Mixing of Highly Viscous Simulated Xanthan Fermentation Broths with the Lightnin A‐315 Impeller

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Abstract
The power characteristics of the Lightnin A-315 impeller have been evaluated in solutions of Xanthan and Carbopol of similar rheological properties, the latter being transparent to allow flow visualization. With the impeller pumping downward (forward mode), at Re600, Po was almost constant ≃1. In addition, for 20≤Re≤250, higher values of Po were found at equal Re, at higher levels of elasticity in the fluids. In the reverse mode, the same trend was found although Po was lower at the same Re. Upon aeration, no significant influence of the aeration rate (0.25−1.0vvm) upon the gassed power was observed. In the forward mode, a decrease in the Pog/Po ratio was observed, reaching levels of about 0.5–0.6 at the highest stirring speed (∼7 s−1, Re=200–800). During operation in the reverse mode, a minimum (or a plateau) Pog/Po value was observed and better gas dispersion was achieved. With the highest Xanthan concentration (35 kg m−3), high torque fluctuations were observed, though these were negligible in the inelastic Carbopol solutions of similar apparent viscosity. Video recordings made with transparent Carbopol solutions revealed very poor gas dispersion. Well-defined “caverns” were also observed in the Carbopol solution. The reverse mode gave cavern volumes up to 60% larger than the forward mode, if the modes are compared at the same power drawn. The previously derived equations for cavern sizes which have been shown to predict well for a range of impellers were not found to be entirely satisfactory with this one in either pumping mode. Although the A-315 impeller may not be a suitable impeller for mixing highly concentrated Xanthan broths because of the drastic power drop and large torque instabilities, the reverse mode is an interesting possibility for moderately concentrated Xanthan broths giving better gas dispersion, less torque fluctuations, lower power drop, and larger cavern volumes than the forward, downward pumping mode.