Rheological Behavior Of Yest Flocs Slurry And Hydrodynamics Of The Suspended-bed Bioreactor

Yeast flocs developed by the self-flocculation of yeast cells are significantly different from free yeast cells. Not only is their size much larger, but their configuration is not uniform. Moreover, they are susceptible to shearing force. These characteristics present a challenge for developing a suitable bioreactor for their immobilization and efficient ethanol production. The objectives of this project are to investigate the rheological property of the yeast flocs slurry, one of the prerequisites for bioreactor design and operation, and the hydrodynamics of the suspended-bed bioreactor (SBB) for the suspension of the yeast flocs.The rheological property of the simulation system with yeast flocs suspended in water was first studied, which exhibited the non-Newtonian fluid behavior and fitted with the power law index model. The model parameters were correlated with the concentration of the yeast flocs. Moreover, the size distribution of the yeast flocs was measured in situ using the focused beam reflectance measurement system, and it was found that the degree of the flocculation of the yeast flocs characterized by their average chord length was the major factor affecting the rheological behavior of the yeast flocs slurry.Furthermore, the rheological property of the real ethanol fermentation system with yeast flocs was investigated under batch fermentation conditions. The experimental results demonstrated that the growth of yeast flocs affected the apparent viscosity and consistency coefficient of the yeast flocs slurry. At the initial stage of the fermentation when the concentration of yeast flocs was low, the rheological property of the yeast flocs slurry exhibited the Newtonian fluid behavior, due to the contribution of the low viscosity fermentation broth. With the massive growth of the yeast flocs, the rheological property was shifted to the non-Newtonian fluid behavior due to the contribution of the yeast flocs.Finally, the hydrodynamics of the SBB was explored using air-fermentation broth biphase fluid as a model system, and the overall circulation velocity was predicted. Based on the 3-D two-phase fluid model and RNGk-εturbulence model, a 3-D computational fluid dynamics (CFD) model was established for the continue-discontinue phase and validated by the experimental data. The gas-liquid fluidic field within the SBB was further simulated using the 3-D CFD model, which will facilitate the identification of the hydrodynamic conditions for the suspension of the yeast flocs within the bioreactor as well as preventing the breaKage of the yeast flocs by the shearing force.