Membrane Gas Separation

4.5.3
Vapour Permeation Measurements 
In this work preliminary vapour permeation measurements were carried out with two 
different species, the rather bulky dichloromethane (DCM) molecules and the much 
smaller methanol molecules. Two typical permeation curves are displayed in Figure 4.8 . 
The transport parameters, determined on the basis of the tangent method and Equations 
(4.9) – (4.11) , are listed in Table 4.3 . It contains the parameters defi ned above as well as 
solubility C in the membrane in equilibrium with the feed pressure of penetrants.
At fi rst sight the behaviour of both vapours is quite similar. Their permeability is of 
the same order of magnitude, with a somewhat higher value for methanol, as expected 
for the smaller molecule in the glassy polymer. The higher methanol permeability is 
indeed mainly due to the faster diffusion of methanol in the PFP matrix, about twice as 
fast as that of DCM. This confi rms the size - sieving character of PFPs, already observed 
for permeation of permanent gases [24] . On the other hand, the solubility and solubility 
coeffi cient of DCM are higher, especially on a weight basis. At the actual feed pressure, 
for DCM the C values and solubility coeffi cient are more than 8 times higher than those 
of methanol, due to a combination of a higher solubility coeffi cient , a higher molar mass 
and a higher vapour pressure. 
More careful analysis of the permeation curves show that the two vapours actually 
display completely different behaviour. The tangent method can be applied without prob-
lems to the DCM permeation curve, resulting in a time lag of ca. 400 s. In contrast, this 
method shows that methanol has an unusually wide transient period. While the extrapo-
Table 4.3  Comparison of the transport properties of dichloromethane and methanol 
vapour in Hyfl on AD80X at 25 ° C
a

Vapour
DCM
MeOH