JPH034057B2 - - Google Patents

Description

[Detailed description of the invention]

This invention is based on methacrylonitrile (hereinafter referred to as MAN
(hereinafter referred to as tertiary butyl alcohol).
This article relates to a method for purifying and recovering MAN. 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 cyanohydrin (boiling point
95℃13mmHg). Therefore, when attempting to obtain the product MAN by distilling the reaction product, methacrolein cyanohydrin is produced in the distillation separation process, which is decomposed into methacrolein and hydrocyanic acid again in the subsequent distillation, and is dissolved in the distillate. Contamination caused the purity of MAN to decrease, making it impossible to obtain a high-purity product. The purification process for MAN is carried out in accordance with the process for acrylonitrile, an example of which is shown in Figure 1. The ammoxide reaction product mainly composed of MAN absorbed in the absorption water was subjected to extractive distillation together with solvent water in the recovery column 1, and the top vapor was condensed in the condenser 2 and separated in the oil-water separator 3. In addition to MAN, the oil layer contains methacrolein,
The recovered liquid contains impurities such as hydrocyanic acid and isobutyronitrile, and a saturated 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 consisting of hydrocyanic acid are separated from the top of the tower. After separation from the organic layer, 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 powders are removed from the top of the column, the bottom liquid is extracted and fed to the product column 7, trace amounts of low-boiling substances and high-boiling substances are removed from the top and bottom of the column, respectively, and products are obtained.
Get MAN from line 8. However, in this process, some of the methacrolein and hydrocyanic acid become methacrolein cyanohydrin, which is removed from the hydrocyanic acid removal/water column 4 and the low-boiling separation column 6.
It cannot be removed in the product column 7 and is mixed into the column bottom liquid.
to go into. This methacrolein cyanohydrin was decomposed into methacrolein and hydrocyanic acid again in the product column 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 the polymerization reaction is difficult to occur, so methacrolein does not disappear and remains at a high concentration in the process. 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,
This is a process in which the entire amount of low-boiling hydrocyanic acid is extracted from the side of the recovery column, and in order to ensure product quality, a large amount of steam is consumed and a distillation column with a large diameter 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 continuous distillation is performed or when cyanohydrin accumulates even during batch distillation, it is necessary to increase the amount of known inorganic or organic acids added. This causes problems in the treatment of these acids and the materials used in the equipment. According to the inventors' study,
In order to be effective in continuous distillation, it is necessary to extract a large amount of the condensing section to prevent the concentration of cyanohydrin from increasing, which is not an economical method. As mentioned above, until now there has been no industrially completed technology for obtaining a highly purified product methacrylonitrile from crude methacronitrile containing methacrolein and hydrocyanic acid. This invention was made in view of the above circumstances, and its purpose is to propose a purification and recovery method that can obtain high quality MAN with extremely low amounts of methacrolein and hydrocyanic acid. The gist of the process is to absorb and recover an ammoxidation reaction product containing MAN as a main component, methacrolein, and hydrocyanic acid using water as a solvent, distill it in a prussic acid removal tower, a dehydration tower, a low-boiling separation tower, and a product tower. In the purification and recovery process to obtain MAN, this MAN recovery and purification method is characterized by introducing the bottom liquid of the low-boiling separation column to a distillation column under reduced pressure and distilling it, and feeding the distillate from the top of the column to the product column. . Figure 2 shows a process to which this recovery and purification method is applied. In this process, a cyanohydrin removal tower 9 is provided attached to the low-boiling separation tower 6, and the bottom liquid of the low-boiling separation towers is distilled under reduced pressure in this removal tower 9.
The top vapor is condensed in the condenser 10 and the distillate is sent to the product column 7.
The bottom liquid is returned to the recovery column 1 through the bottom line 11. The top distillate of the product column 7 is returned to the prussic acid removal/dehydration column 4 via line 12, and the product MAN is extracted from line 8 to obtain it.The bottom liquid is partially blown down and sent to the line. 13 and is returned to the recovery tower 1. This process has the above-mentioned structure, and by distilling under reduced pressure in the cyanohydrin removal column, the bottom temperature is lowered to reduce the decomposition rate of cyanohydrin, suppressing its decomposition, reducing the contamination of decomposed products, and removing it as a gas layer. This makes it possible to remove cyanohydrin, which is a high-boiling substance, to a high degree. In addition, the degree of vacuum in the cyanohydrin removal tower is 400 to 100 mmHg.
A suitable tower bottom temperature is 80 to 40°C. At 80° C. or higher, the effect of inhibiting the decomposition of cyanohydrin is not sufficient, and at less than 100 mmHg, the effect of significantly increasing the decomposition inhibiting effect is not observed, although the equipment cost and proportional cost increase, which is not preferable. Example 1 A hydrocyanic acid/water tower 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. 84 g/Hr is fed to the 45 stages of the boiling separation column, and low-boiling substances such as methacrolein and acrylonitrile are extracted from the top of the column at a rate of 6 g/Hr, and 78 g/Hr are extracted from the bottom of the column.
Crude methacrylonitrile was obtained with Hr. Then 200ml
This crude methacrylonitrile was fed at 200 g/Hr to the bottom of a cyanohydrin removal column (vacuum simple distillation column) equipped with a flask, and the top pressure was 190 mmHg.
The reactor was operated at a bottom liquid temperature of 55° C., and a gas layer containing MAN was extracted from the top of the column at a rate of 196 g/Hr, and a liquid containing cyanohydrin was extracted from the bottom of the column at a rate of 4 g/Hr. This tower top distillate was fed under a pressure of 350 mmHg to 30 stages of a product tower having an inner diameter of 32φ and 55 stages at a rate of 78 g/Hr.
Product MAN at 76g/Hr by side liquid extraction
obtained. Table 1 shows the product MAN and the impurity concentration in the intermediate process liquid.

[Table] Comparative Example 1 The same low-boiling point extraction liquid from the bottoms of the separated columns as in Example 1 was fed to the product column without passing through the cyanohydrin removal column, and the product was operated under the same conditions as in Example 1.
Obtained MAN. Table 2 shows the obtained product MAN and the impurity concentration in the intermediate process liquid.

【table】 [Brief explanation of the drawing]

FIG. 1 is a flow sheet of a conventional MAN purification and recovery process, and FIG. 2 is a flow sheet of a process to which the purification and recovery method of the present invention is applied. 1... Recovery tower, 2... Condenser, 3... Oil-water separator, 4
... Hydrocyanic acid removal/water tower, 4a... Hydrocyanic acid removal tower, 4b... Dehydration tower, 5... Oil/water separator, 6... Low boiling point separation column, 7... Product column, 8... Product MAN take-out line, 9... Cyanohydrin removal column, 10... Condenser, 11... Tower bottom line, 12... Tower top gas line, 13... Tower bottom liquid line.

Claims (1)

[Claims] 1 The ammoxide reaction product containing methacrolein and hydrocyanic acid as a main component is absorbed and recovered using water as a solvent, and distilled in a prussic acid removal tower, a dehydration tower, a low-boiling separation tower, and a product tower to obtain the product methacrylate. In the purification process of methacrylonitrile to obtain lonitrile, the bottom liquid of the low-boiling separation column is led to a distillation column under reduced pressure and distilled, and the distillate from the top of the column is fed to the product column. Nitrile purification method.