Ostwald process

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The Ostwald Process is a key industrial process for producing nitric acid and ammonium nitrate from ammonia.

The process

A 1:9 ratio of dry ammonia gas and air are flowed over a preheated catalyst, producing a mixture of nitrogen oxides and water. The heat generated by the oxidation is sufficient to keep the catalyst heated as long as the process is continued.

Both partial oxidation steps are thermodynamically favored:

4 NH3 + 5 O2 4 NO + 6 H2OΔH=-906kJ
2 NO + O2 2 NO2ΔH=-36.3kJ

This means the reaction will take place over a very large range of operating temperatures, pressures, and reactant velocities. However, the total oxidation is also favored:

2 NO2 N2 + 2 O2ΔH=-51.3kJ

This means the reaction must be monitored and the reaction materials and environment moderated to produce the desired oxides, rather than pure nitrogen.

Managing the reaction

The most important indicator is the color of the gases after the catalyst:

  • Brownish fumes indicate the presense of nitrogen oxides. This is optimal.
  • Whitish fumes indicate underoxidation and the formation of ammonium compounds
    • Perhaps the temperature of the catalyst is too low
    • Perhaps too much ammonia is being injected
    • Perhaps too little air is being injected
    • Perhaps the speed through the catalyst is too high
  • Clear gases indicate overoxidation
    • Perhaps the speed through the catalyst is too low
    • Perhaps not enough ammonia is being injected
    • Perhaps too much air is being injected (very unlikely)
    • Perhaps the temperature of the catalyst is too high

Catalysts

Platinum

Rough

This catalyst operates at flame temperature, ambient pressure and reasonable velocities. Platinum-coated ceramic chips may be produced as follows:

  1. Dissolve platinum in aqua regia
  2. Soak shards of well-fired ceramic in the dissolved platinum, allowing for maximum wetting.
  3. Heat shards until they are dried, distributing the platinum over their surface area

Smooth

Smooth platinum works best at higher pressures and velocities, and has less loss of catalyst material than rough platinum.

  • 200-900°C (800 typical)
  • On-the-order-of 0.01 second contact time
  • Gas mixture speed of 1.5m/s
  • Catalyst depth of 1-1.5cm

Cupric oxide

Copper (II) oxide will catalyze this reaction, less efficiently. A copper mesh is used as the catalyst, and the mesh oxidizes to cupric oxide which is the actual catalyst.

  • 500-800°C
  • On-the-order-of 0.1-1s contact time
  • May require pure oxygen rather than air.

Manganese/Copper oxides

A mixture of 60% MnO2 and 40% CuO has been reported to work effectively.

Cobalt oxide

Cobalt (II,III) Oxide, Co3O4 is the active oxide for the oxidation of ammonia to nitric acid[1] Cobalt is more plentiful than platinum, and it is used in its oxided state, so it doesn't require complicated refining. It also produces less N2O than platinum catalysts[2], so there is somewhat less ammonia lost to waste products.

  • Cobalt (II,III) oxide is only stable in the temperature range 600-900°C.
  • The reaction takes place at 800°C and 1-1.2 atm:
4 NH3(g) + 5 O2(g)
{Co3O4
800°C, 1atm}
4 NO(g) + 6 H2O(g)

References

  1. US patent 4389339
    Link courtesy Google
  2. US patent 3931391
    Link courtesy Google

See Also