Membrane Gas Separation

partition coeffi cient of vapour between the gaseous phase and the polymer, l is the semi -
thickness of the membrane, while q 
n
  are the positive, non - zero solutions of the equation: 
t
g
( q 
n
  ) =
−
 
α
 q 
n
  . By best fi tting to Equation (7.19) the experimental data of mass uptake 
versus time, one obtains the average diffusivity value within the concentration interval 
inspected in the differential sorption step. This apparatus was used to perform experiments 
with pure polymer and mixed matrices based on Tefl on 
® 
AF 2400 and also to measure 
adsorption onto pure fumed silica particles.
7.4
Results and Discussion 
7.4.1
Vapour Sorption in Mixed Matrices Based on AF 2400 
The solubility isotherms obtained for n - butane and n - pentane in mixed matrices of AF 
2400 are shown in Figure 7.4 , as grams of penetrant per grams of total solid, together 
with the solubility isotherms for the pure polymer and the mass adsorption onto the pure 
fi ller.
The addition of fi ller in AF 2400 apparently does not affect the solubility of n - C 
4
per 
unit mass of solid (Figure 7.4 a), in spite of the fact that fumed silica adsorbs much less 
n - C 
4
than the pure polymer can sorb, and infl uences the mass uptake of n - C 
5
(Figure 7.4 b) 
only at high activity where the plasticization effect of the penetrant is signifi cant, as it 
can be seen from the shape of the isotherm that shows a change in concavity. 
Vacuum
reservoir
Pre-chamber
Sample chamber
PI
Figure 7.3 Pressure decay apparatus for vapour sorption

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Vapor Sorption and Diffusion in Mixed Matrices Based on Tefl on® AF 2400 
135
To understand better the underlying mechanism and to model the system behaviour it 
is more meaningful to report the solubility data in terms of mass of penetrant absorbed 
per unit mass of polymer phase alone, C  
i,P
  ; to that aim we apply the pseudo - additive 
approximation embodied by Equation (7.10) and report the values of C 
i,P
  thus obtained 
versus penetrant pressure. The results are shown in Figure
7.5 
, where they are also