POLYMER RECYCLING

Abstract

A method for recycling polyethylene terephthalate (PET) to produce bis(2-hydroxyethyl) terephthalate (BHET) comprises the steps of (a) depolymerizing PET in the presence of ethylene glycol and a catalyst system in a series of preferably two depolymerization reactions to form a depolymerized mixture comprising BHET; (b) crystallizing a precipitate comprising BHET from the depolymerized mixture; (c) dissolving the precipitate in a protic solvent, preferably water, but also optionally methanol, to form a solution comprising BHET; (d) removing impurities from the solution to form a purified solution comprising BHET; and (e) crystallizing a purified product comprising BHET from the purified solution. The apparatus for this method and the use of urea in a catalyst system for the same are also provided.

Description

POLYMER RECYCLING FIELD OF INVENTION The present invention relates to a method and apparatus for recycling polymers, in particular a method for recycling polyethylene terephthalate (PET) to produce bis(2-hydroxyethyl) terephthalate (BHET). The BHET produced using the method and apparatus of the present invention can be of a purity level that makes it suitable for direct use in the preparation of high-quality plastics. BACKGROUND OF THE INVENTION PET is a thermoplastic polymer used in a wide range of materials due to its properties, including strength, moldability, and moisture resistance. Common uses of PET include packaging (e.g., beverage bottles and food containers), fibers (e.g., clothing and carpets), and thin films. Virgin PET can be easily prepared using ethylene glycol and a terephthalate-containing monomer. However, because its raw materials are derived from non-renewable sources such as crude oil, there is a growing concern about the need to recycle PET. When PET waste consists of a single type of PET, such as clear plastic water bottles, recycling can be as simple as melting and remolding flakes of the waste material. However, it is common for waste to comprise a variety of different PET materials, such as a range of bottles of different colors, which, if melted and remolded, could yield a product with a low visual quality. Such materials may be suitable for use in carpet fibers, but are generally not suitable for use in packaging such as clear water bottles. Consequently, there is a need for methods to recycle residual or waste PET into a product that can be used in applications requiring a high degree of visual appeal. The most sophisticated methods for recycling PET involve depolymerizing the residual or waste material to obtain, usually after a number of purification and separation steps, viable raw materials for use in the preparation of a polymer. For example, PET can be depolymerized using a glycolytic agent such as ethylene glycol to form BHET monomers. However, conventional methods for depolymerizing PET tend to produce BHET monomers in a yield of less than 80%, with significant amounts of BHET oligomers, particularly dimers and trimers, produced from the remaining PET. Since the presence of dimers and trimers reduces the quality of a polymer prepared from BHET feedstock, it is standard practice to purify a depolymerization mixture to remove these components. Purification is particularly important where high-quality recycled PET is required, for example, recycled PET suitable for use in clear, colorless bottles. Color spaces are frequently used to denote the grade of a polymer, with the value MA / t / ZUZZ / UÓOZZO de b[h] - a measure of the shade from blue (negative values) to yellow (positive values) - taken as a key indicator of quality. Poor quality recycled PET typically exhibits an undesirable yellow hue. There are numerous disadvantages associated with processes that produce a depolymerization mixture containing significant amounts of dimers and trimers. One of the most significant is the considerable loss of PET raw material from the recycling process when it is removed as dimers and trimers. Unless these dimers and trimers are recycled for further depolymerization, which is itself time-consuming and energy-intensive, the efficiency of typical PET recycling processes is therefore very low. Consequently, there is a need for improved methods for the depolymerization recycling of waste PET. In particular, there is a need for depolymerization recycling methods that yield products suitable for use in high-quality applications, such as clear water bottles. BRIEF DESCRIPTION OF THE INVENTION It has now been surprisingly found that, using a series of depolymerization reactors, a depolymerized mixture can be obtained containing a very high proportion of BHET monomer and relatively low amounts of dimer and trimer, thus allowing conventional purification steps in which the dimers and trimers are removed until they are negligible. This means that solvents that would have previously been rejected as unsuitable for further processing of the crude BHET monomer can now be used. The inventors of the present invention have found that protic solvents are highly effective for recycling the crude depolymerization product. In particular, water is preferred for this purpose, since BHET dimers and trimers are insoluble in water. Thus, the BHET dissolves to form an aqueous phase, while the dimers and trimers remain as solid materials which can be separated from the aqueous phase, for example by filtration, before recrystallization, resulting in a high-purity monomeric product. Methanol may also be preferable since it at least partially decolorizes the product with minimal product loss. Although methanol dissolves and carries the dimers and trimers through the process so that they may be present in the purified product comprising BHET, their concentration can be sufficiently low so that the purified product can nevertheless be used directly in a polymerization reaction. The resulting polymer can be used in high-quality applications, such as clear, colorless water bottles. Furthermore, as detailed below, it is possible to use aprotic and even non-polar solvents to recrystallize the crude depolymerization product, while retaining the advantages of using depolymerization reactors in series according to the present description. Accordingly, the present invention provides a method for recycling polyethylene terephthalate (PET), the method comprising: (a) depolymerize PET in the presence of ethylene glycol and a catalyst system in a series of MA / t / ZUZZ / UÓOZZO MA / depolymerization reactors to form a depolymerized mixture comprising bis(2-hydroxyethyl) terephthalate (BHET); (b) crystallize a precipitate comprising BHET from the depolymerized mixture; (c) dissolving the precipitate in a protic solvent to form a solution comprising BHET; (d) removing impurities from the solution to form a purified solution comprising BHET; and (e) crystallizing a purified product comprising BHET from the purified solution. The present invention further provides a purified product comprising BHET which can be obtained using a method of the present invention. A method for preparing a polymer is also provided, the method comprising carrying out a polymerization reaction using a purified product comprising the BHET of the present invention. In addition, a PET recycling device is provided, the device comprising: (a) a series of depolymerization reactors which are suitable for depolymerizing PET to form a depolymerized mixture comprising BHET, wherein the series of depolymerization reactors is adapted to receive PET, ethylene glycol and a catalyst system; (b) a crystallization unit downstream of the polymerization reactors suitable for crystallizing a precipitate comprising BHET from the depolymerized mixture; (c) a container for receiving the precipitate and which is suitable for dissolving the precipitate in a protic solvent to form a solution comprising BHET; (d) an impurity removal unit to receive the solution comprising BHET and removing impurities from the solution to form a purified solution; and (e) an additional crystallization unit downstream of the purity removal unit suitable for crystallizing a purified product comprising BHET from the purified solution. The present invention also provides for the use of urea in a catalyst system in a polyethylene terephthalate (PET) recycling process to: solubilize metals, in particular a transition metal catalyst component of the catalyst system; and / or form a eutectic salt with a transition metal catalyst component of the catalyst system. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a graph showing the efficiency of depolymerization reactions carried out using different series of reactors. Figure 2 shows photos of BHET samples which are untreated and treated with various bleaching agents, as well as images of PET prepared using the samples. Figure 3 is a diagram of an apparatus for carrying out the method of the present invention. The apparatus includes a series of three depolymerization units (10) for depolymerizing PET to form BHET; a crystallization unit (12) downstream of the polymerization reactors suitable for crystallizing a precipitate comprising BHET from the depolymerized mixture; a vessel (14) for receiving the precipitate and suitable for dissolving the precipitate in methanol to form a solution comprising BHET; an impurity removal unit (16) for receiving the solution comprising BHET and removing impurities from the solution to form a purified solution; and an additional crystallization unit (18) downstream of the impurity removal unit suitable for crystallizing a purified product comprising BHET from the purified solution. Figure 4 is a photo of the representative residue that can be processed using the apparatus shown in Figure 3. Figure 5 is a diagram of an apparatus for carrying out the method of the present invention. The apparatus includes a series of two depolymerization units (100) for depolymerizing PET to form BHET; a crystallization unit (112) downstream of the polymerization reactors suitable for crystallizing a precipitate comprising BHET from the depolymerized mixture; a vessel (114) for receiving the precipitate and suitable for dissolving the precipitate in water to form a solution comprising BHET; an impurity removal unit (116) for receiving the solution comprising 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 comprising BHET from the purified solution. DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for recycling polyethylene terephthalate (PET). PET is a thermoplastic polymer that has the following structure: MA / t / ZUZZ / UÓOZZO The PET used in the method of the present invention will typically be waste or residual PET. Waste PET can be obtained from a wide range of sources, including packaging, bottles, and textiles. Preferably, the PET is obtained from waste or residual bottles. The PET used in step (a) can be waste PET, i.e., PET that has undergone a cleaning process. Waste PET can be PET that has been washed with water, steam purified, solvent cleaned, and / or detergent cleaned. Preferably, the PET used in step (a) is PET that has been washed with water. The PET used in step (a) preferably contains colored PET. The PET may contain colored PET in an amount of at least 5%, preferably at least 10%, and more preferably at least 25% by weight. In some embodiments, the PET may contain colored PET in an amount of at least 50% and more preferably at least 75% by weight. The PET may contain colored PET in an amount of up to 100% by weight. The PET used in step (a) preferably exhibits a b[h] value (i.e., a b value over the Hunter Lab color space) of more than 5, for example, greater than 10, although some PET feeds may have a b[h] value of 100 or even higher. This can be measured using standard techniques, such as a colorimeter. ML / PET is preferably used in step (a) in particulate form, such as flakes. Preferably, at least 80% by weight of the particles (i.e., d80) pass through a mesh having openings with a diameter of 20 mm, preferably 15 mm, and most preferably 12 mm. Even smaller mesh sizes may also be used. Particles of these sizes depolymerize rapidly. Although a range of particle sizes will typically be used in step (a), larger particle sizes are preferably avoided as they can take longer to process. Consequently, 100% by weight of the particles (d100) preferably pass through a mesh having openings with a diameter of 25 mm, preferably 20 mm, and most preferably 12 mm. Even smaller mesh sizes may also be used. Particles that are too small are also preferably avoided, at least if powders are already available through waste collection and separation processes, as the energy and therefore the cost required to shred the PET to this size is unnecessary. Thus, it is preferred that a maximum of 1% by weight of the particles pass through a mesh having openings with a diameter of 0.1 mm, preferably 0.5 mm, and most preferably 1 mm. It will be appreciated that the PET used in step (a) can be fed into the series of reactors in a form in which it is coated with a liquid, for example, wastewater or another solvent that has been used to clean the PET. This liquid coating is considered not to be part of the PET for the purposes of the present invention. In step (a) of the method, PET is depolymerized in a series of depolymerization reactors to form a depolymerized mixture comprising bis(2-hydroxyethyl) terephthalate (BHET). BHET is a monomer having the following structure: PET is partially depolymerized in a first depolymerization reactor and further depolymerized downstream of the first reactor in the reactor series. Using a series of reactors, it has been found that the depolymerized mixture can comprise a high proportion of BHET and a low level of dimers and trimers. The dimers and trimers have the following structure: Higher oligomers will generally not be present in the depolymerized mixture. Thus, in preferred embodiments, the depolymerized mixture is substantially free of higher oligomers (i.e., where n > 4). Surprisingly, a very high quality product can be produced by depolymerizing the PET in a series of only two reactors. Thus, in preferred embodiments, the PET is depolymerized in a series of two depolymerization reactors. This gives high levels of PET conversion and selectivity for BHET. In alternative embodiments, the PET is depolymerized in a series of three or alternatively four or more reactors. Preferably, all the ethylene glycol and catalyst system used in the depolymerization process are added to the first reactor in the series. However, in some embodiments, additional ethylene glycol and / or catalyst system may be added to the reaction mixture downstream of the first reactor as it passes through the series of depolymerization reactors. It will be noted that, although ethylene glycol and / or catalyst system can be added to the reaction mixture downstream of the first reactor, the reaction components are not removed as they pass through the series of reactors. Each of the depolymerization reactors used in step (a) can be operated at a temperature of at least 150°C, preferably at least 170°C, and more preferably at least 190°C. Each of the depolymerization reactors used in step (a) can be operated at a temperature of up to 230°C, preferably up to 220°C, and more preferably up to 210°C. Thus, each of the depolymerization reactors used in step (a) can be operated at a temperature of 150 to 230°C, preferably from 170 to 220°C, and more preferably from 190 to 210°C. Generally, depolymerization reactors will be operated at the same temperature, but this is not necessarily the case. Unlike many prior art processes, PET is not preferably used in a molten state in step (a), meaning the reaction mixture is relatively viscous. Viscosity has typically led to relatively low PET conversion levels. Surprisingly, using a series of depolymerization reactors, excellent conversion levels can be achieved even when step (a) is carried out with PET in a solid state. Each of the depolymerization reactors used in step (a) can be operated at atmospheric pressure, i.e., without the application or removal of pressure. Standard atmospheric pressure is defined as 101,325 Pa. However, since atmospheric pressure varies from place to place, atmospheric pressure, as used herein, is taken to be approximately equal to standard atmospheric pressure, i.e., approximately 101,325 Pa. Each of the depolymerization reactors used in step (a) can be operated for a period of at least 20 minutes, preferably at least 1 hour, and more preferably at least 1.5 hours. Each of the depolymerization reactors used in step (a) can be operated for a period of up to 4 hours, preferably up to 2.5 hours, and more preferably up to 1.75 hours. Thus, each of the depolymerization reactors used in step (a) can be operated from 20 minutes to 4 hours, preferably from 1 to 3 hours, and more preferably from 1.5 to 2.5 hours. The depolymerization reactors can all be operated for the same period, but this is not necessarily the case. PET can be passed to the depolymerization reactor series at a flow rate of PET can be passed to the depolymerization reactor series at a flow rate of up to 100,000 kg, preferably up to 50,000 kg, and more preferably up to 10,000 kg per hour. Thus, PET can be passed to the depolymerization reactor series at a flow rate of 100 to 100,000 kg, preferably from 500 to 50,000 kg, and more preferably from 1,000 to 10,000 kg per hour. Each of the depolymerization reactors used in step (a) is preferably operated with stirring. The size of the reactors used in the depolymerization reactor series can vary depending on the number of reactors used. Each reactor used in step (a) can be at least 5 m³, preferably at least 8 m³, and more preferably at least 10 m³. Alternatively, each reactor used in step (a) can be up to 50 m³, preferably up to 20 m³, and more preferably up to 15 m³. Thus, each reactor used in step (a) can range in size from 5 to 50 m³, preferably from 8 to 20 m³, and more preferably from 10 to 15 m³. Using reactors at this small scale makes it possible to have a series of reactors through which PET can be depolymerized with a minimal residence time. In this way, industrial-scale quantities of PET can be depolymerized into a high-quality product using relatively small reactors. Ethylene glycol is used in step (a) as a glycolytic agent. Ethylene glycol may be used in step (a) in an amount 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 up to 6 times, preferably up to 5 times, and more preferably up to 4.75 times the amount of PET by weight. Thus, ethylene glycol may be used in step (a) in an amount from 2 to 6 times, preferably from 3.25 to 4.75 times, and more preferably from 3.5 to 4.75 times the amount of PET by weight. At least 60%, preferably at least 80%, and most preferably at least 95% by weight of the ethylene glycol may be added to the first reactor. However, as mentioned above, all of the ethylene glycol is preferably added to the first reactor. It will be appreciated that where at least 100% of the ethylene glycol is added to the first reactor, the remainder is added to the series of depolymerization reactors downstream of the first depolymerization reactor. The 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. In some embodiments, the catalyst system consists of a transition metal catalyst. However, in preferred embodiments, the catalyst system comprises a catalyst, for example, as described above, in a carrier. Suitable carriers include nitrogen-containing carriers, such as urea. It has been surprisingly found that urea is highly effective in holding metals (e.g., the transition metal catalyst component of the system) MA / t / ZUZZ / UÓOZZO catalyst; or traces of metal catalysts that were used to produce the PET originally, such as antimony catalysts) and other contaminants in solution, thus allowing these components to be separated from the BHET in step (b). In this way, the present invention also provides for the use of urea in the catalyst system in a PET recycling process to solubilize metals, in particular a transition metal catalyst component of the catalyst system. Urea can also be used to solubilize contaminants in the PET recycling process. It has been surprisingly found that the eutectic salt catalyst system is particularly effective in solubilizing metals and / or contaminants. The carrier can be used in the catalyst system in an amount at least 1 times, preferably at least 2 times, and more preferably at least 3 times the molar amount of transition metal cation in the transition metal catalyst. The carrier can be used in an amount up to 8 times, preferably up to 6 times, and more preferably up to 5 times the molar amount of transition metal cation. Thus, the carrier can be used in an amount from 1 to 8 times, preferably from 2 to 6 times, and more preferably from 3 to 5 times the molar amount of transition metal cation. Such carrier-to-transition metal catalyst ratios have been found to give high reaction rates while retaining the metal ions in solution. As mentioned earlier, the transition metal cation will typically be a zinc cation. The most preferred catalyst systems for use in step (a) are comprising, and preferably consisting of, zinc acetate and urea, and in particular a catalyst system having the formula [4NH2CONH2 ZnOAcj]. This catalyst system advantageously forms a eutectic salt. The present invention also provides for the use of urea in a catalyst system in a PET recycling process to form a eutectic salt with a transition metal catalyst component of the catalyst system. The catalyst system can be in the liquid phase during step (a) and preferably through the method of the present invention. 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. Thus, 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. At least 60%, preferably at least 80%, and most preferably at least 95% by weight of the catalyst system may be added to the first reactor. However, as mentioned above, the entire catalyst system is preferably added to the first reactor. It is appreciated that where less than 100% of the catalyst system is added to the first reactor, the remainder is added to the series of depolymerization reactors downstream of the first depolymerization reactor. Step (a) is generally carried out in the absence of any solvent beyond MA / t / ZUZZ / UÓOZZD ethylene glycol and any carrier that may be present in the catalyst system. It will be noted that there may be some residual liquid, for example water, that has passed into the recovered process as a coating on the PET due to washing; however, this is not considered a solvent for the purposes of the present invention. Thus, the solvent may be present in step (a) in an amount 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. More preferably, substantially no solvent is present in step (a). Preferably, the depolymerized mixture is separated from any insoluble components between steps (a) and (b). Insoluble components include unreacted PET (although levels of this will typically be very low, if present at all) and other inert solids. Other solids may include polymers other than PET, such as polyethylene (PE) and polypropylene (PP). Preferably, the depolymerized mixture is passed through a filter to remove insoluble components, although other techniques, such as centrifugation, may also be used. Centrifugal decanters can be used to achieve very high levels of solid-liquid separation. In step (b) of the method, a precipitate comprising BHET is crystallized from the depolymerized mixture formed in step (a). Step (b) is preferably carried out using cooling crystallization. Suitable crystallizers include stirred or scraped-wall crystallizers. The depolymerized mixture can be allowed to cool naturally, although it is preferably cooled using a coolant. The coolant can be present in a jacket surrounding the crystallizer or can be passed through a series of heat exchangers through which the depolymerized mixture also passes, for example, in countercurrent flow. Step (b) can be carried out by reducing the temperature of the depolymerized mixture to a temperature of at least 5°C, preferably at least 10°C, and more preferably at least 15°C. Step (b) can be carried out by reducing the temperature of the depolymerized mixture to a temperature of up to 50°C, preferably to 40°C, and more preferably to 35°C. Thus, step (b) can be carried out by reducing the temperature of the depolymerized mixture to a temperature of 5 to 50°C, preferably from 10 to 40°C, and more preferably from 15 to 35°C. At these temperatures, incomplete crystallization will likely occur. However, since the amount of active coolant required to reach these temperatures is relatively low, they are nevertheless preferred. Furthermore, in preferred embodiments (discussed below), the liquid remaining after step (b) is recycled to step (a), meaning there is no loss of BHET (or soluble oligomers thereof) in the process. For similar reasons, a single crystallizer may be used to carry out step (b). Where the liquid remaining after step (b) is not recycled, step (b) can, in some cases, be carried out by reducing the temperature of the depolymerized mixture to 5 to 15°C. Step (b) can be carried out at atmospheric pressure, i.e., without the application or removal of pressure. Step (b) can be carried out for a period of at least 10 minutes, preferably at least 20 minutes, and more preferably at least 25 minutes. Step (b) can be carried out for a period of up to 60 minutes, preferably up to 45 minutes, and more preferably up to 35 minutes. Thus, step (b) can be carried out for a period of 10 to 60 minutes, preferably from 20 to 45 minutes, and more preferably from 25 to 35 minutes. The depolymerized mixture can be stirred during step (b). As mentioned above, the liquid remaining at the end of step (b) is preferably recycled for use in step (a). Thus, the method of the present invention preferably comprises isolating the precipitate comprising BHET between steps (b) and (c). The precipitate can be isolated using known methods, for example, by filtration or centrifugation. The residual liquid is preferably recycled for use in step (a) and, more preferably, to the first depolymerization reactor. Typically, the residual liquid will not be further processed since it is recycled to step (a); that is, the composition of the residual liquid will not be altered, although it should be noted that the residual liquid may be pumped and heated. Where the catalyst system comprises a carrier such as urea and a transition metal catalyst, these will also be recycled with the residual liquor. The conditions used in step (a) may lead to a precipitate containing a high proportion of BHET. The BHET may be present in the precipitate in an amount of at least 95%, preferably at least 99%, and most preferably at least 99.5% by weight. The precipitate formed in step (b) comprises BHET but will typically also comprise BHET dimers and trimers, for example, in an amount of at least 0.01 wt%. BHET dimers and trimers may be present in the precipitate in an amount of up to 2%, preferably up to 0.5%, and most preferably up to 0.2 wt. The amount of different components in the precipitate formed in step (b) can be determined using standard techniques, such as high-performance liquid chromatography (HPLC). HPLC can be carried out using the following conditions: instrument: Shimazu LC-20A HPLC; detector: photodiode array detector (PDA), chromatogram center wavelength of 223 nm (slit bandwidth of 4 nm); 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 pL.The samples are quantified by an external standard method. Preferably, in step (c) of the method, the precipitate formed in step (b) is dissolved in water to form a solution comprising BHET. BHET dimers and trimers are insoluble in water; thus, in step (c), the BHET dissolves to form an aqueous phase, while the dimers and trimers remain as solid materials which 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), with the purified product used as a high-quality monomer feed. Alternatively, in step (c) of the method, the precipitate formed in step (b) can also be dissolved in methanol to form a solution comprising BHET. Surprisingly, methanol has also been found to be an excellent solvent for use in step (c). ML / t / ZUZZ / UÓOZZO MA / IZ / ZUZZ / U ÓOZZO because it provides high levels of decolorization of the precipitate formed in step (b) as well as low levels of product loss. However, the use of water is preferred since BHET dimers and trimers are partially soluble in methanol and are consequently retained in detectable amounts in the monomeric product if methanol is used for recrystallization in step (c) of the method. Other alcoholic solvents may also be used in step (c) instead of water or methanol. For example, the solvent in step (c) may consist of or comprise any C1-C12 alcohol. More specifically, the solvent for use in step (c) may be selected from the group consisting of ethanol, propanol (especially isopropanol), and butanols (especially n-butanol and tert-butanol). Thus, the solvent for use in step (c) is preferably a protic solvent, and more preferably a polar protic solvent, and may be selected from the group consisting of water, methanol, ethanol, propanols (especially isopropanol), and butanols (especially n-butanol and tert-butanol). Higher alcoholic solvents may also be considered. In addition, non-alcoholic solvents may also be considered for use in step (c). Although the use of a protic solvent, and in particular a polar protic solvent, is particularly preferred in step (c), in embodiments of the present description, the solvent used in step (c) may instead be a nonpolar protic solvent, for example dimethyl carbonate (DMC), or a nonpolar solvent, for example an ether such as dimethoxyethane (DME) or diisopropyl ether (DIPE). More generally, the solvent in step (c) may consist of or comprise any solvent selected from the group comprising water, methanol, ethanol, propanols (especially isopropanol), butanols (especially n-butanol, tert-butanol), C5 to C12 alcohols (especially heptanols, e.g., n-heptanol, octanols, e.g., n-octanol, iso-octanol, nonanols, e.g., n-nonanol, decanols, e.g., n-decanol, dodecanols, e.g., n-dodecanol), esters (especially DMC), or ethers (especially DME or DIPE). Preferably, the solvent in step (c) is or comprises water, methanol, ethanol, isopropanol, or n-butanol. Mixtures of these and / or any of the solvents mentioned above are also acceptable. Especially when the solvent used is or comprises water, step (c) can be carried out at a temperature of at least 40°C, preferably at least 60°C, and more preferably at least 70°C. Step (c) can be carried out at a temperature up to 95°C, preferably up to 92.5°C, and more preferably up to 90°C. Thus, step (c) can be carried out at a temperature of 40 to 95°C, preferably from 60 to 92.5°C, and more preferably from 70 to 90°C. Alternatively, especially when the solvent used is or comprises methanol, step (c) can be carried out at a temperature of at least 40°C, preferably at least 50°C, and more preferably at least 55°C. Step (c) can be carried out at a temperature up to 80°C, preferably up to 70°C, and more preferably up to 65°C. Thus, step (c) can be carried out at a temperature of 40 to 80°C, preferably from 50 to 70°C, and more preferably from 55 to 65°C. Step (c) can be carried out at atmospheric pressure, i.e., without the application or removal of pressure. Step (c) can be carried out for a period of at least 1 minute, preferably at least 5 minutes, and more preferably at least 10 minutes. Step (c) can be carried out for a period of up to 60 minutes, preferably up to 50 minutes, and more preferably up to 40 minutes. Thus, step (c) can be carried out for a period of 1 to 60 minutes, preferably from 5 to 50 minutes, and more preferably from 10 to 40 minutes. The dissolution of the precipitate can be carried out with stirring. Especially when using water alone as the solvent in step (c), it may be used in an amount 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 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 from 0.1 to 2.5 times, preferably from 0.15 to 1.25 times, and more preferably from 0.2 to 0.5 times the amount of PET used in step (a) by weight. Especially when methanol is used alone as the solvent in step (c), it can be used in an amount at least 1 times, preferably at least 1.5 times, and more preferably at least 2 times the amount of PET used in step (a) by weight. Methanol can be used in step (c) in an amount 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 can be used in step (c) in an amount from 1 to 10 times, preferably from 1.5 to 5 times, and more preferably from 2 to 3 times the amount of PET used in step (a) by weight. In step (d) of the method, impurities are removed from the solution produced in step (c) to give a purified solution comprising BHET. Preferably, step (d) comprises decolorizing the solution. This can be done by contacting the solution with one or more decolorizing agents. Preferably, step (d) is carried out by passing the solution produced in step (c) through a column, and more preferably a plurality of columns in series, packed with one or more decolorizing agents. For example, each column in series can be packed with a different decolorizing agent. Step (d) may also comprise removing other contaminants such as metals and catalyst residues from the solution produced in step (c). One or more of the decolorizing agents used in step (d) may include carbon (e.g., activated carbon, preferably 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, preferably comprising sultanic acid or carboxylic acid groups, with the sultanic acid groups being preferred or alternatively, or in addition to an anion-exchange resin, preferably comprising quaternary ammonium salts, and / or a clay (e.g., activated clays such as bentonite and montmorillonite clays). Preferably, the solution produced in step (c) is contacted with carbon and / or resin, and preferably with an ion-exchange resin. Ion-exchange resins are particularly suitable for decolorizing and removing metal catalyst residues. In particularly preferred forms of the method, the solution produced in step (c) is MA / IZ / ZUZZ / U ÓOZZO contacts a plurality of different decolorizing agents by passing it through a plurality of columns arranged in series. For example, a first column may comprise an activated carbon decolorizing agent, a second column may comprise a cation exchange resin, and a third column may comprise 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 one in step (d). Step (d) can be carried out at a temperature of at least 40°C, preferably at least 55°C and more preferably at least 70°C. Step (d) can be carried out at a temperature up to 110°C, preferably up to 100°C and more preferably up to 90°C. Thus, step (d) can be carried out at a temperature of 40 to 110°C, preferably from 55 to 100°C and more preferably from 70 to 90°C. Step (d) can be carried out at atmospheric pressure, i.e., without the application or removal of pressure. Step (d) can be carried out for a period of at least 10 minutes, preferably at least 25 minutes, and more preferably at least 40 minutes. Step (d) can be carried out for a period of up to 120 minutes, preferably up to 100 minutes, and more preferably up to 60 minutes. Thus, step (d) can be carried out for a period of 10 to 120 minutes, preferably from 25 to 100 minutes, and more preferably from 40 to 80 minutes. Although less preferred, in some embodiments purification step (d) can be omitted. This is because the purification provided as a result of recrystallization, for example in methanol, alone may be sufficient to produce a purified, decolorized product comprising BHET, although such products are typically used in low-grade applications such as carpeting. Thus, in some embodiments, a purified product comprising BHET can be crystallized in step (e) from the solution produced in step (c). 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 BHET dimers and trimers. Although carrying dimers and trimers through a PET recycling process can be avoided by filtering them from an aqueous system, step (a) of the present invention produces dimers and trimers in such low quantities that they can be carried through the recycling process with the BHET. Thus, in some embodiments, a solid-liquid separation step between steps (c) and (e) of the present invention is not performed. 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 the BHET dimers and trimers, which are insoluble in water. Filtering the BHET solution between steps (c) and (d) may also be preferable when solvents other than water or methanol are used. In step (e) of the method, a purified product comprising BHET is crystallized from the purified solution. Step (e) is preferably carried out using cooling crystallization. Suitable crystallizers include stirred or scraped-wall crystallizers. The purified solution produced in step (d) can be allowed to cool naturally, although cooling using a coolant is preferable. The coolant can be present in a jacket surrounding the crystallizer or can be passed through a series of heat exchangers through which the purified solution also passes, for example, in countercurrent flow. Especially when the solvent used in step (c) is water, step (e) can be carried out by reducing the temperature of the purified solution to at least 0°C, preferably at least 10°C, and more preferably at least 20°C. Step (e) can be carried out by reducing the temperature of the purified solution to up to 55°C, preferably to 45°C, and more preferably to 40°C. Thus, step (e) can be carried out by reducing the temperature of the purified solution to a temperature of 0 to 55°C, preferably 10 to 45°C, and more preferably 20 to 40°C. Especially when the solvent used in step (c) is methanol, step (e) can be carried out by reducing the temperature of the purified solution to at least 0°C, preferably at least 5°C, and more preferably at least 8°C. Alternatively, step (e) can be carried out by reducing the temperature of the purified solution to up to 30°C, preferably to 15°C, and more preferably to 10°C. Thus, step (e) can be carried out by reducing the temperature of the purified solution to between 0 and 30°C, preferably between 5 and 15°C, and more preferably between 8 and 12°C. Step (e) can be carried out at atmospheric pressure, i.e. without the application or removal of pressure. Step (e) can be carried out for a period of at least 10 minutes, preferably at least 20 minutes, and more preferably at least 25 minutes. Step (e) can be carried out for a period of up to 60 minutes, preferably up to 45 minutes, and more preferably up to 35 minutes. Thus, step (e) can be carried out for a period of 10 to 60 minutes, preferably from 20 to 45 minutes, and more preferably from 25 to 35 minutes. The purified solution can be stirred during step (e). 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%, preferably at least 99%, and most preferably at least 99.5% by weight. If methanol is used as the solvent in step (c), the purified product formed in step (e) may also comprise BHET dimers and trimers, for example, in an amount of at least 0.01% by weight. The BHET dimers and trimers may be present in the purified product in an amount of up to 2%, preferably up to 0.5%, and more preferably up to 0.2% by weight. Preferably, the amounts of dimers and trimers present in the purified product formed in step (e) are substantially the same as the amounts of dimers and trimers present in the precipitate formed in step (b). The amount of different components in the purified product formed in step (e) can be determined using the methods described above in relation to the precipitate formed in step (b). MA / t / ZUZZ / UÓOZZO A key advantage of the present invention is that it can be used to produce purified products with low b[h] values, particularly b[h] values ​​of 2 or less. The PET prepared from BHET with these color densities is of a very high grade and can be used in applications requiring excellent visual appearance, such as in clear, colorless water bottles. Thus, the purified product formed in step (e) can exhibit a b[h] value of up to 2, for example, from 0 to 2. In some cases, the purified product can be used in lower-grade applications, such as carpets or films, in which case it can have a b[h] value of up to 4, for example, up to 3. The method of the present invention can be used to form a purified product in step (e) with a b[h] value that is 0.5 times, preferably 0.1 times, and more preferably 0.05 times that of the PET used in step (a). Using preferred embodiments of the present invention, even greater reductions in the b[h] value can be obtained, for example, where the PET feed used in step (a) exhibits a high color density. 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). The purified product comprising BHET is preferably isolated after step (e) and, where step (f) is present, before step (f). Precipitates can be isolated using known methods, for example, by filtration or centrifugation. Preferably, the protic solvent used in step (c), typically methanol or water and ethylene glycol, is recovered from the residual liquid remaining after isolation of the purified product, for example, using low-pressure evaporation and condensation. The protic solvent can be recycled to step (c). The ethylene glycol can be recycled for use in step (a) and, more preferably, to the first depolymerization reactor. One of the main advantages of using methanol for step (c), instead of water, is that the methanol and ethylene glycol can be easily recovered. Therefore, methanol and ethylene glycol recovery from the waste liquid can be carried out in a single-stage evaporator. In contrast, when water is used, recovering ethylene glycol and water from the waste liquid can be challenging, as water and ethylene glycol form an azeotropic mixture. Therefore, when water is used in step (c), a multi-stage evaporator is preferred for recovering the water and ethylene glycol from the waste liquid. When methanol is used in step (c), the recovery of methanol and ethylene glycol from the waste liquor can be carried out by heating the waste liquor to a temperature between the boiling points of methanol and ethylene glycol. For example, the waste liquor can be heated to a temperature above 65°C, preferably above 70°C, and more preferably above 75°C. The waste liquor can be heated to a temperature up to 120°C, preferably up to 100°C, and more preferably up to 90°C. Thus, the waste liquor can be heated to a temperature of 65 to 120°C, 70 to 100°C, and more preferably from 70 to 90°C. The recovery of methanol and ethylene glycol from the waste liquid can be carried out at ambient pressure, i.e., without the application or removal of pressure. Typically, the residual liquid will not be further processed if it is processed to recover MA / t / ZUZZ / UÓOZZO methanol and ethylene glycol. Preferably, the methanol is not further processed before being recycled for use in step (c). When water is used in step (c), a two-stage evaporator process is preferred for recovering water and ethylene glycol. In a first evaporator, water can be recovered from the residual liquid by applying low pressure, allowing evaporation at a reduced temperature; for example, operating the evaporator at a pressure of approximately 10 kPa is preferred, with the associated condenser temperature of approximately 46°C and a reboiling temperature of approximately 132°C. The residual ethylene glycol can then be recovered in a second evaporator by applying low pressure, preferably operating at a pressure of approximately 0.08 bar and a temperature of approximately 138°C. The user will appreciate that other operating temperatures and pressures for the first and second evaporators may also be selected.Improved water recovery can be achieved, if desired, by operating the first evaporator at a lower temperature or by using molecular sieves downstream of the first evaporator. Ethylene glycol may, however, undergo further purification before being recycled to step (a). For example, ethylene glycol may be evaporated to separate any organic residue trapped in it. Evaporation can take place at a temperature of at least 130°C, preferably at least 150°C, and more preferably at least 170°C. Evaporation can take place at a temperature of up to 230°C, preferably up to 210°C, and more preferably up to 190°C. Thus, evaporation can take place at a temperature of 130 to 230°C, preferably from 150 to 210°C, and more preferably from 170 to 190°C. Evaporation typically occurs under reduced pressure. For example, evaporation can occur at pressures up to 80,000 Pa, preferably up to 60,000 Pa, and more preferably up to 40,000 Pa. Evaporation can occur at pressures of at least 10,000 Pa, preferably at least 15,000 Pa, and more preferably at least 20,000 Pa. Thus, evaporation can occur at pressures from 10,000 to 80,000 Pa, preferably from 15,000 to 60,000 Pa, and more preferably from 20,000 to 40,000 Pa. When methanol is used in step (c), methanol recovery is so effective (even at industrial scales such as those described herein) that, when the recovered methanol is recycled to step (c), only unrecycled methanol needs to be added to step (c) in an amount up to 0.008 times, preferably up to 0.006 times, and more preferably up to 0.005 times the amount of PET used in step (a) by weight. Unrecycled methanol can be used in step (c) in an amount at least 0.001 times, preferably at least 0.003 times, and more preferably at least 0.004 times the amount of PET used in step (a) by weight. Thus, non-recycled methanol can be used in step (c) in an amount of 0.001 to 0.008 times, preferably 0.003 to 0.006 times, and most preferably 0.004 to 0.005 times the amount of PET used in step (a) by weight.Thus, it will be seen that the amount of methanol lost during the method of the present invention is extremely low and much less than the amount of water that would be lost. ML / t / ZUZZ / UÓOZZO when used instead of methanol in step (c). However, when water is used as the solvent in step (c), it can also be effectively recovered so that at least a majority of the water used in step (c) is recycled, preferably using the two-stage evaporator process described above. The lost water is typically removed from the system as humid air. Given the minimal environmental impact of water loss from the system, compared to the methanol-containing residue and the energy cost associated with water recovery, maximizing water recycling may not be worthwhile. The method of the present invention may further comprise step (f), in which the purified product comprising BHET is dried. The product may be dried by passing air over the purified product, for example, in a fluidized bed dryer. The air can be heated to a temperature of at least 30°C, preferably at least 40°C, and more preferably at least 50°C. The air can be heated to a temperature of up to 100°C, preferably up to 90°C, and more preferably up to 80°C. Thus, the air can be heated to a temperature of 30 to 100°C, preferably from 40 to 90°C, and more preferably from 50 to 80°C. The drying step (f) can be carried out at ambient pressure, i.e., without the application or removal of pressure. The drying step (f) can be carried out for a period of at least 10 minutes, preferably at least 15 minutes, and more preferably at least 20 minutes. The drying step (f) can be carried out for a period of up to 60 minutes, preferably up to 50 minutes, and more preferably up to 40 minutes. Thus, the drying step (f) can be carried out for a period of 10 to 60 minutes, preferably from 15 to 50 minutes, and more preferably from 20 to 40 minutes. The method of the present invention can be operated in a batch mode or in a continuous mode, although it is preferably operated continuously. The method of the present invention is preferably carried out on an industrial scale. In this way, the method can recycle at least 10 tons / day, preferably at least 100 tons / day, and potentially at least 1000 tons / day of PET. The present invention further provides a purified product comprising BHET which can be obtained, and preferably obtained, using a method as described herein. The present invention also provides a method for preparing a polymer. The method comprises carrying out a polymerization reaction using a purified product comprising BHET of the present invention. Preferably, the method comprises preparing the purified product comprising BHET using a method of the present invention. A key advantage of the present invention is that the purified product comprising BHET can be used directly in the polymerization, i.e., it does not undergo further purification before use. The purified product can be used to prepare PET or can be used to prepare copolymers comprising the ethylene terephthalate monomer. The polymer can be further processed into a bottle, packaging, textiles, or similar product. In some embodiments, the polymer can be further processed into a transparent bottle and MA / t / ZUZZ / UÓOZZO preferably, a colorless bottle. The present invention further provides an apparatus for recycling PET, the apparatus comprising: (a) a series of depolymerization reactors which are suitable for depolymerizing PET to form a depolymerized mixture comprising BHET, wherein the series of depolymerization reactors is adapted to receive PET, ethylene glycol and a catalyst system; (b) a crystallization unit downstream of the polymerization reactors suitable for crystallizing a precipitate comprising BHET from the depolymerized mixture; (c) a container for receiving the precipitate and which is suitable for dissolving the precipitate in a protic solvent to form a solution comprising BHET; (d) an impurity removal unit for receiving the solution comprising BHET and removing impurities from the solution to form a purified solution comprising BHET; and (e) an additional crystallization unit downstream of the purity removal unit suitable for crystallizing a purified product comprising BHET from the purified solution. The following non-limiting examples illustrate the present invention. Examples Example 1: depolymerization step (a) The depolymerization reactions were simulated in different series of reactors. The PET:ethylene glycol:catalyst system ratio used in the simulation, by mass, was 1:4:0.005. Each reactor was simulated as operating at 197°C and atmospheric pressure. The simulations were adjusted to provide a 99.0% conversion at the outlet of the final reactor in series. The simulation results are shown in the following table: MA / t / ZUZZ / UÓOZZO Number of depolymerization reactors Residence time per reactor (hours) Total residence time (hours) Conversion at outlet (% of initial PET) 1 54 54 (R1) 99.0% 2 5 10 (R1) 90.2% 5 (R2) 99.0% 3 2 6 (R1) 78.6% 2 (R2) 95.4% 2 (R3) 99.0% 4 1 .18 4.7 (R1) 68.5% 1 1 8 (R2) 90.0% 1 .18 (R3) 96.9% 1 .18 (R4) 99.0% 5 0.83 4.2 (R1) 60.4% 0.83 (R2) 84.4% 0.83 (R3) 93.8% 0.83 (R4) 97.6% 0.83 (R5) 99.0% To achieve a production level of around 10,000 tons per year, the volume of a single reactor would be about 300 m³. Where a series of three reactors is used, the volume per reactor falls to just over 10 m³. A similarly large decrease in the volume per reactor of approximately 11 to 12 m³ can be achieved with a series of only two reactors, as in the most preferred embodiments of the present invention. A graph showing the efficiency of each depolymerization reaction, taking into account the above data, but also the energy and equipment input required in each arrangement, is shown in Figure 1. It can be observed that a dramatic improvement in efficiency is seen when using a series of at least two depolymerization reactors, compared to using a single depolymerization reactor. Example 2: Preferred solvent to use in step (c) BHET recrystallization experiments were carried out in a variety of solvents, including methanol, ethanol, isopropanol, butanols, and alcohols with a longer carbon chain. Specifically, 50 g of BHET were 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 a temperature of 10°C was reached. The recrystallized BHET was analyzed to determine its color density. Weight loss during the recrystallization processes was also measured. The results are shown in the following table: MA / t / ZUZZ / UÓOZZO Solvent Weight Loss (%) Color Density (b[h]) 20 Methanol 28 4.06 Ethanol 57 4.05 Isopropanol 60 3.87 tert-Butanol 25 n-Butanol 56 3.91 4.03 n-Heptanol 22 4.80 Octanol Isooctanol 26 27 4.88 6.07 ΰυ n-Nonanol 15 6.65 It can be observed that each of the lighter solvents resulted in good levels of decolorization. However, the amount of material lost during recrystallization was significantly less with methanol than in any of the other lighter solvent experiments. Methanol, as well as the higher alcohols, is viable for industrial-scale use. Example 3: bleaching step (d) A number of different techniques were used to decolorize an aqueous solution of BHET. Experiments using resins yielded promising results: Resin Type Solution Appearance Weak Acid Cation Exchange High Discoloration Macroporous A Moderate Discoloration Macroporous B Good Discoloration Macroporous C Good-Moderate Discoloration Strong Acid Cation Exchange Very High Discoloration Strong Base Anion Exchange Moderate Discoloration Weak Acid Cation Exchange A Good-Moderate Discoloration Weak Acid Cation Exchange B Good Discoloration MA / t / ZUZZ / UÓOZZD It can be observed that cation exchange resins, and particularly strongly acidic cation exchange resins, give the most promising results. Activated carbon is also highly effective in decolorizing BHET: BHET Sample Color Density (b[h]) Untreated 7.21 Cation exchange resin 4.58 Activated carbon 1.08 Images of the untreated and treated samples and images of the PET prepared using the samples are shown in Figure 2. Although cation exchange resin and activated carbon both give good levels of decolorization, the carbon-treated product gave a better quality polymer product. Further decolorization experiments were carried out. This time a BHET solution in methane was used. The experiments produced similar results to those carried out with aqueous BHET solutions, but with the cation exchange resin giving particularly good results. Example 4: Recycling process using methanol in step (c) A process of the present invention was carried out in the apparatus described in Figure 3. The residue that was used in the process is shown in Figure 4. The residue consists of used blue and green PET flakes. Specifically, PET (2), a zinc acetate and urea catalyst system (4), and ethylene glycol (6) were passed to the first of a series of three depolymerization reactors (10). A sample taken after the series of three depolymerization reactors (10) showed 100% conversion of PET (2) with a selectivity of 99.8% for BHET. The depolymerized mixture was passed through a filter (20) to remove insoluble materials (32), then to a crystallizer (12) in which a precipitate comprising BHET was formed. The precipitate was passed through a filter (20) to one of the two stirred vessels (14). Methanol (8) was added to the containers (14) to dissolve the precipitate, thereby forming a solution comprising BHET. The solution was made through a decolorization stage (16), described in a side-by-side image, to another crystallizer (18) where a purified product comprising BHET was formed. The purified product was passed through another filter (20) to a drying unit (26), and the residual liquor was passed to a methanol and ethylene glycol recovery unit (22). The methanol was recycled from the recovery unit (22) to the stirred vessels (14), while the ethylene glycol was passed through an evaporation unit (24), where the organic residue (34) was removed, before being recycled to the series of depolymerization reactors (10). The purified product was dried by passing hot air (28) through the dryer (26). The hot air (28) was removed from the system through a condenser in which any residual water (36) was removed and an evaporation unit from which methanol was recovered and recycled to the stirred vessel (14). Once dry, the purified product (30) was removed from the system. The purified product (30) had a low color density and was used, without further processing, in the preparation of recycled PET for use in water bottles. Example 5: Recycling process using water in step (c) A process of the present invention was carried out in the apparatus described in Figure 5. Specifically, PET (102), a zinc acetate and urea catalyst system (104), and ethylene glycol (106) were passed to the first of a series of two depolymerization reactors (100). A sample taken after the series of two depolymerization reactors (100) showed 100% conversion of PET (102), with selectivity for BHET at 95.0%; the other 5.0% of the product consisted substantially of BHET oligomers. The excess water (140) was removed by means of an evaporator (138) and the depolymerized mixture was then passed through a filter (120a) to remove the insoluble materials (132), then to a crystallizer (112) in which a precipitate comprising BHET was formed. The precipitate was passed through a filter (120b) to a stirring vessel (114). Water (108) was added to the container (114) to dissolve the precipitate, thereby forming a solution comprising BHET. The solution was passed through a decolorization stage (116). As described, the decolorization stage comprises a filter (120c), followed by a first unit (142) comprising an activated carbon bed, followed in series by a second unit (144) comprising a cation exchange bed, and followed by a third unit (146) comprising an anion exchange bed. After the decolorization stage (116), the solution was passed to another crystallizer (118), in two stages, where a purified product comprising BHET was formed. The purified product was passed through another filter (120d) to a drying unit (126), and the residual liquor was passed to an evaporator (122). The water was recycled from the evaporator (122) to the stirred vessel (114), while the ethylene glycol was passed to an additional evaporator (124), where the organic residue (134) was removed, before being recycled to the series of depolymerization reactors (100). MA / t / ZUZZ / UÓOZZO The purified product was dried by passing hot air (128) through the dryer (126). Once dry, the purified product (130) was removed from the system. The purified product (130) had a low color density and was used, without further processing, in the preparation of recycled PET for use in water bottles.

Claims

1. A method for recycling polyethylene terephthalate (PET), the method being characterized in that it comprises: (a) depolymerizing PET in the presence of ethylene glycol and a catalyst system in a series of depolymerization reactors to form a depolymerized mixture comprising bis(2-hydroxyethyl) terephthalate (BHET); (b) crystallizing a precipitate comprising BHET from the depolymerized mixture; (c) dissolving the precipitate in a protic solvent to form a solution comprising BHET; (d) removing impurities from the solution to form a purified solution comprising BHET; and (e) crystallizing a purified product comprising BHET from the purified solution.

2. The method according to any of the preceding claims, further characterized in that the PET is waste PET, optionally obtained from waste PET bottles, wherein the PET is preferably used in the form of particles where: at least 80% by weight of the particles pass through a mesh having openings with a diameter of 20 mm, preferably 15 mm and more preferably 12 mm; 100% by weight of the particles pass through a mesh having openings with a diameter of 25 mm, preferably 20 mm and more preferably 12 mm; and / or up to 1% by weight of the particles pass through a mesh having openings with a diameter of 0.1 mm, preferably 0.5 mm and more preferably 1 mm.

3. The method in accordance with any of the preceding claims, further characterized in that the PET has a b[h] value of more than 5, for example more than 10.

4. The method in accordance with any of the preceding claims, further characterized in that the PET is depolymerized in a series of two depolymerization reactors.

5. The method according to any of the preceding claims, further characterized in that each of the depolymerization reactors used in step (a) can be operated: at a temperature of 150 to 230°C, preferably 170 to 220°C and more preferably 190 to 210°C; at atmospheric pressure; for a period of 20 minutes to 4 hours, preferably 1 to 3 hours and more preferably 1.5 to 2.5 hours; and / or with stirring.

6. The method according to any of the preceding claims, further characterized in that ethylene glycol is used in step (a) in an amount of 2 to 6, preferably 3 to 4, and more preferably 3.25 to 3.75 times the amount of PET by weight.

7. The method according to any of the preceding claims, further characterized in that the catalyst system comprises a transition metal catalyst, preferably a zinc-containing catalyst and more preferably a zinc acetate catalyst.

8. The method according to any of the preceding claims, further characterized in that the catalyst system comprises a carrier and preferably a carrier containing nitrogen as urea, and wherein the catalyst system preferably comprises zinc acetate and urea and more preferably has the formula [4NH2CONH2-ZnOAc].

9. The method according to any of the preceding claims, further characterized in that the catalyst system is used in step (a) in an amount of 0.001 to 0.5, preferably 0.003 to 0.01 and more preferably 0.004 to 0.005 times the amount of PET by weight.

10. The method according to any of the preceding claims, further characterized in that step (b) is carried out using cooling crystallization and preferably: reducing the temperature of the depolymerized mixture to a temperature of 5 to 50°C, preferably 10 to 40°C and more preferably 15 to 35°C; at atmospheric pressure; for a period of 10 to 60 minutes, preferably 20 to 45 minutes and more preferably 25 to 35 minutes; and / or under stirring.

11. The method according to any of the preceding claims, further characterized in that the protic solvent comprises one or more of water, methanol, ethanol, isopropanol, and n-butanol.

12. The method according to claim 11, further characterized in that the protic solvent is water.

13. The method according to claim 12, further characterized in that step (c) is carried out: at a temperature of 40 to 95°C, preferably 60 to 92.5°C and more preferably 70 to 90°C; at atmospheric pressure; for a period of 1 to 60 minutes, preferably 5 to 50 minutes and more preferably 10 to 40 minutes; and / or under stirring.

14. The method according to claim 12 or 13, further characterized in that the water is used in step (c) in an amount of 0.1 to 2.5, preferably 0.15 to 1.25 and more preferably 0.2 to 0.5 times the amount of PET used in step (a) by weight.

15. The method according to any of the preceding claims, further characterized in that step (d) comprises decolorizing the solution, for example, contacting the solution with carbon (for example, activated carbon), a resin, and preferably an ion-exchange resin (for example, a cation-exchange resin, such as an acidic cation-exchange resin) and / or a clay (for example, activated clays such as bentonite and montmorillonite clays), and preferably contacting the solution with carbon and an ion-exchange resin.

16. The method according to any of the preceding claims, further characterized in that step (e) is carried out using a cooling crystallization and preferably: reducing the temperature of the purified solution to a temperature of 0 to 55°C, preferably 10 to 45°C and more preferably 20 to 40°C; at atmospheric pressure; for a period of 10 to 60 minutes, preferably 20 to 45 minutes and more preferably 25 to 35 minutes; and / or under stirring.

17. The method in accordance with any of the preceding claims, further characterized in that the depolymerized mixture is passed through a filter between steps (a) and (b) to remove the insoluble components.

18. The method according to any of the preceding claims, further characterized in that the precipitate comprising BHET is isolated, for example by filtration, between steps (b) and (c) and wherein the filtrate is preferably recycled as the first depolymerization reactor in step (a).

19. The method according to any of the preceding claims, except where it depends on claim 12, further characterized in that the protic solvent is methanol and the method comprises isolating the purified product comprising BHET, for example by filtration, after step (e), wherein the method preferably comprises processing the filtrate to recover methanol and ethylene glycol and recycling methanol in step (c) and / or ethylene glycol to a depolymerization reactor in step (a).

20. The method according to claim 19, further characterized in that the recovery of methanol and ethylene glycol is carried out in a single-stage evaporator.

21. The method according to any of the preceding claims, further characterized in that the method additionally comprises: (f) drying the purified product comprising BHET, for example with air.

22. The method in accordance with any of the preceding claims, further characterized in that the purified product comprising BHET has a b / [h] value of up to 2.

23. The method according to any of the preceding claims, further characterized in that the purified product comprises: BHET in an amount of at least 95%, preferably at least 99% and more preferably at least 99.5% by weight; and BHET dimers and trimers, for example in an amount of at least 0.01% by weight, preferably in an amount of up to 2%, preferably up to 0.5% and more preferably up to 0.2% by weight.

24. A purified product characterized in that it comprises bis(2-hydroxyethyl) terephthalate (BHET) which is obtained using a method as defined in any of claims 1 to 23.

25. A method for preparing a polymer, the method comprising carrying out a polymerization reaction using a purified product comprising bis(2-hydroxyethyl) terephthalate (BHET) as defined in claim 24, wherein the method preferably comprises preparing the purified product using a method as defined in any one of claims 1 to 23.

26. An apparatus for recycling polyethylene terephthalate (PET), the apparatus being characterized in that it comprises: (a) a series of depolymerization reactors which are suitable for depolymerizing PET to form a depolymerized mixture comprising bis(2-hydroxyethyl) terephthalate (BHET), wherein the series of depolymerization reactors is adapted to receive PET, ethylene glycol, and a catalyst system; (b) a crystallization unit downstream of the polymerization reactors suitable for crystallizing a precipitate comprising BHET from the depolymerized mixture; (c) a vessel for receiving the precipitate and which is suitable for dissolving the precipitate in a protic solvent to form a solution comprising BHET; (d) an impurity removal unit for receiving the solution comprising BHET and which removes impurities from the solution to form a purified solution;and (e) an additional crystallization unit downstream of the purity removal unit suitable for crystallizing a purified product comprising BHET from the purified solution.; 27. Use of urea in a catalyst system in a polyethylene terephthalate (PET) recycling process to: solubilize metals, in particular a transition metal catalyst component of the catalyst system; and / or form a eutectic salt with a transition metal catalyst component of the catalyst system.