Mass transfer in a simulated pseudoplastic fermentation broth

A mycelial fermentation of the filamentous fungus Tolypocladium inflatum exhibited non-Newtonian behavior and pseudoplastic characteristics following the power-law model with the degree of shear thinning increasing with biomass concentration. With an increase in broth apparent viscosity, the volumetric oxygen transfer coefficient (kLa) and gas hold-up (epsilon) decreased. This apparently resulted from promoted bubble coalescence, which led to the formation of a larger bubble size ( dB) and thus decreased the gas-liquid interfacial area per unit volume (a).; A physico-chemical simulated pseudoplastic fermentation (SPF) broth was developed, where Solka Floc cellulose fibre was used to simulate filamentous biomass, and the mixture of 0.1% (w/v) carboxymethyl cellulose and 0.15M aqueous sodium chloride was used to simulate the aqueous phase of the fermentation broth. An investigation of the rheological behavior and hydrodynamics properties of the SPF broth showed that it successfully simulated the mixing and mass transfer performances of an actual T. inflatum fermentation broth. It was shown that although some previous research using CMC solutions achieved similar rheological properties, the predicted mixing and mass transfer performances were much different from that actually achieved in the T. inflatum fermentations and the SPF broth simulations. The presence of the solid phase appears to play an important role in either reducing bubble coalescence or promoting bubble breakup, thus enhancing the oxygen mass transfer.; The SPF broth was used to examine the mixing and mass transfer performance of bioreactors at several scales (50, 500, 1000 L), which employed Rushton turbines, hydrofoil impellers, or hydrofoil impellers with a draft tube. Scale up using the constant kLa criterion was confirmed to be the most reasonable approach. It was shown that hydrofoil impellers offered higher efficiency of liquid bulk mixing and mass transfer performances than that of Rushton turbine in both Newtonian and non-Newtonian fluid systems. In addition, the hybrid device between hydrofoil impellers and a draft tube efficiently improved liquid bulk mixing and mass transfer performance, particularly in highly viscous pseudoplastic non-Newtonian fluids. This hybrid device was well-suited for use with the novel SPF simulation fluid to mimic the T. inflatum morphology and liquid broth, which require good bulk mixing and mass transfer.