Ostwald process

The Ostwald process is chemical process for producing nitric acid, which was developed by Wilhelm Ostwald (patented 1902). It is a mainstay of the modern chemical industry. Historically and practically it is closely associated with the Haber process, which provides the requisite raw material, ammonia.

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Description

Ammonia is converted to nitric acid in two stages. It is oxidized (in a sense "burned") 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 (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
• temperature is about 1173 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 Fritz Haber's method of making ammonia was known, which would have changed the outlook entirely. Ostwald's primary contributions seem to have been practical ones concerning the catalyst and operating conditions, a matter perhaps more of chemical engineering than of basic science.

Prior to this the source of most industrial nitrogen were nitrate ores, primarily imported Chile saltpeter, from which nitric acid was produced by treatment with sulfuric acid. The Ostwald process gained prominence as an adjunct to the fixing of nitrogen (Haber Process).The combined processes undoubtedly prolonged World War I, when Germany's overseas sources were cut off and explosives production was threatened. Subsequently their use of widely available raw materials (and the resulting low cost) led to their general adoption, resulting in vastly increased nitrogen consumption, mostly as 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.