Towards spacer free electrodialysis

In electrodialysis (ED) processes, the operating current density is limited by concentration polarisation. However, in order to get the maximum ion flux per unit membrane area it is desirable to operate at the highest possible current density. The fluid flow in ED modules mainly takes place in flat channels with rectangular cross-sections, where the membranes form the walls and the channels are filled with spacer material. The spacer design itself has great effect on the process costs and has been investigated intensively in recent years. Another way of promoting turbulence in the channel is by using air sparging. This work in fact shows the impact of air sparging on the ED process. The influence of the gas/liquid ratio in combination with various spacer configurations is discussed and compared with the situation where no feed spacer is present at all. This comparison clearly shows that the mass transfer increase is most effective in a spacer free cell where the mass transfer increases linearly with increasing G/L ratio. Of all cases studied here, the spacer free cell combines the highest mass transfer increase with the lowest increase in cell resistance.