Recycling of Polymers

Abstract

A method for recycling polyethylene terephthalate (PET) to produce bis(2-hydroxyethyl) terephthalate (BHET) includes the steps of: (a) depolymerizing PET in the presence of ethylene glycol and a catalyst system, preferably in two depolymerization reactors in series, to form a depolymerized mixture containing BHET; (b) crystallizing a precipitate containing BHET from the depolymerized mixture; (c) dissolving the precipitate in a protic solvent, preferably water but also 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. Apparatus suitable for such a method, and the use of urea in the catalyst system, are also provided accordingly.

Description

Technical Field

[0001] The present invention relates to a method and apparatus for recycling polymers, and more particularly to 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 at a purity level suitable for direct use in the preparation of high-quality plastics.

Background Art

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

[0003] Virgin PET can be easily prepared using ethylene glycol and terephthalate-containing monomers. Nevertheless, since its raw materials are obtained from non-renewable resources, such as crude oil, the need to recycle PET is increasing.

[0004] Recycling of PET waste, such as clear plastic water bottles, when composed of only a single type of PET, can be as simple as melting and remolding the waste flakes. However, in waste, it is common to contain a variety of different PET materials. For example, when various differently colored bottles are melted and remolded, they produce products with a low visual grade. Such materials may be suitable for use in carpet fibers, but they are generally not suitable for use in packaging, such as in clear water bottles.

[0005] Accordingly, there is a need for a method for recycling waste PET into products that can be used in applications requiring a high visual grade.

[0006] More sophisticated methods for recycling PET involve depolymerizing the waste material to obtain a feedstock that is viable for use in the preparation of polymers, typically after several purification and separation steps.

[0007] For example, PET can be depolymerized using a glycolysis agent, such as ethylene glycol, to form the BHET monomer. However, conventional methods for depolymerizing PET tend to produce the BHET monomer in yields of less than 80%, with significant amounts of BHET oligomers, particularly dimers and trimers, being formed from the remainder of the PET.

[0008] The presence of dimers and trimers reduces the quality of the polymer prepared from the BHET feedstock, and it has been conventional to purify the depolymerization mixture to remove these components. Further purification is particularly critical when high-quality recycled PET, such as recycled PET suitable for use in clear, colorless bottles, is required.

[0009] Color spaces are often used to represent the grade of a polymer, and the b[h] value - a measure of the tone from blue (negative values) to yellow (positive values) - is regarded as an important indicator of quality. Recycled PET of poor quality typically exhibits an undesirable yellow hue.

[0010] There are several difficulties associated with processes that produce a depolymerization mixture containing significant amounts of dimers and trimers. One of the most prominent is that a substantial amount is lost from the recycling process when the PET feedstock is removed in the form of dimers and trimers. Further depolymerization, which itself requires time and energy, will not recycle the dimers and trimers, and the efficiency of a typical PET recycling process is, therefore, very poor. SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

[0011] Therefore, there is a need for an improved method for depolymerization recycling of waste PET. In particular, there is a need for a method for depolymerization recycling of waste PET that produces a product suitable for use in high-quality applications, such as transparent water bottles.

Means for Solving the Problems

[0012] By using a series of depolymerization reactors, it was surprisingly found that a depolymerized mixture containing a very high proportion of BHET monomer and relatively small amounts of dimer and trimer can be obtained, thereby making it possible to omit the conventional purification step in which dimer and trimer are removed. This means that the previously discarded solvent can be used as it is inappropriate for further treatment of the crude BHET monomer.

[0013] The inventors have found that protic solvents are highly effective for recrystallizing the crude depolymerization product. In particular, water is preferred for this use because the dimers and trimers of BHET are insoluble in water. Thus, while BHET is dissolved to form an aqueous phase, the dimers and trimers remain as solid materials that can be separated from the aqueous phase, for example by filtration, prior to recrystallization, resulting in a high-purity monomer product.

[0014] Methanol can also preferably be used as it minimizes product loss and at least partially decolorizes the product. Methanol dissolves and transports dimers and trimers throughout the process such that they can be present in the purified product containing BHET, yet their concentrations can be low enough that the purified product can be used directly in the polymerization reaction. The resulting polymer can be used in high-quality applications, such as transparent and colorless water bottles.

[0015] Furthermore, as detailed below in this specification, while maintaining the advantages of using a series depolymerization reactor in accordance with the present disclosure, it is also possible to use aprotic solvents, and even nonpolar solvents, to recrystallize the crude depolymerization product.

[0016] Accordingly, the present invention is a method for recycling polyethylene terephthalate (PET), comprising: (a) depolymerizing PET in a series of depolymerization reactors in the presence of ethylene glycol and a catalyst system to form a depolymerized mixture containing bis(2-hydroxyethyl) terephthalate (BHET); (b) crystallizing a precipitate containing BHET from the depolymerized mixture; (c) dissolving the precipitate in a protic solvent 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.

[0017] The present invention further provides a purified product containing BHET obtainable using the method of the present invention.

[0018] There is also provided a method for preparing a polymer, comprising performing a polymerization reaction using the purified product of the present invention containing BHET.

[0019] An apparatus for recycling PET, comprising: (a) a series of depolymerization reactors suitable for depolymerizing PET to form a depolymerized mixture containing BHET, adapted to receive PET, ethylene glycol, and a catalyst system; (b) a crystallization unit downstream of the polymerization reactor suitable for crystallizing a precipitate containing BHET from the depolymerized mixture; (c) A container suitable for receiving the precipitate and for dissolving the precipitate in a protic solvent to form a solution containing BHET; (d) An impurity removal unit for receiving the solution containing BHET and for removing impurities from the solution to form a purified solution; and (e) Further provided is an apparatus including a further crystallization unit downstream of the impurity removal unit suitable for crystallizing a purified product containing BHET from the purified solution.

[0020] The present invention also provides for the use of urea in a catalyst system in a polyethylene terephthalate (PET) recycling process, the purpose of which is to solubilize metals, in particular the transition metal catalyst component of the catalyst system, and / or to form a eutectic salt with the transition metal catalyst component of the catalyst system.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying out the Invention

[0022] The present invention provides a method for recycling polyethylene terephthalate (PET).

[0023] PET is a thermoplastic polymer having the following structure.

[0024]

Chemical formula

[0025] The PET used in the method of the present invention is typically waste PET. Waste PET can be obtained from a wide range of sources including packaging, bottles and textile products. Preferably, the PET is obtained from waste bottles. The PET used in step (a) may be washed PET, i.e., PET that has passed through a purification process. The washed PET can be PET washed with purified water by steam treatment, a purified solvent and / or a purified detergent. Preferably, the PET used in step (a) is PET washed with water.

[0026] The PET used in step (a) preferably contains colored PET. The PET can contain colored PET in an amount of at least 5% by weight, preferably at least 10% by weight, more preferably at least 25% by weight. In some embodiments, the PET can contain colored PET in an amount of at least 50% by weight, more preferably at least 75% by weight. The PET can contain colored PET in an amount up to 100% by weight.

[0027] The PET used in step (a) preferably exhibits a b[h] value (i.e., the b value in the Hunter Lab color space) greater than 5, for example greater than 10, although some PET feeds can have a b[h] value of 100 or even higher. This can be measured using standard techniques, for example with a colorimeter.

[0028] The PET is preferably used in step (a) in the form of particles, such as flakes. Preferably, at least 80% by weight of the particles (i.e., d80) pass through a mesh having an aperture of 20 mm, preferably 15 mm, more preferably 12 mm. Even smaller mesh diameters can also be used. Particles having these sizes are rapidly depolymerized.

[0029] A range of particle sizes are typically used in step (a), but larger particle sizes are preferably avoided as they take longer to process. Thus, 100% by weight of the particles (d100) preferably pass through a mesh having an aperture of 25 mm, preferably 20 mm, more preferably 12 mm. Even smaller mesh diameters can also be used. Particles that are too small are preferably avoided, except when the powder is already available through the waste collection and separation process, as the energy and thus cost required to grind the PET to this size becomes unnecessary. Thus, preferably at most 1% by weight of the particles pass through a mesh having an aperture of 0.1 mm, preferably 0.5 mm, more preferably 1 mm.

[0030] It will be appreciated that the PET used in step (a) can be transferred to a series of reactors in a form coated with a liquid, such as residual water, or other solvent used to purify the PET. This liquid coating is not considered to form part of the PET for the purposes of the present invention.

[0031] In step (a) of the process, the PET is depolymerized in a series of depolymerization reactors to form a depolymerized mixture containing bis(2-hydroxyethyl) terephthalate (BHET). BHET is a monomer having the following structure.

[0032]

Chemical formula

[0033] The PET is partially depolymerized in the first depolymerization reactor and further depolymerized downstream of the first reactor in a series of reactors. By using a series of reactors, it has been found that the depolymerized mixture can contain a high proportion of BHET and low levels of dimers and trimers. Dimers and trimers have the following structures.

[0034]

Chemical formula

[0035] Higher oligomers are generally not present in the depolymerized mixture. Thus, in a preferred embodiment, the depolymerized mixture is substantially free of higher oligomers (i.e., when n ≥ 4).

[0036] Surprisingly, very high quality products can be produced by depolymerizing the PET with only two reactors in series. Thus, in a preferred embodiment, the PET is depolymerized in two depolymerization reactors in series. This results in high levels of both the conversion of the PET and the selectivity to BHET. In alternative embodiments, the PET is depolymerized in 3, or 4 or 5 or more reactors in series.

[0037] Preferably, all of the ethylene glycol and catalyst system used in the depolymerization process are added to the first reactor in 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.

[0038] It will be appreciated that although ethylene glycol and / or catalyst system may be added to the reaction mixture downstream of the first reactor, there are no components removed from the reaction when passing through the series of reactors.

[0039] 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, more preferably at least 190 °C. Each of the depolymerization reactors used in step (a) can be operated at a temperature up to 230 °C, preferably up to 220 °C, more preferably up to 210 °C. Thus, each of the depolymerization reactors used in step (a) can be operated at a temperature of 150 - 230 °C, preferably 170 - 220 °C, more preferably 190 - 210 °C. Generally, the depolymerization reactors are operated at the same temperature, but this is not necessarily the case.

[0040] Unlike many prior art processes, PET is preferably not used in a molten state in step (a), which means that the reaction mixture is relatively viscous. This viscosity typically results in a relatively low level of PET conversion. It is surprising that by using a series of depolymerization reactors, excellent levels of conversion can be achieved even when step (a) is carried out with solid state PET.

[0041] Each of the depolymerization reactors used in step (a) can be operated under 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 by location, the atmospheric pressure used in this specification is considered to be approximately equal to standard atmospheric pressure, i.e., approximately 101,325 Pa.

[0042] 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, 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, more preferably up to 1.75 hours. Therefore, each of the depolymerization reactors 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. The depolymerization reactors can all be operated for the same period, but this is not necessarily the case.

[0043] PET can be transferred to the series of depolymerization reactors at a flow rate of at least 100 kg per hour, preferably at least 500 kg per hour, more preferably at least 1,000 kg per hour. PET can be transferred to the series of depolymerization reactors at a flow rate of up to 100,000 kg per hour, preferably up to 50,000 kg per hour, more preferably up to 10,000 kg per hour. Therefore, PET can be transferred to the series of depolymerization reactors at a flow rate of 100 to 100,000 kg per hour, preferably 500 to 50,000 kg per hour, more preferably 1,000 to 10,000 kg per hour.

[0044] Each of the depolymerization reactors used in step (a) is preferably operated with stirring.

[0045] The size of the reactors used in the series of depolymerization reactors can vary depending on the number of reactors used. Each of the reactors used in step (a) is at least 5 m 3 , preferably at least 8 m3 and more preferably at least 10 m 3 in size. Each of the reactors used in step (a) can have a size of up to 50 m 3 , preferably up to 20 m 3 , more preferably up to 15 m 3 in size. Thus, each of the reactors used in step (a) can have a size of 5 - 50 m 3 , preferably 8 - 20 m 3 , more preferably 10 - 15 m 3 in size. The use of these small-scale reactors is made possible by having reactors in series through which the PET can be depolymerized with a minimum residence time. Thus, industrial-scale amounts of PET can be depolymerized into high-quality products using relatively small reactors.

[0046] Ethylene glycol is used in step (a) as a glycolysis agent. Ethylene glycol can be used in step (a) in an amount of at least 2 times, preferably at least 3.25 times, more preferably at least 3.5 times the amount of PET by weight. Ethylene glycol can be used in step (a) in an amount of up to 6 times, preferably up to 5 times, more preferably up to 4.75 times the amount of PET by weight. Thus, ethylene glycol can be used in step (a) in an amount of 2 - 6 times, preferably 3.25 - 4.75 times, more preferably 3.5 - 4.75 times the amount of PET by weight.

[0047] At least 60 wt%, preferably at least 80 wt%, more preferably at least 95 wt% of the ethylene glycol can be added to the first reactor. However, as described above, all of the ethylene glycol is most preferably added to the first reactor. It will be appreciated that if less than 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.

[0048] The catalyst system is used in step (a) to improve the depolymerization reaction. The catalyst system preferably includes a transition metal catalyst, such as a zinc-containing catalyst. Suitable zinc catalysts include zinc acetate.

[0049] In some embodiments, the catalyst system consists of a transition metal catalyst. However, in a preferred embodiment, the catalyst system contains a catalyst in a carrier, for example as described above. Suitable carriers include nitrogen-containing carriers, such as urea.

[0050] Surprisingly, urea is highly effective in maintaining metals (e.g., the transition metal catalyst component of the catalyst system; or trace metal catalysts originally used to produce PET, such as antimony catalysts) and other contaminants in solution, thereby making it possible to separate these components from BHET in step (b). Accordingly, the present invention also provides the use of urea in a catalyst system in a PET recycling process to solubilize metals, particularly the transition metal catalyst component of the catalyst system. Urea can also be used to solubilize contaminants in the PET recycling process. Surprisingly, it has been found that eutectic salt catalyst systems are particularly effective in solubilizing metals and / or contaminants.

[0051] The carrier can be used in the catalyst system in an amount of at least 1-fold, preferably at least 2-fold, more preferably at least 3-fold the molar amount of the transition metal cation in the transition metal catalyst. The carrier can be used in an amount up to 8-fold, preferably up to 6-fold, more preferably up to 5-fold the molar amount of the transition metal cation. Thus, the carrier can be used in an amount of 1 - 8-fold, preferably 2 - 6-fold, more preferably 3 - 5-fold the molar amount of the transition metal cation. These ratios of the carrier to the transition metal catalyst have been found to increase the reaction rate while maintaining metal ions in solution. As described above, the transition metal cation is typically a zinc cation.

[0052] Most preferably for use in step (a) is a catalyst system comprising, preferably consisting of, zinc acetate and urea, especially a catalyst system having the formula [4NH2CONH2·ZnOAc]. This catalyst system advantageously forms a eutectic salt. Accordingly, the present invention also provides for the use of urea for forming a eutectic salt with the transition metal catalyst component of the catalyst system in a catalyst system in the PET recycling process.

[0053] The catalyst system may preferably be in the liquid phase in step (a), preferably through the method of the present invention.

[0054] The catalyst system can be used in step (a) in an amount of at least 0.001 times, preferably at least 0.003 times, 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 up to 0.5 times, preferably up to 0.01 times, 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, more preferably 0.004 to 0.005 times the amount of PET by weight.

[0055] At least 60% by weight, preferably at least 80% by weight, more preferably at least 95% by weight of the catalyst system can be added to the first reactor. However, as described above, all of the catalyst system is preferably added to the first reactor. It will be appreciated 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.

[0056] Step (a) is generally carried out in the absence of any solvent other than ethylene glycol and in the absence of any support that may be present in the catalyst system. It will be recognized that there may be present, as a coating on the PET due to washing, some residual liquid, such as water, that has migrated into the claimed process; 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, more preferably up to 0.001 times the amount of PET used in step (a) by weight. Most preferably, the solvent is substantially absent in step (a).

[0057] Preferably, the depolymerized mixture is separated from any insoluble components between steps (a) and (b). The insoluble components include unreacted PET (although, if present, its level is typically very low) and other inert solids. The other solids can include non-PET polymers such as polyethylene (PE) and polypropylene (PP). Preferably, the depolymerized mixture is passed through a filter to remove the insoluble components, although other techniques such as centrifugation can also be used. A tricanter can be used to achieve a very high level of solid-liquid separation.

[0058] In step (b) of the process, the precipitate containing BHET crystallizes from the depolymerized mixture formed in step (a). Step (b) is preferably carried out using cooling crystallization. Suitable crystallization apparatuses include stirred or wall-scraped crystallization apparatuses. The depolymerized mixture can be left to cool naturally, but is preferably cooled using a coolant. The coolant can be present in a jacket surrounding the crystallization apparatus or can be passed through a series of heat exchangers through which the depolymerized mixture is also passed, for example, in countercurrent.

[0059] 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, 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 up to 40°C, more preferably up to 35°C. Accordingly, step (b) can be carried out by reducing the temperature of the depolymerized mixture to a temperature of 5 to 50°C, preferably 10 to 40°C, more preferably 15 to 35°C.

[0060] At these temperatures, incomplete crystallization is expected to occur. Nevertheless, these temperatures are preferred because the amount of active cooling required to reach these temperatures is relatively small. Further, in a preferred embodiment (discussed below), the remaining liquid after step (b) is recycled to step (a), which means that there is no loss of BHET (and its soluble oligomers) in the process. For similar reasons, only a single crystallization apparatus can be used to carry out step (b). If the remaining liquid after step (b) is not recycled, step (b) can, in some instances, be carried out by reducing the temperature of the depolymerized mixture to a temperature of 5 to 15°C.

[0061] Step (b) can be carried out at atmospheric pressure, i.e., without the application or removal of pressure.

[0062] Step (b) can be carried out for a period of at least 10 minutes, preferably at least 20 minutes, 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, more preferably up to 35 minutes. Accordingly, step (b) can be carried out for a period of 10 to 60 minutes, preferably 20 to 45 minutes, more preferably 25 to 35 minutes.

[0063] The depolymerized mixture can be stirred during step (b).

[0064] As described 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 includes a step of isolating a precipitate containing 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), more preferably to the first depolymerization reactor. Typically, it will be appreciated that the residual liquid is recycled to step (a) as it is and is not further processed, i.e., the composition of the residual liquid is not changed, but the residual liquid can pass through a pump and be heated. When the catalyst system includes a carrier, such as urea and a transition metal catalyst, these are also recycled with the residual liquor.

[0065] The conditions used in step (a) can cause a precipitate containing a high proportion of BHET. BHET can 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.

[0066] The precipitate formed in step (b) contains BHET, but typically also contains dimers and trimers of BHET, for example, in an amount of at least 0.01% by weight. The dimers and trimers of BHET can be present in the precipitate in an amount of up to 2% by weight, preferably 0.5% by weight, more preferably 0.2% by weight. The amounts of the 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 - equipment: LC-20A HPLC manufactured by Shimadzu; 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 temp: 35°C; sample: dissolved in methanol; injection volume: 20 μL. The sample is quantified by the external standard method.

[0067] Preferably, in step (c) of the method, the precipitate formed in step (b) is dissolved in water to form a solution containing BHET. The dimers and trimers of BHET are insoluble in water, and thus, in step (c), BHET dissolves and an aqueous phase is formed 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 raw material.

[0068] Alternatively, in step (c) of the method, the precipitate formed in step (b) can be dissolved in methanol to form a solution containing BHET. Methanol has unexpectedly been found to be an excellent solvent for use in step (c) as it exhibits a high level of decolorization of the precipitate formed in step (b) and a low level of product loss. However, the dimers and trimers of BHET are partially soluble in methanol, and thus, they are retained in detectable amounts in the monomer product if methanol is used for recrystallization in step (c) of the method, making the use of water preferable.

[0069] Other alcohol solvents can also be used in step (c) instead of water or methanol. For example, the solvent in step (c) can consist of or contain any C1-C 12 alcohol. More specifically, the solvent for use in step (c) can be selected from the group consisting of ethanol, propanol (especially iso-propanol), and butanol (especially n-butanol, tert-butanol). Thus, the solvent for use in step (c) is preferably a protic solvent, most preferably a polar protic solvent, and can be selected from the group consisting of water, methanol, ethanol, propanol (especially iso-propanol), and butanol (especially n-butanol, tert-butanol). Higher alcohol solvents can also be envisaged.

[0070] Furthermore, non-alcohol solvents can also be envisaged for use in step (c). The use of protic solvents, especially polar protic solvents, is particularly preferred in step (c), but in embodiments of the present disclosure, instead, the solvent used in step (c) can be a polar aprotic solvent such as dimethyl carbonate (DMC), or an apolar solvent such as an ether, for example dimethoxyethane (DME) or diisopropyl ether (DIPE).

[0071] Most commonly, the solvent in step (c) can consist of or include any solvent selected from the group consisting of water, methanol, ethanol, propanol (especially iso-propanol), butanol (especially n-butanol, tert-butanol), C5-C 12 alcohols (especially heptanol, such as n-heptanol, octanol, such as n-octanol, iso-octanol, nonanol, such as n-nonanol, decanol, such as n-decanol, dodecanol, such as n-dodecanol), esters (especially DMC), or ethers (especially DME or DIPE). Preferably, the solvent in step (c) is water, methanol, ethanol, iso-propanol or n-butanol, or includes them. Mixtures of any of these and / or the aforementioned solvents are also conceivable.

[0072] Especially when the solvent used is water or includes it, step (c) can be carried out at a temperature of at least 40°C, preferably at least 60°C, 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, more preferably up to 90°C. Thus, step (c) can be carried out at a temperature of 40 - 95°C, preferably 60 - 92.5°C, more preferably 70 - 90°C.

[0073] Alternatively, especially when the solvent used is methanol or contains it, step (c) can be carried out at a temperature of at least 40 °C, preferably at least 50 °C, more preferably at least 55 °C. Step (c) can be carried out at a temperature up to 80 °C, preferably up to 70 °C, more preferably up to 65 °C. Thus, step (c) can be carried out at a temperature of 40 to 80 °C, preferably 50 to 70 °C, more preferably 55 to 65 °C.

[0074] Step (c) can be carried out at atmospheric pressure, i.e., without the application or removal of pressure.

[0075] Step (c) can be carried out for a period of at least 1 minute, preferably at least 5 minutes, more preferably at least 10 minutes. Step (c) can be carried out for a period up to 60 minutes, preferably up to 50 minutes, more preferably up to 40 minutes. Thus, step (c) can be carried out for a period of 1 to 60 minutes, preferably 5 to 50 minutes, more preferably 10 to 40 minutes.

[0076] The dissolution of the precipitate can be carried out with stirring.

[0077] Especially when water alone is used as the solvent in step (c), this can be used in an amount of at least 0.1 times, preferably at least 0.15 times, more preferably at least 0.2 times the amount of PET used in step (a) by weight. Water can be used in step (c) in an amount up to 2.5 times, more preferably up to 1.25 times, more preferably up to 0.5 times the amount of PET used in step (a) by weight. Thus, water can be used in step (c) in an amount of 0.1 to 2.5 times, preferably 0.15 to 1.25 times, most preferably 0.2 to 0.5 times the amount of PET used in step (a) by weight.

[0078] In particular, when methanol alone is used as the solvent in step (c), it can be used in an amount of at least 1 time, preferably at least 1.5 times, 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, more preferably up to 3 times the amount of PET used in step (a) by weight. Therefore, methanol can be used in step (c) in an amount of 1 to 10 times, preferably 1.5 to 5 times, more preferably 2 to 3 times the amount of PET used in step (a) by weight.

[0079] In step (d) of the method, impurities are removed from the solution produced in step (c) to obtain a purified solution containing BHET. Preferably, step (d) includes a step of decolorizing the solution. This can be done by contacting the solution with one or more decolorizing agents. Preferably, step (d) is carried out by filling a column, most preferably one or more decolorizing agents, in the solution produced in step (c) and passing it through a plurality of columns connected in series. For example, each column connected in series may be filled with a different decolorizing agent. Step (d) may also include a step of removing other contaminants, such as metals and catalyst residues from the solution produced in step (c).

[0080] The 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), resins, such as ion exchange resins, preferably cation exchange resins, e.g., preferably acidic cation exchange resins containing sulfonic acid or carboxylic acid groups, with sulfonic acid groups being preferred, or alternatively or further, preferably anion exchange resins containing quaternary ammonium salts, and / or clay (e.g., activated clay, such as bentonite and montmorillonite clay). Preferably, the solution produced in step (c) is contacted with carbon and / or resin, preferably ion exchange resin. Ion exchange resins are particularly suitable for decolorization and removal of metal catalyst residues.

[0081] In a particularly preferred embodiment of the method, the solution produced in step (c) is contacted with a plurality of different decolorizing agents via a path 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, the third column may contain an anion exchange resin, and the first to third columns may be arranged in series such that the solution produced in step (c) passes through in each step (d).

[0082] Step (d) can be carried out at a temperature of at least 40°C, preferably at least 55°C, more preferably at least 70°C. Step (d) can be carried out at a temperature up to 110°C, preferably up to 100°C, more preferably up to 90°C. Accordingly, step (d) can be carried out at a temperature of 40 - 110°C, preferably 55 - 100°C, more preferably 70 - 90°C.

[0083] Step (d) can be carried out under atmospheric pressure, i.e., without the application or removal of pressure.

[0084] Step (d) can be carried out for a period of at least 10 minutes, preferably at least 25 minutes, more preferably at least 40 minutes. Step (d) can be carried out for a period up to 120 minutes, preferably up to 100 minutes, more preferably up to 60 minutes. Accordingly, step (d) can be carried out for a period of 10 - 120 minutes, preferably 25 - 100 minutes, more preferably 40 - 80 minutes.

[0085] Although less preferred, in some embodiments, the purification step (d) can be omitted. This is because, for example, the purification resulting from recrystallization in methanol alone may be sufficient to produce a decolorized and purified product containing BHET, but typically such products are used for low-grade applications, such as carpets. Accordingly, in some embodiments, the purified product containing BHET can be crystallized in step (e) from the solution produced in step (c).

[0086] One advantage of using methanol in step (c) of the method of the present invention is that the solution formed in step (c) can be purified in step (d) and transferred to step (e) for crystallization without filtration. This is because methanol dissolves dimers and trimers of BHET, unlike BHET and water. The transport of dimers and trimers through the PET recycling process is avoided by filtering them out of the aqueous system, but step (a) of the present invention may produce small amounts of dimers and trimers that may be transported with BHET through the recycling process. Thus, in some embodiments, the solid-liquid separation step is not performed between steps (c) and (e) of the present invention.

[0087] 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 preferable to filter the BHET solution between steps (c) and (d).

[0088] In step (e) of the method, the purified product containing BHET crystallizes from the purified solution.

[0089] Step (e) is preferably carried out using cooling crystallization. Suitable crystallization apparatuses include stirred or wall-scraped crystallization apparatuses. The purified solution produced in step (d) can be allowed to cool naturally by standing, but is preferably cooled using a coolant. The coolant can be present in a jacket surrounding the crystallization apparatus or can be passed through a series of heat exchangers, through which the purified solution can also be passed, for example, in countercurrent.

[0090] Particularly when the solvent used in step (c) is water, step (e) can be carried out by reducing the temperature of the purified solution to a temperature of at least 0 °C, preferably at least 10 °C, more preferably at least 20 °C. Step (e) can be carried out by reducing the temperature of the purified solution to a temperature of up to 55 °C, preferably up to 45 °C, more preferably up to 40 °C. Therefore, step (e) can be carried out by reducing the temperature of the purified solution to a temperature in the range of 0 - 55 °C, preferably 10 - 45 °C, more preferably 20 - 40 °C.

[0091] Particularly when the solvent used in step (c) is methanol, step (e) can be carried out by reducing the temperature of the purified solution to a temperature of at least 0 °C, preferably at least 5 °C, more preferably at least 8 °C. Step (e) can be carried out by reducing the temperature of the purified solution to a temperature of up to 30 °C, preferably up to 15 °C, more preferably up to 10 °C. Therefore, step (e) can be carried out by reducing the temperature of the purified solution to a temperature in the range of 0 - 30 °C, preferably 5 - 15 °C, more preferably 8 - 12 °C.

[0092] Step (e) can be carried out at atmospheric pressure, i.e., without the application or removal of pressure.

[0093] Step (e) can be carried out for a period of at least 10 minutes, preferably at least 20 minutes, 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, more preferably up to 35 minutes. Therefore, step (e) can be carried out for a period in the range of 10 - 60 minutes, preferably 20 - 45 minutes, more preferably 25 - 35 minutes.

[0094] The purified solution can be stirred during step (e).

[0095] 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% by weight, preferably at least 99% by weight, more preferably at least 99.5% by weight.

[0096] When methanol is used as the solvent in step (c), the purified product formed in step (e) may also contain, for example, dimers and trimers of BHET in an amount of at least 0.01% by weight. The dimers and trimers of BHET may be present in the purified product in an amount of up to 2% by weight, preferably 0.5% by weight, more preferably 0.2% by weight. 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 the different components in the purified product formed in step (e) can be determined in relation to the precipitate formed in step (b) using the above method.

[0097] An important advantage of the present invention is that it can be used to produce a purified product having a low b[h] value, particularly a b[h] value of 2 or less. PET prepared from BHET having these color densities is of a very high grade and can be used in applications that require an excellent visual appearance, such as transparent and colorless water bottles. Thus, the purified product formed in step (e) may exhibit a b[h] value of up to 2, for example, from 0 to 2. In some examples, the purified product may be used in lower grade applications, such as carpets or films, in which case it may have a b[h] value of up to 4, for example, up to 3.

[0098] The method of the present invention can be used to form a product purified in step (e) having a b[h] value that is 0.5 times, preferably 0.1 times, more preferably 0.05 times that of the PET used in step (a). By using a preferred embodiment of the present invention, for example, when the supply of PET used in step (a) exhibits a high color density, a further stronger decrease in the b[h] value can be achieved.

[0099] 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).

[0100] The purified product containing BHET is preferably isolated after step (e) and, if step (f) exists, before step (f). The precipitate 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 using, for example, low-pressure evaporation and condensation. The protic solvent can be recycled to step (c). Ethylene glycol can be recycled to its use in step (a), more preferably to the first depolymerization reactor.

[0101] One of the main advantages of using methanol instead of water to carry out step (c) is that methanol and ethylene glycol can be easily recovered. Thus, the recovery of methanol and ethylene glycol from the residual liquid can be carried out 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 azeotrope. Thus, when water is used in step (c), the use of a multi-stage evaporator is preferred for the recovery of the residual liquid of water and ethylene glycol.

[0102] When methanol is used in step (c), the recovery from the residual liquid of methanol and ethylene glycol can be carried out 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 liquor can be heated to a temperature of 65 - 120°C, 70 - 100°C, more preferably 70 - 90°C.

[0103] The recovery from the residual liquid of methanol and ethylene glycol can be carried out at ambient pressure, i.e., without the application or removal of pressure.

[0104] Typically, the residual liquid is not 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) before being recycled.

[0105] When water is used in step (c), a two-stage evaporator process is preferred for recovering water and ethylene glycol. In the first evaporator, water can be recovered from the residual liquid by applying a low pressure and enabling evaporation at a low temperature; for example, operation of the evaporator at a pressure of 10 kPa or about 10 kPa is preferred, the associated condenser temperature is 46°C or about 46°C, and the reboiler temperature is 132°C or about 132°C. The residual ethylene glycol can then be recovered in a second evaporator by applying a low pressure, preferably at a pressure of 0.08 bar or about 0.08 bar, and operating at a temperature of 138°C or about 138°C. Those skilled in the art will recognize that other operating temperatures and pressures can also be selected for the first and second evaporators. Improved water recovery can be achieved, if necessary, through operation of the first evaporator at a low temperature or by using a molecular sieve downstream of the first evaporator.

[0106] However, ethylene glycol can be further purified before being recycled to step (a). For example, ethylene glycol can be flashed to separate any organic waste incorporated therein.

[0107] The flash can be carried out at a temperature of at least 130 °C, preferably at least 150 °C, more preferably at least 170 °C. The flash can be carried out at a temperature up to 230 °C, preferably up to 210 °C, more preferably up to 190 °C. Therefore, the flash can be carried out at a temperature of 130 - 230 °C, preferably 150 - 210 °C, more preferably 170 - 190 °C.

[0108] The flash is typically carried out under reduced pressure. For example, the flash can be carried out at a pressure of up to 80,000 Pa, preferably up to 60,000 Pa, more preferably up to 40,000 Pa. The flash can be carried out at a pressure of at least 10,000 Pa, preferably at least 15,000 Pa, more preferably at least 20,000 Pa. Therefore, the flash can be carried out at a pressure of 10,000 - 80,000 Pa, preferably 15,000 - 60,000 Pa, more preferably 20,000 - 40,000 Pa.

[0109] When methanol is used in step (c), the recovery of methanol is very effective (even on an industrial scale, such as those described herein), so if the recovered methanol is recycled to step (c), the non-recycled methanol only needs to be added in step (c) in an amount up to 0.008 times, preferably up to 0.006 times, more preferably up to 0.005 times, by weight, of the amount of PET used in step (a). The non-recycled methanol can be used in step (c) in an amount of at least 0.001 times, preferably at least 0.003 times, more preferably at least 0.004 times, by weight, of the amount of PET used in step (a). Therefore, the non-recycled methanol can be used in step (c) in an amount of 0.001 - 0.008 times, preferably 0.003 - 0.006 times, more preferably 0.004 - 0.005 times, by weight, of the amount of PET used in step (a). Therefore, it will be recognized that the amount of methanol lost during the process of the present invention is extremely low and much less than the amount of water lost when used instead of methanol in step (c).

[0110] However, when water is used as the solvent in step (c), water can also be efficiently recovered, preferably using the two-stage evaporator process described above herein, such that at least the majority of the water used in step (c) is recycled. The lost water is typically removed from the system as humid air. Considering the minimal environmental impact of water lost from the system and the energy cost associated with water recovery compared to methanol-containing waste, it may not be beneficial to maximize the recycling of water.

[0111] The method of the present invention may further include a step (f) of drying the purified product containing BHET. The product can be dried, for example, in a fluidized bed dryer by passing air over the purified product.

[0112] The air can be heated to a temperature of at least 30°C, preferably at least 40°C, more preferably at least 50°C. The air can be heated to a temperature of up to 100°C, preferably up to 90°C, more preferably up to 80°C. Accordingly, the air can be heated to a temperature of 30 - 100°C, preferably 40 - 90°C, more preferably 50 - 80°C.

[0113] The drying step (f) can be carried out at ambient pressure, i.e., without the application or removal of pressure.

[0114] The drying step (f) can be carried out 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 carried out 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 carried out for a period of 10 - 60 minutes, preferably 15 - 50 minutes, more preferably 20 - 40 minutes.

[0115] The method of the present invention can be operated in a batch mode or a continuous mode, but is preferably operated continuously.

[0116] The method of the present invention is preferably carried out on an industrial scale. Accordingly, the method can recycle at least 10 tons / day, preferably at least 100 tons / day, and potentially at least 1,000 tons / day of PET.

[0117] The present invention further provides a purified product containing BHET, which is preferably obtainable, more preferably obtained, using the method described herein.

[0118] The present invention also provides a method for preparing a polymer, the method including the step of performing a polymerization reaction using the purified product of the present invention containing BHET. Preferably, the method includes the step of preparing a purified product containing BHET using the method of the present invention. An important advantage of the present invention is that the purified product containing BHET can be directly used for polymerization, that is, it does not require further purification before use.

[0119] Using the purified product, PET can be prepared, or using the same product, a copolymer containing ethylene terephthalate monomer can be prepared.

[0120] The polymer can be further processed into bottles, packaging, textile products, etc. In some embodiments, the polymer can be further processed into transparent bottles, preferably colorless bottles.

[0121] The present invention relates to an apparatus for recycling PET, (a) A series of depolymerization reactors suitable for depolymerizing PET to form a depolymerized mixture containing BHET, which is adapted to receive PET, ethylene glycol, and a catalyst system; (b) A crystallization unit downstream of the polymerization reactor suitable for crystallizing a precipitate containing BHET from the depolymerized mixture; (c) A container suitable for receiving the precipitate and for dissolving the precipitate in a protic solvent to form a solution containing BHET; (d) An impurity removal unit suitable for receiving the solution containing BHET and for removing impurities from the solution to form a purified solution containing BHET; and (e) The apparatus further includes a further crystallization unit downstream of the impurity removal unit suitable for crystallizing a purified product containing BHET from the purified solution.

[0122] The following non-limiting examples illustrate the present invention. [Examples] [Examples]

[0123] Depolymerization step (a) The depolymerization reactions in different series of reactors were simulated. The ratio of PET:ethylene glycol:catalyst system used in the simulation was 1:4:0.005 by mass. Each reactor was simulated to operate at a temperature of 197 °C and atmospheric pressure. The simulation was set to achieve 99.0% conversion at the outlet of the final reactor in series.

[0124] The results of the simulation are shown in the following table.

[0125] [Table 1]

[0126] To obtain a product level of around 10,000 tons per year, the volume of a single reactor can be about 300 m 3 . When three reactors in series are used, the volume per reactor fits within just over 10 m 3 . A very large reduction in the volume per reactor to a similar approximately 11 - 12 m 3 can be achieved with only two reactors in series, as in the most preferred embodiment of the present invention.

[0127] A graph showing the efficiency of each depolymerization reaction is shown in Figure 1, taking into account the above data and the energy and equipment inputs required for each configuration.

[0128] It can be seen that when at least two depolymerization reactors in series are used, a dramatic improvement in efficiency is observed compared to the use of a single depolymerization reactor. [Examples]

[0129] Solvents preferred for use in step (c) The recrystallization experiment of BHET was carried out in a variety of solvents including methanol, ethanol, isopropanol, butanol and alcohols with long carbon chains.

[0130] Specifically, 50 g of crude BHET was dissolved in 250 ml of solvent at 80 °C for 1 hour. BHET was recrystallized by cooling at a rate of 7 °C / hour until the temperature reached 10 °C. The recrystallized BHET was analyzed to determine its color density. The weight loss during the recrystallization process was also measured.

[0131] The results are shown in the following table.

[0132]

Table 2

[0133] It can be seen that the level of decolorization was improved by each of the light solvents. However, the amount of material lost during recrystallization was significantly less with methanol than in the experiments with any other light solvent. Methanol, as well as higher alcohols, are feasible for use on an industrial scale.

Example

[0134] Decolorization step (d) Several different techniques were used for the decolorization of the BHET aqueous solution.

[0135] Promising results were obtained from the experiments using resins.

[0136]

Table 3

[0137] It can be seen that the most promising results are obtained with cation exchange resins and especially strongly acidic cation exchange resins.

[0138] Activated carbon was also highly effective in the decolorization of BHET.

[0139]

Table 4

[0140] Images of the untreated and treated samples, as well as the images of the PET prepared using the samples, are shown in Figure 2. With both the cation exchange resin and activated carbon, the level of decolorization is good, while with the carbon-treated product, the quality of the polymer product is better.

[0141] Further decolorization experiments were carried out. At this time, a solution of BHET in methanol was used. The experiments gave results similar to those carried out with the BHET aqueous solution, but particularly good results were obtained with the cation exchange resin.

Example

[0142] Recycling process using methanol in step (c) The process of the present invention was carried out in the apparatus depicted in Figure 3. The typical waste used in this process is shown in Figure 4. The waste consists of blue and green used PET flakes.

[0143] Specifically, PET (2), zinc acetate and urea catalyst system (4), and ethylene glycol (6) were transferred to the first of three depolymerization reactors (10) in series. The sample taken out after the three depolymerization reactors (10) in series showed 100% conversion of PET (2), and the selectivity for BHET was 99.8%.

[0144] The depolymerized mixture was passed through a filter (20) to remove insoluble substances (32), and then transferred to a crystallization device (12), where a precipitate containing BHET was formed. The precipitate passed through the filter (20) and was transferred to one of two stirring vessels (14).

[0145] Methanol (8) was added to the vessel (14) to dissolve the precipitate, thereby forming a solution containing BHET.

[0146] The solution was transferred to another crystallization apparatus (18) in which a purified product containing BHET was formed through a decolorization stage (16) depicted as two parallel units in the image.

[0147] The purified product was passed through another filter (20) and transferred to a drying unit (26), and the residual liquor was transferred to a methanol and ethylene glycol recovery unit (22). Methanol was recycled from the recovery unit (22) to the stirring vessel (14), while ethylene glycol was passed through a flash unit (24) where organic waste (34) was removed and then recycled to the series of depolymerization reactors (10).

[0148] The purified product was dried by passing hot air (28) through a dryer (26). The hot air (28) was removed from the system via a condenser where all wastewater (36) was removed, and a flash unit where methanol was recovered and recycled to the stirring vessel (14). After drying, the purified product (30) was removed from the system.

[0149] The purified product (30) had a low color concentration and was used for the preparation of recycled PET without further treatment for use in water bottles.

Example

[0150] Recycling process using water in step (c) The process of the present invention was carried out in the apparatus depicted in FIG. 5.

[0151] Specifically, PET (102), a zinc acetate and urea catalyst system (104), and ethylene glycol (106) were transferred to the first reactor of two series-connected depolymerization reactors (100). Samples taken after the two series-connected depolymerization reactors (100) showed 100% conversion of PET (102), and the selectivity was 95.0% for BHET; the remaining 5.0% was other products consisting essentially of BHET oligomers.

[0152] Excess water (140) was removed by an evaporator (138), and the depolymerized mixture was then passed through a filter (120a) to remove insoluble substances (132), and then transferred to a crystallization device (112), where a precipitate containing BHET was formed. The precipitate was transferred to a stirring vessel (114) through a filter (120b).

[0153] Water (108) was added to the vessel (114) to dissolve the precipitate, thereby forming a solution containing BHET.

[0154] The solution was passed through a decolorization stage (116). As depicted, the decolorization stage includes a filter (120c), followed by a first unit (142) containing an activated carbon bed, followed by a second unit (144) in series containing a cation exchange bed, followed by a third unit (146) containing an anion exchange bed. Following the decolorization stage (116), the solution was transferred to another crystallization device (118) in two stages, where a purified product containing BHET was formed.

[0155] The purified product was transferred to a drying unit (126) through another filter (120d), and the residual liquor was transferred to an evaporator (122). While water was recycled from the evaporator (122) to the stirring vessel (114), ethylene glycol was further transferred towards an evaporator (124), where organic waste (134) was removed and then recycled to the series of depolymerization reactors (100).

[0156] 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.

[0157] The purified product (130) had a low color concentration and was used in the preparation of recycled PET without further treatment for use in water bottles.

Claims

1. A method for recycling polyethylene terephthalate (PET), comprising: (a) depolymerizing PET in a series of depolymerization reactors in the presence of ethylene glycol and a catalyst system to form a depolymerized mixture containing bis(2-hydroxyethyl) terephthalate (BHET); (b) crystallizing a precipitate containing BHET from the depolymerized mixture; (c) dissolving the precipitate in a protic solvent selected from water, methanol, ethanol, isopropanol, and n-butanol 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. The catalyst system includes a carrier or a nitrogen-containing carrier, or the catalyst system includes zinc acetate and urea, or the method having the formula [4NH ２ CONH ２ ·ZnOAc].

2. The PET is waste PET obtained from waste PET bottles, and the PET is used in the form of particles: At least 80% by weight of the particles pass through a mesh having an opening of 20 mm, 15 mm, or 12 mm in diameter; 100% by weight of the particles pass through a mesh having an opening of 25 mm, 20 mm, or 12 mm in diameter; and / or At most 1% by weight of the particles pass through a mesh having an opening of 0.1 mm, 0.5 mm, or 1 mm in diameter. The method according to claim 1.

3. The PET has a b[h] value greater than 5. The method according to claim 1 or 2.

4. The PET is depolymerized in two series-connected depolymerization reactors. The method according to any one of claims 1 to 3.

5. Each of the depolymerization reactors used in step (a) operates at a temperature of 150 to 230 °C; atmospheric pressure; a period of 20 minutes to 4 hours; and / or with stirring. The method according to any one of claims 1 to 4.

6. Ethylene glycol is used in step (a) in an amount 2 to 6 times the amount of PET by weight. The method according to any one of claims 1 to 5.

7. The catalyst system includes a transition metal catalyst, a zinc-containing catalyst, or a zinc acetate catalyst. The method according to any one of claims 1 to 6.

8. The catalyst system is used in step (a) in an amount 0.001 to 0.5 times the amount of PET by weight. The method according to any one of claims 1 to 7.

9. Step (b) is carried out using cooling crystallization, or under atmospheric pressure; a period of 10 to 60 minutes; and / or with stirring. The method according to any one of claims 1 to 8, which is carried out by lowering the temperature of the depolymerized mixture to a temperature of 5 to 50 °C.

10. The method according to any one of claims 1 to 9, wherein the protic solvent comprises one or more of water, methanol, ethanol, iso-propanol and n-butanol.

11. The method according to claim 10, wherein the protic solvent is water.

12. Step (c) is carried out at a temperature of 40 to 95 °C; under atmospheric pressure; for a period of 1 to 60 minutes; and / or under stirring, the method according to claim 11.

13. The method according to claim 11 or 12, wherein the water is used in step (c) in an amount of 0.1 to 2.5 times the amount of PET used in step (a) by weight.

14. Step (d) comprises decolorizing the solution by contacting the solution with carbon containing activated carbon, a resin, an ion exchange resin containing a cation exchange resin, and / or a clay containing activated clay, or by contacting the solution with carbon and an ion exchange resin, the method according to any one of claims 1 to 13.

15. Step (e) is carried out using cooling crystallization, or at atmospheric pressure; for a period of 10 to 60 minutes; and / or under stirring, by lowering the temperature of the purified solution to a temperature of 0 to 55 °C, the method according to any one of claims 1 to 14.

16. The method according to any one of claims 1 to 15, wherein the depolymerized mixture is passed through a filter between steps (a) and (b) to remove insoluble components.

17. The method according to any one of claims 1 to 16, wherein the precipitate containing BHET is isolated by filtration between steps (b) and (c), and the filtrate is recycled to the first depolymerization reactor in step (a).

18. The protic solvent is methanol, and the method comprises, after step (e), isolating the purified product containing BHET by filtration, or treating the filtrate to recover methanol and ethylene glycol, recycling the methanol to step (c), and / or recycling the ethylene glycol to the depolymerization reactor in step (a), the method according to any one of claims 1 to 17, except when dependent on claim 12.

19. The method according to claim 18, wherein the recovery of methanol and ethylene glycol is carried out in a single-stage evaporator.

20. (f) A step of drying the purified product containing BHET The method according to any one of claims 1 to 19, further comprising.

21. The method according to any one of claims 1 to 20, wherein the purified product containing BHET has a b / [h] value of up to 2.

22. The purified product contains BHET in an amount of at least 95% by weight; and contains the dimer and trimer of BHET in an amount of up to at least 0.01% by weight, The method according to any one of claims 1 to 21.

23. A method for preparing a polymer, comprising the step of carrying out a polymerization reaction using a purified product containing bis(2-hydroxyethyl) terephthalate (BHET), and preparing the purified product using the method according to any one of claims 1 to 22. The said method.

24. An apparatus for recycling polyethylene terephthalate (PET), (a) A series of depolymerization reactors suitable for depolymerizing PET to form a depolymerized mixture containing bis(2-hydroxyethyl) terephthalate (BHET), which is adapted to receive PET, ethylene glycol and a catalyst system; (b) A crystallization unit downstream of the depolymerization reactor suitable for crystallizing a precipitate containing BHET from the depolymerized mixture; (c) A container suitable for receiving the precipitate and dissolving the precipitate in a protic solvent selected from water, methanol, ethanol, isopropanol and n-butanol to form a solution containing BHET; (d) An impurity removal unit for receiving the solution containing BHET and removing impurities from the solution to form a purified solution; and (e) further comprising a crystallization unit downstream of the impurity removal unit suitable for crystallizing a purified product containing BHE T from the purified solution, wherein the catalyst system comprises a carrier, or a nitrogen-containing carrier, or the catalyst system comprises zinc acetate and urea, or has the formula [4NH ２ CONH ２ ·ZnOAc], The said apparatus.

Images