Modeling and Parameterization of the Evaporation and Thermal Decomposition of an Iron(III) Nitrate Nonahydrate/Ethanol Droplet for Flame Spray Pyrolysis

Abstract

Flame spray pyrolysis (FSP) is a promising approach to generate nanoparticles from precursor solutions, where the convective droplet heating and evaporation of the single precursor solution droplet play a key role. Depending on the precursor solution under consideration, reactions inside the liquid may occur. The present numerical study concerns the heating, evaporation, and thermal decomposition of single droplets of iron(III) nitrate nonahydrate (INN) and ethanol at an initial temperature of 293.15 K in hot convective air at atmospheric pressure. If the ambience is below the thermal decomposition temperature (Tth) of the INN, iron nitrate particles are directly formed inside the particle, whereas at ambient temperatures beyond Tth, the iron nitrate thermally decomposes into gaseous Fe2O3 and N2O5. Vaporization and thermal decomposition govern the process, depending on the droplet surface temperature. If the ambient temperature is larger than a specific value T+, thermal decomposition is very fast and vaporization dominates the total process time, whereas at lower ambient temperatures, the vaporization is slower, which causes a lower final droplet surface temperature, leading to considerably longer thermal decomposition, which dominates the total process time under that condition. The ambient temperature at which this reversed behavior occurs depends on initial INN loading of the particle and the relative velocity but is largely independent of the initial droplet size. These new results are very useful in choosing the process temperature, which is recommended to lie beyond the ambient air temperature of T+ to assure that the total process time is kept short. The numerical results are parameterized for use in more complex simulations of FSP.