Title of Invention

"METHOD FOR PRODUCING AROMATIC CARBOXYLIC ACID AND METHOD FOR PRODUCING TEREPHTHALIC ACID"

Abstract

It is an object of the invention to provide a solid-liquid separation method of slurry for enforced filtration without crystal deposition of aromatic carboxylic acid in a filter material, the method never requiring the equipment for pressurizing the slurry. The invention provides a method for producing an aromatic carboxylic acid, including oxidizing an alkyl aromatic compound in a liquid phase in the presence of a catalyst in a solvent of acetic acid to generate the slurry of the aromatic carboxylic acid and crystallizing the slurry, and subsequently separating the solid from the liquid to recover the crystal of the aromatic carboxylic acid, wherein the final step of the crystallization is done at a boiling state under reduced pressure and the slurry is filtered under conditions such that the operation pressure on the side of the slurry is preset higher than that of the reduced pressure state at the following solid-liquid separation step to make the operation temperature less than the boiling point of the solvent.

Full Text

DESCRIPTION Method for producing aromatic carboxylic acid and method for producing terephthalic acid The present invention relates to a solid-liquid separation method for a crystal-containing slurry at a process of producing an aromatic carboxylic acid and a method for producing terephthalic acid. As a method for industrially producing an aromatic carboxylic acid such as terephthalic acid, a method is known, which comprises oxidizing an alkyl aromatic hydrocarbon such as para-xylene in an acetic acid solvent in the presence of a catalyst containing cobalt, manganese and bromine, with molecular oxygen in a liquid phase, to treat a slurry containing crystals of aromatic carboxylic acids such as terephthalic acid as generated thereby with a solid-liquid separator, to recover the crystals. An example of such solid-liquid separator is rotary vacuum filter. The apparatus works for sucking the inside of a cylindrical filter material while rotating the filter material, to sequentially progress the suction filtration process of such slurry and a rinsing process of the resulting cake. However, such rotary vacuum filter has a problem of ready clogging of the filter material (filter such as filter cloth) because when a part of a filtrate evaporates via reduced pressure to lower the filtrate temperature, dissolved impurities are deposited to deposit microfine powders. When the filter material is under way of clogging, the processing capacity is decreased, so that processing is finally impossible. In this case, the apparatus should be stopped and be treated by a rinsing procedure including permeating a rinse solution such as aqueous caustic soda solution into the filter material. When such rinsing procedure is frequently required, the productivity is lowered. As a method for decreasing the occasion requiring the rinsing procedure, a method is known, including a suction filtration process by pressurizing the slurry to maintain conditions of temperature and pressure never causing the supersaturation of the filtrate, to thereby suppress the crystal deposition of aromatic carboxylic acid in the filter material (see for example patent reference 1). [Patent reference 1] The official gazette of JP-A-1-299618 (the claims) However, the solid-liquid separation method of the related art as described above for slurries containing the crystal of aromatic carboxylic acid is problematic in that the method requires the equipment for appropriately pressurizing the outer peripheral face of rotary vacuum filter and the liquid face of such slurry. It is an object of the invention to overcome the problem and provide a solid-liquid separation method of slurry for highly efficient filtration without any crystal deposition of an aromatic carboxylic acid in such filter material, the method never requiring the equipment for pressurizing the slurry. So as to overcome the problem, in accordance with the invention, a method for producing an aromatic carboxylic acid is selected, the method including oxidizing an alkyl aromatic compound in a liquid phase in the presence of a catalyst in a solvent of acetic acid to generate the slurry of the aromatic carboxylic acid and crystallizing the slurry, and subsequently separating the solid from the liquid to recover the crystal of the aromatic carboxylic acid, where the final step of the crystallization is done at a boiling state under reduced pressure and the slurry is filtered under conditions such that the operation pressure on the side of the slurry is preset higher than that of the reduced pressure state at the following solid-liquid separation step to make the operation temperature less than the boiling point of the solvent. The filtration is preferably done in an enforced filtering manner at a difference in pressure between the front and back faces of the filter material being 0.01 to 0.1 MPa. At the solid-liquid separation step, the apparatus for passing the slurry under pressure through a filter material in an enforced manner can thereby be simplified. In such manner, highly efficient enforced filtration can be done, without any specific equipment for integrally pressurizing particularly the outer side of a filter material such as rotary vacuum filter and the slurry, aJjTA o^tjLe^z-tc namely at aojanal pressure so that no crystal deposition of aromatic carboxylic acid might occur in the filter material. Additionally when the pressure is adjusted (reduced) at the final crystallization step so that the operation temperature might be 50 to 100 °C, solid-liquid separation can be done at a state of no boiling at 100 °C or lower at the solid-liquid separation step at a higher pressure than the pressure. As a particularly preferable means of the method for producing terephthalic acid, the method of producing terephthalic acid includes sequentially carrying out the following steps (i) through (iv) and carrying out the steps (iii) and (iv) in the same apparatus: (i) a step of oxidizing para-xylene in the presence of a catalyst including cobalt, manganese and bromine in a solvent of acetic acid in a liquid phase at a pressure of 1 to 2 MPa to generate slurry containing terephthalic acid; (ii) a step of flush cooling the slurry generated at the previous step to crystallize terephthalic acid dissolved in the mother solution; (iii) a step of solid-liquid separation of the slurry of terephthalic acid obtained after the previous step by filtration; and (iv) a step of rinsing the cake filtered at the previous step in acetic acid and/or water, where the flush cooling at the step (ii) is done by reducing the pressure to less than atmospheric pressure using an ejector for cooling to 50 to 100 °C and the slurry is filtered under conditions such that the pressure at the steps (iii) and (iv) is preset higher than the pressure at the step (ii) to make the operation temperature less than the boiling point of the solvent. According to the method for producing terephthalic acid, further, the filtration at the step (iii) is preferably done in an enforced filtration manner at a difference in pressure between the front and back faces of the filter material being 0.01 to 0.1 MPa. The feeding temperature of acetic acid and/or water for use in the rinsing is preferably (T-20) °C or higher, more preferably (T) °C or higher provided that the temperature for the cooling at the step (ii) is defined as less than the boiling point of acetic acid and/or water at d the pressure at the time of the operation. At the step (iii), furthermore, solid-liquid separation is done under the feed of an inert gas, where the temperature of the inert gas is preferably higher than the temperature (T) °C for the cooling at the step (ii) . The upper limit of the temperature of the inert gas is less than the boiling point of the solvent contained in the terephthalic acid slurry at the pressure at the time of the operation. According to the method for producing high-purity terephthalic acid, the filtration at the step (iii) is preferably done by a solid-liquid separation procedure using rotary vacuum filter or horizontal belt filter. Fig.l is a systematic view depicting the solid-liquid separation steps of slurry in an embodiment. Herein, the symbols in the figure are as follows. 1 expresses crystallization tank; 2, flush valve; 3, feed tank of separator; 4, rotary vacuum filter; 6, slurry; 7, filter material; 7a, inside; 8, ejector; 9, condenser; 10, pump; 11, vacuum pump; 12, heater; 13, gas-liquid separator; 14, rinse solution feed line; and 15, inert gas feed line. A method for producing terephthalic acid using para-xylene is described in detail as a typical example of an embodiment for carrying out the invention. Alkyl aromatic compounds for use as raw materials in accordance with the invention are alkylbenezene such as mono-, di- and trialkylbenzene, which are converted to aromatic carboxylic acid such as aromatic monocarboxylic acid, aromatic dicarboxylic acid and aromatic tricarboxylic acid via liquid- phase oxidation. Alkylbenzenes with partially oxidized alkyl groups are also included in the alkyl aromatic compounds. The invention is particularly preferably applied to the production of terephthalic acid. In this case, an alkyl aromatic compound as a raw material is para-xylene. Aliphatic carboxylic solvents for use in accordance with the invention are preferably lower aliphatic carboxylic acids, namely acetic acid. The amount of the solvent to be used is generally 2- to 6-fold the weight of the raw material para-xylene. Acetic acid containing more or less water, specifically water at 10 % by weight or less may also be used as the solvent acetic acid. So as to oxidize alkyl aromatic compounds such as para-xylene, molecular oxygen-containing gas is used. Generally, air is used because air only requires simple equipment at low cost. Additionally, diluted air and oxygen-enriched air may also be used. The catalyst for the oxidation of alkyl aromatic compounds includes catalysts containing cobalt (Co) , manganese (Mn) and bromine (Br) as constitutional elements. Cobalt compounds as specific compounds of such catalyst components include for example cobalt acetate, cobalt naphthenate, and cobalt bromide. Manganese compounds include for example manganese acetate, manganese naphthenate, and manganese bromide. Bromine compounds include for example hydrogen bromide, sodium bromide, cobalt bromide, manganese bromide and bromoethane. The oxidation reaction is done by oxidizing alkyl aromatic compounds in the presence of a catalyst in the solvent acetic acid while continuously feeding molecular oxygen-containing gas at a temperature of 140 to 230 °C, preferably 150 to 210 °C. The reaction pressure is at least a pressure at which the resulting mixture can maintain the liquid phase at the reaction temperature or at a pressure higher than that. The pressure is generally 0.2 to 5 MPa, preferably 1 to 2 MPa. Additional oxidation is also possible. A step of subsequently lowering the additionally oxidized reaction slurry to an appropriate temperature and an appropriate pressure to obtain terephthalic acid slurry is a crystallization step. The crystallization step is preferably done in one to 6 steps, most preferably in 2 to 3 steps. Among the series of the crystallization steps, the final step is done at a boiling state under reduced pressure. As to the specific procedure for the boiling state under reduced pressure, flush cooling is preferable. As shown in the systematic view of the apparatus in Fig.l, the solid-liquid separation method of slurry in the embodiment includes a step of feeding slurry containing terephthalic acid in a solvent of acetic acid from flush valve 2 to final crystallization tank 1 at a boiling state under reduced pressure, and a subsequent solid-liquid separation step using rotary vacuum filter 4 via separator feed tank 3, where enforced filtration is done by setting the outer side of filter material 7 and slurry 6 to an cct-ryuo-X,1> jn2^-^o atmosphere at normal' pressure and setting the inside 7a of the filter material 7 where the slurry 6 is rotated via suction to atmospheric pressure or less, to scrape and recover the cake of terephthalic acid deposited on the filter material 7. In the figure, the symbol 8 expresses ejector; 9, condenser for recovering evaporating acetic acid solvent; and 10, pump. Additionally, 11 expresses vacuum pump; 12, heater; 13, gas-liquid separator; and 14, rinse solution feed line. The filter material is preferably formed of metal gauze (thin metal gauze) or a woven cloth of organic materials for example polyester resin, polypropylene resin, polyethylene ether ketone (PEEK) resin, polyvinylidene fluoride (PVDF) resin and polyphenylene sulfide (PPS) resin. Such filter material is preferably a filter material of an endless belt type, which rotates continuously or intermittently to generate a solid content (cake) containing terephthalic acid as the main component on the belt. When enforced filtration is done as described above, Ojbvv the surface of the filter material is kept at Rormal pressure, while the back face retains the state of reduced pressure in the final crystallization step, with the resulting difference in pressure, so that the pressure on the belt surface with slurry deposited thereon is higher than that on the belt back face. Pressurization if necessary can maintain the surface of the filter material pressure or higher. Preferably, the difference in pressure between the surface and back faces of the filter material is 0.01 to 0.1 MPa. The slurry feed temperature is at least 50 °C, preferably 60 to 100 °C, particularly preferably 70 to 90 °C. When the final crystallization temperature is a low temperature, advantageously, the amount of recovered aromatic carboxylic acid is increased. When the temperature is too low, however, an apparatus for reducing pressure at a high pressure reduction degree is required, disadvantageously. In such manner, the aromatic carboxylic acid slurry obtained through the crystallization step passes through the solid-liquid separation apparatus in a structure of the passage through the filter material (filter), so that the solvent is separated. Subsequently, the filtered cake is rinsed in the acetic acid solvent or water. The separation and rinse step may be done by using not only rotary vacuum filter but also for example belt filter with a filtering face along substantially horizontal direction. In the apparatus for use in the separation and rinse step, herein, the rinse solution is fed through the rinse solution feed line 14 to rinse the filtered cake. The temperature of the rinse solution is preferably a temperature of (T-20) °C or higher, more preferably a temperature of (T) °C or higher, provided that the temperature for cooling at the step (ii), namely the slurry temperature to be fed to the step (iii) is defined as (T) °C. In such manner, the decrease of the temperature of the filtered cake is suppressed, to prevent the crystallization of the mother solution attached on the filtered cake. In the apparatus for use in the separation and rinse step, further, a gas component is discharged, together with the separated solution, from the downstream of the filter material via vacuum pump 11 in case of filtration under suction. Therefore, inert gas is fed from the gas feed line 15 to the upstream of the filter material, so as to constantly maintain the pressure of the apparatus. As such inert gas, for example, nitrogen and process gas discharged at the step (i) are used. The temperature of the gas is preferably a temperature of (T) °C or higher, more preferably a temperature of (T + 10) °C or higher, provided that the temperature for cooling at the step (ii) , namely the slurry temperature to be fed to the step (iii) is defined as (T) °C. In such manner, the decrease of the operation temperature inside the apparatus can be suppressed. The upper limit of the gas temperature is not necessarily too high but is satisfactorily (T + 100) °C or lower, under the provision of (T) °C. [Example 1] Slurry containing terephthalic acid was generated by oxidizing para-xylene in a liquid phase in a solvent acetic acid in the presence of a catalyst containing Co/Mn/Br at a pressure of 1.5 MPa. Then, the slurry was flush-cooled to 90 °C while reducing the pressure to atmospheric pressure or lower (0.05 MPa) using an ejector. For solid-liquid separation of the resulting terephthalic acid slurry by filtration with rotary vacuum filter, the slurry at the boiling point or lower (containing terephthalic acid at 35 % by weight at 90 °C) was filtered in an enforced manner, while setting the side of the rotary vacuum filter in contact to the slurry to atmospheric pressure and sucking the side of the filtrate to the atmospheric pressure or lower (0.05 MPa), to separate crude terephthalic acid. Further, the resulting crude terephthalic acid was dissolved in an aqueous solvent at the next step. 4-Carboxybenzaldehyde in the dissolved material was reduced in the presence of a metal catalyst of the Group 8. The product from the reduction process was crystallized by flush evaporation, to thereby obtain the resulting slurry containing terephthalic acid crystal by solid-liquid separation. The solid content was dried to produce high-purity terephthalic acid. The solid-liquid separation was continuously done by filtration with the rotary vacuum filter. The time period until clogging was 55 hours. The results indicate that a time period capable of continuous filtration was of the same duration or longer as the time period in the general case of pressuring the slurry side of rotary vacuum filter to 0.25 MPa and sucking the filtrate side to 0.20 MPa. [Example 2] In the same manner as in Example 1, para-xylene was oxidized to produce terephthalic acid-containing slurry. The slurry (containing terephthalic acid at 35 % by weight at 91 °C) was filtered using a rotary vacuum filter in an enforced manner, to separate crude terephthalic acid. Then, the temperature of the gas fed to the upstream of the filter was 100 °C. The crude terephthalic acid was rinsed in the rotary vacuum filter apparatus using acetic acid at 92 °C (acetic acid of 30 parts by weight to 100 parts by weight of the crude terephthalic acid). Continuously, high-purity terephthalic acid was produced in the same manner as in Example 1. Solid-liquid separation was continuously done by filtration with the rotary vacuum filter. The time period until clogging was 55 hours. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. This application is based on Japanese patent application filed on January 10, 2003 (Patent Application No. 2003-004800), the entire contents thereof being hereby incorporated by reference. In accordance with the invention, the final step of crystallization is done at a boiling state under reduced pressure, while the following solid-liquid separation step is done under conditions such that the operation temperature is less than the boiling point of the solvent by setting the pressure higher than that at the final step of crystallization. Therefore, the solid-liquid separation step for filtering slurry in an enforced manner can be done at normal pros euro-;—&sr example about- atmospheric pressure. In such manner, enforced filtration can be done under conditions hardly causing the deposition of the crystal of aromatic carboxylic acid in the filter material, without any equipment for pressurizing the outer side of the filter material of a filtering apparatus and the slurry, so that the clogging of the filter material can be prevented to highly efficiently progress solid-liquid separation of slurry in a stable manner, advantageously. 7. Pages 16 to 19, please amend a set of claims as follows. WE CLAIM: 1. A method for producing an aromatic carboxylic acid, comprising oxidizing an alkylbenzenes, of the kind such as herein described in which a part of alkyl groups may be oxidised in a liquid phase in the presence of a catalyst containing cobalt, manganese and bromine in a solvent of acetic acid to generate the slurry of the aromatic carboxylic acid and crystallizing the slurry, subsequently separating the solid from the liquid to recover the crystal of the aromatic carboxylic acid, wherein the crystallization is done at a boiling state under reduced pressure by reducing the pressure to lower than atmospheric pressure and the slurry is filtered under conditions such that the pressure on the side of the slurry is preset higher than that of the reduced pressure state at the following solid-liquid separation step to make the operation temperature less than the boiling point of the solvent. 2. The method for producing an aromatic carboxylic acid as claimed in claim 1, wherein the pressure is adjusted at the final crystallization step so that the operation temperature might be 50 to 100 °C. 3. The method of producing an aromatic carboxylic acid as claimed in claim 1, wherein the pressure on the side of the slurry at the solid-liquid separation step is atmospheric pressure. 4. A method for producing an aromatic carboxylic acid as claimed in claim 1, wherein it comprises producing terephthalic acid comprising sequentially carrying out the following steps (i) through (iv) and carrying out the steps (iii) and (iv) in the same apparatus: (i) a step of oxidizing para—xylene in the presence of a catalyst including cobalt, manganese and bromine in a solvent of acetic acid in a liquid phase at a pressure of 1 to 2 MPa to generate slurry containing terephthalic acid; (ii) a step of flush cooling the slurry generated at the previous step to crystallize terephthalic acid dissolved in the mother solution; (iii) a step of solid-liquid separation of the slurry of terephthalic acid obtained after the previous step by filtration; and (iv) a step of rinsing the cake filtered at the previous step in acetic acid and/or water, wherein the flush cooling at the step (ii) is done by reducing the pressure to less than atmospheric pressure using an ejector for cooling to 50 to 100 °C and the slurry is filtered under conditions such that the pressure at the steps (iii) and (iv) is preset higher than the pressure at the step (ii) to make the operation temperature less than the boiling point of the solvent. 5. The method for producing terephthalic acid as claimed in claim 4, wherein the filtration at the step (iii) is done in an enforced filtration manner at a difference in pressure between the front and back faces of the filter material being 0.01 to 0.1 MPa. 6. The method for producing terephthalic acid as claimed in claim 4 wherein the feeding temperature of acetic acid and/or water for use in the rinsing is (T-20) °C or higher provided that the temperature for the cooling at the step (ii) is defined as (T) °C. 7. The method of producing terephthalic acid as claimed in claim 4 wherein the feeding temperature of acetic acid and/or water for use in the rinsing is (T) °C or higher provided that the temperature for the cooling at the step (ii) is defined as (T) °C 8. The method of producing terephthalic acid as claimed in claim 4, wherein the step (iii) is done for solid-liquid separation under the supply of an inert gas and the temperature of the inert gas is (T) °C or higher provided that the temperature for the cooling at the step (ii) is defined as (T) °C. 9. The method of producing terephthalic acid as claimed in claim 4, wherein the apparatus for use in the step (iii) is rotary vacuum filter or horizontal belt filter. 10. The method of producing terephthalic acid as claimed in claim 4, wherein the pressure on the side of the slurry at the solid-liquid separation step is atmospheric pressure.

Documents:

2892-delnp-2005-abstract.pdf

2892-DELNP-2005-Claims-13-05-2008.pdf

2892-delnp-2005-claims.pdf

2892-DELNP-2005-Correspondence-Others-13-05-2008.pdf

2892-delnp-2005-correspondence-others.pdf

2892-delnp-2005-description (complete)-13-05-2008.pdf

2892-delnp-2005-description (complete).pdf

2892-delnp-2005-drawings.pdf

2892-delnp-2005-form-1.pdf

2892-delnp-2005-form-18.pdf

2892-DELNP-2005-Form-2-13-05-2008.pdf

2892-delnp-2005-form-2.pdf

2892-DELNP-2005-Form-3-13-05-2008.pdf

2892-delnp-2005-form-3.pdf

2892-delnp-2005-form-5.pdf

2892-delnp-2005-gpa.pdf

2892-delnp-2005-pct-308.pdf

2892-delnp-2005-pct-notification.pdf

2892-delnp-2005-pct-search report.pdf