Ostwald Process - Description

Description

Ammonia is converted to nitric acid in two stages. It is oxidized (in a sense "burnt") by heating with oxygen in the presence of a catalyst such as platinum with 10% rhodium, to form nitric oxide and water. This step is strongly exothermic, making it a useful heat source once initiated:

4 NH₃ (g) + 5 O₂ (g) → 4 NO (g) + 6 H₂O (g) (ΔH = −905.2 kJ)

Stage two encompasses two reactions and is carried out in an absorption apparatus containing water. Initially nitric oxide is oxidized again to yield nitrogen dioxide: This gas is then readily absorbed by the water, yielding the desired product (nitric acid, albeit in a dilute form), while reducing a portion of it back to nitric oxide:

2 NO (g) + O₂ (g) → 2 NO₂ (g) (ΔH = −114 kJ/mol)

3 NO₂ (g) + H₂O (l) → 2 HNO₃ (aq) + NO (g) (ΔH = −117 kJ/mol)

The NO is recycled, and the acid is concentrated to the required strength by distillation.

Alternatively, if the last step is carried out in air:

4 NO₂ (g) + O₂ (g) + 2 H₂O (l) → 4 HNO₃ (aq)

Typical conditions for the first stage, which contribute to an overall yield of about 96%, are:

• pressure between 4 and 10 atmospheres (approx. 400-1010 kPa or 60-145 psig) and
• temperature is about 1173 K (approx. 900 °C or 1652 °F.).

A complication that needs to be taken into consideration involves a side-reaction in the first step that reverts the nitric oxide back to N₂:

4 NH₃ + 6 NO → 5 N₂ + 6 H₂O

This is a secondary reaction that is minimised by reducing the time the gas mixtures are in contact with the catalyst.