JP3319032B2 - Method for producing terephthalic acid with excellent powder properties and slurry properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a method for producing terephthalic acid, particularly polyethylene terephthalate, which has a slurry property suitable for use in a so-called direct polymerization method in which terephthalic acid is directly reacted with glycols. The present invention relates to the production of terephthalic acid, which is excellent in fluidity of powder, which is a problem in storage or the like.

[0002]

2. Description of the Related Art In a direct polymerization method, terephthalic acid is mixed with a glycol such as ethylene glycol, sent to a reaction system in a slurry state, and subjected to a reaction. At this time, it is desired that the slurry has good fluidity and uniform reactivity. Further, in handling powder such as terephthalic acid transportation and storage, good powder fluidity is also required.

[0003] In order to obtain good slurry properties and uniformity of the reaction, it is sufficient to use a large amount of glycols with respect to terephthalic acid. However, an excessive amount of glycol increases side reactions during the polycondensation reaction, and the melting point of the polymer, It causes a decrease in the degree of polymerization and further causes coloring. In order to avoid these drawbacks, the glycols should be as close as possible to the stoichiometric amount, but if the amount of glycols used is reduced, a slurry preparation tank and
The power required for stirring the reactor increases. In addition, there are problems such as poor fluidity and reactivity, and a long reaction time. Therefore, terephthalic acid, which forms a slurry having good flowability and reactivity with a necessary minimum amount of glycols, is suitable as a raw material for direct polymerization.

[0004]

As a method for obtaining such terephthalic acid, JP-A-48-29735 discloses a method for obtaining terephthalic acid particles having an average particle diameter of 100 μm or more.
70-85% of terephthalic acid particles of micron or more to 30%
Although a method of mixing at 〜15% is described, this method requires two or more types of crystallization steps, requires equipment for separately storing and mixing the produced terephthalic acids, and is uneconomical. is there. In addition, Japanese Patent Publication No. 49-20303 discloses a method of improving the apparent density while gradually reducing the particle size by performing circulating stirring with a pump in the state of a slurry suspended in a solvent having low solubility. A method for obtaining terephthalic acid having properties is described. According to this method, terephthalic acid having good slurry properties can be obtained, but the flowability of the powder may be deteriorated by reducing the particle size, which is not preferable in handling the powder.

It is generally known that a method of providing a wide range of particle size distribution from a large particle size to a small particle size region can improve slurry properties and powder fluidity. When the amount increases, the reactivity in the polycondensation reaction deteriorates. An object of the present invention is to provide a method for directly obtaining terephthalic acid particles which simultaneously satisfy these slurry properties, powder fluidity, and reactivity without going through complicated steps.

[0006]

Means for Solving the Problems The present inventors dissolve terephthalic acid obtained by liquid-phase oxidation of para-xylene with molecular oxygen in an aqueous medium at a high temperature and a high pressure, and dissolve this in a platinum group metal catalyst. When producing high-purity terephthalic acid by crystallization after contacting and purifying the crystallization, a baffle plate that is placed near the stirring blade and separated from the inner wall of the tank (in this specification, this is referred to as a collision plate) Terephthalic acid excellent in the above-mentioned various characteristics can be produced by performing the reaction in a stirred crystallization tank having the above-mentioned formula, and the present invention has been completed.

The present invention will be described in detail below. Oxidation of para-xylene and purification of an aqueous terephthalic acid solution with a platinum group metal are known, and various methods are known.
As the oxidation reaction of para-xylene, a method is usually employed in which para-xylene is reacted with molecular oxygen in an acetic acid solvent, for example, in the presence of a catalyst containing cobalt, manganese, and bromine at a temperature of 170 to 230 ° C. . Terephthalic acid obtained by this method generally contains 4-carboxybenzaldehyde as an impurity in an amount of 1000 to 10% by weight.
000 ppm. This terephthalic acid is mixed with water to form a slurry of 10 to 40% by weight. Next, this slurry is pressurized to a pressure slightly higher than the reaction pressure by a pressure pump, and sent to a heating and dissolving step to be an aqueous terephthalic acid solution. This terephthalic acid aqueous solution is passed through a column reactor filled with a catalyst containing a platinum group metal. Catalysts containing platinum group metals include palladium, ruthenium,
Rhodium, osmium, iridium, platinum, or the like, or a metal oxide thereof is used. These metals or metal oxides can be used as catalysts as they are, but are used by being insoluble in an aqueous terephthalic acid solution, for example, supported on a carrier such as activated carbon.

[0008] Purification of terephthalic acid with a platinum group metal catalyst can be carried out simply by contacting an aqueous terephthalic acid solution with the catalyst, but is advantageously carried out in the presence of a reducing agent. Normally, hydrogen is used as a reducing agent, an aqueous terephthalic acid solution and hydrogen gas are supplied to the reactor,
The catalyst is contacted at a temperature of 320 ° C., preferably 260 to 300 ° C. Hydrogen gas is terephthalic acid aqueous solution 10
0.05 to 10 Nm ³ , preferably 0.
It may be supplied at a rate of 1 to ³ Nm ³ .

The terephthalic acid that has undergone the purification step is sent to the crystallization step, where terephthalic acid is crystallized by depressurized cooling. The crystallization is usually carried out in multiple stages and finally cooled to a temperature at which most of the terephthalic acid precipitates. The resulting crystals are dried by solid-liquid separation by centrifugation or the like to obtain high-purity terephthalic acid. In the present invention, crystallization of terephthalic acid from this terephthalic acid aqueous solution is performed in a stirred crystallization tank provided with a stirrer and a collision plate. As the blade type of the stirrer, a blade having an appropriate shape such as a fan turbine blade, a disk turbine blade, an inclined fan turbine, or a bull-margin blade can be used. The speed of the stirrer is usually 1 to 10 m as the blade tip speed.
/ S range. The blade diameter is generally in the range of 0.2 to 0.7R, where R is the inner diameter of the crystallization tank.

[0010] It is necessary to provide the impingement plate near the stirring blade. The distance from the tip of the stirring blade to the impingement plate is 0.0
The effect is greatest when it is in the range of 1 to 0.1R. The collision plate is different from a normal baffle plate and is set at least 0.1 R away from the inner wall of the tank. That is, the impingement plate is fixed to the tank, but there is a sufficient space between the impingement plate and the inner wall to allow the slurry flow in the tank to flow freely.

The impingement plate is preferably installed on an extension of the radius of rotation of the stirring blade, that is, along a straight line connecting the rotation axis of the stirrer and the inner wall of the tank. The slurry flow formed by the stirring blade impinges on this plate. As long as the function is not impaired, it may be installed in a direction slightly deviating from this straight line. The width of the impact plate, ie, the length along the installation direction described above, is 0.05
It is preferably in the range of ~ 0.2R. If the width is too large, the flow of the slurry flow formed by the stirring blade is greatly hindered, which may cause local unevenness.
If the width is too small, the effect is reduced.

The height (= length in the vertical direction) of the impingement plate is preferably not less than the height of the stirring blade. Usually, the upper edge of the impingement plate is on or above the same horizontal line as the upper edge of the impeller, and the lower edge of the impingement plate is on the same horizontal line as the lower edge of the impeller, Or below it. When the height and arrangement of the impingement plate are set in this manner in relation to the stirring blade, the effect of the collision plate that the slurry flow formed by the stirring blade collides can be sufficiently exhibited.

When the stirring blades are attached to the stirring shaft in multiple stages, it is necessary to install the impingement plate corresponding to at least the lowermost stirring blade. A collision plate may or may not be provided in the vicinity of the stirring blade above it. In the case where the impact plate is provided, the impact plate may be formed integrally with the impact plate provided corresponding to the lowermost stirring blade, that is, the impact plate may be extended upward. Number of impact plates is 2
As described above, it is usually preferable to provide 4 to 20 sheets at substantially equal intervals. Most preferably, 6 to 12 collision plates are provided at equal intervals.

Although it is unknown why the slurry characteristics and powder characteristics of the obtained terephthalic acid are improved by installing the impingement plate, the slurry flow discharged from the tip of the stirring blade collides with the impingement plate. It is presumed that the crystal immediately after crystallization undergoes some change (probably crushing) due to the impact. From this viewpoint, it is preferable that the stirring blade has a large discharge flow rate in the radial direction.

Usually, crystallization of terephthalic acid is industrially carried out by gradually lowering the temperature in a plurality of crystallization tanks connected in series. In the case of such multi-stage crystallization, the collision plate does not necessarily have to be installed in the entire crystallization tank. In the case of multi-stage crystallization, it is often sufficient to simply install the collision plate in the first crystallization tank or the first crystallization tank and the next crystallization tank. This is because most crystals crystallize at this stage. That is, it is effective to install the collision plate in a crystallization tank having a large amount of crystallization, and it is preferable that 50% of the total crystallization amount be crystallized in the crystallization tank having the collision plate.

As described above, the collision plate is different from the ordinary baffle plate. In addition to the collision plate, a baffle plate is provided on the inner wall of the tank by a conventional method to improve the stirring and mixing of the slurry in the tank. It is optional.

[0017]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the present invention is limited to the following Examples without departing from the scope of the invention. The crystallization of terephthalic acid was performed using an apparatus in which five stirred crystallization tanks having the same structure were connected in series. As shown in FIG. 1, terephthalic acid was continuously supplied to a first crystallization tank via a slurry tank (not shown), a heater and a purification tower. In the figure, reference numeral (1) denotes a heater, which is a device for heating and dissolving a water slurry of terephthalic acid.
(2) is a refining tower, which is filled with a Pd / C catalyst. In the tower (2), hydrogen gas contacts and dissolves in the terephthalic acid aqueous solution. (3) is a first crystallization tank in which a stirrer having a fan blade type stirring blade of four blades having a blade diameter of 0.4R (R is the inside diameter of the tank) is installed. On the inner wall of the tank, five baffles having a width of 0.1R are further installed at equal intervals. On the straight line connecting the center axis of the tank and the inner wall of the tank, a distance of 0.03R from the tip of the stirring blade and a width of 0.1R
And 10 collision plates whose height is twice the height of the stirring blade are installed at equal intervals (the other crystallization tanks have no collision plates). The stirring speed of the stirrer is 6 at the tip speed of the stirring blade.
m / sec.

The first crystallization tank is maintained at 240 ° C., where about 70% of the total crystallization amount is crystallized. The temperature of the second and lower crystallization tanks gradually decreases, and the temperature of the fifth crystallization tank is about 150 ° C. The residence time in each tank is 20 to 30 minutes. The 30% by weight terephthalic acid aqueous solution which had been subjected to the catalytic reduction treatment in the purification tower (2) was continuously supplied to the first crystallization tank. The resulting terephthalic acid slurry was continuously withdrawn from the fifth crystallization tank, and centrifuged while maintaining the crystallization temperature. The results are shown in Table 1.

Comparative Example Terephthalic acid was produced in exactly the same manner as in the example except that the apparatus shown in FIG. 1 was used, with the collision plate removed from the first crystallization tank. The results are shown in Table 1. As is clear from Table 1, according to the present invention, terephthalic acid having a small slurry torque, a small average particle size and good reactivity can be obtained. In addition, the powder discharging property is slightly reduced as the particle diameter is reduced, but is not so large as to hinder practical use.

In Table 1, the slurry torque was calculated as follows: 1 mole of terephthalic acid and 1 mole of ethylene glycol.
This is the stirring torque when the slurry mixed at a ratio of 1 mol is stirred with two blade paddle blades. JIS
This is a value measured by wet classification with a standard sieve. Powder discharge is the time required to discharge 300 grams of terephthalic acid from a hopper having a 64 mm ² opening.

Naturally, it is desirable that the slurry torque be as small as possible. The powder discharging property is desirably 150 seconds or less, and those exceeding 150 seconds have remarkable poor fluidity of the powder. Regarding the particle size distribution, the reactivity deteriorates as the number of large-sized particles exceeding 200 μm increases.

[0022]

[Table 1]

[0023]

According to the present invention, when terephthalic acid obtained by oxidizing para-xylene with oxygen in a liquid phase is dissolved in an aqueous medium, purified and then crystallized, an impingement plate is provided near the stirring blade. By using a stirred crystallization tank, high-purity terephthalic acid excellent in slurry characteristics, reactivity, powder fluidity, and the like can be easily obtained.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of an example of an apparatus for implementing the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Heater 2 Purification tower 3 1st crystallization tank 4 Impact plate 5 Stirrer blade 6 Baffle plate a Width of impact plate b Height of impact plate

Continuation of the front page (72) Inventor Mitsuji Otsuka 1-1 Kurosaki Castle Stone, Yawatanishi-ku, Kitakyushu-shi Kurosaki Plant of Mitsubishi Chemical Co., Ltd. (56) References JP-A-48-71376 (JP, A) JP-A-53- 127431 (JP, A) JP-B 54-20482 (JP, B1) JP-B 46-11001 (JP, B1) JP-B 33-5410 (JP, B1) (58) Fields surveyed (Int. Cl. ⁷ , DB name) C07C 51/43 C07C 63/26

Claims (5)

(57) [Claims] When crystallization of terephthalic acid from an aqueous solution of terephthalic acid is performed, when the inner diameter of the crystallization tank is R, 0.01 to 0.1 mm is provided between the stirrer and the tip of a stirring blade of the stirrer.
A crystallization apparatus including a stirring crystallization tank having a plurality of collision plates arranged in the longitudinal direction of the tank at a distance of at least 0.1R between the inner wall and the inner wall of the tank. A method for producing terephthalic acid. 2. When terephthalic acid is crystallized from an aqueous solution of terephthalic acid, a stirrer is provided.
Terephthalic acid using a crystallization apparatus including a stirring crystallization tank in which a plurality of collision plates having a width of 0.05 to 0.2 R and a plurality of collision plates having a width of 0.05 to 0.2 R are arranged side by side in the longitudinal direction of the tank. Manufacturing method. 3. A crystallization apparatus comprising a plurality of crystallization tanks connected in series, wherein at least 50% of the total crystallization amount is crystallized in a stirred crystallization tank having an impact plate. The method for producing terephthalic acid according to claim 1 or 2, wherein: 4. A stirrer for a stirring crystallization tank having an impingement plate is provided with a plurality of stirring blades, and an impingement plate is present at least between the lowermost stirring blade and the inner wall of the tank. The method for producing terephthalic acid according to claim 1. 5. The agitating blade having an impingement plate between itself and the inner wall of the tank has a blade diameter of 0.2 to 0.7 times the inner diameter of the tank. For producing terephthalic acid.