Membrane Gas Separation

16.3
Results and Discussion 
The results presented here assume the selectivity for water permeation relative to nitrogen 
is 1000 and that for oxygen to nitrogen is 8 which are representative of current commercial 
air dehydration modules. Other fi bre properties and operating conditions are tabulated in 
Table 16.1 and are representative of commercial hollow fi bre dehydration modules.
Centerline 
p = p
0
ρ
ι
 = 
ρ
i0
L
r
m
p = 0
convective flux
axial symmetry 
insulation
(a) 
Centerline 
axial symmetry 
insulation 
(b) 
p = 0
convective flux 
insulation 
insulation 
u = u
0
ρ
ι
 = 
ρ
i0
Figure 16.2 Boundary conditions used in explicit simulations of sweep distribution to 
evaluate module performance: (a) lumen - side boundary conditions and (b) shell - side 
boundary conditions. Note that the boundary conditions for the shell correspond to one of 
the confi gurations used to simulate shell fl ows. Similar boundary conditions apply for the 
others. The shell extensions allow establishment of uniform velocity and concentration 
fi elds as described previously [24]

---

The Effect of Sweep Uniformity on Gas Dehydration Module Performance
341
Table 16.1  Fibre and module properties and operating 
conditions. Literature correlations are used to convert 
dew points to water vapour concentrations [38,39]
Fibre number
10,000
Fibre length (m)
0.1
Fibre inner diameter ( 
μ
m)
200
Fibre outer diameter ( 
μ
m)
150
Fibre packing fraction
0.5
H 
2
O/N 
2
selectivity
1000
O 
2
/N 
2
selectivity
8
N 
2
permeance (GPU)
1
Feed pressure (psia)
147
Shell - side pressure (psia)
14.7
Feed dew point @ 14.7 ( ° F)
45
Operating temperature ( ° F)
115
The simulations assuming a Gaussian sweep distribution were performed using three 
quadrature points based on previous work demonstrating that use of fi ve quadrature points 
gave equivalent results to within less than 1% [30,31] . Simulations with varying number 
of stages indicated that the results did not change by more than 5% if 200 or more stages 
were used. Thus, all simulations were performed with 200 stages. 
The explicit sweep distribution simulations were performed with increasingly refi ned 
meshes until the results did not change by more than 5%. Typical meshes contained 5000 
elements. 
In the following discussion, percent variation prefers to the ratio of the standard devia-
tion to the mean value of a property distribution expressed as a percentage (
100
∗
σ φ
). 
Module performance refers to the dry gas (retentate) fl ow rate and recovery for a given 
purity.