JP2006096710A - Purification method of terephthalic acid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a crystallization method from a purified terephthalic acid aqueous solution which permits the manufacture of terephthalic acid for direct polymerization having good crystalline properties and a low impurity content. <P>SOLUTION: In the recovering method of purified terephthalic acid where crude terephthalic acid obtained by liquid phase oxidation of p-xylene is dissolved in water and is subjected to hydrogenation purification in the presence of a catalyst and the resultant purified aqueous solution is subjected to cooling/crystallization by stages in a serially linked five-staged flash evaporation-cooling crystallization tanks, the operation temperature at each crystallization tank is controlled to a temperature approximated by the correlation equation below between the operation temperature and the concentration of the crude terephthalic acid slurry fed for the hydrogenation purification reaction. Tn=(0.9958*N<SP>3</SP>-8.4268*N<SP>2</SP>-6.8726*N+239.89+S*C)±2.5(wherein Tn is the operation temperature (°C) of each crystallization tank; C is the concentration (wt%) of crude terephthalic acid; N is the stage number of the crystallization tanks (1, 2, 3, 4 and 5 starting from the highest temperature crystallization tank); and S is the coefficient at each crystallization tank (1.48 for the first stage, and 1.19 for the second, the third, the fourth and the fifth stages)). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for purifying terephthalic acid, and in particular, high-purity terephthalic acid is produced by hydrorefining an aqueous solution of crude terephthalic acid (CTA) obtained from the oxidation reaction of P-xylene in the presence of a Group VIII metal. In particular, the present invention relates to a method for purifying terephthalic acid.

  In general, crude terephthalic acid obtained by liquid phase oxidation of P-xylene usually contains 4-carbonylbenzoic acid (4-CBA), p-toluic acid (p-tA) or the like as impurities as impurities. These are refined to produce high-purity terephthalic acid, and the produced high-purity terephthalic acid is used as a polyester raw material.

  In this case, the crude terephthalic acid is purified by mixing crude terephthalic acid and water to form an aqueous solution under high temperature and high pressure, and then hydrogenating in the presence of a Group VIII metal catalyst such as Pd and Pt. There is a method in which purification is performed by reduction treatment, and purified terephthalic acid (PTA) is crystallized and recovered from the treated aqueous solution (Patent Document 1 / Japanese Patent Publication No. 41-16860).

As a method of recovering purified terephthalic acid as crystals from the purified terephthalic acid aqueous solution, there is usually a cooling method by flash evaporation in which the aqueous solution is introduced into a crystallization tank connected in series and the pressure is lowered stepwise. Industrially implemented (Patent Document 2 / Japanese Patent Publication No. 53-
No. 24057).

  The method for recovering terephthalic acid from the crystallized slurry of purified terephthalic acid in the final crystallization tank usually involves washing by filtration or centrifugation at the same temperature and pressure as the final crystallization tank. However, in the washing process, after the solid-liquid separation, the slurry is re-slurried with water, and the solid-liquid separation is performed again for the recovery. Subsequently, a method of sending the recovered terephthalic acid to a drying step and producing it as purified terephthalic acid has been implemented (Patent Document 3 / Japanese Examined Patent Publication No. 47-49049).

  The purified terephthalic acid thus obtained has been produced by a so-called direct polymerization method in which the purified terephthalic acid is directly reacted with glycols. Needless to say, the hue and purity of the purified terephthalic acid, There is also a great demand for crystal properties such as particle size and distribution. In other words, terephthalic acid in the direct polymerization method is mixed with glycols and sent to the reaction system in a slurry state to be subjected to the polycondensation reaction, but good slurry performance by mixing an appropriate amount of purified terephthalic acid and glycols is It brings about effects such as improving transportation, stirring, etc., and improving the uniformity of the polycondensation reaction. However, a large amount of glycols affects the quality of the polyester polymer. Accordingly, there is an increasing demand for crystalline properties from the viewpoint of fluidity and uniformity.

  For this reason, even in the hydrorefining method for the purification of crude terephthalic acid, the obtained purified terephthalic acid has important requirements as a polyester raw material along with the reduction of hue and impurities as well as the crystal grain properties. . A method for producing terephthalic acid described in Patent Document 4 (Japanese Patent Application Laid-Open No. 8-208561) is known as one that meets such a requirement.

In this terephthalic acid production method, crude terephthalic acid is dissolved in an aqueous medium and purified by contacting with a platinum group metal catalyst at a temperature of 260 to 320 ° C., and terephthalic acid is connected in series from the aqueous solution of terephthalic acid. When crystallization is carried out stepwise in the plurality of crystallization tanks, the crystallization temperature in the first crystallization zone is 240 to 260 ° C., and the stirring power is 0.4 to 10 kw / m ³ with a stirring blade. Then, the crystallization temperature in the second crystallization zone is set to 180 to 230 ° C., and the crystallization temperature is set to 20 to 60 ° C. lower than the crystallization temperature in the first crystallization zone. Is performed, followed by solid-liquid separation, and the separated terephthalic acid crystal particles are dried to obtain terephthalic acid particles having a particle size exceeding 210 μm of 10% by weight or less.

  Further, as described in Patent Document 5 (Japanese Patent Application Laid-Open No. 11-228492), a high-purity terephthalic acid satisfying quality is prevented by preventing the eutectic of p-toluic acid contained in terephthalic acid crystals. Methods for recovering terephthalic acid are known.

  In this method of recovering terephthalic acid, an aqueous slurry of crude terephthalic acid having a concentration of 22 to 30 wt% is heated to 270 to 300 ° C. and completely dissolved and then subjected to a hydrogenation reaction. Sequentially supply to the stage crystallization tanks, and select the operation temperature of each stage crystallization tank to a value represented by the following approximate expression: y = 266exp (−0.61x) ± 5; [x: 1 total The value obtained by multiplying the value divided by the number of stages by the stage value of each stage, y: temperature (° C.)], the temperature of the first crystallization tank is less than 240 ° C., and the average residence time of each crystallization tank is 10 -60 minutes, supplying each tank with a terephthalic acid aqueous solution or slurry to the staying liquid phase, and solid-liquid separation of the slurry obtained from the final stage crystallization tank to obtain high-purity terephthalic acid Is the method.

Japanese Patent Publication No.41-16860 Japanese Patent Publication No.53-24057 Japanese Examined Patent Publication No. 47-49049 JP-A-8-208561 Japanese Patent Laid-Open No. 11-228492

  In the production of terephthalic acid for direct polymerization as described above, most of 4-CBA in the terephthalic acid is hydrogenated and disappears by hydrorefining reaction of the crude aqueous terephthalic acid solution, but p-tA is substantially equivalent. Quantity generated. For this reason, it is necessary to prevent p-tA from co-singling in the crystals of terephthalic acid in the cooling and crystallization steps of the purified aqueous solution of terephthalic acid by flash evaporation. In addition, the stepwise crystallization process in which crystals of terephthalic acid are produced is the only control process for the particle properties of the particle size and particle distribution, and at the same time, it is represented by p-tA produced during the process. When impurities are incorporated into the terephthalic acid crystal by eutectic or adsorption, it is difficult to reduce the incorporated p-tA by subsequent separation and washing. Therefore, the cooling and crystallization process is an important process for reducing impurities such as p-tA in purified terephthalic acid and forming crystal grains, and the crystallization process is an important purification that complements the hydrorefining reaction. It becomes the control process of the law.

  Thus, it can be understood that the crystallization temperature greatly affects the crystal properties and the p-tA content. However, the crystal properties and the p-tA content are not adjusted to each. That is, the crystal crystallization ratio in each stage of flash evaporation is determined by the amount of water vapor flashed and the crystallization temperature with respect to the crude terephthalic acid concentration of the slurry introduced into the purification reaction. The crystal properties and the p-tA content do not behave completely differently, but the crystal properties, eutectic and adsorption of the crystals produced in the crystallization ratio for the introduced crude terephthalic acid at each crystallization stage. It has an inseparable behavior between the p-tA content and the like.

  In the prior art described in Patent Document 5 (Japanese Patent Application Laid-Open No. 8-208561), the first and second crystallization temperatures and the stirring conditions are the main operating conditions as means for solving the problem of crystal properties. Also, in Patent Document 2 (Japanese Patent Publication No. 53-24057), in order to reduce the p-tA content, it is first dissolved over a temperature of 360 to 320 degrees Fahrenheit (182 to 160 ° C.) or lower. Terephthalic acid is crystallized at a decreasing rate. Further, in Patent Document 5 (Japanese Patent Laid-Open No. 11-228492), in order to obtain a desired amount of high-purity terephthalic acid with a desired purity by appropriately balancing the amount of p-tA and the amount of precipitated terephthalic acid crystals. In addition, the temperature of each crystallization tank is set to y = 266exp (−0.61x) ± 5.

  The above prior art is an invention that takes into consideration the formation of crystal grains or the prevention of eutectic crystal of p-tA, respectively, but it is still sufficient to solve both of these problems and produce stable purified terephthalic acid. It wasn't easy.

  An object of the present invention is to provide a terephthalic acid which has a low p-tA content in the crystallization process of a hydrorefined terephthalic acid aqueous solution, and which can simultaneously achieve a preferable crystalline property and obtain a stable purified terephthalic acid. It is to provide a purification method.

The object of the present invention is a process of crystallization of a purified aqueous solution having a crude terephthalic acid concentration of 25 to 33% by weight in the aqueous solution to be introduced into the hydrorefining reaction. This is achieved by controlling the operation temperature of each crystallization tank by the crystallization method to a temperature approximated by the following correlation equation with the crude terephthalic acid concentration introduced into the purification reaction. A water slurry having a crude terephthalic acid concentration of 25 to 33% by weight is dissolved in a temperature range of 280 to 295 ° C.

  Another object of the present invention is to dissolve a water slurry having a crude terephthalic acid concentration of 29 to 33 wt%, hydrotreat in the presence of a Group VIII metal catalyst, and then connect the purified aqueous terephthalic acid solution in series. In the method of recovering purified terephthalic acid which has been cooled and crystallized stepwise in a flash evaporation cooling type crystallization tank and recovering the crystallized purified terephthalic acid, the operating temperature of each stage of the crystallization tank is introduced into the coarse slurry. This is achieved by controlling the temperature to be approximated by the following correlation equation with the terephthalic acid concentration, and condensing the water vapor generated in the fifth crystallization tank and refluxing it to the crystallization tank. Further, a water slurry having a crude terephthalic acid concentration of 29 to 33% by weight is dissolved in a temperature range of 290 to 295 ° C.

Tn = (0.9958 * N ^ 3-8.4268 * N ^ 2-6.8726 * N
+ 239.89 + S * C) ± 2.5
Tn: Operating temperature of each crystallization tank (° C)
C: Concentration (% by weight) of crude terephthalic acid
N: Stage value of the crystallization tank (1, 2, 3, 4, 5 from the high temperature side crystallization tank)
S: Coefficient in each crystallization tank (first stage 1.48, second stage, third stage, fourth stage, fifth stage 1.19)

  INDUSTRIAL APPLICABILITY The present invention has an effect that, in the crystallization process of a hydrorefined terephthalic acid aqueous solution, the p-tA content is low, and preferable crystalline properties can be achieved at the same time to obtain a stable purified terephthalic acid. .

  It goes without saying that the purified terephthalic acid obtained by the present invention leads to the production of a high-quality polyester by increasing the uniformity of the reaction, the hue of the polymer product and the degree of polymerization in the production of the polyester by the direct polymerization method. Yes.

  Moreover, the best purification effect can be exhibited in the crystallization tank by controlling the temperature, that is, the crude terephthalic acid is re-slurried into an aqueous solution, and the p-tA content is low at the crystallization stage, and the preferred crystalline properties are obtained. Since purified terephthalic acid can be obtained, even if the quality of the crude terephthalic acid varies, it can be handled at the crystallization stage. Therefore, the reaction intermediate in the crude terephthalic acid is allowed to vary, and the cost of the crude terephthalic acid is reduced.

  Furthermore, since purified terephthalic acid having a low p-tA content and preferable crystalline properties can be stably obtained, the yield of purified terephthalic acid is good, and the production cost of purified terephthalic acid can be reduced.

  The present invention has been made by paying attention to the fact that the conditions of the crystallization stage in which crude terephthalic acid (CTA) is reslurried to purify purified terephthalic acid affect the crystalline properties and p-tA content of purified terephthalic acid. It is. That is, the rate of partial crystallization by stepwise flash evaporation and cooling at each stage affects the crystal properties when forming purified terephthalic acid crystal particles, and also affects the p-tA content. give.

  The present inventor uses a five-stage crystallization tank connected in series with the aim of an average particle size of 130 to 160 μm and a p-tA content of 150 ppm or less, which are preferable as the crystalline properties of purified terephthalic acid, When crystallizing and recovering purified terephthalic acid by cooling by flash evaporation, the concentration of crude terephthalic acid introduced into the purification reaction that can satisfy the target values of both the crystalline properties and the p-tA content and each stage Crystallization temperature conditions were obtained.

  In carrying out the present invention, crude terephthalic acid conventionally produced commercially includes crude terephthalic acid containing 2500 to 3500 ppm, preferably 2700 to 3300 ppm of 4-CBA by oxidation reaction of p-xylene. ing. The crude terephthalic acid is mixed with water to form a slurry. The slurry is heated and dissolved, and then introduced into a hydrotreating reactor. In this case, the crude terephthalic acid concentration is introduced in the range of 25 to 33% by weight, preferably 26 to 33% by weight. More preferably, introduction at 27 to 31% by weight provides stable operation and crystallinity in the subsequent crystallization step. The slurry is heated and melted at a temperature in the range of 280 to 295 ° C. and introduced into the hydrogenation reactor at that temperature for hydrogenation. The temperature is performed at a temperature several degrees higher (ie, 3-12 ° C.) than the saturation temperature at which the crude terephthalic acid concentration is dissolved. For the dissolution of terephthalic acid in water, refer to Table 1 of Patent Document 3 (Japanese Patent Publication No. 47-49049), 276 ° C. at 25% by weight, about 285 ° C. at 29% by weight, and about 285 ° C. at 33% by weight. It is estimated at about 290 ° C.

  Note that unnecessarily dissolved water is not preferable because it leads to a heat load necessary for heating. In addition, a slurry with a high concentration of terephthalic acid causes problems in transportation and handling of the slurry in the final crystallization tank after heating, dissolution and purification. For these reasons, the concentration of terephthalic acid in the slurry is limited. Therefore, in the case of purification treatment with a high concentration of terephthalic acid, for example, 29 to 33% by weight, in order to improve the slurry transport performance especially after purification, a small amount of water is supplied to the final stage crystallization tank to form a slurry. Is diluted (0.5 weight times or less of the supplied terephthalic acid), or the vapor condensate flushed from the final stage crystallization tank is returned to the crystallization tank. Thereby, the performance of slurry transport can be improved without greatly affecting the crystal properties of the purified terephthalic acid. However, since the amount of water increases, there is a slight solid-liquid separation load in the subsequent solid-liquid separation step.

  In the present invention, dissolved terephthalic acid is gradually stepped through partial crystallization and aging by stepwise flash evaporation and cooling from a purified aqueous solution of crude terephthalic acid, and all the terephthalic acid almost dissolved in the final stage is recovered. To be deposited.

In abrupt crystal precipitation by flash evaporation at each stage, if the number of stages is reduced, the crystal particle diameter is reduced and the accompanying reduction in the p-tA content is inhibited. That is, if the number of stages is reduced, the temperature drop per stage is increased and many crystals are precipitated within a short time, so that the crystal grain size is small and the number is large. Therefore, eutectic p-
p- is trapped inside the crystallized terephthalic acid and cannot be removed by washing
It seems that the number of tA increases and inhibits the final reduction of p-tA content. For this reason, in order to obtain the target terephthalic acid, the number of stages of the crystallization tank in which the aqueous solution of terephthalic acid after purification by hydroreduction is flash-evaporated is 5 in the present invention. It was.

  In the crystallization stage, in order to optimize the crystal grain size at each stage, which is a leading factor for inclusion of p-tA, the crystal grain size produced in each crystallization tank based on the saturation solubility. The temperature and the terephthalic acid concentration were changed, and data on the ptA content after crystallization and the average particle diameter shown in Table 1 described later were obtained. Moreover, this is diagrammatically shown as a preferred temperature diagram in FIG. 2 (details will be described later).

  As a result, as a preferable method for obtaining purified terephthalic acid, a relational expression (1) obtained by approximating the temperature diagram shown in FIG. 2 was obtained in relation to the crude terephthalic acid concentration in the slurry introduced after the hydrorefining reaction. .

Tn = (0.9958 * N ^ 3 (N cubed) −8.4268 * N ^ 2 (N squared)
−6.8726 * N + 239.89 + S * C) ± 2.5 (1)
Tn: Operating temperature of each crystallization tank (° C)
C: Concentration (% by weight) of crude terephthalic acid
N: Stage value of the crystallization tank (1, 2, 3, 4, 5 from the high temperature side crystallization tank)
S: Coefficient in each crystallization tank (first stage: 1.48, second to fifth stages: 1.19)
For example, in the case of a terephthalic acid concentration of 25% by weight, the standard operating temperatures of the crystallization tanks in each stage (the allowable range is omitted) are as follows. 1 ° C, 4th stage 171.1 ° C, 5th stage 149.1 ° C. The standard operating temperatures of the crystallization tanks in each stage when the terephthalic acid concentration is 33% by weight (the allowable range is omitted) are the first stage 274.4 ° C, the second stage 239.7 ° C, and the third stage 209.6 ° C. , 4th stage 188.6 ° C., 5th stage 158.6 ° C.

  Thus, the average particle diameter of 130 to 160 μm and p-tA targeted by operating and controlling the crystallization tank by using the operation temperature of each stage of the crystallization tank determined by the relational expression (1). Purified terephthalic acid having a content of 150 ppm or less can be purified.

  The calculated crystallization ratio at each stage in the relational expression (1) is 34 to 46% by weight in the first stage, 82 to 86% by weight in the second stage, and 82 to 86% by weight in the second stage with respect to the introduced crude terephthalic acid amount. 95.2 to 96.3% by weight, the fourth stage is 98.7 to 99.1% by weight, the fifth stage is 99.56 to 99.65% by weight, and it is estimated that the crystallization ratio is controlled. It was. Further, regarding the relationship between the temperatures of the obtained crystallization stages, it is presumed that the crystallization ratio in each stage affects the next crystallization stage and affects the crystal particle diameter and the ptA content. The In the present invention, since the operating temperature of the first stage in the conventional crystallization method shown in Patent Document 4 or Patent Document 5 is set higher, the crystallization ratio in the first stage crystallization tank is suppressed compared to the conventional method. Therefore, it is considered that this set temperature matched well with the crystallization temperature setting of each subsequent stage, and terephthalic acid having the target property was obtained.

  The operating conditions in each crystallization tank are as follows: (1) Since the crystallization is a cooling method by flash evaporation, the slurry is introduced into each crystallization tank in the slurry phase of each tank. (2) Each tank The average residence time is from 10 to 60 minutes. This is because, in order to alleviate the partial rapid crystal precipitation due to the flash, it is performed in the slurry phase in which the crystal is already present, and is crystallized on the crystal already present. In order to age the crystals, it is necessary to control the residence time to be at least about 10 minutes, preferably at least 20 minutes. Further, even if the residence time is too long, the amount of precipitation does not increase with time, so there is no effect.

  The obtained slurry in the final stage (fifth stage) crystallization tank is subjected to solid-liquid separation in the temperature and pressure state, washed, dried and recovered as terephthalic acid for direct polymerization.

  The solid-liquid separation of the slurry may be either a filtration or a sedimentation type separator. After the solid-liquid separation, the slurry can be washed again by re-slurrying with a new medium to further reduce the p-tA acid. Further, a washable solid-liquid separator can be used instead of washing by reslurry.

  Hereinafter, although one Example of this invention is shown below, it is not limited to a following example in the range which does not exceed the claim of this invention.

  FIG. 1 shows an embodiment of the present invention and is a system diagram of a plant for purifying terephthalic acid.

  As shown in FIG. 1, an apparatus for producing purified terephthalic acid after re-slurry of crude terephthalic acid is as follows: slurry adjustment tank 1, pump 2, heater 3, dissolution tank 4, hydrogenation reaction tank 5, first crystallization Tank 6, second crystallization tank 7, third crystallization tank 8, fourth crystallization tank 9, fifth crystallization tank 10, pump 12, solid-liquid separator 13, reslurry tank 14, pump 15, solid It consists of a liquid separator 16 (for example, a centrifugal separator) and a dryer 17, and is a five-stage crystallization tank system in which these are sequentially connected. The reflux condenser 11 condenses and refluxs part of the water vapor discharged from the fifth crystallization tank 10 to adjust the slurry concentration in the fifth crystallization tank 10.

  Using the apparatus configured in this manner, a crude terephthalic acid having a 4-CBA content of 0.30 to 0.33 wt% (3000 to 3300 ppm) and pure water are supplied to the slurry adjusting tank 1 to adjust the slurry. The slurry adjusted to a predetermined concentration in the tank 1 is pressurized and fed by the pump 2 and heated to a predetermined temperature by the heater 3 and sent to the dissolution tank 4 to completely dissolve the crude terephthalic acid in the aqueous solution. . Next, an aqueous solution of crude terephthalic acid is sent to the hydrogenation reaction tank 5, and the aqueous solution is subjected to a hydrorefining reaction (hydrogenation reaction) at a predetermined temperature and pressure in the presence of a Group VIII metal catalyst. 4-CBA in acid is converted to p-tA and other impurities are hydrocracked and purified.

  Thereafter, the aqueous solution is sequentially fed to the five-stage crystallization tanks 6 to 10 connected in series, and flash-evaporated at a pressure that reaches the temperature obtained by the relational expression (1) described above. Further, the residence time in the crystallization tanks 6 to 10 in each stage is set to about 25 minutes to partially crystallize terephthalic acid, and almost the entire amount of terephthalic acid is precipitated in the fifth crystallization tank 10. Here, each crystallization tank 6 to 10 is provided with a temperature sensor 21 and a pressure sensor 22, and the pressure / temperature state of each crystallization tank is monitored by the control device 30 and provided at the inlet of each crystallization tank. The pressure reducing valve 20 is controlled to adjust the pressure in each crystallization tank, and the operation temperature of each crystallization tank is controlled to a predetermined temperature.

  Moreover, the control apparatus 30 adjusts the supply amount of the pure water to the slurry adjustment tank 1, and controls the terephthalic acid density | concentration of the slurry output from the slurry adjustment tank 1 to a predetermined density | concentration. The control device 30 adjusts the pressurization by the pump 2 and the supply amount of the slurry, and adjusts the heating temperature by the heater 3.

  The reflux condenser 11 provided in the fifth crystallization tank 10 condenses a part of the water vapor (for example, 0 to 100%) discharged from the fifth crystallization tank 10 and returns it to the fifth crystallization tank 10. Control of the amount of reflux by the reflux condenser 11 is performed by measuring the terephthalic acid concentration in the slurry discharge line of the fifth crystallization tank 10 and providing it in the water vapor discharge line of the fifth crystallization tank 10 by the controller 30 according to the concentration. The flow rate control valve 23 is adjusted, the amount of water vapor discharged is adjusted, and the amount of water vapor supplied to the reflux condenser 11 is controlled. The terephthalic acid concentration may be measured by extracting a part of the slurry flowing through the slurry discharge line of the fifth crystallization tank 10 or by directly providing a concentration meter on the slurry discharge line. . Thereby, it can suppress that the terephthalic acid density | concentration in a 5th crystallization tank becomes high, and the flow of a slurry worsens, and can perform the favorable driving | operation of a refinery plant.

  For example, when a crude terephthalic acid slurry is prepared in a 29% by weight slurry and subjected to a hydrorefining reaction at a temperature of 290 ° C. and then sequentially crystallized, the terephthalic acid concentration in the fifth crystallization tank is about 40%, In the case of preparing a 33% by weight slurry of crude terephthalic acid and conducting a hydrorefining reaction at a temperature of 290 ° C., followed by sequential crystallization, the terephthalic acid concentration in the fifth crystallization tank is about 45%. In this way, in the slurry prepared with 29% by weight or more of crude terephthalic acid, since the slurry concentration in the fifth crystallization tank exceeds 40%, in the fifth crystallization tank, the slurry flow is improved. It is desirable to condense the generated water vapor and return it to the fifth crystallization tank.

  Next, the slurry after crystallization is sent to the solid-liquid separator 13 by the pump 12 while maintaining the temperature and pressure of the fifth crystallization tank 10 (final stage crystallization tank) for the first stage solid-liquid separation. And separate the purified terephthalic acid from the slurry. Furthermore, the separated purified terephthalic acid is put into the reslurry tank 14 and reslurried with 1.5 to 2.0 times the pure water of the purified terephthalic acid, and p-tA adhering to the purified terephthalic acid is removed. It is dissolved in pure water to remove p-tA from the purified terephthalic acid, and the purified terephthalic acid is washed. The purified terephthalic acid washed slurry is sent to a solid-liquid separator 16 by a pump 15 for solid-liquid separation, and the purified terephthalic acid thus separated is sent to a dryer 17 for drying.

  In the apparatus configured as described above, by setting the temperature of each crystallization tank at the temperature obtained by the relational expression (1) described above, terephthalic acid having a target particle size and ptA content is obtained. Can be purified.

  In this example, the purified terephthalic acid after crystallization is solid-liquid separated in the solid-liquid separator 13, the purified terephthalic acid is separated from the slurry, and the separated purified terephthalic acid is reslurried and washed, The p-tA was dissolved and removed, and the purified terephthalic acid was separated from the slurry by the solid-liquid separator 16 and dried by the dryer 17 to produce purified terephthalic acid, but after crystallization without using the reslurry method The purified terephthalic acid is directly supplied to a washing type filter, such as a pressure type rotary filter or a filter cell type rotary filter, the purified terephthalic acid is washed and filtered, and the filtered purified terephthalic acid is dried in a dryer 17. The terephthalic acid may be produced by drying with Needless to say, various filters can be used as the solid-liquid separator 13.

  In the terephthalic acid purified as described above, the properties of the purified terephthalic acid were measured by taking a sample of the cake after the first-stage solid-liquid separation and drying it. The collected purified terephthalic acid state was evaluated by measuring the particle size distribution by the wet sieving method as a preferable crystal property, and evaluating the average particle size, and the impurity content was evaluated by the p-tA content by the liquid chromatogram method. .

In order to optimize the crystal grain size at each stage, which is a leading factor for inclusion of ptA in the crystallization stage, in the crystallization stage, The temperature and the terephthalic acid concentration were changed, and the data of the p-tA content and average particle diameter after crystallization shown in Table 1 were obtained. In Table 1, those shown in Comparative Examples 1 to 24 are those in which any crystallization tank was operated at a temperature outside the range of the relational expression (1), and purified terephthalic acid having undesirable properties and It has become.

[Examples 1-3, Comparative Examples 1-3]
Crude terephthalic acid (CTA) obtained by oxidizing P-xylene with molecular oxygen in the presence of cobalt, manganese and bromine in an acetic acid solvent was mixed with water to prepare a 28 wt% slurry of crude terephthalic acid. Then, pressurize and supply with a supply pump, and dissolve terephthalic acid in water while heating to 288 ° C. The terephthalic acid aqueous solution was supplied together with hydrogen for purification to a purification reaction column packed with a Pd-Charcoal catalyst having a pressure of 7.3 MpG and a temperature of 288 ° C. to carry out a hydrorefining reaction. The terephthalic acid aqueous solution subjected to the reaction treatment was supplied to a five-stage crystallization tank connected in series to gradually reduce the pressure, and crystals of terephthalic acid were precipitated at the temperatures shown in Table 1. Table 1 shows the average particle diameters of 4-carbonylbenzoic acid (4-CBA), p-toluic acid (p-tA) and terephthalic acid crystal particles in the purified terephthalic acid obtained.

[Examples 4 to 6, Comparative Examples 4 to 7]
Under the same preconditions as in Examples 1 to 3, except that a 25 wt% crude terephthalic acid slurry was prepared, subjected to a hydrorefining reaction, and then sequentially crystallized at the temperatures shown in Table 1. The properties of the obtained purified terephthalic acid were as shown in Table 1.

[Examples 7 to 9, Comparative Examples 8 to 12]
Under the same preconditions as in Examples 1 to 3, except that a 31% by weight crude terephthalic acid slurry was prepared, subjected to a hydrorefining reaction, and then sequentially crystallized at the temperatures shown in Table 1. However, in these crystallizations, the final stage (fifth stage) crystallization tank was supplied with 0.2 parts by weight of warm water (70 ° C.) equivalent to the crude terephthalic acid supplied to the hydrorefining reaction. It was.

  Properties of the obtained purified terephthalic acid are as shown in Table 1.

[Examples 10 to 12, Comparative Examples 13 to 15]
Under the same preconditions as in Examples 1 to 3, except that a crude 27% by weight slurry of terephthalic acid was prepared and heated to 290 ° C. and subjected to a hydrorefining reaction at a pressure of 7.5 MpG and a temperature of 290 ° C. Crystallization was performed sequentially at the temperatures shown in Table 1. The properties of the obtained purified terephthalic acid were as shown in Table 1.

[Examples 13 and 14, Comparative Examples 16 to 19]
Under the same preconditions as in Examples 10 to 12 described above, however, a crude 30% by weight slurry of terephthalic acid was prepared, subjected to a hydrorefining reaction, and then sequentially crystallized at the temperatures shown in Table 1. However, in these crystallizations, the final stage (fifth stage) crystallization tank was supplied with 0.2 parts by weight of warm water (70 ° C.) equivalent to the crude terephthalic acid supplied to the hydrorefining reaction. It was.

  Properties of the obtained purified terephthalic acid are as shown in Table 1.

[Examples 15 to 17, Comparative Examples 20 to 22]
Under the same preconditions as in Examples 10 to 12 above, a 33% by weight crude terephthalic acid slurry was prepared, subjected to a hydrorefining reaction, and then sequentially crystallized at the temperatures shown in Table 1. However, in these crystallizations, the final stage (fifth stage) crystallization tank was supplied with hot water (70 ° C.) equivalent to 0.4 parts by weight with respect to the crude terephthalic acid supplied to the hydrorefining reaction. It was.

  Properties of the obtained purified terephthalic acid are as shown in Table 1.

[Comparative Examples 23 and 24]
The same preconditions as in Examples 1 to 3 above, except that a crude 23% by weight slurry of terephthalic acid was prepared and heated to 285 ° C. and subjected to a hydrorefining reaction at a pressure of 7.1 MpG and a temperature of 285 ° C. Crystallization was performed sequentially at the temperatures shown in Table 1. The properties of the obtained purified terephthalic acid were as shown in Table 1.

  FIG. 2 is a diagram showing the concentration of crude terephthalic acid introduced into the hydrorefining reaction and the temperature of each crystallization tank, and shows the temperature of each example and comparative example shown in Table 1 above. In FIG. 2, the temperature of the comparative example in Table 1 is dotted with a cross, and it is shown as a case where purified terephthalic acid having undesirable properties is generated. In each comparative example, the crystallization tank is operated at a temperature outside the range of the purification method of the present invention. FIG. 2 shows a preferable temperature diagram in each stage of the crystallization tank obtained from these data, and the above-mentioned relational expression (1) is an approximation of this temperature diagram.

  It is suitable for producing terephthalic acid having preferable properties as a raw material for producing polyester.

It is a systematic diagram which shows one Example of the plant for enforcing the purification method of the terephthalic acid of this invention. It is a figure which shows the relationship between the crude terephthalic acid concentration introduce | transduced into a hydrorefining reaction, and each crystallization tank temperature.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Slurry adjustment tank, 2, 12, 15 ... Pump, 3 ... Heater, 4 ... Dissolution tank, 5 ... Hydrogenation reaction tank, 6 ... 1st crystallization tank, 7 ... 2nd crystallization tank, 8 ... 1st 3 crystallization tanks, 9 ... 4th crystallization tank, 10 ... 5th crystallization tank, 11 ... Reflux condenser, 13, 16 ... Solid-liquid separator, 14 ... Reslurry tank, 17 ... Dryer, 20 ... Pressure reducing valve, 21 ... temperature sensor, 22 ... pressure sensor, 23 ... flow rate control valve, 30 ... control device.

Claims (5)

A five-stage flash evaporative cooling type crystallization method in which an aqueous slurry having a crude terephthalic acid concentration of 25 to 33 wt% is dissolved and hydrorefined in the presence of a Group VIII metal catalyst, and then the purified terephthalic acid aqueous solution is connected in series. In the method of recovering purified terephthalic acid which has been cooled and crystallized stepwise in the tank and crystallized purified terephthalic acid, the following correlation formula with the crude terephthalic acid concentration of the slurry to which the operating temperature of each stage of the crystallization tank is introduced: A method for purifying terephthalic acid, characterized in that the temperature is controlled to an approximate temperature.
Tn = (0.9958 * N ^ 3-8.4268 * N ^ 2-6.8726 * N
+ 239.89 + S * C) ± 2.5
Tn: Operating temperature of each crystallization tank (° C)
C: Concentration of crude terephthalic acid (wt%)
N: Stage value of the crystallization tank (1, 2, 3, 4, 5 from the high temperature side crystallization tank)
S: Coefficient in each crystallization tank (first stage 1.48, second stage, third stage, fourth stage, fifth stage 1.19) The method for purifying terephthalic acid according to claim 1, wherein the crude terephthalic acid concentration is 25 to 25.
A method for purifying terephthalic acid, in which a 33 wt% water slurry is dissolved in a temperature range of 280 to 295 ° C. and hydropurified in the presence of a Group VIII metal catalyst.   The method for purifying terephthalic acid according to claim 1, wherein the purified terephthalic acid is separated and recovered at the temperature of the fifth stage of the crystallization tank. A five-stage flash evaporative cooling type crystallization in which an aqueous slurry having a crude terephthalic acid concentration of 29 to 33 wt% is dissolved and hydrorefined in the presence of a Group VIII metal catalyst, and then the purified terephthalic acid aqueous solution is connected in series. In the method of recovering purified terephthalic acid which has been cooled and crystallized stepwise in the tank and crystallized purified terephthalic acid, the following correlation formula with the crude terephthalic acid concentration of the slurry to which the operating temperature of each stage of the crystallization tank is introduced: A method for purifying terephthalic acid, which comprises controlling the temperature to be approximated, condensing water vapor generated in the fifth stage of the crystallization tank, and refluxing the condensed water to the crystallization tank.
Tn = (0.9958 * N ^ 3-8.4268 * N ^ 2-6.8726 * N
+ 239.89 + S * C) ± 2.5
Tn: Operating temperature of each crystallization tank (° C)
C: Concentration of crude terephthalic acid (wt%)
N: Stage value of the crystallization tank (1, 2, 3, 4, 5 from the high temperature side crystallization tank)
S: Coefficient in each crystallization tank (first stage 1.48, second stage, third stage, fourth stage, fifth stage 1.19) 5. The method for purifying terephthalic acid according to claim 4, wherein the crude terephthalic acid concentration is 29 to 29.
A method for purifying terephthalic acid, in which a 33 wt% water slurry is dissolved in a temperature range of 290 to 295 ° C. and hydropurified in the presence of a Group VIII metal catalyst.

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