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What is haber process? it is used in manufacturing ammonia
The Haber process (also known as Haber?Bosch process) is the reaction of nitrogen and hydrogen, over an iron-substrate, to produce ammonia.
The Haber process is important because ammonia is difficult to produce, on an industrial scale. Even though 78.1% of the air we breathe is nitrogen, the gas is relatively inert due to the strength of the triple bond that keeps the molecule together. It was not until the start of the twentieth century that this method was developed to harness the atmospheric abundance of nitrogen to create ammonia, which can then be oxidised to make the nitrates and nitrites essential for the production of nitrate fertilizer and munitions.
In the Haber Process, nitrogen (N2) and hydrogen (H2) gases are reacted over an iron catalyst (Fe3+) in which aluminium oxide (Al2O3) and potassium oxide (K2O) are used as promoters. The reaction is carried out under conditions of 150-250 atmospheres (atm), 450-500 °C; resulting in a yield of 10-20%:
N2(g) + 3H2(g) ? 2NH3(g) ?Ho = -92.4 kJ/mol
(Where ?Ho is the standard heat of reaction or standard enthalpy change)
These conditions are chosen due to the high reaction rate which they foster despite the poor relative amount of ammonia produced.
The process was first patented by Fritz Haber. In 1910 Carl Bosch, while working for chemical company BASF.
The process
The bulk of the chemical technology consists in getting the hydrogen from methane or natural gas using heterogeneous catalysis and then reacting it with the atmospheric nitrogen.
[edit] Synthesis gas preparation
First, the methane is cleaned, mainly to remove sulphur impurities that would poison the catalysts. This is done by turning sulphur into hydrogen sulphide:
CH3SH + H2 ? CH4 + H2S
and then reacting with zinc oxide to form zinc sulphide:
H2S + ZnO ? ZnS + H2O
The clean methane is then reacted with steam over a catalyst of nickel oxide. This is called steam reforming and occurs in two steps:
First step: (one mole of methane in)
CH4 + H2O ? CO + 3H2 (3 moles of hydrogen out)
CO + H2O ? CO2 + H2 (1 extra mole of hydrogen out)
Note that 4 moles of hydrogen are produced per mole of methane
Secondary reforming then takes place with the addition of air:
2 H2 + O2 + N2 ? 2H2O + N2
This now gives a ratio of nitrogen to hydrogen of 1:5
Then occur two??shifts?? which take CO to CO2 again by reaction with steam, one at high temperature, then one at low temperature:
CO + H2O ? CO2 + H2 high temperature 1:6
? the catalyst here is a mixture of iron, chromium and copper
CO + H2O ? CO2 + H2 low temperature 1:7
? the catalyst here is a mixture of copper, zinc and aluminum
The removal of carbon dioxide is easily done by reaction with potassium carbonate.
K2CO3 + H2O + CO2? 2KHCO3
The gas mixture is now passed into a methanator which converts any remaining CO2 into methane for recycling:
CO2 + 4H2 ? CH4 + 2H2O 1:3
We now have a gas mixture containing nitrogen and hydrogen in the correct ratio of 1:3. This is called synthesis gas.
[edit] Ammonia synthesis
The final stage is the crucial synthesis of ammonia using promoted magnetite, iron oxide, as the catalyst:
N2(g) + 3H2(g) ? 2NH3(g), ?Ho = -92.4 kJ/mol
This is done at 150 - 250 atmospheres (atm) and between 300 and 550 °C, passing the gases over four beds of catalyst, with cooling between each pass to maintain a reasonable equilibrium constant. On each pass only about 15% conversion occurs, but any unreacted gases will be recycled, so that eventually an overall yield of 98% can be achieved.
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