(513e) Numerical Analisys of Multicomponent Catalysts Production By Double Flame Spray Pyrolysis (DFSP)

Authors: 
Noriler, D., Regional University of Blumenau
Hodapp, M. J., University of Blumenau
Meierhofer, F., University od Bremen
Meier, H. F., University of Blumenau
Fritsching, U., University of Bremen

The synthesis of nanoparticles by Flame Spray Pyrolysis (FSP) is a newer but already well developed process in terms of product diversity. Many elements of the periodic table can be turned into oxides, salts or even metal NPs in laboratory-scale reactors with production rates of a few grams per hour. In these reactors, low cost metallic precursors (e.g. zirconium(IV) propoxide) dissolved in a liquid fuel (e.g. ethanol, xylene) are supplied into a twin-fluid atomizer. Pure oxygen is utilized as dispersion gas and atomizes the liquid precursor in fine droplets with mass median diameter of ~10 µm. A support flame feed by a mixture of methane and oxygen allow a permanent ignition of the spray. Droplet evaporation, nucleation, cluster formation, coalescence, and finally agglomeration of product nanoparticles are determined as respective process steps in FSP occurring along the spray’s trajectory. The combustion of organic solvents and precursors causes a huge release of thermal energy with temperature larger than 2000 K within the flame. Particle agglomerates can be thermophoretically deposited above the flame on temperature-controlled substrates resulting in homogenous films with high porosity (98 %) and tailored thickness. A further step to the FSP process is to apply it to multicomponent catalysts production, which commonly consist of a supporting material (e.g. TiO2) whose surface is coated with a catalyst (e.g. Pt). However some limitations in the synthesis of such nanomaterials arouse because the particle design and distribution of the catalyst on the support can only be hardly controlled. An approach to overcome such limitations is the Double Flame Spray Pyrolysis (DFSP), in which the individual precursors are separately atomized and burned. The fabricated nanoparticles are subsequently mixed in the downstream of the two intersecting jets at a defined temperature. Therefore the objective of the present work is to apply computational fluid dynamics (CFD) to investigate the nanoparticle evolution in the mixing of two jets. These are defined by two nozzle parameters: intersectional nozzle angle and nozzle distance. The former is determined by the angle between the nozzle centerline and the vertical axis, while the latter one determines the displacement of the two nozzle outlets on a horizontal axis. The mathematical model include an Eulerian description of the gas phase mass, energy and momentum balances, coupled with a Lagrangian tracking of the evaporating fuel and precursor droplets. On the converged solution a simple monodispersed population balance model (PBM) is applied, which describes the nucleation and growth, by coagulation and sintering, of the particles. On this work identical precursors and flow rates were investigated for both nozzles. The simulation results show that on lower height above the burner the temperature profiles of the individual jets are still visible because both are not yet well mixed. The fuel and precursor droplet evaporate in this region, and the nanoparticle nucleation and growth process also begins. When approaching the intersection point, both jets unite and form a temperature distribution with a single maximum and forming a single jet. The temperatures are still high enough to allow further coagulation and sintering of nanoparticle agglomerates. Varying the intersection point the flow behavior and temperature profile changes. Accordingly the nanoparticle formation is also affected by the intersection point. Finally, this work was set to determine the viability of the numerical investigations on DFSP systems. Considering the complexity of the multiphase and multiscale system, the numerical model applied showed good results. Furthermore, with the application of CFD simulations, several geometrical configurations and different materials could be efficiently tested, guiding experimental investigations.
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