Process modeling of very-high-gravity fermentation system under redox potential-controlled conditions
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The objective of this study is to evaluate and compare, both technically and economically, various glucose feeding concentrations and different redox potential settings on ethanol production under very-high-gravity (VHG) conditions. Laboratory data were collected for process modeling and two process models were created by two individual process simulators. The first one is a simplified model created and evaluated by Superpro Designer. The second one is an accurate model created by Aspen Plus and evaluated by Aspen Icarus Process Evaluator (Aspen IPE). The simulation results of the two models were also compared. Results showed that glucose feeding concentration at 250±3.95 g/L to the fermentor resulted in the lowest unit production cost (1.479 $/kg ethanol in the Superpro model, 0.764 $/kg ethanol in the Aspen Plus model), with redox potential control effects accounted. Controlling redox potential at -150 mV increased the ethanol yield under VHG fermentation conditions while no significant influences were observed when glucose feeding concentration was less than 250 g/L. Results of product sales analysis indicated that for an ethanol plant with a production rate of 85~130 million kg ethanol/year, only maintaining the glucose feeding concentration to the fermentor at around 250 g/L resulted in the shortest payout period of 5.33 years in average,, with or without redox potential control. If 300±6.42 g/L glucose feeding concentration to the fermentor is applied, it is essential to have the redox potential only controlled at -150 mV in the fermentor to limit the process payout period within 6 years. In addition, fermentation processes with glucose feeding concentration at around 200 g/L to the fermentor were estimated to be unprofitable under all studied conditions. For environmental concerns, two disposal alternatives were presented for CO2 produced during fermentation process rather than emission into atmosphere. One is to sell CO2 as byproduct, which brought 1.52 million $/year income for an ethanol plant with a capacity of 100 million kg ethanol/year. Another option is to capture and transport CO2 to deep injection sites for geological underground storage, which is already a safe and mature technology in North America, and also applicable to many other sites around the world. This would roughly add 4.78 million dollars processing cost annually in the studied scenario. Deep injection of captured CO2 from ethanol plants prevents emission of CO2 into the atmosphere, thus makes it environmental friendly.
DegreeMaster of Science (M.Sc.)
CommitteeWang, Hui; Chan, Daniel X. B.; Peng, Jian
Redox potential control