JPS60233032A - Production of terephthalic acid suitable for direct polymerization - Google Patents

Abstract

PURPOSE:To obtain terephthalic acid of higher purity without increasing the amount of acetic acid solvent to be burned in purifying an oxidation product of p-xylene with molecular oxygen by blowing oxygen thereinto in two stages, by inserting a step of seperating liquids with a liquid cyclone between both steps. CONSTITUTION:p-Xylene is reacted with molecular oxygen in the presence of a heavy metal and bromine compound in acetic acid solvent to oxidize >=90% thereof, and the resultant oxidation product, preferably controlled at 2,000- 30,000ppm oxidation intermediate concentration is crushed at 140-230 deg.C while blowing molecular oxygen thereinto to give >=0.5vol%, particularly 1-8vol% oxygen concentration in the off-gas. The resultant slurry is then separated into liquids in a liquid cyclone. The underflow is circulated through the above-mentioned step, and the overflow is treated at a temperature (180-300 deg.C)>=10 deg.C higher than the temperature in the first purification step while blowing molecular oxygen to give 0.05-5vol%, particularly 0.1-2vol% oxygen concentration in the off-gas to recover the aimed terephthalic acid from the formed slurry.

Description

Detailed Description of the Invention The present invention relates to a method for producing high-purity terephthalic acid, and more specifically, it can be directly polymerized with a glycol component and used as a raw material for obtaining high-quality polyester. The present invention relates to a method for producing high purity terephthalic acid.

Today, as a method for producing terephthalic acid, a method is employed industrially in which paraxylene is oxidized with a molecular oxygen-containing gas in the presence of a heavy metal catalyst and a bromine compound. However, the telephoto level obtained by this method is
Acids contain impurities such as aldehydes and colored substances in their crystals, and these terephthalic acids cannot be directly used for polymerization as they are. Furthermore, it is extremely difficult to separate these impurities from terephthalic acid crystals.

Many methods have been proposed for purifying terephthalic acid.The first is a purification method by sublimation, and the second is a method in which terephthalic acid is first converted into a salt or ester lvK, and in this state, conventional methods such as recrystallization and distillation are used. The third method is to purify terephthalic acid by extraction or recrystallization with a suitable solvent such as water or acetic acid, and the fourth method is to convert terephthalic acid into a solvent such as water, acetic acid, or propionic acid. Examples include a method of purifying the substance in a suspended state by contacting it with a molecular oxygen-containing gas. Among these methods, the first method causes quality problems in the terephthalic acid decomposition pond due to the high temperature, and it cannot be said to be an advantageous method in terms of equipment.

Although it is possible to obtain terephthalic acid with excellent quality in the second method, it is not always suitable for industrial use because it requires the use of alcohol or alkali, and the process is complicated. I can't say it's an advantageous method. The third extraction method does not yield high-quality products, and the recrystallization method requires a large amount of solvent or must be heated to high temperatures to completely dissolve terephthalic acid in the solvent. Therefore, it is difficult to say that it is an industrially advantageous method. Furthermore, in the fourth method, it is possible to obtain terephthalic acid of considerably superior quality, but the obtained terephthalic acid has a small particle size and apparent density, and the particle shape is also suitable for direct esterification with ethylene glycol. It is inappropriate for This also applies to the third method. In addition, in all of these methods, it is necessary to separate terephthalic acid from the reaction mixture and then subject it to a purification step, which is disadvantageous in terms of equipment.In this respect, too, there are no industrially superior methods. It's hard to say.

As a method to solve the drawbacks of the above-mentioned conventional methods, in recent years, the oxidation reaction of para-xylene has been used directly as a raw material for polyester, for example, the content of 4-carboxybenzaldehyde, a typical impurity, is so
Several methods have been proposed for producing economically advantageous high-purity terephtha/l/- of less than ppm in one plant. For example, methods have been proposed in which the oxidation reaction mixture is re-oxidized at low temperatures (JP-A-52-85136, JP-A-53-57656, etc.), but in these methods, the amount of acetic acid solvent burned during the oxidation reaction is The problem is that there are too many. Also, a method of additionally oxidizing the oxidation reaction mixture at high temperature (Japanese Unexamined Patent Publication No. 5
No. 5-55138, No. 57-18647, etc.) have also been proposed, but although this method allows for the production of high-purity terephthalic acid with a relatively small amount of acetic acid combustion, it generates a large amount of heat due to the extremely high temperature. It has the disadvantage of requiring energy, and even though the amount has been reduced, the combustion of acetic acid solvent is still quite large and cannot be ignored.

In view of the above circumstances, the present inventors have made efforts to develop an economical method for producing high-purity TE-7-lytic acid that can be directly reacted with Glyco-P components to produce high-quality fiber string polyester P. As a result of repeated studies, we found that by adopting a specific process, 7μ dehydes and colored substances could be reduced despite the fact that the amount of acetic acid solvent burnt and the amount of heat energy added in the purification process following the silica reaction were small. Moreover, when mixed with glycos and made into a slurry, the viscosity is low.
It was discovered that high purity terephthalic acid suitable for direct polymerization could be obtained, and the results were published in patent application No. 57-212881.
No. 1 has already been filed, but the inventors will further research
As a result of repeated efforts, it was found that by adding improvements to the invention filed as Patent Application No. 57-212881, it was possible to obtain terephthalic acid of higher purity than K, and also to increase the amount of acetic acid solvent burned. They discovered that there is no such thing and completed the present invention.

That is, the present invention provides a method for producing terephthalic acid by reacting paraxylene with molecular oxygen in the presence of a heavy metal and a bromine compound in an acetic acid solvent.

After the main reaction step of oxidizing 90% or more of para-xylene, 1) Main reaction step oxidation reaction product is oxidized with 140~ The oxidation reaction product was crushed under a temperature condition of 250'Q to reduce the average particle size of terephthalic acid to 2.
2) Supply the slurry crushed in the first purification process to a liquid cyclone, supply the overflow slurry of the liquid cyclone to the following second purification process, and remove the underflow of the liquid cyclone. Step of circulating to the first purification step, 3) While blowing molecular oxygen into the overflow slurry of the liquid cyclone so that the oxygen concentration in the off gas becomes 0.05 to 5 vol%,
A second purification step of treating the crushed slurry at a temperature within the range of 100°C and at least 10°C higher than that of the first purification step.

This is a method for producing terephthalic acid suitable for direct polymerization, which is characterized by sequentially performing the following steps and solid-liquid separation of the obtained purified slurry to obtain terephthalic acid.

The invention filed as Japanese Patent Application No. 57-212881 is an invention that requires combining the first purification step and the second purification step in this order after the main reaction step. It is characterized in that a liquid separation step using a liquid cyclone is inserted between the first purification step and the second purification step. By adding such a step, the purity of the finally obtained TEL-7 acid is improved. Furthermore, the addition of such a step does not result in any increase in the amount of acetic acid solvent burned.

Of course, in the present invention, both the first purification step and the second purification step are essential steps, and if either one of these purification steps is not adopted, it is natural that the purpose of the present invention will not be achieved. For example, if the second purification step is not performed, not only will the reduction rate of 4-capoxybenzaldehyde and colored substances be small, but also the resulting terephthalic acid will have a high viscosity when mixed with glycols, and therefore direct polymerization will not be possible. terephthalic acid suitable for producing polyester/L' advantageously cannot be obtained. Furthermore, if the first purification step is not carried out, the purification effect is difficult to achieve and treatment must be carried out at high temperatures for a long time, which is not only economically disadvantageous but also contains a large amount of colored substances in the resulting terephthalic acid.

Regarding the main reaction step, first purification step, and second purification step carried out in the present invention, the above-mentioned Japanese Patent Application No. 57-212
The method for producing terephthalic acid by oxidizing puff xylene with molecular oxygen, which is the main reaction step of the method of the present invention, is almost the same as that described in No. Any method may be used as long as xylene is reacted with molecular oxygen in an acetic acid solvent in the presence of a catalyst containing a heavy metal and a bromine compound. The heavy metal compound is a compound of one or more metals selected from Coba/l/), mankan, cerium, nickel, iron, chromium, etc., and the bromine compound is an alkali metal salt of bromine,
Alkaline earth metal salts, heavy metal salts, organic compounds such as hydrogen bromide or tetrabromoethane are used. Combinations of these catalyst components include manganese and cobalt.
The composition ratio is preferably in a range such that, for example, the amount of manganese metal is 0.2 to 3 times the weight of Coba A/T metal. Specific examples of bromine compounds include sodium bromide,
Hydrogen bromide, tetrabromoethane or heavy metal bromides such as cobalt bromide, manganese bromide are preferred.

There are no particular restrictions on the reaction conditions for the main reaction, but the catalyst composition should be such that at least 90 mo/L/ti of paraxylene undergoes oxidation.
It is necessary to select the reaction temperature and residence time according to the amount.

The reaction temperature is 150~2SO-C, 180~2%
50°C is preferred. Also, the retention time is 10 to 240
A range of minutes is usually used.

If appropriate conditions are selected from the catalyst composition, reaction temperature, and residence time described above, the reaction rate can easily reach 9° or higher than K, but in general, as the catalyst concentration increases and the reaction temperature increases, Alternatively, as the residence time increases, the reaction rate increases, so it is preferable to take these factors into consideration when determining the reaction conditions. Of course, even under conditions outside the above-mentioned preferred conditions, it is possible to achieve a reaction rate of 90 or more, but this is not satisfactory in terms of economy and the quality of the terephthalic acid obtained. In the method of the present invention, if the conversion rate of paraxylene in the main reaction step is less than 90%, the load of the oxidation reaction in the first purification step will become too high, and the crushing effect will be poor, which is not preferable. On the other hand, increasing the reaction rate too much impairs economic efficiency, so the reaction rate is preferably 99.99% or less.

In this way, this main oxidation reaction step yields a nullary oxidation reaction mixture consisting of the produced terephthalic acid crystals and a mother liquor containing catalyst components, unreacted xylene, xylene oxidation reaction intermediates, acetic acid solvent, etc. This product can be directly subjected to the first purification step described below.

Here, xylene oxidation reaction intermediates refer to compounds in various oxidation states that are generated intermediately in the oxidation reaction process from xylene to terephthalic acid, and the main one is 4-carboxybenzaldehyde (hereinafter "4-CBA"). ”), buff I-/phyllic acid (hereinafter abbreviated as “PTA”), and para-acetoxymethylbenzoic acid (hereinafter “PAM”).
PAMBA is represented by the following structural formula (1), and together with buffed hydroxymethylbenzoic acid (PHMBA) represented by the following structural formula (1). exists in the mother liquor of paraxylene oxidation reaction, and there is an equilibrium relationship of (1) + H2O- = (1) + CHsCOOH in the mother liquor, so here we include (1) and 1) and represent them with PAMBA. Please understand.In addition, (
1), the abundance of (1) varies greatly depending on the oxidation reaction conditions, but in general, (1) is approximately the same amount as 4-CBA, and (1) is 115 to 1/1 of the amount of (1).

In addition to these, there are other oxidation reaction intermediates such as para-tolualdehyde, para-methylbenzyl alcohol-μ, para-methylbenzyl acetate, and benzyl bromide derivatives; PTA and 4-CBA and PAM in the oxidation reaction mixture
The total concentration of BA is an important factor. In other words, the total concentration of the three types of oxidation reaction intermediates floating in the mother liquor of the oxidation reaction mixture in the form of a cloth-like oxidation reaction mixture subjected to the first purification step is generally 1.
．． The effects of the present invention are further enhanced when the content is adjusted to a range of 500 ppm or more and 40,000 ppm or less, preferably 2.000 to 30,000 ppm. The method for adjusting the concentration of the intermediate includes (2) appropriately selecting the reaction conditions in the main reaction step, such as the supply ratio of paraxylene and solvent, that is, the paraxylene concentration, its supply rate, reaction temperature, catalyst, etc. Method: Q3) There is a method of diluting, substituting or concentrating the obtained oxidation reaction mixture with a solvent.90 There is a method of adding or removing the anti-reactant, but method C) above is not economical due to the complicated process. It is advantageous to adjust the concentration of the reaction intermediate within the above range by using the methods described above or (6) alone or in appropriate combinations.

If the total concentration of the oxidation reaction intermediates is less than 1,500 ppm, terephthalate 1/
I/The acid crystals are difficult to crush and the purification efficiency is low. Therefore, even if the terephthalic acid obtained through the second purification step is highly pure, the economic efficiency is evaluated by the sum of the amount of acetic acid burned and energy consumption. There is a tendency for industrial usefulness to decrease. 4 0,000 ppm
Even if it exceeds 100%, the terephthalic acid crystals become difficult to crush in the first refining step, and moreover, there is a tendency for acetic acid combustion to occur simultaneously with reoxidation, resulting in a decrease in economic efficiency.

Therefore, according to a preferred embodiment of the present invention, at least 90 mo/L'% of para-xylene to be subjected to the main reaction step
In addition to oxidizing PTA, 4-
Total concentration of CBA and PAMBA ゛Concentration 1,500-4
0,000ppm, preferably 2,000-30,00
A method of controlling the amount within the range of 0 ppm is adopted. In addition,
When the oxidation reaction mixture obtained from the main reaction step is subjected to the first purification step as it is, the total concentration of intermediates in the mother liquor of the reaction mixture obtained from the main reaction step during the supernatant reaction is 1.
．． It is preferable to select the reaction conditions of the main reaction step so that 50Ω to 41), tloo ppfn.

In selecting such conditions, the catalyst composition, reaction temperature, and feed concentration and feed rate of paraxylene described above should be carefully selected.

For example, for a given catalyst composition and catalyst concentration, lowering the reaction temperature will increase the total concentration of the oxidation reaction intermediates;
Furthermore, under a constant reaction temperature, the intermediate concentration increases as the catalyst concentration decreases. Furthermore, the water concentration in the reaction mixture can also affect the intermediate concentration. That is, under a constant catalyst composition and reaction temperature, the total concentration of oxidation reaction intermediates tends to increase as the water concentration increases. Thus, reaction conditions may be set taking these relationships and trends into account.

Next, the first purification step of the present invention will be explained.

This process involves crushing and reoxidation. That is, the reaction mixture obtained in the main reaction step mainly consists of the produced terephthalic acid crystals and a mother liquor and is in a slurry state, but in the first purification step, the terephthalic acid crystals and mother liquor contained in the mother liquor are in a slurry state.゛The amount of oxidation reaction intermediates is reduced by oxidizing most of the oxidation reaction intermediates contained in the terephthalic acid crystals and converting them into terephthalic acid. This is to prepare for the transition to the second purification step by crushing the terephthalic acid particles and reducing the terephthalic acid particle size.

The reaction mixture obtained in the main reaction step may be directly subjected to the first purification step, or may be diluted with a solvent and then subjected to the reaction mixture after removing a portion of the solvent. The solid content concentration in the slurry in the first purification step may be any value as long as it maintains fluidity, but in order to efficiently perform crushing and purification, it is 20 to 70% by weight, particularly 22 to 65% by weight. is preferred.

The crushing method in the first refining step is a blade crushing method using a high-speed stirring using an oak with an agitator, a method using the crushing force of a centrifugal pump, a method using a known wet crusher, and a method using the main reactor. Any method may be used, such as a method that utilizes the shock caused by the drop in temperature and pressure while reaching the purification tank, or a method that utilizes the shock caused by the pressure difference after releasing the external pressure. The average particle size of terephthalic acid obtained in the main reaction is 30% depending on the reaction method.
It must be more than μ, usually more than 100 μ.

The goal of crushing is to reduce the average particle size by 20% or more (
This is called a crushing degree of 20% or more), preferably 4
It is 0chi or more. The effect of the present invention increases as the degree of crushing increases, but from the perspective of equipment and energy efficiency, the degree of crushing cannot be increased to 95%.
It is not a good idea to do more than that. The average particle size of terephthalic acid referred to in the present invention is the weight average particle size, and can be determined by classifying and measuring using a sieve while showering with water.

The treatment temperature in the first purification step is 140-230°C, preferably 160-210°C. If the temperature exceeds 230°C, the reoxidation rate of the reaction intermediate in the J mother liquor will increase, but the combustion of acetic acid will increase rapidly, and the amount of colored substances in the obtained terephthalic acid will tend to increase, so it is preferable. do not have. On the other hand, when the temperature is lower than 140°C, the reoxidation rate of the reaction intermediate in the mother liquor is slow, it is crushed and becomes K<<, and a large amount of thermal energy is required to raise the temperature for the next second purification step I don't like Tonashi.

The molecular oxygen-containing gas required for the oxidation in the first purification step includes air or a mixture of oxygen and pond gas (eg off-gas from the main reaction step). The amount of molecular oxygen introduced varies depending on the amount of oxidation reaction intermediate in the main reaction mixture subjected to the first purification step and the processing conditions,
The oxygen concentration in the off-gas from the first purification step is 0.5 v
It is necessary to introduce an amount such that the amount becomes 1% or more. If the oxygen concentration is less than o, 5 vol 4, the first
This is not preferable because colored components increase in the purification step, which adversely affects the subsequent purification in the second purification step. The preferred oxygen concentration in the off-gas is 1 to avoi%.

The method for introducing molecular oxygen in the first purification step may be to introduce it below the liquid level of the reaction mixture subjected to the step, but the oxygen-deficient state occurs during the transition from the main reaction step to the first purification step. It is advisable to be careful not to overdo it. Therefore, when using an apparatus that continuously performs the method of the present invention, for example, a molecular oxygen-containing gas is added to the transfer fins so that molecular oxygen is also present in the transfer line from the main reaction step to the first purification step. It is preferable to use a method of introducing a method in combination to ensure the effects of the present invention.

In the first purification step, the crushing part and the reoxidation part may be carried out in the same apparatus, for example, by introducing molecular oxygen while stirring at high speed with an agitator equipped with a stirrer, or in separate apparatus.
For example, a method may be used in which a Japanese cypress tree equipped with a pump is used as a crusher, and the pump part crushes the Japanese cypress tree, and molecular oxygen is introduced into the Japanese cypress tree to reoxidize it. When using the latter separated device, the reaction mother liquor present in the crushing section should be kept for a short period of time or allowed to coexist with molecular oxygen so that it is not exposed to an oxygen-deficient state for a long time. I hope it's jin. By this method, the concentration of the reaction intermediate in the reaction mother liquor of the slurry subjected to the second purification step is 4-CB.
A is 10 to 1,000 ppm, preferably 20 to 40
It is preferable to adjust the amount to 0 ppm.

The first purification step is carried out under the conditions described above, and in particular, in the present invention, the oxidation reaction intermediates PTA and 4-CB in the mother liquor of the slurry during the first purification step are
When the total concentration of A and PAMBA is within the above-mentioned specific range, the effects of the present invention are extremely large. That is, reoxidation in the first purification step is likely to proceed, and the particles are also likely to be crushed, resulting in a phenomenon that the effect of purifying impurities in the terephthalic acid particles due to crushing is increased. Furthermore, such a phenomenon also occurs when a small amount of alkylbenzene is newly supplied to the first purification step. The cause of this phenomenon is unknown, but in the first purification step, the catalyst in the reaction mixture is activated by the oxidation reaction intermediate present in an appropriate amount or the fusical species generated from a small amount of newly added furkylbenzene.
It is presumed that the intermediates in the mother liquor are rapidly oxidized. In addition, for crushing, PTA and 4-CBA
Due to the presence of an appropriate amount of PAMBA or PAMBA, or due to the oxidation of a small amount of newly added alkylbenzene, the coexisting terephthalic acid particles temporarily become coarse particles with weak binding strength and are easily crushed. However, it is estimated that the refining efficiency associated with crushing will increase.

In the first purification step of the present invention, the alkylbenzene added to the oxidation reaction mixture as necessary
such as ene, ethylbenzene, xylenes, propylbenzenes, t/I/7/L/dehydes, toluic acid, etc.
Examples include benzene derivatives having at least one lower alkyl group in the benzene skeleton, and among these, para-xylene and para-toaldehyde are particularly preferred because they have great effects and their oxidation product is terephthalic acid.

The amount of alkylbenzene supplied is preferably an amount corresponding to 0.01 to 57 μm of paraxylene introduced into the main reaction step. When the concentration of the reaction intermediate in the reaction mother liquor transferred from the main reaction step to the first purification step is low, it is advantageous to supply a relatively large amount of alkylbenzene. In addition, the total concentration of reaction intermediates is within the range of the present invention from 1,500 to 40.
,000 ppm but if the catalytic activity is lowered for some reason, the catalytic activity can be improved by supplying a small amount of alkylbenzene. In this case, a new catalyst component is supplied together with the alkylbenzene. If you do so, the effect will be even more certain. However, if the amount of alkylbenzene supplied is too large, the reaction load in the first purification step will increase, and at the same time, the crushing purification efficiency will also tend to decrease, thus reducing the amount of paraxylene introduced into the main reaction. Particularly preferred is 02 to 2 mo/L'%.

Although the alkylbenzene may be supplied intermittently to the first purification step, it is preferable to supply the alkylbenzene continuously.

In addition, as for the supply method, it may be supplied in liquid form or as a gas together with oxygen-containing gas. In particular, when supplied in liquid form, it may be supplied as it is, diluted with a solvent, or mixed with a solution of a new catalyst component. It's okay. The catalyst component in this case may be a catalyst having the same composition as the main reaction, or a bromine compound may be mainly supplied to supplement the bromine component consumed and inactivated in the oxidation reaction.

The slurry thus treated in the first purification step is then supplied to a hydrocyclone. The main purpose of liquid separation in a liquid siphon is to increase the efficiency of crushing. Specifically, the slurry crushed in the first purification step is supplied to a liquid cyclone installed between the first and second purification steps, and the overflow slurry of the liquid cyclone (i.e., the slurry classified in the cyclone is In this method, the slurry (slurry containing small particles of terephthalic acid) is supplied to the second purification step, and the underflow slurry (slurry IJ - containing large particles of terephthalic acid) of a hydrocyclone is circulated to the first purification step. The use of such hydrocyclones is particularly useful when applied to the continuous practice of the present invention. In this case, the result is that the slurry concentration in the first purification step is higher than the slurry concentration in the second purification step, but in the method of the present invention, the solid content concentration of the slurry in the first purification step is 70%. There is no particular problem as long as it does not exceed %. The liquid cyclone that can be applied to the method of the present invention may have a known structure, and a plurality of liquid cyclones may be connected and used. In addition, if a liquid cyclone is attached between the first purification process and the second purification process, the average particle size of the terephthalic acid in the first purification process and the average particle size of the terephthalic acid supplied to the second purification process will be reduced. However, in this case, the degree of crushing is determined by the average particle size of the terephthalic acid crystals at the time of supply to the second refining step.

Next, the second purification step of the present invention will be explained.

tl! The slurry, which has been crushed and reoxidized in the first refining process and classified with a hydrocyclone, is subjected to the first refining process in the second refining process.
It is further contacted with a molecular oxygen-containing gas at 80-300°C, preferably 210-260"C and at least 10'Q higher than the first purification step. In this case, the higher the treatment temperature, the better the Terephthalic acid is glyco-ρ
It is preferable because the viscosity when mixed with other substances is low, and the amount of oxidation intermediates represented by 4-CBA is also low, but it is also undesirable to raise the temperature to high temperatures because it tends to increase the amount of colored substances. Furthermore, at temperatures lower than 180°C, the obtained terephthalic acid has a high slurry viscosity when mixed with glycols, making it undesirable for direct polymerization.
It is within the range of 0 to 260°C. Also, the temperature used in the second purification step is at least 10° higher than the temperature in the first purification step.
It is necessary to have a high carbon temperature, preferably 20 to 100°C. If the temperature difference is less than 10° C., the purification effect in the second purification step will be K<.

The molecular oxygen-containing gas used in the second purification step is 1
For example, air or oxygen and nitrogen, oxygen and an inert gas from a pond, a mixture of air and an inert gas, and off-gas from the main reaction process are used.

The amount of molecular oxygen supplied to the second purification step also varies depending on the amount of oxidation reaction intermediates in the slurry mother liquor and terephthalic acid crystals subjected to this step and the processing conditions, but the oxygen concentration in the off gas is 0.05 to 5 vol. , preferably 0.1
It is necessary to set K so that ~2 vol To,
Moreover, PTA in the slurry mother liquor subjected to this step, 4
- It is preferable to supply 1a to 500 times, particularly 20 to 100 times, the theoretical amount of oxygen required for the oxidation of CBA and PAMBA. When the oxygen concentration in the off-gas is lower than Q, o 5 vol 1, the amount of colored substances in the resulting terephthalic acid increases. On the other hand, if the amount of oxygen supplied is excessively increased, the amount of acetic acid burned increases, which is economically unfavorable. Therefore, as mentioned above, it is advantageous in terms of operation control to use the pressurized off-gas from the main reaction process or to use diluted air to use K so that a very small amount of oxygen is present in the off-gas. It is usually sufficient to carry out the treatment in the second purification step for 5 to 240 minutes. Further, the second purification step can be carried out using a conventional stirring tank, bubble column, or the like.

As detailed above, in the method of the present invention, the main reaction step, the first
It is essential to carry out the purification step and the second purification step sequentially, but methods for doing this include carrying out each step in a four-part method, or conducting one or two of each step in a batch method, etc. Preferably, the entire process is carried out in a continuous manner, each step is carried out in a separate reactor or treatment tank, and an amount of liquid corresponding to the amount of slurry sent from each reactor or treatment tank is continuously or This is a system in which slurry is intermittently received from the previous process or raw material tank, and the amount of slurry in each reactor and processing tank is kept approximately constant at all times.

Following the second purification step, the slurry is separated into terephthalic acid and mother liquor based on a known method, but prior to solid-liquid separation, it is preferable to pass the slurry through a receiving tank with lower temperature and pressure than in the second purification step. . By drying the solid-liquid separated terephthalic acid, a substance having quality and physical properties that allows direct polymerization with the glyco-μ component can be obtained.

In the present invention, the oxygen concentration in the off-gas refers to the oxygen concentration in the dry gas obtained by removing condensable components such as acetic acid and water from the off-gas discharged from the process.

PTA (baftluic acid), PAMBA (
(paraacetoxymethylbenzoic acid) refers to the concentration in the reaction mother liquor after separating terephthalic acid crystals from the reaction mixture. It is the value to be measured. Furthermore, 4-CBA (4-carboxybenz7/I/dehyde) is measured using a polarograph.

Among terephthalic acid qualities, optical density is terephthalic acid 4
This is the absorbance measured at a wavelength of 340 nm using a cell with an optical path length of 5α after dissolving g in 50 ml of 2N potassium hydroxide solution.

In addition, slurry viscosity is obtained by dispersing terephthalic acid in 1.6 times the mole of ethylene glycol and using a B-type viscometer.
Values measured in °C.

Next, the present invention will be explained with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples.

The main reaction mixture used in the following Examples and Comparative Examples was prepared as follows.

Sample 1 Titanium autoclave with SLFI stirrer and slave unit, H2O
4 wt%, paraxylene 20 wt%, cobalt acetate tetrahydrate 0.08 wt%, manganese acetate tetrahydrate 0. iwt
%, 2.8 Jli of sodium bromide 0.1 wt% acetic acid solution
/hr, air 401/II! Supplied with I+ and temperature 210
°C1 pressure 22 kg7cd.

The reaction mixture was reacted for 60 minutes, and the reaction mixture was taken out together with the reaction gas into a receiving tank with a condenser and quickly cooled to avoid high temperature oxygen deficiency.It has the following quality composition.

Amount of terephthalic acid in mixture 1# 250g□, PTA concentration in mother liquor 6100ppm 4CB in mother liquor
A concentration: 1510 ppm Total concentration: as4 oppm 10 og of the terephthalic acid cake in the reaction mixture was washed with hot acetic acid, and the quality was acBA 2100 ppm □ Optical density 0.525 □ Average particle size 138 μm. Also, the reaction rate of paraxylene was It was 988%.

゛Comparative Example A 5Il titanium autoclave A and A and a similar device B equipped with a reflux cooling device, a stirring device, a slurry charging device, a bulk 7-alkylbenzene supply device, and a treated waste outlet □
were connected with a slurry transfer pipe.

AK￥ Reaction mixture sample 1 was continuously fed so that the average residence time was 1 hour, and the stirring blade 180 was fed while feeding it to the next B.
It rotates at 0r, p, m, and supplies air so that the oxygen concentration in the off-gas is 4 to 5vo1%, and then the temperature is 190℃.
Crush at 9 degrees Celsius at 12 degrees Celsius, send it to B through the Lee transfer pipe, rotate at 230'Q, 351 g/d pressure and stirring blades at 850 r, p, m, and remove the oxygen in the off-gas. Nitrogen so that the concentration is 0.6-0.9vo1% 9
The treatment was carried out for a retention time of 30 minutes while supplying a mixed gas of 5 vol. 5 vol. 1% and oxygen 5 vol. 1%. After treating in this manner for 7 hours, apparatuses A and B were cooled to room temperature while being careful not to become oxygen deficient. The average particle size of the solids taken out from device A was 45 μ, and the 4CBA concentration in the mother liquor was 26 pp.
It was m. In addition, the slurry taken out from apparatus B was separated into solid and liquid, and washed with hot acetic acid (4 times the weight of the solid), and the quality of the purified terephthalic acid obtained was as follows.

4CBA 210ppm Optical density 0.079 Average particle size 136μ Slurry viscosity 765c,p Example In the comparative example above, a small hydrocyclone was installed between autoclaves A and B, and the contents of A were supplied to the hydrocyclone, and the liquid A cyclone struck. The average particle size of the solid content of the slurry extracted from A during continuous processing is 85
μ, the 4CBA concentration in the mother liquor was 40 ppm. Terephthalic acid in mixture 1 kQ was 350 g. In addition, the average particle size of the solid content of the slurry extracted from the overflow of the hydrocyclone during the treatment was 20μ, terephthalic acid in mixture 1 and 4 was 255g, and B in the mother liquor
The quality of purified terephthalic acid was as follows.

4CBA I70ppm Optical density 0.078 Average particle size 140μ Slurry viscosity 750c,p Note that no difference was observed between the example and the comparative example regarding the amount of acetic acid burned. The amount of acetic acid burned can be obtained by measuring the amount of carbon components (carbon dioxide, carbon oxide, methane, methyl acetate) in the gas discharged during the main reaction and during purification.

Patent applicant: Kuraray Yuka Co., Ltd. Agent: Patent Attorney Ken Honda

Claims (1)

[Claims] 1. In a method for producing terephthalic acid by reacting paraxylene with molecular oxygen in an acetic acid solvent in the presence of a heavy metal and a bromine compound, 90% or more of paraxylene is oxidized. After the main reaction step, 1) The main reaction step oxidation reaction product is blown into the oxidation reaction product under a temperature condition of 140 to 250°C while blowing oxygen into the off-gas so that the oxygen concentration in the off-gas is 0.5' VO 1% or more. The oxidation reaction product is crushed to reduce the average particle size of terephthalic acid to 2
A first purification step of reducing the amount by 0 or more. 2) First refining step A: Supply the crushed slurry to a liquid cyclone, supply the overflow of the liquid cyclone to the second refining step below, and circulate the under 70- of the liquid cyclone to the first refining step. 3) While blowing molecular oxygen into the over flow free of the liquid cyclone so that the oxygen concentration in the off gas is 0.05 to 5 vol%, the temperature is within the range of 180 to 300 C and at least lower than the first purification step. A second purification step of treating the above-mentioned crushed slurry under a temperature condition of 10°C higher is carried out sequentially, and the resulting purified slurry is subjected to solid-liquid separation to obtain terephthalic acid. Suitable method for producing terephthalic acid.