Modelling the effect of bioreactor height on stripping fermentation products from the engineered bacterium Geobacillus thermoglucosidasius

Mass transfer into microbubbles is important for stripping fermentation products from bioreactors. The effect of bioreactor height on stripping the ethanol produced by the thermophilic bacterium Geobacillus thermoglucosidasius is explored. Being thermophilic, this bacterium allows operation near the boiling point of ethanol. First, a model is developed for a tall bioreactor with a well-mixed liquid phase. An expression relating off-gas concentration, dissolved gas measurements and the mass transfer coefficient is derived. This gives a novel method for estimating the mass transfer coefficient. Second, this approach is adapted to formulate and numerically solve a model for fermentation with Geobacillus thermoglucosidasius in a tall bubble column. For validation, the model is shown to be consistent with an experimentally-validated model of fermentation in a tall bubble column in the literature. The model predicts that under typical operation with 100 µm diameter microbubbles, mass transfer is not limiting: the gas-phase product concentrations are metabolically driven. The model is run with different bubble sizes, obtaining a universal curve for the gas product concentrations. This curve indicates a previously unexplored mass transfer limited regime in which, for a 20 m column, the outlet ethanol gas-phase concentration could be increased by 45% compared to its equilibrium value.