Abstract:

With on-line monitoring size distribution of the self-flocculating yeast with the focused beam reflectance measurement (FBRM)system, the impact of mass transfer limitation was eliminated by increasing stirring speed of the fermentor to reduce the size of yeast flocs, and an intrinsic kinetic model for yeast growth was established. Furthermore, based on the shell mass balance theory, an observed kinetic model was developed and model parameters were determined with experimental data collected under the condition of different substrate concentrations and yeast floc size distributions. The analytical results of yeast flocs kinetic behavior indicated that size distribution should be controlled properly in the tanks-in-series system for continuous ethanol fermentation. Large yeast flocs were preferred for the main tanks where higher sugar concentrations could prevent the occurrence of mass transfer limitation, while small yeast flocs should be used for the rear tanks to maintain yeast flocs viability and fermentability in which sugar depletion significantly increased the risk of mass transfer limitation on yeast flocs. The engineering aspects, such as separation of yeast flocs from the fermentation broth by sedimentation, effective immobilization of yeast flocs within the fermentation system by retention and balancing size distributions.