JPS6158466B2 - - Google Patents

Description

[Detailed description of the invention]

This invention is based on methacrylonitrile (hereinafter referred to as MAN
It relates to a method for producing MAN (abbreviated as ), and more specifically to a method for producing MAN by an ammoxidation reaction of isobutylene or tert-butyl alcohol. MAN is produced by a gas phase contact reaction between isobutylene, etc., ammonia and oxygen, that is, an ammoxidation reaction. This reaction product is
Mainly composed of MAN, it also contains acetonitrile, methacrolein, hydrocyanic acid, acrylonitrile, isobutyronitrile, etc. Among these by-products, methacrolein and hydrocyanic acid combine to form an unstable high-boiling condensate, methacrolein cyanhydrin (boiling point 95
℃, 13mmHg). Therefore, when attempting to separate the reaction mixture by distillation to obtain the product MAN,
During the separation and purification process, methacrolein cyanohydrin is generated, which decomposes into methacrolein and hydrocyanic acid again during the subsequent distillation and mixes into the distillate, reducing the purity of MAN and making it difficult to obtain a high-purity product. I couldn't help it. Purification of MAN is carried out according to the conventional process for acrylonitrile, an example of which is shown in FIG. The ammoxide reaction product mainly composed of MAN absorbed in the absorbed water is subjected to extractive distillation together with solvent water in the recovery column 1.
The top vapor is condensed in a condenser 2, and the oil layer separated in a decanter 3 becomes a liquid containing MAN, impurities such as methacrolein, hydrocyanic acid, isobutyronitrile, and a saturated dissolved amount of water. This recovered liquid is fed into the middle stage of the hydrocyanic acid removal tower 4a located at the upper part of the hydrocyanic acid removal tower 4, and low-boiling components mainly composed of hydrocyanic acid are separated from the top of the tower. After separation, the organic layer is fed to the upper stage of the lower dehydration tower 4b. The top liquid of the dehydration tower 4b is returned to the bottom of the prussic acid removal tower 4a, and the bottom liquid is extracted and fed to the middle stage of the low boiling point separation tower 6. In the low-boiling separation column 6, low-boiling substances are removed from the top of the column, and the bottom liquid is taken out and fed to the product column 7, where trace amounts of low-boiling substances,
and high-boiling substances are removed from the top and bottom of the tower, respectively,
Get product MAN from line 8. However, a portion of methacrolein and hydrocyanic acid becomes methacrolein cyanohydrin, which cannot be removed in the prussic acid removal/water column 4 and low boiling point separation column 6, and enters the product column 7 as a mixture in the bottom liquid. This methacrolein cyanohydrin was again decomposed into methacrolein and hydrocyanic acid in the product column 7 and mixed into the product MAN, reducing its purity. The reason why methacrolein is particularly problematic during the production of methacrylonitrile compared to acrolein, which is produced as a by-product during the production of acrylonitrile, is that methacrolein produces a large amount of by-product compared to acrolein, and is also susceptible to hydration reactions. This is because methacrolein does not disappear and remains at a high concentration in the process because the polymerization reaction is difficult to occur. Several proposals have been made to solve this problem. For example, Japanese Patent Publication No. 50-23017 proposes removing hydrocyanic acid and carbonyl compounds as a side stream in the middle stage of the recovery column to prevent methacrolein cyanohydrin from being carried into the subsequent distillation process. but,
In the process of extracting the entire amount of low-boiling hydrocyanic acid from the recovery column side, in order to ensure product quality, a large amount of steam is consumed and a large-diameter distillation column is required, increasing equipment costs. Furthermore, when hydrocyanic acid is used, it is necessary to increase the number of stripping pots or to further increase the amount of steam consumed in order to separate impurities. In addition, in order to stabilize cyanohydrin and prevent acrolein and hydrocyanic acid from being mixed into the product due to decomposition, oxalic acid (Special Publication
10112), sulfamic acid or acidic ammonium sulfate (Japanese Patent Publication No. 39-28316), sulfonic acid or aromatic sulfonic acid. However, this method is effective when the cyanohydrin content is low, but when the content is high, especially when continuous distillation is carried out, cyanohydrin accumulates and the content increases.
The effect of improving product purity is low. Japanese Patent Publication No. 43-18126 describes that in the first step, chemicals are added to decompose cyanohydrin, and acrolein and hydrocyanic acid are separated by distillation, and in the second step, chemicals are added to stabilize the remaining cyanohydrin, and cyanohydrin is separated by distillation. I have some suggestions. This method is effective when cyanohydrin is not concentrated during batch distillation, but when performing continuous distillation or when cyanohydrin has accumulated even during batch distillation, it is necessary to increase the amount of known inorganic or organic acids added. This poses problems in the treatment of these acids and the materials used in the equipment. According to studies conducted by the present inventors, in order to obtain an effect in continuous distillation, it is necessary to extract a large amount of the condensing part so as to prevent the concentration of cyanohydrin from increasing, which is not an economical method. As stated above, until now there has been no industrially completed technology for obtaining a highly purified product methacrylonitrile from crude methacrylonitrile containing methacrolein and hydrocyanic acid. This invention was made in view of the above circumstances,
The purpose is to propose a method for producing high-quality MAN with extremely low amounts of methacrolein and hydrocyanic acid. The gist of the project is to absorb and recover an ammoxidation reaction product containing MAN as a main component, methacrolein, and hydrocyanic acid using water as a solvent;
In the MAN production method in which product MAN is obtained by distillation in a dehydration tower, low-boiling separation tower, and product tower, the MAN is characterized in that the bottom liquid of the low-boiling separation tower under reduced pressure is extracted as vapor and fed to the product tower. It is a manufacturing method. Figure 2 shows a process to which this production method is applied. The low boiling point separation column 6 is operated under reduced pressure to suppress the decomposition of methacrolein cyanohydrin mixed at the bottom of the column, and the bottom liquid is passed through the line. Steam is extracted from 9 and fed to product column 7, and the bottom liquid in which high-boiling substances such as cyanohydrin are concentrated is sent to line 1.
It is extracted in liquid form from 0 and returned to the recovery tower 1.
The bottom liquid in which high-boiling substances such as isobutyronitrile in the crude MAN fed to the product column 7 are concentrated is extracted in liquid form from the line 11 and returned to the recovery column 1 while being partially blown down. . Also product
MAN is obtained by draining liquid from line 8. This process has the above-mentioned structure. The low-boiling separation column is operated under reduced pressure to suppress the decomposition of cyanohydrin, and the bottom liquid is extracted as vapor, so that the vapor contains very little cyanohydrin decomposition products and cyanohydrin, which is a high-boiling substance. The product can be fed into the tower. Therefore, the concentration of methacrolein cyanohydrin in the product tower is reduced, and the amount of methacrolein and hydrocyanic acid, which are decomposition products thereof, mixed into the product MAN is reduced, and the purity of MAN is improved. Example 1 A hydrocyanic acid/water column bottoms containing MAN as a main component and containing methacrolein, methacrolein cyanohydrin, acrylonitrile and isobutyronitrile was heated under a pressure of 270 mmHg in a low-pressure tank with an inner diameter of 32φ and 70 trays. The 45 stages of the boiling separation column were fed at a rate of 84 g/Hr, and low-boiling substances such as methacrolein and acrylonitrile were extracted from the top of the column at a rate of 6 g/Hr.
By extruding the gas phase at a rate of g/Hr, crude MAN with reduced high-boiling substances such as cyanohydrin was obtained. The liquid phase of the bottom pot containing high boiling substances such as cyanohydrin is
The system was operated at a high boiling concentration ratio (=feed amount/bottom liquid extraction amount) of 6 times. The liquid taken out from the bottom of the column was recycled to the recovery column feed liquid. The resulting crude MAN is then immediately adjusted to 350 mmHg
of a product column with an inner diameter of 32φ and 55 trays under the pressure of
Feed at 64g/Hr to the 30th stage, remove high boiling substances such as isobutyronitrile, and reduce the product MAN from the 50th stage to 59
Obtained by liquid withdrawal at g/hr. ProductMAN
Table 1 shows the impurity concentrations in the process liquid.

[Table] Example 2 Using the same feed liquid and equipment as in Example 1, the low boiling point separation column was operated at a high boiling point concentration ratio of 3 times.
In other words, the feed amount to the low-boiling separation column is 84g/Hr.
The bottom gas phase of the low-boiling separation column is 50g/Hr, and the liquid phase is 28g/Hr.
It was extracted at g/hr. The obtained column bottom gas phase was then fed to the product column at a rate of 50 g/Hr, and product MBN was obtained at a rate of 46 g/Hr by side liquid withdrawal. Table 2 shows the product MAN and the impurity concentration in the intermediate process liquid.

[Table] Comparative Example The same feed liquid as in Example 1 was used, and the bottom of the low-boiling separation column was operated by removing the entire liquid phase. In other words, the amount of feed to the low-boiling separation column was 84.
The liquid phase was extracted from the bottom of the column at a rate of 78 g/Hr to obtain crude MAN with reduced low-boiling substances. Next, this crude MAN was fed to the product column at 78 g/Hr, and the product MAN was obtained at 71 g/Hr by drawing out the side liquid. Table 3 shows the product MAN and the impurity concentration in the intermediate process liquid.

【table】 [Brief explanation of the drawing]

Figure 1 is a flow sheet of the conventional MAN manufacturing process, and Figure 2 is related to the manufacturing method of this invention.
This is a flow sheet of the MAN manufacturing process. 1... Recovery tower, 2... Condenser, 3... Decanter, 4... Hydrocyanic acid removal/water tower, 4a... Hydrocyanic acid removal tower, 4
b... Dehydration tower, 5... Decanter, 6... Low boiling point separation column, 7... Product column, 8... Liquid line, 9... Gas line, 10... Liquid line, 11... Liquid line.

Claims (1)

[Claims] 1. An ammoxide reaction product containing methacrylonitrile as a main component, methacrolein, and hydrocyanic acid is absorbed and recovered using water as a solvent,
Methacrylonitrile characterized in that in the process of distilling in a dehydration tower, a low boiling point separation tower and a product tower to obtain the product methacrylonitrile, the bottom liquid of the low boiling point separation tower under reduced pressure is extracted as vapor and fed to the product tower. manufacturing method.