Remoção de etanol por stripping empregando dióxido de carbono

Abstract

In the ethanol fermentation processes, the product inhibition phenomenon limits the ethanol concentration in the fermented wine around 10% v.v-1, which results in a large volume of vinasse (around 12Lvinasse/Lethanol) and high level of energy consumption by distillation (around 2,5 Kgvapor/Lethanol). Alternative technologies designed to remove ethanol from the fermentation broth have been studied in the literature. One example is the operation of ethanol entrainment by a carrier gas, known as stripping, which in addition to the removal of the ethanol, a reduction of the temperature of the medium is expected due to the vaporization. In this context, the stripping operation could be used to control the ethanol concentration and cooling the fermentation process, as it could be used after the fermentation to promote the ethanol separation, being an alternative to distillation, reducing the energy consumption. In this study it was analyzed the stripping operation using CO2 (generated during the fermentation) in pilot plant. A factorial design was applied to evaluate the influence of the initial temperature of the solution (T0) and specific flow rate of the carrier gas (CO2) on the following stripping performance parameters: entrainment factor (FA), concentration factor (FC), and temperature reduction factor (FT). Furthermore, a mathematical model was proposed to describe the ethanol mass (mET) and the temperature (T) changes during the stripping operation. It has evaluated the performance of the ethanol recovery system and an extractive fermentation. The results showed that FA and FT were positively affected by T0, because the temperature increasing leads to greater vaporization, promoting greater entrainment of the ethanol and the high extraction of energy from the medium. The FA was also positively influenced by CO2, because it promotes greater mass transfer rate and contact area between the bubbles and liquid phase. The CO2 influenced the FT positively, but only for smaller CO2. Suppose that, for larger CO2, the equilibrium was not reached due the lower residence time of the bubbles in the wine, by removing smaller amount of energy from the liquid phase. The FC was negatively influenced by T0, because of the entrainment of a larger amount of water together with the ethanol has occurred. From the simulated results it was observed that, for smaller CO2, the expected results for the proposed model were close to the experimental, with maximum errors of 10%, concluding that the model is valid, i.e., the equilibrium was reached and the effect of mechanical entrainment was negligible. However, for larger CO2, the errors were higher (20%), concluding that the system has not reached the equilibrium and the effect of mechanical entrainment was significant. The ethanol recovery system of the CO2 stream presented 66.88 and 40.93% efficiency for the assays carried out at 33 and 65ºC, respectively, therefore with a low performance. In the extractive fermentation it was observed that the removal of the ethanol from the medium reduced the product inhibition and increased the substrate consumption rate. The CO2 flowthrough the fermentation medium (CO2 of 1.0 vvm) contributed to the reduction in the temperature of the medium in 41%. Furthermore, there was a decrease in the values of the volumetric flow rates of the cooling water, around 34%, in extractive fermentation when compared to conventional fermentation, indicating a potential savings of the 34% on inputs in the cooling towers and, also, saving energy in pumping this water.