Regeneration cycle, water softener

The principles at play in water softeners are the same as those in precipitation reactions. Water is run over a dense material saturated with sodium ions like a gigantic network of salts. The sodium salts are soluble, whereas the calcium analogs of the same salts are not. So the calcium replaces the sodium and becomes trapped on or in the solid. The water, now enriched in sodium instead of calcium, can be used without worry that it will precipitate soap or form the crusty deposits of scale. In the regeneration cycle of the water softener, the solid support is backwashed with water saturated with so much sodium salt that the sodium eventually replaces the calcium on the solid, and the water softener is ready to start softening again. 

Softeners can be a microbial concern. A dark and moist column interior can provide a growth environment. The regeneration cycle which uses concentrated brine solution and a backwash cycle aids in reducing the bioburden. Softeners should be regenerated based on a time clock set for twice weekly regenerations and on a volumetric flow of water, whichever is shorter. Since the regeneration cycle removes the softener bed from operation, a dual bed system is often specified. 

Determine the amount of water to be treated per cycle and the amount of hardness to be removed. Softening of water requires use of a cation-exchange resin operated in sodium form to exchange divalent hardness cations for sodium regenerated with aqueous sodium chloride solution. Total amount of water to be treated is (100 gal/min)(60 min/h)(8 h/cycle) = 48,000 gal/cycle (182 m3/ cycle). 

The partial demineralization of water by the dealkalization-degassing-softening route may be described as a combined cycle process meaning that two different regenerants are used. Dealkalization by itself is a single cycle process and is widely used to remove alkaline hardness from waters used for cooling, and in the beverage industries where uncontrolled alkaline hardness can deleteriously affect the quality of product. 

Sodium cycle softeners with co-flow regeneration softened water hardness = 1-3 mg/1. 

Water treatment uses Na cycle softening Decationization Mixed bed D.I. Fixed number of exchange sites Rate of neutralization fast ri/2 < 5 minutes Regeneration mass-action controlled Excess of regenerant required Operating capacity dominated by equilibrium factors Water treatment uses Acid neutralization Silica removal Mixed bed D.I. 

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. ... 

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