WO2005115956A1 - Process for producing high-purity terephthalic acid - Google Patents

Abstract

A process for producing high-purity terephthalic acid which comprises: an oxidation step (a) in which p-xylene is oxidized to obtain crude terephthalic acid; a dissolution step (b) in which the crude terephthalic acid is dissolved in a water solvent to obtain an aqueous solution of the crude terephthalic acid; a reduction step (c) in which the aqueous solution of the crude terephthalic acid is brought into contact with hydrogen to obtain a liquid resulting from a reduction reaction; a crystallization step (d) in which the liquid resulting from a reduction reaction is cooled to crystallize high-purity terephthalic acid; a solid-liquid separation step (e) in which the slurry obtained in the crystallization step (d) is subjected to solid-liquid separation; and a crystallization step (f) in which the isolated mother liquor obtained in the solid-liquid separation step (e) is cooled to form crystals consisting mainly of the terephthalic acid and p-toluic acid contained in the mother liquor, wherein the cooling of the mother liquor is conducted using two or more cooling tanks which have a reduced pressure so as to vaporize the water solvent contained in the mother liquor and in which the last cooling tank is regulated so as to have a pressure below the atmospheric pressure and a temperature of 40-70°C.

Description

 Method for producing high-purity terephthalic acid

 Technical field

 The present invention relates to a method for producing high-purity terephthalic acid.

 Background art

 [0002] In the high-purity terephthalic acid production process, in order to produce high-purity terephthalic acid with noraxylene power, first, raw para-xylene is oxidized to generate crude terephthalic acid, and then an intermediate contained therein is produced. The product, 4 carboxybenzaldehyde, is reduced to paratoluic acid and removed to produce high-purity terephthalic acid.

 [0003] The process is as follows. The raw material para-xylene is air-oxidized to terephthalic acid with a catalyst in a high-temperature, high-pressure acetic acid solvent. At this time, an intermediate 4 carboxybenzaldehyde is by-produced together with terephthalic acid. The slurry containing these is crystallized and separated into solid and liquid to obtain crude terephthalic acid crystals. Next, the crude terephthalic acid crystals are dissolved in water under high-temperature and high-pressure conditions to form an aqueous solution, and 4-carboxybenzaldehyde contained in the crude terephthalic acid is hydrogen reduced to paratoluic acid having high water solubility. Thereafter, the mixture is depressurized and cooled, and terephthalic acid with low water solubility is crystallized from the aqueous solution and recovered as high-purity terephthalic acid.

 [0004] Here, active components such as terephthalic acid and paratoluic acid are dissolved in the primary separation mother liquor from which high-purity terephthalic acid has been separated, and crystallization is performed to improve productivity and reduce drainage load. It is known that a secondary crystal is recovered by cooling in a tank while stirring to precipitate a secondary crystal (Patent Documents 1 and 2).

[0005] However, when the primary separated mother liquor is cooled to a predetermined final temperature at once, the crystal particle diameter becomes finer, and when the secondary crystal is recovered using a recovery device such as a filter, the filter is clogged. In addition, problems such as a decrease in the recovery rate occur. In addition, this secondary crystal cannot be sufficiently agitated until near the inner wall of the tank by crystallization using a propeller-type stirring blade or paddle-type stirring blade, which has high adhesion, so Is formed. Furthermore, fine powder entrained in the vapor generated by the above-mentioned crystallization treatment is formed on the side of the gas phase in the crystallization tank. Adheres to If the deposits on these devices grow and become detached, they may cause clogging of pipes and the like. Therefore, it was necessary to periodically perform cleaning work such as stopping and cleaning the facilities. Patent Document 1: JP-A-52-128344

 Patent Document 2: JP-A-5-58948

 Disclosure of the invention

 Problems to be solved by the invention

[0006] Therefore, in the production of high-purity terephthalic acid, the present invention increases the recovery rate of the secondary crystals precipitated by cooling the primary separation mother liquor and increases the secondary separation mother liquor having a low turbidity (SS concentration). It is an object of the present invention to provide a method for producing high-purity terephthalic acid, which can obtain terephthalic acid. Another object of the present invention is to provide a method for producing high-purity terephthalic acid that can suppress formation of deposits in the system that cause blockage and generation of lumps during cooling treatment.

 Means for solving the problem

[0007] The present inventors have conducted intensive studies to solve the above problems, and as a result, found that the above problems can be solved by depressurizing and cooling the primary separated mother liquor in multiple stages and crystallizing it, and completed the present invention. Was. That is, the gist of the present invention resides in the following (1) to (11).

[0008] (1) An oxidation step of oxidizing para-xylene to obtain crude terephthalic acid containing 4 carboxybenzaldehyde (a),

 A dissolving step of dissolving the above crude terephthalic acid in an aqueous solvent under high temperature and pressure to obtain a crude terephthalic acid aqueous solution (b)

 A reduction step (c) of obtaining a reduction reaction solution obtained by reducing the carboxybenzaldehyde to paratoluic acid by bringing the crude terephthalic acid aqueous solution into contact with hydrogen in the presence of a catalyst.

 A crystallization step (d) of evaporating the reduction reaction solution under reduced pressure and cooling to 120 to 200 ° C to crystallize high-purity terephthalic acid crystals.

 A solid-liquid separation step (e) of solid-liquid separation of the slurry obtained in the crystallization step (d) into crystals comprising the high-purity terephthalic acid crystals and a separation mother liquor;

Cooling the separated mother liquor, and crystallizing a crystal mainly composed of terephthalic acid or paratoluic acid contained in the separated mother liquor obtained in the solid-liquid separation step (e) (f), The method for producing high-purity terephthalic acid having

 Cooling is performed by using a plurality of cooling tanks for cooling by evaporating the water solvent in the mother liquor by lowering the pressure, the last of which is reduced to below atmospheric pressure, and the temperature of the last cooling tank is reduced to 40 to A method for producing high-purity terephthalic acid at 70 ° C.

[0009] (2) The first of the cooling tanks used in the crystallization step (f) is depressurized and evaporated.

The method for producing high-purity terephthalic acid according to the above (1), wherein the temperature is cooled to a temperature lower than the temperature at the time of solid-liquid separation in the solid-liquid separation step (e).

(3) The method for producing high-purity terephthalic acid according to the above (2), wherein the first cooling tank is depressurized and evaporated to atmospheric pressure.

 (4) The method for producing high-purity terephthalic acid according to any one of (1) to (3) above, wherein the cooling tank used in the crystallization step (f) has an anchor-type stirring blade.

[0012] (5) At least one of the cooling tanks used in the crystallization step (f) has a stirring blade having a distance from the inner wall of the cooling tank of 10 mm or more and 50 mm or less. )) The method for producing high-purity terephthalic acid according to any one of the above).

(6) A filtration step (g) of solid-liquid separation of the slurry obtained in the crystallization step (f) into crystals and a separated mother liquor using a filter,

 The method for producing high-purity terephthalic acid according to any one of the above (1) to (5), comprising:

(7) The method for producing high-purity terephthalic acid according to (6) above, wherein the crystals separated in the filtration step (g) are provided to the oxidation step (a).

(8) In the filtration step (g), the downstream side of the filter of the filter is set at a pressure higher than the atmospheric pressure, and the upstream side of the filter of the filter is set at a pressure higher than that of the downstream side of the filter, and the filtration is performed. The method for producing high-purity terephthalic acid according to (6) or (7), wherein solid-liquid separation is performed by cake filtration in a forward direction of the machine.

(9) The method according to (6) to (8), wherein the separated mother liquor obtained in the filtration step (g) is directly or indirectly provided to the dissolution step (b). A method for producing high-purity terephthalic acid.

(10) The separated mother liquor obtained in the filtration step (g) is brought into contact with a synthetic adsorbent to remove paratoluic acid, and then provided to the dissolution step (b) in the above (6) to (8). The method for producing highly pure terephthalic acid according to any one of the above. (11) The method for producing high-purity terephthalic acid according to any one of (6) to (10), wherein the concentration of the suspended substance in the separated mother liquor obtained in the filtration step (g) is 200 mg ZL or less. .

 The invention's effect

 According to the present invention, in producing high-purity terephthalic acid, the primary separation mother liquor is cooled to increase the recovery rate of secondary crystals precipitated, and a secondary separation mother liquor with low turbidity (SS concentration) is obtained. And a method for producing high-purity terephthalic acid. In addition, it is possible to provide a method for producing high-purity terephthalic acid, which can suppress formation of deposits in the system that cause blockage and generation of lumps during cooling treatment.

 Brief Description of Drawings

FIG. 1 is a flowchart showing an example of a method for producing terephthalic acid which is effective in the present invention.

 Explanation of symbols

[0021] 12 Slurry tank

 12a pump

 12b heater

 13 Hydrogenation reactor

 14 Crystallization tank

 15 Solid-liquid separation and washing equipment

 16 Drying equipment

 17 First pressure relief cooling tank

 18 Final pressure relief cooling tank

 19 Filtration machine

 A para-xylene

 B Oxygen-containing gas

 C Crude terephthalic acid

 D water

 E Starting slurry

 E 'aqueous solution

F hydrogen G reduction anti)

 H slurry

 I Cleaning liquid

 J Primary separation mother liquor

 K Wash drainage

 L High purity terephthalic acid cake

 M High purity terephthalic acid crystal

 N Intermediate slurry

 O Secondary slurry

 P Secondary separation mother liquor

 Q secondary crystal

 BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in detail.

 [0023] The method for producing high-purity terephthalic acid of the present invention comprises the steps of: (a) preparing a crude terephthalic acid containing 4-carboxybenzaldehyde by oxidizing noraxylene;

 A dissolving step of dissolving the above crude terephthalic acid in an aqueous solvent under high temperature and pressure to obtain a crude terephthalic acid aqueous solution (b)

 A reduction step (c) of obtaining a reduction reaction solution obtained by reducing the carboxybenzaldehyde to paratoluic acid by bringing the crude terephthalic acid aqueous solution into contact with hydrogen in the presence of a catalyst.

 A crystallization step (d) of evaporating the reduction reaction solution under reduced pressure and cooling to 120 to 200 ° C to crystallize high-purity terephthalic acid crystals.

 A solid-liquid separation step (e) of solid-liquid separation of the slurry obtained in the crystallization step (d) into crystals comprising the high-purity terephthalic acid crystals and a separation mother liquor;

 Cooling the separated mother liquor and crystallizing a crystal mainly composed of terephthalic acid or paratoluic acid contained in the separated mother liquor obtained in the solid-liquid separation step (e) (f), In the method for producing the acid,

For multiple stages of cooling tanks for cooling by evaporating the water solvent in the mother liquor by lowering the pressure And the pressure in the last cooling tank is reduced to below atmospheric pressure, and the temperature of the last cooling tank is set to 40 to 70 ° C.

 First, in the crude terephthalic acid producing step (a), crude terephthalic acid is produced by subjecting para-xylene to liquid phase oxidation with molecular oxygen in an acetic acid solvent in the presence of a catalyst. This step is well-known, and as the catalyst, a catalyst which is conventionally known to be usable in this reaction is used. Specifically, a heavy metal such as a cobalt compound, a manganese compound, an iron compound, and a chromium compound is used. And bromine compounds. These are present in the reaction system in a dissolved state. Among them, a combination of a cobalt compound or a manganese compound and a bromine compound is preferred. In this case, these compounds are usually used in such a manner that the concentration of SlO in the Connort reactor is 10 to 5000 ppm, the manganese atom is 10 to 5000 ppm, and the bromine atom is 10 to 10000 ppm.

 As the molecular oxygen, a mixed gas of an inert gas and oxygen is usually used, and for example, air or oxygen-enriched air is used. The molar ratio of molecular oxygen to paraxylene supplied to the reactor is usually 3 to 20 times, preferably 2 to 4 times.

 [0026] The ratio of para-xylene to acetic acid supplied to the reactor is usually 1 to 50% by weight. The water concentration in the reaction system is usually 5 to 20% by weight, preferably 5 to 15% by weight.

[0027] The temperature of the oxidation reaction is usually from 160 to 260 ° C, preferably from 170 to 210, and the pressure is usually from 0.5 to higher as long as the reaction system can maintain a liquid phase at a reaction temperature or higher. 5 MPa, preferably 1-2 MPa, and the residence time is usually 10-200 minutes.

[0028] Since terephthalic acid is hardly soluble in acetic acid as a solvent, terephthalic acid generated in the oxidation reaction step usually precipitates as crystals to form a slurry. However, depending on the amount of solvent, reaction temperature and pressure, terephthalic acid may be dissolved. In this case, a crystallization step of cooling the reaction solution or the like is provided to precipitate terephthalic acid to form a slurry. The slurry is subjected to solid-liquid separation to obtain crude terephthalic acid crystals. Although the terephthalic acid slurry obtained in the oxidation reaction step is in a pressurized state, the terephthalic acid slurry may be subjected to solid-liquid separation as it is, or may be subjected to depressurized cooling or the like, followed by solid-liquid separation. As a method for solid-liquid separation, if it is possible to separate the crystal and the mother liquor, filtration, centrifugation and the like can be mentioned. Wash and dry as necessary After drying, crude terephthalic acid crystals (crude terephthalic acid C) are obtained.

 [0029] The "crude terephthalic acid" in the present invention means terephthalic acid containing 1000 to 1 OOOOppm of 4 carboxybenzaldehyde.

 [0030] In the oxidation step (a), when the paraxylene A is oxidized, the oxidization reaction of one of the alkyl groups, which is different from the terephthalic acid alone, completely proceeds. 4 By-products including carboxybenzaldehyde (hereinafter abbreviated as "4CBA") are produced. Crude terephthalic acid C power also removes these by-products and performs the following steps to obtain high-purity terephthalic acid.

 [0031] In the dissolving step (b), the crude terephthalic acid C is slurried with water D in the slurrying tank 12, and the starting slurry E is heated to a high temperature and high pressure by a pump 12a and a heater 12b to be dissolved in water. E '. The above terephthalic acid has a low solubility in water. The above high temperature and high pressure require that the terephthalic acid can be dissolved in water. This temperature is preferably 230 ° C or more and 320 ° C or less depending on the slurry concentration. 2 If the temperature is lower than 30 ° C, the solubility is not sufficient. If the temperature exceeds 320 ° C, energy is wasted. If the temperature is too high, terephthalic acid may be decomposed to produce another substance. Further, this pressure needs to be a pressure that can maintain the liquid phase in the above temperature range, and is desirably 2.8 MPa or more and 11.3 MPa or less.

 [0032] The concentration of the slurry obtained in the dissolving step (b) is usually 20 to 40 wt%, preferably 25 to 35 wt%. If the slurry concentration is too high, clogging in the apparatus will occur, and if the slurry concentration is too low, the amount of mother liquor will increase, and the equipment corresponding to the production volume will increase in size. From the viewpoint of preventing clogging, it is preferable that the slurry concentration is kept constant.

Next, as the reduction step (c), the aqueous solution E ′ of terephthalic acid obtained in the above-mentioned dissolution step is sent to the hydrogenation reactor 13 and catalytically reduced with the introduced hydrogen F in the presence of a catalyst, A reduction reaction solution G is obtained. The conditions of this catalyst and the conditions in the hydrogenation reactor 13 must be such that the above 4CBA is reduced and the above terephthalic acid is not reduced. This is because the above 4CBA contained in the aqueous solution E 'is reduced to paratoluic acid having high water solubility. It is desirable to carry out this reduction at the highest possible rate. This hydrogenation is also well known, and examples of the hydrogenation catalyst include ruthenium, rhodium, palladium, platinum, osmium, etc., groups 8 to 10 (IUPAC inorganic A metal catalyst is used (according to the revised chemical nomenclature (1998)), and is usually used as a fixed bed supported on a carrier such as activated carbon. Of these, palladium supported on activated carbon is preferred. The hydrogenation temperature is usually 260-320. C, preferably 270-300. The partial pressure of C and hydrogen is usually 0.5 to ²⁰ kgZcm ² G.

 [0034] Further, as a first crystallization step (d), the reduction reaction solution G obtained in the reduction step (c) is introduced into the crystallization tank 14, and the temperature is maintained within a range where the above-mentioned paratoluic acid remains dissolved. And the pressure is reduced to crystallize the above terephthalic acid to form slurry H. Here, it is more desirable to provide a plurality of crystallization tanks 14 in series, preferably 3 to 6 stages, and gradually lower the pressure to cool (decompression evaporative cooling) to crystallize the above terephthalic acid. . The temperature of the last crystallization tank 14 may be controlled to a temperature condition such that paratoluic acid does not co-crystallize with terephthalic acid. Specifically, the temperature must be 120 ° C or higher and 200 ° C or lower. It is desirable that the temperature be between 180 ° C and 180 ° C. The pressure at this time needs to be 0.20 MPa or more and 1.56 MPa or less, and preferably 0.27 MPa or more and 1. OOMPa or less. If the temperature and pressure are lower than those conditions, paratoluic acid, not only terephthalic acid alone, will co-crystallize, lowering the purity of the resulting high-purity terephthalic acid crystals. On the other hand, if the temperature and pressure are kept higher than these conditions, the crystallization amount of terephthalic acid obtained is reduced, and the efficiency is reduced.

 Next, in the solid-liquid separation step (e), the slurry H is introduced into a solid-liquid separator, and the slurry H power is used as a primary separation mother! ¾ to obtain a high-purity terephthalic acid cake containing the high-purity terephthalic acid crystals. This high-purity terephthalic acid cake is preferably washed with a washing device and then dried to obtain primary crystals of high-purity terephthalic acid crystals. It is more preferable that the processes are performed collectively in the solid-liquid separation and washing device 15 because the process can be simplified.

[0036] As described above, the operation when the slurry H is separated into solid and liquid by one solid-liquid separation and washing device 15 and washed is as follows. The slurry H and the washing liquid I are introduced into the solid-liquid separation and washing device 15. As the cleaning solution I, water is more preferable. The slurry H is subjected to solid-liquid separation, and the separated cake is washed with a washing solution I to obtain a high-purity terephthalic acid cake L as a primary separation mother! ¾ [Separate from the primary mother. ¾ Discharge the cleaning effluent K, which mainly consists of the components of the cleaning liquid I. Here, the primary separation discharged from the solid-liquid separation and washing device 15 The temperature of the mother! ¾ [is the same as the crystallization conditions in the first crystallization step (d), and is preferably 120 ° C or more and 200 ° C or less, more preferably 130 ° C or more and 180 ° C or less. The pressure should be higher than the pressure of the final crystallization tank in the crystallization step (d) in order to suppress the temperature decrease due to the pressure release. Specifically, the pressure is desirably 0 to 1 MPa higher than the pressure of the final crystallization tank in the crystallization step (d). In the solid-liquid separation step (e), it is desirable to operate so that the supplied slurry H is not cooled. Examples of the solid-liquid separation and washing device 15 capable of performing solid-liquid separation and washing in a batch as described above include, for example, a screen bowl type centrifuge, a rotary vacuum filter, a horizontal belt filter, etc., and particularly preferably a screen. It is a bowl type centrifuge.

[0037] The high-purity terephthalic acid crystal L obtained by drying the high-purity terephthalic acid cake L in the drying device 16 to remove the remaining adhesion liquid can be obtained. The drying device 16 is, for example, a rotary drier or a fluidized bed drier, and is operated at a drying outlet operation temperature of 70 ° C to 180 ° C using a heat source such as steam in the presence of ventilation gas. .

 [0038] On the other hand, the above primary separation mother! ¾ [and the washing effluent K still contain active ingredients, and it is necessary to recover as much of these as possible to make high-purity terephthalic acid crystals. The active ingredient refers to the terephthalic acid and another compound that can be converted to the terephthalic acid by acidification or the like such as paratoluic acid. Including both. In addition, the solid content refers to a precipitated component of the active ingredient.

 First, since the washing discharge K has a low content of paratoluic acid, it is desirable to return it directly as the solvent in the dissolving step (b).

[0040] Further, when the washing effluent K contains a solid, solid-liquid separation may be performed by another solid-liquid separator before returning to the dissolving step (b). This is because if the solid-liquid separation and washing are performed at the same time by the solid-liquid separation and washing device 15 as described above, the omission is likely to occur, and the washing effluent K may contain solids. . Further, if crystallization is performed in advance before separation by the above-mentioned solid-liquid separation device, the amount of components that can be recovered by solid-liquid separation is improved. At that time, the solid content may be collected by sending it to the crystallization tank 14 or the like, or separated. The liquid component may be used as the solvent used in the slurrying tank 12 described above.

Next, as for the primary separated mother bite, in the second crystallization step (f), the primary separated mother bite is cooled using a plurality of pressure relief cooling tanks, and the terephthalic acid contained in the primary separated mother bite is contained. Crystallize secondary crystals that have the same strength as acid-paratoluic acid and collect them. The multi-stage means that two or more pressure relief cooling tanks are installed in series, and in each tank, dissolved components are deposited by sequentially lowering the respective temperatures according to the pressure.

[0042] The pressure-relieving cooling tank is a tank whose pressure is lower than the pressure of the liquid to be introduced, and the boiling point of the main component of the liquid at the pressure in the tank is equal to or lower than the temperature of the liquid before introduction. It means something that is When the liquid is introduced into the pressure-release cooling tank, a part of the liquid evaporates, and the rest of the liquid is cooled to the boiling point under the changed pressure. At this time, if the liquid is a solution, the solute that exceeds the solubility after cooling is crystallized.

[0043] By using a plurality of stages of the above-mentioned pressure-reducing cooling tanks to perform stepwise cooling and crystallization, crystallization is performed by only one-stage pressure-reducing cooling or a combination of one-stage pressure-reducing cooling and cooling by heat exchange. As compared with the case where the crystallization is performed, non-uniformity of cooling and Z or crystallization can be suppressed, so that a more thorough crystallization can be performed, so that the amount of solids obtained and increased The properties of the solid are easy to handle. Further, it is also possible to suppress the crystal of the active ingredient from being fixed in the pressure-reducing cooling tank or in a pipe introduced into the pressure-reducing cooling tank.

[0044] For this reason, among the above-described multiple-stage pressure-release cooling tanks, the first pressure-release cooling tank 17, which is the first pressure-release cooling tank, has a pressure equal to or higher than the atmospheric pressure and the solid-liquid separation step ( e) Primary separation mother discharged from!温度 The desired temperature is below 100 ° C and the primary separation mother discharged from the solid-liquid separation step (e)! (E.g., if the pressure is below 100 ° C (e.g., the pressure is reduced to 100 ° C and the temperature is lower than the temperature at the time of solid-liquid separation in the solid-liquid separation step (e)). Is preferable.). If the pressure is reduced to a pressure lower than atmospheric pressure at once, the width of the pressure reduction is too large to make the crystallization non-uniform, and the size of the obtained crystals is becoming smaller. I will. The temperature is higher because the boiling point of water at atmospheric pressure is 100 ° C. On the other hand, crystallization cannot be performed unless the pressure and temperature of the primary separation mouthpiece are lower than the pressure and temperature. [0045] Further, at least one of the above-described multi-stage pressure-reducing cooling tanks is desirably provided with a stirring blade having a distance from the inner wall of the pressure-reducing cooling tank of 10 mm or more and 50 mm or less. Better. In addition, it is more desirable that the stirring blade is an anchor-type stirring blade that is more desirable as the portion adjacent to the inner wall of the pressure-release cooling tank at the above-mentioned interval is longer. Since the purity of the terephthalic acid is relatively low in the above-mentioned pressure-reducing cooling bath, terephthalic acid is crystallized. There is. When the liquid in the vicinity of the wall is appropriately flowed by the stirring blade rotating near the wall of the tank, the adhesion of the terephthalic acid or the like to the wall can be suppressed. However, the distance between the stirring blade and the wall surface is desirably 10 mm or more, because if it is too close, the pressure relief cooling tank itself may be damaged. In particular, the first pressure relief cooling tank 17, which is the first pressure relief cooling tank, is most likely to have the largest amount of crystallization. More desirable. In addition, the above-described stirring blades may be provided in all of the plurality of pressure-reducing cooling tanks.

 [0046] In the above-mentioned pressure-reducing cooling tank, the primary separation mother using the anchor-type stirring blade! ¾ [force] When crystallizing the secondary crystal, the rotation speed of the anchor-type stirring blade is 3.Orpm or more, The speed is preferably 30 rpm or less, more preferably 5 rpm or more and 20 rpm or less. 3. If the rotation speed is less than Orpm, the effect of stirring cannot be fully exhibited, and the crystallized crystals may adhere to the inner wall side surface below the liquid surface as a lump. On the other hand, a speed of 30 rpm is sufficient for agitation, and even if the speed is higher than that, it is a waste of energy. ¾ is scattered in the cooling tank, causing lumps to adhere above the liquid level in the tank.

 [0047] Further, in the above-described multi-stage pressure-reducing cooling tank, it is desirable to crystallize vigorously as much as possible, not only the terephthalic acid but also the paratolic acid and other by-products. Components that cannot be crystallized in the second crystallization step (f) cannot be recovered in the filtration step (g) described below and are discharged as a secondary mother liquor P. Not only terephthalic acid but also all active ingredients should be crystallized as much as possible so that they can be recovered.

[0048] For this reason, in the present invention, the pressure in the final pressure-reducing cooling tank 18, which is the last pressure-reducing cooling tank among the above-described multiple pressure-releasing cooling tanks, is reduced to below atmospheric pressure, and the temperature of the cooling tank is reduced. To 40 to 80 ° C, preferably 50 to 70 ° C. Specifically, the pressure in the final pressure relief cooling tank 18 is 0.00 It is 7 MPa or more and 0.03 MPa or less, preferably 0.02 to 0.03 MPa. If the pressure exceeds 0.03 MPa and the temperature is too high, the crystallization may not be thorough and the recovery may be insufficient. On the other hand, if the pressure is less than 0.007 MPa and the temperature is too low, the pressure on the final pressure-reducing cooling tank 18 may be too large because the degree of pressure reduction is high.

 [0049] A single or a plurality of pressure-release cooling tanks may be provided between the first pressure-release cooling tank 17 and the final pressure-release cooling tank 18 to perform stepwise crystallization. ! ヽ.

 [0050] The secondary slurry O obtained by crystallizing the terephthalic acid or the like in the second crystallization step (f) is introduced into the filter 19 as a filtration step (g), and the secondary separation mother liquor is obtained. P and secondary crystal Q are separated into solid and liquid.

 [0051] The filter 19 used here can set the downstream side of the filter to a pressure state higher than the atmospheric pressure, and further, can set the upstream side of the filter to a pressure higher than that of the downstream side of the filter, and can also set the forward direction. It is desirable that solid-liquid separation be performed by cake filtration because filtration can be easily performed. The above-mentioned secondary slurry O is easier to handle and handle by crystallizing it in the above-mentioned multi-stage pressure-reducing cooling tank! / Although it is easy to adhere and has properties, it is difficult for ordinary filtration. This is because it is difficult to progress. The upstream side of the filter refers to the side to which the second crystallization step (f) force has been sent, the downstream side of the filter refers to the side for discharging the secondary separation mother liquor P, and the forward direction refers to the secondary side. This refers to the direction in which the flow of the slurry O or the secondary separation mother liquor P is directed from the upstream side of the filter to the downstream side of the filter. In the above-mentioned cake filtration, particles are trapped on the filter pores by a cross-linking phenomenon, a filter cake layer is formed on one surface of the filter shortly after the start of filtration, and the cake layer is used as a filter for the subsequent filtration. This is a mechanism in which filtration proceeds while acting. Examples of the filter 19 operated in this manner include a Fundaback filter manufactured by Ishikawajima-Harima Heavy Industries, Ltd., and a cricket filter manufactured by Tsukishima Kikai Co., Ltd.

It is desirable to return the secondary crystal Q to any of the above steps to reuse the active ingredient, but it is more preferable to return to the above-mentioned oxidation step (a). As described above, the secondary crystal Q contains not only terephthalic acid but also partially reduced impurities such as paratoluic acid. These cannot be used as such as terephthalic acid products. Therefore, these impurities are removed by the above-mentioned terephthalic acid by oxidizing in the above-mentioned acidifying step (a). Tallic acid can be used, and the overall yield of the method for producing high-purity terephthalic acid according to the present invention can be improved.

 On the other hand, the above-mentioned secondary separated mother liquor P preferably has a suspended substance concentration of 200 mg ZL or less, more preferably 100 mg ZL or less, and still more preferably 50 mg ZL or less. Here, the suspended substance concentration means the weight of suspended matter such as paratoluic acid which is dispersed without being dissolved in the solvent with respect to the total weight of the secondary separation mother liquor P, and is described in JIS K 0101. Analyzed according to the method. At least a part of the secondary separation mother liquor P is discharged out of the system because the concentration of impurities in the system is not too high, so that paratoluic acid contained in the secondary separation mother liquor P is discarded. This is because the overall yield of the method for producing terephthalic acid, which is important for the invention, is reduced. Further, when at least a part of the secondary separation mother liquor P is directly or indirectly reused as a solvent in the production process, it is necessary to suppress the accumulation of noraturilic acid in the system.

[0054] For removal of paratoluic acid contained in P in the secondary separated mother liquor, there is a method of bringing P in the secondary separated mother liquor into contact with the synthetic adsorbent. As the synthetic adsorbent, an organic synthetic adsorbent is usually used. For example, SEPABEADS SP825, SP850, SP207 (SEPABEADS is a registered trademark of Mitsubishi Chemical Corporation), AMBERLITE XAD-4, XAD-16 (AMBERLITΕ is a registered trademark of Rohm & Haas Company) A synthetic adsorbent, an acrylic synthetic adsorbent such as DIAION HP2MG (DIAION is a registered trademark of Mitsubishi Chemical Corporation), AMBERLITE XAD-7, XAD-8, etc. can be used. Preferably, a non-polar organic synthetic adsorbent, especially a synthetic adsorbent having a porous copolymer power of a monovinyl conjugate and a polyvinyl conjugate, especially a styrene-dibutylbenzene synthetic adsorbent is used. Since paratoluic acid has a benzene ring, it is easily adsorbed by a styrene-divinylbenzene-based synthetic adsorbent. The specific surface area of the adsorbent is usually 400~1500m ² Zg, preferably 600~1000m ² Zg, pore volume is typically 0. 5~3mLZg, preferably 1. 0~ 2. OmL / g, pore size is usually 10 to : LOOOA, preferably 50-50〜. The adsorbent is usually packed in the adsorption tower so that the height of the packed bed is about 1.5 to 4. Om. Depending on when the supply of the liquid to be treated is stopped, in general, if the height of the packed bed is too low, the utilization efficiency of the adsorbent decreases. The supply rate of the liquid to be treated to the adsorption tower is typically 0.5 to 30 mZhr for LV. , SV is usually 0. 5~20hr ^_1. The P in the secondary separation mother liquor that has been subjected to the adsorption treatment has a reduced paratoluic acid concentration, so that it can be easily reused in the production step, preferably as a solvent in the dissolution step or a washing liquid for the separation cake after the solid-liquid separation step. In addition, paratoluic acid is an intermediate in the conversion of paraxylene to terephthalic acid, and the paratoluic acid adsorbed on the synthetic adsorbent is recovered using a desorbing solution, and It is preferable to supply to the process 工程.

Hereinafter, the present invention will be described specifically with reference to examples.

 [Method for measuring suspended solids concentration (JIS K 0101)]

 For suspended substances (substances suspended in water), filter the sample and dry the substance remaining on the filter media at 105 ° C as follows:

 (a) When using a glass fiber filter paper, attach it to a filter in advance and thoroughly wash it with water.After that, place this filter material on a watch glass and heat it at 105-110 for about 1 hour. After cooling in the oven, measure its mass.

 (b) Attach a filter medium to the filter, pour an appropriate amount of the sample into the filter, and perform suction filtration. The substance adhering to the sample container and the wall of the filter tube should be washed off on the filter medium with water, adjusted to the residual substance on the filter medium, and washed several times with water.

 (c) Carefully remove the residue with a filter medium using tweezers etc. together with the filter media, transfer to the watch glass used in (a), heat at 105-110 ° C for 2 hours, and place in the desiccator. After cooling in, measure the mass.

 (d) The suspended substance (mgZD is calculated by the following formula.

[0056] S = (a-b) X 1000 / V

 Where, S: Suspended substance (mgZD, a: Weight of filter medium and watch glass containing suspended substance (mg), b: Weight of filter medium and watch glass (mg), V: Sample (ml)

 Example 1

[0057] An acetic acid solution containing para-xylene and a catalyst (an acetic acid solution of cobalt acetate and manganese acetate and hydrogen bromide), a separated mother liquor recycled from a subsequent solid-liquid separation step, and air are continuously supplied to the stirring tank. Then, an oxidizing reaction was performed at an operating temperature of 190 ° C. and an operating pressure of 1.23 MPa (absolute pressure) while adjusting the liquid level so that the residence time was 1 hour. The distillate vapor is finally cooled to 40 ° C by a multi-stage condenser, and the oxygen concentration in the exhaust gas is adjusted to 2.5 vol%. The operation was carried out after setting. The condensate obtained from each condenser was integrated and refluxed to the oxidation reactor, and a part of the condensate was withdrawn so that the concentration of water in the mother liquor of the slurry withdrawn was 10% by weight. The slurry concentration of the slurry from which the reactor power was also extracted was 35% by weight, and the concentration of cobalt Z manganese Z bromine in the reaction mother liquor was 300Z300Z1000ppm by weight.

 [0058] The slurry from which the power of the reactor was also withdrawn was continuously supplied to a stirring tank together with air, and at an operating temperature of 181 ° C, an operating pressure of 1.15 MPa (absolute pressure), and a residence time of 15 minutes. The low-temperature re-oxidation reaction was performed while adjusting the liquid level so as to be as follows. The distillate steam was finally cooled to 40 ° C by a multi-stage condenser, and the operation was performed with the oxygen concentration in the exhaust gas adjusted to 6 vol%. The condensed liquid obtained from each condenser was integrated and refluxed to the low-temperature re-oxidation reactor.

 [0059] The slurry from which the power of the low-temperature refining reactor was extracted was crystallized to 90 ° C, and the slurry obtained by the crystallization was supplied to a rotary vacuum filter to perform solid-liquid separation and washing. I got it. Here, the operating pressure was atmospheric pressure. The separated crude terephthalic acid cake was dried with a steam rotary dryer to obtain crude terephthalic acid crystals.

[0060] This crude terephthalic acid was supplied to the process for producing high-purity terephthalic acid shown in Fig. 1. Using water D as a solvent, an aqueous solution E 'containing 30% by weight of crude terephthalic acid at high temperature and pressure was obtained. In the process shown in FIG. 1, the temperature and pressure of the aqueous solution E ′ sent to the hydrogenation reactor 13 were 290 ° C. and 8.7 MPa (89 kgfZcm ² ′ gauge), respectively.

 [0061] In the subsequent first crystallization step (d), the crystallization tank 14 in which the five crystallization tanks are connected in series is cooled stepwise by depressurization evaporation, and finally cooled to a temperature of 155 ° C. After cooling to C, the solute was crystallized. The slurry H obtained by the crystallization is separated into a high-purity terephthalic acid cake L containing primary crystals and a primary separation mother! ¾ [The high-purity terephthalic acid cake L was washed with washing water, dried in a drier 16 and recovered as high-purity terephthalic acid crystals M.

 [0062] On the other hand, the primary separating mother bite is reduced in pressure to the atmospheric pressure in the first pressure relief cooling tank 17 provided with the anchor type stirring blades, and is then cooled by releasing the pressure to 100 ° C. The crystallization to be the second crystallization step (f) was performed. The rotation speed of the anchor type stirring blade was 10 rpm, and the gap between the anchor type stirring blade and the side surface of the inner wall was 10 mm.

[0063] Next, the obtained intermediate slurry N at 100 ° C was finally discharged with a steam ejector. It was introduced into the pressure cooling tank 18, the operating pressure was set to 0.02 MPa, and the pressure was reduced to 60 ° C. under reduced pressure. The secondary slurry O thus obtained was charged as a filter 19 into a Fundaback filter (R56-86-25) manufactured by Ishikawa-Harima Heavy Industries, Ltd., and was subjected to solid-liquid separation. The suspended substance concentration of the secondary mother liquor P obtained here was measured according to the above method, and was 30 mgZl.

[0064] As a result, in the first pressure-release cooling tank 17 and the final pressure-release cooling tank 18, the precipitates do not adhere to and grow on the inner wall side surface for one month, and the secondary crystal Q The collection process could be performed.

 Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention. is there.

 [0066] This application is based on Japanese Patent Application (No. 2004-159786) filed on May 28, 2004, the contents of which are incorporated herein by reference.

 Industrial applicability

According to the present invention, in producing high-purity terephthalic acid, the primary separated mother liquor is cooled to increase the recovery rate of secondary crystals precipitated, and a secondary separated mother liquor having a low turbidity (SS concentration) is obtained. And a method for producing high-purity terephthalic acid. In addition, it is possible to provide a method for producing high-purity terephthalic acid, which can suppress formation of deposits in the system that cause blockage and generation of lumps during cooling treatment. The industrial value of the present invention is remarkable.

Claims

The scope of the claims  [1] Oxidation step of oxidizing para-xylene to obtain crude terephthalic acid containing 4-carboxybenzaldehyde (a) ゝ  A dissolving step of dissolving the above crude terephthalic acid in an aqueous solvent under high temperature and pressure to obtain a crude terephthalic acid aqueous solution (b)  A reduction step (c) of obtaining a reduction reaction solution obtained by reducing the carboxybenzaldehyde to paratoluic acid by bringing the crude terephthalic acid aqueous solution into contact with hydrogen in the presence of a catalyst.  A crystallization step (d) of evaporating the reduction reaction solution under reduced pressure and cooling to 120 to 200 ° C to crystallize high-purity terephthalic acid crystals.  A solid-liquid separation step (e) of solid-liquid separation of the slurry obtained in the crystallization step (d) into crystals comprising the high-purity terephthalic acid crystals and a separation mother liquor;  Cooling the separated mother liquor and crystallizing a crystal mainly composed of terephthalic acid or paratoluic acid contained in the separated mother liquor obtained in the solid-liquid separation step (e) (f), In the method for producing the acid,  Cooling is performed by using a plurality of cooling tanks for cooling by evaporating the water solvent in the mother liquor by lowering the pressure, the last of which is reduced to below atmospheric pressure, and the temperature of the last cooling tank is reduced to 40 to A method for producing high-purity terephthalic acid at 70 ° C. [2] The first cooling tank among the cooling tanks used in the crystallization step (f) is depressurized and evaporated to a temperature of 100 ° C to a temperature lower than the temperature at the time of solid-liquid separation in the solid-liquid separation step (e). Item 4. The method for producing high-purity terephthalic acid according to Item 1. 3. The method for producing high-purity terephthalic acid according to claim 2, wherein the first cooling tank is depressurized and evaporated to atmospheric pressure.  [4] The method for producing high-purity terephthalic acid according to claim 1, wherein the cooling tank used in the crystallization step (f) has an anchor-type stirring blade. [5] The high-purity terephthalate according to claim 1, wherein at least one of the cooling tanks used in the crystallization step (f) has a stirring blade having a distance from the inner wall of the cooling tank of 10 mm or more and 50 mm or less. Method for producing acid. [6] A filtration step (g) of solidifying the slurry obtained in the crystallization step (f) into crystals and a separated mother liquor using a filter,  2. The method for producing high-purity terephthalic acid according to claim 1, comprising: [7] The method for producing high-purity terephthalic acid according to claim 6, wherein the crystals separated in the filtration step (g) are supplied to the oxidation step (a). [8] In the filtration step (g), the downstream side of the filter of the filter is set to a pressure higher than the atmospheric pressure, the upstream side of the filter of the filter is set to a higher pressure than the downstream side of the filter, and the pressure state is set. 7. The method for producing high-purity terephthalic acid according to claim 6, wherein solid-liquid separation is performed by cake filtration in the direction.  [9] The method for producing high-purity terephthalic acid according to claim 6, wherein the separated mother liquor obtained in the filtration step (g) is directly or indirectly supplied to the dissolution step (b). [10] The purified mother liquor obtained in the filtration step (g) is brought into contact with a synthetic adsorbent to remove paratoluic acid, and then supplied to the dissolution step (b). Production method.  [11] The method for producing high-purity terephthalic acid according to claim 6, wherein the concentration of suspended solids in the separated mother liquor obtained in the filtration step (g) is 200 mgZL or less.