Ostwald process

The Ostwald process is a chemical process used for making nitric acid (HNO₃). Wilhelm Ostwald developed the process, and he patented it in 1902.^[1]^[2] The Ostwald process is a mainstay of the modern chemical industry, and it provides the main raw material for the most common type of fertilizer production.^[3] Historically and practically, the Ostwald process is closely associated with the Haber process, which provides the requisite raw material, ammonia (NH₃).

Description

Stage 1

Ammonia is converted to nitric acid in 2 stages. It is oxidized by heating with oxygen in the presence of a catalyst such as platinum with 10% rhodium, platinum metal on fused silica wool, copper or nickel,^[4] to form nitric oxide (nitrogen(II) oxide) and water (as steam). This reaction is strongly exothermic, making it a useful heat source once initiated:^[5]

{\ce {4 NH3 (g) + 5 O2 (g) -> 4 NO (g) + 6 H2O (g)}}

(ΔH = −905.2 kJ/mol)

Stage 2

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 (nitrogen(IV) oxide).^[5] 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:^[5]

{\ce {2 NO (g) + O2 (g) -> 2 NO2 (g)}}

(ΔH = −114 kJ/mol)

{\ce {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.

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

{\ce {4 NO2 (g) + O2 (g) + 2 H2O (l) -> 4 HNO3 (aq)}}

(ΔH = −348 kJ/mol).[In Absorption Tower].

Typical conditions for the first stage, which contribute to an overall yield of about 98%, are:

pressure is between 4–10 standard atmospheres (410–1,000 kPa; 59–150 psi) and
temperature is about 870–1,073 K (600–800 °C; 1,100–1,500 °F).

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

{\ce {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.^[6]

Overall reaction

The overall reaction is the sum of the first equation, 3 times the second equation, and 2 times the last equation; all divided by 2:

{\ce {2 NH3 (g) + 4 O2 (g) + H2O (l) -> 3 H2O (g) + 2 HNO3 (aq)}}

(ΔH = −740.6 kJ/mol)

Alternatively, if the last step is carried out in air, the overall reaction is the sum of equation 1, 2 times the equation 2, and equation 4; all divided by 2.

Without considering the state of water,

{\ce {NH3 (g) + 2 O2 (g) -> H2O + HNO3 (aq)}}

(ΔH = −370.3 kJ/mol)

References

^ GB 190200698, Ostwald, Wilhelm, "Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides", published January 9, 1902, issued March 20, 1902
^ GB 190208300, Ostwald, Wilhelm, "Improvements in and relating to the Manufacture of Nitric Acid and Oxides of Nitrogen", published December 18, 1902, issued February 26, 1903
^ Kroneck, Peter M. H.; Torres, Martha E. Sosa (2014). The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment. Dordrecht: Springer. p. 215. ISBN 978-94-017-9268-4.
^ Foist, Laura. "The Ostwald Process & Catalytic Oxidation of Ammonia". Study.com. Retrieved 5 January 2019.
^ ^a ^b ^c Alan V. Jones; M. Clemmet; A. Higton; E. Golding (1999). Alan V. Jones (ed.). Access to chemistry. Royal Society of Chemistry. p. 250. ISBN 0-85404-564-3.
^ Harry Boyer Weiser (2007). Inorganic Colloid Chemistry -: The Colloidal Elements. Read Books. p. 254. ISBN 978-1-4067-1303-9.

External links

Nitrogen & Phosphorus (General Chemistry course), Purdue University
Drake, G; "Processes for the Manufacture of Nitric Acid" (1963), International Fertiliser Society (paysite/password)
Manufacturing Nitrates: the Ostwald process Carlton Comprehensive High School; Prince Albert; Saskatchewan, Canada.

[1] GB 190200698, Ostwald, Wilhelm, "Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides", published January 9, 1902, issued March 20, 1902

[2] GB 190208300, Ostwald, Wilhelm, "Improvements in and relating to the Manufacture of Nitric Acid and Oxides of Nitrogen", published December 18, 1902, issued February 26, 1903

[3] Kroneck, Peter M. H.; Torres, Martha E. Sosa (2014). The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment. Dordrecht: Springer. p. 215. ISBN 978-94-017-9268-4.

[4] Foist, Laura. "The Ostwald Process & Catalytic Oxidation of Ammonia". Study.com. Retrieved 5 January 2019.

[jones1-5] Alan V. Jones; M. Clemmet; A. Higton; E. Golding (1999). Alan V. Jones (ed.). Access to chemistry. Royal Society of Chemistry. p. 250. ISBN 0-85404-564-3.

[6] Harry Boyer Weiser (2007). Inorganic Colloid Chemistry -: The Colloidal Elements. Read Books. p. 254. ISBN 978-1-4067-1303-9.

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