4.9 - The Haber Process

Making ammonia in the Haber process

Ammonia (NH₃) is manufactured by the Haber process, and is used mainly to make fertilisers.

The reaction

Nitrogen and hydrogen react reversibly to make ammonia:

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Sources of the raw materials

• Nitrogen - Extracted from the air (which is about 78% nitrogen).
• Hydrogen - Obtained from natural gas (by reacting methane with steam).

The conditions and the plant

• Purified nitrogen and hydrogen are mixed in a 1:3 ratio by volume (one volume of nitrogen to three of hydrogen) and pumped into the reaction vessel.
• The conditions inside the vessel are:
  • Temperature - About 450 °C
  • Pressure - About 200 atmospheres
  • Catalyst - Iron

Separating the ammonia

• Because the reaction is reversible, only some of the nitrogen and hydrogen turns into ammonia.
• The mixture passes into a condenser and is cooled. Ammonia has a much higher boiling point than nitrogen or hydrogen, so it condenses into a liquid and is removed.

Recycling unreacted gases

The unreacted nitrogen and hydrogen stay as gases and are piped back into the reaction vessel, so very little reactant is wasted.