11/9/2022 0 Comments Use high s resin![]() ![]() Figure 8-4 is a schematic illustration of a strong acid cation exchange resin bead, which has ionic sites consisting of immobile anionic (SO 3¯) radicals and mobile sodium cations (Na +). The other ionic group is attached to the bead structure. However, only one of the ionic species is mobile. These functional groups consist of both positively charged cation elements and negatively charged anion elements. In addition to a plastic matrix, ion exchange resin contains ionizable functional groups. In addition to polystyrene-divinylbenzene resins (Figure 8-3), there are newer resins with an acrylic structure, which increases their resistance to organic fouling. Macroreticular anion resins (Figure 8-2) are also more resistant to organic fouling due to their more porous structure. These resins possess a higher physical strength than gels, as well as a greater resistance to thermal degradation and oxidizing agents. Macroreticular resins feature discrete pores within a highly cross-linked polystyrene-divinylbenzene matrix. However, in some applications the physical strength and chemical resistance required of the resin structure is beyond the capabilities of the typical gel structure. This structure meets the chemical and physical requirements of most applications. Standard gelular resins, such as those shown in Figure 8-1, have a permeable membrane structure. One of the most significant changes has been the development of the macroreticular, or macroporous, resin structure. ![]() Although the basic resin components are the same, the resins have been modified in many ways to meet the requirements of specific applications and provide a longer resin life. Polystyrene-divinylbenzene resins are still used in the majority of ion exchange applications. This innovation made the complete demineralization of water possible. The polystyrene-divinylbenzene-based anion exchan-ger could remove all anions, including silicic and carbonic acids. These resins were very stable and had much greater exchange capacities than their predecessors. 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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