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Dealkalization

Definition

Alkalinity is a measure of a water’s ability to neutralize acid. Depending on pH, water can contain three types of alkalinity: carbonate (CO3), bicarbonate (HCO3) and hydroxide (OH). Since bicarbonate alkalinity is the specie that exists at a pH range of 4.3 to 8.3, it predominates in natural waters. The carbonate ion begins to appear at pH >8.3 and hydroxide at pH>10.

Effect on Boiler Operations

Alkalinity, due to its strong relationship to pH, has significance to certain food and beverage production operations and textile dyeing but by far its major importance is in boiler operation. Bicarbonate and carbonate ions can break down during the production of steam to produce hydroxide (OH) and carbon dioxide (CO₂). The carbon dioxide dissolves in the condensed steam in the form of carbonic acid (H2C03) which, due to its corrosively, has a deleterious effect on the condensate return lines. Hydroxide present with the CO₂ / HCO3 also contributes to corrosion. Chemical additives such as certain amine compounds that carry over with the steam and help protect the condensate return lines are necessary, and effective to a point, in mitigating this problem. However, they are expensive and limited in the concentration of H2CO3 they can handle. For this reason, alkalinity is often the limiting factor governing the cycles the boiler can safely be operated at (Definitions: "cycles" is a measure of how concentrated water has become inside the boiler and can be calculated by dividing the conductivity of the boiler blowdown by the conductivity of the feedwater. Since most boilers have a significant amount of condensed steam returned, the concentration of dissolved minerals builds up until chemicals alone cannot prevent scaling or damage and the boiler must be "blown down". Blowdown removes some of this concentrated water and allows more feedwater of lower mineral content to be introduced. However, the blown down water is typically ≥ 212° F or higher, depending on the pressure maintained in the boiler, thus wasting significant amounts of energy). Depending on raw water chemistry, boiler operating pressure and percentage of condensate return, alkalinity may be the limiting factor requiring that the boiler be operated at low cycles and wasting a great deal of energy (and treatment chemicals) due to high blowdown requirements.

Treatment

As a general rule of thumb, dealkalization has applicability for boilers operating at < 700 psi with feedwater containing ≥ 50 ppm alkalinity and make-up water (i.e., the amount of raw water introduced to the boiler to offset the amount lost due to steam loss and blowdown) of 1000 gallons or more daily. Several methods can be used to reduce raw water alkalinity:

• Chloride Anion Delkalizers - Chloride anion dealkalizers operate similar to ion exchange water softeners except the filtration vessels contain a Type II strong base anion resin. There are two types of regeneration for this resin. The first utilizes only salt while the second uses salt and caustic (NaOH). If salt is used, the hardness in the influent water should be less than 10 grains (170 ppm) to prevent the precipitation of CaCO₃. In most cases where dealkalization is necessary, the hardness must be removed anyway to minimize boiler scaling. If salt and caustic are used, the water fed to the dealkalizer, as well as the water used to regenerate it, must be softened. The advantage to using salt and caustic is the higher capacity for alkalinity reduction between regenerations.

• Split Stream Dealkalization – Split stream dealkalization utilizes two beds of strong acid cation (SAC) resin operating in parallel. One is operated in the sodium form and the other in the hydrogen form. The former acts as a simple cation exchange softener and the latter functions exactly like the cation vessel of a demineralizer. This bed is normally regenerated with sulfuric acid. The feedwater flow is divided between the two vessels. Water softened by the sodium form vessel contains all of the influent alkalinity while the stream from the acidified bed contains no alkalinity. The two streams are then blended together and degasified to remove CO₂ resulting from neutralization of the acidity from the acid-regeneration tank with the alkalinity from the cation exchange softener. Controlling the ratio of the blended waters will control alkalinity in the final effluent.

• Weak Acid Dealkalization - When the influent water is high in hardness and alkalinity and has hardness to alkalinity ratios of 1 or more, dealkalization using a weak acid cation (WAC) resin offers significant cost advantages. This process uses weak acid cation resin to exchange hydrogen for hardness that is associated with alkalinity, followed by degasification to remove CO₂. A simple cation exchange water softener removes any permanent hardness remaining in the effluent from the degassifier. A low dose of caustic may be required to raise the final effluent pH to that desired.

• Reverse Osmosis– The use of membrane filtration is rapidly expanding for boiler treatment as the cost of fuel climbs. Unlike the methods described above, reverse osmosis can remove up to 98% of all dissolved minerals form the water. Appropriate pretreatment to the reverse osmosis machine, can also eliminate virtually all CO₂ in the feedwater. Thus, not only is reverse osmosis able to reduce alkalinity, it removes all other minerals that limit blowdown. It is not unusual to increase boiler cycles from 10 or 20 to 50 or more.

Selection and Design:

Each dealkalization method described above has significant advantages and disadvantages. The WaterProfessionals® can evaluate your operation and, using raw water analysis and operating parameters for your boiler operation, model the most cost effective method and provide important payback information.

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