JPS6133813B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for purifying methacrylonitrile (hereinafter abbreviated as MAN). The purpose is to propose a purification method that can obtain a highly pure product MAN with less contamination of methacrolein and hydrocyanic acid. MAN is produced by a gas phase contact reaction between isobutylene or tert-butyl alcohol, etc., ammonia and oxygen, that is, an ammoxidation reaction. This reactant is mainly MAN and contains methacrolein, hydrocyanic acid, acetonitrile, acrylonitrile, isobutyronitrile, etc. Among these, methacrolein and hydrocyanic acid combine to form unstable methacrolein cyanohydrin (boiling point 95°C, 13mmHg). Therefore, when attempting to separate the reactants by distillation to obtain the product MAN, methacrolein cyanohydrin is produced in the distillation purification process, which is decomposed into methacrolein and hydrocyanic acid again in the subsequent distillation, and is then dissolved in the product MAN. was contaminated, reducing the purity of the product. Methacrolein cyanohydrin produced during the production of MAN has a larger amount of by-product than acrolein produced during the production of acrylonitrile, and because hydration and polymerization reactions are difficult to occur, methacrolein is not consumed and the process is delayed. It remained at high concentrations within the body, posing a particular problem. Regarding this problem, the applicant has previously discovered that the presence of isobutyronitrile promotes the decomposition of methacrolein cyanohydrin, and has installed a high-boiling separation column in the pre-process of the product column that distills and separates the product MAN.
In Japanese Patent Application No. 107098/1987, we proposed a purification method that substantially prevents isobutyronitrile from being brought into the product column.
This method is, for example, the process shown in Figure 1, in which the absorption liquid obtained by absorbing the ammoxidation reaction gas with water is subjected to extractive distillation using water in the recovery column 1 to remove acetonitrile, etc., and the reactant is recovered. do. Hydrocyanic acid and water are removed from this recovered liquid in the prussic acid removal/dehydration tower 2, and further distilled in the high boiling separation tower 3 to remove high boiling substances such as isobutyronitrile from the bottom of the tower, and the top distillate is converted into a product. The feed is fed to column 4 and distilled, and low-boiling substances such as acrylonitrile and methacrolein are extracted from the top of the column, and the product is passed through conduit 5 at the bottom of the column.
This is what gets the MAN. According to this method, there is virtually no isobutyronitrile in the product column, so the decomposition of methacrolein cyanohydrin is suppressed, and the amount of methacrolein or hydrocyanic acid, which is a decomposition product, mixed into the product MAN is reduced. reduce However, it is difficult to completely suppress the decomposition of methacrolein cyanohydrin using only this method, and it is not necessarily easy to obtain a product MAN containing 50 ppm or less of methacrolein, for example. This invention was made in view of the above circumstances. The gist is that the product is produced by distillation from an ammoxide reaction product containing MAN as the main component and methacrolein, hydrocyanic acid, isobutyronitrile, etc.
In the purification method to obtain MAN, a high-boiling separation column is installed upstream of the product column to substantially remove high-boiling substances such as isobutyronitrile. This is a MAN purification method characterized by side-cutting and extracting part of the distilled vapor from the MAN, guiding it to a second product column, extracting the product MAN from the bottom of the column, and returning the top distillate to the product column. Figure 2 is an example of a MAN purification process using this purification method. The crude MAN distilled in the recovery column 1, hydrocyanic acid removal/dehydration column 2, and high-boiling separation column 3 is fed to the product column 4 and distilled while removing low-boiling substances such as acrylonitrile and methacrolein from the top of the column. At this time, a part of the steam is extracted from a position between the feed stage 6 of the crude MAN and the bottom of the column via a conduit 7 and guided to the second product column 8, and the condensate at the bottom of the column is collected as a product.
As a MAN, the distillate from the top of the column is extracted from the conduit 9 and sent to the feed stage 6 of the product column 4 via the conduit 10.
This is a method of returning it to a position between the steam extraction conduit 7 and the steam extraction conduit 7. According to this method, a trace amount of methacrolein cyanohydrin is present in the product column 4, and this decomposes.
Even if methacrolein and hydrocyanic acid are mixed into the bottom liquid,
The vapor is transferred to the second product column, where methacrolein and hydrocyanic acid, which have a lower boiling point than MAN, are concentrated at the top of the column, reducing the amount mixed into the bottom liquid and improving the purity of the product MAN. This will be explained below with reference to examples. The crude MAN distilled in the recovery tower, prussic acid removal/dehydration tower, and high-boiling separation tower is fed into the center of a product tower with 60 trays, and distilled steam is side-cut from the one tray position from the bottom of the column. 2nd shelf with 25 pull-out shelves
The low-boiling substances that were led to the product column and concentrated at the top of the column were recycled to the 25th stage from the bottom of the product column below the feed stage, and the product MAN was obtained from the bottom of the column. At this time, the flow rate of each part of the product tower and the second product tower,
The liquid composition was as shown in Table 1.

[Table] The conventional product MAN consists of side cut steam at the bottom of the product column, and the amount of methacrolein is 150~
250 ppm, methacrolein in the product MAN extracted from the bottom of the second product column using this method could be reduced to 50 ppm or less.

[Brief explanation of the drawing]

Figure 1 is a flow sheet showing a conventional MAN purification process in which a high-boiling separation column is installed before the product column, and Figure 2 is a MAN according to the present invention in which a second product column is installed.
This is a flow sheet of the purification process. 1... Recovery tower, 2... Hydrocyanic acid removal/dehydration tower, 3...
High-boiling separation column, 4... Product column, 5... Conduit, 6...
Feed stage, 7... Conduit, 8... Second product column, 9
... conduit, 10 ... conduit.

Claims (1)

[Claims] 1. In a purification method for obtaining methacrylonitrile product by distillation from an ammoxide reaction product containing methacrolein, hydrocyanic acid, isobutyronitrile, etc. as the main component,
A high-boiling separation column is provided upstream of the product column to substantially remove high-boiling substances such as isobutyronitrile, and a portion of the steam is side-cut from a position between the feed stage and the bottom of the column during distillation of the product column. A method for purifying methacrylonitrile, which comprises taking out the methacrylonitrile and guiding it to a second product column, extracting the product methacrylonitrile from the bottom of the column, and returning the top distillate to the product column.