Degasification mass transfer

Air stripping requires packed towers for maximum operational efficiency [13]. Water is pumped to the top of a tower packed with media as shown in Figure 2.6. The water is evenly distributed across the media. As it flows down under gravity it forms a film layer on the packing surfaces. Air is blown upwards from the bottom contacting the large surface areas. The blown air enhances the removal of the volatile species by mass transfer. Degasification is... 

Membrane degasification units are devices that can be used to permit mass transfer between a gaseous phase and a liquid phase without dispersing one phase into another. The gas layer is stabilized within the pores of a hydrophobic microporous filter in membrane degasification units. Solutes that are volatile can pass across these membranes, but nonvolatile solutes and aqueous liquids such as electrolytes are completely retained. High flux occurs when the solute is volatile and when it is relatively insoluble in water. 

Extensive research both in industry and academia has resulted in the innovation of porous membranes, which are gas fiUed, with much smaller mass transfer resistance. Parallel research on microporous membranes has adjusted the pore size and membrane hydrophobicity, again yielding a much smaller mass transfer resistance. However, modules with different geometries perform differentiy. Flow outside of, but perpendicular to, the fiber bundle offers reasonably fast mass transfer. Not surprisingly, this geometry is chosen in most of the commercial membrane degasification units. 

Mass Transfer in Membrane Degasification Commercial membrane degasification units involve mass transfer in hollow-fiber modules where the vacuum and sweep gases are applied inside the fibers, and the water flows outside the flbers in cross flow perpendicular to the fiber axis. The mass transfer involves three sequential steps. First, dissolved gas diffuses out of the water to the membrane surface. Second, it diffuses into vapor pores in the walls of the hydrophobic hollow fibers. Third, when the dissolved gas reaches the other wall of the fibers, it diffuses into the surrounding nitrogen sweep gas in a high vacuum condition. 

The above form of the mass transfer equation is adopted in the design of membrane degasification units. 

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