The reactor-based batch process was an early industrial production technique for isocyanates. Its main advantages include simplicity, ease of operation, and suitability for small-scale manufacturing. However, the process has notable drawbacks such as low equipment utilization, long production cycles, inconsistent product quality, low yields, high raw material consumption, labor-intensive operations, and subpar safety measures.
This process consists of four main systems: a cold phosgenation system, a hot phosgenation system, a distillation system, and a system for recovering and neutralizing excess phosgene.
In another variation of the batch process, the cold and hot phosgenation reactions are combined within a single reactor to overcome the challenges of transporting yellow slurry byproducts formed during cold phosgenation. However, combining these steps results in significant energy loss since both reactions occur in the same vessel.
The general batch production process follows these steps:
(1) Dissolution of Amines and Preparation of Phosgene Solution
Diamines or polyamines are dissolved in solvents such as chlorobenzene, o-dichlorobenzene, toluene, or xylene, with o-dichlorobenzene being the most common due to its high boiling point, which enables higher reaction temperatures and faster reaction rates. Solvent usage is typically 5-10 times the mass of the amine compounds. Phosgene is absorbed into the solvent at low temperatures to create a 20%-25% solution, maintained below 0°C.
(2) Cold Phosgenation Reaction
The reaction begins in a reactor containing a phosgene solution (20%-25%) at temperatures below 0°C. Amines are added dropwise at a molar ratio of 1-5 to phosgene, maintaining a temperature below 70°C with constant stirring. The reaction produces yellow slurry-like byproducts, including carbamoyl chlorides and amine hydrochloride salts. As the reaction is exothermic, efficient heat exchange systems are necessary to remove the heat promptly. The heat released at this stage is approximately 8.368 kJ per mole of amine. Measures such as using dilute solutions, vigorous stirring, and an excess of phosgene help minimize the formation of urea derivatives and ensure a successful reaction.
(3) Hot Phosgenation Reaction
The cold phosgenation products are transferred to a hot phosgenation reactor. Under vigorous stirring, the temperature is gradually increased to 100-180°C over 1-2 hours, optionally introducing additional phosgene. The reaction continues until the slurry decomposes completely, forming a transparent brown liquid. The final reaction temperature depends on the boiling point of the solvent used, with a maximum of 120°C for chlorobenzene.
Careful control of the heating rate is critical; rapid temperature increases can cause significant phosgene loss, reduce the reaction yield, and lead to insoluble precipitates. In some operations, the outlet valve of the hot phosgenation condenser is partially closed to perform the reaction under pressure. This raises the boiling point of phosgene, allowing it to reflux while hydrogen chloride is continuously vented, ensuring sufficient phosgene is available for the reaction. However, pressurized operation demands higher safety standards for equipment.
(4) Degassing and Distillation
Residual phosgene and byproduct hydrogen chloride are removed from the reaction mixture using inert gases (such as nitrogen or methane) at temperatures between 100-200°C, typically under atmospheric pressure or slightly elevated pressures of 0.26-0.28 MPa (for chlorobenzene as the solvent). Operating at higher pressures has advantages such as reduced nitrogen consumption and lower acidity (below 0.03% HCl).
After degassing, the phosgenation solution is sent to a vacuum distillation system for solvent recovery and purification of the isocyanate product. The overall yield of isocyanates, based on the amine input, is approximately 90%.
