Ethanol production from corn starch in a fluidized-bed bioreactor

The production of ethanol from industrial dry-milled torn starch was studie d in a laboratory-scale fluidized-bed bioreactor using immobilized biocatal ysts. Saccharification and fermentation were carried out either simultaneou sly or separately. Simultaneous saccharification and fermentation (SSF) exp eriments were performed using small, uniform kappa-carrageenan beads (1.5-2 .5 mm in diameter) of co-immobilized glucoamylase and Zymomonas mobilis. De xtrin feeds obtained by the hydrolysis of 15% dry-milled corn starch were p umped through the bioreactor at residence times of 1.5-4 h. Single-pass con version of dextrins ranged from 54-89%, and ethanol concentrations of 23-36 g/L were obtained at volumetric productivities of 9-15 g/L-h. Very low lev els of glucose were observed in the reactor, indicating that saccharificati on was the rate-limiting step. In separate hydrolysis and fermentation (SHF ) experiments, dextrin feed solutions of 150-160 g/L were first pumped thro ugh an immobilized-glucoamylase packed column. At 55 degrees C and a reside nce time of 1 h, greater than 95% conversion was obtained, giving product s treams of 162-172 g glucose/L. These streams were then pumped through the f luidized-bed bioreactor containing immobilized Z. mobilis. At a residence t ime of 2 h, 94% conversion and ethanol concentration of 70 g/L were achieve d, resulting in an overall process productivity of 23 g/L-h. At residence t imes of 1.5 and 1 h, conversions of 75 and 76%, ethanol concentrations of 4 9 and 47 g/L, and overall process productivities of 19 and 25 g/L-h, respec tively, were achieved.