Modeling of bioethanol production by yeasts: contribution to a biorefinery

• Autor(a) principal: Dinis, Maria Rita Torres
• Data de Publicação: 2019
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/29413
• topic: Modeling; Bioethanol; Fermentation kinetics; Concentration profiles

Second-generation bioethanol is produced from lignocellulosic biomass, namely sub-products from the pulp and paper industry. This work follows the research done by the group on bioethanol production from fermentation of sulfite spent liquor (HSSL) with the yeast Scheffersomyces stipitis, and it aims to evaluate and model the inhibitory effect of the acetic acid on the ethanol yield and productivity. For this, experimental data of batch fermentations with S. stipitis was used, in synthetic medium with xylose, glucose and acetic acid as substrates. The ethanol fermentation mechanism has been thoroughly studied and described over the years, and it is based on the Monod equation for microorganism growth. Among the available kinetic models, 12 were selected and allowed to develop a mathematical tool able to describe the biomass, the substrates and the product concentration profiles as function of time. Furthermore, the algorithm developed allowed to create a stand-alone application for distribution. With the modeled profiles, it was possible to conclude that the acid presence affects negatively both the length of the lag phase and the maximum ethanol concentration, thus causing a substantial decrease in the ethanol productivity and yield on substrate. Moreover, the increase in the initial acetic acid concentration causes the decrease in the substrate consumption rate, prolonging even further the time to reach the maximum ethanol concentration. This concentration value, however, is only significantly affected when the initial acid concentration is higher than 6 g/L. Comparing the synthetic medium experimental data to a 60% HSSL sample, it was possible to conclude that the acetic acid is not the only inhibitory compound present in the liquor, nor are the three substrates considered the only ones consumed by the yeasts. The change from synthetic medium to HSSL caused a significant extension of the lag phase and decrease of the maximum ethanol concentration, again causing the ethanol productivity to decrease considerably. The maximum biomass concentration, however, was much higher for the 60% HSSL sample. Furthermore, in this sample the xylose concentration never reached zero, contrary to what was observed in all synthetic medium samples, where the sugars were always exhausted. Therefore, in the future, the mathematical tool developed could be adapted and used to model the influence of the other relevant compounds present in the liquor, in order to study accurately and conclusively the HSSL fermentations.