Membrane Gas Separation

13.2.3
Gas Permeability Measurements 
In order to assess the reproducibility of the permeation data obtained for each gas, fi ve 
measurements were carried out per Pebax ® sample. CO 
2
and N 
2
permeation properties 
of fi lms were determined using the permeation apparatus previously described
[28] 
. 
Before measurement, the air present in the permeation cell was completely evacuated by 
applying a vacuum on both sides of the fi lm for one night. The pressure in the permeation 
CH
CH
2
CH
2
CH
2
CH
2
O
O
n
m
Cl
Figure 13.1 Chemical structure of poly(ethylene oxide - co - epichlorhydrine)
(a)
(b)
CH
2
CH
2
NH
O
x
y
m
n
O
Figure 13.2 Chemical structures of (a) polyamide and (b) polyether blocks; polyamide 
block could be PA6 ( x = 5) or PA12 ( x = 11) and polyether block could be poly(ethylene 
oxide) (PEO, y = 2) or poly(tetramethylene oxide) (PTMO, y = 4)

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260
Membrane Gas Separation
cell had a constant value ( 
<
5
×
10 
−
3
mbar) before starting the permeation experiment. Then 
the upstream side of the permeation cell was exposed to a fi xed pressure of the gas under 
test (0.5 bar
<
p
1
<
3 bars in different experiments). 
The increase in the pressure p
2
was measured using a sensitive pressure gauge (0 –
10 mbar, Effa AW - 10 - T4) linked to a data acquisition system. The quantity Q ( t ) of gas 
present in the receiving compartment was calculated using the perfect gas law:
Q
p V
RT
d
=
2
where V
d
is the calibrated downstream volume (98.13 cm 
3
), R is the gas constant and T
is the measurement temperature. 
The permeability coeffi cient P (cm 
3
(STP) cm cm 
−
2
s 
−
1
cmHg 
−
1
) and the ideal selectivity 
 
α
  were determined by using the following equations (assuming p
1
>> p
2
) [29] :
P
Q L
A t p
=
1
α =
P
P
CO
N
2
2
where Q is the quantity of STP gas permeated in a time interval t at the steady state of 
gas fl ow, A is the effective fi lm area for gas permeation (11.64 cm 
2
), L is the sample thick-
ness and
P
CO
2
and
P
N
2
are the permeability coeffi cients of pure CO 
2
and N 
2
, respectively. 
The accuracy on P value is ca. 6%. 
The sorption coeffi cient S was indirectly obtained, by dividing the permeability coef-
fi cient P by the diffusion coeffi cient D . The well - known relationship P = DS with constant 
diffusivity D was assumed to be valid (which is generally the case for gas permeation in 
rubbery polymers at relatively low pressure). The diffusivity
D 
is considered to be 
affected by the free volume, i.e. the unoccupied space in the polymer matrix, and the 
solubility S by physical and chemical interactions between gas and polymer.