Mixing phenomena in a large-scale fermenter of starch to bio-ethanol

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: this broth is a non-Newtonian suspension with viscosity around 0.3–0.7 Pa s, and a solids concentration of about 25–30 wt%. Mixing is commonly achieved by the combined action of an external recycle flow, and 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 tentatively predicted by CFD, the characteristics of the gas bubbles and their mixing action have not yet been fully studied, despite being very important in the design of the fermenters. The research investigates the bubble-induced mixing in a 2-dimensional 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.