Title of Invention

"PROCESS OF PRODUCING HIGH-PURITY TEREPHTHALIC ACID"

Abstract

The life of a reduction catalyst to be used in a reduction step where 4CBA is reduced with hydrogen to p-toluic acid is prolonged. The process for producing high-purity terephthalic acid is characterized in that, in the reduction step, the ratio of the amount of hydrogen to be fed to a hydrogenation reactor to the amount of 4CBA in a solution thereof to be fed to the reactor, hydrogen/4CBA, is from 2 to 10 (by mole) and that the ratio of the amount of crude terephthalic acid to be treated in the reduction step to the amount of the reduction catalyst present in the hydrogenation reactor, [amount of terephthalic acid being treated (t/h)]/[amount of reduction catalyst (t)], is from 400 to 5,000 (1/h).

Full Text

This invention relates to a process of producing high-purity terephthalic acid. In general, crude terephthalic acid obtained by oxidation of p—xylene contains relatively large amounts of a variety of impurities including 4-carboxybenzaldehdyde (hereinafter abbreviated as "4CBA") and after purification, has hitherto been used as a raw material of polyesters. As a method of purifying such crude terephthalic acid, there is employed a method of separation and purification including a dissolution step of dissolving crude terephthalic acid in water, a reduction step of reducing the foregoing 4CBA in the solution obtained in this dissolution step with hydrogen into p-toluic acid, and subsequently, a crystallization step of performing crystallization by utilizing a difference in solubility in water between terephthalic acid and p-toluic acid. Now, in this reduction step, a decarboxylation reaction of converting 4CBA into benzoic acid takes place as a side reaction. In this side reaction, carbon monoxide is generated as a by-product. This carbon monoxide works as a catalyst poison of a reducing catalyst to be used in this reduction step, resulting in progression of the deterioration of this reducing catalyst and a lowering of the activity. Then, this invention is aimed to suppress the deterioration of the reducing catalyst to be used in the reduction step of reducing 4CBA with hydrogen into p-toluic acid, thereby keeping the activity. This invention is to solve the foregoing problems by a process of producing high-purity terephthalic acid including oxidizing p-xylene to produce crude terephthalic acid containing 4CBA as an impurity; subsequently, performing a dissolution step of dissolving it in an aqueous medium; subsequently, continuously feeding a solution obtained in the foregoing dissolution step and hydrogen into a hydrogenation reactor filled with a reducing catalyst to perform a reduction step of reducing 4CBA in the foregoing solution; and sending the reduction treated liquid obtained in this reduction step to a group of crystallization tanks comprising at least two crystallization tanks connected in series and performing a crystallization step of crystallizing terephthalic acid by releasing the pressure and cooling in each crystallization tank step by step, wherein in the foregoing reduction step, a ratio of the amount of the foregoing hydrogen to be fed into the foregoing hydrogenation reactor to the amount of 4CBA in the foregoing solution to be fed into the foregoing hydrogenation reactor is controlled at hydrogen/4CBA = 2 to 10 (molar ratio) , and a ratio of the treatment amount of crude terephthalic acid to be treated in the foregoing reaction step to the amount of the reducing catalyst in the foregoing hydrogenation reaction is controlled at [treatment amount (t/h) of terephthalic acid] / [amount (t) of reducing catalyst] = 400 to 5,000 1/h) . In the foregoing reduction step, when the catalyst amount and the feed amount of hydrogen are made to fall within the foregoing ranges, it is possible to control a ratio of carbon monoxide to be formed in the hydrogenation reactor to the amount of 4CBA to be fed into the hydrogenation reactor preferably at (carbon monoxide)/4CBA = 0.01 to 0.12 (molar ratio). Also, when the amount of monoxide is made to fall within a prescribed range, it is possible to prolong the life of the reducing catalyst to be used in the reduction step. Fig. 1 is a flow diagram to show an example of the production step of high-purity terephthalic acid according to this invention. Fig. 2 is a flow diagram to show another example of the production step of high-purity terephthalic acid according to this invention. Incidentally, with respect to the reference numerals and signs in the drawings, 11 denotes a mixing tank; 12 denotes a fist heat exchanger; 13 denotes a hydrogenation reactor; 14 and 14a denote a crystallization tank; 15 denotes a solid-liquid separator; 16 denotes a second heat exchanger; 21 denotes a measuring column; 22 denotes a pre-heater; 23 denotes a dissolution column; 24 denotes a hydrogenation reactor; 25 denotes a receiver; 26 denotes a first crystallization tank; 27 denotes a second crystallization tank; 28 denotes a pressure separator; A denotes crude terephthalic acid; B denotes an aqueous medium; C denotes a slurry of crude terephthalic acid; D denotes a solution; D' denotes an aqueous solution; E denotes a reduction treated liquid; E' denotes a reduction treated liquid; F denotes a terephthalic acid slurry; G denotes high-purity terephthalic acid; G' denotes a high-purity terephthalic acid crystal; H denotes a separation mother liquor; H' denotes a separation mother liquor; I denotes a slurry; S and SI denote a vapor gas; T and Tl denote a discharge gas; and U denotes a condensate. This invention will be described below in detail. High-purity terephthalic acid according to this invention is produced through the steps shown in Fig. 1. First of all, crude terephthalic acid A containing 4CBA as an impurity, as produced by oxidizing p-xylene in an oxidation step is used as a raw material. This crude terephthalic acid A contains from 1,000 to 5,000 ppm of 4CBA. This crude terephthalic acid A is suspended in an aqueous medium B in a mixing tank 11 to form a slurry C of crude terephthalic acid. Subsequently, a dissolution step is performed in such a manner that this slurry C of crude terephthalic acid is pressurized to a prescribed pressure in the mixing tank 11 and heated by a first heat exchanger 12, thereby completely dissolving the foregoing crude terephthalic acid A in the foregoing aqueous medium B to obtain a solution D. Subsequently, a reduction step is performed in such a manner that the foregoing solution D and hydrogen are continuously fed into a hydrogenation reactor 13 filled with a reducing catalyst, thereby reducing 4CBA in the foregoing solution to obtain p-toluic acid. Then, a crystallization step is performed in such a manner that a reduction treated liquid E obtained in this reduction step is sent to a group of crystallization tanks comprising at least two crystallization tanks 14 connected in series, thereby crystallizing terephthalic acid. At this time, since the solubility of p-toluic acid in the aqueous medium is higher than that of terephthalic acid in the aqueous medium, in this crystallization step, terephthalic acid is deposited in the state that the greater part of p-toluic acid is dissolved, whereby a terephthalic acid slurry can be obtained. The resulting terephthalic acid slurry F is introduced into a solid-liquid separator 15 and subjected to a solid-liquid separation step, thereby separating into a solid comprising high-purity terephthalic acid G and a separation mother liquor H. Then, this high-purity terephthalic acid G is dried and forwarded as a product. The foregoing reducing catalyst is not particularly limited so far as it does not have a reducing ability of a carboxyl group but has a reducing ability of an aldehyde group. Examples thereof include metals belonging to the Groups 8 to 10 of the Periodic Table, such as palladium, platinum, rhodium, iridium, ruthenium, cobalt, and nickel. Also, since this catalyst is of a heterogeneous catalyst system, it is preferable from the standpoint of reaction efficiency that the catalyst is supported on a carrier such as active carbon. In the case where the foregoing supported catalyst is used as the foregoing reducing catalyst, the supporting amount of the foregoing metal component varies depending upon conditions such as pressure, temperature, and flow velocity but is from 0.2 to 10 % by weight, and preferably from 0.3 to 1 % by weight. When the supporting amount is less than 0.2 % by weight, a satisfactory reduction reaction tends to be hardly achieved. On the other hand, when it exceeds 10 % by weight, a rate of occurrence of side reactions described later tends to increase. The foregoing aqueous medium refers to a medium to be used in producing high-purity terephthalic acid using the foregoing crude terephthalic acid and specifically means water. The crystallization in the foregoing crystallization step is a method in which the foregoing reduction treated liquid E is introduced into the crystallization tanks 14 set up under a pressure condition lower than the pressure of the foregoing reduction treated liquid E or the pressure of the preceding crystallization tank 14, and the pressure is released herein, and following this, cooling is performed (hereinafter, this pressure release and cooling operation will be abbreviated as "pressure-release and cooling") . In this way, terephthalic acid is crystallized. At this time, the group of crystallization tanks is constructed of at least two crystallization tanks 14. By performing pressure-release and cooling in each crystallization tank 14 step by step, it is possible to control the particle size distribution of terephthalic acid to be deposited and to inhibit eutectoid of p-toluic acid. For example, the foregoing reduction treated liquid E to be introduced into the first crystallization tank 14a of the foregoing group of crystallization tanks has from 5.5 to 9.8 MPa and from 270 to 300 °C, and when introduced into the foregoing crystallization tank 14a, it is subjected to pressure-release and cooling to from 230 to 260 °C. At this time, the operating pressure is a saturated vapor pressure at this temperature, and specifically, is from 2.8 to 4.7 MPa. In this crystallization step, by performing pressure-release and cooling, a part of the reduction treated liquid E or an aqueous medium of a slurry I in which terephthalic acid has been crystallized in the preceding crystallization tank 14 is vaporized to generate a vapor gas S. This vapor gas S is sent to a second heat exchanger 16 and heat exchanged, thereby separating into a discharge gas T and a condensate U. Now, in the foregoing reduction reaction step, the formation of p-toluic acid by reduction of 4CBA is a principal reaction as described previously. However, besides, a reaction of forming benzoic acid and carbon monoxide by decarboxylation of 4CBA takes place as a side reaction. Carbon monoxide formed in this side reaction becomes a catalyst poison of the reducing catalyst in the foregoing hydrogenation reactor 13 and brings a lowering of the catalyst life. For the sake of inhibiting this lowering of the catalyst life, a ratio of the amount of carbon monoxide to be formed in the hydrogenation reactor 13 to the amount of 4CBA to be fed into the hydrogenation reactor may be controlled at (carbon monoxide)/4CBA = 0.01 to 0.12 (molar ratio), and preferably from 0.03 to 0.1 (molar ratio). When the foregoing amount ratio is less than 0.01, the catalyst amount becomes short so that it is difficult to completely reduce 4CBA, whereas when it exceeds 0.1, a lowering of the catalyst life proceeds so that, for example, the catalyst must be exchanged with a period of time of from about 6 months to less than one year. Incidentally, the amount of carbon monoxide to be formed in the hydrogenation reactor can be determined from [(flow rate of a gas generated from the first crystallization tank) x (concentration of carbon monoxide in the gas)]. Also, in the crystallization tank 14a to which the foregoing reduction treated liquid E is first sent in the crystallization step, a content ratio of carbon monoxide and hydrogen in a vapor gas SI containing the aqueous medium as the major component, which is generated by pressure-release and cooling, may be controlled at from 0.00003 to 0.03 (molar ratio), and preferably from 0.00005 to 0.01 in terms of (carbon monoxide)/[(carbon monoxide) + hydrogen] . As this value, for the purpose of simplicity, a content ratio of carbon monoxide and hydrogen in a discharge gas Tl remaining after condensation of moisture in the foregoing vapor gas SI by the heat exchanger 16 may be employed. This is because a considerable amount of moisture is removed in this discharge gas Tl, and hence, analysis can be achieved more easily. When the foregoing content ratio is less than 0.00003, the hydrogen gas to be fed into the foregoing hydrogenation reactor 13 becomes excessive so that a side reaction to cause hydrogenation of a benzene ring of the foregoing terephthalic acid tends to take place. On the other hand, when it exceeds 0.03, the reducing catalyst in the foregoing hydrogenation reactor 13 is deteriorated due to the catalyst poison of carbon monoxide so that the catalyst life is liable to lower. Examples of methods of adjusting the ratio of the amount of carbon monoxide to be formed in the foregoing hydrogenation reactor 13 to the amount of 4CBA to be fed into the hydrogenation reactor and the content ratio of carbon monoxide and hydrogen in the vapor gas SI (or the discharge gas Tl) so as to fall within the foregoing ranges include a method of adjusting the amount of the hydrogen gas to be fed into the foregoing hydrogenation reactor 13 and a method of adjusting the treatment amount of crude terephthalic acid A to be treated in the foregoing reduction step. Of these, it is preferred to combine a method of adjusting the amount of the hydrogen gas to be fed into the foregoing hydrogenation reactor 13 with a method of adjusting a ratio of the treatment amount of crude terephthalic acid A to be treated in the foregoing reduction step and the amount of the catalyst, thereby adjusting the ratio of the amount of carbon monoxide to be formed in the foregoing hydrogenation reactor 13 to the amount of 4CBA to be fed into the hydrogenation reactor and the content ratio of carbon monoxide and hydrogen in the vapor gas SI (or the discharge gas Tl) so as to fall within the foregoing ranges. The foregoing method of adjusting the amount of the hydrogen gas to be fed into the hydrogenation reactor 13 means adjustment of a ratio of the amount of the foregoing hydrogen to be fed into the foregoing hydrogenation reactor 13 to the amount of 4CBA in the foregoing solution to be fed into the foregoing hydrogenation reactor in the foregoing reduction step. Specifically, the hydrogen/4CBA ratio is from 2 to 10, and preferably 2 to 7 in terms of molar ratio. When this ratio is less than 2, a decarboxylation reaction of 4CBA is liable to take place as a side reaction. For this reason, the reducing catalyst in the foregoing hydrogenation reactor 13 is deteriorated due to a catalyst poison of carbon monoxide formed in this reaction so that the catalyst life is liable to lower. On the other hand, when the foregoing ratio exceeds 10, the hydrogen gas to be fed into the foregoing hydrogenation reactor 13 becomes excessive so that a side reaction to cause hydrogenation of a benzene ring of the foregoing terephthalic acid tends to take place. Also, the method of adjusting the treatment amount of crude terephthalic acid A to be treated in the foregoing reduction step means adjustment of a ratio of the treatment amount (treatment rate) of crude terephthalic acid A to be treated in the foregoing reduction step to the amount of the reducing catalyst in the foregoing hydrogenation reactor, that is, the amount of the metallic component in the reducing catalyst. Specifically, the [treatment amount (t/h) of crude terephthalic acid A]/[amount (t) of reducing catalyst] ratio is from 400 to 5,000 (1/h), and preferably from 1,000 to 3,000 (1/h). When this ratio is less than 400 (1/h) , the treatment efficiency of crude terephthalic acid A lowers. On the other hand, when it exceeds 5,000 (1/h), there may be the case where the reduction treatment cannot be sufficiently achieved. (Example 2) A hydrogenation reaction was performed using a process shown in Fig. 2. Specifically, a measuring tank 21 was charged with 1.2 kg/h of crude terephthalic acid A containing 3,000 ppm of 4CBA and water B to produce a 20 % by weight crude terephthalic acid slurry C. The slurry C was pressurized to a pressure of 7.9 MPa; the temperature was raised to 280 °C by a pre-heater 22; the terephthalic acid was dissolved in a dissolution column 23; and this terephthalic acid aqueous solution D' was then fed into a hydrogenation reactor 24. This hydrogenation reactor 24 had a column diameter of 15 mm and a height of 1,450 mm, 0.5 % by weight palladium supported on active carbon was used as a reducing catalyst, and the charge amount of the catalyst was 120 g. In this hydrogenation reactor 24, hydrogen was fed at the same time to reduce the foregoing terephthalic acid aqueous solution. The resulting reduction treated liquid E' was once kept in a receiver 25 for the purpose of regulating the treatment amount, cooled to 225 °C in a first crystallization tank 26, and then cooled to 150 °C in a second crystallization tank 27 to achieve crystallization. The slurry obtained in the crystallization was subjected to solid-liquid separation by a pressure separator 28 and separated into a crystal G' and a separation mother liquor H' . The crystal G' was dried and then recovered as high-purity terephthalic acid. In the forgoing reduction step, the operation was carried out in such a manner that the hydrogen/4CBA ratio, which is a ratio of the amount of the foregoing hydrogen to be fed into the foregoing hydrogenation reactor 24 to the amount of 4CBA in the foregoing solution to be fed into the foregoing hydrogenation reactor 24, was 6 (molar ratio) and that the [treatment amount (kg/h) of crude terephthalic acid]/[amount (kg) of reducing catalyst] ratio, which is a ratio of the treatment amount of crude terephthalic acid to be treated in the foregoing reduction step to the amount of the reducing catalyst in the foregoing hydrogenation reactor, was 2OOO (1/h) . By this operation, the amount of carbon monoxide to be formed in the foregoing hydrogenation reactor was (carbon monoxide)/4CBA = 0.07 (molar ratio) in terms of a ratio to the amount of 4CBA in the foregoing solution to be fed into the hydrogenation reactor. (Example 3) The operation was carried out under the same conditions as in the foregoing Example 2, except that the hydrogen/4CBA ratio, which is a ratio of the amount of hydrogen to be fed into the hydrogenation reactor 24 to the amount of 4CBA in the solution to be fed into the hydrogenation reactor 24, was 3 (molar ratio). By this operation, the amount of carbon monoxide to be formed in the foregoing hydrogenation reactor was (carbon monoxide)/4CBA = 0.09 (molar ratio) in terms of a ratio to the amount of 4CBA in the foregoing solution to be fed into the hydrogenation reactor. (Comparative Example 2) The operation was carried out under the same conditions as in the foregoing Example 2, except that the hydrogen/4CBA ratio, which is a ratio of the amount of hydrogen to be fed into the hydrogenation reactor 24 to the amount of 4CBA in the solution to be fed into the hydrogenation reactor 24, was 1 (molar ratio). By this operation, the amount of carbon monoxide to be formed in the foregoing hydrogenation reactor was (carbon monoxide) /4CBA = 0.15 (molar ratio) in terms of a ratio to the amount of 4CBA in the foregoing solution to be fed into the hydrogenation reactor, and therefore, the amount of carbon monoxide to be formed largely increased. (Example 4) The operation was carried out under the same conditions as in the foregoing Example 3, except that the [treatment amount (kg/h) of crude terephthalic acid]/[amount (kg) of reducing catalyst] ratio, which is a ratio of the treatment amount of crude terephthalic acid to be treated in the reduction step to the amount of the reducing catalyst in the hydrogenation reactor 24, was 600 (1/h) . By this operation, the amount of carbon monoxide to be formed in the foregoing hydrogenation reactor 24 was (carbon monoxide)/4CBA = 0.08 (molar ratio) in terms of a ratio to the amount of 4CBA in the foregoing solution to be fed into the hydrogenation reactor. (Comparative Example 3) The operation was carried out under the same conditions as in the foregoing Example 3, except that the [treatment amount (kg/h) of crude terephthalic acid]/[amount (kg) of reducing catalyst] ratio, which is a ratio of the treatment amount of crude terephthalic acid to be treated in the reduction step to the amount of the reducing catalyst in the hydrogenation reactor 24, was 200 (1/h) . By this operation, the amount of carbon monoxide to be formed in the foregoing hydrogenation reactor was (carbon monoxide)/4CBA = 0.17 (molar ratio) in terms of a ratio to the amount of 4CBA in the foregoing solution to be fed into the hydrogenation reactor. (Referential Example) [Test of activity of hydrogenating catalyst by microcylinder] A microcylinder was charged with 0.5 g of a 0.5 % by weight Pd/C hydrogenating catalyst, 60 g of pure water, and 15 g of crude terephthalic acid (4CBA concentration: 3,000 ppm), a prescribed composition gas (three kinds: 0 % by volume, 2.5 % by volume, and 50 % by volume of carbon monoxide) was incorporated to a pressure of 0.6 MPa, and the reaction was then started at a reaction temperature of 275 °C. After performing the reaction for 15 minutes, the 4CBA concentration of purified terephthalic acid was measured, from which was then calculated the disappearing rate of 4CBA. [Disappearing rate of 4CBA (1/min)] = In [(4CBA concentration in crude terephthalic acid)/(4CBA concentration in purified terephthalic acid)]/15 As a result, when the composition gas having 0 % by volume of carbon monoxide was used, the disappearing rate of 4CBA was 0.150 (1/min); when the composition gas having 2.5 % by volume of carbon monoxide was used, the disappearing rate of 4CBA was 0.086 (1/min); and when the composition gas having 50 % by volume of carbon monoxide was used, the disappearing rate of 4CBA was 0.027 (1/min), respectively. From this model experiment, it is noted that when the concentration of carbon monoxide in the reduction reactor increases, the catalytic activity lowers, and the disappearing rate of 4CBA lowers. Accordingly, it is estimated that in the case where the amount ratio of hydrogen/4CBA and the [treatment amount (t/h) of terephthalic acid]/[amount (t) of reducing catalyst] ratio do not fall within the ranges of the invention of this application, the concentration of carbon monoxide in the hydrogenation reactor increases to a fixed amount or more, and as a result, the degree of a lowering of the catalytic activity becomes fast so that the catalyst must be exchanged within a period of time of less than one year. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof. This application is based on a Japanese patent application filed January 7, 2003 (Japanese Patent Application No. 2003-001060), the entire contents thereof being hereby incorporated by reference. According to this invention, since the amount of carbon monoxide in a gas from a first crystallization tank is made to fall within a prescribed range, it is possible to suppress the deterioration of a reducing catalyst to be used in a reduction reaction and to keep the activity. Also, when the amount of carbon monoxide in a gas from a first crystallization tank is made to fall within a prescribed range, side reactions caused in a reduction step are inhibited, and therefore, it is possible to produce purified terephthalic acid having a higher purity in a high yield. WE CLAIM: 1. A process of producing high-purity terephthalic acid, which comprises performing a dissolution step of dissolving crude terephthalic acid containing 1000 to 5000 ppm of 4-carboxybenzaldehyde as an impurity in an aqueous medium by suspending the crude terephthalic acid in the aqueous medium to obtain slurry of the crude terephthalic acid and subjecting the crude terephthalic acid slurry to pressurization and heating; subsequently, continuously feeding a solution obtained in said dissolution step and hydrogen into a hydrogenation reactor filled with a reducing catalyst comprising metals belonging to the Groups 8 to 10 of the Periodic Table to perform a reduction step of reducing 4-carboxybenzaldehdyde in said solution wherein carbon monoxide is generated as a by-product; and sending the reduction treated liquid having a pressure of from 5.5 to 9.8 MPa and a temperature of from 270 to 300° C obtained in this reduction step to a group of crystallization tanks comprising at least two crystallization tanks connected in series and performing a crystallization step of crystallizing terephthalic acid by releasing the pressure and cooling in each crystallization tank step by step in the first crystallization tank, the releasing the pressure release and cooling are performed to a pressure of from 2.8 to 4.7 MPa and temperature of from 230 to 260° C wherein, in said reduction step, a ratio of the amount of said hydrogen to be fed into said hydrogenation reactor to the amount of 4-carboxybenzaldehdyde in said solution to be fed into said hydrogenation reactor is controlled at hydrogen/4-carboxybenzaldehdyde = 2 to 10 molar ratio, and a ratio of the treatment amount of crude terephthalic acid to be treated in said reaction step to the amount of the reducing catalyst is controlled at [treatment amount (t/h) of crude terephthalic acid]/amount [t] of reducing catalyst = 400 to 5,000 [1/h]. 2. The process of producing high-purity terephthalic acid as claimed in claim 1, wherein a ratio of the amount of carbon monoxide to be formed in said hydrogenation reactor to the amount of 4-carboxybenzaldehdyde in said solution in said hydrogenation reactor is from 0.01 to 0.12 [molar ratio] in terms of [carbon monoxide] / [4-carboxybenxaldehdyde].

Documents:

2743-delnp-2004-abstract.pdf

2743-delnp-2004-claims.pdf

2743-delnp-2004-complete specification (granted).pdf

2743-DELNP-2004-Correspondence-Others-(17-06-2010).pdf

2743-delnp-2004-correspondence-others.pdf

2743-delnp-2004-correspondence-po.pdf

2743-delnp-2004-description (complete).pdf

2743-delnp-2004-form-1.pdf

2743-delnp-2004-form-19.pdf

2743-delnp-2004-form-2.pdf

2743-DELNP-2004-Form-3-(17-06-2010).pdf

2743-delnp-2004-form-3.pdf

2743-delnp-2004-form-4.pdf

2743-delnp-2004-form-5.pdf

2743-delnp-2004-gpa.pdf

2743-delnp-2004-pct-301.pdf

2743-delnp-2004-pct-304.pdf

2743-DELNP-2004-Petition 137-(17-06-2010).pdf

2743-delnp-2004-petition-137.pdf