Uses and Production of Ammonia (Haber Process)

Haber Process for the Production of Ammonia

In 1909 Fritz Haber established the conditions under which nitrogen, N₂(g), and hydrogen, H₂(g), would combine using

• medium temperature (~500^oC)
• very high pressure (~250 atmospheres, ~351kPa)
• a catalyst (a porous iron catalyst prepared by reducing magnetite, Fe₃O₄).
  Osmium is a much better catalyst for the reaction but is very expensive.

This process produces an ammonia, NH₃(g), yield of approximately 10-20%.

The Haber synthesis was developed into an industrial process by Carl Bosch.

The reaction between nitrogen gas and hydrogen gas to produce ammonia gas is exothermic, releasing 92.4kJ/mol of energy at 298K (25^oC).

N2(g) nitrogen

+

3H2(g) hydrogen

heat, pressure, catalyst

2NH3(g) ammonia

H = -92.4 kJ mol-1

OR

N2(g) nitrogen

+

3H2(g) hydrogen

heat, pressure, catalyst

2NH3(g) ammonia

+ 92.4 kJ mol-1

By Le Chetalier's Principle:

• increasing the pressure causes the equilibrium position to move to the right resulting in a higher yeild of ammonia since there are more gas molecules on the left hand side of the equation (4 in total) than there are on the right hand side of the equation (2). Increasing the pressure means the system adjusts to reduce the effect of the change, that is, to reduce the pressure by having fewer gas molecules.
• decreasing the temperature causes the equilibrium position to move to the right resulting in a higher yield of ammonia since the reaction is exothermic (releases heat). Reducing the temperature means the system will adjust to minimise the effect of the change, that is, it will produce more heat since energy is a product of the reaction, and will therefore produce more ammonia gas as well
  However, the rate of the reaction at lower temperatures is extremely slow, so a higher temperature must be used to speed up the reaction which results in a lower yield of ammonia.

Rate considerations:

• A catalyst such as an iron catalyst is used to speed up the reaction by lowering the activation energy so that the N₂ bonds and H₂ bonds can be more readily broken.
• Increased temperature means more reactant molecules have sufficient energy to overcome the energy barrier to reacting (activation energy) so the reaction is faster at higher temperatures (but the yield of ammonia is lower as discussed above).
  A temperature range of 400-500^oC is a compromise designed to achieve an acceptable yield of ammonia (10-20%) within an acceptable time period.
  

  At 200oC and pressures above 750atm there is an almost 100% conversion of reactants to the ammonia product. Since there are difficulties associated with containing larger amounts of materials at this high pressure, lower pressures of around 200 atm are used industrially. By using a pressure of around 200atm and a temperature of about 500oC, the yield of ammonia is 10-20%, while costs and safety concerns in the biulding and during operation of the plant are minimised

  During industrial production of ammonia, the reaction never reaches equilibrium as the gas mixture leaving the reactor is cooled to liquefy and remove the ammonia. The remaining mixture of reactant gases are recycled through the reactor. The heat released by the reaction is removed and used to heat the incoming gas mixture.

Uses of Ammonia

A Brief History

At the beginning of the 20^th century there was a shortage of naturally occurring, nitrogen-rich fertilisers, such as Chile saltpetre, which prompted the German Chemist Fritz Haber, and others, to look for ways of combining the nitrogen in the air with hydrogen to form ammonia, which is a convenient starting point in the manufacture of fertilisers.This process was also of interest to the German chemical industry as Germany was preparing for World War I and nitrogen compounds were needed for explosives.

The hydrogen for the ammonia synthesis was made by the water-gas process (a Carl Bosch invention) which involves blowing steam through a bed of red hot coke resulting in the separation of hydrogen from oxygen. The nitrogen was obtained by distillation of liquid air, then by cooling and compressing air.

These days, the hydrogen is produced by reforming light petroleum fractions or natural gas (methane, CH₄) by adding steam:

CH4(g) + H2O(g)

Ni catalyst ----------> 700oC

CO(g) + 3H2(g)

Enough steam is used to react with about 45% of the methane (CH₄), the rest of the methane is reacted with air:

2CH4(g) +

O2(g) + 4N2(g) (air)

Ni catalyst --------->

2CO(g) + 4H2(g) + 4N2(g)

All the carbon monoxide (CO) in the mixture is oxidised to CO₂ using steam and an iron oxide catalyst:

CO(g) + H2O(g)

iron oxide catalyst ------------------>

H2(g) + CO2(g)

The carbon dioxide (CO₂) is removed using a suitable base so that only the nitrogen gas (N₂) and hydrogen gas (H₂) remain and are used in the production of ammonia (NH₃).

In ammonia production the hydrogen and nitrogen are mixed together in a ratio of 3:1 by volume and compressed to around 200 times atmospheric pressure.

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