JPS6133811B2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to methacrylonitrile (hereinafter referred to as MAN
More specifically, it relates to a purification method for removing by-product isobutyronitrile from an ammoxide reaction product such as isobutylene or tert-butyl alcohol.

MAN is produced by a gas phase contact reaction, that is, an ammoxidation reaction, between isobutylene, etc., ammonia and oxygen. In addition to MAN, this reactant contains acetonitrile, methacrolein, hydrocyanic acid, isobutyronitrile, etc.

In particular, when isobutyronitrile is mixed into the product MAN, it becomes isobutyric acid without being separated when MAN is used as methyl methacrylate, remaining in the methyl methacrylate and reducing its purity, which poses a problem. In addition, when attempting to separate and purify the product MAN by distilling the ammoxidation reaction product using water as a solvent, methacrolein and hydrocyanic acid combine to form unstable methacrolein cyanohydrin (boiling point 95°C, 13 mm 1 fg). However, it is known that this decomposes in the subsequent distillation process and generates methacrolein and hydrocyanic acid, which are mixed into the product MAN and reduce the purity.In this case, isobutyronitrile accelerates the decomposition of cyanohydrin and causes the production of MAN. It was found that this resulted in a decrease in purity.

Regarding this point, the applicant has previously stated that
57-107098, the presence of isobutyronitrile accelerates the decomposition of cyanohydrin, and the product
By installing a high-boiling separation column in the pre-process of the product column that distills and separates MAN and substantially removes isobutyronitrile, the decomposition of cyanohydrin is suppressed in the process of distilling and separating product MAN, It has been disclosed that the amount of methacrolein or hydrocyanic acid mixed into the product MAN can be reduced.

Generally, when producing MAN by an ammoxidation reaction, isobutyronitrile is present in the reaction product recovery liquid at a concentration of several thousand ppm, and in order to effectively suppress the decomposition of cyanohydrin, it must be fed to the product column. The isobutyronitrile concentration of the solution
It has been found that it is necessary to set the content to 100 ppm or less, more preferably 50 ppm or less.

However, in order to reduce the isobutyronitrile concentration to 100 ppm or less, a distillation column with a large number of plates was required, and the burden of distillation removal was considerable.

This invention was made in view of the above-mentioned problems, and its purpose is to provide a method for purifying high-purity MAN that can minimize the amount of isobutyronitrile mixed in the product MAN and at the same time reduce the amount of methacrolein and hydrocyanic acid mixed in. It is to propose. Its main component is methacrylonitrile, and contains by-products such as methacrolein, hydrocyanic acid, and isobutyronitrile. In separating and purifying the product methacrylonitrile by distilling the ammoxidation reaction product using water as a solvent,
A high-boiling separation column (to remove high-boiling substances such as isobutyronitrile) is installed in the pre-process of the product column that separates the product methacrylonitrile by distillation, and the liquid to be fed to the product column is separated between the feed stage of the high-boiling separation column and the column. This is a method for purifying methacrylonitrile, which is characterized in that the isobutyronitrile is taken out as a side stream from the stage where the concentration of isobutyronitrile between the top and the top is substantially the lowest.

In the MAN purification process using water as a solvent, when a MAN-based liquid containing a small amount of water is distilled in a high-boiling separation column installed in the pre-process of the product column, isobutyronitrile (boiling point 103.5℃) is produced from MAN. Since it is a high-boiling substance, it is concentrated at the bottom of the column, but in the MAN condensing section at the top of the column, isobutyronitrile exhibits the opposite movement of a low-boiling substance, and does not reach its lowest concentration at the top of the column. It was found that the lowest concentration occurs at the stage between
We have arrived at this invention.

FIG. 1 shows a purification process according to this purification method, in which a high-boiling separation column 1 is provided upstream of a product column 2. Although not shown in the figure, the ammoxidation reaction product is absorbed into absorption water in an absorption tower, and the absorption liquid is distilled and recovered in a recovery tower using water as a solvent, passes through a prussic acid removal/dehydration tower, and then passes through conduit 3 to 40 to 60 stages. It is fed to the high boiling separation column 1. High-boiling substances, mainly isobutyronitrile, are extracted from the bottom of the tower, and the concentration of isobutyronitrile is lowest at a position above feed position 3 (stages 10 to 20 counting from the bottom) and below the top of the tower. A liquid mainly consisting of MAN is extracted as a side stream from four trays (5 to 15 counting from the top) and fed to the product column 2. In the product column 2, low boiling substances such as acrylonitrile and methacrolein are taken out from the top of the column, trace amounts of high boiling substances remaining from the bottom of the column are removed, and product MAN is obtained from a conduit 5 near the bottom of the column.

Example: 1.07 (ton/ton) for a plate tower with 49 stages in total.
MAN) isobutyronitrile concentration of 5000ppm
A liquid mainly consisting of MAN was fed into the 12th stage from the bottom and heated and distilled with steam at 1.0 (ton/ton MAN). The isobutyronitrile concentration distribution inside the tower is as shown in Figure 2, with approximately 150wt at the top of the tower.
ppm, the lowest value was 50 ppm at the 42nd step from the bottom. By taking out the liquid as a side stream from this stage, a liquid containing 50 wt ppm of isobutyronitrile was supplied to the product column, and a highly pure product MAN could be obtained.

Comparative Example The operation was carried out under the same conditions as in the previous example except that the top of the column was used for extraction to the product column. At this time, the isobutyronitrile concentration in the feed liquid to the product column was 120wt.
ppm, which was increased from 50wt ppm in the example.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet diagram of the high-boiling separation column and product column according to this purification method, and FIG. 2 is a correlation graph between the trays of the high-boiling separation column and the isobutyronitrile concentration of the example. 1... High boiling separation column, 2... Product column, 3... Conduit, 4... Tray with the lowest isobutyronitrile concentration,
5... Conduit.

Claims (1)

[Claims] 1 In separating and purifying the product methacrylonitrile by distilling an ammoxide reaction product containing methacrolein, hydrocyanic acid, and isobutyronitrile as a by-product using water as a solvent, the product A high-boiling separation column (to remove high-boiling substances such as isobutyronitrile) is installed in the pre-process of the product column that separates methacrylonitrile by distillation, and the liquid to be fed to the product column is passed through the feed stage of the high-boiling separation column and the top of the column. A method for purifying methacrylonitrile, characterized in that it is taken out as a side stream from a stage where the concentration of isobutyronitrile is substantially the lowest between.