Method for reprocessing waste containing polyalkylene terephthalate

Abstract

The invention relates to a method for reprocessing waste containing polyalkylene terephthalate in a continuous process by means of depolymerisation, comprising the following method steps: producing a reaction mixture by adding a hydroxide in solid form and / or an aqueous hydroxide solution to the waste, wherein no further reactive components are added to the reaction mixture, and continuously conveying the reaction mixture for depolymerisation through a planetary roller extruder. In this manner, a particularly efficient method for reprocessing waste containing in the polyalkylene terephthalate, in particular polyethylene terephthalate and / or polybutylene terephthalate, in a continuous process by means of depolymerisation is provided.

Description

[0001] Processes for the reprocessing of waste containing polyalkylene terephthalate

[0002] The invention relates to a process for the reprocessing of waste containing polyalkylene terephthalate in a continuous process by means of depolymerization comprising the following process steps:

[0003] Producing a reaction mixture by adding a hydroxide to the waste, wherein no further reactive components are added to the reaction mixture, and

[0004] Continuous pumping of the reaction mixture for depolymerization.

[0005] The terms "waste containing polyalkylene terephthalate" and "waste containing polyalkylene terephthalate" refer to all materials containing polyalkylene terephthalate that can be reprocessed (recycled).

[0006] EP 3790927 B1 describes a process for the depolymerization of waste containing polyalkylene terephthalate using alkali or alkaline earth hydroxides such as sodium hydroxide. The reprocessing of polyalkylene terephthalate, particularly polyethylene terephthalate (PET), from waste has long been a significant challenge. Previous PET depolymerization methods include techniques such as hydrolysis under elevated pressure and temperature, described, for example, in US documents 3,120,561 and 4,578,502. However, these methods are energy-intensive and require extensive safety monitoring, especially when using chemicals such as ammonium hydroxide (US 4,542,239) or sulfuric acid (US 4,355,175). Furthermore, such processes are often carried out in batches, which further limits their economic viability.

[0007] Recent developments, such as those described in US 2017 / 0152203 A1, involve depolymerization at lower temperatures in solvent mixtures like dichloromethane and methanol. While these approaches require lower temperatures, the solvents used are often environmentally hazardous. Other processes utilize glycolysis in extruders at high temperatures, but without directly recovering terephthalic acid (TPA). The challenge of effectively reprocessing multilayer packaging also remains.

[0008] The document "Michael Batton: Adhesive Processing using a Planetary Extruder," ENTEX Rust & Mischke GmbH, states that the large contact area, highly efficient and precisely controllable heating and cooling capabilities, combined with a modular design concept for optimal process control, make the planetary roller extruder (PRE) particularly suitable for replacing conventional batch mixing processes (e.g., Banbury mixers). The PRE offers further advantages: It enables the separation of thermal energy and mechanical or shear energy, allowing for their independent control. Through individual control of process parameters and its modular design (building block concept), this economical process allows for the production of complex pressure-sensitive adhesives in a single, controlled process step.

[0009] Multilayer packaging materials, commonly used in the food industry, consist of various polymers, such as PET combined with polyethylene (PE) or polyamide (PA). These materials offer mechanical stability and protective functions, but are difficult to recycle due to the strong bonds between the layers. Procedures such as those described in WO 2003 / 104315 A1 or WO 2003 / 070376 A1 propose specific separation methods, but are limited to certain materials or layer systems. Most of this packaging is therefore either thermally recycled or landfilled, resulting in a loss of valuable resources.

[0010] Given the aforementioned limitations and challenges, the development of an economical, sustainable, and efficient process for reprocessing waste containing polyalkylene terephthalates remains a pressing need. The aim is to develop approaches that enable the efficient separation and recovery of the individual components without high temperatures, pressures, or environmentally harmful chemicals. The need for such processes is further reinforced by the growing demand for sustainable recycling methods and regulatory requirements to reduce landfill waste and cross-contamination (CC) emissions.

[0011] Against the background described above, the present invention aims to provide a particularly efficient process for the continuous reprocessing of waste containing polyalkylene terephthalate, in particular polyethylene terephthalate and / or polybutylene terephthalate, by means of depolymerization. This process should be capable of chemically recycling colored and non-colored multilayer / multicomponent systems, with and without additives, almost completely into their starting materials at high throughput, in order to produce new polyalkylene terephthalate products from the recycled materials.

[0012] This problem is solved by the subject matter of claim 1. Preferred embodiments are found in the dependent claims.

[0013] According to the invention, a process for the reprocessing of waste containing polyalkylene terephthalate in a continuous process by means of depolymerization is thus provided with the following process steps:

[0014] Producing a reaction mixture by adding a hydroxide in solid form and / or an aqueous hydroxide solution to the waste, without adding any other reactive components to the reaction mixture, and continuously conveying the reaction mixture through a planetary roller extruder for depolymerization.

[0015] The present invention offers advantages attributable to the specific design of the process using a planetary roller extruder. Integrating a planetary roller extruder into the process enables particularly efficient and homogeneous depolymerization of polyalkylene terephthalate-containing waste. The planetary roller extruder is characterized by its unique design, which facilitates intensive mixing and uniform mass transport. It consists of a central main shaft surrounded by several smaller, planetarily arranged rollers. These planetary rollers rotate not only around their own axis but also around the central shaft, resulting in a complex shearing and mixing motion. The extruder's design ensures optimal mixing of the reaction mixture, allowing the reactants to remain in constant contact and enabling the chemical reactions to proceed efficiently.Furthermore, the high shear force ensures that even poorly soluble components, such as colorants or other additives, can be effectively integrated into the process. In addition, the short residence times and the resulting short reaction times in the planetary roller extruder reduce side reactions of other components, such as colorants or additives.

[0016] Another key advantage of the planetary roller extruder is its ability to achieve high throughput while maintaining process control. The continuous feeding of the reaction mixture creates a consistent reaction environment, increasing the efficiency and reproducibility of the process. Furthermore, the extruder allows for precise temperature and pressure control along the reactor vessel, which is crucial for optimizing the depolymerization reaction. Compared to conventional reactors, which often operate in batches, the continuous operation eliminates the time-consuming filling and emptying steps, significantly improving overall process productivity.

[0017] The planetary roller extruder preferably operates with a central spindle speed of 40 to 200 rpm and at an extrusion temperature of 30 to 197 °C.

[0018] The invention comprises several preferred embodiments that can contribute to further improvement of the process. In particular, it is preferably provided that the polyalkylene terephthalate-containing waste comprises polyethylene terephthalate (PET) and / or polybutylene terephthalate (PBT) and / or polyethylene terephthalate isophthalate. These materials are of particular interest for recycling due to their widespread use in packaging, bottles, and technical components, as they enable high purity and recyclability of the recovered starting materials. Another preferred aspect of the invention is that the hydroxide used is alkali hydroxide and / or alkaline earth hydroxide, which is added to the waste in solid or aqueous solution.

[0019] The invention encompasses numerous further embodiments of the process that further improve its efficiency, flexibility, and sustainability. Each of these embodiments offers specific advantages and can contribute to improving the depolymerization process:

[0020] Comminution of the waste before preparing the reaction mixture significantly improves the reaction kinetics. Among other methods, the following can be used for comminution: mechanical comminution by shredding, cutting, guillotine shearing, tearing, and / or milling. The smaller particle size increases the specific surface area of ​​the waste, which facilitates contact with the reactants and increases the rate of the chemical reactions. This leads to more efficient depolymerization and a more uniform conversion of the polyalkylene terephthalates. In the case of PET recycling from polyester textiles, comminution can be followed by granulation or pelletizing to establish or improve the dosing capability of the waste into the reaction vessel. This can be done, for example, with a flat die press. Furthermore, this improves the conveyability.Preferably, the material is reduced to a maximum size of 10 mm, and most preferably to a maximum size of 3 mm. Alternatively, the following methods can be used: compaction by granulation, pelletizing, agglomeration, compression molding, press agglomeration and / or compaction.

[0021] Carrying out the depolymerization at a temperature below the decomposition point of polyalkylene terephthalate and / or below the boiling point of monoethylene glycol, preferably at about 110 to 160 °C, offers significant advantages. These moderate temperatures minimize the thermal stress on the reaction mixture, prevent unwanted side reactions, and reduce the energy consumption of the process.

[0022] Introducing an inert gas, preferably nitrogen, into the reactor vessel protects the reaction mixture from oxidative influences. This improves the stability of the chemical reactions and increases the yield of high-quality products. Nitrogen as an inert gas is readily available and economical to use.

[0023] Adding water to the reaction discharge to dissolve solid components facilitates the separation and further processing of the products. Dissolved solids can thus be efficiently removed or processed further. This increases the flexibility of the process and contributes to the recovery of high-quality recycled products.

[0024] Filtering out the solids from the reaction discharge allows for the targeted separation of the desired products from byproducts. This significantly improves the purity of the recovered terephthalic acid and other reaction products, optimally preparing them for further applications.

[0025] The addition of an acid to the reaction discharge to convert the formed carboxylate ions into free acids further increases the purity of the products. This measure contributes to the direct recovery of high-quality acids that can be reused or further processed without additional processing steps. Prior to the addition of acid, purification using an adsorbent such as activated carbon is preferably carried out.

[0026] The process can include the following further process step: as a lubricant, wherein the alkylene glycol is preferably an alkylene glycol producible as a product of the desired depolymerization. The addition of alkylene glycol to the reaction mixture in a deficient amount is understood here to be consistent with the patent requirement that no further reactive components be added to the reaction mixture, since the added alkylene glycol, apart from side reactions such as oxidation due to thermal stress, re-emerges unchanged from the extrusion. The alkylene glycol used is preferably monoethylene glycol.

[0027] Alkylene glycol can be removed from the resulting reaction residue by evaporation. This facilitates product recovery and purification. Evaporation is an efficient separation method that improves the purity of both the recycled alkylene glycol and the remaining solids. Alternatively, distillation-based isolation of the alkylene glycol is possible at the end of the process, i.e., after acid precipitation.

[0028] Using the recovered products in a subsequent polymerization step to produce recycled PET, PBT, or polypropylene terephthalate closes the recycling loop. This allows high-quality products to be manufactured from the recycled materials, reducing the need for primary raw materials and sustainably lowering environmental impact.

[0029] The following section describes various application examples to further explain the recycling process. The drawing illustrates this.

[0030] Fig. 1 schematically illustrates the sequence of a method according to a first embodiment of the invention.

[0031] Fig. 2 schematically shows the sequence of a method according to a second embodiment of the invention,

[0032] Fig. 3 schematically illustrates the sequence of a method according to a third embodiment of the invention and

[0033] Fig. 4 schematically illustrates the process according to a fourth embodiment of the invention. The embodiments of the process according to the invention described below enable the recycling of polyethylene terephthalate (PET) waste that was previously not recyclable or only thermally recyclable. The process can also be used for the recycling of other polyalkylene terephthalates such as polybutylene terephthalate.

[0034] As illustrated by example in Fig. 1, PET-containing waste, including multi-layer systems such as beverage bottles, detergent bottles (opaque, clear, or black), or other types of food packaging such as salad trays, sausage and cheese packaging, or PET-containing production waste, as well as PET-containing textile waste such as polyester, polycotton, and other blended fabrics, such as polyester-elastane, polyester-nylon, or others, are washed and reduced to a particle size of less than 10 mm in a first step. The waste can then be pre-dried to reduce the water content of the PET material. Alternatively, as shown in Fig. 1, the material to be processed is directly processed according to the inventive method, thus eliminating the need for drying. In some cases, however, more intensive drying may be advantageous.

[0035] In a second process step, "depolymerization," the waste is fed into a planetary roller extruder. In this extruder, the saponification or depolymerization reaction of the PET is carried out continuously. In the plant described with reference to Fig. 1, for example, 6.66 kg / h of PET-containing waste, 3.33 kg / h of sodium hydroxide, and 2 kg / h of monoethylene glycol (MEG) are processed. Water is also added, in approximately the same quantity as the solids (PET and NaOH). The ratio of sodium hydroxide to PET waste is adjusted during the process to maintain a constant stoichiometric ratio of approximately 2, based on the constitutional repeating unit of PET. The reaction discharge from the planetary roller extruder consists of disodium terephthalate, MEG, water, unreacted portions of the sodium hydroxide, and residual materials from the PET waste, such as... B. dyes, degradation products, and other polymers such as PE, PP and PS.The planetary roller extruder has a modular design and consists of individually temperature-controlled segments, enabling precise control of temperature and reaction conditions. The planetarily arranged rollers rotate around a central main shaft, and their shear action generates intensive mixing of the reaction mixture. This design allows for effective mechanical stress on the waste material, promoting the depolymerization reaction by breaking the bonds between layers in multi-component materials.

[0036] The extruder modules are configured so that the feedstocks are drawn into the extruder and mechanically processed along the zones. At the end of the planetary roller extruder, the reaction discharge is conveyed through a specially designed opening. During the process, MEG is continuously removed by distillation, further increasing the efficiency of the process. The entire apparatus is operated under an inert gas atmosphere to protect the reactions from oxygen exposure and ensure high product purity.

[0037] The specific properties of the planetary roller extruder, particularly its uniform and high shear and mixing action as well as the precise control of reaction conditions, ensure homogeneous mixing of the solids. This leads to almost complete saponification of the PET content in the waste. The average residence time in the extruder is approximately two minutes, during which a conversion of 92–97% of the PET is achieved. The process also enables high chemical selectivity.

[0038] The steps described so far take place in the extruder, which is indicated in the figures by the fact that these steps are framed.

[0039] The MEG vapors are condensed and collected in a condenser. In the "dissolving" step, the reaction residue is dissolved in water in a stirred tank, mixing pump, homogenizer, or screw mixer, allowing insoluble residues (e.g., PE, PP, metals, cardboard) to be separated by filtration. This is followed by the "filtration" and "purification" steps.

[0040] In the subsequent step, "TPA precipitation," the purified solution is treated with sulfuric acid. The precipitated terephthalic acid (TPA) is recovered by filtration, washed with water, and purified of residual sulfuric acid and sodium sulfate. The recovered TPA can be used for the production of new polyalkylene terephthalates. This is followed by the steps "S / L separation" (separation of solid and liquid components), "washing," and another "S / L separation" step, ultimately yielding TPA.

[0041] The invention will now be explained in more detail using several examples.

[0042] Example 1

[0043] In a planetary roller extruder with a 70 mm internally toothed bushing, 15 kg / h of PE-coated PET flakes and 6.5 kg / h of sodium hydroxide are continuously fed under an inert gas atmosphere. These feed rates allow for the maintenance of a constant NaOH / PET molar ratio of approximately 2, based on the constitutional repeating unit of PET. Additionally, 21.5 kg / h of water and 4.2 kg / h of MEG are added to the extruder. It is crucial that the water is added only after the reaction, at the end of the extruder. It is not a reactive component but serves to quench or dissolve the reaction mixture. This applies to all examples described here. The extruder housing temperature is set between 50 and 160 °C. The planetary roller extruder operates at a central spindle speed of 100 rpm. Samples show a PET conversion of over 90%.The extruder discharge is then dissolved in water, followed by solid-liquid separation. The solution is then purified and terephthalic acid (TPA) is precipitated by adding an acid.

[0044] Example 2: Using a similar process, a heterogeneous input stream of waste is processed, containing, among other things, PET mixed with other components such as elastane, polyamide, or cotton. 15 kg / h of the heterogeneous input stream, 6.5 kg / h of sodium hydroxide, 4.2 kg / h of MEG, and 21.5 kg / h of water are fed into the planetary roller extruder. The housing temperature is set to 50–160°C, and the extruder's central spindle rotates at 60 rpm. The PET conversion is over 90%. The extruder discharge is then dissolved in water, followed by solid-liquid separation. The solution is then purified, and terephthalic acid (TPA) is precipitated by adding an acid.

[0045] Example 3

[0046] In a planetary roller extruder with a 30 mm internally toothed bushing, 1.2 kg / h of PE-coated PET flakes and 0.6 kg / h of sodium hydroxide are continuously fed under an inert gas atmosphere. These feed rates allow for the maintenance of a constant NaOH / PET molar ratio of approximately 2, based on the constitutional repeating unit of PET. Additionally, 2 kg / h of water and 0.3 kg / h of MEG are added to the extruder. The extruder housing temperature is set between 50 and 160 °C. The planetary roller extruder operates at a central spindle speed of 120 rpm. Samples show a PET conversion of over 90%. The extruder discharge is then dissolved in water, followed by solid-liquid separation. The solution is then purified, and terephthalic acid (TPA) is precipitated by the addition of an acid.

[0047] Example 4

[0048] In a planetary roller extruder, 45 kg / h of PE-coated PET flakes, 19.8 kg / h of sodium hydroxide, 10.1 kg / h of MEG, and 78 kg / h of water are continuously fed. These feed rates enable a stoichiometric NaOH / PET ratio of at least 2. The housing temperature is set between 10 and 150 °C, and the central spindle speed is 200 rpm. The conversion rate is over 90%. While the previously described examples 1 to 4 correspond to the process shown in Fig. 1, the process of example 5, described below, is shown in Fig. 2.

[0049] Example 5

[0050] A planetary roller extruder with a 70 mm internally toothed bushing is continuously fed 15 kg / h of PE-coated PET flakes and 13.2 kg / h of sodium hydroxide solution under an inert gas atmosphere. Additionally, 15 kg / h of water and 2.2 kg / h of MEG are added to the extruder. The extruder housing temperature is set between 50 and 160 °C. The planetary roller extruder operates at a central spindle speed of 180 rpm. Samples show a PET conversion of over 95%. The extruded material is then dissolved in water, followed by solid-liquid separation. The solution is then purified, and terephthalic acid (TPA) is precipitated by the addition of an acid.

[0051] The process of example 6 described below is shown in Fig. 3.

[0052] Example 6

[0053] A planetary roller extruder with a 70 mm internally toothed bushing continuously feeds 15 kg / h of PE-coated PET flakes and 13.2 kg / h of sodium hydroxide solution under an inert gas atmosphere. An additional 15 kg / h of water is added to the extruder. This example does not use MEG. The extruder housing temperature is set between 50 and 160 °C. The planetary roller extruder operates at a central spindle speed of 180 rpm. Samples show a PET conversion of over 90%. The extruder discharge is then dissolved in water, followed by solid-liquid separation. The solution is then purified, and terephthalic acid (TPA) is precipitated by adding an acid.

[0054] As shown in Fig. 4, it is also possible to avoid the "dissolving" step outside the extruder, i.e., to perform the dissolving process directly within the planetary roller extruder. In a planetary roller extruder with a 70 mm internally toothed bushing, 15 kg / h of PE-coated PET flakes and 6.5 kg / h of sodium hydroxide are continuously fed under an inert gas atmosphere. An additional 110 kg / h of water is added to the end of the extruder. The extruder housing temperature is set between 50 and 160 °C. The planetary roller extruder operates with a central spindle speed of 100 rpm. Samples show a PET conversion rate of over 90%. The extruder discharge is separated using a solid-liquid separation process. The filtrate is then purified, and terephthalic acid (TPA) is precipitated by adding an acid.

[0055] In a planetary roller extruder with a 70 mm internally toothed bushing, 15 kg / h of PE-coated PET flakes and 13.2 kg / h of sodium hydroxide solution are continuously fed under an inert gas atmosphere. An additional 100 kg / h of water is added to the end of the extruder for dissolving. The extruder housing temperature is set between 50 and 160 °C. The planetary roller extruder operates at a central spindle speed of 180 rpm. Samples show a PET conversion of over 90%. The extruder discharge is separated using a solid-liquid separation process. The filtrate is then purified, and terephthalic acid (TPA) is precipitated by adding an acid.

[0056] This process enables the efficient processing of multilayer PET-containing waste and textiles with multiple polymer components at high throughput and high quality. The planetary roller extruder plays a central role here, as its design ensures homogeneous mixing and efficient mechanical processing, which are essential for depolymerization.

[0057] The waste may contain duo- or multi-layer systems and textiles made of one or more polymers.

[0058] The waste can contain other polymers and natural materials, making the process particularly flexible for different waste streams. The process is especially suitable for multilayer materials and textiles containing one or more layers of ethylene-vinyl alcohol copolymer (EVOH), cardboard, polyamide (PA), polyethylene (PE), polypropylene (PP), or polystyrene (PS).

[0059] Saponification takes place under precise control of temperature and pressure conditions in the planetary roller extruder to ensure efficient depolymerization under moderate conditions.

Claims

Patent claims 1. Process for the reprocessing of waste containing polyalkylene terephthalate in a continuous process by means of depolymerization with the following process steps: Producing a reaction mixture by adding a hydroxide in solid form and / or an aqueous hydroxide solution to the waste, without adding any other reactive components to the reaction mixture, and continuously conveying the reaction mixture through a planetary roller extruder for depolymerization.

2. The method of claim 1, wherein the waste is crushed prior to the preparation of the reaction mixture in order to improve its conveyability and increase its surface area.

3. A method according to any of the preceding claims, wherein the depolymerization is carried out at a temperature below the decomposition point of the polyalkylene terephthalate and / or below the boiling point of monoethylene glycol.

4. Method according to one of the preceding claims, wherein inert gas is introduced into the reactor vessel.

5. Method according to any of the preceding claims, wherein water is added to the reaction discharge to dissolve solid components.

6. A method according to any of the preceding claims, wherein solids are filtered out from the reaction discharge obtained.

7. A method according to any one of the preceding claims, wherein an acid is added to the reaction residue obtained in order to convert carboxylate ions formed during depolymerization and contained in the reaction residue into acid.

8. A method according to any one of the preceding claims, wherein the hydroxide comprises alkali hydroxide and / or an alkaline earth hydroxide, preferably in solid or dissolved, particularly aqueous, form.

9. The method of claim 8, wherein the alkali hydroxide or the alkaline earth hydroxide is added in such a mass flow that the stoichiometric ratio of the alkali hydroxide or the alkaline earth hydroxide to the polyalkylene terephthalate, based on a constitutional repeating unit, is at least 2, preferably at least 2.

4.

10. Method according to any of the preceding claims, comprising the following further method step: Addition of alkylene glycol to the reaction mixture in a deficit as a lubricant, wherein the alkylene glycol is preferably an alkylene glycol that can be produced as a product of the desired depolymerization.

11. The method of claim 10, wherein alkylene glycol is removed from the reaction discharge obtained by evaporation.

12. Method according to one of the preceding claims, wherein the obtained products are polymerized in a polymerization step to produce recycled PET products or recycled PBT products or recycled polypropylene terephthalate products.

13. Use of a product obtained by a process according to any of the preceding claims for the production of recycled PET products or recycled PBT products or recycled polypropylene terephthalate products by polymerization.

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