TECHNICAL FIELD

[0001] The present invention relates to a process for producing purified terephthalic acid with stable and reproducible quality through a stable hydrogenation reaction, which hydrogenation reaction is performed for hydropurification of crude terephthalic acid by bringing the aqueous solution thereof into contact with a catalytic noble metal supported on an active carbon. Specifically, the present invention relates to a process for producing purified terephthalic acid with stable quality by feeding an optimum amount of hydrogen necessary for the purification reaction at a stable rate to a hydrogenation reactor. In other words, the present invention relates to a technique for setting hydrogenation conditions for economically producing purified terephthalic acid with stable quality by optimizing the hydrogen partial pressure corresponding to the feeding amount of the crude terephthalic acid as the raw material to be purified, even when the production rate of purified terephthalic acid is increased or decreased, or one or more new reactors are added.

BACKGROUND ART

[0002] Terephthalic acid is industrially prepared and purified in the following process. Specifically, p-xylene as the starting material is subjected to liquid-phase oxidation with an oxygen-containing gas in acetic acid solvent in the presence of an oxidation catalyst and thereby yields crude terephthalic acid. The resulting crude terephthalic acid contains trace amounts of impurities such as 4-carboxybenzaldehyde (4-CBA) as a reaction intermediate, and colored by-products. To remove these impurities, the crude terephthalic acid is dissolved in water at a high temperature under high pressure to give an aqueous solution, the aqueous solution is exposed to an atmosphere of the hydrogen gas to dissolve the hydrogen gas therein, and the aqueous solution is subjected to a hydrogenation reaction by bringing into contact with a catalytic noble metal supported on an active carbon to thereby convert or decompose impurities in the solution into substances soluble in water. Thereafter the reacted aqueous solution is cooled to precipitate crystals of terephthalic acid, from which the solids (crystals) are separated and recovered by solid-liquid separation and thereby purified terephthalic acid as powders having reduced contents of impurities is produced industrially.

With increasing demands and applications as polyesters for fibers, films, and containers, the purified terephthalic acid has been produced industrially on a large scale as a starting material for producing polyesters such as polyethylene terephthalates (PETs) include a trickling system in which a reaction liquid trickles down through the catalyst bed in a flowing atmosphere of a reaction gas (continuous gas phase); a bubbling system in which a reaction gas as bubbles passes in a flowing liquid filled with a reaction liquid through the catalyst bed (continuous liquid phase); and a system in which a reaction gas and a reaction liquid are forcedly fed to the catalyst bed and flow therethrough as a mixed-phase flow.

Patent Literature 1 (U.S. Patent No. 3,639,465) discloses a process for purifying crude terephthalic acid. In this process, a hydropurification reaction is performed by passing an aqueous solution of crude terephthalic acid together with hydrogen gas through a catalyst bed at a temperature (about 440°F to 575°F) above the temperature at which the aqueous solution first precipitates a crystal by about 10°F and at such a pressure that the aqueous solution maintains its liquid phase.

[0004] The document describes that the hydropurification reaction is preferably a purification reaction according to a trickling system in which the aqueous solution trickles down through the catalyst bed in the continuous atmosphere of hydrogen gas; that the hydrogenation treating time or space velocity of the aqueous solution in the catalyst bed is about 0.001 to 10 hours, and preferably 0.01 to 2 hours; and that the hydrogen partial pressure in the gas atmosphere is 14.7 to 150 pounds per square inch (psi), or more.

[0005] Patent Literature 2 (U.S. Patent No. 4,405,809) discloses another hydropurification process, in which a purification reaction is performed under reaction conditions substantially the same as those disclosed in Patent Literature 1, but hydrogen gas is fed so as to be dissolved in an aqueous solution of crude terephthalic acid and to maintain a dissolved hydrogen concentration at about 10% to 75% of the saturated concentration, and the purification reaction is performed according to a liquid single phase fluid (continuous liquid phase) system in which the catalyst bed is filled with the aqueous solution in the absence of a gaseous phase (gas atmosphere phase).

[0006] The inventors of Patent Literature 2 mention that the reaction system helps to reduce the amount of hydrogen gas to be used and to give a purified terephthalic acid product with decreased delta Y values, in which the delta Y value is an index of the amount of carbon (black) particles in purified terephthalic acid.

However, as is describedby the inventors, the saturated amount of dissolved hydrogen in the aqueous solution of crude terephthalic acid varies with temperature, pressure (hydrogen partial pressure), and terephthalic acid concentration and is a factor that is difficult to measure.

[0007] Regarding the amount of hydrogen gas used in the purification process. Patent Literature 3 (U.S. Patent No. 6,407,286) describes that hydrogen is used in an amount exceeding the amount necessary for hydrogenation (reduction) of impurities (such as 4-CBA and colored by-products) dissolved in the reaction aqueous solution and that, specifically, hydrogen is used in 1 to 7 times by mole the stoichiometric amount required to reduce 4-CBA dissolved in the aqueous solution of crude terephthalic acid to p-toluic acid.

[0008] The reduction reaction of 4-CBA to p-toluic acid is as follows:

[0009]

[Chemical Formula 1] 4-CBA p-Toluic acid

[0010] For purifying the aqueous solution of crude terephthalic acid, hydrogen is used in an amount of 3 to 9 moles per mole of 4-CBA contained in the aqueous solution, because amount of the stoichiometric hydrogen required for the hydrogenation of 4-CBA to p-toluic acid is 2 times to the moles of 4-CBA.

[0011] Above-mentioned Patent Literature 3 (U.S. Patent No. 6,407,286) discloses a process for recovering and reusing hydrogen which is present in excess and discharged after the purification reaction. Patent Literature 4 (U.S. Patent No. 4,626,598) has investigated about factors affecting such hydropurification reactions in more detail and has proposed a technique of determining a color scale "b-value" of purified terephthalic acid with respect to the variation in color level of a fed reaction aqueous solution and adjusting the hydrogen concentration (hydrogen gas flow rate, hydrogen partial pressure) in the aqueous reaction solution so as to maintain the color scale "b-value" at a certain level.

[0012] This proposal is a technique of controlling the purified terephthalic acid (as crystals) to have a predetermined color scale. In a hydraulically full reactor including a liquid single phase, in which the hydrogenation reactor housing a catalyst is filled with an aqueous solution of crude terephthalic acid, the control is performed by adjusting the flow rate of hydrogen gas fed to the reactor. In another reaction system, in which the reactor is not filled with an aqueous solution of crude terephthalic acid, the control is performed by adjusting the hydrogen partial pressure. Specifically, the hydrogen flow rate to be fed is controlled corresponding to the purified product, i.e. , hydrogen partial pressure is adjusted corresponding to changing in color scale "b-value".

[0013] These known techniques for the hydropurification reaction of crude terephthalic acid are proposals of the reaction conditions within specific ranges, such as temperature, pressure, mole ratio of hydrogen to 4-CBA, and hydrogenation treating time (space velocity) or are proposals of the control of reaction conditions to produce purified terephthalic acid with stable quality.

Citation List Patent Literature

[0014] Patent Literature 1: U.S.Patent No. 3,639,465 Patent Literature 2: U.S.Patent No. 4,405,809 Patent Literature 3: U.S.Patent No. 6,407,286 Patent Literature 4: U.S.Patent No. 4,626,598

SUMMARY OF INVENTION

Technical Problem

[0015] However, the setting of these reaction conditions within specific ranges is not always the answer to produce purified terephthalic acid with stable quality economically corresponding typically to increase in feeding amount of the material crude terephthalic acid. In addition, these controls adopt a feed-back control system in which the conditions are controlled based on inspection typically of the color scale of the purified terephthalic acid product.

[0016] The known techniques, therefore, may fail to produce purified terephthalic acid with guaranteed quality or may give out-of-specification in purified terephthalic acid products when amount of the production in an existing production unit is changed (mainly increased) or when reaction conditions of a hydropurification reactor to increase the production in a newly revamped unit are to be set. Accordingly, excessive reaction conditions will be set to cause overcapacity in order to set the conditions within safe range.

[0017] Under these circumstances, the present inventors have got back to the basis that the hydropurification reaction is a catalytic reaction between a gaseous phase and a liquid phase through a solid catalyst. As a result, they have recognized that there are not sufficient findings about dynamic conditions such as the flow behavior of the material aqueous solution and the reactivity, assuming that the hydropurification reaction proceeds according to a Langmuir-Hinshelwood mechanism in which the reactant (4-CBA) and hydrogen are each absorbed by and activated by the catalyst respectively.

[0018] Specifically, hydrogen is dissolved and diffused in a reaction aqueous solution and then reaches the catalyst under reaction conditions as specified in the known techniques, whereby the partial pressure of hydrogen, which accelerates the dissolution of hydrogen gas, is grasped as one of reaction factors, but the flow of the aqueous reaction solution on the catalyst is never grasped, which flow of the aqueous reaction solution affects the diffusion of hydrogen in the solution. Likewise, the known techniques fail to give findings about the flow of the solution on the catalyst as a factor affecting the diffusion and adsorption of the reactant (4-CBA) in the aqueous solution and affecting the desorption and diffusion of the reaction product (p-toluic acid).

[0019] This is because the known techniques only specify that the reaction is performed for a hydrogenation treating time or space velocity of the reaction solution of about 0.001 to 10 hours, and preferably about 0.01 to 2 hours, but they fail to specify the reaction system to a trickling reaction system (using a trickling reactor; gaseous continuous phase) or hydraulically full reaction system (using a hydraulically full reactor; continuous liquid phase) . Specifically, the techniques specify the catalytic reaction on a solid catalyst only by apparent residence time, i.e., space time (or space velocity) , defined based on the relation between the feeding amount (volume) of the reaction solution and the catalyst volume.

[0020] In general, the space velocity (hr-1 ) in a catalyst bed (contact) reaction is an index obtained by dividing the feeding amount of the reaction solution (m3 /hr) by the volume of catalyst bed (m3 ) , and the reciprocal thereof, i.e., space time (hr) is used as an apparent treating time. [0021] Thus, the specified range of hydrogenation treating time (apparent residence time) in the catalytic reaction of about 0, 001 to 10 hours, preferably about 0. 01 to 2 hours, corresponds to a wide range of space velocity of about 0.1 to 1,000 (hr-1 ) , preferably about 0.5 to 100 (hr-1 ) . The known techniques therefore fail to make mention of the flow rate of the reaction solution as an effector, which affects the diffusion of liquid on the surface of the catalyst, and as an index of the flow of the reaction solution.

[0022] In this connection, the flow rate of the reaction solution in the catalyst bed and the flow thereof on the surface of the catalyst bed significantly vary depending not only on the reaction system as described above but also on the cross section (diameter) of the reactor even at an identical space velocity (space time).

[0023] The present inventors have recognized that economical production of purified terephthalic acid with good reproducibility, stable reactivity, and stable quality is not guaranteed unless reproduction of the flow of reaction solution in increased production or first production with a new unit is considered, to which the present invention is related. The present inventors have grasped the flow rate of reaction solution as one of reaction conditions, which flow rate is considered to be a factor directly affecting the diffusion, adsorption, reaction, and desorption of the reactant {4-CBA) and hydrogen on the surface of the catalyst, based on the assumed reaction mechanism of the hydropurification reaction. They have therefore aimed to find more precise or specific conditions for hydrogenation reaction as purification reaction of crude terephthalic acid, which conditions can correspond to changes in conditions such as feeding amount of materials.

[0024] Accordingly, an object of the present invention is to provide a process for purifying crude terephthalic acid, which process enables economical production of purified terephthalic acid with good reproducibility, stable reactivity, and stable quality.

Solution to Problem

[0025] Initially, the present inventors have specif led the hydrogenation reaction, a catalytic reaction between a gaseous phase and a liquid phase both flowing in complicated behavior, to a reaction system in which the aqueous reaction solution is fed so as to have a liquid surface level above the catalyst bed, a hydrogen-containing gas phase is maintained over the liquid surface of the aqueous solution, and the aqueous reaction solution, in which hydrogen has been dissolved (absorbed) from the liquid-gas interface (liquid surface), continuously flows downward through the catalyst bed to proceed the purification reaction. This reaction system is chosen as a reaction system according to which the flow of aqueous reaction solution is most stabilized with good reproducibility.

[0026] The purification reaction proceeds further stably by feeding the aqueous solution of crude terephthalic acid and hydrogen gas continuously from the top of the reactor, respectively, to form and maintain the liquid level of the fed aqueous solution over the catalyst bed and to form and maintain a hydrogen-containing gas phase over the liquid surface of liquid level. This is because the stable feed of the aqueous solution ensures a stable flow rate of the aqueous solution flowing downward through the catalyst bed, and this in turn allows hydrogen to be dissolved (absorbed) from the liquid surface by the aqueous solution steadily into an amount corresponding to the hydrogen partial pressure.

[0027] The present inventors have further made intensive investigations on how the relation between the flow rate of the aqueous reaction solution flowing downward through the catalyst bed (superficial linear velocity which the volumetric flow rate of the aqueous solution is divided by the cross section area of reactor (hereinafter simply-referred to as "linear velocity") ) and the hydrogen partial pressure in the above-mentioned reaction system affects the purification reaction, to find conditions to achieve the object of the present invention.

[0028] Initially, the present inventors have made measurements of the vapor pressure of the aqueous reaction solution (aqueous solution of terephthalic acid) to a certain terephthalic acid concentration at different reaction temperatures, because it becomes a key to precisely grasp the hydrogen partial pressure on the purification reaction. [0029] The hydrogen partial pressure in the hydropurif ication reaction can be determined as a difference in pressure between the reactor pressure (reactionpressure) and the vapor pressure (shown in Table 1) of the aqueous reaction solution (aqueous solution of terephthalic acid) at different reactor temperatures (reaction temperatures) . [0030] This is because the vapor pressure of the aqueous reaction solution and the vapor pressure of hydrogen gas differ from each other significantly, and a reactor pressure at a certain reaction temperature is not in accordance with the Raoult's law but is the total sum of the vapor pressure of the aqueous reaction solution and the hydrogen partial pressure.

[0031] Accordingly, the present inventors have made measurements of vapor pressures of aqueous solution of crude terephthalic acid at different concentrations of dissolved terephthalic acid (at concentrations of 23.1 percent by-weight, 27 . 0 percent by weight, and 30 . 0 percent by weight) . The results are shown in Table 1.

[0032] The vapor pressure of an aqueous solution of crude terephthalic acid becomes lower than the vapor pressure of water and decreases with an increasing amount of dissolved terephthalic acid, because the boiling point rises due to the dissolution of terephthalic acid. However, vapor pressures were measured in a state of partial dissolution of terephthalic acid, because terephthalic acid is not completely dissolved, for example, up to 285°C as in a 30 percent by weight aqueous solution of terephthalic acid.

[0033] [Table 1]


[0034] The hydrogen partial pressure was determined assuming that hydrogen gas is fed as pure hydrogen without being diluted typically with an inert gas such as nitrogen gas (N2 gas). However, when the hydrogen gas is fed as a gas diluted typically with N2 gas, the gas pressure is defined as the difference in pressure between the reaction pressure and the vapor pressure of the aqueous solution, and the hydrogen partial pressure can be determined as the product of the gas pressure and the proportion of hydrogen gas in the diluted gas.

[0035] Next, the present inventors have grasped the linear velocity (superficial linear velocity in the reactor) as an index of the flow of the aqueous solution of crude terephthalic acid through the catalyst bed and as a factor of diffusion of the dissolved hydrogen and reactant in flow on the surface of the catalyst, and have analyzed how the purification is affected by the determined hydrogen partial pressure and the reactor linear velocity. Simultaneously, the present inventors have actually measured and made sure the feeding amount of hydrogen, i.e., amount of absorbed hydrogen into this reaction system.

[0036] As a result, the present inventors have succeeded in reproducing a certain amount of absorbed hydrogen gas and whereby reproducing certain effective purification, by bringing the reactor linear velocity (LV =41.1 m/hr or 44.4 m/hr) equal to or near to the hydrogen partial pressure (10.3 Kg/cm2 or 10 . 9 Kg/cm2 ) even when hydropurif ication reactions are performed at largely different feeding amounts of the aqueous solution of crude terephthalic acid (feeding amounts of slurry) in terephthalic acid production units with different production scales (Examples 1 and 2).

[0037] In addition, the present inventors have succeeded in maintaining or reproducing a certain amount of absorbed hydrogen gas and whereby reproducing certain effective purification, by modifying the hydrogen partial pressure corresponding to change in feeding amount of the aqueous solution of crude terephthalic acid (feeding amount of slurry), i.e., corresponding to the reactor linear velocity (see Fig. 2) . Fig. 2 is a graph of the 4-CBA content in purified terephthalic acid purified according to the examples, in which the 4-CBA content is plotted versus the hydrogen partial pressure (H2PP) with the linear velocity (LV) as a parameter. The 4-CBA content of an out-of-specification in purified terephthalic acid product according to a comparative example is plotted at 27 ppm.

[0038] The present inventors have performed production of purified terephthalic acid according to the reaction system, in which the reaction solution continuously flows downward while a gaseous phase is maintained above the catalyst bed as described above. As a result, the present inventors have recognized that the control of the hydrogen partial pressure in the gas phase present above the liquid surface of the reaction aqueous solution and the flow rate (linear velocity) of the aqueous reaction solution brings the amount of absorbed hydrogen gas and the reaction rate of hydrogenation into balance, and this enables stable purification effectively.

[0039] Accordingly the present inventors have performed working examples on purification reactions of aqueous solutions of crude terephthalic acid while grasping the linear velocity (LV) of the aqueous reaction solution and the hydrogen partial pressure (H2.PP) as reaction factors. The working examples were performed with different parameters such as type of the hydrogenation reactor (diameter of the reactor) , amount of absorbed hydrogen gas (molar ratio of hydrogen to 4-CBA), and terephthalic acid concentration. The measured hydrogen partial pressure (H2.PP) was plotted versus the superficial velocity (LV) with different parameters, and the plotted data are shown in Fig. 3. Based on these data, the present inventors have found some relational expressions between the linear velocity (LV) of the aqueous reaction solution flowing downward through the catalyst bed and the hydrogen partial pressure (H2.PP) in purification reactions of crude terephthalic acid.

[0040] The present inventors further have found that a preferred purification process of crude terephthalic acid is a process for purifying crude terephthalic acid through hydropurif ication by passing an aqueous solution of the crude terephthalic acid through a catalyst bed including a catalytic noble metal supported on an active carbon, in which the aqueous solution of crude terephthalic acid is fed so that the catalyst bed is filled with the aqueous solution and a liquid surface level of the aqueous solution is positioned above the catalyst bed while a hydrogen-containing gas phase is maintained on the liquid surface of the aqueous solution; and a hydrogenation reaction is carried out by allowing the aqueous solution to flow downward through the catalyst bed continuously while maintaining a hydrogen partial pressure {H2.PP) in the gas phase to be equal to a partial pressure defined by the following relational expression with respect to the velocity (LV) of the aqueous solution flowing downward through the catalyst bed:

(H2PP)=-0.000550x(LV)^+0.2 99x(LV)-1.4 8 (1) [0041] Specifically, the present inventors have found that purif led terephthalicacidwith stable quality (e.g., having a 4-CBA content of about 15 ppm) can be obtainedby controlling the hydrogen partial pressure (H2PP) of the upper gas phase to satisfy the relational expression (1) corresponding to the flow rate (linear velocity LV) of the aqueous solution of crude terephthalic acid flowing downward through the catalyst bed in the hydrogenation reactor (claim 3). [0042] The present inventors have also recognized from the data that, if the hydrogen partial pressure (H2PP) of the upper gas phase reaches a partial pressure defined by the relational expression (2) below, the amount of absorbed hydrogen gas decreases (molar ratio of hydrogen to 4-CBA: about 3), and this causes the deterioration of quality in a purified terephthalic acid product (e.g., having a 4-CBA content of more than about 20 ppm). (H2PP)=-0.000413x(LV) 2+0.224x(LV)-1.11 (2)

Accordingly, the present inventors have recognized that hydrogenation reactions, if performed at a hydrogen partial pressure less than the partial pressure defined by the relational expression (2) (Region (F) in Fig. 3), may give out-of-specification of purified terephthalic acid products (having 4-CBA contents of more than 25 ppm; Comparative Examples 1, 2, 3, and 4); and that these hydrogenation reactions can cause formation of additional impurities and deterioration of the catalyst due to insufficient hydrogen, because these purification reactions proceed without absorbing amounts of hydrogen required for the reactions. Based on these recognitions, the present inventors have found that a hydrogenation reaction performed at a hydrogen partial pressure equal to or higher than the hydrogen partial pressure defined by the relational expression (2) can give purified terephthalic acid products within the product specifications (claim 1) . [0043] The present inventors have also recognized that, when a hydrogenation reaction is performed whilemaintaining the hydrogen partial pressure (H2.PP) in the hydrogen-containing gas phase to be a partial pressure defined by the following relational expression (3) with respect to the linear velocity (LV) of the aqueous solution flowing downward through the catalyst bed:

(H2.PP)=-0.0020x(LV) 2+0.569x(LV)-1.93 (3) the amount of absorbed hydrogen gas tends to increase (to a molar ratio of hydrogen to 4-CBA of about 9) and the 4-CBA content in the purified terephthalic acid tends to decrease (to about 10 ppm) . The present inventors have further recognized that when the hydrogen partial pressure is maintained beyond the partial pressure defined by the relational expression (3) (Region (E) in Fig. 3), a purified terephthalic acid having a smaller 4-CBA content but having excessive quality may be produced.

[0044] The present inventors have thereby recognized that hydrogenation at a hydrogen partial pressure exceeding the partial pressure defined by the relational expression (3) is a hydropurif ication reaction which consumes hydrogen over the amount required amount for the purification of crude terephthalic acid. They also have recognized that such excessive hydrogen may cause severe hydrogenation. Based on these, they have recognized that a hydrogenation reaction at a hydrogen partial pressure equal to or lower than the partial pressure defined by the relational expression (3) enables economical production of purified terephthalic acid products within the product specifications (claim 2). [0045] Accordingly, a process for purifying crude terephthalic acid is preferably performed by carrying out a hydrogenation reaction while maintaining the hydrogen partial pressure (H2.PP) in the hydrogen-containing gas phase within a range between the relational expression (2) and the relational expression (3) with respect to the linear velocity (LV) of the aqueous solution flowing downward through the catalyst bed (claim 2). The above-specified range is expressed by the relational expressions as follows :
-0.0020x(LV)2+0.569x(LV)-1.93>(H2.PP)>-0.000413x(LV )2+0.224x(LV)-l.ll

In the hydropurification reaction of an aqueous solution of crude terephthalic acid, the hydrogen partial pressure (H2PP) defined by the relational expression (2) is considered to be a lower limit of the hydrogen partial pressure (H2.PP) to produce purified terephthalic acid with preferred quality (4-CBA content of about 20 ppm or less); and the hydrogen partial pressure defined by the relational expression (1) is considered to be a preferred hydrogen partial pressure (H2PP) for the purification of crude terephthalic acid. The hydrogen partial pressure defined by the relational expression (2) corresponds to about 25% below the hydrogen partial pressure defined by the relational expression (1).

[0046] Accordingly, for producing purified terephthalic acid with stable and reproducible quality, hydropurification is preferably performed at a hydrogen partial pressure (H2PP) within the range of about ±25% of the partial pressure defined by the relational expression (1) (Region (B) in Fig. 3) with respect to the linear velocity (LV) of the aqueous solution flowing donward through the catalyst bed (claim 3) . The specified range is expressed by the relational expressions as follows:
(-0.000550x(LV)2+0.299x(LV)-1.48)x0.75<(H2.PP)<(-0. 000550x(LV)^+0.299x(LV)-1.48)xl.25

As the hydrogen partial pressure (H2PP) corresponding to +25% of the relational expression (1) corresponds to a hydrogen partial pressure defined by the following relational expression (4) , the right-hand side of the above inequality expression may be substituted by the relational expression (4): (H2PP)=-0.000688x(LV)2 +0.374x(LV)-1.85 (4)

Consequently, the present inventors have found economical conditions for purification reaction of an aqueous solution of crude terephthalic acid, which reaction conditions are reproducible and stable corresponding to the change in production amount, such as increased production or reduced production, of purified terephthalic acid and do not cause excess hydrogenation reactions consuming excessive hydrogen gas. Specifically, the present inventors have found and determined these reaction conditions by specifying the reaction system to a system in which an aqueous solution of crude terephthalic acid is filled above a catalyst bed, and a hydrogenation reaction is performedby allowing the aqueous solution to continuously flow downward through the catalyst bed while maintaining a hydrogen-containing gas phase above the liquid surface of the aqueous solution; and by controlling a linear velocity of the aqueous reaction solution flowing downward through the catalyst bed, whereas the linear velocity has not been grasped as a reaction factor by known technologies. Advantageous Effects of Invention

[0047] According to the present invention, production of purified terephthalic acid through hydropurification of an aqueous solution of crude terephthalic acid can be performed corresponding to the change in reaction conditions and can give a purified terephthalic acid product with stable quality.

This is achievedby controlling the hydrogen partial pressure (H2.PP) within the range of the pressure based on the relational expressions (1), (2), and (3), and by controlling the hydrogenpartial pressure (H2.PP) basedon the relational expressions (2) and (3), which relational expressions are relations between the hydrogen partial pressure and the linear velocity (LV) of the aqueous solution flowing downward through the catalyst bed. In addition, the production of purified terephthalic acid according to the present invention does not give out-of-specification products, does not cause excessive hydrogenation reactions due to excessive hydrogen, and enables economical production.

[0048] Accordingly, even when a production unit is to be modified for boosted production or a new production unit is to be provided, the production unit can be modified or provided optimally without setting excessive conditions. This enables economical production also in subsequent production performed thereafter. Brief Description of Drawings

[0049] [Fig. 1] Fig. 1 is a schematic diagram of a process for purifying crude terephthalic acid according to an embodiment to embody the present invention.

[Fig. 2] Fig. 2 is a graph showing how the 4-CBA content in purified terephthalic acid products varies depending on the hydrogenpartial pressure (H2PP) with the linear velocity (LV) as a parameter, as obtained in working examples, in which the 4-CBA content of an out-of-specification in purified terephthalic acid product as a comparative example is plotted as 27 ppm.

[Fig. 3] Fig. 3 is a graph in which the hydrogen partial pressure (H2.PP) is plotted versus the linear velocity (LV) based on the data obtained in working examples, with parameters such as type of hydrogenation reactor, amount of absorbed hydrogen gas (molar ratio of hydrogen (H2) to 4-CBA), and terephthalic acid concentration. Description of Embodiments

[0050] Some preferred embodiments of the process for purifying crude terephthalic acid according to the present invention will be illustrated below. A raw material used herein for producing purified terephthalic acid is mainly crude terephthalic acid prepared from staring material p-xylene through liquid-phase oxidation with an oxygen-containing gas in acetic acid solvent in the presence of a catalyst such as cobalt or manganese. The crude terephthalic acid may contain about 2,000 to about 3,500 ppm of 4-CBA.

[0051] The schematic diagram of process as in Fig. 1 is a flow chart of the process for producing purified terephthalic acid by dissolving the crude terephthalic acid as a material in water at a high temperature under high pressure, and subjecting the resulting aqueous solution to a hydropurification reaction in a hydrogenation reactor. An outline of the process and conditions therefor will be illustrated below.

[0052] Initially, a slurry is prepared by feeding crude terephthalic acid and water to a slurry preparation tank Awhile controlling the feed ratio between crude terephthalic acid and water so as to give a slurry having a terephthalic acid concentration of generally from 23 to 30 percent by weight, preferably from 26 to 29 percent by weight. Next, the prepared slurry is fed to a heater C and a dissolving tank D at a pressure exceeding the pressure of a hydrogenation reactor (55 to 100 Kg/cm2G) using a high-pressure pump. The slurry is then heated in the heater C to a predetermined temperature within the range of 275°C to 300°C, is fed to and resides in the dissolving tank D to allow the whole quantity of crude terephthalic acid to be dissolved to give an aqueous solution, and the aqueous solution is fed to the hydrogenation reactor F.

[0053] In the hydrogenation reactor F, the aqueous solution and high-pressure hydrogen gas wetted with high-pressure (high-temperature) steam are respectively fed to above a catalyst bed Fl and are controlled to give a predetermined reactor pressure and a predetermined reaction temperature. Gas-liquid phase separated in two phases of an upper gas phase J containing hydrogen gas and a lower liquid phase comprising the aqueous solution are formed in a space above the catalyst bed.

[0054] The hydrogen gas is dissolved in (absorbed by) the aqueous reaction solution from the liquid surface at the interface between the two phases, in an amount corresponding to the hydrogen partial pressure in the gas phase J. Hydrogen gas in an amount corresponding to the dissolved (absorbed) amount is thereby fed and supplemented by controlling the reactor pressure through a pressure indicating controller (PIC) . Accordingly, the hydrogen gas is fed directly from a feed line 6 (Line 6 in Fig. 1) at the top of the hydrogenation reactor F to maintain the pressure of the gas phase J in the reactor F. Alternatively, the hydrogen gas may be fed to the dissolving tank D and a downstream aqueous reaction solution feed line 4 so as to maintain the reactor pressure. In either case, feeding of the hydrogen gas is preferably performed so that the hydrogen gas can be stably fed at a rapid response rate responding to the reactor pressure.

[0055] The liquid level of the aqueous solution at the interface between the upper gas phase and lower liquid phase above the catalyst bed is controlled and maintained through a liquid-level indicating controller (LIC) , and the aqueous reaction solution in an amount corresponding to the feeding amount flows downward through the catalyst bed and undergoes a hydrogenation reaction with dissolved (absorbed) hydrogen in the presence of the catalyst, and the resulting aqueous reaction solution is drawn out from the bottom of the catalyst bed via a line 9 to a first crystallization tank G. The aqueous reaction solution in the first crystallization tank G is controlled and maintained at a predetermined pressure of about 30 to about 55 Kg/cm2G, and the aqueous reaction solution at an elevated temperature (275°C to 300°C) and a high pressure (55 to 100 Kg/cm2G) is then flushed (relieved) and cooled to precipitate crystals partially to thereby give a slurry of purified terephthalic acid at about 230°C to about 270°C.

[0056] The reaction temperature of the aqueous reaction solution flowing downward through the catalyst bed is preferably a temperature exceeding such a temperature that terephthalic acid crystals in the fed slurry of crude terephthalic acid are completely dissolved (i.e., at such a temperature that crystals are precipitated from the fed aqueous solution of crude terephthalic acid) . Specifically, the reaction is preferably performed on an aqueous reaction solution having a crude terephthalic acid concentration of from 23 to 30 percent by weight at a temperature (about 275°C to about 300°C) exceeding the dissolution temperature thereof. Typically, according to the technique disclosed in Patent Literature 1 (U.S. Patent No. 3,639,465), a purification reaction is performed at a temperature of about 10°F (about 5.6°C) exceeding the deposition point (first precipitating point) of terephthalic acid crystals.

[0057] The pressure of the reactor F upon hydrogenation reaction is determined and set by adding a hydrogen partial pressure to the vapor pressure of the aqueous solution of terephthalic acid corresponding to the terephthalic acid concentration (Table 1) and temperature of the aqueous reaction solution, which hydrogen partial pressure equals the range of hydrogen partial pressure (H2.PP) defined by the relational expressions (1), (2), and (3) and is equal to or higher than the hydrogen partial pressure (H2.PP) defined by the relational expression (2) or is within a range (Region (C) in Fig. 3) between the hydrogen partial pressures (H2PP) defined by the relational expressions (2) and (3), to the different linear velocities. The purification reaction is more preferably a hydrogenation reaction performed at a reactor pressure determined by adding, to the vapor pressure of the aqueous reaction solution, a hydrogen partial pressure within the range of about +25% (Region (B) in Fig. 3) of the hydrogen partial pressure (H2PP) defined by the relational expression (1). In this more preferred embodiment, the reaction is performed generally at a pressure of about 55 to about 100 Kg/cm2G.

[0058] Fig. 3 is a diagram showing how the hydrogen partial pressure (H2.PP) varies depending on the superficial velocity (LV) , based on the data obtained in working examples with parameters such as scale of hydrogenation reactor, amount of absorbed hydrogen gas (molar ratio of hydrogen (H2) to 4-CBA), and terephthalic acid concentration. In this diagram, approximate lines and relational expressions at identical 4-CBA contents are also shown.
The relational expressions are as follows:
4-CBA content: about 15 ppm (H2.PP)=-0.000550x(LV) V0.299x(LV)-1.48 (1)
4-CBA content: about 20 ppm
(H2.PP)=-0.000413x(LV)^+0.224x(LV)-l.ll (2)
4-CBA content: about 10 ppm (H2.PP)=-0.0020x{LV)^+0.569x(LV)-1.93 (3)

The relational expression (2) corresponds to 25% below the relational expression (1), and a hydrogen partial pressure 25% above the relational expression (1) isexpressed by the following relational expression (4): (H2.PP)=-0.000688x(LV) 2+0.374x(LV)-1.85 (4)

Specifically, the relational expressions (1) to (4) correspond to the approximate relational lines in Fig. 3, respectively. In Fig. 3, regions divided by the relational lines are indicated as Regions (A) , (B) , (C) , (D) , (E) , and (F) . Region (A) is a region equal to or less than the relational expression (3); Region (B) is a region around the relational expression (1) and positioned between the relational expressions (2) and (4); Region (C) is a region positioned between the relational expressions (2) and (3) ; Region (D) is a region equal to or more than the relational expression (2); Region (E) is a region more than the relational expression (3) ; and Region (F) is a region less than the relational expression (2).

[0059] The catalyst to be charged in the hydrogenation reactor F is generally a catalyst including a Group VIII noble metal, such as palladium, platinum, or ruthenium, supported on an active carbon carrier, of which a catalytic palladium supported on an active carbon is generally preferred. The amount of each noble metal deposited on the carbon is generally in the range of 0 .1 to 3 percent by weight. Among such catalyst, a catalyst including about 0.5 percent by weight of palladium supported on an active carbon is generally preferably used as the catalyst. The dimensions of the hydrogenation reactor F used in this embodiment, the dimensions of the catalyst bed Fl, the dimensions of the upper and lower separated phases above the catalyst bed, and the liquid level of the aqueous reaction solution in the hydrogenation reactor F are schematically illustrated in the hydrogenation reactor F in Fig. 1. However, it should be noted that these dimensions are not restricted, as long as the hydrogen-containing gas phase J and the liquid surface of the reaction aqueous solution are stably formed and maintained above the catalyst bed.

[0060] Next, the crystallized slurry drawn out from the first crystallization tank G is flushed and cooled via multiple crystallization tanks (not shown) in which the pressure is reduced stepwise, thereafter the slurry is further flushed into a final crystallization tank H held at about 2 to about 5 Kg/cm2G, and thereby yields a crystallized slurry of purified terephthalic acid at a temperature of from about 130°C to about 160°C.

[0061] The stepwise flushing and cooling using multiple crystallization tanks including the first crystallization tank G can be performed according to any procedure such as oneproposedtypically inU. S, PatentNo. 3,391,305, Japanese Opened Patent No. H08(1996)-208561A, Japanese Patent No. 3848372, and Japanese Opened Patent No. 2006-96710A.

[0062] The crystallized slurry obtained from the final crystallization tankH is subjected to separation and washing in a solid-liquid separator I while maintaining the temperature to recover a wet cake of purified terephthalic acid. The recovered wet cake is dried in a drier (not shown) and thereby yields a purified terephthalic acid product.

[0063] In another embodiment, washing in the solid-liquid separator I is not performed, and the crystallized slurry obtained in the final crystallization tank H is recovered only through solid-liquid separation to give a purified terephthalic acid cake. The purified terephthalic acid cake is dispersed in water at a high temperature (about 100°C to about 160°C) to give a slurry again, and the resulting slurry is subjected again to solid-liquid separation using a solid-liquid separator to recover a wet cake of purified terephthalic acid. The wet cake is then dried and thereby yields a purified terephthalic acid product.

[0064] Embodiments of the present invention will be illustrated in further detail with reference to several working examples below. It should be noted, however, that these examples are never construed to limit the scope of the present invention.

[0065] The working examples will be illustrated by using the schematic process chart in Fig. 1, which shows the production unit of purified terephthalic acid in which the crude terephthalic acid is dissolved in water and then subjected to hydropurification. The outlines of the reaction in the hydrogenation reactor and reaction ) conditions will be described in detail in respective examples,

[0066] (EXAMPLE 1)

A hydrogenation reactor used herein was prepared by 5 placing 2.56 m3 of a hydrogenation catalyst into a hydrogenation reactor F (Fig. 1, hydrogenation reactor F, outline) to forma catalyst bed. The hydrogenation catalyst was a catalyst including 0.5 percent by weight palladium (Pd) supported on an active carbon. The hydrogenation reactor F had a cylindrical barrel having an inner diameter of 740 mm and a length of 7,000 mm, and upper and lower (hemispherical) domes. The catalyst bed was charged to a height in the cylindrical barrel of about 5,710 mm (about 1,2 90 mm from the upper end of the cylindrical barrel) , and a catalyst weight including 20-mesh wire netting was placed on the catalyst bed to fix the catalyst bed.

[0067] Initially, a slurry having a terephthalic acid concentration of 26 percent by weight was prepared by feeding crude terephthalic acid having a 4-CBA content of 2,800 ppm from a powder feed hopper B and pure water from a line 1 respectively to the slurry preparation tank A. The prepared slurry was fed using a high-pressure pump via a line 3 to the heater C at a rate of 14,500 kg/hr, heated in the heater C to about 283°C, and fed via the dissolution tank D and line 4 to the hydrogenation reactor F.

[0068] The aqueous solution containing dissolved crude terephthalic acid prepared by heating was dispersed via a branched and perforated pipe equipped at the top of the reactor F and fed onto the liquid surface which is held at a position of about 650 mm above the catalyst bed.

Independently, high-pressure hydrogen gas (about 120 Kg/cm G) was fed via a line 7 through a heater/vapor saturator E to the top of the reactor F. The high-pressure hydrogen gas was fed in such an amount as to maintain the reactor pressure to about 73 Kg/cm2G through pressure control (with a pressure indicating controller (PIC)), whereby hydrogen gas in an amount corresponding to the dissolved (absorbed) amount was supplemented.

[0069] The fed aqueous solution flowed downward through the catalyst bed through liquid-level control with a liquid-level indicating controller (LIC), underwent catalytic hydrogenation with dissolved hydrogen dissolved from the liquid surface, and was drawn out from the bottom of the reactor F and fed to the first crystallization tank G. A cylindrical grid-like collector with an aperture of 0.8 mm was equipped at the bottom of the catalyst bed, and the solution after hydrogenation was drawn out via the collector. In the hydrogenation reactor F, the temperature (as measured with a temperature indicator (TI)) was held at about 282°C, and the reactor pressure was held at 73.2 Kg/cm2G.. Under these conditions, the vapor pressure of the 26 percent by weight aqueous solution of terephthalic acid is 62.9 Kg/cm2G(282°C) and the hydrogen partial pressure was calculated to be 10.3 Kg/cm2, The average feeding (absorbing) amount of hydrogen during the reaction was 8.2 Nm3/hr.

[0070] Based on these data and parameters, the ratio of the amount of absorbed hydrogen to the feeding amount of 4-CBA (ratio of hydrogen to 4-CBA) was calculated to be 5.2 (molar ratio) . The aqueous solution of crude terephthalic acid was fed to the hydrogenation reactor F so as to be hydrogenated at a linear velocity (LV) of 41.1 m/hr. [0071] The aqueous solution after hydrogenation was passed through a line 9 and flushed into the first crystallization tank G at a high pressure (about 48 Kg/cm2G) , sequentially flushed using a series of multiple crystallization tanks (not shown), cooled to about 150°C in the final crystallization tank H, and thereby yielded a slurry of purified terephthalic acid. The resulting slurry was fed via a line 13 and separated and washed in the solid-liquid separator I to recover a wet cake of purified terephthalic acid crystals. The cake was dried in a dryer (not shown) and thereby yielded a purified terephthalic acid product. [0072] A stable operation under the above hydrogenation conditions was performed for about 24 hours. The purified terephthalic acid crystals recovered during this operation had a 4-CBA content of about 15 ppm and a p-toluic acid content of about 125 ppm.

[0073]

(EXAMPLE 2)

A production of purified terephthalic acid was performed by dissolving material crude terephthalic acid in water and performing a hydropurification reaction of the resulting aqueous solution by using a unit system as in Example 1 (outline; Fig. 1) by the procedure of Example 1 under the conditions of Example 1, except that the unit system was equipped with a hydrogenation reactor having a cylindrical barrel having an inner diameter of 3900 mm and a length of 14,000 mm, and upper and lower hemispherical domes and that the following conditions were adopted. The hydrogenation reactor F was charged with 159 m3 of a hydrogenation catalyst (the same catalyst as in Example 1) to give a catalyst bed having a height in the cylindrical barrel of about 12,000 mm. A slurry having a terephthalic acid concentration of 26.5 percent by weight was prepared from crude terephthalic acid (4-CBA content: 2,700 ppm) and pure water. The prepared slurry was fed at a rate of 435 ton/hr to a heater C and a dissolving tank D using a high-pressure pump, for heating and dissolving the crude terephthalic acid to give an aqueous solution, and the aqueous solution was fed to the hydrogenation reactor F by the procedure of Example 1.

[0074] In the hydrogenation reactor F, the reaction solution was drawn out from the bottom of the catalyst bed while maintaining the liquid level at a position of 1,000 mm above the catalyst bed through liquid-level control with a liquid-level indicating controller (LIC); and hydrogen gas was fed and supplemented from the top of the reactor through pressure control with a pressure indicating controller (PIC). These were performed by the procedure of Example 1. During the hydropurification reaction, the temperature (determined with a temperature indicator (TI) ) and the pressure of the hydrogenation reactor F were controlled at 282°C and 73.6 Kg/cm2G, respectively, and the feeding (absorbing) amount of hydrogen gas was 239 Nm2/hr. [0075] During this process, the hydrogen partial pressure was calculated to be 10.8 Kg/cm2 (vapor pressure of aqueous solution: 62.8 Kg/cm2G) , and the ratio of absorbing amount of hydrogen to the feeding amount of 4-CBA was calculated tobeS.l (bymole) . The superficial linear velocity (linear velocity; LV) in feed of the aqueous solution of crude terephthalic acid was 44.4 m/hr. The resulting recovered purified terephthalic acid crystals had a 4-CBA content of about 15 ppm and a p-toluic acid content of about 125 ppm. Table 2 provides a summary of the reaction conditions and properties of purified terephthalic acid products in Examples 1 and 2.


[0077] These data demonstrate that, even when the hydrogenation reactor and the feeding amount of crude terephthalic acid slurry significantly change and/or even when,the space velocity (SV; hr-1) changes, equivalent amounts of absorbed hydrogen gas (molar ratios to 4-CBA) are obtained and whereby the resulting purified terephthalic acid products can have equivalent properties, by controlling the linear velocity of the aqueous solution and the hydrogen partial pressure within specific ranges, respectively.

[0078] (EXAMPLES 3, 4. 5, and 6)

Productions of purified terephthalic acid were performed using the same production unit as in Example 1, by preparing a 26 percent by weight aqueous slurry of crude terephthalic acid (with a 4-CBA content of 2800 ppm) in the slurry preparation tank A; feeding the slurry via the line 3 to the heater C at rates of 25,000 kg/hr, 35,000 kg/hr, 41,000 kg/hr, and 7,500 kg/hr, respectively, using a high-pressure pump; and performing hydrogenation reactions by the procedure of Example 1. The hydrogenation reactions were performed while maintaining the reactor temperature at 2 82°C and the reactor pressure at 7 9.8 Kg/cm 2 G, 85.8 Kg/cm 2 G, 88.8 Kg/cm2G, and 67.3 Kg/cm 2 G, respectively. [0079] As a result, the ratios of the amount of absorbed hydrogen to the feeding amount of 4-CBA were each 5.2 (by mole) , and the produced purified terephthalic acid crystals had 4-CBA contents of about 15 ppm and p-toluic acid contents of about 125 ppm. The hydrogen partial pressures were calculated to be 16.9 Kg/cm2, 22.9 Kg/cm2, 25.9 Kg/cm2, and 4.4 Kg/cm2, and the linear velocities were calculated to be70.9m/hr, 99.3m/hr, 116m/hr, and21.3m/hr, receptively, as shown in Table 3. These data demonstrate that the amount of absorbed hydrogen gas (molar ratio of hydrogen (H2) to 4-CBA) should be maintained at a certain level by increasing the reactor pressure (to increase the hydrogen partial pressure) even when the feeding amount of slurry is increased.

[0080] (COMPARATIVE EXAMPLE 1)

A production of purified terephthalic acid was performed by the procedure of Example 1 using the same production unit as in Example 1, except for feeding the prepared slurry containing 26 percent by weight of terephthalic acid at a rate of 14, 500 kg/hr and maintaining the pressure and temperature of the reactor at 69.3 Kg/cm2G and 282°C, respectively. The feeding (absorbing) amount of hydrogen gas was 3. 1 Nm3 /hr. Asaresult, the 4-CBA content of the purified terephthalic acid crystals began to increase about 15 hours after the setting of the reactor pressure at 69,3 Kg/cm2G, exceeded 25 ppm after 20 hours, and thereby yielded a purified terephthalic acid product having a 4-CBA content out of product specifications,

[0081] In this process, the hydrogen partial pressure was 6.4Kg/cm2, the absorbing amount of hydrogen gas (molar ratio of hydrogen to 4-CBA) was 1.9, and the superficial velocity was 41.1 m/hr, as shown in Table 3. These data demonstrate that reduction of the reactor pressure from one adopted in Example 1 (from 73.2 to 69,3 Kg/cm2 G) causes the hydrogen partial pressure to decrease (from 10.3 to 6.4 Kg/cm ) and thereby reduction of the amount of absorbed hydrogen gas; resulting in a purified terephthalic acid product having a 4-CBA content out of product specifications (4-CBA specification: 25 ppm).

[0082] (COMPARATIVE EXAMPLE 2)

A production of purified terephthalic acid was performed by the procedure of Example 1 using the same production unit as in Example 1, except for feeding the prepared slurry containing 26 percent by weight of terephthalic acid at a rate of 35, 000 kg/hr and maintaining the pressure and temperature of the reactor at 74.9 Kg/cm2G and 282°C, respectively. The feeding (absorbing) amount of hydrogen gas was 7.1 Nm'^/hr.

As a result, the 4-CBA content of the purified terephthalic acid crystals began to increase about 15 hours after the setting of the reactor pressure at 74.9 Kg/cm2G, exceeded 25 ppm after 20 hours, and thereby yielded a purified terephthalic acid product having a 4-CBA content out of product specifications. In this process, the hydrogen partial pressure was 12.0 Kg/cm2, the amount of absorbed hydrogen gas (molar ratio of hydrogen to 4-CBA) was 1.9, and the linear velocity was 99.3 m/hr, as shown in Table 3. These data demonstrate that increase in feeding amount of slurry (from 14,500 to 35,000 kg/hr) causes the purified terephthalic acid product to have a 4-CBA content out of product specifications, even though the reactor pressure was controlled to be higher (74.9 Kg/cm2G) than that (73.2 Kg/cm2G) in Example 1.

[0084] (EXAMPLES 7, 8, and 11)

Productions of purified terephthalic acid were performed using the same production unit as in Example 1, by feeding the prepared slurry having a terephthalic acid concentration of 26 percent by weight each at a rate of 14, 500 kg/hr as inExample 1; and performing hydrogenation reactions by the procedure of Example 1, except for maintaining the reactor pressure at 75.0 Kg/cm2G, 70.6 Kg/cm2G, and 81.0 Kg/cm2G, respectively, as shown in Table 4. The feeding (absorbing) amounts of hydrogen gas were 9.1 Nm3/hr, 4.8 Nm'^/hr, and 14.0 Nm2/hr, respectively.

As a result, the purified terephthalic acid products had 4-CBA contents of about 15 ppm, about 20 ppm, and about 10 ppm, respectively, and p-toluic acid contents of about 125 ppm.

[0085] In these processes, the hydrogen partial pressures were calculated to be 12.1Kg/cm2, 7.7Kg/cm2 and 18 .1 Kg/cm2, respectively; the amounts of absorbed hydrogen gas (molar ratios of hydrogen to 4-CBA) were calculated to be 5. 7, 3.0, and 8.9, receptively; and the linear velocities were calculated to be each 41.1 m/hr. These data demonstrate that, when the slurry is fed at the identical rate (including Example 1) , the reactor pressure (hydrogen partial pressure) varies depending directly on the feeding (absorbing) amount of hydrogen gas and affects the purification effects. In addition, the data indicate that production of purified terephthalic acid according to Example 11 requires an excessive amount of hydrogen to be fed (absorbed).

[0086] [Table 4]

Productions of purified terephthalic acid were performed using the same production unit as in Example 1, by feeding the prepared slurry containing 26 percent by weight of terephthalic acid each at a rate of 25,000 kg/hr as in Example 3 (at a linear velocity of 70.9 m/hr as in Example 3) ; and performing hydrogenation reactions by the procedure of Example 3, except for maintaining the reactor pressure at 91.4 Kg/cm2G and 75.1 Kg/cm2G, respectively, as shown in Table 5. The resulting purified terephthalic acid products had 4-CBA contents of 10 ppm and 20 ppm, respectively, and p-toluic acid contents of 125 ppm. These data demonstrate that purified terephthalic acid products within product specifications can be produced. In these processes, the hydrogen partial pressures were calculated to be 28.5 Kg/cm2 and 12.2 Kg/cm2; the amounts of absorbed hydrogen gas (molar ratios of hydrogen to 4-CBA) were calculated to be 8.8 and 3.1, receptively; and the space velocities and linear velocities are as given in Table 5.

[0088] These data demonstrate that, when the slurry is fed at an identical rate (including Example 3), the reactor pressure varies (hydrogen partial pressure varies) depending directly on the feeding (absorbing) amount of hydrogen gas and affects the purification effects. In addition, the data indicate that production of purified terephthalic acid according to Example 9 requires an excessive amount of hydrogen to be fed (to be absorbed).

[0089] (COMPARATIVE EXAMPLE 3)

A production of purified terephthalic acid was performed by the procedure of Example 3 using the same production unit as in Example 1, while feeding the prepared slurry containing 26 percent by weight of terephthalic acid at a rate of 25,000 kg/hr as in Example 3 (at a linear velocity of 70.9 m/hr as in Example 3), except for maintaining the reactor pressure at 72.3 Kg/cm2G. The feeding (absorbing) amount of hydrogen gas was 5.3 Nm2/hr. As a result, the 4-CBA content of the purified terephthalic acid crystals exceeded 25 ppm about 20 hours after the setting of the reactor pressure at 72.3 Kg/cm2G, and the resulting purified terephthalic acid product had a 4-CBA content out of product specifications. In this process, the hydrogen partial pressure was 9.4 Kg/cm2, the amount of absorbed hydrogen gas (molar ratio of hydrogen to 4-CBA) was 2.0, and the space velocity and linear velocity were as shown in Table 5.

[0090] These data demonstrate that reduction of the reactor pressure (from 7 9.8 to 72.3 Kg/cm2G) from one adopted in Example 3 causes the hydrogen partial pressure to reduce (from 16.9 to 9.4 Kg/cm2) to thereby reduce the amount of absorbed hydrogen gas, and the resulting purified terephthalic acid product had a 4-CBA content out of product specifications (4-CBA specification: 25 ppm).

[0091] (COMPARATIVE EXAMPLE 4)

A production of purified terephthalic acid was performed by the procedure of Example 5 using the same production unit as in Example 1, while feeding the prepared slurry containing 26 percent by weight of terephthalic acid at a rate of 41, 000 kg/hr as in Example 5 (at a linear velocity of 116 m/hr as in Example 5), except for maintaining the reactor pressure at 77.0 Kg/cm2G. The feeding (absorbing) amount of hydrogen gas was 8.7 Nm3/hr. As a result, the 4-CBA content of the purified terephthalic acid crystals exceeded 25 ppm about 20 hours after the setting of the reactor pressure at 77.0 Kg/cm2G, and the resulting purified terephthalic acid product had a 4-CBA content out of product specifications. In this process, the hydrogen partial pressure was 14.1 Kg/cm2, the absorbing amount of hydrogen gas (molar ratio of hydrogen to 4-CBA) was 2.0, and the space velocity and superficial velocity were as shown in Table 5.

[0092] These data demonstrate that reduction of the reactor pressure (from 88.8 to 77.0 Kg/cm2G) from one adopted in Example 5 causes the hydrogen partial pressure to reduce (from 25.9 to 14.1 Kg/cm2) to thereby reduce the amount of absorbed hydrogen gas, and the resulting purified terephthalic acid product had a 4-CBA content out of product specifications (4-CBA specification:25 ppm).

[0094] (EXAMPLE 12)

A production of purified terephthalic acid was performed using the same production unit as in Example 1, by preparing a 29 percent by weight aqueous slurry of crude terephthalic acid (with a 4-CBA content of 2800 ppm) in the slurry preparation tank A; feeding the slurry to the heater C at a rate of 14,500 kg/hr; and performing a hydrogenation reaction by the procedure of Example 1, while maintaining the pressure and temperature of the reactor at 7 6.4 Kg/cm2G and 287°C, respectively. These reaction conditions are as given in Table 6. In this process, the slurry was fed so as to give a linear velocity of 41.1 m/hr, and the hydrogen gas was fed (absorbed) in an amount of 9.0 Nm3/hr. [0095] As the vapor pressure of aqueous solution at 29 percent by weight of terephthalic acid is 66. 9 Kg/cm2G (287°C) , the hydrogen partial pressure was calculated to be 9 . 5 Kg/cm2. The resulting purified terephthalic acid product had a 4-CBA content of about 15 ppm and a p-toluic acid content of about 125 ppm. The amount of absorbed hydrogen gas (molar ratio of hydrogen to 4-CBA) herein was calculated to be 5.1, and the space velocity and linear velocity were as shown in Table 6. These data demonstrate that purified terephthalic acid products having equivalent quality to that in Example 1 can be produced by controlling the reaction pressure (hydrogen partial pressure) corresponding to changes in terephthalic acid concentration and reactor temperature.

[0096] (EXAMPLE 13 and COMPARATIVE EXAMPLE 5)

Productions of purified terephthalic acid were performed using the same production unit as in Example 2, by preparing a 26. 5 percent by weight aqueous slurry of crude terephthalic acid (with a 4-CBA content of 2700 ppm) ; feeding the slurry at rates of 220,000 kg/hr and 435,000 kg/hr, respectively; and performing hydrogenation reactions by the procedure of Example 2, while maintaining the reactor pressures at 67.7 Kg/cm2G and 68.8 Kg/cm2G, respectively. These reaction conditions are as given in Table 6. In these processes, the temperature of hydrogenation reactor was each held at 282°C. As the vapor pressure of aqueous solution at the 26.5 percent by weight of terephthalic acid is 62.8 Kg/cm2G (282°C) , the hydrogen partial pressures were calculated to be 4.9 Kg/cm2 and 6.0 Kg/cm2; and the feeding (absorbing) amounts were calculated to be 124 Nm2/hr and 54 Nm2/hr, respectively.

[0097] The resulting purified terephthalic acid product according to Example 13 had a 4-CBA content of 15 ppm. In contrast, in Comparative Example 5, the 4-CBA content of the purified terephthalic acid exceeded 25 ppm about 20 hours after the setting of the reactor pressure at 68.8 Kg/cm2G, and the resulting purified terephthalic acid product had a 4-CBA content out of product specifications. In these processes, the linear velocities were 22.4 m/hr and 44.4 m/hr, respectively, the amounts of absorbed hydrogen gas (molar ratios of hydrogen to 4-CBA) were 5,2 and 1.2, respectively, and other properties are as shown in Table 6.

[0098] These data demonstrate that the production in Comparative Example 5 was performed without increasing the reactor pressure to 73.6 Kg/cm2G (the reactor pressure adopted in Example 2) corresponding to the increase in slurry feeding amount (from 220 to 435 ton/hr); and the resulting purified terephthalic acid product was out of product specifications due to an insufficient amount of absorbed hydrogen gas (decreased from 124 to 54 Nm"^/hr) .

Reference Signs List [0100]

A slurry preparation tank
B powder feed hopper
C heater
D dissolution tank
E heater/vapor saturator
F hydrogenation reactor
Fl catalyst bed
G first crystallization tank
H final crystallization tank
I solid-liquid separator
J gas phase (hydrogen-containing gas phase)
FIC flow-rate indicating controller
PIC pressure indicating controller
Lie liquid-level indicating controller
FI flow-rate indicator
TI temperature indicator
PIC pressure indicating controller
Lie liquid-level indicating controller
FI flow-rate indicator
TI temperature indicator

We Claim:

1. A process for purifying crude terephthalic acid by dissolving the crude terephthalic acid in water to give an aqueous solution thereof and passing the aqueous solution through a catalyst bed including a noble metal supported on an active carbon to carry out hydropurification, the process comprising:

feeding the aqueous solution of crude terephthalic acid so that the catalyst bed be filled with the aqueous solution and a liquid surface level of the aqueous solution is positioned above the catalyst bed while maintaining a hydrogen-containing gas phase on the liquid surface of the aqueous solution; and carrying out a hydrogenation reaction by allowing the aqueous solution to flow downward through the catalyst bed continuously while maintaining a hydrogen partial pressure

(H2 . PP) in the gas phase to be equal to or more than a partial pressure def inedby the following relational expression with respect to a linear velocity (LV) of the aqueous solution flowing down through the catalyst bed:
(H2. PP)=-0.000413x(LV) 2+0.224x(LV)-1.11

2. The process for purifying crude terephthalic acid, according to claim 1, wherein the hydrogenation reaction is carried out while maintaining the liquid surface of the aqueous solution of crude terephthalic acid above the catalyst bed and maintaining the hydrogen partial pressure (H2. PP) in the gas phase to be equal to or less than a partial pressure def inedby the following relational expression with respect to the superficial linear velocity (LV) of the aqueous solution flowing downward through the catalyst bed: (H2.PP)=-0.0020x(LV)^+0.569x(LV)-1.93

3. The process for purifying crude terephthalic acid, according to claim 1 or 2, wherein the hydrogenation reaction is carried out while maintaining the liquid surface level of the aqueous solution of crude terephthalic acid above the catalyst bed and maintaining the hydrogen partial pressure (H2.PP) in the gas phase within about ±25% of a partial pressure defined by the following relational expression with respect to the superficial linear velocity (LV) of the aqueous solution flowing downward through the catalyst bed: (H2.PP)=-0.000550x(LV)^+0.299x(LV)-1.48

4. The process for purifying crude terephthalic acid, according to any one of claims 1 to 3, wherein the crude terephthalic acid to be subjected to the hydrogenation reaction has been prepared from p-xylene as a material through liquid-phase oxidation with an oxygen-containing gas in acetic acid solvent in the presence of a catalyst, and the prepared crude terephthalic acid has a 4-carboxybenzaldehyde (4-CBA) content of from2, 000 to 3, 500 ppm.

5. The process for purifying crude terephthalic acid, according to any one of claims 1 to 4, wherein the aqueous solution of crude terephthalic acid to be subjected to the hydrogenation reaction has a terephthalic acid concentration of from 23 to 30 percent by weight, and the hydrogenation reaction is carried out by passing the aqueous solution of crude terephthalic acid at a temperature of from 275°C to 300°C through the catalyst bed.