JPS6245219B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for purifying acrylic acid. Specifically, the present invention relates to the production of acrylic acid by cooling and condensing the acrylic acid-containing reaction product gas discharged from the oxidation reactor in the process of producing acrylic acid by catalytic gas phase oxidation reaction of propylene or acrolein. This invention relates to a method for industrially and efficiently purifying and obtaining acrylic acid from an aqueous solution without any trouble.

In recent years, it has been known from numerous publications on the development of oxidation catalysts and process development that acrylic acid can be produced by catalytic gas phase oxidation of compounds having three carbon atoms, such as propylene and acrolein. However, it is a fact that various difficulties are encountered in the process of separating and purifying high purity acrylic acid from the obtained acrylic acid aqueous solution. The reason for this is that even though the catalyst performance is at a high level, various by-products are present in the reaction products. This is because it is extremely difficult to separate and remove these by-products from the intended acrylic acid, and various problems in the acrylic acid purification process have not yet been solved. In addition to acrylic acid, the reaction product gas also contains substances with relatively low boiling points, such as acrolein, propionic acid, acetic acid, acetaldehyde, carbon monoxide, and carbon dioxide, as well as maleic acid, aromatic carboxylic acids, benzoic acid, terephthalic acid, etc. It also contains heavy substances such as tar-like substances and other by-products. Among these, substances with relatively high boiling points are thought to cause various troubles in the collection and purification process of acrylic acid.

Conventionally, a method has been adopted in which acrylic acid is collected in the form of an aqueous solution by cooling and condensing the reaction product gas, and in this process, many of the reaction by-products mentioned above are also collected by condensation or dissolution along with acrylic acid and water. be done. Substances with relatively low boiling points in the aqueous acrylic acid solution are substantially removed by dissipation or distillation in a step before the extraction operation. However, aromatic carboxylic acids,
High-boiling substances such as maleic acid, polymers, and tar-like substances still remain in the acrylic acid aqueous solution, causing various troubles during the extraction of acrylic acid and the distillation process to separate and purify acrylic acid from the extract. It's summery. Various proposals have been made to alleviate these troubles. However, separation and removal of by-products cannot be achieved sufficiently, and in long-term operation, poor interfacial separation during extraction and polymerization caused by these by-products often occur during distillation, which often requires expensive polymerization inhibitors. However, there are some disadvantages such as having to use a large amount of acrylic acid, and a large amount of waste water is generated when cleaning the equipment. It has not become familiar.

An object of the present invention is to provide a method for purifying acrylic acid that can overcome the above-mentioned drawbacks. That is, the present invention is specified as follows.

(1) In the process of producing acrylic acid by catalytic gas phase oxidation of propylene or acrolein, an aqueous acrylic acid solution obtained by cooling and condensing the acrylic acid-containing reaction product gas discharged from an oxidation reactor is first added to the aqueous solution. The light boiling point substances contained in the acrylic acid solution are removed by distillation or dissipation operation, and then bisulfite is added to the resulting acrylic acid aqueous solution and subjected to an extraction process, thereby preventing the generation of scum in the extraction process. A method for purifying acrylic acid.

(2) The method according to (1) above, wherein the bisulfite is at least one selected from the group consisting of alkali metal salts and ammonium salts.

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

When propylene or acrolein is subjected to a single-stage or multi-stage oxidation reaction using an oxidation catalyst prepared based on molybdenum oxide or its composite oxide, a reaction product containing acrylic acid as a main component is obtained. The acrylic acid is then typically collected as a condensed aqueous solution containing acrylic acid. This aqueous acrylic acid solution is then subjected to distillation and dispersion operations to remove light boilers, and further subjected to extraction operations using ketones such as methyl ethyl ketone and acetic esters.

The present inventors investigated various ways to improve the separation state of the interface during extraction and the polymerization stability during distillation of the above-mentioned aqueous acrylic acid solution from which light boilers had been removed, and found that bisulfite was added to the aqueous acrylic acid solution in advance The present inventors have discovered that by doing so, it becomes easier to separate and remove by-products, the separation state at the interface during extraction is improved, and the polymerization stability during distillation is improved, leading to the completion of the present invention.

It has been found that by employing the method of the present invention, not only the separation state at the upper interface of the extraction column is improved, but also the adhesion of scale to the inner wall of the column can be prevented, and long-term continuous operation becomes possible. The inventors
It has been found that aromatic carboxylic acids, tar-like substances, and some polymerized substances cause poor separation at the interface of the extraction tower, but these are caused by bisulfite added to the acrylic acid aqueous solution before extraction. It is not known why the raffinate is removed to the raffinate during the process in the extraction column.

What is even more surprising is that by employing the method of the present invention, it has been found that the generation of acrylic acid polymers and scale generation in the process of purifying acrylic acid from the extract following this process is greatly reduced. It was released. Examples include suppressing the generation of polymers in distillation columns and acrylic acid rectification columns for dissipating solvents in extracts, and reducing scale generation due to high boiling point substances on the heat transfer surface of the acrylic acid purification distillation column reboiler.

Furthermore, it was also found that the amount of maleic acid in the extract was drastically reduced, and thus it became possible to obtain highly pure acrylic acid with a simple process using the method of the present invention.

The bisulfite used in the present invention is selected from alkali metal salts such as sodium, potassium, and cesium, and ammonium salts, and sodium salts, potassium salts, and ammonium salts are particularly preferably used. These are used in the form of a saturated aqueous solution with a concentration of 10% by weight or more, and the amount used is in the range of 0.5 to 15% by weight, preferably 1 to 10% by weight, based on the target acrylic acid. If the amount added is too large, even acrylic acid will turn into an alkali salt and will be lost in the extraction process. Also, if the amount is too small, the effect of addition will decrease, which is not preferable. As for the addition method, it is preferable to mix the acrylic acid aqueous solution and the above-mentioned bisulfite aqueous solution continuously in a line mixer, or to mix them continuously or semi-continuously in a stirring tank and supply the mixture to an extraction column. The temperature does not particularly require a high temperature, and room temperature is sufficient, but it may be heated to 50 to 60°C.

The aqueous acrylic acid solution to which bisulfite has been added is extracted with a conventional solvent such as ketones such as methyl ethyl ketone or esters such as ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, etc. in an extraction tower. The extract obtained from the upper part is directly supplied to a solvent stripping tower, and the solvent is removed to obtain crude acrylic acid. In the extraction operation, the extract can also be washed with water according to a conventional method. In this case, some of the added bisulfite may have decomposed and a small amount of sulfur dioxide may have been generated, so washing the extract with water is an effective method to wash and remove this sulfur dioxide. Ru.

Post-process to obtain high-purity acrylic acid from the extract,
In other words, in the solvent stripping tower, light boiling point stripper, and purification tower, ordinary polymerization inhibitors such as hydroquinone, methoxyhydroquinone,
Distillation in the presence of a phenothiazine or the like and molecular oxygen is preferred.

Example 1 A molybdenum-based composite oxide was used as a catalyst for the first-stage reaction, and a catalyst based on a molybdenum-vanadium-based composite oxide was used as a catalyst for the second-stage reaction.
Propylene was subjected to a catalytic gas phase oxidation reaction with air in the presence of water vapor, and the resulting reaction product gas was cooled and condensed to obtain an aqueous solution containing acrylic acid containing hydroquinone as a polymerization inhibitor. Light boiling substances such as acrolein were removed by distillation from the resulting aqueous solution, resulting in 24% by weight of acrylic acid, 0.8% by weight of acetic acid, 0.5% by weight of phthalic acids (ortho, meta, rose), 0.8% by weight of maleic acid, and 1.0% by weight of tar-like substances. 20Kg/Hr of an aqueous solution containing % by weight was obtained.

To this aqueous solution, a 30% by weight aqueous sodium bisulfite solution was added and mixed at a rate of 0.25 Kg/Hr, and the mixture was supplied from the upper part of the extraction tower. Isopropyl acetate was supplied from the lower part of the extraction column at a rate of 20 kg/hr, and extraction was carried out continuously in countercurrent for 20 days. The extraction operation was performed at room temperature and pressure. The extraction tower has an inner diameter of 70mm and a total height of 1800mm.
It is a rotating disk tower. After sufficiently reaching extraction equilibrium, the extract (organic layer) was heated at 26.9Kg/Hr.
and a raffinate (aqueous layer) were obtained at a rate of 13.4 Kg/Hr. There is almost no foamy intermediate layer due to poor separation at the upper interface of the extraction tower, and the heat transfer surface of each distillation tower column and reboiler during the subsequent steps of solvent separation, light boiling point separation, and heavy substance separation of the extract. Even after 20 days of continuous operation, there was very little adhesion of high-boiling substances or polymers to the reactor, and it was possible to operate for an even longer period of time.

Comparative Example 1 The same acrylic acid aqueous solution used in Example 1 was extracted using the same apparatus and method as in Example 1. However, sodium bisulfite was not added. After reaching extraction equilibrium, 26.7 Kg/Hr of the extract (organic layer) and 13.1 Kg/Hr of the raffinate (aqueous layer) were obtained. There were a large number of foamy intermediate parts at the upper interface part of the extraction column.
In addition, after 10 days of operation, solid matter was observed on the columns and reboilers of the solvent separation tower and light boiling point separation tower, but it was not enough to stop the operation, whereas the reboiler of the heavy material separation tower There was an extremely large amount of solid matter adhering to the heat transfer surface, making it impossible to continue operation.

Example 2 The same acrylic acid aqueous solution used in Example 1 was extracted using the same apparatus and method as in Example 1. However, instead of sodium bisulfite, use an aqueous solution of ammonium bisulfite at a concentration of 30% by weight.
They were added and mixed at a rate of 0.5Kg/Hr. After sufficiently reaching extraction equilibrium, extract the extract (organic layer) to 26.8 kg/
Extraction was carried out for 20 days while obtaining Hr and raffinate (aqueous layer) at a ratio of 13.7 Kg/Hr. There is almost no foamy intermediate layer at the upper interface of the extraction tower, and high-boiling materials and polymers are present in the columns of each distillation tower and on the heat transfer surface of the reboiler during solvent separation, light boiling point separation, and heavy material separation of the extract. There was very little adhesion, and it was possible to operate for a longer period of time.

Claims (1)

[Claims] 1. In a process for producing acrylic acid by catalytic gas phase oxidation of propylene or acrolein,
The acrylic acid aqueous solution obtained by cooling and condensing the acrylic acid-containing reaction product gas discharged from the oxidation reactor is first removed by distillation or dissipation to remove light boiling point substances contained in the aqueous solution. A method for purifying acrylic acid, which comprises adding bisulfite to an aqueous acrylic acid solution and subjecting it to an extraction step, thereby preventing the generation of scum in the extraction step. 2. The method according to claim 1, wherein the bisulfite is at least one selected from the group consisting of alkali metal salts and ammonium salts.