Defoaming Using Packed Beds of Appropriate Wettability

4 DEFOAMING USING PACKED BEDS OF APPROPRIATE WETTABILITY 

An example of an application of this concept is described by Ward et al. [79]. This application concerns fuel cell development. The relevant fuel cell cathode employs an aqueous solution of a redox catalyst, which must be regenerated by aerobic oxygenation (see, e.g., reference [80] for a description of this type of fuel cell). Sparging the solution with air produces an unacceptable foam in a situation where contamination with conventional antifoam would damage the integrity of the process. Use of rotary foam devices, on the other hand, consumes an unacceptable proportion of the output of the fuel cell. The resultant foam is, however, readily destroyed by a packed porous bed consisting of a polytetrafluoroethylene (PTFE) mesh. 

The mesh structure consists of knitted cylindrical fibers. The surface tension of the aqueous solution of the oxygenated redox catalyst is -75 mN m- at 20°C, which implies a negative surface excess of the redox electrolyte. It seems likely therefore that the air-solution contact angle with this system is 90° since that at the air-water surface is 90° and a negative surface excess is to be expected also at the PTFE-water surface. Such high contact angles for the redox catalyst solution are consistent with the destruction of foam by the cylindrical fibers in the mesh. 

---