Gas circulation rate and medium exchange ratio as influential factors affecting ethanol production in carbon monoxide fermentation using a packed-bed reactor†
A packed-bed reactor (PBR) which has recyclable internal gas and medium exchange functions for carbon monoxide (CO) fermentation was operated using an ethanol producing acetogen, Clostridium autoethanogenum DSM 10061. The PBR was designed to circulate the headspace gas, so as to achieve a higher volumetric gas–liquid mass transfer coefficient (kLa). Periodic medium exchange was performed to avoid nutrient limitations. The CO utilization rate and ethanol productivity were maximized when the medium exchange ratio was held constant, and varied linearly with the gas circulation rate. The PBR operation using 2× medium composition resulted in maximum 16.1 g L−1 ethanol titer and 0.13 g L−1 h−1 productivity. Consistent ethanol production was achieved throughout the reactor runs, confirming the stability of the PBR operation. The cost effectiveness of the PBR operation was determined based on the medium prices and the ethanol titer. The most realistic operation parameters were 0.01 h−1 (mean dilution rate) and 1.5 vvm (gas circulation rate) using 12.6 times the nutrient reinforced medium.