Simulation, Analysis and Optimization of Sugar Concentration in an In Situ Gas Stripping Fermentation Process for Bioethanol Production

Ethanol production in Brazil is carried out through biological routes where yeasts are generally employed. For greater flexibility, ethanol production is linked to the sugar production. In this configuration, the main byproduct of sugar production (molasses containing about 50 % wt in sugars) can be widely used for ethanol production. Nonetheless, due to strong ethanol inhibition, molasses dilution is necessary in order to maintain ethanol concentration in the broth below the threshold of toxicity at the end of fermentation. Hence, large amounts of water are added to the process resulting in higher costs of separation and plant design (since larger equipments are necessary). One way to solve these problems is couple fermentation to a continuous product removal technique such as in situ gas stripping fermentation process. This simple and inexpensive technique, which consists basically of a fermentor, a condenser and a pump for circulating gases, is used to remove volatile inhibitors continuously. In this work, in situ gas stripping on lab-scale was studied using ASPEN PLUS r V. 7.3 aiming to evaluate the impact of sugar concentration in feed stream on the overall process. The gas flow rate was varied for three different levels of sugar concentration in the feed (20, 30 and 40 % wt). The main goal was to evaluate the sugar concentration ranges where more ethanol can be stripped compared with water, maintaining the ethanol concentration below the threshold of toxicity in the broth. Optimized sugar concentration were found after analyzing the following output variables: ethanol stripped (%), ethanol concentration in condensate (w/w), ethanol recovered (%), selectivity and ethanol mass fraction in the broth (w/w).