Ostwald process

The Ostwald process is ammonia.

Description

Ammonia is converted to nitric acid in two stages. It is nitric oxide and water. This step is strongly exothermic, making it a useful heat source once initiated (ΔH = -950 kJ):

4NH_3(g) + 5O_2(g) → 4NO_(g) + 6H₂O_(g)

Stage two (combining two reaction steps) is carried out in the presence of water in an absorption apparatus. Initially nitric oxide is oxidized again to yield nitrogen dioxide:

2NO_(g) + O_2(g) → 2NO_2(g)

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:

3NO_2(g) + H₂O_(l) → 2HNO_3(aq) + NO_(g)

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:

4NO_2(g) + O_2(g) + 2H₂O_(l) → 4HNO_3(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
• K (approx. 900°C or 1652°F.).

History

An earlier patent (Kuhlmann, 1838) described the basic chemistry, but ammonia (from animal sources) was scarce at the time, so it was probably of only academic interest, either then or when Ostwald patented his version in 1902. A later date is frequently given for the development of the process (1908), and it may be that by then chemical engineering than of basic science.

Prior to this the source of most industrial nitrogen were fertilizers.

Initially the process was carried out in low pressure stoneware vessels, but the introduction of stainless steel made higher pressures practical, leading to higher yields.