Depolymerization of LDPE under low pressure-hydrothermal processing and pressurized pyrolysis: Effect of the ZSM-5 catalyst

This study compares the performance of autogenous pressurized pyrolysis (P-PYR) and low-pressure hydrothermal liquefaction (LP-HT) for transforming low-density polyethylene (LDPE) into gasoline-range and diesel-range hydrocarbons. Both processes were carried out in a closed reactor, at temperatures from 350 to 450 °C. The effect of ZSM-5 zeolite as a catalyst was also analyzed. The evaluated parameters included the amount of the gas, oil, and wax fractions formed; the chemical composition of the gas and liquid fractions; and the simulated distillations (SIMDIS) of the liquid fractions. The distillation curves obtained by SIMDIS were used to estimate the yield of gasoline range and diesel-range hydrocarbon fractions. In the non-catalyzed processes, all the products had nearly the same composition, indicating that water had a limited effect on the depolymerization process. In the catalyzed processes, there was a slight change in the yield of the recovered products when water was present. The ZSM-5 zeolite also had an impact on the chemical composition of the produced fractions. Without the catalyst, both LP-HT and P-PYR (at 400 °C) produced about 40 % gasoline-range and 25 % diesel-range hydrocarbons, leading to a total yield of fuel-range hydrocarbons of around 65 %. When the ZSM-5 catalyst was used, the recovered oil mostly contained single-ring aromatic compounds, leading to a fuel yield of around 44 % (at 375 °C), with gasoline-range hydrocarbons making up 36–38 % and diesel-range hydrocarbons 6–8 %. The results indicate that ZSM-5 zeolite can alter the makeup of the thermochemical products from LDPE, and at the experimental conditions studied, water only had a limited effect.