DESCRIPTION PROCESS FOR PRODUCING TEREPHTHALIC ACID, AND TEREPHTHALIC
ACID TechnicalField
The present invention relates to a process for producing terephthalic acid. Particularly, the invention relates to a process for producing terephthalic acid which is suitable as a starting material for the production of a polyester such as polyethylene terephthalate. The invention also relates to a terephthalic acid having a narrow particle size distribution, which is produced by using the same process for production.
Background Art
Crude terephthalic acid obtained by oxidation of para-xylene usually contains relatively large amounts of various impurities including 4-carboxybenzaldehyde (hereinafter, referred to simply as "4CBA"). Such terephthalic acid is purified and then used as a starting material for a polyester.
As a purification method for such crude terephthalic acid, a method is known wherein crude terephthalic acid is

dissolved in an aqueous medium and contacted with a platinum group metal catalyst at a high temperature and a high pressure (for example, JP-A No. 6-329583), and several other methods are also known.
For the oxidation reaction of para-xylene, a method in which para-xylene is reacted with molecular oxygen in an acetic acid solvent in the presence of a catalyst containing, for example, cobalt, manganese and bromine, at a temperature of 170 to 230°C, is usually employed. The terephthalic acid obtained by this method typically contains 1,000 to 10,000 ppm by weight of 4-carboxybenzaldehyde as an impurity. This terephthalic acid is mixed with water to form a 10 to 40 wt% slurry. Subsequently, this slurry is pressurized by a pressurizing pump to a pressure slightly higher than the reaction pressure, and sent to a heat dissolving step, whereby it is formed into an aqueous terephthalic acid solution.
This aqueous terephthalic acid solution is passed through a tower reactor packed with a catalyst containing a platinum group metal. As the catalyst containing a platinum group metal, palladium, ruthenium, rhodium, osmium, iridium, platinum or the like, or a metal oxide thereof, is employed. Such a metal or a metal oxide may be used by itself as a catalyst, but may be used as supported on a support such as activated carbon, which is insoluble in the aqueous

terephthalic acid solution.
Purification of terephthalic acid by the platinum group metal catalyst can be carried out simply by contacting the aqueous terephthalic acid solution with the catalyst, but it is advantageous to carry out the purification in the presence of a reducing agent. As the reducing agent, hydrogen is usually used. An aqueous terephthalic acid solution and hydrogen gas are supplied to a reactor and brought into contact with a catalyst at a temperature condition of 220 to 320°C, and preferably 260 to 300°C. The hydrogen gas may be supplied at a rate of 0.05 to 10 Nm3, and preferably 0.1 to 3 Nm3, with respect to 1000 kg of the aqueous terephthalic acid solution.
The terephthalic acid from the purification step is then sent to a crystallization step, where the terephthalic acid is crystallized out by pressure release and cooling. Crystallization is generally carried out in multi-stages, and finally, cooling is carried out to a temperature at which most of the terephthalic acid is crystallized out. The formed crystals are subjected to solid-liquid separation by centrifugation or the like, followed by drying. The mean particle diameter is usually determined by adjusting the crystallization temperatures and the average residence times at a first crystallization tank and a second crystallization tank, and a high purity terephthalic acid typically having a
mean particle diameter of 50 to 150 um is obtained.
As a conventional crystallization apparatus applied to the production of terephthalic acid, known is an apparatus which has a rotating shaft b vertically mounted at the center of the crystallization tank a such that the lower end of the rotating shaft b is located at a predetermined distance away from the inner bottom of the crystallization tank a, the rotating shaft b having an agitating blade c such as, for example, a pitched paddle-type or turbine blade at the lower end part, and which also has baffle plates d placed on the inner walls of the crystallization tank a, as illustrated in Fig. 4; or an apparatus which has a rotating shaft f vertically mounted at the center of the crystallization tank e such that the lower end of the rotating shaft f is located at a predetermined distance away from the inner bottom of the crystallization tank e, the rotating shaft f having an agitating blade g such as a pitched paddle blade at the lower end part, and which has baffle plates d placed on the inner walls of the crystallization tank e, and colliding plates h near the tips of the agitating blade g, as illustrated in Fig. 5.
Conventional crystallization apparatuses have problems such as rotating flows of the aqueous terephthalic acid solution occurring at the tip parts of the agitating blade due to the rotation of the agitating blade, and in turn,
clogging caused by stagnation of the slurry at the center of the inner bottom of the crystallization tank underneath the center of the agitating blade.
Meanwhile, in recent years, it has been common to produce polyester, and particularly polyethylene terephthalate, by a so-called direct polymerization method, wherein terephthalic acid is directly reacted with a glycol. In this direct polymerization method, terephthalic acid is mixed with a glycol such as ethylene glycol to form a slurry, which is then fed to a reaction system and subjected to the reaction. In this process, the handling and transportation of terephthalic acid, the miscibility of the acid with ethylene glycols, and the homogeneity of the reaction are largely affected by the particle size distribution or the mean particle diameter of the terephthalic acid particles.
In general, when the terephthalic acid has the wide particle size distribution which is covered from large to small particle sizes, the characteristics of the slurries of the terephthalic acid with ethylene glycols tend to be improved, and the terephthalic acid is suitable for mixing with ethylene glycols. A mean particle diameter is
typically in the range of 50 to 150 um. To the contrary, if the proportion of particles having large particle sizes
exceeding 250 um increases too much, the terephthalic acid is likely to remain unreacted at the time of direct

polymerization, and consequently, there will be problems such that it will necessarily take a long reaction time, and the generation of by-products will increase. On the other hand, if the proportion of particles having small particle
sizes of less than 50 um increases too much, there will be a problem such that handling or transportation of the product will be time-consuming.
Therefore, it is difficult to produce a terephthalic acid which satisfies all of the conditions such as the handling and transportation of terephthalic acid, the miscibility of the acid with ethylene glycols and the homogeneity of the reaction, and the required product quality may vary depending on the apparatus, scale and operation mode of the so-called direct polymerization facilities in which terephthalic acid is directly reacted with glycols.
Disclosure of the Invention
It is an object of the present invention to provide a process for producing terephthalic acid, wherein terephthalic acid is efficiently obtained and the desired particle size distribution of the terephthalic acid particles can be separately formed directly in the crystallization tank, by preventing stagnation of the slurry
of terephthalic acid occurring at the center of the inner bottom of the crystallization tank. The invention also provides terephthalic acid having a narrow particle size distribution, which is prepared by using the same process for production.
The invention provides, for example, the followings: [1] a process for producing terephthalic acid comprising crystallizing terephthalic acid from an aqueous terephthalic acid solution by a crystallization apparatus, wherein the crystallization apparatus has an agitator whose agitating blade is disposed close to the inner bottom of a
crystallization tank, and has baffle plates extending in the vertical direction on the inner walls of the crystallization tank;
[2] the process for producing terephthalic acid as described in [I] above, wherein the agitating blade has a shape of plate whose vertical length is longest at the center of the crystallization tank and is gradually shortened toward the periphery of the crystallization tank;
[3] the process for producing terephthalic acid as described in [1] or [2] above, wherein the opening of the inlet tube for the aqueous terephthalic acid solution that is connected to the crystallization tank, is formed to face upward in the crystallization tank;
[4] the process for producing terephthalic acid as
described in any one of [1] to [3] above, wherein the ratio of the clearance c between the lower edge of the agitating blade and the inner bottom of the crystallization tank, to the blade diameter R of the agitating blade, c/R, is in the range of 0.005 to 0.10;
[5] the process for producing terephthalic acid as described in any one of [1] to [4] above, wherein a plurality of crystallization tanks are connected in series, and at least 50% by weight of the total amount of the crystallized terephthalic acid is crystallized in a crystallization tank which has an agitator having the agitating blade disposed close to the inner bottom, and has baffle plates extending in the vertical direction on the inner walls of the crystallization tank;
[6] the process for producing terephthalic acid as described in any one of [1] to [5] above, wherein the agitating power of the agitating blade of a first crystallization tank is 0.01 to 10 kW/m3;
[7] the process for producing terephthalic acid as described in [2] above, wherein a plurality of agitating blades are used, the agitating blade having a blade diameter of (0.3 to 0.8)D, a vertical length at the center of (0.1 to 3.0)R and a vertical length at the blade tip of (0 to 0.5)R when the inner diameter of a first crystallization tank is D and the blade diameter is R;

[8] the process for producing terephthalic acid as de-scribed in any one of [I] to [7] above, comprising dissolving crude terephthalic acid in an aqueous medium, contacting it with a platinum group metal catalyst at a temperature condition of 260 to 320°C to purify it, and crystallizing out terephthalic acid from the aqueous terephthalic acid solution by cooling the solution stepwise in a plurality of crystallization tanks connected in series, wherein the crystallization temperature at a first crystallization tank is adjusted to 240 to 260°C, agitating is carried out with an agitating power of the agitating blade in the range of 0.01 to 10 kW/m3, and then the crystallization temperature at a second crystallization tank is adjusted to 180 to 230°C, which is lower than the crystallization temperature at the first crystallization tank by 20 to 60°C;
[9] the process for producing terephthalic acid as described in any one of [1] to [8] above, wherein the particle size distribution of the terephthalic acid particles is separately formed by varying the agitating power to vary the rotational speed of the agitating blade;
[10] a terephthalic acid having a mean particle
diameter of 50 to 150 um with a standard deviation of 30 to 5 0; and
[11] the terephthalic acid having a mean particle

diameter of 50 to 150 um with a standard deviation of 30 to 50, which is produced by the process as described in any one of [1] to [9] above.
The first embodiment of the invention will be described with reference to Fig. 1. Fig. 1 illustrates an example in which a motor is equipped at the top of a crystallization tank, but the motor may also be equipped at the bottom of the crystallization tank.
Reference numeral 1 represents the crystallization tank, in which a rotating shaft 2 is vertically mounted at the center of the crystallization tank 1, and the rotating shaft 2 has an agitating blade 3 at the lower end part.
Here, each of a plurality of blade bodies 3a which constitute agitating blade 3 is an approximately trapezoid-shaped plate, whose vertical width is largest at the base part that is close to the rotating shaft 2 and is gradually decreasing toward the tip. Each blade body 3a is attached to the shaft such that it has its lower edge close to the inner bottom of the crystallization tank 1.
That is, the invention is characterized by using a plurality of the agitating blades such that, when the inner diameter of the crystallization tank is D, and the blade diameter is R, the agitating blade diameter R is (0.3 to 0.8)D, and preferably (0.4 to 0.6)D; and each of the blades has a shape that the vertical length at the center is (0.1
to 3.0)R, preferably (0.3 to 0.7)R, and more preferably (0.4 to 0.6)R, and the vertical length at the blade tip is (0 to 0.5)R, and preferably (0.1 to 0.3)R.
The ratio of the clearance c between the lower edge of the agitating blade and the inner bottom of the crystallization tank, to the blade diameter R of the agitating blade, c/R, is in the range of 0.005 to 0.10, preferably in the range of 0.01 to 0.05, and more preferably in the range of 0.01 to 0.03.
The crystallization tank 1 also has a plurality of baffle plates 4 extending in the vertical direction on the inner walls of the tank, and the lower edges of the baffle plates 4 are close to the inner bottom of the crystallization tank 1.
In the case of producing terephthalic acid in a plurality of crystallization tanks connected in series, it is preferable to crystallize terephthalic acid at least 50% by weight, and preferably 70% by weight or more, of the total crystallized amount in a crystallization tank which is equipped with an agitator having its agitating blade disposed close to the inner bottom, and baffle plates vertically extending on the inner walls of the crystallization tank.
In Fig. 1, reference numeral 5 represents the inlet tube for the aqueous terephthalic acid solution which is
connected to the crystallization tank 1, and reference numeral 6 represents the outlet tube for the aqueous solution which is connected to the crystallization tank 1.
Thus, when the motor (not shown in the figure) is operated to drive the rotating shaft 2 to rotate, and thereby the lower end part of the shaft drives the agitating blade 3 disposed close to the inner bottom of the crystallization tank 1 to rotate, pressure is applied to the aqueous terephthalic acid solution in the crystallization tank 1 at the central lower part of the crystallization tank 1, by each of the blade bodies 3a whose vertical width is largest at the base part, and the aqueous solution forms a flow directed exclusively outward along the inner bottom of the crystallization tank 1 by each of the blade bodies 3a whose vertical width is gradually decreasing toward the tip. Thereafter, the aqueous solution forms a rapid upward flow directed along the baffle plates 4 whose lower edges are disposed close to the inner bottom, and then flows toward the center of the crystallization tank 1 to form a downward flow at the center, thus forming a circulating flow C.
Here, the operating conditions are such that the power required for agitating in the crystallization tank 1 is preferably 0.01 to 10 kW/m3, and more preferably 0.1 to 1.0 kW/m3, while the process of this embodiment was carried out with a power required for agitating in the range of 0.15 to
0.7 kW/m3.
Therefore, the aqueous solution does not form slurry and stagnate at the center of the inner bottom of the crystallization tank 1, and forms a rapid upward flow at the periphery close to the inner walls of the crystallization tank 1. Thus, no non-uniform crystallization takes place and crystals of uniform particle diameter are efficiently obtained.
Furthermore, since there occurs no stagnation of the slurry as described above, stagnation of slurry is not observed even with low-speed rotation of the agitating blade 3. Therefore, it becomes possible to control the rotational speed of the agitating blade 3 within a wide range and thus, to control the residence time or the effect of classification, thereby it being possible to artificially produce crystals having uniform particle size distributions or diverse particle size distributions.
For example, a terephthalic acid having a narrow particle size distribution, with the mean particle diameter in the range of 50 to 150 (jm, preferably in the range of 80 to 110 pirn, and the standard deviation of 30 to 50, can be obtained. For example, in this case, the agitating power is 0.01 to 10 kW/m3, and preferably 0.1 to 1.0 kW/m3.
The standard deviation is determined by the following equations. That is, when the mean value is X, the standard
deviation is s, the representative value is X0, the interval width is h, the frequency is f, the number of data is N, and u = (X-X0)/h,
Mean Value; X = X0 + S(u x f)/N x h
Standard Deviation; s = h x /"(1/(N-1) x {Z(u2 x f)-(Zuf)2/N}) .
Furthermore, although the present embodiment describes an example of using an agitating blade 3 having six blade bodies 3a, the invention is not limited to this embodiment and may use two or more blade bodies 3a.
When crude terephthalic acid is dissolved in an aqueous medium and purified by contacting it with a platinum group metal catalyst at a temperature condition of 260 to 320°C, and terephthalic acid is crystallized out from the aqueous terephthalic acid solution by cooling the solution stepwise in a plurality of crystallization tanks connected in series, it is preferable to use a process for producing terephthalic acid, wherein at the first crystallization tank, the crystallization temperature is 240 to 260°C, agitating is carried out by an agitating blade with an agitating power in the range of 0.01 to 10 kW/m3, and the crystallization temperature at the second crystallization tank is in the range of 180 to 230°C.
In this case, it is preferable to have the crystallization temperature at the second crystallization

tank lower than the crystallization temperature at the first crystallization tank by 20 to 60°C, and to set the crystallization temperature at the second crystallization tank at 180 to 230°C, by varying the agitating power.
Fig. 2 illustrates a second embodiment of the invention. In this embodiment, the end of an inlet tube 5 in the crystallization tank 1 of the first embodiment is bent to face upward inside the crystallization tank 1 so that an opening 5a of the inlet tube 5 is directed upward, and the end of an outlet tube 6 is bent to face upward inside the crystallization tank 1 so that the opening 6a of the outlet tube 6 is directed upward.
Thus, the aqueous terephthalic acid solution is introduced from the opening 5a of the inlet tube 5 to be directed upward in a solution flowing upward at the periphery of the crystallization tank 1, so that the vapor does not directly tangle around the blade, and thus the discharge efficiency for the aqueous solution at the agitating blade 3 is not lowered. Also, since the opening 6a of the outlet tube 6 faces upward, the aqueous solution flowing upward at the periphery of the crystallization tank 1 is not directly introduced to be discharged, and the residence time can be extended. At the same time, the crystals formed in the aqueous solution that reaches the top of the opening 6a, flow downward to enter the opening 6a and
are discharged out from the outlet tube 6. Thus, uniform crystals having a predetermined particle diameter or larger are obtained.
Fig. 3 illustrates a third embodiment of the invention. In this embodiment, the end of the inlet tube 5 in the first embodiment is bent to face upward inside the crystallization tank 1 so that the opening 5a of the inlet tube 5 is directed upward, and an inverse conical flow distributor 5b is disposed at the upper part of the opening 5a. The end of the outlet tube 6 is bent to face downward so that the end face of the end is closed, and the opening 6b is formed at a lateral side of this end in a direction facing to the inner walls of the crystallization tank 1.
Thus, the aqueous terephthalic acid solution is distributed by the inverse conical flow distributor 5b after being discharged upward from the opening 5a of the inlet tube 5, and flows into the aqueous solution in the crystallization tank 1 in a dispersed state, thereby the vapor of the solution being prevented from tangling around the blade. Thus, the discharge efficiency for the aqueous solution by the agitating blade 3 is improved, and an effect of initial dispersion by means of the inflowing energy can be obtained. Also, the opening 6b of the outlet tube 6 is disposed at a position where the particles crystallized at the periphery of the inner bottom of the crystallization
tank 1, are classified into those flowing upward together with the aqueous solution and those not flowing upward. Therefore, the crystal particles having a predetermined particle diameter or larger are assuredly aspirated at the opening 6b under a negative pressure.
Accordingly, the present invention relates to a process for producing terephthalic acid which process comprises dissolving crude terephthalic acid in an aqueous medium, contacting it with a platinum group metal catalyst at a temperature condition of 220 to 320°C to purify it, and crystallizing terephthalic acid from the aqueous terephthalic acid solution by cooling the solution stepwise in a crystallization apparatus having a plurality of crystallization tanks connected in series,
characterized in that a first crystallization tank 1 in the crystallization apparatus has an agitator whose agitating blade 3 is disposed close to the inner bottom of the first crystallization tank 1 and has baffle plates 4 extending in the vertical direction on the inner wall of the first crystallization tank 1, the agitating blade 3 having a shape of plate whose vertical length is longest at the center of the first crystallization tank 1 and is gradually shortened toward the periphery of the first crystallization tank 1,
wherein the crystallization temperature at the first crystallization tank 1 is adjusted to 160 to 260°C, the agitating power of the agitating blade 3 in the first crystallization tank 1 is in the range of 0.01 to 10 kW/m3, and the crystallization temperature at a second crystallization tank is adjusted to a temperature that is in the range of 120
to 230°C and is lower than the crystallization temperature at the first crystallization tank 1 by 20 to 60°C,
and wherein the ratio of the clearance c between the lower edge of the agitating blade 3 and the inner bottom of the first crystallization tank 1, to the blade diameter R of the agitating blade 3, c/R, is in the range of 0.005 to 0.10.
Brief Description of the Drawings
Fig. 1 is a cross-sectional view of the first embodiment of the present invention.
Fig. 2 is a cross-sectional view of the essential parts of the second embodiment of the invention.
Fig. 3 is a cross-sectional view of the essential parts of the third embodiment of the invention.
Fig. 4 is a cross-sectional view of an exemplary production apparatus of the related art.
Fig. 5 is" a cross-sectional view of another exemplary production apparatus of the related art.
Fig. 6 is a graph showing the relationship between the agitating power in the crystallization tank and the particle size distribution of terephthalic acid, in a comparison of the production apparatus of the invention with the production apparatus of the related art.
Reference Numerals 19
1 Crystallization tank
2-, Rotating shaft
3 Agitating blade
3a Blade body
4 Baffle plate
5 Inlet tube
5a Opening
5b Flow distributor
6 Outlet tube
6a, 6b Opening
a Agitating tank
b Rotating shaft
c Agitating blade
d Baffle plate
e Crystallization tank
f Rotating shaft...
g Agitating blade
h Colliding plate
i Inlet tube
j Outlet tube
EXAMPLES
Now, the present invention will be described with
reference to Examples, but it should be construed that the invention is in no way limited to those Examples.
EXAMPLE 1
The experiments of continuously operating the production apparatus of the related art and the production apparatus of the third embodiment of the invention, by the present inventors, will be described in detail.
An aqueous slurry of a high purity terephthalic acid having a,particle size distribution of 74 to 149 µM was adjusted, and this slurry was completely dissolved at 220°C by applying pressure thereto in a dissolution tank to yield an aqueous terephthalic acid solution. From this aqueous solution, crystallization was performed in three crystallization tanks of identical volumes connected in series.
In this case, an agitating blade consisting of six blades was used, and when the inner diameter of the first crystallization tank was D and the blade diameter was R, the agitating blade diameter R was 0.5D, and the shape of the blade was such that the vertical length at the center was 0.5R, and the vertical length at the blade tip was 0.15R. The ratio of the clearance c between the lower edge of the agitating blade and the inner bottom of the crystallization tank, to the blade diameter R of the agitating blade, c/R,
was 0.02. The rotational speed of the blade could be varied, and crystallization was carried out at an agitating power of 0.2 or 0.7 kW/m3. On the other hand, an agitating blade having four pitched paddle blades, operating at a constant agitating power of 0.7 kW/m3 was used in the second crystallization tank.
The temperatures at the first" crystallization tank and the second crystallization tank were maintained at 160°C and 120°C, respectively, so that the first crystallization tank and the second crystallization tank were capable of crystallizing out about 70% by weight and about 99% by weight, respectively, of the total amount of crystallized terephthalic acid. The temperature of the third crystallization tank was maintained at 100°C.
The results for the test of the terephthalic acid particle properties are presented. In Fig. 6, the difference between the Example and the Comparative Example is that a pitched paddle blade having a blade diameter of 0.5D was used in the Comparative Example, with other operating conditions being the same in both cases. The particle diameter was measured by using a particle size distribution measuring apparatus based on laser diffraction and scattering. .
When the agitating power was 0.2 kW/m3, the mean particle diameter was 190.7 µm, and the standard deviation
was 61.1. When the agitating power was 0.7 kW/m3, the mean particle diameter was 134 µm, and the standard deviation was 4 6.9, a terephthalic acid with narrower particle size distribution being obtained in this case.
Industrial Applicability
According to the present invention, crystals of terephthalic acid can be obtained efficiently, and an effect is provided such that the particle diameter of the crystals can be controlled.

WE CLAIM:
1. A process for producing terephthalic acid which process comprises
dissolving crude terephthalic acid in an aqueous medium, contacting it with
a platinum group metal catalyst at a temperature condition of 220 to 320°C
to purify it, and crystallizing terephthalic acid from the aqueous terephthalic
acid solution by cooling the solution stepwise in a crystallization apparatus
having a plurality of crystallization tanks connected in series,
wherein a first crystallization tank 1 in the crystallization apparatus has an agitator whose agitating blade 3 is disposed close to the inner bottom of the first crystallization tank 1 and has baffle plates 4 extending in the vertical direction on the inner wall of the first crystallization tank 1, the agitating blade 3 having a shape of plate whose vertical length is longest at the center of the first crystallization tank 1 and is gradually shortened toward the periphery of the first crystallization tank 1,
wherein the crystallization temperature at the first crystallization tank 1 is adjusted to 160 to 260°C, the agitating power of the agitating blade 3 in the first crystallization tank 1 is in the range of 0.01 to 10 kW/m3, and the crystallization temperature at a second crystallization tank is adjusted to a temperature that is in the range of 120 to 230°C and is lower than the crystallization temperature at the first crystallization tank 1 by 20 to 60°C,
and wherein the ratio of the clearance c between the lower edge of the agitating blade 3 and the inner bottom of the first crystallization tank 1, to the blade diameter R of the agitating blade 3, c/R, is in the range of 0.005 to 0.10.
2. The process for producing terephthalic acid as claimed in claim 1,
wherein the opening 5a of the inlet tube 5 for the aqueous terephthalic acid
solution that is connected to the first crystallization tank 1, is formed to face
upward in the first crystallization tank 1.
3. The process for producing terephthalic acid as claimed in any one of laim 1 or 2, wherein at least 50% by weight of the total amount of the crystallized terephthalic acid is crystallized in the first crystallization tank 1.
4. The process for producing terephthalic acid as claimed in claim 1, wherein a plurality of agitating blades 3 are used, the agitating blade 3 having a blade diameter of (0.3 to 0.8)D, a vertical length at the center of (0.1 to 3.0)R and a vertical length at the blade tip of (0 to 0.5)R when the inner diameter of the first crystallization tank 1 is D and the blade diameter isR.
5. The process for producing terephthalic acid as claimed in any one of claims 1 to 4, wherein the particle size distribution of the terephthalic acid particles is separately formed by varying the agitating power to vary the rotational speed of the agitating blade 3.
6. Terephthalic acid having a mean particle diameter of 50 to 150 µm with a standard deviation of 30 to 50, which is produced by the process as claimed in any one of claims 1 to 5.