Title of Invention

"METHOD OF THE CENTRIFUGAL SEPARATION OF SLURRY CONTAINING TEREPHTHALIC ACID"

Abstract

An object of the invention is to provide a technique in which especially in the step of solid/liquid separation of a slurry with a centrifugal separator in the production of an aromatic carboxylic acid, the solid/liquid separation with a centrifugal separator can be smoothly conducted, and which thus enables the high-purity aromatic carboxylic acid to be efficiently produced. Means for resolution of the invention relates to a process comprising a step in which a slurry comprising acetic acid as a solvent and containing terephthalic acid is supplied through a flash valve 2 to a final crystallization vessel 1 while keeping the slurry in a boiling state under reduced pressure to thereby cause the solvent to undergo flash vaporization and precipitate crystals of terephthalic acid and a subsequent solid/liquid separation step in which a centrifugal separator 4 is operated at atmospheric pressure to recover the crystals of terephthalic acid. The operating pressure in the centrifugal separation is regulated so as to be higher than the operating pressure in the final crystallization vessel 1. Consequently, crystals of the aromatic carboxylic acid are less apt to separate out and the centrifugal separator can be stably operated.

Full Text

DESCRIPTION METHOD OF CENTRIFUGAL SEPARATION OF SLURRY CONTAINING AROMATIC CARBOXYLIC ACID This invention relates to a method of the centrifugal separation of a crystal-containing slurry in steps for producing an aromatic carboxylic acid, and to a process for producing high-purity terephthalic acid. Generally known as an industrial process for producing an aromatic carboxylic acid such as terephthalic acid is a process which comprises subjecting an alkylaromatic hydrocarbon such as p-xylene as a starting material to a liquid-phase oxidation reaction with molecular oxygen in an acetic acid solvent in the presence of a catalyst comprising cobalt, manganese, and bromine and treating the resultant slurry containing crystals of terephthalic acid or another aromatic carboxylic acid with a solid/liquid separator to produce terephthalic acid. Examples of the solid/liquid separator include rotary vacuum filters, belt filters, centrifugal separators, and the like. Of these, the centrifugal separators comprise a rotating cylindrical bowl part disposed in a vertical or horizontal drum and a screw conveyor. The centrifugal separators have a mechanism in which a slurry supplied is dispersed in the rotating cylindrical bowl part and the solid matter sedimented on the bowl wall due to centrifugal force is moved/discharged with the screw conveyor (see patent document 1). [Patent Document 1] JP-T-7-507291 (page 10, left lower column and right lower column; Figs. 3 and 4) However, when a centrifugal separator such as those described above is used to conduct the solid/liquid separation of a slurry, there are cases where the slurry is cooled by a decrease in pressure in the centrifugal separator to thereby undergo local crystallization. There is hence a problem that the crystals generated make the weight distribution in the rotating bowl uneven and the stability (balance) of the rotation axis of the bowl becomes poor, resulting in vibration and in a decrease in rotational speed, etc. to impair the efficiency of the solid/liquid separation treatment. Likewise, there is the following problem. When the mother liquor separated is cooled in, e.g., a mother liquor overflow storage chamber for temporarily storing the mother liquor separated, crystallization occurs and crystal deposition and growth occurs on the inner surface of the separator. Namely, scaling occurs and this is a factor which retards the separation operation. Such problems are common to the cases where the solid/liquid separation of a slurry is conducted with a centrifugal separator in the production of terephthalic acid. Accordingly, an object of the invention is to provide a method in which especially in the step of solid/liquid separation of a slurry with a centrifugal separator in the production of an aromatic carboxylic acid, the solid/liquid separation with a centrifugal separator can be smoothly conducted to thereby eliminate the problems described above. Another object is to efficiently produce high-purity terephthalic acid using this method. In order to accomplish those objects, the invention provides a method of the centrifugal separation of a slurry containing an aromatic carboxylic acid which comprises subjecting a water-based or acetic-acid-based slurry saturated with an aromatic carboxylic acid to solid/liquid separation with a centrifugal separator, characterized in that the operating pressure in the centrifugal separation is regulated so as to be higher than the saturated vapor pressure of the mother liquor of the aromatic-carboxylic-acid-containing slurry supplied as a feed to be separated. In the method of the invention for the centrifugal separation of an aromatic-carboxylic-acid-containing slurry, which has the constitution described above, the operating pressure in the centrifugal separation is regulated so as to be higher than the saturated vapor pressure of the mother liquor of the slurry in the step which preceds introduction into the centrifugal separator. Because of this, although the slurry before centrifugal separation contains a large amount of the aromatic carboxylic acid in a saturation state, the saturation concentration of the aromatic carboxylic acid increases during the centrifugal separation and, hence, crystals of the aromatic carboxylic acid are less apt to separate out. Consequently, local crystal deposition does not occur in the centrifugal separator and the rotation mechanism retains its balance. The solid/liquid separation with the centrifugal separator can hence be smoothly conducted. Furthermore, by utilizing this method, high-purity terephthalic acid can be efficiently produced. The step which precedes introduction into the centrifugal separator where the effect described above is produced may be a step in which an aromatic carboxylic acid is yielded by an oxidation reaction in an acetic acid solvent or may be a step in which an aromatic carboxylic acid is crystallized by the flash cooling of a slurry. Likewise, a process for producing crude terephthalic acid using the method of the solid/liquid separation of a slurry described above can be employed. High-purity terephthalic acid can be produced by further purifying it. Namely, as the process for producing crude terephthalic acid which eliminates the problems described above can be employed a process for producing terephthalic acid which comprises successively conducting the following steps (i) to (iii) : (i) a step in which p-xylene is subjected to liquid-phase oxidation at a pressure of 1-2 MPa in acetic acid as a solvent in the presence of a catalyst comprising cobalt, manganese, and bromine to yield a slurry containing terephthalic acid, (ii) a step in which the terephthalic-acid-containing slurry obtained is subjected to solid/liquid separation with a centrifugal separator, and (iii) a step in which the solid matter obtained by the centrifugal separation and containing terephthalic acid is washed with acetic acid and/or water, characterized in that the operating pressure in the centrifugal separation in step (ii) is regulated so as to be higher than the saturated vapor pressure of the mother liquor of the terephthalic-acid-containing slurry supplied as a feed to be separated. In the process described above, it is preferred that step (ii) and step (iii) be conducted in the same apparatus and that the temperature of the acetic acid and/or water to be supplied for washing be (T-20)°C or higher, provided that T is the temperature (°C) of the slurry yielded in step (i ) . In the process described above, the operating pressure in the centrifugal separation is preferably regulated so as to be higher by 0.01-0.2 MPa than the saturated vapor pressure of the mother liquor of the terephthalic-acid-containing slurry supplied as a feed to be s epa ra ted. Furthermore, as the process for producing high-purity terephthalic acid which eliminates the problems described above can be employed a process for producing high-purity terephthalic acid which comprises successively conducting the following steps (i) to (v): (i) a step in which p-xylene is subjected to liquid-phase oxidation in acetic acid as a solvent in the presence of a catalyst to yield crude terephthalic acid, (ii) a step in which the crude terephthalic acid is dissolved in water and the 4-carboxy benzaldehyde in the resultant solution is subjected to a reduction treatment at 230-350°C in the presence of a catalyst, (iii) a step in which the slurry which has undergone the reduction treatment is subjected to stepwise flash cooling to 120-180°C with two or more crystallization vessels to conduct crystallization, (iv) a step in which the terephthalic-acid-containing slurry obtained is subjected to solid/liquid separation with a centrifugal separator, and (v) a step in which the solid matter obtained by the centrifugal separation and containing terephthalic acid is washed with water, characterized in that the operating pressure in the centrifugal separation in step (iv) is regulated so as to be higher than the final crystallization pressure in the step of crystallization by flash cooling. In the process described above, it is preferred that step (iv) and step (v) be conducted in the same apparatus and that the temperature of the water to be supplied for washing be (T-20)°C or higher, provided that T is the temperature (°C) of the slurry yielded in step (iii). This process for producing high-purity terephthalic acid preferably is one in which the operating pressure in the centrifugal separation is regulated so as to be higher by 0.01-0.2 MPa than the final crystallization pressure in the step of crystallization by flash cooling. Fig. 1 is an apparatus flow diagram illustrating a first embodiment of the method of the solid/liquid separation of a slurry; and Fig. 2 is an apparatus flow diagram illustrating a second embodiment of the method of the solid/liquid separation of a slurry. In the figures, numeral 1 denotes a crystallization vessel, 2 a flash valve, 4 a centrifugal separator, 5 an ejector, 6 a condenser, 7 a pump, 8 a flash valve, 9 a crystallization vessel, 10 a pump, 11 a centrifugal separator, 12 a suspension/washing vessel, 13 a decompression vessel, 14 a centrifugal separator, and 15 a dryer. The invention will be explained below in detail. A process for producing terephthalic acid from p-xylene is explained as a typical example. An alkylaromatic compound is used as a starting material in the invention. This alkylaromatic compound is an alkylbenzene, e.g., a mono-, di-, or trialkylbenzene, which is converted to an aromatic carboxylic acid, e.g., an aromatic monocarboxylic acid, aromatic dicarboxylic acid, or aromatic tricarboxylic acid, through liquid-phase oxidation, and may be such aromatic compound in which the one or more alkyl groups have been partly oxidized. This invention is especially preferably applied to the production of terephthalic acid, and examples of the alkylaromatic compound to be used as a starting material in this case include p-xylene. The solvent to be used in the invention preferably is acetic acid, which is a lower aliphatic carboxylic acid. The amount of this solvent to be used is generally 2-6 times by weight the amount of the p-xylene as a starting material. An acetic acid solvent which contains water in a small amount, e.g., up to 10% by weight, can also be used. For oxidizing an alkylaromatic compound such as p-xylene, a gas containing molecular oxygen is used. In general, air is used because of apparatus simplicity and low cost. Also usable besides air are diluted air, oxygen-enriched air, and the like. A catalyst containing cobalt (Co), manganese (Mn), and bromines (Br) as constituent elements is generally used for oxidizing an alkylaromatic compound. Specific examples of compounds of such catalyst ingredients are as follows. Example of cobalt compounds include cobalt acetate, cobalt naphthenate, and cobalt bromide. Examples of manganese compounds include manganese acetate, manganese naphthenate, and manganese bromide. Examples of bromine compounds include hydrogen bromide, sodium bromide, cobalt bromide, manganese bromide, and bromoethane. The oxidation reaction of an alkylaromatic compound in a liquid phase is conducted by oxidizing the alkylaromatic compound, e.g., p-xylene, in an acetic acid solvent while continuously supplying a molecular-oxygen-containing gas at a temperature of 140-230°C, preferably 150-210°C, in the presence of a catalyst. The reaction pressure is either the minimum pressure which enables the mixture to retain a liquid phase at the reaction temperature or a pressure higher than that. The pressure is generally 0.2-5 MPa, preferably 1-2 MPa. In a crystallization step, the reaction slurry is cooled and decompressed to an appropriate temperature and pressure to obtain an aromatic-carboxylic-acid slurry. The number of stages of the crystallization is preferably from 1 to 6. Most preferred is 2-stage or 3-stage crystallization. In this series of crystallization stages, the final stage is conducted while keeping the reaction mixture in a boiling state under reduced pressure. For example, a preferred operation for attaining a boiling state under reduced pressure is flash cooling. Flash cooling is cooling by release from a pressure. In the step of centrifugal slurry separation in that embodiment of the process for producing terephthalic acid which is shown by the apparatus flow diagram of Fig. 1, a slurry containing terephthalic acid in acetic acid as a solvent is supplied to a final crystallization vessel 1 through a flash valve 2. Namely, this step is conducted while keeping the slurry in a boiling state under reduced pressure to thereby cause the solvent to undergo flash vaporization and precipitate crystals of terephthalic acid. In the subsequent solid/liquid separation step, a centrifugal separator 4 is operated at atmospheric pressure to recover the crystals of terephthalic acid. Numeral 5 in the figure denotes an ejector, 6 a condenser for recovering the vaporized solvent ingredient, and 7 a pump. In conducting the centrifugal slurry separation in the manner described above, the operating pressure, i.e., the pressure which the slurry receives from the atmosphere in the centrifugal separator drum, is regulated so as to be higher than the saturated vapor pressure of the mother liquor of the slurry. For example, the operating pressure, i.e., the pressure which the slurry receives from the atmosphere in the centrifugal separator drum, may be ordinary pressure. This pressure is higher than the pressure which the slurry receives from the atmosphere just before introduction into the centrifugal separator (the pressure obtained by decompression by flashing is lower than atmospheric pressure). Because of this, although the slurry in a step which precedes centrifugal separation is saturated with the aromatic carboxylic acid, crystals of the aromatic carboxylic acid are less apt to separate out during the centrifugal separation. It is desirable that the pressure of the slurry or of the atmosphere in contact therewith be regulated so as to be higher by 0.01-0.2 MPa, more preferably by 0.03-0.1 MPa, than the pressure of the slurry or atmosphere in contact therewith in the step which precedes introduction into the centrifugal separator. The temperature of the slurry to be supplied is desirably at least 50°C, preferably 60-100°C, especially 70-90°C. Low final-crystallization temperatures are advantageous because the aromatic carboxylic acid is recovered in an increased amount. However, too low temperatures are disadvantageous because it becomes necessary to use a vacuum device attaining a higher degree of vacuum. There are cases where the solid matter thus separated is washed with acetic acid and/or water. It is desirable that the solid/liquid separation and the washing be conducted in the same apparatus. A preferred solid/liquid separator for this use is a solid bowl type centrifugal separator or a screen bowl type centrifugal separator. The temperature of the acetic acid and/or water to be supplied for washing is preferably (T-20)°C or higher, provided that T is the temperature (°C) of the slurry to be supplied. The upper limit of the temperature of this washing liquid is lower than the boiling point thereof at the operating pressure so that the washing liquid retains its liquid state after supply. However, since higher temperatures result in an increase in the amount of terephthalic acid dissolving in the washing liquid, the temperature of the washing liquid is preferably (T+30)°C or lower, provided that T is the temperature (°C) of the slurry to be supplied. By thus conducting temperature regulation, the slurry and the cake separated are inhibited from decreasing in temperature and operational troubles accompanying mother liquor crystallization are prevented. The centrifugal slurry separation steps in another embodiment are conducted in the following manner as shown in the apparatus flow diagram of Fig. 2. The 4-carboxy benzaldehyde (4CBA) contained in crude terephthalic acid are subjected to a hydrogenation (reduction) reaction in a reactor not shown in the figure, in a water solvent at 230-350 °C in the presence of a catalyst. Subsequently, crystallization vessels disposed in 3- to 6-stage arrangement are used to conduct a crystallization step in which the resultant reaction mixture is passed through each flash valve 8 to vaporize the water by flash cooling. The slurry saturated with terephthalic acid is supplied from the final crystallization vessel 9 to a centrifugal separator 11 through a pump 10. Therefore, the cake separated is introduced into a suspension/washing vessel 12, in which the cake is suspended in and washed with water. The resultant slurry is subjected to flash cooling in a decompression vessel 13 having a pressure around atmospheric pressure, and then introduced with a pump 10 into a centrifugal separator 14, where second solid/liquid separation is conducted. Thereafter, the solid matter is supplied to a dryer 15 to produce crystals of high-purity terephthalic acid. In these steps also, the operating pressure for the centrifugal separators 11 and 14 is regulated so as to be higher than the saturated vapor pressure of the mother liquor of the feed slurry. Because of this, although the slurry before centrifugal separation is saturated with the aromatic carboxylic acid, crystals of the aromatic carboxylic acid are less apt to separate out during the centrifugal separation due to the increased pressure difference. It is desirable that the pressure of the slurry or of the atmosphere in contact therewith be regulated so as to be higher by 0.01-0.2 MPa, more preferably by 0.03-0.1 MPa, than the pressure of the slurry or atmosphere in contact therewith in the step which precedes introduction into the centrifugal separator. The temperature of the slurry to be supplied to the centrifugal separator 11 is desirably 120-180°C, preferably 130-180°C, especially 140-170°C. Low final-crystallization temperatures are advantageous because the aromatic carboxylic acid is recovered in an increased amount. However, too low temperatures result in accelerated formation of eutectic crystals including the p-toluic acid generated by the reduction of 4CBA, leading to a decrease in the quality of the high-purity terephthalic acid obtained. Likewise, the temperature of the slurry to be supplied to the centrifugal separator 14 is desirably at least 50°C, preferably 60-120°C, especially 90-110°C. There are cases where the solid matter separated with the centrifugal separator is washed with water. It is desirable that the solid/liquid separation with the centrifugal separator and the washing be conducted in the same apparatus. A preferred solid/liquid separator for this use is a solid bowl type centrifugal separator or a screen bowl type centrifugal separator. The temperature of the water to be supplied for washing is preferably (T-20)°C or higher, provided that T is the temperature (°C) of the slurry to be supplied. The upper limit of the temperature of this washing liquid is lower than the boiling point thereof at the operating pressure so that the washing liquid retains its liquid state after supply. However, since higher temperatures result in an increase in the amount of terephthalic acid dissolving in the washing liquid, the temperature of the washing liquid is preferably (T+30)°C or lower, provided that T is the temperature (°C) of the slurry to be supplied. By thus conducting temperature regulation, the slurry and the cake separated are inhibited from decreasing in temperature and operational troubles accompanying mother liquor crystallization are prevented. [EXAMPLE 1] p-Xylene was subjected to liquid-phase oxidation in acetic acid as a solvent at a pressure of 1.5 MPa in the presence of a catalyst comprising Co/Mn/Br to yield a slurry containing terephthalic acid. This slurry was flashed with an ejector to a pressure (0.05 MPa) lower than atmospheric pressure. The slurry was thus flash-cooled to 90°C. The terephthalic acid slurry obtained was subjected to solid/liquid separation with a centrifugal separator in which the atmosphere in the drum was kept at atmospheric pressure. Thus, crude terephthalic acid was separated. Furthermore, the crude terephthalic acid obtained btececbi^ in the »e*fe step was dissolved in water, and the 4-carboxy benzaldehyde in the resultant solution was reduced in the presence of a Group 8 metal catalyst. This mixture which had undergone the reduction treatment was subjected to 5-stage crystallization by flash vaporization. The final crystallization was conducted under the conditions of 160°C and 0.62 MPa. The slurry thus obtained, which contained terephthalic acid crystals, was subjected to solid/liquid separation with a centrifugal separator in the steps shown in Fig. 2 at an operating pressure of 0.69 MPa. The cake separated was washed in the suspension/washing vessel, and the resultant slurry was cooled by flashing to atmospheric pressure. This slurry was further supplied to the second centrifugal separator and subjected to solid/liquid separation at an operating pressure of 0.12 MPa. The cake separated was dried to produce high-purity terephthalic acid. The solid/liquid separation with the centrifugal separators was continuously conducted for 200 hours. As a result, the centrifugal separators retained a satisfactory balance in rotation and no vibration occurred. The solid/liquid separation could be smoothly conducted. It is presumed from these results that no local deposition of terephthalic acid occurred in the centrifugal separators. [EXAMPLE 2] p-Xylene was subjected to liquid-phase oxidation in acetic acid as a solvent at a pressure of 1.5 MPa in the presence of a catalyst comprising Co/Mn/Br to yield a slurry containing terephthalic acid. This slurry was flashed with an ejector to a pressure (0.05 MPa) lower than atmospheric pressure. The slurry was thus flash-cooled to 90°C. The terephthalic acid slurry obtained was subjected to solid/liquid separation with a centrifugal separator in which the atmosphere in the drum was kept at atmospheric pressure. Thus, crude terephthalic acid was separated. In the operation described above, a screen bowl type centrifugal separator was used as the centrifugal separator. In the screen part thereof, 95°C acetic acid was used as a washing liquid to wash the solid matter. The solid/liquid separation and washing with the screen bowl type centrifugal separator was continuously conducted for 150 hours. As a result, the centrifugal separator retained a satisfactory balance in rotation and no vibration occurred. The solid/liquid separation could be smoothly conducted. It is presumed from these results that no local deposition of terephthalic acid occurred in the centrifugal separator. [EXAMPLE 3] The same procedure as in Example 2 was conducted, except that the temperature of the acetic acid for use as a washing liquid in Example 2 was changed to 85°C. The solid/liquid separation and washing with the screen bowl type centrifugal separator was continuously conducted for 110 hours. As a result, the centrifugal separator retained a satisfactory balance in rotation and no vibration occurred. The solid/liquid separation could be smoothly conducted. It is presumed from these results that no local deposition of terephthalic acid occurred in the centrifugal separator. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. This application is based on a Japanese patent application filed on January 10, 2003 (Application No. 2003-004846), the entire contents thereof being hereby incorporated by reference. The invention has the following advantages. In the method for the centrifugal separation of aromatic carboxylic acid crystals generated in a slurry, the pressure of the slurry during the centrifugal separation is regulated so as to be higher than the slurry pressure in the preceding step in which the slurry was brought into the state of being saturated with the aromatic carboxylic acid. Because of this, no local deposition of crystals occurs in the centrifugal separator and the rotation mechanism retains its balance. The solid/liquid separation with the centrifugal separator can hence be smoothly conducted. Furthermore, by utilizing this method, high-purity terephthalic acid can be efficiently produced. CLAIMS 1. A method of the centrifugal separation of a slurry containing an aromatic carboxylic acid which comprises subjecting a water-based or acetic-acid-based slurry saturated with an aromatic carboxylic acid to solid/liquid separation with a centrifugal separator, characterized in that the operating pressure in the centrifugal separation is regulated so as to be higher than the saturated vapor pressure of the mother liquor of the aromatic-carboxylic-acid-containing slurry supplied as a feed to be separated. The method of the centrifugal separation of an aromatic-carboxylic-acid-containing slurry of claim 1, characterized in that the slurry saturated with an aromatic carboxylic acid is a slurry obtained by yielding an aromatic carboxylic acid by an oxidation reaction in an acetic acid solvent. The method of the centrifugal separation of an aromatic-carboxylic-acid-containing slurry of claim 1, characterized in that the slurry saturated with an aromatic carboxylic acid is a slurry containing crystals of the aromatic carboxylic acid which have been formed by flash cooling. The method of the centrifugal separation of an aromatic-carboxylic-acid-containing slurry of any one of claim, 1 characterized in that the operating pressure in the centrifugal separation is regulated so as to be higher by 0.01-0.2 MPa than the saturated vapor pressure of the mother liquor of the aromatic-carboxylic-acid- containing slurry supplied as a feed to be separated. The method of the centrifugal separation of an aromatic-carboxylic-acid-containing slurry of any one of claim 1 characterized in that the centrifugal separator is a solid bowl type centrifugal separator or a screen bowl type centrifugal separator. 6. A process for producing terephthalic acid which comprises successively conducting the following steps (i) to (iii) : (i) a step in which p-xylene is subjected to liquid-phase oxidation at a pressure of 1-2 MPa in acetic acid as a solvent in the presence of a catalyst comprising cobalt, manganese, and bromine to yield a slurry containing terephthalic acid, (ii) a step in which the terephthalic-acid-containing slurry obtained is subjected to solid/liquid separation with a centrifugal separator, and (iii) a step in which the solid matter obtained by the centrifugal separation and containing terephthalic acid is washed with acetic acid and/or water, characterized in that the operating pressure in the centrifugal separation in step (ii) is regulated so as to be higher than the saturated vapor pressure of the mother liquor of the terephthalic-acid-containing slurry supplied as a feed to be separated. The process for producing terephthalic acid of claim 6, characterized in that step (ii) and step (iii) are conducted in the same apparatus and that the temperature of the acetic acid and/or water to be supplied for washing is (T-20)°C or higher, provided that T is the temperature (°C) of the slurry yielded in step (i ). The process for producing terephthalic acid of claim 6 characterized in that the operating pressure in the centrifugal separation is regulated so as to be higher by 0.01-0.2 MPa than the saturated vapor pressure of the mother liquor of the terephthalic-acid-containing slurry supplied as a feed to be separated. The process for producing terephthalic acid of any one of claim,' 6 characterized in that the centrifugal separator is a solid bowl type centrifugal separator or a screen bowl type centrifugal separator. 10. A process for producing high-purity terephthalic acid which comprises successively conducting the following steps (i) to (v): (i) a step in which p-xylene is subjected to liquid-phase oxidation in acetic acid as a solvent in the presence of a catalyst to yield crude terephthalic acid, (ii) a step in which the crude terephthalic acid is dissolved in water and the 4-carboxy benzaldehyde in the resultant solution is subjected to a reduction treatment at 230-350°C in the presence of a catalyst, (iii) a step in which the slurry which has undergone the reduction treatment is subjected to stepwise flash cooling to 120-180°C with two or more crystallization vessels to conduct crystallization, (iv) a step in which the terephthalic-acid-containing slurry obtained is subjected to solid/liquid separation with a centrifugal separator, and (v) a step in which the solid matter obtained by the centrifugal separation and containing terephthalic acid is washed with water, characterized in that the operating pressure in the centrifugal separation in step (iv) is regulated so as to be higher than the final crystallization pressure in the step of crystallization by flash cooling. The process for producing high-purity terephthalic acid of claim 10, characterized in that step (iv) and step (v) are conducted in the same apparatus and that the temperature of the water to be supplied for washing is (T-20) °C or higher, provided that T is the temperature (°C) of the slurry yielded in step (iii) . The process for producing high-purity terephthalic acid of claim 10 characterized in that the operating pressure in the centrifugal separation is regulated so as to be higher by 0.01-0.2 MPa than the final crystallization pressure in the step of crystallization by flash cooling. The process for producing high-purity terephthalic acid of any one of claims 10 . . , characterized in that the centrifugal separator is a solid bowl type centrifugal separator or a screen bowl type centrifugal separator.

Documents:

2843-DELNP-2005-Abstract (14-02-2008).pdf

2843-delnp-2005-abstract.pdf

2843-DELNP-2005-Claims (14-02-2008).pdf

2843-delnp-2005-claims.pdf

2843-DELNP-2005-Correspondence-Others (14-02-2008).pdf

2843-delnp-2005-correspondence-others.pdf

2843-delnp-2005-description (complete).pdf

2843-delnp-2005-drawings.pdf

2843-delnp-2005-form-1.pdf

2843-delnp-2005-form-18.pdf

2843-DELNP-2005-Form-2 (14-02-2008).pdf

2843-delnp-2005-form-2.pdf

2843-delnp-2005-form-3.pdf

2843-delnp-2005-form-5.pdf

2843-delnp-2005-gpa.pdf

2843-delnp-2005-pct-308.pdf

2843-delnp-2005-pct-notification.pdf

2843-delnp-2005-pct-search report.pdf