Title of Invention	"PROCESS FOR PRODUCING HIGH-PURITY TEREPHTHALIC ACID"
Abstract	A subject for the invention is to provide a process for producing high purity terephthalic acid with a high level of energy efficiency by suppressing the loss of the heat energy possessed by the condensate obtained as a distillate during crystallization and by washings discharged after washing. The invention relates to a process for producing high purity terephthalic acid which includes the steps of slurry preparation, dissolution, reduction, crystallization, separation, washing and drying, and employs the washings at a temperature of from 120°C to 200°C as an aqueous solvent for the step of slurry preparation.

Title of Invention

"PROCESS FOR PRODUCING HIGH-PURITY TEREPHTHALIC ACID"

Abstract

A subject for the invention is to provide a process for producing high purity terephthalic acid with a high level of energy efficiency by suppressing the loss of the heat energy possessed by the condensate obtained as a distillate during crystallization and by washings discharged after washing. The invention relates to a process for producing high purity terephthalic acid which includes the steps of slurry preparation, dissolution, reduction, crystallization, separation, washing and drying, and employs the washings at a temperature of from 120°C to 200°C as an aqueous solvent for the step of slurry preparation.

Full Text	DESCRIPTION PROCESS FOR PRODUCING HIGH-PURITY TEREPHTHALIC ACID Technical Field [0001] The present invention relates to a process for producing high-purity terephthalic acid. Background Art [0002] For obtaining crystals of high-purity terephthalic acid, it is necessary that intermediate impurities which have generated as by-products of the oxidation of p-xylene and include 4-carboxybenzaldehyde as a major component should be removed as much as possible from crude terephthalic acid crystals obtained by the oxidation. For this purpose, purification steps are conducted in which the 4-carboxybenzaldehyde is reduced to p-toluic acid, which has higher water solubility than terephthalic acid, and terephthalic acid crystallization is then conducted to obtain high-purity terephthalic acid crystals. [0003] First, in a slurrying step (a1), the crude terephthalic acid is mixed with a water solvent in a slurrying vessel to obtain a slurry. Fresh water or recovery water discharged from the purification steps and comprising water as the main component is used as the water solvent. Usually, the slurry concentration is 20-30% by weight. In low-temperature regions, the terephthalic acid which is the main component of the crude terephthalic acid a has insufficient solubility in water. Because of this, the slurry obtained in the slurring vessel is heated to about 250-350°C to dissolve the terephthalic acid. In order to maintain the liquid state in the high-temperature region, the slurry is pressurized beforehand to or above the water vapor pressure as measured at the final heating temperature. Specifically, the slurry is pressurized with a pump and then heated with a preheater to dissolve the crude terephthalic acid and obtain an aqueous solution. [0004] A reduction step (b') is then conducted in which the aqueous solution obtained by dissolving the terephthalic acid under a condition of a high temperature and a high pressure is introduced into a hydrogenation reactor and brought into contact with hydrogen in the presence of a catalyst to thereby reduce the 4-carboxybenzaldehyde into p-toluic acid. The liquid reduction reaction mixture thus obtained, which contains p-toluic acid and terephthalic acid as major ingredients, is subjected to a crystallization step (cf), in which the reaction mixture is introduced into crystallization vessels to cause crystallization. Since terephthalic acid has lower water solubility than p-toluic acid, high-purity crystals of terephthalic acid can be separated out in this step by controlling crystallization conditions. In the case where pressure-release evaporation vessels are employed as the crystallization vessels in this step, the water vapor generated as a distillate is recovered and used as the water solvent. Furthermore, the heat energy possessed by the vapor generated can be used for the dissolution. [0005] Although crystallization by pressure-release evaporation is employed here, pressure release to atmospheric pressure and cooling to 100°C result in the crystallization of" not only terephthalic acid but also p-toluic acid and, hence, make it impossible to obtain high-purity terephthalic acid crystals. It is therefore necessary to control the pressure of the reaction mixture to a pressure higher than atmospheric pressure so as to result in a final crystallization temperature of 120°C to 180°C, preferably 130°C to 170°C. [0006] A separation step (d1) is subsequently conducted, in which the crystallization slurry containing the high-purity terephthalic acid crystals which have separated out is introduced into a solid-liquid separator. The slurry is subjected to solid-liquid separation in which the slurry in a pressurized state is separated into a high-purity terephthalic acid cake j and a reduction-reaction mother liquor. This high-purity terephthalic acid cake is subjected to a washing step (e1), in which a washing operation with a cleaning fluid is performed in a washing apparatus and solid-liquid separation is conducted again to obtain a washed cake. Furthermore, a drying step (f) is conducted in which the washed cake obtained is dried with a dryer to obtain high-purity terephthalic acid crystals. [0007] On the other hand, since the reduction-reaction mother liquor discharged from the separation step (d1) still contains effective ingredients including terephthalic acid and p-toluic acid, these effective ingredients are recovered as much as possible. The solvent ingredient is reused as the water solvent either directly or indirectly after some treatment. This can be accomplished by introducing the reduction-reaction mother liquor into a cooling vessel, precipitating the effective ingredients as a solid matter, returning the solid matter to the p-xylene oxidation step for yielding crude terephthalic acid, removing the metallic ingredients, organic substances, and other effective ingredients dissolved in the residual liquid by ion-exchange or adsorption or with a membrane, etc., and then reusing the liquid as a water solvent in the slurrying vessel in the slurrying step (a') or as a cleaning fluid in the washing step (e?). [0008] Furthermore, the washings discharged from the washing step (e?) are supplied, either directly or after the removal of solid matters contained therein, as a water solvent to the slurrying vessel in the slurrying step (a')-[0009] Moreover, an apparatus for conducting both the separation step (d1) and the washing step (e1) therein may be employed to conduct solid-liquid separation and washing while maintaining a pressurized state. Thus, step simplification can be attained. [0010] Examples of documents which describe a process for terephthalic acid production employing such a technique include patent document 1 (WO 93/24440). The washings obtained in this prior-art process are pressurized. [0011] The washings are pressurized and have the same temperature as the final crystallization temperature. Although the distillate vapor condensate obtained in the crystallization step (c1) and the washings have heat energy as stated above, returning the condensate and the washings as they are to the slurrying step (a1) brings the slurrying vessel, which is operated at atmospheric pressure, into a boiling state. It is therefore necessary to subject the condensate and the washings to a cooling treatment before being supplied. In particular, it is necessary to adopt an operating temperature at which the pump for sending the slurry from the slurrying vessel to the next step can be prevented from suffering cavitation (the phenomenon in which when the liquid flowing through the impeller is accelerated, the static pressure of the liquid locally decreases to or below the saturated water vapor pressure to form bubbles and this generates a high impact pressure to cause mechanical damage) . Because of this, the temperature in the slurrying vessel is generally controlled to about 90°C. [0012] However, cooling the washings before supplying them to the slurrying vessel means a loss of heat energy and results in a poor efficiency because the slurry in the slurrying vessel should be heated to a given temperature for dissolution as stated above. Patent Document 1: WO 93/24440 Disclosure of the Invention Problem that the Invention is to Solve [0013] An object of the invention is to provide a process for producing high-purity terephthalic acid in which the energy possessed by the condensate obtained as a distillate in crystallization and by washings discharged after washing in purification steps for producing high-purity terephthalic acid is inhibited from being lost and which attains a higher energy efficiency. Means for Solving the Problem [0014] The present inventors made intensive investigations in order to overcome the problem described above. As a result, they have found that the problem can be eliminated by keeping the temperature of washings discharged from a washing step high. The invention has been thus completed. Essential points of the invention reside in the following (1) to (6) . [0015] (1) A process for producing high-purity terephthaiic acid which comprises a slurrying step (a) in which crude terephthaiic acid containing 4-carboxybenzaldehyde is supplied to a water solvent to obtain a slurry having a slurry concentration of 10-50% by weight, a dissolution step (b) in which the slurry is pressurized and heated to dissolve the crude terephthalic acid contained in the slurry in the water solvent and thereby obtain an aqueous crude-terephthalic-acid solution, the pressurizing being conducted under such conditions that the slurry is pressurized to a pressure higher than the vapor pressure of the heated aqueous solution, the heating being conducted after the pressurizing, a reduction step (c) in which the aqueous solution is brought into contact with hydrogen in the presence of a catalyst to reduce the 4-carboxybenzaldehyde contained in the aqueous solution and thereby obtain a liquid reduction reaction mixture, a crystallization step (d) in which the liquid reduction reaction mixture is cooled to 120-200°C to cause crystals of high-purity terephthalic acid to separate out and thereby obtain a high-purity terephthalic acid slurry, a separation step (e) in which the high-purity terephthalic acid slurry obtained in the crystallization step (d) is subjected to solid-liquid separation while substantially maintaining a pressure not lower than the vapor pressure of the mother liquor of the slurry and the temperature of the high-purity terephthalic acid slurry to thereby obtain a high-purity terephthalic acid cake and a reduction-reaction mother liquor, a washing step (f) in which the high-purity terephthalic acid cake is washed, while substantially maintaining the operating pressure in the separation step (e) , with a cleaning fluid having a temperature which is not lower than the operating temperature in the separation step (e) and is lower than the boiling point of water as measured at the operating pressure in the separation step (e) , and a drying step (g) in which the cake washed is dried to obtain crystals of high-purity terephthalic acid, wherein washings discharged after washing in the washing step (f) are sent, in the state of having a temperature of from 120°C to 200°C, to the slurrying step (a) and used as part or all of the water solvent. [0016] (2) The process for producing high-purity terephthalic acid as described under (1) above wherein the washings discharged after washing in the washing step (f) are sent to the slurrying step (a) conducted at a pressure not lower than the vapor pressure of the cleaning fluid. [0017] (3) The process for producing high-purity terephthalic acid as described under (1) or (2) above wherein the cooling in the crystallization step (d) is cooling by pressure-release evaporation, and a condensate of a crystallization vapor is obtained in the crystallization step (d) and this condensate in the state of having a temperature of from 120°C to 200°C is sent to the slurrying step (a) and used as a water solvent. [0018] (4) The process for producing high-purity terephthalic acid as described under (3) above wherein the condensate of a crystallization vapor obtained in the crystallization step (d) is sent to the slurrying step (a) conducted at a pressure not lower than the vapor pressure of the condensate. [0019] (5) The process for producing high-purity terephthalic acid as described under any one of (1) to (4) above wherein the operating temperature in the slurrying step (a) is from 120°C to 200°C and the operating pressure therein is from 0.2 MPa to 2.1 MPa. [0020] (6) The process for producing high-purity terephthalic acid as described under any one of (1) to (5) above wherein the reduction-reaction mother liquor obtained in the separation step (e) is sent to the washing step (f) after the metallic ingredients and/or organic ingredients contained in the reduction-reaction mother liquor are removed therefrom at least partly, and is used as part or all of the cleaning fluid. Advantage of the Invention [0021] The invention can provide a process for producing high-purity terephthalic acid in which the energy possessed by the condensate obtained as a distillate in crystallization and by washings discharged after washing in purification steps for producing high-purity terephthalic acid is inhibited from being lost and which attains a higher energy efficiency. Brief Description of the Drawing [0022] [Fig. 1] Fig. 1 is a flow diagram illustrating an embodiment of the process for producing high-purity terephthalic acid according to the invention. Description of the Reference Numerals and Signs [0023] 11 slurrying vessel lla pump lib heat exchanger 13 hydrogenation reactor 14 crystallization vessel 14a pump 14b heat exchanger 15 solid-liquid separator 16 washing apparatus 17 solid-liquid separator 18 solid-liquid separation/washing apparatus 19 dryer 20 cooling vessel 21 solid-liquid separator A crude terephthalic acid B water C slurry D aqueous solution E hydrogen F liquid reduction reaction mixture G water vapor H high-purity terephthalic acid slurry I reduction-reaction mother liquor J high-purity terephthalic acid cake K cleaning fluid L washed slurry M washings N washed cake O high-purity terephthalic acid crystals P cooled slurry Q solid matter R liquid matter Best Mode for Carrying Out the Invention [0024] The invention will be explained below in detail. [0025] The process of the invention for producing high-purity terephthalic acid comprises a slurrying step (a) in which crude terephthalic acid containing 4-carboxybenzaldehyde is supplied to a water solvent to obtain a slurry having a slurry concentration of 10-50% by weight, a dissolution step (b) in which the slurry is pressurized and heated to dissolve the crude terephthalic acid contained in the slurry in the water solvent and thereby obtain an aqueous crude-terephthalic-acid solution, the pressurizing being conducted under such conditions that the slurry is pressurized to a pressure higher than the vapor pressure of the heated aqueous solution, the heating being conducted after the pressurizing, a reduction step (c) in which the aqueous solution is brought into contact with hydrogen in the presence of a catalyst to reduce the 4 -carboxybenzaldehyde contained in the aqueous solution and thereby obtain a liquid reduction reaction mixture, a crystallization step (d) in which the liquid reduction reaction mixture is cooled to 120-200°C to cause crystals of high-purity terephthalic acid to separate out and thereby obtain a high-purity terephthalic acid slurry, a separation step (e) in which the high-purity terephthalic acid slurry obtained in the crystallization step (d) Is subjected to solid-liquid separation while substantially maintaining a pressure not lower than the vapor pressure of the mother liquor of the slurry and the temperature of the high-purity terephthalic acid slurry to thereby obtain a high-purity terephthalic acid cake and a reduction-reaction mother liquor, a washing step (f) in which the high-purity terephthalic acid cake is washed, while substantially maintaining the operating pressure in the separation step (e) , with a cleaning fluid having a temperature which is not lower than the operating temperature in the separation step (e) and is lower than the boiling point of water as measured at the operating pressure in the separation step (e), and a drying step (g) in which the cake washed is dried to obtain crystals of high-purity terephthalic acid, the process being characterized in that washings discharged after washing in the washing step (f) are sent, in the state of having a temperature of from 120°C to 200°C, to the slurrying step (a) and used as part or all of the water solvent. [0026] The term "high-purity terephthalic acid" in the invention means the terephthalic acid recovered after crude terephthalic acid is subjected to a reduction reaction treatment with hydrogenation. [0027] The "crude terephthalic acid" in the invention is one obtained by oxidizing p-xylene in an acetic acid solvent. It contains, as by-products, intermediates in which one of the alkyl groups has not been completely oxidized, such as, e.g., 4-carboxybenzaldehyde (the intermediates are effective ingredients convertible to terephthalic acid but are impurities if they get mixed into a terephthalic acid product.) A general process for producing crude terephthalic acid is explained below. [0028] Crude terephthalic acid is produced by oxidizing p- xylene in a liquid phase with molecular oxygen in an acetic acid solvent in the presence of a catalyst. This step is well known. As the catalyst, use may be made of a catalyst which has been known to be usable in this reaction. Examples thereof include compounds of heavy metals, such as cobalt compounds, manganese compounds, iron compounds, and chromium compounds, and bromine compounds. In the reaction system, these compounds are present in a dissolved state. Preferred of these is a combination of a cobalt compound or manganese compound with a bromine compound. In this case, these compounds are generally used in such amounts that the amounts of cobalt atoms, manganese atoms, and bromine atoms are 10-5,000 ppm, 10-5,000 ppm, and 10-10,000 ppra, respectively, of the solvent. [0029] As the molecular oxygen is generally used a gas mixture comprising an inert gas and oxygen. For example, air or oxygen-enriched air is used. The molar proportion of the molecular oxygen to the p-xylene to be fed to the reactor is generally 3-20 times by mole, preferably 2-4 times by mole. [0030] The proportion of the p-xylene to the acetic acid to be supplied to the reactor is generally 1-50% by weight. The concentration of water in the reaction system is generally 5-20% by weight, preferably 5-15% by weight. [0031] The temperature in the oxidation reaction is generally 160-260°C, preferably 170-210°C. The reaction may be conducted at any pressure at which the reaction system can retain a liquid phase at the reaction temperature. The pressure is generally 0.5-5 MPa, preferably 1-2 MPa. The residence time is generally 10-200 minutes. [0032] Terephthalic acid is less apt to dissolve in the acetic acid serving as a solvent. Because of this, the terephthalic add yielded in the oxidation reaction step precipitates as crystals to form a slurry. However, terephthalic acid may be in a dissolved state depending on the amount of the solvent, reaction temperature, and pressure. In this case, a crystallization step for, e.g., cooling the liquid reaction mixture is conducted to precipitate the terephthalic acid and thereby form a slurry. This slurry is subjected to solid-liquid separation to obtain crude terephthalic acid crystals. Although the terephthalic acid slurry obtained in the oxidation reaction step is in a pressurized state, this slurry may be subjected as it is to solid-liquid separation or may be subjected to pressure-release cooling or the like and then to solid-liquid separation. For the solid-liquid separation, any method may be used as long as the crystals can be separated from the mother liquor. Examples thereof include filtration and centrifugal separation. According to need, washing and drying are conducted. Thus, crude terephthalic acid crystals are obtained. [0033] The term "crude terephthalic acid" in the invention means terephthalic acid containing 4-carboxybenzaldehyde in an amount of 1,000-10,000 ppm. [0034] For producing high-purity terephthalic acid, it is necessary that those intermediates should be removed as much as possible from the crude terephthalic acid. Steps for this removal are explained by reference to Fig. 1. [0035] First, in the slurrying step (a), crude terephthalic acid A is supplied to a slurrying vessel 11 and slurried with a water solvent. It is necessary that washings M discharged after washing in the washing step (f) be used as the water solvent while keeping the washings M in the state of having a temperature of from 120°C to 200°C, preferably from 130°C to 180°C. The term washings in the invention means the liquid discharged from the washing step. For maintaining the temperature of the washings in supplying the washings to the slurrying step (a) , it is preferred that the operating pressure in the slurrying step (a) be controlled so as to be not lower than the vapor pressure of the washings to thereby avoid flash cooling. Incidentally, flash cooling by pressure-release evaporation is a technique in which a liquid to be treated is introduced into an environment having a pressure lower than that of the liquid to be introduced to thereby regulate the boiling point of the liquid as measured at the pressure of this environment so as to be not higher than the temperature of the liquid before introduction. It means that the liquid is partly vaporized and the remainder is cooled to the boiling point as measured at the pressure. [0036] When a condensate of a crystallization vapor is obtained in the crystallization step (d) and is sent to the slurrying step (a) and used as a water solvent, then the operating conditions in the slurry ing step (a) are determined by the temperature in the washing step (f) . For maintaining the temperature of the washings, it is preferred to use an operating pressure not lower than the vapor pressure of the washings. [0037] In the case where the cooling in the crystallization step (d) is cooling by pressure-release evaporation, the condensate of water vapor G obtained as a distillate in the crystallization step (d) (crystallization vapor condensate) may be sent, in the state of having a temperature of from 120°C to 200°C, to the slurrying step (a) and used as part of the water solvent. Water B may be freshly added in order to control the total feed amount of the water solvent. For condensing the water vapor G, known methods can be used. In general, however, a multitubular condenser is preferred. It is preferred to use the heat of this condensation as an energy for heating in the dissolution step (b). [0038] The slurry C obtained in the slurrying step (a) has a slurry concentration of 10-50% by weight, preferably 20-40% by weight. [0039] The operating temperature of the slurrying vessel 11 for slurrying the crude terephthalic acid A is desirably from 12OC to 200°C, more desirably from 130°C to 180°C, even more desirably from 130°C to 170°C. This operating temperature is determined from the temperature of the water solvent to be supplied. The purpose of keeping the washings M and/or the condensate of the water vapor G in the state of having a temperature of from 120°C to 200°C is to control the slurrying vessel 11 so as to have a temperature higher than those heretofore in use. [0040] The operating pressure of the slurrying vessel 11 should be not lower than the vapor pressure of water as measured at the operating temperature thereof. It is desirably from 0.2 MPa to 2.1 MPa. This operating pressure is a pressure at which the slurry mother liquor in the slurrying vessel 11 can retain the liquid state, and should be higher than the vapor pressure of the slurry mother liquor. It is also necessary to set pressure conditions so as not to cause cavitation (bubble generation) within a pump lla for transporting the slurry to the dissolution step (b) . The pressure conditions are determined by various conditions including the liquid level in the slurrying vessel 11 and the specifications of the pump. [0041] In the dissolution step (b) , the slurry C obtained in the dissolution step (b) is pressurized and heated to dissolve the crude terephthalic acid contained in the slurry in the water solvent and thereby obtain an aqueous crude-terephthalic-acid solution D. This pressurizing is conducted under such conditions that the slurry is pressurized to a pressure higher than the vapor pressure of the heated aqueous solution, and the heating is conducted after the pressurizing. The term "vapor pressure of the heated water solvent" means the vapor pressure of the water solvent heated to a "temperature at which the crude terephthalic acid in the slurry C can be dissolved to give an aqueous solution" after the pressurizing. The "pressure higher than the vapor pressure of the heated water solvent" preferably is from 0.1 MPa to 1 MPa, more preferably from 0.2 MPa to 0.5 MPa. The "temperature at which the crude terephthalic acid in the slurry C can be dissolved to give an aqueous solution" is a temperature higher than the temperature at which the acid can be dissolved and which is calculated from the solubility of terephthalic acid in water. This temperature preferably is (theoretical temperature)+(0-20°C). [0042] An explanation is given on an example. The slurry C is pressurized to 2.8-11.8 MPa with a pump and this slurry C is heated to 230-320°C with a heat exchanger lib to completely dissolve the terephthalic acid in the water solvent. Since terephthalic acid has low solubility in water, it is generally necessary to heat the slurry C to or above a given temperature to thereby heighten the solubility. This heating temperature is determined by the slurry concentration in the slurrying step (a) . In case where the temperature is too low, the solubility is insufficient for complete dissolution. However, excessive heating is unnecessary when the slurry concentration and solubility are taken into account. Excessive heating is undesirable from the standpoint of reactions because it presents a possibility that side reactions might be accelerated. Furthermore, excessive heating leads to an increase in apparatus cost. With respect to pressure, it is necessary to elevate the pressure to above the water vapor pressure in order to keep the water solvent in the liquid state while keeping the slurry C in the heated state. However, elevating the pressure to an unnecessarily high degree leads to an increase in apparatus cost. [0043] Specifically, a method is used in which the slurry C obtained in the slurrying step (a) is pressurized to a given pressure with a pump and heated to a given temperature with a heat exchanger lib. Although steam, heat medium oil, or the like is usable as a heat source for the heat exchanger lib, it is desired to utilize the crystallization vapor generated in the crystallization step (d) which will be described later. [0044] In the subsequent reduction step (c) / the aqueous crude-terephthalic-acid solution D obtained in the dissolution step (b) is sent to a hydrogenation reactor 13 and catalytically reduced in the presence of a catalyst with hydrogen E introduced to thereby obtain a liquid reduction reaction mixture F. The catalyst and the conditions in the hydrogenation reactor 13 should be ones which reduce the 4-carboxybenzaldehyde and do not reduce the terephthalic acid. This is intended to reduce the 4-carboxybenzaldehyde contained in the aqueous solution D to p-toluic acid, which has higher solubility in water than the terephthalic acid. It is desirable that this reduction reaction should be conducted to the highest possible level. This reduction reaction also is well known. As the catalyst may be used a catalyst comprising a Group 8-10 (according to TUPAC Nomenclature of Inorganic Chemistry, revised edition (1998)) metal such as ruthenium, rhodium, palladium, platinum, or osmium. Usually, such a catalyst is supported on a support, e.g. , activated carbon, and used as a fixed bed. Preferred of those is palladium supported on activated carbon. The temperature in the reduction reaction is generally 260-320°C, preferably 270-300°C, and the partial pressure of hydrogen is generally 0.5-20 Jtg/cm2G. [0045] Furthermore, in the crystallization step (d) , the liquid reduction reaction mixture F obtained in the reduction step (c) is introduced into a crystallization vessel 14. The temperature is reduced to such a degree that the p-toluic acid remains dissolved. Thus, high-purity terephthalic acid is crystallized to obtain a high-purity terephthalic acid slurry H. Specifically, the reaction mixture F is cooled to 120-200°C, preferably 130-180°C, to crystallize high-purity terephthalic acid. Too low temperatures are undesirable from the standpoint of quality because p-toluic acid forms a eutectic to increase the p-toluic acid concentration in the high-purity terephthalic acid. On the other hand, too high temperatures result in a decrease in the amount of terephthalic acid precipitated and, hence, in a decrease in the amount of high-purity terephthalic acid obtained (recovered) . In addition, the impurity content in the high-purity terephthalic acid is reduced to an unnecessarily low level, resulting in a decrease in production efficiency- It is more desirable that two or more crystallization vessels 14 be arranged in series to cool the reaction mixture F by pressure-release evaporation in stages to thereby crystallize the terephthalic acid. Although two crystallization vessels 14 have been disposed in Fig. 1, the number thereof may be 1 or may be 3 or larger. [0046] The crystallization vessels 14 desirably are pressure-release evaporation vessels because the energy possessed by the liquid reduction reaction mixture F can be effectively utilized therewith. The pressure-release evaporation vessels are vessels regulated so as to have a pressure which is lower than the pressure of the liquid to be introduced thereinto and at which the boiling point of the main component of the liquid is not higher than the temperature of the liquid before introduction. When a liquid is introduced into this pressure-release evaporation vessel, pressure-release evaporation occurs in which part of the liquid vaporizes and the remainder of the liquid is cooled to the boiling point of the liquid as measured at the pressure after the change. In this operation, when the liquid is a solution, the solute in the liquid remaining unvaporized crystallizes in an amount by which the solute amount exceeds the solubility after the cooling. [0047] The temperature in the final crystallization vessel may be controlled so as to result in temperature conditions under which the p-toluic acid does not form a eutectic with the terephthalic acid. Specifically, the temperature is desirably from 120°C to 200°C, more desirably from 130°C to 180°C. The pressure in this case is desirably from 0.20 MPa to 1.56 MPa, more desirably from 0.27 MPa to 1.00 MPa. In case where the temperature is too low, not only the terephthalic acid crystallizes but also the p-toluic acid also is apt to crystallize therewith and there is hence a possibility that the high-purity terephthalic acid obtained might have a reduced purity. On the other hand, in case where the temperature is too high, the amount of the terephthalic acid obtained by crystallization is reduced, resulting in an impaired efficiency. [0048] Besides the use of the crystallization vessels 14 in the crystallization step (d) , use may be made of a method in which the reaction mixture F is gradually cooled by heat exchange or another technique to thereby crystallize the terephthalic acid. [0049] When the pressure-release evaporation vessels are employed as the crystallization vessels 14, part of the water solvent of the liquid reduction reaction mixture F is vaporized as water vapor G by the pressure-release evaporation. This water vapor G is condensed with a heat exchanger 14b, and this water in the state of condensate is introduced into the slurrying vessel 11 in the slurrying step (a) and used as the water solvent. This method is more desirable because the amount of water B to be used can be reduced and the energy for heating it can be saved. In this method, the temperature of the condensate which is being introduced into the slurrying vessel 11 may be from 120°C to 200°C, and is preferably from 130°C to 180°C. The operating pressure in the slurrying step (a) in this operation should be not lower than the vapor pressure of the condensate. [0050] Subsequently, in the separation step (e) , the slurry H is introduced into a solid-liquid separator 15 to separate a reduction-reaction mother liquor I and obtain a high-purity terephthalic acid cake J. Examples of the solid-liquid separator 15 include a centrifugal separator and a filter. Especially preferred is a centrifugal separator. The operating conditions for the solid-liquid separator 15 include a. pressure not lower than the vapor pressure of the mother liquor of the high-purity terephthalic acid slurry obtained in the crystallization step (d) and a state in which the temperature of the high-purity terephthalic acid slurry is substantially maintained. The term "pressure not lower than the vapor pressure of the mother liquor of the high-purity terephthalic acid slurry obtained in the crystallization step (d)" means to control the pressure so as not to decrease by maintaining the operating pressure of the final crystallization vessel or adding a slight pressure. The term "state in which the temperature of the high-purity terephthalic acid slurry is substantially maintained* means that when the high-purity terephthalic acid slurry obtained in the crystallization step (d) is sent to the separation step (e) , the temperature of the slurry is maintained although there are cases where the slurry suffers pressure release due to a pressure loss caused by the piping, apparatus, valves, etc. and decreases in temperature due to the pressure release or other factors. Specifically, it is desirable that the pressure be elevated beforehand to a value higher than the final crystallization pressure ^V 0.05-1 MPa with a pump 14a in order to inhibit the slurry from being cooled by such pressure release. In case where the pressure in the solid-liquid separator 15 is lower than the pressure in the final crystallization vessel or the final crystallization pressure in the crystallization step (d) , the slurry in the solid-liquid separator 15 comes to have a reduced temperature. There is hence a possibility that part of the terephthalic acid and p-toluic acid dissolved in the slurry mother liquor might crystallize within the solid-liquid separator 15 to arouse a trouble in the operation stability of solid-liquid separation. [0051] Furthermore, the washing step (f) is conducted, in which the high-purity terephthalic acid cake J is washed with a cleaning fluid K in a washing apparatus 16, This cleaning fluid K desirably is water. The washed cake N which has been washed is taken out, and washings M comprising the ingredient of the cleaning fluid K as a major component are discharged. The operating conditions in the washing step (f) include a state in which the operating pressure in the separation step (e) is substantially maintained. The term "state in which the operating pressure in the separation step (e) is substantially maintained" means that the operating pressure in the separation step (e) is maintained without purposely changing the pressure although a pressure decrease occurs due to a pressure loss in the piping and apparatus. The temperature of the cleaning fluid K with which the high-purity terephthalic acid cake J is washed is a temperature which is not lower than the operating temperature in the separation step (e) and is lower than the boiling point of water as measured at the operating pressure in the separation step (e) . In case where the temperature of the cleaning fluid K is lower than the operating temperature in the separation step (e) , this cleaning fluid K lowers the washing operation temperature and this may lead to scaling in the apparatus. In addition, the washings also have a lowered temperature and this reduces the effect of the invention. On the other hand, in case where the operating temperature in the separation step (e) is not lower than the boiling point of water as measured at the operating pressure in the separation step (e) , flashing occurs in the washing apparatus, resulting in a decrease in washing function. Specifically, the operating conditions for the washing step (f) include a state in which a temperature of 120-200°C is maintained at a pressure not lower than the vapor pressure of the high-purity terephthalic acid slurry H obtained in the crystallization step (d) . This state preferably is the same as in the separation step (e). [0052] In the invention, the separation step (e) and the washing step (f) may be conducted with a solid-liquid separation/washing apparatus (corresponding to the part 18 surrounded by the broken line in the figure) in which the two steps can be simultaneously conducted. Examples of such a solid-liquid separation/washing apparatus include a screen bowl type centrifugal separator, rotary vacuum filter, and horizontal belt filter. Especially preferred is a screen bowl type centrifugal separator. The operating temperature in the case of simultaneously conducting the separation step (e) and the washing step (f) may be the same as that of the final crystallization vessel in the crystallization step (d) , and is desirably from 120°C to 200°C, more desirably from 130°C to 180°C. On the other hand, the pressure therein should be higher than the pressure in the final crystallization vessel in the crystallization step (d) in order to inhibit a temperature decrease caused by pressure release. Specifically, it is desirable that the pressure in the apparatus be higher by 0.05-1 MPa than the pressure in the final crystallization vessel in the crystallization step (d) . In case where the reaction mixture which has undergone crystallization in the final crystallization vessel further undergoes a pressure reduction, cooling by pressure-release evaporation occurs and ingredients dissolved in the mother liquor may precipitate. This may arouse a trouble in the separation step (e) in the solid-liquid separation/washing apparatus. On the other hand, to heighten the pressure to an unnecessarily high degree requires the apparatus to be reinforced so as to withstand it. [0053] The washed cake N thus obtained is dried with a dryer 19 in the drying step (g) . By the drying, the adherent liquid remaining in the washed cake N is removed, whereby high-purity terephthalic acid crystals O can be obtained. Examples of the dryer 19 to be employed here include a steam tube dryer and a fluidized-layer dryer. Two or more dryers 19 may be employed in combination. It is also possible to use a method in which the washed cake N having a high temperature and a high pressure is released from the pressure to thereby vaporize at least part of the solvent ingredient adherent to the cake. Drying with the dryer 19 is conducted in the presence of a flowing gas at a drying outlet operating temperature of 70-180°C using steam or the like as a heat source. [0054] On the other hand, the reduction-reaction mother liquor I separated, which consists mainly of a water solvent, still contains effective ingredients therein. The term effective ingredients means terephthalic acid and other compounds convertible into terephthalic acid through, e.g., oxidation/reduction in terephthalic acid production steps and purification steps, and includes both dissolved ingredients and precipitated ingredients. The term solid matter means precipitated ingredients among the effective ingredients. In case where this reduction-reaction mother liquor I is discarded as it is, the effective ingredients are wasted. On the other hand, in case where the mother liquor I is returned as it is to any of the steps described above, there is a possibility that part of the effective ingredients might accumulate as an impurity in the system. It is therefore desirable that the organic ingredients contained in the reduction-reaction mother liquor I be removed at least partly and this mother liquor be then returned to the slurrying vessel 11 in the slurrying step (a) optionally after it is further subjected to some treatment. For example, it is desirable to use a method which comprises employing a cooling vessel 20 to precipitate the effective ingredients and thereby obtain a cooled slurry P, subjecting this slurry P to solid-liquid separation with a solid-liquid separator 21, subsequently removing the solid matter Q, further treating the residual liquid matter R to obtain pure water, and using this pure water in place of the cleaning fluid K in the washing step (f) or together with the cleaning fluid K. It is also desirable that the solid matter Q, which includes the effective ingredients, be returned to an oxidation reaction step for producing crude terephthalic acid. [0055] There are cases where the reduction-reaction mother liquor I separated contains metallic ingredients (examples include cobalt and manganese) derived from the catalyst used for the oxidation reaction for producing the crude terephthalic acid. At least part of these metallic ingredients may be taken out with an ion-exchange resin from the reduction-reaction mother liquor I or from the cooled slurry P in which the effective ingredients have been precipitated with the cooling vessel 20 or the like. Thus, the loss of catalyst metals can be diminished. [0056] Furthermore, the washings M, which are discharged from the solid-liquid separator 17 or the solid-liquid separation/washing apparatus 18 and comprise the ingredient of the cleaning fluid K as a major component, also contain the effective ingredients. Especially when the united solid-liquid separation/washing apparatus 18 is employed to conduct filtration and washing as in a screen bowl type centrifugal separator, then the effective Ingredients are apt to come as the solid matter. In case where the resultant washings M are reused as they are as a solvent in the slurring step (a) , this leads to an increase in equipment size according to the increase in solid matter amount. It is therefore desirable that the washings M be used as the water solvent in the slurrying step (a) after the solid matter is removed therefrom. It should, however, be noted that since the slurrying vessel 11 in the slurrying step (a) is desirably operated in an environment of from 120°C to 200°C, it is necessary that the washings M discharged from the solid-liquid separator 17 or solid- liquid separation/washing apparatus 18 should be made to retain a temperature of from 120 °C to 200°C and used as the water solvent in the slurrying step (a) . In this case, it is desirable to not only keep the washings M in that temperature range but also minimize the temperature decrease. The purpose of this is to effectively utilize the heat energy possessed by the washings M. [0057] By thus using the washings M as the water solvent while maintaining the temperature thereof, the slurrying vessel 11 in the slurrying step (a) can be operated at a higher temperature and a higher pressure than those heretofore in use. Consequently, the energy required for obtaining a temperature and pressure necessary for dissolving the slurry C in the subsequent dissolution step (b) can be reduced. This leads to an improvement in productivity in producing high-purity terephthalic acid. [0058] 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. [0059] This application is based on a Japanese patent application filed on June 28, 2004 (Application No. 2004- 189158), the contents thereof being herein incorporated by reference. Industrial Applicability [0060] The invention can provide a process for producing high-purity terephthalic acid in which the energy possessed by the condensate obtained as a distillate in crystallization and by washings discharged after washing in purification steps for producing high-purity terephthalic acid is inhibited from being lost and which attains a higher energy efficiency- CLAIMS [1] A process for producing high-purity terephthalic acid which comprises a slurrying step (a) in which crude terephthalic acid containing 4-carboxybenzaldehyde is supplied to a water solvent to obtain a slurry having a slurry concentration of 10-50% by weight, a dissolution step (b) in which the slurry is pressurized and heated to dissolve the crude terephthalic acid contained in the slurry in the water solvent and thereby obtain an aqueous crude-terephthalic-acid solution, the pressurizing being conducted under such conditions that the slurry is pressurized to a pressure higher than the vapor pressure of the heated aqueous solution, the heating being conducted after the pressurizing, a reduction step (c) in which the aqueous solution is brought into contact with hydrogen in the presence of a catalyst to reduce the 4-carboxybenzaldehyde contained in the aqueous solution and thereby obtain a liquid reduction reaction mixture, a crystallization step (d) in which the liquid reduction reaction mixture is cooled to 120-200°C to cause crystals of high-purity terephthalic acid to separate out and thereby obtain a high-purity terephthalic acid slurry, a separation step (e) in which the high-purity terephthalic acid slurry obtained in the crystallization step (d) is subjected to solid-liquid separation while substantially maintaining the temperature of the high-purity terephthalic acid slurry at a pressure not lower than the vapor pressure of the mother liquor of the slurry to thereby obtain a high-purity terephthalic acid cake and a reduction-reaction mother liquor, a washing step (f) in which the high-purity terephthalic acid cake is washed, while substantially maintaining the operating pressure in the separation step (e) , with a cleaning fluid having a temperature which is not lower than the operating temperature in the separation step (e) and is lower than the boiling point of water as measured at the operating pressure in the separation step (e) , and a drying step (g) in which the cake washed is dried to obtain crystals of high-purity terephthalic acid, wherein washings discharged after washing in the washing step (f) are sent, in the state of having a temperature of from 120°C to 200°C, to the slurrying step (a) and used as a water solvent. [2] The process for producing high-purity terephthalic acid as claimed in claim 1, wherein the washings discharged after washing in the washing step (f) are sent to the slurrying step (a) conducted at a pressure not lower than the vapor pressure of the cleaning fluid. [3] The process for producing high-purity terephthalic acid as claimed in claim 1 or 2, wherein the cooling in the crystallization step (d) is cooling by pressure-release evaporation, and a condensate of a crystallization vapor is obtained in the crystallization step (d) and this condensate in the state of having a temperature of from 120°C to 200°C is sent to the slurrying step (a) and used as a water solvent. [4] The process for producing high-purity terephthalic acid as claimed in claim 3, wherein the condensate of a crystallization vapor obtained in the crystallization step (d) is sent to the slurrying step (a) conducted at a pressure not lower than the vapor pressure of the condensate. [5] The process for producing high-purity terephthalic acid as claimed in any one of claims 1 to 4, wherein the operating temperature in the slurrying step (a) is from 120°C to 200°C and the operating pressure therein is from 0.2 MPa to 2.1 MPa. [6] The process for producing high-purity terephthalic acid as claimed in any one of claims 1 to 5, wherein the reduction-reaction mother liquor obtained in the separation step (e) is sent to the washing step (f) after the metallic ingredients and/or organic ingredients contained in the reduction-reaction mother liquor are removed therefrom at least partly, and is used as part or all of the cleaning fluid. [7] A process for producing high-purity terephthalic acid substantially as herein described with reference to the foregoing examples and accompanying drawing.

Full Text

DESCRIPTION
PROCESS FOR PRODUCING HIGH-PURITY TEREPHTHALIC ACID Technical Field [0001]
The present invention relates to a process for producing high-purity terephthalic acid. Background Art [0002]
For obtaining crystals of high-purity terephthalic acid, it is necessary that intermediate impurities which have generated as by-products of the oxidation of p-xylene and include 4-carboxybenzaldehyde as a major component should be removed as much as possible from crude terephthalic acid crystals obtained by the oxidation. For this purpose, purification steps are conducted in which the 4-carboxybenzaldehyde is reduced to p-toluic acid, which has higher water solubility than terephthalic acid, and terephthalic acid crystallization is then conducted to obtain high-purity terephthalic acid crystals. [0003]
First, in a slurrying step (a1), the crude terephthalic acid is mixed with a water solvent in a slurrying vessel to obtain a slurry. Fresh water or recovery water discharged from the purification steps and comprising water as the main component is used as the water solvent. Usually, the slurry concentration is 20-30% by
weight. In low-temperature regions, the terephthalic acid which is the main component of the crude terephthalic acid a has insufficient solubility in water. Because of this, the slurry obtained in the slurring vessel is heated to about 250-350°C to dissolve the terephthalic acid. In order to maintain the liquid state in the high-temperature region, the slurry is pressurized beforehand to or above the water vapor pressure as measured at the final heating temperature. Specifically, the slurry is pressurized with a pump and then heated with a preheater to dissolve the crude terephthalic acid and obtain an aqueous solution. [0004]
A reduction step (b') is then conducted in which the aqueous solution obtained by dissolving the terephthalic acid under a condition of a high temperature and a high pressure is introduced into a hydrogenation reactor and brought into contact with hydrogen in the presence of a catalyst to thereby reduce the 4-carboxybenzaldehyde into p-toluic acid. The liquid reduction reaction mixture thus obtained, which contains p-toluic acid and terephthalic acid as major ingredients, is subjected to a crystallization step (cf), in which the reaction mixture is introduced into crystallization vessels to cause crystallization. Since terephthalic acid has lower water solubility than p-toluic acid, high-purity crystals of terephthalic acid can be separated out in this
step by controlling crystallization conditions. In the case where pressure-release evaporation vessels are employed as the crystallization vessels in this step, the water vapor generated as a distillate is recovered and used as the water solvent. Furthermore, the heat energy possessed by the vapor generated can be used for the dissolution. [0005]
Although crystallization by pressure-release evaporation is employed here, pressure release to atmospheric pressure and cooling to 100°C result in the crystallization of" not only terephthalic acid but also p-toluic acid and, hence, make it impossible to obtain high-purity terephthalic acid crystals. It is therefore necessary to control the pressure of the reaction mixture to a pressure higher than atmospheric pressure so as to result in a final crystallization temperature of 120°C to 180°C, preferably 130°C to 170°C. [0006]
A separation step (d1) is subsequently conducted, in which the crystallization slurry containing the high-purity terephthalic acid crystals which have separated out is introduced into a solid-liquid separator. The slurry is subjected to solid-liquid separation in which the slurry in a pressurized state is separated into a high-purity terephthalic acid cake j and a reduction-reaction mother
liquor. This high-purity terephthalic acid cake is subjected to a washing step (e1), in which a washing operation with a cleaning fluid is performed in a washing apparatus and solid-liquid separation is conducted again to obtain a washed cake. Furthermore, a drying step (f) is conducted in which the washed cake obtained is dried with a dryer to obtain high-purity terephthalic acid crystals.
[0007]
On the other hand, since the reduction-reaction mother liquor discharged from the separation step (d1) still contains effective ingredients including terephthalic acid and p-toluic acid, these effective ingredients are recovered as much as possible. The solvent ingredient is reused as the water solvent either directly or indirectly after some treatment. This can be accomplished by introducing the reduction-reaction mother liquor into a cooling vessel, precipitating the effective ingredients as a solid matter, returning the solid matter to the p-xylene oxidation step for yielding crude terephthalic acid, removing the metallic ingredients, organic substances, and other effective ingredients dissolved in the residual liquid by ion-exchange or adsorption or with a membrane, etc., and then reusing the liquid as a water solvent in the slurrying vessel in the slurrying step (a') or as a cleaning fluid in the washing step (e?).
[0008]
Furthermore, the washings discharged from the washing step (e?) are supplied, either directly or after the removal of solid matters contained therein, as a water solvent to the slurrying vessel in the slurrying step (a')-[0009]
Moreover, an apparatus for conducting both the separation step (d1) and the washing step (e1) therein may be employed to conduct solid-liquid separation and washing while maintaining a pressurized state. Thus, step simplification can be attained. [0010]
Examples of documents which describe a process for terephthalic acid production employing such a technique include patent document 1 (WO 93/24440). The washings obtained in this prior-art process are pressurized. [0011]
The washings are pressurized and have the same temperature as the final crystallization temperature. Although the distillate vapor condensate obtained in the crystallization step (c1) and the washings have heat energy as stated above, returning the condensate and the washings as they are to the slurrying step (a1) brings the slurrying vessel, which is operated at atmospheric pressure, into a boiling state. It is therefore necessary to subject the condensate and the washings to a cooling treatment before being supplied. In particular, it is necessary to adopt an
operating temperature at which the pump for sending the slurry from the slurrying vessel to the next step can be prevented from suffering cavitation (the phenomenon in which when the liquid flowing through the impeller is accelerated, the static pressure of the liquid locally decreases to or below the saturated water vapor pressure to form bubbles and this generates a high impact pressure to cause mechanical damage) . Because of this, the temperature in the slurrying vessel is generally controlled to about 90°C. [0012]
However, cooling the washings before supplying them to the slurrying vessel means a loss of heat energy and results in a poor efficiency because the slurry in the slurrying vessel should be heated to a given temperature for dissolution as stated above. Patent Document 1: WO 93/24440 Disclosure of the Invention Problem that the Invention is to Solve [0013]
An object of the invention is to provide a process for producing high-purity terephthalic acid in which the energy possessed by the condensate obtained as a distillate in crystallization and by washings discharged after washing in purification steps for producing high-purity terephthalic acid is inhibited from being lost and which
attains a higher energy efficiency. Means for Solving the Problem [0014]
The present inventors made intensive investigations in order to overcome the problem described above. As a result, they have found that the problem can be eliminated by keeping the temperature of washings discharged from a washing step high. The invention has been thus completed. Essential points of the invention reside in the following (1) to (6) . [0015]
(1) A process for producing high-purity terephthaiic acid which comprises
a slurrying step (a) in which crude terephthaiic acid containing 4-carboxybenzaldehyde is supplied to a water solvent to obtain a slurry having a slurry concentration of 10-50% by weight,
a dissolution step (b) in which the slurry is pressurized and heated to dissolve the crude terephthalic acid contained in the slurry in the water solvent and thereby obtain an aqueous crude-terephthalic-acid solution, the pressurizing being conducted under such conditions that the slurry is pressurized to a pressure higher than the vapor pressure of the heated aqueous solution, the heating being conducted after the pressurizing,
a reduction step (c) in which the aqueous solution
is brought into contact with hydrogen in the presence of a catalyst to reduce the 4-carboxybenzaldehyde contained in the aqueous solution and thereby obtain a liquid reduction reaction mixture,
a crystallization step (d) in which the liquid reduction reaction mixture is cooled to 120-200°C to cause crystals of high-purity terephthalic acid to separate out and thereby obtain a high-purity terephthalic acid slurry,
a separation step (e) in which the high-purity terephthalic acid slurry obtained in the crystallization step (d) is subjected to solid-liquid separation while substantially maintaining a pressure not lower than the vapor pressure of the mother liquor of the slurry and the temperature of the high-purity terephthalic acid slurry to thereby obtain a high-purity terephthalic acid cake and a reduction-reaction mother liquor,
a washing step (f) in which the high-purity
terephthalic acid cake is washed, while substantially
maintaining the operating pressure in the separation step
(e) , with a cleaning fluid having a temperature which is
not lower than the operating temperature in the separation
step (e) and is lower than the boiling point of water as
measured at the operating pressure in the separation step
(e) , and
a drying step (g) in which the cake washed is dried to obtain crystals of high-purity terephthalic acid,
wherein washings discharged after washing in the washing step (f) are sent, in the state of having a temperature of from 120°C to 200°C, to the slurrying step (a) and used as part or all of the water solvent. [0016]
(2) The process for producing high-purity terephthalic acid
as described under (1) above wherein the washings
discharged after washing in the washing step (f) are sent
to the slurrying step (a) conducted at a pressure not lower
than the vapor pressure of the cleaning fluid.
[0017]
(3) The process for producing high-purity terephthalic acid
as described under (1) or (2) above wherein the cooling in
the crystallization step (d) is cooling by pressure-release
evaporation, and a condensate of a crystallization vapor is
obtained in the crystallization step (d) and this
condensate in the state of having a temperature of from
120°C to 200°C is sent to the slurrying step (a) and used
as a water solvent.
[0018]
(4) The process for producing high-purity terephthalic acid
as described under (3) above wherein the condensate of a
crystallization vapor obtained in the crystallization step
(d) is sent to the slurrying step (a) conducted at a
pressure not lower than the vapor pressure of the condensate.
[0019]
(5) The process for producing high-purity terephthalic acid
as described under any one of (1) to (4) above wherein the
operating temperature in the slurrying step (a) is from
120°C to 200°C and the operating pressure therein is from
0.2 MPa to 2.1 MPa.
[0020]
(6) The process for producing high-purity terephthalic acid
as described under any one of (1) to (5) above wherein the
reduction-reaction mother liquor obtained in the separation
step (e) is sent to the washing step (f) after the metallic
ingredients and/or organic ingredients contained in the
reduction-reaction mother liquor are removed therefrom at
least partly, and is used as part or all of the cleaning
fluid.
Advantage of the Invention [0021]
The invention can provide a process for producing high-purity terephthalic acid in which the energy possessed by the condensate obtained as a distillate in crystallization and by washings discharged after washing in purification steps for producing high-purity terephthalic acid is inhibited from being lost and which attains a higher energy efficiency. Brief Description of the Drawing [0022]
[Fig. 1] Fig. 1 is a flow diagram illustrating an embodiment of the process for producing high-purity terephthalic acid according to the invention. Description of the Reference Numerals and Signs [0023]
11 slurrying vessel lla pump lib heat exchanger
13 hydrogenation reactor
14 crystallization vessel
14a pump
14b heat exchanger
15 solid-liquid separator
16 washing apparatus
17 solid-liquid separator
18 solid-liquid separation/washing apparatus
19 dryer
20 cooling vessel
21 solid-liquid separator
A crude terephthalic acid
B water
C slurry
D aqueous solution
E hydrogen
F liquid reduction reaction mixture
G water vapor
H high-purity terephthalic acid slurry
I reduction-reaction mother liquor
J high-purity terephthalic acid cake
K cleaning fluid
L washed slurry
M washings
N washed cake
O high-purity terephthalic acid crystals
P cooled slurry
Q solid matter
R liquid matter
Best Mode for Carrying Out the Invention
[0024]
The invention will be explained below in detail. [0025]
The process of the invention for producing high-purity terephthalic acid comprises
a slurrying step (a) in which crude terephthalic acid containing 4-carboxybenzaldehyde is supplied to a water solvent to obtain a slurry having a slurry concentration of 10-50% by weight,
a dissolution step (b) in which the slurry is pressurized and heated to dissolve the crude terephthalic acid contained in the slurry in the water solvent and thereby obtain an aqueous crude-terephthalic-acid solution, the pressurizing being conducted under such conditions that
the slurry is pressurized to a pressure higher than the vapor pressure of the heated aqueous solution, the heating being conducted after the pressurizing,
a reduction step (c) in which the aqueous solution is brought into contact with hydrogen in the presence of a catalyst to reduce the 4 -carboxybenzaldehyde contained in the aqueous solution and thereby obtain a liquid reduction reaction mixture,
a crystallization step (d) in which the liquid reduction reaction mixture is cooled to 120-200°C to cause crystals of high-purity terephthalic acid to separate out and thereby obtain a high-purity terephthalic acid slurry,
a separation step (e) in which the high-purity terephthalic acid slurry obtained in the crystallization step (d) Is subjected to solid-liquid separation while substantially maintaining a pressure not lower than the vapor pressure of the mother liquor of the slurry and the temperature of the high-purity terephthalic acid slurry to thereby obtain a high-purity terephthalic acid cake and a reduction-reaction mother liquor,
a washing step (f) in which the high-purity terephthalic acid cake is washed, while substantially maintaining the operating pressure in the separation step (e) , with a cleaning fluid having a temperature which is not lower than the operating temperature in the separation step (e) and is lower than the boiling point of water as
measured at the operating pressure in the separation step (e), and
a drying step (g) in which the cake washed is dried to obtain crystals of high-purity terephthalic acid,
the process being characterized in that washings discharged after washing in the washing step (f) are sent, in the state of having a temperature of from 120°C to 200°C, to the slurrying step (a) and used as part or all of the water solvent. [0026]
The term "high-purity terephthalic acid" in the invention means the terephthalic acid recovered after crude terephthalic acid is subjected to a reduction reaction treatment with hydrogenation. [0027]
The "crude terephthalic acid" in the invention is one obtained by oxidizing p-xylene in an acetic acid solvent. It contains, as by-products, intermediates in which one of the alkyl groups has not been completely oxidized, such as, e.g., 4-carboxybenzaldehyde (the intermediates are effective ingredients convertible to terephthalic acid but are impurities if they get mixed into a terephthalic acid product.) A general process for producing crude terephthalic acid is explained below. [0028]
Crude terephthalic acid is produced by oxidizing p-
xylene in a liquid phase with molecular oxygen in an acetic acid solvent in the presence of a catalyst. This step is well known. As the catalyst, use may be made of a catalyst which has been known to be usable in this reaction. Examples thereof include compounds of heavy metals, such as cobalt compounds, manganese compounds, iron compounds, and chromium compounds, and bromine compounds. In the reaction system, these compounds are present in a dissolved state. Preferred of these is a combination of a cobalt compound or manganese compound with a bromine compound. In this case, these compounds are generally used in such amounts that the amounts of cobalt atoms, manganese atoms, and bromine atoms are 10-5,000 ppm, 10-5,000 ppm, and 10-10,000 ppra, respectively, of the solvent.
[0029]
As the molecular oxygen is generally used a gas mixture comprising an inert gas and oxygen. For example, air or oxygen-enriched air is used. The molar proportion of the molecular oxygen to the p-xylene to be fed to the reactor is generally 3-20 times by mole, preferably 2-4 times by mole.
[0030]
The proportion of the p-xylene to the acetic acid to be supplied to the reactor is generally 1-50% by weight. The concentration of water in the reaction system is generally 5-20% by weight, preferably 5-15% by weight.
[0031]
The temperature in the oxidation reaction is generally 160-260°C, preferably 170-210°C. The reaction may be conducted at any pressure at which the reaction system can retain a liquid phase at the reaction temperature. The pressure is generally 0.5-5 MPa, preferably 1-2 MPa. The residence time is generally 10-200 minutes.
[0032]
Terephthalic acid is less apt to dissolve in the acetic acid serving as a solvent. Because of this, the terephthalic add yielded in the oxidation reaction step precipitates as crystals to form a slurry. However, terephthalic acid may be in a dissolved state depending on the amount of the solvent, reaction temperature, and pressure. In this case, a crystallization step for, e.g., cooling the liquid reaction mixture is conducted to precipitate the terephthalic acid and thereby form a slurry.
This slurry is subjected to solid-liquid separation to obtain crude terephthalic acid crystals. Although the terephthalic acid slurry obtained in the oxidation reaction step is in a pressurized state, this slurry may be subjected as it is to solid-liquid separation or may be subjected to pressure-release cooling or the like and then to solid-liquid separation. For the solid-liquid separation, any method may be used as long as the crystals
can be separated from the mother liquor. Examples thereof include filtration and centrifugal separation. According to need, washing and drying are conducted. Thus, crude terephthalic acid crystals are obtained. [0033]
The term "crude terephthalic acid" in the invention means terephthalic acid containing 4-carboxybenzaldehyde in an amount of 1,000-10,000 ppm. [0034]
For producing high-purity terephthalic acid, it is necessary that those intermediates should be removed as much as possible from the crude terephthalic acid. Steps for this removal are explained by reference to Fig. 1. [0035]
First, in the slurrying step (a), crude terephthalic acid A is supplied to a slurrying vessel 11 and slurried with a water solvent. It is necessary that washings M discharged after washing in the washing step (f) be used as the water solvent while keeping the washings M in the state of having a temperature of from 120°C to 200°C, preferably from 130°C to 180°C. The term washings in the invention means the liquid discharged from the washing step. For maintaining the temperature of the washings in supplying the washings to the slurrying step (a) , it is preferred that the operating pressure in the slurrying step (a) be controlled so as to be not lower than the vapor

pressure of the washings to thereby avoid flash cooling. Incidentally, flash cooling by pressure-release evaporation is a technique in which a liquid to be treated is introduced into an environment having a pressure lower than that of the liquid to be introduced to thereby regulate the boiling point of the liquid as measured at the pressure of this environment so as to be not higher than the temperature of the liquid before introduction. It means that the liquid is partly vaporized and the remainder is cooled to the boiling point as measured at the pressure. [0036]
When a condensate of a crystallization vapor is obtained in the crystallization step (d) and is sent to the slurrying step (a) and used as a water solvent, then the operating conditions in the slurry ing step (a) are determined by the temperature in the washing step (f) . For maintaining the temperature of the washings, it is preferred to use an operating pressure not lower than the vapor pressure of the washings. [0037]
In the case where the cooling in the crystallization step (d) is cooling by pressure-release evaporation, the condensate of water vapor G obtained as a distillate in the crystallization step (d) (crystallization vapor condensate) may be sent, in the state of having a temperature of from 120°C to 200°C, to the slurrying step
(a) and used as part of the water solvent. Water B may be freshly added in order to control the total feed amount of the water solvent. For condensing the water vapor G, known methods can be used. In general, however, a multitubular condenser is preferred. It is preferred to use the heat of this condensation as an energy for heating in the dissolution step (b). [0038]
The slurry C obtained in the slurrying step (a) has a slurry concentration of 10-50% by weight, preferably 20-40% by weight. [0039]
The operating temperature of the slurrying vessel 11 for slurrying the crude terephthalic acid A is desirably from 12O*C to 200°C, more desirably from 130°C to 180°C, even more desirably from 130°C to 170°C. This operating temperature is determined from the temperature of the water solvent to be supplied. The purpose of keeping the washings M and/or the condensate of the water vapor G in the state of having a temperature of from 120°C to 200°C is to control the slurrying vessel 11 so as to have a temperature higher than those heretofore in use. [0040]
The operating pressure of the slurrying vessel 11 should be not lower than the vapor pressure of water as measured at the operating temperature thereof. It is
desirably from 0.2 MPa to 2.1 MPa. This operating pressure is a pressure at which the slurry mother liquor in the slurrying vessel 11 can retain the liquid state, and should be higher than the vapor pressure of the slurry mother liquor. It is also necessary to set pressure conditions so as not to cause cavitation (bubble generation) within a pump lla for transporting the slurry to the dissolution step (b) . The pressure conditions are determined by various conditions including the liquid level in the slurrying vessel 11 and the specifications of the pump. [0041]
In the dissolution step (b) , the slurry C obtained in the dissolution step (b) is pressurized and heated to dissolve the crude terephthalic acid contained in the slurry in the water solvent and thereby obtain an aqueous crude-terephthalic-acid solution D. This pressurizing is conducted under such conditions that the slurry is pressurized to a pressure higher than the vapor pressure of the heated aqueous solution, and the heating is conducted after the pressurizing. The term "vapor pressure of the heated water solvent" means the vapor pressure of the water solvent heated to a "temperature at which the crude terephthalic acid in the slurry C can be dissolved to give an aqueous solution" after the pressurizing. The "pressure higher than the vapor pressure of the heated water solvent" preferably is from 0.1 MPa to 1 MPa, more preferably from
0.2 MPa to 0.5 MPa. The "temperature at which the crude terephthalic acid in the slurry C can be dissolved to give an aqueous solution" is a temperature higher than the temperature at which the acid can be dissolved and which is calculated from the solubility of terephthalic acid in water. This temperature preferably is (theoretical temperature)+(0-20°C). [0042]
An explanation is given on an example. The slurry C is pressurized to 2.8-11.8 MPa with a pump and this slurry C is heated to 230-320°C with a heat exchanger lib to completely dissolve the terephthalic acid in the water solvent. Since terephthalic acid has low solubility in water, it is generally necessary to heat the slurry C to or above a given temperature to thereby heighten the solubility. This heating temperature is determined by the slurry concentration in the slurrying step (a) . In case where the temperature is too low, the solubility is insufficient for complete dissolution. However, excessive heating is unnecessary when the slurry concentration and solubility are taken into account. Excessive heating is undesirable from the standpoint of reactions because it presents a possibility that side reactions might be accelerated. Furthermore, excessive heating leads to an increase in apparatus cost. With respect to pressure, it is necessary to elevate the pressure to above the water
vapor pressure in order to keep the water solvent in the liquid state while keeping the slurry C in the heated state. However, elevating the pressure to an unnecessarily high degree leads to an increase in apparatus cost. [0043]
Specifically, a method is used in which the slurry C obtained in the slurrying step (a) is pressurized to a given pressure with a pump and heated to a given temperature with a heat exchanger lib. Although steam, heat medium oil, or the like is usable as a heat source for the heat exchanger lib, it is desired to utilize the crystallization vapor generated in the crystallization step (d) which will be described later. [0044]
In the subsequent reduction step (c) / the aqueous crude-terephthalic-acid solution D obtained in the dissolution step (b) is sent to a hydrogenation reactor 13 and catalytically reduced in the presence of a catalyst with hydrogen E introduced to thereby obtain a liquid reduction reaction mixture F. The catalyst and the conditions in the hydrogenation reactor 13 should be ones which reduce the 4-carboxybenzaldehyde and do not reduce the terephthalic acid. This is intended to reduce the 4-carboxybenzaldehyde contained in the aqueous solution D to p-toluic acid, which has higher solubility in water than the terephthalic acid. It is desirable that this reduction
reaction should be conducted to the highest possible level.
This reduction reaction also is well known. As the
catalyst may be used a catalyst comprising a Group 8-10
(according to TUPAC Nomenclature of Inorganic Chemistry, revised edition (1998)) metal such as ruthenium, rhodium, palladium, platinum, or osmium. Usually, such a catalyst is supported on a support, e.g. , activated carbon, and used as a fixed bed. Preferred of those is palladium supported on activated carbon. The temperature in the reduction reaction is generally 260-320°C, preferably 270-300°C, and the partial pressure of hydrogen is generally 0.5-20 Jtg/cm2G.
[0045]
Furthermore, in the crystallization step (d) , the liquid reduction reaction mixture F obtained in the reduction step (c) is introduced into a crystallization vessel 14. The temperature is reduced to such a degree that the p-toluic acid remains dissolved. Thus, high-purity terephthalic acid is crystallized to obtain a high-purity terephthalic acid slurry H. Specifically, the reaction mixture F is cooled to 120-200°C, preferably 130-180°C, to crystallize high-purity terephthalic acid. Too low temperatures are undesirable from the standpoint of quality because p-toluic acid forms a eutectic to increase the p-toluic acid concentration in the high-purity terephthalic acid. On the other hand, too high
temperatures result in a decrease in the amount of terephthalic acid precipitated and, hence, in a decrease in the amount of high-purity terephthalic acid obtained (recovered) . In addition, the impurity content in the high-purity terephthalic acid is reduced to an unnecessarily low level, resulting in a decrease in production efficiency- It is more desirable that two or more crystallization vessels 14 be arranged in series to cool the reaction mixture F by pressure-release evaporation in stages to thereby crystallize the terephthalic acid. Although two crystallization vessels 14 have been disposed in Fig. 1, the number thereof may be 1 or may be 3 or larger. [0046]
The crystallization vessels 14 desirably are pressure-release evaporation vessels because the energy possessed by the liquid reduction reaction mixture F can be effectively utilized therewith. The pressure-release evaporation vessels are vessels regulated so as to have a pressure which is lower than the pressure of the liquid to be introduced thereinto and at which the boiling point of the main component of the liquid is not higher than the temperature of the liquid before introduction. When a liquid is introduced into this pressure-release evaporation vessel, pressure-release evaporation occurs in which part of the liquid vaporizes and the remainder of the liquid is
cooled to the boiling point of the liquid as measured at the pressure after the change. In this operation, when the liquid is a solution, the solute in the liquid remaining unvaporized crystallizes in an amount by which the solute amount exceeds the solubility after the cooling. [0047]
The temperature in the final crystallization vessel may be controlled so as to result in temperature conditions under which the p-toluic acid does not form a eutectic with the terephthalic acid. Specifically, the temperature is desirably from 120°C to 200°C, more desirably from 130°C to 180°C. The pressure in this case is desirably from 0.20 MPa to 1.56 MPa, more desirably from 0.27 MPa to 1.00 MPa. In case where the temperature is too low, not only the terephthalic acid crystallizes but also the p-toluic acid also is apt to crystallize therewith and there is hence a possibility that the high-purity terephthalic acid obtained might have a reduced purity. On the other hand, in case where the temperature is too high, the amount of the terephthalic acid obtained by crystallization is reduced, resulting in an impaired efficiency. [0048]
Besides the use of the crystallization vessels 14 in the crystallization step (d) , use may be made of a method in which the reaction mixture F is gradually cooled by heat exchange or another technique to thereby
crystallize the terephthalic acid. [0049]
When the pressure-release evaporation vessels are employed as the crystallization vessels 14, part of the water solvent of the liquid reduction reaction mixture F is vaporized as water vapor G by the pressure-release evaporation. This water vapor G is condensed with a heat exchanger 14b, and this water in the state of condensate is introduced into the slurrying vessel 11 in the slurrying step (a) and used as the water solvent. This method is more desirable because the amount of water B to be used can be reduced and the energy for heating it can be saved. In this method, the temperature of the condensate which is being introduced into the slurrying vessel 11 may be from 120°C to 200°C, and is preferably from 130°C to 180°C. The operating pressure in the slurrying step (a) in this operation should be not lower than the vapor pressure of the condensate. [0050]
Subsequently, in the separation step (e) , the slurry H is introduced into a solid-liquid separator 15 to separate a reduction-reaction mother liquor I and obtain a high-purity terephthalic acid cake J. Examples of the solid-liquid separator 15 include a centrifugal separator and a filter. Especially preferred is a centrifugal separator. The operating conditions for the solid-liquid
separator 15 include a. pressure not lower than the vapor pressure of the mother liquor of the high-purity terephthalic acid slurry obtained in the crystallization step (d) and a state in which the temperature of the high-purity terephthalic acid slurry is substantially maintained. The term "pressure not lower than the vapor pressure of the mother liquor of the high-purity terephthalic acid slurry obtained in the crystallization step (d)" means to control the pressure so as not to decrease by maintaining the operating pressure of the final crystallization vessel or adding a slight pressure. The term "state in which the temperature of the high-purity terephthalic acid slurry is substantially maintained** means that when the high-purity terephthalic acid slurry obtained in the crystallization step (d) is sent to the separation step (e) , the temperature of the slurry is maintained although there are cases where the slurry suffers pressure release due to a pressure loss caused by the piping, apparatus, valves, etc. and decreases in temperature due to the pressure release or other factors. Specifically, it is desirable that the pressure be elevated beforehand to a value higher than the final crystallization pressure ^V 0.05-1 MPa with a pump 14a in order to inhibit the slurry from being cooled by such pressure release. In case where the pressure in the solid-liquid separator 15 is lower than the pressure in the final crystallization vessel or the final crystallization
pressure in the crystallization step (d) , the slurry in the solid-liquid separator 15 comes to have a reduced temperature. There is hence a possibility that part of the terephthalic acid and p-toluic acid dissolved in the slurry mother liquor might crystallize within the solid-liquid separator 15 to arouse a trouble in the operation stability of solid-liquid separation. [0051]
Furthermore, the washing step (f) is conducted, in which the high-purity terephthalic acid cake J is washed with a cleaning fluid K in a washing apparatus 16, This cleaning fluid K desirably is water. The washed cake N which has been washed is taken out, and washings M comprising the ingredient of the cleaning fluid K as a major component are discharged.
The operating conditions in the washing step (f) include a state in which the operating pressure in the separation step (e) is substantially maintained. The term "state in which the operating pressure in the separation step (e) is substantially maintained" means that the operating pressure in the separation step (e) is maintained without purposely changing the pressure although a pressure decrease occurs due to a pressure loss in the piping and apparatus. The temperature of the cleaning fluid K with which the high-purity terephthalic acid cake J is washed is a temperature which is not lower than the operating
temperature in the separation step (e) and is lower than the boiling point of water as measured at the operating pressure in the separation step (e) . In case where the temperature of the cleaning fluid K is lower than the operating temperature in the separation step (e) , this cleaning fluid K lowers the washing operation temperature and this may lead to scaling in the apparatus. In addition, the washings also have a lowered temperature and this reduces the effect of the invention. On the other hand, in case where the operating temperature in the separation step (e) is not lower than the boiling point of water as measured at the operating pressure in the separation step (e) , flashing occurs in the washing apparatus, resulting in a decrease in washing function. Specifically, the operating conditions for the washing step (f) include a state in which a temperature of 120-200°C is maintained at a pressure not lower than the vapor pressure of the high-purity terephthalic acid slurry H obtained in the crystallization step (d) . This state preferably is the same as in the separation step (e). [0052]
In the invention, the separation step (e) and the washing step (f) may be conducted with a solid-liquid separation/washing apparatus (corresponding to the part 18 surrounded by the broken line in the figure) in which the two steps can be simultaneously conducted. Examples of
such a solid-liquid separation/washing apparatus include a screen bowl type centrifugal separator, rotary vacuum filter, and horizontal belt filter. Especially preferred is a screen bowl type centrifugal separator. The operating temperature in the case of simultaneously conducting the separation step (e) and the washing step (f) may be the same as that of the final crystallization vessel in the crystallization step (d) , and is desirably from 120°C to 200°C, more desirably from 130°C to 180°C. On the other hand, the pressure therein should be higher than the pressure in the final crystallization vessel in the crystallization step (d) in order to inhibit a temperature decrease caused by pressure release. Specifically, it is desirable that the pressure in the apparatus be higher by 0.05-1 MPa than the pressure in the final crystallization vessel in the crystallization step (d) . In case where the reaction mixture which has undergone crystallization in the final crystallization vessel further undergoes a pressure reduction, cooling by pressure-release evaporation occurs and ingredients dissolved in the mother liquor may precipitate. This may arouse a trouble in the separation step (e) in the solid-liquid separation/washing apparatus. On the other hand, to heighten the pressure to an unnecessarily high degree requires the apparatus to be reinforced so as to withstand it. [0053]
The washed cake N thus obtained is dried with a dryer 19 in the drying step (g) . By the drying, the adherent liquid remaining in the washed cake N is removed, whereby high-purity terephthalic acid crystals O can be obtained. Examples of the dryer 19 to be employed here include a steam tube dryer and a fluidized-layer dryer. Two or more dryers 19 may be employed in combination. It is also possible to use a method in which the washed cake N having a high temperature and a high pressure is released from the pressure to thereby vaporize at least part of the solvent ingredient adherent to the cake. Drying with the dryer 19 is conducted in the presence of a flowing gas at a drying outlet operating temperature of 70-180°C using steam or the like as a heat source. [0054]
On the other hand, the reduction-reaction mother liquor I separated, which consists mainly of a water solvent, still contains effective ingredients therein. The term effective ingredients means terephthalic acid and other compounds convertible into terephthalic acid through, e.g., oxidation/reduction in terephthalic acid production steps and purification steps, and includes both dissolved ingredients and precipitated ingredients. The term solid matter means precipitated ingredients among the effective ingredients. In case where this reduction-reaction mother liquor I is discarded as it is, the effective ingredients
are wasted. On the other hand, in case where the mother liquor I is returned as it is to any of the steps described above, there is a possibility that part of the effective ingredients might accumulate as an impurity in the system. It is therefore desirable that the organic ingredients contained in the reduction-reaction mother liquor I be removed at least partly and this mother liquor be then returned to the slurrying vessel 11 in the slurrying step (a) optionally after it is further subjected to some treatment. For example, it is desirable to use a method which comprises employing a cooling vessel 20 to precipitate the effective ingredients and thereby obtain a cooled slurry P, subjecting this slurry P to solid-liquid separation with a solid-liquid separator 21, subsequently removing the solid matter Q, further treating the residual liquid matter R to obtain pure water, and using this pure water in place of the cleaning fluid K in the washing step (f) or together with the cleaning fluid K. It is also desirable that the solid matter Q, which includes the effective ingredients, be returned to an oxidation reaction step for producing crude terephthalic acid. [0055]
There are cases where the reduction-reaction mother liquor I separated contains metallic ingredients (examples include cobalt and manganese) derived from the catalyst used for the oxidation reaction for producing the crude
terephthalic acid. At least part of these metallic ingredients may be taken out with an ion-exchange resin from the reduction-reaction mother liquor I or from the cooled slurry P in which the effective ingredients have been precipitated with the cooling vessel 20 or the like. Thus, the loss of catalyst metals can be diminished. [0056]
Furthermore, the washings M, which are discharged from the solid-liquid separator 17 or the solid-liquid separation/washing apparatus 18 and comprise the ingredient of the cleaning fluid K as a major component, also contain the effective ingredients. Especially when the united solid-liquid separation/washing apparatus 18 is employed to conduct filtration and washing as in a screen bowl type centrifugal separator, then the effective Ingredients are apt to come as the solid matter. In case where the resultant washings M are reused as they are as a solvent in the slurring step (a) , this leads to an increase in equipment size according to the increase in solid matter amount. It is therefore desirable that the washings M be used as the water solvent in the slurrying step (a) after the solid matter is removed therefrom. It should, however, be noted that since the slurrying vessel 11 in the slurrying step (a) is desirably operated in an environment of from 120°C to 200°C, it is necessary that the washings M discharged from the solid-liquid separator 17 or solid-
liquid separation/washing apparatus 18 should be made to retain a temperature of from 120 °C to 200°C and used as the water solvent in the slurrying step (a) . In this case, it is desirable to not only keep the washings M in that temperature range but also minimize the temperature decrease. The purpose of this is to effectively utilize the heat energy possessed by the washings M.
[0057]
By thus using the washings M as the water solvent while maintaining the temperature thereof, the slurrying vessel 11 in the slurrying step (a) can be operated at a higher temperature and a higher pressure than those heretofore in use. Consequently, the energy required for obtaining a temperature and pressure necessary for dissolving the slurry C in the subsequent dissolution step
(b) can be reduced. This leads to an improvement in productivity in producing high-purity terephthalic acid.
[0058]
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.
[0059]
This application is based on a Japanese patent application filed on June 28, 2004 (Application No. 2004-
189158), the contents thereof being herein incorporated by
reference.
Industrial Applicability
[0060]
The invention can provide a process for producing high-purity terephthalic acid in which the energy possessed by the condensate obtained as a distillate in crystallization and by washings discharged after washing in purification steps for producing high-purity terephthalic acid is inhibited from being lost and which attains a higher energy efficiency-

CLAIMS
[1] A process for producing high-purity terephthalic acid which comprises
a slurrying step (a) in which crude terephthalic acid containing 4-carboxybenzaldehyde is supplied to a water solvent to obtain a slurry having a slurry concentration of 10-50% by weight,
a dissolution step (b) in which the slurry is pressurized and heated to dissolve the crude terephthalic acid contained in the slurry in the water solvent and thereby obtain an aqueous crude-terephthalic-acid solution, the pressurizing being conducted under such conditions that the slurry is pressurized to a pressure higher than the vapor pressure of the heated aqueous solution, the heating being conducted after the pressurizing,
a reduction step (c) in which the aqueous solution is brought into contact with hydrogen in the presence of a catalyst to reduce the 4-carboxybenzaldehyde contained in the aqueous solution and thereby obtain a liquid reduction reaction mixture,
a crystallization step (d) in which the liquid reduction reaction mixture is cooled to 120-200°C to cause crystals of high-purity terephthalic acid to separate out and thereby obtain a high-purity terephthalic acid slurry,
a separation step (e) in which the high-purity terephthalic acid slurry obtained in the crystallization
step (d) is subjected to solid-liquid separation while substantially maintaining the temperature of the high-purity terephthalic acid slurry at a pressure not lower than the vapor pressure of the mother liquor of the slurry to thereby obtain a high-purity terephthalic acid cake and a reduction-reaction mother liquor,
a washing step (f) in which the high-purity
terephthalic acid cake is washed, while substantially
maintaining the operating pressure in the separation step
(e) , with a cleaning fluid having a temperature which is
not lower than the operating temperature in the separation
step (e) and is lower than the boiling point of water as
measured at the operating pressure in the separation step
(e) , and
a drying step (g) in which the cake washed is dried to obtain crystals of high-purity terephthalic acid,
wherein washings discharged after washing in the washing step (f) are sent, in the state of having a temperature of from 120°C to 200°C, to the slurrying step (a) and used as a water solvent.
[2] The process for producing high-purity terephthalic acid as claimed in claim 1, wherein the washings discharged after washing in the washing step (f) are sent to the slurrying step (a) conducted at a pressure not lower than the vapor pressure of the cleaning fluid. [3] The process for producing high-purity terephthalic
acid as claimed in claim 1 or 2, wherein the cooling in the crystallization step (d) is cooling by pressure-release evaporation, and a condensate of a crystallization vapor is obtained in the crystallization step (d) and this condensate in the state of having a temperature of from 120°C to 200°C is sent to the slurrying step (a) and used as a water solvent.
[4] The process for producing high-purity terephthalic acid as claimed in claim 3, wherein the condensate of a crystallization vapor obtained in the crystallization step (d) is sent to the slurrying step (a) conducted at a pressure not lower than the vapor pressure of the condensate.
[5] The process for producing high-purity terephthalic acid as claimed in any one of claims 1 to 4, wherein the operating temperature in the slurrying step (a) is from 120°C to 200°C and the operating pressure therein is from 0.2 MPa to 2.1 MPa.
[6] The process for producing high-purity terephthalic acid as claimed in any one of claims 1 to 5, wherein the reduction-reaction mother liquor obtained in the separation step (e) is sent to the washing step (f) after the metallic ingredients and/or organic ingredients contained in the reduction-reaction mother liquor are removed therefrom at least partly, and is used as part or all of the cleaning fluid.
[7] A process for producing high-purity terephthalic acid substantially as herein described with reference to the foregoing examples and accompanying drawing.

Documents:

7716-DELNP-2006-Abstract-(29-08-2011).pdf

7716-DELNP-2006-Abstract.pdf

7716-delnp-2006-claims.pdf

7716-DELNP-2006-Correspondence Others-(28-11-2011).pdf

7716-DELNP-2006-Correspondence Others-(29-08-2011).pdf

7716-delnp-2006-Correspondence-Others-(31-03-2011).pdf

7716-DELNP-2006-Correspondence-Others-1.pdf

7716-delnp-2006-correspondence-others.pdf

7716-delnp-2006-description (complete).pdf

7716-DELNP-2006-Drawings-(29-08-2011).pdf

7716-delnp-2006-drawings.pdf

7716-DELNP-2006-Form-1.pdf

7716-delnp-2006-form-13.pdf

7716-delnp-2006-form-18.pdf

7716-delnp-2006-form-2.pdf

7716-delnp-2006-form-26.pdf

7716-DELNP-2006-Form-3-(28-11-2011).pdf

7716-delnp-2006-Form-3-(31-03-2011).pdf

7716-DELNP-2006-Form-3.pdf

7716-delnp-2006-form-5.pdf

7716-delnp-2006-Form13-(19-12-2006).pdf

7716-delnp-2006-pct-304.pdf

7716-delnp-2006-pct-308.pdf

7716-delnp-2006-pct-search report.pdf

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Patent Number

250036

Indian Patent Application Number

7716/DELNP/2006

PG Journal Number

48/2011

Publication Date

02-Dec-2011

Grant Date

30-Nov-2011

Date of Filing

19-Dec-2006

Name of Patentee

MITSUBISHI CHEMICAL CORPORATION

Applicant Address

14-1, SHIBA 4-CHOME, MINATO-KU, TOKYO 108-0014, JAPAN

Inventors:

#	Inventor's Name	Inventor's Address
1	FUKUI, KATSUHIKO	C/O MITSUBISHI CHEMICAL CORPORATION, 1-1, KUROSAKISHIROISHI, YAHATANISHI-KU, KITAKYUSHU-SHI,FUKUOKA 806-0004 JAPAN
2	OGATA, TOMOHIKO	C/O MITSUBISHI CHEMICAL CORPORATION, 1-1, KUROSAKISHIROISHI, YAHATANISHI-KU, KITAKYUSHU-SHI,FUKUOKA 806-0004 JAPAN
3	NUMATA, MOTOKI	MITSUBISHI CHEMICAL CORPORATION, 1-1, KUROSAKISHIROISHI, YAHATANISHI-KU, KITAKYUSHU-SHI,FUKUOKA 806-0004 JAPAN

PCT International Classification Number

C07C 51/487

PCT International Application Number

PCT/JP2005/011830

PCT International Filing date

2005-06-28

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2004-189158	2004-06-28	Japan