A Method to Enable Recycling of Polyester Waste Material and a System for Applying the Method

Abstract

A process for recycling a stream of polyester waste material by depolymerising the polyester by alcoholysis, includes first and second consecutive stages of depolymerisation, in which the polymer waste material is subjected in a continuous manner, in the first stage, the polyester waste material is continuously fed to an extruder operated at a temperature above the polyester melting temperature, while a first amount of alcohol is co-fed to the extruder, to produce a fluid mixture including a melt of the at least partly depolymerised polyester, and in the second stage, the fluid mixture is continuously fed to a continuously stirred tank reactor (CSTR) operated at a temperature above the polyester melting temperature, while co-feeding a second amount of alcohol to the CSTR, a residence time in the CSTR being used to provide at the CSTR outlet a continuous stream of polyester depolymerised into an oligomeric ester.

Claims

1-24. (canceled)

25. A process to enable the recycling of a stream of polyester waste material by depolymerizing the polyester by alcoholysis, the process comprising: at least a first stage of depolymerization and a separate second consecutive stage of depolymerization, to which first and second stages the stream of polymer waste material is subjected in a continuous manner, in the first stage of the two consecutive stages, continuously feeding the stream of polyester waste material to an extruder operated at a temperature above the melting temperature of the polyester, while co-feeding a first amount of alcohol to the extruder, in order to produce a fluid mixture comprising a melt of the at least partly depolymerized polyester; and in the second stage, continuously feeding said fluid mixture to a continuously stirred tank reactor (CSTR) operated at a temperature above the melting temperature of the polyester, while co-feeding a second amount of alcohol to the CSTR, wherein a residence time in the CSTR is used to provide at the outlet of the CSTR a continuous stream of polyester depolymerized into an oligomeric ester.

26. A process according to claim 25, wherein the total amount of alcohol of the first and second amount of alcohol, is between 3 and 12% w/w with respect to the stream of polyester material.

27. A process according to claim 26, wherein the first amount of alcohol is between 1 and 5% w/w with respect to the stream of polyester material, and the second amount of alcohol is between 2 and 8% w/w with respect to the stream of polyester material.

28. A process according to claim 25, wherein the first stage comprises first and second separate consecutive sub-stages, wherein each of the first and second sub-stages comprises a corresponding extruder, and further comprising the step of: operating each extruder at a temperature above the melting temperature of the polyester, while co-feeding an amount of alcohol to each extruder, to produce the fluid mixture fed to the CSTR.

29. A process according to claim 28, further comprising the step of feeding the alcohol to the extruder of the first sub-stage, in one of: a distal 20% of the extruder length, a distal 5-15% of the extruder length, or at 10% of the extruder length.

30. A process according to claim 28, further comprising the step of feeding the alcohol to the extruder of the second sub-stage, in one of: the middle 30-70% of the extruder length, the middle 40-60% of the extruder length, or at 50% of the extruder length.

31. A process according to claim 28, further comprising the step of feeding the amount of alcohol in the extruder of the first sub-stage between 0 and 2% w/w with respect to the stream of polyester material and the amount of alcohol fed in the extruder of the second sub-stage between 1 and 5% w/w with respect to the stream of polyester material.

32. A process according to claim 28, wherein the extruder of the second sub-stage is a single screw extruder.

33. A process according to claim 25, further comprising, prior to the two consecutive stages, a step of drying the stream of polyester waste material to reduce the amount of water in the stream to less than 5000 ppm.

34. A process according to claim 33, wherein an amount of water in the stream is one of: between 10 and 1000 ppm, between 20 and 100 ppm, or around 50 ppm.

35. A process according to claim 25, further comprising the step of controlling the amount of alcohol fed in the first stage of the process by measuring a viscosity of the fluid mixture fed to the CSTR and adjusting the amount of alcohol to arrive at a predetermined value for said viscosity, which predetermined value is an intrinsic viscosity (IV) between 0.1 and 0.2 dl/g.

36. A process according to claim 25, further comprising the step of controlling the amount of alcohol fed in the second stage of the process by measuring a viscosity of the stream of oligomeric ester and adjusting the amount of alcohol to arrive at a predetermined value for the viscosity, which predetermined value is an intrinsic viscosity (IV) between 0.09 and 0.1.

37. A process according to claim 25, wherein the polyester is polyethylene terephthalate (PET), wherein residence time in the CSTR is such that the oligomeric ester for over any one of 50% w/w, 60% w/w, 70% w/w, 80% w/w or even 90% w/w, comprises oligomers of one of: 4 to 16 Bis(2-Hydroxyethyl) terephthalate (BHET) units, 6 to 14 (BHET) units, or 8 to 10 (BHET) units.

38. A process according to claim 25, further comprising the step of heating the continuous stream of oligomeric ester to an elevated temperature above the melting temperature of the polyester while subjected to a vacuum below one of: 10 mbar, 5 mbar, between 0.5 and 2 mbar, or around 1 mbar, to induce repolymerisation, to arrive at a repolymerised amorphous polyester, having an IV between 0.4 to 0.6.

39. A system for recycling a stream of polyester waste material by depolymerising the polyester by alcoholysis, the system comprising: an in line arrangement of first through third consecutive reactors for conducting a continuous process inducing alcoholysis of the polyester, wherein the first reactor is a twin screw extruder having a feeding entrance for feeding an alcohol in the distal 20% of the twin screw extruder length, wherein the second reactor is a single screw extruder having a feeding entrance for feeding an alcohol in the middle 30-70% of the single screw extruder length, and wherein the third reactor is a continuous stirred tank reactor having a feeding entrance for feeding an alcohol.

Description

EXAMPLES

[0052] FIG. 1 schematically depicts an overview of a process according to the invention.

[0053] FIG. 2 schematically depicts a screw for use in a single screw reactive extrusion process.

[0054] Example 1 provides various experiments in a single screw extruder alone.

[0055] Example 2 provides various experiments, including a two stage depolymerisation process according to the invention.

[0056] FIG. 1

[0057] FIG. 1 schematically depicts an overview of a process according to the invention. In the shown process, PET is depolymerised and repolymerised in a continuous process through steps 1-8. Step 9 is an additional repolymerisation step including solid-stae polymerisation, to arrive at a required IV above 0.6.

[0058] The process is based on the commonly known equilibrium reaction of PET in an alcoholysis based on mono ethylene glycol (MEG):

BHET .Math.[PET].sub.x+½×MEG

[0059] By adding MEG to the polyester melt, the equilibrium shift to the left, resulting in shorter polymer chains, ultimately oligomers (less than 100 repeating BHET units, in particular less than 50, 40, 30, 20 or even 10 units) and decreasing viscosity. By removing MEG, for example by using a vacuum or nitrogen, the short chains react with each other to form a polyester again. By controlling the depolymerisation rate, and thus the oligomer length, the viscosity of the material is controlled.

[0060] In step 1, a polyester waste material comprising pieces of (pure) PET carpet and flakes of PET bottles is dried to a moisture level of 50 ppm. Then, in step 2 the dried stream of waste material is fed to a conical co-rotating twin screw extruder. Due to the conical shape of the extruder the opening for feeding the material is bigger than with a conventional twin-screw extruder so feeding is easier and it generates less shear due to a more gentle natural compression giving less thermal damage to the polymer. Thermal degradation generates undesirable side reactions and formation of end groups giving an inferior end-product quality. The extruder is operated at 280° C. to melt the polyester completely. Adjacent the end of the conical twin screw extruder (at 10% of its length) an injection point for dosing MEG (indicated as arrow 50) is provided to obtain the first step in depolymerization, thus reducing the viscosity. For this, about 1% of MEG (w/w) is dosed. The reduction of the IV also helps to minimize the pressure difference over the first filtration step 3 to make it possible to filtrate with a mesh size of 80 micrometre. The filter also acts as a static mixer to homogenize the mixture and distribute the added glycol with the molten polymer to react completely with shorter polymer chains and a molecular weight distribution at equilibrium (dispersion grade of about 2) as a result. Process parameters are chosen such that the MEG is (almost) fully reacted and no (hardly any) free MEG is present anymore.

[0061] The partly depolymerized and filtered material is fed to a single screw extruder in step 4. The screw design is schematically depicted in FIG. 2. This design aims at maximising the percentage glycol which can be dosed (preventing the melt from becoming inhomogeneous). This maximum is increased by adjusting common process parameters like screw speed, pressure build up, etc. Typically, about 3-4% of MEG is dosed in this extruder (indicated by arrow 50′). At the end of the extruder the viscosity of the melt is measured. The level of the viscosity is controlled by an automated control loop (not indicated in FIG. 1) controlling the level of MEG being dosed in the single screw extruder. This automated control loop results in a consistent viscosity, typically an IV between 0.1 and 0.2, independent of the IV of the starting material. Due to the inherent transesterification reaction which takes place in the extruder, the polydispersity may remain low, preferably around 2-3, depending mainly on the residence time in the extruder (which may be adjusted in the process by controlling initial feed and extruder speed).

[0062] The depolymerized material filtered for a second time in step 5. Due to the IV of about 0.15 the filtration size can be reduced in comparison to the first filter, preferable being 40 micrometre, without a pressure difference over the filter being too high.

[0063] The material with an IV of about 0.15 (0.1-0.2) is continuously added to the CSTR in step 6. In this CSTR also MEG is added (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 difference in viscosity with the added MEG is not so big that homogeneous mixing is critical. 4-6% of MEG can be homogeneously mixed easily. The residence time in the CSTR is long enough (typically 25-45 minutes) to depolymerize the material to the required oligomer length, but also to have enough time for the transesterification reaction to obtain a polydispersity of 2. At the end of the reactor the viscosity is measured and with an automated control loop controlling the addition of MEG in the reactor. This results in an extremely stable continuous process hardly dependent on the type (IV) of the starting material.

[0064] In the CSTR, decolouration takes place by adding activated carbon, indicated by arrow 60. The activated carbon may be pre-selected for the best performance to absorb the colourants present in the polyester waste. After the CSTR, the low viscous oligomer/activated carbon mixture is pumped through a three-step micro filtration (20/10/5 micrometre) step 7 to remove the carbon particles loaded with colourant from the oligomers. A parallel set of three filters is installed so that in case of a pressure difference over the filter that is too high, the melt can be pumped through the parallel set, while the first filter set can be cleaned.

[0065] After the filtration while the melt is still an elevated temperature of about 250° C., the melt is pumped to the polycondensation reactor (step 8) which is operated under vacuum at 1 mbar, at a temperature of about 260° C., to remove the MEG, with the result that the equilibrium of the BHET/PET equilibrium shifts to the right forming the PET polymer. In this reactor also a significant purification step takes place since any volatiles with a boiling point up to 250-260° C. (e.g. benzene and Bisphenol A) are removed from the material. Due to the processing conditions, a polyester with an IV between 0.4 and 0.6 can be obtained. The polymer is removed from the reactor and pumped through a die-plate provide with holes, thus generating polymer strands. These strands are cooled down and cut into amorphous granules.

[0066] The amorphous granules undergo an off-line crystallisation process by subjecting the granules to a temperature of 130-180° C. which leads to a crystallisation process. The partly crystallised granules are subjected to a solid-state polymerisation process wherein the polyester is heated to an elevated temperature below the melting temperature of this polyester while subjected to a vacuum or an inert gas. This way a solid state additional repolymerisation is induced, to arrive at an IV above 0.6. The obtained IV can be adjusted by processing parameters in a way that the IV matches the required IV for the intended application, having any value between 0.65 and 1.0 typically.

[0067] FIG. 2

[0068] FIG. 2 schematically depicts a screw 10 for use in a single screw reactive extrusion process. The basic design of the screw is standard, but the screw has four non-standard zones designed for optimizing the polyester/oligomer and volatile reactant. At the middle section of the screw, where the alcohol reactant is dosed, the screw is provided with a double screw design, indicated as 11. In downstream direction, this zone is followed by a zone 12 in which the screw also is provided with a double screw design, but at which zone the screw is partially milled through (as such a known set-up for improved mixing). In further downstream direction there is an energy transfer mixing zone 13. The distal zone 14 lastly is of a so-called Saxton type.

Example 1

[0069] Example 1 provides various experiments in a single screw extruder alone.

[0070] Depolymerization of PET to oligomers with monoethylene glycol (MEG) by reactive extrusion has been performed in a range of 10-20 kg/h. It was found that up to 3% MEG could be properly dosed in a single screw extruder with a standard screw design (not optimsed for dosing MEG). Control of the moisture content of the PET flakes was achieved by pre-drying and applying degassing by vacuum to the extruder. This limited the hydrolysis of PET during the melting process, with the carboxyl end groups increasing by 7 mmol/kg.

[0071] The results are listed in Table 1. The amount of MEG used in the extruder for depolymerization depends i.a. on the capacity of the extruder and it screw design. This has to do with the residence time in the extruder and the effectiveness of mixing MEG with PET. This effect can also be seen if the PET is too moist. The increase in hydrolytic depolymerization means that the amount of MEG that can be added decreases. This phenomenon is caused by too low pressure in the extruder which causes the vapor pressure of MEG and the low viscosity of the oligomer to push it out of the extruder and cause an unstable depolymerization process. Increasing the pressure in the extruder will contribute positively to the stability of the depolymerization process and the amount of MEG that can be added while still arriving at a homogenous mixture.

TABLE-US-00001 TABLE 1 Influence of PET dosing 10 kg PET/h 15 kg PET/h 20 kg PET/h % MEG IV IV IV 0.0 0.76  0.76  0.76  0.7 n.d. 0.276 n.d. 1.1 n.d. 0.233 n.d. 1.2 0.209 n.d. n.d. 1.3 n.d. n.d. 0.234 1.4 n.d. 0.207 0.198 1.7 n.d. n.d. 0.174 1.8 0.173 n.d. n.d. 2.0 0.176 0.175 n.d. 2.2 0.157 n.d. 0.165 2.4 0.161 n.d. n.d. 2.5 0.157 n.d. n.d. 2.6 0.153 n.d. n.d. 2.7 n.d. 0.153 n.d. 2.8 0.145 n.d. n.d. 3.0 0.124 n.d. n.d.

[0072] The speed of the extruder screw contributes to the mixing of MEG with PET. Table 2 shows this at 10 and 15 kg/h. At a higher capacity (20 kg/h) and the same percentage of MEG, an opposite effect is seen. It is believed that a higher screw speed, leading to more transport, results in a residence time in the extruder that is too short for a complete depolymerisation. The mixing effect of the screw is seen in particular at speed below <70 rpm (results not shown). Then, depolymerization becomes unstable and MEG is released from the extruder as vapor.

TABLE-US-00002 TABLE 2 Influence of screw speed PET kg/h MEG kg/h IV Extruder rpm 10 0.20 0.166  70 10 0.20 0.161 135 15 0.30 0.166 113 15 0.30 0.164 163 20 0.35 0.177 139 20 0.35 0.181 169 20 0.40 0.169 139 20 0.40 0.184 189

[0073] As long as a good mixing of MEG with PET takes place and sufficient reaction time is given, almost all MEG will react with the polyester. Table 3 shows measurements performed on oligomer collected in water. Virtually no MEG is present in the water phase.

TABLE-US-00003 TABLE 3 Free MEG in product PET MEG Polymer MEG MEG in MEG in kg/h kg/h sample Water w/w water PET 10.0 0.27 79.0 g 0.49 l 2.63% 30.0 mg/l 0.018%

[0074] Table 4 provides typical properties for dried PET flakes. As can be seen the amount of diethylene glycol is about 1.5-1.6%. Since at the end of the extruder the amount measured typically was about 1.5-1.6% it is reasonable to conclude that no DEG was produced in the extruder.

TABLE-US-00004 TABLE 4 Typical values for dried PET flakes Moisture IV Ec mmol/kg DEG AA mg/kg 0.02% 0.75-0.85 28-30 1.5-1.6% 0.8-2.5 DEG = diethylene glycol; Ec = amount of free carboxyl groups; AA = acetaldehyde

[0075] In summary, depolymerization of PET to oligomers with monoethylene glycol by reactive extrusion has been performed in a range of 10-20 kg/h. The limits within which an efficient depolymerization process can take place have been established for these different capacities. Under the existing conditions, up to 3% MEG could be properly dosed on the standard extruder. Control of the moisture content of the PET flakes has been achieved by pre-drying and applying degassing by vacuum to the extruder. This limited the hydrolysis of PET during the melting process, with the carboxyl end groups increasing by +7 mmol/kg.

[0076] Additional diethylene glycol formation could not be demonstrated in the test series. The added MEG was almost completely taken up during depolymerization. The speed of the extruder screw has an influence on the process. At a capacity of 20 kg/h, a higher speed gives a higher viscosity, believed to be due to the fact that the increase in transport counteracts the effective reaction with MEG. At a lower capacity, a higher screw speed contributes to better mixing with MEG. When the speed of the screw is too low, instability of the depolymerization process is seen, and MEG may be released.

Example 2

[0077] Example 2 provides various experiments, including a two stage depolymerisation process according to the invention. In particular, this example provides results achieved with glycolysis using MEG as a reactant for depolymerising PET in a single screw extruder (see FIG. 2) and in combination with a CSTR configured as a continuous process.

[0078] Clear PET flakes made of PET bottles were used for the experiments. The basic properties of the PET waste are listed in Table 5. When not dried, the moisture content is about 1% w/w. This moisture is present mainly because of the pre-treatment of the flakes for cleaning.

TABLE-US-00005 TABLE 5 Analysis of the starting material moisture viscosity Ec DEG AA Nr % m/m IV Meq/kg % % Remarks 1 0.757 0.755 27.5 1.57 2.45 Not dried 2 0.023 0.750 28.5 1.50 1.27 Dried 3 0.170 0.764 28.4 n.d. n.d. After 24 h of exposure to air Ec = amount of free carboxyl groups; DEG = diethyleneglycol; AA = acetaldehyde

[0079] Table 6 shows the results of PET melting using dried and undried material. The more PET is dried, hydrolysis, and so the formation of carboxyl end groups (Ec), will have less influence on the glycolysis process and ultimate repolymerization process.

[0080] Because any (small) amount of moisture may already cause hydrolysis, the PET is kept dry during dosing of PET in the extruder. This is done by keeping the PET under dry nitrogen. To underline this, in table 5 (see no. 3) the effect of moisture absorption from the air can be seen after sample no. 2 is exposed for 24 hours to regular air.

TABLE-US-00006 TABLE 6 Hydrolysis of dried and non-dried PET (Tm = 268° C.) Sample PET MEG Viscosity Ec Nr Kg/h Kg/h IV meq/kg Description samples 1 10.0 0 0.38 83 Not dried, no vacuum 2 10.0 0 0.57 44 Not dried, vacuum 3 10.0 0 0.60 39 Dried, vacuum

[0081] Reactive extrusion tests were performed using up to 3% w/w MEG with dried PET flakes. The glycolysis process took place very rapidly. The PET was depolymerized into oligomers within 30 seconds. The dosed MEG had almost completely reacted in this time frame. Over 98% of the dosed MEG had been used for the glycolysis process. Table 7 shows the analysis results of the glycolysis process. To increase the performance larger amounts of MEG have been dosed continuously in combination with a Reactor (indicated as “+CSTR”).

[0082] About 2% w/w MEG is added to the extruder and an additional amount in the CSTR, if applicable. By choosing a filling of 10 kg in the CSTR, an average residence time of 1 hour was realized at 270° C. at a capacity of 10 kg/h PET. Steady state was reached by a total dosing of ±12% MEG w/w. The entire glycolysis process was carried out without additional catalyst. A viscosity (IV) as low as 0.07 could be achieved. However, even a viscosity of about 0.09-0.1 is sufficiently low to allow (micro) filtration. At a dosing of 12%, a percentage of 2-3 free MEG could still be detected in the oligomer ester. The relatively high proportion of carboxyl end groups caused by hydrolysis was probably due to moisture present in the MEG. This can be avoided by drying the MEG.

TABLE-US-00007 TABLE 7 Glycolysis of dried, clear PET flakes (Tm 266° C.) Viscosity after Ec MEG Sample PET MEG CSTR meq/ free Nr Kg/h Kg/h IV kg % Remarks 1 10.0 0.12 0.209 35.8 n/a 2 10.0 0.18 0.173 35.3 n/a 3 10.0 0.22 0.157 34.8 n/a 4 10.0 0.29 0.145 35.1 n/a 5 10.0 0.31 0.124 32.8 n/a (3%) 6 10.0 >2 0.072 112 2.9 +CSTR (>20%) 7 10.0 1.23 0.071 95 2.3 Two extruders + (12%) CSTR

[0083] The HPLC analysis results in Table 8 show a significant amount of BHET to tetramer when applying 12% MEG or more. However, in order to be able and filter the melt, it is not necessary to depolymerize this far. A viscosity of (IV=±) 0.1 is sufficient for the filtration process. This corresponds to a 8-10 (octa-deca) oligomer. This will require 5% to 8% MEG w/w.

TABLE-US-00008 TABLE 8 Analysis of the oligomers (relative to pure BHET) tri- tet- pen- hex- oc- Sample BHET dimer mer ramer tamer amer heptamer tamer 6 12.6 17.1 14.3 12.0 6.1 1.8 0.6 0.4 7 10.3 14.9 13.8 10.2 5.1 1.4 0.4 0.3