4,4'-Methylene bis (phenyl isocyanate) (MDI) is an important chemical raw material widely used in polyurethane, coatings, sealants, elastomers and other fields. In this paper, we will introduce the manufacturing methods of MDI, including various process routes such as photogasification, nitration and carbonization, and discuss their advantages, disadvantages and scope of application.
I. Photogasification
Photogasification is a common MDI manufacturing method, the principle is to make diphenylmethane diisocyanate (MDI) react with carbon monoxide under the light condition to generate MDI.The specific steps are as follows:
1. Mix diphenylmethane diisocyanate (MDI) with solvent, and pass it into the photogasification reactor.
2. Carbon monoxide gas is introduced and the photogasification reaction is carried out.
3. At the end of the reaction, the reaction solution is post-treated to obtain the MDI product.
The advantage of the photogasification method is that a high purity MDI product can be obtained while the reaction conditions are relatively mild. However, the method requires the use of high-energy light source, equipment investment is larger, while the production process will produce a large amount of waste gas and wastewater, environmental protection costs are higher.
II. Nitrification method
Nitrification method is a method to prepare MDI by nitrification reaction, the principle is to react diphenylmethane with nitric acid to generate nitrodiphenylmethane under acidic condition, and then to react with urea to generate MDI.The specific steps are as follows:
1. Mix diphenylmethane with nitric acid and carry out nitrification reaction to generate nitrodiphenylmethane.
2. Nitrodiphenylmethane is mixed with urea, and a reduction reaction is carried out to form MDI.
3. The reaction solution is post-treated to obtain the MDI product.
The advantages of the nitrification method are the relative simplicity of the process, the easy availability of raw materials, and the low cost. However, the production process of this method will produce a large amount of acidic wastewater and exhaust gas, serious corrosion of equipment, environmental protection management is difficult.
Third, carbonization method
Carbonization method is a method of preparing MDI through carbonization reaction, the principle is that under high temperature and high pressure conditions, diphenylmethane and carbon dioxide reaction to generate MDI. the specific steps are as follows:
1. Mix diphenylmethane and carbon dioxide, and pass it into the high-temperature and high-pressure reactor.
2. Under the action of a catalyst, a carbonization reaction is carried out to generate MDI.
3. The reaction solution is post-treated to obtain the MDI product.
The advantages of the carbonization method are that the reaction conditions are relatively simple and do not require the use of large amounts of solvents and catalysts, while the production process generates less waste and has lower environmental costs. However, the method requires high-temperature and high-pressure reaction conditions and a large investment in equipment, while the activity of the catalyst has a large impact on the reaction and requires strict control of the reaction conditions.
In summary, different manufacturing methods have their own advantages and disadvantages and scope of application, and the selection of a suitable manufacturing method needs to be considered comprehensively according to the specific product requirements, production conditions and environmental requirements and other factors. At the same time, with the increasing awareness of environmental protection and the continuous progress of chemical technology, the manufacturing methods of MDI also need to be updated and improved to improve product quality and reduce environmental costs.
