Pyrolysis of waste polypropylene plastics for energy recovery: Investigation of operating parameters and process development at pilot scale

Thesis (MEng)--Stellenbosch University, 2019.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.AFRIKAANSE OPSOMMING: Polipropolien (PP) is uitgeken as die plastiek wat wêreldwyd die tweede meeste voorkom in vullisterreine en dit is die vinnigste groeiende plastiekmateriaalafval in Suid Afrika. Die groeiende syfers word toegeken aan die die feit dat konvensionele meganiese herwinning beperkte kapasiteit het om afvalplastiekmateriaal wat met organiese afval gekontamineer is, the hanteer. Herwinning via pirolise is geïdentifiseer as ʼn belowende opsie om plastiekafval te bestuur, as gevolg van pirolise se vermoë om beduidende vlakke van kontaminasie te hanteer, asook die oplewering van produkte met belowende brandstof eienskappe. Pirolise onder vakuumkondisies van ander organiese afvalmateriaal, soos biomass, toon belowende opbrengste van kondenseerbare produkte. Ongelukkig, vind pirolise van plastiek onder vakuumkondisies baie min aandag. Verder, kan dit ingewikkeld wees om van banktoetsskaal na industriële skaal oor te gaan en lootsskaal prosesse kan hulpvaardig wees om tussen die twee vlakke te bemiddel om volhoubare kommersialisering van plastiekpirolise om brandstof the maak, te bekom. Die doelwit van die studie was om die effekte van sluetel-proses-parameters (insluitend temperatuur en verhittingstempo) op die verspreiding en kwaliteit van kondenseerbare produkte vanaf die pirolise van afval PP-plastiek by banktoetsskaal onder atmosferiese en vakuumdruk kondisies te ondersoek. Vier temperature (450, 488, 525 en 600 ᵒC) was onderoek by twee duidelike verhittingstempo’s van 15 ᵒC /min (stadig) en 175 ᵒC /min (vining). As deel van die doelwitte van die studie, is ‘n 5 kg/h pirolise lootsaanleg ook ontwerp en opgedra, waarna toetse verkry vanaf atmosferiese vinnige verhittingstempo’s, opgeskaleer is na die lootsaanleg. Pirolise van PP onder atmosferiese stadige en vinnige verhittingstempo’s het maksimale kondenseerbare produkte (olie en was) opbrengste gelewer van 85.6 wt.% en 84.5 wt.% onderskeidelik, als verkry by 488 °C, waarna ʼn verdere toename in temperatuur sekondêre kraking reaksies tot gevolg gehad het wat opbrengste van permanente gasse teenoor kondenseerbare produkte bevorder. Kraking was egter hewiger onder vinnige verhittingstempo as gevolg van die gekombineerde gevolge van hoër temperatuur en vinniger verhittingstempo. GC/MS-analise van kondenseerbare produkte wat onder hierdie toestande verkry is het ook gewys dat produksie van petrol bestek samestellings is meestal bevoordeel waar sekondêre kraking reaksies voorkom. Verder, was die hoërverhittingswaarde (HHV) van herwinde kondenseerbare produkte onder atmosferiese kondisies in bestek van 41 - 45 MJ/kg. HHVs het egter verminder by geweldige temperature en verhittingstempo kondisies a.g.v die toenemende vervaardiging van aromatiese verbindings. Maksimale kondenseerbare produkte opbrengste vir stadige en vinnige verhittingstempo’s onder vakuum is gerapporteer as 92.7 wt.% (by 525 °C) en 91.8 wt.% (by 488 °C) onderskeidelik. Totale opbrengste van kondenseerbare produkte onder vakuum was hoër as die ooreenstemmende opbrengste van olies onder atmosferiese toestande. Anders as atmosferiese toestande, het diesel bestek verbindings alle kondenseerbare produkte wat onder vakuum pirolise herwin is oorheers. Verder, het kondenseerbare produkte wat onder vakuum herwin is, HHV’s wat in die bestek van tussen 42 - 46 MJ/kg geval het en dit was gevind om hoog te bly selfs onder geweldige kondisies van temperatuur en verhittingstempo. Laastens, is toetse onder atmosferiese vinnige verhittingstempo’s op banktoetsskaal nageboots op die lootsaanleg (want hulle altwee benut ‘n voorverhitte reaktor). In vergelyking met die banktoetsskaal toetse, het die temperatuur waarby maksimum opbrengs van kondenseerbare produkte herwin is, met 28 °C verminder. Die maksimum opbrengs van kondenseerbare produkte het ook met 6% verminder. Hierde verskille is toegeken aan die verskil in lengte konfigurasies van die reaktore. Physico-chemiese eienskappe van olies herwin uit die lootsaanleg, as vergelyk word met kommersiële diesel en petrol brandstowwe, het gewys dat die samestelling van PP afgeleide olies in die bestek van beide diesel en petrol samestellings val