Process for operating a gas phase phosgenation plant

A method of operating a gas phase phosgenation plant that is configured to produce an isocyanate by reacting an amine with phosgene in a stoichiometric excess in relation to the primary amino groups of the amine to give the corresponding isocyanate, and that comprises at least

• (i) an apparatus for providing a gaseous phosgene stream, which optionally comprises an inert substance in addition to phosgene,
• (ii) an apparatus for providing a gaseous amine stream, which optionally comprises an inert substance in addition to amine,
• (iii) a mixing zone for mixing the gaseous phosgene stream and the gaseous amine stream, with the mixing zone being connected by connecting devices for the phosgene stream and for the amine stream to the apparatus for providing a gaseous phosgene stream and the apparatus for providing a gaseous amine stream,
• (iv) a reaction zone arranged downstream of the mixing zone for further conversion of the previously mixed streams and,
• (v) a reaction stopping zone arranged downstream of the reaction zone to end the reaction,
• and optionally
• (vi) a workup section which comprises devices for recovery and recycling of unconverted phosgene and devices for obtaining the isocyanate prepared in pure form,
• wherein
• the gas phase phosgenation plant is shut down by:
• (I) reducing the amine mass flow rate introduced into the apparatus for providing a gaseous amine stream to zero, while maintaining a gaseous inert gas stream through the connecting device for the amine stream, the mixing zone, the reaction zone and the reaction stopping zone;
• (II) reducing the mass flow rate of phosgene leaving the apparatus for providing a gaseous phosgene stream to zero, while maintaining a gaseous inert gas stream through the connecting device for the phosgene stream, the mixing zone, the reaction zone and the reaction stopping zone only after a period corresponding to at least 10 times the residence time of phosgene from exit from the apparatus for providing a gaseous phosgene stream to exit from the reaction stopping zone in the regular operation of the gas phase phosgenation plant has elapsed, calculated from the moment from which the amine mass flow rate is zero;
• and
• (III) maintaining at least the inert gas stream from (I), at least for a period corresponding to three times the residence time of the inert gas stream from entry into the mixing zone to exit from the reaction stopping zone, calculated from the moment from which the mass flow rate of phosgene leaving the apparatus for providing a gaseous phosgene stream is zero.