Depolymerization of Household Plastic Waste Via Catalytic Hydrothermal Liquefaction

An unprecedented use of plastics has caused many environmentalissues, and as usual, there is a growing interest in recycling andreusing single-use household plastics. In this work, a mixture offive prominent plastic polymers, as simulated household waste, wasdepolymerized via the hydrothermal liquefaction (HTL) process usinga pretreated red mud catalyst (RM) for the liquid product at 430 & PLUSMN;20 & DEG;C reaction temperature for an average 2 h residence time.The selected plastics were polyethylene terephthalate (PET), high-densitypolyethylene (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) asa simulated household plastic waste. Additionally, each plastic typewas treated individually for control experiments. Among the singleplastics, HDPE generated a maximum crude oil yield of 76 wt %, whereasPET produced mainly solid (80 wt %) and gaseous products. The crudeoil yield production from noncatalytic reactions followed this trend:HDPE > PS > PP > LDPE. The plastic crude oil possessed 36-92wt % gasoline-range compounds. Without a catalyst, HDPE decomposedinto straight-chain alkanes, whereas PP- and PS-derived products consistedof cyclic compounds. The noncatalytic PM HTL reaction produced 23wt % liquid crude product and 23 wt % solid from PET. Though the useof 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 by5-80%. The use of the RM catalyst increased the crude yieldof PM by 63%, decreased solid output from PET by 10%, improved energyrecovery 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 PMcrude yield. This liquefaction study can help in mitigating plasticrecycling issues with liquid fuel production.