Improved process for depolymerization of polyethylene terephthalate

Abstract

The invention relates to a method for depolymerizing polyethylene terephthalate (= "PET"), wherein PET is reacted in a mixture containing ethylene glycol and MAOR to form bis (2-hydroxyethyl) terephthalate (= "BHET"; cAS No.: 959-26-2) in which MA is an alkali metal selected from the group consisting of sodium, potassium, lithium, and in which R is an alkyl group having 1 to 6 carbon atoms. The MAOR is obtained by reactive distillation. The method according to the invention is characterized in that the proportion of unwanted cleavage products mono (2-hydroxyethyl) terephthalate (= "MHET") and terephthalate (= "TS") is particularly low relative to the proportion of BHET. Thus, the process according to the invention provides a high yield of BHET, which can be directly used in regenerative PET production. The invention also relates to a process for recovering PET in which BHET obtained in the PET depolymerization process is re-polymerized to form PET, optionally after further cleaning.

Description

[0001] The present invention relates to a method for depolymerizing polyethylene terephthalate (= "PET"), in which PET is converted to bis(2-hydroxyethyl) terephthalate (= "BHET"; CAS No.: 959-26-2) in a mixture comprising ethylene glycol and M A OR, where M A is an alkali metal selected from sodium, potassium, and lithium, and where R is an alkyl group having 1 to 6 carbon atoms. M A OR is obtained herein by reactive distillation.

[0002] The method according to the invention is characterized in that, based on the proportion of BHET, there is a particularly low proportion of unwanted mono(2-hydroxyethyl) terephthalate (= "MHET") and terephthalate (= "TS") cleavage products. As a result, the method according to the invention provides a high yield of BHET, which can be directly used for new PET production.

[0003] Accordingly, the present invention 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 obtain PET. Background Art

[0004] Polyethylene terephthalate (= "PET") is one of the most important plastics, which is used in textile fibers, as films, and as materials for plastic bottles. In 2007 alone, the amount used in plastic bottles was ~10 7 t (W. Caseri, Polyethylenterephthalate, RD-16-03258 (2009), in F. B. Dill, G. Eisenbrand, F. Faupel, B. Fugmann, T. Gamse, R. Matissek, G. Pohnert, A. Rühling, S. Schmidt, G. Sprenger, [Online], Stuttgart, Georg Thieme Verlag, January 2022).

[0005] Due to the persistence and quantity of waste derived from PET, it constitutes one of the greatest current environmental challenges. The solution to this problem lies in avoiding PET and the effective reuse of PET.

[0006] The prior art has proposed various PET cleavage methods.

[0007] GB784,248A describes the methanolysis of PET.

[0008] The hydrolysis method for PET depolymerization is described in JP2000-309663A, US4,355,175A and T. Yoshioka, N. Okayama, A. Okuwaki, Ind. Chem. Res. 1998, 37, 336-340.

[0009] The reaction of PET with ethylene glycol is described in EP0723951 A1, US3,222,299A, WO2020 / 002999A2, Journal of Applied Polymer Science 2005, 97, 513-517 by S. R. Shukla, A. M. Harad (hereinafter referred to as "Shukla & Harad") and European Polymer Journal 2008, 44, 4151-4156 by N. D. Pingale, S. R. Shukla.

[0010] Shukla & Harad stated that the glycolysis of PET produces bis(2-hydroxyethyl) terephthalate (="BHET"). This cleavage product can be used simultaneously as a reactant for producing new PET. In contrast, certain by-products such as mono(2-hydroxyethyl) terephthalate (="MHET") monoester or free terephthalic acid or the corresponding carboxylic acid esters, terephthalate (="TS") are disadvantageous because these by-products cannot be used as reactants for producing new PET.

[0011] Therefore, there is an interest in methods for PET depolymerization in which the largest possible proportion of BHET is obtained in the cleavage product while the proportions of unwanted by-products such as MHET and TS should be minimized.

[0012] The problem solved by the present invention is to provide such a method. Summary of the Invention

[0013] It has been found, surprisingly, that the reaction of PET with M A OR obtained by reactive distillation in ethylene glycol provides a lower proportion of unwanted MHET and TS by-products than BHET-based in conventional methods.

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

[0015] Therefore, the present invention relates to a method for depolymerizing polyethylene terephthalate PET, comprising the following steps:

[0016] (a) Converting M A OH and ROH into M A OR in reactive distillation.

[0017] Here M A is an alkali metal selected from sodium, potassium, lithium, in particular an alkali metal selected from sodium, potassium, and preferably M A = sodium.

[0018] R is an alkyl group having 1 - 6, in particular 1 - 5, preferably 1 - 4, more preferably 1 - 3 carbon atoms. Even more preferably, R is methyl or ethyl. Most preferably, R = methyl.

[0019] (b) converting PET to bis(2 - hydroxyethyl) terephthalate BHET in a mixture comprising ethylene glycol and at least a portion of M A OR obtained in step (a).

[0020] On the other hand, the present invention relates to a method for recycling PET, wherein the BHET obtained in the depolymerization method according to the present invention is polymerized in step (ζ) to obtain PET. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The figure shows a comparison of the contents of mono(2 - hydroxyethyl) terephthalate ("MHET"; "1") and terephthalic acid ("TS"; "2") during depolymerization using sodium methoxide obtained by the method according to the present invention and using sodium methoxide obtained by a conventional method. The contents of the corresponding by - products are reported herein based on the content of BHET (in mole percentage).

[0022] In the PET depolymerization according to Example E1 of the present invention (wherein the sodium methoxide for depolymerization is obtained by reactive distillation), the respective contents of MHET and TS in the reactor output are each represented by a shaded bar ("\\\\\") in each case. Only a significant proportion of MHET was found, which is why there is no shaded bar at "2".

[0023] The black bars show the corresponding contents of MHET and TS in the reactor output during PET depolymerization according to Comparative Example V1, where the sodium methoxide for depolymerization is obtained by mixing NaOH and methanol in a reactor. DETAILED DESCRIPTION OF THE INVENTION

[0025] It has now been found, surprisingly, that when PET is cracked in a mixture of ethylene glycol and M A OR obtained by reactive distillation, the proportion of the desired BHET cracking product in the glycolysis of PET is increased relative to the proportion of the unwanted TS and MHET cracking products.

[0026] Thus, the method according to the present invention is superior to the prior art methods, in which, for example, cracking is carried out in a mixture obtained by dissolving an alkali metal hydroxide in ethylene glycol and ROH.

[0027] 1. Step (a): Preparation of M by reactive distillation A OR

[0028] According to the present invention, the alkali metal alkoxide M used in the method according to the present invention A OR obtained by the conversion of M A OH and ROH by reactive distillation.

[0029] M A is an alkali metal selected from lithium, sodium, potassium, especially selected from sodium and potassium. M A is preferably sodium.

[0030] R is an alkyl group having 1 to 6 carbon atoms, especially 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.

[0031] The alkyl group having 1 to 6 carbon atoms is especially selected from 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-dimethylbutyl, 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 methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, n-hexyl, even more preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, n-pentyl, n-hexyl.

[0032] In the context of the present invention, the alkyl group having 1 to 5 carbon atoms is especially selected from 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 methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, more preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, n-pentyl.

[0033] In the context of the present invention, an alkyl group having 1 to 4 carbon atoms is particularly selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, tert-butyl, and more preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl.

[0034] In the context of the present invention, an alkyl group having 1 to 3 carbon atoms is particularly selected from methyl, ethyl, n-propyl, isopropyl, and is preferably selected from methyl, ethyl, isopropyl.

[0035] "Ethylene glycol" in the context of the present invention is to be understood as referring to ethylene-1,2-diol having the chemical formula HO-CH 2 -CH 2 -OH (CAS No. 107-21-1).

[0036] 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 many chemicals, such as in the production of active pharmaceuticals or agrochemical components, and as catalysts in transesterification and amidation reactions.

[0037] Alkali metal alkoxides (MOR) are generally prepared by reactive distillation of alkali metal hydroxides (MOH) and alcohols (ROH) in a countercurrent distillation column, where the water of reaction formed according to the following reaction <1> is removed together with the distillate:

[0038]

[0039] The principle of this method is described, for example, in US2,877,274A, where an aqueous solution of alkali metal hydroxide and gaseous methanol proceed in a rectification column in countercurrent. This method is described again in WO01 / 42178A1 in substantially the same form.

[0040] The most important alkali metal alkoxides industrially are those of sodium and potassium, especially methoxides and ethoxides. There are many descriptions of their synthesis in the prior art, such as in EP1997794 A1, WO2021 / 148174A1, and WO2021 / 148175A1.

[0041] Similar methods are described in GB377,631A and US1,910,331A, but an entrainer, such as benzene, is additionally used therein.

[0042] Accordingly, DE968903C describes a process for the continuous preparation of alkali metal alcoholates in a reaction column, in which the water-alcohol mixture removed at the top of the column is condensed and then phase-separated. The aqueous phase is discarded and the alcohol phase is returned together with fresh alcohol to the top of the column. EP0299577 A2 describes a similar process in which water in the condensate is separated by means of a membrane.

[0043] In a preferred embodiment of the process according to the invention, in step (a), a reaction stream S containing ROH AE1 is reacted countercurrently in a reactive distillation column RR A with a reaction stream S containing M AE2 OH to give a crude product RP containing M A OR, water, ROH, M A OH, where a bottoms product stream S containing M A OR and ROH is withdrawn at the lower end of RR A , and a vapor stream S containing water with or without ROH is withdrawn at the upper end of RR A . A AP A AB

[0044] According to the invention, a "reactive distillation column" is defined as a distillation column in which the reaction of step (a) of the process according to the invention takes place at least in some parts. It may also be abbreviated as "reaction column".

[0045] The reaction stream S AE1 contains ROH. In a preferred embodiment, the mass fraction of ROH in S AE1 is ≥95 wt%, more preferably ≥99.5 wt%, where S AE1 additionally contains especially water.

[0046] The ROH used as the reaction stream S AE1 in a preferred embodiment of the process according to the invention can also be a commercially available alcohol ROH having an ROH mass proportion of greater than 99.5 wt% and a water mass proportion of at most 0.03 wt%.

[0047] In a specific embodiment of the invention, the reaction stream S AE1 is added in vapor form to the reactive distillation column RR A .

[0048] In an alternative preferred embodiment of the process according to the invention, the ROH is first charged to the bottom of the reactive distillation column RR A before step (a) and then heated to boiling in step (a), which in the reactive distillation column RR AReactant S with a constant flow rate is generated AE1 If necessary, ROH is supplemented at the bottom of the reactive distillation column RR A during the execution of step (a).

[0049] The reaction stream S AE2 contains M A OH. In a preferred embodiment, in addition to M A OH, S AE2 further comprises at least one additional compound selected from water and ROH. Even more preferably, S AE2 in addition to M A OH further comprises water. In this case, S AE2 is an aqueous solution of M A OH.

[0050] When the reaction stream S AE2 contains M A OH and water, based on the total weight of the aqueous solution forming S AE2 the mass ratio of M A OH is especially in the range of 10% to 75% by weight, preferably in the range of 15% to 54% by weight, more preferably in the range of 30% to 53% by weight, still more preferably in the range of 40% to 52% by weight, and most preferably 50% by weight.

[0051] Step (a) of the process according to the invention is preferably carried out in a reactive distillation column (or "reaction column") RR A .

[0052] The reaction column RR A preferably comprises internals. Suitable internals are, for example, trays, structured packings or random packings. When the reaction column RR A comprises trays, suitable trays are bubble-cap trays, valve trays, trough bubble-cap trays, Thormann trays, cross-slit bubble-cap trays or sieve trays. When the reaction column RR A contains trays, trays are preferably selected in which no more than 5% by weight, more preferably less than 1% by weight, of the liquid drips through the respective trays. The construction measures required to minimize the dripping of the liquid are familiar to those skilled in the art. For example, in the case of valve trays, a particularly tightly closed valve design is selected. Reducing the number of valves also allows the vapor velocity in the tray openings to be increased to twice the normally established value. When using sieve trays, it is particularly advantageous to reduce the diameter of the tray openings and maintain or even increase the number of openings.

[0053] When using regular or irregular packings, regular packings are preferred in terms of the uniform distribution of the liquid.

[0054] Step (a) of the process according to the invention can be carried out continuously or batchwise. It is preferably carried out continuously.

[0055] In one embodiment according to the invention, "the reaction stream S containing ROH AE1 and the reaction stream S containing M A OH AE2 reacting in countercurrent in the reactive distillation column RR A is ensured in particular by the following: the feed point of at least a part of the reaction stream S containing ROH in the reaction column RR A is lower than the feed point of the reaction stream S containing M AE1 OH. A AE2 "

[0056] In this embodiment, the reaction column RR A preferably contains at least 2, in particular 15 - 40, theoretical plates between the feed point of the reaction stream S AE1 and the feed point of the reaction stream S AE2 ".

[0057] The reaction column RR A can be operated as a pure stripping column. In this case, the reaction stream S containing ROH AE1 is introduced in vapor form into the lower region of the reaction column RR A ".

[0058] Optionally, a part of the reaction stream S containing ROH AE1 is added in vapor form below the feed point of the reaction stream S containing the alkali metal hydroxide solution M A OH but still in the upper end or upper region of the reaction column RR AE2 ". This makes it possible to reduce the size of the lower region of the reaction column RR A ". When a part of the reaction stream S containing ROH A is added, especially in vapor form, in the upper end or upper region of the reaction column RR AE1 it is preferred to feed only a part, i.e. 10 wt% to 70 wt%, preferably 30 wt% to 50 wt% (in each case based on the total amount of ethylene glycol used) at the lower end of the reaction column RR A and the remaining part in vapor form in a single stream or divided into a plurality of sub - streams, preferably 1 to 10 theoretical plates, more preferably 1 to 3 theoretical plates, below the reaction stream S containing M A OH A AE2 ​​is added at the feed point.

[0059] In an alternative embodiment of step (a) of the process according to the invention, the reaction stream S comprising ROH AE1 is reacted with the reaction stream S comprising M A OH AE2 in the reactive distillation column RR A in countercurrent in the following way is particularly ensured: ROH is present at the bottom of the reactive distillation column RR A and the feed point of the reaction stream S comprising M A OH is above the bottom. During step (a) of the process according to the invention, the ROH is then heated to boiling at the bottom of the RR AE2 and a reaction stream S comprising ROH is generated A . Then S AE1 and S AE1 are led countercurrent to each other. AE2

[0060] In the reaction column RR A the reaction stream S comprising ROH AE1 then reacts according to the above reaction <1> with the reaction stream S comprising M A OH to give M AE2 OR and H A O, and since the reaction is an equilibrium reaction, these products are present in a mixture with the ROH and M 2 OH reactants. Thus, in step (a) in the reaction column RR A a crude product RP A is obtained which, in addition to the M A OR and water products, also contains ROH and M A OH. A

[0061] Then a bottoms product stream S A comprising ROH and M AP OR is obtained and withdrawn at the lower end of the RR A .

[0062] At the upper end of the RR A , preferably at the top of the RR A column, in a preferred embodiment of the process according to the invention, a water stream which may or may not contain ROH is withdrawn, hereinafter referred to as "vapour stream S AB comprising water, with or without ROH".

[0063] If the vapour stream S ABContains not only water but also ROH. For example, in a distillation column, ROH is preferably also obtained by distillation. In this embodiment, at least a part of the ROH obtained in the distillation can be used as the reaction stream S AE1 The feed returns to the reaction column RR A .

[0064] In a preferred embodiment, when S AB contains water and ROH, water and ROH are at least partially separated from each other in the distillation column RD A (hereinafter referred to with reference to point 2).

[0065] Preferably select the amount of ROH contained in the reaction stream S AE1 such that the ROH is simultaneously used as a solvent for M AP obtained in the bottom product stream S A OR. Preferably select the amount of ROH in the reaction stream S AE1 such that the required concentration of the bottom product stream S containing ROH and M A OR is taken out as the M AP OR solution is present at the bottom of the reaction column. A

[0066] In a preferred embodiment of the process according to the invention, and especially in the case where S AE2 contains water in addition to M A OH, the total weight (mass; unit: kg) of the ROH used as the reaction stream S AE1 and the total weight (mass; unit: kg) of the M AE2 OH used as the reaction stream S A have a ratio of 1:1 to 50:1, more preferably 2:1 to 40:1, even more preferably 3:1 to 30:1, still more preferably 5:1 to 10:1.

[0067] 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.

[0068] "With reflux" means that the vapor stream S containing water with or without ROH taken out at the upper end of the corresponding column (especially the reaction column RR A ) is not completely removed. Therefore, the relevant vapor stream S AB is at least partially, preferably partially, fed back as reflux to the corresponding column, especially the reaction column RR AB A ​​In the case of establishing such a reflux, the reflux ratio is preferably from 0.01 to 1, more preferably from 0.02 to 0.9, still more preferably from 0.03 to 0.34, particularly preferably from 0.04 to 0.27, very particularly preferably from 0.05 to 0.24, and most preferably 0.2.

[0069] The reflux ratio is generally understood in the context of the present invention to mean the ratio of the proportion of the mass flow rate (kg / h) withdrawn from the column removed from the corresponding column in liquid or gaseous form to the proportion of this mass flow rate (kg / h) returned to the column (reflux) in liquid form.

[0070] The reflux can be established by installing a condenser at the top of the corresponding column. For this purpose, in particular, the condenser K RRA is installed on the reaction column RR A above. In the condenser K RRA the vapor stream S AB is at least partially condensed and fed back to the corresponding column, in particular the reaction column RR A .

[0071] In an embodiment in which a reflux is established in the reaction column RR A in a preferred embodiment of the process according to the invention, the M AE2 used as the reaction stream S A OH can also be at least partially mixed with the reflux stream, and the resulting mixture can thus be supplied to the reaction column RR A .

[0072] In a preferred embodiment of the process according to the invention, step (a) is carried out in particular under the distillation conditions of the ROH reflux.

[0073] Step (a) is carried out in particular 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 an absolute pressure in the range from 0.5 bar absolute to 40 bar absolute, preferably 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, more preferably at an absolute pressure from 1.0 bar absolute to 3 bar absolute.

[0074] In a more preferred embodiment, the reaction column RR A comprises at least one evaporator selected in particular from the intermediate evaporator V ZA and the bottom evaporator V SA . The reaction column RR A more preferably comprises at least one bottom evaporator V SA .

[0075] According to the invention, the "intermediate evaporator" V ZRefers to an evaporator above the bottom of the corresponding column, in particular an evaporator above the bottom of reaction column RR A (in which case they are referred to as "V ZA ") or above the bottom of distillation column RD A (in which case they are referred to as "V ZRD "), distillation column RD A is used in a preferred embodiment and is further described in detail below. In the case of RR A the evaporator in particular evaporates the crude product RP A which is withdrawn from the column as a side stream S ZAA .

[0076] According to the invention, "bottom evaporator" V S refers to an evaporator that heats the bottom of the corresponding column, in particular the bottom of reaction column RR A or the bottom of distillation column RD A (in which case they are referred to as "V SRD " or "V SRD' "). In the case of RR A the evaporator in particular evaporates at least a portion of the bottom product stream S AP . In the case of RD A the evaporator in particular evaporates a portion of the bottom product stream S UA or S UA i.e. S UA1 .

[0077] Evaporators are generally arranged outside the corresponding reaction column or distillation column.

[0078] Suitable evaporators that can be used as intermediate evaporators and bottom evaporators include, for example, natural circulation evaporators, forced circulation evaporators, forced circulation flash evaporators, kettle evaporators, falling film evaporators or thin film evaporators. Heat exchangers for evaporators commonly used in the case of natural circulation evaporators and forced circulation evaporators are shell-and-tube or plate-type devices. When using a shell-and-tube exchanger, the heat carrier can flow through the tubes and the mixture to be evaporated around the tubes, or the heat carrier flows around the tubes and the mixture to be evaporated through the tubes. In the case of a falling film evaporator, the mixture to be evaporated is generally introduced as a film on the inside of the tube, and the tube is heated on the outside. Compared with a falling film evaporator, a thin film evaporator additionally includes a rotor with scraping blades that distributes the liquid to be evaporated on the inner wall of the tube to form a film.

[0079] In addition to those mentioned, any other desired types of evaporators known to those skilled in the art and suitable for distillation columns can also be used.

[0080] In a preferred embodiment of the process according to the invention, a bottoms product stream S comprising ROH and M A OR AP is withdrawn as the bottoms product stream at the lower end of the reaction column RR A .

[0081] In a preferred embodiment of the optional step (a*) described below, the reaction column RR A comprises at least one reboiler V SA , and the bottoms product stream S AP is then passed partially through this reboiler and the ROH is partially removed therefrom, thereby providing a bottoms product stream S AP having a reduced ROH mass fraction compared to S AP* .

[0082] In particular, in the process according to the invention, S AP or S AP* (if at least one reboiler V through which at least some of the bottoms product stream S AP is passed and the ROH is at least partially removed therefrom) has an M SA OR mass fraction in the ROH in the range from 1 wt% to 50 wt%, preferably in the range from 5 wt% to 35 wt%, more preferably in the range from 15 wt% to 35 wt%, most preferably in the range from 20 wt% to 35 wt%, in each case based on the total mass of S A or S AP or S AP* .

[0083] Based on the total mass of S AP / S AP* , the mass fraction of residual water in S AP / S AP* is preferably <1 wt%, preferably <0.8 wt%, more preferably <0.5 wt%.

[0084] Based on the total mass of S AP / S AP* , the mass fraction of the reactant M A OH in S AP / S AP* is preferably <1 wt%, preferably <0.8 wt%, more preferably <0.5 wt%.

[0085] As described above, in a preferred embodiment of the process according to the invention, a vapor stream S comprising water with or without ROH is withdrawn at the upper end of RR A . AB

[0086] 2. Vapor stream S​AB In distillation column R DA the distillation (preferably)

[0087] In a preferred embodiment of the process according to the invention, when the vapor stream S AB contains water and ROH, these (i.e., the water contained in the vapor stream S AB and the ROH contained in the vapor stream S AB are at least partially separated from each other in distillation column RD A .

[0088] In particular, S containing water and ROH AB is led to distillation column RD A and separated in RD A into at least one stream S containing water 1 and at least one stream S containing ROH 2 .

[0089] It is obvious here that the boiling point ratio of water and ROH determines which of the two streams S 1 (containing water) and S 2 (containing ROH) is obtained as the vapor stream and the bottom stream:

[0090] (i) If ROH has a lower boiling point than water, especially when R = an alkyl group having 1 to 3, preferably 1 to 2 carbon atoms, then S 2 (containing ROH) is obtained as the vapor stream and taken off at the upper end of RD A , and S 1 (containing water) is obtained and taken off at the lower end of RD A ;

[0091] (ii) If water has a lower boiling point than ROH, especially when R = an alkyl group having 5 to 6 carbon atoms, then S 1 (containing water) is obtained as the vapor stream and taken off at the upper end of RD A , and S 2 (containing ROH) is obtained and taken off at the lower end of RD A .

[0092] The vapor stream S AB can be led to distillation column RD A via one or more feed points. In an embodiment of the invention in which the vapor stream S AB is led to distillation column R DA as two or more separate streams, it is advantageous when the feed points of the individual streams are at substantially the same height on distillation column RD A .

[0093] Another term for the "upper end of the distillation column" is "top".

[0094] Another term for the "lower end of the distillation column" is "bottom" or "foot".

[0095] The distillation column RD used A can be any distillation column known to those skilled in the art.

[0096] The distillation column RD A preferably includes internals. Suitable internals are, for example, trays, random packings or structured packings. The trays used are usually bubble-cap trays, sieve trays, valve trays, trough bubble-cap trays or strip valve perforated trays. Random packings are usually beds of random packing elements. The random packing elements used are usually Raschig rings, Pall rings, Berl saddles or saddle packings. Structured packings are, for example, sold under the Sulzer trade name. In addition to the internals mentioned, other suitable internals are known to those skilled in the art and can equally be used.

[0097] The preferred internals have a low pressure drop per theoretical plate. Structured packings and random packing elements have, for example, a significantly lower pressure drop per theoretical plate than trays. This has the advantage that the pressure drop in the distillation column RD A remains as low as possible, and the mechanical power of the compressor and the temperature of the ROH / water mixture to be evaporated are thus kept low.

[0098] When the distillation column RD A contains structured packings or random packings, these packings can be separate or in the form of uninterrupted packings. However, usually at least two packings are provided, one packing above the feed point of the vapor stream S AB and one packing below the feed point of the vapor stream S AB . It is also possible to provide one packing above the feed point of the vapor stream S AB and two or more trays below the feed point of the vapor stream S AB . If random packings, such as random packing elements, are used, the random packing elements are usually arranged on a suitable support grid (such as a sieve tray or a mesh tray).

[0099] In this preferred embodiment, then S A or S 1 or S 2 is taken out as the vapor stream at the upper end of the distillation column RD A and S 2 or S 1Withdrawn as bottom stream.

[0100] Water in S 1 The preferred mass ratio is ≥ 96.0% by weight, more preferably ≥ 99.6% by weight, still more preferably ≥ 99.9% by weight, and the remainder is especially ROH.

[0101] S 2 Includes ROH, where S 2 May preferably include < 1% by weight, more preferably ≤ 5000 ppm by weight, still more preferably ≤ 1000 ppm by weight, and more preferably ≤ 100 ppm by weight of water.

[0102] In the context of the present invention, withdrawing at least one vapor stream from the top of the rectification column RD A More specifically means that at least one vapor stream is withdrawn as a top stream or as a side stream above the internals in the rectification column RD A

[0103] In the context of the present invention, withdrawing at least one stream from the bottom of the rectification column RD A More specifically means that at least one stream is withdrawn as a bottom stream or from the lower trays of the rectification column RD A

[0104] Rectification column RD A Operates with or without reflux, preferably with reflux.

[0105] "With reflux" means that the vapor stream withdrawn from the upper end of the rectification column RD A Is not completely discharged, but is partially condensed and returned to the rectification column RD A . In the case of establishing such reflux, the reflux ratio is preferably 0.01 to 1, more preferably 0.02 to 0.9, still more preferably 0.03 to 0.34, particularly preferably 0.04 to 0.27, very particularly preferably 0.05 to 0.24, and most preferably 0.2.

[0106] Reflux can be established by installing a condenser K A At the top of the rectification column RD RD . The corresponding vapor stream S OA Is partially condensed in the condenser K RD And returned to the rectification column RD A .

[0107] 3. Optional step (a*): Removing ROH from S AP

[0108] In a preferred embodiment of step (a) of the process according to the invention, wherein the reaction stream S containing ROH AE1 Is contacted with the stream containing M A ​​​Reaction stream S of OH AE2 In reactive distillation column RR A a countercurrent reaction occurs to obtain a crude product RP containing M A OR, water, ROH, M A OH A ,

[0109] and wherein at the lower end of RR A a bottoms product stream S containing M A OR and ROH is withdrawn, AP and wherein at the upper end of RR A a vapor stream S containing water with or without ROH is withdrawn AB ,

[0110] According to the present invention, in an optional step (a*), ROH can be at least partially removed from S AP , which provides a solution S containing M A OR and ROH AP *or M OR in solid form F* A OR, wherein S AP *has a reduced ROH mass fraction compared to S AP .

[0111] Whether the preferred embodiment (a*) provides solution S AP *or solid material F* depends on whether ROH is removed from S AP partially or substantially completely.

[0112] At least partial removal of ROH from S AP *in the optional step (a*) can be carried out by methods known to those skilled in the art. For example, as described above, the reaction column RR A can include at least one reboiler V SA , and the bottoms product stream S AP is then partially passed through the reboiler V SA , and ROH is partially removed therefrom, thereby providing a bottoms product stream S with a reduced ROH content compared to S AP . AP* .

[0113] Alternatively and preferably, ROH can also be substantially completely removed from S AP , for example, in a distillation apparatus known to those skilled in the art. In this case, M A OR in solid form F* is obtained.

[0114] If step (a*) is carried out, based on the total mass of S AP *or F*, S APThe water content in *or F* is in particular < 1% by weight, preferably < 0.8% by weight, more preferably < 0.5% by weight.

[0115] 4. Step b): Conversion of PET into BHET

[0116] In step (b) of the process according to the invention, PET is converted into BHET in a mixture comprising ethylene glycol and at least a portion of M A OR obtained in step (a).

[0117] 4.1 PET feedstock

[0118] The PET used in step (b) of the process according to the invention can be any PET that has to be depolymerized. Generally, such PET occurs as waste, especially in households, in industry, in the sanitary system (e.g., hospitals, doctors' surgeries) or in agriculture.

[0119] In one embodiment of the process according to the invention, the PET to be depolymerized is thus in a mixture with other plastics, especially at least one plastic selected from polyethylene ("PE"), polyvinyl chloride ("PVC"). This is usually the case when depolymerizing PET from plastic waste in the process according to the invention. In this embodiment, before being subjected to step (b) of the process according to the invention, the PET is at least partially separated from the other plastics, preferably by sorting.

[0120] In one embodiment of the process according to the invention, the PET is subjected to at least one pretreatment step.

[0121] Such pretreatment steps are described, for example, in DE10032899 C2.

[0122] According to the invention, the PET is subjected to at least one pretreatment step selected from chemical pretreatment steps, comminution steps before being used in step (b).

[0123] In the case where the PET is in a mixture with other plastics, before being used in step (b), the PET is preferably subjected to at least one pretreatment step selected from at least partial separation from the other plastics, preferably by sorting, chemical pretreatment steps, comminution steps.

[0124] In the case where the PET is in a mixture with other plastics, more preferably the PET is first at least partially separated from the other plastics and then subjected to at least one chemical pretreatment and finally comminuted.

[0125] The chemical pretreatment step is especially the washing step. The advantage of such a washing step is that any impurities, especially food residues, cosmetic residues and / or body secretions (such as blood, sperm, feces), are removed before step (b). Such impurities can reduce the efficiency of the reaction in step (b) and / or deteriorate the purity of the BHET obtained thereby.

[0126] In the chemical pretreatment step, especially in the washing step, the waste is heated, especially in the washing solution, at a temperature in the range of 30 °C to 99 °C, preferably in the range of 50 °C to 90 °C, more preferably in the range of 70 °C to 85 °C.

[0127] Typical washing solutions are familiar to those skilled in the art and are preferably selected from:

[0128] - Surfactants, preferably aqueous solutions of non-ionic surfactants;

[0129] - Aqueous solutions of alkali metal hydroxides or alkaline earth metal hydroxides; preferably an aqueous NaOH solution.

[0130] The treatment time in the chemical pretreatment step, especially in the washing step, is especially 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 to 90 minutes, and most preferably 60 minutes.

[0131] After treating the PET by the chemical pretreatment step (especially the washing step), the aqueous solution is separated, for example, by filtration, and the cleaned PET is preferably washed with water at least once to remove the residues of the washing solution.

[0132] Then the PET waste obtained thereby is dried, especially in a drying oven.

[0133] The temperature used for drying here is especially 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.

[0134] The comminution step has the advantage of increasing the surface area of the PET available for the reaction in step (b). This increases the reaction rate of the reaction in step (b). Comminution can be carried out in equipment known to those skilled in the art, such as a pulverizer or a cutting mill.

[0135] In another embodiment of the method according to the invention, the PET is decolorized or colored in a controlled manner before step (b). This can be carried out by methods known to those skilled in the art, such as decolorization with hydrogen peroxide or dyeing with dyes.

[0136] 4.2 Conversion conditions in step (b)

[0137] In step (b) of the process according to the invention, PET is converted to BHET in a mixture comprising ethylene glycol and at least a portion of M A OR obtained in step (a).

[0138] It is evident that "PET is converted to bis(2-hydroxyethyl) terephthalate BHET in a mixture comprising ethylene glycol and at least a portion of M A OR obtained in step (a)" means that step (b) is carried out in a mixture comprising PET, ethylene glycol and at least a portion of M A OR obtained in step (a). In the conversion of step (b), PET undergoes a formal transesterification with ethylene glycol at the internal ester bond [see structure (≡) shown below, where the alkoxide anion of M A OR acts as a catalyst].

[0139] Without being bound by any particular theory, the mechanism for the cleavage of PET to BHET first involves a nucleophilic attack of the alkoxide anion RO - on the ester bond and the cleavage of the polymer PET, which results in the intermediate formation of an ester of the terephthalic acid unit with the alcohol ROH, followed by transesterification of this ester with ethylene glycol. This is schematically shown below for the ester bond of PET:

[0140]

[0141] Step (b) of the process according to the invention can be carried out in any manner familiar to those skilled in the art. Generally, in step (b), the components PET, ethylene glycol and M A OR obtained in step (a) are mixed in any order, and the reaction conditions for cleaving PET to BHET in step (b) are established.

[0142] In particular, in step (b), PET is mixed with ethylene glycol and at least a portion of M A OR obtained in step (a) (in a preferred embodiment of step (a), the M A OR is obtained in the form of solution S AP or solution S AP * or in the form of solid material F*), to obtain a mixture M A comprising PET, ethylene glycol and M 1 OR, and the PET in mixture M 1 is reacted at least in part with ethylene glycol and M A OR to obtain bis(2-hydroxyethyl) terephthalate BHET. After the end of step (b), this preferably provides a mixture M 2, and which particularly further comprises ethylene glycol, M A OR and any unreacted PET, with or without MHET and with or without TS.

[0143] In a preferred embodiment of step (b), one or two of the three components selected from PET, ethylene glycol, and M A OR obtained in step (a) are first added, reaction conditions are established therein, and then the other components selected from PET, ethylene glycol, and M A OR obtained in step (a) are finally added. After adding this last component, a mixture M 1 is then immediately obtained, wherein since the reaction conditions have been established, PET is then immediately cracked into BHET in step (b), and at the end of step (b), a mixture M 2 is then obtained, which contains BHET, and which particularly further comprises ethylene glycol, M A OR and any unreacted PET, with or without MHET and with or without TS.

[0144] In another alternative embodiment of step (b), which is particularly carried out in a continuous process scheme, at least one, preferably two, preferably all three of the components PET, ethylene glycol, and M A OR obtained in step (a) are fed into a mixture M A containing PET, ethylene glycol, M 1 OR obtained in step (a), and BHET, which means that during the addition of at least one of the three components PET, ethylene glycol, and M A OR obtained in step (a), the conversion of PET to BHET in step (b) is carried out in this mixture M 1 . In the case where at least two of the components PET, ethylene glycol, and M A OR obtained in step (a) are added to this mixture M 1 , these are particularly added separately from each other. After the end of step (b), this preferably provides a mixture M 2 containing BHET, and which particularly further comprises ethylene glycol, M A OR and any unreacted PET, with or without MHET and with or without TS.

[0145] In a preferred embodiment of step (a) according to the invention, wherein a reaction stream S AE1 containing ROH is reacted countercurrently with a reaction stream S A containing M AE2 OH in a reactive distillation column RR A to obtain a product containing M A OR, water, ROH, MA The crude product RP of OH A , wherein at the lower end of RR A , a bottoms product stream S containing M A OR and ROH is withdrawn AP , and wherein at the upper end of RR A , a vapor stream S containing water is withdrawn AB , with or without ROH,

[0146] and wherein, in an optional step (a*), at least a portion of ROH is removed from S AP such that M A OR is obtained in solid form F* or as a solution S containing M A OR and ROH, AP wherein S AP * has a reduced mass fraction of ROH compared to S AP

[0147] This particularly includes, in step (b), converting PET to bis(2-hydroxyethyl) terephthalate BHET in a mixture containing ethylene glycol and at least a portion of M AP contained in S A OR, or at least a portion of M A contained in F* AP OR or at least a portion of M A contained in S

[0148] "At least a portion of M AP contained in S A OR, or at least a portion of M A contained in F* AP OR or at least a portion of M A contained in S A * (if step (a*) is carried out)" implies the addition of S AP containing M A OR or F* containing M A OR or S AP * containing M A OR. Thus, in this specific embodiment, a mixture particularly produced contains PET, ethylene glycol, and S AP containing M A OR, or F* containing M A OR or S AP * (if step (a*) is carried out), and then in step (b), PET is reacted with it and with ethylene glycol to obtain BHET.

[0149] ​The reaction in step (b) is carried out especially at a temperature of at least 100 °C, preferably in the temperature range from 100 °C to 197 °C, more preferably in the temperature range from 130 °C to 197 °C, more preferably in the temperature range from 150 °C to 197 °C, more preferably in the temperature range from 175 °C to 197 °C.

[0150] The reaction in step (b) is preferably carried out at the boiling temperature of ethylene glycol.

[0151] Even more preferably, the ethylene glycol is refluxed, which means that the ethylene glycol evaporates from the reaction, condenses, and then returns to the reaction. This reflux can be established in a manner familiar to those skilled in the art, for example, in a distillation apparatus.

[0152] This embodiment is advantageous especially when M A OR is added to the mixture as a solution in ROH, i.e., especially in the form of S AP or S AP *, because the excess alcohol ROH with a boiling point lower than that of ethylene glycol then evaporates from the mixture. This additionally reduces the occurrence of by-products.

[0153] Preferably, the reaction in step (b) is carried out until, i.e., 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%, even more preferably at least P = 99% of the PET used in step (b) is converted at time t b .

[0154] This percentage P is calculated by the following formula:

[0155] P = (n TS + n MHET + n BHET ) / n PET .

[0156] Here, n PET is the molar amount of the repeating unit of the following structure (≡) in the PET used in step (b):

[0157]

[0158] n TS is the molar amount of TS formed in step (b) from the start of step (b) until time t b .

[0159] n MHETfrom step (b) until time t b The molar amount of MHET formed in step (b).

[0160] n BHET from step (b) until time t b The molar amount of BHET formed in step (b).

[0161] The structures of the compounds BHET, MHET, and TS are as follows:

[0162]

[0163] "MHET" also includes the corresponding carboxylic acid esters of the shown structure.

[0164] "TS" also includes the corresponding monocarboxylic acid esters and dicarboxylic acid esters of the shown structure.

[0165] In step (b), in particular, a sufficient amount of M A OR is used such that, based on the total weight of PET used in step (b) of the method according to the invention, the total weight of M A OR used in step (b) of the method according to the invention 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, more preferably 3.5% to 5.0%, and most preferably 3.9%.

[0166] Based on the weight [kg] of PET used in the method according to the invention, the ratio of the weight [kg] of ethylene glycol used in step (b) of the method according to the invention is especially in the range of 1:1 to 100:1, preferably in the range of 2:1 to 50:1, more preferably in the range of 3:1 to 40:1, more preferably in the range of 4:1 to 30:1, more preferably in the range of 5:1 to 20:1, more preferably in the range of 6:1 to 10:1, more preferably in the range of 7:1 to 9:1, and most preferably 8:1.

[0167] The reaction in step (b) can be carried out using equipment familiar to those skilled in the art.

[0168] After the completion of step (b) of the method according to the invention, the molar amount of BHET (n BHET ) in the mixture obtained after step (b) and the molar amounts 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, still more preferably 11:1 to 60:1, and still more preferably 13:1 to 24:1 in terms of the molar ratio η of the sum of the molar amounts. In a particularly preferred embodiment, the molar amount of TS in the mixture obtained after step (b) is undetectable, i.e., = 0.

[0169] η = n BHET / (n MHET + n TS )

[0170] 4.3 Preferred step (c)

[0171] In a further preferred step (c), BHET is at least partially separated from the mixture obtained after the end of step (b), in particular mixture M 2 . This is even more preferably achieved by crystallization and / or distillation. Even more preferably, the BHET in step (c) is filtered and then crystallized from the mixture obtained after the end of step (b).

[0172] 5. Method for recycling PET

[0173] The BHET obtained after the end of step (b) in the method according to the invention is preferably polymerized into PET in step (ζ) in a method for recycling polyethylene terephthalate PET.

[0174] This polymerization is the "polycondensation" known to those skilled in the art and is described, for example, in EP0723951A1 and by Th. Rieckmann and S. described on page 92 of the second chapter "Poly(ethylene terephthalate) Polymerization - Mechanisms, Catalysis, Kinetics, Mass Transfer, and Reactor Design" in 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").

[0175] In particular, for this purpose, BHET is polymerized back into PET in step (ζ) in the presence of a catalyst, and the catalyst is particularly selected from antimony compounds, preferably Sb 2 O 3 .

[0176] Preferably, the polymerization of BHET to PET in step (ζ) is carried out at least at the boiling temperature of ethylene glycol. In particular, during the polymerization in step (ζ), ethylene glycol is removed from the reaction mixture in order to shift the reaction equilibrium to the polymer PET side.

[0177] More preferably, the polymerization of BHET to PET in step (ζ) is carried out at the boiling temperature of ethylene glycol. Even more preferably, in this case, during the polymerization in step (ζ), ethylene glycol is removed from the reaction mixture in order to shift the reaction equilibrium to the polymer PET side.

[0178] This is achieved in particular by distillation at a pressure of <1 bar, preferably 0.1 mbar, at the simultaneous boiling temperature of ethylene glycol at the corresponding pressure. Examples

[0179] 1. Invention Example E1:

[0180] PET depolymerization using a methanol solution of sodium methoxide from reactive distillation

[0181] By the method described in Example 1.1 of EP1997794 A1, a methanol solution of 30% sodium methoxide was obtained by reactive distillation.

[0182] Then, first, 150 g of PET and 1200 g of ethylene glycol were added to the autoclave. Then the solution was heated to 175 °C with stirring. Once the temperature of 175 °C was reached, 12.3 g (corresponding to 0.068 mol NaOCH 3 ) of the 30% sodium methoxide methanol solution obtained by reactive distillation was added. The reaction was carried out for 12 hours, and the reactor output was analyzed after cooling. The conversion obtained was determined by gas chromatography. The amounts of the secondary components mono(2-hydroxyethyl) terephthalate (= "MHET") (1) and terephthalic acid (= "TS") (2) relative to the amount of the main product (BHET) formed are shown in the figure (in % relative to the obtained BHET; from upper left → lower right diagonal lines: "\")

[0183] 2. Comparative Example V1:

[0184] PET depolymerization using a conventionally prepared methanol solution of sodium methoxide

[0185] In the comparative experiment, first, 150 g of PET and 1200 g of ethylene glycol were charged into the autoclave. Then the solution was heated to 175 °C with stirring. Once the temperature of 175 °C was reached, 11 g of methanol (corresponding to 0.068 mol NaOCH 32.7 g of solid NaOH in ). The reaction was carried out for 5 hours, and the reactor output was analyzed after cooling. The amounts of the secondary components mono(2-hydroxyethyl) terephthalate (="MHET") (1) and terephthalic acid (="TS") (2) relative to the amount of the main product formed (BHET) are shown in the figure (bars: "■").

[0186] 3. Results

[0187] A comparison of the contents of BHET, MHET, and TS in the depolymerization products in Example E1 and Comparative Example V1 of the present invention shows that the amount of BHET obtained in the two experiments is approximately the same. However, in the depolymerization using a methanol solution of sodium methoxide obtained by reactive distillation, a smaller proportion of the unwanted MHET and by-products was obtained based on BHET. In addition, in the method according to the present invention, no TS was found, while the proportion of this unwanted by-product from V1 in the reaction mixture was high.

[0188] Therefore, in the process of the present invention, a larger proportion of the cleavage product BHET is obtained, and this can be advantageously directly converted into new PET products in polycondensation.

Claims

1. Method for depolymerizing polyethylene terephthalate (PET), comprising the following steps: (a)M A OH and ROH are converted to M A OR in reactive distillation, where M A is an alkali metal selected from sodium, potassium, and lithium, and where R is an alkyl group having 1 to 6 carbon atoms (b) In a mixture comprising ethylene glycol and at least a portion of M obtained in step (a) A convert PET to bis(2-hydroxyethyl) terephthalate BHET.

2. The method according to claim 1, wherein in step (a), a reaction stream S containing ROH AE1 is reacted countercurrently with a reaction stream S A containing M AE2 OH in a reactive distillation column RR A to obtain a crude product RP A containing M A OR, water, ROH, M A , Among them, in RR A At the lower end, a bottom product stream S containing M A OR and ROH is taken out AP and among them, in RR A At the upper end, a vapor stream S with or without ROH and containing water is taken out AB , and wherein, In optional step (a*), at least partially remove ROH from S AP such that M A OR is obtained in solid form F* or as a solution S A containing M AP OR and ROH, where S AP * has a reduced mass proportion of ROH compared to S AP ​ And wherein, in step (b), PET is converted to bis(2-hydroxyethyl) terephthalate BHET in a mixture comprising ethylene glycol and at least a portion of M contained by S AP comprising M A OR, or if step (a*) is carried out, at least a portion of M contained by F* A OR or at least a portion of M contained by S AP *comprising M A OR.

3. The method according to claim 2, wherein S AB comprises water and ROH, and these are at least partially separated from each other in the rectification column RD A ​ 4. The method according to claim 2 or 3, wherein the water content in S AP is < 1% by weight, or if step (a*) is carried out, the water content in S AP * or F* is < 1% by weight.

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

6. The method according to any one of claims 1 to 5, wherein M A is an alkali metal selected from sodium and potassium.

7. The method according to any one of claims 1 to 6, wherein step (b) is carried out until at least P = 10% of the PET used in step (b) has been converted.

8. The method according to any one of claims 1 to 7, wherein in step (b), a sufficient amount of M A OR is used such that, based on the total weight of the PET used in step (b), the total weight of the M A OR used in step (b) is in the range of 0.1% by weight to 100% by weight.

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

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

11. The method according to any one of claims 1 to 10, wherein the PET is subjected to at least one pretreatment step selected from chemical pretreatment steps and comminution steps before being used in step (b).

12. Method for recycling polyethylene terephthalate (PET), wherein BHET is obtained by the method according to any one of claims 1 to 11, and the thus obtained BHET is polymerized into PET in step (ζ).

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

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

15. The method according to claim 14, wherein the catalyst is selected from antimony compounds.

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