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[Chemical Knowledge]：Manufacturing method and process of pure MDI (diphenyl methane diisocyanate)

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Manufacturing method
of pure MDI (diphenylmethane diisocyanate)
pure MDI is an important chemical raw material, which is widely used in polyurethane, rubber, coating and other fields. This paper will introduce the manufacturing methods of pure MDI, including phosgenation, nitration and carbodiimide.

1. phosgenation

phosgenation is the main method for the production of pure MDI, the process includes the reaction of aniline and formaldehyde to produce two phenyl methane diamine (MDA),MDA and phosgene reaction to produce pure MDI. The specific steps are as follows:

1. aniline and formaldehyde reaction

aniline and formaldehyde are added into the reactor according to a certain proportion, and under the action of acidic catalyst, the reaction generates diphenylmethanediamine (MDA).

2. MDA and phosgene reaction

MDA and phosgene are added into the reactor according to a certain proportion, under the protection of inert gas, the reaction generates pure MDI. The temperature and pressure need to be controlled during the reaction to ensure the smooth progress of the reaction.

3. Purification
After the
reaction is completed, the product needs to be purified to remove impurities therein. Purification is usually carried out by distillation to obtain pure MDI products with higher purity.

2. nitration

nitration is an earlier pure MDI manufacturing method, the process includes the reaction of aniline and nitric acid to produce nitroaniline, nitroaniline and formaldehyde to produce dinitrodiphenylmethane, and finally reduced to obtain pure MDI. The specific steps are as follows:

1. aniline nitration

aniline and nitric acid are added into a reactor according to a certain proportion, and under the action of an acidic catalyst, nitroaniline is generated by reaction.

2. Nitroaniline and formaldehyde reaction

Nitroaniline and formaldehyde are added into the reactor according to a certain proportion, and under the action of alkaline catalyst, the reaction generates dinitrodiphenylmethane.

3. Reduction

Dinitrodiphenylmethane is subjected to reduction reaction to obtain pure MDI product. As the reduction method, chemical reduction, catalytic hydrogenation, or the like can be used.

3. carbodiimide method

carbodiimide method is a new type of pure MDI manufacturing method, its process includes aniline and urea reaction of carbodiimide, carbodiimide and aniline reaction of pure MDI. The specific steps are as follows:

1. Aniline and urea reaction

Aniline and urea are added into the reactor according to a certain proportion, and under the conditions of high temperature and high pressure, the reaction generates carbodiimide.

2. carbodiimide and aniline reaction

carbodiimide and aniline according to a certain proportion into the reactor, under the protection of inert gas, reaction to produce pure MDI. The temperature and pressure need to be controlled during the reaction to ensure the smooth progress of the reaction.

3. Purification
After the
reaction is completed, the product needs to be purified to remove impurities therein. Purification is usually carried out by distillation to obtain pure MDI products with higher purity.

The above introduces three manufacturing methods of pure MDI, each method has its advantages and disadvantages and scope of application. In actual production, the appropriate manufacturing method should be selected according to the specific situation to ensure the quality and output of the product. At the same time, the safety operation procedures should be strictly observed in the production process to ensure the smooth progress of production safety.

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