Boiler feedwater oxygen removal

Removing oxygen from boiler feedwater in deaerators... 

Corrosion of pipes in boilers by oxygen can be alleviated through the use of sodium sulfite. Sodium sulfite removes oxygen firom boiler feedwater by the following reaction ... 

Palladium loaded resins can be utilized for the process of catalytically removing oxygen from boiler feedwater (Scheme 11.9). Scientists from Lanxess have reported that their basic polymeric resins can be used in a fixed bed reactor for this application [43]. 

ORGANIC OXYGEN SCAVENGERS - These are organic compounds such as hydroquinone and ascorbate to remove dissolved oxygen from the boiler feedwater and condensate. 

Corrosion of boiler and superheater tubes is sometimes a problem on the hot combustion gas side, especially if vanadium-containing oils are used as fuel. This matter is discussed in Sections 11.8 and 11.9. On the steam side, since modern boiler practice ensures removal of dissolved oxygen from the feedwater, a reaction occurs between H2O and Fe, resulting in a protective film of magnetite (Fe304) as follows ... 

The softened water is now in its most corrosive state, being still saturated with DO and having no hardness to form a protective scale. For reasons of economy, the removal of DO is usually first effected in part by thermomechanical deaeration. The boiler feedwater is preheated, then flashed in a deaerator to remove any free carbon dioxide and most of the dissolved oxygen. Then the last traces of DO are chemically scavenged with sodium sulfite (Na SO ) that reacts with oxygen to form sodium sulfate (Na SO ) as described in Eq. (8.22) ... 

Deaerator Used to remove oxygen from boiler feedwater. 

Sulfite ion, SOj, is found in some industrial wastewaters. Sodium sulfite is commonly added to boiler feedwaters as an oxygen scavenger where it reacts to remove potentially corrosive dissolved molecular oxygen fi om the water ... 

In order to limit thermal stresses and to reduce corrosion of the steel vessel, the reflector temperature was regulated near 350°F. About 50 kw of heat conducted from the fuel core to the reflector liquid was removed by circulating the heavy water with a 30-gpm canned-rotor pump through a reflector cooler which acted as a boiler feedwater preheater. A jet was located in this high-pressure circulating loop, the suction of which drew a continuous stream of gas from the vapor space above the reflector to a catalytic recombiner so that the concentration of deuterium and oxygen gases in this vapor space could be kept below explosive limits. 

The removal of each milligram of oxygen requires about 8 mg of sodium sulfite, so that the dose should be adjusted to suit the amount of oxygen introduced, which corresponds roughly to 80mg/l of cold make-up. A high level of condensate return therefore reduces the scavenger demand, and this is not only an economy in itself but can also mean a considerable reduction in the amount of total dissolved solids introduced into the boiler with the feedwater. 

Oxygen can also be removed from feedwater by thermal de-aeration, or partially removed by skilful design of the feed heating system and blowdown recovery. These processes run without cost to the operator, but save chemicals, and, by reducing the required dose of sulfite into the system, decrease the amount of non-volatile solids added into the boiler. 

Oxygen, even in very small amounts, may cause serious corrosion in feedwater lines, stage heaters, economizers, boiler metal, steam operated equipment and condensable piping. It must, therefore, be removed from the closed system. The solubility of oxygen varies with both pressure... 

Oxygen, even in trace amoimts, must be eliminated. High chelant residuals must not be allowed under any circumstances to build in boilers. All hot spots must be removed and the quality of feedwater must be carefully controlled. Copper-base alloys must not be exposed to chelants. The protecting layer of magnetite Fc304 is removed and corrosion initiated. 

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