Boiler water chemicals
Boiler water chemicals include all chemicals that are used for the following applications:
· Oxygen scavenging;
· Scale inhibition;
· Corrosion inhibition;
· Alkalinity control.
When referring to coagulants, positive ions with high valence are preferred. Generally aluminium and iron are applied, aluminium as Al2(SO4)3- (aluin) and iron as either FeCl3 or Fe2(SO4)3-. One can also apply the relatively cheap form FeSO4, on condition that it will be oxidised to Fe3+ during aeration.
Coagulation is very dependent on the doses of coagulants, the pH and colloid concentrations. To adjust pH levels Ca(OH)2 is applied as co-flocculent. Doses usually vary between 10 and 90 mg Fe3+/ L, but when salts are present a higher dose needs to be applied.
Corrosion is a general term that indicates the conversion of a metal into a soluble compound.
Corrosion can lead to failure of critical parts of boiler systems, deposition of corrosion products in critical heat exchange areas, and overall efficiency loss.
That is why corrosion inhibitors are often applied. Inhibitors are chemicals that react with a metallic surface, giving the surface a certain level of protection. Inhibitors often work by adsorbing themselves on the metallic surface, protecting the metallic surface by forming a film.
There are five different kinds of corrosion inhibitors. These are:
1) Passivity inhibitors (passivators). These cause a shift of the corrosion potential, forcing the metallic surface into the passive range. Examples of passivity inhibitors are oxidizing anions, such as chromate, nitrite and nitrate and non-oxidizing ions such as phosphate and molybdate. These inhibitors are the most effective and consequently the most widely used.
2) Cathodic inhibitors. Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult. Other cathodic inhibitors, ions such as calcium, zinc or magnesium, may be precipitated as oxides to form a protective layer on the metal.
3) Organic inhibitors. These affect the entire surface of a corroding metal when present in certain concentration. Organic inhibitors protect the metal by forming a hydrophobic film on the metal surface. Organic inhibitors will be adsorbed according to the ionic charge of the inhibitor and the charge on the surface.
4) Precipitation inducing inhibitors. These are compounds that cause the formation of precipitates on the surface of the metal, thereby providing a protective film.
The most common inhibitors of this category are silicates and phosphates.
5) Volatile Corrosion Inhibitors (VCI). These are compounds transported in a closed environment to the site of corrosion by volatilisation from a source. Examples are morpholine and hydrazine and volatile solids such as salts of dicyclohexylamine, cyclohexylamine and hexamethylene-amine. On contact with the metal surface, the vapour of these salts condenses and is hydrolysed by moist, to liberate protective ions.
Disinfectants kill present unwanted microrganisms in water. There are various different types of disinfectants:
· Chlorine (dose 2-10 mg/L)
· Chlorine dioxide
Chlorine dioxide disinfection
ClO2 is used principally as a primary disinfectant for surface waters with odor and taste problems. It is an effective biocide at concentrations as low as 0.1 ppm and over a wide pH range. ClO2 penetrates the bacterial cell wall and reacts with vital amino acids in the cytoplasm of the cell to kill the organisms. The by-product of this reaction is chlorite.
Chlorine dioxide disinfects according to the same principle as chlorine, however, as opposed to chlorine, chlorine dioxide has no harmful effects on human health.
Hypochlorite is aplied in the same way as chlorine dioxide and chlorine. Hypo chlorination is a disinfection method that is not used widely anymore, since an environmental agency proved that the Hypochlorite for disinfection in water was the cause of bromate consistence in water.
Ozone is a very strong oxidation medium, with a remarkably short life span. It consists of oxygen molecules with an extra O-atom, to form O3. When ozone comes in contact with odour, bacteria or viruses the extra O-atom breaks them down directly, by means of oxidation. The third O-atom of the ozone molecules is than lost and only oxygen will remain.
Disinfectants can be used in various industries. Ozone is used in the pharmaceutical industry, for drinking water preparation, for treatment of process water, for preparation of ultra-pure water and for surface disinfection.
Chlorine dioxide is used primarily for drinking water preparation and disinfection of piping.
Every disinfection technique has its specific advantages and its own application area. In the table below some of the advantages and disadvantages are shown:
To promote the formation of flocs in water that contains suspended solids polymer flocculants (polyelectrolytes) are applied to promote bonds formation between particles. These polymers have a very specific effect, dependent upon their charges, their molar weight and their molecular degree of ramification. The polymers are water-soluble and their molar weight varies between 105 and 106 g/ mol.
There can be several charges on one flocculent. There are cationic polymers, based on nitrogen, anionic polymers, based on carboxylate ions and polyampholytes, which carry both positive and negative charges.
Neutralizing agents (alkalinity control)
In order to neutralize acids and basics we use either sodium hydroxide solution (NaOH), calcium carbonate, or lime suspension (Ca(OH)2) to increase pH levels. We use diluted sulphuric acid (H2SO4) or diluted hydrochloric acid (HCl) to decline pH levels. The dose of neutralizing agents depends upon the pH of the water in a reaction basin. Neutralization reactions cause a rise in temperature.
Chemical oxidation processes use (chemical) oxidants to reduce COD/BOD levels, and to remove both organic and oxidisable inorganic components. The processes can completely oxidise organic materials to carbon dioxide and water, although it is often not necessary to operate the processes to this level of treatment
A wide variety of oxidation chemicals are available. Examples are:
· Combined ozone & peroxide;
Hydrogen peroxide is widely used thanks to its properties; it is a safe, effective, powerful and versatile oxidant. The main applications of H2O2 are oxidation to aid odour control and corrosion control, organic oxidation, metal oxidation and toxicity oxidation. The most difficult pollutants to oxidize may require H2O2 to be activated with catalysts such as iron, copper, manganese or other transition metal compounds.
Ozone cannot only be applied as a disinfectant; it can also aid the removal of contaminants from water by means of oxidation. Ozone then purifies water by breaking up organic contaminants and converting inorganic contaminants to an insoluble form that can then be filtered out. The Ozone system can remove up to twenty-five contaminants.
Chemicals that can be oxidized with ozone are:
· Absorbable organic halogens;
· Nitrogen oxides;
· Odorous substances;
· Chlorinated hydrocarbons;
Oxygen can also be applied as an oxidant, for instance to realize the oxidation of iron and manganese. The reactions that occur during oxidation by oxygen are usually quite similar.
These are the reactions of the oxidation of iron and manganese with oxygen:
2 Fe2+ + O2 + 2 OH– -> Fe2O3 + H2O
2 Mn2+ + O2 + 4 OH– -> 2 MnO2 + 2 H2O
Oxygen scavenging means preventing oxygen from introducing oxidation reactions. Most of the naturally occurring organics have a slightly negative charge. Due to that they can absorb oxygen molecules, because these carry a slightly positive charge, to prevent oxidation reactions from taking place in water and other liquids.
Oxygen scavengers include both volatile products, such as hydrazine (N2H4) or other organic products like carbohydrazine, hydroquinone, diethylhydroxyethanol, methylethylketoxime, but also non-volatile salts, such as sodium sulphite (Na2SO3) and other inorganic compounds, or derivatives thereof. The salts often contain catalysing compounds to increase the rate of reaction with dissolved oxygen, for instance cobalt chloride.
Municipal water is often pH-adjusted, in order to prevent corrosion from pipes and to prevent dissolution of lead into water supplies. During water treatment pH adjustments may also be required. The pH is brought up or down through addition of basics or acids. An example of lowering the pH is the addition of hydrogen chloride, in case of a basic liquid. An example of bringing up the pH is the addition of natrium hydroxide, in case of an acidic liquid.
The pH will be converted to approximately seven to seven and a half, after addition of certain concentrations of acids or basics. The concentration of the substance and the kind of substance that is added, depend upon the necessary decrease or increase of the pH.
Ion exchange resins need to be regenerated after application, after that, they can be reused. But every time the ion exchangers are used serious fouling takes place. The contaminants that enter the resins will not be removed through regeneration; therefore resins need cleaning with certain chemicals.
Chemicals that are used are for instance sodium chloride, potassium chloride, citric acid and chlorine dioxide.
Chlorine dioxide cleansing serves the removal of organic contaminants on ion exchange resins. Prior to every cleaning treatment resins should be regenerated. After that, in case chlorine dioxide is used, 500 ppm of chlorine dioxide in solution is passed through the resin bed and oxidises the contaminants.
Scale is the precipitate that forms on surfaces in contact with water as a result of the precipitation of normally soluble solids that become insoluble as temperature increases. Some examples of scale are calcium carbonate, calcium sulphate, and calcium silicate.
Scale inhibitors are surface-active negatively charged polymers. As minerals exceed their solubility’s and begin to merge, the polymers become attached. The structure for crystallisation is disrupted and the formation of scale is prevented. The particles of scale combined with the inhibitor will than be dispersed and remain in suspension.
Examples of scale inhibitors are phosphate esters, phosphoric acid and solutions of low molecular weight polyacrylic acid.