Effect of Coalescence on the Performance of a Continuous Foam Fractionation Column

A model for the hydrodynamics of coalescing foam bed is proposed which accounts for liquid drainage through thin films and plateau borders, bubble coalescence, and mixing of liquid between thin films and plateau borders. Foam bed is assumed to consist of equal sized dodecahedral bubbles at any cross section, and the variation of bubble size with foam height due to coalescence is described through phenomenological models. This model is employed to investigate the effect of coalescence on enrichment and recovery for the nonanol-water system in a continuous foam fractionation column for different inlet bubble sizes, superficial gas velocities, and inlet concentrations. Coalescence is found to yield higher enrichments and lower recoveries, possibly because of the predominant effect of increased liquid drainage due to larger bubble sizes resulting in a significant decrease in liquid holdup with foam height. Larger bubble sizes and lower superficial gas velocities were found to result in higher enrichments and lower recoveries. There exists an optimum inlet concentration for which enrichment was found to be maximum. Enrichment was found to be highest for coalescence frequency proportional to the surface area of bubbles, intermediate for frequency proportional to the bubble size, and lowest for constant coalescence frequency.