Urea Manufacturing Process

Urea is manufactured by reacting ammonia with carbon dioxide. In a gas-based plant, methane-rich natural gas supplies hydrogen for ammonia and carbon dioxide for urea, while air supplies nitrogen. Natural gas also provides process heat, so it functions as both feedstock and fuel.

Production of synthesis gas

Natural gas is first converted into a purified mixture of hydrogen and nitrogen suitable for ammonia synthesis.

• Sulfur compounds are removed because they poison the reforming and ammonia-synthesis catalysts.
• In primary reforming, methane reacts with steam over a nickel catalyst: CH₄ + H₂O ⇌ CO + 3H₂.
• Air is introduced during secondary reforming; its oxygen supports further conversion, while its nitrogen enters the synthesis-gas stream.
• The water-gas shift reaction, CO + H₂O ⇌ CO₂ + H₂, increases hydrogen output. Carbon dioxide is then separated for use in urea synthesis, and residual carbon oxides are removed.

Ammonia synthesis

Purified nitrogen and hydrogen are compressed in approximately a 1:3 ratio and passed over an iron-based catalyst at high temperature and pressure. The Haber-Bosch reaction, N₂ + 3H₂ ⇌ 2NH₃, produces ammonia; cooling condenses the ammonia, while unreacted gases are recycled.

Conversion of ammonia into urea

Recovered carbon dioxide reacts with ammonia at high pressure through two main steps.

• First, 2NH₃ + CO₂ ⇌ NH₂COONH₄ forms ammonium carbamate.
• The carbamate then dehydrates: NH₂COONH₄ ⇌ NH₂CONH₂ + H₂O, producing urea.
• Unconverted ammonia and carbamate are recovered and recycled; the urea solution is concentrated and converted into prills or granules for agricultural use.

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