Alkalinity is a measurement of water's ability to neutralize acid. There are 3 types of alkalinity, depending on pH of the water: bicarbonate, carbonate and hydroxide. Bicarbonate (HCO3) has a pH ranging from 4.3 to 8.3. Bicarbonate is the most prevalent alkalinity in natural bodies of water. Carbonate (CO3) has a pH ranging from greater than 8.3 to 10. Hydroxide (OH) alkalinity exists when the pH is greater than 10.
How Alkalinity Affects Boiler Performance
Alkalinity is a significant factor in the production of many goods including food and beverages, and textile dyes, but it plays an even more significant role in boiler operations. Hydroxide (OH) and carbon dioxide (C02) are both produced from the breakdown of carbonate and bicarbonate ions during steam production. As the steam condenses, the carbon dioxide dissolves and forms carbonic acid (H2C03) – a highly corrosive compound that deteriorates condensate return lines. The presence of hydroxide with carbon dioxide and bicarbonate may also lead to further corrosion.
Certain amine compounds and other chemical additives are necessary to help protect the condensate return lines from corrosion. However, these chemicals are costly, and their effectiveness limited by the amount of carbonic acid they can control. For this reason, alkalinity is often the defining factor in determining the cycles at which a boiler can be operated safely. High alkalinity may require that a boiler operate at shorter cycles, wasting energy and chemicals due to high blowdown.
- A cycle is the measure of how concentrated water has become inside a boiler. Concentration levels may be measured by dividing the conductivity of the boiler blowdown by the conductivity of the feedwater.
- Blowdown is a term used to describe the process in which concentrated water is replaced by new feedwater. As condensed steam returns to the boiler, minerals concentrate in the water until chemicals alone can no longer prevent scaling or damaging corrosion. The boiler must then be blown down to reduce the mineral- rich water and allow more feedwater of a lower mineral content to enter. There is, however, one drawback to the blowdown process: loss of energy. The water discharged to sewer is, typically, in excess of 212° F and in high pressure boilers may be significantly hotter.
Generally, dealkalization can be used to treat water in boilers operating at less than 700 psi, with feedwater containing less than or equal to 50 ppm alkalinity, and with make-up of 1,000 gallons or more per day. Makeup water is the water added to the boiler to offset water lost due to steam and blowdown.
Raw water alkalinity may be reduced using several different methods:
- Reverse Osmosis - Membrane filtration has become the popular option for boiler water treatment. With appropriate pretreatment, nearly all carbon dioxide can be eliminated from RO-treated feedwater. Reverse osmosis can simultaneously remove up to 98% of all dissolved minerals, greatly reducing alkalinity and blowdown- limiting minerals. Boiler cycles may be increased to 50 or more with reverse osmosis.
- Chloride Anion Dealkalizers – Chloride anion dealkalizers operate similar to ion exchange water softeners, except the filtration vessels contain a Type II strong-base anion resin. Two methods may be used for resin regeneration. The first uses salt and the second uses a salt-caustic combination (NaOH). If just salt is used, water hardness should be 10 grains or less (<170 ppm) to prevent the precipitation of CaCO3. If a salt-caustic combination is used, water must be softened prior to being fed to the dealkalizers. In most cases where a dealkalizer is required, water should be softened to prevent boiler scale buildup. Salt-caustic combinations have a higher capacity for alkalinity before regeneration is required.
- Weak Acid Dealkalization – When the ratio of water hardness to alkalinity is 1 or greater, a weak acid cation (WAC) resin offers significant cost advantages. A WAC resin exchanges hydrogen for the hardness associated with the water’s alkalinity. Then degasification is used to remove carbon dioxide. Caustic may be added in low doses to raise the final pH level if desired.
- Split Stream Dealkalization – Split stream dealkalization utilizes two beds of strong acid cation (SAC) operating in parallel. One bed operates in the sodium form, acting as a cation exchange softener. The other bed operates in the hydrogen form and acts like the cation vessel of a demineralizer. Sulfuric acid is typically used to regenerate this bed. Feedwater flow is divided between the two vessels. Water softened by the sodium-based vessel contains all of the alkalinity, while the stream from the hydrogen vessel contains no alkalinity. The streams from each vessel are then blended together and degasified to remove carbon dioxide. The controlled ratio at which each stream is blended will determine the final alkalinity level in the effluent.
A dealkalizer works similar to a water softener, in that it utilizes ion exchange to remove unwanted ions from a water supply. However, rather than removing calcium and magnesium ions, dealkalization removes carbonate ions, exchanging them for chloride ions. Like water softeners, dealkalizers make use of salt during the regeneration process. Unlike water softeners, a dealkalizer resin must also be treated with an additional caustic solution. This caustic solution boosts pH levels and enhances the resins efficiency.
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.