Effect of the Type of Impellers on Mixing in an Electrochemical Reactor

Abstract

The electrochemical process is an alternative method to remove hexavalent chromium Cr(VI) from industrial wastewaters process. However, in electrochemical plug flow reactors with no liquid mixing or static electrodes, iron salt film is formed on the electrodes surface (electrode passivation) because of the poor diffusion and mass transfer that reduces the Cr (VI) removal efficiency and causes greater energy consumption. Therefore, the electrochemical reactors require to be designed to provide high mass transfer between the bulk liquid and the electrodes. Electrochemical reactors with electrodes such as the rotating electrodes have been studied to reduce the passivation effect. It has been demonstrated that in this kind of electrochemical reactors, the flow velocity field and turbulence intensity are not and there is a zone inside the rotating ring electrodes, about 35 % of the reactor volume, with low velocity, turbulence and vorticity that reduces the mass transfer and process efficiency. In this study, the performance of two kinds of impellers (four internal fins and one internal pitched blade turbine) located inside the ring electrodes zone were evaluated in an electrochemical reactor with rotating ring electrodes with a volume capacity of 18 L. Their performance was compared with the reactor without impellers inside the ring electrode. Experimental test at different rotational speeds were performed with both kind of impellers to evaluate the mixing time and the power consumption. CFD tools were used to describe their performance. The mixing time was reduced up to 33 % for the 4 internal fins reactor and 52 % with the pitched blade central (PB4) turbine in comparison with rotating ring electrode without internal impellers. The highest mean turbulence intensity and vorticity was reached with the pitched blade central turbine and the power consumption was the same in all the cases.