Method for recycling polyester waste material streams and system for applying the method - Patents.com

Abstract

The present invention relates to a process for recycling a stream of polyester waste material by depolymerizing polyesters by alcoholysis to a liquid mixture comprising oligomeric esters having 5 to 20 subunits, and continuously feeding said liquid mixture of oligomeric esters to a reactor in which said liquid mixture is present in combination with a gas headspace, said reactor having means for increasing the interface between said liquid mixture and said gas headspace by at least 5 times relative to a horizontal cross section of said reactor, and maintaining a pressure in said headspace of less than 20 mbar, whereby said oligomeric esters are repolymerized to polyesters having an intrinsic viscosity of at least 0.3 dL / g during a residence time of the mixture in said reactor of less than 120 minutes. The present invention also relates to a system for applying this process.

Description

[Technical Field]

[0001] The present invention relates to the field of recycling polyester waste materials, in particular materials comprising semi-crystalline polyesters such as polyethylene terephthalate (PET), and to systems for applying the recycling method. [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 environmental pressures to improve waste management. Another driving factor for the polyester recycling industry is the slow rate of natural decomposition of polyester products. Many polyesters are non-degradable plastics under normal conditions because there are no known organisms capable of consuming their relatively large molecules. Biological degradation of polyester requires complex and expensive procedures.

[0003] The first global efforts to recycle polyester waste materials (i.e., post-consumer polyester objects or materials) occurred in the 1970s, but the development of suitable recycling methods has progressed rapidly. As an 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 flake must meet certain minimum requirements. The primary factor affecting the suitability of post-consumer PET flake for recycling is the level and nature of contaminants present in the flake. Minimizing the amount of these contaminants improves the quality of rPET (i.e., recycled PET). PET becomes contaminated with many substances, such as acid-forming contaminants, water, color contaminants, and acetaldehyde, due to the use of PET bottles to store other contaminants, such as detergents, fuel, and pesticides.

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

[0005] The first class of methods for recycling polyester waste materials is so-called energy recycling, such as pyrolysis and carbonization. Pyrolysis of polyester waste was first described in the early 1980s as an alternative to PET waste 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 replacement for fossil fuels or as a source of chemicals. Carbonization is the second method for pyrolyzing polyester waste materials.

[0006] The second class of methods for recycling polyester waste materials involves simple separation of the polyester waste material, followed by use as an additive in crushed stone mastic, cementitious materials, mortar, or concrete composites. Because polyester waste can be supplied in admixture with other polymers, the polyester material must be separated from these polymers before reprocessing. Therefore, several methods have been developed, including froth flotation, wet vibrating tables, swelling, or thermomechanical procedures.

[0007] Next comes a class of methods based on (thermo)mechanical methods for recycling polyester waste materials. The simplest method of thermomechanical recycling is the remelting of separated polyester waste. This method is applied, for example, to the bottle-to-bottle technique, in which separated PET bottles are remelted in their crushed form and reprocessed into bottles for beverage packaging. Some research has been conducted on thermally reprocessed PET. During this process, the polymer is exposed to high temperatures, shear forces, and pressure. This results in thermal degradation of PET. As a result, a decrease in the thermal and mechanical properties of the reprocessed material usually occurs. 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 the mixture of polyester materials of different colors, such thermally reprocessed materials result in undesirable coloration of the recycled polyester. Therefore, the addition of complementary colors to polyester waste has been applied to hide discoloration. Although an approved procedure, this method severely limits the use of recycled polyester, especially as a food packaging material.

[0008] A fourth class of methods for recycling polyester waste is the so-called chemical recycling (chemical decomposition) method, in which the recycling of polyester waste material 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 reactants used for the chemical decomposition.

[0009] One example is the application of ionic liquids for depolymerization, first described around 2000. This method was developed to provide an environmentally friendly polymer decomposition agent and enable decomposition under moderate 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.

[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 is 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] The degradation of polyester polymers using enzymes was first described in the 1970s. This biochemical method, using ionic liquids and castor oil, was developed to provide an environmentally friendly procedure for polymer recycling, in contrast to traditional chemical recycling methods. However, the efficiency is quite low with respect to complete depolymerization of polyesters, and therefore quantitative recovery of homogeneous reaction products for reuse is not possible.

[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 and provide a renewable, more environmentally friendly decomposition agent for the polymer. Typically, polyesters are depolymerized with an excess of alcohol to yield the corresponding acid and the corresponding ester of ethylene glycol. Among alcoholysis methods, reactions using methanol have become particularly important due to methanol's low cost and availability. Ethylene glycol (a diol that falls under the alcoholysis category but whose use is sometimes categorized separately as "glycolysis") 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 also been described as useful. Other diols besides 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 art for depolymerizing polyesters by alcoholysis. However, in all cases, an undefined mixture of low molecular weight oligomers is typically obtained. Another significant downside of alcoholysis is the need for a catalyst. The most important catalysts have been zinc acetate and manganese acetate. Further catalysts include cobalt and lead acetate. The need for such a catalyst hinders the widespread application of alcoholysis for recycling polyester waste materials.

[0013] Because the reactivity of amine groups is higher than that of hydroxyl groups or alcohols used in the alcoholysis of polyesters, aminolysis and ammonolysis have been developed for recycling polyesters. However, for alcoholysis, the need for a metal catalyst remains.

[0014] Finally, an alternative chemical recycling approach for polyesters is the controlled depolymerization of polyesters using chain cleavage 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 sorted polyester material that must be free of contaminants.

[0015] While energetic and thermomechanical recycling involve downcycling of materials, chemical recycling requires significant amounts of chemicals and has the major drawbacks of high cost, variability in the end product (highly dependent on the type of waste material), the need for the use of (metal) catalysts, and the difficulty of removing colorants. Therefore, there is a need for improved methods, systems, and methods to enable the recycling of polyester waste materials. [Prior art documents] [Non-patent literature]

[0016] [Non-Patent Document 1] Geyer et al.in 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 method and system that allows for the recycling of polyester waste material into high grade polyester in an economically advantageous manner. [Means for solving the problem]

[0018] In order to meet the objectives of the present invention, a process has been developed for recycling a stream of polyester waste material by first depolymerizing polyester by alcoholysis to a liquid mixture comprising oligomeric esters having 5 to 20 subunits, and then continuously feeding this liquid mixture of oligomeric esters to a reactor in which the liquid mixture is present in combination with a gas headspace, the reactor having means for increasing the interface between the liquid mixture and the gas headspace by at least 5 times relative to a horizontal cross section of the reactor, the pressure in the headspace being maintained at less than 20 mbar, whereby the oligomeric esters are repolymerized to polyesters having an intrinsic viscosity of at least 0.3 dL / g during a residence time of the mixture in the reactor of less than 120 minutes.

[0019] The process of depolymerizing polyesters by alcoholysis to give a liquid mixture containing oligomeric esters having 5 to 20 subunits is known per se from the prior art, for example from WO 2022 / 003084 (assigned to CureTechnology BV). It is also described that for repolymerization of the oligomers, the mixture can be subjected to low pressure in a so-called condensation reactor. However, as shown, a combination of two of these condensation reactors is typically required. In the present invention, a precondensation step is no longer necessary.

[0020] The present inventors have recognized several drawbacks of known methods. First, the repolymerization process in a condensation reactor requires significant time, typically about 8 to 10 hours or more. This means that the process, in which the mixture must be maintained above its melting point, requires a large amount of energy and is therefore relatively expensive to implement. However, this has always been considered an inherent problem that cannot be avoided: a large amount of alcohol monomer must be removed to induce repolymerization, and the process simply takes time. Importantly, however, the present invention has recognized another major drawback. The oligomers resulting from the depolymerization reaction appear to be more susceptible to thermal degradation than oligomers made from virgin material. While the reason for this is unclear, the known process can result in a partially decomposed polymer that exhibits a slight yellowish discoloration, making the material less suitable, or even unsuitable, for high-end applications.

[0021] The present inventors have now surprisingly found that, despite the low intrinsic viscosity (iv) of oligomeric esters having 5 to 20 subunits, i.e., about 0.1 dL / g, such mixtures can be repolymerized by feeding them directly into a so-called finishing reactor. In traditional industrial polycondensation processes, a common feature is the use of multiple reaction stages using several reactors in series. This continuous reaction procedure involves: (a) the synthesis of monomers, low molecular weight oligomers, and low molecular weight prepolymers in a conventional condensation reactor; and (b) the prepolymers are then polymerized into high molecular weight polymers in a finishing polymerization reactor (often a rotating disk reactor; see, for example, U.S. Pat. No. 3,728,083, assigned to Allied Chemical Corporation), which provides a large vapor-liquid surface area for extracting the final alcohol (increasing the interface between the liquid mixture and the gas headspace by at least five times relative to the horizontal cross section of the reactor). It is generally known that the first stage is necessary to increase the viscosity (reduce the amount of volatile monomer) to an iv of at least 0.3, otherwise the required low pressures cannot be obtained in the finisher. A stepwise increase in viscosity to at least 0.3 is considered necessary for a stable and effective polymerization process.

[0022] However, it has now been found that the oligomeric products resulting from alcoholysis can be fed directly to such a finishing reactor, even when the polymerization level is between 5 and 20 subunits, and thus the IV is very low (approximately 0.1). Even more surprisingly, the total time required to remove volatiles can be kept below 120 minutes. This means that the new process is much more economical and requires substantially less energy, while the time required to expose materials to high melting temperatures (above 250°C) is significantly shorter, and therefore thermal degradation of these materials, if any, is at all minimal.

[0023] The reason why low-viscosity oligomeric esters can be fed directly to a finishing reactor and (re)polymerized into oligomers in a relatively short time is not fully understood. One reason is believed to be that the oligomeric esters are products of depolymerization alcoholysis, such as those known from the '084 patent application identified above. Another reason is believed to be that the process is limited to continuous feeding and repolymerization steps. Discontinuous processes have been found to be less stable and may even lead to repolymerization failure within a short time frame. The amount of polymerization of the oligomers, i.e., the number of subunits, is also important. Below 5, the amount of volatiles is too high to be removed in a single step, let alone in a finisher with a high liquid-gas interface. Above 20, the oligomers may be too viscous to be prepolymerized and purified, resulting in a low-quality product.

[0024] In the finisher, an iv of up to 0.65 can be reached in a relatively short time frame, making the material ideally suited for direct use or as a starting material for even higher grade polyester. The iv reached is only a matter of time. Also, the finisher design (pump type and capacity, outlet diameter, etc.) allows for even higher processable maximum iv.

[0025] It should be noted that instead of one large volume finisher, the finisher can consist of two or more sub-finishers in a line, as long as each of these sub-finishers has a means to increase the interface between the liquid mixture and the gas headspace by at least five times relative to the horizontal cross section of the reactor (i.e., each sub-finisher), and the pressure in the headspace is maintained below 20 mbar.

[0026] The present invention has been found to be effective for different types of polyesters because their chemical properties are essentially the same. Correspondingly, the type of alcohol is not essential for the depolymerization process itself, and therefore for obtaining 5 to 20 units of oligomeric ester for repolymerization in the finisher. Depending on the type of polyester and the type and amount of alcohol, different levels of depolymerization and repolymerization can be achieved (assuming other conditions, such as residence time, are the same), or conditions can be varied to achieve a predetermined level of depolymerization or repolymerization. This can be controlled by measuring, among other things, the viscosity of the (partially depolymerized) polyester mixture, the residence time in various reactors, and other conditions, such as temperature and pressure. For any type of polyester at various stages of depolymerization and repolymerization with a specific alcohol, a reference curve can be prepared in advance, defining, for example, the relationship between viscosity and polymerization grade, so that the required level of polymerization can be obtained.

[0027] It should be noted that methods partially utilizing the features of the present invention are known in the art. For example, CZ299244 (assigned to Sirek Milan) discloses a process for 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, 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 under continuous injection of an aqueous solution of alkali metal hydroxide and / or ammonium hydroxide. The resulting product is then used for repolymerization.

[0028] US 4,620,032 (assigned to Celanese Corporation) also discloses a two-stage depolymerization process for repolymerization into high-grade polyester, but this process is based on hydrolysis. Specifically, in the known process, polyester is intimately 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 in the first stage for a time sufficient to reduce the molecular weight of the polyester by at least 50%. The low molecular weight treated condensation polymer is then subjected to neutral hydrolysis in the second stage, resulting in complete hydrolytic depolymerization into monomeric materials that can be used for repolymerization.

[0029] WO 9720886 (assigned to Eastman Chemical Company) discloses a one-step batch process in which post-consumer or scrap polyester is reacted with glycol to depolymerize the polyester to produce monomers or low molecular weight oligomers. The monomers or oligomers can then be optionally purified using one or more of several steps including filtration, crystallization, and optionally possible adsorbent treatment or evaporation, and used in a subsequent repolymerization step to arrive at polyester.

[0030] The present invention also relates to a method for repolymerizing an oligomeric ester having 5 to 20 subunits, comprising continuously feeding a liquid mixture comprising the oligomeric ester to a reactor in which the liquid mixture is present in combination with a gas headspace, the reactor having means for increasing the interface between the liquid mixture and the gas headspace by at least 5 times relative to a horizontal cross section of the reactor, and maintaining a pressure in the headspace of less than 20 mbar, thereby repolymerizing the oligomeric ester into a polyester having an intrinsic viscosity of at least 0.3 dL / g for a residence time of the mixture in the reactor of less than 120 minutes.

[0031] The present invention also relates to a system for recycling a stream of polyester waste material comprising, in sequential order: 1) an extruder; 2) a continuous stirred tank reactor (CSTR); and 3) a finishing reactor within which liquid contents may reside in combination with a gas headspace; further comprising means for feeding a first amount of alcohol to the extruder and a second amount of alcohol into the CSTR; the reactor having means for increasing the interface between the liquid and gas headspace by at least five times relative to a horizontal cross-section of the reactor; the pressure within the headspace can be maintained at less than 20 mbar; and the extruder, CSTR, and finishing reactor are operatively connected such that a stream of polyester waste material can flow continuously from an inlet of the extruder to an outlet of the finishing reactor.

[0032] definition Polyesters are polymers in which monomer units are linked together by ester groups. They are typically formed by polymerizing polyhydric alcohols with polybasic acids and are primarily used in the production 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 component" (including its ester-forming derivatives) reacts with a monomer providing a "hydroxyl component." If desired, polyesters can contain other linking groups, such as a proportion of carbonylamino linking groups, -C(=O)-NH- (i.e., amide linking groups), or -C(=O)-NR 2-(tertiary amide linking group). Polyesters used in daily life can be aliphatic, semi-aromatic, or aromatic. Typical examples are polyglycolic acid (PGA), polylactic acid (PLA), polycaprolactone (PCL), polyhydroxyalkanoates (PHAs), 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), polyethylene furanoate (PEF), and Vectran, a polycondensation product of 4-hydroxybenzoic acid and 6-hydroxynaphthalene-2-carboxylic acid. Of these, PETE, also abbreviated as PETP or PET-P, is the most common thermoplastic polymer resin in the polyester family, and the virgin material is considered one of the most important engineering polymers of the past few decades. It is considered an excellent material for many applications, and is used in textiles for clothing, containers for liquids and food, thermoforming for manufacturing, and in combination with glass fibers for engineering resins. Polyester is also known by trade names such as Terylene, Arnite, Eastapac, Mylar, Lavsan, and Dacron. While still referred to as polyester material, polyester may 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%).

[0033] 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 high end of a fairly wide temperature range over which the polymer slowly becomes "leathery," then "sticky," and finally liquid.

[0034] Polyester waste material is post-consumer material at or after the end of its consumer life, i.e., the time when it is used by a consumer for practical or aesthetic purposes, which 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.

[0035] Depolymerization refers to the reduction of molecular weight by breaking down the original polyester molecules into molecules of shorter length, eg, oligomers.

[0036] A continuous process is an assembly line process used to process materials without interruption. In such a process, the material, e.g., dry bulk or fluid, is in continuous motion while undergoing chemical reactions and / or mechanical or thermal treatments. A continuous process is contrasted with a batch process.

[0037] An alcohol is a hydrocarbon substance or mixture of such substances formed when a hydrogen atom in a hydrocarbon is replaced with a hydroxyl group. Alcohols can be monohydric, dihydric (i.e., diols), etc.

[0038] Intrinsic viscosity is a measure of the contribution of a solute to the viscosity η of a solution; see "Progress in Biophysics and Molecular Biology" (Harding 1997). IV (or iv) can be measured according to DIN / ISO 1628. Typically used at a concentration of 1% relative to the polymer, m-cresol is used as the solvent, and IV can be expressed in dL / g (although the latter magnitude is often not given). A practical way to determine intrinsic viscosity is by using an Ubbelohde viscometer.

[0039] Two consecutive stages means that the respective first and second stages follow one another consecutively and therefore without interruption, however, this does not exclude that one or more additional intermediate process steps take place between the two stages.

[0040] A mixture is a composition in which two or more different substances are combined or blended into one mass. Mixing two compounds does not exclude the compounds from reacting while being mixed to form other compounds in the mixture.

[0041] An oligomer having 5 to 20 subunits is any oligomer from a pentamer to a decamer.

[0042] The interface between the gas and the liquid is a surface area and therefore has units of m 2 is.

[0043] The cross section of a reactor is its footprint and therefore its measurement when projected onto a horizontal plane. DETAILED DESCRIPTION OF THE INVENTION

[0044] In a first further embodiment of the process according to the invention, the liquid mixture contains oligomeric esters having 5 to 14 subunits (hence 5, 6, 7, 8, 9, 10, 11, 12, 13, or 14 subunits), preferably 8 to 10 subunits (8, 9, or 10). A higher lower limit means that the repolymerization time can be further reduced, improving the economics of the process and the quality of the resulting polymer. A lower upper limit is advantageous from the standpoint of handling the feed stream to the finishing reactor, including any purification treatments. It was even more surprising to find that the material could be directly repolymerized in the finisher within 120 minutes, despite the very low polymerization rate of 14 or fewer subunits.

[0045] In another embodiment of the method according to the invention, the pressure in the headspace is maintained at less than 10 mbar. In this way, the time required for repolymerization can be further reduced. Preferably, the pressure in the headspace is maintained at 1 to 3 mbar, for example 2 mbar.

[0046] In yet another embodiment of the method according to the present invention, the reactor has a means for increasing the interface between the liquid mixture and the gas headspace by at least 10 times, preferably at least 20-100 times, relative to the cross section of the reactor. This increase in the interface between the liquid mixture and the gas headspace means that repolymerization can occur at a faster rate. The drawback is that the reactor is typically more complex and requires a higher pumping capacity. This means the process is more expensive. Furthermore, this may be compensated for by the fact that the quality of the final product is higher, meaning a higher price can be obtained.

[0047] In yet another embodiment of the process according to the invention, the oligomeric ester is repolymerized to a polyester having an intrinsic viscosity of at least 0.3 dL / g in less than 100 minutes, preferably less than 80 minutes, and most preferably less than 60 minutes. Shorter times correspond to less thermal degradation and are therefore advantageous. The time can be gradually reduced (in the range of 1 to 60 minutes) by various means such as lower pressure, wider liquid-gas interface, etc. This is a matter of finisher design within the toolbox of a skilled artisan.

[0048] In yet another embodiment of the process according to the invention, the oligomeric ester is repolymerized to a polyester having an intrinsic viscosity of at least 0.4 dL / g, preferably at least 0.5 or even 0.6 dL / g. For a given finisher configuration, a higher iv can be obtained by applying a longer residence time. To increase the iv for a given residence time, the finisher configuration must be adapted (higher pumping capacity; increasing the liquid-gas interface, etc.).

[0049] Advantageously, the method for depolymerizing polyester in a stream of polyester waste material by alcoholysis comprises a first stage depolymerization and a separate second continuous stage depolymerization to which the first and second stage polymer waste material streams are continuously subjected, wherein in the first stage of the two continuous stages, the stream of polyester waste material 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 is co-fed to the CSTR, wherein the residence time in the CSTR is adapted to provide a continuous stream of polyester depolymerized to oligomeric esters at the outlet of the CSTR.

[0050] The process known from the above-identified '084 International Patent Application has been found to be advantageously suitable for arriving at the required liquid oligomeric ester mixture.

[0051] Preferably, in the second stage of this process, the amount of alcohol fed to the CSTR is less than 6 wt. %, preferably less than 3.5 wt. %, and even more preferably 0-2 wt. The total amount of alcohol in the first and second amounts of alcohol (thus adding up both amounts) is 3-12 wt. % based on the polyester material stream. While the '084 patent states that the upper alcohol limit is less critical in the CSTR than in the extruder, it has been found that the preferred total amount of alcohol used to depolymerize the polyester in the CSTR is preferably less than 6 wt. %, more preferably less than 3.5 wt. %, and even more preferably 0-2 wt. % based on the polyester waste material stream used in the process. However, it should be noted that the total amount of alcohol used in the entire process (and thus all alcohol used in the alcoholysis of the polyester) is preferably greater than 2.5 wt. %, particularly when MEG is used, as it is believed that the less alcohol used, the faster the repolymerization process will be. The preferred minimum amount of alcohol (i.e., 2.5%) is a feature of the present invention in its broadest scope and is therefore not limited to any particular embodiment of the use of the extruder and CSTR.

[0052] In yet another embodiment of the method according to the present invention, the two successive stages are preceded by a drying step in which the polyester waste stream is subjected to a drying process to reduce the amount of water in the stream to less than 5000 ppm (i.e., less than 0.5 wt.% water relative to the polyester waste stream), preferably 10-1000 ppm, and more preferably 20-100 ppm. The presence of water during this process inevitably results in the generation of free carboxyl groups due to hydrolysis. High levels of free carboxyl groups, i.e., more than 45 mmol / kg at the end of the first repolymerization step (i.e., the step in which the oligomeric ester is repolymerized to a polyester with an IV of typically 0.4-0.6), are less advantageous in this process. The type of pre-drying step is not particularly important. Pre-drying can be carried out in a variety of ways, for example, by simply blowing hot air at 120°C through a bed of polyester flakes or aggregates, which may already be sufficient to reduce the water content to less than 5000 ppm. However, this may still require a very low vacuum in the extruder to remove some additional moisture if necessary, so it is preferable to use more powerful methods such as a desiccant air dryer with hot air at 150°C, or a dryer applying a (low) vacuum. Advantageously, the amount of water in the stream is between 10 and 1000 ppm, preferably between 20 and 100 ppm, most preferably about 50 ppm.

[0053] In one embodiment, the alcoholysis involves the use of a diol, preferably a diol selected from the group consisting of ethylene glycol, 1,3-propanediol, butanediol, cyclohexanedimethanol, and neopentyl glycol. The use of a diol results in a further reduction in the time required for repolymerization in the finisher.

[0054] In still yet a further embodiment, in this embodiment the polyester is polyethylene terephthalate (PET) and greater than 50 w / w%, preferably greater than 60, 70, 80 or even 90 w / w% of the oligomeric ester comprises oligomers of 5 to 20 bis(2-hydroxyethyl) terephthalate (BHET) units, preferably 6 to 15 (BHET) units, most preferably 8 to 10 BHET units.

[0055] The above does not exclude any modifications to the process or additional process steps. For example, additional (pre)purification / filtration steps could be added. Furthermore, the present invention is not limited to a specific type of filter, but a screen changer-type filter is considered ideally suited to the claimed continuous process, since the filter may need to be changed every few hours depending on the amount of particulate matter present in the polyester waste material. Furthermore, if a filter is applied, it may be very well suited to a cascade of two, three, or more separate, continuous filters of decreasing mesh size to withstand the pressure difference across the filter. Additionally, to remove all colorants and dyes as completely as possible, the molten material may advantageously be pumped through a bed filled with (activated) carbon granules, SiO granules, or any other small molecule absorbent material.

[0056] It is also possible to introduce copolymerization during repolymerization in the finisher. For example, a slurry of pure diacids and pure diols can be prepared, and / or other monomers can be formed that can be added to the finisher to which the purified oligomer mixture is added. If the diacids and / or diols are different from the monomers present in the polyester waste material, copolymers are produced. Such monomers can be, for example, bio-based to further reduce the CO2 footprint, or can be, for example, isophthalic acid, succinic acid, or neopentyl glycol to achieve other product properties in the resulting polyester that are tailored to the intended application. Typically, the comonomers are prepolymerized and then fed to the finisher where they are mixed with the recycled oligomers.

[0057] Any of the above further embodiments may also be embodied in the system of the present invention.The present invention will now be further described using the following non-limiting figures and examples. [Brief explanation of the drawings]

[0058] [Figure 1] FIG. 1 shows a schematic overview of methods for depolymerization and repolymerization known in the art. [Figure 2] FIG. 1 shows a schematic diagram of a typical repolymerization setup for industrial applications. [Figure 3] FIG. 1 is a schematic diagram of a prior art configuration for depolymerizing and repolymerizing a polyester waste stream. [Figure 4] FIG. 1 is a schematic diagram of a depolymerization and repolymerization configuration according to the present invention. [Example]

[0059] Example 1 provides various experiments comparing the prior art with the present invention.

[0060] Figure 1 Figure 1 shows a schematic overview of the process for depolymerization and repolymerization known from WO 2022 / 003084. In this process, PET is depolymerized and repolymerized in a continuous process through steps 1 to 8. Step 9 is an additional repolymerization step involving solid-state polymerization to reach the desired IV well above 0.6.

[0061] This process is based on the commonly known equilibrium reaction of PET in alcoholysis based on monoethylene glycol (MEG): BHET←→[PET]x+1 / 2xMEG

[0062] In this process, adding MEG to the polyester melt shifts the equilibrium to the left, resulting in shorter polymer chains and ultimately oligomers (fewer than 100 repeating BHET units, particularly 50, 40, 30, 20, or even 10 units), which reduces viscosity. By removing the MEG, for example using vacuum or nitrogen, the short chains react with each other to form polyester again. By controlling the depolymerization rate, and therefore the oligomer length, the viscosity of the material is controlled. This chemistry is, in principle, the same for any industrial polymerization process to arrive at polyester or the corresponding oligomer.

[0063] In step 1, polyester waste material, including (pure) PET carpet scraps and PET bottle flakes, is dried to a moisture level of 50 ppm. Then, in step 2, the dried waste material stream is fed into a conical co-rotating twin-screw extruder. Due to the extruder's conical shape, the opening for feeding the material is larger than in conventional twin-screw extruders, making feeding easier and generating less shear due to the relatively gentle natural compression, which reduces thermal damage to the polymer. Thermal degradation can result in undesirable side reactions and the formation of end groups, resulting in reduced quality of the final product. The extruder is operated at 280°C to completely melt the polyester. An injection point (shown as arrow 50) for MEG injection is provided adjacent to the end of the conical twin-screw extruder (10% of its length) to achieve the first step in depolymerization and reduce viscosity. For this purpose, approximately 1% MEG (w / w) is injected. The reduction in IV also serves to minimize the pressure difference across the first filtration step 3, allowing filtration with a mesh size of 80 micrometers. The filter also acts as a static mixer to homogenize the mixture, distributing the added glycol into the molten polymer and allowing it to react completely with the shorter polymer chains, resulting in an equilibrium molecular weight distribution (dispersion grade approximately 2). The process parameters are selected so that the MEG is (almost) completely reacted and (almost) no free MEG is anymore present.

[0064] The partially depolymerized and filtered material is fed into a single-screw extruder in step 4. The screw design (see WO 2022 / 003084, Figure 2) aims to maximize the percentage of glycol that can be added (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% of the MEG is added to this 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 added to 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 that occurs in the extruder, the polydispersity can remain low, preferably around 2-3, depending primarily on the residence time in the extruder (which can be adjusted during the process by controlling the initial feed rate and extruder speed).

[0065] The depolymerized material was filtered a second time in step 5. Due to the IV of about 0.15, the filter size can be small compared to the first filter without the pressure difference across the filter being too high, preferably 40 micrometers.

[0066] 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 this CSTR (indicated by arrow 50) to further depolymerize the material to the required viscosity / oligomer length. Due to the fact that the material already has a low (controlled) viscosity upon entry, the viscosity difference with the added MEG is not so great that uniform mixing is critical. 4-6% MEG can be easily mixed uniformly. The residence time in the CSTR is long enough (typically 25-45 minutes) to depolymerize the material to the required oligomer length, but also long enough for the transesterification reaction to have sufficient time to achieve polydispersity 2. At the end of the reactor, the viscosity is measured, and an automatic control loop is used to control the addition of MEG in the reactor. This results in a very stable, continuous process that is largely independent of the type (IV) of the starting material.

[0067] In the CSTR, decolorization is accomplished by adding activated carbon, as indicated by arrow 60. The activated carbon can also be pre-selected for best performance in adsorbing 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) step 7 to remove colorant-laden carbon particles from the oligomer. A parallel set of three filters is installed, and if the pressure differential across the filters becomes too high, the first filter set can be washed while the melt is pumped through the parallel set.

[0068] After filtration while the melt, with an IV of approximately 0.1 (the number of subunits in the oligomer mixture is approximately 10 on average), is still at a high temperature of approximately 250 °C, the melt is pumped into a condensation reactor (Step 8) operated at a temperature of approximately 260 °C under a vacuum of 2 mbar to remove MEG, resulting in a shift of the BHET / PET equilibrium to the right to form PET polymer. Within the reactor, polyesters with IVs of 0.4 to 0.6 can be obtained. The polymer is removed from the reactor and pumped through a die plate with holes, thus producing polymer strands. These strands are cooled and cut into amorphous granules.

[0069] Amorphous granules can be subjected to an offline crystallization process by subjecting the granules to temperatures of 130-180°C, which results in a crystallization process. Partially crystallized granules can then be subjected to a solid-state polymerization process, in which the polyester is heated to a temperature below the polyester's melting point while under vacuum or inert gas. This induces additional solid-state repolymerization, resulting in an IV greater than 0.6. The resulting IV can be adjusted by processing parameters to match the IV required for the intended application, typically between 0.65 and 1.0.

[0070] Figure 2 Figure 2 shows a schematic of a typical repolymerization configuration for industrial applications. This configuration is the so-called two-reactor polycondensation technique, where the polymerization is carried out in two sequential polymerization reactors. The first reactor, designated by reference numeral 8, is a standard condensation reactor. The second reactor, designated by reference numeral 30, is a so-called finisher, which has a means for increasing the liquid-gas interface within the reactor by at least five times relative to the reactor footprint.

[0071] Reactor 8 is the same as that known from WO 2022 / 003084. For laboratory testing, time is not a fundamental issue, so using such a standard condensation reactor alone is often sufficient. However, in industrial applications, a standard condensation reactor is always followed by a finisher to allow for faster repolymerization. In this typical configuration, an oligomer mixture having a typical IV of about 0.1 is fed to reactor 8 (22), forming a gas-liquid interface 23 midway through the reactor. The reactor is equipped with mixing means 21 driven by motor 20. Alcohol is pumped out via means 24. In this way, the oligomer mixture can be polymerized to a higher grade (25 units or more) to an IV of at least 0.2-0.3 and then fed to, for example, finisher 30 (25) to form gas-liquid interface 230. In this example, the finisher comprises a rotary disk mixer 210 driven by motor 200. Because the gas-liquid interface is established midway through the rotating disk, the rotation of the disk and the resulting rise of the liquid film (on either side of the disk) into the open gas space results in a substantial increase in the gas-liquid interface, in this case by a factor of about 10 relative to the footprint of finishing reactor 30. In this way, the alcohol can be removed more quickly (240), resulting in a polyester stream 250 with an IV of about 0.6 in a relatively short time frame.

[0072] This two-stage arrangement is commonly used because it is known that mixtures with low iv's of about 0.1 cannot be fed directly to the finisher: most of the alcohol must be removed before entering the finisher, otherwise low pressure cannot be achieved in the finisher and therefore little polycondensation will occur. Often, even a three-stage arrangement (adding an additional condensation reactor 8 before the finisher 30) is used in industry to gradually reduce the amount of alcohol and therefore gradually increase the IV until the mixture is suitable for feeding into the final stage finisher.

[0073] Figure 3 Figure 3 shows a schematic diagram of a prior art depolymerization and repolymerization configuration for a polyester waste stream. This configuration utilizes depolymerization techniques known from WO 2022 / 003084. Specifically, as described in the Examples section of WO 2022 / 003084 (which section is incorporated herein by reference), the waste stream is fed to a first extruder 2, then to a second extruder 4, and then to a CSTR 6. This corresponds to steps 1-6 in Figure 1. The resulting oligomer mixture, having an IV of approximately 0.1, is then fed to a condensation reactor 8. The remainder of the process is identical to that shown in Figure 2.

[0074] Figure 4 Figure 4 shows a schematic diagram of a depolymerisation and repolymerisation arrangement according to the invention. In principle the arrangement is the same as that of Figure 3, although the oligomer mixture from the CSTR 6 is fed directly to the finisher 30.

[0075] [Example 1] In this example, a direct comparison is made between the prior art process shown in Figure 3 and a process using the arrangement shown in Figure 4, in which the oligomer mixture resulting from the depolymerization of a polyester waste stream is fed directly to the finisher. For this comparison, a laboratory-type finisher capable of processing only liquids up to an IV of 0.3-0.35 was used. It is estimated that approximately twice as long would be required to reach an IV of approximately 0.6 in an industrial finisher. However, this extra time is expected to be fully compensated for by the fact that lower pressures, on the order of 1-2 mbar, can be achieved in the industrial finisher, whereas in the laboratory setup this was approximately 10 mbar. This means that the time required to reach an IV of approximately 0.3 in the laboratory finisher is expected to be the same as the time required to reach an IV of approximately 0.6 in the industrial finisher.

[0076] For comparative experiments, polyester material consisting of PET granules (RamaPET N180; IV 0.8 dL / g) was melted and depolymerized using monoethylene glycol (MEG; fiber grade, obtained from Vivochem) to a liquid mixture of oligomeric esters with an average polymerization grade of 5–20 (5–20 BHET subunits per molecule) at a throughput of approximately 30 kg / h. The loading level in the CSTR was maintained at approximately 48%, so the average residence time of the partially depolymerized polyester material in the CSTR was 30 min. For these depolymerization and repolymerization trials, virgin polyester material was used as is and therefore not pre-dried.

[0077] In the first experiment, the resulting oligomers were repolymerized in the configuration shown in Figure 2, so the process was consistent with that shown in Figure 3. In a second series of experiments, the resulting oligomers were repolymerized by feeding the oligomer mixture directly into a finisher, as shown in Figure 4. In the latter series, the amount of MEG used in the depolymerization step was varied to achieve oligomers with various grades of polymerization. In this regard, it should be noted that the use of approximately 10% MEG results in x (=degree of polymerization=number of subunits in the oligomer) being approximately 8-10; with 7.5% MEG, x is approximately 11-12; with 4.5-6% MEG, x is approximately 15-16; and with 2.5% MEG, x is approximately 20.

[0078] Melting and depolymerization of the polyester material, and subsequent repolymerization of the liquid depolymerized polyester material, were carried out under the conditions shown in Table 1. For each of these experiments, the depolymerization process described in WO 2022 / 003084 was used. In the first extruder, the temperature was maintained at about 288°C in each case, and no MEG was added. The amount of MEG added to the second extruder was 2.4% in each case, and the temperature was maintained at about 268°C. The amount of MEG added to the CSTR is shown in the table above. The temperature in the decondensation reactor was about 258°C. In the finisher, the temperature was maintained at about 271°C.

[0079] [Table 1]

[0080] In Experiment 1, partial depolymerization of molten virgin polyester material proceeded smoothly over a time frame of approximately 20–25 min in the presence of 10 w / w% MEG relative to PET in total for Extruder II and the CSTR. Intermediate sampling before repolymerization indicated the formation of a white depolymerized polyester material with a degree of polymerization (DP) of approximately 8–10 (or a molecular weight Mw of 1500–1950 Daltons). The liquid depolymerized polyester oligomer was repolymerized using the configuration shown in Figure 2 to yield a pale yellow liquid recycled polyester material with an intrinsic viscosity (IV) of 0.31 dL / g (above the threshold Lab value) after approximately 9 h of polymerization. The yellow coloration indicated significant thermal degradation.

[0081] In Runs 2–16, various continuous depolymerization–repolymerization experiments were conducted by eliminating the standard condensation reactor and feeding the liquid oligomer mixture directly to the finisher, thus eliminating the contribution of a prepolymerization reactor. The resulting liquid recycled polyester material was cooled in a water bath and chopped into solid polyester granules. In these experiments, apart from assessing the total time required to reach the same degree of polymerization as in the first experiment, the effects of the amount of MEG during the depolymerization step and the residence time in the finisher on the physical properties of the resulting polymer, expressed as intrinsic viscosity (IV), and color value, were investigated. The conditions and settings of the individual reactors in the continuous depolymerization–repolymerization line, as well as the results of these experiments in terms of intrinsic viscosity (IV) and color value (L*a*b*), are listed in the table where relevant.

[0082] Runs 2 and 3 in the table show that reducing the amount of MEG in the CSTR to 5.1 wt / w%, and thus the total amount of MEG to 7.5 wt / w%, allows for the production of recycled polyester material from liquid polyester oligomer with a residence time of approximately 1 hour in the finisher. Aside from omitting the prepolymerization reactor, the 1-hour residence time is significantly shorter than the 5 hours in Run 1. Polyester granules were obtained from the recycled polyester material with an intrinsic viscosity of approximately 0.3 dL / g (the level of viscosity of the recycled polyester material obtained in the repolymerization process including the prepolymerization reactor and finisher; Run 1), but in a significantly shorter time and with significantly reduced color, indicating less thermal degradation.

[0083] For the partial depolymerization reaction to polyester oligomers in Extruder II and CSTR, shortening the residence time in the finisher to 0.5 h (see Experiments 4-8) while maintaining the total amount of MEG at 7.5 w / w% results in recycled polyester granules with higher intrinsic viscosity when compared to Experiments 2 and 3. Reducing the residence time in the finisher improves the color of the recycled polyester granules, as indicated by the lower b* value of about 5.

[0084] In Runs 9 and 10-14, the amount of MEG in the CSTR was reduced to 3.4 and 1.7 w / w%, respectively, thereby reducing the total amount of MEG used in the partial depolymerization reaction to 5.8 w / w% or less. Repolymerization of these polyester oligomers in a finisher for 1 hour and subsequent granulation resulted in recycled polyester granules with intrinsic viscosities varying from about 0.33 dL / g, slightly higher than in the preceding runs, and even higher b* values.

[0085] Increasing the residence time for the repolymerization step to 2 hours was investigated in Runs 15 and 16. The amount of MEG in the CSTR was also reduced to 0 wt%. Repolymerization of depolymerized polyester material with a DP of 20-25 for 2 hours showed a further increase in the intrinsic viscosity of the resulting recycled polyester granules to 0.35 dL / g. The polyester granules appeared yellower than in Runs 4-9, as confirmed by the higher b* value of approximately 9.

[0086] These experiments demonstrate that faster repolymerization can be achieved when using only a finisher in the repolymerization of polyester oligomers. Recycled polyester granules exhibit comparable viscosities at significantly shorter residence times. Color values ​​of recycled polyester granules indicate that shorter residence times of polyester oligomers in the finisher result in less thermal degradation.

Claims

1. 1. A process for recycling a stream of polyester waste material by depolymerizing polyester by alcoholysis to a liquid mixture comprising oligomeric esters having 5 to 20 subunits, and continuously feeding said liquid mixture of oligomeric esters to a reactor in which said liquid mixture is present in combination with a gas headspace, said reactor having means for increasing the interface between said liquid mixture and said gas headspace by at least a factor of 5 relative to a horizontal cross section of said reactor, and maintaining a pressure in said headspace of less than 20 mbar, whereby said oligomeric esters are repolymerized to polyesters having an intrinsic viscosity of at least 0.3 dL / g during a residence time of the mixture in said reactor of less than 120 minutes.

2. 10. The method of claim 1, wherein the liquid mixture comprises an oligomeric ester having 5 to 14 subunits.

3. 3. The method of claim 1, wherein the liquid mixture comprises an oligomeric ester having 8 to 10 subunits.

4. A method according to any one of claims 1 to 3, characterized in that the pressure in the headspace is maintained below 10 mbar.

5. 5. The method of claim 4, wherein the pressure in the headspace is maintained at 1 to 3 mbar.

6. 6. A method according to any one of claims 1 to 5, characterized in that the reactor has means for increasing the interface between the liquid mixture and the gas headspace over the cross section of the reactor by a factor of at least 10, preferably at least 20 to 100.

7. 7. The method of any one of claims 1 to 6, characterized in that the oligomeric ester is repolymerized to a polyester having an intrinsic viscosity of at least 0.3 dL / g in less than 100 minutes, preferably less than 80 minutes, and most preferably less than 60 minutes.

8. 8. A method according to any of claims 1 to 7, characterized in that the oligomeric ester is repolymerized to a polyester having an intrinsic viscosity of at least 0.4 dL / g, preferably at least 0.5 or even 0.6 dL / g.

9. a process for depolymerizing polyester from a stream of polyester waste material by alcoholysis, the process comprising a first stage of depolymerization and a separate, consecutive second stage of depolymerization, the stream of polymer waste material being subjected to the first and second stages successively; - in the first of the two successive stages, the stream of polyester waste material is continuously fed to an extruder operated at a temperature above the melting temperature of the polyester to produce a fluid mixture comprising a melt of at least partially depolymerized polyester, and a first amount of alcohol is co-fed to the extruder; 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, and a second amount of alcohol is co-fed to the CSTR, the residence time in the CSTR being used to provide a continuous stream of polyester depolymerized to oligomeric esters at the outlet of the CSTR; The method according to any one of claims 1 to 8.

10. 10. The method according to claim 9, characterized in that in the second stage the amount of alcohol fed to the CSTR is less than 6 w / w%, preferably less than 3.5 w / w%, even more preferably between 0 and 2 w / w%.

11. 11. A method according to claim 9 or 10, characterized in that before the two successive stages, a step is carried out in which the stream of polyester waste material is subjected to a drying process in order to reduce the amount of water in the stream to less than 5000 ppm, preferably between 10 and 1000 ppm, more preferably between 20 and 100 ppm.

12. 12. A method according to any of claims 1 to 11, characterized in that the alcoholysis comprises the use of a diol, preferably a diol selected from the group of ethylene glycol, 1,3-propanediol, butanediol, cyclohexanedimethanol and neopentyl glycol.

13. 13. A method according to any of the preceding claims, characterized in that the polyester is polyethylene terephthalate (PET) and more than 50 w / w%, preferably more than 60, 70, 80 or even 90 w / w% of the oligomeric ester comprises oligomers of 5 to 20 bis(2-hydroxyethyl) terephthalate (BHET) units, preferably 6 to 15 (BHET) units, most preferably 8 to 10 BHET units.

14. 1. A method for repolymerizing an oligomeric ester having 5 to 20 subunits, comprising continuously feeding a liquid mixture comprising said oligomeric ester to a reactor in which said liquid mixture is present in combination with a gas headspace, said reactor having means for increasing an interface between said liquid mixture and said gas headspace by at least 5 times relative to a horizontal cross section of said reactor, and wherein the pressure in said headspace is maintained at less than 20 mbar, thereby repolymerizing said oligomeric ester to a polyester having an intrinsic viscosity of at least 0.3 dL / g for a residence time of said mixture in said reactor of less than 120 minutes.

15. 1. A system for recycling a stream of polyester waste material, comprising, in sequential order: 1) an extruder; 2) a continuous stirred tank reactor (CSTR); and 3) a finishing reactor in which liquid contents may be present in combination with a gas headspace; further comprising means for feeding a first amount of alcohol to the extruder and a second amount of alcohol into the CSTR; the reactor having means for increasing the interface between the liquid and the gas headspace by at least five times relative to a horizontal cross section of the reactor; the pressure in the headspace can be maintained at less than 20 mbar; and the extruder, CSTR, and finishing reactor are operatively connected such that a stream of polyester waste material can flow continuously from an inlet of the extruder to an outlet of the finishing reactor.

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