Method for depolymerising a polyester comprising polyethylene terephthalate by recirculating an oligomer effluent

Abstract

The invention relates to a method for depolymerising a polyester feedstock comprising PET. The method comprises a step a) of conditioning the polyester feedstock, a depolymerisation step b) which implements a plurality of reaction sections (A) (B) (N), a step (c) of separating the diol, a step (d) of separating a heavy impurity effluent and, optionally, a bleaching step (f) followed by a crystallisation step (g). The heavy impurity effluent undergoes a separation step (e) in order to produce two fractions: a first fraction of which at least 70 wt% is sent to the second reactor (B) and a second fraction which is at least partially removed from the process.

Description

[0001]PROCESS FOR DEPOLYMERIZING A POLYESTER COMPRISING POLYETHYLENE TEREPHTHALATE WITH RECYCLE OF AN OLIGOMERIC EFFLUENT Technical field The invention relates to a process for depolymerizing a polyester, in particular polyester terephthalate, comprising polyethylene terephthalate (PET), with a view to recycling it. More particularly, the invention relates to a process for depolymerizing a polyester feedstock comprising PET, to produce a diester monomer, said process having an optimized recycle of an oligomer effluent. Prior art The chemical recycling of polyester, in particular polyethylene terephthalate (PET), has been the subject of numerous studies aimed at decomposing the polyester recovered in the form of waste into monomers which can again be used as feedstock for a polymerization process. Many polyesters come from material collection and sorting circuits. In particular, polyester, in particular PET,can come from the collection of bottles, trays, films, resins and / or fibers composed of polyester (such as textile fibers, tire fibers). Polyester from collection and sorting channels is called polyester for recycling. PET for recycling can be classified into four main categories: - clear PET, consisting mainly of colorless transparent PET (generally at least 60% by weight) and azure transparent PET, which does not contain pigments and can be used in mechanical recycling processes, - dark PET, or colored (green, red, etc.), which can generally contain up to 0.1% by weight of dyes or pigments but remains transparent, or translucent; - opaque PET, which contains a significant quantity of pigments at levels typically varying between 0.25 and 5.0% by weight to opacify the polymer. Opaque PET is increasingly used, for example, in the manufacture of food containers, such as milk bottles,in the composition of cosmetic, phytosanitary or dye bottles; - multi-layer PET, which comprises layers of polymers other than PET or a layer of recycled PET between layers of virgin PET (i.e. PET that has not been recycled), or an aluminum film for example. Multi-layer PET is used after thermoforming to make packaging such as trays. The collection channels, which feed the recycling channels, are structured differently depending on the country. They evolve in such a way as to maximize the quantity of plastic recovered in the waste depending on the nature and quantity of the flows and the sorting technologies. The recycling channel for these flows generally consists of an initial packaging stage in the form of flakes during which bales of raw packaging are unpacked and the containers washed,sorted and crushed, then purified and sorted again to produce a flake stream generally containing less than 1% by mass of "macroscopic" impurities (glass, metals, other plastics, wood, cardboard, mineral elements). The clear PET flakes can then undergo an extrusion-filtration step to produce extrudates that are then reusable in a mixture with virgin PET to make new products (bottles, fibers, films). A solid-state vacuum polymerization step (known by the acronym SSP) is necessary for food uses. This type of recycling is called mechanical recycling. Dark (or colored) PET flakes are also mechanically recyclable. However,The coloring of extrudates formed from colored streams limits uses: dark PET is most often used to produce fibers or packaging strips. The outlets are therefore more limited compared to those of clear PET. The presence of opaque PET containing pigments at high levels in the PET to be recycled poses problems for recyclers because opaque PET alters the mechanical properties of the recycled PET. Opaque PET is currently collected with colored PET and ends up in the colored PET stream. Given the development of uses for opaque PET, the opaque PET contents in the colored PET stream to be recycled are currently between 5-20% by weight and tend to increase further. In a few years, it will be possible to achieve opaque PET contents in the colored PET stream higher than 20-30% by weight. However, it has been shown that beyond 10-15% of opaque PET in colored PET streams,the mechanical properties of recycled PET are altered (see “Impact of the development of white opaque PET on the recycling of PET packaging”, preliminary COTREP note of 5 / 12 / 13) and prevent recycling in the form of fibers, the main outlet for colored PET. Dyes are natural or synthetic substances, soluble in particular in polyester material and used to color the material into which they are introduced. The dyes generally used are of different natures and often contain heteroatoms of type O and N, and conjugated unsaturations, such as quinone, methine, azo functions, or molecules such as pyrazolone and quinophthalone. Pigments are finely divided substances, insoluble in particular in polyester material, used to color and / or opacify the material into which they are introduced. The main pigments used to color and / or opacify polyesters,in particular PET, are metal oxides such as TiO2, CoAl2O4, Fe2O3, silicates, polysulfides, and carbon black. Pigments are particles generally between 0.1 and 10 µm in size, and mostly between 0.4 and 0.8 µm. The total removal of these pigments by filtration, necessary to consider recycling opaque PET, is technically difficult because they are extremely clogging. Mechanical recycling of colored and opaque PET is therefore extremely delicate. Document FR 3053691 A1 describes a process for depolymerizing a polyester filler comprising opaque PET and in particular 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 specific separation and purification steps. This document considers the possibility of reactive extrusion in a first stage of conditioning the feedstock to initiate the depolymerization reaction. It also discusses the recycling of heavy residues separated during the purification steps, to be treated with the polyester feedstock. Document FR 3105236 describes an improvement to the process for depolymerization by glycolysis of a polyester feedstock comprising PET and in particular that of document FR 3053691. The improvement consists in particular of optimizing the conditioning phase of the polyester feedstock by mixing the feedstock with at least one recycled oligomer residue effluent in the presence of diol,upstream of its introduction into the depolymerization step. The present application proposes to increase the yield of the depolymerization process by optimizing the use of the heavy impurity effluent obtained during the separation of the effluent rich in liquid monomers. Summary of the invention The present invention relates to a process for depolymerizing a polyester feedstock comprising PET, said process comprising: a) a conditioning step, supplied at least by said polyester feedstock, to produce a conditioned feedstock stream; b) a depolymerization step implementing a first reaction section and at least one second reaction section, said at least one second reaction section operating at a temperature strictly lower than the temperature of said first reaction section, the first reaction section being supplied at least by the conditioned feedstock stream and optionally by a first diol addition,said at least one second reaction section being fed with the effluent from the first reaction section, and with a recycled oligomer effluent and optionally with a second diol supplement, so that the total quantity of diol feeding said step b) is adjusted to 1 to 20 moles of diol per mole of diester feeding said step b), said step b) being carried out at a temperature of between 180 and 300°C, and with a residence time of between 0.334 and 10 h; c) a diol separation step, fed at least with the effluent from step b), carried out at a temperature of between 100 and 250°C, at a pressure lower than that of step b) and producing a diol effluent and an effluent rich in liquid monomers; d) a step of separating the liquid monomer-rich effluent from step c) into a heavy impurity effluent and a pre-purified monomer effluent,operated at a temperature less than or equal to 250°C and a pressure less than or equal to 0.001 MPa with a liquid residence time less than or equal to 10 min, e) a step of separating said heavy impurity effluent into two fractions: a first fraction of which at least 70% by weight makes up the recycled oligomer effluent which feeds step b) and a second fraction which is at least partly removed from the process; f) optionally a step of decolorization of the pre-purified monomer effluent, carried out at a temperature between 100 and 250°C, and at a pressure between 0.1 and 1.0 MPa in the presence of an adsorbent, and producing a purified monomer effluent, g) optionally a step of crystallization of the purified monomer effluent, using at least one solid production section, operated at a temperature between 0 and 100°C and at a pressure between 0.00001 and 1.00 MPa, followed by a solid-liquid separation section,producing a decolorized and purified monomer effluent. Recycling the oligomers obtained in step e) in the second reaction section of the depolymerization step b) which operates at a lower temperature than that of the first reaction section and at a lower temperature than that used in the conditioning step a) has several advantages, including: - an improvement in the quality of the monomer produced, in particular a lowering of the proportion of co-monomer, in particular BHETdeg, in the monomer product, responsible for the greatest loss of yield after the oligomers. Indeed, recycling the oligomers makes it possible to accelerate the rate of formation of the diester monomer (monomer of interest), in particular BHET,while maintaining a BHETdeg formation rate identical to that of the case without recycling of the oligomers; - a reduction in thermo-degradation phenomena by avoiding the presence of monomers and oligomers in the hottest zones of the process, in particular in the conditioning step (a) and in the first reaction section of step (b). This effect contributes to improving the quality of the final product; - a reduction in the temperatures of the reaction sections which also makes it possible to improve the quality of the product and to improve the energy efficiency of the process. According to the invention, said polyester feedstock may comprise at least colored PET, opaque PET or their mixtures. The conditioning step a) may implement at least one conditioning section (a1) to produce a fluid feed stream, and one mixing section (a2) to produce a mixed stream, said mixed stream corresponding to the conditioned feed stream,said conditioning section (a1) being supplied at least by said polyester feed and being implemented at a temperature between 150 and 300°C, said mixing section (a2) being at least supplied by said fluid feed stream from the conditioning section and a diol stream, at least a portion of said diol stream preferably being composed of at least a fraction of the diol effluent from step c), said mixing section (a2) being operated at a temperature between 150 and 300°C, with a residence time between 0.5 seconds and 20 minutes. The conditioning section (a1) may be operated in an extruder, said mixing section (a2) of the polyester feed from step a) possibly also being implemented within said extruder. The mixing section (a2) may implement at least one static or dynamic mixer. In step a), the weight ratio of the diol stream introduced in step a) relative to the polyester feed,may be between 0.03 and 6.00, preferably between 0.05 and 5.00, preferentially between 0.10 and 4.00, preferably between 0.50 and 3.00. Said at least one second reaction section may be operated at a temperature 5 to 50°C lower than the temperature of said first reaction section. The recycled oligomer effluent which feeds said at least one second reaction section may comprise the entire first fraction from step e). According to a particular embodiment, in step e), the second fraction may be split into two parts, a first part being injected into one of the reaction sections of step b) downstream of the first reaction section, the second part being removed from the process. According to the invention, the depolymerization step may comprise two reaction sections. Alternatively, according to the invention, the depolymerization step may comprise three reaction sections,the second reaction section and the third reaction section each being fed with a portion, varying between 0 and 100% by weight, of the recycled oligomer effluent, the sum of said portions constituting 100% by weight of said recycled oligomer effluent. The depolymerization step may comprise at least one internal recirculation loop implementing at least the following operations: the withdrawal of a fraction of the reaction system from one of the reaction sections, the filtration of said fraction, and the reinjection of said fraction into one of the reaction sections. The separation step e) may be carried out so as to obtain a first fraction enriched in oligomers and a second fraction enriched in heavy impurities,the first fraction enriched in oligomers having an oligomer content strictly greater than the oligomer content of said heavy impurity effluent and the second fraction having a heavy impurity content strictly greater than the heavy impurity content of said heavy impurity effluent. The first fraction obtained in step e) may be mixed with a diol stream, preferably with a fraction of the diol effluent obtained in step c) before being recycled to step b) and / or optionally to step a). The diol separation step c) may be carried out in 1 to 5 successive gas-liquid separation sections each producing a gas effluent and a liquid effluent, the liquid effluent from the previous section feeding the subsequent section, the liquid effluent from the last gas-liquid separation section constituting the liquid monomer-rich effluent,all of the gas effluents being recovered to constitute the diol effluent. According to an improvement of the invention, the conditioning section can be supplied, in addition, with a diol flow. List of figures Figure 1 represents a diagram of the process according to the invention. Figure 1 is explained below. Figure 2 represents a particular diagram of the process according to the invention and illustrates the process according to the invention described in Example 1. Figure 3 represents a diagram of a process for depolymerizing a polyester not in accordance with the invention, comprising a recycle to the conditioning step a), and illustrates the process described in comparative Example 2. Figure 4 represents a diagram of a process for depolymerizing a polyester not in accordance with the invention, comprising a recycle to reactor A and illustrates the process described in comparative Example 3. Figure 5 represents a diagram of a process for depolymerizing a polyester not in accordance with the invention,and illustrates the method described in Comparative Example 4. The references used in Figures 2 to 5 identical to the references used in Figure 1 represent and designate the same elements. Description of the embodiments Figure 1 illustrates a process diagram according to the invention without limiting its scope. In the embodiment described with reference to Figure 1, the method implements a step (a) of conditioning the load (1) comprising PET. The conditioning step (a) implements a conditioning section (a1) (for example an extruder) to condition the load (1) and obtain a fluid load, at least one mixing section (a2) (for example a static or dynamic mixer,or an extruder portion) fed with the fluid feed and a diol stream (2) which may advantageously be a fraction of the diol effluent (3) recovered in step (c). The mixed stream (or conditioned feed stream) obtained at the end of step (a) is introduced into the depolymerization step (b) which uses several reaction sections, for example: a first reaction section (A) and a second reaction section (B), or even other reaction sections. In Figure 1, only the reaction sections (A), (B) and (N) are shown. Without departing from the scope of the invention, there may be other reaction sections between the reaction section (B) and the reaction section (N). The mixture passes successively into the reaction section (A) then into the reaction section (B), possibly into a reaction section not shown,then into the reaction section (N). Each of the reaction sections can be supplied with diol (3) from step (c). The effluent obtained at the end of the depolymerization step (b) is introduced into the diol separation step (c) making it possible to recover a diol effluent (3) and a monomer-rich effluent. The monomer-rich effluent is introduced into the separation step (d) making it possible to obtain a pre-purified monomer effluent and to eliminate a heavy impurity effluent, comprising oligomers and heavy impurities, which is sent to the separation step (e). In the separation step (e), the heavy impurity effluent is separated into two fractions: a first fraction (6) constituting the recycled oligomer effluent which feeds step (b) and a second fraction (7) which is discharged from the process according to the invention, in other words which is purged. According to the invention,at least 70% by weight of the first fraction constituting the recycled oligomer effluent which feeds step (b) is injected into the second reaction section (B). Before being recycled to step (b), the recycled oligomer effluent may be mixed in a static or dynamic mixer (a3) ​​with a diol stream (2) which may advantageously be a fraction of the diol effluent (3) recovered in step (c). The pre-purified monomer effluent obtained at the end of step d) may be sent to a step (f) of decolorization by adsorption, then a crystallization step (g) to recover a decolorized purified diester monomer effluent (4). The diol effluent (3) obtained in step (c) is advantageously recycled, in whole or in part, to step (b) in one or more of the reaction sections (A),(B) and (N) or a possible additional reaction section between section (B) and (N). In addition, a fraction of the diol effluent (3) obtained in step (c) can be recycled to one or more of the following steps: to step (f), to step (a), as diol stream (2), for example in the static mixer (a2) and / or in step (e), by being mixed with the heavy impurity effluent before separation and / or in the mixer (a3) ​​to be mixed with the recycled oligomer effluent (6) before being sent to step (b). According to the invention, the polyethylene terephthalate or poly(ethylene terephthalate), also simply called PET, has an elementary repeating unit of formula:, Conventionally, PET is obtained by polycondensation of terephthalic acid (PTA), or dimethyl terephthalate (DMT), with ethylene glycol. In the remainder of the text, the expression "per mole of diester in said polyester feedstock" corresponds to the number of moles of unit –[O- CO-O-(C6H4)-CO-O-CH2-CH2]-, which is the diester unit resulting from the reaction of PTA and ethylene glycol, in the PET included in said polyester feedstock. According to the invention, the term "monomer" or "diester monomer" advantageously designates the monomer of interest. Preferably, the monomer of interest is bis(2-hydroxyethyl) terephthalate (BHET) of chemical formula HOC2H4-CO2-(C6H4)-CO2-C2H4OH, in which - (C6H4)- represents an aromatic cycle, and which is the diester unit resulting from the reaction of PTA and ethylene glycol, in the PET included in said polyester filler. There are co-monomers, defined by the occurrence of a single terephthalic unit, but different from BHET.The most predominant is 2-(2-hydroxyethoxy) ethyl 2-hydroxyethyl terephthalate (BHETdeg) with the chemical formula HOC2H4-CO2-(C6H4)-CO2-C2H4O-C2H4OH and CAS# 65133-69-9. The term "oligomer" typically refers to a small polymer, generally consisting of 2 to 20 elementary repeating units. According to the invention, the term "ester oligomer" or "diester oligomer" or "BHET oligomer" refers to a terephthalate ester oligomer, comprising between 2 and 20, preferably between 2 and 5, elementary repeating units of the formula –[O-CO-(C6H4)-CO-O-C2H4]-, with -(C6H4)- an aromatic ring. The term "heavy impurities" refers in particular to pigments, polymers, possibly present, other than polyester and polymerization catalysts. According to the invention, the terms "diol" and "glycol" are used interchangeably and correspond to compounds comprising 2 hydroxyl groups -OH.The preferred diol is ethylene glycol, also called mono-ethylene glycol or MEG. The diol streams or diol effluents used in the steps of the process of the invention thus preferably comprise ethylene glycol (or MEG) in a very large quantity, i.e. such that the MEG represents 95% by weight or more of the total weight of said diol stream or diol effluent. The term "dye" defines a substance soluble in the polyester material and used to color it. The dye may be of natural or synthetic origin. According to the invention, the term "pigment", more particularly coloring and / or opacifying pigment, defines a finely divided substance, insoluble in particular in the polyester material. The pigments are in the form of solid particles, generally between 0.1 and 10 µm in size, and mostly between 0.4 and 0.8 µm. They are often of mineral nature.Pigments generally used, in particular for opacifying, are metal oxides such as TiO2, CoAl2O4, Fe2O3, silicates, polysulfides, and carbon black. According to the present invention, the expressions "between ... and ..." and "between ... and ..." mean that the limit values ​​of the interval are included in the range of values ​​described. If this is not the case and the limit values ​​are not included in the range described, such clarification will be provided by the present description. For the purposes of the invention, the different parameter ranges for a given step such as pressure ranges and temperature ranges can be used alone or in combination. For example, for the purposes of the present invention, a range of preferred pressure values ​​can be combined with a range of more preferred temperature values. In the following, particular and / or preferred embodiments of the invention are described.They can be implemented separately or combined with each other without limitation of combinations when this is technically feasible. According to the present invention, the pressures are absolute pressures. They are given in MPa (or MPa). According to the invention, the times and durations are expressed in hours (h), minutes (min) and / or seconds (sec). The terms "upstream" and "downstream" are to be understood according to the general flow of the fluid(s) or stream(s) in question in the process. Feed The process according to the invention is supplied with a polyester feed comprising at least one polyester, i.e. a polymer whose main chain repeating unit contains an ester function, and comprising polyethylene terephthalate (PET), preferably comprising at least colored PET and / or opaque PET, or mixtures thereof.Said polyester filler is advantageously a polyester filler to be recycled, originating from waste collection and sorting channels, in particular plastic waste. Said polyester filler may originate, for example, from the collection of bottles, trays, films, resins and / or fibers made of polyethylene terephthalate. Advantageously, the polyester filler comprises at least 50% by weight, preferably at least 70% by weight, more preferably at least 90% by weight of polyethylene terephthalate (PET). Preferably, said polyester filler comprises at least one PET chosen from colored, opaque, dark, multi-layer PET and their mixtures. Very particularly, said polyester filler comprises at least 10% by weight of opaque PET, very preferably at least 15% by weight of opaque PET, said opaque PET being advantageously opaque PET to be recycled, i.e. originating from collection and sorting channels.Said polyester filler advantageously comprises from 0.1% to 10% by weight of pigments, advantageously from 0.1 to 5% by weight. It may also comprise in particular from 0.05% to 1% of dyes, preferably from 0.05 to 0.2% by weight. In the collection and sorting channels, the polyester resin waste is washed and ground before constituting the polyester filler of the process according to the invention. The polyester fiber waste is densified in the form of granules or popcorn so that it can be mechanically introduced into the process according to the invention. The polyester filler may therefore be, in whole or in part, in the form of granules or flakes, the greatest length of which is less than 10 cm, preferably between 5 and 25 mm or in the form of a micronized solid, that is to say in the form of particles preferably having a size of between 10 microns and 1 mm.The filler may also comprise “macroscopic” impurities, preferably less than 5% by weight, preferably less than 3% by weight of “macroscopic” impurities, such as glass, metal, plastics other than polyester (for example PP, HDPE, etc.), wood, cardboard, mineral elements. Said polyester filler may also be, in whole or in part, in the form of fibers, such as textile fibers, optionally pretreated to remove cotton fibers, polyamide fibers, or any other textile fiber other than polyester, or such as tire fibers, optionally pretreated to remove in particular polyamide fibers or rubber or polybutadiene residues. Said polyester filler may, in addition, comprise polyester from the production waste of the polymerization and / or transformation processes of the polyester material.The polyester filler may also comprise elements used as polymerization catalysts and as stabilizing agents in PET production processes, such as antimony, titanium, tin. Conditioning step a) Said method according to the invention comprises a conditioning step a) which is supplied at least with the polyester filler and optionally a diol stream, to produce a conditioned feed stream. Conditioning step a) makes it possible to make the polyester filler at least partly liquid. Indeed, step a) makes it possible to bring the polyester filler to the operating conditions, of temperature and pressure, of depolymerization step b) and to make it at least partly liquid. Preferably, said polyester conditioning step a) is carried out at a temperature between 150 and 300°C, preferably between 225 and 275°C.This temperature is high enough to be close to or even slightly higher than the melting temperature of the polyester, in particular PET, of the polyester filler, so that the latter is at least partly liquid, but is kept as low as possible to minimize degradation of the polyester. Preferably, the conditioning step a) is carried out under an inert atmosphere to limit the introduction of oxygen into the system and the oxidation of the polyester filler. Preferably, said conditioning step a) uses at least one conditioning section and one mixing section, said conditioning section being supplied at least by said polyester filler and producing a fluid filler stream, said mixing section being at least supplied by said fluid filler stream and at least one diol stream, said mixing section producing a mixed stream.In this preferred embodiment, the mixed flow corresponds to the flow of conditioned feed which is recovered at the outlet of step a) and which advantageously feeds the depolymerization step b). Said conditioning section of step a) makes it possible to heat and pressurize said polyester feed at the operating conditions of the depolymerization step b). In the conditioning section, the polyester feed is gradually heated to a temperature close to or even slightly above its melting temperature so as to become at least partly liquid. Advantageously, at least 70% by weight of the polyester feed, very advantageously at least 80% by weight, preferably at least 90% by weight, preferentially at least 95% by weight of the polyester feed is in liquid form at the end of the conditioning section of step a).The temperature at which the conditioning section of step a) is carried out is advantageously between 150 and 300°C, preferably between 225 and 275°C. Preferably, the conditioning section is operated under an inert atmosphere to limit the introduction of oxygen into the system and the oxidation of the polyester feedstock. The conditioning section may be operated with the addition of a diol flow to the polyester feedstock. In this case, the weight ratio of diol to polyester feedstock in the conditioning section is between 0.001 and 0.1, preferably between 0.002 and 0.05, more preferably between 0.003 and 0.03. According to a preferred embodiment of the invention, said conditioning section is an extrusion section which corresponds to a screw conveyor section. In other words, the conditioning section is operated in an extruder.The residence time in said extrusion section, defined as the volume of said section divided by the volume flow rate of the feedstock, is advantageously less than or equal to 5 hours, preferably less than or equal to 1 hour, preferably less than or equal to 30 minutes, preferably less than or equal to 10 minutes and preferably greater than or equal to 2 minutes. Advantageously, the extrusion section makes it possible to condition the polyester feedstock such that the fluid feedstock flow is at a temperature between 150 and 300°C, preferably between 225 and 275°C, and at a pressure between atmospheric pressure (i.e. 0.1 MPa) and 20 MPa. Said extrusion section is advantageously connected to a vacuum extraction system so as to remove impurities such as dissolved gases, light organic compounds and / or moisture present in the feedstock.Said extrusion section may also advantageously comprise a filtration system for removing solid particles larger than 40 µm, and preferably smaller than 2 cm, such as sand particles. The polyester feedstock is advantageously fed into the extruder by any method known to those skilled in the art, for example via a feed hopper, and is advantageously inerted to limit the introduction of oxygen into the system. The mixing section is at least fed by said fluid feedstock flow from the conditioning section and by a diol flow. Said mixing section advantageously makes it possible to bring said polyester feedstock, previously treated in the conditioning section, into contact with a diol flow. This contacting has the effect of initiating the depolymerization reaction of the polyester feedstock, before introduction into depolymerization step b).It also makes it possible to significantly reduce the viscosity of the feedstock, which facilitates its transport in particular to the depolymerization step b). Very advantageously, said diol stream which feeds the mixing section is at least partly composed of at least a fraction of the diol effluent from step c) of the process. In this preferred embodiment, said mixing section of step a) (also called the polyester feedstock mixing section) is advantageously carried out at a temperature between 150 and 300°C, preferably between 225 and 275°C, at a residence time between 0.5 seconds and 20 minutes, preferably 1 second and 5 minutes, preferably between 3 seconds and 3 minutes, and such that the weight ratio of the diol stream introduced in step a) (i.e.in the mixing section and optionally in the conditioning section) relative to the polyester feedstock is between 0.03 and 6.00, preferably between 0.05 and 5.00, preferentially between 0.10 and 4.00, more preferably between 0.50 and 3.00. The residence time is defined here as the ratio between the volume of liquid in said mixing section relative to the volume flow rate of diester feedstock in said mixing section. In a first embodiment, said mixing section can implement at least one static or dynamic mixer, preferably between one and five successive static or dynamic mixers. In a second very advantageous embodiment and when the conditioning section is operated in an extruder, the polyester feedstock mixing section can thus be implemented within the extruder. In this case, it is a reactive extrusion phase.The diol stream and the diol stream, if used, may each be composed of a diol supplement external to the process according to the invention, a fraction of the diol effluent from step c), or their mixtures, preferably a fraction of the diol effluent from step c). Preferably, the diol stream and / or the diol stream is (are) advantageously heated prior to its (their) supply in step a) in order to facilitate the heating of the polyester feedstock.Depolymerization step b) The process according to the invention comprises a depolymerization step using a first reaction section and at least one second reaction section, said at least one second reaction section operating at a temperature strictly lower than said first reaction section, the first reaction section being fed at least by the conditioned feed stream and optionally by a first diol addition, said at least one second reaction section being fed by the effluent from the first reaction section, a recycled oligomer effluent and optionally by a second diol addition. Said recycled oligomer effluent comprises at least 70% by weight of the first fraction separated in step e). The depolymerization reaction corresponds to a depolymerization reaction by alcoholysis, preferably by glycolysis, i.e. preferably in the presence of diol.Advantageously, the depolymerization step is carried out so that the total quantity of diol feeding said step b), corresponding to the sum of the quantities of diol introduced in step a) and in step b), is adjusted from 1 to 20 moles, preferably from 3 to 15 moles, preferably from 5 to 10 moles of diol per mole of diester feeding said step b). In other words, the depolymerization step is advantageously carried out so that the weight ratio between the total quantity of diol introduced in step a) and step b) relative to the total quantity of diester contained in the conditioned feed stream and optionally the fraction of the recycled oligomer effluent from step e) recycled to step b) is respectively between approximately 0.3 and 6.7, preferably between approximately 1.0 and 5.0, preferably between 1.7 and 3.3.Advantageously, said depolymerization step b) uses at least two reaction sections, for example between 2 and 4 reaction sections, preferably two or three reaction sections. The reaction sections operate in series, that is to say that the effluent from one reaction section feeds a downstream reaction section. For example, the effluent from the first reaction section feeds the second reaction section, the effluent from the second reaction section feeds the third reaction section, and so on. Each reaction section can be implemented in any type of reactor known to those skilled in the art for carrying out a depolymerization or transesterification reaction, preferably in a reactor stirred by a mechanical stirring system and / or by a recirculation loop and / or by fluidization. Said reactor can comprise a conical bottom for purging impurities.According to the invention, the reaction section(s) from the second reaction section is operated at a temperature strictly lower than the temperature of the first reaction section, preferably at a temperature 5 to 50°C lower, preferably 10 to 50°C lower, or even 20 to 40°C lower, compared to the temperature of the first reaction section. Said reaction sections are operated at a temperature between 180 and 300°C, preferably between 190 and 300°C, preferably between 200°C and 280°C, in particular in the liquid phase. The residence time in the depolymerization step, i.e. the cumulative residence time in the reaction sections implemented in the depolymerization step, is between 0.334 and 10 h, preferably between 0.5 and 8 h, and preferentially between 1 and 6 h.The residence time is defined as the ratio of the volume of liquid in said reaction sections to the volume flow rate of the stream leaving the last reaction section. The operating pressure of said reaction section(s) of step b) is determined so as to maintain the reaction system in the liquid phase. This pressure is advantageously at least 0.1 MPa, preferably at least 0.4 MPa, and preferably less than 5 MPa. The term “reaction system” means all of the constituents and phases present in said step b) resulting from the feed of said step. The diol is advantageously monoethylene glycol. The first diol make-up which optionally feeds the first reaction section of step b) may be composed of a diol make-up external to the process according to the invention, a fraction of the diol effluent resulting from step c), or mixtures thereof, and preferably a fraction of the diol effluent resulting from step c).The second diol supplement which optionally feeds said at least one second reaction section of step b) may be composed of a diol supplement external to the process according to the invention, a fraction of the diol effluent from step c), or mixtures thereof, and preferably a fraction of the diol effluent from step c). The depolymerization reaction may be carried out in the presence or absence of a catalyst. When the depolymerization reaction is carried out in the presence of a catalyst, the latter may be homogeneous or heterogeneous and chosen from esterification catalysts known to those skilled in the art such as oxide complexes and salts of antimony, tin, titanium, alkoxides of metals from groups (I) and (IV) of the periodic table of elements, organic peroxides, acid-base metal oxides.A preferred heterogeneous catalyst advantageously comprises at least 50% by mass relative to the total mass of the catalyst, preferably at least 70% by mass, advantageously at least 80% by mass, very advantageously at least 90% by mass, and even more advantageously at least 95% by mass of a solid solution consisting of at least one spinel of formula Z. x Al2O (3+x)in which x is between 0 (limit excluded) and 1, and Z is chosen from Co, Fe, Mg, Mn, Ti, Zn, and comprising at most 50% by mass of alumina and oxide of element Z. Said preferred heterogeneous catalyst advantageously contains at most 10% by mass of dopants chosen from silicon, phosphorus and boron taken alone or in a mixture. For example, and in a non-limiting manner, said solid solution may consist of a mixture of ZnAl2O4 spinel and CoAl2O4 spinel, or consist of a mixture of ZnAl2O4 spinel, MgAl2O4 spinel and FeAl2O4 spinel, or consist solely of ZnAl2O4 spinel. Preferably, said depolymerization step is carried out without adding an external catalyst to the polyester feedstock.Said depolymerization step may advantageously be carried out in the presence of a solid adsorbent agent in powder form or in a shaped form, the function of which is to capture at least part of the colored impurities, thus alleviating the bleaching step f). Said solid adsorbent agent is advantageously an activated carbon. The depolymerization reaction advantageously makes it possible to convert the polyester feedstock into ester monomers and oligomers, advantageously PET into at least one diester monomer and ester oligomers, in particular the bis(2-hydroxyethyl) terephthalate (BHET) monomer and BHET oligomers. The conversion of the polyester feedstock in said depolymerization step is greater than 50%, preferably greater than 70%, more preferably greater than 85%. The molar yield of diester monomer, in particular BHET, is greater than 50%, preferably greater than 70%, more preferably greater than 85%.The molar yield of diester monomer, in particular BHET, corresponds to the molar flow rate of diester monomer, in particular BHET, at the outlet of said step b) over the number of moles of diester (i.e. diester units) in the polyester feedstock feeding said step b). An internal recirculation loop may advantageously be implemented in step b), i.e. the withdrawal of a fraction of the reaction system from one of the reaction sections, preferably the filtration of this fraction, and the reinjection of said fraction into one of the reaction sections. The reaction system corresponds to the fluid contained in each of the reaction sections. According to one embodiment, the reinjection of the fraction may be carried out in the same reaction section from which the fraction was withdrawn.Alternatively, the fraction may be withdrawn from one of the reaction sections and the reinjection of said fraction may be carried out in another reaction section different from that from which the withdrawal was carried out. The method according to the invention may comprise several withdrawal loops. For example, each of the reaction sections is equipped with a recirculation loop, each of the recirculation loops being able to carry out a withdrawal and a reinjection in the same reaction section. This (these) internal loop(s) makes it possible to eliminate the solid, “macroscopic” impurities possibly included in the reaction liquid. Advantageously, the depolymerization step b) makes it possible to obtain a reaction effluent which is sent to a step c) of separation of the diol.Step c) of separation of the diol The process according to the invention comprises a step c) of separation of the diol, supplied at least with the effluent from step b), carried out at a temperature of between 100 and 250°C, at a pressure lower than that of step b) and producing a diol effluent and an effluent rich in liquid monomers. The main function of step c) is to recover all or part of the unreacted diol. Step c) is carried out at a pressure lower than that of step b) so as to vaporize a fraction of the effluent from step b) into a gas effluent and a liquid effluent. Said liquid effluent constitutes the effluent rich in liquid monomers. The gas effluent, consisting of more than 50% by weight of diol, preferably more than 70% by weight, more preferably more than 90% by weight, constitutes a diol effluent.Step c) is advantageously implemented in a gas-liquid separation section or a succession of gas-liquid separation sections, advantageously from 1 to 5 successive gas-liquid separation sections, very advantageously from 3 to 5 successive gas-liquid separation sections. Each of the gas-liquid separation sections produces a liquid effluent and a gas effluent. The liquid effluent from the previous section feeds the subsequent section. All of the gas effluents are recovered to constitute the diol effluent. The liquid effluent from the last gas-liquid separation section constitutes the liquid monomer-rich effluent. Advantageously, one or even at least two gas-liquid separation sections can be implemented in a falling film evaporator or a scraped film evaporator or a short-path distillation.Step c) is carried out in such a way that the temperature of the liquid effluents is maintained above the value below which the monomer, the polyester oligomers precipitate, and below a high value, depending on the diol / monomer molar ratio, above which the monomer re-polymerizes significantly. The temperature in step c) is between 100 and 250°C, preferably between 110 and 220°C, more preferably between 120 and 210°C. The operation in a succession of gas-liquid separations, advantageously in a succession of 2 to 5, preferably 3 to 5 successive separations, is particularly advantageous because it makes it possible to adjust in each separation the temperature of the liquid effluent meeting the aforementioned constraints. A re-polymerization inhibitor can advantageously be mixed with the liquid monomer-rich effluent before feeding said step c).The pressure in step c) is lower than that of step b) and is advantageously adjusted to allow evaporation of the diol at a temperature while minimizing re-polymerization and allowing optimal energy integration. It is preferably between 0.00001 and 0.2 MPa, preferably between 0.00004 and 0.15 MPa, more preferably between 0.00004 and 0.1 MPa. The separation section(s) are advantageously agitated by any method known to those skilled in the art. The diol effluent may contain other compounds such as dyes, acetaldehyde, dioxane, dioxolane, light alcohols, water, diamine monomers, urethane, diols including diethylene glycol or cyclomethane diol.At least a fraction of the diol effluent is very advantageously recycled, in liquid form (i.e. after condensation), to step b) and / or optionally to step a), and optionally to step f), optionally in a mixture with a diol supply external to the process according to the invention. All or part of said diol effluent may be treated in a purification step (3) prior to its recycling, in liquid form (i.e. after condensation), advantageously to step b) and / or optionally to step a) and / or its use in a mixture in step e). This purification step may comprise, in a non-exhaustive manner, adsorption on a solid (for example on activated carbon) to remove the dyes and one or more distillations to separate impurities such as diethylene glycol, water and other alcohols. The monomer-rich effluent obtained in step c) is sent to the separation step d).Step d) of separation of the monomer The process according to the invention comprises a step d) of separation of the monomer-rich effluent from step c) producing a heavy impurity effluent and a pre-purified monomer effluent. Said step d) is advantageously carried out at a temperature less than or equal to 250°C, preferably less than or equal to 230°C, and very preferably less than or equal to 215°C, and preferably greater than or equal to 110°C, and a pressure less than or equal to 0.001 MPa, preferably less than or equal to 0.0005 MPa, preferably greater than or equal to 0.000001 MPa, with a liquid residence time less than or equal to 10 min, preferably less than or equal to 5 min, preferably less than or equal to 3 min, and preferably greater than or equal to 0.1 second. The liquid residence time is defined as the ratio of the liquid volume in step d) to the volume flow rate of the liquid stream exiting step d).The purpose of this separation step d) is to separate all or part of the monomer, in particular the BHET, which is vaporized, from the oligomers, not entirely converted, which remain liquid and therefore carry oligomers corresponding to partially converted polyester polymer and also heavy impurities, in particular pigments, other polymers possibly present and polymerization catalysts, while minimizing the loss of monomers by re-polymerization. Some oligomers may possibly be carried along with the monomer, in particular those of small size. These heavy impurities are found with the oligomers in the heavy impurity effluent. Due to the possible presence of polymerization catalysts in the polyester feedstock, the separation must be carried out with very short liquid residence times and at a temperature not exceeding 250°C, in order to limit any risk of re-polymerization of the monomer during this step.Separation by simple atmospheric distillation is therefore not feasible. Separation step d) is advantageously carried out in a falling film or scraped film evaporation system or by short-path falling film or scraped film distillation. A very low operating pressure is required to be able to carry out step d) at a temperature below 250°C, preferably below 230°C, while still allowing vaporization of the monomer. A polymerization inhibitor may advantageously be mixed with the liquid monomer-rich effluent before feeding said step d). A fluxing agent may also advantageously be mixed with the liquid monomer-rich effluent before feeding said step d), so as to facilitate the removal of heavy impurities, in particular pigments, at the bottom of the evaporation or short-path distillation system.This flux must have a boiling point much higher than the monomer, in particular BHET, under the operating conditions of step d). It may be, for example, polyethylene glycol, or PET oligomers. Said heavy impurity effluent comprises in particular pigments, oligomers and unseparated monomer. Said heavy impurity effluent obtained in step d) is sent to separation step e). Step e) of separation of the heavy impurity effluent The process according to the invention comprises a step e) of separation of said heavy impurity effluent from step d) so as to produce two fractions: a first fraction of which at least 70% by weight composes the recycled oligomer effluent which feeds step b) and a second fraction which is at least partly removed from the process according to the invention.The heavy impurity effluent obtained at the end of step d) comprises monomer, oligomers and heavy impurities, in particular pigments, unconverted polyester polymer, possibly other polymers and polymerization catalysts. Preferably, the first fraction comprises at least 50% by weight, preferably at least 70%, more preferably at least 80% by weight, or even at least 90% by weight of the heavy impurity effluent obtained at the end of step d). According to a first embodiment, step e) of separating said heavy impurity effluent consists of a simple division into two fractions, the two fractions having the same chemical composition.According to a second embodiment, step e) of separating said heavy impurity effluent comprising the monomer, the oligomers and the heavy impurities is carried out so as to obtain a first fraction enriched in monomer and oligomers and a second fraction enriched in heavy impurities, that is to say that the first fraction enriched in oligomers comprises a content of monomer and oligomers strictly greater than the content of monomer and oligomers of said heavy impurity effluent from step d) and that the second fraction comprises a content of heavy impurities strictly greater than the content of heavy impurities of said heavy impurity effluent.For example, the first fraction enriched in monomer and oligomers has a content of at least 10% by weight, preferably at least 50% by weight, preferably at least 100% by weight of monomer and oligomers greater than the content of monomer and oligomers in said heavy impurity effluent from step d). For example, the second fraction enriched in heavy impurities has a content of at least 10% by weight, preferably at least 50% by weight, preferably at least 100% by weight greater than the content of heavy impurities in said heavy impurity effluent from step d). In the second embodiment, the separation of said heavy impurity effluent can be carried out by filtration, by decantation, by MEG extraction, by centrifugation, etc.For example, said heavy impurity effluent from step d) may advantageously undergo at least one separation step, for example by filtration so as to reduce the quantity of pigments and / or other solid impurities in the first fraction and recover a second fraction enriched in pigments and / or other solid impurities. Preferably, the entirety of the first fraction obtained in step e) may be recycled to the depolymerization step b). More particularly, at least 70% by weight of this first fraction is injected into the or one of the reaction sections downstream of the first reaction section and the remainder, i.e., less than 30% by weight, of the first fraction is injected into another reaction section of step b), for example into the first reaction section.For example, in the embodiment of step b) with two reaction sections, the first reaction section is fed with less than 30% by weight, preferably less than 20% by weight, more preferably less than 10% by weight of the first fraction, and the second reaction section is fed with at least 70% by weight, preferably at least 80% by weight, more preferably at least 90% by weight of the first fraction.For example, in the embodiment of step b) with three reaction sections, the first reaction section is fed with less than 30% by weight, preferably less than 20% by weight, more preferably less than 10% by weight of the first fraction, and the second reaction section and the third reaction section are each fed with a percentage of the first fraction varying between 0% by weight and 70% by weight, preferably between 0% by weight and 80% by weight, more preferably between 0% by weight and 90% by weight, the sum of the percentages of the first fraction feeding the second and third sections being respectively at least 70% by weight, preferably at least 80% by weight, preferably at least 90% by weight of the first fraction. According to a particular embodiment of the invention, the entirety of the first fraction obtained in step e) is recycled by injection into the or one of the reaction sections downstream of the first reaction section.In other words, the first reaction section is not fed with the first fraction. In order to facilitate separation, the heavy impurity effluent may be mixed with a diol effluent, for example with a portion of the diol effluent from step c), a diol supplement external to the process according to the invention, or mixtures thereof. Before being recycled to step b), the first fraction may advantageously be mixed with a diol effluent, for example with a portion of the diol effluent from step c), a diol supplement external to the process according to the invention, or mixtures thereof. This mixing may be carried out in a static or dynamic mixer.Mixing the first fraction with a diol effluent facilitates the recycling of ester oligomers, in particular BHET oligomers, since the mixing makes it possible on the one hand to fluidize the first fraction, which potentially concentrates solid particles such as pigments and polymer compounds such as polyolefins, polyamides, polyurethanes present in the treated polyester feedstock and contributing to increasing the viscosity and fouling power of said residues, and therefore to simplify the operability of their transport, and on the other hand to reduce the viscosity of said first fraction and therefore promote its mixing with the polyester feedstock in the depolymerization step. According to one embodiment, the entirety of the second solid-enriched fraction can advantageously be purged from the process, for example by being sent to an incineration system.Alternatively, according to a second embodiment, a portion of the second solid-enriched fraction may be recycled to the depolymerization step, by injecting it into the or one of the reaction sections downstream of the first reaction section, the other remaining portion of the second fraction being purged from the process, for example by being sent to an incineration system. In this second embodiment, the portion of the second fraction that is purged may represent at least 50% by weight, or even at least 70%, preferably at least 80% of the second fraction.Optional decolorization step f) Advantageously, the process according to the invention may comprise a step of decolorization of the pre-purified monomer effluent from step d), carried out at a temperature of between 70 and 180°C, preferably between 80 and 150°C, and preferably between 90 and 120°C, and at a pressure of between 0.05 and 1.0 MPa, preferably between 0.05 and 0.8 MPa, and preferably between 0.1 and 0.5 MPa in the presence of at least one adsorbent and producing a purified monomer effluent. Said adsorbent may be any adsorbent known to those skilled in the art capable of capturing dyes, such as activated carbon, clays, advantageously an activated carbon. The pre-purified monomer effluent may be advantageously mixed with a solvent. The contacting of the pre-purified monomer effluent with the solvent can be carried out prior to contacting with said adsorbent.Said solvent optionally introduced is for example a fraction of the diol effluent from step c) optionally previously treated in a purification step or for example with a diol supplement external to the process according to the invention or for example with water. Optional crystallization step g) The purified monomer effluent obtained in optional step f) can optionally be introduced into a crystallization step g). When implemented, crystallization step g) advantageously implements at least one solid production section and at least one solid-liquid separation section. Crystallization step g), when implemented, makes it possible to obtain a decolorized purified diester monomer effluent and rejects a used solvent effluent. Advantageously, crystallization step g) implements one or more crystallization or precipitation operations and one or more solid-liquid separation operations.According to a particular embodiment, crystallization step g) implements a solid production section as described below, followed by a solid-liquid separation section as detailed below. According to another particular embodiment, crystallization step g) implements several solid production sections, preferably between two and five solid production sections, as described below, each of the solid production sections being followed by a solid-liquid separation section as detailed below. The solid production section of step g) is advantageously fed with the purified monomer effluent from optional step f). Optionally, the solid production section may also be fed with a crystallization solvent, which is identical to or different from the solvent used in optional step f).The crystallization solvent is advantageously chosen from water, mono-alcohols, diols, ethers, aldehydes, esters, hydrocarbons and mixtures of at least two of these compounds belonging to the same chemical family or different chemical families. Preferably, said crystallization solvent is chosen from water, mono-alcohols having between 1 and 12 carbon atoms, such as methanol or ethanol, diols having between 1 and 12 carbon atoms. Very advantageously, said crystallization solvent is water, a mono-alcohol having between 1 and 12 carbon atoms, such as methanol or ethanol, a diol having between 1 and 12 carbon atoms, such as ethylene glycol. Preferably, the crystallization solvent is water, a diol having between 1 and 12 carbon atoms, preferably ethylene glycol, or mixtures thereof.Preferably, when steps f) of decolorization and g) of crystallization are carried out and crystallization solvent is introduced in step g), the amount of crystallization solvent introduced into the solid production section is adjusted so that the purified monomer effluent which feeds step f) represents between 1 and 75% by weight, preferably between 5 and 45% by weight, more preferably between 15 and 35% by weight, of the total weight of the mixture in said solid production section (i.e. the mixture comprising the purified monomer effluent, the solvent introduced in step f) and the crystallization solvent introduced in step g)).Prior to its introduction into the solid production section, all or part of the crystallization solvent may be heated, preferably to the temperature at which the adsorption section is operated, or cooled and in particular brought to a temperature preferably between 0 and 120°C, preferably between 5 and 100°C, and preferably between 10 and 90°C. Advantageously, the solid production section of optional step g) is operated at a temperature (i.e. such that the temperature of the effluent from said solid production section is) between 0 and 100°C, preferably between 5 and 80°C, and preferably between 10 and 70°C.More specifically, in the solid production section, the purified monomer effluent pretreated by adsorption, optionally mixed with the crystallization solvent, is cooled from the temperature at which the adsorption section is operated, i.e. from a temperature between 70 and 180°C, preferably between 80°C and 150°C, preferentially between 90 and 120°C, to a temperature between 0 and 100°C, preferably between 5 and 80°C, and preferably between 10 and 70°C. The cooling can be carried out according to any method known to those skilled in the art.For example, in particular in discontinuous mode (batch mode according to the established Anglo-Saxon term), the cooling of the temperature can be carried out without regulation of the drop in temperature (i.e. without an imposed temperature ramp; thus only the initial and final temperatures are controlled) or according to at least one decreasing temperature ramp, in particular according to a decreasing temperature ramp of between 5 and 30°C / hour and more particularly between 8 and 15°C / hour, or even according to the two modes which follow one another successively, i.e. without control for a part of the cooling and according to a decreasing temperature ramp, for another part of the cooling.According to another example, the cooling may be simply due to the introduction of the flow to be cooled, i.e. the monomer effluent pretreated by adsorption from the adsorption step f) or the mixture comprising the monomer effluent pretreated by adsorption and the crystallization solvent, into a capacity, the volume of which is advantageously adapted to the flow rate of the flow to be cooled, maintained at a temperature of between 0 and 100°C, preferably between 5 and 80°C, and preferably between 10 and 70°C. The solid production section is advantageously operated at a pressure of between 0.00001 and 1.00 MPa, preferably between 0.0001 and 0.50 MPa, and preferably between 0.001 and 0.20 MPa. According to a particular embodiment of the invention, the solid production section is operated under vacuum, preferably at a pressure between 0.0001 and 0.10 MPa, preferably between 0.001 and 0.01 MPa.According to another particular embodiment, the solid production section is advantageously operated in a double-jacketed reactor, at a pressure between 0.01 and 1.00 MPa, preferably between 0.05 and 0.20 MPa, preferably at atmospheric pressure, i.e. at 0.10 MPa. Advantageously, the solid production section aims to make solid, i.e. to crystallize or precipitate, at least in part the diester monomer, preferably BHET, present in the purified monomer effluent pretreated by adsorption and resulting from step f). Thus, the solid production section comprises, preferably consists of, a precipitation or crystallization phase carried out by any precipitation or crystallization techniques known to those skilled in the art.Preferably, the solid production section is a crystallization section, for example by cooling or by concentration, implemented in any equipment known to those skilled in the art, as for example defined in the journal Techniques de L'ingénieur "Industrial Crystallization - Practical Aspects", ref. J2788 V1, followed by liquid-solid separation. According to a preferred embodiment, water as crystallization solvent is mixed with the purified monomer effluent from step f) and the solid production section in step g) is operated under conditions such that the temperature of the effluent from said solid production section is between 5 and 50°C, preferably between 10 and 40°C.According to another preferred embodiment, the crystallization solvent introduced and mixed with the purified monomer effluent from step f) is ethylene glycol and the solid production section is operated under conditions such that the temperature of the effluent from said solid production section is between 5 and 50°C, preferably between 10 and 40°C. Advantageously, said solid production section, preferably by crystallization, comprises one or more crystallization operations, operating in series or in parallel, carried out in batch or continuously, preferably continuously. The solid production section makes it possible to obtain a heterogeneous effluent, comprising a solid phase of diester monomer and a liquid phase. The heterogeneous effluent is advantageously sent to the solid-liquid separation section.In the solid-liquid separation section of the optional step g), the diester monomer, preferably BHET, advantageously in solid form, in particular in the form of crystals, is separated from the liquid phase comprising all or part of the solvent introduced in step f) and the crystallization solvent optionally introduced into the solid production section. The solid-liquid separation section advantageously uses any solid-liquid separation means known to those skilled in the art, in particular at least one filtration, decantation and / or centrifugation system. The solid diester monomer thus separated constitutes the decolorized purified diester monomer effluent, the liquid phase constituting the used solvent effluent.According to a particular embodiment, the purified decolorized diester monomer effluent, recovered at the end of the optional step g) in solid form, preferably by filtration or centrifugation, can also advantageously undergo all or some of the following operations, carried out one or more times without pre-defined chronological order: rinsing with a solvent, identical to or different from the solvent feeding step f) or possibly the solid production section of step g); additional filtration or centrifugation; removal of the residual solvent by any method known to those skilled in the art, for example by drying by evaporation; shaping, for example into powder or granules; and storage of the solid.According to another embodiment, the decolorized purified diester monomer effluent is recovered, preferably by filtration or centrifugation, in the solid-liquid separation section and is then directly sent (i.e. without a solid storage phase) to a polymerization step known to those skilled in the art, with optionally, prior to the polymerization reaction, rinsing with water or a diol effluent, for example an ethylene glycol effluent, preferably rinsing with water, of the solid effluent of decolorized purified diester monomer, then heating the rinsed solid to be melted. According to another embodiment, the decolorized purified diester monomer effluent is recovered, preferably by filtration or centrifugation, in the solid-liquid separation section and then the monomer is dried by one of the means known to those skilled in the art before being stored or conveyed to a polymerization step known to those skilled in the art.According to another embodiment, the decolorized purified diester monomer effluent is recovered, preferably by filtration or centrifugation, in the solid-liquid separation section and then the monomer is shaped after optionally having been dried by one of the means known to those skilled in the art (granulation, pelletization) before being stored or conveyed to a polymerization step known to those skilled in the art. The decolorized purified monomer effluent from the optional step g) can advantageously feed a polymerization step known to those skilled in the art in order to produce PET that is indistinguishable from virgin PET. The decolorized purified monomer effluent can advantageously be mixed with ethylene glycol, terephthalic acid or dimethyl terephthalate depending on the polymerization step chosen, prior to the polymerization reaction.Feeding the decolorized purified monomer effluent into a polymerization step makes it possible to reduce the feed of dimethyl terephthalate or terephthalic acid by an equivalent flow rate. EXAMPLES Example 1 described with reference to Figure 2 illustrates the invention without limiting its scope, Examples 2 to 4 described with reference to Figures 3 to 5 respectively present examples not in accordance with the invention. Example 1 (according to the invention) The process of Example 1 is shown diagrammatically in Figure 2. The feed (1) composed of 100% PET is supplied continuously at a flow rate of 2500 kg / h, which corresponds to a recycling capacity of 20 KTA (kilotons per year) of PET. As illustrated in Figure 2, the conditioning step (a) uses: - An extruder (a1) to condition the PET load (1), by melting it, - A static mixer (a2) to pre-mix the load from the extruder with a flow (2) of ethylene glycol and obtain a mixed flow.The mixed flow is introduced into the depolymerization step (b) which uses two reaction sections (A) and (B) arranged in series, each section being composed of a perfectly stirred reactor (CSTR type reactor, i.e. "Continuous Stirred Tank Reactor" or Closed Continuous Stirred Reactor). The useful volumes of the reactors are: (A): 6 m. 3 , (B): 42 m 3The reactor temperatures are: (A): 250°C, (B): 210°C. The diol separation section (c) extracts a diol effluent comprising mainly ethylene glycol. The flow rate of the separated diol effluent is 9800 kg / h. The diol effluent is then purified, then mixed with a fresh ethylene glycol flow rate corresponding to the depolymerization consumption, i.e. 700 kg / h in unit (3), and the whole is reinjected into reactor (A) for a flow rate of 7500 kg / h and into reactor (B) for a flow rate of 3000 kg / h. The diol flow sent to reactor (B) is used to dilute the recycled oligomer effluent (6) from separation section (e).The separation section (d) of the monomer-rich effluent extracts the monomers from a heavy impurity effluent which notably includes the heavier oligomers, with a yield of 75%, thus generating a flow of pre-purified monomers having a flow rate of 2815 kg / h and a heavy impurity effluent comprising heavier compounds (910 kg / h) as well as a lost portion of monomers (940 kg / h). The heavy impurity effluent (1850 kg / h) consisting of dimers and heavier compounds as well as lost monomers and heavy impurities, is directed to the separation section (e).The pre-purified monomer stream is discharged from (d) and is introduced into the decolorization step (f) and then into the crystallization step (g) to produce a purified and decolorized diester monomer stream (4) consisting of the monomer of interest BHET (2650 kg / h), the undesired monomer BHETdeg (150 kg / h) as well as compounds consisting in particular of monomer altered by thermal or thermo-oxidative degradation reactions (15 kg / h), which corresponds to a desired monomer purity degree of 94.1%. The heavy impurity effluent separation section (e) is a simple non-selective division of the stream, which leads to two fractions: a first fraction (6) representing 80% of the feed stream of the separation section (e) which constitutes the recycled oligomer effluent and a second fraction (7). The recycled oligomer effluent (6) (1480 kg / h) is fully introduced into the reactor (B), and the remainder (7) is purged (370 kg / h), i.e. evacuated from the process.Example 1 shows that directing the recycled oligomer effluent (6) to reactor (B) leads to the production of 15 kg / h of oxidized or thermally degraded species which are colored and difficult to extract from the final monomer. Example 2 (comparative: with recycle at conditioning step a) The process of example 2 is shown schematically in figure 3. The feed (1) composed of 100% PET is supplied continuously at a flow rate of 2500 kg / h which corresponds to a recycling capacity of 20 KTA (kilotons per year) of PET.As illustrated in Figure 3, the conditioning step (a) uses: - An extruder (a1) to condition the PET feedstock (1), by melting it, - A static mixer (a3) ​​to pre-mix the recycled oligomer effluent (8), comprising the oligomers from the separation step (d) with a stream (2) of ethylene glycol, and to obtain a mixture of residues (9) - A static mixer (a2) to pre-mix the feedstock from the extruder (a1) with a stream (2) of ethylene glycol and with the mixture of residues (9) to obtain a mixed stream. The mixed stream is introduced into the depolymerization step (b) which uses two reaction sections (A) and (B) arranged in cascade, each section being composed of a perfectly stirred reactor. The useful volumes of the reactors are: (A): 6 m. 3 , (B): 42 m 3The reactor temperatures are: (A): 250°C, (B): 210°C. The diol separation section (c) perfectly extracts the diols, i.e. a flow rate of 9800 kg / h, from the terephthalic acids. The diols are then purified, mixed with a fresh flow rate corresponding to the consumption of the depolymerization, i.e. 700 kg / h in unit (3) and the majority is reinjected into reactor (A), the remainder into section (a3) ​​and (a2), to accompany the recycled oligomer effluent (8). The separation section (d) of the monomer-rich effluent extracts the monomers from the heavier dimers and oligomers with an efficiency of 75%, thus generating a flow of pre-purified monomers from the dimers with a flow rate of 2827 kg / h and a heavy impurity effluent comprising heavier compounds (870 kg / h) as well as a lost portion of monomers (940 kg / h).The effluent consisting of dimers and heavier monomers as well as lost monomers and heavy impurities, called heavy impurity effluent (1810 kg / h) is directed to the separation section (e). The purified monomer stream discharged from (d) is introduced into the decolorization step (f) and then into the crystallization step (g) to produce a purified and decolorized diester monomer stream (4) consisting of the monomer of interest BHET (2660 kg / h), the undesired monomer BHETdeg (140 kg / h) as well as monomer altered by thermal or thermo-oxidative degradation reactions (27 kg / h), which corresponds to a desired monomer purity level of 94.1%. The heavy impurity effluent separation section (e) partially extracts heavy terephthalic impurities from the recycled residue stream with a yield of 20% to form a first fraction (8) which constitutes the recycled oligomer effluent and a second fraction (7).The recycled oligomer effluent (8) (1500 kg / h) is introduced into the static mixer (a3), and the remainder (7) is purged (375 kg / h), i.e. removed from the process. Example 2 shows that directing the recycled oligomer effluent to the static mixer (a4) then (a2) of the conditioning step (a) increases the thermo-oxidative degradation phenomena by 80% relative to Example 1, producing 27 kg / h of colored species that are difficult to extract from the final monomer (while the process according to Example 1 produces only 15 kg / h of colored species that are difficult to extract from the final monomer). Example 3 (comparison: with recycling to reactor A) The process of example 3 is shown diagrammatically in figure 4. The feed (1) composed of 100% PET is supplied continuously at a flow rate of 2500 kg / h which corresponds to a recycling capacity of 20 KTA (kilotons per year) of PET.As illustrated in Figure 4, the conditioning step (a) uses: - An extruder (a1) to condition the PET feed (1), by melting it, - A static mixer (a2) to pre-mix the feed from the extruder with a flow (2) of ethylene glycol and obtain a mixed flow. The mixed flow is introduced into the depolymerization step (b) which uses two reaction sections (A) and (B) arranged in cascade, each section being composed of a perfectly stirred reactor. The useful volumes of the reactors are: (A): 6 m. 3 , (B): 42 m 3The reactor temperatures are: (A): 250°C, (B): 210°C. The diol separation section (c) perfectly extracts the diols, i.e. a flow rate of 9800 kg / h, from the terephthalic acids. The diols are then purified, mixed with a fresh flow rate corresponding to the consumption of the depolymerization, i.e. 700 kg / h in unit (3) and the whole is reinjected into reactor (A) for a flow rate of 10500 kg / h. The diol flow prepared in unit (3) is sent entirely to reactor (A) to accompany the mixed flow obtained in the conditioning step (a) and the recycled oligomer effluent (6) from the separation section (e).The separation section (d) of the monomer-rich effluent extracts the monomers from the heavier dimers and oligomers with an efficiency of 75%, thus generating a flow of pre-purified monomers from the dimers with a flow rate of 2826 kg / h and a heavy impurity effluent comprising heavier compounds (870 kg / h) as well as a lost portion of monomers (940 kg / h). The effluent consisting of the dimers and heavier compounds as well as the lost monomers and heavy impurities, called the heavy impurity effluent (1810 kg / h) is directed to the separation section (e).The purified monomer stream discharged from (d) is introduced into the decolorization step (f) and then into the crystallization step (g) to produce a purified and decolorized diester monomer stream (4) consisting of the monomer of interest BHET (2664 kg / h), the undesired monomer BHETdeg (144 kg / h) as well as monomer altered by thermal or thermo-oxidative degradation reactions (18 kg / h), which corresponds to a desired monomer purity degree of 94.3%. The heavy impurity effluent separation section (e) partially extracts the heavy impurities from the terephthalic compounds from the recycled residue stream with a yield of 20% to form a first fraction (10) which constitutes the recycled oligomer effluent and a second fraction (7). The recycled oligomer effluent (10) (1450 kg / h) is introduced into the reactor (A), and the remainder (7) is purged (370 kg / h), i.e. evacuated from the process.Example 3 shows that directing the recycled oligomer effluent (10) to the reactor (A) increases the thermo-oxidative degradation phenomena by 20% relative to Example 1, producing 18 kg / h of colored species that are difficult to extract from the final monomer (while the process according to Example 1 only produces 15 kg / h of colored species that are difficult to extract from the final monomer). Example 4 (comparative: without recycling of the heavy impurities effluent) The process of Example 4 is shown schematically in Figure 5. The feed (1) composed of 100% PET is supplied continuously at a flow rate of 2500 kg / h, which corresponds to a recycling capacity of 20 KTA (kilotons per year) of PET.As illustrated in Figure 5, the conditioning step (a) uses: - An extruder (a1) to condition the PET feed (1), by melting it, - A static mixer (a2) to pre-mix the feed from the extruder with a flow (2) of ethylene glycol and obtain a mixed flow. The mixed flow is introduced into the depolymerization step (b) which uses two reaction sections (A) and (B) arranged in cascade, each section being composed of a perfectly stirred reactor. The useful volumes of the reactors are: (A): 6 m. 3 , (B): 42 m 3The reactor temperatures are: (A): 250°C, (B): 210°C. The diol separation section (c) perfectly extracts the diols, i.e. a flow rate of 6900 kg / h, from the terephthalic acids. The diols are then purified, mixed with a fresh flow rate corresponding to the consumption of the depolymerization, i.e. 600 kg / h in unit (3) and the whole is reinjected into reactor (A) for a flow rate of 7500 kg / h. The separation section (d) of the monomer-rich effluent extracts the monomers from the heavier dimers and oligomers with an efficiency of 75%, thus generating a flow of pre-purified monomers from the dimers with a flow rate of 2380 kg / h and a heavy impurity effluent comprising heavier compounds (710 kg / h) as well as a lost portion of monomers (600 kg / h). The effluent (11) consisting of the dimers and heavier compounds as well as the lost monomers and heavy impurities, called the heavy impurity effluent (1310 kg / h) is discharged from the process, i.e. purged.The purified monomer stream discharged from (d) is introduced into the decolorization step (f) and then into the crystallization step (g) to produce a purified and decolorized diester monomer stream (4) consisting of the monomer of interest BHET (2176 kg / h), the undesired monomer BHETdeg (189 kg / h) as well as monomer altered by thermal or thermo-oxidative degradation reactions (15 kg / h), which corresponds to a desired monomer purity of 91.4%. Example 4 shows that not recycling part of the heavy impurity effluent (10) leads to obtaining a purity of 91.4% significantly lower than that of Example 1 (94.1%), accompanied by an overall monomer yield (71.1%) also significantly lower than that of Example 1 (84.5%).Furthermore, the content of species resulting from thermal degradation reactions is similar to example 1 (same flow rate, 15 kg / h, but represents a fraction of 0.6% by weight instead of 0.5% by weight according to example 1 compared to the pre-purified monomer).

Claims

Claims 1. Process for depolymerizing a polyester filler comprising PET, said process comprising: a) a conditioning step, supplied at least with said polyester filler, to produce a flow of conditioned filler;b) a depolymerization step implementing a first reaction section (A) and at least one second reaction section (B), said at least one second reaction section (B) operating at a temperature strictly lower than the temperature of said first reaction section (A), the first reaction section (A) being fed at least by the conditioned feed stream and optionally by a first diol supplement, said at least one second reaction section (B) being fed by the effluent from the first reaction section, and by a recycled oligomer effluent and optionally by a second diol supplement, so that the total quantity of diol feeding said step b) is adjusted to 1 to 20 moles of diol per mole of diester feeding said step b), said step b) being operated at a temperature between 180 and 300°C, and with a residence time between 0.334 and 10 h;c) a diol separation step supplied at least by the effluent from step b), operated at a temperature between 100 and 250°C, at a pressure lower than that of step b) and producing a diol effluent and an effluent rich in liquid monomers, the diol effluent being at least partly recycled to compose at least part of one of the following streams: the first diol make-up and the second diol make-up which optionally supply step b);d) a step of separating the liquid monomer-rich effluent from step c) into a heavy impurity effluent and a pre-purified monomer effluent, carried out at a temperature less than or equal to 250°C and a pressure less than or equal to 0.001 MPa with a liquid residence time less than or equal to 10 min, e) a step of separating said heavy impurity effluent into two fractions: a first fraction (6) of which at least 70% by weight makes up the recycled oligomer effluent which feeds step b) and a second fraction (7) which is at least partly removed from the process;f) optionally a step of decolorization of the pre-purified monomer effluent, carried out at a temperature between 100 and 250°C, and at a pressure between 0.1 and 1.0 MPa in the presence of an adsorbent, and producing a purified monomer effluent, g) optionally a step of crystallization of the purified monomer effluent, using at least one solid production section, operated at a temperature between 0 and 100°C and at a pressure between 0.00001 and 1.00 MPa, followed by a solid-liquid separation section, producing a decolorized and purified monomer effluent.; 2. Method according to claim 1 wherein said polyester filler comprises at least colored PET, opaque PET or their mixtures. 3.

4. Method according to one of the preceding claims, in which the conditioning step a) uses at least one conditioning section (a1) to produce a fluid feed stream, and a mixing section (a2) to produce a mixed stream, said mixed stream corresponding to the conditioned feed stream, said conditioning section (a1) being supplied at least by said polyester feed and being operated at a temperature between 150 and 300°C, said mixing section (a2) being at least supplied by said fluid feed stream from the conditioning section and a diol stream, at least a portion of said diol stream preferably being composed of at least a fraction of the diol effluent from step c), said mixing section (a2) being operated at a temperature between 150 and 300°C, with a residence time between 0.5 seconds and 20 minutes.Method according to claim 3, wherein the conditioning section (a1) of step a) is operated in an extruder, said mixing section (a2) of the polyester feedstock of step a) optionally also being implemented within said extruder.

5. Method according to claim 3, wherein the mixing section (a2) of step a) uses at least one static or dynamic mixer.

6. Method according to one of claims 3 to 5 wherein, in step a), the weight ratio of the diol flow introduced in step a) relative to the polyester feedstock is between 0.03 and 6.00, preferably between 0.05 and 5.00, preferentially between 0.10 and 4.00, preferably between 0.50 and 3.

00.

7. Process according to one of the preceding claims, in which said at least one second reaction section (B) is operated at a temperature 5 to 50°C lower than the temperature of said first reaction section (A). 8.Process according to one of the preceding claims, in which the recycled oligomer effluent which feeds said at least one second reaction section (B) comprises the entirety of the first fraction originating from step e).

9. Process according to one of the preceding claims, in which in step e) the second fraction (7) is split into two parts, a first part being injected into one of the reaction sections of step b) downstream of the first reaction section, the second part being removed from the process.

10. Process according to one of the preceding claims, in which the depolymerization step comprises two reaction sections.

11. Process according to one of claims 1 to 9, in which the depolymerization step comprises three reaction sections, the second reaction section and the third reaction section each being fed with a portion, varying between 0 and 100% by weight, of the recycled oligomer effluent, the sum of said portions constituting 100% by weight of said recycled oligomer effluent.

12. Process according to one of the preceding claims in which the depolymerization step comprises at least one internal recirculation loop implementing at least the following operations: the withdrawal of a fraction of the reaction system from one of the reaction sections, the filtration of said fraction, and the reinjection of said fraction into one of the reaction sections. 13.

14. Method according to one of the preceding claims, in which the separation step e) is carried out so as to obtain a first fraction (6) enriched in oligomers and a second fraction (7) enriched in heavy impurities, the first fraction enriched in oligomers having an oligomer content strictly greater than the oligomer content of said heavy impurity effluent and the second fraction having a heavy impurity content strictly greater than the heavy impurity content of said heavy impurity effluent. 15.

16. Method according to one of the preceding claims, in which the diol separation step c) is carried out in 1 to 5 successive gas-liquid separation sections, each producing a gas effluent and a liquid effluent, the liquid effluent from the previous section feeding the subsequent section, the liquid effluent from the last gas-liquid separation section constituting the liquid monomer-rich effluent, all of the gas effluents being recovered to constitute the diol effluent.

16. Method according to one of claims 3 to 15, in which the conditioning section is further supplied with a diol stream.

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