EFFECTS OF MIXING ON THE PRODUCT DISTRIBUTION OF PYROLYSED PLASTIC WASTES

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Department of Engineering

ABSTRACT
The effect of mixing different plastic types on the product distribution of pyrolysed plastic wastes was successfully investigated. Samples of waste low density polyethylene (LDPE), high density polyethylene (HDPE), polystyrene (PS), and polypropylene (PP) were subjected to thermal pyrolysis giving non-condensable gases, liquid/wax and char as products. A semi-batch reactor with an external heating source was used to first pyrolyse waste PS and PP at a low heating rate of 1oC/min. Aromatic compounds and fatty acid impurities components dominated respectively, and waste PS and PP were deemed unsuitable for higher heating rate studies. An electric tubular furnace was used as the heating source for higher heating rates. Heating rate values of 22, 26 and 32oC/min were used and the effect on the volume of non-condensable gases produced showed a direct relationship for HDPE and an inverse relationship for LDPE, with a maximum residence time of 25 minutes for both samples. A temperature range of 480oC – 600oC and a heating rate of 22oC/min was observed for maximum gas production, and was therefore used for the investigation of the effect of mixing. Mixtures containing 0%, 20%, 40%, 50%, 60%, 80% and 100% LDPE in HDPE were used and the gaseous and liquid product were analyzed by gas chromatography and gas chromatography-mass spectrometry respectively. Results obtained showed a similar hydrocarbon product distribution for both LDPE and HDPE gas products - 100% LDPE showed a composition of 21.84%, 47.39%, 20.78%, 8.40%, and 1.59% and 100% HDPE showed a composition of 18.88%, 46.91%, 22.89%, 9.59%, and 1.73% for C1, C2, C3, C4, and C5+ respectively. The presence of LDPE in the LDPE-HDPE mixture favoured the production of C1 up to 99 mol.% and no trace of carbon monoxide or carbon dioxide was found in the gas samples. The liquid products showed hydrocarbons with carbon number ranging from C9 – C27 with pentadecane (C15H32) dominating. The energy content of the gas products calculated based on the lower heating values (LHV) using Aspen HYSYS® v8.0 ranged from 47.03 – 49.99 MJ/kg and were comparable with energy values for coal, natural gas and gasoline.