Comparison of the technical and economic feasibility of devulcanisation processes for recycling waste tyres in South Africa

Edwards, Devon William (2016-03)

Thesis (MEng)--Stellenbosch University, 2016

Thesis

ENGLISH ABSTRACT: The problem of the accumulation of waste tyres is receiving increased attention in the 21st century as a result of environmental concerns as well as the economic undesirability of discarding the valuable materials still present in tyres at the end of their useful life. The difficulty associated with recycling waste tyres is linked to the stable thermoset network structure of the vulcanised rubber comprising the majority of a waste tyre’s mass. The recovery of value from tyres via incineration for their relatively high calorific value has been a popular method of diverting tyres from landfills and stockpiles in the past, although newer methods such as pyrolysis and devulcanisation aim to recover more value than merely the energy content of tyres. Pyrolysis processes aim to recover and purify valuable chemicals generated by the thermal decomposition of the rubber compounds in tyres. Devulcanisation processes aim for the controlled breakdown of the vulcanised rubber network in such a way that the rubber regains its thermoplastic properties and can be moulded and revulcanised into new products, without a significant loss of the important mechanical properties associated with vulcanised rubber products. The shortcomings identified in the literature include the lack of comparable technical data between devulcanisation technologies, and the near absence of any form of energy consumption and economic data associated with devulcanisation processes. This study aimed to identify promising devulcanisation technologies and address the shortcomings identified in the literature by generating comparable technical and economic data for the selected devulcanisation technologies. The devulcanisation technologies identified for further analysis included the extrusion-based mechanical and mechanochemical devulcanisation processes. The experimental work showed that increasing extrusion temperature has a strong effect on increasing the extent of devulcanisation in both devulcanisation processes. Varying screw speed in the mechanical devulcanisation process showed a very weak effect on the extent of the devulcanisation reaction. Increasing concentration of the devulcanisation chemical in the mechanochemical devulcanisation process caused an increase in the extent of the reaction, although the effect was rather weak. Overall, the mechanochemical devulcanisation process resulted in a substantially higher selectivity for crosslink scission and therefore higher product quality in comparison to the mechanical devulcanisation process. Economic analysis of the processes was conducted assuming various scales of operation from approximately 400 tons/year to 7000 tons/year, using scaled-up power consumption data generated during the experimental work. The mechanical devulcanisation process was found to be likely to outperform the mechanochemical devulcanisation process from an economic perspective due to the high costs of the extra chemicals required for the mechanochemical devulcanisation process. It should be noted, however, that the economic analysis did not take into account the potentially higher market value of the reclaimed rubber produced by the mechanochemical devulcanisation process. Therefore, further market research will be required in order to come to a firm conclusion as to which process will be more economically viable. A sensitivity analysis also showed that the economics of both processes are very sensitive to the power consumption, which could be a major problem for devulcanisation processes in South Africa.

AFRIKAANS OPSOMMING: Die probleem van die opeenhoping van afval bande ontvang baie aandag in die 21ste eeu as gevolg van hulle effek op die omgewing, sowel as die ekonomiese onwenslikheid van die weggooi van die waardevolle materiaal wat nog steeds teenwoordig in die bande is. Die probleem wat verband hou met die herwinning van afval bande is gekoppel aan die stabiele termoset netwerk struktuur van die gevulkaniseerde rubber in die bande. Die herwinning van waarde van afval bande volg gewoonlik ʼn roete van verbranding vir hulle hoë energie-inhoud, maar nuwe metodes soos pirolise en devulkanisasie probeer om meer waarde te herwin as net die energie-inhoud. Devulkanisasie prosesse mik vir die beheerde uiteensetting van die gevulkaniseerde rubber netwerk in so 'n manier dat die rubber sy termoplastiese eienskappe herwin en kan gevorm word en hervulkaniseerd in nuwe produkte, sonder om die meganiese eienskappe te verminder. Die gapings wat in die devulkanisasie literatuur voorkom is die skaarsheid van vergelykbare tegniese data tussen devulkanisasie prosesse, asook die afwesigheid van metings van die elektriese krag verbruik. Die doel van hierdie werk was om die vereisde data te skep vir twee onderskeide devulkanisasie prosesse, en daarna die prosesse te vergelyk. Die prosesse wat gekies is vir die studie was meganiese devulkanisasie en meganochemiese devulkanisasie. Die eksperimentele werk het getoon dat die verhoging van ekstrusie temperatuur 'n sterk invloed op die verhoging van die mate van devulkanisasie in beide devulcanisation prosesse. Wisselende skroef spoed in die meganiese devulkanisasie proses het 'n baie swak invloed op die mate van die devulkanisasie reaksie. Toenemende konsentrasie van die chemikalieë in die meganochemiese devulcanisation proses veroorsaak 'n toename in die mate van die reaksie, alhoewel die effek swak is. Oor die algemeen het die meganochemiese devulkanisasie proses gelei tot 'n aansienlik hoër selektiwiteit vir kruis-skakel verdeling en dus hoër kwaliteit van die produk in vergelyking met die meganiese devulkanisasie proses. Ekonomiese analise het gewys dat die meganiese devulkanisasie proses meer waarskynlik winsgewend sal wees in vergelyking met die duurder meganochemiese proses op ʼn skaal van 400 tot 7000 ton/jaar. Dit moet egter daarop gelet word dat die ekonomiese analise nie rekening gehou het met die potensieele hoër markwaarde van die herwonne rubber vervaardig deur die mechanochemical devulcanisation proses nie. Daarvoor sal verdere marknavorsing benodig word om by 'n vaste gevolgtrekking te kom. A sensitiwiteitsanalise het ook getoon dat die ekonomie van beide prosesse baie sensitief is vir die krag verbruik, wat 'n groot probleem vir devulkanisasie prosesse in Suid-Afrika kan wees.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98807
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