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.

Automated summary

The widespread use of plastics has caused environmental issues, leading to a growing interest in recycling single-use household plastics. In this study, a mixture of five prominent plastic polymers was depolymerized using a hydrothermal liquefaction process with a pretreated red mud catalyst. The use of the catalyst decreased the yield of single plastics but increased low boiling products in the liquid crude oil.