Membrane Gas Separation

16 
The Effect of Sweep Uniformity 
on Gas Dehydration 
Module Performance
Pingjiao Hao and G. Glenn Lipscomb
Chemical and Environmental Engineering Department, University of Toledo, Toledo, Ohio, USA
16.1
Introduction 
Membrane gas separation for air dehydration (AD) differs from other commercial applica-
tions in several ways. First, the component to be removed (water) possesses a permeability 
that may be more than three orders of magnitude greater than the other components in 
the feed (oxygen and nitrogen). Second, the feed concentration is small, less than 1% on 
a molar basis. Third, the product concentration may be two orders of magnitude less than 
the feed concentration. 
Membranes in the form of hollow fi bre modules are used commonly for gas dehydra-
tion. In comparison to spiral wound modules made from fl at sheet membranes, hollow 
fi bre membrane modules contain more membrane surface area per unit volume thereby 
reducing the size of the module. Additionally, hollow fi bre module manufacturing costs 
are lower [1] and hollow fi bre designs easily permit permeate sweep. 
A hollow fi bre module can be operated in three different modes: co - current, cross, or 
counter - current fl ow. In co - current fl ow, the permeate fl ows in the same direction as 
the feed and retentate. In cross - fl ow, the permeate fl ows perpendicularly to the feed 
and retentate while in countercurrent fl ow the permeate fl ows in the opposite direction. 
The countercurrent fl ow pattern gives the best performance as the driving force for trans-
port is maximized along the module length. One can produce an arbitrarily high purity 

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Membrane Gas Separation
retentate product with the countercurrent design but the maximum permeate purity is 
limited by the intrinsic separation properties of the membrane [2,3] . Production of a high -
purity permeate requires module staging. Thus, the production of a high - purity product 
is easier when the product is the retentate. 
Wang et al. [4,5] demonstrate that for a countercurrent hollow fi bre module the mem-
brane resistance to water transport is negligible – the overall mass transfer coeffi cient is 
controlled by lumen and shell - side concentration boundary layers. Returning part of the 
retentate product as a permeate sweep increases the rate of water removal. The required 
membrane area decreases dramatically as the fraction of the retentate used as sweep is 
increased (sweep fraction). However, for sweep fractions greater than
∼
0.1, the increase 
in productivity is offset by an increase in consumption of product gas as sweep. 
The patent literature describes three primary methods for introducing sweep from the 
retentate product into the shell of a hollow fi bre module. 
First, Skarstrom and Kertzman [6] teach the use of conduits and valves external to the 
module to return a portion of the retentate product to the shell as sweep. The sweep may 
be introduced either through an external port on a case enclosing the fi bre bundle or 
through a tube that extends from outside the module through the tube sheet into the shell. 
Similarly, Makino and Nakagawa [7] teach the use of valves and conduits to feed the 
sweep to the fi bre lumens in a shell - fed module. Friesen et al. [8,9] teach the use of the 
sweep stream that is mixed with the water - containing permeate at a point generally oppo-
site the feed to the module, preferably through a port near the retentate product (i.e. the 
dehydrated air product) end of the module. 
Second, Stookey [10] teaches the use of fi bres that possess reduced selectivity and 
increased permeance near the retentate product end. If the fi bres used in the module are 
composite membranes, one can change their transport properties simply by removing the 
discriminating coating in a region near the tube sheet. This allows introduction of the 
sweep from the fi bres themselves but does not allow control of the sweep rate. 
Third, Morgan et al. [11] teach the use of a plurality of passages (fi bres, tubes, or other 
conduits) embedded in the retentate end tube sheet that allow fl uid communication 
between the retentate header and the shell. The pressure difference between the header 
and shell drives a portion of the retentate product back into the shell. The sweep fl ow rate 
is determined by the number and size of the passages and cannot be regulated externally. 
This ‘ internal sweep ’ design is used extensively. Burban et al. [12] describe a modifi ca-
tion in which a diffuser is used to distribute the sweep more uniformly in the shell. A 
channel or conduit extends from the retentate header through the tube sheet into the fi bre 
bundle. The channel end in the header is left as an open orifi ce while the channel end in 
the fi bre bundle is capped by a porous diffuser. 
The patent literature also teaches the importance of countercurrent contacting [13] and 
recycle confi gurations to improve process performance [14] . Auvil et al. [14] describe 
two confi gurations in which the wet permeate is recycled to eliminate feed air losses. If 
the feed is at ambient pressure, the permeate is sent to the inlet of the feed compressor. 
If the feed is already at pressure, a recycle compressor is used to increase the permeate 
pressure to the feed pressure. In both confi gurations, water is removed from the process 
in the chiller/condenser that follows the compressor. 
More recent literature addresses other issues associated with membrane dehydration. 
Vallieres and Favre [15] demonstrate that use of a permeate vacuum may be preferable 

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