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The effects of solids loading on gas hold-up and oxygen transfer in external-loop airlift bioreactors with non-Newtonian fermentation media are discussed. Experiments were performed in two model external-loop airlift bioreactors with aqueous solutions of carboxymethyl cellulose (CMC) and xanthan gum representing non-Newtonian flows. Low-density plastic particles of 1030 and 1300 kg m-3 were used and the solids loading was varied in the range 0-20% (v/v). For the inelastic non-Newtonian CMC aqueous solutions, the presence of low-density solid particles slightly increased the riser gas hold-up, &gr, but decreased the volumetric mass transfer coefficient, k,a. On the other hand, #gr decreased but k,a increased with solids loading in the viscoelastic non-Newtonian xanthan gum aqueous solution. The extent of these effects depended on non-Newtonian flow behavior. Theoretical models of riser gas hold-up and volumetric mass transfer coefficient have been developed. The capability of the proposed models was examined using the present experimental data obtained in the model external-loop airlift bioreactors and the available data in the literature. The data were successfully correlated by the proposed correlations except the results for k,a coefficient in the xanthan gum solution.Key words: three-phase external-loop airlift bioreactor, non-Newtonian fermentation broths, gas hold-up, volumetric mass transfer coefficient. a A d Ar C CAL D Dd D r EXP 9 Hd kI. NOTATION Specific surface area (m-') Downcomer cross-sectional area (m2) Riser cross-sectional area (m2) Coefficient Calculated Diffusivity (m2 s -l ) Downcomer diameter (m) Riser diameter (m) Experimental Gravitational acceleration (m s -2, Length of vertical portion of downcomer (m) Liquid-phase mass transfer coefficient (m s -') K R n Usgr V L 3 & PI P S PI 0 z +v * To whom correspondence should be addressed. 325 Consistency index in a power-law model (Pa s") Effective consistency index in a power-law model for (solid-non-Newtonian liquid) two-phase system (eqn (2)) (Pa s") Flow index in a power-law model Superficial gas velocity in the riser (m s -l ) Liquid volume (m3) Shear rate (SKI) Energy dissipation rate per unit mass (W kg-') Liquid density (kg m-3) Solid density (kg m-3) Effective density of (solid-liquid) two-phase system (eqn (2)) (kg m-3) Surface tension (N m-') Shear stress (Pa) Riser gas hold-up J . Chem. Tech. Biotechnol. 0268-2575/96/$09.00 0 1996 SCI. Printed in Great Britain

The effects of solids loading on gas hold‐up and oxygen transfer in external‐loop airlift bioreactors with non‐Newtonian fermentation media are discussed. Experiments were performed in two model external‐loop airlift bioreactors with aqueous solutions of carboxymethyl cellulose (CMC) and xanthan gum representing non‐Newtonian flows. Low‐density plastic particles of 1030 and 1300 kg m−3 were used and the solids loading was varied in the range 0–20% (v/v). For the inelastic non‐Newtonian CMC aqueous solutions, the presence of low‐density solid particles slightly increased the riser gas hold‐up, ϕgr, but decreased the volumetric mass transfer coefficient, kLa. On the other hand, ϕgr decreased but kLa increased with solids loading in the viscoelastic non‐Newtonian xanthan gum aqueous solution. The extent of these effects depended on non‐Newtonian flow behavior. Theoretical models of riser gas hold‐up and volumetric mass transfer coefficient have been developed. The capability of the proposed models was examined using the present experimental data obtained in the model external‐loop airlift bioreactors and the available data in the literature. The data were successfully correlated by the proposed correlations except the results for kLa coefficient in the xanthan gum solution.

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