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Ostwald process

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The Ostwald process is a chemical process used for making nitric acid (HNO3).[1] 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.[2] Historically and practically, the Ostwald process is closely associated with the Haber process, which provides the requisite raw material, ammonia (NH3).

Reactions edit

Ammonia is converted to nitric acid in 2 stages.

Oxidation of ammonia edit

The Ostwald process begins with burning ammonia. Ammonia burns in oxygen at temperature about 900 °C (1,650 °F) and pressure up to 8 standard atmospheres (810 kPa)[3] in the presence of a catalyst such as platinum gauze with 10% rhodium, platinum metal on fused silica wool, copper or nickel to form nitric oxide (nitrogen(II) oxide) and water (as steam). This reaction is strongly exothermic, making it a useful heat source once initiated:[4]

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

Side reactions edit

A number of side reactions compete with the formation of nitric oxide. Some reactions convert the ammonia to N2, such as:

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.[5] Another side reaction produces nitrous oxide:

4 NH3 + 4O2 → 2 N2O + 6 H2O (ΔH = −1105 kJ/mol)

Conversion of nitric oxide edit

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).[4] 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:[4]

2 NO + O2 → 2 NO2 (ΔH = −114 kJ/mol)
3 NO2 + H2O → 2 HNO3 + NO (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:

4 NO2 + O2 + 2 H2O → 4 HNO3 (ΔH = −348 kJ/mol).[In Absorption Tower].

Overall reaction edit

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:

2 NH3 + 4 O2 + H2O → 3 H2O + 2 HNO3 (ΔH = −740.6 kJ/mol)

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

Without considering the state of the water,

NH3 + 2 O2 → H2O + HNO3 (ΔH = −370.3 kJ/mol)

History edit

Wilhelm Ostwald developed the process, and he patented it in 1902.[6][7]

See also edit

References edit

  1. ^ Thiemann, Michael; Scheibler, Erich; Wiegand, Karl Wilhelm (2005). "Nitric Acid, Nitrous Acid, and Nitrogen Oxides". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a17_293. ISBN 978-3-527-30673-2.
  2. ^ 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.
  3. ^ Considine, Douglas M., ed. (1974). Chemical and process technology encyclopedia. New York: McGraw-Hill. pp. 769–72. ISBN 978-0-07-012423-3.
  4. ^ 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.
  5. ^ Harry Boyer Weiser (2007). Inorganic Colloid Chemistry -: The Colloidal Elements. Read Books. p. 254. ISBN 978-1-4067-1303-9.
  6. ^ GB 190200698, Ostwald, Wilhelm, "Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides", published January 9, 1902, issued March 20, 1902 
  7. ^ 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 

External links edit

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