Improved method for depolymerizing polyethylene terephthalate

Abstract

The present invention provides a method for depolymerizing polyethylene terephthalate ("PET"), which comprises depolymerizing PET with glycols and M A In a mixture containing OR, it is converted to bis(2-hydroxyethyl) terephthalate (BHET; CAS number: 959-26-2), and M A is an alkali metal selected from sodium, potassium, and lithium, and R is an alkyl group having 1 to 6 carbon atoms. A OR is obtained by reactive distillation. The process according to the invention is characterized by a particularly low proportion of the undesired decomposition products mono(2-hydroxyethyl) terephthalate (= "MHET") and terephthalate (= "TS") relative to the proportion of BHET. As a result, the process according to the invention provides a high yield of BHET, which can be used directly for the production of new PET. The invention therefore also relates to a process for recycling PET, in which BHET obtained in a process for depolymerizing PET is polymerized back into PET, optionally after further purification.

Description

[Technical Field]

[0001] The present invention provides a method for depolymerizing polyethylene terephthalate ("PET"), which comprises depolymerizing PET with glycols and M A In a mixture containing OR, it is converted to bis(2-hydroxyethyl) terephthalate (BHET; CAS number: 959-26-2), and M A is an alkali metal selected from sodium, potassium, and lithium, and R is an alkyl group having 1 to 6 carbon atoms. A OR is obtained by reactive distillation.

[0002] The process according to the invention is characterized by a particularly low proportion of the undesired degradation products mono(2-hydroxyethyl) terephthalate (= "MHET") and terephthalate (= "TS") relative to the proportion of BHET. As a result, the process according to the invention provides a high yield of BHET, which can be used directly in new PET production.

[0003] The present invention therefore also relates to a method for recycling PET, in which the BHET obtained in the method for depolymerizing PET, optionally after further purification, is polymerized again to PET.

[0004] Background of the Invention Polyethylene terephthalate (PET) is one of the most important plastics used in textiles, as films, and as a material for plastic bottles. In 2007 alone, approximately 10 7(W. Caseri, Polyethylenterephthalate, RD 16 03258 (2009) in F. Boeckler, B. Dill, G. Eisenbrand, F. Faupel, B. Fugmann, T. Gamse, R. Matissek, G. Pohnert, A. Ruehling, S. Schmidt, G. Sprenger, ROEMPP [Online], Stuttgart, Georg Thieme Verlag, January 2022).

[0005] The durability of PET and the amount of waste it generates make it one of the biggest environmental problems of our time. The solution to this problem lies in avoiding and efficiently recycling PET.

[0006] The prior art has proposed several methods for degrading PET.

[0007] GB 784,248 describes the methanolysis of PET.

[0008] Hydrolysis methods for depolymerizing PET are described in JP 2000-309663, U.S. Pat. No. 4,355,175, and T. Yoshioka, N. Okayama, A. Okuwaki, Ind. Eng. Chem. Res. 1998, 37, 336-340.

[0009] The reaction of PET with glycols is described in EP 0 723 951 A1, U.S. Pat. No. 3,222,299, WO 2020 / 002999, SR Shukla, AM Harad, Journal of Applied Polymer Science 2005, 97, 513-517 (hereinafter "Shukla & Harad"), and ND Pingale, SR Shukla, European Polymer Journal 2008, 44, 4151-4156.

[0010] Shukla & Harad describe the production of bis(2-hydroxyethyl) terephthalate (= "BHET") during the glycolysis of PET. This degradation product can simultaneously be used as a starting material for the production of new PET. In contrast, certain by-products, such as the monoester mono(2-hydroxyethyl) terephthalate (= "MHET") or free terephthalic acid or the corresponding carboxylate, terephthalate (= "TS"), are disadvantageous because they cannot be used directly as starting materials for the production of new PET.

[0011] Therefore, in processes for depolymerizing PET, there is an interest in achieving as high a proportion of BHET among the degradation products as possible, while keeping the proportion of undesirable by-products such as MHET and TS as low as possible.

[0012] The object of the present invention was to provide such a method.

[0013] BRIEF DESCRIPTION OF THE INVENTION Surprisingly, M obtained by reactive distillation A It was found that the reaction of OR with PET in glycol gave lower proportions of the undesirable by-products MHET and TS relative to BHET than conventional processes.

[0014] Therefore, surprisingly, a method for solving the problem according to the present invention has been found.

[0015] Accordingly, the present invention provides a method for depolymerizing polyethylene terephthalate (PET), comprising the steps of: (a)M A OR and ROH were reacted to form M A Convert to OR, Regarding the method.

[0016] M A is an alkali metal selected from sodium, potassium and lithium, particularly an alkali metal selected from sodium and potassium, preferably M A = sodium.

[0017] R is an alkyl group having 1 to 6, especially 1 to 5, preferably 1 to 4, more preferably 1 to 3 carbon atoms. Even more preferably, R is methyl or ethyl. Most preferably, R=methyl.

[0018] (b) PET is mixed with glycol and the M obtained in step (a). A and at least a portion of the OR is converted to bis(2-hydroxyethyl) terephthalate, BHET, in a mixture containing the OR.

[0019] In a further embodiment, the present invention relates to a method for recycling PET, in which in step (ζ) the BHET obtained by the method for depolymerization according to the present invention is polymerized to PET. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 shows a comparison of the content of mono(2-hydroxyethyl)terephthalic acid ("MHET," "1") and terephthalic acid ("TS," "2") in depolymerizations using sodium methoxide obtained according to a method of the present invention and sodium methoxide obtained according to a conventional method. The content of each by-product is shown based on the BHET content (mol percent).

[0021] The respective contents of MHET and TS in the reactor effluent during the depolymerization of PET according to Example E1 according to the invention, in which the sodium methoxide used for the depolymerization was obtained by reactive distillation, are shown in each case by hatched bars ("\\\\\"). Only a significant proportion of MHET was detected, therefore no hatched bars are shown in "2".

[0022] The black bars show the respective contents of MHET and TS in the reactor effluent in the depolymerization of PET according to Comparative Example V1, in which the sodium methoxide used for the depolymerization was obtained by mixing NaOH and methanol in the reactor.

[0023] Detailed Description of the Invention Thus, surprisingly, glycol and M obtained by reactive distillation A It was found that when degradation of PET is carried out in a mixture with OR, the proportion of the desired degradation product BHET increases during glycolysis of PET compared to the proportion of the undesired degradation products TS and MHET.

[0024] The process according to the invention is therefore superior to prior art processes in which the decomposition is carried out in a mixture obtained by dissolving, for example, an alkali metal hydroxide in glycol and ROH.

[0025] Step 1(a): M by reactive distillation A OR manufacturing According to the invention, the alkali metal alkoxide M used in the process according to the invention A OR is M by reactive distillation A It is obtained by the reaction of OH with ROH.

[0026] M A is an alkali metal selected from lithium, sodium and potassium, in particular sodium and potassium. A is preferably sodium.

[0027] R is an alkyl group having 1 to 6 carbon atoms, particularly an alkyl group having 1 to 5 carbon atoms, preferably an alkyl group having 1 to 4 carbon atoms, more preferably an alkyl group having 1 to 3 carbon atoms. Even more preferably, R is selected from ethyl and methyl. Most preferably, R=methyl.

[0028] Alkyl groups having 1 to 6 carbon atoms are in particular methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl, 2,3-dimethyl butyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl, 1-ethyl-2-methylpropyl, preferably selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, n-hexyl, even more preferably selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, n-pentyl, n-hexyl.

[0029] In the context of the present invention, an alkyl group having 1 to 5 carbon atoms is in particular selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl, 1-ethylpropyl, preferably selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, even more preferably selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, n-pentyl.

[0030] In the context of the present invention, an alkyl group having 1 to 4 carbon atoms is in particular selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, preferably selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl, even more preferably selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, n-butyl.

[0031] In the context of the present invention, alkyl groups having 1 to 3 carbon atoms are in particular selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, preferably selected from the group consisting of methyl, ethyl, isopropyl.

[0032] In the context of the present invention, "glycol" is understood to mean 1,2-ethylenediol, having the chemical formula HO-CH2-CH2-OH (CAS number 107-21-1).

[0033] Reactive distillation for the preparation of alkali metal alkoxides is an important industrial process because alkali metal alkoxides are used as strong bases in the synthesis of numerous chemicals, for example in the preparation of pharmaceutical or agrochemical active ingredients, and as catalysts in transesterification and amidation reactions.

[0034] Alkali metal alkoxides (MOR) are produced using reactive distillation, typically from alkali metal hydroxides (MOH) and alcohols (ROH) in a countercurrent distillation column, by the following reaction: <1> The water of reaction formed according to is removed with the distillate.

[0035] [ka]

[0036] Such a process principle is described, for example, in U.S. Pat. No. 2,877,274, in which an aqueous alkali metal hydroxide solution and gaseous methanol are run countercurrently in a rectification column. This process is again described, essentially unchanged, in WO 01 / 42178.

[0037] The most important industrially alkali metal alkoxides are sodium and potassium alkoxides, in particular methoxide and ethoxide, the synthesis of which is frequently described in the prior art, for example in EP 1997794, WO 2021 / 148174 and WO 2021 / 148175.

[0038] A similar process, but in which an additional azeotroping agent such as benzene is used, is described in GB Patent No. 377,631 and US Patent No. 1,910,331.

[0039] Similarly, German Patent No. 968903 describes a process for the continuous production of alkali metal alkoxides in a reaction column, in which a water-alcohol mixture withdrawn from the top is condensed and subsequently subjected to phase separation. The aqueous phase is discarded, while the alcohol phase is returned to the top of the column together with fresh alcohol. A similar process is described in European Patent Application No. 0299577, in which water is separated from the condensate using a membrane.

[0040] In a preferred embodiment of the process according to the invention, in step (a), a reactant stream S comprising ROH is AE1 M A OH-containing reactant stream S AE2 and reactive rectification column RR A reacts in countercurrent at M A OR, Water, ROH, M A Crude product RP containing OH A is obtained, and M A Bottoms product stream S containing OR and ROH AP RR A A steam stream S containing water and possibly ROH is taken from the lower end of the AB RR A is taken from the top end of the

[0041] According to the present invention, a "reactive rectification column" is defined as a rectification column in which the reaction according to step (a) of the process according to the invention takes place in at least some parts. It may also be called a "reaction column" for short.

[0042] Reactive Logistics S AE1 In a preferred embodiment, S AE1 The mass fraction of ROH in S is ≧95 wt.%, and even more preferably ≧99.5 wt.%. AE1 In addition, it contains, inter alia, water.

[0043] In a preferred embodiment of the process according to the invention, the reactant stream S AE1 The ROH used as may be a commercially available alcoholic ROH having a mass proportion of ROH of more than 99.5% by weight and a mass proportion of water of up to 0.03% by weight.

[0044] In certain embodiments of the present invention, reactant stream S AE1 is the reactive rectification column RR A is added in vapor form to

[0045] In an alternative preferred embodiment of the process according to the invention, the ROH is fed to a reactive rectification column RR prior to step (a). Ais initially charged into the bottom of the reaction rectification column RR A At a constant reactant flow rate S AE1 is produced. Optionally, during step (a), a reactive rectification column RR A The bottom of the tank is filled with ROH.

[0046] Reactive Logistics S AE2 is M A In a preferred embodiment, S AE2 is M A In addition to OH, it contains at least one further compound selected from water, ROH. Even more preferably, S AE2 is M A In addition to OH, it contains water, in which case S AE2 is M A It is an aqueous solution of OH.

[0047] Reactive Logistics S AE2 M A When OH and water are included, S AE2 M based on the total weight of the aqueous solution forming A The mass proportion of OH is in particular in the range from 10 to 75% by weight, preferably in the range from 15 to 54% by weight, more preferably in the range from 30 to 53% by weight, even more preferably in the range from 40 to 52% by weight and most preferably 50% by weight.

[0048] Step (a) of the process according to the invention preferably comprises a reactive rectification column (or "reaction column") RR A It will be carried out in.

[0049] RR A The reaction column preferably comprises internals. Suitable internals are, for example, trays, structured packing or unstructured packing. A If trays are included, they may be bubble cap trays, valve trays, tunnel trays, Thormann trays, cross-slit bubble cap trays or sieve trays. AWhen trays are included, advantageously, trays are selected such that a maximum of 5% by weight of liquid, preferably less than 1% by weight, drips through each tray. The construction measures necessary to minimize dripping are well known to those skilled in the art. For example, in the case of valve trays, a particularly tight valve design is selected. Reducing the number of valves can further increase the vapor velocity at the tray openings by twice the commonly established value. When sieve trays are used, it is particularly advantageous to reduce the diameter of the tray openings and maintain or increase the number of openings.

[0050] When structured or unstructured packing is used, structured packing is preferred in view of uniform distribution of the liquid.

[0051] Step (a) of the process according to the invention can be carried out continuously or discontinuously, preferably continuously.

[0052] In one embodiment according to the present invention, the reactant stream S containing ROH AE1 and M A OH-containing reactant stream S AE2 Reaction rectification column RR A in In countercurrent The "reaction" refers to, in particular, the reaction tower RR A Reactant streams containing ROH in AE1 At least some of the supply points of M A OH-containing reactant stream S AE2 This is guaranteed by the fact that it is below the supply point of

[0053] In this embodiment, the reaction column RR A is advantageously a reactant stream S AE1 supply point and reactant flow S AE2 At least two theoretical plates, particularly 15 to 40 theoretical plates, are included between the feed point and the column.

[0054] Reactor RR A can be operated as a pure stripping column. In this case, the reactant stream S containing ROH is AE1 is released into the reactor RR in the form of steam. Ais supplied to the lower region of the

[0055] Optionally, a reactant stream S containing ROH AE1 Part of the alkaline solution A OH-containing reactant stream S AE2 Although the feed point of the reactor RR A The reaction mixture is added in the form of steam to the top or top region of the reactor RR. A The size of the lower region of the reactant flow S containing ROH can be reduced. AE1 A part of the reaction mixture, especially in the form of steam, is transferred to the reactor RR A If the glycol is added to the top or in the top region of the reaction column RR, then preferably only a partial amount of 10 to 70% by weight, preferably 30 to 50% by weight (in each case based on the total amount of glycol used) is added to the reaction column RR. A The remaining part is fed into the lower end of M A OH-containing reactant stream S AE2 Preferably, the feed point is below 1 to 10 theoretical plates, more preferably below 1 to 3 theoretical plates, and is added in the form of vapor.

[0056] In an alternative embodiment of step (a) of the method according to the invention, the reactant stream S AE1 and M A OH-containing reactant stream S AE2 Reaction rectification column RR A Countercurrent reaction in the reaction rectification column RR A Located at the bottom of M A OH-containing reactant stream S AE2 This is particularly ensured by the fact that the feed point of RR is above the bottom. During step (a) of the method according to the invention, ROH is then fed to RR A The reactant stream S containing ROH is heated to boiling at the bottom of the reactor. AE1 is then generated. AE1 and S AE2 and are directed countercurrently to each other.

[0057] Reactor RR A Then, a reactant stream S containing ROH is added.AE1 is the reaction described above <1> According to M A OH-containing reactant stream S AE2 In response, M A The reaction forms OR and HO, and since this is an equilibrium reaction, these products are converted to the reactants ROH and M. A OH. Therefore, in step (a), the reaction column RR A In the crude product RP A This gives the product M A In addition to OR and water, ROH and M A Includes OH.

[0058] RR A At the lower end of ROH and M A Bottoms product stream S containing OR AP is obtained and extracted.

[0059] RR A The upper end of the A In a preferred embodiment of the process according to the invention, the vapor stream S comprising water and optionally ROH is AB A water stream, possibly containing ROH, called "recycle" is removed.

[0060] Steam flow S AB If S contains ROH in addition to water, the ROH is preferably obtained by distillation, for example in a rectification column. In this embodiment, at least a portion of the ROH obtained during distillation is added to the reactant stream S AE1 As the reactor RR A can be supplied again to

[0061] In a preferred embodiment, S AB contains water and ROH, the water and ROH are A (described in item 2 below) are at least partially separated from one another.

[0062] Reactive Logistics S AE1 The amount of ROH contained in the bottom product stream S AP M obtained byA Advantageously, the reactant stream S is selected to simultaneously function as a solvent for the reactant stream S. AE1 The amount of ROH in the A Bottoms product stream S containing OR AP M is extracted as A The desired concentration of OR solution is selected to be present at the bottom of the reactor column.

[0063] In a preferred embodiment of the method according to the invention, in particular S AE2 M A If the reactant stream S contains water in addition to OH, AE2 Used as M A The total weight (mass; unit: kg) of OH is AE1 The ratio of the total weight (mass; unit: kg) of the ROH used as the raw material to the total weight (mass; unit: kg) of the ROH used as the raw material is 1:1 to 50:1, more preferably 2:1 to 40:1, even more preferably 3:1 to 30:1, and even more preferably 5:1 to 10:1.

[0064] In a preferred embodiment of the process according to the invention, the reaction column RR A is operated with or without reflux, preferably with reflux.

[0065] "With reflux" means that each tower, especially the reactor RR A A steam stream S containing water and possibly ROH is taken off from the top of the AB This means that the steam flow S AB is at least partly, preferably partly, again as reflux in the respective column, in particular in the reaction column RR A When such reflux is established, the reflux ratio is preferably 0.01 to 1, more preferably 0.02 to 0.9, even more preferably 0.03 to 0.34, especially preferably 0.04 to 0.27, very preferably 0.05 to 0.24, and most preferably 0.2.

[0066] In the context of the present invention, reflux ratio is generally understood to mean the ratio of the proportion of the mass flow (kg / h) taken off from the respective column that is discharged in liquid or gaseous form to the proportion of this mass flow (kg / h) that is returned to the column in liquid form (reflux).

[0067] Reflux can be established by installing a condenser at the top of each column. RRA The condenser is used for this purpose in the reactor RR A The steam flow S AB is the condenser K RRA and at least partially condensed in the respective columns, in particular the reaction column RR A is supplied again.

[0068] Reflux is in the reactor RR A In the embodiment established in the above, in a preferred embodiment of the method according to the invention, the reactant stream S AE2 Used as M A OH may be at least partially mixed with the reflux, and the resulting mixture is then passed through reactor RR A may be supplied to

[0069] In a preferred embodiment of the process according to the invention, step (a) is carried out under distillation conditions, in particular with reflux of ROH.

[0070] Step (a) is especially carried out at a temperature in the range from 45°C to 150°C, preferably from 47°C to 120°C, more preferably from 60°C to 110°C, and at a pressure of from 0.5 bar (absolute) to 40 bar (absolute), preferably in the range from 0.7 bar (absolute) to 5 bar (absolute), more preferably in the range from 0.8 bar (absolute) to 4 bar (absolute), more preferably in the range from 0.9 bar (absolute) to 3.5 bar (absolute), even more preferably from 1.0 bar (absolute) to 3 bar (absolute).

[0071] In a more preferred embodiment, the reaction column RR A is especially suitable for intermediate evaporators. ZA and bottom evaporator VSA The reaction column RR includes at least one evaporator selected from the group consisting of: A is particularly preferably at least one bottom evaporator V SA Includes.

[0072] According to the present invention, the "intermediate evaporator" V Z This refers to the area above the bottom of each column, especially the reaction column RR. A (In this case, "V ZA "), or the rectification column RD used in the preferred embodiment and described in further detail below. A (In this case, "V ZRD This refers to the evaporator located above the bottom of the refrigerant tank (called the RR). A In the case of , among them, especially the lateral flow S ZAA The crude product RP is taken out from the column as A is evaporated.

[0073] According to the invention, the "bottom evaporator" V S The bottom of each column, especially the reaction column RR A or the bottom of the rectification column RD, which is used in the preferred embodiment and is described in further detail below. A The evaporator (in this case, "V SRD " or "V SRD’ RR A In the case of AP At least a portion of the RD is evaporated. A In the case of UA or S UA S is part of UA1 is evaporated.

[0074] The evaporator is generally located outside the respective reaction column or fractionation column.

[0075] Suitable evaporators that can be used as intermediate and bottom evaporators are, for example, natural circulation evaporators, forced circulation evaporators, forced circulation flash evaporators, boiler evaporators, falling film evaporators, or thin film evaporators. In the case of natural circulation evaporators and forced circulation evaporators, tube bundles or plate devices are generally used as the heat exchangers of the evaporators. When using tube bundle heat exchangers, the heat transfer medium flows through the tubes and the mixture to be evaporated can flow around the tubes, or the heat transfer medium flows around the tubes and the mixture to be evaporated flows through the tubes. In the case of falling film evaporators, the mixture to be evaporated is generally added as a thin film to the inside of the tubes, and the tubes are heated from the outside. In contrast to falling film evaporators, thin film evaporators additionally have a wiper rotor that distributes the liquid to be evaporated on the inner wall of the tubes to form a thin film.

[0076] However, in addition to those mentioned, any other evaporator design known to those skilled in the art that is suitable for use in a rectification column can also be used.

[0077] In a preferred embodiment of the method according to the invention, ROH and M A Bottoms product stream S containing OR AP is the reactor RR A The bottom product stream is removed from the bottom end of the column.

[0078] In a preferred embodiment of the optional step (a*) described below, the reaction column RR A At least one bottom evaporator V SA whereby the bottoms product stream S AP is then partially delivered, from which ROH is partially removed, thereby S AP a bottoms product stream S with a reduced mass fraction of ROH compared to AP * is obtained.

[0079] In particular, in the method according to the invention, S AP , or at least one bottom evaporator V SA is used, whereby the bottom product stream S APS when is at least partially delivered and ROH is at least partially removed from it AP * indicates S in each case AP or S AP M in ROH in the range of 1 to 50 wt %, preferably in the range of 5 to 35 wt %, more preferably in the range of 15 to 35 wt %, and most preferably in the range of 20 to 35 wt %, based on the total mass of * A The mass proportion of OR

[0080] S AP or S AP *The mass fraction of residual water in the AP or S AP * is <1% by weight, preferably <0.8% by weight, more preferably <0.5% by weight, based on the total weight of the component (s).

[0081] S AP or S AP *M of reactants A The mass proportion of OH is preferably S AP or S AP * is <1% by weight, preferably <0.8% by weight, more preferably <0.5% by weight, based on the total weight of the component (s).

[0082] As previously described, in a preferred embodiment of the process according to the invention, a steam stream S comprising water and optionally ROH is AB RR A is taken from the top end of the

[0083] 2. Rectifier RD A Steam flow at S AB Rectification of (preferred) In a preferred embodiment of the method according to the invention, the vapor stream S AB If the steam flow S AB Water and steam flow S contained in AB ROH contained in the rectification column RD A are at least partially separated from one another in

[0084] In particular, S containing water and ROH AB is the rectification tower RD A sent to RD A In the step S1, the stream is separated into at least one stream S1 containing water and at least one stream S2 containing ROH.

[0085] It is obvious that the ratio of the boiling points of water and ROH determines which of the two streams S1 (containing water) and S2 (containing ROH) is obtained as vapor stream and bottom stream: (i) When ROH has a boiling point lower than that of water (this is especially true for R=alkyl groups having 1 to 3, preferably 1 to 2 carbon atoms), S2 (including ROH) is RD A is obtained as a vapor stream at the upper end of the RDA and is withdrawn therefrom, and S1 (containing water) is obtained at the lower end of the RDA and is withdrawn therefrom; (ii) If water has a lower boiling point than ROH (this is especially true when R=an alkyl group having 5-6 carbon atoms), then S1 (including water) is RD A S2 (containing ROH) is obtained as a vapor stream at the top of the RD A is obtained at the lower end of the

[0086] Steam flow S AB is fed to the rectification column RD via one or more feed points. A The steam flow S AB are separated into two or more streams from the rectification column RD A In the embodiment of the present invention where the feed points of the individual streams are fed to rectification column RD A It is advantageous if the values ​​are at substantially the same level.

[0087] Another name for the "top of the rectification column" is the "top."

[0088] Another name for the "lower end of the rectification column" is the "bottom" or "lower part."

[0089] Rectification tower RD AAs such, any arbitrary rectification column known to those skilled in the art can be used.

[0090] Rectification tower RD A The packing preferably comprises internals. Suitable internals are, for example, trays, unstructured packings or structured packings. As trays, bubble cap trays, sieve trays, valve trays, tunnel cap trays or slotted trays are generally used. The unstructured packings are generally layers of random packing elements. As random packing elements, Raschig rings, Pall rings, Berl saddles or Intalox® saddles are generally used. Structured packings are, for example, sold by Sulzer under the trade name Mellapack®. In addition to the internals mentioned above, further suitable internals are known to those skilled in the art and can be used as well.

[0091] Preferred internals have a low specific pressure drop per theoretical separation stage. For example, structured packing and random packing elements have a significantly lower pressure drop per theoretical separation stage than trays. This is especially true for rectification column RDs. A This has the advantage that the pressure loss in the compressor is kept as low as possible, and therefore the mechanical power of the compressor and the temperature of the ROH / water mixture to be evaporated are kept low.

[0092] Rectification tower RD A When S contains structured or unstructured packing, they may be divided or may be one continuous packing. However, generally, at least two packings are provided, and the vapor flow S AB One packing above the feed point of the steam flow S AB There is one packing below the feed point of the steam flow S AB One packing above the feed point of the steam flow S ABThere may be multiple trays below the feed point. When non-structured packing, such as random packing element packing, is used, the random packing elements are generally mounted on a suitable support grid (e.g., sieve trays or grid trays).

[0093] In this preferred embodiment, S1 or S2 is then fed to the rectification column RD A S2 or S1 is taken out as a vapor stream from the top of the rectification column RD A The bottom stream is taken out from the bottom end of the column.

[0094] The preferred mass proportion of water in S1 is ≧96.0% by weight, more preferably ≧99.6% by weight, even more preferably ≧99.9% by weight, the remainder being in particular ROH.

[0095] S2 includes ROH, and S2 can preferably have <1 wt. %, more preferably ≦5000 wt. ppm, even more preferably ≦1000 wt. ppm, more preferably ≦100 wt. ppm water.

[0096] Within the scope of the present invention, the rectification column RD A In particular, the at least one vapor stream is taken off at the top of the rectification column RD A This means that the oil is withdrawn above the internals as a top stream or as a side stream.

[0097] Within the scope of the present invention, the rectification column RD A The removal of at least one stream at the bottom of the rectification column RD can be achieved in particular by the removal of at least one stream as a bottoms stream or a A This means that the liquid is taken from the bottom of the container.

[0098] Rectification tower RD A is operated with or without reflux, preferably with reflux.

[0099] "With reflux" means that the rectification column RD AThe vapor stream taken from the top of the rectification column RD is not completely discharged, but is partially condensed and discharged into the rectification column RD. A When such reflux is established, the reflux ratio is preferably 0.01 to 1, more preferably 0.02 to 0.9, even more preferably 0.03 to 0.34, particularly preferably 0.04 to 0.27, very preferably 0.05 to 0.24, and most preferably 0.2.

[0100] Reflux is the rectification column RD A Condenser K at the top of RD The steam flow S can be established by installing OA is the condenser K RD It is partially condensed in the rectification column RD A is supplied again.

[0101] 3. Optional step (a*): S AP Removal of ROH from In a preferred embodiment of step (a) of the process according to the invention, the reactant stream S comprising ROH AE1 , M A OH-containing reactant stream S AE2 and reactive rectification column RR A The reaction is carried out in countercurrent at M A OR, Water, ROH, M A Crude product RP containing OH A Forming M A Bottoms product stream S containing OR and ROH AP RR A A steam stream S containing water and possibly ROH is taken from the lower end of the AB RR A is taken from the top of According to the present invention, ROH is prepared in optional step (a*) by the addition of S AP can be at least partially removed from M A Solution S containing OR and ROH AP *(S AP *S AP The mass fraction of ROH is reduced compared to M AOR is obtained as a solid F*.

[0102] In this preferred embodiment (a*), solution S AP * or solid F* is obtained by S AP depending on whether the ROH is partially or substantially completely removed from the

[0103] S in optional step (a*) AP At least partial removal of ROH from reactor RR can be carried out according to methods known to those skilled in the art. For example, as previously described, A At least one bottom evaporator V SA whereby the bottoms product stream S AP is then partially passed through, from which ROH is partially removed, thereby forming S AP a bottoms product stream S having a reduced content of ROH compared to AP * is obtained.

[0104] Alternatively and preferably, the ROH may be distilled by distillation using a distillation apparatus such as S AP It can also be substantially completely removed from M A OR is obtained as a solid F*.

[0105] When step (a*) is performed, S AP * or F* moisture content is S AP In particular, it is <1% by weight, preferably <0.8% by weight, more preferably <0.5% by weight, based on the total mass of F* or F*.

[0106] 4. Step (b): Conversion of PET to BHET In step (b) of the process according to the invention, PET is reacted with glycol and the M obtained in step (a). A and at least a portion of the OR is converted to BHET in a mixture containing the OR.

[0107] 4.1 PET starting material The PET used in step (b) of the process according to the invention may be any PET that has to be depolymerized. Typically, such PET is generated as waste, especially in households, industries, healthcare systems (e.g. hospitals, clinics) or agriculture.

[0108] In one embodiment of the process according to the invention, the PET to be depolymerized is present in a mixture with other plastics, in particular at least one plastic selected from polyethylene ("PE"), polyvinyl chloride ("PVC"). This is typically the case when PET is depolymerized from plastic waste in the process according to the invention. In this embodiment, the PET is at least partially separated from other plastics, preferably by sorting, before being subjected to step (b) of the process according to the invention.

[0109] In one embodiment of the method according to the invention, the PET is subjected to at least one pretreatment step.

[0110] Such a pretreatment step is described, for example, in DE 100 32 899 C1.

[0111] According to the present invention, the PET is subjected to at least one pretreatment step selected from a chemical pretreatment step and a grinding step before being used in step (b).

[0112] If the PET is present in a mixture with other plastics, the PET is preferably subjected to at least one pretreatment step selected from at least partial separation from the other plastics, preferably by sorting, a chemical pretreatment step, a grinding step, before it is used in step (b).

[0113] If the PET is present in a mixture with other plastics, the PET is preferably first at least partially separated from the other plastics, then chemically pretreated at least once, and finally shredded.

[0114] The chemical pretreatment step is in particular a washing step. Such a washing step has the advantage that, before step (b) is carried out, possible impurities, in particular food residues, cosmetic residues and / or bodily fluids (e.g., blood, semen, feces), are removed. Such impurities may reduce the efficiency of the reaction in step (b) and / or impair the purity of the resulting BHET.

[0115] In the chemical pretreatment step, especially the washing step, the waste is heated, especially in the washing liquid, at a temperature in the range of 30°C to 99°C, preferably in the range of 50°C to 90°C, even more preferably in the range of 70°C to 85°C.

[0116] Typical cleaning solutions are well known to those skilled in the art and are preferably: - an aqueous solution of a surfactant, preferably a non-ionic surfactant; - an aqueous solution of an alkali metal hydroxide or alkaline earth metal hydroxide, preferably an aqueous solution of NaOH is selected from.

[0117] The treatment time of the chemical pretreatment step, especially the washing step, is in particular in the range of 1 minute to 12 hours, preferably in the range of 10 minutes to 6 hours, more preferably in the range of 30 minutes to 2 hours, even more preferably in the range of 45 minutes to 90 minutes, and most preferably 60 minutes.

[0118] After the PET has been treated with chemical pretreatment, particularly washing, the aqueous solution is separated, for example by filtration, and the cleaned PET is preferably washed at least once with water to remove any residues of the washing solution. The PET waste thus obtained is then dried, particularly in a drying cabinet. The temperature used for drying is particularly in the range of 30°C to 120°C, preferably in the range of 50°C to 100°C, more preferably in the range of 60°C to 90°C, and most preferably 80°C.

[0119] The grinding step has the advantage of increasing the surface area of ​​the PET available for reaction in step (b), thereby increasing the reaction rate of the reaction in step (b). Grinding can be carried out in equipment known to those skilled in the art, such as a shredder or cutting mill.

[0120] In a further embodiment of the process according to the invention, the PET is bleached or suitably colored before it is subjected to step (b), which can be carried out using methods known to those skilled in the art, for example bleaching with hydrogen peroxide or coloring with dyes.

[0121] 4.2 Conversion conditions in step (b) In step (b) of the process according to the invention, PET is reacted with glycol and the M obtained in step (a). A and at least a portion of the OR to BHET.

[0122] "PET is mixed with glycol and the M obtained in step (a) A "Converting bis(2-hydroxyethyl) terephthalate BHET into a mixture containing PET, glycol and at least a portion of the M obtained in step (a)" means that step (b) converts bis(2-hydroxyethyl) terephthalate BHET into a mixture containing PET, glycol and at least a portion of the M obtained in step (a). A It is self-evident that the conversion according to step (b) is carried out in a mixture containing at least a portion of PET, M A The alkoxide anion of OR is catalyzed by glycol, which undergoes formal transesterification at an intramolecular ester bond [see structure (≡) shown below].

[0123] Without being bound by any particular theory, the mechanism of decomposition of PET to BHET is as follows: first, the alcohol anion RO - The process involves nucleophilic attack on the ester bond of PET and degradation of the polymer PET, resulting in the intermediate formation of an ester of the terephthalic acid unit with the alcohol ROH, followed by transesterification of this ester with glycol. This is shown schematically below using the ester bond of PET: [ka]

[0124] Step (b) of the process according to the invention can be carried out in any manner familiar to those skilled in the art. Typically, in step (b), the components PET, glycol and the M obtained in step (a) are mixed. A OR are mixed in any order and reaction conditions are established, so that PET is decomposed into BHET according to step (b).

[0125] In particular, in step (b), PET is mixed with glycol and the M obtained in step (a). A At least a portion of the OR (which in a preferred embodiment of step (a) is solution S AP Or Solution S AP * or as solid F*) to produce PET, glycol and M A forming a mixture M1 containing OR, and dissolving the PET in the mixture M1 in glycol and M A and at least partially reacting with OR to form bis(2-hydroxyethyl) terephthalate BHET. Preferably, thereby, after completion of step (b), a bis(2-hydroxyethyl) terephthalate (BHET) containing BHET is obtained, in particular a glycol, M A A mixture M2 is obtained which additionally comprises OR and optionally unconverted PET and optionally MHET and optionally TS.

[0126] In a preferred embodiment of step (b), PET, glycol, M obtained in step (a) A One or two of the three components selected from OR are initially charged, and reaction conditions are established therein. Finally, PET, glycol, and M obtained in step (a) are mixed. A OR are added. Immediately after the addition of this last component, a mixture M1 is then obtained, in which, since the reaction conditions have already been established, the PET is then immediately decomposed into BHET according to step (b), and at the end of step (b), a mixture M2 is then obtained, which mixture M2 contains BHET and in particular glycol, M AIt additionally comprises OR and optionally unconverted PET and optionally MHET and optionally TS.

[0127] In a further alternative embodiment of step (b), which is particularly carried out in a continuous process operation, the components PET, glycol, M obtained in step (a) A At least one, preferably two, preferably all three of the ORs are PET, glycol, M obtained in step (a), A is added to a mixture M1 containing OR and BHET, i.e. in this mixture M1, the conversion of PET to BHET according to step (b) is carried out by mixing the three components PET, glycol, M obtained in step (a), A The addition of at least one of the components PET, glycol, and M obtained in step (a) is carried out. A If at least two of the ORs are added to this mixture M1, they are preferably added separately from each other. Preferably, this allows, after completion of step (b), the addition of the BHET, in particular the glycol, M A A mixture M2 is obtained which additionally comprises OR and optionally unconverted PET and optionally MHET and optionally TS.

[0128] In a preferred embodiment of step (a) according to the present invention, the reactant stream S AE1 , M A OH-containing reactant stream S AE2 and reactive rectification column RR A The reaction is carried out in countercurrent at M A OR, Water, ROH, M A Crude product RP containing OH A is formed, and M A Bottoms product stream S containing OR and ROH AP RR A A steam stream S containing water and possibly ROH is taken from the lower end of the AB RR A is taken from the top of In optional step (a*), ROH is S AP and in so doing, at least partially removed from M AOR as solid F*, or M A Solution S containing OR and ROH AP * is obtained as S AP *S AP The mass ratio of ROH is reduced compared to In step (b), in particular, PET is reacted with glycol and S AP M included in A At least a part of OR, or M included in F* if step (a*) is performed A At least part of OR or S AP *Includes M A and at least a portion of the OR is converted to bis(2-hydroxyethyl) terephthalate, BHET, in a mixture containing the OR.

[0129] "S AP M included in A At least a part of OR, or M included in F* if step (a*) is performed A At least part of OR or S AP *Includes M A "At least part of the OR" means A S containing OR AP or M A F* or M with OR A S containing OR AP * indicates that the PET, glycol, and M A S containing OR AP or, if step (a*) is performed, M A F* or M with OR A S containing OR AP * is produced and then PET is reacted with it and glycol according to step (b) to form BHET.

[0130] The reaction in step (b) is in particular carried out at a temperature of at least 100°C, preferably at a temperature in the range of 100°C to 197°C, more preferably at a temperature in the range of 130°C to 197°C, more preferably at a temperature in the range of 150°C to 197°C, more preferably at a temperature in the range of 175°C to 197°C.

[0131] The reaction in step (b) is preferably carried out at the boiling point of the glycol.

[0132] More preferably, the glycol is refluxed, i.e., the glycol is evaporated from the reaction, condensed, and then returned to the reaction. This reflux can be established by means known to those skilled in the art, for example, in a distillation apparatus.

[0133] This embodiment is particularly characterized in that in step (b), M A OR as a solution in ROH, i.e., S AP or S AP It is advantageous if the glycol is added to the mixture in the form of *, because the excess alcohol ROH, which has a lower boiling point than the glycol, then evaporates from the mixture, which further reduces the generation of by-products.

[0134] Preferably, the reaction in step (b) is of a fixed length, i.e., from a certain time t b until at least P=10%, preferably at least P=20%, more preferably at least P=25%, more preferably at least P=30%, more preferably at least P=40%, more preferably at least P=50%, more preferably at least P=60%, more preferably at least P=70%, more preferably at least P=80%, more preferably at least P=90%, more preferably at least P=95%, and even more preferably at least P=99% of the PET used in step (b) is converted.

[0135] This percentage P is calculated using the following formula: P=(n TS +n MHET +nBHET ) / n PET It is calculated as follows.

[0136] n PET represents the following structure (≡) in the PET used in step (b): [ka] is the amount of substance in the repeating unit.

[0137] n TS is the time from the start of step (b) to time t b is the amount of TS formed in step (b).

[0138] n MHET is the time from the start of step (b) to time t b is the amount of MHET formed in step (b).

[0139] n BHET is the time from the start of step (b) to time t b is the amount of substance of BHET formed in step (b).

[0140] The structures of compounds BHET, MHET, and TS are as follows: [ka] As follows.

[0141] "MHET" also includes the corresponding carboxylate of the structure shown.

[0142] "TS" also includes the corresponding mono- and dicarboxylates of the structure shown.

[0143] In step (b), in particular the M used in step (b) of the method according to the invention AA sufficient amount of M is added so that the total weight of OR is in the range of 0.1 to 100% by weight, preferably in the range of 0.5 to 80% by weight, more preferably in the range of 1.0 to 50% by weight, more preferably in the range of 1.5 to 25% by weight, more preferably in the range of 2.0 to 10% by weight, more preferably in the range of 2.5 to 6.0% by weight, especially preferably in the range of 3.5 to 5.0% by weight, and most preferably 3.9% by weight, based on the total weight of PET used in step (b) of the process according to the invention. A OR is used.

[0144] The ratio by weight (in kg) of glycol used in step (b) of the process according to the invention based on the weight (in kg) of PET used in step (b) of the process according to the invention is in particular in the range from 1:1 to 100:1, preferably in the range from 2:1 to 50:1, more preferably in the range from 3:1 to 40:1, more preferably in the range from 4:1 to 30:1, more preferably in the range from 5:1 to 20:1, more preferably in the range from 6:1 to 10:1, especially preferably in the range from 7:1 to 9:1 and most preferably 8:1.

[0145] The conversion in step (b) can be carried out using equipment well known to those skilled in the art.

[0146] After completion of step (b) of the process according to the invention, preferably in the mixture obtained after step (b), the amount of substance of BHET (n BHET ) and the sum of the amount of substance of MHET and TS (n MHET +n TS ) is in the range of 1:1 to 1000:1, preferably 2:1 to 500:1, more preferably 4:1 to 300:1, even more preferably 10:1 to 100:1, even more preferably 11:1 to 60:1, even more preferably 13:1 to 24:1. In a particularly preferred embodiment, the amount of substance of TS in the mixture obtained after step (b) is undetectable, i.e. = 0. η=n BHET / (n MHET +n TS )

[0147] 4.3 Preferred Step (c) In a preferred further step (c), BHET is at least partially separated from the mixture obtained after completion of step (b), in particular from mixture M2. This is even more preferably carried out by crystallization and / or distillation. Even more preferably, in step (c), BHET is filtered off from the mixture obtained after completion of step (b) and then crystallized.

[0148] 5. Methods for recycling PET The BHET obtained after completion of step (b) in the process according to the invention is preferably polymerized to PET in a process for recycling polyethylene terephthalate PET in step (ζ).

[0149] This polymerization is known to those skilled in the art as "polycondensation" and is described, for example, in EP 0 723 951 A1 and in Chapter 2 "Poly(Ethylen Terephthalate) Polymerization - Mechanism, Catalysis, Kinetics, Mass Transfer and Reactor Design," page 92 of the book "Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters. Edited by J. Scheirs and T.E. Long, 2003, John Wiley & Sons, Ltd. ISBN: 0-471-49856-4" by Th. Rieckmann and S. Voelker.

[0150] In particular, BHET is polymerized in step (ζ) back to PET in the presence of a catalyst, in particular selected from the group consisting of antimony compounds, preferably Sb2O3.

[0151] Preferably, the polymerization of BHET to PET in step (ζ) is carried out at least at the boiling point of the glycol, and in particular, glycol is removed from the reaction mixture during polymerization in step (ζ) to shift the reaction equilibrium toward polymer PET.

[0152] More preferably, the polymerization of BHET to PET in step (ζ) is carried out at the boiling point of the glycol. Even more preferably, glycol is removed from the reaction mixture during polymerization in step (ζ) to shift the reaction equilibrium toward polymer PET.

[0153] This is achieved in particular by distillation at a pressure of <1 bar, preferably 0.1 mbar, at the simultaneous boiling points of the glycols at the respective pressures.

[0154] Example 1. Inventive Example E1: Depolymerization of PET using methanolic sodium methoxide solution from reactive distillation A 30% methanolic sodium methoxide solution was obtained by reactive distillation according to the method described in Example 1.1 of EP 1997794 A1.

[0155] 150 g of PET was then initially charged into the autoclave together with 1200 g of ethylene glycol. The solution was then heated to 175 °C while stirring. As soon as a temperature of 175 °C was reached, 12.3 g of a 30% methanolic sodium methoxide solution (corresponding to 0.068 mol of NaOCH3) obtained by reactive distillation was added. The reaction was carried out for 12 hours, and after cooling, the reactor effluent was analyzed. The conversion obtained was determined by gas chromatography. The amounts of the minor components mono(2-hydroxyethyl)terephthalic acid (= "MHET") (1) and terephthalic acid (= "TS") (2) relative to the main product (BHET) formed are shown in the figure (hatched bars from upper left to lower right, in % of the obtained BHET).

[0156] 2. Comparative Example V1: Depolymerization of PET using conventionally prepared methanolic sodium methoxide solutions In a comparative experiment, 150 g of PET was initially charged into the autoclave together with 1200 g of ethylene glycol. The solution was then heated to 175 °C with stirring. As soon as a temperature of 175 °C was reached, 2.7 g of solid NaOH (corresponding to 0.068 mol of NaOCH3) in 11 g of methanol was added. The reaction was carried out for 5 hours, and after cooling, the reactor effluent was analyzed. The amounts of the minor components mono(2-hydroxyethyl)terephthalic acid (= "MHET") (1) and terephthalic acid (= "TS") (2) relative to the main product (BHET) formed are shown in the figure (bar graph "filled squares").

[0157] 3.Results Comparing the contents of BHET, MHET, and TS in the depolymerized products of Example E1 of the present invention and Comparative Example V1, it can be seen that the amount of BHET obtained in both experiments was almost the same. However, in the case of depolymerization using a methanolic sodium methoxide solution obtained by reactive distillation, the ratio of the undesired by-product MHET to BHET was lower. Furthermore, in the process according to the present invention, TS could not be detected, while the ratio of this undesired by-product in the reaction mixture from V1 was high.

[0158] Accordingly, in the procedure of the present invention, a higher proportion of the degradation product BHET is obtained, which can advantageously be converted directly into the new product PET in polycondensation.

Claims

1. 1. A method for depolymerizing polyethylene terephthalate (PET), comprising: (a) M Ａ OH and ROH are reacted to distill Ａ OR, Ａ is an alkali metal selected from sodium, potassium, and lithium, and R is an alkyl group having 1 to 6 carbon atoms; (b) PET is mixed with glycol and the M obtained in step (a). Ａ and at least a portion of the OR to bis(2-hydroxyethyl) terephthalate BHET. A method comprising:

2. In step (a), a reactant stream S containing ROH is ＡＥ１ A, M Ａ OH-containing reactant stream S ＡＥ２ and reactive rectification column RR Ａ By reacting in countercurrent at M Ａ OR, water, ROH, M Ａ Crude product RP containing OH Ａ Forming M Ａ a bottoms product stream S containing OR and ROH; ＡＰ RR Ａ and a vapor stream S containing water and optionally ROH is taken from the lower end of ＡＢ RR Ａ Remove from the top of In optional step (a*), ROH is converted to S ＡＰ and thereby at least partially removing M Ａ OR as solid F*, or M Ａ Solution S containing OR and ROH ＡＰ * and S ＡＰ * indicates S ＡＰ The mass ratio of ROH is reduced compared to In step (b), PET is mixed with glycol and S ＡＰ M included in Ａ At least a part of OR, or M included in F* when step (a*) is performed Ａ At least a part of OR or S ＡＰ *M included in Ａ and at least a portion of the OR to bis(2-hydroxyethyl) terephthalate BHET. The method of claim 1.

3. S ＡＢ contains water and ROH, and these are separated into the rectification column RD Ａ 3. The method of claim 2, wherein the plurality of electrodes are at least partially separated from one another in a direction parallel to the axis of rotation.

4. S ＡＰ the content of water in the solution, or S if step (a*) is carried out ＡＰ 4. The method according to claim 2 or 3, wherein the content of water in F* or F* is <1% by weight.

5. 5. The process of any one of claims 1 to 4, wherein R is selected from methyl and ethyl.

6. M Ａ 6. The method of claim 1, wherein is an alkali metal selected from sodium and potassium.

7. 7. The process according to claim 1, wherein step (b) is carried out until at least P=10% of the PET used in step (b) has reacted.

8. In step (b), the amount of M used in step (b) relative to the total weight of PET used in step (b) is Ａ A sufficient amount of M to provide a total weight of OR in the range of 0.1 to 100% by weight Ａ 8. The method according to claim 1, wherein OR is used.

9. 9. The process according to any one of claims 1 to 8, wherein BHET is at least partially separated from the mixture obtained after completion of step (b) in a further step (c).

10. 10. The process according to claim 9, wherein the separation of BHET is carried out by crystallization and / or distillation.

11. 11. The method according to any one of claims 1 to 10, wherein prior to use in step (b), the PET is subjected to at least one pretreatment step selected from a chemical pretreatment step, a grinding step.

12. 12. A process for recycling polyethylene terephthalate (PET), comprising obtaining BHET by the process of any one of claims 1 to 11 and polymerizing the BHET so obtained to PET in step (ζ).

13. 13. The method of claim 12, wherein the polymerization of BHET to PET in step (ζ) is carried out at least at the boiling point of the glycol.

14. 14. The process according to claim 12 or 13, wherein the polymerization in step (ζ) is carried out in the presence of a catalyst.

15. 15. The method of claim 14, wherein the catalyst is selected from the group consisting of antimony compounds.

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