Continuous process for the preparation of isocyanates from urethanes

A continuous process for the production of an organic mono- or di-isocyanate from the corresponding urethane, according to the following reaction:

• ##STR2## where R represents a C3-C18 alkyl radical, with a linear or branched chain, optionally having one or more ethylenic unsaturations;
• a C5-C7 cycloalkyl radical;
• or a monocyclic or polycyclic aryl radical, with condensed or uncondensed rings;
• said radicals optionally having one or more substituents chosen from the group consisting of C1-C4 alkyl, C1-C4 alkoxy, nitro, chloro, fluoro and bromo groups;
• R' represents a C1-C20 alkyl group;
• and
• x is 1 or 2;
• said process being characterized by the fact that:
• (a) a flow of urethane (I) is continuously fed to a decomposition reactor containing a liquid mixture, having a substantially constant volume and composition and without any catalytic substances for the decomposition reaction, composed of a high boiling, inert, organic solvent containing unaltered urethane (I) in a concentration of from 0.05 to 1.0 equivalent urethane functions and isocyanate (II) in a concentration of from 0.02 to 0.9 equivalent isocyanate functions for every kg of the liquid mixture, the above reactor being kept at the boiling temperature of the solvent and at the decomposition temperature of the urethane (I) in isocyanate (II) and alcohol (III), the decomposition rate of the urethane (I) corresponding to the rate at which it is fed to the reactor, the alcohol (III) being evaporated at a rate which corresponds more or less to that of its formation;
• (b) the liquid mixture is continuously withdrawn from the reactor in (a) at a rate which does not substantially alter the volume of the mixture in (a) and is fed to a flash apparatus kept at a temperature which is lower than the decomposition temperature of the urethane (I), in said apparatus, the isocyanate (II) being partially evaporated at a rate which corresponds to that of its formation in the reactor in (a), the residual liquid mixture of the flash being recycled to the reactor in (a);
• and
• (c) the flow of vapor generated in (b) is continuously withdrawn, condensed and distilled, in a distillation column, to separate the isocyanate (II), which is the product at the head of the column, in a pure or substantially pure form.