Electrodialysis is used to transport salt from one solution, the diluate, to another solution (concentrate) by applying an electric current. This is done in an electrodialysis cell providing all necessary elements for this process. The conentrate and diluate are separated by the membranes into the two different process streams (concentrate and diluate), as shown in the figure below. An electric current is applied, moving the salt over the membranes.

Applications:

Inside an electrodialysis unit, the solutions are separated by alternately arranged anion exchange membranes, permeable only for anions and cation exchange membranes, permeable only for cations. By this, the two kinds of compartments are formed, distinguishing in the membrane type facing the cathode's direction. Applying a current, cations within the diluate (blue compartment set) move toward the cathode passing the cation exchange membrane facing this side and anions move towards the anode passing the anion exchange membrane. A further transport of these ions, now being in a chamber of the concentrate (red compartments), is stopped by the respective next membrane:

The following picture shows an electrodialysis cell:

The cell consists of two electrode-end blocks (PP, grey) and the membranes stacked between them. The end blocks contain the in- and outlet adapters and the electrical connections. They are pressed together by a steel frame.

The general construction principle of an electrodialysis cell is shown in the following sketch:

The membranes are separated by spacers (5) consisting of a fabric in the active area filled with the electrolyte combined with a sealing around it. The spacer net prevents the membranes from touching each other. The stacked spacers form with their holes tubes, which are arranged in a way to build two different channel systems. By this way, the concentrate and diluate circuit is built.

Electrodialysis makes it possible to transport ionic compounds from one solution to another. Therefore, its application covers the transfer of salts and acids from one solution to another. One common example is sea water desalination.

Not only salt solutions can be desalted and concentrated, but also acids. Examples illustrating this important application field are given in the recovery of pickling acid (German) and the recycling of rinsing solution (German) from the hot dip galvanizing (English review )

As shown before, the electrodialysis process takes place inside the cell (stack). The solutions are circulated through the cells from a storage vessel. Each circuit needs a pump, a storage vessel and piping. By passing the stack one time, the solution is usually not finally treated (desalted from the initial value to the target value). The solution needs to pass the stack several times.

The simplest case, a batch desalination process, is carried out by circulating the solution through the stack until the conductivity of the tank solution has its target conductivity. As a result, the power consumption rises also within the process because the voltage drop over the cell increases.

It is also possible to run an ED process continuously (see Figure above) or in the feed and bleed mode. Both process schemes are shown below. To decide whether a batch or a continuous process should be performed, the stack design has to be taken into account. To run in a continuous mode, the module has to treat the solution in one go. Because you need a certain time and a certain velocity of solution, this corresponds with a certain process length.

A running ED process means that the ions within the cell are moved over the membrane. This key process has to be hold up by all the other tools around the membrane: the stack, the feed flow, the current and the temperature.

One important effect is the polarization of the ions on the membrane surface: Within the solution, all ions move in the extent of their concentration and mobility. On the membrane's surface, both mobiltiy and concentration change dramatically. This means that there is a boundary layer of ions depletion or concentration. An important point is ionic depletion, which has to be prevented because it leads to a high ohms resistance and to water splitting and it may burn the membranes.

Application Examples

Figure 3 shows an example of a batch desalination (conductivity of diluate against time). The effect of a single pass desalination at the start and stop time result in a conductivity jump. It depends on the current, flowing and other factors. The plot shows, that it is - more ore less - proportional to the current. The next diagram shows the salt (calculated as NaCl) removal in dependence of the current at theoretical current efficiency (ce) and at 85% ce. With the PCCell ED 200 you can expect for sodium chloride ce's in the range between 90 and 95 %. It depends on current denity, concentration and other factors. The amount is given per cell pair. A 25 cell pair- unit will make 25 times of this.