Improved method 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 with sodium or potassium glycolate obtained by reactive distillation to obtain a mixture M1 containing bis-2-hydroxyethyl terephthalate (= "BHET"). The process according to the invention is characterized in that BHET makes up a particularly high proportion of the cleavage products in the mixture M1. As a result, the process according to the invention provides a high yield of BHET, which can be used directly for the production of recycled PET. The present invention therefore also relates to a process for the regeneration of PET, in which the BHET obtained in the depolymerization of PET, if appropriate after further purification from M1, is repolymerized to produce PET.

Description

[Technical field]

[0001] The present invention relates to a process for the depolymerization of polyethylene terephthalate (= "PET"), which comprises reacting PET with sodium or potassium glycolate obtained by reactive distillation to obtain a mixture M1 comprising bis-2-hydroxyethyl terephthalate (= "BHET"; CAS number: 959-26-2).

[0002] The process according to the invention is characterized in that BHET constitutes a particularly high proportion of the cleavage products in mixture M1, and as a result, the process according to the invention provides a high yield of BHET, which can be used directly in the production of recycled PET.

[0003] The present invention therefore also relates to a process for the regeneration of PET, in which the BHET obtained by the depolymerization of PET is repolymerized to produce PET, optionally after further purification from M1.

[0004] 2. Background of the Invention Polyethylene terephthalate (PET) is one of the most important plastics used in textile fibers, as films and as a material for plastic bottles. In 2007 alone, the amount used in plastic bottles was about 10 7 (W. Caseri, Polyethylenterephthalate, RD-16-03258 (2009) in F. Boeckler, B. Dill, G. Eisenbrand, F. Faupel, B. Fugmann, T. Gamse, R. Matissek, G. Pohnert, A. Ruehling, S. Schmidt, G. Sprenger, ROEMPP [Online], Stuttgart, Georg Thieme Verlag, January 2022).

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

[0006] Several methods for cleaving PET have been proposed in the prior art.

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

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

[0009] The reaction of PET with glycols is described in EP 0723951, U.S. Pat. No. 3,222,299, WO 2020 / 002999, SR Shukla, AM Harad, Journal of Applied Polymer Science 2005, 97, 513-517 (hereinafter "Shukla & Harad") and ND Pingale, SR Shukla, European Polymer Journal 2008,44, 4151-4156.

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

[0011] Therefore, there is growing interest in methods for depolymerizing PET that will yield the highest possible proportion of BHET among the cleavage products.

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

[0013] Brief description of the invention Surprisingly, a method has now been found which solves the problem according to the invention.

[0014] The present invention relates to the following: (a)M A OH and glycol are reacted by reactive distillation to produce glycol and M A Solution S containing glycolate AP wherein M A is an alkali metal selected from sodium and potassium, preferably M A = sodium; (b) Solution S AP with PET to produce a mixture M1 comprising bis-2-hydroxyethyl terephthalate (= "BHET"); The present invention relates to a method for depolymerizing polyethylene terephthalate (PET), comprising the steps of:

[0015] Preferably, S AP is a glycol-containing reactant stream S in step (a). AE1 and M. A Reactant stream S containing OH AE2 and the reaction rectification column RR A The reaction proceeds in a countercurrent manner in M A Glycolate, Water, Glycol, M A Crude RP containing OH A is obtained by generating S AP RR A The bottom product stream is removed at the lower end of the reactor.

[0016] a vapor stream S optionally containing water and optionally glycol; AB But, RR A It is taken out at the top of the

[0017] In a further aspect, the present invention relates to a method for the regeneration of PET, comprising, in step (ζ), polymerizing BHET obtained by the depolymerization method according to the present invention to produce PET.

[0018] S obtained by reactive distillation of PET AP It has surprisingly been found that reacting with a glycol in the corresponding alkali metal hydroxide gives a higher percentage of BHET than the conventional method of obtaining an alkaline alkali metal glycolate solution by mixing the glycol in the corresponding alkali metal hydroxide. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 shows a comparison of the contents of BHET ("1"), 2-hydroxyethyl terephthalic acid ("MHET"; "2") and terephthalic acid ("TS"; "3") in the depolymerization using sodium glycolate obtained by the method according to the invention with the contents of BHET ("1"), 2-hydroxyethyl terephthalic acid ("MHET"; "2") and terephthalic acid ("TS"; "3") in the depolymerization using sodium glycolate obtained by a conventional method.

[0020] The bars hatched with "\\\\\" indicate the respective contents of BHET, MHET and TS in the reactor effluent during the depolymerization of PET according to example E1 according to the invention, in which the sodium glycolate used for the depolymerization was obtained by reactive distillation.

[0021] The black bars show the BHET, MHET and TS contents in the reactor effluent in the depolymerization of PET according to Comparative Example V1, where only glycol was used for the depolymerization.

[0022] The bars hatched with " / / / / / " indicate the BHET, MHET and TS contents in the reactor effluent in the depolymerization of PET according to comparative example V2, in which the sodium glycolate used for the depolymerization was obtained by mixing NaOH and glycol in the reactor.

[0023] Detailed Description of the Invention It has now surprisingly been found that the glycolysis of PET proceeds particularly efficiently with sodium or potassium glycolate obtained by reactive distillation. In the reactive distillation according to the invention, the glycolate is reacted with the corresponding alkali metal hydroxide M A It has now been found that the process according to the invention gives a higher proportion of BHET in the cleavage products compared to the prior art process using glycolates obtained by dissolving an alkali metal hydroxide in glycol.

[0024] 1. Step (a): Reactive distillation of glycol and M A Solution S containing glycolate AP Get The glycols and M used in the process according to the invention A Solution S containing glycolate AP According to the present invention, M A It is obtained by reactive distillation by reacting OH with glycol.

[0025] M A is an alkali metal selected from sodium and potassium. A is preferably sodium.

[0026] Reactive distillation for the production of alkali metal alkoxides is an important industrial process because they are used as strong bases in the synthesis of many chemicals, e.g. in the production of pharmaceuticals and agricultural active substances, and as catalysts for transesterification and amidation reactions.

[0027] Alkali metal alkoxides (MORs) are produced from alkali metal hydroxides (MOHs) and alcohols (ROHs) by reactive distillation, usually in a countercurrent distillation column, according to the following reaction: <1> The water of reaction produced by this is removed together with the distillate.

[0028] [ka]

[0029] Such a process principle is described, for example, in US Pat. No. 2,877,274, where an aqueous alkali metal hydroxide solution and gaseous methanol are used countercurrently in a rectification column. This process is also described in essentially unchanged form in WO 01 / 42178.

[0030] The most important industrial alkali metal alkoxides are those of sodium and potassium, in particular the methylates and ethylates, whose synthesis is often described in the prior art, for example in EP 1997794, WO 2021 / 148174 and WO 2021 / 148175.

[0031] British Patent No. 377,631 and US Pat. No. 1,910,331 describe similar processes, but in addition a further entrainer such as benzene is used.

[0032] Likewise, DE 968 903 A1 describes a process for the continuous preparation of alkali metal alkoxides in a reaction column, in which a water-alcohol mixture taken off at the top is condensed and then subjected to phase separation. The water phase is then discarded, and the alcohol phase is returned to the top of the column together with fresh alcohol. A similar process is described in EP 0 299 577 A1, in which water is separated from the condensate by means of a membrane.

[0033] In a preferred embodiment of the method according to the invention, S AP is a glycol-containing reactant stream S in step (a). AE1 and M. A Reactant stream S containing OH AE2 and the reaction rectification column RR A The reaction proceeds in a countercurrent manner in M A Glycolate, Water, Glycol, M A Crude RP containing OHA is obtained by generating S AP RR A The bottom product stream is removed at the lower end of the reactor.

[0034] Even more preferably, the vapor stream S comprises water and optionally glycol. AB But, RR A It is taken out at the top of the

[0035] A "reactive rectification column" is defined according to the present invention as a rectification column in which at least a part of the reaction according to step (a) of the process according to the present invention takes place. It may also be abbreviated as "reaction column".

[0036] According to a preferred embodiment of the method according to the invention, RR A At the lower end of A A bottom product stream S AP A vapor stream S containing water and optionally glycol is withdrawn. AB But, RR A It is taken out at the top of the

[0037] By "glycol" is understood, within the meaning of the present invention, 1,2-ethylenediol, having the chemical formula HO-CH2-CH2-OH (CAS number 107-21-1).

[0038] "M A "Glycolate" means, for the purposes of the present invention, a glycol and M A It is understood to be a salt of "M A The term "glycolate" includes A O-CH2-CH2-OH and M A O-CH2-CH2-OM A At least one of, preferably at least M A O-CH2-CH2-OH, most preferably M A O-CH2-CH2-OH and M A O-CH2-CH2-OM A is included.

[0039] MA is an alkali metal selected from sodium and potassium, preferably sodium.

[0040] Reactive Logistics S AE1 In a preferred embodiment, S AE1 The mass fraction of glycol in S is ≧95% by weight, more preferably ≧99.5% by weight. AE1 Among other things, these include water and diethylene glycol.

[0041] In a preferred embodiment of the process according to the invention, the reactant stream S AE1 The glycols used as may be commercially available glycols having a glycol mass proportion of more than 99.5% by weight and a water mass proportion of up to 0.03% by weight and up to 0.05% by weight of diethylene glycol.

[0042] Reactive Logistics S AE1 In one embodiment of the present invention, the reaction rectification column RR A It is added in vapor form to the

[0043] In an alternative preferred embodiment of the process according to the invention, the glycol is fed to a reaction rectification column RR prior to step (a). A is charged to the bottom of the reaction rectification column RR A At a constant reactant flow rate S AE1 If necessary, glycol is then fed to the reaction rectification column RR during step (a). A The bottom of the container is then filled.

[0044] Reactive Logistics S AE2 is M A In a preferred embodiment, S AE2 is M A In addition to OH, it contains at least one further compound selected from water, glycol. Even more preferably, S AE2 is M A In addition to OH, water is included. In that case, S AE2 is M AIt is an aqueous solution of OH.

[0045] Reactive Logistics S AE2 M A If OH and water are included, S AE2 M relative to the total weight of the aqueous solution formed A The mass proportion of OH is in particular in the range from 10 to 75% by weight, preferably from 15 to 54% by weight, more preferably from 30 to 53% by weight, even more preferably from 40 to 52% by weight and most preferably 50% by weight.

[0046] Step (a) of the process according to the invention preferably comprises a reactive rectification column (or "reaction column") RR A It will be carried out.

[0047] Preferably, the reaction column RR A includes internals. Suitable internals are, for example, trays, structured packing or random packing. A If reactor RR contains trays, bubble cap trays, valve trays, tunnel cap trays, turman trays, cross slit bubble cap trays or sieve trays are suitable. A If the comprises trays, they are advantageously selected such that a maximum of 5% by weight of the liquid, preferably less than 1% by weight, drips through each tray. The design measures necessary to minimize dripping of liquid are well known to those skilled in the art. In the case of valve trays, for example, a particularly tight valve design is selected. By reducing the number of valves, the steam velocity at the tray openings can further be increased up to twice the value normally set. 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.

[0048] When structured or random packing is used, structured packing is preferred from the viewpoint of uniform distribution of liquid.

[0049] Step (a) of the process according to the invention can be carried out continuously or discontinuously. Preferably, step (a) of the process according to the invention is carried out continuously.

[0050] "Reactant stream containing glycol S AE1 and M. A Reactant stream S containing OH AE2 and the reaction rectification column RR A In the embodiment according to the invention, the term "countercurrent reaction in the reaction column RR A The glycol-containing reactant stream S AE1 At least some of the supply points of M A Reactant stream S containing OH AE2 This is ensured by being located below the supply point.

[0051] Reactor RR A is advantageously the reactant flow S in this embodiment. AE1 Supply points and reactant flows S AE2 Between the feed points there are at least 2, particularly 15 to 40 theoretical plates.

[0052] Reactor RR A can be operated as a pure stripping column, in which case the glycol-containing reactant stream S AE1 is the reactor RR A The gas is supplied in vapor form in the lower region of the gas turbine.

[0053] Optionally, a reactant stream S containing glycol AE1 A part of the alkaline solution M A Reactant stream S containing OH AE2 The reactor RR is located below the feed point of A The gas is added in vapor form at the top or in the region of the top of the reactor RR. A The size of the lower region of the reactant stream S containing glycol can be reduced. AE1 Part of the reactor RR AIn particular, if added in vapor form, preferably only a fractional amount of 10 to 70% by weight, preferably 30 to 50% by weight (in each case based on the total amount of glycol used), is added to the top or top region of the reaction column RR. A The remaining portion is fed in a single stream or divided into several partial streams at 1 to 10 theoretical plates, particularly preferably at 1 to 3 theoretical plates, at the lower end of M A Reactant stream S containing OH AE2 The gas is added in vapor form below the feed point of the gas.

[0054] In an alternative embodiment of step (a), "a reactant stream S AE1 and M. A Reactant stream S containing OH AE2 and the reaction rectification column RR A In particular, the term "countercurrent reaction in the reaction rectification column RR" refers to the reaction of glycol with the reaction rectification column RR. A Located at the bottom of M A Reactant stream S containing OH AE2 This is ensured by the fact that the feed point of RR is located above the bottom. In that case, during step (a) of the method according to the invention, A The glycol at the bottom of the reaction stream S is heated to boiling and the glycol-containing reactant stream S AE1 In that case, S AE1 and S AE2 are countercurrents to each other.

[0055] Next, the reactor RR A In the above reaction <1> ("ROH" stands for "glycol"), reactant stream S containing glycol AE1 and M. A Reactant stream S containing OH AE2 and reacted, M A Glycolate and HO are produced, and since this is an equilibrium reaction, these products are the same as the reactants glycol and M A Therefore, in step (a), the reaction tower RP A In the above, the product M A In addition to glycolate and water, glycol and M A Crude product RP containing OH Ais obtained.

[0056] RR A At the lower end of the range, glycol and M A A bottom product stream S AP is obtained, which is then extracted.

[0057] RR A The upper end of the RR A In a preferred embodiment of the process according to the invention, the above-mentioned "vapor stream S comprising water and optionally glycol" is AB A water stream, which may still contain glycol, is removed, designated "."

[0058] Steam flow S AB In the case where reactant stream S contains glycol in addition to water, the glycol is preferably recovered by distillation, for example in a fractionator. In this embodiment, at least a portion of the glycol obtained during distillation is recovered as reactant stream S AE1 Reactor RR A can be supplied again.

[0059] In a preferred embodiment, S AB If contains glycol in addition to water, S AB is the rectification tower RD A sent to RD A In R.D. A At least one steam stream S containing water is withdrawn at the top end of OA and R.D. A At least one glycol-containing stream S is withdrawn at the lower end of UA and are separated into

[0060] Reactive Logistics S AE1 The amount of glycol contained in the bottom product stream S AP M obtained by A Advantageously, the reactant stream S is selected to also act as a solvent for the glycolate. AE1 The amount of glycol in the solution is adjusted to the desired concentration of M A The glycolate solution is present at the bottom of the reactor and is mixed with glycol and M AA bottom product stream S AP is selected to be retrieved as

[0061] In a preferred embodiment of the method according to the invention, in particular S AE2 M A If the reactant stream S contains water in addition to OH, AE1 The total weight of glycol used as the reactant stream (weight; unit: kg) and the total weight of the reactant stream S AE2 Used as M A The ratio to the total weight of OH (weight; unit: kg) is 1:1 to 50:1, more preferably 2:1 to 40:1, even more preferably 3:1 to 30:1, and even more preferably 5:1 to 10:1.

[0062] In a preferred embodiment of the process according to the invention, the reaction column RR A is operated with or without reflux, preferably with reflux.

[0063] "With reflux" means that each column, especially the reaction column RR A A vapor stream S containing water and optionally glycol is withdrawn at the top end of AB However, this means that the steam flow S AB At least a part, preferably a part, of each column, in particular the reaction column RR A When such a reflux is set up, the reflux ratio is preferably 0.01 to 1, more preferably 0.02 to 0.9, even more preferably 0.03 to 0.34, particularly preferably 0.04 to 0.27, very particularly preferably 0.05 to 0.24, and most preferably 0.2.

[0064] The reflux ratio is generally, and within the meaning of the present invention, understood to be the ratio of the proportion of the mass flow (kg / h) withdrawn from each column in liquid or gaseous form to the proportion of the mass flow (kg / h) returned to the column in liquid form (reflux).

[0065] The reflux can be set by installing a condenser at the top of each column. A Cooler K RRA Cooler K is installed. RRA In the vapor flow S AB is at least partially condensed, and A is supplied again to

[0066] Reactor RR A In an embodiment in which reflux is set up in the preferred embodiment of the process according to the invention, the reactant flow S AE2 Used as M A OH is at least partially mixed with the reflux stream and the resulting mixture is passed through reaction column RR A can be supplied to.

[0067] In a preferred embodiment of the process according to the invention, step (a) is in particular carried out under distillation conditions with glycol reflux.

[0068] Step (a) is particularly carried out at a temperature in the range of 80°C to 197°C, preferably 100°C to 197°C, more preferably 120°C to 140°C, and at a pressure in the range of 0.01 bar abs. to 1 bar abs., preferably 0.05 bar abs. to 1 bar abs., more preferably 0.05 bar abs. to 0.15 bar abs., more preferably 0.05 bar abs. to 0.10 bar abs.

[0069] In a more preferred embodiment, the reaction column RR A Especially the intermediate evaporator V ZA and bottom evaporator V SA The reaction column RR includes at least one evaporator selected from the group consisting of: A Particularly preferably, at least one bottom evaporator V SA Includes.

[0070] "Intermediate evaporator" V Z According to the present invention, above the bottom of each column, in particular the reaction column RR AThe evaporator located above the bottom of the ZA ") or the rectification column RD used in the preferred embodiment and described in more detail below. A The evaporator located above the bottom of the ZRD " RR A In the case of ZAA Crude product RP extracted as A Allow to evaporate.

[0071] "Bottom evaporator" V S According to the present invention, the bottom of each column, in particular the reaction column RR A or the bottom of the rectification column RD, which is used in the preferred embodiment and described in more detail below. A The evaporator that heats the bottom of the SRD " or "V SRD‘ " RR A In the case of AP At least a portion of the RD is evaporated. A In the case of UA , or S UA S is part of UA1 Allow to evaporate.

[0072] The evaporator is usually located outside each reaction column or fractionation column.

[0073] Suitable evaporators that can be used as intermediate and bottom evaporators are, for example, natural circulation evaporators, forced circulation evaporators, forced circulation evaporators with expansion, kettle-type reboilers, falling film evaporators or thin film evaporators. In natural circulation evaporators and forced circulation evaporators, tube bundles or plate devices are usually used as the heat transfer device of the evaporator. When using tube bundle heat transfer devices, the heat transfer medium can flow through the tubes and the mixture to be evaporated can flow around the tubes, or the heat transfer medium can flow around the tubes and the mixture to be evaporated can flow through the tubes. In the case of falling film evaporators, the mixture to be evaporated is usually added as a thin film on the inside of the tubes, and the tubes are heated from the outside. In contrast to falling film evaporators, thin film evaporators are additionally provided with a rotor with wipers, which distributes the liquid to be evaporated on the inner wall of the tubes to form a thin film.

[0074] However, in addition to those mentioned above, any other type of evaporator known to those skilled in the art that is suitable for use in a rectification column may also be used.

[0075] In a preferred embodiment of the process according to the invention, the reaction column RR A At the lower end of A Glycolate and S AP is removed as a bottom product stream.

[0076] Reactor RR A At least one bottom evaporator V SA and a bottom product stream S AP is partially passed through and glycol is partially removed therefrom, thereby AP Compared to M A Bottom product stream S with an increased mass fraction of glycolate AP* It is preferred that

[0077] In particular, in the method according to the invention, S AP or at least one bottom evaporator V SA is used, which is supplemented with a bottom product stream S APis at least partially passed through and glycol is at least partially removed therefrom, AP* is S AP % by weight, preferably 5 to 35% by weight, more preferably 15 to 35% by weight, and most preferably 20 to 35% by weight, based on the total weight of A It has a weight proportion of glycolate.

[0078] Here, S AP Or S AP* The mass fraction of residual water in the AP <1% by weight, preferably <0.8% by weight, more preferably <0.5% by weight, based on the total weight of the

[0079] Here, S AP Or S AP* Reactant M in A The mass proportion of OH is preferably S AP <1% by weight, preferably <0.8% by weight, more preferably <0.5% by weight, based on the total weight of the

[0080] In an even more preferred embodiment of the process according to the invention, a vapour stream S comprising water and optionally glycol is AB But, RR A It is taken out at the top of the

[0081] 2. Rectification tower R DA Steam flow in S AB Rectification of (preferred) Steam flow S AB When S comprises water and glycol, in a further preferred embodiment, AB is the rectification tower RD A sent to RD A In R.D. A At least one steam stream S containing water is withdrawn at the top end of OA and R.D. A At least one glycol-containing stream S is withdrawn at the lower end of UA and are separated into

[0082] "RD A At least one steam stream S containing water is withdrawn at the top end of OA " means RD A This means that the steam obtained at the top end of the boiler can be removed there as one or more steam streams.

[0083] "RD A At least one glycol-containing stream S is withdrawn at the lower end of UA " means RD A This means that the glycol obtained at the lower end of the column can be removed there as one or more streams.

[0084] In this case, the steam flow S AB through one or more feed points to a rectification column RD A The steam flow S AB are separated from each other as two or more streams in the rectification column R DA In the embodiment of the present invention in which the individual streams are fed to the rectification column RD A Advantageously, the electrodes are at substantially the same height.

[0085] In a preferred embodiment of the method according to the invention, the vapor stream S AB If contains water and glycol, S AB is the rectification tower RD A In R.D. A A steam flow containing water is taken out at the top of OA and R.D. A A glycol-containing stream S is taken off at the bottom of the UA and are separated into

[0086] Another term for the "top of the rectification column" is the "top."

[0087] Alternative terms for the "lower end of the rectification column" are the "bottoms" or "foot."

[0088] Rectification tower RD A As such, any rectification column known to those skilled in the art can be used.

[0089] Preferably, the rectification column RD A includes internals. Suitable internals are, for example, trays, random packings or structured packings. As trays, bubble cap trays, sieve trays, valve trays, tunnel cap trays or slit trays are usually used. The random packings are generally random packings. As random packings, Raschig rings, Pall rings, Burl saddles or Intalox® saddles are usually used. Structured packings are, for example, sold by Sulzer under the trade name Mellapack®. In addition to the mentioned internals, other suitable internals are known to the person skilled in the art and can be used as well.

[0090] Preferred internals have low specific pressure drop per theoretical stage. For example, structured and random packings have significantly lower pressure drop per theoretical stage than trays. These include rectification column RDs. A The advantages of this approach are that the pressure drop in the compressor is kept as low as possible, the mechanical performance of the compressor, and the temperature of the evaporating glycol / water mixture are kept low.

[0091] Rectification tower RD A If packings S2 contain structured or random packings, they may be split or may be one continuous packing. Usually, however, at least two packings are provided and the vapor flow S2 is AB One packing above the feed point of the steam flow S AB One packing is provided below the feed point of the steam flow S AB and provide one packing above the feed point of the steam flow S AB A plurality of trays may also be provided below the feed point of the packing. When random packing, e.g., random packing, is used, the random packing is typically resting on a suitable support grid (e.g., sieve trays or mesh trays).

[0092] In this preferred embodiment, at least one steam flow S containing water is OA Then, the rectification column RDA This vapor flow S OA The preferred mass proportion of water therein is ≧96.0% by weight, more preferably ≧99.6% by weight, even more preferably ≧99.9% by weight, the remainder being, in particular, glycol.

[0093] RD A At the lower end of the at least one glycol-containing stream S UA is removed, which may preferably contain <1 wt.-%, more preferably ≦5000 ppm, even more preferably ≦1000 ppm, more preferably ≦100 ppm, by weight of water.

[0094] Rectification tower RD A At least one steam flow S containing water at the top of OA In the context of the present invention, the term "removing" refers in particular to removing at least one steam flow S OA However, the rectification tower RD A In the instant embodiment, this means that the gas is removed as a top stream or as a side draw above the internals.

[0095] Rectification tower RD A At the bottom of the at least one glycol-containing stream S UA In the context of the present invention, the term "removing" refers in particular to removing at least one flow S UA as the bottom stream or from the rectification column RD A This means that the liquid is removed at the lower bottom of the container.

[0096] Rectification tower RD A is operated with or without reflux, preferably with reflux.

[0097] "With reflux" means that the rectification column RD A The steam flow S taken off at the top of OA However, instead of being completely discharged, it is partially condensed and discharged to each rectification column RD AWhen such a reflux is set up, the reflux ratio is preferably 0.01 to 1, more preferably 0.02 to 0.9, even more preferably 0.03 to 0.34, particularly preferably 0.04 to 0.27, very particularly preferably 0.05 to 0.24, and most preferably 0.2.

[0098] Reflux is the rectification tower RD A A cooler K is placed on top of the RD This can be set by installing the cooler K RD Now, for each steam flow S OA is partially condensed and A is supplied again to

[0099] 3. Step (b): PET and solution S AP Reaction with In step (b) of the process according to the invention, the glycol obtained in step (a) is reacted with M A Solution S containing glycolate AP is reacted with PET to produce a mixture M1 containing BHET.

[0100] 3.1 PET starting material As PET used in step (b) of the process according to the invention, any PET that needs to be depolymerized can be used. Typically, such PET arises as waste, in particular in households, industries or agriculture.

[0101] In one embodiment of the process according to the invention, the PET to be depolymerized according to the invention is mixed with other plastics, in particular at least one plastic selected from polyethylene ("PE"), polyvinyl chloride ("PVC"). This is typical when PET is depolymerized from plastic waste in the process according to the invention. In this embodiment, the PET is at least partially separated from other plastics, preferably by sorting, before being subjected to step (b) of the process according to the invention.

[0102] In one embodiment of the method according to the invention, the PET is subjected to at least one pretreatment step.

[0103] Such a pretreatment step is described, for example, in DE 100 32 899 C1.

[0104] According to the invention, the PET is subjected to at least one pretreatment step selected from a chemical pretreatment step, a shredding step, before being used in step (b).

[0105] If the PET is mixed with other plastics, the PET is preferably subjected to at least one pre-treatment step selected from at least partial separation from other plastics, preferably by sorting, a chemical pre-treatment step, a shredding step, before being used in step (b).

[0106] If the PET is mixed with other plastics, the PET is more preferably first at least partially separated from the other plastics, then chemically pre-treated at least once, and finally shredded.

[0107] The chemical pretreatment step is in particular a washing step. Such a washing step has the advantage that before step (b) is carried out, impurities possibly present are removed, in particular food residues, cosmetic residues and / or bodily secretions (e.g. blood, semen, feces). Such impurities may reduce the efficiency of the reaction in step (b) and / or impair the purity of the BHET obtained.

[0108] In the chemical pretreatment step, especially in the washing step, the waste is heated, especially in the washing liquid, at a temperature between 30°C and 99°C, preferably between 50°C and 90°C, even more preferably between 70°C and 85°C.

[0109] Typical washing solutions are well known to those skilled in the art and are preferably selected from the following: - an aqueous solution of a surfactant, preferably a non-ionic surfactant; - An aqueous solution of an alkali metal hydroxide or an alkaline earth metal hydroxide, preferably an aqueous NaOH solution.

[0110] The treatment time of the chemical pretreatment step, in particular the washing step, is in this case in particular 1 minute to 12 hours, preferably 10 minutes to 6 hours, more preferably 30 minutes to 2 hours, even more preferably 45 minutes to 90 minutes, most preferably 60 minutes.

[0111] After treatment of the PET by the chemical pretreatment steps, in particular the washing steps, the aqueous solution is separated, for example by filtration, and the washed PET is preferably washed at least once with water to remove residues of the washing liquid.

[0112] The PET waste thus obtained is then dried, in particular in a drying cabinet, the temperature used for drying being in this case in particular in the range from 30 to 120°C, preferably from 50 to 100°C, more preferably from 60 to 90°C, most preferably 80°C.

[0113] The shredding step has the advantage that the surface area of ​​the PET available for reaction in step (b) is increased, thereby increasing the reaction rate of the reaction in step (b). Shredding can be carried out in equipment known to those skilled in the art, such as a shredder or cutting mill.

[0114] In a further embodiment of the process according to the invention, the PET is bleached or targeted colored before being subjected to step (b), which can be carried out using methods known to the skilled artisan, for example bleaching with hydrogen peroxide or coloring with dyes.

[0115] 3.2 Reaction conditions PET, glycol and M A Solution S containing glycolate AP The formation of mixture M1 by reaction with can then be carried out under conditions well known to those skilled in the art.

[0116] Preferably, the reaction of step (b) is carried out at a time tb and for a period of time until at least P=10%, preferably at least P=20%, more preferably at least P=25%, more preferably at least P=30%, more preferably at least P=40%, more preferably at least P=50%, more preferably at least P=60%, more preferably at least P=70%, more preferably at least P=80%, more preferably at least P=90%, more preferably at least P=95%, and even more preferably at least P=99% of the PET used in step (b) has reacted, at a time t b This will be carried out until

[0117] This percentage P is calculated according to the following formula: P = (n TS +n MHET +n BHET ) / n PET where n PET is the amount of repeating units of the following structure (≡) in the PET used in step (b): [ka]

[0118] n TS is the time from the start of step (b) to time t b is the amount of TS formed up to that point.

[0119] n MHET is the time from the start of step (b) to time t b is the amount of MHET substance formed up to that point.

[0120] n BHET is the time from the start of step (b) to time t b is the amount of BHET material formed up to that point.

[0121] The structures of the compounds BHET, MHET, and TS are as follows: [ka]

[0122] "MHET" also encompasses the corresponding carboxylates of the structures shown.

[0123] "TS" also encompasses the corresponding mono- and dicarboxylates of the structures shown.

[0124] The reaction of step (b) is in this case especially 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.

[0125] The reaction of step (b) is preferably carried out at the boiling temperature of the glycol. Even more preferably, the glycol is refluxed in this case, i.e. the glycol is evaporated from the reaction, condensed and then returned to the reaction again. This reflux can be set up using means well known to those skilled in the art, for example in a distillation apparatus.

[0126] The ratio of the total weight of PET used in the method to the mass of A The total weight of glycolate is in particular 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, particularly preferably in the range of 3.5 to 5.0% by weight, and most preferably 3.9% by weight.

[0127] The reaction of step (b) can be carried out using equipment well known to those skilled in the art.

[0128] After completion of step (b) of the method according to the invention, the amount of substance of BHET (n BHET ) and the sum of the amount of substance of MHET and the amount of substance of TS (n MHET +n TS) is in the range 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, even more preferably 13:1 to 60:1, and even more preferably 13:1 to 24:1. η=n BHET / (n MHET +n TS )

[0129] 3.3 Preferred step (c) In a preferred further step (c), BHET is at least partially separated from M1. This is still more preferably carried out by crystallization and / or distillation. Still more preferably, BHET is filtered off from M1 in step (c) and then crystallized.

[0130] 4. How to recycle PET The BHET obtained in the mixture M1 in the process according to the invention is preferably polymerized in step (ζ) in a process for the reproduction of polyethylene terephthalate to produce PET.

[0131] This polymerization is known to those skilled in the art as "polycondensation" and is described, for example, in EP 0 723 951 A1 and in the book "Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters. Edited by J. Scheirs and TE Long, 2003, John Wiley & Sons, Ltd ISBN: 0-471-49856-4" by Th. Rieckmann and S. Voelker, Chapter 2 "Poly(Ethylen Terephthalate) Polymerization - Mechanism, Catalysis, Kinetics, Mass Transfer and Reactor Design", page 92.

[0132] In particular, for this purpose, BHET is polymerized in step (ζ) in the presence of a catalyst to again give PET, this catalyst being in particular a catalyst selected from the group consisting of antimony compounds, preferably Sb2O3.

[0133] Preferably, the polymerization of BHET to PET in step (ζ) is carried out at at least the boiling temperature of the glycol, in particular, glycol is removed from the reaction mixture during the polymerization in step (ζ) to shift the reaction equilibrium towards the polymer PET.

[0134] More preferably, the polymerization of BHET to PET in step (ζ) is carried out at the boiling temperature of the glycol. Even more preferably, glycol is subsequently removed from the reaction mixture during the polymerization in step (ζ) to shift the reaction equilibrium towards the polymer PET.

[0135] This is in particular achieved by distillation at a pressure <1 bar, preferably 0.1 mbar, at the simultaneous boiling temperatures of the glycols at the respective pressures.

[0136] Working Example 1. Example E1 according to the invention: 1.1 Preparation of glycol solution of sodium glycolate by reactive distillation The following distillation equipment was used: A heatable 2.5 liter double jacketed vessel equipped with a temperature sensor and a vacuum sealed stirrer served as the receiver or bottom of the distillation apparatus. Above this there was a 25 cm column equipped with Multifill packing and a silver mirror (stripping section). The NaOH feed was carried out at the top of the column by a dropping funnel. Above the feed section there was a further column for separating ethylene glycol and water vapor (rectifying section). The reflux ratio could be set in the vapor separator at the top of the column and the distillate was collected in a round bottom flask. The round bottom flask could be separated and replaced from the distillation system via a pressure equalizing dropping funnel. A vacuum-connected reflux condenser was connected to the rectifying section, which allowed the whole apparatus to be evacuated. The vacuum was generated by a rotary vane pump, which was connected to the distillation apparatus by two cold traps and a safety bottle. The pressure in the distillation apparatus was measured by a safety flask (Buechi vacuum controller) and aeration could also be carried out here. To ensure a constant temperature in the reactor / tower, the bottom receiver and the tower equipped with the Multifill packing were completely covered with aluminum foil for thermal insulation.

[0137] Ethylene glycol was charged at the bottom and the whole apparatus was evacuated to 50 mbar. The bottom was then heated to boiling temperature so that reflux from the condensing section occurred. Then caustic soda solution (50% in water) was fed in through a dropping funnel. The feed rate was chosen so that the caustic soda solution did not reach the bottom (approximately 2 mL / min).

[0138] The water added or produced was separated from the ethylene glycol by distillation in the retentate and collected in a round-bottom flask. The reflux ratio was 5:1 (5 parts reflux, 1 part distillate). The amount of distillation had to be at least equal to the amount of water added. After distillation, the sodium ethylene glycolate was redistilled at the bottom for another approx. 2 hours. At the same constant vacuum and temperature, the water present in the retentate was removed to prevent backflow to the bottom.

[0139] After the experiment was completed and the flask was cooled, the bottom was opened with a drain valve, and an ethylene glycol solution of about 20% by weight of sodium glycolate was taken out.

[0140] 1.2 Depolymerization of PET using glycol solution of sodium glycolate obtained by reactive distillation In the process according to the invention, 100 g of PET were charged into an autoclave together with 800 g of ethylene glycol. The solution was then heated to 150° C. with stirring. As soon as a temperature of 150° C. was reached, 19.5 g (corresponding to 0.046 mol) of a 20% by weight solution of sodium glycolate in ethylene glycol obtained by reactive distillation were added. The reaction was carried out for 5 hours and the reactor discharge was examined after cooling. The conversions obtained were determined by gas chromatography. The conversions of BHET (1) and 2-hydroxyethyl terephthalic acid ("MHET") (2) and terephthalic acid ("TS") (3) are shown in the figure (hatched bars from upper left to lower right in % relative to the repeat unit (≡) of the PET used: "\\\\\\\").

[0141] 2. Comparative Example V1: In a comparative test, 100 g of PET was charged into the autoclave together with 800 g of ethylene glycol. The solution was then heated to 150 °C with stirring. The reaction was carried out for 5 h and the reactor effluent was examined after cooling. The resulting conversions of BHET (1) and MHET (2) as well as TS (3) are shown in the figure (black "□").

[0142] 3. Comparative Example V2: In a comparative test, 100 g of PET was charged into the autoclave together with 800 g of ethylene glycol. The solution was then heated to 150 °C under stirring. As soon as a temperature of 150 °C was reached, 3.7 g of 50 wt. % aqueous NaOH solution (corresponding to 0.046 mol) was added. The reaction was carried out for 5 h and the reactor effluent was examined after cooling. The obtained conversions of BHET (1) and MHET (2) as well as TS (3) are shown in the figure (hatched bars from upper right to lower left: " / / / / ").

[0143] 4.Results Comparing the contents of BHET, MHET and TS in the depolymerization products of Example E1 according to the invention and Comparative Examples V1 and V2, it can be seen that the depolymerization with the glycol solution of sodium glycolate obtained by reactive distillation gives a higher percentage of BHET, which is advantageous since it means that more products are obtained that can be directly converted into new PET products by polycondensation.

Claims

1. below: (a) M Ａ OH and glycol are reacted by reactive distillation to produce glycol and M Ａ Glycolate and solution S ＡＰ wherein M Ａ is an alkali metal selected from sodium and potassium; (b) the solution S ＡＰ is reacted with PET to obtain a mixture M containing bis-2-hydroxyethyl terephthalate BHET. １ and generating A method for depolymerizing polyethylene terephthalate (PET), comprising:

2. S ＡＰ is a reactant stream S containing glycol in the step (a). ＡＥ１ and M Ａ OH-containing reactant stream S ＡＥ２ and the reactive rectification column RR Ａ The reaction is carried out in a countercurrent manner in M Ａ Glycolate, Water, Glycol, M Ａ Crude RP containing OH Ａ is obtained by generating S ＡＰ RR Ａ 2. The process of claim 1, wherein the bottom product stream is removed at the lower end of

3. a vapor stream S comprising water and optionally glycol ＡＢ RR Ａ 3. The method of claim 2, wherein the mixture is removed at the top end of the container.

4. S ＡＢ contains water and glycol, and S ＡＢ Rectification column RD Ａ Send to RD Ａ In RD Ａ At least one steam stream S containing water is withdrawn at the top end of ＯＡ and RD Ａ at least one glycol-containing stream S ＵＡ 4. The method of claim 3, wherein the separation is performed as follows:

5. 3. The process of claim 1 or 2, wherein step (b) is carried out for a time until at least P=10% of the PET used in step (b) has reacted.

6. S ＡＰ 3. The method of claim 1, wherein the water content is <1% by weight.

7. 3. The process of claim 1 or 2, wherein step (b) is carried out at the boiling temperature of the glycol.

8. The weight of M used in step (b) relative to the total weight of PET used in step (b) Ａ S in an amount such that the total weight of glycolate is in the range of 0.1 to 100% by weight ＡＰ 3. The method according to claim 1 or 2, wherein in step (b)

9. In a further step (c), BHET is reacted with M １ 3. The method of claim 1, wherein the at least partially separates the

10. M in step (c) １ 10. The process of claim 9, wherein at least partial separation of BHET from the ester is carried out by crystallization and / or distillation.

11. 3. The method of claim 1 or 2, wherein the PET is subjected to at least one pretreatment step selected from a chemical pretreatment step, a shredding step, before being used in step (b).

12. A method for recycling polyethylene terephthalate (PET), comprising obtaining BHET by the method of claim 1 and polymerizing the BHET thus obtained in step (ζ) to produce PET.

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

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

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

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