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Process modelling and economic evaluation of waste tyres to limonene via pyrolysis

Mulaudzi, Lusani (2017-12)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH SUMMARY: It is estimated that there are 60 million waste tyres disposed of across South Africa, with approximately 11 million waste tyres added each year. Most of the waste tyres end up being dumped in landfills and stockpiles; the dumps and stockpiles present a series of environmental and human health problems. Processes such as incineration, material recovery, re-treading and energy recovery have mostly been used as current pathways to deal with the waste tyre problem. Current processes have shown to be environmentally unfriendly and/or economically unattractive due to emissions, low demand and low market prices of their associated products. Pyrolysis has emerged as a potential process that can be used to tackle the problem of waste tyre disposal by valorisation through conversion into gas, liquid, and char products. The liquid product of tyre pyrolysis contains compounds like limonene, benzene, toluene, xylene, and styrene, which could be valuable chemical feedstock due to their market values. Pyrolysis processes that focus on recovery of valuable products are greatly desired to improve the economics of waste tyre pyrolysis. The main objective of this study was to investigate the economic feasibility of using the pyrolysis technology for upgrading low-value waste tyres to high-value chemicals. Limonene was chosen as the valuable compound of interest in this study. Using literature sources, a seven-step/level hierarchical method with mostly Douglas approach logic was used to develop and evaluate the process for upgrading the waste tyres into limonene. A literature-based Aspen Plus® simulation model was developed to evaluate the technical performance of the process, and the model was also used as a tool to ascertain the economic feasibility of the process. The PR-BM and NRTL property models were used for conventional components in the simulation model, with the UNIFAC property model used to estimate missing binary parameters for the NRTL model. The HCOALGEN and DCOALIGT property models were used for non-conventional components in the simulation model. The discounted cash flow method was used to evaluate the economic feasibility of a 30 tons/day waste tyres to limonene process, producing limonene at a rate of 672 kg/day and a purity of 95 wt.%. The residual TDO from the waste tyres to limonene process (at 523 L/hr) was also sold to generate income. The waste tyres to limonene process was then compared with a 30 tons/day conventional process of tyre pyrolysis for TDO production on the basis of economic performance. The tyres to limonene process was found to be more economically feasible than the tyres to TDO process at the end of a 10 year plant life. The tyres to limonene process had an IRR of 30%, NPVs of 6.3 and 1.1 MM$ at 12% and 25% discount rates respectively, and a payback period of just under 3 years, at a current limonene selling price of $12/kg. The process had capital investment requirements of 7.6 MM$. Sensitivity analysis showed that the process is most sensitive to changes in the cost of distillation columns, limonene selling price, and the yield of limonene. To achieve 25% IRR for economic attractiveness, a maximum column cost of 2.5 MM$, a minimum limonene selling price of $10/kg, or a minimum limonene yield of 2.1 wt.% are required. For the process to achieve the minimum required IRR of 12% to ensure feasibility, a maximum column cost of 5.3 MM$, a minimum limonene selling price of $5/kg, or a minimum limonene yield of 1.1 wt.% are required. The tyres to TDO process showed that an IRR of 17% can be achieved, with a payback period of 4.4 years and an NPV of 0.71 MM$ at 12% discount rate, at a current TDO selling price of $0.27/L. A capital investment of 3.3 MM$, and annual total operating cost of $525 323 will be required for the process.

AFRIKAANS OPSOMMING: Dit word beraam dat daar 60 miljoen bande bestaan regoor Suid-Afrika en elke jaar word daar ongeveer 11 miljoen afval bande bygevoeg. Meeste van die afval bande beland in stortingsterreine; hierdie stortingsterreine veroorsaak n reeks van omgewings- en menslike gesondheidsprobleme. Prosesse soos verbranding, die herwinning van materiale, “re-treading” en die herwinning van energie word huidiglik uitgelig as behandelingsmetodes van afval bande. Dit is wel bewys dat huidiglike prosesse, as gevolg van emissies, lae vraag en lae markpryse van verwante produkte, onaantreklik is vanaf beide n omgewings en ekonomiese standpunt. Pirolise word uitgelig as n potensiele proses om die afval band probleem te behandel deur die omskakeling daarvan na n gas, vloeistof of “char” produkte. Die vloeistof produk vanaf die pirolise van bande bevat potensiele waardevolle verbindings soos limonene, benseen, tolueen, xileen en stireen as gevolg van hul hoe markwaardes. Die winsgewendheid van die pirolise van afval bande word verbeter deur fokus te verskuif na die herwinning van waardevolle produkte. Die hoofdoel van hierdie studie was om die ekonomies vatbaarheid van pirolise tegnologie te ondersoek vir die opgradering van afval bande na waardevolle chemikaliee. Die verbinding van belang vir hierdie studie was gekies as limonene. Die opgraderingsproses van afval bande na limonene was ontwikkel en ondersoek deur gebruik te maak van n sewe-stap/vlak hierargiese metode, gebaseer op die Douglas benadering. Die tegniese en ekonomiese vatbaarheid van die proses was evalueer deur gebruik te maak van n literatuur-gebaseerde Aspen Plus® simulasie model. Die PR-BM en NRTL eienskap modelle was gebruik vir die bepaling van konvensionele komponente in the die simulasie model. Die UNIFAC eienskap model was gebruik om die onbekende parameters in die NRTL eienskap model te bepaal. Die HCOALGEN en DCOALIGT eienskap modelle was gebruik vir die bepaling van onkonvensionele komponente in die simulasie model. Die ekonomiese vatbaarheid van die afval band na limonene proses was evalueer deur die ‘discounted cash flow method’ met a voer- en produksietempo van onderskeidelik 30 ton/dag en 672 kg/day, met a suiwerheid van 95 wt%. Die TDO oorblyfsel (523 L/hr) vanaf die afval band na limonene proses was verkoop teen n wins. Die proses was vergelyk met die konvensionele proses van TDO produksie deur afval band verbranding vanaf n ekonomiese perspektief. Dit was bevind dat vanuit n ekonomiese oogpunt, die omskakeling van afval bande na limonene haalbaar was aan die einde van n 10 jaar aanleg leeftyd. Dit was bevind dat die afval band na limonene proses n IOK van 30%, n NPV van 6.3 en 1.1 MM$ het teen n afslagkoers van 12% en 25%, onderskeidelik, en n terugbetaal periode van net onder 3 jaar met die huidiglike limonene verkoopsprys van $12/kg. Die proses vereis n kapitalbelegging van 7.6 MM$. Dit was bewys deur sensitiwiteitsanalise dat die proses meer sensitief is teenoor veranderinge in the prys van distillasiekolomme, die verkoopsprys van limonene en die proses opbrengs van limonene. Ekonomiese aantreklikheid deur n 25% IRR kan bereik word, deur n maksimum kolom koste van 2.5 MM$, n minimum limonene verkoopsprys van $10/kg, of n minimum limonene proses opbrengs van 2.1 wt.% te vereis. Vir die proses om vatbaarheid te verseker deur a minimum IRR van 12% te bereik, n maksimum kolom koste van 5.3 MM$, n minimum limonene verkoopsprys van $5/kg, of n minimum limonene proses opbrengs van 1.1 wt.% word vereis. Daar was bevind dat die pirolise van bande vir die produksie van TDO n IOK van 17% en n NPV van 0.71 MM$ kan behaal met n terugbetaal periode van 4.4 jaar met die huidiglike TDO verkoopsprys van $0.27/L. Hierdie proses vereis n kapitaalbelegging van 3.3 MM$ en n total bedryfskoste van $525 323.

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