Supercritical Alcoholysis Process Of Waste Plastics PET/PC And Its Process Strengthening

Solid waste recycling is an inevitable requirement for sustainable development and environmental friendliness.Waste plastics account for a large proportion of solid waste,commonly known as white waste.How to recycle and reuse has always been the focus of the industry.This paper introduces the present situation of waste plastics treatment,common recycling processes and research status at home and abroad,summarizes the application of supercritical fluids in the chemical recycling of waste plastics,and points out the existing problems.On this basis,it is proposed to use supercritical（subcritical）methanol（ethanol）for alcoholysis of polyethylene terephthalate（PET）and polycarbonate（PC）,and CO₂ is used to strengthen the alcoholysis process.The waste PET was alcoholized with supercritical methanol（ethanol）.The reaction temperature,reaction time and the mass ratio of methanol（ethanol）to PET were investigated and the optimum reaction conditions were obtained.Under the optimal reaction conditions,CO₂ was used to strengthen the alcoholysis process of methanol（ethanol）.The optimal CO₂pressurization condition was 1.5 MPa（2.0 MPa）.The effects of reaction temperature,reaction time and the mass ratio of methanol（ethanol）to PET on the yield of monomer DMT（DET）were investigated under the optimal CO₂ pressurization conditions.It was found that the yield of monomer DMT（DET）was greatly improved under the enhancement of CO₂.The waste PC polyester was alcoholized with subcritical ethanol.The reaction temperature,reaction time and the mass ratio of ethanol to PC were investigated.Through the comprehensive analysis of the depolymerization rate of PC polyester and the yield of BPA,the optimal reaction conditions for subcritical ethanol alcoholysis were obtained as follows:the reaction temperature was 200℃,the mass ratio of ethanol to PC was 6,the reaction time was 15 min and the rotational speed was 1000 rpm.In order to better understand the role of CO₂ in the alcoholysis of pet with supercritical methanol（ethanol）,a comparative experiment of N₂ was carried out.It was found that under the same conditions,the yield of monomer could not be improved by using N₂.The process mechanism of CO₂ enhanced alcoholysis of PET was further discussed in combination with p-x,y phase diagrams,which also provided guidance for the industrialized large-scale recovery of PET.