Chemistry unit 2 ( Water treatment and Boiler problems)

Chapter 6: Water Treatment and Boiler Problems

Boiler Problems

  • Common issues in boilers include:

    • Caustic Embrittlement

    • Sludge Formation

    • Boiler Scale Formation

    • Corrosion in Boiler

Sludge and Scale Formation

  • Sludge:

    • Soft, loose precipitate in the boiler.

    • Forms when soluble salts of calcium (Ca) and magnesium (Mg) precipitate and get deposited in the boiler.

    • Easily removed with a wire brush.

  • Scale:

    • Hard deposits that coat the inner surface of the boiler.

    • Difficult to remove, often needing mechanical means.

    • Common examples include calcium compounds (CaCl2, CaSO4, CaCO3).

Causes of Sludge and Scale Formation

  • Water evaporation in the boiler leads to:

    • Soluble salts of Ca and Mg transforming into insoluble salts.

    • These deposits accumulate on the boiler's inner surface.

Disadvantages of Sludge and Scale Formation

  1. Wastage of fuel - As more energy is needed to heat the boiler due to insulation effects of the layers.

  2. Decrease in efficiency - Loss of heating efficiency due to surface deposits.

  3. Lowered boiler safety - Increase in pressure and risks due to fouled heaters.

  4. Operational disturbances - Improper functioning due to accumulation of sludge or scale, leading to the potential for boiler failure.

  5. Explosion risks - Significant scale can cause overheating, leading to catastrophic failures.

Prevention of Sludge and Scale Formation

  • Employ techniques to mitigate formation:

    1. Use soft water to prevent hard scale.

    2. Conduct frequent blow down operations to remove sediments.

    3. Employ chemical additives such as sodium phosphate and sodium carbonate to reduce hardness.

    4. Regular cleaning/brushing of the boiler system.

Caustic Embrittlement

  • Occurs due to highly alkaline water present in the boiler (often through sodium carbonate).

  • Leads to corrosion from caustic agents contributing to hairline cracks in the boiler material.

  • Prevention:

    • Use of sodium phosphate as a softening agent.

Corrosion in Boilers

  • Types of corrosion affecting boilers:

    1. Dissolved Oxygen (O2):

      • Reacts with iron in the presence of water to form rust.

      • Removal involves chemical treatment (e.g. hydrazine).

    2. Dissolved Carbon Dioxide (CO2):

      • Forms carbonic acid, causing slow corrosiveness.

      • Removal can be counteracted using ammonium hydroxide.

    3. Acids from Dissolved Salts:

      • Hydrolysis of salts creates acids that react with boiler materials.

      • Removal via adding alkaline substances to neutralize acidity.

Carry Over Water Phenomena

  • Priming and Foaming:

    • Priming refers to droplets of water carried along with steam.

    • Foaming is when bubbles form in the boiler, both leading to the carry-over of impurities.

Water Softening Methods

  1. External Water Treatment:

    • Methods include: i. Lime-Soda Process ii. Zeolite/Permutit Process iii. Ion Exchange Process

  2. Internal Water Treatment:

    • Conducted within the boiler to alleviate existing hardness.

Lime-Soda Process

  • Converts soluble impurities into insoluble precipitates for removal.

  • Chemical Reactions:

    • Temporary and permanent hardness removal through reactions involving lime (Ca(OH)2) and soda (Na2CO3).

  • Advantages:

    • Produces alkaline water, reducing corrosiveness.

  • Disadvantages:

    • Sludge disposal can be challenging, and operational care desired.

Zeolite or Permutit Process

  • Zeolites are hydrated sodium aluminosilicates that exchange sodium ions with hardness ions.

  • Regeneration involves passing brine solution to restore zeolite capacity.

  • Advantages:

    • High efficiency in reducing hardness, low sludge production.

  • Disadvantages:

    • Can lead to increased sodium content in treated water.

Ion Exchange Process

  • Removes cations (Ca, Mg) and anions (Cl, SO4) responsible for hardness using resin.

  • Results in deionized or demineralized water with nearly zero hardness.

  • Regeneration of Resins:

    • Cation exchange resin uses acid while anion exchange resin uses alkali for regeneration.

Conclusion

  • Proper understanding of modulation in boiler chemistry is vital for efficient operation and maintenance. Regular monitoring and treatment reduce risks associated with water hardness and corrosion.