Catalytic Thermal Cracking of Postconsumer Waste Plastics to Fuels. 1. Kinetics and Optimization

Thermogravimetric analysis (TGA) was used to investigate thermal and catalytic pyrolysis of waste plastics such as prescription bottles (polypropylene/PP), high density polyethylene, landfill liners (polyethylene/PE), packing materials (polystyrene/PS), and foams (polyurethane/PU) into crude plastic oils. In the first phase of this investigation, a statistical design experiments approach identified reaction temperature and time as the most important factors influencing product oil yield. Kinetic parameters including activation energy determined for both catalytic and noncatalytic processes showed a reduction in activation energy for the catalytic reactions. In the second phase, the interactions of reaction temperature and time with a number of catalysts were investigated to determine the effect on the yield of crude plastic oil. It was found that Y-zeolites increased conversion at reduced temperature for PP and PE while spent fluid catalytic cracking and sulfated zirconia catalysts supported pyrolytic decomposition of PS and PU foams. Response surface methodology (RSM) was utilized to optimize TGA conditions for pyrolytic decomposition of PP. The results were then validated through batch scale experiments, and the resulting crude oils were characterized and distilled into motor gasoline, diesel #1, diesel #2, and vacuum gas oil fractions. Catalysts enhanced cracking at lower temperatures and narrowed the molecular weight (hydrocarbon) distribution in the crude oils. Chemical characterization of the crude oils indicated an increased gasoline-range fraction in oils obtained in the presence of catalyst while the distillate fractions were more evenly distributed among gasoline-range and diesel-range hydrocarbons in the absence of catalyst. The distillates obtained were characterized for fuel properties, elemental composition, boiling point, and molecular weight distribution. The fuel properties of the diesel-range distillate (diesel fraction) were comparable to those of ultralow sulfur diesel (ULSD).