PURPOSE: To obtain the subject compound minimized in the content of hydrolyzable chlorine by using a thin-film type evaporator having a condenser on the outside of the first and/or third distillation steps in distilling operation composed of the three steps.
CONSTITUTION: Diphenylmethane diisocyanate is separated from a crude polyisocyanate obtained by phosgenating a condensational product of aniline with formaldehyde by distillation. In the process, the diphenylmethane diisocyanate isomer is separated from the polyisocyanate in the first distillation step using a thin-film type evaporator having a condenser on the outside or inside. The 2,2'- and 2,4'-isomers are then separated as an overhead product in the second distillation step. Bottoms from the second distillation step are purified by using the thin-film type evaporator having a condenser on the outside to continuously take out the objective pure 4,4'-diphenylmethane diisocyanate. Entrainment in the evaporator can be suppressed by using the external condensing type thin-film evaporator.
TSURUTA SATOSHI
FUJINO HIROSHI
KURODA KAZUMOTO
NITTA KAZUNARI
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