Method for recycling polyester containers
Patent Information
- Application Number
- EP2023723604
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2022-05-13
- Filing Date
- 2023-05-12
- Publication Date
- 2025-03-19
AI Technical Summary
Existing polyester recycling processes using SSP technology are limited in removing non-volatile contaminants like PVC and PS, leading to benzene formation, and have long residence times, resulting in suboptimal rPET quality and increased costs.
Integrating a liquid phase polycondensation (LSP) process between extrusion and granulation steps, followed by SSP, to degrade benzene-forming precursors and reduce residence times, ensuring almost benzene-free rPET pellets with improved intrinsic viscosity.
The combined LSP-SSP process significantly reduces residence times by at least 33%, produces high-quality, almost benzene-free rPET pellets, and lowers investment costs, making the process more efficient and cost-effective while maintaining the quality of rPET suitable for food containers.
Smart Images
Figure IMGF000011_0001
Abstract
Description
[0001] Process for recycling polyester containers
[0002] Field of the invention
[0003] The invention relates to a method for recycling polyester containers according to the preamble of claim 1 and a plant for implementing the method, as well as recycled polyester pellets which are produced by the method.
[0004] State of the art
[0005] Previous PET recycling processes typically use SSP (solid-state polycondensation) technology to decontaminate rPET or build up the intrinsic viscosity (IV) of the recycled material. Volatile substances such as acetaldehyde can be efficiently removed, and yellowing of the resulting rPET granules is minimal. However, SSP technology can only remove non-volatile contaminants, especially so-called precursor substances that can lead to the formation of benzene, to a limited extent, even though the process takes at least 360 minutes. The most common precursor substances are the polymers PVC and PS.
[0006] Object of the invention
[0007] The disadvantages of the described prior art give rise to the task of improving the known recycling process, which uses SSP technology, in such a way that non-volatile substances are increasingly depleted or reacted and are thus considered unproblematic in the further process.
[0008] Description
[0009] The stated problem is solved in a process for recycling polyester containers, in particular PET containers, by the features stated in the characterizing portion of patent claim 1. Further developments and / or advantageous embodiments are the subject of the dependent patent claims.
[0010] The invention is preferably characterized in that between the process steps of extrusion of the flakes, including a first melt filtration e, and granulation of the melt to form pellets f, a liquid state polycondensation (LSP) i of the melt is applied as a further process step, whereby the process comprises a combination of an LSP and an SSP. The decontamination in the LSP (t < 30 min, T > 260°C) leads to the fact that, in addition to the free benzene, the formation reactions (= degradation reactions of, for example, PVC and PS) to form benzene are also further advanced. This process thus actively promotes or enforces the formation of benzene and depletes the precursor substances accordingly so that they are no longer found in subsequent process steps. At the same time, an initial purification of the free benzene components takes place.Subsequent decontamination in the SSP can remove any remaining free benzene (boiling point = 80°C) and acetaldehyde (boiling point = 20°C) from the granulate. The arrangement of the LSP between the process steps of extrusion (first melt filtration) and granulation of the melt is essential, as this is the only way for the process to function. Fundamentally, a melt is required for the LSP reactor to operate - in this respect, extrusion is necessary beforehand. However, this melt must only lie within a certain viscosity range so that the mass can be handled at all. If the viscosity were too high (e.g. after a previous SSP treatment), the melt could not be handled mechanically because it would be too viscous. If the arrangement were SSP - LSP (instead of LSP - SSP), there would be two extrusion processes includingGranule production requires: First, the flakes would have to be melted and granulated before SSP so that SSP can be carried out on the granules. Second, this granule would have to be melted again before LSP and granulated again after LSP.
[0011] The described arrangement of the LSP is therefore essential so that the process can run as energy-efficiently as possible and, above all, so that investment costs can be kept low.
[0012] In the process according to the invention, the residence times of the SSP and the LSP do not add up synergistically, but can be significantly reduced. The residence times previously required for both technologies can be reduced by at least 33% (ti_sp < 20 min; tssp < 4h), which means that the process quickly pays for itself despite the increased investment for an SSP and an LSP reactor. Furthermore, the produced rPET pellets are virtually benzene-free and therefore of significantly better quality than standard rPET granules. In this context, "benzene-free" not only describes that the rPET granules contain hardly any benzene, but also that products made from these granules (e.g., PET blanks) have only slightly higher benzene levels than products made from virgin material.In a particularly preferred embodiment of the invention, the intrinsic viscosity (IV) of the melt is increased by the LSP (i) to up to 0.95 dl / g and a further increase in the IV of the pellets occurs by the SSP (h) to up to 1.3 dl / g. The increase in IV can therefore take place in 2 stages and still achieve the target viscosity with an acceptable residence time. For this special application, the invention is characterized in that the LSP i lasts a maximum of 1 h and the SSP h preferably lasts 14 h and particularly preferably 10 h. With these selected residence times, which are longer than in the "standard process", the IV can be increased to up to 1.3 dl / g.The residence time can therefore be significantly reduced compared to the processes used according to the state of the art to achieve a final IV of 0.8-0.9 dl / g (standard applications) and final IV of 1.2-1.3 dl / g (EBM-PET applications), making the present process efficient and accordingly cost-optimized.
[0013] It has proven useful to subject the melt to a second melt filtration step with a mesh size of 50-150 pm after LSP (i). This allows for the additional removal of any remaining solid contaminants or those newly formed during LSP, thus improving the optical quality of the material.
[0014] The first melt filtration after extrusion is preferably carried out with a mesh size of 30 - 100 pm, which reliably removes solid impurities from the melt.
[0015] In a further preferred embodiment of the invention, the LSP i before the second melt filtration lasts 10 to 15 minutes. The residence time can be kept particularly short because further polycondensation takes place in the SSP reactor. This minimizes the buildup of acetaldehyde.
[0016] It is particularly preferred if degradation reactions of benzene precursors take place in the melt during LSP i, thereby preventing the formation of benzene in the pellets and the polyester containers produced from them. The formation reactions (= degradation reactions of PVC and PS) are driven forward during LSP. The process actively promotes benzene formation in order to prevent it in subsequent process steps. For example, no benzene can form in the preform injection-molded from the rPET pellets because no precursors are present. Advantageously, any benzene present is removed from the pellets during SSP h. As a result, the produced rPET pellets are almost benzene-free because the LSP causes the benzene precursors to react to form benzene beforehand, and all of the benzene is depleted by the SSP.
[0017] A further aspect of the invention relates to a plant for implementing the process as described above. The invention is also characterized in that the plant comprises an LSP reactor and an SSP, each with a throughput of 450 to 2500 kg / h. To achieve such a throughput, the SSP reactor has a volume of approximately 11 m 3 At this throughput in the continuous process, the increased space requirements and increased investment costs are manageable. The investment is quickly offset by the shortened residence times in the two reactors and the improved quality of the rPET pellets, which can also be used in the food industry.
[0018] Another aspect of the invention relates to recycled polyester pellets, in particular rPET pellets, produced by the process described above. Since the pellets are virtually benzene-free, they are particularly well suited for the production of food containers, especially rPET bottles for sensitive applications such as mineral water.
[0019] Preforms can be advantageously injection molded from the recycled polyester pellets, which are then stretch blow molded into containers.
[0020] Detailed description of the state of the art and an embodiment
[0021] Known processes for producing recycled polyester pellets are based on solid-state polycondensation (SSP) in a reactor. This increases the intrinsic viscosity and effectively depletes any benzene or acetaldehyde present in the return flow from polyester containers.
[0022] Some return streams for post-consumer PET recycling are contaminated with non-PET components, particularly foreign polymers such as PS and PVC. Despite all the sorting and washing steps in the recycling plant, it repeatedly happens that components (< 50 ppm) of PVC and PS penetrate into the extrusion and decontamination stages. The degradation of these foreign polymers during the recycling process can, among other things, cause benzene formation, which mainly occurs in the melt phase. Subsequent decontamination in the SSP (t = 6 h; T > 195°C, p < 10 mbar) means that any free benzene present (boiling temperature = 80°C) can be removed from the granulate - but no further depletion of the benzene-forming substances (precursors) PS and PVC can take place.
[0023] Typical process values of SSP technology are:
[0024] Process flow:
[0025] • Washed flakes as input
[0026] • Extrusion (optional including degassing)
[0027] • Melt filtration
[0028] • Crystallization
[0029] • SSP reactor (T » 195 - 220°C, t > 6h, p < 10 mbar)
[0030] • Final granulate is available
[0031] IV-Build:
[0032] • rather slow, typically 0.02 dl / g per hour for granules
[0033] • Inlet IV before extrusion ~ 0.65 - 0.75 dl / g
[0034] • IV after extrusion ~ 0.60 - 0.72 dl / g
[0035] • IV after SSP ~ 0.72 - 0.88 dl / g (with a residence time of 6 - 8h)
[0036] Benzene after decontamination:
[0037] • Very low, typically < 0.1 ppm according to the Fraunhofer method
[0038] • Temperature in SSP is too low to degrade PVC, for example
[0039] Benzene in the preform:
[0040] • Low to high (if benzene forming substances are present in the input stream) - strongly dependent on the quality of the flakes used.
[0041] • Benzene typically 0.1 - 0.9 ppm according to the Fraunhofer method
[0042] • “Precursor” substances such as PVC are degraded during the injection molding process and lead to renewed benzene formation
[0043] The SSP process has been on the market for decades and is therefore highly sophisticated. To gain market share, new companies entering the recycling market developed liquid-phase polycondensation (LSP). LSP shares similarities with the melt-to-resin process used to produce new PET. While LSP technology struggles with yellowing and higher acetaldehyde levels, the process is very fast (» 30 min).
[0044] Typical process values of LSP technology are:
[0045] Process flow:
[0046] • Washed flakes as input
[0047] • Extrusion (optional including degassing)
[0048] • Melt filtration
[0049] • Melt pump for transport to the LSP reactor
[0050] • LSP reactor including agitator (T > 265°C, p < 10 mbar, t ~ 25 - 30 min)
[0051] • Melt pump for discharging the material
[0052] • Filtration
[0053] • Granulation
[0054] • Crystallization
[0055] • Post-treatment in “dealdehydization”: Flow of hot air through the material (200 - 500m 3 / h; 150 - 165°C; 10 - 20h)
[0056] • Final granulate is available
[0057] IV structure:
[0058] • very fast, up to 0.01 dl / g per minute
[0059] • Inlet IV before extrusion ~ 0.65 - 0.75 dl / g
[0060] • IV after extrusion ~ 0.60 - 0.72 dl / g
[0061] • IV after LSP ~ 0.78 - 0.90 dl / g (with a residence time of 25 - 30 min)
[0062] Yellowing:
[0063] • Is rather high due to long residence time in the melt + oxidation in dealdehydization
[0064] • Typically 2 - 5 in b* higher than the same material from the SSP process (depending on the quality of the flakes used)
[0065] AA values (acetaldehyde content; according to Fraunhofer method): 4 - 12 ppm before dealdehydization
[0066] • 0.5 - 4 ppm after dealdehydization (10 - 20 h residence time)
[0067] • Target should be < 2ppm
[0068] Benzene after decontamination:
[0069] • Very low and typically < 0.1 ppm according to the Fraunhofer GC method
[0070] • Long residence time in the melt phase triggers certain reactions, for example the degradation of PVC
[0071] Benzene in the preform:
[0072] • Very low, since all benzene-forming substances react in the melt reactor
[0073] • Very low benzene build-up in the injection molding process
[0074] • Typically < 0.2 ppm according to the Fraunhofer method
[0075] The present procedure comprises the following steps:
[0076] In step a, the input stream of collected polyester containers, particularly PET bottles, is sorted, and in step b, the sorted containers are ground into flakes. The flakes are washed in step c and sorted in step d. Subsequently, in step e, the flakes are extruded after being pre-dried. During extrusion, the flakes are filtered in a first melt filtration step with a mesh size of 30-100 pm to remove foreign matter from the melt. After granulation of the melt into pellets (step f) and crystallization of the pellets (step g), the pellets are transferred to an SSP reactor (step h).
[0077] Surprisingly, the process can be combined with liquid-phase polycondensation (step i), with the LSP being performed between process steps e and f. This leads to synergistic effects and almost completely eliminates the disadvantages of SSP and LSP. Combining steps h and i is not obvious because both the space requirements of an SSP reactor and an LSP reactor, as well as the investment costs of both reactors, are considerable. Furthermore, economies of scale are very small, since a large LSP or SSP reactor is cheaper than a small LSP reactor and a small SSP reactor combined. From a cost perspective, a combination of SSP and LSP is pointless, and amortization would take a long time.
[0078] However, the investment in performing a combined SSP and LSP is worthwhile, as the required residence times for EBM-PET can be significantly reduced with an I of 1.10–1.30 dl / g: In the first step in the LSP reactor, an IV increase of up to a maximum of 0.95 dl / g is possible. The IV can be further increased to 1.10–1.30 dl / g in the SSP reactor. Instead of 24 hours in the SSP reactor, the process duration can be significantly shortened with a residence time of 1 hour in the LSP reactor and 10–14 hours in the SSP reactor.
[0079] Further advantages of the combined implementation of an LSP and a downstream SSP are:
[0080] • The extruded melt can be transferred directly to the LSP reactor after filtration. This makes the process extremely energy-efficient, as no unnecessary liquid / solid phase transition occurs.
[0081] • Slightly condensed melt after LSP (residence time approx. 15 min) can be filtered a second time. Due to the shorter residence time, the IV after LSP is lower than in the conventional LSP process (residence time 25-30 min), resulting in less AA buildup.
[0082] • The granulation and crystallization of the rPET is carried out in the “semi-finished state” to enable the SSP
[0083] • The IV of the granulated rPET is then further built up during the SSP and volatile components, especially AA, are removed.
[0084] • The final recycled polyester, especially rPET, can be further processed, for example, using preform injection molding. The preform is highly pure because both benzene and benzene-forming substances are removed during the process by the combination of SSP and LSP. Such high-quality preforms can be stretch blow molded into containers that can also be used for filling food products.
[0085] The following values can be achieved with this process for recycling polyester containers:
[0086] IV structure:
[0087] Input IV before extrusion: 0.65 - 0.75 dl / g • IV after extrusion: 0.60 - 0.72 dl / g
[0088] • IV according to LSP: 0.74 - 0.85 dl / g (for standard applications such as water bottles, soft drinks, cosmetic products...) / 0.85 - 0.95 dl / g (for EBM-PET applications)
[0089] • IV according to SSP: 0.80 - 0.90 dl / g (for standard applications) / IV > 1.2 - 1.3 dl / g (for EBM-PET applications) Yellowing:
[0090] • Comparable to previous material from the SSP process
[0091] • SSP (under vacuum or nitrogen atmosphere) instead of dealdehydization leads to less oxidative material damage
[0092] AA values: • Comparable to previous material from the SSP process
[0093] • Lower AA build-up after LSP due to reduced intrinsic viscosity
[0094] • Increased reduction of AA by SSP due to higher temperatures and applied vacuum (compared to dealdehydization)
Claims
1. A process for recycling polyester containers, in particular PET containers, comprising the following process steps (a) Container sorting, (b) crushing the containers into flakes, (c) Washing the flakes, (d) Flake sorting, (e) Extrusion of the flakes including a first melt filtration, (f) Granulation of the melt into pellets, (g) Crystallization of the pellets and (h) Treatment of the pellets with a solid-phase polycondensation (SSP) characterized in that a liquid-phase polycondensation (LSP) (i) of the melt is applied as a further process step between process steps (e) and (f), whereby the process comprises a combination of an SSP (h) and an LSP (i).
2. Method according to claim 1, characterized in that the intrinsic viscosity (IV) of the melt is increased by the LSP (i) to up to 0.95 dl / g and a further increase of the IV of the pellets is carried out by the SSP (h) to up to 1.3 dl / g.
3. Method according to claim 1 or 2, characterized in that the LSP (i) lasts at most 1 h and the SSP (h) preferably lasts 14 h and particularly preferably 10 h.
4. Method according to one of the preceding claims, characterized in that the melt undergoes a second melt filtration with a mesh size of 50 - 150 pm after the LSP (i).
5. Method according to one of the preceding claims, characterized in that the first melt filtration is carried out with a mesh size of 30 - 100 pm. A process according to claim 4 or 5, characterized in that the LSP (i) lasts 10 to 15 minutes before the second melt filtration. A process according to any of the preceding claims, characterized in that during the LSP (i), degradation reactions of precursors of benzene take place in the melt, thereby minimizing the formation of benzene in the pellets and polyester containers produced therefrom. A process according to any of the preceding claims, characterized in that benzene present in the pellets is removed during the SSP (h). An installation for implementing the process according to any of claims 1 to 8, characterized in that the installation comprises an LSP reactor and an SSP reactor, each with a throughput of 450 to 2500 kg / h. Recycled polyester pellets, in particular rPET pellets, produced according to any of claims 1 to 8.Preform for stretch blow molding of containers, which preform is injection molded from recycled polyester pellets according to claim 10.