Various methods of removing water oxygen from the boiler

Oxygen scavenger

In their working process, boiler systems encounter two gases, oxygen and carbon dioxide, dissolved in the water entering them. The presence of these gases causes corrosion in boilers and heating systems and will disable the device in the long term. Usually, dissolved oxygen in water is removed in two ways, physically and chemically. Mechanical and thermal deaeration, including physical methods and the use of chemicals such as DEHA, sodium sulfite, carbohydrazide and hydrazine, are chemical methods for removing oxygen.

Oxygen separation with a deaerator

A deaerator can remove most of the oxygen in the water by increasing the temperature of the water before it enters the boiler, but it is necessary that the remaining oxygen is separated and deactivated through chemicals.

Chemical deaeration (Scavenging)

Although mechanical and thermal deaerators are able to remove significant amounts of dissolved oxygen, the remaining dissolved oxygen must be removed by chemicals.

Review of oxygen scavenging chemicals

Water-soluble oxygen scavenging chemicals can be reviewed from 7 perspectives:

• Reaction with oxygen
• Passivation of the metal surface
• Decomposition under pressure and heat
• Volatility or Distribution rate
• Water-soluble solids
• Toxicity and environment
• Cost to the consumer

Sodium sulfite (Na₂SO₃)

The first method of deoxygenating boiler water that is used and its mechanism of action is as follows:

Na₂SO₃ + _O2 + Heat 2Na₂SO₄

The above reaction is accelerated by increasing temperature, increasing pH, and the presence of a catalyst.

Sodium sulfite does not show any metal surface deactivation properties.

Boiler water soluble solids (TDS)

Sodium sulfite and sodium sulfate remain in the boiler water and increase the amount of water-soluble solids (TDS). In fact, with increasing TDS, the boiler concentration coefficient is at a lower level, requiring more make-up water, higher sub-water, and more energy loss. Higher TDS will also increase the possibility of carry-over in the boiler.

Effect of sodium sulfite on boiler corrosion

Sodium sulfite prevents oxygen corrosion in the inlet water path to the boiler, but it has no effect on the parts after the boiler. Sodium sulfite is converted into sulfur and sulfur dioxide (SO2) when decomposed. When SO2 dissolves in water, sulfuric acid is formed, which causes corrosion. Sodium sulfite is not able to prevent or reduce corrosion in the parts after the boiler due to its lack of volatility. It is also a product of the sulfate reaction, which can create sulfate salts and cause problems. This substance is not highly toxic, and since it is not volatile, it does not exist in the vapor or condensate phase, and can be used to produce steam for the textile and food industries.

How much sodium sulfite is used in a boiler?

The recommended dosage of 98% sodium sulfite is 8 parts sodium sulfite per part oxygen. However, for boilers below 600 psi, the residual sodium sulfite in the boiler water should be maintained in the range of 30 ppm to 50 ppm based on S03. It is usually used in boilers with pressures below 900 psi.

Hydrazine Oxygen Scavenger

Introduced to the market in the 1950s for oxygen removal, it has been widely used since the 1960s as a successful oxygen scavenger for high pressures. The rate of hydrazine reaction is directly proportional to the temperature, pH of the water, and the presence of a catalyst.

Hydrazine's effect on metals

Hydrazine is an excellent passivator for metal surfaces, in other words, it creates magnetite Fe3O4 with a dense and cohesive crystalline structure on the metal surface. This material acts as a barrier between water and the metal surface and minimizes the risk of corrosion.

3 N₂H₄ +6Fe₃O₄ → 4NH₃+N₂

When decomposed by pressure or heat, it turns into ammonia and nitrogen.

3N₂H₄+ Heat → 4NH₃+N₂

Although ammonia can attack copper parts and its alloys in the boiler feedwater or condensate path in the presence of oxygen, thus increasing the corrosion rate, the small amount of hydrazine remaining in the system can be used to escape oxygen, and this problem can be overcome.

Hydrazine volatility

The rate of dispersion is the ratio of vapor to liquid phase, i.e. for every 1 ppm of hydrazine in the liquid phase, there is 8% hydrazine in the vapor phase. It can be concluded that hydrazine is not likely to be present at greater distances downstream of the boiler, whereas ammonia is likely to be present at greater distances due to its dispersion coefficient of 10.

• Hydrazine does not increase the TDS of water.
• It is toxic (carcinogenic) and flammable, and anhydrous hydrazine is very unstable.
• Hydrazine is typically used in boilers with pressures of 900 psi and above.

DEHA (Diethyl Hydroxyl Amine)

It was introduced to the market in 1981 for use in boiler systems.

4 (C₂ H5)₂ NOH+ 9O₂ → 8CH₃COOH + 2N₂ + 6H2O

If hydroxide is present in the boiler water, the acetic acid formed is neutralized and removed as sodium acetate by the water bath.

The relationship between the reaction rate of DEHA with oxygen

The reaction rate of DEHA with oxygen is directly proportional to the increase in temperature, pH, and the presence of a catalyst. DEHA begins to decompose at a pressure of about 21 bar. As a result of the decomposition of DEHA, two dialkylamines,

a) diethylamine and

b) ethylmethylamine

and also ammonia is formed, which products of decomposition increase the pH in the condensate phase and slightly reduce the amount of morpholine consumed.

DEHA Volatility Rate

In terms of volatility, DEHA, like other amines, is very volatile and has a dispersion rate of 16.2, so it goes with the steam phase to the stages after the boiler, preventing the corrosion of dissolved oxygen. The dispersion rate of 16 times DEHA compared to hydrazine causes better passivation of the areas after the boiler by it.

DEHA Toxicity

Dermal LD50- 1300 mg/kg for Rabbits

DEHA Standard Consumption Rate

DEHA is usually provided at a concentration of 85%. Its practical consumption rate in steam boilers is about 3.3 parts per part of oxygen.

HDMA is usually recommended as a suitable technical substitute for sodium sulfite up to pressures of 1100 PSI.

In selecting a chemical for absorbing oxygen in water, parameters such as reaction rate, temperature, operating pressure, residence time, pH of the feed water, etc. are important, appropriate to the industry in which the oxygen scavenger is used.

What is carbohydrazide?

Carbohydrazide is the chemical compound with the formula (N₂H₃) OC. It is a white solid soluble in water. It decomposes on melting. A number of carbazides are known in which one or more N-H groups are replaced by other atoms. They are widely used in pharmaceuticals, herbicides, plant growth regulators and vegetable dyes.

This compound is produced by the reaction of urea with hydrazine:

OC(NH₂)₂+2 N₂H₄ OC(N₂H₃)₂+2 NH₃

It can also be prepared by the reaction of other C1-precursors with hydrazine, such as carbonate esters. This can be prepared from fusogen, but this route produces the hydrazinium salt [N2H5Cl]. Carbohydrazide is also a suitable precursor. They can be obtained by the condensation of carbonic acid with hydrazine. Carbohydrazide is the simplest carbazide. Diphenylcarbazide is used as a reagent.

The structure of carbohydrazide

is non-planar molecular. All nitrogen centers are at least partially pyramidal, indicating a weakening of the C-N bond. The C-N and C-O distances are 1.36 and 1.25 Å, respectively.

Oxygen Removal with Carbohydrazide

Carbohydrazide is used to remove oxygen in boiler systems and prevent corrosion. A suitable precursor to polymers, carbohydrazide can be used as a hardener for epoxy-type resins.

What are the dangers of carbohydrazide?

Heating carbohydrazide may cause an explosion. Carbohydrazide is harmful to the eyes, respiratory system, and skin if swallowed. Carbohydrazide is toxic to aquatic organisms.

Mitreh Oxygen Depleting Solution

The selection of the appropriate oxygen scavenger depends on the operating conditions of the boiler. Also, for more information, please refer to the Abrizan Company products page and the oxygen scavenger section. The following are the oxygen scavenging solutions produced by Abrizan Company: