Performance of turbine impellers in sparger‐aerated fermentation vessels

The gas‐handling capacity and influence on mass transfer of turbine impellers have been investigated in baffled 5–1, vessels, mainly in a three‐phase mixture containing a high proportion of fibrous solids. The ability of the impellers to disperse the gas was strongly dependent on the speed of rotation and to a smaller extent on blade size. In the non‐Newtonian systems investigated, the correlation of mass transfer and ‘impeller factor’ showed two regions which were characterised by different constants of proportionality. Within each region, mass transfer was directly related to the impeller factor, regardless of the impeller design. The range of power inputs of importance in many industrial fermentation processes lies in the region characterised by the lower constant of proportionality. It is suggested that bubble coalescence is the dominant factor limiting impeller performance in this region. When the baffles were not in contact with the walls of the vessels, partially‐baffled conditions were produced in Newtonian systems of low viscosity. This resulted in a reduction of the power required for a given rate of mass transfer. This advantage was less marked in non‐Newtonian systems.