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Hollow-fiber membrane contactors have proven themselves to be an effective technology for removing gases and vapors from liquid streams. The membranes provide accelerated mass transfer rates due to their unique ability to expand the liquid/gas contact interface. Membrane contactors used to degasify liquid streams have been used for decades in industrial applications. With the recent design improvements to the contactors, the hollow-fiber membranes are now capable of handling larger liquid flows and, with the expanded ability to remove entrained air, trihalomethanes (THM) and volatile organic compounds (VOC).
The theory of gas transfer in hollow-fiber membrane contactors has been studied over the past few decades by various groups. The theory describing this process is rooted in fundamental mass transfer principles and is similar to conventional air stripping using a packed tower design.
The membranes used in the Liqui-Cel contactors are porous, polypropylene membranes with a pore size less than 0.05 micron. Membrane contactors typically operate with the water flow on the outside of a hydrophobic membrane and a sweep gas and/or vacuum on the inside (lumen side) of the membrane. At the pore opening, a gas/water interface is established where the gas phase is in direct contact with the liquid phase. As a result, a gas liquid interface for mass transfer is established. Similar to a packed bed in an air tower, the membrane acts as a medium to allow efficient contact between the liquid and gas phases, and allows mass transfer to occur at the membrane pore. Since the membrane is hydrophobic, water does not fully penetrate through the membrane.
The hollow-fiber can be bundled inside the contactor. This minimizes the hydraulic losses and allows for a greater flux (ratio of volume vs. surface area), making the systems a very attractive method for removing undesirable gases and volatile compounds from liquid streams. An illustration of a membrane contactor can be seen above.
One major advantage of the Liqui-Cel membrane contactor is the patented center baffle on the shell-side (liquid side). This baffle directs water outward from the center of the contactor, thereby providing maximum contact of the liquid with the membrane surface, ultimately minimizing short-circuiting. This baffle also provides and maximizes hydraulic turbulence, which enhances mass transfer between the liquid and gas phases.
Membrane contactors can be used to effectively remove a variety of gases from water. The Liqui-Cel systems have a variety of advantages over conventional treatment, as summarized below.
Pilot Studies Recently Completed
Layne completed a pilot study near Fabens, Texas in May 2012. The primary objective of the pilot study was to demonstrate the effectiveness of the membrane technology for THM removal and to obtain design parameters for full-scale application.
The testing during the pilot utilized a membrane contactor system provided by Layne. The skid-mounted system consisted of: two Liqui-Cel 4 x 28 contactors, and a 3 hp liquid-ring vacuum pump.
During the study, water and air flow rates were varied to determine the impact and efficiency on THM removal. Various combinations of water flow and air flow rates were tested ranging from 6 to 12 gal per minute (gpm) and 5 to 12 cu ft per minute (cfm) per contactor, respectively. The membrane contactors each had 215 ft2 of membrane area, resulting in membrane flux rates (per contactor) of approximately 27 to 107 gal/day-ft2 (gfd) and air:water (A:W) ratios of approximately 1 to 22 (ft3/ft3) for this THM removal pilot study.
Layne completed another pilot study near Glendale, Ariz., in April 2012. The primary objective of this pilot study was to demonstrate the effectiveness of the membrane technology for entrained air removal and to obtain design parameters for full-scale application. Again, water and air flow rates were varied ranging from 5 gpm to 15 gpm, and 5 to 16 cfm per contactor, respectively.
Pilot Study Results
THM Removal. Based on the data collected, Layne achieved up to 77% THM removal from the water supply by designing a multiple contactor Liqui-Cel Membrane Degasifier System to operate in a parallel configuration with an A:W ratio of 5 using an air/vacuum combination sweep.
Entrained Air Removal. To assess entrained gas removal, a visual inspection of samples was taken before the membrane units and after the membrane units. In addition, samples were collected for dissolved oxygen, dissolved nitrogen and carbon dioxide.
As can be seen in the example below, entrained gas removal was accomplished by using the Liqui-Cel system. Only a very minute quantity of bubbles could be detected after passing through the Membrana Liqui-Cel contactors. These results were consistent for all flow rates that were tested. In addition, these results were achieved without the use of a vacuum pump.
The results produced by the pilot tests show that the Liqui-Cel membrane contactors will reduce entrained air to acceptable levels, either visually or quantitatively. The THM pilot tests show that the Liqui-Cel contactors will remove a significant amount of THM contaminants from the initial feed source. Other VOC contaminants were also removed during the THM pilot; however, THM was the contaminant of concern in the pilot study.
The Membrana Liqui-Cel contactor’s compact design allows the equipment to be installed in areas where space is limited. The Liqui-Cel contactors can be installed so that pressure is maintained, and because of the closed design, there is no need for additional pumps to maintain the flow rate. The Liqui-Cel contactors also allow for variable flow rates, and existing systems can be easily modified to treat higher or lower flow rates. These pilot studies and results show that Liqui-Cel contactors have some definitive advantages to other conventional treatment technologies.
Victor Wong is an applications engineer for Layne Christensen. Wong can be reached at [email protected]