JPS6331458B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for obtaining purified acetonitrile by removing allyl alcohol contained in crude acetonitrile without adding any chemicals. General industrial acetonitrile produced in the petrochemical industry, such as propylene or isobutylene, is usually reacted with ammonia in the presence of oxygen and a catalyst, and acetonitrile, which is produced as a by-product along with acrylonitrile or methacrylonitrile, is separated from this product. Contains allyl alcohol, which is thought to be a by-product during the above reaction process. This allyl alcohol has a strong irritating odor, and its presence in even a small amount can seriously harm the working environment. However, since the boiling point of allyl alcohol is close to that of acetonitrile, it can be easily removed by rectification. It is very difficult to completely remove this. Until now, the methods for removing allyl alcohol from acetonitrile include adding hypochlorite and mineral acid and distilling the mixture (Japanese Patent Publication No. 52-4531), and reacting hydrogen chloride to convert allyl alcohol into allyl alcohol. Known methods include converting it into chloride and then distilling it to separate it (Japanese Unexamined Patent Publication No. 52-156815), and adding sulfuric acid to react and then distilling it (Japanese Unexamined Patent Application No. 51-23218). However, in this treatment method that involves adding chemicals, the equipment used must be made of special materials because these chemicals will attack them, and special considerations must be taken in terms of wastewater treatment. There is a drawback that it must be done. Therefore,
It has become an important issue in this technical field to remove allyl alcohol from acetonitrile with a simple operation without adding any chemicals. As a result of various studies on the purification of crude acetonitrile, the present inventors found that acetonitrile containing allyl alcohol can be extracted and distilled in a rectification column in the presence of water, and acetonitrile can be distilled out from the top of the column along with a small amount of water. It was discovered that it can be easily separated from allyl alcohol present in this, and based on this finding, the present invention was accomplished. That is, in purifying crude acetonitrile containing allyl alcohol, the present invention supplies the crude acetonitrile to the middle part of the rectification column and performs extractive distillation under reduced pressure of 80 to 600 Torr in the presence of water. , provides a method for recovering purified acetonitrile, which is characterized in that acetonitrile is distilled out along with a small amount of water from the top of the column, and allyl alcohol is separated and removed from the bottom of the column along with water and a small amount of acetonitrile. Crude acetonitrile which can be treated according to the process of the invention is, for example, that obtained by ammoxidation of propylene or isobutylene. This crude acetonitrile is usually
Contains 0.3-1.0% allyl alcohol and other impurities 1.0-2.0% by weight, but water 25-70%
When present as a crude acetonitrile aqueous solution containing % by weight, it can be used as a stock solution in the method of the present invention without any particular pretreatment. In the process of the invention it is necessary to subject crude acetonitrile to extractive distillation in the presence of water. In this extractive distillation operation, acetonitrile is recovered from the top of the column as an azeotropic mixture with water or a mixture with a similar composition, and allyl alcohol is concentrated and separated from the bottom of the column as a mixture with water and a small amount of acetonitrile. That is, under such conditions, the specific volatility of acetonitrile and allyl alcohol becomes extremely high, making it possible to separate acetonitrile and allyl alcohol by distillation, which has not been able to be completely separated by conventional distillation methods. The boiling point difference between allyl alcohol (boiling point 96.9℃) and acetonitrile (boiling point 81.5℃) is 15.4℃, while the allyl alcohol-water azeotrope (boiling point 88.9℃) is 15.4℃.
°C) and acetonitrile-water azeotrope (boiling point 76.0
It was completely unexpected that they could be separated so easily, even though the difference in boiling point between the two molecules was as small as 12.9°C, which would make separation more difficult. Although the amount of water in the distillation operation according to the method of the present invention can be arbitrarily selected, the amount of water greatly affects the separation efficiency of allyl alcohol and the recovery efficiency of acetonitrile. In other words, if the amount of water present is the amount required to make acetonitrile and allyl alcohol each an azeotropic composition with water, or an amount close to that amount, the separation efficiency of allyl alcohol will improve and the recovery rate of acetonitrile will be greatly increased. If the amount of water present is less than the amount required to bring acetonitrile and allyl alcohol into an azeotropic composition with water, the separation efficiency of allyl alcohol will decrease, resulting in a reduction in purified acetonitrile. The recovery rate will decrease. Furthermore, if the amount of water present is in large excess compared to the amount required to make acetonitrile and allyl alcohol into an azeotropic composition, the separation efficiency of allyl alcohol will decrease, resulting in a decrease in the recovery rate of purified acetonitrile. . However, under any conditions, the recovery rate of acetonitrile and the separation efficiency of allyl alcohol are contradictory under the same conditions. As mentioned above, the amount of water present greatly affects the separation efficiency of allyl alcohol, that is, the recovery efficiency of purified acetonitrile. However, in industrial implementation, it is necessary to separate the impurity allyl alcohol from crude acetonitrile. It is possible to select the most efficient conditions depending on the composition, use of purified acetonitrile, etc. In addition, in order to perform the separation of allyl alcohol in crude acetonitrile more quickly and efficiently, the method of the present invention is performed under reduced pressure conditions, that is, at a pressure of 80°C.
Extractive distillation at ~600 Torr, preferably 100-200 Torr is required. As described above, by using reduced pressure conditions, the separation efficiency of allyl alcohol is greatly improved and purified acetonitrile from which allyl alcohol has been completely separated can be obtained at a higher recovery rate than when carried out under normal pressure. In the method of the present invention, crude acetonitrile and water are continuously supplied to the middle part of the rectification column, either separately or in a premixed state with water, and an azeotropic composition mixture of acetonitrile and water or a composition close to that is produced. Purified acetonitrile can be continuously recovered by continuously withdrawing a mixture from the top of the column and a mixture of allyl alcohol, water, and a small amount of acetonitrile from the bottom of the column. Next, embodiments of the present invention will be described in accordance with the accompanying drawings. The drawings depict one preferred apparatus for carrying out the invention.
It is an explanatory diagram showing an example, and the raw material mixture is in the introduction pipe 2.
It is supplied to the middle part of the rectification column 1 through. The number of distillation plates in the rectification column 1 is not particularly limited as long as it is 10 or more, but 30 to 50 plates are preferred. Rectification tower 1
Heating steam is sent to the bottom of the column through conduit 3 and heated to 30-95°C, preferably 40-60°C. In this way, the top temperature is 20-80℃, preferably 30-50℃, the pressure is 80-600Torr, preferably
If vacuum distillation is continued under conditions of 100 to 200 Torr,
A mixture of water and acetonitrile is distilled out from the discharge pipe 4 at the top of the column. After cooling, this mixture passes through a reflux tank 6, a part of which is refluxed into the rectification column through a conduit 7, and the remainder is recovered through a branch pipe 8 as purified acetonitrile. On the other hand, a mixture of allyl alcohol, water and a small amount of acetonitrile is extracted from the bottom of the tower through a discharge pipe 5, and a part of it is sent to a reboiler 9.
The steam is regenerated into steam and sent to the rectification column via conduit 3 for heating, and the remainder is drained. In this way, allyl alcohol in crude acetonitrile can be almost completely removed, and acetonitrile can be stably recovered at a recovery rate of 96 to 97.5% or more. On the other hand, the remaining acetonitrile that is not recovered is taken out from the bottom of the column, and good operation is achieved when the acetonitrile content of the bottom liquid is 1 to 5% or more. Next, the present invention will be explained in more detail with reference to Examples. Example 1 In the apparatus shown in the drawing, a rectification column 1 with 50 perforated plates was used, and in the fifth stage from the bottom of the rectification column, 31.2% by weight of water, 0.6% by weight of allyl alcohol, and other impurities were added. Crude acetonitrile containing 1.5% by weight was supplied at a rate of 600g/hour, and the top pressure
By operating at 110Torr, bottom pressure of 210Torr, top temperature of 40℃, bottom temperature of 55℃, and reflux ratio of 2.7,
Purified acetonitrile having the composition shown below was obtained from the top of the column at a rate of 438 g/hour. Component weight% Acetonitrile 89.2 Water 9.2 Allyl alcohol Trace Others 1.6 Example 2 The same crude acetonitrile as in Example 1 was added to Example 1.
The top pressure is 350 Torr and the bottom pressure is 350 Torr.
450Torr, top temperature 50℃, bottom temperature 65℃, reflux ratio
By operating under the conditions of 2.7, acetonitrile having the composition shown below was recovered from the top of the column at a rate of 452 g/hour. Ingredient weight% Acetonitrile 85.8 Water 12.6 Allyl alcohol Trace Other 1.6 Reference example Diameter 500 with 40 sieve trays and reboiler
3.6% water by weight in the lower liquid reservoir of the mm cylindrical distillation column.
After charging 9000 kg of acetonitrile containing 0.3% by weight of allyl alcohol and continuing total reflux for 5 hours,
Distillate was extracted at a rate of 510 kg/hour, and further extracted at a rate of 900 kg/hour from the fourth stage counting from the top of the tower.The composition of the liquid was analyzed every two hours and found to be as follows.

[Table] In addition, the concentrations (wt%) of water and allyl alcohol in each part of the distillation column during this period are shown below.

[Table] As shown, when water is insufficient, allyl alcohol is distributed throughout the distillation column, making it impossible to perform effective separation.

[Brief explanation of the drawing]

The drawing is an explanatory view showing one example of an apparatus suitable for carrying out the method of the present invention, and in the drawing, reference numeral 1 is a rectification column, 2 is a stock solution introduction pipe, 3 is a steam introduction pipe, and 4 is a discharge pipe.

Claims (1)

[Claims] 1. When purifying crude acetonitrile containing allyl alcohol, the crude acetonitrile is supplied to the middle part of the rectification column, and in the presence of water, the crude acetonitrile is heated at a pressure of 80 to
Extractive distillation is performed under reduced pressure of 600 Torr, and acetonitrile is distilled out from the top of the column along with a small amount of water.
A method for recovering purified acetonitrile, which comprises separating and removing allyl alcohol together with water and a small amount of acetonitrile from the bottom of the column. 2. The recovery method according to claim 1, wherein the amount of water present is sufficient to form an azeotropic composition between acetonitrile and allyl alcohol and water.