Title of Invention	"METHOD OF SUPPLYING TEREPHTHALIC ACID SLURRY"
Abstract	In the hydrogenation purification of crude terephthalic acid, at the stage of mixing crude terephthalic acid with water into a slurry, conducting heating/dissolving operation to thereby convert the same to an aqueous solution and subjecting the aqueous solution to a hydrogenation purification reaction operation, the variation of concentration of prepared/supplied slurry has often caused disordering of the continuous operation of the purification reaction. Thus, it has been demanded to prepare and supply a slurry of stable concentration wherein the range of variation of slurry concentration is reduced. The amount of crude terephthalic acid supplied is regulated at given ratio (given concentration) on the basis of the amount of water fed into a slurry preparation tank with agitator, and mixing thereof is conducted to thereby prepare a slurry (1st regulation). While conducting external circulation of the slurry by means of a pump, the concentration thereof is measured, and the amount of water fed from the side of pump suction is regulated into a given concentration so as to attain regulation of the slurry concentration (2nd regulation). Thus, a slurry with narrowed range of concentration variation is obtained. Thereafter, in this process, by means of a high-pressure pump branched from circulation line, the prepared slurry is continuously supplied to heating/dissolving operation.

Title of Invention

"METHOD OF SUPPLYING TEREPHTHALIC ACID SLURRY"

Abstract

In the hydrogenation purification of crude terephthalic acid, at the stage of mixing crude terephthalic acid with water into a slurry, conducting heating/dissolving operation to thereby convert the same to an aqueous solution and subjecting the aqueous solution to a hydrogenation purification reaction operation, the variation of concentration of prepared/supplied slurry has often caused disordering of the continuous operation of the purification reaction. Thus, it has been demanded to prepare and supply a slurry of stable concentration wherein the range of variation of slurry concentration is reduced. The amount of crude terephthalic acid supplied is regulated at given ratio (given concentration) on the basis of the amount of water fed into a slurry preparation tank with agitator, and mixing thereof is conducted to thereby prepare a slurry (1st regulation). While conducting external circulation of the slurry by means of a pump, the concentration thereof is measured, and the amount of water fed from the side of pump suction is regulated into a given concentration so as to attain regulation of the slurry concentration (2nd regulation). Thus, a slurry with narrowed range of concentration variation is obtained. Thereafter, in this process, by means of a high-pressure pump branched from circulation line, the prepared slurry is continuously supplied to heating/dissolving operation.

Full Text	DESCRIPTION METHOD OF SUPPLYING TEREPHTHALIC ACID SLURRY TECHNICAL FIELD [0001] The present invention relates to a method for preparing and supplying a raw material slurry in the crude terephthalic acid purification process by dissolving the crude terephthalic acid in water and subsequently carrying out hydrogenation purification by the noble metal catalyst. Specifically, it relates to a method for mixing the crude terephthalic acid with water to give a slurry, and supplying the prepared slurry as a slurry having a stable concentration to the heating/dissolving process. BACKGROUND ART [0002] A hydrogenation purification process of a crude terephthalic acid includes mixing the crude terephthalic acid and water to give a slurry, heating and dissolving the slurry to give an aqueous solution of terephthalic acid, bringing the aqueous solution into contact with the catalyst which palladium metal supported by active carbon in the presence of hydrogen gas to hydrogenate impurities (e.g., 4-carboxybenzaldehyde, fluorenones, and diketones) contained in the crude terephthalic acid to thereby give a purified terephthalic acid. This process is industrially widely carried out. In this process, a powdery crude terephthalic acid and water are continuously fed to a slurry preparation tank, and stirred and mixed therein to give a slurry. The prepared slurry of terephthalic acid should be heated and thereby dissolved into an aqueous solution containing no powdery or solid terephthalic acid before being continuously supplied to a purification catalyst bed. [0003] In an industrially operated hydrogenation purification process, a slurry of a crude terephthalic acid and water is prepared at a concentration of about 20 to 30 percent by weight, and the slurry is supplied to a heating/dissolving process to give an aqueous solution of crude terephthalic acid heated at high temperatures (280°C to 300°C) . Terephthalic acid has low solubility in water as given in the table below, and dissolution thereof should be performed at high temperatures in commercial-scale plant. In particular, in recent years, the concentration of terephthalic acid in slurry has been often set at a high concentration (a concentration of 25 percent by weight or more or a concentration of more than 30 percent by weight), and the dissolution of terephthalic acid should therefore be performed at higher temperatures (about 275°C or higher). [0004] The solubility of terephthalic acid at high temperatures is significantly depended on the temperature, and the dissolution saturation temperature varies depending on even a slight change of concentration, and control of the heating/dissolving temperature is naturally one of important control points. In addition, the preparation and supply of a slurry of crude terephthalic acid so as to have a stable concentration is also one of industrially important factors in operation. [0005] [Table 1] Table 1: Solubility of Terephthalic Acid in Water (Table Removed) [0006] Specifically, if the concentration of crude terephthalic acid in the prepared slurry to be supplied varies and is thereby supplied to the catalyst bed with incompletely dissolved terephthalic acid, the residual solids cause precipitation, deposition, stoppage, or drift in the heater/dissolver and the catalyst bed to impede the flow of the aqueous solution to thereby interfere a stable hydrogenation purification reaction and continuous operation. Accordingly, the control of temperature should be carried out by setting a dissolution temperature, i.e., a catalyst bed temperature, so as to perform dissolution even when the concentration of the supplied slurry is varied within a certain range. [0007] In the previously known processes for the hydrogenation purification of crude terephthalic acid, the difference between the temperature of the dissolved aqueous solution of crude terephthalic acid (catalyst bed temperature) and the saturation temperature is generally set at 2°C to 8°C. The difference is, for example, set at 10°F (5.6°C) in Patent Publication 1 (U.S. Pat. No. 3,639,465), at 3°F to 10°F (1.7°C to 5.6°C) in Patent Publication 2 (U.S. Pat. No. 4,405,809), and at about 8°C in Patent Publication 3 (Japanese Patent Application Laid-Open Publication No. H06-321857). [0008] As a process for preparing and supplying the slurry of a stable concentration, the concentration of the slurry in a line extracted from a slurry preparation tank, to which the crude terephthalic acid and water are fed and mixed therein, or in a line for circulating the extracted slurry is measured, and the feed rates of the raw materials (crude terephthalic acid and water) to the slurry preparation tank are regulated so as to regulate the measured concentration to a target predetermined concentration. These techniques can be seen, for example, in Patent Publication 4 (Japanese Patent Application Laid-Open Publication No. H05-116131), Patent Publication 5 (Japanese Patent Application Laid-Open Publication No. H08-143504), and Patent Publication 6 (Chinese Patent No. CN 1539809A). [0009] Typically, regulation of the concentration (density) of the prepared slurry to a predetermined concentration is performed by regulating the feed rate of either one of the slurry raw materials (powder and water) in the technique disclosed in Patent Publication 4 or by regulating the feed rate of water in the technique disclosed in Patent Publication 7. The regulation is performed by predicting the concentration at an outlet of the slurry preparation tank based on the ratio between raw materials fed for the preparation of slurry (predictive control operation low) and controlling the feed rate of the raw material powder (terephthalic acid) in the technique disclosed in Patent Publication 6. Though some considerations are made, these proposed regulation techniques cannot be said to have quick regulation response, in which the concentration of the slurry (outlet) extracted from the slurry preparation tank is regulated by the raw material (inlet) fed to the slurry preparation tank. [0010] A delay of the regulation response therefore appears in the concentration of the prepared slurry (concentration of the slurry extracted from the slurry preparation tank), i.e., in the measured concentration, and this causes a variation or fluctuation in concentration. According to the purification process of the present invention, however, the slurry having a stable concentration is prepared and supplied, and this reduces the allowable width (difference in temperature) with respect to the dissolution saturation temperature of the slurry, leading to stable operation at a relatively low catalyst bed temperature. [0011] Patent Publication 1: U.S. Pat. No. 3,639,465 Patent Publication 2: U.S. Pat. No. 4,405,809 Patent Publication 3: Japanese Patent Application Laid- Open Publication No. H06-321857 Patent Publication 4: Japanese Patent Application Laid- Open Publication No. H05-116131 Patent Publication 5: Japanese Patent Application Laid- Open Publication No. H08-143504 Patent Publication 6: Chinese Patent Application Publication No. CN 1539809A Patent Publication 7: Japanese Patent Application Laid- Open Publication No. H08-143505 Patent Publication 8: Japanese Examined Patent Application Publication No. S39-10119 Patent Publication 9: Japanese Patent Application Laid- Open Publication No. S 58-189135 DISCLOSURE OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION [0012] According to known processes, an aqueous solution of terephthalic acid is supplied to a noble metal catalyst bed, and a hydrogenation purification reaction is performed in the presence of hydrogen gas at a temperature of from 280°C to 300°C, and a pressure of 70 to 100 Kg/cm2 gauge pressure so as to maintain the aqueous solution in a liquid form. In most of the processes, the concentration of terephthalic acid in the aqueous solution is set at 25 percent by weight (33.3 gr/100grH20) or more. The purification has been performed at a higher and higher concentration of terephthalic acid, for the sake of industrial advantages. This invites an elevated saturation temperature of the aqueous solution of terephthalic acid and thereby narrows the difference in temperature between the saturation temperature and the catalyst bed temperature (temperature of aqueous solution of terephthalic acid). This often impedes stable operations. A first problem is present in the step of continuously feeding a crude terephthalic acid and water to a slurry preparation tank, mixing them using an agitator, and regulating the concentration of slurry. [0013] While continuously supplying a powdery terephthalic acid and water to the slurry preparation tank, the regulation of the concentration of slurry is performed by a technique of measuring the concentration of the slurry through an external circulation line of the slurry preparation tank (measuring the outlet concentration), and controlling the feed rate of water fed to the slurry preparation tank based on the measured concentration (see Patent Publication 5); or by a technique of controlling the feed rate of either one of the raw materials (powder and liquid) fed to the slurry preparation tank (see Patent Publication 4). According to another technique, the regulation is performed not by directly controlling the feed rate of the raw material based on the measured concentration but by the feed rate of the powdery crude terephthalic acid based on calculation. In this technique, the concentration in the slurry preparation tank (outlet slurry concentration) is predicted according to a predictive control computation from the inlet slurry concentration and a change (converted value) thereof based on the feed rates of the crude terephthalic acid and water (see Patent Publication 6). [0014] Continuous feeding of the powdery terephthalic acid is adapted to arranging a feeder typically of a rotary valve type, screw feeder type, or a table feeder type at the bottom of a powder feeding hopper, and controlling the feed rate of the powder typically by controlling the rotational feed speed of the feeder mechanism. The feed rate is determined or measured by a technique of previously grasping the correlation between a feed rate and a rotational feed speed of the feeder, and finding the feed rate from the rotational speed; or by a technique of using a powder feed hopper with a weighing device and finding the feed rate directly from the rate of weight reduction measured by the weighing device of the hopper. However, smooth feeding of a powder is believed to be difficult as compared with feeding of a liquid, because the feed rate in the feed part suffers from fluctuations or pulsations to some extent due typically to the physical properties of the powder, size and control mechanism of the feed machine, and properties as a powder. The powdery crude terephthalic acid has a size distribution as equivalent diameters of from several micrometers to several hundreds of micrometers, and, if mixed in the agitation tank (preparation tank with an agitator), suffers from local separation due to the (natural or centrifugal) difference in specific gravity among up and down directions, radial directions, and rotational directions of agitating blades. This inevitably causes local ununiformity of the concentration of powdery terephthalic acid in the slurry preparation tank. [0015] Specifically, the first problem is present in that the concentration as measured at the outlet or circulation line of the slurry preparation tank is regulated by the feed rate of raw material fed to the slurry preparation tank, but the feed rate suffers from fluctuations or pulsations because of unsmooth feed of the powdery crude terephthalic acid, and the measured concentration suffers from local ununiformity occurring upon mixing of the powdery crude terephthalic acid and water. A second problem is present in the step of extracting the prepared slurry from the slurry preparation tank and continuously supplying the extracted slurry to the heater for heating and dissolving. In practice for extracting the slurry from the slurry preparation tank and transferring the extracted slurry stably, the slurry to be transferred is externally circulated and is then supplied via a branched line to the heater (see Patent Publication 8). When such a branched slurry is supplied to a process performed under high pressure, the slurry is pressurized with a high-pressure pump and is then supplied (see Patent Publication 9). [0016] The slurry is therefore generally supplied to the heater typically through the circulation line, branched line, and pump. Terephthalic acid in the slurry suffers again from uneven distribution and ununiformity, as affected by the flow (drifting), flow velocity, and the pipe diameter and the piping arrangement. The slurry is supplied through the branched line to the heater by the action of the high-pressure pump. In this process, the mechanics of heater, and the flow velocity and flow (drifting) of the slurry in the heater may affect the slurry concentration, and ununiformity, and fluctuations of the slurry concentration may appear more than the measured concentration. This impedes stable operations in the catalyst bed. The problem of fluctuations in concentration has become apparent as operations have been performed at higher slurry concentrations. According to the known methods for preparing slurry, therefore, fluctuations of about ±5% in terms of the amount of the powdery terephthalic acid are inevitable, as a total typically of uneven distribution and local ununiformity of the powdery terephthalic acid or fluctuations in concentration of terephthalic acid. The fluctuations in concentration appear as fluctuations in measured concentration of about ±1.5 percent by weight in a slurry of high terephthalic acid concentration (25 percent by weight or more). Such high terephthalic acid concentration is generally operated in commercial-scale plant for the purification of crude terephthalic acid. The fluctuations appear as fluctuations in terms of saturation temperature in a range of about ±2°C at a slurry concentration of 30 percent by weight. This requires a catalyst bed temperature (dissolution temperature) to be higher than the saturation temperature by 3°C or more and preferably by 4°C or more. Even in operations carried out with such difference in temperature, the supply pressure of the slurry may be elevated due to occlusions in the heater and the catalyst bed when the high-concentration slurry is purified. The elevated pressure may impede the stability of the system for the purification of crude terephthalic acid. MEANS OF SOLVING THE PROBLEMS [0017] The present inventors made measurements and analyses in detail about factors for fluctuations in concentration of crude terephthalic acid to be supplied from the hydrogenation purification tank to the catalyst bed. As a result, they have found that fluctuations of about ±5% in terms of the amount of powdery terephthalic acid are inevitable according to the known methods for regulating the concentration, as described above. Consequently, they have found the following method which does not impede a stable operation of hydrogenation purification reaction by supplying a slurry with narrower fluctuations in its concentration to the heating/dissolving process to melt the slurry to an aqueous solution, and supplying the aqueous solution at a stable concentration to the catalyst bed. Specifically, there is provided a method for acquiring purified terephthalic acid by continuously feeding a crude terephthalic acid and water to a slurry preparation tank equipped with an agitator to give a slurry at a predetermined concentration and heating and dissolving the said slurry therein to give a solution and continuously supplying the solution to a hydrogenation purification reactor. The method comprises the steps of: 1) controlling the feed rate of the crude terephthalic acid based on the feed rate of water to the slurry preparation tank so as to attain a predetermined ratio between the crude terephthalic acid and water (a predetermined concentration), and mixing them to give a slurry; 2) measuring the concentration of the slurry with a slurry concentration meter arranged on a discharge side of an external circulation pump of the slurry preparation tank while circulating the prepared slurry; 3) controlling the water feed rate supplied from a water feed line to a line on the suction side of the circulation pump so that the above-measured concentration becomes a predetermined concentration to thereby regulate the concentration of the slurry; and 4) supplying the prepared slurry at the predetermined concentration to the heating/dissolving process with a high-pressure pump through a line branched from the circulation line on the discharge side of the circulation pump. This method enables preparation and supply of a slurry of crude terephthalic acid with narrower fluctuations in concentration and thereby enables stable operations of hydrogen purification reactions. [0018] In general, a liquid (water) can be fed quantitatively, but a powder (crude terephthalic acid) is fed with some fluctuations or pulsations in feed rate, as described above. Accordingly, regulation of the feed rate of the crude terephthalic acid to a predetermined ratio (predetermined concentration) based on the feed rate of water is preferred as a macro regulation method (primary regulation) of the slurry concentration in the slurry preparation tank when the powdery crude terephthalic acid and water are continuously and quantitatively fed to the slurry preparation tank. In other words, regulation of the feed rate of water to the preparation tank to be predetermined concentration as proposed in above-mentioned Patent Publication 5. It cannot be said as a preferred regulation method. This is because, in this method, the feed rate of water is regulated based on the feed rate of the powdery crude terephthalic acid that is difficult to be fed continuously smoothly. The prepared slurry at a predetermined concentration is continuously supplied to the heater which generally uses a shell and tube heat exchanger. To avoid precipitation of terephthalic acid powder particles in the slurry, the flow velocity of the slurry in the slurry transfer pipe and in circulation of the slurry in the heater should always be 1.5 m/hr or more and is preferably 2 m/hr or more. Consequently, the method according to the present invention uses regulation of the feed rate of the powdery crude terephthalic acid to a predetermined ratio (predetermined concentration) based on the feed rate of water. This regulation is preferred from the viewpoint of ensuring a stable slurry concentration, because flows of water in predetermined amounts are always ensured during circulation and transfer, and in the heater. [0019] A common procedure for stable transfer of a high-concentration slurry is measurement of a slurry concentration while performing external circulation of the slurry. The circulation line is branched, and a slurry the same as the slurry whose concentration has been measured is supplied to the heating/dissolving process through a high-pressure pump. According to the method of the present invention, the concentration is measured with a slurry concentration meter arranged on the discharge side of the circulation pump to detect a difference in concentration with respect to the predetermined concentration, i.e., a fluctuation in slurry concentration; water is directly fed from a water feed line arranged on the suction side line of the circulation pump to thereby promptly regulate the fluctuation in concentration of the slurry at the discharge side of the pump. The slurry at the regulated concentration is then supplied to the heating/dissolving process. This regulation is a micro regulation (secondary regulation) in the method according to the present invention. The resulting slurry with further narrower fluctuations in concentration is supplied as intact through the high-pressure pump to the heating/dissolving process, in which the slurry is melted to an aqueous solution at a stable terephthalic acid concentration, and is then supplied to the hydrogenation purification tank. [0020] In the method according to the present invention, the feed rate of the powdery crude terephthalic acid is regulated in the primary regulation based on the feed rate of water that can be fed quantitatively, so as to prepare a slurry; and fluctuations in concentration at the inlet of the heating/dissolving process is promptly corrected by controlling the feed rate of water in the secondary regulation, as mentioned above. According to the known methods, the feed rate of raw material crude terephthalic acid or water to be fed to the slurry preparation tank is regulated based on the measured slurry concentration in the slurry preparation tank. The measured slurry concentration is, however, a value after the elapse of residence in the slurry preparation tank (residence time of from 10 to 100 minutes), and this causes response delay. In contrast, the method according to the present invention can respond and regulate fluctuations in concentration immediately and can supply a slurry with narrower fluctuations in concentration to the heating/dissolving process. This leads to more stable operations in process for purifying crude terephthalic acid. The stable operation in purification process, an object of the present invention, can be more effectively achieved by the method according to the present invention when only fluctuations in slurry concentration in which measured slurry concentration is higher than the predetermined concentration, are controlled. [0021] Specifically, in a preferred method of the present invention, crude terephthalic acid is fed at a predetermined ratio based on the feed rate of water to prepare a slurry at a predetermined concentration in the primary regulation; the concentration of the prepared slurry is measured; and feed of water under control is performed in the secondary regulation only to fluctuations in which the measured slurry concentration is higher than the predetermined concentration. The primary regulation and secondary regulation are fully independently performed or controlled according to this method, and it is desirable as a high-precision control. [0022] The system according to a preferred method, in which the secondary regulation is performed only to fluctuations in which the measured concentration is higher than the predetermined concentration, is also desirable as a method that enables smooth transit to a stable operation typically at the beginning of operation. Specifically, in this method, feed of water at predetermined feed rates to the slurry preparation tank and to the heating/dissolving process is initiated; conditions such as temperature and pressure in the hydrogenation purification tank are set based only on the feed rate of water; thereafter feed of the powdery crude terephthalic acid to the slurry preparation tank is initiated so as to achieve a feed rate of the powder at a predetermined ratio gradually or quickly to thereby raise the slurry concentration to a predetermined concentration. This method is preferably used, because the operation can shift to a smooth continuous operation while ensuring predetermined flow velocities typically in the pipe and heater. EFFECTS OF THE INVENTION [0023] In the method according to the present invention, the primary regulation and secondary regulation are performed in the preparation of a slurry of crude terephthalic acid and water. The resulting slurry is stable with narrower fluctuations in concentration with respect to a predetermined target concentration. The fluctuations are about one-fifth the fluctuations according to the known methods. Thus, the slurry with a stable concentration can be supplied to the heating/dissolving process, whereby the hydrogenation purification reaction can be carried out more stably. This eliminates system troubles such as drift and occlusion in systems or devices such as the catalyst bed and thereby improves production efficiency. These effects are also obtained and thereby higher production efficiency is obtained by the system according to a preferred method, in which the second regulation to feed water under control is performed only when the measured concentration is higher than the predetermined concentration. [0024] The purified terephthalic acid through hydrogenation purification of crude terephthalic acid has been used in a wider and wider various ways. Along with this, the concentration of the slurry of crude terephthalic acid is set higher and higher (25 percent by weight or more) in industrial plant. The method according to the present invention enables operations of the hydrogenation purification tank and related facilities thereof in these industrial plant at not so high temperatures and whereby at not so high pressures. This eliminates the need of reinforcement in facilities. This also eliminates the need of increasing the heating energy and enables energy-saving processes, because the plant can be operate at not so high temperatures. BEST MODE FOR CARRYING OUT THE INVENTION [0025] Some systems of the method according to the present invention will be illustrated below with reference to the flow chart of a process and its system shown in Fig. 1. The process in Fig. 1 is by no means intended to limit the scope of the present invention. Initially, water is fed from the pure water line [1] to the slurry preparation tank 4 at a predetermined flow rate; and a crude terephthalic acid is fed from the crude terephthalic acid hopper 1 via the powder feeder 3 and the supply line [2] to the slurry preparation tank 4, while the feed rate of the crude terephthalic acid is regulated by the action of the powder feed rate adjuster 2 based on the feed rate of water, so as to attain a predetermined ratio (predetermined concentration). Feed of water from the pure water line [1] is carried out at a predetermined feed rate. This predetermined feed rate of water is calculated from the amount and concentration of crude terephthalic acid to be purified in the purification process of crude terephthalic acid. The predetermined feed rate of water is a flow rate set also in consideration of such a flow velocity as to prevent the precipitation in the transfer line and heater. The feed rate of the powdery crude terephthalic acid is adjusted by the action of the powder feed rate adjuster 2 so as to attain a predetermined ratio (predetermined concentration) with respect to the predetermined feed rate of water based on the setting in the feed rate controller (FFC1). Thus the powder is fed under regulation to the slurry preparation tank 4 (primary regulation). The crude terephthalic acid and water fed to the slurry preparation tank 4 are agitated and mixed with each other to give a slurry, and the slurry is extracted from the bottom of the slurry preparation tank 4 through the circulation pump 5 and is circulated via lines [4] and [6]. The slurry concentration meter 6 is arranged at some midpoint in the circulation lines [4] and [6] to measure the concentration of the circulating slurry. Then, the feed rate of the water from the water feed line [3] is controlled through the setting of the flow rate controller (FFC2) so as to allow the measured concentration to be a predetermined concentration. Thus, a fluctuation in concentration is immediately regulated so as to attain the predetermined concentration (secondary regulation). The secondary regulation can be performed according to two procedures. Specifically, in one procedure, water is fed so as to regulate only fluctuations in which the measured concentration is higher than the predetermined concentration; and in the other process, water is fed so as to regulate both fluctuations in which the measured concentration is higher than the predetermined concentration and fluctuations in which the measured concentration is lower than the predetermined concentration. [0026] Specifically, the concentration regulation according to the primary regulation suffers from fluctuations of up to about ±5% in the amount of powdery crude terephthalic acid, as described above. Water should be fed for the secondary regulation at a feed rate of up to about 10% and preferably up to about 20% with respect to the feed rate of water from the water line [1]. In the procedure in which only a fluctuation higher than the predetermined concentration is regulated, the system is configured to feed water at a feed rate of up to about 10% with respect to the predetermined feed rate of water from the line [1]; and water is fed from the line [3] to regulate the concentration, only when the measured concentration is higher than the predetermined concentration. When a fluctuation in which the measured concentration is lower than the predetermined concentration, water feed is stopped and concentration regulation is not performed. [0027] In the other procedure, regulation is performed to fluctuations in which the measured concentrations are both higher than and lower than the predetermined concentration. In this case, water is fed from the line [1] at a predetermined feed rate corresponding to about 90% to 95% of the predetermined total feed rate of water; and water is fed from the line [3] at a predetermined feed rate corresponding to about 10% to 5% of the predetermined total feed rate of water. Thus, the system is configured so as to enable control of the feed rate of water from the line [3] corresponding to fluctuations in which the measured concentrations are both higher than and lower than the predetermined concentration. For this purpose, the regulation of the feed rate of water from the line [3] is preferably set so as to enable regulation of feed rate of water from the line [3] of up to about 20% of the predetermined total feed rate of water. In regulation by the method according to the present invention, the primary regulation (control of feed rate of crude terephthalic acid) and secondary regulation (control of feed rate of water) can be performed separately and independently. In a system, the primary regulation and the secondary regulation can be performed fully independently when the second regulation is performed only to fluctuations in which the measured concentration is higher than the predetermined concentration. The method according to this system of the present invention can also achieve the objects of the present invention. [0028] The concentration of slurry in the circulation line is measured with the slurry concentration meter 6 by continuously or intermittently measuring a density with a densitometer, and converting the measured density to a concentration of terephthalic acid which varies depending on the density. The densitometer usable herein is one that can measure the density of slurry, such as a mechanical oscillation densitometer, a radiation densitometer, or a coriolis densitometer. When the measured concentration with the slurry concentration meter 6 differs from the predetermined concentration, the feed rate of water from the line [3] is controlled through the setting of the flow rate controller (FFC2) to thereby regulate the slurry concentration. The control of the feed rate of water from the line [3] through the setting of the flow rate controller (FFC2) is performed according to proportional, integral, and derivative (PID) control. The slurry concentration meter 6 may be arranged either downstream or upstream from the branched portion of the line at which the slurry is extracted for supplying. The concentration of slurry is directly measured in the circulation line or is measured through the bypass line of a suitable size that is branched from the circulation line, depending on the type of the slurry concentration meter 6 and the size of piping arranged. The measuring mode may be chosen according typically to the type and location of the meter. [0029] The slurry whose concentration has been regulated is branched (line [5]) from the circulating line [4], and the branched slurry is supplied in a predetermined amount (flowmeter 8) through the slurry supply pump 7 to the heater 9 and the dissolution tank 10. Thus, the slurry of crude terephthalic acid is heated and dissolved. The supplied slurry is heated and dissolved at a temperature of from 280°C to 300°C to give an aqueous solution of terephthalic acid. In this process, a pressure of from 70 to 100 Kg/cm2 gauge pressure is required to maintain the resulting aqueous solution as a liquid, and the supply pump 7 requires a high-pressure pump head. Such a high-pressure pump for use herein is preferably one that can quickly raise the pressure to attain a high pressure, such as a multi-stage plunger pump or a highspeed single-stage centrifugal pump. When the supply flow rate of slurry is measured with the flowmeter 8 (e.g., electromagnetic flowmeter or coriolis flowmeter) and the slurry is supplied by the action of a volume pump, the flow rate is regulated to a predetermined flow rate by controlling the driving speed of the plunger. When a centrifugal pump is used, the flow rate is regulated to the predetermined flow rate through the flow rate control valve. Excess slurry is returned via the line [8] to the slurry preparation tank 4 by the action of pressure controller (PIC; to maintain at a predetermined pressure) that is under constant pressure control. [0030] Accordingly, the slurry is prepared to a predetermined concentration by the action of the flowmeter 8 and is supplied at a predetermined flow rate. This stabilizes the amount of crude terephthalic acid to be purified in the purification reactor (i.e., the amount of purified terephthalic acid). In other words, continuous supply of the slurry under regulation ensures stable operation of the system and stable quality of the product. When the fluid level in the slurry preparation tank 4 rises, the rise is corrected by the control of the supply flow rate of slurry by the action of the flowmeter 8 so as to stabilize the fluid level. Such rise of fluid level occurs due to imbalance between the amount of slurry preparation and the feed rate (amount) of slurry supply through the high-pressure pump. The amount of slurry preparation is determined by the feed rate of crude terephthalic acid (line [2]), the predetermined feed rate of water (line [1] or line [1] plus line [3]), and, in addition, the feed rate of water from the line [3] fed for the regulation of concentration. The heater 9 serves to heat the slurry (at a temperature of 100°C or lower) under normal atmospheric pressure to a temperature of from 280°C to 300°C and to pressurize the slurry. The heating is performed with two or more heaters using heating media of different temperatures. When the temperature of slurry reaches the predetermined dissolution temperature, the slurry is supplied to the dissolution tank 10 while maintaining the slurry at the dissolution temperature. After the elapse of the suitable dissolution time (5 to 20 minutes), the resulting aqueous solution containing no powdery terephthalic acid is supplied to the hydrogenation purification tank in which a hydrogenation purification reaction of crude terephthalic acid is performed. EXAMPLES [0031] Next, some specific systems of the method according to the present invention will be illustrated in detail with reference to several examples below. [0032] The slurry of crude terephthalic acid and water was prepared, subjected to a heating/dissolving process, and supplied to the hydrogenation purification reaction according to the flow chart illustrated in Fig. 1. While water was fed at the predetermined feed rate (8.9 ton/hr) from the line [1] to the slurry preparation tank 4 (2,500 mm in diameter and 4,000 mm in height) equipped with an agitator, a powdery crude terephthalic acid was extracted from the crude terephthalic acid hopper 1 so as to attain a feed rate of 3.8 ton/hr by the action of a rotary-valve powder feed rate adjuster 2 and continuously fed via the powder feeder 3 and the line [2] to the slurry preparation tank 4, in which the both materials were agitated and mixed to give a slurry. In this process, the relation between the feed speed (rotational speed) of the powder feed rate adjuster 2 and the feed rate of the crude terephthalic acid had been grasped, and the feed speed by the adjuster 2 was controlled through the setting of the feed rate controller (FFC1) so that the feed rate of crude terephthalic acid is 3.8 ton/hr, corresponding to the feed rate of water (8.9 ton/hr) (primary regulation). [0033] The prepared slurry was extracted from the bottom of the preparation tank through the circulation pump 5 and circulated (at about 20 ton/hr) through the lines [4] and [ 6 ] . The concentration of the slurry was measured in the circulation line with a slurry concentration meter 6, in which the density was measured with the mechanical oscillation densitometer and converted to a concentration. When a fluctuation in concentration occurred, in which the measured concentration of slurry was higher than 30 percent by weight, water was fed from the line [3] through the setting of the flow rate controller (FFC2) so as to regulate the slurry concentration to 30 percent by weight (secondary regulation). Next, the prepared slurry in the circulating line [4] was branched as the line [5] at some midpoint of the line [4] (upstream of the slurry concentration meter), raised in pressure (to about 112 Kg/cm2 gauge pressure) by the action of the slurry supply pump 7 (high-speed single-stage centrifugal pump; under the trade name of Sundyne Pump), and supplied via the flowmeter 8 (coriolis flowmeter) and the line [7] to the heaters 9. The feed rate of the slurry was adjusted to a predetermined flow rate of 12.7 ton/hr by controlling the control valve (FIC) based on an indication from the flowmeter 8. [0034] The supplied slurry was heated through the heaters 9 with superheated steam while setting the target temperature at 284°C, supplied to the dissolution tank 10 to give the aqueous solution, and the aqueous solution was supplied to the hydrogenation purification tank 11. The hydrogenation purification reaction was performed in the hydrogenation purification tank 11 at a temperature of about 284°C and a pressure of about 74 Kg/cm2 gauge pressure. The pressure was set as above by injection of hydrogen gas. The hydrogenation purification reaction could be continuously smoothly performed over about half a year. The exemplary graphic plot of concentrations in stationary state during this process measured by the slurry concentration meter 6 (every 30 minutes for 24 hours) is shown as data indicated by "Example 1" in Fig. 3. [0035] As seen from the results, the slurry having a variation in concentration higher than 30 percent by weight of 0.2 percent by weight (relative variation of 0.67 percent by weight) or less could be supplied to the heaters 9, and the hydrogenation purification reaction could therefore be stably continued. The fluid level in the slurry preparation tank tended to rise during the operation. In this case, the fluid level was maintained by increasing the supply rate of slurry to some extent to balance between the supply rate of slurry and the feed rates of the raw materials. [0036] The procedure of Example 1 was repeated, except for the followings. Water was fed from the line [1] at a feed rate of 8.0 ton/hr, and crude terephthalic acid was continuously fed from the terephthalic acid hopper 1 to the slurry preparation tank 4 at the feed rate of 3.8 ton/hr, in which the feed rate was adjusted by the powder feed rate adjuster 2 based on the feed rate of water of 8.9 ton/hr. This feed rate of water was set assuming that water would be fed from the line [3] to the circulation line at the feed rate of 0.9 ton/hr. The crude terephthalic acid and water were then agitated and mixed in the slurry preparation tank 4 and circulated to give a slurry (primary regulation). During this process, water was fed from the line [3] at the feed rate of 0.9 ton/hr, the resulting slurry was circulated through the circulation pump 5, the concentration of the circulation slurry was measured with the slurry concentration meter 6, and the feed rate of water from the line [3] was controlled corresponding to the excess and deficiency of the concentration. Thus, the concentration of the slurry was regulated so that the measured concentration is 30 percent by weight (secondary regulation). [0037] The above-prepared slurry was extracted from the branched line [5], raised in pressure (about 112 Kg/cm2 gauge pressure) by the action of the slurry supply pump 7, supplied to the heaters 9, and a hydrogenation purification reaction was performed as in Example 1. As a result, a smooth operation could be continued. The exemplary graphic plot of concentrations in stationary state during this process measured by the slurry concentration meter 6 (every 30 minutes for 24 hours) is shown as data indicated by "Example 2" in Fig. 3. As seen from the results, a slurry having a fluctuation range in concentration, in which the measured concentration is higher than 30 percent by weight, of ±0.2 percent by weight (relative variation of 0.67 percent by weight) could be supplied to the heaters 9, and a hydrogenation purification reaction could therefore be stably continued. Fluctuations in fluid level in the slurry preparation tank 4 were regulated by the control of the supply rate of slurry by the flowmeter 8 (FIC). [0038] The slurry of crude terephthalic acid and water was prepared, subjected to a heating/dissolution process, and supplied to a hydrogenation purification reaction according to the flow chart illustrated in Fig. 2. In this process, units and systems were substantially the same as those in the flow chart of Fig. 1 (as above examples), except for the secondary regulation unit of the slurry concentration. The crude terephthalic acid was fed from the crude terephthalic acid hopper 1 to the slurry preparation tank 4 at the predetermined feed rate of 3.8 ton/hr, which feed rate was adjusted by the powder feed rate adjuster 2. Water was fed from the line [1] to the slurry preparation tank 4 at the feed rate of about 8.9 ton/hr which was regulated based on the feed rate of crude terephthalic acid, and water was agitated and mixed with the crude terephthalic acid to give a slurry. The prepared slurry was extracted from the bottom of the preparation tank, circulated at about 20 ton/hr through the circulating pump 5, the concentration of the circulation slurry was measured with the slurry concentration meter 6, and the feed rate of water from the water feed line [1] was controlled through the setting of the flow rate controller (FFC1) so that the measured concentration is 30 percent by weight. Thus, the slurry concentration was regulated (primary regulation). The above-prepared slurry was extracted from the branched line [5], raised in pressure (about 112 Kg/cm2 gauge pressure) by the action of slurry supply pump 7, supplied to the heaters 9 and the dissolution tank 10 at the predetermined flow rate of 12.7 ton/hr under the control by the control valve (FIC) as indicated by the flowmeter 8. Thus, the hydrogenation purification reaction was performed. As a result, the degree of opening of the feed rate control valve (generally about 40%) increased from one month later, and became 100% on the 45th day, and the operation became out of control. Specifically, the pressure of the heater inlet (control valve outlet side) was increased in order to supply the slurry at the predetermined flow rate (12.7 ton/hr), because occlusion occurred at the heaters 9 and the downstream thereof. Thereafter, the feed of crude terephthalic acid was stopped, and an operation was performed with the feed of water alone, and the hydrogenation purification operation was then stopped. The graphic plot of concentrations in stationary state during this process measured by the slurry concentration meter 6 (every 30 minutes for 24 hours) is shown as data indicated by "Comparative Example" in Fig. 3. The data demonstrate that the slurry was supplied with a fluctuation range of about ±1.4% (relative variation of ±4.7%) with respect to the target slurry concentration of 30 percent by weight. BRIEF DESCRIPTION OF THE DRAWINGS [0039] [Fig. 1] Fig. 1 is the flow chart of processes and systems, in which the slurry of powdery crude terephthalic acid and water is prepared, the slurry is supplied to a heating/dissolving process to give an aqueous solution, and the aqueous solution is supplied to the hydrogenation purification reaction, according to the method of the present invention. [Fig. 2] Fig. 2 is the flow chart of processes and systems, in which the slurry of powdery crude terephthalic acid and water is prepared according to known method, and the prepared slurry is supplied in the same way as in the method according to the present invention (Fig. 1). [Fig. 3] Fig. 3 is the diagram of concentrations of slurries of powdery crude terephthalic acid and water as measured by the slurry concentration meter (mechanical oscillation densitometer) every 30 minutes for 24 hours, respectively, in which the slurries were prepared according to the examples and the comparative example, and supplied to the heating/dissolving process. EXPLANATION OF REFERENCE NUMERALS AND SYMBOLS [0040] 1: crude terephthalic acid hopper, 2: powder feed rate adjuster, 3: powder feeder, 4: slurry preparation tank, 5: slurry circulation pump, 6: slurry concentration meter, 7: slurry supply pump, 8: flowmeter, 9: heater, 10: dissolver, 11: hydrogenation purification reactor. CLAIMS 1. A method for acquiring purified terephthalic acid by continuously feeding a crude terephthalic acid and water to a slurry preparation tank equipped with an agitator to give a slurry at a predetermined concentration and heating and dissolving the said slurry therein to give a solution and continuously supplying the solution to a hydrogenation purification reactor, the method comprises the steps of: 1) controlling the feed rate of the crude terephthalic acid based on the feed rate of water to the slurry preparation tank so as to attain a predetermined ratio between the crude terephthalic acid and water, and mixing them to give a slurry; 2) measuring the concentration of the slurry with a slurry concentration meter arranged on a discharge side of an external circulation pump of the slurry preparation tank while circulating the prepared slurry; 3) controlling the water feed rate supplied from a water feed line to a line on the suction side of the circulation pump so that the above-measured concentration becomes a predetermined concentration to thereby regulate the concentration of the slurry; and 4) supplying the prepared slurry at the predetermined concentration to the heating/dissolving process with a high-pressure pump through a line branched from the circulating line on the discharge side of the circulation pump. 2. The method for supplying a slurry of crude terephthalic acid, according to claim 1, wherein water is supplied from the water feed line in section 3) when the above-measured concentration is higher than the predetermined concentration.

Full Text

DESCRIPTION METHOD OF SUPPLYING TEREPHTHALIC ACID SLURRY
TECHNICAL FIELD [0001]
The present invention relates to a method for preparing and supplying a raw material slurry in the crude terephthalic acid purification process by dissolving the crude terephthalic acid in water and subsequently carrying out hydrogenation purification by the noble metal catalyst. Specifically, it relates to a method for mixing the crude terephthalic acid with water to give a slurry, and supplying the prepared slurry as a slurry having a stable concentration to the heating/dissolving process. BACKGROUND ART [0002]
A hydrogenation purification process of a crude terephthalic acid includes mixing the crude terephthalic acid and water to give a slurry, heating and dissolving the slurry to give an aqueous solution of terephthalic acid, bringing the aqueous solution into contact with the catalyst which palladium metal supported by active carbon in the presence of hydrogen gas to hydrogenate impurities (e.g., 4-carboxybenzaldehyde, fluorenones, and diketones) contained in the crude terephthalic acid to thereby give a
purified terephthalic acid. This process is industrially widely carried out.
In this process, a powdery crude terephthalic acid and water are continuously fed to a slurry preparation tank, and stirred and mixed therein to give a slurry. The prepared slurry of terephthalic acid should be heated and thereby dissolved into an aqueous solution containing no powdery or solid terephthalic acid before being continuously supplied to a purification catalyst bed. [0003]
In an industrially operated hydrogenation purification process, a slurry of a crude terephthalic acid and water is prepared at a concentration of about 20 to 30 percent by weight, and the slurry is supplied to a heating/dissolving process to give an aqueous solution of crude terephthalic
acid heated at high temperatures (280°C to 300°C) . Terephthalic acid has low solubility in water as given in the table below, and dissolution thereof should be performed at high temperatures in commercial-scale plant. In particular, in recent years, the concentration of terephthalic acid in slurry has been often set at a high concentration (a concentration of 25 percent by weight or more or a concentration of more than 30 percent by weight), and the dissolution of terephthalic acid should therefore be performed at higher temperatures (about 275°C or higher).
[0004]
The solubility of terephthalic acid at high temperatures is significantly depended on the temperature, and the dissolution saturation temperature varies depending on even a slight change of concentration, and control of the heating/dissolving temperature is naturally one of important control points. In addition, the preparation and supply of a slurry of crude terephthalic acid so as to have a stable concentration is also one of industrially important factors in operation. [0005] [Table 1]
Table 1: Solubility of Terephthalic Acid in Water

(Table Removed)
[0006]
Specifically, if the concentration of crude
terephthalic acid in the prepared slurry to be supplied varies and is thereby supplied to the catalyst bed with incompletely dissolved terephthalic acid, the residual solids cause precipitation, deposition, stoppage, or drift in the heater/dissolver and the catalyst bed to impede the flow of the aqueous solution to thereby interfere a stable hydrogenation purification reaction and continuous operation.
Accordingly, the control of temperature should be carried out by setting a dissolution temperature, i.e., a catalyst bed temperature, so as to perform dissolution even when the concentration of the supplied slurry is varied within a certain range. [0007]
In the previously known processes for the hydrogenation purification of crude terephthalic acid, the difference between the temperature of the dissolved aqueous solution of crude terephthalic acid (catalyst bed temperature) and the saturation temperature is generally set at 2°C to 8°C. The difference is, for example, set at 10°F (5.6°C) in Patent Publication 1 (U.S. Pat. No. 3,639,465), at 3°F to 10°F (1.7°C to 5.6°C) in Patent Publication 2 (U.S. Pat. No. 4,405,809), and at about 8°C in Patent Publication 3 (Japanese Patent Application Laid-Open Publication No. H06-321857).
[0008]
As a process for preparing and supplying the slurry of a stable concentration, the concentration of the slurry in a line extracted from a slurry preparation tank, to which the crude terephthalic acid and water are fed and mixed therein, or in a line for circulating the extracted slurry is measured, and the feed rates of the raw materials (crude terephthalic acid and water) to the slurry preparation tank are regulated so as to regulate the measured concentration to a target predetermined concentration. These techniques can be seen, for example, in Patent Publication 4 (Japanese Patent Application Laid-Open Publication No. H05-116131), Patent Publication 5 (Japanese Patent Application Laid-Open Publication No. H08-143504), and Patent Publication 6 (Chinese Patent No. CN 1539809A). [0009]
Typically, regulation of the concentration (density) of the prepared slurry to a predetermined concentration is performed by regulating the feed rate of either one of the slurry raw materials (powder and water) in the technique disclosed in Patent Publication 4 or by regulating the feed rate of water in the technique disclosed in Patent Publication 7. The regulation is performed by predicting the concentration at an outlet of the slurry preparation tank based on the ratio between raw materials fed for the
preparation of slurry (predictive control operation low) and controlling the feed rate of the raw material powder (terephthalic acid) in the technique disclosed in Patent Publication 6.
Though some considerations are made, these proposed regulation techniques cannot be said to have quick regulation response, in which the concentration of the slurry (outlet) extracted from the slurry preparation tank is regulated by the raw material (inlet) fed to the slurry preparation tank. [0010]
A delay of the regulation response therefore appears in the concentration of the prepared slurry (concentration of the slurry extracted from the slurry preparation tank), i.e., in the measured concentration, and this causes a variation or fluctuation in concentration. According to the purification process of the present invention, however, the slurry having a stable concentration is prepared and supplied, and this reduces the allowable width (difference in temperature) with respect to the dissolution saturation temperature of the slurry, leading to stable operation at a relatively low catalyst bed temperature. [0011]
Patent Publication 1: U.S. Pat. No. 3,639,465 Patent Publication 2: U.S. Pat. No. 4,405,809
Patent Publication 3: Japanese Patent Application Laid-
Open Publication No. H06-321857
Patent Publication 4: Japanese Patent Application Laid-
Open Publication No. H05-116131
Patent Publication 5: Japanese Patent Application Laid-
Open Publication No. H08-143504
Patent Publication 6: Chinese Patent Application
Publication No. CN 1539809A
Patent Publication 7: Japanese Patent Application Laid-
Open Publication No. H08-143505
Patent Publication 8: Japanese Examined Patent Application
Publication No. S39-10119
Patent Publication 9: Japanese Patent Application Laid-
Open Publication No. S 58-189135
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0012]
According to known processes, an aqueous solution of terephthalic acid is supplied to a noble metal catalyst bed, and a hydrogenation purification reaction is performed in the presence of hydrogen gas at a temperature of from 280°C to 300°C, and a pressure of 70 to 100 Kg/cm2 gauge pressure so as to maintain the aqueous solution in a liquid form. In most of the processes, the concentration of terephthalic
acid in the aqueous solution is set at 25 percent by weight (33.3 gr/100grH20) or more. The purification has been performed at a higher and higher concentration of terephthalic acid, for the sake of industrial advantages. This invites an elevated saturation temperature of the aqueous solution of terephthalic acid and thereby narrows the difference in temperature between the saturation temperature and the catalyst bed temperature (temperature of aqueous solution of terephthalic acid). This often impedes stable operations.
A first problem is present in the step of continuously feeding a crude terephthalic acid and water to a slurry preparation tank, mixing them using an agitator, and regulating the concentration of slurry. [0013]
While continuously supplying a powdery terephthalic acid and water to the slurry preparation tank, the regulation of the concentration of slurry is performed by a technique of measuring the concentration of the slurry through an external circulation line of the slurry preparation tank (measuring the outlet concentration), and controlling the feed rate of water fed to the slurry preparation tank based on the measured concentration (see Patent Publication 5); or by a technique of controlling the feed rate of either one of the raw materials (powder and
liquid) fed to the slurry preparation tank (see Patent Publication 4).
According to another technique, the regulation is performed not by directly controlling the feed rate of the raw material based on the measured concentration but by the feed rate of the powdery crude terephthalic acid based on calculation. In this technique, the concentration in the slurry preparation tank (outlet slurry concentration) is predicted according to a predictive control computation from the inlet slurry concentration and a change (converted value) thereof based on the feed rates of the crude terephthalic acid and water (see Patent Publication 6). [0014]
Continuous feeding of the powdery terephthalic acid is adapted to arranging a feeder typically of a rotary valve type, screw feeder type, or a table feeder type at the bottom of a powder feeding hopper, and controlling the feed rate of the powder typically by controlling the rotational feed speed of the feeder mechanism. The feed rate is determined or measured by a technique of previously grasping the correlation between a feed rate and a rotational feed speed of the feeder, and finding the feed rate from the rotational speed; or by a technique of using a powder feed hopper with a weighing device and finding the feed rate directly from the rate of weight reduction
measured by the weighing device of the hopper. However, smooth feeding of a powder is believed to be difficult as compared with feeding of a liquid, because the feed rate in the feed part suffers from fluctuations or pulsations to some extent due typically to the physical properties of the powder, size and control mechanism of the feed machine, and properties as a powder. The powdery crude terephthalic acid has a size distribution as equivalent diameters of from several micrometers to several hundreds of micrometers, and, if mixed in the agitation tank (preparation tank with an agitator), suffers from local separation due to the (natural or centrifugal) difference in specific gravity among up and down directions, radial directions, and rotational directions of agitating blades. This inevitably causes local ununiformity of the concentration of powdery terephthalic acid in the slurry preparation tank. [0015]
Specifically, the first problem is present in that the concentration as measured at the outlet or circulation line of the slurry preparation tank is regulated by the feed rate of raw material fed to the slurry preparation tank, but the feed rate suffers from fluctuations or pulsations because of unsmooth feed of the powdery crude terephthalic acid, and the measured concentration suffers from local
ununiformity occurring upon mixing of the powdery crude terephthalic acid and water.
A second problem is present in the step of extracting the prepared slurry from the slurry preparation tank and continuously supplying the extracted slurry to the heater for heating and dissolving.
In practice for extracting the slurry from the slurry preparation tank and transferring the extracted slurry stably, the slurry to be transferred is externally circulated and is then supplied via a branched line to the heater (see Patent Publication 8). When such a branched slurry is supplied to a process performed under high pressure, the slurry is pressurized with a high-pressure pump and is then supplied (see Patent Publication 9). [0016]
The slurry is therefore generally supplied to the heater typically through the circulation line, branched line, and pump. Terephthalic acid in the slurry suffers again from uneven distribution and ununiformity, as affected by the flow (drifting), flow velocity, and the pipe diameter and the piping arrangement. The slurry is supplied through the branched line to the heater by the action of the high-pressure pump. In this process, the mechanics of heater, and the flow velocity and flow (drifting) of the slurry in the heater may affect
the slurry concentration, and ununiformity, and fluctuations of the slurry concentration may appear more than the measured concentration. This impedes stable operations in the catalyst bed. The problem of fluctuations in concentration has become apparent as operations have been performed at higher slurry concentrations. According to the known methods for preparing slurry,
therefore, fluctuations of about ±5% in terms of the amount of the powdery terephthalic acid are inevitable, as a total typically of uneven distribution and local ununiformity of the powdery terephthalic acid or fluctuations in concentration of terephthalic acid. The fluctuations in concentration appear as fluctuations in measured
concentration of about ±1.5 percent by weight in a slurry of high terephthalic acid concentration (25 percent by weight or more). Such high terephthalic acid concentration is generally operated in commercial-scale plant for the purification of crude terephthalic acid. The fluctuations appear as fluctuations in terms of saturation temperature in a range of about ±2°C at a slurry concentration of 30 percent by weight. This requires a catalyst bed temperature (dissolution temperature) to be higher than the
saturation temperature by 3°C or more and preferably by 4°C or more.

Even in operations carried out with such difference in temperature, the supply pressure of the slurry may be elevated due to occlusions in the heater and the catalyst bed when the high-concentration slurry is purified. The elevated pressure may impede the stability of the system for the purification of crude terephthalic acid. MEANS OF SOLVING THE PROBLEMS [0017]
The present inventors made measurements and analyses in detail about factors for fluctuations in concentration of crude terephthalic acid to be supplied from the hydrogenation purification tank to the catalyst bed. As a
result, they have found that fluctuations of about ±5% in terms of the amount of powdery terephthalic acid are inevitable according to the known methods for regulating the concentration, as described above.
Consequently, they have found the following method which does not impede a stable operation of hydrogenation purification reaction by supplying a slurry with narrower fluctuations in its concentration to the heating/dissolving process to melt the slurry to an aqueous solution, and supplying the aqueous solution at a stable concentration to the catalyst bed.
Specifically, there is provided a method for acquiring purified terephthalic acid by continuously feeding a crude
terephthalic acid and water to a slurry preparation tank equipped with an agitator to give a slurry at a predetermined concentration and heating and dissolving the said slurry therein to give a solution and continuously supplying the solution to a hydrogenation purification reactor. The method comprises the steps of:
1) controlling the feed rate of the crude terephthalic acid based on the feed rate of water to the slurry preparation tank so as to attain a predetermined ratio between the crude terephthalic acid and water (a predetermined concentration), and mixing them to give a slurry;
2) measuring the concentration of the slurry with a slurry concentration meter arranged on a discharge side of an external circulation pump of the slurry preparation tank while circulating the prepared slurry;
3) controlling the water feed rate supplied from a water feed line to a line on the suction side of the circulation pump so that the above-measured concentration becomes a predetermined concentration to thereby regulate the concentration of the slurry; and
4) supplying the prepared slurry at the predetermined concentration to the heating/dissolving process with a high-pressure pump through a line branched from the circulation line on the discharge side of the circulation pump.
This method enables preparation and supply of a slurry of crude terephthalic acid with narrower fluctuations in concentration and thereby enables stable operations of hydrogen purification reactions. [0018]
In general, a liquid (water) can be fed quantitatively, but a powder (crude terephthalic acid) is fed with some fluctuations or pulsations in feed rate, as described above. Accordingly, regulation of the feed rate of the crude terephthalic acid to a predetermined ratio (predetermined concentration) based on the feed rate of water is preferred as a macro regulation method (primary regulation) of the slurry concentration in the slurry preparation tank when the powdery crude terephthalic acid and water are continuously and quantitatively fed to the slurry preparation tank.
In other words, regulation of the feed rate of water to the preparation tank to be predetermined concentration as proposed in above-mentioned Patent Publication 5. It cannot be said as a preferred regulation method. This is because, in this method, the feed rate of water is regulated based on the feed rate of the powdery crude terephthalic acid that is difficult to be fed continuously smoothly. The prepared slurry at a predetermined concentration is continuously supplied to the heater which generally uses a
shell and tube heat exchanger. To avoid precipitation of terephthalic acid powder particles in the slurry, the flow velocity of the slurry in the slurry transfer pipe and in circulation of the slurry in the heater should always be 1.5 m/hr or more and is preferably 2 m/hr or more. Consequently, the method according to the present invention uses regulation of the feed rate of the powdery crude terephthalic acid to a predetermined ratio (predetermined concentration) based on the feed rate of water. This regulation is preferred from the viewpoint of ensuring a stable slurry concentration, because flows of water in predetermined amounts are always ensured during circulation and transfer, and in the heater. [0019]
A common procedure for stable transfer of a high-concentration slurry is measurement of a slurry concentration while performing external circulation of the slurry. The circulation line is branched, and a slurry the same as the slurry whose concentration has been measured is supplied to the heating/dissolving process through a high-pressure pump.
According to the method of the present invention, the concentration is measured with a slurry concentration meter arranged on the discharge side of the circulation pump to detect a difference in concentration with respect to the
predetermined concentration, i.e., a fluctuation in slurry concentration; water is directly fed from a water feed line arranged on the suction side line of the circulation pump to thereby promptly regulate the fluctuation in concentration of the slurry at the discharge side of the pump. The slurry at the regulated concentration is then supplied to the heating/dissolving process. This regulation is a micro regulation (secondary regulation) in the method according to the present invention. The resulting slurry with further narrower fluctuations in concentration is supplied as intact through the high-pressure pump to the heating/dissolving process, in which the slurry is melted to an aqueous solution at a stable terephthalic acid concentration, and is then supplied to the hydrogenation purification tank. [0020]
In the method according to the present invention, the feed rate of the powdery crude terephthalic acid is regulated in the primary regulation based on the feed rate of water that can be fed quantitatively, so as to prepare a slurry; and fluctuations in concentration at the inlet of the heating/dissolving process is promptly corrected by controlling the feed rate of water in the secondary regulation, as mentioned above. According to the known methods, the feed rate of raw
material crude terephthalic acid or water to be fed to the slurry preparation tank is regulated based on the measured slurry concentration in the slurry preparation tank. The measured slurry concentration is, however, a value after the elapse of residence in the slurry preparation tank (residence time of from 10 to 100 minutes), and this causes response delay. In contrast, the method according to the present invention can respond and regulate fluctuations in concentration immediately and can supply a slurry with narrower fluctuations in concentration to the heating/dissolving process. This leads to more stable operations in process for purifying crude terephthalic acid. The stable operation in purification process, an object of the present invention, can be more effectively achieved by the method according to the present invention when only fluctuations in slurry concentration in which measured slurry concentration is higher than the predetermined concentration, are controlled. [0021]
Specifically, in a preferred method of the present invention, crude terephthalic acid is fed at a predetermined ratio based on the feed rate of water to prepare a slurry at a predetermined concentration in the primary regulation; the concentration of the prepared slurry is measured; and feed of water under control is
performed in the secondary regulation only to fluctuations
in which the measured slurry concentration is higher than
the predetermined concentration.
The primary regulation and secondary regulation are fully
independently performed or controlled according to this
method, and it is desirable as a high-precision control.
[0022]
The system according to a preferred method, in which the secondary regulation is performed only to fluctuations in which the measured concentration is higher than the predetermined concentration, is also desirable as a method that enables smooth transit to a stable operation typically at the beginning of operation.
Specifically, in this method, feed of water at predetermined feed rates to the slurry preparation tank and to the heating/dissolving process is initiated; conditions such as temperature and pressure in the hydrogenation purification tank are set based only on the feed rate of water; thereafter feed of the powdery crude terephthalic acid to the slurry preparation tank is initiated so as to achieve a feed rate of the powder at a predetermined ratio gradually or quickly to thereby raise the slurry concentration to a predetermined concentration. This method is preferably used, because the operation can shift to a smooth continuous operation while ensuring
predetermined flow velocities typically in the pipe and
heater.
EFFECTS OF THE INVENTION
[0023]
In the method according to the present invention, the primary regulation and secondary regulation are performed in the preparation of a slurry of crude terephthalic acid and water. The resulting slurry is stable with narrower fluctuations in concentration with respect to a predetermined target concentration. The fluctuations are about one-fifth the fluctuations according to the known methods. Thus, the slurry with a stable concentration can be supplied to the heating/dissolving process, whereby the hydrogenation purification reaction can be carried out more stably. This eliminates system troubles such as drift and occlusion in systems or devices such as the catalyst bed and thereby improves production efficiency. These effects are also obtained and thereby higher production efficiency is obtained by the system according to a preferred method, in which the second regulation to feed water under control is performed only when the measured concentration is higher than the predetermined concentration. [0024]
The purified terephthalic acid through hydrogenation
purification of crude terephthalic acid has been used in a wider and wider various ways. Along with this, the concentration of the slurry of crude terephthalic acid is set higher and higher (25 percent by weight or more) in industrial plant. The method according to the present invention enables operations of the hydrogenation purification tank and related facilities thereof in these industrial plant at not so high temperatures and whereby at not so high pressures. This eliminates the need of reinforcement in facilities.
This also eliminates the need of increasing the heating energy and enables energy-saving processes, because the plant can be operate at not so high temperatures. BEST MODE FOR CARRYING OUT THE INVENTION [0025]
Some systems of the method according to the present invention will be illustrated below with reference to the flow chart of a process and its system shown in Fig. 1. The process in Fig. 1 is by no means intended to limit the scope of the present invention.
Initially, water is fed from the pure water line [1] to the slurry preparation tank 4 at a predetermined flow rate; and a crude terephthalic acid is fed from the crude terephthalic acid hopper 1 via the powder feeder 3 and the supply line [2] to the slurry preparation tank 4, while the
feed rate of the crude terephthalic acid is regulated by the action of the powder feed rate adjuster 2 based on the feed rate of water, so as to attain a predetermined ratio (predetermined concentration).
Feed of water from the pure water line [1] is carried out at a predetermined feed rate. This predetermined feed rate of water is calculated from the amount and concentration of crude terephthalic acid to be purified in the purification process of crude terephthalic acid. The predetermined feed rate of water is a flow rate set also in consideration of such a flow velocity as to prevent the precipitation in the transfer line and heater. The feed rate of the powdery crude terephthalic acid is adjusted by the action of the powder feed rate adjuster 2 so as to attain a predetermined ratio (predetermined concentration) with respect to the predetermined feed rate of water based on the setting in the feed rate controller (FFC1). Thus the powder is fed under regulation to the slurry preparation tank 4 (primary regulation). The crude terephthalic acid and water fed to the slurry preparation tank 4 are agitated and mixed with each other to give a slurry, and the slurry is extracted from the bottom of the slurry preparation tank 4 through the circulation pump 5 and is circulated via lines [4] and [6]. The slurry concentration meter 6 is arranged at some
midpoint in the circulation lines [4] and [6] to measure the concentration of the circulating slurry. Then, the feed rate of the water from the water feed line [3] is controlled through the setting of the flow rate controller (FFC2) so as to allow the measured concentration to be a predetermined concentration. Thus, a fluctuation in concentration is immediately regulated so as to attain the predetermined concentration (secondary regulation). The secondary regulation can be performed according to two procedures. Specifically, in one procedure, water is fed so as to regulate only fluctuations in which the measured concentration is higher than the predetermined concentration; and in the other process, water is fed so as to regulate both fluctuations in which the measured concentration is higher than the predetermined concentration and fluctuations in which the measured concentration is lower than the predetermined concentration. [0026]
Specifically, the concentration regulation according to the primary regulation suffers from fluctuations of up to about ±5% in the amount of powdery crude terephthalic acid, as described above. Water should be fed for the secondary regulation at a feed rate of up to about 10% and preferably up to about 20% with respect to the feed rate of water from the water line [1].
In the procedure in which only a fluctuation higher than the predetermined concentration is regulated, the system is configured to feed water at a feed rate of up to about 10% with respect to the predetermined feed rate of water from the line [1]; and water is fed from the line [3] to regulate the concentration, only when the measured concentration is higher than the predetermined concentration. When a fluctuation in which the measured concentration is lower than the predetermined concentration, water feed is stopped and concentration regulation is not performed. [0027]
In the other procedure, regulation is performed to fluctuations in which the measured concentrations are both higher than and lower than the predetermined concentration. In this case, water is fed from the line [1] at a predetermined feed rate corresponding to about 90% to 95% of the predetermined total feed rate of water; and water is fed from the line [3] at a predetermined feed rate corresponding to about 10% to 5% of the predetermined total feed rate of water. Thus, the system is configured so as to enable control of the feed rate of water from the line [3] corresponding to fluctuations in which the measured concentrations are both higher than and lower than the predetermined concentration. For this purpose, the
regulation of the feed rate of water from the line [3] is preferably set so as to enable regulation of feed rate of water from the line [3] of up to about 20% of the predetermined total feed rate of water. In regulation by the method according to the present invention, the primary regulation (control of feed rate of crude terephthalic acid) and secondary regulation (control of feed rate of water) can be performed separately and independently. In a system, the primary regulation and the secondary regulation can be performed fully independently when the second regulation is performed only to fluctuations in which the measured concentration is higher than the predetermined concentration. The method according to this system of the present invention can also achieve the objects of the present invention. [0028]
The concentration of slurry in the circulation line is measured with the slurry concentration meter 6 by continuously or intermittently measuring a density with a densitometer, and converting the measured density to a concentration of terephthalic acid which varies depending on the density. The densitometer usable herein is one that can measure the density of slurry, such as a mechanical oscillation densitometer, a radiation densitometer, or a coriolis densitometer. When the measured concentration
with the slurry concentration meter 6 differs from the predetermined concentration, the feed rate of water from the line [3] is controlled through the setting of the flow rate controller (FFC2) to thereby regulate the slurry concentration. The control of the feed rate of water from the line [3] through the setting of the flow rate controller (FFC2) is performed according to proportional, integral, and derivative (PID) control.
The slurry concentration meter 6 may be arranged either downstream or upstream from the branched portion of the line at which the slurry is extracted for supplying. The concentration of slurry is directly measured in the circulation line or is measured through the bypass line of a suitable size that is branched from the circulation line, depending on the type of the slurry concentration meter 6 and the size of piping arranged. The measuring mode may be chosen according typically to the type and location of the meter. [0029]
The slurry whose concentration has been regulated is branched (line [5]) from the circulating line [4], and the branched slurry is supplied in a predetermined amount (flowmeter 8) through the slurry supply pump 7 to the heater 9 and the dissolution tank 10. Thus, the slurry of crude terephthalic acid is heated and dissolved. The
supplied slurry is heated and dissolved at a temperature of from 280°C to 300°C to give an aqueous solution of terephthalic acid. In this process, a pressure of from 70 to 100 Kg/cm2 gauge pressure is required to maintain the resulting aqueous solution as a liquid, and the supply pump 7 requires a high-pressure pump head.
Such a high-pressure pump for use herein is preferably one that can quickly raise the pressure to attain a high pressure, such as a multi-stage plunger pump or a highspeed single-stage centrifugal pump.
When the supply flow rate of slurry is measured with the flowmeter 8 (e.g., electromagnetic flowmeter or coriolis flowmeter) and the slurry is supplied by the action of a volume pump, the flow rate is regulated to a predetermined flow rate by controlling the driving speed of the plunger. When a centrifugal pump is used, the flow rate is regulated to the predetermined flow rate through the flow rate control valve. Excess slurry is returned via the line [8] to the slurry preparation tank 4 by the action of pressure controller (PIC; to maintain at a predetermined pressure) that is under constant pressure control. [0030]
Accordingly, the slurry is prepared to a predetermined concentration by the action of the flowmeter 8 and is supplied at a predetermined flow rate. This stabilizes the

amount of crude terephthalic acid to be purified in the purification reactor (i.e., the amount of purified terephthalic acid). In other words, continuous supply of the slurry under regulation ensures stable operation of the system and stable quality of the product.
When the fluid level in the slurry preparation tank 4 rises, the rise is corrected by the control of the supply flow rate of slurry by the action of the flowmeter 8 so as to stabilize the fluid level. Such rise of fluid level occurs due to imbalance between the amount of slurry preparation and the feed rate (amount) of slurry supply through the high-pressure pump. The amount of slurry preparation is determined by the feed rate of crude terephthalic acid (line [2]), the predetermined feed rate of water (line [1] or line [1] plus line [3]), and, in addition, the feed rate of water from the line [3] fed for the regulation of concentration.
The heater 9 serves to heat the slurry (at a temperature of 100°C or lower) under normal atmospheric pressure to a temperature of from 280°C to 300°C and to pressurize the slurry. The heating is performed with two or more heaters using heating media of different temperatures. When the temperature of slurry reaches the predetermined dissolution temperature, the slurry is supplied to the dissolution tank 10 while maintaining the slurry at the dissolution
temperature. After the elapse of the suitable dissolution time (5 to 20 minutes), the resulting aqueous solution containing no powdery terephthalic acid is supplied to the hydrogenation purification tank in which a hydrogenation purification reaction of crude terephthalic acid is performed. EXAMPLES [0031]
Next, some specific systems of the method according to the present invention will be illustrated in detail with reference to several examples below. [0032]

The slurry of crude terephthalic acid and water was prepared, subjected to a heating/dissolving process, and supplied to the hydrogenation purification reaction according to the flow chart illustrated in Fig. 1. While water was fed at the predetermined feed rate (8.9 ton/hr) from the line [1] to the slurry preparation tank 4 (2,500 mm in diameter and 4,000 mm in height) equipped with an agitator, a powdery crude terephthalic acid was extracted from the crude terephthalic acid hopper 1 so as to attain a feed rate of 3.8 ton/hr by the action of a rotary-valve powder feed rate adjuster 2 and continuously fed via the powder feeder 3 and the line [2] to the slurry preparation
tank 4, in which the both materials were agitated and mixed to give a slurry.
In this process, the relation between the feed speed (rotational speed) of the powder feed rate adjuster 2 and the feed rate of the crude terephthalic acid had been grasped, and the feed speed by the adjuster 2 was controlled through the setting of the feed rate controller (FFC1) so that the feed rate of crude terephthalic acid is 3.8 ton/hr, corresponding to the feed rate of water (8.9 ton/hr) (primary regulation). [0033]
The prepared slurry was extracted from the bottom of the preparation tank through the circulation pump 5 and circulated (at about 20 ton/hr) through the lines [4] and [ 6 ] . The concentration of the slurry was measured in the circulation line with a slurry concentration meter 6, in which the density was measured with the mechanical oscillation densitometer and converted to a concentration. When a fluctuation in concentration occurred, in which the measured concentration of slurry was higher than 30 percent by weight, water was fed from the line [3] through the setting of the flow rate controller (FFC2) so as to regulate the slurry concentration to 30 percent by weight (secondary regulation). Next, the prepared slurry in the circulating line [4] was
branched as the line [5] at some midpoint of the line [4] (upstream of the slurry concentration meter), raised in pressure (to about 112 Kg/cm2 gauge pressure) by the action of the slurry supply pump 7 (high-speed single-stage centrifugal pump; under the trade name of Sundyne Pump), and supplied via the flowmeter 8 (coriolis flowmeter) and the line [7] to the heaters 9. The feed rate of the slurry was adjusted to a predetermined flow rate of 12.7 ton/hr by controlling the control valve (FIC) based on an indication from the flowmeter 8. [0034]
The supplied slurry was heated through the heaters 9 with superheated steam while setting the target temperature at 284°C, supplied to the dissolution tank 10 to give the aqueous solution, and the aqueous solution was supplied to the hydrogenation purification tank 11. The hydrogenation purification reaction was performed in the hydrogenation purification tank 11 at a temperature of about 284°C and a pressure of about 74 Kg/cm2 gauge pressure. The pressure was set as above by injection of hydrogen gas. The hydrogenation purification reaction could be continuously smoothly performed over about half a year. The exemplary graphic plot of concentrations in stationary state during this process measured by the slurry concentration meter 6 (every 30 minutes for 24 hours) is
shown as data indicated by "Example 1" in Fig. 3. [0035]
As seen from the results, the slurry having a variation in concentration higher than 30 percent by weight of 0.2 percent by weight (relative variation of 0.67 percent by weight) or less could be supplied to the heaters 9, and the hydrogenation purification reaction could therefore be stably continued.
The fluid level in the slurry preparation tank tended to rise during the operation. In this case, the fluid level was maintained by increasing the supply rate of slurry to some extent to balance between the supply rate of slurry and the feed rates of the raw materials. [0036]

The procedure of Example 1 was repeated, except for the followings. Water was fed from the line [1] at a feed rate of 8.0 ton/hr, and crude terephthalic acid was continuously fed from the terephthalic acid hopper 1 to the slurry preparation tank 4 at the feed rate of 3.8 ton/hr, in which the feed rate was adjusted by the powder feed rate adjuster 2 based on the feed rate of water of 8.9 ton/hr. This feed rate of water was set assuming that water would be fed from the line [3] to the circulation line at the feed rate of 0.9 ton/hr. The crude terephthalic acid and
water were then agitated and mixed in the slurry preparation tank 4 and circulated to give a slurry (primary regulation).
During this process, water was fed from the line [3] at the feed rate of 0.9 ton/hr, the resulting slurry was circulated through the circulation pump 5, the concentration of the circulation slurry was measured with the slurry concentration meter 6, and the feed rate of water from the line [3] was controlled corresponding to the excess and deficiency of the concentration. Thus, the concentration of the slurry was regulated so that the measured concentration is 30 percent by weight (secondary regulation). [0037]
The above-prepared slurry was extracted from the branched line [5], raised in pressure (about 112 Kg/cm2 gauge pressure) by the action of the slurry supply pump 7, supplied to the heaters 9, and a hydrogenation purification reaction was performed as in Example 1. As a result, a smooth operation could be continued.
The exemplary graphic plot of concentrations in stationary state during this process measured by the slurry concentration meter 6 (every 30 minutes for 24 hours) is shown as data indicated by "Example 2" in Fig. 3. As seen from the results, a slurry having a fluctuation
range in concentration, in which the measured concentration is higher than 30 percent by weight, of ±0.2 percent by weight (relative variation of 0.67 percent by weight) could be supplied to the heaters 9, and a hydrogenation purification reaction could therefore be stably continued. Fluctuations in fluid level in the slurry preparation tank 4 were regulated by the control of the supply rate of slurry by the flowmeter 8 (FIC). [0038]

The slurry of crude terephthalic acid and water was prepared, subjected to a heating/dissolution process, and supplied to a hydrogenation purification reaction according to the flow chart illustrated in Fig. 2. In this process, units and systems were substantially the same as those in the flow chart of Fig. 1 (as above examples), except for the secondary regulation unit of the slurry concentration. The crude terephthalic acid was fed from the crude terephthalic acid hopper 1 to the slurry preparation tank 4 at the predetermined feed rate of 3.8 ton/hr, which feed rate was adjusted by the powder feed rate adjuster 2. Water was fed from the line [1] to the slurry preparation tank 4 at the feed rate of about 8.9 ton/hr which was regulated based on the feed rate of crude terephthalic acid, and water was agitated and mixed with the crude
terephthalic acid to give a slurry.
The prepared slurry was extracted from the bottom of the preparation tank, circulated at about 20 ton/hr through the circulating pump 5, the concentration of the circulation slurry was measured with the slurry concentration meter 6, and the feed rate of water from the water feed line [1] was controlled through the setting of the flow rate controller (FFC1) so that the measured concentration is 30 percent by weight. Thus, the slurry concentration was regulated (primary regulation).
The above-prepared slurry was extracted from the branched line [5], raised in pressure (about 112 Kg/cm2 gauge pressure) by the action of slurry supply pump 7, supplied to the heaters 9 and the dissolution tank 10 at the predetermined flow rate of 12.7 ton/hr under the control by the control valve (FIC) as indicated by the flowmeter 8. Thus, the hydrogenation purification reaction was performed. As a result, the degree of opening of the feed rate control valve (generally about 40%) increased from one month later, and became 100% on the 45th day, and the operation became out of control.
Specifically, the pressure of the heater inlet (control valve outlet side) was increased in order to supply the slurry at the predetermined flow rate (12.7 ton/hr), because occlusion occurred at the heaters 9 and the
downstream thereof.
Thereafter, the feed of crude terephthalic acid was stopped,
and an operation was performed with the feed of water alone,
and the hydrogenation purification operation was then
stopped. The graphic plot of concentrations in stationary
state during this process measured by the slurry
concentration meter 6 (every 30 minutes for 24 hours) is
shown as data indicated by "Comparative Example" in Fig. 3.
The data demonstrate that the slurry was supplied with a
fluctuation range of about ±1.4% (relative variation of
±4.7%) with respect to the target slurry concentration of
30 percent by weight.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039]
[Fig. 1] Fig. 1 is the flow chart of processes and systems,
in which the slurry of powdery crude terephthalic acid and
water is prepared, the slurry is supplied to a
heating/dissolving process to give an aqueous solution, and
the aqueous solution is supplied to the hydrogenation
purification reaction, according to the method of the
present invention.
[Fig. 2] Fig. 2 is the flow chart of processes and systems,
in which the slurry of powdery crude terephthalic acid and
water is prepared according to known method, and the
prepared slurry is supplied in the same way as in the
method according to the present invention (Fig. 1). [Fig. 3] Fig. 3 is the diagram of concentrations of slurries of powdery crude terephthalic acid and water as measured by the slurry concentration meter (mechanical oscillation densitometer) every 30 minutes for 24 hours, respectively, in which the slurries were prepared according to the examples and the comparative example, and supplied to the heating/dissolving process. EXPLANATION OF REFERENCE NUMERALS AND SYMBOLS [0040]
1: crude terephthalic acid hopper, 2: powder feed rate adjuster, 3: powder feeder, 4: slurry preparation tank, 5: slurry circulation pump, 6: slurry concentration meter, 7: slurry supply pump, 8: flowmeter, 9: heater, 10: dissolver, 11: hydrogenation purification reactor.

CLAIMS
1. A method for acquiring purified terephthalic acid by continuously feeding a crude terephthalic acid and water to a slurry preparation tank equipped with an agitator to give a slurry at a predetermined concentration and heating and dissolving the said slurry therein to give a solution and continuously supplying the solution to a hydrogenation purification reactor, the method comprises the steps of:
1) controlling the feed rate of the crude terephthalic acid based on the feed rate of water to the slurry preparation tank so as to attain a predetermined ratio between the crude terephthalic acid and water, and mixing them to give a slurry;
2) measuring the concentration of the slurry with a slurry concentration meter arranged on a discharge side of an external circulation pump of the slurry preparation tank while circulating the prepared slurry;
3) controlling the water feed rate supplied from a water feed line to a line on the suction side of the circulation pump so that the above-measured concentration becomes a predetermined concentration to thereby regulate the concentration of the slurry; and
4) supplying the prepared slurry at the predetermined concentration to the heating/dissolving process with a high-pressure pump through a line branched from the

circulating line on the discharge side of the circulation pump.
2. The method for supplying a slurry of crude terephthalic acid, according to claim 1, wherein water is supplied from the water feed line in section 3) when the above-measured concentration is higher than the predetermined concentration.

Documents:

15682-356.pdf

29-10-2014_Claim.pdf

29-10-2014_Others.pdf

29-10-2014_Reply.pdf

5328-delnp-2009-Abstract-(24-04-2014).pdf

5328-delnp-2009-abstract.pdf

5328-delnp-2009-Claims-(24-04-2014).pdf

5328-delnp-2009-claims.pdf

5328-DELNP-2009-Correspondance Others-(23-01-2015).pdf

5328-delnp-2009-Correspondence Others-(24-04-2014).pdf

5328-delnp-2009-Correspondence Others-(26-02-2014).pdf

5328-delnp-2009-Correspondence Others-(30-01-2014).pdf

5328-DELNP-2009-Correspondence-Others-(28-10-2014).pdf

5328-delnp-2009-correspondence-others.pdf

5328-delnp-2009-Description (Complete)-(24-04-2014).pdf

5328-delnp-2009-description (complete).pdf

5328-delnp-2009-drawings.pdf

5328-delnp-2009-form-1.pdf

5328-delnp-2009-form-18.pdf

5328-delnp-2009-form-2.pdf

5328-delnp-2009-Form-3-(24-04-2014).pdf

5328-delnp-2009-form-3.pdf

5328-delnp-2009-form-5.pdf

5328-DELNP-2009-GPA-(23-01-2015).pdf

5328-DELNP-2009-GPA-(28-10-2014).pdf

5328-delnp-2009-pct-210.pdf

Claim.pdf

FORM 13 2nd inventor.pdf

FORM 13 applicant.pdf

FORM 13 first inventor.pdf

Merger Document.pdf

Others.pdf

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Patent Number

265605

Indian Patent Application Number

5328/DELNP/2009

PG Journal Number

10/2015

Publication Date

06-Mar-2015

Grant Date

28-Feb-2015

Date of Filing

19-Aug-2009

Name of Patentee

HITACHI PLANT TECHNOLOGIES, LTD.

Applicant Address

5-2, Higashi-lkebukuro 4-chome, Toshima-ku, Tokya 170-8466, Japan

Inventors:

#	Inventor's Name	Inventor's Address
1	HARA NORIAKI	Hitachi Plant Technologies Ltd., 5-2, Higashi-lkebukuro 4-chome, Toshima-ku, Tokya 170-8466, Japan
2	ITO TOSHINOBU	Hitachi Plant Technologies Ltd.,5-2, Higashi-lkebukuro 4-chome, Toshima-ku, Tokya 170-8466, Japan
3	YAMAZAKI HATSUTARO	92-1-402, TAKANOSHIMIZU-CHO, SAKYO-KU, KYOTO-SHI, KYOTO 606-8102, JAPAN.

PCT International Classification Number

C07C 51/487

PCT International Application Number

PCT/JP2007/053778

PCT International Filing date

2007-02-28

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	NA	1900-01-01	IB