Browsing by Author "Parku, George Kofi"
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- ItemPyrolysis of waste polypropylene plastics for energy recovery: Investigation of operating parameters and process development at pilot scale(2019-04) Parku, George Kofi; Gorgens, Johann F.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.ENGLISH ABSTRACT: Polypropylene (PP) has been identified as the second most abundant plastic waste in landfills globally and the fastest growing plastic waste in South Africa. The increasing numbers have been attributed to the limitation of conventional mechanical recycling not being able to handle plastic wastes contaminated with other organic wastes. Recycling via pyrolysis has been identified as a promising option to managing these plastic wastes due to its ability to handle significant levels of contamination and also yielding products with huge fuel prospects from plastics. Pyrolysis under vacuum conditions for other organic wastes such as biomass has demonstrated promising yields of condensable products. Unfortunately, pyrolysis of plastics under vacuum has received very little attention. Also, transition from bench to industrial scale applications of pyrolysis processes could be complicated and pilot scale processes to mediate between both levels are very helpful towards attaining the sustainable commercialisation of plastics pyrolysis into fuels. The aim of the study was to investigate the effects of key process parameters (that include temperature and heating rate) on products yield distribution and quality of condensable products from the pyrolysis of waste PP plastics at bench scale under atmospheric and vacuum conditions. Four temperatures (450, 488, 525 and 600 ᵒC) were investigated at two distinct heating rates of 15 ᵒC/min (slow), 175 ᵒC/min (fast). As part of the aims of the study, a 5 kg/h pyrolysis pilot plant was also designed and commissioned after which tests obtained from atmospheric fast heating rates were scaled-up to the commissioned pilot. Pyrolysis of PP under atmospheric slow and fast heating rates revealed maximum condensable products (oil and wax) yields of 85.6 and 84.5 wt.% respectively all attained at 488 ᵒC after which further increase of temperature resulted in secondary cracking reactions which promoted yields of permanent gases against condensables. Cracking was however more severe under fast heating rates due to the combined effects of higher temperature and faster heating rate. Gas Chromatography/ Mass Spectrometry (GC/MS) analysis of condensable products obtained under these conditions also revealed that production of gasoline range compounds was favoured mostly under reactions where some cracking reactions occur. Also, Higher Heating Values (HHVs) of condensable products recovered under atmospheric conditions ranged between 41 - 45 MJ/kg. HHVs were however seen to decrease at severe temperature and heating rate conditions due to the increased production of aromatics. Maximum yields of condensable products for slow and fast heating rates under vacuum conditions were reported to be 92.7 wt.% (at 525 ᵒC) and 91.8 wt.% (at 488 ᵒC) respectively. Total yields of condensable products under vacuum were observed to be higher than the corresponding yields under atmospheric conditions. Unlike atmospheric conditions, diesel range compounds predominated all condensable products recovered under vacuum pyrolysis. In addition, HHVs of condensable products retrieved under vacuum ranged between 42 - 46 MJ/kg and were seen to remain high even at severe conditions of temperature and heating rate. Lastly, tests under atmospheric fast heating rates at bench were mimicked on the commissioned pilot plant (because they both employ a pre-heated reactor). Compared to the bench scale test, temperature at which maximum yield of condensable products were retrieved on the pilot decreased by 28 ᵒC with the maximum condensable products yield also decreasing by 6%. These differences were blamed on different reactor length configurations. Physico-chemical properties of oils recovered from the pilot when compared to commercial diesel and gasoline fuels disclosed that PP derived oils contained compositions of both diesel and gasoline range compounds.