The Haber Process (Cambridge O Level Chemistry)

• Ammonia is manufactured in an exothermic reaction called the Haber process which occurs in five stages:
• Stage 1: H₂ and N₂ are obtained from methane in natural gas and the air respectively and are pumped into the compressor through pipe
• Stage 2: The gases are compressed to about 200 atmospheres (20,000 kPa) inside the compressor
• Stage 3: The pressurised gases are pumped into a tank containing layers of catalytic iron beds at a temperature of 450 °C. Some of the hydrogen and nitrogen react to form ammonia:

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

• Stage 4: Unreacted H₂ and N₂ and the product ammonia pass into a cooling tank. The ammonia is liquefied and removed to pressurised storage vessels
• Stage 5: The unreacted H₂ and N₂ gases are recycled back into the system and start over again

 

The production of ammonia by the Haber process

 

• Reaction conditions such as temperature and pressure affect the rate of a reaction
• If the reaction is reversible then the position of equilibrium is also affected by changes in these conditions and often we must consider a trade-off between the rate of reaction and product yield
• The graph below illustrates the effects of changing temperature and pressure on the yield of ammonia obtained
• By following any of the curved lines on the graph it can be seen that as the pressure increases, so too does the yield at any given temperature
• By following any vertical line upwards from the x-axis, the graph shows that as the temperature decreases, the yield actually increases
• The actual conditions used must be chosen depending on a number of economical, chemical and practical considerations

The yield of ammonia produced changes with changes made to temperature and pressure

Economic Considerations

• Like all industries, companies that manufacture and sell chemical goods do so to make a profit
• Part of the industrial process is the economic decision on how and where to design and implement a manufacturing site
• The availability and cost of raw materials is a major consideration which must be studied well before any decisions are taken
• In the Haber Process the raw materials are readily available and inexpensive to purify:
  • Nitrogen - from the air
  • Hydrogen- from methane in natural gas

• If the cost of extraction of raw materials is too high or they are unavailable then the process is no longer economically viable
• Many industrial processes require huge amounts of heat and pressure which is very expensive to maintain
• Production energy costs are also a factor to be considered carefully and alongside the raw materials issue

Temperature: 450 ºC

• A higher temperature would favour the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made
• If a lower temperature is used it favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be made
• However at a lower temperature the rate of reaction is very slow
• So 450 ºC is a compromise temperature between having a lower yield of products but being made more quickly

Pressure: 200 atm

• A lower pressure would favour the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be made
• A higher pressure would favour the forward reaction as it will try to decrease the pressure by creating less molecules (2 molecules of gaseous products) so a higher yield of products will be made
• However high pressures can be dangerous and very expensive equipment is needed
• So 200 atm is a compromise pressure between a lower yield of products being made safely and economically

Catalyst: Iron

• The presence of a catalyst does not affect the position of equilibrium but it does increase the rate at which equilibrium is reached
• This is because the catalyst increases the rate of both the forward and backward reactions by the same amount (by providing an alternative pathway requiring lower activation energy)
• As a result, the concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst.

  So a catalyst is used as it helps the reaction reach equilibrium quicker

• It allows for an acceptable yield to be achieved at a lower temperature by lowering the activation energy required
• Without it the process would have to be carried out at an even higher temperature, increasing costs and decreasing yield as the higher temperature decomposes more of the NH₃ molecules