Membrane Gas Separation

7.4.4
Permeability and Selectivity 
From the solubility and diffusivity data collected in the sorption experiments the value 
for the permeability of the two penetrants in the different mixed matrices was calculated. 
If the solution - diffusion model is considered to hold true, and Fick ’ s law is suitable to 
represent the diffusive fl ux, the permeability can be calculated as the product of diffusivity 
and solubility coeffi cient. The calculated permeability is shown in Figures 7.8 a and b, 
where it is clear that the addition of fi ller increases the permeability of the membrane for 
both penetrants.
In Figure 7.8 c the ratio between the permeability in the composite and that in the pure 
polymer at infi nite dilution for the two penetrants is reported as calculated from Equation 
(7.18) , and clearly the addition of inorganic fi ller to the polymer enhances the permeability 
of the material. 
From the knowledge of the permeability isotherms, the behaviour of the ideal selectiv-
ity of the composite can thus be calculated; as shown in Figure 7.9 there is an increase 
in the permeability of the larger penetrant as the initial FFV increases in the membrane. 
The case of 25% of fi ller in AF 2400 is peculiar, since a change in the selectivity is 
observed: the membrane is n - butane selective at low activity and becomes n - pentane 
selective at higher activity values.

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Vapor Sorption and Diffusion in Mixed Matrices Based on Tefl on® AF 2400 
139
(a) 
10
-9
10
-8
10
-7
10
-6
2.90
2.95
3.00
3.05
3.10
3.15
D
M
0
t