Water in the Laboratory

 
Purificación de agua
 
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Elix® Continuous Deionization

This technology is a combination of electrodialysis and ion exchange, resulting in a process which effectively deionizes water, while the ion-exchange resins are continuously regenerated by the electric current in the unit. This electrochemical regeneration replaces the chemical regeneration of conventional ion-exchange systems.

Benefits & Limitations
Benefits
  • Removes dissolved inorganics effectively, allowing resistivity above 5 Ω•cm @ 25° to be reached (which corresponds to a total ionic contamination level in water of approximately 50 ppb).
  • Environment-friendly:
    • no chemical regeneration
    • no chemical disposal
    • no resin disposal
  • Inexpensive to operate.
  • Safe: no heating element.
Limitations
  • Removes only a limited number of charged organics.
  • Requires feed by good quality water (for instance, reverse osmosis-treated water) for economically efficient operation.

The Elix® module consists of a number of “cells” sandwiched between two electrodes. Each cell consists of a polypropylene frame onto which are bonded a cation-permeable membrane on one side, and an anion-permeable membrane on the other.
Merck:/Freestyle/LW-Lab-Water/water-lab/LW-Elix-Continuous-Deionization-Water-456x560.jpg
The space in the center of the cell, between the ion-selective membranes, is filled with a thin bed of ion-exchange resins. The cells are separated from one another by a screen separator. 

The feed water entering the module is split into three parts. A small percentage flows over the electrodes, 65-75% of the feed passes through the resin beds in the cell, and the remainder passes along the screen separator between the cells.

The ion-exchange resins capture dissolved ions in the feed water at the top of the cell. Electric current applied across the module pulls those ions through the ion-selective membrane towards the electrodes. Cations are pulled through the cation-permeable membrane towards the cathode, and anions through the anion-selective membrane towards the anode. These ions, however, are unable to travel all the way to their respective electrodes since they come to the adjacent ion-selective membrane which is of the opposite charge. This prevents further migrations of ions, which are then forced to concentrate in the space between the cells. This space is known as the “concentrate” channel, and the ions concentrated in this area are flushed out of the system to the drain.



The channel running through the resin bed in the center of the cell is known as the “dilute” channel. As water passes down this channel, it is progressively deionized. At the lower end of the dilute channel, where water is free of ions, splitting of H2O occurs in the electric field. This generates H+ and OH- which regenerate the ion-exchange resins, effectively eliminating chemical regeneration.

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