JPH0648982A - Production of high-purity terephthalic acid - Google Patents

Abstract

PURPOSE:To obtain high-purity terephthalic acid having excellent whiteness by oxidizing p-xylene with molecular oxygen in a solvent in the presence of a specific catalyst in a liquid phase, replacing the mother liquor in the slurry with acetic acid, dissolving under heating and bringing the resultant substance into contact with a catalyst of a noble metal of the group VIII. CONSTITUTION:p-Xylene is oxidized with molecular oxygen in a solvent of acetic acid in the presence of a cobalt or manganese-containing catalyst in a liquid phase at 170-230 deg.C under 10-30 atmospheric pressure and the mother liquor in the slurry of oxidation reaction outflow is replaced with acetic acid having 1-50wt.% (preferably 3-20wt.% water content). The prepared terephthalic acid/acetic acid slurry is dissolved under heating at 230-320 deg.C (preferably 240-300 deg.C), brought into contact with a noble metal of the group VIII in the presence of hydrogen under 20-80kg/cm<2>.G, cooled and crystal of terephthalic acid is precipitated, separated and dried to give the objective terephthalic acid. The mother liquor obtained in the separation of crystal of terephthalic acid is circulated, returned to the first process and used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for producing high-purity terephthalic acid having excellent whiteness, which is useful as an intermediate raw material for polymers such as polyester resins, fibers and films.

[0002]

BACKGROUND OF THE INVENTION Terephthalic acid is produced by oxidizing para-xylene, and is usually a catalyst of a heavy metal salt of cobalt, manganese or the like using acetic acid as a solvent, or a catalyst containing a bromine compound or a promoter such as acetaldehyde. Is used.
Crude terephthalic acid obtained by liquid-phase oxidation usually contains a large amount of impurities such as 4-carboxybenzaldehyde (4CBA), and is not suitable for reacting with glycol as it is to form polyester. 4CBA like this
As a method for producing high-purity terephthalic acid from crude terephthalic acid containing impurities such as, there are known many methods of purifying by a reaction such as oxidation or reduction. As a hydrogenation reduction purification method, for example, Japanese Patent Publication No. 41-16860 describes a method in which an aqueous solution of crude terephthalic acid is subjected to contact treatment with hydrogen at high temperature (hereinafter, referred to as "catalytic hydrogenation"). In addition, Japanese Patent Publication No. 51-32618 and Japanese Patent Publication No. 5
An improved method is shown in 1-38698.

[0003]

High-purity terephthalic acid can be obtained by purifying the crude terephthalic acid aqueous solution obtained by oxidation by the above-mentioned hydrogenation reduction purification method, but this is carried out on an industrial scale. If you do, there are the following problems. (1) Since an aqueous solution of crude terephthalic acid crystals obtained by liquid phase oxidation is used, the overall steps are liquid phase oxidation → crystallization → separation → drying → redissolution → catalytic hydrogenation → crystallization → separation → drying. There are many processes, and large investment and complicated operation control related to these are problems. (2) Since water is used as a solvent for hydrorefining, toluic acid in the mother liquor from which the crystals have been separated and terephthalic acid accompanying it are discharged together with the mother liquor, so that a wastewater treatment facility is required.

[0004]

[Means for Solving the Problems] The inventors of the present invention have made extensive studies to overcome the above-mentioned problems in the production of terephthalic acid, and as a result, obtained by liquid-phase oxidation of paraxylene in an acetic acid solvent. Without crystallizing the slurry-like oxidation reaction effluent, the mother liquor in the oxidation reaction effluent was replaced with fresh acetic acid to give a solution of crude terephthalic acid in acetic acid, and by catalytic hydrogenation, It was found that the production process of terephthalic acid is simplified, and that the mother liquor obtained when separating terephthalic acid crystals after catalytic hydrogenation is circulated and used, the amount of wastewater is significantly reduced and the yield is improved. The present invention has been reached.

That is, according to the present invention, when para-xylene is liquid-phase oxidized with molecular oxygen to produce terephthalic acid, (a) para-xylene is added to molecular oxygen in the presence of a catalyst containing cobalt and manganese in an acetic acid solvent. (B) Substituting the mother liquor in the oxidation reaction effluent slurry from step (a) with acetic acid containing 1-50 wt% water (c) The terephthalate obtained in step (b) High-purity terephthalic acid, which comprises the steps of heating and dissolving an acid / acetic acid slurry, contacting it with a Group VIII noble metal catalyst in the presence of hydrogen, and then cooling to precipitate, separate, and dry terephthalic acid crystals. Is a manufacturing method of.

Further, as described above, by recycling the mother liquor obtained when separating the terephthalic acid crystals in the step (c) to the step (a), the amount of waste water discharged from the terephthalic acid production apparatus is remarkably reduced, and the terephthalic acid is also reduced. Yield is improved. Hereinafter, the present invention will be described in the order of steps.

(A) Liquid Phase Oxidation Step Usually, cobalt, manganese and bromine compounds are used as a catalyst in the liquid phase oxidation step. A method of using an accelerator such as acetaldehyde or methyl ethyl ketone instead of the bromine compound is also performed. This liquid phase oxidation step usually takes 3 to 2
An acetic acid solvent containing about 0% water is used. Air or oxygen is usually used as the molecular oxygen for the oxidation reaction. The reaction temperature of the liquid phase oxidation is 170 to 230 ° C,
The pressure is 10 to 30 atmospheres.

(B) Liquid Replacement Step The slurry-like reaction effluent after the liquid phase oxidation step contains 4CBA, toluic acid, a catalyst and various other impurities in addition to terephthalic acid crystals. In this step, this reaction effluent is introduced into an ascending flow of acetic acid, and the mother liquor of the oxidation reaction is taken out upward together with the ascending liquid flow of acetic acid, and terephthalic acid crystals are precipitated in acetic acid and taken out together with part of acetic acid. .
The water concentration in acetic acid used in this step is in the range of 1 to 50% by weight, preferably 3 to 20% by weight. The temperature of acetic acid, that is, the temperature of the liquid replacement step is not particularly limited, but if liquid replacement is performed at a high temperature, the impurities in the extracted terephthalic acid crystals will be smaller. Further, in order to carry out liquid replacement at a temperature significantly lower than the oxidation reaction temperature, it is necessary to install an appropriate crystallization tank between the oxidation step and the liquid replacement step. Therefore, it is preferable that the temperature of the liquid replacement step is generally within the oxidation reaction temperature −50 ° C.
The pressure is sufficient to keep the solvent in the liquid phase. The flow rate of the ascending liquid flow of acetic acid varies depending on the structure of the apparatus, the size of the crystal, etc., but is preferably about 0.001 to 0.01 m / sec. If the flow rate is too low, the replacement of the mother liquor will be insufficient. On the other hand, if the flow rate is too high, some terephthalic acid crystals will not fully settle and will be taken out upward together with the rising liquid flow, which is not preferable.

The structure of the apparatus for separating the crystal and the reaction solvent is preferably columnar in order that the rising liquid flow of acetic acid rises at a certain linear velocity without backmixing. In order to achieve such a purpose, a tower having baffles inside or a perforated plate tower is suitable. A stirrer is not necessarily required in the column, but a stirrer should be provided in order to improve the contact between acetic acid and crystals and to remove the reaction mother liquor entrained in the crystals in the process of suspending and settling the crystals in acetic acid. It is effective. In order to achieve such an object, a so-called RDC tower (Rotary)
Use of the Disc Contactor is recommended. The liquid stream obtained from the top of the liquid displacement tower is mainly as it is,
Alternatively, after appropriate treatment such as concentration, it is reused in step (a) as an oxidation reaction solvent. The acetic acid solvent slurry liquid stream of terephthalic acid extracted from the bottom of the column is guided to step (c).

(C) Catalytic hydrogenation step The acetic acid solvent slurry of terephthalic acid obtained above is dissolved by heating, and catalytic hydrogenation treatment is carried out in the presence of hydrogen with a Group VIII noble metal catalyst of the periodic table. Before introducing the acetic acid solvent slurry liquid stream of terephthalic acid extracted from the bottom of the step (b) to the catalytic hydrogenation device, if necessary, in order to make the slurry concentration equal to or lower than the solubility of terephthalic acid at the treatment temperature described later. The amount of acetic acid must be added to adjust. This step is performed under high pressure and high temperature. Since the solubility of terephthalic acid in acetic acid greatly depends on the temperature, only a low concentration terephthalic acid solution can be processed at low temperatures, which is industrially disadvantageous.
It is necessary to keep at 0 ° C or higher. However, if the temperature is too high, side reactions may occur and hydrogenolysis of acetic acid may occur. Therefore, the temperature is usually 230 to 320 ° C., preferably 24.
A temperature of 0-300 ° C is chosen. The concentration of the terephthalic acid solution is preferably in the range of 10 to 30% by weight, and may be a concentration below the concentration at which the terephthalic acid is completely dissolved at the selected temperature. The pressure may be a pressure that is sufficient to maintain the liquid phase of the solvent and has a hydrogen partial pressure suitable for catalytic hydrogenation, and is usually in the range of ^{20 to} 80 kg / cm ² G. It is desirable that the amount of hydrogen is at least twice the molar amount of 4 CBA. The treatment time may be a time that is substantially sufficient for the catalytic reaction to proceed, and is usually in the range of 1 to 300 minutes, preferably 2 to 120 minutes.

After the catalytic hydrogenation, the terephthalic acid solution is filtered at a high temperature to remove a small amount of insoluble components such as fragments of the hydrogenation catalyst, and then cooled to crystallize terephthalic acid, and then the crystals are solidified. Separate the liquid. Highly pure terephthalic acid with excellent whiteness can be obtained by washing the separated crystals and drying them. The mother liquor from which the crystals were separated was refiltered and then part of it was removed.
It is recycled to the oxidation step (a), and the rest is recycled to the catalytic hydrogenation step (c). Since the terephthalic acid and p-toluic acid are dissolved in the refiltered filtrate and recovered as crude terephthalic acid in the oxidation step, the yield is improved.

The re-filtration of the separated mother liquor must be carried out at a temperature at which crystals do not precipitate, and a method of passing a porous filter medium is preferable. The reason why such filtration treatment is necessary is that the Group VIII noble metal catalyst used for catalytic hydrogenation exerts an inhibitory action on the liquid phase oxidation reaction. Open holes for filtration
A preferred method is to pass a filter made of a porous material having a diameter of 0.1 to 5 μm. In particular, it is desirable to use a porous material such as carbon, ceramics, glass, and sintered metal as the anticorrosive base material when filtering at high temperature.

[0013]

EXAMPLES The present invention will now be described with reference to examples. However, the present application is not limited to these examples. The OD ₃₄₀ value indicating a coloring impurity was determined by dissolving 2 g of terephthalic acid crystal in 25 ml of 2N potassium hydroxide solution and then measuring the wavelength at a wavelength of 34 with a 50 mm wide glass cell.
Absorbance at 0 mμ.

Example 1 (a) Oxidation step Acetic acid and cobalt acetate were placed in a pressure-resistant titanium reactor equipped with a stirrer, a reflux cooling device, and a heating device, and having an air inlet, an exhaust gas outlet, and a reflux liquid reflux inlet. After manganese acetate and hydrogen bromide were charged and the temperature was raised, raw material paraxylene and air were introduced to carry out an oxidation reaction. The reaction temperature was 200 ° C.

(B) Liquid replacement step A cylindrical separation column made of titanium (inner diameter 25 mm, effective length) having an oxidation reaction effluent inlet directly connected to the oxidation reactor, a thermal acetic acid inlet and outlets from the upper and lower portions. 1500 mm)
Is installed, and the oxidation reaction effluent inlet is 750 mm from the bottom.
And the hot acetic acid inlet was 200 mm from the bottom. To the upper and lower outlets of this separation column, receivers each equipped with a reflux condenser having an internal volume of 50 L, a stirring device and a liquid level detector were connected. When introducing the oxidative reaction effluent, the separation tower is heated in advance so as to be maintained at 200 ° C, and hot acetic acid at 200 ° C (containing 10% by weight of water) is supplied from the hot acetic acid inlet at a rate of 5.7 kg per hour. Then, when the tower was filled with hot acetic acid, it was discharged to the receiver through the upper and lower discharge ports. The oxidation reaction effluent was introduced into the separation column at a rate of 7.1 kg / hr, and 6.2 kg / hr from the top of the separation column and 6.6 from the bottom of the column.
Withdraw kg / hr (internal terephthalic acid 1.8 kg / hr). The acetic acid slurry of terephthalic acid received in the receiver at the bottom of the tower was cooled to room temperature and extracted.

(C) Catalytic Hydrogenation Process As the catalytic hydrogenation device, a flow-through type device comprising a hydrogen supply port, a remelting tube for terephthalic acid crystals, a catalyst layer, a crystallization tank and a heating device was used. The catalyst was charged with 200 ml of a granular palladium catalyst (supporting 0.5 wt% of palladium) in which palladium was supported on coconut shell activated carbon (4 to 8 mesh). The reaction temperature was kept at 240 ° C. A mixture obtained by adding 200 parts by weight of acetic acid (containing 10% by weight of water) to 100 parts by weight of the terephthalic acid acetic acid slurry extracted in the step (b) is supplied at a rate of 1200 kg / h and hydrogen at a rate of 2.4 nL / h. Then reacted. The reaction product was transferred to a crystallizer at 100 ° C. to precipitate terephthalic acid crystals and then extracted.
The resulting slurry was washed with acetic acid, separated by filtration, and dried to obtain terephthalic acid. As a result of analyzing the obtained terephthalic acid by polarogov, 4CBA was not detected, and p-toluic acid was analyzed by gas chromatography after methyl esterification, but this was also not detected. OD
_{The 340} was 0.1.

Example 2 In Example 1, the filtered mother liquor from which the terephthalic acid crystals were separated was heated to 100 ° C. and refiltered with a 0.5 μm carbon filter, and then the resulting acetic acid solution was subjected to a liquid phase oxidation step. Used as the solvent. As a result, the reaction conditions of the liquid phase oxidation and catalytic hydrogenation steps were completely the same as when fresh acetic acid was used, and an equivalent product was obtained.

[0018]

By using the method of the present invention, the crystallization, separation, drying and re-dissolution steps after the oxidation reaction are unnecessary in the above-mentioned terephthalic acid production step. Therefore, the manufacturing process of terephthalic acid is simplified, the investment amount is reduced and the operation is simplified. In addition, the separated mother liquor after catalytic hydrogenation can be reused as a solvent for the liquid-phase oxidation reaction, so the amount of wastewater is significantly reduced, and by this, terephthalic acid and p-toluic acid in the separated mother liquor are recovered as terephthalic acid. The rate is improved.

Claims (2)

[Claims] 1. When liquid phase oxidation of para-xylene with molecular oxygen to produce terephthalic acid, (a) liquid phase oxidation of para-xylene with molecular oxygen in the presence of a catalyst containing cobalt and manganese in an acetic acid solvent. Step of oxidizing (b) Step of replacing mother liquor in the oxidation reaction effluent slurry from step (a) with acetic acid containing 1 to 50% by weight of water (c) Terephthalic acid / acetic acid obtained in step (b) A method for producing high-purity terephthalic acid, which comprises the steps of dissolving a slurry by heating, subjecting the slurry to contact treatment with a Group VIII noble metal catalyst in the presence of hydrogen, and then cooling to precipitate, separate, and dry terephthalic acid crystals. 2. The mother liquor obtained when separating the terephthalic acid crystals of step (c) is recycled to step (a).
Of high-purity terephthalic acid