Method of preparing BIS (2-hydroxyethyl) terephthalate monomer crystals and reactor system for preparing the same

Abstract

Described is a method of preparing bis (2-hydroxyethyl) terephthalate (BHET) monomer crystals comprising the steps of providing a mixture of BHET monomers and oligomers thereof, a carrier liquid, and impurities; providing BHET seed crystals in the mixture; forming BHET monomer crystals from the mixture comprising the BHET seed crystals, thereby obtaining BHET monomer crystals in a mother liquor; and separating the BHET monomer crystals from the mother liquor; wherein the powder X-ray diffraction spectrum of the BHET seed crystals at least has characteristic intensity peaks at 2Ɵ about equal to 10.7°, 14.4°, 18.9° and 21.6°, where Ɵ is the angle of diffraction, and / or the IR spectrum of the BHET seed crystals at least has characteristic absorption peaks at λ about equal to 3260 cm-1 and 897 cm-1, where λ is the ordinary wavenumber. The obtained BHET monomer crystals have superior filtration properties. Also described is a method of preparing the BHET seed crystals, a method of depolymerizing a terephthalate polymer using the BHET seed crystals, as well as a corresponding reactor system for depolymerizing said terephthalate polymer.

Description

[0001] METHOD OF PREPARING BIS (2-HYDROXYETHYL) TEREPHTHALATE MONOMER CRYSTALS AND REACTOR SYSTEM FOR PREPARING THE SAME

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a method of preparing bis (2-hydroxyethyl) terephthalate (BHET) monomer crystals using BHET seed crystals. The invention further relates to a method of preparing BHET seed crystals. The invention moreover relates to a method of depolymerizing a terephthalate polymer into reusable raw material, wherein use is made of the BHET seed crystals. The invention finally relates to a reactor system for depolymerizing a terephthalate polymer into reusable raw material.

[0004] BACKGROUND OF THE INVENTION

[0005] Bis (2-hydroxyethyl) terephthalate (BHET) may be conveniently used as the main raw material for the production of polyethylene terephthalate (PET). Suitable production methods of PET based on BHET may comprise transesterification methods (DMT method) and direct esterification methods (PTA method). The BHET may be synthesized as such or may be obtained by PET depolymerization methods.

[0006] A suitable depolymerization method of this kind is for instance disclosed in WO2017 / 111602A1 in the name of the applicant. Herein, a method of degrading a condensation polymer such as a terephthalate polymer in a depolymerization reaction catalyzed by a heterogeneous depolymerization catalyst is disclosed, in which method a reactive solvent, such as an alkanediol, alkanetriol, glycol, glycerol, or propylene glycol, acts as a reactant in the degradation reaction. The depolymerization reaction results in a mixture comprising monomer, oligomers, reactive solvent and heterogeneous depolymerization catalyst. A polar medium, such as water or an aqueous solution, is added to this mixture to obtain a hydrophilic solution comprising monomer and a second phase comprising oligomers and heterogeneous depolymerization catalyst. The hydrophilic solution is then separated from the second phase using a centrifuge for instance. The hydrophilic solution comprising monomer is subsequently treated with an adsorption medium, such as active carbon, to remove pigments and dyes. The monomer, such as BHET monomer, is finally obtained in a crystallization step. The obtained BHET monomer may be reused to synthesize PET, thereby realizing a green recycling and reuse of PET.

[0007] Although the method disclosed in the prior art may be used to obtain BHET monomer crystals to a satisfactory extent, being able to better control the BHET crystal properties, such as grain size, crystal morphology and particle size distribution remains an important goal, in particular in view of subsequent processing, such as filtering, washing, and drying of the obtained BHET monomer crystals. In this respect, it has further been noted that impurities present during BHET crystallization may significantly influence crystal properties. It would be desirable therefore to provide a method of preparing bis (2-hydroxyethyl) terephthalate (BHET) monomer crystals that is less sensitive to the presence of impurities. The above is even more desirable when the capacity of the preparation method is scaled up to industrial scale, and / or when using a continuous preparation method. Indeed, in the latter impurities may actually accumulate.

[0008] Tu Lee at al., “Chemical recycling development of poly(ethylene terephthalate) by glycolysis and cooling crystallization with water”, Ind. Eng. Chem. Res. 2023, 62, 19873-19883 discloses a study of cooling crystallization of BHET with various cooling rates and volumes of water added to the ethylene glycol (EG) containing depolymerized mixture after glycolysis of transparent PET bottles. A volumetric water / EG ratio of 1:1 produced stable a-form BHET monomer crystals, which were shown to lead to relatively large filtration times. An increase in water volume up to a volumetric water / EG ratio of 4:1 decreased the viscosity of the depolymerized mixture which gave rise to faster nucleation rates of BHET monomer crystals. The increased water volume also had an influence on BHET monomer crystal morphology and favored the production of metastable 5-form BHET monomer crystals. Due to the lower viscosity, shorter filtration times were observed. However, a lower crystal yield was obtained with the 5-form BHET crystals and also poor flowability. Further, one skilled in the art will appreciate that water / EG ratios of 2 and more are less desirable on an industrial scale for efficiency and environmental reasons.

[0009] It is therefore an object of the invention to provide an improved method of preparing bis (2- hydroxyethyl) terephthalate (BHET) monomer crystals, wherein the improvement relates to at least partly overcoming the disadvantages of the prior art method.

[0010] A particular object relates to providing a method of preparing bis (2-hydroxyethyl) terephthalate (BHET) monomer crystals, which method is less sensitive to the presence of impurities during crystallization.

[0011] It is another object of the invention to provide a method of preparing BHET seed crystals to be used in the improved method. Yet another object of the invention relates to providing BHET seed crystals for use in a depolymerisation process of a terephthalate polymer into reusable raw material.

[0012] It is yet another object of the invention to provide a method of depolymerizing a terephthalate polymer into reusable raw material, wherein use is made of the improved BHET monomer crystals.

[0013] Another object of the invention is to provide a corresponding reactor system for depolymerizing a terephthalate polymer into reusable raw material.

[0014] SUMMARY OF THE INVENTION

[0015] It has been understood by the inventors in investigations leading to the present invention that one or more of the objects may be met by preparing bis (2-hydroxyethyl) terephthalate (BHET) monomer crystals from a mixture consisting of BHET monomers and oligomers thereof, a carrier liquid, impurities, and further ingredients, by contacting the mixture with BHET seed crystals with a specific morphology and crystallizing said mixture. It was found that the BHET monomer crystals that are grown on the invented BHET seed crystals show an improved behavior in downstream processing, such as in separating the BHET crystals from the crystallization mother liquor, for instance by filtering.

[0016] It has further been found that crystallization of the BHET monomers in the mixture appears to be less sensitive to the amount of impurities present in the mixture during BHET crystallization. In other words, BHET monomer crystals of the desired morphology may be obtained by seeding the mixture with the invented seed crystals, even at relatively high amounts of impurities.

[0017] The relative insensitivity of the BHET crystallization to impurities makes the invention particularly useful in cases wherein the BHET monomer results from depolymerization of a terephthalate polymer waste stream, in particular PET. It is rather likely indeed that impurities will be present in the depolymerized mixture, due to the origin of the feedstock and also due to side reactions that occur in depolymerization of the terephthalate polymer. The mixture of BHET monomers and oligomers thereof and the carrier liquid may typically comprise impurities including iso-BHET (i.e. BHEI or bis-hydroxyethylene-isophthalate), mono-2-hydroxyethyl terephthalate (MHET) and 2- hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate (BHEET, a monomer including more ethylene glycol units than BHET). Further ingredients may also be present, but these further ingredients are believed to have a lesser influence on the BHET monomer crystallization than the impurities. In the context of the present invention, the “impurities” are defined as the group consisting of iso- BHET (also referred to as BHEI), MHET and BHEET. It is not excluded that in the mixture the group of impurities may be incomplete in that the amount of one or two members is zero. It is further noted that BHET dimers and higher oligomers for instance are not included in the impurities as defined according to the present disclosure.

[0018] In the context of the present invention, the “further ingredients” are defined as the group that comprises ingredients believed to have a lesser influence on the BHET monomer crystallization. The further ingredients may for instance include crystallizing agents other than the BHET seed crystals, colorants, pigments, rheological and other additives commonly present in terephthalate polymers, and other substances that may optionally originate from the depolymerization of PET, such as BHET dimers and higher oligomers for instance. The further ingredients account for at most 10 wt.% of the total weight of the mixture.

[0019] It has been found by the inventors that the specific BHET seed crystals may be obtained by crystallizing the mixture consisting of BHET monomers and oligomers thereof, the carrier liquid, the impurities, and the further ingredients, wherein the concentration of the impurities is below 10 wt.%, relative to the weight of the BHET monomers in the mixture.

[0020] In a first aspect of the invention a method of preparing bis (2 -hydroxyethyl) terephthalate (BHET) monomer crystals is provided, said method comprising the steps of:

[0021] (a) providing a mixture consisting of BHET monomers and oligomers thereof, a carrier liquid, impurities and at most 10 wt.%, relative to the total weight of the mixture, of further ingredients, wherein the carrier liquid consists of at least 80 wt.% of a water / ethylene glycol mixture and at most 20 wt.% of other liquids, wherein the water / ethylene glycol weight ratio is from 20:80 to 60:40;

[0022] (b) providing BHET seed crystals in the mixture;

[0023] (c) forming BHET monomer crystals from the mixture comprising the BHET seed crystals, thereby obtaining BHET monomer crystals in a mother liquor; and

[0024] (d) separating the BHET monomer crystals from the mother liquor; wherein the BHET seed crystals have at least one of a powder X-ray diffraction spectrum with at least one characteristic intensity peak at 20 chosen from about equal to 10.7°, 14.4°, 18.9° and 21.6°, where 0 is the angle of diffraction; an IR spectrum with at least one characteristic absorption peak at I chosen from about equal to 3260 cm1and 897 m1, where I is the ordinary wavenumber.

[0025] According to a second aspect of the invention, there is provided a method of preparing BHET seed crystals having at least one of: a powder X-ray diffraction spectrum with at least one characteristic intensity peak at 20 chosen from about equal to 10.7°, 14.4°, 18.9° and 21.6°, where 0 is the angle of diffraction; an IR spectrum with at least one characteristic absorption peak at I chosen from about equal to 3260 cm1and 897 cm'1, where I is the ordinary wavenumber, the method comprising the steps of: i. providing a mixture consisting of BHET monomers and oligomers thereof, a carrier liquid, impurities and at most 10 wt.%, relative to the total weight of the mixture, of further ingredients, wherein the carrier liquid consists of at least 80 wt.% of a water / ethylene glycol mixture and at most 20 wt.% of other liquids, wherein the water / ethylene glycol weight ratio is from 20:80 to 60:40, wherein further the concentration of impurities is below 10 wt.% relative to the total weight of the BHET monomers in the mixture; ii. forming BHET seed crystals from the mixture thereby obtaining BHET seed crystals and a seeded mother liquor; and iii. separating the BHET seeding crystals from the seeded mother liquor.

[0026] According to a third aspect of the invention, there is provided a method of depolymerising a terephthalate polymer, preferably PET, into reusable raw material, such as BHET monomers, the method comprising the steps of: a) depolymerizing the terephthalate polymer in a reactive solvent comprising at least 80 wt.% ethylene glycol and optionally water, and at most 20 wt.% of other solvents; b) forming BHET monomer crystals from the depolymerized mixture by a method according to the first aspect of the invention thereby obtaining BHET crystals in a mother liquor; c) separating the BHET crystals from the mother liquor.

[0027] Step b) of the method according to the third aspect requires adjusting the mixture obtained from the depolymerization step a) to provide a mixture consisting of BHET monomers and oligomers thereof, the carrier liquid, the impurities and at most 10 wt.%, relative to the total weight of the mixture, of the further ingredients, as required by the first aspect of the invention. Moreover, the carrier liquid should consist of at least 80 wt.% of a water / ethylene glycol mixture and at most 20 wt.% of other liquids, wherein the water / ethylene glycol weight ratio is from 20:80 to 60:40. It will be apparent to one skilled in the art that the carrier liquid may comprise ethylene glycol that originates from the reactive solvent. The other liquids of the carrier liquid may originate from the other solvents of the reactive solvent. Likewise, the further ingredients may originate from PET depolymerization products, and from ingredients of the waste PET polymer, such as colorants for instance. Adjusting the mixture may be carried out by adding components such as water, by removing components such as an amount of the other ingredients that surpasses 10 wt.%, or by a combination thereof.

[0028] According to a fourth aspect of the invention, there is provided a reactor system for depolymerising a terephthalate polymer into reusable raw material, said reactor system comprising: a depolymerization reactor comprising at least one inlet for a stream of terephthalate waste polymer and a stream of reactive solvent, wherein said depolymerization reactor is configured for depolymerizing the terephthalate waste polymer into a depolymerized mixture by using the reactive solvent; a BHET monomer crystallization unit fluidly connected to an outlet of the depolymerization reactor and arranged for forming BHET monomer crystals from the depolymerized mixture thereby obtaining BHET monomer crystals in a mother liquor; a separator fluidly connected to an outlet of the monomer crystallization unit and arranged for receiving the BHET monomer crystals and separating them from the mother liquor; and further a collection vessel fluidly connected to an inlet of the BHET monomer crystallization unit and arranged for providing BHET seed crystals to the BHET monomer crystallization unit.

[0029] According to a fifth aspect of the invention, there are provided BHET seed crystals having at least one of:

[0030] (i) a powder X-ray diffraction spectrum with at least one characteristic peak at 20 chosen from about equal to 10.7°, 14.4°, 18.9° and 21.6°; and

[0031] (ii) an IR spectrum with at least one characteristic absorption peak at I chosen from about equal to 3260 cm1and 897 cm'1, preferably for use in depolymerising a terephthalate polymer into reusable raw material.

[0032] The BHET seed and monomer crystals according to the invention (all five aspects thereof) differ from a-form BHET crystals. In particular, the BHET seed and monomer crystals have a powder X- ray diffraction spectrum which lack characteristic intensity peaks at 20 about equal to 16.4° and 35.2°, where 0 is the angle of diffraction. Alternatively, the IR spectrum of the BHET seed and monomer crystals lack a characteristic absorption peak at I about equal to 910 cm1. This does not exclude that the invention according to the first, third, and fourth aspect may in addition use other BHET crystal polymorphs than the claimed polymorph, such as a-form BHET crystals for instance, as long as the desired BHET monomer crystal morphology is formed.

[0033] DETAILED DESCRIPTION

[0034] In a first aspect, the invention concerns a method of preparing bis (2-hydroxyethyl) terephthalate (BHET) monomer crystals, said method comprising the steps of:

[0035] (a) providing a mixture consisting of BHET monomers and oligomers thereof, a carrier liquid, impurities and at most 10 wt.%, relative to the total weight of the mixture, of further ingredients, wherein at least 80 wt.% of the carrier liquid consists of a water / ethylene glycol mixture and at most 20 wt.% of the carrier liquid of other liquids, wherein the water / ethylene glycol weight ratio is from 20:80 to 60:40;

[0036] (b) providing BHET seed crystals in the mixture;

[0037] (c) forming BHET monomer crystals from the mixture comprising the BHET seed crystals, thereby obtaining BHET monomer crystals in a mother liquor; and

[0038] (d) separating at last a part of the BHET monomer crystals from the mother liquor; wherein the BHET seed crystals have at least one of a powder X-ray diffraction spectrum with at least one characteristic intensity peak at 20 chosen from about equal to 10.7°, 14.4°, 18.9° and 21.6°, where 0 is the angle of diffraction;

[0039] -an IR spectrum with at least one characteristic absorption peak at I chosen from about equal to 3260 cm1and 897 m ', where I is the ordinary wavenumber.

[0040] It turned out that by seeding the mixture of BHET monomers and oligomers thereof, a carrier liquid as claimed according to the first aspect, impurities and further ingredients with the BHET seed crystals, BHET monomer crystals may be obtained that need less time to separate (for instance by filtration) from the mother liquor. It indeed has been found that typical separation times may be reduced with a factor of five or more.

[0041] The method according to the first aspect is also less sensitive, or even insensitive, to the presence of impurities during crystallization. The concentration of the impurities in the mixture may indeed be higher than 8 wt.% relative to the weight of BHET monomers in the mixture, more preferably higher than 9 wt.%, even more preferably higher than 10 wt.%, even more preferably higher than 11 wt.%, and most preferably higher than 12 wt.%, relative to the weight of BHET monomers in the mixture, without substantially affecting the formation of the desired BHET monomer crystal morphology. The desired BHET monomer crystal morphology results in separation times that may be reduced with respect to those known in the art.

[0042] It has also been found that up to about 15 wt.% of impurities relative to the weight of the BHET monomers in the mixture, the BHET monomer crystal morphology conforms to the morphology of the BHET seed crystals and reduced separation times may be observed.

[0043] The concentration of BHEET in the mixture may be higher than 3 wt.% relative to the weight of BHET monomers in the mixture, more preferably higher than 4 wt.%, even more preferably higher than 5 wt.%, even more preferably higher than 6 wt.%, even more preferably higher than 7 wt.%, and most preferably higher than 8 wt.%, relative to the weight of BHET monomers in the mixture, without substantially affecting the formation of the desired BHET monomer crystal morphology. It has also been found that up to about 10 wt.% of BHEET relative to the weight of the BHET monomers in the mixture, the BHET monomer crystal morphology conforms to the morphology of the BHET seed crystals and reduced separation times may be observed.

[0044] Therefore, the produced BHET monomer crystals in mixtures having up to 15 wt.% of impurities relative to the weight of the BHET monomers may actually be used for seeding subsequent BHET monomer crystallization. The same holds for produced BHET monomer crystals in mixtures having up to about 10 wt.% of BHEET relative to the weight of the BHET monomers.

[0045] It is noted in this context that the “BHET seed crystals ” are defined as the BHET crystals obtained by the method according to the second aspect of the invention. The “BHET monomer crystals ” are defined as the BHET crystals obtained by the method according to the first aspect of the invention. As explained above, the BHET monomer crystals may, under the conditions disclosed above, be used for seeding subsequent mixtures to be crystallized and still produce the advantages of the invention, such as reduced separation times.

[0046] The powder X-ray diffraction spectrum of the BHET seed crystals has at least one characteristic intensity peak at 20 chosen from about equal to 10.7°, 14.4°, 18.9° and 21.6°. The wording ‘about’ in the context of the angle of diffraction indicates that slight variation may be seen in the exact positions of the intensity peaks. In particular, the positions 20 may show a variation of ± 0.5° around the claimed values for 20. More preferably, the powder X-ray diffraction spectrum of the BHET seed crystals has at least two, even more preferably at least three characteristic intensity peaks at 20 chosen from about equal to 10.7°, 14.4°, 18.9° and 21.6°. Most preferably, the powder X-ray diffraction spectrum of the BHET seed crystals has characteristic intensity peaks at 20 about equal to 10.7°, 14.4°, 18.9° and 21.6°, or at 20 about equal to 10.7° and 18.9°.

[0047] The IR spectrum of the BHET seed crystals has at least one characteristic absorption peak at I chosen from about equal to 3260 cm1and 897 cm'1. The wording ‘about’ in the context of the ordinary wave number indicates that slight variation may be seen in the exact positions of the absorption peaks. In particular, the positions I may show a variation of ± 15 cm1around the claimed values for I. Preferably, the IR spectrum of the BHET seed crystals has characteristic absorption peaks at I about equal to 3260 cm1and 897 cm '.

[0048] In an embodiment of the invention, a method according to the first aspect is provided wherein the BHET seed crystals have the powder X-ray diffraction spectrum of Figure 11 , and / or the BHET seed crystals have the IR spectrum of Figure 12.

[0049] Step (a) of the method according to the first aspect provides a mixture consisting of BHET monomers and oligomers thereof, a carrier liquid, impurities, and further ingredients. The impurities and further ingredients were defined hereinabove. The mixture contains the further ingredients in an amount of at most 10 wt.% relative to the total weight of the mixture. More preferred amounts of the further ingredients range from 0 to 8 wt.%, even more preferred from 0 to 5 wt.% and most preferred from 0 to 3 wt.% relative to the total weight of the mixture. To crystallize the BHET monomers, these monomers are typically dissolved in the carrier liquid at a suitably high temperature, taking advantage of the fact that most solids are more soluble at higher temperatures.

[0050] Step a) of the method according to the first aspect further requires that the carrier liquid consists of at least 80 wt.% of a water / ethylene glycol mixture and at most 20 wt.% of other liquids, wherein the water / ethylene glycol weight ratio is from 20:80 to 60:40. The relatively low water / ethylene glycol weight ratio used in the method according to the first aspect of the invention permits the use of a relatively low volume of water which is advantageous on an industrial scale. Even with this relatively low water / ethylene glycol weight ratio, the use of the BHET seed crystals in the method according to the first aspect allows forming the BHET monomer crystals having the desired morphology. The water / ethylene glycol weight ratio in more preferred embodiments ranges from 30:70 to 60:40, more preferably from 35:65 to 60:40, even more preferably from 40:60 to 60:40. Other preferred ranges of the water / ethylene glycol wight ratio are from 41:59 to 59:41, more preferably from 42:58 to 58:42, more preferably from 43:57 to 57:43, more preferably from 44:56 to 56:44, more preferably from 45:55 to 55:45, more preferably from 46:54 to 54:46, more preferably from 47:53 to 53:47, more preferably from 48:52 to 52:48, more preferably from 49:51 to 51:49, and most preferably 50:50.

[0051] In an embodiment, the other liquids of the carrier liquid may comprise alkanols, alkanediols, alkanetriols or combinations thereof, methanol, diethylene glycol, propylene glycol, dipropylene glycol, 1 ,4-butanediol, 1,5-pentanediol, glycerol or combinations thereof.

[0052] The carrier liquid preferably consists of at least 85 wt.% of the water / ethylene glycol mixture and at most 15 wt.% of the other liquids, more preferably of at least 90 wt.% of the water / ethylene glycol mixture and at most 10 wt.% of the other liquids, more preferably of at least 95 wt.% of the water / ethylene glycol mixture and at most 5 wt.% of the other liquids, and most preferably of about 100 wt.% of the water / ethylene glycol mixture.

[0053] The BHET seed crystals are introduced to the mixture in step (b) of the method. “Seeding" in the context of the present invention disclosure is defined as the introduction of seed crystals into a supersaturated liquid at a point, at which these seed crystals do not substantially dissolve, but rather may grow and proliferate. Seeding obviates the need for initial nucleation, i.e. the formation of a first stable crystal in the supersaturated liquid. Rather, a seed crystal is added to serve as a parent crystal for subsequent crystals to be formed.

[0054] The BHET seed crystals may be introduced to the mixture by admixing the BHET seed crystals into the mixture. The BHET seed crystals may be added into the mixture as dry seeds or as a slurry in a saturated solution. During the addition of dry seeds, care should be given to ensure that the seeds have been dispersed uniformly in the solution before cooling. Dry powder adhesion is more prone to particle clumping or aggregation if not properly dispersed. Wet loading or slurry loading of the BHET seed crystals attempts to overcome this difficulty by dispersing the BHET seed crystals in a small volume of liquid prior to their addition into the mother liquor. The slurry could be prepared in a saturated solution of the same solvent as the carrier liquid or in a different solvent. Preferably, the dried BHET seed crystals are slurried (re-slurried) by admixing with water before introducing them to the mixture. A preferred concentration of the BHET seed crystals in the slurry ranges from 1-50 wt.%, more preferably from 2-40 wt.% and most preferably from 3-30 wt.%, or from 5-25 wt.%.

[0055] Alternatively, the mixture may be added to the BHET seed crystals or BHET seed crystal slurry. In a preferred embodiment of the invention according to the first aspect, a part of the BHET monomer crystals formed in a crystallization unit in step c) is left in the crystallization unit in which they were crystallized. Since the BHET monomer crystals formed by the method according to the first aspect were found to have substantially the same morphology as the BHET seed crystals, the BHET monomer crystals formed then serve as BHET seed crystals for a subsequent mixture added to the crystallization unit, as will be elaborated further below. An important asset of the invention therefore is that formed BHET monomer crystals may, under the conditions regarding the level of the impurities disclosed above, be used as BHET seed crystals since they were found to have substantially the same morphology as the BHET seed crystals.

[0056] The concentration of the BHET seed crystals in the mixture may be very low. In fact, even one BHET seed crystal may suffice to nucleate and grow further crystallization in the mixture. Typically, adding a relatively low amount of BHET seed crystals to the mixture may promote a relatively high yield (the ratio of the amount of formed crystals to the amount of seed crystals) but may take time for nucleation to occur. Adding a relatively high amount of BHET seed crystals may promote the speed of crystallization at the expense of yield. Suitable amounts of BHET seed crystals in the mixture may be from 0.01 to 50 wt.%, more preferably from 0.2 to 20 wt.%, relative to the amount of BHET monomer in the mixture,

[0057] Step (c) of the method according to the first aspect further relates to forming BHET monomer crystals from the mixture comprising the BHET seed crystals, thereby obtaining BHET monomer crystals in a mother liquor. This crystallization process in which BHET monomer crystals are formed does not exclude forming dimer crystals, trimer crystals and crystals of even higher oligomers as well.

[0058] During the crystallization step (c), the mixture in which the BHET monomers are dissolved is typically subjected to cooling from a temperature at which they are typically introduced in a crystallization unit. As the mixture cools, the solubility of the BHET monomers in the carrier liquid will gradually decrease. The resultant mixture may be supersaturated at a given temperature, meaning that there is more BHET solute dissolved in the mixture than would be predicted by its solubility at that temperature. Crystallization is then induced from this supersaturated mixture (solution) and the resultant BHET monomer crystals are removed by separation, according to step (d) of the claimed method.

[0059] The cooling curve used in crystallization may be designed according to known procedures. In unseeded crystallization for instance, overcooling may generate an unfavourable amount of oversaturation, and starting with slow cooling before crystallization is preferred to create the appropriate environment for nucleation to occur. The method according to the first aspect that uses seeding with the BHET seed crystals does not suffer from this downside. Cooling can be rather rapid, and the cooling rate is slowed down at the time when the BHET seed crystals are introduced in the mixture. After the BHET seed crystals have been introduced, a cooling curve may be started. This approach may allow for a significant decrease of crystallization time with respect to known methods. For instance, this could likely save 30 min. to 1 hour on crystallization time. Seeding not only allows choosing the moment of starting the crystallization, but also to choose the degree of oversaturation of the mixture. The cooling rate may then be used to control the degree of oversaturation throughout the crystallization process in step (c).

[0060] The mixture is typically introduced in the crystallization unit at a temperature above 80°C to dissolve the BHET monomers in the mixture. A maximum temperature may be set at 140°C for instance. Although the invention is not limited to using a particular cooling curve, an efficient embodiment of the method involves a step (a) comprising bringing the mixture from a temperature To of above 80°C to a temperature Ti of between 60 and 75°C. In another embodiment, step (a) comprises cooling the mixture from the temperature Ti to a different temperature T2 of between 25 and 60°C at a rate of between 0-10°C / h.

[0061] It has further turned out that a preferred embodiment relates to a method wherein step (b) comprises providing the BHET seed crystals in the mixture at a temperature T3 of between 25 and 60°C.

[0062] In another embodiment, the crystallization step (c) comprises cooling the mixture further to a temperature T4 of between 15 and 25°C at a rate of between 2-20°C / h.

[0063] The above embodiments are preferably combined in a preferred embodiment in which step (a) comprises cooling the mixture from the temperature Ti to a temperature T2 of between 25 and 45°C at a rate of between 2-10°C / h, wherein step (b) comprises providing the BHET seed crystals in the mixture at a temperature T3 of between 35 and 45°C, and wherein the crystallization step (c) comprises cooling the mixture further to a temperature T4 of between 15 and 25 °C at a rate of between 10-20°C / h.

[0064] The mixture may further comprise a crystallizing agent as part of the further ingredients. Crystallizing agents (also referred to as crystallants or precipitants) in the context of the present invention disclosure are defined as those chemical species, distinct from the carrier liquid and from the BHET seed and monomer crystals, that can precipitate the soluble BHET monomers and oligomers thereof and hence bring them into a supersaturated state promoting their crystallization. Typical examples are salts, long-chain and low-molecular-mass polymers, and non-volatile organic compounds.

[0065] The invention according to another embodiment has the advantage that adding a crystallizing agent other than the carrier liquid and / or the BHET seed crystals to the mixture is not needed and therefore may be excluded or is excluded. Crystallizing agents to be specifically excluded consist of the group of at least one of polysorbate 80 (tradename Tween 80), sorbitan monooleate (tradename Span 80) and alkyl polyglycosides.

[0066] In an embodiment of the invention, the method may be performed batchwise by first separating the BHET monomer crystals from the mother liquor and provide them as BHET seed crystals in a subsequent batch operation, wherein, optionally, the BHET monomer crystals separated from the mother liquor during the batch operation are re-slurried by mixing with a re-slurrying liquid, preferably water, and provided as the BHET seed crystals in the subsequent batch operation.

[0067] Alternatively, in another embodiment of the invention according to the first aspect, the process can be performed continuously in that it comprises the steps of:

[0068] (a) continuously providing the mixture consisting of BHET monomers and oligomers thereof, the carrier liquid, the impurities, and at most 10 wt.% relative to the total weight of the mixture of the further ingredients, wherein the carrier liquid consists of at least 80 wt.% of a water / ethylene glycol mixture and at most 20 wt.% of other liquids, wherein the water / ethylene glycol weight ratio is from 20:80 to 60:40;

[0069] (b) providing the BHET seed crystals in the mixture at least at start-up of the continuous method; (c) continuously forming BHET monomer crystals from the mixture comprising the BHET seed crystals, thereby obtaining BHET monomer crystals in a mother liquor; and

[0070] (d) continuously separating the BHET monomer crystals from the mother liquor.

[0071] The skilled person will understand that in this embodiment the seeding of the mixture with the invented BHET seed crystals must be carried out at least at the start of the continuous operation, or at the start of the continuous operation only. The presence of the BHET seed crystals according to the invention will entail the formation of subsequent BHET monomer crystals having the same or similar morphology. These BHET monomer crystals may then act as nucleating sites for future BHET monomer crystals, if desired.

[0072] As already elucidated above, a particularly preferred embodiment of the invention according to the first aspect relates to a method in which a part of the BHET monomer crystals formed in a crystallization unit in step c) is left in the crystallization unit in which they were crystallized (this part is referred to as the “heel”). The heel then serves as BHET seed crystals for a subsequent mixture added to the crystallization unit. Such an embodiment may be applicable in a batch and in a continuous operation. In the context of this embodiment, it is important to avoid dissolving the heel in the mixture. This may be achieved in an embodiment by maintaining the heel below a temperature at which the BHET monomer crystals dissolve, which may be in the range of from 46 to 52°C°C, by cooling. In this context, it is helpful to introduce the mixture to be crystallized, which may have a temperature of 85 °C for instance, in a first part of a crystallizing unit and keep the heel in a second part thereof at a distance from the first part. For instance, the first part may be a top part of a crystallizing unit, whereas the second part may be a bottom part of the crystallizing unit. Adequate cooling capacity may then be provided at the second part to be able to keep the heel at a temperature below the dissolution temperature of the BHET monomer crystals. The temperature may for instance be kept from 15 to 1 °C lower than the dissolution temperature of the BHET monomer crystals, more preferably from 10 to 2°C lower than the dissolution temperature of the BHET monomer crystals, and most preferably from 6 to 3°C lower than the dissolution temperature of the BHET monomer crystals. Adequate cooling may be provided, as known to one skilled in the art.

[0073] The above does not rule out that additional BHET seed crystals are added to the mixture in a continuous operation. In an embodiment of the method therefore, a part of the BHET monomer crystals separated in step (d) is stored in a collection vessel and provided as BHET seed crystals in step (b). One skilled in the art will appreciate that BHET dimers, trimers and even higher BHET oligomers may be present in the mixture to be crystallized besides BHET monomers. In embodiments of the invention in which the BHET dimers and other oligomers do not appreciably affect the crystallization of the BHET monomers due to the seeding, the BHET dimers and other oligomers preferably remain in the mixture during crystallization, and may also crystallize.

[0074] However, there may be circumstances in which the BHET dimers and other oligomers are preferably removed. In such circumstances, the BHET dimers and other oligomers may be removed from the mixture at least partly by crystallizing the BHET dimers and other oligomers first before crystallizing the BHET monomers. A suitable method is disclosed in WO2021 / 032821 Al in the name of the applicant, incorporated herein by reference in its entirety.

[0075] An embodiment of the invention provides a method wherein the BHET monomer crystals are recovered in solid form, and wherein preferably the method further comprises at least one of the steps of washing the BHET monomer crystals and drying the BHET monomer crystals.

[0076] Another embodiment relates to a method wherein the separating of the BHET monomer crystals from the mother liquor is performed by means of at least one of filtration and centrifugation, preferably filtration.

[0077] According to a second aspect of the invention, the BHET seed crystals are prepared using a method comprising the steps of: i. providing a mixture consisting of BHET monomers and oligomers thereof, a carrier liquid, impurities and at most 10 wt.%, relative to the total weight of the mixture, of further ingredients, wherein the carrier liquid consists of at least 80 wt.% of a water / ethylene glycol mixture and at most 20 wt.% of other liquids, wherein the water / ethylene glycol weight ratio is from 20:80 to 60:40, wherein further the concentration of impurities is below 10 wt.% relative to the total weight of the BHET monomers in the mixture; ii. forming BHET seed crystals from the mixture thereby obtaining BHET seed crystals and a seeded mother liquor; and iii. separating the BHET seeding crystals from the seeded mother liquor.

[0078] It turned out that the method according to the second aspect is able to produce BHET seed crystals having at least one of: a powder X-ray diffraction spectrum with at least one characteristic intensity peak at 20 chosen from about equal to 10.7°, 14.4°, 18.9° and 21.6°, where 0 is the angle of diffraction; an IR spectrum with at least one characteristic absorption peak at I chosen from about equal to 3260 cm1and 897 cm1, where I is the ordinary wavenumber, at relatively low water consumption because the concentration of the impurities is kept relatively low, or even zero. As already disclosed above, the impurities, in the context of the present invention, are defined as the group consisting of iso-BHET, MHET and BHEET. The further ingredients also have the same meaning as disclosed in the context of the first aspect.

[0079] The carrier liquid used in the invented method according to the second aspect may be the same as the carrier liquid used in the context of the first aspect of the invention. The composition of the carrier liquid and its preferred ranges are the same as those disclosed in the context of the first aspect of the invention. The same holds for the amount and preferred amounts of the further ingredients.

[0080] According to the second aspect of the invention, the concentration of the impurities in the mixture is below 10 wt.% relative to the weight of the BHET monomers in the mixture, more preferably below 9 wt.%, even more preferably below 8 wt.%, even more preferably below 7 wt.%, even more preferably below 6 wt.%, and most preferably below 5 wt.%, relative to the weight of the BHET monomers in the mixture, thereby promoting the formation of the desired BHET seed crystal morphology.

[0081] In other embodiments of the invention, the concentration of BHEET in the mixture is below 5 wt.% relative to the weight of the BHET monomers in the mixture, more preferably below 4 wt.%, even more preferably below 3 wt.%, even more preferably below 2 wt.%, and most preferably below 1 wt.%, relative to the weight of the BHET monomers in the mixture.

[0082] In order to provide a mixture having a relatively low concentration of impurities, or even no impurities at all, a method according to an embodiment comprises the step of purifying at least the mixture before the crystallization thereof. Purification of the mixture is preferably performed by contacting the mixture with an adsorption means, preferably active coal, such as by adding the adsorption means, preferably, active coal particles to it, or by contacting the mixture with the adsorption means, preferably active coal in an active coal column. Suitable adsorption means to be optionally used before crystallization comprise adsorption means that are configured to remove nonpolar compounds from the mixture at least partly.

[0083] The crystallization conditions for obtaining the BHET seed crystals as claimed may be selected within a broad range, making use of the general knowledge of one skilled in the art of crystallization.

[0084] The inventors however have found out that particular cooling conditions may produce the BHET seed crystals in a surprisingly effective way, for instance by decreasing the total cooling cycle to a substantial extent and still obtain the desired BHET seed crystal morphology. An embodiment of the invention provides a method wherein step (i) comprises bringing the mixture from a temperature Tso above 80°C to a temperature Tsiof between 60 and 75°C. In another preferred embodiment, step (i) comprises cooling the mixture from the temperature Tsito a different temperature TS2 of between 25 and 60°C at a rate of between 0-10°C / h. Yet another embodiment provides a method wherein step (ii) comprises providing the BHET seed crystals in the mixture at a temperature TS3 of between 25 and 60°C. According to yet another embodiment, step (iii) comprises cooling the mixture further to a temperature TS4 of between 15 and 25 °C at a rate of between 2-20°C / h.

[0085] A more preferred embodiment relates to a method wherein step (i) comprises bringing the mixture to a temperature Tsiof between 45 and 65°C, wherein step (i) further comprises cooling the mixture from the temperature Tsito a temperature TS2 of between 25 and 45 °C at a rate of between 2-10°C / h, wherein step (ii) comprises providing the BHET seed crystals in the mixture at a temperature TS3 of between 35 and 45°C, and wherein step (iii) comprises cooling the mixture further to a temperature TS4 of between 15 and 25 °C at a rate of between 10-20°C / h.

[0086] According to the claimed method, BHET seed crystals may be obtained having at least one of: a powder X-ray diffraction spectrum with at least one characteristic intensity peak at 20 chosen from about equal to 10.7°, 14.4°, 18.9° and 21.6°, where 0 is the angle of diffraction; an IR spectrum with at least one characteristic absorption peak at I chosen from about equal to 3260 cm-1 and 897 cm-1, where I is the ordinary wavenumber.

[0087] A fifth aspect of the invention therefore relates to BHET seed crystals obtainable by the method as claimed according to the second aspect having at least one of: (i) a powder X-ray diffraction spectrum with at least one characteristic peak at 20 chosen from about equal to 10.7°, 14.4°, 18.9° and 21.6°; and

[0088] (ii) an IR spectrum with at least one characteristic absorption peak at I chosen from about equal to 3260 cm-1 and 897 cm-1, preferably for use in depolymerizing a terephthalate polymer into reusable raw material, preferably BHET monomers and oligomers thereof.

[0089] According to an embodiment of the first aspect of the invention, the BHET seed crystals provided in step (b) of the method of preparing bis (2 -hydroxyethyl) terephthalate (BHET) monomer crystals according to the first aspect are obtained by the method of preparing BHET seed crystals according to the second aspect.

[0090] A third aspect of the invention relates to a method of depolymerizing a terephthalate polymer, preferably PET, into reusable raw material, such as BHET monomers, the method comprising the steps of: a) depolymerizing the terephthalate polymer in a reactive solvent comprising at least 80 wt.% ethylene glycol and optionally water, and at most 20 wt.% of other solvents; b) forming BHET monomer crystals from the depolymerized mixture by a method according to the first aspect of the invention thereby obtaining BHET crystals in a mother liquor; c) separating the BHET crystals from the mother liquor.

[0091] In the third aspect, the mixture consisting of the BHET monomers and oligomers thereof, the carrier liquid, the impurities and the further ingredients, as all defined in the first aspect of the invention, may at least partly originate from a depolymerization reaction wherein the condensation polymer that is depolymerized is preferably polyethylene terephthalate (PET). PET may include further comonomers, such as isophthalic acid, diethylene glycol (DEG), polyethylene naphthalate (PEN) and cyclohexane dimethanol (CHDM), to improve its properties, as is known in the art.

[0092] The depolymerization reaction is typically performed in a reactive solvent. This reactive solvent can be water, resulting in hydrolysis of the condensation polymer. Depolymerization of condensation polymers in a reactive solvent other than water is referred to as solvolysis. The reactive solvent is typically selected to be a solvent for the condensation polymer and / or for the monomers and oligomers thereof obtained from said condensation polymer by depolymerisation. Such reactive solvents are known to the person skilled in the art. In the context of the invention, the term ‘reactive solvent' also encompasses mixtures of a solvent that is reactive per se and a non-reactive solvent. Depolymerisation of polyesters by solvolysis is typically performed in alkanols, alkanediols, alkanetriols or in combinations thereof. For the depolymerisation of PET, the use of ethylene glycol as reactive solvent (‘glycolysis’) leads to the monomer bis(2 -hydroxyethyl) terephthalate (BHET) as primary depolymerisation product. The use of ethylene glycol as reactive solvent is therefore preferred. Dimers, trimers and further oligomers are typically also obtained. For the depolymerisation of PET, the use of methanol as reactive solvent leads to the formation of ethylene glycol and the monomer dimethyl terephthalate (DMT) as primary depolymerisation products.

[0093] According to the third aspect of the invention, any reactive solvent remaining after the depolymerization reaction and any solvent formed during the depolymerization reaction, such as ethylene glycol during methanolysis, may form at least a part of the carrier liquid in the mixture consisting of the BHET monomers, the carrier liquid, the impurities and the further ingredients that originates from a depolymerization reaction as provided in step (a) of the method according to the first aspect. This does, however, not mean that the carrier liquid in said mixture originating from the depolymerization reaction can only comprise liquids already present during the depolymerization reaction. As explained in for example W02023 / 008997A2, water may be added to the depolymerization reaction mixture comprising ethylene glycol as reactive solvent to improve downstream crystallization of the BHET monomers, as well as separation of a heterogeneous depolymerization catalyst from BHET monomers and oligomers thereof.

[0094] In an embodiment, the other solvents of the reactive solvent may comprise alkanols, alkanediols, alkanetriols or combinations thereof, preferably methanol, diethylene glycol, propylene glycol, dipropylene glycol, 1 ,4-butanediol, 1,5-pentanediol, glycerol or combinations thereof. The other solvents may constitute at most 20 wt.% of the reactive solvent, more preferably at most 15 wt.% of the reactive solvent, even more preferably at most 10 wt.% of the reactive solvent, even more preferably at most 5 wt.% of the reactive solvent, and most preferably 0 wt.% of the reactive solvent. In the latter embodiment, the reactive solvent consists of 100 wt.% ethylene glycol and optionally water. The amount of water preferably is below 1 wt.% of the reactive solvent.

[0095] In a preferred embodiment, the weight ratio of BHET monomers and oligomers thereof to carrier liquid in the mixture originating from a depolymerization reaction provided in step (a) is from 10:20 to 10:100, more preferably from 10:40 to 10:90.

[0096] The depolymerization reaction may be catalyzed by a suitable catalyst. Depolymerization catalysts for depolymerization of polycondensation polymers are well-known in the art. In this respect, reference is made to S. Thiyagarajan et al., RSC Adv., 2022, 12, pp 947-970 (DOI: 10.1039 / dlra08217e), which is hereby incorporated by reference in its entirety.

[0097] In a preferred embodiment, the weight ratio of heterogeneous depolymerization catalyst to monomers and oligomers thereof in the mixture originating from a depolymerization reaction provided in step (a) ranges from 0.001 : 10 to 1 : 10, more preferably from 0.005: 10 to 0.3: 10, and even more preferably from 0.008:10 to 0.015:10. The heterogeneous depolymerization catalyst may be removed from the mixture before the crystallization steps b) and c).

[0098] Preferred heterogeneous depolymerization catalysts are chosen from the group consisting of metal particles and oxides thereof, solid metal salts, magnetic particles that are optionally functionalized, particles based on earth alkali metals and hydrotalcites. Examples of magnetic particles include particles based on ferromagnetic materials, ferrimagnetic materials, anti-ferromagnetic materials, synthetic magnetic materials, paramagnetic materials, superparamagnetic materials and combinations thereof.

[0099] In a preferred embodiment, the heterogeneous depolymerization catalyst comprises transition metal particles, such as particles comprising transition metals chosen from the first series of transition metals, also known as the 3d orbital transition metals, more preferably transition metals chosen from iron, nickel, cobalt and combinations thereof, even more preferably transition metals chosen from iron, nickel and combinations thereof. Most preferred is a heterogeneous depolymerization catalyst comprising iron-containing particles.

[0100] If a heterogeneous depolymerization catalyst is made of metal, it may be provided with an oxide surface, which may further enhance catalysis. The oxide surface may be formed by itself, in contact with air, in contact with water, or the oxide surface may be applied deliberately.

[0101] In another embodiment, the heterogeneous depolymerization catalyst comprises earth alkali metal particles, such as particles comprising earth alkali metals chosen from the group consisting of beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr) barium (Ba), their oxides and combinations thereof. A preferred earth alkali metal oxide is magnesium oxide (MgO).

[0102] In yet another embodiment, the heterogeneous depolymerization catalyst comprises (non-magnetic) metal particles and oxides thereof, such as particles comprising metals chosen from the group consisting of titanium (Ti), zirconium (Zr), manganese (Mn), zinc (Zn), aluminum (Al), germanium (Ge), antimony (Sb), their oxides, and combinations thereof.

[0103] The heterogeneous depolymerization catalyst material is substantially insoluble in the carrier liquid, also at temperatures of more than 100°C.

[0104] The heterogeneous depolymerization catalyst preferably has an average particle size of between 5 nm and 200 pm, more preferably between 10 nm and 5 pm, even more preferably between 25 nm and 500 nm, still more preferably between 30 and 450 nm. The term ‘particle' as used herein includes both single particles and agglomerates and aggregates thereof if present in the heterogeneous depolymerization catalyst. The average particle size as defined herein thus concerns an average size of single particles and agglomerates and aggregates thereof, if present.

[0105] In a very preferred embodiment, the heterogeneous depolymerization catalyst comprises functionalized magnetic particles comprising a catalyst complex (ABC), said functionalized magnetic particles having an average particle size of between 25 and 500 nm, as measured with DLS, more preferably between 30 and 450 nm, wherein the catalyst complex comprises three distinguishable elements: (nano)particle (A), bridging moiety / linking group (B) chemically attached to the particle (A), such as by a covalent bond, or physically, such as by adsorption, and catalyst entity (C) that is associated with the particles (A), such as by being chemically bonded, for instance covalently bonded, to the linking group. The linking group preferably does not fully cover the (nano)particle surface, such as in a core-shell particle. The (nano)particles of this catalyst complex are preferably based on ferromagnetic and / or ferrimagnetic materials. Also, antiferromagnetic materials, synthetic magnetic materials, paramagnetic materials, superpar amagnetic materials, such as materials comprising at least one of Fe, Co, Ni, Gd, Dy, Mn, Nd, Sm, and preferably at least one of O, B, C, N, such as iron oxide, such as ferrite, such as magnetite, hematite, and maghemite can be used.

[0106] The functional groups of the bridging moiety (B) are for instance weak organic acids, such as a carboxylic acid or a dicarboxylic acid, but preferably silanols, including silanediols and silanetriols. The bridging moiety may be introduced as a reactant in the form of a silyl comprising group, such as silyl ethers, such as triethoxy silylpropylhalide. The linking group is for instance an alkylene chain, with the alkylene typically between C2 and C10, preferably C3-C5, i.e. propylene, butylene, pentylene. Propylene is preferred. The bridging moiety is suitably provided as a reactant in which the linking group is functionalized for chemical reaction with the catalyst entity, whereas the functional group may be protected. For instance, a suitable functionalization of the linking group is the provision as a substituted alkyl halide. A suitable protection of the functional group may be in the form of an ester or alkoxysilane. The alkoxy-group is preferably ethoxy, though methoxy or propoxy are not excluded.

[0107] In an embodiment, the alkoxysilane is provided as a trialkoxysilane, having one alkylene group that constitutes the linking group. In an alternative embodiment, use is made of dialkyl-dialkoxysilanes, with one of the alkyl groups being the linking group. In again another embodiment, use is made of monoalkoxy-trialkylsilanes, with one of the alkyl groups being the linking group. In the latter cases, the alkyl groups are preferably lower alkyl, such as C1-C4 alkyl, thus methyl, ethyl, propyl, / / -butyl and isobutyl. At least one of the alkyls is then functionalized, for instance with a halide, as specified above. Linear alkyls appear preferable to limit steric hindrance.

[0108] The use of a dialkyl-dialkoxysilane and / or a monoalkoxy-trialkylsilane is understood to be beneficial to create a better separation when the carrier liquid comprises ethylene glycol and water. It is believed that not all alkoxy-groups of the trialkoxy silanes bond to the surface of the nanoparticle aggregate. Some of the alkoxygroups may even remain protected. The protective groups may however be removed upon addition of water to the complex. As a result, the hydrophilicity of the complex may increase. By using silanes with less alkoxy-groups, the remaining groups are inherently non-polar and cannot become unprotected. The entire complex thus becomes more hydrophobic. Rather than merely one type of bridging moiety (B), also known as a silane coupling agent, a mixture of those may be used, for instance a mixture of alkyltrialkoxysilane and dialkyl-dialkoxysilane, wherein one of the alkyl-groups is functionalized as a halide to react to the catalytic entity, and subsequently - after the reaction of both - carries the catalytic entity. The addition of dialkyldialkoxysilanes may well reduce the size of the layer of groups bonded to the surface. This is not deemed a disadvantage.

[0109] The catalyst entity (C) may be aromatic or aliphatic, and heterocyclic. An aromatic heterocyclic moiety suitably comprises a heterocycle having at least one, preferably at least two nitrogen atoms. The heterocycle may have 5 or 6 atoms, preferably 5 atoms. Suitable aromatic heterocycles are pyrimidines, imidazoles, piperidines, pyrrolidine, pyridine, pyrazol, oxazol, triazol, thiazol, methimazol, benzotriazol, isoquinol and viologen-type compounds (having for instance two coupled pyridine -ring structures). Particularly preferred is an imidazole structure, which results in an imidazolium ion. The negatively charged moiety may relate to an anionic complex, but alternatively a simple ion, such as a halide. Preferably, the reaction of the alkylhalide of the bridging moiety with an uncharged aromatic heterocyclic moiety including at least one nitrogen atom generates the positive charge on the aromatic moiety, particularly on the nitrogen atom therein, as well as the creation of the negative halide. The negatively charged halide may thereafter be strengthened by addition of a Lewis acid to form a metal salt complex. One example is the conversion of chloride to FeCl4. The aromatic moiety has in one example at least one tail. The at least one tail preferably has a length of C1-C10, such as C2-C4, the at least one tail suitably being attached to a nitrogen atom. This tail is more particularly a tail extending into the carrier liquid and away from the bridging moiety. A longer tail is deemed beneficial to increase the hydrophobicity of the complex. This may counteract tendencies of complex to enter a hydrophilic phase.

[0110] The bridging moiety (B) and the catalyst entity (C) bonded thereto are preferably present in an amount of (mole bridging moiety / gr magnetic particle) 5406-0.1, more preferably in an amount of 1405-0.01, even more preferably in an amount of 2405-103, such as in an amount 4405-104. It is preferred to have a relatively large amount available in terms of an effective optional recovery of the catalyst complex, whereas, in terms of amount of catalyst and costs thereof, a somewhat smaller amount may be preferred.

[0111] It has been found that limited coverage of the surface of the (nano)particles, or aggregates of such (nano)particles, with the catalyst entity (C) is sufficient to obtain an effective catalyst.

[0112] A fourth aspect of the invention relates to a reactor system for depolymerizing a terephthalate polymer into reusable raw material, said reactor system comprising: a depolymerization reactor comprising at least one inlet for a stream of terephthalate waste polymer and a stream of reactive solvent, wherein said depolymerization stage is configured for depolymerizing the terephthalate waste polymer into a depolymerized mixture by using the reactive solvent; a BHET monomer crystallization unit fluidly connected to an outlet of the depolymerization reactor and arranged for forming BHET monomer crystals from the depolymerized mixture thereby obtaining BHET monomer crystals in a mother liquor; a separator fluidly connected to an outlet of the monomer crystallization unit and arranged for receiving the BHET monomer crystals and separating them from the mother liquor; and further a collection vessel fluidly connected to an inlet of the BHET monomer crystallization unit and arranged for providing BHET seed crystals to the BHET monomer crystallization unit. In an embodiment, the reactor system is characterized in that the collection vessel is fluidly connected to an outlet of the monomer crystallization unit and arranged for receiving BHET crystals from the monomer crystallization unit.

[0113] In accordance with the invention, the formation of BHET monomer crystals precedes a separation step in which the corresponding mother liquor is removed. The formation of the BHET monomer crystals preferably occurs in a crystallisation reactor, such as a vessel. The separation step is carried out with filtration, centrifugation, a cyclone or another type of separator as known in the art.

[0114] It is not excluded that the crystallisation reactor includes the separator, which is for instance activated after a predefined residence time. However, a separate separator is deemed preferable. In case that the crystals are to be recovered, a washing step is preferably carried out after the separation step. A band filter is deemed one practical arrangement for performing a separation step and a subsequent washing step. The characteristic size of the separation means can be chosen in dependence of the size of the generated crystals and a desired duration for the separation step. In an implementation, recovering the BHET crystals comprises separating the BHET crystals from the mother liquor by means of filtration using a filter element.

[0115] The monomer is preferably recovered in solid form from the mother liquor. It is deemed appropriate that the recovery may be followed by a washing step and a drying step. Preferably, at least 80 wt.%, more preferably at least 85 wt.% or even at least 90wt.% of the recovered product consists of 5wt%, BHET monomer crystals. The recovered product may also comprise up to 20 wt.% dimer, more preferably at most 15wt% of dimer, and most preferably at most 10wt% of dimer.

[0116] This reactor system is configured for performing the method according to the third aspect of the invention. Typically, the at least one crystallisation unit is embodied as at least one vessel with an inlet and an outlet. Preferably a controller is present for controlling process conditions in each of said vessels. Sensors may be available thereto, as known to those skilled in the art. The crystallisation units, and the separator stage may be configured for batch operation or for continuous operation. Alternatively, the system is semi-continuous, in that the crystallisation units are of a batch type but the streams from a further processing stage and beyond are continuous. It may also be desirable that a plurality of crystallisation units is arranged in parallel to load one crystallisation unit while performing the crystallisation treatment in another one. In an embodiment, the BHET monomer crystal recovering stage comprises a filtration unit configured to separate the BHET monomer crystals from the mother liquor by means of filtration, and wherein the filtration unit is configured to carry out the washing of the separated BHET monomer crystals inside the filtration unit.

[0117] An adsorption means may also be present and arranged downstream of the separator, if desired. Suitable adsorption means to be optionally used after crystallization comprise adsorption means that are configured to remove polar compounds from the mixture at least partly, such as an ion exchange apparatus for instance.

[0118] It is observed for clarity that any of the embodiments discussed hereinabove and / or hereinafter with reference to the figures or in the context of the examples or as defined in the dependent claims with respect to one aspect of the invention is also applicable and deemed disclosed in relation to any other aspect of the invention, which aspects are further defined in the claims as filed.

[0119] BRIEF DESCRIPTION OF THE FIGURES

[0120] The above and other advantages of the features and objects of the 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:

[0121] Fig. 1 schematically illustrates a reactor system for preparing bis (2 -hydroxyethyl) terephthalate (BHET) monomer crystals according to the state of the art;

[0122] Fig. 2 schematically illustrates a reactor system for preparing bis (2 -hydroxyethyl) terephthalate (BHET) monomer crystals according to an embodiment of the invention;

[0123] Fig. 3 schematically illustrates a reactor system for preparing bis (2 -hydroxyethyl) terephthalate (BHET) monomer crystals according to another embodiment of the invention;

[0124] Fig. 4 schematically illustrates a reactor system for preparing BHET seed crystals according to an embodiment of the invention;

[0125] Fig. 5 schematically illustrates a reactor system for preparing BHET seed crystals according to another embodiment of the invention; and

[0126] Fig. 6 shows the morphology obtained by microscopy of BHET seed crystals according to an embodiment of the invention;

[0127] Fig.7-10 show the morphology obtained by microscopy of BHET monomer crystals prepared from an increasingly fouled solution using the BHET seed crystals according to an embodiment of the invention; Fig. 11 shows an X-ray diffraction pattern of BHET crystals, obtainable by an embodiment of the invention;

[0128] Fig. 12 shows an IR spectrum of BHET crystals, obtainable by an embodiment of the invention;

[0129] Fig. 13 shows an X-ray diffraction pattern of BHET crystals, obtainable by the state of the art;

[0130] Fig. 14 shows an IR spectrum of BHET crystals, obtainable by the state of the art;

[0131] Fig. 15 shows the combined X-ray diffraction patterns of figures 11 and 12;

[0132] Fig. 16 shows the combined IR spectra of figures 13 and 14;

[0133] Fig. 17 shows a detail of the combined X-ray diffraction patterns of figure 15; and

[0134] Fig. 18 shows yet another detail of the combined X-ray diffraction patterns of figure 15.

[0135] DESCRIPTION OF EMBODIMENTS

[0136] The accompanying figures are used to illustrate preferred non-limiting exemplary embodiments of reactor systems of the present invention. The figures are schematic and not drawn to scale. The same reference numerals in different figures refer to equal or corresponding elements.

[0137] Figure 1 illustrates schematically a reactor system 10 according to the state of the art. The shown reactor system 10 essentially comprises a depolymerization reactor 1 and four separation units 2, 3, 4 and 5. Inlet streams A, B and C to the depolymerization reactor 1, as well as feedback streams X and Y are indicated which respectively recycle catalyst and solvent, in particularly ethylene glycol. A purge stream Z is defined for produced BHEET.

[0138] The reactor system 10 is provided with an input stream A comprising polymeric material the bulk thereof being a terephthalate polymer for depolymerization, more particularly polyethylene terephthalate (PET). The input stream A may be in solid form, such as in the form of flakes. However, the input stream may also be supplied in liquid form, such as a dispersion or a solution.

[0139] The input stream A enters the depolymerization reactor 1 , together with other streams entering the depolymerization reactor 1. These include a stream B of reactive solvent, such as ethylene glycol, and a stream of a suitable catalyst C for catalysing the depolymerization reaction. The stream C may also comprise an optional recycled stream X of catalyst. A recycled stream Y of reactive solvent, such as ethylene glycol, also enters the reactor 1 in this example. The input streams A, B, C, and the recycle streams X and Y may be arranged as individual inlets or may be combined into one or more inlets. The depolymerization reactor 1 may be of a batch type or a continuous type, the latter being preferred. While it is indicated as a single reactor, it is not excluded that a combination of reactor vessels is used, such as the combination of a tank reactor and a plurality of plug flow reactors. Also, a plurality of vessels may be arranged in parallel within one unit. While not indicated, it will be understood that the reactor system 10 is provided with a controller and that sensors may be present as well as valves for setting flow rates into the reactor and for setting residence times in the reactor. Furthermore, the depolymerization reactor 1 and the separation units 2, 3, 4 and 5 may be provided with heating means and / or other temperature regulation means to prevent deviations from predefined temperatures and other variables.

[0140] Following the depolymerization in reactor 1 , the depolymerized reaction mixture Sd is pumped to a separation unit 2, which may be provided with an inlet for water D. The water D may alternatively be provided as an aqueous solution. It is not excluded that one or more further additives are added thereto to facilitate the phase separation intended to occur in the separation unit 2.

[0141] The separation unit 2 serves to cool down the depolymerized mixture from a depolymerization temperature, typically in the range of 160-250°C, to a processing temperature, for instance around 100°C. The optional stream of water D may contribute to the cooling process, and also to the generation of a two-phase mixture in the separation unit 2. A first phase may at least comprise BHET monomer and oligomers, a carrier liquid, impurities such as BHEET as solutes in a mixture of ethylene glycol and optionally water, and possibly at least a part of the other ingredients. A second phase may comprise BHET oligomers, catalyst, water, and / or a part of the other ingredients. The two-phase mixture is separated in the separation unit 2 which thereto comprises a first separator, for instance a centrifuge. The second phase containing catalyst may thereafter be recycled to the depolymerization reactor 1 as stream X. While the separation unit 2 is shown as one unit, it is not excluded that this unit 2 comprises several separate units, such as a cooling vessel, the first separator, and a filtration unit for instance. Alternatively, a cooling means may be incorporated in the depolymerization reactor 1, particularly in case of using a batch process. Also, in other embodiments, further purification units may be provided.

[0142] The first phase leaving the separation unit 2 is also referred to as a solution S. Rather than a pure solution, the solution S may be a colloidal solution or a dispersion. The solution S is transferred to a BHET crystallization unit 3 in which BHET is crystallized and subsequently recovered in a separator unit 4 as solid BHET monomer product I. Rather than or in addition to lowering the temperature relative to the separation unit 2, an anti-solvent such as water E may be added to the solution S in the crystallization stage 3, as indicated in the figure by means of the arrow E. This may reduce the solubility of BHET and enable crystallization at a higher temperature. Upon crystallization of the BHET, the solution S is transformed into a mother liquor M that comprises solid BHET, as well as impurities such as BHEET. The mother liquor M enters a solid / liquid separation unit 4 in which the solid BHET monomer product I is separated from the mother liquor M. The remaining mother liquor Ml that may also contain impurities such as BHEET is then led to a processing unit 5, which preferably includes at least one distillation column. In the processing unit 5, the mother liquor Ml is processed to reduce its water content. The BHEET content in the mother liquor Ml may also be reduced through a BHEET purge Z, if desired. The resulting purified carrier liquid may then be returned to the depolymerization reactor 1 as recycle stream Y.

[0143] Referring now to figure 2, a reactor system for preparing bis (2 -hydroxyethyl) terephthalate (BHET) monomer crystals according to an embodiment of the fourth aspect of the invention is schematically shown. The units referred to with the same reference numerals as in figure 1 refer to equal or corresponding elements. As an example, unit 1 refers to a depolymerization reactor while units 2, 3, 4 and 5 are similar units as disclosed in the context of figure 1. It should be added that the first phase leaving the separation unit 2 as solution S is adjusted to conform to a mixture consisting of BHET monomers and oligomers thereof, the carrier liquid, the impurities and at most 10 wt.%, relative to the total weight of the mixture, of the further ingredients, wherein the carrier liquid consists of at least 80 wt.% of a water / ethylene glycol mixture and at most 20 wt.% of other liquids, wherein the water / ethylene glycol weight ratio is from 20:80 to 60:40, in accordance with the first aspect of the invention. Adjusting the solution S may be carried out by adding water through the inlet for water E of the crystallization unit 3, or upstream of the crystallization unit 3 (not shown). Also, one or more separating units (not shown) may optionally be provided upstream of the crystallization stage 3 to remove at least a part of the impurities and / or the other ingredients.

[0144] The solution S is transferred to a BHET crystallization unit 3 in which BHET is crystallized and subsequently recovered in a separator unit 4 as solid BHET monomer product I. Rather than or in addition to lowering the temperature relative to the separation unit 2, an anti-solvent such as water E may be added to the solution S in the crystallization stage 3, as indicated in the figure by means of the arrow E. This may reduce the solubility of BHET and enable crystallization at a higher temperature.

[0145] The embodiment shown in figure 2 comprises an additional unit representing a re-slurrying vat or vessel 6. The re-slurrying vessel 6 has an inlet through which BHET seed crystals sc may be entered. The BHET seed crystals have a powder X-ray diffraction spectrum as defined in the context of the first and fifth aspect of the invention and may be obtained from any suitable source. A possible method for preparing the BHET seed crystals as claimed is disclosed further below. The re-slurrying vessel 6 is further provided with an inlet for water F, preferably demineralized water. The water F may alternatively be provided in the form of an aqueous solution of salts for instance. In the re-slurrying vessel 6, a BHET seed crystal slurry is prepared by re-slurrying an amount of the BHET seed crystals entering the re-slurrying vessel 6 in the added water F. The re-slurrying vessel 6 is thereto preferably provided with suitable mixing equipment, such as one or more stirrers. The BHET seed crystal slurry scs may be stored for future use, in particular when performing a batch operation. In a continuous operation, a continuous stream or an intermittent amount of the BHET seed crystal slurry scs is entered into the BHET crystallization unit 3, preferably at a bottom part thereof, as shown in figure 2. Entrance may be regulated by valve 8. In continuous operation, it is also possible to add an initial amount of the BHET seed crystal slurry scs to the crystallization unit 3 once. Such seeding initiates and steers the BHET crystallization in the crystallization unit 3 to obtain BHET crystals with favourable morphology. The favourable morphology appears as a reduced filtration time in the subsequent separator unit 4.

[0146] Upon crystallization of the BHET, the solution S is transformed into a mother liquor M that comprises solid BHET, as well as impurities such as BHEET. The mother liquor M enters a solid / liquid separation unit 4 in which the solid BHET monomer product I is separated from the mother liquor M. A suitable separator unit 4 may for instance be embodied as a filtration unit. It is important to minimize processing times in the separator unit 4, such as filtration times in an embodiment wherein the separator unit 4 is a filtration unit.

[0147] The remaining mother liquor Ml that may also contain impurities such as BHEET is then led to a processing unit 5, which preferably includes at least one distillation column. In the processing unit 5, the mother liquor Ml is processed to reduce its water content. The BHEET content in the mother liquor Ml may also be reduced through a BHEET purge Z, if desired. The resulting purified carrier liquid may then be returned to the depolymerization reactor 1 as recycle stream Y.

[0148] By means of the reactor system disclosed hereinabove, it has turned out feasible to carry out a method of forming BHET crystals and arrive at BHET monomer crystals I having the desired morphology, even when a relatively high amount of impurities, such as BHEET, is present during crystallization.

[0149] It should be noted that the purge of impurities (for instance BHEET) in the processing unit 5, shown in figure 2, is not essential to the invention, and may be omitted. In another embodiment, the recycle loop for the carrier liquid Y (such as ethylene glycol for instance) may also be omitted. Omitting the processing unit 5 and the recycle loop for the carrier liquid Y effectively turns the reactor system 10 into a batch process.

[0150] It is indeed one of the advantages of the invention that BHET monomer crystals having a morphology that enables low separation (in an embodiment comprising filtration) times may be produced, even with relatively high amounts of impurities, such as BHEET in the mixture to be crystallized. Purging of BHEET and other impurities may therefore not be needed or may be needed to a lesser extent.

[0151] The reactor system embodied in figure 2 may also be used in a preferred embodiment of the invention according to the first aspect, wherein a part of the BHET monomer crystals formed in the crystallization unit 3 is left in the crystallization unit 3 in which they were crystallized. Since the BHET monomer crystals formed by the method according to the first aspect were found to have substantially the same morphology as the BHET seed crystals, the BHET monomer crystals formed and left behind in the crystallization unit 3 then serve as BHET seed crystals for a subsequent mixture added to the crystallization unit 3. An important asset of the invention therefore is that formed BHET monomer crystals may be used as BHET seed crystals since they were found to have substantially the same morphology as the BHET seed crystals. This embodiment may be used in a batch process, but also in a continuous operation.

[0152] Referring now to figure 3, a reactor system 10 for preparing bis (2-hydroxyethyl) terephthalate (BHET) monomer crystals according to yet another embodiment of the invention is schematically shown. The units referred to with the same reference numerals as in figure 1 refer to equal or corresponding elements. Unit 1 again refers to a depolymerization reactor while units 2, 3, 4 and 5 are similar units as disclosed in the context of figure 1. Unit 6 also refers to the re-slurrying vessel 6 of the embodiment shown in figure 2.

[0153] The embodiment shown in figure 3 re-uses BHET monomer crystals I obtained after separating in separator unit 4 as BHET seed crystals. Surprisingly, re-using the BHET monomer crystals I formed as seed crystals yields the formation of BHET monomer crystals having the desired morphology as claimed. Thereto, the separator unit 4 is provided with an outlet for the BHET monomer crystals I, which outlet is connected to an inlet of the re-slurrying vessel 6. Optionally, a drying unit 7 is provided between the separator unit 4 and the re-slurrying vessel 6 to produce dried BHET crystals Id. The drying unit 7 may also act as a buffer vessel for the dried BHET crystals Id. Like the embodiment shown in figure 2, the re-slurrying vessel 6 has an inlet through which at least a part of the dried BHET crystals Id may be entered. Optionally, an inlet provided with shutoff valve 11 is instrumental in supplying further seed crystals sc to the re-slurrying vessel 6. The dried BHET crystals Id act as seed crystals for the crystallization in the crystallization unit 4, and preferably also have a powder X-ray diffraction spectrum as defined in the context of the first and fifth aspect, or similar to this. Another part If of the BHET crystals is stored as a final BHET crystal product If, to be used as a monomer for PET polymerization for instance.

[0154] The re-slurrying vessel 6 is further provided with an inlet for water F, preferably demineralized water and / or an aqueous solution of a salt for instance. In the re-slurrying vessel 6, a BHET seed crystal slurry is prepared by re-slurrying an amount of the dried BHET monomer crystals Id entering the re-slurrying vessel 6 in the added water F. As already disclosed above, the re-slurrying vessel 6 may thereto be provided with suitable mixing equipment, such as one or more stirrers. In a continuous operation, a continuous stream or an intermittent amount of the BHET seed crystal slurry scs is entered into the BHET crystallization unit 3, preferably at a bottom part thereof (also referred to as ‘heel seeding’). Even in continuous operation, it is also possible to add an initial amount of the BHET seed crystal slurry scs to the crystallization unit 3 once. Such seeding initiates and steers the BHET crystallization in the crystallization unit 3 to obtain BHET crystals with favourable morphology. The favourable morphology appears as a reduced filtration time in the subsequent separator unit 4. A suitable separator unit 4 may for instance be embodied as a filtration unit.

[0155] As with the embodiment of figure 2, it should be noted that the processing unit 5, shown in figure 3, is not essential to the invention, and may be omitted. In such an embodiment, the recycle loop for the carrier liquid Y (such as ethylene glycol for instance) is also omitted. Omitting the processing unit 5 and the recycle loop for the carrier liquid Y effectively turns the reactor system 10 into a batch process.

[0156] By means of the reactor system disclosed in figure 3, it has turned out feasible to carry out a method of forming BHET seed crystals and arrive at BHET monomer crystals If having the desired morphology, even when a relatively high amount of impurities, such as BHEET, is present during crystallization, and, surprisingly, using at least a part Id of the produced BHET monomer crystals I as a source for seeding.

[0157] Now turning to figure 4, a reactor system 20 for use in a method of preparing BHET seed crystals according to an embodiment of the second aspect is schematically illustrated. An embodiment of a reactor system 20 able to produce such BHET seed crystals comprises a container 19 for a mixture of BHET monomers and oligomers thereof, the carrier liquid, the impurities, such as BHEET, and the other ingredients. In one embodiment, the concentration of the impurities in the mixture is below 10 wt.% relative to the total weight of the BHET monomers in the mixture. In an improved embodiment that may achieve this, the mixture S is pumped to a purifying unit 18, preferably an active coal column. It is noted that the symbol S in figures 4 and 5 is not necessarily equal to the solution S used in figures 1-3. While the purifying unit 18 is shown as one unit, it is not excluded that this unit 18 may comprise several purifying units, such as active coal columns, provided in series. Alternative to the purifying unit 18, active coal may be added to the seeding solution S and separated therefrom, for instance by centrifugation.

[0158] Due to contacting the mixture S with active coal, a purified solution Sp is obtained which is transferred to a BHET crystallization unit 13 in which BHET is crystallized and subsequently recovered in a separator unit 14 as solid BHET monomer seed crystals sc. Rather than or in addition to lowering the temperature relative to the separation unit 2, an anti-solvent such as water E may be added to the purified solution Sp in the crystallization stage 13, as indicated in figure 4 by means of the arrow E in order to obtain a mixture consisting of BHET monomers and oligomers thereof, the carrier liquid, the impurities and at most 10 wt.%, relative to the total weight of the mixture, of the further ingredients, wherein the carrier liquid consists of at least 80 wt.% of a water / ethylene glycol mixture and at most 20 wt.% of other liquids, wherein the water / ethylene glycol weight ratio is from 20:80 to 60:40. Adjusting the solution S may be carried out by adding water through the inlet for water E of the crystallization unit 13, or upstream of the crystallization unit 13 (not shown). Upon crystallization of the BHET, the purified solution Sp is transformed into a mother liquor M that comprises solid BHET and may comprise the impurities albeit at a concentration below 10 wt.% relative to the total weight of the BHET monomers. The mother liquor M then enters a solid / liquid separation unit 14 in which the solid BHET monomer seed crystals sc are separated from the mother liquor M. The remaining mother liquor Ml may optionally be led to a processing unit 15, which preferably includes at least one distillation column. In the processing unit 15, the mother liquor Ml may be processed to reduce its water content. The BHEET content in the mother liquor Ml may also be reduced through a BHEET purge Z, if desired. The resulting purified carrier liquid may then be used for other purposes, such as preparing the mixture to enter container 19.

[0159] Referring to figure 5, BHET seed crystals may also be prepared by a reactor system 20 which provides a mixture of BHET monomers, a carrier liquid such as ethylene glycol, and impurities wherein the mixture originates from depolymerization of a terephthalate polymer. The reactor system 20 is similar to the one of figure 1 and therefore explicit reference is made to the description of figure 1. The reactor system 20 differs from the one of figure 1 in that the solution S is pumped to a purifying unit 18, preferably an active coal column. While the purifying unit 18 is shown as one unit, it is not excluded that this unit 18 may comprise several purifying units, such as active coal columns, provided in series. Alternative to the purifying unit 18, active coal may be added to the seeding solution S and separated therefrom, for instance by centrifugation.

[0160] Due to contacting the mixture S with active coal, a purified solution Sp is obtained which is transferred to a BHET crystallization unit 13 in which BHET is crystallized and subsequently recovered in a separator unit 14 as solid BHET monomer seed crystals sc. Rather than or in addition to lowering the temperature relative to the separation unit 2, an anti-solvent such as water E may be added to the purified solution Sp in the crystallization stage 13, as indicated in the figure by means of the arrow E.. Upon crystallization of the BHET, the purified solution Sp is transformed into a mother liquor M that comprises solid BHET, as well as the impurities albeit at a concentration below 10 wt.% relative to the total weight of the BHET monomers. The mother liquor M then enters a solid / liquid separation unit 14 in which the solid BHET monomer seed crystals sc are separated from the mother liquor M. The remaining mother liquor Ml is then led to a processing unit 15, which preferably includes at least one distillation column. In the processing unit 15, the mother liquor Ml may be processed to reduce its water content. The BHEET content in the mother liquor Ml may also be reduced through a BHEET purge Z, if desired. The resulting purified carrier liquid may then be returned to the depolymerization reactor 1 as recycle stream Y.

[0161] EXAMPLES

[0162] Methods

[0163] IR spectroscopy was carried out with a Thermo Fischer Nicolet ISS FTIR with ATR iD7, using OMNIC software. A background spectrum was taken with air on the ATR crystal. Then a few milligrams of a sample were placed on the ATR crystal and pushed down with a rotating disk. A number of 32 spectra was taken at a resolution of 4 cm1and averaged for each sample (and background). The rotating disk was then removed, and the ATR crystal was cleaned with acetone and wiped dry.

[0164] X-ray diffraction was used to determine a sample's crystalline structure. XRD was carried out with a Bruker D2 Phaser. Powdered XRD samples were prepared by crushing BHET samples with a mortar and pestle. Proper sample preparation is important and was designed to achieve a sufficient randomness of crystallite orientations, a sufficient number of crystallites to get a representative intensity distribution for the sample, and sufficient diffraction intensity to meet satisfy counting statistics.

[0165] These criteria were met by grounding the samples to obtain a fine powder having particle sizes of at most 44 microns.

[0166] Each sample was then transferred to a PMMA sample holder and gently packed into the 25 mm cavity of the holder of the Bruker D2 Phaser using a microscopy glass, taking care to pack the sample flat. The XRD measurement was performed with the sample continuously rotating while scanning the 20 angle stepwise over the desired values. The laser source used was a copper filament. According to known procedures, the angle of diffraction 0 can be used to determine the difference between atomic planes using Bragg’ s law, sin 0 = nZ / 2d where I (lambda) is the wavelength of the incident X-ray beam, and d is the distance between atomic planes. The distance between atomic plates can then be used to determine crystalline structure.

[0167] The result of XRD plots the intensity of the signal for various angles of diffraction at their respective two theta (20) positions. The two theta positions correspond to a certain spacing between the crystals or atoms in the samples, determined by the angle of diffraction from the incident x-ray beam sent into the sample. The intensity of the peaks relates to the amount of molecules with that spacing. The width of the peaks is said to be inversely proportional to the crystal size. A thinner peak may correspond to a bigger crystal, while a broader peak may be indicative of a smaller crystal, a defect in the crystalline structure, or the sample being amorphous. The patterns determined using XRD analysis may be used to determine a sample’s crystal morphology, for instance by comparing with data from the literature or from a database of crystal morphologies, elements, compounds, and minerals that contain the diffraction patterns for elements, compounds, and minerals.

[0168] Microscopy was carried out with a Bresser Erudit DLX transmission-type microscope. This microscope has a standard magnification of lOx, with an additional magnification of 4x for a total magnification of 40x. A drop of sample slurry was placed on a microscope slide with a pipette and covered with a cover plate. Filtration time was determined using a BHS pocket leaf filter. The device was set up using 3 bar overpressure, with a flow through the empty device of 2400 Nl / hour. Approximately 90-110 grams of a sample slurry was loaded, the device was then pressurized, and a tap was opened while a timer was started. The filtration time was recorded when the top of the filter cake felt dry. This filtration was carried out at room temperature.

[0169] A second method for determining filtration time used a sample slurry provided in a Buchner funnel setup, using filtration paper with openings of 12-15 pm. This filtration was carried out at room temperature.

[0170] Gas chromatography (GC) was used for quantifying EG and DEG in dry BHET.

[0171] A sample was prepared as follows. 100 mg dry BHET was weighed in a 10 ml flask. After this, the flask was filled with acetonitrile to the mark on the flask. The whole was then placed in an ultrasonic bath for 30 minutes to dissolve the BHET (this was checked visually).

[0172] The Equipment used was an Agilent 8860, equipped with a DB wax Agilent 122-7032UI column, using nitrogen as carrier gas.

[0173] The settings were as follows:

[0174] Injection: split - 1:40

[0175] Injection temperature: 200°C

[0176] Injection volume: 1.0 pl

[0177] Oven temp.: 1 minute at 80°C , 80- 200 °C ramp 25°C / min, 5 minutes at 200°C

[0178] Detector: FID Detector temp.: 300 °C

[0179] The amount of EG and DEG that is present in the sample was calculated based on the calibration line for EG and DEG.

[0180] High-performance liquid chromatography (HPLC) was used for quantifying BHET, dimer, BHEET, iso-BHET, and MHET.

[0181] A sample was prepared as follows. 25 mg dry BHET is weighed in a 50 mL flask. After this, the flask is filled with acetonitrile to the mark on the flask. The whole is then placed in an ultrasonic bath for 30 minutes to dissolve the BHET (this was checked visually). The equipment used was an Agilent Technologies 1200 series instrument, employing an XBridge C8 3.5 pm, 4.6x150 mm column. The mobile phase was

[0182] A: Water and 0.05% Formic Acid (A)

[0183] B: Acetonitrile and 0.05% Formic Acid (B)

[0184] The gradient was as follows:

[0185] 0 min = 80% (A) & 20% (B)

[0186] 3 min = 80% (A) & 20% (B)

[0187] 8 min = 0% (A) & 100% (B)

[0188] 11 min = 80% (A) & 20% (B)

[0189] 12 min = 80% (A) & 20% (B)

[0190] The injection volume was 2.0 pl, and the oven temperature was set at 45°C.

[0191] The detector used was a Diode Array Detector (DAD). The DAD wavelength was 242 nm (180- 640 nm).

[0192] The amount of a component that is present in the sample is calculated based on the calibration line for the component concerned.

[0193] Karl Fischer volumetric method (KF) was used for quantifying H2O.

[0194] Sample preparation was as follows: 100- 200 mg of dry BHET was added to a KF solvent in a reaction vessel of the 870 KF Titrino plus and dissolved by stirring.

[0195] The equipment used was a 870 KF Titrino plus, Metrohm.

[0196] Liquids used were Titrant 2 (Methanol + Iodine) and KF solvent (50 %vol Hydranal + 50%vol IPA).

[0197] After the BHET was dissolved in the KF solvent, the solution was titrated with the Titrant 2 solution. Based on the equivalence point, the composition of the Titrant 2 solution and the amount of BHET that was weighed in, the % of H2O in the sample could be calculated according to well- known procedures.

[0198] Example 1

[0199] In a first experiment, a single crystallisation was carried out to make BHET seed crystals. A first solution was prepared by mixing 200 g of a BHET / EG (ethylene glycol) mixture with 150 g water. A second clean solution was prepared by mixing 36 g pure BHET with 164.4 g EG. To ensure that the solutions are sufficiently pure, at least one of them is treated by means of contact with active coal. The first and second solution were prepared to allow mixing them in different proportions to obtain dissolved mixtures with different purity. Purity here relates to the amount of impurities, relative to the amount of BHET in the dissolved mixtures. A high purity BHET mixture has a relatively low amount of impurities, while a low purity BHET mixture has a relatively high amount of impurities.

[0200] A mixture was then prepared of 226.8 g of the first solution, 150 g of the second clean solution and 127.5 g of water at a temperature of 60°C as set with a heating bath to dissolve the BHET. The ingredients of the composition were hereto added to a 500 ml flask, provided with a lid. Subsequently, an overhead stirrer was provided on the flask. The flask was placed on a heating plate and heated to approximately 90 °C, so as to dissolve all solid material (i.e. the BHET). The closed flask was then placed in the heating bath that was prepared to have the temperature of 60°C. The dissolved mixture was then transferred to a crystallisation vat kept at a temperature of 60°C. The crystallisation vat was then subject to a cooling curve involving natural cooling to 44°C, a controlled cooling at a rate of 6°C / h to 28°C, a further controlled cooling at a rate of 16°C / h to 20°C, and a stay at 20°C for 30 minutes. This cooling curve is given as an example only, and other cooling curves may also produce satisfactory results. The crystallisation was performed under stirring at a tip speed of 0.5 m / sec.

[0201] A sample of 75 g was taken from the cooled mother liquor (ML I) containing the BHET crystals. The sample of mother liquor containing the BHET crystals was then filtrated over a Buchner funnel setup, using filtration paper with openings of 12-15 pm. This filtration was carried out at room temperature. The resulting BHET filtrate appeared as a white cake weighing 8 g. The cake was washed with demineralized water and dried. The filtration time was 10 sec.

[0202] The composition of the ML and the crystallized filtrate was then determined by HPLC / KF / GC and is shown in Table 1. The amounts of all ingredients sum up to about 100 wt.%,.

[0203] Table 1 - composition of ML and filtrate

[0204] Because the solutions used in preparing the mixture to be crystallized are sufficiently pure, the ML typically contains impurities including iso-BHET (i.e. BHEI), MHET and BHEET (a monomer including more ethylene glycol units than BHET) of about 4.4 wt.%, relative to the amount of BHET monomers, which is below the impurity concentration of below 10 wt.% relative to the weight of the BHET monomer, in accordance with the second aspect of the invention. Moreover, the amount of BHEET is lower than 3 wt.%, relative to the amount of BHET monomer. The impurities may for instance arise from side reactions in depolymerising PET. If the mixture to be crystallized is too high in impurity concentration, the mixture may be contacted with active coal according to some embodiments to reduce the impurity concentration to the desired level.

[0205] The morphology obtained by microscopy of the obtained BHET crystals is shown in figure 6. The formed BHET seed crystals substantially exhibit a needle morphology.

[0206] The first example shows that BHET crystals of the desired morphology may be obtained from a relatively clean solution, containing less than 10 wt.% of impurities in total, more preferably less than 5 wt.% relative to the amount of BHET monomer.

[0207] The relatively low filtration time observed during filtration of the mother liquor shows that the formed BHET seed crystals are advantageous.

[0208] A BHET seed crystal slurry was prepared by re-slurrying 8 g of the BHET seed crystals in water by adding 32 g of demineralized water to obtain a BHET seed crystal slurry. The BHET seed crystal slurry was stored for future use, as described hereinbelow.

[0209] Example 2 The BHET seed crystal slurry prepared in Example 1 was used to seed BHET crystallisation in mixtures of BHET monomers and oligomers thereof, ethylene glycol (EG) as carrier liquid and impurities in a higher concentration than in Example 1 (fouled solutions).

[0210] In this second example, a mixture comprising 9.8 g of BHET, 1 g of BHEET, 34 g of demineralized water and 47 g of ethylene glycol (EG) was prepared by depolymerisation of PET by means of catalysed glycolysis and subsequent removal of oligomers and catalyst by addition of water and a centrifuge treatment. Use was made of the catalyst specified in WO2017111602, which is included herein by reference. However, the use of alternative catalysts is not excluded. It typically also comprises other depolymerisation products from PET in small amounts, including BHET trimer, potentially BHET tetramer (in lesser quantities), isoBHET and an amount of 1 g of BHEET of the mixture. The mixture was diluted further with 10 g of demineralized water in a ratio of 65 / 10.

[0211] The diluted mixture in an amount of 75 g was added to a crystallization vat at a temperature of 60°C as set with a heating bath to dissolve the BHET. The crystallisation vat was then subject to a cooling curve involving natural cooling to 44°C, a controlled cooling at a rate of 6°C / h to 28°C, a further controlled cooling at a rate of 16°C / h to 20°C, and a stay at 20°C for 30 minutes. Again, this cooling curve is given as an example only, and other cooling curves may also produce satisfactory results. When the mixture reached a temperature of 40°C, 2 g of the BHET seed crystal slurry prepared in Example 1 was added to the mixture (500 g) and the mixture was allowed to crystallize during cooling.

[0212] A sample of 75 g was taken from the cooled mother liquor II (ML II) containing BHET crystals. The sample of mother liquor containing the BHET crystals was then filtrated over a Buchner funnel setup, using filtration paper with openings of 12-15 pm. This filtration was carried out at room temperature. The resulting BHET appeared as a white cake weighing 8 g. The cake was washed with demineralized water and dried. The filtration time was 11 sec.

[0213] The composition of the ML II and the crystallized filtrate was then determined by HPLC / KF / GC and is shown in Table 2. The amounts of all ingredients sum up to about 100 wt.%.

[0214] Table 2 - composition of ML II and filtrate

[0215] The morphology obtained by microscopy of the obtained BHET crystals is shown in figure 7. From this, a nice needle like morphology may be observed, giving rise to good filtration properties.

[0216] The second example shows that BHET crystals of the desired morphology may also be obtained from a slightly fouled solution, containing 7.4 wt.% of impurities relative to the amount of BHET monomer. This advantageous effect may be attributed to the use of the BHET seed crystals, as claimed.

[0217] The relatively low filtration time observed during filtration of the mother liquor shows that the formed BHET seed crystals are advantageous.

[0218] A BHET crystal slurry was prepared by re-slurrying 8 g of the BHET crystals obtained in Example 2 in water by adding 32 g of demineralized water to obtain a second BHET crystal slurry. The second BHET seed crystal slurry was stored for future use, as described hereinbelow.

[0219] Example 3

[0220] The second BHET crystal slurry prepared in Example 2 was then used to seed BHET crystallisation in mixtures of BHET monomers and oligomers thereof, ethylene glycol (EG) as carrier liquid and impurities in a higher concentration than in Example 1 (fouled solutions).

[0221] In this third example, the same mixture prepared by depolymerisation of PET by means of catalysed glycolysis, as described in Example 2, was diluted further with 10 g of demineralized water in a ratio of 65 / 10. The diluted mixture in an amount of 75 g was added to a crystallization vat at a temperature of 60°C as set with a heating bath to dissolve the BHET. The crystallisation vat was then subject to the same cooling curve as in Example 2. When the mixture reached a temperature of 40°C, the second BHET crystal slurry prepared in Example 2 was added to the mixture and the mixture was allowed to crystallize during cooling. A sample of 75 g was taken from the cooled mother liquor III (ML III) containing the BHET crystals. The sample of mother liquor containing the BHET crystals was then filtrated over a Buchner funnel setup, using filtration paper with openings of 12-15 pm. This filtration was carried out at room temperature. The resulting BHET appeared as a white cake weighing 8 g. The cake was washed with demineralized water and dried. The filtration time was 9.7 sec.

[0222] The composition of the ML III and the crystallized material was then determined by HPLC / KF / GC and is shown in Table 3. The amounts of all ingredients sum up to about 100 wt.%.

[0223] Table 3 - composition of ML III and filtrate

[0224] The morphology obtained by microscopy of the obtained BHET crystals is shown in figure 8. Even though the BHET crystals were prepared from a solution with a relatively high amount of impurities, the observed morphology still gives rise to low filtration times. Although an increased amount of smaller crystals are observed, they still have the desired needle like morphology.

[0225] The third example shows that BHET crystals of the desired morphology may also be obtained from a slightly more fouled solution, containing about 10.2 wt.% of impurities relative to the amount of BHET monomer. Surprisingly, this effect also occurs when using the BHET crystals obtained in Example 2 as BHET seed crystals. This advantageous effect may be attributed to the initial use of the BHET seed crystals (Examples 1 and 2), as claimed. It appears that the beneficial properties, such as morphology, of the invented BHET seed crystals is prolonged or continued when using BHET crystals that were formed by seeding their crystallization with the invented BHET seed crystals. In the case of a continuous process therefore, adding the invented BHET seed crystals at the start-up of the crystallization operation is sufficient to produce the desired BHET crystal morphology. Another beneficial consequence is that, once the crystallization process is properly started (with the invented BHET seed crystals) subsequently produced BHET crystals may be used as BHET seed crystals in future crystallization. This means that BHET crystals having the desired morphology, as claimed, may be taken from the crystallisation process to generate a BHET seed crystal generation loop, once properly started. The relatively low filtration time observed during filtration of the mother liquor shows that the formed BHET seed crystals are advantageous.

[0226] A BHET crystal slurry was prepared by re-slurrying 8 g of the BHET crystals obtained in Example 3 in water by adding 32 g of demineralized water to obtain a third BHET crystal slurry. The third BHET seed crystal slurry was stored for future use, as described hereinbelow.

[0227] Example 4

[0228] The third BHET crystal slurry prepared in Example 3 was then used to seed BHET crystallisation in mixtures of BHET monomers and oligomers thereof, ethylene glycol (EG) as carrier liquid and impurities in a higher concentration than in Example 1 (fouled solutions).

[0229] In this fourth example, the same mixture prepared by depolymerisation of PET by means of catalysed glycolysis, as described in Example 2, was diluted further with 10 g of demineralized water in a ratio of 65 / 10. The diluted mixture in an amount of 75 g was added to a crystallization vat at a temperature of 60°C as set with a heating bath to dissolve the BHET. The crystallisation vat was then subject to the same cooling curve as in Example 2. When the mixture reached a temperature of 40°C, the third BHET crystal slurry prepared in Example 3 was added to the mixture and the mixture was allowed to crystallize during cooling.

[0230] A sample of 75 g was taken from the cooled mother liquor IV (ML IV) containing the BHET crystals. The sample of mother liquor containing the BHET crystals was then filtrated at room temperature over a Buchner funnel setup, using filtration paper with openings of 12-15 pm. The resulting BHET appeared as a white cake weighing 8 g. The cake was washed with demineralized water and dried. The filtration time was 9.5 sec.

[0231] The composition of the ML IV and the crystallized material was then determined by HPLC / KF / GC and is shown in Table 4. The amounts of all ingredients sum up to about 100 wt.%.

[0232] Table 4 - composition of ML IV and filtrate The morphology obtained by microscopy of the obtained BHET crystals is shown in figure 9. The desired needle like morphology is observed, although smaller crystals increasingly appear.

[0233] The fourth example shows that BHET crystals of the desired morphology may also be obtained from a slightly more fouled solution, containing about 11 wt.% of impurities, relative to the amount of BHET monomer. Surprisingly, this effect also occurs when using the BHET crystals obtained in Example 3 as BHET seed crystals. This advantageous effect may be attributed to the initial use of the BHET seed crystals (Examples 1 and 2), as claimed.

[0234] The relatively low filtration time observed during filtration of the mother liquor shows that the formed BHET seed crystals are advantageous.

[0235] A BHET seed crystal slurry was prepared by re-slurrying 8 g of the formed BHET crystals in water by adding 32 g of demineralized water to obtain a fourth BHET crystal slurry. The fourth BHET crystal slurry was stored for future use, as described hereinbelow.

[0236] Example 5

[0237] The fourth BHET crystal slurry prepared in Example 4 was then used to seed BHET crystallisation in mixtures of BHET monomers and oligomers thereof, ethylene glycol (EG) as carrier liquid and impurities in a higher concentration than in Example 1 (fouled solutions).

[0238] In this fourth example, the same mixture prepared by depolymerisation of PET by means of catalysed glycolysis, as described in Example 2, was diluted further with 10 g of demineralized water in a ratio of 65 / 10. The diluted mixture in an amount of 75 g was added to a crystallization vat at a temperature of 60°C as set with a heating bath to dissolve the BHET. The crystallisation vat was then subject to the same cooling curve as in Example 2. When the mixture reached a temperature of 40°C, the fourth BHET crystal slurry prepared in Example 4 was added to the mixture and the mixture was allowed to crystallize during cooling.

[0239] A sample of 75 g was taken from the cooled mother liquor V (ML V) containing the BHET crystals. The sample of mother liquor containing the BHET crystals was then filtrated at room temperature over a Buchner funnel setup, using filtration paper with openings of 12-15 pm. The resulting BHET appeared as a white cake weighing 8 g. The cake was washed with demineralized water and dried. The filtration time was about 10 sec. The composition of the ML V and the crystallized material was then determined by HPLC / KF / GC and is shown in Table 5.

[0240] Table 5 - composition of ML V and filtrate - nd=not determined

[0241] The morphology obtained by microscopy of the obtained BHET crystals is shown in figure 10. Still, the desired needle like morphology is apparent. Apart from the smaller crystals, also some clustering of crystals seems to appear. Yet, filtration times are still low.

[0242] The fifth example shows that BHET crystals of the desired morphology may also be obtained from a rather strongly fouled solution, containing about 15 wt.% of impurities relative to the amount of BHET monomer. Surprisingly, this effect also occurs when using the BHET crystals obtained in Example 4 as BHET seed crystals. This advantageous effect may be attributed to the initial use of the BHET seed crystals (Examples 1 and 2), as claimed.

[0243] The relatively low filtration time observed during filtration of the mother liquor shows that the formed BHET seed crystals are advantageous.

[0244] Comparative Experiment A

[0245] In this comparative experiment, the same mixture prepared by depolymerisation of PET by means of catalysed glycolysis, as described in Example 5, was diluted further with 10 g of demineralized water in a ratio of 65 / 10. The diluted mixture in an amount of 75 g was added to a crystallization vat at a temperature of 60°C as set with a heating bath to dissolve the BHET. The crystallisation vat was then subject to the same cooling curve as in Example 5. The mixture was then allowed to crystallize during cooling without adding any BHET seed crystals.

[0246] A sample of 75 g was taken from the cooled mother liquor containing the BHET crystals. The sample of mother liquor containing the BHET crystals was then filtrated at room temperature over a Buchner funnel setup, using filtration paper with openings of 12-15 pm. The resulting BHET appeared as a white cake weighing 8 g. The cake was washed with demineralized water and dried.

[0247] The observed filtration time was about 22 sec, and the mother liquor was very sticky which hindered the filtration process to a substantial extent.

[0248] A BHET crystal slurry prepared in Example 4 was then used to seed BHET crystallisation in mixtures of BHET monomers and oligomers thereof, ethylene glycol (EG) as carrier liquid and impurities in a higher concentration than 3 wt.% relative to the total weight of the mixture (fouled solutions).

[0249] In this comparative experiment, a single crystallisation was carried out to make BHET seed crystals according to the state-of-the-art. The same mixture prepared by depolymerisation of PET by means of catalysed glycolysis, as described in Example 2, was diluted with 10 g of demineralized water in a ratio of 65 / 10. The diluted mixture in an amount of 75 g was added to a crystallization vat at a temperature of 60°C as set with a heating bath to dissolve the BHET. The crystallisation vat was then subject to the same cooling curve as in Example 2, and the mixture was allowed to crystallize during cooling. Please note that the mixture from which the known BHET seed crystals were made contains a relatively high amount of impurities.

[0250] A sample of 75 g was taken from the cooled mother liquor containing the BHET seed crystals. The sample of mother liquor containing the BHET crystals was then filtrated at room temperature over a Buchner funnel setup, using filtration paper with openings of 12-15 pm. The resulting BHET appeared as a white cake weighing 8 g. The cake was washed with demineralized water and dried. The filtration time was more than 25 sec.

[0251] The observed filtration time of undesirable length hindered the filtration process to a substantial extent.

[0252] Table 6 summarizes the filtration times obtained with the different mother liquors as a function of BHEET concentration (acting as a measure of the level of contamination in the mother liquors). It shows that using the invented BHET seed crystals or BHET crystals that were formed using the BHET seed crystals to crystallise BHET yields excellent filtration times, independent of the amount of contamination (for instance BHEET). This is surprising since one would expect a worse crystallization with increasing amount of contamination. From comparative examples A and B can be inferred that the known seed BHET crystals do not have this beneficial effect. Indeed, filtration times are more than doubled, which is indicative of a poor crystallization.

[0253] Table 6 - filtration time in dependency of BHEET concentration.

[0254] Example 6

[0255] The BHET seed crystal slurry prepared in Example 1 was used to seed BHET crystallisation in mixtures of BHET monomers and oligomers thereof, ethylene glycol (EG) as carrier liquid and impurities in a higher concentration than that of Example 1 (fouled solutions).

[0256] In this sixth example, the same mixture prepared by depolymerisation of PET by means of catalysed glycolysis, as described in Example 2, was added to a crystallization vat in an amount of 75 g at a temperature of 60°C as set with a heating bath to dissolve the BHET. The crystallisation vat was then subject to the same cooling curve as in Example 2 while stirring with an overhead stirrer operated at a tip speed of 0.5 m / sec. When the mixture reached a temperature of 40°C, the BHET seed crystal slurry prepared in Example 1 was added to the mixture and the mixture was allowed to crystallize during cooling.

[0257] A sample of 75 g was taken from the cooled mother liquor VI (ML VI) containing BHET crystals. The composition of the ML VI was then determined by HPLC / KF / GC and is shown in Table 7. The total amounts sum up to 100 wt.%. The amounts of BHET and the impurities are given as wt.% of the total amount of the mixture.

[0258] Table 7 - composition of ML VI

[0259] The sample of mother liquor containing the BHET crystals was then filtrated using a BHS pocket leaf filter. The device was set up using 3 bar overpressure, with a flow through the empty device of 2400 Nl / hour. Approximately 90-110 grams of a sample slurry was loaded, the device was then pressurized, and a tap was opened while a timer was started. The filtration time was recorded when the top of the filter cake felt dry. This filtration was carried out at room temperature. The resulting cake was washed with demineralized water and dried.

[0260] The filtration measurements were repeated 5 times. The resulting filtration times are given in Table 8.

[0261] Table 8 - filtration times

[0262] The X-ray diffraction pattern of the obtained BHET crystals is shown in figure 11 and the characteristic values thereof are tabulated in Table 10. The IR spectrum is shown in figure 12 and the characteristic values thereof are tabulated in Table 11.

[0263] The sixth example shows that BHET crystals of the desired morphology may also be obtained from a slightly fouled solution, containing about 11.6 wt.% of impurities relative to the amount of BHET monomer. This advantageous effect may be attributed to the use of the BHET seed crystals, as claimed.

[0264] The relatively low filtration time observed during filtration of the mother liquor shows that the formed BHET seed crystals are advantageous.

[0265] Comparative Experiment C

[0266] In this comparative experiment, the same mixture prepared by depolymerisation of PET by means of catalysed glycolysis, as described in Example 2, was added to a crystallization vat in an amount of 75 g at a temperature of 60°C as set with a heating bath to dissolve the BHET. The crystallisation vat was then subject to the same cooling curve as in Example 2. The mixture was allowed to crystallize during cooling without adding any seeding crystals.

[0267] A sample of 75 g was taken from the cooled mother liquor containing BHET crystals and filtrated using a BHS pocket leaf filter, according to the procedure described in the Methods section and in Example 6. The resulting cake was washed with demineralized water and dried.

[0268] The filtration measurements were repeated 3 times. The resulting filtration times are given in Table 9.

[0269] Table 9

[0270] The observed filtration times are more than 5 times longer than obtained by the invention.

[0271] The X-ray diffraction pattern of the obtained BHET crystals is shown in figure 13, while the IR spectrum is shown in figure 14.

[0272] Comparison of the BHET crystals obtained in Example 6 and in Comparative Experiment C In figure 15, the X-ray diffraction pattern of Example 6 and Comparative Experiment C are shown in combination to clarify the differences. Figure 16 shows the IR spectrum of Example 6 and Comparative Experiment C. The characteristic 20 values of the XRD spectra of the BHET crystals of Example 6 and those of Comparative Experiment C are tabulated in Table 10. Table 11 tabulates the characteristic wavenumbers X of the IR spectrum of the BHET crystals of Example 6 and those of Comparative Experiment C. Table 10: Characteristic 20 values of XRD spectra for polymorphs a and according to the invention, respectably. invention, respectably, in cm-1 The characteristic 20 values of the XRD spectra of the BHET crystals in Table 10 correspond to intensity (in arbitrary units = a.u.) peaks of the XRD spectra exceeding 100. Alternatively, the highest intensity peaks are considered, for instance the 6 highest intensity peaks.

[0273] The characteristic wavenumbers I of the IR spectrum of the BHET crystals in Table 11 correspond to the highest absorbance peaks of the XRD spectra. The observed IR spectra may be used to identify structures because functional groups give rise to characteristic bands both in terms of intensity and position (wave number or frequency). These functional groups are shown in Table 11 between brackets.

[0274] The XRD and IR data clearly show two different spectra, and therefore two different polymorphs of the produced BHET crystals.

[0275] The unseeded BHET crystals obtained in Comparative Experiment C (also denoted in figures 11- 16 as VSS037-01-07) can be matched to a known XRD spectrum and may be identified as the BHET a polymorph. This is confirmed by comparing the IR spectrum with a known spectrum. It has been shown by the inventors that the a polymorph forms in a BHET solution under the influence of impurities, and yields poorly filterable crystal forms, in particular when BHET dimers are also present.

[0276] The seeded BHET crystals obtained in Example 6 (also denoted in figures 11-16 as VSS037-01- 06) cannot be convincingly matched to a known XRD spectrum, nor to a known IR spectrum. The differences in XRD spectrum between the known BHET crystals and the BHET crystals according to the invention are however clearly apparent from Table 10 and figure 15. The differences in IR spectrum are demonstrated by Table 11 and figure 16 and may be even made more apparent by considering figures 17 and 18 which show detailed views of the IR spectrum of figure 16.

[0277] Focusing on the wavenumber area around 1700 cm'1, figure 17 shows that the known BHET crystals (the a polymorph) have two peaks. A first peak at -1720 cm1may be attributed to unbound C=O, whereas a second peak at -1695 cm1is known to indicate a C=O involved in hydrogen bonding. In contrast herewith, the BHET crystals seeded according to the invention do not exhibit the split peak but instead has one peak at -1720 cm'1. This may indicate that the C=O bond is not involved in hydrogen bonding. Another difference is clearly shown by figure 18 which focusses on the wavenumber area around 850-920 cm'1. The BHET crystals seeded according to the invention have peaks at 897cm1and -870cm1, whereas the known a polymorph has peaks at 860, 875, 898, and 910 cm1. It is further noted that the peak corresponding to the characteristic wavenumber at about 910 cm1is absent in the IR spectrum of the BHET crystals of Example 6.

[0278] It has been shown that the desired polymorph according to the invention forms in relatively clean BHET solutions with a low amount or even no impurities at all. This polymorph was shown to form filterable needle like crystals. It has further been established that this polymorph is stable enough not to transform into the a polymorph by itself. Seeding thus seems a feasible method for the generation of better filterable crystals having the desired polymorph. Indeed, am improvement of filterability by a factor 5 has been observed.

[0279] The BHET crystals formed with the desired morphology according to the invention, as exemplified by the spectra of figures 11 and 12, may thus serve as original parent crystals to all subsequent crystals. The subsequent crystals will also exhibit the desired low filtration times.

[0280] During crystallization, it may be desirable to not emptying the crystallizer completely (to retain the claimed seeding crystals) and further to fill the BHET solution at a temperature 15 to 1°C below the solubility temperature of the BHET monomers, for instance at 40°C, to prevent a situation where all BHET crystals having the desired polymorph would dissolve. The solubility (or dissolution) temperature of the BHET monomer crystals varies with BHET and impurities concentration, and solvent ratio for instance, and in some embodiments may range from 46 to 52°C. As a result, there are BHET crystals of the desired polymorph present for other BHET crystals to grow on, thereby hindering or even preventing nucleation of BHET crystals of the undesired a morphology.

[0281] It has further been shown that seeded BHET crystals having the desired morphology may be retained at least up to levels of 10 wt.% of impurities, and / or up to 7.5 wt% of BHEET, relative to the amount of BHET in the solution to be crystallized.

Claims

Claims1. A method of preparing bis (2 -hydroxyethyl) terephthalate (BHET) monomer crystals is provided, said method comprising the steps of:(a) providing a mixture consisting of BHET monomers and oligomers thereof, a carrier liquid, impurities and at most 10 wt.%, relative to the total weight of the mixture, of further ingredients, wherein the carrier liquid consists of at least 80 wt.% of a water / ethylene glycol mixture and at most 20 wt.% of other liquids, wherein the water / ethylene glycol weight ratio is from 20:80 to 60:40;(b) providing BHET seed crystals in the mixture;(c) forming BHET monomer crystals from the mixture comprising the BHET seed crystals, thereby obtaining BHET monomer crystals in a mother liquor; and(d) separating the BHET monomer crystals from the mother liquor; wherein the BHET seed crystals have at least one of a powder X-ray diffraction spectrum with at least one characteristic intensity peak at 20 chosen from about equal to 10.7°, 14.4°, 18.9° and 21.6°, where 0 is the angle of diffraction;-an IR spectrum with at least one characteristic absorption peak at I chosen from about equal to 3260 cm1and 897 cm1, where I is the ordinary wavenumber.

2. The method as claimed in claim 1, wherein the BHET seed crystals have the powder X-ray diffraction spectrum of Figure 11.

3. The method as claimed in claim 1 or 2, wherein the BHET seed crystals have the IR spectrum of Figure 12.

4. The method as claimed in any one of the preceding claims, wherein step (a) comprises bringing the mixture bringing the mixture from a temperature To of above 80°C to a temperature Ti of between 60 and 75 °C.

5. The method as claimed in any one of the preceding claims, wherein step (a) comprises cooling the mixture from the temperature Ti to a temperature T2 different from Ti of between 25 and 60°C at a rate of between 0-10°C / h. .

6. The method as claimed in any one of the preceding claims, wherein step (b) comprises providing the BHET seed crystals in the mixture at a temperature T3 of between 25 and 60°C.

7. The method as claimed in any one of the preceding claims, wherein step (c) comprises cooling the mixture further to a temperature T4 of between 15 and 25°C at a rate of between 2-20°C / h.

8. The method as claimed in any one of the preceding claims, wherein adding a crystallizing agent other than the BHET seed crystals to the mixture is excluded.

9. The method as claimed in any one of the preceding claims, wherein providing the BHET seed crystals in the mixture in step b) is embodied by keeping a part of the BHET monomer crystals formed in a previous crystallization step c) in a crystallization unit in which they were crystallized and maintain said part at a temperature below about 45 °C by cooling to avoid dissolving said part.

10. The method as claimed in any one of the preceding claims, wherein the method is carried out in batch operations, and the BHET monomer crystals separated from the mother liquor during a batch operation are provided as BHET seed crystals in a subsequent batch operation, wherein, optionally, the BHET monomer crystals separated from the mother liquor during the batch operation are re-slurried by mixing with a re-slurrying liquid, preferably water, and provided as the BHET seed crystals in the subsequent batch operation.

11. The method as claimed in any one of claims 1-9, wherein the method is carried out continuously in that it comprises the steps of:(a) continuously providing the mixture consisting of BHET monomers and oligomers thereof, the carrier liquid, the impurities, and at most 10 wt.% relative to the total weight of the mixture of the further ingredients, wherein the carrier liquid consists of at least 80 wt.% of a water / ethylene glycol mixture and at most 20 wt.% of other liquids, wherein the water / ethylene glycol weight ratio is from 20:80 to 60:40;(b) providing the BHET seed crystals in the mixture at least at start-up of the continuous method;(c) continuously forming BHET monomer crystals from the mixture comprising the BHET seed crystals, thereby obtaining BHET monomer crystals in a mother liquor; and(d) optionally, continuously separating the BHET monomer crystals from the mother liquor.

12. The method as claimed in claim 11, wherein a part of the BHET monomer crystals separated in step d) are stored in a collection vessel and provided as BHET seed crystals in step b).

13. The method as claimed in any one of the preceding claims, wherein the other liquids of the carrier liquid comprise alkanols, alkanediols, alkane triols or combinations thereof, methanol, diethylene glycol, propylene glycol, dipropylene glycol, 1 ,4-butanediol, 1,5- pentanediol, glycerol or combinations thereof.

14. The method as claimed in any one of the preceding claims, wherein the mixture consisting of BHET monomers and oligomers thereof, the carrier liquid, the impurities and the further ingredients results from a depolymerization of PET.

15. The method as claimed in any one of the preceding claims, wherein the BHET monomer crystals are recovered in solid form, and wherein preferably the method further comprises at least one of the steps of washing the BHET monomer crystals and drying the BHET monomer crystals.

16. The method as claimed in any one of the preceding claims, wherein the separating of the BHET monomer crystals from the mother liquor is performed by means of filtration.

17. A method of preparing BHET seed crystals having at least one of:- a powder X-ray diffraction spectrum of the BHET seed crystals at least has characteristic intensity peaks at 20 about equal to 10.7°, 14.4°, 18.9° and 21.6°, where 0 is the angle of diffraction, and- an IR spectrum of the BHET seed crystals at least has characteristic absorption peaks at I about equal to 3260 cm-1 and 897 cm-1, where I is the ordinary wavenumber, the method comprising the steps of: i. providing a mixture consisting of BHET monomers and oligomers thereof, a carrier liquid, impurities and at most 10 wt.%, relative to the total weight of the mixture, of further ingredients, wherein the carrier liquid consists of at least 80 wt.% of a water / ethylene glycol mixture and at most 20 wt.% of other liquids,wherein the water / ethylene glycol weight ratio is from 20:80 to 60:40, wherein further the concentration of impurities is below 10 wt.% relative to the total weight of the BHET monomers in the mixture; ii. forming BHET seed crystals from the mixture thereby obtaining BHET seed crystals and a seeded mother liquor; and iii. separating the BHET seeding crystals from the seeded mother liquor.

18. The method as claimed in claim 17, wherein at least the mixture is purified by contacting the mixture with an adsorption means such as active coal.

19. The method as claimed in claim 17 or 18, wherein step (i) comprises bringing the mixture from a temperature Tso above 80°C to a temperature Tsiof between 60 and 75°C, and cooling the mixture from the temperature Tsito a temperature TS2 different from Tsiof between 25 and 60°C at a rate of between 0-10°C / h.

20. The method as claimed in any one of claims 17-19, wherein step (ii) comprises providing the BHET seed crystals in the mixture at a temperature TS3 of between 25 and 60°C.

21. The method as claimed in any one of claims 17-20, wherein step (iii) comprises cooling the mixture further to a temperature TS4 of between 15 and 25 °C at a rate of between 2- 20°C / h.

22. The method as claimed in any one of claims 1-16, wherein the BHET seed crystals provided in step (b) are obtained by a method as claimed in any one of claims 17-21.

23. A method of depolymerising a terephthalate polymer, preferably PET, into reusable raw material comprising at least BHET monomers, the method comprising the steps of: a) depolymerizing the terephthalate polymer in a reactive solvent comprising at least 80 wt.% ethylene glycol and optionally water, and at most 20 wt.% of other solvents; b) forming BHET monomer crystals from the depolymerized mixture by a method as claimed in any one of claims 1-17 thereby obtaining BHET crystals in a mother liquor; c) separating the BHET crystals from the mother liquor.

24. The method as claimed in claim 23, wherein the depolymerizing step is carried out by means of a heterogeneous depolymerization catalyst comprising functionalized magnetic particles comprising a catalyst complex (ABC), wherein the catalyst complex comprises three distinguishable elements: (nano)particle (A), bridging moiety or linking group (B) chemically or physically attached to the particle, and catalyst entity (C) that is associated with the particles (A), such as by being chemically bonded, for instance covalently bonded, to the linking group.

25. A reactor system for depolymerising a terephthalate polymer into reusable raw material, said reactor system comprising: a depolymerization reactor comprising at least one inlet for a stream of terephthalate waste polymer and a stream of reactive solvent, wherein said depolymerization reactor is configured for depolymerizing the terephthalate waste polymer into a depolymerized mixture by using the reactive solvent; a BHET monomer crystallization unit fluidly connected to an outlet of the depolymerization reactor and arranged for forming BHET monomer crystals from the depolymerized mixture thereby obtaining BHET monomer crystals in a mother liquor; a separator fluidly connected to an outlet of the monomer crystallization unit and arranged for receiving the BHET monomer crystals and separating them from the mother liquor; and further a collection vessel fluidly connected to an inlet of the BHET monomer crystallization unit and arranged for providing BHET seed crystals to the BHET monomer crystallization unit.

26. The reactor system as claimed in claim 25, wherein the collection vessel is fluidly connected to an outlet of the monomer crystallization unit and arranged for receiving BHET crystals from the monomer crystallization unit.

27. BHET seed crystals having at least one of:(i) a powder X-ray diffraction spectrum with at least one characteristic peak at 20 chosen from about equal to 10.7°, 14.4°, 18.9° and 21.6°; and(ii) an IR spectrum with at least one characteristic absorption peak at I chosen from about equal to 3260 cm-1 and 897 cm-1, preferably for use in depolymerising a terephthalate polymer into reusable raw material.

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