Ostwald process

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). This method is preferred over other methods of nitric acid production, in that it is less expensive and more efficient.^[3]

Reactions

Ammonia is converted to nitric acid in 2 stages.

Initial oxidation of ammonia

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)^[4] in the presence of a catalyst such as platinum gauze, alloyed with 10% rhodium to increase its strength and nitric oxide yield, 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:^[5]

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

Side reactions

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

4NH₃ + 6NO → 5N₂ + 6H₂O

This is a secondary reaction that is minimised by reducing the time the gas mixtures are in contact with the catalyst.^[6] Another side reaction produces nitrous oxide:

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

Platinum-rhodium catalyst

The platinum and rhodium catalyst is frequently replaced due to decomposition as a result of the extreme conditions which it operates under, leading to a form of degradation called cauliflowering.^[7] The exact mechanism of this process is unknown, the main theories being physical degradation by hydrogen atoms penetrating the platinum-rhodium lattice, or by metal atom transport from the centre of the metal to the surface.^[7]

Secondary oxidation

The nitric oxide (NO) formed in the prior catalysed reaction is then cooled down from around 900˚C to roughly 250˚C to be further oxidised to nitrogen dioxide (NO₂)^[8] by the reaction:

2NO + O₂ → 2NO₂ (ΔH = -114.2 kJ/mol)^[9]

The reaction:

2NO₂ → N₂O₄ (ΔH = -57.2 kJ/mol)^[10]

also occurs once the nitrogen dioxide has formed.^[11]

Conversion of nitric oxide

Stage two encompasses the absorption of nitrous oxides in water and is carried out in an absorption apparatus, a plate column containing water.^{[citation needed]} This gas is then readily absorbed by the water, yielding the desired product (nitric acid in a dilute form), while reducing a portion of it back to nitric oxide:^[5]

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

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

This is only one of over 40 absorption reactions of nitrous oxides recorded,^[11] with other common reactions including:

3N₂O₄ + 2H₂O → 4HNO₃ + 2NO

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

4NO₂ + O₂ + 2H₂O → 4HNO₃ (ΔH = −348 kJ/mol).

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:

2NH₃ + 4O₂ + H₂O → 3H₂O + 2HNO₃ (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 + 2O₂ → H₂O + HNO₃ (ΔH = −370.3 kJ/mol)

History

Wilhelm Ostwald developed the process, and he patented it in 1902.^[12][13]

See also

• Birkeland–Eyde process

References

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. "Ostwald Process". Unacademy. Retrieved 2024-09-05.
4. Considine, Douglas M., ed. (1974). Chemical and process technology encyclopedia. New York: McGraw-Hill. pp. 769–72. ISBN 978-0-07-012423-3.
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.
7. Hannevold, Lenka; Nilsen, Ola; Kjekshus, Arne; Fjellvåg, Helmer (2005-04-28). "Reconstruction of platinum–rhodium catalysts during oxidation of ammonia". Applied Catalysis A: General. 284 (1): 163–176. doi:10.1016/j.apcata.2005.01.033. ISSN 0926-860X.
8. Afolayan Ayodele S (7 December 2007). "Design of a Plant to Produce 20,000 Litres per Day of Nitric Acid From Ammonia and Air (Using Oswald Process)" (PDF). Repository Futminna. Retrieved 24 May 2024.
9. Grande, Carlos A.; Andreassen, Kari Anne; Cavka, Jasmina H.; Waller, David; Lorentsen, Odd-Arne; Øien, Halvor; Zander, Hans-Jörg; Poulston, Stephen; García, Sonia; Modeshia, Deena (2018-08-08). "Process Intensification in Nitric Acid Plants by Catalytic Oxidation of Nitric Oxide". Industrial & Engineering Chemistry Research. 57 (31): 10180–10186. doi:10.1021/acs.iecr.8b01483. ISSN 0888-5885.
10. "21.1 The Effect of Temperature on the NO2/N2O4 Equilibrium". chemed.chem.purdue.edu. 24 May 2024. Retrieved 24 May 2024.
11. Liu, Yunda; Bluck, David; Brana-Mulero, Francisco (2014-01-01), Eden, Mario R.; Siirola, John D.; Towler, Gavin P. (eds.), "Static and dynamic simulation of NOx absorption tower based on a hybrid kinetic-equilibrium reaction model", Computer Aided Chemical Engineering, Proceedings of the 8 International Conference on Foundations of Computer-Aided Process Design, vol. 34, Elsevier, pp. 363–368, doi:10.1016/b978-0-444-63433-7.50045-6, ISBN 978-0-444-63433-7, retrieved 2024-05-24
12. GB 190200698, Ostwald, Wilhelm, "Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides", published January 9, 1902, issued March 20, 1902
13. 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

• 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.