Conditions for the Manufacture of Ammonia
For the manufacture of ammonia, the reaction is usually carried out at a temperature of 450º C and a pressure 200 atmospheres. To see why these conditions are used look at Figure 2.18. This shows the effect of pressure on the amount of ammonia in the equilibrium mixture at five different temperatures.
The graph shows that:
• the higher the pressure, the greater is the percentage of ammonia at equilibrium, and
• the higher the temperature, the smaller is the percentage of ammonia at equilibrium.
Therefore, to get the maximum amount of ammonia we need a very high pressure and a very low temperature.
In practice, a high pressure of about 200 atmospheres is used. Using higher pressures would produce even more ammonia, but this involves extra expense, and there is a safety risk.
It is not realistic to use a very low temperature of about 100 C, because the reaction would be far too slow and it would take a very long time to reach equilibrium. Therefore, a moderately high temperature of 450 ºC is used as a compromise, even though the amount of ammonia at equilibrium is reduced.
At the conditions of and a pressure atm, only about 15% of the and hydrogen react to form ammonia. However, because unreacted nitrogen and hydrogen are passed over the catalyst again, about 98% of the reactants are eventually changed into ammonia.