Membrane Gas Separation

11.3.4
Carbon Monoxide 
Carbon monoxide is present in post - combustion carbon capture as the product of partial 
oxidation due to reduced availability of oxygen, along with the reduced environment in 
iron smelting. Importantly, it is a major component of syngas and exists in signifi cant 
quantities due to the equilibrium nature of the water gas shift reaction [41] . Hence, there 
is considerable research into membrane reactors that can be incorporated into the water 
gas shift reaction to drive hydrogen and carbon dioxide production. The performance of 
CO in polymeric membranes is less documented, due to their limitations in processing 
syngas at high temperatures. The majority of polymeric membranes will favour CO 
2
over 
CO because the kinetic diameter of CO is greater [5] , hence a sieving mechanism will 
favour CO 
2
, while the critical temperature of CO is lower than CO 
2
.
11.3.5
Water 
In the majority of industrial processes where carbon dioxide capture can occur, the feed 
stream for separation is saturated with water vapour [42] . Therefore, competitive water 
sorption in the membrane, as well as plasticization and ageing effects, will have a much 
stronger infl uence on membrane performance compared to the previously mentioned 
Figure 11.8 N 
2
permeability through polysulfone, 55 ° C 600 kPa, along with the CO 
2
 /N 
2

selectivity [38]

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218
Membrane Gas Separation
Figure 11.9 Water permeability within polymeric membranes and selectivity relative to 
CO 
2
 . Reprinted with permission from Separation & Purifi cation Reviews, Effects of minor 
components in carbon dioxide capture using polymeric gas separation membranes by 
Scholes, C. A., S. E. Kentish, and G. W. Stevens, 38: 1 – 44, Copyright (2009) Taylor and 
Francis
components. The permeability of water for a range of polymeric membranes and selectiv-
ity with respect to CO 
2
is shown in Figure 11.9 . Water permeability is almost always 
greater than CO 
2
, in many cases substantially, because water has both a smaller kinetic 
diameter than CO 
2
, therefore diffuses faster, and also has a higher critical temperature, 
meaning greater sorption in the membrane. Hence, for membrane CO 
2
separation the 
permeate stream will generally have a higher water percentage than the feed. If the 
permeate temperature is below the dew point, i.e. supersaturated, there is the possibility 
of condensation formation, which will create an additional transfer layer for gases to cross, 
reducing performance. Membrane support structures can also be severely impacted by 
water condensation. Specifi cally, the capillary forces that act upon later evaporation of 
the water can cause porous supports to collapse irreversibly.
Plasticization due to water sorption has been observed, with a polyethersulfone mem-
brane experiencing a dramatic fl ux increase of
∼
250% upon exposure to water, with a 
corresponding decrease in selectivity [43] . This is also of critical concern in membrane 
processing since it can permanently alter the membrane structure, meaning performance 
does not always return once the membrane is dried [44] . 

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The Effects of Minor Components on the Gas Separation Performance
219
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
560
550
540
530
520
510
500
720
710
700
690
680
670