(331a) Novel Feedback Control Method for Enhanced Ethanol Production Via Syngas Fermentation

Syngas made by gasification of biomass, coal, petcoke and waste materials or comparable gases present in industrial waste streams can be used for production of fuels and chemicals. The biological conversion of CO, CO₂ and H₂ to fuels and chemicals at commercial feasibility requires high conversion efficiency, product specificity and operation stability. Acetogenic bacteria convert CO, CO₂ and H₂ via the Wood-Ljungdahl pathway, first to acetic acid and then to ethanol by reduction of the acid. In a novel control of the fermentation process, the supply of CO, CO₂ and H₂ is regulated to maintain constant fermentation pH through formation of weak acetate buffer and reduction of nearly all produced acetic acid to ethanol. Gas flow is automatically adjusted by a standard PID controller using pH as the input parameter. Results showed that over 95% of the moles of CO plus H₂ uptake was converted into ethanol during continuous syngas fermentation in a 3-L CSTR. Continuous fermentations were operated for over 4000 h with ethanol production up to 25 g/L. In addition, a ratio of ethanol to acetic acid of greater than 30 moles per mole was demonstrated. The novel feedback control technique allowed stabilized syngas fermentation with sustained high conversion efficiency and selectivity to ethanol. The developed control method can be applied in process design for commercial syngas fermentation.