Optimization method for depolymerizing a polyester containing polyethylene terephthalate

Abstract

The present invention relates to a process for depolymerizing a polyester feedstock containing PET, which comprises an improved feedstock conditioning step prior to the steps of depolymerization by glycolysis and purification of the depolymerization effluent, in which the polyester feedstock is temperature and pressure conditioned and then mixed with at least a recycled residue stream and a diol effluent, which, inter alia, significantly reduces the viscosity of the feedstock.

Description

Technical Field

[0001] The present invention relates to a method for depolymerizing a terephthalate polyester containing polyester, particularly polyethylene terephthalate (PET), for the purpose of recycling it into a polymerization unit. More specifically, the present invention relates to a method for depolymerizing a polyester feedstock containing PET, the method having an optimized step of conditioning the feedstock.

Background Art

[0002] Chemical recycling of polyesters, particularly polyethylene terephthalate (PET), has been the subject of numerous studies aimed at decomposing polyesters recovered in the form of waste into monomers that can be reused as feedstocks in polymerization processes.

[0003] Many polyesters are derived from circuits for collecting and sorting materials. In particular, polyesters, particularly PET, may originate from the collection of bottles, containers, films, resins, and / or fibers made of polyester (e.g., fabrics, cords, or fibers). Polyester derived from the collection and sorting channels is known as the polyester to be recycled.

[0004] Recyclable PET can be classified into four main categories: - Clear PET; mainly composed of colorless and transparent PET (generally at least 60% by weight) and transparent blue-colored PET, which contains no pigments and may be used in mechanical recycling methods; - Dark or colored (green, red, etc.) PET; this may generally contain up to 0.1% by weight of dyes or pigments, but retains transparency or translucency; - Opaque PET; this contains a significant amount of pigment, typically in a content range of 0.25% to 5.0% by weight to make the polymer opaque; opaque PET is increasingly used, for example, in the manufacture of food containers such as milk bottles, in the composition of cosmetic, plant protection or dye bottles. - Multilayer PET; this includes a layer of recycled PET between layers of polymers other than PET or layers of virgin PET (i.e., PET that has not undergone recycling), or includes a film of, for example, aluminum; multilayer PET is used to manufacture packaging such as container trays after thermoforming.

[0005] The collection channels feeding the recycling channels are constructed differently from country to country. They vary to maximize the amount of plastic upgraded from waste depending on the nature and amount of the feed stream and the separation technology. The channels for recycling these feed streams generally consist of a first step of conditioning in the form of flakes, during which bales of raw packaging are washed, purified, separated, shredded, and then purified and separated again to produce a flake stream, which generally contains "macroscopic" impurities (glass, metal, other plastics, wood, paper, cardboard, inorganic elements) of less than 1% by mass, preferably less than 0.2% by mass, and even more preferably less than 0.05% by mass.

[0006] Clear PET flakes then typically pass through an extrusion - filtration step to produce an extrudate, which is then reused as a mixture with virgin PET to produce new products (bottles, fibers, films). The process of solid state polymerization (known by the acronym SSP) under vacuum is necessary for food applications. This type of recycling is known as mechanical recycling.

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

[0008] The presence of opaque PET with a high pigment content in the PET to be recycled poses a problem for recyclers. This is because opaque PET has an adverse effect on the mechanical properties of the PET being recycled. Opaque PET is currently collected with colored PET and is found in the colored PET feed stream. From the perspective of developing applications for opaque PET, the content of opaque PET in the colored PET feed stream for recycling is currently 5 - 20 wt%, and it is further increasing. It will likely be possible to achieve a content of opaque PET in the colored PET feed stream of over 20 - 30 wt% within a few years. However, it has been shown that when the content of opaque PET in the colored PET feed stream exceeds 10 - 15%, the mechanical properties of the PET being recycled are adversely affected (see Impact du developpement 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 5 / 12 / 13), preventing recycling in the form of fibers, which is the main outlet for the colored PET channel.

[0009] Dyes are natural or synthetic substances, especially soluble in polyester materials and used to color the materials into which they are introduced. Commonly used dyes have various properties and often contain O- and N-type heteroatoms and conjugated unsaturations, such as quinone, methine or azo groups, or molecules, such as pyrazolone and quinophthalone. Pigments are finely divided substances, especially insoluble in polyester materials and used to color and / or opacify the materials into which they are introduced. The main pigments used to color and / or opacify polyester, especially PET, are metal oxides, such as TiO2, CoAl2O4 or Fe2O3, silicates, polysulfides and carbon black. Pigments are generally particles with a size of 0.1 - 10 μm, mainly 0.4 - 0.8 μm. Complete removal of these pigments by filtration is necessary to envisage recycling of opaque PET, but complete removal of these pigments is technically difficult because they have a very high blocking capacity.

[0010] Recycling of colored PET and opaque PET is, therefore, extremely problematic.

[0011] A patent application (Patent Document 1) describes a method for glycolic depolymerization of materials derived from the recovery of colored PET, especially green-colored PET bottles. The feedstock treated by this method is in the form of PET flakes and is placed in a reactor to be in contact with ethylene glycol at a temperature of 180 - 280 °C for several hours. The BHET obtained at the end of the glycolysis process is purified on activated carbon to separate and remove a predetermined dye, such as a blue dye, and then the extraction of residual dyes, such as yellow dyes, is carried out by extraction with alcohol or water. The BHET is crystallized in the extraction solvent and then separated and removed for the purpose of being able to be used in the PET polymerization process.

[0012] In the patent application (Patent Document 2), post-consumer PET contains various colored PETs, such as clear PET, blue PET, green PET, and / or a mixture of white PET, in the form of flakes, which are glycolyzed and depolymerized in a reactor in batch mode at 150 to 250 °C in the presence of ethylene glycol, an amine catalyst, and an alcohol. The resulting diester monomer is purified by filtration, ion exchange, and / or passage over activated carbon, and then crystallized and recovered by filtration.

[0013] In the patent (Patent Document 3), a method for depolymerizing a polyester, particularly a colored polyester such as green PET, includes a step of depolymerization in a reactor at a temperature of 180 to 240 °C in the presence of a diol, an optional step of evaporation in a thin-film evaporator (however, the conditions under which this evaporator should be operated are not specified), and a step of dissolving the mixture in a high-temperature solvent. A filtration step is performed to separate and remove insoluble impurities with a size greater than 50 μm after high-temperature dilution. The low proportion of pigments in the colored PET allows separation by filtration. However, this technique cannot operate on the amount of pigment present in opaque PET. These pigments quickly clog the filter.

[0014] The patent (Patent Document 4) describes the production of purified BHET from PET in the form of flakes. The depolymerization step consists of glycolysis of the pre-treated PET flakes, which are pre-treated by removing residual water at 180 °C in a stirred reactor in the presence of ethylene and a catalyst, and then washing with water in solid form at 195 - 200 °C. Following depolymerization, steps of preliminary purification by cooling, filtration, adsorption and treatment on ion exchange resins are carried out, which are presented as being very important and are carried out prior to the evaporation of glycol and the purification of BHET. Preliminary purification makes it possible to prevent the repolymerization of BHET in subsequent purification steps. However, it is extremely problematic to proceed through the steps of filtration and treatment on ion exchange resins when the feedstock contains a large amount of very small solid particles, such as pigments, and / or polymer compounds other than PET, such as polyolefins or polyamides, which is the case especially when the feedstock to be treated contains a significant proportion (more than 10% by weight of opaque PET and / or multilayer preformed PET) of opaque PET and / or multilayer preformed PET.

[0015] In parallel, the patent (Patent Document 5) discloses a method for depolymerizing a polyester comprising a step of glycolysis in the presence of ethylene glycol and a method for purifying a solution of bis(2-hydroxyethyl) terephthalate on cation exchange resins and anion exchange resins.

[0016] Finally, Patent Application (Patent Document 6) describes a method for depolymerizing opaque PET, particularly a polyester feedstock containing a pigment at 0.1% to 10% by weight, by glycolysis in the presence of ethylene glycol. A purified bis(2-hydroxyethyl) terephthalate (BHET) effluent is obtained after a specific separation and purification step by adsorption. In the said patent application, the possibility of reactive extrusion in the first step of conditioning the feedstock to initiate the depolymerization reaction is envisaged. It is also mentioned that the recycling of the heavy residues separated and removed during the purification step should be treated with the polyester feedstock.

[0017] The present invention aims to improve these methods for the depolymerization by glycolysis of PET, particularly of polyester feedstocks containing the PET of Patent Application (Patent Document 6), in particular by optimizing, upstream of its introduction into the depolymerization step, the phase for conditioning the polyester feedstock and the phase for mixing it with at least one recycled oligomer residue effluent in the presence of a diol.

Prior Art Documents

Patent Documents

[0018]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Summary of the Invention

Means for Solving the Problems

[0019] (Summary of the Invention) The subject of the present invention is, therefore, a method for depolymerizing a polyester feedstock containing PET, said method comprising at least the following steps: a) A conditioning step; performing at least one conditioning section that generates a conditioned feedstock stream and a mixing section that generates a mixed stream, feeding at least the polyester feedstock to the conditioning section and performing it at a temperature of 150 to 300 °C, feeding at least the stream of the conditioned feedstock obtained from the conditioning section, the recycled oligomer residue effluent, and at least one diol effluent to the mixing section, and the mixing section mixing the polyester feedstock at a temperature of 150 to 300 °C and a residence time of 0.5 seconds to 20 minutes such that the total weight ratio of the recycled oligomer residue effluent and the at least one diol effluent relative to the polyester feedstock is 0.03 to 3.0, and including at least one zone; b) A step of depolymerization by glycolysis; feeding at least the mixed stream and optionally a diol feed, adjusting the total amount of diol fed to step b) to 1 to 20 mol of diol per mole of diester fed to step b), and performing it at a temperature of 180 to 400 °C and a residence time of 0.1 to 10 hours; c) Step of separating and removing the diol; feeding at least the effluent from step b), carried out at a temperature of 100 to 250 °C and a pressure lower than the pressure in step b), to produce a diol effluent and an effluent rich in liquid monomers, performing the diol separation step in 1 to 5 consecutive gas-liquid separation sections, each of which produces a gas effluent and a liquid effluent, feeding the liquid effluent from the preceding section to the next section, the liquid effluent obtained from the last gas-liquid separation section constitutes an effluent rich in liquid monomers, and all the gas effluents are recovered to constitute a diol effluent; d) Step of separating the effluent rich in liquid monomers obtained from step c) into a heavy impurity effluent and a preliminarily purified monomer effluent; carried out at a temperature of 250 °C or lower and a pressure of 0.001 MPa or lower with a liquid residence time of 10 minutes or less, at least one part of the heavy impurity effluent constitutes an oligomer effluent to be recycled, and this is fed to the mixing section in step a); and e) Step of decolorizing the preliminarily purified monomer effluent; carried out at a temperature of 100 to 250 °C and a pressure of 0.1 to 1.0 MPa in the presence of an adsorbent to produce a purified monomer effluent is included.

[0020] One advantage of the present invention is to optimize the process of conditioning the polyester feedstock and to promote the homogenization of a mixture of the polyester feedstock and at least one recycled oligomer residue effluent, preferably containing at least a diester oligomer and at least one diol effluent, preferably containing at least ethylene glycol, in the reaction section, and to obtain an effective viscosity in the reaction section, particularly in a reactor directly connected to the conditioning unit, thereby obtaining a reasonable stirring force in this reactor, particularly 3000 W / m 3It becomes possible to use less than. By this method, therefore, it becomes possible to improve the homogenization of the mixture of the feedstock and at least one recycled oligomer residue effluent and at least one diol effluent in the reaction section, thereby making it possible to improve the efficiency of depolymerization, while at the same time reducing the stirring force required for this homogenization in the reaction section.

[0021] To ensure good mixing and homogenization of the reagents in the depolymerization reactor(s), it is necessary to provide optimal stirring, in particular, at the highest possible ratio of residence time to mixing time (t * =ts / tm), preferably, t * is greater than 10 (t * >10). The mixing time depends on a plurality of parameters, such as the type of stirring head, the viscosity of the mixture, and the stirring force. Due to the short residence time, often it is necessary to provide a high stirring force to meet the criterion t * >10. The present invention gives flexibility to the method and enables a significant reduction in the viscosity of the feedstock upstream of the depolymerization reactor(s) and achieves substantially complete homogenization up to (or more than) 95% of the mixing between the products, i.e., by achieving substantially complete homogenization of the compounds upstream of the reactor, ensuring that the criterion t * >10 is met. The stirring of the reaction medium is dedicated to maintaining the homogeneity in the reactor rather than dispersing one product into another. Therefore, by the present invention, it is also possible to use a reasonable stirring force (P) in the depolymerization reactor(s), preferably less than 3000 W / m 3 (P<3000 W / m 3 ), which is considered acceptable by those skilled in the art. In particular, the stirring force is 500 - 2000 W / m 3 .

[0022] The present invention also makes it possible to simplify the introduction of the feedstock into the depolymerization reactor. When the feedstock is very viscous (500 - 1000 Pa·s), as in the case of molten PET, its introduction into the reactor requires certain precautions, particularly the installation of a suitable system, such as a deflocculator or a dedicated dispersion stirring head. The present invention makes it possible to simplify the introduction system by improving the homogenization and reducing the viscosity of the product in the conditioning step. The present invention also makes it possible to simplify the transfer operability of the highly viscous feedstock and oligomer residues to the reaction section.

[0023] Another advantage of the present invention is to promote the transfer of residues containing diester oligomers, obtained from the diester effluent purification step and intended for recycling them, and residues separated during the purification of the diester effluent, to the reaction section, by premixing at least a part of said residues with the diol effluent, prior to the improved treatment of said mixture with the polyester feedstock in the conditioning step. In addition to the diester oligomers, said residues potentially concentrate solid particles present in the polyester feedstock, such as pigments, and polymer compounds, such as polyolefins or polyamides, which is the cause of increasing the viscosity and adhesion ability of said residues. By the method according to the present invention, it is thus possible to fluidize the residues separated and removed during the purification of the diester effluent and reduce the risk of clogging and blockage of the equipment during their transfer, particularly the transfer recycled to the reaction section. By premixing the residues separated and removed during the purification of the diester effluent with the diol effluent, it is also possible to enhance the mixing of said residues with the polyester feedstock for the purpose of recycling at least a part of said residues. As a result, by promoting the transfer of the residues and the mixing of the residues with the polyester feedstock, the method according to the present invention makes it possible to promote the recycling of at least a part of said residues containing diester oligomers and thus improve the overall yield of the method.

[0024] Finally, one advantage of the present invention is that it can process any type of polyester waste, including an increasing number of pigments, dyes and other polymers, such as blue, colored, opaque and multi-layer PET. The method according to the invention is capable of processing opaque PET, and according to the invention, it is possible to remove pigments, dyes and other polymers and return them to diester monomers by chemical reaction. This monomer can then be repolymerized into a polymer, which shows no difference from virgin polyester, in particular virgin PET, and thus allows all uses of virgin PET.

DETAILED DESCRIPTION OF THE INVENTION

[0025] (List of Drawings) (Figure 1) Figure 1 shows one embodiment of the method according to the present invention. In this embodiment, the method comprises the steps of: conditioning a feedstock (1) containing PET (a); depolymerization (b); a diol separation step (c) of recovering a diol effluent (3); a step (d) of separating and removing BHET diester to remove heavy impurities (5); and a step (e) of decolorizing by adsorption and recovering a purified BHET effluent (4). The conditioning step (a) includes an extruder (a1) for conditioning the feedstock (1), a static mixer (a3) for feeding heavy impurities, particularly those containing oligomers that have not been completely depolymerized, and a diol stream (2), and a static mixer (a2) for feeding the conditioned feedstock exiting the extruder (a1), the residue mixture (6) obtained from the mixer (a3), and the diol effluent (2). The diol stream (2) may advantageously be a portion of the diol effluent (3) recovered in step (c) and giving rise to the residue mixture (6). The static mixer (a2) is fed with the conditioned feedstock leaving the extruder (a1), the residue mixture (6) obtained from the mixer (a3), and the diol stream (2), which may advantageously be a portion of the diol effluent (3) recovered in step (c). The diol effluent (3) obtained in step (c) is advantageously recycled to steps (b) and (e) and optionally to step (a) as the diol stream (2).

[0026] (Figure 2) Figure 2 is a specific embodiment of the method according to the present invention and is carried out as illustrated in Example 1. In this embodiment, the method comprises the steps of: conditioning a feedstock (1) containing PET (a); depolymerization (b); a diol separation step (c) for recovering a diol effluent (3); a step (d) for separating and removing BHET diester to remove heavy impurities (5); and a step (e) for decolorizing by adsorption to recover a purified BHET effluent (4). The conditioning step (a) includes an extruder (a1) for conditioning the feedstock (1), a static mixer (a3) for feeding heavy impurities, in particular those containing oligomers that have not been fully depolymerized, and an ethylene glycol (or MEG) stream (2), and a static mixer (a2) for feeding the conditioned feedstock exiting the extruder (a1) and the residue mixture (6) obtained from the mixer (a3). The ethylene glycol (or MEG) stream (2) may advantageously be a portion of the diol effluent (3) recovered in step (c) and giving rise to the residue mixture (6). The diol effluent (3) obtained in step (c) is advantageously recycled to steps (b) and (e) and, optionally, to step (a) as an ethylene glycol (or MEG) stream (2).

[0027] (Description of the Embodiment) According to the present invention, polyethylene terephthalate or poly(ethylene terephthalate) (also simply known as PET) has a basic repeating unit of the following formula.

[0028] [Chemical formula]

[0029] Conventionally, PET is obtained by polycondensation of terephthalic acid (PTA) or dimethyl terephthalate (DMT) with ethylene glycol. In the following description of this specification, the expression "per mole of diester in the polyester feedstock" corresponds to the number of moles of the -[O-CO-O-(C6H4)-CO-O-CH2-CH2]- unit in the PET contained in the polyester feedstock, which is the diester unit obtained from the reaction of PTA and ethylene glycol in the PET contained in the polyester feedstock.

[0030] According to the present invention, the term "monomer" or "diester monomer" preferably denotes bis(2-hydroxyethyl) terephthalate (BHET) of the chemical formula HOC2H4-CO2-(C6H4)-CO2-C2H4OH, wherein -(C6H4)- represents an aromatic ring, which is the diester unit obtained from the reaction of PTA and ethylene glycol in the PET contained in the polyester feedstock.

[0031] The term "oligomer" typically denotes a small-sized polymer, generally composed of 2 to 20 basic repeating units. According to the present invention, the term "ester oligomer" or "BHET oligomer" denotes a terephthalate ester oligomer containing 2 to 20, preferably 2 to 5, basic repeating units of the formula -[O-CO-(C6H4)-CO-O-C2H4]-, where -(C6H4)- is an aromatic ring.

[0032] According to the present invention, the terms "diol" and "glycol" are used interchangeably and correspond to a compound containing two hydroxyl groups -OH. A suitable diol is ethylene glycol, which is also known as monoethylene glycol or MEG.

[0033] The diol or diol effluent stream used in the process of the method of the present invention, therefore, preferably contains ethylene glycol (or MEG) in a very predominant amount, i.e., MEG represents 95% by weight or more relative to the total weight of said diol or diol effluent stream.

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

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

[0036] According to the present invention, the expression "between A and B, or A to B" means that the two limit values of the interval (A, B) are included in the described range of values. If this were not the case and the two limit values were not included in the described range, such an explanation would be given by the present invention.

[0037] In the following description of the present specification, specific and / or preferred embodiments of the present invention may be described. They may be implemented separately or in combination with each other, and there are no restrictions on the combination when this is technically feasible.

[0038] (Feedstock) The method according to the invention is fed by a polyester feedstock, which includes at least one polyester, i.e., a polymer having repeating units in the main chain containing ester groups, and includes polyethylene terephthalate (PET), preferably including at least colored PET and / or opaque PET.

[0039] The polyester feedstock is preferably a feedstock of polyester for recycling obtained from a waste collection and sorting channel, in particular plastic waste. The polyester feedstock may come, for example, from the collection of bottles, container trays, films, resins and / or fibers made of polyethylene terephthalate.

[0040] Advantageously, the polyester feedstock contains at least 50% by weight, preferably at least 70% by weight, suitably at least 90% by weight of polyethylene terephthalate (PET).

[0041] Preferably, the polyester feedstock contains at least one PET selected from colored, opaque, dark-colored and multilayer PET, and mixtures thereof. More specifically, the polyester feedstock contains at least 10% by weight of opaque PET, more preferably at least 15% by weight of opaque PET, and the opaque PET is preferably opaque PET for recycling, i.e., opaque PET obtained from the collection and sorting channel.

[0042] The polyester feedstock preferably contains 0.1% to 10% by weight of pigments, preferably 0.1% to 5% by weight of pigments. In particular, it may contain 0.05% to 1% by weight of dyes, preferably 0.05% to 0.2% by weight of dyes.

[0043] In the collection and sorting channel, polyester waste is washed and ground before constituting the polyester feedstock of the method according to the invention.

[0044] The polyester feedstock may be in the form of flakes, either wholly or in part, the longest length of which is less than 10 cm, preferably 5 - 25 mm, or may be in the form of micronized solids, i.e., in the form of particles, which particles preferably have a size of 10 microns to 1 mm. The feedstock may contain macroscopic impurities, preferably less than 5 wt%, more preferably less than 3 wt% of macroscopic impurities, such as glass, metal, plastics other than polyester (e.g., PP, PEHD, etc.), wood, paper, cardboard or inorganic elements. The polyester feedstock may be in the form of fibers, either wholly or in part, for example, a fabric, which may in some cases be pretreated to remove cotton or polyamide fibers, or any fabric other than polyester, or a terya fiber, etc., which may in some cases be specifically pretreated to remove polyamide fibers or rubber or polybutadiene residues. The polyester feedstock may contain polyester obtained from manufacturing rejects of polyester polymerization and / or conversion processes. The polyester feedstock may contain elements used as polymerization catalysts and stabilizers in PET manufacturing methods, such as antimony, titanium or tin.

[0045] (Conditioning step a)) The method according to the invention includes a conditioning step a), which at least implements a conditioning section and a mixing section. The conditioning section is fed at least with the polyester feedstock to produce a stream of conditioned feedstock. The mixing section is fed at least with the stream of conditioned feedstock, the recycled oligomer residue effluent and at least one diol effluent to produce a mixed stream.

[0046] By the conditioning section of step a), it becomes possible to heat the polyester feedstock under the operating conditions of the depolymerization step b) and maintain it under pressure. In the conditioning section, the polyester feedstock is gradually heated to a temperature close to or slightly above its melting point so that it becomes at least partially liquid. Advantageously, at least 70% by weight of the polyester feedstock, very advantageously at least 80% by weight of the polyester feedstock, preferably at least 90% by weight, and preferentially at least 95% by weight is in liquid form when leaving the conditioning section of step a). The temperature at which the conditioning section of step a) is operated is advantageously 150 - 300 °C, preferably 225 - 275 °C. This temperature is kept as low as possible to minimize thermal degradation of the polyester. Preferably, the conditioning section is operated under an inert atmosphere to limit the introduction of oxygen into the system and the oxidation of the polyester feedstock.

[0047] 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 is operated in an extruder. The residence time in the extrusion section is defined as the volume of the section divided by the volumetric flow rate of the feedstock and is 5 hours or less, preferably 1 hour or less, preferentially 30 minutes or less, preferably 10 minutes or less, and preferably 2 minutes or more. Advantageously, the extrusion section makes it possible to condition the polyester feedstock so that the flow of the conditioned feedstock is at a temperature of 150 - 300 °C, preferably 225 - 275 °C, and at a pressure between atmospheric pressure (i.e., 0.1 MPa) and 20 MPa.

[0048] The extrusion section is advantageously connected to a vacuum extraction system to remove impurities present in the feedstock, such as dissolved gases, light organic compounds and / or moisture. The extrusion section may include a filtration system, and advantageously removes solid particles, such as sand particles, having 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 inactivated to limit the introduction of oxygen into the system.

[0049] The mixing section is fed at least the conditioned feedstock stream obtained from the conditioning section, the recycled oligomer residue effluent and at least one diol effluent. According to the invention, the recycled oligomer residue effluent comprises, preferably consists of, part or all of the heavy impurity effluent obtained at the end of separation step d). Preferably, the diol effluent(s) each comprise, preferably consist of, a portion of the diol effluent obtained from step c), a supply of diol external to the process according to the invention, or a mixture thereof, preferably a portion of the diol effluent obtained from step c).

[0050] The mixing section includes at least one zone for mixing the polyester feedstock, in which the polyester feedstock, pre-conditioned in the conditioning section, is preferably placed in contact with at least the recycled oligomer residue effluent in the presence of a diol. The effect of placing it in this contacting state is to initiate the depolymerization reaction of the polyester feedstock before introduction into the depolymerization step b). Thereby, it becomes possible to significantly reduce the viscosity of the feedstock, which particularly facilitates its conveyance to the depolymerization step b). The temperature when the polyester feedstock mixing zone is operated is preferably 150 to 300 °C, preferably 225 to 275 °C, and the residence time at that time is defined as the ratio of the volume of the liquid in the polyester feedstock mixing zone, preferably in the mixer, to the volumetric flow rate of the diester feedstock in the polyester feedstock mixing zone, and is 0.5 seconds to 1 hour, preferably 0.5 seconds to 30 minutes, preferentially 0.5 seconds to 20 minutes, preferably 1 second to 5 minutes, preferably 3 seconds to 1 minute, and the total weight ratio of the recycled oligomer residue effluent and the at least one diol effluent relative to the polyester feedstock is made to be 0.03 to 3.0, preferably 0.05 to 2.0, preferably 0.1 to 1.0.

[0051] The polyester feedstock mixing zone may be carried out in a static or dynamic mixer. In a highly advantageous embodiment, when the conditioning section is operated in an extruder, the polyester feedstock mixing zone may therefore be carried out in the extruder. In this case, it is a reactive extrusion phase is.

[0052] The polyester feedstock mixing zone is preferably fed at least with the stream of conditioned feedstock obtained from the conditioning section, the recycled oligomer residue effluent as a mixture with the diol effluent, if any, and optionally another diol effluent. In other words, the diol effluent may be introduced into the polyester feedstock mixing zone directly, indirectly, or either directly or indirectly. If it is introduced indirectly into the polyester feedstock mixing zone, the diol effluent obtained from step c), preferably consisting of a portion of the diol effluent, is injected into the polyester feedstock mixing zone. If it is introduced indirectly into the polyester feedstock mixing zone, this means that the diol effluent obtained from step c), preferably consisting of a portion of the diol effluent, is pre-mixed with the recycled oligomer residue effluent, at least in part, in a residue mixing zone prior to introduction into the polyester feedstock mixing zone. If it is introduced directly and indirectly into the polyester feedstock mixing zone, the diol effluent obtained from one of the steps c), preferably consisting of a portion of the diol effluent, is injected directly into the polyester feedstock mixing zone, and the diol effluent obtained from the other step c), preferably consisting of a second portion of the diol effluent, different from the diol effluent preferably injected directly, is pre-mixed with the recycled oligomer residue effluent, in particular in a residue mixing zone, prior to introduction into the polyester feedstock mixing zone.

[0053] According to a preferred embodiment of the invention, the polyester feedstock mixing zone is fed with the stream of conditioned feedstock obtained from the conditioning section, the recycled oligomer residue effluent and the diol effluent obtained from step c), preferably consisting of a portion of the diol effluent.

[0054] According to another preferred embodiment of the present invention, the polyester feedstock mixing zone is fed a residue mixture comprising the stream of conditioned feedstock obtained from the conditioning section, the recycled oligomer residue effluent and the diol effluent obtained from step c), preferably a portion of the diol effluent.

[0055] According to a third preferred embodiment of the present invention, the polyester feedstock mixing zone is fed a mixture of residues comprising the stream of conditioned feedstock obtained from the conditioning section, the diol effluent obtained from step c), preferably a portion of the diol effluent and the recycled oligomer residue effluent and another diol effluent obtained from step c), preferably a second portion of the diol effluent.

[0056] According to at least one embodiment of the present invention, particularly according to the above-described second and third preferred embodiments, the mixing section in step a) advantageously also includes a residue mixing zone, which is configured to contact all or part of the heavy impurity effluent obtained at the end of the separation step d) with at least one diol effluent obtained from step c), preferably a portion of the diol effluent, a supply of diol external to the method according to the present invention, or a mixture thereof, preferably a portion of the diol effluent obtained from step c). Placing it in this contacting manner promotes the recycling of BHET oligomers. The residue mixing zone makes it possible, firstly, to fluidize the residue, which is present in the polyester feedstock to be treated and contributes to increasing the viscosity and adhesion ability of the residue, potentially concentrating solid particles such as pigments and polymer compounds such as polyolefins or polyamides, and thus facilitating their handling during conveyance, and secondly, to reduce the viscosity of the residue, and thus enhance their mixing with the polyester feedstock. Preferably, the residue mixing zone is fed with a part or all of the heavy impurity effluent constituting the recycled oligomer residue effluent and the diol effluent obtained at the end of step d).

[0057] Advantageously, the temperature at which the residue mixing zone is operated is 150 to 300 °C, preferably 180 to 220 °C, and the residence time at that time is defined as the ratio of the volume of the liquid in the zone for mixing the recycled oligomer residue effluent introduced into the mixing zone to the volume flow rate of the recycled oligomer residue effluent introduced into the mixing zone, preferably the volume of the liquid in the mixer, and is 0.5 seconds to 20 minutes, preferably 1 second to 5 minutes, preferably 3 seconds to 1 minute, and the weight ratio of the diol to the weight of the heavy impurity effluent introduced into the residue mixing zone (i.e., the weight of the recycled oligomer residue effluent) is made to be 0.03 to 3.0, preferably 0.1 to 2.0, preferably 0.5 to 1.0.

[0058] Preferably, the residue mixing zone comprises, preferably consists of, a static or dynamic mixer, preferably a static mixer.

[0059] Advantageously, the residue mixing zone gives rise to a residue mixture which comprises at least part of the heavy impurity effluent obtained from step d), which constitutes the recycled oligomer residue effluent, and the diol obtained from the diol effluent introduced into said zone, which is fed to the polyester feedstock mixing zone of step a).

[0060] The heavy impurity effluent obtained at the end of the separation step d) is a BHET oligomer, in particular resulting from an incomplete depolymerization of the PET of the polyester feedstock, and potentially other heavy impurities, such as pigments and / or polymer compounds, such as polyolefins, polyamides, etc., originating from the polyester feedstock. All or part of said heavy impurity effluent may advantageously be sent, for example, to a separation step, which may be by filtration, upstream of the feeding treatment of all or part of said heavy impurity effluent to the mixing section of step a) or downstream of the residue mixing zone of said mixing section of step a), with a reduction in the content of solid impurities.

[0061] According to another embodiment, at least part of the heavy impurity effluent obtained at the end of step d) is fed to the mixing section of step a) and recycled to the depolymerization step b) without prior separation of the impurities. In this embodiment, an accumulation of impurities may occur in the process. To limit this accumulation, a purge treatment of a portion of the heavy impurity effluent obtained at the end of step d) is carried out.

[0062] Advantageously, the portion of the heavy impurity effluent obtained at the end of step d) is recycled directly to the reaction section of step b), either alone or after mixing with the diol stream in the residue mixing zone.

[0063] Preferably, the diol effluent(s) obtained from step c), in particular a portion of the diol effluent(s), is / are advantageously superheated to promote the establishment of the temperature of the polyester feedstock and / or the residue before being fed to step a).

[0064] According to one embodiment, the mixing section of step a) is fed only with the conditioned stream of the feedstock obtained from the conditioning section and the recycled oligomer residue effluent, which consists of at least a portion of the heavy impurity effluent obtained at the end of the separation step d).

[0065] (Depolymerization step b)) The process according to the invention comprises a step of depolymerization by glycolysis, and the total amount of diol fed to step b), which is at least the mixed stream obtained from the conditioning step a), optionally the supply of diol alone or as a mixture with the diol effluent, and optionally a portion of the heavy impurity effluent obtained at the end of step d), is contained in the mixed stream obtained from step a) which contains the polyester feedstock and at least a portion of the heavy impurity effluent obtained from step d), and optionally in a portion of the heavy impurity effluent obtained from step d) which is directly recycled to step b), and is carried out such that the molar amount of diol per mole of diester is adjusted to 1 to 20 mol, preferably 3 to 15 mol, preferably 5 to 10 mol, i.e., the weight ratio of the total amount of diol introduced into steps a) and b) to the total amount of diester contained in the mixed stream and optionally in a portion of the heavy impurity effluent obtained from step d) which is directly recycled to step b) is 0.3 to 6.7, preferably about 1.0 to 5.0, preferably 1.7 to 3.3, respectively.

[0066] Advantageously, the depolymerization step b) comprises one or more reaction sections, preferably at least two reaction sections, preferably 2 to 4 reaction sections, and preferably functions 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 a recirculation loop and / or fluidization. The reactor may include a conical bottom for purging impurities. Preferably, the depolymerization step b) comprises at least two reaction sections, preferably 2 to 4 reaction sections, which function in series, and the reaction section(s) starting from the second reaction section onwards operate at the same or different temperatures and at a temperature below that of the first reaction section, preferably below the first reaction section, preferably 10 to 50 °C lower, more preferably 20 to 40 °C lower, relative to the temperature of the first operating section.

[0067] The said reaction section(s) operate at a temperature of 180 to 400 °C, preferably 200 to 300 °C, preferably 210 °C to 280 °C, especially in the liquid phase, and the residence time in the reaction section is 0.1 to 10 hours, preferably 0.25 to 8 hours, 0.5 to 6 hours. The residence time is defined as the ratio of the volume of liquid in the said reaction section to the volumetric flow rate of the stream leaving the said reaction section.

[0068] The operating pressure of the said reaction section(s) of step b) is determined so as to maintain the reaction system in the liquid phase. This pressure is advantageously at least 0.1 MPa, preferably at least 0.4 MPa, and preferably less than 5 MPa. The term "reaction system" means all of the components and phases present in step b) obtained from the feed of the said step.

[0069] The diol is advantageously monoethylene glycol.

[0070] The glycolysis reaction may be carried out in the presence or absence of a catalyst.

[0071] When the glycolysis reaction is carried out in the presence of a catalyst, the catalyst may be homogeneous or heterogeneous and is selected from esterification catalysts known to those skilled in the art, for example, complexes, oxides and salts of antimony, tin or titanium, alkoxides of metals from Groups (I) and (IV) of the Periodic Table of the Elements, organic peroxides or acidic / basic metal oxides.

[0072] A preferred heterogeneous catalyst advantageously contains at least 50% by weight, preferably at least 70% by weight, advantageously at least 80% by weight, very advantageously at least 90% by weight, even more advantageously at least 95% by weight of a solid solution relative to the total mass of the catalyst. This solid solution has the formula Z x Al2O (3+x) and consists of at least one spinel, where x is between 0 (excluding the limit value) and 1, Z is selected from Co, Fe, Mg, Mn, Ti and Zn, and contains up to 50% by weight of alumina and oxides of element Z. The preferred heterogeneous catalyst advantageously contains up to 10% by weight of a dopant. This dopant is selected from silicon, phosphorus and boron and these are used alone or as a mixture. For example, without limitation, the solid solution may consist of a mixture of spinel ZnAl2O4 and spinel CoAl2O4, or alternatively, may consist of a mixture of spinel ZnAl2O4, spinel MgAl2O4 and spinel FeAl2O4, or alternatively, may consist of spinel ZnAl2O4 only.

[0073] Preferably, the depolymerization step is carried out without adding an external catalyst to the polyester feedstock.

[0074] The depolymerization step may advantageously be carried out in the presence of a solid adsorbent in powder form or in a shaped form, the function of which is to absorb at least part of the colored impurities and thus to facilitate the decolorization step e). The solid adsorbent is advantageously activated carbon.

[0075] By means of the glycolysis reaction, it is possible to convert the polyester feedstock into ester monomers and oligomers, advantageously at least the monomer of PET, bis(2-hydroxyethyl) terephthalate (BHET) and BHET oligomers. The conversion rate of the polyester feedstock in the depolymerization step is more than 50%, preferably more than 70%, suitably more than 85%. The BHET molar yield is more than 50%, preferably more than 70%, suitably more than 85%. The BHET molar yield corresponds to the molar flow rate of BHET at the outlet of step b) to the number of moles of diester in the polyester feedstock fed to step b).

[0076] An internal recirculation loop is advantageously used in step b). That is, the internal recirculation loop is the extraction of a part of the reaction system, the filtration of this part and the reinjection of this part into step b). This internal loop makes it possible to remove macroscopic solid impurities that may be present in the reaction solution.

[0077] Advantageously, the depolymerization step b) makes it possible to obtain a reaction effluent which is sent to a diol separation step c).

[0078] (Step c) of separation of diol) The method according to the invention includes a diol separation step c), is fed at least with the effluent from step b), is carried out at a temperature of 100 to 250 °C and at a pressure lower than the pressure of step b), and produces a diol effluent and an effluent rich in liquid monomers.

[0079] The main role of step c) is to recover all or part of the unreacted diol.

[0080] Step c) is carried out at a pressure lower than the pressure of step b) so as to evaporate a part of the effluent from step b) to give a gas effluent and a liquid effluent. The liquid effluent constitutes an effluent rich in liquid monomers. The gas effluent consisting of more than 50% by weight, preferably more than 70% by weight, preferably more than 90% by weight of diol constitutes the diol effluent.

[0081] Step c) is advantageously carried out in one or a series of a plurality of gas-liquid separation sections, advantageously 1 to 5 consecutive gas-liquid separation sections, and very advantageously a series of 3 to 5 gas-liquid separation sections. Each of the gas-liquid separation sections gives rise to a liquid effluent and a gas effluent. The liquid effluent from the preceding section is fed to the next section. All of the gas effluents are recovered and constitute the diol effluent. The liquid effluent obtained from the last gas-liquid separation section constitutes an effluent rich in liquid monomer.

[0082] Advantageously, at least one of the gas-liquid separation sections may be carried out in a falling-film evaporator or a thin-film evaporator or a short-path distillation apparatus.

[0083] Step c) is carried out such that the temperature of the liquid effluent is maintained above a value at which the polyester monomer precipitates if it is below this value and below a high value at which the monomer significantly repolymerizes depending on the molar ratio of diol / monomer. The temperature in step c) is 100 to 250 °C, preferably 110 to 220 °C, preferably 120 to 210 °C. A series of a plurality, advantageously a series of 2 to 5, preferably a series of 3 to 5 gas-liquid separations as operations is particularly advantageous because it enables the temperature of the liquid effluent corresponding to the above-mentioned constraints to be adjusted in each separation.

[0084] The pressure in step c) is lower than the pressure in step b) and is advantageously adjusted to a temperature that enables the evaporation of the diol while minimizing repolymerization and enabling optimal energy integration. It is preferably 0.00001 to 0.2 MPa, preferably 0.00004 to 0.15 MPa, preferably 0.00004 to 0.1 MPa.

[0085] The separation section(s) is advantageously agitated via any method known to those skilled in the art.

[0086] The diol effluent may contain other compounds such as dyes, light alcohols, water or diethylene glycol. At least one part of the diol effluent is preferably recycled, in liquid form (i.e., after condensation), to step a) and / or step b), and optionally to step e), optionally as a mixture with an external supply of diol to the process according to the invention.

[0087] All or part of the diol effluent may be treated in a purification step in liquid form (i.e., after condensation) before being recycled to step a) and / or b) and / or before being used as a mixture in step e). This purification step may include, in a non-exhaustive manner, absorption onto a solid (such as activated carbon) to remove dyes and one or more distillations to separate and remove impurities such as diethylene glycol, water and other alcohols.

[0088] (Monomer separation step d)) The process according to the invention includes a step d) of separating and removing the monomer-rich effluent obtained from step c), yielding a heavy impurity effluent and a pre-purified monomer effluent.

[0089] Said step d) is preferably carried out at a temperature of 250 °C or less, preferably 230 °C or less, very preferably 200 °C or less, and preferably 110 °C or more, and a pressure of 0.001 MPa or less, preferably 0.0005 MPa or less, preferably 0.000001 MPa or less, and the liquid residence time is 10 minutes or less, preferably 5 minutes or less, preferably 1 minute or less, and preferably 0.1 second or more.

[0090] The purpose of this separation step d) is to separate the evaporated monomers, in particular BHET, from the oligomers that have not been converted at all and remain liquid, and thus absorb heavy impurities, in particular pigments, from the unconverted polyester polymer, from other polymers that may be present, and from the polymerization catalyst, while minimizing monomer loss due to repolymerization at the same time. Minor oligomers may, in some cases, be entrained by monomers, in particular those of small size. These heavy impurities are found in the heavy impurity effluent together with the oligomers.

[0091] Due to the possible presence of the polymerization catalyst in the polyester feedstock, the separation has to be carried out at a temperature not exceeding 250 °C with a very short liquid residence time in order to limit any risk of monomer repolymerization during this step. Separation by simple atmospheric distillation is, therefore, not conceivable.

[0092] The separation step d) is preferably carried out in a falling film or thin film evaporation system or by short path falling film or thin film distillation. A very low operating pressure is required in order to enable the vaporization of the monomers while allowing step d) to be carried out at a temperature below 250 °C, preferably below 230 °C.

[0093] The polymerization inhibitor may preferably be mixed with the liquid monomer-rich effluent before being fed to said step d).

[0094] A solvent (flux) may preferably be mixed with the liquid monomer-rich effluent before being fed to said step d) so as to facilitate the removal of heavy impurities, in particular pigments, at the bottom of the short path distillation or evaporation system. This solvent may have a boiling point far higher than that of the monomers, in particular BHET, under the operating conditions of step d). It may be, for example, polyethylene glycol or a PET oligomer.

[0095] The heavy impurity effluent contains, in particular, pigments, oligomers and optionally BHET that has not been separated out. The heavy impurity effluent is advantageously recycled, in whole or in part, to conditioning step a), in particular to the mixing section. A part of the heavy impurity effluent may advantageously be recycled directly to step b), either alone or as a mixture with the diol effluent. The heavy impurity effluent may advantageously pass through at least one separation step, for example by filtration, before being recycled, reducing the amount of pigment and / or other solid impurities. A part of the heavy impurity effluent that has been separated out and has a high solid content may advantageously be purged from the process and sent to an incineration system.

[0096] Preferably, all or part of the heavy impurity effluent is recycled to step a) and optionally to step b) without prior separation of solid impurities.

[0097] The pre-purified monomer effluent is preferably sent to a gas-liquid separation section and carried out at a temperature of 100 to 250 ° C, preferably 110 to 200 ° C, preferably 120 to 180 ° C and a pressure of 0.00001 to 0.1 MPa, preferably 0.00001 to 0.01 MPa, preferably 0.00001 to 0.001 MPa in any equipment known to those skilled in the art. The separation section enables the separation of the gaseous diol effluent and the pre-purified liquid monomer effluent. By the gas-liquid separation, more than 50% by weight, preferably more than 70% by weight, suitably more than 90% by weight of the diol entrained with the pre-purified monomer effluent in step d) is recovered, so that the amount of diol remaining in the pre-purified monomer effluent can be further reduced. The amount of monomer entrained in the gaseous diol effluent is preferably less than 1% by weight, preferably less than 0.1% by weight, more preferably less than 0.01% by weight of the amount of monomer present in the pre-purified monomer effluent. Then, the gaseous diol effluent is preferably condensed and optionally pretreated in a purification step and recycled to step a) and / or step b) and / or step e) as a mixture with the diol effluent obtained from step c).

[0098] (Decolorization step e)) The process according to the invention comprises a step of decolorizing the pre-purified monomer effluent and is carried out at a temperature of 100 to 200 ° C, preferably 100 to 170 ° C, preferably 120 to 150 ° C and a pressure of 0.1 to 1.0 MPa, preferably 0.1 to 0.8 MPa, preferably 0.2 to 0.5 MPa in the presence of at least one adsorbent to produce a purified monomer effluent.

[0099] The adsorbent may be any adsorbent known to those skilled in the art that can absorb dyes, for example, activated carbon or clay, preferably activated carbon.

[0100] The pre-purified monomer effluent is preferably mixed with a portion of the diol effluent obtained from step c) or with a supply of diol external to the process according to the invention. This diol effluent is optionally pretreated in a purification step.

[0101] The purified monomer effluent is preferably fed downstream to a polymerization process known to those skilled in the art, preferably after the selected polymerization step, where ethylene glycol, terephthalic acid or dimethyl terephthalate is fed. This is intended to produce PET that cannot be distinguished from virgin PET. Feeding the purified monomer effluent in the polymerization process makes it possible to reduce the feed of dimethyl terephthalate or terephthalic acid at an equivalent flow rate.

[0102] The following drawings and examples illustrate the invention but do not limit the scope of the invention.

[0103] (Example) (Example 1: Conforming to the invention) In this example, only the conditioning step a) and the depolymerization step b) are described for a process for the depolymerization of a continuous 100% PET feedstock, and the recycling capacity is 20 KTY (kilotonnes per year) of PET (i.e., 2500 kg / h). The process of this example is schematically presented in Figure 2.

[0104] As illustrated in Figure 2, the conditioning step (a) includes the following: - An extruder (a1) for conditioning the PET feedstock (1) by melting it, - A static mixer (a3) for pre-mixing the residue containing the oligomers obtained from the separation step (d) with a stream (2) of ethylene glycol (or MEG) to obtain a residue mixture (6), and - A static mixer (a2) for pre-mixing the conditioned feedstock obtained from the extruder with the residue mixture obtained from the mixer (a3).

[0105] The reaction section consists of two continuously stirred tank reactors in cascade. The working volume of the reactors is R1: 3.75 m 3 , R2: 22.4 m 3 . The reactors are mechanically stirred. Reactor R1 is equipped with a spiral ribbon type stirring head. This stirring head is well known to those skilled in the art and is particularly suitable for mixing at high viscosities.

[0106] The operating conditions in the extruder, the two mixers (a2) and (a3), and the first reactor R1 are summarized in Table 1 below.

[0107]

Table 1

[0108] By using such a pre-mixing section, it is thus possible to obtain a residue stream containing oligomers at a low viscosity (3 Pa·s), and to facilitate its conveyance to mixer (a2) for the purpose of recycling the oligomers that could not be completely converted. Thereby, it is also possible to significantly reduce the viscosity of the feedstock before entering the reaction unit, particularly before entering the first reactor, from 530 Pa·s in the case of only the molten PET feedstock to about 10 Pa·s for the mixture (feedstock + residue + MEG).

[0109] To check such an effect of viscosity on the quality of mixing in the first reactor, the stirring force required to satisfy the criterion t * > 10 is calculated for reactor R1.

[0110] Due to the viscosity at the inlet of reactor R1 on the order of 10 Pa·s, for the absorbed stirring force of less than 1500 W / m in reactor R1, it is possible to ensure that the stirring criterion t 3 > 10, whereas with only the molten PET feedstock, 1500 W / m * > 10, whereas with only the molten PET feedstock, 1500 W / m 3The stirring force below does not guarantee to satisfy the stirring standard t * > 10.

[0111] Therefore, premixing the feedstock with a mixture containing recycled oligomers and MEG upstream of the reaction section gives flexibility to the method for depolymerizing the PET feedstock and ensures good quality mixing in the depolymerization reactor while at the same time being overall reasonable in response to the stirring force and promoting the conveyance of the recycled oligomers.

Brief Description of the Drawings

[0112]

Figure 1

Figure 2

Claims

1. A method for depolymerizing a polyester feedstock containing PET, comprising at least the following steps: a) A conditioning step of implementing at least one conditioning section and a mixing section; the conditioning section generates a conditioned feedstock stream, and the mixing section generates a mixed stream, feeding the polyester feedstock to at least the conditioning section and implementing it at a temperature of 225 - 275 °C, feeding at least the stream of the conditioned feedstock obtained from the conditioning section, the recycled oligomer residue effluent, and at least one diol effluent to the mixing section, and the mixing section includes at least one zone for mixing the polyester feedstock, to the mixing section of the polyester feedstock, - the stream of the conditioned feedstock obtained from the conditioning section, the diol effluent, and the recycled oligomer residue effluent, - the stream of the conditioned feedstock obtained from the conditioning section, and the residue mixture containing the recycled oligomer residue effluent and the diol effluent, or - the stream of the conditioned feedstock obtained from the conditioning section, the diol effluent, and the residue mixture of the recycled oligomer residue effluent and another diol effluent are fed, the mixing section is at a temperature of 225 - 275 °C, the residence time is 0.5 seconds to 20 minutes, and the total weight ratio of the recycled oligomer residue effluent and the at least one diol effluent relative to the polyester feedstock is made to be 0.03 - 3.0; b) A step of depolymerization by glycolysis; feeding at least the mixed stream and optionally feeding a diol supply, adjusting the total amount of diol fed to step b) to 1 - 20 mol of diol per mole of diester fed to step b), and performing it at a temperature of 180 - 400 °C and a residence time of 0.1 - 10 hours; c) a step of separating and removing the diol; feeding at least the effluent from step b), carried out at a temperature of 100 to 250 ° C and a pressure lower than the pressure of step b), to produce a diol effluent and a liquid monomer-rich effluent, carried out in 1 to 5 consecutive gas-liquid separation sections, each of which produces a gas effluent and a liquid effluent, feeding the liquid effluent from the previous section to the next section, and the liquid effluent obtained from the last gas-liquid separation section constitutes a liquid monomer-rich effluent, and all the gas effluents are recovered to constitute a diol effluent; d) a step of separating the liquid monomer-rich effluent obtained from step c) into a heavy impurity effluent and a preliminarily purified monomer effluent; carried out at a temperature of 250 ° C or lower and a pressure of 0.001 MPa or lower with a liquid residence time of 10 minutes or less, at least one part of the heavy impurity effluent is an oligomer residue effluent to be recycled, and this oligomer residue effluent is fed to the mixing section of step a); and e) a step of decolorizing the preliminarily purified monomer effluent; carried out in the presence of an adsorbent at a temperature of 100 to 250 ° C and a pressure of 0.1 to 1.0 MPa to produce a purified monomer effluent A method comprising.

2. The method according to claim 1, wherein the polyester feedstock comprises at least colored PET, opaque PET or a mixture thereof.

3. The method according to claim 1 or 2, wherein the polyester feedstock comprises at least 10% by weight of opaque PET.

4. The method according to any one of claims 1 to 3, wherein the polyester feedstock comprises 0.1% to 10% by weight of a pigment.

5. The method according to any one of claims 1 to 4, wherein the conditioning section of step a) is carried out in an extruder.

6. The method according to any one of claims 1 to 5, wherein the polyester feedstock mixing zone of step a) is carried out in a static or dynamic mixer.

7. The method according to claim 5, wherein the polyester feedstock mixing zone of step a) is carried out in the extruder.

8. The method according to any one of claims 1 to 7, wherein the total weight ratio of the recycled oligomer residue effluent and the diol effluent to the polyester feedstock in the polyester feedstock mixing zone is 0.05 to 2.

0. **Claim 9**: The method according to any one of claims 1 to 8, wherein in the polyester feedstock mixing zone, the total weight ratio of the recycled oligomer residue effluent and the diol effluent relative to the polyester feedstock is 0.1 to 1.

0. **Claim 10**: The method according to any one of claims 1 to 9, wherein the diol effluent fed to the mixing section in step a) is a portion of the diol effluent obtained from step c), and the other diol effluent is a second portion of the diol effluent obtained from step c). **Claim 11** The mixing section in step a) includes a residue mixing zone, and feeds a part or all of the heavy impurity effluent obtained at the end of step d) and the diol effluent, and is carried out at a temperature of 150 to 300 °C and a residence time of 0.5 seconds to 20 minutes, so that the weight ratio of the diol relative to the amount of the heavy impurity effluent introduced into the residue mixing zone is 0.03 to 3.0, to produce a residue mixture. The method according to any one of claims 1 to 10. **Claim 12**: The method according to claim 11, wherein the residence time when carrying out the residue mixing zone is 1 second to 5 minutes. **Claim 13**: The method according to claim 11 or 12, wherein the weight ratio of the diol relative to the amount of the heavy impurity effluent introduced into the residue mixing zone is 0.1 to 2.0, to produce a residue mixture. **Claim 14**: The method according to any one of claims 11 to 13, wherein the weight ratio of the diol relative to the amount of the heavy impurity effluent introduced into the residue mixing zone is 0.5 to 1.0, to produce a residue mixture. **Claim 15** A part of the heavy impurity effluent obtained at the end of step d) is recycled directly after being mixed with the diol stream alone or in the residue mixing zone in the reaction section of step b). The method according to any one of claims 1 to 14.

Images