Method using polyester depolymerization for producing highly pure terephthalic acid and highly pure terephthalic acid obtained thereby

Abstract

The present invention provides a method using polyester depolymerization for producing highly pure recycled terephthalic acid, and highly pure recycled terephthalic acid obtained thereby. The present invention relates to a method for recovering highly pure terephthalic acid, the method having higher purification efficiency due to the use of an alkylated aromatic compound that is stable even under alkaline conditions, and improved terephthalic acid conversion rate due the swelling of waste plastic having been improved by controlling solvent polarity.

Description

Method for producing high-purity terephthalic acid using polyester depolymerization and high-purity terephthalic acid obtained thereby

[0001] The present invention relates to a method for obtaining high-purity terephthalic acid from waste polyester. Furthermore, the present invention relates to the regeneration of high-purity terephthalic acid by improving the conversion rate to terephthalic acid by increasing purification efficiency using an alkylated aromatic compound with a stable structure even under alkaline conditions and controlling the polarity of the solvent to facilitate the swelling process of waste plastic.

[0002] Polyester is the most widely purchased and widely used polymer worldwide, with a production volume of well over 75 million tons annually. Its relatively low cost of manufacture and its exceptional physical, chemical, and thermal properties make it a versatile material, including clothing, carpets, and films. Polyethylene terephthalate (PET) is one of the most widely used polyesters, with a wide range of applications, including single-use applications like beverage containers. The commercial success of polyester, particularly PET, has driven efforts to recover materials from post-consumer, post-industrial, post-scrap, and other sources, and to reuse them as an alternative to primary disposal methods like landfills. As the annual global volume of polyester-based plastic waste grows increasingly unmanageable, interest in recycling or reusing waste polyester has grown. Among waste polyester, colorless and transparent PET beverage containers can be physically recycled. However, colored and composite materials, such as automotive interior materials or textiles, are difficult to recycle and have been incinerated or landfilled. Consequently, technologies are being developed to recover waste polyester and chemically recycle it as an alternative to conventional treatment methods like incineration or landfilling.

[0003] Depolymerization, a chemical recycling method for waste polyester, involves depolymerizing polyester into monomers and then re-polymerizing the resulting monomers to create new products. These depolymerization techniques can be categorized into glycolysis, methanolysis, and hydrolysis.

[0004] The above glycolysis is a method of monomerizing polyester into bis(2-hydroxyethyl) terephthalate (BHET) through a transesterification reaction using a glycol such as ethylene glycol (EG) or propylene glycol (PG) and a catalyst. This is the simplest, oldest, and easiest process to try, and many companies are developing this technology. However, glycolysis is very slow, and complete depolymerization of PET into BHET cannot be achieved unless a catalyst such as a metal salt, zeolite, or ionic catalyst is used. Since glycolysis produces a final product containing a significant amount of other oligomers in addition to the BHET monomer, it is difficult to recover the BHET monomer when it is the desired product. Since the reaction proceeds at high temperatures above 150℃, glycolysis poses problems in terms of energy efficiency and reduction of carbon emissions. Furthermore, prior to glycolysis, a pretreatment step is required to remove heavy metals and moisture from the waste, and the significant amount of activated carbon used during purification reduces economic feasibility. Furthermore, while BHET produced through glycolysis is suitable for PET synthesis, it requires an additional hydrolysis reaction to produce high-value-added polyesters such as polybutylene terephthalate (PBT), limiting its economic feasibility and versatility.

[0005] Methanollysis is a process that depolymerizes polyester into dimethyl terephthalate (DMT) and ethylene glycol through a transesterification reaction using methanol and a catalyst. While this process shares the same reaction mechanism as glycolysis, it also has the advantage of being able to react at low temperatures, below 100°C, leading to extensive commercial development. It offers the advantage of being able to produce a variety of polyesters, and the resulting monomer, DMT, is highly versatile. However, methanolysis also requires a pretreatment step to remove heavy metals and moisture from the waste prior to reaction. Furthermore, it requires sublimation refining at temperatures exceeding 200°C, making it uneconomical and environmentally undesirable. Furthermore, most polyester manufacturing facilities worldwide currently utilize polymerization facilities using terephthalic acid (TPA), posing usability challenges.

[0006] Hydrolysis is a method for depolymerizing polyester into terephthalic acid and ethylene glycol through hydrolysis using water or a solvent and a catalyst. It is broadly divided into acid hydrolysis, neutral hydrolysis, and alkaline hydrolysis. Acid hydrolysis uses water and sulfuric acid, heating the reaction to 100°C to directly produce terephthalic acid. This reaction has the advantage of producing terephthalic acid directly and eliminating the need for organic solvents. However, it carries the risk of using over 85% sulfuric acid, and the terephthalic acid produced must be re-alkaliized for purification, making it uneconomical. Neutral hydrolysis utilizes high-temperature steam to produce terephthalic acid at temperatures ranging from 200 to 300°C or using microwaves. However, this method requires specialized equipment and consumes significant energy. Similarly to acid hydrolysis, it also requires a re-alkaliization step for purification. Alkaline hydrolysis involves forming an intermediate, a terephthalate salt, using an alkaline catalyst and water or a solvent. This is followed by a purification process and acid treatment to form the final terephthalic acid. While the reaction conditions and facilities are relatively simple and mild, the purification process uses large amounts of water and solvent, making it less economical and environmentally friendly. Furthermore, the removal of unspecified impurities, such as dyes, requires large amounts of activated carbon, and high conversion rates require expensive solvents, leading to high unit costs. In other words, hydrolysis requires multiple steps to recover the terephthalic acid monomer.

[0007] Therefore, there is a continuing need for a method for regenerating waste polyester using a low-cost solvent composition that is easy to recover terephthalic acid, can recover terephthalic acid with high purity, and is highly stable even after repeated use.

[0008] [Prior Art Literature]

[0009] [Patent Document]

[0010] Domestic Patent No. 10-2462599

[0011] The problem to be solved by the present invention is to provide a method for producing terephthalic acid that can obtain high-purity terephthalic acid from waste polyester, reduce the amount of solvent used for hydrolysis, solvent recovery energy and manufacturing cost, and increase the stability of the solvent.

[0012] In order to solve the above problem, the present invention provides a method for producing high-purity regenerated terephthalic acid using polyester depolymerization, the method comprising the steps of: preparing a solvent by mixing a basic hydrolysis solvent comprising an alkylated aromatic compound and a polarity adjusting compound and an alkaline catalyst; treating waste polyester with the solvent to obtain a polyester depolymerization product; filtering the polyester depolymerization product to obtain a sludge cake; and adding purified water to the filtered sludge cake to prepare a sludge cake aqueous solution containing a terephthalate metal salt; treating the sludge cake aqueous solution with an organic solvent to obtain an aqueous solution containing a terephthalate metal salt; treating the aqueous solution with an adsorbent to purify the terephthalate metal salt aqueous solution; and treating the purified terephthalate metal salt aqueous solution with an acidic solution to precipitate solid terephthalic acid.

[0013] The method for producing terephthalic acid using polyester depolymerization according to the present invention provides a very high conversion rate to terephthalic acid, and the regenerated terephthalic acid contains virtually no impurities, thereby providing highly pure terephthalic acid. Furthermore, the amount of solvent used in the conversion to terephthalic acid and the energy required for solvent recovery can be minimized, and the solvent's high stability allows for repeated use.

[0014] Figure 1 is a graph showing the conversion rate of toluene used in Example 17 of the present invention.

[0015] Figure 2 is a graph showing the conversion rate of toluene used in Example 18 of the present invention.

[0016] Figure 3 is a graph showing the conversion rate of toluene used in Example 19 of the present invention.

[0017] Figure 4 is a graph showing the conversion rate of anisole used in comparative example 3 of the present invention.

[0018] Figure 5 is a graph showing the conversion rate of MIBK used in Comparative Example 4 of the present invention.

[0019] Hereinafter, various embodiments of the present invention will be described with reference to the accompanying drawings. It should be understood that the present invention is not limited to specific embodiments, but rather encompasses various modifications, equivalents, and / or alternatives of the embodiments of the present invention. In the description of the drawings, similar reference numerals may be used for similar components.

[0020] In this document, the expressions "have", "may have", "include", or "may include" indicate the presence of a feature (e.g., a number, function, operation, or component such as a part), and do not exclude the presence of additional features.

[0021] In this document, the expressions "A or B," "at least one of A and / or B," or "one or more of A and / or B" can include all possible combinations of the listed items. For example, "A or B," "at least one of A and B," or "at least one of A or B" can all refer to cases where (1) at least one A is included, (2) at least one B is included, or (3) at least one A and at least one B are included.

[0022] The expression "configured to" as used herein can be used interchangeably with, for example, "suitable for", "having the capacity to", "designed to", "adapted to", "made to", or "capable of". The term "configured to" does not necessarily mean "specifically designed to".

[0023] The terms used in this document are used only to describe specific embodiments and may not be intended to limit the scope of other embodiments. The singular expression may include the plural expression unless the context clearly indicates otherwise. Terms used herein, including technical or scientific terms, may have the same meaning as commonly understood by those of ordinary skill in the art described in this document. Terms defined in general dictionaries among the terms used in this document may be interpreted as having the same or similar meaning in the context of the relevant technology, and shall not be interpreted in an idealized or overly formal sense unless explicitly defined in this document. In some cases, even if a term is defined in this document, it cannot be interpreted to exclude the embodiments of this document.

[0024] The embodiments disclosed in this document are presented for the purpose of explaining and understanding the disclosed technical content and are not intended to limit the scope of the present invention. Therefore, the scope of this document should be interpreted to include all modifications or various other embodiments based on the technical concept of the present invention.

[0025] Hereinafter, preferred embodiments of the present invention will be described in detail. Prior to this, it should be noted that the terms and words used in this specification and claims should not be construed as limited to their conventional or dictionary meanings. Based on the principle that the inventor can appropriately define the concept of a term to best explain his or her invention, they should be interpreted in a way that aligns with the technical spirit of the present invention.

[0026] Accordingly, the configurations of the embodiments described in this specification are only some of the most preferred embodiments of the present invention and do not represent all of the technical ideas of the present invention, so it should be understood that there may be various equivalents and modified examples that can replace them at the time of filing this application.

[0027] Throughout the specification, whenever a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0028] Hereinafter, the present invention will be described in detail.

[0029]

[0030] A method for producing high-purity recycled terephthalic acid using polyester depolymerization according to one embodiment of the present invention may include the steps of: preparing a solvent by mixing a basic hydrolysis solvent comprising an alkylated aromatic compound and a polarity adjusting compound and an alkaline catalyst; treating waste polyester with the solvent to obtain a polyester depolymerization product; filtering the polyester depolymerization product to obtain a sludge cake; adding purified water to the filtered sludge cake to prepare a sludge cake aqueous solution containing a terephthalate metal salt; treating the sludge cake aqueous solution with an organic solvent to obtain an aqueous solution containing a terephthalate metal salt; purifying the terephthalate metal salt aqueous solution by treating the aqueous solution with an adsorbent; and treating the purified terephthalate metal salt aqueous solution with an acidic solution to precipitate solid terephthalic acid.

[0031] The step of preparing a solvent by mixing an alkaline catalyst and a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity-controlling compound is for preparing a solvent having a stable structure capable of sufficiently swelling a polyester structure and increasing the conversion rate to terephthalic acid.

[0032] Polyester is a polymer that has an ester (RO-C(=O)-R') chemical functional group in its main chain and is also called polyester.

[0033] The above polyester dicarboxylic acid may be a polymer formed by condensation polymerization with a dialcohol. The dicarboxylic acid may be any one selected from the group consisting of terephthalic acid, naphthalene dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenoxyethane dicarboxylic acid, succinic acid, adipic acid, sebacic acid, azelaic acid, decanedicarboxylic acid, cyclohexanedicarboxylic acid, trimellitic acid, and pyromellitic acid. Preferably, the dicarboxylic acid may be terephthalic acid.

[0034] The dialcohol may be any one selected from the group consisting of ethylene glycol, trimethylene glycol, 1,2-propanediol, tetramethylene glycol, neopentyl glycol, hexamethylene glycol, decanmethylene glycol, dodecamethylene glycol, 1,4-cyclohexanedimethanol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, polypropylene glycol, di(tetramethylene) glycol, tri(tetramethylene) glycol, polytetramethylene glycol, pentaerythritol, and 2,2-bis(4-βhydroxyethoxyphenyl)propane. Preferably, the dialcohol may be ethylene glycol.

[0035] The polyester may be at least one selected from the group consisting of polyethylene terephthalate (PET), polypropylene terephthalate (PPT), polyglycolide or polyglycolic acid (PGA), polylactic acid (PLA), polycaprolactone (PCL), polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), polyethylene adipate (PEA), polybutylene succinate (PBS), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), and Vectran. Preferably, the polyester may be polyethylene terephthalate (PET).

[0036] In the step of preparing a solvent by mixing an alkaline catalyst and a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity adjusting compound, the solvent is used for the basic hydrolysis of polyester and can sufficiently dissolve the alkaline catalyst and can be reused. The above-mentioned alkylated aromatic compound has a structure that is highly stable against the basic hydrolysis reaction of polyester and can produce high-purity terephthalic acid. The above-mentioned polarity adjusting compound can increase the solubility of the catalyst and have a favorable effect on the destruction of the ester functional group. The alkaline catalyst mixed in the step of preparing the solvent forms a form of a terephthalate metal salt, which is dissolved in the purified water added after filtration to create an alkaline environment, thereby helping to completely decompose monoesters, which are by-products of the hydrolysis reaction, and lead to complete hydrolysis.

[0037] In the step of preparing a solvent by mixing a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity adjusting compound and an alkaline catalyst, the above-mentioned alkylated aromatic compound may be at least one compound selected from the group consisting of toluene, xylene, trimethylbenzene, ethylbenzene, diethylbenzene, propylbenzene, dipropylbenzene, and butylbenzene. Preferably, the above-mentioned alkylated aromatic compound may be toluene. The above-mentioned alkylated aromatic compound has a stable structure even under basic conditions, so that it does not generate contaminants even through a chain reaction and enables solvent reuse.

[0038] In the step of preparing a solvent by mixing a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity-controlling compound and an alkaline catalyst, the polarity-controlling compound may be at least one compound selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, undecanol, dodecanol, ethylene glycol, propylene glycol, and butylene glycol. Preferably, the polarity-controlling compound may be ethanol. The polarity-controlling compound may serve to increase the solubility of the alkaline catalyst and control the polarity of the solvent to facilitate swelling of the polyester.

[0039] In the step of preparing a solvent by mixing a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity adjusting compound with an alkaline catalyst, the alkaline catalyst may be at least one selected from the group consisting of potassium hydroxide, sodium hydroxide, lithium hydroxide, potassium oxide, sodium oxide, and lithium oxide. The alkaline catalyst prevents a portion of the terephthalic acid being prepared from acting as an acid catalyst and forms a terephthalate-metal salt form so that it can be easily dissolved in water.

[0040] In the step of preparing a solvent by mixing a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity adjusting compound and an alkaline catalyst, the weight ratio of the above-mentioned alkylated aromatic compound and the polarity adjusting compound may be 1:0.1 to 10. Preferably, the weight ratio may be 1:0.5 to 5. Most preferably, the weight ratio may be 1:0.6 to 3. The solvent introduced for the basic hydrolysis of polyester must be able to sufficiently dissolve the basic catalyst and is affected by the polarity of the solvent. Therefore, when the weight of the above-mentioned polarity adjusting compound is less than the above-mentioned numerical range, the conversion rate into terephthalic acid is low, and the reaction must be performed under high temperature conditions to increase the conversion rate, and when it exceeds the above-mentioned numerical range, the polarity increases, making it difficult to swell the plastic.

[0041] In the step of preparing a solvent by mixing a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity adjusting compound and an alkaline catalyst, the weight ratio of the basic hydrolysis solvent and the alkaline catalyst may be 1:0.01 to 0.5. Preferably, the weight ratio may be 1:0.05 to 0.3. If the weight of the alkaline catalyst is less than the above-mentioned numerical range, the metal content derived from the catalyst may be low, thereby lowering the yield of terephthalic acid, and if it exceeds the above-mentioned numerical range, the polarity adjusting compound may be excessively mixed to dissolve the catalyst, thereby lowering the conversion rate to terephthalic acid.

[0042] In the step of preparing a solvent by mixing a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity adjusting compound and an alkaline catalyst, the basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and the polarity adjusting compound is characterized in that the conversion rate of the alkylated aromatic compound at the 10th time as a solvent reuse rate is 90% or more (see Figures 1 to 3).

[0043] The step of treating the above-mentioned waste polyester with the above-mentioned solvent to obtain a polyester depolymerization product is for eluting the terephthalate metal salt, and by treating the above-mentioned solvent, an environmentally friendly, high-purity terephthalic acid can be produced. The step may involve a saponification reaction in addition to a hydrolysis reaction using water generated by a basic hydrolysis solvent and an alkaline catalyst as a medium.

[0044] In the step of treating the above-mentioned waste polyester with the solvent to obtain a polyester depolymerization product, waste polyester such as bulk waste that has not undergone a separate pulverization step and whose ester functional group is destroyed by the treating solvent can also be depolymerized. In addition, in the process of treating the above-mentioned waste polyester with the solvent to perform basic hydrolysis, conditions such as temperature and pressure can be applied without limitation to the conditions applied in the previous hydrolysis method.

[0045] In the step of treating the waste polyester with the solvent to obtain a polyester depolymerization product, the weight ratio of the waste polyester to the solvent may be 1:1 to 50. Preferably, the weight ratio may be 1:5 to 25. Most preferably, the weight ratio of the waste polyester to the solvent may be 1:5 to 20. If the weight of the solvent is less than the above numerical range, the viscosity of the waste polyester treated with the solvent increases, so that depolymerization cannot occur smoothly, and if it exceeds the above numerical range, an excessive amount of solvent is added, which is not economical.

[0046] In the step of treating the above-mentioned waste polyester with the above-mentioned solvent to obtain a polyester depolymerization product, the above-mentioned solvent may be added so that the pH of the entire reaction becomes 7.1 or higher.

[0047] The step of filtering the polyester depolymerization product to obtain a sludge cake and adding purified water to the filtered sludge cake to prepare an aqueous sludge cake solution containing a terephthalate metal salt is to remove a solvent and dissolved impurities from the polyester depolymerization product. Since the filtered sludge cake contains the obtained terephthalate metal salt and non-reactive impurities, by adding purified water thereto, an aqueous solution containing a first-purified terephthalate metal salt can be obtained.

[0048] The above terephthalate metal salt is a combination of an alkali metal derived from a catalyst, and may be any one selected from the group consisting of dipotassium terephthalate (K2-TPA), disodium terephthalate (Na2-TPA), and dilithium terephthalate (Li2-TPA). Preferably, the above terephthalate metal salt may be disodium terephthalate (Na2-TPA).

[0049] In the step of filtering the polyester depolymerization product to obtain a sludge cake and adding purified water to the filtered sludge cake to prepare a sludge cake aqueous solution containing a terephthalate metal salt, the weight ratio of the filtered sludge cake to the purified water may be 1:2 to 20. Preferably, the weight ratio may be 1:4 to 15. Most preferably, the weight ratio may be 1:4 to 10. If the amount of the purified water added is less than the above numerical range, the filtered sludge cake becomes supersaturated, and if it exceeds the above numerical range, it has a negative impact on the environment and is economically unfeasible.

[0050] The step of treating the above sludge cake aqueous solution with an organic solvent to obtain an aqueous solution containing a terephthalate metal salt is to separate the phases by adding an organic solvent for purification.

[0051] In the step of treating the above sludge cake aqueous solution with an organic solvent to obtain an aqueous solution containing a terephthalate metal salt, the organic solvent may be at least one selected from the group consisting of pentane, hexane, heptane, octane, nonane, decane, and undecane. The organic solvent may be used alone as a hydrophobic solvent or may be used in combination with other hydrophobic solvents. In some cases, the organic solvent may be used alone as a water-soluble solvent or in combination with a hydrophobic solvent. The organic solvent may be used for the purpose of removing impurities or reaction solvents present in trace amounts in the aqueous layer after phase separation.

[0052] In the step of treating the above sludge cake aqueous solution with an organic solvent to obtain an aqueous solution containing a terephthalate metal salt, the weight ratio of the sludge cake aqueous solution and the organic solvent may be 1:1 to 20. Preferably, the weight ratio may be 1:2 to 15. Most preferably, the weight ratio may be 1:2 to 10. If the organic solvent is added in an amount less than the above numerical range, the purity of the terephthalic acid finally obtained may be lowered due to an emulsion layer between the aqueous phase and the organic phase, and if the organic solvent is added in an amount exceeding the above numerical range, terephthalic acid may be separated due to the organic phase, thereby lowering the purity. In addition, if the amount of organic solvent for purification that is discarded increases, the economic feasibility decreases and environmental pollution occurs.

[0053] The step of purifying the terephthalate metal salt aqueous solution by treating the above-mentioned aqueous solution with an adsorbent is to remove trace amounts of impurities and organic solvents.

[0054] In the step of purifying the terephthalate metal salt aqueous solution by treating the above-mentioned aqueous solution with an adsorbent, the adsorbent may be at least one selected from the group consisting of incinerator ash, activated carbon, zeolite, silicate, calcium carbonate, calcium oxide, calcium hydroxide, magnesium carbonate, magnesium oxide, magnesium hydroxide, sodium carbonate, sodium bicarbonate, and alumina.

[0055] The step of treating the above-mentioned purified terephthalate metal salt aqueous solution with an acidic solution to precipitate solid terephthalic acid is to obtain terephthalic acid by reacting the metal salt with an acid. The acidic solution may be a strong acid, and preferably sulfuric acid is used to precipitate terephthalic acid with high purity.

[0056] According to another embodiment of the present invention, the terephthalic acid produced by any one of the above methods for producing high-purity regenerated terephthalic acid using polyester depolymerization may have a purity of 99.00% or higher. Preferably, the terephthalic acid may have a purity of 99.50% or higher. The regenerated terephthalic acid produced by any one of the above methods is terephthalic acid with extremely low impurity content.

[0057] The method for calculating the conversion rate of terephthalic acid by the above manufacturing method is as follows.

[0058]

[0059] The method for calculating the yield of terephthalic acid by the above manufacturing method is as follows.

[0060]

[0061] Hereinafter, the present invention will be described in more detail through examples. These examples are intended solely to illustrate the present invention more specifically, and it will be apparent to those skilled in the art that the scope of the present invention is not limited by these examples, in accordance with the gist of the present invention.

[0062]

[0063] Examples and Comparative Examples

[0064] [Comparison of depolymerization reactions according to the weight of polarity-controlling compounds]

[0065] Example 1

[0066] A solvent was prepared by adding 8.3 g of NaOH to a solvent containing 40 g of toluene and 60 g of ethanol in a 250 mL flask (S1).

[0067] 10 g of finely crushed PET waste was added to the solvent manufactured in step S1 and stirred at 60°C for 3 hours (S2).

[0068] The PET depolymerization product after the above stirring was filtered to obtain a sludge cake, and 80 g of water was added to the filtered sludge cake to obtain an aqueous solution from which disodium terephthalate was eluted (S3).

[0069] After treating the above aqueous solution with 20 g of hexane to separate the phases, only the aqueous layer was recovered (S4).

[0070] The above-mentioned aqueous layer was treated with 0.1 g of activated carbon, stirred for 2 hours, filtered, and the purified aqueous layer was recovered (S5).

[0071] The recovered aqueous layer was treated with sulfuric acid to adjust the pH to 3 or lower, and a white solid was obtained. This was filtered, washed three times with 30 g of water, and then dried to obtain white terephthalic acid (S6).

[0072] Example 2

[0073] Terephthalic acid was obtained in the same manner as in Example 1, except that 26 g of ethanol was applied in the above step S1.

[0074] Example 3

[0075] Terephthalic acid was obtained in the same manner as in Example 1, except that 40 g of ethanol was applied in the above step S1.

[0076] Example 4

[0077] Terephthalic acid was obtained in the same manner as in Example 1, except that 100 g of ethanol was applied in the above S1 step.

[0078] Example 5

[0079] Terephthalic acid was obtained in the same manner as in Example 1, except that 4 g of ethanol was applied in the above S1 step.

[0080] Example 6

[0081] Terephthalic acid was obtained in the same manner as in Example 1, except that 20 g of ethanol was applied in the above S1 step.

[0082] Example 7

[0083] Terephthalic acid was obtained in the same manner as in Example 1, except that 200 g of ethanol was applied in the above S1 step.

[0084] Example 8

[0085] Terephthalic acid was obtained in the same manner as in Example 1, except that 400 g of ethanol was applied in the above step S1.

[0086] Comparative Example 1

[0087] Terephthalic acid was obtained in the same manner as in Example 3, except that 40 g of anisole was used instead of toluene in the above S1 step.

[0088] Comparative Example 2

[0089] Terephthalic acid was obtained in the same manner as in Example 3, except that 40 g of MIBK was used instead of toluene in the above S1 step.

[0090] PET(g)Toluene(g)EtOH(g)NaOH(g)Time(h)Obtained TPA(g)Yield(%)Conversion(%)Purity%(Acid value)Example 11040608.338.6299.71100.0099.67Example 21040268.338.5999.36100.0099.88Example 31040408.338.6299.71100.0099.86Example 410401008.338.5699.02100.0099.56Example 5104048.338.1195.6598.0699.63Example 61040208.338.1494.2699.8799.74Example 710402008.337.1496.3885.6999.66Example 810404008.336.1794.8775.2699.54PET(g)Anisole(g)EtOH(g)NaOH(g)Time(h)Obtained TPA(g)Yield(%)Conversion(%)Purity%(Acid value)Comparative Example 11040408.338.2596.5898.7899.23PET(g)MIBK(g)EtOH(g)NaOH(g)Time(h)Obtained TPA(g)Yield(%)Conversion(%)Purity%(Acid value)Comparative Example 21040408.338.1295.6598.2099.14

[0091] [Comparison of depolymerization reactions according to alkaline catalyst weight]

[0092] Example 9

[0093] Terephthalic acid was obtained in the same manner as in Example 1, except that 1 g of NaOH was applied in the above S1 step.

[0094] Example 10

[0095] Terephthalic acid was obtained in the same manner as in Example 1, except that 5 g of NaOH was applied in the above S1 step.

[0096] Example 11

[0097] Terephthalic acid was obtained in the same manner as in Example 1, except that 30 g of NaOH was applied in the above step S1.

[0098] Example 12

[0099] Terephthalic acid was obtained in the same manner as in Example 1, except that 50 g of NaOH was applied in the above step S1.

[0100] PET(g)Toluene(g)EtOH(g)NaOH(g)Time(h)Obtained TPA(g)Yield(%)Conversion(%)Purity%(Acid value)Example 11040608.338.6299.71100.0099.67Example 9104060131.0195.3512.2199.61Example 10104060533.6994.2245.3299.65Example 111040603038.5899.2110099.54Example 121040605038.6199.6110099.89

[0101] [Comparison of depolymerization reactions according to solvent weight]

[0102] Example 13

[0103] Terephthalic acid was obtained in the same manner as in Example 1, except that in the above step S1, a solvent was prepared by adding 0.8 g of NaOH to a mixture of 3.7 g of toluene and 5.5 g of ethanol.

[0104] Example 14

[0105] Terephthalic acid was obtained in the same manner as in Example 1, except that in the above step S1, a solvent was prepared by adding 3.8 g of NaOH to a mixed solvent of 18.5 g of toluene and 27.7 g of ethanol.

[0106] Example 15

[0107] Terephthalic acid was obtained in the same manner as in Example 1, except that in the above step S1, 15 g of NaOH was added to a solvent mixed with 74 g of toluene and 111 g of ethanol to prepare a solvent.

[0108] Example 16

[0109] Terephthalic acid was obtained in the same manner as in Example 1, except that in the above step S1, 20 g of NaOH was added to a solvent mixed with 92 g of toluene and 138 g of ethanol to prepare a solvent.

[0110] PET(g)Toluene(g)EtOH(g)NaOH(g)Time(h)Obtained TPA(g)Yield(%)Conversion(%)Purity%(Acid value)Example 11040608.338.6299.71100.0099.67Example 13103.75.50.830.7281.2410.2499.68Example 141018.527.73.835.4084.2174.2199.74Example 1510741111538.6199.6310099.87Example 1610921382038.6199.5810099.68

[0111] Example 17

[0112] A solvent was prepared by adding 8.3 g of NaOH to a solvent containing 40 g of toluene and 40 g of ethanol in a 250 mL flask (S1').

[0113] 10 g of finely crushed PET waste was added to the solvent manufactured in step S1 and stirred at 60°C for 3 hours (S2').

[0114] The PET depolymerization product after the above stirring was filtered to obtain a sludge cake, and 80 g of water was added to the filtered sludge cake to obtain an aqueous solution from which disodium terephthalate was eluted. After obtaining the sludge cake, the solvent filtrate containing a mixture of toluene and ethanol was stored for use in the next reaction (S3').

[0115] After treating the above aqueous solution with 20 g of hexane to separate the phases, only the aqueous layer was recovered (S4').

[0116] The above-mentioned aqueous layer was treated with 0.1 g of activated carbon, stirred for 2 hours, filtered, and the purified aqueous layer was recovered (S5').

[0117] The recovered aqueous layer was treated with sulfuric acid to adjust the pH to 3 or lower, and a white solid was obtained. This was filtered, washed three times with 30 g of water, and then dried to obtain white terephthalic acid (S6').

[0118] A solvent was prepared by adding the solvent filtrate of the mixed toluene and ethanol stored in step S3' and 4.5 g of NaOH to a 250 mL flask, and the solvent was treated with 10 g of finely crushed PET waste and stirred at 60°C for 3 hours. Thereafter, the process was repeated 10 times in total by repeating steps S3' to S6' (S7'). The conversion rate of toluene was graphed and shown in Fig. 1.

[0119] Number of turns PET(g)Toluene(g)EtOH(g)NaOH(g)Time(h)Obtained TPA(g)Yield(%)Conversion(%)11040408.338.6199.60100.00210Repeated use4.538.5298.55100.003104.538.5999.36100.004104.538.6099.48100.005104.538.5899.25100.006104.538.5598.90100.007104.538.5999.5799.798104.538.5599.2299.689104.538.5098.9199.4110104.538.5399.6399.04

[0120] Example 18

[0121] Terephthalic acid was obtained in the same manner as in Example 17, except that in the above step S1, 8.3 g of NaOH was added to a solvent containing 40 g of toluene and 28 g of ethanol to prepare a solvent. The conversion rate of toluene was graphed and shown in Fig. 2.

[0122] Number of turns PET(g)Toluene(g)EtOH(g)NaOH(g)Time(h)Obtained TPA(g)Yield(%)Conversion(%)11040288.338.6499.91100.00210Repeated4.538.6299.82100.003104.538.6299.68100.004104.538.5999.36100.005104.538.5799.2199.906104.538.6099.5899.877104.538.5999.6899.718104.538.6099.8599.669104.538.5799.6399.5110104.538.5699.9299.12

[0123] Example 19

[0124] Terephthalic acid was obtained in the same manner as in Example 17, except that in the above step S1, 8.3 g of NaOH was added to a solvent containing 40 g of toluene and 52 g of ethanol to prepare a solvent. The conversion rate of toluene was graphed and shown in Fig. 3.

[0125] Number of repetitions PET(g)Toluene(g)EtOH(g)NaOH(g)Time(h)TPA obtained(g)Yield(%)Conversion(%)11040528.338.5799.14100.00210Repeated4.538.5799.12100.003104.538.6299.68100.004104.538.6499.9 4100.005104.538.5899.23100.006104.538.6399.81100.007104.538.6099.45100.008104.538.6199.63100.009104.538.5899.21100.0010104.538.5999.36100.00

[0126] Comparative Example 3

[0127] Terephthalic acid was obtained in the same manner as in Example 17, except that 40 g of anisole was used instead of toluene in the S1' step of Example 17. The conversion rate of anisole was graphed and shown in Figure 4.

[0128] Number of turns PET(g) Anisole(g) EtOH(g) NaOH(g) Time(h) Obtained TPA(g)Yield(%)Conversion(%)11040408.338.3597.8198.79210Repeated4.538.3597.7798.783104.538.3397.9498.444104.538.1997.3897.315104.537.9596.8794.886104.537.8597.5393.097104.537.7398.0791.168104.537.4797.7588.449104.537.0696.9284.3110104.536.8097.0681.04

[0129] Comparative Example 4

[0130] Terephthalic acid was obtained in the same manner as in Example 17, except that 40 g of MIBK was used instead of toluene in the S1' step of Example 17. The conversion rate of MIBK was graphed and shown in Figure 5.

[0131] Number of turns PET(g)MIBK(g)EtOH(g)NaOH(g)Time(h)Obtained TPA(g)Yield(%)Conversion(%)11040408.338.4799.8498.13210Repeated use4.538.4499.8697.773104.538.0298.6694.034104.537.7498.2791.115104.536.9897.5282.796104.536.6498.9277.657104.535.2195.5763.068104.535.1295.7461.869104.535.0699.6358.7510104.534.8998.7057.31

Claims

1. A step of preparing a solvent by mixing a basic hydrolysis solvent comprising an alkylated aromatic compound and a polarity adjusting compound and an alkaline catalyst; A step of treating waste polyester with the solvent to obtain a polyester depolymerization product; A step of filtering the polyester depolymerization product to obtain a sludge cake and adding purified water to the filtered sludge cake to prepare a sludge cake aqueous solution containing a terephthalate metal salt; A step of treating the above sludge cake aqueous solution with an organic solvent to obtain an aqueous solution containing a terephthalate metal salt; A step of purifying a terephthalate metal salt aqueous solution by treating the above-mentioned aqueous solution with an adsorbent; A step of treating the above purified terephthalate metal salt aqueous solution with an acid solution to precipitate solid terephthalic acid. A method for producing high-purity recycled terephthalic acid using polyester depolymerization comprising:

2. In claim 1, A method for producing high-purity regenerated terephthalic acid using polyester depolymerization, wherein in the step of producing a solvent by mixing a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity-controlling compound and an alkaline catalyst, the above-mentioned alkylated aromatic compound is at least one compound selected from the group consisting of toluene, xylene, trimethylbenzene, ethylbenzene, diethylbenzene, propylbenzene, dipropylbenzene, and butylbenzene.

3. In claim 1, A method for producing high-purity regenerated terephthalic acid using polyester depolymerization, wherein in the step of producing a solvent by mixing a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity-controlling compound and an alkaline catalyst, the polarity-controlling compound is at least one compound selected from the group consisting of methanol, ethanol, propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol, decanol, undecanol, dodecanol, ethylene glycol, propylene glycol, and butylene glycol.

4. In claim 1, A method for producing high-purity regenerated terephthalic acid using polyester depolymerization, wherein in the step of producing a solvent by mixing a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity-controlling compound and an alkaline catalyst, the alkaline catalyst is at least one selected from the group consisting of potassium hydroxide, sodium hydroxide, lithium hydroxide, potassium oxide, sodium oxide, and lithium oxide.

5. In claim 1, A method for producing high-purity regenerated terephthalic acid using polyester depolymerization, wherein in the step of treating the above sludge cake aqueous solution with an organic solvent to obtain an aqueous solution containing a terephthalate metal salt, the organic solvent is at least one selected from the group consisting of pentane, hexane, heptane, octane, nonane, decane, and undecane.

6. In claim 1, A method for producing high-purity regenerated terephthalic acid using polyester depolymerization, wherein in the step of purifying the terephthalate metal salt aqueous solution by treating the above-mentioned aqueous solution with an adsorbent, the adsorbent is at least one selected from the group consisting of incinerator ash, activated carbon, zeolite, silicate, calcium carbonate, calcium oxide, calcium hydroxide, magnesium carbonate, magnesium oxide, magnesium hydroxide, sodium carbonate, sodium bicarbonate, and alumina.

7. In claim 1, A method for producing high-purity regenerated terephthalic acid using polyester depolymerization, wherein in the step of producing a solvent by mixing a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity-controlling compound and an alkaline catalyst, the weight ratio of the above-mentioned alkylated aromatic compound and the polarity-controlling compound is 1:0.1 to 10.

8. In claim 1, A method for producing high-purity regenerated terephthalic acid using polyester depolymerization, wherein in the step of producing a solvent by mixing a basic hydrolysis solvent comprising the above-mentioned alkylated aromatic compound and a polarity-controlling compound and an alkaline catalyst, the weight ratio of the basic hydrolysis solvent and the alkaline catalyst is 1:0.01 to 0.

5.

9. In claim 1, A method for producing high-purity recycled terephthalic acid using polyester depolymerization, wherein the weight ratio of the waste polyester and the solvent in the step of treating the waste polyester with the solvent to obtain a polyester depolymerization product is 1:1 to 50.

10. Regenerated terephthalic acid having a purity of 99.50% or higher, produced by any one of the production methods of claims 1 to 9.

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