Multistage stirrers – one five-stage and two three-stage stirrers – of the pilot fermenter are discussed in this master's thesis. Rushton turbine (8RT), asymmetric blades turbine (6ABT), four intermig impellers (IM) and two pitch blades turbines (6PBT) were used. Mixing power P, gas holdup α and mixing time t_90 measurements were performed for different operating regimes in Newtonian and non-Newtonian fluid. The measured average values of the power number in the turbulent regime were 18.91 for five-stage stirrer, and 7.37 and 2.73 for three-stage stirrers. Gas holdup has risen up to ~10% by dispersing in the water, to ~8% by dispersing in the fermentation broth and to ~7% by dispersing in the xanthan aqueous solution. The lowest power ratio (P_g/P) value was 0.32 by dispersing 10.8 m^3/h of air in the water at a rotational speed of 200 min^-1. By dispersing in the broth, the lowest power ratio was ~0.6; by dispersing in the xanthan aqueous solution the lowest value was ~0.5. The flooding transition could not be detected by the method of the abrupt change. Mixing time was determined by disorder/response method and varied between 40 s and 120 s by mixing in the water, while mixing times were scattered around 50 s by air dispersing. A scale-down procedure is also shown where we want to achieve similar flow field of the two phase gas/liquid system on the pilot fermenter from the actual measured characteristic quantities at the industrial fermenter.
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