PROCESS FOR DISTILLING A MIXTURE OF ISOMERIC DIISOCYANATODIPHENYLMETHANES

Preparation of 2,4'-methylene diphenylene diisocyanate (MDI) comprises distillation of a diisocyanatodiphenylmethane isomer mixture from 2,2'-diisocyanatodiphenylmethane, 2,4'-diisocyanatodiphenylmethane and 4,4'-diisocyanatodiphenylmethane to give a mixture comprising 2,4'-diisocyanatodiphenylmethane (85-99 wt.%), 4,4'-diisocyanatodiphenylmethane (up to 15 wt.%) and 2.2'-diisocyanatodiphenylmethane (up to 0.2 wt.%). Preparation of 2,4'-methylene diphenylene diisocyanate (MDI) (with less production of 2,4'-MDI) comprises distillation of a diisocyanatodiphenylmethane isomer mixture from 2,2'-diisocyanatodiphenylmethane, 2,4'-diisocyanatodiphenylmethane and 4,4'-diisocyanatodiphenylmethane to give a mixture comprising 2,4'-diisocyanatodiphenylmethane (85-99 wt.%), 4,4'-diisocyanatodiphenylmethane (up to 15 wt.%) and 2.2'-diisocyanatodiphenylmethane (up to 0.2 wt.%) (where: distillation is carried out in a single-step; a partition column is used in at least one distillation stage; a destillative before separation is upstreamed to the partition column before distillation to give a mixture (comprising 2,2'-diisocyanatodiphenylmethane (0-15 wt.%), 2,4'-diisocyanatodiphenylmethane (12-60 wt.%) and 4,4'-diisocyanatodiphenylmethane (25-88 wt.%)), which is used as feed stream to the partition column; and the vapor at the top of the partition column is condensed and 98-99.6% of it returns to the top zone).