| 摘要 |
The invention relates to a method for producing isocyanates by thermally splitting carbamates, also known as carbamide acid ester or urethane. The carbamate splitting increases in significance as a phosgene-free method for producing isocyanates. Various apparatuses are proposed for technically performing the carbamate splitting, in particular columns (in EP 0 795 543), fluidized bed reactors (in EP 555 628 and in DE 199 07 648), falling film evaporators or thin film evaporators (in EP 0 092 738). The carbamate splitting can be carried out in the liquid or gas phase. The formation of high molecular weight byproducts is problematic when thermally splitting carbamates, said byproducts arising from continued reaction of the splitting products with themselves or with the initial materials. Said byproducts can lead to deposits in the apparatuses, thus limiting continuous operation and leading to loss of yield. The residues comprise in particular allophanates and isocyanurates. The byproducts also arise by the reaction of half-urethanes (semicarbamates, that is, a difunctional compound comprising a urethane and an isocyanate function, intermediate to splitting bisurethanes) with themselves. In order to prevent said problems, the split products isocyanate and alcohol must be separated from the carbamate split gas as quickly as possible. It is further known that the problems of back reaction and continued reaction are reduced in the course of splitting, in that the carbamate split is performed in the presence of solvents, because the reaction speed of the back reaction of isocyanate and alcohol (urethanization) and the continued reactions is dependent on the type of solvent and the dilution by the solvent. For example, J.H. Saunders and K.C. Frisch: Polyurethanes, Chemistry and Technology, 1962, P. 146, Table 10, Data on the reactivity of isocyanates having alcohols in the presence of different solvents. |
