Patent ID: 7358388

Claim:
A process for purifying an isocyanate, comprising: (a) separating a crude isocyanate stream ( 1 ) containing isocyanate, higher- and lower-boiling components and unvaporizable residue into a part-stream ( 2 ) which contains the unvaporizable residue and isocyanate, and a vapor stream ( 3 ) containing isocyanate and low boilers; wherein the weight ratio of stream ( 2 ) to stream ( 3 ) ranges from 20:1 to 1:1; and wherein said separation is performed by distillation using at least one theoretical plate; (b) keeping part-stream ( 2 ) separate from the vapor stream ( 3 ) and/or from streams which at least partly contain the vapor stream ( 3 ); (c) separating part-stream ( 2 ) into residue stream ( 8 ) which comprises unvaporizable residue and less than 2.5% isocyanate and into vapor stream ( 4 ) which contains isocyanate; (d) separating vapor streams ( 3 ) and ( 4 ) in gaseous form or after condensation into high-( 5 ), medium-( 6 ) and low-boiling ( 7 ) streams, wherein ( 5 ) and ( 7 ) contain a substantial part of the low- or high-boiler content of crude isocyanate stream ( 1 ), and wherein ( 6 ) contains purified isocyanate.