On a Dual-Chamber Ultrasonic Separator for the Filtration of Suspensions Containing Yeast Cells and Oil

The principle of ultrasonically enhanced settling (UES) is successfully applied in biotechnology for filtration purposes, e.g. as cell-filter. Particles (biological cells) are concentrated in certain regions of a sonicated volume by an ultrasonic standing wave field (~2 MHz). Due to the dependence of the final settling velocity on the diameter of an object (Stoke's law), the agglomerates formed by sonication settle more quickly than single cells. This principle, which relies on a sufficiently large difference in mass densities of the particles and the host liquid, is also applicable to dispersed material lighter than the liquid. Up to now, no feasible solution was at hand when a suspension contained both heavier and lighter particles. For this demand, a novel setup, the UES dual-chamber separator, was tested on suspensions of yeast cells and oil droplets as a model system. Here the volume exposed to the standing wave field is divided into two parts by an ultrasonically transparent thin foil oriented perpendicularly to the direction of sound propagation. Experiments have shown that the UES dual-chamber separator was able to deliver oil- enriched medium at an upper collecting outlet of the first chamber and a cell-enriched medium at a lower collecting outlet of the second chamber. Data for the over-all separation efficiency of above 90 % at inlet throughputs of approximately 18 L/d will be shown and discussed.