Two-Stage Phosgenation Process for Preparing Aromatic Isocyanates (1966)
U.S. Patent No. 3,234,253, granted on February 8, 1966, to John Richard Cooper and assigned to E. I. du Pont de Nemours and Company, details an improved chemical engineering process for manufacturing aromatic mono-, di-, and polyisocyanates. Aromatic isocyanates, such as toluene diisocyanate (TDI), serve as essential precursors in the large-scale production of polyurethanes, coatings, and synthetic foams.
This invention resolved two major bottlenecks in industrial isocyanate synthesis: the drastic drop in chemical yield and buildup of intractable nonvolatile residues when attempting to increase amine concentration, and the severe mechanical clogs caused by thick, viscous intermediate slurries in low-temperature reaction stages.
The Innovation: The “Stoichiometric Excess HCl” Technique
Conventional two-stage phosgenation relied on reacting aromatic amines with phosgene at low temperatures (below 90°C) before heating the mixture to complete conversion. Increasing production rates by raising reactant concentrations produced viscous slurries that were difficult to pump and led to undesirable side reactions that sharply reduced overall yield.
Cooper discovered that maintaining an excess of hydrogen chloride (HCl) gas in the second reaction stage—at concentrations significantly higher than predicted by chemical stoichiometry—dramatically suppressed by-product formation and lifted finished yields, even at elevated throughput.
Why Excess Hydrogen Chloride?
- By-Product Suppression: It shifts reaction equilibria away from forming insoluble, nonvolatile polymeric residues that trap product and foul processing gear.
- Viscosity Control: Operating the primary stage at elevated temperatures (above 90°C, ideally 100–110°C) alongside controlled gas recirculation produces a fluid, manageable reaction slurry that transfers easily between reactors.
- Concentration Independence: It prevents the typical steep loss in conversion efficiency when amine concentrations in the inert solvent are pushed up to expand manufacturing plant capacity.
Key Chemical Components
The multi-stage continuous reaction utilizes a precise balance of active reactants, carrier solvents, and recycling gases:
| Component | Function |
| Aromatic Primary Amine (e.g., Tolylenediamine) | The base feedstock providing amine groups (-NH2) to be converted into target isocyanate groups (-NCO). |
| Phosgene (COCl2) | The primary reactive carbonylating agent, fed in stoichiometric excess to drive intermediate conversion. |
| Hydrogen Chloride (HCl) Gas | The yield-protecting agent, fed to the second zone in excess of the theoretical primary off-gas ratio: 200A / (A + P) mole percent. |
| Carbamyl Chlorides & Amine Hydrochlorides | Reactive chemical intermediates formed in Zone 1 that are cleanly cracked to final isocyanates in Zone 2. |
| Inert Organic Solvent (e.g., o-Dichlorobenzene) | The high-boiling liquid carrier that suspends reactants, dissolves product, and controls slurry mechanics. |
Performance: Boosting Isocyanate Yields
Cooper demonstrated that enriching the second reaction zone with surplus hydrogen chloride consistently delivered superior yields compared to standard phosgenation setups across multiple amine feedstocks.
Test Results Across Industrial Runs:
- Toluene Diisocyanate (TDI) at Baseline: Reached 91.6% yield with standard phosgene feed in Stage 2.
- TDI with Excess HCl Gas: Yield climbed to 94.3%–95.5% of theoretical recovery.
- High-Concentration TDI Run (12.5% concentration): Standard method fell to 86.9% yield; adding surplus HCl restored conversion to 90.7%.
- Phenyl Isocyanate (from Aniline): Increased from 88.5% yield to 91.5% under enriched HCl atmosphere.
- 1,3-Phenylene Diisocyanate: Jumped from 79.0% with phosgene alone to 88.0% with added hydrogen chloride.
The Manufacturing Process
Cooper specified a continuous two-stage flow system with strict thermal and gas-composition controls:
- Feed and Heat: Continuously feed aromatic amine solution and excess phosgene into primary Reactor 1, holding the reaction mass between 100°C and 110°C to form a thin, pumpable intermediate slurry.
- Limit Primary Holdup: Restrict dwell time in the first zone to between several seconds and 30 minutes to prevent premature secondary reactions before the mass enters the high-yield secondary environment.
- Overflow to Secondary Zone: Transfer the fluid reaction mass continuously into secondary Reactor 2, maintained at an elevated temperature between 150°C and 170°C (up to 190°C).
- Inject Excess HCl: Supply supplemental hydrogen chloride gas directly into Stage 2—or recycle a tailored fraction of Stage 1 off-gases—so the HCl gas concentration exceeds the stoichiometric formula 200A / (A + P) mole percent.
- Condense and Degas: Vent off-gases through a reflux condenser to return solvent vapors to the reactor, flash off volatile acidic gases, and vacuum-distill the liquid effluent to recover purified isocyanate.
About the Inventor: John Richard Cooper
John Richard Cooper was an industrial research chemist based in Hockessin, Delaware, working for E. I. du Pont de Nemours and Company.
Patents and Chemistry: Cooper developed specialized manufacturing processes for complex organic intermediates, focusing heavily on continuous flow engineering, reaction kinetics, and commercial scaling of volatile and toxic reagents like phosgene.
Impact: During the post-war industrial boom, demand for synthetic polymers, flexible foams, and elastomeric fibers surged worldwide. Cooper’s continuous-flow phosgenation engineering allowed chemical producers to run higher throughputs at elevated reactant concentrations without suffering capacity-killing slurry clogs or yield drops.
Legacy: His optimization of gaseous equilibrium controls in multi-stage reactors helped establish standard continuous chemical manufacturing techniques that remain foundational to modern polyurethane industrial plants.
Summary of Claims
The patent explicitly claims:
- A continuous two-stage process for converting aromatic primary mono-, di-, or polyamines into corresponding isocyanates in an inert organic solvent.
- Operating the second reaction zone at 130°C to 190°C while contacting intermediates with a gaseous mixture of phosgene and hydrogen chloride.
- Enforcing a gas concentration in the second zone where hydrogen chloride strictly exceeds 200A / (A + P) mole percent, where A equals primary amine equivalents and P equals moles of phosgene fed to the first stage.
- Operating the primary reaction zone at temperatures between 90°C and 170°C (optimally 100°C to 110°C) and system pressures between 1 and 5 atmospheres.
