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.
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