Heat transfer evaluation for conventional and extractive ethanol fermentations: Saving cooling water

Abstract

Ethanol production in Brazil has two major drawbacks, related to the maximum product concentration and control of the process temperature. During the fermentation, yeast metabolism completely ceases when the ethanol concentration reaches 95 g.L−1, while high temperatures increase both the risk of bacterial contamination and the sensitivity of the yeast to ethanol toxicity. In order to address these problems, extractive ethanol fermentation with carbon dioxide stripping offers a way to overcome the toxic effect of ethanol on yeast cell growth, while also avoiding increase of the broth temperature during the fermentation. Although ethanol removal by gas stripping has a positive effect on ethanol volumetric productivity, the application of this technique for economization of cooling water during the ethanol fermentation process requires a technical analysis. Therefore, in the present work, evaluation was made of heat removal from the culture broth during extractive fermentation with ethanol stripping by CO2. First, ethanol fermentations were carried out using a 10-L bench-scale bubble column bioreactor. The mass and energy balances were used to obtain kinetic parameters, with the heat generated during fermentation being close to theoretical values. Extractive batch fermentations were performed using two carbon dioxide specific flow rates (0.5 and 1.0 vvm), resulting in efficient removal of heat from the fermentation broth, as well as reducing cooling water consumption by up to 62.7%. These results were then validated in a pilot-scale (100-L) extractive fed-batch fermentation performed at 1.0 vvm, resulting in a 63.1% reduction of cooling water consumption. In an industrial distillery, this lower consumption of cooling water would allow substantial savings of chemicals needed for cooling water treatment, making the ethanol production process in Brazil more competitive in the near future.