Kinetic Models Of A Silicone Rubber Bioreactor With Cell Recycling For Continuous Ethanol Fermentation

A composite polydimethylsiloxane (PDMS) membrane is suitable to separate actual fermentation broth due to it's selectivity to ethanol and nontoxicity to microorganism. A silicone membrane bioreactor with cell recycling was constructed based on concepts of cell regeneration and product separation in situ. Equilibriums of cell growth and ethanol production were realized by gravitational settling of cells and pervaporation of ethanol. Cell, glucose and ethanol concentrations in the membrane bioreactor were nearly constant in the duration of 300h operation with the microorganism of Thermophilic Alcohol Active Dry Yeast(TH-AADY). Cell concentration of 10.02~13.2g?L(1 was achieved under the cell discharge rate of 0.17g?L(1h(1 and mortality ratio of cells kept in the bioreactor was less than 20%. The average ethanol yield during continuous fermentation with ethanol concentration under 56.2g?L(1 was 3.83g?L(1h(1, which was much higher than that of a batch fermentation. A concentrated ethanol permeate of 17.9wt% was collected downstream the membrane when the ethanol concentration in the fermentation broth was 31.4g?L(1. pH in the bioreactor was stabilized at 4.0±0.5 by autoregulation after the coupling of fermentation and separation.Cells, ethanol and glucose interacted with each other when the TH-AADY metabolized in the bioreactor. Product inhibition and cell inhibition could not be neglected under the condition of the bioreactor. Fermentation kinetics including effects of cell, product and substrate were set up by nonlinear regression based on orthogonal experiments of metabolism. Glucose affected kinetic performance weakly when it was sufficient (much more than 1.6g?L(1) and the substrate inhibition didn't appear when glucose concentration was under 100g?L(1. The specific growth rate of cells decreased with cell concentration or ethanol concentration. The maximum concentrations of cell and ethanol at which cells are capable of continuous propagation theoretically are 15.2 and 69.6g?L(1. The maximum cell growth rate, ethanol yield rate and glucose consumption rate under the condition of bioreactor are 0.912, 5.43 and 12.74g?L(1h(1 respectively. An optimization of operating parameters could give appropriate technology for cell culture or ethanol production.The composite PDMS membrane prepared in the laboratory exhibited stable flux and selectivity in the pervaporation of aqueous ethanol solution (model solution) and fermentation broth. Fluxes increased with feed ethanol concentration but selectivities decreased with feed concentration slightly. The total fluxes and selectivities resulting from fermentation broth were much higher than those characterized with model solution under similar feed concentrations. The overall transfer coefficient of ethanol for model solution changed little under various feed concentrations as long as temperature and flow rate were constant. The transfer coefficient of 1.34×10(6m?s(1 in the certain experiment indicated that effects of boundary layer resistance and membrane resistance on the ethanol transfer were almost equivalent. The boundary layer transfer coefficient increased with circulating flow rate and also changed with configuration of a membrane module. At the same flow rate, a boundary layer transfer coefficient in a spiral channel module was higher than that in a free vortex module. The overall transfer coefficients of ethanol for fermentation broth were higher than model solution and increased with active cell concentrations. At 35℃ the overall transfer coefficient for fermentation broth was 2.38×10(6m?s(1corresponding to cell concentration of 14.1g?L(1 and flow rate of 100L?h(1, which indicated that the membrane transfer was a dominating factor in pervaporation. The correlation of the overall transfer coefficient and the active cell concentration could be characterized by a modified power law. Kinetic models based on analysis of the fermentation and the membrane transfer give mass balances of substrate, product and cells. The cell and ethanol concentrations predict...