TW202502939A - Process for depolymerization of a polyester comprising polyethylene terephthalate with recycling of oligomer effluent - Google Patents

Abstract

The invention relates to a process for depolymerizing a polyester feedstock comprising PET. The process comprises a step a) of conditioning the polyester feedstock, a depolymerization step b) which implements several reaction sections (A) (B) (N), a step of separation (c) of the diol, a step of separation (d) of a heavy impurities effluent, optionally a decolourization step (f) followed by a crystallization step (g). The heavy impurities effluent undergoes a separation step (e) to produce two fractions:a first fraction, at least 70% by weight of which is sent to the second reactor (B), and a second fraction which is at least partially discharged from the process.

Description

Process for depolymerizing polyesters comprising polyethylene terephthalate with recycling of oligomer effluent

The present invention relates to a process for depolymerizing polyesters, particularly terephthalate polyesters including polyethylene terephthalate (PET), for the purpose of recycling. More particularly, the present invention relates to a process for depolymerizing polyester feedstocks including PET to produce diester monomers, which process optimizes the recycling of the oligomer effluent.

The chemical recycling of polyesters, in particular polyethylene terephthalate (PET), has been the subject of much research aimed at breaking down polyesters recovered as waste into monomers that can be used again as feedstock for polymerization processes.

Many polyesters originate from networks for collecting and sorting materials. In particular, polyester, in particular PET, may originate from the collection of bottles, trays, films, resins and/or fibers (e.g. textile fibers, tire fibers) made of polyester. The polyester originating from the collection and sorting channels is referred to as polyester to be recycled.

PET to be recycled can be classified into four main categories: - transparent PET, which consists mainly of transparent colorless PET (usually at least 60% by weight) and transparent sky-blue PET, does not contain pigments and can be used in mechanical recycling processes, - dark or colored PET (green, red, etc.), which can usually contain up to 0.1% by weight of dyes or pigments but remain transparent or translucent; - opaque PET, which contains a large amount of pigment (in a content that usually varies between 0.25% and 5.0% by weight) to make the polymer opaque. Opaque PET is increasingly used, for example, in compositions for the manufacture of food containers (e.g. milk bottles) and bottles for cosmetics, plant protection or dyes; - multilayer PET, which consists, for example, of layers of polymers other than PET or of recycled PET layers or aluminum films between layers of virgin PET (i.e. PET that has not been recycled). Multi-layer PET is used to make packaging such as trays after thermoforming.

The structure of the collection channels providing the recycling channels varies from country to country. Depending on the nature and quantity of the streams and the sorting technologies, they are constantly being developed to maximize the amount of plastic recovered from the waste. The recycling channels of these streams usually consist of a first conditioning stage in the form of flakes, during which the bales of original packaging are opened and the containers are cleaned, sorted and shredded and then further purified and sorted to produce a flake stream that usually contains less than 1% by mass of "macro" impurities (glass, metals, other plastics, wood, paper, mineral elements).

The transparent PET flakes can then undergo an extrusion filtration step to produce an extrudate which is then reused as a mixture with virgin PET to make new products (bottles, fibers, films). A solid state vacuum polymerization step (known by the abbreviation SSP) is necessary for food applications. This type of recycling is called mechanical recycling.

Dark (or colored) PET flakes can also be recycled mechanically. However, the coloring of the extrudate formed from the colored stream limits the applications: dark PET is usually used to produce packaging strips or fibers. Compared with transparent PET, the export is therefore more limited.

The presence of opaque PET with a high content of pigment in the PET to be recycled causes problems for recyclers, since the opaque PET impairs 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 the uses of opaque PET, the content of opaque PET in the colored PET stream to be recycled is currently between 5-20% by weight and tends to increase further. In a few years, it will be possible to reach an opaque PET content of more than 20-30% by weight in the colored PET stream. 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 impaired (see “ Impact of the development of opaque white PET on the recycling of PET packaging ”, preliminary presentation of COTREP, May 12, 2013) and recycling in fiber form (the main outlet for the channel of colored PET) is hindered.

Dyes are natural or synthetic substances that are soluble in (particularly) polyester materials and used to color the material into which they are introduced. Commonly used dyes are of different kinds and usually contain O and N type impurity atoms and conjugated unsaturated groups (e.g. quinone, methine or azo functional groups) or molecules (e.g. pyrazolone and quinophthalone). Pigments are fine substances that are insoluble in (particularly) polyester materials and used to color and/or opacify the material into which they are introduced. The main pigments used to color and/or opacify polyester, particularly PET, are metal oxides (e.g. TiO ₂ , CoAl ₂ O ₄ , Fe ₂ O ₃ ), silicates, polysulfides and carbon black. Pigments are particles whose size is usually between 0.1 and 10 µm and mainly between 0.4 and 0.8 µm. Complete removal of these pigments by filtration, which is necessary for envisaging recycling of opaque PET, is technically difficult due to their extremely high clogging capacity.

Therefore, the mechanical recycling of colored and opaque PET is extremely complex.

Document FR 3053691 A1 describes a process for depolymerizing a polyester raw material comprising opaque PET and, in particular, 0.1% to 10% by weight of pigments by glycolysis in the presence of ethylene glycol. After specific separation and purification steps, an effluent of purified bis(2-hydroxyethyl)terephthalate (BHET) is obtained. The document envisages the possibility of carrying out reactive extrusion in the first phase of conditioning the raw material in order to initiate the depolymerization reaction. It also mentions recycling the heavy residue separated during the purification step in order to be treated with the polyester raw material.

Document FR 3105236 describes a process for depolymerization of polyester feedstocks comprising PET by glycolysis and in particular an improvement of the process of document FR 3053691. The improvement consists in particular in optimizing the stage of conditioning the feedstock by mixing the feedstock with at least one recycled oligomer residue effluent in the presence of a diol upstream of introducing the polyester feedstock into the depolymerization step.

The present application proposes to increase the productivity of the depolymerization process by optimizing the use of the heavy impurities effluent obtained during the separation of the effluent rich in liquid monomers.

The present invention relates to a method for depolymerizing a polyester feedstock containing PET, the method comprising: a) a conditioning step, which at least supplies the polyester feedstock to produce a conditioned feedstock stream; b) carrying out a depolymerization step of a first reaction section and at least one second reaction section, the at least one second reaction section being operated at a temperature strictly lower than the temperature of the first reaction section, the first reaction section being supplied with at least a regulated feed stream and, if appropriate, a first diol supply, the at least one second reaction section being supplied with the effluent of the first reaction section and the recycled oligomer effluent and, if appropriate, a second diol supply, so that the total amount of diol supplied to the step b) is regulated to 1 to 20 mol diol/mol diester supplied to the step b), the step b) being operated at a temperature between 180 and 300°C and a residence time between 0.334 and 10 h; c) a step of separating diols, which is supplied with at least the effluent of step b), operates at a temperature between 100 and 250°C and a pressure lower than the pressure of step b) and produces a diol effluent and an effluent rich in liquid monomers; d) a step of separating the effluent rich in liquid monomers from step c) into a heavy impurity effluent and a pre-purified monomer effluent, which is operated at a temperature less than or equal to 250°C and a pressure less than or equal to 0.001 MPa, and the liquid residence time is less than or equal to 10 min, e) a step of separating the impurity effluent into two fractions: a first fraction (of which at least 70% by weight constitutes the recycled oligomer effluent supplied to step b)) and a second fraction (which is at least partially discharged from the process); f) a step of decolorizing the pre-purified monomer effluent, as appropriate, operating in the presence of an adsorbent at a temperature between 100 and 250°C and a pressure between 0.1 and 1.0 MPa, and producing a purified monomer effluent, g) a step of crystallizing the purified monomer effluent, as appropriate, using at least one solid generating stage, at a temperature between 0 and 100°C and a pressure between 0.00001 and 1.00 MPa, followed by a solid-liquid separation stage to produce a decolorized and purified monomer effluent.

Recirculation of the oligomers obtained in step e) to the second reaction stage of the depolymerization step b), which is operated at lower temperature values than the first reaction stage and at a lower temperature than that used in the regulation step a), has several advantages, in particular: - Improvement of the quality of the monomers produced, in particular reduction of the proportion of comonomers, in particular BHETdeg, in the monomer product (which is the cause of the maximum yield loss after the oligomers). In fact, recycling the oligomers makes it possible to accelerate the formation rate of the diester monomer (monomer of interest), in particular BHET, while maintaining the same formation rate of BHETdeg as in the case where the oligomers are not recycled; - Reduction of thermal degradation phenomena by avoiding the presence of monomers and oligomers in the hottest areas of the process, in particular in the conditioning step (a) and in the first reaction stage of step (b). This effect contributes to improving the quality of the final product; - Reducing the temperature of the reaction stage, which also makes it possible to improve the quality of the product and to improve the energy efficiency of the process.

According to the present invention, the polyester raw material may include at least colored PET, opaque PET or a mixture thereof.

The adjustment step a) can be implemented with at least one adjustment section (a1) for producing a fluid feed stream and a mixing section (a2) for producing a mixed stream, wherein the mixed stream corresponds to the adjusted feed stream, at least the polyester feed stream is used to supply the adjustment section (a1) and is implemented at a temperature between 150 and 300° C., at least the fluid feed stream and the glycol stream from the adjustment section are used to supply the mixing section (a2), at least a portion of the glycol stream is preferably composed of at least a portion of the glycol effluent from step c), the mixing section (a2) is implemented at a temperature between 150 and 300° C., and the residence time is between 0.5 seconds and 20 minutes.

The conditioning section (a1) may be operated in an extruder, and the mixing section (a2) for the polyester raw material of step a) may also be implemented in the extruder as appropriate.

The mixing section (a2) may be implemented with at least one static or dynamic mixer.

In step a), the weight ratio of the diol stream introduced in step a) to the polyester raw material may be between 0.03 and 6.00, preferably between 0.05 and 5.00, more preferably between 0.10 and 4.00, and most preferably between 0.50 and 3.00.

The at least one second reaction stage may be operated at a temperature 5 to 50°C lower than the temperature of the first reaction stage.

The recycled oligomer effluent supplied to the at least one second reaction stage may comprise the entire first stream originating from step e).

According to a particular embodiment, in step e), the second stream can be divided into two parts (a first part which is injected into one of the reaction stages of step b) downstream of the first reaction stage, a second part which is discharged from the process).

According to the present invention, the depolymerization step may comprise two reaction sections. Alternatively, according to the present invention, the depolymerization step may comprise three reaction sections: the second reaction section and the third reaction section are supplied with a portion of the recycled oligomer effluent (varying between 0% by weight and 100% by weight), the sum of these portions constituting 100% by weight of the recycled oligomer effluent.

The depolymerization step may comprise at least one internal recirculation loop for performing at least the following operations: withdrawing a portion of the reaction system from one of the reaction sections, filtering the portion, and reinjecting the portion into one of the reaction sections.

The separation step e) can be carried out so as to obtain a first oligomer-enriched stream and a second heavies-enriched stream, the first oligomer-enriched stream comprising oligomers strictly greater than the content of oligomers of the heavies outflow and the second stream comprising heavy impurities strictly greater than the content of heavy impurities of the heavies outflow.

The first stream obtained in step e) can be mixed with the glycol stream, preferably with a portion of the glycol effluent obtained in step c), and then recycled to step b) and/or, as appropriate, to step a).

The glycol separation step c) may be carried out in 1 to 5 consecutive gas-liquid separation stages, each of which produces a gaseous effluent and a liquid effluent, the liquid effluent from the preceding stage being supplied to the subsequent stage, the liquid effluent from the last gas-liquid separation stage constituting the effluent rich in liquid monomer, and all the gaseous effluent being recovered to constitute the glycol effluent.

According to one refinement of the invention, the regulating section can also be supplied with a glycol flow.

FIG. 1 illustrates a schematic diagram of the method of the present invention without limiting its scope.

In the embodiment explained with reference to FIG. 1 , the method implements a step (a) of conditioning a feedstock (1) comprising PET. The conditioning step (a) uses a conditioning section (a1) (e.g. an extruder) for conditioning the feedstock (1) and obtaining a fluid feedstock; at least one mixing section (a2) (e.g. a static or dynamic mixer or extruder section) supplied with a fluid feedstock and a diol stream (2) (which may advantageously be a portion of the diol effluent (3) recovered in step (c)). The mixed stream (or conditioned feedstock stream) obtained at the end of step (a) is introduced into a depolymerization step (b) using several reaction sections (e.g. a first reaction section (A) and a second reaction section (B) and even further reaction sections). In FIG. 1 , only reaction sections (A), (B) and (N) are shown. Without departing from the scope of the invention, further reaction stages may be present between reaction stage (B) and reaction stage (N). The mixture successively enters reaction stage (A) and then reaction stage (B), optionally a reaction stage not shown, and then reaction stage (N). Each of the reaction stages can be supplied with the diol (3) from step (c). The effluent obtained at the end of the depolymerization step (b) is introduced into the diol separation step (c), thereby enabling the recovery of the diol effluent (3) and the effluent enriched in monomers.

The monomer-enriched effluent is introduced into the separation step (d), thereby enabling the pre-purified monomer effluent to be obtained and the removal of the heavy impurities effluent comprising oligomers and heavy impurities which is sent to the separation step (e).

In the separation step (e), the heavy impurities effluent is separated into two fractions: a first fraction (6) which constitutes the recycled oligomer effluent supplied to step (b) and a second fraction (7) which is discharged (in other words purged) from the process of the invention. According to the invention, at least 70% by weight of the first fraction, which constitutes the recycled oligomer effluent supplied to step (b), is fed to the second reaction section (B).

Before recycling 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 the diol effluent (3) recovered in step (c)).

The pre-purified monomer effluent obtained at the end of step d) can be sent to a decolorization step (f) by adsorption and then to a crystallization step (g) to recover the decolorized and purified diester monomer effluent (4).

The diol effluent (3) obtained in step (c) is advantageously recycled (in whole or in part) to one or more of the reaction sections (A), (B) and (N) of step (b) or to an optional additional reaction section between sections (B) and (N). In addition, a portion 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 static mixer (a2); and/or in step (e) to mix with the heavy impurities effluent before separation; and/or in mixer (a3) to mix with the recycled oligomer effluent (6) before being sent to step (b).

According to the present invention, polyethylene terephthalate or poly(ethylene terephthalate) (also referred to as PET) has a basic repeating unit of the following formula: Traditionally, PET is obtained by condensing terephthalic acid (PTA) or dimethyl terephthalate (DMT) with ethylene glycol. In the following, the expression "per mole of diester in the polyester raw material" corresponds to the molar number of -[O-CO-O-( _C6H4 )-CO-O- _CH2 - _CH2 ]- units (which are diester units obtained from the reaction of PTA with ethylene glycol) in the PET included in the polyester raw _material .

According to the present 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 the formula HOC2H4 _{- CO2-} ( _C6H4 ) _-CO2 - _C2H4OH , wherein -( _{C6H4 )} - represents an aromatic ring, and which is a diester unit obtained from the reaction of PTA with ethylene glycol in the PET included in the polyester raw material. Comonomers are present (defined as the presence of a single terephthalic acid unit), but are different from BHET. The most prominent one is _2- (2- _{hydroxyethoxy} )ethyl terephthalate (BHETdeg) with the chemical formula _{HOC2H4 - CO2-} ( _{C6H4 ) -CO2 - C2H4OC2H4OH} and CAS# 65133-69-9.

The term "oligomer" generally refers to a small polymer usually composed of 2 to 20 basic repeating units. According to the present invention, the term "ester oligomer" or "diester oligomer" or alternatively "BHET oligomer" refers to a terephthalate oligomer comprising 2 to 20, preferably 2 to 5 basic repeating units of the formula -[O-CO-(C ₆ H ₄ )-CO-OC ₂ H ₄ ]-, wherein -(C ₆ H ₄ )- is an aromatic ring.

The term "heavy impurities" designates, in particular, pigments, if present, polymers other than polyesters, and polymeric catalysts.

According to the present invention, the terms "diol" and "glycol" are used interchangeably and correspond to compounds containing two hydroxyl -OH groups. A preferred diol is ethylene glycol, also known as monoethylene glycol or MEG.

The glycol or glycol effluent stream used in the steps of the process of the invention therefore preferably comprises a predominant amount of ethylene glycol (or MEG), i.e. such that MEG represents 95% by weight or more relative to the total weight of the glycol or glycol effluent stream.

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

According to the invention, the term "pigment", more particularly coloring and/or devitrifying pigment, defines fine substances which are insoluble in (particularly) polyester materials. Pigments are in the form of solid particles which are usually between 0.1 and 10 μm in size and mainly between 0.4 and 0.8 μm. They are usually of mineral nature. Commonly used, especially devitrifying pigments are metal oxides (e.g. TiO ₂ , CoAl ₂ O ₄ or Fe ₂ O ₃ ), 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 specified. If this is not the case and the limit values are not included in the specified range, the accuracy will be provided by the present description.

In the sense of the present invention, various parameter ranges (such as pressure ranges and temperature ranges) for a given step can be used alone or in combination. For example, in the sense of the present invention, a range of preferred pressure values can be combined with a range of more preferred temperature values.

In the following, specific and/or preferred embodiments of the present invention are described. Where technically feasible, they can be implemented alone or in combination with each other without limiting the combination.

According to the present invention, pressure is absolute pressure, which is given in MPa (or MPa).

According to the present invention, time and duration are expressed in hours (h), minutes (min) and/or seconds (sec).

The terms "upstream" and "downstream" are to be understood as varying with the general flow of the fluid or streams under consideration in the process.

Raw Materials The process of the present invention is provided with a polyester raw material comprising at least one polyester (i.e., a polymer whose main chain repeating units contain ester functional groups) and polyethylene terephthalate (PET), preferably comprising at least colored PET and/or opaque PET or a mixture thereof.

The polyester raw material is advantageously a polyester raw material to be recycled originating from a waste collection and sorting channel, in particular plastic waste. The polyester raw material may, for example, come from the collection of bottles, trays, films, resins and/or fibers made of polyethylene terephthalate.

Advantageously, the polyester raw material comprises at least 50% by weight, preferably at least 70% by weight and in a preferred manner at least 90% by weight of polyethylene terephthalate (PET).

Preferably, the polyester raw material comprises at least one PET selected from colored, opaque, dark and multi-layered PET and mixtures thereof. In a very specific manner, the polyester raw material comprises at least 10% by weight of opaque PET, very preferably at least 15% by weight of opaque PET, which is advantageously opaque PET to be recycled (i.e. originating from a collection and sorting channel).

The polyester raw material advantageously contains 0.1% to 10% by weight, advantageously 0.1% to 5% by weight of pigment. It may also particularly contain 0.05% to 1% by weight, preferably 0.05% to 0.2% by weight of dye.

In the collecting and sorting channels, the polyester resin waste is washed and ground before constituting the polyester raw material of the process of the invention. The polyester fiber waste is densified in the form of granules or popcorn in order to be able to be mechanically introduced into the process of the invention.

The polyester raw material may thus be wholly or partly in the form of granules or flakes, the maximum length of which is less than 10 cm, preferably between 5 and 25 mm, or in the form of micronized solids, i.e. in the form of particles preferably having a size between 10 microns and 1 mm. The raw material may also contain "macro" impurities, preferably less than 5% by weight, more preferably less than 3% by weight of "macro" impurities (e.g. glass, metal, plastics other than polyester (e.g. PP, HDPE, etc.), wood, paperboard, mineral elements). The polyester raw material may also be in whole or in part in the form of fibers (e.g. textile fibers), optionally pretreated to remove cotton fibers, polyamide fibers or any other textile fiber other than polyester or, for example, tire fibers, optionally pretreated to remove in particular polyamide fibers or rubber or polybutadiene residues. The polyester raw material may further comprise polyester from production wastes of polyester material polymerization and/or conversion processes. The polyester raw material may also contain elements (e.g. antimony, titanium, tin) used as polymerization catalysts and stabilizers in the PET production process.

Conditioning step a) The process of the invention comprises a conditioning step a) which is supplied with at least a polyester feedstock and, if appropriate, a diol stream to produce a conditioned feedstock stream. Conditioning step a) enables the polyester feedstock to be at least partially liquid. In practice, step a) enables the polyester feedstock to be brought to the operating conditions (temperature and pressure) of the depolymerization step b) and to be at least partially liquid.

Preferably, the 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 slightly above the melting temperature of the polyester, in particular PET, of the polyester raw material so that the latter is at least partially liquid, but is kept as low as possible to minimize degradation of the polyester. Preferably, the conditioning step a) is operated under an inert atmosphere in order to limit the introduction of oxygen into the system and oxidation of the polyester raw material.

Preferably, the conditioning step a) implements at least one conditioning section and one mixing section, the conditioning section is supplied with at least the polyester feedstock and produces a fluid feed stream, the mixing section is supplied with at least the fluid feed stream and at least one diol stream, and the mixing section produces a mixed stream. In the preferred embodiment, the mixed stream corresponds to the conditioned feed stream recovered at the outlet of step a) and advantageously supplied to the depolymerization step b).

The conditioning section of step a) makes it possible to heat and pressurize the polyester raw material to the operating conditions of the depolymerization step b). In the conditioning section, the polyester raw material is gradually heated to a temperature close to or slightly above its melting temperature so as to at least partially become liquid. Advantageously, at least 70% by weight of the polyester raw material, very advantageously at least 80% by weight, preferably at least 90% by weight, preferentially at least 95% by weight of the polyester raw material are in liquid form when leaving the conditioning section of step a). The temperature of the conditioning section for implementing step a) 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 in the system and oxidation of the polyester raw material. The adjustment section can be operated with the diol stream added to the polyester feedstock. In this case, the weight ratio of diol to polyester feedstock in the adjustment section is between 0.001 and 0.1, preferably between 0.002 and 0.05, and more preferably between 0.003 and 0.03.

According to a preferred embodiment of the present invention, the regulating section is the extrusion section corresponding to the screw conveyor section. In other words, the regulating section operates in the extruder. The residence time in the extrusion section (defined as the volume of the section divided by the raw material volume flow rate) is advantageously less than or equal to 5 h, preferably less than or equal to 1 h, more preferably less than or equal to 30 min, extremely preferably less than or equal to 10 min and preferably greater than or equal to 2 min. Advantageously, the extrusion section enables the polyester raw material to be regulated so that the fluid raw material 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.

The extrusion section is advantageously connected to a vacuum extraction system in order to remove impurities present in the feedstock (e.g. dissolved gases, light organics and/or water). The extrusion section may also advantageously comprise a filtration system for removing solid particles (e.g. sand) with a size greater than 40 μm and preferably less than 2 cm. The polyester feedstock is advantageously fed to the extruder by any method known to those skilled in the art, for example via a feed hopper, and is advantageously inertized to limit the introduction of oxygen into the system.

The mixing section is supplied with at least the fluid feed stream and the glycol stream coming from the conditioning section. The mixing section advantageously enables the polyester feed previously treated in the conditioning section to be brought into contact with the glycol stream. This contact has the effect of starting the depolymerization reaction of the polyester feed before introduction into the depolymerization step b). It also enables a substantial reduction in the viscosity of the feed, which facilitates its transport, in particular, into the depolymerization step b). Very advantageously, the glycol stream supplied to the mixing section is at least partially constituted by at least a portion of the glycol effluent coming from step c) of the process.

In the preferred embodiment, the mixing section of step a) (also referred to as the mixing section of the polyester raw material) is advantageously carried out at a temperature between 150 and 300°C, preferably between 225 and 275°C and a residence time between 0.5 seconds and 20 minutes, preferably between 1 second and 5 minutes, more preferably between 3 seconds and 3 minutes, and the weight ratio of the diol stream introduced in step a) (i.e. in the mixing section and, if appropriate, in the adjustment section) to the polyester raw material is between 0.03 and 6.00, preferably between 0.05 and 5.00, more preferably between 0.10 and 4.00, and extremely preferably between 0.50 and 3.00. The residence time is defined here as the ratio of the volume of liquid in the mixing section to the volume flow rate of the diester feed in the mixing section.

In a first embodiment, the mixing section may implement at least one static or dynamic mixer, preferably one to five continuous static or dynamic mixers.

In a second very advantageous embodiment and when the conditioning section is operated in the extruder, the mixing section of the polyester raw material can therefore be implemented in the extruder. In this case, it is a reactive extrusion section.

The diol stream and the diol stream (if used) may each consist of a diol supply external to the process of the invention, a portion of the diol effluent from step c) or a mixture thereof, preferably a portion of the diol effluent from step c). Preferably, the diol stream and/or the diol stream is advantageously heated before being supplied in step a) in order to facilitate heating of the polyester feedstock.

Depolymerization step b) The process of the invention comprises a depolymerization step of carrying out a first reaction stage supplied with at least a conditioned feed stream and optionally a first diol feed and at least one second reaction stage operated at a temperature strictly lower than that of the first reaction stage, the first reaction stage being supplied with an effluent from the first reaction stage, a recycled oligomer effluent and optionally a second diol feed. The recycled oligomer effluent comprises at least 70% by weight of the first stream separated in step e).

The depolymerization reaction corresponds to a depolymerization reaction performed by alcoholysis, preferably by glycolysis, ie preferably in the presence of a diol.

Advantageously, the depolymerization step is operated so that the total amount of diol supplied to step b) (corresponding to the sum of the amounts of diol introduced in step a) and step b) is adjusted to 1 to 20 mol, preferably 3 to 15 mol, more preferably 5 to 10 mol of diol per mole of diester supplied to step b). In other words, the depolymerization step is advantageously operated so that the weight ratio of the total amount of diol introduced in step a) and step b) relative to the total amount of diester contained in the regulated feed stream and, if appropriate, the recycled oligomer effluent fraction recycled from step e) to step b) is between about 0.3 and 6.7, preferably between about 1.0 and 5.0, more preferably between 1.7 and 3.3, respectively.

Advantageously, the depolymerization step b) is carried out in at least two reaction stages (for example 2 to 4 reaction stages, preferably two or three reaction stages). The reaction stages are operated in series, i.e. the effluent from one reaction stage is fed to a downstream reaction stage. For example, the effluent from the first reaction stage is fed to the second reaction stage, the effluent from the second reaction stage is fed to the third reaction stage and so on. Each reaction stage can be used in any type of reactor known to those skilled in the art, so that the depolymerization or transesterification reaction can preferably be carried out in a reactor agitated by a mechanical stirring system and/or by a recirculation loop and/or by a fluidized bed. The reactor may comprise a conical bottom for removing impurities.

According to the present invention, the reaction stages starting from the second reaction stage are operated at a temperature strictly lower than the temperature of the first reaction stage, preferably at a temperature 5 to 50°C lower, preferably 10 to 50°C lower or even 20 to 40°C lower than the temperature of the first reaction stage.

The reaction stages are operated at between 180 and 300° C., preferably between 190 and 300° C., more preferably between 200 and 280° C., in particular in liquid phase.

The residence time in the depolymerization step (i.e. the cumulative residence time in the reaction sections used in the depolymerization step) is between 0.334 and 10 h, preferably between 0.5 and 8 h and more preferably between 1 and 6 h. The residence time is defined as the ratio of the volume of liquid in the reaction section to the volume flow rate of the stream leaving the last reaction section.

The operating pressure of the reaction section(s) of step b) is determined so as to maintain the reaction system in the liquid phase. The 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 components and phases present in step b) obtained from the feed of the step.

The diol is advantageously monoethylene glycol.

The first diol feed, which is optionally supplied to the first reaction stage of step b), may consist of a diol feed external to the process of the invention, of a portion of the diol effluent from step c) or a mixture thereof and preferably of a portion of the diol effluent from step c).

The second diol feed, optionally supplied to the at least one reaction stage of step b), may consist of a diol feed external to the process of the invention, a portion of the diol effluent from step c) or a mixture thereof and preferably a portion of the diol effluent from step c).

The depolymerization reaction can 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 may be selected from esterification catalysts known to those skilled in the art (e.g., oxide complexes and salts of antimony, tin and titanium, alkoxides of metals of Groups (I) and (IV) of the Periodic Table of the Elements, organic peroxides and acid-base metal oxides).

The preferred heterogeneous catalyst advantageously comprises at least 50 mass%, preferably at least 70 mass%, advantageously at least 80 mass%, very advantageously at least 90 mass% and even more advantageously at least 95 mass% of a solid solution, relative to the total mass of the catalyst, the solid solution consisting of at least one spinel of the formula _{ZxAl2O (3+x)} , wherein x is between 0 (exclusive limit) and 1, _and Z is selected from Co, Fe, Mg, Mn, Ti, Zn, and contains not more than 50 mass% of aluminum oxide and oxide of the element Z. The preferred heterogeneous catalyst advantageously contains at most 10 mass% of a dopant selected from silicon, phosphorus and boron (used alone or as a mixture). For example, and in a non-limiting manner, the solid solution may consist of a mixture of _{ZnAl2O4 spinel and CoAl2O4 spinel} , or it may consist of a mixture of _ZnAl2O4 spinel, _{MgAl2O4 spinel} and _{FeAl2O4 spinel} , or it may consist only of _{ZnAl2O4 spinel} .

Preferably, the depolymerization step is carried out without adding an external catalyst to the polyester raw material.

The depolymerization step may advantageously be carried out in the presence of a solid adsorbent in powdered or shaped form, the function of which is to capture at least part of the colored impurities, thus obviating the decolorization step f). The solid adsorbent is advantageously activated carbon.

The depolymerization reaction advantageously enables the conversion of the polyester raw material into ester monomers and oligomers, advantageously the conversion of PET into at least one diester monomer and ester oligomer, in particular bis(2-hydroxyethyl)terephthalate monomer (BHET) and BHET oligomers. The conversion rate of the polyester raw material in the depolymerization step is greater than 50%, preferably greater than 70%, more preferably greater than 85%. The molar yield of the diester monomer, in particular BHET, is greater than 50%, preferably greater than 70%, more preferably greater than 85%. The molar yield of the diester monomer, in particular BHET, corresponds to the molar flow rate of the diester monomer, in particular BHET, at the outlet of the step b) relative to the molar number of diesters (i.e. diester units) in the polyester raw material supplied to the step b).

An internal recirculation loop can advantageously be used in step b), i.e. to withdraw a portion of the reaction system from one of the reaction sections, preferably filter this portion and reinject it 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 portion can be reinjected into the same reaction section from which it was withdrawn. Alternatively, the portion can be withdrawn from one of the reaction sections and can be reinjected into another reaction section different from the one in which the withdrawal was carried out. The method of the invention can comprise several withdrawal loops. For example, each of the reaction sections is equipped with a recirculation loop, each of which is capable of withdrawing and reinjecting into the same reaction section. The internal loop(s) make it possible to eliminate solid, "macro" impurities that may be included in the reaction liquid.

Advantageously, the depolymerization step b) makes it possible to obtain a reaction effluent which is sent to the diol separation step c).

Diol separation step c) The process of the invention comprises a diol separation step c), which is supplied with at least the effluent from step b), is carried out at a temperature between 100 and 250° C. and at a pressure lower than that of step b) and produces 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 the pressure of step b) so as to vaporize a portion of the effluent from step b) to obtain a gaseous effluent and a liquid effluent. The liquid effluent constitutes an effluent rich in liquid monomers. The gaseous effluent composed of more than 50% by weight, preferably more than 70% by weight, more preferably more than 90% by weight of glycol constitutes a glycol effluent.

Step c) is advantageously carried out in a gas-liquid separation section or in a series of gas-liquid separation sections, advantageously 1 to 5 consecutive gas-liquid separation sections, very advantageously 3 to 5 consecutive gas-liquid separation sections. Each of the gas-liquid separation sections produces a liquid effluent and a gaseous effluent. The liquid effluent from the preceding section is fed to the subsequent section. The entire gaseous effluent is recovered to constitute a glycol effluent. The liquid effluent obtained from the last gas/liquid separation section constitutes an effluent rich in liquid monomer.

Advantageously, one or even at least two gas-liquid separation stages can be used in a falling film evaporator or wiped film evaporator or short path distillation.

Step c) is carried out in such a way that the temperature of the liquid effluent is maintained above a value below which the monomers and polyester oligomers precipitate and below a high value (molar ratio of endoprosthenic diol to monomer) above which the monomers significantly repolymerize. The temperature in step c) is between 100 and 250° C., preferably between 110 and 220° C., more preferably between 120 and 210° C. Operating in a series of gas-liquid separations, advantageously in a series of 2 to 5, preferably 3 to 5 consecutive separations, is particularly advantageous, since it enables the temperature of the liquid effluent to be adjusted in each separation to meet the above constraints.

A repolymerization inhibitor may advantageously be mixed with the liquid monomer-rich effluent before being fed to step c).

The pressure in step c) is lower than that in step b) and is advantageously adjusted to allow evaporation of the diol at a temperature while minimizing repolymerization and achieving optimal energy integration. It is preferably between 0.00001 and 0.2 MPa, preferentially between 0.00004 and 0.15 MPa, preferably between 0.00004 and 0.1 MPa.

The separation stage is advantageously agitated by any method known to those skilled in the art.

The diol effluent may contain other compounds (e.g. dyes, acetaldehyde, dioxane, dioxane cyclopentane, light alcohols, water, diamine monomers, carbamate monomers, diols including diethylene glycol or cyclomethanediol). At least a portion of the diol effluent, in liquid form (i.e. after condensation), is very advantageously recycled to step b) and/or, as appropriate, to step a) and, as appropriate, to step f), optionally mixed with the diol added externally in the process of the invention.

All or part of the glycol effluent may be treated in a purification step (3) before advantageously being recycled in liquid form (i.e. after condensation) to step b) and/or, as the case may be, to step a) and/or used in the form of a mixture in step e). The purification step may comprise, in a non-limiting manner, adsorption on a solid (e.g. on activated carbon) in order to remove dyes and one or more distillates, and in order to separate out impurities (e.g. diethylene glycol, water and other alcohols).

The monomer-enriched effluent obtained in step c) is sent to separation step d).

Monomer Separation Step d) The method of the present invention comprises a step d) of separating the monomer-enriched effluent obtained from step c) to produce a heavy impurities effluent and a pre-purified monomer effluent.

The step d) is advantageously carried out at a temperature of 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 of 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 of 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 leaving step d).

The purpose of the separation step d) is to separate all or part of the monomers, in particular the vaporized BHET, from the incompletely converted oligomers (which remain liquid and therefore carry with them the oligomers corresponding to the partially converted polyester polymer and also heavy impurities, in particular pigments, other polymers and polymerization catalysts if applicable), while minimizing the loss of monomers by repolymerization. Some oligomers may carry with them monomers, in particular those of smaller size. These heavy impurities appear together with the oligomers in the heavy impurities effluent.

Due to the possible presence of polymerization catalysts in the polyester raw materials, the separation must be carried out at extremely short liquid residence times and at temperatures less than or equal to 250°C in order to limit any risk of repolymerization of the monomers during this step. Separation by simple atmospheric distillation is therefore not conceivable.

The separation step d) is advantageously carried out in a falling or wiped film evaporation system or by short-path falling or wiped film distillation. Very low operating pressures are necessary in order to be able to carry out step d) at temperatures below 250° C., preferably below 230° C., while allowing vaporization of the monomer.

A polymerization inhibitor may advantageously be mixed with the liquid monomer-rich effluent before being fed to step d).

It may also be advantageous to mix a flux with the effluent rich in liquid monomer before supplying step d) in order to promote the removal of heavy impurities, especially pigments, at the bottom of the evaporation or short-path distillation system. Under the operating conditions of step d), the flux must have a much higher boiling point than the monomer, in particular BHET. It may be polyethylene glycol (for example) or a PET oligomer.

The heavy impurities effluent specifically comprises pigments, oligomers and unseparated monomers. The heavy impurities effluent obtained in step d) is sent to the separation step e).

Step e) of separating the heavy impurities effluent The process of the invention comprises a step e) of separating the heavy impurities effluent from step d) so as to produce two fractions: a first fraction, at least 70% by weight of which constitutes the recycled oligomer effluent supplied to step b) and a second fraction which is at least partially discharged from the process of the invention.

The heavy impurities effluent obtained at the end of step d) comprises monomers, oligomers and heavy impurities (especially pigments), unconverted polyester polymer, optionally other polymers and polymerization catalysts.

Preferably, the first stream comprises at least 50% by weight, preferably at least 70% by weight, more preferably at least 80% by weight or even at least 90% by weight of the heavy impurities effluent obtained at the end of step d).

According to a first embodiment, step e) of separating the heavy impurities effluent comprises a simple division into two fractions, the two fractions having the same chemical composition.

According to a second embodiment, step e) of separating the heavy impurities effluent comprising monomers, oligomers and heavy impurities is performed so as to obtain a first stream enriched in monomers and oligomers and a second stream enriched in heavy impurities, which means that the first stream enriched in oligomers comprises a monomer and oligomer content strictly greater than the monomer and oligomer content of the heavy impurities effluent from step d) and the second stream comprises a heavy impurities content strictly greater than the heavy impurities content of the heavy impurities effluent. For example, the first stream enriched in monomers and oligomers comprises a content of monomers and oligomers of at least 10% by weight, preferably at least 50% by weight, more preferably at least 100% by weight, which is greater than the content of monomers and oligomers of the heavy impurities effluent from step d). For example, the second stream enriched in heavy impurities comprises a content of at least 10% by weight, preferably at least 50% by weight, more preferably at least 100% by weight, which is greater than the content of heavy impurities of the heavy impurities effluent from step d). In a second embodiment, the separation of the heavy impurities effluent is carried out by filtration, by decanting, by MEG extraction, by centrifugation, etc. For example, the heavy impurities effluent from step d) may advantageously be subjected to at least one separation step (e.g. by filtration) in order to reduce the amount of pigment and/or other solid impurities in the first stream and to recover a second stream enriched in pigment and/or other solid impurities.

Preferably, the entire first stream obtained in step e) can be recycled to the depolymerization step b). More particularly, at least 70% by weight of the first stream is injected into one of the reaction sections downstream of the first reaction section and the remainder of the first stream (i.e., less than 30% by weight) is injected into the other reaction section of step b), for example, into the first reaction section. For example, in the embodiment of step b) with two reaction sections, less than 30% by weight, preferably less than 20% by weight, more preferably less than 10% by weight of the first stream is used to supply the first reaction section, and at least 70% by weight, preferably at least 80% by weight, more preferably at least 90% by weight of the first stream is used to supply the second reaction section. For example, in an embodiment of step b) with three reaction stages, less than 30% by weight, preferably less than 20% by weight, more preferably less than 10% by weight of the first stream is used to supply the first reaction stage, and the first stream is used to supply each of the second and third reaction stages with a percentage 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, and the sum of the percentages of the first stream supplied to the second and third stages is at least 70% by weight, preferably at least 80% by weight, more preferably at least 90% by weight of the first stream, respectively. According to a specific embodiment of the present invention, the entire first stream obtained in step e) is recycled by injecting it into one of the reaction stages downstream of the first reaction stage. In other words, the first stream is not used to supply the first reaction stage.

To facilitate separation, the heavy impurities effluent may be mixed with the glycol effluent, for example with a portion of the glycol effluent from step c), glycol feed external to the process of the invention, or a mixture thereof.

Before recycling to step b), the first stream may advantageously be mixed with a diol effluent, for example with a portion of the diol effluent from step c), a diol supply external to the process of the invention or a mixture thereof. The mixing is carried out in a static or dynamic mixer. Mixing the first portion with the diol effluent is advantageous for recycling ester oligomers, in particular BHET oligomers, since the mixing, on the one hand, makes it possible to fluidize the first stream (which potentially concentrates the solid particles (e.g. pigments) and polymeric compounds (e.g. polyolefins, polyamides, polyurethanes) present in the treated polyester raw material and helps to increase the viscosity and the polluting capacity of these residues and thus simplifies the operability of their transport), and on the other hand, reduces the viscosity of the first stream and thus facilitates its mixing with the polyester raw material in the depolymerization step.

According to one embodiment, the entire solids-rich second stream may advantageously be purged from the process, for example by sending it to an incineration system.

Alternatively, according to a second embodiment, a portion of the second stream rich in solids can be recycled to the depolymerization step by injecting it into one of the reaction stages downstream of the first reaction stage, the other remaining portion of the second stream being purged from the process by, for example, sending it to an incineration system. In this second embodiment, the portion of the second stream purged can represent at least 50% by weight of the second stream or even at least 70%, preferably at least 80%.

Optional decolorization step f) Advantageously, the process of the invention may comprise a step of decolorizing the pre-purified monomer effluent from step d), which step is operated at a temperature between 70 and 180° C., preferably between 80 and 150° C. and more preferably between 90 and 120° C. and a pressure between 0.05 and 1.0 MPa, preferably between 0.05 and 0.8 MPa and more preferably between 0.1 and 0.5 MPa in the presence of at least one adsorbent and produces a purified monomer effluent.

The adsorbent may be any adsorbent known to those skilled in the art that is capable of absorbing dyes (eg activated carbon or clay, advantageously activated carbon).

The pre-purified monomer effluent can advantageously be mixed with a solvent. Before contacting with the adsorbent, the pre-purified monomer effluent can be contacted with a solvent. The solvent introduced optionally is, for example, part of the glycol effluent from step c) which has been treated previously in a purification step, optionally, or, for example, with a glycol supply external to the process of the invention, or, for example, with water.

Optional crystallization step g) Optionally, the purified monomer effluent obtained in optional step f) can be introduced into the crystallization step g). When implementing it, the crystallization step g) advantageously implements at least one solid generation stage and at least one solid-liquid separation stage. The crystallization step g) (when implemented) makes it possible to obtain a decolorized and purified diester monomer effluent and to discharge the used solvent effluent.

Advantageously, the crystallization step g) implements one or more crystallization or precipitation operations and one or more solid-liquid separation operations. According to a particular embodiment, the crystallization step g) uses a solid generation section as explained below, followed by a solid-liquid separation section as described in detail below. According to another particular embodiment, the crystallization step g) uses several solid generation sections as explained below, preferably two to five solid generation sections, each of which is followed by a solid-liquid separation section as described in detail below.

Advantageously, the purified monomer effluent from the optional step f) is used to supply the solid generation section of step g). Optionally, a crystallization solvent that is the same as or different from the solvent used in the optional step f) can also be used to supply the solid generation section. The crystallization solvent is advantageously selected from water, monoalcohols, diols, ethers, aldehydes, esters, hydrocarbons and mixtures of at least two such compounds belonging to the same chemical family or different chemical families. Preferably, the crystallization solvent is selected from water, monoalcohols having 1 to 12 carbon atoms (such as methanol or ethanol) and diols having 1 to 12 carbon atoms. Very advantageously, the crystallization solvent is water, monoalcohols having 1 to 12 carbon atoms (such as methanol or ethanol) or diols having 1 to 12 carbon atoms (such as ethylene glycol). Preferably, the crystallization solvent is water, a diol having 1 to 12 carbon atoms (preferably ethylene glycol) or a mixture thereof.

Preferably, when the decolorization and crystallization steps f) and g) are performed and a crystallization solvent is introduced in step g), the amount of the crystallization solvent introduced into the solid generation section is adjusted so that the purified monomer effluent supplied to step f) accounts for 1% to 75% by weight, preferably 5% to 45% by weight, and more preferably 15% to 35% by weight of the total weight of the mixture in the solid generation section (i.e., a mixture comprising the purified monomer effluent, the solvent introduced in step f) and the crystallization solvent introduced in step g).

Before introduction into the solid generation stage, all or some of the crystallization solvent is preferably heated to the operating temperature of the adsorption stage or cooled and in particular reaches a temperature preferably between 0 and 120° C., preferably between 5 and 100° C. and more preferably between 10 and 90° C.

Advantageously, the solid generation section of optional step g) is operated at a temperature (i.e. so that the temperature of the effluent from the solid generation section is) between 0 and 100° C., preferably between 5 and 80° C. and more preferably between 10 and 70° C. More precisely, in the solid generation section, the purified monomer effluent pretreated by adsorption, optionally mixed with a crystallization solvent, is cooled from the operating temperature of the adsorption section (i.e. from between 70 and 180° C., preferably between 80 and 150° C., more preferably between 90 and 120° C.) to a temperature between 0 and 100° C., preferably between 5 and 80° C. and more preferably between 10 and 70° C.

The cooling can be carried out according to any method known to the person skilled in the art. For example, in particular in batch mode, the temperature cooling can be carried out without regulating the temperature reduction (i.e. without applying a temperature ramp; thus only the initial and final temperatures are controlled) or according to at least one temperature reduction ramp, in particular according to a temperature reduction ramp between 5 and 30° C./hour and more particularly between 8 and 15° C./hour or according to two continuous modes connected together (i.e. without controlling one part of the cooling) and according to a temperature reduction ramp of another part of the cooling. According to another example, cooling can be simplified by introducing the stream to be cooled (i.e., the monomer effluent pretreated by adsorption from the adsorption step f) or a mixture comprising the monomer effluent pretreated by adsorption and a crystallization solvent) into a tank of a volume advantageously adapted to the flow rate of the stream to be cooled, the tank being maintained at a temperature between 0 and 100°C, preferably between 5 and 80°C and more preferably between 10 and 70°C.

The solid generation section is advantageously operated at a pressure 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 generation section is operated in a vacuum, preferably between 0.0001 and 0.10 MPa, preferentially between 0.001 and 0.01 MPa. According to another particular embodiment, the solid generation 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, more preferably at atmospheric pressure (i.e. at 0.10 MPa).

Advantageously, the purpose of the solid generation stage is to at least partially generate a solid (i.e., to crystallize or precipitate the diester monomer, preferably BHET, present in the purified monomer effluent by adsorption pretreatment or originating from step f)). Thus, the solid generation stage comprises and preferably consists of a precipitated or crystallized phase implemented by any precipitation or crystallization technique known to those skilled in the art. The solid generation stage is preferably a stage for crystallization, for example by cooling or by concentration, which is implemented in any apparatus known to those skilled in the art, as defined, for example, in the engineers' technical journal "Cristallisation industrielle – Aspects pratiques" [Industrial Crystallization – Practical Aspects], No.: J2788 V1; liquid-solid separation is then carried out.

According to a preferred embodiment, water as crystallization solvent is mixed with the purified monomer effluent from step f) and the solid generation section in step g) is operated under conditions such that the temperature of the effluent from the solid generation 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 generation section is operated under conditions such that the temperature of the effluent from the solid generation section is between 5 and 50°C, preferably between 10 and 40°C.

Advantageously, the solid generation stage (preferably by a crystallization operation) comprises one or more crystallization operations (operated in series or in parallel) carried out batchwise or continuously, preferably continuously.

The solid generation section makes it possible to obtain a heterogeneous effluent comprising a solid phase of the diester monomer and a liquid phase. The heterogeneous effluent is advantageously sent to a solid-liquid separation section.

In the solid-liquid separation section of the optional step g), the diester monomer, advantageously in solid form, in particular in crystalline form, preferably BHET, is separated from the liquid phase of all or part of the solvent introduced in step f) and, if appropriate, the crystallization solvent introduced into the solid generation section. The solid-liquid separation section advantageously implements any solid-liquid separation means known to those skilled in the art, in particular at least one filtration, decanting and/or centrifugation system. The solid diester monomer thus separated constitutes the decolorized diester monomer effluent, and the liquid phase constitutes the used solvent effluent.

According to a particular embodiment, the decolorized diester monomer effluent, which is preferably recovered in solid form at the end of the optional step g), can also advantageously be subjected to all or some of the following operations (performed one or more times without a predetermined time sequence): washing with a solvent that is the same or different from that of the solvent supply step f) or, as the case may be, the solid generation stage of step g), additional filtration or centrifugation; removal of residual solvent by any method known to those skilled in the art (e.g., by evaporative drying); shaping (e.g.) into powder or granules; and storage of the solid.

According to another embodiment, the decolorized diester monomer effluent is preferably recovered by filtration or centrifugation in the solid-liquid separation stage and then sent directly (i.e. without a solid storage phase) to a polymerization step known to those skilled in the art. Optionally, before the polymerization reaction, the solid effluent of the decolorized diester monomer is rinsed with water or a glycol effluent (e.g., ethylene glycol effluent), preferably with water, and the rinsed solid is then heated to melt it.

According to another embodiment, the decolorized diester monomer effluent is recovered in the solid-liquid separation stage, preferably by filtration or centrifugation, and the monomer is then dried by one of the means known to those skilled in the art before being stored or transported to a polymerization step known to those skilled in the art.

According to another embodiment, the decolorized diester monomer effluent is recovered in the solid-liquid separation stage, preferably by filtration or centrifugation, and then the monomer is shaped after drying, if appropriate, by one of the means known to those skilled in the art (granulation, pelletizing) and before storage or transport to a polymerization step known to those skilled in the art.

The decolorized monomer effluent from the optional step g) can advantageously be fed to a polymerization step known to those skilled in the art to produce PET indistinguishable from nascent PET. Depending on the polymerization step used, the decolorized monomer effluent can advantageously be mixed with ethylene glycol, terephthalic acid or dimethyl terephthalate before the polymerization reaction. Feeding the decolorized monomer effluent to the polymerization step makes it possible to reduce the feed of dimethyl terephthalate or terephthalic acid at an equal flow rate.

Examples The example 1 illustrated with reference to FIG. 2 illustrates the present invention without limiting its scope; examples 2 to 4 illustrated with reference to FIGS. 3 to 5 respectively present examples that do not conform to the present invention.

Example 1 ( according to the present invention ) The method of Example 1 is schematically shown in Figure 2.

A raw material consisting of 100% PET is continuously supplied at a flow rate of 2500 kg/h (1), which corresponds to a recycling capacity of 20 KTA (thousand tons per year) of PET.

As illustrated in Figure 2, the conditioning step (a) involves: - an extruder (a1) for conditioning the PET raw material (1) by melting, - a static mixer (a2) for premixing the raw material from the extruder with the ethylene glycol stream (2) and obtaining a mixed stream.

The mixed stream is introduced into the depolymerization step (b) using two reaction sections (A) and (B) in a series configuration, each section consisting of a fully stirred reactor (reactor of the CSTR type (i.e., continuously stirred tank reactor type)). The working volumes of the reactors are: (A): 6 m ³ , (B): 42 m ³ . The 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 and then mixed with a fresh ethylene glycol stream having a flow rate corresponding to the depolymerization consumption (i.e. 700 kg/h in unit (3)), and the whole is reinjected into reactor (A) at a flow rate of 7500 kg/h and into reactor (B) at a flow rate of 3000 kg/h. The diol stream sent to reactor (B) enables dilution of the recycled oligomer effluent (6) from the separation section (e).

The separation section (d) of the monomer-rich effluent extracts monomers from the heavy impurities effluent containing, in particular, heavier oligomers at a yield of 75%, thereby generating a pre-purified monomer stream with a flow rate of 2815 kg/h and an effluent of heavy impurities containing heavier compounds (910 kg/h) and also monomer loss fractions (940 kg/h). The effluent of heavy impurities (1850 kg/h) (consisting of dimers and heavier compounds and also lost monomers and heavy impurities) is directed to the separation section (e). The pre-purified monomer stream is discharged from (d) and introduced into a decolorization step (f) and then into a crystallization step (g) to produce a purified and decolorized diester monomer stream (4), which consists of the monomer of interest BHET (2650 kg/h), the undesired monomer BHETdeg (150 kg/h) and also compounds of monomer composition modified in particular by thermal or thermooxidative degradation reactions (15 kg/h), which corresponds to a purity of 94.1% of the desired monomer.

The section for separating the heavy impurities effluent (e) is a simple non-selective separation of the stream, which results in two fractions: a first fraction (6) (80% of the feed stream to the separation section (e), which constitutes the recycle oligomer effluent) and a second fraction (7). The recycle oligomer effluent (6) (1480 kg/h) is introduced completely into the reactor (B), and the remainder (7) (370 kg/h) is purged, i.e. discharged from the process.

Example 1 shows that directing the recycled oligomer effluent (6) to reactor (B) results in the production of 15 kg/h of oxidative or thermal degradation products which are colored and difficult to extract from the final monomer.

Example 2 ( Comparison : with recirculation to adjustment step a)) The method of Example 2 is schematically shown in FIG. 3 .

A raw material consisting of 100% PET is continuously supplied at a flow rate of 2500 kg/h (1), which corresponds to a recycling capacity of 20 thousand tons of PET per year.

As illustrated in FIG3 , the conditioning step (a) involves: - an extruder (a1) for conditioning the PET feedstock (1) by melting, - a static mixer (a3) for premixing the recycled oligomer effluent (8) containing oligomers from the separation step (d) with the ethylene glycol stream (2) and obtaining a residue mixture (9), - a static mixer (a2) for premixing the feedstock from the extruder (a1) with the ethylene glycol stream (2) and with the residue mixture (9) to obtain a mixed stream.

The mixed stream was introduced into the depolymerization step (b) using two reaction sections (A) and (B) in a cascade configuration, each consisting of a fully stirred reactor. The working volumes of the reactors were: (A): 6 m ³ , (B): 42 m ³ . The reactor temperatures were: (A): 250°C, (B): 210°C.

The diol separation section (c) fully extracts the diol from the terephthalic acid (i.e. a flow rate of 9800 kg/h). The diol is then purified, mixed with a fresh stream having a flow rate corresponding to the depolymerization consumption (i.e. 700 kg/h in unit (3)), and most of it is reinjected into reactor (A), the rest is injected into sections (a3) and (a2) to accompany the recycled oligomer effluent (8).

The separation section (d) of the monomer-rich effluent extracts monomers from dimers and heavier oligomers at a yield of 75%, thereby generating a monomer stream from dimer prepurification and having a flow rate of 2827 kg/h and a heavy impurities effluent containing heavier compounds (870 kg/h) and also monomer loss fractions (940 kg/h). The effluent consisting of dimers and heavier compounds and also lost monomers and heavy impurities (referred to as heavy impurities effluent) (1810 kg/h) is directed to the separation section (e). The pre-purified monomer stream discharged from (d) is introduced into a decolorization step (f) and then into a crystallization step (g) to produce a purified and decolorized diester monomer stream (4), which consists of the monomer of interest BHET (2660 kg/h), the undesirable monomer BHETdeg (140 kg/h) and also monomers changed by thermal or thermooxidative degradation reactions (27 kg/h) (which corresponds to a purity of 94.1% of the desired monomer).

The heavy impurities effluent separation section (e) extracts heavy impurities from the terephthalic acid portion of the recycle residue stream at a yield of 20% to form a first stream (8) and a second stream (7) constituting the recycle oligomer effluent. The recycle oligomer effluent (8) (1500 kg/h) is introduced into the static mixer (a3), and the remaining portion (7) (375 kg/h) is purged, i.e. discharged from the process.

Example 2 shows that directing the recycled oligomer effluent to a static mixer (a4) and then to (a2) of the conditioning step (a) can increase the thermooxidative degradation phenomenon by 80% relative to Example 1, due to the production of 27 kg/h of color substances that are difficult to extract from the final monomer (whereas the method according to Example 1 only produces 15 kg/h of color substances that are difficult to extract from the final monomer).

Example 3 ( Comparative : with recycle to reactor A) The process of Example 3 is schematically shown in FIG. 4 .

A raw material consisting of 100% PET is continuously supplied at a flow rate of 2500 kg/h (1), which corresponds to a recycling capacity of 20 thousand tons of PET per year.

As illustrated in Figure 4, the conditioning step (a) involves: - an extruder (a1) for conditioning the PET raw material (1) by melting, - a static mixer (a2) for premixing the raw material from the extruder with the ethylene glycol stream (2) and obtaining a mixed stream.

The mixed stream was introduced into the depolymerization step (b) using two reaction sections (A) and (B) in a cascade configuration, each consisting of a fully stirred reactor. The working volumes of the reactors were: (A): 6 m ³ , (B): 42 m ³ . The reactor temperatures were: (A): 250°C, (B): 210°C.

The diol separation section (c) fully extracts diol from terephthalic acid (i.e., a flow rate of 9800 kg/h). The diol is then purified, mixed with a fresh stream having a flow rate corresponding to the depolymerization consumption (i.e., 700 kg/h in unit (3)), and the whole is reinjected into the reactor (A) at a flow rate of 10500 kg/h. The diol stream prepared in unit (3) is sent entirely to the reactor (A) to accompany the mixed stream 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 monomers from dimers and heavier oligomers at a yield of 75%, thereby generating a monomer stream from dimer prepurification and having a flow rate of 2826 kg/h and a heavy impurities effluent containing heavier compounds (870 kg/h) and also monomer loss fractions (940 kg/h). The effluent consisting of dimers and heavier compounds and also lost monomers and heavy impurities (referred to as heavy impurities effluent) (1810 kg/h) is directed to the separation section (e). The pre-purified monomer stream discharged from (d) is introduced into a decolorization step (f) and then into a crystallization step (g) to produce a purified and decolorized diester monomer stream (4), which consists of the monomer of interest BHET (2664 kg/h), the undesirable monomer BHETdeg (144 kg/h) and also monomers changed by thermal or thermooxidative degradation reactions (18 kg/h) (which corresponds to a purity of 94.3% of the desired monomer).

The heavy impurities effluent separation section (e) extracts heavy impurities from the terephthalic acid portion of the recycle residue stream at a yield of 20% to form a first stream (10) and a second stream (7) constituting the recycle oligomer effluent. The recycle oligomer effluent (10) (1450 kg/h) is introduced into the reactor (A), and the remaining portion (7) (370 kg/h) is purged, i.e. discharged from the process.

Example 3 shows that directing the recycled oligomer effluent (10) to the reactor (A) can increase the thermo-oxidative degradation by 20% compared to Example 1, due to the production of 18 kg/h of color substances that are difficult to extract from the final monomer (whereas the method according to Example 1 only produces 15 kg/h of color substances that are difficult to extract from the final monomer).

Example 4 ( Comparative : where the heavy impurities effluent is no longer recycled ) The method of Example 4 is schematically shown in FIG. 5 .

A raw material consisting of 100% PET is continuously supplied at a flow rate of 2500 kg/h (1), which corresponds to a recycling capacity of 20 thousand tons of PET per year.

As illustrated in Figure 5, the conditioning step (a) involves: - an extruder (a1) for conditioning the PET raw material (1) by melting, - a static mixer (a2) for premixing the raw material from the extruder with the ethylene glycol stream (2) and obtaining a mixed stream.

The mixed stream was introduced into the depolymerization step (b) using two reaction sections (A) and (B) in a cascade configuration, each consisting of a fully stirred reactor. The working volumes of the reactors were: (A): 6 m ³ , (B): 42 m ³ . The reactor temperatures were: (A): 250°C, (B): 210°C.

The diol separation section (c) fully extracts diol from terephthalic acid (i.e., a flow rate of 6900 kg/h). The diol is then purified, mixed with a fresh stream having a flow rate corresponding to the depolymerization consumption (i.e., 600 kg/h in unit (3)), and all is reinjected into reactor (A) at a flow rate of 7500 kg/h.

The separation section (d) of the monomer-rich effluent extracts monomers from dimers and heavier oligomers at a yield of 75%, thereby generating a monomer stream from dimer prepurification and having a flow rate of 2380 kg/h and a heavy impurities effluent containing heavier compounds (710 kg/h) and also monomer losses (600 kg/h). The effluent (11) consisting of dimers and heavier compounds and also lost monomers and heavy impurities, referred to as heavy impurities effluent (1310 kg/h), is discharged from the process, i.e., purged.

The pre-purified monomer stream discharged from (d) is introduced into a decolorization step (f) and then into a crystallization step (g) to produce a purified and decolorized diester monomer stream (4), which consists of the monomer of interest BHET (2176 kg/h), the undesirable monomer BHETdeg (189 kg/h) and also monomers changed by thermal or thermooxidative degradation reactions (15 kg/h) (which corresponds to a purity of 91.4% of the desired monomer).

Example 4 shows that the fact that part of the effluent of the heavy impurities (10) is no longer recycled leads to a purity of 91.4% obtained, which is significantly lower than the purity of Example 1 (94.1%), and is accompanied by a total monomer yield (71.1%) which is also significantly lower than the purity of Example 1 (84.5%). In addition, the content of substances originating from the thermal degradation reaction is similar to Example 1 (same flow rate, 15 kg/h, but a fraction of 0.6% by weight relative to the pre-purified monomer instead of 0.5% by weight according to Example 1).

(1): Raw material (2): Diol stream (3): Diol effluent (4): Diester monomer effluent (6): Recycled oligomer effluent/first stream (7): Second stream/remainder (8): Recycled oligomer effluent (9): Residue mixture (10): First stream/recycled oligomer effluent (11): Heavy impurity effluent (a): Adjustment step (a1): Adjustment section (a2): Mixing section (a3): Static or dynamic mixer (a4): Static mixer (b): Depolymerization step (c): Diol separation section/diol separation step (d): Separation section/separation step (e): Separation section/separation step (f): decolorization step (g): crystallization step A: first reaction section/reactor B: second reaction section N: reaction section

FIG. 1 represents a diagram of the process of the present invention. FIG. 1 is explained below. FIG. 2 represents a specific diagram of the process of the present invention and illustrates the process of the present invention as described in Example 1. FIG. 3 represents a diagram of a process for depolymerizing polyesters not in accordance with the present invention, including recycling to the adjustment step a), and illustrates the process described in Comparative Example 2. FIG. 4 represents a diagram of a process for depolymerizing polyesters not in accordance with the present invention, including recycling to reactor A, and illustrates the process described in Comparative Example 3. FIG. 5 represents a diagram of a process for depolymerizing polyesters not in accordance with the present invention, and illustrates the process described in Comparative Example 4. The same numbers used in FIGS. 2 to 5 as in FIG. 1 represent and designate the same elements.

(1): Raw materials

(2): Diol flow

(3): Diol effluent

(4): Diester monomer effluent

(6): Recirculating oligomer effluent/first flow

(7): The second stream/the rest

(a): Adjustment steps

(a1):Regulatory segment

(a2): Mixing section

(a3): Static or dynamic mixer

(b): Depolymerization step

(c): Diol separation section/diol separation step

(d): Separation section/separation step

(e): Separation section/separation step

(f): Decolorization step

(g): Crystallization step

A: First reaction section/reactor

B: Second reaction stage

N: Reaction section

Claims (16)

A method for depolymerizing a polyester feedstock containing PET, the method comprising: a) a conditioning step, which is at least supplied with the polyester feedstock to produce a conditioned feedstock stream; b) carrying out a depolymerization step of a first reaction section (A) and at least one second reaction section (B), the at least one second reaction section (B) being operated at a temperature strictly lower than the temperature of the first reaction section (A), the first reaction section (A) being supplied with at least the regulated feed stream and, if appropriate, a first diol supply, the at least one second reaction section (B) being supplied with the effluent of the first reaction section and the recycled oligomer effluent and, if appropriate, a second diol supply, so that the total amount of diol supplied to the step b) is regulated to 1 to 20 mol diol/mol diester supplied to the step b), the step b) being operated at a temperature between 180 and 300°C and a residence time between 0.334 and 10 h; c) a step of separating the diol, which is supplied with at least the effluent of step b), operated at a temperature between 100 and 250°C and a pressure lower than the pressure of step b) and produces a diol effluent and an effluent rich in liquid monomers; d) a step of separating the effluent rich in liquid monomers from step c) into a heavy impurity effluent and a pre-purified monomer effluent, which is operated at a temperature less than or equal to 250°C and a pressure less than or equal to 0.001 MPa, and the liquid retention time is less than or equal to 10 min, e) a step of separating the heavy impurities effluent into two fractions: at least 70% by weight of which constitutes a first fraction (6) of the recycled oligomer effluent supplied to step b) and a second fraction (7) at least partially discharged from the process; f) a step of decolorizing the pre-purified monomer effluent, as appropriate, operating in the presence of an adsorbent at a temperature between 100 and 250°C and a pressure between 0.1 and 1.0 MPa, and producing a purified monomer effluent; g) a step of crystallizing the purified monomer effluent, as appropriate, using at least one solids generating stage, at a temperature between 0 and 100°C and a pressure between 0.00001 and 1.00 MPa, followed by a solid-liquid separation stage to produce a decolorized and purified monomer effluent. The method of claim 1, wherein the polyester raw material comprises at least colored PET, opaque PET or a mixture thereof. A method as claimed in any of the preceding claims, wherein the regulating step a) is implemented with at least one regulating section (a1) for producing a fluid feed stream and a mixing section (a2) for producing a mixed stream, the mixed stream corresponding to the regulated feed stream, the regulating section (a1) is at least supplied with the polyester feed and implemented at a temperature between 150 and 300°C, the mixing section (a2) is at least supplied with the fluid feed stream and a diol stream from the regulating section, at least a portion of the diol stream is preferably composed of at least a portion of the diol effluent from step c), the mixing section (a2) is operated at a temperature between 150 and 300°C and a retention time between 0.5 seconds and 20 minutes. A method as claimed in claim 3, wherein the adjustment section (a1) of step a) is operated in an extruder, and the section (a2) for mixing the polyester raw material of step a) is also implemented in the extruder as appropriate. The method of claim 3, wherein the mixing section (a2) of step a) is implemented by at least one static or dynamic mixer. A method as in any one of claims 3 to 5, wherein in step a), the weight ratio of the diol stream introduced in step a) to the polyester raw material is between 0.03 and 6.00, preferably between 0.05 and 5.00, more preferably between 0.10 and 4.00, and most preferably between 0.50 and 3.00. The process of any of the preceding claims, wherein the at least one second reaction stage (B) is operated at a temperature which is 5 to 50° C. lower than the temperature of the first reaction stage (A). The process of any of the preceding claims, wherein the recycled oligomer effluent supplied to the at least one second reaction section (B) comprises the entire first stream originating from step e). A method as claimed in any of the preceding claims, wherein, in step e), the second stream (7) is divided into two parts: a first part is injected into one of the reaction stages of step b) downstream of the first reaction stage, and a second part is discharged from the method. The method of any of the preceding claims, wherein the depolymerization step comprises two reaction stages. A method as claimed in any one of claims 1 to 9, wherein the depolymerization step comprises three reaction sections: the second reaction section and the third reaction section are each supplied with a portion of the recycled oligomer effluent varying between 0 and 100 wt %, the sum of these portions constituting 100 wt % of the recycled oligomer effluent. The method of any of the preceding claims, wherein the depolymerization step comprises at least one internal recirculation loop implementing at least the following operations: withdrawing a portion of the reaction system from one of the reaction sections, filtering the portion, and reinjecting the portion into one of the reaction sections. A method as claimed in any of the preceding claims, wherein the separation step e) is carried out so as to obtain a first oligomer-enriched stream (6) and a second oligomer-enriched stream (7), wherein the first oligomer-enriched stream comprises an oligomer content strictly greater than the oligomer content of the heavy impurities effluent and the second oligomer content comprises a heavy impurities content strictly greater than the heavy impurities content of the heavy impurities effluent. A process as claimed in any of the preceding claims, wherein the first stream (6) obtained in step e) is mixed (a3) with a diol stream, preferably with a portion of the diol effluent obtained in step c), and then recycled to step b) and/or, as the case may be, to step a). A method as in any of the preceding claims, wherein the glycol separation step c) is carried out in 1 to 5 consecutive gas-liquid separation stages, each separation stage produces a gaseous effluent and a liquid effluent, the liquid effluent from the previous stage is supplied to the subsequent stage, the liquid effluent from the last gas-liquid separation stage constitutes the liquid monomer-rich effluent, and all the gaseous effluent is recovered to constitute the glycol effluent. A method as claimed in any one of claims 3 to 15, wherein the regulating section is additionally supplied with a glycol stream.