Power consumption, local and average volumetric mass transfer coefficient in multiple-impeller stirred bioreactors for xanthan gum solutions

• Highly viscous non-Newtonian fluids in gas liquid system were studied.
• Various impellers including small- or large-scale impellers were employed.
• The resulting heterogeneous kLa distribution were observed and investigated.
• Correlations useful for design of XG fermentation process were proposed.

Mass transfer and mixing performances are very critical for xanthan gum fermentation process. Power consumption, local and average volumetric mass transfer coefficient (kLa) were compared for six impeller combinations in a 50 L perspex tank with xanthan gum solutions. Impellers used in various combinations can be distinguished as two categories: “small-diameter” impeller, which include Rushton turbine, hollow blade turbine and wide-blade hydrofoil impeller and “large-diameter” including ellipse gate impeller, Intermig and double helical ribbon. The results show that in order to gain the same power input, the rotating speed of “small-diameter” impeller combinations increases as the concentration of xanthan gum increases, while it decreases for “large-diameter” impeller combinations. The two categories also show distinguished mass transfer rates. For the “small-diameter” impeller combinations, the kLa values near the wall region drop faster than that in other areas as the concentration of xanthan gum increases. While for the “large-diameter” impeller combinations, the distribution of kLa is homogenous except in the bottom area but with poor gas dispersion capabilities as concentration of xanthan gum increases. The averaged kLa for each impller combination was correlated well with the specific gassed power input, superficial gas velocity and effective viscosity. The obtained correlation shows that the kLa strongly depends on specific power input and viscosity, but is less influenced by the gas flow rate.