Production of bioethanol under high pressure of CO2: The effect of process conditions

The aim of this work is to investigate the possibility of producing ethanol by glucose fermentation under high pressure of carbon dioxide up to 48 bar, in order to exploit both ethanol and denser CO2 as a by-product of the process. The fermentation is carried out using Ethanol Red™-Lesaffre Saccharomyces cerevisiae yeast strain, which is commonly applied in industrial bioethanol production. The experiments were performed in six small reactors (2 mL of volume each) connected in parallel, to investigate the effect of the process variables at the same conditions of temperature and pressure, and in one pilot reactor (1 L of volume) to confirm the results obtained at the lower scale. The influence of operative variables, such as carbon dioxide pressure (0–48 bar), temperature (32 and 36 °C), glucose (150–250 g/L), inositol (0–400 mg/L) and biomass concentration (OD 2 and 3.5), was measured in terms of ethanol concentration (by gas chromatography) and ethanol productivity (expressed as grams of ethanol per CFU of yeast). Both of these parameters were found to be strongly dependent on glucose concentration and CO2 pressure, which negatively affects the fermentation. Nevertheless, also at 50 bar it is possible to produce appreciable amounts of ethanol.