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 sulfur impurities that would poison the catalysts. This is done by turning sulfur into hydrogen sulfide:
CH3SH + H2 -> CH4 + H2S
and then reacting with zinc oxide to form zinc sulfide:
H2S + ZnO -> ZnS + H2O
The clean methane is then reacted with steam over a catalyst of nickel oxide. This is called steam reforming:
CH4 + H2O -> CO + 3H2
Secondary reforming then takes place with the addition of air to convert the methane that did not react during steam reforming. The carbon monoxide formed is also cause catalyst poisoning, which would react with the iron catalyst, forming an iron compound, thus affecting the reaction.[citation needed] The air added during this step also serves as a nitrogen source for ammonia synthesis:
2CH4 + O2 -> 2CO + 4H2
CH4 + 2O2 -> CO2 + 2H2O
Then occur two’’shifts’’ which convert CO to CO2 by reaction with steam, one at high temperature, then one at low temperature:
CO + H2O -> CO2 + H2 high temperature
-> the catalyst here is a mixture of iron, chromium and copper
CO + H2O -> CO2 + H2 low temperature
-> the catalyst here is a mixture of copper, zinc and aluminium
Then the carbon dioxide is removed 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
We now have a gas mixture containing nitrogen and hydrogen in the correct ratio of 1:3.
[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 are recycled, so that eventually an overall conversion of 98% can be achieved.
|
Linear Mode
