[Technical Field]

[1] The present invention concerns a method for recovering crystals from a crystallized slurry for improving the purity of recovered crystals upon recovery of crystals by continuous pressured filtration in repeating each of steps of filtration from a crystallized slurry, washing, dehydration and flaking.

[Background Art]

[2] A method for producing a highly pure terephthalic acid purified by dissolving a crude terephthalic acid in water at high temperature, reducing 4-CBA (4- carboxybenzaldehyde) contained in the crude terephthalic acid to p-toluic acid by hydrogenation under the presence of a reduction catalyst, then lowering the pressure stepwise by passing the solution through a plurality of crystallization tanks connected in series and recovering crystals by solid-liquid separation from a crystallized slurry has generally been conducted commercially.

[3] This is because p-toluic acid contained in the aqueous solution of the purified terephthalic acid is separated more easily from terephthalic acid crystals compared with 4-CBA due to solubility, molecular and crystal structures, etc. in the course of crystallization, separation and recovery of the crystallized crystals. Accordingly, a purification method for reducing 4-CBA by hydrogenation to p-toluic acid having good separability and then recovering terephthalic acid by crystallization has been adopted.

[4] For the method described above, improvement method of the separability of p-toluic acid in the crystallization step by forming crystals of terephthalic acid from an aqueous solution of terephthalic acid has been proposed in Patent Documents 1, 2, 3 (Japanese Patent Examined Publication JP S47 (1972)-49049B, JP S53(1978)-24057B, Japanese Patent Unexamined Publication JP Hll(1999)-228492A, etc.) and improvement method of the separability of p-toluic acid in the recovery step for recovering crystals of terephthalic acid by solid-liquid separation of formed crystals has been proposed in Patent Documents 4, 5 (International Publication JP H06(1994)- 506461A, JP H07(1995)-507291A, etc.)

[5] Among them, since the separability from p-toluic acid in the recovery step is improved more by separating terephthalic acid crystals at higher temperature due to the difference of the solubility between terephthalic acid and p-toluic acid (Table 1 of Japanese Patent Examined Publication JP S47(1972)-49049B is shown in Fig. 5), terephthalic acid crystals have been recovered by solid-liquid separation of a crystallized slurry while maintaining the temperature and the pressure of the crystallized slurry higher than in the final crystallization tank, without lowering the temperature (pressure).

[6] Therefore, International Publication JP H06- 506461A proposed a precise filtration and washing system of conducting filtration by charging a purified terephthalic acid slurry in a temperature range of about 38 to 205°C into a filter cell type filtration zone at a pressure of a filtration system (recovery system) within a range from atmospheric pressure to 235 psig (0 to 16.5 kg/cm2G) , and conducting the wash by forming a water sump layer that covers the filter cake in the filter cell by a wash liquor at a temperature of about 38 to 205°C, thereby reliably replacing and washing the filtrate containing p-toluic acid remained in the filter cake.

[7] International Publication JP H07(1995)-507291A proposes a process for producing a purified terephthalic acid of conducting filtration/separation and washing at such a high pressure (pressure on the side of the filtration surface) that the pressure on the lower pressure side at the filtration surface is not substantially lower than the pressure for forming the crystallized slurry, in which it is proposed pressured filtration, that is, high temperature filtration of crystallized terephthalic acid at a pressure of 1.5 to 15 bar in the final crystallization stage, with the surface side pressure on the side of the filtration surface preferably at 2 to 15 bar.

[8] Then, it is described that a wash liquor for removing a liquid filtrate containing p-toluic acid that remains in the filter cake of terephthalic acid crystals is introduced at a substantially identical temperature with the temperature of the filter cake so as to avoid the problem of flashing or cooling (which may cause a danger to form crystallization impurity). However, while any of the proposals described above proposes filtration/separation of a crystallized slurry at high temperature and high pressure for recovering crystals of purified terephthalic acid, washing is actually conducted by using a wash liquor at a temperature identical with or lower than that of the crystallized slurry.

[9] In the continuous filtration of the crystallized slurry containing terephthalic acid crystals, with a view point of decreasing clogging caused by crystal deposition on a filter medium (filter cloth), it is considered that a pressured filtration method of conducting filtration while pressurizing the high pressure side of the filtration surface (pressure on the side of the slurry) to a pressure exceeding the vapor pressure of the filtered slurry such that the pressure on the lower pressure side of the filtration layer maintains temperature and pressure that do not result in supersaturation vapor pressure of the liquid filtrate is a preferred method for the continuous filtration method (Patent Document 6: Japanese Patent Unexamined Publication JP HOI(1989)-299618A).

[10] Further, as a method for avoiding the problem of clogging in the filter medium (filter cloth) upon recovery of the crystals, recovery of terephthalic acid crystals is conducted also by a two step centrifugal separation method of replaced-washing by re-slurrifying crystals separated in the first stage, in which crystals are recovered by centrifugal settling separation while maintaining the temperature and the pressure of the final crystallization tank (centrifugal settling system not using filter medium), and the separated crystals (moistened cake) are re-slurrified by a wash liquor (water) and then separated again by centrifugal separation (settling) (Japanese Patent Examined Publication JP S47(1972)-49049B).

[Prior Art Documents] [Patent Documents]

[11]
Patent Document 1 : Japanese Patent Examined Publication JP S47 (1972)-49049B
Patent Document 2 : Japanese Patent Examined Publication JP S53(1978)-24057B
Patent Document 3 : Japanese Patent Unexamined Publication JP Hll(1999)-228492A Patent Document 4 : International Publication JP H06(1994)-506461A
Patent Document 5 : International Publication JP H07(1995)-507291A
Patent Document 6 : Japanese Patent Unexamined Publication JP HOI(1989)-299618A

[Summary of the Invention]

[Problem to be solved by the Invention]

[12] With the backgrounds as described above, a high temperature and high pressure (pressurized) separation system has usually been adopted for the filtration/separation method of improving the separability of p-toluic acid and lowering the p-toluic acid content in the recovered terephthalic acid in the continuous separation method for recovering crystals from the slurry of purified terephthalic acid crystals.
Accordingly, in the recovery of the purified terephthalic acid crystals, it is preferred in view of the separability of p-toluic acid to separate the crystals while maintaining the slurry in the final crystallization tank at a temperature and a pressure as high as possible (205°C at the highest) and this required expensive facilities for high temperature and high pressure use for the separator (filter, and centrifuge) and systems therefor (systems for the recovery of separated wet cake, separation liquor/wash liquor receiving tank, and pressurized circulating gas).

[13] However, in the high temperature separation of the crystallized slurry of purified terephthalic acid, since the amount of terephthalic acid dissolved in the separation liquor increases abruptly along with temperature (refer to Fig. 5), this issues a problem of dissolution loss of terephthalic acid in the separation liquor, or a problem that the burden on the post treatment for the separation liquor containing terephthalic acid is included therein.

[14] Further, the two stage centrifugal separation method involves a problem of requiring a great amount of wash water so as to re-slurrify separated crystals by using water by 3 to 5 times as much as that of the separated crystals (Japanese Patent Examined Publication JP S47(1952)-49049B).

[15] As described above, even when the aqueous solution of the crude terephthalic acid containing 4-CBA is reduced by hydrogenation to convert 4-CBA p-toluic acid of easily solubility in water, production of the highly pure terephthalic acid has been conducted so far while involving problems in view of facilities and processing conditions described above in the course of the crystallization step, the recovery step, and the crystal washing step in the recovery process as the subsequent steps.

[16] In view of the present situations described above in the production technique of highly pure terephthalic acid, the present invention intends to provide a filtration/washing method more improved for the separability of the impurity content (p-toluic acid) by a usual amount of a wash liquor while using a filtration system designed to have a relatively low pressure in the recovery of crystals from a crystallized slurry of purified terephthalic acid.

[17] Accordingly, it has been intended to improve the separability of p-toluic acid by processing the crystallized slurry of terephthalic acid using a filter and a system therefor at the (operating) pressure of the filtration system (filter) of 6 kg/cm2G at the highest, and at a temperature of 160°C (vapor pressure: 5.3 kg/cm2G) or lower that causes relatively less terephthalic acid loss into the filtered and separated liquor.

[18] That is, the high temperature/high pressure filtration system is designed so as to enable filtration by installing an existent rotary vacuum cylindrical filter (rotary vacuum filter), and filter of a vacuum belt filter type in a pressure casing (high pressure vessel) at a pressure of 6 kg/cm2G (design pressure: 9 kg/cm2G) and using pressure proof equipments of a relatively low pressure specification at a maximum operating pressure of 6 kg/cm2G also for relevant equipments and accessory parts for filtration, thereby suppressing the installation cost.

[19] On the other hand, since the terephthalic acid loss due to dissolution into the filtrate in the filtration/separation (0.33 g/100 g H20) is about 1% by weight or less based on the amount of recovered terephthalic acid crystals at a temperature of 160°C of the final crystallization tank for the purified crystallized slurry of terephthalic acid (filtration/separation temperature), it is defined that crystals are recovered from the crystallized slurry of terephthalic acid while setting the highest operating temperature of the final crystallization tank at 160°C.

[20] Further, since filtration is usually practiced at a differential pressure of 0.2 to 0.9 kg/cm2 (about 0.5 kg/cm2) between the high pressure side and the low pressure side of a filtration layer in a filter, the crystallized slurry at the highest temperature of 160°C (vapor pressure: 5.3 kg/cm2G) can be filtered in a pressured filtration system at the highest operating pressure of 6 kg/cm2G and this is a preferred method for continuous filtration while suppressing clogging in the filter medium (filter cloth).

[21] Further, it is preferred that the
filtration/separation temperature is not lowered in view of the separability of p-toluic acid in the recovery step of purified terephthalic acid crystals, and it is intended in the method of the invention to conduct filtration/separation at least at a temperature (130°C or higher) that provides the solubility ratio (p-toluic acid/terephthalic acid) of 30 or more shown in Fig. 5.

[22] Therefore, the method of the invention can provide the effect of improving the separability of the impurity for the crystallized slurry of purified terephthalic acid at about 130 to 160°C in a recovery step at higher temperature by using a pressure type filtration system in which the pressure of the filtration system (filter, filter cake take-out mechanism, liquid filtrate receiving tank, wash liquor receiving tank, and pressure gas circulation system) is within a range of about 2.5 to 6 kg/cm2G (design pressure: 9 kg/cm2G) .

[Means for solving the problem]

[23] The present invention provides a method for recovering crystals from a slurry comprising a solvent and crystallized crystals by continuous filtration of repeating each of the steps of filtration, washing and flaking sequentially, in which the method includes; filtering the slurry, in the filtration step, by pressurizing the high pressure side of a filtration surface (on the side of slurry) using a circulating solvent vapor-containing gas, and washing the filtered and separated crystals by a wash liquor accompanying generation of vapors, in the washing step, by using a wash liquor superheated so as to have a vapor pressure exceeding the pressure on the high pressure side of the filtration surface.

[24] In the method for recovering the crystals of the invention described above, a crystal slurry which is obtained by subjecting an aqueous solution of terephthalic acid formed by dissolving a crude terephthalic acid in water at high temperature and high pressure to hydrogenation reduction under the presence of a reduction catalyst and then crystallizing terephthalic acid crystals through a plurality of crystallization tanks connected in series for lowering the pressure stepwise is filtered, in the filtration step, under pressurization at a pressure of 2.5 to 6 kg/cm2G on the high pressure side of the filtration surface.

[25] Then, in the washing step, a wash liquor (water) superheated to have a vapor pressure exceeding the pressure on the high pressure side of the filtration surface is fed, and crystals filtered and separated are washed by the wash liquor accompanying generation of vapors.

[26] For this purpose, it is characterized that wash water superheated so as to have a vapor pressure of about 2.7 to 7 kg/cm2G (about 140 to 170°C) or higher that exceeds each of the pressures on the filtration surface mentioned above is fed and the filtered and separated crystals are washed by the wash water accompanying generation of vapors.

[27] Further, in the recovery of crystals from the crystal slurry at about 151°C (about 4 kg/cm2G), wash water superheated so as to have a vapor pressure of about 7 kg/cm2G (about 170°C) or higher is fed and the filtered and separated crystals are washed to provide the effect of decreasing the impurity content (p-toluic acid) and, further, the decreasing effect becomes more remarkable by using wash water superheated so as to have a vapor pressure of about 9 kg/cm2G (about 180°C) or higher.

[28] The ratio of the generated steams from the superheated wash water corresponds to about 2% or more and, more preferably, about 4% or more.

However, even in a case where the vapor pressure of the superheated wash water is higher than 16.5 kg/cm2G (about 250°C, steam generation ratio: 9.6%) or higher, the effect of decreasing the impurity content (p-toluic acid) tends to change no more and this requires more cost for higher pressure facilities.

[29] Accordingly, in the washing step for the recovery of crystals from the crystallized slurry by continuous pressured filtration of conducting each of the steps of filtration, washing and flaking, a superheated wash liquor having a vapor pressure exceeding the pressure on the high pressure side of the filtration surface is fed and the filtered and separated crystals are cleaned by the wash liquor accompanying generation of vapors of about 2 to 10%.

[30] Further, the invention provides a method for recovering purified terephthalic acid crystals by filtration/separation while maintaining the temperature(about 130 to 160°C) of the final crystallization tank of the crystallized slurry obtained by a crystallization method of releasing and lowering the pressure stepwise and, at the same time, gradually lowering the temperature of the aqueous solution of a purified terephthalic acid, feeding the crystallized slurry to a continuous filter of a filtration system pressurized to about 2.5 to 6 kg/cm2G, wherein a wash liquor (water) superheated so as to have a vapor pressure exceeding the pressure of the filtration system is fed to the washing zone of the filter, and the filtered and separated cake is washed at least accompanying generation of vapors. This can improve the separability of p-toluic acid and can decrease the p-toluic acid content to the recovered terephthalic acid crystals compared with the existent case of recovering crystals by filtration/washing when a temperature of the wash liquor (wash water not accompanying generation of steams) is equal with or lower than that of the crystallized slurry.

[31] In Japanese Patent Examined Publication JP
S47 (1972)-49049B as the existent filtration/separation method, it was devoted only to the replaced-washing of a liquid containing p-toluic acid remaining in the cake after filtration by wash liquor and it was considered preferable to conduct washing by the wash liquor at a temperature substantially equal with or lower than that of the separated cake. However, it may be considered in this case that the impurity (p-toluic acid) remains at the crystal surface of the cake layer due to physical and chemical affinity of crystals such as deposition, adsorption, etc. and the amount thereof that can be removed only by the replaced-washing is limited. The present inventors have made an intensive study for further decreasing the amount of limit and have accomplished the invention.

[32] That is, in accordance with the invention, the amount of the impurity deposited and adsorbed to the crystal surface of the cake layer corresponding to the concentration of the impurity in the liquid filtrate (p- toluic acid) in the filtration/separation is washed by the wash liquor in the form of a mixed vapor-liquor phase accompanying vapors at a temperature at least higher than that upon filtration/separation by the introduced superheated water. It is considered in this washing that the flow rate of the wash liquor is increased by the vapors and the crystal surface is activated by the high temperature, so that desorption and detachment of the impurity are promoted and the impurity is dissolved and removed, to provide the effect of decreasing p-toluic acid. As a result, dehydration from the recovered terephthalic acid crystal cake is also promoted and the decreasing trend of the humidity is observed.

[33] Then, the processing operation of the invention described above can be attained by using a facility capable of using a pressured filtration system that can be pressurized to 2.5 to 6 kg/cm2G and capable of feeding a wash liquor superheated so as to have a vapor pressure exceeding the system pressure and feeding the wash liquor at least accompanying generation of vapors to the washing zone of the filter. Further, the effect of decreasing the impurity content (p-toluic acid) becomes further remarkable by feeding a wash liquor superheated such that the ratio of vapor generation is more than about 2%, preferably, about 4% or more.

[34] Accordingly, separability of p-toluic acid enhanced by the improvement of using the superheated wash liquor accompanying generation of vapors can attain the same effect as filtration/separation effect at higher pressure (higher temperature) in the existent method, so that the burden for the pressure reinforcement on facilities for the filtration system in order to decrease the content of p-toluic acid can be suppressed.

[35] Production of highly pure terephthalic acid by the method according to the invention can be made more effective by practicing the method of the invention, by forming a crystallized slurry by stepwise flash crystallization method proposed, for example, in Japanese Patent Examined Publication JP S53-24057B, Japanese Patent Unexamined Publication JP2006-96710A, etc. in the crystallization step for the aqueous solution of purified terephthalic acid, conducting pressure filtration proposed by Japanese Patent Unexamined Publication JP H01-299618A, International Publication JP H07-507291A, etc. in the crystal recovery step, and feeding the superheated wash liquor (water) accompanying the generation of vapors from the system of feeding the superheated liquid (water) described above in the washing step.

[36] Further, the method of the invention is not limited to the production of the highly pure terephthalic acid but is applicable also to the recovery of crystals from a crystallized slurry of a crude terephthalic acid, as well as filtration/washing upon recovery of crystals from a crystallized slurry by re-crystallization intended for decreasing the impurity content by crystallization, and the preferred effects can be expected.

[37] That is, the method can be made more effective by decreasing the impurity in the recovered crystals in the pressurized filtration/separation method from the crystallized slurry by feeding a wash liquor superheated so as to have a vapor pressure exceeding the pressure on the high pressure side of the filtration surface and washing the filter cake by the wash liquor at least accompanying the generation of vapors.

[Effect of the Invention]

[38] According to the invention, separability of the impurity can be further improved to recover the crystals at high purity by using a pressured filtration system at about 2.5 to 6 kg/cm2G and feeding superheated water accompanying generation of vapors to the crystallized slurry at a predetermined temperature, thereby contributing to the improvement of product value.

[39] Further, the separation effect equal with that when the filtration temperature (system pressure) is increased can be obtained only by improving the wash water feed system without increasing the design pressure of the filter and the filtration system and the burden on the facilities intending for pressure reinforcement over the entire filtration system can be mitigated. At the same time, the burden on drying in the subsequent step can also be mitigated by the decrease in the wetting ratio of the recovered cake.

[40] Further, in a case of using terephthalic acid for the crystallized slurry, improvement obtained for the degree of purification for highly pure terephthalic acid (purified terephthalic acid) (decrease in the content of p-toluic acid) can contribute, in turn, to the mitigation of the burden on the production of a crude terephthalic acid. That is, the effect of decreasing the content of p-toluic acid (impurity) in the highly pure terephthalic acid can be transformed to the permission for the increasing amount of the 4-CBA content in the crude terephthalic acid and this can contribute to the mitigation of the burden on the production of the crude terephthalic acid.

[41] It is considered that not only the effect described above is obtained for the recovery step of crystals of purified terephthalic acid but also the same effect can be obtained also in the crystal recovery process from similar other crystallized slurries.

[Brief Description of the Drawings]

[42]
Fig. 1 is a flow chart for a filtration system according to an embodiment of the invention;
Fig. 2 is a schematic cross sectional view of a pressure type rotary cylindrical filter of the embodiment;
Fig. 3 is a characteristic figure showing a relation of the wetting ratio (wt%) of terephthalic acid cake and the content of p-toluic acid (ppm) to the temperature of wash water of the embodiment;
Fig. 4 is a table showing a relation of the wetting ratio (wt%) of the discharged cake of terephthalic acid and the content of p-toluic acid (ppm) to the temperature of wash water of the embodiment; and
Fig. 5 is a table showing the solubility of terephthalic acid and p-toluic acid.

[Mode for carrying out the Invention]

[43] Fig. 1 and Fig. 2 show schematic views for an example of a filtration system and a filter for conducting the method of the present invention respectively.

[44] A pressure type rotary cylindrical filter 2 is pressurized at the inside of the casing to about 2.5 to 6 kg/cm2G by using a pressurized circulating inert gas (containing vapors). A cylindrical rotating filter medium 11 is rotationally disposed in the inside of the casing and recovers a filter cake by way of each of the steps of filtration, washing and flaking sequentially while rotating clockwise from a filtration zone 13 at the bottom to a wash-dehydration zone 14 at the upper portion, and then to a flaking zone 15.

[45] A crystallized slurry is fed from a slurry supply tank 1 to the bottom 12 of the casing of the filter 2 by a pressurized pump or the like, and filtered by pressurization and suction in the filtration zone 13 at the bottom of the rotating filter medium 11 in which the liquid level of the crystallized slurry is maintained. The excess slurry during filtration is discharged through an overflow pipe from the filter 2 and returned to the slurry supply tank 1, etc. Since this is discharged to a zone at a lower pressure than the inner pressure of the casing, pressure interception and discharge control of the slurry are controlled by a discharge valve for maintaining the inner pressure of the casing and the temperature of the slurry.

[4 6] On the other hand, the filter cake separated by the pulsative blowing of an inert gas in the flaking zone 15 of the rotating filter medium 11 is discharged and recovered through a pressure buffer zone M-2 such as a two stage valve (slide valve) or a rotary valve for maintaining the inner pressure of the casing. [0047] The rotating cylindrical filter medium 11 moves to the washing-dehydration zone 14 while sucking (differential pressure: 0.2 to 0.9 kg/cm2) and removing the liquid filtrate while rotating a deposited cake which is filtered by pressurization (about 2.5 to 6 kg/cm2G) in the filtration zone 13 at the bottom. In the washing- dehydrating zone 14, a wash liquor (water) 17a which is superheated till it has a vapor pressure exceeding the inner pressure of the casing is introduced through a flash valve 17, and the introduced wash liquor 17a is sucked and removed together with the filtrate remained in the inside of the cake at least accompanying the generation of vapors under a pressure of the circulating gas (about 2.5 to 6 kg/cm2G) .

[48] The effect due to the ratio of vapor generation from the wash liquor (washing effect, wetting ratio of cake) is developed by feeding the wash liquor superheated to a temperature accompanying the generation of vapors in an amount exceeding about 2% based on the feeding amount of the wash liquor and a preferred effect is obtained by the wash liquor at a temperature accompanying the generation of vapors that exceeds about 4%. However, the effect of vapor generation of the superheated wash liquor tends to be decreased at a temperature where the amount of generated vapors exceeds 10%. Accordingly, it is effective to conduct washing by the wash liquor superheated to a temperature providing the amount of generated vapors of from 2 to 10% based on the wash liquor.

[49] It is considered that the phenomenon described above is attributable to that desorption and detachment of the impurity are promoted and the impurity is dissolved and removed due to the increase in the flow speed of the wash liquor by the vapor and activation of the crystal surface due to the high temperature. Further, it is considered that since a vapor at the vapor generation amount of 2 to 10% of the wash liquor has high humidity and high viscosity, it shows a large effect of scraping out the impurity on the crystal surface, whereas since a vapor at the vapor generation amount of more than 10% becomes low humidity and low viscosity, it shows less scraping effect of the impurity on the crystal surface.

[50] The wash liquor superheated through a heater E-2 to a liquid temperature having a vapor pressure exceeding the inner pressure of the casing is introduced into the casing by the flash valve 17 controlled to a constant pressure exceeding the inner pressure of the casing at a pressure exceeding the control pressure of the flash valve control pressure and released over the cake in the washing zone 14 while flashing through the flash valve 17.

[51] The rotating filter medium 11 further rotates and the deposited cake is transferred to the flaking zone 15, and the filter cake is flaked by the blow of a pulsative gas at a gas pressure higher than the inner pressure of the casing (+ about 0.1 to 0.5 kg/cm2) from the rear side of the filtration cake layer. The separated filter cake is transferred through a specially designed valve that suppresses the fluctuation of the inner pressure of the casing described above to a drier which is usually about at an atmospheric pressure and then discharged and produced as a dried powder product (terephthalic acid).

[52] Then, in Fig. 1, the sucked and removed filtrate and discharged wash liquor are once stored together with sucked vapors individually in a liquid filtrate receiving tank 4 and a discharged wash liquor receiving tank (not illustrated), or in an identical filtrate/discharged wash liquor receiving tank 4 for separation of incondensible gases and then transferred to respective treating steps.

[53] The vapor gas mixture (incondensible gas) separated in the receiving tank 4 is slightly cooled in a cooler E-3 for temperature stabilization and then the condensate in the gas-liquid separation tank 5 is recovered to the receiving tank 4. The vapor gas mixture ingredient that cannot be condensed is fed on the suction side of a compressor M-4 and, after supplementing shortage (inert gas) or releasing surplus by a pressure regulator PIC-2 for the exhaust pressure of the compressor M-4, fed as a circulating gas to the filter 2.

[54] The circulating gas from the compressor M-4 is controlled at a system pressure by a pressure regulator PIC-2 for pressurizing the filtration system and then sent to a pressure buffer zone M-2 for discharging the inner pressure of the filter 2 and the flaked cake. Meanwhile, the circulating gas is controlled at a temperature equal with or higher than the temperature of the fed slurry by a heater E-4 (TIC-2). For feeding the circulating gas to the filter 2, it is preferably blown to the zone before the washing zone 14 for suppressing the evaporation of the filtrate.

[55] The method described above is applicable not only for the recovery of crystals from the purified terephthalic acid crystal slurry in the production of the highly pure terephthalic acid but also for decreasing the content of 4-CBA on the recovery of a crude terephthalic acid crystals from a crystallized slurry obtained by oxidizing reaction. Further, it is also applicable for decreasing the impurity content upon recovery of crystals from other crystallized slurries comprising crystals formed by crystallization and a solvent.

[56] The method adopted herein is conducted at a relatively low pressure in which the system pressure of the pressured filtration system used for the method is at a pressure not exceeding 6 kg/cm2G.

[57] A preferred embodiment of the invention is to be described by way of an example more specifically. This is an example of a preferred embodiment and the invention is not limited thereto.

[Embodiment]

[58] In this example, a slurry of crystallized terephthalic acid slurry is used as a crystallized slurry and the slurry of crystallized terephthalic acid is obtained from a production plant for a highly pure terephthalic acid. An aqueous solution of terephthalic acid formed by dissolving a crude terephthalic acid in water at high temperature and high pressure is subjected to hydrogenation reduction under the presence of a reduction catalyst. Then, it is crystallized by passing through a plurality of crystallization tanks connected in series for lowering the pressure stepwise. A method for recovering the crystals is conducted by feeding the slurry of the crystallized terephthalic acid at high temperature obtained from the final crystallization tank (slurry supply tank 1 in Fig.l : at 151°C) at the end of the serially connected crystallization tanks to a pressured filtration system and then the crystallized slurry is filtered and separated under pressure.

[59] The pressured type filtration system in Fig. 1 uses a rotating cylindrical filter 2 having a filtration area of 0.2 m2 attached in a casing having a design pressure of 7 kg/cm2G and has a cake receiving tank 3, a mother liquor filtrate/discharged wash liquor receiving tank 4, a gas-liquid separation tank 5, a compressor M-4, a wash liquor heater E-2, etc. which is shown as a flow system surrounded by a dotted line in Fig. 1.

[60] The crystallized slurry of terephthalic acid from the slurry supply tank 1 (final crystallization tank) is fed to the bottom of the filter 2 while being pressurized by a compressor M-4 to 5.3 kg/cm2G (steam-containing N2 gas), and filtered under pressure (suction) in a dipping portion of the cylindrical rotating filter medium 11 (bottom 12 in Fig. 2) by the suction of the compressor M- 4 through the mother liquor filtrate/discharged wash liquor receiving tank 4.

[61] The rotating filter medium 11 rotates clockwise at a rate of 2 rpm and the filter cake deposited in the slurry dipping portion is cleaned under suction by the wash liquor 17a introduced from the upper portion of the rotating filter medium 11 while conducting dehydration of the mother liquor filtrate under suction and, after successive dehydration under suction, the filter cake is flaked by a pulse pressure (5.5 kg/cm2G) of a gas feed pulser M-l applied from the inside of the filter medium. The pressure to the flaked filter cake is controlled by a pressure buffer valve M-2 intercepted by two slide valves and then discharged to a cake receiving tank 3 which is at a normal pressure.

[62] Wash water is fed under pressure by the flash valve 17 set to about 20 kg/cm2G and injected to the washing zone of the filter 2. In this case, the filter cake is washed while heating the wash water by a heater E-2 to each of the temperatures set for the wash water (TIC-1) described in Fig. 4, and flashing the wash water 17a (releasing pressure) to the washing zone 14. The wash water exceeding the wash water temperature of 160°C (steam pressure; 5.3 kg/cm2G) forms a wash water (vapor-liquid mixed phase) at least accompanying generation of steams at the same time with flashing and the filter cake is washed with the mixed phase water.

[63] The (mother) liquor filtrate filtered under suction in the filtration zone 13 at the bottom of the rotating filter medium 11 and the discharged wash liquor drew under suction in the washing-dehydration zone 14 in the upper portion of the filter medium are simultaneously transferred under suction to the mother liquor filtrate/discharged wash liquor receiving tank 4 and discharged as a liquid mixture from the lower portion of the receiving tank 4.

[64] The suction gas containing vapors separated in the mother liquor filtrate/discharged wash liquor receiving tank 4 is cooled to about 145°C being passed through a cooler (condenser) E-3 from above, and the condensate is recovered in the gas-liquid separation tank 5. An uncondensed vapor/gas component is fed as a circulating gas to the suction side of the compressor M-4. The circulating gas from the compressor M-4 is controlled to 5.3 kg/cm2G by a pressure regulator PIC-2 for pressurizing the filtration system and to 151°C by a temperature regulator TIC-2 and served for circulation.

[65] The filter cake in the cake receiving tank 3 was recovered by discharging the cake of terephthalic acid under an atmospheric pressure by a screw type discharger M-3 attached to the lower portion of the tank 3. The discharged cake was sampled and the wetting ratio and the amount of p-toluic acid contained in the terephthalic acid crystals were measured.

[66] The amount of the wash liquor in this example was about 0.7 times by weight based on the discharged amount of the terephthalic acid crystals and the temperature and the vapor pressure of the wash water are shown in Fig. 4 by corresponding example numbers. The calculation ratio for the generation of vapors from the wash water (evaporation ratio) and the wetting ratio and the p- toluic acid content of the discharged cake of terephthalic acid in each of the example numbers are as shown in Fig. 4. Fig. 3 is a characteristic figure showing the relation of the wetting ratio (wt%) and the p-toluic acid content (ppm) of the discharged cake of terephthalic acid relative to the temperature, the vapor pressure, and the evaporation ratio of the wash water for each of the examples shown in Fig. 4.

[67] As a result, it has been found that the p-toluic acid content is decreased along with increase of the temperature of the wash water, and the p-toluic acid content is decreased greatly at a superheated temperature of 170°C or higher which is considered to accompany the generation of vapors.

Then, in the production plant for the highly pure terephthalic acid to be separated and collected from the crystallized slurry in this example, a highly pure terephthalic acid at a p-toluic acid content of about 125 ppm was produced. Accordingly, it has been found that the same effect as that obtained by the re-slurrifying washing method by the two stage centrifugation (for highly pure terephthalic acid production plant) can be obtained by the washing method of invention using the wash water superheated to a temperature of the wash water at 190°C (evaporation ratio: 6.3%) in Example 3.

[68] Further, it has been found that the p-toluic acid content of 150 ppm or less, which is specified as the production specification for the production of highly- pure terephthalic acid can be attained at a temperature of the wash water in Example 2 (wash water temperature: 180°C, evaporation ratio: 4.2%) or higher.

[Description of Reference Numerals]
[69]
1 slurry feed tank (final crystallization tank)
2 rotating cylindrical filter
3 cake receiving tank
4 filtrate/discharged wash liquor receiving tank
5 gas-liquid separation tank
6 solid-liquid separator 11 filter medium
13 filtration zone
14 washing-dehydration zone
15 flaking zone
M-l gas feed pulser
M-2 pressure buffer zone
M-3 screw type discharger
M-4 compressor
E-l, E-3 cooler
E-2, E-4 heater

We claim:

1. A method for recovering crystals from crystallized slurry comprising a solvent and crystallized crystals by continuous filtration of repeating each of the steps of filtration, washing, and flaking sequentially, in which the method includes;

filtering the slurry, in the filtration step, by pressurizing the high pressure side of a filtration surface (on the side of slurry) using a circulating solvent vapor-containing gas, and

feeding a wash liquor superheated so as to have a vapor pressure exceeding the pressure on the high pressure side of the filtration surface and washing the filtered and separated crystals by the wash liquor accompanying generation of vapors, in the washing step.

2. A method for recovering crystals from crystallized slurry according to claim 1, wherein a crystallized slurry, which is obtained by subjecting an aqueous solution of terephthalic acid formed by dissolving a crude terephthalic acid in water at high temperature and high pressure to hydrogenation reduction under the presence of a reduction catalyst and then crystallizing terephthalic acid crystals through a plurality of crystallization tanks connected in series for lowering the pressure stepwise, is filtered, in the filtration step, under pressurization at a pressure of 2.5 to 6 kg/cm2G on the high pressure side of the filtration surface.

3. A method for recovering crystals from crystallized slurry according to claim 2, wherein a wash water superheated so as to have a vapor pressure of about 2.7 to 7 kg/cm2G (about 140 to 170°C) exceeding each of pressures of 2.5 to 6 kg/cm2G on the high pressure side of the filtration surface is fed, and filtered and separated crystals are washed by the wash water accompanying the generation of vapors in the washing step.

4. A method for recovering crystals from crystallized slurry according to claim 2 or 3, wherein a wash water superheated so as to have a vapor pressure of about 7.0 to 16.5 kg/cm2G is fed and the crystals are washed by the wash water accompanying the generation of vapors in the washing step.

5. A method for recovering crystals from crystallized slurry according to any one of claims 1 to 4, wherein crystals are washed by a wash liquor superheated such that the ratio of generated vapors is about 2% to 10% in the washing step.