A PLANT AND METHOD FOR PURIFYING AN ISOMERIC MONOMER, SUCH AS 4,4'-METHYLENE DIPHENYL DIISOCYANATE, FROM A CRUDE MIXTURE

The present invention relates to a plant for preparing a purified isomeric monomer from a mixture of different isomeric monomers, wherein the plant comprises a distillation apparatus, which comprises:a) a distillation column including a structured packing,b) a source for a mixture of different isomeric monomers,c) an evaporator,d) an overhead vapor condenser,e) optionally, an overhead vacuum system andf) a flow-controlled reflux system,wherein the overhead vapor condenser comprises a shell and tube arrangement and is embodied so as to directly subcool the condensate to less than 47°C, and wherein the flow-controlled reflux system comprises a heater, which is embodied so as to reheat a partial stream of the condensate formed in the overhead vapor condenser up to 190 °C.Moreover, the present invention relates to a process for preparing a purified isomeric monomer and in particular to a process for purifying 4,4'-methylene diphenyl diisocyanate from a mixture of different isomers of methylene diphenyl diisocyanate, which contains 4,4'-methylene diphenyl diisocyanate and at least one of 2,2'-methylene diphenyl diisocyanate and 2,4'-methylene diphenyl diisocyanate, wherein the process is performed in the plant as described above.