Crude MDI with a specific ring distribution is fed into a vacuum separation column. This yields diphenylmethane diisocyanate mixture A1 and polymeric MDI. Mixture A1 is then introduced into a distillation column where parameters are adjusted to obtain mixture B1 with a specific composition. B1 undergoes free chlorine concentration via a crystallization unit, controlling the endpoint temperature and residual liquid ratio, to yield intermediate C1. Intermediate C1 is blended with another diphenylmethane diisocyanate mixture D1 of specific composition. Stirring gives the final MDI-50 product.
