Process for Producing High Purity Isophthalic Acid

Europa,schesP_ MM M II M M MM I M I M Ml M Ml J European Patent Office _ ^ _ _ © Publication number: 0 465 100 B1 Office europeen- desj brevets^ .

@ Date of filing: 25.06.91

® Priority: 25.06.90 JP 164217/90 Inventor: Yoshida, Terumasa, Mizushima 23.04.91 JP 117896/91 Works of Mitsubishi Gas Chem. Co. Inc, @ Date of publication of application: 10 Mizushima Kaigantori 3-chome 08.01.92 Bulletin 92/02 Kurashiki-shi, Okayama-ken (JP) Inventor: Okoshi, Fumio, c/o Mizushima © Publication of the grant of the patent: Works of Mitsubishi 28.09.94 Bulletin 94/39 Gas Chem. Co. Inc, 10 Mizushima Kaigantori 3-chome © Designated Contracting States: Kurashiki-shi, Okayama-ken (JP) BE DE ES FR GB IT Inventor: Motoyama, Ichihei, Mizushima Works of Mitsubishi © References cited: Gas Chem. Co. Inc, EP-A- 0 021 747 10 Mizushima Kaigantori 3-chome GB-A- 1 555 246 Kurashiki-shi, Okayama-ken (JP) GB-A- 1 577 019 Inventor: Ohta, Tazuo, c/o Mitsubishi Gas US-A- 3 859 344 Chemical Co. Inc. 5-2 Marunouchi 2-chome © Proprietor: MITSUBISHI GAS CHEMICAL COM- Chiyoda-ku, Tokyo (JP) PANY, INC. Inventor: Abe, Toshiaki 5-2, Marunouchi 2-chome 38-3, Yokodai 6-chome, Chiyoda-Ku Isogo-ku Tokyo, 100 (JP) Yokohama-shi, Kanagawa-ken (JP)

00 @ Inventor: Tanaka, Kazuo, c/o Mizushima Works of Mitsubishi Gas Chem. Co. Inc, 10 Mizushima Kaigantori 3-chome Kurashiki-shi, Okayama-ken (JP) CO Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid (Art. 99(1) European patent convention). Rank Xerox (UK) Business Services (3. 10/3.09/3.3.3) EP 0 465 100 B1

0 Representative: Myerscough, Philip Boyd et al J.A. KEMP & CO. 14 South Square Gray's Inn London WC1R 5LX (GB)

2 EP 0 465 100 B1

Description

The present invention relates to a process for producing isophthalic acid by liquid phase oxidation of m- xylene. High purity isophthalic acid is useful as an intermediate material for polymers such as an 5 unsaturated polyester resin, an alkyd resin, a modified polyester fiber, heat-resistant polyamide, etc.

Prior Art of the Invention

As a process for producing an aromatic carboxylic acid, there is known a process in which an aromatic io hydrocarbon having an aliphatic substituent is subjected to liquid phase oxidation with molecular oxygen in a solvent of an aliphatic carboxylic acid such as acetic acid, etc., in the presence of a catalyst comprising a heavy metal and bromine. This process can be applied to the production of isophthalic acid. British Patent 1555246 specifically discloses a process for oxidizing m-xylene with air in an acetic acid solvent in the presence of a catalyst comprising cobalt, manganese and bromine, and this process has 75 been widely industrially practiced. Since, however, the isophthalic acid produced by this process contains a large amount of impurities including 3-carboxybenzaldehyde (to be sometimes referred to as 3CBA hereinafter), a polymer directly produced from this isophthalic acid as a material has no excellent hue, nor is it suitable for high-function use. In particular, with an advance in industrial technology in recent years, requirements of qualities of polyester products as a highly functional material are increasingly severe, and 20 therefore, isophthalic acid as a material for polyester is desired to have high purity and excellent whiteness. In order to highly purify crude isophthalic acid produced by oxidation, a method disclosed in British Patents 1,152,575 and 1,152,576 can be employed in which an aqueous solution of a crude isophthalic acid is subjected to hydrogenation and purification at a high temperature in the presence of a palladium catalyst. On the other hand, terephthalic acid is produced by oxidizing p-xylene in the same manner as in the 25 production of m-xylene, and it is general practice to employ a process in which a high-purity terephthalic acid is directly produced without carrying out the above hydrogenation, purification, etc. That is, Japanese Patent Publication No. 36732/1970 discloses a process for producing terephthalic acid usable for direct polymerization, which comprises oxidizing p-xylene in the presence of a cobalt/manganese/bromine system catalyst mainly comprising cobalt and containing 1 to 20 % by weight, based on the cobalt, of manganese. 30 This process uses a large amount of expensive cobalt. Therefore, many processes for producing high-purity terephthalic acid have been proposed in which the catalyst amount and the acetic acid combustion amount are decreased by improvements in the oxidation conditions and oxidation method (U.S. Patent 4,160,108, U.S. Patent 4,053,506, British Patents 1,511,181 and 1,511,182, U.S. Patent 4,051,178, U.S. Patent 4,197,412, British Patent 1,454,478, Bristish Patent 1,577,019 and U.S. Patent 3,859,344). 35 The conventional process for purifying crude isophthalic acid by hydrogenation in the presence of a palladium catalyst requires separate apparatus for a hydrogenation reaction, crystallization, etc., and a high investment for apparatus for purification and much labors are required, which causes an increase in the production cost. The present inventors attempted to apply the above process for producing high-purity terephthalic acid 40 by oxidation of p-xylene to a process for producing isophthalic acid by oxidation of m-xylene. As a result, it has been found that the conventional process for producing terephthalic acid cannot be directly applied the process for producing isophthalic acid due to the following problems. (1) Even in the oxidation of m-xylene, intermediates such as m-toluic acid and 3CBA can be almost fully removed by carrying out post oxidation after a main oxidation reaction as is described in U.S. Patent 45 3,859,344. However, if the amount of 3CBA is decreased to become smaller than a certain amount, no isophthalic acid having excellent whiteness can be obtained, and the amount of coloring impurities, by contraries, increases to deteriorate the quality of isophthalic acid. (2) When acetic acid is recovered by evaporation of a reaction liquid from which isophthalic acid has been separated (i.e. mother liquor), the recovery of acetic acid is poor and a loss of acetic acid is large. 50 The reason therefor is that the solubility of isophthalic acid in acetic acid around the boiling point of acetic acid is about 10 times higher than that of terephthalic acid, and the mother liquor obtained after separation of isophthalic acid crystals contains a larger amount of isophthalic acid dissolved. When the recovery of acetic acid is increased, a high-boiling substance in the state of a high-temperature melt, i.e. a residue in an evaporator, shows a decrease in fluidity, and the operationability of the evaporator is 55 deteriorated extremely. In particular, therefore, such an evaporator as a film evaporator cannot be suitably used.

3 EP 0 465 100 B1

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process for producing isophthalic acid having both high purity and excellent whiteness. 5 It is another object of the present invention to provide a process for producing isophthalic acid having both high purity and excellent whiteness at high yields. It is further another object of the present invention to provide an industrially advantageous process for producing isophthalic acid having both high purity and excellent whiteness with simple apparatus, which process also permits high recovery of acetic acid. io Further, it is another object of the present invention to provide a process for producing isophthalic acid having high purity and excellent whiteness at high yields in the presence of an inexpensive catalyst. According to the present invention, there is provided a process for producing high-purity isophthalic acid by oxidizing m-xylene with an oxygen-containing gas in a solvent of hydrous acetic acid in the presence of a cobalt/manganese/bromine system catalyst, the process comprising: is a first step of feeding an oxidation reactor with the solvent, m-xylene of which the amount is 1 to 1/15 times the weight of the solvent and the catalyst in which the total amount of cobalt and manganese is in the range between 300 ppm and 1 ,500 ppm based on the weight of the solvent, the atomic ratio of manganese to cobalt (manganese/cobalt) is 0.5 to 5 and the atomic ratio of bromine to the total amount of cobalt and manganese (bromine/total of cobalt and manganese) is 0.5 to 1 .5, and carrying out oxidation of m-xylene at 20 a temperature between 180°C and 210 °C with maintaining 2 to 8 % by volume of an oxygen concentration in a discharge gas until crude isophthalic acid produced in this first step has a 3-carboxybenzaldehyde concentration in the range between 500 ppm and 10,000 ppm and the total amount of isophthalic acid and intermediates produced by the oxidation is 10 to 35 % by weight based on the weight of the solvent, b. a second step of further oxidizing a mixture formed by the oxidation in the first step until the crude 25 isophthalic acid has a 3-carboxybenzaldehyde concentration in the range between 100 ppm and 800 ppm, separating the crude isophthalic acid, evaporating remaining mother liquor to obtain a condensate, and purifying the condensate by distillation to recover acetic acid, and c. a third step of mixing the crude isophthalic acid separated in the second step with purified acetic acid, treating the resultant mixture by stirring it at a temperature of not less than 100°C, and separating 30 purified isophthalic acid.

DETAILED DESCRIPTION OF THE INVENTION

Having focussed on characteristics of the oxidation of m-xylene, the present inventors have further 35 made a diligent study of application of the above process for producing high-purity terephthalic acid, and as a result, found the following. Main oxidation and post oxidation are separately carried out in the presence of a specific amount of a hydrous acetic acid solvent and a catalyst up to predetermined limits of oxidation, and specific purification treatment is carried out, whereby (1) high-purity isophthalic acid having excellent whiteness can be obtained, (2) acetic acid can be effectively recovered from mother liquid from which 40 isophthalic acid crystals have been separated, and can be recycled, and (3) the combustion amount of acetic acid in a reactor can be decreased. The present invention has been completed on this finding. In the process of the present invention, in a first step, m-xylene is oxidized with an oxygen-containing gas in a hydrous acetic acid solvent in the presence of a catalyst comprising a cobalt compound, a manganese compound and a bromine compound. 45 As a starting material, m-xylene is generally used. However, substituents on a benzene ring are not limited to methyl groups, and may be ethyl, propyl or isopropyl groups, or a combination of these. Further, the substituent may be a group which can be oxidized to a carboxyl group. One of the two substituents may also be a carboxyl group. m-Xylene is fed to an oxidation reactor continuously in this step, the feed rate of m-xylene per liter of a 50 solvent volume retained in the reactor is 0.05 to 0.2 kg/hour, and the reaction time is 10 to 120 minutes, preferably 20 to 90 minutes. Hydrous acetic acid is used as a solvent for the reaction. The hydrous acetic acid to be fed to the oxidation reactor has a water concentration of 3 to 15 % by weight, preferably 5 to 12 % by weight. When the water concentration is too low, the resultant isophthalic acid is liable to be deteriorated in color quality, 55 and the combustion amount of acetic acid increases. When the water concentration is too high, the oxidation activity in the reaction decreases, the reaction takes longer, and the resultant isophthalic acid shows degradation in quality.

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Hydrous acetic acid as a solvent is used in such an amount that the total amount of isophthalic acid and intermediates (m-toluic acid and 3CBA) produced by the oxidation in the oxidation reactor is 10 to 35 % by weight. In this case, in general, the feed amount of the solvent is 1 to 15 times, preferably 3 to 12 times the weight of m-xylene. When the total amount of the isophthalic acid and intermediates is less than 10 % by 5 weight in concentration, the amount of the solvent is relatively large. Uneconomically, therefore, a large- sized apparatus for separation of crystals is required, and the amount of the solvent to be treated in a solvent recovery step is large. When the above total amount is more than 35 % by weight in concentration, a tube from a post oxidation reactor to a crystallizer of crude isophthalic acid and the crystallizer are liable to be clogged. io The cobalt compound, manganese compound and bromine compound are selected from known compounds used for liquid phase oxidation, such as carbonates, acetic acid salt tetrahydrates, bromides, etc., of cobalt and manganese. As a bromine compound, preferred is an inorganic compound which is dissolved in the solvent and readily dissociated to form ions. Particularly preferred are hydrobromic acid, cobalt bromide, manganese bromide, etc. is In a catalyst metal concentration, the total amount of cobalt and manganese based on the solvent weight is in the range of 300 ppm and 1,500 ppm, preferably between 400 ppm and 1,200 ppm, and the atomic ratio of manganese to cobalt is 0.5 to 5, preferably 0.8 to 4. Bromine is used in such a concentration that the atomic ratio of bromine to the total amount of cobalt and manganese is 0.5 to 1 .5, preferably 0.6 to 1.2. When these metal components are used in the above ranges, the 3CBA concentration is suitably 20 retained through the main oxidation and the post oxidation, an isophthalic acid having excellent hue is obtained, and the combustion amount of acetic acid is remarkably small. The reaction temperature in the first step is in the range between 180°C and 210°C, and the pressure is set at a level sufficient to maintain a liquid phase of the solvent. In general, the reaction is carried out at a pressure between 10 kg/cm2 and 25 kg/cm2. 25 As an oxygen-containing gas, air is generally used. Air is fed into the reaction liquid such that the a discharge gas from the oxidation reactor has an oxygen concentration of 2 to 8 % by volume, preferably 3 to 6 % by volume. In the main oxidation in the first step, the operational factors such as a residence time, a reaction temperature, etc., are controlled such that crude isophthalic acid produced under the conditions in the 30 above ranges has a 3CBA concentration in the range between 500 ppm and 10,000 ppm. When the 3CBA concentration is too low at this stage, the oxidation degree is too high. As a result, the amount of coloring impurities increases, and even the post oxidation produces no effect. Therefore, the resultant isophthalic acid shows no improvement in color quality. Further, this means that the combustion amount of acetic acid increases, which is very disadvantageous economically. When the 3CBA concentration is higher than the 35 above upper limit, 3CBA is not sufficiently oxidized in the post oxidation, and no high-purity isophthalic acid is obtained. The above 3CBA concentration is determined by measuring crude isophthalic acid crystals obtained by taking out an oxidation reaction mixture from a reactor used in the first step and subjecting it to solid-liquid separation. 40 In the second step, at first, a mixture of oxidation reaction products produced in the first step is post- oxidized with an oxygen-containing gas. In general, in the case of a continuous method, this post oxidation is carried out in a crystallizer vessel into which the above mixture has been recharged. The post oxidation may be also carried out during the step when the mixture is cooled after the main reaction in the first step. The post oxidation reaction is carried out at a temperature between the main oxidation reaction temperature 45 in the first step and a temperature which is lower than the main oxidation reaction temperature by about 30 °C. It is industrially disadvantageous to set the temperature for the post oxidation reaction at a level higher than that for the main oxidation reaction, since heating is required. Moreover, such a higher temperature for the post oxidation is liable to cause degradation in color quality. When the post oxidation reaction temperature is too low, the post oxidation is insufficient, and no high-purity isophthalic acid is 50 obtained. As an oxygen-containing gas for the post oxidation, air can be used. A discharge gas from the oxidation in the first step may be used. The oxygen concentration in a discharge gas in the second step is so maintained as to be 2 to 7 % by volume. The post oxidation is carried out for such a period of time that an intended oxidation substantially 55 proceeds, i.e. for 1 to 300 minutes, preferably for 2 to 120 minutes. The most important point in the second step is that the oxidation reaction liquid obtained by the post oxidation has a 3CBA concentration in the range between 100 ppm to 800 ppm. The reasons therefore are as follows.

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(1) The reaction mixture obtained by the post oxidation is cooled to crystallize isophthalic acid, crystals of isophthalic acid are separated from a liquid of the reaction mixture, and the crystals are purified in the next third step. When the oxidation reaction liquid has a 3CBA concentration in the above range, a high- purity isophthalic acid having excellent whiteness can be obtained. 5 In addition, when the oxidation reaction liquid obtained by the post oxidation has a 3CBA concentration of higher than 800 ppm, no high-purity isophthalic acid is obtained even after the purification is carried out in the third step. When the 3CBA concentration is lower than 100 ppm, no purified isophthalic acid having excellent whiteness is obtained. (2) Mother liquor remaining after the separation of isophthalic acid from the oxidation reaction liquid io contains high-boiling substances such as a coloring substance, etc., in addition to acetic acid, water, a catalyst composition, benzoic acid, m-toluic acid, 3CBA, isophthalic acid, phthalic acid and trimellitic acid. A detailed study by the present inventors has showed that the fluidity of a mother liquor residue in an evaporator at a high temperature after the isophthalic acid has been separated greatly depends upon contents of acetic acid, water, a catalyst composition, benzoic acid, 3CBA, m-toluic acid, trimellitic acid, is etc., relative to the content of isophthalic acid. That is, when the 3CBA in the post oxidation reaction liquid is lower than the above lower limit, the fluidity of the mother liquor residue in an evaporator at a high temperature is greatly reduced due to a change in the composition of the residue, and acetic acid therefore cannot be effectively evaporated. When the 3CBA concentration is in the above range, acetic acid can be effectively recovered. 20 The oxidation reaction liquid obtained by the post oxidation is recharged into a crystallizer, and cooled to a temperature between about 110°C and about 80 °C to finally precipitate isophthalic acid crystals. Then, the crystals were recovered by filtering, and subjected to the third step. The filtrate mother liquor is evaporated to separate high-boiling substances in the state of a melt, and a condensate of recovered vapor is purified by distillation to recover purified acetic acid. For this evaporation, a film evaporator can be 25 suitably used. The mother liquor may be recycled to the oxidation reactor in the first step, and in this case, a remaining portion of the mother liquor is evaporated as above, and the above purification by distillation is carried out to recover acetic acid. When a film evaporator is used, it is preferred to distill the mother liquor partially or wholly with a distillation column before the evaporation in order to reduce a load of the mother liquor on the film 30 evaporator, and a column bottom liquid is evaporated with the film evaporator. As a film evaporator, a film evaporator of a type supplied by Luwa is suitably usable in the present invention, and the temperature of the condensate is adjusted to a temperature between 130°C and 230 °C. It is preferred to further rectify an acetic acid aqueous solution separated from the distillation column and acetic acid from the film evaporator, and formed water is separated by this rectification. Acetic acid recovered can be used as a solvent in the 35 first step or as a wash liquid in the third step. In the third step, isophthalic acid crystals recovered from the reaction mixture in the second step are suspended in purified acetic acid, and the resultant suspension is stirred at a temperature of about 100°C or higher. Thereafter, isophthalic acid crystals are separated. For this stirring, a retention temperature of about 100°C or higher is sufficient, and it is not necessary to increase the retention temperature so as to 40 dissolve isophthalic acid completely. The retention time is 10 to 60 minutes. In the present invention, the "purified acetic acid" refers to acetic acid which substantially does not contain water. The isophthalic acid crystals separated above are washed with purified acetic acid. In this case, acetic acid recovered in the second step or industrial acetic acid may be used. The acetic acid for washing the above isophthalic acid crystals may be hydrous acetic acid. The amount of purified acetic acid for use is 1 45 to 5 times the weight of the isophthalic acid crystals. After the washing, the isophthalic acid crystals are dried, whereby purified isophthalic acid having excellent whiteness can be obtained. Mother liquor remaining after the isophthalic acid crystals have been separated in the third step can be used as a solvent for the liquid phase oxidation in any of the first and second step. Due to this, isophthalic acid dissolved in the mother liquor is recovered in the oxidation step. 50 According to the present invention, high-purity isophthalic acid having excellent color quality useful as a polymer material for an unsaturated polyester resin, an alkyd resin, heat-resistant polyamide, etc., for high functional use can be industrially and very easily produced. In the process of the present invention, the cost for purification can be decreased since no expensive purification step is necessary, the amount of acetic acid for use can be decreased, and high-purity isophthalic acid can be obtained at high yields. 55 EP 0 465 100 B1

Examples

The present invention will be described further in detail below by reference to Examples, to which, however, the present invention shall not be limited. 5 Example 1

A continuous oxidation reaction of m-xylene was carried out by the use of a first reactor made of pressure-resistant titanium, which was equipped with a stirring device, a reflux condenser and a heating io device and had a raw material introducing port, an air introducing port, a gas discharging port and a liquid refluxing port, a second reactor made of pressure-resistant titanium, which was similarly equipped with a stirring device, a reflux condenser and a heating device and had an air introducing port, a gas discharging port, a liquid refluxing port, an inlet port for recharging a reaction product and an outlet port for drawing out a reaction product, and a crystallizer. is [First step] The first reactor was continuously charged, as a feed liquid, with m-xylene, cobalt acetate tetrahydrate, manganese acetate tetrahydrate, hydrobromic acid and acetic acid. The rate of charging m- xylene was 100 parts by weight, and based on this, the rates of charging the other components of the feed liquid were 0.561 part for cobalt acetate tetrahydrate, 1.656 parts for manganese acetate tetrahydrate, 1.117 parts for hydrobromic acid (47 %), and 896.7 parts for acetic acid (water content 10 %). (The catalyst 20 concentration based on the solvent weight was as follows: cobalt = 147 ppm, manganese = 412 ppm, and bromine = 576 ppm). m-Xylene was oxidized while air was blown into the first reactor. While keeping temperature inside the reactor at 200 °C and the pressure at 16.5 kg/cm2 G, the feed amount of air was adjusted such that an oxidation discharge gas had an oxygen concentration of 5 % by volume. In this case, the rate of feeding m-xylene was 0.17 kg/hour per liter of the solvent volume within the first reactor, the 25 average residence time was about 30 minutes, and the concentration of the total of isophthalic acid and intermediates within the first reactor was 15.6 %. [Second step] The resultant oxidation reaction mixture obtained in the first reactor was recharged into the second reactor, and subjected to post oxidation with air at 195°C under a pressure of 12 kg/cm2 G for an average residence time of 50 minutes. The oxygen concentration in a discharge gas was maintained at 3 30 % by volume. The amount of CO2 + CO formed in this oxidation step and contained in the discharge gas (which is an index for oxidation of acetic acid) was 0.41 mole per mole of the raw material m-xylene. The oxidation reaction mixture obtained by the post oxidation in the second reactor was transferred to the crystallizer at 100° C to precipitate isophthalic acid crystals. Isophthalic acid was recovered by filtering the resultant slurry with washing it with acetic acid. The remaining mother liquor was concentrated by 35 distillation with a distillation column, and acetic acid was recovered as follows. A column bottom liquid was continuously fed to a film evaporator (in which a heat medium having a temperature of 240 °C was circulating) for 6 hours while the flow amount of the heat medium was adjusted such that a concentrated liquid had a temperature of 230 0 C, whereby high-boiling substances in a residue could be well separated (total acetic acid content in a residue liquid in the film evaporator 4.3 %). Acetic acid recovered from the 40 distillation column and from the film evaporator was purified with a rectifying column, and used as a washing liquid in the next step. [Third step] A suspension prepared by adding purified acetic acid (water content 10 %) of the isophthalic acid crystals separated above (the amount of purified acetic acid was 1 .5 times the weight of the isophthalic acid crystals) was transferred to a washing vessel, and kept at 150°C for 30 minutes with 45 stirring. The resultant suspension was cooled to 100°C and filtered, and the .resultant solid was dried to give 146 parts by weight, based on the m-xylene, of purified isophthalic acid having high purity and excellent whiteness. The yield of the isophthalic acid based on the raw material m-xylene was 93.0 mol.

Example 2 50 In Example 1, the catalyst concentration was changed to 1.5 times, and an oxidation reaction was carried out at a temperature of 190°C. Post oxidation treatment was carried out in the same manner as in Example 1 , and crystallization and washing treatment was carried out in the same manner as in Example 1 . As a result, purified isophthalic acid having high purity and excellent whiteness was obtained. A filtrate 55 mother liquor was treated with a distillation column and a film evaporator in the same manner as in Example 1 , whereby acetic acid could be smoothly recovered (total acetic acid content in residue liquid in the film evaporator 5.5 %).

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Comparative Example 1

In Example 2, an oxidation reaction in the first step was carried out at a temperature of 215°C. As a result, crude isophthalic acid obtained by post oxidation in the second step had a low 3CBA concentration. 5 A filtrate mother liquor was subjected to acetic acid recovery with a film evaporator in the same manner as in Example 1 to show that the fluidity of a residue was poor and the film evaporator was not operable.

Example 3 io A first reactor was continuously charged, as a feed liquid, with m-xylene, cobalt acetate tetrahydrate, manganese acetate tetrahydrate, hydrobromic acid and acetic acid. The rate of charging m-xylene was 60 parts by weight, and based on this, the rates of charging the other components for the feed liquid were 0.561 part for cobalt acetate tetrahydrate, 1 .656 parts for manganese acetate tetrahydrate, 1 .551 parts for hydrobromic acid (47 %), and 496.2 parts for acetic acid (water content 10 %). (The catalyst concentration is based on the solvent weight was as follows: cobalt = 276 ppm, manganese = 773 ppm, and bromine = 1 ,500 ppm). m-Xylene was oxidized while air was blown into the first reactor. While keeping temperature inside the reactor at 190°C and the pressure at 16.5 kg/cm2 G, the feed amount of air was adjusted such that an oxidation discharge gas had an oxygen concentration of 5 % by volume. In this case, the rate of feeding m-xylene was 0.09 kg/hour per liter of the solvent volume within the first reactor, the average 20 residence time was about 55 minutes, and the concentration of the total of isophthalic acid and intermediates within the first reactor was 19.0 %. The resultant oxidation reaction mixture obtained in the first reactor was recharged into a second reactor, and subjected to post oxidation with air at 185°C under a pressure of 12 kg/cm2 G for an average residence time of 50 minutes. The oxygen concentration in a discharge gas was maintained at 3 % by 25 volume. Thereafter, the post oxidation reaction product was treated in the same manner as in Example 1 to give isophthalic acid having high purity and excellent whiteness A filtrate mother liquor was treated with a distillation column and a film evaporator in the same manner as in Example 1 , whereby acetic acid could be smoothly recovered (total acetic acid content in residue liquid in the film evaporator 5.0 %). 30 Comparative Example 2

Example 3 was repeated except that the rates of charging cobalt acetate tetrahydrate and manganese acetate tetrahydrate were changed to 1 .795 parts and 0.442 parts (ratio of manganese to cobalt 0.25). 35 As a result, isophthalic acid crystals obtained by post oxidation in the second step had a low 3CBA concentration. A filtrate mother liquor was subjected to acetic acid recovery with a film evaporator in the same manner as in Example 1 to show that the fluidity of a residue was poor and the film evaporator was not operable.

40 Comparative Example 3

The first step in Example 1 was repeated, and post oxidation in a second step was carried out at 165°C under a pressure of 7 kg/cm2 G. As a result, a filtrate mother liquor was smoothly treated with a distillation column and a film evaporator to recover acetic acid (total acetic acid content in residue liquid in the film 45 evaporator 4.8 %). However, purified isophthalic acid had a high 3CBA concentration and was colored.

Comparative Example 4

Example 1 was repeated except that the washing temperature in the third step was changed to 90 ° C. 50 As a result, coloring of purified isophthalic acid was observed. Table 1 shows main operational conditions and results of analysis of the isophthalic acid crystals obtained in Examples and Comparative Examples. In addition, each crude isophthalic acid in Table 1 was obtained by directly recharging an oxidation reaction mixture from the first or second reactor into a pressure sample container, and separating a solid 55 with washing it with acetic acid at 100 ° C. 3CBA and a resin color were measured as follows. (1) 3CBA: determined by polarography. For high-purity isophthalic acid, the 3CBA concentration is required to be not more than 50 ppm.

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(2) Resin color: Isophthalic acid, fumaric acid, neopentyl glycol and propylene glycol were polymerized in an isophthalic acid/fumaric acid/neopentyl glycol/propylene glycol molar ratio of 57/43/50/53, and the resultant resin was measured for a Hazen color number by dissolving it in styrene (resin content 60 % by weight). A resin showing a smaller Hazen color number has a better resin color. For high-purity isophthalic acid, the Hazen color number is required to be not more than 30. EP 0 465 100 B1

(3) m-Toluic acid: determined by gas chromatography of an esterified sample.

Table 1

Examp 1 e/Compar a t i ve Example Ex.1 Ex.2 CEx.l Ex.3 CEx.2 CEx.3 CEx.4

Feed amount of m-xylene, kg 100 100 100 60 60 100 100 Feed rate of m-xylene, kg/H/1 0.17 0.17 0.17 0.09 0.09 0.17 0.17 Acetic acid/m-xylene weight ratio 9.0 9.0 9.0 8.0 8.0 9.0 9.0 Cobalt (Co) concentration, ppm- 147 221 221 276 884 147 147 Manganese (Mg) concentration, ppm 412 618 618 773 206 412 412 Bromine (Br) concentration, ppm 576 866 866 1,500 1,500 576 576 Mn/Co atomic ratio 3.0 3.0 3.0 3.0 0.25 3.0 3.0 Br/(Co+Mn) atomic ratio 0.72 0.72 0.72 1.0 1.0 0.72 0.72

Main oxidation temperature, °C 200 190 215 190 190 200 200 Residence time, min. 30 30 30 55 55 30 30 Oxygen concentration in discharge gas. voU 5.0 5.0 5.0 5.0 5.0 5.0 5.0

Post oxidation temperature, °C 195 185 195 185 185 165 195 Amount of (CO+C02) formed, mol/mol 0.41 0.35 0.59 0.50 0.65 0.39 0.41

2nd step, treatment with film evaporator good good No good No good good

3rd step, temperature for washing wi th acetic acid. 4C 150 150 150 150 150 150 90

3CBA concentration, ppm Crude isophthalic acid (after main oxidation) 6,000 7,400 2,850 2,080 900 6,000 6,000 Crude isophthalic acid (after post oxidation) 188 240 90 150 90 900 188 Purified isophthalic acid 34 44 16 27 16 164 34

Resin color (purified isophthalic acid) 30 20 50 30 40 60 40

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Claims

1. A process for producing high-purity isophthalic acid by oxidizing m-xylene with an oxygen-containing gas in a solvent of hydrous acetic acid in the presence of a cobalt/manganese/bromine system catalyst, 5 the process comprising: a. a first step of feeding an oxidation reactor with the solvent, m-xylene of which the amount is 1 to 1/15 times the weight of the solvent and the catalyst in which the total amount of cobalt and manganese is in the range between 300 ppm and 1 ,500 ppm based on the weight of the solvent, the atomic ratio of manganese to cobalt (manganese/cobalt) is 0.5 to 5 and the atomic ratio of bromine io to the total amount of cobalt and manganese (bromine/total of cobalt and manganese) is 0.5 to 1 .5, the m-xylene being fed continuously, and carrying out oxidation of m-xylene at a temperature between 180°C and 210 °C with maintaining 2 to 8 % by volume of an oxygen concentration in a discharge gas until crude isophthalic acid produced in this first step has a 3-carboxybenzaldehyde concentration in the range between 500 ppm and 10,000 ppm and the total amount of isophthalic is acid and intermediates produced by the oxidation is 10 to 35 % by weight based on the weight of the solvent, b. a second step of further oxidizing a mixture formed by the oxidation in the first step until the crude isophthalic acid has a 3-carboxybenzaldehyde concentration in the range between 100 ppm and 800 ppm, separating the crude isophthalic acid, evaporating remaining mother liquor to obtain a 20 condensate, and purifying the condensate by distillation to recover acetic acid, and c. a third step of mixing the crude isophthalic acid separated in the second step with purified acetic acid, treating the resultant mixture by stirring it at a temperature of not less than 100°C, and separating purified isophthalic acid.

25 2. A process according to claim 1, wherein in step a. the intermediates are m-toluic acid and 3- carboxybenzaldehyde.

3. A process according to claim 1 or 2, wherein in step a. the solvent of hydrous acetic acid contains 3 to 15 % by weight of water and 85 to 97 % by weight of acetic acid. 30 4. A process according to claim 1 , 2 or 3 wherein in step a. the amount of m-xylene is one third to one twelfth of the weight of the solvent.

5. A process according to any one of the preceding claims, wherein in step a. the co- 35 balt/manganese/bromine system catalyst contains carbonate, acetic acid salt tetrahydrate or bromide of cobalt.

6. A process according to any one of the preceding claims, wherein in step a. the cobalt/manganese/bromine system catalyst contains carbonate, acetic acid salt tetrahydrate or bromide of 40 manganese.

7. A process according to any one of the preceding claims, wherein in step a. the cobalt/manganese/bromine system catalyst contains a bromine compound selected from hydrobromic acid, cobalt bromide and manganese bromide. 45 8. A process according to any one of the preceding claims, wherein in step a. the total amount of cobalt and manganese based on the solvent weight is between 400 ppm and 1,200 ppm.

9. A process according to any one of the preceding claims, wherein in step a. the atomic ratio of 50 manganese to cobalt is 0.8/1 to 4/1 .

10. A process according to any one of the preceding claims, wherein in step a. the atomic ratio of bromine to the total amount of cobalt and manganese is 0.6/1 to 1 .2/1 .

55 11. A process according to any one of the preceding claims, wherein in step b. the mixture is oxidized at a temperature between the temperature for the oxidation in step a. and a temperature which is 30 °C lower than the temperature for the oxidation in step a.

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12. A process according to any one of the preceding claims, wherein discharge gas from step a. has an oxygen concentration of 3 to 6 % by volume.

13. A process according to any one of the preceding claims, wherein discharge gas from step b. has an 5 oxygen concentration of 2 to 7 % by volume.

14. A process according to any one of the preceding claims, wherein step b. further includes a step of subjecting the crude isophthalic acid to crystallization and precipitation and a step of separating the resultant isophthalic acid crystals by filtering. 10 15. A process according to claim 14, wherein after the isophthalic acid crystals have been separated by filtering the mother liquor remaining is evaporated to provide a condensate which is distilled to recover purified acetic acid. is 16. A process according to claim 14, wherein after the isophthalic acid crystals have been separated by filtering part of the mother liquor remaining is fed to the oxidation reactor in step a.

17. A process according to claim 14, wherein after the isophthalic acid crystals have been separated by filtering the mother liquor remaining is distilled and a column bottom liquid from said distillation is 20 evaporated to recover acetic acid.

18. A process according to any one of the preceding claims, wherein the purified acetic acid in step c. is used in an amount which is 1 to 5 times the weight of the crude isophthalic acid.

25 19. A process according to any one of the preceding claims, wherein in step c. acetic acid remaining after the purified isophthalic acid has been separated is used as a solvent in step a. or step b.

Patentanspruche

30 1. Verfahren zur Herstellung von hochreiner Isophthalsaure durch Oxydation von m-Xylol mit einem Sauerstoff-enthaltenden Gas in einem Losemittel aus wassriger Essigsaure in Gegenwart eines Co- balt/Mangan/Brom Katalysatorsystems, wobei das Verfahren umfaBt: a) Eine erste Stufe mit Beschickung eines Oxydationsreaktors mit dem Losemittel m-Xylol, dessen Menge das 1 bis 1/15 fache des Losemittelgewichts betragt, und dem Katalysator, in dem die 35 Gesamtmenge an Cobalt und Mangan im Bereich zwischen 300 ppm und 1 500 ppm basierend auf dem Losemittelgewicht liegt, das atomare Verhaltnis von Mangan zu Cobalt (Mangan/Cobalt) 0,5 bis 5 betragt und das atomare Verhaltnis von Brom zur Gesamtmenge an Cobalt und Mangan (Brom/Gesamtmenge an Cobalt und Mangan) 0,5 bis 1,5 betragt, das m-Xylol kontinuierlich zuge- fuhrt wird und die Oxydation von m-Xylol bei einer Temperatur zwischen 180°C und 210 °C unter 40 Aufrechterhaltung von 2 bis 8 Volumenprozent Sauerstoffkonzentration im ausstromenden Gas ausgefuhrt wird, bis die rohe in dieser ersten Stufe gebildete Isophthalsaure eine 3-Carboxybenzal- dehydkonzentration im Bereich zwischen 500 ppm und 10 000 ppm aufweist und die Gesamtmenge an Isophthalsaure und der durch die Oxydation gebildeten Zwischenprodukte 10 bis 35 Gewichtspro- zent basierend auf dem Losemittelgewicht betragt, 45 b) eine zweite Stufe der weiteren Oxydation eines durch die Oxydation in der ersten Stufe gebildeten Gemisches bis die rohe Isophthalsaure eine 3-Carboxybenzaldehydkonzentration im Bereich zwischen 100 ppm und 800 ppm aufweist, Abtrennung der rohen Isophthalsaure, Verdamp- fen der verbleibenden Mutterlauge unter Erhaltung eines Kondensats, und Reinigung des Konden- sats durch Destination, urn die Essigsaure wiederzugewinnen, und 50 c) eine dritte Stufe mit Mischen der in der zweiten Stufe abgetrennten rohen Isophthalsaure mit der gereinigten Essigsaure, Behandlung des entstehenden Gemisches durch Ruhren bei einer Tempera- tur von nicht weniger als 100°C, und Abtrennung der gereinigten Isophthalsaure.

2. Verfahren nach Anspruch 1 , worin in Stufe a) die Zwischenprodukte m-Toluylsaure und 3-Carboxyben- 55 zaldehyd sind.

3. Verfahren nach Anspruch 1 oder 2, worin in Stufe a) das Losemittel aus wassriger Essigsaure 3 bis 15 Gewichtsprozent Wasser und 85 bis 97 Gewichtsprozent Essigsaure enthalt.

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4. Verfahren nach Anspruch 1, 2 oder 3, worin in Stufe a) die Menge an m-Xylol ein Drittel bis ein Zwolftel des Losemittelgewichts betragt.

5. Verfahren nach einem der vorangehenden Anspruche, worin in Stufe a) das Cobalt/Mangan/Brom 5 Katalysatorsystem Carbonat, Essigsauresalztetrahydrat oder ein Bromid von Cobalt enthalt.

6. Verfahren nach einem der vorangehenden Anspruche, worin in Stufe a) das Cobalt/Mangan/Brom Katalysatorsystem Carbonat, Essigsauresalztetrahydrat oder ein Bromid von Mangan enthalt.

io 7. Verfahren nach einem der vorangehenden Anspruche, worin in Stufe a) das Cobalt/Mangan/Brom Katalysatorsystem eine Bromverbindung enthalt, die aus Bromwasserstoffsaure, Cobaltbromid und Manganbromid ausgewahlt wird.

8. Verfahren nach einem der vorangehenden Anspruche, worin in Stufe a) die Gesamtmenge an Cobalt is und Mangan basierend auf dem Losemittelgewicht zwischen 400 ppm und 1 200 ppm liegt.

9. Verfahren nach einem der vorangehenden Anspruche, worin in Stufe a) das atomare Verhaltnis von Mangan zu Cobalt 0,8/1 bis 4/1 betragt.

20 10. Verfahren nach einem der vorangehenden Anspruche, worin in Stufe a) das atomare Verhaltnis von Brom zur Gesamtmenge an Cobalt und Mangan 0,6/1 bis 1 ,2/1 betragt.

11. Verfahren nach einem der vorangehenden Anspruche, worin in Stufe b) das Gemisch bei einer Temperatur oxydiert wird, die zwischen der Temperatur fur die Oxydation in Stufe a) und einer 25 Temperatur liegt, die 30 °C niedriger als die Temperatur fur die Oxydation in Stufe a) ist.

12. Verfahren nach einem der vorangehenden Anspruche, worin das ausstromende Gas von Stufe a) eine Sauerstoffkonzentration von 3 bis 6 Volumenprozent aufweist.

30 13. Verfahren nach einem der vorangehenden Anspruche, worin das ausstromende Gas von Stufe b) eine Sauerstoffkonzentration von 2 bis 7 Volumenprozent aufweist.

14. Verfahren nach einem der vorangehenden Anspruche, worin die Stufe b) ferner einen Schritt, in dem die rohe Isophthalsaure einer Kristallisation und einer Fallung unterzogen wird, und einen Schritt zur 35 Abtrennung der entstehenden Isophthalsaurekristalle durch Filtration beinhaltet.

15. Verfahren nach Anspruch 14, worin nach der Abtrennung der Isophthalsaurekristalle durch Filtration die verbleibende Mutterlauge unter Bereitstellung eines Kondensats verdampft wird, das zur Wiedergewin- nung der Essigsaure destilliert wird. 40 16. Verfahren nach Anspruch 14, worin nach der Abtrennung der Isophthalsaurekristalle durch Filtration ein Teil der verbleibenden Mutterlauge in den Oxydationsreaktor in Stufe a) gegeben wird.

17. Verfahren nach Anspruch 14, worin nach der Abtrennung der Isophthalsaurekristalle durch Filtration, die 45 verbleibende Mutterlauge destilliert wird und die Saulenbodenflussigkeit von dieser Destination unter Wiedergewinnung der Essigsaure verdampft wird.

18. Verfahren nach einem der vorangehenden Anspruche, worin die gereinigte Essigsaure in Stufe c) in einer Menge verwendet wird, die das 1 bis 5 fache des Gewichts der rohen Isophthalsaure betragt. 50 19. Verfahren nach einem der vorangehenden Anspruche, worin die in Stufe c) nach der Abtrennung der reinen Isophthalsaure zuruckbleibende Essigsaure als Losemittel in Stufe a) oder Stufe b) verwendet wird.

55 Revendicatlons

1. Procede de production d'acide isophtalique de purete elevee, par oxydation du m-xylene avec un gaz contenant de I'oxygene, dans un solvant constitue d'acide acetique et d'eau, en presence d'un systeme

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catalytique cobalt/manganese/brome, ledit precede comprenant : a) une premiere etape dans laquelle on introduit dans un reacteur d'oxydation, le solvant, une quantite de m-xylene representant 1 a 1/15 de fois le poids du solvant, et le catalyseur en une quantite telle que la quantite totale de cobalt et de manganese soit comprise entre 300 ppm et 1500 5 ppm par rapport au poids du solvant, le rapport atomique du manganese au cobalt (manganese/cobalt) etant de 0,5 a 5 et le rapport atomique du brome a la quantite totale de cobalt et de manganese (brome/total de cobalt et de manganese) etant de 0,5 a 1 ,5, le m-xylene etant introduit de maniere continue, et on effectue I'oxydation du m-xylene a une temperature comprise entre 180°C et 210°C, en maintenant a 2 a 8% en volume la concentration de I'oxygene dans le gaz io evacue, jusqu'a ce que I'acide isophtalique brut, produit dans cette premiere etape, presente une concentration de 3-carboxybenzaldehyde comprise entre 500 ppm et 10 000 ppm et la quantite totale d'acide isophtalique et de produits intermediaires, produits par I'oxydation, represente 10 a 35% en poids par rapport au poids du solvant, b) une seconde etape dans laquelle on oxyde encore plus le melange forme par oxydation dans la is premiere etape, jusqu'a ce que I'acide isophtalique brut presente une concentration de 3-carboxybenzaldehyde comprise entre 100 ppm et 800 ppm, on separe I'acide isophtalique brut, on evapore la liqueur-mere restante pour obtenir un condensat et on purifie le condensat par distillation pour recuperer de I'acide acetique, et c) une troisieme etape dans laquelle on melange I'acide isophtalique brut, separe dans la seconde 20 etape, avec de I'acide acetique purifie, on traite le melange resultant en I'agitant a une temperature non inferieure a 100°C, et on separe de I'acide isophtalique purifie.

2. Procede selon la revendication 1, dans lequel, dans I'etape a), les produits intermediaires sont I'acide m-toluique et le 3-carboxybenzaldehyde. 25 3. Procede selon la revendication 1 ou 2, dans lequel, dans I'etape a), le solvant constitue d'acide acetique et d'eau contient 3 a 15% en poids d'eau et 85 a 97% en poids d'acide acetique.

4. Procede selon I'une quelconque des revendications 1 a 3, dans lequel, dans I'etape a), la quantite de 30 m-xylene represente 1/3 a 1/12 du poids du solvant.

5. Procede selon I'une quelconque des revendications precedentes, dans lequel, dans I'etape a), le systeme catalytique cobalt/manganese/brome contient du carbonate, du tetrahydrate d'acetate ou du bromure de cobalt. 35 6. Procede selon I'une quelconque des revendications precedentes, dans lequel, dans I'etape a), le systeme catalytique cobalt/manganese/brome contient du carbonate, du tetrahydrate d'acetate ou du bromure de manganese.

40 7. Procede selon I'une quelconque des revendications precedentes, dans lequel, dans I'etape a), le systeme catalytique cobalt/manganese/brome contient un derive du brome, choisi parmi I'acide bromhydrique, le bromure de cobalt et le bromure de manganese.

8. Procede selon I'une quelconque des revendications precedentes, dans lequel, dans I'etape a), la 45 quantite totale de cobalt et de manganese est comprise entre 400 ppm et 1200 ppm, par rapport au poids du solvant.

9. Procede selon I'une quelconque des revendications precedentes, dans lequel, dans I'etape a), le rapport atomique du manganese au cobalt est de 0,8/1 a 4/1 . 50 10. Procede selon I'une quelconque des revendications precedentes, dans lequel, dans I'etape a), le rapport atomique du brome a la quantite totale de cobalt et de manganese est de 0,6/1 a 1 ,2/1 .

11. Procede selon I'une quelconque des revendications precedentes, dans lequel, dans I'etape b), le 55 melange est oxyde a une temperature comprise entre la temperature utilisee pour I'oxydation dans I'etape a) et une temperature qui est inferieure de 30 ° C a la temperature utilisee pour I'oxydation dans I'etape a).

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12. Procede selon I'une quelconque des revendications precedentes, dans lequel le gaz evacue provenant de I'etape a), presente une concentration d'oxygene de 3 a 6% en volume.

13. Procede selon I'une quelconque des revendications precedentes, dans lequel le gaz evacue provenant 5 de I'etape b) presente une concentration d'oxygene de 2 a 7% en volume.

14. Procede selon I'une quelconque des revendications precedentes, dans lequel I'etape b) comprend en outre une etape consistant a soumettre I'acide isophtalique brut a une cristallisation et une precipitation, et une etape consistant a separer les cristaux d'acide isophtalique resultant par filtration. 1070 15. Procede selon la revendication 14, dans lequel, apres separation des cristaux d'acide isophtalique par filtration, on evapore la liqueur-mere restante pour obtenir un condensat que Ton distille pour recuperer de I'acide acetique purifie.

75 16. Procede selon la revendication 14, dans lequel, apres separation des cristaux d' acide isopthalique par filtration, on introduit une partie de la liqueur-mere restante dans le reacteur d'oxydation de I'etape a).

17. Procede selon la revendication 14, dans lequel, apres separation des cristaux d'acide isophtalique par filtration, on distille la liqueur-mere restante et on evapore un liquide de fond de colonne, provenant de 20 la distillation, pour recuperer de I'acide acetique.

18. Procede selon I'une quelconque des revendications precedentes, dans lequel, dans I'etape c), on utilise I'acide acetique purifie en une quantite qui represente 1 a 5 fois le poids de I'acide isophtalique brut.

25 19. Procede selon I'une quelconque des revendications precedentes, dans lequel I'acide acetique restant dans I'etape c) apres separation de I'acide isophtalique purifie, est utilise comme solvant dans I'etape a) ou I'etape b).