Performance of a continuous screw kiln reactor for the thermal and catalytic conversion of polyethylene–lubricating oil base mixtures

Both thermal and catalytic cracking of mixtures consisting of low-density polyethylene (LDPE) and a lubricating oil base have been investigated in a continuous screw kiln reactor, provided with two reaction zones operating at different temperatures (T1/T2). The incorporation of the lubricating oil into the plastic leads to a significant decrease in its viscosity which favors the mixture flow through the reactor. Thermal cracking at 450/500 °C of LDPE–lubricating oil base mixtures with compositions ranging from 40/60–70/30 (%, w/w) led in all cases to their almost complete conversion (∼90%) towards a broad spectrum of C1–C40 hydrocarbons. Catalytic cracking of a 70/30 (%, w/w) LDPE–lubricating oil base mixture over mesoporous Al-MCM-41 catalysts at 400/450 °C proceeded with lower activity with regards to the pure LDPE catalytic cracking. This result is related to both the lower reactivity of the oil compared to the pure polyolefin and to the poisoning of the catalyst acid sites by sulfur/nitrogen-containing compounds present in the lubricant oil base. However, at higher temperatures (450/500 °C), complete conversion of the mixture was attained over both Al-MCM-41 and nanocrystalline HZSM-5 (n-HZSM-5) catalysts. Al-MCM-41 materials leads mainly to C5–C12 products (65% selectivity) whereas lighter hydrocarbons were formed preferentially over n-HZSM-5 (63% selectivity towards C3–C5 compounds). These results demonstrate that the screw kiln reactor is an efficient system for the continuous processing of plastic–lubricant oil mixtures by thermal and catalytic treatments.