The Häber process is the industrial method of creating ammonia, developed just before the first world war it was a process that allowed the Germans to produce explosives. The British had access to natural nitrate reserves in Chile, it’s thought that without this process the war would have finished many years earlier.
The Häber process is still one of the most important industrial methods of time as ammonium salts are a key ingredient of ammonium phosphate salt fertilisers which are in regular use throughout the world.
The key step in this process is the direct synthesis of ammonia from nitrogen and hydrogen in which an equilibrium can be set up.
N2(g) + 3H2(g) –> NH3(g)
Iron catalyst, 400oC, 200atm
Economic factors to be considered
- Containers operating at a high pressure are expensive to build and operate
- The forward reaction is exothermic so lower temperatures favour a higher yield of Ammonia
- A reduction in temperature lowers the rate at which the equilibrium can be attained
- Unreacted gases can be recycled if the product can be removed
- Catalysts are used in order to increase the rate of reaction
- Catalysts are often susceptible to poisoning by impurities present, especially sulphur compounds and carbon monoxide in the original feedstock
- Catalysts usually last longer at lower temperatures.
In practise a compromise set of conditions is applied so that the requirement to push the equilibrium position to the right is balanced against the cost of doing so.
The conditions generally used are –
- Pressure of 200 atm
- Temperature of 400oC
- Finely divided Iron based catalyst
Why these conditions?
These values keep the rate of reaction at a reasonable level as well as giving long life to the catalyst
A finely divided catalyst increases the rate of attainment of equilibrium and the incoming gases are purified before entering the catalyst chamber in order to avoid poisoning the catalyst.
Although a conversion of 40% should be possible under these conditions the gases do not spend long enough in thee reaction chamber to reach equilibrium and a yield of only 15% is often achieved.
The gas mixture is cooled in order to liquefy the ammonia and remove it from the remaining gases. The unreacted gases are then passed through the reaction chamber again maintaining a continuous circulation.
Removed from methane by reacting it with steam
CH4(g) + H2O(g) –> CO(g) + 3H2(g)
Removed from the air by fractional distillation