JPH0357887B2 - - 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, since terephthalic acid produced by the SD method contains 1000 to 3000 ppm of 4-carboxybenzaldehyde (hereinafter abbreviated as 4CBA) as an impurity, it can be used as it is, for example, for textiles and films. It cannot be used as a raw material for polyester. 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 constant temperature below 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 this method, re-oxidation is carried out at a high temperature, so if too much molecular oxygen is supplied in this step, combustion of the acetic acid solvent tends to increase. Therefore, in reoxidation,
A predetermined amount of molecular oxygen is used in consideration of the content of the substance to be oxidized in the additional oxidation mixture, the desired quality of the product terephthalic acid, etc. However, if the amount of molecular oxygen supplied is too small, the oxidizing power becomes weak and a sufficient re-oxidation effect cannot be obtained. This oxidizing power can be compensated for by, for example, increasing the treatment temperature, but this is not an industrially advantageous method as it increases the energy cost. In view of the above-mentioned circumstances, the present inventors investigated various methods for effectively re-oxidizing with a small amount of molecular oxygen even at relatively low temperatures, and found that the first oxidation reaction was completed. The present invention was completed based on the discovery that re-oxidation at high temperatures can be carried out well by adding a catalyst component after the oxidation process. That is, the gist of the present invention is to oxidize para-xylene to terephthalic acid in an acetic acid solvent in the presence of a catalyst consisting of heavy metals and bromine with molecular oxygen at a temperature of 180 to 230° C. The first oxidation reaction mixture obtained by In a continuous production method for terephthalic acid in which the reaction mixture is layered after oxidation and then solid-liquid separated, heavy metals or A method for producing high-purity terephthalic acid, which comprises adding heavy metals or bromine in an amount of 0.05 to 2, respectively, to bromine. The present invention will be explained in detail below. The method for producing terephthalic acid that is the object of the present invention includes 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 and bromine. In the present invention, firstly, in a first reactor of a stirred tank type, para-xylene is contained in an amount of 95% by weight or more, preferably 98% by weight.
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 on the other hand, if the reaction temperature is too high, not only will high purity terephthalic acid not be obtained, but the combustion loss of the acetic acid solvent will increase, which is not preferred. 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.
It is about 200 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 150 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 ratio is too small, stirring in the reactor will not be carried out well. Furthermore, high temperature reoxidation, which will be described later, is not carried out satisfactorily, which is not preferable. 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, with a ratio of 3 to 100 moles of molecular oxygen to paraxylene, and the oxygen concentration in the oxidation exhaust gas is usually
It is supplied at a concentration of 1.5 to 8% by volume. In the above-mentioned first oxidation reaction, a part of the condensate obtained by cooling the condensable gas from the reactor is extracted from the system without being refluxed to the reactor, thereby reducing the water concentration in the reactor, for example. , the concentration may be adjusted to as low as 5 to 15% by weight. In addition, in the present invention, the 4CBA concentration in the reaction mother liquor can be adjusted to 1000 to 3000 ppm by adjusting the temperature, pressure, residence time, catalyst composition, etc. in the first reactor.
It is preferable to keep it at In this reaction mother liquor
The lower the 4CBA concentration, the more pure terephthalic acid can be obtained, but on the other hand, when the oxidation reaction is carried out under such conditions, the combustion of the acetic acid solvent tends to increase. However, in the present invention, even if the 4CBA concentration in this reaction mother liquor is kept high to some extent, high-purity terephthalic acid can be finally obtained through the additional oxidation and re-additional oxidation treatments described below. Combustion of acetic acid solvent in the reactor can be significantly suppressed. Next, the slurry containing terephthalic acid obtained in the first reactor is fed to a second reactor, for example in a stirred tank type, and the slurry is then fed to a second reactor, usually at a temperature lower than the reaction temperature of the first reactor, without feeding para-xylene. Additional oxidation treatment is carried out using molecular oxygen at a temperature lower than 0 to 50°C, preferably 2 to 30°C. This treatment mainly oxidizes the oxidized intermediates in the mother liquor, but if this temperature is too low, the oxidized intermediates contained in the reaction slurry cannot be sufficiently oxidized;
If the temperature is higher than the reaction temperature of the first reactor, impurities that become coloring components of the product terephthalic acid will be produced, which is not preferable. Further, the time for this additional oxidation treatment is usually 20 to 90 minutes, preferably 30 to 60 minutes. The amount of molecular oxygen used in this additional oxidation is approximately 1/10 to 1/1000 of the amount supplied to the first reactor, since the oxidized substance contained in the reaction mixture is small. Usually, the oxygen concentration in the oxidizing exhaust gas is 1~
An amount of 6% by volume is preferred. Generally, air or air diluted with an inert gas may be used as molecular oxygen. The reaction mixture that has undergone the additional oxidation described above is then fed to another stirred tank reactor and treated with molecular oxygen at a temperature higher than that of the first oxidation reaction, usually without feeding paraxylene. Perform reoxidation treatment. This treatment extracts the oxidized intermediates in the terephthalic acid particles into the mother liquor and oxidizes them. If the treatment temperature at this time is too low, the oxidized intermediates in the terephthalic acid particles will not be extracted well, and if the treatment temperature is too high, it will not only be energy-economical but also cause colored impurities to be produced. ℃ or higher, preferably 235 to 290℃.
The reaction pressure must be able to maintain the mixture in the liquid phase, and is usually industrially 30 to 100
Kg/ ^cm2 . Further, the residence time is 5 to 120 minutes, preferably 10 to 60 minutes. In this re-oxidation process, the amount of oxidized substances is extremely small and the temperature is high. The amount is 0.003 to 0.3 times the terephthalic acid in the mixture,
It is preferably 0.01 to 0.1 mole times, and it is preferable that the oxygen concentration in the oxidizing exhaust gas is usually 0.5% by volume or less. The molecular oxygen used here usually includes air or air diluted with an inert gas. As mentioned above, in the present invention, a mixture obtained by oxidizing 95% by weight or more of paraxylene to terephthalic acid is additionally oxidized at a low temperature, and then additionally oxidized at a high temperature. The reactor is usually of the stirred tank type with a reflux condenser at the top. The number of reaction stages in each step is usually one, but if necessary, the additional oxidation step or the additional oxidation step may be performed in multiple stages. Furthermore, in order to perform further oxidation on the mixture after additional oxidation, the mixture is usually pumped into a high-pressure section and heated to a predetermined temperature. This can be done using a tube heat exchanger. In the present invention, it is essential to add heavy metals and/or bromine between the end of the first oxidation reaction and the re-oxidation treatment.
The additional addition of this catalyst component significantly improves the oxidizing power in the re-oxidation step at high temperatures, so that efficient oxidation can be carried out even with a very small amount of molecular oxygen. In the present invention, it is important to additionally add a catalyst component as described above, and even if the amount of catalyst used in the first oxidation reaction is increased in advance, the expected effects of the present invention cannot be obtained. do not have. The point at which the catalyst component is added is usually the first
The period after the completion of the oxidation reaction and before the low-temperature additional oxidation treatment step is preferable. In this case, since the added catalyst component acts continuously, not only the re-oxidation but also the content of the additional oxidation is improved, which is preferable. Further, the amount of the catalyst component to be added is usually 0.05 to 2 times, preferably 0.05 to 2 times, each of the heavy metal or bromine used in the first oxidation reaction.
It is 0.1 to 1 times. If the amount of this catalyst component is too small, the expected effects of the present invention cannot be obtained; on the other hand, even if the amount is too large, there is no difference in the effect and it is not economical. The additional catalyst component may be a heavy metal alone or bromine alone, but it is preferable to add both a heavy metal and bromine. In particular, it is preferable to add a catalyst having substantially the same composition as that used in the first oxidation reaction. The reaction mixture that has been reoxidized at high temperature 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. Furthermore, in the present invention, by-products in the reaction mother liquor, especially impurities that interfere with the oxidation reaction, are extremely small, so that 10 to 80% by weight of the reaction mother liquor can be recycled as is 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 since 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 1 A reaction was carried out using the reaction apparatus shown in the flow sheet of FIG. A pressure-resistant titanium first reactor 1 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 parasylene and acetic acid containing 5% water from a pipe 7. A mixture consisting of 4.5 parts/hour, cobalt acetate tetrahydrate 0.0025 parts/hour, manganese acetate tetrahydrate 0.0027 parts/hour and 47% hydrobromic acid 0.0039 parts/hour was fed, and the temperature
Under the conditions of 200℃, pressure 18Kg/cm ² G, residence time 90 minutes,
Air is used as the oxidizing gas, and is supplied through pipe 10 so that the oxygen concentration in the exhaust gas of the oxidation reaction is 4% by volume, and 1.5 parts/hour of reflux liquid is extracted from pipe 8 to reduce the water concentration in reactor 1 to approximately 10%. A liquid phase oxidation reaction of para-xylene was carried out by controlling the weight %. The mixture from the first reactor 1 was continuously fed through a pipe 13 to a second reactor 2 equipped with the same equipment as the first reactor 1. At this time, a catalyst having exactly the same composition as the catalyst supplied to the first reactor was added into the pipe 13 from the pipe 9 in an amount equivalent to 0.25 times. In the second reactor 2, the temperature was 185℃, the pressure was 11Kg/cm ² G,
Under conditions of a residence time of 30 minutes, additional oxidation was performed by supplying air from the pipe 11 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 14 and is pumped to a pressure of 55 Kg/cm ² by a booster pump (not shown).
The pressure of the mixture was increased to 260°C, and the mixture was further supplied to a monotube heater 3 to raise the temperature of the mixture to 260°C. Furthermore, the mixture leaving the heater 3 was then fed to a third reactor 4 with the same equipment as the first reactor 1. In the third reactor 4, the temperature is 260℃ and the pressure is 55Kg/cm ²
G. Under conditions of a residence time of 30 minutes, 0.07 parts/hour of air was supplied from pipe 12 to carry out re-oxidation. After sequentially performing additional oxidation and re-oxidation in this manner, the mixture was cooled and crystallized in a crystallizer 5, and then passed through a centrifuge 6 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 shown in Table 1 were obtained. Comparative Example 1 Table 1 shows the results when terephthalic acid was produced in exactly the same manner as in Example 1 without supplying the catalyst from pipe 9. Comparative Example 2 In the method of Example 1, the supply of catalyst from pipe 9 was stopped, and a corresponding amount of catalyst was added to the first pipe.
The oxidation reaction was carried out by supplying 4CBA to the reactor from the beginning and adjusting the temperature and pressure of the first reactor so that the level of 4CBA in terephthalic acid at the outlet of the second reactor was the same as in Example 1. Table 1 shows the results when terephthalic acid was produced in a similar manner.

[Table] (Note) The amount of catalyst is the weight ratio of atoms to the solvent.

[Brief explanation of drawings]

FIG. 1 is a flow sheet showing the reactor used in the examples of the present invention, 1 is the first reactor, 2 is the second reactor, 3 is the heater, 4 is the third reactor, and 5 is the crystallizer. The analyzer is shown.

Claims (1)

[Claims] 1. The first oxidation reaction obtained by oxidizing para-xylene to terephthalic acid in an acetic acid solvent at a temperature of 180 to 230°C at a temperature of 95% or more of para-xylene to terephthalic acid. The mixture is additionally oxidized with molecular oxygen at a temperature lower than the temperature of the first oxidation reaction, then re-oxidized with molecular oxygen at a temperature higher than the temperature of the first oxidation reaction, and then the reaction mixture is crystallized. In the continuous production method of terephthalic acid, which involves separation and solid-liquid separation,
After the completion of the first oxidation reaction and during the re-oxidation treatment,
For the heavy metal or bromine used in the first oxidation reaction,
A method for producing high-purity terephthalic acid, which comprises adding heavy metals or bromine in an amount of 0.05 to 2 times each.