Chemical Treatments for Boiler Deposits - Part 5
Phosphate treatment results are improved by organic supplements. Naturally occurring organics such as lignins, tannins, and starches were the first supplements used. The organics were added to promote the formation of a fluid sludge that would settle in the mud drum. Bottom blowdown from the mud drum removed the sludge.
There have been many advances in organic treatments. Synthetic polymers are now used widely, and the emphasis is on dispersion of particles rather than fluid sludge formation. Polymers alter the surface area and the surface charge to mass ratio of typical boiler solids. The surface charge on the particle can be favorably altered with proper polymer selection and application.
Many synthetic polymers are used in phosphate precipitation programs. Most are effective in dispersing magnesium silicate and magnesium hydroxide as well as calcium phosphate. The polymers are usually low in molecular weight and have numerous active sites. Some polymers are used specifically for hardness salts or for iron; some are effective for a broad spectrum of ions.
TABLE: Phosphate/polymer performance can be maintained at high heat transfer rates through the selection of the appropriate polymer.
|Treatment Type||Boiler Treatment Concentration (ppm)||Heat Transfer Rate (Btu/ft²/hr)||Operating Pressure (psig)||% Scale Reduction|
|Synthetic Polymer A||10||185,000||300||44|
|Synthetic Polymer B||10||185,000||300||93|
|Synthetic Polymer C||10||185,000||300||94|
|Synthetic Polymer B||5||185,000||300||56|
|Synthetic Polymer C||5||185,000||300||94|
|Synthetic Polymer B||10||185,000||900||64|
|Synthetic Polymer C||10||185,000||900||92|
|Synthetic Polymer B||10||300,000||900||44|
|Synthetic Polymer C||10||300,000||900||86|
|Synthetic Polymer B||10||300,000||1200||30|
|Synthetic Polymer C||10||240,000||1200||90|
|Synthetic Polymer C||10||300,000||1200||83|
Chelants are the prime additives in a solubilizing boiler water treatment program. Chelants have the ability to complex many cations. They accomplish this by locking metals into a soluble organic ring structure. The chelated cations do not deposit in the boiler. When applied with a dispersant, chelants produce clean waterside surfaces.
The greatest challenge with chelants, and any material, is to understand the proper application.
Chelants are weak organic acids that are injected into boiler feedwater in the neutralized sodium salt form. The water hydrolyzes the chelant, producing an organic anion. The degree of hydrolysis is a function of pH; full hydrolysis requires a relatively high pH.
The anionic chelant has reactive sites that attract coordination sites on cations (hardness and heavy metal contaminants). Coordination sites are areas on the ion that are receptive to chemical bonding. For example, iron has six coordination sites, as does EDTA (ethylenediaminetetraacetic acid). Iron ions entering the boiler (e.g., as contamination from the condensate system) combine with EDTA. All coordination sites on the iron ion are used by the EDTA, and a stable metal chelate is formed.
NTA (nitrilotriacetic acid), another chelant applied to boiler feedwater, has four coordination sites and does not form as stable a complex as EDTA. With NTA, the unused coordination sites on the cation are susceptible to reactions with competing anions.
Chelants combine with cations that form deposits, such as calcium, magnesium, iron, and copper. The metal chelate formed is water-soluble. When the chelate is stable, precipitation does not occur. Although there are many substances having chelating properties, EDTA and NTA are, to date, the most suitable chelants for boiler feedwater treatment.