Ostwald process

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A laboratory setup illustrating the consecutive steps of the Ostwald process for making nitric acid. Ostwaldverfahren Laboraufbau.jpeg
A laboratory setup illustrating the consecutive steps of the Ostwald process for making nitric acid.

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 because it is less expensive and more efficient. [3]

Contents

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 twice that 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

The Ostwald process was developed at the turn of the 20th Century by Wilhelm Ostwald, who demonstrated that ammonia could be catalytically oxidised to nitric oxide using platinum-based catalysts, providing a practical route to nitric acid without the use of natural nitrate sources. [12] In 1902, he patented this process, and early industrial implementations revealed the importance of catalyst composition and operating conditions to maximise yield while limiting ammonia decomposition (mainly high temperatures and short contact times). [13] [14] [15] Subsequent improvements included the adoption of platinum-rhodium gauze catalysts; these increased mechanical stress, reduced platinum losses, and were more stable long-term. [16] [17]

Large-scale deployment of the Ostwald process became economically viable only after the development and industrial implementation of the Haber-Bosch process in 1913, which supplied a continuous, synthetic ammonia feedstock in sufficient quantities to sustain nitric acid production. [15] The integration of the Haber-Bosch process during the First World War enabled Germany to maintain nitric acid production despite Allied blockades of Chilean nitrate imports, making the combines processes strategically significant in the manufacture of explosives as well as fertiliser production. [16] [18] Continued post-war refinements in catalyst design included secondary catchment gauzes containing palladium or gold, which were introduced downstream of the primary catalyst to recover lost platinum. [16] [19]

See also

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. 1 2 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. 1 2 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. Bibcode:2005AppCA.284..163H. 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. 1 2 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. ↑ Kent, James Albert; Riegel, Emil Raymond; Riegel, Emil Raymond (2006). Kent and Riegel's handbook of industrial chemistry and biotechnology (11th ed. / edited by James A. Kent ed.). New York: Springer. ISBN   978-0-387-27842-1.
  13. ↑ GB 190200698, Ostwald, Wilhelm,"Improvements in the Manufacture of Nitric Acid and Nitrogen Oxides",published January 9, 1902,issued March 20, 1902
  14. ↑ 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
  15. 1 2 Smil, Vaclav; Haber, Fritz; Bosch, Carl (2004). Enriching the earth: Fritz Haber, Carl Bosch, and the transformation of world food production. Cambridge, Mass. London: MIT. ISBN   978-0-262-69313-4.
  16. 1 2 3 Weissermel, Klaus; Arpe, Hans-Jürgen, eds. (2003). Industrial organic chemistry (4th ed.). Weinheim: Wiley-VCH. ISBN   978-3-527-30578-0.
  17. ↑ Housecroft, Catherine E. (2018). Inorganic chemistry (5th ed.). Harlow, England ; New York: Pearson. ISBN   978-1-292-13414-7.
  18. ↑ Travis, A. S. “The Synthetic Nitrogen Industry in World War I.” Chemical Heritage, 6, 2, 1988, pp. 1–11.
  19. ↑ Bartholomew, Calvin H.; Farrauto, Robert J. (2006). Fundamentals of industrial catalytic processes (2nd ed.). Hoboken, N.J: Wiley. ISBN   978-0-471-73007-1.
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