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Bubble size and liquid-side mass transfer coefficient measurements in aerated stirred tank reactors with non-Newtonian liquids

Abstract : Oxygen transfer is a key element in aerobic fermentations, especially if the culture broth's rheology is non-Newtonian, as in the case of cultures of fil-amentous fungi. It is well known that viscosity negatively affects the volu-metric mass transfer coefficient (k L a), but mechanisms involved in terms of change of interfacial area (a) and liquid-side mass transfer coefficient (k L) have still not been clearly identified. This lack of knowledge is in part due to the difficulty in measuring bubble size in aerated media, especially in vis-cous fluids. In the present study, a recently developed dual-probe method was validated and then used to measure bubble Sauter mean diameter (d 32) in water and in xanthan gum solutions, which exhibit rheological behaviors similar to filamentous fungi's broths. Ethanol was used to modify the coales-cence of the dispersed phase and the results were compared to data obtained in a filtered fermentation culture of Trichoderma reesei. Additional experiments were carried out to obtain the volumetric mass transfer coefficient and the global gas holdup (α G). It was then possible to obtain the liquid-side mass transfer coefficient and to compare it with existing models. Finally, empirical correlations were proposed to estimate d 32 , α G , k L a and k L in a mechanically stirred tank. It was observed that k L depended only on the apparent viscosity of the system, unlike all the other measured properties (k L a, d 32 , α G) which were very sensitive to power consumption, superficial gas velocity, viscosity and the presence of additives.
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Vincenzo Cappello, Cécile Plais, Christophe Vial, Frédéric Augier. Bubble size and liquid-side mass transfer coefficient measurements in aerated stirred tank reactors with non-Newtonian liquids. Chemical Engineering Science, Elsevier, 2020, 211, pp.115280. ⟨10.1016/j.ces.2019.115280⟩. ⟨hal-02447708⟩

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