JP2002179618A - Method for purifying methacrylic acid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for purifying methacrylic acid from its aqueous solution in a stably operable way for a long period. SOLUTION: This method for purifying methacrylic acid comprises the step of adding an N-oxyl compound as the polymerization inhibitor and an azeotropic solvent capable of forming an azeotropic mixture with water to an aqueous solution of methacrylic acid followed by performing a distillation and then separating and removing water from the azeotropic mixture.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for purifying methacrylic acid from an aqueous solution of methacrylic acid while preventing polymerization of methacrylic acid. Especially when removing water from the aqueous solution of methacrylic acid obtained by the dehydration reaction of 2-hydroxyisobutyric acid, it effectively prevents the polymerization of methacrylic acid in the distillation column, and thus the blockage due to the polymer, and stabilizes it for a long time. And a method for purifying methacrylic acid.

[0002]

2. Description of the Related Art It is known that methacrylic acid is produced by a dehydration reaction of 2-hydroxyisobutyric acid. For example, JP-B-44-5859 and JP-B-63-45659 can be cited.

It is also generally known that methacrylic acid has a property of being easily polymerized by light or heat. Therefore, in order to prevent polymerization during the methacrylic acid production step or during storage, various polymerization inhibitors are added to the monomer either alone or in combination of several kinds.

The use of N-oxyl compounds as one of the polymerization inhibitors for preventing the polymerization of methacrylic acid and effectively stabilizing methacrylic acid with the above polymerization inhibitors has been proposed. For example, in Japanese Patent Publication No. 45-1054,
Tertiary butyl nitroxide or 4-hydroxy-2,2,6,6
-A method using an N-oxyl compound such as tetramethylpiperidinooxyl alone, Japanese Patent Publication No. 54-3853,
A method using 2,2,6,6-tetramethylpiperidinooxyl or 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl as a polymerization inhibitor when producing methacrylic acid from methacrolein Etc. are described.

However, according to the study of the present inventors,
In the methacrylic acid production and purification step by the 2-hydroxyisobutyric acid dehydration reaction, even if a normal amount of these effective N-oxyl compounds is used, the polymerization cannot be completely suppressed, and the polymerization of the N-oxyl compound is prohibited. In order to sufficiently exert the effect, a large amount of a polymerization inhibitor is required, and it has been difficult to use it in an actual device.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and a method for distilling and purifying methacrylic acid from an aqueous methacrylic acid solution obtained by a dehydration reaction of 2-hydroxyisobutyric acid is disclosed. It is an object of the present invention to find a method for effectively preventing the polymerization of methacrylic acid in the reactor and performing a stable distillation for a long period of time.

[0007]

Means for Solving the Problems As a result of intensive studies by the present inventors to solve the above problems, unreacted 2-hydroxyisobutyric acid and by-products derived from 2-hydroxyisobutyric acid are added to methacrylic acid in the presence of water. When contained, it was found that it was extremely easy to polymerize, and by using an N-oxyl compound as a polymerization inhibitor and using an azeotropic agent that forms an azeotropic mixture with water, water and methacrylic acid could be quickly removed at low temperature. It has been found that the above problem can be solved by separating.

That is, in the present invention, when methacrylic acid is purified from an aqueous solution of methacrylic acid obtained by a dehydration reaction of 2-hydroxyisobutyric acid, an N-oxyl compound is added as a polymerization inhibitor and an azeotropic solvent with water is used. Thus, it has been found that what has heretofore been impossible for long-term continuous distillation becomes possible, and the present invention has been accomplished. Hereinafter, the present invention will be described in detail.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described in detail. The composition of the reaction solution obtained by the dehydration reaction of 2-hydroxyisobutyric acid varies depending on the catalyst and reaction conditions used, but contains 50 to 80% of methacrylic acid, 20 to 50% of water, and many other by-products. .

An embodiment of the present invention will be described with reference to FIG. The methacrylic acid aqueous solution is introduced into the dehydration distillation column (A) via the conduit 1. In the dehydration distillation column, an azeotropic solvent is introduced from the conduit 3 as necessary, and an azeotropic solvent, a polymerization inhibitor, water and a portion of methacrylic acid-containing top gas are generated from the top of the column by azeotropic dehydration distillation. This is cooled to obtain an overhead distillate. The distillate is separated into two phases, an organic solvent phase and an aqueous phase. The organic solvent phase containing methacrylic acid (azeotropic solvent phase) is refluxed by the conduit 4, and all aqueous phases are discharged out of the system via the conduit 5. Is done. The bottom liquid of the dehydration distillation column is introduced into the methacrylic acid purification step through the conduit 6. At this time, the water concentration in the bottom liquid is controlled by the reflux amount of the azeotropic solvent. The reflux amount of the azeotropic solvent is determined from the azeotropic composition of water and the azeotropic solvent, and it is possible to perform dehydration distillation until the mass concentration of water in the bottom liquid is 0.01% or less.

The azeotropic solvent which forms an azeotropic mixture with water used in the present invention is preferably one which gives an azeotropic point lower than the azeotropic point of water-methacrylic acid and can be separated from water quickly. For example, toluene, ethylbenzene, heptane, hexane, xylene, a mixture thereof and the like can be mentioned, but xylene is particularly preferable. As xylene
Any of o-xylene, m-xylene, p-xylene or a mixture of these isomers may be used.

By using these azeotropic solvents,
The azeotrope of water-methacrylic acid is suppressed. Therefore, when an azeotropic dehydration distillation column is used, the mass concentration of methacrylic acid contained in the distillate at the top of the azeotropic dehydration distillation column is reduced, and the distillate is entrained in the phase-separated aqueous phase. The amount of methacrylic acid discharged to the outside can be reduced.

Since the azeotropic solvent has a lower azeotropic boiling point with water than the azeotropic boiling point of water-methacrylic acid, the azeotropic dehydration distillation column is operated at a lower operating temperature than when methacrylic acid is used as the azeotropic solvent. The methacrylic acid, which can be operated and is extremely polymerizable, can be subjected to dehydration distillation without exposing it to high temperatures. Furthermore, since the N-oxyl compound used as a polymerization inhibitor is returned to the column together with the azeotropic solvent, even in an azeotropic separation column in which polymerization of methacrylic acid is extremely likely to occur due to the inclusion of water or other acids, In order to sufficiently exhibit a stabilizing effect with a small amount used without requiring a large amount of polymerization inhibitor,
The operation of purifying methacrylic acid can be stably performed over a long period of time.

The N-oxyl compound used in the present invention includes 2,2,6,6-tetramethylpiperidyloxyl and
And hydroxy-2,2,6,6-tetramethylpiperidinooxyl.

2,2,6,6-tetramethylpiperidyloxyl or 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl may be used alone or in combination of two or more. Even when used in combination with another polymerization inhibitor, for example, a phenolic polymerization inhibitor, a more excellent polymerization prevention effect may be obtained due to the synergistic effect of these polymerization inhibitors.

When the polymerization inhibitor is supplied into the distillation column, the supply place of the distillation column is not particularly limited, and a solid or powder may be directly added, or may be added in a form dissolved in another solvent. You may. These are supplied by dissolving them in an aqueous methacrylic acid solution supplied to the distillation apparatus or an azeotropic agent refluxed in the column, or from the top of the azeotropic distillation column.

When used in combination with molecular oxygen, the effect of preventing polymerization is further improved. As the molecular oxygen, oxygen itself or a substance diluted with an inert gas such as nitrogen may be used, but a method using air is economically preferable. Regarding the method of supplying molecular oxygen, it is common to indirectly mix molecular oxygen into methacrylic acid by bubbling from the bottom of a distillation column or a stripper, or from a reboiler.
The molecular oxygen is 0.1 to 0.1 with respect to the distillation vapor amount of methacrylic acid.
It is preferable to supply about 1.0% by volume.

The weight of the N-oxyl compound used in the present invention is 1 to 1000 ppm, more preferably 5 to 50 ppm, based on the weight of methacrylic acid in the methacrylic acid-water separation tower.
It is. If the amount is too small, a sufficient polymerization inhibiting effect cannot be obtained,
Too much is not economical. The aqueous solution of methacrylic acid applied to the method of the present invention is not limited to one obtained by a dehydration reaction of 2-hydroxyisobutyric acid, but may be an aqueous solution of methacrylic acid of any origin.

[0019]

According to the present invention, the azeotropic solvent forming an azeotrope with water and the use of a polymerization inhibitor comprising an N-oxyl compound can be used for a long-term continuous operation of the production apparatus. become.

[0020]

EXAMPLES The present invention will be described in more detail with reference to the following examples. However, the following examples do not limit the present invention, and all modifications that do not depart from the spirit of the present invention will be described below. Included in the scope. Here, ppm is based on weight. Example 1 A packed tower having a 1000 ml SUS 316L kettle at the bottom of the tube, a reflux tank at the top, and a raw material supply pipe at the center was used. The operating state during steady-state operation is that the top temperature of the azeotropic separation tower is
31 ° C, tower low temperature 91 ° C, tower top pressure 6.8kPa. An aqueous methacrylic acid solution (containing 20 ppm of 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl) obtained by the dehydration reaction of 2-hydroxyisobutyric acid is azeotropically dehydrated using m-xylene as an azeotropic solvent. The distillation was run continuously for 3 months. After the operation was stopped, in order to measure the amount of the polymer in the azeotropic separation column, the amount of polymethacrylic acid eluted in methanol after the total reflux with methanol was quantified. Table 1 shows the results.

Example 2 An azeotropic distillation operation of an aqueous methacrylic acid solution was carried out in the same manner as in Example 1 except that p-xylene was used as an azeotropic solvent. Was measured in the same manner as in Example 1. Table 1 shows the results.

Example 3 An azeotropic distillation operation of an aqueous methacrylic acid solution was carried out in the same manner as in Example 1 except that ethylbenzene was used as an azeotropic solvent. The product was measured in the same manner as in Example 1. Table 1 shows the results.

Comparative Example 1 An azeotropic distillation operation of an aqueous methacrylic acid solution was carried out in the same manner as in Example 1 except that 40 ppm of p-methoxyphenol was used as a polymerization inhibitor. The polymer inside was measured in the same manner as in Example 1.
Table 1 shows the results.

Comparative Example 2 In Example 1, phenothiazine 20p was used as a polymerization inhibitor.
An azeotropic distillation operation of the methacrylic acid aqueous solution was performed in the same manner as in Example 1 except that pm was used. After the operation was stopped, the amount of the polymer in the azeotropic separation column was determined by the same method as in Example 1. Table 1 shows the results.

Example 4 In Example 1, 4-hydroxy-2,2,2 was used as a polymerization inhibitor.
An azeotropic distillation operation of an aqueous methacrylic acid solution was performed in the same manner as in Example 1 except that 20 ppm of 6,6-tetramethylpiperidinooxyl, 40 ppm of p-methoxyphenol, and 20 ppm of phenothiazine were used. The polymer in the column was measured in the same manner as in Example 1. Table 1 shows the results.

Example 5 In Example 1, 4-hydroxy-2,2,2 was used as a polymerization inhibitor.
An azeotropic distillation operation of an aqueous methacrylic acid solution was carried out in the same manner as in Example 1 except that 20 ppm of 6,6-tetramethylpiperidinooxyl, 40 ppm of p-methoxyphenol and 20 ppm of phenothiazine were used. The polymer in the column was measured in the same manner as in Example 1. Table 1 shows the results.

Table 1 Polymerization inhibitor azeotropic solvent Continuous operation Polymethacryl 4H-TEMPO MQ PTZ Days Acid amount (g) (ppm) (ppm) (ppm) Example 1 20--m-xylene 90 1.5 Example 2 20 - - p-xylene 90 1.8 example 3 20 - - ethylbenzene 90 2.1 Comparative example 1 - 40 - m-xylene 25 180.7 Comparative example 2 - - 20 m-xylene 47 156.2 example 4 20 40 - m-xylene 90 0.8 embodiment Example 5 20 40 20 m-xylene 90 1.1 4H-TEMPO: 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl MQ: p-methoxyphenol PTZ: phenothiazine

[0028]

[Brief description of the drawings]

FIG. 1 is a diagram showing a flow when distillation is performed using a distillation column.

Claims (6)

[Claims] 1. An aqueous methacrylic acid solution is added with an N-oxyl compound as a polymerization inhibitor and an azeotropic solvent which forms an azeotropic mixture with water, and distilled to separate and remove water as an azeotropic mixture. A method for purifying methacrylic acid, comprising: 2. An aqueous methacrylic acid solution obtained by a dehydration reaction of 2-hydroxyisobutyric acid.
A method for purifying methacrylic acid according to the above. 3. The method for purifying methacrylic acid according to claim 1, wherein the distillation is performed in a distillation column. 4. The method for purifying methacrylic acid according to claim 3, wherein the azeotropic mixture is separated into an aqueous layer and an organic solvent layer, and then the organic solvent layer composed of the azeotropic solvent is reused as the azeotropic solvent. 5. The method according to claim 1, wherein the N-oxyl compound is 2,2,6,6-tetramethylpiperidinooxyl or 4-hydroxy-2,2,6,6-tetramethylpiperidinooxyl. Methacrylic acid polymerization inhibitor composition 6. The purification method according to claim 1, wherein the azeotropic solvent is xylene.