Oxygen Transfer and Fragmentation of Aspergillus niger Pellets in Stirred Tank and Concentric-Duct Airlift Bioreactors

Abstract

The hydrodynamic environment in bioreactors is an important parameter because cultivation conditions that provide satisfactory oxygen transfer rates tend to be harmful to microorganisms, due to the effect of shear on the cellular structure. Consequently, it is a challenge to find the ideal set of bioreactor operating conditions that favor production of the metabolites of interest. The present work evaluated the fragmentation of Aspergillus niger pellets in two different types of bioreactor: a conventional stirred tank bioreactor (STB) and a concentric-duct airlift (CDA) bioreactor. Both were operated under similar oxygen transfer or shear conditions. Oxygen transfer was quantified by means of the volumetric oxygen transfer coefficient (k_La). The shear condition was determined using the average shear rate ( ${\dot{\gamma }}_{av}$ ), obtained using computational fluid dynamics (CFD), together with pellet fragmentation. The findings showed that the CDA bioreactor caused less fragmentation of the pellets, compared to the STB, when both systems were operated with the same k_La values. For similar fragmentation levels, the k_La value for the CDA bioreactor was 38% higher than observed for the STB. Considering the results for both bioreactor models, the fragmentation of A. niger pellets, represented by the variation of the average normalized A. niger pellet equivalent diameter (D_eq/D_eq,0) after fragmentation assays of 4 h duration (Δ_frag), exhibited a linear relation with ${\gamma }_{av}$ (R² = 0.97). The CDA bioreactor also presented advantages in terms of specific power consumption and mass transfer efficiency. These results should assist in the selection of suitable bioreactor models and operating conditions, in order to improve the efficiency of industrial production of bio-based products.