JP2899927B2 - Method for producing purified terephthalic acid - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing purified terephthalic acid, and more particularly, to a method for producing crude terephthalic acid by subjecting alkylbenzene to liquid phase oxidation with molecular oxygen in a solvent comprising a lower aliphatic carboxylic acid. Was dissolved in hot water, and this solution was brought into contact with a Group 8 metal, cooled, and subjected to crystallization twice to separate purified terephthalic acid and aromatic carboxylic acids as other impurities from the mother liquor. The amount of organic substances in the mother liquor was reduced, and then the mother liquor was extracted with p-xylene at a low temperature to obtain purified terephthalic acid and the production of purified terephthalic acid in which the organic substances remaining in the mother liquor were reduced. About the method.

[0002]

2. Description of the Related Art The production of terephthalic acid by liquid phase continuous oxidation of alkylbenzene with molecular oxygen in the presence of a transition metal compound and a bromine compound in a solvent composed of a lower aliphatic carboxylic acid is an industrial process. It is being conducted on a large scale. The crude terephthalic acid thus obtained is usually
Since it contains a considerable amount of impurities, it is conventionally purified and then used as a raw material for polyester. As a method for purifying such crude terephthalic acid, for example, as described in JP-B-41-16860, crude terephthalic acid is dissolved in hot water, and this solution is dissolved in palladium or the like in the presence of hydrogen. It is known to contact a Group VIII metal and then cool the solution to crystallize purified terephthalic acid.

However, according to such a method, the mother liquor after separation of purified terephthalic acid contains valuable substances such as aromatic carboxylic acid, which is a terephthalic acid derivative, in addition to terephthalic acid equivalent to solubility. It contains a significant amount of p-toluic acid, which can be converted to terephthalic acid by oxidation. Therefore, discarding such a mother liquor as it is results in the loss of valuable resources such as p-toluic acid and the like. The high oxygen demands of the environment lead to environmental pollution.

Generally, as a method of recovering valuable resources from a mother liquor after separating a target substance and reducing entrainment of an organic substance which is a source of environmental pollution, conventionally, the mother liquor is brought into contact with an organic solvent and dissolved in the mother liquor. A method of extracting an organic substance in an organic solvent is well known. In particular, in order to recover p-toluic acid from the mother liquor in the above-mentioned industrial production of terephthalic acid, the mother liquor is treated with p-xylene, as described in U.S. Patent No. 4,500,732.
A method of extracting p-toluic acid into p-xylene is useful. However, according to this method, a part of p-xylene is entrained in the mother liquor during extraction of the mother liquor with p-xylene, and p-xylene is lost. There is a problem that it cannot substantially reduce environmental pollutants accompanying the mother liquor because it becomes a pollution source.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems in the conventional production of purified terephthalic acid, and is intended to be used for extracting a mother liquor after separating purified terephthalic acid. -Reduce the amount of xylene,
Further, the present invention provides a method for producing purified terephthalic acid, which can reduce the amount of p-xylene entrained in the mother liquor by this extraction treatment, and thus can reduce the amount of organic substances, which are environmental pollution sources discharged together with the mother liquor. The purpose is to do.

[0006]

According to the present invention, there is provided a process for producing purified terephthalic acid, comprising the steps of reacting an alkylbenzene with a molecular oxygen in the presence of a transition metal compound and a bromine compound in a solvent comprising a lower aliphatic carboxylic acid. The crude terephthalic acid obtained by the phase oxidation is dissolved in hot water, and this solution is brought into contact with a Group VIII metal, then cooled to a temperature of 100 to 160 ° C., and subjected to a first crystallization. Separating the purified terephthalic acid and then cooling the mother liquor to a temperature of 20-80 ° C.
A second crystallization is performed to separate an aromatic carboxylic acid containing terephthalic acid, and thereafter the mother liquor is brought into contact with p-xylene at a temperature of 20 to 80 ° C. to extract and remove impurities in the mother liquor. It is characterized by.

As described above, terephthalic acid is obtained by subjecting alkylbenzene to liquid phase oxidation with molecular oxygen in the presence of a transition metal compound and a bromine compound as a catalyst in a solvent comprising a lower aliphatic carboxylic acid. The method of production is already well known and is performed industrially on a large scale. In such a method for producing terephthalic acid, p-xylene is usually used as alkylbenzene, and acetic acid is used as a solvent. As a transition metal compound as a catalyst, manganese, cobalt, iron,
Chromium, nickel, etc. transition metal bromates, benzoates,
Naphthenate, carboxylate such as acetate, acetylacetonate and the like, and as the bromine compound, manganese,
Bromine salts such as cobalt, iron, chromium, and nickel, hydrobromic acid, dibromoethylene, tetrabromoethane, and the like are used. As molecular oxygen for oxidizing alkylbenzene, pure oxygen, air, a mixture of pure oxygen and an inert gas, or the like is used.

More specifically, for example, when terephthalic acid is produced by oxidizing p-xylene, the raw material p-xylene is usually used at a ratio of 1 to 50% by weight based on the solvent. When a cobalt compound or a manganese compound and a bromine compound are used as a catalyst, these compounds may contain 10 to 5000 ppm by weight of a cobalt atom, 10 to 5000 ppm by weight of a manganese atom, and 10 to 5000 ppm by weight of a bromine atom in a solvent. It is used so that it may be 10,000 ppm by weight. When air is used as the gas containing molecular oxygen, the air is usually added to 1 kg of p-xylene.
It is supplied to the oxidation tank at a rate of 0.5 to 15N cubic meters. The reaction is usually performed at 160 to 260 ° C. and at a reaction pressure of 4 to 5
The reaction is carried out at 0 kg / square centimeter G with a residence time of 10 to 200 minutes.

The crude terephthalic acid thus obtained contains 4-carboxybenzaldehyde as an impurity.
It is contained in an amount of 0 to 10,000 ppm by weight, and is purified for use as a raw material for polyester. That is, the crude terephthalic acid is dissolved in hot water, brought into contact with a Group 8 metal at high temperature and high pressure in the presence of hydrogen, and then cooled to obtain purified terephthalic acid by crystallization. More specifically, for example, water 1
1 to 60 parts by weight of crude terephthalic acid per 1 part by weight
Dissolve at a temperature of 80 to 320 ° C., and in the presence of pure hydrogen, a mixture of hydrogen and an inert gas, and the like, under the conditions of 10 to 120 kg / cm 2 G, ruthenium, rhodium, palladium, platinum, osmium, etc. Contact with metal for 1-100 minutes. These Group VIII metals are usually used by supporting them on a carrier insoluble in a hot aqueous terephthalic acid solution, for example, activated carbon. Among these, palladium supported on activated carbon is particularly preferably used in terms of purification effect.

The hot aqueous solution of crude terephthalic acid thus treated is then subjected to a first crystallization treatment. In the first crystallization step, the hot aqueous solution is heated to 100 to 160 ° C.
And the majority of the terephthalic acid in the aqueous solution is crystallized from purified terephthalic acid, which is separated from the mother liquor. At this time, the impurity 4-carboxybenzaldehyde contained in the crude terephthalic acid, which may be volatilized and distilled off a part of the solvent, is mostly converted into p-toluic acid which is more easily removed by crystallization in the purification step. And thus,
Most of p-toluic acid remains in the mother liquor in the first crystallization step of terephthalic acid.

The mother liquor after the first crystallization step contains an aromatic carboxylic acid containing terephthalic acid in an amount of 1,000 to 10,000.
Contains about ppm by weight. Here, the aromatic carboxylic acid includes, in addition to terephthalic acid, p-toluic acid,
It shall include carboxybenzaldehyde, hydroxymethylbenzoic acid, benzoic acid, isophthalic acid, phthalic acid, trimellitic acid, and the like.

Therefore, according to the method of the present invention, the mother liquor after the first crystallization step is subjected to the second crystallization step.
In the second crystallization step, the mother liquor is cooled to a temperature of 20 to 80 ° C. to precipitate an aromatic carboxylic acid containing terephthalic acid, which is separated by an appropriate means such as filtration and sedimentation. According to the method of the present invention, the mother liquor after the second crystallization step is kept at the temperature after crystallization, that is, at 20 to 80 ° C.,
It is brought into contact with xylene for about 1 to 30 minutes, and the aromatic carboxylic acid containing terephthalic acid remaining in the mother liquor is extracted into p-xylene. Normally, p-xylene is used in the range of 0.05 to 2 liters per 1 liter of the mother liquor.

In the method of the present invention, it is important that the second crystallization step is performed at a temperature of 20 to 80 ° C. as described above, and that the subsequent extraction with p-xylene is also performed within the temperature range. is there. 80 ° C. for the second crystallization step
If carried out at a temperature exceeding 80 ° C., a large amount of aromatic carboxylic acid remains in the mother liquor, so that a large amount of p-xylene is required for the extraction treatment, and the subsequent extraction treatment is carried out at a temperature exceeding 80 ° C. Therefore, the amount of p-xylene entrained in the mother liquor also increases, so that the amount of organic substances in the mother liquor cannot be reduced. Furthermore, benzene and toluene are not suitable for use in the present invention because of their large amounts accompanying the mother liquor in the mother liquor extraction process.

After that, p-xylene may be used again as an extraction solvent after separating an aromatic carboxylic acid containing terephthalic acid by a method such as distillation, or may be used as an extraction solvent. It may be used as a raw material for producing crude terephthalic acid while containing the aromatic carboxylic acid. On the other hand, the aromatic carboxylic acid containing terephthalic acid obtained by the second crystallization step is used, if necessary, by mixing with a raw material for producing crude terephthalic acid.

[0015]

According to the present invention, crude terephthalic acid obtained by liquid-phase oxidation of alkylbenzene with molecular oxygen in the presence of a transition metal compound and a bromine compound in a solvent comprising a lower aliphatic carboxylic acid. After dissolving the acid in hot water and contacting this solution with the Group VIII metal, a first crystallization step gives purified terephthalic acid and then a second step at a lower temperature.
An aromatic carboxylic acid containing terephthalic acid is precipitated and separated by a single crystallization step.

Therefore, according to the method of the present invention, the amount of aromatic carboxylic acid in the mother liquor can be remarkably reduced, so that the amount of p-xylene used in the subsequent extraction step with p-xylene can be reduced. Since the extraction with p-xylene is performed at a low temperature, the amount of p-xylene entrained in the mother liquor can be reduced. Therefore, according to the present invention, the load in the wastewater treatment of the mother liquor can be reduced.

[0017]

EXAMPLES The present invention will be described below with reference to examples.
The present invention is not limited by these examples. Aromatic carboxylic acid in the aqueous solution was quantified by liquid chromatography, and p-xylene entrained in the aqueous solution was extracted with benzene (special grade reagent manufactured by Wako Pure Chemical Industries, Ltd.), followed by gas chromatography. It quantified by e. After extracting toluene accompanying the aqueous solution with Mitsui Hexane (manufactured by Mitsui Petrochemical Industry Co., Ltd.) and benzene with p-xylene (special grade reagent manufactured by Wako Pure Chemical Industries, Ltd.),
It was quantified by gas chromatography. In the examples, the ventilation gas amount is a value at 0 ° C. and 1 atm. Example 1 In acetic acid solvent, p-xylene was liquid-phase oxidized with air in the presence of a catalyst comprising a cobalt compound, a manganese compound and a bromine compound to obtain crude terephthalic acid. This was dissolved in hot water and brought into contact with palladium metal in the presence of hydrogen, and then cooled to 150 ° C. as a first crystallization step, and the purified terephthalic acid crystallized was separated from the mother liquor. The concentration of terephthalic acid and aromatic carboxylic acid as impurities in this mother liquor was 5300 ppm by weight.

Next, the mother liquor is cooled to 20 ° C.
In the second crystallization step, the precipitated aromatic carboxylic acid containing terephthalic acid was separated by filtration. Then, p-xylene was brought into contact with 1 liter of the mother liquor at a rate of 0.17 liter at 20 ° C. for 15 minutes in countercurrent to extract. Table 1 shows the amounts of residual aromatic carboxylic acid and p-xylene in the mother liquor. Examples 2 to 5 In Example 1, each of the second crystallization
At 40 ° C., 40 ° C., 70 ° C. and 80 ° C., and after separating the precipitated aromatic carboxylic acid by filtration, p-xylene was added at a ratio of 0.17 liter to 1 liter of the mother liquor at the above temperature. Extracted by contacting counter-current for minutes.
Table 1 shows the amounts of residual aromatic carboxylic acid and p-xylene in the mother liquor. Examples 6 to 8 In Example 1, the second crystallization step was performed at 70 ° C., and the precipitated aromatic carboxylic acid was separated by filtration.
At the above-mentioned temperature, p-xylene was brought into contact with 1 liter of the mother liquor at a rate of 0.1 liter, 0.25 liter, and 0.5 liter, respectively, for 15 minutes in countercurrent and extracted. Table 1 shows the amounts of residual aromatic carboxylic acid and p-xylene in the mother liquor.
Shown in Comparative Examples 1 and 2 In Example 1, the second crystallization step
At 140.degree. C. and the precipitated aromatic carboxylic acid was separated by filtration. At the above-mentioned temperature, p-xylene was added in a ratio of 0.17 liter to 1 liter of the mother liquor.
Extraction was performed by contacting countercurrent for 5 minutes. Table 1 shows the amounts of residual aromatic carboxylic acid and p-xylene in the mother liquor. Comparative Examples 3 and 4 In Example 1, the second crystallization step was performed at 70 ° C., and the precipitated aromatic carboxylic acid was separated by filtration.
At the above-mentioned temperature, benzene and xylene were respectively brought into contact with one liter of the mother liquor at a rate of 0.17 liter in a countercurrent for 15 minutes to extract. Table 1 shows the amount of residual aromatic carboxylic acid and the amount of benzene or xylene in the mother liquor. Comparative Examples 5 to 7 In Example 1, the second crystallization step was performed for 20 times each.
C., 40.degree. C. and 70.degree. C., and the amount of residual aromatic carboxylic acid in the mother liquor as it was separated by filtration of the precipitated aromatic carboxylic acid is shown in Table 1.

[0019]

[Table 1]

Continuation of front page (56) References JP-A-55-33458 (JP, A) U.S. Pat. No. 4,500,732 (US, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C07C 63/26 B01J 23 / 44 B01J 27/128 C07C 51/42 C07B 61/00 300

Claims (1)

(57) [Claims] A crude terephthalic acid obtained by subjecting alkylbenzene to liquid phase oxidation with molecular oxygen in the presence of a transition metal compound and a bromine compound in a solvent comprising a lower aliphatic carboxylic acid is dissolved in hot water. After contacting the solution with a Group VIII metal, the solution is cooled to a temperature of 100 to 160 ° C., and a first crystallization is performed to separate purified terephthalic acid. After cooling to a temperature of 80 ° C. and performing a second crystallization to separate an aromatic carboxylic acid containing terephthalic acid, the mother liquor is brought into contact with p-xylene at a temperature of 20 to 80 ° C. A method for producing terephthalic acid, comprising extracting and removing impurities therein.