1. A method for treating a mother liquor separated from a purified terephthalic acid in a process of dissolving a crude terephthalic acid produced by liquid phase oxidation of paraxylene in water solvent at high temperature and high pressure, subjecting the solution to hydrogenative purification and then cooling the said solution to form a crystal slurry of a purified terephthalic acid, thereby producing a highly purified terephthalic acid from the crystal slurry, in which the method comprises: a first separation step of separating the crystal slurry of the purified terephthalic acid into a crystals of the purified terephthalic acid (first crystals) and a mother liquor (first mother liquor), a step of forming a cooled liquid suspension by adding terephthalic acid crystals to the separated mother liquor (first mother liquor), then supplying them to a cooling tank controlled at a reduced pressure of lower than an atmospheric pressure, and performing pressure-release cooling thereby forming the cooled liquid suspension, and a second separation step of subjecting the cooled liquid suspension to filtration-separation under pressure of an atmospheric pressure or higher, and separating and recovering the terephthalic acid crystals at a less purity (second crystals).

2. A method of treating a mother liquor separated according to claim 1, wherein the mother liquor (first mother liquor) is separated from the crystal slurry of the purified terephthalic acid at a temperature of about 120 to 170°C and at a vapor pressure of the slurry or higher (0.2 to 0.9 MPa) in the first separation step, and a terephthalic acid crystals is added to the separated mother liquor (first mother liquor), then they are supplied to a cooling tank controlled at a reduced pressure of 1.3 to 53 kPa (10 - 400 mmHg), and then subjected to pressure-release cooling thereby forming the cooled liquid suspension in the step of forming the cooled liquid suspension.

3. A method of treating a mother liquor according to claim 1 or 2, wherein the terephthalic acid crystals formed as a slurry in water is added at a ratio of 0.05 to 1% by weight to the separated mother liquor (first mother liquor) in the step of forming the cooled liquid suspension.

4. A method of treating a mother liquor according to any one of claims 1 to 3, wherein the terephthalic acid crystals to be added to the separated mother liquor (first mother liquor) is in the form of a crystal powder formed by drying the crystals of the purified terephthalic acid in the step of forming the cooled liquid suspension.

5. A method of treating a mother liquor according to any one of claims 1 to 4, wherein the crystals of terephthalic acid at a less purity separated and recovered from the cooled liquid suspension (second crystals) in the second separation step is used as a portion of the starting material for the liquid phase oxidation reaction of paraxylene.

6. A method of treating a mother liquid according to any one of 1 to 5, wherein the mother liquor separated from the cooled liquid suspension (second mother liquor) in the second separation step is used as a portion of water for dissolving the crude terephthalic acid. Dated this the 13th Day of August, 2013

TECHNICAL FIELD

[0001] The present invention concerns a method for efficiently treating a mother liquor (first mother liquor) obtained by solid-liquid separation of a crystal slurry of a purified terephthalic acid in a process for producing a highly purified terephthalic acid (PTA) by dissolving crystals of a crude terephthalic acid produced by liquid phase oxidation reaction of paraxylene into water solvent at a high temperature and a high pressure, subjecting the solution to hydrogenative purification and then cooling thereby forming a crystal slurry of a purified terephthalic acid, and then subjecting the slurry to a solid-liquid separation step and a cleaning step, followed by drying.

BACKGROUND ART

[0002] After producing a crude terephthalic acid (CTA) from paraxylene as a starting material by liquid phase oxidation with an oxygen-containing gas in an acetic acid solvent under the presence of an oxidation catalyst, the crude terephthalic acid is dissolved in a water solvent at a high temperature and high pressure and purified by hydrogenation-purifying the solution with a hydrogen-containing gas in the presence of a reduction catalyst to obtain an aqueous solution of a purified terephthalic acid and then a slurry of a purified terephthalic acid crystals is formed by way of a crystallization step of performing pressure-release cooling while reducing the pressure stepwise. Then, the slurry of the purified terephthalic acid crystals is subjected to solid-liquid separation to recover the crystals and the crystal powder of the purified terephthalic acid (PTA) by way of a drying step is produced as a highly purified terephthalic acid for production of polyesters used for fibers, films, and bottles.

[0003] In the production process as described above, highly purified terephthalic crystals (first crystals) are formed and, at the same time, impurities such as 4-carboxybenzaldehyde (4-CBA) contained in the crude terephthalic acid are reduced by hydrogenation into paratoluic acid, etc. After subjected the hydrogenated solution to pressure-release cooling and solid-liquid separation, the separated mother liquor (first mother liquor) containing a most portion of paratoluic acid dissolved therein is formed. Further, terephthalic acid which is saturated at a separation temperature is also dissolved in the first mother liquor.

[0004] accordingly, methods for effectively utilizing the first mother liquor by further cooling the same to precipitate the dissolved terephthalic acid and the dissolved impurities at the same time, recovering the less pure crystals of terephthalic acid at low impurity (second crystals), and incorporating them together in the oxidation reaction of paraxylene have been proposed (JP-S56-35175B and JP-H05-58948A).

[0005] On the other hand, there have also been proposed methods of effectively utilizing the mother liquor (second mother liquor) separated from the less pure crystals of terephthalic acid (second crystals) as a water solvent for dissolving a crude terephthalic acid crystals directly as it is or indirectly by a treatment of fractional distillation, etc.

[0006] However, in this proposal, since much paratoluic acid which are impurities in the crystals of the purified terephthalic acid (PTA) is contained in the second mother liquor, the direct use of the second mother liquor as the water solvent for dissolving the crystals of the crude terephthalic acid (CTA) is restricted only to a certain amount. The recovered mother liquor is used as water solvent for dissolving the crude terephthalic acid or wash water for cleaning crystals of the purified terephthalic acid (PTA) (first crystals) after the treatment by fractional distillation at a reflux ratio of 2 to 10 and at 25 to 125 NTP (a number of theoretical plates) (JP-H05-58948A).

[0007] Such proposals have now been put to practical use in commercial units for producing highly purified terephthalic acids along with the development of the demand for polyesters and the recent trend of resource-saving and reducing the environmental load. Accordingly, along with increase in the requirement for utilizing a most amount of the first mother liquor, the problems in the practical use have now been getting obvious.

[0008] As one of the problems, when the first mother liquor is cooled, it is cooled at once to a final separation temperature. As the results, the grain size of the second crystals is decreased. This is resulted in a large load on a recovery apparatus of the second crystals and in practical troubles due to deposition and clogging of fine crystals in the cooling equipment. Therefore, it has been proposed for crystallizing the second crystals to be improved by using a plurality of cooling tanks for lowering the pressure stepwise upon pressure-release cooling of the first mother liquor (JP2006-8671A).

[0009] Further, for increasing the amount of using the second mother liquor, a method for removing paratoluic acid by further bringing the second mother liquor into contact with a synthetic adsorbent such as SEPABEADS (manufacturing by Mitsubishi Chemical) has been proposed (JP2006-8671A). Under the circumstances described above, the present inventors have proposed a method of treating the first mother liquor for increasing the amount of reusing the mother liquor by subjecting the first mother liquor under the presence of terephthalic acid crystals to flash-evaporation (pressure-releasing) and/or heating evaporation, condensing and recovering the evaporated solvent vapors to obtain the recovered water at a reduced content of impurities. This is a method capable of recovering water solvent at a reduced content of paratoluic acid by obtaining a liquid condensate by an evaporation operation without a fractional distillation using a distillation column and having 25 to 125 of NTP at 2 to 10 reflux ratio (WO2009/141968). Related

Art Document Patent Document [0010]

[Patent Document 1] JP-B-Sho56-35275

[Patent Document 2] JP-A-Hei05-58948

[Patent Document 3] JP-A-2006-8671

[Patent Document 4] WO2009/141968

SUMMARY OF INVENTION TECHNICAL PROBLEM

[0011] When the method for separating the mother liquor (first mother liquor) separated from a purified terephthalic acid into the second crystals and the second mother liquor respectively and utilizing them effectively is applied to practical unit such as described above, the e liquid suspension formed by cooling the first mother liquor involves a handling problem due to fine and adhesive suspended particles (second crystals) and it is considered that separation of the suspended particles (second crystals) causes clogging in a filter or lowering of the recovery rate. On the other hand, when the mother liquor is used recyclically as the dissolving water for hydrogenative purification, a treatment such as evaporation or distillation is necessary due to the dissolved impurities left in the second mother liquor, and there is a problem that the application and the amount of use of the second mother liquor are restricted when the mother liquor is reused as it is without treatment.

[0012] The present invention intends to separate and recover the less pure crystals of terephthalic acid (second crystals) with the excellent filterability by a simple treating step of the mother liquor (first mother liquor) separated with the first crystals in producing a purified terephthalic acid.

[0013] The present invention further intends to improve the yield of terephthalic acid by recyclically using the less pure crystals of terephthalic acid (second crystals) separated and recovered at a excellent filterability to a portion of starting materials for producing crude terephthalic acid and further intends to form water for the re-use and reduce the treatment load on the waste water by reducing the amount of impurities dissolved in the first mother liquor.

BRIEF DESCRIPTION OF THE INVENTION

[0014] Generally, the crystal slurry of a purified terephthalic acid is subjected to solid-liquid separation under a pressure (0.2 to 0.9 MPa) and the separated mother liquor (first mother liquor) is obtained at a temperature of about 120 to 170°C under the pressure. The present inventors have attained to a treating method capable of obtaining a cooled liquid suspension improved in the adhesiveness and in the excellent filtrability by adding and mixing a terephthalic acid crystals to the first mother liquor and then supplying them to a cooling tank controlled to a reduced pressure of lower than an atmospheric pressure, and crystallizing a crystals by pressure-release cooling.

[0015] For solving the problems described above, the present invention provides a method of treating a mother liquor separated from a purified terephthalic acid in a process of dissolving a crude terephthalic acid produced by liquid phase oxidation of paraxylene in water at a high temperature and a high pressure, subjecting the solution to hydrogenative purification and then cooling the solution to form the crystal slurry of a purified terephthalic acid, thereby producing a highly purified terephthalic acid from the crystal slurry, in which the method comprises:
a first separation step of separating the crystal slurry of the purified terephthalic acid into a crystals of the purified terephthalic acid (first crystals) and a mother liquor (first mother liquor),
a step of forming a cooled liquid suspension by adding a terephthalic acid crystals to the separated mother liquor (first mother liquor), then supplying them to a cooling tank controlled to a reduced pressure of lower than an atmospheric pressure, and performing pressure-release cooling, thereby forming the cooled liquid suspension, and a second separation step of subjecting the cooled liquid suspension to filtration-separation under pressure of an atmospheric pressure or higher, and separating and recovering the terephthalic acid crystals at a less purity (second crystals).

[0016] Further, according to the invention, the said mother liquor (first mother liquor) is separated from the crystals slurry of the purified terephthalic acid at a temperature of about 120 to 170°C and at a vapor pressure of the slurry or higher (about 0.2 to 0.9 MPa) in the first separation step, and a terephthalic acid crystals are added to the separated mother liquor (first mother liquor), then they are supplied to a cooling tank controlled to a reduced pressure of about 1.3 to 53 kPa (about 10 -400 mmHg), and then formed a cooled liquid suspension by pressure-release cooling in the step of forming the cooled liquid suspension.

[0017] Further, according to the invention, a terephthalic acid crystals formed as a suspended crystals in water are added at a ratio of 0.05 to 1% by weight to the separated mother liquor (first mother liquor) in the step of forming the cooled liquid suspension.

[0018] Further, according to the invention, the terephthalic acid crystals to be added to the separated mother liquor (first mother liquor) are in the form of a crystal powder prepared by drying the crystals of the purified terephthalic acid before the step of forming the cooled liquid suspension.

[0019] Further, according to the invention, the crystals of terephthalic acid at a less purity (second crystals) separated and recovered from the cooled liquid suspension in the second separation step are used as a portion of a starting material for the liquid phase oxidation reaction of paraxylene.

[0020] Further, according to the invention, the mother liquor (second mother liquor) separated from the cooled liquid suspension in the second separation step is used as a portion of water solvent for dissolving the crude terephthalic acid.

[0021] It is considered that the adhesiveness and the filterability are improved in the invention since the crystallizable materials dissolved in the first mother liquor, particularly, the crystallizable impurities such as terephthalic acid and paratoluic acid are crystallized or adsorbed on the surface of the crystal around the added crystal particles of terephthalic acid as nucleus and formed as a result of growing of the added crystal particles of terephthalic acid by pressure-release cooling of the suspended first mother liquor to which a terephthalic acid crystals have been previously added and mixed.

[0022] Then, the cooled liquid suspension is recovered by filtration-separation as crystals of terephthalic acid at a less purity (second crystals) in which impurities are deposited, and the liquid suspension in which the size distribution of the suspended crystal particles is displaced greatly is improved in the filtrability and the depositability of the second crystals is also improved to obtain a liquid suspension for the practical use.

[0023] The recovered secondary crystals is supplied as a portion of the starting material for producing terephthalic acid by the liquid phase oxidation of paraxylene. Since the reaction intermediates such as paratoluic acid as the impurities are oxidized, the impurities contribute to the improvement of the yield of terephthalic acid, not to mention the crystallized terephthalic acids. The added and mixed terephthalic acid crystals are also recycled and recovered, and have not an effect on the production loss.

[0024] On the other hand, in the separated second mother liquor, the content of paratoluic acid and a trace amount of dissolved metals as dissolved impurities are greatly decreased, and the amount of recyclically using the same as the water solvent for hydrogenative purification can also be increased, when compared with the second mother liquor separated by the existent method without the addition of terephthalic acid crystals. Further, the decrease in the amount of the dissolved impurities in the second mother liquor reduces the environmental load such as chemical oxygen demand (COD) upon subjecting the second mother liquor to the waste water treatment as industrial waste water.

[0025] While any form of terephthalic acid is effective for terephthalic acid crystals to be added and mixed, the dried terephthalic acid is preferred and, particularly, the use of the dried crystals of purified terephthalic acid (PTA), that is, a product of highly purified terephthalic acid can provide a desirable effect for reducing the content of paratoluic acid. This is considered to be attributable to that the surface of the crystals is increased by drying the terephthalic acid crystals (increasing the roughness in the pores and on the surface) or activated by drying to enhance the adsorptive force and quantity. The dissolved impurities having physical and chemical affinity to the surface of the added terephthalic acid crystals are adsorbed more to the surface of the added terephthalic acid crystals to decrease the residual impurities in the second mother liquor.

EFFECT OF THE INVENTION

[0026] Referring to the effects described above, while the content of impurities in the second crystals increases, the intermediates such as paratoluic acid eventually provides a preferred result of increasing the yield of terephthalic acid along with increase in the recovery ratio.

[0027] By the simple treating process which comprises adding a terephthalic acid crystals to the first mother liquor, pressure-release cooling to a reduced pressure of lower than the atmospheric pressure and then separating them by filtration, the invention provides a significant practical effect in the simple treating method such as improvement in the filterability for the separation of the formed crystals, improvement in the recovery rate of the second crystals, and decrease in the content of impurities in the second mother liquor. That is, improvement in the filterability can increase the filtration speed and filtration stability, to decrease the size of the filter and enable the stable continuous or semi-continuous operation. Further, the recovery rate for the second crystals is improved by the stable operation, and the installation cost and the maintenance cost for the filter and labor works can be saved.

[0028] Further, since the recovered second crystals is used again as a portion of the starting material for the oxidation reaction, improvement in the recovery rate of the second crystals greatly contributes to the improvement in the yields of the crude terephthalic acid and the purified terephthalic acid.

[0029] Further, the decrease in the dissolved impurities (paratoluic acid) in the second mother liquor results in the improvement in the recovery ratio of the second crystals and, at the same time, can increase the ratio of using the second mother liquor recyclically as water solvent for dissolving the crude terephthalic acid to save the amount of additional dissolving water. Accordingly, the amount of the waste water to be disposed is decreased and mitigated the environmental load.

[0030] Further, the decrease in the amount of the second mother liquor unused recyclically results in not only the decrease in the amount of waste water to be disposed but also mitigation of the load on COD (Chemical Oxygen Demand) of waste water, which can decrease the total load on the waste water disposal to provide a remarkable practical effect.

[0031] Further, the method of the invention is a simple treating method capable of obtaining the effect of the invention together, with the treatment for the first mother liquor unnecessary for the additional treatment of contacting the mother liquor liquid with a synthetic absorbent for decreasing the dissolved impurities (paratoluic acid) in the second mother liquor (JP2006-8671A).

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]

Fig. 1 is an explanatory diagram showing the whole process for producing a highly purified terephthalic acid according to a preferred embodiment of the invention.

Fig. 2 is an explanatory diagram view showing a production step of highly purified terephthalic acid from a crystals slurry of a purified terephthalic acid and a treating step for a separated first mother liquor according to the preferred embodiment of the invention.

Fig. 3 is an explanatory diagram for performing a treating test of a separated first mother liquor according to the preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0033] Preferred embodiments of the invention are to be described with reference to the drawings.

[0034] Fig. 1 shows an whole process for producing a highly purified terephthalic acid by way of the production of a crude terphthalic acid using paraxylene as a starting material, which shows the location of a first mother liquor upon separation from crystals of a purified terephthalic acid, the location of secondcrystals and a second mother liquor after treatment of the first mother liquor, and the outlined flow of the treatment according to the invention. In a production step for a crude terephthalic acid (CTA), paraxylene as a starting material is subjected to oxidation reaction and crystallized and then crystals of a crude terephthalic acid (CTA) are produced in a solid-liquid separator. In the production step for the purified terephthalic acid (PTA), crude terephthalic acid (CTA) crystals is formed into a slurry in a CTA slurry control tank, and then dissolved, subjected to hydrogenative purification, and cooled to obtain a crystal slurry of the purified terephthalic acid and then, separated into PTA crystals and the first mother liquor in a solid-liquid separator 1.

[0035] Terephthalic acid crystals (TA crystals) is added to the first mother liquor separated as described above, then they are formed into a cooled liquid suspension in a pressure-release cooling tank 13, and separated into a second crystals and a second mother liquor in a filter 15.

[0036] The separated second crystals is utilized again for a portion of paraxylene as the starting material for producing the crude terephthalic acid, and the separated secondary mother liquid is used again for a portion of a water solvent for dissolving the crystals of the crude terephthalic acid in the production process for the purified terephthalic acid. A portion of the second mother liquor that is not used again is treated in a waste water treating step. A flow of using crystals of the purified terephthalic acid (PTA) as the crystals of terephthalic acid (TA crystals) added to the first mother liquor is shown by a dotted line.

[0037] Fig. 2 shows the process for producing the highly purified terephthalic acid (PTA) from slurry of purified terephthalic acid in the preferred embodiment of the invention. In the production of the highly purified terephthalic acid after the hydrogenative purification of crude terephthalic acid by way of the crude terephthalic acid using paraxylene as the starting material by liquid phase oxidation, a slurry of the purified terephthalic acid crystals is subjected to solid-liquid separation under pressure in a solid-liquid separator 1, the crystals of the purified terephthalic acid (first crystals) is recovered, and then supplied to a flow schematically shown for producing the highly purified terephthalic acid by way of a crystal cleaning tank 2.

[0038] A schematic flow for the treating method according to the preferred embodiment of the invention using a mother liquor (first mother liquor) separated from the crystals of the purified terephthalic acid (first crystals) is shown as a flow of : first mother liquor tank 10 slurry preparation tank 12 pressure-release cooling tank 13 filter 15 second mother liquid tank 16. Referring to the indication of a stirring blade in each of vessels (tanks), the stirrer is provided since the content (in the tank) is a slurry or a liquid suspension.

[0039] Fig. 3 is an explanatory diagram for performing a treating test on the separated first mother liquor according to the preferred embodiment of the invention. Fig. 3 illustrates a flow of practicing the method of the invention by sampling a part of first mother liquor from a high temperature/high pressure first mother liquor tank (test area for primary mother liquid treatment) separately to the flow of the treatment for the first mother liquor in the production process (purified terephthalic acid production area), after the crystal slurry of the purified terephthalic acid formed in the production process for the purified terephthalic acid is subjected to the solid-liquid separation to obtain the crystals of purified terephthalic acid and the separated mother first (first mother liquor). The filtration test was performed by sampling the steady flow returned from the pressure-release cooling tank (in first mother liquor test area) to the liquid waste treating step (in purified terephthalic acid production area).

[0040] An example of the preferred embodiment of the invention is to be described with reference to Fig. 2. The solid-liquid separation system for the crystal slurry of purified terephthalic acid is a system for obtaining the first crystals carried out by using a centrifugal separation or a filtration separation under pressure (about 0.2 to 0.9 MPa), reslurrying the crystals to clean with water again, and recovering the crystals of highly purified terephthalic acid (first crystals) with a separation apparatus, as described in JP-S47-49049B. The system is shown as a flow of solid-liquid separator 1 crystal cleaning tank 2 flash cooling tank 3 slurry supply tank 4 solid-liquid separator 5 drier 6. The method according to the preferred embodiment of the invention is performed by using the first mother liquid obtained from the above system as the starting material.

[0041] Further, a first mother liquid obtained by the system of recovering the crystals of a purified terephthalic acid (first crystals) by carefully washing terephthalic acid crystals with water after filtration-separation in an identical filter as described in JP-H06-506461T can also be used as the starting material.

[0042] In any of the filtration-separation methods described above, since the leaked suspended matters in the mother liquor (crystal particles of terephthalic acid) are contained only slightly (about 0.01% by weight or less) in the first mother liquor obtained by filtration-separation through a filter medium, when the primary mother liquid obtained by filter-separation is selected as the starting material, the effect of the method according to the preferred embodiment of the invention by adding and mixing the terephthalic acid crystals is expected.

[0043] That is, in the production of the purified terephthalic acid, a practical requirement of improving the filterability of the second crystals and the residual amount of the dissolved impurities in the second mother liquor as the problems of the invention has been increased for the treatment of the first mother liquor obtained by filtration-separation of the crystal slurry of the purified terephthalic acid. In the first mother liquor obtained under pressure (about 0.2 to 0.9 MPa) at about 120 to 170°C (first mother liquor tank 10), about 0.1 to 0.5% by weight of terephthalic acid is dissolved, and paratoluic acid is dissolved as the impurity at a ratio about 0.03 to 0.15% by weight.

[0044] In the preferred embodiment of the invention, terephthalic acid crystals are added to and mixed with the first mother liquor. While the crystals may be added directly and mixed, it is preferred to once prepare the terephthalic acid crystals to be added as a slurry in water at an appropriate ratio (about 10 to 40% by weight) (slurry preparation tank 12), and then it is supplied and mixed in a suspended state to the first mother liquor under pressure (about 0.2 to 0.9 MPa). The crystals are supplied to the first mother liquor in an amount of about 0.05 to 1% by weight and, preferably, at about 0.1 to 0.5% by weight in view of the improvement of the filterability relative to the addition amount of the terephthalic acid crystals.

[0045] While the amount of paratoluic acid dissolved in the second mother liquor is reduced by increasing the addition amount, it should be noted that the process load on the filtration-separation and on the recycled amount of the added terephthalic acid crystals is increased.

[0046] The suspended first mother liquor mixed with the terephthalic acid crystals is supplied directly to a pressure-release cooling tank 13 which is depressurized to about 1.3 to 53 kPa (about 10 to 400 mmHg), i.e., lower than an atmospheric pressure by using a vacuum pump or an ejector, and subjected to pressure-release and evaporation at once to form a liquid suspension at about 10 to 80°C. Further, the cooled liquid suspension may be formed also by stepwise pressure-release in a plurality of cooling tanks such that the pressure in the final cooling tank is lowered to about 1.3 to 53 kPa (about 10 to 400 MMHg), which is lower than the atmospheric pressure.

[0047] The effect of this embodiment is obtained so long as the cooled liquid suspension can be formed by pressure-releasing and cooling the final cooling tank under a reduced pressure of about 1.3 to 53 kPa (about 10 to 400 mmHg), which is lower than the atmospheric pressure.

[0048] The temperature in the final pressure-release cooling tank 13 under a reduced pressure of lower than the atmospheric pressure is preferably lower in view of crystallization and recovery of dissolved matters and decrease of the dissolved impurities in the second mother liquor after recovery and separation. While the temperature is about 10 to 80°C, it is practically preferred to obtain a cooled liquid suspension at about 30 to 65°C under a reduced pressure of about 4 to 26 kPa (about 30 to 200 mmHg).

[0049] Further, the evaporated vapors are condensed and refluxed to the pressure-release cooling tank 13 and can also be used again as a condensate of evaporated water and, as a treating water at a further reduced content of paratoluic acid proposed by the present inventors in WO 2009/141968.

[0050] Then, the liquid suspension in the pressure-release cooling tank 13 is separated by a filter into the second crystals and the second mother liquor. For filtration-separation of the liquid suspension containing the fine adhesive particles, it is preferred to perform the clarifying filtration at an atmospheric pressure or an elevated pressure that exceeds the pressure of the final cooling tank (at a pressure lower than the atmospheric pressure), and a pressured filter such as a press filter or Fundbac filter (manufactured by IHI Co.), Cricket filter (manufactured by Tsukishima Kikai Co.), etc. are used.

[0051] The separated second crystals are prepared as the starting material for performing oxidation reaction of paraxylene, slurrified in an acetic acid solvent and then used as a portion of the starting material for producing terephthalic acid (flow of second crystals in Fig. 1).

[0052] The second mother liquor is re-used as a portion of dissolving water for dissolving a crude terephthalic acid for the hydrogenative purification, and the amount for re-use is controlled depending on the residual amount of dissolved impurities (paratoluic acid) remaining in the secondary mother liquid. The unused second mother liquor is treated directly as waste water, for example, by an activated sludge process practiced so far, in which the amount of use and the load on the treatment of waste water to be treated can be further decreased (flow of second mother liquor in Fig. 1).

[0053] A water solvent further reduced in the amount of impurities (paratoluic acid content) can be recovered by a treatment such as fractional distillation proposed in JP-H05-58948A.

[0054] A preferred embodiment of the invention is to be described specifically with reference to the following examples. However, the method of the invention is not restricted to the following examples. (Example 1) In the production of a highly purified terephthalic acid (PTA) after producing a crude terephthalic acid (CTA) by liquid phase air oxidation of p-xylene, then dissolving the obtained crude terephthalic acid in water, and performing hydrogenative purification at high temperature and high pressure, the crystals of the purified terephthalic acid (PTA) were produced by subjecting a crystal slurry of the purified terephthalic acid (PTA)
at high temperature and high pressure (about 145°C, about 0.4 MPa) obtained from the crystallization step to solid-liquid separation, and obtaining the crystals of the purified terephthalic acid (PTA) by way of cleaning and drying steps.

[0055] In this process, a separated mother liquor (first mother liquor) obtained from the solid-liquid separation step was transferred by way of a storage tank at high temperature and high pressure (primary mother liquid storage tank at high temperature/high pressure) to a step of a liquid waste treatment under an atmospheric pressure and treated. In practicing the method of the invention, a portion of the first mother liquor in the first mother liquor tank at high temperature and high pressure in the production area for the purified terephthalic acid (Fig. 3) was dispensed by a branched pipe at high temperature and high pressure (about 145°C, about 0.4 MPa) (first mother liquor treating test area in Fig. 3), and continuously supplied at a ratio of 100 parts by weight per unit hour to a pressure-release cooling tank (provided with refluxing condenser) equipped with a stirrer and controlled by a vacuum pump to a pressure of about 20 kPa (about 150 mmHg) which is lower than an atmospheric pressure and the first mother liquor was cooled under pressure-release (temperature of about 60°C). After crystallizing the dissolved matters such as terephthalic acid and paratoluic acid dissolved in the first mother liquor by pressure-release cooling and forming a cooled liquid suspension, it was returned to the liquid waste treating step for the mother liquor (first mother liquor) in the production of the purified terephthalic acid (PTA) (purified terephthalic acid production area in Fig. 3) and discharged continuously.

[0056] In this process, a slurry of terephthalic acid previously prepared into an aqueous 20 wt% slurry of terephthalic acid crystals (dried crystals of purified terephthalic acid (PTA powder)) was supplied continuously at a ratio of 0.75% by weight to the first mother liquor (the addition ratio of PTA crystals was 0.15% by weight to the first mother liquor). After mixing with the first mother liquor, they were supplied to the cooling tank and cooled under pressure-releasing. The cooled liquid suspension at about 60°C formed in the cooling tank was returned to the waste water treating step for the first mother liquor and discharged.

[0057] In this process, 100 parts of the cooled liquid suspension discharged continuously was sampled, a filtration test was performed by using a Buchner funnel facility equipped with a heat insulator to measure the filtration time and the content of paratoluic acid in the liquid filtrate (second mother liquor). As a result, the filtration time was 42 sec and the content of paratoluic acid in the second mother liquor was 0.025 wt%. (Table 1) (Comparative Example 1)In the method of Example 1, pressure-release cooling was performed while stopping the addition of terephthalic acid of crystals (supply of 20 wt% aqueous slurry of terephthalic acid crystals) to form a cooled liquid suspension.In the same manner as in Example 1, the cooled liquid suspension was sampled and a filtration test was performed. As a result, the filtration time was 620 sec and the content of paratoluic acid was 0.048 wt%.(Table 1) (Example 2) In the method of Example 1, pressure-release cooling was performed while supplying an aqueous 20 wt% slurry of terephthalic acid crystals to the first mother liquor at a ratio of 0.35 wt% (the addition ratio of PTA crystals was 0.07% by weight to the first mother liquor) to form a cooled liquid suspension.
In the same manner as in Example 1, the cooled liquid suspension was sampled and a filtration test was performed. As a result, the filtration time was 113 sec and the content of paratoluic acid was 0.033 wt%, respectively.(Table 1) (Example 3)

In the method of Example 1, pressure-release cooling was performed while supplying an aqueous 20 wt% slurry of terephthalic acid crystals to the first mother liquor at a ratio of 1.5 wt% (the addition ratio of PTA crystals was 0.3% by weight to the first mother liquor) to form a cooled liquid suspension. In the same manner as in Example 1, the cooled liquid suspension was sampled and a filtration test was performed. As a result, the filtration time was 38 sec and the content of paratoluic acid was 0.017 wt%, respectively. (Table 1) (Example 4) In the method of Example 1, pressure-release cooling was performed while supplying an aqueous 20 wt% slurry of terephthalic acid crystals to the first mother liquor at a ratio of 2.25 wt% (the addition ratio of PTA crystals was 0.45% by weight to the first mother liquor) to form a cooled liquid suspension.
In the same manner as in Example 1, the cooled liquid suspension was sampled and a filtration test was performed. As a result, the filtration time was 41 sec and the content of paratoluic acid was 0.011 wt% respectively.(Table 1) (Example 5) In the method of Example 1, an aqueous 20 wt% slurry of terephthalic acid crystals was prepared by using a wet cake separated from the slurry of the purified terephthalic acid crystals (PTA crystals before drying), which was supplied to the first mother liquor at a ratio of 0.75% by weight (the addition ratio of PTA crystals was 0.15% by weight to the first mother liquor), and pressure-release cooling was performed to form a cooled liquid suspension. In the same manner as in Example 1, the cooled liquid suspension was sampled and a filtration test was performed. As a result, the filtration time was 40 sec and the content of paratoluic acid was 0.032 wt% respectively.(Table 1) (Example 6)

In the method of Example 1, pressure-release cooling was performed while controlling the pressure for the pressure-release cooling tank at about 7.3 kPa (about 55 mmHg, temperature about 40°C) to form a cooled liquid suspension. In the same manner as in Example 1, the cooled liquid suspension was sampled and a filtration test was performed. As a result, the filtration time was 59 sec and the content of paratoluic acid was 0.008 wt% respectively.(Table 1) (Example 7)
In the method of Example 1, pressure-release cooling was performed while controlling the pressure for the pressure-release cooling tank at about 47 kPa (about 355 mmHg, temperature about 80°C) , and supplying an aqueous 20 wt% slurry of terephthalic acid crystals to the first mother liquor at a ratio of 1.5 wt% (the addition ratio of PTA crystals was 0.3% by weight to the first mother liquor) to form a cooled liquid suspension. In the same manner as in Example 1, the cooled liquid suspension was sampled and a filtration test was performed. As a result, the filtration time was 28 sec and the content of paratoluic acid was 0.031 wt% respectively.(Table 1) (Comparative Example 2) In the method of Example 7, pressure-release cooling was performed while stopping the addition of terephthalic acid crystals (supplying of an aqueous 20 wt% slurry of terephthalic acid crystals) to form a cooled liquid suspension. In the same manner as in Example 1, the cooled liquid suspension was sampled and a filtration test was performed. As a result, the filtration time was 540 sec and the content of paratoluic acid was 0.092 wt% respectively.(Table 1) (Example 8) In the method of Example 1, pressure-release cooling was performed while controlling the pressure for the pressure-release cooling tank to about 2.4 kPa (about 18 mmHg, temperature about 20°C) to form a cooled liquid suspension. In the same manner as in Example 1, the cooled liquid suspension was sampled and a filtration test was performed. As a result, the filtration time was 94 sec and a content of paratoluic acid was 0.003 wt% respectively.(Table 1)

Table 1

DESCRIPTION OF REFERENCES

[0058]

1, 5 solid-liquid separator

2 crystal cleaning tank

3 flash cooling tank

4 slurry supply tank

6 drier

7 cleaning and discharge tank

8,9,11,14 gas-liquid separator

10 primary mother liquid tank

12 slurry conditioning tank

13 pressure-release cooling tank

15 filter

16 secondary mother liquid tank

We claim:

1. A method for treating a mother liquor separated from a purified terephthalic acid in a process of dissolving a crude terephthalic acid produced by liquid phase oxidation of paraxylene in water solvent at high temperature and high pressure, subjecting the solution to hydrogenative purification and then cooling the said solution to form a crystal slurry of a purified terephthalic acid, thereby producing a highly purified terephthalic acid from the crystal slurry, in which the method comprises: a first separation step of separating the crystal slurry of the purified terephthalic acid into a crystals of the purified terephthalic acid (first crystals) and a mother liquor (first mother liquor), a step of forming a cooled liquid suspension by adding terephthalic acid crystals to the separated mother liquor (first mother liquor), then supplying them to a cooling tank controlled at a reduced pressure of lower than an atmospheric pressure, and performing pressure-release cooling thereby forming the cooled liquid suspension, and a second separation step of subjecting the cooled liquid suspension to filtration-separation under pressure of an atmospheric pressure or higher, and separating and recovering the terephthalic acid crystals at a less purity (second crystals).

2. A method of treating a mother liquor separated according to claim 1, wherein the mother liquor (first mother liquor) is separated from the crystal slurry of the purified terephthalic acid at a temperature of about 120 to 170°C and at a vapor pressure of the slurry or higher (0.2 to 0.9 MPa) in the first separation step, and a terephthalic acid crystals is added to the separated mother liquor (first mother liquor), then they are supplied to a cooling tank controlled at a reduced pressure of 1.3 to 53 kPa (10 - 400 mmHg), and then subjected to pressure-release cooling thereby forming the cooled liquid suspension in the step of forming the cooled liquid suspension.

3. A method of treating a mother liquor according to claim 1 or 2, wherein the terephthalic acid crystals formed as a slurry in water is added at a ratio of 0.05 to 1% by weight to the separated mother liquor (first mother liquor) in the step of forming the cooled liquid suspension.

4. A method of treating a mother liquor according to any one of claims 1 to 3, wherein the terephthalic acid crystals to be added to the separated mother liquor (first mother liquor) is in the form of a crystal powder formed by drying the crystals of the purified terephthalic acid in the step of forming the cooled liquid suspension.

5. A method of treating a mother liquor according to any one of claims 1 to 4, wherein
the crystals of terephthalic acid at a less purity separated and recovered from the cooled liquid suspension (second crystals) in the second separation step is used as a portion of the starting material for the liquid phase oxidation reaction of paraxylene.

6. A method of treating a mother liquid according to any one of 1 to 5, wherein the mother liquor separated from the cooled liquid suspension (second mother liquor) in the second separation step is used as a portion of water for dissolving the crude terephthalic acid.
Dated this the 13th Day of August, 2013