Nitric acid is a colourless liquid that is used in the manufacture of inorganic and organic nitrates and nitro compounds for fertilizers, dye intermediates, explosives, and many different organic chemicals. Nitric acid is most commonly manufactured by Ostwald process. The Ostwald process converts ammonia to nitric acid and involves two steps.
In step 1, ammonia is oxidized to form nitric oxide and also nitrogen dioxide. Then in step 2, the nitrogen dioxide that was formed is absorbed in water. This in-turn forms nitric acid.
The Ostwald process has many well-known uses in both the industrial and health field. Through the Ostwald process, nitric acid is commonly used in fertilizers and pharmaceuticals, and because of it’s chemical reaction with some compounds it is used in rocket fuel and explosives like trinitrotoluene (TNT).
Step 1 - Primary oxidation (formation of nitric acid)
Oxidation of ammonia is carried out in a catalyst chamber in which one part of ammonia and eight parts of oxygen by volume are introduced. The temperature of chamber is about 600oC. This chamber contains a platinum gauze which serves as catalyst.
Oxidization of ammonia is reversible and exothermic process. Therefore according to Le- chatelier’s principle, a decrease in temperature favours reaction in forward direction. In primary oxidization 95 per cent of ammonia is converted into nitric oxide (NO).
4NH3 + 5O2 ↔ 4NO + 6H2O ?H -24.8 Kcal/mol
Step 2 - Secondary oxidation (formation of nitrogen dioxide)
Nitric oxide gas obtained by the oxidation of ammonia is very hot. In order to reduce its temperature, it is passed through a heat exchanger where the temperature of nitric oxide is reduces to 150oC. Nitric oxide after cooling is transferred to another oxidizing tower where at about 50oC it is oxidized to nitrogen dioxide (NO2).
2NO + O2 ↔2 NO2
Step 3 - Absorption of NO2 (formation of HNO3)
Nitrogen dioxide from secondary oxidation chamber is introduced into a special absorption tower. NO2 gas passed through the tower and water is showered over it. By the absorption, nitric acid is obtained.
3NO2 + H2O -> 2HNO3 + NO
Nitric acid so obtain is very dilute. It is recycled in absorption tower so that more and more NO2 get absorbed. HNO3 after recycle becomes about 68 per cent concentrated.
Step 4 - Concentration
In order to increase the concentration of HNO3, vapours of HNO3 are passed over concentrated H2SO4. Being a dehydrating agent, H2SO4 absorbs water from HNO3 and concentrated HNO3 is obtained.
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