Crystallization method for purified terephthalic acid

Abstract

The present invention relates to a method for crystallizing purified terephthalic acid (PTA) to obtain crystalline PTA with an average crystal size of approximately 50 to 150 μm, the method comprising the following steps in the following order: a) preparing a PTA solution containing 7 to 13% by weight of purified metallized terephthalic acid in water in a reactor and adjusted to pH 7; b) heating the PTA solution to the maximum crystallization temperature; c) adjusting the pH of the PTA solution to 5 using an acid; d) subjecting the PTA solution to a cooling-heating cycle at least once, preferably three times, more preferably five to eight times, to obtain growing PTA crystals. In this case, the solution is cooled to the minimum crystallization temperature before being heated to the maximum crystallization temperature; e) adjusting the pH of the PTA solution to 1 to 3 using an acid to precipitate the crystalline PTA at the maximum crystallization temperature; f) filtering the PTA solution at the maximum crystallization temperature to recover the crystalline PTA; g) washing the filtered crystalline PTA with cold water; h) drying the washed crystalline PTA.

Description

Technical Field

[0001] The present invention relates to a method for crystallizing purified terephthalic acid (PTA) to obtain crystalline PTA having an average crystal size of about 50 to 150 μm.

Background Art

[0002] Poly(ethylene terephthalate), known as PET, is a semi-crystalline thermoplastic polyester used in various industries in the form of fibers, sheets, films and bottles. Due to its stability, high mechanical strength, high resistance to atmospheric and biological factors, and excellent aesthetics, it is widespread in both commercial and industrial fields. While PET has become an indispensable part of our lives, environmental concerns include pollution in the ocean and landfill sites. The environmental impact of PET production is not limited to post-consumer PET polluting landfill sites. It has been reported that while other industrial sectors can reduce carbon emissions, the petrochemical industry that produces PET will ultimately increase greenhouse gas emissions with the increase in PET production volume, thereby compromising the climate benefits.

[0003] The most common industrial synthesis route of PET is through polycondensation of ethylene glycol (EG) with dimethyl terephthalate (DMT) or purified terephthalic acid (PTA) using a continuous melt polymerization process at about 280°C. The basic raw materials of this process (EG, DMT and PTA) are typical bulk chemicals obtained in the process of the petrochemical industry obtaining paraxylene by catalytic reforming of petroleum naphtha. Therefore, closing the recycling loop will have continuous effects starting from removing post-consumer waste from the environment, reducing greenhouse gas emissions due to an increase in the supply of PTA in the market, and reducing the dependence on PTA from the petroleum industry.

[0004] After commercial or industrial consumption, users typically recycle the product through four different methods, which are referred to as primary, secondary, and quaternary recycling depending on the quality of the recycled material. Primary recycling deals only with industrial PET scrap and recovered materials, while secondary recycling physically reprocesses consumer PET by crushing, washing, drying, and reprocessing. However, the quality of PET obtained through secondary recycling is not virgin, so much of it is incinerated for energy recovery (quaternary recycling). Ultimately, the ideal way to close the recycling loop is tertiary recycling, which involves breaking down PET back into its starting monomers, allowing the monomers to be resold to the chemical industry to produce virgin PET and other products.

[0005] An effective, inexpensive, robust, and practical technique for decomposing plastic waste such as PET material and simultaneously generating terephthalic acid (TPA) and / or ethylene glycol (EG) and / or other monomers constituting plastic materials is described in International Publication No. 2020 / 173961 by the same applicant.

[0006] International Publication No. 2020 / 173961 (which is incorporated herein by reference in its entirety) provides a method for alkaline hydrolysis of one or more plastic polymers to terephthalic acid (TPA) and / or ethylene glycol (EG) and / or monomers constituting one or more plastic polymers. This method may produce terephthalic acid of a quality that is contaminated or requires additional purification before further processing or specification. Current methods for purifying TPA are energy-intensive and economically inefficient.

[0007] U.S. Provisional Application No. 63 / 425771 by the same applicant (which is incorporated herein by reference in its entirety) describes a method for purifying terephthalic acid. The method includes the steps of: contacting unpurified terephthalic acid with graphite, activated carbon and a molecular sieve to prepare a reaction mixture; stirring the reaction mixture for a first predetermined time; filtering the reaction mixture to obtain a reaction mixture filtrate; adding graphite, activated carbon and a molecular sieve to the reaction mixture filtrate; stirring the reaction mixture filtrate for a second predetermined time; filtering the reaction mixture filtrate to obtain a reaction product; and precipitating purified terephthalic acid (PTA) from the reaction product.

[0008] To process PTA into polyethylene terephthalate (PET) and other polyester polymers containing TPA monomers, crystalline PTA with a crystal size of 50-150 μm is required. This is to ensure efficient handling and fluidity, as well as to achieve the optimal ratio of PTA to other monomers (ethylene glycol in the case of PET) and form a paste suitable for the polymerization process.

[0009] A common method for purifying TPA to obtain purified terephthalic acid (PTA) typically involves a recrystallization process, which requires high temperatures (>100°C) and high pressures (>1 atm) to bring the PTA crystals to a size (average 100 μm) suitable for the subsequent polycondensation process. This crystallization process is energy-intensive and requires the use of hazardous solvents such as dimethylformamide. Such solvents can pose a safety risk to workers, and their handling and management require complex, time-consuming procedures.

[0010] Industrial crystallization is an effective method for purifying many organic and inorganic compounds. There are two types of nucleation in crystallization: primary and secondary nucleation. Primary nucleation proceeds rapidly and often produces small, irregularly shaped particles. This is not commonly used industrially because the risk of impurities being trapped within the crystal due to nucleation on foreign particles exists. Secondary nucleation occurs when the degree of saturation is sufficiently high, and crystals are formed in the presence of other crystals of the same substance. Secondary nucleation is commonly used industrially and is also seen in the purification process of crude terephthalic acid (CTA).

[0011] Industrial PTA is produced by a two-step process: (1) The first step of the process involves oxidizing p-xylene to produce powdered CTA. The impurity 4-CBA is a byproduct of this process. The CTA powder is mixed with deionized water to form a slurry, which is then sent to the next step, the purification process. (2) The CTA purification process is carried out under high temperature and pressure. Powdered CTA (29 wt%) is mixed with deionized water (71 wt%) to form a slurry. This slurry is heated to 287°C and transferred to a hydrogenation reactor operating at 79 bar. The slurry reacts with nitrogen and hydrogen in the presence of a carbon-coated palladium catalyst to convert 4-CBA to p-toluic acid. The slurry containing acceptable levels of 4-CBA (<25 ppm) undergoes multi-stage flash crystallization to remove the main impurities, p-toluic acid and benzoic acid (Non-Patent Literature 1 and 2).

[0012] The residence time in each crystallizer has been reported to be 20 to 50 minutes in order to achieve a crystal size of 118 μm (Non-Patent Literature 1). Therefore, it is estimated that the five-step crystallization process takes approximately 1.5 to 4 hours.

[0013] Novel crystallization processes introduce more solvents to shorten the time required to reach the desired purity. Commonly used solvents include acetic acid, DMSO, DMF, DMA, and alcohols, while catalysts such as Mn, Br, Co, and Zn are used in some steps. However, in all cases, the conversion from CTA to PTA involves the use of high temperatures and pressures, leading to increased energy costs.

[0014] Key elements in the production of purified terephthalic acid include forming crystals of a size and shape that allow for good handling, washability, and filtration in the PTA manufacturing process, as well as improving ease of handling and processability in the polyester process. [Prior art documents] [Non-patent literature]

[0015] [Non-Patent Document 1] Online dual updating with recursive PLS model and its application predicting crystal size of PTA process (Mu et al, 2005, China). DOI:10.1016 / j.jprocont.2005.11.004. [Non-Patent Document 2] Control structure design of a crude terephthalic acid hydropurification process with catalyst deactivation (Li et al, 2016, China). DOI:10.1016 / j.compchemeng.2016.01.017 [Overview of the project]

[0016] The object of the present invention is to reduce energy consumption in the preparation of purified terephthalic acid (PTA) obtained by the chemical recycling of PET, thereby reducing the overall energy consumption of the PET recycling process.

[0017] At least one of the objectives of the present invention is achieved by the crystallization method described in claim 1. A method for crystallizing purified terephthalic acid (PTA) to obtain crystalline PTA with an average crystal size of about 50 to 150 μm comprises the following steps in the following order: (a) preparing a PTA solution containing 7 to 13% by weight of purified metallized salt of terephthalic acid (M-PTA) in water in a reactor and adjusted to pH 7; (b) heating the PTA solution to the maximum crystallization temperature; (c) adjusting the pH of the PTA solution to 5 using an acid; (d) subjecting the PTA solution to at least one cooling-heating cycle, preferably at least (a) A step of obtaining growing PTA crystals by repeating the process three times, more preferably five to eight times, wherein the PTA solution is cooled to the lowest crystallization temperature and then heated to the highest crystallization temperature; (b) a step of adjusting the pH of the PTA solution to 1 to 3 in order to completely precipitate the crystal PTA at the highest crystallization temperature; (c) a step of filtering the PTA solution at the highest crystallization temperature to recover the crystal PTA; (d) a step of washing the filtered crystal PTA with cold water; (e) a step of drying the washed crystal PTA.

[0018] The inventors have found that by using several cooling-heating cycles, the crystallization method and the growth of PTA crystals to a desired size can be carried out at low temperatures in the range of 40-85°C. Furthermore, this method can be carried out under standard atmospheric pressure (approximately 10¹³ mbar). Therefore, the energy consumption of the crystallization process can be reduced compared to known crystallization processes that use high temperatures (>100°C) and high pressures (>1 atm). After the cooling-heating cycles, the grown crystals precipitate completely at pH 1-3 at the maximum crystallization temperature, are immediately filtered, and washed with cold water to remove crystallized salts (derived from M-PTA) and water-soluble impurities.

[0019] Cooling can be achieved by active cooling of the PTA solution or by passive cooling without energy input that only removes the heat source to cool the PTA solution. Good results were achieved by heating at a rate of about 5°C per 10 minutes and cooling at a rate of about 5°C per 15 minutes. Preferably, in each cycle, the maximum crystallization temperature can be maintained for about 1 - 2 minutes before the start of the next cycle or the next step. Heating may start as soon as the PTA solution reaches the minimum crystallization temperature.

[0020] It should be noted that the maximum crystallization temperature and the minimum crystallization temperature can be selected by setting the heating and / or cooling system of the reactor to the desired temperature range. Thus, the set maximum crystallization temperature and the set minimum crystallization temperature mean the settings of the heating and cooling system of the reactor. Due to the inertia of the system, the actual measured temperature of the PTA solution may be up to about 3°C higher or lower than the set maximum or minimum temperature. Usually, the difference between the set temperature and the actual temperature is within the range of 0 - 3°C. It should not exceed 5°C.

[0021] Steps (b) to (e) are usually carried out while stirring the PTA solution.

[0022] Further embodiments of the present invention are described in the dependent claims.

[0023] In some embodiments, the minimum crystallization temperature and the maximum crystallization temperature of the cooling - heating cycle are selected and set between 40 - 85°C, preferably selected from the ranges of 40 - 45°C, 45 - 50°C, 50 - 55°C, 55 - 60°C, 60 - 65°C, 65 - 70°C, 70 - 75°C and 75 - 80°C. The actual temperature in the reactor may deviate by several degrees from the set temperature.

[0024] The set maximum crystallization temperature may be in the range of 45 - 85°C. The set minimum crystallization temperature may be about 5°C lower than the maximum crystallization temperature. In other words, good crystal growth can be obtained with only a slight temperature difference of about 5°C between the set maximum crystallization temperature and the minimum crystallization temperature.

[0025] Due to the inertia of the system, the actual minimum crystallization temperature of the PTA solution in the reactor may be up to about 3 °C lower than the set minimum crystallization temperature, and / or the actual maximum crystallization temperature in the reactor may be up to about 3 °C higher than the set maximum crystallization temperature.

[0026] In some embodiments, the difference between the set or actual minimum crystallization temperature and the set or actual maximum crystallization temperature may be in the range of 5 - 40 °C, 5 - 20 °C, 5 - 15 °C or 5 - 10 °C.

[0027] In some embodiments, this method can be carried out under atmospheric pressure.

[0028] In some embodiments, cooling and / or heating can be carried out at a rate of about 5 °C per 10 - 20 minutes.

[0029] In some embodiments, one cooling-heating cycle can be continued for about 10 - 40 minutes.

[0030] In some embodiments, the cooling time and / or the heating time can be continued for about 5 - 20 minutes.

[0031] In some embodiments, the maximum crystallization temperature in each cooling-heating cycle can be maintained for about 1 - 3 minutes before the start of the next cooling-heating cycle or the next step.

[0032] In some embodiments, in each cooling-heating cycle, heating can be started immediately when the minimum crystallization temperature is reached.

[0033] In some embodiments, the metalized salt of terephthalic acid (M-PTA) can be sodium terephthalate (Na-PTA) or potassium terephthalate (K-PTA).

[0034] In some embodiments, M-PTA can be obtained from depolymerization by alkaline hydrolysis of waste PET, preferably by the method described in International Publication No. 2020 / 173961, which is incorporated in whole by reference herein. Depolymerization is achieved by alkaline hydrolysis of one or more plastic polymers to terephthalic acid (TPA) and ethylenediol (EG) and / or other monomers constituting one or more plastic polymers, the method comprising: a) contacting one or more plastic polymers with a metal oxide in solution in the presence of a base to obtain a reaction mixture; b) stirring the reaction mixture for a suitable time under ultraviolet irradiation; and c) recovering terephthalic acid, ethylene glycol and / or other monomers from the reaction mixture.

[0035] One or more plastic polymers may be selected from the group including polylactic acid (PLA), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene isosorbide terephthalate (PEIT), polyethylene furanate (PEP), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), or combinations thereof. Preferably, polyethylene terephthalate (PET). The base is NaOH, NaO t The group can be selected from those containing Bu and KOH.

[0036] In some embodiments, M-PTA obtained from the depolymerization of waste PET may be further purified using graphite, activated carbon, and molecular sieves to remove organic and inorganic contaminants. Purification may be carried out by the method described in U.S. Provisional Application No. 63 / 425771 by the same applicant, the entire literature of which is incorporated herein by reference.

[0037] The purification method may include contacting unpurified terephthalic acid with graphite, activated carbon, and a molecular sieve to obtain a reaction mixture; stirring the reaction mixture for a first predetermined time; filtering the reaction mixture to obtain a reaction mixture filtrate; adding graphite, activated carbon, and a molecular sieve to the reaction mixture filtrate; stirring the reaction mixture filtrate for a second predetermined time; filtering the reaction mixture filtrate to obtain a reaction product; and precipitating purified terephthalic acid from the reaction product.

[0038] In relation to the purification method, the following features can be realized individually or in any combination with the features described above. - The reaction mixture can be stirred at pH 14. - The reaction mixture filtrate can be stirred at pH 7. -Graphite, activated carbon, and molecular sieves may be provided in a ratio of 1:6:2 for contact with unrefined terephthalic acid. -Graphite, activated carbon, and molecular sieves may be added to the reaction mixture filtrate in a ratio of 1:6:2. -Purified terephthalic acid can be precipitated from the reaction product solution using an acid, preferably hydrochloric acid or sulfuric acid. - The first predetermined time may be 10 to 120 minutes, preferably 30 minutes. - The second predetermined time may be 10 to 120 minutes, preferably 30 minutes. -This method can be carried out at room temperature. - The graphite may include amorphous, crystalline, or flake graphite, and may have a purity of 99% and a particle size range of 5 to 30 μm. - Activated carbon has a surface area of ​​500-1500 m² 2 It contains granular activated carbon with a particle size of less than 1 mm, and its concentration is / g. - The molecular sieve may include aluminosilicate crystalline molecular sieves such as zeolite 13X molecular sieves. Zeolite 13X has an average pore size of 9 Å and can adsorb molecules with a dynamic diameter of less than 9 Å.

[0039] The purification method may be carried out as a batch operation, preferably in a reaction vessel equipped with a stirrer and a plug drain filter. In some embodiments, the method described herein may be carried out as a batch operation in a configuration in which multiple reaction vessels are arranged in series. In some embodiments, the method described herein may be modified to be carried out as a continuous process.

[0040] The purification method allows for the stable acquisition of purified terephthalic acid (PTA), which is up to 1% purer and 5% brighter white than virgin terephthalic acid when dried.

[0041] The product is usually a metallized salt of terephthalic acid (M-PTA, where "M" is Na from the hydrolysis process). * or K * The product is a solution of metals (such as , , etc.) dissolved in water, at a concentration of 7-13% by weight, close to the maximum solubility in the M-PTA solution. The product can then be subjected to crystallization.

[0042] In some embodiments, the acid in step c) and / or step e) may be selected from the group consisting of acetic acid, HCl, H2SO4, formic acid, propionic acid, and butyric acid.

[0043] In some embodiments, the cold water in step g) may have a temperature of about 10-20°C. [Brief explanation of the drawing]

[0044] The present invention will be described in more detail below with reference to embodiments shown in the drawings. [Figure 1] Figure 1 is a microscopic image of a PTA crystal. [Figure 2] Figure 2 is a graph showing the temperature profile of sample 27 within the set temperature range of 60-65°C. [Modes for carrying out the invention]

[0045] Unpurified terephthalic acid can be obtained from the products of polymer recycling processes, such as the alkali polymer hydrolysis process at room temperature described in International Publication No. 2020 / 173961.

[0046] Such unrefined terephthalic acid may contain contaminants including solid pigments such as carbon black, isomorphic monomers such as phthalic acid and isophthalic acid, and molecules that may be present as contaminants in PET, such as benzoic acid, p-tolueneic acid, bisphenol A, 4-carboxybenzaldehyde, metal ions, and / or other contaminants derived from waste PET. To remove impurities, unrefined terephthalic acid may be subjected to the purification process described in U.S. Provisional Application No. 63 / 425771.

[0047] The purified terephthalic acid obtained from the purification process is M-TPA (where "M" is derived from the Na hydrolysis process). * or K * These are metals (representing metals such as [specific metals]), and they exist in a dissolved form in water at a concentration of approximately 13% by weight, close to the maximum solubility in the M-TPA solution. [Examples]

[0048] Examples of the crystallization process are shown in Table 1. Samples 1-18 were processed without a cooling-heating cycle, while samples 19-27 were processed with a cooling-heating cycle.

[0049] [Table 1] JPEG2026523193000003.jpg242156JPEG2026523193000004.jpg174156

[0050] After the adsorption purification process, a neutral (pH 7) Na-PTA liquid phase (PTA solution) is supplied to all samples and heated to the maximum crystallization temperature (the higher temperature in the "Temperature" column). At the maximum crystallization temperature, the pH of the PTA solution is adjusted to 5 using acetic acid (in the "First Acid Addition (pH 5)" column). The temperature in the "Temperature" column refers to the set temperature without considering the difference due to system inertia.

[0051] The PTA solutions of samples 19-27 were subjected to a cooling-heating cycle (samples 19-27), in which the temperature of the PTA solution was lowered from the maximum crystallization temperature to the minimum crystallization temperature (the lower temperature in the "Temperature" column), and immediately after reaching the minimum crystallization temperature, it was reheated to the maximum crystallization temperature. The cooling and reheating process is considered one cycle. Different samples were assigned different cycle numbers ("Cycles" column). Before starting the next cycle or step, the maximum crystallization temperature was maintained for 1 minute (the specified heating time). The heating rate was set to approximately 5°C per 10 minutes. The cooling rate was approximately 5°C per 15 minutes without active cooling. Active cooling may be required depending on the scale-up.

[0052] Sample 19 differs from samples 20-27 in that it underwent only one cycle. The maximum crystallization temperature was maintained for 1 hour before cooling. The minimum crystallization temperature was maintained for 10 minutes before reheating.

[0053] After the final cycle, the pH of the PTA solutions for samples 19-27 was adjusted to pH 1-3 using sulfuric acid (H2SO4) (see "Second Acid Addition pH 1-3" section) to completely precipitate the crystalline PTA. The samples were then filtered at the highest crystallization temperature to recover the crystalline PTA, which was immediately washed with cold water (10-20°C) to remove crystallized salts, such as those derived from M-PTA, and water-soluble impurities (see "Washing Step" section).

[0054] Samples 1-18 differed from samples 19-27 in that they did not undergo a cooling-heating cycle and followed a slightly different protocol (see "Method Details" and "Washing Steps" sections). In the "Method Details" section, AA refers to acetic acid. "Normal precipitation method": The second acid is added directly to the Na-TPA solution to precipitate TPA. "Normal addition, then precipitation": The first acid is added to bring the pH to 5, then an additional amount of the second acid is added to bring the pH to 1 and precipitate TPA. "AA for xx hours, then H2SO4": Acetic acid is added to adjust the pH to 5, then the mixture is stirred for xx hours. Sulfuric acid is then added to adjust the pH to 1 and precipitate TPA.

[0055] After filtration and washing, all samples (1-27) were dried and tested for crystal shape (visual inspection), the MEG:TPA ratio required for preliminary production paste, and density (Table 2).

[0056] [Table 2] JPEG2026523193000006.jpg77158

[0057] Crystal shape (visual observation): Samples 14-27 showed a "thick needle-like" shape within or close to the desired size.

[0058] The required MEG:TPA ratio for the preliminary production paste was determined by weighing 0.1 moles of TPA and adding a sufficient amount of (mono)ethylene glycol (EG or MEG) to produce a paste with fluidity in the powder. The weight of the added EG was calculated from the volume and density of the EG, and the molar ratio of MEG to TPA was calculated. The MEG:TPA ratio commonly used in the industrial production of PET is in the range of 1.2 to 1.5. Generally, a lower ratio is preferred. Samples 20 to 27, which underwent at least one cycle, showed a lower ratio and larger particle size compared to samples 1 to 18, which had not undergone any cycles. Samples 21 to 28 were close to the range commonly used in the industrial production of PET.

[0059] Density: TPA was injected into a space of known volume without compression, and the amount of TPA required to fill the space was weighed. The density was then calculated in g / cm³. 3 The calculation was performed using the following method. The density commonly used in the industrial production of PET is 0.8 g / cm³. 3 It exceeds [value]. Samples 20–27, which have at least one cycle, show increased density compared to samples 1–18, which have no cycles.

[0060] Figure 1 shows a microscopic image of a PTA crystal. The lower left of the image shows a crystal produced by the method described. The upper right of the image shows a commercially available PTA crystal for comparison.

[0061] Figure 2 shows the temperature profile of the cooling-heating cycle for sample 27 in the set temperature range of 60–65°C. The thick line at the top indicates the maximum crystallization temperature at the set temperature of 65°C. The thick line at the bottom indicates the minimum crystallization temperature at the set temperature of 60°C. As is evident from the temperature profile, the maximum and minimum crystallization temperatures actually measured in the PTA solution in the reactor were up to 3°C higher or lower than the set temperature. This difference is due to system inertia, which causes heating or cooling to stop or reverse as soon as the PTA solution reaches the set temperature. [Explanation of Symbols]

[0062] 1. PTA crystals obtained by the described method 2. Commercially available PTA crystals

Claims

1. A method for crystallizing purified terephthalic acid (PTA) to obtain crystalline PTA with an average crystal size of approximately 50 to 150 μm, the method comprising the following steps in the following order: a. A step of preparing a PTA solution containing 7 to 13% by weight of purified metallized terephthalic acid (M-PTA) in water in a reactor, and adjusting the pH to 7; b. Heating the PTA solution to the highest crystallization temperature; c. Adjusting the pH of the PTA solution to 5 using an acid; d. A step of obtaining growing PTA crystals by subjecting the PTA solution to a cooling-heating cycle at least once, preferably at least three times, more preferably five to eight times, wherein the PTA solution is cooled to the lowest crystallization temperature and then heated to the highest crystallization temperature; e. A step of adjusting the pH of the PTA solution to 1-3 using an acid in order to completely precipitate the crystalline PTA at the highest crystallization temperature; f. A step of filtering the PTA solution at the highest crystallization temperature to recover the crystalline PTA; g. A step of washing the filtered crystalline PTA with cold water; h. A step to dry the washed crystalline PTA.

2. The method according to claim 1, wherein the minimum crystallization temperature and the maximum crystallization temperature of the cooling-heating cycle are selected and set between 40 and 85°C, preferably selected from the ranges of 40-45°C, 45-50°C, 50-55°C, 55-60°C, 60-65°C, 65-70°C, 70-75°C, and 75-80°C.

3. The method according to claim 2, wherein the actual minimum crystallization temperature of the PTA solution in the reactor is at most about 3°C ​​lower than the set minimum crystallization temperature, and / or the actual maximum crystallization temperature of the PTA solution in the reactor is at most about 3°C ​​higher than the set maximum crystallization temperature.

4. The method according to any one of claims 1 to 3, wherein the difference between the minimum crystallization temperature and the maximum crystallization temperature is in the range of 5 to 40°C, preferably 5 to 20°C, and more preferably 5 to 10°C.

5. The method according to any one of claims 1 to 4, carried out under standard atmospheric pressure.

6. The method according to any one of claims 1 to 5, wherein the cooling and / or heating is performed at a rate of about 5°C per 10 to 20 minutes.

7. The method according to any one of claims 1 to 6, wherein one cooling-heating cycle is performed for approximately 10 to 40 minutes.

8. The method according to any one of claims 1 to 7, wherein the cooling time and / or heating time is continued for about 5 to 20 minutes.

9. The method according to any one of claims 1 to 8, wherein the maximum crystallization temperature in each of the cooling-heating cycles is maintained for about 1 to 3 minutes before the start of the next cooling-heating cycle or the next step.

10. The method according to any one of claims 1 to 9, wherein in each of the cooling-heating cycles, the heating is started immediately when the minimum crystallization temperature is reached.

11. The method according to any one of claims 1 to 10, wherein the purified metallated salt (M-PTA) of terephthalic acid is sodium terephthalate (Na-PTA) or potassium terephthalate (K-PTA).

12. The method according to any one of claims 1 to 11, wherein the M-PTA is obtained from the depolymerization of waste PET by alkaline hydrolysis.

13. The method according to any one of claims 1 to 12, wherein the M-PTA is purified using graphite, activated carbon, and a molecular sieve to remove organic and inorganic contaminants.

14. The acid in step c and / or step e is acetic acid, HCl, H ２ SO ４ The method according to any one of claims 1 to 13, selected from the group consisting of formic acid, propionic acid, and butyric acid.

15. The method according to any one of claims 1 to 14, wherein the cold water in step g is at a temperature of about 10 to 20°C.

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