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

[摘要] ENGLISH SUMMARY: It is estimated that there are 60 million waste tyres disposed of across South Africa, with approximately11 million waste tyres added each year. Most of the waste tyres end up being dumped in landfills andstockpiles; 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 usedas current pathways to deal with the waste tyre problem. Current processes have shown to beenvironmentally unfriendly and/or economically unattractive due to emissions, low demand and lowmarket prices of their associated products.Pyrolysis has emerged as a potential process that can be used to tackle the problem of waste tyre disposalby valorisation through conversion into gas, liquid, and char products. The liquid product of tyre pyrolysiscontains compounds like limonene, benzene, toluene, xylene, and styrene, which could be valuablechemical feedstock due to their market values. Pyrolysis processes that focus on recovery of valuableproducts 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 pyrolysistechnology for upgrading low-value waste tyres to high-value chemicals. Limonene was chosen as thevaluable compound of interest in this study. Using literature sources, a seven-step/level hierarchicalmethod with mostly Douglas approach logic was used to develop and evaluate the process for upgradingthe waste tyres into limonene.A literature-based Aspen Plus® simulation model was developed to evaluate the technical performanceof 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. TheHCOALGEN and DCOALIGT property models were used for non-conventional components in thesimulation model.The discounted cash flow method was used to evaluate the economic feasibility of a 30 tons/day wastetyres to limonene process, producing limonene at a rate of 672 kg/day and a purity of 95 wt.%. Theresidual 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 wasfound 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 ratesrespectively, and a payback period of just under 3 years, at a current limonene selling price of $12/kg. Theprocess had capital investment requirements of 7.6 MM$. Sensitivity analysis showed that the process ismost sensitive to changes in the cost of distillation columns, limonene selling price, and the yield oflimonene. To achieve 25% IRR for economic attractiveness, a maximum column cost of 2.5 MM$, aminimum limonene selling price of $10/kg, or a minimum limonene yield of 2.1 wt.% are required. For theprocess to achieve the minimum required IRR of 12% to ensure feasibility, a maximum column cost of 5.3MM$, a minimum limonene selling price of $5/kg, or a minimum limonene yield of 1.1 wt.% are required.The tyres to TDO process showed that an IRR of 17% can be achieved, with a payback period of 4.4 yearsand an NPV of 0.71 MM$ at 12% discount rate, at a current TDO selling price of $0.27/L. A capitalinvestment of 3.3 MM$, and annual total operating cost of $525 323 will be required for the process.

[发布机构] Stellenbosch University