JP2006008671A - Method for producing high-purity terephthalic acid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing high-purity terephthalic acid, increased in recovery of secondary crystals precipitated by cooling a first separated mother liquor and capable of yielding a second separated mother liquor having low turbidity (SS content) in production of the high-purity terephthalic acid. <P>SOLUTION: The method comprises a oxidation step (a), a dissolving step (b), a reduction step (c), a first crystallization step (d), a solid-liquid separation step (e) and a second crystallization step (f), wherein cooling in the second crystallization step (f) is performed by decreasing pressure, and using multiple steps of cooling vessels for performing the cooling, in which vessels the final cooling vessel is decompressed to be lower than atmospheric pressure and 40-70°C temperature. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

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

  In the high-purity terephthalic acid production process, in order to produce high-purity terephthalic acid from paraxylene, first, raw paraxylene is oxidized to produce crude terephthalic acid, and then intermediate product 4- High purity terephthalic acid is produced by reducing carboxybenzaldehyde and removing it as p-toluic acid.

  The process is as follows. The raw material para-xylene is air-oxidized to terephthalic acid with a catalyst in acetic acid solvent at high temperature and high pressure. At this time, 4-carboxybenzaldehyde as an intermediate is by-produced together with terephthalic acid. The slurry containing these is crystallized and solid-liquid separated to obtain crude terephthalic acid crystals. Next, this crude terephthalic acid crystal is 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 reduced with hydrogen to highly soluble p-toluic acid. Thereafter, it is cooled by releasing pressure, and terephthalic acid having low water solubility is crystallized from the aqueous solution and recovered as high-purity terephthalic acid.

  Here, the primary separation mother liquor from which high-purity terephthalic acid has been separated contains active ingredients such as terephthalic acid and p-toluic acid, and while stirring in the crystallization tank to improve productivity and reduce drainage load It is known to recover secondary crystals by cooling (Patent Documents 1 and 2).

  However, when the primary separation mother liquor is cooled to a predetermined final temperature all at once, the crystal grain size becomes fine, and when collecting secondary crystals using a recovery device such as a filter, the filter is clogged and the recovery rate Inconveniences such as reduction of In addition, this secondary crystal has high adhesion, and in the crystallization using a propeller type stirring blade or paddle type stirring blade, sufficient stirring cannot be performed up to the vicinity of the inner wall of the tank, and there is an adhesion on the side surface of the inner wall. It is formed. Furthermore, fine powder entrained in the vapor generated by the crystallization treatment adheres to the side surface of the gas phase portion in the crystallization tank. If these device deposits grow and detach, it may cause clogging of pipes and the like, and it has been necessary to perform a cleaning operation such as periodically stopping and cleaning the equipment.

JP-A-52-128344 JP-A-5-58948

  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, thereby obtaining a secondary separation mother liquor having a low turbidity (SS concentration). An object of the present invention is to provide a method for producing high-purity terephthalic acid. It is another object of the present invention to provide a method for producing high-purity terephthalic acid capable of suppressing the formation of deposits in the system that cause clogging and the generation of lumps during cooling treatment.

  As a result of intensive studies to solve the above-mentioned problems, the present inventors have found that the above-mentioned problems can be solved by releasing and cooling the primary separation mother liquor in multiple stages, and the present invention has been completed. That is, the gist of the present invention resides in the following (1) to (11).

(1) (a) an oxidation step of oxidizing paraxylene to obtain crude terephthalic acid containing 4-carboxybenzaldehyde;
(B) a dissolving step in which the crude terephthalic acid is dissolved in an aqueous solvent under high temperature and pressure to obtain a crude terephthalic acid aqueous solution;
(C) A reduction step of obtaining a reduction reaction solution obtained by reducing the 4-carboxybenzaldehyde to p-toluic acid by bringing the crude aqueous terephthalic acid solution into contact with hydrogen in the presence of a catalyst;
(D) a first crystallization step of evaporating the reduction reaction solution under reduced pressure, cooling the temperature to 120 to 200 ° C., and crystallizing a primary crystal made of high-purity terephthalic acid;
(E) a solid-liquid separation step in which the slurry obtained by the first crystallization step (d) is solid-liquid separated into the primary crystal and the primary separation mother liquor,
(F) Production of high-purity terephthalic acid having a second crystallization step of crystallization of secondary crystals mainly composed of terephthalic acid and paratoluic acid contained in the primary separation mother liquor by cooling the primary separation mother liquor In the method
The cooling in the second crystallization step (f) is performed by lowering the pressure, and a plurality of cooling tanks are used for this cooling, of which the last cooling tank is reduced to less than atmospheric pressure and the temperature is set to 40 to A method for producing high-purity terephthalic acid at 70 ° C.

(2) Among the cooling tanks used in the second crystallization step (f), the first cooling tank is subjected to pressure release evaporation, so that the temperature is 100 ° C. to the solid-liquid separation in the solid-liquid separation step (e). The method for producing high-purity terephthalic acid according to (1), wherein the method is cooled to less than the temperature of 1.
(3) The method for producing high-purity terephthalic acid according to (2), wherein the first cooling tank is evaporated under pressure to atmospheric pressure.

(4) The method for producing high-purity terephthalic acid according to any one of (1) to (3), wherein the cooling tank used in the second crystallization step (f) is a cooling tank having an anchor type stirring blade.
(5) Among the cooling tanks used in the second crystallization step (f), at least one has a stirring blade having an interval with the inner wall of the cooling tank of 10 mm or more and 50 mm or less (1) to (4 ). The manufacturing method of the high purity terephthalic acid in any one of.

(6) (g) A filtration step in which the slurry obtained in the second crystallization step (f) is solid-liquid separated into a secondary crystal and a secondary separation mother liquor using a filter;
The method for producing high-purity terephthalic acid according to any one of (1) to (5).
(7) The method for producing high-purity terephthalic acid according to (6), wherein the secondary crystal separated in the filtration step (g) is donated to the oxidation step (a).

(8) In the filtration step (g), the filter downstream side of the filter is set to a pressure state of atmospheric pressure or higher, and the filter upstream side of the filter is set to a pressure state higher than the filter downstream side. The method for producing high-purity terephthalic acid according to (6) or (7), wherein solid-liquid separation is performed by cake filtration in the forward direction.
(9) The secondary separation mother liquor obtained in the filtration step (g) is provided directly or indirectly to the dissolution step (b), according to any one of (6) to (8) Of high purity terephthalic acid.

(10) The secondary separation mother liquor obtained in the filtration step (g) is contacted with a synthetic adsorbent to remove paratoluic acid, and then provided to the dissolution step (b) (6) to (8) The manufacturing method of the highly purified terephthalic acid in any one of.
(11) The high-purity terephthalic acid according to any one of (4) to (7), wherein the suspension concentration in the secondary separation mother liquor obtained in the filtration step (g) is 200 mg / L or less. Production method.

  According to the present invention, in producing high-purity terephthalic acid, the primary separation mother liquor can be cooled to increase the recovery rate of secondary crystals deposited, and a secondary separation mother liquor with low turbidity (SS concentration) can be obtained. A method for producing high-purity terephthalic acid can be provided. Moreover, the manufacturing method of the high purity terephthalic acid which can suppress formation of the deposit | attachment in a system | system | group which becomes a cause of obstruction | occlusion at the time of a cooling process, and generation | occurrence | production of a lump can be provided.

Hereinafter, the present invention will be described in detail.
This invention has an oxidation step (a), a dissolution step (b), a reduction step (c), a first crystallization step (d), a solid-liquid separation step (e), and a second crystallization step (f). In the method for producing high-purity terephthalic acid, the cooling in the second crystallization step (f) is performed by lowering the pressure, and a plurality of cooling tanks are used for this cooling, of which the last cooling tank is below atmospheric pressure. This is a method for producing high-purity terephthalic acid under reduced pressure and at a temperature of 40 to 70 ° C. Said process (a)-(f) shows the following content.

Oxidation step (a): A step of oxidizing crude paraxylene to obtain crude terephthalic acid containing 4-carboxybenzaldehyde.
Dissolving step (b): A step of dissolving the crude terephthalic acid in an aqueous solvent under high temperature and high pressure to obtain a crude terephthalic acid aqueous solution.

Reduction step (c): A step of obtaining a reduction reaction solution obtained by reducing the 4-carboxybenzaldehyde to p-toluic acid by bringing the crude terephthalic acid aqueous solution into contact with hydrogen in the presence of a catalyst.
First crystallization step (d): a step of evaporating the reduction reaction solution under reduced pressure, cooling the temperature to 120 to 200 ° C., and crystallizing a primary crystal composed of high-purity terephthalic acid.

Solid-liquid separation step (e): A step of solid-liquid separation of the slurry obtained in the first crystallization step (d) into the primary crystal and the primary separation mother liquor.
Second crystallization step (f): A step of crystallizing secondary crystals mainly composed of terephthalic acid and p-toluic acid contained in the primary separation mother liquor by cooling the primary separation mother liquor.

Hereinafter, the detail of the manufacturing method of this invention is demonstrated using FIG.
First, in the oxidation step (a), crude terephthalic acid is produced by liquid phase oxidation of paraxylene A with molecular oxygen B in an acetic acid solvent in the presence of a catalyst. This step is well known, and as the catalyst, a catalyst conventionally known to be usable for this reaction is used. Specifically, a heavy metal compound such as a cobalt compound, a manganese compound, an iron compound, a chromium compound, and the like are used. Examples thereof include bromine compounds. These are present in the reaction system in a dissolved state. Among these, a combination of a cobalt compound or a manganese compound and a bromine compound is preferable. In this case, these compounds are usually used so that the cobalt atom is 10 to 5000 ppm, the manganese atom is 10 to 5000 ppm, and the bromine atom is 10 to 10,000 ppm with respect to the solvent.

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

  The ratio of para-xylene to acetic acid fed 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.

The temperature of the oxidation reaction is usually 160 to 260 ° C., preferably 170 to 210 ° C., and the pressure is not less than the pressure at which the reaction system can maintain the liquid phase at the reaction temperature, and usually 0.5 to 5 MPa, preferably 1 ˜2 MPa, residence time is usually 10 to 200 minutes.

  Since terephthalic acid is difficult to dissolve in acetic acid as a solvent, terephthalic acid produced in the oxidation reaction step usually precipitates as crystals to form a slurry. However, terephthalic acid may be dissolved depending on the amount of solvent, reaction temperature, and pressure. In this case, a crystallization step for cooling the reaction solution is provided to deposit terephthalic acid to form a slurry. This 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, it may be solid-liquid separated as it is, or may be solid-liquid separated after it is cooled by releasing pressure. Any solid-liquid separation method may be used as long as the crystal and the mother liquor can be separated, and examples thereof include filtration and centrifugation. Washing and drying are performed as necessary to obtain crude terephthalic acid crystals (crude terephthalic acid C).

  In addition, "crude terephthalic acid" in the present invention means terephthalic acid containing 1000 to 10,000 ppm of 4-carboxybenzaldehyde.

  When paraxylene A is oxidized in the oxidation step (a), not only the terephthalic acid but also 4-carboxybenzaldehyde (hereinafter referred to as “4CBA”) in which the oxidation reaction of one alkyl group has not completely proceeded. By-products such as abbreviated) are produced. In order to remove these by-products from the crude terephthalic acid C and obtain high purity terephthalic acid, the following steps are performed.

  In the dissolution step (b), the crude terephthalic acid C is slurried with water D in the slurrying tank 12, and this starting slurry E is dissolved in water at a high temperature and high pressure by the pump 12a and the heater 12b, and is dissolved in an aqueous solution E ′. To do. The terephthalic acid has low solubility in water, and the high temperature and high pressure need to be in a state where the terephthalic acid can be dissolved in water. Although this temperature depends on the slurry concentration, it is preferably 230 ° C. or higher and 320 ° C. or lower. If the temperature is lower than 230 ° C., the solubility is not sufficient. If the temperature exceeds 320 ° C., energy is wasted, and if the temperature is too high, terephthalic acid may decompose to produce another substance. Moreover, this pressure needs to be a pressure which can maintain a liquid phase in said temperature range, and it is desirable that it is 2.8 MPa or more and 11.3 MPa or less.

  In addition, the density | concentration of the slurry obtained in a melt | dissolution process (b) is 20 to 40 weight% normally, Preferably it is 25 to 35 weight%. If the slurry concentration is too high, it will cause clogging in the apparatus, and if it is too low, the amount of the mother liquor will increase and the equipment for the production volume will be enlarged. From the viewpoint of preventing clogging, the slurry concentration is preferably kept constant.

  Next, as the reduction step (c), the aqueous solution E ′ of terephthalic acid obtained in the dissolution step is sent to the hydrogenation reactor 13 and contact-reduced with the introduced hydrogen F in the presence of the catalyst to obtain a reduction reaction solution. Get G. The catalyst and the conditions in the hydrogenation reactor 13 need to reduce the 4CBA and not the terephthalic acid. This is because the 4CBA contained in the aqueous solution E ′ is reduced to p-toluic acid having high water solubility. It is desirable to perform this reduction at a rate as high as possible.

This hydrogenation is also well known. As the hydrogenation catalyst, a metal catalyst such as ruthenium, rhodium, palladium, platinum, osmium (in accordance with the revised IUPAC inorganic chemical nomenclature (1998)) metal catalyst is used. It is supported on a carrier such as activated carbon and used as a fixed bed. Among these, palladium supported on activated carbon is preferable. The hydrogenation temperature is usually 260 to 320 ° C., preferably 270 to 300 ° C., and the hydrogen partial pressure is usually 0.5 to 20 kg / cm ² G.

  Further, as the 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 and pressure are maintained within the range in which the paratoluic acid remains dissolved. And the primary crystal composed of the terephthalic acid is crystallized to make slurry H. Here, it is more desirable that the crystallization tank 14 is provided in a plurality of stages, preferably 3 to 6 stages in series, and the temperature and pressure are gradually lowered to cool (pressure-evaporative cooling) to crystallize the terephthalic acid. . The temperature of the last crystallization tank 14 may be controlled to a temperature condition in which p-toluic acid does not co-crystallize with terephthalic acid. Specifically, the temperature needs to be 120 ° C. or higher and 180 ° C. or lower, and 130 ° C. or higher. , 170 ° C. or less is desirable. The pressure at this time needs to be 0.20 MPa or more and 1.56 MPa or less, and is preferably 0.27 MPa or more and 1.00 MPa or less. When the temperature and pressure are lower than these conditions, not only the terephthalic acid but also p-toluic acid are co-crystallized, and the purity of the resulting high-purity terephthalic acid crystal is lowered. On the other hand, if the temperature and pressure are kept higher than these conditions, the amount of crystallization of terephthalic acid obtained decreases and the efficiency deteriorates.

  Next, as a solid-liquid separation step (e), the slurry H is introduced into a solid-liquid separator, the primary separation mother liquor J is separated from the slurry H, and the high purity containing the high-purity terephthalic acid crystals is obtained. Get a terephthalic acid cake. The high-purity terephthalic acid cake is preferably washed and then dried to form primary crystals composed of high-purity terephthalic acid crystals, and the step performed by the solid-liquid separator and the washing device is performed as one solid-liquid. It is more desirable that the separation / cleaning apparatus 15 collectively perform the process because the process can be simplified.

  As described above, the operation when the slurry H is solid-liquid separated and washed by one solid-liquid separation / washing device 15 is as follows. Slurry H and cleaning liquid I are introduced into the solid-liquid separation / cleaning device 15. As the cleaning liquid I, water is more desirable. The slurry H is separated into solid and liquid, the separated cake is washed with the washing liquid I, and the high-purity terephthalic acid cake L is separated from the primary separation mother liquor J and taken out from the primary separation mother liquor J and mainly the components of the washing liquid I. The cleaning discharge liquid K is discharged. Here, the temperature of the primary separation mother liquor J discharged from the solid-liquid separation / washing device 15 is equivalent to the crystallization conditions in the first crystallization step (d), preferably 120 ° C. or more and 200 ° C. or less, and 130 More preferably, the pressure is higher than or equal to 180 ° C. and lower than or equal to 180 ° C., and the pressure needs to be higher than the pressure of the final crystallization tank in the crystallization step (d) in order to suppress the temperature drop due to the release pressure. is there. Specifically, the pressure is preferably 0 to 1 MPa higher than the pressure in the final crystallization tank in the crystallization step (d). In this 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 / washing device 15 that can collectively perform solid-liquid separation and washing include a screen bowl type centrifuge, a rotary vacuum filter, a horizontal belt filter, and the like, and particularly preferably a screen. A bowl-type centrifuge.

  The high-purity terephthalic acid crystal M can be obtained by drying the high-purity terephthalic acid cake L thus obtained with the drying device 16 to remove the remaining adhering liquid. Examples of the drying device 16 include a rotary dryer, a fluidized bed dryer, and the like. In the presence of aeration gas, the drying device 16 is used as a heat source such as water vapor at a drying outlet operating temperature of 70 ° C. to 180 ° C.

  On the other hand, the primary separation mother liquor J and the washing effluent K described above still contain active ingredients, and it is necessary to collect these as much as possible to obtain high-purity terephthalic acid crystals. The active ingredient refers to the terephthalic acid and other compounds that can be converted to the terephthalic acid by oxidation or the like, such as p-toluic acid, and includes both dissolved components and solids. Moreover, the said solid content shows the component which has precipitated among the said active ingredients.

  First, since the washing | cleaning discharge liquid K has little content of paratoluic acid, it is desirable to return directly as a solvent of said melt | dissolution process (b).

  Moreover, when the washing | cleaning discharge liquid K contains solid content, before returning to a melt | dissolution process (b), you may carry out solid-liquid separation with another solid-liquid separation apparatus. This is because when the solid-liquid separation and washing are performed together in the solid-liquid separation / washing device 15, since leakage easily occurs, the washing discharge liquid K may contain a solid content. . In addition, if the crystallization is performed in advance before the separation by the other solid-liquid separation device, the amount of components that can be recovered by the solid-liquid separation is improved. At that time, the solid content may be sent to the crystallization tank 14 and recovered, or the separated liquid content may be used as the solvent used in the slurrying tank 12.

  Next, for the primary separation mother liquor J, as the second crystallization step (f), the primary separation mother liquor J is cooled by lowering the pressure using a plurality of cooling tanks, and contained in the primary separation mother liquor J. The secondary crystals mainly composed of the terephthalic acid and p-toluic acid are crystallized and recovered. The above-mentioned plurality of stages refers to a configuration in which two or more cooling tanks are installed in series. In each tank, the dissolved components are precipitated by sequentially lowering the respective temperatures in accordance with the pressure.

  In addition, a pressure relief cooling tank is used as the cooling tank. This pressure-release 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 set to be equal to or lower than the temperature of the liquid before introduction. Say things. When the liquid is introduced into the pressure-reducing 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, when the liquid is a solution, the solute that exceeds the solubility after cooling is crystallized.

  When crystallization is carried out step by step using the above-mentioned multiple pressure relief cooling tanks, and crystallization is carried out by combining only one-stage pressure relief cooling, or one-stage pressure relief cooling and cooling by heat exchange. Compared with cooling and crystallization, it is possible to suppress unevenness of cooling and crystallization, so that more thorough crystallization can be performed and the amount of solids obtained is increased, and the properties of the obtained solids are easy to handle. It becomes. Moreover, it can also suppress that the crystal | crystallization of the said active ingredient adheres in the said discharge pressure cooling tank or piping introduced into the said pressure reduction cooling tank.

  For this reason, the first cooling tank 17, which is the first pressure-reducing cooling tank among the plurality of stages of pressure-reducing cooling tanks, has a pressure of atmospheric pressure or higher and is discharged from the solid-liquid separation step (e). The primary separation mother liquor J is preferably less than the pressure of the primary separation mother liquor J, the temperature is 100 ° C. or higher, and is less than the temperature of the primary separation mother liquor J discharged from the solid-liquid separation step (e). It may be cooled to 100 ° C. to a temperature lower than that at the time of solid-liquid separation in the solid-liquid separation step (e), and it is desirable that the pressure-release evaporation is performed by pressure-release to atmospheric pressure. If the pressure is reduced to less than atmospheric pressure all at once, the pressure reduction width becomes too large and the crystallization becomes non-uniform, or the size of the resulting crystal becomes small, and the significance of providing a multi-stage pressure-reducing cooling bath is diminished. End up. Further, the temperature becomes 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 mother liquor J are lower.

  Moreover, it is desirable that at least one of the plurality of stages of the pressure-reducing cooling tanks has a stirring blade whose distance from the inner wall of the pressure-reducing cooling tank is 10 mm or more and 50 mm or less. In addition, it is desirable that the stirring blade has a longer portion close to the inner wall of the pressure-reducing cooling tank at the above-described interval, and more desirably an anchor-type stirring blade. Since the terephthalic acid having a relatively low purity is crystallized in the pressure relief cooling tank, there is a possibility that crystals such as terephthalic acid adhere to the wall surface and solidify only by using a normal stirring blade. Therefore, when the liquid near the wall surface is appropriately flowed by the stirring blade that rotates in the vicinity of the wall surface of the tank, adhesion of the terephthalic acid or the like to the wall surface can be suppressed. However, if the pressure is too close, the pressure relief cooling tank itself may be damaged. Therefore, the distance between the stirring blade and the wall surface is preferably 10 mm or more. In particular, the first cooling tank 17, which is the first pressure-reducing cooling tank, is more likely to have the largest amount of crystallization. Therefore, it is more desirable that the first discharging tank 17 is provided with the above-described stirring blade. In addition, the above-described stirring blades may be provided in all of the multiple-stage pressure-release cooling tanks.

  When the secondary crystal is crystallized from the primary separation mother liquor J using the anchor-type stirring blade in the pressure-release cooling tank, the rotation speed of the anchor-type stirring blade is 3.0 rpm or more and 30 rpm or less. Is preferable, and is more desirably 5 rpm or more and 20 rpm or less. If it is less than 3.0 rpm, the effect of stirring cannot be fully exerted, and crystals crystallized on the inner wall side surface below the liquid level may adhere as a lump. On the other hand, a speed of 30 rpm is sufficient for stirring, and not only is it wasteful of energy, but also the primary separation mother liquor J is scattered in the pressure-reducing cooling tank, so that the liquid in the tank It becomes a factor that clumps adhere to the surface.

  Furthermore, it is desirable to crystallize not only the terephthalic acid but also the p-toluic acid and other by-products as much as possible in the multi-stage pressure-relief cooling tank. The components that cannot be crystallized even in the second crystallization step (f) cannot be recovered in the filtration step (g) described later and are discharged as the secondary separation mother liquor P. In addition to the above, all active ingredients are crystallized as much as possible so that they can be recovered.

  For this reason, the pressure of the last cooling tank 18 which is the last cooling tank (releasing pressure cooling tank) among the above-mentioned cooling tanks (releasing pressure cooling tanks) is reduced to less than atmospheric pressure, and the temperature of the cooling tank is reduced. 40 to 80 ° C., preferably 50 to 70 ° C. Specifically, the pressure in the final cooling bath 18 is 0.007 MPa or more and 0.03 MPa or less, preferably 0.021 MPa or more and 0.031 MPa or less. If the pressure exceeds 0.03 MPa and the temperature is too high (above 80 ° C.), the crystallization is not thorough and there is a possibility that the recovery becomes insufficient. On the other hand, if the pressure is less than 0.007 MPa and the temperature is too low (below 40 ° C.), the degree of pressure reduction is high, so the burden on the final cooling bath 18 may be too great.

  Note that one or more pressure-release cooling tanks may be provided between the first cooling tank 17 and the final cooling tank 18 to perform stepwise crystallization.

  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 the filtration step (g), and the secondary separation mother liquor P and the second slurry O The next crystal Q is solid-liquid separated.

  The filter 19 used here is capable of setting the downstream side of the filter to a pressure state equal to or higher than atmospheric pressure, and further enabling the upstream side of the filter to be in a higher pressure state than the downstream side of the filter, and by cake filtration in the forward direction. It is desirable that solid-liquid separation is performed because filtration is facilitated. This is because the secondary slurry O is easy to handle by being crystallized in the above-described multiple-stage pressure-reducing cooling tanks, but is easy to adhere, and thus is difficult to proceed by normal filtration. The upstream side of the filter means the side sent from the second crystallization step (f), the downstream side of the filter means the side from which the secondary separation mother liquor P is discharged, and the forward direction is the secondary direction. The direction of the flow of the slurry O and the secondary separation mother liquor P is from the upstream side of the filter toward the downstream side of the filter. Also, the above cake filtration means that particles are trapped by the cross-linking phenomenon on the filter pores, and soon after the start of filtration, a filter cake layer is formed on the filter surface, and the cake layer acts on the filter for subsequent filtration. It is a mechanism in which filtration proceeds while performing. Examples of the filter 19 to be operated in this way include a Fundaback filter manufactured by Ishikawajima-Harima Heavy Industries, Ltd., a cricket filter manufactured by Tsukishima Kikai Co., Ltd., and the like.

  The secondary crystal Q is preferably returned to any of the above steps to reuse the active ingredient, but more preferably returned to the oxidation step (a). As described above, the secondary crystal Q contains not only the terephthalic acid but also partially reduced impurities such as p-toluic acid, which cannot be used as they are as the terephthalic acid product. Then, by oxidizing in said oxidation process (a), these impurities can be made into the said terephthalic acid, and the yield of the whole manufacturing method of the high purity terephthalic acid concerning this invention can be improved.

  On the other hand, the concentration of the suspension of the secondary separation mother liquor P is desirably 200 mg / L or less, more desirably 100 mg / L or less, and particularly desirably 50 mg / L or less. Here, the suspension concentration means the weight of suspended matter such as p-toluic acid dispersed without dissolving in the solvent with respect to the total weight of the secondary separation mother liquor P, and is a method described in JIS K 0101. Analyzed according to At least a portion of the secondary separation mother liquor P is discharged out of the system because the concentration of impurities in the system is not excessively high, so that the paratoluic acid contained in the secondary separation mother liquor P is discarded. This is because the yield of the entire process for producing terephthalic acid according to the invention will be lowered. Furthermore, in the case where at least a part of the secondary separation mother liquor P is reused directly or indirectly as a solvent in the production process, it is necessary to suppress the accumulation of p-toluic acid in the system.

  For removal of paratoluic acid contained in the secondary separation mother liquor P, there is a method of bringing the secondary separation mother liquor P into contact with a synthetic adsorbent. As the synthetic adsorbent, an organic synthetic adsorbent is usually used. For example, synthetic adsorption of styrene-divinylbenzene system such as SEPABEADS SP825, SP850, SP207 (SEPABEADS is a registered trademark of Mitsubishi Chemical Corporation), AMBERLITE XAD-4, XAD-16 (AMBERLITE is a registered trademark of Rohm & Haas) And acrylic synthetic adsorbents such as DIAION HP2MG (DIAION is a registered trademark of Mitsubishi Chemical Corporation), AMBERLITE XAD-7, and XAD-8. Preferably, a nonpolar organic synthetic adsorbent, particularly a synthetic adsorbent composed of a porous copolymer of a monovinyl compound and a polyvinyl compound, in particular, a styrene-divinylbenzene synthetic adsorbent is used. Since p-toluic acid has a benzene ring, it is easily adsorbed by a synthetic adsorbent based on styrene-divinylbenzene.

The specific surface area of the adsorbent is usually 400~1500m ² / g, preferably 600~1000m ² / g, a pore volume of usually 0.5-3 ml / g, preferably 1.0~2.0mL / g, fine The pore diameter is usually 10 to 1000 mm, preferably 50 to 500 mm. The adsorbent is usually packed in the adsorption tower so that the packed bed height is about 1.5 to 4.0 m. Although depending on at which point the supply of the liquid to be treated is stopped, generally, if the packed bed height is too low, the utilization efficiency of the adsorbent is lowered.

Regarding the supply speed of the liquid to be treated to the adsorption tower, LV is usually 0.5 to 30 m / hr, and SV is usually 0.5 to 20 hr ⁻¹ . The adsorbed secondary separation mother liquor P has a reduced p-toluic acid concentration, and is preferably easily reused in the production process as a solvent in the dissolution process or as a separation cake washing liquid after the solid-liquid separation process. In addition, paratoluic acid is an oxidation intermediate when oxidized from paraxylene to terephthalic acid, and it is preferable that the paratoluic acid adsorbed on the synthetic adsorbent is recovered using a desorbing solution and supplied to the oxidation step.

Hereinafter, the present invention will be specifically described by way of examples.
[Method for Measuring Suspension Concentration (JIS K 0101)]
The suspended substance (substance suspended in water) is filtered according to the following operation, the substance remaining on the filter medium is dried at 105 to 110 ° C., and the mass is measured.
(A) When using glass fiber filter paper, attach it to a filter in advance and thoroughly wash it with water, and then place this filter medium on a watch glass and heat it at 105-110 ° C for about 1 hour, in a desiccator. After standing to cool, measure the mass.
(B) A filter medium is attached to the filter, and an appropriate amount of the sample is poured into the filter and suction filtered. The material adhering to the wall of the sample container and the filter tube is washed off on the filter medium with water, matched with the residual substance on the filter medium, and washed several times with water.
(C) The residue is carefully removed from the filter using tweezers together with the filter medium, transferred onto the watch glass used in (a), heated at 105-110 ° C. for 2 hours, and then in the desiccator. After standing to cool, measure the mass.
(D) A suspended substance (mg / L) is calculated by the following formula <1>.
S = (a−b) × 1000 / V (1)
Here, S is a suspended substance (mg / L), a is a mass of the filter medium and watch glass (mg) containing the suspended substance, b is a mass of the filter medium and the watch glass (mg), V Means sample (mL).

Example 1
[Step (a)]
Acetic acid solution containing para-xylene, catalyst (cobalt acetate, manganese acetate acetic acid solution and hydrogen bromide), separation mother liquor recycled from the subsequent solid-liquid separation process, and air are continuously supplied to the stirring tank, and the operating temperature The oxidation reaction was carried out while adjusting the liquid level so that the residence time was 1 hour at 190 ° C. and an operating pressure of 1.23 MPa (absolute pressure). The distillate vapor was finally cooled to 40 ° C. by a multistage condenser, and the operation was carried out with the oxygen concentration in the exhaust gas adjusted to 2.5 vol%. 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 water concentration in the mother liquor of the reaction withdrawal slurry was 10% by weight. The slurry concentration of the slurry extracted from the oxidation reactor was 35% by weight, and the cobalt / manganese / bromine concentration in the reaction mother liquor was 300/300/1000 ppm by weight.

  The slurry extracted from the oxidation reactor is continuously supplied to the stirring tank together with air, and the liquid level is adjusted so that the residence time is 15 minutes at an operating temperature of 181 ° C. and an operating pressure of 1.15 MPa (absolute pressure). The low temperature additional oxidation reaction was performed. The distillate vapor was finally cooled to 40 ° C. by a multistage condenser, and the operation was carried out with the oxygen concentration in the exhaust gas adjusted to 6 vol%. In addition, the condensate obtained from each condenser was integrated and refluxed to the low temperature additional oxidation reactor.

  The slurry extracted from the low temperature additional oxidation reactor was crystallized to 90 ° C., and then the slurry obtained by this crystallization was supplied to a rotary vacuum filter to perform solid-liquid separation and washing. 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.

[Step (b) to Step (c)]
The obtained crude terephthalic acid was donated to the production process of high purity terephthalic acid shown in FIG. Using water D as a solvent, an aqueous solution E ′ having a high temperature and high pressure containing 30% by weight of crude terephthalic acid was obtained. In the process shown in FIG. 1, the temperature and pressure of the aqueous solution E ′ fed to the hydrogenation reactor 13 were 290 ° C. and 8.7 MPa (89 kgf / cm ² · gauge), respectively.

[Step (d) to Step (e)]
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 pressure release evaporation, and finally cooled to a temperature of 155 ° C. The solute was crystallized. The slurry H obtained by crystallization is separated into a high-purity terephthalic acid cake L containing primary crystals and a primary separation mother liquor J by a solid-liquid separation / washing device 15, and the high-purity terephthalic acid cake L is washed. After washing with water and drying with a drying device 16, it was recovered as high-purity terephthalic acid crystals M.

[Step (f)]
On the other hand, the primary separation mother liquor J is subjected to first-stage second crystallization step (f) by reducing the pressure to atmospheric pressure and releasing it to 100 ° C. in the first cooling tank 17 provided with an anchor type stirring blade. Was crystallized. The rotation speed of the anchor type stirring blade was 10 rpm, and the gap between the anchor type stirring blade and the inner wall side face was 10 mm.

  Subsequently, the obtained intermediate slurry N at 100 ° C. was introduced into a final cooling bath 18 equipped with a steam ejector, the operation pressure was set to 0.02 MPa, and the mixture was cooled to 60 ° C. under reduced pressure. The secondary slurry O obtained in this way was put into a Fondaback filter (R56-86-25 type) manufactured by Ishikawajima-Harima Heavy Industries, Ltd. as a filter 19 and separated into solid and liquid. The suspension concentration of the secondary separation mother liquor P obtained here was measured according to the method described in JIS K 0101 and found to be 30 mg / L.

  As a result, in the first cooling tank 17 and the final cooling tank 18, the secondary crystal Q could be stably recovered without deposits growing on the inner wall side surface for one month. .

Flow chart showing an example of a method for producing terephthalic acid according to the present invention

Explanation of symbols

12 Slurry tank 12a Pump 12b Heater 13 Hydrogenation reactor 14 Crystallization tank 15 Solid-liquid separation / washing apparatus 16 Drying apparatus 17 First cooling tank 18 Final cooling tank 19 Filter A Paraxylene B Molecular oxygen C Crude terephthalic acid D Water E Starting slurry E 'Aqueous solution F Hydrogen G Reduction reaction liquid H Slurry I Cleaning liquid J Primary separation mother liquor K Cleaning discharge liquid L High purity terephthalic acid cake M High purity terephthalic acid crystal N Intermediate slurry O Secondary slurry P Secondary separation mother liquid Q secondary crystal

Claims (11)

(A) an oxidation step of oxidizing paraxylene to obtain crude terephthalic acid containing 4-carboxybenzaldehyde;
(B) a dissolving step in which the crude terephthalic acid is dissolved in an aqueous solvent under high temperature and pressure to obtain a crude terephthalic acid aqueous solution;
(C) A reduction step of obtaining a reduction reaction solution obtained by reducing the 4-carboxybenzaldehyde to p-toluic acid by bringing the crude aqueous terephthalic acid solution into contact with hydrogen in the presence of a catalyst;
(D) a first crystallization step of evaporating the reduction reaction solution under reduced pressure, cooling the temperature to 120 to 200 ° C., and crystallizing a primary crystal made of high-purity terephthalic acid;
(E) a solid-liquid separation step in which the slurry obtained by the first crystallization step (d) is solid-liquid separated into the primary crystal and the primary separation mother liquor,
(F) Production of high-purity terephthalic acid having a second crystallization step of crystallization of secondary crystals mainly composed of terephthalic acid and paratoluic acid contained in the primary separation mother liquor by cooling the primary separation mother liquor In the method
The cooling in the second crystallization step (f) is performed by lowering the pressure, and a plurality of cooling tanks are used for this cooling, of which the last cooling tank is reduced to less than atmospheric pressure and the temperature is set to 40 to A method for producing high-purity terephthalic acid at 70 ° C.   Among the cooling tanks used in the second crystallization step (f), the first cooling tank is subjected to pressure-pressure evaporation, so that the temperature is 100 ° C. to less than the temperature during solid-liquid separation in the solid-liquid separation step (e). The method for producing high-purity terephthalic acid according to claim 1, wherein the method is cooled to a low temperature.   The method for producing high-purity terephthalic acid according to claim 2, wherein the first cooling bath is evaporated under reduced pressure to atmospheric pressure.   The method for producing high-purity terephthalic acid according to any one of claims 1 to 3, wherein the cooling tank used in the second crystallization step (f) is a cooling tank having an anchor type stirring blade.   5. The cooling tank used in the second crystallization step (f), wherein at least one of the cooling tanks has a stirring blade having an interval with the inner wall of the cooling tank of 10 mm or more and 50 mm or less. The manufacturing method of high-purity terephthalic acid as described. (G) A filtration step in which the slurry obtained in the second crystallization step (f) is solid-liquid separated into a secondary crystal and a secondary separation mother liquor using a filter;
The method for producing high-purity terephthalic acid according to any one of claims 1 to 5.   The method for producing high-purity terephthalic acid according to claim 6, wherein the secondary crystals separated in the filtration step (g) are donated to the oxidation step (a).   In the filtration step (g), the filter downstream side of the filter is set to a pressure state equal to or higher than the atmospheric pressure, and the filter upstream side of the filter is set to a pressure state higher than the filter downstream side in the forward direction of the filter. The method for producing high-purity terephthalic acid according to claim 6 or 7, wherein solid-liquid separation is performed by cake filtration.   The high-purity terephthalic acid according to any one of claims 6 to 8, wherein the secondary separation mother liquor obtained in the filtration step (g) is directly or indirectly provided to the dissolution step (b). Manufacturing method.   The secondary separation mother liquor obtained in the filtration step (g) is contacted with a synthetic adsorbent to remove paratoluic acid, and then supplied to the dissolution step (b). Of high purity terephthalic acid.   The method for producing high-purity terephthalic acid according to any one of claims 4 to 7, wherein the suspension concentration in the secondary separation mother liquor obtained in the filtration step (g) is 200 mg / L or less.

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