Oxygen scavengers

Types of Oxygen Scavengers

• Sodium Sulfite (Na₂SO₃)

  • Mechanism: Reacts with dissolved oxygen to form sodium sulfate (Na₂SO₄).
  • Reaction: Na2SO3+O2→Na2SO4\text{Na}_2\text{SO}_3 + \text{O}_2 \rightarrow \text{Na}_2\text{SO}_4Na2​SO3​+O2​→Na2​SO4​
  • Applications: Commonly used in low to medium pressure boilers. Requires a catalyst (usually cobalt) at lower temperatures.

• Sodium Bisulfite (NaHSO₃)

  • Mechanism: Similar to sodium sulfite, reacts with oxygen to form sodium bisulfate (NaHSO₄).
  • Reaction: 2NaHSO3+O2→2NaHSO42 \text{NaHSO}_3 + \text{O}_2 \rightarrow 2 \text{NaHSO}_42NaHSO3​+O2​→2NaHSO4​
  • Applications: Often used in combination with sodium sulfite or where rapid reaction with oxygen is needed.

• Hydrazine (N₂H₄)

  • Mechanism: Reacts with oxygen to form nitrogen and water, leaving no dissolved solids.
  • Reaction: N2H4+O2→2H2O+N2\text{N}_2\text{H}_4 + \text{O}_2 \rightarrow 2 \text{H}_2\text{O} + \text{N}_2N2​H4​+O2​→2H2​O+N2​
  • Applications: Used in high-pressure boilers. Effective in high-temperature environments but is toxic and carcinogenic, requiring careful handling.

• Carbohydrazide (CH₆N₄O)

  • Mechanism: Breaks down to form hydrazine and carbon dioxide, reacts with oxygen similar to hydrazine.
  • Reaction: CH6N4O+2O2→3N2+3H2O+CO2\text{CH}_6\text{N}_4\text{O} + 2 \text{O}_2 \rightarrow 3 \text{N}_2 + 3 \text{H}_2\text{O} + \text{CO}_2CH6​N4​O+2O2​→3N2​+3H2​O+CO2​
  • Applications: Safer alternative to hydrazine, used in high-pressure boilers.

• DEHA (Diethylhydroxylamine, C₄H₁₁NO)

  • Mechanism: Reacts with oxygen to form water and organic byproducts.
  • Reaction: C4H11NO+2O2→3H2O+CO2+Byproducts\text{C}_4\text{H}_{11}\text{NO} + 2 \text{O}_2 \rightarrow 3 \text{H}_2\text{O} + \text{CO}_2 + \text{Byproducts}C4​H11​NO+2O2​→3H2​O+CO2​+Byproducts
  • Applications: Suitable for low to high-pressure systems, provides passivation of metal surfaces.

• Erythorbic Acid and Isoascorbic Acid

  • Mechanism: Reacts with oxygen to form dehydroascorbic acid and water.
  • Reaction: C6H7O6−+O2→C6H5O6−+H2O\text{C}_6\text{H}_7\text{O}_6^- + \text{O}_2 \rightarrow \text{C}_6\text{H}_5\text{O}_6^- + \text{H}_2\text{O}C6​H7​O6−​+O2​→C6​H5​O6−​+H2​O
  • Applications: Non-toxic, used in food and pharmaceutical applications, suitable for low to medium pressure boilers.

Factors to Consider When Choosing Oxygen Scavengers

• Boiler Pressure and Temperature: Different scavengers are more effective at specific pressures and temperatures.
• Feedwater Chemistry: The presence of other chemicals in the feedwater can influence the effectiveness of oxygen scavengers.
• System Design and Operation: The choice of scavenger can depend on system design, including whether there is a deaerator in place.
• Safety and Environmental Impact: Some oxygen scavengers, like hydrazine, have significant safety and environmental concerns.

Benefits of Using Oxygen Scavengers

• Corrosion Prevention: Removing dissolved oxygen prevents oxidation and corrosion of boiler and condensate return system components.
• Extended Equipment Life: Reduced corrosion leads to a longer lifespan for the boiler and associated equipment.
• Improved Efficiency: Less corrosion translates to better heat transfer and overall system efficiency.
• Reduced Maintenance Costs: Minimizing corrosion lowers maintenance frequency and costs.

Application Methods

• Continuous Dosing: Oxygen scavengers are continuously added to the feedwater to ensure consistent removal of dissolved oxygen.
• Batch Dosing: In some systems, oxygen scavengers are added in batches based on water usage and operating conditions.
• Automated Dosing Systems: Advanced systems use sensors and automated dosing pumps to maintain optimal levels of oxygen scavengers.