Purification of Bioethanol Using Microbubbles Generated by Fluidic Oscillation: A Dynamical Evaporation Model

(Graph Presented) A computational model of a single gas microbubble immersed in a liquid of ethanol-water mixture is developed and solved numerically. This complements earlier binary distillation experiments in which the ethanol-water mixture is stripped by hot air microbubbles achieving around 98% vol. ethanol from the azeotropic mixture. The proposed model has been developed using Galerkin finite element methods to predict the temperature and vapor content of the gas microbubble as a function of its residence time in the liquid phase. This model incorporates a novel rate law that evolves on a time scale related to the internal mixing of microbubbles of 10-3s. The model predictions of a single bubble were shown to be in very good agreement with the existing experimental data, demonstrating that the ratio of ethanol to water in the microbubble regime are higher than the expected ratios that would be consistent with equilibrium theory for all initial bubble temperatures and all liquid ethanol mole fractions considered and within the very short contact times appropriate for thin liquid layers. Our previous experiments showed a decrease in the liquid temperature with decreasing liquid depth in the bubble tank, an increase in the outlet gas temperature with decreasing liquid depth, and an improvement in the stripping efficiency of ethanol upon decreasing the depth of the liquid mixture and increasing the temperature of the air microbubbles, all of which are consistent with the predictions of the computational model.