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[Chemical Knowledge]：Production method and process flow of toluene diisocyanate

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Toluene diisocyanate (TDI) is an important organic chemical raw material, which is widely used in polyurethane foam, coatings, elastomers, adhesives and other fields. Next, the production method of tolylene diisocyanate will be described.

1. Raw Material Preparation

The main raw materials for the manufacture of toluene diisocyanate are toluene, nitric acid and urea. Among them, toluene requires high purity, generally need to use industrial grade toluene, nitric acid using dilute nitric acid, urea using industrial grade urea.

2. toluene nitration

toluene with dilute nitric acid at a certain temperature and pressure to produce methyl nitrobenzene (MNB). The nitration reaction generally uses a kettle reactor or a tubular reactor, the reaction temperature is controlled at 350-400°C, and the pressure is controlled at 0.3-0.5MPa. In order to ensure the smooth progress of the nitration reaction, it is necessary to control the appropriate acidity, temperature and residence time.

3. methyl nitrobenzene hydrogenation reduction

methyl nitrobenzene and hydrogen hydrogenation reduction reaction at a certain temperature and pressure to produce methyl aniline (MA). The hydrogenation reduction reaction generally adopts a fixed bed reactor or a slurry bed reactor, the reaction temperature is controlled at 180-220 ℃, and the pressure is controlled at 2.0-3.0MPa. The catalyst is generally a copper-chromium or nickel-based catalyst. In this process, it is necessary to control the appropriate hydrogen flow, reaction temperature and residence time to ensure the complete reduction reaction.

4. methyl aniline phosgenation

methyl aniline and phosgene under certain conditions for the formation of toluene diisocyanate. The phosgenation reaction generally adopts phosgene method or gas phase method, the reaction temperature is controlled at 80-120 ℃, and the pressure is controlled at 0.1-0.3MPa. In this process, it is necessary to pay attention to control the flow rate of phosgene and the reaction temperature to avoid the occurrence of side reactions. At the same time, the generated exhaust gas needs to be effectively treated to avoid environmental pollution.

5. post-treatment

After the phosgenation reaction, the obtained crude toluene diisocyanate needs to be post-treated, including distillation, crystallization, centrifugation and other steps, in order to obtain a higher purity toluene diisocyanate product. In the post-treatment process, it is necessary to control appropriate operating conditions to ensure the quality and yield of the product.

In summary, the manufacturing method of toluene diisocyanate involves multiple reaction steps and post-treatment processes, and the operating conditions of each step need to be strictly controlled to ensure the quality and yield of the product. At the same time, it is necessary to pay attention to safety production and environmental protection requirements to ensure the smooth progress of the production process.

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