The Haber Process

The Haber Processvideo

The Haber Process

• 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

Explaining the Conditions in the Haber Process

Explaining the Conditions in 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