Sonoprocessing of oil: Asphaltene declustering behind fine ultrasonic emulsions

Despite the transition toward carbon-free energy carriers, liquid fossil fuels are expected to occupy an important market share in the future. Therefore, it is crucial to develop innovative technology for better combustion reducing the emissions of pollutants associated with their utilization. Water in oil (w/o) emulsions contribute to greener combustion, increasing carbon efficiency and reducing emissions. Water content, emulsions stability, and droplet size distributions are key parameters in targeting the efficient use of emulsions as combustibles. In particular, for fixed water content, the finer the emulsion, the better its beneficial effect on combustion. In this work, two emulsions, mechanically and ultrasonically generated, were compared. Cryogenic scanning electron microscopy (cryo-SEM) allowed the visualization of water droplets inside the oily matrix. No surfactants were added to the oil, due to its high asphaltenic content. Asphaltene molecular aggregates, namely clusters, act as natural surfactants stabilizing the emulsions by arranging at w/o interface and forming a rigid film. The asphaltenic rigid film is clearly visualized in this work and compared for the two emulsions. The results showed finer water droplets in the ultrasonically generated emulsion, together with a reduction in the thickness of the asphaltenic film. Ultrasonically induced cavitation favored the de-clustering (breakage of intermolecular forces) of asphaltene molecules. Thus, smaller clusters allowed to stabilize smaller water droplets resulting in an ultra-fine emulsion, which improves the combustion performances of the fuel

Numerical model of an ultrasonically induced cavitation reactor and application to heavy oil processing

This study describes a numerical approach to model ultrasonically induced cavitation (UIC) reactors. UIC forms vapour-filled cavities in a liquid medium due to an applied acoustic field and their eventual collapse. UIC reactors are characterized by the presence of a vibrating probe that generates pressure waves by high-frequency oscillations (>20 kHz), which control the formation, dynamics, and eventual collapse of the vapour cavities. Those vapour cavities eventually enhance mixing and favour the occurrence of gas-liquid reactions. The zones of high mixing and reactivity coincide with the presence of the bubble cloud, which depends on the shape of the vessel and sonotrode. The development of advanced computational fluid dynamics (CFD) models is crucial to optimizing UIC processes’ geometry and operation parameters. A new algorithm for modelling UIC has been implemented within the OpenFOAM framework in the present study. The volume-of-fluid (VoF) method employs a diffuse interface approach for the volume fraction transport equation. The bubble dynamics are solved with sub-grid models, and the coupling between the main flow field and the sub-grid scales is performed through source terms in the transport equations. The source terms are de-coupled from convective and diffusive components of the volume fraction equation. The history of the bubbles is considered to consist of nucleation, oscillations, and collapse. The oscillations are resolved via the Rayleigh–Plesset equation. The concluding part of the work demonstrates the application of the algorithm to simulate the operation of an UIC reactor, which was designed to desulfurize fuels using the oxidative (ODS) process