Techno-economic evaluation of redox potential-controlled ethanol fermentation processes

Nine redox potential-controlled operating conditions during ethanol fermentation were evaluated by means of a process simulator. Results show that the lowest unit production cost (0.764 $/kg ethanol) was estimated for ∼250 g glucose/L among all studied conditions. Controlling the redox potential at −150 mV increases the ethanol yield when glucose concentrations are greater than 250 g/L; while no significant effects were observed at glucose feeds below 250 g/L. For a facility with an annual production capacity of ∼85–130 million kg ethanol, the shortest payout period of ∼5.33 years was obtained under ∼250 g glucose/L conditions, either with or without redox potential control. If ∼300 g glucose/L is applied, controlling the redox potential at −150 mV is required to limit the process payout period to be less than 6 years. Carbon dioxide disposal options are presented. Selling CO2 as a byproduct can bring in 1.43 million $/year income for an ethanol plant with a capacity of 100 million kg ethanol/year. Capture and transport of CO2 to deep injection sites for geological underground storage is economically unprofitable and adds ∼4.78 million $/year in processing costs; however, this option results in a net removal of CO2 from the atmosphere, making it environmentally preferable. The presented process model is available upon request.

► An Aspen model was used to evaluate redox potential-controlled ethanol fermentation process. ► The lowest unit ethanol production cost is obtained when operating at ∼250 g glucose/L. ► Three CO2 disposal plans were proposed and compared for their impacts on the overall profitability.