Fractional condensation of pyrolysis volatiles produced from desulphurised waste tyre feedstock

Date
2022-04
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Stellenbosch : Stellenbosch University
Abstract
ENGLISH SUMMARY: The world has always struggled with pollution and finding sustainable ways to manage waste disposal. Thus, waste valorisation strategies have become of the utmost importance in the engineering industry. There are currently millions of tons of waste tyres produced each year. Depolymerisation of these tyres can produce valuable products which provide a promising sustainable waste valorisation strategy. One of the most prominent depolymerisation strategies currently being employed is pyrolysis. The pyrolysis of waste tyres produces lumped oil, char and non-condensable gas. The lumped oil is commonly referred to as tyre derived oil (TDO). TDO has been investigated as a potential fuel replacement, however, several problems have been identified concerning TDO. TDO has a large boiling point range, high sulphur content, high aromatic content, low flashpoint and a large concentration of heavy boiling point (> 350˚C) molecules. Several studies have aimed to improve the quality of TDO through fractionation through distillation. Through these studies, it became apparent that TDO possessed three distinctive fractions (light, medium and heavy). These fractionations share some similarities with crude derived fuel products such as gasoline, diesel and marine bunker oil (MBO). This study had two distinct goals: (i) To achieve significant desulphurisation of the waste tyre feedstock and subsequently reduce the sulphur content of the produced oils. (ii) To fractionate the waste tyre pyrolysis vapours to produce three distinctive pyrolysis oil fractions, which were comparable to commercial crude derived fuels. The desulphurisation was achieved by treating the waste tyre feedstock through low temperature (180 - 220˚C) pyrolysis. The maximum desulphurisation was achieved at a treatment temperature of 220˚C, producing a feedstock that had a reduction in elemental sulphur of 54.1 wt%. Feedstock for the pyrolysis experiments was desulphurised at 190˚C (35.6 wt% elemental sulphur reduction from 2.342 wt% to 1.508 wt%) to avoid any significant rubber degradation which occurs at temperatures above 200˚C. The oil yield was optimised for reactor temperature and volatile residence time (nitrogen gas flowrate) through a two-factor central composite inscribed design. The reactor temperature was identified to significantly influence the oil yield for the parameter ranges investigated, while the nitrogen gas flowrate did not. The optimal oil yield (42.50 wt%) was achieved for parameter set points of 524˚C for reactor temperature and 1.9 l/min (volatile residence time of 3 min) for nitrogen gas flowrate. The fractional condensation system consisted of three controlled temperature condensers, which operated at 200˚C, 160˚C and 10˚C. Each condenser produced a distinctive fuel fraction in terms of boiling point range. Significant fractionation was achieved as the boiling point range of the lumped pyrolysis oil (54.41 – 246.23˚C) nearly covered the combined boiling point ranges of the obtained light-cut (48.99 – 77.32˚C), medium-cut (74.98 – 225.25˚C) and heavy-cut (133.12 – 288.75˚C) pyrolysis oil fractions. Overlap of the boiling point ranges for the produced fractions were ascribed to insufficient volatile residence times within condensers 1 and 2 of the system. The obtained fractions were analysed to determine their respective fuel properties (density, viscosity, flashpoint, ash content, moisture content and sulphur content). The fractions were compared to commercial crude derived products and other pyrolysis fractions obtained through distillation of lumped pyrolysis oil. The heavy-cut fraction was comparable to commercial MBO products (Engen 180, Engen 150) for all of the fuel properties tested. The sulphur content (0.79 wt%) did however not comply with IMO regulations published in January 2020, which reduced the sulphur content of marine fuels to 0.5 wt%. The medium-cut fraction was comparable to Chevron No.4 diesel for viscosity, moisture content, ash content and sulphur content. The light-cut fraction compared favourably with regards to viscosity, ash and moisture content, while the fraction was unfavourable in terms of density and sulphur content. It was concluded that the fractional condensation of waste tyre pyrolysis volatiles is possible without significant degradation of the produced oil fractions. The TDO fractions’ fuel characteristics can be manipulated through varying condenser operating temperatures, similar to distillation. The TDO fractions will however require extensive processing to reduce the sulphur content to within automotive fuel regulations.
AFRIKAANS OPSOMMING: Die wêreld lei onder besoedeling en sukkel om volhoubare praktyke te ontwikkel om afvalverwydering te bestuur. As gevolg hiervan, het die valorisering van afval deur herwinning strategieë uiters belangrik geraak in die ingenieursbedryf. Daar word jaarliks miljoene tonne afvalbande gegenereer regoor die wêreld. Depolimerisasie van afvalbande produseer waardevolle produkte. Depolimerisasie word dus as ‘n belowende volhoubare afvalverwyderingspraktyk gesien. Een van die mees belowende depolimerisasie strategieë was tans in praktyk gebruik word is piroliese. Die piroliese van afvalbande produseer ‘n mengsel van olie, brandkrummel en gas. Die olie megsel word ook band afgeleide olie (TDO) genoem. TDO was al deeglik ondersoek om die potensiaal daarvan te bepaal om as ‘n alternatiewe brandstof gebruik te word. Ongelukkig is daar al verskeie probleme met TDO geïdentifiseer as ‘n alternatiewe brandstof. Die probleme is TDO se wye kookpuntreeks, hoë swaelinhoud en lae flitspunt asook ‘n hoë konsentrasie van swaar kookpunt (> 350˚C) molekules. Verskeie studies het al gepoog om die kwalitiet van TDO te verbeter deur middel van distillasie. Hierdie studies het gevind dat TDO uit drie unieke fraksies (lig, medium en swaar) bestaan. Hierdie fraksies deel sekere ooreenkomste met ru-olie produkte soos petrol, diesel en mariene bunker olie (MBO). Hierdie studie het twee kenmerkende doelwitte gehad, naamlik: (i) Om beduidende ontswaeling van afvalbande te beryk wat sodoende die swael inhoud van die geproduseerde TDO verlaag. (ii) Om die geproduseerde piroliese dampe te fraksioneer om drie unieke olie fraksies te produseer wat vergelykbaar is met kommersiële ru-olie brandstof produkte. Die ontswaeling was verrig deur die afvalbande bloot te stel aan lae temperatuur (180 - 220˚C) piroliese. The maksimale ontswaeling was behaal teen ‘n temperatuur van 220˚C, wat ‘n voermateriaal geproduseer met ‘n swaelinhoud wat 54.1 wt% laer was as die rou voermateriaal. Voermateriaal vir die piroliese eksperimente was ontswael teen ‘n temperatuur van 190˚C wat die beduidende afbreek van die voermateriaal rubber voorkom het wat dikwels by temperature hoër as 200°C plaasvind. Die voermateriaal (1.508 wt% swael) het ‘n swaelinhoud gehad wat 35.6 wt% minder swael bevat het as die rou voermateriaal (2.342 wt%). Die olie opbrengs was geoptimiseer vir reaktor temperatuur en vlugtige damp verblyftyd (stikstofgas vloeitempo) duer ‘n twee faktor sentrale saamgestelde eksperimentele ontwerp. Die reaktor temperatuur was as die enigste faktor wat ‘n beduidende invloed gehad het op die olie opbrengs. Die stikstofgas vloeitempo het geen beduidende invloed gehad vir die reeks van kondisies wat ondersoek was nie. Die optimale kondisies vir maksimale olie opbrengs was ‘n reaktor temperatuur van 524°C en ‘n stikstofgas vloeitempo van 1.9 l/min (vlugtige verblyftyd van 3 min). Die fraksionele kondensasie sisteem het bestaan uit drie kondensers met verstelbare temperatuurbeheer. Hierdie kondensers was teen temperature van 200°C, 160°C en 10°C bedryf. Elke kondeser het ‘n unieke olie fraksie geproduseer in terme van kookpunt reeks. Beduidede fraksionasie was behaal aangesien die kookpunt reeks van die mengsel van TDO (54.41 – 246.23˚C) byna die gekombineerde kookpunt reekse van die ligte-snit (48.99 – 77.32˚C), medium-snit (74.98 – 225.25˚C) en swaar-snit (133.12 – 288.75˚C) TDO fraksies gespan het. Oorvleuling van kookpunt reekse was geïdentifiseer vir die geproduseerde TDO fraksies. Hierdie oorvleuling was toegeskryf aan ‘n tekort van verblyftyd in kondensers 1 en 2 van die fraksionele kondensasie sisteem. The geproduseerde fraksies was geanaliseer om hul branstof eienskappe (digtheid, viskositiet, flitspunt, asinhoud, voginhoud en swaelinhoud) te bepaal. Die fraksie was vergelyk met kommersiële re-olie produkte en ander TDo fraksie wat geproduseer is deur die distillasie van piroliese olie mengsels. Die swaar-snit fraksie was vergelykbaar met kommersiële MBO produkte (Engen 180, Engen 150) vir al die eienskappe wat geanaliseer is. Die swaelinhoud (0.79 wt%) het egter nie voldoen aan die IMO (Internasionale Mariene Organisasie) regulasie wat in Januarie 2020 gepubliseer is nie. Hierdie regulasies vereis ‘n maksimale swaelinhoud in MBO van 0.5 wt%. Die medium-snit fraksie was vergelykbaar aan Chevron No.4 diesel vir viskositiet, voginhoud en asinhoud, terwyl die fraksie ongepas was in terme van digtheid en swaelinhoud. Die ligte-snit fraksie was vergelykbaar in terme van viskositiet, asinhoud en voginhoud, terwyl die fraksie ongunstig in terme van digtheid en swaelinhoud was. Die was vasgestel dat die fraksionele kondensasie van afvalband piroliese dampe moontlik is, sonder die beduidende degradasie van die geproduseerde olie fraksies. Manipulasie van die geproduseerde olie fraksies se brandstof eienskappe is moontlik deur die verstelling van die kondenser temperature, soortgelyk aan distillasie. Die TDO fraksies gaan egter additionele prosessering benodig om ‘n swaelinhoud te kry wat voldoen aan motorbrandtsof regulasies.
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Thesis (MEng)--Stellenbosch University, 2022.
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