Method for producing bis-2-hydroxyethyl terephthalate via continuous depolymerization

Abstract

The present invention relates to a method for producing bis-2-hydroxyethyl terephthalate, comprising the steps of: (1) adding waste polyester feedstock to a co-extruder to obtain a co-extrudate; (2) adding the co-extrudate to a reactor equipped with a stirring shaft to depolymerize the co-extrudate to obtain a first reaction product; (3) adding the first reaction product to a first continuous stirred tank reactor to depolymerize the first reaction product to obtain a second reaction product; and (4) adding the second reaction product to a second continuous stirred tank reactor to depolymerize the second reaction product to obtain a third reaction product.

Description

[Technical Field]

[0001] The present invention relates to a method for efficiently preparing high-purity bis(2-hydroxyethyl) terephthalate (BHET) using waste polyester. [Background technology]

[0002] Among polymers, polyester is used as a material in various fields due to its excellent mechanical strength, heat resistance, transparency, and gas barrier properties. In particular, polyester sheets or plates have good transparency and excellent mechanical strength, and as a result, polyester sheets or plates are widely used for cases, boxes, partitions, shelves, panels, packaging materials, building materials, interior materials, exterior materials, etc.

[0003] As a result, plastic waste, including polyester, is generated at unmanageable levels worldwide each year. Recently, countries around the world have been developing regulations and programs to recycle waste plastic resources, including waste polyester.

[0004] Physical and chemical methods are used to recycle waste polyester, but physical recycling is not widely used because it cannot guarantee purity. Chemical recycling, on the other hand, involves depolymerizing the waste polyester by cleaving its ester bonds. Reactions such as glycolysis, hydrolysis, methanolysis, and aminolysis are used. Glycolysis involves decomposing the waste polyester by adding glycols such as ethylene glycol or diethylene glycol at high temperatures. The resulting reaction product contains primarily bis(2-hydroxyethyl) terephthalate (BHET). After crystallization or purification, the bis(2-hydroxy) terephthalate contained in the reaction product can be used as a raw material for preparing unsaturated polyesters or ester polyols.

[0005] In order to use bis(2-hydroxyethyl) terephthalate as the raw material, it is necessary to increase the purity of bis(2-hydroxyethyl) terephthalate by minimizing the production of by-products such as diethylene glycol esters (DEG esters) during the depolymerization process. For this purpose, a method of carrying out depolymerization using a multi-stage continuous stirred tank reactor (CSTR) is currently being adopted.

[0006] However, this method requires more than twice the time required for depolymerizing waste polyester compared to using a batch reactor, resulting in a problem of reduced process efficiency. Furthermore, this method of depolymerizing waste polyester does not sufficiently suppress the generation of by-products, and there is a limit to obtaining bis(2-hydroxyethyl) terephthalate with the desired purity. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Korean Patent Publication No. 2022-0068991 Summary of the Invention [Problem to be solved by the invention]

[0008] The present inventors have conducted various studies to solve the above-mentioned conventional problems, and as a result, have found that high-purity bis(2-hydroxyethyl) terephthalate can be prepared with high efficiency (improved productivity) when waste polyester is subjected to molecular weight reduction and short-term depolymerization before depolymerization in a continuous stream reactor (CSTR).

[0009] Therefore, an object of the present invention is to provide a method for preparing bis(2-hydroxyethyl) terephthalate by continuous depolymerization of waste polyester, which method makes it possible to improve the purity and production efficiency (productivity) of bis(2-hydroxyethyl) terephthalate. [Means for solving the problem]

[0010] To achieve the above object, the present invention provides a method for preparing bis(2-hydroxyethyl) terephthalate, the method comprising the steps of: (1) feeding waste polyester raw materials into a co-extruder to obtain a co-extrudate; (2) feeding the co-extrudate into an agitated shaft reactor and depolymerizing the co-extrudate to obtain a first reactant; (3) feeding the first reactant into a first continuous reactor and depolymerizing the first reactant to obtain a second reactant; and (4) feeding the second reactant into a second continuous reactor and depolymerizing the second reactant to obtain a third reactant. [Effects of the Invention]

[0011] According to the preparation method of the present invention, waste polyester is subjected to coextrusion and short-term depolymerization in a stirred shaft reactor to reduce the molecular weight, followed by depolymerization in a continuous multi-stage reactor (CSTR). This allows the preparation (production) of bis(2-hydroxyethyl) terephthalate (BHET) in a relatively short time while minimizing the formation of by-products (e.g., DEG and DEG esters) that could be considered impurities. Therefore, the present invention can provide high-purity bis(2-hydroxyethyl) terephthalate (BHET) with high efficiency, and the bis(2-hydroxyethyl) terephthalate (BHET) prepared as described above can be used as a raw material for preparing polyesters having excellent quality and products using the same. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 illustrates an example method for preparing bis(2-hydroxyethyl) terephthalate according to an embodiment of the present invention. [Figure 2] FIG. 1 is a diagram showing methods for preparing bis(2-hydroxyethyl) terephthalate according to Comparative Examples 1 to 3. [Figure 3]FIG. 1 is a diagram showing methods for preparing bis(2-hydroxyethyl) terephthalate according to Comparative Examples 4 to 6. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in detail below. The present invention in this specification is not limited to the disclosure content shown below, and can be modified in various forms as long as the gist of the present invention is not changed.

[0014] As used herein, the term "comprising" is intended to specify certain features, regions, steps, processes, elements, and / or components, and does not exclude the presence or addition of other features, regions, steps, processes, elements, and / or components, unless specifically stated to the contrary.

[0015] Throughout this specification, the terms first, second, etc. are used to distinguish one element from another, but the elements should not be limited by the terms.

[0016] All numerical values ​​and expressions relating to amounts of ingredients, reaction conditions, and the like used in this specification should be understood to be modified by the term "about" unless otherwise specified.

[0017] The number average molecular weight and weight average molecular weight of the compounds (reactants or products) described herein are, as is well known, carbon-12 ( 12 C). Its unit is not stated, but if necessary, it may be understood as the molar mass (g / mol) of the same value.

[0018] For illustrative purposes, the size of individual elements in the accompanying drawings may be exaggerated and may differ from the actual size.

[0019] Method for preparing bis(2-hydroxyethyl) terephthalate The present invention relates to a method for preparing bis(2-hydroxyethyl) terephthalate, one of the raw materials for preparing recycled resins (post-consumer resins), by depolymerizing waste polyester. The method is characterized in that the waste polyester is subjected to molecular weight reduction and short-term depolymerization, followed by multi-stage depolymerization in a continuous reactor.

[0020] Specifically, the method for preparing bis(2-hydroxyethyl) terephthalate according to the present invention includes the steps of: (1) feeding waste polyester raw materials into a co-extruder to obtain a co-extrudate; (2) feeding the co-extrudate into a stirred shaft reactor and depolymerizing the co-extrudate to obtain a first reactant; (3) feeding the first reactant into a first continuous reactor and depolymerizing the first reactant to obtain a second reactant; and (4) feeding the second reactant into a second continuous reactor and depolymerizing the second reactant to obtain a third reactant.

[0021] The method for preparing bis(2-hydroxyethyl) terephthalate according to the present invention may further comprise step (5) of purifying the third reactant of step (4).

[0022] Each step of this method is explained in detail below with reference to FIG.

[0023] Step (1): Obtaining a co-extrusion According to the present invention, in step (1), waste polyester raw material is fed into a co-extruder (10) to obtain a co-extrudate. Specifically, in step (1), the molecular weight of the waste polyester raw material is reduced by physical and / or chemical methods through the co-extruder (10).

[0024] The waste polyester raw material can be obtained from polyester material products discarded after use. Specifically, the waste polyester can be obtained by pre-treating waste products such as beverage bottles, fabrics, films, cases, boxes, partitions, shelves, protective panels, packaging materials, building materials, interior and exterior materials, etc., which contain various polyester materials (e.g., polyethylene terephthalate (PET) materials) that are discarded after use by consumers.

[0025] Pretreatment may be performed by removing other plastics, metals, and foreign matter from the waste, washing the waste polyester, and then crushing it in a crusher. As a result of pretreatment, the waste polyester raw material may be in the form of flakes. Furthermore, the waste polyester raw material may have a fine structure like fibers.

[0026] In step (1), the first glycol-based compound may be continuously supplied to the co-extruder (10). When the first glycol-based compound is supplied to the co-extruder (10), the molecular weight of the waste polyester raw material can be reduced more efficiently.

[0027] The first glycol compound is not particularly limited, and specifically, the first glycol compound may be at least one selected from the group consisting of ethylene glycol (monoethylene glycol), propylene glycol, and diethylene glycol.

[0028] The amount of the first glycol compound to be fed may be 0.01 to 100 parts by weight, 1 to 80 parts by weight, 3 to 60 parts by weight, or 5 to 50 parts by weight, relative to 100 parts by weight of the waste polyester raw material. When the amount of the first glycol compound to be fed is within the above range, the molecular weight of the waste polyester raw material can be reduced to the utmost limit through the co-extruder (10).

[0029] On the other hand, co-extrusion may be carried out at 170 to 290°C, specifically 173 to 275°C, 175 to 250°C, 180 to 230°C, 185 to 215°C, or 190 to 200°C. When co-extrusion is carried out within the above temperature range, the molecular weight of the waste polyester raw material can be stably reduced. Furthermore, the extrusion speed (screw rpm) during co-extrusion may be 130 to 250 rpm, 135 to 220 rpm, 140 to 200 rpm, or 145 to 185 rpm.

[0030] The co-extruder (10) is not particularly limited as long as it is capable of co-extruding the waste polyester raw material. Specifically, the co-extruder (10) may be a conventionally known single-screw co-extruder or a multi-screw (e.g., twin-screw) co-extruder.

[0031] The coextrudate obtained through step (1) may have a relatively low weight-average molecular weight and number-average molecular weight. That is, the coextrudate may have a weight-average molecular weight of 3,000 to 36,000, specifically 3,500 to 30,000, 3,800 to 25,000, or 4,000 to 20,000. Furthermore, the coextrudate may have a number-average molecular weight of 500 to 10,000, 800 to 8,000, 1,000 to 6,500, or 1,100 to 5,500.

[0032] Because the coextrudate obtained through step (1) has a relatively low molecular weight, as described above, the time required for the depolymerization procedure in steps (2) to (4) can be reduced, and the production of by-products (e.g., DEG and DEG esters) is minimized.

[0033] Step (2): Obtaining the first reactant through a stirred shaft reactor According to the present invention, in step (2), the coextrudate is fed to a stirred shaft reactor and depolymerized (first depolymerization) to obtain a first reactant. Specifically, in step (2), a glycolysis reaction in which polymer chains and the like present in the coextrudate are decomposed by the first glycol-based compound can be carried out in a short period of time.

[0034] In step (1), if the first glycol-based compound is not fed to the co-extruder (10), the first glycol-based compound may be continuously fed to the stirred shaft reactor (20).

[0035] To promote the depolymerization of the coextrudate through the stirred shaft reactor (20), a catalyst for promoting the depolymerization reaction may be further supplied to the stirred shaft reactor (20). The catalyst is not particularly limited as long as it is a known catalyst. Specifically, the catalyst may be a catalyst containing a metal acetate, an anhydrous acetate, or a hydrate of an acetate. More specifically, the catalyst may be at least one acetate selected from the group consisting of zinc acetate, sodium acetate, cobalt acetate, and manganese acetate, or a hydrate or anhydrous thereof.

[0036] The amount of catalyst supplied to the stirring shaft reactor (20) may be 0.01 to 5 parts by weight, 0.1 to 3 parts by weight, or 0.2 to 1 part by weight relative to 100 parts by weight of the waste polyester raw material.

[0037] The depolymerization of the coextrudate may be carried out at 180 to 210°C (specifically, 183 to 208°C, 185 to 205°C, 186 to 204°C, 188 to 203°C, 190 to 200°C, or 193 to 198°C) for 50 minutes or less (specifically, 5 to 50 minutes, 10 to 50 minutes, 20 to 50 minutes, 22 to 45 minutes, 25 to 40 minutes, or 30 to 35 minutes). In particular, the depolymerization temperature of the coextrudate may be higher than the depolymerization temperature in steps (3) and (4), and the time spent on depolymerization may be shorter than the time spent on depolymerization in steps (3) and (4). As a result, high-purity bis(2-hydroxyethyl) terephthalate can be prepared with high efficiency.

[0038] On the other hand, the stirred shaft reactor (20) is not particularly limited as long as it is used to mix the coextrudate, the first glycol-based compound, and the catalyst. Specifically, the stirred shaft reactor (20) 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, the stirred shaft reactor may be a kneader or a paddle mixer.

[0039] The first reaction product obtained through step (2) may have a peak area fraction of bis(2-hydroxyethyl) terephthalate (BHET) of 50 to 75%, specifically 55 to 75%, 58 to 73%, or 65 to 70%, when analyzed by high performance liquid chromatography (HPLC).

[0040] Furthermore, when analyzed by gel permeation chromatography (GPC), the first reaction product may have a peak area fraction of oligomers having a weight average molecular weight of less than 2,000 (>Mw2,000) of 11.0% or less, specifically, 0.5 to 10.5%, 0.8 to 10%, 1.0 to 8.0%, 1.3 to 7.5%, or 1.5 to 7.3%.

[0041] Step (3): Obtaining the second reactant through the first continuous reactor According to the present invention, in step (3), the first reactant is supplied to a first continuous reactor and depolymerized (second depolymerization) to obtain a second reactant. Specifically, in step (3), a second glycol-based compound may also be continuously supplied to the first continuous reactor (30). As a result, a glycolysis reaction can be carried out in which polymer chains and the like present in the first reactant are decomposed by the second glycol-based compound.

[0042] The second glycol compound is not particularly limited, and specifically may be at least one selected from the group consisting of ethylene glycol (monoethylene glycol), propylene glycol, and diethylene glycol.

[0043] The amount of the second glycol-based compound supplied to the first continuous reactor (30) may be 50 to 340 parts by weight relative to 100 parts by weight of the first reactant. Specifically, the second glycol-based compound may be continuously supplied to the first continuous reactor (30) in an amount of 50 to 300 parts by weight, 50 to 250 parts by weight, 50 to 200 parts by weight, or 50 to 100 parts by weight relative to 100 parts by weight of the first reactant. When the supply amount of the second glycol-based compound is within the above range, the depolymerization of the first reactant can be carried out efficiently, and thereby the proportion of oligomers, dimers, or trimers contained in the second reactant obtained via step (3) can be significantly reduced.

[0044] The depolymerization of the first reactant may be carried out at 170 to 195°C (specifically, 173 to 194°C, 175 to 193°C, 177 to 192°C, 180 to 191°C, 183 to 191°C, or 185 to 190°C) for 30 to 50 minutes (specifically, 32 to 45 minutes, 35 to 43 minutes, or 38 to 40 minutes). When the depolymerization of the first reactant is carried out under the above conditions, the depolymerization is carried out efficiently and the overall process time is shortened, thereby improving the purity and preparation efficiency (productivity) of bis(2-hydroxyethyl) terephthalate.

[0045] The depolymerization of the first reactant may be carried out in the presence of a catalyst that is continuously fed to the stirred shaft reactor (20) in step (2) or directly fed to the first continuous reactor (30). The catalyst may be a catalyst comprising a metal acetate, its anhydride, or its hydrate.

[0046] On the other hand, the first continuous reactor (30) is not particularly limited as long as it is a conventional continuous flow tank reactor designed to carry out depolymerization.

[0047] The second reaction product obtained through step (3) may have a peak area fraction of bis(2-hydroxyethyl) terephthalate (BHET) of 50 to 85%, specifically 55 to 84%, 65 to 83.5%, or 75 to 83%, when analyzed by high performance liquid chromatography (HPLC). Here, the HPLC peak area fraction of bis(2-hydroxyethyl) terephthalate (BHET) in the second reaction product can vary depending on the amount of the second glycol compound supplied to the first continuous reactor (30).

[0048] Step (4): Obtaining the third reactant through the second continuous reactor According to the present invention, in step (4), the second reactant is supplied to the second continuous reactor (40) and depolymerized (third depolymerization) to obtain a third reactant. Specifically, in step (4), the unreacted second glycol-based compound discharged from the first continuous reactor (30) in step (1) and supplied to the second continuous reactor (40) can carry out a glycolysis reaction in which polymer chains and the like present in the second reactant are decomposed. Furthermore, in case the unreacted second glycol-based compound is not supplied to the second continuous reactor (40) to an extent sufficient for depolymerization to be carried out, or in case the purity of the unreacted second glycol-based compound decreases, a third glycol-based compound may be additionally supplied to the second continuous reactor (40) to prepare for a decrease in depolymerization efficiency.

[0049] The third glycol compound is not particularly limited, and specifically may be at least one selected from the group consisting of ethylene glycol (monoethylene glycol), propylene glycol, and diethylene glycol.

[0050] The amount of the third glycol-based compound supplied to the second continuous reactor (40) may be 50 to 150 parts by weight relative to 100 parts by weight of the second reactant. Specifically, the third glycol-based compound may be continuously supplied to the second continuous reactor (40) in an amount of 50 to 130 parts by weight, 55 to 110 parts by weight, 60 to 90 parts by weight, or 65 to 80 parts by weight relative to 100 parts by weight of the second reactant. When the supply amount of the third glycol-based compound is within the above range, the depolymerization of the second reactant can be carried out efficiently, and thus the proportion of oligomers, dimers, or trimers contained in the third reactant obtained via step (4) can be significantly reduced.

[0051] The depolymerization of the second reactant may be carried out at 140 to 170°C (specifically, 143 to 168°C, 145 to 165°C, 148 to 160°C, 149 to 158°C, or 150 to 155°C) for 30 to 50 minutes (specifically, 35 to 45 minutes, 38 to 43 minutes, or 40 to 42 minutes). When the depolymerization of the second reactant is carried out under the above conditions, the depolymerization is carried out efficiently and the overall process time is shortened, thereby improving the purity and preparation efficiency (productivity) of bis(2-hydroxyethyl) terephthalate.

[0052] The depolymerization of the second reactant may be carried out in step (2) in the presence of a catalyst continuously fed to the stirred shaft reactor (20), continuously fed to the first continuous reactor (30), or fed directly to the second continuous reactor (40). The catalyst may be a catalyst comprising a metal acetate, an anhydride thereof, or a hydrate thereof.

[0053] On the other hand, the second continuous reactor (40) is not particularly limited as long as it is a conventional continuous flow tank reactor designed to carry out depolymerization.

[0054] The third reaction product obtained via step (4) may have a peak area fraction of bis(2-hydroxyethyl) terephthalate (BHET) of 80 to 90%, specifically 83.5 to 90%, 84 to 89.5%, or 84.5 to 89%, when analyzed by high-performance liquid chromatography (HPLC).

[0055] Furthermore, when analyzed by gel permeation chromatography (GPC), the third reaction product may have a peak area fraction of oligomers having a weight average molecular weight of less than 2,000 (>Mw2,000) of 2.0% or less, specifically, 0.0 to 2.0%, 0.0 to 1.8%, 0.0 to 1.7%, 0.0 to 1.5%, or 0.0 to 1.0%.

[0056] Meanwhile, the third reactant is subjected to a filtration step to remove unreacted materials and impurities (contaminants). In this case, the third reactant contains almost no oligomers, which significantly affect the time required for the filtration step. Therefore, in the present invention, the filtration step can be performed in a relatively short time. That is, in the present invention, before the waste polyester is directly fed to the continuous reactor for multistage depolymerization, the waste polyester raw material is reduced in molecular weight through a coextruder (10) in step (1), and then depolymerized in a short period of time through a stirred shaft reactor (20) in step (2). The resulting reactant is then depolymerized in multiple stages. Therefore, the third reactant obtained by the final depolymerization contains almost no oligomers, which would otherwise increase the filtration time or delay the filtration step. Furthermore, when multistage depolymerization is performed through steps (3) and (4), the residence time of the reactants obtained in each step during high-temperature reaction is shortened, minimizing the generation of by-products. Therefore, high-purity bis(2-hydroxyethyl) terephthalate can be prepared in high yield.

[0057] Specifically, when the third reactant is filtered using a membrane filter having a pore size of 0.1 μm, the flow rate through the membrane filter (filtration flow rate) may be 10 kg / hr or more, specifically, 10 to 100 kg / hr, 12 to 90 kg / hr, 13 to 80 kg / hr, or 14 to 70 kg / hr. When the filtration flow rate is within the above range, it is possible to significantly improve the preparation efficiency (productivity) of bis(2-hydroxyethyl) terephthalate.

[0058] Furthermore, the third reactant may have a filtration loss rate according to the following formula 1 of less than 8 wt %, specifically 0.1 to 7.5 wt %, 0.5 to 7.0 wt %, 1.0 to 6.0 wt %, or 1.3 to 5.0 wt %. [Formula 1] Filtration loss rate (weight%)=(m1-m2 / m1)×100 m1: initial weight of the third reactant m2: weight of the third reactant that passed through a filter membrane with a pore size of 0.1 μm

[0059] Step (5): Purification of the third reactant According to the present invention, in step (5), the third reactant is purified. Step (5) can be optionally performed as needed.

[0060] The third reactant may be purified by a commonly known method, specifically, by one or more of the following steps: filtration, ion exchange, distillation, decolorization, and adsorption.

[0061] The filtration step may include processes such as membrane filtration, filter aid filtration, reduced pressure flash (cooling), solid-liquid separation, etc. When such a filtration step is performed, fine particles and insoluble foreign matter contained in the third reaction liquid can be removed.

[0062] Ion exchange is a step performed using a commonly known ion exchange resin. Examples of ion exchange resins include cation exchange resins, anion exchange resins, amphoteric ion exchange resins, and chelating resins. Specifically, the cation exchange resin may be a strongly acidic cation exchange resin having a sulfonic acid group (-SO3H) or a weakly acidic cation exchange resin having a carboxyl group (-COOH). The anion exchange resin may be a strongly basic anion exchange resin in the form of a quaternary ammonium salt or a weakly basic anion exchange resin having an amino group. When such an ion exchange step is performed, catalysts and foreign metals can be removed.

[0063] The distillation may include steps such as vacuum distillation, thin film evaporation, falling film evaporation, short path evaporation, etc. When such a distillation step is performed, unreacted glycol-based compounds can be removed.

[0064] Bleaching is a step that is generally carried out using a known bleaching agent, such as activated carbon, activated clay, or diatomaceous earth. When such a bleaching step is carried out, colored substances can be removed.

[0065] The adsorption step is carried out using a conventionally known adsorbent, and can remove other foreign substances and obtain the crystallized final reaction product (bis(2-hydroxyethyl) terephthalate).

[0066] Method of Invention The present invention will be described in more detail below with reference to the following embodiments, but these examples are provided for illustrative purposes only and are not intended to limit the scope of the present invention.

[0067] Preparation Example 1 Crushed waste PET (flake-like PET) and monoethylene glycol (MEG) were fed into a single-screw co-extruder at a feed rate of 15.5 kg / hr, respectively, and co-extruded at a temperature of 180°C and 150 rpm (to reduce molecular weight) to obtain a co-extrudate.

[0068] Preparation Examples 2 to 7 Each coextrudate was obtained in the same manner as in Preparation Example 1, except that the feed flow rate and coextrusion conditions were adjusted as shown in Table 1 below.

[0069] Test Example 1 The coextrusion products obtained in Preparation Examples 1 to 7 were analyzed by gel permeation chromatography (GPC) under the following conditions, and the results are shown in Table 1 below. GPC analyzer: HLC-8420GPC Elite of TOSOH Mobile phase: Chloroform / phenol mixed solvent Temperature: 40℃ Column: Individual gel type x 4EA Sample pretreatment: The sample concentration in the solution was adjusted to 0.5 wt / v% using a chloroform / phenol mixed solvent.

[0070] [Table 1]

[0071] Example 1 Application of a co-extruder, a kneader, a first continuous reactor and a second continuous reactor Following the coextrusion process of Preparation Example 1, depolymerization was continuously carried out via a kneader, a first continuous reactor (CSTR-1), and a second continuous reactor (CSTR-2) under the design conditions shown in Figure 1. Specifically, the coextrudate of Preparation Example 1 (feed rate: 31.0 kg / hr) and zinc acetate anhydride as a catalyst (feed rate: 0.065 kg / hr) were fed to the kneader, and a first depolymerization reaction was carried out at 195°C for 35 minutes to obtain a first reaction product.

[0072] The first reaction product thus obtained and additional monoethylene glycol (MEG-2) (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 reaction product.

[0073] The second reaction product thus obtained and additional monoethylene glycol (MEG-3) (feed rate: 31.0 kg / hr) were supplied to a second continuous reactor (CSTR-1), where a third depolymerization reaction was carried out at 150°C for 40 minutes to obtain a third reaction product.

[0074] The third reaction product thus obtained was purified by a conventional method to prepare bis(2-hydroxyethyl) terephthalate.

[0075] Examples 2 to 7 Application of the Co-Extruder, Kneader, First Continuous Reactor, and Second Continuous Reactor Bis(2-hydroxyethyl) terephthalate was prepared in the same manner as in Example 1, except that the feed flow rate and reaction temperature in each depolymerization reaction were adjusted as shown in Table 2 below.

[0076] Comparative Examples 1 to 3 Application of the Kneader, First Continuous Reactor, and Second Continuous Reactor According to the design conditions in Figure 2, depolymerization of shredded waste PET (flake-like PET) was carried out continuously in the presence of anhydrous zinc acetate catalyst via a kneader, a first continuous reactor (CSTR-1), and a second continuous reactor (CSTR-2), without subjecting the shredded waste PET to a co-extrusion process. Bis(2-hydroxyethyl) terephthalate was prepared by adjusting the feed flow rate and reaction temperature in each depolymerization reaction as shown in Table 3 below.

[0077] Comparative Examples 4 to 6 Application of the first continuous reactor, the second continuous reactor, and the third continuous reactor According to the design conditions in Figure 3, depolymerization of shredded waste PET (flake-like PET) was carried out continuously in the presence of zinc acetate anhydride catalyst through the first continuous reactor (CSTR-1), the second continuous reactor (CSTR-2), and the third continuous reactor (CSTR-3), without going through the coextrusion process of the shredded waste PET or depolymerization using a kneader. Bis(2-hydroxyethyl) terephthalate was prepared by adjusting the feed flow rate and reaction temperature in each depolymerization reaction as shown in Table 3 below.

[0078] [Table 2]

[0079] [Table 3]

[0080] Test Example 2 The reaction products obtained in Examples 1 to 7 and Comparative Examples 1 to 6 were analyzed by gel permeation chromatography (GPC) under the following conditions. The results are shown in Tables 4 and 5 below. GPC analyzer: HLC-8420GPC Elite of TOSOH Mobile phase: Chloroform / phenol mixed solvent Temperature: 40℃ Column: Individual gel type x 4EA Sample pretreatment: The sample concentration in the solution was adjusted to 0.5 wt / v% using a chloroform / phenol mixed solvent.

[0081] Test Example 3 The third reaction products obtained by depolymerization in the second continuous reactor (CSTR-2) in Examples 1 to 7 and Comparative Examples 1 to 3, and the third reaction products obtained by depolymerization in the third continuous reactor (CSTR-3) in Comparative Examples 4 to 6 were each filtered through a circular glass fiber filter, and the filtration time and flow rate were measured. The results are shown in Tables 4 and 5 below. The evaluation conditions and criteria were as follows: Glass fiber filter pore size: 0.1 μm Diameter of glass fiber filter: 320mm Filtration temperature: 100~150℃ Judgment of success or failure of filtration - ◎: Filtration time within 20 minutes, ○: Filtration time from 20 minutes to 60 minutes or less, ×: Filtration time over 60 minutes

[0082] Test Example 4 The reaction products (filtrates) obtained in Test Example 3 were analyzed by high performance liquid chromatography (HPLC) under the following conditions. The results are shown in Tables 4 and 5 below. Pretreatment: Approximately 0.01 g of a sample was diluted with approximately 20 mL of methanol and measured by HPLC. HPLC analyzer (model): Waters e2695 Column: C18 (4.6 x 250 mm), 5 μm UV detector: 242 nm Injection volume: 10μL Eluent (gradient) - A: H2O + H3PO4, B: acetonitrile

[0083] Test Example 5 After carrying out Test Example 3, the filtration loss rate of the reaction product (filtrate) was calculated according to the following formula 1. [Formula 1] Filtration loss rate (weight%)=(m1-m2 / m1)×100 m1: initial weight of the third reactant (initial weight of the third reactant obtained by depolymerization in the second continuous reactor (CSTR-2) in Examples 1 to 7 and Comparative Examples 1 to 3, and initial weight of the reactant obtained by depolymerization in the third continuous reactor (CSTR-3) in Comparative Examples 4 to 6). m2: weight of the third reaction product that has passed through a filtration membrane having a pore size of 0.1 μm (in Examples 1 to 7 and Comparative Examples 1 to 3, this is the weight of the third reaction product obtained by depolymerization in the second continuous reactor (CSTR-2) that has passed through a filtration membrane having a pore size of 0.1 μm, and in Comparative Examples 4 to 6, this is the weight of the reaction product obtained by depolymerization in the third continuous reactor (CSTR-3) that has passed through a filtration membrane having a pore size of 0.1 μm).

[0084] [Table 4]

[0085] [Table 5]

[0086] Referring to Table 4, in Examples 1 to 7, which applied the preparation method of the present invention, multi-stage depolymerization was performed after coextrusion and kneader depolymerization. By performing depolymerization in a relatively short time, the generation of by-products that could be considered impurities was minimized, and high-purity bis(2-hydroxyethyl) terephthalate was produced. In addition, because the waste PET was successfully depolymerized and there was almost no residual oligomer, the filtration process for removing unreacted materials and impurities could be carried out efficiently.

[0087] In contrast, referring to Table 5, in Comparative Examples 1 to 3, in which the coextrusion process was not performed, the molecular weight of the waste PET was not reduced as compared to Examples 1 to 7, resulting in an increase in residual oligomers and an increase in the amount of by-products produced. Also, in Comparative Examples 4 to 6, in which multi-stage depolymerization was performed without reducing the molecular weight of the waste PET, the amount of residual oligomers was large and the filtration loss rate was high, resulting in a significant deterioration in the efficiency of the method for preparing bis(2-hydroxyethyl) terephthalate. [Explanation of symbols]

[0088] 10: Co-extruder 20: Stirred shaft reactor 30: First continuous reactor 40: Second continuous reactor

Claims

1. (1) feeding waste polyester raw material into a co-extruder to obtain a co-extrudate; (2) feeding the co-extrudate into a stirred shaft reactor and depolymerizing the co-extrudate to obtain a first reactant; (3) feeding the first reactant into a first continuous reactor and depolymerizing the first reactant to obtain a second reactant; (4) feeding the second reactant into a second continuous reactor and depolymerizing the second reactant to obtain a third reactant; (5) purifying the third reactant to obtain bis(2-hydroxyethyl) terephthalate; Including, The co-extrusion in step (1) is carried out at 170 to 290°C; The depolymerization in step (2) is carried out at 180 to 210°C; The depolymerization in step (3) is carried out at 170 to 195°C; The depolymerization in step (4) is carried out at 143 to 168°C; A method for preparing bis(2-hydroxyethyl) terephthalate, wherein the depolymerization in steps (2), (3), and (4) is a glycolysis reaction with a glycol-based compound.

2. 2. The method for preparing bis(2-hydroxyethyl) terephthalate according to claim 1, wherein a first glycol-based compound is continuously supplied to the co-extruder in step (1) in an amount of 0.01 to 100 parts by weight based on 100 parts by weight of the waste polyester raw material.

3. 2. The method for preparing bis(2-hydroxyethyl) terephthalate according to claim 1, wherein the coextrudate has a weight average molecular weight of 3,000 to 36,000.

4. 2. The method for preparing bis(2-hydroxyethyl) terephthalate according to claim 1, wherein the depolymerization in step (2) is carried out for 20 to 50 minutes.

5. 2. The method for preparing bis(2-hydroxyethyl) terephthalate according to claim 1, wherein in step (2), a catalyst comprising a metal acetate or an anhydride or hydride thereof is further supplied to the stirred shaft reactor.

6. 2. The method for preparing bis(2-hydroxyethyl) terephthalate according to claim 1, wherein the first reaction product obtained through step (2) has a peak area fraction of bis(2-hydroxyethyl) terephthalate (BHET) of 50 to 75% when analyzed by high-performance liquid chromatography (HPLC).

7. 2. The method for preparing bis(2-hydroxyethyl) terephthalate according to claim 1, wherein the stirred shaft reactor comprises at least one selected from the group consisting of a kneader, a paddle mixer, a plowshare mixer, a screw mixer, and a ribbon blender.

8. 2. The method for preparing bis(2-hydroxyethyl) terephthalate according to claim 1, wherein a second glycol-based compound is continuously supplied to the first continuous reactor in step (3) in an amount of 50 to 340 parts by weight per 100 parts by weight of the first reactant.

9. 2. The method for preparing bis(2-hydroxyethyl) terephthalate according to claim 1, wherein the third reaction product obtained through step (4) has a peak area fraction of bis(2-hydroxyethyl) terephthalate (BHET) of 80 to 90% when analyzed by high-performance liquid chromatography (HPLC).

10. 2. The method for preparing bis(2-hydroxyethyl) terephthalate according to claim 1, wherein the depolymerization in step (3) is carried out for 30 to 50 minutes, and the depolymerization in step (4) is carried out for 30 to 50 minutes.

11. 2. The method for preparing bis(2-hydroxyethyl) terephthalate according to claim 1, wherein a third glycol-based compound is continuously fed to the second continuous reactor in step (4) in an amount of 50 to 150 parts by weight per 100 parts by weight of the second reactant.

12. 2. The method for preparing bis(2-hydroxyethyl) terephthalate according to claim 1, wherein when the third reactant in step (4) is filtered using a membrane filter having a pore size of 0.1 μm, the flow rate through the membrane filter is 10 kg / hr or more.

13. The third reactant in step (4) is a compound represented by the following formula 1: [Formula 1] Filtration loss rate (wt%) = (m １ -m ２ / m １ )×100 m １ : initial weight of the third reactant m ２ : Weight of the third reactant that passed through a filter membrane having a pore size of 0.1 μm 2. A method for preparing the bis(2-hydroxyethyl) terephthalate of claim 1, wherein the bis(2-hydroxyethyl) terephthalate has a filtration loss rate of less than 8% by weight according to the formula:

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