Patent ID: 7649108

Claim:
A process for the distillation of a mixture of isomeric diisocyanatodiphenylmethanes said mixture comprising 2,2′-diisocyanatodiphenyl-methane, 2,4′-diisocyanatodiphenylmethane and 4,4′-diisocyanato-diphenylmethane in which the distillation is performed in at least one stage using a divided-wall column wherein (a) a bottoms product of 4,4′-diisocyanatodiphenylmethane having a purity of at least 98% by weight is obtained in the divided wall column and (b) a mixture of (i) 50-60% by weight of 2,4′-diisocyanatodiphenyl-methane and (ii) 40-50% by weight of 4,4′-diisocyanatodiphenyl-methane is drawn off in a side stream in the divided wail column.