PROCESS VARIABLES IN MICROBIAL ETHANOL PRODUCTION (FERMENTATION, CELL RECYCLE, REMOVAL, REDOX POTENTIAL, OXYGEN UPTAKE)

This dissertation deals with four major research tasks involving process variables in ethanol production by microbial fermentation. The summary of the results for each research task is presented collectively in this section.;2. In-situ removal of ethanol during fermentation of xylose by Pachysolen tannophilus was found effective in raising the fermentation rate. The improvement in fermentation rate was primarily due to rapid cell growth and high cell concentration achieved under reduced ethanol inhibition.;3. The significance of redox potential and oxygen uptake on D-xylose fermentation by non-growing P. tannophilus was also investigated. Redox potential was a useful index in monitoring and controlling low-level dissolved oxygen during the oxygen-limited phase of the fermentation. The redox potential itself, however, was not a parameter directly influencing the fermentation, only an indirect factor in the sense that it is related to dissolved oxygen.;4. A cell recycle bioreactor equipped with hollow fiber membranes was used to carry out the continuous fermentation of xylose using growing and non-growing cells of P. tannophilus. Employing 4% xylose feed, a maximum ethanol productivity of 2.43 g ethanol/l * h was achieved at the dilution rate of 0.26 hr('-1). With growing cells a maximum yield of ethanol was attained at 0.32, whereas with non-growing cells it occurred at 0.35. However, growing cells showed higher rates of ethanol production. The specific xylose consumption for cell maintenance was 0.011 g xylose/g cells * h.;1. High-level yeast inocula was investigated as a means of overcoming the toxicity problem in ethanol fermentation of acid hydrolyzate of wood cellulose. When the inoculum level exceeded 10('8) initial viable cells/ml, 50% of the yeast cells (Saccharomyces cerevisiae) survived the initial cell death period during which furfural and hydroxymethyl furfural were depleted. Continuous fermentation with cell recycle was superior to batch fermentation in that there was no overall cell decline and the ethanol yield was substantially higher. The maximum ethanol productivity in continuous fermentation was 4.9 g/liter * hr at the dilution rate of 0.24 hr('-1).