In the middle 1940's, ion exchange resins were developed based on the copolymerization of styrene cross-linked with divinylbenzene. However, early anion exchangers were unstable and could not remove such weakly ionized acids as silicic and carbonic acid. The new anion resin was used with the hydrogen cycle cation resin in an attempt to demineralize (remove all dissolved salts from) water. Soon, an anion exchange resin (a condensation product of polyamines and formaldehyde) was developed. The development of a sulfonated coal cation exchange medium, referred to as carbonaceous zeolite, extended the application of ion exchange to hydrogen cycle operation, allowing for the reduction of alkalinity as well as hardness. Microscopic view of cellular resin beads (20-50 mesh) of a sulfonated styrene-divinylbenzene strong acid cation exhcanger. It is usually expressed in kilograins per cubic foot as calcium carbonate.įigure 8-1. Capacity is defined as the amount of exchangeable ions a unit quantity of resin will remove from a solution. Greensand had a lower exchange capacity than the synthetic material, but its greater physical stability made it more suitable for industrial applications. The synthetic zeolite exchange material was soon replaced by a naturally occurring material called Greensand. Although aluminosilicate materials are rarely used today, the term "zeolite softener" is commonly used to describe any cation exchange process.
In 1905, Gans, a German chemist, used synthetic aluminosilicate materials known as zeolites in the first ion exchange water softeners. For example, in a sodium zeolite softener, scale-forming calcium and magnesium ions are replaced with sodium ions. In an ion exchange system, undesirable ions in the water supply are replaced with more acceptable ions. Ion exchangers exchange one ion for another, hold it temporarily, and then release it to a regenerant solution. Ion exchange systems are used for efficient removal of dissolved ions from water. For high-pressure boiler feedwater systems and many process systems, nearly complete removal of all ions, including carbon dioxide and silica, is required. Hardness ions, such as calcium and magnesium, must be removed from the water supply before it can be used as boiler feedwater. Overheating caused by the buildup of scale or deposits formed by these impurities can lead to catastrophic tube failures, costly production losses, and unscheduled downtime. Ionic impurities can seriously affect the reliability and operating efficiency of a boiler or process system. Positively charged ions are called cations negatively charged ions are called anions. All natural waters contain, in various concentrations, dissolved salts which dissociate in water to form charged ions.
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