Method and reactor system for depolymerizing terephthalate polymers into reusable raw materials

Abstract

A method for depolymerizing a terephthalate polymer into a reusable raw material, comprising: depolymerizing a terephthalate polymer into a depolymerized mixture comprising at least one monomer and at least one dimer by using ethylene glycol, the at least one monomer comprising BHET, and the at least one dimer comprising BHET-dimer; removing solid compounds from the depolymerized mixture in a first separator to obtain a composition comprising at least one dimer and at least one monomer, the at least one dimer and at least one monomer as solutes in a mixture of ethylene glycol and water; crystallizing the BHET-dimer from a solution to obtain a mixture of BHET-dimer crystals and a first mother liquor; separating the crystallized BHET-dimer from the first mother liquor, thereafter: forming BHET crystals from the first mother liquor to obtain a mixture of BHET crystals and a second mother liquor; and recovering the BHET crystals.

Description

Technical Field

[0001] The present invention relates to a process for depolymerizing terephthalate polymers into reusable raw materials. The present invention also relates to a reactor system for depolymerizing terephthalate polymers into reusable raw materials. Background Art

[0002] Terephthalate polymers are a group of polyesters containing terephthalate in the main chain. The most common example of terephthalate polymers is polyethylene terephthalate, also known as PET. Alternative examples include polybutylene terephthalate, polytrimethylene terephthalate, polyethylene isophthalate, polypentaerythritol terephthalate and copolymers thereof, such as copolymers of ethylene terephthalate and polyglycols, such as polyethylene oxide and poly(butylene glycol), polyethylene oxide / polybutylene terephthalate copolymers. PET is one of the most common plastics, and it is very desirable to recycle PET by depolymerizing it into reusable raw materials. A preferred depolymerization mode is glycolysis, which is preferably catalyzed. Typically, due to the use of ethylene glycol, a reaction mixture comprising at least one monomer can be formed, the monomer comprising bis(2-hydroxyethyl) terephthalate (BHET). An example of depolymerization is known from WO2016 / 105200 under the name of the applicant. According to the method, the terephthalate polymer is depolymerized by glycolysis in the presence of a catalyst. At the end of the depolymerization process, water is added and phase separation occurs. This enables the separation of a first phase comprising BHET monomers from a second phase comprising catalyst, oligomers and additives. The first phase may include impurities in dissolved form and as dispersed particles. The BHET monomer may be obtained by crystallization.

[0003] Reuse of the depolymerized raw material requires high purity. It is well known that any contaminants may have an impact on the subsequent polymerization reaction from the raw material. In addition, since terephthalate polymers are used in food and medical applications, strict regulations are required to prevent health problems.

[0004] Although the applicant's method leads to a very high conversion rate of terephthalate polymers and also facilitates the separation of various additives from the monomer, it was observed in further studies that the crystallized BHET monomer was not colorless but rather pale yellow or reddish. This monomer could not be reused as a monomer. Summary of the invention

[0005] Therefore, there remains a need for a method of depolymerizing terephthalate polymers into high purity reusable raw materials suitable for preparing fresh terephthalate polymers.

[0006] There is also a need for a reactor system in which such a process can be practiced.

[0007] According to a first aspect of the present invention, there is provided a method for depolymerizing terephthalate polymers into reusable raw materials, the method comprising the following steps:

[0008] - depolymerizing a terephthalate polymer by using ethylene glycol into a depolymerized mixture comprising at least one monomer and at least one dimer, wherein the at least one monomer comprises bis(2-hydroxyethyl)terephthalate (BHET) and the at least one dimer comprises bis(2-hydroxyethyl)terephthalate dimer (BHET-dimer);

[0009] - removing solid compounds from the depolymerized mixture to obtain a composition comprising at least one dimer and at least one monomer as solutes in a mixture of ethylene glycol and water;

[0010] - crystallizing the BHET dimer from the solution, thereby obtaining a mixture of BHET dimer crystals and a first mother liquor;

[0011] - separating the crystallized BHET dimer from the first mother liquor, after which:

[0012] - forming BHET crystals from the first mother liquor, thereby obtaining a mixture of BHET crystals and a second mother liquor; and

[0013] - Recover BHET crystals.

[0014] According to a second aspect of the present invention, there is provided a reactor system for carrying out the method of the present invention, which will be discussed in more detail below.

[0015] According to a third aspect, the present invention relates to a solid composition which is a polymerizable raw material obtained from depolymerization and which comprises at least 70% by weight, preferably at least 80% by weight, of BHET dimer in crystalline form.

[0016] In the research leading to the present invention, the inventors have understood that the contamination of crystallized BHET is due to the simultaneous crystallization of dimers and the inclusion of pollutants in the solution that crystallizes BHET. The results show that at a given dimer concentration in the solution, the dimer has a higher crystallization temperature, so that under the optimal crystallization conditions of the BHET monomer, the dimer is violently crystallized under the effect of forming inclusions with any other pollutants (such as, for example, iron ions or iron oxide). By performing a dimer crystallization step before BHET crystallization, the dimer concentration in the first mother liquor is significantly reduced, and any violent dimer crystallization (even precipitation) during BHET crystallization can be prevented. Therefore, a crystallized BHET monomer product that meets the purity requirements of subsequent polymerization is obtained.

[0017] To the surprise of the inventor, dimer crystallization and BHET monomer crystallization can be separated in time, so that dimer can be effectively removed before the crystallization of BHET monomer. However, it is observed that in one embodiment, some crystallization may occur in BHET monomer together with dimer. This co-crystallization is not necessarily problematic, because dimer can return to the depolymerization stage to further depolymerize into monomer, or dimer can be sent to a further depolymerization stage to convert dimer and any optional oligomer into monomer. In the embodiment in which dimer is further processed into monomer, it is considered that preferably after removing the first mother liquor, the crystallized dimer is dissolved or dispersed in a solvent such as ethylene glycol. However, this is not considered to be necessary. In one embodiment, the separation of crystallized BHET-dimer is carried out by a filtration unit, and the crystallized BHET-dimer separated by further processing includes removing the crystallized BHET-dimer separated by flushing flow (it includes ethylene glycol) from a filtration unit.

[0018] In addition to solving the described problem of coloration of the resulting monomer by selective crystallization of the dimer, it has been found that the filtration time for filtering the BHET monomer is significantly reduced by reducing the dimer content in the BHET crystalline material. As the dimer content is reduced from 5% to 3.5%, the filtration time is reduced by 60%, and as the dimer content is reduced from 5.0% to 1.5%, the filtration time is reduced by nearly 75%. In addition, it is found that the reduced dimer content reduces the amount of liquid remaining in the residue, and therefore as part of the filter cake. This reduction in liquid in the filter cake is associated with a reduction in the time and effort required for drying. Therefore, in another aspect, the present invention relates to a method for purifying a crude composition containing bis(2-hydroxyethyl)terephthalate (BHET), the method comprising the following steps: (1) providing a crude composition containing BHET and bis(2-hydroxyethyl)terephthalate dimer (BHET-dimer), BHET and bis(2-hydroxyethyl)terephthalate dimer as solutes in a mixture of ethylene glycol and water; (2) pretreating the crude composition to obtain a first mother liquor having a BHET-dimer content of up to 3 weight % relative to the BHET content; (3) forming BHET crystals from the first mother liquor to obtain a mixture of BHET crystals and a second mother liquor, and; (4) separating the BHET crystals from the second mother liquor, preferably by filtration.

[0019] For the sake of clarity, it is observed that in an advantageous embodiment the pretreatment is the crystallization of the BHET-dimer. However, another pretreatment is not excluded to reach conditions suitable for the crystallization of BHET and the separation of the formed crystals from the second mother liquor. For example, the content of the crude composition can be changed by diluting with a solvent and / or the crude composition can be mixed with a stream comprising BHET and a lower amount of BHET-dimer to reach a specified BHET-content.

[0020] Furthermore, it was observed that filtering the BHET crystals from the second mother liquor yielded beneficial results. However, the use of alternative separation techniques (such as centrifugation) is not excluded. Also in centrifugation, this effect can be expected, since wet BHET crystals (referred to as cakes) have a tendency to clog the fluid outlet in the centrifuge.

[0021] It is observed that the crude composition comprising BHET is more preferably equal to the composition comprising at least BHET and BHET-dimer as solutes. However, this is not necessarily the case, the crude composition can be obtained from different processes, while it is preferably obtained from a depolymerization process. For example, it is not excluded that the crude composition contains solids, which are preferably removed in a pretreatment step, more preferably before the BHET-dimer content is reduced (e.g. by crystallization of BHET-dimer).

[0022] For the sake of completeness, it is further observed that in a preferred embodiment, the purification method as defined above can be carried out in a reactor system comprising a dimer crystallization unit for crystallizing the dimer from the crude product, wherein the remaining composition constitutes a first mother liquor, wherein a separator is arranged to separate the crystallized dimer from the first mother liquor; a monomer crystallization unit arranged to crystallize the monomer from the first mother liquor; and a monomer crystallization recovery section arranged downstream of the monomer crystallization unit. Further embodiments of the reactor system will be apparent to those skilled in the art. For example, it is preferred that the reactor system comprises a controller for controlling the process steps performed, including the setting of temperature, residence time, flow rate, etc., as is common in the field of chemical engineering.

[0023] In an alternative and very interesting embodiment, the dimer crystallized is recovered, in particular in solid form, and further processed to be used as a raw material for fresh polymerization. The resulting polymerizable raw material contains at least 70% by weight of dimer, even without any further optimization. This optimization of crystallization, as well as the variation of the dimer content to be crystallized in the composition, will produce preferably at least 80% by weight, and most preferably significantly higher dimer concentrations, such as at least 90% by weight, at least 92% by weight, at least 94% by weight or even at least 95% by weight of dimer. Depending on the customer's requirements, the desired purity can be obtained. An advantage of recovering the BHET-dimer as a product is that a higher ratio between terephthalate polymer and solvent (ethylene glycol) can be used in the depolymerization stage.

[0024] In another embodiment of the present invention, solid composition also comprises both monomer and trimer of BHET.Preferably, the amount of monomer and trimer is equal based on mole % within the tolerance range, and this tolerance range is for example within 20% or even within 10%, and the percentage is based on the amount of monomer.With respect to dimer, the content of monomer and trimer can be optimized and can be less than 5 mole %, between 5 and 10 mole % or even exceed 10 mole %, and the percentage is based on the molar amount of dimer.It is understood that any precipitation-particularly crystallization-of monomer and trimer is harmless to solid composition, as long as their amount (in mole) is roughly equal.In this case, the total mean value corresponds to the dimer of BHET.Therefore, although the exact amount of monomer, trimer and dimer is different, it is feasible to supply a predetermined amount of solid composition for subsequent polymerization.

[0025] The subsequent polymerization can be carried out on the basis of the dimer alone or in combination with a monomer, such as the BHET obtained from the depolymerization. The advantageous ratio between monomer and dimer in the subsequent polymerization can be determined individually by a person skilled in the art of terephthalate polymerization. The need for less ethylene glycol is seen as an advantage of using BHET-dimer in the polymerization. Thus, less ethylene glycol is to be removed at the end of the polymerization than when starting with BHET-monomer. The amount of ethylene glycol used can be reduced by up to one third, or even by up to 50%.

[0026] A particular advantage of the polymerization based on the combination of monomers and dimers of BHET is that this combination has a lower melting point than pure BHET. The reduction in melting point may be up to 30 to 40°C. Therefore, the polymerization can be carried out at a lower temperature. This is obviously efficient, thereby reducing energy consumption. In addition, the lower temperature may result in a lower reaction rate, which enables better control of the polymerization.

[0027] Although BHET-dimer is preferred, preferably in combination with BHET for the preparation of PET, its use in the preparation of other reactions, including the preparation of polyurethanes, is not excluded.

[0028] Preferably, therefore, after dimer crystallization, the dimer concentration in the first mother liquor is at most 3 wt %, and more preferably at most 2.5 wt % or even at most 2 wt % relative to the monomer concentration in the first mother liquor. These dimer concentrations in the first mother liquor have been found to be acceptable, thereby achieving BHET crystallization without forming inclusions. In an embodiment, at the beginning of the step of forming BHET crystals, the concentration of bis(2-hydroxyethyl)terephthalate in the first mother liquor is controlled in the range of 1 wt % to 30 wt % of the mother liquor, preferably in the range of 5 wt % to 25 wt % of the mother liquor. In another embodiment, the BHET concentration is in the range of 10 to 20 wt %.

[0029] Preferably, the crystallized BHET monomer comprises an iron impurity in a concentration of at most 30 ppm, and more preferably at most 25 ppm or even at most 20 ppm. In addition, the dimer concentration in the crystallized BHET monomer is preferably at most 3.0 wt.-%, more preferably at most 2.5 wt.-% or even at most 2.0 wt.-%. A dimer concentration of 1.5 wt.-% has been obtained in experiments. The BHET monomer obtained according to the invention is crystallized in the shape of needles.

[0030] In an embodiment, the crystallization of BHET-dimer from the first mother liquor is carried out without substantially forming BHET crystals. The term "substantially without forming BHET crystals" herein means that as part of dimer crystals, the amount of pure BHET crystals is less than 20% by weight, preferably less than 15% by weight, or even less than 10% by weight. More preferably, the amount of BHET crystals in the dimer product is less than 5% of the BHET in the initial composition, more preferably less than 3% or even less than 2%. It can crystallize on its own or on the dimer core to form mixed crystals.

[0031] In an embodiment, relative to the initial total weight of BHET-dimer in the solution, crystallizing the BHET-dimer comprises crystallizing at least 60 wt%, preferably at least 70 wt% of the BHET-dimer. In experiments, at least 80 wt% of the BHET dimer in the initial composition was crystallized, such as in the range of 83 to 86 wt%.

[0032] The process conditions can be further controlled to achieve the occurrence of dimer crystallization different from BHET crystallization. It has been found that it is feasible to grow dimers to a size in the micrometer range so that they can be separated by filtering with a filter, the filter opening being in the range of 5 to 20 μm, for example 10 to 15 μm. If the dimer is transported for further depolymerization, it seems preferred to keep the size relatively small. If the dimer is recovered, a larger size is considered to be preferred. In the latter case, a second filter is preferably used for the washing step. In particular, but not exclusively, in the embodiment where the dimer is transported back for further depolymerization, it is considered to be a favorable option to use a centrifuge instead of a filter. The phase left in the centrifuge will be a slurry rather than a solid matter as in the filtration, which is more easily transported and recycled.

[0033] In a first embodiment, the BHET-dimer crystallization step is carried out at a higher temperature than the step of forming BHET crystals. It has been found that it is a reliable and practical embodiment for a dimer concentration of 5 to 15% by weight relative to the BHET concentration in the solution. However, it is also applicable to dimer concentrations exceeding the 15% by weight, such as up to 25% by weight or up to 30% by weight or even higher. In an advantageous embodiment, the crystallization temperature T2 of BHET crystallization is in the range of 70°C to 30°C, preferably in the range of 60°C to 40°C. Preferably, the crystallization temperature T1 of dimer crystallization is at least 5°C higher than the BHET crystallization temperature T2, preferably at least 10°C, more preferably at least 15°C. The crystallization temperature is defined herein as the temperature defined at the beginning of the crystallization step, and therefore nucleation usually occurs at this temperature. It is not excluded that the temperature changes or is actively modified during crystallization.

[0034] Preferably, the process conditions during the dimer crystallization are controlled. Possible control parameters include the concentration of bis(2-hydroxyethyl)terephthalate in the composition at the beginning of the step of forming BHET crystals; and the volume ratio between water and ethylene glycol in the composition during the step of forming BHET crystals; and the duration of crystallization, in particular by controlling the temperature within a predetermined range and for a predetermined residence time, such as 2 minutes to 120 minutes, preferably within the range of 5 minutes to 60 minutes.

[0035] In a second embodiment, an antisolvent is added to the first mother liquor before forming BHET crystals. The antisolvent is preferably water or an aqueous solution, such as a saline solution. In this second embodiment, the temperature of BHET crystallization does not have to be lower than the temperature of dimer crystallization. More specifically, adding an antisolvent reduces the solubility of BHET. In a further variant, an antisolvent is added to the first mother liquor and the temperature of BHET crystallization is controlled to be lower than the temperature of the dimer variant.

[0036] More generally, the process conditions may be controlled to control the solution prior to the crystallization step and / or the first mother liquor with respect to the concentration of the compound to be crystallized, the volume ratio between water and ethylene glycol and the temperature control over a predetermined time.

[0037] According to the present invention, the formation of crystals (dimer and BHET) precedes the separation step of removing the corresponding mother liquor. The formation of crystals preferably occurs in a crystallization reactor (such as a container). The separation step is carried out by filtration, centrifugation, cyclone separator or another type of separator known in the art.

[0038] In a first embodiment, the separation of the crystallized dimer and the first mother liquor occurs in the first separator. In other words, the composition comprising dimer and monomer as solute is recycled to the first separator by a feedback loop. The advantage of doing so is that a separate separator is not required for dimer crystallization. This seems to be practical, especially when the crystallized dimer is undesirable as a product anyway and is further depolymerized into monomers. The dimer crystallization unit can be arranged only downstream of the first separator, or arranged in the feedback loop. In another embodiment, a switch can be foreseen to select between the feedback loop and the forward path toward the monomer crystallization unit. As known to those skilled in the art of reactor engineering, such a switch can be implemented as one or more valves. The switch is usually (although not strictly necessary) implemented as under the control of a control unit. When the depolymerization section is implemented as a batch reactor, such an embodiment is considered to be particularly advantageous. In another further embodiment, a recirculation ratio is set between the flow rate entering the feedback loop and the flow rate flowing to the monomer crystallization unit. Thus, the dimer concentration in the first mother liquor can be set and controlled. Preferably, the recycle ratio is controlled by a control unit and can be modified when necessary.This embodiment is considered applicable to both batch and continuous operation of the depolymerization.

[0039] In a second embodiment, the separation of the crystallized dimer and the first mother liquor occurs in a second separator, which is arranged downstream of the dimer crystallization unit and upstream of the monomer crystallization unit. By using the second separator, the crystallized dimer can be separated and, if desired, recovered in solid form with a higher purity than when the first separator is used for it.

[0040] It is not excluded that the crystallization reactor comprises a separator that is activated, for example, after a predetermined residence time. However, a separate separator is considered to be preferred. In the case where the crystals are to be recovered, a washing step is preferably performed after the separation step. A belt filter is considered to be a practical arrangement for performing the separation step and the subsequent washing step. The characteristic dimensions of the separation device can be selected according to the size of the crystals produced and the desired duration of the separation step. In one embodiment, recovering the BHET crystals comprises separating the BHET crystals from the second mother liquor by filtration using a filter element.

[0041] The monomer is preferably recovered in solid form. It is considered appropriate to carry out a washing step and a drying step after the recovery. Preferably, the BHET monomer crystals are essentially composed of BHET, such as at least 95 wt %, more preferably at least 96 wt % or even at least 97 wt % BHET. More preferably, the BHET monomer crystals contain a dimer of at most 2.0 wt %, a dimer of at most 1.5 wt % or even a dimer of at most 1.0 wt %. It has been found that this significantly reduces the risk of cationic contamination.

[0042] In an embodiment, the depolymerization step comprises controlling the molar ratio between the terephthalate polymer and ethylene glycol to control the molar ratio between BHET-dimer and BHET-monomer in the depolymerized mixture. Since the dimer is crystallized separately, the molar ratio between the dimer and the monomer of BHET can be increased. This corresponds to using less ethylene glycol and therefore a higher ratio between terephthalate polymer and ethylene glycol. The depolymerization step involves glycolysis, wherein the ethylene glycol solvent is also a reactant to obtain BHET and BHET-dimer, rather than obtaining, for example, terephthalic acid produced in the hydrolysis. The depolymerization step is preferably catalyzed by a catalyst. Various catalysts known in the art are used for the depolymerization of PET. One group of catalysts is based on the use of iron and / or iron oxide, which can be Fe 2+ or Fe 3+ . Such catalysts have proven to be effective, but may lead to leakage of iron ions into the solution to be crystallized. Preferably, a heterogeneous catalyst is used which can be at least partially recovered by phase separation. More preferably, the heterogeneous catalyst is a particulate catalyst dispersed in a solvent (ethylene glycol). A most preferred example is a catalyst known from WO2017 / 111602A1, which is included herein by reference. Another example is a catalyst known from WO2018 / 143798A1, which is also included herein by reference.

[0043] In a further embodiment, an adsorption treatment is carried out downstream of the further processing step of obtaining the solution. This adsorption treatment is used to remove any dispersed particles and / or any dissolved organic compounds in the solution. Preferably, according to one embodiment of the present invention, the adsorption treatment is carried out on the first mother liquor, therefore downstream of dimer crystallization and separation. The results show that this prolongs the life of the adsorption filter and increases the total product yield. In fact, it was observed in the study that a large amount of dimers were adsorbed and therefore lost. In addition, the dispersed particles can serve as cores for dimer crystallization, which further promotes the crystallization of dimers. For example, adsorption treatment is carried out using activated carbon as an adsorption medium.

[0044] According to another aspect of the present invention, there is provided a reactor system for depolymerizing terephthalate polymers into reusable raw materials, the reactor system comprising:

[0045] a depolymerization section comprising at least one inlet for a terephthalate polymer stream and an ethylene glycol stream; wherein the depolymerization section is configured to depolymerize the terephthalate polymer into a depolymerized mixture by using ethylene glycol, wherein the depolymerized mixture comprises at least one monomer and at least one dimer, wherein the at least one monomer comprises bis(2-hydroxyethyl)terephthalate (BHET) and the at least one dimer comprises bis(2-hydroxyethyl)terephthalate dimer (BHET-dimer);

[0046] - a first separator provided with an outlet, the first separator being configured to separate the solid compounds in the depolymerized mixture from a composition comprising at least one dimer and at least one monomer as solutes in a mixture of ethylene glycol and water;

[0047] - a dimer crystallization unit for crystallizing a dimer from said composition, wherein the remaining composition constitutes a first mother liquor, wherein a separator is arranged to separate said crystallized dimer from said first mother liquor;

[0048] - a monomer crystallization unit arranged to crystallize monomer from said first mother liquor; and

[0049] - A monomer crystal recovery section arranged downstream of the monomer crystallization unit.

[0050] The reactor system is configured to perform the method of the present invention. Typically, each crystallization unit is implemented as at least one container with an import and an export. Preferably, as known to those skilled in the art, there is a controller for controlling the process conditions in each of the containers, in which sensors can be used. The crystallization unit, separator and recovery section can be configured to be intermittently operated or continuously operated. Alternatively, the system is semi-continuous, wherein the crystallization unit is intermittent, but the logistics from the further processing section and thereafter are continuous. In this embodiment, it is considered to be preferred that a plurality of crystallization units are arranged in parallel to load a crystallization unit while performing a crystallization process in another crystallization unit.

[0051] The advantage of the integrated reactor system is that heat losses are reduced to a minimum, thereby preventing unexpected precipitation. Another advantage is that the second mother liquor remaining after the BHET crystallization can be recycled for the depolymerization stage. Therefore, it is preferred to distill it to reduce the water content in the ethylene glycol. A preferred embodiment thereof is described in the unpublished application NL2022037 in the name of the applicant, which is included herein by reference.

[0052] As described above, the temperature of the dimer crystallization section may be higher than the temperature of the monomer crystallization section. It is considered preferred that the dimer crystallization section includes a temperature sensor for determining the temperature of the solution and the monomer crystallization section includes a temperature sensor for determining the temperature of the first mother liquor. At least one controller unit of the reactor system is configured therein to control the first temperature T1 of the solution and / or the second temperature T2 of the first mother liquor in response to a sensor signal of at least one of the first temperature sensor and the second temperature sensor. Alternatively or additionally, an inlet may be present in the monomer crystallization section and / or the dimer separation section for adding an anti-solvent such as water.

[0053] In one embodiment, water or an aqueous solution is added to the depolymerized mixture from the depolymerization stage before the first separator. The water is more preferably water having a temperature of at least 85°C. The addition is preferably carried out before the added water or aqueous solution is mixed with the depolymerized mixture. The addition may occur in a depolymerization reactor, in a separate mixing container or in a container that is part of the first separator, especially but not exclusively when the first separator is implemented as a centrifuge. The addition of water and / or an aqueous solution results in the precipitation of at least part of the solid compounds, which are then removed in the first separator. Preferably, the addition is controlled to produce a first phase and a second phase, wherein the first phase is a composition comprising monomers and dimers and the second phase comprises solid compounds. The second phase may be a slurry or a suspension, etc. The composition may be a solution, but alternatively may be a dispersion, such as a colloidal solution comprising invisible or almost invisible particulate matter. The particulate matter therein preferably comprises nanoparticles, etc., such as particles having a size less than 0.1, 0.5, 1 or 5 microns as determined by sieving. The size limit may depend on the embodiment of the first separator. It is considered preferred that the concentration of particulate material in the composition is lower, for example up to 5% or even up to 3% by weight.

[0054] As described above, several embodiments are envisioned for separating the crystallized dimer from the first mother liquor. In a first main embodiment, a first separator is used for said separation of the crystallized dimer and the first mother liquor. In a second main embodiment, there is a second separator for said separation. In the first main embodiment, a switch or - a predetermined but modifiable - recycle ratio can be used to achieve that the dimer-rich composition is recycled (before or after dimer crystallization) while the remaining first mother liquor (relatively lacking dimer) is transported to the monomer crystallization unit. In particular, but not exclusively, in the second main embodiment, it is feasible to recover the crystallized dimer in addition to or instead of producing a stream comprising dimers to be further depolymerized.

[0055] In one embodiment, the second separator comprises an inlet for a flushing stream comprising ethylene glycol, wherein the separator for dimers is configured to process the separated BHET-dimer crystals to obtain a stream comprising BHET-dimer and ethylene glycol. The stream is preferably recycled to the depolymerization section. However, it is not excluded to transfer such a stream to a separate reaction vessel, in which the reaction conditions are optimized for the depolymerization of oligomers.

[0056] In a further embodiment, the second separator comprises a filtration unit configured to separate the BHET-dimer crystals from the first mother liquor by filtration, and wherein the dimer separation section is configured to convey a flushing flow through the filtration unit in a flow direction opposite to the flow direction of the first mother liquor of the filtration step.

[0057] In embodiments, the monomer crystal recovery section is further configured to wash the separated BHET crystals via a wash stream and is configured to dry the separated BHET crystals.

[0058] In an embodiment, the monomer crystal recovery section comprises a filtration unit configured to separate the BHET crystals from the second mother liquor by filtration, and wherein the filtration unit is configured to wash the separated BHET crystals inside the filtration unit.

[0059] Advantageously, an adsorption device is present and arranged downstream of the second separator. It will be appreciated that such an arrangement results in a longer life of the adsorption device (such as a column or filter).

[0060] For the sake of clarity, it is observed that any embodiment discussed above and / or below with reference to the drawings or in the context of the embodiments or as defined in the dependent claims with respect to one aspect of the invention is also applicable and deemed to be disclosed with respect to any other aspect of the invention, which aspects are further defined in the submitted claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] The above and other advantages of the features and objects of the present invention will become more apparent and the invention will be better understood from the following detailed description when read in conjunction with the accompanying drawings, in which:

[0062] Figure 1 Schematically illustrates a first embodiment of a reactor system of the present invention, which includes a dimer crystallization stage and a monomer crystallization stage;

[0063] Figure 2 A second embodiment of the reactor system of the present invention is schematically illustrated;

[0064] Figure 3 Figure 2 shows pictures of recovered BHET crystals (after a washing step) with and without Fe 2+ or Fe 3+ element;

[0065] Figure 4 The molecular structure of BHET is shown;

[0066] Figure 5 The molecular structure of BHET-dimer is shown;

[0067] Figure 6 HPLC of BHET-dimer is shown. DETAILED DESCRIPTION

[0068] The accompanying drawings are used to illustrate the currently preferred non-limiting exemplary embodiments of the device of the present invention. These figures are not drawn to scale. The same reference numerals in different figures refer to the same or corresponding elements.

[0069] Figure 1 A first embodiment of a reactor system 10 of the present invention is schematically illustrated. The reactor system 10 shown essentially comprises a depolymerization reactor 1, three units 2, 4 and 7 in which phase separation occurs and four separation devices 3, 5, 6, 8 for solid-liquid separation. Feedback streams X, Y and Z are indicated which recycle catalyst, intermediate products (such as oligomers) and solvent (especially ethylene glycol). It should be understood that Figure 1 The description is highly schematic and does not exclude any variations or modifications.

[0070] The reactor system 10 is provided with an input stream A comprising a polymer material. Preferably, the polymer material has been pre-separated so that at least a large part of it is a terephthalate polymer for depolymerization, more particularly PET. The input stream may be in solid form, such as in the form of flakes. However, it is not excluded that the input stream is in the form of a dispersion or even a solution.

[0071] Input stream A enters the depolymerization reactor 1. Other streams entering the depolymerization reactor include a recycle stream Z of the solvent (such as ethylene glycol), a recycle stream X of oligomers and catalysts. A recycle stream Y of dimers is optional and is therefore represented by a dotted line. Additional fresh ingredients (such as fresh catalyst and fresh solvent) are represented as input stream B. The input streams A, B, X, Y, Z may be arranged as separate inlets or may be combined into one or more inlets. The depolymerization reactor 1 may be of the intermittent or continuous type. Although it is represented as a single reactor, the use of a combination of reactor vessels is not excluded, such as a combination of a tank reactor and a plurality of plug flow reactors as disclosed in WO2016 / 105200A1. As Figure 1 As shown, multiple vessels may also be arranged in parallel within a unit. Although not shown, it is understood that the reactor system 10 is provided with a controller and there may be sensors and valves for setting the flow rate into the reactor and for setting the residence time in the reactor. In addition, reactors 1, 2, 4 and 7 will be provided with heating devices and / or other temperature regulation devices to prevent deviations from predetermined temperatures and other variables.

[0072] After depolymerization, the depolymerized reaction mixture is pumped into a cooling vessel 2, which is provided with an inlet for water C. Water C can alternatively be provided as an aqueous solution. It is not excluded that one or more additional additives are added thereto to promote phase separation. The cooling vessel 2 is used to cool the depolymerized mixture from the depolymerization temperature (usually in the range of 160 to 200° C.) to the processing temperature (e.g., about 100° C.). Water may contribute to the cooling process. Due to the addition of water, a two-phase mixture is produced, the first phase being monomers and dimers as solutes in a mixture of ethylene glycol and water, and the second phase comprising oligomers, catalysts, additives. Thereafter, the two-phase mixture is separated in a first separator 3 (e.g., a centrifuge). Thereafter, the second phase is recycled to the depolymerization reactor as logistics X. Although the separator 3 and the cooling vessel 2 are represented herein as separate units, it is not excluded that the cooling step will occur in the first separator 3. Alternatively, the depolymerization reactor 1 and the cooling vessel 2 can be physically a single unit, especially in the case of using a batch process.

[0073] In the context of the present invention, the first phase leaving the first separator 3 is also referred to as solution S. Solution S may be a colloidal solution or dispersion, rather than a pure solution. The solution is transferred to the dimer crystallization section 4. This is preferably a mixing container provided with a temperature regulating device for bringing the crystallization solution to and maintaining it at a predetermined temperature, for example in the range of 50 to 70°C, such as 57 to 64°C, for example 60°C. The exact temperature will depend on the concentration of the dimer, the ratio between water and ethylene glycol, and the desired residence time. After a sufficient degree of crystallization, the resulting combination of the first mother liquor M1 and dimer crystals II is transferred to the second separator 5. The second separator 5 may use a centrifuge, a cyclone separator, and a filtration unit, and other separators are not excluded. In addition, this embodiment will depend on any desire to collect the dimer crystals II as a separate product or only for recycling to the depolymerization reactor 1. When recycled, this preferably occurs in the form of a dispersion or solution in ethylene glycol. This embodiment may also depend on the amount of dimer crystals recovered. Although BHET monomers are traditionally considered as the starting point for fresh polymerization of terephthalate polymers, the ability to recover the dimer enables its use in polymerization. Thus, it becomes feasible to control the depolymerization to produce significantly more dimer than traditionally expected. For example, traditionally, the concentration of dimer is in the range of 5 to 10 weight % of the amount of BHET. According to one embodiment of the present invention, where the dimer is recovered as a product, the depolymerization can be controlled to achieve a dimer concentration of 15 to 50 weight % relative to the amount of BHET.

[0074] Thereafter, the first mother liquor M1 is conveyed to an adsorption processor 6, which is preferably arranged as a column or filter, such as an activated carbon column. The adsorption processor is used to eliminate any solid particles in the mother liquor. Placing the adsorption processor downstream of the dimer crystallizer prevents the adsorption of dimers thereon. In order to prevent the crystallization of BHET during this adsorption treatment, it is considered beneficial to arrange the adsorption processor in a room maintained at a sufficiently high temperature. It is not excluded that the first mother liquor will be heated and / or the first mother liquor will be diluted with additional ethylene glycol before passing through the adsorption processor to prevent any undesirable BHET crystallization. It is not excluded that the adsorption tower 6 is arranged at different positions within the reactor system 10.

[0075] Downstream thereof, BHET is crystallized in the BHET crystallization section 7 and recovered as a monomer product I in a separator 8. Instead of lowering the temperature relative to the dimer crystallization section 4 or in addition thereto, water may be added to the first mother liquor (as shown by the dashed line C in the figure). This will reduce the solubility of BHET and enable crystallization and higher temperatures. After BHET crystallization, the first mother liquor M1 is converted into a second mother liquor M2, which is recycled after recovering the BHET monomer product I. Recycling appropriately includes processing of the mother liquor M2 to reduce its water content. This occurs in a processing section 9, which preferably includes at least one distillation column. The resulting upgraded ethylene glycol is returned to the depolymerization reactor 1 as logistics Z.

[0076] By the method of the present invention, it has been proved that it is feasible to obtain a white BHET monomer product I that does not contain major pollutants. Figure 2 This is shown in the accompanying drawings and will be discussed in more detail in the following examples. Figure 2 A photograph of four samples including 50 ppm of iron (Fe) ions, which are Fe 2+ or Fe 3+ Iron is a common part of the catalyst. In the presence of the dimer, the BHET crystals turn yellow to reddish. In the absence of the dimer, the BHET crystals are white.

[0077] Figure 2 A second embodiment of the reactor system 11 of the present invention is schematically shown. Figure 1Compared with the first embodiment shown, the reactor system 11 according to the second embodiment does not include the second separator 5. More specifically, the first separator 3 is used to separate the dimer crystals II and the first mother liquor M1. In the illustrated embodiment, there is a control switch 14 in the output stream 13 leaving the first separator 3. The control switch 14 is configured so that the solution S flows into the feedback loop 15 including the dimer crystallization unit. The switch is also configured so that the first mother liquor M1 flows to the adsorption device 6. The control switch 14 is appropriately under the control of the control unit (not shown). The switch 14 can be arranged at the output end of the first separator 3, rather than in the output stream 13 at a position downstream of the first separator 3. The switch 14 can be configured as a pair of valves to selectively open or close the feedback loop 15 or, more specifically, to open or close the path to the monomer crystallization unit. The switch can also be configured as one or more devices for setting the flow rate. Combinations thereof are also feasible. The feedback loop 15 can end in the cooling container 2, rather than in the first separator 3.

[0078] The skilled person can envisage further variations. For example, the recycling of one or more of the streams X, Y, Z including a (further) purification step, a heating or cooling step is feasible. It is not excluded that the streams X, Y and / or Z are combined before entering the depolymerization section, or even that the streams X and Y are combined with the second mother liquor M2 to pass through the process section 9.

[0079] Example

[0080] Example 1

[0081] A first experiment was conducted in which the initial composition was enriched with 50 ppm Fe 2+ or Fe 3+ Contamination. In this first preliminary experiment, a single crystallization was performed to crystallize BHET without any previous separate crystallization of BHET dimer. The crystallization temperature was 50° C., set with a heating bath. After crystallization, the mother liquor was removed by filtration and the crystalline material was dried. Compositions comprising dimer-enriched BHET (8 wt. % dimer relative to BHET) or pure BHET were prepared. These compositions all included 230 g of demineralized water and 230 g of ethylene glycol and 40 g of BHET or dimer-enriched BHET. The results are shown in Table 1.

[0082] Surprisingly, the crystals including BHET-dimer were yellow, whereas pure BHET was white. Thus, the results demonstrate that the presence of BHET dimer leads to an undesired coloration of BHET crystals.

[0083] Table 1 - Results of the first preliminary experiment

[0084]

[0085] Solutions of Embodiment 2 and Further Embodiments

[0086] A composition for crystallization experiments is prepared. The composition includes 40 g of dimer-rich BHET, 230 g of demineralized water and 230 g of ethylene glycol (EG). Dimer-rich BHET is obtained from a depolymerization experiment, in which PET is depolymerized by catalytic glycolysis, followed by the addition of water and centrifugation to remove oligomers and catalysts. The catalyst described in WO2017111602 is used, which is included herein by reference. However, the use of alternative catalysts is not excluded. In order to ensure that dimer-rich BHET is pure enough, it is treated multiple times by activated carbon and deionized resin-this treatment is feasible for experiments on a laboratory scale, but cannot be used for industrial production. Unless otherwise stated, the dimer-rich BHET used in the experiment generally contains about 8% by weight dimer relative to the amount of BHET. It also typically contains small amounts (less than 1% of each contamination) of other depolymerization products from PET, including BHET trimers, possible BHET tetramers (in smaller amounts), isoBHET (i.e., isomers of BHET), and BHEET (monomers of ethylene glycol that include more units than BHET). Iron ions were added as a specific contaminant. The iron dosage used in the experiments was 50 ppm.

[0087] The ingredients of the composition are added to a 500 ml flask provided with a lid. Subsequently, a (magnetic) stirring bean is added to the flask. The flask is placed on a hot plate and heated to about 90°C to dissolve all solid material (i.e., dimer-enriched BHET). The closed flask is then placed in a prepared heating bath at a temperature of 60°C. It remains there for 20 minutes in order to selectively crystallize the dimer.

[0088] The resulting mother liquor with BHET-dimer crystals was separated by a Buchner funnel apparatus using a filter paper with an opening of 12 to 15 μm. The Buchner funnel apparatus was preheated to avoid additional crystallization of BHET. In addition, 100 g of demineralized water were boiled and poured directly through the Buchner funnel apparatus before slowly adding the dimer crystals and the mother liquor. A magnet was maintained against the bottom of the flask to ensure that the stirring bean remained in the flask. The filtered mother liquor was a clear, colorless liquid that began to crystallize over time. The residue in the Buchner funnel was a white filter cake. Thereafter the residue was dried in an oven overnight.

[0089] The filtered mother liquor is then reheated to 80°C to ensure that all crystals are redissolved. This is done to perform laboratory-scale experiments. Thereafter, the mother liquor is crystallized by reducing the temperature in a controlled manner, for example by using a series of heating baths at mutually decreasing temperatures. Thereafter, the second mother liquor containing BHET crystals is filtered on another Buchner funnel apparatus using filter paper with an opening of 12 to 15 μm. The filtration is performed at room temperature. The resulting BHET is a white filter cake. The filter cake is washed with demineralized water. The washed filter cake is dried.

[0090] Example 2

[0091] An experiment was conducted to evaluate dimer crystallization under different conditions. The crystallization temperature (i.e., the temperature of the bath) was 60° C. in all cases. As shown in Table 2, it was found that dimer crystallization resulted in a dimer content of approximately 2.0 wt % (i.e., 1.5 to 2.5 wt %) relative to BHET in the first mother liquor used to crystallize BHET.

[0092] Table 2 – Dimer crystallization

[0093]

[0094] Example 3

[0095] Pretreatment was carried out as described in the above scheme, starting with 50ppm iron contamination. Measurements were taken to determine the amount of crystals and the amount of iron contaminants. Comparative experiments were performed in which the dimer-rich BHET contained only 5% by weight of dimer relative to BHET, but there was no separate dimer crystallization step. In addition, a reference example was included, which contained 0% dimer. Table 3 shows the content of BHET, iron and dimer for the initial composition, the second mother liquor and the obtained BHET. It also shows the amount of BHET obtained. From these data, it can be concluded that dimer crystallization significantly reduces the dimer content in the BHET crystals, and the iron content is reduced in a corresponding manner.

[0096] Table 3 – Dimer and iron content

[0097]

[0098] Example 4

[0099] Figure 6The HPLC graph of the BHET dimer formed according to the above scheme is shown. The identification was performed in duplicate. The peak at 3.75 minutes represents BHET. The peak at 7.62 minutes represents the dimer. The peak at 8.28 minutes represents the BHET trimer. It is assumed that the peak at 8.69 minutes corresponds to the BHET tetramer. The relative contents (weight %) of the different compounds are shown in Table 4:

[0100] Table 4 - Content of BHET, BHET dimer and other oligomers in dimer crystals, all in wt %

[0101] Compound Time (min) First HPLC Second HPLC average value BHET 3.75 11.9 11.8 11.9 BHET-Dimer 7.62 77.0 76.5 76.7 BHET-Trimer 8.28 7.7 11.7 9.7 BHET-tetramer 8.69 3.4 0 1.7

[0102] It is observed that the value of BHET is higher than the value obtained when the calibration standard of BHET is used. Then the average value ends at 10.4% by weight, and the standard deviation is 0.7. It is also observed that these contents are the results of the first experiment that is not optimized. In addition, it is known that the amount of dimer in the composition can be changed by depolymerization conditions (such as the ratio between ethylene glycol and polymer (PET)) relative to the amount of BHET monomer. Therefore, the monomer and dimer content of the obtained dimer crystals are likely to change. Effectively, in the absence of further optimization of crystallization and the composition to be crystallized without significant changes, the content of BHET will be in the range of 8 to 12% by weight (based on calibration), and the content of dimer is in the range of 70 to 85% by weight. Using different settings, the monomer content is likely to vary in the range of 5 to 15% by weight, and the dimer content varies in the range of 67 to 90% by weight. When crystallization is further optimized to obtain pure dimer, for example, by optimizing the crystallization temperature and the changes therein, the dimer content is likely to exceed 90% by weight, for example, 90 to 95% by weight or even higher than 95% by weight, such as up to 97% by weight. As previously mentioned, the dimer content in the composition can be further optimized so that the amount of BHET and BHET trimer is equal within a tolerance range (such as up to 3 wt %). More specifically, the amount is equal relative to the molar amount, i.e., the dimer crystals include, for example, 0.1 to 0.2 moles of BHET and 0.1 to 0.2 moles of trimer per mole of dimer.

[0103] Example 5: Filtration process BHET

[0104] The duration of the filtration process of the second mother liquor including BHET crystals was studied. In each case, the same filter paper with an opening of 12 to 15 μm was used. Comparisons were made between different compositions with different concentrations of dimers in the BHET crystals and the second mother liquor. For all compositions, the iron content in the second mother liquor was approximately the same. Table 5 shows the results.

[0105] Table 5 – Filtration times depending on dimer concentration.

[0106]

[0107] Depending on the dimer concentration, it was found that the filtration time increased significantly. Several conclusions can be drawn from the data in this table. First, the weight of wet BHET crystals (usually in the form of filter cake) depends significantly on the dimer concentration. Here, the filter cake weights of compositions A and B are similar, and the filter cake weights of compositions C and D are similar, almost twice the filter cake weights of compositions A and B. The filter cake weight seems to depend mainly on the dimer concentration in the second mother liquor, while the dimer concentrations of compositions C and D are almost equal. Secondly, the filtration time is highly dependent on the dimer concentration. Here, the dimer concentration in the BHET crystals seems to be very important. The filtration time of composition B is relatively long. But in further experiments, the filtration time of the mother liquor and BHET crystals obtained with the present invention can be further reduced to less than 30 seconds.

Claims

1. A method for depolymerizing terephthalate polymers into reusable raw materials, the method comprising the following steps: a) depolymerizing a terephthalate polymer by using ethylene glycol into a depolymerized mixture comprising at least one monomer and at least one dimer, wherein the at least one monomer comprises bis(2-hydroxyethyl)terephthalate (BHET), and the at least one dimer comprises bis(2-hydroxyethyl)terephthalate dimer (BHET-dimer); b) removing solid compounds from the depolymerized mixture in a first separator to obtain a composition comprising the at least one dimer and the at least one monomer as solutes in a mixture of ethylene glycol and water; c) crystallizing the BHET-dimer from the solution, thereby obtaining a mixture of BHET-dimer crystals and the first mother solution; d) separating the crystallized BHET dimer from the first mother liquor, after which: e) forming BHET crystals from the first mother liquor, thereby obtaining a mixture of BHET crystals and a second mother liquor; and f) recovering the BHET crystals, wherein at least one of the following is applied: - wherein the step of crystallizing the BHET-dimer is performed at a higher temperature than the step of forming the BHET crystals; and - wherein after said step of crystallizing said BHET-dimer and before said step of forming BHET crystals, an anti-solvent is added to said first mother liquor to enhance the formation of BHET crystals from said first mother liquor.

2. The method according to claim 1, wherein the step of crystallizing the BHET-dimer comprises controlling at least one of the following: i. controlling the concentration of BHET-dimer in the solution at the beginning of the step of crystallizing the BHET-dimer; and ii. controlling the volume ratio between water and ethylene glycol in the solution during the step of crystallizing the BHET-dimer; and iii. controlling the solution at the first temperature T1 for a time in the range of 2 minutes to 120 minutes.

3. The method according to claim 1, wherein crystallizing the BHET dimer from the solution is performed without forming BHET crystals.

4. The method of claim 1, wherein the crystallized BHET is recovered in solid form, and wherein the method further comprises at least one of the steps of washing the BHET crystals and drying the BHET crystals.

5. The process according to claim 1, wherein the separated crystallized BHET-dimer is further processed to obtain a stream comprising the BHET-dimer and ethylene glycol.

6. The method according to claim 5, wherein the separation of the crystallized BHET-dimer is performed by a filtration unit, and wherein further processing of the separated crystallized BHET-dimer comprises removing the separated crystallized BHET-dimer from the filtration unit by a flushing stream, the flushing stream comprising ethylene glycol.

7. The method according to claim 5 or 6, wherein the stream is processed to convert the BHET-dimer into BHET.

8. The method of claim 1, wherein the crystallized BHET-dimer is recovered in solid form.

9. The method of claim 1, wherein the depolymerization step comprises controlling the molar ratio between the terephthalate polymer and ethylene glycol to control the molar ratio between BHET-monomer and BHET-dimer in the depolymerized mixture, wherein the molar ratio is controlled to obtain at least 10 wt% dimer.

10. The method of claim 1, wherein the removal of the solid compound is preceded by a precipitation step to precipitate at least a portion of the solid compound, wherein the precipitation step comprises the addition of water or an aqueous solution.

11. The method of claim 1, wherein the separation of the crystallized BHET-dimer occurs in the first separator. 12 . The method according to claim 1 , wherein the separation of the crystallized BHET-dimer occurs in a second separator, which is arranged downstream of the crystallization unit of the BHET-dimer and upstream of the crystallization unit of the monomer.

13. The method according to claim 12, wherein after separating the crystallized BHET-dimer from the first mother liquor and before forming BHET crystals, the first mother liquor is subjected to an adsorption treatment to remove dissolved BHET-dimer from the first mother liquor.

14. The process of claim 1, wherein the depolymerization step is performed over a catalyst comprising elemental Fe, and wherein the depolymerized mixture comprises elemental Fe in a concentration of at least 5 ppm relative to the weight concentration of bis(2-hydroxyethyl)terephthalate (BHET) in the depolymerized mixture.

15. A reactor system for depolymerizing terephthalate polymers into reusable raw materials, the reactor system comprising: - a depolymerization section comprising at least one inlet for a terephthalate-containing polymer stream and an ethylene glycol stream; wherein the depolymerization stage is configured to depolymerize the terephthalate-containing polymer into a depolymerized mixture by using the ethylene glycol, wherein the depolymerized mixture comprises at least one monomer and at least one dimer, wherein the at least one monomer comprises bis(2-hydroxyethyl)terephthalate (BHET), and the at least one dimer comprises bis(2-hydroxyethyl)terephthalate dimer (BHET-dimer); - a first separator provided with an outlet, the first separator being configured to separate the solid compounds in the depolymerized mixture from a composition comprising the at least one dimer and the at least one monomer as solutes in a mixture of ethylene glycol and water; - a dimer crystallization unit for crystallizing the dimer in the composition, wherein the remaining composition constitutes a first mother liquor, wherein the separator is arranged to separate the crystallized dimer from the first mother liquor; - a monomer crystallization unit arranged to crystallize monomer from said first mother liquor; and - a monomer crystal recovery section arranged downstream of the monomer crystallization unit. 16 . The reactor system according to claim 15 , wherein the reactor system comprises at least one controller unit configured to control the temperature T1 in the dimer crystallization unit to be higher than the temperature T2 in the monomer crystallization unit.

17. The reactor system according to claim 15 or 16, wherein there is an inlet for an anti-solvent for the first mother liquor downstream of the dimer crystallization unit to enhance the formation of BHET crystals at the monomer crystallization unit.

18. The reactor system of claim 15, wherein the reactor system comprises at least one controller unit configured to control at least one of: i. a molar ratio between the terephthalate polymer and the ethylene glycol; ii. the concentration of BHET-dimer in the solution stream at the inlet of the dimer crystallization unit; and iii. The volume ratio between water and ethylene glycol in the solution stream at the inlet of the dimer crystallization unit.

19. The reactor system of claim 15, wherein the dimer crystallization unit is arranged in a feedback loop such that the first separator serves as the separator to separate the crystallized dimer from the first mother liquor.

20. The reactor system according to claim 19, wherein a switch is present at the outlet of the first separator, under the control of a control unit, to direct the outlet stream from the first separator to the dimer crystallization unit or the monomer crystallization unit.

21. The reactor system of claim 15, wherein the first separator is a centrifuge.

22. The reactor system according to claim 15, wherein the dimer crystallization unit is arranged downstream of the first separator, and wherein the separator for separating the crystallized dimer from the first mother liquor is implemented as a second separator arranged downstream of the dimer crystallization unit and upstream of the monomer crystallization unit.

23. The reactor system of claim 22, wherein the second separator comprises an inlet for a flushing stream, and wherein the second separator is configured such that the crystalline dimer is dispersed and / or dissolved into the flushing stream, wherein the flushing stream is recycled to the depolymerization section.

24. The reactor system of claim 22, wherein the second separator is configured to recover the crystalline dimer in solid crystalline form.

25. A solid composition, the solid composition being a polymerizable raw material obtained from the process for depolymerizing terephthalate polymers into reusable raw materials according to any one of claims 1 to 14, comprising at least 70% by weight of dimer of bis(2-hydroxyethyl)terephthalate (BHET) in crystalline form.

26. The solid composition of claim 25, further comprising monomers and trimers of bis(2-hydroxyethyl)terephthalate (BHET), wherein the total amount of monomers, dimers and trimers of BHET in the solid composition is at least 90 wt%.

27. The solid composition of claim 26, wherein the amount of monomer and trimer of BHET in the solid composition is each at least 5 wt%, and wherein the difference between the amount of monomer and trimer in the solid composition is at most 5 wt%.

28. A method for purifying a crude composition comprising bis(2-hydroxyethyl)terephthalate (BHET), the method comprising the steps of: a) providing the crude composition comprising BHET and bis(2-hydroxyethyl)terephthalate dimer (BHET-dimer), the BHET and the bis(2-hydroxyethyl)terephthalate dimer as solutes in a mixture of ethylene glycol and water; b) pre-treating the crude composition to obtain a first mother liquor having a BHET dimer content of at most 3 wt. % relative to the BHET content; c) forming BHET crystals from the first mother liquor, thereby obtaining a mixture of BHET crystals and a second mother liquor; and d) separating the BHET crystals from the second mother liquor; The pretreatment of the crude composition comprises at least one of the following: - wherein the step of crystallizing the BHET-dimer is performed at a higher temperature than the step of forming the BHET crystals; and - wherein after said step of crystallizing said BHET-dimer and before said step of forming BHET crystals, an anti-solvent is added to said first mother liquor to enhance the formation of BHET crystals from said first mother liquor.

29. The method of claim 28, further comprising the step of drying the separated BHET crystals.

30. The method according to claim 28 or 29, wherein the method is configured to obtain BHET crystals having at most 3.5 wt.% BHET-dimer, wherein the percentage of BHET-dimer is relative to BHET in the BHET crystals.

31. The method of claim 28, wherein the pretreatment of the crude composition comprises crystallizing BHET-dimer and separating the crystallized BHET-dimer from the remaining first mother liquor.

32. The method of claim 28, wherein the crude composition is derived from depolymerization of PET.

33. The method of claim 28, wherein the concentration of BHET-dimer in the second mother liquor is at most 2.0 wt% relative to BHET in the second mother liquor.

34. The method according to claim 33, wherein the BHET-dimer content in the first mother solution is at most 2.0 wt. % relative to BHET.

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