Liquid-liquid dispersions from in-line rotor-stator mixers

Abstract

Experiments were performed on an industrial scale Silverson in-line rotor-stator mixer to investigate its liquid-liquid dispersion capabilities. A non-coalescing kerosene-water system was used in the tests and the effect of stator geometry, rotor-speeds, flowrates and dispersed phase concentrations on the droplet size distribution was investigated. The rotor-speed and the dispersed phase concentration were found to have the most influence on droplet size. Drop sizes were also seen to increase with an increase in open area. No differences were observed in the droplet sizes or power draw of the stators with the smallest and similar open areas (Emulsor Screens and Square Hole High Shear screen). The power supplied to the fluid was proportional to N3 indicating the mixer was operating under turbulent conditions. d32 was correlated against the rotor speed and dispersed phase and the relationship was found to be close to that described by Chen and Middleman (1967). This analysis suggested that inertial stresses in the viscous sub-range were mainly responsible for drop break-up. d32 = 2x109 (1 + 20(D)(We Re4)-'" An estimation of the average energy dissipation rate was made in order to determine the Kolomogov length scale. The droplet sizes ranged from below the Kolmogorov length scale to significantly higher, suggesting that droplet break-up is due to inertial and viscous sub-range eddies. The Re could be defined in different regions within the mixer these values were used along with the nominal residence times in each region to determine where in the mixer the main drop break-up was occurring. The residence times for each region were in the following order: Shear Gap < Stator < Inlet < Rotor < Volute. The relatively long residence times and the magnitude of the stresses indicated that droplet break-up in a single pass through the in-line rotor-stator mixer is predominately determined by the viscous and inertial stresses in the rotor region.