Nitrogenous fertilisers: urea and ammonium nitrate

Why it matters

Urea is the most widely used fertiliser in India and the world because it carries the most nitrogen per tonne of any solid fertiliser (46.6 % N), which cuts transport and handling cost. Ammonium nitrate and its safer form, calcium ammonium nitrate (CAN), supply nitrogen in a form plants take up quickly. Both processes sit directly downstream of the ammonia plant and use its CO₂ and NH₃, so they are designed as an integrated complex.

Key ideas

Urea chemistry. Urea forms in two steps in the same high-pressure reactor:

1. 2NH₃ + CO₂ ⇌ NH₂COONH₄ (ammonium carbamate). Fast and strongly exothermic.
2. NH₂COONH₄ ⇌ NH₂CONH₂ + H₂O. Slow, mildly endothermic and equilibrium-limited; this step sets the conversion.

The heat from step 1 drives step 2, so the reactor is roughly autothermal. Typical synthesis conditions: 140–160 bar, 180–190 °C, NH₃ : CO₂ molar feed ratio about 3–4 (excess NH₃ raises CO₂ conversion and keeps carbamate in solution), CO₂ conversion per pass about 60–65 %. Water in the feed (from recycled carbamate solution) lowers conversion, since it is a product of step 2.

Process routes. Because conversion is incomplete, unconverted carbamate must be decomposed back to NH₃ and CO₂ and recycled.

• Total recycle with stripping is modern practice. In the CO₂ stripping process (Stamicarbon) the reactor effluent flows down a falling-film stripper at synthesis pressure against fresh CO₂, which lowers the partial pressure of CO₂ and decomposes carbamate. In the NH₃ (self) stripping process (Snamprogetti) ammonia or heat does the stripping. The stripped gases are condensed back to carbamate in a high-pressure condenser that raises steam.
• Downstream: medium- and low-pressure decomposers recover the rest, then the 70–75 % urea solution is concentrated by vacuum evaporation to a melt of about 99.7 %, and finished by prilling (melt sprayed down a tall tower against rising air) or granulation (larger, stronger granules).

Biuret. At high temperature and long residence time, 2NH₂CONH₂ → NH₂CONHCONH₂ + NH₃. Biuret is toxic to some crops, so evaporation is done quickly under vacuum, and the fertiliser specification limits biuret to a small percentage (take the exact limit from the Fertiliser Control Order).

Corrosion. Carbamate solutions are very corrosive; reactors use stainless-steel or titanium/zirconium liners, and a little passivation air is added with the CO₂.

Indian practice. Urea is neem-coated; the neem oil slows nitrification and discourages diversion to industrial use.

Ammonium nitrate (AN). NH₃ (gas) + HNO₃ (55–60 %) → NH₄NO₃, ΔH ≈ −146 kJ/mol. The neutraliser heat evaporates much of the water; the solution is concentrated to a melt and prilled or granulated. AN has 35 % N, half as ammonium and half as nitrate. It is an oxidiser that can detonate when contaminated, confined or heated, so pure AN is tightly regulated. Mixing the melt with finely ground limestone or dolomite gives calcium ammonium nitrate (CAN), with about 25–26 % N, which is safer and less acidifying to soil. Process control keeps pH slightly alkaline and excludes chlorides and organics.

Ammonium sulphate (21 % N) is made by neutralising H₂SO₄ with NH₃ or as a by-product of caprolactam and coke ovens.

Formulas

2NH₃ + CO₂ ⇌ NH₂COONH₄ (exothermic, fast) NH₂COONH₄ ⇌ NH₂CONH₂ + H₂O (endothermic, slow) Overall: 2NH₃ + CO₂ → NH₂CONH₂ + H₂O 2NH₂CONH₂ → NH₂CONHCONH₂ + NH₃ (biuret) NH₃ + HNO₃ → NH₄NO₃

%N = (number of N atoms × 14.007 / M) × 100

• M: molar mass of the fertiliser (g/mol). Urea 60.06 → 46.6 % N; NH₄NO₃ 80.04 → 35.0 % N; (NH₄)₂SO₄ 132.14 → 21.2 % N.

X_CO₂ = CO₂ converted to urea / CO₂ fed to reactor

• Per-pass conversion; overall conversion in a total-recycle plant approaches 100 %.

Worked examples

Example 1 (standard): raw material use. A urea plant makes 1000 t/day. Find the theoretical NH₃ and CO₂ needed and the nitrogen content of the product.

1. Overall: 2NH₃ + CO₂ → urea. Molar masses: NH₃ 17.03, CO₂ 44.01, urea 60.06 g/mol.
2. NH₃ = 1000 × 2 × 17.03 / 60.06 = 567.1 t/day.
3. CO₂ = 1000 × 44.01 / 60.06 = 732.8 t/day.
4. %N = 2 × 14.007 / 60.06 × 100 = 46.6 %, so N delivered = 466 t/day.

Answer: 567 t/day NH₃, 733 t/day CO₂; 46.6 % N.

Example 2 (GATE level): reactor effluent composition. The urea reactor receives NH₃ : CO₂ : H₂O = 3.5 : 1 : 0.5 (molar). Per-pass CO₂ conversion is 62 %. Counting unconverted carbamate as free NH₃ and CO₂, find the mass fraction of urea in the effluent.

1. Basis 100 kmol CO₂: feed NH₃ 350, CO₂ 100, H₂O 50 kmol.
2. Urea formed = 62 kmol; NH₃ used = 124; H₂O formed = 62.
3. Effluent: urea 62, NH₃ 226, CO₂ 38, H₂O 112 kmol.
4. Masses: urea 62 × 60.06 = 3724 kg; NH₃ 226 × 17.03 = 3849 kg; CO₂ 38 × 44.01 = 1672 kg; H₂O 112 × 18.015 = 2018 kg; total 11 263 kg.
5. Urea mass fraction = 3724 / 11 263 = 0.331.

Answer: about 33 mass % urea (the rest must be stripped and recycled).

Common mistakes

• Treating urea formation as a single fast reaction; the carbamate dehydration is the slow, equilibrium-limited step.
• Adding water to "help" the reaction; water is a product and lowers conversion.
• Using too much heat or residence time in evaporation and forming biuret.
• Quoting CAN as 35 % N; dilution with limestone brings it to about 25–26 %.
• Forgetting that excess NH₃ is recycled, so per-pass and overall conversion differ.

For GATE CH

Questions test the two-step mechanism and which step is rate- and equilibrium-limiting, the effect of NH₃ : CO₂ ratio, water and temperature on conversion, the purpose of stripping, biuret formation and its control, prilling versus granulation, and the hazards of ammonium nitrate. Numericals cover nitrogen content, raw-material consumption and reactor balances on a 100 kmol basis.

Quick check

1. Which step of urea synthesis is endothermic?
2. Why is excess ammonia used in the reactor?
3. What is biuret and why is it limited?
4. What is the nitrogen content of NH₄NO₃?

Answers: 1. dehydration of ammonium carbamate to urea; 2. it raises CO₂ conversion and suppresses side reactions; 3. a condensation product of two urea molecules (with NH₃ released), harmful to some crops; 4. 35 % N.