Crystallization method of purified terephthalic acid

Abstract

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

Description

TECHNICAL FIELD

[0001] The present invention relates to a crystallization process of purified terephthalic acid (PTA) to obtain crystalline PTA with an average crystal size of about 50 to 150 microns. BACKGROUND

[0002] Poly(ethylene terephthalate), widely known as PET, is a semi-crystalline thermoplastic polyester used in various industries in the form of fibers, sheets, films, and bottles. Its stability, high mechanical strength, high resistance to atmospheric and biological agents, and good aesthetic appearance lead to its widespread use in commercial and industrial sectors. While PET has become an integral part of our lives, its pollution in our oceans and landfills raises environmental concerns. The environmental impact of PET production is not limited to post-consumer PET polluting landfills. It has also been reported that while other industrial sectors can reduce their carbon footprint, the petrochemical industry that produces PET will eventually increase greenhouse gas emissions as PET production increases, thus eroding the climate benefits.

[0003] The most common industrial synthesis route for 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 at a temperature of about 280°C. The base chemicals used for this process (EG, DMT, and PTA) are typical bulk chemicals obtained by the petrochemical industry from the catalytic reforming of petroleum naphtha into para-xylene. Therefore, a closed-loop recycling would have a cascading effect: from removing post-consumer waste from our environment, to reducing greenhouse gas emissions by increasing the supply of PTA in the market, and reducing our dependence on PTA from the oil industry.

[0004] After consuming PET either commercially or industrially, users typically recover one or more products through four different methods, which are referred to as primary to quaternary recycling depending on the quality of the recovered product. Primary recycling specifically deals with industrial PET scrap and salvage, while secondary recycling physically reprocesses post-consumer PET through grinding, washing, drying, and reprocessing. However, the quality of the PET obtained through secondary recycling is not pristine, so most of it ends up being incinerated to recover the energy content (quaternary recycling). Ultimately, tertiary recycling or depolymerizing PET into its starting monomers is an ideal method for a closed-loop recycling, as the monomers can be resold to the chemical industry to form pristine PET or other products.

[0005] The same applicant describes in WO2020173961 an efficient, inexpensive, robust, and practical technology for degrading plastic waste (e.g., PET material) and simultaneously producing terephthalic acid (TPA) and / or ethylene glycol (EG) and / or other monomers that form plastic materials.

[0006] WO2020173961 (incorporated by reference in its entirety) provides a method for the alkaline hydrolysis of one or more plastic polymers into terephthalic acid (TPA) and / or ethylene glycol (EG) and / or other monomers that form one or more plastic polymers. The method can output terephthalic acid that is contaminated or of a quality that requires further purification before further processing and use. Current methods of purifying TPA are energy intensive and economically inefficient.

[0007] U.S. Provisional Application No. 63425771 of the same applicant (incorporated by reference in its entirety) describes a method for purifying terephthalic acid. The method includes contacting unpurified terephthalic acid with graphite, activated carbon, and molecular sieves to provide a reaction mixture, agitating the reaction mixture for a first specified period of time, filtering the reaction mixture to provide a reaction mixture filtrate, providing graphite, activated carbon, and molecular sieves to the reaction mixture filtrate, agitating the reaction mixture filtrate for a second specified 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.

[0008] To process PTA into polyethylene terephthalate (PET) and other polyester-based polymers with TPA monomers, crystalline PTA with a crystal size of 50 to 150 microns is required. This is to allow for efficient handling and flowability, and also to allow for the optimal ratio of PTA and other monomers (ethylene glycol in the case of PET) to form a paste suitable for polymerization processes.

[0009] Common methods of purifying TPA to obtain purified terephthalic acid (PTA) typically include a recrystallization step that requires high temperatures (>100 °C) and high pressures (>1 atm) to bring PTA crystals to a suitable size (average of 100 microns) for subsequent polycondensation processes. The crystallization process is energy intensive and requires the use of hazardous solvents, such as dimethylformamide. Such solvents can endanger the safety of operators and require expensive, time-consuming, and complex procedures for handling and management.

[0010] Industrial crystallization is an efficient purification means for many organic and inorganic compounds. Under crystallization, there are two types of nucleation: primary nucleation and secondary nucleation. Primary nucleation is fast and usually produces small, irregularly shaped particles, which means that primary nucleation is usually not used in industry because of the risk of nucleating on foreign particles, thereby trapping impurities into the crystals. Secondary nucleation occurs when the level of supersaturation is high enough to form crystals in the presence of other crystals of the same material. Secondary nucleation is usually used in industry and is also found in the crude terephthalic acid (CTA) purification process.

[0011] Therefore, industrial PTA is produced in a two-step process: (1) The first stage of the process includes the oxidation of para-xylene to produce CTA in powder form. The impurity 4-CBA is a byproduct of the process. The CTA powder is mixed with deionized water to form a slurry, which is passed to the next stage, purification. (2) The CTA purification process is carried out at high temperature and high pressure. The powder CTA (29 wt%) is mixed with deionized 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 is reacted with nitrogen and hydrogen in the presence of a carbon-coated palladium catalyst, thereby converting 4-CBA to p-toluic acid. The slurry with an acceptable level of 4-CBA (<25 ppm) is passed through multi-stage flash crystallization, in which the main contaminants, p-toluic acid and benzoic acid, are removed.[1], [2]

[0012] The residence time in each crystallizer is reported to be 20 to 50 minutes to obtain a crystal size of 118 microns.[1] Therefore, it is estimated that the five-stage crystallization step takes about 1.5 to 4 hours.

[0013] In newer iterations of the crystallization process, more solvents are introduced to reduce the time required to reach the required purity. These solvents commonly used are acetic acid, DMSO, DMF, DMA, alcohols, and in some iterations involve the use of catalysts such as Mn, Br, Co, Zn. However, in all cases, there is an energy loss associated with the use of high temperatures and high pressures in the conversion of CTA to PTA.

[0014] An important factor in the production of purified terephthalic acid is the formation of crystals that have a size and shape that allow good handling, washing, and filtration performance in the PTA production process, as well as easier handling and better processability in the polyester process.

[0015] References:

[0016] [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.

[0017] [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. SUMMARY

[0018] It is an object of the present invention to reduce the energy consumption in the production of purified terephthalic acid (PTA) obtained by chemical recycling of PET, thereby reducing the energy consumption of the overall recycling process of PET.

[0019] At least one object of the present invention is achieved by a crystallization method according to claim 1. The crystallization method of purified terephthalic acid (PTA) for obtaining crystalline PTA having an average crystal size of about 50 to 150 microns comprises the following steps in the following order: (a) providing a PTA solution of 7 to 13 wt% purified metalized salt of terephthalic acid (M-PTA) in water having a pH of 7 in a reactor; (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 at least once, preferably at least 3 times, more preferably 5 to 8 times, to obtain grown PTA crystals, wherein the solution is cooled to a minimum crystallization temperature and subsequently heated to the maximum crystallization temperature; (e) adjusting the pH of the PTA solution to 1 to 3 at the maximum crystallization temperature for complete precipitation of crystalline PTA; (f) filtering the PTA solution at the maximum crystallization temperature to collect the crystalline PTA; (g) washing the filtered crystalline PTA using cold water; (h) drying the washed crystalline PTA.

[0020] The inventors have realized that by using several cooling-heating cycles, the crystallization process can be performed at low temperatures in the range of 40 to 85 °C and the PTA crystals can be grown to the desired size. Furthermore, the process can be performed at normal atmospheric pressure (about 1013 mbar). Thus, the energy consumption of the crystallization process can be reduced compared to known crystallization processes using high temperatures (> 100 °C) and high pressures (> 1 atm). After the cooling-heating cycle, the grown crystals are completely precipitated at a pH of 1 to 3 at the highest crystallization temperature and are immediately filtered and washed with cold water to remove any salts (from M-PTA) and any water-soluble impurities that have also crystallized.

[0021] Cooling can be achieved by active cooling of the PTA solution or by passive cooling by simply removing the heat source and letting the PTA solution cool without energy input. Good results are achieved by heating at a rate of about 5 °C / 10 minutes and cooling at a rate of about 5 °C / 15 minutes. Preferably, the highest crystallization temperature can be maintained for about 1 to 2 minutes before the next cycle or step is started in each cycle. Once the PTA solution reaches the lowest crystallization temperature, heating can be started.

[0022] It should be noted that the highest and lowest crystallization temperatures can be selected by setting the heating and / or cooling system of the reactor to the desired temperature range. The highest crystallization temperature set thereby and the lowest crystallization temperature set refer to the settings of the heating and cooling system of the reactor. Due to system inertia, the actual measured temperature of the PTA solution can be up to about 3 °C higher or lower than the set highest or lowest temperature. Typically, the difference between the set temperature and the actual temperature is in the range of 0 °C to 3 °C. The difference should not be higher than 5 °C.

[0023] Typically, steps b) to e) are performed while the PTA solution is stirred.

[0024] Further embodiments of the present application are set out in the dependent claims.

[0025] In some embodiments, the lowest and highest crystallization temperatures of the cooling-heating cycle can be selected and set between 40 to 85 °C, preferably selected from the range of 40 to 45 °C, 45 to 50 °C, 50 to 55 °C, 55 to 60 °C, 60 to 65 °C, 65 to 70 °C, 70 to 75 °C and 75 to 80 °C. The actual temperature in the reactor can differ from the set temperature by a few degrees.

[0026] The set highest crystallization temperature can be in the range of 45 °C to 85 °C. The set lowest crystallization temperature can be about 5 °C lower than the highest crystallization temperature. In other words, good crystal growth can be achieved with a small temperature change of about 5 °C between the set highest crystallization temperature and the set lowest crystallization temperature.

[0027] Due to system inertia, the actual minimum crystallization temperature of the PTA solution in the reactor can be up to about 3 °C lower than the set minimum crystallization temperature, and / or the actual maximum crystallization temperature of the PTA solution in the reactor can be up to about 3 °C higher than the set maximum crystallization temperature.

[0028] In some embodiments, the difference between the set or actual minimum crystallization temperature and the set or actual maximum crystallization temperature can be in the range of 5 to 40 °C, 5 to 20 °C, 5 to 15 °C, or 5 to 10 °C.

[0029] In some embodiments, the method can be carried out at ambient pressure.

[0030] In some embodiments, the cooling and / or heating can be carried out at a rate of about 5 °C per 10 to 15 minutes.

[0031] In some embodiments, one cooling-heating cycle lasts for about 10 to 40 minutes.

[0032] In some embodiments, the cooling period and / or the heating period can last for about 5 to 20 minutes.

[0033] In some embodiments, in each cooling-heating cycle, the maximum crystallization temperature can be held for about 1 to 3 minutes before the next cooling-heating cycle or step is started.

[0034] In some embodiments, in each cooling-heating cycle, the heating can start immediately upon reaching the minimum crystallization temperature.

[0035] In some embodiments, the metalized salt of terephthalic acid (M-PTA) can be sodium terephthalate (Na-PTA) or potassium terephthalate (K-PTA).

[0036] In some embodiments, the M-PTA can be obtained from depolymerization of waste PET by alkaline hydrolysis, preferably by the method described in WO2020173961 and incorporated herein by reference in its entirety. The depolymerization can be achieved by a method for the alkaline hydrolysis of one or more plastic polymers into terephthalic acid (TPA) and ethylene glycol (EG) and / or other monomers forming the one or more plastic polymers, the method comprising a) contacting one or more plastic polymers with a metal oxide in the presence of a base in solution to provide a reaction mixture; b) stirring the reaction mixture under UV light for a suitable time; c) recovering terephthalic acid, ethylene glycol, and / or other monomers from the reaction mixture.

[0037] The one or more plastic polymers can be selected from the group comprising polylactic acid (PLA), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polytrimethylenesorbitol terephthalate (PEIT), polyethylene furanoate (PEP), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC) or combinations thereof. Preferably, polyethylene terephthalate (PET). The base can be selected from the group consisting of NaOH, NaO t Bu, KOH.

[0038] In some embodiments, the M-PTA that can be obtained from depolymerization of waste PET can be further purified to remove organic and inorganic contaminants using graphite, activated carbon and molecular sieves. The purification can be performed using the method described in U.S. Provisional Application No. 63425771, of the same applicant and incorporated herein by reference in its entirety.

[0039] The purification method can comprise contacting the unpurified terephthalic acid with graphite, activated carbon and molecular sieves to provide a reaction mixture, stirring the reaction mixture for a first specified period of time, subjecting the reaction mixture to filtration to provide a reaction mixture filtrate, providing graphite, activated carbon and molecular sieves to the reaction mixture filtrate, stirring the reaction mixture filtrate for a second specified period of time, subjecting the reaction mixture filtrate to filtration to provide a reaction output solution, and precipitating purified terephthalic acid from the reaction output solution.

[0040] In connection with the purification method, the following features can be implemented individually or in any combination with the preceding features:

[0041] - The reaction mixture can be stirred at a pH of 14.

[0042] - The reaction mixture filtrate can be stirred at a pH of 7.

[0043] - The graphite, activated carbon and molecular sieves can be provided in contact with the unpurified terephthalic acid in a ratio of 1 :6:2.

[0044] - The graphite, activated carbon and molecular sieves can be provided to the reaction mixture filtrate in a ratio of 1 :6:2.

[0045] - The purified terephthalic acid can be precipitated from the reaction output solution using an acid, preferably hydrochloric acid or sulfuric acid.

[0046] - The first period of time can be between 10 and 120 minutes, preferably 30 minutes.

[0047] - The second period of time can be between 10 and 120 minutes, preferably 30 minutes.

[0048] - The method can be performed at room temperature.

[0049] - Graphite can comprise amorphous, crystalline or flaky graphite with a purity of 99% and a particle size range of 5 to 30 microns.

[0050] - Activated carbon can comprise activated carbon in particulate form having a surface area of 500 to 1500 square meters per gram and a particle size of less than 1 micron.

[0051] - Molecular sieves can comprise aluminosilicate crystalline molecular sieves, such as zeolite 13X molecular sieves. Zeolite 13X comprises an average pore size of 9 Angstroms as measured and can adsorb molecules having a kinetic diameter of less than 9 Angstroms.

[0052] The purification process can be implemented as a batch operation and carried out in a reactor vessel, which is preferably equipped with a stirrer and a plug drain filter. In some embodiments, the process described herein can be implemented as a batch operation across multiple reactor vessels in a series configuration. In some embodiments, the process described herein can be modified to operate as a continuous process.

[0053] The purification process can reliably output purified terephthalic acid (PTA) that, after drying, has a purity that is up to 1% higher and a whiteness that is up to 5% higher than the original terephthalic acid.

[0054] The output is typically a metalized salt of terephthalic acid (M-PTA, where "M" represents a metal that can result from the hydrolysis process, such as Na* or K*) dissolved in water, which has a solubility close to the maximum solubility of M-PTA in solution of 7 to 13 wt%. The output can then be subjected to a crystallization process.

[0055] In some embodiments, the acid of step c) and / or step e) can be selected from the group of acetic acid, HC1, H2SO4, formic acid, propionic acid, and butyric acid.

[0056] In some embodiments, the temperature of the cold water of step g) can be about 10 to 20 °C. BRIEF DESCRIPTION OF DRAWINGS

[0057] The present application is described in more detail below with reference to embodiments shown in the drawings. The drawings show:

[0058] Figure 1 A microscope image of PTA crystals is shown;

[0059] Figure 2 A temperature profile of sample 27 is shown, where the temperature range is set to 60 to 65 °C. DETAILED DESCRIPTION

[0060] The unpurified terephthalic acid can be derived from the output of a polymer recycling process, such as the room temperature alkaline polymer hydrolysis process described in PCT Application Publication No. WO2020173961A1.

[0061] Such unpurified terephthalic acid can contain contaminants, including solid pigments (e.g., carbon black), structural isomeric monomers (e.g., o-phthalic acid, m-phthalic acid), and molecules that can be present in PET as contaminants (e.g., benzoic acid, p-methyl benzoic acid, bisphenol A, 4-carboxybenzaldehyde, metal ions) and / or other contaminants derived from waste PET. To remove impurities, the unpurified terephthalic acid can be subjected to the purification process described in U.S. Provisional Application No. 63425771.

[0062] The purified terephthalic acid resulting from the purification process is in the form of M-TPA (where “M” represents a metal that can result from the hydrolysis process, such as Na+or K+) dissolved in water, which solubility is close to the maximum solubility of M-TPA in solution of about 13 wt%.

[0063] Embodiments:

[0064] Examples of the crystallization process are given in Table 1, in which samples 1-18 were not subjected to a cool-heat cycle, and samples 19-27 were subjected to at least one cool-heat cycle.

[0065]

[0066] Table 1

[0067] After the adsorption purification process, a neutral (pH 7) Na-PTA liquid phase (PTA solution) was provided for all samples and heated to the maximum crystallization temperature (higher temperature in the “Temperature” column). At the maximum crystallization temperature, the pH of the PTA solution was corrected to pH 5 using acetic acid (column “First acid addition (pH 5)”). The temperature in the “Temperature” column refers to the set temperature, not accounting for differences due to system inertia.

[0068] The PTA solutions of samples 19 to 27 were then subjected to a cool-heat cycle (samples 19 to 27) in which the temperature of the PTA solution was decreased from the maximum crystallization temperature to the minimum crystallization temperature (the lower temperature in the "Temperature" column), and once the minimum crystallization temperature was reached, heated back to the maximum crystallization temperature. The process of cooling and reheating was considered one cycle. Different samples were subjected to different numbers of cycles ("Cycles" column). The maximum crystallization temperature was held for 1 minute (the prescribed heating time) before starting the next cycle or next step. The heating rate was set to about 5 °C per 10 minutes. The cooling rate was about 5 °C per 15 minutes without active cooling. Depending on the scale-up, active cooling can be required.

[0069] Sample 19 was different from samples 20 to 27 in that sample 19 only experienced one cycle. The maximum crystallization temperature was held for 1 hour before cooling. The minimum crystallization temperature was held for 10 minutes before reheating.

[0070] After the last cycle, the pH of the PTA solutions of samples 19 to 27 was adjusted to a pH of 1 to 3 using sulfuric acid (H2SO4) ("Second acid addition (pH 1 to 3)" column) to completely precipitate the 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 to 20 °C) to remove any salts (e.g., salts derived from M-PTA) and any water-soluble impurities that also crystallized ("Washing step" column).

[0071] Samples 1 to 18 were different from samples 19 to 27 in that samples 1 to 18 were not subjected to a cool-heat cycle and followed a slightly different protocol ("Method description" and "Washing step" columns). In the "Method description" column, AA stands for acetic acid. "Normal precipitation method": a second acid was added directly to the Na-TPA solution, allowing the TPA to precipitate. "Normal addition, then precipitation": a first acid was added to bring the pH to 5, and then an additional amount of a second acid was added to bring the pH to a pH of 1, allowing the TPA to precipitate. "AA xx time, then H2SO4": acetic acid was added to bring the pH to 5, and then the stirring was allowed to continue for xx time. After that, an additional amount of sulfuric acid was added to bring the pH to a pH of 1, allowing the TPA to precipitate.

[0072] After filtration and washing, all samples (1 to 27) were dried, and the properties of these samples with respect to the crystal shape (visually), the MEG:TPA ratio required for a pre-production paste, and the density were tested (Table 2).

[0073]

[0074] Table 2

[0075] Crystal shape (visual): Samples 14 to 27 show the desired size or close to the desired size of "coarse needles".

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

[0077] Density: TPA is poured into a known volume of space without any compression and the amount of TPA required to fill the space is weighed. The density is then calculated (unit: g / cm 3 ). The usual density for industrial production of PET is greater than 0.8 g / cm 3 . Samples 20 to 27 with at least one cycle show an increased density compared to samples 1 to 18 without a cycle.

[0078] Figure 1 Microscope images of PTA crystals are shown. In the lower left of the image are crystals produced by the method. In the upper right of the image are crystals of commercially obtained PTA for comparison.

[0079] Figure 2 Temperature profile of the cool-heat cycle of sample 27 is shown, where the temperature range set is 60 to 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. From the temperature profile, it can be seen that the actual measured maximum and minimum crystallization temperatures of the PTA solution in the reactor are up to 3 °C higher or lower than the set temperatures. This difference is due to system inertia, i.e. once the set temperature in the PTA solution is reached, heating or cooling is stopped or reversed.

[0080] Reference signs

[0081] 1 PTA crystals obtained by the method

[0082] 2 Commercialized PTA crystals

Claims

1. A crystallization process of purified terephthalic acid (PTA) for obtaining crystalline PTA having an average crystal size of about 50 to 150 microns, said process comprising the steps in the following order: a. providing a PTA solution of 7 to 13 wt% purified metalized salt of terephthalic acid (M-PTA) in water having a pH of 7 in a reactor; 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 at least once, preferably at least 3 times, more preferably 5 to 8 times, to obtain grown PTA crystals, wherein the solution is cooled to a minimum crystallization temperature and subsequently heated to a maximum crystallization temperature; e. adjusting the pH of the PTA solution to 1 to 3 at the maximum crystallization temperature using an acid for complete precipitation of crystalline PTA; f. subjecting the PTA solution to filtration at the maximum crystallization temperature to collect crystalline PTA; g. washing the filtered crystalline PTA using cold water; h. drying the washed crystalline PTA.

2. The process 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 to 85 °C, preferably selected from the range of 40 to 45 °C, 45 to 50 °C, 50 to 55 °C, 55 to 60 °C, 60 to 65 °C, 65 to 70 °C, 70 to 75 °C and 75 to 80 °C.

3. The process according to claim 2, wherein the actual minimum crystallization temperature of the PTA solution in the reactor is at most 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 3 °C higher than the set maximum crystallization temperature.

4. The process according to any one of the preceding claims, 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, more preferably 5 to 10 °C.

5. The process according to any one of the preceding claims, wherein the process is carried out at normal atmospheric pressure.

6. The process according to any one of the preceding claims, wherein cooling and / or heating is carried out at a rate of about 5 °C per 10 to 15 minutes.

7. The process according to any one of the preceding claims, wherein one cooling-heating cycle lasts for about 10 to 40 minutes.

8. The process according to any one of the preceding claims, wherein the cooling period and / or the heating period lasts for about 5 to 20 minutes.

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

10. The process according to any one of the preceding claims, wherein at each cooling-heating cycle, heating starts immediately upon reaching the minimum crystallization temperature.

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

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

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

14. The process according to any one of the preceding claims, wherein the acid of step c) and / or step e) is selected from the group of acetic acid, HC1, H2SO4, formic acid, propionic acid and butyric acid.

15. The process according to any one of the preceding claims, wherein the temperature of the cold water of step g) is about 10 to 20 °C.

Images