The present invention relates to a process for the production of phosgene by reacting carbon monoxide (CO) and chlorine on activated charcoal in a shell-and-tube reactor, in which the dissipation of the heat of reaction is accomplished by evaporative cooling with water under reduced pressure. The invention also relates to an apparatus for the production of phosgene, comprising a shell-and-tube reactor, a steam/water separator, a heat exchanger (condenser), a closable vacuum line as well as at least one monitoring device by means of which a leakage of phosgene into the coolant circulation can be detected.
The production of phosgene from CO and chlorine on activated charcoal catalysts in a shell-and-tube reactor is known from the prior art. See, for example, Ullmann's Encyclopedia of Industrial Chemistry, 5th Ed., Vol. A 19, p. 413 ff., VCH Verlagsgesellschaft mbH, Weinheim, 1991. An essential objective in this process is the safe dissipation of the considerable heat of reaction generated. This dissipation of heat is normally accomplished by direct cooling. Evaporative cooling is, however, also mentioned in the literature. For example, U.S. Pat. No. 4,231,959 mentions that, apart from direct cooling with water, cooling with boiling water is also possible, in which usable steam is generated. EP-A-134 506 describes the production of phosgene under vapor cooling with a high boiling point medium, which is then used to generate useful steam. Evaporative cooling is also mentioned in general there as a method. At the same time, however, it is stated that a gas temperature in the reactor outlet of below 100° C. must be achieved in order to obtain the necessary low chlorine content of below 50 ppm in the phosgene. Therefore, the process of this invention is also carried out in two stages. In the first stage of this process, the production of phosgene is carried out at high temperature and with evaporative cooling using a high boiling point coolant, and the second stage of this process is carried out at 70° to 100° C. under direct cooling. Finally, WO-A-03/072237 describes a reactor and a process for the production of phosgene, in which the reactor, in a special tubular arrangement, is cooled with a liquid coolant, namely monochlorobenzene. For reasons of corrosion prevention, stainless steel is specified for the reactor tubing. At the same time, however, the problems that can cause corrosion in the event of non-uniform or insufficient cooling are described therein.
Accordingly, the following process-technology objectives arise in the production of phosgene:
On the one hand, a safe and uniform dissipation of heat with the best possible heat transfer coefficients should be ensured so that a gas temperature of less than 100° C. is established in the reactor outlet and a chlorine content of less than 50 ppm is thereby obtained in the phosgene produced by the process. On the other hand, safety technology problems which arise if there is a leakage of heat transfer medium into the reaction space due to corrosion damage must be solved. In this connection, the use of water as heat transfer medium is problematical for safety technology reasons. If water enters the reaction space through, for example, a hole in a reaction tube which was caused or created by corrosion, the water reacts very vigorously with the formed phosgene on the activated charcoal catalyst. Such a reaction between water and phosgene forms hydrochloric acid, and this (i.e. hydrochloric acid), in turn, leads to further corrosion.
Accordingly, the object of the present invention is to provide a process and an apparatus for the production of phosgene in which, as a result of efficient heat dissipation, a gas temperature of below 100° C. and a concentration of chlorine in the phosgene of below 50 ppm is ensured at the outlet of the reactor, and which is simultaneously safe to operate.