Polymer recycling

Abstract

A method for producing bis(2-hydroxyethyl) terephthalate (BHET) by recycling polyethylene terephthalate (PET) comprises: (a) depolymerizing PET in the presence of ethylene glycol and a catalyst system in a series, preferably two, of depolymerization reactors to form a depolymerization mixture containing BHET; (b) crystallizing a precipitate containing BHET from the depolymerization mixture; (c) dissolving the precipitate in a protonic solvent, preferably water, but also optionally methanol, to form a solution containing BHET; (d) removing impurities from the solution to form a purified solution containing BHET; and (e) crystallizing a purified product containing BHET from the purified solution. An apparatus suitable for this method and the use of urea in a catalyst system for this method are also provided.

Description

Technology Field

[0001] The present invention relates to a method and apparatus for recycling polymers, in particular to a method for producing bis(2-hydroxyethyl) terephthalate (BHET) by recycling polyethylene terephthalate (PET). The BHET produced using the method and apparatus of the present invention may have a purity level suitable for direct use in the manufacture of high-quality plastics. Background Technology

[0002] PET is a thermoplastic polymer used in a wide range of materials, particularly due to its properties of strength, moldability, and moisture impermeability. Common applications of PET include packaging materials (e.g., beverage bottles and food containers), fibers (e.g., clothing and carpets), and thin films.

[0003] Virgin PET can be easily manufactured using ethylene glycol and terephthalate-containing monomers. Nevertheless, because raw materials are obtained from non-renewable sources such as crude oil, awareness of the need for PET recycling is increasing.

[0004] When PET waste consists solely of a single type of PET, such as clear plastic water bottles, recycling can be simple, involving melting and remolding the waste flakes. However, waste typically contains various types of PET materials, such as bottles of different colors, which can result in low-visual-grade products when melted and remolded. While these materials may be suitable for use in carpet fibers, they are generally not suitable for packaging materials like clear water bottles.

[0005] Therefore, there is a need for a method to recycle waste PET into products that can be used for applications requiring high visual grades.

[0006] A more sophisticated method for recycling PET includes the step of depolymerizing the waste to obtain a viable raw material for use in the manufacture of a polymer, generally after a number of purification and separation steps.

[0007] For example, PET can be depolymerized using a glycolysis agent such as ethylene glycol to form BHET monomers. However, conventional methods for depolymerizing PET produce BHET monomers in a yield of less than 80%, and a significant amount of BHET oligomers, particularly dimers and trimers, tend to be produced from the remaining PET.

[0008] Since the presence of dimers and trimers reduces the quality of polymers produced from BHET raw materials, it is common practice to purify the depolymerization mixture to remove these components. Further purification is particularly important when high-quality recycled PET is required, for example, when recycled PET suitable for use in clear and colorless bottles.

[0009] Color spaces are often used to indicate the grade of polymers, and the b[h] value—a scale of blue (negative values) to yellow (positive values) tones—is taken as a primary indicator of quality. Poor quality recycled PET generally exhibits an undesirable yellow hue.

[0010] There are several disadvantages associated with the process of producing depolymerization mixtures containing significant amounts of dimers and trimers. One of the most significant disadvantages is the loss of a substantial amount of PET raw material from the recycling process when removed in the form of dimers and trimers. Unless the dimers and trimers are recycled for further depolymerization, the efficiency of the general PET recycling process is very low because they require time and energy in themselves.

[0011] Therefore, an improved method for the depolymerization recycling of waste PET is required. In particular, a method is needed for the depolymerization recycling of waste PET that provides products suitable for high-quality applications, such as clear water bottles.

[0012] Surprisingly, it was found that by using a series of depolymerization reactors, a depolymerization mixture containing a very high proportion of BHET monomer and relatively small amounts of dimers and trimers can be obtained, thereby eliminating the conventional purification step for removing dimers and trimers. This implies that solvents previously rejected as unsuitable for further processing of crude BHET monomer can be used.

[0013] The inventors have discovered that protic solvents are highly effective for recrystallizing crude depolymerization products. In particular, since the dimers and trimers of BHET are insoluble in water, water is preferred for this application. Accordingly, BHET dissolves to form an aqueous phase, while the dimers and trimers remain as solid materials that can be separated from the aqueous phase, for example by filtration, before recrystallization, thereby producing a high-purity monomer product.

[0014] Methanol can also be preferably used because it at least partially decolorizes the product while minimizing product loss. Although methanol may dissolve and transport dimers and trimers through the process to be present in the purified product containing BHET, their concentrations can nevertheless be low enough so that the purified product can be used directly in the polymerization reaction. The resulting polymer can be used for high-quality applications, such as clear, colorless water bottles.

[0015] In addition, as described in detail below, it is also possible to use aprotic and even non-polar solvents to recrystallize the crude depolymerization product while maintaining the advantages of using a serial depolymerization reactor according to the present disclosure.

[0016] Accordingly, the present invention provides a method for recycling polyethylene terephthalate (PET), said method comprising the following steps:

[0017] (a) depolymerizing PET in the presence of ethylene glycol and a catalyst system in a series of depolymerization reactors to form a depolymerization mixture comprising bis(2-hydroxyethyl) terephthalate (BHET);

[0018] (b) a step of crystallizing a precipitate containing BHET from a depolymerization mixture;

[0019] (c) a step of dissolving the precipitate in a protonic solvent to form a solution containing BHET;

[0020] (d) a step of removing impurities from the solution to form a purified solution containing BHET; and

[0021] (e) A step of crystallizing a purified product containing BHET from a purified solution.

[0022] The present invention further provides a purified product comprising BHET obtainable using the method of the present invention.

[0023] A method for manufacturing a polymer is also provided, and the method comprises the step of performing a polymerization reaction using a purified product containing BHET of the present invention.

[0024] A PET recycling device is additionally provided, and said device includes the following:

[0025] (a) A series of depolymerization reactors suitable for depolymerizing PET to form a depolymerization mixture containing BHET (wherein the series of depolymerization reactors is configured to accommodate PET, ethylene glycol, and a catalyst system);

[0026] (b) a crystallization unit downstream of a polymerization reactor suitable for crystallizing a precipitate containing BHET from a depolymerization mixture;

[0027] (c) A vessel suitable for receiving a precipitate and dissolving the precipitate in a protonic solvent to form a solution containing BHET;

[0028] (d) an impurity removal unit that receives a solution containing BHET and removes impurities from the solution to form a purified solution; and

[0029] (e) An additional crystallization unit downstream of an impurity removal unit suitable for crystallizing a purified product containing BHET from a purified solution.

[0030] The present invention also provides a use of urea in a catalyst system in a polyethylene terephthalate (PET) recycling process for solubilizing a metal, particularly a transition metal catalyst component of the catalyst system, and / or forming a eutectic salt together with the transition metal catalyst component of the catalyst system. Brief explanation of the drawing

[0031] Fig. 1 This is a graph showing the efficiency of a depolymerization reaction performed using a different series of reactors. Fig. 2 It shows photos of untreated and treated BHET samples with various decolorizing agents, as well as photos of PET prepared using the samples. Fig. 3[Image] is a diagram of an apparatus for carrying out the method of the present invention. The apparatus comprises a series of three depolymerization units (10) for depolymerizing PET to form BHET; a crystallization unit (12) located downstream of a polymerization reactor suitable for crystallizing a precipitate containing BHET from a depolymerization mixture; a vessel (14) suitable for receiving the precipitate and dissolving the precipitate in methanol to form a solution containing BHET; an impurity removal unit (16) suitable for receiving the solution containing BHET and removing impurities from the solution to form a purified solution; and an additional crystallization unit (18) located downstream of the impurity removal unit suitable for crystallizing a purified product containing BHET from the purified solution. Fig. 4 This is a photograph of a typical waste product that can be processed using the device shown in Fig. 3. Fig. 5 is a diagram of an apparatus for carrying out the method of the present invention. The apparatus comprises: a series of two depolymerization units (100) for depolymerizing PET to form BHET; a crystallization unit (112) downstream of a polymerization reactor suitable for crystallizing a precipitate containing BHET from a depolymerization mixture; a vessel (114) suitable for receiving the precipitate and dissolving the precipitate in water to form a solution containing BHET; an impurity removal unit (116) suitable for receiving the solution containing BHET and removing impurities from the solution to form a purified solution; and an additional crystallization unit (118) downstream of the impurity removal unit suitable for crystallizing a purified product containing BHET from the purified solution. Specific details for implementing the invention

[0032] The present invention provides a method for recycling polyethylene terephthalate (PET).

[0033] PET is a thermoplastic polymer with the following structure:

[0034] .

[0035] The PET used in the method of the present invention will generally be waste PET. Waste PET can be obtained from a wide range of sources, including packaging materials, bottles, and fabrics. Preferably, PET is obtained from waste bottles. The PET used in step (a) may be washed PET, i.e., PET that has undergone a washing process. Washed PET may be PET that has been washed with water, purified by steaming, and solvent-washed and / or detergent-washed. Preferably, the PET used in step (a) is PET that has been washed with water.

[0036] The PET used in step (a) preferably contains colored PET. The PET may contain colored PET in an amount of at least 5 weight%, preferably at least 10 weight%, more preferably at least 25 weight%. In some embodiments, the PET may contain colored PET in an amount of at least 50 weight%, more preferably at least 75 weight%. The PET may contain colored PET in an amount of up to 100 weight%.

[0037] The PET used in step (a) preferably has a b[h] value greater than 5, for example, greater than 10 (i.e., a b-value in the Hunter Lab color space), but some PET feeds may have a b[h] value greater than 100. This can be measured using standard techniques such as a colorimeter.

[0038] PET is preferably used in step (a) in the form of flake-like particles. Preferably, at least 80 weight percent of the particles (i.e., d80) pass through a mesh having an opening with a diameter of 20 mm, preferably 15 mm, more preferably 12 mm. Even smaller mesh sizes may also be used. Particles of these sizes are rapidly depolymerized.

[0039] Although a range of particle sizes is generally used in step (a), larger particle sizes are preferably avoided because they may take longer to process. Accordingly, 100 weight percent of particles (d100) preferably pass through a mesh having an opening with a diameter of 25 mm, preferably 20 mm, and more preferably 12 mm. Even smaller mesh sizes may also be used. Unless the powder is already available through a waste collection and separation process, excessively small particles are also preferably avoided, as the energy and thus the cost required to grind PET to this size are not required. Up to 1 weight percent of particles preferably pass through a mesh having an opening with a diameter of 0.1 mm, preferably 0.5 mm, and more preferably 1 mm.

[0040] It will be understood that the PET used in step (a) may be transferred to a series of reactors in a form coated with the liquid used to clean the PET, for example, residue or other solvent. This liquid coating is not considered to form part of the PET for the purposes of the present invention.

[0041] In step (a) of the method, PET is depolymerized in a series of depolymerization reactors to form a depolymerization mixture containing bis(2-hydroxyethyl) terephthalate (BHET). BHET is a monomer having the following structure:

[0042] .

[0043] PET is partially depolymerized in a first depolymerization reactor and further depolymerized downstream of the first reactor in a series of reactors. By using a series of reactors, it was found that the depolymerization mixture may contain a high proportion of BHET and low levels of dimers and trimers. The dimers and trimers have the following structures:

[0044] .

[0045] Higher oligomers will generally not be present in the depolymerization mixture. Therefore, in a preferred embodiment, there are substantially no higher oligomers in the depolymerization mixture (i.e., where n ≥ 4).

[0046] Surprisingly, a very high-quality product can be produced by depolymerizing PET in only a series of two reactors. Thus, in a preferred embodiment, PET is depolymerized in a series of two depolymerization reactors. This results in both the conversion rate of PET and the selectivity for BHET being at a high level. In an alternative embodiment, PET is depolymerized in a series of three or alternatively four or more reactors.

[0047] Preferably, all ethylene glycol and catalyst systems used in the depolymerization process are added to the first reactor of the series. However, in some embodiments, additional ethylene glycol and / or catalyst systems may be added to the reaction mixture downstream of the first reactor as it passes through the series of depolymerization reactors.

[0048] You will understand that while ethylene glycol and / or a catalyst system may be added to the reaction mixture downstream of the first reactor, no components are removed from the reaction as it passes through a series of reactors.

[0049] Each depolymerization reactor used in step (a) can be operated at a temperature of at least 150°C, preferably at least 170°C, more preferably at least 190°C. Each depolymerization reactor used in step (a) can be operated at a temperature of up to 230°C, preferably up to 220°C, more preferably up to 210°C. Thus, each depolymerization reactor used in step (a) can be operated at a temperature of 150 to 230°C, preferably 170 to 220°C, more preferably 190 to 210°C.

[0050] Generally, depolymerization reactors operate at the same temperature, but this is not necessarily the case.

[0051] Unlike many prior art processes, PET is preferably not used in a molten state in step (a), which implies that the reaction mixture is relatively viscous. This viscosity has generally led to relatively low PET conversion rates. It is surprising that by using a series of depolymerization reactors, an excellent level of conversion rate can be obtained even when step (a) is performed using solid-state PET.

[0052] Each depolymerization reactor used in step (a) can be operated at atmospheric pressure, that is, without applying or removing pressure. Standard atmospheric pressure is defined as 101,325 Pa. However, since atmospheric pressure varies by location, atmospheric pressure as used herein is considered to be approximately the same as standard atmospheric pressure, i.e., about 101,325 Pa.

[0053] Each depolymerization reactor used in step (a) can be operated for a period of at least 20 minutes, preferably at least 1 hour, more preferably at least 1.5 hours. Each depolymerization reactor used in step (a) can be operated for a period of up to 4 hours, preferably up to 2.5 hours, more preferably up to 1.75 hours. Accordingly, each depolymerization reactor used in step (a) can be operated for 20 minutes to 4 hours, preferably 1 to 3 hours, more preferably 1.5 to 2.5 hours. All depolymerization reactors may be operated for the same period, but are not necessarily so.

[0054] PET can be delivered to a series of depolymerization reactors at a flow rate of at least 100 kg per hour, preferably at least 500 kg, more preferably at least 1,000 kg. PET can be delivered to a series of depolymerization reactors at a flow rate of up to 100,000 kg per hour, preferably up to 50,000 kg, more preferably up to 10,000 kg. Thus, PET can be delivered to a series of depolymerization reactors at a flow rate of 100 to 100,000 kg per hour, preferably 500 to 50,000 kg, more preferably 1,000 to 10,000 kg.

[0055] Each depolymerization reactor used in step (a) is preferably operated while stirring.

[0056] The size of the reactors used in the series of depolymerization reactors may vary depending on how many reactors are used. The size of each reactor used in step (a) is at least 5 m 3 , preferably at least 8 m 3 , more preferably at least 10 m 3 It may be. The size of each reactor used in step (a) is up to 50 m 3 , preferably up to 20 m 3 , more preferably up to 15 m 3 It may be. Therefore, the size of each reactor used in step (a) is 5 to 50 m 3 , preferably 8 to 20 m 3 , more preferably 10 to 15 m 3 This is possible. The use of these small-scale reactors is made possible by having a series of reactors in which PET can be depolymerized with a minimum residence time. Therefore, industrial-scale quantities of PET can be depolymerized into high-quality products using relatively small reactors.

[0057] Ethylene glycol is used as a saccharifier in step (a). Ethylene glycol may be used in step (a) in an amount of at least 2 times, preferably at least 3.25 times, and more preferably at least 3.5 times, the amount of PET by weight. Ethylene glycol may be used in step (a) in an amount of up to 6 times, preferably up to 5 times, and more preferably up to 4.75 times, the amount of PET by weight. Accordingly, ethylene glycol may be used in step (a) in an amount of 2 to 6 times, preferably 3.25 to 4.75 times, and more preferably 3.5 to 4.75 times, the amount of PET by weight.

[0058] At least 60 weight%, preferably at least 80 weight%, more preferably at least 95 weight% of ethylene glycol may be added to the first reactor. However, as mentioned above, all ethylene glycol is most preferably added to the first reactor. It will be understood that if less than 100% of ethylene glycol is added to the first reactor, the remainder is added to a series of depolymerization reactors downstream of the first depolymerization reactor.

[0059] A catalyst system is used in step (a) to enhance the depolymerization reaction. The catalyst system preferably comprises a transition metal catalyst, such as a zinc-containing catalyst. Suitable zinc catalysts include zinc acetate.

[0060] In some embodiments, the catalyst system consists of a transition metal catalyst. However, in a preferred embodiment, the catalyst system comprises, for example, the catalyst described above within a carrier. Suitable carriers include nitrogen-containing carriers, such as urea.

[0061] Urea was found to be surprisingly effective in retaining metals (e.g., transition metal catalyst components of a catalyst system; or trace amounts of metal catalysts originally used to produce PET, e.g., antimony catalysts) and other contaminants in solution, thereby allowing these components to be separated from BHET in step (b). Thus, the present invention also provides for the use of urea in catalyst systems in PET recycling processes for solubilizing metals, particularly transition metal catalyst components of catalyst systems. Urea can also be used to solubilize contaminants in PET recycling processes. Surprisingly, eutectic salt catalyst systems were found to be particularly effective in solubilizing metals and / or contaminants.

[0062] The support can be used in the catalyst system in an amount of at least 1 time, preferably at least 2 times, and more preferably at least 3 times, the molar amount of the transition metal cation in the transition metal catalyst. The support can be used in an amount of up to 8 times, preferably up to 6 times, and more preferably up to 5 times, the molar amount of the transition metal cation. Thus, the support can be used in an amount of 1 to 8 times, preferably 2 to 6 times, and more preferably 3 to 5 times, the molar amount of the transition metal cation. The ratio of the support to these transition metal catalysts has been found to provide a high reaction rate while retaining the metal ions in solution. As mentioned above, the transition metal cation will generally be a zinc cation.

[0063] The most desirable catalyst system for use in step (a) is a catalyst system comprising zinc acetate and urea, preferably composed of the same, in particular a catalyst system having the chemical formula [4NH2CONH2ㆍZnOAc]. This catalyst system advantageously forms a eutectic salt. Accordingly, the present invention also provides a use of urea in a catalyst system in a PET recycling process for forming a eutectic salt together with the transition metal catalyst component of the catalyst system.

[0064] The catalyst system may be in a liquid phase during step (a), preferably throughout the method of the present invention.

[0065] The catalyst system can be used in step (a) in an amount of at least 0.001 times, preferably at least 0.003 times, and more preferably at least 0.004 times, the amount of PET by weight. The catalyst system can be used in step (a) in an amount of up to 0.5 times, preferably up to 0.01 times, and more preferably up to 0.005 times, the amount of PET by weight. Accordingly, the catalyst system can be used in step (a) in an amount of 0.001 to 0.5 times, preferably 0.003 to 0.01 times, and more preferably 0.004 to 0.005 times, the amount of PET by weight.

[0066] At least 60 weight percent, preferably at least 80 weight percent, more preferably at least 95 weight percent of the catalyst system may be added to the first reactor. However, as mentioned above, all of the catalyst system is preferably added to the first reactor. It will be understood that if less than 100% of the catalyst system is added to the first reactor, the remainder is added to a series of depolymerization reactors downstream of the first depolymerization reactor.

[0067] Step (a) is generally performed in the absence of any carrier that may be present in any solvent other than ethylene glycol and catalyst systems. Some residual liquid, e.g., water, passed through the process claimed as a coating on PET due to washing may be present; however, it will be understood that this is not considered a solvent for the purposes of the present invention. Accordingly, the solvent may be present in an amount of up to 0.1 times, preferably up to 0.01 times, and more preferably up to 0.001 times, the amount of PET used in Step (a) by weight in Step (a). Most preferably, substantially no solvent is present in Step (a).

[0068] Preferably, the depolymerization mixture is separated from any insoluble components between steps (a) and (b). Insoluble components include unreacted PET (if present, its levels are generally very low) and other inert solids. Other solids may include non-PET polymers, such as polyethylene (PE) and polypropylene (PP). Preferably, the depolymerization mixture is passed through a filter to remove insoluble components, but other techniques such as centrifugation may also be used. A tricant may be used to achieve a very high level of solid-liquid separation.

[0069] In step (b) of the method, a precipitate containing BHET is crystallized from the depolymerization mixture formed in step (a). Step (b) is preferably carried out using cooling crystallization. Suitable crystallizers include stirred or wall-scraping crystallizers. The depolymerization mixture may be left to cool naturally, but is preferably cooled using a coolant. The coolant may be present in a jacket surrounding the crystallizer, or, for example, may pass through a series of heat exchangers through which the depolymerization mixture also passes in countercurrent.

[0070] Step (b) can be performed by reducing the temperature of the depolymerization mixture to at least 5°C, preferably at least 10°C, more preferably at least 15°C. Step (b) can be performed by reducing the temperature of the depolymerization mixture to a maximum of 50°C, preferably at least 40°C, more preferably at least 35°C. Accordingly, Step (b) can be performed by reducing the temperature of the depolymerization mixture to 5 to 50°C, preferably 10 to 40°C, more preferably 15 to 35°C.

[0071] At these temperatures, incomplete crystallization will occur. However, these temperatures are nevertheless preferred because the amount of active cooling required to reach these temperatures is relatively small. Additionally, in a preferred embodiment (discussed below), the liquid remaining after step (b) is recycled to step (a), which means there is no loss of BHET (and its soluble oligomers) during the process. For similar reasons, only a single crystallizer may be used to perform step (b). If the liquid remaining after step (b) is not recycled, step (b) may be performed in some cases by reducing the temperature of the depolymerization mixture to a temperature of 5 to 15°C.

[0072] Step (b) can be performed at atmospheric pressure, that is, without applying or removing pressure.

[0073] Step (b) can be performed for a period of at least 10 minutes, preferably at least 20 minutes, more preferably at least 25 minutes. Step (b) can be performed for a period of up to 60 minutes, preferably up to 45 minutes, more preferably up to 35 minutes. Accordingly, Step (b) can be performed for a period of 10 to 60 minutes, preferably 20 to 45 minutes, more preferably 25 to 35 minutes.

[0074] The depolymerization mixture can be stirred during step (b).

[0075] As mentioned above, the liquid remaining at the end of step (b) is preferably recycled for use in step (a). Accordingly, the method of the present invention preferably includes a step of isolating a precipitate containing BHET between steps (b) and (c). The precipitate may be isolated using known methods, e.g., filtration or centrifugation. The residual liquid is preferably recycled to a first depolymerization reactor for use in step (a), more preferably. Generally, it will be understood that the residual liquid will not be further processed when recycled to step (a), that is, the composition of the residual liquid will not be modified, but the residual liquid may pass through a pump and be heated. If the catalyst system includes a carrier, e.g., urea and a transition metal catalyst, these will also be recycled along with the residual liquid.

[0076] The conditions used in step (a) may result in a precipitate containing a high proportion of BHET. BHET may be present in the precipitate in an amount of at least 95% by weight, preferably at least 99% by weight, more preferably at least 99.5% by weight.

[0077] The precipitate formed in step (b) contains BHET, but generally will also contain dimers and trimers of BHET, for example, in an amount of at least 0.01 wt%. The dimers and trimers of BHET may be present in the precipitate in an amount of up to 2 wt%, preferably up to 0.5 wt%, and more preferably up to 0.2 wt%. The amounts of different components in the precipitate formed in step (b) can be determined using standard techniques, such as high-performance liquid chromatography (HPLC). HPLC may be performed using the following instrument under the following conditions: Shimazu LC-20A HPLC; detector: photodiode array (PDA) detector, chromatogram center wavelength of 223 nm (4 nm 'slit' bandwidth); column: C18; mobile phase: 30% water 70% methanol; flow rate: 0.5 ml / min; oven temperature: 35°C; sample: dissolved in methanol; Injection volume: 20 uL. The sample is quantified by an external standard method.

[0078] Preferably, in step (c) of the method, the precipitate formed in step (b) is dissolved in water to form a solution containing BHET. Since the dimers and trimers of BHET are insoluble in water, in step (c), BHET is dissolved to form an aqueous phase, while the dimers and trimers remain as solid materials that can be separated from the aqueous phase, for example, by filtration, at the end of step (c). The aqueous solution can then be recrystallized in step (e), and the purified product is used as a high-quality monomer feedstock.

[0079] Alternatively, in step (c) of the method, the precipitate formed in step (b) may be dissolved in methanol to form a solution containing BHET. Surprisingly, methanol was found to be an excellent solvent for use in step (c) because it provides a high level of decolorization of the precipitate formed in step (b) as well as a low level of product loss. However, since the dimers and trimers of BHET are partially soluble in methanol, the use of water is preferred, and thus, when methanol is used for recrystallization in step (c) of the method, they are maintained in detectable amounts in the monomer product.

[0080] In step (c), other alcohol solvents may also be used instead of water or methanol. For example, the solvent in step (c) can be any C1-C 12 It may consist of or include alcohol. More specifically, the solvent for use in step (c) may be selected from the group consisting of ethanol, propanol (particularly iso-propanol), and butanol (particularly n-butanol, tert-butanol). Thus, the solvent for use in step (c) is preferably a protonic solvent, most preferably a polar protonic solvent, and may be selected from the group consisting of water, methanol, ethanol, propanol (particularly iso-propanol), and butanol (particularly n-butanol, tert-butanol). Higher alcohol solvents may also be considered.

[0081] Additionally, a non-alcoholic solvent for use in step (c) may be considered. Although the use of a protonic solvent, particularly a polar protonic solvent, is particularly preferred in step (c), in an embodiment of the present disclosure, the solvent used in step (c) may instead be a polar non-protonic solvent, e.g., dimethyl carbonate (DMC), or a non-polar solvent, e.g., ether such as dimethoxyethane (DME) or diisopropyl ether (DIPE).

[0082] Most commonly, the solvent in step (c) is water, methanol, ethanol, propanol (particularly iso-propanol), butanol (particularly n-butanol, tert-butanol), C5 to C 12 It may consist of or include any solvent selected from the group comprising alcohols (particularly heptanol, e.g., n-heptanol; octanol, e.g., n-octanol; iso-octanol; nonanol, e.g., n-nonanol; decanol, e.g., n-decanol; dodecanol, e.g., n-dodecanol), esters (particularly DMC); or ethers (particularly DME or DIPE). Preferably, the solvent in step (c) is water, methanol, ethanol, iso-propanol, or n-butanol, or includes the same. Mixtures thereof and / or any of the aforementioned solvents may also be considered.

[0083] In particular, when the solvent used is water or includes it, step (c) can be performed at a temperature of at least 40°C, preferably at least 60°C, more preferably at least 70°C. Step (c) can be performed at a temperature of up to 95°C, preferably up to 92.5°C, more preferably up to 90°C. Accordingly, step (c) can be performed at a temperature of 40 to 95°C, preferably 60 to 92.5°C, more preferably 70 to 90°C.

[0084] Alternatively, particularly when the solvent used is methanol or includes it, step (c) can be performed at a temperature of at least 40°C, preferably at least 50°C, more preferably at least 55°C. Step (c) can be performed at a temperature of up to 80°C, preferably up to 70°C, more preferably up to 65°C. Thus, step (c) can be performed at a temperature of 40 to 80°C, preferably 50 to 70°C, more preferably 55 to 65°C.

[0085] Step (c) can be performed at atmospheric pressure, that is, without applying or removing pressure.

[0086] Step (c) can be performed for a period of at least 1 minute, preferably at least 5 minutes, more preferably at least 10 minutes. Step (c) can be performed for a period of up to 60 minutes, preferably up to 50 minutes, more preferably up to 40 minutes. Accordingly, Step (c) can be performed for a period of 1 to 60 minutes, preferably 5 to 50 minutes, more preferably 10 to 40 minutes.

[0087] The dissolution of the precipitate can be performed while stirring.

[0088] In particular, when water is used alone as a solvent in step (c), it may be used in an amount of at least 0.1 times, preferably at least 0.15 times, and more preferably at least 0.2 times, the amount of PET used in step (a) by weight. Water may be used in step (c) in an amount of up to 2.5 times, more preferably up to 1.25 times, and more preferably up to 0.5 times, the amount of PET used in step (a) by weight. Thus, water may be used in step (c) in an amount of 0.1 to 2.5 times, preferably 0.15 to 1.25 times, and most preferably 0.2 to 0.5 times, the amount of PET used in step (a) by weight.

[0089] In particular, when methanol is used alone as a solvent in step (c), it may be used in an amount of at least 1 time, preferably at least 1.5 times, and more preferably at least 2 times, the amount of PET used in step (a) by weight. Methanol may be used in step (c) in an amount of up to 10 times, preferably up to 5 times, and more preferably up to 3 times, the amount of PET used in step (a) by weight. Thus, methanol may be used in step (c) in an amount of 1 to 10 times, preferably 1.5 to 5 times, and more preferably 2 to 3 times, the amount of PET used in step (a) by weight.

[0090] In step (d) of the method, impurities are removed from the solution produced in step (c) to provide a purified solution containing BHET. Preferably, step (d) includes a step of decolorizing the solution. This can be performed by contacting the solution with one or more decolorizing agents. Preferably, step (d) is performed by passing the solution produced in step (c) through a column packed with one or more decolorizing agents, most preferably a plurality of columns connected in series. For example, each column connected in series may be packed with a different decolorizing agent. Step (d) may also include a step of removing other contaminants, such as metal and catalyst residues, from the solution produced in step (c).

[0091] One or more decolorizing agents used in step (d) may include carbon (e.g., preferably activated carbon having a high pore volume and surface area), a resin, such as an ion exchange resin, preferably a cation exchange resin, such as an acidic cation exchange resin containing preferably a sulfonic acid group or a carboxylic acid group (sulfonic acid groups are preferred), or alternatively or additionally preferably an anion exchange resin containing a quaternary ammonium salt and / or clay (e.g., active clay such as bentonite and montmorillonite clay). Preferably, the solution produced in step (c) is brought into contact with the carbon and / or resin, preferably an ion exchange resin. Ion exchange resins are particularly suitable for decolorizing and removing metal catalyst residues.

[0092] In a particularly preferred embodiment of the method, the solution produced in step (c) is brought into contact with a plurality of different decolorizing agents through a passage passing through a plurality of columns arranged in series. For example, the first column may contain an activated carbon decolorizing agent, the second column may contain a cation exchange resin, and the third column may contain an anion exchange resin, and the first to third columns may be arranged in series so that the solution produced in step (c) passes through each of them in step (d).

[0093] Step (d) can be performed at a temperature of at least 40°C, preferably at least 55°C, more preferably at least 70°C. Step (d) can be performed at a temperature of up to 110°C, preferably up to 100°C, more preferably up to 90°C. Accordingly, Step (d) can be performed at a temperature of 40 to 110°C, preferably 55 to 100°C, more preferably 70 to 90°C.

[0094] Step (d) can be performed at atmospheric pressure, that is, without applying or removing pressure.

[0095] Step (d) can be performed for a period of at least 10 minutes, preferably at least 25 minutes, more preferably at least 40 minutes. Step (d) can be performed for a period of up to 120 minutes, preferably up to 100 minutes, more preferably up to 60 minutes. Accordingly, Step (d) can be performed for a period of 10 to 120 minutes, preferably 25 to 100 minutes, more preferably 40 to 80 minutes.

[0096] Although less desirable, the purification step (d) may be omitted in some embodiments. This is because purification provided, for example, as a result of recrystallization in methanol alone may be sufficient to produce a decolorized purified product containing BHET, but generally, such a product will be used for low-grade applications such as carpets. Accordingly, in some embodiments, the purified product containing BHET may be crystallized in step (e) from the solution produced in step (c).

[0097] One of the advantages of using methanol in step (c) of the method of the present invention is that the solution can be formed in step (c), purified in step (d), and passed to step (e) for crystallization without filtration. This is because methanol dissolves BHET and, unlike water, also dissolves the dimers and trimers of BHET. Although transporting the dimers and trimers through the PET recycling process can be avoided by filtration in an aqueous system, step (a) of the present invention produces dimers and trimers in small amounts that can be transported through a recycling process using BHET. Therefore, in some embodiments, the solid-liquid separation step is not performed between steps (c) and (e) of the present invention.

[0098] However, when water is used in step (c) of the method of the present invention, it is advantageous to filter the BHET solution between steps (c) and (d) to remove BHET dimers and trimers that are insoluble in water. When a solvent other than water or methanol is used, it may also be desirable to filter the BHET solution between steps (c) and (d).

[0099] In step (e) of the method, the purified product containing BHET is crystallized from the purified solution.

[0100] Step (e) is preferably performed using cooling crystallization. Suitable crystallizers include stirred or wall-scraping crystallizers. The purified solution produced in step (d) may be left to cool naturally, but it is preferable to cool it using a coolant. The coolant may be present in a jacket surrounding the crystallizer, or, for example, may pass through a series of heat exchangers through which the purified solution also passes in countercurrent.

[0101] In particular, when the solvent used in step (c) is water, step (e) can be performed by reducing the temperature of the purified solution to at least 0°C, preferably at least 10°C, more preferably at least 20°C. Step (e) can be performed by reducing the temperature of the purified solution to up to 55°C, preferably up to 45°C, more preferably up to 40°C. Accordingly, step (e) can be performed by reducing the temperature of the purified solution to 0 to 55°C, preferably 10 to 45°C, more preferably 20 to 40°C.

[0102] In particular, when the solvent used in step (c) is methanol, step (e) can be performed by reducing the temperature of the purified solution to at least 0°C, preferably at least 5°C, more preferably at least 8°C. Step (e) can be performed by reducing the temperature of the purified solution to up to 30°C, preferably up to 15°C, more preferably up to 10°C. Accordingly, step (e) can be performed by reducing the temperature of the purified solution to 0 to 30°C, preferably 5 to 15°C, more preferably 8 to 12°C.

[0103] Step (e) can be performed at atmospheric pressure, that is, without applying or removing pressure.

[0104] Step (e) can be performed for a period of at least 10 minutes, preferably at least 20 minutes, more preferably at least 25 minutes. Step (e) can be performed for a period of up to 60 minutes, preferably up to 45 minutes, more preferably up to 35 minutes. Accordingly, Step (e) can be performed for a period of 10 to 60 minutes, preferably 20 to 45 minutes, more preferably 25 to 35 minutes.

[0105] The purified solution can be stirred during step (e).

[0106] The purified product formed in step (e) may contain a high proportion of BHET. BHET may be present in the purified product in an amount of at least 95 weight%, preferably at least 99 weight%, more preferably at least 99.5 weight%.

[0107] If methanol is used as a solvent in step (c), the purified product formed in step (e) may also contain dimers and trimers of BHET, for example, in an amount of at least 0.01 wt%. The dimers and trimers of BHET may be present in the purified product in an amount of up to 2 wt%, preferably up to 0.5 wt%, more preferably up to 0.2 wt%. Preferably, the amount of dimers and trimers present in the purified product formed in step (e) is substantially the same as the amount of dimers and trimers present in the precipitate formed in step (b). The amounts of different components in the purified product formed in step (e) can be determined using the method described above in relation to the precipitate formed in step (b).

[0108] A major advantage of the present invention is that it can be used to produce a purified product with a low b[h] value, particularly a b[h] value of 2 or less. PET produced from BHET having these color densities is of very high grade and can be used in applications requiring excellent visual appearance, such as transparent colorless water bottles. Thus, the purified product formed in step (e) can exhibit a b[h] value of up to 2, for example, 0 to 2. In some cases, the purified product can be used for lower grade applications, for example, carpets or films, in which case the b[h] value can be up to 4, for example, up to 3.

[0109] The method of the present invention can be used to form a purified product in step (e) in which the b[h] value of the PET used in step (a) is 0.5 times, preferably 0.1 times, and more preferably 0.05 times. By using a preferred embodiment of the present invention, a much greater reduction in the b[h] value can be obtained, for example, when the PET feed used in step (a) exhibits a high color density.

[0110] The color density of the purified product formed in step (e) can be measured as described above in relation to the PET used in step (a).

[0111] The purified product containing BHET is preferably isolated after step (e), and if step (f) is present, isolated before step (f). The precipitate may be isolated using known methods, for example by filtration or centrifugation. Preferably, the protonic solvent used in step (c), generally methanol or water, and ethylene glycol are recovered from the residual liquid remaining after isolation of the purified product, for example by low-pressure evaporation and condensation. The protonic solvent may be recycled to step (c). The ethylene glycol may be recycled for use in step (a), and more preferably recycled to the first depolymerization reactor.

[0112] One of the main advantages of performing step (c) using methanol instead of water is that methanol and ethylene glycol can be easily recovered. Therefore, the recovery of methanol and ethylene glycol from the residual liquid can be performed in a single-stage evaporator. In contrast, when water is used, the recovery of ethylene glycol and water from the residual liquid can be difficult because water and ethylene glycol form an azeotropic mixture. Therefore, when water is used in step (c), the use of a multi-stage evaporator to recover water and ethylene glycol from the residual liquid is preferred.

[0113] When methanol is used in step (c), the recovery of methanol and ethylene glycol from the residual liquid can be performed by heating the residual liquid to a temperature between the boiling points of methanol and ethylene glycol. For example, the residual liquid can be heated to a temperature above 65°C, preferably above 70°C, more preferably above 75°C. The residual liquid can be heated to a temperature up to 120°C, preferably up to 100°C, more preferably up to 90°C. Thus, the residual liquid can be heated to a temperature of 65 to 120°C, 70 to 100°C, more preferably 70 to 90°C.

[0114] The recovery of methanol and ethylene glycol from the residual liquid can be carried out at ambient pressure, that is, without applying or removing pressure.

[0115] Generally, the residual liquid will not be further processed before being processed to recover methanol and ethylene glycol. Preferably, methanol is not further processed before being recycled for use in step (c).

[0116] When water is used in step (c), a two-stage evaporator process is preferred to recover water and ethylene glycol. In the first evaporator, water can be recovered from the residual liquid by applying low pressure and evaporated at a reduced temperature; for example, it is preferable to operate the evaporator at a pressure of 10 kPa or about 10 kPa, the associated condenser temperature is 46°C or about 46°C, and the reboiler temperature is 132°C or about 132°C. Subsequently, the residual ethylene glycol can be recovered in the second evaporator by applying low pressure, preferably at a pressure of 0.08 bar or about 0.08 bar and a temperature of 138°C or about 138°C. Those skilled in the art will understand that other operating temperatures and pressures may also be selected for the first and second evaporators. If desired, improved water recovery can be achieved by operating the first evaporator at a lower temperature or by using a molecular sieve downstream of the first evaporator.

[0117] However, ethylene glycol may undergo additional purification before being recycled to step (a). For example, ethylene glycol may be flashed to separate any organic waste incorporated therein.

[0118] Flashing can occur at a temperature of at least 130°C, preferably at least 150°C, and more preferably at least 170°C. Flashing can occur at a temperature of up to 230°C, preferably up to 210°C, and more preferably up to 190°C. Thus, flashing can occur at a temperature of 130 to 230°C, preferably 150 to 210°C, and more preferably 170 to 190°C.

[0119] Flashing generally occurs under reduced pressure. For example, flashing can occur at a pressure of up to 80,000 Pa, preferably up to 60,000 Pa, and more preferably up to 40,000 Pa. Flashing can occur at a pressure of at least 10,000 Pa, preferably at least 15,000 Pa, and more preferably at least 20,000 Pa. Thus, flashing can occur at a pressure of 10,000 to 80,000 Pa, preferably 15,000 to 60,000 Pa, and more preferably 20,000 to 40,000 Pa.

[0120] When methanol is used in step (c), the recovery of methanol is so effective (even on an industrial scale as described herein) that when the recovered methanol is recycled to step (c), the non-recycled methanol needs to be added in step (c) in an amount of at most 0.008 times, preferably at most 0.006 times, and more preferably at most 0.005 times, by weight, the amount of PET used in step (a). The non-recycled methanol may be used in step (c) in an amount of at least 0.001 times, preferably at least 0.003 times, and more preferably at least 0.004 times, by weight, the amount of PET used in step (a). Accordingly, non-recycled methanol may be used in step (c) in an amount of 0.001 to 0.008 times, preferably 0.003 to 0.006 times, and more preferably 0.004 to 0.005 times by weight of the amount of PET used in step (a). Thus, it will be understood that the amount of methanol lost during the method of the present invention is very small and much less than the amount of water lost when used instead of methanol in step (c).

[0121] However, when water is used as a solvent in step (c), at least most of the water used in step (c) can also be effectively recovered so that it is recycled using the previously described two-stage evaporator process. The lost water is generally removed from the system as humid air. Given that the environmental impact of water loss from the system is minimized compared to methanol-containing waste, and considering the energy costs associated with water recovery, maximizing water recycling may not be beneficial.

[0122] The method of the present invention may further include a step (f) in which a purified product containing BHET is dried. The product may be dried, for example, by passing air over the purified product in a fluidized bed dryer.

[0123] Air can be heated to a temperature of at least 30°C, preferably at least 40°C, more preferably at least 50°C. Air can be heated to a temperature of up to 100°C, preferably up to 90°C, more preferably up to 80°C. Thus, air can be heated to a temperature of 30 to 100°C, preferably 40 to 90°C, more preferably 50 to 80°C.

[0124] The drying step (f) can be performed at ambient pressure, that is, without applying or removing pressure.

[0125] The drying step (f) can be performed for a period of at least 10 minutes, preferably at least 15 minutes, more preferably at least 20 minutes. The drying step (f) can be performed for a period of up to 60 minutes, preferably up to 50 minutes, more preferably up to 40 minutes. Accordingly, the drying step (f) can be performed for a period of 10 to 60 minutes, preferably 15 to 50 minutes, more preferably 20 to 40 minutes.

[0126] The method of the present invention may be operated in batch mode or continuous mode, but preferably operates continuously.

[0127] The method of the present invention is preferably carried out on an industrial scale. Accordingly, the method can recycle at least 10 ton / day, preferably at least 100 ton / day, and potentially at least 1,000 ton / day of PET.

[0128] The present invention further provides a purified product comprising BHET that can be obtained using the method as described herein, preferably obtained.

[0129] The present invention also provides a method for preparing a polymer, said method comprising the step of performing a polymerization reaction using a purified product containing BHET of the present invention. Preferably, said method comprises the step of preparing a purified product containing BHET using the method of the present invention. A key advantage of the present invention is that the purified product containing BHET can be used directly for polymerization, that is, without undergoing additional purification before use.

[0130] The purified product can be used to manufacture PET or to manufacture copolymers containing ethylene terephthalate monomers.

[0131] The polymer can be further processed into bottles, packaging materials, fabrics, etc. In some embodiments, the polymer can be further processed into transparent bottles, preferably colorless bottles.

[0132] The present invention further provides a PET recycling device, said device comprising the following:

[0133] (a) A series of depolymerization reactors suitable for depolymerizing PET to form a depolymerization mixture containing BHET (wherein the series of depolymerization reactors is configured to accommodate PET, ethylene glycol, and a catalyst system);

[0134] (b) a crystallization unit downstream of a polymerization reactor suitable for crystallizing a precipitate containing BHET from a depolymerization mixture;

[0135] (c) a vessel suitable for receiving a precipitate and dissolving the precipitate in a protonic solvent to form a solution containing BHET;

[0136] (d) an impurity removal unit that receives a solution containing BHET and removes impurities from the solution to form a purified solution containing BHET; and

[0137] (e) An additional crystallization unit downstream of an impurity removal unit suitable for crystallizing a purified product containing BHET from a purified solution.

[0138] The following non-limiting embodiments illustrate the invention.

[0139] Examples

[0140] Example 1: Depolymerization step (a)

[0141] Depolymerization reactions were simulated in a series of different reactors. The mass ratio of the PET:ethylene glycol:catalyst system used in the simulation was 1:4:0.005. Each reactor was simulated to operate at a temperature of 197°C and atmospheric pressure. The simulation was set to provide a conversion rate of 99.0% at the outlet of the final reactor in the series of reactors.

[0142] The results of the simulation are shown in the following table:

[0143]

[0144] To achieve an annual production level of approximately 10,000 tons, the volume of a single reactor is approximately 300 m³ 3 It will be. If a series of three reactors is used, the volume per reactor is 10 m³ 3It slightly exceeds. As in the most preferred embodiment of the present invention, using only a series of two reactors, the volume per reactor is about 11 to 12 m³. 3 A very large reduction similar to this can be achieved.

[0145] Figure 1 shows a graph representing the efficiency of each depolymerization reaction, taking into account not only the data above but also the energy and equipment input required for each array.

[0146] Compared to the use of a single depolymerization reactor, it can be seen that a dramatic improvement in efficiency is observed when a series of at least two depolymerization reactors are used.

[0147] Example 2: Preferred solvent for use in step (c)

[0148] BHET recrystallization experiments were performed in various solvents including methanol, ethanol, isopropanol, butanol, and alcohols with longer carbon chains.

[0149] Specifically, 50 g of crude BHET was dissolved in 250 ml of solvent at 80°C for 1 hour. The BHET was recrystallized by cooling at a rate of 7°C / hour until it reached a temperature of 10°C. The recrystallized BHET was analyzed to determine its color density. Weight loss during the recrystallization process was also measured.

[0150] The results are shown in the following table:

[0151]

[0152] It can be seen that each lighter solvent provides an excellent level of decolorization. However, the amount of material lost during recrystallization was significantly lower in methanol than in any other of the lighter solvent experiments. Higher alcohols, as well as methanol, are available for use on an industrial scale.

[0153] Examples 3: decolorization Step (d)

[0154] A number of different techniques were used to decolorize the aqueous solution of BHET.

[0155] Experiments using resin provided promising results:

[0156]

[0157] It can be seen that cation exchange resins, particularly strong acid cation exchange resins, provide the most promising results.

[0158] Activated carbon was also very effective for BHET decolorization:

[0159]

[0160] Photographs of untreated and treated samples and a photograph of PET prepared using the samples are shown in Fig. 2. Both the cation exchange resin and the activated carbon provided excellent levels of decolorization, whereas the carbon-treated product provided a polymer product of better quality.

[0161] Additional decolorization experiments were performed. This time, a solution of BHET in methanol was used. The experiment yielded results similar to those performed on an aqueous solution of BHET, but the cation exchange resin provided particularly superior results.

[0162] Examples 4: Recycling process using methanol in step (c)

[0163] The process of the present invention was carried out in the apparatus shown in FIG. 3. Representative waste used in the process is shown in FIG. 4. The waste consists of used PET flakes of blue and green.

[0164] Specifically, PET (2), a zinc acetate and urea catalyst system (4) and ethylene glycol (6) were passed through the first of a series of three depolymerization reactors (10). A sample taken after the series of three depolymerization reactors (10) showed a 100% conversion rate of PET (2) with 99.8% selectivity for BHET.

[0165] The depolymerization mixture was passed through a filter (20) to remove insoluble material (32) and then passed to a crystallizer (12) where a precipitate containing BHET was formed. The precipitate was passed through the filter (20) to one of two stirred vessels (14).

[0166] A solution containing BHET was formed by adding methanol (8) to the vessel (14) to dissolve the precipitate.

[0167] The solution was passed through a decolorization stage (16), depicted as two units parallel to the drawing, to another crystallizer (18) where a purified product containing BHET was formed.

[0168] The purified product was passed through another filter (20) to a drying unit (26), and the residue was passed to a methanol and ethylene glycol recovery unit (22). The methanol was recycled from the recovery unit (22) to a stirring vessel (14), while the ethylene glycol was passed to a flash unit (24), where organic waste (34) was removed before being recycled to a series of depolymerization reactors (10).

[0169] The purified product was dried by passing warm air (28) through a dryer (26). The warm air (28) was removed from the system through a condenser where any wastewater (36) is removed, and a flash unit that recovers methanol and recycles it to a stirring vessel (14). Once dried, the purified product (30) was removed from the system.

[0170] The purified product (30) had a low color density and was used to manufacture recycled PET for use in water bottles without additional processing.

[0171] Example 5: Recycling process using water in step (c)

[0172] The process of the present invention was performed in the apparatus shown in Fig. 5.

[0173] Specifically, PET (102), a zinc acetate and urea catalyst system (104), and ethylene glycol (106) were passed through the first of a series of two depolymerization reactors (100). A sample taken after the series of two depolymerization reactors (100) showed a selectivity of 95.0% for BHET; and a 100% conversion rate of PET (102), with the remaining 5.0% being a product substantially composed of BHET oligomers.

[0174] Excess water (140) is removed by an evaporator (138), then the depolymerization mixture is passed through a filter (120a) to remove insoluble material (132), then passed through a crystallizer (112) where a precipitate containing BHET is formed. The precipitate is passed through a filter (120b) to a stirring vessel (114).

[0175] A solution containing BHET was formed by adding water (108) to the vessel (114) to dissolve the precipitate.

[0176] The solution was passed through a decolorization stage (116). As illustrated, the decolorization stage includes a filter (120c), followed by a first unit (142) including an activated carbon bed, followed by a second unit (144) including a cation exchange bed in series, followed by a third unit (146) including an anion exchange bed. After the decolorization stage (116), the solution was passed through another crystallizer (118) in two stages where a purified product containing BHET was formed.

[0177] The purified product was passed through another filter (120d) to a drying unit (126), and the residue was passed to an evaporator (122). Water was recycled from the evaporator (122) to a stirring vessel (114), while ethylene glycol was passed to an additional evaporator (124), where organic waste (134) was removed before being recycled to a series of depolymerization reactors (100).

[0178] The purified product was dried by passing warm air (128) through a dryer (126). Once dried, the purified product (130) was removed from the system.

[0179] The purified product (130) had a low color density and was used to manufacture recycled PET for use in water bottles without additional processing.

Claims

Claim 1 A method for recycling polyethylene terephthalate (PET), comprising: (a) depolymerizing PET in the presence of an ethylene glycol and a catalyst system in a series of depolymerization reactors, such as two depolymerization reactors, to form a depolymerization mixture containing bis(2-hydroxyethyl) terephthalate (BHET) — wherein the series of depolymerization reactors is arranged such that one or more depolymerization reactors are located downstream of a first depolymerization reactor —; (b) crystallizing a precipitate containing BHET from the depolymerization mixture; (c) dissolving the precipitate in a protonic solvent containing methanol or ethanol to form a solution containing BHET; (d) removing impurities from the solution to form a purified solution containing BHET; and (e) crystallizing a purified product containing BHET from the purified solution. Claim 2 A method according to claim 1, wherein the PET is optionally waste PET obtained from waste PET bottles, wherein the PET is used in the form of particles in which at least 80 weight percent of the particles pass through a mesh having an opening with a diameter of 20 mm and / or; 100 weight percent of the particles pass through a mesh having an opening with a diameter of 25 mm and / or; or up to 1 weight percent of the particles pass through a mesh having an opening with a diameter of 0.1 mm. Claim 3 A method according to claim 1 or 2, wherein the b[h] value of the PET is greater than 5. Claim 4 A method according to claim 1, wherein each depolymerization reactor used in step (a) is operated: at a temperature of 150 to 230°C; at atmospheric pressure; for a period of 20 minutes to 4 hours; and / or while stirring. Claim 5 A method according to claim 1, wherein ethylene glycol is used in an amount 2 to 6 times the amount of PET by weight in step (a). Claim 6 A method according to claim 1, wherein the catalyst system comprises a transition metal catalyst. Claim 7 A method according to claim 1, wherein the catalyst system comprises a carrier. Claim 8 A method according to claim 1, wherein the catalyst system is used in an amount of 0.001 to 0.5 times the amount of PET by weight in step (a). Claim 9 A method according to claim 1, wherein step (b) is performed using cooling crystallization and by reducing the temperature of the depolymerization mixture to a temperature of 5 to 50°C; at atmospheric pressure; for a period of 10 to 60 minutes; and / or under stirring. Claim 10 delete Claim 11 A method according to claim 1, wherein step (d) comprises a step of decolorizing the solution. Claim 12 A method according to claim 1, wherein step (e) is performed using cooling crystallization and by reducing the temperature of the purified solution to a temperature of 0 to 55°C; at atmospheric pressure; for a period of 10 to 60 minutes; and / or under stirring. Claim 13 The method of claim 1, wherein the depolymerization mixture is passed through a filter between steps (a) and (b) to remove insoluble components and / or; the precipitate containing BHET is isolated between steps (b) and (c), the protonic solvent is methanol, the method comprises the step of isolating the purified product containing BHET after step (e), optionally, the recovery of methanol and ethylene glycol is performed in a single-stage evaporator and / or; the method further comprises the step of drying the purified product containing BHET (f). Claim 14 The method of claim 1, wherein the b / [h] value of the purified product containing BHET is at most 2 and / or; the purified product comprises: BHET in an amount of at least 95 weight%; and dimers and trimers of BHET. Claim 15 As a recycling apparatus for polyethylene terephthalate (PET), the apparatus comprises: (a) a series of depolymerization reactors suitable for depolymerizing PET to form a depolymerization mixture containing bis(2-hydroxyethyl) terephthalate (BHET) — said series of depolymerization reactors configured to accommodate PET, ethylene glycol, and a catalyst system, said series of depolymerization reactors arranged such that one or more depolymerization reactors are located downstream of a first depolymerization reactor —; (b) a crystallization unit located downstream of a polymerization reactor suitable for crystallizing a precipitate containing BHET from said depolymerization mixture; (c) a vessel suitable for receiving said precipitate and dissolving said precipitate in a protonic solvent containing methanol or ethanol to form a solution containing BHET; (d) an impurity removal unit receiving said solution containing BHET and removing impurities from said solution to form a purified solution; and (e) an additional crystallization unit downstream of the impurity removal unit suitable for crystallizing a purified product containing BHET from the purified solution; comprising an apparatus. Claim 16 delete Claim 17 delete Claim 18 delete Claim 19 delete Claim 20 delete Claim 21 delete Claim 22 delete Claim 23 delete Claim 24 delete Claim 25 delete Claim 26 delete Claim 27 delete

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