Urea Synthesis from Ammonia and Carbon Dioxide
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The world's most consumed solid nitrogen fertilizer
Overview
Urea is the world's most widely used solid nitrogen fertilizer, produced by reacting ammonia with carbon dioxide at high temperature and pressure. The process conveniently uses the CO2 byproduct from hydrogen production in ammonia synthesis, making urea plants typically co-located with ammonia plants. Urea contains 46% nitrogen (the highest of any solid fertilizer), is non-explosive, highly water-soluble, and easy to handle and apply. It is also used in animal feed, urea-formaldehyde resins, and diesel exhaust fluid (DEF/AdBlue).
Chemical Process
Ammonia and CO2 (molar ratio 3-4:1) react at 180-210 degrees C and 140-250 bar in a high-pressure reactor to form ammonium carbamate, which dehydrates to urea. Single-pass conversion is approximately 65%. Unconverted carbamate is decomposed and recycled. The urea solution is concentrated by evaporation and formed into prills (spray tower) or granules (fluid bed).
NH2COONH4 <=>[dehydration] CO(NH2)2 + H2O (urea formation, slow, equilibrium limited)
Raw Materials
-
Ammonia (NH3) — Haber-Bosch process (Nitrogen source)
-
Carbon dioxide (CO2) — Byproduct of steam methane reforming for H2 (Carbon source)
End Products
-
Urea prills or granules (CO(NH2)2) — Nitrogen fertilizer, animal feed, DEF/AdBlue, resins (46% N, the highest of any solid fertilizer)
Environmental Impact
Urea application to soil generates N2O (a potent greenhouse gas) through nitrification and denitrification. Ammonia volatilization from surface-applied urea wastes nitrogen and contributes to particulate matter formation. Urease inhibitors and controlled-release coatings can reduce these losses by 30-50%.
Safety Considerations
- ⚠ High-pressure reactor operation (140-250 bar) with corrosive ammonium carbamate
- ⚠ Ammonia toxicity and flammability risks
- ⚠ Urea dust in prill towers can cause respiratory irritation
- ⚠ Molten urea at 130+ degrees C during prilling causes burn hazard
Recent Innovations
Urease inhibitors (NBPT) applied to urea granules reduce ammonia volatilization by 50%.
Polymer-coated urea provides controlled nitrogen release matching crop demand.
CO2 capture from flue gas for urea production could expand supply while reducing emissions.
Production Scale
190000000
tons/year
$60 billion
market value
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