JOURNAL OF APPLIED ELECTROCHEMISTRY 21 (1991) 537-542 Mass transfer at the gas evolving inner electrode of a concentric cylindrical reactor M. F. EL-SHERBINY, A. A. ZATOUT, M. HUSSIEN, G. H. SEDAHMED* Chemical Engineering Department, Faculty of Engineering, Alexandria University, Alexandria, Egypt Received 1 October 1990; revised 10 December 1990 Mass transfer coefficients for an oxygen evolving vertical PbO 2 coated cylinder electrode were measured for the anodic oxidation of acidified ferrous sulphate above the limiting current. Variables studied included the ferrous sulphate concentration, the anode height, the oxygen discharge rate and the anode surface roughness. The mass transfer coefficient was found to increase with increasing 02 discharge rate, V, and electrode height, h, according to the proportionality expression K oc V~176 Surface roughness with a peak to valley height up to 2.6 mm was found to increase the rate of mass transfer by a modest amount which ranged from 33.3 to 50.8% depending on the degree of rough- ness and oxygen discharge rate. The present data, as well as previous data at vertical oxygen evolving electrodes where bubble coalescence is negligible, were correlated by the equation J = 7.63 (Re. Fr) oA2, where J is the mass transfer J factor (St. 8c~ Notation r al, a2 A C d D e F g h IFe2+ I0 2 K m P P Q R constants t electrode area (cm2) T concentration of Fe2+ (M) /~ bubble diameter (cm) V diffusivity (cm2 s- ~ ) electrochemical equivalent (g C- l ) Z Faraday's constant acceleration due to gravity (cm s -2) Sh electrode height (cm) Re current consumed in Fe 2+ oxidation A Sc current consumed in 02 evolution, A J mass transfer coefficient (cm s- ~ ) St amount of Fe 2+ oxidized (g) Fr gas pressure (atm) p pitch of the threaded surface (cm) v volume of oxygen gas passing any point q5 at the electrode surface (cm3 s -~) gas constant (atm em3mol- ~K- ~ ) 0 1. Introduction Although much work has been done on the effect of gas evolution on the rate of mass transfer, current distribution and ohmic drop in the parallel plate reac- tor [1, 2], little has been done on other geometries of practical importance such as the annular geometry which has the advantage of uniform primary current and potential distribution. The object of the present work is to study the effect of oxygen evolution on the rate of mass transfer at a vertical cylinder anode surrounded by a vertical tubular anode in relation to anode height, oxygen discharge rate and surface * Author to whom all correspondence should be addressed. 0021-891X/91 $03.00 + .12 1991 Chapman and Hall Ltd. peak-to-valley height of the threaded surface (cm) time of electrolysis (s) temperature (K) solution viscosity (g cm- l s-~ ) oxygen discharge velocity as defined by Equation 3 (cm s- 1) number of electrons involved in the reaction Sherwood number (Kd/D) Reynolds number (p Vd/It) Schmidt number (v/D) mass transfer J factor (St. Sc TM) Stanton number (K/V) Froude number (V 2/dg) Solution density, g cm -3 Kinematic viscosity (cm2 s- I) bubble geometrical parameter defined in [31] fractional surface coverage diffusion layer thickness (cm) roughness of the anode. This would assist in predict- ing the rate of mass transfer, not only at stationary vertical cylinders, but also at gas evolving rotating cylinders and gas evolving annular flow reactors used in conducting diffusion controlled reactions accom- panied by gas evolution, as in the case of electrosyn- thesis and electrochemical waste water management. Vogt [3] has shown that for gas evolving rotating cylinders the overall mass transfer coefficient may be predicted in terms of the mass transfer coefficient due to gas evolution and the mass transfer coefficient due to rotation, the same rule applies to gas evolving flow reactors [4-6]. 537