Quantitative assessment of H2 and CO2 supersaturation during thermophilic cellobiose fermentation with Clostridium thermocellum

摘要

Off-gas analysis is a common method for assessing the performance of many bioprocesses, but often little regard is given to the dissolved gas concentrations to which the microbes are actually exposed. In this work, a membrane-inlet mass spectrometer (MIMS) was used to quantitatively assess dissolved gas (H2 and C02) concentrations during thermophilic fermentation of 2 g L1 cellobiose with Clostridium thermocellum in a N2-sparged bioreactor. During baseline operating conditions (100 rpm mixing and 10mL min-1 N2 sparging) H2(aq), and to a lesser extent C02(aq), were detected in levels higher than would be predicted using gas-phase measurements and the assumption of equilibrium with the liquid phase. Under these conditions, H2 supersaturation ratios (RH2) as high as 22.8 were observed, and C02 supersaturation ratios were measured to be as high as 1.6. In an effort to improve liquid-to-gas mass transfer and reduce supersaturation, a second condition was evaluated in which stirring was increased to 200rpm, N2 sparging was increased to 100 mL min 1, and surface mixing was applied. Under this condition, H2 supersaturation was reduced (maximum RH2= 9.6), while C02(aq) measurements showed no significant supersaturation. Accounting for the elevated [H2](aq) in the measured with the MIMS led to an improved electron balance, and in a similar way the additional C02(aq) (when supersaturated) led to improved carbon recovery. This work shows how liquid-to-gas mass transfer can influence carbon flux in microbial metabolism, which is an important process engineering parameter in scale-up.