JPH0442389B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for collecting pyromellitic anhydride from a high temperature mixed gas containing pyromellitic anhydride. Specifically, the present invention includes:
The present invention relates to a method for collecting pyromellitic anhydride in the form of crystals or a solution using an organic medium from a high-temperature mixed gas containing pyromellitic anhydride produced by catalytic gas-phase oxidation or a high-temperature mixed gas containing a carrier gas produced by sublimation.

<Prior art> Conventionally, pyromellitic anhydride is diurene (1,
2,4,5-tetramethylbenzene) or other 1,2,4,5-tetramethylbenzenes by catalytic gas phase oxidation with a molecular oxygen-containing gas. In such catalytic gas phase oxidation, pyromellitic anhydride is obtained as a high-temperature mixed gas, and therefore it is necessary to collect pyromellitic anhydride from the mixed gas. Furthermore, when pyromellitic anhydride is purified by sublimation, it is necessary to collect pyromellitic anhydride from the pyromellitic anhydride-containing gas generated together with the transport gas.

Conventionally, when separating and collecting pyromellitic anhydride from such a high-temperature mixed gas containing pyromellitic anhydride, industrially, the high-temperature mixed gas was cooled by letting it cool or forcedly cooled so that it could be collected on the wall of a cooler. An indirect cooling method in which crystals are crystallized, and a direct cooling method in which a cold gas or cold liquid is introduced into a high-temperature mixed gas to bring it into gas-liquid contact.

However, the collection method using indirect cooling method is
In order to remove the crystallized crystals, it is important to stop the equipment and scrape off the crystals that have formed on the wall of the cooler.
Alternatively, it is necessary to stop the process, heat it, melt it, and take it out, which has the disadvantage that if you want to collect it continuously, you have to alternately switch between at least two cooling devices to cool and take it out. . Furthermore, the scraping method requires a great deal of labor and equipment, which is economically disadvantageous, and when carried out manually, it causes hygiene problems.
On the other hand, the heat melting method is difficult to apply because pyromellitic anhydride has a high melting point of 285 to 287° C. and requires heat melting with a high-temperature heat medium, resulting in large energy loss and easy deterioration.

On the other hand, the direct cooling collection method using cold gas or cold liquid has the problem of not only the collection rate of the target object but also difficult problems in recovering or processing the cold gas or cold liquid. The difficulty increases as the concentration of pyromellitic anhydride in the hot gas mixture decreases.

On the other hand, in order to collect pyromellitic anhydride from a high-temperature mixed gas, a method using water or an aqueous medium, which is inexpensive and easy to recover or treat, has been proposed (Japanese Unexamined Patent Publication No. 47-39045). These methods involve injecting water or an aqueous medium into a high-temperature mixed gas in the form of raindrops or a film, so the high-temperature mixed gas is rapidly cooled, and the pyromellitic anhydride is
It becomes microscopic particles and becomes atomized. The atomized pyromellitic anhydride particles are suspended in the gas of the device, and their collection is extremely difficult due to the nature of the atomized material, and cannot be collected in an economical manner.

In order to solve this problem of atomization, the pyromellitic anhydride is introduced into a condenser that is maintained at a temperature below the precipitation temperature of the pyromellitic anhydride and has an aqueous medium flowing downward so as to cover the wall thinly. A collection method has been proposed by depositing it on the condenser wall. However, in this method, there is a large temperature difference between the dew point of pyromellitic anhydride in the high-temperature mixed gas and the water or aqueous medium. , crystals of pyromellitic anhydride crystallize and grow in lumps, causing problems such as closing the device and lowering the quality of pyromellitic anhydride, making it difficult to collect pyromellitic anhydride stably and efficiently. That's not the point.

<Problems to be Solved by the Invention> Since all of these methods use water or an aqueous medium, what is obtained during collection is only hydrated pyromellitic acid, and if one attempts to obtain anhydrous pyromellitic acid, The heating dehydration reaction must be performed again, and there are many points that need to be improved even when only considering energy consumption.

Therefore, an object of the present invention is to provide an economical method for collecting pyromellitic anhydride, which has extremely excellent collection efficiency without converting a high-temperature mixed gas containing pyromellitic anhydride into water.

<Means for Solving the Problems> The present invention has been made in order to eliminate the various drawbacks of the conventional methods as described above. The high temperature mixed gas is introduced into a column, and the high temperature mixed gas is heat exchanged with an organic medium to lower the temperature of the high temperature mixed gas below the dew point, and the mixed gas is introduced into a washing column and brought into contact with an organic medium to produce anhydrous pyromellitate. A method for collecting pyromellitic anhydride is characterized in that the pyromellitic anhydride is collected as a slurry or solution from the bottoms of the wet wall tower and the washing tower.

That is, the present invention introduces a hot mixed gas stream containing pyromellitic anhydride into a wetted wall tower formed of an organic medium, heat exchanges the hot mixed gas with the organic medium, and then passes the stream to a washing tower. The purpose is to introduce pyromellitic anhydride and collect it in an organic medium.

Since an organic medium is used, the pyromellitic anhydride thus collected is collected as an anhydride as a slurry or an organic solution.

The advantage of the wet wall column in the present invention is that, firstly, when the high temperature mixed gas is heat exchanged with the cooling medium, the high temperature mixed gas gradually falls, so that the crystallized crystals grow, and the pyromellitic anhydride There is almost no atomization and collection is easy. In other words, when a cooling medium is sprayed onto a high-temperature mixed gas in the form of raindrops or a film, the high-temperature mixed gas comes into direct contact with the low-temperature cooling medium in a short period of time, so the temperature of the mixed gas drops rapidly and crystallization occurs. The resulting crystals remain fine and become atomized, making collection difficult. However, according to the wet wall cooling method of the present invention, collectable crystals of pyromellitic anhydride can be obtained.

The second advantage is that since the inside of the wet wall column is covered with a fluid organic medium, crystalline substances such as pyromellitic anhydride do not precipitate or settle on the vessel wall surface, and the crystalline substances are absorbed together with the organic medium. Since it is continuously transported to the wet wall tower, it can be operated continuously.

The third advantage is that a considerable distance and space can be provided between the high-temperature mixed gas inlet and the low-temperature organic medium. will not precipitate and make operations difficult.

The fourth advantage is that since it is a wet wall tower, it has good heat exchange efficiency and the apparatus can be made smaller compared to an air-cooled crystallizer.

Although the shape of the wetted wall tower used in the present invention is not particularly limited, it is desirable that the organic medium forming the wetted wall not form raindrops or mist. In addition, the size of the wetted wall tower can be determined as long as it can cool the high temperature mixed gas to the temperature at which crystals are precipitated, and can be appropriately determined depending on the amount and characteristics of the high temperature mixed gas introduced. .

The high-temperature mixed gas containing pyromellitic anhydride introduced into the wet wall column is diurene (1,
2,4,5-tetramethylbenzene) or other 1,2,4,5-tetramethylbenzene with a molecular oxygen-containing gas, a mixed gas containing pyromellitic anhydride or pyromellitic anhydride There are mixed gases that use an inert gas such as nitrogen as a carrier gas when refining by sublimation. Such a high temperature mixed gas is introduced into the wetted wall tower at a temperature higher than the dew point of the mixed gas, taking into account blockages in gas flow pipes, precipitation of crystals at the wetted wall inlet, etc. Considering that the column is a crystallizer and the separation operation of the collected pyromellitic anhydride is performed, a temperature as close to the dew point as possible and up to 100°C higher than the dew point, preferably about 20°C higher than the dew point, is suitable. be.

The organic medium that forms the wetted wall is not particularly limited, but hydrocarbons, aromatic hydrocarbons, heterocycles, ketones, ethers, alcohols, nitriles, etc., which are liquid at room temperature, are preferred for handling. is particularly preferably an aromatic nitrile, more preferably benzonitrile. This benzonitrile is particularly superior in its ability to dissolve pyromellitic anhydride compared to ketones, ethers, etc. (solubility 50g/100g benzonitrile at 125℃,
90g/100g benzonitrile at 90°C), it is also an excellent solvent when recrystallizing pyromellitic anhydride in the solid-liquid separator 5 described later. The organic medium forming the wetted wall may be a slurry containing the collected pyromellitic anhydride or a separated liquid obtained by separating the collected pyromellitic anhydride.

The temperature of the organic medium is not particularly limited as long as it is liquid and flows through the vessel wall. In particular, when using benzonitrile, take into account the solubility of its pyromellitic anhydride and use it at room temperature to 190°C, preferably at 100°C to
Preferably used at 190°C. The flow rate of organic medium is
The amount should cover the entire wall surface of the wetted wall tower, and is determined by the type of wetted wall tower and the amount of evaporation associated with heat exchange.

The temperature of the high temperature mixed gas containing pyromellitic anhydride is lowered in the wet wall tower, and the pyromellitic anhydride contained therein crystallizes. Although some of the crystallized solids are collected by the organic medium flowing down the wet wall tower, most of the solids remain in the mixed gas, so
Contact with an organic medium in a washing tower. The pyromellitic anhydride-containing gas that has passed through the wet wall column can be easily collected, since the pyromellitic anhydride no longer becomes atomized when it comes into contact with an organic medium in the form of rain drops or spray.

Next, one embodiment of the present invention will be described with reference to the drawings. That is, a high-temperature mixed gas containing pyromellitic anhydride is introduced through line 3 to the top of wetted wall column 2, to which an organic medium is supplied near the top through nozzle 1. While passing through the wet wall tower 2, the introduced high-temperature mixed gas comes into contact with the film-like organic medium that forms the wet wall, exchanges heat, and cools while crystallizing a portion of pyromellitic anhydride. The released pyromellitic anhydride is collected in a film of organic medium. As a result, the resulting slurry or solution is discharged from the bottom of the column through line 4 to solid-liquid separator 5. The heat-exchanged mixed gas is supplied to the bottom of the cleaning tower 7 through line 6, and while rising inside the tower, it is brought into countercurrent contact with the organic medium supplied from the top of the tower through the shower nozzle 8, and pyromellitic anhydride is be captured. As a result, the resulting slurry or solution is discharged from the bottom of the column through line 9 to solid-liquid separator 5. Note that the exhaust gas is discharged from the top of the column via line 10 to the outside of the system after recovering the organic medium contained therein.
The recovered organic medium can also be returned to line 12. Note that the contact between the organic medium and the mixed gas may be a cocurrent contact.

The solid and liquid are separated in the solid-liquid separator 5, and the pyromellitic anhydride is passed through the line 11 as a solid.
is discharged from. The separated liquid is supplied via line 12 to wet wall column 2 and washing column 7. Further, a part of the liquid is discharged from the run 13 to the outside of the system. Note that in the liquid in line 12, line 1
4, the organic medium is replenished.

<Effects of the Invention> The present invention introduces a high-temperature mixed gas containing pyromellitic anhydride into a wetted wall tower formed of an organic medium, heat-exchanges the high-temperature mixed gas and the organic medium, and then performs cleaning. Since the pyromellitic anhydride was introduced into the column and collected using an organic medium, the pyromellitic anhydride was collected in an anhydrous state without being hydrated, resulting in a collection method with significantly lower energy consumption. Moreover, it is an excellent method for collecting pyromellitic anhydride with almost no atomization.

In addition, when aromatic nitriles, especially benzonitrile, are used as organic media, their use in the purification step by recrystallization of pyromellitic acid after collection, since these compounds are excellent solvents for pyromellitic anhydride. The amount of solvent can be significantly reduced, and the resulting pyromellitic anhydride can be of high purity.

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

Example 1 A glass tube with an inner diameter of 60 mm and a height of 350 mm was used as a wet wall tower, and benzonitrile was allowed to flow down at a rate of 2 liters/min. A glass tube with an inner diameter of 60 mm and a height of 350 mm was filled with glass beads as a cleaning tower. Benzonitrile was flowed down at a rate of 2 L/min, and these were connected in a 2 L flask. A pyromellitic anhydride-containing gas obtained by catalytic gas phase oxidation of Diurene with air was introduced into this, and pyromellitic anhydride was collected.

The introduced gas was 720 l/Hr (standard condition) and contained 13 g of pyromellitic anhydride. The temperature of the collection liquid
The temperature was maintained at 100°C and collected for 1 hour. The collection liquid was slightly colored but transparent. When this was measured by liquid chromatography, it contained 12.9 g of pyromellitic anhydride. The collection rate of pyromellitic anhydride was 99.2%.

Comparative Example 1 The same operation as in Example 1 was carried out except that the wet wall tower in Example 1 was removed and the reaction gas was introduced directly into the washing tower. Immediately after introduction, white smoke was observed at the outlet of the cleaning tower, and white solid matter was observed on the exhaust gas line. After collection, pyromellitic anhydride was measured by liquid chromatography and found to be 5.4 g. The collection rate of pyromellitic anhydride was 41.5%.

Comparative Example 2 The same operation as in Example 1 was carried out except that the liquid supply to the wet wall tower in Example 1 was stopped. White crystals adhered to the wet wall tower to which no liquid was supplied. Although less than in Comparative Example 1, fumes were observed at the outlet of the washing tower. After collection, pyromellitic anhydride was measured by liquid chromatography and found to be 9.9 g. The collection rate of pyromellitic anhydride was 76.2%.

[Brief explanation of drawings]

The drawing is a flow sheet showing one embodiment of the present invention. 1... Nozzle, 2... Wet wall tower, 7... Washing tower, 8... Shower nozzle, 5... Solid-liquid separation device.

Claims (1)

[Scope of Claims] 1. A high-temperature mixed gas containing pyromellitic anhydride is introduced into a wetted wall tower formed of an organic medium, and heat is exchanged between the high-temperature mixed gas and the organic medium to reduce the high-temperature mixed gas. The temperature is lowered below the dew point, the mixed gas is introduced into a washing tower and brought into contact with an organic medium to collect pyromellitic anhydride, and pyromellitic anhydride is collected from the bottom of the wetted wall tower and the washing tower as a slurry or solution. 1. A method for collecting pyromellitic anhydride, the method comprising recovering pyromellitic anhydride. 2 The temperature of the high temperature mixed gas containing pyromellitic anhydride introduced into the wet wall tower is at or below the dew point.
The method for collecting pyromellitic anhydride according to claim 1, wherein the temperature is adjusted to a high temperature range of 100°C. 3. The method for collecting pyromellitic anhydride according to claim 1 or 2, wherein the organic medium is an aromatic nitrile. 4. The method for collecting pyromellitic anhydride according to claim 1 or 2, wherein the organic medium is a liquid obtained by collecting pyromellitic anhydride and separating crystals of pyromellitic anhydride in the liquid. 5 Claim 1 in which the organic medium contains solid content produced by collecting pyromellitic anhydride
The method for collecting pyromellitic anhydride according to item 1 or 2. 6. The method for collecting pyromellitic anhydride according to any one of claims 1 to 5, wherein the pyromellitic anhydride-containing mixed gas is a product gas obtained by a catalytic gas phase oxidation reaction. 7. The method for collecting pyromellitic anhydride according to any one of claims 1 to 5, wherein the mixed gas containing pyromellitic anhydride is a gas produced by heating and sublimating crude pyromellitic anhydride.