This study investigated the hydrothermal liquefaction (HTL) approach to chemically recycle polyethylene (PE) with different weight percentages (wt. %, ranging from 1% to 19%) of calcium carbonate (CaCO₃). The role and the fate of CaCO₃in HTL was elucidated under different reaction conditions (425-450 °C for 1-2 h). At 450°C-2 h, the oil yield (~80%) obtained was higher than that from 425°C for 1.5 h (~60%). However, with higher reaction temperatures (450°C) and longer reaction times (2 h), a higher gas yield (25%) was also achieved. The gas chromatography mass spectrometry (GC-MS) analysis revealed that the oil product obtained from all samples with different wt.% of CaCO₃ exhibited similar composition at 425°C for 1.5 and 2 h, mostly composed of paraffin and olefins with carbon numbers ranging from C₇ to C₁₇. Moreover, at 450°C-2 h, the oil product was predominantly aromatics for all PE samples with different wt.% of CaCO₃. This is expected as the literature suggests that more aromatics can be formed at a higher temperature under HTL because of different reaction pathways experienced by PE. Furthermore, GC-MS analysis of post-HTL aqueous products indicated the presence of acids, suggesting potential for further refinement for chemical applications. Elemental analysis of carbon, hydrogen, and oxygen in PE feedstock and post-HTL solid residue revealed that dehydrogenation of PE with higher CaCO₃ loadings (10% and 19%) took place under HTL. Additionally, inductively coupled plasma optical emission spectroscopy analysis showed that over 90% of Ca was distributed to the post-HTL aqueous products, indicating that HTL can simultaneously remove contaminants such as CaCO₃ while chemically recycle PE waste into valuable products like oil or aromatic chemicals.

Wan-Ting Chen

University of Massachussets Lowell

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