Membrane Gas Separation

11.3
Minor Components 
11.3.1
Sulfur Oxides 
Sulfur oxides, a combination of SO 
2
and SO 
3
, are generated by combustion of sulfur -
containing fuels such as coal [25] . Their emission is a major environmental problem, 
Figure 11.1 Langmuir affi nity constants ( b ) for a range of gases in various polymeric 
membranes relative to gas critical temperature. 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

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The Effects of Minor Components on the Gas Separation Performance
211
Figure 11.2 SO 
2
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
being the major cause of acid rain. A large range of non - porous polymeric membranes 
have been tested for SO 
2
separation, with the permeability of SO 
2
relative to SO 
2
/CO 
2
selectivity for a range of membranes with potential in CO 
2
capture shown in Figure 11.2 . 
The selectivity almost always favours SO 
2
, implying that it permeates faster than CO 
2
and therefore enriches the permeate stream. This is due to the higher condensability of 
SO 
2
within the membrane, due to a larger critical temperature compared to CO 
2
, given 
that the kinetic diameter difference does not favour diffusion selectivity.
Sulfur dioxide has also been reported to plasticize polymeric membranes, which pro-
duces a more rubbery material and increases the diffusivity of penetrant gases [26 – 28] . 
Plasticization also reduces the mechanical integrity of the membrane, meaning it is more 
likely the membrane will rupture. However, plasticization is a strongly pressure dependent 
phenomenon, for example it has been reported in polyvinylidene membranes to occur at 
SO 
2
pressures greater than 10 psi [29] . For many of the processes in carbon capture, such 
high partial pressures of SO 
2
are not observed (Table 11.1 ), and therefore only minor 
plasticization by SO 
2
is likely to occur. 
A critical factor for membranes and SOx is the presence of high water vapour in 
many industrial processes. The mixture of SOx and water allows for the generation of 
sulfuric acid, which can chemically degrade the polymeric membrane, its support or the 

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212
Membrane Gas Separation
Figure 11.3 H 
2
 S permeability within polymeric membranes and selectivity relative to CO 
2

surrounding module structure. This has been observed for cellafan, a polysaccharide -
based polymer, where SO 
2
permeability experiences a 30 - fold increase with water present, 
undoubtedly due to sulfuric acid degradation of the polymeric matrix [30] .