Improved process for depolymerization of polyethylene terephthalate

Abstract

The present invention relates to a process for the depolymerization of polyethylene terephthalate ("PET") in which PET is reacted in a mixture containing ethylene glycol and MAOR to form bis (2-hydroxyethyl) terephthalate ("BHET"; cAS number: 959-26-2) in which MA is an alkali metal selected from sodium, potassium, lithium, and in which R is an alkyl group having 2 to 7 carbon atoms. The MAOR is obtained by carrying out transalcoholization on short-chain alkoxide MAOR '. 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 of the present invention provides a high yield of BHET, which can be directly used in the production of regenerated PET. 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 into 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 2 to 7 carbon atoms. Here, M A OR is obtained by transalcoholization from a shorter-chain alcoholate M A OR'.

[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. Thus, the method according to the invention provides a high yield of BHET, which can be directly used for new PET production.

[0003] Therefore, the present invention also relates to a method for recycling PET, in which the BHET obtained in the PET depolymerization method is optionally repolymerized after further purification to produce PET. Background Art

[0004] Polyethylene terephthalate (= "PET") is one of the most important plastics used in textile fibers, as films, and as plastic bottle materials. 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 amount of waste derived from PET, it poses one of the greatest environmental challenges at present. The solution to this problem lies in avoiding PET and effectively reusing PET.

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

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

[0008] Hydrolysis methods for PET depolymerization are described in JP2000 - 309663A, US4,355,175A, and T. Yokshioka, N. Okayama, A. Okuwaki, Ind. Eng. Chem. Res. 1998, 37, 336 - 340.

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

[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 the production of new PET. In contrast, specific by - products such as mono(2 - hydroxyethyl) terephthalate (="MHET") monoester or free terephthalic acid or the corresponding carboxylate, terephthalate (="TS") are disadvantageous because they cannot be used as reactants for the production of new PET.

[0011] Therefore, there is an interest in methods for PET depolymerization in which the highest possible proportion of BHET is obtained in the cleavage products, 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 PET in ethylene glycol reacts with an alkali metal alkoxide M A OR' obtained by reactive distillation to an alkali metal alkoxide M A OR, providing a lower proportion of unwanted MHET and TS by - products based on BHET than conventional methods, where R' is an alkyl group having fewer carbon atoms than the alkyl group R. Thus, a method for solving the problem solved by the present invention has now been surprisingly found.

[0014] Accordingly, the present invention relates to a method for the depolymerization of polyethylene terephthalate PET, comprising the following steps:

[0015] (a) In reactive distillation, M AOR′ and ROH are converted to M A OR.

[0016] M A Here is an alkali metal selected from sodium, potassium, and lithium, especially an alkali metal selected from sodium and potassium, and 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] R is an alkyl group having 2 - 7, especially 2 - 6, preferably 2 - 5, more preferably 2 - 4 carbon atoms. Even more preferably, R = n - propyl, isopropyl or ethyl. Most preferably, R = ethyl. This alkyl group R has at least one more carbon atom than the alkyl group R’.

[0019] (b) Then convert PET to bis(2 - hydroxyethyl) terephthalate BHET in a mixture containing 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, in which BHET obtained in the depolymerization method according to the present invention is polymerized in step (ζ) to produce PET. Detailed Description of the Invention

[0022] It has now been found, surprisingly, that when the cleavage of PET is carried out in a mixture of ethylene glycol and M A OR obtained by reactive distillation, the proportion of the desired BHET cleavage product in the glycolysis of PET is increased relative to the proportion of the unwanted TS and MHET cleavage products.

[0023] In the reactive distillation according to the present invention, M A OR is obtained by the reaction of the corresponding alkali metal alkoxide M A OR’ with ROH, where M A is selected from lithium, potassium, and sodium, where R is an alkyl group having 2 - 7 carbon atoms, and R’ is an alkyl group having 1 - 6 carbon atoms. This alkyl group R has at least one more carbon atom than the alkyl group R’.

[0024] Therefore, the method according to the present invention is superior to the prior art methods, where, for example, the cleavage is carried out in a mixture obtained by dissolving an alkali metal hydroxide in ethylene glycol and ROH.

[0025] 1. Step (a): Prepare M A OR

[0026] According to the present invention, the alkali metal alkoxide M used in the method according to the present invention A OR is obtained by reactive distillation, by the conversion of M A OR' and ROH. Thus, step (a) of the method according to the present invention is the transesterification of M A OR′ to M A OR. In step (a), M A OR' and ROH are converted to M A OR in reactive distillation.

[0027] Here, 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 methyl and ethyl. Most preferably, R' = methyl.

[0028] Here, R is an alkyl group having 2 to 7 carbon atoms, especially an alkyl group having 2 to 6 carbon atoms, preferably an alkyl group having 2 to 5 carbon atoms, more preferably an alkyl group having 2 to 4 carbon atoms. Even more preferably, R is selected from n-propyl and isopropyl. Even more preferably, R is selected from ethyl and isopropyl. Most preferably, R = ethyl.

[0029] The alkyl group R has at least one more carbon atom than the alkyl group R'; the alkyl group R preferably has one or two more carbon atoms than the alkyl group R'. More preferably, the alkyl group R has one more carbon atom than the alkyl group R'.

[0030] Therefore, the alkyl group R is a "higher" alkyl group than the alkyl group R'. The alkyl group R' is a "lower" alkyl group than the alkyl group R. The terms "higher" and "lower" relate to the number of carbon atoms in the alkyl group.

[0031] M A is an alkali metal selected from lithium, sodium, potassium, especially sodium and potassium. M A is preferably sodium.

[0032] According to the present invention, the alkyl group having 1 to 6 carbon atoms is particularly 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, and even more preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, n-pentyl, n-hexyl.

[0033] In the context of the present invention, the alkyl group having 1 to 5 carbon atoms is particularly 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, and more preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, n-pentyl.

[0034] In the context of the present invention, the 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.

[0035] In the context of the present invention, the alkyl group having 1 to 3 carbon atoms is particularly selected from methyl, ethyl, n-propyl, isopropyl, preferably selected from methyl, ethyl, isopropyl.

[0036] The alkyl group having 2 to 7 carbon atoms is particularly selected from 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, n-heptyl, preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, n-hexyl, n-heptyl, and even more preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, n-pentyl, n-hexyl, n-heptyl.

[0037] The alkyl group having 2 to 6 carbon atoms is particularly selected from 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, and even more preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, n-pentyl, n-hexyl.

[0038] In the context of the present invention, the alkyl group having 2 to 5 carbon atoms is particularly selected from 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 ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, n-pentyl, and more preferably selected from ethyl, n-propyl, isopropyl, n-butyl, n-pentyl.

[0039] In the context of the present invention, an alkyl group having 2 to 4 carbon atoms is particularly selected from ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, preferably selected from ethyl, n-propyl, isopropyl, n-butyl, tert-butyl, and more preferably selected from methyl, ethyl, n-propyl, isopropyl, n-butyl.

[0040] In the context of the present invention, an alkyl group having 2 to 3 carbon atoms is particularly selected from ethyl, n-propyl, isopropyl, and is preferably selected from ethyl, isopropyl.

[0041] "Ethylene glycol" in the context of the present invention is understood to be ethylene-1,2-diol having the chemical formula HO-CH 2 -CH 2 -OH (CAS No. 107-21-1).

[0042] 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, for example in the production of active pharmaceutical or agrochemical ingredients, and as catalysts in transesterification and amidation reactions.

[0043] 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 formed by the following reaction <1> is removed together with the distillate:

[0044]

[0045] For example, the principle of this method is described in US2,877,274A, where an aqueous solution of alkali metal hydroxide and gaseous methanol are carried out countercurrently in a rectification column. This method is described again in substantially the same form in WO01 / 42178A1.

[0046] The most important alkali metal alkoxides industrially are those of sodium and potassium, especially methoxides and ethoxides. In the prior art, there are many descriptions of their synthesis, for example in EP1997794 A1, WO2021 / 148174A1, and WO2021 / 148175A1.

[0047] Similar methods in which an entrainer (such as benzene) is additionally used are described in GB377,631A and US1,910,331A.

[0048] Correspondingly, DE968903C describes a method for continuously preparing alkali metal alkoxides in a reaction column, where the water-alcohol mixture taken out at the top of the column is condensed and then phase-separated. The aqueous phase is discarded, and the alcohol phase and fresh alcohol are returned together to the top of the column. EP0299577 A2 describes a similar method in which the water in the condensate is separated by means of a membrane.

[0049] In addition to alkali metal hydroxides, different alkali metal alkoxides MOR' can also be used in the "transesterification" by reactive distillation and reacted with an alcohol ROH to obtain the desired alkali metal alkoxide MOR and alcohol R'OH. R'OH is generally an alcohol with a lower boiling point than ROH, which is usually the case for monohydric alcohols when the alkyl group has fewer carbon atoms than in the alkyl group of ROH. Generally, alkali metal alkoxides of methanol, i.e., alkali metal methoxides M A OR'.

[0050] Such transesterifications are described, for example, in DE2726491 A1, EP0776995 A1 or WO2021 / 122702A1.

[0051] In a preferred embodiment of the process according to the invention, in step (a), ROH and M A OR' are fed, separately or as a mixture, preferably separately, to a reactive distillation column RR A and converted in RR A into a crude product containing ROH, R'OH and M A OR, with or without M A OR'.

[0052] wherein a bottoms product stream S A containing M A OR and ROH is withdrawn at the lower end of RR AP and wherein a vapor stream S A containing R'OH, with or without ROH, is withdrawn at the upper end of RR AB .

[0053] "Separately" according to the invention means that ROH and M A OR' are fed to the reactive distillation column RR A at different feed points. This also encompasses embodiments in which only a part of the ROH used in step (a) is fed to the reactive distillation column RR 1 at a first feed point (= "Z A ") and mixed with M A OR' and the remaining part of the ROH used in step (a) is fed to the column at a feed point Z 1 other than Z 2 . In this embodiment, Z 1 is especially below Z 2 .

[0054] In a more preferred embodiment of the process according to the invention, in step (a), a reaction stream S AE1 containing ROH is combined with a stream containing M AThe reaction stream S of OR' AE2 is subjected to a countercurrent reaction in the reactive distillation column RR A to obtain a crude product RP containing M A OR, ROH and R'OH, with or without M A OH, wherein at the lower end of RR A , a bottoms product stream S containing M A OR and ROH is withdrawn, and wherein at the upper end of RR A , a vapor stream S containing R'OH, with or without ROH is withdrawn AP . A . AB .

[0055] According to the present invention, a "reactive distillation column" is defined as a distillation column in which the reaction according to step (a) of the process according to the present invention takes place at least in some parts. It may also be abbreviated as "reaction column".

[0056] In a preferred embodiment of the process according to the present invention, at the lower end of RR A , a bottoms product stream S containing ROH and M A OR is withdrawn. At the upper end of RR AP , a vapor stream S containing ROH, with or without R’OH is withdrawn A . AB .

[0057] The reaction stream S AE1 contains ROH. In a preferred embodiment, the mass fraction of ROH in S AE1 is ≥ 95% by weight, but more preferably ≥ 99.5% by weight, wherein S AE1 especially additionally contains water.

[0058] The ROH used as the reaction stream S AE1 in a preferred embodiment of the process according to the present invention may also be a commercially available alcohol ROH, which has an ROH proportion by mass of greater than 99.5% by weight and a water proportion by mass of at most 0.03% by weight.

[0059] In a particular embodiment of the present invention, the reaction stream S AE1 is added to the reactive distillation column RR A in vapor form.

[0060] In an alternative preferred embodiment of the process according to the present invention, ROH is initially charged to the bottom of the reactive distillation column RR A before step (a), and then heated to boiling in step (a), which generates a reactant S A at a constant flow rate in the reactive distillation column RR AE1If desired, then during the execution of step (a), ROH is supplemented at the bottom of the reactive distillation column RR A The bottom of

[0061] The reaction stream S AE2 Contains M A OR'. In a preferred embodiment, S AE2 Not only contains M A OR', but also contains at least one alcohol selected from ROH, ROH'. This is then a solution of M A OR' in ROH or R'OH.

[0062] When the reaction stream S AE2 Contains M A OR' and R'OH and / or ROH, based on the total weight of all alcohols R'OH and / or ROH in S AE2 The mass ratio of M A OR' in S AE2 Is particularly 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.

[0063] Step (a) of the process according to the invention is preferably carried out in a reactive distillation column (or "reaction column") RR A Inside.

[0064] The reaction column RR A Preferably contains internals. Suitable internals are, for example, trays, structured packings or random packings. When the reaction column RR A Contains 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, it is preferred to select trays in which no more than 5% by weight, more preferably less than 1% by weight, of the liquid drips through the corresponding 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 makes it possible to increase the vapor velocity in the tray openings 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.

[0065] When using structured or random packings, structured packings are preferred in terms of the uniform distribution of the liquid.

[0066] Step (a) of the process according to the invention can be carried out continuously or batchwise. It is preferably carried out continuously.

[0067] In one embodiment according to the invention, "the reaction stream S containing ROH AE1 is reacted countercurrently with the reaction stream S A containing M AE2 OR in the reactive distillation column RR A ″ is ensured in particular when the feed point of at least a part of the reaction stream S A containing ROH in the reaction column RR AE1 is lower than the feed point of the reaction stream S A containing M AE2 OR'.

[0068] 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 .

[0069] The reaction column RR A can be operated as a pure stripping column. In this case, the reaction stream S containing ROH is introduced in vapor form into the lower region of the reaction column RR A . AE1 .

[0070] 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 alkoxide M A OR' 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 , in particular in vapor form, is added in the upper end or upper region of the reaction column RR AE1 , preferably only a part, i.e. 10 wt% - 70 wt%, preferably 30 wt% - 50 wt% (in each case based on the total amount of ethylene glycol used), is fed at the lower end of the reaction column RR A , and the remaining part is added in vapor form in a single stream or divided into a plurality of sub-streams, preferably 1 - 10 theoretical plates, more preferably 1 - 3 theoretical plates, below the feed point of the reaction stream S A containing M A OR. AE2

[0071] In an alternative and more preferred embodiment of step (a) of the process according to the invention, the reaction stream S containing ROH AE1 is reacted countercurrently with a reaction stream S A containing M AE2 OR in a reactive distillation column RR A is particularly ensured by having ROH present at the bottom of the reactive distillation column RR A and the feed point of the reactant stream S A containing M AE2 OR' being above the bottom. During step (a) of the process according to the invention, the ROH is then heated to boil at the bottom of the RR A and a reaction stream S containing ROH AE1 is generated. The S AE1 and the S AE2 are then led countercurrently to each other.

[0072] In the reaction column RR A the reaction stream S containing ROH AE1 then reacts with the reaction stream S A containing M AE2 OR′ to give M A OR and R′OH, and these products are generally present in the mixture with ROH, in particular the M A OR′ reactant, since the reaction is an equilibrium reaction. Thus, in step (a) in the reaction column RR A a crude product RP A is obtained which contains, in addition to the M A OR and R′OH products, also ROH, with or without M A OR′.

[0073] Then a bottoms product stream S A containing ROH and M A OR is obtained and withdrawn at the lower end of the RR AP .

[0074] At the upper end of the RR A and preferably at the top of the column of the RR A in a preferred embodiment of the process according to the invention, a stream of R'OH which may or may not contain ROH (hereinafter referred to as "the vapor stream S AB containing R'OH, with or without ROH") is withdrawn.

[0075] If the vapor stream S AB contains not only R′OH but also ROH, in an advantageous embodiment the ROH is also preferably obtained by distillation, for example in a rectification column. In this embodiment, at least a part of the ROH obtained in the distillation can be used as the reaction stream SAE1 The feed is returned to the reaction column RR A .

[0076] In a preferred embodiment, when S AB contains ROH and R’OH, these two alcohols (i.e., ROH and R’OH) are at least partially separated from each other in the rectification column RD A (referred to hereinafter with reference to point 2).

[0077] The amount of ROH contained in the reaction stream S is preferably selected AE1 such that the ROH is simultaneously used as a solvent for M AP obtained in the bottoms stream S A OR. The amount of ROH in the reaction stream S is preferably selected AE1 such that the bottoms stream S containing ROH and M A OR with the required concentration contains M AP removed A OR solution present at the bottom of the reaction column.

[0078] In a preferred embodiment of the process according to the invention, the ratio of the total weight (mass; unit: kg) of ROH used in step (a) to the total weight (mass; unit: kg) of M A OR’ used in step (a) is from 1:1 to 50:1, more preferably from 2:1 to 40:1, even more preferably from 3:1 to 30:1, still more preferably from 5:1 to 10:1.

[0079] The reaction column RR in a preferred embodiment of the process according to the invention A is operated with or without reflux, preferably with reflux.

[0080] "With reflux" means that the vapor stream S containing R’OH, with or without ROH, withdrawn at the upper end of the corresponding column (in particular the reaction column RR A ) is not completely removed. Accordingly, the relevant vapor stream S AB is at least partially, preferably partially, fed back as reflux to the corresponding column, in particular the reaction column RR AB . In the case of establishing such reflux, the reflux ratio is preferably from 0.01 - 1, more preferably from 0.02 - 0.9, even more preferably from 0.03 - 0.34, particularly preferably from 0.04 - 0.27, and very particularly preferably from 0.05 - 0.24, most preferably 0.2. A .

[0081] The reflux ratio is generally understood in the context of the present invention to be the ratio of the ratio of the mass flow rate (kg / h) withdrawn from the corresponding column in liquid or gaseous form to the ratio of the mass flow rate (kg / h) returned to the column (reflux) in liquid form.

[0082] 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 . In the condenser K RRA , the vapor stream S AB is at least partially condensed and fed back into the corresponding column, in particular the reaction column RR A .

[0083] In an embodiment in which 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 be supplied to the reaction column RR A .

[0084] In a preferred embodiment of the process according to the invention, step (a) is carried out in particular under the distillation conditions of ROH reflux.

[0085] Step (a) is carried out in particular at a temperature of 45 °C - 150 °C, preferably 47 °C - 120 °C, more preferably 60 °C - 110 °C and a pressure in the range from 0.5 bar absolute pressure to 40 bar absolute pressure, preferably in the range from 0.7 bar absolute pressure to 5 bar absolute pressure, more preferably in the range from 0.8 bar absolute pressure to 4 bar absolute pressure, more preferably in the range from 0.9 bar absolute pressure to 3.5 bar absolute pressure, more preferably in the range from 1.0 bar absolute pressure to 3 bar absolute pressure.

[0086] 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 .

[0087] According to the invention, the "intermediate evaporator" V Z refers to an evaporator above the bottom of the corresponding column, in particular an evaporator above the bottom of the reaction column RR A (in which case they are referred to as "V ZA ") or the rectification column RD AAn evaporator above the bottom (in this case, they are referred to as "V" ZRD "), which is used in a preferred embodiment and is described in further detail. In the case of RR A , the evaporator particularly evaporates the crude product RP ZAA taken out from the column as a sidestream S A .

[0088] According to the present invention, the "bottom evaporator" V S refers to an evaporator that heats the bottom of the corresponding column, particularly the bottom of the reaction column RR A or the bottom of the rectification column RD A , which is used in a preferred embodiment and is described in further detail (in this case they are referred to as "V" SRD " or "V" SRD' "). In the case of RR A , the evaporator particularly evaporates at least a part of the bottom product stream S AP . In the case of RD A , the evaporator particularly evaporates a part of the bottom product stream S UA or S UA , namely S UA1 .

[0089] The evaporator is usually arranged outside the corresponding reaction column or rectification column.

[0090] 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 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 usually introduced as a film on the inside of the tubes, and the tubes are heated on the outside. Contrary to the falling film evaporator, the thin film evaporator additionally includes a rotor with scraping blades that distribute the liquid to be evaporated on the inner wall of the tubes to form a film.

[0091] In addition to those mentioned, any other desired types of evaporators known to those skilled in the art and suitable for rectification columns can also be used.

[0092] In a preferred embodiment of the method according to the present invention, S A containing ROH and M AP OR is taken out at the lower end of the reaction column RR A as the bottom product stream.

[0093] 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 partially directed through the reboiler V SA , and ROH is partially removed therefrom, thereby providing a bottoms product stream S AP having a reduced proportion of ROH by mass compared to S AP* .

[0094] 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 directed and ROH is at least partially removed therefrom) has an M SA in ROH of A OR a proportion by mass 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 10 wt% to 30 wt%, most preferably in the range from 20 wt% to 25 wt%, most preferably 21 wt%, in each case based on the total mass of S AP or S AP* .

[0095] The proportion by mass of R′OH in S AP / S AP* is preferably < 1 wt%, preferably < 0.8 wt%, more preferably < 0.5 wt%, based on the total mass of S AP / S AP* .

[0096] Based on the total mass of S AP / S AP* , the proportion by mass of the reactant M A OR’ in S AP / S AP* is preferably < 1 wt%, preferably < 0.8 wt%, more preferably < 0.5 wt%.

[0097] As described above, in a preferred embodiment of the process according to the invention, a vapor stream S containing ROH, with or without R′OH, is withdrawn at the upper end of RR A . AB

[0098] 2. Rectification (preferred) of the vapor stream S AB in the rectification column R DA

[0099] In a preferred embodiment of the process according to the invention, when the vapor stream S​​AB When ROH and R’OH are included, these are at least partially separated from each other in the rectification column RD A

[0100] In particular, S containing ROH and R’OH AB is introduced into the rectification column RD A and separated in RD A into at least one stream S of ROH withdrawn at the lower end of RD A and at least one stream S of R’OH withdrawn at the upper end of RD 1 A 2 .

[0101] Since R’OH is a lower alcohol compared to ROH and the alkyl group R’ has at least one fewer carbon atom than the alkyl group R, the boiling point of R’OH will be higher than the boiling point of ROH.

[0102] Accordingly, S 2 (containing R’OH) is obtained as a vapor stream and withdrawn at the upper end of RD A and S 1 (containing R’OH) is obtained and withdrawn at the lower end of RD A .

[0103] The vapor stream S AB can be led into the rectification column RD A via one or more feed points. In embodiments of the invention in which the vapor stream S AB is led into the rectification 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 the rectification column RD A .

[0104] In a preferred embodiment of the process according to the invention, when the vapor stream S AB contains ROH and R’OH, it is separated in the rectification column RD A into a vapor stream S containing ROH 1 and a stream S containing R’OH 2 . In this preferred embodiment, S 2 is then withdrawn as a vapor stream at the upper end and S 1 is withdrawn as a bottoms stream at the lower end of the rectification column RD A .

[0105] Another term for "the upper end of the rectification column" is "the top".

[0106] Another term for "the lower end of the rectification column" is "the bottom" or "the foot".​​​

[0107] Rectification column RD used A can be any rectification column known to those skilled in the art.

[0108] Rectification column RD A Preferably includes internal components. Suitable internal components 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 slotted 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 sold, for example, under the Sulzer trade name. In addition to the internal components mentioned, other suitable internal components are known to those skilled in the art and can equally be used.

[0109] Preferred internal components have a low specific pressure drop per theoretical plate. Structured packings and random packing elements have, for example, a pressure drop per theoretical plate significantly lower than that of trays. This has the advantage that the pressure drop in rectification column RD A remains as low as possible, and the mechanical power of the compressor and the temperature of the ROH / R′OH mixture to be evaporated are thus kept low.

[0110] When rectification column RD A contains structured packings or random packings, they can be separate or in the form of an uninterrupted packing. However, usually, at least two packings are provided, one packing above the feed point of vapor stream S AB and one packing below the feed point of vapor stream S AB of. It is also possible to provide one packing above the feed point of vapor stream S AB and two or more trays below the feed point of 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 perforated tray).

[0111] The preferred proportion of ROH in S 1 by mass is ≥96.0% by weight, more preferably ≥99.6% by weight, still more preferably ≥99.9% by weight, and the remainder is especially R’OH.

[0112] S 2 contains R’OH, where S 2 may preferably include <1% by weight, more preferably ≤5000 ppm by weight, but more preferably ≤1000 ppm by weight, still more preferably ≤100 ppm by weight of ROH.

[0113] In the context of the present invention, at least one vapor stream S comprising R’OH is withdrawn at the top of the rectification column RD A More specifically, this means that at least one vapor stream S 2 is withdrawn as a top stream or as a side stream above the internals in the rectification column RD 2 A

[0114] In the context of the present invention, at least one stream S comprising ROH is withdrawn at the bottom of the rectification column RD A More specifically, this means that at least one stream S 1 is withdrawn as a bottoms stream or at the lower trays of the rectification column RD 1 A

[0115] The rectification column RD A is operated with or without reflux, preferably with reflux.

[0116] "With reflux" means that the vapor stream withdrawn at 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 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, and very particularly preferably from 0.05 to 0.24, most preferably 0.2.

[0117] The reflux can be established by installing a condenser K RD at the top of the rectification column RD A . The corresponding vapor stream S OA is partially condensed in the condenser K RD and returned to the rectification column RD A .

[0118] 3. Optional step (a*): Removal of ROH from S AP

[0119] In a preferred embodiment of step (a) of the process according to the invention, wherein ROH and M A OR’ are each directly or as a mixture fed to the reactive distillation column RR A and converted in the RR A into a crude product comprising ROH, R’OH, M A OR, in particular by countercurrent reaction of a reaction stream S comprising ROH AE1 with a reaction stream S comprising M A OR’ in the reactive distillation column RR AE2 to give a product comprising M A A ​​​​​​OR, ROH and R’OH, with or without M A The crude product RR with OH A ,

[0120] and wherein in RR A at the lower end, a bottom product stream S containing M A OR and ROH is withdrawn AP , and wherein in RR A at the upper end, a vapor stream S containing R’OH, with or without ROH, is withdrawn AB ,

[0121] According to the invention, in an optional step (a*), ROH can be at least partially removed from S AP such that M OR is obtained in solid form F* or as a solution S containing M A OR and ROH AP *, where S A has a reduced proportion of ROH by mass compared to S AP *. AP Whether the preferred embodiment (a*) provides the solution S

[0122] * or the solid material F* depends on whether ROH is partially or substantially completely removed from S AP *. AP Removed.

[0123] In the optional step (a*), the at least partial removal of ROH from S AP * 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 bottom product stream S AP is then partially directed through the at least one reboiler V SA , and ROH is partially removed therefrom, thereby providing a bottom product stream S with a reduced ROH content compared to S AP . AP* .

[0124] 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 OR is obtained in solid form F* A .

[0125] 4. Step b): Conversion of PET to BHET

[0126] In step (b) of the process according to the invention, PET is reacted in the presence of ethylene glycol and at least a portion of M obtained in step (a) AIn the mixture of OR, it is converted into BHET.

[0127] 4.1 PET raw materials

[0128] The PET used in step (b) of the method according to the invention can be any PET that has to be depolymerized. Generally, such PET appears as waste, especially in households, in industry, in the sanitary system or in agriculture.

[0129] In one embodiment of the method according to the invention, the PET to be depolymerized is thus in a mixture with at least one other plastic, especially selected from polyethylene ("PE"), polyvinyl chloride ("PVC"). This is usually the case when PET from plastic waste is depolymerized in the method according to the invention. In this embodiment, before being subjected to step (b) of the method according to the invention, the PET is preferably at least partially separated from the other plastics by sorting.

[0130] In one embodiment of the method according to the invention, the PET is subjected to at least one pretreatment step.

[0131] Such pretreatment steps are described, for example, in DE10032899 C2.

[0132] According to the invention, before the PET is used in step (b), the PET is subjected to at least one pretreatment step selected from chemical pretreatment steps, grinding steps.

[0133] In the case where the PET is in a mixture with other plastics, the PET is especially subjected to at least one pretreatment step selected from at least partially separating from the other plastics, preferably by sorting, chemical pretreatment steps, grinding steps, and then used in step (b).

[0134] In the case where the PET is in a mixture with other plastics, the PET is more preferably first at least partially separated from the other plastics, then subjected to at least one chemical pretreatment and finally ground.

[0135] The chemical pretreatment step is especially a washing step. Such a washing step has the advantage of removing any impurities, especially residues of foodstuffs, cosmetics and / or body secretions (such as blood, sperm, feces) before performing step (b). Such impurities can reduce the efficiency of the reaction in step (b) and / or deteriorate the purity of the BHET obtained thereby.

[0136] In the chemical pretreatment step, especially the washing step, the waste is heated in the washing solution at a temperature in the range of 30 °C to 99 °C, preferably 50 °C to 90 °C, more preferably 70 °C to 85 °C.

[0137] Typical washing solutions are familiar to those skilled in the art and are preferably selected from:

[0138] - Surfactants, preferably aqueous solutions of non-ionic surfactants;

[0139] - Aqueous solutions of alkali metal hydroxides or alkaline earth metal hydroxides, preferably aqueous NaOH.

[0140] In the chemical pretreatment step, especially the washing step, the treatment time is especially from 1 min to 12 h, preferably from 10 min to 6 h, more preferably from 30 min to 2 h, even more preferably from 45 min to 90 min, and most preferably 60 min.

[0141] After treating 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.

[0142] Then the PET waste thus obtained is dried, especially in an oven.

[0143] 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.

[0144] The comminution step has the following advantages: the surface area of the PET available for the reaction in step (b) is increased. This increases the reaction rate of the reaction in step (b). Comminution can be achieved in devices known to those skilled in the art, such as crushers or cutting mills.

[0145] In another embodiment of the method according to the invention, the PET is decolorized or colored in a controlled manner before being subjected to 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.

[0146] 4.2 Conversion conditions in step (b)

[0147] In step (b) of the method according to the invention, the PET is converted to BHET in a mixture comprising ethylene glycol and at least a portion of M A OR obtained in step (a).

[0148] It is obvious 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" means that step (b) is carried out in a mixture comprising PET, ethylene glycol and at least a portion of M AIt is carried out in a mixture of OR. In the conversion of step (b), PET is formally transesterified with ethylene glycol at the internal ester bond [see structure (Ξ) shown below, where M A The alkoxide anion of OR acts as a catalyst.

[0149] Without being bound to any particular theory, the cleavage mechanism of PET to BHET first involves the alkoxide anion RO - Nucleophilic attack on the ester bond and cleavage of the polymer PET, which results in the formation of an intermediate of the ester of terephthalic acid unit with alcohol ROH, and subsequently this ester is transesterified with ethylene glycol. This is schematically shown below using the ester bond of PET:

[0150]

[0151] 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 obtained in step (a) A OR are mixed in any order, and the reaction conditions for the cleavage of PET to BHET are established in step (b).

[0152] In particular, in step (b), PET is mixed with ethylene glycol and at least a part of M obtained in step (a) A OR, wherein, in a preferred embodiment of step (a), it 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 containing PET, ethylene glycol and M A OR, and the PET in the mixture M 1 is reacted at least in part with ethylene glycol and M 1 to obtain bis(2-hydroxyethyl) terephthalate BHET. After the end of step (b), this preferably provides a mixture M A containing BHET, which especially additionally contains ethylene glycol, M 2 OR and any unreacted PET, with or without MHET and with or without TS. A OR and any unreacted PET, with or without MHET and with or without TS.

[0153] In a preferred embodiment of step (b), one or two of the three components selected from PET, ethylene glycol and M obtained in step (a) A OR are first added, the reaction conditions are established therein, and then the other components selected from PET, ethylene glycol and M obtained in step (a) A OR are finally added. After the addition of this last component, the mixture M 1, 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 is then obtained 2 , which contains BHET and especially additionally contains ethylene glycol, M A OR and any unconverted PET, with or without MHET and with or without TS.

[0154] In another alternative embodiment of step (b), which is carried out especially in a continuous process scheme, the components PET, ethylene glycol, and M obtained in step (a) A OR, at least one of which, preferably two, preferably all three, are fed into a mixture M containing PET, ethylene glycol, M obtained in step (a) A OR, and BHET 1 , which means that the conversion of PET to BHET in step (b) takes place in this mixture M 1 during the addition of at least one of the three components PET, ethylene glycol, and M obtained in step (a). In the case of adding at least two of the components PET, ethylene glycol, and M obtained in step (a) A to this mixture M A , these components are especially added separately from each other. After the end of step (b), this preferably provides a mixture M containing BHET 1 , which especially additionally contains ethylene glycol, M 2 OR, and any unconverted PET, with or without MHET and with or without TS. A

[0155] In a preferred embodiment of step (a) according to the invention, the reaction stream S containing ROH AE1 reacts countercurrently with the reaction stream S containing M A OR' in a reactive distillation column RR AE2 to obtain a crude product RP containing M A OR, ROH, and R'OH, with or without M A OH A , wherein at the lower end of RR A , a bottoms product stream S containing M A OR and ROH is withdrawn A , and wherein at the upper end of RP AP , a vapor stream S containing R'OH, with or without ROH, is withdrawn A , AB

[0156] , wherein, in an optional step (a*), from S APAt least partially remove ROH such that F* in solid form or as a solution S containing M A OR and ROH is obtained AP *M A OR, where compared with S AP S AP *has a reduced proportion of ROH by mass,

[0157] This particularly includes, in step (b), converting PET in the mixture into bis(2-hydroxyethyl) terephthalate BHET, the mixture comprising ethylene glycol and at least a part of M contained by S AP contained M A OR, or at least a part of M contained by F* A OR or at least a part of M contained by S AP *M A OR (if step (a*) is carried out).

[0158] "At least a part of M contained by S AP contained M A OR, or at least a part of M contained by F* A OR or at least a part of M contained by S AP *M A OR (if step (a*) is carried out)" implies that S containing M A OR or F* containing M AP OR or S containing M A OR is added.* A OR Therefore, in this particular embodiment, a mixture is especially produced which contains PET, ethylene glycol and S containing M AP *, or F* containing M A OR or S containing M AP *, or F* containing M A OR or S containing M A * (if step (a*) is carried out), and then in step (b) PET is reacted with it and with ethylene glycol to obtain BHET. AP

[0159] The reaction in step (b) is particularly 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.

[0160] ​The reaction in step (b) is preferably carried out at the boiling temperature of ethylene glycol. Even more preferably, ethylene glycol refluxes, which means that 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.

[0161] This embodiment is particularly 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 having a boiling point lower than that of ethylene glycol then evaporates from the mixture. This additionally reduces the occurrence of by-products.

[0162] 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) has been converted at the moment t b .

[0163] This percentage P is calculated by the following formula:

[0164] P = (n TS + n MHET + n BHET ) / n PET .

[0165] Here, n PET is the molar amount of the repeating unit of the following structure (Ξ) in the PET used in step (b):

[0166]

[0167] n TS is the molar amount of TS formed from the start of step (b) until the moment t b in step (b).

[0168] n MHET is the molar amount of MHET formed from the start of step (b) until the moment t b in step (b).

[0169] n BHET is the molar amount of BHET formed from the start of step (b) until the moment t b in step (b).

[0170] The structures of the compounds BHET, MHET, and TS are as follows:

[0171]

[0172] "MHET" also includes the corresponding carboxylic acid esters of the shown structure.

[0173] "TS" also includes the corresponding monocarboxylic acid esters and dicarboxylic acid esters of the shown structure.

[0174] Based on the total weight of PET used in step (b) of the process according to the invention, the total weight of M A OR is especially 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% by weight, and most preferably 3.9% by weight.

[0175] Based on the weight of PET [in kg] used in the process according to the invention, the ratio of the weight of ethylene glycol [in kg] used in step (b) of the process 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.

[0176] The reaction in step (b) can be carried out using equipment familiar to those skilled in the art.

[0177] After the end of step (b) of the process according to the invention, the molar amount of BHET (n BHET ) in the mixture obtained after step (b) and the sum of the molar amounts of MHET and TS (n MHET +n TS ) have a molar ratio η 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 a particularly preferred embodiment, the molar amount of TS in the mixture obtained after step (b) is undetectable, i.e., = 0.

[0178] η = n BHET / (nMHET +n TA )

[0179] 4.3 Preferred step (c)

[0180] In a further preferred step (c), BHET is separated at least in part from the mixture obtained after the end of step (b), in particular from 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).

[0181] 5. Method for recycling PET

[0182] The BHET obtained after the end of step (b) in the process according to the invention is preferably polymerized to PET in step (ζ) in a process for recycling polyethylene terephthalate PET

[0183] This polymerization is known to the person skilled in the art as "polycondensation" 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").

[0184] In particular, for this purpose, BHET is polymerized back to PET in step (ζ) in the presence of a catalyst, said catalyst being in particular selected from antimony compounds, preferably Sb 2 O 3 catalysts.

[0185] Preferably, the polymerization of BHET to PET in step (ζ) is carried out at least at the boiling point temperature of ethylene glycol. In particular, during the polymerization in step (ζ), ethylene glycol is removed from the reaction mixture to shift the reaction equilibrium to the polymer PET side.

[0186] 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 to shift the reaction equilibrium to the polymer PET side.

[0187] 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 respective pressure. Example

[0188] 1. Invention Example E1:

[0189] 1.1 Preparation of sodium ethoxide ethanol solution by reactive distillation

[0190] The following equipment is used as the distillation equipment:

[0191] The storage container or bottom used in the distillation equipment is a heatable 2.5 l jacketed container with a temperature sensor and a vacuum-sealed stirrer. Above this is a 25 cm column (stripping section) with Multifill packing and a silver mirror. Sodium ethoxide is metered in above the column through a dropping funnel. Above the metering point is another column (rectifying section) for separating methanol and ethanol. The reflux ratio can be established with the help of a vapor distributor in the upper part of the column, and the distillate is collected in a round-bottom flask. The round-bottom flask can be separated from the distillation system and exchanged via a pressure-equalizing dropping funnel. In the rectifying section, a reflux condenser with a vacuum connection is connected, and the entire equipment can be evacuated through this reflux condenser. A vacuum is generated by a rotary vane pump, which is connected to the distillation equipment through two cold traps and a guard bottle. The pressure in the distillation equipment is measured in the guard bottle (Büchi vacuum controller), and ventilation is also possible. The bottom reservoir and the column with Multifill packing are completely surrounded by aluminum foil for heat insulation to ensure a uniform temperature in the reactor / column.

[0192] The bottom is first filled with ethanol, and the entire equipment is evacuated to 50 mbar. Subsequently, the bottom is heated to the boiling temperature so that reflux from the rectifying section is established. Subsequently, sodium methoxide (30% by weight in methanol, abbreviated as "NM30", obtained by the method described in Example 1.1 of EP1997794A1) is metered in with the help of the dropping funnel. The metering rate is selected so that the sodium methoxide does not reach the bottom (about 2 ml / min).

[0193] The methanol added / formed is separated by distillation with ethanol in the rectifying section and collected in the round-bottom flask. The reflux ratio is 5:1 (5 parts as reflux and 1 part as distillate). The amount distilled off must at least correspond to the amount of methanol added. After distillation, the sodium ethoxide in the bottom is distilled for about another 2 hours. At the same constant vacuum and temperature, the methanol present in the rectifying section is removed to prevent it from refluxing to the bottom.

[0194] After the experiment is completed and cooled, the bottom is opened through the outlet valve, and an ethanol solution of sodium ethoxide (abbreviated as "NE21") of about 21% by weight is removed.

[0195] 1.2 Depolymerization of PET with sodium ethoxide ethanol solution from reactive distillation

[0196] In the process according to the invention, first 150 g of PET are charged into an autoclave together with 1200 g of ethylene glycol. Then the solution is heated to 175 °C while stirring. Once the temperature of 175 °C is reached, 22.1 g of the ethanol solution of 21% sodium ethoxide from the transesterification described in part 1.1 (corresponding to 0.068 mol of sodium ethoxide) are added. The reaction is carried out over a period of 12 hours and the reactor output is analyzed after cooling. The conversion obtained and the amounts of the minor components mono(2-hydroxyethyl) terephthalate (= "MHET") and terephthalic acid (= "TS") relative to the amount of the main product BHET formed are determined by gas chromatography (= "GC").

[0197] 2. Comparative example C1

[0198] Depolymerization of PET with a conventionally produced sodium ethoxide ethanol solution

[0199] In the comparative experiment, first 150 g of PET are charged into an autoclave together with 1200 g of ethylene glycol. Then the solution is heated to 175 °C while stirring. Once the temperature of 175 °C is reached, 2.7 g of solid NaOH in 11 g of ethanol (equivalent to 0.068 mol of sodium ethoxide) are added. The reaction is carried out over a period of 5 hours and the reactor output is analyzed after cooling. The amounts of the minor components MHET and TS relative to the main product BHET formed are determined by GC.

[0200] 3. Results

[0201] Comparison of the contents of BHET, MHET and TS in the depolymerization products in example E1 of the invention and comparative example V1 shows that the amounts of BHET obtained in the two experiments are approximately the same. However, in the depolymerization using the sodium ethoxide ethanol solution obtained by reactive distillation, a smaller proportion of the unwanted MHET and TS by-products based on BHET was found.

Claims

1. Method for depolymerizing polyethylene terephthalate (PET), comprising the following steps: (a) Convert M A OR′ and ROH to M A OR, where M A is an alkali metal selected from sodium, potassium, and lithium, wherein R’ is an alkyl group having 1 to 6 carbon atoms, wherein R is an alkyl group having 2 to 7 carbon atoms and having at least one more carbon atom than the alkyl group R’, (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 in a mixture of OR.

2. The method according to claim 1, wherein, In step (a), ROH and M A OR' are fed individually or as a mixture into the reactive distillation column RR A and converted in RR A into a crude product containing ROH, R'OH, M A OR Among them, at the lower end of RR A a bottoms product stream S containing M A OR and ROH is withdrawn, and AP among them, at the upper end of RR A a vapor stream S containing R'OH, with or without ROH, is withdrawn AB , And wherein, in the optional step (a*), ROH is at least partially removed from S AP such that M OR and ROH are obtained in solid form F* or as a solution S A containing M OR and ROH AP * such that M OR is obtained, A wherein, compared to S AP S* has a reduced proportion by mass of ROH, AP ​ 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 R’OH and ROH, and these are at least partially separated from each other in the rectification column RD A in which.

4. The method according to any one of claims 1 to 4, wherein R’ = methyl and R is an alkyl group having 2 to 7 carbon atoms.

5. The method according to claim 4, wherein R is selected from ethyl, n-propyl, isopropyl, sec-butyl, 2-methyl-2-butyl, tert-butyl, 2-methyl-2-pentyl, 3-methyl-3-pentyl, 3-ethyl-3-pentyl, 2-methyl-2-hexyl, 3-methyl-3-hexyl, especially selected from ethyl, n-propyl, isopropyl, 2-methyl-2-butyl, 3-methyl-3-pentyl, 3-ethyl-3-pentyl.

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 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 achieved by crystallization and / or distillation.

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

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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