Method for recycling polyester containers

Abstract

The present invention relates to a method for recycling polyester containers, in particular PET containers, comprising the steps of (a) sorting the containers, (b) comminuting the containers into flakes, (c) washing the flakes, (d) sorting the flakes, (e) extruding the flakes, comprising a first melt filtration, (f) granulating the melt into pellets, (g) crystallizing the pellets, and (h) treating the pellets with solid state polycondensation (SSP). Between the method steps (e) and (f), liquid polycondensation (LSP) (i) of the melt is used as a further step, whereby the method has a combination of SSP (h) and LSP (i).

Description

Technical Field

[0001] The invention relates to a method for recycling polyester containers according to the preamble of claim 1 and to a plant for carrying out the method, as well as to recycled polyester pellets produced using the method. Prior art

[0002] Previous PET recycling methods usually use SSP (solid state polycondensation) technology to purify rPET or to establish the intrinsic viscosity (IV) of the recycled material. Volatile substances such as acetaldehyde can be effectively removed, and the yellowing of the resulting rPET particles is minimal. However, the SSP technology can only purify non-volatile contaminants to a certain extent, especially the so-called precursor substances that can lead to the formation of benzene, although the process takes at least 360 minutes. The best-known precursor substances are the polymers PVC and PS.

[0003] Purpose of the invention

[0004] The disadvantages of the prior art have led to the aim of improving the known recycling methods using SSP technology in such a way that the non-volatile substances are gradually depleted or reacted away and are therefore considered to be non-problematic in the further process.

[0005] describe

[0006] The stated object is achieved with a method for recycling polyester containers, in particular PET containers, by the features listed in the characterizing part of claim 1. Improvements and / or advantageous alternative embodiments form the subject matter of the dependent claims.

[0007] Preferably, the invention is characterized in that between the method step e of extruding the sheet including the first melt filtration and the method step f of granulating the melt into pellets, a liquid polycondensation (LSP) i of the melt is applied as a further step, whereby the method has a combination of LSP and SSP. The purification in the LSP (t<30 min, T>260° C.) leads to further promotion of reactions forming benzene (=e.g. degradation reactions of PVC and PS) in addition to free benzene. This process actively promotes or forces the formation of benzene and correspondingly depletes the precursor substances so that they are no longer present in the subsequent process steps. At the same time, an initial purification of the free benzene component is carried out. With the help of the subsequent purification in the SSP, any residual free benzene (boiling temperature=80° C.) and also acetaldehyde (boiling temperature=20° C.) can be removed from the pellets. It is important to arrange the LSP between the process steps of extrusion (first melt filtration) and granulating the melt, because this is the only way in which the process can function at all. Basically, a melt is required to operate an LSP reactor - in this regard, pre-extrusion is required. However, such a melt may only lie within a certain viscosity range such that the material can be processed at all. If the viscosity is too high (e.g. after a previous SSP treatment), the melt cannot be processed mechanically because it is too viscous. If the arrangement is SSP-LSP (instead of LSP-SSP), two extrusion processes including pellet production will be required: First, the flakes must be melted and pelletized before SSP so that the pellets can be subjected to SSP. Secondly, such pellets must be melted again before LSP and pelletized again after LSP.

[0008] The described arrangement of the LSP is therefore necessary so that the process can be operated as energy-efficiently as possible and above all so that the investment costs can be kept low.

[0009] In a synergistic manner, in the method according to the invention, the SSP and LSP residence times are not additive, but can be significantly reduced. The residence times previously required for both technologies can be reduced by at least 33% (t LSP <20min;t SSP <4h), which means that despite the added investment in SSP and LSP reactors, the process pays for itself very quickly. In addition, the rPET pellets produced contain virtually no benzene and are therefore of significantly better quality than standard rPET pellets. In this context, benzene-free does not only mean that the rPET pellets contain virtually no benzene, but also that the products made from these pellets (e.g. PET blanks) have only slightly higher benzene levels than products made from virgin material.

[0010] In a particularly preferred embodiment of the present invention, the intrinsic viscosity (IV) of the melt is increased to a maximum of 0.95 dl / g by LSP (i), and the IV of the pellets is further increased to a maximum of 1.3 dl / g by SSP (h). Thus, the IV can be increased in two stages and the target viscosity can still be achieved with an acceptable residence time. For this specific application, the present invention is characterized in that LSP i lasts for a maximum of 1 h and SSP h preferably lasts for 14 h, and particularly preferably 10 h. Using these longer residence times compared to the "standard process", the IV can be increased to a maximum of 1.3 dl / g. Thus, compared to the processes used according to the prior art for achieving a final IV of 0.8 dl / g to 0.9 dl / g (standard application) and a final IV of 1.2 dl / g to 1.3 dl / g (EBM-PET application), the residence time can be significantly reduced, making the process of the present invention effective and therefore cost-optimized.

[0011] It has proven useful to subject the melt to a second melt filtration after LSP (i) with a mesh size of 50 to 150 μm. This allows any solid contaminants still present or newly formed during LSP to be removed, thereby improving the optical quality of the material.

[0012] It is advisable to carry out a first melt filtration after extrusion with a mesh size of 30 μm to 100 μm, as a result of which solid impurities are reliably removed from the melt.

[0013] In a further preferred embodiment of the invention, the LSP before the second melt filtration lasts from 10 to 15 minutes. The residence time can be kept particularly short, since further polycondensation takes place in the SSP reactor. This keeps the accumulation of acetaldehyde to a minimum.

[0014] It is particularly preferred that during LSP i, the degradation reaction of the benzene precursor takes place in the melt, thereby preventing the formation of benzene in the pellets and the polyester container made from the pellets. The formation reaction (= degradation reaction of PVC and PS) is promoted during LSP. The process actively promotes the formation of benzene to prevent the formation of benzene in subsequent process steps. For example, no benzene can be formed in a preform injection molded from rPET pellets, because no precursor is present any more.

[0015] Advantageously, any benzene present in the pellets is removed during the SSP h. The rPET pellets produced are therefore virtually benzene-free, since the LSP causes the benzene precursors to react to preform benzene and the SSP consumes all the benzene.

[0016] Another aspect of the invention relates to a facility for implementing the method as described above. The invention is also characterized in that the facility comprises an SSP reactor and an LSP reactor, each of which has a throughput of 450 kg / h to 2500 kg / h. To achieve such a throughput, the volume of the SSP reactor is about 11 m 3 With this throughput in the continuous process, the increased required space and the increased investment costs are manageable. The investment is quickly compensated by the shortened residence time in both reactors and the improved quality of the rPET pellets, which can also be used in the food sector.

[0017] Another aspect of the invention relates to recycled polyester pellets, in particular rPET pellets, produced according to the above method. Since the pellets contain almost no benzene, they are particularly suitable for the production of food containers, in particular rPET bottles for sensitive applications such as mineral water.

[0018] Preforms may advantageously be injection molded from recycled polyester pellets and then the preforms are stretch blow molded into containers.

[0019] Detailed description of prior art and exemplary embodiments

[0020] 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 the return stream from the polyester vessel of any benzene or acetaldehyde present.

[0021] Some return streams for the recycling of post-consumer PET are contaminated with non-PET components, in particular foreign polymers such as PS and PVC. Despite all sorting and washing steps carried out in the recycling facility, penetration of components of PVC and PS (<50 ppm) into the extrusion and purification still occurs. The degradation of these foreign polymers during the recycling process can lead in particular to benzene formation, which occurs mainly in the melt phase. Subsequent purification in the SSP (t=6h; T>195°C, p<10 mbar) means that free benzene (boiling temperature=80°C) can be removed from the granules - but no further depletion of the benzene precursors PS and PVC can occur.

[0022] Typical process values ​​for SSP technology are:

[0023] Process :

[0024] · Washed flakes as input

[0025] Extrusion (optionally including degassing)

[0026] Melt Filtration

[0027] ·crystallization

[0028] SSP reactor (T≈195℃ to 220℃, t>6h, p<10mbar)

[0029] Final pellets are available

[0030] IV accumulation :

[0031] · Quite slow, typically 0.02 dl / g for granules

[0032] Initial IV before extrusion ≈ 0.65 dl / g to 0.75 dl / g

[0033] IV after extrusion ≈ 0.60 dl / g to 0.72 dl / g

[0034] IV after SSP≈0.72dl / g to 0.88dl / g (residence time 6h to 8h)

[0035] Purified benzene :

[0036] Very low, typically <0.1ppm according to Fraunhofer method

[0037] The temperature in SSP is too low to degrade e.g. PVC

[0038] Benzene in preforms :

[0039] Low to high (if benzene precursor is present in the input stream) - strongly dependent on the quality of the flakes used

[0040] Benzene is typically 0.1ppm to 0.9ppm according to the Fraunhofer method

[0041] "Precursor" substances such as PVC decompose during injection molding and lead to the reformation of benzene

[0042] The SSP process has been on the market for decades and is therefore well developed. To gain market share, new companies entering the recycling market developed liquid state polycondensation (LSP). LSP is shown to be in parallel with the "melt to resin" process used to make new PET goods. LSP technology must deal with yellowing and higher acetaldehyde levels, but the process is very fast (≈30 min).

[0043] Typical process values ​​for LSP technology are:

[0044] Process :

[0045] · Washed flakes as input

[0046] Extrusion (optionally including degassing)

[0047] Melt Filtration

[0048] Melt pump for transport to LSP reactor

[0049] LSP reactor including agitator (T>265°C, p<10 mbar, t≈25 min to 30 min)

[0050] Melt pump for discharging material

[0051] ·filter

[0052] Granulation

[0053] ·crystallization

[0054] Post-treatment in "dealdehyde removal": hot air flows through the material (200m 3 / h to 500m 3 / h; 150℃ to 165℃; 10h to 20h)

[0055] Final pellets are available IV accumulation :

[0056] Very fast, up to 0.01dl / g per minute

[0057] Initial IV before extrusion ≈ 0.65 dl / g to 0.75 dl / g

[0058] IV after extrusion ≈ 0.60 dl / g to 0.72 dl / g

[0059] IV after LSP≈0.78dl / g to 0.90dl / g (residence time 25min to 30min)

[0060] Yellowing :

[0061] · Quite high due to long residence time in the melt + oxidation during dealdehydeation

[0062] Typically b* 2 to 5 higher than the same material from the SSP process (depending on the quality of the flakes used)

[0063] AA value (acetaldehyde content; according to Fraunhofer method) :

[0064] ·4ppm to 12ppm before dealdehyde removal

[0065] ·After dealdehyde removal, the concentration is 0.5ppm to 4ppm (residence time is 10h to 20h)

[0066] The target should be <2ppm

[0067] Purified benzene :

[0068] ·Very low and typically <0.1 ppm according to Fraunhofer GC method

[0069] Long residence time in the melt phase sets certain reactions (e.g., degradation of PVC) in motion

[0070] Benzene in preforms :

[0071] Very low, since all benzene precursors are reacted in the melt reactor

[0072] Very low benzene accumulation in injection molding processes

[0073] According to Fraunhofer method, usually <0.2ppm

[0074] The method of the present invention comprises the following steps:

[0075] In step a, the collected input stream of polyester containers, in particular PET bottles, is sorted and in step b, the sorted containers are ground into flakes. These flakes are washed in step c and sorted in step d. The flakes are then extruded in step e after pre-drying. During extrusion, the flakes are filtered in a first melt filtration with a mesh size of 30 μm to 100 μm to remove foreign matter from the melt. After the melt has been granulated into pellets (step f) and the pellets have been crystallized (step g), the pellets are transferred to an SSP reactor (step h).

[0076] Surprisingly, the process can be combined with a liquid polycondensation (step i), whereby LSP is carried out between step e and step f. This results in a synergistic effect and almost completely eliminates the disadvantages of SSP and LSP. The combination of step h and step i is not obvious, since the space required for the SSP reactor and the LSP reactor as well as the investment costs for both reactors are considerable. In addition, the 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 together. From a cost point of view, the combination of SSP and LSP is meaningless and the payback on investment will take a long time.

[0077] However, the investment in combined SSP and LSP is worthwhile, since the necessary residence time of EBM-PET can be significantly shortened when using an IV of 1.10 dl / g to 1.30 dl / g: In the first step, an IV increase of up to a maximum of 0.95 dl / g is possible in the LSP reactor. The IV can be further increased to 1.10 dl / g to 1.30 dl / g in the SSP reactor. Instead of 24 hours in the SSP reactor, the duration of the process can be significantly shortened with a residence time of 1 hour in the LSP reactor and a residence time of 10 to 14 hours in the SSP reactor.

[0078] Additional advantages of the combination of LSP and downstream SSP are:

[0079] The extruded melt can be transferred directly to the LSP reactor after filtration. This makes the process very energy efficient, since no unnecessary liquid / solid phase transitions occur.

[0080] • The slightly condensed melt can be filtered a second time after LSP (residence time of about 15 min). Due to the shorter residence time, the IV after LSP is lower than that using the conventional LSP process (residence time of 25 min to 30 min), so that less AA accumulation occurs.

[0081] • Pelletization and crystallization of rPET is done in a "semi-finished state" to allow SSP.

[0082] • The IV of the granulated rPET is then further accumulated during SSP and the volatile components, especially AA, are purified.

[0083] The final recycled polyester, in particular rPET, can be further processed, for example in preform injection molding. Because both benzene and benzene precursors are purified during the process by the combination of SSP and LSP, the preforms have a high level of purity. Such high-quality preforms can be stretch blow molded into containers, which can also be filled with food products.

[0084] The following values ​​can be obtained with the process of the invention for recycling polyester containers: IV accumulation :

[0085] Initial IV before extrusion: 0.65 dl / g to 0.75 dl / g

[0086] IV after extrusion: 0.60 dl / g to 0.72 dl / g

[0087] IV after LSP: 0.74 dl / g to 0.85 dl / g (for standard applications such as water bottles, soft drinks, cosmetics, ...) / 0.85 dl / g to 0.95 dl / g (for EBM-PET applications)

[0088] IV after SSP: 0.80 dl / g to 0.90 dl / g (for standard applications) / IV> 1.2 dl / g to 1.3 dl / g (for EBM-PET applications) Yellowing:

[0089] Comparable to previous materials from the SSP process

[0090] SSP (in vacuum or under nitrogen atmosphere) rather than de-aldehydeation results in less oxidative material damage

[0091] AA Value :

[0092] Comparable to previous materials from the SSP process

[0093] Lower AA accumulation after LSP due to lower intrinsic viscosity

[0094] Increased AA reduction by SSP due to higher temperatures and applied vacuum (compared to dealdehydeation)

Claims

1. A method for recycling polyester containers, in particular PET containers, the method comprising: The following steps are involved: (a) sorting the container, (b) crushing the container into flakes, (c) washing the sheet, (d) sorting the flakes, (e) extruding the sheet, including a first melt filtration, (f) granulating the melt into pellets, (g) crystallizing the pellets, and (h) treating the pellets with solid state polycondensation (SSP) Features Between process steps (e) and (f), liquid state polycondensation (LSP) (i) of the melt is applied as a further step, whereby the process has a combination of SSP (h) and LSP (i).

2. The method according to claim 1, It is characterized in that The intrinsic viscosity (IV) of the melt is increased to a maximum of 0.95 dl / g by LSP (i), and the IV of the pellets is further increased to a maximum of 1.3 dl / g by SSP (h).

3. The method according to claim 1 or claim 2, It is characterized in that The LSP (i) lasts for a maximum of 1 h, and the SSP (h) lasts preferably for 14 h and particularly preferably for 10 h.

4. The method according to any one of the preceding claims, It is characterized in that After LSP (i), the melt was subjected to a second melt filtration using a mesh size of 50 μm to 150 μm.

5. The method according to any one of the preceding claims, It is characterized in that The first melt filtration was performed with a mesh size of 30 μm to 100 μm.

6. The method according to claim 4 or claim 5, It is characterized in that The LSP (i) before the second melt filtration lasts from 10 to 15 minutes.

7. The method according to any one of the preceding claims, It is characterized in that During LSP (i), degradation reactions of benzene precursors occur in the melt, thereby minimizing benzene formation in the pellets and the polyester containers produced from the pellets.

8. The method according to any one of the preceding claims, It is characterized in that Any benzene present in the pellets is removed during SSP(h).

9. A plant for carrying out the method according to any one of claims 1 to 8, It is characterized in that The facility comprises an LSP reactor and an SSP reactor, each of the LSP reactor and the SSP reactor having a throughput of 450 kg / h to 2500 kg / h.

10. Recycled polyester pellets, in particular rPET pellets, produced according to any one of claims 1 to 8.

11. A preform for stretch blow molding a container, the preform being injection molded from the recycled polyester pellets according to claim 10.