Method for producing bis(2-hydroxyethyl) terephthalate using recycled ethylene glycol

Abstract

In the process of preparing bis(2-hydroxyethyl) terephthalate, after the glycolysis of waste polyester, the ethylene glycol from which acetate compounds have been removed is recovered and reused. Therefore, even if the ethylene glycol is reused in a continuous process, the acetate compounds are not concentrated, and bis(2-hydroxyethyl) terephthalate with excellent purity and quality can be obtained.

Description

[Technical Field]

[0001] Technical Field The present invention relates to a process for preparing bis(2-hydroxyethyl) terephthalate using recycled ethylene glycol in the depolymerization process of waste polyester. [Background technology]

[0002] Background technology Due to its excellent mechanical strength, heat resistance, transparency, and gas barrier properties, polyester is widely used as a material for beverage containers, packaging films, audio and video films, etc. In addition, polyester is widely produced worldwide as an industrial material for medical fibers, tire cords, etc. In particular, polyester sheets or plates have good transparency and excellent mechanical strength, so they are widely used as raw materials for cases, boxes, partitions, shelves, panels, packaging materials, building materials, interior and exterior materials, etc.

[0003] As a result, plastic waste, such as polyester, is generated at uncontrollable levels globally every year. Recently, countries around the world have created regulations and plans to recycle waste plastic resources, including waste polyester. Physical or chemical methods are used to recycle waste polyester, but physical recycling methods are not widely used because they cannot guarantee purity.

[0004] On the other hand, in chemical recycling methods, the ester bonds of waste polyesters are cleaved to depolymerize them. Reactions such as glycolysis, hydrolysis, methanolysis, and aminolysis are used. Among these, glycolysis involves decomposing waste polyesters by adding glycols such as ethylene glycol or diethylene glycol at high temperatures. The resulting reaction product contains primarily bis(2-hydroxyethyl) terephthalate (BHET). The bis(2-hydroxyethyl) terephthalate contained in the reaction product can be used as a raw material for preparing unsaturated polyester or ester polyols after crystallization or purification.

[0005] To use bis(2-hydroxyethyl) terephthalate as a raw material, it is necessary to increase the purity of bis(2-hydroxyethyl) terephthalate by minimizing the formation of by-products such as diethylene glycol esters (DEG esters) during the depolymerization process. To this end, a method of carrying out depolymerization by designing a multi-stage continuous stirred tank reactor (CSTR) is currently employed.

[0006] [Prior art documents] (Patent Document 1) Korean Patent Application Publication No. 2022-0068991 Summary of the Invention [Problem to be solved by the invention]

[0007] Disclosure of the Invention technical issues For the production of bis(2-hydroxyethyl) terephthalate, an excess amount of ethylene glycol is added to the glycolysis reaction, so it is common to recover and reuse it to increase cost efficiency, which requires controlling the purity of the ethylene glycol recycled in this process.

[0008] However, the content of by-products in ethylene glycol increases during this continuous process, and these by-products are converted to esters via glycolysis reactions, leading to quality degradation problems in the final bis(2-hydroxyethyl) terephthalate.

[0009] The present inventors have focused on the accumulation of acetate-based compounds originating from the catalyst in the ethylene glycol reused in the continuous glycolysis process, and have been able to solve this problem by removing them during the ethylene glycol recovery process.

[0010] In particular, the acetate compounds accumulated in the above process have boiling points similar to those of ethylene glycol, making it difficult to separate them through a distillation column, but it has been possible to do so through side-draw purging of a specific fraction from the stage with the highest acetate concentration in a multi-stage distillation column.

[0011] Therefore, it is an object of the present invention to provide a process for the preparation of bis(2-hydroxyethyl) terephthalate to provide bis(2-hydroxyethyl) terephthalate of excellent purity and quality while preventing the concentration of acetate compounds when ethylene glycol is reused in a continuous glycolysis process. [Means for solving the problem]

[0012] Solution to the problem According to one aspect of the present invention, there is provided a process for preparing recycled bis(2-hydroxyethyl) terephthalate, the process comprising: (a) depolymerizing waste polyester by glycolysis to obtain a crude bis(2-hydroxyethyl) terephthalate solution; (b) distilling the crude bis(2-hydroxyethyl) terephthalate solution to separate a distillate containing ethylene glycol; (c) removing acetate compounds from the distillate to recover ethylene glycol; and (d) reusing the recovered ethylene glycol for glycolysis.

[0013] In a specific embodiment, the glycolysis in step (a) comprises reacting the waste polyester with ethylene glycol in the presence of an acetate-based catalyst; the separation of the distillate in step (b) is carried out using a distillation column at a pressure of 0.1 Torr to 300 Torr and a temperature of 70°C to 170°C; the removal of acetate-based compounds in step (c) is carried out by side-draw purging of a specific fraction in the distillation column; and in step (d), the resulting recovered ethylene glycol is reused for the glycolysis in step (a), which can be repeated in a continuous process.

[0014] According to another aspect of the present invention, there is provided recycled bis(2-hydroxyethyl) terephthalate obtained by depolymerizing waste polyester, and when measured by high-performance liquid chromatography (HPLC), the peak area fraction of bis(2-hydroxyethyl) terephthalate is 97% or more, and the peak area fraction of acetate ester compounds is 1% or less.

[0015] Advantageous Effects of the Invention In the process for preparing bis(2-hydroxyethyl) terephthalate of the present invention, after glycolysis of waste polyester, ethylene glycol from which acetate compounds have been removed is recovered and reused. Therefore, even if ethylene glycol is reused in a continuous process, acetate compounds are not concentrated, and bis(2-hydroxyethyl) terephthalate with excellent purity and quality can be obtained.

[0016] According to a specific embodiment, in a process for separating low-boiling compounds including ethylene glycol by feeding a crude product solution from glycolysis to a multi-stage distillation column, acetate-based compounds are removed via side-draw purging, which makes it possible to prevent acetate-based ester compounds from concentrating in the finally obtained bis(2-hydroxyethyl) terephthalate.

[0017] The resulting recycled bis(2-hydroxyethyl) terephthalate has excellent purity and color, as well as high crystallinity, and can be used to prepare environmentally friendly, high melting polyester products. [Brief explanation of the drawings]

[0018] BRIEF DESCRIPTION OF THE DRAWINGS [Figure 1] 1 illustrates a distillation column used in a process for preparing bis(2-hydroxyethyl) terephthalate according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] Best Mode for Carrying Out the Invention In this specification, the terms used to describe each component are used to distinguish them from one another and are not intended to limit the scope of the present embodiments. In addition, in this specification, the singular expressions "a," "an," and "the" are to be construed as covering the plural unless the context clearly dictates otherwise.

[0020] In this specification, terms such as first, second, etc. are used to describe various components. However, the components should not be limited by the terms. The terms are used only to distinguish elements from one another.

[0021] As used herein, the term "comprising" is intended to specifically indicate certain features, regions, steps, processes, elements, and / or components, and does not in any way exclude the presence or addition of other features, regions, steps, processes, elements, and / or components, unless specifically stated to the contrary.

[0022] The molecular weights of the compounds or polymers described herein, for example, number average molecular weights or weight average molecular weights, are relative masses based on carbon-12, as is well known. Although the units are not given, if necessary, they can be understood as molar masses (g / mol) of the same numerical value.

[0023] According to one aspect of the present invention, there is provided a process for preparing bis(2-hydroxyethyl) terephthalate, in which waste polyester is depolymerized and the ethylene glycol, from which acetate-based compounds have been removed, is recovered and reused.

[0024] According to one aspect of the present invention, there is provided a process for preparing bis(2-hydroxyethyl) terephthalate, which involves depolymerizing waste polyester and recovering and reusing ethylene glycol from which acetate-based compounds have been removed. The process for preparing bis(2-hydroxyethyl) terephthalate according to one embodiment includes: (a) depolymerizing waste polyester by glycolysis to obtain a crude bis(2-hydroxyethyl) terephthalate solution; (b) distilling the crude bis(2-hydroxyethyl) terephthalate solution to separate a distillate containing ethylene glycol; (c) removing acetate-based compounds from the distillate to recover ethylene glycol; and (d) reusing the recovered ethylene glycol for glycolysis.

[0025] According to the above process, after glycolysis of waste polyester, ethylene glycol from which acetate compounds have been removed is recovered and reused. Therefore, even if ethylene glycol is reused in a continuous process, the acetate compounds are not concentrated, making it possible to obtain bis(2-hydroxyethyl) terephthalate of excellent purity and quality. According to a specific embodiment, in the process of separating low-boiling compounds including ethylene glycol by supplying the crude product solution of glycolysis to a multi-stage distillation column, the acetate compounds are removed via side-draw purging, making it possible to prevent acetate ester compounds from concentrating in the final bis(2-hydroxyethyl) terephthalate obtained.

[0026] Hereafter, each step of the process for preparing bis(2-hydroxyethyl) terephthalate will be described in detail.

[0027] Depolymerization of waste polyester First, waste polyester is depolymerized by glycolysis to obtain a crude bis(2-hydroxyethyl) terephthalate solution (step (a)).

[0028] The waste polyester raw material can be obtained from post-consumer discarded polyester material products. Specifically, the waste polyester can be obtained by pre-processing various post-consumer discarded polyester materials (e.g., polyethylene terephthalate (PET) materials) including waste products such as beverage bottles, fabrics, films, cases, boxes, partitions, shelves, protective panels, packaging materials, building materials, and interior and exterior materials.

[0029] The pre-treatment can be carried out by removing other plastics, metals, and other foreign materials mixed in the waste, washing it, and then crushing it through a crusher. As a result of the pre-treatment, the waste polyester raw material can be in a flake form. In addition, the waste polyester raw material can have a fibrous microstructure.

[0030] The pretreated waste polyester is then subjected to a depolymerization process. The depolymerization process may include, for example, a glycolysis reaction. As is well known, a glycolysis reaction is a chemical reaction in which a polymer chain is cleaved by a glycol such as ethylene glycol. The total weight of the glycols added may be 1, 2, or 3 times or more the weight of the waste polyester resin, and may be less than 7, 5, or 4 times the weight of the waste polyester resin. For example, the weight of the glycols added may be 1 to 7 times, specifically 2 to 5 times, and more specifically 3 to 4 times the weight of the waste polyester resin.

[0031] A catalyst can be used in the glycolysis reaction. The catalyst can be a metal catalyst, such as a metal salt catalyst or a metallic organic catalyst. Specifically, the catalyst can be a metal acetate, carbonate, oxide, or hydroxide, and the metal can be an alkali metal, alkaline earth metal, or transition metal. Specific examples of the catalyst include a metal acetate, an anhydride, or a hydride thereof. More specifically, the catalyst can be at least one selected from the group consisting of zinc acetate, sodium acetate, cobalt acetate, and manganese acetate, or a hydrate or anhydrous form thereof. Additionally, the weight of the catalyst added can be 0.01 parts by weight or more, 0.1 parts by weight or more, 0.2 parts by weight or more, or 0.3 parts by weight or more, and 5 parts by weight or less, 1 part by weight or less, 0.7 parts by weight or less, 0.5 parts by weight or less, or 0.4 parts by weight or less, based on 100 parts by weight of the waste polyester resin. For example, the weight of the catalyst added may be 0.1 to 1 part by weight, specifically 0.2 to 0.7 parts by weight, based on 100 parts by weight of the waste polyester resin. More specifically, the catalyst may be used in an amount of 0.2 to 0.4 parts by weight, based on 100 parts by weight of the waste polyester.

[0032] As a specific example, the glycolysis in step (a) may involve reacting the waste polyester with ethylene glycol in the presence of an acetate-based catalyst.

[0033] The depolymerization may include, for example, a multi-stage depolymerization reaction at low temperature. As a specific example, the depolymerization may include subjecting waste polyester to depolymerization via a first glycolysis reaction at a high temperature (180 to 200°C), and subjecting the product to depolymerization via a second glycolysis reaction at a low temperature (150 to 170°C).

[0034] The depolymerization in step (a) can be carried out in a continuous process. Specific examples of the continuous process include: (1) feeding waste polyester raw materials to a co-extruder to obtain a co-extrudate; (2) feeding the co-extrudate to an agitator shaft reactor to depolymerize it to obtain a first reactant; (3) feeding the first reactant to a first continuous reactor to depolymerize it to obtain a second reactant; and (4) feeding the second reactant to a second continuous reactor to depolymerize it to obtain a third reactant, and ethylene glycol can be added to at least one of steps (1) to (4).

[0035] In step (1) of the continuous process, molecular weight reduction of the waste polyester raw material is achieved physically and / or chemically via a co-extruder. First, a first glycol-based compound can be continuously fed into the co-extruder. The first glycol-based compound is not particularly limited. Specifically, it can be at least one selected from the group consisting of ethylene glycol (i.e., monoethylene glycol), propylene glycol, and diethylene glycol. The amount of the first glycol-based compound fed can be 0.01 to 100 parts by weight based on 100 parts by weight of the waste polyester raw material. Meanwhile, co-extrusion can be carried out at 170°C to 290°C. The co-extruder can be any suitable type as long as it is designed to co-extrude the waste polyester raw material. Specifically, the co-extruder can be a conventional single-screw co-extruder or a multi-screw (e.g., twin-screw) co-extruder. For example, the co-extruded product can have a weight average molecular weight of 3,000 to 36,000. Because the coextrudate obtained via step (1) has a relatively low molecular weight as described above, it is possible to reduce the time spent in the depolymerization procedure in steps (2) to (4) while minimizing the formation of by-products (e.g., DEG and DEG esters).

[0036] In step (2) of the continuous process, a glycolysis reaction, in which polymer chains present in the coextrudate are cleaved by the first glycol-based compound, can be carried out within a short period of time. If the first glycol-based compound is not fed to the coextruder in step (1), it can be continuously fed to the agitation shaft reactor. To promote the depolymerization reaction, a catalyst can be further added to the agitation shaft reactor. As exemplified above, the catalyst can be a catalyst containing a metal acetate, its anhydride, or its hydrate. The amount of catalyst fed to the agitation shaft reactor can be 0.01 to 5 parts by weight based on 100 parts by weight of the waste polyester raw material. The depolymerization of the coextrudate can be carried out at 180°C to 210°C for 20 to 50 minutes. The agitation shaft reactor is not particularly limited, as long as it is designed to mix the coextrudate, the first glycol-based compound, and the catalyst. Specifically, the agitation shaft reactor may include at least one selected from the group consisting of a kneader, a paddle mixer, a plow shear mixer, a screw mixer, and a ribbon blender. More specifically, it may be a kneader or a paddle mixer.

[0037] In step (3) of the continuous process, a second glycol-based compound can be continuously supplied to the first continuous reactor. As a result, a glycolysis reaction can be carried out in which polymer chains present in the first reactant are cleaved by the second glycol-based compound. The second glycol-based compound is not particularly limited. Specifically, it can be at least one selected from the group consisting of ethylene glycol (monoethylene glycol), propylene glycol, and diethylene glycol. The amount of the second glycol-based compound supplied to the first continuous reactor can be 50 to 340 parts by weight based on 100 parts by weight of the first reactant. The depolymerization of the first reactant can be carried out at 170°C to 195°C for 30 to 50 minutes. The depolymerization of the first reactant can be carried out in the presence of a catalyst continuously supplied to the agitation shaft reactor in step (2) or a catalyst directly supplied to the first continuous reactor. As exemplified above, the catalyst can be a catalyst including a metal acetate, its anhydride, or its hydrate. On the other hand, the first continuous reactor is not particularly limited as long as it is a continuous flow tank reactor designed to carry out depolymerization.

[0038] In step (4) of the continuous process, a glycolysis reaction can be carried out in which polymer chains present in the second reactant are cleaved by the unreacted second glycol-based compound discharged from the first continuous reactor in step (1) and supplied to the second continuous reactor. A third glycol-based compound can be additionally supplied to the second continuous reactor to prevent a decrease in depolymerization efficiency when the unreacted second glycol-based compound is not supplied to the second continuous reactor to an extent sufficient for depolymerization to be achieved or when the purity of the unreacted second glycol-based compound decreases. The third glycol-based compound is not particularly limited. Specifically, it can be at least one selected from the group consisting of ethylene glycol (monoethylene glycol), propylene glycol, and diethylene glycol. The amount of the third glycol-based compound supplied to the second continuous reactor can be 50 to 150 parts by weight based on 100 parts by weight of the second reactant. Depolymerization of the second reactant can be carried out at 140°C to 170°C for 30 to 50 minutes. The depolymerization of the second reactant can be carried out in the presence of a catalyst continuously fed to the agitation shaft reactor in step (2), a catalyst continuously fed to the first continuous reactor, or a catalyst directly fed to the second continuous reactor. As exemplified above, the catalyst can be a catalyst containing a metal acetate, its anhydride, or its hydrate. On the other hand, the second continuous reactor is not particularly limited as long as it is a continuous flow tank reactor designed to carry out depolymerization.

[0039] Removal of acetate compounds and recovery of ethylene glycol via distillation The crude bis(2-hydroxyethyl) terephthalate solution obtained above is distilled to separate low-boiling distillates containing ethylene glycol (step (b)).

[0040] The distillation of step (b) can be carried out, for example, by vacuum distillation, using a distillation column for this purpose.

[0041] Figure 1 illustrates a distillation column used in a process for preparing bis(2-hydroxyethyl) terephthalate according to an embodiment of the present invention. Referring to Figure 1, the distillation column may have multiple stages, for example, 3 or more, 5 or more, 6 or more, or 7 or more, and may have 20 or less, 15 or less, or 10 or more stages. Specifically, it may have 7 to 15 stages.

[0042] The temperature at which the distillate is separated in step (b) may be, for example, 60°C or higher, 70°C or higher, 80°C or higher, 90°C or higher, or 100°C or higher, and may be 200°C or lower, 180°C or lower, 170°C or lower, 150°C or lower, or 130°C or lower. Additionally, the pressure at which the distillate is separated may be, for example, 500 Torr or lower, 400 Torr or lower, 300 Torr or lower, or 270 Torr or lower, and may be 0.1 Torr or higher, 1 Torr or higher, 10 Torr or higher, 100 Torr or higher, or 200 Torr or higher. Specifically, the distillate may be separated in step (b) using a distillation column at a pressure of 0.1 Torr to 300 Torr and a temperature of 70°C to 170°C.

[0043] The acetate-based compound in step (c) may have one glycol group in the molecule and may not contain an aromatic group in the molecule.

[0044] Specifically, the acetate-based compound in step (c) may include 2-hydroxyethyl acetate.

[0045] As shown in Reaction Scheme 1 below, acetic acid (AA), derived from metal acetates that are primarily used as catalysts, reacts with ethylene glycol (EG) to produce acetate compounds such as 2-hydroxyethyl acetate (HA) and water (HO). [Reaction Scheme 1] [ka]

[0046] The acetate compounds are then removed from the distillate to recover ethylene glycol (step (c)).

[0047] The removal of acetate compounds in step (c) can be carried out by side-draw purging in a distillation column.

[0048] Referring to FIG. 1, the distillate, which mainly contains low-boiling compounds, specifically ethylene glycol (EG) and other trace compounds, separated from the crude BHET solution, may contain acetate-based compounds (e.g., HA) at high concentrations at certain stages while passing through several stages of the distillation column. Therefore, acetate-based compounds such as 2-hydroxyethyl acetate (HA) may be discharged from the system via side-draw purging. As a result, ethylene glycol (EG) from which acetate-based compounds have been removed is recovered, effectively preventing the accumulation of acetate-based compounds during ethylene glycol circulation. For this purpose, the distillation column may be equipped with a pipe for discharging acetate-based compounds from the system at a specific stage and a nozzle for controlling the discharge rate.

[0049] As a specific example, the inside of the distillation column may be divided into a total of 5 to 20 stages, and side-draw purging may be performed on the second to tenth stages from the top. As another specific example, the inside of the distillation column may be divided into a total of 10 to 15 stages, and side-draw purging may be performed on the second to eighth stages from the top. In addition, the internal temperature of the stage where side-draw purging is performed may be 70°C to 170°C, more specifically 90°C to 150°C.

[0050] The side-draw purging fraction may be, for example, 3 wt % or less, 1 wt % or less, 0.7 wt % or less, 0.5 wt % or less, or 0.3 wt % or less, and may be 0.001 wt % or more, 0.01 wt % or more, 0.05 wt % or more, 0.1 wt % or more, 0.2 wt % or more, 0.3 wt % or more, or 0.4 wt % or more, based on the total weight of the crude bis(2-hydroxyethyl) terephthalate solution. As a specific example, the side-draw purging may be carried out at a fraction of 0.05 wt % to 1 wt % based on the total weight of the crude bis(2-hydroxyethyl) terephthalate solution.

[0051] Generally, ethylene glycol recovered from the depolymerization of waste polyester via glycolysis, i.e., recycled ethylene glycol (REG), may contain by-products formed by side reactions with reagents or solvents used in various chemical steps. Therefore, such recycled ethylene glycol must be understood as something separate from virgin ethylene glycol. For this reason, recycled ethylene glycol can be considered a composition containing two or more components, i.e., an ethylene glycol composition. Recycled ethylene glycol recovered by conventional depolymerization methods may not be highly pure because it contains organic and inorganic impurities in addition to ethylene glycol as the main component.

[0052] However, the ethylene glycol recovered in step (d) of the process of the present invention may have a high purity, particularly a low content of acetate-based compounds. For example, the recovered ethylene glycol, i.e., recycled ethylene glycol, may have a purity of 95% or more, 97% or more, 98% or more, 99% or more, or 99.5% or more. The purity or content of specific components in the recycled ethylene glycol may be determined by quantifying the peak area fraction obtained by gas chromatography (GC) analysis using a calibration curve of a standard substance.

[0053] When analyzed by gas chromatography (GC), the ethylene glycol recovered in step (d), i.e., recycled ethylene glycol, may have, for example, an acetate content of 3 wt. % or less, or 2 wt. % or less. Specifically, the recycled ethylene glycol may have an acetate content of 1 wt. % or less. More specifically, the acetate content may be 0.7 wt. %, 0.5 wt. %, 0.3 wt. %, or 0.1 wt. %.

[0054] For example, the content of monoethylene glycol (MEG) in the recycled ethylene glycol recovered in step (d) may be 95% by weight or more, 97% by weight or more, 98% by weight or more, 99% by weight or more, or 99.5% by weight or more. Additionally, the content of diethylene glycol (DEG) in the recycled ethylene glycol may be, for example, 1% by weight or less, 0.5% by weight or less, or 0.3% by weight or less. Additionally, the content of triethylene glycol (TEG) in the recycled ethylene glycol may be, for example, 0.3% by weight or less, 0.1% by weight or less, or 0.05% by weight or less.

[0055] The ethylene glycol thus recovered, i.e., recycled ethylene glycol, can be reused in the glycolysis in step (a) (step (d)). The number of times that ethylene glycol is reused can be, for example, 2 or more, 3 or more, 4 or more, or 5 or more times, and can be 100 or less, 50 or less, 30 or less, 20 or less, or 10 or less times. Specific examples include 2 to 20 times or 3 to 10 times.

[0056] By recovering excess ethylene glycol from the glycolysis reaction product, the recycled ethylene glycol can be recycled in continuous reactions for repeated use several times, preventing the accumulation of impurities, particularly acetate compounds, and thus maintaining purity and quality.

[0057] On the other hand, the recycled bis(2-hydroxyethyl) terephthalate obtained by the first separation of the distillate containing ethylene glycol from the crude bis(2-hydroxyethyl) terephthalate solution can be subjected to additional purification.

[0058] As a specific example, recycled bis(2-hydroxyethyl) terephthalate can be prepared by at least one thin film distillation under reduced pressure and further performing dissolution in water and adsorption-crystallization.

[0059] Thin-film distillation is a distillation method that separates a mixture into a thin film to increase its surface area in contact with a heat source. Specifically, the mixture fed into the evaporator of a thin-film evaporator is formed into a thin film on the inner wall of the thin-film evaporator by a wiper rotor. Distillation is then performed under appropriate temperature conditions by heating. Alternatively, a condenser for recovering evaporated materials may be provided inside the thin-film evaporator. Thin-film evaporation can be performed by short-path evaporation. Such short-path and thin-film evaporation has a short residence time and allows vacuum distillation using a high vacuum, minimizing the thermal changes of reactants. Another advantage is that reducing the pressure inside the thin-film evaporator reduces the vapor pressure of the material, allowing evaporation to occur at a temperature lower than its original boiling point. Specifically, reactants from the previous step are fed into a short-path and thin-film evaporator, and the wiper for forming the thin film is rotated at 300 rpm or more. As a result, vaporized and non-vaporized materials can be separated from each other. The internal thin film temperature of the upper thin film evaporator during thin film evaporation may be, for example, 150° C. to 250° C., 190° C. to 250° C., or 180° C. to 220° C. Additionally, the internal pressure of the upper thin film evaporator during thin film evaporation may be, for example, 0.005 Torr to 5.0 Torr, 0.05 Torr to 5.0 Torr, 0.05 Torr to 1.5 Torr, or 0.05 Torr to 1 Torr.

[0060] Adsorption-crystallization can be carried out, for example, by adding an adsorbent to water as a solvent, filtering, and then crystallizing. Various solvents can be used for adsorption-crystallization, but a solvent capable of dissolving bis(2-hydroxyethyl) terephthalate is preferably used as the solvent. For example, to obtain the final reactant, water is added as a solvent to recycled bis(2-hydroxyethyl) terephthalate, which is dissolved by heating, and an adsorbent is added thereto. The solution obtained by filtration is then subjected to cooling-crystallization and final filtration. As a result, high-purity bis(2-hydroxyethyl) terephthalate can be obtained. Water can be added in an amount of 100 to 500 parts by weight, specifically 200 to 400 parts by weight, and more specifically 300 to 350 parts by weight, based on 100 parts by weight of recycled bis(2-hydroxyethyl) terephthalate. In addition, the dissolution temperature may be 50°C to 95°C, specifically 60°C to 85°C, and more specifically 70°C to 75°C. The adsorbent added may function to adsorb and remove other foreign matter. It may be added in an amount of 0.1 to 3 parts by weight based on 100 parts by weight of recycled bis(2-hydroxyethyl) terephthalate. The type and form of the adsorbent are not particularly limited. For example, activated carbon may be used.

[0061] Recycled bis(2-hydroxyethyl) terephthalate The present invention provides recycled bis(2-hydroxyethyl) terephthalate obtained by the process described above.

[0062] Bis(2-hydroxyethyl) terephthalate (BHET) is an ester of two ethylene glycols and one terephthalic acid. For example, it is a compound formed as an intermediate in the preparation of polyesters such as polyethylene terephthalate (PET) via the polymerization of ethylene glycol with terephthalic acid or its esters.

[0063] On the other hand, recycled bis(2-hydroxyethyl) terephthalate (referred to as recycled BHET) obtained by the depolymerization of waste polyester described above may contain by-products formed by side reactions with reagents or solvents used in various chemical steps during the depolymerization of waste polyester. Therefore, recycled bis(2-hydroxyethyl) terephthalate obtained by the depolymerization of waste polyester described above must be understood as something distinguishable from pure BHET compounds. For this reason, recycled BHET can also be considered a composition containing two or more components, i.e., a BHET composition. Such recycled BHET can be used as a raw material for the polymerization of polyester resins. BHET recycled by conventional depolymerization processes contains organic and inorganic impurities in addition to the main component, BHET, and therefore has a low purity.

[0064] However, the recycled bis(2-hydroxyethyl) terephthalate according to the present invention has excellent purity and quality, despite being obtained by depolymerization of waste polyester.

[0065] The purity of recycled BHET can be measured using liquid chromatography, etc. Specifically, the purity of recycled BHET can be calculated by measuring the fraction (%) of the peak area of ​​BHET relative to the total peak area in a spectrum obtained using high-performance liquid chromatography (HPLC).

[0066] For example, the purity of the recycled BHET can be 95% or more, 97% or more, 98% or more, 99% or more, or 99.5% or more, specifically 95% to 100% or 97% to 100%.

[0067] The recycled bis(2-hydroxyethyl) terephthalate according to some embodiments can have a peak area fraction of bis(2-hydroxyethyl) terephthalate of 97% or greater, more specifically 98% or greater, 99% or greater, or 99.5% or greater, as measured by high performance liquid chromatography (HPLC).

[0068] On the other hand, recycled bis(2-hydroxyethyl) terephthalate may contain compounds other than BHET, specifically, BHET analogs, BHET oligomers (e.g., dimers, trimers), esters (e.g., DEG esters), and acetate-based compounds (such as HAET).

[0069] As shown in Reaction Scheme 2 below, 2-hydroxyethyl acetate (HA) can be converted to ester compounds such as 2-hydroxyethyl(2-acetoxyethyl)terephthalate (HAET) and ethylene glycol (EG) via a transesterification reaction with bis(2-hydroxyethyl)terephthalate. [Reaction Scheme 2] [ka]

[0070] The recycled bis(2-hydroxyethyl) terephthalate has a peak area fraction of acetate-based ester compounds of 1% or less when measured by high-performance liquid chromatography (HPLC). Specifically, the recycled bis(2-hydroxyethyl) terephthalate may have a peak area fraction of acetate-based ester compounds of 0.7% or less, 0.5% or less, 0.3% or less, or 0.1% or less when measured by high-performance liquid chromatography (HPLC). The acetate-based ester compound may be a compound containing an aromatic group, such as a 2-hydroxyethyl acetate ester compound or terephthalate. Specifically, it may include at least one selected from the group consisting of 2-hydroxyethyl (2-acetoxyethyl) terephthalate (HAET), 2-acetoxyethyl [2-(2-hydroxyethoxy)ethyl] terephthalate, and 2-hydroxyethyl [2-[2-(2-hydroxyethoxy)ethoxy]ethyl] terephthalate. As a more specific example, the acetate-based ester compound may include 2-hydroxyethyl (2-acetoxyethyl) terephthalate (HAET).

[0071] According to another embodiment, the recycled bis(2-hydroxyethyl) terephthalate may have a peak area fraction of diethylene glycol (DEG) esters of 2% or less, 1.5% or less, 1% or less, or 0.5% or less, when measured by high-performance liquid chromatography (HPLC). Specifically, the recycled bis(2-hydroxyethyl) terephthalate may have a peak area fraction of diethylene glycol (DEG) esters of 0.5% or less, when measured by high-performance liquid chromatography (HPLC). The diethylene glycol ester compounds may include 2-hydroxyethyl [2-(2-hydroxyethoxy)ethyl] terephthalate and bis[2-(2-hydroxyethoxy)ethyl] benzene-1,4-dicarboxylate.

[0072] According to another embodiment, the recycled bis(2-hydroxyethyl) terephthalate can have a peak area fraction of monohydroxyethyl terephthalate (MHET) of 2% or less, 1.5% or less, 1% or less, or 0.5% or less, as measured by high performance liquid chromatography (HPLC). As a specific example, the recycled bis(2-hydroxyethyl) terephthalate can have a peak area fraction of monohydroxyethyl terephthalate (MHET) of 1% or less, as measured by high performance liquid chromatography (HPLC).

[0073] Recycled bis(2-hydroxyethyl) terephthalate has excellent crystallinity, resulting in a high melting point, and also has excellent qualities, such as color.

[0074] For example, recycled bis(2-hydroxyethyl) terephthalate may have a melting point (mp) of 107°C to 117°C, or more specifically, 107°C to 115°C.

[0075] Additionally, the recycled bis(2-hydroxyethyl) terephthalate may have a yellow index (YID) of 5.0 or less when measured by a colorimeter in a 25% by weight solution. Specifically, the yellow index may be 5.0 or less, 4.5 or less, 4.0 or less, 3.5 or less, 3.0 or less, 2.5 or less, 2.0 or less, 1.5 or less, or 1.0 or less.

[0076] As a specific example, recycled bis(2-hydroxyethyl) terephthalate may have a melting point of 107°C to 115°C and a yellow index (YID) of 5.0 or less when measured in a solution dissolved in dimethylformamide at a concentration of 25% by weight.

[0077] Mode of Invention Hereinafter, preferred embodiments are presented for understanding the present invention. However, the following examples are provided only to help the reader understand the present invention, and are not intended to limit the scope of the present invention. [Example]

[0078] Example 1 Step (1) Preparation of crude BHET solution by glycolysis Shredded PET waste and monoethylene glycol (MEG) were fed into a single-screw coextruder at a feed rate of 15.5 kg / hr, respectively, and coextrusion (to reduce molecular weight) was performed at 180 °C and 150 rpm to obtain a coextrudate. Following this coextrusion process, depolymerization was continuously carried out via a kneader, a first continuous reactor (CSTR-1), and a second continuous reactor (CSTR-2). Specifically, the coextrudate (feed rate: 31.0 kg / hr) and zinc acetate anhydride (feed rate: 0.065 kg / hr) as a catalyst were fed into the kneader, and a first depolymerization reaction was carried out at 195 °C for 35 minutes to obtain the first reactant. The first reactant thus obtained and additional monoethylene glycol (MEG) (feed rate: 15.5 kg / hr) were supplied to a first continuous reactor (CSTR-1), where a second depolymerization reaction was carried out at 190°C for 40 minutes to obtain a second reactant. The second reactant thus obtained and additional monoethylene glycol (MEG) (feed rate: 31.0 kg / hr) were supplied to a second continuous reactor (CSTR-2), where a third depolymerization reaction was carried out at 150°C for 40 minutes to obtain a third reactant, i.e., a crude bis(2-hydroxyethyl) terephthalate solution.

[0079] Step (2) Recovery and reuse of EG via distillation As shown in Figure 1, a total of 15 distillation columns were prepared for the purification of low-boiling substances, and side-draw discharge pipes were installed on the second to eighth columns. The crude bis(2-hydroxyethyl) terephthalate solution prepared in Step 1 was continuously fed into the distillation column for purification. The internal pressure between the bottom and top of the distillation column was set to 200 to 270 Torr, and the distillation temperature between the bottom and top of the column was set to 70 to 170°C. First, low-boiling substances distilled from the crude bis(2-hydroxyethyl) terephthalate solution were elevated from plates 1 to 15 of the distillation column. Side-draw purging was performed at a fraction of 0.4 wt% on plates 2 to 8 (internal temperature: 90-150 °C), where the 2-hydroxyethyl acetate (HA) concentration was highest in the section containing a mixture of monoethylene glycol (MEG) and 2-hydroxyethyl acetate (HA). This prevented 2-hydroxyethyl acetate (HA) from accumulating in the distillation column. Subsequently, glycol containing residual ethylene glycol was separated and sent to the top, while high-boiling substances containing bis(2-hydroxyethyl) terephthalate were recovered at the bottom of the distillation column. Through these distillations, recycled ethylene glycol (REG-1) with a 2-hydroxyethyl acetate (HA) content of 0.6 wt% was recovered. The recycled ethylene glycol (REG-1) thus recovered was reused in Step 1 of the continuous glycolysis process (total number of repeated uses: 10 times).

[0080] Step (3) Obtain recycled BHET after further purification The high-boiling substances, including BHET, recovered in Step 2 were subjected to thin-film evaporation using a thin-film evaporator (VTA VKL70-4S) at 220°C and 0.08 Torr to obtain a product from which dimers and higher oligomers had been removed. Subsequently, for adsorption-crystallization, the product and distilled water were charged into a glass reactor and dissolved at 70°C. Activated carbon was then added, followed by stirring for 30 minutes and filtration. The filtrate was cooled to room temperature, filtered, and dried in a vacuum oven to obtain recycled bis(2-hydroxyethyl) terephthalate.

[0081] Example 2 Recycled bis(2-hydroxyethyl) terephthalate was prepared through the same procedures as steps (1) to (3) in Example 1, except that the side-draw purging fraction was adjusted to 0.35 wt % and recycled ethylene glycol (REG-2) was recovered and reused in step (1) of the continuous glycolysis process (a total of 10 times).

[0082] Example 3 Recycled bis(2-hydroxyethyl) terephthalate was prepared through the same procedure as steps (1) to (3) in Example 1, except that the side-draw purging fraction was adjusted to 0.28 wt % and recycled ethylene glycol (REG-3) was recovered and reused in step (1) of the continuous glycolysis process (a total of 10 times).

[0083] Example 4 Recycled bis(2-hydroxyethyl) terephthalate was prepared through the same procedures as steps (1) to (3) in Example 1, except that the side-draw purging fraction was adjusted to 0.12 wt % and recycled ethylene glycol (REG-4) was recovered and reused in step (1) of the continuous glycolysis process (a total of 10 times).

[0084] Example 5 Recycled bis(2-hydroxyethyl) terephthalate was prepared through the same procedure as steps (1) to (3) in Example 1, except that the side-draw purging fraction was adjusted to 0.06 wt % and recycled ethylene glycol (REG-5) was recovered and reused in step (1) of the continuous glycolysis process (a total of 10 times).

[0085] Comparative Example 1 Recycled bis(2-hydroxyethyl) terephthalate was prepared via the same procedure as steps (1) to (3) in Example 1, except that side-draw purging was not performed in step (2) and recycled ethylene glycol (REG-6) was recovered and reused in step (1) of the continuous glycolysis process (a total of three times).

[0086] Comparative Example 2 Recycled bis(2-hydroxyethyl) terephthalate was prepared via the same procedure as steps (1) to (3) in Example 1, except that side-draw purging was not performed in step (2) and recycled ethylene glycol (REG-7) was recovered and reused in step (1) of the continuous glycolysis process (a total of seven times).

[0087] Comparative Example 3 Recycled bis(2-hydroxyethyl) terephthalate was prepared via the same procedure as steps (1) to (3) in Example 1, except that side-draw purging was not performed in step (2) and recycled ethylene glycol (REG-8) was recovered and reused in step (1) of the continuous glycolysis process (a total of 10 times).

[0088] Test Example The recycled ethylene glycol or recycled bis(2-hydroxyethyl) terephthalate of the Examples and Comparative Examples were each tested as follows.

[0089] (1) Composition of recycled ethylene glycol - Gas chromatography (GC) Approximately 0.1 g of each recycled ethylene glycol (REG) was diluted with 10 ml of chloroform, filtered (0.45 μm), and analyzed by gas chromatography (Model: Agilent 7890B, Column: DB-624 (30 m × 0.25 mm × 1.4 μm), Oven Temperature: 60 °C (2 min) - 10 °C / min - 260 °C (5 min), Injector Temperature: 250 °C, Detector Temperature: 250 °C, Flow Rate: 1.5 ml / min (N), Split Ratio: 1 / 50). The peak area fractions obtained by gas chromatography (GC) analysis were quantified using calibration curves of standard materials to obtain the respective contents (wt%) of monoethylene glycol (MEG), diethylene glycol (DEG), triethylene glycol (TEG), and 2-hydroxyethyl acetate (HA).

[0090] (2) Composition of recycled BHET - HPLC Approximately 0.01 g of each recycled bis(2-hydroxyethyl)terephthalate before and after purification was diluted with approximately 20 ml of methanol and analyzed by high-performance liquid chromatography (HPLC) (model: Waters e2695, column: C18 (4.6 × 250 mm), 5 μm, UV detector: 242 nm, injection volume: 10 μl, eluent (gradient) A: HO + HPO, B: acetonitrile). The peak area fractions (%) of the following components were then obtained from the total HPLC peak area. The composition of the recycled BHET was determined for crude BHET and purified BHET, respectively. -MHET: Monohydroxyethyl terephthalate -BHET: Bis(2-hydroxyethyl) terephthalate -DEG-ester-1: 2-hydroxyethyl [2-(2-hydroxyethoxy)ethyl] terephthalate -DEG-ester-2: bis[2-(2-hydroxyethoxy)ethyl]benzene-1,4-dicarboxylate -DEG-ester-3: 1,4-Benzenedicarboxylic acid, 1,1'-(1,2-ethanediyl)-4-[2-(hydroxyethoxy)ethyl], 4'-(2-hydroxyethyl) ester -DEG-ester-4: 1,4-benzenedicarboxylic acid, 1,1'-(1,2-ethanediyl)-4,4'-bis((2-hydroxyethoxy)ethyl) ester -HA-ester: 2-hydroxyethyl (2-acetoxyethyl) terephthalate -Dimer: BHET dimer -Trimmer: BHET trimmer -Tetramer: BHET tetramer

[0091] (3) Yellow Index (YID) The purified bis(2-hydroxyethyl) terephthalate was dissolved in dimethylformamide at room temperature at a concentration of 25% by weight, and the yellow index was measured after 30 minutes. Transmission data were obtained using a Hunterlab Color Flex EZ with illuminant D65 at a 2° observer angle. The yellow index (YID) value was calculated using the color analyzer software.

[0092] (4) Melting point (mp) - Differential scanning calorimetry (DSC) The melting points (mp) were measured using a differential scanning calorimeter (DSC, TA Instruments Q20) while heating from 30°C to 280°C at a rate of 10°C / min.

[0093] (5) Moisture analysis The moisture content (wt%) of the recycled ethylene glycol was measured using a Karl Fischer moisture analyzer (Metro Toledo, Model: V20).

[0094] The configurations of the Examples and Comparative Examples and the compositions of the recycled ethylene glycol and recycled BHET analyzed through the Test Examples are summarized in Tables 1 and 2 below.

[0095] [Table 1]

[0096] [Table 2]

[0097] As can be seen from Tables 1 and 2 above, the accumulation of 2-hydroxyethyl acetate (HA) was prevented through side-draw purging during vacuum distillation in Examples 1 to 5. As a result, the concentration of 2-hydroxyethyl acetate (HA) in the ethylene glycol reused 10 times was very low, at 0.6 wt% or less. As a result, the concentration of 2-hydroxyethyl acetate ester compounds (HA esters) in the final purified bis(2-hydroxyethyl) terephthalate was less than 0.5 wt%, resulting in good quality.

[0098] In contrast, in Comparative Examples 1 to 3, in which side-draw purging was not performed during vacuum distillation, 2-hydroxyethyl acetate (HA) continued to accumulate, resulting in a high concentration. Specifically, in Comparative Example 3, in which ethylene glycol was reused 10 times, the concentration reached 2.4 wt%. As a result, the concentration of 2-hydroxyethyl acetate ester compounds (HA esters) in the final purified bis(2-hydroxyethyl) terephthalate was 1.68 wt%, resulting in poor quality.

Claims

1. (a) depolymerizing waste polyester by glycolysis to obtain a crude bis(2-hydroxyethyl) terephthalate solution; (b) distilling the crude bis(2-hydroxyethyl) terephthalate solution to separate a distillate comprising ethylene glycol; (c) removing acetate compounds from the distillate to recover the ethylene glycol; and (d) reusing the recovered ethylene glycol in the glycolysis; A process for preparing recycled bis(2-hydroxyethyl) terephthalate, comprising:

2. 2. The process for preparing recycled bis(2-hydroxyethyl) terephthalate according to claim 1, wherein the glycolysis in step (a) comprises reacting waste polyester with ethylene glycol in the presence of an acetate-based catalyst.

3. 2. The process for preparing recycled bis(2-hydroxyethyl) terephthalate according to claim 1, wherein the acetate-based compound comprises 2-hydroxyethyl acetate.

4. 10. The process for preparing recycled bis(2-hydroxyethyl) terephthalate according to claim 1, wherein the depolymerization in step (a) is carried out in a continuous process.

5. The continuous process comprises: (1) feeding waste polyester raw material into a co-extruder to obtain a co-extrudate; (2) feeding the coextrudate into an agitation shaft reactor and depolymerizing it to obtain a first reactant; (3) feeding the first reactant into a first continuous reactor and depolymerizing it to obtain a second reactant; and (4) feeding the second reactant to a second continuous reactor and depolymerizing it to obtain a third reactant; Including, 5. The process for preparing recycled bis(2-hydroxyethyl) terephthalate according to claim 4, wherein ethylene glycol is added to at least one of steps (1) to (4).

6. 2. The process for preparing recycled bis(2-hydroxyethyl) terephthalate according to claim 1, wherein the separation of the distillate in step (b) is carried out using a distillation column at a pressure of 0.1 Torr to 300 Torr and a temperature of 70°C to 170°C.

7. 7. The process for preparing recycled bis(2-hydroxyethyl) terephthalate according to claim 6, wherein the removal of the acetate compounds in step (c) is carried out by side-draw purging in the distillation column.

8. 8. The process for preparing recycled bis(2-hydroxyethyl) terephthalate according to claim 7, wherein the side-draw purging is carried out at a fraction of 0.05 wt% to 1 wt%, based on the total weight of the crude bis(2-hydroxyethyl) terephthalate solution.

9. 2. The process for preparing recycled bis(2-hydroxyethyl) terephthalate according to claim 1, wherein the content of acetate compounds in the ethylene glycol recovered in step (d) is 1% by weight or less.

10. 2. The process for preparing recycled bis(2-hydroxyethyl) terephthalate according to claim 1, wherein the recycled bis(2-hydroxyethyl) terephthalate is prepared by further performing at least one thin film distillation under reduced pressure, dissolving in water, and adsorption-crystallization.

11. A recycled bis(2-hydroxyethyl) terephthalate obtained by depolymerization of waste polyester, which has a peak area fraction of bis(2-hydroxyethyl) terephthalate of 97% or more and a peak area fraction of acetate ester compounds of 1% or less, when measured by high performance liquid chromatography (HPLC).

12. The recycled bis(2-hydroxyethyl) terephthalate of claim 11, wherein the acetate-based ester compound comprises 2-hydroxyethyl (2-acetoxyethyl) terephthalate.

13. 12. The recycled bis(2-hydroxyethyl) terephthalate of claim 11, wherein the recycled bis(2-hydroxyethyl) terephthalate has a peak area fraction of diethylene glycol esters totaling 0.5% or less, as measured by high performance liquid chromatography (HPLC).

14. 14. The recycled bis(2-hydroxyethyl) terephthalate of claim 13, wherein the diethylene glycol ester compounds include 2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl]terephthalate and bis[2-(2-hydroxyethoxy)ethyl]benzene-1,4-dicarboxylate.

15. 12. The recycled bis(2-hydroxyethyl) terephthalate of claim 11, wherein the recycled bis(2-hydroxyethyl) terephthalate has a melting point of 107°C to 115°C and a yellowness index (YID) of 5.0 or less, as measured in a 25% by weight solution in dimethylformamide.

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