Effect of operating parameters on monomer production from depolymerization of waste polyethylene terephthalate in supercritical ethanol

Waste plastic presents a high risk for environment and ecosystem, and it is essential to recycle the discarded plastic. In this study, supercritical ethanol depolymerization (SCED) is used to recover monomer from waste polyethylene terephthalate (PET). The effect of reaction temperature (230–350 °C), reaction time (2–60 min), and liquid/solid (ethanol/waste PET) mass ratio (L/S ratio = 8:1, 10:1 and 12:1) on depolymerization of waste PET in supercritical ethanol (SCE) were investigated by experiments. The solid residue after depolymerization process was characterized by FTIR, TG-DSC, and SEM. It was found that the main components of liquid product include diethyl terephthalate (DET), and ethylene glycol (EG), etc. The optimum depolymerization efficiency (100%), DET yield (98%), and EG yield (89.8%) could be obtained from SCED of transparent waste PET at 310 °C, 60 min, and L/S ratio= 10:1. Such optimal condition was also applied for depolymerization of various waste PET. When recycled ethanol from preceding depolymerization was reused in subsequent depolymerization, the yield of DET and EG decreased by 6.0–7.5% and 3.5–4.0%, respectively. The potential reaction pathway of waste PET in SCE was also proposed. Generally, SCED is potential technology for monomer recovery from waste PET.