Limiting current density of an ion-exchange membrane and of an electrodialyzer

The limiting current density of an ion-exchange membrane was measured using a small-scale electrodialysis apparatus and was expressed by the function of NaCl concentration and linear velocity of a solution in a desalting cell. The limiting current density under a flowing solution in a desalting cell was understandable based on the Nernst-diffusion model assuming that the thickness of a boundary layer is equivalent to that of a diffusion layer, and expressed by the function of NaCl concentration and linear velocity of a solution in a desalting cell. A spacer is usually considered to function as turbulence promoter. However, it seems block the main stream of laminar flow in a desalting cell, generate dead spaces between the spacer and a membrane and decrease the limiting current density. In order to increase the limiting current density, the solution velocity and the Reynolds number must be increased and create turbulent flow. In an ion-exchange membrane electrodialyzer, solution velocities in desalting cells are not uniform between the cells incorporated in a stack. This event produces electrolyte concentration distribution between the cells and current density distribution in an electrodialyzer. When an electric current reaches the limiting current density of an ion-exchange membrane at the outlet of a desalting cell in which velocity and electrolyte concentration are the least, the average current density applied to an electrodialyzer is defined as the limiting current density of the electrodialyzer. In this study, the solution velocity distribution was measured using a practical and a semi-practical electrodialyzer. Further, we computed the relationship between the standard deviation of normal distribution of linear velocities in desalting cells and electrolyte concentration at the outlet of the desalting cell in which velocity and electrolyte concentration are the least. Based on the above computation and the evaluation of the limiting current of an ion-exchange membrane, the limiting current density of an electrodialyzer was determined.