Title of Invention

METHOD OF FEEDING A SOLUTION OF CRUDE TEREPHTHALIC ACID INTO A REACTOR

Abstract

THE DISCLOSURE In order to prevent local thinning or destruction of an inner wall of a reactor when an aqueous solution of crude terephthalic acid is supplied into the reactor, according to this invention, in a method for feeding a solution of crude terephthalic acid into the reaction during a process for producing high-purity terephthalic acid in which crude terephthalic acid obtained by oxidizing paraxylene in an acetic acid solvent is dissolved in water, the liquid thus obtained is subjected to reduction treatment with hydrogen in the reactor in the presence of a catalyst, and the reduced product thus obtained is subjected to crystallization and solid-liquid separation, the solution of crude terephthalic acid is fed into the reactor at a flow rate not exceeding 1 meter per second.

Full Text

Specification Method of feeding solution of crude terephthalic acid to reactor Technical field This invention relates to a method for feeding a solution of crude terephthalic acid into a reactor during a process for producing high-purity terephthalic acid. Background art In producing high-purity terephthalic acid, which is a raw material for polyesters, crude terephthalic acid crystals are first produced by dissolving paraxylene in an acetic acid solvent and oxidizing it. Then, an aqueous solution of the thus produced crude terephthalic acid is passed through a reactor of the packed tower type which includes a catalyst layer containing a metal of the platinum group under high temperature, high pressure conditions. In the reactor, the solution is subjected to hydrogenation to purify the crude terephthalic acid, thereby producing high-purity terephthalic acid. During the above production steps, hydrogen which has been pressurized to a pressure higher than the reaction pressure is supplied into the packed tower reactor, together with the high-temperature, high-pressure aqueous solution of crude terephthalic acid. If the aqueous solution of terephthalic acid fed into the catalyst layer in the reactor contains any undissolved terephthalic acid crystals, the reactor cannot be operated in a stable manner. Thus, the aqueous solution is fed into a buffer tank before being fed into the reactor to completely dissolve any terephthalic acid crystals. Otherwise, the following alternative measures are taken. That is, as shown in Fig. 4, a tubular overflow wall 3 is provided at the upper portion of the packed tower reactor 11 to define a retention zone 4, and an aqueous solution of crude terephthalic acid is fed through a supply port 12 into the retention zone 4 to cause the solution to overflow the tubular overflow wall so that any terephthalic acid crystals contained in the solution are completely dissolved when the solution is passed through the catalyst layer 2 (as disclosed e.g. in patent document 1). To a port 5, a hydrogen supply pipe is connected. (Patent document 1- Japanese patent No. 3232700 (claims)) Disclosure of the invention However, in the above-described conventional method of feeding a crude terephthalic acid solution into a reactor, if an aqueous solution of crude terephthalic acid is fed into the reactor at a high flow rate, local thinning of the wall of the reactor (e.g. at its portion shown by chain line in Fig. 4), or in the worst case, destruction of the wall of the reactor may result due to what is known as erosion-corrosion, which is the synergetic effect of mechanical action, i.e. erosion due to pressure when the high-pressure crude terephthalic acid solution collides against the inner wall of the reactor, the overflow wall, and other structures in the reactor, and the chemical action by high-temperature corrosive components. Actually, when reactors of which the inner walls are made of stainless steel or a known corrosion-resistant alloy (such as titanium steel or hastelloy steel) are operated for several months, their inner walls tend to suffer local degradation. An object of the present invention is to solve the abovementioned problems, more specifically to prevent local thinning or destruction of the inner wall of a reactor when an aqueous solution of crude terephthalic acid is fed into the reactor. According to the present invention, there is provided a method of feeding a solution of crude terephthalic acid into a reactor during a process for producing high-purity terephthalic acid, the method comprising dissolving crude terephthalic acid obtained by oxidizing paraxylene in an acetic acid solvent into water to prepare a solution, subjecting the solution to reduction treatment with hydrogen in the reactor in the presence of a catalyst to produce a reduced product, and subjecting the reduced product to crystallization and solid-liquid separation, characterized in that the solution of crude terephthalic acid is fed into the reactor at a flow rate not exceeding 1 meter per second. With this arrangement, since the solution of crude terephthalic acid is brought into contact with the inner wall of the reactor at the flow rate of the solution, which is not more than 1 meter per second, the pressure and the flow of the solution of crude terephthalic acid, which is normally heated to 230 degrees Celsius or over, will place no significant burden on the inner wall of the reactor. This significantly reduces the possibility of local thinning, or in the worst case, destruction, of the inner wall of the reactor due to erosion-corrosion. In order to achieve this object more reliably, in the method of feeding the solution of crude terephthalic acid into the reactor, a dispersing device for dispersing fluid flow is preferably provided in the reactor to slow down the flow rate of the solution of crude terephthalic acid fed into the reactor. With this arrangement, it is possible to reliably reduce the burden on the inner wall of the reactor. Preferably, the dispersing device includes a supply port to which a supply pipe through which the solution of crude terephthalic acid is fed, and a plurality of dispersing ports through which the solution is dispersed, with the ratio SA/S set to be greater than 1, where SA is the total sectional area of the plurality of dispersing ports, and S is the internal sectional area of the supply port. With this arrangement, it is possible to significantly slow down the flow rate at the dispersing ports compared to the flow rate at the supply port. Thus, by adjusting the ratio SA/S, it is possible to reliably slow down the solution of crude terephthalic acid that has passed the dispersing device. Preferably, the dispersing device is an annular pipe connected to the supply pipe, and the plurality of dispersing ports are through holes formed in a peripheral wall of the annular pipe. With this arrangement, since the solution of crude terephthalic acid can be dispersed through the peripheral wall of the annular pipe in every direction toward the inner wall of the pipe, the flow pressure of the solution is also dispersed. Thus, its flow rate drops. According to the present invention, during the production process of high-purity terephthalic acid, a solution of crude terephthalic acid is fed into the reactor at a flow rate not exceeding 1 meter per second. Thus, the pressure and the flow of the solution of crude terephthalic acid, which is normally heated to 230 degrees Celsius or over, will place no significant burden on the inner wall of the reactor. This significantly reduces the possibility of local thinning, or in the worst case, destruction, of the inner wall of the reactor. By providing the dispersing device for dispersing fluid flow in the reactor to reduce the flow rate of the solution of crude terephthalic acid fed into the reactor, it is possible to more reliably achieve the object of the present invention. By providing the dispersing device for dispersing fluid flow in the reactor, and further by setting the ratio of the total sectional area SA of the plurality of dispersing ports to the internal sectional area S of the supply port so as to exceed 1, it is possible to reliably reduce the burden on the inner wall of the reactor. Brief description of the drawings Fig. 1 is a schematic view of a reactor embodying the invention; Fig. 2 is a vertical sectional view of an upper portion of the reactor of the embodiment; Fig. 3 is a sectional view taken along line III-III of Fig. 2; Fig. 4 is a schematic view of a conventional reactor; Fig. 5 is a cross-sectional view of an upper portion of another reactor embodying the invention; and Fig. 6 is a cross-sectional view of an upper portion of another conventional reactor. Best mode for embodying the invention The embodiments of the invention are now described with reference to the drawings. In the process for producing high-purity terephthalic acid, crude terephthalic acid obtained by oxidizing paraxylene in a liquid phase is dissolved into water, and the thus obtained aqueous solution of crude terephthalic acid is subjected to reduction treatment by passing the solution through the catalyst layer 2 in the reactor 1. The reduced product thus obtained is subjected to crystallization and solid-liquid separation. According to the invention, the solution of crude terephthalic acid is fed into the reactor at a flow rate not exceeding 1 meter per second, preferably at a flow rate not exceeding 0.9 meters per second, more preferably at a flow rate not exceeding 0.8 meters per second. If the flow rate is too low, a larger dispersing device is needed. Thus, the flow rate is preferably not less than 0.1 meters per second, more preferably not less than 0.2 meters per second. Paraxylene is oxidized by what is known as the SD process in which paraxylene is reacted with molecular oxygen typically in an acetic acid solvent in the presence of a catalyst containing e.g. cobalt, manganese and bromine at a temperature of typically 170 to 230 degrees Celsius. The crude terephthalic acid thus produced is crystalline and contains 4-carboxybenzaldehyde (hereinafter "4CBA") as an impurity typically by 1000 to 5000 ppm in weight. Terephthalic acid is low in solubility at normal temperature and normal pressure. Thus, in order to increase the solubility of terephthalic acid, it is necessary to increase the temperature and pressure. The following is a typical method of obtaining an aqueous solution of crude terephthalic acid. First, crude terephthalic acid is mixed into water by 10 to 40 percent by weight to produce a slurry. The slurry is pressurized to the reaction pressure plus a (which is a value equivalent to pressure loss until the slurry reaches the reactor) with a pressurizing pump, and is supplied to a heating/dissolving step in which a multitubular heat exchanger is combined. The slurry is heated in a stepwise manner to a predetermined reaction temperature using preferably a plurality of heat exchangers. The solution of crude terephthalic acid is thus heated to 230 degrees Celsius or over. The aqueous solution of crude terephthalic acid thus obtained is passed through the reactor 1, which is of the packed tower type and in which is received the catalyst layer 2, which contains a metal of the platinum group. In the reactor 1, the crude terephthalic acid is purified by hydrogenation. Specifically, 4CBA in the aqueous solution of crude terephthalic acid is reduced with hydrogen into paratoluic acid. The catalyst containing a metal of the platinum group is one selected from palladium, ruthenium, rhodium, osmium, iridium, platinum, etc. and their metallic oxides. Such a metal or metallic oxide may be used as it is as a catalyst, but is preferably carried on a carrier such as activated carbon by 0.2 to 10 percent by weight. The reaction temperature is 200 to 400 degrees Celsius, preferably 230 to 350 degrees Celsius. The reaction pressure should be kept at a level at which the slurry can maintain its liquid state. Specifically, the reaction pressure should be not less than 1.6 MPa, preferably 2.8 to 16.5 MPa. Hydrogen is supplied into the reactor after being pressurized to the reaction pressure or higher. As shown in Figs. 1 to 3, the packed tower reactor 1 of the embodiment has a retention zone 4 for an aqueous solution of crude terephthalic acid which is defined by an overflow wall 3 at the top of the reactor 1 near its inlet. The catalyst layer 2 as the reaction zone is disposed under the retention zone 4. Figs. 2 and 3 show the detailed structure of the top of the packed tower reactor 1 near its inlet. As shown, the reactor 1 has a dome-shaped top space having a hydrogen supply pipe connecting port 5. A disk-shaped partitioning plate 6 partitions the dome-shaped top space into the retention zone 4 and the underlying reaction zone 10. The tubular overflow wall 3 extends vertically from the central portion of the disk-shaped partitioning plate 6. A crude terephthalic acid solution supply pipe 7 extends through the side wall defining the retention zone 4, and is connected to an annular pipe 8 as a dispersing device. The annular pipe 8 is a circular ring having a circular cross-section and formed with many small through holes 9 in its peripheral wall. But instead, the annular pipe 8 may be a polygonal pipe having a circular or a polygonal cross-section. The ratio of the total sectional area SA of the small holes 9 formed in the annular pipe 8 to the internal cross-sectional area S of the supply pipe 7, i.e. the ratio (SA/S) is greater than 1 so that even if the crude terephthalic acid solution is fed through the supply pipe 7 at a flow rate of 1.1 meters per second or higher, the solution will flow through the holes 9 at a flow rate not exceeding 1 meter per second. The ratio SA/S is preferably not less than 1.1, more preferably not less than 1.5. But if the ratio SA/S is too large, a larger dispersing device will be needed. Thus, the ratio SA/S is preferably not more than 10, more preferably not more than 5. With this arrangement, the solution supplied into the retention zone 4 rises along the overflow wall 3, overflows the wall 3, and is supplied into the reaction zone 10, which is disposed under the partitioning plate 6. Even if the solution contains undissolved crystal grains, such grains will settle in the retention zone 4 and will be mixed and dissolved into the flow of solution supplied into the retention zone 4 before overflowing the wall 3. Since the solution of crude terephthalic acid is controlled to flow through the holes 9 in the annular pipe 8 at a flow rate not exceeding 1 meter per second, the pressure and the flow of the high-temperature solution of crude terephthalic acid will place no significant burden on the inner wall of the reactor. This significantly reduces the possibility of local thinning, or in the worst case, destruction, of the inner wall of the reactor 1. The terephthalic acid solution thus normally overflows, is purified when passing through the catalyst layer 2 in the reaction zone 10, and is discharged from the system through an outlet port formed in the bottom of the reactor. The solution thus discharged is typically subjected to crystallization, solid-liquid separation and drying, and is recovered as purified terephthalic acid crystals. [Example l] A slurry containing crude terephthalic acid as a raw material by 30 percent by weight of the entire aqueous solution was pressurized to 9 MPa, and heated to 285 degrees Celsius with the multitubular heat exchanger. The slurry was then supplied to the solution supply pipe of the packed tower type shown in Figs. 1 to 3. The reactor measured 1.26 meters in diameter and 10 meters high. The catalyst layer was 7 meters high. The overflow wall was 0.7 meters high with the downcomer having a diameter of 0.3 meters. The reactor body was formed of clad steel comprising carbon steel on which an SUS304 layer having a thickness of 7 mm was laminated. The overflow wall was formed of titanium steel. A solution of crude terephthalic acid was fed through many holes formed in the peripheral wall of an annular dispersing device formed of titanium (SA/S = 1.8), at a flow rate of 0.5 to 0.7 meters per second and dispersed into the retention zone, which was provided at the top of the reactor, to let the solution in the retention zone overflow. Simultaneously, hydrogen gas was fed through the retention zone into the reaction zone. Reaction was carried out at a pressure of 8.0 MPa, a temperature of 285 degrees Celsius, and a hydrogen partial pressure of 0.8 MPa. As a catalyst, 0.5% palladium/carbon was used. Under these conditions, the reactor was operated continuously for about 150 days. During this period, no local thinning or destruction of the inner wall of the reactor due to erosion-corrosion was observed, and high-quality purified terephthalic acid was obtained. [Example 2] Except that a hexagonal pipe (SA/S = 1.6) as shown in Fig. 5 was used as the dispersing device, hydrogenation was carried out in the same manner as in Example 1. A solution of crude terephthalic acid was fed into the retention zone at a flow rate of about 0.7 meters per second. Under these conditions, the reactor was operated continuously for 150 days. During this period, no local thinning or destruction of the inner wall of the reactor due to erosion-corrosion was observed, and high-quality purified terephthalic acid was obtained. [Comparative Example l] Except that no annular pipe as the dispersing device was used, and a solution of crude terephthalic acid was fed through a solution supply port directly into the reactor at a flow rate of 1.1 meters per second, the reactor was operated continuously for about 90 days under exactly the same conditions as in Examples 1 and 2. Local thinning of the overflow wall in the reactor due to erosion-corrosion was observed. Specifically, the 7-mm-thick SUS304 layer had partially gone. [Comparative Example 2] Except that no annular pipe as the dispersing device was used, and a solution of crude terephthalic acid was fed through a solution supply port into the reactor in the tangential direction as shown in Fig. 6 at a flow rate of 1.1 meters per second, hydrogenation reaction was carried out in exactly the same manner as in Example 1. After the reactor had been operated continuously for 150 days, local thinning of the inner wall of the reactor due to erosion-corrosion was observed near the solution supply port. Specifically, the 7-mnrthick SUS304 layer was partially missing. [Comparative Example 3] Except that the reactor was formed of clad steel comprising carbon steel on which an SUS304 layer having a thickness of 5 mm was laminated, and a titanium layer having a thickness of 2 mm was further laminated, hydrogenation was carried out in exactly the same manner as in Comparative Example 2. After the reactor had been operated continuously for 150 days, local thinning of the inner wall of the reactor due to erosion-corrosion was observed near the solution supply port. Specifically, the 2-mm-thick titanium layer and the 5-mm-thick SUS304 layer were partially missing. What is claimed is 1. A method of feeding a solution of crude terephthalic acid into a reactor during a process for producing high-purity terephthalic acid, said method comprising dissolving crude terephthalic acid obtained by oxidizing paraxylene in an acetic acid solvent into water to prepare a solution, subjecting the solution to reduction treatment with hydrogen in the reactor in the presence of a catalyst to produce a reduced product, and subjecting the reduced product to crystallization and solid-liquid separation, characterized in that the solution of crude terephthalic acid is fed into the reactor at a flow rate not exceeding 1 meter per second. 2. The method of feeding the solution of crude terephthalic acid into the reactor of claim 1 wherein the solution of crude terephthalic acid is a solution of crude terephthalic acid which has been heated to 230 degrees Celsius or over. 3. The method of feeding the solution of crude terephthalic acid into the reactor of clkim 1 or 2 wherein a dispersing device for dispersing fluid flow is provided in the reactor to slow down the flow rate of the solution of crude terephthalic acid fed into the reactor. 4. The method of feeding the solution of crude terephthalic acid into the reactor of claim 3 wherein said dispersing device includes a supply port to which a supply pipe through which the solution of crude terephthalic acid is fed, and a plurality of dispersing ports through which the solution is dispersed, and wherein the ratio SA/S is greater than 1, where, SA is the total sectional area of said plurality of dispersing ports, and S is the internal sectional area of said supply port. 5. The method of feeding the solution of crude terephthalic acid into the reactor of claim 4 wherein said dispersing device is an annular pipe connected to said supply pipe, and wherein said plurality of dispersing ports are through holes formed in a peripheral wall of said annular pipe. 6. A process for producing terephthalic acid wherein the method of any of claims 1 to 5 is used to feed a solution of crude terephthalic acid into said reactor.

Documents:

1159-chenp-2005 amended pages of specification 30-06-2011.pdf

1159-chenp-2005 amended claims 30-06-2011.pdf

1159-CHENP-2005 CORRESPONDENCE OTHERS 21-06-2011.pdf

1159-chenp-2005 form-1 30-06-2011.pdf

1159-CHENP-2005 AMENDED PAGES OF SPECIFICATION 10-03-2011.pdf

1159-CHENP-2005 AMENDED CLAIMS 10-03-2011.pdf

1159-CHENP-2005 CORRESPONDENCE OTHERS 30-06-2011.pdf

1159-chenp-2005 correspondence others 11-04-2011.pdf

1159-CHENP-2005 CORRESPONDENCE OTHERS 16-07-2010.pdf

1159-chenp-2005 form-1 10-03-2011.pdf

1159-chenp-2005 form-3 10-03-2011.pdf

1159-chenp-2005 form-3 11-04-2011.pdf

1159-CHENP-2005 OTHER PATENT DOCUMENT 10-03-2011.pdf

1159-CHENP-2005 POWER OF ATTORNEY 10-03-2011.pdf

1159-CHENP-2005 EXAMINATION REPORT REPLY RECEIVED 10-03-2011.pdf

1159-chenp-2005-abstract.pdf

1159-chenp-2005-claims.pdf

1159-chenp-2005-correspondnece-others.pdf

1159-chenp-2005-description(complete).pdf

1159-chenp-2005-drawings.pdf

1159-chenp-2005-form 1.pdf

1159-chenp-2005-form 18.pdf

1159-chenp-2005-form 26.pdf

1159-chenp-2005-form 3.pdf

1159-chenp-2005-form 5.pdf

1159-chenp-2005-pct.pdf