Method for producing isocyanates
The invention relates to a method for preparing isocyanates by phosgenating the corresponding amines, wherein low-boiling secondary components, excess phosgene, and the co-product hydrogen chloride are separated from the crude liquid isocyanate stream, which is obtained after the phosgenation has occurred, within a maximum of 60 minutes, and wherein the crude liquid isocyanate stream is not exposed to temperatures above 250° C. until said separation.
1. A continuous method for preparing an isocyanate by
(i) reacting the corresponding primary amine with phosgene in stoichiometric excess in a reaction chamber, wherein the reaction is carried out either in the liquid phase in the presence of an inert solvent or in the gas phase, such that a crude product 1 is obtained comprising the desired isocyanate, inert solvent, secondary components having a boiling point below that of the isocyanate (low boilers), secondary components having a boiling point above that of the isocyanate (high boilers), hydrogen chloride and unreacted phosgene,
(ii) separating the crude product 1 into a liquid product stream 2 containing the desired isocyanate and into a gaseous product stream 3,
(iii) working-up the liquid product stream 2, wherein inert solvent, low boilers, high boilers, hydrogen chloride and phosgene are removed from the desired isocyanate,
characterized in that
the low boilers, the hydrogen chloride and the phosgene are removed in step (iii) within a period of 30 second to 60 minutes following the separation of the crude product 1 in step (ii) into the product streams 2 and 3, and the temperature of the product stream 2 is always maintained below or equal to 250° C., and
wherein in the case where the reaction is carried out in the gas phase, the isocyanate is liquefied after leaving the reaction chamber by adding an inert solvent to the process product.
2. The method as claimed in claim 1 , in which the workup step (iii) is carried out in more than one stage, wherein inert solvent, low boilers, hydrogen chloride and phosgene are removed from the product stream 2 by distillation in a first stage (iii.a), such that a product stream 4 depleted in inert solvent, low boilers, hydrogen chloride and phosgene is obtained, and in at least one further stage (iii.b) pure isocyanate 5 is obtained from the product stream 4 by distillation.
3. The method as claimed in claim 2 , in which step (iii.a) is carried out in a dephosgenation column and step (iii.b) is carried out in a column for final purification, and in which the residence time of isocyanate formed in step (i) after the separation of the crude product in step (ii) up to the effluent of the product stream 4 depleted in inert solvent, low boilers, hydrogen chloride and phosgene from the dephosgenation column in step (iii.a) is 30 seconds to 60 minutes.
4. The method as claimed in claim 1 , in which the reaction of the primary amine with phosgene in the reaction chamber in step (i) is carried out in the gas phase.
5. The method of claim 1 , in which the primary amine used is toluylenediamine.