Average shear rate in three pneumatic bioreactors

A method proposed in recent literature was applied to evaluate the average shear rate \( \left( {\dot{\gamma }_{\rm av} } \right) \) in three pneumatic bioreactors of 5-dm3 working volume: bubble column, split airlift, and concentric-tube airlift. The volumetric oxygen transfer coefficient (k L a) is the appropriate characteristic parameter to assess the average shear rate \( \left( {\dot{\gamma }_{\rm av} } \right) \) in this methodology. Correlations for \( \dot{\gamma }_{\rm av} \) as a function of superficial gas velocity in the riser region (U GR) and rheological fluid properties (consistency index, K, and flow index, n) were obtained for each model of pneumatic bioreactor studied. The \( \dot{\gamma }_{\rm av} \) values estimated by the proposed methodology lay within the range of values calculated by classical correlations. The proposed correlations were utilized to predict the \( \dot{\gamma }_{\rm av} \) during the Streptomyces clavuligerus cultivations carried out at the same specific air flow rate (3.5 vvm) in the different types of pneumatic bioreactors. The lowest values of \( \dot{\gamma }_{\rm av} \) related to the highest values of consistency index (K) were found for the bubble column bioreactor, and the highest values of \( \dot{\gamma }_{\rm av} \) related to the lowest values of K were found for the concentric-tube airlift bioreactor. Intermediate values were found for the split airlift bioreactor. The results showed that high \( \dot{\gamma }_{\rm av} \) values affect the structural health of the mycelia by the rupture of the hipha.