Sustainability assessment of sugarcane residues valorization to biobutadiene by exergy and exergoeconomic evaluation

Butadiene is one of the essential platform chemicals for producing a broad spectrum of synthetic rubbers, polymers, copolymers, and latexes. Among the various routes developed to make bio-based butadiene, dehydrogenation/dehydration of bioethanol has exhibited great promise regarding economic viability and ecological soundness. In this study, a process producing bioethanol from sugarcane residues and then converting the synthesized bioethanol to biobutadiene was simulated and analyzed. The process included biomass pretreatment, enzyme production, saccharification/fermentation, bioethanol separation/distillation, ethanol dehydrogenation, acetaldehyde liquefaction/compression, biobutadiene synthesis, biobutadiene liquefaction/compression, and biobutadiene recovery/purification subsystems. Exergy analysis was first used to identify the causes and sources of thermodynamic losses of the process. In continuation, exergoeconomic analysis was performed based on the obtained exergetic results to reveal the hotspots of the cost losses of the process. The highest exergy destruction rate was recorded for the bioethanol separation/distillation subsystem, amounting to 34.9% of the total exergy destruction rate of the process. The exergy efficiency of the process was 17.5% by considering only biobutadiene as the main product of the process, while this value was increased to 87.5% by considering all useful or potentially useful outputs. The saccharification/fermentation subsystem showed the highest total hourly exergoeconomic cost (504 USD/h), corresponding to 23.1% of the total hourly exergoeconomic cost of the process. The unit costs of exergy for the biobutadiene (main product) and stillage (main by-product) streams were 6.2 and 4.2 USD/GJ, respectively. Overall, efforts to improve the process exergoeconomically should be mainly directed towards reducing the investment cost of the biomass pretreatment subsystem while diminishing the exergy destruction cost of the saccharification/fermentation subsystem.