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Making fertilisers industrially

The Ostwald process for making nitric acid

Ammonium nitrate is perhaps the most common compound found in fertilisers. It is made by reacting an alkaline solution of ammonium hydroxide with nitric acid. The equation for the reaction is given below:

ammonium hydroxide(aq) + nitric acid(aq) → ammonium nitrate(aq) + water(l)
NH4OH(aq) + HNO3(aq) → NH4NO3(aq) + H2O(l)
Ammonium hydroxide is simply made by dissolving ammonia in water:
ammonia(g) + water(l) ⇌ ammonium hydroxide(aq)
NH3(s) + H2O(g) ⇌ NH4OH(aq)
Obtaining large amounts of ammonium hydroxide will be straight forward. Ammonia is readily obtained from the Haber process. The other reactant; nitric acid is also obtained thanks to the Haber process. Recall that to make an acid a non-metal oxide is dissolved in water e.g.
Non-metal oxide + water → acid
Carbon dioxide + water → carbonic acid
Sulfur dioxide + water → sulfurous acid
Sulfur trioxide + water → sulfuric acid
Nitrogen dioxide + water →nitric acid

The problem with making nitric acid is actually getting the nitrogen dioxide gas. Nitrogen gas is a very unreactive gas so simply burning nitrogen gas in oxygen (often called burning air!) is really not feasible since large amounts of energy are needed.

nitrogen(g) + oxygen(g) → nitrogen dioxide (g)

So what is needed is another, easier way of preparing nitrogen dioxide gas. What about burning ammonia? Ammonia burns in oxygen with a yellowish coloured flame; as shown below: Ammonia burns with a yellow flame to form nitrogen gas and water vapour.

However there is a problem, ammonia burns to produce nitrogen gas and water. No nitrogen dioxide is produced as might have been expected:

ammonia(g) + oxygen(g) → nitrogen(aq) + water(l)
However by altering the conditions above we can obtain nitrogen dioxide gas, the gas needed to make nitric acid. All that is needed is the introduction of a platinum catalyst and some heat. The apparatus is shown below:

Oxidation of ammonia in presence of a platinum catalyst to form nitrogen dioxide gas.

In the presence of a platinum catalyst the ammonia is oxidised to give:

ammonia(g) + oxygen(g) → nitrogen monoxide(g) + water(l)
4NH3(s) + 5O2(g) → 4NO(g) + 6H2O(l)
Nitrogen monoxide gas which is often called nitric oxide is a colourless gas that forms inside the combustion tube. However on exposure to air/oxygen nitrogen monoxide is immediately oxidised to form brown nitrogen dioxide gas.
nitrogen monoxide(g) + oxygen(g) → nitrogen dioxide(g)
2NO(g) + O2(g) → 2NO2(g)
Nitrogen dioxide is a reddish-brown toxic gas with a bleachy smell. It dissolves in water to form nitric acid:
nitrogen dioxide(g) + water(l) + oxygen(g) → nitric acid(aq)
4NO2(g) + 2H2O(l) + O2(g) → 4HNO3(aq)
The industrial process for making nitric acid is called the Ostwald process, after Wilhelm Ostwald; a German Nobel prize winning scientist. He developed a process based on the reactions above to manufacture nitric acid. An outline of the Ostwald process is shown below:

Ostwald process for making nitric acid, oxidation of ammonia in the presence of a hot catalyst.

Starting from the left hand-side of the image:

  1. Oxygen from the air is compressed to between 4-10 atmospheres pressure and then pre-heated before it enters the reactor.
  2. Liquid ammonia from the Haber process enters the vaporizer where it is turned into a gas. Next the oxygen and gaseous ammonia enter the reactor. The ammonia is oxidised to nitrogen monoxide gas in a high temperature catalysed reaction. A platinum/rhodium catalyst is used and temperatures are in the range 800-950OC. This reaction is highly exothermic and releases a large amount of heat energy. This heat can be used to generate electricity or used as a heat source to pre-heat gases elsewhere in the reaction.
    ammonia(g) + oxygen(g) → nitrogen monoxide(g) + water(l)
    4NH3(s) + 5O2(g) → 4NO(g) + 6H2O(l)
  3. The nitrogen monoxide gas leaves the reactor and is cooled in the cooler. Here cold water is turned into steam as the hot nitrogen monoxide loses heat energy. This cool nitrogen monoxide gas now joins with oxygen to form nitrogen dioxide gas:
    nitrogen monoxide(g) + oxygen(g) → nitrogen dioxide(g)
    2NO(g) + O2(g) → 2NO2(g)
  4. The final vessel in the image is called the absorption tower. Here a shower of water falls from the top of the tower and meets the nitrogen dioxide gas as it rises up the tower. The nitrogen dioxide gas dissolves in the shower of water to form nitric acid. The nitric acid leaves at the base of the absorption tower and is collected in a large tank. The nitric acid produced can then be reacted with ammonium hydroxide solution, made by dissolving ammonia in water. This neutralisation reaction will form ammonium nitrate:
    ammonium hydroxide(aq) + nitric acid(aq) → ammonium nitrate(aq) + water(l)
    NH4OH (aq) + HNO3(aq) → NH4NO3(aq) + H2O(l)

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