Performance of tapered column packed-bed bioreactor for ethanol production.
A tapered column type of bioreactor system packed with immobilized Saccharomyces cerevisiae was used to study the bioreactor performance as a function of design and operating variables. The performance of tapered column bioreactor system was found to be better than that of the conventional cylindrical column reactor system for the ethanol fermentation. The new bioreactor design alleviated problems associated with carbon dioxide evolution and provided a significantly better flow pattern for both liquid and gas phases in the bioreactor without local channelling. A mathematical simulation model, which takes into account of the axial convection and dispersion, interphase mass transfer, and apparent kinetic design parameters, was developed. The effect of radial concentration gradients on the bioreactor performance was found to be insignificant. For the reactor system studied, the maximum ethanol productivity obtained was 60 g ethanol/L gel/h, and the maximum glucose assimilation rate was 140 g glucose/L gel/h. One of the most important findings from this study was that the apparent kinetic parameters change at the glucose concentration of 2 g/L This change was found to be due to the changes in yeast physiology and metabolism. The values of V(m) (') and V(m) (') decreased from 0.8 to 0.39 g ethanol/g cell/h and from 97mM to 11mM, respectively. The substrate inhibition constant was estimated as 0.76M and the product inhibition constant was determined as 113 g ethanol/L The degree of product inhibition showed practically a linear relationship with an increasing ethanol concentration. Based on the hydro-dynamic analysis of the bioreactor system, it was found that the Peclet number, N(Pe) was not a strong function of the flow velocity at low flow rates through the bioreactor system, but its value decreased somewhat at an interstitial velocity greater than 0.03 cm/s. The tapered column bioreactor system showed a much better flow pattern of gas and liquid phases within the reactor, thereby providing a more homogeneous distribution of gas-liquid-solid phases in the reactor without any phase separation.
Hamamci
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University of California, Davis, California 95616.
Ryu
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