Evaluation of the Performance of a Three-Phase Fluidized Bed Reactor with Immobilized Yeast Cells for the Biotechnological Production of Xylitol

Abstract

This work deals with the evaluation of the hydrodynamic characteristics and fermentation parameters of a bench-scale three-phase fluidized bed reactor (FBR) with cells of the yeast Candida guilliermondii immobilized calcium alginate beads for xylitol production. The effects of the following hydrodynamic variables on cell immobilization were evaluated: total particle density, terminal velocity, particles drag force, minimum fluidization velocity and bed porosity (?). According to the results obtained, the reactor was shown to operate similar to a fixed-bed bioreactor at ? < 0.5 and a fluidized bed bioreactor at ? > 0.5, respectively. The maximum flowrate needed to obtain maximum bed fluidization in the reactor was equal to the terminal velocity of the immobilized cell particles. As for the fermentation parameters, the effects of the aeration rate and fermentation time on the process productivity and yield were analyzed. The maximum xylitol concentration of 28.9 g xylitol/l was obtained at a high aeration rate of 600ml/min after 70h of fermentation, indicating that the use of a high aeration rate in this system is favored for better oxygen transfer into the immobilized cells. The specific xylitol productivity and the xylitol yield were of 0.41 g xylitol/l.h and 0.582 g xylitol/g xylose respectively. After 90h of fermentation xylitol productivity and yield decreased to 0.256 g xylitol/l.h and 0.47 g xylitol/g xylose respectively, indicating the beginning of xylitol consumption by the yeast.