolites dehydrated in vacuo at
◦
450 C are given in Figure 10.12. The results showed that the addition of increas-
ing amounts of Ag resulted in a change in the general aspect of isotherm toward
that of the nearly fully Ag -exchanged zeolite. For practical applications in
+
VSA/PSA cycles, the relative linearity of the isotherms given by zeolites with
1-3 Ag per unit cell is desirable.
+
The isosteric heats of adsorption of N 2 on almost fully Li and Ag exchanged
LSX are shown in Figure 10.13, along with that of LSX with 1.1 Ag and 94.2
Li. The first N 2 molecule apparently adsorbed on Ag + at SII* with higher
interactions than with Li + and Ag + at other sites. The results with mixed
Li,Ag-LSX zeolites clearly pointed to the potential advantage of using the mixed
form with only 1-3 Ag/unit cell for air separation. In Figure 10.14, the N 2 /O 2
isotherms are compared for Li-LSX and that with about 1% Ag cation. The
◦
amount of N 2 adsorbed at 25 C and 1 atm was significantly increased with the
addition of 1.1 Ag/u.c. The increased amount was approximately 12%. The O 2
isotherms were both very low and hence did not significantly impact the VSA
performance.