(91a) Simulation and Experimental Studies on Hydrothermal Synthesis of Nanoparticles in a Batch Reactor

Hydrothermal synthesis of nanoparticles has been gaining attention because it utilizes an inexpensive solvent (water), involves simple and efficient operation, and usually requires short process times. In this process, water and a metal salt are used as the reaction medium and the precursor, respectively. The metal salt undergoes hydrolysis to produce the metal hydroxide which subsequently condenses to form metal oxide or metal molecules. The metal oxide molecules undergo nucleation, growth through diffusion and coagulation, and Ostwald ripening to form ultrafine particles. The resulting product particle characteristics such as particle size distribution (PSD), morphology, and phase composition determine their end application. It is possible to control the product particle characteristics by tuning the process variables like temperature, pressure, precursor concentration, pH, ionic strength, aging time, and stirring speed.

Several research groups have attempted, through both experimental and numerical investigations, to understand the effect of process variables on the PSD, one of the key product characteristics. However, it is not well understood due to the complex nature of the process and difficulties associated with sample collection. The temperature and velocity profiles inside the reactor under different process conditions have been studied using computational fluid dynamics (CFD) simulations. Similarly, population balance models (PBMs) have been used to predict the PSD. But coupling of CFD with PBM to understand the effect of hydrodynamics and heat transfer on PSD and its validation with experimental data has not been demonstrated so far.

We have synthesized ceria nanoparticles using a laboratory-scale, 250 ml autoclave reactor at different temperatures, ranging from 373 to 573 K, and at a constant precursor concentration [Ce(NO₃)₃=5 mM]. The samples were collected at six hour intervals for a period of 24 hours and analysed for reaction kinetics, PSD, morphology and phase composition using UV Visible spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). It has been observed that mean ceria particle size decreases as temperature increases whereas it increases with aging time at a constant temperature. This can be attributed to high nucleation rates at elevated temperatures due to the large product monomer concentration resulting from high reaction rates. The ionic strength and pH of the nanoparticle slurry has been measured for all the samples to understand the hydrolysis and condensation reactions and resulting particle formation and growth.

Further, we have developed a coupled CFD-PBM to study the effect of process variables on the PSD. The CFD model consists of mass, momentum, turbulence, species, and energy balance equations while the PBM accounts for particle nucleation, diffusional growth and growth by particle coagulation and coalescence. The coupled CFD-PBM was implemented in a commercial CFD software (FLUENT 14.5) and tested with the PSD data of ceria nanoparticles obtained from our experiments at different reactor temperatures and aging times. The predicted PSD is in close agreement with the experimental data obtained at different temperatures and with different aging times. The product PSD was found to be a strong function of the mixing index, estimated using the velocity and temperature profiles obtained from CFD simulation results at different locations of the reactor. The coupling of CFD with PBM and reaction kinetics from the experimental studies will be useful for design and scale-up the hydrothermal reactor for synthesis of nanoparticles.