Carbonic acid diester such as dimethyl carbonate are attracting attention as a raw material substituted for phosgene or dimethyl sulfate in the production of isocyanates, polycarbonates and various drugs and agricultural chemicals. Further, the use thereof as an additive to automobile fuel is being considered.
Carbonic acid diester has long been produced by reacting phosgene with an alcohol. However, the use of phosgene has drawbacks in that phosgene is highly toxic and that the reaction forms hydrochloric acid as a by-product, which causes corrosion of the apparatus. Therefore, processes for producing the carbonic acid diester without the use of phosgene have been developed and are now being executed on industrial scales.
For example, Japanese Patent Laid-open Publication No. 54(1979)-24827 discloses a process for producing a carbonic acid diester, in which an alcohol, carbon monoxide and oxygen react in the liquid phase in the presence of a copper halide catalyst. Furthermore, Japanese Patent Laid-open Publication No. 50(1975)-40528, Japanese Patent Publication No. 61(1986)-8816 and Japanese Patent Laid-open Publication Nos. 62(1987)-81356 and 1(1989)-287062 disclose processes for producing the carbonic acid diester in liquid phase, in which copper and palladium halides are used as catalytic components.
Although having the advantage of using no phosgene, the above processes for producing a carbonic acid diester in the liquid phase have drawbacks in that (1) the reaction apparatus is likely to corrode by the action of a halide catalyst which is used as a solution (2), the catalyst activity is deteriorated rapidly by the water formed during the reaction and (3) it is difficult to separate the catalyst dissolved in the reaction product. In particular, it is requisite that the reaction apparatus is constructed of a corrosion resistant high-quality material to thereby prevent the corrosion of the apparatus and any disaster attributed to the corrosion. This inevitably causes the construction cost of the apparatus to be extremely high.
Processes for producing carbonic acid diester by vapor phase reaction have been proposed as substitutes for the liquid phase processes (for example, Published Japanese Translation of PCT Patent Applications from Other States, No. 63(1988)-503460, Japanese Patent Laid-open Publication No. 4(1992)-89458 and International Application Publication WO 90/15791).
In the vapor phase processes disclosed in these publications, vaporized alcohol, carbon monoxide and oxygen react in the vapor phase in the presence of a catalyst, and the resultant gaseous reaction product is withdrawn from the reactor. The withdrawn gas is cooled to thereby separate it into a condensed liquid and a noncondensable gas and the carbonic acid diester is separated from the liquid. These vapor phase processes are substantially free from the above drawbacks of the liquid phase processes.
Apart from the above, the oxidative carbonylation of an alcohol is an exothermic reaction. The heat of reaction must be removed for maintaining an appropriate reaction temperature. For example, the calorific values are about 71 kcal/mol in both of the following reactions in which carbonic acid diesters are prepared from methanol and ethanol: EQU 2CH.sub.3 OH+CO+1/2O.sub.2 .fwdarw.(CH.sub.3 O).sub.2 CO+H.sub.2 O, and EQU 2C.sub.2 H.sub.5 OH+CO+1/2O.sub.2 .fwdarw.(C.sub.2 H.sub.5 O).sub.2 CO+H.sub.2 O.
When a catalyst consists of, for example, a copper oxychloride supported on an active carbon, the above oxidative carbonylation of an alcohol in the vapor phase is conducted at a temperature as relatively low as about 130 to 170.degree. C.
When it is attempted to synthesize a carbonic acid diester by the use of a fixed-bed reactor, hot spots are likely to occur because the reaction is highly exothermic, so that the dangers of decrease of reaction selectivity, runaway of reaction and catalyst deactivation are increased.
By contrast, the heat of reaction can be removed much more easily in a fluidized-bed reactor than in the fixed-bed reactor, so that the temperature can be controlled without the occurrence of hot spots.
In the above vapor phase reaction in the fluidized-bed reactor, generally, cooling pipes are inserted in the fluidized bed so as to effect heat removal and water is generally used as the heat transfer medium. For example, International Application Publication WO 95/21692 discloses a fluidized-bed reactor for vapor-phase exothermic reaction which is provided with cooling pipes capable of feeding a cooling medium at a steady rate and cooling pipes capable of feeding a cooling medium at a variable rate. In the publication, water is mentioned as the cooling medium fed at a steady rate and steam is mentioned as the cooling medium fed at a variable rate.
If steam obtained by the vaporization resulting from the removal of the heat of reaction generated in the fluidized bed can be used as, for example, a heat source for a reboiler of a distillation column, effective utilization of the heat of reaction would be attained. When the difference between the reaction temperature and the temperature of the heat transfer medium flowing through the cooling pipes is large, the removal of the heat of reaction would be easy.
In the oxidative carbonylation conducted at relatively low temperatures, the difference between the reaction temperature and the temperature of steam available as a heat source (for example, about 125.degree. C. with respect to reboiler heat source) is so small that the number of cooling pipes must be increased to thereby enlarge the heat-transfer area in order to obtain the steam of the above temperature.
However, the number of cooling pipes which can be inserted in a fluidized bed of a given volume is so limited that it is likely that the heat-transfer area required for the removal of heat cannot be satisfactorily secured. Further, when the removal of heat is attempted by increasing the difference between the reaction temperature and the temperature of heat transfer medium under the condition of the limited number of cooling pipes, only low-temperature steam or water can be obtained which has little value.
On the other hand, the attempt to increase the heat-transfer area leads to an unnecessary expansion of the volume of the fluidized bed.
Therefore, there has been a demand for the development of a process capable of producing a carbonic acid diester in high energy efficiency with the effective utilization of the heat of reaction in the vapor-phase oxidative carbonylation of an alcohol which is conducted at relatively low temperatures.