Direct liquefaction of waste plastics and coliquefaction of coal-plastic mixtures

Abstract

We have conducted further investigations of the direct liquefaction reactions of waste plastics, medium and high density polyethylene (PE), polypropylene (PPE) and coal-plastic mixtures, varying the catalyst, temperature, gas, pressure, time and solvent. The experiments used four types of catalysts: a commercial HZSM-5 zeolite catalyst, and three catalysts synthesized in our laboratory, ferrihydrite treated with citric acid, coprecipitated Al 2O 3SiO 2, and a ternary ferrihydrite-Al 2O 3SiO 2. For direct liquefaction of plastics alone, a solid acid catalyst such as HZSM-5 or Al 2O 3SiO 2 markedly improves oil and total liquid yields, as determined by pentane and THF solubility, respectively. Yields are higher when using either a waste oil solvent or no solvent than using tetralin as the solvent. For PE, temperatures of 430 °C or higher are required for good yields, while PPE gives excellent yields at 420 °C. A commingled plastic provided by the American Plastics Council (APC) exhibited peak oil and total liquid yields at 445–460 °C. The oil yields and total liquid from PE (HZSM-5, 430 °C) and the APC commingled waste plastic decreased only slightly with decreasing hydrogen pressure (from 800 to 100 psig H 2 (cold)). Furthermore, yields were as high under nitrogen (200–600 psig, cold) as under hydrogen. Coliquefaction experiments were conducted on 50-50 mixtures of PE, PPE and the APC plastic with Black Thunder coal. For these experiments, the best results were obtained when the solvent was tetralin or a mixture of tetralin and waste oil. Lower yields were observed with only waste oil or with no solvent. Either HZSM-5 or Al 2O 3SiO 2-ferrihydrite increased oil and total yields by approximately 10% at 460 °C. Under the same conditions, yields from a PPE-coal mixture were substantially higher than those from a PE-coal mixture.