Potential of flotation as alternative separation process in biotechnology with focus on cost and energy efficiency

Abstract

Flotation is a density separation process which is influenced by physico-chemical interaction between a particle and a bubble. In biotechnology, it finds application in the solid-liquid separation of biomass. The process mechanism depends on the formation of stable microorganism-bubble-complexes by collision and adsorption. The difference in density of these complexes and the surrounding liquid medium causes them to rise to the liquid surface, where a foam develops. This microorganism enriched foam can then be removed. An economical comparison of the separation technologies centrifugation, cross-flow-filtration and flotation showed, that flotation is a financially and energetically interesting alternative process step for the harvest of microorganisms. In this work separation results with a model microorganism S. cerevisiae are presented. Experiments were conducted batch-wise in a 6L-laboratory dissolved air flotation setup, where bubble generation classically results from expanding air from supersaturated medium. Goal was the optimization of the flotation rate and the energy efficiency of the flotation process by adaptation of operational parameters, such as gas- and recycle flow rate and saturation pressure. Flotation models show proportional dependency between the flotation rate and the bubble size and bubble surface area flow rate. Therefore, the effect of operational parameters, such as saturation pressure, recycle and gas flow rate, on the bubble size and surface area flow rate was analyzed. The positive effect of an increasing bubble surface area was possible with increasing gas flow rate, recycle flow rate and saturation pressure. Decreasing bubble sizes were measured for increasing recycle flow rate and saturation pressure, meanwhile the contrary was the case with an increasing gas flow rate. Evaluation of the flotation rate and flotation energy efficiency showed, increasing the recycle flow rate and the saturation pressure both improved the flotation rate, but had low effect on the energy efficiency. Increasing the gas flow rate improved both flotation rate and energy efficiency. It is the most interesting operational parameter for enhancing the process efficacy.