JPH0434556B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION 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 aqueous 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.

Conventionally, pyromellitic anhydride is diurene (1,
2,4,5-tetramethylbenzene) or other 1,2,4,5-tetraalkylbenzenes by catalytic gas phase oxidation with a molecular oxygen-containing gas. In such a gas phase oxidation reaction, pyromellitic anhydride is obtained as a high-temperature mixed gas, so it is necessary to collect pyromellitic anhydride from the mixed gas. Further, when pyromellitic anhydride is purified by sublimation, it is necessary to collect pyromellitic anhydride from the gas containing pyromellitic anhydride generated together with the carrier 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 and melt it, and take it out. Therefore, if continuous collection is desired, cooling and removal must be performed by alternately switching between at least two cooling devices. . 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, in the heat melting method, pyromellitic anhydride has a high melting point of 285 to 287° C. and requires heating and melting with a high-temperature heat medium, resulting in large energy loss and being susceptible to deterioration, making it difficult to apply.

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 spraying 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 extremely fine particles and becomes atomized. The atomized pyromellitic anhydride particles float in the gas of the device, and their collection is extremely difficult due to the nature of the atomized material, and cannot be collected by economical methods.

In order to solve this problem of atomization, the pyromellitic anhydride is introduced into a condenser which is maintained at a temperature below the precipitation temperature of pyromellitic anhydride and has a wall surface through which an aqueous medium flows down so as to thinly cover the wall surface. 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, so the contact point between the high-temperature mixed gas and water or aqueous medium in the collection device is However, it is difficult to collect pyromellitic anhydride stably and efficiently, as crystals of pyromellitic anhydride crystallize and grow in lumps, clogging the device and reducing the quality of pyromellitic anhydride. It's not a thing.

The present invention was made in order to eliminate the various drawbacks of the conventional method as described above, and involves introducing a high temperature mixed gas containing pyromellitic anhydride into a wet wall tower formed of water or an aqueous medium, and Heat exchange is performed with the mixed gas and water or an aqueous medium to lower the temperature of the high-temperature mixed gas below the dew point, and the mixed gas is introduced into a washing tower and brought into contact with water or an aqueous medium to collect anhydrous pyromellits. A method for collecting pyromellitic anhydride is characterized in that pyromellitic anhydride and pyromellitic acid are recovered as a slurry or an aqueous solution from the bottom of the wet wall column and the washer.

That is, the present invention introduces a hot mixed gas stream containing pyromellitic anhydride into a wetted wall tower formed of water or an aqueous medium, and after heat exchange between the hot mixed gas and the water or aqueous medium. , to collect pyromellitic anhydride in water or an aqueous medium.

Since the pyromellitic anhydride collected in this way uses water or an aqueous medium, part or all of the anhydride becomes dihydrate and becomes pyromellitic acid as a slurry or aqueous solution of a mixture of the remaining pyromellitic anhydride and pyromellitic acid. be captured.

The advantage of the wetted wall column in the present invention is that, firstly, when high temperature mixed gas is heat exchanged with water or an aqueous medium, the temperature of the high temperature mixed gas gradually decreases, so that crystallized crystals grow, and anhydrous There is almost no atomization of pyromellitic acid, and it can be easily collected. In other words, when water or an aqueous 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 water or aqueous medium in a short period of time, resulting in a rapid drop in the mixed gas temperature. The crystallized crystals remain fine and become atomized,
Although collection is difficult, according to 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 fluid water or an aqueous medium, crystals such as pyromellitic anhydride do not precipitate or settle on the wall surface of the vessel. Alternatively, since it is continuously carried out of the wet wall tower together with the aqueous medium, it can be operated continuously.

The third advantage is that a considerable distance and space can be provided between the hot mixed gas inlet and the cold water or aqueous medium. Crystals do not precipitate at the supply port 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 wet wall tower used in the present invention is not particularly limited, it is desirable to have a structure in which the water or aqueous medium forming the wet wall does not form raindrops or mist. In addition, the size of the wetted wall tower can be appropriately determined depending on the amount and characteristics of the high-temperature mixed gas introduced, as long as it can cool the high-temperature mixed gas to the temperature at which crystals are precipitated. .

The high-temperature mixed gas containing pyromellitic anhydride introduced into the wet wall tower is diurene (1,
2,4,5-tetramethylbenzene) or other 1,2,4,5-tetraalkylbenzene with a molecular oxygen-containing gas, or 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 during sublimation purification. 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.

Water or aqueous media forming wet walls include:
In addition to water such as pure water, tap water, and industrial water, it may also be an aqueous slurry containing collected pyromellitic anhydride, or a separated liquid obtained by separating collected pyromellitic anhydride.

The temperature of the water or aqueous medium is not particularly limited as long as it is in liquid form and flows through the vessel walls, but when using the water or aqueous medium in circulation, the calorific value of the high-temperature mixed gas and the evaporated water are released outside the system. Usually 40~
Operated at 80℃. The flow rate of water or aqueous medium is
The amount may 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 accompanying 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 water or aqueous medium flowing down the wet wall column, the majority remains in the gas mixture and is brought into contact with the water or aqueous medium in the washing column. The pyromellitic anhydride-containing gas which has passed through the wetted wall column can be easily collected, since the pyromellitic anhydride no longer becomes atomized when it comes into contact with water or an aqueous 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 a line 3 to the top of a wetted wall tower 2 to which water or an aqueous medium is supplied near the top by a nozzle 1. While passing through the wet wall tower 2, the introduced high-temperature mixed gas comes into contact with a film of water or an aqueous medium that forms the wet wall, exchanges heat, and cools while crystallizing a portion of pyromellitic anhydride. The crystallized pyromellitic anhydride is collected in a membrane of water or an aqueous medium. As a result, the resulting slurry or aqueous solution is discharged from the bottom of the tower 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 water or an aqueous medium supplied from the top of the tower through shower nozzle 8, and pyromellitic anhydride is is collected. As a result, the resulting slurry is discharged from the bottom of the tower through line 9 to solid-liquid separator 5. Note that the exhaust gas is discharged from the top of the tower through line 10 to the outside of the system. Note that the contact between the water or the aqueous 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 and pyromellitic acid are discharged from the line 11 as solids. 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 system through line 13. In addition, line 12
Water or an aqueous medium is replenished into the liquid from a line 14.

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 water was allowed to flow down at a rate of 2/min.
A glass tube with an inner diameter of 60 mm and a height of 350 mm filled with glass beads and in which water was allowed to flow down at a rate of 2/min was used as a washing tower, and these were connected by two flasks. 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/Hr (standard condition) and contained 13 g of pyromellitic anhydride. The temperature of the collection liquid
The temperature was maintained at 60°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 cleaning 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 5.2 g. The collection rate of pyromellitic anhydride was 40%.

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 9.8 g. The collection rate of pyromellitic anhydride was 75.4%.

[Brief explanation of drawings]

The drawing is a flow sheet illustrating one embodiment of the 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 water or an aqueous medium, and heat exchange is performed between the high-temperature mixed gas and water or an aqueous medium to produce the pyromellitic anhydride. The temperature of the hot mixed gas is lowered below the dew point, the mixed gas is introduced into a washing tower and brought into contact with water or an aqueous medium to collect pyromellitic anhydride, and a slurry is collected from the bottom of the wet wall tower and the washing tower. Alternatively, a method for collecting pyromellitic anhydride, which comprises recovering pyromellitic anhydride and pyromellitic acid as an aqueous solution. 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 aqueous medium is water. 4. Claim 1, wherein the aqueous medium is a liquid obtained by collecting pyromellitic anhydride and separating pyromellitic anhydride and pyromellitic acid crystals in the liquid.
The method for collecting pyromellitic anhydride according to item 1 or 2. 5 Claim 1 in which the aqueous medium contains solid content generated 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.