Static mixers were introduced in the draught tube of a three-phase, concentric tube type airlift bioreactor, resulting ina 30% increase in ethanol productivity during glucose fermentation with a highly flocculent strain of Saccharomycescerevisiae. Increased productivity was obtained as a consequence of the floc size reduction provoked by the new designof the draught tube and of the smaller applied aeration rates, allowing a higher dilution rate to be used. Steady state dataat different dilution rates were measured for both systems and the results were compared in terms of specific consumption/ production rates and ethanol productivity. An approach based on heterogeneous catalysis principles was used inorder to consider the presence of porous yeast flocs. Total glucose conversion was achieved at a dilution rate of 0.34 hˉ¹.The smaller floc size contributed to the higher observed reaction rates by reducing diffusional limitations inside the flocs.The respiratory quotient had a constant value (around 23) at all dilution rates, meaning that the metabolic state of thecells in the flocs remained constant, having a strong fermentative metabolism.
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