Oxygen Transfer Characteristics of Multiple-phase Dispersions Simulating Water-in-oil Xanthan Fermentations

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Publication Date

Fall 1999


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 confined within the aqueous droplets without significantly 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 coefficient for oxygen transfer from gas bubbles to the oil, i.e., (k L a)g/o, was much smaller than that for transfer from the bulk oil phase to the droplet surface, i.e., (k L a)o/w. 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 (G/V) – 0.25, 0.5, and 0.875 vvm; aqueous-phase volume fraction (φw) – 0.2, 0.3, 0.4 and 0.5; and aqueous-phase xanthan concentration (Xn) – 10, 20, and 40 kg/m3. The correlations developed for the power input of agitation (P g, in W), droplet diameter (d p , in μm), and (k L a)g/oHo (in kg mol/m3 s atm) are: where Ho is the Henry's law solubility for oxygen in the oil phase (kg mol/m3 atm) and v s is the superficial 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 φw are less clear.





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