Process of Urea Production from Ammonia: A Detailed Guide

Urea is one of the most important nitrogen-based fertilizers and industrial chemicals in the world. It is synthesized from ammonia (NH₃) and carbon dioxide (CO₂) in a highly efficient and economical process. This guide explores the production of urea in detail, covering process stages, critical equipment, operating conditions, and a process flow diagram (P&ID).

Overview of Urea Production

Urea (CO(NH₂)₂) is synthesized through the ammonia stripping process, which leverages the reaction of ammonia and carbon dioxide under high temperature and pressure. The main stages of the process include:

1. Ammonia and Carbon Dioxide Reaction
2. Urea Synthesis in the Reactor
3. Ammonium Carbamate Decomposition
4. Urea Solution Concentration
5. Prilling or Granulation

The process is exothermic and produces urea as the final product while recycling unreacted ammonia and carbon dioxide.

Read More: The Lummus Ammonia Process: A Detailed Guide

Reaction of Ammonia and Carbon Dioxide

• Objective: Form ammonium carbamate (NH₂COONH₄), the intermediate for urea synthesis.
• Reaction: 2NH₃ + CO₂ ↔ NH₂COONH₄
• Conditions:
  • Temperature: 180-210°C
  • Pressure: 140-180 bar
• Critical Equipment:
  • High-pressure ammonia and CO₂ feed pumps.
  • High-pressure reaction vessel.
• Special Metallurgy:
  • 316L stainless steel or duplex stainless steel to resist corrosion from ammonium carbamate.

Urea Synthesis in the Reactor

• Objective: Convert ammonium carbamate to urea and water through a dehydration reaction.
• Reaction: NH₂COONH₄ ↔ CO(NH₂)₂ + H₂O
• Conditions:
  • Temperature: 180-190°C
  • Pressure: 140-180 bar
• Critical Equipment:
  • Urea reactor with multiple trays or stages for enhanced conversion efficiency.
  • Heat exchangers to control temperature.
• Special Metallurgy:
  • Stainless steel with special linings to resist urea corrosion.

Ammonium Carbamate Decomposition

• Objective: Recover unreacted ammonia and carbon dioxide for recycling.
• Process:
  • Decomposition of ammonium carbamate occurs under controlled heating in a stripper column.
• Reaction: NH₂COONH₄ → NH₃ + CO₂
• Conditions:
  • Temperature: ~200°C
  • Pressure: ~140 bar
• Critical Equipment:
  • High-pressure stripper column.
  • Condensers to separate ammonia and CO₂.
• Special Metallurgy:
  • Duplex stainless steel to withstand high pressure and corrosive environment.

Urea Solution Concentration

• Objective: Increase the urea concentration by removing water from the solution.
• Process:
  • Urea solution is heated in evaporators or concentrators to remove water and produce a melt with 95-99% urea content.
• Conditions:
  • Temperature: ~150-180°C
  • Vacuum pressure for effective evaporation.
• Critical Equipment:
  • Multi-stage vacuum evaporators.
  • Vacuum pumps and condensers.
• Special Metallurgy:
  • High-grade stainless steel.

Prilling or Granulation

• Objective: Solidify the concentrated urea into prills or granules for storage and transportation.
• Process:
  • Prilling: Urea melt is sprayed from a tower, solidifying as it falls through a counter-current air stream.
  • Granulation: Urea melt is agglomerated with seed granules in rotating drums.
• Critical Equipment:
  • Prilling tower or granulation drum.
  • Air scrubbers for dust and emission control.
• Special Metallurgy:
  • Stainless steel for high wear resistance.

Critical Aspects of Urea Production

1. Energy Efficiency: Heat integration reduces energy consumption, especially in the recycling loop for unreacted NH₃ and CO₂.

2. Recycling Loop: Up to 85% of unreacted materials are recycled, minimizing raw material waste.

3. Emission Control: Scrubbers and absorbers limit ammonia emissions and ensure environmental compliance.

4. Corrosion Resistance: The highly corrosive nature of ammonium carbamate and urea necessitates special materials like duplex and super duplex stainless steel.

Process Flow Diagram (PFD)

Below is a simplified PFD representing the urea production process:

Key Components:

1. Ammonia feed pump.
2. CO₂ compressor.
3. High-pressure reactor.
4. High-pressure stripper.
5. Evaporators.
6. Prilling or granulation equipment.
7. Heat exchangers and condensers.

Conclusion

The production of urea from ammonia is a highly optimized and sustainable process. By recycling unreacted materials, integrating heat recovery, and adopting advanced materials, modern urea plants achieve high efficiency and low environmental impact. The use of prilled or granulated urea ensures its safe handling and effective use in agriculture and industry. With continuous advancements in technology, the urea production process continues to evolve, meeting global demands sustainably.