2-PHASE MASS-TRANSFER IN CHANNEL ELECTROLYZERS WITH GAS-LIQUID FLOW

The electrochemical limiting current method was employed to study the mass transfer to a solid electrode in cocurrent gas-liquid flow through a vertical parallel-plate electrolyzer. Three systems were investigated: aqueous ferricyanide, aqueous ferricyanide containing a dispersion of nitrogen bubbles, and aqueous electrolyte containing a dispersion of oxygen bubbles in equilibrium with the liquid phase. The total mass-transfer rate was found to be the sum of three contributions: (i) the one-phase convective rate associated with the liquid as if it were flowing alone through the cell; (ii) the enhancement of mass transfer owing to disruption of the mass-transfer boundary layer, even by bubbles containing inert gas; (iii) the further enhancement owing to penetration of the mass-transfer boundary layer by bubbles containing reactive gas. A series of controlled experiments was conducted to determine the dependence of these enhancement mechanisms upon operating variables such as gas and liquid flow rates, bubble size, and electrode material. It was found that, although the conversion per pass through the cell was negligible, a sevenfold increase in the mass transfer, as compared to one-phase flow with the same liquid velocity, was obtained with a reactive gas void fraction as low as 10%.