Sodium softeners cation resin

Sodium softeners are used to treated RO influent water to remove soluble hardness (calcium, magnesium, barium, and strontium) that can form scale on RO membranes. Once known as sodium zeolite softeners, zeolites have been replaced with synthetic plastic resin beads. For sodium softeners, these resin beads are strongly acidic cation (SAC) polystyrene resin in the sodium form. The active group is benzene sulfonic acid, in the sodium, not free acid, form. Figure 8.12 shows styrene-divinylbenzene gel cation resin. Equation 8.4 shows the softening reaction for calcium exchange ... 

Resins containing these functional groups are generally available in one or more forms for example, strong acid cation resin is typically a sulfonated polystyrene type, available either in the hydrogen form (H+ ) or in the sodium form (Na+). The sodium form of this resin is used for water softening. 

Replacement of positive ions of a soluble salt by passing sodium through a cation resin (softening or base exchange) or hydrogen through an acidic resin (as in one half of the demineralization process). 

While displacement is the more correct term, the best known application is that of softening.. A cation resin in the sodium form is used to soften tap water. The residual hardness is tested by EDTA titration or standard soap solution. 

Figure 21. Schematic representation of the softening of a hard water feedstream on an ideally continuous bed of strong cation resin in the sodium form.f ...

Softened and Demineralized Water. Ionic contaminants are removed from water by softening or demineralization. The softening process removes the hardness elements (and other multivalent cations) that are of major concern to the chlor-alkali producer. Chemical softening was the first process developed, but modem plants use a cation-resin process. Exposing water to a resin containing labile sodium ions allows the removal of divalent cations and their replacement with sodium. The chemistry is that discussed in Section 7.5.5.1. When the resin is nearly saturated with hardness ions. 

These affinity relationships are reversed in concentrated solutions. This is what makes regeneration of exhausted resins possible. An exhausted cation resin used for softening is predominantly in the calcium and magnesium form, both divalent ions. The resin is restored to its regenerated condition, the sodium form, by the introduction of 10 percent sodium chloride. This sodium chloride solution is concentrated enough (10,000 ppm) to reverse the selectivity. The driving force of the monovalent sodium ion then converts the resin back to the sodium form. 

Ion exchange resins are used in columns 8 to 10 feet high and 6 to 10 feet in diameter holding between 100 and 300 ft of resin. The resin bed depth is only 2 to 4 feet. Flow rates are rapid (3000-4500 gal/hr), and the cycle is short (8-16 hours). Regeneration is accomplished in the column with a 10 to 15 percent salt solution at 140 to 160°F. The chloride form of a strong anionic resin decolorizes the liquor, and the sodium form of a strong cation resin softens the liquor. The sweet water from the various adsorbent columns are recovered by returning them to the melter. ... 

Water is softened by removing calcium and magnesium ions from hard water in exchange for sodium ions at sites on cation-exchange resin. Water softeners typically use a gel polystyrene sulfonate cation-exchange resin regenerated with a 10% salt brine solution (25). 

Part of a softener regeneration process whereby the cation ion exchange resin is converted to the sodium form by the application of a, typically, 10 to 15% W/V strength of brine (sodium chloride). 

Sodium ion exchange. Sodium ion exchange on zeolites (Section 7.3) or on synthetic organic cation-exchange resins such as Dowex-50 (a sulfonated polystyrene Fig. 14.1), in most circumstances, is superior to the above softening methods.13 The exchange process favors binding of Ca2+ or Mg2+ over Na+ in the solid resin phase ... 

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