JP2004202318A - Crystallization tank having anchor type stirring blade - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To control the adhesion of a deposit to the inner wall side of a crystallization tank when a solute is crystallized from a solution or slurry in the crystallization tank by bursting and cooling. <P>SOLUTION: By the crystallization of the solution or the slurry by using the crystallization tank 1 having an anchor type stirring blade 2 rotating in the vicinity of the inner wall side, by sufficiently stirring the solution in the vicinity of the wall surface, the adhesion of the deposit to the inner wall side is controlled, and the slurry can be discharged efficiently and stably. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a crystallization tank by depressurized cooling and a method for recovering crystals containing paratoluic acid in a terephthalic acid production process using the crystallization tank.
[0002]
[Prior art]
In the terephthalic acid production process, in order to produce high-purity terephthalic acid from p-xylene, the raw material p-xylene is first oxidized to produce crude terephthalic acid, and then the intermediate product 4- High-purity terephthalic acid is produced by reducing carboxybenzaldehyde and removing it with paratoluic acid.
[0003]
The steps are as follows. The raw material p-xylene is air-oxidized to terephthalic acid by a catalyst in an acetic acid solvent at high temperature and pressure. At this time, an intermediate 4-carboxybenzaldehyde is by-produced together with terephthalic acid. The slurry containing these is crystallized and solid-liquid separated to obtain crude terephthalic acid crystals. Next, the crude terephthalic acid crystals are dissolved in water under high-temperature and high-pressure conditions to form an aqueous solution, and 4-carboxybenzaldehyde contained in the crude terephthalic acid is hydrogen reduced to highly water-soluble paratoluic acid. Thereafter, the mixture is depressurized and cooled, and only terephthalic acid having low water solubility is crystallized from the aqueous solution and recovered as high-purity terephthalic acid. In addition, the primary separated mother liquor from which high-purity terephthalic acid has been separated is depressurized and cooled while stirring in a crystallization tank to crystallize and collect secondary crystals containing paratoluic acid. Here, a propeller type stirring blade or a paddle type stirring blade is usually used for stirring in the crystallization tank.
[0004]
[Problems to be solved by the invention]
However, when crystallization by depressurized cooling is performed while stirring using the propeller type stirring blades or paddle type stirring blades, sufficient stirring cannot be performed near the inner wall of the tank, and precipitates adhere to the inner wall side surfaces. . In addition, fine powder entrained in the steam generated by the above-mentioned depressurized cooling adheres to the wall surface of the gas phase in the crystallization tank. In order to remove these deposits, it was necessary to perform a cleaning operation such as stopping and cleaning the apparatus.
[0005]
Therefore, an object of the present invention is to suppress the attachment of precipitates on the inner wall side surface of the crystallization tank when solute is crystallized in the crystallization tank.
[0006]
[Means for Solving the Problems]
The present invention has achieved the above object by using a crystallization tank having an anchor type stirring blade.
[0007]
When a solute is crystallized from a solution or slurry using a crystallization tank having an anchor-type stirring blade that rotates near the inner wall side, precipitates adhere to the inner wall side by sufficiently stirring the solution near the wall. And the slurry can be efficiently discharged. Further, it is possible to suppress the fine powder accompanying the steam generated by the depressurized cooling from adhering to and growing on the inner wall surface of the gas phase portion in the crystallization tank, and to prevent the lump from being mixed.
[0008]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described.
[0009]
As shown in FIG. 1, the present invention is a crystallization tank 1 by pressure-reduced cooling provided with an anchor type stirring blade 2 for preventing lump from adhering to the inner wall side surface of the tank.
[0010]
Here, the depressurized cooling means that the pressure applied to the liquid is reduced to a value equal to or lower than its vapor pressure, and flash evaporation is rapidly performed, whereby the temperature is also reduced to the boiling point at the pressure. When the liquid is a solution, the solute is crystallized in excess of the solubility due to the decrease in the amount of the solvent and the decrease in the temperature.
[0011]
In addition, if the liquid before entering the crystallization tank 1 is at a high temperature and a high pressure, the depressurized cooling is possible even if the inside of the tank is at normal temperature and normal pressure. . However, when the pressure is reduced to 1 atm or less, a pressure reducing device is used.
[0012]
FIG. 1 shows an example of the crystallization tank 1 described above. The crystallization tank 1 needs to have both the supply port 1a and the discharge port 1c for continuous operation. A supply port 1a for putting a high-temperature and high-pressure liquid into the crystallization tank 1 is generally provided at a position where it is supplied into the slurry. The outlet 1c is for discharging a slurry containing a liquid that has not evaporated and a solid that has crystallized.
[0013]
The crystallization tank 1 is provided with a gas outlet 1b. The gas outlet 1b is for discharging gas evaporated due to a pressure drop in the tank. In addition, it may be equipped with a decompression device.
[0014]
The crystallization tank 1 includes an anchor type stirring blade 2. The anchor-type stirring blade 2 has a stirring plate 2a mounted along the side surface of the inner wall of the crystallization tank 1. The stirring plate 2a and the side peripheral surface of the crystallization tank 1 are rotated so as to rotate in the center of the crystallization tank 1. The rotation support rod 2c joins the stirrer shaft 2b and the stirring shaft 2b mounted in parallel. The stirring shaft 2b is operated by a motor 2e, whereby the stirring plate 2a rotates to stir the inside of the crystallization tank 1.
[0015]
There may be a lower bearing 2d that connects the stirring plate 2a and the stirring shaft 2b in order to minimize the blurring that occurs when the anchor-type stirring blade 2 rotates. Further, the stirring plate 2a includes not only a plate attached vertically to the inner wall but also a plate attached with an inclination. The stirring plate 2a in FIG. 1 has two plates, which are attached at a position of 180 degrees with the stirring shaft 2b interposed therebetween, and have a shape similar to an anchor. However, it is not always necessary to use two plates, and three or more plates can be used. Good.
[0016]
The gap X between the anchor-type stirring blade 2 and the inner wall of the crystallization tank 1 is desirably not more than 1/20 of the inner diameter Y of the tank. If the gap X is wider than 1/20 of the inner diameter Y of the tank, the liquid in the crystallization tank 1 cannot be sufficiently stirred to the vicinity of the wall surface, and even if the anchor-type stirring blade 2 is used, the liquid in the crystallization tank 1 Solids may adhere. Further, the gap X is more desirably 1/100 or more of the inner diameter Y of the tank. If the gap X is smaller than 1/100 of the inner diameter Y of the tank, the anchor-type stirring blade 2 may cut the wall surface in some cases, which may hinder operation. Further, it is preferable that the upper portion of the stirring plate 2a is provided to be higher than the liquid level. For this reason, for example, in the crystallization tank 1 having a cylindrical side surface and a curved surface with both ends bulging, the stirring plate 2a may reach a TL (tangent line) which is a boundary between the upper portion of the cylindrical portion and the curved upper end portion. preferable.
[0017]
By using the anchor-type stirring blade 2 as described above, the stirring plate 2a passes near the wall surface, and the solution is sufficiently stirred up to the wall surface portion, and a substance that crystallizes when the solution is depressurized and cooled. Is greatly reduced on the side surface of the inner wall, and in some cases, the adhesion is eliminated. Further, even if fine powder entrained in the steam generated by the depressurized cooling adheres to the wall of the gas phase in the crystallization tank, the lump is scraped off by the anchor-type stirring blade 2 before the lump grows large. The amount is greatly reduced. As a result, even when the slurry by the solution and the crystallizing substance is generated by the depressurized cooling, the slurry can be smoothly discharged from the discharge port 1c without causing the crystallizing substance to adhere to the inner wall side surface. Further, the number of times of stopping the crystallization operation and cleaning the inside of the crystallization tank 1 to remove the deposits can be reduced.
[0018]
The above-mentioned crystallization tank 1 can be used for crystallization from a mixed solution or a slurry generated in the process of producing an aromatic dicarboxylic acid. In that case, the mixed solution or slurry having a pressure of 0.27 to 1.0 MPa is supplied to the crystallization tank 1 having a pressure of 0.01 to 0.27 MPa to dissolve the mixed solution or slurry. The product is crystallized, and a slurry composed of the mixed solution and the crystallized precipitate can be discharged from the outlet 1c.
[0019]
When the melt is crystallized from the mixed solution or slurry in the crystallization tank 1, the rotation speed of the anchor-type stirring blade is preferably 3 to 30 rpm, more preferably 5 to 20 rpm. If the rotation speed is less than 3 rpm, the effect of stirring cannot be fully exerted, and there is a possibility that the above-mentioned lumps of the deposited crystals adhere to the inner wall side surface below the liquid level. On the other hand, a speed of 30 rpm is sufficient for stirring, and a speed higher than 30 rpm is not only wasteful of energy, but also the mixed solution or slurry for crystallizing the above-mentioned dissolved material in the crystallization tank 1. Is a factor that causes the deposit to adhere above the liquid level in the tank.
[0020]
When the aromatic dicarboxylic acid is terephthalic acid, the crystallization tank 1 having the anchor-type stirring blades 2 performs solid-liquid separation of the slurry obtained by reduction and purification in the terephthalic acid production process, and mainly performs The present invention can be applied to a process of obtaining a primary crystal containing terephthalic acid and a primary mother liquor, crystallizing the primary mother liquor from the primary mother liquor by depressurized cooling, and collecting a secondary crystal containing paratoluic acid.
[0021]
The step of collecting the secondary crystals in the terephthalic acid production process will be described with reference to FIG. FIG. 2 shows a process for recovering crude terephthalic acid produced by oxidizing p-xylene into high-purity terephthalic acid by reduction and purification. In order to increase the efficiency of this process, the above anchor type is used. The crystallization tank 1 having the stirring blade 2 is applied.
[0022]
Crude terephthalic acid A containing 4-carboxybenzaldehyde, obtained through an oxidation step of oxidizing p-xylene, is mixed with aqueous solvent B in slurrying tank 11 to obtain a slurry, and the slurry is pressurized by pump 16. Then, a dissolution step of dissolving the slurry by heating in the heat exchanger 17 is performed. The first composite solution C obtained in this dissolving step is sent to the hydrogenation reactor 12.
[0023]
In the hydrogenation reactor 12, 4-carboxybenzaldehyde is hydrogen reduced to paratoluic acid. The first composite solution C generally contains 1000 to 5000 ppm of 4-carboxybenzaldehyde, most of which becomes paratoluic acid after reduction, and the amount of 4-carboxybenzaldehyde becomes 30 ppm or less. At this time, the temperature to the hydrogenation reactor 12 is a temperature at which the crude terephthalic acid completely dissolves in the aqueous solvent B, and is generally preferably 230 to 320 ° C., and the pressure is 2.8 to 11.3 MPa in order to maintain a liquid phase. Is desirable.
[0024]
The second composite solution D subjected to the hydrogen reduction treatment in the hydrogenation reactor 12 is crystallized using the stirring tank 13. This crystallization step comprises a group of stirring tanks having a plurality of stirring tanks 13 connected in series, and terephthalic acid is crystallized sequentially using these. Although two stirring tanks 13 are shown in FIG. 2, three or more stirring tanks may be used. At this time, the temperature of the last stirring tank 13 is preferably 130 to 180 ° C., and the pressure is preferably 0.27 to 1.0 MPa. When the temperature and pressure are further lowered, para-toluic acid as well as terephthalic acid co-crystallizes, and the purity of the obtained terephthalic acid crystals decreases. Further, if the temperature and the pressure are kept higher than this, the crystallization amount of terephthalic acid is reduced, and the yield is deteriorated. A more desirable temperature condition at which the purity of the obtained terephthalic acid crystals is high and the yield is good is 140 to 170 ° C.
[0025]
The slurry E containing the crystallized terephthalic acid crystals is sent to the solid-liquid separator 14, where the solid high-purity terephthalic acid crystals F are separated from the mother liquor and washed as needed. The high-purity terephthalic acid crystal F is dried and then shipped as a product. On the other hand, the primary separation mother liquor G after the separation of the high-purity terephthalic acid crystal F is a mixed solution of mainly paratoluic acid, terephthalic acid not crystallized in the stirring tank group 13, and the aqueous solvent B.
[0026]
The primary separation mother liquor G, which is this mixed solution, is sent to the crystallization tank 1 and cooled under pressure. The temperature in the crystallization tank 1 is preferably 50 to 130 ° C., and the pressure is preferably 0.01 to 0.27 MPa. As a result, crystals mainly composed of paratoluic acid and terephthalic acid are precipitated from the primary separated mother liquor G to form a slurry H. The precipitate having a relatively large amount of paratoluic acid easily adheres to the inner wall surface of the crystallization tank 1. At this time, since the crystallization tank 1 with the anchor-type stirring blade 2 is used, the rate of deposits adhering to the inner wall is reduced, and the slurry H can be more reliably discharged from the discharge port. This reduces the number of times that the crystallization tank 1 is stopped and the removal work is bothersome, and the operation of the crystallization process is more smoothly performed.
[0027]
The slurry H is sent to the solid-liquid separator 15, and the secondary crystal I mainly composed of paratoluic acid and terephthalic acid is separated from the mother liquid J for secondary separation. The secondary separation mother liquor J is an aqueous solution containing paratoluic acid and terephthalic acid corresponding to the solubility at the operating temperature. Paratoluic acid is obtained by reducing 4-carboxybenzaldehyde. If the amount is small, mixing with paratoluic acid immediately before reduction of 4-carboxybenzaldehyde does not hinder the reduction. Therefore, not only the secondary separation mother liquor J is treated as waste water, but also a part or the whole is sent to the above-mentioned dissolving step, and a part or all of the aqueous solvent B may be used as the solvent of the first composite solution C. It is possible. Since the secondary separation mother liquor J, which is wastewater containing organic matter, is reused, the amount of wastewater to be treated can be reduced.
[0028]
The secondary crystal I mainly comprises paratoluic acid and terephthalic acid. Paratoluic acid is a substance in which one of the two methyl groups of p-xylene, which is a raw material of terephthalic acid, is substituted with a carboxyl group, and oxidizes the methyl group to form terephthalic acid in the same manner as p-xylene. Is possible. Then, the recovered secondary crystal I is sent to the above oxidation step, and is air-oxidized with a catalyst in an acetic acid solvent together with p-xylene, thereby oxidizing paratoluic acid contained in the secondary crystal I to terephthalic acid, Terephthalic acid can be produced. As a result, the terephthalic acid contained in the secondary crystal I is recovered while 4-carboxybenzaldehyde, which is an intermediate product of the above oxidation step, is converted into paratoluic acid and reused as a raw material, so that the yield with respect to the raw material p-xylene is improved. Leads to.
[0029]
【Example】
Hereinafter, examples will be described more specifically.
[0030]
(Example 1)
Raw material p-xylene was oxidized in an acetic acid solvent using air to produce crude terephthalic acid. This crude terephthalic acid was supplied to the high-purity terephthalic acid production process shown in FIG. A slurry containing 30% by weight of crude terephthalic acid was obtained in the slurrying tank 11 using the aqueous solvent B. In the step shown in FIG. 2, the temperature and pressure of the solution sent to the hydrogenation reactor 12 were 290 ° C. and 7.9 MPa (80 kgf / cm ² · gage), respectively. In the subsequent crystallization step, cooling was performed stepwise by using a stirring tank 13 in which the five stirring tanks were connected in series, and finally cooled to a temperature of 155 ° C. to crystallize a solute. The slurry obtained by the crystallization was separated into a primary crystal and a primary separated mother liquor by a solid-liquid separator 14, and the primary crystal was dried and recovered as high-purity terephthalic acid. On the other hand, the primary separated mother liquor was crystallized by lowering the pressure to normal pressure in the crystallization tank 1 provided with the anchor type stirring blade 2 shown in FIG. The volume of the crystallization tank 1 was 1 m ³ , the number of revolutions of the anchor type stirring blade was 10 rpm, and the gap between the anchor type stirring blade and the side surface of the inner wall was 10 mm.
As a result, in the crystallization tank 1, the slurry could be discharged for one month without deposits and growth on the inner wall side surface, and high-purity terephthalic acid could be produced stably.
[0031]
【The invention's effect】
When the mixed solution or slurry is depressurized and cooled in a crystallization tank having an anchor-type stirring blade to perform crystallization, the slurry is sufficiently stirred up to near the wall surface, and the precipitate hardly adheres to the wall surface, The slurry can be discharged stably. As a result, it is possible to reduce the number of cleanings in the tank due to deposition and growth of the precipitate on the side surface of the inner wall, and a long-term continuous operation of the plant becomes possible.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view illustrating an example of a crystallization tank that performs depressurized cooling. FIG. 2 is a schematic view illustrating a manufacturing process of high-purity terephthalic acid.
DESCRIPTION OF SYMBOLS 1 Crystallization tank 1a Supply port 1b Gas discharge port 1c Discharge port 2 Anchor type stirring blade 2a Stirring plate 2b Stirring shaft 2c Rotation support rod 2d Lower bearing 2e Motor 11 Slurry tank 12 Hydrogenation reactor 13 Stirring tank 14 Solid-liquid separation Machine 15 Solid-liquid separator 16 Pump 17 Heat exchanger A Crude terephthalic acid B Aqueous solvent C First composite solution D Second composite solution E Slurry F High-purity terephthalic acid crystal G Primary separated mother liquor H Slurry I Secondary crystal J Secondary Separation mother liquor X Clearance Y between anchor type stirring blade and crystallization tank Inner diameter of crystallization tank

Claims (8)

A crystallization tank equipped with a supply port, a discharge port and an anchor-type stirring blade for performing depressurized cooling. The crystallization tank according to claim 1, wherein a gap between the anchor-type stirring blade and an inner wall of the tank is 1/20 or less of an inner diameter of the tank. The crystallization according to claim 1 or 2, wherein the mixed solution having a pressure of 0.27 to 1.0 MPa obtained in the process of producing the aromatic dicarboxylic acid has an internal pressure of 0.01 to 0.27 MPa. A crystallization method in which the mixture is supplied to a tank and cooled under pressure. The crystallization method according to claim 3, wherein the rotation speed of the anchor-type stirring blade is 3 to 30 rpm. The crystallization method according to claim 3 or 4, wherein a melt is crystallized from the mixed solution by depressurized cooling, and a slurry of the mixed solution and the crystallized product is discharged from the outlet. A method for recovering paratoluic acid, comprising using the crystallization tank according to claim 1 or 2 to precipitate crystals mainly composed of paratoluic acid through the following (Step A) to (Step E).
(Step A) A dissolution step of heating and dissolving a slurry mainly composed of terephthalic acid, 4-carboxybenzaldehyde and an aqueous solvent is performed, and the solution is subjected to a temperature of 230 to 320 ° C. and a pressure of 2.8 to 11.3 MPa. Reducing 4-carboxybenzaldehyde to paratoluic acid,
(Step B) Next, the solution mainly composed of terephthalic acid, paratoluic acid and the above-mentioned aqueous solvent which has been subjected to the reduction treatment is depressurized and cooled to a temperature of 130 to 180 ° C. and a pressure of 0.27 to 1.0 MPa, and is mainly subjected to Crystallizing a primary crystal containing
(Step C) a step of separating the primary crystals obtained in Step B from a mother liquor,
(Step D) The primary separated mother liquor obtained in Step C is depressurized and cooled to a temperature of 50 to 130 ° C. and a pressure of 0.01 to 0.27 MPa using the above-mentioned crystallization tank to obtain a secondary liquid mainly containing paratoluic acid. Crystallizing the next crystal,
(Step E) a step of separating the secondary crystals crystallized in Step D from a secondary separation mother liquor. The method for recovering paratoluic acid according to claim 6, wherein the secondary separated mother liquor obtained in the step E is sent to a dissolving step in a step A and used as a part or all of the aqueous solvent. The secondary crystal separated in the step E according to claim 6 is sent to an oxidation step of oxidizing p-xylene, which is a raw material of terephthalic acid, and the paratoluic acid contained in the secondary crystal is converted to the p-xylene. And a method for producing terephthalic acid which is oxidized to terephthalic acid.

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