JPH0328410B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing terephthalic acid, and more particularly, to a method for producing high-purity terephthalic acid that can produce polyester by directly reacting with a glycol component. Terephthalic acid is useful as a raw material for polyester, and is usually produced by the so-called SD method in which paraxylene is reacted with molecular oxygen in an acetic acid solvent in the presence of a catalyst containing heavy metals and bromine.
However, terephthalic acid produced by the SD method usually contains 1000 to 3000 ppm of 4-carboxybenzaldehyde (hereinafter abbreviated as 4CBA).
is contained as an impurity, so
For example, it cannot be used as a polyester raw material for fibers, films, etc. Therefore, conventionally, terephthalic acid is reacted with methanol to purify dimethyl terephthalate and then reacted with a glycol component, or terephthalic acid is dissolved in an aqueous solvent under high temperature and pressure.
For example, a method has been adopted in which the material is purified by contacting with a noble metal catalyst such as palladium, and then used as a raw material for polyester. However, all of these methods
In addition to the crude terephthalic acid production plant by
There was a problem that required a separate plant for purification. Therefore, in recent years, it has become possible to directly produce high-purity terephthalic acid in one plant by employing specific catalysts, oxidation conditions, or oxidation methods when oxidizing paraxylene. The applicant previously proposed that directly in one plant,
As an industrially advantageous method for producing high-purity terephthalic acid with a 4CBA content of 500 ppm or less, a mixture containing terephthalic acid obtained by oxidizing paraxylene is subsequently additionally oxidized at a temperature lower than the oxidation reaction temperature. After that, we proposed a method of re-oxidizing at a high temperature of 235℃ or higher. (Japanese Patent Application Laid-Open No. 55-55138) This method is industrially advantageous because not only can high-purity terephthalic acid be obtained in one plant, but the combustion loss of the acetic acid solvent that occurs during the production of terephthalic acid is small. This is a great method. However, in the above method, if the re-oxidation treatment at high temperature is extremely high, there is a risk that coloring components will be produced as by-products, and the energy cost will be too high for industrial use. Therefore, when carrying out the above method industrially, it is desirable to carry out the re-oxidation at a temperature of about 240 to 290°C using the usual amount of solvent, but under these conditions, the terephthalic acid particles It is not completely dissolved in the solvent, but only a portion of it, for example, about 10 to 40%, is dissolved. Under these conditions, the re-oxidation reaction causes the surface layer of the terephthalic acid particles to dissolve, small cracks appear in the particles, oxidation intermediates inside the particles are extracted into the solvent, and oxidation intermediates dissolved in the solvent undergoes additional oxidation. Therefore, in order to effectively perform high-temperature additional oxidation, it is necessary to smoothly extract the oxidized intermediate from the terephthalic acid particles. In view of the above-mentioned circumstances, the present inventors conducted various studies to provide a method for performing the high-temperature additional oxidation in the above-mentioned method more effectively. It was discovered that the subsequent high-temperature additional oxidation treatment can be carried out more effectively by carrying out the oxidation treatment using the above method, and the present invention has been completed. In other words, the gist of the present invention is to use a tank-type method for continuously producing high-purity terephthalic acid by reacting paraxylene with molecular oxygen in an acetic acid solvent in the presence of a catalyst containing heavy metals and bromine. Paraxylene and molecular oxygen are supplied to the first reactor, and 180~
After oxidizing at least 95% by weight of paraxylene to terephthalic acid at a temperature of 230°C, the reaction mixture from the first reactor is transferred to a tank-type second reactor at a temperature of 0 to 50°C above the temperature of the first reactor. Additional oxidation is performed by supplying molecular oxygen without supplying paraxylene at a low temperature, and the reaction mixture from the second reactor is further supplied with paraxylene at a temperature of 235 to 300°C in a third reactor. In a method for recovering terephthalic acid by supplying molecular oxygen and re-oxidizing the mixture, and then crystallizing and filtering the mixture, the second reactor has an external circulation line, and A method for producing terephthalic acid characterized by using a reactor having a wet pulverizer for pulverizing terephthalic acid particles in the middle of the line. The present invention will be explained in detail below. Examples of the method for producing terephthalic acid, which is the object of the present invention, include a method in which paraxylene is reacted with molecular oxygen in an acetic acid solvent in the presence of a catalyst containing a heavy metal. In the present invention, first, the first reactor, which is usually a stirred tank type, contains 95% by weight or more of paraxylene, preferably 98% by weight or more.
More than % by weight is oxidized to terephthalic acid, and the reaction temperature is usually 180-230℃, preferably 190-210℃
The pressure is several kg/cm ² to 100 kg/cm ² , preferably 10 to 30 kg/cm ² . If the reaction temperature is too low, paraxylene cannot be sufficiently oxidized, and conversely, if the reaction temperature is too high, not only will high purity terephthalic acid not be obtained, but also the combustion loss of the acetic acid solvent will increase, which is not preferable. In addition, the reaction time in the first reactor is required to allow at least 95% by weight of paraxylene to be oxidized to terephthalic acid.
minutes, preferably about 40 to 150 minutes. The catalyst used in the present invention usually contains three elements: cobalt, manganese, and bromine, for example,
120-600ppm as cobalt metal relative to solvent,
Preferably 200 to 400 ppm of cobalt compound, 0.5 to 1.5 times of manganese compound as manganese metal to cobalt, and 500 to 500 ppm of bromine to solvent.
2000 ppm, preferably 600 to 1500 ppm of bromine compounds are used. Specific examples of these compounds include cobalt compounds such as cobalt acetate and cobalt naphthenate, manganese compounds such as manganese acetate and manganese naphthenate, and bromine compounds such as hydrogen bromide, sodium bromide, cobalt bromide, and manganese bromide. can be mentioned. Note that when manganese bromide and cobalt bromide are used, they can also serve as two types of catalyst components. The ratio of paraxylene and solvent supplied to the first reactor is usually 2 to 6 times the weight of paraxylene, and if the solvent is too small, stirring in the reactor will not be carried out well, and This is not preferable because the high-temperature additional oxidation described later cannot be carried out well. Further, the acetic acid solvent may contain, for example, 20% by weight or less of water. The molecular oxygen supplied to the liquid phase of the first reactor may normally be air, and is supplied at a ratio of 3 to 100 times the mole of molecular oxygen to para-xylene. In the first oxidation reaction described above, the water concentration in the reactor is reduced by cooling the condensable gas from the reactor and extracting a part of the condensate from the system without refluxing it into the reactor. For example, the concentration may be adjusted to as low as 5 to 15% by weight. Further, it is preferable to maintain the 4CBA concentration in the reaction mother liquor in the first reactor at 2000 ppm or less by adjusting the reaction temperature, pressure, time, catalyst, etc., since the treatment described below can be performed satisfactorily. The slurry containing terephthalic acid obtained in the first reactor is then withdrawn and fed to a separate second reactor, usually of the stirred tank type, and the slurry in the first reactor is 0-50 than temperature
The first additional oxidation treatment is carried out at a lower temperature, preferably 2-30°C. If this temperature is too low, the oxidized intermediate contained in the reaction slurry cannot be sufficiently oxidized, while if it is higher than the reaction temperature of the first reactor, the product terephthalic acid This is not preferable because impurities that become coloring components are generated. Further, the time for this first additional oxidation treatment is usually 20 to 90 minutes, preferably 30 to 60 minutes. Since the molecular oxygen used in this additional oxidation has a small amount of oxidized material, the amount supplied is about 1/10 to 1/1000 of the amount supplied to the first reactor, and the oxygen concentration in the oxidation exhaust gas is usually is preferably 1 to 6% by volume. Generally, air or air diluted with an inert gas may be used as molecular oxygen. In the present invention, it is essential to carry out the reaction in the second reactor described above using a reactor having an external circulation line and a wet crusher for crushing the terephthalic acid particles in the middle of the line. This is a requirement. In the additional oxidation reaction, most of the terephthalic acid produced in the first reactor is precipitated as particles, so it is the intermediate dissolved in the solvent that undergoes additional oxidation. When the additional oxidation reaction is carried out while crushing the acid particles,
Grinding dissolves some of the intermediates in the particles and produces particles with an overall smaller diameter, which makes it easier for the intermediates in the particles to be extracted during the subsequent high-temperature additional oxidation. . In the pulverization treatment of terephthalic acid particles in the present invention, the average particle size of the terephthalic acid discharged from the second reactor is usually 0.8 times or less than that of the untreated case.
Preferably, the size is reduced to 0.7 to 0.2 times. The amount of external circulation in the second reactor varies depending on the degree of pulverization of the terephthalic acid particles and the capacity of the wet pulverizer, but is usually 1 to 10 times the amount of the reaction mixture discharged from the reactor. External circulation is usually carried out by withdrawing the reaction mixture from the bottom of the reactor, sending it to a wet mill using a circulation pump, and returning it to the reactor. The type of wet pulverizer is not particularly limited, but typically a shear grinder type, cutting shear mill type, colloid mill type, ball mill type, impact type pulverizer type, etc. are used. The mixture from the second reactor is then heated to a temperature of 235-300°C, preferably 240-290°C, by passing through a heater after increasing the pressure. Pressurization of the mixture is usually carried out by forcing the mixture from the second reactor into the high pressure section through a pump. The pressure after increasing the pressure is when the mixture is heated to the above temperature,
The pressure is such that the mixture can sufficiently maintain a liquid phase, and is usually 30 to 100 kg/cm ² in industrial terms. In order to maintain this pressure, for example, a method of pressurizing using an inert gas such as nitrogen gas is adopted. In addition, at least a portion of the terephthalic acid crystals in the mixture are dissolved by the heat treatment, but if the heating temperature is lower than the above range, the terephthalic acid cannot be well dissolved in the solvent, and vice versa. , if it is too high, it is not only uneconomical, but also there is a risk that colored impurities will be produced. As the heater, a monotube or multitube heat exchanger is usually used. As mentioned above, the mixture after increasing the temperature and pressure is usually supplied to a stirred tank type third reactor to perform reoxidation.
The reactor is usually a stirred tank type similar to the first and second reactors, and includes a reflux condenser at the top. The reaction temperature in the third reactor is the same as the heating temperature described above, and the residence time is 5 to 120 minutes, preferably 10 to 60 minutes. The amount of molecular oxygen used in this additional oxidation is usually 0.003 to 0.3 mole times, preferably 0.01 to 0.3 times the amount of terephthalic acid in the mixture.
It is 0.1 mole times. Molecular oxygen is usually air, and the oxygen concentration in the oxidation exhaust gas is substantially zero. Furthermore, in the present invention, additional oxidation can be carried out even more efficiently by supplying molecular oxygen to the mixture from the second reactor in the flow path during the temperature rise after increasing the pressure, and performing additional oxidation also in the heater. preferable. In this case, the molecular oxygen is usually air, and the amount of supplied air is equal to the amount of terephthalic acid in the mixture.
The amount is 0.003 to 0.3 times by mole, preferably 0.01 to 0.1 times. The mixture that has been reoxidized is crystallized in a conventional manner. It is usually preferable to carry out the crystallization treatment in multiple stages and gradually lower the temperature and pressure. Next, for example, solid-liquid separation such as centrifugation can be performed to recover crystals of terephthalic acid. If necessary, the terephthalic acid crystals are washed with water or acetic acid, and then dried to become a product. On the other hand, the reaction mother liquor is usually sent to a distillation column to remove produced water, catalyst, and byproducts, and recover acetic acid. In addition, in the present invention, there are very few by-products in the reaction mother liquor, especially impurities that interfere with the oxidation reaction, so 10% of the reaction mother liquor is
~80% by weight can also be recycled directly to the first reactor. As described above, according to the present invention, the 4CBA content is
When producing high-purity terephthalic acid of 500 ppm or less in one plant, it is extremely advantageous industrially and economically because higher-purity terephthalic acid can be easily produced. Next, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to the following Examples unless the gist thereof is exceeded. In addition, in the examples, "part" means "part by weight"
represents. Example A reaction was carried out using the reaction apparatus shown in the flow sheet of FIG. A first reactor 1 made of pressure-resistant titanium and equipped with a reflux cooling device, a stirring device, a raw material and solvent inlet, an air inlet, and a reaction slurry outlet is supplied with 1 part/hour of paraxylene and 5% water from a pipe 10. Acetic acid 4.5
parts/hour and cobalt acetate tetrahydrate 0.0025 parts/hour,
supplying a mixture consisting of 0.0027 parts/hour of manganese acetate tetrahydrate and 0.0039 parts/hour of 47% hydrobromic acid;
At a temperature of 200°C, a pressure of 18 kg/cm ² G, and a residence time of 90 minutes, air was used as the oxidizing gas and was supplied from pipe 11 so that the oxygen concentration in the exhaust gas of the oxidation reaction was 4% by volume. 1.5 parts of reflux solution/
Remove the water and reduce the water concentration in reactor 1 to approximately 10% by weight.
The liquid phase oxidation reaction of para-xylene was carried out under the control of The mixture from the first reactor 1 was continuously fed to the second reactor 2 through a pipe 16. Second reactor 2
In addition to the same equipment as the first reactor, the reactor has an external circulation line for the mixture, and the circulation line is equipped with a circulation pump 3 and a shear grinding type wet crusher 4. In the second reactor 2, the reaction mixture was circulated at a temperature of 185° C., a pressure of 11 Kg/cm ² G, a residence time of 30 minutes, and the amount of external circulation was three times the amount of extraction.
Additional oxidation was performed by supplying air so that the oxygen concentration in the exhaust gas from the oxidation reaction was 4% by volume. The mixture from the second reactor 2 passes through a pipe 17 and is then brought to a pressure of 65 Kg/cm ² G by a booster pump 5.
After increasing the pressure to pipe 13 in the middle of pipe 17
After adding 0.02 part/hour of air, the mixture was supplied to a monotube type heater 6, and additional oxidation was carried out in the heater 6, and the temperature of the mixture was raised to 275°C. Furthermore, the mixture leaving the heater 6 was then fed to a third reactor 7 with the same equipment as the first reactor 1. In the third reactor 7, the temperature is 275℃ and the pressure is 65Kg/cm ²
G. Under conditions of residence time of 30 minutes, 0.05 part/hour of air was supplied from pipe 14 to carry out re-oxidation. After performing additional oxidation in this manner, the mixture was cooled and crystallized in a crystallizer 8, and then passed through a centrifuge 9 to recover crystals of terephthalic acid. For the terephthalic acid obtained as described above,
The 4CBA content and transmittance (T ₃₄₀ ) were measured and the results are shown in Table 1. For reference, the slurry in the slurry transport pipe of pipe 17 was sampled, and the average particle size of the terephthalic acid particles was determined by the 50% value (D ₅₀ ) of the weight cumulative distribution by the wet sieving method. Shown in the table. Comparative Example A test was carried out in the same manner as in the example except that the second reactor 2 was a device in which the wet grinder 4 in the external circulation line was omitted, and the results are shown in Table 1. I got it. 【table】

[Brief explanation of the drawing]

FIG. 1 is a flow sheet showing the reaction equipment used in the examples of the present invention, 1 is a first reactor, 2 is a second reactor, 3 is a circulation pump, 4 is a wet crusher,
7 is a third reactor, and 8 is a crystallizer.

Claims (1)

[Claims] 1. In order to continuously produce high-purity terephthalic acid by reacting paraxylene with molecular oxygen in an acetic acid solvent in the presence of a catalyst containing heavy metals and bromine, paraxylene and paraxylene are placed in a tank-type first reactor. After supplying molecular oxygen to oxidize at least 95% by weight of paraxylene to terephthalic acid at a temperature of 180 to 230°C, the reaction mixture from the first reactor is subjected to the first reaction in a tank-type second reactor. Additional oxidation is performed by supplying molecular oxygen without supplying paraxylene at a temperature 0 to 50 °C lower than the temperature of the reactor, and the reaction mixture from the second reactor is heated to 235 to 300 °C in the third reactor. In a method for recovering terephthalic acid by supplying molecular oxygen and re-oxidizing without supplying paraxylene at a temperature of A method for producing terephthalic acid, which comprises using a reactor having a circulation line and a wet pulverizer for pulverizing terephthalic acid particles in the middle of the line.