Riser Design in Foam Fractionation

The growth of biotechnology has led to renewed interest in the possibility of enriching valuable bioproducts using foam fractionation. This is a separation process in which surface active molecules are separated from a solution by sparging through bubbles of gas. Adsorption of such molecules at the gas/liquid interface causes enrichment in the foam formed above the liquid pool. Previous work has shown that liquid flow and mixing in the foam riser are crucial features determining the degree of separation. Theoretical considerations suggest that diffusion in the Plateau borders of the rising foam is the principal cause of mass transfer between the rising liquid and the reflux. This diffusion is one element of the well-known Taylor dispersion, and it enhances enrichment in the riser. An experimental study has measured the velocities of the counterflowing streams within a riser, and the axial dispersion. Fitted dispersivity values are similar to those previously published. Further modelling of the dispersion processes will enable these experimental values to be related to the enrichment within the riser, enabling confident design and control of foam fractionation columns.