Use of hydrodynamic cavitation for volatile removal compound

Abstract

Hydrodynamic cavitation and its feasibility for volatile compound removal in enclosed channels is discussed in this paper. Very high Reynolds numbers are needed to rupture liquid by decreasing its pressure below its saturated vapour pressure. Hence, a simple stratified flow, at which the two phases separate, is precluded in vertical and horizontal tubes, where turbulence stresses will be much larger than the buoyant forces. The most probable flow regime at this high turbulence regime is a bubble- or annular flow, where the volatile matter tends to concentrate in the centre of the pipe because of the lift force resulting from the unequal flow of the viscous liquid around the bubbles in the presence of the pipe wall. Therefore, boiling the volatile matter for volatile compound removal is not enough if hydrodynamic cavitation is pursued. The attainable efficiency must also be assessed. An expression for the volatile removal efficiency and the main parameters affecting this efficiency were derived by utilising a simplified geometrical and physical model. The efficiency was found to approximate a power law as a function of the volatile concentration and its strong dependence on the size of the volatile bubble reasonably well. This result implied the need of bubble growth and the limitation of the process for highly concentrate compounds to a few percent concentrations. With regard to energetic requirements, both thermal and hydrodynamic cavitations are quantitatively similar. Furthermore, the choice of one or another corresponds more to the kind of energy source available.