METHOD FOR ENABLED RECYCLING OF POLYESTER WASTE AND SYSTEM FOR APPLYING THE METHOD

Abstract

The present invention relates to a process for recycling polyester waste streams by depolymerizing polyesters by alcoholysis, the process comprising at least a first stage of depolymerization and another second consecutive stage of depolymerization, wherein the polymer waste stream is continuously subjected to the first and second stages, wherein in the first of the two consecutive stages, the polyester waste stream is continuously fed to an extruder operated at a temperature above the melt temperature of the polyester to produce a fluid mixture comprising a melt of at least partially depolymerized polyester, while co-feeding a first amount of alcohol to the extruder, and in the second stage, the fluid mixture is continuously fed to a continuous stirred tank reactor (CSTR) operated at a temperature above the melt temperature of the polyester, while co-feeding a second amount of alcohol to the CSTR, using residence time in the CSTR to provide a continuous stream of polyester depolymerized into oligomeric esters at the outlet of the CSTR. The present invention also relates to a system for applying this method.

Description

[Technical Field]

[0001] The present invention relates to the field of recycling polyester waste, in particular materials containing semi-crystalline polyesters such as polyethylene terephthalate (PET), and to systems for applying the recycling process. [Background technology]

[0002] Polyesters, such as PET, commonly used in soda bottles and yarn for textiles, are commonly recycled. The post-consumer polyester recycling industry began as a result of external pressure to improve waste management. Another driving force behind the polyester recycling industry is the slow rate of natural decomposition of polyester products. Many polyesters are non-degradable plastics under normal conditions, as there are no known organisms capable of consuming their relatively large molecules. For polyester to biodegrade, complex and expensive procedures are required.

[0003] The first global recycling efforts for polyester waste (i.e., post-consumer polyester objects or materials) occurred in the 1970s, but the development of suitable recycling processes has evolved rapidly. For example, in 2000, total PET consumption in Australia was 88,258 tons, of which 28,113 tons were recovered, representing a recovery rate of approximately 32%. Many researchers have reported that for PET recycling to be successful, PET flakes must meet certain minimum requirements. The primary factor affecting the suitability of post-consumer PET flakes for recycling is the level and nature of contaminants present in the flakes. Minimizing the amount of these contaminants leads to better rPET (i.e., recycled PET) quality. PET is contaminated with many substances, including acid-forming contaminants, water and color contaminants, acetaldehyde, and other contaminants from detergents, fuels, pesticides, etc., due to the use of PET bottles to store these materials.

[0004] A variety of different types of processes have been applied to recycle polyester waste, each with its own advantages and disadvantages.

[0005] The primary process for recycling polyester waste is so-called energy recycling, such as pyrolysis and carbonization. Pyrolysis of polyester waste was first described in the early 1980s. It is an alternative to the disposal of PET in landfills. Generally, polyester waste is pyrolyzed without further purification of the plastic waste. Most pyrolysis is carried out to produce aliphatic and aromatic hydrocarbons as a substitute for fossil fuels or as a source of chemicals. Carbonization is the second method for pyrolyzing polyester waste.

[0006] A secondary process for recycling polyester waste is simple separation of the polyester waste and then use of the separated material as an additive in stone mastic asphalt, cementitious materials, mortar, or concrete composites. Because polyester waste can be supplied in mixtures with other polymers, the polyester material must be separated from these polymers before reprocessing. For this reason, several methods have been developed, including froth flotation, wet shaker table, swelling, or thermodynamic techniques.

[0007] Next to this, there is a group of processes based on (thermo)mechanical methods for recycling polyester waste. The simplest method of thermodynamic recycling is to remelt sorted polyester waste. This method is applied, for example, to bottle-to-bottle technology, in which sorted PET bottles are remelted in crushed form and reprocessed into bottles for beverage packaging. Several studies have been conducted on thermally reprocessed PET. During this process, the polymer is exposed to high temperatures, shear forces, and pressure. This causes thermal degradation of PET. As a result, the thermal and mechanical properties of the reprocessed material usually deteriorate. Therefore, repeated thermal reprocessing of polyester waste leads to downcycling of the material. The problem of undesirable coloration in recycled polyester materials is typically overcome by using additives. Because the collection of polyester waste generally involves a mixture of polyester materials of different colors, such thermally reprocessed materials result in undesirable coloration in the recycled polyester. Therefore, the addition of complementary colors to polyester waste has been applied to conceal the coloration. Although this method is an approved method, it severely limits the use of recycled polyester, especially as a food packaging material.

[0008] A fourth class of processes for recycling polyester waste is the so-called chemical recycling (chemical degradation) processes, in which the recycling of polyester waste is made possible by depolymerization into monomers and / or oligomers. This class can be divided into a number of subclasses depending on the type of reactant used for the chemical degradation.

[0009] One example is the application of ionic liquids for depolymerization, which was first described around 2000. This method was developed to provide an environmentally friendly decomposition agent for polymers and to enable decomposition under mild reaction conditions, avoiding the drawbacks of other methods such as alcoholysis (high pressure and temperature and heterogeneous reaction products) or acidic and alkaline hydrolysis (pollution issues). However, no applications of the resulting reaction products have been described to date.

[0010] Alternatively, castor oil is applied for depolymerization. This method was developed to provide a renewable alternative to petrochemical agents (e.g., glycol) for PET depolymerization. After depolymerization, the reaction product was used for the preparation of polyurethane systems. However, due to the excessive amount of castor oil applied, it appears very difficult to determine the characteristic molecular weight. Furthermore, even with precise control of the reaction temperature, a heterogeneous mixture of reaction products is obtained.

[0011] Enzymatic degradation of polyester polymers was first described in the 1970s. This biochemical method, using ionic liquids and castor oil, offered an environmentally friendly procedure for polymer recycling, in contrast to traditional chemical recycling methods. However, the efficiency of this method was quite low for complete depolymerization of polyesters, making quantitative recovery of homogeneous reaction products for reuse impossible.

[0012] Alcoholysis for the depolymerization of PET was first described in the early 1990s. This method was developed to avoid the drawbacks (pollution issues) of acidic and alkaline hydrolysis to provide a renewable and more environmentally friendly decomposition agent for the polymer. Generally, polyesters are depolymerized with an excess of alcohol to yield the corresponding acid and the corresponding ester of ethylene glycol. Among alcoholysis methods, the reaction with methanol has been particularly important due to the low cost and availability of methanol. Ethylene glycol (a diol, sometimes classified separately as "glycolysis," but still included in the alcoholysis category) is primarily used in reactive extrusion to produce low-molecular-weight oligomers. However, the crude reaction product consists of a heterogeneous mixture of monomer, oligomer, and polymer, and these oligomers must be separated and purified for further processing. Various other alcohols, such as pentaerythritol, 1-butanol, 1-pentanol, 1-hexanol, and 2-ethyl-1-hexanol, have been described as useful. Diols other than ethylene glycol, such as BHET, neopentyl glycol (NPG), tetraethylene glycol (TEEG), polyethylene glycol, polytetramethylene oxide, and terpoly[poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene)], have also been described in the field of depolymerizing polyesters by alcoholysis. However, in all cases, a poorly defined mixture of low molecular weight oligomers is typically obtained. Another serious disadvantage of alcoholysis is the need for a catalyst. The most important catalysts have been zinc acetate and manganese acetate. Further catalysts are cobalt and lead acetate. The need for such a catalyst hinders the widespread application of alcoholysis for the reuse of polyester waste.

[0013] Aminolysis and ammonolysis have been developed for polyester recycling because of the higher reactivity of amine groups compared to the hydroxyl groups or alcohols used in the alcoholysis of polyesters, but as with alcoholysis, the need for a metal catalyst remains.

[0014] Finally, an alternative chemical recycling approach for polyesters is the controlled depolymerization of polyesters using blocking chain scission with a defined amount of depolymerization agent (see Geyer et al. in eXPRESS Polymer Letters, Vol. 10, No. 7 (2016) 559-586). This method produces polyester oligomers with a broader range of well-defined molecular weights than existing chemical methods, such as alcoholysis. However, this method requires the polyester material to be fractionated and free of contaminants.

[0015] While energetic and thermodynamic recycling involves downcycling of materials, chemical recycling requires significant amounts of chemicals, and its main drawbacks are its high cost, variability in the final product (highly dependent on the type of waste), the need for the use of (metal) catalysts, and the difficulty in removing colorants.Therefore, there is a need for improved methods, systems, and processes that allow for the recycling of polyester waste. [Prior art documents] [Non-patent literature]

[0016] [Non-Patent Document 1] Geyer et al., eXPRESS Polymer Letters, Vol. 10, No. 7 (2016) 559-586 Summary of the Invention [Problem to be solved by the invention]

[0017] The object of the present invention is to devise a process and system that will allow the recycling of polyester waste by depolymerizing the polyester to a predetermined oligomer length without the need for added catalyst, regardless of the molecular weight of the polyester in the waste (and hence the intrinsic viscosity, which typically varies from 0.65 dL / g for textiles and tray materials, to 0.80 dL / g for bottle-grade products, to 0.95 dL / g for strapping, and to 1.0 dL / g or more for tenacity yarns), and regardless of the amount of contaminants such as colorants, pigments, and other particulate matter, and at the same time purifying it so that it is suitable for repolymerization into higher polyesters of any required molecular weight. [Means for solving the problem]

[0018] In order to meet the objectives of the present invention, a process has been developed that allows for the recycling of polyester waste streams by depolymerizing the polyesters by alcoholysis, the process comprising at least a first stage of depolymerization and another second consecutive stage of depolymerization, the polymer waste stream being continuously subjected to the first and second stages, wherein in the first of the two consecutive stages, the polyester waste stream is continuously fed to an extruder operated at a temperature above the melting temperature of the polyester, while a first amount of alcohol is co-fed to the extruder to produce a fluid mixture comprising a melt of at least partially depolymerized polyester, and in the second stage, the fluid mixture is continuously fed to a continuous stirred tank reactor (CSTR) operated at a temperature above the melting temperature of the polyester, while a second amount of alcohol, which is not necessarily the same as the alcohol fed to the CSTR, is co-fed to the CSTR, using residence time in the CSTR to provide a continuous stream of polyester depolymerized into oligomeric esters at the outlet of the CSTR.

[0019] In this way, the depolymerization of polyesters, such as PET, can be precisely controlled in a continuous, energy-saving process, regardless of the IV of the starting (waste) material. As a result, even polyesters with very low polydispersities (typically around 2) can be decomposed into oligomeric esters of a given oligomer length without the need for added catalysts. At the same time, the intermediate viscosity during the process or after the CSTR is low enough to remove any particulate matter, such as pigments and fillers, by simple methods such as filtration. This provides the option of using the depolymerized material to obtain higher-grade polyesters of any desired molecular weight. Therefore, polyester waste can ultimately be upcycled into higher-grade polyesters in a low-energy process, even without the need for added catalysts.

[0020] The present invention is based on a combination of features that together provide the described advantageous effects. Importantly, the two-stage feature, operating at temperatures where the alcohol can depolymerize the polyester, provides the option of adjusting the amount of depolymerization in the first stage down to the molecular weight (and therefore IV) of the starting material. In this way, the input to the second stage is less variable, facilitating control of the output oligomeric ester. The first stage must then include an extruder so that the starting materials are thoroughly mixed to form a high-density, homogeneous melt that allows for proper control of depolymerization, largely independent of their starting composition. Applicant has recognized that the residence time within the CSTR can be used to achieve low polydispersity levels through inherent transesterification. By controlling the CSTR input through the proper dosing of alcohol in the extruder, the CSTR output can also be properly controlled based on temperature and residence time. Another important feature is the use of alcoholysis in both stages. While hydrolysis is most commonly used in the art, it has been found to lead to oligomeric products that are difficult to repolymerize. Hydrolysis essentially introduces free carboxyl groups, which appear to negatively hinder repolymerization of the oligomeric material, at least compared to the oligomeric products resulting from depolymerization using alcohol. Surprisingly, the process can operate without the addition of a catalyst. Without being bound by theory, this is believed to be due in part to the fact that polyester waste contains trace amounts of metals (catalysts) and that the process is continuous, so the waste is continuously maintained at elevated temperatures until it is sufficiently decomposed. Another important aspect is the applicant's recognition that depolymerization does not necessarily have to lead to monomers. While monomers offer the advantage of being clean, well-defined starting materials and are therefore generally recognized as ideal for repolymerization, the applicant recognized that the same is true for well-defined oligomers, given the nature of how polymerization must be carried out.A problem encountered thus far has always been the heterogeneous nature of any oligomeric products resulting from the depolymerization of (various) polymer wastes, and therefore the desire to depolymerize them into monomers. However, that problem can be overcome using the process of the present invention. Thus, applicants have confirmed that it is sufficient to depolymerize polyesters into oligomeric esters, as long as their composition can be controlled, while at the same time ensuring that the viscosity between the two stages or after the CSTR is low enough to apply simple techniques such as filtration to remove any particulate contaminants, such as pigments and fillers, or even any non-polyester polymers not decomposed in the process. By controlling the level of depolymerization at the various stages, and thus the resulting viscosity, in the continuous process of the present invention, it is possible to apply a line filter to remove any non-polyester material with little additional energy cost.

[0021] The present invention has been found to be effective for different types of polyesters. Furthermore, the amount and type of alcohol, as is generally known, is not critical to the depolymerization process. Depending on the type of polyester and the type and amount of alcohol, different levels of polymerization can be reached (assuming other conditions, such as residence time, are the same) or the conditions for reaching a given level of depolymerization can be changed. This can be controlled by measuring the viscosity of the (partially depolymerized) polyester mixture. Reference curves can be prepared in advance to define the relationship between viscosity and depolymerization for all types of polyesters at various stages of depolymerization with a specific alcohol.

[0022] It is noted that processes are known in the art that partially utilize the features of the present invention. For example, CZ299244 (assigned to Sirek Milan) discloses a process for the basic hydrolysis of waste PET based on the principle of two-stage chemical decomposition into terephthalate and ethylene glycol, in which in the first stage, the PET waste is decomposed by simultaneous extrusion hydrolysis and glycolysis, and in the second stage, the melt of the resulting oligomeric product of PET reactive extrusion leaving the first stage is subjected to basic hydrolysis in the presence of a catalyst in a sequential order and with the continuous administration of an aqueous solution of alkali metal hydroxide and / or ammonium hydroxide.

[0023] US 4,620,032 (assigned to Celanese Corporation) also discloses a two-stage process, but the process is based on hydrolysis. Specifically, in the known process, polyester is thoroughly mixed with a depolymerization agent, which is either water or a product resulting from the complete hydrolytic depolymerization of the condensation polymer. The depolymerization agent is mixed with the polyester for a time sufficient to reduce the molecular weight of the polyester by at least 50% in the first stage. The reduced molecular weight treated condensation polymer is then subjected to neutral hydrolysis in a second stage to achieve complete hydrolytic depolymerization to monomeric materials that can be used for repolymerization.

[0024] WO9720886 (assigned to Eastman Chemical Company) discloses a one-step batch process in which used or scrap polyester is reacted with glycol to produce monomers or low molecular weight oligomers by depolymerization of the polyester. The monomers or oligomers can then optionally be purified using one or more of a number of steps, including filtration, crystallization, and optionally, adsorbent treatment or evaporation.

[0025] The present invention also relates to a system for recycling polyester waste streams by depolymerizing the polyester via alcoholysis, the system comprising a line arrangement of three reactors in series for carrying out a continuous process for inducing alcoholysis of polyester, the first reactor being a twin-screw extruder having a feed inlet for supplying alcohol in the terminal 20% of its length, the second reactor being a single-screw extruder having a feed inlet for supplying alcohol in the central 30-70% of its length, and the third reactor being a continuous stirred tank reactor having a feed inlet for supplying alcohol.

[0026] <Definition> Polyesters are polymers in which monomer units are linked by ester groups. They are typically formed by polymerizing polyhydric alcohols with polybasic acids and are primarily used in the manufacture of resins, plastics, and textile fibers. It is well known that polyesters can be prepared by a condensation polymerization process in which a monomer providing an "acid moiety" (including its ester-forming derivatives) reacts with a monomer providing a "hydroxyl moiety." If desired, polyesters can contain a proportion of, for example, -C(=O)-NH (i.e., amide linking groups) or -C(=O)-NR 2Other linking groups, such as -(tertiary amide linking group) or carbonylamino linking group, may also be included. Polyesters used in daily life can be aliphatic, semi-aromatic, or aromatic. Typical examples are polyglycolic acid (PGA), polylactic acid (PLA), polycaprolactone (PCL), polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), polyethylene adipate (PEA), polybutylene succinate (PBS), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), and Vectra, a polycondensation product of 4-hydroxybenzoic acid and 6-hydroxynaphthalene-2-carboxylic acid. Of these, PETE, or PETP or PET-P as it's more commonly known, is the most common thermoplastic polymer resin in the polyester family, and its virgin form is considered one of the most important engineering polymers of the past few decades. It's considered an excellent material for many applications, including clothing fibers, liquid and food containers, thermoforming for manufacturing, and in combination with glass fibers in engineering resins. It's also known by brand names such as Terylene, Arnite, Eastapac, Mylar, Lavsan, and Dacron. While still referred to as a polyester material, polyester can contain up to 50% (w / w) of non-polyester polymer chains (e.g., 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, or 50%).

[0027] The melting temperature of a polyester is the temperature above which the polyester assumes liquid properties. Because many polymers typically do not have very sharp melting points, the melting temperature can be the highest temperature over a fairly wide range of temperatures above which the polymer slowly becomes "leathery," then "sticky," and finally liquid.

[0028] Polyester waste is post-consumer material at or after the end of its useful life, i.e., the time when it is used by the consumer for practical or aesthetic purposes, and consists essentially of polyester and up to 10% (e.g., 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10) of additives such as fillers (e.g., fibrous or particulate materials), stabilizers, colorants, etc.

[0029] Depolymerization refers to the reduction of molecular weight by breaking down the original polyester molecules into molecules of shorter length, eg, oligomers.

[0030] A continuous process is a flow production process used to process material without interruption. In such a process, the material, e.g., dry bulk or fluid, is continuously in motion, chemically reacts, and / or undergoes mechanical or thermal treatment. Continuous processes are contrasted with batch processes.

[0031] An alcohol is a hydrocarbon substance or mixture of such substances produced when a hydrogen atom in a hydrocarbon is replaced with a hydroxyl group. Alcohols can be monohydric, dihydric (i.e., diols), etc.

[0032] Mixing means combining or blending into one mass. Mixing two compounds does not exclude the compounds from reacting as they are being mixed to form other compounds in the mixture.

[0033] Intrinsic viscosity is a measure of the solute's contribution to the viscosity η of a solution; see "Progress in Biophysics and Molecular Biology" (Harding 1997). IV can be measured according to DIN / ISO 1628. Typically, a 1% polymer concentration is used, m-cresol is used as the solvent, and IV can be expressed in dl / g (often presented without the latter dimension). A practical way to determine intrinsic viscosity is by using an Ubbelohde viscometer.

[0034] Two consecutive stages means that the respective first and second stages follow one another in a continuous manner and thus without interruption, but this does not exclude that one or more additional intermediate processing steps are carried out between the two stages. [Brief explanation of the drawings]

[0035] [Figure 1] 1 shows a schematic overview of the process according to the present invention. [Figure 2] 1 shows a schematic of a screw used in a single-screw reactive extrusion process. DETAILED DESCRIPTION OF THE INVENTION

[0036] In a further embodiment of the process according to the present invention, the total amount of alcohol in the first and second amounts of alcohol (hence the sum of both amounts) is 3-12% w / w relative to the polyester material stream. It has been found that below 3%, depolymerization may be so low that a viscosity that allows for easy purification (e.g., by filtration) of the oligomeric material is not achieved. Above 12%, a significant amount of alcohol may not react, which is detrimental to the repolymerization process. Furthermore, at such high amounts, depolymerization may lead to significant amounts of monomer or low levels of oligomers that are simply not needed, resulting in extra effort in decomposition and repolymerization. Preferably, the amount of the first alcohol is 1-5% w / w relative to the polyester material stream, and the amount of the second alcohol is 2-8% w / w relative to the polyester material stream. It has been found that smaller amounts of alcohol can be advantageously applied in the extruder, since in this way the first stage controlled depolymerization to a predetermined viscosity level while avoiding alcohol loss due to evaporation within the extruder. A higher amount of alcohol in the second stage allows for finer control of depolymerization. In a CSTR, the upper alcohol limit is less critical than in an extruder. The preferred total amount of alcohol used to depolymerize the polyester is believed to be 5-8% w / w of the polyester waste stream.

[0037] In another embodiment, the first stage comprises first and second separate consecutive substages, each of which comprises a corresponding extruder, each operated at a temperature above the melting temperature of the polyester while co-feeding a certain amount of alcohol to each extruder to produce a fluid mixture fed to the CSTR. It has been found advantageous to use two separate extruders (arranged in line). This appears to be advantageous because the residence time in the extruder is relatively short, thus limiting the length for mixing volatile agents, let alone significant amounts of such agents. A further advantage is that the functionality and thus operation of each extruder can be better tailored to the depolymerization (sub)stage. The first extruder is operated to optimally convert the polymer waste into a dense melt, while the second extruder is operated to optimally control the depolymerization to a constant IV level, regardless of the IV of the starting material. Preferably, the alcohol fed to the extruder of the first sub-stage is fed in the terminal 20% of the extruder length (i.e., the last 20% of the extruder length observed in the process direction), preferably in the terminal 5-15% of the extruder length, and preferably about 10% (i.e., 9-11%) of the extruder length. This minimizes the risk of alcohol loss due to evaporation and ensures at least partial initial depolymerization, making the second extruder more controllable. In contrast, the alcohol fed to the extruder of the second sub-stage is advantageously fed in the central 30-70% of the extruder length, preferably in the central 40-60% of the extruder length, and preferably about 50% (i.e., 49-51%) of the extruder length. In this way, it is easy to obtain a melt feed of polymer waste at a predetermined IV.

[0038] Advantageously, the amount of alcohol fed in the first sub-stage extruder is 0-2% w / w (i.e., greater than 0 and up to 2%) with respect to the polyester material flow, and the amount of alcohol fed in the second sub-stage extruder is 1-5% w / w with respect to the polyester material flow, a ratio that appears to be very suitable for the process of the present invention.

[0039] In a further embodiment, the extruder of the first sub-stage is a twin-screw extruder. Because some polyesters, especially PET, are sensitive to shear, it is advantageous to use a twin-screw extruder to reduce the amount of shear. Even more advantageously, the extruder of the first sub-stage is a conical twin-screw extruder. Conical extruders can better accommodate the large variations in polyester waste, which is often fluffy or at least low density.

[0040] In another embodiment, the extruder of the second sub-stage is a single-screw extruder. A single-screw extruder is better able to achieve high pressures, which are advantageous for mixing larger amounts of volatile reagents. The fact that a high-density melt is already produced by the first extruder minimizes the need to accommodate materials of different densities.

[0041] In yet another embodiment, a polyester waste stream is subjected to a drying process in two successive stages to reduce the amount of water in the stream to less than 5000 ppm, i.e., less than 0.5% w / w water based on the polyester waste stream. The presence of water in the process inevitably results in the generation of free carboxyl groups through hydrolysis. High levels of free carboxyl groups, i.e., greater than 45 mmol / kg at the end of the initial repolymerization process (i.e., the process in which the oligomeric ester is repolymerized to an amorphous polyester, typically having an IV of 0.4-0.6), are disadvantageous in this process, which aims to produce oligomeric polyesters that are susceptible to repolymerization in a further solid-state repolymerization process. The type of pre-drying process is not critical. Pre-drying can be accomplished in a variety of ways. For example, simply blowing hot air at 120°C onto a bed of polyester flakes or agglomerates can reduce the moisture content to less than 5000 ppm. However, this can require a significant low vacuum in the extruder to remove additional moisture, if desired, so it is preferable to use more aggressive methods, such as a desiccant air dryer using hot air at 150°C or a dryer applying a (low) vacuum. Advantageously, the amount of water in the stream is 10 to 1000 ppm, preferably 20 to 100 ppm, and most preferably about 50 ppm. Surprisingly, it has been found that for a simple solid-state repolymerization process, very low levels of free carboxyl groups, i.e., levels below 10 mmol / kg, also result in a decrease in polymerization rate. Therefore, it has been established that there is a preferred range of humidity levels for the waste stream entering the first extruder, i.e., a range of 10 to 1000 ppm. 50 ppm is considered to be the optimal level.

[0042] In yet another embodiment of the process according to the present invention, the amount of alcohol fed to the first stage of the process is controlled by measuring the viscosity of the fluid mixture fed to the CSTR and adjusting the amount of alcohol to achieve a predetermined viscosity value, preferably an intrinsic viscosity (IV) of 0.1 to 0.2 dL / g. In this embodiment, a so-called loop control is introduced into the process to ensure that the material fed to the CSTR has a predetermined viscosity (depolymerization) level, at least within a reasonably controllable range. This supports the production of a predetermined oligomeric ester within a narrow viscosity / oligomer range in the CSTR. If the viscosity level of the fluid mixture is too high (and therefore the depolymerization rate is too low), more alcohol is fed to the first stage to increase the depolymerization rate. Conversely, if the viscosity level is too low, the amount of alcohol fed to the first stage can be reduced.

[0043] Similarly, a loop control is advantageously implemented to supply the amount of alcohol to the CSTR. The viscosity of the oligomeric ester stream exiting the CSTR is measured and the amount of alcohol supplied to the CSTR is adjusted to achieve a predetermined viscosity value. This predetermined value is preferably an intrinsic viscosity (IV) of 0.08 to 0.11, more preferably 0.09 to 0.1.

[0044] In yet another embodiment, the fluid mixture fed to the CSTR is pumped through a first filter positioned between the first and second stages, preferably having a mesh size of 10-80 μm, preferably 30-50 μm, and most preferably about 40 μm. Such a filter can be advantageously used to remove any particulate matter or non-polyester polymers (not degraded by the alcoholysis process) while simultaneously serving as a means to homogenize the fluid mixture. This filter option is possible because the method allows for monitoring and control of local viscosity. When the first stage comprises two separate, sequential extruders, a second filter is positioned between the first substage of the first stage and the second substage of the first stage. The second filter (upstream of the aforementioned first filter) preferably has a mesh size of 40-120 μm, preferably 60-100 μm, and most preferably about 80 μm, to remove typical contaminant particulate matter. The second filter also functions as a homogenizer. If deemed necessary, for example if one wishes to introduce more than 1% alcohol before entering the second extruder, a static mixer can be placed immediately before or after the second filter.

[0045] Similarly, the oligomeric ester stream is pumped through a third filter placed after the CSTR, preferably with a mesh size of less than 20 μm, more preferably 5-10 μm. This is to remove any remaining fine particulate matter, such as pigments, or any remaining non-oligomeric polymeric materials. Also, in this manner, activated carbon, which may be added to the CSTR to bind any colorants, can be removed by filtration. Given the relatively long residence time in the CSTR (compared to the extruder), activated carbon is ideally suited for colorant removal.

[0046] The alcohol preferably comprises a diol, preferably at least a diol selected from the group consisting of ethylene glycol, 1,3-propanediol, (1,4-)butanediol, cyclohexanedimethanol, and neopentyl glycol, such that the oligomeric ester is a hydroxylated oligomeric ester. The use of a diol results in oligomers that are prone to repolymerization.

[0047] In particular, when the polyester is polyethylene terephthalate (PET), the residence time in the CSTR is selected (controlled) so that greater than 50% w / w, preferably greater than 60, 70, 80, or even greater than 90% w / w of the oligomeric ester comprises oligomers of 4 to 16 bis(2-hydroxyethyl) terephthalate (BHET) units, preferably 6 to 14 (BHET) units, and most preferably 8 to 10 BHET units. This material is highly defined, has a low enough viscosity to allow filtration through very fine filters, and is easily repolymerized by application of a simple vacuum at elevated temperatures.

[0048] For the initial repolymerization step, a continuous stream of oligomeric ester is heated (either as a further step in a continuous process or batchwise) to a high temperature (typically above 240°C or even above 250°C) above the melting temperature of the polyester while being subjected to a vacuum of less than 10 mbar, preferably less than 5 mbar, preferably 0.5-2 mbar, and most preferably about 1 mbar, to induce repolymerization by removing the alcohol (shifting the equilibrium toward the polymer), leading to a repolymerized amorphous polyester with an IV of preferably 0.4-0.6. An additional advantage of the high vacuum is that it simultaneously removes any volatile contaminants that may be present (such as benzene and bisphenol A). It may be necessary to combine two condensation reactors (depending primarily on the amount of volatiles present in the oligomeric material) to be able to reach a low pressure of about 1 mbar. In either case, the resulting material is stable, highly pure, and well-defined, and therefore can be easily upgraded to higher polyesters of any IV. For this purpose, the repolymerized polyester can be heated to an elevated temperature below the melting temperature of the polyester (i.e., below 250°C, or even below 240, 230, or 220°C, typically about 210°C) while exposed to a vacuum (i.e., below 0.5 bar, preferably below 0.1 bar, or even below 50 mbar, 40 mbar, 30 mbar, 20 mbar, or 10 mbar) or an inert gas to induce further repolymerization, preferably to an IV greater than 0.6. Preferably, the additional repolymerization step is preceded by a solid-state crystallization step by cooling the repolymerized amorphous polyester to a temperature of 130-180°C to reach solid amorphous polyester and holding the solid amorphous polyester at that temperature until the polyester is at least partially crystallized.

[0049] The above does not preclude any modifications to the method or additional processing steps. For example, additional purification / filtration steps could be added. Furthermore, the present invention is not limited to a particular type of filter, but since filters may need to be replaced every few hours depending on the amount of particulate matter present in the polyester waste, a screen-changing filter is believed to be ideally suited to the claimed continuous process. Furthermore, if a filter is used, it is very likely to be a cascade of two, three, or more separate successive filters of decreasing mesh size to withstand the pressure differential across the filter. Following this, the molten material can be advantageously pumped through a bed filled with (activated) carbon granules, SiO2 granules, or any other small molecule absorbent material to ensure complete removal of all colorants and dyes.

[0050] It is also anticipated that copolymerization may be introduced in one or more repolymerization steps, typically the first repolymerization step. For example, slurries of pure diacids and pure diols can be prepared, and / or other monomers can be added to the initial polycondensation reactor to which the purified ester oligomers obtained by the process of the present invention are added. Copolymers are produced when the diacids and / or diols are different from the monomers present in the polyester waste. Such monomers may be biobased, for example, to further reduce the CO2 footprint, or may be, for example, isophthalic acid, succinic acid, or neopentyl glycol, to achieve other product properties of the final polyester to be produced that are suited to the intended use.

[0051] Any of the above further embodiments may be embodied in the system of the present invention.

[0052] The invention is further illustrated by the following non-limiting figures and examples. [Example]

[0053] Figure 1 shows a schematic overview of the process according to the present invention. Figure 2 shows a schematic of the screw used in the single screw reactive extrusion process.

[0054] Example 1 provides various runs with a single screw extruder alone. Example 2 provides various runs involving a two-stage depolymerization process according to the present invention.

[0055] <Figure 1> Figure 1 shows a schematic overview of the process according to the present invention. In the method shown, PET is depolymerized and repolymerized in a continuous process from steps 1 to 8. Step 9 is an additional repolymerization step involving solid-state polymerization to reach the required IV above 0.6.

[0056] This process is based on the commonly known equilibrium reaction of PET in alcoholysis based on monoethylene glycol (MEG).

[0057]

number

[0058] Adding MEG to a polyester melt shifts the equilibrium to the left, shortening the polymer chains and ultimately forming oligomers (fewer than 100 repeating BHET units, particularly 50, 40, 30, 20 units, or even less than 10 units), reducing the viscosity. By removing the MEG, for example by using vacuum or nitrogen, the short chains react with each other and form the polyester again. Controlling the depolymerization rate, and thus the oligomer length, controls the viscosity of the material.

[0059] In step 1, polyester waste, including (pure) PET carpet shreds and PET bottle flakes, is dried to a moisture level of 50 ppm. The dried waste stream is then fed into a conical co-rotating twin-screw extruder. Due to the extruder's conical design, the opening for feeding the material is larger than in a conventional twin-screw extruder, making it easier to feed, less likely to cause thermal damage to the polymer, and less shear due to gentler natural compression. Thermal degradation can lead to undesirable side reactions, forming end groups that result in inferior quality in the final product. The extruder is operated at 280°C to fully melt the polyester. Adjacent to the end of the conical twin-screw extruder (10% of its length), an injection point is provided for administering MEG (shown as arrow 50) to reduce the viscosity and achieve the first stage of depolymerization. Approximately 1% (w / w) MEG is administered for this purpose. Reducing the IV also helps minimize the pressure difference across the first filtration step 3 to allow filtration through an 80 micrometer mesh size. The filter also acts as a static mixer, homogenizing the mixture and distributing the added glycol with the molten polymer, allowing it to react completely with the shorter polymer chains, resulting in an equilibrium molecular weight distribution (dispersity grade of about 2). The process parameters are selected so that the MEG is (almost) completely reacted and free MEG is no longer present (or is almost absent).

[0060] The partially depolymerized and filtered material is fed into a single-screw extruder in step 4. Figure 2 shows a schematic of the screw design. This design aims to maximize the dosable glycol percentage (preventing the melt from becoming inhomogeneous). This maximum can be increased by adjusting general process parameters such as screw speed and pressure buildup. Typically, approximately 3-4% MEG is dosed into the extruder (indicated by arrow 50'). The viscosity of the melt is measured at the end of the extruder. The viscosity level is controlled by an automatic control loop (not shown in Figure 1) that controls the level of MEG injected into the single-screw extruder. This automatic control loop results in a consistent viscosity, typically an IV of 0.1-0.2, regardless of the IV of the starting material. Due to the inherent transesterification reaction occurring in the extruder, polydispersity can remain low, preferably around 2-3, depending primarily on the residence time in the extruder (which can be adjusted in the process by controlling the initial feed and extruder speed).

[0061] This depolymerized material was filtered a second time in step 5. Due to the IV of about 0.15, the filter size can be smaller than the first filter, preferably 40 micrometers, without creating too much pressure difference across the filter.

[0062] Material with an IV of approximately 0.15 (0.1-0.2) is continuously added to the CSTR in step 6. MEG is also added to the CSTR (indicated by arrow 50) to further depolymerize the material to the desired viscosity / oligomer length. Because the material already has a low (controlled) viscosity at the inlet, the viscosity difference with the added MEG is not so great that homogeneous mixing is difficult. 4-6% MEG can be easily mixed homogeneously. The residence time in the CSTR is long enough (typically 25-45 minutes) to depolymerize the material to the desired oligomer length, while also allowing sufficient time for the transesterification reaction to achieve a polydispersity of 2. Viscosity is measured at the end of the reactor, and an automatic control loop controls the addition of MEG within the reactor. This results in an extremely stable continuous process that is largely independent of the type (IV) of starting material.

[0063] In the CSTR, decolorization is achieved by adding activated carbon, indicated by arrow 60. The activated carbon can be pre-selected for best performance in absorbing colorants present in the polyester waste. After the CSTR, the low-viscosity oligomer / activated carbon mixture is pumped through a three-stage microfiltration (20 / 10 / 5 micrometer) process 7 to remove colorant-laden carbon particles from the oligomer. Three parallel sets of filters are installed so that if the pressure differential across the filters becomes too high, the melt can be pumped through the parallel sets while the first set of filters is washed.

[0064] After filtering the melt while it is still hot (approximately 250°C), the melt is pumped to a polycondensation reactor (step 8) operated under vacuum at 1 mbar and approximately 260°C to remove MEG and shift the BHET / PET equilibrium to the right, forming PET polymer. This reactor also performs an important purification step, since all volatiles with boiling points up to 250-260°C (e.g., benzene and bisphenol A) are removed from the material. Depending on the processing conditions, polyesters with IVs of 0.4-0.6 can be obtained. The polymer is removed from the reactor and pumped through a die plate equipped with holes, producing polymer strands. These strands are cooled and chopped into amorphous granules.

[0065] The amorphous granules are subjected to an offline crystallization process by subjecting the granules to temperatures of 130-180°C, which results in a crystallization process. The partially crystallized granules are then subjected to a solid-state polymerization process, in which the polyester is heated to a temperature below the polyester's melting point while subjected to a vacuum or inert gas. This induces further repolymerization of the solid, resulting in an IV greater than 0.6. The resulting IV typically ranges anywhere from 0.65 to 1.0 and can be adjusted by processing parameters to match the IV required for the intended application.

[0066] <Figure 2> FIG. 2 shows a schematic of a screw 10 for use in a single-screw reactive extrusion process. The basic screw design is standard, but it has four non-standard zones designed to optimize the polyester / oligomer and volatile reactants. In the middle of the screw, where the alcohol reactant is dosed, the screw is provided with a double-screw design, shown as 11. Downstream, this zone is followed by zone 12, where the screw also has a double-screw design, but in this zone the screw is partially milled (as in a known configuration for improving mixing). Further downstream is energy transfer mixing zone 13. Finally, terminal zone 14 is of the so-called Saxton type.

[0067] [Example 1] Example 1 presents various experiments using a single-screw extruder alone. Depolymerization of PET to oligomers with monoethylene glycol (MEG) by reactive extrusion was performed at rates ranging from 10 to 20 kg / h. It was found that up to 3% MEG could be adequately dosed in a single-screw extruder with a standard screw design (not optimized for MEG dosing). Control of the moisture content of the PET flakes was achieved by pre-drying and applying vacuum degassing to the extruder. This limited hydrolysis of the PET during the melting process, resulting in an increase of 7 mmol / kg of carboxyl end groups.

[0068] The results are listed in Table 1. The amount of MEG used in the extruder for depolymerization depends on the capacity of the extruder and its screw design. This is related to the residence time in the extruder and the effectiveness of the mixing of MEG with PET. This effect is also observed if the PET is too wet. An increase in hydrolytic depolymerization means that the amount of MEG that can be added decreases. This phenomenon occurs because the pressure in the extruder is too low, which creates MEG vapor pressure and pushes the oligomer out of the extruder due to its low viscosity, causing an unstable depolymerization process. Increasing the pressure in the extruder positively contributes to the stability of the depolymerization process and the amount of MEG that can be added while still achieving a homogeneous mixture.

[0069] [Table 1]

[0070] The extruder screw speed contributes to the mixing of MEG and PET. Table 2 shows this at 10 and 15 kg / h. At higher capacities (20 kg / h) and the same proportion of MEG, the opposite effect is seen. It is believed that a faster screw speed leads to more transport, resulting in a residence time in the extruder that is too short for complete depolymerization. The mixing effect of the screw is particularly evident at speeds below 70 rpm (results not shown). After that, the depolymerization becomes unstable and MEG is released from the extruder as vapor.

[0071] [Table 2]

[0072] As long as there is good mixing of the MEG with the PET and sufficient reaction time, almost all of the MEG will react with the polyester. Table 3 shows measurements made on oligomers collected in water. Virtually no MEG is present in the aqueous phase.

[0073] [Table 3]

[0074] Table 4 shows typical properties of dried PET flakes. As shown, the amount of diethylene glycol is approximately 1.5-1.6%. Since the amount typically measured at the end of the extruder was approximately 1.5-1.6%, it is reasonable to conclude that no diethylene glycol was produced in the extruder.

[0075] [Table 4]

[0076] In summary, the depolymerization of PET to oligomers using monoethylene glycol by reactive extrusion was performed at rates ranging from 10 to 20 kg / h. Limits were established for these different capacities within which an efficient depolymerization process could occur. Under the existing conditions, up to 3% MEG could be adequately dosed into a standard extruder. Control of the moisture content of the PET flakes was achieved by pre-drying and applying vacuum degassing to the extruder. This limited the hydrolysis of PET during the melting process, resulting in an increase of 7 mmol / kg of carboxyl end groups.

[0077] No additional diethylene glycol formation was evident in this test series. The added MEG was almost completely incorporated during depolymerization. The extruder screw speed influences the process. At a capacity of 20 kg / h, higher speeds lead to higher viscosity, likely because increased transport opposes effective reaction with the MEG. At smaller capacities, higher screw speeds result in better mixing with the MEG. If the screw speed is too slow, the depolymerization process may become unstable, potentially resulting in the release of MEG.

[0078] [Example 2] Example 2 provides various experiments involving a two-stage depolymerization process in accordance with the present invention. In particular, this example provides results achieved with glycolysis using MEG as a reactant to depolymerize PET in a single-screw extruder (see FIG. 2) and in combination with a CSTR configured as a continuous process.

[0079] The experiments used transparent PET flakes made from PET bottles. The basic properties of PET waste are listed in Table 5. Without drying, its moisture content is approximately 1% w / w. This moisture is present mainly due to the pre-treatment of the flakes for washing.

[0080] [Table 5]

[0081] The results of PET melting using dried and undried materials are shown in Table 6. The drier the PET, the less hydrolysis and therefore the formation of carboxyl end groups (Ec) affects the glycolysis and eventual repolymerization processes.

[0082] Since any (small amounts of) moisture may already have caused hydrolysis, the PET is kept dry during its dosing in the extruder. This is done by keeping the PET under dry nitrogen. To emphasize this, in Table 5 (see No. 3) you can see the effect of moisture absorption from the air on sample No. 2 after 24 hours of exposure to normal air.

[0083] [Table 6]

[0084] Reactive extrusion tests were performed on dried PET flakes with up to 3% w / w MEG. The glycolysis process occurred very rapidly. PET depolymerized into oligomers within 30 seconds. The administered MEG reacted almost completely within this time frame. More than 98% of the administered MEG was used in the glycolysis process. Table 7 shows the analysis of the glycolysis process. To improve performance, a larger amount of MEG was administered continuously in combination with a reactor (labeled "+CSTR").

[0085] Approximately 2% w / w MEG was added to the extruder, with additional amounts added to the CSTR if applicable. By selecting a 10 kg charge in the CSTR, an average residence time of 1 hour was achieved at 270 °C for a PET capacity of 10 kg / h. Steady state was reached at a total MEG dosage of ±12% w / w. The entire glycolysis process was carried out without additional catalyst. Viscosities (IV) as low as 0.07 were achieved. However, even viscosities of approximately 0.09–0.1 were low enough to allow (micro)filtration. At a dosage of 12%, 2–3% free MEG was still detected in the oligomeric esters. The relatively high proportion of carboxyl end groups generated by hydrolysis was likely due to the presence of moisture in the MEG. This could be avoided by drying the MEG.

[0086] [Table 7]

[0087] The HPLC analysis results in Table 8 show significant amounts of BHET relative to the tetramer when 12% or more MEG is applied. However, depolymerization to this extent is not necessary to allow filtration of the melt. A viscosity of (IV = ±) 0.1 is sufficient for the filtration process. This corresponds to 8-10 (octa-deca) oligomers. This requires 5%-8% w / w MEG.

[0088] [Table 8]

Claims

1. 1. A process for enabling the recycling of polyester waste streams by depolymerizing polyesters by alcoholysis, comprising at least a first stage of depolymerization and a second, consecutive stage of depolymerization, the polyester waste stream being subjected to the first and second stages in succession; - in a first of said two successive stages, said polyester waste stream is continuously fed to an extruder operating at a temperature above the melt temperature of said polyester while co-feeding a first amount of alcohol to said extruder to produce a fluid mixture comprising a melt of at least partially depolymerized polyester; - in the second stage, the fluid mixture is continuously fed to a continuous stirred tank reactor (CSTR) operated at a temperature above the melting temperature of the polyester while co-feeding a second amount of alcohol to the CSTR, using residence time in the CSTR to provide a continuous stream of polyester depolymerized to oligomeric ester at the outlet of the CSTR; process.

2. 2. The process of claim 1, wherein the total amount of alcohol in the first and second alcohol amounts is 3 to 12% w / w relative to the polyester material stream.

3. 3. The process of claim 2, wherein the amount of the first alcohol is 1-5% w / w relative to the polyester material stream and the amount of the second alcohol is 2-8% w / w relative to the polyester material stream.

4. 4. The process of any one of claims 1 to 3, wherein the first stage comprises first and second distinct successive sub-stages, each of the first and second sub-stages comprising a corresponding extruder, each of which is operated at a temperature above the melting temperature of the polyester with an amount of alcohol co-fed to each extruder to produce the fluid mixture that is fed to a CSTR.

5. 5. The process of claim 4, wherein the alcohol fed to the extruder of the first sub-stage is fed at the terminal 20% of the extruder length.

6. 6. The process according to claim 4, wherein the alcohol fed to the extruder of the second sub-stage is fed into the central 30 to 70% of the extruder length.

7. 7. The process according to any one of claims 4 to 6, characterized in that the amount of alcohol fed to the extruder of the first sub-stage is 0 to 2% w / w relative to the flow of polyester material, and the amount of alcohol fed to the extruder of the second sub-stage is 1 to 5% w / w relative to the flow of polyester material.

8. A process according to any one of claims 4 to 7, characterized in that the extruder of the first sub-stage is a twin-screw extruder.

9. 9. The process of claim 8, wherein the extruder of the first sub-stage is a conical twin-screw extruder.

10. A process according to any one of claims 4 to 9, characterized in that the extruder of the second sub-stage is a single screw extruder.

11. 11. A process according to any one of claims 1 to 10, characterized in that the two successive stages are preceded by a step in which the polyester waste stream is subjected to a drying process to reduce the amount of water in said stream to less than 5000 ppm.

12. 12. The process of claim 11, wherein the amount of water in the stream is between 10 and 1000 ppm.

13. 13. A process according to any one of claims 1 to 12, characterized in that the amount of alcohol fed in the first stage of the process is controlled by measuring the viscosity of the fluid mixture fed to a CSTR and adjusting the amount of alcohol to reach a predetermined value of said viscosity.

14. 14. The process according to any one of claims 1 to 13, characterized in that the amount of alcohol fed in the second stage of the process is controlled by measuring the viscosity of the oligomeric ester stream and adjusting the amount of alcohol to reach a predetermined value of said viscosity.

15. 15. The process of any one of claims 1 to 14, characterized in that the fluid mixture fed to a CSTR is pumped through a first filter located between the first stage and the second stage.

16. A method according to any one of claims 4 to 10, or a process according to any one of claims 11 to 14 when relying on claim 4, characterized in that a second filter is placed between the first sub-stage and the second sub-stage of the first stage.

17. 17. The process of any one of claims 1 to 16, characterized in that the oligomeric ester stream is pumped through a third filter located after the CSTR.

18. 18. The process of claim 17, wherein activated carbon is added to the CSTR and the carbon is removed from the oligomeric ester after the ester exits the CSTR.

19. The process of any one of claims 1 to 18, wherein the alcohol comprises a diol.

20. 20. The process of any one of claims 1 to 19, characterized in that the polyester is polyethylene terephthalate (PET) and the residence time in a CSTR is such that more than 50% w / w of the oligomeric ester comprises oligomers of 4 to 16 bis(2-hydroxyethyl) terephthalate (BHET) units.

21. 21. The process of any one of claims 1 to 20, characterized in that a continuous stream of the oligomeric ester is heated to an elevated temperature above the melting temperature of the polyester while being subjected to a vacuum of less than 10 mbar to induce repolymerization, leading to a repolymerized amorphous polyester.

22. 22. The process of claim 21, wherein the polyester is heated to an elevated temperature below the melting temperature of the polyester while exposed to vacuum or an inert gas to induce further repolymerization.

23. 23. The process of claim 22, characterized in that the further repolymerization step is preceded by a solid state crystallization step by cooling the repolymerized amorphous polyester to a temperature of 130-180°C to reach a solid amorphous polyester and maintaining the solid amorphous polyester at that temperature until the polyester is at least partially crystallized.

24. 1. A system for recycling polyester waste streams by depolymerizing the polyesters by alcoholysis, comprising an in-line arrangement of three consecutive reactors for carrying out a continuous process for inducing alcoholysis of the polyester, wherein the first reactor is a twin-screw extruder having a feed inlet for feeding alcohol in the terminal 20% of its length, the second reactor is a single-screw extruder having a feed inlet for feeding alcohol in the central 30-70% of its length, and the third reactor is a continuous stirred tank reactor having a feed inlet for feeding alcohol.

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