DEVELOPMENT OF A MATHEMATICAL MODEL FOR THE SIMULTANEOUS SACCHARIFICATION AND FERMENTATION OF CELLULOSE WITH THERMOMONOSPORA FUSCA YX CELLULASES AND CLOSTRIDIUM THERMOHYDROSULFURICUM (DIAUXIE)

Producing ethanol from renewable biomass is one of the main processes being investigated as a possible alternative to the high cost and limited supply of crude oil. This process requires four steps: (1) Pretreatment of the biomass, (2) Enzymatic breakdown of the cellulose into soluble sugars, (3) Fermentation of these sugars into alcohol, (4) Alcohol recovery. One possible method to alleviate the problem of low rates of hydrolysis due to end-product inhibition of the enzymes is to perform the saccharification and fermentation simultaneously in the same vessel. Such a process is called a simultaneous saccharification and fermentation (SSF). The goal of this work was to investigate, both experimentally and theoretically, a SSF system utilizing Thermomonospora enzymes as the cellulose hydrolysis agent and Clostridium thermohydrosulfuricum as the fermenting organism. A model was developed which predicted the cellobiose and glucose concentrations during Avicel hydrolysis at 65(DEGREES)C. Cellobiose, a hydrolysis end-product, was found to non-competitively inhibit the cellulases. Batch and continuous growth of Cl. thermohydrosulfuricum on both single and multiple substrates (products of the hydrolysis process) was studied in depth. Single sugar experiments were used to determine growth rates, cell and product yields, K(,s)\u27 and maintenance coefficients on cellobiose, glucose and xylose. In mixed sugar systems, glucose repressed utilization of cellobiose, xylose and xylobiose, all of which could be catabolized simultaneously in the absence of glucose. A mathematical model was developed which described the effect of glucose on cellobiose uptake by Cl. thermohydrosulfuricum. Simultaneous saccharification and fermentation, using Thermomonospora enzymes and Cl. thermohydrosulfuricum, of either Avicel or pretreated poplar wood showed a drastic decrease in the level of soluble sugars over that seen in a normal saccharification system. A model which generally predicted the soluble sugar and fermentation products level was developed by combining the Avicel hydrolysis and cellobiose/growth models. This model was used to simulate a continuous SSF system and investigate the effect of various parameters on ethanol productivity on such a system