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Static mixers were introduced in the draught tube of a three-phase, concentric tube type airlift bioreactor, resulting in a 30% increase in ethanol productivity during glucose fermentation with a highly flocculent strain of Saccharomyces cerwisiae. Increased productivity was obtained as a consequence of the floc size reduction provoked by the new design of the draught tube and of the smaller applied aeration rates, allowing a higher dilution rate to be used. Steady state data at 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 in order to consider the presence of porous yeast flocs. Total glucose conversion was achieved at a dilution rate of 0.34 h-'.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 the cells in the flocs remained constant, having a strong fermentative metabolism.Des melangeurs statiques ont ete introduits dans un tube d'aspiration d'un bioreacteur triphasique a courant ascendant de type tube concentrique, ce qui entraine une augmentation de 30 % de la productivite de I'ethanol lors de la fermentation du glucose avec une souche hautement floculante de Saccharomyces cerevisiae. Une productivite accrue a ete obtenue consecutivement a la reduction de la taille des flocs provoquee par la nouvelle conception du tube d'aspiration et les plus petites vitesses d'aeration appliquees, ce qui permet d'utiliser une plus grande vitesse de dilution. Les donnees en regime permanent a differentes vitesses de dilution ont ete mesurees pour les deux systemes et les resultats ont Cte compares en termes de vitesses specifiques de consommation/production et de productivite de I'ethanol. Une approche basee sur les principes de catalyse heterogene a ete utilisee pour tenir compte de la presence de flocs de levure poreux. La conversion totale du glucose a ete realisee a une vitesse de dilution de 0,34 h-I. La taille plus petite des flocs contribue aux plus grandes vitesses de reaction observees en reduisant les limitations diffusionnelles a I'interieur des flocs. Le quotient respiratoire a une valeur constante (environ 23) a toutes les vitesses de dilution, ce qui signifie que I'etat metabolique des cellules dans les flocs demeure constant, car elles ont un fort metabolisme fermentatif.

Limitations in the diffusion of substrates into the¯ocs will condition cell metabolic behaviour, having obvious consequences on growth and product formation. Polymeric additives have been used aiming the reduction of those limitations. The knowledge of the concentration pro®les and metabolic¯uxes of glucose and oxygen inside the¯ocs would bring valuable information about the conditions under which a fermentation should run. Direct measurement of such pro®les is rather dif®cult but their simulation has been performed and is presented in this work.Calculations were made for different possible sizes of the yeast¯ocs, considering also the presence or absence of a polymeric additive. Only a small percentage of the cells in the¯ocs metabolise glucose oxidatively due to severe oxygen limitations. The presence of the polymeric additive increases the ratio of cells operating under respiratory metabolism over those under fermentative metabolism: from 0.4% to 5.7% without additive to 1.2% to 8.5% with additive, depending on the bulk glucose concentration. Also, based on this data, it is possible to justify the yeast oc natural shape.List of symbols a speci®c mass transfer area, 1/m

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