Membrane Gas Separation

16.3.4
Effect of Fibre Packing Variation Along Case 
The literature indicates that large variations in packing of spheres [34] or fabrics [35] can 
occur at the boundary between the packing and a solid case enclosing the packing. At the 

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348
Membrane Gas Separation
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
Radial distance (cm)
Molar density (mol/m
3
)
Figure 16.14 Variation in water concentration in the radial direction along the retentate 
outlet (i.e. the dry gas product end): solid – lumen and short dash – shell. The results are 
for operating conditions that give a dry gas dew point of
−
 30 ° F
0.84
0.85
0.86
0.87
0.88
0.89
0.90
-40
-30
-20
-10
0
10
20
30
40
Dew Point (°F, 1 atm)
Dry Gas Reco
very
Figure 16.13 Effect of sweep confi guration on dry gas recovery as a function of dew 
point: diamond – internal, circle – shell, triangle – offset. The solid line corresponds to 
uniform sweep distribution. Note the diamond and circle symbols overlap

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The Effect of Sweep Uniformity on Gas Dehydration Module Performance
349
0
1
2
3
4
5
6
7
8
9
-40
-30
-20
-10
0
10
20
30
40
Dew Point (°F, 1 atm)
Dr
y Gas
 Flow (SCFM)
Figure 16.15 Effect of variable fi bre packing near the module case on dry gas fl ow rate 
as a function of dew point. The symbols correspond to different values of  
α
 : diamond – 0, 
square – 200, triangle – 400, circle – 600. The upper solid line corresponds to uniform 
sweep distribution. The lower solid line is provided as a guide for the reader
packing/case interface, the packing fraction can decrease (i.e. the void volume increase) 
relative to the bulk packing. Increased void volume will lead to signifi cant increases in 
hydraulic permeability and preferential fl ow at the packing/case interface. Such preferen-
tial fl ow is referred to as race tracking since fl uid fl ows are much higher than the super-
fi cial fl ow within the packing and can impact heat and mass transfer [36,37] . 
The observation of large shell concentration gradients at the periphery of the fi bre 
bundle suggests race tracking phenomena may have a signifi cant impact on module per-
formance. To test this hypothesis, the hydraulic permeability for the shell side fl ow was 
assumed to have the following dependence on the radial coordinate:
k
k
r
=
( )
0
exp
α
(16.18)
where k
0
is the hydraulic permeability of the fi bre bundle and  
α
  characterizes the extent 
and magnitude of the variation
– 
as
 
α
  
increases the extent and magnitude increase. 
Equation (16.18) introduces an exponentially increasing hydraulic permeability with r . 
The maximum value occurs at the bundle/case boundary which will lead to race tracking 
for suffi ciently large  
α
  . 
Figures 16.15 and 16.16 illustrate the effect of the variable hydraulic permeability on 
module performance. For a fi xed dew point, dry gas fl ow rate and recovery decrease as 
 
α
  increases. The fl ow rate decreases by up to 50% for the largest value of  
α
  but recovery 
decreases by less than 10%.
Reduced fi bre packing at the bundle/case boundary is detrimental to performance. 
However, characterization of fi bre packing in actual modules is required to determine the 
appropriate value for  
α
  and the impact of race tracking on performance.

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350
Membrane Gas Separation