Liquid-Solid Mass Transfer at a Fixed Bed of Lessing Rings, in Relation to Electrochemical Reactor Design

The mass-transfer behavior of a fixed bed of Lessing rings, under single- and two-phase flow, was studied by measuring the limiting current of the cathodic reduction of the ferricyanide ion. Variables studied included ring diameter (d), solution flow rate, gas flow rate, physical properties of the solution, and the effect of drag-reducing polymers. The single-phase mass-transfer data were correlated for the conditions of 1390 < Sc < 4763 (where Sc is the Schmidt number, which represents a dimensionless kinematic viscosity/molecular diffusivity coefficient), 166 < Re < 722 (where Re is the solution Reynolds number, which represents a dimensionless flow parameter), and 1 cm < d < 1.4 cm by the equation Sh = 1.57Sc0.33Re0.46 (where Sh is the Sherwood number, which represents a dimensionless mass-transfer coefficient). The two-phase mass-transfer data were correlated for the conditions 1390 < Sc < 4763, 144 < Re < 748, 60 < Reg < 818 (where Reg is the gas Reynolds number), and 1 cm < d < 1.4 cm by the equation Sh = 1.93Sc0.33Re0.34Reg0.11. Polyox WSR-301 drag-reducing polymer was found to reduce the rate of mass transfer by an amount of 17.2%-32.7% for single-phase flow; for two-phase flow, the decrease in the rate of mass transfer was 27%-34%. Implication of the present results for the design and operation of electrochemical reactors-especially those involving gas absorption-was noted.