Urea synthesis

Industrial urea manufacture involves reacting ammonia with carbon dioxide under elevated pressure, initially forming ammonium carbamate, which subsequently dehydrates to yield urea and water. Both stages proceed in the liquid phase within a single reactor under equilibrium conditions. Carbon dioxide conversion typically reaches 50 to 80%. Temperature enhances this yield, as does a high NH3/CO2 ratio, whereas a substantial H2O/CO2 proportion reduces it. Standard industrial conditions range between 140 and 250 bar, at 180-210 °C, with a molar NH3/CO2 ratio of 2.8:1 to 4:1 and a residence time of 20 to 30 minutes^[1].

Side reactions and quality constraints

Several equilibrium reactions accompany urea formation. Urea hydrolysis, the reverse of synthesis, occurs in the presence of water and may be accelerated by acidic or alkaline conditions. Biuret forms through the condensation of two urea molecules with release of ammonia; its content must be limited to a maximum of 1.2% in fertiliser-grade urea, as this compound can cause phytotoxicity, particularly during foliar applications. For technical-grade urea, the specification generally tightens to 0.3-0.4%, or even below 0.15% depending on customer requirements. Isocyanic acid constitutes another by-product, favoured by low ammonia concentrations and high temperatures, notably in the evaporation section^[1].

Total recycle processes

To improve overall efficiency, the industry has developed "total recycle" processes that separate urea from unconverted feedstocks for reinjection into the reactor. Four main routes coexist: conventional processes without stripping (older plants), CO2 stripping (Stamicarbon, Toyo ACES), NH3 stripping (Snamprogetti), and the IDR (Isobaric Double Recycling) process from Montedison combining both stripping agents^[2].

The IDR process illustrates this sophistication: synthesis takes place at 180-200 bar and 185-190 °C with an NH3:CO2 ratio of approximately 3.5-4, enabling 70% CO2 conversion per pass. Two strippers in series, operating at reactor pressure and heated by 25 bar steam, perform the separation. The first decomposes carbamate, the second removes residual ammonia using CO2 as an entraining agent. Carbamate condensation heat is recovered as 7 bar steam and reused in the process. Only 40% of fresh ammonia passes through the carbamate condenser, the surplus being injected directly into the reactor to control temperature^[3].

Energy consumption and cost structure

An efficient site using natural gas requires approximately 35 GJ to produce one tonne of ammonia, equivalent to 42 GJ per tonne of nitrogen. Conversion to urea adds 3 to 4 MMBtu per tonne of product, bringing total consumption to roughly 51 GJ/t of nitrogen. Coal-based plants, predominant in China, consume 50 to 100% additional energy per unit of nitrogen produced^[4].

Ammonia represents the dominant cash cost item. Some 0.58 tonne of ammonia is needed per tonne of urea. By way of illustration for the US Gulf, with gas at USD 7/MMBtu and consumption of 36 MMBtu/t NH3, the ammonia cost works out at USD 169/t of urea. Adding the process gas specific to urea (5.2 MMBtu/t) and other production costs brings the total cash cost to USD 251/t of urea, excluding freight and loading estimated at USD 8/t. Natural gas thus constitutes approximately 72% of the production cost^[5].

Finishing and non-fertiliser applications

Urea solution from the synthesis and recycle stages is concentrated by evaporation or crystallisation, then melted for solidification by prilling or granulation. Some units produce exclusively for melamine manufacture^[2].

Industrial outlets absorb approximately 19% of global urea demand (2023 data), compared with 23% for nitrogen overall. Urea alone accounts for nearly 48% of industrial nitrogen consumption. Urea-formaldehyde resins, employed as binders, represent 46% of non-fertiliser urea use. Diesel exhaust fluid constitutes a rapidly growing segment, driven by regulation of NOx emissions from heavy-duty vehicles and the power sector^[6].