Membrane Gas Separation

16.4
Conclusion 
An analysis of sweep uniformity on the performance of hollow fi bre gas dehydration 
modules is presented. The analysis is based on the governing conservation of mass 
and momentum equations and appropriate expressions for the overall mass transfer 
coeffi cient. 
In all cases, for a given product dew point, the dry gas fl ow rate increases as the frac-
tion of the product used as permeate sweep increases. However, the dry gas recovery (the 
fraction of the wet feed recovered as product) simultaneously decreases. The optimal 
sweep fraction will depend on the trade - off between increased productivity and fractional 
loss of the dry, high pressure product. 
The effect of non - uniform sweep distribution is examined by: (1) assuming a Gaussian 
variation in the sweep fl ow around each fi bre and (2) explicitly calculating the sweep 
distribution within the bundle for specifi ed sweep inlet and outlet locations. In both cases, 
non - uniform sweep fl ows has little effect on module performance. The explicit sweep 
distribution calculations indicate large radial concentration gradients are present in the 
module. Surprisingly, these concentration gradients are not detrimental to performance. 
If fi bre packing is allowed to decrease at the interface between the fi bre bundle and the 
enclosing case, preferential fl ow will occur along the interface relative to the centre of 
the bundle. Such fl ow is detrimental to performance for suffi ciently large variation in 
hydraulic permeability. To determine the magnitude of this effect in commercial modules, 
fi bre packing variations need to be characterized. 
0.78
0.8
0.82
0.84
0.86
0.88
0.9
0.92
-40
-30
-20
-10
0
10
20
30
40
Dew Point (°F, 1 atm)
Dry Gas Recovery
Figure 16.16 Effect of variable fi bre packing near the module case on dry gas recovery 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 lines are provided as guides for the reader

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The Effect of Sweep Uniformity on Gas Dehydration Module Performance
351
We believe the analysis presented here will be useful in evaluating alternative methods 
for introducing sweep in membrane modules for both gas and liquid separations. Although 
uniform sweep distribution appears not to be critical for air dehydration, other applications 
may be more sensitive.