Improved process for depolymerization of polyethylene terephthalate

Abstract

The present invention relates to a process for the depolymerization of polyethylene terephthalate (i.e. "PET") in which PET is reacted with sodium or potassium ethylene glycol obtained via reactive distillation to form a mixture M1 comprising bis (2-hydroxyethyl) terephthalate ("BHET"). The method according to the invention is characterized in that a particularly high proportion of BHET is formed in the cleavage product of the mixture M1. Thus, the process of the present invention provides a high yield of BHET, which can be directly used to re-produce PET. Thus, the invention also relates to a process for recovering PET wherein BHET obtained in the process for depolymerizing PET, optionally, after further purification from M1, is polymerized again to form PET.

Description

[0001] The present invention relates to a process for the depolymerization of polyethylene terephthalate (= "PET"), wherein PET is reacted with sodium glycolate or potassium glycolate obtained by reactive distillation to give a mixture M1 comprising bis-2-hydroxyethyl terephthalate (= "BHET"; CAS No.: 959-26-2).

[0002] The process according to the invention is characterized in that a particularly high proportion of BHET is formed in the cleavage products in the mixture M1. As a result, the process according to the invention results in a high yield of BHET which can be used directly in the new production of PET.

[0003] The present invention therefore also relates to a process for recycling PET, wherein BHET obtained in a depolymerization process of PET is polymerized again to give PET, optionally after further purification from M1. Background Art

[0004] Polyethylene terephthalate (= "PET") is one of the most important plastics and is used in textile fibers, as films and as a material for plastic bottles. In 2007 alone, the volume 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 its persistence and the amount of waste derived from PET, it constitutes one of the biggest environmental challenges currently. The solution to this problem lies in avoiding and efficiently reusing PET.

[0006] The prior art proposes a variety of methods for cracking PET.

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

[0008] JP 2000-309663 A, US Pat. No. 4,355,175 A and T. Yoshioka, N. Okayama, A. Okuwaki, Ind. Eng. Chem. Res. 1998, 37, 336-340 describe hydrolysis methods for the depolymerization of PET.

[0009] EP 0723951 A1, US 3,222,299 A, WO 2020 / 002999 A2, SR Shukla, AM Harad, Journal of Applied Polymer Science 2005, 97, 513-517 (hereinafter “Shukla & Harad”) and ND Pinale, SR Shukla, European Polymer Journal 2008, 44, 4151-4156 describe the reaction of PET with ethylene glycol.

[0010] Shukla & Harad point out that PET glycolysis produces bis-2-hydroxyethyl terephthalate (="BHET"). This cleavage product can also be used as a reactant for the production of new PET.

[0011] Therefore, there is interest in PET depolymerization processes in which a maximum proportion of BHET is obtained in the cleavage products.

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

[0013] Surprisingly, a solution to the problem set out in the present invention has now been found.

[0014] The present invention relates to a depolymerization method of polyethylene terephthalate (PET), which comprises the following steps:

[0015] (a) Conversion of M in reactive distillation A OH and ethylene glycol to obtain a A Salt (M A glycolate) solution AP , where M A is an alkali metal selected from sodium and potassium, preferably M A =Sodium,

[0016] (b) Make solution S AP Reaction with PET gives a mixture M1 comprising bis-2-hydroxyethyl terephthalate (="BHET").

[0017] Preferably, S APIn step (a), the reaction material stream S comprising ethylene glycol is AE1 With M A The reactant stream S of OH AE2 In the reactive distillation column RR A The countercurrent reaction obtains ethylene glycol M A Salt, water, ethylene glycol, M A Crude product RP of OH A And obtained,

[0018] Where S AP In RR A The lower end is taken off as a bottom product stream.

[0019] Optionally, a vapor stream S comprising water, with or without ethylene glycol AB In RR A The upper end is removed.

[0020] In another aspect, the present invention relates to a process for recovering PET, wherein in step (ζ) BHET obtained in the depolymerization process according to the present invention is polymerized to give PET.

[0021] Surprisingly, it was found that PET and S obtained by reactive distillation AP The reaction results in a higher proportion of BHET than in the conventional process in which a basic alkali metal glycolate solution is obtained by mixing ethylene glycol in the corresponding alkali metal hydroxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings show a comparison of the contents of BHET ("1"), 2-hydroxyethyl terephthalate ("MHET"; "2"), and terephthalic acid ("TS"; "3") in the depolymerization of sodium ethylene glycol obtained by the process of the present invention and sodium ethylene glycol obtained by a conventional process.

[0023] The bars with shading “\\\\\” show the respective contents of BHET, MHET and TS in the reactor output in the depolymerization of PET according to Example E1 of the present invention, wherein the sodium ethylene glycol used for the depolymerization was obtained by reactive distillation.

[0024] The black bars show the respective contents of BHET, MHET and TS in the reactor output in the depolymerization of PET according to comparative example V1, wherein only ethylene glycol was used in the depolymerization reaction.

[0025] The bar graph with shading “ / / / / / / ” shows the respective contents of BHET, MHET and TS in the reactor output in the depolymerization of PET according to Comparative Example V2, wherein the sodium ethylene glycol used for the depolymerization was obtained by mixing NaOH and ethylene glycol in the reactor. DETAILED DESCRIPTION

[0026] Surprisingly, it has now been found that the glycolysis of PET proceeds particularly efficiently when sodium or potassium ethylene glycolate obtained by reactive distillation is used. A OH reacts with ethylene glycol to give glycolates. It has been observed that in the process according to the invention a higher proportion of BHET is obtained in the cleavage product than in the prior art process using glycolates obtained by dissolving an alkali metal hydroxide in ethylene glycol.

[0027] 1. Step (a): Reactive distillation to obtain a mixture containing ethylene glycol and ethylene glycol M A Salt Solution S AP

[0028] According to the invention, the polyol used in the method according to the invention comprises ethylene glycol and ethylene glycol M A Salt solution S AP By means of reactive distillation, M A Obtained by conversion of OH and ethylene glycol.

[0029] M A M is an alkali metal selected from sodium and potassium. A Sodium is preferred.

[0030] Reactive distillation for the preparation of alkali metal alkoxides is an important industrial process since alkali metal alkoxides are used as strong bases in the synthesis of a wide variety of chemicals, for example in the production of active pharmaceuticals or agrochemical ingredients, and as catalysts in transesterification and amidation reactions.

[0031] Alkali metal alkoxides (MOR) are usually prepared in a countercurrent distillation column from alkali metal hydroxides (MOH) and alcohols (ROH) by means of reactive distillation, wherein the removal of <1> The reaction water formed and the distillate:

[0032]

[0033] The principle of such a process is described, for example, in US Pat. No. 2,877,274 A, in which an aqueous alkali metal hydroxide solution and gaseous methanol flow countercurrently in a rectification column. This process is described again in essentially unchanged form in WO 01 / 42178 A1.

[0034] The most important alkali metal alkoxides in industry are sodium and potassium alkoxides, especially methoxides and ethoxides. There are many descriptions of their synthesis in the prior art, for example in EP 1 997 794 A1, WO 2021 / 148174 A1 and WO 2021 / 148175 A1.

[0035] Similar processes are described in GB 377,631 A and US Pat. No. 1,910,331 A, but in these additionally an introducing agent, for example benzene, is used.

[0036] Accordingly, DE 968903C describes a process for the continuous preparation of alkali metal alkoxides in a reaction column, wherein the water-alcohol mixture taken off at the top of the column is condensed and then phase separated. The aqueous phase is discarded and the alcohol phase is returned to the top of the column together with fresh alcohol. EP 0299577A2 describes a similar process, wherein the water in the condensate is separated with the help of a membrane.

[0037] In a preferred embodiment of the method according to the invention, S AP In step (a), the reaction material stream S comprising ethylene glycol is AE1 With M A The reactant stream S of OH AE2 In the reactive distillation column RR A The countercurrent reaction obtains ethylene glycol M A Salt, water, ethylene glycol, M A Crude product RP of OH A And obtained, where S AP In RR A The lower end is taken off as a bottom product stream.

[0038] Even more preferably, the vapor stream S comprising water, with or without ethylene glycol AB In RR A The upper end is removed.

[0039] 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 is carried out at least in some parts. It can also be referred to as a "reaction column" for short.

[0040] In a preferred embodiment of the method according to the invention, the composition comprises ethylene glycol and ethylene glycol M A Salt bottom product stream S AP In RR A A vapor stream S comprising water, with or without ethylene glycol AB In RR A The upper end is removed.

[0041] In the context of the present invention, "ethylene glycol" is understood to mean 1,2-ethylene glycol having the chemical formula HO-CH2-CH2-OH (CAS No. 107-21-1).

[0042] In the context of the present invention, "ethylene glycol M A The term "salt" is understood to mean ethylene glycol and M A The term "ethylene glycol MA Salt includes M A O-CH2-CH2-OH and M A O-CH2-CH2-OM A At least one of M A O-CH2-CH2-OH,

[0043] The most preferred A O-CH2-CH2-OH and M A O-CH2-CH2-OM A .

[0044] M A It is an alkali metal selected from sodium and potassium, preferably sodium.

[0045] Reactant Flow S AE1 In a preferred embodiment, S AE1 The mass proportion of ethylene glycol is ≥95% by weight, and more preferably ≥99.5% by weight, wherein S AE1 Furthermore, water and diethylene glycol are included, among others.

[0046] In a preferred embodiment of the process according to the invention, the reactant stream S AE1 The ethylene glycol may also be commercially available ethylene glycol, wherein the mass proportion of ethylene glycol is greater than 99.5% by weight, the mass proportion of water is at most 0.03% by weight, and the mass proportion of diethylene glycol is at most 0.05% by weight.

[0047] In one embodiment of the present invention, the reactant stream S AE1 Added in the form of vapor to the reactive distillation column RR A middle.

[0048] In an alternative preferred embodiment of the process according to the invention, before step (a), ethylene glycol is first charged into a reactive distillation column RR A The bottom of the column is then heated to boiling in step (a), which is in the reactive distillation column RR A A constant reactant flow S is generated in AE1 If necessary, then during the performance of step (a) in the reactive distillation column RR A Add ethylene glycol to the bottom.

[0049] Reactant Flow S AE2 Contains M A OH. In a preferred embodiment, S AE2 Not only M A OH, and further comprising at least one other compound selected from water and ethylene glycol. Even more preferably, S AE2 Not only M A OH, also contains water, in this case SAE2 It is M A An aqueous solution of OH.

[0050] When the reactant flow S AE2 Contains M A OH and water, based on the formation of S AE2 The total weight of the aqueous solution, M A The mass proportion of OH is especially 10% to 75% by weight, preferably 15% to 54% by weight, more preferably 30% to 53% by weight, even more preferably 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 is being carried out.

[0052] Reaction tower RR A Preferably, the reaction column RR contains internals. Suitable internals are, for example, trays, structured packings or unstructured packings. A When containing trays, suitable trays are bubble-cap trays, valve trays, tunnel-cap trays, Thormann trays, cross-slit bubble-cap trays or sieve trays. A When containing trays, preferably trays are selected in which no more than 5 wt %, more preferably less than 1 wt %, of liquid trickles through the respective trays. The construction measures required to minimize the trickling of liquid are familiar to those skilled in the art. In the case of valve trays, for example, 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 value usually established. When using sieve trays, it is particularly advantageous to reduce the diameter of the tray openings while maintaining or even increasing the number of openings.

[0053] When structured packing or non-structured packing is used, structured packing is preferred in terms of uniform distribution of the liquid.

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

[0055] In one embodiment of the present invention, the “reactant stream S comprising ethylene glycol” AE1 With M A The reactant stream S of OH AE2 In the reactive distillation column RR AIn particular, the reaction mixture S comprising ethylene glycol is reacted in a countercurrent manner. AE1 At least part of the feed point is located in the reaction column RR A Includes M A The reactant stream S of OH AE2 This is achieved below the feeding point.

[0056] In this embodiment, the reaction column RR A Preferably, in the reactant stream S AE1 Feed point and reactant flow S AE2 The feed points of the column preferably contain at least 2, in particular 15 to 40, theoretical plates.

[0057] Reaction tower RR A It can be operated as a pure stripping column. In this case, the reactant stream S comprising ethylene glycol is AE1 Introduced into the reaction tower RR in the form of vapor A in the lower area of ​​.

[0058] Optionally, in a solution comprising an alkali metal hydroxide A The reactant stream S of OH AE2 Below the feed point, but still in the reaction column RR A At the upper end or in the upper region of the AE1 This makes it possible to reduce the reaction tower RR A The size of the lower area of ​​the reaction tower RR A The upper end or upper region of the reactor is preferably fed with a reactant stream S comprising ethylene glycol in the form of vapor. AE1 In the reaction column RR, preferably only a portion of 10 to 70% by weight, preferably 30 to 50% by weight (based in each case on the total amount of ethylene glycol used) is introduced. A The remaining part is fed into the lower end of the A The reactant stream S of OH AE2 The feed point of NH4Cl2O3 is added as a single stream in vapor form or divided into a plurality of substreams, preferably 1 to 10 theoretical plates, more preferably 1 to 3 theoretical plates.

[0059] In an alternative embodiment of step (a) of the process according to the invention, the "reactant stream S comprising ethylene glycol" AE1 With M A The reactant stream S of OH AE2 In the reactive distillation column RR A The countercurrent reaction is particularly caused by the presence of ethylene glycol in the reactive distillation column RR. A At the bottom, and including M A The reactant stream S of OHAE2 During step (a) of the process according to the invention, then at RR A The bottom of the reactor heats the ethylene glycol to boiling and produces a reactant stream S comprising ethylene glycol. AE1 Then S AE1 and S AE2 Guiding each other upstream.

[0060] In the reaction tower RR A In the reaction material stream S containing ethylene glycol AE1 Then with the inclusion of M A The reactant stream S of OH AE2 According to the above reaction <1> Reaction (where "ROH" is ethylene glycol) to obtain ethylene glycol M A salt and H2O, these products exist in the presence of ethylene glycol and M A OH reactant mixture, because the reaction is an equilibrium reaction. Therefore, step (a) in the reaction tower RR A The crude product RP was obtained A , which contains not only ethylene glycol M A Salt and water products, also containing ethylene glycol and M A OH.

[0061] In RR A The lower end is then obtained and taken out containing ethylene glycol and ethylene glycol M A Salt bottom product stream S AP .

[0062] In a preferred embodiment of the method according to the invention, in RR A The upper end, preferably at RR A A water stream which may or may not still contain ethylene glycol is taken out from the top of the tower, which is referred to as "a vapor stream S containing water and containing or not containing ethylene glycol" AB ”.

[0063] If the vapor stream S AB If the reaction material stream S contains not only water but also ethylene glycol, the ethylene glycol is preferably obtained by distillation, for example in a rectification column. In this embodiment, at least a portion of the ethylene glycol obtained in the distillation can be used as the reaction material stream S AE1 Return to Reaction Tower RR A middle.

[0064] In a preferred embodiment, when S AB If it contains not only water but also ethylene glycol, it is introduced into the distillation column RD A and in RD A Separate into at least one A The steam stream S containing water taken out from the upper end OA, and at least one in RD A The material stream S containing ethylene glycol taken out from the lower end UA .

[0065] Preferably, the reactant stream S is selected AE1 The amount of ethylene glycol contained is such that it is simultaneously used as the bottom product stream S AP Ethylene glycol M obtained from A The solvent of the salt is preferably selected AE1 The amount of ethylene glycol is such that the ethylene glycol and ethylene glycol M A Salt bottom product stream S AP Take out the desired concentration of ethylene glycol M A The salt solution is present at the bottom of the reaction column.

[0066] In a preferred embodiment of the method according to the invention, and in particular in S AE2 Not only M A OH also contains water, the reactant stream S is used as AE1 The total weight (mass; unit: kg) of ethylene glycol used as the reactant stream S AE2 M A The ratio of the total weight (mass; unit: kg) of OH is 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 It is operated with or without reflux, preferably with reflux.

[0068] "With reflux" means that in the corresponding column, especially the reaction column RR A The vapor stream S containing water and containing or not containing ethylene glycol is taken out from the upper end AB Therefore, the relevant vapor stream S AB At least partly, preferably partly, is returned as reflux to the corresponding column, in particular to the reaction column RR A In the case where such a reflux is established, 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, most preferably 0.2.

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

[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 Installed in the reaction tower RR A On the condenser K RRA In the steam stream S AB At least partly condensed and returned to the corresponding column, in particular the reaction column RR A middle.

[0071] In the reaction tower RR A In a preferred embodiment of the process according to the invention, the reactant stream S is used as AE2 M 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 distillation conditions with refluxing ethylene glycol.

[0073] Step (a) is especially carried out at a temperature of 80°C to 197°C, preferably 100°C to 197°C, more preferably 120°C to 140°C and a pressure of 0.01 bar absolute to 1 bar absolute, preferably 0.05 bar absolute to 1 bar absolute, more preferably 0.05 bar absolute to 0.15 bar absolute, more preferably 0.05 bar absolute to 0.10 bar absolute.

[0074] In a more preferred embodiment, the reaction column RR A It comprises at least one evaporator, which is especially selected from the intermediate evaporator V ZA and bottom evaporator V SA . Reaction tower RR A More preferably, at least one bottom evaporator V is included SA .

[0075] According to the invention, the "intermediate evaporator" V Z This refers to the area above the bottom of the corresponding column, especially in the reaction column RR A (In this case they are called "V ZA ”) or in the distillation column RD used in a preferred embodiment and described in further detail below A (In this case they are called "V ZRD ”) above the bottom of the evaporator. A In the case of ZAA The crude product RP taken out from the column A .

[0076] According to the invention, the "bottom evaporator" V S This refers to heating the bottom of the corresponding column, especially the reaction column RR A or the bottom of the distillation column RD used in a preferred embodiment and described in further detail below A (In this case they are called "V SRD ” or “V SRD' ”) at the bottom of the evaporator. In RR A In the case of AP At least part of the RD A In the case of UA or S UA Part of UA1 .

[0077] The evaporator is usually 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. The heat exchanger of the evaporator commonly used in the case of natural circulation evaporators and forced circulation evaporators is a shell and tube or plate type device. When using a shell and tube exchanger, a heat carrier can flow through the tube together with the mixture to be evaporated flowing around the tube, or a heat carrier can flow around the tube together with the mixture to be evaporated flowing through the tube. In the case of a falling film evaporator, the mixture to be evaporated is usually introduced into the inside of the tube in the form of a film, and the tube is heated from the outside. Compared with a falling film evaporator, a thin film evaporator additionally includes a rotor with wipers, which distributes the liquid to be evaporated on the inner wall of the tube to form a film.

[0079] In addition to the evaporator types described, it is also possible to use any desired other evaporator types which are known to the person skilled in the art and which are suitable for distillation columns.

[0080] In a preferred embodiment of the method according to the invention, the composition comprises ethylene glycol and ethylene glycol M A Salt S AP In the reaction tower RR A The lower end is taken off as a bottom product stream.

[0081] Preferably, the reaction tower RR A Comprising at least one bottom evaporator V SA , some of the bottom product stream S AP Then through the bottom evaporator V SA and partially removing ethylene glycol therefrom, which provides a bottom product stream S AP* , and SAP In comparison, its ethylene glycol M A The mass ratio of salt increases.

[0082] In particular, in the process according to the invention, if at least one bottom evaporator V is used SA , at least some of the bottom product stream S AP Through the bottom evaporator V SA and at least partially removing ethylene glycol therefrom, then based on S AP The total mass in each case, S AP or S AP* Ethylene glycol M A The mass proportion of the salt in the ethylene glycol is 1% to 50% by weight, preferably 5% to 35% by weight, more preferably 15% to 35% by weight, most preferably 20% to 35% by weight.

[0083] Based on S AP The total mass, S AP / S AP* The mass proportion of residual water in the mixture is preferably <1% by weight, preferably <0.8% by weight, more preferably <0.5% by weight.

[0084] Based on S AP The total mass, S AP / S AP* Reactant M A The mass proportion of OH is preferably <1% by weight, preferably <0.8% by weight, more preferably <0.5% by weight.

[0085] In an even more preferred embodiment of the process according to the invention, the vapor stream S comprising water, with or without ethylene glycol AB In RR A The upper end is removed.

[0086] 2. In the distillation column R DA Middle distillation vapor stream S AB (Preferred)

[0087] In a further preferred embodiment, when the vapor stream S AB When it contains water and ethylene glycol, it is introduced into the distillation column RD A and in RD A Separate into at least one A The steam stream S containing water taken out from the upper end OA , and at least one in RD A The material stream S containing ethylene glycol taken out from the lower end UA .

[0088] “At least one in RD A The steam stream S containing water taken out from the upper end OA ” shall be understood to meanA The vapor obtained at the upper end can be withdrawn there as one or more vapor streams.

[0089] “At least one in RD A The material stream S containing ethylene glycol taken out from the lower end UA ” shall be understood to mean A The ethylene glycol obtained at the lower end can be removed there as one or more streams.

[0090] Steam stream S AB can be introduced into the distillation column RD via one or more feed points A In the steam stream S AB As two or more separate streams are introduced into the distillation column R DA In the embodiment of the present invention, when the feed point of each stream is in the distillation column RD A It is advantageous when the upper and lower surfaces are at substantially the same height.

[0091] In a preferred embodiment of the process according to the invention, when the vapor stream S AB When it contains water and ethylene glycol, it is distilled in the distillation tower RD A Separated into RD A The steam stream S containing water taken out from the upper end OA , and in RD A The material stream S containing ethylene glycol taken out from the lower end UA .

[0092] Another term for the "upper end of a distillation column" is "head".

[0093] Another term for the "lower end of a distillation column" is the "bottom" or "foot."

[0094] The distillation column RD used A It may be any distillation column known to a person skilled in the art.

[0095] Distillation tower RD A Preferably, internal components are present. Suitable internal components are, for example, trays, unstructured packings or structured packings. The trays used are typically bubble cap trays, sieve plate trays, valve trays, grooved bubble cap trays or slotted trays. Unstructured packings are typically beds of random packing elements. The random packing elements used are typically Raschig rings, Pall rings, Berl saddles or Saddle packing. Structured packing such as Sulzer Besides the interior components mentioned, other suitable interior components are known to the person skilled in the art and can likewise be used.

[0096] Preferred internals have a low specific pressure drop per theoretical plate. Structured packing and random packing elements have, for example, a significantly lower pressure drop per theoretical plate than trays. This has the following advantages: A The pressure drop in the reactor is kept as low as possible and the mechanical power of the compressor and the temperature of the glycol / water mixture to be evaporated are therefore kept low.

[0097] When the distillation tower RD A When structured or unstructured packings are present, they may be in the form of separate or continuous packings. However, generally at least two packings are provided, one packing in the vapor stream S AB Above the feed point, a packing is placed in the vapor stream S AB It is also possible to add steam stream S AB A packing is provided above the feed point of the vapor stream S AB Two or more trays are provided below the feed point of . If unstructured packing, such as random packing, is used, the random packing elements are generally arranged on a suitable support grid, such as a sieve tray or a mesh tray.

[0098] In this preferred embodiment, at least one steam stream S comprising water OA Then in the distillation tower RD A The vapor stream S OA The preferred mass fraction of water is ≥99% by weight, more preferably ≥99.6% by weight, even more preferably ≥96.0% by weight, the remainder being in particular ethylene glycol.

[0099] In this preferred embodiment, in RD A At least one stream S containing ethylene glycol is taken out from the lower end of UA , which may preferably comprise <1% by weight, more preferably ≤5000 ppm by weight, still more preferably ≤1000 ppm by weight, more preferably ≤100 ppm by weight of water.

[0100] In the context of the present invention, in the distillation column RD A At least one vapor stream S comprising water is taken from the top of OA This is to be understood in particular as meaning at least one vapor stream S OA In the distillation column RD A The liquid is taken off above the internal components in the tank as a top stream or as a side stream.

[0101] In the context of the present invention, in the distillation column RD A At least one stream S comprising ethylene glycol is taken from the bottom of UA It is to be understood in particular as meaning at least one stream S UA As bottom stream or in the distillation column RD A is taken out from the lower tray.

[0102] Distillation tower RD A It is operated with or without reflux, preferably with reflux.

[0103] "With reflux" is understood to mean that in the distillation column RD A The vapor stream S taken out from the upper end OA Not completely discharged, but partially condensed and returned to the corresponding distillation column RD A In the case where such a reflux is established, 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, most preferably 0.2.

[0104] Reflux can be achieved by distillation column RD A Top mounted condenser K RD The corresponding steam stream S OA In the condenser K RD The middle part is condensed and returned to the distillation column RD A middle.

[0105] 3. Step (b): PET and solution S AP Reactions

[0106] In step (b) of the process according to the invention, the mixture obtained in step (a) comprising ethylene glycol and ethylene glycol M A Salt solution S AP Reaction with PET gave a mixture M1 containing BHET.

[0107] 3.1 PET raw materials

[0108] The PET used in step (b) of the process according to the invention may be any PET which needs to be depolymerized. Typically, such PET occurs as waste, especially in households, industry or agriculture.

[0109] In one embodiment of the process according to the invention, the PET to be depolymerized is therefore mixed with other plastics, in particular at least one plastic selected from polyethylene ("PE"), polyvinyl chloride ("PVC"). This is generally the case when PET from plastic waste is to be depolymerized in the process according to the invention. In this embodiment, the PET is at least partially separated from the other plastics, preferably by sorting, before carrying out step (b) of the process according to the invention.

[0110] In one embodiment of the method according to the invention, the PET is subjected to at least one pretreatment step.

[0111] Such a pretreatment step is described, for example, in DE 10032899 C2.

[0112] According to the invention, before being used in step (b), the PET is subjected to at least one pretreatment step selected from a chemical pretreatment step, a comminution step.

[0113] In case PET is mixed 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, a chemical pretreatment step, a comminution step.

[0114] In case PET is mixed 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 comminuted.

[0115] The chemical pretreatment step is in particular a washing step. Such a washing step has the advantage of removing any impurities, in particular food residues, cosmetic residues and / or body secretions (e.g. blood, semen, feces) before carrying out step (b). Such impurities can reduce the reaction efficiency in step (b) and / or reduce the purity of the BHET obtained thereby.

[0116] In the chemical pretreatment step, especially the washing step, the waste is heated, especially in a washing solution, at a temperature of 30 to 99°C, preferably 50 to 90°C, more preferably 70 to 85°C.

[0117] Typical washing solutions are familiar to those skilled in the art and are preferably selected from:

[0118] - an aqueous solution of a surfactant, preferably a nonionic surfactant;

[0119] - an aqueous solution of an alkali metal hydroxide or an alkaline earth metal hydroxide, preferably an aqueous solution of NaOH.

[0120] The treatment time in the chemical pretreatment step, especially the washing step, is especially 1 min to 12 h, preferably 10 min to 6 h, more preferably 30 min to 2 h, even more preferably 45 to 90 min, most preferably 60 min.

[0121] After the PET has been treated by a chemical pretreatment step, in particular a washing step, the aqueous solution is separated off, for example by filtration, and the cleaned PET is preferably washed at least once with water in order to remove residues of the washing solution.

[0122] The PET waste thus obtained is then dried, in particular in a drying cabinet.

[0123] The temperature used for drying here is especially 30 to 120°C, preferably 50 to 100°C, more preferably 60 to 90°C, most preferably 80°C.

[0124] The comminution step has the advantage that 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). The comminution can be carried out in equipment known to those skilled in the art, such as a shredder or a cutter.

[0125] In a further embodiment of the method according to the invention, the PET is decolorized or colored in a controlled manner before step (b) is carried out. This can be carried out by methods known to those skilled in the art, such as decolorization with hydrogen peroxide or coloring with dyes.

[0126] 3.2 Reaction conditions

[0127] PET can then be reacted with a mixture containing ethylene glycol and ethylene glycol M under conditions familiar to those skilled in the art. A Salt solution S AP The reaction was carried out to obtain a mixture M1.

[0128] Preferably, the reaction in step (b) is carried out until time t b , at which point 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.

[0129] The percentage P is calculated by the following formula:

[0130] P=(n TA +n MHET +n BHET ) / n PET .

[0131] Here n PET is the molar amount of repeating units of the following structure (Ξ) in the PET used in step (b):

[0132]

[0133] n TA is the time from step (b) to time t b The molar amount of TA formed.

[0134] n MHET is the time from step (b) to time t b Molar amount of MHET formed.

[0135] n BHET is the time from step (b) to time t b Molar amount of BHET formed.

[0136] The structures of compounds BHET, MHET and TA are as follows:

[0137]

[0138] "MHET" also encompasses the corresponding carboxylate salts of the indicated structure.

[0139] "TA" also encompasses the corresponding mono- and di-carboxylates of the shown structure.

[0140] The reaction in step (b) is especially carried out at a temperature of at least 100°C, preferably at a temperature of ≥100°C to ≤197°C, more preferably at a temperature of ≥130°C to ≤197°C, more preferably at a temperature of ≥150°C to ≤197°C, more preferably at a temperature of ≥175°C to ≤197°C.

[0141] The reaction in step (b) is preferably carried out at the boiling point of ethylene glycol. 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 set up in a manner familiar to those skilled in the art, for example in a distillation apparatus.

[0142] The ethylene glycol M used in the process is based on the total weight of PET used in the process. A The total weight of the salt is especially from 0.1% to 100% by weight, preferably from 0.5% to 80% by weight, more preferably from 1.0% to 50% by weight, more preferably from 1.5% to 25% by weight, more preferably from 2.0% to 10% by weight, more preferably from 2.5% to 6.0% by weight, more preferably from 3.5% to 5.0% by weight, most preferably 3.9% by weight.

[0143] The reaction in step (b) can be carried out using apparatus familiar to those skilled in the art.

[0144] After step (b) of the process according to the invention has been completed, a mixture M1 is obtained in which the molar amount of BHET (n BHET ) and the total molar amount of MHET and TA (n MHET +n TA ) has a molar ratio η of 1:1 to 1000:1, preferably 2:1 to 500:100, more preferably 4:1 to 300:1, even more preferably 10:1 to 100:1, still more preferably 13:1 to 60:1, still more preferably 13:1 to 24:1.

[0145] η=n BHET / (n MHET +n TA )

[0146] 3.3 Preferred step (c)

[0147] In a further preferred step (c), BHET is at least partially separated from M1. This is even more preferably carried out by crystallization and / or distillation. Even more preferably, the BHET in step (c) is filtered off from M1 and then crystallized.

[0148] 4. Process of recycling PET

[0149] In the process according to the invention, the BHET obtained in the mixture M1 is preferably polymerized to PET in the process for recovering polyethylene terephthalate in step (ζ).

[0150] This type of polymerization is known to the person skilled in the art as "polycondensation" and is described, for example, in EP 0 723 951 A1 and in Th. Rieckmann and S. "Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters". Edited by J.Scheirs and TELong, 2003, John Wiley & Sons, Ltd ISBN: 0-471-49856-4, Page 92 Chapter 2 "Poly(Ethylene Terephthalate) Polymerization–Mechanism, Catalysis, Kinetics, Mass Transfer and Reactor Design".

[0151] In particular, for this purpose, BHET is polymerized back to PET in step (ζ) in the presence of a catalyst, in particular a catalyst selected from antimony compounds, preferably Sb2O3.

[0152] Preferably, the polymerization of BHET to PET in step (ζ) is carried out at least at the boiling point of ethylene glycol. In particular, during the polymerization in step (ζ), ethylene glycol is removed from the reaction mixture so as to shift the reaction equilibrium toward the polymer PET.

[0153] More preferably, the polymerization of BHET to PET in step (ζ) is carried out at the boiling point of ethylene glycol. Even more preferably, in this case, during the polymerization in step (ζ), ethylene glycol is removed from the reaction mixture so as to shift the reaction equilibrium to the polymer PET side.

[0154] This is achieved in particular by distillation at a pressure of <1 bar, preferably 0.1 mbar, at the boiling temperature of the ethylene glycol at the corresponding pressure.

[0155] Example

[0156] 1. Invention Example I1:

[0157] 1.1 Preparation of sodium glycolate solution by reactive distillation

[0158] The following equipment is used as distillation equipment:

[0159] The storage container or bottom used in the distillation apparatus is a heatable 2.5l jacketed container with a temperature sensor and a vacuum-tight agitator. Above is a 25cm tower with Multifill filler and a silver mirror (stripping section). NaOH is metered into the tower by means of a dropping funnel. Above the metering point is another tower (rectification section) for separating ethylene glycol and water vapor. The reflux ratio can be established with the help of a vapor distributor in the upper part of the tower, where the distillate is collected in a round-bottomed flask. The round-bottomed flask can be separated and exchanged from the distillation system via a pressure-equalizing dropping funnel. In the rectification section, a reflux condenser with a vacuum connection is attached, with the help of which the entire apparatus can be evacuated. A vacuum is generated by means of a rotary vane pump, which is connected to the distillation apparatus via two cold traps and a safety bottle. The pressure in the distillation apparatus is measured in a safety bottle (Büchi vacuum controller), which can also be ventilated. The bottom storage and the tower with Multifill filler are completely surrounded by aluminum foil for insulation to ensure uniform temperature in the reactor / tower.

[0160] The bottom is first filled with ethylene glycol and the entire apparatus is evacuated to 50 mbar. Subsequently, the bottom is heated to boiling point, thereby establishing a reflux from the rectification section. Subsequently, sodium hydroxide solution (50% by weight in water) is metered in with the help of a dropping funnel. The metered addition rate is selected so that the sodium hydroxide solution does not reach the bottom (approximately 2 ml / min).

[0161] The added / formed water is separated from the ethylene glycol by distillation in the rectification section and collected in a round-bottom flask. The reflux ratio is 5:1 (5 parts reflux, 1 part distillate). The amount distilled off must at least correspond to the amount of water added. After the distillative removal, the sodium glycolate in the bottom is distilled further for about 2 hours. The water present in the rectification section is removed under constant vacuum and temperature in order to prevent reflux into the bottom.

[0162] After the experiment was over and cooling, the bottom was opened by means of an outlet valve and about 20% by weight of a solution of sodium glycolate in ethylene glycol was removed.

[0163] 1.2 Depolymerization of PET with sodium ethylene glycol solution from reactive distillation

[0164] In the process according to the invention, the autoclave is first charged with 100 g of PET and 800 g of ethylene glycol. The solution is then heated to 150° C. with stirring. Once the temperature has reached 150° C., 19.5 g of a 20% solution of sodium ethylene glycol in ethylene glycol from a reactive distillation (corresponding to 0.046 mol) are added. The reaction is carried out for 5 hours and the reactor output is analyzed after cooling. The resulting conversion is determined by gas chromatography. The conversions of BHET (1) and 2-hydroxyethyl terephthalate (= “MHET”) (2) and terephthalic acid (= “TA”) (3) are shown in the figure (% relative to the repeating unit (Ξ) used in PET; shaded bars from the upper left corner to the lower right corner: “\\\\\\”).

[0165] 2. Comparative Example V1:

[0166] In the comparative experiment, the autoclave was first charged with 100 g of PET and 800 g of ethylene glycol. The solution was then heated to 150° C. with stirring. The reaction was carried out for 5 hours and the reactor output was analyzed after cooling. The resulting conversions of BHET (1) and MHET (2) and TA (3) are shown in the figure (black “■”).

[0167] 3. Comparative Example V2:

[0168] In the comparative experiment, the autoclave was initially charged with 100 g of PET and 800 g of ethylene glycol. The solution was then heated to 150° C. with stirring. Once the temperature reached 150° C., 3.7 g of a 50% by weight aqueous NaOH solution (corresponding to 0.046 mol) were added. The reaction was carried out for 5 hours and the reactor output was analyzed after cooling. The resulting conversions of BHET (1) and MHET (2) and TA (3) are shown in the figure (hatched bar from the upper right corner to the lower left corner: “ / / / ”).

[0169] 4. Results

[0170] Comparison of the contents of BHET, MHET and TA in the depolymerization products of Inventive Example E1 and Comparative Examples V1 and V2 shows that the use of a sodium ethylene glycol solution obtained by reactive distillation for depolymerization results in a higher proportion of BHET. This is advantageous because more product is obtained as a result, which can be directly converted into new PET products in the polycondensation.

Claims

1. A method for depolymerizing polyethylene terephthalate (PET), comprising the following steps: (a) Conversion of M in reactive distillation A OH and ethylene glycol to obtain a A Salt solution S AP , where M A is an alkali metal selected from sodium and potassium, (b) making the solution S AP Reaction with PET gives a mixture M1 comprising bis-2-hydroxyethyl terephthalate BHET.

2. The method according to claim 1, wherein S AP In step (a), the reaction material stream S comprising ethylene glycol is AE1 With M A The reactant stream S of OH AE2 In the reactive distillation column RR A The countercurrent reaction obtains ethylene glycol M A Salt, water, ethylene glycol, M A Crude product RP of OH A And obtained, Where S AP In RR A The lower end is taken off as a bottom product stream.

3. The method according to claim 2, wherein in RR A A vapor stream S containing water, with or without ethylene glycol is taken out from the upper end AB .

4. The method according to claim 3, wherein the S comprising water and ethylene glycol AB Introducing distillation column RD A and in RD A Separate into at least one A The steam stream S containing water taken out from the upper end OA , and at least one in RD A The material stream S containing ethylene glycol taken out from the lower end UA .

5. The process according to any one of claims 1 to 4, wherein step (b) is performed until at least P = 10% of the PET used in step (b) has been converted.

6. The method according to any one of claims 1 to 5, wherein S AP The water content is less than 1% by weight.

7. The process according to any one of claims 1 to 6, wherein step (b) is carried out at the boiling point of ethylene glycol.

8. The method according to any one of claims 1 to 7, wherein in step (b) a sufficient amount of S is used. AP , so that based on the total weight of PET used in step (b), the ethylene glycol M used in step (b) A The total weight of the salt is from 0.1 wt% to 100 wt%.

9. The process according to any one of claims 1 to 8, wherein in a further step (c), BHET is at least partially separated from M1.

10. The process according to claim 9, wherein the at least partial separation of BHET from M1 in step (c) is carried out by crystallization and / or distillation.

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

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

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

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

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

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