Computational fluid dynamics simulation of chemical vapor synthesis of WC nanopowder from tungsten hexachloride

The chemical vapor synthesis (CVS) reactor for the preparation of WC nanopowder from tungsten hexachloride was simulated by a two-dimensional multiphase computational fluid dynamics (CFD) model. The model solves the gas-phase governing equations of overall continuity, momentum, energy, and species mass transport inside a tubular reactor system. The population balance model is coupled with the gas-phase equations to describe the formation and growth of WC nanoparticles. The model has been validated with experimental data in terms of average particle size and concentration of unreacted precursor at the outlet. The contours of temperature, velocity, species concentration and particle size distribution (PSD) inside the tubular reactor were computed.     

Effects of pressure and precursor loading in the flame synthesis of titania nanoparticles

The synthesis of TiO₂ nanoparticles is investigated experimentally and computationally in low-pressure H₂/O₂/N₂ burner-stabilized flat stagnation flames, using titanium tetra-iso-propoxide as precursor. The flow field is modeled with detailed chemical kinetics and transport, and is compared with measurements using laser-induced fluorescence to map gas-phase temperature and OH-radical species concentration. A sectional model, coupled with the simulated flow field and flame structure, is employed to model particle growth dynamics, computing aggregate particle size distribution, geometric standard deviation, and average primary particle size. The computations are compared with the experiments, for which in situ characterization of the nanoparticles in the flow field is accomplished by a low-pressure aerosol sampling system connected to a nano-scanning mobility particle sizer. Effects of operating pressure and precursor-loading rate on particle growth are examined experimentally and are compared with computational modeling. Different from other works, temperature profiles, rather than mass flow rate, are fixed using strategic dilution to base the comparisons. Higher pressures produce larger aggregate particles, but also smaller primary particles, due to longer residence times, as seen in both the experiments and computations. For higher precursor-loading rates, aggregate particle size is larger (for both experiments and computations), while primary particle size remains constant for the experiments and decreases slightly for the computations in the corresponding range.     

Large-eddy-simulation-based multiscale modeling of TiO2 nanoparticle synthesis in a turbulent flame reactor using detailed nucleation chemistry

In this work, we present a multiscale computational model for flame synthesis of TiO₂ nanoparticles in a turbulent flame reactor. The model is based on large-eddy simulation (LES) methodology in conjunction with detailed gas-phase chemical kinetics to accurately model the highly complicated combustion and nucleation processes in a turbulent flame. A flamelet-based model is used to model turbulence–chemistry interactions. In particular, the transformation of TiCl₄ to the solid primary nucleating TiO₂ nanoparticles is represented using an unsteady kinetic model considering 30 species and 69 reactions in order to accurately describe the important event of nanoparticle formation. The evolution of the TiO₂ number density function is tracked using the quadrature method of moments (QMOM). For validation purposes, the detailed computational model is compared against experimental data and reasonable agreement is obtained.     

Population Balance-Monte Carlo Simulation for Gas-to-Particle Synthesis of Nanoparticles

The process simulation of nanoparticle synthesis via the gas-phase method is essential to understanding the detailed dynamic evolution of nanoparticles within a very short time period under high temperature. The task is, however, very challengeable up to now as the conversion of the gaseous precursor to the end-use nanoparticle is a complex physicochemical process involving nucleation of the particulate phase, agglomeration between particles and sintering under industrial production conditions. In this article, we extended the differentially weighted Monte Carlo method for population balance to simulate the dynamic evolution of titania (TiO₂) nanoparticles synthesized by gas-to-particle conversion in a single aerosol reactor, considering simultaneous nucleation, agglomeration, and sintering. The simulated size distribution of TiO₂ agglomerate and primary particles produced by the thermal decomposition of titanium tetraisoproxide agreed well with the experimental data. In the simulation, the fast population balance-Monte Carlo method was utilized to accelerate the process simulation on a desktop PC. Results were obtained up to 178 times faster than that of a normal Monte Carlo method. The inhomogeneous internal structure of primary particles was considered through solving population balance of polydisperse primary particles within agglomerate. It was found the polydisperse model could predict the primary particle size distribution better. Simulation results revealed a complex competition relation among nucleation, agglomeration and sintering.

Copyright 2013 American Association for Aerosol Research     

Simulation of nanoparticle synthesis in an aerosol flame reactor using a coupled flame dynamics–monodisperse population balance model

Flame aerosol synthesis is one of the commonly employed techniques for producing ultra fine particles of commodity chemicals such as titanium dioxide, silicon dioxide and carbon black. Large volumes of these materials are produced in industrial flame reactors. Particle size distribution of product powder is the most important variable and it depends strongly on flame dynamics inside the reactor, which in turn is a function of input process variables such as reactant flow rate and concentration, flow rates of air, fuel and the carrier gas and the burner geometry. A coupled flame dynamics–monodisperse population balance model for nanoparticle synthesis in an aerosol flame reactor is presented here. The flame dynamics was simulated using the commercial computational fluid dynamics software CFX and the particle population dynamics was represented using a monodisperse population balance model for continuous processes that predicts the evolution of particle number concentration, particle volume and surface area. The model was tested with published experimental data for synthesis of silica nanoparticles using different burner configurations and with different reactor operating conditions. The model predictions for radial flame temperature profiles and for the effects of process variables like precursor concentration and oxygen flow rate on particle specific surface area and mean diameter are in close agreement with published experimental data.     

Sinterable Ceramic Powders from Laser-Driven Reactions: II, Powder Characteristics and Process Variables

Si, Si₃N₄, and SiC powders which possess a unique set of characteristics were produced by a laser-driven gas-phase synthesis process. The powders have a fine particle size (<0.1 μm), are spherical, have a narrow range of particle sizes, are free of hard agglomerates, have a high degree of phase purity, and have a high absolute purity (<0.1% including oxygen). A detailed analysis of the physical, chemical, and crystalline characteristics of Si and Si₃N₄ is presented. A brief discussion of our initial work with SiC is also included. The dependence of particle characteristics on the various process parameters (laser power, cell pressure, gas composition) is discussed and related to a model of the powder-synthesis process.     

Preparation of Al2O3-SiO2 fine particles by CVD method in tube flow reactor

The aim of this study was the synthesis of mixed or coated multicomponent alumina-silica particles by chemical vapour deposition method in a tube flow reactor. The particles were produced by simultaneous thermal decomposition of aluminum tri-sec-butoxide and tetraethylorthosilicate in one reactor. The particle production was monitored by Differential Mobility Particle Sizer, composition of particles was analysed by energy dispersive X-ray analysis, morphology by scanning/transmission electron microscopy and crystallinity by selected area electron diffraction. In dependence on experimental conditions, the particles produced were either alumina particles with intermixture of silica, or they were coated by silica, or it was a mixture of particles of various compositions. The particles were often agglomerates of the primary nanoparticles and were partially crystalline.     

On the prediction of internal particle morphology in suspension polymerization of vinyl chloride. Part I:: The effect of primary particle size distribution

The present study provides a comprehensive investigation on the determination of the primary particle size distribution in the suspension “powder” polymerization of vinyl chloride. The primary particle size distribution inside the polymerizing monomer droplets is determined by the solution of a population balance equation governing the nucleation, growth, and aggregation of the primary particles. The stability of the colloidal primary particles is expressed in terms of the electrostatic and steric stabilization forces. The primary particle stability model includes the effects of agitation, temperature, electrolyte as well as primary and secondary stabilizer concentrations. It also includes both diffusive and shear-induced particle destabilization mechanisms. The proposed stability model is shown to accurately describe existing experimental data on particle number, mean particle size and particle size distribution for both bulk and suspension vinyl chloride polymerizations. The primary particle population balance model can predict the critical monomer conversion at which massive particle aggregation occurs leading to the formation of a continuous network of primary polymer particles inside the polymerizing monomer droplets. A detailed investigation on the predicted critical monomer conversion is carried out including its dependence on the rate of agitation, temperature, electrolyte concentration, as well as the concentrations of the primary and secondary stabilizers.     

Experimental study of gas-dynamically induced nanoparticle synthesis by use of adapted sampling probes

A new process for the synthesis of nanoparticles in the gas-phase is experimentally investigated. The gas-dynamically induced nanoparticle synthesis uses the initiation of the chemical reaction by gasdynamic shock and the quenching of high temperature gas by accelerating the flow from subsonic flow speed to supersonic speed. Therefore, the design of the reactor consists of two Laval nozzles. The process provides high heating and cooling rates, an adjustable reaction time and a particle synthesis at constant thermodynamic conditions to obtain non-aggregated nanoparticles. In order to analyze the synthesized SiO₂ particles during their growth, samples are taken in the reaction volume and downstream of the quenching. The particles from the reaction chamber were extracted with the help of a specially designed water-cooled probe. The geometry of the probe is optimized by CFD simulations. The particles downstream the quenching are extracted by a heated and isolated probe. The particles are collected on TEM grids. The experimental investigations show that the synthesized particles are spherical and non-aggregated in the reaction chamber and after quenching. The possibility to synthesize a non-aggregated product in the novel process is thus demonstrated. The mean particle size is defined by the process conditions and varies from 25 to 37 nm after quenching.     

Computational Modeling of Silicon Nanoparticle Synthesis: II. A Two-Dimensional Bivariate Model for Silicon Nanoparticle Synthesis in a Laser-Driven Reactor Including Finite-Rate Coalescence

In this work, a two-dimensional model was developed for silicon nanoparticle synthesis by silane thermal decomposition in a six-way cross laser-driven aerosol reactor. This two-dimensional model incorporates fluid dynamics, laser heating, gas phase and surface phase chemical reactions, and aerosol dynamics, with particle transport and evolution by convection, diffusion, thermophoresis, nucleation, surface growth, coagulation, and coalescence processes. Because of the complexity of the problem at hand, the simulation was carried out via several sub-models. First, the chemically reacting flow inside the reactor was simulated in three dimensions in full geometric detail, but with no aerosol dynamics and with highly simplified chemistry. Second, the reaction zone was simulated using an axisymmetric two-dimensional CFD model, whose boundary conditions were obtained from the first step. Last, a two-dimensional aerosol dynamics model was used to study the silicon nanoparticle formation using more complete silane decomposition chemistry, together with the temperature and velocities extracted from the reaction zone CFD simulation. A bivariate model was used to describe the evolution of particle size and morphology. The aggregates were modeled by a moment method, assuming a lognormal distribution in particle volume. This was augmented by a single balance equation for primary particles that assumed locally equal number of primary particles per aggregate and fractal dimension. The model predicted the position and size at which the primary particle size is frozen in, and showed that increasing the peak temperature was a more effective means of improving particle yield than increasing silane concentration or flowrate.     

A Novel Porous Tube Reactor for Nanoparticle Synthesis with Simultaneous Gas-Phase Reaction and Dilution

A novel porous tube reactor that combines simultaneous reactions and continuous dilution in a single-stage gas-phase process was designed for nanoparticle synthesis. The design is based on the atmospheric pressure chemical vapor synthesis (APCVS) method. In comparison to the conventional hot wall chemical vapor synthesis reactor, the APCVS method offers an effective process for the synthesis of ultrafine metal particles with controlled oxidation. In this study, magnetic iron and maghemite were synthesized using iron pentacarbonyl as a precursor. Morphology, size, and magnetic properties of the synthesized nanoparticles were determined. The X-ray diffraction results show that the porous tube reactor produced nearly pure iron or maghemite nanoparticles with crystallite sizes of 24 and 29 nm, respectively. According to the scanning mobility particle sizer data, the geometric number mean diameter was 110 nm for iron and 150 nm for the maghemite agglomerates. The saturation magnetization value of iron was 150 emu/g and that of maghemite was 12 emu/g, measured with superconducting quantum interference device (SQUID) magnetometry. A computational fluid dynamics (CFD) simulation was used to model the temperature and flow fields and the decomposition of the precursor as well as the mixing of the precursor vapor and the reaction gas in the reactor. An in-house CFD model was used to predict the extent of nucleation, coagulation, sintering, and agglomeration of the iron nanoparticles. CFD simulations predicted a primary particle size of 36 nm and an agglomerate size of 134 nm for the iron nanoparticles, which agreed well with the experimental data.

Copyright 2015 American Association for Aerosol Research     

Synthesis of Silica Nanoparticles by Ultrasound‐Assisted Sol‐Gel Method: Optimized by Taguchi Robust Design

Silica nanoparticles were prepared by ultrasound‐assisted and conventional sol‐gel method. The synthesis procedures were designed and optimized by the Taguchi experimental design method. Molar concentrations of TEOS, H₂O, NH₄OH, and reaction temperature were chosen as main factors. The results showed that the molar concentration of ammonia is the main factor which affects the particle size of the silica nanoparticles. The chemical structure, size, and morphology of the product were investigated by X‐ray diffraction, Fourier transform infrared spectroscopy, laser light scattering, and scanning electron microscopy. By the optimum conditions of the ultrasound‐assisted sol‐gel method, silica nanoparticles with an average particle size of 13 nm were prepared.     

Effect of reaction temperature on CVD-made TiO2 primary particle diameter

The effect of chemical reaction rate on the generation of titania nanoparticles by chemical vapor deposition using two different precursors was investigated by FTIR, XRD, and microscopy. The size of the primary particle exhibited a minimum with increasing reactor temperature. At lower reaction temperatures, the continuous and gradual formation of titania monomers occurred followed by coagulation and/or surface reaction on the existing particles. In addition, unreacted precursor condensed at the reactor exit. As the reaction temperature increased, the rate of monomer production increased, the dominant characteristics of particle growth were coagulation and sintering. The reactor temperature where the minimum primary particle diameter was produced was different for the two precursors due to differences in chemical reaction rates. Phase composition as well as the primary particle diameter of product titania were affected by the chemical reaction rate. Particle-laden reactor wall enhanced the precursor conversion at low reactor temperatures, where surface reactions compete effectively with gas-phase precursor conversion.     

Nanoparticle formation through solid‐fed flame synthesis: Experiment and modeling

The preparation of silica nanoparticles through solid‐fed flame synthesis was investigated experimentally and theoretically. Monodispersed submicrometer‐ and micrometer‐sized silica powders were selected as solid precursors for feeding into a flame reactor. The effects of flame temperature, residence time, and precursor particle size were investigated systematically. Silica nanoparticles were formed by the nucleation, coagulation, and surface growth of the generated silica vapors due to the solid precursor evaporation. Numerical modeling was conducted to describe the mechanism of nanoparticle formation. Evaporation of the initial silica particles was considered in the modeling, accounting for its size evolution. Simultaneous mass transfer modeling due to the silica evaporation was solved in combination with a general dynamics equation solution. The modeling and experimental results were in agreement. Both results showed that the methane flow rate, carrier gas flow rate, and initial particle size influenced the effectiveness of nanoparticle formation in solid‐fed flame synthesis. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Approaches to increasing yield in evaporation/condensation nanoparticle generation

With the recent interest in the chemical, electronic and optical properties of nanometer scale metal particles, there is now interest in manufacturing these materials in larger quantities. Since both small particle size and high particle number concentrations are sought, there is a need for improved particle generation reactors that can realize both goals. Here, results are presented for the synthesis of indium metal nanoparticles in an evaporation/condensation aerosol generator. Size distributions were measured for metal nanoparticles formed using a standard flow configuration, as well as using several variations on the standard configuration. The aim of the modifications is to increase the cooling rate and thus, to increase the nucleation rate of the nanoparticles. An increase in the number concentration of particles and, in some cases, a significant decrease in average particle size was observed when the modified reactor configurations were used. These results can be explained by the changes in the time–temperature history of the nanoparticles resulting from the modifications to the aerosol generator. A monodisperse model of nanoparticle formation and growth, accounting for nucleation, condensation and coagulation, was used to describe particle formation in the standard flow configuration, to guide the modifications, and to describe particle formation in one of the modified configurations, with qualitative agreement seen between measured and predicted particle sizes.     

Theoretical Model of Phosphorus Incorporation in Silica in Modified Chemical Vapor Deposition

Based on experiment, a theoretical model is developed of phosphorus incorporation in modified chemical vapor deposition. In the glass formation stage of this process, gas-phase reaction of silicon tetrachloride and phosphorus oxychloride generates submicrometer particles of phosphorus-doped silica. These particles deposit in a thin porous soot layer which is then viscously sintered. Equations for thermal and dopant transport through the soot voids and particles during sintering predict that gradients in each layer are due to inhibited diffusion of dopant through the gaseous voids of the soot, not to gas-phase gradients. Thermal gradients create variations in gas and solid diffusion and dopant solubility which enhance concentration gradients. These results apply to viscous sintering of glasses produced by flame hydrolysis and sol-gel processes.     

碳化合成纳米SiO2颗粒比表面积的控制机理

以Na2SiO3为前驱物,研究了碳化合成纳米SiO2颗粒比表面积的控制机理. 正交实验结果表明,影响其比表面积的次序为Na2SiO3浓度>PEG6000添加量>反应温度>CO2/N2混合气流量. 当反应温度80℃、Na2SiO3浓度40 g/L、PEG6000 4 g/L、CO2/N2混合气流量1.2 L/min时,所制SiO2颗粒的比表面积为318.9 m2/g. 随Na2SiO3 浓度增大,SiO2胶粒在pH>7的碳化液中Ostwald凝聚生长过程相对延长,颗粒的一次粒径增大,比表面积却随之减小. 由于温度对SiO2溶解度和胶粒Brownian运动的影响,降低反应温度可促进SiO2成核、减少Si(OH)4和≡SiOSi(OH)3的聚合,SiO2颗粒的比表积增大. 添加剂PEG6000的空间架构效应抑制纳米SiO2颗粒团聚,从而促进SiO2颗粒比表面积增大;但当其浓度大于7 g/L时,Na2SiO3分子进入缠绕着的PEG长链架构内部,所得SiO2颗粒比表面积有所降低.     

On the interaction of coagulation and coalescence during gas-phase synthesis of Fe-nanoparticle agglomerates

A numerical model was applied to the synthesis of iron nanoparticles and compared to experimental findings. The experiments comprised the production of iron particles from iron pentacarbonyl in a wall-heated aerosol reactor. The wall temperature of the reactor was varied between 400°C and 800°C. The size and morphology of the so-formed particles were investigated via transmission electron microscopy (TEM). The images show strongly agglomerated particles with mean primary particle diameters in the range of 10-, depending on the experimental conditions.The numerical model used accounts for the various physical and chemical processes taking place in the described aerosol reactor. Beside convection, nucleation, and coagulation, the model particularly accounts for the influence of coalescence on the particle size and morphology. With an extension of the sintering time for the nanometer size range, the experimental primary particle size could be well predicted by the computer simulations.