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US-12624188-B2 - Method for producing bis-2-hydroxyethyl terephthalate through continuous depolymerization

US12624188B2US 12624188 B2US12624188 B2US 12624188B2US-12624188-B2

Abstract

A method for producing bis-2-hydroxyethyl terephthalate is disclosed. The method includes the steps of: (1) adding a waste polyester raw material into a co-extruder to obtain a co-extrudate; (2) adding the co-extrudate into a reactor with an agitation shaft and depolymerizing same to obtain a first reaction product; (3) adding the first reaction product into a first continuous stirred tank reactor and depolymerizing same to obtain a second reaction product; and (4) adding the second reaction product into a second continuous stirred tank reactor and depolymerizing same to obtain a third reaction product.

Inventors

  • Ji-Hun Kim
  • Kwang-Woo PARK
  • Seong-Ki Lee
  • Joong Ki LEE
  • Yuntae JIN

Assignees

  • SK CHEMICALS CO., LTD.

Dates

Publication Date
20260512
Application Date
20230615
Priority Date
20220711

Claims (15)

  1. 1 . A process for preparing bis(2-hydroxyethyl) terephthalate, which comprises steps of: (1) feeding a waste polyester raw material and a glycol based compound to a co-extruder to obtain a co-extrudate; (2) feeding the co-extrudate to an agitated 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, wherein, in step (3), a second glycol-based compound is continuously fed to the first continuous reactor in an amount of 50 to 340 parts by weight relative to 100 parts by weight of the first reactant, wherein the depolymerization in step (2) is carried out at 180 to 210° C., wherein the depolymerization in step (4) is carried out at 140 to 170° C., and wherein a temperature of the depolymerization in step (2) is higher than a temperature of the depolymerization in steps (3) and (4).
  2. 2 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 1 , wherein a first glycol-based compound is continuously fed to the co-extruder 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.
  3. 3 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 1 , wherein the co-extrusion in step (1) is carried out at 170 to 290° C.
  4. 4 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 1 , wherein the co-extrudate has a weight average molecular weight of 3,000 to 36,000.
  5. 5 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 1 , wherein the depolymerization in step (2) is carried out for 20 to 50 minutes.
  6. 6 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 1 , wherein a catalyst comprising a metal acetate or an anhydride or a hydrate thereof is further fed to the agitated shaft reactor in step (2).
  7. 7 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 1 , wherein the first reactant obtained through step (2) has a peak area fraction of bis(2-hydroxyethyl) terephthalate (BHET) of 50 to 75% as measured by high-performance liquid chromatography (HPLC).
  8. 8 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 1 , wherein the agitated shaft reactor comprises one or more selected from the group consisting of a kneader, a paddle mixer, a plow shear mixer, a screw mixer, and a ribbon blender.
  9. 9 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 1 , wherein the third reactant obtained through step (4) has a peak area fraction of bis(2-hydroxyethyl) terephthalate (BHET) of 80 to 90% as measured by high-performance liquid chromatography (HPLC).
  10. 10 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 1 , wherein the depolymerization in step (3) is carried out at 170 to 195° C. for 30 to 50 minutes.
  11. 11 . The process for preparing bis(2-hydroxyethyl) terephthalate of 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 relative to 100 parts by weight of the second reactant.
  12. 12 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 1 , wherein, when the third reactant of step (4) is filtered using a filtration membrane having a pore size of 0.1 μm, the flow rate passing through the filtration membrane is 10 kg/hr or more.
  13. 13 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 1 , wherein, when the third reactant of step (4) is filtered using a filtration membrane having a pore size of 0.1 μm, the third reactant in step (4) has a filtration loss rate of less than 8% by weight according to the following Equation 1: Filtration ⁢ loss ⁢ rate ⁢ ( % ⁢ by ⁢ weight ) = ( m 1 - m 2 / m 1 ) × 100 [ Equation ⁢ 1 ] m 1 : Initial weight of the third reactant m 2 : Weight of the third reactant that has passed through the filtration membrane having a pore size of 0.1 μm.
  14. 14 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 1 , which further comprises step (5) purifying the third reactant of step (4).
  15. 15 . The process for preparing bis(2-hydroxyethyl) terephthalate of claim 2 , 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 relative to 100 parts by weight of the second reactant.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This application is a National Stage of International Application No. PCT/KR2023/008268 filed Jun. 15, 2023, claiming priority based on Korean Patent Application No. 10-2022-0085160 filed Jul. 11, 2022. TECHNICAL FIELD The present invention relates to a process for preparing bis(2-hydroxyethyl) terephthalate (BHET) with high purity in high efficiency using waste polyester. BACKGROUND ART Polyester among polymers is used as a material in various fields by virtue of its excellent mechanical strength, thermal resistance, transparency, and gas barrier properties. In particular, polyester sheets or plates have good transparency and excellent mechanical strength, so that they are widely used for cases, boxes, partitions, shelves, panels, packaging materials, building materials, interior and exterior materials, and the like. As a result, waste of plastics such as polyester is generated globally at an unmanageable level every year. Recently, countries around the world are preparing regulations and plans for recycling waste plastic resources, including waste polyester. Although physical and chemical methods are used as methods of recycling waste polyester, physical recycling methods cannot guarantee purity and, thus, are not widely used. Meanwhile, in chemical recycling methods, the ester bond of waste polyester is cleaved to depolymerize it. Reactions such as glycolysis, hydrolysis, methanolysis, and aminolysis are used. Glycolysis among them is to decompose waste polyester by adding a glycol such as ethylene glycol or diethylene glycol at high temperatures. A reaction product mainly containing bis(2-hydroxyethyl) terephthalate (BHET) is obtained. The bis(2-hydroxyethyl) terephthalate contained in the reaction product may be used as a raw material for preparing unsaturated polyester or ester polyol after the crystallization or purification thereof. In order to use bis(2-hydroxyethyl) terephthalate as the above 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 performing depolymerization by designing continuous stirred tank reactor (CSTRs) in multiple stages is currently adopted. However, this method has a problem in that the process efficiency is reduced since the time required to depolymerize waste polyester is twice or more the case in which a batch reactor is used. In addition, the formation of by-products is not well controlled in the process of depolymerizing waste polyester; thus, there is a limit to obtaining bis(2-hydroxyethyl) terephthalate having a desired purity. PRIOR ART DOCUMENT Patent Document (Patent Document 1) Korean Laid-open Patent Publication No. 2022-0068991 DISCLOSURE OF INVENTION Technical Problem The present inventors have conducted various studies in order to solve the above-mentioned conventional problems. As a result, it has been discovered that, as waste polyester is subjected to a reduction in molecular weight and short-term depolymerization prior to the depolymerization through a continuous reactor (CSTR), bis(2-hydroxyethyl) terephthalate with high purity can be prepared in high efficiency (increased productivity). Accordingly, an object of the present invention is to provide a process 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 enhanced. Solution to Problem In order to accomplish the above object, the present invention provides a process for preparing bis(2-hydroxyethyl) terephthalate, which comprises (1) feeding a waste polyester raw material to a co-extruder to obtain a co-extrudate; (2) feeding the co-extrudate to an agitated 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. Advantageous Effects of Invention According to the preparation process of the present invention, waste polyester is subjected to a reduction in molecular weight through co-extrusion and short-term depolymerization with an agitated shaft reactor, followed by depolymerization through continuous reactors (CSTRs) in multiple stages; thus, it is possible to prepare (produce) bis(2-hydroxyethyl) terephthalate (BHET) in a relatively short period of time while minimizing the formation of by-products (e.g., DEG and DEG esters) considered as impurities. Accordingly, the present invention can provide bis(2-hydroxyethyl) terephthalate (BHET) with high purity in high efficiency, and the bis(2-hydroxyethyl) terephthalate (BHET) prepared as described above can be used as a raw material to prepare a pol