Membrane Gas Separation

16.3.1
Module Performance with Ideal Sweep Distribution 
Figures 16.3 and 16.4 illustrate the effect of sweep fraction (i.e. fraction of the dry product 
returned as sweep to the shell) on module performance assuming uniform, ideal sweep 
distribution. For all sweep fractions, the product gas fl ow rate and recovery decrease as 
the dew point decreases since increased water removal is accompanied by increased loss 
of oxygen and nitrogen.
Increasing the sweep fraction dramatically increases the product gas fl ow rate. For 
example, Figure 16.3 indicates the dry gas fl ow rate increases by a factor of nearly 20 at 
a dew point of 0 ° F if the sweep fraction is increased to 0.2. This increase in fl ow rate 
comes at the cost of reduced recovery. For high selectivities, recovery is reduced by an 
amount approximately equal to the sweep fraction. Figure 16.4 indicates recovery decreases 
from 0.88 to 0.79 at a dew point of 0 ° F as the sweep fraction is increased to 0.2. 
For the higher dew points, fl ow rates are high and signifi cant pressure drops can 
occur. For the results in Figures 16.3 and 16.4 , the highest pressure drops were
∼
8 kPa. 
Large pressure drops reduce the partial pressure driving force for water transport and 
consequently are detrimental to recovery. 

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Membrane Gas Separation
The changes in performance for a lower water/nitrogen selectivity of 100 are illustrated 
in Figures 16.5 and 16.6 . Relative to a selectivity of 1000, dry gas recovery decreases 
slightly but dry gas fl ow rates are signifi cantly lower. A decrease in recovery is expected 
with a decrease in selectivity since more dry air will permeate along with the water. For 
a dew point of 0 ° F and sweep fraction of 0.2, the recovery decreases from 0.79 to 0.75.
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