Concentration Polarization and Limiting Current Density

Concentration polarization occurs in all mass separation processes and is the result of changes in mass transport properties at an interface. In electrodialysis, separation of ions is the result of differences in their transport numbers in solution and in the membranes. At the surface of an ion exchange membrane facing the diluate the concentration of counterions is reduced and at the surface facing the concentrate the concentration of counterions is increased because of the lower transport number of the counterions in the solution than that in the membrane. Because of the electroneutrality requirement, the co-ions in the boundary layers are transported in the opposite direction. Thus, salt concentration gradients are established in the boundary layers at membrane surfaces, which leads to an additional mass transport towards the membrane surface in the diluate and away from it in the concentrate solution. Due to turbulent mixing m .mig cou m .mig cou

Laminar boundary layer

Figure 3 Schematic drawing illustrating concentration profiles of a salt in the boundary layer on both sides of an ion exchange membrane and the fluxes of cations and anions in the boundary layer and the membrane surface. For abbreviations, see Table 1.

Laminar boundary layer

Figure 3 Schematic drawing illustrating concentration profiles of a salt in the boundary layer on both sides of an ion exchange membrane and the fluxes of cations and anions in the boundary layer and the membrane surface. For abbreviations, see Table 1.

of the bulk solutions, the concentration gradients are limited to a relatively thin laminar boundary layer at the membrane surfaces, as indicated in Figure 3, which shows the salt concentration profiles in the solutions near the surface of an anion exchange membrane.

The concentration profiles at the membrane surface can be determined by a mass balance based on the so-called Nernst film model, which assumes static boundary layers at the membrane surfaces, where concentration and electrical potential gradients perpendicular to the membrane surfaces are the only driving forces for the mass transport. The bulk solution between the laminar boundary layer is well mixed and has a uniform concentration. It can be assumed that the transport of ions through an ion exchange membrane is the result of migration caused only by an electrical potential gradient, while in the solution ions are transported by both migration and diffusion. In a steady state the ion flux through the membrane is identical to that through the boundary layer. For a strictly permselective membrane it is given by:

The current density can be obtained from eqn [21] by integration over the boundary layer. For the boundary layer at the membrane surfaces adjacent to the diluate the current density is:

FDs d md

Cs Cs i

When the hydrodynamic flow conditions are kept constant the boundary layer thickness, Az, will be constant and the current will reach a maximum value independent of the electrical potential gradient if the salt concentration at the membrane surface, mdCs, becomes 0. This maximum current is referred to as the limiting current density, illm, which is given by:

Exceeding the limiting current density in practical applications of electrodialysis can affect the efficiency of the process severely by increasing the electrical resistance of the solution and by causing water splitting, which leads to changes in the pH values of the solutions, causing precipitation of metal hydroxides on the membrane surface.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment