The Haber process

The raw materials for the process of making ammonia are hydrogen and nitrogen.

Hydrogen is obtained by reacting natural gas (mostly methane) with steam, or from cracking oil fractions.

CH4 + 2H2O → CO2 + 2H2

Nitrogen is obtained from the air. Air is 78 per cent nitrogen and nearly all the rest is oxygen. When hydrogen is burned in air, the oxygen combines with the hydrogen leaving nitrogen behind.

In the Haber process, nitrogen and hydrogen react together under these conditions:

  • a high temperature – ranging from 350°C to 450°C
  • a high pressure – ranging from 150 to 200 atmospheres (150 – 200 times normal pressure)
  • an iron catalyst

In addition, any unreacted nitrogen and hydrogen are recycled. The reaction is reversible. In a chemical equation, the symbol ⇌ is used instead of an ordinary arrow if the reaction is reversible:

This equation summarises the Haber process:

nitrogen + hydrogen ⇌ ammonia

N2 + 3H2 ⇌ 2NH3

Hydrogen from natural gas and nitrogen from air are combined at a pressure of 200 atmospheres and a temperature of 450 °C using an iron catalyst, to produce ammonia.
Stage oneHaving obtained the hydrogen and nitrogen gases (from natural gas and the air respectively), they are pumped into the compressor through pipes.
Stage twoThe gases are pressurised to about 200 atmospheres of pressure inside the compressor.
Stage threeThe pressurised gases are pumped into a tank containing beds of iron catalyst at about 450°C. In these conditions, some of the hydrogen and nitrogen will react to form ammonia.
Stage fourThe unreacted nitrogen and hydrogen, together with the ammonia, pass into a cooling tank. The cooling tank liquefies the ammonia, which can be removed into pressurised storage vessels.
Stage fiveThe unreacted hydrogen and nitrogen gases are recycled by being fed back through pipes to pass through the hot iron catalyst beds again.

The reaction mixture contains some ammonia, plus a lot of unreacted nitrogen and hydrogen. The mixture is cooled and compressed, causing the ammonia gas to condense into a liquid. The liquefied ammonia is separated and removed. The unreacted nitrogen and hydrogen are then recycled back into the reactor.