Bioreactor Scale‐Up for the Oxygen‐Sensitive Culture Bacillus subtilis: The Influence of Stirrer Shaft Geometry

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Abstract
The relative amounts of the metabolites acetoin and 2,3‐butanediol, formed in a Bacillus subtilis culture, depend strongly on the dissolved oxygen concentration (DO). Hence any factor determining DO can influence the product ratio Ac/Bu. Experiments in a 0.045 m3 stirred bioreactor, fitted with three Rushton turbines, have shown different product ratios when the diameter of the drive shaft was changed and all other conditions were kept constant. This unexpected result was attributed to changes in the power consumption and the pumping and gas handling capacities of the turbines. B. subtilis was cultivated in three geometrically similar vessels (0.045, 0.45, and 4.5 m3) using a constant superficial gas velocity (0.0075 m s−1). The product ratio and the volumetric mass transfer coefficient were essentially independent of scale when the gassed power input per unit volume was constant. This scale‐up result reflects the importance of adequate oxygenation of this culture, which was violated when the shaft diameter was not scaled with the vessel diameter. Geometrical similarity was an important factor in the scale‐up of this fermentation.