Comparison of liquid-liquid dispersions formed by a stirred tank and electrostatic spraying

Two methods of producing liquid-liquid dispersions are compared in terms of the dispersed phase drop-size, energy requirements, and other properties. In the first method, a stirred-tank contactor, used for laboratory bioprocessing studies, was employed. Experiments were conducted using a 10 cm-diameter cylindrical tank, stirred by one or two 5 cm-diameter 6-blade Rushton-turbine impellers. The transient drop-size distribution of kerosene in water was measured by a video technique. It was found that (i) the drop-size had not reached steady state even after 10 hrs of agitation, and (ii) the drop-size produced by one impeller was smaller than that produced by two impellers. In the second method, aqueous droplets were electrohydrodynamically generated at the tip of a metal capillary under the influence of a pulsed, direct-current (dc) voltage. The capillary tube was located co-axially at the center of another tube made of a dielectric (teflon) wall. Kerosene was pumped between the capillary and the outer tube. An electric field was formed between the electrically-grounded capillary tube and an electrified electrode mounted on the external surface of the outer dielectric tube. Positive, sinusoidal-type voltage pulses in the range of 10-25 kV at frequency between 3.4 and 3.7 kHz were applied and the electric current was measured. The size of the drops ejected from the capillary was measured by a laser light scattering facility and found to be in the range 1 to 100 μm. Single and multiple spraying cones were observed depending on the aqueous-phase flow-rate. Smaller drop-size was obtained when multiple-cone spraying occurred. Energy calculations showed that dilute dispersions can be produced more efficiently by electrostatic spraying than by mechanical agitation.