Patent ID: 8779182

Claim:
A process for the simultaneous and continuous production of a first isomer mixture and a second isomer mixture wherein each of the mixtures comprises predominantly of 4,4′-diphenylmethane diisocyanate and 2,4′-diphenylmethane diisocyanate comprising the step of separating a mixed diphenylmethane diisocyanate isomer feed stream [MIx] by a single-step suspension melt crystallisation process into two mixed isomer diphenylmethane diisocyanate streams [MIy and MIz] thereby forming a slurry in a crystallisation section, wherein the melt crystallisation process comprises a crystallization step wherein crystals [MIy] are formed in a mother liquid [MIz], wherein the slurry exits the crystallisation section and the mother liquid is subjected to a subsequent separation step whereby one or more crystals are melted out of the mother liquid to form the first isomer mixture and the remaining mother liquid is split into the second isomer mixture and a recycling stream, wherein the recycling stream is fed back into the crystallisation section, wherein x is 80 to 92, y is 97 to 99 and z is 60 to 80 where x, y and z are percentages by weight of the 4,4′-diphenylmethane diisocyanate isomer contained in the diphenylmethane diisocyanate isomer mixture; and wherein the temperature of the slurry exiting the crystallisation section is controlled to about 25° C. to 27° C.