Phosgene is an important organic intermediate which is widely used in pesticides, pharmaceuticals, engineering plastics, polyurethane materials and military. Phosgene can be prepared with various methods. A primary method for industrial production of phosgene at present uses carbon monoxide and chlorine as raw materials, with activated carbon as a catalyst for the synthesis of phosgene wherein the commonly used activated carbon is cocoanut charcoal and coal-based charcoal. 80% of phosgene from industrial production is currently used for the production of isocyanates, mainly used for the production of MDI (diphenylmethane diisocyanate), TDI (toluene diisocyanate) and polycarbonate. Production of isocyanates has very strict requirements on the quality of the raw material phosgene, wherein a content of the free chlorine in the phosgene is controlled within 200 ppm, and 5-10% of CO excess relative to chlorine in industry is generally used to control the content of the free chlorine in the synthesized phosgene. At the late stage of phosgene synthesis, with pulverization of the catalyst activated carbon and decrease of the catalytic ability, the content of the free chlorine in the synthesized phosgene is increased, and the catalyst in the phosgene synthesizing tower need to be replaced.
Phosgene, as a highly toxic chemical, has an allowable maximum concentration of 0.5 ppm in the air. It is necessary to purge the phosgene remained in the synthesizing tower and adsorbed in the activated carbon, before replacing the activated carbon in the phosgene synthesizing tower. Activated carbon has a relatively large saturate adsorption rate on phosgene, and it takes a long time to purge so as to completely desorb the phosgene from the activated carbon, which prolongs the time for replacing the catalyst in the phosgene synthesizing tower, and it is also necessary to consume a lot of nitrogen gas.
Chinese Patent No. CN102502700A discloses a method of replacing a catalyst in an ammonia gas synthesis system. Described by the patent is that carbon dioxide gas is introduced into a synthesis system after the synthesis system is decompressed, the carbon dioxide gas is used to replace the gas in the system, which however, is less effective in desorbing a small amount of phosgene from the activated carbon, resulting in longer purging time finally.
Chinese Patent CN202199338U discloses a solid catalyst replacing apparatus. Described by the patent is to arrange a vacuum valve and a feed inlet at the top of the catalyst storage tank wherein the vacuum valve is connected to a vacuum pump via a vacuum tube and the feed inlet is communicated with reaction tubes packing the catalyst in the reactor via a feed pipe. However, the activated carbon in the phosgene synthesizing tower has a relatively large adsorption rate on the phosgene, and the outlet of the synthesizing tower requires an extremely low concentration of phosgene. It is difficult to completely remove the phosgene from the phosgene synthesizing tower directly through the vacuum system in a comparatively short time.
Chinese Patent No. CN101829526A discloses a catalyst replacing system, a catalyst replacing method, and a rectifying tower having the system. Described by the patent is that a catalyst is processed by a rectification system and then pumped to a reaction system. However, the phosgene synthesizing tower is a gas-solid reaction system, and the activated carbon cannot be regenerated after reaching its service life.
The method of replacing the catalyst in the synthesizing tower described by the above patent has disadvantages of long phosgene purging time and complicated device for catalyst replacement. It has not been reported any process for quickly purging the phosgene synthesizing tower so as to improve the catalyst replacement rate of the catalyst in the phosgene synthesizing tower. Therefore, in view of the characteristics of the phosgene synthesizing tower system and the nature of the phosgene, it is necessary to develop a method of quickly removing phosgene from the phosgene synthesizing tower with facilitating safe operation.
In the prior art, only nitrogen gas is used to directly purge the phosgene synthesizing tower, and it is necessary to consume a large amount of nitrogen gas when phosgene concentration is low in the later stage of purging, and the purging rate is very slow. Some of the phosgene remains in the synthesizing tower, and there are still great risks in replacing the catalyst in the synthesizing tower.