Membrane Gas Separation

time (s)
Permeate pressure (mbar)
205
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
200
400
600
800
1000
1200
time (s)
Permeate pressure (mbar)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
500
1000
1500
2000
2500
time (s)
Permeate pressure (mbar)
Exp.
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
200
400
600
800
1000
1200
time (s)
Permeate pressure (mbar)
Eq.(6)
Exp.
Eq.(6)
Figure 4.8 Permeation curves of dichloromethane and methanol vapour in Hyfl on 
AD80X membranes at 25 ° C. Time lag calculation by the tangent method (top) and by a 
direct least squares fi t of the entire permeation curve according to Eq. (4.6) (bottom). The 
thick dark line represents the experimental data; the thin brighter line gives the tangent 
(top) or the least squares fi t. The experimental and fi tted lines superimpose completely in 
the case of DCM, and only one curve can be distinguished, whereas MeOH gives a poor 
fi t. See text for further explanation
With this equation the diffusion coeffi cient can be obtained from the time lag if the 
membrane thickness is known. This way to calculate the diffusion coeffi cient will be 
further referred to as the ‘ tangent method ’ and the procedure is schematically displayed 
in Figure 4.8 , top section. The permeability follows from the steady state pressure increase 
rate and can be defi ned as:
P
V V
l
RT A p
dp dt
dp dt
P
m
f
=
⋅ ⋅
⋅ ⋅
⋅
(
)
−
(
)
[
]
0
(4.10)

---

76
Membrane Gas Separation
If the simple solution - diffusion model applies, the penetrant solubility can be calculated 
from P and D from the simple equation:
P
D S
= ⋅
(4.11)
This procedure thus allows the determination of all three fundamental transport 
parameters, P , D and S .