Process for producing bis-2-hydroxyethyl terephthalate by continuous depolymerization

Abstract

The present invention relates to a method for producing bis-2-hydroxyethyl terephthalate, the method comprising the steps of: (1) adding a waste polyester raw material to a coextruder to obtain a coextrudate; (2) adding the coextrudate to a reactor having a stirring shaft and depolymerizing it to obtain a first reaction product; (3) adding the first reaction product to a first continuously stirred tank reactor and depolymerizing it to obtain a second reaction product; and (4) adding the second reaction product to a second continuously stirred tank reactor and depolymerizing it to obtain a third reaction product.

Description

Technical Field

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

[0002] Polyester among polymers 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, making them widely used in boxes, boxes, partitions, racks, panels, packaging materials, building materials, interior and exterior materials, etc.

[0003] As a result, plastic (eg, polyester) waste is generated globally at an uncontrollable level each year. Recently, countries around the world are developing regulations and programs for recycling waste plastic resources, including waste polyester.

[0004] Although physical methods and chemical methods are used as methods for recovering waste polyester, physical recovery methods cannot ensure purity and are therefore not widely used. Meanwhile, in chemical recovery methods, the ester bonds of waste polyester are cleaved to depolymerize it. Reactions such as glycolysis, hydrolysis, methanolysis and aminolysis are used. Wherein glycolysis decomposes waste polyester at high temperatures by adding glycols such as ethylene glycol or diethylene glycol. A reaction product mainly comprising bis(2-hydroxyethyl)terephthalate (BHET) is obtained. The bis(2-hydroxyethyl)terephthalate contained in the reaction product can be used as a raw material for preparing unsaturated polyester or ester polyol after its crystallization or purification.

[0005] In order to use bis(2-hydroxyethyl)terephthalate as the above raw material, the purity of bis(2-hydroxyethyl)terephthalate must be improved by minimizing the formation of by-products such as diethylene glycol ester (DEG ester) during the depolymerization process. For this reason, a method of depolymerizing by designing a multi-stage continuous stirred tank reactor (CSTR) is currently adopted.

[0006] However, this method has a problem in that since the time required to depolymerize the waste polyester is twice or more as long as that in the case of using a batch reactor, the process efficiency is reduced. In addition, in the process of depolymerizing the waste polyester, the formation of by-products cannot be well controlled; therefore, there is a limit to obtaining bis(2-hydroxyethyl)terephthalate having a desired purity.

[0007] [Prior art literature]

[0008] [Patent Document]

[0009] (Patent Document 1) Korean Patent Publication No. 2022-0068991 Summary of the invention

[0010] Technical issues

[0011] The present inventors have conducted various studies to solve the above conventional problems. As a result, it was found that since waste polyester is subjected to molecular weight reduction and short-term depolymerization before being depolymerized by a continuous reactor (CSTR), bis(2-hydroxyethyl)terephthalate having high purity can be prepared with high efficiency (improved productivity).

[0012] Therefore, an object of the present invention is to provide a method for preparing bis(2-hydroxyethyl)terephthalate by continuous depolymerization of waste polyester, in which the purity and production efficiency (productivity) of bis(2-hydroxyethyl)terephthalate can be improved.

[0013] Technical Solution

[0014] To achieve the above object, the present invention provides a method for preparing bis(2-hydroxyethyl)terephthalate, which comprises (1) feeding a waste polyester raw material to a coextruder to obtain a coextrudate; (2) feeding the coextrudate to a stirred shaft reactor and depolymerizing it to obtain a first reactant; (3) feeding the first reactant to 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.

[0015] Beneficial Effects

[0016] According to the preparation method of the present invention, waste polyester is subjected to molecular weight reduction by coextrusion and short-term depolymerization with a stirred shaft reactor, and then depolymerized by a multi-stage continuous reactor (CSTR); therefore, bis(2-hydroxyethyl)terephthalate (BHET) can be prepared (produced) in a relatively short period of time while minimizing the formation of by-products (e.g., DEG and DEG esters) considered as impurities.

[0017] Therefore, the present invention can provide bis(2-hydroxyethyl)terephthalate (BHET) having high purity with high efficiency, and the bis(2-hydroxyethyl)terephthalate (BHET) prepared as described above can be used as a raw material to prepare polyester having excellent quality and products using the same. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 An example of a method for preparing bis(2-hydroxyethyl)terephthalate according to one embodiment of the present invention is shown.

[0019] Figure 2Methods for preparing bis(2-hydroxyethyl)terephthalate according to Comparative Examples 1 to 3 are shown.

[0020] Figure 3 Methods for preparing bis(2-hydroxyethyl)terephthalate according to Comparative Examples 4 to 6 are shown. DETAILED DESCRIPTION

[0021] Hereinafter, the present invention will be described in detail. The present invention is not limited to the disclosure given below herein, but may be modified into various forms as long as the gist of the present invention is not changed.

[0022] In this specification, the term "comprising" is intended to specify specific features, regions, steps, processes, elements and / or components. Unless there is a specific description to the contrary, it does not exclude the existence or addition of any other features, regions, steps, processes, elements and / or components.

[0023] Throughout the specification, the terms first, second, etc. are used for the purpose of distinguishing one element from another element. However, the components should not be limited by the terms.

[0024] Unless otherwise indicated, all numbers and expressions relating to quantities of components, reaction conditions, and so forth used herein are to be understood as modified by the term "about."

[0025] The number average molecular weight and weight average molecular weight of the compounds / complexes (reactants or products) described in this specification are as known based on carbon-12 ( 12 Although the unit is not described, it can be understood as the molar mass (g / mole) of the same numerical value if necessary.

[0026] For the purpose of description, the sizes of individual elements in the drawings may be described exaggeratedly and they may be different from the actual sizes.

[0027] Method for preparing bis(2-hydroxyethyl)terephthalate

[0028] The present invention relates to a method for preparing bis(2-hydroxyethyl)terephthalate, one of the raw materials for preparing recycled resin (post-consumer resin), by depolymerization of waste polyester. The present invention is characterized in that the waste polyester undergoes molecular weight reduction and short-term depolymerization, and then undergoes multi-stage depolymerization through a continuous reactor.

[0029] Specifically, the method for preparing bis(2-hydroxyethyl)terephthalate according to the present invention includes (1) feeding a waste polyester raw material to a coextruder to obtain a coextrudate; (2) feeding the coextrudate to a stirred shaft reactor and depolymerizing it to obtain a first reactant; (3) feeding the first reactant to 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.

[0030] The method for preparing bis(2-hydroxyethyl)terephthalate according to the present invention may further include (5) purifying the third reactant of step (4).

[0031] In the following, reference will be made to Figure 1 Each step of the method is described in detail.

[0032] Step (1): Obtaining a co-extrudate

[0033] According to the present invention, in step (1), the waste polyester raw material is fed to a coextruder (10) to obtain a coextrudate. Specifically, in step (1), the molecular weight of the waste polyester raw material is reduced physically and / or chemically by the coextruder (10).

[0034] The waste polyester raw material can be obtained from polyester material products discarded after use. Specifically, the waste polyester can be obtained by pre-processing waste products containing various polyester materials (e.g., polyethylene terephthalate (PET) materials) discarded after use by consumers, such as beverage bottles, fabrics, films, boxes, boxes, partitions, racks, protective panels, packaging materials, building materials, and interior and exterior materials.

[0035] The pretreatment may be performed by removing other plastics, metals and foreign materials mixed in the waste, washing it, and then crushing it through a crusher. As a result of the pretreatment, the waste polyester raw material may have a flake form. In addition, the waste polyester raw material may have a fiber-like fine structure.

[0036] In step (1), the first diol-based compound may be continuously fed to the coextruder (10). When the first diol-based compound is fed to the coextruder (10), the molecular weight of the waste polyester raw material may be more effectively reduced.

[0037] The first glycol-based compound is not particularly limited, and may be at least one selected from ethylene glycol (monoethylene glycol), propylene glycol, and diethylene glycol.

[0038] The feed amount of the first diol-based compound 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 feed amount of the first diol-based compound is within the above range, the molecular weight of the waste polyester raw material can be reduced to the greatest extent by the coextruder (10).

[0039] Meanwhile, coextrusion can be performed at 170° C. to 290° C., specifically 173° C. to 275° C., 175° C. to 250° C., 180° C. to 230° C., 185° C. to 215° C., or 190° C. to 200° C. When coextrusion is performed within the above temperature range, the molecular weight of the waste polyester raw material can be stably reduced. In addition, the extrusion speed (screw rpm) during coextrusion can be 130 rpm to 250 rpm, 135 rpm to 220 rpm, 140 rpm to 200 rpm, or 145 rpm to 185 rpm.

[0040] The coextruder (10) may not be particularly limited as long as it is designed to coextrude the waste polyester raw material. Specifically, the coextruder (10) may be a conventionally known single-screw coextruder or a multi-screw (eg, twin-screw) coextruder.

[0041] The coextrudate obtained by step (1) may have a relatively low weight average molecular weight and number average molecular weight. That is, the weight average molecular weight of the coextrudate may be 3,000 to 36,000, specifically 3,500 to 30,000, 3,800 to 25,000, or 4,000 to 20,000. In addition, the number average molecular weight of the coextrudate may be 500 to 10,000, 800 to 8,000, 1,000 to 6,500, or 1,100 to 5,500.

[0042] When the coextrudate obtained through step (1) has a relatively low molecular weight as described above, the time spent in the depolymerization process in steps (2) to (4) can be shortened while minimizing the formation of by-products (eg, DEG and DEG esters).

[0043] Step (2): Obtaining a first reactant by a stirred shaft reactor

[0044] According to the present invention, in step (2), the coextrudate is fed to a stirred shaft reactor (20) 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 diol-based compound can be performed in a short time.

[0045] If the first diol-based compound is not fed to the coextruder (10) in step (1), the first diol-based compound may be continuously fed to the stirred shaft reactor (20).

[0046] In order to promote the depolymerization of the co-extrudate through the stirred shaft reactor (20), a catalyst for promoting the depolymerization reaction may be further fed into the stirred shaft reactor (20). The catalyst is not particularly limited as long as it is a known catalyst. Specifically, it may be a catalyst comprising a metal acetate, an anhydride of an acetate, or a hydrate of an acetate. More specifically, the catalyst may be an acetate selected from at least one of zinc acetate, sodium acetate, cobalt acetate, and manganese acetate, or a hydrate or anhydride thereof.

[0047] The amount of the catalyst fed to the stirred 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.

[0048] The depolymerization of the coextrudate may be performed at 180° C. to 210° C. (specifically, 183° C. to 208° C., 185° C. to 205° C., 186° C. to 204° C., 188° C. to 203° C., 190° C. to 200° C., or 193° C. to 198° C.) for 50 minutes or less (specifically, 5 minutes to 50 minutes, 10 minutes to 50 minutes, 20 minutes to 50 minutes, 22 minutes to 45 minutes, 25 minutes to 40 minutes, or 30 minutes to 35 minutes). In particular, the depolymerization temperature of the coextrudate may be higher than the depolymerization temperature in step (3) and step (4), and the time spent for depolymerization may be shorter than the time spent for depolymerization in step (3) and step (4). Therefore, bis(2-hydroxyethyl)terephthalate having high purity can be prepared with high efficiency.

[0049] Meanwhile, the stirred shaft reactor (20) may not be particularly limited as long as it is designed to mix the co-extrudate, the first diol-based compound and the catalyst. Specifically, the stirred shaft reactor (20) may include at least one selected from a kneader, a paddle mixer, a plough shear mixer, a screw mixer and a ribbon mixer. More specifically, it may be a kneader or a paddle mixer.

[0050] When analyzed by high-performance liquid chromatography (HPLC), the peak area fraction of bis(2-hydroxyethyl)terephthalate (BHET) of the first reactant obtained by step (2) may be 50% to 75%, specifically 55% to 75%, 58% to 73%, or 65% to 70%.

[0051] In addition, when analyzed by gel permeation chromatography (GPC), the peak area fraction of oligomers having a weight average molecular weight of less than 2,000 (>Mw 2,000) of the first reactant can be 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%.

[0052] Step (3): Obtaining a second reactant through a first continuous reactor

[0053] According to the present invention, in step (3), the first reactant is fed to the first continuous reactor (30) and depolymerized (second depolymerization) to obtain the second reactant. Specifically, in step (3), the second diol-based compound may also be continuously fed to the first continuous reactor (30). Thus, a diol hydrolysis reaction in which the polymer chains and the like present in the first reactant are decomposed by the second diol-based compound may be performed.

[0054] The second glycol-based compound is not particularly limited, and may be at least one selected from ethylene glycol (monoethylene glycol), propylene glycol, and diethylene glycol.

[0055] The amount of the second diol-based compound fed 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 diol-based compound may be continuously fed 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 feed amount of the second diol-based compound is within the above range, the depolymerization of the first reactant may be effectively performed, thereby significantly reducing the ratio of oligomers, dimers, or trimers contained in the second reactant obtained by step (3).

[0056] The depolymerization of the first reactant may be performed at 170° C. to 195° C. (specifically, 173° C. to 194° C., 175° C. to 193° C., 177° C. to 192° C., 180° C. to 191° C., 183° C. to 191° C., or 185° C. to 190° C.) for 30 minutes to 50 minutes (specifically, 32 minutes to 45 minutes, 35 minutes to 43 minutes, or 38 minutes to 40 minutes). When the depolymerization of the first reactant is performed under the above conditions, the depolymerization is effectively performed and the total process time is shortened, thereby improving the purity and preparation efficiency (productivity) of bis(2-hydroxyethyl)terephthalate.

[0057] The depolymerization of the first reactant may be carried out in the presence of a catalyst 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, an anhydride thereof, or a hydrate thereof.

[0058] Meanwhile, the first continuous reactor (30) may not be particularly limited as long as it is a conventional continuous flow tank reactor designed to perform depolymerization.

[0059] When analyzed by high performance liquid chromatography (HPLC), the peak area fraction of bis(2-hydroxyethyl)terephthalate (BHET) of the second reactant obtained by step (3) may be 50% to 85%, specifically 55% to 84%, 65% to 83.5%, or 75% to 83%. Here, the HPLC peak area fraction of bis(2-hydroxyethyl)terephthalate (BHET) in the second reactant may vary depending on the amount of the second diol-based compound fed to the first continuous reactor (30).

[0060] Step (4): Obtaining a third reactant through a second continuous reactor

[0061] According to the present invention, in step (4), the second reactant is fed to the second continuous reactor (40) and depolymerized (third depolymerization) to obtain the third reactant. Specifically, in step (4), a glycolysis reaction in which the polymer chains and the like present in the second reactant are decomposed by the unreacted second diol-based compound discharged from the first continuous reactor (30) of step (3) and supplied to the second continuous reactor (40) may be performed. When the unreacted second diol-based compound is not supplied to the second continuous reactor (40) to a sufficient extent for depolymerization, or when the purity of the unreacted second diol-based compound is reduced, the third diol-based compound may be additionally fed to the second continuous reactor (40) to prepare for a reduction in depolymerization efficiency.

[0062] The third glycol-based compound is not particularly limited, and may be at least one selected from ethylene glycol (monoethylene glycol), propylene glycol, and diethylene glycol.

[0063] The amount of the third diol-based compound fed 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 diol-based compound may be continuously fed 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 feeding amount of the third diol-based compound is within the above range, the depolymerization of the second reactant can be effectively performed, thereby significantly reducing the ratio of oligomers, dimers, or trimers contained in the third reactant obtained by step (4).

[0064] The depolymerization of the second reactant may be performed at 140° C. to 170° C. (specifically, 143° C. to 168° C., 145° C. to 165° C., 148° C. to 160° C., 149° C. to 158° C., or 150° C. 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 performed under the above conditions, the depolymerization is effectively performed and the total process time is shortened, thereby improving the purity and preparation efficiency (productivity) of bis(2-hydroxyethyl)terephthalate.

[0065] The depolymerization of the second reactant can be carried out in the presence of the catalyst continuously fed to the stirred shaft reactor (20), the catalyst continuously fed to the first continuous reactor (30), or the catalyst directly fed to the second continuous reactor (40) in step (2). The catalyst can be a catalyst comprising a metal acetate, an anhydride thereof, or a hydrate thereof.

[0066] Meanwhile, the second continuous reactor (40) may not be particularly limited as long as it is a conventional continuous flow tank reactor designed to perform depolymerization.

[0067] When analyzed by high performance liquid chromatography (HPLC), the peak area fraction of bis(2-hydroxyethyl)terephthalate (BHET) of the third reactant obtained by step (4) can be 80% to 90%, specifically 83.5% to 90%, 84% to 89.5%, or 84.5% to 89%.

[0068] In addition, when analyzed by gel permeation chromatography (GPC), the peak area fraction of oligomers having a weight average molecular weight of less than 2000 (>Mw 2,000) of the third reactant can be 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%.

[0069] At the same time, the third reactant is subjected to a filtration step to remove unreacted materials and impurities (contaminants). In such a case, since the third reactant contains almost no oligomers that significantly affect the time required for the filtration step, the present invention can perform the filtration step in a relatively short time. That is, in the present invention, before multi-stage depolymerization is performed by directly feeding the waste polyester to a continuous reactor, the molecular weight of the waste polyester raw material is reduced by the coextruder (10) in step (1), and the reactant obtained by short-term depolymerization via the stirred shaft reactor (20) in step (2) is subjected to multi-stage depolymerization; therefore, the third reactant obtained by the final depolymerization can contain almost no oligomers that would otherwise increase the filtration time or delay the filtration step. In addition, when the multi-stage depolymerization is performed through the above-mentioned steps (3) and (4), the residence time of the high temperature reaction of the reactants obtained in each step is shortened, and the formation of by-products is minimized; therefore, bis(2-hydroxyethyl)terephthalate with high purity can be prepared in high yield.

[0070] Specifically, when the third reactant is filtered using a filter membrane having a pore size of 0.1 μm, the flow rate (filtration flow rate) passing through the filter membrane can be 10 kg / hour or more, specifically 10 kg / hour to 100 kg / hour, 12 kg / hour to 90 kg / hour, 13 kg / hour to 80 kg / hour, or 14 kg / hour to 70 kg / hour. When the flow rate is within the above range, the preparation efficiency (productivity) of bis(2-hydroxyethyl)terephthalate can be significantly improved.

[0071] In addition, the filtration loss rate of the third reactant according to the following Equation 1 may be less than 8 wt %, specifically 0.1 wt % to 7.5 wt %, 0.5 wt % to 7.0 wt %, 1.0 wt % to 6.0 wt %, or 1.3 wt % to 5.0 wt %.

[0072] [Equation 1]

[0073] Filtration loss rate (weight %) = (m1-m2 / m1) × 100

[0074] m1: initial weight of the third reactant

[0075] m2: The weight of the third reactant that passes through the filter membrane with a pore size of 0.1 μm

[0076] Step (5): Purification of the third reactant

[0077] According to the present invention, in step (5), the third reactant is purified. Step (5) can be performed optionally as required.

[0078] The purification of the third reactant can be carried out by a known method. Specifically, the purification can include one or more steps of filtration, ion exchange, distillation, decolorization and adsorption.

[0079] The filtration step may include processes such as membrane filtration, filter-aid filtration, reduced pressure flash evaporation (cooling), and solid-liquid separation. When such a filtration step is performed, particles and insoluble foreign matter contained in the third reactant can be removed.

[0080] Ion exchange is a step performed using a known ion exchange resin. Ion exchange resins may include cation exchange resins, anion exchange resins, amphoteric ion exchange resins, chelating resins, etc. Specifically, cation exchange resins may be strongly acidic cation exchange resins with sulfonic acid groups (-SO3H) or weakly acidic cation exchange resins with carboxyl groups (-COOH). Anion exchange resins may be strongly basic anion exchange resins in the form of quaternary ammonium salts or weakly basic anion exchange resins with amino groups. When such an ion exchange step is performed, catalysts and metallic foreign matter may be removed.

[0081] The distillation may include steps such as vacuum distillation, thin film evaporation, falling film evaporation, and short path evaporation. When such a distillation step is performed, unreacted diol-based compounds may be removed.

[0082] Decolorization is a step performed using a known decolorizing agent. Specifically, the decolorizing agent may include activated carbon, activated clay, diatomaceous earth, etc. When such a decolorization step is performed, colored substances can be removed.

[0083] The adsorption is a step performed using a conventionally known adsorbent. When such an adsorption step is performed, other foreign substances can be removed to obtain a crystalline final reactant (bis(2-hydroxyethyl)terephthalate).

[0084] Embodiments of the invention

[0085] Hereinafter, the present invention will be described in more detail with reference to embodiments. However, these embodiments are provided for illustrative purposes only, and the present invention is not limited thereto.

[0086] [Preparation Example 1]

[0087] Crushed waste PET (PET in the form of flakes) and monoethylene glycol (MEG) were respectively fed to a single screw coextruder at a feed rate of 15.5 kg / hour, and coextruded at a temperature of 180° C. and 150 rpm (for molecular weight reduction) to obtain a coextrudate.

[0088] [Preparation Examples 2 to 7]

[0089] Each coextrudate was obtained by the same procedure as in Preparation Example 1, except that the feed rate and coextrusion conditions were adjusted as shown in Table 1 below.

[0090] [Test Example 1]

[0091] The coextrudates obtained in Preparation Examples 1 to 7 were each analyzed by gel permeation chromatography (GPC) under the following conditions. The results are shown in Table 1 below.

[0092] GPC analysis equipment: TOSOH's HLC-8420GPC Elite

[0093] Mobile phase: Chloroform / phenol based mixture

[0094] Temperature: 40℃

[0095] Column: Single gel type × 4EA

[0096] Sample pretreatment: The sample concentration in the solution was adjusted to 0.5 wt / vol % using a chloroform / phenol based mixture solvent.

[0097] [Table 1]

[0099] [Example 1] Application of a co-extruder, a kneader, a first continuous reactor and a second continuous reactor

[0100] according to Figure 1 According to the designed conditions, after the coextrusion process of Preparation Example 1, depolymerization was continuously performed through a kneader, a first continuous reactor (CSTR-1) and a second continuous reactor (CSTR-2). Specifically, the coextrudate of Preparation Example 1 (feed rate: 31.0 kg / hour) and acetic zinc anhydride as a catalyst (feed rate: 0.065 kg / hour) were fed to the kneader, and a first depolymerization reaction was performed at 195° C. for 35 minutes to obtain a first reactant.

[0101] The first reactant thus obtained and additional monoethylene glycol (MEG-2) (feed rate: 15.5 kg / hour) were fed to the first continuous reactor (CSTR-1), and a second depolymerization reaction was performed at 190° C. for 40 minutes to obtain a second reactant.

[0102] The second reactant thus obtained and additional monoethylene glycol (MEG-3) (feed rate: 31.0 kg / hour) were fed to the second continuous reactor (CSTR-2), and a third depolymerization reaction was performed at 150° C. for 40 minutes to obtain a third reactant.

[0103] The third reactant thus obtained is purified by a conventional method to prepare bis(2-hydroxyethyl)terephthalate.

[0104] [Examples 2 to 7] Application of a co-extruder, a kneader, a first continuous reactor, and a second continuous reactor

[0105] Bis(2-ethyl)terephthalate was prepared in the same manner as in Example 1, except that the feed rate and the reaction temperature in each depolymerization reaction were adjusted as shown in Table 2 below.

[0106] [Comparative Examples 1 to 3] Application of a kneader, a first continuous reactor, and a second continuous reactor

[0107] according to Figure 2 According to the designed conditions, depolymerization of crushed waste PET (PET in the form of flakes) was continuously performed in the presence of acetic zinc anhydride catalyst through a kneader, a first continuous reactor (CSTR-1) and a second continuous reactor (CSTR-2) without a co-extrusion process of crushed waste PET (PET in the form of flakes). While adjusting the feed rate and reaction temperature in each depolymerization reaction as shown in Table 3 below, bis(2-hydroxyethyl)terephthalate was prepared.

[0108] [Comparative Examples 4 to 6] Application of the first continuous reactor, the second continuous reactor and the third continuous reactor

[0109] according to Figure 3 According to the design conditions of , depolymerization of crushed waste PET (PET in the form of flakes) was continuously performed in the presence of acetic zinc anhydride catalyst through the first continuous reactor (CSTR-1), the second continuous reactor (CSTR-2) and the third continuous reactor (CSTR-3) without the co-extrusion process of crushed waste PET (PET in the form of flakes) and the depolymerization through the kneader. While adjusting the feed rate and the reaction temperature in each depolymerization reaction as shown in the following Table 3, bis(2-hydroxyethyl)terephthalate was prepared.

[0110] [Table 2]

[0113] [Table 3]

[0115] [Test Example 2]

[0116] The reactants obtained in Examples 1 to 7 and Comparative Examples 1 to 6 were each analyzed by gel permeation chromatography (GPC) under the following conditions. The results are shown in Tables 4 and 5 below.

[0117] GPC analysis equipment: TOSOH's HLC-8420GPC Elite

[0118] Mobile phase: Chloroform / phenol based mixture

[0119] Temperature: 40℃

[0120] Column: Single gel type × 4EA

[0121] Sample pretreatment: A chloroform / phenol based mixture solvent was used to adjust the sample concentration in the solution to 0.5 weight / volume %.

[0122] [Test Example 3]

[0123] 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 the third reactant obtained by depolymerization in the third continuous reactor (CSTR-3) in Comparative Examples 4 to 6 were each filtered through a round glass fiber filter, and the filtration time and flow rate were measured. The results are shown in Tables 4 and 5 below. Here, the evaluation conditions and evaluation criteria are as follows:

[0124] Pore ​​size of glass fiber filter: 0.1 μm

[0125] Diameter of glass fiber filter: 320mm

[0126] Filtration temperature: 100℃ to 150℃

[0127] Whether the filtration is successful -◎: filtration time is within 20 minutes;○: filtration time is more than 20 minutes to 60 minutes;×: filtration time is more than 60 minutes

[0128] [Test Example 4]

[0129] The reaction products (filtrates) obtained in Test Example 3 were each analyzed by high performance liquid chromatography (HPLC) under the following conditions. The results are shown in Tables 4 and 5 below.

[0130] Pretreatment: About 0.01 g of sample was diluted in about 20 mL of methanol and then measured by HPLC.

[0131] HPLC analysis equipment (model): Waters e2695

[0132] Column: C18 (4.6 mm × 250 mm), 5 μm

[0133] UV detector: 242nm

[0134] Injection volume: 10 μL

[0135] Eluent (gradient) - A: H2O+H3PO4, B: acetonitrile

[0136] [Test Example 5]

[0137] After performing Test Example 3, the filtration loss rate of the reactant (filtrate) was calculated by the following equation (1). The results are shown in Tables 4 and 5 below.

[0138] [Equation 1]

[0139] Filtration loss rate (weight %) = (m1-m2 / m1) × 100

[0140] m1: initial weight of the third reactant (the 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 the initial weight of the reactant obtained by depolymerization in the third continuous reactor (CSTR-3) in Comparative Examples 4 to 6)

[0141] m2: weight of the third reactant passing through the filter membrane with a pore size of 0.1 μm (the weight of the third reactant obtained by depolymerization in the second continuous reactor (CSTR-2) passing through the filter membrane with a pore size of 0.1 μm in Examples 1 to 7 and Comparative Examples 1 to 3; and the weight of the reactant obtained by depolymerization in the third continuous reactor (CSTR-3) passing through the filter membrane with a pore size of 0.1 μm in Comparative Examples 4 to 6)

[0142] [Table 4]

[0144] [Table 5]

[0147] Referring to Table 4, in Examples 1 to 7 in which the preparation method according to the present invention is applied, when depolymerization is performed by coextrusion and a kneader and then multi-stage depolymerization is performed, depolymerization is performed in a relatively short time, which minimizes the formation of by-products regarded as impurities and produces bis(2-hydroxyethyl)terephthalate with high purity. In addition, since the molecular weight of the waste PET is well reduced, there is almost no residual oligomer, so that the filtering process for removing unreacted substances and contaminants is effectively performed.

[0148] In contrast, referring to Table 5, in Comparative Examples 1 to 3 in which the coextrusion process was not performed, the molecular weight reduction of the waste PET was not achieved compared to Examples 1 to 7; therefore, the residual oligomers increased, and the amount of by-products formed increased. In addition, in Comparative Examples 4 to 6 in which multi-stage depolymerization was performed without molecular weight reduction of the waste PET, a large amount of residual oligomers and a high filtration loss rate were caused; therefore, the efficiency of the method for preparing bis(2-hydroxyethyl)terephthalate was significantly deteriorated.

[0149] [Explanation of Reference Numerals]

[0150] 10: Co-extruder

[0151] 20: Stirred shaft reactor

[0152] 30: First continuous reactor

[0153] 40: Second continuous reactor

Claims

1. A method for preparing bis(2-hydroxyethyl)terephthalate, the method comprising: (1) feeding the waste polyester raw material into a coextruder to obtain a coextrudate; (2) feeding the co-extrudate into a stirred shaft reactor and depolymerizing it to obtain a first reactant; (3) feeding the first reactant to a first continuous reactor and depolymerizing it to obtain a second reactant; as well as (4) feeding the second reactant to a second continuous reactor and depolymerizing it to obtain a third reactant, wherein the first diol-based compound is continuously fed into the coextruder in step (1) in an amount of 0.01 to 100 parts by weight relative to 100 parts by weight of the waste polyester raw material, wherein the second diol-based compound is continuously fed into the first continuous reactor in step (3) in an amount of 50 to 340 parts by weight relative to 100 parts by weight of the first reactant, The coextrusion in step (1) is carried out at 170°C to 290°C, The depolymerization in step (2) is carried out at 180°C to 210°C, The depolymerization in step (3) is carried out at 170°C to 195°C for 30 minutes to 50 minutes, and the depolymerization in step (4) is carried out at 140°C to 170°C for 30 minutes to 50 minutes. 2 . The method for preparing bis(2-hydroxyethyl)terephthalate according to claim 1 , wherein the weight average molecular weight of the coextrudate is 3,000 to 36,000.

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

4. The process for producing bis(2-hydroxyethyl)terephthalate according to claim 1, wherein a catalyst comprising a metal acetate or anhydride or hydrate thereof is further fed into the stirred shaft reactor in step (2).

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

6. 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 plow shear mixer, a screw mixer, and a ribbon mixer.

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

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

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

10. The method for preparing bis(2-hydroxyethyl)terephthalate according to claim 1, wherein the filtration loss rate of the third reactant in step (4) according to the following equation 1 is less than 8 wt %: [Equation 1] Filtration loss rate (weight %) = (m1-m2 / m1) × 100 m1: initial weight of the third reactant m2: the weight of the third reactant passing through the filter membrane having a pore size of 0.1 μm.

11. The method for preparing bis(2-hydroxyethyl)terephthalate according to claim 1, further comprising (5) purifying the third reactant of step (4).

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