How to avoid the Boilers corrosion using Oxygen scavengers In Water treatment plants.
In Industrial power plants for steam generation, pretreatment of water is very essential. It is one of the operations on which the whole industrial applications are based on. The efficiency of the boilers and steam equipment depends on the levels of contaminants in the water that is being used. The major contaminant which effects the Boiled feed water is Oxygen, as it attacks the steel and leads to corrosion. In Boilers the water supply systems such as pre boiler systems, condensate return lines, and steam blanketed boiler surface are prone to oxygen attack at elevated temperatures and low pH. Therefore, removal of dissolved oxygen from feed water to prevent oxygen attack is accomplished either mechanically or chemically. Deaerators and degasifiers can control the oxygen levels mechanically whereas; chemically removal of oxygen is achieved by using Oxygen scavenger chemical.
These Oxygen scavengers decrease the oxygen levels in the feed water by absorption mechanism. Boiler water oxygen scavenger is fed either upstream or downstream in the Dearator to control the corrosion and also sometimes in the condensate. Based on the Oxygen scavenging capacities and oxygen scavenging methods at varying temperatures and pressures, there are many types of Oxygen scavenger available. Whereas boiled feed water treatment in boilers for steam generation is performed using Oxygen scavenger such as-
- Sodium Sulfide(Na2SO3): It is mostly preferred in low and medium pressure plants as it acts as a catalyst and enhances the reactivity with oxygen even at low temperature and pressures. It is non-volatile in nature, therefore 8ppm (parts per million) of Na2SO3 is the dosage needed to dissolve 1ppm of dissolved oxygen present in the boiler feed water.
- Hydrazine(N2H4): It is a colorless liquid and is mostly preferred for high temperature and high-pressure plants as it acts as a catalyst and enhances the reactivity with oxygen without producing corrosive gases. It is also claimed as a safe and effective as it protects the boiler by forming a protective magnetite film inside the tubes and drums of a boiler thereby converting the red iron dust if present into magnetite.
- DEHA(Diethyl hydroxy amine): It is colorless to pale yellow transparent liquid, volatile in nature and shows excellent reactivity with oxygen when compared with Sodium sulfide and Hydrazine. Nearly 4ppm (parts per million) of DEHA is the dosage needed to dissolve 1ppm of dissolved oxygen present in the boiler feed water. The toxicity is also very low when compared with Hydrazine.
Apart from these, many other alternatives catalyzed O2 scavenger were developed which include –MEKO(Methyl ethyl ketoxime), Hydroquinone, Erythorbic acid, NTA(Nitrilotriacetic acid).
Prior to use of these Oxygen scavenger by Industries and in several other applications, a formal document under the name as Oxygen scavenger MSDS (Material Safety data sheet) is included which defines the features such as-Identification and preparation by, composition, first aid measures, accidental release measures, holding and storage, physical and chemical properties, Stability and reactivity, Disposal consideration, Transport information and Regulatory information.
Oxygen scavengers are available in different forms such as-
- Sachets: Theses are non-toxic and non-editable as they contain ferrous iron powder along with activated carbon and salt. These are stable in open air when kept dry, therefore handled easily. Each sachet contains 7grams of iron, which is less than a lethal dose and hence used in packing food items.
- Scavenging films: It is bio-based multilayer packaging film which remains brownish red in color, but on the absorption of oxygen changes its color to greenish black. They are used for vacuum packaged products
- Scavenging bottle closures: They are made up of permeable material composed of anti-oxidant compounds and catalysts, to improve the storage ability. They are used for bottles as a plastic caps and crowns for beverages.
Oxygen scavenger lowers the level of oxygen in the headspace and also used in a wide array of applications including pharmaceuticals to prevent the ingress of oxygen.
The following are some of the guidelines to be followed to increase the effectiveness of Oxygen scavengers-
- Selection of Packaging containers or Films with oxygen barrier
- Films with oxygen permeability not more than 20ml/atm should be used for packaging
- Selection of appropriate type and size of the scavenger
Apart from Industrial power plants and Food packagings, Oxygen scavengers are extensively used in Oil refineries, Petrochemical industries and in gas industries to entrap the impurities such as peroxide, hydrogen peroxide, and carbonyl groups which are formed due to the presence of oxygen. Oxygen scavenger oil & gas react selectively with the impurities thus, formed and help in avoiding corrosion and scale formation in the cooling system as well as in boilers. Therefore, there is an increasing interest in the Oxygen Scavenging field as they play a major role in extending the shelf life of packages, including Food, liquids as well as equipment.