Abstract:

The low-carbon and clean disposal of plastic wastes has great significance to environmental protection and the development of circular economy. A method of thermal decomposition to liquefy plastics in low pressure and superheated solvent steam system was proposed. The solvent thermal decomposition feasibility and characteristics of polypropylene (PP) and high density polyethylene (HDPE) under low pressure (≤5MPa) were studied. Two kinds of degradation mechanisms of plastic thermal decomposition process were proposed. The results showed that the low-pressure organic solvent vapor system could achieve efficient thermal decomposition of plastics under relatively mild temperature and 330℃ was the initial temperature for rapid plastic liquefaction. At 330℃, the thermal liquefaction rates of PP in methanol, ethanol and acetone all were 100%, and the ones of HDPE in methanol, ethanol and acetone were 100%, 72.9% and 71.03%, respectively. In such conditions, small molecules generated in plastic liquefaction were more likely to occur polycondensation reaction, which released a lot of heat and made the reaction system appear different amplitude rise of temperature, so as to provide energy for further depolymerization of unreacted plastics to smaller molecules and improve the thermal liquefaction rate of plastics. In PP liquefied oil, the content of oxygenates was more than 40%, while the content of oxygenates in HDPE liquefied oil was lower and the content of hydrocarbons was more than 80%, demonstrating that HDPE had great potential to be used as fuel.

Key words: plastics, solvents, low pressure and superheated solvent steam, thermal decomposition, liquefaction, recovery, hydrocarbons

摘要：

低碳清洁处理塑料垃圾对环境保护和循环经济发展具有重要意义。本文提出了一种在低压过热溶剂蒸汽体系中高效热裂解液化塑料的方法，以聚丙烯（PP）和高密度聚乙烯（HDPE）为原料，研究了塑料在溶剂低压过热蒸汽环境中（≤5MPa）热裂解液化过程特性，并提出了两种塑料热液化过程中的降解机制。结果表明：低压有机溶剂蒸汽体系能在相对温和的条件下实现塑料的高效液化，330℃是本研究中实现塑料快速热裂解液化的起始温度。在此温度下，PP在甲醇、乙醇、丙酮中的热裂解液化率均为100%，HDPE在三种溶剂中的热裂解液化率分别为100%、72.9%和71.03%。在本研究条件下，塑料液化中小分子更容易发生缩聚反应，放出大量热并使反应体系温度出现不同幅度的飞升，从而为塑料进一步解聚为小分子提供能量，提高塑料的热裂解液化率。PP热裂解液化油中含氧化合物的质量分数大于40%，而HDPE的热裂解液化油中氧化物含量低，且烃类的质量分数大于80%，作为燃料使用的潜力大。

关键词: 塑料, 溶剂, 低压过热蒸汽, 热裂解, 液化, 回收, 碳氢化合物