Purification method for terephthalic acid diester monomer by adsorption

Abstract

The present invention relates to a method for purifying a crude diester monomer feed, the method comprising the steps of: a) mixing the crude diester monomer feed with an aqueous solvent at a temperature of 60-150°C to obtain an aqueous mixture of diester monomers, adjusting the amount of aqueous solvent injected so that the crude diester monomer feed accounts for 20-90% of the total weight of the aqueous mixture of diester monomers; b) adsorbing the aqueous mixture of diester monomers at a temperature of 60-150°C and a pressure of 0.1-1.0 MPa to obtain a purified monomer effluent.

Description

[Technical field]

[0001] The present invention relates to a process for purifying diester monomers, in particular terephthalic diester monomers, in particular bis(2-hydroxyethyl) terephthalate (BHET), by adsorption. More particularly, the present invention relates to a process for purifying a crude diester monomer feedstock, in particular a crude diester monomer feedstock comprising terephthalic diester monomers, especially bis(2-hydroxyethyl) terephthalate (BHET), by adsorption of a mixture of said feedstock with an aqueous solvent with at least one adsorbent to obtain a purified and decolorized diester monomer effluent. The crude diester monomer feedstock may for example be obtained by depolymerization of a polyester feedstock, in particular consisting of polyester waste and post-consumer plastics. [Background technology]

[0002] Chemical recycling of polyesters, and in particular polyethylene terephthalate (PET), has been the subject of numerous studies aimed at breaking down polyesters recovered in waste form into monomers that could be used again as feedstock for polymerization processes.

[0003] A large number of polyesters are obtained from networks for collecting and sorting materials. In particular, polyester, and in particular PET, may originate from the collection of bottles, container trays, films, resins and / or fibres (e.g. textile fibres, tyre fibres) made of polyester. Polyesters obtained from collection and sorting channels are called polyesters eligible for recycling.

[0004] PET eligible for recycling can be divided into four main categories: - Clear PET: composed mainly of colorless transparent PET (generally at least 60% by weight) and pale blue transparent PET, does not contain pigments and can be used in mechanical recycling processes; - dark or colored (green, red, etc.) PET: generally may contain up to 0.1% by weight of a dye or pigment, but remains transparent or translucent; - Opaque PET: contains a significant amount of pigment, typically with a content varying between 0.25% and 5.0% by weight, which opacifies the polymer. Opaque PET is increasingly used, for example, in the manufacture of food containers, such as milk bottles, in the composition of cosmetic, plant protection or dye bottles. - Multilayer PET: containing layers of polymers other than PET or layers of recycled PET with virgin PET (i.e. not recycled PET) or films, for example of aluminium, between the layers. Multilayer PET is used after thermoforming to produce packaging materials, for example container trays.

[0005] After collection and sorting, recycling of these streams generally consists of a first step of conditioning, during which the bales of raw packaging material are washed, refined, sorted, crushed and then refined again and sorted to produce a flake stream containing generally less than 1% by weight of "macroscopic" impurities (glass, metals, other plastics, wood, paper, cardboard, inorganic elements), preferentially less than 0.2% by weight of "macroscopic" impurities and even more preferentially less than 0.05% by weight of "macroscopic" impurities.

[0006] The clear PET flakes may then undergo an extrusion-filtration process to produce extrudates that can then be reused in mixtures with virgin PET to produce new products (bottles, fibers, films). A process of solid state polymerization (known by the abbreviation SSP) under vacuum is often required for food applications. This type of recycling is known as mechanical recycling.

[0007] Dark (or colored) PET flakes can also be recycled mechanically. However, the color of the extrudates formed from the colored stream limits the applications: dark PET is generally used to manufacture packaging straps or fibers. The outlets are therefore more limited compared to those of clear PET.

[0008] The presence of opaque PET with a high pigment content in the recycled PET presents a problem for recyclers, since opaque PET has a negative effect on the mechanical properties of the recycled PET. Opaque PET is currently collected together with colored PET and is found in the colored PET stream. In view of the development of applications for opaque PET, the content of opaque PET in the colored PET stream to be recycled is currently 5-20% by weight and is tending to increase. It will be possible to reach a content of opaque PET in the colored PET stream of 20-30% by weight or more within a period of a few years. However, it has been shown that with more than 10-15% opaque PET in the colored PET stream, the mechanical properties of the recycled PET are negatively affected (see Impact du developpement du PET opaque blanc sur le recyclage des emballages en PET [Impact of the increase in white opaque PET on the recycling of PET packaging], preliminary report of COTREP of 5 / 12 / 13), preventing recycling in the form of fibers to the main outlet of the channel for colored PET.

[0009] Dyes are natural or synthetic substances, particularly soluble in polyester materials, that are used to color the materials they are introduced into. Commonly used dyes have different properties and often contain heteroatoms of the O and N type and conjugated unsaturation, such as quinone, methine or azo groups, or molecules such as pyrazolones and quinophthalones.

[0010] Pigments are finely divided substances, particularly insoluble in polyester materials, which are used to colour and / or opacify the materials into which they are introduced. The main pigments used to colour and / or opacify polyesters, and in particular PET, are metal oxides, such as TiO2, CoAl2O4 or Fe2O3, silicates, polysulfides and carbon black. Pigments are particles generally with a size between 0.1 and 10 μm, mainly between 0.4 and 0.8 μm. The complete removal of these pigments is necessary to envisage recycling opaque PET, but the complete removal of these pigments by filtration is technically difficult because they have an extremely high blocking capacity.

[0011] Recycling of colored and opaque PET is therefore extremely problematic.

[0012] Patent document 1 describes a method for the depolymerization by glycolysis of colored PET, especially that resulting from the recovery of green colored PET bottles. The PET feedstock treated by this method is contacted with ethylene glycol at a temperature between 180° C. and 280° C. for several hours. The product of glycolysis is obtained at the end of the depolymerization step, which is purified at a temperature above 170° C. on activated carbon, either directly or after filtration, and then extraction of the residual dyes, especially yellow colored dyes, is carried out with a solvent, which may be an alcohol, for example methanol, or a glycol, for example ethylene glycol, and then crystallization of BHET in the extraction solvent is carried out by lowering the temperature. BHET is then separated by filtration.

[0013] In WO 2005 / 023111, post-consumer PET includes a mixture of various PETs, such as clear PET and colored PETs, such as blue PET, green PET and / or amber PET, which are depolymerized by glycolysis in a batchwise manner in a reactor at 150-250° C. in the presence of ethylene glycol and an amine catalyst and an alcohol. The resulting diester monomer is purified by direct filtration, then adsorption on activated carbon and finally by passage over an ion exchange resin, in particular at a temperature of 80-90° C., before being crystallized and recovered by filtration. WO 2005 / 023111 discloses another method for the purification of the resulting diester monomer by short path distillation at 200° C.

[0014] Patent document 3 likewise describes the purification of a solution of crude BHET dissolved in methanol or ethylene glycol, which comprises at least a series of contacts between said solution and activated carbon, an anion exchange resin and a cation exchange resin at temperatures between 40° C. and 120° C., in particular equal to 60° C., 65° C. or 80° C. In particular, this patent describes that contact with activated carbon alone under the above conditions is not sufficient to completely decolorize the solution, since, while residual colors, in particular yellow colors, persist, after successive passes over activated carbon and anion and cation exchange resins the yellow coloring is no longer visible.

[0015] In patent 4, the method comprises the steps of depolymerization of a colored polyester, for example green PET, in a reactor in the presence of a diol, in particular ethylene glycol, at temperatures between 180° C. and 240° C., optionally evaporation in a thin-film evaporator, dissolution in a hot solvent, and filtration to separate out insoluble impurities with a size of more than 50 μm. The low proportion of pigment in the colored PET allows separation by filtration. However, this technique cannot be operated with higher amounts of pigment, for example pigments present in opaque PET, because these pigments quickly clog the filters.

[0016] US Pat. No. 5,399,633 describes the production of purified BHET from PET. The process comprises: glycolysis of PET flakes in solid form, previously pretreated by washing with water, in a stirred reactor in the presence of ethylene glycol and a catalyst at 180° C. and then at 195-200° C., followed by prepurification of the reaction effluent by cooling, filtration, adsorption and treatment on ion exchange resins. This prepurification step is presented as important to prevent repolymerization of BHET in the subsequent purification steps, and is carried out before evaporation of the glycol and purification of BHET. However, going through the filtration and ion exchange resin steps can be extremely problematic when the feedstock contains a large amount of very small solid particles, e.g. pigments.

[0017] Finally, US Pat. No. 6,399,666 describes a method for depolymerizing opaque PET, in particular polyester feedstocks containing 0.1% to 10% by weight of pigments, by glycolysis in the presence of ethylene glycol. A purified bis(2-hydroxyethyl) terephthalate (BHET) effluent is obtained after certain steps of separation and purification by adsorption. However, the BHET effluent obtained by the depolymerization method described in US Pat. No. 6,399,666 can have a defect: the obtained BHET effluent undergoes a particularly rapid coloration, despite having passed through a column of adsorbent.

[0018] The present invention seeks to improve the purification of crude diester monomers, the purification step of the prior art processes, such as those mentioned above, to improve the decolorization of diester monomers, in particular BHET monomer, especially obtained after depolymerization of polyester feedstocks containing PET. The object of the present invention is specifically to obtain high purity and decolorized diester monomers, in particular BHET monomer, from crude diester monomer feedstocks, in particular those resulting from the reaction of depolymerization by glycolysis of polyester waste, in particular PET waste. [Prior art documents] [Patent documents]

[0019] [Patent Document 1] US Patent Application Publication No. 2006 / 0074136 [Patent Document 2] US Patent Application Publication No. 2015 / 0105532 [Patent Document 3] U.S. Patent No. 6,642,350 [Patent Document 4] European Patent No. 0865464 [Patent Document 5] Patent No. 3715812 [Patent Document 6] French Patent Application Publication No. 3053691 Summary of the Invention [Means for solving the problem]

[0020] (Summary of the invention) An object of the present invention is therefore a process for purifying a crude diester monomer feedstock, comprising: a) a mixing step, feeding a crude diester monomer feedstock and an aqueous solvent at a temperature between 60° C. and 150° C., obtaining an aqueous mixture of diester monomers, and adjusting the amount of aqueous solvent introduced so that the crude diester monomer feedstock represents 20% to 90% by weight of the total weight of the aqueous mixture of diester monomers; b) an adsorption step, carried out by contacting the aqueous diester monomer mixture with at least one adsorbent at a temperature between 60° C. and 150° C. and a pressure between 0.1 and 1.0 MPa; a purified monomer effluent is obtained.

[0021] One advantage of the present invention is to obtain a purified and decolorized diester monomer effluent, in particular a BHET effluent, from a crude diester monomer feedstock, in particular a crude BHET feedstock. The purified diester monomer effluent obtained at the end of the process according to the invention, when it is in liquid form, is advantageously colorless or almost colorless in appearance, and when it is processed to obtain an effluent in solid form, the purified diester monomer effluent in solid form is of white solid appearance. Advantageously, the process according to the present invention makes it possible to obtain a purified diester monomer effluent which, when characterized by UV-visible spectroscopy, does not show any significant absorption bands (i.e. cannot be distinguished from background noise) in the range of visible wavelengths, i.e. between 400 and 800 nm. Highly advantageously, the purified diester monomer effluent obtained at the end of the process according to the present invention, preferably in solid form, is of CIE 1976 L * a * b * It has color parameters expressed in a reference system and determined by colorimetry (in accordance with the ASTM D6290 2019 method), preferably as follows: - Lightness (or luminance) parameter L * : close to 100, more particularly greater than 90.00, preferably greater than 92.00 (100.00 being the maximum); - Parameter a * (corresponding to the green-red axis); close to 0, more particularly between -1.50 and +1.50, preferably between -1.00 and +1.00; and - Parameter b * (corresponding to the blue-yellow axis); closer to 0, more precisely -2.50 to +2.50, even more precisely -1.00 to +1.50.

[0022] The advantage of the present invention therefore consists in obtaining a purified and decolorized diester monomer effluent from a crude diester monomer stream which in particular originates from a process for the depolymerization by glycolysis of polyester waste, typically comprising colored, opaque and even multi-layer PET, and thus consequently comprising pigments and dyes. The process according to the invention thus makes it possible to remove residual impurities, such as dyes and / or organic or inorganic salts, which could not be eliminated during the separation steps downstream of the depolymerization of the polyester.

[0023] Such diester monomers may be subsequently (re)polymerized to give polyester polymers indistinguishable from virgin polyester, and especially from virgin PET, thus enabling access to the full range of applications for virgin PET.

[0024] The process according to the invention is very flexible and can be easily incorporated downstream of any process for the depolymerization, in particular by glycolysis, of polyesters, including opaque and / or colored PET, such as PET, as a step for purifying the diester monomer effluent obtained after a step for separating diols and / or heavy impurities, such as incompletely converted oligomers and pigments, either obtained directly by the depolymerization reaction or introduced in excess for glycolysis or generated during the depolymerization. For example, the process according to the invention can be easily incorporated instead of the decolorization step of the process described in patent application FR 3053691. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] (List of Drawings) FIG. 1 shows the UV-visible spectra obtained from 550 nm to 700 nm for the effluents generated by the methods described in Examples 1, 2 and 3.

[0026] FIG. 2 shows the UV-visible spectrum obtained from 350 nm to 850 nm for the effluent generated by the method described in Example 1.

[0027] (Description of the embodiment) According to the present invention, polyethylene terephthalate or poly(ethylene terephthalate), also simply referred to as PET, has a base repeat unit comprising a terephthalic acid diester of the formula:

[0028] [ka]

[0029] Traditionally, PET is obtained by the polycondensation of terephthalic acid (PTA) or dimethyl terephthalate (DMT) with ethylene glycol.

[0030] According to the present invention, the term "monomer" or "diester monomer" advantageously refers to a repeating unit of a polyester polymer and defines a diester of a dicarboxylic acid, preferably an aromatic dicarboxylic acid, preferentially terephthalic acid, and a diol, preferably a diol containing 2 to 12 carbon atoms, preferentially 2 to 4 carbon atoms (the preferred diol is ethylene glycol). More specifically, the "monomer" or "diester monomer" corresponds to the product targeted by the method of the present invention. Thus, according to an embodiment of the present invention, the "monomer" or "diester monomer" (the product targeted by the method of the present invention) is a diester of HOC n H 2n -CO2-(Aro)-CO2-C n H 2n OH type chemical formula, where n=2-12, preferably n=2-4, and -(Aro)-=-(C6H4)- represents an aromatic ring. Preferably, the term "monomer" or "diester monomer" refers to bis(2-hydroxyethyl) terephthalate (BHET) of chemical formula HOC2H4-CO2-(C6H4)-CO2-C2H4OH, where -(C6H4)- represents an aromatic ring.

[0031] The term "oligomer" typically refers to a small sized polymer, generally consisting of 2-20 basic repeat units. According to the present invention, the term "ester oligomer" or "BHET oligomer" refers to a terephthalate ester oligomer containing 2-20, preferably 2-5, basic repeat units of the formula -[O-CO-(CH)-CO-O-CH]-, where -(CH)- is an aromatic ring.

[0032] According to the present invention, the terms "diol" and "glycol" are used interchangeably and correspond to compounds containing two hydroxyl groups -OH and preferably containing from 2 to 12 carbon atoms, preferentially from 2 to 4 carbon atoms. A suitable diol is ethylene glycol, also called monoethylene glycol or MEG.

[0033] The diol or diol effluent stream optionally used in a step of the process of the invention thus preferably comprises ethylene glycol (or MEG) in an amount of more than 40% by weight, preferentially more than 50% by weight, preferably 60% by weight or more, of the total weight of said diol or diol effluent stream.

[0034] The term "dye" defines a substance that is soluble in polyester material and is used to color it. Dyes may be of natural or synthetic origin.

[0035] According to the present invention, the term "pigment", more particularly opacifying and / or coloring pigments, defines finely divided substances that are particularly insoluble in polyester materials. Pigments are in the form of solid particles and generally have a size between 0.1 and 10 μm, predominantly between 0.4 and 0.8 μm. They are often of inorganic nature. Commonly used pigments, especially those used for opacifying, are metal oxides, such as TiO2, CoAl2O4 or Fe2O3, silicates, polysulfides and carbon black.

[0036] According to the present invention, the expressions "of between A and B" and "between A and B" are equivalent and mean that both limits of the interval (A, B) are included in the stated range of values. If this is not the case and if both limits are not included in the stated range, such an explanation will be given by the present invention.

[0037] For the purposes of the present invention, various ranges of parameters for a given process, such as pressure ranges and temperature ranges, may be used alone or in combination, for example, a range of preferred pressure values ​​may be combined with a more preferred range of temperature values ​​within the meaning of the present invention.

[0038] In the following text, specific embodiments of the present invention may be described, which may be implemented separately or, where technically feasible, in combination together without any restriction on the combination.

[0039] The terms "upstream" and "downstream" should be understood according to the general flow of the streams in the process.

[0040] According to the invention, pressure is absolute pressure and is given in MPa or MPa (absolute) (or MPa (absolute)).

[0041] (Feed material) The process according to the present invention is fed with a crude diester monomer feedstock.

[0042] Said crude diester monomer feedstock advantageously comprises at least 70.0% by weight, preferably at least 85.0% by weight, preferentially at least 95.0% by weight, preferably at least 99.0% by weight, highly preferably at least 99.9% by weight of diester monomer, preferably bis(2-hydroxyethyl) terephthalate (or BHET), with 100.0% by weight representing the maximum of diester monomer in the feedstock. The crude diester monomer feedstock may also comprise impurities, preferably impurities soluble in the diester monomer. These impurities may also be referred to as residual impurities. These are compounds of the type of dyes, such as dyes typically used to color polyester polymer materials, organic or inorganic salts or esters of dicarboxylic acids and at least one dimer or trimer of a diol (said diol being the one forming the targeted diester monomer), the dicarboxylic acid being preferably an aromatic dicarboxylic acid, preferentially terephthalic acid, the dimer or trimer of the diol preferably containing from 4 to 36 carbon atoms, preferentially from 4 to 8 carbon atoms, for example diethylene glycol (one such impurity is, for example, 2-(2-hydroxyethoxy)ethyl 2-hydroxyethyl terephthalate, which is an ester of terephthalic acid with ethylene glycol and diethylene glycol). More particularly, the dye-type impurities may represent up to 1% by weight (i.e. less than 1% by weight), preferably up to 0.1% by weight, preferentially up to 0.05% by weight, and in particular at least 1 ppm by weight of the total weight of the crude diester monomer feedstock. Other impurities, in particular impurities of the type of esters of dicarboxylic acids and dimers or trimers of at least one diol, may represent in total up to 15.0% by weight, preferably up to 10.0% by weight, preferentially up to 5.0% by weight, of the total weight of the crude diester monomer feedstock and in particular a minimum of 10 ppm by weight of the total weight of the crude diester monomer feedstock.

[0043] The crude diester monomer feedstock may optionally further comprise a solvent, for example a monoalcohol, in particular methanol or ethanol, or a diol, more particularly ethylene glycol. The crude diester monomer feedstock may in particular comprise up to 30.0% by weight, preferably up to 15.0% by weight, preferentially up to 5.0% by weight, preferably up to 1.0% by weight, highly preferably up to 0.1% by weight of a solvent, more particularly a diol, for example ethylene glycol, or only traces of solvent, in particular less than 500 ppm by weight of a solvent, preferably a diol, in particular ethylene glycol. The crude diester monomer feedstock may be free of any solvent.

[0044] According to one preferred embodiment of the present invention, the crude diester monomer feedstock is obtained from a process for the depolymerization by glycolysis of a polyester feedstock, in particular a polyester waste feedstock, typically comprising colored and / or opaque PET, and optionally multi-layer PET. More specifically, the crude diester monomer feedstock may originate directly or indirectly from the depolymerization of a polyester feedstock comprising colored and / or opaque PET in the presence of a diol, preferably ethylene glycol, the term "indirectly originating" meaning that the depolymerization process comprises a step of preliminary purification of the reaction effluent obtained by the depolymerization reaction in the presence of a diol, for example a step of separating the diol used in excess for glycolysis or generated during the depolymerization and / or a step of separating heavy impurities, for example incompletely converted oligomers and / or a step of separating ionic species, for example by passing through an ion exchange resin, and the term "directly originating" meaning that the depolymerization process does not comprise such a preliminary purification step.

[0045] According to one very particular embodiment of the invention, the crude diester monomer feedstock fed to the purification process according to the invention is obtained from a depolymerization process, for example the process described in patent application FR 3053691, in which the purification process according to the invention replaces the decolorization step described.

[0046] (purification method) Purification of the crude diester monomer feedstock is advantageously carried out by adsorption of an aqueous solution of the crude diester monomer.

[0047] The process according to the invention therefore comprises at least a step a) for mixing a diester monomer feedstock with an aqueous solvent and an adsorption step b) for obtaining a purified diester monomer effluent by contacting the resulting diester monomer aqueous mixture with at least one adsorbent.

[0048] The process according to the invention may optionally comprise a step c) for separating the diester monomer to obtain a separated and purified diester monomer effluent and a spent aqueous solvent effluent. The process according to the invention may optionally comprise an additional step c) for crystallizing the diester monomer. * ), preferably located downstream of the adsorption step b) and advantageously upstream of the optional separation step c).

[0049] (Mixing step a)) Advantageously, the mixing step a) comprises feeding a crude diester monomer feedstock and an aqueous solvent, the crude diester monomer feedstock preferably comprising BHET. Step a) makes it possible to obtain an aqueous diester monomer mixture.

[0050] The aqueous solvent advantageously comprises water, preferably at least 50% by weight of water, preferably at least 75% by weight of water, preferentially at least 90% by weight of water, even more preferentially at least 97% by weight of water, preferably at least 99% by weight of water, and up to 100% by weight of water (i.e. the aqueous solvent advantageously comprises up to 100% by weight of water, in particular 50% to 100% by weight, preferably 75% to 100% by weight, preferentially 90% to 100% by weight, more preferentially 97% to 100% by weight, preferably 99% to 100% by weight of water). In some cases, the aqueous solvent may comprise, apart from water, other water-miscible compounds, for example compounds of the type alcohols, diols, acids, etc. The aqueous solvent may comprise small amounts of ions and / or inorganic salts, typically in amounts less than 1% by weight. Preferably, the aqueous solvent comprises at least 99% by weight of water, in particular up to 100% by weight of water, and in some cases may comprise ions and / or inorganic salts. According to a preferred embodiment of the present invention, the aqueous solvent fed to the mixing step a) comprises, preferably consists of, all or part of the aqueous solvent effluent, which is optionally purified and obtained from the spent aqueous solvent effluent obtained at the outlet of the optional separation step c), optionally supplemented with a supply of solvent external to the process according to the invention.

[0051] Preferably, the amount of aqueous solvent introduced is adjusted so that the crude diester monomer feedstock represents from 20% to 90% by weight, preferentially from 30% to 80% by weight, preferably from 40% to 75% by weight, even more preferably from 40% to 60% by weight of the total weight of the aqueous diester monomer mixture.

[0052] Advantageously, the mixing step a) is carried out at a temperature of 60°C to 150°C, preferably 70°C to 120°C, preferably 75°C to 110°C, and preferably at a pressure of 0.1 to 1.0 MPa, preferably 0.1 to 0.8 MPa, and suitably 0.1 to 0.5 MPa.

[0053] The aqueous solvent may be heated prior to said mixing step a), preferably to the temperature at which mixing step a) is carried out, in particular to a temperature between 60°C and 150°C, preferably between 70°C and 120°C, preferably between 75°C and 110°C. Preferably, the crude diester monomer feedstock is fed to mixing step a) at a temperature at which said crude diester monomer feedstock is at least partially, preferably completely, in liquid or molten form. Highly advantageously, the crude diester monomer feedstock may be heated prior to mixing step a), preferably to a temperature of at least 110°C, preferably at least 120°C and preferably at most 220°C, preferentially at most 200°C. Thus, the crude diester monomer feedstock, preferably the crude diester monomer feedstock comprising BHET, is advantageously fed to mixing step a) at a temperature (or inlet temperature) of at least 110°C, preferably at least 120°C and preferably at most 220°C, preferentially at most 200°C.

[0054] The mixing step a) may use any mixing equipment known to the person skilled in the art, such as, for example, a static or dynamic mixer, in particular a static mixer.

[0055] The aqueous diester monomer mixture obtained at the end of step a) is advantageously a homogeneous mixture in which the diester monomers, in particular BHET, are soluble.

[0056] (Adsorption step b)) The aqueous diester monomer mixture obtained at the end of step a) is fed to an adsorption step b), which is carried out by contacting the aqueous diester monomer mixture with at least one adsorbent, in particular a solid one, at a temperature advantageously between 60° C. and 150° C., preferably between 70° C. and 120° C. and preferentially between 75° C. and 110° C., and at a pressure highly advantageously between 0.1 and 1.0 MPa, in particular between 0.1 and 0.8 MPa and more particularly between 0.1 and 0.5 MPa.

[0057] In the adsorption step b), advantageously at least one adsorption unit (also called adsorption train), preferably 1 to 10 adsorption units, preferably 1 to 4 adsorption units, are used, each adsorption unit advantageously operating in parallel with one another. Advantageously, each adsorption section comprises at least one adsorber, and preferably up to 4 adsorber, each adsorber being for example a reactor or a column. Highly advantageously, the residence time in each adsorber of the adsorption step is preferably between 20 min and 40 h, preferably between 1 h and 30 h, preferably between 1 h and 20 h. The residence time is defined here as the ratio between the internal volume of the adsorber in question and the volumetric flow rate of the diester monomer aqueous mixture from the mixing step a).

[0058] In the adsorption step b), at least one adsorbent, in particular one that is solid, preferably up to five different adsorbents are used. According to a very particular embodiment, in the adsorption step b), one or two different adsorbents are used. According to the invention, the adsorbents are said to be different if they differ in their nature and / or their composition and / or their particle size and / or their textural properties, such as pore volume. Preferably, the different adsorbents are of different nature. In particular, it may be advantageous to combine several different adsorbents, in particular adsorbents of different nature, to optimize the removal of residual impurities, in particular residual dyes or residual salts, which may be of very different natures in themselves. In particular, polyester waste, for example PET packaging or plastic bottle waste, from which the crude diester monomer feedstock to be treated by the method according to the invention may be obtained by depolymerization of said waste, may contain a very large number of colored and / or opaque PET and therefore a very large number of different dye compounds. Color in the crude diester monomer feedstock may originate from decomposition or conversion of compounds contained in the polyester waste during various steps of the depolymerization process from which the crude diester monomer feedstock may be obtained (e.g., the waste conditioning step and / or the depolymerization reaction step).

[0059] When 2 to 5 different adsorbents are used in the adsorption step b), the different adsorbents are in a mixture or are arranged in series in one or more adsorbers, advantageously in one and the same adsorption unit. Preferably, when 2 to 5 different adsorbents are used in the adsorption step b), the different adsorbents are in series with one another, advantageously in one and the same adsorption unit, and more preferentially each of the adsorbents is in different adsorbents arranged in series or in parallel, preferably in series, advantageously in one and the same adsorption unit.

[0060] Advantageously, the adsorbent or adsorbents, especially those that are solid, are chosen from activated carbon, alumina and clay. The activated carbon is obtained, for example, from petroleum coke, bituminous coal or any other fossil origin, or from biomass, for example wood, coconut or any other biomass source. Different raw materials may be mixed to obtain an activated carbon that may be used as an adsorbent in said adsorption step b). The clay may be a layered double hydroxide or a natural or converted clay, for example, as known to the skilled person as a bleaching earth. Preferably, at least one adsorbent is activated carbon. Thus, when a single type of adsorbent is used in the adsorption step b), said adsorbent is activated carbon, and when two or more different adsorbents are used in the adsorption step b), one adsorbent is activated carbon and one or more other adsorbents are another activated carbon, alumina and / or clay, preferably activated carbon and / or clay, more particularly clay.

[0061] Preferably, the pore volume (Vp) of each adsorbent used in adsorption step b), determined by mercury porosimetry, is greater than or equal to 0.25 mL / g, preferentially greater than or equal to 0.40 mL / g, more preferably greater than or equal to 0.50 mL / g and preferably less than or equal to 5 mL / g.

[0062] Preferably, the adsorption step b) is advantageously carried out in each adsorption unit as follows: - in a flow-through fixed bed (or fixed bed) mode, i.e. in at least one adsorber containing a fixed bed of one or more adsorbents, in particular in at least one column of one or more adsorbents; which may be operated in upflow or downflow mode, or - A stirred system, advantageously in at least one continuously stirred reactor, also known as a continuous stirred tank reactor (CSTR).

[0063] When the adsorption step b) is carried out in stirred mode in at least one stirred reactor of the CSTR type, the reactor or reactors are followed by a filtration system for recovering said adsorbent or adsorbents present in suspension in the treated liquid. Preferably, the adsorption step b) is carried out in flow-through fixed bed mode, advantageously in each adsorption unit.

[0064] Preferably, in the adsorption step b), advantageously in each adsorption unit, when at least two different adsorbents are used in a flow-through fixed bed mode, the adsorbents may be: - all present in each adsorber or column, used as a mixture or in a series of fixed beds; or - each is used in an adsorption section, the sections being placed in series with one another, each adsorption section consisting of 1 to 4, preferably 2 to 4, fixed bed adsorbent columns, advantageously in each adsorption unit.

[0065] Highly advantageously, in the adsorption step b), advantageously in each adsorption unit or adsorption section, several fixed-bed columns of the same adsorbent or adsorbents are used, in particular at least two fixed-bed columns, preferably 2 to 4 fixed-bed columns. When in the adsorption step b), advantageously in each adsorption unit or in an adsorption section, two columns of the same adsorbent or adsorbents are used, the adsorption step b) may advantageously be operated in each adsorption unit or adsorption section according to a "swing" operation mode, in which one of the columns is online while the other column is in reserve. When the adsorbent in the online column is exhausted, this column is isolated, while the in-reserve column is placed online. The used adsorbent of the isolated column may then be regenerated in situ and / or replaced with fresh adsorbent and placed back online again where the other column was isolated. Another mode of operation of the adsorbent columns is to have at least two columns, advantageously in each adsorption unit, operating in series: when the adsorbent of the leading column (i.e. the first column in the series) is worn out, this first column is isolated, the used adsorbent is regenerated in situ or replaced with fresh adsorbent, said column is then put back on-line at the last position in the series of columns, etc. This operation is called "lead-lag". Highly preferably, in the adsorption step b), advantageously at least two columns of the same adsorbent, preferably 2 to 4 columns of the same adsorbent, preferentially 2 columns of the same adsorbent, are used in "lead-lag" mode in each adsorption unit or each adsorption section.

[0066] The combination of at least two columns of the same adsorbent advantageously makes it possible to improve in particular the possibly rapid saturation and / or clogging of the adsorbent in each adsorption unit. In particular, the presence of at least two adsorbent columns advantageously facilitates the replacement and / or regeneration of the adsorbent without shutting down the adsorption unit and thus the treatment process, thus reducing the risk of clogging, preventing unit shutdowns due to adsorbent saturation, controlling costs and limiting adsorbent consumption, while ensuring continuous production of purified diester monomer. This combination of at least two adsorbent columns also advantageously makes it possible to maximize the adsorption capacity of said adsorbent in each adsorption unit, especially when operating in the "lead-lag" mode.

[0067] In a very particular embodiment of the invention, in the adsorption step b), advantageously two different adsorbents are used in each adsorption unit, which very preferentially comprises a first adsorption section comprising at least two, preferably from two to four, fixed bed columns of activated carbon and a second adsorption section comprising at least two, preferably from two to four, fixed bed columns of another adsorbent, preferably chosen from another activated carbon or a clay, the fixed bed columns of the first adsorption section being operated in swing or lead-lag mode and the fixed bed columns of the second adsorption section being operated in particular in swing or lead-lag mode and being arranged upstream or downstream of the first section of fixed bed activated carbon columns.

[0068] Each adsorbent used in the adsorption step b) is preferably in the form of granules, extrudates or powder. Preferably, each adsorbent is in the following form: in the form of granules or extrudates, if the adsorption step b) is carried out in a flow-through fixed bed mode, and in the form of a powder, if the adsorption step b) is carried out in a stirred reactor of the CSTR type.

[0069] The size of the at least one sorbent, especially when it is in the form of granules or extrudates, is such that the smallest dimension of the at least one sorbent (corresponding to the diameter of the circle circumscribing the basis of the granules or lobe extrudates, or the diameter of the cylinder circumscribing the basis of the cylinder type extrudates; this dimension is also called "diameter") is preferably between 0.1 and 5 mm, preferentially between 0.3 and 2 mm. For example, activated carbon extrudates with a diameter of 0.8 mm sold by Cabot Norit, or granules in the size range of 0.4 to 1.7 mm sold by Chemviron may be suitable as sorbents.

[0070] The process according to the invention may advantageously comprise a step of regenerating said adsorbent or adsorbents of adsorption step b).

[0071] A purified monomer effluent is obtained at the end of the adsorption step b). It can be subjected to an optional separation step c) or possibly a crystallization step c). * ) may be fed.

[0072] (Optional crystallization step c * )) The process according to the invention optionally further comprises a step c) for crystallizing the diester monomer. * ), preferably located downstream of the adsorption step b) and highly advantageously upstream of the optional separation step c). Advantageously, the crystallization step c) * In the optional crystallization step c), at least one solid-producing section is used. * ) makes it possible to obtain an aqueous suspension of solid diester monomers.

[0073] This optional crystallization step has a dual effect: it makes it possible to facilitate the separation of the purified diester monomer in solid form from the spent aqueous solvent effluent, and it also makes it possible to improve the purification of the diester monomer.

[0074] According to one preferred embodiment of the invention, the process comprises a crystallization step c) downstream of the adsorption step b) and highly advantageously upstream of the optional separation step c). * According to another embodiment of the invention, the method comprises, downstream of the adsorption step b), 2 to 4 crystallization steps c). * ), each step being followed by an optional separation step c) as described below.

[0075] According to another embodiment of the invention, the process further comprises at least one crystallization step c) upstream of the adsorption step b) and advantageously upstream of the mixing step a). * In the latter embodiment, the crystallization step c * a) comprises a solids production section fed with the optionally filtered crude diester monomer feedstock and a crystallization solvent as described below, and a solid-liquid separation section separating the solids formed in the solids production section, the separated solids then being sent to the mixing step a) where they will be dissolved in an aqueous solvent.

[0076] Optional Crystallization Step c * ), more particularly the solids production section, advantageously arranged downstream of the adsorption step b), is fed with the purified monomer effluent from the adsorption step b). * ) may optionally further comprise a section for filtering the purified monomer effluent from the adsorption step b), which section is located upstream of the solids production section.

[0077] Optionally, the solids-producing section may be fed with a crystallization solvent that is the same as or different from the aqueous solvent introduced in the mixing step a), which is advantageously chosen from: water, an aqueous solvent containing at least 50% by weight, preferably at least 75% by weight, preferentially at least 90% by weight, even more preferentially at least 97% by weight, preferably at least 99% by weight of water; monoalcohols, preferably monoalcohols having 1 to 12 carbon atoms, such as methanol or ethanol; diols, preferably diols having 1 to 12 carbon atoms; ethers; aldehydes; esters; hydrocarbons, preferably aromatic hydrocarbons, such as monoaromatics; and mixtures of at least two of these compounds belonging to the same or different chemical families. Preferably, the crystallization solvent is chosen from: water, an aqueous solvent containing at least 50% by weight, preferably at least 75% by weight, preferentially at least 90% by weight, even more preferentially at least 97% by weight, preferably at least 99% by weight of water; monoalcohols having 1 to 12 carbon atoms, such as methanol or ethanol; diols having 1 to 12 carbon atoms, preferably ethylene glycol; monoaromatic compounds, such as xylene; and mixtures thereof. Preferably, the crystallization solvent is the same as the aqueous solvent introduced in the mixing step a).

[0078] According to a preferred embodiment of the invention, the crystallization solvent comprises, preferably consists of, all or part of the aqueous solvent effluent, which optionally originates from the spent aqueous solvent effluent obtained at the outlet of the optional separation step c), optionally purified, and which is supplemented by a supply of solvent external to the process according to the invention.

[0079] Preferably, when a crystallization solvent is introduced, the amount of crystallization solvent introduced into the solids production section is adjusted so that the crude diester monomer feedstock fed to the process, in particular to the mixing step a) of the process, represents between 1% and 75% by weight, preferentially between 5% and 45% by weight, preferably between 15% and 35% by weight of the total weight of the mixture in said solids production section (i.e. the mixture comprising the crude diester monomer feedstock, the aqueous solvent introduced in mixing step a) and the crystallization solvent).

[0080] Prior to said solid production section, all or part of the crystallization solvent may preferably be heated or cooled to the temperature at which the adsorption step b) is carried out, in particular to a temperature of preferably between 0°C and 120°C, preferably between 5°C and 100°C, preferably between 10°C and 90°C.

[0081] Advantageously, an optional crystallization step c * The temperature at which the solids production section of step b) is operated is between 0° C. and 100° C., preferably between 5° C. and 80° C., preferably between 10° C. and 70° C. (i.e. the temperature of the effluent from said solids production section is such). More precisely, in the solids production section the purified monomer effluent from adsorption step b), optionally in a mixture with the crystallization solvent, is cooled from the temperature at which the adsorption step b) is carried out, i.e. a temperature of 60° C. to 150° C., preferably between 70° C. to 120° C., preferentially between 75° C. to 110° C., to a temperature of 0° C. to 100° C., preferably between 5° C. to 80° C., preferably between 10° C. to 70° C.

[0082] The cooling may be carried out according to any method known to the skilled person. For example, in a batch mode, the realization of the cooling of the temperature may be without regulation of the temperature drop (without an imposed temperature ramp; therefore, only the initial and final temperatures are controlled) or according to at least one decreasing temperature gradient, in particular according to a decreasing temperature gradient of 5°C to 30°C / h, more particularly 8°C to 15°C / h, or else according to both modes linked together in series, i.e. without control for one part of the cooling and according to a decreasing temperature gradient for another part of the cooling. According to another example, the cooling may simply result from the introduction of the stream to be cooled from the adsorption step b), i.e. the purified monomer effluent or the mixture comprising the purified monomer effluent and the crystallization solvent, into a reservoir having a volume advantageously adapted to the flow rate of the stream to be cooled and kept at a temperature between 0°C and 100°C, preferably between 5°C and 80°C, preferably between 10°C and 70°C.

[0083] The pressure at which the solids-producing section is operated is advantageously between 0.00001 and 1.00 MPa, preferably between 0.0001 and 0.50 MPa, and suitably between 0.001 and 0.20 MPa. According to a particular embodiment of the invention, the solids-producing section is operated under vacuum, preferably at a pressure between 0.0001 and 0.10 MPa, preferentially between 0.001 and 0.01 MPa. According to another particular embodiment, the solids-producing section is advantageously operated in a jacketed reactor, at a pressure between 0.01 and 1.00 MPa, preferably between 0.05 and 0.20 MPa, and preferably at atmospheric pressure, i.e. 0.10 MPa.

[0084] According to a preferred embodiment of the present invention, water is mixed as crystallization solvent with the purified monomer effluent from step b) and the solids production section is operated under conditions such that the temperature of the effluent from said solids production section is between 5°C and 50°C, preferably between 10°C and 40°C.

[0085] According to another preferred embodiment of the present invention, the crystallization solvent introduced and mixed with the purified monomer effluent from step b) is ethylene glycol and the solids production section is operated under conditions such that the temperature of the effluent from said solids production section is between 5°C and 50°C, preferably between 10°C and 40°C.

[0086] Advantageously, the purpose of the solids-producing section is to solidify, i.e. at least partially crystallize or precipitate, the diester monomer, preferably BHET. The solids-producing section therefore comprises, and preferably consists of, a precipitation or crystallization stage carried out by any precipitation or crystallization technique known to the skilled person. The solids-producing section is preferably a section for crystallization, for example by cooling or concentration, carried out in any equipment known to the skilled person, as defined, for example, in the journal Techniques de l'Ingenieur, "Cristallisation industrielle - Aspects pratiques" [Industrial Crystallization - Practical Aspects], ref. J2788 V1, followed by liquid-solid separation.

[0087] Advantageously, the section for generating solids, preferably by crystallization, comprises one or more crystallization operations, operating in series or in parallel, carried out batchwise or continuously, preferably continuously.

[0088] Optional Crystallization Step c * The solids-producing section of step b) makes it possible to obtain a heterogeneous effluent, more particularly an aqueous suspension of solid diester monomer, which comprises a solid phase of diester monomer and a liquid phase which may contain residual impurities, such as residual dyes, that may still be present in the purified monomer effluent obtained from the adsorption step c). Advantageously, the optional crystallization step c) is * The aqueous suspension of solid diester monomer obtained at the end of step c) is sent to a separation step c).

[0089] (Optional separation step c)) The purification process may comprise, and preferably comprises, a separation step c) which is arranged downstream of step b). The optional separation step is advantageously arranged downstream of the purified monomer effluent from the adsorption step b) or downstream of the optional crystallization step c) of the process according to the invention. * ), the aqueous suspension of solid diester monomer obtained at the end of such a process is fed.

[0090] If a separation step c) is incorporated into the process according to the invention, it advantageously makes it possible to separate the purified diester monomer effluent and the spent aqueous solvent effluent.

[0091] In the optional separation step c), advantageously any separation technique known to the person skilled in the art may be used. In the optional separation step c), for example, a separation by distillation and / or evaporation of the aqueous solvent may be used to obtain, on the one hand, a separated and purified diester monomer effluent and, on the other hand, a spent aqueous solvent effluent comprising the aqueous solvent. According to another embodiment, in particular the process according to the invention further comprises a crystallization step c). * In the case of the adsorption step b), particularly downstream of the adsorption step c), the optional separation step c) may involve solid-liquid separation, for example by filtration, decantation and / or centrifugation, to separate the solid diester monomer, advantageously in crystalline form, preferably BHET crystals, from the crystallization step c). * The solid diester monomer thus separated may be separated from the liquid phase of the aqueous suspension of solid diester monomer obtained at the end of step (a). The solid diester monomer thus separated constitutes the separated and purified diester monomer effluent, and the liquid phase constitutes the spent aqueous solvent effluent.

[0092] Highly advantageously, the temperature and pressure in optional step c) are adjusted by the skilled person to obtain a sufficient separation of the purified diester monomer effluent and the spent aqueous solvent effluent. According to the embodiment in which solid-liquid separation is used in optional separation step c), the temperature at which step c) is carried out varies from 0° C. to 100° C., preferably from 5° C. to 80° C., preferably from 10° C. to 50° C., and the pressure preferably varies from 0.0001 to 0.50 MPa, preferably from 0.001 to 0.20 MPa.

[0093] According to a particular embodiment of the invention, the separated and purified diester monomer effluent, preferably recovered in solid form by filtration or centrifugation, may further advantageously undergo all or some of the following operations, carried out one or more times without a predefined chronological sequence: rinsing with a solvent identical or different to that fed to the mixing section or, optionally, to the solid-producing section; additional filtration or centrifugation; removal of residual solvent by any method known to the skilled person, for example by evaporative drying; shaping, for example into powder or granules; and storage of the solid.

[0094] According to another embodiment of the invention, the separated and purified diester monomer effluent is recovered, preferably by filtration or centrifugation, and then sent directly (i.e. without a solids storage step) to a polymerization process known to those skilled in the art, optionally followed by an operation of rinsing the solid purified diester monomer effluent with water or a diol effluent, e.g. ethylene glycol effluent, preferably with water, prior to the polymerization reaction, and subsequently heating the rinsed solids for melting purposes.

[0095] Highly advantageously, the purified diester monomer effluent or the separated purified diester monomer effluent obtained at the end of the process according to the invention is: colourless or almost colourless to the naked eye if it is in liquid form; white if it is in solid form. The purification process according to the invention comprises a step b) of adsorbing the aqueous diester monomer solution and, optionally, a step c) of crystallizing said diester monomer.* ), the purification process according to the invention thus makes it possible to sufficiently purify and decolorize the crude diester monomer feedstock, even if the crude diester monomer feedstock originates from a process for the polymerization of a polyester feedstock containing a significant amount of colored and / or opaque PET. In particular, impurities present in the diester monomer feedstock, e.g. dyes, remain at least partially captured by the adsorbent and / or for at least one other portion dissolved in the aqueous solvent or mixture of solvents introduced during the process and may also be found in the optionally separated spent aqueous solvent effluent.

[0096] Highly advantageously, the purified diester monomer effluent or the separated purified diester monomer effluent obtained at the end of the process according to the invention preferably comprises, on a dry weight basis (i.e. relative to the solids contained in said purified or separated diester monomer effluent), at least 90% by weight, preferentially at least 95% by weight, preferably at least 98% by weight of diester monomer (i.e. the product targeted by the process according to the invention), preferably BHET. Highly advantageously, the purified diester monomer effluent or the separated purified diester monomer effluent obtained at the end of the purification process according to the invention may comprise, on a dry weight basis (i.e. relative to the solids of said effluent), less than 5% by weight, preferably less than 1% by weight, preferentially less than 0.5% by weight of impurities of ester type of dicarboxylic acid and dimer or trimer of at least one diol, for example ester compounds obtained from diethylene glycol.

[0097] The purified diester monomer effluent or the separated purified diester monomer effluent obtained at the end of the method may be characterized by UV-visible spectroscopy to confirm the presence of an absorption band in the visible range, in particular from 400 to 800 nm. According to this characterization method, the purified diester monomer effluent or the separated purified diester monomer effluent is preferably characterized by UV-visible spectroscopy, in particular at 400 to 800 nm, advantageously after dilution or dissolution in a liquid medium, i.e. advantageously in a suitable solvent, preferably at 0.1% to 10% by weight, at ambient temperature, using conventional laboratory benchtop UV-visible spectroscopy. Ethanol may be used as a suitable solvent, allowing dilution or dissolution of the purified diester monomer effluent or the separated purified diester monomer effluent sample. A cuvette with a conventional 1 cm or 1 inch optical path length may be used. Preferably, the UV-visible spectrum of the diester monomer effluent or the separated and purified diester monomer effluent is determined using a solution of said diester monomer effluent prepared at 5% by weight in ethanol and a cuvette with an optical path length of 1 inch. According to this method, the spectrum of the purified or separated diester monomer effluent obtained by the method according to the invention does not display any significant absorption bands (i.e. cannot be distinguished from background noise) within the visible wavelength range (400-800 nm), in particular within the range of 550-650 nm. Indeed, it is surprising that the method according to the invention, which includes a step of adsorption in water, makes it possible to effectively eliminate blue dyes that typically absorb visible light in the range of 550-650 nm.

[0098] The purified diester monomer effluent or the separated purified diester monomer effluent obtained at the end of the process may be characterized according to the colorimetric method as described in ASTM D6290 2019. The light source chosen is Illuminant D65 and the measurements are made in reflection, in specular excluded mode, with a standard observer at 10°. The measurements are made according to the CIE L * a * b *According to the colorimetry method, the diester monomer effluent or the separated and purified diester monomer effluent obtained by the method according to the invention is preferably in solid form and advantageously has a CIE L * a * b * Indicate the frame of reference: - Lightness (or luminance) parameter L * close to 100, more particularly greater than 90.00, preferably greater than 92.00 (100.00 being the maximum); - Parameter a * (corresponding to the green-red axis); close to 0, more particularly between -1.50 and +1.50, preferably between -1.00 and +1.00; and - Parameter b * (corresponding to the blue-yellow axis); close to 0, more specifically -2.50 to +2.50, more specifically -1.00 to +1.50.

[0099] The spent aqueous solvent effluent obtained at the end of the optional step c) is a mixture of the aqueous solvent introduced in the mixing step a) and the optional crystallization step c). * ), which may also contain dyes and / or other residual impurities. Preferably, the spent aqueous solvent effluent contains less than 20% by weight, preferentially less than 15% by weight, preferably less than 10% by weight, preferably less than 5% by weight of diester monomers (i.e. the targeted products), preferably BHET monomers.

[0100] The spent aqueous solvent effluent is then at least partially subjected to the mixing step a) and / or the optional crystallization step c) of the method. * The spent aqueous solvent effluent may be at least partially treated to separate, in particular, dyes and / or impurities, for example by adsorption, and thus recover a purified aqueous solvent, which may then be at least partially recycled to the mixing step a) and / or the optional crystallization step c) of the method. *The spent aqueous solvent effluent may, in addition to the separation of dyes and / or impurities, also undergo an operation for separation of the solvent, for example by distillation or decantation, if a crystallization solvent is introduced and the crystallization solvent is different from the solvent introduced in the mixing step a), to obtain two separate solvents, one of which can be recycled to the mixing step a) and the second of which can be recycled to the crystallization step c). * ) can be recycled to the solids producing section of the

[0101] The purified diester monomer effluent or the separated purified diester monomer effluent obtained at the end of the process according to the invention may therefore be fed directly or indirectly to a polymerization process known to the person skilled in the art for the purpose of producing polyester polymers, preferably PET or PET-based copolyesters, which are indistinguishable from the corresponding virgin resins. Said polymerization process may be fed, in addition to the purified diester monomer effluent or the separated purified diester monomer effluent, with ethylene glycol, terephthalic acid or dimethyl terephthalate or any other monomer depending on the targeted (co)polymer.

[0102] The following figures and examples are illustrative of the present invention and are not intended to limit the scope of the invention.

[0103] (Example) In the following examples, the steps leading to the production of crude BHET feedstock are identical and are described below.

[0104] A polyester feedstock is obtained for processing from the collection and sorting channel. This polyester feedstock contains, in particular, 20% by weight of opaque PET. 4 kg / h of flakes of said polyester feedstock containing 20% ​​by weight of opaque PET (containing 6.2% by weight of TiO2 pigment) are brought to a temperature of 250°C and then injected into the first stirred reactor together with 11.5 kg / h of ethylene glycol (MEG) and then into the second and third stirred reactors. The first stirred reactor is maintained at 250°C and the second and third stirred reactors are maintained at 220°C. The reactors are maintained at a pressure of 0.4 MPa. The residence time is set to 20 minutes in the first reactor and 2.1 hours in the second and third reactors. The residence time is defined as the ratio of the liquid volume in the reactor to the sum of the liquid volumetric flow rates entering the reactor. At the outlet of the third reactor, the reaction effluent consists of 67.7 wt.% diol, 25.8 wt.% diester monomer, 0.32 wt.% TiO2, and 6.1 wt.% heavy compounds, the diol being predominantly composed of ethylene glycol (MEG) (i.e., containing 95 wt.% or more MEG), the diester monomer being predominantly composed of bis(2-hydroxyethyl) terephthalate (BHET) (i.e., containing 95 wt.% or more BHET), and the heavy compounds including, inter alia, dimers and / or oligomers.

[0105] The diols present in the reaction effluent are separated by evaporation in two successive flash vessels at temperatures ranging from 180° C. to 120° C. and pressures ranging from 0.04 MPa to 0.004 MPa, followed by a thin-film evaporator operated at 175° C. and 0.0005 MPa. At the end of this evaporation step, 10.46 kg / h of a MEG-rich stream and 5.02 kg / h of a BHET-rich liquid stream are recovered. The BHET-rich liquid stream corresponds to the liquid monomer effluent and consists of 79.6% by weight of BHET diester monomer, 0.6% by weight of MEG, 1.0% by weight of TiO2 and 18.8% by weight of heavy compounds, including, inter alia, dimers of BHET.

[0106] The BHET-rich liquid stream is then injected into a short-path evaporator, also called short-path distillation, which is operated at a pressure of 20 Pa. The hot oil at 215° C. allows the evaporation of BHET, which is subsequently condensed in the short-path evaporator at 130° C. to obtain a liquid stream of BHET as the distillate of the short-path evaporator, which has a flow rate of 3.8 kg / h. The residence time in the short-path evaporator is 1 min. The liquid stream of HET recovered at the outlet of the short-path evaporator corresponds to crude BHET, which is the feedstock for the purification process described in Examples 1, 2 and 3 below. It consists of 99% by weight of BHET diester monomer and does not contain any traces of TiO2. A heavy residue having a flow rate of 1.19 kg / hr was recovered as the bottoms from the short path evaporator, which contained 16.7 wt. % BHET diester monomer, 79.2 wt. % BHET oligomers, and 4.1 wt. % TiO2.

[0107] Example 1 - In accordance with the invention The crude BHET is pressurized to 0.15 MPa and fed to a mixing section, to which a stream of water is also fed. The feed rate of water is adjusted so that the crude BHET represents 50% by weight of the mixture (crude BHET + water). The mixing section is operated at 90°C and a pressure of 0.15 MPa.

[0108] The resulting mixture is then fed to an adsorption section consisting of two columns, each of which is packed with adsorbent (i.e. has a fixed bed of adsorbent). The adsorption section is operated at 90° C. and a pressure of 0.15 MPa. One column is on stream (i.e. it is in operation) and the other column remains in reserve. The adsorbent used to pack the two columns is activated carbon (reference ROY 0.8 from Cabot Norit) consisting of cylindrical extrudates with a diameter of 0.8 mm.

[0109] In one column, the residence time is fixed at 40 min. The linear velocity of the empty column is 2.4 cm / min.

[0110] The effluent, consisting of BHET at 50% by weight in a BHET-water mixture, is collected over time at the outlet of the column.

[0111] UV-Visible spectroscopy measurements are performed on the BHET solution. The BHET solution is prepared by using a sample of the effluent obtained at 40 hours of operation, which is then dissolved in ethanol to reach a concentration of 5% by weight of BHET in the final solution. UV-Visible spectroscopy measurements are performed in a cuvette with a 1 inch path length using a Hach DR3900 laboratory benchtop UV-Visible spectrometer.

[0112] The spectrum obtained (see FIG. 1) does not display any significant absorption bands in the wavelength range of 550-650 nm.

[0113] Example 2 - In accordance with the invention The crude BHET is pressurized to 0.15 MPa and fed to a mixing section, to which a stream of water is also fed. The feed rate of water is adjusted so that the crude BHET represents 50% by weight of the mixture (crude BHET + water). The mixing section is operated at 90°C and a pressure of 0.15 MPa.

[0114] The resulting mixture is then fed to an adsorption section consisting of two columns, each of which is packed with an adsorbent in a fixed bed. The adsorption section is operated at 90° C. and a pressure of 0.15 MPa. One column is on stream (i.e. in operation) and the other column remains in reserve. The adsorbent used to pack the two columns is activated carbon (reference ROY 0.8 from Cabot Norit) consisting of cylindrical extrudates with a diameter of 0.8 mm.

[0115] In one column, the residence time is fixed at 40 min. The linear velocity of the empty column is 2.4 cm / min.

[0116] 780 g of the liquid stream from the adsorption step are mixed with water in a stirred tank to reach a final content of 20 wt. % BHET and 80 wt. % water and a final temperature of 60° C. The mixture is kept under stirring and cooled to 50° C. over 1 hour, then gradually cooled to 20° C. according to a gradient of 12° C. / h.

[0117] During the cooling process, solid particles are formed, resulting in a suspension of solids in a liquid that contains primarily water. The resulting suspension is then filtered at 20° C. to recover a solid cake and a colored liquid filtrate. The solid cake is rinsed with 1.5 L of water. The rinsed solid cake is recovered and then dried overnight at 40° C. under vacuum to yield 320 g of a white solid containing 99% by weight of BHET diester (composition determined by liquid chromatography).

[0118] The recovered solid is white in color. UV-Visible spectroscopy is performed on the BHET solution. A BHET solution is prepared by dissolving a sample of the obtained white solid in ethanol at 5% by weight. UV-Visible spectroscopy is performed using a Hach DR3900 laboratory benchtop UV-Visible spectrometer in a cuvette with a 1 inch path length.

[0119] The spectrum obtained (see Figures 1 and 2) does not display any significant absorption bands in the wavelength range of 400-800 nm.

[0120] Chromaticity measurements are also performed on the obtained solid BHET according to the method of ASTM D6290 2019. A 5 g sample of the solid BHET product is ground in a mortar with a pestle. 5 g of the ground BHET is placed in a cuvette made of optical quality glass with a diameter of 34 mm. Measurements are performed in reflection using a Konica Minolta CM-2300d colorimeter and SpectraMagic NX software under the following conditions: illuminant D65, specular reflection excluded, standard observer at 10°. Measurements are performed according to the CIE L * a * b *The results are expressed in the reference frame. The results were obtained by averaging the values ​​obtained in 10 measurements performed on the sample. The results are shown in Table 1.

[0121] [Table 1]

[0122] The resulting color measurements are in agreement with the target values.

[0123] Example 3 - Not in accordance with the invention The crude BHET is pressurized to 0.15 MPa and fed to a mixing section, to which a stream of ethylene glycol is also fed. The feed rate of ethylene glycol is adjusted so that the crude BHET represents 50% by weight of the mixture (crude BHET + ethylene glycol). The mixing section is operated at 120° C. and a pressure of 0.15 MPa.

[0124] The resulting mixture is then fed to an adsorption section consisting of two columns, each of which is packed with an adsorbent in a fixed bed. The adsorption section is operated at 150° C. and a pressure of 0.15 MPa. One column is on stream (i.e. in operation) and the other column remains in reserve. The adsorbent used to pack the two columns is activated carbon (reference ROY 0.8 from Cabot Norit) consisting of cylindrical extrudates with a diameter of 0.8 mm.

[0125] In one column, the residence time is fixed at 40 min. The linear velocity of the empty column is 2.4 cm / min.

[0126] The effluent, consisting of 50% by weight BHET in a BHET-ethylene glycol mixture, is collected over time at the outlet of the column.

[0127] The product obtained after 40 hours of operation has a bluish tint.

[0128] UV-Visible spectroscopy measurements are performed on the BHET solution. The BHET solution is prepared by using a sample of the effluent obtained at 40 hours of operation, which is then dissolved in ethanol to reach a concentration of 5% by weight of BHET in the final solution. UV-Visible spectroscopy measurements are performed using a Hach DR3900 laboratory benchtop UV-Visible spectrometer in a cuvette with a 1 inch path length.

[0129] The spectrum obtained (see Figure 1) has a significant absorption band in the wavelength range of 550-650 nm, which is consistent with the bluish tint of the product. [Brief description of the drawings]

[0130] [Figure 1] 1 shows the UV-visible spectra obtained from 550 nm to 700 nm for the effluents generated by the methods described in Examples 1, 2 and 3. [Diagram 2] 1 shows the UV-visible spectrum obtained from 350 nm to 850 nm for the effluent produced by the method described in Example 1.

Claims

1. A method for purifying a crude diester monomer feedstock, the method comprising the following steps: a) A mixing step; feeding a crude diester monomer feedstock and an aqueous solvent, carried out at a temperature of 60°C to 150°C; obtaining an aqueous mixture of diester monomers; adjusting the amount of the aqueous solvent introduced such that the crude diester monomer feedstock accounts for 20% to 90% by weight of the total weight of the aqueous diester monomer mixture. b) An adsorption step; carried out by contacting the aqueous crude diester monomer mixture with at least one adsorbent at a temperature of 60°C to 150°C and a pressure of 0.1 to 1.0 MPa; obtaining a purified monomer effluent.

2. The method according to claim 1, wherein the aqueous solvent contains at least 50% by weight of water, preferably at least 75% by weight of water, preferentially at least 90% by weight of water, even more preferentially at least 97% by weight of water, preferably at least 99% by weight of water.

3. The method according to claim 1 or 2, wherein the adjustment of the amount of the aqueous solvent introduced in step a) is carried out such that the crude diester monomer feedstock accounts for 30% to 80% by weight, preferably 50% to 75% by weight, preferably 40% to 60% by weight of the total weight of the aqueous diester monomer mixture.

4. The method according to claim 1, wherein the temperature during the mixing step a) is 70°C to 120°C, preferably 75°C to 110°C, and the pressure at that time is preferably 0.1 to 1.0 MPa, preferentially 0.1 to 0.8 MPa, preferably 0.1 to 0.5 MPa.

5. The method according to claim 1, wherein the temperature during the adsorption step b) is 70°C to 120°C, preferentially 75°C to 110°C.

6. The method according to claim 1, wherein the pressure during the adsorption step b) is 0.1 to 0.8 MPa, more specifically 0.1 to 0.5 MPa.

7. The method according to claim 1, wherein the at least one adsorbent is selected from activated carbon, alumina, and clay.

8. In the adsorption step b), 1 to 5 adsorbents, preferably 1 or 2 adsorbents, are used, the method according to claim 1.

9. The method according to claim 8, wherein the at least one adsorbent is activated carbon.

10. The method according to claim 1, wherein the adsorption step b) is carried out in a flow-through fixed bed mode.

11. The separation step c) is included downstream of the step b) to obtain a separated and purified diester monomer effluent and a used aqueous solvent effluent. In the separation step c), preferably, separation by solid-liquid separation, such as filtration, decantation, and / or centrifugation, is used, the method according to claim 1.

12. Step c) of crystallizing the diester monomer ＊ ) is included, preferably arranged downstream of the adsorption step b) and particularly upstream of the separation step c). In the crystallization step c ＊ ), at least one solid formation section is used, and the purified monomer effluent from the adsorption step b) is fed to the solid formation section to produce an aqueous suspension of solid diester monomer, the method according to claim 1.

13. A crystallization solvent identical to or different from the aqueous solvent introduced into the mixing step a) is fed to the solid formation section, and the amount of the introduced crystallization solvent is adjusted such that the crude diester monomer feedstock fed to the mixing step a) accounts for 1 wt% to 75 wt%, preferably 5 wt% to 45 wt%, and more preferably 15 wt% to 35 wt% of the total weight of the mixture in the solid formation section, the method according to claim 12.

14. Crystallization step c ＊The temperature when operating the solid formation section of ( ) is from 0 °C to 100 °C, preferably from 5 °C to 80 °C, preferably from 10 °C to 70 °C, and the pressure at that time is preferably from 0.00001 to 1.00 MPa, preferentially from 0.0001 to 0.50 MPa, preferably from 0.001 to 0.20 MPa, the method according to claim 12 or 13.

15. In separation step c), feed the purified monomer effluent from adsorption step b) or the aqueous suspension of the solid diester monomer obtained at the end of crystallization step c ＊ ) to obtain a separated and purified diester monomer effluent and a used aqueous solvent effluent, the method according to claim 11.

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