Industrial Production of Urea

Industrial urea manufacture converts methane-rich natural gas first into hydrogen, then into ammonia, which reacts with carbon dioxide to form urea. Thus, an integrated plant normally contains an ammonia section and a urea section; natural gas serves both as chemical feedstock and as process fuel.

Producing synthesis gas from natural gas

Natural gas is purified to remove sulphur compounds that can poison catalysts. Its methane is then converted into a hydrogen-rich synthesis gas.

• In steam reforming, methane reacts with steam: CH4 + H2O ⇌ CO + 3H2.
• In secondary reforming, process air supports further conversion and introduces the nitrogen required for ammonia synthesis.
• The water-gas shift reaction, CO + H2O ⇌ CO2 + H2, raises hydrogen output and produces carbon dioxide.
• Carbon dioxide is separated for use in urea manufacture, while residual carbon oxides are removed or converted before ammonia synthesis.

Synthesising ammonia

The purified hydrogen is combined with nitrogen in approximately the stoichiometric ratio required by N2 + 3H2 ⇌ 2NH3. Under the Haber-Bosch process, the compressed gas mixture passes over an iron-based catalyst at elevated temperature and pressure.

• Ammonia is separated by cooling, while unreacted nitrogen and hydrogen are recycled through the synthesis loop.
• The nitrogen originates from air, whereas most hydrogen originates from natural gas and steam.

Converting ammonia into urea

Ammonia reacts with the recovered carbon dioxide at high pressure to form ammonium carbamate, which then loses water to form urea, CO(NH2)2.

• The overall reaction is 2NH3 + CO2 ⇌ CO(NH2)2 + H2O.
• Unconverted ammonia and carbon dioxide are recovered and recycled to improve material efficiency.
• The urea solution is concentrated and solidified as prills or granules suitable for storage and agricultural application.

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