Purification method for bis(2-hydroxyethyl) terephthalate

Abstract

A process for purifying bis(2-hydroxyethyl) terephthalate (BHET) obtained from the depolymerization of polyethylene terephthalate (PET) waste, comprising the steps of treating the solution with at least one oxidizing agent at a temperature of 30° C. to 100° C., preferably 50° C. to 90° C., to obtain an oxidized solution, treating the oxidized solution with at least one adsorbent to obtain a purified oxidized solution, and separating the at least one adsorbent from the purified oxidized solution to obtain a purified BHET solution. The process is particularly effective in removing organic dyes and other low-molecular-weight organic contaminants, thereby improving the purity of the recovered BHET.

Description

[Technical Field]

[0001] The present invention relates to a purification process for bis(2-hydroxyethyl) terephthalate (BHET), in particular BHET obtained by depolymerization of polyethylene terephthalate (PET) waste. [Background technology]

[0002] Polyethylene terephthalate (PET) is a widely used semi-crystalline thermoplastic polyester with high strength and transparency. Its physical and chemical properties have led to several applications, particularly in packaging and textile manufacturing. While PET poses no safety issues, its increasing consumption, accumulation in wastewater streams, and poor biodegradability have raised environmental and economic concerns. Therefore, there has been growing interest in PET recycling technologies.

[0003] PET is considered an easily recyclable polymeric material, and its recycling is the most widely practiced of all polymeric materials. Techniques can be divided into two broad categories: mechanical recycling and chemical recycling.

[0004] Mechanical recycling mainly involves separating, decontaminating, crushing and grinding waste to obtain flakes, which are then sent directly to extrusion to produce new molded products. The main problems with this technology are the heterogeneity of the solid waste and the poor quality of the final product, as the properties of PET deteriorate with each recycling.

[0005] Chemical recycling involves breaking down the polyester using reagents capable of depolymerizing the PET chains to yield the starting monomers. Chemical depolymerization of PET is usually achieved by solvolysis, especially hydrolysis, or methanolysis or glycolysis.

[0006] Hydrolysis involves the reaction of PET with water to break it down into terephthalic acid (TPA) and ethylene glycol (EG) (also known as monoethylene glycol - MEG). Methanolysis involves the reaction of PET with methanol to break it down into dimethyl terephthalate (DMT) and EG. Glycolysis involves the depolymerization of PET with EG to produce bis(2-hydroxyethyl) terephthalate (BHET), an intermediate formed in the first step of PET production from the starting monomer.

[0007] To date, mechanical recycling has been the most widely used method for treating PET waste, with few industrial applications of chemical recycling. However, chemical recycling technology is gaining interest because it is in line with sustainable development principles and returns virgin PET raw material of much higher quality than mechanically recycled PET.

[0008] To better understand the fundamentals of chemical recycling, the basics of PET production and glycolysis are reported below.

[0009] Industrially, PET is obtained in two separate reaction steps: in the first step, the starting products react to form BHET and oligomers, followed by another reaction step where chain growth occurs.

[0010] PET can be produced from two types of raw materials: (a) TPA and EG react via an acid to produce BHET and water, and (b) dimethyl terephthalate (DMT) and EG react via an ester to produce BHET and methanol. The resulting BHET is then reacted through an esterification or transesterification step to produce PET via a polycondensation reaction. EG is generated during this step and can be separated from the reaction mixture.

[0011] [ka]

[0012] Polycondensation is an equilibrium reaction, and EG is produced as a by-product along with PET. Therefore, the generated EG must be evaporated to shift the equilibrium toward PET.

[0013] When PET reacts with EG, it attacks the ester bond, resulting in chain scission, so the reverse reaction can be used to produce BHET from PET. PET is produced by a reversible polycondensation reaction, so adding EG can shift the reaction in the opposite direction, converting the polymer back into a monomer or oligomer.

[0014] In this reverse reaction, called glycolysis, the polymer chains are transformed by solvolytic chain scission, theoretically either completely depolymerized back to monomers (BHET) or partially depolymerized to monomers along with oligomers.

[0015] [ka]

[0016] Because the reaction is slightly endothermic and reversible, it can be driven towards the formation of monomers / oligomers by using an excess of EG relative to the starting PET.

[0017] The entire glycolysis reaction can be divided into two steps: (i) EG cleaves the ester bonds of the PET chains, forming oligomers that are solubilized in EG itself; and (ii) because the oligomers are in equilibrium with BHET, adding excess EG shifts the reaction toward BHET formation.

[0018] Without a catalyst, the glycolysis reaction is slow, making it difficult to completely depolymerize PET to BHET in an acceptable time for industrial processing. Therefore, a transesterification catalyst is typically used, which ensures a shorter reaction time at a lower temperature than the uncatalyzed reaction, thereby increasing yield by eliminating or minimizing side reactions that can affect the overall process. Potential catalysts are disclosed, for example, in U.S. Pat. No. 6,630,601 and WO 2017 / 111602. The glycolysis catalysis reaction is typically carried out at temperatures between 190°C and 250°C.

[0019] To be used in the production of virgin PET, BHET obtained by glycolysis of PET waste must be thoroughly purified because it contains various contaminants that inhibit the polymerization reaction, such as organic dyes, antistatic agents, stabilizers, and UV absorbers used in colored PET. Furthermore, BHET may contain contaminants that are insoluble in the glycolysis solvent, such as aluminum flakes, polyolefins, fillers (such as titanium dioxide, carbon black, silicates, and other pigments), and adhesives. Insoluble contaminants can be separated by filtration. Other contaminants are barrier polymers used in the production of multilayer materials for food applications.

[0020] Regarding organic dyes and other low molecular weight organic contaminants, they may be present in PET waste as a mixture of different compounds in varying amounts from batch to batch. Their removal may present different difficulties and therefore require different treatments to obtain purified BHET of similar final quality.

[0021] A possible approach to obtain purified BHET is to treat the starting PET waste to separate or decompose undesirable materials such as dyes.

[0022] For example, EP 2 784 110 A1 relates to a method for pretreating PET flakes prior to depolymerization by glycolysis and / or methanolysis and / or hydrolysis and / or saponification, which involves extracting organic colorants from the PET flakes and simultaneously embrittling the PET flakes by contacting them with EG.

[0023] US2009 / 0133200 relates to a method for recovering useful components from dyed polyester fibers, which includes a dye extraction step, a solid-liquid separation step, a depolymerization reaction step, a transesterification reaction step, and a useful component separation step. The dye extraction step is a step in which dye is extracted and removed from dyed polyester fibers using an extraction solvent containing xylene and alkylene glycol at a temperature above the glass transition temperature of polyester and below 220°C.

[0024] US 2019 / 0016860 relates to a method for decolorizing a polymer by mixing a solution of the polymer with a photocatalyst and irradiating the mixture with ultraviolet light.

[0025] Such processes are rarely effective in achieving high purity of the final BHET, and furthermore, they are very cumbersome and can cause undesirable degradation of the polymer with the formation of degradation by-products.

[0026] Various methods have been proposed to purify BHET obtained by depolymerization of PET waste.

[0027] For example, US 2004 / 0182782 A1 relates to a method for purifying BHET by subjecting a solution obtained by decomposing PET to crystallization and solid-liquid separation under specific temperature conditions. The solution has previously been subjected to impurity removal treatments, including removal of solid foreign matter (e.g., filtration under heat), decolorization with activated carbon, and deionization with ion exchange resins.

[0028] EP 1 234 812 A1 describes a method for purifying crude BHET obtained by decomposing PET with alcohol or glycol in the presence of a catalyst, which includes a filtration step, treatment with activated carbon, and treatment with an ion exchange resin.

[0029] US 6,630,601 B1 relates to a method for producing BHET, which comprises contacting a BHET solution composition containing EG, BHET, and cations and / or anions as impurities with a cation exchanger and / or anion exchanger to reduce the total content of cations and anions as impurities to 50 ppm or less based on BHET. The BHET solution thus treated is then pre-evaporated or distilled to remove compounds with a boiling point lower than that of BHET, and then evaporated or distilled under reduced pressure to obtain purified BHET. Summary of the Invention [Problem to be solved by the invention]

[0030] Applicant was faced with the problem of providing a process for purifying bis(2-hydroxyethyl) terephthalate (BHET), particularly BHET obtained from the depolymerization of PET waste, that is particularly effective in removing organic dyes and other low molecular weight organic contaminants, and that yields highly pure recovered BHET. It would be desirable for such a process to be effective against a wide range of contaminants and not result in unacceptable losses of recovered BHET, which can be used as a monomer in polymerization reactions, particularly to produce virgin PET with high clarity. Furthermore, the process should be effective on large quantities of BHET without unacceptable increases in production costs. [Means for solving the problem]

[0031] Thus, according to a first aspect, the present invention relates to a process for purifying bis(2-hydroxyethyl) terephthalate (BHET), which process comprises: providing a crude BHET solution obtained from the depolymerization of polyethylene terephthalate (PET) waste; treating said solution with at least one oxidizing agent at a temperature of from 30°C to 100°C, preferably from 50°C to 90°C, to obtain an oxidized solution; treating the oxidized solution with at least one adsorbent to obtain a purified oxidized solution; separating the at least one adsorbent from the purified oxidized solution to obtain a purified BHET solution; Includes:

[0032] The crude BHET solution is preferably obtained by depolymerizing PET waste through glycolysis, which is typically carried out by reacting PET waste with ethylene glycol (EG).

[0033] The glycolysis reaction is usually carried out in the presence of a heterogeneous transesterification catalyst, which can be selected from, for example, carbonates, fatty acid salts or borates of Na, Mg, Zn, Cd, Mn, Co, Ca or Ba (e.g., zinc borate, zinc acetate, sodium carbonate).

[0034] Preferably, the glycolysis reaction is carried out at a temperature of 170°C to 270°C, more preferably 195°C to 210°C.

[0035] In the glycolysis reaction, EG is generally used in an amount of 1.0 to 10.0 parts by weight, preferably 1.5 to 6.0 parts by weight, based on the parts by weight of the PET waste.

[0036] The glycolytic reaction time can vary within a wide range depending on reaction conditions such as temperature, agitation, and type of reactor. Typically, the reaction time is 1 to 8 hours, preferably 1.5 to 3 hours. The reaction can be carried out batchwise or continuously. The reaction pressure is usually atmospheric, but reduced or increased pressure can also be used.

[0037] Further details regarding the glycolysis of PET waste to recover BHET are reported, for example, in US 3,222,299, US 4,609,680 and EP 0 723 951 A1.

[0038] The PET waste may be post-consumer and / or post-industrial PET waste, which may come from a wide variety of uses such as: - Transparent and / or colored PET bottles for water, soft drinks, carbonated drinks, etc. - PET opaque products, where the PET contains fillers such as titanium dioxide, carbon black, silicates, and other pigments. - Multilayer PET moldings, typically for the food industry, in which a PET layer is bonded to a layer of gas barrier polymer (e.g. nylon, polyvinyl alcohol (EVOH), polyvinyl acetate (EVA)) or metal foil (e.g. aluminum foil) or polyolefin foil. -Printed PET foil. -PET fiber.

[0039] The product of the glycolysis reaction is a crude BHET solution, in which BHET is dissolved in EG along with various contaminants derived from the specific composition of the PET waste. Typically, the crude BHET solution also contains BHET oligomers, preferably dimers and / or trimers.

[0040] In this specification and the appended claims, the total amount of BHET in a BHET solution is calculated based on BHET (monomer) and all of its oligomers.

[0041] The contaminants are components of PET waste or derivatives obtained by glycolysis of such components, such as: -Dyes, usually organic dyes. -ink. -Adhesives and glues. Polyolefins, such as polyethylene or polypropylene, used in the manufacture of caps. -PET-G. Biodegradable polymers, for example PLA. Gas barrier polymers, such as polyamides, polyvinyl alcohol (EVOH), polyvinyl acetate (EVA). -UV absorber. Fillers such as titanium dioxide, carbon black, silica, silicates and other pigments. Metal foils and fragments thereof, for example aluminium foil.

[0042] The contaminants may be dissolved in EG or, if not soluble in EG, may be in a suspended state.

[0043] Contaminants that are not soluble in EG, such as fillers, adhesives, polyolefins, and aluminum films, are preferably separated from the crude BHET solution, usually by filtration, before subjecting the crude BHET solution to the oxidation step.

[0044] To reduce the solubility of some contaminants in the crude BHET solution, such as polyamides and glues, water is preferably added to the crude BHET solution to allow precipitation of such insoluble contaminants while maintaining BHET and its oligomers in solution. The insoluble contaminants are then separated from the crude BHET solution, usually by filtration. This is advantageous for reducing the amount of contaminants subjected to the oxidation step, thus allowing for a reduction in the amount of oxidizing agent used. In the crude BHET solution obtained after filtration, BHET and its oligomers are dissolved in a mixture of water and EG.

[0045] After separation of the insoluble contaminants, the crude BHET solution may be directly subjected to the oxidation step. Alternatively, the crude BHET solution may be cooled, typically to a temperature of 10°C to 30°C, preferably 15°C to 20°C, to cause precipitation of BHET and its oligomers, which may be separated (e.g., by filtration), dissolved in hot water (typically at 75°C), and then subjected to the oxidation step. In this way, EG is removed, and the aqueous solution of BHET and its oligomers is subjected to the oxidation step.

[0046] The main contaminants dissolved in crude BHET solutions are organic dyes. Typically, they are disperse dyes, which are hydrophobic and dissolve in the polymer matrix to form solid-solid solutions. Examples of disperse dyes include Disperse Blue 165, Direct Red 81, and Pigment Yellow 14. These are aromatic compounds that can absorb light in the visible wavelength range (400 to 700 nm).

[0047] Another type of dye used to color PET is the azo dyes, which are characterized by the presence of a nitrogen double bond (-N=N-) connecting the aromatic rings.

[0048] To assess the amount of dye in the crude BHET solution being processed, the total absorbance of the sample at a given wavelength (Σabs 0 ) can be used, which can be measured by a spectrophotometer operating in the desired wavelength range. Typically, the wavelengths chosen to cover the visible range are 475, 510, 590, and 650 nm. Thus, Σabs 0 =abs 475 +abs 510 +abs 590 +abs 650 This becomes:

[0049] The spectrum of a sample of BHET solution is registered as follows: a certain amount of BHET solution is heated on a thermobalance to evaporate the solvent. The resulting solid is weighed and dissolved in dimethyl sulfoxide (DMSO) in a BHET:DMSO weight ratio of 1:1. The solution thus obtained is placed in an optical glass tube (light path: 1.5 cm) and inserted into a spectrometer to measure the UV-visible spectrum.

[0050] In Table 1 below, some typical Σabs values are shown for crude BHET solutions obtained by glycolysis of several post-industrial and post-consumer PET wastes. 0 The values reported are:

[0051] [Table 1]

[0052] More specifically, the absorbance values reported in Table 1 refer to the following crude BHET solutions: (i) Aqueous solutions of crude BHET (products 1 to 6). These are obtained by adding water to the crude solution obtained after glycolysis of PET waste, filtering to remove insoluble contaminants, cooling to precipitate BHET and its oligomers, separating them, and then dissolving BHET and its isomers in hot water. (ii) Solution of crude BHET in water and EG (product 7): This is obtained by adding water to the crude solution obtained after glycolysis of the same PET waste as product 6, and removing insoluble contaminants. In this case, BHET and its oligomers are not precipitated, separated, and dissolved as reported above. The amount of contaminants is higher than that of product 6, because BHET and its isomers are not purified by precipitation. (iii) Crude BHET in EG (product 8): This is a crude solution obtained after glycolysis of the same PET waste as products 6 and 7, but without adding water and / or precipitating BHET and its oligomers to achieve higher purity. The amount of contaminants is virtually identical to product 7.

[0053] As shown in Table 1, the crude BHET solution exhibited a Σabs depending on the starting PET waste and the possibility of replacing EG with water. 0 The degree of coloration, as measured by the coloring agent, may vary within wide limits.

[0054] Typically, the total absorbance (Σabs 0 ) is 1.5 to 12.0, preferably 2.0 to 10.0.

[0055] The crude BHET solution, optionally pretreated as described above, is then subjected to an oxidation step. The oxidizing agent is preferably selected from inorganic oxidizing agents, such as alkali metal or alkaline earth metal chlorites or hypochlorites, alkali metal or alkaline earth metal persalts, preferably persulfates, potassium permanganate, hydrogen peroxide, ozone, chlorine gas, or mixtures thereof. Preferably, the alkali metal is sodium or potassium, and the alkaline earth metal is magnesium, calcium, or barium.

[0056] Particularly preferred are sodium or potassium hypochlorite or chlorite, chlorine gas, hydrogen peroxide, hydrogen peroxide, or mixtures thereof.

[0057] Chlorine gas has the same oxidizing effect as hypochlorite and chlorite, but has the advantage of not introducing metal ions into the reaction mixture as hypochlorite and chlorite, which are used in salt form. However, because chlorine gas cannot disperse into the atmosphere, a device to mitigate reaction outgassing is required, such as a trap containing a reducing agent such as sodium metabisulfite.

[0058] To enhance the effectiveness of hydrogen peroxide, it can be used in combination with ultraviolet irradiation. For this purpose, the treatment solution is irradiated with ultraviolet lamps, usually with an emission wavelength of 150 nm to 400 nm.

[0059] The oxidizing agent is desirably used in an amount of 0.005 to 5% by weight, preferably 0.01 to 3% by weight, based on the weight of BHET in the crude BHET solution.

[0060] According to a preferred embodiment, in the case of crude BHET solutions with high contaminant contents, it may be advantageous to use a combination of oxidizing agents that can be used together or, preferably, sequentially to achieve complete oxidation of the contaminants. Particularly preferred is the combination of sodium hypochlorite (or sodium chlorite) and hydrogen peroxide, used sequentially.

[0061] After the oxidation reaction is completed, the resulting oxidized solution is treated with at least one adsorbent to obtain a purified oxidized solution.

[0062] The term "adsorbent" refers to a solid material, usually in powder form, capable of adsorbing organic molecules dispersed or solubilized in a liquid medium onto its surface, and sequestering and removing such molecules by separating the adsorbent from the liquid medium.

[0063] The purpose of the treatment with at least one sorbent is to remove decomposition products formed in the solution during the oxidation step, as well as other contaminants that may be present in such solution, such as residual additives that were not decomposed during oxidation. Additionally, the sorbent may be useful in removing by-products derived from the oxidizing agent, such as chloride ions.

[0064] Preferably, the adsorbent is activated carbon or silica.

[0065] Preferably, the adsorbent has a particle size of from 50 to 500 mesh, more preferably from 100 to 350 mesh, measured according to ASTM standard D2862 / 16.

[0066] Preferably, the adsorbent has a viscosity of 250 to 5000 m, measured according to ASTM standard D6556 / 19. 2 / g, more preferably 500 to 3000m 2 / g surface area (BET).

[0067] The amount of adsorbent added depends mainly on the nature of the adsorbent and the concentration of the contaminants to be separated. For example, activated carbon is added in an amount of preferably 0.05 to 3% by weight, more preferably 0.1 to 1% by weight, based on the weight of BHET present in the solution to be purified. The purification step is preferably carried out at a temperature of 40 to 100°C, more preferably 70 to 90°C.

[0068] After the purification step is completed, the resulting solution is treated to separate the adsorbent from the solution, which can be done according to known techniques, preferably by filtration.

[0069] From the purified oxidized solution thus obtained, BHET and its oligomers may be recovered, preferably by precipitation. For this purpose, the solution may be cooled to a temperature of 10° C. to 30° C., more preferably 15° C. to 20° C. BHET and its oligomers may precipitate in an amorphous state or in at least partially crystalline form, depending mainly on the conditions applied for the precipitation.

[0070] The decolorization yield (Y) of the process according to the present invention can be calculated as follows: Y=100-(Σabs / Σabs 0 )x100 where Σabs 0 is the total absorbance at a given wavelength (see above) of the crude BHET solution, and Σabs is the total absorbance of the purified BHET solution. [Example]

[0071] The following examples are provided purely for the purpose of illustration of the present invention and should not be considered as limiting the scope of protection defined by the enclosed claims.

[0072] Examples 1 to 12 A series of tests were carried out on different PET wastes and different solvents, as reported in Table 1 above.

[0073] PET waste was subjected to glycolysis according to the following method.

[0074] PET waste was dissolved in EG at 200 °C and reduced pressure (p = 0.4 MPa) to make the weight ratio of EG / PET equal to about 3. The glycolysis reaction was carried out in the presence of Na2CO3 as a heterogeneous catalyst.

[0075] The resulting crude BHET solution was then subjected to a purification process using two different embodiments, as reported below.

[0076] (Process A: BHET in EG+H2O solution) 400 g of the crude BHET in EG solution was diluted with percolating HO to cause some precipitation of contaminants (especially polyamides), which were separated by filtration. The resulting solution of BHET in EG and HO, containing 20 wt. % BHET, was then heated at a temperature of T ox Under stirring, for a time T ox During this time, the quantity Q ox was treated with an oxidizing agent.

[0077] (Process B: BHET in H2O) 400 g of crude BHET in EG solution was diluted with percolating H2O to induce precipitation of some of the contaminants (especially polyamides), which were then separated by filtration. The resulting mixed solution of BHET in EG and H2O was then cooled to 20 °C to induce precipitation of BHET and its oligomers. The solid was filtered and washed with percolating H2O to remove residual EG. A BHET cake was obtained, which was dried at 70 °C / 0.9 bar.

[0078] To 80 g of dry BHET powder, 148 g of osmotic H2O was added at 75-80 °C. The resulting solution containing 35 wt% BHET was heated to a temperature of T ox Under stirring, for a time t ox During this time, the quantity Q ox was treated with an oxidizing agent.

[0079] (Process C: BHET in EG solution) 1000 g of crude BHET in EG containing 25 wt. % BHET was added to the EG solution at temperature T ox Under stirring, for a time t ox During this time, the quantity Q ox was treated with an oxidizing agent.

[0080] In all of the above processes A, B, and C, the solution obtained at the end of the oxidation treatment is heated to the same temperature T ox While maintaining the quantity Qac Activated carbon (particle size: 325 mesh; surface area BET: approximately 2000 m 2 1 / g) was added and stirred for 15 minutes. The mixture was then filtered at 75-80°C to remove the activated carbon, and the clear solution was cooled to 20°C to obtain crystals of BHET. To increase the purity, the crystallized BHET was washed with percolated water and filtered. A BHET cake was obtained by drying at 70°C / 0.9 bar.

[0081] The experimental conditions for each test are reported in Table 2. ox and Q ac is expressed as weight percent relative to the weight of BHET.

[0082] [Table 2]

[0083] As for the oxidizing agents, NaClO was used in the form of an aqueous solution with a concentration of 17% by weight, H2O2 was used in the form of an aqueous solution with a concentration of 32% by weight, and NaClO2 was used in the form of an aqueous solution with a concentration of 31% by weight. In Example 12, the ultraviolet light had a wavelength of 254 nm.

[0084] With respect to the amount of oxidizing agent, Examples 6, 7, and 8 used redox probes whose redox potential could be controlled, thereby significantly reducing the amount of oxidizing agent that had to be used to obtain results substantially similar to those obtained without controlling the redox potential.

[0085] (Color evaluation) Two different tests were performed on each purified BHET compared with crude BHET obtained from glycolysis.

[0086] (Test 1: Spectrophotometer) A sample of dry BHET powder was dissolved in DMSO (weight ratio 1:1), and the solution was introduced into a spectrophotometer to register the UV-VIS spectrum. The values of Σabs and Y, as defined above, were determined and reported in Table 3.

[0087] (Test 2: Hunter Lab color space) The color of the dry BHET powder was measured using the Hunter Lab color scale according to the Hunter Lab L, a, b color space (details about this measurement are reported on the website https: / / support.hunterlab.com).

[0088] Measurements were performed on cylindrical samples (diameter: 50 mm, height: 10 mm) obtained by compressing the BHET dry powder in a cylindrical container at 400 bar. The L, a, and b parameters were determined as reported in Table 3.

[0089] [Table 3]

[0090] From the above results, it is clear that the process according to the present invention can achieve very high decolorization yields (>90%), whereas treatment with activated carbon alone without oxidation results in low decolorization yields even at the economically unsuitable large amounts.

[0091] (Examples 13 to 14) Treatment according to the present invention was carried out using chlorine gas as the oxidizing agent.

[0092] PET waste with the nominal composition reported in Table 4 was subjected to glycolysis according to the same process used in Examples 1 to 12. From the glycolysis stock solution, after cooling at room temperature, a crude BHET solution (product 9) was obtained by filtration and washing (BHET = 25 wt %, EG = 75 wt %).

[0093] Another PET waste with the nominal composition reported in Table 4 was glycolyzed and then diluted with HO (BHET = 20 wt%, HO = 20 wt%, EG = 60 wt%), and after cooling at room temperature, filtered and washed to obtain another crude BHET solution (Product 10).

[0094] The spectroscopic absorption characteristics of products 9 and 10 before the oxidation treatment are reported in Table 4.

[0095] [Table 4]

[0096] The two crude solutions have slightly different colorants due to unavoidable variations in the composition of waste materials.

[0097] Example 13 1475 g of crude BHET in EG solution 9 (25 wt % BHET, 75 wt % EG) was heated to 84°C and charged into a reactor. 0.83 wt % chlorine gas was added for 20 minutes under stirring. The oxidation solution was then diluted with HO to obtain a solution with the following composition: 25 wt % BHET, 20 wt % HO, and 55 wt % EG. This solution was heated to 85°C, and 0.5 wt % (based on the weight of BHET) of the same activated carbon used in Examples 2 to 12 was added. The mixture was then stirred for 30 minutes. The activated carbon was then removed by filtration, and the decolorized solution was cooled to 15°C to precipitate BHET. The precipitated BHET was recovered by filtration, washed with water, and then subjected to the spectrophotometric color evaluation test reported above. The Σabs and Y% values were measured and are reported in Table 5.

[0098] Example 14 A crude BHET solution (1995 g) containing 20 wt. % BHET, 20 wt. % HO, and 60 wt. % EG was treated with chlorine gas (0.73 wt. %) at 84°C for 20 minutes under stirring. The same activated carbon used in Examples 2 to 12 was then added to the oxidized solution at 0.5 wt. % (based on the weight of BHET) and the mixture was stirred for 30 minutes. The activated carbon was then removed by filtration, and the decolorized solution was cooled to 15°C to precipitate BHET. The precipitated BHET was recovered by filtration, washed with water, and then subjected to the spectrophotometric color evaluation test reported above. The Σabs and Y% values were measured and are reported in Table 5.

[0099] [Table 5]

Claims

1. 1. A process for purifying bis(2-hydroxyethyl) terephthalate (BHET), comprising: providing a crude BHET solution obtained from the depolymerization of polyethylene terephthalate (PET) waste; treating the solution with at least one oxidizing agent at a temperature between 30°C and 100°C to obtain an oxidized solution; treating the oxidized solution with at least one adsorbent to obtain a purified oxidized solution; separating the at least one adsorbent from the purified oxidized solution to obtain a purified BHET solution; Including, the at least one oxidizing agent is an inorganic oxidizing agent selected from an alkali metal chlorite or hypochlorite; an alkali metal persalt; potassium permanganate; hydrogen peroxide; ozone; chlorine gas; or a mixture thereof; The process wherein at least one adsorbent is activated carbon.

2. 10. The process of claim 1, wherein the solution is treated with at least one oxidizing agent at a temperature of from 50°C to 90°C.

3. 3. The process of claim 1 or 2, wherein the crude BHET solution is obtained from the depolymerization of PET waste by glycolysis.

4. 4. The process of claim 3, wherein the glycolysis reaction is carried out by reacting PET waste with ethylene glycol (EG), optionally in the presence of a heterogeneous transesterification catalyst.

5. PET waste, - PET clear and / or coloured bottles for water, soft drinks, carbonated drinks etc., - Opaque articles of PET, wherein the PET contains fillers such as titanium oxide, carbon black, silicates and other pigments; - multilayer PET mouldings, typically for the food industry, in which a PET layer is bonded to a layer of gas barrier polymer or metal foil or polyolefin foil; - printed PET foil, -PET fiber 5. The process of claim 1, comprising:

6. 6. The process of any one of claims 1 to 5, wherein the crude BHET solution is pretreated to separate contaminants that are not soluble in EG.

7. 7. The process of claim 1, wherein water is added to the crude BHET solution beforehand to cause precipitation of insoluble contaminants while maintaining BHET and its oligomers in solution, and the insoluble contaminants are then separated from the crude BHET solution.

8. 8. The process of any one of claims 1 to 7, wherein the crude BHET solution is pre-cooled to cause precipitation of BHET and its oligomers, which are separated and dissolved in water before being subjected to the oxidation step.

9. The crude BHET solution has a total absorbance Σabs ０ =abs ４７５ +abs ５１０ +abs ５９０ +abs ６５０ is 1.5 to 12.0, In the formula, abs ４７５ , abs ５１０ , abs ５９０ and abs ６５０ are the absorbance values measured at radiation wavelengths equal to 475, 510, 590 and 650 nm for a solution of BHET (obtained by drying a sample of crude BHET solution) dissolved in dimethyl sulfoxide (DMSO) (weight ratio BHET:DMSO equal to 1:1), 9. The process of any one of claims 1 to 8.

10. The crude BHET solution has a total absorbance Σabs ０ =abs ４７５ +abs ５１０ +abs ５９０ +abs ６５０ 10. The process of claim 9, wherein is from 2.0 to 10.

0.

11. 11. The process of any one of claims 1 to 10, wherein the alkali metal persalt is a persulfate.

12. 11. The process of any one of claims 1 to 10, wherein the at least one oxidizing agent is selected from sodium or potassium hypochlorite or chlorite, chlorine gas, hydrogen peroxide, or mixtures thereof.

13. 11. The process of any one of claims 1 to 10, wherein the at least one oxidizing agent is hydrogen peroxide used in combination with UV irradiation.

14. 11. The process of any one of claims 1 to 10, wherein the at least one oxidizing agent is a combination of sodium hypochlorite (or sodium chlorite) and hydrogen peroxide, which are used sequentially.

15. 15. The process of any one of claims 1 to 14, wherein the at least one oxidizing agent is used in an amount of 0.005 wt. % to 5 wt. %, based on the weight of BHET in the crude BHET solution.

16. 16. The process of claim 15, wherein the at least one oxidizing agent is used in an amount of 0.01 wt. % to 3 wt. %, based on the weight of BHET in the crude BHET solution.

17. 17. The process of any one of claims 1 to 16, wherein the at least one adsorbent has a particle size of 50 to 500 mesh, measured according to ASTM standard D2862 / 16.

18. 18. The process of claim 17, wherein the at least one adsorbent has a particle size of 100 to 350 mesh, measured according to ASTM standard D2862 / 16.

19. At least one adsorbent has a viscosity of 250 to 5000 m, measured according to ASTM standard D6556 / 19. ２ 19. The process of any one of claims 1 to 18, wherein the surface area (BET) of the catalyst is 1 / g.

20. At least one adsorbent has a viscosity of 500 to 3000 m, measured according to ASTM standard D6556 / 19. ２ 20. The process of claim 19, wherein the surface area (BET) of the SiO2 nanoparticles is 0.1 g / g.

21. 21. The process of any one of claims 1 to 20, wherein the at least one adsorbent is added in an amount of 0.05% to 3% by weight relative to the weight of BHET present in the solution to be purified.

22. 22. The process of claim 21, wherein the at least one adsorbent is added in an amount of 0.1% to 1% by weight relative to the weight of BHET present in the solution to be purified.

23. 23. The process of any one of claims 1 to 22, wherein the at least one adsorbent purification step is carried out at a temperature of from 40°C to 100°C.

24. 24. The process of claim 23, wherein the at least one adsorbent purification step is carried out at a temperature of from 70°C to 90°C.

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