Improved performance in viscous mycelial fermentations by agitator retrofitting.

For viscous mycelial fermentations it was demonstrated at the pilot-plant scale that the replacement of standard radial flow Rushton turbines with larger diameter axial-flow Prochem hydrofoil impellers significantly improved oxygen transfer efficiency. It was also determined that the Streptomyces broth under evaluation is highly shear thinning. Separate experiments using a Norcardia broth with similar Theological properties demonstrated that the oxygen transfer coefficient, K(L)a, can be greatly increased by use of water additions to reduce broth viscosity. These observations are consistent with the hypothesis that the improvement in oxygen transfer by changing agitator types is primarily due to an improvement in bulk mixing. A model is presented, based on the concepts of Bajpai and Reuss, which explains this improvement in performance in terms of enlargement of the well mixed micromixer region for viscous mycelial broths.