The condensation of aniline and formaldehyde under acidic conditions is a fully mature technical route for MDI production abroad. The reactants were neutralized with alkali and then distilled to obtain diphenylmethane diamine (MDA). After MDA is dissolved in solvent, it is phosgenated to produce 4.4-MDI 2.4-MDI 2.2-MDI or mixed PMDI, and then purified by distillation to obtain pure MDI.
The reaction of free amine with phosgene is the most important method of MDI. Before the reaction, amine was dissolved in an inert solvent, and excess phosgene of the same solvent was continuously added at low temperature to form carbamoyl chloride and amine hydrochloride slurry, then heated to high temperature, and excess phosgene was introduced until a clear solution was obtained. The reaction is carried out in about 20% solution. In order to reduce side reactions, phosgene must be above 50%. Phosgene is toxic and fatal to human body, so the central government strictly controls it. Moreover, the phosgene production investment is large, and transportation and storage are inconvenient. The generated hydrogen chloride has serious corrosion to equipment, demanding production requirements, high operational risk and difficult equipment maintenance. Nowadays, people are actively developing methods to replace phosgene.
Monsanto has issued a patent. TDI and MDI are produced by non-phosgene methods, such as amine method, carbon dioxide method and dehydrating agent method. The reaction produces carbamate at almost normal pressure and low pressure, and then dehydrates with phosphorus pentoxide and triethylamine as dehydrating agents to produce isocyanate. From the whole reaction, the dominant direction in the future is to catalyze carbonyl group to synthesize carbamate, and then pyrolyze it into MDI. It is reported that BASF has established industrial production facilities for carbamate process in Belgium and the United States. According to the data reported by Asahi Glass Company, the production cost of non-phosgene method is 20% lower than that of phosgene method.
The carbamate method is that aniline and carbamate are first made into aniline carbamate, then mixed with nitrobenzene in the presence of sulfuric acid to form MDI mixture, and then distilled to get the finished product.
Aniline first reacts with carbon monoxide, ethanol and oxygen to produce ethyl aniline formate (EPC). Then, the solution of EPC and formaldehyde was concentrated to produce binuclear methylene diphenyl carbamate (MDV), and the product was pyrolyzed to produce MDI and ethanol, which were recycled for carbonylation.
In the reaction process, the presence of aniline can reduce the carbonylation of nitrobenzene and improve the yield of carbamate. In order to make the reaction go smoothly. Methanol is usually in excess. The feed ratio of raw materials is methanol: aniline: nitrobenzene: catalyst13.5:1.0:1.0: 0.002, and the reaction is carried out at CO pressure of 6.87Mpa and 160℃ for 3.5h to generate EPC. The catalyst used is a new carbonyl compound, and the reaction liquid is quickly discharged and sent to the lower tank.
Excess carbon monoxide and by-product carbon dioxide are sent to a stirred reactor to promote the mixing of the solution and organic substances. The reaction is heated by induction coil and reacts at the interface of formaldehyde solution/sulfuric acid layer and EPC/ organic layer. The reaction temperature is controlled at 75℃, and the intermediate product MDV/PMDV is generated under normal pressure.
Then the reactants enter the organic matter/solution separator, and most of the H2SO4 catalyst is separated for recycling. The organic layer was washed with water to remove residual sulfuric acid and unreacted formaldehyde.
The reaction mixture contains unreacted EPC, MCV/PMDV, organic solvent and reaction intermediate, effluent from organic/solution separator and liquid accelerator, which is converted into MDV/PMDV at 75℃ and atmospheric pressure for about 20min minutes.
After purification, the generated MDV/PMDV enters the degradation device. In the presence of inert solvent, the reaction humidity is controlled at 250℃, the pressure is controlled at 20Pa, and the residence time is 65438±0h h.. Nitrogen was continuously introduced to remove excess methanol from the reactor. The bottom product is sent to MDI extraction tower to separate MDI and by-product polyisocyanate.