Depolymerization of Household Plastic Waste via Catalytic Hydrothermal Liquefaction

Abstract

An unprecedented use of plastics has caused many environmental issues, and as usual, there is a growing interest in recycling and reusing single-use household plastics. In this work, a mixture of five prominent plastic polymers, as simulated household waste, was depolymerized via the hydrothermal liquefaction (HTL) process using a pretreated red mud catalyst (RM) for the liquid product at 430 ± 20 °C reaction temperature for an average 2 h residence time. The selected plastics were polyethylene terephthalate (PET), high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS), which were blended at a ratio of 42, 20, 20, 4, and 14 wt %, respectively, to form a plastic mixture (PM) as a simulated household plastic waste. Additionally, each plastic type was treated individually for control experiments. Among the single plastics, HDPE generated a maximum crude oil yield of 76 wt %, whereas PET produced mainly solid (80 wt %) and gaseous products. The crude oil yield production from noncatalytic reactions followed this trend: HDPE > PS > PP > LDPE. The plastic crude oil possessed 36–92 wt % gasoline-range compounds. Without a catalyst, HDPE decomposed into straight-chain alkanes, whereas PP- and PS-derived products consisted of cyclic compounds. The noncatalytic PM HTL reaction produced 23 wt % liquid crude product and 23 wt % solid from PET. Though the use of a catalyst decreased the single plastic crude yield by 5–60%, it reduced viscosity by 20–80%, minimized acidity by 14–57%, and increased low boiling products (gasoline range) of HTL oil by 5–80%. The use of the RM catalyst increased the crude yield of PM by 63%, decreased solid output from PET by 10%, improved energy recovery by 4.7%, promoted aromatization in PM-derived crudes by 11.4%, and increased the gasoline boiling range compounds by 18.3%. Additionally, the RM catalyst was recycled without significant change in the PM crude yield. This liquefaction study can help in mitigating plastic recycling issues with liquid fuel production.