Sequential hydrothermal dechlorination and liquefaction of PVC

Poly(vinyl chloride) (PVC) is globally the third most produced plastic, but recycling PVC waste through mechanical means is difficult. Due to the high presence of chlorine in PVC, it is unsuitable for both pyrolysis and combustion. Consequently, there is a need to explore innovative methods for harnessing its energy potential. Hydrothermal liquefaction (HTL) is a topic of growing interest in the literature for chemical recycling of waste plastic or conversion into liquid energy carriers. In this work an innovative sequential hydrothermal processing strategy was tested for oil production from PVC. PVC at high loadings (20 wt.%) was first processed under subcritical conditions to produce a chlorine-free solid, which was then liquefied in supercritical conditions. Different temperatures were tested for the dechlorination stage, and at 300 °C 99% of chlorine was removed, while 94% of carbon remained in the solid residue. The mechanism behind the solid's formation was investigated by assessing activation energies, both with the addition of KOH as a neutralizing agent (120 kJ/mol) and without it (186 kJ/mol). Different temperatures were screened for an HTL supercritical stage (420-480 °C), but the maximum conversion of the dechlorinated solid was 47%, and the maximum mass yield of oil was 10%. Overall, the subcritical phase demonstrated its potential as a technology for producing chlorine-free solid material with a high energy density. The subsequent supercritical step partially valorized the dechlorinated solid by producing an aromatic-rich oil phase.