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ansesterification of vegetable oil with methanol, promoted by alkaline catalysts, has been tested in many different reactors and the reaction time to obtain a complete conversion, in some of these reactors, resulted in the range of few seconds. By using static mixers, micro-reactors, oscillatory-flow reactors, cavitation-inducing reactors, microwave reactors or centrifugal contactors, it is possible to obtain high biodiesel yields in a much shorter time than in stirred tank reactors. As these reactors are characterized by a very high liquid-liquid interface we can conclude that higher is the interface area shorter is the reaction time. This behavior cannot be described with the monophasic kinetic model reported in the literature. Therefore, we have developed a biphasic kinetic model that is able to describe any type of reactor such as: more or less stirred batch reactors, more or less efficient continuous structured reactors, micro-reactors etc. We have also experimentally observed that it is possible to obtain a complete conversion in less than 10 seconds by using a very efficient micro-mixer followed by a void settling tube of an opportune volume size. This reactor has been simulated also with the mentioned liquid-liquid biphasic model. This model is based on a reliable transesterification mechanism and takes into account both the intrinsic kinetic laws, mass transfer and reactants and products two phases partition. Some of the unexplained kinetic aspects, observed in batch runs, like the difference in the final conversions for different catalyst concentrations have been clarified with the biphasic model. This final conversion is due to catalyst deactivation and not to chemical equilibrium as suggested in the monophasic kinetic models normally employed in the literature. In this work, the developed biphasic approach will be applied on different kinds of laboratory reactors characterized by a different micromixing efficiency. The kinetic parameters for the intrinsic occurring reactions will be given and discussed. Moreover, the role of the mass transfer effect will be considered and the consequence of the methanol and glycerol partition on the conversion and yield will be described. By concluding, the possibility to produce biodiesel in a micro-device with residence times of few seconds will be experimentally shown and theoretically interpreted.

Diffusion plays a central part in many unit operations. The Maxwell–Stefan model is the dominant model for both gaseous and liquid diffusion. However, it was developed from the kinetic theory of gases, raising the question of whether it can be extended to non-ideal liquid systems. The dynamic fluctuation model is an alternative model based on the Cussler theory and predicts a smaller thermodynamic influence relative to the linear influence of the Maxwell–Stefan model due to dynamic concentration fluctuations. Since the dynamic fluctuation model, which uses the scaling factor α, had improved performance relative to the Maxwell–Stefan model for a wide range of binary systems, it is postulated that this improved performance should also be observed for a ternary system. In this work, the dynamic molecular fluctuation model was extended to a highly non-ideal ternary system, using the same scaling factor α, through matrix manipulation. Using self-diffusion data measured by NMR, mutual diffusion predictions of the developed model and the Maxwell–Stefan model were compared to experimental mutual diffusion data of the partially miscible system ethanol/toluene/n-decane. It is demonstrated that the dynamic fluctuation model gives improved predictions relative to the Maxwell–Stefan approach, consistent with previous observations on binary systems, showing that the reduced thermodynamic influence of the dynamic fluctuation model is an improvement. In addition, we show that the use of local mole fractions, to account for molecular association, in both the dynamic fluctuation and Maxwell–Stefan models, results in improved diffusion predictions for the ternary system. The results confirm that the dynamic fluctuation model improves predictions of mutual diffusion in liquid mixtures, suggesting a non-linear correction to the thermodynamic correction factor. The results also suggest that that the key assumptions in the Maxwell–Stefan model and its derivation, rooted in the kinetic theory of gases, are not entirely accurate for highly non-ideal liquid systems. The optimum α for the ternary system studied here is approximately 0.45, similarly to the optimum α of 0.40 to 0.80 for a range of binary systems previously studied, suggesting that the use of the α scaling factor, which is grounded in scaling laws theory, is of general validity.

In this chapter, the state-of-the-art catalysts used to produce the most common liquid biofuels are reviewed. In particular, the catalysts used to produce biofuels from either oils and fats or cellulose lignin feedstocks will be discussed. Attention will be paid to discussing the recent advances in the current catalytic technologies, showing for each the related drawbacks and possible perspectives.