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 similar to 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 similar to 85-130 million kg ethanol, the shortest payout period of similar to 5.33 years was obtained under similar to 250 g glucose/L conditions, either with or without redox potential control. If similar to 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 similar to 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. (C) 2012 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.