Evolution of drop size distribution in liquid–liquid dispersions for various impellers*

Abstract The drop size distribution is one of the most important characteristics of liquid–liquid dispersions. Several shapes of the distribution have been proposed by previous investigators. This work investigates the drop size distribution in very dilute (0.03% by volume) liquid–liquid dispersions over a wide range of rotational speeds, using different impellers with varying diameters and off-bottom clearances. Four impellers were used: one radial flow impeller (Rushton turbine); and three axial flow impellers [pitched blade turbine, HE3 turbine and fluidfoil (A310) turbine]. Drop sizes were measured using a phase Doppler particle analyzer (PDPA) in both the bulk and impeller regions in an agitated tank. It was found that the drop size distribution changes with an increase in the rotational speed. Typically, four types of drop size distribution evolve with increasing rotational speed: long tail, double peak, skew and skew-normal distribution. A new scaling parameter; the product of average power input per unit mass P ρV T and ND 2 is proposed to define the regions where the four types of drop size distribution occur.