JPS62120341A - Purification of methacrylic acid - Google Patents

Abstract

PURPOSE:Isobutylene, tert.-butanol, methacrolein or isobutyl aldehyde is catalytically oxidized with O2 in the presence of steam, and the resultant methacrylic acid is efficiently purified by combination of treatment with a semipermeable membrane and extraction with isopropyl acetate. CONSTITUTION:The catalytic oxidation of isobutylene, tert.-butanol, methacrolein or isobutyl aldehyde is carried out with an oxygen-containing gas in the presence of steam, the resultant reaction mixture gas (containing acetic acid as a major impurity) is cooled down to form aqueous methacrylic acid. The solution is brought into contact with a semipermeable membrane at 5-50 deg.C at 20-60kg/cm<2>. The filtrate is extracted with isopropyl acetate to collect methacrylic acid acetic acid simultaneously and methacrylic acid is purified.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] This invention relates to an improved method for purifying methacrylic acid. More specifically, isobutylene, tertiary butanol, methacrolein or inbutyraldehyde F
: A method for efficiently purifying methacrylate obtained by catalytic oxidation with a molecular oxygen-containing gas in the presence of water vapor using a combination of semi-permeable membrane treatment with MIFr and extraction with isopropyl acetate.

[Prior art and problems to be solved by the invention] Isobutylene, tertiary butanol, methacrolein, or isobutyraldehyde (hereinafter referred to simply as raw materials) are obtained by gas phase catalytic oxidation in one or two stages of reaction. The methacrylic acid obtained can be made into a highly pure product by conventional purification means such as extraction and distillation.

However, when implementing these methods industrially, there are many problems in terms of operability and economy. That is, when producing methacrylic acid from the above raw materials, the oxidation reaction product gas contains, in addition to methacrylic acid, methacrolein, carbon dioxide, carbon dioxide, unreacted 04 hydrocarbons including water vapor, oxygen, Furthermore, by-products such as aldehydes, ketones, organic acids, and polymers are included.

This oxidation reaction product gas is generally cooled with water to form an aqueous solution of methacrylic acid, then methacrylic acid is extracted using an organic solvent, and the organic solvent is removed and recovered from the extract to obtain methacrylic acid. There are already many documents regarding this method.

In many cases, the main purpose of these methods is to selectively extract the main product, methacrylic acid, but the main by-product, acetic acid, is also extracted at the same time. A method of using it as a collection solvent for methacrolein has also been proposed (Japanese Patent Application Laid-open No. 59-93028).

Since acetic acid by-produced during the production of methacrylic acid by gas-phase air oxidation amounts to about 10 to 30% of the methacrylic acid, various proposals have been made to recover this acetic acid. in this way,
The latter proposal, in which acetic acid is extracted at the same time as methacrylic acid, is an economical method both in terms of equipment and calorific value.When obtaining the methacrylic acid aqueous solution, many of the above-mentioned by-products are also extracted together with methacrylic acid and water. Condensed and collected. Among these, substances with relatively low boiling points can be easily removed by distillation, etc., and aromatic carboxylic acids with high boiling points and low solubility, such as terephthalic acid, are mostly precipitated as solids, so they can be removed by appropriate means. Can be removed separately. However, in addition to these aromatic carboxylic acid residues, maleic acid, citraconic acid, and polymers of methacrolein and methacrylic acid (hereinafter referred to as nonvolatile residues) are dissolved in the methacrylic acid aqueous solution, and methacrylic acid During liquid-liquid extraction of acid, this appears as scum at the liquid-liquid interface, contamination and blockage by cool substances in the extraction tower, and/or precipitation of carbon-like substances on the raffinate phase side and blockage of piping. My driving is impaired.

In particular, when using a solvent that precipitates methacrylic acid and acetic acid at the same time, such as isopropyl acetate, the present inventors found that due to the strong extraction power of the solvent, not only methacrylic acid and acetic acid but also the non-volatile residue is used. 50-6 minutes
0% is extracted, which not only generates a large amount of scum at the liquid-liquid interface, but also causes polymer formation and blockage in the distillation column boiler and column bottom in the next solvent recovery step. It was discovered that

Possible methods for solving this problem include preventing or suppressing the generation of non-volatile residues that are thought to be the causative agent, or removing generated non-volatile residues as a symptomatic treatment. As for the former proposal, for example,
8-10452, JP-A-48-26717, JP-A-49
-104884, JP-A-49-54317, etc. However, as stated in the specification of JP-A-52-48609, it is virtually impossible to completely suppress the generation of non-volatile residues. be.

On the other hand, the latter proposals include (1) a method of treating an aqueous methacrylic acid solution with activated carbon or a special ion exchange resin before extraction (Japanese Patent Application Laid-Open No. 50-52021), and (2) a method in which methacrylic acid is extracted by the extraction operation. In order to prevent non-volatile residue from precipitating in the raffinate phase (aqueous phase),
A method in which additives are added in advance to form a water-soluble adduct for the purpose of increasing I. (Unexamined Japanese Patent Publication No. 50-151805, No. 58-1
28337, 58-99434, 59-44337
, 59-44338), (3) A method for forcibly generating scum and separating it in a furnace before extraction (Japanese Patent Laid-Open No. 54-68617)
, 56-16438, 56-79634) , (4
) A method of selectively dissolving and removing the reaction product gas with hot water before making it into an aqueous solution of methacrylic acid (JP-A-52-4860)
9) etc.

However, method 1) complicates the process, causes various troubles during long-time operation, and generates a lot of waste water for cleaning the equipment, so it is difficult to adopt method 2 industrially. Although the method simplifies the operation, even if highly acidic aromatic carboxylic acids etc. can be modified with alkali, it is difficult to sufficiently modify many polymers, and therefore the above problems cannot be fundamentally solved; In addition, in the study of the present inventors,
There are still some difficulties in industrial use, such as a new problem of alkali-modified substances being partially entrained in the organic phase and precipitated, which causes pipe blockage. Although method (3) requires sufficient extraction of methacrylic #i to generate the entire amount of scum, the effect is only small (satisfactory) because the solvent is mixed in one step. Not only the processing but also the operation is complicated. (4) Since method 4 is a treatment before quenching the reaction product gas, there is no means for removing the polymer newly generated during quenching. The present inventors have determined that the amount of polymer produced during the reaction process and that produced during the subsequent quenching process are about 50/50. Therefore, it cannot be a fundamental countermeasure for long-term extraction operations.

[Means for solving problems]

The present inventors focused on this point and developed a method for purifying methacrylic acid! As a result of extensive studies, we have found that by bringing the above methacrylic acid aqueous solution into contact with a semipermeable membrane made of polysulfone, polyacrylonitrile, or polyamide under pressure for membrane treatment, we can achieve a high permeability and eliminate an exclusion rate of 80% or more. Obtaining a permeate from which non-volatile residues have been removed and extracting it liquid-liquid eliminates contamination and clogging in the extraction column as described above, and also allows for recovery of isopropyl acetate, which is the extraction agent, in the distillation column. The present invention has been completed based on the discovery that polymerization, formation, and blockage of polyurethane particles do not occur.

That is, the method for purifying methacrylic acid of the present invention uses isobutylene,
A methacrylic acid aqueous solution obtained by catalytically oxidizing tertiary butanol, methacrolein or isobutyraldehyde with a molecular oxygen-containing gas in the presence of water vapor and cooling the reaction product gas containing vinegar M as the main impurity from the catalytic oxidation. This method is characterized in that methacrylic acid and acetic acid are simultaneously extracted by contacting a semipermeable membrane under pressure and using isopropyl acetate as an extraction agent from the semipermeable membrane permeate.

The aqueous methacrylic acid solution in the present invention is obtained by catalytically oxidizing the raw material in one or two stages with a molecular oxygen-containing gas in the presence of water vapor, and the methacrylic acid aqueous solution is obtained by catalytically oxidizing the raw material in one or two stages with a molecular oxygen-containing gas in the presence of water vapor. It is used in the form of an aqueous solution containing an acid or an aqueous methacrylic acid solution obtained by removing low-boiling components such as acetone, ketones such as methacrolein, and aldehydes from the aqueous solution by stripping or the like. However, in consideration of the life span of the semipermeable membrane, an aqueous methacrylic acid solution from which low-boiling components have been removed is desirable.

The semipermeable membrane used to treat the methacrylic acid aqueous solution in the present invention preferably has a molecular weight cut-off of 500 to 2,000.
0. This semipermeable membrane is made of polysulfone,
Preferably, those made of polyacrylonitrile or polyamide... In the present invention, the methacrylic acid aqueous solution is generally
The semipermeable membrane is contacted under pressure at ~50°C.

In general, the higher the treatment temperature, the larger the amount of permeate can be obtained. Considering the stability of the methacrylic acid aqueous solution and the temperature of the subsequent extraction treatment with inpropyl acetate,
Temperatures higher than 5° C. are not preferred, and temperatures lower than 5° C. are not preferred because the amount of permeate is small from a practical standpoint, and the extraction efficiency is also lowered as an extraction treatment temperature.

The pressure in membrane treatment varies depending on the form of the semipermeable membrane used.
0 to 60 q/cm (gauge pressure, below); if the pressure is too low, the liquid permeation rate will be low, while if the pressure is too high, the membrane may be easily consolidated or damaged. I don't like it because it is.

Furthermore, in the present invention, the methacrylic acid aqueous solution is continuously circulated through the semipermeable membrane under the above conditions until at least 50% or more, preferably 70 to 98%, of the methacrylic acid aqueous solution is recovered as the membrane permeate. It is desirable to pressurize the contact while keeping the water flowing.If necessary, add water appropriately to compensate for the permeation.The flow velocity with respect to the membrane surface is set to 0.
．． The speed is preferably 5 to 8 m/sec, preferably 1 to 4 m/sec.

If the linear velocity is too low, the concentration polarization of impermeable components on the membrane surface becomes large, which impedes the permeation of methacrylic acid, and if it is too high, the power cost of the pump increases undesirably.

In this way, the methacrylic acid aqueous solution from which most of the non-volatile residue has been removed is free from the above-mentioned problems in the solvent extraction process and the solvent recovery distillation process, so it is selective to the target components methacrylic acid and acetic acid. Isopropyl acetate, which is a high extraction solvent, can be used initially.

The advantages of isopropyl acetate as an extraction solvent are described in JP-A-59
As described in the specification of Japanese Patent No. 93028, under the conditions that extract 99% or more of methacrylic acid, 70% or more of acetic acid can also be extracted.

Isopropyl acetate has a large partition coefficient of 0.7 in an aqueous solution with an acetic acid concentration of 5%, but its solubility in water is low (2.9%,
20°C), so the cost of recovering from the raffinate water is low6.Also, the boiling point is 89°C, which is a large difference from the boiling point of acetic acid, and since it does not create an azeotrope, it is easy to separate the solvent.

Among the same acetic esters, ethyl acetate has a high solubility in water and is easily hydrolyzed by water and odour, making it undesirable.Also, n-propyl acetate has a higher boiling point than isopropyl acetate, approaching the boiling point of acetic acid, which is undesirable. Both butyl and isobutyl acetate have boiling points close to acetic acid, making it difficult to separate them from acetic acid.

〔Example〕

Next, the present invention will be explained by examples.

Example 1 Using a heteropolyacid-based oxidation catalyst, methacrolein was catalytically oxidized in the vapor phase in the presence of sudden air and water vapor. The reaction product gas was cooled to 250°C in an indirect cooler, and then transferred to a quenching tower. introduced into.

The quenching tower has a tower diameter of 6 inches, a height of 3 m, and a Raschig ring of 2 m.
A part of the oxidation reaction product condensed in the tower is cooled by a cooler and then circulated to the top of the tower, where it is brought into parallel flow contact with the reaction product gas, and the target product, methacrylic acid, is condensed and collected. I did it. The obtained aqueous solution of methacrylic acid contained 21.0 parts of methacrylic acid, 3.7 parts of a polymeric substance, and 0.5 parts of an aldehyde.
1 part, 6.0 parts of organic acids, and the remainder water. This methacrylic acid aqueous solution was applied to a polyamide membrane flat module with a membrane area of 0.2rrf and a molecular weight cutoff of 500 at 40°C for 101/Hr.
The treatment was carried out at an operating pressure of 40 q/c. The amount of permeated liquid was 6511 tri, and the composition of the permeated liquid was 19.2 parts of methacrylic acid, 0.05 parts of high molecular weight substances, 0.5 parts of aldehydes, and 5.0 parts of organic acids. The obtained aqueous solution was supplied to the upper part of a methacrylic acid extraction column (column diameter: 75 mm, rotating disk field with 70 disks) at a rate of 2.8 to 9/Hr. Inpropyl acetate was supplied from the bottom of the column at 2.2 Kq/Hr to extract methacrylic acid.

Extraction of each acid was methacrylic M99.9%, acrylic acid M99.9%,
2.0% and acetic acid 75.1%. This methacrylic acid extract was fed into a solvent recovery column (a perforated plate column with a column diameter of 70 mm and 30 plates) at a supply rate of 3.0 q/Hr.

Isopropyl acetate and water are recovered from the top of the tower and placed in a decanter.
After separating the aqueous layer, it was recycled to an extraction column. A liquid from which most of the isopropyl acetate and water had been removed was obtained from the bottom of the column. This liquid was passed sequentially through an acetic acid separation column and a methacrylic acid purification column, and purified methacrylic acid could be obtained from the top of the column.

During this period, there was no occurrence of scum or clogging due to tar-like substances at the interface of the extraction tower, and stable operation was possible without any clouding or polymerization of the liquid at the bottom of the solvent recovery tower.

Comparative Example 1 Methacrylic acid was extracted from the same aqueous solution of methacrylic acid as in Example 1 under the same conditions as in Example 1 using isopropyl acetate as a solvent without treating it with a semipermeable membrane. Shortly after the start of operation, a large amount of scum was generated at the interface, making it difficult to identify the interface, so operation was continued while extracting the scum along with the liquid from the nozzle near the interface. The color of the extract is brown, and the composition is 21.3 parts of methacrylic acid, 0.2 parts of aldehydes, 1.9 parts of polymeric substances, 5.0 parts of organic acids, 7.6 parts of water, The remainder was solvent. When this extract was sent to a solvent recovery tower and distilled,
After an hour, the bottom of the tower became cloudy, and 10 hours later, the pressure of the boiler steam rose and the bottom liquid was not being extracted properly, so the operation was stopped and the inside +! When I13 was inspected, it was found that a considerable amount of viscous polymer had adhered to the upper part of the boiler and the perforated plate in the lower part of the column.

〔Effect of the invention〕

According to the present invention, methacrylic acid and acetic acid are extracted after removing nonvolatile residues from a methacrylic acid solution containing acetic acid as the main impurity by semipermeable membrane treatment, so that the selectivity for methacrylic acid is high; Non-volatile residue 50~
Isobuvir acetate, which extracts up to 60%, can be used as an extractant. As a result, extraction, recovery of the oil agent, and separation of the extractant and acetic acid are facilitated, making it extremely efficient in terms of equipment and recovery energy. In addition, the occurrence of carbon-like substances and/or tar-like substances that precipitate during the extraction process and scum floating on the liquid-liquid interface is no longer observed,
The problems that had been a major hindrance to continued operation over a long period of time have been eliminated.

Claims (1)

[Claims] Methacrylic acid obtained by catalytically oxidizing isobutylene, tertiary butanol, methacrolein, or isobutyraldehyde with a molecular oxygen-containing gas in the presence of water vapor, and cooling the reaction product gas containing acetic acid as the main impurity from the catalytic oxidation. A method for purifying methacrylic acid, which comprises bringing an aqueous solution into contact with a semipermeable membrane under pressure, and simultaneously extracting methacrylic acid and acetic acid from the semipermeable membrane permeate using isopropyl acetate as an extractant.