Improved method for depolymerizing polyester containing polyethylene terephthalate

Abstract

The present invention relates to a process for depolymerizing polyester feedstocks, including PET, which comprises an improved step of conditioning the feedstock prior to the step of depolymerization by glycolysis and prior to the step of purification of the depolymerization effluent, in which the polyester feedstock is conditioned at temperature and pressure and then mixed with a diol effluent in a static or dynamic mixer, in particular to significantly reduce the viscosity of the feedstock.

Description

[Technical Field]

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

[0002] The chemical recycling of polyester, particularly polyethylene terephthalate (PET), has been the subject of much research aimed at breaking down polyester recovered in waste form into monomers that can be reused as feedstock in polymerization methods.

[0003] Much polyester originates from collection and sorting channels for materials. In particular, polyester, especially PET, may originate from the collection of bottles, containers, films, resins, and / or fibers composed of polyester (e.g., textiles, linoleum fibers). Polyester originating from collection and sorting channels is known as recyclable polyester.

[0004] PET for recycling can be classified into the following four main categories: • Clear PET; consisting mainly of colorless, 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-colored or colored (green, red, etc.) PET; these may generally contain up to 0.1% by weight of dye or pigment, while retaining transparency or translucency; • Opaque PET; this contains a significant amount of pigment, typically ranging from 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, and 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 unused PET (i.e., PET that has not undergone recycling), or, for example, an aluminum film; multilayer PET is used to manufacture packaging, such as container trays, after thermoforming.

[0005] Collection channels supplying recycling channels are constructed differently from country to country. They are modified to maximize the amount of plastic upgraded from waste, depending on the nature and volume of the supply flow and sorting technology. Channels for recycling these supply flows generally consist of a first step of conditioning into flake form, during which the unprocessed packaging packs are washed, purified and sorted, ground, and then purified and sorted again to produce a flake flow, which generally contains less than 1% by mass of "macroscopic" impurities (glass, metal, other plastics, wood, paper, cardboard, inorganic elements), preferably less than 0.2% by mass of "macroscopic" impurities, and even more preferably less than 0.05% by mass of "macroscopic" impurities.

[0006] Clear PET flakes may then undergo an extrusion-filtration process to produce an extruded product, which can then be reused as a mixture with unused PET to produce new products (bottles, fibers, films). A solid-state polymerization process under vacuum (known by the acronym SSP) is required for food applications. This type of recycling is known as mechanical recycling.

[0007] Dark-colored (or colored) PET flakes can also be mechanically recycled. However, the coloration of extruded materials formed from colored feed streams limits their applications: dark-colored PET is commonly used to manufacture packaging straps or fibers. Its uses are therefore more limited compared to those of clear PET.

[0008] The presence of opaque PET containing high pigment content in PET intended for recycling presents a problem for recyclers because opaque PET negatively affects the mechanical properties of the recycled PET. Currently, opaque PET is collected together with colored PET and is found in the colored PET supply stream. From the perspective of developing applications for opaque PET, the content of opaque PET in the colored PET supply stream for recycling is currently 5-20% by weight and is showing a tendency to increase. Within a few years, it will be possible to achieve a content of over 20-30% by weight of opaque PET in the colored PET supply stream. However, studies have shown that when opaque PET exceeds 10-15% in the colored PET supply stream, the mechanical properties of recycled PET are adversely affected (see: Impact du developmentpement 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 colored PET channels.

[0009] Dyes are natural or synthetic substances that are particularly soluble in polyester materials and are used to color the materials into which they are introduced. Commonly used dyes are of various properties and often contain heteroatoms of the O and N type, and conjugated unsaturated groups or molecules, such as quinone, methine, or azo groups, or molecules, such as pyrazolone and quinophthalone. Pigments are finely divided substances that are particularly insoluble in polyester materials and are used to color and / or opaque the materials into which they are introduced. The main pigments used to color and / or opaque 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 for the purpose of recycling opaque PET, but complete removal of these pigments is technically difficult because they have extremely high occlusion capacity.

[0010] The recycling of colored and opaque PET is therefore extremely problematic.

[0011] A patent application (Patent Document 1) describes a method for glycolysis and depolymerization of colored PET, particularly from recovered green colored PET bottles. The feed material to be processed by this method is in the form of PET flakes and is placed in a reactor at a temperature of 180-280°C for several hours in contact with ethylene glycol. The BHET obtained at the end of the glycolysis step is purified on activated carbon to separate and remove a predetermined dye, such as a blue dye, and then the residual dye, such as a yellow dye, is extracted with alcohol or water. The BHET is crystallized in the extraction solvent and then separated and removed for use in a PET polymerization method.

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

[0013] In a patent (Patent Document 3), a method for depolymerizing polyester, particularly colored polyester, such as green PET, includes a step of depolymerization in a reactor at a temperature of 180-240°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. After high-temperature dilution, a filtration step is performed to separate and remove insoluble impurities larger than 50 μm. The low proportion of pigment in colored PET makes separation by filtration possible. However, this technique cannot be operated with the amount of pigment present in opaque PET, because these pigments quickly clog the filter.

[0014] The patent (Patent Document 4) describes the production of purified BHET from PET in flake form. The depolymerization step consists of glycolysis of pretreated PET flakes by washing with water in solid form at 180°C to remove residual water, and then at 195-200°C in a stirred reactor in the presence of ethylene glycol and a catalyst. Following depolymerization, steps of pre-purification by cooling, filtration, adsorption, and treatment on an ion exchange resin are performed, which are presented as being of great importance and are carried out before the evaporation of glycol and purification of BHET. Pre-purification makes it possible to prevent the repolymerization of BHET in the subsequent purification step. However, the preceding steps of filtration and treatment on an ion exchange resin may be extremely problematic if the feedstock contains a large amount of very small solid particles, e.g., pigments, and / or polymer compounds other than PET, e.g., polyolefins or polyamides, which is particularly the case when the feedstock to be processed contains a substantial amount (more than 10% by weight of opaque PET and / or multilayer preformed PET).

[0015] In parallel, a patent (Patent Document 5) discloses a method for depolymerizing 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 a cation exchange resin and an anion exchange resin.

[0016] Finally, a patent application (Patent Document 6) describes a method for depolymerizing opaque PET, particularly a polyester feedstock containing 0.1% to 10% by weight of pigment, by glycolysis in the presence of ethylene glycol. A purified bis(2-hydroxyethyl) terephthalate (BHET) effluent is obtained after specific separation and purification steps. The aforementioned patent application envisions the possibility of reactive extrusion in a first step of conditioning the feedstock to initiate the depolymerization reaction.

[0017] The present invention aims to improve these methods for the depolymerization by glycolysis of polyester feedstocks containing PET, particularly the PET of patent application (Patent Document 6), and in particular to improve the phase in which it is conditioned and the phase in which it is mixed with at least one diol effluent, upstream of the introduction of the polyester feedstock into the depolymerization step.

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, the method comprising: a) a conditioning step implementing at least one conditioning section generating a flow of conditioned feedstock and a mixing section generating a mixed flow, feeding the polyester feedstock at least to the conditioning section and carrying out the process at a temperature of 150 to 300 °C, The process involves supplying at least the flow of conditioned feed material obtained from the conditioning section and the diol effluent to the mixing section, and operating the mixture in a static or dynamic mixer at a temperature of 150 to 300°C with a residence time of 0.5 seconds to 20 minutes so that the weight ratio of diol relative to polyester feed material is 0.03 to 3.0; b) A step of depolymerization by glycolysis, wherein the mixed flow is optionally supplied with a diol feed, and the total amount of diol supplied to step b) is adjusted to 1 to 20 moles of diol per mole of diester supplied to step b), and the step is carried out at a temperature of 180 to 400°C and with a residence time of 0.1 to 10 hours; c) A step of separating and removing diols, wherein the effluent from step b) is fed at least, the process is carried out at a temperature of 100-250°C and a pressure lower than that of step b), producing a diol effluent and a liquid monomer-rich effluent, the process is carried out in 1-5 consecutive gas-liquid separation sections, the liquid effluent from the previous section is fed to the next section, and a diol effluent and a liquid monomer-rich effluent is produced; d) A step of separating the abundant effluent from the liquid monomer obtained in step c) into heavy impurity effluent and pre-purified monomer effluent, the step being carried out at a temperature of 250°C or less and a pressure of 0.001 MPa or less, with a liquid residence time of 10 minutes or less; e) A step of decolorizing the pre-purified monomer effluent, which is 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, thereby producing purified monomer effluent.

[0020] One advantage of the present invention is that it facilitates the homogenization of the mixture of polyester feed material and at least one diol effluent in the reaction section, and improves the process of conditioning the polyester feed material in the reaction section, particularly in a reactor directly connected to a conditioning unit, to obtain an effective viscosity, thereby enabling a reasonable stirring force in this reactor, especially 3000 W / m 3It becomes possible to use a stirring force less than this. By this method, therefore, it becomes possible to improve the homogenization of the mixture of the feedstock and at least one diol effluent in the reaction section, thereby making it possible to improve the depolymerization efficiency while at the same time reducing the stirring force required for this homogenization in the reaction section.

[0021] In order to ensure good mixing and homogenization of the reagents in the depolymerization reactor(s), it is necessary to provide optimal stirring, especially, with a residence time to mixing time ratio (t * =ts / tm) that is as high as possible, 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 reference 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 a substantially complete homogenization of up to 95% (or more) of the mixing between the products, that is, by achieving a substantially complete homogenization of the compounds upstream of the reactor, ensuring that the reference t * >10 is satisfied. The stirring of the reaction medium is dedicated to maintaining the homogeneity in the reactor rather than dispersing one product into another. By the present invention, therefore, 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, and in particular, the stirring force is 500 - 2000 W / m 3 .

[0022] The present invention also makes it possible to simplify the introduction of 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 by the installation of an appropriate 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 viscosity reduction of the product in the conditioning process.

[0023] Finally, one advantage of the present invention is that it can process all types of polyester waste containing an increasing number of pigments, dyes, and other polymers, such as blue, colored, opaque, and multilayer PET. The method according to the present invention can process opaque PET, and the present invention makes it 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 raw polyester, especially raw PET, and therefore allows all applications of raw PET. [Modes for carrying out the invention]

[0024] (List of drawings) (Figure 1) Figure 1 shows one embodiment of the method according to the present invention, which includes: a step (a) of conditioning a feed material (1) containing PET; an extruder (a1) for conditioning the feed material (1) and a static mixer (a2) thereafter which ethylene glycol (2) is also fed; a depolymerization step (b): feeding the mixture obtained from the conditioning step and the diol effluent (3); a diol separation step (c) for recovering the diol effluent (3); a step (d) for separating and removing BHET diester and removing heavy impurities (5); and a step (e) for recovering purified BHET effluent (4) by decolorization by adsorption.

[0025] (Description of the embodiment) According to the present invention, polyethylene terephthalate or poly(ethylene terephthalate) (also known simply as PET) has the following basic repeating unit.

[0026] [ka]

[0027] Conventionally, PET is obtained by polycondensation of terephthalic acid (PTA) or dimethyl terephthalate (DMT) with ethylene glycol. Hereafter in this specification, the expression "per mole of diester in the polyester feedstock" corresponds to the number of moles of -[O-CO-O-(C6H4)-CO-O-CH2-CH2]- units, which are diester units obtained from the reaction of PTA and ethylene glycol in the PET contained in the polyester feedstock.

[0028] According to the present invention, the term “monomer” or “diester monomer” advantageously represents bis(2-hydroxyethyl) terephthalate (BHET) of the chemical formula HOC2H4-CO2-(C6H4)-CO2-C2H4OH, where -(C6H4)- represents an aromatic ring, which is a diester unit obtained from the reaction of PTA and ethylene glycol in PET contained in the polyester feedstock.

[0029] The term "oligomer" typically refers to a small-sized polymer, generally consisting of 2 to 20 basic repeating units. According to the present invention, the term "ester oligomer" or "BHET oligomer" refers to 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.

[0030] According to the present invention, the terms “diol” and “glycol” are used interchangeably and correspond to compounds containing two hydroxyl groups (-OH). A preferred diol is ethylene glycol, also known as monoethylene glycol or MEG.

[0031] The diol or diol effluent flow used in the steps of the method of the present invention preferably contains ethylene glycol (or MEG) in a very dominant amount, i.e., MEG accounts for 95% by weight or more relative to the total weight of the diol or diol effluent flow.

[0032] The term "dye" refers to a substance that is soluble in polyester materials and used to color them. Dyes may be of natural or synthetic origin.

[0033] According to the present invention, the term “pigment,” more specifically, a pigment for coloring and / or opacity, refers to a finely divided substance, in particular, insoluble in polyester materials. Pigments exist in the form of solid particles, generally having a size of 0.1–10 μm, predominantly 0.4–0.8 μm. They are often of mineral nature. Commonly used pigments, particularly those used for opacity, are metal oxides, e.g., TiO2, CoAl2O4 or Fe2O3, silicates, polysulfides, and carbon black.

[0034] According to the present invention, the expression "between A and B" means that both limit values ​​of the interval (A, B) are within the specified range of values. If this is not the case, and both limit values ​​are not within the specified range, such explanation will be given by the present invention.

[0035] Specific and / or preferred embodiments of the present invention may be described in the following description herein. They may be carried out separately or in combination, and there are no restrictions on the combination as long as it is technically feasible.

[0036] (Feed material) The method according to the present invention is fed by a polyester supply material, which comprises at least one polyester, i.e., a polymer in which the repeating units of the main chain contain ester groups, and comprises polyethylene terephthalate (PET), preferably comprising at least colored PET and / or opaque PET.

[0037] The polyester feedstock is, advantageously, a polyester feedstock for recycling obtained from waste collection and sorting channels, particularly plastic waste. The polyester feedstock may come from the collection of, for example, bottles, container trays, films, resins and / or fibers made of polyethylene terephthalate.

[0038] Advantageously, the polyester supply material contains at least 50% by weight, preferably at least 70% by weight, and preferably at least 90% by weight of polyethylene terephthalate (PET).

[0039] Preferably, the polyester feedstock comprises at least one PET selected from colored, opaque, dark, and multilayer PET and mixtures thereof. More specifically, the polyester feedstock comprises at least 10% by weight of opaque PET, more preferably at least 15% by weight of opaque PET, wherein the opaque PET is advantageously opaque PET for recycling, i.e., opaque PET obtained from collection and sorting channels.

[0040] The polyester supply material preferably contains 0.1% to 10% by weight of pigment, preferably 0.1% to 5% by weight of pigment. In particular, it may also contain 0.05% to 1% by weight of dye, preferably 0.05% to 0.2% by weight of dye.

[0041] In the collection and sorting channels, polyester waste is washed and ground before it constitutes the polyester feedstock for the method according to the present invention.

[0042] The polyester feedstock may be in the form of flakes, either entirely or partially, with a maximum length of less than 10 cm, preferably 5 to 25 mm, or in the form of finely milled solids, i.e., particles, preferably 10 microns to 1 mm in size. The feedstock may contain macroscopic impurities, preferably less than 5% by weight, preferably less than 3% by weight, 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 entirely or partially, such as woven fabrics, which may optionally be pre-treated to remove cotton or polyamide fibers, or any woven fabric other than polyester, or woven fibers, which may optionally be pre-treated to particularly remove polyamide fibers or rubber or polybutadiene residues. The polyester feedstock may also contain polyester obtained from defective products of a polyester polymerization and / or conversion method. The polyester supply material may contain elements used as polymerization catalysts and stabilizers in PET manufacturing methods, such as antimony, titanium, or tin.

[0043] (Conditioning process a)) The method according to the present invention comprises a conditioning step a), which comprises at least one, preferably a single, conditioning section and a mixing section, wherein the conditioning section is fed the polyester feed material to produce a flow of conditioned feed material, and the mixing section is fed at least the flow of conditioned feed material and diol effluent to produce a mixed flow.

[0044] The conditioning section of step a) makes it possible to heat the polyester feedstock to the operating conditions of 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, much more advantageously at least 80% by weight, preferably at least 90% by weight, and preferably at least 95% by weight of the polyester feedstock is in liquid form when it leaves 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 oxidation of the polyester feedstock.

[0045] 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 feed material, which is advantageously 5 hours or less, preferably 1 hour or less, preferredly 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 feed material so that the flow of the conditioned feed material is at a temperature of 150 to 300°C, preferably 225 to 275°C, and at a pressure between atmospheric pressure (i.e., 0.1 MPa) and 20 MPa.

[0046] The extrusion section is advantageously connected to a vacuum extraction system to remove impurities present in the feed material, such as dissolved gases, light organic compounds, and / or moisture. The extrusion section may also advantageously include a filtration system to remove solid particles larger than 40 μm and preferably less than 2 cm in size, such as sand particles. The polyester feed material is advantageously fed to the extruder, for example, via a feed hopper, by any method known to those skilled in the art, and advantageously inactivated to limit the introduction of oxygen into the system.

[0047] The mixing section is fed at least, preferably only, the flow of the conditioned feed material obtained from the conditioning section and the diol effluent obtained from step c), preferably the portion of the diol effluent. In the mixing section, the polyester feed material, which has been pre-conditioned in the conditioning section, is advantageously placed in contact with the diol effluent. The effect of this contact is to initiate the depolymerization reaction of the polyester feed material before its introduction to the depolymerization step (b). This also makes it possible to significantly reduce the viscosity of the feed material, thereby facilitating its transport, particularly to the depolymerization step b). Advantageously, the mixing section includes a static mixer or a dynamic mixer, preferably a static mixer. The mixing section is preferably operated in a static mixer, at a temperature of preferably 150-300°C, more preferably 225-275°C, with a residence time defined as the ratio of the volume of liquid in the static mixer to the volumetric flow rate of the diester feedstock, which is 0.5 seconds to 20 minutes, more preferably 1 second to 5 minutes, more preferably 3 seconds to 1 minute, and the weight ratio of the weight of the diol to the weight of the polyester feedstock is 0.03-3.0, more preferably 0.05-2.0, more preferably 0.1-1.0. This weight ratio of diol to polyester feedstock corresponds to a molar ratio of 0.09-9.0, more preferably 0.15-6.0, and more preferably 0.3-3.0, of the molars of diol to the molars of diester in the polyester feedstock.

[0048] Preferably, the diol effluent obtained from step (c) and introduced into the mixing section of step (a), preferably the portion of the diol effluent, is advantageously superheated before being introduced into the mixing section of step (a) to facilitate the establishment of the temperature of the polyester feedstock.

[0049] If the conditioning section is operated in the extruder, the mixing section may be carried out in the extruder. In this case, it is the reactive extrusion phase. The temperature during the operation is 150-300°C, preferably 225-275°C, and the residence time is defined as the ratio of the volume of liquid in the mixing section to the volumetric flow rate of the diester feedstock, which is 0.5 seconds to 1 hour, preferably 0.5 seconds to 30 minutes, preferably 1 second to 20 minutes, or 3 seconds to 10 minutes, or 1 minute to 5 minutes, and the weight ratio of the weight of the diol to the weight of the polyester feedstock is 0.03-3.0, preferably 0.33-2.0, or preferably 0.35-1.0. This weight ratio of the diol to the polyester feedstock corresponds to a molar ratio of 0.09-9.0, preferably 1.0-6.0, and preferably 1.05-3.0, of the moles of diol to the moles of diester in the polyester feedstock.

[0050] In some cases, at least one portion of the heavy impurity effluent obtained at the end of step d) may be recycled as appropriate to the conditioning step a), particularly to the mixing section, or directly to the reaction section of step b), and the said portion of the heavy impurity effluent may be filtered before recycling.

[0051] (Depolymerization step b)) The method according to the present invention includes a step of depolymerization by glycolysis, wherein the mixed flow obtained from conditioning step a) is optionally supplied with a supply of diols, and the total amount of diol supplied to step b) is equivalent to the sum of the amounts of diols introduced into step a) and optionally step b), that is, the amount of diol per mole of diester contained in the mixed flow obtained from step a) is adjusted to 1 to 20 mol, preferably 3 to 15 mol, preferably 5 to 10 mol. That is, the weight ratio between the total amount of diols introduced into step a) and optionally step b) relative to the total amount of diester contained in the mixed flow is approximately 0.3 to 6.7, preferably approximately 1.0 to 5.0, preferably 1.7 to 3.3.

[0052] Advantageously, step b) comprises one or more reaction sections, preferably at least two, preferably two to four, and preferably operating in series. Each reaction section may be used in any type of reactor known to those skilled in the art that enables the depolymerization or transesterification reaction, preferably 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, step b) comprises at least two, preferably two to four, reaction sections operating in series, and the reaction sections (one or more) from the second reaction section onward are operated at the same or different temperatures among them and at or below the temperature of the first reaction section, preferably lower than the first reaction section, and preferentially 10 to 50°C lower, and even 20 to 40°C lower, relative to the temperature of the first operating section.

[0053] The reaction section(s)

[0054] The operating pressure of the reaction section(s)(1 or more) in step b) is determined 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 more preferably less than 5 MPa. The term “reaction system” means all the components and phases present in step b) obtained from the feed of step b).

[0055] The diol is preferably monoethylene glycol.

[0056] The glycolysis reaction may be carried out in or without the catalyst.

[0057] When glycolysis 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, such as complexes, oxides and salts of antimony, tin or titanium, alkoxides of metals from groups (I) and (IV) of the periodic table, organic peroxides or acidic / basic metal oxides.

[0058] A suitable heterogeneous catalyst preferably contains a solid solution in an amount of at least 50% by mass, preferably at least 70% by mass, preferably at least 80% by mass, very preferably at least 90% by mass, and even more preferably at least 95% by mass, relative to the total mass of the catalyst. This solid solution is given by formula Z x Al2O (3+x) The catalyst consists of at least one type of spinel, where x is between 0 (except for a limiting value) and 1, and Z is selected from Co, Fe, Mg, Mn, Ti, and Zn, and contains 50% by mass or less of alumina and an oxide of element Z. The preferred heterogeneous catalyst preferably contains 10% by mass or less of a dopant. This dopant is selected from silicon, phosphorus, and boron, and these are used alone or in mixtures. For example, the solid solution may consist of a mixture of spinel ZnAl2O4 and spinel CoAl2O4, or otherwise a mixture of spinel ZnAl2O4, spinel MgAl2O4, and spinel FeAl2O4, or otherwise consist of spinel ZnAl2O4 alone.

[0059] Preferably, the depolymerization step is carried out without adding an external catalyst to the polyester supply material.

[0060] The depolymerization step may be advantageously carried out in the presence of a solid adsorbent, which is either in powder form or molded, and whose function is to absorb at least some of the colored impurities, and therefore to facilitate the decolorization step e). The solid adsorbent is advantageously activated carbon.

[0061] The glycolysis reaction makes it possible to convert the polyester feedstock into ester monomers and oligomers, and more favorably, PET into at least the monomer bis(2-hydroxyethyl) terephthalate (BHET) and BHET oligomers. The conversion rate of the polyester feedstock in the depolymerization step is greater than 50%, preferably greater than 70%, and preferably greater than 85%. The BHET molar yield is greater than 50%, preferably greater than 70%, and preferably greater than 85%. The BHET molar yield corresponds to the molar flow rate of BHET at the outlet of step b) versus the number of moles of diester in the polyester feedstock supplied to step b).

[0062] An internal recirculation loop is advantageously used in step b). That is, the internal recirculation loop involves withdrawing a portion of the reaction system, filtering this portion, and reinjecting the portion into step b). This internal loop makes it possible to remove any macroscopic solid impurities that may be present in the reaction mixture.

[0063] Advantageously, the depolymerization step b) makes it possible to obtain reaction effluent, which is then sent to the diol separation step c).

[0064] (Step c of the diol separation) The method according to the present invention comprises a diol separation step c), to which the effluent from step b) is fed at least, and is carried out at a temperature of 100 to 250°C and at a pressure lower than that of step b), thereby producing a diol effluent and a liquid monomer-rich effluent.

[0065] The main role of step c) is to recover all or part of the unreacted diol.

[0066] Step c) is carried out at a pressure lower than that of step b) to evaporate the portion of the effluent from step b) to yield a gaseous effluent and a liquid effluent. The liquid effluent constitutes a effluent rich in liquid monomers. The gaseous effluent, consisting of more than 50% by weight, preferably more than 70% by weight, preferably more than 90% by weight of diols, constitutes a diol effluent.

[0067] Step c) is carried out, advantageously, in one or a series of gas-liquid separation sections, advantageously in 1 to 5 series of gas-liquid separation sections, and more advantageously in 3 to 5 series of gas-liquid separation sections. Each gas-liquid separation section produces a liquid effluent and a gas phase. The liquid effluent from the preceding section is fed to the next section. All gas effluent is recovered to form a diol effluent. The liquid effluent obtained from the last gas-liquid separation section forms a liquid monomer-rich effluent.

[0068] Advantageously, at least one of the gas-liquid separation sections may be carried out in a falling membrane evaporator, a thin-film evaporator, or a short-path distillation apparatus.

[0069] Step c) is carried out such that the temperature of the liquid effluent is maintained above a value below which the polyester monomer precipitates, and below a high value above which the monomer significantly repolymerizes, depending on the molar ratio of the diol / monomer. The temperature in step c) is 100 to 250°C, preferably 110 to 220°C, preferably 120 to 210°C. Operation as multiple consecutive gas-liquid separations, preferably 2 to 5 consecutive, and preferably 3 to 5 consecutive, is particularly advantageous because it allows for adjustment of the temperature of the liquid effluent in each separation to address the aforementioned constraints.

[0070] The pressure in step c) is lower than the pressure in step b) and is advantageously adjusted to allow evaporation of the diol at temperature while simultaneously minimizing repolymerization and enabling optimal energy integration. It is preferably 0.00001 to 0.2 MPa, preferredly 0.00004 to 0.15 MPa, and preferably 0.00004 to 0.1 MPa.

[0071] The separation section(s)(s)(s)(s)(s)(s) are advantageously agitated by any method known to those skilled in the art.

[0072] The diol effluent may contain other compounds, such as dyes, light alcohols, water, or diethylene glycol. At least one portion of the diol effluent may be advantageously recycled in liquid form (i.e., after condensation) in steps a) and / or b), and optionally in step e), optionally as a mixture with an external diol feed for the method according to the present invention.

[0073] All or part of the diol effluent may be treated in liquid form in a purification step before being recycled to steps a) and / or b) and / or used as a mixture in step e). This purification step may include, in a non-comprehensive manner, absorption onto a solid (e.g., activated carbon) to remove the dye, and one or more distillations to separate and remove impurities, such as diethylene glycol, water, and other alcohols.

[0074] (Monomer separation step d)) The method according to the present invention includes step d) of separating and removing monomer-rich effluent obtained from step c), thereby producing heavy impurity effluent and pre-purified monomer effluent.

[0075] Step d) is advantageously 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 higher, and at a pressure of 0.001 MPa or less, preferably 0.0005 MPa or less, and preferably 0.000001 MPa or lower, with a liquid residence time of 10 minutes or less, preferably 5 minutes or less, preferably 1 minute or less, and preferably 0.1 seconds or more.

[0076] The purpose of this separation step d) is to separate the evaporated monomer, particularly BHET, from oligomers that were not converted at all, remain liquid, and are therefore heavy impurities, especially those absorbing pigments, from the unconverted polyester polymer, from any other polymers that may be present, and from the polymerization catalyst, while simultaneously minimizing monomer loss due to repolymerization. A small amount of oligomers may be present, in some cases, along with monomers, particularly small ones. These heavy impurities are found together with the oligomers in the heavy impurity effluent.

[0077] Due to the possible presence of polymerization catalysts in the polyester feedstock, separation must be carried out at a temperature not exceeding 250°C with a very short liquid residence time to limit any risk of monomer repolymerization during this process. Separation by simple atmospheric distillation is therefore not feasible.

[0078] The separation step d) is advantageously carried out in a falling membrane or thin film evaporation system or by short-path falling membrane or thin film distillation. Very low operating pressure is required to allow step d) to be carried out at a temperature of less than 250°C, preferably less than 230°C, while simultaneously enabling vaporization of the monomer.

[0079] The polymerization inhibitor may, advantageously, be mixed with the liquid monomer-rich effluent before being fed to step d).

[0080] The solvent (flux) may, advantageously, be mixed with the liquid monomer-rich effluent before being fed to step d), which facilitates the removal of heavy impurities, particularly pigments, at the bottom of the short-path distillation or evaporation system. The solvent may have a boiling point much higher than that of the monomer, particularly BHET, under the operating conditions of step d). It may be, for example, polyethylene glycol or PET oligomer.

[0081] The heavy impurity effluent particularly includes pigments, oligomers, and optionally BHET that were not separated and removed. The heavy impurity effluent may, advantageously, undergo at least one separation step, for example by filtration, before being recycled, to reduce the amount of pigments and / or other solid impurities. A portion of the heavy impurity effluent that has been separated and has a high solid content may, advantageously, be purged from the method and sent to an incineration system.

[0082] The pre-purified monomer effluent is advantageously sent to a gas-liquid separation section, where it is carried out in any equipment known to those skilled in the art at a temperature of 100-250°C, preferably 110-200°C, preferably 120-180°C, and a pressure of 0.00001-0.1 MPa, preferably 0.00001-0.01 MPa, preferably 0.00001-0.001 MPa. The separation section makes it possible to separate the gaseous diol effluent from the pre-purified liquid monomer effluent. The gas-liquid separation makes it possible to further reduce the amount of diol remaining in the pre-purified monomer effluent by recovering more than 50% by weight, preferably more than 70% by weight, and preferably more than 90% by weight, of the diol encombusted with the pre-purified monomer effluent in step d) in the gaseous diol effluent. The amount of monomer encombined in the gaseous diol effluent is preferably less than 1% by weight, preferably less than 0.1% by weight, and more preferably less than 0.01% by weight, of the amount of monomer present in the pre-purified monomer effluent. The gaseous diol effluent is then advantageously condensed and optionally pre-treated in a purification step and recycled to step a) and / or step b) and / or as a mixture to step e) together with the diol effluent obtained from step c).

[0083] (Bleaching process e)) The method according to the present invention includes a step of decolorizing the pre-purified monomer effluent, which is carried out in the presence of an adsorbent at a temperature of 100 to 250°C, preferably 110 to 200°C, preferably 120 to 180°C, and at a pressure of 0.1 to 1.0 MPa, preferably 0.2 to 0.8 MPa, preferably 0.3 to 0.5 MPa, to produce the purified monomer effluent.

[0084] The adsorbent may be any adsorbent known to those skilled in the art that can absorb dyes, such as activated carbon or clay, and preferably activated carbon.

[0085] The pre-purified monomer effluent is advantageously mixed with a portion of the diol effluent obtained from step c) and optionally pre-treated in the purification step, or with an external diol feed for the method according to the present invention.

[0086] The purified monomer effluent is, advantageously, fed downstream to a polymerization process known to those skilled in the art, which feeds ethylene glycol, terephthalic acid, or dimethyl terephthalate after a selected polymerization process, for the purpose of producing PET that is indistinguishable from raw PET. Feeding the purified monomer effluent in the polymerization process makes it possible to reduce the feeding of dimethyl terephthalate or terephthalic acid at an equivalent flow rate.

[0087] The following drawings and embodiments illustrate the present invention, but do not limit its scope.

[0088] The following figures and examples illustrate the present invention, but do not limit its scope.

[0089] (Examples) (Example 1 - Conforms to the present invention) In this embodiment, only conditioning step a) and depolymerization step b) are described, and a method for continuously depolymerizing 100% PET supply material is described. The recycling capacity is PET 20 KTY (kilotons / year) (i.e., 2500 kg / hour).

[0090] The conditioning and pre-mixing step (a) is carried out in an extruder for melting the PET feed material and a static mixer for pre-mixing the molten PET feed material with ethylene glycol (MEG), as shown in Figure 1.

[0091] The reaction section consists of two cascaded, fully agitated reactors. The working volume of the reactor is R1: 3.75 m³. 3 R2: 22.4m 3 The reactor is 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 viscosity.

[0092] The operating conditions for the extruder, static mixer, and the first reactor R1 are summarized in Table 1 below.

[0093] [Table 1]

[0094] By using a premix of molten feedstock and ethylene glycol at a weight ratio of 0.23, it is possible to reduce the viscosity of the feedstock from 530 Pa·s for the molten PET feedstock alone to approximately 10 Pa·s for the mixture, particularly at the inlet of the first reactor R1. Therefore, by conditioning step a) according to the method of the present invention, it is possible to significantly reduce the viscosity of the feedstock before it enters the first reactor R1.

[0095] To check the effect of such viscosity on the quality of the mixture in the first reactor, reference t * Calculate the stirring force required for reactor R1 to satisfy >10.

[0096] Due to the viscosity at the inlet of reactor R1 being on the order of 10 Pa·s, the load in reactor R1 is 1500 W / m 3 For absorbed stirring force less than t, stirring standard t* While it becomes possible to ensure >10, using only molten PET raw material, 1500 W / m 3 A stirring force less than t is based on the stirring standard t. * We do not guarantee that the condition will be met >10.

[0097] Therefore, it is understood that pre-mixing the feedstock with a solvent, such as MEG, upstream of the reaction section provides flexibility in the method for depolymerizing the PET feedstock and ensures good quality mixing in the depolymerization reactor while simultaneously allowing for a reasonable overall stirring force. [Brief explanation of the drawing]

[0098] [Figure 1] One embodiment of the method according to the present invention is shown.

Claims

1. A method for depolymerizing polyester supply material containing PET, a) A conditioning step comprising at least one conditioning section that generates a flow of conditioned feedstock and a mixing section that generates a mixed flow, The polyester supply material is supplied to the conditioning section at least, and the process is carried out in an extruder at a temperature of 225 to 275°C. The process involves supplying the aforementioned flow of conditioned feed material obtained from the conditioning section and a diol effluent containing 95% by weight or more of ethylene glycol to the mixing section, and operating the extruder at a temperature of 225 to 275°C with a residence time of 0.5 seconds to 20 minutes so that the weight ratio of diol relative to the polyester feed material is 0.1 to 1.0; b) A step of depolymerization by glycolysis, wherein the mixed flow is optionally supplied with a diol feed, and the total amount of diol supplied to step b) is adjusted to 1 to 20 moles of diol per mole of diester supplied to step b), and the process is carried out at a temperature of 180 to 400°C and for a residence time of 0.1 to 10 hours; c) A step of separating and removing diols, wherein the effluent from step b) is fed at least, the process is carried out at a temperature of 100 to 250°C and at a pressure lower than that of step b), producing diol effluent and liquid monomer-rich effluent, the process is carried out in 1 to 5 consecutive gas-liquid separation sections, each section producing liquid effluent, gas effluent, and liquid monomer-rich effluent, the liquid effluent from the previous section is fed to the next section, the liquid effluent obtained from the last gas-liquid separation section constitutes liquid monomer-rich effluent, and all gas effluent is recovered to constitute diol effluent; d) A step of separating the abundant effluent from the liquid monomer obtained in step c) into heavy impurity effluent and pre-purified monomer effluent, the step being carried out at a temperature of 250°C or less and a pressure of 0.001 MPa or less, with a liquid residence time of 10 minutes or less; Here, at least a portion of the heavy impurity effluent is recycled directly to the reaction section of step b); and e) A step of decolorizing the pre-purified monomer effluent, which is 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, thereby producing purified monomer effluent. Methods that include...

2. The method according to claim 1, wherein the polyester supply material contains at least 10% by weight of opaque PET.

3. The method according to claim 1 or 2, wherein the polyester supply material includes colored PET.

4. The method according to any one of claims 1 to 3, wherein the polyester supply material contains 0.1% to 10% by weight of pigment.

5. The method according to any one of claims 1 to 4, wherein the mixing section is carried out with a residence time of 3 seconds to 1 minute.

6. The method according to any one of claims 1 to 5, wherein the diol effluent introduced into the mixing section of step a) is superheated before being introduced into the mixing section of step a).

7. The method according to any one of claims 1 to 6, wherein the diol effluent introduced into the mixing section of step a) is the portion of the diol effluent obtained from step c).

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