Membrane Gas Separation

Dew Point (°F, 1 atm)
Dr
y Gas R
eco
v
er
y
Figure 16.6 Dry gas recovery as a function of dew point for a water/nitrogen selectivity 
of 100 and various sweep fractions: solid – 0; short dash – 0.1; long dash – 0.2

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344
Membrane Gas Separation
a dew point of 0 ° F and sweep fraction of 0.2, the dry gas fl ow decreases by a factor of 
∼
6 (from 2 to 0.3 SCFM) as the selectivity and water permeance decrease by a factor of 
10. The decrease in fl ow rate is less than the decrease in permeance because more dry air 
permeates with the water. This reduces the water concentration in the permeate which in 
turn increases the water partial pressure difference that drives transport across the mem-
brane. Consequently, the rate of water removal and the amount of humid gas that can be 
dried increase. Similar changes with selectivity have been reported previously [4,5] .
16.3.2
Effect of Sweep and ID Variation 
Figures 16.7 and 16.8 illustrate the effect of sweep variation on module performance. An 
average sweep fraction of 0.1 was used for the calculations.
Variability in the sweep fl ow around individual fi bres has little effect on module per-
formance. Only slight decreases in dry gas recovery and fl ow rate occur as the variability 
in sweep fl ow increases. The dry gas fl ow rate drops by less than 10% over the range of 
dew points considered as the variability in sweep fl ow rate increases to 20%. The dry gas 
recovery changes by less than 1%. 
Figures 16.9 and 16.10 compare the effects of inner diameter and sweep variation on 
module performance. Like sweep variability, inner diameter variability is detrimental to 
performance
– 
dry gas recovery and fl ow rate decrease as the variability increases. 
However, the effect of inner diameter variability is signifi cantly larger. The dry gas fl ow 
rate decreases by a factor of two with 20% inner diameter variation for the lower dry gas 
dew points. Changes in dry gas recovery are smaller and do not exceed 5% at the lowest 
dew points considered.
0
1
2
3
4
5
6
7
8
9
10
-40
-30
-20
-10
0
10
20
30
40