JPS5993027A - Method for purifying methacrylic acid - Google Patents

Abstract

PURPOSE:To produce methacrylic acid of high purity and quality, by oxidizing a 4C compound, e.g. isobutylene or tert-butanol, etc. catalytically in the vapor phase, separating the resultant crude methacrylic acid by the two layer separation method, and adding an alkali bisulfite thereto to remove impurities. CONSTITUTION:Isobutylene, tert-tutanol, methacrolein or isobutyl aldehyde is catalytically oxidized in the vapor phase to give methacrylic acid. In the process, a reaction product gas containing the resultant methacrylic acid is cooled and condensed to afford an aqueous solution of the methacrylic acid, which is then extracted and/or distilled to separate water and give crude methacrylic acid. The resultant crude methacrylic acid is then brought into contact with an aqueous solution of a bisulfite in the presence of a water-soluble organic solvent, e.g. benzene or n-hexane, to remove inpurities. The impurities, e.g. high-boiling aldehydes or protoanemonin, form an adduct with the bisulfite. Thus, the methacrylic acid is purified.

Description

[Detailed description of the invention] The present invention relates to a method for purifying methacrylic acid.

Specifically, the present invention provides a method for producing methacrylic acid by a catalytic gas phase oxidation reaction of imbutylene, ternary butanol, methacrolein, or inobutyraldehyde (hereinafter referred to as C4), which provides high purity methacrylic acid,
This article relates to a method for purifying high-quality methacrylic acid.

The method of catalytically oxidizing the above-mentioned Type 04 in a reaction of one stage and two or more id stages to obtain methacrylic acid is already known in many documents, and some of them are being commercialized. However, the oxidation yield has not yet reached a sufficiently high level, and as a result, various by-products are included in the reaction product gas in addition to the target product, methacrylic acid. At present, it is unavoidable that a considerable amount of by-products are mixed into the crude methacrylic acid obtained. The main by-products contained in this crude methacrylic acid include acetic acid, acrolein, acetaldehyde, methacrolein, acrylic acid, and trace amounts of aromatic compounds and tar-like compounds such as terephthalic acid, toluic acid, and benzoic acid. Examples include substances.

Among these, compounds with relatively low boiling points can be separated relatively easily in a conventional distillation process, but the aromatic compounds and tar-like substances mentioned above are separated in various ways during the purification process of crude methacrylic acid. As a cause of trouble, the inventors of the present invention proposed a special method to overcome these corridors.
The methods described in No. 6952 and No. 57-10242 have been proposed. However, although these methods have certainly solved the problem of scale precipitation in the distillation column, they have not completely suppressed the occurrence of polymerization on the heat transfer surface of the distillation column and boiler. It was also found that the quality of the purified methacrylic acid obtained was not completely satisfactory. Polymerization in the reboiler does not occur even in the presence of a considerable amount of polymerization inhibitor, for example, twice the normal amount, and this is caused by tar-like substances (mainly 1. It was found that the product contained high-boiling aldehydes and grotoanemonin, albeit in trace amounts, with regard to quality.

The present inventors investigated various methods for separating and removing high-boiling aldehydes and protoanemonin contained in the crude methacrylic acid, and found that when crude methacrylic acid and an alkali salt of bisulfite are brought into contact, these The present invention has been achieved by discovering that most of the high-boiling aldehydes and protoanemonin contained in the above react easily with alkali bisulfite salts to form water-soluble compounds (considered to be addition reaction compounds).

The present invention is characterized in that aldehydes and glotoanemonin, which are impurities in crude methacrylic acid, are reacted with an alkali bisulfite salt, and these impurities are separated and removed from methacrylic acid. A two-layer separation method is adopted to carry out this process. That is, methacrylic acid is transferred to the oil layer, the reaction product of impurities and alkali bisulfite is transferred to the water layer, high-boiling aldehydes and grotoanemonin are separated and removed from the crude methacrylic acid, and the oil layer is distilled ◆High purity methacrylic acid This method adopts a process that allows for The present invention is thus specified as follows.

Methacrylic acid obtained by cooling and condensing the methacrylic acid-containing reaction product gas discharged from the oxidation reactor in the process of producing methacrylic acid by catalytic gas phase oxidation of isobutylene, butanol, methacrolein, or isobutyraldehyde. A method for purifying methacrylic acid characterized by subjecting an aqueous solution to extraction and/or distillation operations to separate water, and contacting the resulting crude methacrylic acid with an aqueous bisulfite solution in the coexistence of a water-insoluble organic solvent. , the method of the present invention will be explained in more detail.

The crude methacrylic acid referred to in unexploded ψJ refers to a methacrylic acid aqueous solution obtained by a catalytic gas-phase oxidation reaction of C4 group, which is first separated by a dissipation operation to remove light boilers, and then the aqueous solution is directly distilled or azeotropically distilled to remove water. This is the distillation column bottom liquid from which all or part of the solvent has been separated by distillation or the like from the expelled bottom liquid or the methacrylic acid extract obtained by solvent extraction. Therefore, this crude methacrylic acid or its solution usually contains a small amount of acetic acid, acrylic acid, a small amount of aromatic compound, a tar-like substance, and a trace amount of grotoanemonin. In this case, high-boiling aldehydes are 0.5 to 5% higher by weight than methacrylic acid (calculated as tacrolein), and glotoanemonin is 200 to 600 ppm.
It is usually included. The amount of organic solvent contained in the crude methacrylic acid is not particularly limited, but is preferably 50% by weight or less. This is because it is preferable that the organic solvent to be present in the treatment with the following bisulfite aqueous solution be equal to or less than the weight of methacrylic acid. This organic solvent is necessary to separate the two liquids into two layers after mixing, and the amount of coexistence varies depending on the type of organic solvent.
This is because an amount equal to or less than that of methacrylic acid is usually sufficient.

In addition to the above crude methacrylic acid or its solution,
In the present invention, it is possible to use a methacrylic acid solution containing high-boiling aldehydes or protoanemonin, for example, a distillate obtained by applying the bottom liquid after stopping the supply of the extract solution to a light boiling point stripping tower. can.

The bisulfite used in the present invention includes alkali metal salts such as sodium, potassium, and cesium, and ammonium salts, and sodium, potassium, and ammonium salts are particularly preferably used. These salts are usually added in the form of an aqueous solution with a saturation level of 5 to 11 by weight, and the addition point varies depending on the content of target impurities in the crude methacrylic acid solution, but is 0.1 to 15% by weight, preferably 1 to 15% by weight
8% by weight (as sulfite). When adding the crude methacrylic acid solution + CM nitrite aqueous solution, the presence of a water-insoluble organic solvent is essential in order to separate the two layers into two layers. It is necessary to determine the amount of water to be added with the bisulfite so that the solvent layer remains, and the amount of water to be added with the bisulfite is determined so that the proportion of methacrylic acid that migrates to the aqueous layer is 10% by weight or less, preferably 5% by weight or less. -There is a point. therefore,
For this purpose, the amount of organic solvent should range from 10 to 50% by weight,
Preferably (20 to 40 weight-related weight should be increased).

Mixing with the crude methacrylic acid solution can be carried out continuously or batchwise using a mixer such as a line mixer or a stirring mixing tank. After adding an organic solvent as necessary to the methacrylic acid solution and further adding an aqueous solution of bisulfite,
A batch method in which the above three liquids are continuously introduced into a mixer, the mixture is continuously stirred and mixed, and the oil and water layers are continuously taken out from a static tank is also adopted. can do. The mixing temperature ranges from room temperature to 60°C.

The oil layer is then separated from the solvent in a distillation column, and after the light boiling point substances are separated, the high boiling point substances are removed to obtain high purity and high quality methacrylic acid as a distillate. The aqueous layer can still be discharged to the outside of the system, or if necessary, the methacrylic acid dissolved in the aqueous layer can be extracted and recovered with a solvent.

Commonly known organic solvents may be used as the organic solvents used in the present invention. For example, aromatic compounds such as benzene, toluene, xylene, ethylbenzene, inglovir acetate,
Esters such as isobutyl acetate, n-butyl acetate, and methyl methacrylate, hydrocarbons such as n-hexane and n-heptane, and ketones such as intaron and methyl isobutyl ketone can be used, and in particular, the above-mentioned aromatic compounds and Aliphatic hydrocarbons are preferably used.

The purified methacrylic acid obtained from the rice methacrylic acid solution by the method of the present invention has a glotoanemonin content of 20%.
ppm or less, resulting in excellent polymerization properties.

Next, the method of the present invention will be specifically explained with reference to Examples.

Example] A catalytic gas phase oxidation reaction of isobutylene with air in the presence of water vapor using a molybdenum-based composite oxide as a catalyst for the first stage reaction and a catalyst based on a molybdenum-phosphorus heteropolymer as a catalyst for the second stage reaction. The resulting reaction product gas was cooled and condensed using hydroquinone as a polymerization inhibitor to obtain an aqueous solution containing methacrylic acid. Light boiling substances such as methacrolein were removed by distillation from the resulting aqueous solution, and the methacrylic acid aqueous solution was subjected to continuous countercurrent extraction using toluene as an extraction solvent to obtain a methacrylic acid-containing extract. Next, the solvent toluene was separated from this extract in a solvent separation column operated under reduced pressure to obtain a crude methacrylic acid solution. This contained impurities such as 0.1% by weight of acetic acid, 0.14% by weight of acrylic acid, 30% by weight of toluene, and 500 ppm by weight of protoanemonin. To this crude methacrylic acid solution 10 was added 23% by weight (+) bisulfite aqueous solution 11 (9) based on 9, and after stirring well, it was left to stand and separated into two layers. The methacrylic acid concentration therein was 12% by weight.

Next, the oil layer was fed to a distillation column to carry out continuous distillation to separate light boilers, yielding 6.8 KQ as a bottom liquid.

During the distillation period of about 10 hours, there was no drop in the liquid due to polymerization of the bottom liquid. However, a polymerization inhibitor was added to the can liquid so that the phenothiazine concentration was 1000 ppm. Further, this tank liquid was rectified with 15 stages and a reflux ratio of 0.5 to obtain a product with a ratio of 6.5 to 9.

This official methacrylic acid contains protoannenin 1:
j,' p pm, aldehydes (methacrolein equivalent value) 70 weight-jit ppm are included, and the induction period of polymerization is 3
It was 0 minutes. The conditions for measuring the polymerization period are as follows.

Example 2 An aqueous methacrylic acid solution was obtained by distilling off light-boiling substances such as methacrolein by the same method as in Example. Next, the methacrylic acid aqueous solution was subjected to continuous countercurrent extraction using isobutyl acetate as an extraction solvent to obtain a methacrylic acid-containing extract. Further, this extract was subjected to vacuum distillation to separate the solvent, isobutyl acetate, to obtain crude methacrylic acid. This contains impurities such as acetic acid with a concentration of 0.8% and acrylic acid with a concentration of 0.
．． It contained 255% by weight protoannenin, 400ppms by weight, and 0.2% by weight of isobutyl acetate.

To this crude methacrylic acid 10 was added 2.17 Kj of an aqueous bisulfite aqueous solution to 155% by weight based on 9 and mixed, and then 6 Kq of inbutyl acetate was added and mixed with stirring, then allowed to stand to separate into two layers. At this time, the water layer weighed 2.2 kg and contained methacrylic acid concentration of 9 weight cycles.

Next, the oil layer was subjected to vacuum distillation, and continuous distillation was performed for about 10 hours to remove light boiling point substances such as solvents. I was able to get the liquid in the can to 9.0, but there was no liquid in the can. However, a polymerization inhibitor was added to the can liquid so that the phenothianone concentration was 1000 ppm by weight. Further, this bottom liquid was rectified with 15 stages and a reflux ratio of 0.5 to obtain product 8, '6.9. The brodoanenine in the product is 1.5 ppm-aldehyde, 90 weight ppm (in terms of methacrolein), and the induction period of polymerization is 32.
It was a minute.

Comparative Example 1 Crude methacrylic acid was obtained in exactly the same manner as in Example 1. This crude methacrylic acid solution was distilled under reduced pressure in the same manner as in Example 1 without adding an aqueous sodium bisulfite solution to remove light boiling point substances such as the solvent. The liquid in the can at this time was 6.5 ni.
Phenothiazine was also added so that the concentration at 0 degrees was 2000 ppm by weight, but the liquid in the can was cloudy. Further, this bottom liquid was distilled using the same apparatus and under the same conditions as in Example 1. Distillation was stopped when the distillate was reduced to 4.2% because scale due to polymers was attached to the can. The resulting product contained 60 tppm by weight of grotoanemonin and 200 ppm by weight of aldehydes, and the induction period of polymerization was 40 minutes.

Example 3 The distillate from the light boiling point separation column in Example 1 was taken as a crude methacrylic acid solution. The analytical values of this distillate were 82.6% by weight of methacrylic acid, 12.4% by weight of acrylic acid over 3.4M of acetic acid, 1.3% by weight of metaxylene, and 0.21% by weight of protoanemonin. This distillate 10
to 9 to 30 weight ratio i1i sodium sulfite aqueous solution I
Add K, and then add 4 Kq of m-xylene and stir.
After mixing, the mixture was allowed to stand and separated into two layers. The water layer is 1. The methacrylic acid concentration was 111% by weight in IKg.

Further, light boiling point substances and heavy substances were separated from the oil layer using the same equipment and conditions as in Example 1 to obtain a product with 7 HP. The protoannenin content in the product methacrylic acid was 3 ppm by weight, and the induction period of polymerization was 31 minutes.

Patent applicant: Agent of Nippon Shokubai Chemical Co., Ltd.
Tsuyoshi Yamaguchi: ゛Hato゛-zo゛

Claims (1)

[Claims] Methacrylic acid obtained by cooling and condensing the methacrylic acid-containing reaction product gas discharged from the oxidation reactor in the process of producing methacrylic acid by catalytic gas phase oxidation of isobutylene, tertiary-butanol, methacrolein, or isobutyraldehyde. A method for purifying methacrylic acid, which comprises subjecting an aqueous solution to extraction and/or distillation operations to separate water, and contacting the resulting crude methacrylic acid with an aqueous bisulfite solution in the coexistence of a water-insoluble organic solvent.