Oxygen transfer characteristics of multiple-phase dispersions simulating water-in-oil xanthan fermentations

Abstract

A water-in-oil (W/O) cultivation technology has the potential of overcoming the problems related with high broth viscosity in xanthan fermentations. The aqueous broth is dispersed in a continuous oil phase. Consequently, the broth thickening mechanisms are con®ned within the aqueous droplets without signi®cantly increasing the overall viscosity. To better characterize the mixing and oxygen transfer in the complex multiple-phase (G-O-W) systems involved, the W/O dispersions of xanthan solutions in either n-hexadecane or vegetable oil were examined in this study. The experiments with n-hexadecane indicated that the coef®cient for oxygen transfer from gas bubbles to the oil, i.e., k L a gao , was much smaller than that for transfer from the bulk oil phase to the droplet surface, i.e., k L a oaw . The oxygen partial pressure at the surface of aqueous droplets, p R , was therefore close to that in the bulk oil phase.The experiments with vegetable oil were conducted under various combinations of operating conditions: agitation speed (N) ± 400, 600, and 775 rpm; aeration rate (GaV) ± 0.25, 0.5, and 0.875 vvm; aqueous-phase volume fraction (/ w ) ± 0.2, 0.3, 0.4 and 0.5; and aqueous-phase xanthan concentration (Xn) ± 10, 20, and 40 kg/m 3 . The correlations developed for the power input of agitation (P g , in W), droplet diameter (d p , in lm), and k L a gao H o (in kg mol/m 3 s atm) are:where H o is the Henry's law solubility for oxygen in the oil phase (kg mol/m 3 atm) and v s is the super®cial gas velocity. The dependencies associated with P g , d p , and N are consistent with those reported in the literature for simpler systems although no previous correlations exist for complex G-L-L systems. The dependencies associated with Xn are intuitively plausible while the responsible mechanisms for the observed dependencies on / w are less clear. List of symbolsA integration constant in Eq. (5) a speci®c interfacial area, m À1 C i empirical constant: C 1 , C 2 and C 3 in Eqs. (11)±(13), C 5 in Eq. (24), and C 6 in Eq. (26) are dimensional while C 4 in Eq. (17) is dimensionless D i impeller diameter, m D w oxygen diffusion coef®cient in the aqueous phase, m 2 /s d p droplet diameter, lm g gravitational constant, m 2 /s G volumetric aeration rate, m 3 /s H Henry's law solubility constant for oxygen, de®ned as the ratio of dissolved concentration to partial pressure, kg mol/(m 3 atm) k L a volumetric oxygen transfer coef®cient, s À1 k s rate constant for the second-order reaction of oxygen consumption in Eq. (3), kg mol/(m 3 s atm 2 ) N impeller agitation speed, rpm N CD minimum agitation speed required for complete dispersion of gas bubbles (N CDYG ) or liquid dispersed phase (N CDYL ), rpm N JS minimum agitation speed at which solid particles or liquid droplets are just fully suspended, rpm N JSG minimum agitation speed at which particles are fully suspended under gassed conditions, rpm P power input for agitation, W p oxygen partial pressure, atm q O speci®c oxygen uptake rate of microorganisms, kg mol/[(kg dry cells) s] q p...

A restructured framework for modeling oxygen transfer in two‐phase partitioning bioreactors

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Effect of n-dodecane on Crypthecodinium cohnii fermentations and DHA production

“…Several research groups have applied oxygen vectors to enhance oxygen supply and, as a consequence, to increase the biomass in different culture systems [10,25,26]. The main oxygen vectors used in biotechnology are hydrocarbons such as n-dodecane and n-hexadecane [27][28][29][30][31][32], perfluorocarbons [26,33,34] as well as vegetable oils [35]. In addition, biocompatible organic solvents have been used in milking microalgae products such as b-carotene from Dunaliella salina in two-phase bioreactors.…”

Techniques for oxygen transfer measurement in bioreactors: a review

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