JPH0257528B2 - - 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 a solvent under high temperature and high 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 with the need for a separate plant for purification. Therefore, in recent years, when oxidizing para-xylene, by adopting specific catalysts, oxidation conditions, or oxidation methods, it is possible to directly oxidize para-xylene in one plant.
Methods for producing high purity terephthalic acid are known. 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 additional oxidation at a high temperature of 230°C 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 acetic acid solvent that occurs during the production of terephthalic acid is small. This is a great method. However, this method
In the process of performing additional oxidation at a high temperature of 230℃ or higher,
Since combustion of the acetic acid solvent is likely to occur, there is still room for improvement to further reduce the amount of combustion loss of the acetic acid solvent. In view of the above-mentioned circumstances, the present inventors have investigated various methods for lowering the amount of combustion loss of acetic acid solvent when producing terephthalic acid by the combination of oxidation of paraxylene, low-temperature additional oxidation, and high-temperature re-oxidation. As a result, they discovered that by employing a certain method, the amount of combustion loss of acetic acid solvent could be reduced, and the present invention was completed. That is, the gist of the present invention is a 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. oxidizing at least 95% by weight of the paraxylene to terephthalic acid by supplying paraxylene and molecular oxygen to a tank-type first reactor maintained at It is supplied to a tank-type second reactor maintained at a temperature 0 to 50°C lower than the first reactor, and molecular oxygen is supplied to the first reactor.
After increasing the pressure of the mixture obtained in the second reactor, the mixture is heated to a temperature of 235°C or higher through a tube heater, and after the pressure is increased, the mixture is heated during the temperature increase. 0.003 to terephthalic acid
carrying out a second additional oxidation in the heater by supplying 0.3 moles of molecular oxygen; the mixture passing through the heater is then heated to 235°C;
The third additional oxidation is carried out by supplying molecular oxygen in an amount of 0.003 to 0.3 times the mole of terephthalic acid in the mixture without supplying paraxylene. A method for producing terephthalic acid, comprising: recovering the terephthalic acid by crystallizing and filtering the mixture from the third reactor. 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, 95% by weight or more, preferably 98% by weight or more of paraxylene is oxidized to terephthalic acid in a tank-type first reactor, and the reaction temperature is usually 180-230°C, preferably 190-230°C. 210°C, and the pressure is several Kg/cm ² to 100 Kg/cm ² , preferably 10 to 30
Kg/ ^cm2 . 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. Further, the reaction time in the first reactor is required to allow 95% by weight or more of paraxylene to be oxidized to terephthalic acid, and is usually about 30 to 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 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. Next, the slurry containing terephthalic acid obtained in the first reactor is extracted and supplied to another tank-type second reactor, and the slurry is 0 to 50% higher than the reaction temperature of the first reactor.
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. The amount of molecular oxygen used in this additional oxidation is about 1/10 to 1/1000 of the amount supplied to the first reactor, since the amount of oxidized material is small _. The amount is preferably such that the concentration is 1 to 6 vol%.
Generally, air or air diluted with an inert gas may be used as molecular oxygen. Furthermore, the above-mentioned first reactor and second reactor, as well as the third reactor described later, are all tank-type reactors, and usually a stirred tank type having a reflux condenser at the top is used. . In this first additional oxidation step, it is preferable to carry out the reaction using only the slurry from the first reactor and molecular oxygen, but if desired, a small amount of paraxylene may be supplied thereto. In other words, the purpose of this first additional oxidation step is to additionally oxidize the slurry in which more than 95% by weight of paraxylene has been oxidized to terephthalic acid in the first reactor, so a large amount of paraxylene is newly supplied here. This is equivalent to additionally oxidizing the slurry with a low reaction rate in the first reactor, and the effect of this process cannot be exhibited. The mixture from the second reactor is then pressurized and then heated to a temperature above 235°C, preferably from 240 to 290°C, by passing through a heater. 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 N ₂ 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 tube heat exchanger is usually used. In the present invention, it is an essential requirement to supply molecular oxygen to the mixture from the second reactor in the flow path after the pressure rise and during the temperature rise, and to perform the second additional oxidation in the heater. It is. The amount of molecular oxygen supplied is 0.003 to 0.3 per terephthalic acid in the mixture.
It is twice the molar amount, preferably 0.01 to 0.1 times the molar amount. If the amount of molecular oxygen used is too small, a high purity product cannot be obtained unless a large amount of molecular oxygen is supplied in the third additional oxidation process, which will be described later.As a result, the amount of combustion loss of the acetic acid solvent will decrease. On the other hand, if the amount of molecular oxygen used is too large, the amount of combustion loss of the acetic acid solvent in the second additional oxidation will increase, which is not preferable. The molecular oxygen used in this second additional oxidation and the third additional oxidation described later is usually air. The mixture heated through the heater and subjected to the second additional oxidation is then supplied to a third reactor,
A third additional oxidation is carried out at a heated temperature above 235°C, preferably between 240 and 290°C. As mentioned above, the pressure in this heating zone is usually 30 to 100 kg/cm ²
It is. The third reactor is of the tank type like the first and second reactors, and the residence time in the third reactor is usually 5 to 120 minutes, preferably 10 to 60 minutes.
The amount of molecular oxygen used in the third additional oxidation is 0.003 to 0.3 times, preferably 0.01 to 0.1 times by mole, the amount of terephthalic acid in the mixture. In the present invention, as mentioned above, the second additional oxidation is performed in the heater, so even if the amount of molecular oxygen supplied in the third additional oxidation is reduced, high purity terephthalic acid can be produced. As a result, the overall amount of acetic acid solvent combustion loss is reduced. Furthermore, molecular oxygen is usually air, and the O ₂ concentration in the oxidizing exhaust gas is substantially zero. The mixture from the third reactor is crystallized according to conventional methods. 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
Since the combustion loss of acetic acid solvent when producing high purity terephthalic acid of 500 ppm or less in one plant is small, it is extremely advantageous industrially and economically. 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: Acetic acid containing 1 part of paraxylene and 5% water is supplied from pipe 8 to the first reactor 1 made of pressure-resistant titanium, which is equipped with a reflux cooling device, a stirring device, a raw material and solvent inlet, an air inlet, and a reaction slurry outlet. 4.5 parts, cobalt acetate (tetrahydrate) 0.0025 parts, manganese acetate (4
hydrate) 0.0027 parts and hydrobromic acid (47% aqueous solution)
Feed a mixture consisting of 0.0039 parts, residence time 90
minutes, temperature 200℃, pressure 18Kg/ ^cm2G , air was used as the oxidizing gas, and the exhaust gas from the oxidation reaction was
Supply from pipe 9 so that the O ₂ concentration is 4 vol%, and extract 1.5 parts of reflux liquid from pipe 10 to control the water concentration in the reactor 1 to about 10%.
A liquid phase oxidation reaction of paraxylene was carried out. The mixture from the first reactor 1 was continuously fed through a pipe 14 to a second reactor 2 equipped similarly to the first reactor. In the second reactor 2, the temperature is 185℃,
Under conditions of a pressure of 11 Kg/cm ² G and a residence time of 30 minutes, air was supplied from the pipe 11 to perform the first additional oxidation so that the O ₂ concentration in the exhaust gas from the oxidation reaction was 4 vol %. The mixture from the second reactor 2 passes through the pipe 15, and then the pressure is increased to 65 kg/cm ² G by the pump 3. After 0.02 part of air is added from the pipe 12 in the middle of the pipe 16, the mixture is passed through the monotube heater 4.
A second additional oxidation was carried out in a heater, and the temperature of the mixture was raised to 275°C. Furthermore, the mixture leaving the heater 4 passes through a pipe 17 to a third reactor 5 equipped with the same equipment as the first reactor.
supplied. In the third reactor 5, the temperature is 275℃ and the pressure is
65Kg/ ^cm2G , residence time 30 minutes, air 0.05
A third additional oxidation was carried out by supplying the same amount through pipe 13. After performing additional oxidation in this manner, the mixture was cooled and crystallized in a crystallizer 6, and then passed through a centrifugal separator 7 to recover crystals of terephthalic acid. For the terephthalic acid obtained as described above,
Measure the 4CBA content, transmittance (T ₃₄₀ ), and also
The amount of combustion loss of the acetic acid solvent due to the second and third additional oxidation was measured, and the results shown in Table 1 were obtained. Comparative Example 1 A reaction was carried out in exactly the same manner as in the example except that the air supply from pipe 12 was stopped and the second additional oxidation was not performed. Comparative Examples 2 and 3 In the method of the example, the air supply from the pipe 12 was stopped and the second additional oxidation was not performed, but instead:
The reaction was carried out in the same way when the amount of air supplied from the pipe 13 to the third reactor 5 was increased to 0.07 part (Comparative Example 2), and when it was increased to 0.21 part, which is three times that amount (Comparative Example 3). Ta.

【table】

【table】 [Brief explanation of the drawing]

FIG. 1 is a flow sheet showing the reactor used in the examples of the present invention, where 1 is the first reactor, 2 is the reactor, 3 is the pump, 4 is the heater, and 5 is the third reactor. .

Claims (1)

[Claims] 1. A 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, at a temperature of 180 to 230°C. oxidizing at least 95% by weight of the paraxylene to terephthalic acid by supplying paraxylene and molecular oxygen to a tank-type first reactor maintained at a temperature of the first reactor; The second tank-type reactor is maintained at a temperature 0 to 50°C lower than the first reactor, and molecular oxygen is supplied to the second reactor.
After increasing the pressure of the mixture obtained in the second reactor, the mixture is heated to a temperature of 235°C or higher through a tube heater, and after the pressure is increased, the mixture is heated during the temperature increase. 0.003 to terephthalic acid
carrying out a second additional oxidation in the heater by supplying 0.3 moles of molecular oxygen; the mixture passing through the heater is then heated to 235°C;
The third additional oxidation is carried out by supplying molecular oxygen in an amount of 0.003 to 0.3 times the mole of terephthalic acid in the mixture without supplying paraxylene. A method for producing terephthalic acid, comprising: recovering the terephthalic acid by crystallizing and filtering the mixture from the third reactor.