US-12617925-B2 - Method for producing bis(2-hydroxyethyl)terephthalate by using recycled ethylene glycol

Abstract

A method for producing bis(2-hydroxyethyl)terephthalate is disclosed. The method includes recovering and reusing ethylene glycol with an acetate removed, after the glycolysis of a waste polyester. According to the method, an acetate is not concentrated even though ethylene glycol is reused in a continuous process, leading to bis(2-hydroxyethyl)terephthalate with excellent purity and quality.

Inventors

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

Assignees

• SK CHEMICALS CO., LTD.

Dates

Publication Date

20260505

Application Date

20230904

Priority Date

20220905

Claims (8)

1. 1 . A process for preparing recycled bis(2-hydroxyethyl) terephthalate, which comprises: (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 an acetate-based compound from the distillate to recover the ethylene glycol; and (d) reusing the recovered ethylene glycol for the glycolysis, wherein a content of the acetate-based compound in the ethylene glycol recovered in step (d) is 1% by weight or less, wherein the separation of the distillate in step (b) is carried out using a distillation column, and wherein the removal of the acetate-based compound in step (c) is carried out by side draw purging in the distillation column.
2. 2 . The process for preparing recycled bis(2-hydroxyethyl) terephthalate of claim 1 , wherein the glycolysis in step (a) comprises a reaction of waste polyester and ethylene glycol in the presence of an acetate-based catalyst.
3. 3 . The process for preparing recycled bis(2-hydroxyethyl) terephthalate of claim 1 , wherein the acetate-based compound comprises 2-hydroxyethyl acetate.
4. 4 . The process for preparing recycled bis(2-hydroxyethyl) terephthalate of claim 1 , wherein the depolymerization in step (a) is carried out in a continuous process.
5. 5 . The process for preparing recycled bis(2-hydroxyethyl) terephthalate of claim 4 , wherein the continuous process 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, wherein ethylene glycol is added to at least one of steps (1) to (4).
6. 6 . The process for preparing recycled bis(2-hydroxyethyl) terephthalate of claim 1 , wherein the separation of the distillate in step (b) using the distillation column is carried out 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 of claim 1 , wherein the side draw purging is performed at a fraction of 0.05% by weight to 1% by weight based on a total weight of the crude bis(2-hydroxyethyl) terephthalate solution.
8. 8 . The process for preparing recycled bis(2-hydroxyethyl) terephthalate of claim 1 , wherein the recycled bis(2-hydroxyethyl) terephthalate is prepared by further carrying out at least one of thin-film distillation under reduced pressure; and dissolution in water and adsorption-crystallization.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This application is a National Stage of International Application No. PCI/KR2023/013141 filed Sep. 4, 2023, claiming priority based on Korean Patent Application No. 10-2022-0112190 filed Sep. 5, 2022. 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 ART Polyester is widely used as a material for beverage-filling containers, packaging films, audio and video films, and the like by virtue of its excellent mechanical strength, thermal resistance, transparency, and gas barrier properties. In addition, polyester is widely produced worldwide as an industrial material such as medical fibers and tire cords. In particular, polyester sheets or plates have good transparency and excellent mechanical strength, so that they are widely used as raw materials 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 have prepared regulations and plans for recycling waste plastic resources, including waste polyester. Although physical or 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 severed 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 comprising mainly 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 reactors (CSTRs) in multiple stages is currently adopted. PRIOR ART DOCUMENT (Patent Document 1) Korean Laid-open Patent Publication No. 2022-0068991 DISCLOSURE OF INVENTION Technical Problem An excessive amount of ethylene glycol is added to a glycolysis reaction for the production of bis(2-hydroxyethyl) terephthalate. Thus, it is common to recover and reuse it to increase cost efficiency, and it is required to manage the purity of ethylene glycol circulated in this process. However, the content of by-products in ethylene glycol increases during this continuous process. These by-products are converted to esters through a glycolysis reaction, causing a problem of deteriorating the quality of a final bis(2-hydroxyethyl) terephthalate. The present inventors paid attention to the accumulation of acetate-based compounds originating from the catalysts in ethylene glycol reused in a continuous glycolysis process and have been able to solve this problem by removing them during the recovery process of ethylene glycol. In particular, the acetate-based compounds accumulated in the above process have similar boiling points to that of ethylene glycol; thus, it is not easy to separate them through a distillation column. However, it has been possible to do so by side draw purging at a specific fraction from the stages with the highest concentration of acetate-based compounds in a multi-stage distillation column. Accordingly, an object of the present invention is to provide a process for preparing bis(2-hydroxyethyl) terephthalate while preventing acetate compounds from being concentrated when ethylene glycol is reused in a continuous glycolysis process and to provide bis(2-hydroxyethyl) terephthalate with excellent purity and quality. Solution to Problem According to an aspect of the present invention, there is provided a process for preparing recycled bis(2-hydroxyethyl) terephthalate, which comprises (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 an acetate-based compound from the distillate to recover the ethylene glycol; and (d) reusing the recovered ethylene glycol for the glycolysis. In a specific embodiment, the glycolysis in step (a) comprises a reaction of waste polyester and ethylene glycol in the presence of an acetate-based catalyst; the separation of the distillate in step