Technology Type

Historical Development

The phosgenation route to isocyanates was first developed in the early 20^th century and became the dominant commercial method by the 1940s. Key historical milestones include:

• 1930s-1940s: Initial commercial development of liquid-phase phosgenation processes by IG Farben (Germany), with Werner Siefken's 1936 breakthrough establishing the foundation for industrial isocyanate production
• 1950s-1960s: Scale-up to large industrial production for TDI and MDI by Bayer, BASF (Germany), DuPont, Mobay Chemical, Dow Chemical, and Union Carbide (USA)
• 1980s: Development of gas-phase phosgenation technology by Bayer (now Covestro), offering significant energy savings for volatile amine applications
• 1990s-2000s: Process intensification and environmental improvements led by BASF, Covestro, Huntsman, and Dow, with technology licensing expansion to Asian markets
• 2010s-Present: Advanced process control, energy efficiency improvements, and alternative route research, with market leadership shifting to Wanhua Chemical (China) while BASF and Covestro maintain technology leadership through continued innovation and global licensing

Process Summary and Chemistry

Core Chemistry

The phosgenation process involves the reaction of organic amines with phosgene to produce isocyanates via a two-step mechanism (example of a difunctional amine):

Step 1: Carbamoyl Chloride Formation

H₂N—R—NH₂ + 2 O=CCl₂ → Cl—C(=O)—NH—R—NH—C(=O)—Cl + 2 H—Cl

Step 2: Thermolysis to Isocyanate

Cl—C(=O)—NH—R—NH—C(=O)—Cl → O=C=N—R—N=C=O + 2 H—Cl

Overall Reaction for Diisocyanates:

H₂N—R—NH₂ + 2 O=CCl₂ → O=C=N—R—N=C=O + 4 H—Cl

Figure 1 - TDA reaction with phosgene to form TDI^[7]

Process Types and Applicability

Modern isocyanate production employs two main phosgenation approaches with different applicability ranges:

Liquid-Phase Process (Historical): Universal applicability to all amines

• Operating Temperature: TDI: 65-95°C, MDI: ~200°C, HDI: 130-170°C (optimum 150°C)
• Operating Pressure: 1-5 bar gauge (typically 1-3 bar)
• Applicability: All amine types - aromatic, aliphatic, alicyclic, specialty amines
• Solvents: Monochlorobenzene, orthodichlorobenzene, toluene

Gas-Phase Process: Limited to volatile amines only

• Operating conditions: 300-400°C (typically ~320°C practical operation)
• Applicability: Restricted to volatile amines that can be vaporized without decomposition
• Successfully applied: TDA (for TDI), select aliphatic diamines like HDA (for HDI)
• Not suitable: High molecular weight diamines, thermally unstable amines, complex aromatic diamines

Figure 2 - Conceptual description of the amine phosgenation process

Detailed Process Description with Practical Operating Conditions

Liquid-Phase Phosgenation Process

Reactor Systems

• Reactor Type: Stirred tank reactors with agitation systems
• Configuration: Single-stage or multi-stage continuous tanks
• Temperature Control: Steam-circulating jackets for precise temperature management
• Pressure: Standard vessels operating 1-5 bar gauge
• Residence Time: Several hours (3-15 hours typical)

Figure 3 - Process for the manufacture of organic isocyanates in liquid phase^[5]

Specialized Equipment

• Salt-forming reactors: For amine hydrochloride preparation (HDI process)
• Agitating blades: High-speed mixing (2.5-6 m/sec circumferential velocity)
• Feed systems: Dual nozzle systems for simultaneous amine/HCl introduction
• Recovery systems: Conventional condensers and gas-liquid separators

Practical Operating Conditions (Based on Patent Examples)

• TDI Production: 65-80°C for carbamoyl chloride formation, hot phosgenation to ~90°C
• MDI Production: ~200°C for phosgenation with 6-8 molar excess phosgene
• HDI Production: 130-170°C (optimum 150°C), 1-3 kg/cm²G pressure
• Residence Time: 3.3-15 hours depending on amine type and conversion requirements

Reactor Configuration Examples

• Two-stage HDI process: First stage 3.3 hours (70-95% conversion), second stage 6.9 hours (>99% conversion)
• Pressure optimization: 2 kg/cm²G optimal for phosgene recovery vs. by-product formation
• Phosgene flow rate: 5-12 molar times per hour the total HDA·HCl + HDI content

Gas-Phase Phosgenation Process

Reactor Systems

• Reactor Type: Specialized high-temperature tubular reactors
• Configuration: Short residence time reactors with rapid mixing nozzles
• Temperature Control: High-temperature heating systems (up to 400°C capability)
• Pressure: Atmospheric to slightly elevated pressure
• Residence Time: <1 minute (vs. hours for liquid phase)

Figure 4 - Production of TDI by the gas phase phosgene method^[6]

Specialized Equipment

• Feed vaporization: High-temperature evaporators for amine and phosgene
• Rapid mixing: Specialized reactor nozzles for instantaneous gas-phase contact
• Quench systems: Immediate cooling with solvent spray to prevent side reactions
• Heat integration: Advanced heat recovery systems for energy efficiency

Practical Operating Conditions (Based on Commercial Examples)

• TDI Production: 320°C preheating, reactor operation at 300°C, 0.1 MPa pressure
• Contact Time: <1 minute residence time in reactor
• Phosgene Ratio: High molar ratios to maintain reaction kinetics in gas phase
• Quench Temperature: Immediate cooling to prevent product decomposition

Gas-Phase Process Limitations and applicability

• Maximum Operating Temperature: ~400°C before most amines decompose
• Challenging Applications: MDI, PMDI due to low amine volatilty, viscosity challenges, thermal sensitivity associated with decomposition risks
• TDI production: Primary commercial success with proven large-scale operation
• Select HDI production: Where thermal stability permits high-temperature operation
• Economic drivers: Large-scale plants where energy savings justify technology complexity

Commercial Performance

• Covestro Dormagen TDI plant: 300,000 t/y capacity using gas-phase technology
• Energy Savings: 60% energy reduction vs. liquid-phase process
• Solvent Reduction: 80% less solvent requirement
• Investment Savings: 20% lower capital costs for new plants

Process Efficiency

Conversion and Yields

• Liquid Phase: 85-95% amine conversion typically achieved, 96% HDI yield demonstrated
• Gas Phase: Up to 99% theoretical yields possible for suitable amines
• Overall Efficiency: Modern processes achieve >90% carbon efficiency across both technologies

Energy and Environmental Performance

• Gas-Phase Benefits: 60% energy reduction, 22,000 tons CO₂/year savings (300,000 t/y plant)
• Liquid-Phase Advantages: Universal applicability, established technology with proven reliability
• Solvent Management: Gas-phase 80% solvent reduction vs. liquid-phase comprehensive solvent recovery

Process Economics

Capital Investment

• Liquid Phase: Standard equipment costs, established supply chains, higher solvent system costs
• Gas Phase: 20% capital reduction for suitable applications, specialized high-temperature equipment
• Scale Economics: >200,000 t/y plants required for competitive gas-phase implementation

Operating Costs

• Energy: Gas-phase significant advantage through process intensification
• Raw Materials: 40% phosgene savings per kg TDI in gas-phase process
• Solvent: Liquid-phase higher solvent costs, gas-phase reduced handling costs
• Maintenance: Liquid-phase conventional maintenance, gas-phase specialized high-temperature equipment

Isocyanate Industry: Producers and Technology Providers

*ADI: Aliphatic Diisocyanates; **XDI: Xylylene Diisocyanate.

Summary Statistics

• Total Active Major Producers: 13 companies
• Combined MDI Capacity: ~12,130 kt/y
• Market Leaders: Top 5 producers control ~85% of global capacity
• Technology Distribution: Mix of liquid-phase (dominant) and gas-phase processes
• Geographic Distribution: Asia-Pacific leads with 60% of global capacity

Key Technology Providers

• Gas-Phase Technology: Covestro (pioneer), BASF
• Liquid-Phase Technology: Most producers use proven liquid-phase processes
• Specialty Technologies: Evonik (IPDI), Vencorex (HDI), Mitsui (integrated systems)
• Licensing Leaders: BASF, Covestro, Mitsui Chemicals for comprehensive technology packages

References

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