For many laboratory and industrial applications, high-purity water which is essentially free form ionic contaminants is required. Water of this quality can be Produced by deionization.
The two most common types of deionization are:
The two-bed deionizer consists of two vessels - one containing a Cation-exchange resin in the hydrogen(H+) form and the other containing an anion resin in the hydroxyl (OH-) form. Water flows through the caution column, whereupon all the captions are exchanged for hydrogen ions. To keep the water electrically belanced, for every monovalent Caution, e.g. Na+, one hydrogen ion is exchanged and for every divalent caution, e.g. Ca2+ , or Mg2+, two hydrogen ions are exchanged. The same principle applies when considering anion exchange. The desalinized water then flows through the anion column. This time all the negatively changed ions are exchanged for hydroxide ions which then combine with the hydrogen ions to form water (H2O).
In mixed-bed deionizers the caution-exchange and anion-exchange resins are intimately mixed and contained in a single pressure vessel. The thorough mixture of Caution-exchangers and anion-exchangers in a single column makes a mixed-bed deionizer equivalent to a lengthy series of two-bed plants. As a result, the water quality obtained from a mixed-bed deionizer is appreciably higher than that produced by a two-bed plant.
Although more efficient in purifying the incoming feed water, mixed-bed plants are more sensitive to impurities in the water Supply and involve a more complicated regeneration process. Mixed-bed deionizers are normally used to 'polish' the Water to higher levels of purity after it has been initially treated by either a two-bed deionizer or a reverse osmosis unit.
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