Below is the production of PAPI .
PAPI (crude MDI) is produced in the same manner as MDI except that the ratio of the amount of aniline to hydrochloric acid is smaller. The synthesis of PAPI and MDI-like are carried out in two steps. First, the aniline and formaldehyde solution (formalin) is condensed into a polyamine mixture containing a certain amount of diamine under the action of a Lewis acid, and then the polyamine compound is subjected to phosgenation reaction to obtain PAPI. In the synthesis of polyamine compounds, the content of diamine is controlled to be about 50%. Generally, it can be achieved by adjusting the ratio of the amount of input of the two raw materials of aniline and formaldehyde, that is, the amount of the substance. It has been experimentally determined that when the molar ratio of aniline to formaldehyde is 1:75, about 50% of the diamine in the synthesized polyamine compound is present. Therefore, the raw material group distribution ratio, generally the ratio of the amount of aniline and formaldehyde is 1.6 to 2. The range of O changes, other operating conditions for polyamine synthesis are similar to those of MDA. The polyamine required for the production process of the MDI and PAPI processes must be a polyamine compound containing 60% to 75% diamine, and the key is also achieved by changing the ratio of the amount of the aniline and formaldehyde charged materials. When the ratio of the amount of the aniline/formaldehyde substance is 4/1.4, the synthesized polyamine contains about 75% of the diamine compound. Usually, the process of co-production of MDI and PAPI is adopted.