Ionic Liquid Membranes for Carbon Dioxide Separation
191
The CO
2
/H
2
selectivities for supported [hmim][Tf
2
N] decrease progressively with
increasing temperature, as shown in Figure 10.5 , as anticipated
for a solution diffusion
mechanism. Differences in the selectivity for varying CO concentrations are quite similar
ranging from 6.46 to 1.44, 6.20 to 1.30, 6.13 to 1.34, and 4.70 to 1.01 in the temperature
range 310
K to 423
K for the 10
ppm, 100
ppm, 500
ppm, and 5% CO, respectively.
The trends are strongly Arrhenius over the entire temperature range with coeffi cients of
determination of 0.998, 0.998, 0.999, and 0.999 for the 10 ppm, 100 ppm, 500 ppm, and
5% CO, respectively.
The membrane performance of nylon - supported [hmim][Tf
2
N] was measured in gas
mixtures with
varying concentrations of H
2
S, as shown in Figure 10.6 . The CO
2
perme-
ability for mixtures containing 10
ppm H
2
S ranged from 3.33
×
10
−
15
to 6.04
×
10
−
15
m
2
s
−
1
Pa
−
1
in the temperature range from 310 K to 423 K, a slight decrease compared with mixtures
not containing H
2
S. At H
2
S concentrations of 100 ppm, the CO
2
permeability
increased
from 3.62
×
10
−
15
to 5.58
×
10
−
15
m
2
s
−
1
Pa
−
1
in the temperature range from 310 K to 373 K,
closely matching CO
2
permeability without H
2
S. At 423 K, the CO
2
permeability in the
presence of 100 ppm H
2
S, 7.03
×
10
−
15
m
2
s
−
1
Pa
−
1
, was increased as compared to the per-
meability measured in the absence of H
2
S. In mixtures containing 0.9% H
2
S, the CO
2
permeability increased from 3.00
×
10
−
15
to 5.40
×
10
−
15
m
2
s
−
1
Pa
−
1
over the temperature
range 310 K to 423 K, a uniform decrease from the permeability
in mixtures not containing
H
2
S.
473 K
423 K
373 K
348 K
323 K 310 K
CO
2
/H
2
Selectivity
10
1
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
1000T
-1
/K
-1
Figure 10.5 Carbon dioxide selectivity of [hmim][Tf
2
N] on cross - linked nylon;
䊏
→
without CO;
䊐
→
5% CO;
䉫
→
10 ppm CO;
䊊
→
100 ppm CO;
Δ
→
500 ppm CO
192
Membrane Gas Separation
The CO
2
/H
2
selectivity for supported [hmim][Tf
2
N], as shown in Figure 10.7 , decreases
with increasing temperature as anticipated for a solution diffusion mechanism. The CO
2
/
H
2
selectivities
for varying H
2
S concentrations are quite similar ranging from 6.53 to 1.37,
6.71 to 1.33, 6.82 to 1.42, and 7.24 to 1.57 in the temperature range 310 K to 423 K for
H
2
S concentrations of 10 ppm, 100 ppm, 500 ppm, and 0.9%, respectively. The trends are
strongly Arrhenius over the entire temperature range with coeffi cients of determination
of 0.999, 0.995, 0.996, and 0.999 for H
2
S concentrations of 10 ppm, 100 ppm, 500 ppm,
and 0.9%, respectively.
The membrane performance of [H
2
NC
3
H
6
mim][Tf
2
N], which
contains a primary amine
functionality, was also examined and the results shown in Figure
10.8
. The CO
2
permeability exhibits Arrhenius behaviour with coeffi cient of determination 0.998 in the
temperature range from 310 K to 368 K. Little change is observed in permeability from
368 K to 448 K. The CO
2
/H
2
selectivity for supported [H
2
NC
3
H
6
mim][Tf
2
N]
increases
from 9.5 to 13.8 in the temperature range from 310 K to 348 K then decreases from 10.5
to 3.11 in the range from 373 K to 423 K, creating a selectivity maximum. Ionic liquids
which contain primary amines such as [H
2
NC
3
H
6
mim][Tf
2
N] are thought to form chemi-
cal complexes with CO
2
[21] . It has been previously suggested that the observed trends
in permeability and selectivity may be explained by a change in
rate - limiting step from
decomplexation of the CO
2
at low temperature to diffusion of complexed CO
2
at high
temperatures [20] .
The CO
2
permeability of supported [H
2
NC
3
H
6
mim][Tf
2
N] in the presence of 5% CO
shows an Arrhenius behaviour as it increases from 2.84
×
10
−
15
to 5.85
×
10
−
15
m
2
s
−
1
Pa
−
1
in the temperature range from 310 K to 423 K with a coeffi cient of determination of 0.971,
2.0
2.2
2.4
2.6
2.8
3.0
3.2
3.4
CO
2
Permeability/m
2
s
-1
Pa
-1
1000T
-1
/K
-1
423 K
373 K
348 K
323 K 310 K
473 K
1.00E-14
1.00E-15
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