Introduction to the Optical Chemical Process - Kettle-Type Continuous Production Process

The kettle-type continuous production process is an advancement based on the kettle-type batch production process. This process is characterized by high output (compared to the batch method), high equipment utilization, easy centralized control through instruments, and relatively stable product quality. However, it still has disadvantages such as high consumption of phosgene, low yield, a high amount of by-products like ureas, and multiple steps for phosgene recovery.

The chosen solvents are still inert solvents like o-dichlorobenzene and chlorobenzene, with the amount of solvent being 5-10 times the weight of the organic amine. The molar ratio of phosgene to amine is 2-3. In the first cold phosgenation kettle, more than 1.25 mol of phosgene is required. The residence time of the material in the first phosgenation kettle is about 30-60 minutes. In the second hot phosgenation kettle, an additional 0.75 mol or more of phosgene is supplemented, and the material stays there for 60-120 minutes.

The process flow of the kettle-type continuous production process is shown in the diagram below. From the diagram, it can be seen that the flow direction of the materials in each kettle is from the bottom of the kettle, through stirring and mixing, and then overflowing from the top of the kettle. Generally, for the design of the kettle, a high aspect ratio is preferred. However, due to the structural limitations of the kettle itself and the back-mixing phenomenon of materials during stirring, isocyanate groups (NCO) formed during the reaction come into contact with raw organic amines or their hydrochlorides to form urea compounds, thereby reducing the yield of the finished product.

The process is roughly as follows: Polyamine solution from storage tank 1 and phosgene solution from storage tank 5 are proportionally fed into the cold phosgenation kettle 3. The temperature of the cold phosgenation kettle is controlled at 100-160°C by the brine cooling coil 6, with a molar ratio of phosgene to amine being 1.1-1.4. The slurry-like products formed, such as carbamoyl chloride, overflow into intermediate storage tank 7. The slurry is then pumped to high-temperature kettle 12.

Kettle-Type Continuous Production Process Flow Diagram

The temperature is maintained at 100-160°C for 30-60 minutes, stirring and supplementing with phosgene, with the amount of phosgene being 1-2 mol per mole of carbamoyl chloride hydrochloride fed. It then enters the hot phosgenation kettle 13, stirring and supplementing with 0.1-0.5 mol of phosgene, at a temperature higher than that of the high-temperature phosgenation kettle, around 130-160°C, for 30-50 minutes. The solvent vapor generated during the reaction is refluxed through a condenser, and hydrogen chloride gas and residual phosgene are recovered as liquid phosgene through the brine condenser (low-temperature condenser) 10 and sent to storage tank 14 for recycling. The transparent reaction liquid discharged from the hot phosgenation kettle goes to the degassing kettle 9 to remove residual HCl and phosgene. Storage tank 11 holds the semi-finished products, which are then sent to the distillation section for processing.

As mentioned in previous articles, during the cold phosgenation reaction stage, the main products of organic amines and phosgene are organic carbamoyl chlorides and organic ammonium salts. During the hot phosgenation reaction stage, isocyanate compounds are mainly formed. To reduce side reactions and improve yield, the key is to avoid contact between organic ammonium salts and isocyanates during the reaction and prevent back-mixing of materials during flow. Therefore, some manufacturers have modified the high-temperature hot phosgenation kettle to a long and narrow hot phosgenation tower structure, eliminating the stirring device. The characteristics of this reaction flow are: for low-temperature phosgenation, considering the high viscosity of materials in the low-temperature region and the difficulty of mixing, a kettle-type cold phosgenation is used; for high-temperature phosgenation, considering the phenomenon of urea precipitation caused by back-mixing of materials, a tower-type hot phosgenation is chosen. This improved process is more reasonable, resulting in fewer solid precipitates and tar-like substances in the product, and thereby improving product quality and yield.