Optimized process for glycolysis depolymerization of polyesters containing polyethylene terephthalate

Abstract

The present invention relates to a process for depolymerizing a polyester feedstock comprising PET, comprising: a) a conditioning step; b) a glycololysis step in the presence of glycols; c) a glycol separation step producing at least a gaseous glycol effluent, at least a liquid glycol effluent and a liquid monomer effluent; d) a step of separating the liquid monomer effluent into a heavy impurities effluent and a pre-purified monomer effluent, the step being carried out with a residence time of less than 10 minutes; e) a step of decolorizing the pre-purified monomer effluent in the presence of at least one adsorbent; and f) a glycol purification step supplied with at least the gaseous glycol effluent and at least the liquid glycol effluent obtained from step c) and producing a purified glycol effluent and one or more impurity effluents.

Description

Technical Field

[0001] The present invention relates to an optimized process for the glycolysis depolymerization of polyesters, more particularly polyester terephthalates comprising polyethylene terephthalate (PET), in the presence of glycols, in order to recover said polyester in a polymerization unit. More specifically, the present invention relates to a process for the depolymerization of a polyester raw material, particularly comprising at least colored and / or opaque and / or multilayered PET, by glycolysis in the presence of glycols, more particularly ethylene glycol (or monoethylene glycol, MEG), said process having a specific step of purifying the glycol effluent recovered after depolymerization, thereby obtaining a glycol stream, more particularly a purified MEG stream, which can be recycled in particular to the glycolysis reaction step. Prior art

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

[0003] Many polyesters come from circuits for collecting and sorting materials. In particular, polyester, especially PET, can originate from the collection of bottles, pallets, films, resins and / or fibers (e.g., textile fibers, tire fibers) composed of polyester. The polyester generated by the collection and sorting sector is referred to as recycled polyester.

[0004] PET to be recycled can be divided into four major categories:

[0005] - transparent PET, consisting mainly of colorless transparent PET (usually at least 60% by weight) and sky-blue transparent PET, which contains no pigments and can be used in mechanical recycling processes;

[0006] - Dark or colored (green, red, etc.) PET, which can typically contain up to 0.1% by weight of dye or pigment but is still transparent or translucent;

[0007] - Opaque PET, which contains significant amounts of pigments, usually in an amount of 0.25% to 5.0% by weight, which renders the polymer opaque. Opaque PET is increasingly being used, for example, in the manufacture of food containers (e.g. milk bottles), in the composition of cosmetic, plant-protection or dye bottles;

[0008] - Multilayer PET comprising layers of polymers other than PET, or layers of recycled PET between layers of virgin PET (ie PET which has not undergone recycling), or for example aluminium films. After thermoforming, multilayer PET is used to produce packaging, such as trays.

[0009] The structure of the collection departments that feed the recycling sector varies from country to country. They vary with the nature and volume of the streams and with the sorting techniques, so as to maximize the amount of plastics recovered from the waste. The departments for recycling these streams generally include a first step of conditioning in the form of flakes, during which the bales of raw packaging are washed, purified and sorted, ground, and then purified and sorted again to produce a flake stream that generally contains less than 1% by weight of "macro" impurities (glass, metals, other plastics, wood, paper, cardboard, inorganic elements), preferably less than 0.2%, even more preferably less than 0.05%.

[0010] The transparent PET flakes can then be subjected to an extrusion-filtration step which can produce an extrudate which can then be reused as a mixture with virgin PET to produce new products (bottles, fibers, films). A solid phase polymerization (known by the acronym SSP) step under vacuum is necessary for food uses. This type of recycling is known as mechanical recycling.

[0011] Dark (or colored) PET flakes can also be mechanically recycled. However, the coloration of the extrudate formed from the colored stream limits the uses: dark PET is usually used to produce packaging tapes or fibers. Therefore, the outlets for dark PET are more limited than those for transparent PET.

[0012] The presence of opaque PET with a high content of pigments in the PET to be recycled is problematic for recyclers, since the opaque PET adversely affects the mechanical properties of the recycled PET. Opaque PET is currently collected together with the colored PET and is present in the colored PET stream. In view of the development of uses for opaque PET, the content of opaque PET in the colored PET stream to be recycled is currently 5-20% by weight and has a tendency to increase further. In a few years, it will be possible to achieve an opaque PET content of more than 20-30% by weight in the colored PET stream. In fact, 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 adversely affected (see Impact du développement du PET opaque blanc sur le recyclage des emballages en PET [Impact of the growth of white opaque PET on the recycling of PET packaging], preliminary report of COTREP of May 12, 2013) and recycling in fiber form (the main outlet for the colored PET industry) is hindered.

[0013] Dyes are natural or synthetic substances that are soluble, in particular soluble in polyester materials and are used to color the materials into which they are introduced. Commonly used dyes have different properties and usually contain O and N type heteroatoms and conjugated unsaturated groups, such as quinone, methine or azo functional groups, or molecules such as pyrazolone and quinophthalone. Pigments are insoluble, in particular finely divided substances that are insoluble in polyester materials and are used to color and / or opacify the materials into which they are introduced. The main pigments used to color and / or opacify polyester, in particular PET, are metal oxides, such as TiO2, CoAl2O4 or Fe2O3, silicates, polysulfides and carbon black. Pigments are particles with a size of usually 0.1 to 10 μm, mainly 0.4 to 0.8 μm. It is technically difficult to completely remove these pigments by filtration (which is necessary for the envisioned recycling of opaque PET) because they have an extremely high blocking capacity.

[0014] Therefore, the recycling of colored PET and opaque PET is extremely problematic.

[0015] For certain applications, such as making PET resins for bottles, comonomers are intentionally added to the composition of PET during its production to enhance certain properties of PET and facilitate its processability. For example, 0.1 wt % to 10 wt %, preferably 1 wt % to 5 wt %, of diethylene glycol (DEG) may be added relative to the weight of the PET polyester.

[0016] Patent application US 2006 / 0074136 describes a process for depolymerizing colored PET, in particular colored PET resulting from the recycling of green PET bottles, by glycololysis. The feedstock treated by this process is in the form of PET flakes and is contacted with ethylene glycol in a reactor at a temperature of 180 to 280° C. for several hours. The glycololysis product obtained at the end of the depolymerization step is purified on activated carbon at a temperature greater than 170° C., and the residual dye, in particular the yellow dye, is then extracted with a solvent, which may be an alcohol, such as methanol, or a glycol, such as ethylene glycol. BHET, for its part, crystallizes in the extraction solvent and is subsequently separated by filtration.

[0017] In patent application US 2015 / 0105532, post-consumer PET, which comprises a mixture of different colored PET in the form of flakes, such as transparent PET, blue PET, green PET and / or amber PET, is depolymerized by glycolysis in a reactor in a batch mode in the presence of ethylene glycol and an amine catalyst at 150-250° C. The diester monomers obtained are purified by filtration, ion exchange and / or passing through activated carbon, in particular at a temperature of 80-90° C., and then crystallized and recovered by filtration.

[0018] In patent EP0865464, the process for depolymerizing polyester, more particularly colored polyester, such as green PET, comprises the following steps: a depolymerization step in a reactor in the presence of a diol at a temperature of 180 to 240°C; an optional evaporation step in a thin film evaporator, although the conditions under which the evaporator must be operated are not specified; and a step of dissolving the mixture in a hot solvent. The hot dilution is followed by a filtration step that allows the separation of insoluble impurities with a size greater than 50µm. The low proportion of pigments in colored PET can be separated by filtration. However, this technology cannot operate at the amount of pigments present in opaque PET, which quickly clog the filter.

[0019] Patent JP3715812 describes obtaining purified bis(2-hydroxyethyl)terephthalate (BHET) from PET in flake form. The depolymerization step consists in the glycolysis of PET flakes in a stirred reactor in the presence of ethylene glycol and a catalyst at 180° C. to remove water, then at 195-200° C., having previously been pretreated in solid form by washing with water. The depolymerization is followed by a prepurification step by cooling, filtering, adsorption and treatment on ion exchange resins, which is very important and is carried out before evaporating the glycol and purifying the BHET. The prepurification prevents the repolymerization of the BHET in the subsequent purification step. However, when the feedstock contains a large amount of very small solid particles (for example pigments and / or polymer compounds other than PET, such as polyolefins and polyamides), the steps of passing through the filtration and ion exchange resins can be extremely problematic, as is the case when the feedstock being processed contains opaque PET and / or multilayer preformed PET, especially in significant proportions, in particular when the filler accounts for more than 10% by weight of the opaque PET and / or multilayer preformed PET.

[0020] Meanwhile, patent EP 1 120 394 discloses a process for depolymerizing polyesters comprising a glycolysis step in the presence of ethylene glycol and a process for purifying a bis(2-hydroxyethyl)terephthalate solution on a cation exchange resin and an anion exchange resin.

[0021] Finally, patent application FR 3053691 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, a purified bis(2-hydroxyethyl)terephthalate (BHET) effluent is obtained. This patent application considers the possibility of sending some or all of the glycol effluent containing unreacted ethylene glycol in liquid form to a purification step and then recycling it, but does not provide details of this step for treating the glycol effluent, nor does it take into account the diethylene glycol that may be present in non-negligible amounts as a comonomer in the PET chains and that is produced during the depolymerization of these PET chains.

[0022] The object of the present invention is to perfect these processes for glycolysis depolymerization of polyester raw materials comprising PET, more particularly the process of application FR 3053691, so as to improve the recycling of the glycol effluent recovered at the end of the glycolysis, in particular by a purification step integrated in an optimal way into the depolymerization process. The object of the present invention is to obtain a high-purity diester stream, more particularly a BHET stream, by depolymerizing a polyester raw material comprising PET, more particularly colored and / or opaque PET, while reducing the energy consumption of the process. SUMMARY OF THE INVENTION

[0024] The subject of the present invention is therefore a process for depolymerizing a polyester feedstock comprising polyethylene terephthalate (PET), said process comprising and preferably consisting of the following steps:

[0025] a) a conditioning step comprising at least a conditioning section supplied with said polyester feedstock to produce a conditioned stream;

[0026] b) a step of depolymerization by glycolysis, which step is fed with at least the conditioned stream and is carried out at a temperature of 180 to 400° C., with a residence time of 0.1 to 10 hours, in the presence of glycols in such a way that the total molar amount of glycols is adjusted to 1 to 20 moles of glycol per mole of diester in the polyester raw material;

[0027] c) a glycol separation step, which is fed with at least the effluent from step b) and is carried out at a temperature of 60 to 250° C., at a pressure lower than the pressure of step b), and produces at least a gaseous glycol effluent, at least a liquid glycol effluent and a liquid monomer effluent, wherein the glycol separation step is carried out in one gas-liquid separation stage or in a series of 2 to 5 consecutive gas-liquid separation stages, each separation stage producing a gaseous effluent and a liquid effluent, wherein the liquid effluent from the preceding stage is fed to the subsequent stage and the liquid monomer effluent is formed from the liquid effluent obtained from the final gas-liquid separation stage,

[0028] The one or more gaseous effluents obtained from step c) at least partially form one or more gaseous glycol effluents,

[0029] The one or more gaseous effluents obtained from step c) are at least partially condensed to form one or more liquid glycol effluents;

[0030] d) a step of separating the liquid monomer effluent obtained from step c) into a heavy impurity effluent and a pre-purified monomer effluent, the step being carried out at a temperature below 250° C. and a pressure below 0.001 MPa with a liquid residence time of less than 10 minutes;

[0031] e) subjecting the pre-purified monomer effluent obtained from step d) to a decolorization step using at least one adsorption stage comprising at least one adsorber, operating in the presence of at least one adsorbent at a temperature ranging from 100 to 200° C., so as to produce a purified monomer effluent;

[0032] f) a glycol purification step which is fed with at least the gaseous glycol effluent and at least the liquid glycol effluent obtained from step c) and is conducted at a temperature of 40 to 250° C. and a pressure of 0.001 to 0.2 MPa and produces at least a purified glycol effluent and one or more impurity effluents.

[0033] The present invention makes it possible to obtain a diester effluent, more particularly a BHET stream, with high purity from a polyester raw material comprising PET and originating from a collection and sorting sector, by subjecting a polyester raw material, more particularly polyethylene terephthalate, to glycolysis in the presence of a glycol, more particularly ethylene glycol (MEG), obtained at least in part from a glycol effluent recovered at the end of depolymerization and purified before recycling. Thus, one advantage of the present invention is that an optimized process scheme is proposed, by combining a step c) of separation of unreacted glycols and / or glycols released during a previous depolymerization step by glycolysis with a step f) of purification of said glycols, said step f) treating at least one gaseous glycol effluent and at least one liquid glycol effluent directly obtained from said glycol separation step c), thereby making it particularly possible to reduce the energy consumption of said glycol separation step c) and of said purification step f), respectively. Energy gain is advantageously obtained by varying the amounts of gaseous glycol effluent and liquid glycol effluent obtained in separation step c) and sent to purification step f), which cannot be run independently of step c). Thus, the process according to the invention makes it possible to reduce the overall process energy consumption.

[0034] Another advantage of the present invention is the separation of impurities released during the depolymerization reaction and carried by the diols into the diol separation step c), thereby obtaining a purified diol effluent, which can then be sent in whole or in part to the depolymerization step b) and / or the optional conditioning step a) and / or the decolorization step e). In fact, during the depolymerization reaction, compounds lighter than the diester monomers, more particularly the BHET monomers, may be released and / or formed in the reaction mixture, such as diethylene glycol (DEG) (which can be used as a comonomer for the production of PET) or water, which are optionally present in the polyester raw material obtained from plastic waste or formed during the depolymerization step by side reactions (such as etherification of ethylene glycol). These compounds are separated from the diols, more particularly ethylene glycol, in the diol separation step c) and may be detrimental to the performance of the depolymerization step b) and / or may modify the composition of the purified monomer effluent obtained at the end of step e). Therefore, the process described can improve the overall performance of the process in terms of the quality of the diester product obtained, by recycling the purified diols to the depolymerization stage.

[0035] Another advantage of the present invention is that it can process any type of polyester waste, such as sky blue PET, colored PET, opaque PET and multilayer PET, which contain more and more pigments, dyes and other polymers. The method according to the present invention is suitable for processing opaque PET and is able to remove pigments, dyes and other polymers and recover diester monomers, especially BHET bis(2-hydroxyethyl) terephthalate monomers, by chemical reaction. This monomer can then be repolymerized into a polymer that behaves indistinguishably from virgin polyester, more especially virgin PET, thereby enabling all uses of virgin PET to be realized. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1

[0037] Figure 1 represents a specific embodiment of the present invention. It schematically illustrates the method detailed in Example 1.

[0038] In step a), a polyester feedstock (1) comprising opaque PET is conditioned, in particular at a temperature of 250° C. The conditioned feedstock (2) is introduced into a depolymerization step b) by glycolysis, which step is also fed with an ethylene glycol (MEG) stream (10) consisting of a portion of the purified MEG effluent (9) obtained from step f) and a fresh MEG make-up (17) from outside the process. The depolymerization step is carried out at a temperature of 220° C. to 250° C. and a pressure of 0.4 MPa.

[0039] The reaction effluent (3) obtained is then sent to a glycol separation step c) using a series of two flash tanks at a temperature of 180°C to 120°C and a pressure of 0.04 MPa to 0.004 MPa, followed by a thin film evaporator operating at 175°C and 0.0005 MPa.

[0040] At the end of this evaporation step (step c), a liquid stream and three gaseous streams rich in MEG are recovered. The liquid stream forms a liquid BHET effluent (4) and is sent to a BHET separation step d). The gaseous stream obtained at the outlet of the first flash tank is maintained in gaseous form and forms a gaseous MEG effluent (7). The gaseous stream obtained at the outlet of the second flash tank is condensed and forms a liquid MEG effluent (8). The gaseous stream obtained at the outlet of the thin film evaporator is also condensed and forms another liquid MEG effluent (8'). The gaseous MEG effluent (7) and the liquid MEG effluents (8) and (8') are sent to a glycol purification step f).

[0041] In step d), the liquid BHET effluent (4) is injected into a short-path evaporator, also known as short-path distillation, which is operated at a pressure of 20 Pa and heated at 215° C. A pre-purified BHET effluent (5) and a heavy impurity effluent (14) are obtained at the outlet of the short-path evaporator. A portion (16) of the heavy impurity effluent (14) is discharged, while another portion (15) of the heavy impurity effluent (14) is recycled to the reaction step (step b). The pre-purified BHET effluent (5) is compressed to 0.15 MPa before being sent to the decolorization step e).

[0042] In step e), the pre-purified BHET effluent (5) is used to feed a mixing section operated at 150°C and a pressure of 0.15 MPa and also fed with a portion (11) of the purified MEG effluent (9) obtained from the diol purification step f). The flow rate of the purified MEG stream (11) is adjusted so that the liquid BHET stream represents 50% by weight of the final mixture. The mixture obtained is then used to feed an adsorption section consisting of two towers, each filled with an adsorbent, more particularly activated carbon, operated at 150°C and a pressure of 0.15 MPa. One tower is placed online and the other is kept on standby (switching operation). The purified BHET effluent is recovered at the outlet of step e).

[0043] The diol purification step f) uses a distillation column including a condenser, a reboiler and 30 theoretical plates, and is operated at 0.025 MPa at the top. The gaseous MEG effluent (7) is directly used to supply the column at the 23rd tray. The liquid MEG effluent (8') is compressed to 0.1 MPa and preheated to 112°C in a heat exchanger by heat exchange with the residue taken out at the bottom of the distillation column. The preheated liquid MEG effluent (8') is then mixed with the liquid MEG effluent (8) previously compressed to 0.1 MPa, and the liquid MEG mixture is then used to supply the distillation column at the 23rd tray. A water-rich liquid distillate is taken out at a temperature of 65°C at the top of the column, after the condenser, and it forms a light impurity effluent (12). A DEG-rich liquid residue is taken out at a temperature of 200°C at the bottom of the column, at the reboiler, and it forms a heavy impurity effluent (13). A purified liquid MEG stream is withdrawn at the 4th tray at a temperature of 154°C and forms the purified MEG effluent (9).

[0044] Thus, a portion (10) of the purified glycol effluent (9) can be recycled to the depolymerization reactor (step b) as a mixture with fresh MEG make-up (17) from outside the process, and another portion can be sent to the decolorization step (step e).

[0045] Description of the implementation plan

[0046] According to the present invention, polyethylene terephthalate or polyethylene terephthalate, also referred to as PET, has a basic repeating unit of the following formula:

[0047] .

[0048] Conventionally, PET is obtained by polycondensation of terephthalic acid (PTA) or dimethyl terephthalate (DMT) with ethylene glycol. Hereinafter, the expression "diester per mole of the polyester raw material" corresponds to the number of moles of -[O-CO-(C6H4)-CO-O-CH2-CH2]- units in the PET contained in the polyester raw material, which are diester units obtained by the reaction of PTA and ethylene glycol.

[0049] According to the present invention, the term "monomer" or "diester monomer" advantageously refers to bis(2-hydroxyethyl)terephthalate (BHET) having the chemical formula HOC2H4-CO2-(C6H4)-CO2-C2H4OH, which is a diester unit obtained by reacting PTA and ethylene glycol in the PET contained in the polyester raw material, wherein -(C6H4)- represents an aromatic ring.

[0050] The term "oligomer" generally refers to a polymer of small size, which generally comprises 2 to 20 basic repeating units. According to the present invention, the term "ester oligomer" or "BHET oligomer" refers to a terephthalate oligomer comprising 2 to 20, preferably 2 to 5, basic repeating units of the formula -[O-CO-(C6H4)-CO-O-C2H4]-, wherein -(C6H4)- is an aromatic ring.

[0051] According to the invention, the terms "diol" and "glycol" are used equivalently and correspond to compounds comprising two -OH hydroxyl groups. A preferred diol is ethylene glycol, also known as monoethylene glycol or MEG.

[0052] Therefore, the diol or diol effluent stream used in the process step of the present invention preferably comprises ethylene glycol (or MEG) in an amount greater than 40 wt% of the total weight of the diol or diol effluent stream. They may also optionally comprise diethylene glycol (DEG).

[0053] The term "dye" is understood to mean a substance which is soluble in the polyester material and which is used to color it. The dye may be of natural or synthetic origin.

[0054] According to the invention, the term "pigment", more particularly opaque and / or coloring pigments, refers to finely divided substances which are insoluble, particularly insoluble in the polyester material. The pigments are in the form of solid particles, the size of which is generally between 0.1 and 10 μm, mainly between 0.4 and 0.8 μm. They are generally of an inorganic nature. Commonly used, in particular for opacifying, pigments are metal oxides, for example TiO2, CoAl2O4 or Fe2O3, silicates, polysulfides and carbon black.

[0055] According to the invention, the expressions "between ... and ..." and "... to ..." mean that the limits of the interval are included in the described range of values. If this is not the case, and if a limit is not included in the described range, the invention will give such clarification.

[0056] Specific and / or preferred embodiments of the present invention may be described below. They may be implemented individually or in combination with each other, as long as such a combination is technically feasible, and there is no restriction on their combination.

[0057] raw material

[0058] The method according to the invention is supplied with a polyester raw material comprising at least one polyester (which is a polymer whose main chain repeating units contain ester functional groups), comprising polyethylene terephthalate (PET), preferably comprising at least colored PET and / or opaque PET and / or multilayer PET, preferably at least opaque PET.

[0059] The polyester raw material is advantageously a polyester raw material to be recycled, obtained from the collection and sorting sectors of waste, more particularly plastic waste. The polyester raw material may originate from the collection of bottles, trays, films, resins and / or fibers composed, for example, of polyethylene terephthalate.

[0060] Advantageously, the polyester raw material comprises at least 50 wt%, preferably at least 70 wt%, more preferably at least 90 wt% of polyethylene terephthalate (PET).

[0061] Preferably, the polyester raw material comprises at least one PET selected from opaque, dark or colored and multilayer PET and mixtures thereof. Very particularly, the polyester raw material comprises at least opaque PET, preferably 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. PET obtained from the collection and sorting sector.

[0062] The polyester raw material advantageously contains 0.1% to 10% by weight, advantageously 0.1% to 5% by weight of pigments. Preferably, it also contains 0.05% to 1% by weight, in particular 0.05% to 0.2% by weight of dyes.

[0063] In the collection and sorting sectors, the polyester waste is washed and ground and then forms the polyester raw material for the process according to the invention.

[0064] The polyester raw material may be in the form of flakes, whose maximum length is less than 10 cm, preferably 5 to 25 mm, or in the form of micronized solids, i.e. in the form of particles preferably having a size of 10 microns to 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, such as glass, metal, plastics other than polyester (such as PP, HDPE, etc.), wood, paper, cardboard or inorganic elements. The polyester raw material may also be in the form of fibers, such as textile fibers, optionally pretreated to remove cotton or polyamide fibers, or any other textile fibers other than polyester, or such as tire fibers, optionally pretreated to remove polyamide fibers, or rubber, or polybutadiene residues in particular. The polyester raw material may also contain polyester obtained from production waste of a polymerization and / or conversion process of polyester materials. The polyester raw material may also contain elements used as polymerization catalysts and stabilizers in the PET production process, such as antimony, titanium and tin.

[0065] Conditioning step a)

[0066] The process according to the invention comprises a conditioning step a) comprising at least one conditioning section which is fed with at least the polyester raw material and produces a conditioned stream.

[0067] The conditioning section of step a) can heat and pressurize the polyester raw material to the operating conditions of the depolymerization step b).

[0068] In the conditioning section, the polyester raw material is gradually heated to a temperature close to or even slightly above its melting point, so as to render the raw material at least partially liquid. Advantageously, at the end of the conditioning section of step a), at least 70% by weight of the polyester raw material, very advantageously at least 80% by weight, preferably at least 90% by weight, more preferably at least 95% by weight of the polyester raw material are in liquid form. The temperature of the conditioning section, in which step a) is carried out, is advantageously between 150 and 300° C., preferably between 225 and 275° C. This temperature is kept as low as possible, so as to minimize thermal degradation of the polyester.

[0069] Advantageously, the conditioning section may be operated under an inert atmosphere to limit the introduction of oxygen into the system and oxidation of the polyester feedstock.

[0070] According to a preferred embodiment of the present invention, the conditioning section is an extrusion section, which corresponds to a screw conveying section. In other words, the conditioning section operates in an extruder.

[0071] The residence time in the extrusion section, defined as the volume of the section divided by the volume flow of the feedstock, is advantageously less than or equal to 5 hours, preferably less than or equal to 1 hour, more preferably less than or equal to 30 minutes, very preferably less than or equal to 10 minutes, and preferably more than or equal to 2 minutes. Advantageously, the extrusion section can condition the polyester feedstock so that the conditioned stream is at a temperature of 150 to 300° C., preferably 225 to 275° C., and a pressure of atmospheric pressure (i.e. 0.1 MPa) to 20 MPa.

[0072] The extrusion section is advantageously connected to a vacuum extraction system in order to remove impurities, such as dissolved gases, light organic compounds and / or moisture present in the feedstock. The extrusion section may also advantageously include a filtration system to remove solid particles, such as sand, having a size greater than 40 μm, preferably less than 2 cm. The polyester feedstock is advantageously fed to the extruder by any method known to those skilled in the art, such as via a feed hopper, and may advantageously be inerted to limit the introduction of oxygen into the system.

[0073] According to an optional embodiment of the invention, the conditioning section may comprise a conditioning zone, advantageously as described above, carried out at a temperature of 150 to 300° C., preferably 225 to 275° C., preferably in an extruder, followed by a feedstock-diol contacting stage, wherein the polyester feedstock is advantageously contacted with a diol stream, preferably comprising at least a portion of the purified diol effluent obtained from step f), optionally as a mixture with an external diol supplement and / or a portion of the liquid diol effluent obtained from step c), preferably such that the number of moles of diol in the diol stream is from 0.05 to 10, preferably from 0.15 to 6.0, more preferably from 0.3 to 3.0 relative to the number of moles of diester in the polyester feedstock, the diol stream preferably comprising said at least a portion of the purified diol effluent obtained from step f), optionally as a mixture with an external diol supplement and / or a portion of the liquid diol effluent obtained from step c). The effect of this contact is to initiate the depolymerization reaction of the polyester feedstock before it is introduced into the depolymerization step b). It also enables the viscosity of the polyester feedstock to be reduced, thus particularly facilitating its conveying to the depolymerization step b). The optional feedstock-diol contacting stage can be implemented in a static or dynamic mixer. The feedstock-diol contacting stage can be implemented in an extruder when the conditioning zone is implemented in an extruder; in this case, it constitutes a reactive extrusion stage. Preferably, the diol stream, more particularly comprising at least a portion of the purified diol effluent obtained from step f), optionally as a mixture with an external diol supplement and / or a portion of the liquid diol effluent obtained from step c), can advantageously be superheated before being supplied to the feedstock-diol contacting stage, in order to more easily bring the polyester feedstock to temperature.

[0074] The polyester feed can also advantageously be mixed with at least a portion of the heavy impurities effluent obtained from step d) before or in the conditioning stage.

[0075] The conditioned stream obtained from the conditioning section is advantageously sent to a depolymerization step b).

[0076] Depolymerization step b)

[0077] The process according to the invention comprises a step of depolymerization by glycolysis, which step is fed with at least the conditioned stream obtained from step a) and is carried out in particular in the liquid phase, in the presence of glycols, advantageously at a temperature ranging from 180 to 400° C., preferably from 200 to 300° C., more preferably from 210 to 280° C., with a residence time in said step b) of 0.1 to 10 hours, preferably from 0.25 to 8 hours, more preferably from 0.5 to 6 hours and in such a way that the total molar amount of glycol introduced into step a) and / or step b) is adjusted to 1 to 20 mol, preferably from 3 to 15 mol, more preferably from 5 to 10 mol of glycol per mole of diester in said polyester raw material (corresponding to a weight ratio of glycol introduced into step a) and / or step b) relative to the weight of the polyester raw material of 0.3 to 6.7, preferably from 1.0 to 5.0, more preferably from 1.7 to 3.3).

[0078] Thus, said step b) of depolymerization by glycolysis is optionally supplied with a glycol supplement, depending on the amount of glycol optionally introduced in step a) during the optional feedstock-glycol contacting stage. This optional glycol supplement advantageously comprises at least said portion of the purified glycol effluent obtained from step f), optionally as a mixture with an external glycol supplement and / or a portion of the liquid glycol effluent obtained from step c). Very preferably, the optional glycol supplement is an optional ethylene glycol supplement, advantageously comprising at least a portion of the purified glycol effluent obtained from step f), preferably consisting of ethylene glycol (MEG).

[0079] The operating pressure of step b) is determined so that the reaction system is kept 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. "Reaction system" refers to all components and phases present in step b).

[0080] The residence time is defined as the ratio of the volume of liquid in the reaction zone to the volume flow rate of the stream entering the reaction zone.

[0081] The diol is advantageously monoethylene glycol.

[0082] Advantageously, the depolymerization step b) is implemented in one or more reaction sections, preferably at least two reaction sections, more preferably two to four reaction sections, the reaction sections being operated in series. Each reaction section can be used in any type of reactor known to those skilled in the art capable of carrying out a depolymerization or transesterification reaction, preferably in a reactor stirred by a mechanical stirring system and / or by a recycling loop and / or by fluidization. The reactor may comprise a conical bottom capable of discharging impurities. Preferably, the depolymerization step b) comprises at least two reaction sections, more preferably two to four reaction sections, the reaction sections being operated in series, wherein, starting from the second reaction section, one or more reaction sections are operated at a temperature that is the same or different from one another and is lower than or equal to the temperature of the first reaction section, preferably at a temperature lower than the first reaction section, more preferably at a temperature that is 10 to 50° C., or even 20 to 40° C. lower than the temperature of the first operable section.

[0083] The glycolysis reaction can be carried out in the presence or absence of a catalyst. When the glycolysis reaction is carried out in the presence of a catalyst, the catalyst can be homogeneous or heterogeneous and is selected from esterification catalysts known to those skilled in the art, such as complexes, oxides and salts of antimony, tin or titanium, alkoxides of metals from Groups (I) and (IV) of the Periodic Table of Elements, organic peroxides or acidic / basic metal oxides. Relative to the total mass of the catalyst, a 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 %, more advantageously at least 95 mass % of a catalyst having the formula Z x Al2O (3+x) A solid solution of at least one spinel of wherein x is 0 (excluding limiting values) to 1, Z is selected from Co, Fe, Mg, Mn, Ti and Zn, and contains up to 50% by mass of alumina and oxides of the Z element. The preferred heterogeneous catalyst advantageously contains up to 10% by mass of a dopant selected from silicon, phosphorus and boron, used alone or as a mixture. For example, and without limitation, the solid solution may be composed of a mixture of spinel ZnAl2O4 and spinel CoAl2O4, or a mixture of spinel ZnAl2O4, spinel MgAl2O4 and spinel FeAl2O4, or only spinel ZnAl2O4.

[0084] Preferably, the depolymerization step is carried out without adding an external catalyst to the polyester feedstock.

[0085] The depolymerization step can advantageously be carried out in the presence of a solid adsorbent in powder or shaped form, the function of which is to capture at least part of the colored impurities, thereby relieving the pressure of the decolorization step e). The solid adsorbent is advantageously activated carbon.

[0086] The glycolysis reaction can convert the polyester raw material into ester oligomers and monomers, and advantageously converts PET into at least bis(2-hydroxyethyl)terephthalate (BHET) monomers and BHET oligomers. In the depolymerization step, the conversion rate of the polyester raw material is greater than 50%, preferably greater than 70%, preferably greater than 85%. The molar yield of BHET is greater than 50%, preferably greater than 70%, preferably greater than 85%. The molar yield of BHET corresponds to the molar flow rate of BHET at the outlet of step b) to the mole number of diesters in the polyester raw material supplied to step b).

[0087] In step b) an internal recycling loop is advantageously used, ie a portion of the reaction system is withdrawn, filtered and reinjected into said step b). This internal loop makes it possible to remove solid "macro" impurities that may be present in the reaction liquid.

[0088] The depolymerization step b) may advantageously result in an effluent, also called reaction effluent, which is sent to the glycol separation step c).

[0089] Diol separation step c)

[0090] The process according to the invention comprises a glycol separation step c), which is fed with at least the effluent from step b) and is operated at a temperature of 60 to 250° C., at a pressure lower than the pressure of step b), and produces at least a gaseous glycol effluent, at least a liquid glycol effluent and a liquid monomer effluent.

[0091] The main purpose of step c) is to recover some or all of the unreacted diol, as well as compounds lighter than the monomers, which are released or generated during the depolymerization reaction.

[0092] Step c) is advantageously carried out at a pressure lower than that of step b) so as to vaporize a portion of the effluent from step b) in order to obtain at least a gaseous effluent and a liquid effluent.

[0093] Advantageously, step c) is implemented in a gas-liquid separation section or a series of gas-liquid separation sections, advantageously two to five continuous separation sections, very advantageously three to five continuous separation sections. Each gas-liquid separation section produces a liquid effluent and a gaseous effluent. The liquid effluent from the previous section is supplied to the subsequent section. The liquid effluent obtained by the final gas-liquid separation section forms a liquid monomer effluent. Each gaseous effluent produced by one of the gas-liquid separation sections advantageously comprises a glycol greater than 40% by weight, preferably greater than 50% by weight, more preferably greater than 60% by weight, preferably ethylene glycol (MEG). One or more gaseous effluents may also contain other compounds, such as dyes, light alcohols, water or diethylene glycol. At least the gaseous effluent obtained from the series of gas-liquid separation sections, or at least a portion of the gaseous effluent obtained from the gas-liquid separation sections, or each gaseous effluent obtained from the gas-liquid separation sections is maintained in a gaseous state to form at least one gaseous glycol effluent and sent directly to the glycol purification step f). One or more gaseous effluents obtained from step c) at least partially form one or more gaseous glycol effluents. Step c) preferably produces at least one gaseous glycol effluent, preferably up to four gaseous glycol effluents. At least one gaseous effluent obtained from the series of gas-liquid separation sections, or at least a portion of the gaseous effluent obtained from the gas-liquid separation sections, or from the series of gas-liquid separation sections is condensed to form at least one liquid glycol effluent, preferably up to four liquid glycol effluents, which are subsequently sent to the glycol purification step f). One or more gaseous effluents obtained from step c) are at least partially condensed to form one or more liquid glycol effluents, preferably up to four liquid glycol effluents.

[0094] According to a preferred embodiment of the present invention, step c) advantageously comprises at least three, preferably three, consecutive gas-liquid separation stages. The gaseous effluent obtained from the first gas-liquid separation stage forms a gaseous glycol effluent and is sent directly to the glycol purification step f). The gaseous effluent obtained from the final gas-liquid separation stage is condensed to form a liquid glycol effluent, which is subsequently sent to step f). The gaseous effluent(s) obtained from one or more gas-liquid separation stages between the first gas-liquid separation stage and the final gas-liquid separation stage may be condensed or kept in a gaseous state before being sent to the glycol purification step f).

[0095] When step c) produces a plurality of gaseous glycol effluents, the gaseous glycol effluents obtained from step c) may be mixed with each other before being supplied to step f), or may be supplied to said step f) independently. When step c) produces a plurality of liquid glycol effluents, the liquid glycol effluents obtained from step c) may be mixed with each other before being supplied to step f), or may be supplied to said step f) independently, or at least one liquid glycol effluent may be supplied directly to step f), independently or as a mixture, and the other one or more liquid glycol effluents may be optionally recycled in whole or in part to step a) and / or step b) and / or step e).

[0096] Advantageously, at least one gas-liquid separation stage can be carried out in a falling-film evaporator or a thin-film evaporator or in a short-path distillation.

[0097] Step c) is carried out in such a way that the temperature of the liquid effluent is kept above a certain value below which the polyester monomers precipitate and below a certain high value above which the monomers significantly repolymerize, depending on the diol / monomer molar ratio. The temperature in step c) is between 60 and 250° C., preferably between 90 and 220° C., more preferably between 100 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 this makes it possible to adjust the temperature of the liquid effluent in each separation to meet the above constraints. It also allows to distribute the feed to step f) into one or more gaseous diol effluents and one or more liquid diol effluents, so as to limit the energy consumption of the process while optimizing the separation of the diols and the purification of the diols.

[0098] The pressure in step c) is advantageously adjusted to enable evaporation of the diol at a temperature that minimizes repolymerization and optimizes energy integration. The pressure is generally between 0.00001 and 0.2 MPa, preferably between 0.00004 and 0.15 MPa, more preferably between 0.00004 and 0.1 MPa.

[0099] The gas-liquid separation stage or stages are advantageously agitated by any method known to a person skilled in the art.

[0100] Optionally, some or all of the at least one gaseous glycol effluent, preferably after condensation, and / or some or all of the at least one liquid glycol effluent can advantageously be recycled directly to step a) and / or step b) and / or step e), optionally as a mixture with glycol make-up outside the process according to the invention. Preferably, all of the gaseous glycol effluent and the liquid glycol effluent obtained from step c) is sent to a glycol purification step f), before recycling the glycol to step a) and / or step b) and / or using it in step e).

[0101] Monomer separation step d)

[0102] The process according to the invention comprises a step d) of separating the liquid monomer effluent obtained from step c) into a heavy impurities effluent and a pre-purified monomer effluent.

[0103] Said step d) is advantageously carried out at a temperature of less than 250° C., preferably less than 230° C., very preferably less than 200° C., and preferably greater than 110° C., at a pressure of less than 0.001 MPa, preferably less than 0.0005 MPa, preferably greater than 0.000001 MPa, with a liquid residence time of less than 10 minutes, preferably less than 5 minutes, more preferably less than 1 minute, and preferably greater than 0.1 seconds.

[0104] The purpose of this separation step d) is to separate the monomers, more particularly the vaporized BHET, from the still liquid and therefore also trapped heavy impurities, particularly the oligomers of pigment, from the unconverted polyester polymer, from other polymers that may be present in the polyester raw material, from the polymerization catalyst, which are not completely converted during the depolymerization step, while minimizing the losses of monomers due to repolymerization. These heavy impurities are found together with the oligomers in the heavy impurities effluent. However, some oligomers may optionally be entrained with the monomers, particularly those with small sizes.

[0105] Due to the possible presence of a polymerization catalyst in the polyester raw material, the separation must be carried out with very short liquid residence times and at temperatures not higher than 250° C., so as to limit any risk of repolymerization of the monomers, more particularly BHET, during this step. Therefore, separation by simple atmospheric distillation cannot be considered.

[0106] The separation step d) is advantageously carried out in a separation stage which comprises a falling film or thin film evaporation system or an evaporation system involving a falling film or thin film short path distillation, more particularly by a falling film or thin film short path distillation.

[0107] In order to operate step d) at a temperature below 250° C., preferably below 230° C., while vaporizing the monomers, it is necessary to adopt a very low operating pressure, advantageously below 0.001 MPa, preferably below 0.0005 MPa, preferably above 0.000001 MPa.

[0108] A polymerization inhibitor may advantageously be mixed with the liquid monomer effluent before being used to feed said step d).

[0109] Before being used to supply said step d), it may also be advantageous to mix a diluent with the liquid monomer effluent in order to facilitate the removal of heavy impurities, in particular pigments, at the bottom of the short-path distillation or evaporation system. Under the operating conditions of step d), this diluent needs to have a much higher boiling point than the monomers, more particularly BHET. For example, it may be polyethylene glycol, or a PET oligomer.

[0110] The heavy impurity effluent comprises in particular pigments, oligomers and optionally unseparated BHET. The heavy impurity effluent is advantageously recycled in whole or in part to the conditioning step a). A portion of the heavy impurity effluent can advantageously be directly recycled to step a) and / or step b), either alone or as a mixture with the glycol effluent. The heavy impurity effluent can advantageously be subjected to at least one purification step, preferably a filtration step, before recycling to reduce the amount of pigments and / or other solid impurities. The separated portion of the heavy impurity effluent with a high pigment content can advantageously be discharged from the method and fed to an incineration system.

[0111] Part of the heavy impurities effluent is preferably recycled to step a) and / or step b) without prior separation of solid impurities.

[0112] Advantageously, said prepurified monomer effluent obtained from the separation section of step d) is sent to step e).

[0113] Said pre-purified monomer effluent obtained from the separation section of step d) may optionally be sent to a gas / liquid separation section, which is operated in any apparatus known to a person skilled in the art at a temperature of 100 to 250° C., preferably 110 to 200° C., more preferably 120 to 180° C., at a pressure of 0.00001 to 0.1 MPa, preferably 0.00001 to 0.01 MPa, more preferably 0.00001 to 0.001 MPa. In a preferred embodiment of the process, wherein the separation step d) is carried out in an evaporation system by falling film or thin film short path distillation, said optional gas-liquid separation section is integrated in the evaporation system. Said optional gas-liquid separation section is capable of separating a gaseous glycol effluent and a pre-purified liquid monomer effluent. By recovering more than 50% by weight, preferably more than 70% by weight, more preferably more than 90% by weight of the glycols and advantageously optional light compounds that may be entrained by the pre-purified monomer effluent in step d), the gas-liquid separation can further reduce the amount of glycols, preferably ethylene glycol (MEG), remaining in the pre-purified monomer effluent, and advantageously reduce the amount of optional light compounds (such as DEG), or even remove residual glycols. The amount of monomers entrained in the gaseous glycol effluent is preferably less than 1% by weight, more preferably less than 0.1% by weight, and even more preferably less than 0.01% by weight relative to the amount of monomers present in the pre-purified monomer effluent. The gaseous glycol effluent is then advantageously condensed and sent to step f). Optionally, some or all of the condensed glycol effluent can be recycled to step a) and / or step b) and / or enter step e) as a mixture with the glycol effluent obtained from step f). If the process comprises this optional gas-liquid separation stage, the stream sent to step e) is the pre-purified liquid monomer effluent obtained at the end of said optional gas-liquid separation stage.

[0114] Decolorization step e)

[0115] The process according to the invention comprises a step of decolorizing the pre-purified monomer effluent obtained from step d) to produce a purified monomer effluent.

[0116] Said step e) advantageously enables to remove residual dyes, more particularly dyes having a boiling point below the cut-off point, from the pre-purified monomer effluent, i.e. in particular under the temperature and pressure conditions employed in the monomer separation step d). The reason is that these residual dyes entrained with the pre-purified monomer effluent dyed by the dyes can be effectively removed in this way by adsorption in said step e).

[0117] Said step e) advantageously employs at least one adsorption stage comprising at least one adsorber and is operated in the presence of at least one adsorbent, preferably up to five different adsorbents, at a temperature of 100 to 200° C., preferably 100 to 170° C., more preferably 120 to 150° C. and preferably at a pressure of 0.1 to 1.0 MPa, more particularly 0.1 to 0.8 MPa, very particularly 0.2 to 0.5 MPa.

[0118] When the adsorption stage comprises two or more different adsorbents, the different adsorbents are in the adsorption stage in a mixture or arranged in series, preferably in series, more preferably each adsorbent is in a different adsorber (eg reactor or column) arranged in series.

[0119] According to the present invention, when the nature of the adsorbent and / or its composition and / or its particle size and / or its textural characteristics (such as pore volume) are different, the adsorbents are called different adsorbents. The one or more adsorbents can be any adsorbent known to those skilled in the art that has the ability to capture dyes, such as activated carbon, alumina and clay. Advantageously, at least one adsorbent is selected from activated carbon.

[0120] The adsorption stage is advantageously implemented in flow-through fixed bed mode, i.e. in at least one adsorber with a fixed bed of one or more adsorbents, more particularly in at least one tower of one or more adsorbents, which can be operated in upflow or downflow mode, preferably in upflow mode, or in at least one continuously stirred reactor, also called "continuous stirred tank reactor" (CSTR). When this stage is implemented in at least one CSTR stirred reactor, the reactor or reactors are followed by a filtration system for recovering the one or more adsorbents suspended in the treated liquid. Preferably, the adsorption stage is implemented in flow-through fixed bed mode.

[0121] Advantageously, the adsorption stage comprises two or more towers with one or more adsorbents in the form of a fixed bed, more particularly at least two towers with one or more adsorbents, preferably two to four towers with the same adsorbent, more particularly two towers with the same adsorbent. When the adsorption stage comprises two towers with the same adsorbent, an operating mode may be referred to as "switching", in which one tower is online and the other tower is on standby. When the adsorbent in the online tower is exhausted, the tower is isolated and the standby tower is placed online. The spent adsorbent can then be regenerated in situ and / or replaced with fresh adsorbent to be replaced online again when the other tower is isolated. Another mode of operating the adsorbent tower is to have at least two towers operating in series; when the adsorbent of the lead tower (i.e., the first tower in the series) is exhausted, the first tower is isolated, and the spent adsorbent is regenerated in situ or replaced with fresh adsorbent. Then, the tower is replaced online at the last position in the tower train, and so on. This operation is referred to as "lead-lag".

[0122] Said step e) also advantageously employs a stage for mixing the pre-purified monomer effluent obtained from step d) with the diol effluent, preferably a portion of the purified diol effluent obtained from step f), optionally as a mixture with external diol make-up and / or a portion of the liquid diol effluent obtained from step c).

[0123] The mixing section, when present, is preferably located upstream of the adsorption section. Preferably, the amount of diol effluent introduced into the mixing section, which diol effluent is very advantageously obtained from a portion of the diol effluent obtained in step f), optionally as a mixture with external diol make-up and / or a portion of the liquid diol effluent obtained in step c), is adjusted so that the pre-purified monomer effluent represents from 20 to 90 wt.-%, more preferably from 30 to 80 wt.-%, very preferably from 50 to 75 wt.-%, of the total weight of the mixture in the mixing section. The mixing section is advantageously operated at a temperature of from 100 to 250° C., preferably from 110 to 200° C., more preferably from 120 to 180° C., and a pressure of from 0.1 to 1.0 MPa, preferably from 0.1 to 0.8 MPa, more preferably from 0.2 to 0.5 MPa. The glycol effluent, preferably that part of the glycol effluent obtained from step f), optionally as a mixture with external glycol supplement and / or a part of the liquid glycol effluent obtained from step c), may be heated before the mixing section, preferably to the operating temperature of the mixing section. The mixing section advantageously employs a static or dynamic mixer, preferably a static mixer.

[0124] The decolorization step e) may also advantageously comprise a regeneration phase of the adsorbent or adsorbents.

[0125] The purified monomer effluent obtained at the end of step e) of the process of the invention is almost colorless, or even colorless to the naked eye. Preferably, the purified monomer effluent is characterized by UV-visible spectroscopy to determine the presence of an absorption band in the visible range, in other words from 400 to 800 nm. Preferably, by this method, in other words, by UV-visible spectroscopy from 400 to 800 nm and with an optical path length of 5 mm at a temperature above the melting point of the characterized effluent (in other words, in a liquid medium, or preferably from 120° C. to 150° C.), the purified monomer effluent obtained according to the invention exhibits a spectrum in which there is no significant absorption band (i.e. no band that can be distinguished from background noise) in the visible wavelength range (i.e. from 400 to 800 nm).

[0126] The purified monomer effluent is advantageously fed to a polymerization step known to those skilled in the art to produce PET indistinguishable from virgin PET, advantageously downstream of the feed point of ethylene glycol, of terephthalic acid or of dimethyl terephthalate, depending on the polymerization step employed. Feeding the purified monomer effluent in the polymerization step makes it possible to reduce the feed of dimethyl terephthalate or terephthalic acid by an equivalent flow rate.

[0127] Diol purification step f)

[0128] The process according to the invention comprises a diol purification step f), which is fed with at least the gaseous diol effluent and at least the liquid diol effluent obtained from the diol separation step c), and is carried out at a temperature of 40 to 250° C., preferably 70 to 220° C., at a pressure of 0.001 to 0.2 MPa, preferably 0.004 to 0.1 MPa, to produce at least a purified diol effluent and one or more, preferably one to three, preferably two impurity effluents. The purified diol effluent contains more than 90% by weight, preferably more than 95% by weight, more preferably more than 99% by weight of diol, preferably ethylene glycol. The purification step is capable of recovering more than 80% by weight, preferably more than 90% by weight, more preferably more than 95% by weight of diol, preferably ethylene glycol, contained in the one or more gaseous diol effluents and the one or more liquid diol effluents fed to the step f).

[0129] The main function of step f) is to separate some or all of the impurities entrained by the unreacted glycol (preferably ethylene glycol) separated in step c) and to obtain the desired purity of the glycol effluent before recycling the glycol effluent to step a) and / or step b) and / or step e).

[0130] The diol purification step f) advantageously comprises at least a purification section using a separation based on volatility differences, based on gas-liquid separation, or based on distillation, preferably a separation based on distillation. According to the present invention, the term "distillation" refers to a separation based on volatility differences, which is preferably carried out in one or more columns with or without a reboiler and / or a condenser.

[0131] Said purification section advantageously comprises at least a stage for separation of impurities, advantageously a stage for separation of impurities lighter than the glycol, preferably ethylene glycol, contained in the gaseous glycol effluent and the liquid glycol effluent obtained from step c) and intended to be fed to step f), and a stage for separation of impurities heavier than the glycol, preferably ethylene glycol, contained in said gaseous effluent and the liquid effluent.

[0132] The purification stage is preferably operated by distillation, using at least one distillation column equipped with internal components known to those skilled in the art for performing distillation operations. For example, one or more distillation columns are advantageously equipped with trays or random or structured packings.

[0133] According to a preferred embodiment of the present invention, the purification section comprises a distillation column, preferably equipped with a condenser at the top of the column and a reboiler at the bottom of the column, the column being fed with at least the gaseous glycol effluent and at least the liquid glycol effluent obtained from step c) on the same theoretical plate or on different theoretical plates, and producing a distillate effluent more particularly rich in light impurities, which is advantageously withdrawn at the top of the column and forms a light impurity effluent; a residue effluent more particularly rich in heavy impurities, which is advantageously withdrawn at the bottom of the column and forms a heavy impurity effluent; and a purified glycol effluent, which is advantageously withdrawn at the side of the column, between the positions where the distillate effluent and the residue effluent are withdrawn. In this embodiment, the distillation column advantageously comprises from 10 to 50 theoretical plates, preferably from 15 to 35 theoretical plates.

[0134] According to another preferred embodiment of the present invention, the purification section comprises at least two distillation columns, preferably up to 4 distillation columns, more preferably two distillation columns, more particularly at least one topping column comprising 3 to 25 theoretical plates, preferably 8 to 20 theoretical plates, and a tailing column comprising 10 to 50 theoretical plates, preferably 15 to 35 theoretical plates, wherein the topping column is capable of separating impurities lighter than glycol, preferably ethylene glycol, and the tailing column is preferably equipped with a condenser and a reboiler, and is capable of separating impurities heavier than glycol, preferably ethylene glycol. The topping column may be arranged upstream of the tailing column, or vice versa. In any case, the distillate taken out at the top of the topping column is a light impurity effluent, and the residue taken out at the bottom of the tailing column is a heavy impurity effluent. Advantageously, at least the gaseous glycol effluent, at least the liquid glycol effluent, or at least the gaseous glycol effluent and the liquid glycol effluent are used to supply the tailing column and / or the topping column on the same theoretical plate or on different theoretical plates.

[0135] A configuration in which the one or more distillation columns in the purification section are fed with at least a gaseous glycol effluent and at least a liquid glycol effluent, optionally in stages, meaning that the feeding is carried out on different theoretical plates of the one or more distillation columns in the purification section, is particularly advantageous, since such a configuration makes it possible to reduce the energy consumption of step f) and, more generally, of the process, while ensuring optimal purification of the glycol effluent, preferably of ethylene glycol, in order to recycle it to step a) and / or step b) and / or step e) of the process according to the invention.

[0136] Purification stage f) may advantageously comprise one or more filtration operations.

[0137] Advantageously, purification section f) may also comprise a decolorization section, for example by adsorption on a solid, for example on activated carbon, to remove the dyes optionally entrained with the glycol.

[0138] Advantageously, said step f) may also comprise a stage for removing volatile organic compounds by thermal or catalytic combustion of said compounds, in order to prevent them from being emitted into the environment. Non-exhaustively, said stage for treating impurities comprises filtration, if solid particles are present, and catalytic or non-catalytic combustion systems.

[0139] The purified glycol effluent obtained at the end of step f) of the process according to the invention can then be sent in whole or in part to at least step a), step b) and step e) of the process according to the invention.

[0140] The drawings and the following examples illustrate the invention without limiting its scope. Example

[0141] In the following examples, the conditioning step a), the depolymerization step b), the monomer separation step d) and the decolorization step e) are identical and are described below. The only changes between Example 1 (according to the invention) and Example 2 (not according to the invention) are the diol separation step c) and the diol purification step f). Figure 1 The depolymerization method according to Example 1 of the present invention is schematically shown.

[0142] The polyester raw material (1) to be processed is obtained from the collection and sorting sector and contains in particular 20% by weight of opaque PET.

[0143] 4 tons / hour of said polyester raw material (1) flakes containing 20 wt.% of opaque PET and thus 6.2 wt.% of TiO2 pigment were brought to a temperature of 250°C (step a) and then injected into a first stirred reactor maintained at 250°C together with 11.5 tons / hour of ethylene glycol (MEG) (10) obtained from the purified MEG effluent fraction (9) obtained in step f) and fresh MEG replenishment (17) maintained at 220°C and then into a second stirred reactor and a third stirred reactor (step b). The reactors were maintained at a pressure of 0.4 MPa. The residence time, defined as the ratio of the liquid volume in the reactor to the sum of the liquid volume flows entering the reactors, was set to 20 minutes in the first reactor and 2.1 hours in the second and third reactors. At the outlet of the third reactor, the reaction effluent (3) consisted of 64.2 wt% of MEG (monoethylene glycol), 25.8 wt% of diester monomers, most of which consisted of BHET (containing 95 wt% or more of BHET), 3.1 wt% of DEG (diethylene glycol), 0.48 wt% of water, 0.32 wt% of TiO2, and 6.1 wt% of heavy compounds containing dimers and / or oligomers of BHET.

[0144] The ethylene glycol present in the reaction effluent (3) is separated by evaporation in a series of two flash tanks at a temperature of 180° C. to 120° C. and a pressure of 0.04 MPa to 0.004 MPa and in a thin film evaporator operated at 175° C. and 0.0005 MPa (step c). At the end of this evaporation step (step c), 5.00 tons / hour of a liquid stream (4) rich in BHET and three gaseous streams rich in MEG are recovered. At the outlet of the first flash tank, 8.54 tons / hour of a gaseous MEG stream 1 containing 96.4% by weight of MEG is recovered. At the outlet of the second flash tank, 0.85 tons / hour of a gaseous MEG stream 2 containing 96.8% by weight of MEG is recovered. At the outlet of the thin film evaporator, 1.11 tons / hour of a gaseous MEG stream 3 containing 79.4% by weight of MEG is recovered. The BHET-rich liquid stream forms the liquid BHET effluent (4) and consists of 79.7 wt% of BHET diester monomer, 0.17 wt% of MEG, 0.19 wt% of DEG and 1.0 wt% of TiO2 and 18.94 wt% of heavy compounds including BHET dimers.

[0145] The liquid BHET effluent (4) is then injected into a short-path evaporator, also known as short-path distillation, which is operated at a pressure of 20 Pa (step d). Hot oil at 215°C evaporates the BHET, which is subsequently condensed in a short-path evaporator at 130°C to obtain a pre-purified BHET liquid stream (5) corresponding to the pre-purified monomer effluent. The residence time in the short-path evaporator is 1 minute. The flow rate of the pre-purified BHET liquid stream (5) is 3.8 tons / hour and is recovered from the short-path evaporator as a distillate. It contains 99% by weight of BHET diester monomers and does not contain trace amounts of TiO2. The heavy residue (14) with a flow rate of 1.2 tons / hour, corresponding to the heavy impurity effluent, is recovered from the short-path evaporator as a residue and consists of: 18.5% by weight of BHET diester monomers, 77.4% by weight of BHET oligomers and 4.1% by weight of TiO2. A portion (16) of the heavy residue (14) is discharged, while another portion (15) can be recycled to the reaction step (step b).

[0146] The gaseous effluent was condensed at 130°C to obtain a pre-purified BHET liquid stream (5).

[0147] The pre-purified BHET liquid stream (5) containing 99% by weight of BHET diesters is compressed to 0.15 MPa and sent to the decolorization step e). It is first used to supply a mixing section which also receives a supply of a portion (11) of the purified MEG effluent (9) obtained from the diol purification step f). The supply rate of the portion of purified MEG effluent (11) is adjusted so that the BHET liquid stream accounts for 50% by weight of the final mixture. The section is operated at 150°C and a pressure of 0.15 MPa.

[0148] The mixture obtained is then used to feed an adsorption stage (step e) consisting of two towers, each filled with an adsorbent. The adsorption stage is operated at 150° C. and a pressure of 0.15 MPa. One tower is placed on line and the other is kept on standby. The adsorbent used to fill the two towers is an activated carbon consisting of cylindrical extrudates with a diameter of 0.8 mm, ROY 0.8 from Cabot Norit being mentioned. The residence time is set at 3 hours. A purified monomer effluent (6) is obtained at the outlet of the adsorption tower.

[0149] The coloration of the effluent (6) at the outlet of the tower was monitored in situ by UV-visible spectroscopy using a UV-visible probe of the Falcata series of the Hellma brand, with an optical path length of 5 mm. The coloration was monitored at 400 to 800 nm by the appearance of a distinct absorption band in the wavelength range of 400 to 800 nm.

[0150] The appearance of coloration, as determined by the appearance of an absorption band at about 450 nm, was observed after 7 days. This time defines the operating life of the column before the stream to be treated is switched to the standby column.

[0151] When the standby tower is exhausted, the tower containing the spent adsorbent is drained or regenerated before starting online operation again.

[0152] Example 1 - According to the present invention

[0153] In Example 1 according to the invention, the gaseous MEG stream 1 obtained from the first flash tank is not condensed. It forms a gaseous MEG effluent (7).

[0154] The gaseous MEG stream 2 obtained from the second flash tank was completely condensed at 85°C and 0.0011 MPa to obtain a liquid MEG effluent (8).

[0155] The gaseous MEG stream 3 obtained from the thin film evaporator is completely condensed at 72° C. and 0.0004 MPa to obtain a liquid MEG effluent ( 8 ′).

[0156] The gaseous MEG effluent (7) and the liquid MEG effluents (8) and (8') are sent to the glycol purification step f).

[0157] The diol purification step f) uses a distillation column including a condenser, a reboiler and 30 theoretical plates, and is operated at 0.025 MPa at the top. The gaseous MEG effluent (7) is directly used to feed the column at the 23rd tray. The liquid MEG effluent (8') is compressed to 0.1 MPa and preheated to 112°C in a heat exchanger by heat exchange with the residue taken out at the bottom of the distillation column. The preheated liquid MEG effluent (8') is then mixed with the liquid MEG effluent (8) previously compressed to 0.1 MPa, and the liquid MEG mixture is then used to feed the distillation column at the 23rd tray.

[0158] A water-rich liquid distillate is taken off at the top of the column, after the condenser, at a temperature of 65° C. The water-rich liquid distillate contains 93.5% by weight of water. It forms the light impurities outflow ( 12 ).

[0159] A DEG-rich liquid residue is taken off at the bottom of the column, at the level of the reboiler, at a temperature of 200° C. The DEG-rich liquid residue contains 95% by weight of DEG. It forms the heavy impurities effluent ( 13 ).

[0160] A purified MEG liquid stream is taken off from the 4th tray at a temperature of 154° C. The purity of the purified MEG stream is 99.8 wt % MEG. It forms the purified glycol effluent (9).

[0161] Thus, a portion (10) of the purified glycol effluent (9) can be recycled to the depolymerization reactor (step b) as a mixture with fresh MEG make-up (17) outside the process, another portion being sent to the decolorization step (step e).

[0162] The energy consumption regarding hot heat transfer fluid for the glycol separation step c) and the glycol purification step f) as well as for the entire process is reported in Table 1 below.

[0163] Table 1

[0164] Energy consumption Step c) Step f) The whole method MW 1.72 0.92 4.02

[0165] Feeding a gaseous stream and a liquid stream to a distillation column in the glycol purification step f) produces a purified MEG stream containing 99.8 wt% MEG (ethylene glycol), while the energy consumption regarding hot heat transfer fluid is reasonable, especially for said step f) (0.92 MW).

[0166] Example 2 - Not According to the Invention

[0167] In Example 2 which is not according to the present invention, the gaseous MEG stream 1 obtained from the first flash tank was completely condensed at 140° C. and 0.024 MPa to obtain a liquid MEG stream 1 .

[0168] The gaseous MEG stream 2 obtained from the second flash tank is completely condensed at 85° C. and 0.0011 MPa to obtain a liquid MEG stream 2.

[0169] The gaseous MEG stream 3 obtained from the thin film evaporator is completely condensed at 72° C. and 0.0004 MPa to obtain a liquid MEG stream 3.

[0170] The diol purification step f) involves a distillation column including a condenser, a reboiler and 30 theoretical plates, and is operated at 0.025 MPa at the top. Liquid MEG stream 1 and liquid MEG stream 2 are compressed to 0.1 MPa. Liquid MEG stream 3 is compressed to 0.1 MPa and preheated to 112° C. in a heat exchanger by heat exchange with the residue taken out at the bottom of the distillation column. The preheated liquid MEG stream 3 is then mixed with liquid MEG streams 1 and 2, and the liquid MEG mixture is then used to feed the distillation column of the diol purification step f) at the 23rd tray.

[0171] A water-rich liquid distillate was taken off at the top of the column, after the condenser, at a temperature of 65° C. The water-rich liquid distillate contained 93.5% by weight of water.

[0172] A DEG-rich liquid residue was taken off at the bottom of the column, at the level of the reboiler, at a temperature of 200° C. The DEG-rich liquid residue contained 95% by weight of DEG.

[0173] A purified MEG liquid stream was taken from the 4th tray at a temperature of 154° C. The purity of the purified MEG stream was 99.8 wt % MEG.

[0174] The energy consumption regarding hot heat transfer fluid for the glycol separation step c) and the glycol purification step f) as well as for the entire process is reported in Table 2 below.

[0175] Table 2

[0176] Energy consumption Step c) Step f) The whole method MW 1.72 2.84 6.03

[0177] By supplying a gaseous stream (gaseous MEG effluent) and a liquid stream (a mixture of liquid MEG effluents (8) and (8')) to the distillation tower of the diol purification step f), as described in the method of Example 1 according to the present invention (energy consumption regarding heat transfer fluid regarding part of the entire process is 4.02 MW), the overall energy consumption regarding heat transfer fluid regarding part of the process can be reduced relative to the method according to Example 2 which is not according to the present invention and in which the distillation tower of the diol purification step f) is supplied only with a liquid stream (a mixture of liquid MEG streams 1, 2 and 3) (energy consumption regarding heat transfer fluid regarding part of the entire process is 6.03 MW).

Claims

1. A method for depolymerizing a polyester feedstock comprising polyethylene terephthalate (PET), the method comprising: a) a conditioning step, said step comprising at least a conditioning section supplied with said polyester raw material to produce a conditioned stream; b) a step of depolymerization by glycolysis, said step being fed with at least a conditioned stream and being carried out at a temperature of 180 to 400° C., at a pressure of at least 0.1 MPa, at a residence time of 0.1 to 10 hours, in the presence of glycols in such a manner that the total molar amount of glycols is adjusted to 1 to 20 moles of glycol per mole of diester in the polyester raw material; c) a glycol separation step, which is fed with at least the effluent from step b) and is carried out at a temperature of 60 to 250° C., at a pressure lower than the pressure of step b), and produces at least a gaseous glycol effluent, at least a liquid glycol effluent and a liquid monomer effluent, wherein the glycol separation step is carried out in one gas-liquid separation stage or in a series of 2 to 5 consecutive gas-liquid separation stages, each separation stage producing a gaseous effluent and a liquid effluent, wherein the liquid effluent from the preceding stage is fed to the subsequent stage and the liquid monomer effluent is formed from the liquid effluent obtained from the final gas-liquid separation stage, The one or more gaseous effluents obtained from step c) at least partially form one or more gaseous glycol effluents, The one or more gaseous effluents obtained from step c) are at least partially condensed to form one or more liquid glycol effluents; d) a step of separating the liquid monomer effluent obtained from step c) into a heavy impurity effluent and a pre-purified monomer effluent, said step being carried out at a temperature below 250° C. and a pressure below 0.001 MPa with a liquid residence time of less than 10 minutes; e) subjecting the pre-purified monomer effluent obtained from step d) to a decolorization step using at least one adsorption stage comprising at least one adsorber, said step being operated in the presence of at least one adsorbent at a temperature ranging from 100 to 200° C., so as to produce a purified monomer effluent; f) a glycol purification step, which is fed with at least the gaseous glycol effluent and at least the liquid glycol effluent obtained from step c), and is carried out at a temperature of 40 to 250° C. and a pressure of 0.001 to 0.2 MPa, and produces at least a purified glycol effluent and one or more impurity effluents, Wherein the diol purification step f) comprises a purification section, wherein the purification section comprises: - a distillation column which is supplied with at least the gaseous glycol effluent and at least the liquid glycol effluent obtained from step c) on the same theoretical plate or on different theoretical plates and produces a distillate effluent forming a light impurity effluent, a residue effluent forming a heavy impurity effluent and a purified glycol effluent, the distillation column comprising 10 to 50 theoretical plates; or - at least a topping column comprising 3 to 25 theoretical plates and a tailings column comprising 10 to 50 theoretical plates, wherein a light impurity effluent is taken at the top of the topping column and a heavy impurity effluent is taken at the bottom of the tailings column.

2. The process according to claim 1, wherein the polyester starting material comprises at least 50% by weight of polyethylene terephthalate.

3. The process according to claim 1, wherein the polyester starting material comprises at least 90% by weight of polyethylene terephthalate.

4. The method according to any one of claims 1 to 3, wherein the polyester raw material comprises at least opaque PET.

5. The process according to any one of claims 1 to 3, wherein the polyester raw material comprises at least 10% by weight of opaque PET.

6. The process according to any one of claims 1 to 3, wherein the polyester raw material comprises at least 15% by weight of opaque PET.

7. The process according to any one of claims 1 to 3, wherein the polyester raw material comprises 0.1% to 10% by weight of pigment.

8. The process according to any one of claims 1 to 3, wherein step c) is operated at a temperature of 90 to 220°C.

9. The process according to any one of claims 1 to 3, wherein step c) is operated at a temperature of 100 to 210°C.

10. The process according to any one of claims 1 to 3, wherein the pressure in step c) is 0.00001 to 0.2 MPa.

11. The process according to any one of claims 1 to 3, wherein the pressure in step c) is from 0.00004 to 0.1 MPa.

12. The process according to any one of claims 1 to 3, wherein step c) comprises at least three consecutive gas-liquid separation stages, wherein the gaseous effluent obtained from the first gas-liquid separation stage forms the gaseous glycol effluent which is sent directly to step f), and the gaseous effluent obtained from the final gas-liquid separation stage is condensed to form a liquid glycol effluent which is then sent to step f), and one or more gaseous effluents from one or more gas-liquid separation stages between the first gas-liquid separation stage and the final gas-liquid separation stage are condensed or kept in a gaseous state before being sent to step f).

13. The process according to any one of claims 1 to 3, wherein the diol purification step f) is carried out at a temperature of 70 to 220°C.

14. The process according to any one of claims 1 to 3, wherein the diol purification step f) is carried out at a pressure of 0.004 to 0.1 MPa.

15. The process according to claim 1, wherein the purification section comprises a distillation column, wherein the distillation column comprises 15 to 35 theoretical plates.

16. The process according to claim 1, wherein the purification section comprises a topping column and a tailings column, wherein at least the topping column comprises 8 to 20 theoretical plates and the tailings column comprises 15 to 35 theoretical plates.

17. The process according to any one of claims 1 to 3, wherein the conditioning section of step a) is operated in an extruder with a residence time of less than or equal to 5 hours and greater than or equal to 2 minutes, at a temperature ranging from 150 to 300°C and a pressure ranging from 0.1 MPa to 20 MPa.

18. The process according to any one of claims 1 to 3, wherein the conditioning section of step a) comprises a conditioning zone carried out at a temperature of from 150 to 300°C, followed by a feed-diol contacting stage, wherein the polyester feed is contacted with a diol stream comprising at least a portion of the purified diol effluent obtained from step f), optionally as a mixture with external diol make-up and / or a portion of the liquid diol effluent obtained from step c), such that the number of moles of diol in the diol stream relative to the number of moles of diester in the polyester feed is from 0.05 to 10.

19. The process according to any one of claims 1 to 3, wherein said step e) employs a section for mixing the pre-purified monomer effluent obtained from step d) with a diol effluent, said diol effluent being a portion of the purified diol effluent obtained from step f), optionally as a mixture with an external diol make-up and / or a portion of the liquid diol effluent obtained from step c), such that the pre-purified monomer effluent represents 20% to 90% by weight of the total weight of the mixture, said mixing section being operated at a temperature of 100 to 250° C. and a pressure of 0.1 to 1.0 MPa.

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