Crystallization method for purified terephthalic acid

Abstract

The invention relates to a method for preparing crystalline purified terephthalic acid (PTA) with an average crystal size of approx. 50-150 microns, comprising, in the following order, the steps of: a) providing an aqueous PTA solution of purified metalated salt of terephthalic acid (pH 7); b) heating the solution to a maximum crystallization temperature; c) adjusting the solution to a pH of 5; d) subjecting the solution to a cooling-heating cycle to obtain growing PTA crystals, wherein the solution is cooled to a minimum crystallization temperature and subsequently heated to the maximum crystallization temperature; e) at the maximum crystallization temperature adjusting the pH of the PTA solution to 1 – 3 for full precipitation of crystalline PTA, and f) filtering the PTA solution to collect crystalline PTA; g) washing the filtered crystals with cold water; h) drying the crystals.

Description

[0001] Crystallization method for purified terephthalic acid

[0002] Technical Field

[0003] The invention relates to crystallization method for purified terephthalic acid (PTA) to obtain crystalline PTA with an average crystal size of approx. 50-150 microns.

[0004] Technical Background

[0005] Poly(ethylene terephthalate), widely known as PET, is a semicrystalline thermoplastic polyester that is used in a variety of industries in the form of fibres, sheets, films, and bottles. Its stability, high mechanical strength, high resistance to atmospheric and biological agents, and good aesthetic appearance has led to its prevalence in both the commercial and industrial sectors. While PET has become an inextricable part of our lives, environmental concerns have been raised about its pollution in our ocean and landfills. The environmental effects of PET production are not only limited to postconsumer PET contaminating the landfills It has also been reported that while other industrial sectors can lower their carbon footprint, the petrochemical industry which produces PET will ultimately increase their greenhouse gas emissions with increased PET production, thereby eroding climate benefits.

[0006] The most common industrial synthesis route of PET is through the polycondensation of ethylene glycol (EG) and dimethyl terephthalate (DMT) or purified terephthalic acid (PTA) using a continuous melt-phase polymerization process with temperatures of approximately 280°C. The base chemicals for this process (EG, DMT, and PTA) are typical bulk chemicals that the petrochemical industry obtains from catalytic reforming of petroleum naptha to paraxylene. Therefore, closing the recycling loop has a cascading effect from removing post-consumer waste from our environment to lowering greenhouse gas emission by increasing the supply of PTA within the market, and decreasing our reliance of PTA from the petroleum industry.

[0007] Following the consumption of PET, either commercially or industrially, users typically recycle the product(s) via four distinct methods, which are referred to as primary through quaternary recycling depending on the quality of the recycled product While primary recycling exclusively deal with industrial PET scrap and salvage, secondary recycling physically reprocesses consumer PET through grinding, washing, drying and reprocessing. However, the quality of PET obtained through secondary recycling is not virgin, and therefore much of it ends up being incinerated to recovery the energy content (quaternary recycling). Ultimately, tertiary recycling, or the depolymerization of PET to its starting monomers is the ideal method to close the recycling loop, as the monomers can be resold back to the chemical industry to form virgin PET, or other products.

[0008] An effective, inexpensive, robust and practical technology for degrading plastic waste, such as PET material, and simultaneously producing terephthalic acid (TPA), and / or ethylene glycol (EG) and / or other monomers that form plastic material is described in W02020173961 by the same applicant.

[0009] W02020173961 (incorporated herein by reference in its entirety) provides a method of alkaline hydrolysis of one or more plastic polymers into terephthalic acid (TPA) and / or ethylene glycol (EG) and / or other monomers that form the one or more plastic polymers. The method may output terephthalic acid that is contaminated or otherwise of a quality that requires further purification before further processing and use. Current methods of purifying TPA are energy intensive, and economically inefficient.

[0010] U.S. provisional application No. 63425771 by the same applicant (incorporated herein by reference in its entirety) describes a method of purifying terephthalic acid. The method includes contacting unpurified terephthalic acid with graphite, activated carbon, and molecular sieve to provide a reaction mixture, stirring the reaction mixture for a first certain period of time, filtering the reaction mixture, to provide a reaction mixture filtrate, providing graphite, activated carbon, and molecular sieve to the reaction mixture filtrate, stirring the reaction mixture filtrate for a second certain period of time, filtering the reaction mixture filtrate, to provide a reaction output solution and precipitating purified terephthalic acid (PTA) from the reaction output solution.

[0011] For PTA to be processed into polyethylene terephthalate (PET) and other polyester based polymers with the TPA monomer, crystalline PTA with a crystal size of 50 to 150 microns is required. This is to allow efficient handling and flowability, and also to allow the optimal ratio of the PTA and other monomer (ethylene glycol in the case of PET) to form a paste suitable for the polymerisation process.

[0012] Common methods of purifying TPA to obtain purified terephthalic acid (PTA) typically include recrystallization steps, which require high temperatures (>100 °C) and pressures (>1 atm) to get the crystal of PTA to the right size (avg. 100 microns) for a later polycondensation process. The crystallization process is energy intensive and requires the use of harmful solvents, such as dimethylformamide. Such solvents may risk operator safety and require expensive, time consuming and complicated procedures to handle and manage.

[0013] Industrial crystallization is an effective means of purification for many organic and inorganic compounds. Under crystallization, there are two types of nucleation, primary nucleation and secondary nucleation. Primary nucleation is rapid and often results in small, irregularly shaped particles, meaning it is not often used in industry because of the risk of nucleation on foreign particles, thus occluding impurities into the crystal. Secondary nucleation occurs when the level of supersaturation is high enough for crystals to form in the presence of other crystals of the same material. Secondary nucleation is generally used in Industry and is also found in the crude terephthalic acid (CTA) purification process.

[0014] Industrial PTA is thus produced in a two-step process: (1) The first stage of the process includes p-xylene oxidation to produce CTA in a powder form. The impurity 4-CBA is a byproduct of this process. CTA powder is mixed with de-ionised water to form a slurry that is passed through the next stage i.e. purification. (2) The CTA purification process is carried out at high temperature and high pressure. Powdered CTA (29 wt%) is mixed with de-ionised water (71 wt%) to form a slurry. The slurry is heated to 287°C and pumped 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, with acceptable levels of 4-CBA (<25ppm) is passed through multi-stage flash crystallization where the primary contaminants p-toluic Acid and benzoic acid are removed. [1], [2]

[0015] The retention time in each crystallizer is reported as 20 - 50 minutes in order to target a crystal size of 118 microns [1], The 5-stage crystallization step is thus estimated to take approximately 1.5 - 4 hours.

[0016] In newer iterations of the crystallization process, more solvents have been introduced to decrease the time required to reach the required purity. These solvents typically used are acetic acid, DMSO, DMF, DMA, alcohols, and in some iterations involve the use of a catalysts such as Mn, Br, Co, Zn. However in all cases, there is an energy penalty associated with the use of high temperatures and pressures in the conversion of CTA to PTA.

[0017] An important factor in the production of purified terephthalic acid is the formation of crystals of a size and shape that allow good handling, washing and filtration properties in the PTA production process, as well as easier handling and better processability in the polyester process.

[0018] References:

[0019] [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.

[0020] [2] Control structure design of a crude terephthalic acid hydropurification process with catalyst deactivation (Li et al, 2016, China). D0l:10.1016 / j.compchemeng.2016.01.017

[0021] Summary of the Invention

[0022] It is an objective of the invention to reduce energy consumption in the preparation of purified terephthalic acid (PTA) obtained by chemical recycling of PET, thus lowering the energy consumption of the overall recycling process of PET.

[0023] At least one of the objectives of the present invention is achieved by a crystallization method according to claim 1. The crystallization method for purified terephthalic acid (PTA) to obtain crystalline PTA with an average crystal size of approx. 50-150 microns comprises in the following order the steps of: (a) providing in a reactor a PTA solution of 7-13 wt% of purified metalated salt of terephthalic acid (M-PTA) in water at a pH of 7; (b) heating the PTA solution to a maximum crystallization temperature; (c) adjusting the PTA solution to a pH of 5 using an acid; (d) subjecting the PTA solution to a cooling-heating- cycle for at least 1 time, preferably at least 3 times, more preferably 5 to 8 times, to obtain growing PTA crystals, wherein the solution is cooled to a minimum crystallization temperature and subsequently heated to the maximum crystallization temperature; (e) at the maximum crystallization temperature adjusting the pH of the PTA solution to 1 - 3 for full precipitation of crystalline PTA; (f) at the maximum crystallization temperature filtering the PTA solution to collect crystalline PTA; (g) washing the filtered crystalline PTA with cold water; (h) drying the washed crystalline PTA.

[0024] The inventors realized that by using a few cooling-heating-cycles the crystallization method and the growing of PTA crystals to the required size may be performed at low temperatures in the range of 40 to 85°C. In addition, the method can be performed under normal atmospheric pressure (about 1013 mbar). Thus, the energy consumption of the crystallization process could be reduces compared to the known crystallization processes using high temperatures (>100 °C) and pressures (>1 atm). After the cooling-heating- cycles, the grown crystals are fully precipitated at pH 1-3 at the maximum crystallization temperature and immediately filtered and washed with cold water to remove any salts (from the M-PTA) that also crystallized, as well as any water-soluble impurities.

[0025] The cooling may be realized with active cooling of the PTA solution or with passive cooling without energy input by simply removing the heat source and letting the PTA solution to cool. Good results were achieved by heating at a rate of approx. 5°C per 10 minutes and cooling at a rate of approx. 5°C per 15 minutes. Preferably, in each cycle the maximum crystallization temperature may be kept for about 1 to 2 minutes before the next cycle or next step starts. The heating may start as soon as the PTA solution reaches the minimum crystallization temperature.

[0026] It is noted that the maximum and minimum crystallization temperature may be chosen by setting the heating and / or cooling system of the reactor to a desired temperature range. The thereby set maximum crystallization temperature and the set minimum crystallization temperature refers to the setting of the heating and cooling system of the reactor. Due to system inertia the actual measured temperature of the PTA solution may be up to approx. 3°C higher or lower than the set maximum or minimum temperature. Typically, the difference between set and actual temperature is in the range of 0°C to 3°C. It should be not higher than 5°C.

[0027] The steps b) to e) are typically performed while stirring the PTA solution.

[0028] Further embodiments of the invention are set forth in the dependent claims.

[0029] In some embodiments the minimum crystallization temperature and the maximum crystallization temperature of the cooling-heating-cycle may be chosen and set between 40-85°C, preferably selected from the range 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 defer by a few degrees from the set temperature.

[0030] The set maximum crystallization temperature may be in the range of 45-85°C. The set minimum crystallization temperature may be approx. 5°C lower than the maximum crystallization temperature. In other words, already with small temperature changes of approx. 5°C between the set maximum and set minimum crystallization temperature good growth of crystals could be obtained.

[0031] Due to system inertia an actual minimum crystallization temperature of the PTA solution in a reactor may be up to approx. 3°C lower than the set minimum crystallization temperature and / or an actual maximum crystallization temperature of the PTA solution in a reactor may be up to approx. 3°C higher than the set maximum crystallization temperature. In some embodiments the difference between the set or actual minimum crystallization temperature and the set or actual maximum crystallization temperature may be in a range of 5-40°C, 5-20°C, 5-15°C, or 5-10°C.

[0032] In some embodiments the method may be performed at ambient pressure.

[0033] In some embodiments the cooling and / or heating may be performed at a rate of approx. 5°C per 10-20 minutes.

[0034] In some embodiments one cooling-heating-cycle lasts approx. 10 to 40 minutes.

[0035] In some embodiments a cooling period and / or a heating period may last approx. 5 to 20 minutes.

[0036] In some embodiments in each cooling-heating-cycle the maximum crystallization temperature may be kept for approx. 1 to 3 minutes before the next cooling-heating-cycle or the next step starts.

[0037] In some embodiments in each cooling-heating-cycle the heating may start immediately when the minimum crystallization temperature is reached.

[0038] In some embodiments the metalated salt of terephthalic acid (M-PTA) may be either sodium terephthalic acid (Na-PTA) or potassium terephthalic acid (K-PTA).

[0039] In some embodiments the M-PTA may be obtained from depolymerization of waste PET via alkaline hydrolysis, preferably via the method as described in W02020173961 and incorporated herein by reference in its entirety. The depolymerization may be realized by a method of alkaline hydrolysis of one or more plastic polymer into terephthalic acid (TPA) and ethylene glycol (EG) and / or other monomers that form the one or more plastic polymers, the method comprising a) contacting the one or more plastic polymers with a metal oxide in a solution in the presence of a base to provide a reaction mixture; b) stirring the reaction mixture during appropriate time under UV light; c) recovering terephthalic acid, ethylene glycol and / or the other monomers from the reaction mixture.

[0040] The one or more plastic polymers may be selected from the group comprising poly lactic acid (PLA), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene isosorbide terephthalate (PEIT), polyethylene furanoate (PEP), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC) or combinations thereof. Preferably, polyethylene terephthalate (PET). The base may be selected from the group comprising NaOH, NaO‘Bu, KOH.

[0041] In some embodiments the M-PTA, which may be obtained from depolymerization of waste PET, may be further purified using graphite, activated carbon and molecular sieve to remove organic and inorganic contaminants. The purification may be performed with the method as described in U.S. provisional application No. 63425771 by the same applicant and incorporated herein by reference in its entirety.

[0042] The purification method may include contacting unpurified terephthalic acid with graphite, activated carbon, and molecular sieve to provide a reaction mixture, stirring the reaction mixture for a first certain period of time, filtering the reaction mixture, to provide a reaction mixture filtrate, providing graphite, activated carbon, and molecular sieve to the reaction mixture filtrate, stirring the reaction mixture filtrate for a second certain period of time, filtering the reaction mixture filtrate, to provide a reaction output solution and precipitating purified terephthalic acid from the reaction output solution.

[0043] In connection with the purification method, the following features can be realised individually or in any combination with the aforementioned features:

[0044] - The reaction mixture may be stirred at a pH of 14.

[0045] - The reaction mixture filtrate may be stirred at a pH of 7.

[0046] - Graphite, activated carbon, and molecular sieve may be provided for contacting with the unpurified terephthalic acid at a 1:6:2 ratio.

[0047] - Graphite, activated carbon, and molecular sieve may be provided to the reaction mixture filtrate at a 1 :6:2 ratio.

[0048] - Purified terephthalic acid may be precipitated from the reaction output solution using an acid, preferably hydrochloric acid or sulfuric acid.

[0049] - The first period of time may be between 10 and 120 minutes, preferably 30 minutes.

[0050] - The second period of time may be between 10 and 120 minutes, preferably 30 minutes.

[0051] - The method may be performed at room temperature.

[0052] - The graphite may comprise amorphous, crystalline or flake graphite, of a purity of 99% and a particle size range of 5-30 microns.

[0053] - The activated carbon may comprise particulate form activated carbon with a surface area of 500-1500 m2 per gram, and a particle size less than 1mm.

[0054] - The molecular sieve may comprise an aluminosilicate crystal molecular sieve, such as Zeolite 13X molecular sieve. Zeolite 13X comprises average pores measuring 9 angstrom, and may absorb molecules with a kinetic diameter smaller than 9 angstrom.

[0055] The purification method may be conducted as a batch operation, and performed in a reactor vessel, preferably equipped with an agitator, and a plug drain filter. In some examples, the method described herein may be conducted as a batch operation, across multiple reaction vessels, in a series configuration. In some examples, the method described herein may be modified to be operated as a continuous process.

[0056] The purification method may reliably output a purified terephthalic acid (PTA) which - when dryed - is up to 1% purer and 5% brighter white in color than virgin terephthalic acid.

[0057] The output is typically a metalated salt of terephthalic acid (M-PTA, with “M” denoting a metal such as Na* or K* that may originate from a hydrolysis process) dissolved in water, at close to the maximum solubility of M-PTA in solution at 7 to 13% by weight. The output may then be subjected to the crystallization method.

[0058] In some embodiments wherein the acid of step c) and / or step e) may be selected from the group of acetic acid, HCI, H2SO4, formic acid, propionic acid and butyric acid.

[0059] In some embodiments the cold water of step g) may have a temperature of approx. 10 to 20°C.

[0060] Brief Explanation of the Figures

[0061] The invention is described in greater detail below with reference to embodiments that are illustrated in the figures. The figures show:

[0062] Fig. 1 a microscopy image of PTA crystals;

[0063] Fig. 2 a graph showing a temperature profile of sample 27 with a set temperature range of 60 - 65°C.

[0064] Embodiments of the Invention

[0065] The unpurified terephthalic acid may be sourced from the output of a polymer recycling process, such as the room temperature alkaline polymer hydrolysis process described in PCT Application Publication No. W02020173961 A1.

[0066] Such unpurified terephthalic acid may comprise contaminants including, solid pigments such as carbon black, isostructural monomers such as phthalic acid, isophthalic acid, as well as molecules that can be present as contaminants in PET such as benzoic acid, p- toluic acid, bisphenol A, 4-carboxybenzaldehyde, metal ions and / or other contaminants originating from the waste PET. To remove the impurities, the unpurified terephthalic acid may be subject of a purification process described in U.S. provisional application No. 63425771. The purified terephthalic acid resulting from the purification process is in the form of M- TPA (with “M” denoting a metal such as Na* or K* that may originate from a hydrolysis process) dissolved in water, at close to the maximum solubility of M-TPA in solution at ~ 13% by weight. Examples:

[0067] Examples of a crystallization process are given in Table 1 whereof Samples 1 to 18 are without cooling-heating-cycle and Samples 19 to 27 were subjected to at least one cooling-heating-cycle.

[0068] Table 1

[0069] Following the adsorption purification process, a neutral (pH 7) Na-PTA liquid phase is provided (PTA solution) for all Samples and heated to a maximum crystallization temperature (higher temperature in column "Temp."). At the maximum crystallization temperature, the pH of the PTA solution is corrected to a pH of 5 using an acetic acid (column "1st Acid Addition (pH 5)"). The temperatures in column "Temp." refer to set temperatures not taking account of difference due to system inertia.

[0070] The PTA solution of Samples 19-27 were then put through a cooling-heating-cycle (Samples 19 - 27), in which the temperature of the PTA solution is decreased from the maximum crystallization temperature to a minimum crystallization temperature (lower temperature in column "Temp.") and heated back to the maximum crystallization temperature as soon as the minimum crystallization temperature is reached. The process of cooling and re-heating is considered as one cycle. Different samples were subject to different number of cycles (column "Cycles"). The maximum crystallization temperature was kept for 1 minute (defined heating period) before the next cycle or next step started. The heating rate was set to approx. 5°C per 10 minutes. The cooling rate was approx. 5°C per 15 minutes without active cooling. Depending on the scale-up active cooling may be required.

[0071] Sample 19 differs from Samples 20-27 as it went through only one cycle. The maximum crystallization temperature was held for 1 hour before cooling. The minimum crystallization temperature was held for 10 minutes before reheating.

[0072] After the last cycle, the pH of the PTA solutions of Samples 19-27 was adjusted to a pH of 1-3 using sulfuric acid (H2SO4) (column "2nd Acid Addition (pH 1-3)") to fully precipitate crystalline PTA. The samples were then filtered at the maximum crystallization temperature to collect the crystalline PTA, which was immediately washed with cold water (10 - 20°C) to remove any salts, e.g. originating from the M-PTA, that also crystallized, as well as any water-soluble impurities (column "Washing Step").

[0073] Samples 1 to 18 differ from Samples 19 to 27 in that they were not subjected to a cooling- heating-cycle and followed a slightly different protocol (column "Method Description" and "Washing Step"). In column "Method Description", AA stands for acetic acid. "Normal precipitation method": Second acid directly added to the Na-TPA solution, allowing the precipitation of TPA. "Normal addition, then precipitation": First acid was added to bring the pH to 5, then an additional amount of second acid was added to bring the pH to pH1 , allowing the precipitation of TPA. "AA xx time, then H2SO4": Acetic acid was added to bring the pH to 5, then allowed to stir for xx amount of time. After this, an additional amount of sulfuric acid was added to bring the pH to pH1, allowing the precipitation of TPA. After filtering and washing, all samples (1 to 27) were dried and their properties were tested regarding crystal shape (Visual), Ratio of MEG:TPA required for pre-production paste, and density (Table 2).

[0074] Table 2

[0075] Crystal Shape (Visual): Samples 14 to 27 showed "large needles" in or close to the desired size.

[0076] Ratio of MEG:TPA required for pre-production paste: 0.1 moles of TPA was weighed out and enough (mono-)ethylene glycol (EG or MEG) was added to the powder to produce a flowable paste. The weight of EG added was calculated from the volume and density of EG, and the molar ratio of MEG to TPA was calculated. The ratio of MEG:TPA often used for the industrial production of PET is in the range of 1.2 - 1.5. In general, lower ratios are preferred. Samples 20-27 with at least one cycle show this ratio decreasing as compared to samples 1-18 without cycles, which is indicative that the particle size is getting larger. Sample 21-28 were close to the range often used by the industrial production of PET.

[0077] Density: TPA was poured into a known volume of space, without any compression, and the amount of TPA required to fill the space was weighed. The density in g / cm3was then calculated. The density often used for the industrial production of PET is greater than 0.8 g I cm3. Samples 20-27 with at least one cycle show an increasing density as compared to samples 1-18 without cycles.

[0078] Fig. 1 shows a microscopy image of PTA crystals. In the lower left of the image are crystals produced by the described method. In upper right of the image are crystals of commercially obtained PTA for comparison.

[0079] Fig. 2 shows a temperature profile of the cooling-heating-cycle of sample 27 with a set temperature range of 60 - 65°C. The upper thick line marks the set maximum crystallization temperature of 65°C. The lower thick line marks the set minimum crystallization temperature of 60°C. As can be seen in the temperature profile the actual measured maximum and minimum crystallization temperature of the PTA solution in the reactor is up to 3°C higher or lower as the set temperature. The difference is due to the system inertia in that the heating or cooling is stopped or reversed as soon as the set temperature in the PTA solution is reached.

[0080] Reference signs

[0081] 1 PTA crystals obtained by described method

[0082] 2 commercial PTA crystals

Claims

Claims1. A crystallization method for purified terephthalic acid (PTA) to obtain crystalline PTA with an average crystal size of approx. 50-150 microns, the method comprising in the following order the steps of: a. providing in a reactor a PTA solution of 7-13 wt% of purified metalated salt of terephthalic acid (M-PTA) in water at a pH of 7; b. heating the PTA solution to a maximum crystallization temperature; c. adjusting the PTA solution to a pH of 5 using an acid; d. subjecting the PTA solution to a cooling-heating-cycle for at least 1 time, preferably at least 3 times, more preferably 5 to 8 times, to obtain growing PTA crystals, wherein the solution is cooled to a minimum crystallization temperature and subsequently heated to the maximum crystallization temperature; e. at the maximum crystallization temperature adjusting the pH of the PTA solution to 1 - 3 using an acid for full precipitation of crystalline PTA; f. at the maximum crystallization temperature filtering the PTA solution to collect crystalline PTA; g. washing the filtered crystalline PTA with cold water; h. drying the washed crystalline PTA.

2. Method according to claim 1, wherein the minimum crystallization temperature and the maximum crystallization temperature of the cooling-heating-cycle are chosen and set between 40-85 °C, preferably selected from the range 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. Method according to claim 2, wherein an actual minimum crystallization temperature of the PTA solution in the reactor is up to approx. 3°C lower than the set minimum crystallization temperature and / or an actual maximum crystallization temperature of the PTA solution in the reactor is up to approx. 3°C higher than the set maximum crystallization temperature.

4. Method according to one of the preceding claims, wherein the difference between the minimum crystallization temperature and the maximum crystallization temperature is in a range of 5-40°C, preferably 5-20°C, more preferably 5-10°C.

5. Method according to one of the preceding claims, wherein the method is performed under normal atmospheric pressure.

6. Method according to one of the preceding claims, wherein the cooling and / or heating is performed at a rate of approx. 5°C per 10-20 minutes.

7. Method according to one of the preceding claims, wherein one cooling-heating-cycle lasts approx. 10 to 40 minutes.

8. Method according to one of the preceding claims, wherein a cooling period and / or a heating period last approx. 5 to 20 minutes.

9. Method according to one of the preceding claims, wherein in each cooling-heating- cycle the maximum crystallization temperature is kept for approx. 1 to 3 minutes before the next cooling-heating-cycle or the next step starts.

10. Method according to one of the preceding claims, wherein in each cooling-heating- cycle the heating starts immediately when the minimum crystallization temperature is reached.

11. Method according to one of the preceding claims, wherein the metalated salt of terephthalic acid (M-PTA) is either sodium terephthalic acid (Na-PTA) or potassium terephthalic acid (K-PTA).

12. Method according to one of the preceding claims, wherein the M-PTA is obtained from depolymerization of waste PET via alkaline hydrolysis.

13. Method according to one of the preceding claims, wherein the M-PTA is purified using graphite, activated carbon and molecular sieve to remove organic and inorganic contaminants.

14. Method according to one of the preceding claims, wherein the acid of step c) and / or step e) is selected from the group of acetic acid, HCI, H2SO4, formic acid, propionic acid and butyric acid.

15. Method according to one of the preceding claims, wherein the cold water of step g) has a temperature of approx. 10 to 20°C.

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