Study On The Chlorine Migration Characteristics During The Hydrothermal Pretreatment Of Waste Plastics

The production of waste plastics are increasing,and the existence of chlorine increases the technical difficulty of plastics resource utilization and harmless disposal.In this paper,PE,PP and PVC were used as raw materials and the existing hydrothermal test bench was improved to investigate the chlorine existing morphology and product distribution characteristics in real time.The key factors and mechanisms affecting the chlorine removal were revealed,and the kinetic model of hydrothermal dechlorination was hunted according to Arrhenius law.This work could provide a theoretical basis for the harmless treatment and resource utilization of chlorinated waste plastics.The main results were listed as follows:(1)During the hydrothermal pretreatment of chlorinated waste plastics,chlorine was mainly removed by removing HCl,and chlorine could be found in gas,liquid and solid products.HCl gas was rapidly dissolved in water during cooling,therefore inorganic chlorine was mainly observed in liquid products after hydrothermal dechlorination reactions.(2)Hydrothermal temperature was the most significant factor affecting the chlorine migration,followed by hydrothermal reaction time.Due to the difference in the plastic thermal stability,the chlorine removal was not obvious when the hydrothermal temperature was lower than 205 °C.205～235 ℃ was the rapid dechlorination stage,the dechlorination efficiency(DE)was increased rapidly from4.0 % to 77.9 %,with an increase of 73.9 % as the hydrothermal temperature was increased from 205 ℃ to 235 ℃.The distribution rate of chlorine in gas products was increased from 1.1% to 5.5%,with an increase of 4.4%.The distribution rate of chlorine in liquid products was highly increased from 2.8% to 72.4%,with an increase of 69.6%.At a hydrothermal temperature of 220 ℃,the extending of residence time could obviously promote the chlorine removal.The DE was increased from 19.8% to37.6%,with an increase of 17.8% as the residence time was extended from 15 min to60 min.When the hydrothermal temperature was higher than 235 °C,further improving the hydrothermal temperature and reaction time could not significantly promote the chlorine removal.(3)During the hydrothermal pretreatment of waste plastics,other components such as PE and PP did not change the chlorine migration morphology,but they affected the chlorine removal and chlorine distribution in the three-phase products.Due to the differences in melting characteristics and thermal stability of various plastics,the addition of PE and PP could hinder the agglomeration of PVC,thus promoting the chlorine removal when the hydrothermal temperatures were 205 °C and220 °C.The co-hydrothermal DE of PE/PVC,PP/PVC,PE/PP/PVC was 11.9%,9.5%and 10.3% higher than that of PVC DE,respectively at a hydrothermal temperature of220 ℃ for 30 min.The coating formed by the deep melting of PE and PP was wrapped outside the PVC hydrochar,which hindered the timely escape of HCl gas and inhibited the migration of chlorine when the hydrothermal temperature was higher than 235 °C.The co-hydrothermal DE of PE/PVC,PP/PVC,PE/PP/PVC was 2.1%,3.6% and 24.1% lower than that of PVC DE,respectively at a hydrothermal temperature of 235 ℃ for 30 min.(4)The chlorine migration during the hydrothermal pretreatment of waste plastics conformed to the first-order reaction kinetics.According to Arrhenius law,the reaction activation energies of PVC,PE/PVC,PP/PVC and PE/PP/PVC for hydrothermal dechlorination were 217.4 k J/mol,249.1 k J/mol,135.6 k J/mol and154.7 k J/mol,respectively.Hydrothermal dechlorination reaction was controlled by chemical reaction,and high temperature could accelerate the reaction process.However,when the hydrothermal temperature was higher than 250 °C,the melting mucosa of PE and PP could hinder the mass transfer of the reaction process and inhibit the removal of chlorine,resulting in the decrease of the co-hydrothermal DE of PE/PVC,PP/PVC and PE/PP/PVC with the increase of hydrothermal temperature in this study.This study can provide a theoretical basis for the harmless treatment and resource utilization of chlorinated organic waste.