Visualisation of electrolyte flow fields in an electrolysis cell

Abstract

The performance and efficiency of an electrochemical system with gas evolution can be related to the mass transfer effects which are influenced by the resulting two-phase flow. The aim of this investigation was to develop a better understanding in the effects of current density, anode height and inter-electrode spacing on the electrolyte flow patterns and to validate Computational Fluid Dynamic (CFD) model predictions of the electrolyte flow patterns. The CFD model was developed in a previous study and was applied to the experimental rig developed for this study, in which the electrolysis of copper sulphate was studied. A direct flow visualisation technique was used as the method of investigation in the experimental work. To facilitate the visual observation of the electrolyte flow patterns, O2 gas bubbles evolved on the anode surface were used as the flow followers to track the electrolyte flow patterns. At the bottom of the anode where there was no gas evolution, polyamide seeding particles (PSP) were used as the flow followers. A Photron FASTCAM SA4 high speed camera with a capability of recording up to 5000 fps was used to record the electrolyte flow patterns and circulation. The Photron FASTCAM Viewer (PFV) camera software was used for the post analysis of the recordings and for measuring bubble size, bubble speed and the speed of the PSP tracking particles. The experimental results were then compared with the results obtained from the CFD model simulation in order to validate the CFD model. The electrolysis cell was approximated by a simplified planar two-dimensional domain. The fluid flow patterns were assumed to be affected only by the change in momentum of the two fluids. To simplify the model, other physical, chemical and electro-magnetic phenomena were not modelled in the simulation. The Eulerian multiphase flow model was used to model the multiphase flow problem investigated. The flow fields observed in the experiments and predicted by the model were similar in some of the positions of interest. The gas bubble flow field patterns obtained in the experiment and model were similar to each other in Position A (the top front of the anode), C (the area at the bottom of the cell below the anode), and D (the gap between the anode and the diaphragm), with the only exception being Position B (slightly above the anode top back). The experimental results showed an accumulation of the smaller gas bubbles in Position B with a resulting circulation loop across that region. On the other hand, the model predictions did not show this gas bubble accumulation and circulation in Position B. All the flow patterns predicted for the electrolyte flow illustrated similar flow patterns to the ones observed in the experimental results, including the circulation loop in Position B. The bubble speeds measured at Position A in the experimental work had a reasonable agreement with the bubble speeds predicted by the model. The error between the two results ranged from 6% to 29% for the various cases which were tested. An increase in the current density generated more gas bubbles which resulted in an increase in the bubble speed. Increasing the anode height increased the amount of gas bubbles generated as well as bubble speed while the bubble speed was decreased with an increasing inter-electrode distance.