Economic recovery of biobutanol-a platform chemical for the sugarcane biorefinery.
Chikava, Frederick Kudzanai.
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In recent years, the South African sugar industry has faced challenges, such as drought, low prices and labour issues that have impacted negatively on the perceived sustainability. The adoption of the sugarcane biorefinery concept by the sugar industry is a possible solution to improving the sustainability of the industry amid these challenges. In this envisioned biorefinery, multiple products are created within an integrated system that maximises sustainability, as opposed to relying on producing one or very few products. In this study, the potential economic viability of the recovery of biobutanol was explored with the ultimate intention of using this biobutanol as a platform chemical for the production of higher value products to include in the biorefinery’s product portfolio. Biobutanol is produced from biomass via the ABE (acetone, butanol, and ethanol) fermentation process. Biobutanol production is characterised by very low butanol concentrations in the fermentation broth (around 2 wt. %) due to high inhibition, resulting in a very high cost of recovery (distillation) and the need for several downstream purification steps. Following a literature search on technologies that have been proposed and previously implemented for biobutanol production, processes integrating gas stripping and extraction were simulated on Aspen Plus® and techno economic analyses performed to determine the profitability based on cash flows over a 25 year period. Gas stripping and liquid-liquid extraction experiments were first carried out in order to have a way of validating simulation results. Gas stripping experiments created scenario-based results of the expected butanol concentration in the gas phase once a steady state butanol concentration can be maintained in the fermenter. The extraction experiments were conducted to establish a quick way of evaluating the extractive properties of a solvent based on the distribution coefficients and selectivities with respect to butanol. Five solvents were evaluated including hexyl acetate and diethyl carbonate, which have not been reported on but have been previously applied in biomass processing. Distribution coefficients of 3.57 and 6.15 and selectivities of 367.09 and 396.00, with respect to butanol, were obtained for hexyl acetate and diethyl carbonate, respectively. Four processes were then simulated on Aspen Plus® and they all assumed a fermentation process that make use of 281.67 t/h clear juice from a South African generic sugar mill iv model. A study estimate type economic evaluation, accurate within ±30% error, was performed with profitability being assessed in terms of the Net Present Value (NPV) and the Internal Rate of Return (IRR) over the 25 year period. Process Scheme 1 was the benchmarking case and consists of the conventional series of five distillation columns. For this process a Total Capital Investment (TCI) of US$124.85 million was obtained and based on the sales and production costs a negative NPV of US$3.80 million was obtained. This indicates a non-viable process under the current economic conditions. Process Scheme 2 included in situ recovery by gas stripping and final purification using distillation. Five distillation columns were still required to purify the condensate from the stripper due to a large amount of water that is carried in. The increased productivity in the fermenter and the reduction the downstream column sizes in this process, compared to the benchmarking case, resulted in a reduced capital cost of US$67.43 million. This recovery process also yielded a potential to be profitable with a positive NPV of US$505.88 million and an IRR of 31%. This was attributed to the reduced TCI as well as the ability of the process to yield all the three ABE solvents to sellable purities. Process Scheme 3 that included gas stripping and liquid-liquid extraction had almost the same TCI as Process Scheme 2 (US$68.94 million) but could only yield butanol to sellable quality due to the selective property of the solvent used (2-ethyl-hexanol). This reduction in sales led to an IRR of 6% which is below the discounted rate used (10%) although a positive NPV of US$82.38 million resulted. Process Scheme 4, making use of a two-stage gas stripping and distillation, was the most profitable process and it was concluded it would be the process to attach to the sugar mill model and also to be considered for the higher value chemical production. An NPV of US$524.09 and an IRR of 32% were realised for this process. Sensitivity analyses on these four processes showed that the cost of the substrate (clear juice) and the butanol selling price have the major effects on the profitability. It was, therefore, recommended that other streams from the sugar mill be considered as substrates for higher value chemical products which can attract higher prices than butanol which is regulated by the petro based butanol. Finally, a structure of a functionalised ionic liquid was suggested based on group contribution methods to be a potential reactive extraction reactant for converting butanol to a higher value ester product.