JP2001288139A - Method for producing high-purity terephthalic acid - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To provide a method for reducing the amount of new water used in producing high-purity terephthalic acid without deteriorating the quality of a product. SOLUTION: Para-xylene is oxidized by molecular oxygen in a liquid reaction medium to obtain a mixture containing crude terephthalic acid, and the mixture is heated at a temperature higher than the reaction zone without supplying para-xylene. Additional oxidation is carried out with oxygen to produce a slurry containing terephthalic acid of higher purity, terephthalic acid is separated from the slurry as a solid, and the terephthalic acid solid is dissolved in a liquid containing water. A solution is prepared, hydrogenated in the presence of a catalyst, and the resulting reaction product is subjected to solid-liquid separation to produce high-purity terephthalic acid. Further, at least a part of the liquid obtained by the solid-liquid separation is supplied to a hydrogenation treatment step.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for producing high-purity terephthalic acid. In particular, by reducing the amount of impurities contained in the water discharged (purged) from the reaction system, the processing load of the wastewater is reduced, and the amount of fresh water used and the amount of discharged water are reduced, which is economically advantageous. The present invention relates to a method for producing highly pure terephthalic acid, which is environmentally friendly. Here, “fresh water” is water newly supplied to the production process when producing high-purity terephthalic acid, and has been used in the reaction in the hydrogenation treatment step of the production. It means water without water, and water generated in the oxidation step is sometimes used as "fresh water".

[0002]

2. Description of the Related Art Generally, high-purity terephthalic acid is produced by purifying crude terephthalic acid obtained by oxidizing para-xylene. As a method for producing crude terephthalic acid,
Generally, paraxylene is oxidized by molecular oxygen in the reaction medium in the presence of a catalyst. As a general example of a method for producing crude terephthalic acid by purifying crude terephthalic acid, a crude terephthalic acid is made into a slurry in water, which is heated and dissolved, and then subjected to hydrogenation purification to produce terephthalic acid with higher purity. There is a method of obtaining. Generally, a product of high-purity terephthalic acid has a content of 4-carboxybenzaldehyde (hereinafter, referred to as “4CBA”), which is one of the intermediates in the oxidation reaction of para-xylene, and paratoluic acid (hereinafter, referred to as a hydrogenated product of 4CBA). , "P-TA") and the transmittance of light having a wavelength of 340 nm when the product is dissolved in an aqueous alkaline solution (hereinafter referred to as "transmittance"). Judgment and control are performed.

[0003] In the step of hydrogenating and refining the crude terephthalic acid to produce high-purity terephthalic acid, the liquid separated from solid high-purity terephthalic acid by solid-liquid separation after the hydrogenation reaction usually contains para-xylene. Contains a large amount of impurities generated during a reaction such as an oxidation reaction of hydrogen or a hydrogenation reaction of crude terephthalic acid. As impurities, 4CBA,
p-TA, benzoic acid (hereinafter referred to as “BA”) and the like. Therefore, water is generally discarded without being recycled.

[0004] Japanese Patent Application Laid-Open No. 5-58948 discloses that, when producing high-purity terephthalic acid, a liquid containing water is separated from terephthalic acid precipitated after the hydrogenation reaction to dissolve crude terephthalic acid. A method has been proposed for reducing the amount of fresh water used by recycling the liquid as it is. However, if the recycling of the liquid containing water is performed by the method proposed in the publication, when the recycling rate is increased, it is inevitable that a large amount of impurities are accumulated in the liquid, and as a result, In addition, the amount of the impurities mixed into the product is greatly increased. This allows
The transmittance also deteriorates, and the quality of the product high-purity terephthalic acid decreases.

[0005]

As described above, in the conventional method, in the production of high-purity terephthalic acid, the liquid containing water separated from terephthalic acid precipitated after the purification step contains a large amount of impurities, and is difficult to use in wastewater treatment. Load is applied. Further, even if the liquid is recycled, the recycling rate must be set low in order to maintain the quality of the product, and the economic benefit of applying the method industrially is extremely small. As a result of intensive studies, the present inventors have found the following findings. That is, between the step of supplying the paraxylene and the molecular oxygen-containing gas to carry out the oxidation and the subsequent hydrotreating step, the temperature is higher than the temperature of the oxidation step without supplying the paraxylene. By introducing an oxidation step, the content of impurities in the material sent to the hydrorefining step can be reduced very effectively. Also, for the liquid containing water, which is recovered by solid-liquid separation after the hydrorefining, the introduction of the above-mentioned additional oxidation step can extremely effectively reduce the content of impurities that may cause deterioration in product quality. Can be done. Further, the liquid can be recycled in the hydrorefining step while keeping the amount of impurities that may cause quality deterioration into the product to be extremely small. An object of the present invention is to provide a method for reducing the amount of fresh water used in producing high-purity terephthalic acid without deteriorating the quality of the product, high-purity terephthalic acid.

[0006]

The gist of the present invention is as follows. ~ 5. And a process for producing high-purity terephthalic acid. 1. 1. oxidation of para-xylene with molecular oxygen in a liquid reaction medium to obtain a mixture containing crude terephthalic acid; The mixture is subjected to an additional oxidation with molecular oxygen at a temperature higher than the one reaction zone without supplying para-xylene to obtain a slurry containing terephthalic acid which is higher in purity than the terephthalic acid obtained in 1 above. 2. manufacturing; 3. separating terephthalic acid from the slurry as a solid; 4. a solution is prepared by dissolving the above terephthalic acid solid in a liquid containing water, and hydrogenated in the presence of a catalyst; The reaction product obtained in the above 4 is subjected to solid-liquid separation to produce high-purity terephthalic acid. Still further, the gist of the present invention is more preferably a method of producing high-purity terephthalic acid, wherein at least a part of the liquid obtained by the solid-liquid separation in step 5 is supplied to the hydrogenation treatment step in step 4. Be in the way.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. First, in step 1, para-xylene is oxidized by a molecular oxygen-containing gas in a liquid reaction medium to obtain a crude terephthalic acid slurry. At this time, it is usually desirable to oxidize 90% by weight or more, particularly 95% by weight or more of para-xylene to terephthalic acid. As a liquid reaction medium for the oxidation, a lower aliphatic carboxylic acid is usually used. Acetic acid is particularly preferred.

Hereinafter, a case where acetic acid is used as a solvent will be described as an example. The amount of the acetic acid solvent to be used 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 and the like are used.
Usually, the use of air is preferable from the viewpoint of facilities and cost.

A catalyst is usually used for the oxidation of para-xylene. The catalyst generally contains cobalt, manganese and bromine, and specific examples thereof include cobalt compounds such as cobalt acetate, cobalt naphthenate,
Cobalt bromide and the like are exemplified. 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 use amount of the catalyst is such that the use amount of the cobalt component is 120 to 3,000 weight ppm, preferably 200 to 2,000 weight ppm, based on acetic acid in terms of cobalt metal. The use amount of the manganese component is 0.001 to 2 times in atomic ratio to cobalt. In terms of the absolute amount of the manganese component, it is generally 1 to 300 ppm by weight, preferably 5 to 280 parts by weight, based on acetic acid in terms of manganese metal.
m. 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, or the combustion of acetic acid will increase, and the effect will be hardly obtained. In particular, the amount of the manganese component used is important, and the atomic ratio of cobalt to cobalt is 0.1%.
If it is less than 001 times, the reaction activity tends to decrease significantly. If it exceeds twice, a manganese component precipitates, which is mixed into terephthalic acid, which is likely to cause disadvantages such as deterioration of the quality of terephthalic acid as a product or an increase in acetic acid loss.

The reaction temperature for the oxidation of paraxylene in step 1 is usually 140 to 210 ° C., preferably 170 to 200 ° C.
C., particularly preferably 175-198.degree. C. If the temperature is lower than 140 ° C., the reaction rate decreases. If the temperature exceeds 210 ° C., the loss due to the combustion of the acetic acid solvent tends to increase. The reaction pressure is at least a 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 3
0-300 minutes. The water concentration in the reaction medium is usually 5
The concentration of water is usually adjusted by extracting a gas volatilized in the reactor and condensing the gas to obtain a part of a reflux liquid of a condensable component. It can be performed by purging outside the system.

The reactor used in the present invention is usually a tank equipped with a stirrer, but the stirrer is not always necessary, and may be a bubble column type. 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 was 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 acetic acid was condensed and separated in a reflux condenser. Later, it is discharged as oxidizing exhaust gas. Part of the condensate is purged out of the system for moisture control,
The remainder is refluxed to the reactor.

In the present invention, when carrying out the oxidation reaction of para-xylene in step 1, as described in JP-A-9-278709, condensable components are condensed and removed from the gas extracted from the reactor. Oxidation exhaust gas obtained by
It may be branched into two streams, one discharged outside the system and the other continuously circulated to the reactor.

In the present invention, after the above oxidation reaction, additional oxidation may be performed immediately at a temperature higher than the reaction temperature, but if necessary, other additional treatment may be performed prior to this. As the additional treatment, for example, para-xylene is supplied in a second reaction zone in which the reaction mixture of the oxidation reaction (first reaction zone) is maintained at a temperature lower than the first reaction zone, usually at 140 to 195 ° C. It is effective to perform additional oxidation (hereinafter referred to as “low temperature supplemental oxidation”). The reaction pressure is at least a 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 5 to 120 minutes. As in the case of the first reaction zone, air, oxygen diluted with an inert gas, oxygen-enriched air, or the like is used as the molecular oxygen-containing gas to be supplied to the low-temperature re-oxidation. Usually, the use of air is preferred. The supply amount is usually 1/5 to 1 / 10,000 of the amount supplied to the oxidation reaction in the first reaction zone.
It is about. As a reactor for performing the low-temperature reoxidation reaction,
Any one that can be used for the first reaction zone can be used.

In the present invention, from the first reaction zone,
Direct oxidation or through an oxidation reaction in the second reaction zone, in a reaction zone (third reaction zone) at a higher temperature than the first reaction zone, additional oxidation without supplying para-xylene (hereinafter, “high-temperature additional oxidation”)
To obtain medium-purity terephthalic acid. In the present invention, "medium purity" refers to a higher purity than the reaction product in the first reaction zone or a substance which has been subjected to further low-temperature post-oxidation, and a terephthalic acid finally obtained. Also means low purity. For example, medium purity
4CBA content is 500ppm or less, transmittance is 80%
The quality level is above. When the oxidation reaction prior to the high-temperature additional oxidation is performed under normal conditions to produce crude terephthalic acid, the 4CBA content is 800 to 5,000 ppm and the transmittance is less than 80%. If an attempt is made to produce medium-purity terephthalic acid without performing high-temperature additional oxidation, it is necessary to increase the reaction temperature and / or the catalyst concentration in the oxidation reaction prior to this, and in this case, the combustion amount of acetic acid also increases. Since it increases, it is disadvantageous in terms of cost.

The temperature of the third reaction zone is usually 200 ° C. or more, preferably 210-280 ° C. The reaction pressure is
The pressure is at least the pressure at which the mixture can maintain the liquid phase at least at the reaction temperature, and is usually 5 to 10 MPa. The reaction is usually carried out continuously, and the reaction time (average residence time) is 5
~ 120 minutes. As in the case of the first reaction zone, air, oxygen diluted with an inert gas, oxygen-enriched air, or the like is used as the molecular oxygen-containing gas to be supplied to the high-temperature re-oxidation. Usually, the use of air is preferred. The supply amount is usually about 1/5 to 1 / 10,000 of the supply amount for the oxidation reaction in the first reaction zone. The processing time is usually 5 to 120 minutes.
Any reactor that can be used in the first reaction zone can be used as a reactor for performing the high-temperature re-oxidation reaction.

The high-temperature additional oxidation treatment may be performed without adding a catalyst, or may be performed after adding a small amount of a catalyst. When a catalyst is added, usually, a catalyst having the same composition as that used in the first reaction zone is used. However, in consideration of the concentration in the high-temperature reoxidation reactor, the composition of the catalyst to be added is slightly changed. Is also good. Further, the amount of the catalyst to be added is usually 10% or less of the amount used in the first reaction zone. By crystallizing the reaction mixture subjected to the high-temperature reoxidation treatment, a terephthalic acid solid having a medium purity can be obtained. The terephthalic acid is usually dried after the crystallization treatment and sent to the hydrogenation and purification step. However, the drying step is omitted, and
It may be sent to the hydrorefining step.

In the present invention, by increasing the purity of terephthalic acid to be sent to the hydrogenation purification step described below to a medium purity, the content of impurities contained in the water-containing liquid recovered after the purification step can be reduced. did it. As a result, even if the liquid is recycled in the hydrorefining step, the amount of accumulated impurities in the step can be reduced, and the recycling rate of the liquid can be reduced while maintaining high product quality. It became possible to raise.

Various methods can be used for the subsequent hydrorefining step. As an example, a liquid containing water is added to the above crude terephthalic acid to form a slurry, which is then heated and dissolved, and then subjected to a hydrogenation treatment in the presence of a hydrogenation catalyst. And a method for producing high-purity terephthalic acid by performing solid-liquid separation. At this time, the crude terephthalic acid is usually supplied to the hydrogenation reaction system as a slurry of 20 to 35% by weight based on the liquid containing water.

As the hydrogenation catalyst, any known catalyst can be used. For example, palladium, ruthenium, rhodium, osmium, iridium, platinum, iron, cobalt, nickel, etc. supported on activated carbon can be used. Is mentioned. These catalysts may be used in combination.

The reaction conditions for the hydrorefining reaction are usually a reaction temperature of 255-300 ° C. and a reaction pressure of 1-12.
In general, the pressure is set at 0.05 MPa to 3 MPa, but the present invention is not limited thereto.

The terephthalic acid purified by the hydrogenation reaction is generally separated as a solid by crystallization and solid-liquid separation. The crystallization conditions are selected in consideration of the yield of the solid content to be precipitated, the purity of the solid content, and the like.
The operation is performed at 1.5 MPa. Preferably, the crystallization is performed in a plurality of stages, and the crystallization conditions in the final stage are 150 to 1
The method is carried out in such a manner that the temperature becomes 80 ° C. and 0.3 to 1.4 MPa. At this time, the residence time in each crystallization tank is 5 to 2
It takes about 00 minutes. In particular, when the temperature of the crystallization tank in the final stage is lower than the above range, the amount of impurities such as p-TA rapidly increases, so that the purity of terephthalic acid separated as a solid content decreases. Tends to occur.

In general, by performing solid-liquid separation after crystallization, terephthalic acid precipitated in the crystallization step is separated from a liquid whose main component is water. The temperature for solid-liquid separation and the pressure conditions are 100 to 190 ° C. and 0.1 to
1.5 MPa, preferably 150 to 180 ° C., 0.3
Performed at ~ 1.4 MPa. Normally, when the above-mentioned conditions of the crystallization tank and the crystallization step are performed in multiple stages, conditions substantially equal to those of the final crystallization tank are selected. As a device for performing solid-liquid separation, a device such as a decanter centrifuge, a vacuum belt filter, and a drum filter is generally used, but is not limited thereto. It is also possible to use a combination of two or more of these devices.

The terephthalic acid obtained by separating solids precipitated after the purification step by solid-liquid separation may be dried as it is to obtain a product, or may be slurried with fresh water in a suspension washing tank and washed. Alternatively, the solid content separated by solid-liquid separation may be dried to obtain a product. The water used for the washing is separated as a liquid containing water by solid-liquid separation, and has a sufficiently low content of impurities, so that it can be recycled as it is as a reaction medium when hydrogenating and purifying crude terephthalic acid. .

After the above-described oxidation reaction step and subsequent hydrogenation and purification step, the liquid containing precipitated high-purity terephthalic acid and water separated by solid-liquid separation may be used as such or may be subjected to additional treatment. Later, the medium-purity terephthalic acid is used for dissolving and recycled to the hydrotreating step again. As additional processing, for example,
There is a method in which impurities are further precipitated by lowering the temperature of the liquid, and the impurities are separated into solid and liquid. For example, the liquid is sent to a tank depressurized to 9.3 kPa (70 torr) with a steam ejector or the like, and the generated steam is condensed by a condenser and cooled to 40 to 50 ° C. Thereafter, the liquid from which the precipitated impurities have been removed by solid-liquid separation is recycled again to the hydrogenation purification step. For this separation, a centrifuge, a Buchner, a belt filter, a back filter, or the like is used, but the invention is not limited to this.

At this time, the liquid can be continuously circulated and supplied for dissolving terephthalic acid, or it can be stored in another place once and taken out in a required amount according to demand. It is. As the amount of recycling used,
It is selected in consideration of the concentration of impurities contained in the liquid, economics, etc., but the total amount of liquid including water separated by solid-liquid separation from terephthalic acid precipitated after the purification step, 2
It is 0 to 100% by weight, preferably 40 to 90% by weight. In selecting the amount of recycling, the concentration of impurities remaining in the high-purity terephthalic acid as the final product, in particular, the concentration of p-TA, is used as a guide.
It is preferable to select the recycling amount so as to be 0 ppm or less.

Next, one embodiment of the present invention will be described with reference to FIG. FIG. 1 schematically illustrates an example of a process in the present invention. First, a mixture (1) of a catalyst, a solvent, and paraxylene and a molecular oxygen-containing gas (2) are supplied to the oxidation reactor (3). The gas component extracted from the reactor (3) is condensed and separated from acetic acid and the like in the heat exchanger (4), and then discharged as oxidized exhaust gas (5). The condensate is
After a part thereof is discharged out of the system as a purge amount (6) for moisture control, it is refluxed to the reactor (3). The oxidized substance (7) in the reactor (3) is transferred to a low-temperature re-oxidation tank (9) and oxidized by a small amount of a molecular oxygen-containing gas (8). The low-temperature oxidized substance (1
0) is preliminarily heated to a higher temperature than the reactor (3) in the heat exchanger (11) and then transferred to the high-temperature post-oxidation tank (13), where a small amount of the molecular oxygen-containing gas (12 ). The substance subjected to the high temperature oxidation treatment (14)
Is crystallized in the crystallization tank (15). In FIG.
Although an example is shown in which crystallization is performed in two stages after the high-temperature additional oxidation treatment, the number of crystallization stages is not limited to this,
Any number of stages or more may be used. The crystallized substance (1
6) is separated into a liquid component (18) and a medium-purity terephthalic acid (19) by a solid-liquid separator (17). The medium-purity terephthalic acid (19) is made into a slurry with a liquid (20) containing water in a mixing tank (21), and the slurry (2)
2), together with hydrogen (23), a hydrorefining reactor (2)
Transferred to 4). The hydrotreated material (2
5) is crystallized in the crystallization tank (26). FIG. 1 shows an example in which crystallization is performed in one stage after the high-temperature additional oxidation treatment. However, the number of crystallization stages is not limited to this.
One or more stages may be used. The crystallized substance (2
7) is separated into a liquid component (30) and a solid component (29) by a solid-liquid separator (28), and the solid component (29) is
The product is dried in a dryer (31) to produce a high-purity terephthalic acid (32) as a product. On the other hand, a part of the liquid component (30) separated by the solid-liquid separator (28) is discharged to the outside as a purge part (33) as needed, and the remainder is recycled. The liquid (34) to be recycled is supplied to the mixing tank (21) as a liquid (20) containing water together with fresh water (35) supplemented in the system as needed, and is recycled.

[0028]

EXAMPLES Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to the examples unless it exceeds the gist. Example 1 A catalyst containing three components of Co, Mn, and Br was used as a catalyst, air was used as a molecular oxygen-containing gas, and acetic acid was used as a solvent. Obtained. First, the oxidation reaction in the first reaction zone
The concentrations of Co, Mn, and Br in the reactor were maintained at 270 ppm, 170 ppm, and 500 ppm, respectively, and the temperature and pressure were set at 197 ° C. and 1.68, respectively.
The operation was performed so that the residence time was 87 minutes as MPa. Subsequent low-temperature re-oxidation treatment was performed by increasing the temperature and pressure in the reactor to 191 ° C. and 1.42, respectively.
The operation was performed so that the residence time was 33 minutes as MPa. In the subsequent high-temperature reoxidation treatment, the temperature and pressure in the reactor were raised to 265 ° C. and 5.49, respectively.
The operation was performed so that the residence time was 39 minutes as MPa. Subsequently, crystallization and solid-liquid separation were performed to analyze the medium-purity terephthalic acid.
The CBA content was 248 ppm, and the transmittance was 89.0%. The medium-purity terephthalic acid was purified by a series of continuous reactions maintaining the conditions detailed below. The solid to be treated is transferred to the continuous reaction system as a 30% by weight slurry. In the continuous reaction system, the temperature and pressure in the reactor are set to 290 and 290, respectively, in the hydrotreating step.
C., 8.73 MPa, and the residence time was 9 minutes. As a result of analyzing the terephthalic acid product produced by a series of purification steps in which the above condition was maintained, the content of 4CBA was 1.7.
ppm, p-TA content 15 ppm, transmittance 97.
A good value of 3% was shown.

Example 2 The same procedure as in Example 1 was carried out until the hydrogenation and purification step.
50% of the liquid component collected by crystallization and solid-liquid separation of the treated substance is discharged out of the system, and the remaining 50%
Was recycled into the above-mentioned slurry of medium-purity terephthalic acid. At this time, in order to compensate for the discharge of the liquid,
The required amount of fresh water was supplied. As a result of analyzing the terephthalic acid product manufactured by a series of purification steps in which the above-mentioned state was maintained, a good value was obtained with a 4CBA content of 2.1 ppm, a p-TA content of 33 ppm, and a transmittance of 96.9%. showed that. Thus, in this example, it was found that the quality of the product was reduced only slightly by recycling 50% of the liquid containing water used in the hydrorefining reaction.

Comparative Example Terephthalic acid was produced in the same manner as in Example 1 except that the reaction temperature and the catalyst concentration in the oxidation reaction in the first reaction zone were increased, and the high-temperature re-oxidation step was not performed. did. In the oxidation reaction in the first reaction zone, the concentration of Co, Mn, and Br in the reactor was set to 330 p
pm, 215 ppm, and 660 ppm, and the temperature and pressure were set to 202 ° C. and 1.57 MPa, respectively, so that the residence time was 62 minutes. The subsequent low-temperature post-oxidation treatment was performed at a temperature and pressure in the reactor of 189 ° C. and 1.27 MPa, respectively, so that the residence time was 34 minutes. It is not essentially different from the low-temperature re-oxidation treatment conditions in Examples 1 and 2. In this comparative example, as a result of analyzing medium-purity terephthalic acid before performing the hydrorefining step,
The 4CBA content is 1,865 ppm, and the transmittance is 37.0.
%Met. As a result of analyzing the terephthalic acid product obtained by subjecting it to hydrogenation purification, crystallization, and solid-liquid separation in the same manner as in the examples, the 4CBA content was 6 ppm and the p-
The TA content was 87 ppm, and the transmittance was as low as 94.4%. Thereby, even if the reaction temperature and the catalyst concentration in the step are increased for the purpose of reducing the amount of impurities generated in the oxidation reaction in the first reaction zone, as in Examples and Reference Examples, It has been found that the quality of the product is significantly reduced unless the high-temperature additional oxidation treatment is performed. In this comparative example, the liquid obtained by the crystallization and the solid-liquid separation after the hydrorefining step is all converted into a slurry of medium-purity terephthalic acid with fresh water without recycling. Further, if the liquid is recycled, the quality of the product is necessarily deteriorated.

Table 1 shows each condition of the above experimental example.
Table 2 summarizes the experimental results.

[0032]

[Table 1]

(*) Concentration of crude terephthalic acid or medium-purity terephthalic acid sent to the hydrorefining step (**) (flow rate of liquid to be recycled) / ((flow rate of liquid to be recycled) + (purge) Liquid flow rate))

[0034]

[Table 2]

[0035]

According to the method of the present invention described above, when producing high-purity terephthalic acid, impurities contained in water discharged from the reaction system can be reduced, and the load of wastewater treatment can be reduced. . In addition, the supply amount of fresh water and the purge amount of water in the hydrorefining of terephthalic acid can be reduced without deteriorating the quality of the product, and the production method is economically advantageous and environmentally friendly. , High-purity terephthalic acid can be produced.

[0036]

[Brief description of the drawings]

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

[Explanation of symbols]

1: Mixture of catalyst, solvent and para-xylene 2: Molecular oxygen-containing gas 3: Oxidation reactor 4: Heat exchanger 5: Oxidation exhaust gas 6: Condensate purge 7: Oxidized substance 8: Molecular oxygen-containing Gas 9: Low-temperature reoxidation reactor 10: Low-temperature reoxidation-treated substance 11: Heat exchanger 12: Molecular oxygen-containing gas 13: High-temperature re-oxidation reactor 14: High-temperature re-oxidation-treated substance 15: Crystallization tank 16: Crystallized substance 17: Solid-liquid separator 18: Liquid component separated at 17: 19 Solid component separated at 19: 17 (medium purity terephthalic acid) 20: Liquid containing water 21: Mixing tank 22: Medium purity terephthalic acid slurry 23: Hydrogen 24: Hydrotreating reactor 25: Hydrotreated material 26: Crystallization tank 27: Crystallized material 28: Separated by solid-liquid separator 29:28 Solid component 3 : 28 the separated liquid component 31: Dryer 32: Product high purity part of the liquid component separated terephthalic acid 33:28 purge fraction 34:28 recycled portion of the separated liquid component 35: fresh water

 ──────────────────────────────────────────────────続 き Continued on front page F term (reference) 4H006 AA02 AC46 AD15 AD17 AD31 BB31 BC10 BC14 BC37 BD42 BD53 BD60 BE20 BE30 BJ50 BS30 4H039 CA65 CC30

Claims (5)

[Claims] 1. The following 1. ~ 5. A method for producing high-purity terephthalic acid, comprising the steps of: 1. 1. oxidation of para-xylene with molecular oxygen in a liquid reaction medium to obtain a mixture containing crude terephthalic acid; The mixture is subjected to an additional oxidation with molecular oxygen at a temperature higher than the one reaction zone without supplying para-xylene to obtain a slurry containing terephthalic acid which is higher in purity than the terephthalic acid obtained in 1 above. 2. manufacturing; 3. separating terephthalic acid from the slurry as a solid; 4. a solution is prepared by dissolving the above terephthalic acid solid in a liquid containing water, and hydrogenated in the presence of a catalyst; The reaction product obtained in the above 4 is subjected to solid-liquid separation to produce high-purity terephthalic acid. 2. The method for producing high-purity terephthalic acid according to claim 1, wherein at least a part of the liquid obtained by the solid-liquid separation in step 5 is supplied to the hydrogenation treatment step in step 4. 3. The high-purity terephthalic acid according to claim 1, wherein the oxidation of paraxylene in the step 1 is carried out in a reaction zone at 140 to 210 ° C., and the additional oxidation in the step 2 is carried out in a reaction zone at 210 to 280 ° C. Manufacturing method. 4. The method for producing high-purity terephthalic acid according to claim 1, wherein the transmittance of terephthalic acid separated in step 3 is 80% or more. 5. The high-purity terephthalic acid according to claim 1, wherein the content of 4-carboxybenzaldehyde in the terephthalic acid separated in the step 3 is 500 ppm by weight or less. Method for producing acid.