Continuous electrodeionization

Continuous electrodeionization is widely used at present for the preparation of high-quality deionized water for the electronic industry or in analytical laboratories [30]. Compared to the deionization by conventional ion-exchange techniques, the continuous electrodeionization has the advantage that no chemicals are needed for the regeneration of the ion-exchange resins, which is time-consuming, labor-intensive, and generates a salt containing wastewater. There are some variations of the basic design as far as the distribution of the ion-exchange resin is concerned, and recently bipolar membranes are also used in the process [6].

4.3.1 System components and process design aspects

The process design in electrodeionization is very similar to that of conventional electrodialysis. The main difference is that in a continuous electrodeionization stack, the diluate cells and sometimes also the concentrate cells are filled with ion-exchange resins. There are different concepts used for the distribution of the cation- and anion-exchange resins in the cell. Two more frequently applied stack designs are illustrated in Fig. 18a and b.

Fig. 18a shows a conventional electrodialysis stack in which the diluate cell is filled with a mixed-bed ion-exchange resin. Both cations and anions are adsorbed by the ion-exchange resins and then transported by an electrical potential gradient through the corresponding ion-exchange resin toward the adjacent concentrate cells facing the cathode and the anode, respectively. Since the ion conductivity in the ion-exchange resin is several orders of magnitude higher than in the deionized water, the stack can be operated economically at relatively high current density compared to conventional electrodialysis.

However, the use of a mixed-bed ion-exchange resin in continuous electrodeionization results in a rather poor removal of weak acids and bases such as boric or silicic acid. Much better removal of weakly dissociated electrolytes can be obtained in a system in which the cation- and anion-exchange resins are placed in separate beds with a bipolar membrane placed in between as illustrated in Fig. 18b, which shows a diluate cell filled with a cation-exchange resin facing toward the cathode separated by a bipolar membrane from a diluate cell facing the anode. A cation-exchange membrane, a cation-exchange resin, a bipolar membrane, an anion-exchange resin, an anion-exchange resin, and a concentrate cell form a repeating unit between two electrodes.

Feed

Feed

Continuous ElectrodeionizationContinuous Electrodeionization

Figure 18 Schematic drawing illustrating different stack concepts used in continuous electrodeionization: (a) a conventional stack with diluate cells filled with a mixed-bed ion-exchange resin and (b) a stack with cation-exchange and anion-exchange resins in different diluate cells and regeneration of the ion-exchange resins by H+ and OH~ ions generated in a bipolar membrane.

Figure 18 Schematic drawing illustrating different stack concepts used in continuous electrodeionization: (a) a conventional stack with diluate cells filled with a mixed-bed ion-exchange resin and (b) a stack with cation-exchange and anion-exchange resins in different diluate cells and regeneration of the ion-exchange resins by H+ and OH~ ions generated in a bipolar membrane.

The main difference between the electrodeionization system with the mixed-bed ion-exchange resins and the system with separate beds is that in mixed-bed electrodeionization systems, anions and cations are simultaneously removed from the feed while the solution leaving the diluate cell is neutral. In the electrodeionization system with separate ion-exchange beds and bipolar membranes, the cations will first be exchanged by the protons generated in the bipolar membrane with the result that the solution leaving the cation-exchange bed is acidic. This solution is then passed through the cell with the anion-exchange resin where the anions are exchanged by the OH~ ions generated in the bipolar membrane and the solution is neutralized, and at the exit of the anion-exchange-filled cell, the solution is also neutral.

4.3.2 Operational problems in practical application of electrodeionization

The main problem of an electrodeionization system with a mixed-bed ionexchange resin is the incomplete removal of weak acids or bases. But also electrodeionization systems with separate resin beds are affected by uneven flow distribution in the resin bed, which leads to poor utilization of the ionexchange resins. The fouling of the ion-exchange resins by organic components such as humic acids and bacterial growth on the surface of the resin is another problem that requires a very thorough pretreatment of the feed solution to guarantee a long-term stability of the system.

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