The ion exchange water softener is one of the most common tools of water treatment. Its function is to remove scale-forming calcium and magnesium ions from hard water. In many cases soluble iron (ferrous) can also be removed with softeners. A standard water softener has four major components: a resin tank, resin, a brine tank and a valve or controller.
Ion exchange is an effective, versatile means of conditioning boiler feed water. The term “ion exchange” describes the process: as water flows through a bed of ion exchange material, undesirable ions are removed and replaced with less objectionable ones.
For example, in softening processes, calcium and magnesium ions (hardness) are exchanged for sodium ions. In dealkalization, the ions contributing to alkalinity (carbonate, bicarbonate, etc.) are removed and replaced with chloride ions. Other dealkalization processes utilizing weak acid cation resin or strong acid cation resin in a split stream process, exchange cations with hydrogen. This forms carbonic acid which can be removed in a decarbonator tower. Demineralization is simply replacing all cations with hydrogen ions (H+) and all anions with hydroxide ions (OH–). Ion exchange materials are like storage batteries; they must be recharged (regenerated) periodically to restore their exchange capacity. With proper design and operation, ion exchange processes are capable of removing selected ions almost completely (in some cases to a fraction of a part per million).
There has been constant improvement in ion exchange materials since the first use of natural and synthetic inorganic products. Sulfonated coal, styrene-base resins, phenolic resins and acrylic resins are some that have been developed. Exchange capacities were greatly increased with the development of the styrene-base exchangers. These resins are manufactured in spherical, stress and strain-free form to resist physical degradation. They are stable at temperatures as high as 300°F and are applicable over a wide pH range. More dense resins, those having greater degrees of copolymer crosslinking, were specially developed for heavy duty industrial applications. These products are more resistant to degradation by oxidizing agents such as chlorine, and withstand physical stresses that fracture lighter duty materials. Weakly acidic cation exchange resins contain carboxylic and phenolic groups. They remove alkalinity by exchanging their hydrogen ions for the cations associated with the bicarbonate ion (calcium, magnesium, and sodium bicarbonates). Being weakly acidic, they will not affect the cations associated with the anions of strong acids (chlorides or sulfates). Because of almost 100% utilization of the regenerant acid, chemical operating costs will be at a minimum, and there will be little excess acid to produce objectionable waste effluents.
Anion exchange materials are classified as either weak base or strong base depending on the type of exchange group. Weak base resins act as acid adsorbers, efficiently removing strong acids such as sulfuric and hydrochloric. However, they will not remove carbon dioxide or silica. They are used in systems where strong acids predominate, where silica reduction is not required, and where carbon dioxide is removed in degasifiers. Preceding strong base units in demineralizing processes, weak base resins give more economical removal of sulfates and chlorides. These are two general classes of strong base anion exchangers, Types I and II, denoting differences in chemical nature. Both remove silica and carbon dioxide as well as other anions. Type I is more effective in removing silica, and is used when the combined silica and carbon dioxide content of the water contacting the exchanger is more than 25% of the total anions. When there is contamination of the water with organic matter, a more porous form of Type I resin is recommended. The Type II anion material is used in treating waters where the combined carbon dioxide and silica content is less than 25% of the total anions. This is often the case when carbon dioxide is taken out in a degasifier ahead of the anion exchanger unit.
|Ion exchange Process|
Ion exchange processes fall into several categories: softening (including removal of iron and manganese), dealkalization, and demineralization. Examples of these processes are listed below:
Water softening is a simple, well-documented ion exchange process. It solves a very common form of water contamination: hardness. Regeneration with sodium chloride is simple, inexpensive and can be automatic, with no strong chemicals required.
The disadvantages of water softening become apparent when high-quality water is required. Softening merely exchanges the hardness ions in the water supply for normally less-troublesome sodium ions. Since the treated water contains sodium instead of calcium or magnesium, it is still unsuitable for many uses.
The use of ion exchange processes affords numerous efficient and effective means of conditioning feedwater. The proper selection of the specific ion exchange process depends on water quality needs, operating convenience, and economic considerations. For effective results, the system must be carefully selected, designed, operated and maintained. Because the decision is complex, an experienced ion exchange engineer should be consulted to assist in selection and design.