Membrane Gas Separation

11.3.6
Hydrocarbons 
In natural gas sweetening, hundreds of heavy hydrocarbons, both paraffi nic and aromatic, 
are present in the feed gas [48] and can infl uence CO 
2
separation through polymeric 
membranes with a detrimental loss in performance. It is alleged that low quantities of 
condensable hydrocarbons can also cause pre - mature ageing and membrane failure [49] . 
The competitive sorption effects of hexane on CO 
2
permeability and selectivity against 
CH 
4
in the glassy polyimide membrane poly (4,4 
′
- hexafl uoroisopropylidene diphthalic 
anhydride - 2,3,5,6 - tetramethyl - 1,4 - phenylenediamine) (6FDA - TMPDA, a polyimide) is 
shown in Figures 11.15 and 11.16 respectively [50] .
There is a decrease in the permeability of both CO 
2
and CH 
4
as a result of exposure to 
hexane at different partial pressures (Figure 11.15 ) as the result of competitive sorption. 
The decrease is more pronounced for CO 
2
, because the solubility of this gas within the 
polymer is more strongly infl uenced by Langmuir adsorption. Indeed at the hexane con-
centrations used, almost all the carbon dioxide is displaced from the Langmuir voids by 
hexane with the permeability approaching that predicted by Henry ’ s Law sorption alone. 
The greater reduction in CO 
2
permeability results in a net loss of selectivity from 16 to 
around 10. 
The use of Equation (11.37) allows for an estimation of the b value for hexane, based 
on known dual sorption parameters [50] . The resulting value of 240
±
100 atm 
−
1
is two 
orders of magnitude greater than that available for CO 
2
(0.51 atm 
−
1
[51] ), and indicates 
that hexane has a much stronger affi nity for the micro - voids than CO 
2
. This is consistent 

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The Effects of Minor Components on the Gas Separation Performance
223
CO
2
Permeability (barrer)
CO
2
/CH
4
Selectivity
500
(a)
(b)
(c)
450
400
350
300
250
200
150
100
50
0
0
5
10
15
20
25