Modelling current efficiency in an electrochemical hypochlorite reactor

Abstract

© 2014 The Institution of Chemical Engineers. A mathematical model was set up for anodic, cathodic and overall current efficiencies of an electrochemical cell for hypochlorite production acting as an ideal stirred reactor. A 0.06-0.20moldm-3 NaClO hypochlorite solution was obtained in the cell by the electrolysis of 0.25-0.50moldm-3 sodium chloride solution at a temperature of 20°C, at a current density of 100mAcm-2 and at 8.3<pH<8.7. Anodic current losses occur through hypochlorite and water oxidation at the DSA anode, and cathodic current losses result from hypochlorite reduction at the titanium cathode. Theoretical dependences of the anodic current efficiency on the overall anodic current density, hypochlorite concentration and partial current density for water oxidation were established. Theoretically derived relations and the corresponding experimental results showed that oxygen evolution due to water and hypochlorite oxidation leads to reduced thickness of the anodic diffusion layer and increases the limiting diffusion current for hypochlorite oxidation. Mathematical models for the dependence of the cathodic current efficiency on the cathodic current density and hypochlorite concentration in the bulk solution were also established. The expression for the overall current efficiency of the electrochemical cell for hypochlorite production was derived on the basis of both the anodic and the cathodic current efficiency. The good agreement between experimental results and the values provided by the mathematical models has confirmed the correctness of the proposed models, suggesting that the model can be used to optimise electrolysis parameters. (The term hypochlorite is used to include both hypochlorite and undissociated hypochlorous acid.). The concentration of dissolved elementary chlorine in the slightly alkaline environment is negligible compared to hypochlorite concentration.