Hydrothermal Liquefaction of Polycarbonate (PC) Plastics in Sub-/Supercritical Water and Reaction Pathway Exploration

Abstract

Plastic pollution is one of the most important environmental issues being faced today. In this work, the hydrothermal liquefaction performance of polycarbonate (PC) in sub-/supercritical water was investigated using a quartz tube reactor. Response surface methodology (RSM) was employed to demonstrate the correlation between reaction conditions and liquefaction efficiency of PC. The conversion selectivity and product recovery efficiency of PC were also analyzed. Moreover, according to the experimental results, the liquefaction reaction pathways of PC were speculated. In addition, based on the established reaction pathways, the lumped parameter method was also used to calculate the liquefaction kinetics of PC. The results showed that phenol was the largest liquefied product of PC, followed by 4-isopropylphenol (IPrP) and 4-isopropenylphenol (IPP), and the recovery efficiency of these three components determined the level of liquefaction efficiency. The PC liquefaction favored a mild reaction temperature, long residence time, and low feedstock concentration. Finally, for identified products, a 57.70 wt % carbon liquefaction efficiency was obtained with a 5 wt % feedstock concentration at 400 °C for 60 min. The liquefaction pathways showed that in the initial stage of depolymerization, 4-tert-butylphenol was first formed as a structural regulator, and phenol and IPP were the primary liquefied products. The IPrP, 4-ethylphenol, and p-cresol were the secondary products. The kinetic results indicated that the liquefied intermediates were easily converted to form phenol and IPP, and the IPP and IPrP were not directly converted to phenol, but there was a clear conversion relationship between them.

Expanding Plastics Recycling Technologies: Chemical Aspects, Technology Status and Challenges

“…Some step-growth polymers formed by condensation polymerization, such as PET, PC, polyamides, and polyurethane (PUR), have ether, ester, or acid amide linkages. 546,553,[556][557] These functional groups can be hydrolyzed to rapidly depolymerize the plastics with high chemical selectivity and liquefaction efficiencies even under sub-critical conditions. 546,[556][557] There is pronounced support for hydrogen bonding and ion solvation due to the higher density of the sub-critical water medium.…”

“…546,553,[556][557] These functional groups can be hydrolyzed to rapidly depolymerize the plastics with high chemical selectivity and liquefaction efficiencies even under sub-critical conditions. 546,[556][557] There is pronounced support for hydrogen bonding and ion solvation due to the higher density of the sub-critical water medium. 546,556 HTL of PUR mainly promotes monomer production.…”

“…Increasing temperature and treatment time during HTL often facilitated conversion to increase liquid and gas yields. 532,542,549,[557][558] However, it has been noted that high reaction temperatures and a prolonged residence time can reduce the desired product yield and increase coke formation due to over-cracking of the liquefied products. 548,550,554 The feedstock-to-solvent mass ratio also affected the HTL process.…”

“…Increasing the solid mass loading to beyond the corresponding optimum led to increased heat transfer limitations and decreased mobility of the macromolecular free-radical fragments in the solution. 557 Such limitations favored undesired secondary reactions like repolymerization. Jin 557 et al also reported that increasing temperature and mass ratio would eventually result in decreased liquefaction efficiency for the same reaction time.…”

Hydrothermal Co-Liquefaction of Synthetic Polymers and Miscanthus giganteus: Synergistic and Antagonistic Effects

“…Another possibility is that oligomers yielded from PC and Epoxy HTL are more easily converted to bisphenol-A in the presence of miscanthus HTL products, compensating the consumption of this compound in the formation of the byproducts and keeping concentrations proportional. The latter option follows chemical equilibrium principles, and recently published kinetics 28 suggests bisphenol-A is present…”