Mass Transport to Cylindrical Electrodes Rotating in Suspensions of Inert Microspheres

Abstract

Mass transfer enhancement produced by the addition of inert microspheres was investigated on a rotating cylinder electrode operating in the turbulent regime. The effects of rotation speed, rotor radius, particle size, solids volume fraction, and particle density on the rate of mass transfer were determined by limiting current measurements for ferricyanide ion reduction. In comparison to transport rates observed with clear electrolytes, up to two and a half fold higher limiting currents were obtained in concentrated suspensions containing 5–80 μm diameter microspheres with densities ranging from 0.7 to 2.1 g/cm3. Transport enhancement is attributed to the microconvective eddies produced by particle rotation in the shear field adjacent to the spinning electrode, and to the increased shear rate caused by the formation of a particle‐free wall layer. Experimental data could be correlated in the formwhere Sh, Re, and Sc are the Sherwood, Reynolds, and Schmidt numbers, respectively, and α and β are empirical functions of solids volume fraction determined from transport rate measurements. The addition of appreciable volume fractions of inert particles to the electrolyte results in significant improvements in production capacity when the latter is limited by transport rates. The use of suspended particles to achieve a given increase in limiting current density is shown to require much less power than simply increasing electrode rotation speed to enhance mass transport.