(703b) Hydrothermal Liquefaction for Co-Valorization of Plastics and Biopolymers

Hydrothermal liquefaction (HTL) takes place in water at near- and supercritical conditions (T_c = 374 ˚C, P_c = 22 MPa). HTL has been studied to valorize components of municipal solid waste (MSW), namely synthetic and biopolymers. Biopolymers such as those in food waste and wood are broken down to produce energy-dense oil under subcritical conditions (280-374˚C) while synthetic polymers generally decompose under supercritical conditions (374-500˚C). HTL of plastics (polypropylene (PPE), polyethylene terephthalate (PET), polystyrene (PS), polycarbonate (PBC)) can give higher yields of energy dense (33 – 91 wt% oil yield) than HTL of biopolymers (10 – 50 %). The heating value of the oil produced from some plastics is higher than that from biopolymers and comparable to petroleum. The oil, solid, and aqueous products obtained contain moieties similar to monomeric units of plastics that could be recycled.

Separation of biopolymers from plastics for recycling has been a challenge due to contamination, source management and enormity of waste produced. This limitation is extended to most chemical and mechanical recycling techniques. HTL offers a unique platform to valorize together most components of municipal solid waste to obtain useful products while bypassing the need for separation. The products obtained are separated into 4 phases – non-polar liquid, polar liquid, solid, and gas. HTL can be hence be viewed as an intensified step that combines reaction and separation.

Co-liquefaction experiments were performed with mixtures of nine model compounds (5 biopolymers, 4 plastics) to mimic the composition of MSW. Experiments were conducted from 300 – 425 °C with a batch holding time of 30 min. Synergistic interactions were calculated by comparing the oil yield of the mixture with the oil yields from HTL of the biopolymer and plastics individually. Note that synergistic interactions are observed among plastic, biopolymers in their respective mixtures. The percentage synergy in the nine-component mixture exceeds the synergies obtained among the separate mixtures of plastic alone and biopolymer alone. This indicates the presence of positive interactions between plastics and biopolymers that increase the oil yield and hence the energy recovered from the process. A high energy recovery of 45 percent was obtained at the lowest operated temperature (300 °C). The largest percentage synergy (105 %) is also observed at the same temperature. The presence of these synergistic behaviors at a low operating temperature aids at obtaining higher energy recoveries at lower operating temperatures which is economically and environmentally beneficial.

To further pinpoint the binary interactions between biopolymers and plastics that cause such synergy – more experiments were conducted by manipulating the mixture composition. We will also report on a mathematical model that would predict the energy recovered from isothermal operation of HTL for any given mixture of synthetic polymers and biopolymers. This would further aid in enabling feedstock engineering to increase the net energy recovered to more than 45% i.e. adding waste feedstocks to optimize yield obtained from available waste feedstock.