Ammonia and Urea Production by J. C. Copplestone and Dr. C. M. Kirk

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Urea (NH2CONH2) is of great importance to the agriculture industry as a nitrogen-rich fertiliser. In Kapuni, Petrochem manufacture ammonia and then convert the majority of it into urea. The remainder is sold for industrial use.

Ammonia synthesis
Ammonia is synthesised from hydrogen (from natural gas) and nitrogen (from the air).

Natural gas contains some sulfurous compounds which damage the catalysts used in this process. These are removed by reacting them with zinc oxide, e.g.

ZnO + H2S → ZnS + H2O

The methane from the natural gas is then converted to hydrogen:

CH4 + H2O → 3H2 + CO
CH4 + 2H2O → 4H2 + CO2
CO + H2O → H2 + CO2

Air is mixed in with the gas stream to give a hydrogen:nitrogen ratio of 3:1.

Water, carbon monoxide and carbon dioxide (all of which poison the iron catalyst used in the ammonia synthesis) are removed. The carbon monoxide is converted to carbon dioxide for use in urea production, and the carbon dioxide removed:

CO + H2O → CO2 + H2

The remaining traces of CO and CO2 are converted to methane and then the gases cooled until the water becomes liquid and can be easily removed.

The nitrogen and hydrogen are then reacted at high temperature and pressure using an iron catalyst to form ammonia:

N2 + 3H2 → 2NH3

Urea synthesis

Urea is made from ammonia and carbon dioxide. The ammonia and carbon dioxide are fed into the reactor at high pressure and temperature, and the urea is formed in a two step reaction

2NH3 + CO2 → NH2COONH4 (ammonium carbamate)
NH2COONH4 → H2O + NH2CONH2 (urea)

The urea contains unreacted NH3 and CO2 and ammonium carbamate. As the pressure is reduced and heat applied the NH2COONH4 decomposes to NH3 and CO2. The ammonia and carbon dioxide are recycled.

The urea solution is then concentrated to give 99.6% w/w molten urea, and granulated for use as fertiliser and chemical feedstock.