JP2001139514A - Method for producing aromatic dicarboxylic acid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an industrially extremely economical method having merits of requiring no drier, enabling the energy used for the drying to be saved and the like, especially in the method for producing terephthalic acid by a liquid- phase oxidation of p-xylene. SOLUTION: This method for producing an aromatic dicarboxylic acid by carrying out the liquid phase oxidation of a dialkyl aromatic hydrocarbon with molecular oxygen in a reaction medium, and removing the reaction medium from the obtained slurry to provide the product of the aromatic dicarboxylic acid comprises carrying out the solid-liquid separation of the slurry under pressurization, and subjecting the aromatic dicarboxylic acid obtained as a solid to a flashing treatment without reheating the one by moving the one from the pressurized condition to a separation vessel kept at a low pressure to provide the aromatic dicarboxylic acid from which the reaction medium has been removed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an aromatic dicarboxylic acid by subjecting a dialkyl aromatic hydrocarbon to liquid phase oxidation, and particularly to a method for producing terephthalic acid by subjecting para-xylene to liquid phase oxidation. The present invention relates to a method for producing an aromatic dicarboxylic acid which is economically advantageous by reducing the number of machines and the energy used for drying.

[0002]

2. Description of the Related Art Usually, terephthalic acid is produced by using paraxylene as a raw material, a metal salt such as cobalt or manganese and bromine as a catalyst, a fatty acid such as acetic acid as a reaction medium, and a liquid phase of a molecule. It is generally oxidized by oxygen. After this production step, after water purification as a purification step, hydrogenation and purification may be performed to obtain terephthalic acid with higher purity, or additional oxidation may be performed at a higher temperature in the oxidation step, In some cases, high-purity terephthalic acid can be obtained without going through the hydrorefining step. In any terephthalic acid production process, it is essential that the product terephthalic acid be separated from acetic acid (which may contain a small amount of water) or the like used as a reaction medium and taken out as a dry powder.

[0003] That is, it was confirmed that acetic acid was removed from a terephthalic acid cake containing acetic acid by flashing under high temperature and high pressure, and drying was performed. Conventionally, in order to dry terephthalic acid, a method has been employed in which a liquid is removed by a separator and then dried using a steam or the like as a heating medium in a non-condensable gas stream such as nitrogen in a rotary dryer. According to this method, the residual amount of the reaction medium in terephthalic acid after drying can be reduced to about 0.1% by weight. However, this drying method has a low drying speed, a long residence time in the dryer, a large-sized apparatus, and problems such as terephthalic acid sticking to the inside of the dryer and the piping of the heating medium. There are many problems such as this. Moreover, a large amount of energy is consumed for the steam of the heating medium used for drying. As a method for drying terephthalic acid without using a dryer, a method described in JP-A-55-164650 has been proposed. In this method, a slurry of terephthalic acid and acetic acid is heated to evaporate a reaction medium such as acetic acid to separate terephthalic acid and vaporized acetic acid in a solid portion. In this case, a large amount of energy is required. Also, in the method proposed in Japanese Patent Application Laid-Open No. 10-36313, in which the solvent is replaced with water, the number of dryers can be reduced. Large energy is required in towers and the like.

[0004]

SUMMARY OF THE INVENTION The present inventors have made intensive studies for the purpose of reducing the number of dryers for producing terephthalic acid and the energy for drying the terephthalic acid. After liquid phase oxidation of para-xylene,
Solid-liquid separation under high temperature and high pressure, and found that it is extremely effective to remove the separated acetic acid and water by flashing the cake containing a small amount of the solvent to a lower pressure, which led to the present invention. It is.

[0005]

SUMMARY OF THE INVENTION That is, the gist of the present invention is to remove a reaction medium from a slurry obtained by subjecting a dialkyl aromatic hydrocarbon to liquid phase oxidation in a reaction medium with molecular oxygen to remove product aromatics. In the method for producing an aromatic dicarboxylic acid to obtain a dicarboxylic acid, the slurry is subjected to solid-liquid separation under pressure, and the aromatic dicarboxylic acid obtained as a solid phase, without reheating, is subjected to a lower pressure from the pressure. To obtain an aromatic dicarboxylic acid from which a reaction medium has been removed by flushing into a separation tank kept at a constant temperature.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. In the present invention, examples of the dialkyl aromatic hydrocarbon include para-xylene. As a reaction medium,
Lower aliphatic carboxylic acids are preferred, specifically acetic acid. In the present invention, the liquid-phase oxidation reaction of the dialkyl aromatic hydrocarbon in the reaction medium can be carried out according to a conventional method, and is usually, for example, a catalyst containing a heavy metal such as cobalt, iron or manganese, preferably the heavy metal. In the presence of salt and bromine, 90% by weight or more of para-xylene is oxidized in liquid phase to terephthalic acid by a molecular oxygen-containing gas. The following describes an example in which acetic acid is used as a reaction medium in the production of terephthalic acid, but the present invention is not particularly limited thereto. The amount of the acetic acid solvent used as a reaction medium in the liquid phase oxidation is usually 2 to 6 times the weight of para-xylene. The acetic acid solvent may contain a small amount of water, for example, 10% by weight or less. As the molecular oxygen-containing gas, air, oxygen diluted with an inert gas, oxygen-enriched air, or the like is used, but air is usually used from the viewpoint of equipment and cost.

The catalyst used for the liquid phase oxidation contains cobalt, manganese and bromine, and specific examples thereof include cobalt acetate and cobalt acetate.
Examples thereof include cobalt naphthenate and cobalt bromide.
Examples of the manganese compound include manganese acetate, manganese naphthenate, and manganese bromide. Examples of the bromide compound include hydrogen bromide, sodium bromide, cobalt bromide, manganese bromide, tetrabromoethane, and the like. These compounds may be used in combination. The amount of the catalyst used is such that the amount of the cobalt component is 1 to acetic acid in terms of cobalt metal.
20 to 3,000 weight ppm, preferably 200 to 2,
000 ppm by weight. The amount of the manganese component used is 0.001 to 0.4 times in atomic ratio to cobalt.
In terms of the absolute amount of the manganese component, it is usually 1 to 250 ppm by weight, preferably 5 to 200 ppm by weight, based on acetic acid in terms of manganese metal. The use amount of the bromine component is 0.1 to 5 times, preferably 0.2 to 2 times, the atomic ratio to cobalt. If the amount of the catalyst used is outside the above range, the purity or transmittance of the obtained terephthalic acid will be insufficient, and the combustion of acetic acid will increase, and the effect will not be obtained. In particular, the amount of the manganese component used is important. If the atomic ratio to cobalt is less than 0.001 times, the reaction activity is significantly reduced, and if it exceeds 0.4 times, the manganese component precipitates and is mixed into terephthalic acid. However, the quality of the product terephthalic acid deteriorates, or the loss of acetic acid increases.

The reaction is preferably carried out at a temperature of 140 to 210 ° C., preferably 170 to 200 ° C., particularly preferably 175 to 195 ° C. If the temperature is lower than 140 ° C., the reaction rate decreases. If the temperature exceeds 210 ° C., the loss due to combustion of the acetic acid solvent increases, which is not preferable. The reaction pressure is
The pressure is at least the pressure at which the mixture can maintain a liquid phase at least at the reaction temperature, and is usually 0.2 to 5 MPa. The reaction is usually carried out continuously, and the reaction time (average residence time) is 30 to 300 minutes. The water concentration in the reaction mother liquor is usually 5 to 25% by weight, preferably 7 to 20% by weight, and the water concentration is adjusted usually by extracting the gas volatilized in the reactor and condensing the gas. This can be performed by discharging (purging) a part of the reflux liquid of the condensable component out of the system. The reactor used in the present invention usually has a tank equipped with a stirrer, but a stirrer is not required, and a bubble column type may be used. A cooler is provided at the upper part of the reactor, and a molecular oxygen-containing gas supply port is provided at the lower part. Then, the molecular oxygen-containing gas supplied from the lower part is extracted from the reactor as a gas component accompanied by a large amount of acetic acid vapor after being used for the oxidation reaction, and then the acetic acid is condensed and separated by a reflux condenser. ,
Emitted as oxidizing exhaust gas. A part of the condensate is purged out of the system, and the remainder is refluxed to the reactor.

In the present invention, when carrying out the oxidation reaction of para-xylene, the oxidation obtained by condensing and removing condensable components from the gas extracted from the reactor as described in Japanese Patent Publication No. 5-32381. The exhaust gas may be branched into two streams, one discharged to the system and the other continuously circulated to the reactor. In the present invention, solid-liquid separation may be performed immediately after the above oxidation reaction, but additional treatment may be performed as necessary. As the additional treatment, for example, in the second reaction zone where the reaction mixture of the oxidation reaction (first reaction zone) is maintained at a temperature lower than the reaction temperature of the first reaction zone (normally 140 to 190 ° C.), It is effective to perform additional oxidation (hereinafter referred to as “low temperature additional oxidation”) without supplying xylene. The supply amount of the molecular oxygen-containing gas supplied to the second reaction zone is usually about 1/5 to 1/1000 of the amount supplied to the oxidation reaction in the first reaction zone, and the treatment time is 5 to 120 minutes. It is. As another method of additional treatment, the oxidation reaction mixture is further subjected to the first step.
It is also effective to perform additional oxidation in a reaction zone (third reaction zone) at a temperature higher than the reaction zone without supplying para-xylene (hereinafter, referred to as “high-temperature additional oxidation”). The temperature of the third reaction zone is usually 210 ° C. or higher, preferably 220 to 280 ° C. The amount of the molecular oxygen-containing gas supplied to the high-temperature additional oxidation is usually about 1/5 to 1/1000 of the initial oxidation reaction.

The reaction mixture after the oxidation reaction or the additional treatment step of low-temperature re-oxidation or high-temperature re-oxidation,
Solid-liquid separation may be performed immediately after each step, or solid-liquid separation may be performed through a crystallization treatment step. As a device for performing the solid-liquid separation, a device such as a decanter, a centrifugal separator, a vacuum belt filter, and a drum filter is used, but the device is not limited thereto. After the solid-liquid separation, the reaction medium attached to the surface of terephthalic acid is replaced and washed with acetic acid or acetic acid containing water. The washing may be performed by providing a suspension washing tank and then performing the solid-liquid separation again, or the washing and the solid-liquid separation may be performed in the single-stage operation described above. In view of the reduction in the number of devices, it is preferable to perform the washing and separation in one step.
As a liquid used for washing, fresh acetic acid may be used, or a liquid condensed in a reactor or a condenser above a low-temperature reoxidation tank may be used. Important operating factors in the present invention, the conditions for carrying out the above-mentioned washing and solid-liquid separation, the pressure region higher than normal pressure,
The temperature is preferably 19.6 kPa (gauge pressure) or more, more preferably 49 kPa (gauge pressure) or more, and the temperature range is preferably 140 ° C. or more, more preferably 165 ° C. or more, and even more preferably 170 ° C. to low-temperature reoxidation. Separation of the wet cake (terephthalic acid containing some reaction medium) at a temperature lower than the solid-liquid separation pressure without externally heating the separated wet cake (terephthalic acid containing some reaction medium) ") (Flash from high pressure to low pressure). That is, the drying is performed by utilizing the vapor pressure of the high-temperature solvent at the time of pressure separation. Usually, the pressure of the flash tank is preferably normal pressure, but may be either under pressure or under reduced pressure as long as the vaporized components do not condense. The temperature of the flash tank is not particularly limited, but in order to obtain a product in a dry state, a high temperature,
Desirably, the temperature ranges from 100 ° C. to about the temperature of pressure separation. The pressure difference before and after the flash is preferably 19.6k
It is at least Pa (gauge pressure), more preferably at least 49 kPa (gauge pressure).

As the valve used at the inlet of the flash tank, it is desirable to use a discharge valve. However, the valve is not limited as long as it is a valve capable of continuously supplying a high-concentration slurry. The reaction medium vaporized in the flash tank is removed by entraining with a non-condensable inert gas such as nitrogen, and the gas is cooled and condensed to recover the reaction medium, and then supplied to the reactor again or a water recovery step. Good to send to. On the other hand, the inert gas excluding the reaction medium can be used again as an entraining gas. The thus obtained dried terephthalic acid may be temporarily transferred to a silo by vigorous transfer, or may be immediately converted into a water slurry and sent to a hydrotreating step. Next, an example of a specific embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic example of a process according to the present invention.

First, a catalyst, a reaction medium, and para-xylene are supplied from line 1 to the oxidation reactor 3, and molecular oxygen is supplied from line 2. The liquid after the oxidation reaction is transferred to a low-temperature re-oxidation tank 5 through a line 4. A small amount of molecular oxygen is supplied from the line 6 to the low-temperature reoxidation tank. After the low-temperature additional oxidation, it is transferred to the pressurized solid-liquid separator 9 through the line 7. After the reaction medium in the slurry is separated by the separator 9 through the line 10, the remaining cake is washed with a part of the liquid generated by condensing the gas generated in the reactor 3 (withdrawn and supplied by the line 8), and again. Perform solid-liquid separation (the separated liquid line is 1
1). The reaction medium obtained by the first stage separation (line 1
0), and mainly the washing liquid (line 11) obtained by the second-stage separation is recycled to the reactor 3 again through the line 12. Some of the liquid passing through line 12
Through to the water separation step. The cake subjected to pressure separation and washing separation is flushed from a discharge valve 14 to a flush tank 15. Nitrogen is flowed from the flash tank 15 into the gas phase section 16, and the reaction medium evaporated in the flash tank 15 is accompanied and discharged from the line 17.
The discharged gas is cooled, and the condensed liquid 18 is supplied to the reactor 3. The gas phase 19 is transferred again to 16 and reused.
The dried terephthalic acid discharged from the flash tank 15 may be transferred to, for example, a slurrying tank, slurried with water, and then transferred to a hydrotreating step.

[0013]

EXAMPLE A model experiment was performed using the apparatus shown in FIG. 2 under the following conditions. That is, the configuration of the apparatus in FIG. 2 is equivalent to the pressurized solid-liquid separator 9 in FIG. 1, and instead of the sample holder 21 and the discharge valve 14 to which pressure can be applied, a ball valve 23 and a flash tank 15 are used. Corresponds to the flash tank 24. As shown in Table 1 below, the water / acetic acid-containing cake 22 in the sample holder was flushed through the ball valve 23 into the flash tank 24 using the apparatus shown in FIG. The liquid content in the acetic acid-containing cake 22 was determined by the following equation. The liquid composition (weight ratio) in the water / acetic acid-containing cake 22 was acetic acid: water = 80: 2.
0. The flash tank 24 is at normal pressure and 15 ° C. That is, it was confirmed that acetic acid was removed from the terephthalic acid cake containing acetic acid by flashing from high temperature and high pressure, and drying was performed.

[0014]

## EQU1 ## Liquid content (%) in cake of 22 = liquid / (dry cake + liquid) × 100

[0015]

[Table 1]

[0016]

According to the present invention, in particular, in the method for producing terephthalic acid by liquid-phase oxidation of para-xylene, a dryer is not required, and the energy used for drying can be reduced. It is an economical method.

[0017]

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an example process for implementing the present invention.

FIG. 2 is a schematic diagram of an example of a flash tank useful for practicing the present invention.

[Explanation of symbols]

1, 2, 4, 6, 7, 8, 10, 11, 12, 13, 1
6, 17, 18, and 19 are pipes a, b, and c are heat exchangers 3 are oxidation reactors 5 are low-temperature reoxidation reactors 9 are pressure separators / washers 14 are discharge valves 15 are flash tanks 21 are sample holders 22 is a cake containing water and acetic acid 23 is a ball valve 24 is a flash tank PG is a pressure gauge

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Toshiya Iwasaki 1-1 Kurosaki Castle Stone, Yawatanishi-ku, Kitakyushu-shi, Fukuoka F-term in the Kurosaki Works of Mitsubishi Chemical Corporation (reference) 4D076 AA02 AA14 BB18 EA12Y EA12Z FA22 HA11 4H006 AA02 AC46 AD10 AD17 BA03 BA16 BA20 BA28 BA32 BA37 BA49 BC10 BC14 BD82 BE30 BJ50 BS30 4H039 CA65 CC30

Claims (4)

[Claims] 1. A method for producing an aromatic dicarboxylic acid by removing a reaction medium from a slurry obtained by subjecting a dialkyl aromatic hydrocarbon to liquid phase oxidation in a reaction medium with molecular oxygen to obtain a product aromatic dicarboxylic acid. In the above, the slurry is subjected to solid-liquid separation under pressure, and the aromatic dicarboxylic acid obtained as a solid phase is flushed into the separation tank kept at a lower pressure from the pressure without reheating. To obtain an aromatic dicarboxylic acid from which the reaction medium has been removed. 2. A liquid phase oxidation of a dialkyl aromatic hydrocarbon,
The method for producing an aromatic dicarboxylic acid according to claim 1, wherein the temperature of the solid phase at the inlet of the separation tank is in the range of 140 to 210C. 3. The method for producing an aromatic dicarboxylic acid according to claim 1, wherein a discharge valve is used for flushing the separation tank. 4. The method for producing an aromatic dicarboxylic acid according to claim 1, wherein the dialkyl aromatic hydrocarbon is para-xylene and the aromatic dicarboxylic acid is terephthalic acid.