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Published: Kem. Ind. 51 (12) (2002) 503–514
Paper reference number: KUI-10/2002
Paper type: Original scientific paper

Electrochmical Reactor with Three-Dimensional Electrode: Part I. Forced convection

A. Meštrović-Markovinović and T. Todorović


Mass transfer was studied in a vertical flow-through pipe reactor with three dimensional electrodes. The current and electrolyte flows were parallel. The electrolyte was a 0.5 mol dm-3 solution of NaOH with varying ferri-ferrocyanide concentrations, c = 5.5; 10; 15; 30 and 50 mol m-3 . The electrode was composed of metallic sphere particles, dk = 2.65; 3.42 and 4.35 mm, arranged in 1-5 layers. Under potentiostatic experimental conditions, mass transfer was regulated by the limiting current; the change in the limiting current was monitored as dependent on the size of the particles, the number of layers, and the concentration of the electrolyte, with the electrolyte flow velocity ranging from 10-5 m s-1 to 3 x 10-2 m s-1, 0,02 < Re < 200. The experimental data have been successfully correlated by the equation for mass transfer: Sh = 1.14 Re0.51 Sc0.33 (h/dk)1.5 which is in good agreement with the literature data for the same type of reactor with parallel current and electrolyte flow. It has also been determined that a significant influence of natural convection exists. The reactor was set up in such a way that the direction of natural and forced convection is the same, which resulted in increased mass transfer. The region of mixed influence depends on electrolyte concentration, that is, on the intensity of natural convection and the number of layers of the electrode. Analysis of results showed that in the region where natural convection exerts additional influence on the rate, the logarithmic relation for the calculation of kL (relation 15, Figure 6) cannot be employed. In this paper, the mass transfer coefficient was calculated from a linear relation, directly from the measured current values. Besides the surface area of the electrode (which in the case of a three dimensional electrode is defined as the area of the reactor cross section), the calculation takes into account the specific area of the electrode, As, and the sphere diameter, dk, (relation 16). For the region in which only forced convection acts, the value of the mass transfer coefficient no longer depends on the calculation procedure. At a concentration of c ≤ 10 mol m-3, the region of mixed influence of natural and forced convection on total mass transfer extends to Re < 10, regardless of the number of layers (Figure 8), however, at higher concentrations, for example at c ≥ 30 mol m-3, the region of mixed influence extends to the entire investigated region of Re, Figure 10.

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packed bed electrode, flow through reactor, mass transfer, forced convection, mixed convection-additive effect