Flocculation Characters And Observed Growth Kinetic Model For The Self-flocculating Yeast BHL01

Ethanol fermentation with self-flocculating yeast immobilized within bioreactor is considered as a prospective method to improve the performance of fuel ethanol production. The process development is necessary for industrial application. Process design and amplification is based on the dynamics to quantitatively analyze and predicate the parameters during the ethanol fermentation. Besides ethanol, residual sugar and biomass, the size distribution of flocs is also concerned in the fermentation system. The genetically modified self-flocculating yeast BHL01 was selected as a model strain in this study, and the focused beam reflectance measurement system (FBRM) was used for on-line monitoring size distribution of flocs. In this work, we1. Compared the performances of continuous ethanol fermentation systems using flocculating yeast BHL01 and free yeast ATCC4126Under the same conditions, the fermentation parameters and main by-products were analyzed. It was observed that both strains showed similar fermentation performance. Meanwhile, due to the rector's retention, the biomass of BHL01 was higher and the ethanol production capacity of BHL01 was more efficient. However, the specific growth rate of BHL01 was significantly lower because of the mass transfer resistance caused by flocs.2. Discussed the effects of physical and chemical factors on the floculation of BHL01The impacts of stirring speed, biomass, temperature, pH, monovalent and bivalent cations, saccharides and ethanol concentration on flocculation of BHL01 was investigated. It was found that most of the experimental results were very similar with its parent strain SPSC01. Moreover, new experiment phenomenons was identified, such as pH range extended to 3-7.5, no influence on the flocs below 30% (v/v) ethanol concentration, only sensitive to mannose. The strain BHL01 is classified as Flol flocculation and the mechanism corresponds with zymolectin hypothesis.3. Developed observed growth kinetic model for BHL01 continuous ethanol fermentationAn observed growth kinetic model for flocculating yeast BHL01 was developed through two parameter estimating strategies. First, the impact of mass transfer resistance was eliminated by increasing stirring speed of the fermentor to reduce the size of yeast flocs, and an intrinsic growth kinetic model was established. Based on the shell mass balance theory, an observed growth kinetic model was then proposed and model parameters which were verified reliability were obtained through experimental data collecting under the condition of different substrate concentrations and size distribution of yeast flocs. The effects of dead zone which caused by shortfall of glucose and toxicity of ethanol were analyzed, and the operating diagrams were mapped based on the data which generated from the growth model simulation.The analytical results of the yeast flocs kinetic behavior indicate that the size distribution in each tank should be controlled separately in the tanks-in-series system for continuous ethanol fermentation. Large yeast flocs are preferred for the main tanks where higher sugar concentrations could prevent the occurrence of mass transfer resistance, while small yeast flocs should be applied for the rear tanks to maintain yeast flocs' viability and fermentability in which the sugar depletion significantly increases the risk of mass transfer resistance on yeast flocs. Therefore, comprehensive consideration of all the engineering aspects such as the separation of yeast flocs from the fermentation broth by sedimentation, the effective immobilization of yeast flocs within the tanks by retention and their size distribution are necessary.In conclusion, self-flocculating yeast BHL01 is adopted competitively in the ethanol industry. The observed growth kinetics which is established by combining with the intrinsic growth kinetics and classic mass transfer theory provides beneficial reference for the ethanol process optimization.