1. Field of the Invention
The present invention relates to a process for preparing alkanolamines such as ethanolamines and isopropanolamines. More specifically, it relates to a process for particularly and selectively preparing monoalkanolamines.
2. Description of the Related Art
Alkanolamines can easily be obtained by reacting an alkylene oxide with aqueous ammonia, and also in an industrial scale, they have been manufactured by this process.
However, the alkanolamines obtained by the above-mentioned process are present in the state of a mixture of a monoalkanolamine, a dialkanolamine and a trialkanolamine, and it is an important theme to control the production ratio of these components. Particularly in recent years, the demand for the monoalkanolamine is larger as compared with that of the dialkanolamine and the trialkanolamine, and a technique for preparing the monoalkanolamine in a high production ratio has been required.
In general, with regard to a method for manufacturing the monoalkanolamine in a high production ratio by, for example, the reaction of ethylene oxide and ammonia, the following matters are known. That is to say, the reactivity between ethylene oxide and ammonia is lower as compared with the reactivity between ethylene oxide and monoethanolamine or diethanolamine, and therefore the ratio of the reaction products depends upon the ratio of ammonia to ethylene oxide. Thus, as ammonia is used in large excess, the production ratio of the monoethanolamine increases (K. Weissermel and H. J. Arpe, Translation supervised by Mitsuaki Mukouyama, "Industrial Organic Chemistry--Main Materials and Intermediates--", Tokyo Kagaku Dojin, p. 149 (1978).
However, if aqueous ammonia is used and a molar ratio of ammonia/the alkylene oxide is high, the production ratio of the monoalkanolamine can be increased, but the volumetric efficiency of a reactor deteriorates, and excess aqueous ammonia is required to be collected and recycled. In consequence, the energy unit requirement decreases and the load of the ammonia collection system and the water collection system increases inconveniently.
In order to solve such problems, there is a method in which the water content of the aqueous ammonia is minimized, but since the reaction of the alkylene oxide and ammonia proceeds by virtue of the function of water as a catalyst, such a method causes the deterioration of activity. Thus, as some measures to the above-mentioned inconvenience, there have been suggested a method of using a solid acid catalyst and increasing the reaction temperature (e.g., Japanese Patent Application Laid-open No. 47728/1974; Zh. Prikl. Khim., Vol. 56, p. 1966 (1983); and U.S. Pat. No. 4,438,281) and another method in which the reaction is carried out in a supercritical state (Japanese Patent Application Laid-open Nos. 13751/1984 and 33247/1984), but these methods have a problem that reaction pressure is high and so a high-pressure reactor is required, and another problem that the load of the ammonia collection system increases.
On the other hand, there are known methods in which when ammonia reacts with an alkylene oxide to prepare alkanolamines, a carbonate of ammonia is used, whereby the production ratio of the monoalkanolamine can be increased under relatively gentle conditions (British Patent No. 497,093 and U.S. Pat. No. 2,186,392).
The present inventors have intensively investigated a process in which ammonia reacts with an alkylene oxide in the presence of a carbonate of ammonia in accordance with the above-mentioned U.S. Pat. No. 2,186,392 to prepare the alkanolamines, and it has been confirmed that even if ammonia is not used in large excess relative to the alkylene oxide, the production ratio of the monoalkanolamine can be remarkably increased.
Next, it was tried to separate, from a reaction solution, the monoalkanolamine, the dialkanolamine and the trialkanolamine which were products. That is to say, after ammonia, most of the carbon dioxide and water had been distilled off, the alkanolamines which were the products were separated from the resultant bottoms, and at this time, there was elucidated a problem, which was heretofore unknown, which impurities such as nitrogen-containing compounds such as carbamic acid and carbamic acid esters and nitrogen-containing heterocyclic compounds such as oxazolidone and N-hydroxyethylpiperazine were present in the alkanolamines.