POWER-CONSUMPTION OF 3 IMPELLER COMBINATIONS IN MIXING XANTHAN FERMENTATION BROTHS

Xanthan gum fermentation represents a good model for the study of the mixing of rheologically complex culture broths. Most of the previous work on power consumption dealt with 'standard', single impellers and used model fluids to simulate xanthan broths. This work describes the characterization of three dual-impeller combinations (D/T = 0.53) for the mixing of dehydrated-reconstituted fermentation broths of Xanthomonas campestris that had matched rheology to the actual broths. The bottom impeller was a Rushton turbine (RT) and the top impeller was another RT, a 45-degrees pitched blade turbine (PT) or an A-310 Lightnin mixer (A310). The experiments were carried out in a tank of 0.0094 m3 working volume equipped with an air bearing dynamometer. The power was measured in a wide range of xanthan concentrations (5-40 kg m-3) in aerated (0.25, 0.5 and 1.0 vvm) and unaerated conditions. Unaerated power number (Po) vs. Reynolds number (Re) curves showed similar trends for the three combinations. Exponents close to -1 were obtained in the laminar region. A minimum in Po (Po(min)) occurred at Re = 30-40, then increasing to a plateau value which was evident at Re > 200. In the transition region Po(min) values were 4.3 (RT and RT), 3.6 (RT and PT) and 2.4 (RT and A310). The aerated power data for (RT and PT) and (RT and A-310) showed higher torque instabilities than the dual RT combinations at higher xanthan concentrations. The higher the xanthan concentrations, the higher the drop in power and the less important the effect of the aeration rate. Among the combinations tested, when using Rushton turbines, the well-mixed 'cavern' reached the tank wall (i.e., fluid motion was observed) at the lowest volumetric power input. High productivity was achieved when xanthan fermentations were carried out with the dual, D/T = 0.53 Rushton turbines in a fermenter of identical geometry to that of the mixing tank. The dual RT system turned out to be 2.5 times more efficient (in terms of kg xanthan/kW h) than a single RT system (D/T = 0.62) reported in the literature.