COMPARISON OF LIQUID-LIQUID DISPERSIONS FORMED BY A STIRRED-TANK AND ELECTROSTATIC SPRAYING

Two methods of producing liquid-liquid dispersions are compared in ter ms of the dispersed phase drop-size, energy requirements, and other pr operties. In the first method, a stirred-tank contactor, used for labo ratory bioprocessing studies, was employed. Experiments were conducted using a 10cm-diameter cylindrical tank, stirred by one or two 5cm-dia meter 6-blade Rushton-turbine impellers. The transient drop-size distr ibution 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 w as smaller than that produced by two impellers. In the second method a queous droplets were electrohydrodynamically generated at the tip of a metal capillary under the influence of a pulsed, direct-current (de) voltage. The capillary tube was located co-axially at the center of an other tube made of a dielectric (teflon) wall. Kerosene was pumped bet ween the capillary and the outer tube. An electric field was formed be tween the electrically-grounded capillary tube and an electrified elec trode mounted on the external surface of the outer dielectric tube. Po sitive, sinusoidal-type voltage pulses in the range of 10-25 kV at fre quency between 3.4 and 3.7 kHz were applied and the electric current w as measured. The size of the drops ejected from the capillary was meas ured by a laser light scattering facility and found to be in the range 1 to 100 mu m. Single and multiple spraying cones were observed depen ding on the aqueous-phase flow-rate. Smaller drop-size was obtained wh en multiple-cone spraying occurred. Energy calculations showed that di lute dispersions can be produced more efficiently by electrostatic spr aying than by mechanical agitation.