By Brian Miller, Jorge Munoz, And Fred Wiesler
(presented at the 2005 International Water Conference)
Membrane Contactors have been used to remove dissolved oxygen in boiler feed water systems for years. This report reviews various system designs and discusses how Membrane Contactors can be optimized in boiler feed water applications for removal of dissolved oxygen as well as carbon dioxide. The report also highlights what advantages membrane systems have over conventional oxygen and carbon dioxide removal technologies. New methods of degassing that improve the overall performance of Membrane Contactors in boiler feed water systems will also be introduced.
Membrane Contactors are devices that provide very efficient mass transfer between gases and liquids. These devices utilize microporous, hydrophobic hollow fiber membranes that contain a large surface area, which promotes ideal mass transfer. By controlling process conditions within the contactor, systems can be designed to remove gasses from or dissolve gases into water. These devices have been commercially available for industrial use for over fifteen years.
First developed as a means to remove dissolved gases from small flow applications, the technology has rapidly evolved and is suitable for very large-flow water systems in the power, pharmaceutical, food and beverage and microelectronics markets. Membrane Contactors are currently processing systems with flow rates as high as 1,696 m3/hr (7,462 gpm) and as small as a few ml/min.
In a typical design, water flows on one side of the microporous hollow fiber membrane. Gas flows on the other side of the membrane. Since the membrane is hydrophobic and the pores are very small, liquid will not pass through the pores. Pores in the membrane fiber provide a very stable gas/liquid interface. Manipulation of partial pressures at the interface allows gases to be added to or removed from the bulk water flow.
Manufacturing practices can vary as to how the fibers are packaged and assembled into a complete contactor device. One way is to knit the fibers into a fiber array sheet and roll the sheet around a perforated "distribution tube".