Environmental Engineering Reference
In-Depth Information
table 21.1
selected physical and spectroscopic data for branched pei-sWNts
CO
2
absorption at
75°C (%)
CO
2
efficiency, mol
of CO
2
mol
-1
of pEI
pEI (
M
W
) (Da)
pEI:C
SWNT
Raman D:G
600
1:155
0.448
4.1
0.0217
1,800
1:405
0.291
4.8
0.0827
10,000
1:1910
0.273
5.2
0.143
25,000
1:2065
0.273
7.2
0.159
Reprinted with permission from Ref. [51]. © 2008, American Chemical Society.
5
4
3
2
20
30
40 50
Temperature (°C)
60
70
80
fiGure 21.13
A plot of CO
2
adsorption efficiency as a function of temperature for pEI(10000)-SWNT. Reprinted with permission from
Ref. [51]. © 2008, American Chemical Society.
21.3.1.2.3 Effect of Moisture
Due to the presence of moisture in the flue gas streams, it is important to evaluate its effect on
the adsorption capacity. Water vapor plays a significant role in the CO
2
adsorption reaction. It may inhibit the CO
2
adsorption
capacity of hydrophilic materials [52] while increasing that of hydrophobic materials, for example, CNTs. Su et al. reported that
the adsorption capacity (
q
e
) of CNT(ApTS) increased from 85.7 to 108 mg g
-1
sorbent as the water vapor increased from 0 to
2.2%, but decreased from 108 to 87.5 mg g
-1
sorbent as the water vapor further increased from 2.2 to 8.9% (Fig. 21.14). Similar
results were reported by Ye et al. [49] (Fig. 21.15). This could be explained as a further increase in water vapor led to competitive
adsorption between CO
2
and H
2
O at the same sorption sites.
This improvement in the adsorption of CO
2
in the presence of moisture may be due to several reasons. First, the surface of
CNT(ApTS) contains a primary amine (RNH
2
), which can react with CO
2
and, in the absence of water, lead to the formation
of a carbamate ion by reactions 21.3 and 21.4. The presence of water then regenerates amine molecules by reactions 21.5
and 21.6 [53].
+
−
CO
+
2
RNHR RNHCOO
�
ΝΗ
+
(21.3)
2
2
3
+
−
CO RNHR RNCOO
2
+
2
ΝΗ
+
(21.4)
2
2
2
2
−
−
RNHCOO
+
ΗΟ ΝΗ
2
R CO
+
(21.5)
2
3
−
−
RNCOO
+
ΗΟ
R
ΝΗ
+
HCO
(21.6)
2
2
2
2
3
Second, the amine groups can also directly react with CO
2
and water to form a bicarbonate ion ( HCO
−
), as shown in reactions
21.7 and 21.8.
