Simultaneous fermentation and separation in an immobilized cell trickle bed reactor: Acetone-butanol-ethanol (ABE) and ethanol fermentation
Abstract
A novel process employing immobilized cells and in-situ product removal was studied for acetone-butanol-ethanol (ABE) fermentation by Clostridium acetobutylicum and ethanol fermentation by Saccharomyces cerevisiae. Experimental studies of ABE fermentation in a trickle bed reactor without product separation showed that solvent production could be improved by one order of magnitude compared to conventional batch fermentation. Control of effluent pH near 4.3 and feed glucose concentrations higher than 10 g/L were the necessary conditions for cell growth and solvent production. Nutrient dosing after cell growth allowed solvent production at a glucose concentration of 10 g/L. Lower nutrient supply and pH induced degeneration. Upon degeneration glucose consumption and solvent yield decreased with a simultaneous increase in acid yield. Steady long term operation was accomplished without clogging or degeneration in a reactor with improved packing. The best solvent production was obtained at 40 g/L of feed glucose concentration. Solvents were produced simultaneously with organic acids unlike batch fermentation where acid formation preceded solvent production. Most of the pH drop occurred within the first two inches of the column. A mathematical model using an equilibrium staged model predicted efficient separation of butanol from the fermentation broth. Activity coefficients of multicomponent system were estimated by Wilson's equation or the ASOG method. Inhibition by butanol and organic acids was incorporated into the kinetic expression. Butanol productivity could be increased further by using in-situ product separation. Theory predicted that glucose concentration higher than 60 g/L could be fermented with an extended length of reactor. Experimental performance of simultaneous fermentation and separation in an immobilized cell trickle bed reactor showed that glucose conversion was improved as predicted by mathematical modeling and analysis. The fermentation gas stripped solvents preferentially and butanol removal was as efficient as acetone removal. Stripping of organic acids was far less efficient than solvents. Glucose conversion was improved by 54.7% with product separation. The effect of pH and temperature on ethanol fermentation by Saccharomyces cerevisiae was studied in free and immobilized cell reactors. Conditions for the highest glucose conversion, cell viability and least glycerol yield were determined.
Degree
Ph.D.
Advisors
Okos, Purdue University.
Subject Area
Chemical engineering
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