Section Seven
NEW EQUIPMENT APPLICATION
63    MEMBRANE AIR STRIPPING: OPERATING PROBLEMS
AND COST ANALYSIS
Sylvia E. Schwarz, Graduate Research Assistant
Michael J. Semmens, Professor
Civil and Mineral Engineering Department
University of Minnesota
Minneapolis, Minnesota 55455
Karl J. Froelich, Technical Manager
Hoechst Celanese
Charlotte, North Carolina 28217
INTRODUCTION
Volatile organic compounds (VOCs) traditionally have been removed from water by packed tower
air stripping. Substantial literature is available concerning this method.'2,3
In packed tower air stripping, VOC laden water flows down from the top of a tower packed with
small ceramic, metal or plastic packing material. Air flows counter-currently from the bottom of the
tower. As the water flows around the packing elements, it is broken into small streams and droplets
thus creating a large surface area. VOCs transfer easily from the water phase into the air phase and are
carried out the top of the tower. The efficiency of this method of treatment depends on the value of
the Henry's constant, H, for the compound to be stripped, the air to water ratio, and the specific
surface area for mass transfer provided by the packing.
Microporous hollow fiber membranes recently have been investigated as an alternative to packed
tower air stripping.4 These fibers are made of non-wetting microporous polypropylene with pores
covering 40 percent of their surface area. Because the pores cover so much of the fiber surface area,
and because the liquid phase will not wet the pores, gases may transfer from one side of the membrane
through the pores to the other side with negligible membrane resistance. In addition the fibers provide
a very high surface area for mass transfer.5
In laboratory-scale air stripping studies using microporous hollow fiber membranes it was shown
that overall mass transfer coefficients (KLa values) for VOC removal were 3 to 7 times higher than
those commonly observed in packed tower air strippers.5 In addition the mass transfer rate was
observed to be liquid film diffusion controlled.
Zander et al.6 compared the VOC removal performance of a small commercial hollow fiber membrane module with a pilot-scale air stripping tower. The membrane system compared favorably for the
six compounds studied. It was shown that equivalent removals could be achieved in the membrane
module at lower air flow rates. The study was of short term duration however, and did not address
aspects of long-term fiber usage such as fouling of membrane surfaces.
Zander et al.6 also observed that some compounds were removed more poorly than they should be
based on theoretical predictions. These poor removals corresponded to conditions when the stripping
constant, R (= QgH/Qw) was less than 2. The authors speculated that the high packing density of the
fibers in the commercially-made modules may have led to air flow channeling and poor flow characteristics on the shell side of the module.
An important advantage of the hollow fiber membrane modules is that they are smaller than
packed towers and can be mounted either vertically or horizontally. If mounted horizontally, the need
to pump water 30 feet to the top of a tower is eliminated. Water pressure drops across a single module
are therefore much lower than those required of packed towers. However, very large air pressure
45th Purdue Industrial Waste Conference Proceedings, © 1991 Lewis Publishers, Inc., Chelsea, Michigan 48118.
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