Modelling of Droplet Combustion in Flame Synthesis of Nanomaterials

Over the past few decades, nanomaterials have proven to be important for energy and chemical production as well as environmental remediation. Our research focuses on synthesizing nanomaterial catalysts, using the flame spray pyrolysis (FSP). FSP is a continuous one-step process and could potentially save costs, energy and time during the synthesis of catalysts. Spray droplet combustion is a critical step for nanoparticle formation in flame synthesis as it influences the physical and chemical properties of the catalyst. In order to understand the synthesis process, a single droplet combustion model has been developed based on the conservation of mass, energy and momentum along with thermodynamic relationships. The film theory that accounts for the heat and mass transfer resistance between the gas and droplet surface has also been taken into consideration. The model quantitatively shows that the initial droplet conditions, solvent type and solvent mixture composition affect the droplet combustion occurring during flame synthesis. For a single droplet undergoing combustion, a high heat transfer rate was observed initially due to the large temperature difference between the flame and droplet. The high heat transfer rate allows for solvent vaporization and increasing droplet temperature, decreasing the temperature gradient over time. The reduced temperature gradient led to a decreasing heat transfer rate with time. Droplet temperature increased over time resulting in a decreasing droplet size. Several initial droplet sizes were also tested. It was observed that the evaporation time and droplet temperature decreased with decreasing initial droplet diameters. The heat transfer rate also decreased with decreasing droplet size, while the maximum vaporization rate increased. In addition, solvent composition significantly affects the final droplet temperature; a droplet temperature between 340K to 480 K can be achieved depending on the EHA/Toluene solvent mixing ratio. Spray and combustion diagnostics are being carried out using Phase Doppler Anemometry (PDA) to experimentally determine initial spray conditions from synthesis conditions and for model validation. Future work will include the investigation of the effect of the heating rate of the precursor solution on the properties of the synthesized catalysts. This model will be specifically used for the design of synthesis of stable and active single atom catalysts (SACs).