Hydrothermal liquefaction of polyethylene terephthalate plastic and co-liquefaction with palm empty fruit bunch

Abstract

Hydrothermal liquefaction (HTL) at subcritical conditions produces biocrude oil and hydrochar with low emissions of CO₂. HTL of polyethylene terephthalate (PET) plastic waste under the subcritical region provided hydrochar as the major product. Pyrolysis kinetics were conducted on PET hydrochar, which exhibited a lower pyrolysis activation energy than raw PET. The addition of 7 vol% H₂O₂ as an oxidant in hydrothermal media resulted in hydrochar having the lowest activation energy. In another strategy, palm empty fruit bunch (EFB), an abundant biomass waste from the palm oil industry, was mixed with PET to produce value-added chemicals through HTL. In this work, hydrothermal liquefaction of PET and EFB was investigated at different PET ratios, in the presence of H₂O₂ and in the presence of an ethanol-water mixture, with a reaction time of 30 min and an initial pressure of 10 bar. FT-IR, GC-MS, and TGA were used to characterize the properties of the biocrude oil, hydrochar, and aqueous phase. The highest biocrude oil yield of 31.2 wt% was obtained at 20 % PET in the feedstock mixture with 40 vol% ethanol co-solvent. GC-MS analysis indicated that the biocrude oil produced at these conditions was rich in phenolic and ester compounds. GC-MS analysis revealed that the relative abundance of phenolic compounds increased from 28 % to 38 % with an increase in PET content from 20 wt% to 60 wt% in the feed mixture. In HTL using 40 vol% ethanol as the solvent, the biocrude oil had significantly higher ester content, which was approximately 82.7 % for pure PET and 25.8 % for a feed containing 20 wt% PET.