KR100881166B1 - Method of preparing terephthalic acid - Google Patents

Abstract

The present invention relates to a method for producing terephthalic acid, the method comprising the step of oxidizing paraxylene with a gas containing molecular oxygen at a temperature of 165 to 180 ℃ in the presence of a solvent containing a catalyst comprising the solvent Preparing a crude terephthalic acid composition, performing a solvent exchange in the crude terephthalic acid composition using a hydrocyclone, heating the solvent-exchanged crude terephthalic acid composition to 220 to 250 ° C, and purifying oxidation using oxygen in air. Include.

The method for producing terephthalic acid of the present invention can produce high purity terephthalic acid with a low content of impurities such as 4-carboxybenzaldehyde in a simple process.

Terephthalic Acid, Oxidation, Solvent Exchange

Description

Method for producing terephthalic acid {METHOD OF PREPARING TEREPHTHALIC ACID}

1 is a diagram schematically showing a process for producing terephthalic acid of the present invention.

[Industrial use]

The present invention relates to a method for producing terephthalic acid, and more particularly, to a method for producing terephthalic acid which can produce terephthalic acid in high purity.

[Prior art]

Terephthalic acid is commercially prepared by an oxidation process in which paraxylene is reacted with oxygen in the air using a catalyst, usually in an acetic acid solvent. Cobalt, manganese, bromine, and the like are used as the catalyst. However, in this case, the produced terephthalic acid is low purity crude terephthalic acid (crude terephthalic acid), and contains a large amount of organic impurities such as high concentration of 4-carboxybenzaldehyde is not suitable for direct use in the production of polyester films and fibers.

Therefore, it is necessary to purify and use the above terephthalic acid. As a purification method which is commercially performed, a hydrogenation purification method and an oxidation method are known.                         

The hydrogenation purification method is a method of preparing terephthalic acid of high purity through the hydrogenation reaction after dissolving the crude terephthalic acid under high temperature and high pressure, and then under a noble metal catalyst such as palladium. The oxidation method is a method of producing terephthalic acid with high purity by purifying the produced crude terephthalic acid through a secondary or tertiary oxidation reaction. Among the hydrogenation purification method and the oxidation method, in general, the oxidation method is excellent in terms of economics because the raw material consumption and the initial investment cost is less than the hydrogenation method, many studies have been made.

Known oxidation methods are classified into two types according to the reaction temperature: first high temperature method and second low temperature method.

In the high temperature method, paraxylene is oxidized at a high temperature of 180 ° C. or higher, and then the oxidation product is subjected to secondary oxidation. Subsequently, the secondary oxidation reaction product is heated to a high temperature of 250 ° C. or higher to carry out the tertiary oxidation reaction and subjected to cooling crystallization. The obtained crystallization product is a method of obtaining a purified terephthalic acid by performing a two-step solvent wash through a separator such as centrifugation or vacuum filtration.

The low temperature process oxidizes paraxylene at a relatively low temperature of up to 160 ° C. and solvent exchanges the oxidized product at high temperature using a centrifuge. Subsequently, the obtained product is subjected to a secondary oxidation reaction and cooled crystallized. The crystallized product is centrifuged or vacuum filtered and then dried to obtain purified terephthalic acid.

The high temperature method, the first of the two methods described above, has a good effect of increasing the particle size due to the high temperature of paraxylene oxidation, while removing impurities co-precipitated in particles such as 4-carboxybenzaldehyde during particle formation. It has a hard disadvantage. Therefore, the 4-carboxybenzaldehyde content is present in a large amount, such as 250 ppm in the final product through a conventional purification process. In addition, in order to convert 4-carboxybenzaldehyde in the particles into terephthalic acid during purification, the purification oxidation temperature must be maintained at a high temperature, thereby increasing the energy cost and performing an oxidative purification reaction at a high temperature without solvent exchange, thereby causing side reactions such as burning of acetic acid. There is a disadvantage in that the consumption of raw materials and subsidiary materials is large.

In addition, since the second low temperature method oxidizes the para xylene at low temperature, the particle size is small and the solvent exchange is performed at a high temperature, so that an expensive centrifuge made of a special alloy capable of performing solvent exchange at a high temperature is used. It is disadvantageous because the equipment cost is expensive.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide a method for producing terephthalic acid which can produce high purity terephthalic acid in a simple process.

Another object of the present invention is to provide a method for producing terephthalic acid which can economically produce high purity terephthalic acid.

In order to achieve the above object, the present invention, in the presence of a solvent containing a catalyst, oxidized paraxylene with a gas containing molecular oxygen at a temperature of 165 to 180 ℃ to prepare a joterephthalic acid composition comprising the solvent and; Conducting solvent exchange in said joterephthalic acid composition using a hydrocyclone; It provides a method for producing terephthalic acid comprising the step of heating the solvent- exchanged crude terephthalic acid composition to 220 to 250 ℃ purified purification reaction.

Hereinafter, the present invention will be described in more detail.

The present invention relates to a method for producing terephthalic acid, which is cost-competitive and can improve particle size by solving the disadvantages of the conventional method for producing terephthalic acid using additional oxidation, and thus, the initial investment cost is small and the amount of raw materials and subsidiary materials used is small.

Hereinafter, a method for producing terephthalic acid of the present invention will be described with reference to FIG. 1.

1 is a schematic diagram illustrating a commercial process for preparing terephthalic acid. A unit process for exchanging paraxylene, which is the main subject of the present invention, with an acetic acid solvent and an acetic acid solvent from a slurry of crude terephthalic acid and acetic acid, which is an oxidation reactor product, is illustrated. Is a diagram of a high-purity terephthalic acid manufacturing process mainly composed of a secondary oxidation reaction process, which is a purification of crude terephthalic acid.

Paraxylene is oxidized with a gas containing molecular oxygen at a temperature of 165 to 180 ° C. in the presence of a solvent comprising a catalyst. The oxidation reaction is performed by paraxylene reacting with oxygen in the air. The reactor used in the oxidation reaction process uses an oxidation reactor 1, such as a single or a plurality of oxidation vessels, which promote the oxidation reaction using a stirrer.                     

At this time, the particle size of the resulting crude terephthalic acid is maintained in 70㎛ to 90㎛. In general, when the particle size of the crude terephthalic acid produced in the oxidation reactor of para xylene is large, the purification reaction requires a higher temperature condition in order to oxidize impurities in the particles, thus increasing the amount of raw materials and subsidiary materials such as solvent burning. In addition, if the particles of the Zoterephthalic acid is too small, the growth of the particle size in the subsequent purification and crystallization process is limited because it is limited to its use should maintain a certain size or more. Therefore, in the present invention, the para-styrene oxidation reaction conditions are optimized to have a size of 70 to 90 μm, which is an optimum particle size in consideration of these physical properties.

Cobalt, manganese and bromine are used as the catalyst, and bromine also serves as a reaction initiator. The cobalt concentration in the solvent is preferably 400 to 2500 ppm, and the manganese concentration is preferably 20 to 80 ppm. When the concentration of cobalt catalyst is higher than 2500ppm, the conversion to terephthalic acid is improved, but the combustion side reaction of acetic acid and paraxylene increases, which increases the production cost.In the case of manganese catalyst, when the concentration is higher than 80ppm, the oxidation reaction is inhibited. .

In addition, the ratio of bromine to the total weight of the cobalt and manganese is preferably 1: 0.3 to 1 weight ratio. If the proportion of bromine is lower than 0.3 weight ratio, the reactivity is lowered. If the proportion of bromine is higher than 1 weight ratio, it is not preferable because it increases the side reaction and the corrosion of the equipment.

Acetic acid may be mainly used as the solvent.

Due to the reaction heat generated during the oxidation reaction, the solvent and the reaction product water and the like boil together with gas such as nitrogen and carbon dioxide and are discharged to the upper portion of the reactor to pass through the heat exchanger 11. In general, low pressure steam is produced using the heat of reaction, which is used as a steam source in the dehydration distillation and drying process, and low pressure steam is used as a power source for driving an air compressor.

The aqueous solvent solution condensed through the heat exchanger 12 at the top of the reactor is sent to the dehydration process 13, while the gaseous components are sent to the absorption tower 12 for low boiling point separation. The solvent aqueous solution is recovered and sent to the dehydration process 13 at the lower part of the absorption tower, and the gas component from which the low boiling point organic substances are removed is discharged to the upper part. Meanwhile, the terephthalic acid discharged from the bottom of the reactor is discharged to the crude terephthalic acid composition in the form of a slurry together with the solvent.

The aqueous solution of the solvent contains not only a large amount of catalyst components but also many organic impurities such as by-products generated during the reaction. For this reason, the content of the catalyst and the organic impurities should be lowered below a certain level in order to prevent the increase of solvent burning by the high concentration of cobalt catalyst and the reduction of the quality due to the organic impurities.

To this end, a solvent exchange process is performed in which a part of the solvent contained in the slurry discharged from the reactor 1 is replaced with a purified solvent. The solvent exchange process uses a hydrocyclone (2) to exchange the oxidized solvent contained in the crude terephthalic acid composition with a purified solvent. The solvent exchange step is carried out at the same high temperature as the oxidation reaction, that is, 165 to 180 ° C. As such, the solvent exchange process at a high temperature can not only increase the organic impurity cleaning effect but also lower the catalyst concentration in the solvent, thereby reducing the burning of the solvent during the high temperature oxidative purification reaction in a later process. .

The hydrocyclone has the advantage of low operating costs and easy maintenance because it uses only the power of the input pump is low equipment cost. Hydrocyclones used for solvent exchange are usually composed of multiple stages, and the solvent exchange is carried out through such multi-stage hydrocyclones to purified solvents. The liquid component separated from the solid phase of terephthalic acid in the latter stage is mixed with the continuously introduced terephthalic acid composition and introduced into the hydrocyclone.

Purifying solvents used for solvent exchange, preferably dehydrated acetic acid, are used. At this time, the solvent exchange rate can be more than 60% and the solvent exchange rate can be adjusted by the amount of purified solvent inflow according to the catalyst concentration of the para xylene oxidation reaction.

A portion of the high temperature mother liquor separated from the hydrocyclone is recycled to the oxidation reactor without cooling, and the other portion removes the organic impurities dissolved in the solvent and recovers the catalyst.

The solvent exchanged crude terephthalic acid composition is heated to a range of 220 to 250 ° C. via a preheater 4 via a buffer tank 3. Subsequently, the heated joterephthalic acid composition is introduced into a purification oxidation reactor (5) and subjected to purification oxidation reaction. The heat source for raising the composition temperature is supplied from the fruit boiler and the heated slurry is mixed with air on the reactor input line and introduced into the reactor. In the reactor, some of the terephthalic acid particles are dissolved and organic impurities such as 4-carboxybenzaldehyde present in the particles react with oxygen to be converted to terephthalic acid, the final material, so that the content of 4-carboxybenzaldehyde in the final product can be maintained at 150 ppm or less. Do.

The gaseous components discharged to the top of the reactor are condensed and sent to the solvent dehydration process 13 for solvent purification. Subsequently, the terephthalic acid slurry discharged from the purification oxidation reactor 5 is crystallized. This crystallization process is carried out using two stage crystallizers (6, 3), the first crystallizer (6) is maintained at a lower pressure than the purified oxidation reactor (5) to evaporate the solvent through flashing Crystallization is carried out. In addition, the second crystallizer 3 performs the role of cooling to the temperature at which the filtration process 8, which is a post process, is carried out as well as particle formation in a vacuum state. Through this crystallization process, the particle size of terephthalic acid is improved. The average particle size in the final crystallizer is maintained at the level of about 90 μm. The subsequent process separates the solid and liquid from the terephthalic acid and the mother liquor aqueous solution purified in a conventional manner through a vacuum filtration apparatus, using a conventional rotary vacuum filter. The obtained filtrate is put into a drying apparatus in order to remove the aqueous solvent solution from the obtained filtrate to obtain dried terephthalic acid. As the drying apparatus used, a general rotary dryer may be used. This process yields high purity terephthalic acid.

The present invention will be described in more detail with reference to the following examples and comparative examples. In addition, the experimental method and terms used in the present specification are described in detail below, but the examples do not limit the scope of the present invention.

(Example 1)                     

Acetic acid solvent and paraxylene, including the liquid reactants cobalt, bromine and manganese catalyst, were introduced into the oxidation reactor at a constant flow rate using a metering pump. Air, which is a gaseous reactant, was compressed to a desired pressure using a compressor, and then injected into the oxidation reactor by a predetermined amount through a flow control valve.

The temperature of the oxidation reactor was heated to a reaction temperature of 175 ° C using a heating jacket, and a pressure control valve was installed at the rear end of the condenser to adjust the reaction temperature and the pressure.

Zoterephthalic acid slurry that was oxidized in the oxidation reactor was collected. The slurry was subjected to filtration, washing, and drying, followed by quality analysis of organic impurities (4-carboxybenzaldehyde, paratoluic acid, etc.) content and color of terephthalic acid.

(Comparative Example 1)

The same procedure as in Example 1 was carried out except that the reaction temperature was changed to 150 ° C.

(Comparative Example 2)

The same procedure as in Example 1 was carried out except that the reaction temperature was changed to 190 ° C.

The content of the catalyst and the content of 4-carboxybenzaldehyde, which is an impurity, contained in the crude terephthalic acid prepared in Example 1 and Comparative Examples 1 and 2 are shown in Table 1 below. In Table 1 below, the concentration of cobalt and manganese refers to the weight concentration of each atom (cobalt, manganese) based on the solvent weight, and the concentration of bromine, which serves as a reaction initiator, is also based on the solvent. Refers to the weight of bromine atom contained.

Comparative Example 1 Example 1 Comparative Example 2 Reaction temperature [℃] 150 175 190 catalyst Cobalt [ppm] 2500 1000 500 Cobalt / manganese 20 20 1.2 Bromine / (cobalt + manganese) 0.7 0.7 1.0 Response result 4-carboxybenzaldehyde [ppm] 670 510 420 Average particle size [㎛] 63 82 95

As shown in Table 1, the crude terephthalic acid of Comparative Example 1 subjected to the oxidation reaction at 150 ℃ high concentration of 4-carboxybenzaldehyde contained and the size of the particles are small. Therefore, the purification process for removing 4-carboxybenzaldehyde which is an impurity takes a lot of load. In addition, the particle size is so small that it is difficult to produce high quality terephthalic acid using this terephthalic acid.

Zoterephthalic acid of Comparative Example 2 oxidizes the para xylene at high temperature, so the content of 4-carboxybenzaldehyde is low, but the particle size is large, and the purification process should be performed at 250 ° C. or higher. This is because impurities such as 4-carboxybenzaldehyde are co-precipitated in the terephthalic acid particles, so that 4-carboxybenzaldehyde and dissolved oxygen can be contacted only through dissolution.

On the other hand, the crude terephthalic acid of Example 1 has a low content of 4-carboxybenzaldehyde, the particle size is a condition that can obtain a high-quality high-purity terephthalic acid through the purification process and crystallization and has a physical property that does not place a great burden on the purification process. Can be.                     

(Example 2 and Comparative Example 3-4)

Purified oxidation was carried out using the crude terephthalic acid produced in Example 1. The same apparatus as used in Example 1 used for this reaction was used. In the case of the crude terephthalic acid used in Comparative Example 3 and Example 2, the solvent in the crude terephthalic acid slurry produced in Comparative Examples 1 and 1 was used by solvent exchange with 80% by weight. In the case of Comparative Example 4, the slurry produced in Comparative Example 2 was used as it was in the purification reaction.

Comparative Example 3 Example 2 Comparative Example 4 Reaction temperature [℃] 210 240 260 4-carboxybenzaldehyde [ppm] 670 510 420 Catalytic conditions Cobalt [ppm] 500 200 500 Cobalt / manganese 20 20 1.2 Bromine / (cobalt + manganese) 0.7 0.7 1.0 Reaction result 4-carboxybenzaldehyde [ppm] 35 38 46 Average particle size [㎛] 68 91 102

It can be seen that the terephthalic acid of Comparative Example 3 did not grow properly even after the purification reaction. In addition, although the terephthalic acid of Comparative Example 4 used the low content of 4-carboxybenzaldehyde in Comparative Example 2, the content of 4-carboxybenzaldehyde contained in the final terephthalic acid is rather higher than that of Example 2 and Comparative Example 3. . This is due to the large size of the Zoterephthalic acid particles of Comparative Example 2, 4-carboxybenzaldehyde co-precipitated inside the particles seems to be present in an unreacted state without melting in acetic acid solvent.

The terephthalic acid of Example 2 is slightly higher than Comparative Example 3 in the case of 4-carboxybenzaldehyde, but it can be seen that much better conditions in the average particle size.                     

That is, the method of Example 2 can maintain a lower catalyst concentration than other comparative examples has the advantage of reducing the acetic acid combustion reaction during the metal content and purification reaction in the final product.

As described above, the method for producing terephthalic acid of the present invention can produce high purity terephthalic acid having a small content of impurities such as 4-carboxybenzaldehyde in a simple process.

Claims (3)

In the presence of a solvent comprising a catalyst, oxidizing paraxylene with a gas containing molecular oxygen at a temperature of 165 to 180 ° C. to prepare a crude terephthalic acid composition comprising the solvent; Conducting solvent exchange in said joterephthalic acid composition using a hydrocyclone; The solvent-exchanged joterephthalic acid composition is heated to 220 to 250 ° C to purify and oxidize using oxygen in the air. A method for producing terephthalic acid comprising a step. The process according to claim 1, wherein cobalt, bromine and manganese are used as the catalyst. The method according to claim 2, wherein the cobalt concentration in the solvent is in the range of 400 to 2500 ppm, the manganese concentration is in the range of 20 to 80 ppm, and the weight ratio of bromine to the cobalt and manganese is in the range of 1: 0.3 to 1.

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