Exergy-based performance analysis of a continuous stirred bioreactor for ethanol and acetate fermentation from syngas via Wood–Ljungdahl pathway

• Exergy analysis as a decision-making tool for sustainability appraisal of continuous ethanol and acetate fermentation.
• Effect of liquid media and syngas flow rates as well as agitation speed on exergetic performance parameters of the bioreactor.
• 450 rpm agitation speed, 0.55 ml/min liquid media flow rate, and 8 ml/min syngas A flow rate as the best operational condition.
• Potential application of the developed approach to facilitate ongoing attempts to improve the performance of bioreactors.

In this work, a thermodynamic framework was proposed to achieve improved process understanding of ethanol and acetate fermentation in a continuous stirred tank bioreactor from syngas through the Wood–Ljungdahl pathway. The bioreactor performance was evaluated using both conventional exergy and eco-exergy principles to identify the effect of different operational parameters i.e. agitation speeds and liquid media flow rates as well as syngas volume flow rates and its composition on the sustainability and renewability of the process. The exergy efficiency of the bioreactor was found to be in the range of 8.14–89.51% and 8.86–89.52% using the conventional exergy and eco-exergy concepts, respectively. The maximum exergetic productivity index was found to be 6.82 and 6.90 using the conventional exergy and eco-exergy concepts, respectively, at agitation speed of 450 rpm, liquid media flow rate of 0.55 ml/min, and syngas volume flow rate of 8 ml/min containing 10% CO2, 15% Ar, 20% H2, and 55% CO. In general, the exergetic performance parameters computed using both concepts under the studied conditions did not display significant differences because of the low volume of the bioreactor and slow growth rate of the microorganisms. The results of the present study showed that exergy concept and its extensions could undoubtedly play a strategic role in assessing biofuel production pathways with respect to the issues currently of major interest in the renewable energy industry, i.e., sustainability and productively.