Sodium Zeolite Softening

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  • Topic: Water, Sodium, Water softening
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  • Published : October 10, 2012
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Sodium Zeolite Softening

Introduction
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Ion exchange is the process in which materials exchange one ion for another, hold it temporarily, and release it to a regenerating solution. These materials are widely used to treat raw water supplies that contain dissolved salts. Today, the most commonly used material is an ion exchange resin. Resins are plastic beads to which a favorable ion has been chemically attached which can be exchanged for unwanted ions dissolved in the raw in the water supply. Once the resin has given up or exchanged most of its favorable ions, it is said to be exhausted and needs to be regenerated by coming in contact with a strong solution of ions called the regenerant.

Sodium zeolite softeners historical used natural zeolite materials but have now been replaced with exchange resins made of polystyrene. These resins have sodium ions loosely attached and will readily give up the sodium for a more desirable ion such as calcium and magnesium. This exchange is only for cations of positively charged ions. This is why sodium zeolite resin is referred to as a cation exchange resin. . The primary exchange reactions occurring at the surface of a sodium cation exchange resin are : 2RSO3-Na+ ⇔ (RSO3-)2Ca2+

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Ca(HCO3)2

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2Na(HCO3)2

Hardness Breakthrough at End of Softener Run
End of Run

...where "R" represents the solid resin. The water to be softened is passed through the vessel containing resin. Calcium and magnesium ions are exchanged for the sodium ions in and on the resin beads. The sodium then takes the place of the calcium and magnesium in associating with the anionic .

Na+
Effluent Level

TH

Volume Through Softener

Steam Generation Systems, Inc.

1108 Lavaca St., Suite 110-309 • Austin, Texas 78701 USA • 832-725-7662 •www.SteamGenerationSystems.com

Sodium Zeolite Softening

species in solution to provide charge balance. Although sodium zeolite treated water is nearly free of all hardness, some hardness leakage normally does occur. The amount of leakage depends primarily on the raw water hardness, sodium concentration and softener regeneration level. A plot of the softener effluent profile shows a low, nearly constant effluent hardness level until the ion exchange resin nears exhaustion. At this point, the hardness level usually increases quite rapidly and resin regeneration is required. The resin bed is regenerated by reversing the softening reactions. Exhausted sodium zeolite resin is regenerated by exposure to a concentrated brine solution - sodium chloride. Analogously, strong acid cation resin is regenerated by exposure to a strong acid (H2SO4). Concentrated brine used for sodium zeolite regeneration is 26% by weight sodium chloride. However, because of the dilution of the brine as it is forced through the softener vessel, the actual application strength of the brine is closer to Cation Relative Affinity 10% when it reaches the resin bed. αi/Na Various cations are exchanged at resins to different degrees. The order of preference or selectivity for a specific cation to be exchanged is a complex function of charge and strength of solvation. In dilute solutions, the general rule is that ions with the highest charge and the lowest strength of solvation are preferentially exchanged. A partial list of relative affinities for ion exchanged at cation exchange resins are contained in the adjacent table. Ba2+ Pb2+ Cu2+ Ca2+ Zn2+ Fe2+ Mg2+ Na+ H+ 5.80 5.00 2.60 1.90 1.80 1.70 1.67 1.0 0.67

Equipment
Softener systems consist of a tank, valves and a means of transporting the brine to the softener tank. Usually the softener tank is a steel, vertical shell, pressure vessel with dished heads. The interior of the tanks are sometimes lined with corrosion resistant coatings to prevent attack by the brine regenerating solutions. The tank includes a service and...
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