Membrane Gas Separation

Figure 11.11 PDMS permeability of N 
2
upon exposure to wet mixed gas at different 
water partial pressures. Reprinted with permission from Journal of Membrane Science, The 
effect of hydrogen sulfi de, carbon monoxide and water on the performance of a PDMS 
membrane in carbon dioxide/nitrogen separation by Colin Scholes, Sandra Kentish and 
Geoff Stevens, 350, 1 – 2, 189 – 199. Copyright (2010) Elsevier Ltd
Permeability (barrer)
Permeability (barrer)
0
1000 1500 2000
500
Water Partial Pressure (Pa)
2500
0
1000 1500 2000
500
Water Partial Pressure (Pa)
2500
35°C
55°C
2000
1980
1960
1940
1920
1900
1800
1860
1840
1820
1800
2240
2220
2200
2180
2140
2160
2100
2120
2080
2060
2040
2020
2000
1980
Figure 11.10 PDMS permeability of CO 
2
upon exposure to wet mixed gas at different 
water partial pressures. Reprinted with permission from Journal of Membrane Science, The 
effect of hydrogen sulfi de, carbon monoxide and water on the performance of a PDMS 
membrane in carbon dioxide/nitrogen separation by Colin Scholes, Sandra Kentish and 
Geoff Stevens, 350, 1 – 2, 189 – 199, Copyright (2010) Elsevier Ltd
11.3.5.1
Water Competitive Sorption into Polydimethylsiloxane (PDMS) 
Figures 11.10 and 11.11 show the results of our recent work where the permeability 
of the N 
2
/CO 
2
mixture through PDMS was recorded upon exposure to different partial 
pressures of H 
2
O.

---

220
Membrane Gas Separation
CO
2
Permeability (barrer)
3.5
0.7 kPa
Water Partial
Pressure
Wet Feed Gas
0.85 kPa
1.18 kPa
1.61 kPa
2.17 kPa
3.0
2.5
2.0
0
50
100
150
Time (min)
200
250
300
350
Figure 11.12 Polysulfone CO 
2
permeability from a 90% N 
2
– 10% CO 
2
gas mixture upon 
exposure to wet feed gas at 35 ° C and 800 kPa [45]
Upon exposure to the wet feed, the permeability of both CO 
2
and N 
2
reduces, due to 
competition from water, with a trend of low permeability with increased humidity of the 
feed.
11.3.5.2 Water Competitive Sorption into Polysulfone and Matrimid 5218 
The infl uence of water on glassy polymeric membrane performance is important in many 
applications. The effect on the glassy polymeric membranes polysulfone and Matrimid 
5218 is shown in Figures 11.12 and 11.13 respectively, taken from a 90% CH 
4
– 10% CO 
2
mixture, to simulate natural gas processing.
The change in CO 
2
permeability upon initial exposure to wet feed gas for both poly-
sulfone and Matrimid is indicative of competitive sorption of water within the glassy 
membrane reducing the fl ux of CO 
2
. For Matrimid this behaviour is clearly dependent on 
the relative humidity of the feed gas and indicates that the amount of water within the 
microvoids is dependent on the relative humidity. Polysulfone does not display the same 
behaviour, with CO 
2
permeability reduction most signifi cant for the lowest water pressure. 
However, it is clear from the increase in permeability at latter time for polysulfone that 
plasticization is occurring, and for some water pressures, the plasticization is of suffi cient 
magnitude to overcome any loss in permeability due to competitive sorption. A similar 
effect is observed for CH 
4
permeability (Figure 11.14 ), with the initial reduction in per-
meability due to com 
petitive sorption, followed by plasticization. The correlation of 
selectivity demonstrates the signifi cance of plasticization on performance. Initially, when 
only competitive sorption occurs, the selectivity falls; however, as plasticization of 

---

Figure 11.14 Polysulfone CH 
4
permeability with water present as a function of time. 
Data at 55 ° C and 800 kPa [45]
Wet Feed Gas
Increasing
Relative Humidity
CO
2
Permeability (barrer)
10.5
10.0
9.5
9.0
8.5
8.0
7.5
7.0
0
50
100
150
Time (min)
200
250
300
Figure 11.13 Matrimid CO 
2
permeability from a 90% N 
2
– 10% CO 
2
gas mixture upon 
exposure to wet feed gas at 35 ° C and 800 kPa [45]

---

222
Membrane Gas Separation
polysulfone occurs, the selectivity rises and becomes greater than the non - plasticized 
membrane.
In the case of water, the hydrogen bonding potential between molecules allows for 
multilayer adsorption to occur within the microvoids of the membrane and clustering 
of water molecules has been observed for a number of polymers. Hence, additional 
adsorption is occurring and the Langmuir model presented as Equations (11.29) to (11.37)
may not be appropriate. Park [46] puts forward a simple model for the concentration of 
water within the glassy membrane when clustering occurs:
C
K f
bf
bf
K K
f
nf
n
n
=
+ ′
+
+
D
H
C
D
C
1
0
(11.38)
where n is the number of water molecules within the cluster, K
C
the equilibrium constant 
for the reaction between monomeric water and cluster water and f
0
the saturation fugacity. 
Alternatively, a multi - layer Brunauer – Emmett – Teller (BET) model may be a more appro-
priate isotherm to apply [47] :
C
K f
C b
f
f
n
f
nf
b
f
b
f
n
n
n
=
+
′
−
⎛
⎝⎜
⎞
⎠⎟
− +
(
)
+
+
−
(
)
−
+
+
D
H BET
BET
BET
1
1
1
1
1
1
1
⎛⎛
⎝⎜
⎞
⎠⎟
(11.39)
The parameter n can be correlated with the number of multilayers formed. If n is unity 
the isotherm converts to the Langmuir model, and if n is infi nity it becomes the standard 
BET isotherm.