Methyl isocyanate can be synthesized by reacting monomethylamine and phosgene at 240.degree. to 250.degree. C in a continuous vapor phase tubular reactor: ##STR1## Methyl isocyanate and HCl react in an equilibrium reaction (b) to form methyl carbamyl chloride: ##STR2## In equilibrium reaction (b), dissociation (dehydrochlorination) of methyl carbamyl chloride to methyl isocyanate and HCl approaches 100% at 240.degree. C, but is almost negligible at 45.degree. C. Thus, when the gases leaving the continuous hot tube reactor of synthesis reaction (a) are scrubbed in a packed column with refluxing solvent (e.g., chloroform or toluene) or refluxing phosgene plus solvent, excess phosgene and one mole of HCl (of the two moles formed) can be removed overhead, and a liquid stream consisting mainly of methyl carbamyl chloride and solvent, with a small percentage of methyl isocyanate, can be drawn off from the bottom of the column.
The boiling point of methyl isocyanate is 39.degree. C., whereas the melting point of methyl carbamyl chloride is about 45.degree. C. (and its boiling point is 92.degree. C.) The equilibrium in the aforesaid dissociation reaction is shifted strongly to the right (to reassociation) at 39.degree.-45.degree. C. Consequently, it is impossible to separate pure methyl isocyanate from pure methyl carbamyl chloride simply by condensing methyl isocyanate in the presence of HCl.
Merz, in U.S. Pat. No. 3,388,145, discloses thermally dihydrochlorinating a solution of methyl carbamyl chloride to produce a mixture of solvent, methyl carbamyl chloride, methyl isocyanate and HCl, and removing HCl from that mixture by reflux in a condenser which is connected directly to the dehydrochlorination reactor. The condenser is cooled to a temperature substantially below the boiling point of the isocyanate so that only HCl is removed from that mixture. Moreover, Merz requires that a fractionating column be connected directly to the reactor and that the fractionating column and the reflux condenser be operated simultaneously with one another.
Slocombe et al., in U.S. Pat. No. 2,480,088, disclose preparation of methyl isocyanate by dehydrochlorinating methyl carbamyl chloride in the presence of an HCl acceptor. Slocombe et al. also disclose thermally dehydrochlorinating other carbamyl chloride compounds in the absence of an HCl acceptor, but only where the isocyanate (e.g., phenyl isocyanate) possesses a boiling point higher than the decomposition temperature of the carbamyl chloride.