Membrane Gas Separation

12.4
Composite Membranes 
The novel developed membrane materials should have potential applications on a large 
scale. The composite membrane preparation and its evaluation therefore must present 
promising results before testing the membranes in a real gas mixture. Single gas (CO 
2
, 
H 
2
, N 
2
and CH 
4
) measurements of Pebax  
 ® 
  /PEG blend membrane at high pressure (up to 
20 bar) and at 293 K presented higher CO 
2
fl uxes than pristine copolymer (Figure 12.14 ). 
(b)
(a)
0
1.00 mm 0
1.00 mm
(d)
(c)
Figure 12.13 AFM height images (surface) for: (a) pristine Polyactive
 
®
 
, (b) Polyactive
 
®
 
/
PEG blend, (c) pristine Pebax
 
®
 
 and (d) Pebax
 
®
 
/PEG blend [Reprinted with permission 24 ]

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246
Membrane Gas Separation
This fi gure shows the CO 
2
fl ux as a function of fugacity, and in all samples (Pebax  
 ® 
  and 
blend membranes) the CO 
2
fl ux increased when the fugacity was increased from 5 to 
20 bar. This is expected because in rubbery polymers, the solubility coeffi cient is the 
determinant and it strongly depends on gas condensability, especially when strong per-
meant – polymer interactions exist.
After single gas measurements, mixed gas experiments were carried out with 50/50 vol.% 
in the feed for CO 
2
/H 
2
and CO 
2
/CH 
4
, and 25/75 vol.% for CO 
2
/N 
2
at 293 K and a total 
feed pressure up to 20 bar. The fl uxes of CO 
2
as a function of the total fugacity are plotted 
in Figure 12.15 . The pressure increase enhances the CO 
2
fl ux for the CO 
2
/H 
2
and CO 
2
/
CH 
4
mixture, which is a consequence of membrane swelling. However, the fl ux of CO 
2
in the CO 
2
/N 
2
mixture slightly decreases with a pressure increase. This behaviour has 
been attributed to the gas composition in the feed, since high concentration of N 
2
would 
decrease the CO 
2
fl ux due to the plasticization effect, thus the N 
2
fl ux would increase and 
simultaneously the CO 
2
/N 
2
selectivity is reduced. When a membrane is plasticized by one 
component from a mixture, the permeability or fl ux of the other one is increased [92] ; a 
stronger plasticization effect could be observed in CO 
2
/CH 
4
mixture (30 – 50 wt.% of PEG) 
due to methane presence, since it has stronger sorption behaviour than hydrogen and 
nitrogen. However, all CO 
2
fl uxes of the membrane measured with mixed gases show 
good agreement with those obtained by single gas measurements. Although the CO 
2
fl ux 
is not signifi cantly increased with the feed pressure, the Pebax