Published: Kem. Ind. 52 (2) (2003) 45–52
Paper reference number: KUI-20/2002
Paper type: Original scientific paper
Electrochemical Reactor with Three-Dimensional Electrode: Part III Mixed convection
A. Meštrović-Markovinović and A. Marijić
The mixed influence of natural and forced convection on 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. The electrode was composed of metallic sphere particles, arranged in 1-5 layers. Measurements were made potentiostatically at the limiting current potential. 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. Investigations were conducted under the conditions without the electrolyte flow rate and at various electrolyte flow velocity through the layer: 0 < e < 3 x 10-2 m s-1. Measurements have shown that mixed convection takes place in the region of slower electrolyte flow. The same direction of flow of natural and forced convection leads to a decrease in the thickness of the boundary layer, which results in increased mass transfer. From figures 2(a), (b), (c) and (d) it is evident that the measured values of the limiting current, Ig are greater than the corresponding expected values of Ig , if only forced convection was acting. Comparing the figures 2(a) and (c) (dk = 2.65 mm), as well as 2(b) and (d) (dk = 4.35 mm), which depict the measured Ig values for different concentrations and the same sphere size, it is evident, from the increased intensity of natural convection, that the region of mixed convection is translocated to a greater electrolyte flow by one order of magnitude. This indicates that the width of the mixed region primarily depends on the intensity of natural convection. At electrolyte concentration c > 10 mol m-3, independent of the sphere particle size, it is evident that the region of mixed convection depends on the number of layers, that is, with an increase in the number of layers, the region of mixed convection expands towards greater rates of forced convection, figure 4. A relation has been set up that describes mass transfer under the conditions of mixed natural and forced convection, for the additive effect: Sh = 137 Reu0,47 Sc0,33 (h/dk)1,5 which holds true in the investigated region: za 0 < Re < 100 i 105 < Gr Sc < 108. The relation is in good accordance with experimentally obtained results and the deviations are less than 4 %, Figures 5. and 6. These investigations in the region of mixed convection are important for the correct orientation of the reactor with three-dimensional electrode where the processes of metal deposition take place, independent of whether we deal with the purification of water or metal extraction from diluted solutions. These investigations may also be applied in reactors with an immobile layer of the solid catalyst, when the flow of the solution by natural convection is established due to a change in temperature of the catalyst.
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packed bed electrode, parallel current and electrolyte flow, mass transfer, natural convection, forced convection, mixed convection, aiding flow