Process Optimization and Mass Transfer Enhancement for Biological Syngas Conversion Process

Abstract

Technological solutions to reduce greenhouse gas (GHG) emissions from anthropogenic sources are required. Heavy industrial processes, such as steel making, contribute considerably to GHG emissions. Fermentation of carbon monoxide (CO)-rich off gases with wild-type acetogenic bacteria can be used to produce ethanol, acetate, and 2,3-butanediol, thereby, reducing the carbon footprint of heavy industries. Here, the processes for the production of ethanol from CO-rich off gases are discussed and a perspective on further routes towards an integrated biorefinery at a steel mill is given. Recent achievements in gas fermentation as well as integration of other biotechnology platforms to increase the product portfolio are summarized. In this study, to utilize waste CO for mixed culture gas fermentation, carbon sources (CO, CO2) and pH were optimized in the batch system to find out the center point and boundary of response surface method (RSM) for higher acetic acid production (center points: 25% CO, 40% CO2, and pH 8). The concentrations of CO and CO2, and pH had significant effects on acetate production, but the pH was the most significant on the acetic acid production. The optimum condition for acetic acid production in the gas fermentation was 20.81% CO, 41.38% CO2, 37.81% N2, and pH 7.18. The continuous gas fermentation under the optimum condition obtained 1.66 g/L of cell DW, 23.6 g/L Acetic acid, 3.11 g/L propionate, and 3.42 g/L ethanol. The CO mass transfer was important issue in gas fermentation. Because the aqueous solubility of CO and H2 are low, synthesis-gas fermentations are typically limited by the rate of gas-to-liquid mass transfer. activated carbon can be useful a gas delivery mediator in this study. To improve CO mass transfer in gas fermentation, activated carbon were added to mixed culture fermentation system and the particle size and concentration were optimized to maximize acetic acid production yields in a mixed culture. The optimized activated carbon was applied to a continuous gas fermentation system with optimized condition to construct a high-speed CO conversion system. Various type of activated carbon was added to a synthesis gas fermentation process to investigate their influence on syngas fermentation. CO was fermented using mixed culture, resulting in an enhanced acetate concentration and production rate as a result of enhanced the CO-water volumetric mass transfer by activated carbon addition. The CO water volumetric mass transfer coefficients were enhanced by activated carbon. acetate production was increased by introduction activated carbons. The highest condition for acetate production was surface area = 700 m2/L and particle size = 6 mesh, at which the acetate production was 709.2 mg/L and CO consumption rate was 1.11 mg/L/h. The continuous gas fermentation with activated carbons 23.8 g/L of acetate, 5.20 g/L of propionate, and 2.60 g/L of ethanol. This systematic approach improved productivity and can be applied to other biological production systems.