JPS6312460B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for purifying methacrylic acid. Specifically, the present invention is a process for producing methacrylic acid by a catalytic gas phase oxidation reaction of isobutylene, tert-butanol or methacrolein, by cooling and condensing the methacrylic acid-containing reaction product gas discharged from an oxidation reactor. The present invention relates to a method for industrially and efficiently purifying and obtaining methacrylic acid from a prepared aqueous methacrylic acid solution.

Research and development of a method for producing methacrylic acid by subjecting isobutylene, tert-butanol, or methacrolein to a catalytic oxidation reaction in the gas phase has been underway for some time. However, the current situation is that it has not yet been industrialized. A possible reason for this is that oxidation catalysts have not been sufficiently developed and yields worthy of industrialization cannot be obtained. This means that the methacrylic acid-containing reaction product gas discharged from the oxidation reactor contains many impurities in addition to methacrylic acid, such as methacrolein, acrylic acid, maleic acid, acetic acid, acrolein, acetone, and acetaldehyde. ,carbon dioxide,
In addition to carbon monoxide, there are aromatic carboxylic acids and tar-like substances that are difficult to separate from methacrylic acid and cause major problems in the subsequent purification process, and are known to cause various problems. Traditionally, a method has been used to collect methacrylic acid in the form of an aqueous solution by cooling and condensing the reaction product gas.
At this time, many of the above-mentioned by-products are condensed and collected together with methacrylic acid and water. Substances with relatively low boiling points in the aqueous methacrylic acid solution can be substantially removed by distillation or dispersion operation before being subjected to extraction operation. However, impurities such as aromatic carboxylic acids, maleic acid, polymers, and tar-like substances still remain in the methacrylic acid aqueous solution, causing various problems when extracting methacrylic acid or distilling the extract. . Various measures have been proposed to alleviate these troubles.

For example, JP-A-50-52021 proposes a method of treating an aqueous methacrylic acid solution with activated carbon or a special ion exchange resin before extraction;
Japanese Patent No. 56-16438 proposes a method in which scum that is deposited at the interface between the solvent and the aqueous layer during extraction is mixed in advance with a solvent and an adsorption treatment is performed as a pretreatment in order to separate the scum.

However, all of these methods complicate the process and do not provide a fundamental solution. That is, the method causes various troubles due to long-time operation, generates a large amount of waste water for cleaning the equipment, and is not a method that can be stably adopted industrially.

An object of the present invention is to provide a method for purifying methacrylic acid that can overcome the above-mentioned drawbacks.

That is, the present invention is specified as follows.

An aqueous methacrylic acid solution 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 reaction of isobutylene, tert-butanol, or methacrolein. A method for purifying methacrylic acid, which comprises first removing light boilers contained in the aqueous solution by distillation or dispersion operation, adding and mixing a basic substance to the obtained methacrylic acid aqueous solution, and then subjecting it to an extraction operation. .

Next, the present invention will be explained in more detail.

When isobutylene, tertiary butanol, or methacrolein is subjected to a step-oxidation or multi-step oxidation reaction using a catalyst prepared based on molybdenum oxide or its composite oxide, a reaction product consisting mainly of methacrylic acid is obtained. . It is usually collected as a condensed aqueous solution containing methacrylic acid. This aqueous methacrylic acid solution is then subjected to distillation or diffusion operations to remove light boiling point substances, and further subjected to extraction operations using a solvent such as xylene or toluene.

According to the findings of the present inventors, when a basic compound is added and mixed into an aqueous methacrylic acid solution after removal of light boilers and extraction treatment is performed, the various impurities mentioned above hardly migrate to the extract side and most of them are removed. was found to remain in the raffinate. Therefore, impurities that are difficult to separate from methacrylic acid, such as aromatic carboxylic acids, maleic acid, polymers, and tar-like substances, do not accompany methacrylic acid, which greatly reduces troubles during purification of methacrylic acid. It became clear. In addition, it was discovered that by treating the aqueous methacrylic acid solution with a basic compound, the amount of scum deposited on the interface during extraction was significantly reduced, and major troubles would not occur in the subsequent purification process. be. This is because aromatic carboxylic acids, polymers, and tar-like substances undergo decomposition and other denaturation due to the addition of basic compounds, and substances that are more acidic than methacrylic acid form salts, making them difficult to extract. It is presumed that this is because it remains on the raffinate side.

The basic compounds used in the present invention include alkali metals such as sodium, potassium, and alkaline earth metals such as calcium and magnesium hydroxide carbonates and bicarbonates, with sodium hydroxide and sodium carbonate being particularly preferred. .
These are advantageously used in the form of 5-50% by weight aqueous solutions. The amount used is in the range of 1 to 10 mol%, preferably 2 to 5 mol%, based on the target amount of methacrylic acid. If the amount added is too large, even methacrylic acid will be neutralized and cannot be extracted, resulting in a loss. If the amount is too small, the effect of addition will be reduced and troubles in the refining process will not be sufficiently prevented. Preferably, the addition method is to mix the methacrylic acid aqueous solution and the basic compound aqueous solution continuously using a line mixer, or to introduce them into a stirring tank and mix them thoroughly. A high temperature is not necessary, and room temperature is sufficient. The thoroughly mixed methacrylic acid aqueous solution is subjected to an extraction operation, the extract is distilled to remove the solvent, light boiling substances are distilled off, and finally the heavy components are separated to produce a product.

Through these purification steps, the method of the present invention is highly effective in reducing the amount of scum generated at the interface of the extraction tower and reducing scale adhesion to the heat transfer surface of the reboiler of the distillation tower, greatly contributing to long-term stable operation. do.

The present invention will be explained in more detail below by giving examples.

Example 1 In the presence of two types of oxidation catalysts containing molybdenum oxide as the main component, tertiary-butanol is oxidized in a two-stage contact gas phase with water vapor and air, and the resulting reaction product gas is cooled and condensed, and collected as an aqueous methacrylic acid solution. In addition, 24% by weight of methacrylic acid, obtained by removing light boilers such as methacrolein, and acetic acid.
3.6% by weight, phthalic acids, (o-, m-, p-)
To 180 kg of an aqueous solution containing 1.4% by weight, 0.8% by weight of maleic acid, and 1% by weight of tar-like substances, 3.7 kg of a 20% by weight aqueous sodium hydroxide solution was added and mixed with stirring at 25° C. for 10 minutes in a tank. Next, this liquid was supplied to the upper part of the extraction tower at a rate of 20 kg/hr, and xylene was supplied from the lower part at a rate of 20 kg/hr.
Continuous countercurrent feeding was carried out at a rate of Kg/hr, and continuous extraction was carried out for 2 days. The extraction operation was performed at normal temperature and pressure. The extraction column is a rotating disk column with an inner diameter of 70 mm and a total height of 1800 mm. After sufficient extraction equilibrium was reached, the extract (organic phase) was produced at a rate of 26 kg/hr and the raffinate (aqueous phase) at a rate of 14 kg/hr. There is very little precipitation of solid scum at the upper interface of the extraction tower, and there is no risk of high-boiling substances adhering to the heat transfer surfaces of the solvent separation tower, light boiling point separation tower, and reboiler of the heavy material separation tower. It was also possible to operate for long periods of time.

Comparative Example 1 The methacrylic acid aqueous solution obtained in Example 1 was used as it was without adding sodium hydroxide to 20 kg/
It is supplied to the top of the extraction column at a rate of 20
Continuously supply xylene countercurrently at a rate of Kg/hr,
Extracted. The operating conditions for this extraction and the extraction column used were the same as in Example 1. After reaching sufficient extraction equilibrium, the extract liquid (organic phase) is added at 25.5Kg/hr.
and raffinate (aqueous phase) were obtained at a rate of 14.5 kg/hr.

A large amount of scum was deposited at the upper interface of the extraction tower, and after several hours of operation, scum was deposited on the upper viewing window, and the operation was stopped after 10 hours. In addition, although there were no problems caused by adhesion of solids to the extraction liquid solvent separation tower and the reboiler in the light-boiling point separation tower, there was severe adhesion of solids to the heat transfer surface of the reboiler of the heavy substance separation tower. It turned out that it could not be used for long-distance driving.

Claims (1)

[Claims] 1. 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 reaction of isobutylene, tert-butanol or methacrolein. A method for producing methacrylic acid, which is characterized in that the aqueous solution is first removed by distillation or dispersion to remove light-boiling substances contained in the aqueous solution, and then a basic substance is added to and mixed with the resulting methacrylic acid aqueous solution, and then subjected to an extraction operation. Purification method.