CN-116606201-B - Method for chemically recycling PET (polyethylene terephthalate) into monomer

Abstract

The invention belongs to the technical field of chemical recycling, and discloses a method for chemically recycling polyethylene terephthalate (PET) into monomers. The steps are crushing, washing, drying, depolymerizing, filtering, rectifying, filtering, acidifying and filtering in turn. The invention uses 1, 2-dimethyl-2-imidazolidone (DMI) or other solvent which can be used as hydrogen bond acceptor but can not be used as hydrogen bond donor, and the mixed anhydrous solution composed of Ethylene Glycol (EG) and sodium hydroxide (NaOH) as degradation liquid, thereby ensuring the solubility of NaOH, greatly weakening the solvent effect of hydroxyl and having excellent PET degradation efficiency.

Inventors

• HU HAOQUAN
• CHEN HONGJIE

Assignees

• 大连理工大学

Dates

Publication Date

20260505

Application Date

20230417

Claims (1)

1. 1. A method for chemically recycling PET into monomers is characterized by using waste PET materials subjected to crushing, washing and drying treatment, and comprises the following steps: s1, adding the treated waste PET material into a reactor containing DMI, EG, naOH mixed solution for depolymerization; s2, filtering the product treated in the step S1 for the first time, enabling filtrate to enter a rectifying tower, separating EG added in the depolymerization process to be recovered as a product, and enabling the rest solution to enter a depolymerization reactor in the step S1 for recycling; s3, adding a small amount of water into filter residues generated after the filtering in the step S2 to dissolve sodium terephthalate, and performing secondary filtering; S4, acidifying the filtrate filtered in the step S3 for the third time; The depolymerization reaction temperature in the step S1 is 50-80 ℃; the volume ratio of DMI in DMI, EG, naOH mixed solution added in the step S1 is 60% -98%.

Description

Method for chemically recycling PET (polyethylene terephthalate) into monomer Technical Field The invention belongs to the technical field of chemical recycling, and particularly relates to a method for chemically recycling ethylene terephthalate (PET) into a monomer. Background At present, most PET products are disposable consumer products, and a large amount of PET waste is generated and leaked into the nature, so that a huge burden is brought to the environment. In China, the main PET recycling mode is mechanical recycling, however, during the mechanical recycling process, the PET molecular structure is obviously degraded, and the viscosity and mechanical strength of the molten material are reduced. Chemical recycling is considered a more promising option because it can degrade the polymer into monomers, producing a product of comparable quality to the original PET. Depending on the nucleophile used, the chemical recovery of PET can be divided into glycolysis, methanolysis and hydrolysis. The glycolysis and the alcoholysis belong to transesterification reactions, and have the problems of more side reactions, difficult product purification and the like. Hydrolysis of PET can be classified into acidic hydrolysis, neutral hydrolysis and basic hydrolysis. Acid hydrolysis typically has high yields of terephthalic acid (TPA) monomer, but the use of large amounts of acid makes the process very expensive and adversely affects the purity of Ethylene Glycol (EG). Ecological environment-friendly neutral hydrolysis is usually carried out at high temperature and high pressure, has high requirements on equipment materials, and is difficult to realize continuous operation. The alkaline hydrolysis is carried out in two steps, the PET is depolymerized to alkali metal salts of EG and TPA, and the reaction solution is acidified to obtain TPA. The main advantages of alkaline hydrolysis are the degradation of highly contaminated waste PET, such as magnetic tape, metallized PET film or photo film, and the like, and the relatively simple process, lower cost than methanol hydrolysis, and the like. Conventional alkaline hydrolysis is carried out in an alkaline aqueous solution, typically requiring high temperatures above 200 ℃, and the time for complete depolymerization of PET is long. Many researchers report that PET can be rapidly depolymerized in alkaline mixed solutions under mild conditions. S. Ugduer et al report [ GREENCHEMISTRY,22 (2020) 5376-5394] that after hydrolysis of PET in alkaline ethanol-water binary solution and degradation at 80℃for 20min, TPA yields up to 95%. However, to precipitate the alkali metal salt of TPA, the process also requires a large amount of acid to neutralize the alkaline reaction solution. And the water added in the process greatly increases the energy consumption of EG separation. Recently, S.Zhang et al [ GREEN CHEMISTRY,24 (2022) 3284-3292] developed a non-aqueous alkaline PET hydrolysis method that automatically precipitated terephthalate from degradation solutions using EG-Tetrahydrofuran (THF) -potassium hydroxide (KOH) systems. The consumption of acid and waste water production can be reduced by at least five times relative to conventional aqueous alkaline hydrolysis PET processes, but the KOH used in this system is too expensive and it is relatively cumbersome to separate EG produced by the depolymerization process from the EG-THF-KOH solution system. In addition, no literature reports the relationship between solvent properties and degradation effects, and little is known about solvent systems suitable for alkaline hydrolysis of PET. Thus, there is a need in the art for a method of selecting a solvent system that is clear and economically valuable. Disclosure of Invention In order to overcome the defects of the prior art, the invention provides a method for chemically recycling PET into monomers, which can rapidly depolymerize PET into EG and disodium terephthalate (TPA-Na 2) through 1, 2-dimethyl-2-imidazolidinone (DMI) or other mixed anhydrous solution system which can be used as a hydrogen bond acceptor but can not be used as a hydrogen bond donor and consists of EG and sodium hydroxide (NaOH). Filtering the product, separating EG added by PET depolymerization from filtrate which is mixed solution of DMI-EG and residual NaOH by rectification, supplementing NaOH consumed in the depolymerization process, recycling the mixture for depolymerizing PET, adding a small amount of water to dissolve residue TPA-Na 2, adding hydrochloric acid or sulfuric acid for acidification, and filtering the solution to obtain TPA, wherein the residual sodium chloride or sodium sulfate solution is the only byproduct in the whole process. The invention aims at realizing the technical scheme that the method for chemically recycling PET into the monomer comprises the following steps: 1. sequentially carrying out crushing, washing and drying procedures on the waste PET material; 2. Adding the waste PET mater