This study aims to elucidate the mass transfer behavior and reaction mechanisms governing the depolymerization of polyethylene (PE) during hydrothermal processing (HTP). It specifically focuses on the comparative effects of superheated steam (SHS, defined as water heated beyond its boiling point at a given pressure without undergoing condensation) and supercritical water (SCW, formed above the critical point of water) conditions on PE depolymerization, providing an unprecedented analysis of these two environments. The effects of pressure ranging from 5.5 to 23 MPa, polymer concentrations between 30–83 wt%, and temperatures of 425 and 450 °C. The oil, gas, and solid products obtained were analyzed by quantifying their total yields, and the chemical compositions of the oil, aqueous phase products and gas were analyzed using gas chromatography mass spectrometry (GC–MS) and gas chromatography coupled to a thermal conductivity detector (GC-TCD) respectively. The results show comparable oil yields (∼83 %) between reactions conducted at lower pressures with SHS and those operated at 23 MPa with SCW. Notably, the chemical composition of the oils − primarily olefins and paraffins − remained identical regardless of changes in pressure; the same was observed when increasing polymer concentration under conditions with SHS. At higher temperatures (450 °C), complete conversion of PE into valuable chemicals, including benzene, toluene, ethylbenzene, and xylene (BTEX) in the oil was observed. The chemical composition of the gas revealed the absence of CO2 from reactions conducted with SHS, representing a notable advantage for this technology considering CO2 and its role as a major greenhouse gas. By understanding the differences between SHS and SCW, this study provides insights into optimizing HTP for efficient PE conversion and the development of sustainable technologies for plastic waste management and resource recovery.

中文翻译：

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聚乙烯在过热蒸汽和超临界水中进行水热加工成燃料和化学品


本研究旨在阐明水热处理 （HTP） 过程中控制聚乙烯 （PE） 解聚的传质行为和反应机制。它特别关注过热蒸汽（SHS，定义为在给定压力下加热到超过沸点而不发生冷凝的水）和超临界水（SCW，在水的临界点以上形成）条件对 PE 解聚的比较影响，对这两种环境进行了前所未有的分析。压力范围为 5.5 至 23 MPa，聚合物浓度在 30–83 wt% 之间，温度为 425 至 450 °C。 通过定量总产率分析获得的石油、天然气和固体产物，并分别使用气相色谱质谱 （GC-MS） 和气相色谱与热导检测器联用 （GC-TCD） 分析油、水相产物和气体的化学成分。结果表明，在较低压力下使用 SHS 进行的反应与在 23 MPa 下使用 SCW 进行的反应之间的产油率相当 （∼83 %）。值得注意的是，无论压力如何变化，油的化学成分（主要是烯烃和石蜡）都保持不变;在 SHS 条件下增加聚合物浓度时观察到相同的结果。在较高温度 （450 °C） 下，观察到 PE 完全转化为有价值的化学品，包括油中的苯、甲苯、乙苯和二甲苯 （BTEX）。气体的化学成分显示，与 SHS 进行的反应中没有 CO2，考虑到 CO2 及其作为主要温室气体的作用，这代表了该技术的显着优势。 通过了解 SHS 和 SCW 之间的差异，本研究为优化 HTP 以实现高效的 PE 转化以及开发用于塑料废物管理和资源回收的可持续技术提供了见解。