Q. What do you understand by dissolved oxygen in water? How does it create trouble in boiler? Give chemical methods with suitable reactions to remove it from the water.

Ans.

Presence of dissolved oxygen: At room temperature about 8 ppm of dissolved oxygen is present in water. Oxygen enters the boiler through raw make up water. When water containing the free gas is evolved under prevailing high temperature of boiler and attacks the boiler material.

2Fe + 2H₂O + O₂ = 2Fe (OH) ₂

4Fe (OH) ₂ + O₂ + 2H₂O = 2[Fe₂O₃.2H₂O]

Removal of dissolved oxygen:

1. Chemical means: Oxygen can be removed by adding calculated amounts of certain chemicals such as Na₂SO₃, Na₂S and N₂H₄.

         Na₂SO₃ + O₂ = 2Na₂SO₄

      Na₂S + 2O₂ = Na₂SO₄

However, the sodium sulphate so formed may decompose in high pressure to form H₂SO₄. Hydrazine (N₂H₄) is found to be an ideal compound for removing dissolved oxygen since the products are water and nitrogen gas

                                          N₂H₄ + O₂ = N₂ + 2H₂o

Mechanical means: It works on Henry's law and Dalton's law. Solubility of gas is directly proportional to pressure. Mechanical desecrator is used