Process modelling of an alkaline water electrolyzer

In this paper a model for the prediction of the product gas purity in alkaline water electrolysis is proposed. For the estimation of the exhaust gas compositions the operating conditions, such as current density, electrolyte flow rate, concentration and temperature as well as process management possibilities are considered. The development of the model relies on a classical process engineering approach and depicts the electrolysis cell through coupled continuously stirred-tank reactors. Furthermore, the mass transport phenomena between the phases are considered through the application of Reynolds and Sherwood correlations. Finally, the validation of the model is performed through experiments, which are carried out in a lab-scale electrolyzer with a 150 cm² zero-gap cell and KOH electrolyte at atmospheric pressure. This investigation reveals that gas purity in alkaline water electrolysis is mainly affected by mixing the anodic and cathodic electrolyte cycles, which transport dissolved electrolysis products into the opposite half cell compartments. However, this transport mechanism can be significantly reduced by adjustment of the operating conditions of the electrolyzer.