Overpotential phenomena on charged membranes in flow conditions. Transport mechanism in polarization conditions with KCl and HCl solutions and analysis of boundary layer overpotentials.

Abstract

Previous experiments on a laboratory electrodialysis cell with KCl solutions, conducted in laminar flow conditions, had shown that, when limit current densities are reached, mass transport through the membrane becomes significantly affected by interfacial phenomena not exhaustively considered in the mathematic models generally used. Further determinations with HCl solutions have been carried out in order to eliminate acid-base generation (water splitting) effects in KCl solutions under high current densities. The results obtained with KCl and HCl solutions on cation membranes with the current interruption technique are analyzed and a consistent influence of competitive H + transport in KCl solutions in high polarization conditions is found while in HCl solutions Nernst-Plank equations hold for interfacial concentrations as low as 10 -7N. In previous work great attention had beed dedicated to the study of concentration overpotential η c as a function of current density and of hydrodynamic conditions. However, η c must be determined with the current interruption technique and is a parameter of limited practical use, as it takes into account only concentration polarization phenomena. An overall overpotential difference across the two concentration boundary layers (defined as boundary layer overpotential E(i)) can be directly determined also in large scale units; it includes all potential drops other than those associated to concentration polarization. E(i) correlates satisfactorily with process parameters such as current density, bulk and interfacial concentrations; hence it could be used for better ED process design and control.