Dissolved oxygen, nitrogen, and carbon dioxide are naturally present in water. Dissolved gases should be carefully monitored and controlled, as they can affect the products and processes in which the water is used. For example, dissolved oxygen can react with metais, forming an oxide Iayer on the surface of finished productsorpipingmaterial. AIso,asthe water temperature is increased, dissolved gases can form bubbles that may lead to incomplete wetting of surfaces. The removal of dissolved gases is an important processing step and is routinely accomplished with a vacuum degasification column or with chemicais. Removal of specific gases, such as oxygen, can also be accomplished by stripping the gases from the water using avacuum and!
Membranes have been used to degasify waterfor more than 20 years in laboratory environments. One major drawback of the membranes has been their inability to handle industrial-size flowrates efficiently. Because of this low capacity, membranes havebeenviewed as small-scale devices. Over the last several years, new designs have recently been developed to overcomethe capacity limitations. Degasification membrane modules, also called membrAnA contActors. and designed to handle industrial-size flowrates, are now commercially available.
This paper discusses the theory behind membrane degasification and its evolution overthe last few years.
In order to best explain the operation of membrane-based degasification systems, it is important to review the laws governing ideal gases. These relationships govern the operation of the membrane contactors as well as that of conventional degasification columns.
The partial pressure component of each gas can be rewritten as in Equation 2:
taI = aiYi + aIY2 + taIY3 + Eq. 2 where y = the mole fraction of the component.