ABSTRACT
Bubble production in the process of foam fractionation is centrally important, not only because the whole process is based on the principle that surfaceactive materials adsorb to the surface of bubbles, but also because the bubble size determines the final efficiency of the process, in terms of enrichment and recovery. A smaller bubble size gives more interfacial area available for the target substance to adsorb. Smaller bubbles also rise more slowly in the liquid, allowing for more adsorption time (see Chapter 5). Thus, a smaller bubble size favors interfacial adsorption and gives a higher recovery rate. On the other hand, liquid drainage from a foam made of small bubbles is slower, resulting in a wet foam that contains a large amount of low-concentration interstitial liquid (see Chapter 3) that reduces the final foamate concentration, thereby compromising enrichment. Figure 7.1 is a simulation of the effect of bubble size on the enrichment and recovery of 0.5 g.L−1 SDS (sodium dodecyl sulfate) solution using the hydrodynamic theory of rising foam (Stevenson, 2007). Of course, many experimental studies show the same effect (e.g., Brown et al., 1999; Crofcheck and Gillette, 2003; Lee and Maa, 1986). It should be noted that if the bubble size increase is caused by an increase in gas flowrate, then the effect of superficial gas velocity on the enrichment and recovery of the process can override the effect of bubble size, as observed by Du et al. (2002). The effect of gas flowrate on bubble size distribution is discussed in the following sections.
