Hydrothermal liquefaction of diverse plastics using water and seawater

Abstract

In this study, a variety of plastics are converted to their monomers, fuel range hydrocarbons and other platform chemicals using hydrothermal liquefaction (HTL) at sub-critical (350 °C), near-critical (400 °C) and super-critical (450 °C) temperatures. The effect of seawater as a medium on the HTL of plastics is also studied. Polymers like polycarbonate (PC), acrylonitrile–butadiene–styrene (ABS), polyamides (PA-6, PA-66), and polyoxymethylene (POM) liquefied effectively at 350 °C, while polyolefins like polyethylene (PE) and polypropylene (PP) required ≥ 400 °C. HTL of polystyrene (PS) resulted in the highest oil yield (93 wt%) and energy recovery from oil (93.5 %) at 450 °C. HTL of PC, PS and ABS led to the formation of phenols and benzene derivatives, while polyethylene terephthalate (PET) and PA-6 formed their monomers terephthalic acid and caprolactam, respectively. Crude oils from PP and PE contained fuel grade aliphatic hydrocarbons with carbon chain length ranging between C₇ and C₃₆. Highest calorific value was recorded for the oil from PP (46.3 MJ kg⁻¹) at 400 °C. The use of seawater generally reduced the oil yields from all the plastics, except ABS, PET and PP. The production of benzoic acid from PET was enhanced, while the selectivity to caprolactam from PA-6 decreased when seawater was used, possibly due to the enhanced decarboxylation and aromatization reactions induced by the ions in the aqueous phase. In general, the crude oil from HTL of PC, PS, ABS and polyamides are suitable for sustainable production of specialty chemicals and monomers, while that from PE and PP are good candidates for sustainable transportation fuels.