The bubble-induced mixing in starch-to-ethanol fermenters

China launched an important production of fuel-ethanol by fermentation of non-grain feedstock, with cassava mostly used as starch source. The industrial fermenters have diameters and liquid levels between 6 and 16 m. The mixing of the fermentation broth is important for the efficient operation. Mixing is commonly achieved by the combined action of (i) an external recycle flow, and (ii) the gas-induced mixing by CO2-bubbles formed during the bio-reaction. To avoid solids sedimentation, flat-bottom fermenters add mechanical impellers. Whereas the effects of impellers and external recycle flow can be predicted by CFD, the characteristics of the gas bubbles, and their mixing action have not yet been fully studied, despite being of paramount importance in the design of the fermenters. The research investigates the bubble-induced mixing in a two-dimensional scaled-down experimental rig. Experimental results are used to define the dominant parameters of a model approach to bubble-induced mixing. The liquid mixing data can moreover be used to validate complex CFD approaches to the overall mixing.

► Fermenters are widely used to produce bio-ethanol from starch-biomass.
► The microbial conversion produces significant quantities of CO2 bubbles.
► These bubbles induce a liquid mixing in the fermenter.
► Experiments are used to define the dominant bubble-induced parameters, and are transformed into a simplified mixing model.