Mixed bed deionizers

Example 2 Calculation of Variance In mixed-bed deionization of a solution of a single salt, there are 8 concentration variables 2 each for cation, anion, hydrogen, and hydroxide. There are 6 connecting relations 2 for ion exchange and 1 for neutralization equilibrium, and 2 ion-exchanger and 1 solution electroneutrality relations. The variance is therefore 8 — 6 = 2. 

The progress of the experiment is followed by adding the aliquot to a flask containing about 50 ml of ctK)l mixed bed deionized or distilled water. I he acidity is titrated at once using phenolphthalcin indicator and standardized... 

The two basic deionizer configurations are the two-bed and mixed-bed. Two-bed deionizers have separate vessels for the cation and anion resins. In mixed-bed deionizers, the anion and cation resins are blended in a single vessel. Generally, mixed-bed systems produce higher-quahty water but have a lower total capacity compared with two-bed systems. Figures 3 and 4 are schematics of typical ion exchange demineralization processes. 

Application to Mixed-Bed Deionization. The removal of a salt M+X" from water in a single bed utilizes the three reactions ... 

The presence of alumma in the surface of colloidal sdica particles not only provides for charge sites independent of pH but also inhibits the conversion to cristobalite. Hence it is possible to add alumina to the surface of colloidal silica by reacting with sodium aluminate. The manufacture of such products is discussd above in the first part of this chapter. After alumina is added to the particle surface, the particles are deionized to remove the sodium. This provides a product that is both stable, low in sodium, and has a pH of 3-5. Adjusting the pH with ammonia is not necessary. The product is stable at the lower pH that results from being deionized because the alumina provides sufficient charge density to avoid coagulation. If mixed bed deionization is used then the absence of salts further enhances the stability of such a product. 

A major effort was initiated to deionize all three reactor basins on a continuous basis using portable deionizing systems, which had been in use since the mid-1960s. This effort resulted in some improvements in all three basins, but because of the hmited number and availabihty of portable deionizers for the three basins, the improvements were slow, and new mixed bed deionizers were acquired for the K and L basins. 

Janus, J.W., Kenchington, A.W. and Ward, A.G. (1951) A rapid method for the determination of the isoelectric point of gelatin using mixed bed deionization. Research, 4, 247. 

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