Local bubble size distribution in a pilot scale stirred tank containing a mycelial (three phases) dispersion as a function of constant retrofitted gassed power input

Abstract

Data about the effects of mycelium on local air dispersion in stirred tanks are very limited. Available information has been focused in laboratory-scale bioreactors and under constant stirring speed conditions. However, aeration and the biomass cause indirect changes in the actual power consumption. Therefore, studies keeping constant the gassed power input per unit volume (Pg/V) are necessary to identify effects not ascribable to this variable. We used the mycelial biomass of the filamentous fungus Trichoderma harzianum, and analyzed its effect on the local bubble size distribution (BSD) and on Sauter mean diameter (d32) in a pilot scale proto-fermenter having three ports for local measurements. Total Pg/V was kept constant by adjusting impeller speed. Without biomass, the local BSD presented a bimodal shape in all locations analyzed. Statistically significant changes in the BSD and its shape were observed when the biomass concentration increased, becoming unimodal in its shape. The local analysis showed that the bubble dispersion had a different pattern in the three locations analyzed indicating that different hydrodynamic conditions prevail in each impeller. The upper and middle impellers behaved similarly: the bubble size decreased as the Pg/V increased, whereas, the bottom impeller behaved differently, suggesting a flooding condition.