Fluid Properties and Droplet Evaporation Modeling for Spray Flame Synthesis

Spray flame synthesis is a versatile process for producing nanoparticles. In that process, a mixture of a combustible solvent with a metal-containing substance, the so-called precursor solution, is sprayed through a nozzle. The droplets are then ignited by contact with a pilot flame, eventually resulting in the formation of metal oxide nanoparticles. Thermophysical property data are essential for improving the understanding of the process and its rational design, but are unavailable for many precursor solutions. In the present work, fluid properties of mixtures of 2-ethylhexanoic acid (EHA) with ethanol or p-xylene, which are common solvents for precursor solutions, are studied. Moreover, fluid properties of mixtures of tin(II) 2-ethylhexanoate and p-xylene, which are e.g. used for the spray flame synthesis of gas sensor materials, are investigated. For these mixtures, the investigated properties include the density, viscosity, thermal conductivity, isobaric heat capacity, and self-diffusion coefficients of both components at temperatures between 293.15 and 353.15 K. Furthermore, isobaric vapor-liquid equilibria of mixtures of EHA with ethanol or p-xylene are measured at pressures below atmospheric pressure. Liquid-liquid equilibria in mixtures of EHA, ethanol, and water are measured at temperatures between 283 and 333 K. Correlations are established for all properties and provide a basis for process analysis and optimization e.g. by process simulations. In the spray flame process, micro-explosions of the evaporating droplets are believed to be key for achieving a homogeneous product quality, but their causes are under debate in the literature. A possible explanation for these micro-explosions is superheating inside the droplet, which can occur if heat conduction inside the droplet is faster than diffusive mass transport, i.e., for high Lewis numbers. For all studied systems, the Lewis numbers are high, indicating that superheating may occur inside the evaporating droplets. To study this in more detail, a one-dimensional evaporating droplet model that considers both the energy transport and the mass transport inside the droplet and the corresponding convective boundary conditions is developed and implemented. Simulations of the evaporation of droplets of titanium(IV) isopropoxide and p-xylene are carried out. The simulation results show that superheating sets in inside the droplet. Hence, after reaching a certain threshold, a sudden evaporation will occur inside the droplet – and cause the micro-explosion. This behavior is to be expected in all systems with high Lewis numbers.