The Lummus Ammonia Process: A Detailed Guide

Ammonia production is a cornerstone of the chemical industry, with widespread applications in fertilizers, chemicals, and energy storage. The Lummus Ammonia Process, a modern adaptation of the Haber-Bosch process, exemplifies the industry’s drive for efficiency, sustainability, and performance. This article explores the intricate stages of the Lummus process, the critical equipment used, operational parameters, and material considerations.

Overview of the Lummus Ammonia Process

The Lummus Ammonia Process synthesizes ammonia (NH₃) by reacting nitrogen (N₂) and hydrogen (H₂) under high-pressure and high-temperature conditions. What sets the Lummus process apart is its focus on energy efficiency, process integration, and environmental sustainability.

The key stages of the process include:

1. Feedstock Desulfurization
2. Primary Reforming
3. Secondary Reforming
4. CO Shift Conversion
5. CO₂ Removal
6. Methanation
7. Ammonia Synthesis
8. Ammonia Recovery

Feedstock Desulfurization

• Objective: Remove sulfur compounds from natural gas to protect catalysts downstream.
• Process:
  • Sulfur compounds (e.g., H₂S) are hydrogenated to H₂S.
  • H₂S is absorbed in zinc oxide beds.
• Critical Equipment:
  • Hydrodesulfurization reactors.
  • Zinc oxide absorbers.
• Conditions:
  • Temperature: ~350°C.
  • Pressure: 30-50 bar.
• Materials:
  • Stainless steel and carbon steel, resistant to H₂S corrosion.

Primary Reforming

• Objective: Convert methane (CH₄) into syngas (a mixture of H₂ and CO).
• Process:
  • Methane reacts with steam over a nickel-based catalyst:
    CH₄ + H₂O → CO + 3H₂
• Critical Equipment:
  • Fired tubular reformers with nickel-based catalysts.
• Conditions:
  • Temperature: 850-900°C.
  • Pressure: 25-40 bar.
• Materials:
  • High-alloy steel (e.g., 25Cr/20Ni with Nb or Ti micro-alloying) to withstand high temperatures.

Secondary Reforming

• Objective: Complete methane conversion and introduce nitrogen.
• Process:
  • Air is added for partial combustion, providing nitrogen for ammonia synthesis.
  • Remaining methane is converted to CO and H₂.
• Critical Equipment:
  • Autothermal reformer.
• Conditions:
  • Temperature: ~1000°C.
  • Pressure: 30-40 bar.
• Materials:
  • Refractory-lined reactors for high-temperature resistance.

CO Shift Conversion

• Objective: Maximize hydrogen production by converting CO to CO₂.
• Process:
  • High-temperature shift (HTS) reaction:
    CO + H₂O → CO₂ + H₂
  • Low-temperature shift (LTS) reaction for fine-tuning.
• Critical Equipment:
  • HTS and LTS reactors.
• Conditions:
  • HTS: 350-450°C, 25-30 bar.
  • LTS: 200-250°C, 20-25 bar.
• Materials:
  • Stainless steel to resist acidic condensates.

CO₂ Removal

• Objective: Remove CO₂ from syngas to purify hydrogen.
• Process:
  • Chemical absorption using MDEA or Benfield solution.
  • Stripping to regenerate the absorbent.
• Critical Equipment:
  • Absorption towers and strippers.
• Conditions:
  • Absorption: Ambient temperature, 20-30 bar.
  • Stripping: ~120°C.
• Materials:
  • Stainless steel to resist CO₂ and amine corrosion.

Methanation

• Objective: Remove trace amounts of CO and CO₂ to prevent catalyst poisoning.
• Process:
  • CO and CO₂ react with hydrogen to form methane:
    CO + 3H₂ → CH₄ + H₂O
• Critical Equipment:
  • Methanators with nickel-based catalysts.
• Conditions:
  • Temperature: ~300°C.
  • Pressure: 25-30 bar.
• Materials:
  • Carbon steel or low-alloy steel.

Ammonia Synthesis

• Objective: Synthesize ammonia from nitrogen and hydrogen.
• Process:
  • Reaction:
    N₂ + 3H₂ ↔ 2NH₃
  • Multi-bed reactors with inter-bed cooling.
• Critical Equipment:
  • Ammonia converters.
  • Heat exchangers.
• Conditions:
  • Temperature: 400-450°C.
  • Pressure: 150-300 bar.
• Materials:
  • 2.25Cr-1Mo steel to handle high pressure and NH₃ embrittlement.

Ammonia Recovery

• Objective: Separate ammonia from unreacted gases.
• Process:
  • Cooling and condensation of ammonia.
• Critical Equipment:
  • Refrigeration systems.
• Conditions:
  • Temperature: -30 to -40°C.
  • Pressure: Same as synthesis loop.
• Materials:
  • Low-temperature carbon steel.

Critical Aspects of the Lummus Process

1. Energy Efficiency:

   • Heat recovery systems.
   • Optimized flow paths to reduce energy losses.

2. Environmental Integration:

   • Carbon capture and storage (CCS).

3. Advanced Catalysts:

   • Enhanced activity and longevity.

4. Material Challenges:

   • Creep-resistant alloys in reformers.
   • High-pressure resistant materials in synthesis loop.

The Lummus Ammonia Process showcases innovation in ammonia production, addressing both industrial demands and environmental concerns. Its integration of advanced catalysts, energy recovery, and sustainable technologies ensures its relevance in the modern chemical industry.