| Summary: | With increasing energy demand and environmental concerns associated with the use of fossil-based fuels, the use of renewable sources of energy, such as biomass, has attracted considerable attention. Biofuels, such as bioethanol and bio-oil which are derived from the pyrolysis of biomass, are potential candidates to replace conventional fuels. However, the utilization of these fuels poses some challenges. In the case of bioethanol, it must have a composition higher than 98% to be used as an additive to gasoline in automobile engines. Pyrolysis oils, on the other hand, suffer from thermal instability, low heating values due to high water content and high acidity due to high acid content. In both cases, conventional distillation is not a feasible method for separation due to the azeotropic barrier, the high operating temperatures and the long residence times associated with its operation. The current work is a serious attempt to address these concerns by using a novel distillation technique mediated by hot microbubbles. The study suggests injecting a hot carrier gas in the form of microbubbles to remove the volatile components from the liquid phase and thus minimizing the sensible heat transfer to the liquid. Preliminary experiments were carried out with a 50 vol/vol ethanol-water mixture to evaluate the separation ability of microbubble mediated distillation. The experiments were planned based on a central composite rotatable design method, from which an empirical model was developed, giving an inference about the optimum operating conditions of the process. The results from the binary distillation experiments showed that upon decreasing the height of the liquid mixture in the bubble tank and increasing the temperature of air microbubbles, the separation efficiency of ethanol was improved significantly. Furthermore, it was demonstrated that separation can be achieved with only a small rise in the temperature of the liquid mixture, making this system suitable for treating thermally sensitive mixtures. Microbubble mediated distillation was successful for breaking the equilibrium barrier in separating liquid mixtures by traditional distillation. The enrichment of ethanol in the vapor phase was found to be higher than that predicted from equilibrium conditions for all liquid ethanol mole fractions considered, including the azeotrope, and within a very short contact time for the microbubbles in the liquid phase (i.e. thin liquid levels). Ethanol with a purity of 98.2% vol. was obtained using a thin liquid level of 3 mm in conjunction with a microbubble air temperature of 90C. Microbubble distillation was used to isolate the major problematic components, water and carboxylic acids, from a model bio-oil mixture. The model mixture was chosen to contain water, acetic acid and hydroxy propanone with concentrations close to those in real bio-oil mixtures. It was found that 84% of the water content and 75% of the corrosive acid content were removed from the model mixture after 150 min. These reductions, in turn, will increase the calorific value, reduce the corrosivity and improve the stability of the bio-oil mixture. This upgrading was accomplished with only a slight increase in the liquid temperature of about 5C under conditions of 3 mm liquid depth and 100C microbubble air temperature making this technique convenient for separating bio-oil mixtures without affecting their quality. A computational model of a single gas microbubble was developed using a Galerkin finite element method to complement the binary distillation experiments of ethanol-water mixtures. This model incorporates a novel rate law that evolves on a timescale related to the internal mixing of the microbubbles of 10-3 s. The model predictions were shown to be in very good agreement with the experimental data, demonstrating that the ratios of ethanol to water in the microbubble regime are higher than those predicted from equilibrium theory for all initial bubble temperatures and all liquid ethanol mole fractions considered. Furthermore, these ratios were achieved within very short contact times in the liquid mixture. The modelling data demonstrate that at shorter residence times, microbubbles are more efficient than fine bubbles in the separation process, however, as time passes the effect of bubble size diminishes. The modelling also showed improvements in the stripping efficiency of ethanol upon increasing the temperature of the air microbubbles, and an increase in the gas temperature with decreasing the residence time of the microbubbles. All of these results are consistent with experimental findings.
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