Optical Chemical Technology - Kettle Batch Production Process

The kettle batch method was used in the early stages of industrial production of isocyanates. Its characteristics include a simple process, easy mastery of the technology, and suitability for small-batch production. However, it has drawbacks such as low equipment utilization, long production cycles, low product yield, unstable quality, high raw material consumption, high labor intensity, and poor safety conditions.

The process mainly consists of four parts: a cold phosgenation system, a hot phosgenation system, a distillation system, and an excess phosgene recovery and destruction system.

In another batch method process, the cold phosgenation reaction and the hot phosgenation reaction are combined in a single reactor to overcome the difficulty of transporting the yellow slurry produced by the cold phosgenation reaction. However, this process has higher energy loss because both "cold" and "hot" reactions occur in the same kettle.

The batch production process generally follows these steps:

1. Preparation of Amine Solution and Phosgene Solution

Dissolve diamine or polyamine compounds in a solvent, generally using aromatic hydrocarbon inert solvents such as chlorobenzene, o-dichlorobenzene, toluene, xylene, etc., with o-dichlorobenzene being the most commonly used. This is because o-dichlorobenzene has a high boiling point, which allows for higher temperatures in the hot phosgenation process, thereby speeding up the reaction. The solvent amount is 5 to 10 times the mass of the amine compounds. Phosgene is absorbed by the solvent at low temperatures to form a solution of about 20% to 25%. The temperature is kept below 0°C.

2. Cold Phosgenation Reaction

In the cold phosgenation kettle, a 20% to 25% phosgene solution at below 0°C is prepared. The molar ratio of phosgene to amine is 1 to 5. Then, the amine solution is added dropwise at a temperature below 70°C with stirring, maintaining the reaction for about 30 minutes. The reaction products are yellow slurries of carbamoyl chloride and amine hydrochloride. The low-temperature reaction between amine and phosgene is exothermic and occurs very rapidly, releasing a large amount of heat instantaneously. Sufficient heat exchange equipment is necessary to remove the reaction heat promptly and ensure the smooth progress of the cold phosgenation reaction. The heat released during the low-temperature reaction stage is about 8.368 kJ/mol of amine. Measures such as using dilute solutions, vigorous stirring, and an excess of phosgene are required to minimize the production of urea derivatives during the low-temperature reaction. The dispersion and fineness of the materials directly affect the subsequent hot phosgenation reaction and the overall yield of the product.

3. Hot Phosgenation Reaction

The cold phosgenation reaction mixture is transferred to the hot phosgenation kettle. Under vigorous stirring, the temperature is gradually increased while adding more phosgene if necessary. The reaction mixture is maintained at a temperature of 100°C to 180°C for about 1 to 2 hours until the slurry completely decomposes into a transparent brown liquid. The reaction temperature is determined by the boiling point of the solvent used, typically controlled below the solvent's boiling point. If chlorobenzene is used as the solvent, the final temperature for hot phosgenation is 120°C. The heating rate is crucial during hot phosgenation, generally controlled to slowly increase the temperature to about 100°C within 2 hours. A rapid heating rate will result in significant phosgene loss, directly affecting the further reaction between ammonium salt compounds and phosgene, reducing the yield, and increasing insoluble precipitates. Even with a large amount of additional phosgene later, it is difficult to turn the reaction liquid transparent. To overcome the substantial phosgene loss due to vaporization during the hot phosgenation process, in practice, the outlet valve behind the hot phosgenation kettle condenser is partially closed to operate the hot phosgenation kettle under pressure. This raises the boiling point of phosgene, causing it to reflux while continuously expelling hydrogen chloride, ensuring sufficient phosgene for the reaction. However, operating under pressure requires higher safety standards for the equipment.

4. Gas Stripping and Distillation

Use an inert gas (such as nitrogen or methane) to strip residual phosgene and some HCl gas from the reaction mixture within a temperature range below the solvent's boiling point (100°C to 200°C). This is usually done at atmospheric pressure, but sometimes under pressure, increasing the stripping operation pressure to 0.26 to 0.28 MPa (using chlorobenzene as the solvent) and the stripping temperature to 175°C to 180°C. Practice has shown that pressure stripping has many advantages: reduced nitrogen usage, lower acidity (below 0.03% calculated as HCl). In other words, due to the high stripping temperature, pressure stripping more thoroughly removes phosgene and hydrogen chloride gas compared to atmospheric pressure stripping. The phosgene-free and hydrogen chloride-free phosgenation liquid is sent to a vacuum distillation system to remove the solvent and refine the isocyanate product. The overall yield of the final isocyanate product is about 90% based on the amine