Manufacture of nitric acid

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Manufacture of nitric acid

Overview:

The manufacture of nitric acid is a three-stage process each having distinct reaction:

a) Ammonia oxidation

4NH3 + 5O2 4NO + 6H2O H = - 900 kJmol-1

b) Nitric oxide oxidation

2NO + O2 2NO2 H = - 115 kJmol-1

2NO2 N2O4 H = - 58 kJmol-1

c) Dinitrogen tetroxide absorption

3N2O4 + 2H2O 4HNO3 + 2NO H = - 103 kJmol-1

Industrial Production of Nitric Acid

Nitric acid is produced by 2 methods.

  1. The first method involves oxidation, condensation, and absorption and yields a weak nitric acid with concentrations ranging from 30 to 70 percent nitric acid.

  1. The second method combines dehydrating, bleaching, condensing, and absorption and produces a high-strength nitric acid from a weak nitric acid. High-strength nitric acid contains more than 90 percent nitric acid.

Weak Nitric Acid Production

This is done by the high-temperature catalytic oxidation of ammonia as shown in Figure. This process consists of 3 steps. Each step corresponds to a distinct chemical reaction

  • Ammonia oxidation.
  • Nitric oxide oxidation.
  • Absorption.

.1. Ammonia Oxidation:

A mixture of Ammonia and air is oxidized at a temperature of 1380 to 1470 F while it passes through a catalytic converter.

4NH3 + 5O2 4NO + 6H2O H = - 900 kJmol-1

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The catalyst used is made of 90 percent platinum and 10 percent rhodium gauze.

Electron micrograph of platinum/rhodium catalyst

Effect of Temperature:

The oxidation of ammonia to nitric oxide (NO) is an exothermic reaction and yields 93 to 98 percent. A higher catalyst temperature increases NO production whereas lower catalyst temperatures produce nitrogen (N) and nitrous oxide (NO). Nitric oxide is a pollutant whereas nitrous oxide is a global warming gas.

2. Nitric Oxide Oxidation -

The nitric oxide formed during the ammonia oxidation must be oxidized. This is achieved ...

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