Generalized kinetics of ethanol fermentation and ethanol production in a Ca-alginate plate reactor

Abstract

Ethanol from renewable sources has great potential for the alternatives of fossil fuels and commodity chemicals. Increasing the efficiency of ethanol fermentation processes and using low cost and stable feedstocks can make fermentation ethanol competitive with synthetic ethanol on a cost basis. The goals of this research were to investigate a fermentation system capable of handling of a particulate material and to develop a universal mathematical model for ethanol fermentation. An immobilized cell reactor system was developed for continuous ethanol fermentation of a feedstock containing small particulate matter. Polyester fiber cloth coated with a mixture of Ca-alginate and yeast cells was used for immobilized cell support. The cloth plates were inserted vertically in the reactor to prevent blocking the flow path of particulates in the feedstock. Immobilized cell density in the Ca-alginate gel coated polyester fabric was investigated, and ethanol productivities were studied for different fermentation conditions. Average immobilized cell loading of 0.006 g/cm² was obtained using 1% alginate solution. At 0.456 h⁻¹ dilution rate, ethanol productivity was 8.37 g/Lh using a plate surface area to reactor volume ratio of 1.471 cm² (10 plates). Using data for ethanol fermentation reported in the literature, a kinetic model for ethanol fermentation was developed. The model fit the literature data better than other models which have been suggested. Using the new mathematical model, computer simulations were performed for fermentations using various feed glucose concentrations and various dilution rates with 10 plates in the reactor. The simulation followed experimental data during both steady and transient states. The characteristics of batch fermentation in the new plate reactor were investigated using both a synthetic glucose medium and the liquid fraction of screened sorghum mashes having different particle size distributions. As the particle size of the feedstock increased, the ethanol fermentation rate decreased. Using the liquid fraction screened through 0.5 mm openings, continuous fermentation was carried out. The new immobilized plate reactor successfully fermented this feedstock.