Composition of products from the pyrolysis of polyethylene and polystyrene in a closed batch reactor: Effects of temperature and residence time

The compositions of the pyrolysis products of pure low-density polyethylene (LDPE) and polystyrene (PS) and their mixtures have been investigated over a temperature range from 300 to 500 °C. The pyrolysis experiments were carried out in a closed batch reactor under inert nitrogen atmosphere to study the effects of reaction temperature and residence time. LDPE was thermally degraded to oil at 425 °C however, beyond this temperature the proportion of oil product decreased as a result of its conversion to char and hydrocarbon gas. Compositional analysis of the oil products showed that aliphatic hydrocarbons were the major components, but the proportion of aromatic compounds increased at higher temperatures and residence times. On the other hand, PS degraded at around 350 °C, mainly into a viscous dark-coloured oil. The formation of char only increased marginally until 425 °C, but was dramatically enhanced at 450 and 500 °C, reaching up to 30 wt.%. The oil product from PS even at 350 °C consisted almost entirely of aromatic compounds especially toluene, ethylbenzene and styrene. Under increasing temperatures and residence times, the oil product from PS was preferentially converted to char, while gas formation was preferred for the oil from LDPE. For instance at 500 °C, PS produced about twice the amount of char obtained from LDPE indicating the role of aromatic compounds in char formation via condensation of the aromatic ring structure. During the co-pyrolysis of a 7:3 mixture of LDPE and PS, wax product was observed at 350 °C leading to oil at 400 °C, indicating that the presence of PS influenced the conversion of LDPE by lowering its degradation temperature. The mixture produced more oil and less char than the individual plastics at 450 °C.