Theory And Practice Of Ion Exchange

Water can contain varying concentrations of dissolved salts which dissociate to form charged particles called ions. These ions are the positively charged cations and negatively charged anions that permit the water or solution to conduct electrical currents and are therefore called electrolytes. Electrical conductivity is thus a measure of water purity, with low conductivity corresponding to a state of high purity. The process of ion exchange is uniquely suited to the removal of ionic species from water supplies for several reasons. First, ionic impurities may be present in rather low concentrations. Second, modern ion-exchange resins have high capacities and can remove unwanted ions preferentially. Third, modern ion-exchange resins are stable and readily regenerated, thereby allowing their reuse. Other advantages ion exchange offers are: (1) the process and equipment are a proven technology. Designs are well developed into pre-engineered units that are rugged and reliable, with well-established applications; (2) fully manual to completely automatic units are available; (3) there are many models of ion-exchange systems on the market which keep costs competitive; (4) temperature effects over a fairly wide range (from 0° to 35° C) are negligible; (5) the technology is excellent for both small and large installations, from home water

Ion exchange is a well-known method for softening or for demineralizing water. Although softening could be useful in some instances, the most likely application for ion exchange in wastewater treatment is for demineralization. Many ion-exchange materials are subject to fouling by organic matter. It is possible that treatment of secondary effluent for suspended-solids removal and possibly soluble organic removal will be required before carrying out ion exchange. Many natural materials and, more importantly, certain synthetic materials have the ability to exchange ions from an aqueous solution for ions in the material itself. Cation-exchange resins can, for example, replace cations in solution with hydrogen ions. Similarly, anion-exchange resins can either replace anions in solution with hydroxyl ions or absorb the acids produced from the cation-exchange treatment. A combination of these cation-exchange and anion-exchange treatments results in a

Since the exchange capacity of ion-exchange materials is limited, they eventually become exhausted and must be regenerated. The cation resin is regenerated with an acid; the anion resin is regenerated with a base. Important considerations in the economics of ion exchange are the type and amounts of chemicals needed for regeneration. Often, water to be demineralized is first passed through a cation-exchange material requiring a strong acid, usually sulfuric, for regeneration. The exchange material is referred to as strong acid resin. The amount of acid regenerant is somewhat more than the stoichiometric amount, possibly 100 percent excess or more. If sulfuric acid is the regenerating acid, a waste brine is produced consisting of sulfates of the various actions in the water being treated. Because the partially treated water contains mineral acids, it is common to pass it next through an acid-absorbing resin or weak base resin. This resin can be regenerated with either a weak or strong base. The efficiency of regenerant use is quite high with these resins. If sodium hydroxide is the regenerating base, a waste brine is produced consisting of the sodium salts of the various anions in the water being treated. Certain anionic materials are not removed by the weak-base resin and must be further treated with strong-base resin if thorough demineralization is desired. Regenerant usage by the strong-base resins is poorer than for the weak base resins. The reasons for applying this technology in the removal of mineral species should be quite apparent to those of you who work with applications involving heat exchange. Water problems in cooling, heating, steam generation, and manufacturing are caused in large measure from the kinds and concentrations of dissolved solids, dissolved gases, and suspended matter in the makeup water supplied. Table 1 lists the major objectionable ionic constituents present in many water supplies that can be removed by demineralization. Prevention of scale and other deposits in cooling and boiling waters is best accomplished by removal of dissolved solids. Whereas in municipal water purification such removal is limited to the partial reduction of hardness and the removal of iron and manganese, in industrial water treatment it is often carried much further and may include the complete removal of hardness, the reduction or removal of alkalinity, the removal of silica, or even the complete removal of all dissolved solids.

Table 1. Common Ionic Constituents Contained in Water

Constituent of concern

Chemical designation

Resultant problems

Hardness

Calcium and magnesium salts in the forms of CaC02, Ca, Mg.

This is the primary source of scaling in heat exchange equipment, boilers, pipelines/transfer lines, etc. Tends to form curds with soap and interferes with dyeing applications as well.

Alkalinity

Bicarbonate (HC03), carbonate (C03), and hydrate (OH), expressed as CaC03.

Causes foaming and carryover of solids with steam. Can cause embrittlement of boiler steel. Biocarbonate and carbonate generate C02 in steam, a source of corrosion.

Constituent of concern

Chemical designation

Resultant problems

Free mineral acidity

H2S04, HC1, and other acids, expressed as CaCOj.

Causes rapid corrosion and deterioration of surfaces.

Chloride

ci-

Interferes with silvering processes and increase TDS.

Sulfates

(S04)=

Results in the formation of calcium sulfate scale.

Iron and manganese

Fe+2 (ferrous) FeT3 (ferric) Mn+2

Discolors water, and results in the formation of deposits in water lines, boilers and other heat exchangers. Can interfere with dying, tanning, paper manufacture and various process works.

Carbon dioxide

C02

Results in the corrosion of water lines, especially steam and condensate lines.

Silica

Si02

Results in the formation of scale in boilers and cooling water systems, can produce insoluble scale on turbine blades due to silica vaporization in high pressure boilers (usuallu over 600 psi).

The two most frequently encountered water problems-scale formation and corrosion-are common to cooling, heating, and steam-generating systems. Hardness (calcium and magnesium), alkalinity, sulfate, and silica all form the main source of scaling in heat-exchange equipment, boilers, and pipes. Scales or deposits formed in boilers and other exchange equipment act as insulation, preventing efficient heat transfer and causing boiler tube failures through overheating of the metal. Free mineral acids (sulfates and chlorides) cause rapid corrosion of boilers, heaters, and other metal containers and piping. Alkalinity causes embrittlement of boiler steel, and carbon dioxide and oxygen cause corrosion, primarily in steam and condensate lines. Low-quality steam can produce undesirable deposits of salts and alkali on the blades of steam turbines; much more difficult to remove are silica deposits which can form on turbine blades even when steam is satisfactory by ordinary standards. At steam pressures above 600 psi, silica from the boiler water actually dissolves in the gaseous steam and then reprecipitates on the turbine blades

In the operation of every cooling, heating, and steam-generating plant, the water changes temperature. Higher temperatures, of course, increase both corrosion rates and scale-forming tendency. Evaporation in process steam boilers and in evaporative cooling equipment increases the dissolved-solids concentration of the

In addition to the formation of scale or corrosion of metal within boilers, auxiliary equipment is also susceptible to similar damage. Attempts to prevent scale formation within a boiler can lead to makeup line deposits if the treatment chemicals are improperly chosen. Thus, the addition of normal phosphates to an unsoftened feed water can cause a dangerous condition by clogging the makeup line with precipitated calcium phosphate. Deposits in the form of calcium or magnesium stearate deposits, otherwise known as "bathtub ring" can be readily seen, and are caused by the combination of calcium or magnesium with negative ions of soap

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