IMPROVED PROCESS FOR DEPOLYMERISING POLYETHYLENE TEREPHTHALATE

Abstract

The present invention relates to a process for depolymerising polyethylene terephthalate (PET), in which method PET is reacted with sodium glycolate or potassium glycolate, which has been obtained by reactive distillation, to form a mixture M.sub.1 including Bis(2-hydroxyethyl) terephthalate (BHET). The process according to the invention is distinguished by the fact that BHET makes up a particularly high proportion of the decomposition products in the mixture M.sub.1. The process according to the invention thus provides a high yield of BHET that can be used directly for producing PET again. The present invention thus also relates to a process for recycling PET, in which the BHET obtained in the process for depolymerising PET is polymerised again to form PET, optionally after being further purified from M.sub.1.

Claims

1-15. (canceled)

16. A method for depolymerizing polyethylene terephthalate (PET), comprising the following steps: (a) converting M.sub.AOH and glycol in a reactive distillation to obtain a solution S.sub.AP comprising glycol and M.sub.A glycolate, where M.sub.A is sodium or potassium; (b) reacting the solution S.sub.AP with PET to give a mixture M.sub.1 comprising bis-2-hydroxyethyl terephthalate (BHET).

17. The method of claim 16, wherein S.sub.AP is obtained in step (a) by reacting a reactant stream S.sub.AE1 comprising glycol with a reactant stream S.sub.AE2 comprising M.sub.AOH in countercurrent, in a reactive rectification column RR.sub.A, to give a crude product RP.sub.A comprising M.sub.A glycolate, water, glycol, and M.sub.AOH; wherein S.sub.AP is withdrawn as a bottom product stream at the lower end of RR.sub.A.

18. The method of claim 17, wherein a vapour stream S.sub.AB comprising water, with or without glycol, is withdrawn at the upper end of RR.sub.A.

19. The method of claim 18, wherein S.sub.AB comprises water and glycol, and wherein S.sub.AB is directed into a rectification column RD.sub.A where it is separated into at least one vapour stream S.sub.OA, comprising water which is withdrawn at the upper end of RD.sub.A, and at least one stream S.sub.UA, comprising glycol which is withdrawn at the lower end of RD.sub.A.

20. The method of claim 16, wherein the reaction of step (b) is conducted until at least P=10% of the PET used in step (b) has been converted.

21. The method of claim 16, wherein the content of water in S.sub.AP is <1% by weight.

22. The method of claim 16, wherein step (b) is performed at the boiling temperature of the glycol.

23. The method of claim 16, wherein a sufficient amount of S.sub.AP is used in step (b) so that the total weight of the M.sub.A glycolate used in step (b), based on the total weight of the PET used in step (b), is in the range of from 0.1% to 100% by weight.

24. The method of claim 16, wherein BHET is at least partly separated from M.sub.1 in a further step (c).

25. The method of claim 24, wherein the at least partial separation of BHET from M.sub.1 in step (c) is effected by crystallization and/or distillation.

26. The method of claim 16, wherein the PET is subjected to at least one pretreatment step selected from a chemical pretreatment step, or a comminution step, before being used in step (b).

27. The method of claim 19, wherein step (b) is conducted until at least P=10% of the PET used in step (b) has been converted.

28. The method of claim 27, wherein the content of water in S.sub.AP is <1% by weight.

29. The method of claim 28, wherein step (b) is performed at the boiling temperature of the glycol.

30. The method of claim 29, wherein a sufficient amount of S.sub.AP is used in step (b) so that the total weight of the M.sub.A glycolate used in step (b), based on the total weight of the PET used in step (b), is in the range from 0.1% to 100% by weight.

31. The method of claim 30, wherein BHET is at least partly separated from M.sub.1 in a further step (c).

32. A method of recycling polyethylene terephthalate PET, in which BHET is obtained by the method of claim 16 and the BHET thus obtained is polymerized to PET in a step ().

33. The method of claim 32, wherein the polymerization of BHET to PET in step () is conducted at least at the boiling temperature of the glycol.

34. The method of claim 33, wherein the polymerization in step () is performed in the presence of a catalyst.

35. The method of claim 34, wherein the catalyst is an antimony compound.

Description

FIGURE

[0022] The FIGURE shows the comparison of the content of BHET (1), 2-hydroxyethylterephthalic acid (MHET; 2) and terephthalic acid (TS; 3) in depolymerization with sodium glycolate obtained by the process according to the invention and sodium glycolate obtained by conventional processes.

[0023] The bar with the hatching \\\\\ shows the respective content of BHET, MHET and TS in the reactor output in the depolymerization of PET according to inventive example E1, in which the sodium glycolate used for the depolymerization was obtained by reactive distillation.

[0024] The black bars show the respective content of BHET, MHET and TS in the reactor output in the depolymerization of PET according to comparative example V1, in which merely glycol was used in the depolymerization.

[0025] The bar with the hatching ///// shows the respective content of BHET, MHET and TS in the reactor output 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.

DETAILED DESCRIPTION OF THE INVENTION

[0026] It has now been found that, surprisingly, the glycolysis of PET proceeds particularly efficiently when sodium glycolate or potassium glycolate that has been obtained by reactive distillation is used. In the reactive distillation according to the invention, the glycolate is obtained by reaction of the corresponding alkali metal hydroxide M.sub.AOH with glycol. It has now been observed that, in the method according to the invention, by comparison with the prior art methods in which glycolate that has been obtained by dissolution of the alkali metal hydroxides in glycol is used, a higher proportion of BHET is obtained in the cleavage product.

1. Step (a): Reactive Distillation to Obtain the Solution S.SUB.AP .Comprising Glycol and M.SUB.A .Glycolate

[0027] According to the invention, the solution S.sub.AP comprising glycol and M.sub.A glycolate which is used in the method according to the invention is obtained by means of reactive distillation, by conversion of M.sub.AOH and glycol.

[0028] M.sub.A is an alkali metal selected from sodium, potassium. M.sub.A is preferably sodium.

[0029] Reactive distillation for preparation of alkali metal alkoxides is an important industrial process since alkali metal alkoxides are used as strong bases in the synthesis of numerous chemicals, for example in the production of active pharmaceutical or agrochemical ingredients, and as catalysts in transesterification and amidation reactions.

[0030] Alkali metal alkoxides (MOR) are prepared by means of reactive distillation, typically in a countercurrent distillation column, from alkali metal hydroxides (MOH) and alcohols (ROH), with removal of the water of reaction formed according to the following reaction <1> together with the distillate:

MOH+ROHMOR+H.sub.2O.

[0031] Such a method principle is described, for example, in U.S. Pat. No. 2,877,274 A, wherein aqueous alkali metal hydroxide solution and gaseous methanol are conducted in countercurrent in a rectification column. This method is described again in basically unchanged form in WO 01/42178 A1.

[0032] The most industrially important alkali metal alkoxides are those of sodium and potassium, especially the methoxides and ethoxides. There are many descriptions of the synthesis thereof in the prior art, for example in EP 1 997 794 A1, WO 2021/148174 A1 and WO 2021/148175 A1.

[0033] Methods that are similar, but in which an introducing agent, for example benzene, is additionally used, are described in GB 377,631 A and U.S. Pat. No. 1,910,331 A.

[0034] Correspondingly, DE 96 89 03 C describes a method of continuous preparation of alkali metal alkoxides in a reaction column, wherein the water-alcohol mixture withdrawn at the top of the column is condensed and then subjected to a phase separation. The aqueous phase is discarded and the alcoholic phase is returned to the top of the column together with the fresh alcohol. EP 0 299 577 A2 describes a similar method, wherein the water in the condensate is separated off with the aid of a membrane.

[0035] In a preferred embodiment of the method according to the invention, S.sub.AP is obtained in step (a) by reacting a reactant stream S.sub.AE1 comprising glycol with a reactant stream S.sub.AE2 comprising M.sub.AOH in countercurrent in a reactive rectification column RR.sub.A to give a crude product RP.sub.A comprising M.sub.A glycolate, water, glycol, M.sub.AOH, wherein S.sub.AP is withdrawn as bottom product stream at the lower end of RR.sub.A.

[0036] Even more preferably, a vapour stream S.sub.AB comprising water, with or without glycol, is withdrawn at the upper end of RR.sub.A.

[0037] According to the invention, a reactive rectification column is defined as a rectification column in which the reaction according to step (a) of the method according to the invention proceeds at least in some parts. It may also be referred to as reaction column for short.

[0038] In the preferred embodiment of the method according to the invention, a bottom product stream S.sub.AP comprising glycol and M.sub.A glycolate is withdrawn at the lower end of RR.sub.A. A vapour stream S.sub.AB comprising water, with or without glycol, is withdrawn at the upper end of RR.sub.A.

[0039] Glycol in the context of the invention is understood to mean ethylene-1,2-diol having the chemical formula HOCH.sub.2CH.sub.2OH (CAS No. 107-21-1).

[0040] M.sub.A glycolate in the context of the invention is understood to mean the salt of the glycol with M.sub.A. The term M.sub.A glycolate comprises at least one of M.sub.AOCH.sub.2CH.sub.2OH and M.sub.AOCH.sub.2CH.sub.2OM.sub.A, preferably at least M.sub.AOCH.sub.2CH.sub.2OH, most preferably M.sub.AOCH.sub.2CH.sub.2OH and M.sub.AOCH.sub.2CH.sub.2OM.sub.A.

[0041] M.sub.A is an alkali metal selected from sodium, potassium, and is preferably sodium.

[0042] The reactant stream S.sub.AE1 comprises glycol. In a preferred embodiment, the proportion by mass of glycol in S.sub.AE1 is 95% by weight, still more preferably 99.5% by weight, where S.sub.AE1 otherwise comprises especially water, diethylene glycol.

[0043] The glycol used as reactant stream S.sub.AE1 in the preferred embodiment of the method according to the invention may also be commercial glycol having a proportion by mass of glycol of more than 99.5% by weight and a proportion by mass of water of up to 0.03% by weight, up to 0.05% by weight, of diethylene glycol.

[0044] In one embodiment of the present invention, the reactant stream S.sub.AE1 is added in vaporous form to the reactive rectification column RR.sub.A.

[0045] In an alternative, preferred embodiment of the method according to the invention, glycol is initially charged in the bottom of the reactive rectification column RR.sub.A prior to step (a), and then heated to boiling in step (a), which produces a constant reactant stream S.sub.AE1 in the reactive rectification column RR.sub.A. If necessary, glycol is then replenished in the bottom of the reactive rectification column RR.sub.A during the performance of step (a).

[0046] The reactant stream S.sub.AE2 comprises M.sub.AOH. In a preferred embodiment, S.sub.AE2 comprises not only M.sub.AOH but also at least one further compound selected from water, glycol. Even more preferably, S.sub.AE2 comprises not only M.sub.AOH but also water, in which case S.sub.AE2 is an aqueous solution of M.sub.AOH.

[0047] When the reactant stream S.sub.AE2 comprises M.sub.AOH and water, the proportion by mass of M.sub.AOH based on the total weight of the aqueous solution that forms S.sub.AE2 is especially in the range from 10% to 75% by weight, preferably from 15% to 54% by weight, more preferably from 30% to 53% by weight and even more preferably from 40% to 52% by weight and most preferably 50% by weight.

[0048] Step (a) of the method according to the invention is preferably performed in a reactive rectification column (or reaction column) RR.sub.A.

[0049] The reaction column RR.sub.A preferably contains internals. Suitable internals are, for example, trays, structured packings or unstructured packings. When the reaction column RR.sub.A contains trays, suitable trays are bubble-cap trays, valve trays, tunnel-cap trays, Thormann trays, cross-slit bubble-cap trays or sieve trays. When the reaction column RR.sub.A contains trays, it is preferable to choose trays where not more than 5% by weight, more preferably less than 1% by weight, of the liquid trickles through the respective trays. The construction measures required to minimize trickle-through of the liquid are familiar to those skilled in the art. In the case of valve trays, particularly tightly closing valve designs are selected for example. Reducing the number of valves also makes it possible to increase the vapour velocity in the tray openings to twice the value typically 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.

[0050] When using structured or unstructured packings, structured packings are preferred in terms of uniform distribution of the liquid.

[0051] Step (a) of the method according to the invention may be carried out either continuously or batchwise. It is preferably effected continuously.

[0052] Reaction of a reactant stream S.sub.AE1 comprising glycol with a reactant stream S.sub.AE2 comprising M.sub.AOH in countercurrent in a reactive rectification column RR.sub.A is achieved in one embodiment of the invention, in particular, by virtue of the feed point for at least a portion of the reactant stream S.sub.AE1 comprising glycol being located in the reaction column RR.sub.A below the feed point for the reactant stream S.sub.AE2 comprising M.sub.AOH.

[0053] In this embodiment, the reaction column RR.sub.A preferably comprises at least 2, in particular 15 to 40, theoretical plates between the feed point for the reactant stream S.sub.AE1 and the feed point for the reactant stream S.sub.AE2.

[0054] The reaction column RR.sub.A may be operated as a pure stripping column. In that case, the reactant stream S.sub.AE1 comprising glycol is introduced in vaporous form in the lower region of the reaction column RR.sub.A.

[0055] Optionally, a portion of the reactant stream S.sub.AE1 comprising glycol is added in vaporous form below the feed point for the reactant stream S.sub.AE2 comprising alkali metal hydroxide solution M.sub.AOH, but nevertheless at the upper end or in the region of the upper end of the reaction column RR.sub.A. This makes it possible to reduce the dimensions of the lower region of the reaction column RR.sub.A. When a portion of the reactant stream S.sub.AE1 comprising glycol is added, especially in vaporous form, at the upper end or in the region of the upper end of the reaction column RR.sub.A, preferably only a portion of 10% to 70% by weight, preferably of 30% to 50% by weight, (based in each case on the total amount of glycol used) is fed in at the lower end of the reaction column RR.sub.A, and the remaining portion is added, in vaporous form in a single stream or divided into a plurality of substreams, preferably 1 to 10 theoretical plates, more preferably 1 to 3 theoretical plates, below the feed point for the reactant stream S.sub.AE2 comprising M.sub.AOH.

[0056] In an alternative embodiment of step (a) of the method according to the invention, reaction of a reactant stream S.sub.AE1 comprising glycol with a reactant stream S.sub.AE2 comprising M.sub.AOH in countercurrent in a reactive rectification column RR.sub.A is achieved, in particular, by virtue of glycol being present in the bottom of the reactive rectification column RR.sub.A and the feed point for the reactant stream S.sub.AE2 comprising M.sub.AOH being located above the bottom. During step (a) of the method according to the invention, glycol is then heated to boiling in the bottom of RR.sub.A and a reactant stream S.sub.AE1 comprising glycol is produced. S.sub.AE1 and S.sub.AE2 are then directed in countercurrent to one another.

[0057] In the reaction column RR.sub.A, the reactant stream S.sub.AE1 comprising glycol then reacts with the reactant stream S.sub.AE2 comprising M.sub.AOH according to the above-described reaction <1> (in which ROH is then glycol) to give M.sub.A glycolate and H2O, with these products being present in a mixture with the glycol and M.sub.AOH reactants since the reaction is an equilibrium reaction. Accordingly, step (a) affords a crude product RP.sub.A in the reaction column RR.sub.A that comprises not only the M.sub.A glycolate and water products but also glycol and M.sub.AOH.

[0058] At the lower end of RR.sub.A, the bottom product stream S.sub.AP comprising glycol and M.sub.A glycolate is then obtained and withdrawn.

[0059] At the upper end of RR.sub.A, preferably at the top of the column of RR.sub.A, in a preferred embodiment of the method according to the invention, a stream of water that may or may not still contain glycol, referred to above as vapour stream S.sub.AB comprising water, with or without glycol is withdrawn.

[0060] If the vapour stream S.sub.AB contains not only water but also glycol, glycol is obtained, preferably by distillation, for example in a rectification column. In this embodiment, at least a portion of the glycol obtained in the distillation can be fed back to the reaction column RR.sub.A as reactant stream S.sub.AE1.

[0061] In a preferred embodiment, S.sub.AB, when it comprises not only water but also glycol, is directed into a rectification column RD.sub.A and is separated in RD.sub.A into at least one vapour stream S.sub.OA comprising water which is withdrawn at the upper end of RD.sub.A, and at least one stream S.sub.UA comprising glycol which is withdrawn at the lower end of RD.sub.A.

[0062] The amount of glycol encompassed by the reactant stream S.sub.AE1 is preferably chosen such that said glycol simultaneously serves as a solvent for the M.sub.A glycolate obtained in the bottom product stream S.sub.AP. The amount of glycol in the reactant stream S.sub.AE1 is preferably chosen such that the desired concentration of the M.sub.A glycolate solution which is withdrawn as bottom product stream S.sub.AP comprising glycol and M.sub.A glycolate is present in the bottom of the reaction column.

[0063] In a preferred embodiment of the method according to the invention, and especially in the cases in which S.sub.AE2 comprises not only M.sub.AOH but also water, the ratio of the total weight (mass; unit: kg) of glycol used as reactant stream S.sub.AE1 to the total weight (mass; unit: kg) of M.sub.AOH used as reactant stream S.sub.AE2 is 1:1 to 50:1, more preferably 2:1 to 40:1, even more preferably 3:1 to 30:1, yet more preferably 5:1 to 10:1.

[0064] The reaction column RR.sub.A in the preferred embodiment of the method according to the invention is operated with or without, preferably with, reflux.

[0065] What is meant by with reflux is that the vapour stream S.sub.AB comprising water, with or without glycol, that is withdrawn at the upper end of the respective column, especially the reaction column RR.sub.A, is not removed completely. The relevant vapour stream S.sub.AB is thus fed at least partly, preferably partly, back to the respective column as reflux, especially to the reaction column RR.sub.A. In the cases where such a reflux is established, the reflux ratio is preferably 0.01 to 1, more preferably 0.02 to 0.9, yet more preferably 0.03 to 0.34, especially preferably 0.04 to 0.27 and very especially preferably 0.05 to 0.24, most preferably 0.2.

[0066] A reflux ratio is understood generally and in the context of this invention to mean the ratio of the proportion of the mass flow rate withdrawn from the column (kg/h) which is removed from the respective column in liquid form or gaseous form to the proportion of this mass flow rate (kg/h) which is returned back to the column in liquid form (reflux).

[0067] A reflux can be established by mounting a condenser at the top of the respective column. For this purpose, in particular, a condenser K.sub.RRA is mounted on the reaction column RR.sub.A. In the condenser K.sub.RRA, the vapour stream S.sub.AB is at least partly condensed and fed back to the respective column, especially to the reaction column RR.sub.A.

[0068] In the embodiment in which a reflux is established in the reaction column RR.sub.A, the M.sub.AOH used as reactant stream S.sub.AE2 in the preferred embodiment of the method according to the invention may also be at least partly mixed with the reflux stream, and the resulting mixture may thus be supplied to the reaction column RR.sub.A.

[0069] In a preferred embodiment of the method according to the invention, step (a) is especially conducted under distillative conditions under which glycol is refluxed.

[0070] Step (a) is performed especially at a temperature in the range from 80 C. to 197 C., preferably 100 C. to 197 C., more preferably 120 C. to 140 C., and at a pressure of 0.01 bar abs. to 1 bar abs., preferably in the range from 0.05 bar abs. to 1 bar abs., more preferably in the range from 0.05 bar abs. to 0.15 bar abs., more preferably in the range from 0.05 bar abs. to 0.10 bar abs.

[0071] In a more preferred embodiment, the reaction column RR.sub.A comprises at least one evaporator which is especially selected from intermediate evaporators V.sub.ZA and bottom evaporators V.sub.SA. The reaction column RR.sub.A more preferably comprises at least one bottom evaporator V.sub.SA.

[0072] According to the invention, intermediate evaporators V.sub.Z refer to evaporators above the bottom of the respective column, especially above the bottom of the reaction column RR.sub.A (in which case they are referred to as V.sub.ZA) or of the rectification column RD.sub.A which is used in the preferred embodiment and is described in detail further down (in which case they are referred to as V.sub.ZRD ). In the case of RR.sub.A, said evaporators especially evaporate crude product RP.sub.A which is withdrawn from the column as side stream S.sub.ZAA.

[0073] According to the invention, bottom evaporators V.sub.S refer to evaporators that heat the bottom of the respective column, especially the bottom of the reaction column RR.sub.A or the bottom of the rectification column RD.sub.A which is used in the preferred embodiment and is described in detail further down (in which case they are referred to as V.sub.SRD or V.sub.SRD.). In the case of RR.sub.A, said evaporators especially evaporate at least a portion of the bottom product stream S.sub.AP. In the case of RD.sub.A, said evaporators especially evaporate bottom product stream S.sub.UA or a portion of S.sub.UA, S.sub.UA1.

[0074] An evaporator is typically arranged outside the respective reaction column or rectification column.

[0075] Suitable evaporators employable as intermediate evaporators and bottom evaporators include for example natural circulation evaporators, forced circulation evaporators, forced circulation flash evaporators, kettle evaporators, falling-film evaporators or thin-film evaporators. Heat exchangers for the evaporator typically employed in the case of natural circulation evaporators and forced circulation evaporators are shell and tube or plate apparatuses. When using a shell and tube exchanger the heat carrier may flow through the tubes with the mixture to be evaporated flowing around the tubes or else the heat carrier may flow around the tubes with the mixture to be evaporated flowing through the tubes. In the case of a falling-film evaporator, the mixture to be evaporated is typically introduced as a thin film on the inside of a tube and the tube is heated externally. In contrast to a falling-film evaporator, a thin-film evaporator additionally comprises a rotor with wipers which distributes the liquid to be evaporated on the inner wall of the tube to form a thin film.

[0076] In addition to the recited evaporator types it is also possible to employ any desired further evaporator type known to those skilled in the art and suitable for use on a rectification column.

[0077] In the preferred embodiment of the method according to the invention, S.sub.AP comprising glycol and M.sub.A glycolate is withdrawn as bottom product stream at the lower end of the reaction column RR.sub.A.

[0078] It is preferable that the reaction column RR.sub.A comprises at least one bottom evaporator V.sub.SA through which some of the bottom product stream S.sub.AP is then passed and glycol is partly removed therefrom, which affords a bottom product stream S.sub.AP* having an elevated proportion by mass of M.sub.A glycolate compared to S.sub.AP.

[0079] In particular, in the method according to the invention, S.sub.AP, or S.sub.AP* if at least one bottom evaporator V.sub.SA through which at least some of the bottom product stream S.sub.AP is passed is used and glycol is removed at least partly therefrom, has a proportion by mass of M.sub.A glycolate in glycol in the range from 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, in each case based on the total mass of S.sub.AP.

[0080] The proportion by mass of residual water in S.sub.AP/S.sub.AP* is preferably <1% by weight, preferably <0.8% by weight, more preferably <0.5% by weight, based on the total mass of S.sub.AP.

[0081] The proportion by mass of reactant M.sub.AOH in S.sub.AP/S.sub.AP* is preferably <1% by weight, preferably <0.8% by weight, more preferably <0.5% by weight, based on the total mass of S.sub.AP.

[0082] In an even more preferred embodiment of the method according to the invention, a vapour stream S.sub.AB comprising water, with or without glycol, is withdrawn at the upper end of RR.sub.A.

2. Rectification of the Vapour Stream S.SUB.AB .in a Rectification Column RD.SUB.A .(Preferred)

[0083] In a further preferred embodiment, the vapour stream S.sub.AB, when it comprises water and glycol, is directed into a rectification column RD.sub.A and is separated in RD.sub.A into at least one vapour stream S.sub.OA comprising water which is withdrawn at the upper end of RD.sub.A, and at least one stream S.sub.UA comprising glycol which is withdrawn at the lower end of RD.sub.A.

[0084] At least one vapour stream S.sub.OA comprising water which is withdrawn at the upper end of RD.sub.A shall be understood to mean that the vapour obtained at the upper end of RD.sub.A may be withdrawn there as one or more vapour streams.

[0085] At least one stream S.sub.UA comprising glycol which is withdrawn at the lower end of RD.sub.A shall be understood to mean that glycol obtained at the lower end of RD.sub.A may be withdrawn there as one or more streams.

[0086] The vapour stream S.sub.AB may be directed into the rectification column RD.sub.A via one or more feed points. In the embodiments of the present invention in which the vapour stream S.sub.AB is directed into the rectification column RD.sub.A as two or more separate streams, it is advantageous when the feed points for the individual streams are at substantially the same height on the rectification column RD.sub.A.

[0087] In a preferred embodiment of the method according to the invention, the vapour stream S.sub.AB, when it comprises water and glycol, is separated in a rectification column RD.sub.A into a vapour stream S.sub.OA comprising water which is withdrawn at the upper end of RD.sub.A and a stream S.sub.UA comprising glycol which is withdrawn at the lower end of RD.sub.A.

[0088] Another term for upper end of a rectification column is head.

[0089] Another term for lower end of a rectification column is bottom or foot.

[0090] The rectification column RD.sub.A used may be any rectification column known to those skilled in the art.

[0091] The rectification column RD.sub.A preferably contains internals. Suitable internals are, for example, trays, unstructured packings or structured packings. Trays used are typically bubble-cap trays, sieve trays, valve trays, tunnel-cap trays or slotted trays. Unstructured packings are generally beds of random packing elements. Random packing elements used are typically Raschig rings, Pall rings, Berl saddles or Intalox saddles. Structured packings are for example sold under the Sulzer Mellapack trade name. Apart from the internals mentioned, further suitable internals are known to a person skilled in the art and can likewise be used.

[0092] Preferred internals have a low specific pressure drop per theoretical plate. Structured packings and random packing elements have, for example, a significantly lower pressure drop per theoretical plate than trays. This has the advantage that the pressure drop in the rectification column RD.sub.A remains as low as possible and the mechanical power of the compressor and the temperature of the glycol/water mixture to be evaporated therefore remain low.

[0093] When the rectification column RD.sub.A contains structured packings or unstructured packings, these may be divided or in the form of an uninterrupted packing. Typically, however, at least two packings are provided, one packing above the feed point for the vapour stream S.sub.AB and one packing below the feed point for the vapour stream S.sub.AB. It is also possible to provide one packing above the feed point for the vapour stream S.sub.AB and two or more trays below the feed point for the vapour stream S.sub.AB. If an unstructured packing is used, for example a random packing, the random packing elements are typically disposed on a suitable support grid (for example sieve tray or mesh tray).

[0094] In this preferred embodiment, the at least one vapour stream S.sub.OA comprising water is then withdrawn at the upper end of the rectification column RD.sub.A. The preferred mass fraction of water in this vapour stream S.sub.OA is 96.0% by weight, more preferably 99.6% by weight, yet more preferably 99.9% by weight, with the remainder being especially glycol.

[0095] Withdrawn at the lower end of RD.sub.A in this preferred embodiment is at least one stream S.sub.UA comprising glycol which may preferably include <1% by weight, more preferably 5000 ppm by weight, yet more preferably 1000 ppm by weight, more preferably 100 ppm by weight of water.

[0096] The withdrawal of at least one vapour stream S.sub.OA comprising water at the top of the rectification column RD.sub.A shall in particular be understood in the context of the present invention to mean that the at least one vapour stream S.sub.OA is withdrawn above the internals in the rectification column RD.sub.A as a top stream or as a side stream.

[0097] The withdrawal of the at least one stream S.sub.UA comprising glycol at the bottom of the rectification column RD.sub.A shall in particular be understood in the context of the present invention to mean that the at least one stream S.sub.UA is withdrawn as bottom stream or at the lower tray of the rectification column RD.sub.A.

[0098] The rectification column RD.sub.A is operated with or without, preferably with, reflux.

[0099] With reflux shall be understood to mean that the vapour stream S.sub.OA withdrawn at the upper end of the rectification column RD.sub.A is not completely discharged but rather partly condensed and returned to the respective rectification column RD.sub.A. In the cases where such a reflux is established, the reflux ratio is preferably 0.01 to 1, more preferably 0.02 to 0.9, yet more preferably 0.03 to 0.34, especially preferably 0.04 to 0.27 and very especially preferably 0.05 to 0.24, most preferably 0.2.

[0100] A reflux may be established by mounting a condenser K.sub.RD at the top of the rectification column RD.sub.A. The respective vapour stream S.sub.OA is partly condensed in the condenser K.sub.RD and returned to the rectification column RD.sub.A.

3. Step (b): Reaction of PET With the Solution S.SUB.AP

[0101] In step (b) of the method according to the invention, the solution S.sub.AP obtained in step (a), comprising glycol and M.sub.A glycolate, is reacted with PET to give a mixture M.sub.1 comprising BHET.

3.1 PET Starting Material

[0102] The PET which is used in step (b) of the method according to the invention may be any PET which has to be depolymerized. Typically, such PET occurs as waste, especially in the home, in industry or in agriculture.

[0103] In one embodiment of the method according to the invention, the PET to be depolymerized is thus in a mixture with other plastics, especially at least one plastic selected from polyethylene (PE), polyvinylchloride (PVC). This is typically the case when PET from plastic wastes is to be depolymerized in the method according to the invention. In this embodiment, the PET is at least partly separated from the other plastics, preferably by sorting, before being subjected to step (b) of the method according to the invention.

[0104] In one embodiment of the method according to the invention, the PET is subjected to at least one pretreatment step.

[0105] Such pretreatment steps are described, for example, in DE 10032899 C2.

[0106] According to the invention, the PET is subjected to at least one pretreatment step selected from chemical pretreatment step, comminution step, before being used in step (b).

[0107] In the cases in which the PET is in a mixture with other plastics, the PET is preferably subjected to at least one pretreatment step selected from at least partial separation from other plastics, preferably by sorting, chemical pretreatment step, comminution step, before being used in step (b).

[0108] In the cases in which the PET is in a mixture with other plastics, the PET is more preferably first separated at least partly from other plastics, then subjected to at least one chemical pretreatment and finally comminuted.

[0109] The chemical pretreatment step is especially a wash step. Such a wash step has the advantage that any impurities, especially food residues, residues of cosmetics and/or bodily secretions (e.g. blood, sperm, faeces), are removed prior to the performance of step (b). Such impurities can lower the efficiency of the reaction in step (b) and/or worsen the purity of the BHET thus obtained.

[0110] In the chemical pretreatment step, especially the wash step, the waste is especially heated in a wash solution at a temperature of 30 C. to 99 C., preferably 50 C. to 90 C., more preferably 70 C. to 85 C.

[0111] Typical wash solutions are familiar to the person skilled in the art and are preferably selected from: [0112] aqueous solution of a surfactant, preferably a nonionic surfactant; [0113] aqueous solution of an alkali metal hydroxide or alkaline earth metal hydroxide, preferably aqueous NaOH.

[0114] The treatment time in the chemical pretreatment step, especially the wash 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.

[0115] After the treatment of the PET by the chemical pretreatment step, especially the wash 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 wash solution.

[0116] The PET waste thus obtained is then dried, especially in a drying cabinet.

[0117] The temperature used for drying here is especially in the range of 30 to 120 C., preferably 50 C. to 100 C., more preferably 60 C. to 90 C., most preferably 80 C.

[0118] 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 effected in apparatuses known to the person skilled in the art, for example a shredder or a cutting mill.

[0119] In a further embodiment of the method according to the invention, the PET is decolorized or coloured in a controlled manner before being subjected to step (b). This can be conducted by methods known to the person skilled in the art, for example decolorization with hydrogen peroxide or dyeing with a dye.

3.2 Reaction Conditions

[0120] The reaction of the PET with a solution S.sub.AP comprising glycol and M.sub.A glycolate to give a mixture M.sub.1 can then be effected under the conditions that are familiar to the person skilled in the art.

[0121] Preferably, the reaction in step (b) is conducted until, i.e. up to a juncture t.sub.b at which, 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.

[0122] This percentage P is calculated by the following formula:

[00001]$P=\left(n_{TA}+n_{\mathrm{MHET}}+n_{BHET}\right)/n_{PET}.$ [0123] n.sub.PET here is the molar amount of repeat units of the following structure (E) in the PET used in step (b):

##STR00001## [0124] n.sub.TA is the molar amount of TA formed in step (b) from commencement of step (b) up to the juncture t.sub.b. [0125] n.sub.MHET is the molar amount of MHET formed in step (b) from commencement of step (b) up to the juncture t.sub.b. [0126] n.sub.BHET is the molar amount of BHET formed in step (b) from commencement of step (b) up to the juncture t.sub.b.

[0127] The structures of compounds BHET, MHET, TA are as follows:

##STR00002## [0128] MHET also encompasses the corresponding carboxylate of the structure shown. [0129] TA also encompasses the corresponding mono- and dicarboxylate of the structure shown.

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

[0131] The reaction in step (b) is preferably conducted at the boiling temperature of the glycol. Even more preferably, glycol is refluxed, meaning that glycol is evaporated out of the reaction, condenses and is then returned to the reaction. This refluxing can be established by means familiar to the person skilled in the art, for example in a distillation apparatus.

[0132] The total weight of the M.sub.A glycolate used in the method based on the total weight of the PET used in the method is especially in the range from 0.1% to 100% by weight, preferably in the range from 0.5% to 80% by weight, more preferably in the range from 1.0% to 50% by weight, more preferably in the range from 1.5% to 25% by weight, more preferably in the range from 2.0% to 10% by weight, more preferably in the range from 2.5% to 6.0% by weight, more preferably 3.5% to 5.0% by weight, most preferably 3.9% by weight.

[0133] The reaction in step (b) can be effected with devices familiar to the person skilled in the art.

[0134] After step (b) of the method according to the invention has ended, a mixture M.sub.1 is obtained, in which the molar ratio of the molar amount of BHET (n.sub.BHET) to the sum total of the molar amounts of MHET and TA (n.sub.MHET+n.sub.TA) 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, yet more preferably 13:1 to 60:1, yet more preferably 13:1 to 24:1.

[00002]$=n_{\mathrm{BHET}}/\left(n_{\mathrm{MHET}}+n_{\mathrm{TA}}\right)$

3.3 Preferred Step (c)

[0135] In a preferred further step (c), BHET is at least partly separated from M.sub.1. This is even more preferably effected by crystallization and/or distillation. Even more preferably, BHET in step (c) is filtered out of M.sub.1 and then crystallized.

4. Process for Recycling of PET

[0136] The BHET obtained in the mixture M.sub.1 in the method according to the invention is preferably polymerized to PET in a method of recycling of polyethylene terephthalate in a step ().

[0137] This polymerization is known to the person skilled in the art as polycondensation and is described, for example, in EP 0 723 951 A1 and by Th. Rieckmann and S. Vlker in chapter 2 Poly(Ethylene Terephthalate) PolymerizationMechanism, Catalysis, Kinetics, Mass Transfer and Reactor Design on page 92 of the book Modern Polyesters: Chemistry and Technology of Polyesters and Copolyesters. Edited by J. Scheirs and T. E. Long, 2003, John Wiley & Sons, Ltd ISBN: 0-471-49856-4.

[0138] In particular, for this purpose, BHET is polymerized back to PET in step () in the presence of catalysts, which are especially catalysts selected from the group consisting of antimony compounds, preferably Sb.sub.2O.sub.3.

[0139] Preferably, the polymerization of BHET to PET in step () is conducted at least at the boiling temperature of the glycol. In particular, during the polymerization in step (), glycol is removed from the reaction mixture in order to shift the reaction equilibrium to the side of the polymer PET.

[0140] More preferably, the polymerization of BHET to PET in step () is conducted at the boiling temperature of the glycol. Even more preferably, in that case, during the polymerization in step (), glycol is removed from the reaction mixture in order to shift the reaction equilibrium to the side of the polymer PET.

[0141] This is especially achieved by distillation at a pressure of <1 bar, preferably 0.1 mbar, at the simultaneous boiling temperature of the glycol at the respective pressure.

EXAMPLES

1. Inventive Example 11

1.1 Preparation of the Glycolic Sodium Glycolate Solution by Reactive Distillation

[0142] The following apparatus was utilized as distillation apparatus:

[0143] The reservoir vessel or bottom used in the distillation apparatus was a heatable 2.5 I jacketed vessel with temperature sensor and vacuum-tight stirrer. Above that was a 25 cm column with Multifill packing and silver mirror (stripping section). NaOH was metered in above the column by means of a dropping funnel. Above the metering point was a further column that served to separate off ethylene glycol and water vapour (rectifying section). A reflux ratio was able to be established with the aid of a vapour divider in the upper part of the column, with collection of the distillate in a round-bottomed flask. The round-bottomed flask was able to be separated from the distillation system via a pressure-equalizing dropping funnel and exchanged. In the rectifying section, a reflux condenser with vacuum connections was attached, by means of which the entire apparatus could be evacuated. The vacuum was generated by means of a rotary vane pump which was connected to the distillation apparatus by two cold traps and a safeguard bottle. The pressure in the distillation apparatus was measured in the safeguard bottle (Buchi vacuum controller), where ventilation was also possible. The bottom reservoir and the column with the Multifill packing were completely surrounded by aluminium foil for insulation in order to assure a uniform temperature in the reactor/column.

[0144] The bottom was initially charged with the ethylene glycol and the entire apparatus was evacuated to 50 mbar. Subsequently, the bottoms were heated to boiling temperature, such that a reflux from the rectifying section was established. Subsequently, sodium hydroxide solution (50% by weight in water) was metered in with the aid of a dropping funnel. The metering rate was chosen such that the sodium hydroxide solution did not reach the bottom (about 2 ml/min).

[0145] The water added/formed was separated by distillation from ethylene glycol in the rectifying section and collected in the round-bottomed flask. The reflux ratio was 5:1 (5 parts as reflux, 1 part as distillate). The amount distilled off had to correspond at least to the amount of water added. After the distillative removal, the sodium ethyleneglycolate in the bottoms was subjected to continued distillation for about another 2 hours. The water present in the rectifying section was removed at constant vacuum and temperature in order to prevent backflow into the bottom.

[0146] After the experiment had ended and been cooled down, the bottom was opened by means of an outlet valve and about 20% by weight solution of sodium glycolate in ethylene glycol was removed.

1.2 PET Depolymerization With Glycolic Sodium Glycolate Solution From the Reactive Distillation

[0147] In the method according to the invention, an autoclave was initially charged with 100 g of PET together with 800 g of ethylene glycol. The solution was then heated to 150 C. while stirring. As soon as the temperature of 150 C. had been attained, 19.5 g of 20% sodium glycolate solution in ethylene glycol (corresponding to 0.046 mol) from the reactive distillation was added. The reaction was conducted over the course of five hours, and the reactor output was analysed after cooling. The resultant conversion was determined by gas chromatography. The conversion of BHET (1) and 2-hydroxyethyl terephthalate (=MHET) (2) and of terephthalic acid (=TA) (3) is shown in the FIGURE (in % relative to the repeat unit () used in the PET; bars hatched from top left.fwdarw.bottom right: \\\\\\).

2. Comparative Example V1

[0148] In a comparative experiment, an autoclave was initially charged with 100 g of PET together with 800 g of ethylene glycol. The solution was then heated to 150 C. while stirring. The reaction was conducted over the course of five hours, and the reactor output was analysed after cooling. The resultant conversion of BHET (1) and MHET (2) and of TA (3) is illustrated in the FIGURE (black, .square-solid.).

3. Comparative Example V2

[0149] In a comparative experiment, an autoclave was initially charged with 100 g of PET together with 800 g of ethylene glycol. The solution was then heated to 150 C. while stirring. As soon as the temperature of 150 C. had been attained, 3.7 g of 50% by weight NaOH solution in water (corresponding to 0.046 mol) was added. The reaction was conducted over the course of five hours, and the reactor output was analysed after cooling. The resultant conversion of BHET (1) and MHET (2) and of TA (3) is shown in the FIGURE (bars hatched from top right.fwdarw.bottom left: ///).

4. Result

[0150] Comparison of the content of BHET, MHET and TA in the depolymerized product in Inventive Example E1 and Comparative Examples V1, V2 shows that the depolymerization using the glycolic sodium glycolate solution obtained by reactive distillation affords a higher proportion of BHET. This is advantageous since more product is available as a result, which can be converted directly in a polycondensation to new PET product.