Homogeneous nucleation of particles from the vapor phase in thermal plasma synthesis

Particle nucleation and growth are simulated for iron vapor in a thermal plasma reactor with an assumed one-dimensional flow field and decoupled chemistry and aerosol dynamics. Including both evaporation and coagulation terms in the set of cluster-balance rate equations, a sharply defined homogeneous nucleation event is calculated. Following nucleation the vapor phase is rapidly depleted by condensation, and thereafter particle growth occurs purely by Browntan coagulation. The size and number of nucleated particles are found to be affected strongly by the cooling rate and by the initial monomer concentration. An explanation is presented in terms of the response time of the aerosol to changing thermodynamic conditions.This work appears in abbreviated from in the proceedings of the International Symposium on Combustion and Plasma Synthesis of High Temperature Materials, San Francisco, Oct. 24–26, 1988, to be published asCombustion and Plasma Synthesis of Hig Temperature Materials, Z. A. Munir and J. B. Holt (eds.), VCH, New York (in press).     

Modeling of a counterflow plasma reactor

Modeling of a counterflow plasma reactor is presented, using liquid injection for the synthesis of fine particles. An experimental reactor has been developed in this laboratory, and feasibility has been demonstrated for synthesizing advanced ceramic powders. The flow field calculations show two major recirculating regions which are of importance for increasing the particles' residence time inside of the reactor. In addition, the temperature within these recirculation zones remains relatively uniform. For simulation, water droplet trajectories have been calculated for droplets produced by an injection probe. It is shown that the droplets in a size range below 50 m in diameter will follow the streamlines and evaporate completely within a short traveling distance. This finding suggests that this reactor configuration provides a favorable environment for the synthesis of fine particles using liquid precursors.     

Titanium production in a plasma reactor: A feasibility investigation

The feasibility of producing titanium metal from titanium tetrachloride in a thermal plasma under equilibrium and adiabatic expansion conditions has been theoretically investigated. Free energy minimization and adiabatic expansion calculations to simulate a nozzle expansion were used to study the practicality of production. The crucial requirements for the production of titanium powder from TiCl₄ and H₂ appear to be rapid quenching of the plasma gas at high temperature (e.g., 3700 K) and appropriate reactant concentrations. Quenching of tire plasma gas and production of titanium powder can be achieved by adiabatic expansion through a nozzle. Preliminary experimental data indicate that titanium powder of approximately 5 nm in size can be produced in an argon plasma rising a nozzle expansion approach.     

Hydrogen production by catalytic decomposition of methane using a Fe-based catalyst in a fluidized bed reactor

Catalytic decomposition of methane using a Fe-based catalyst for hydrogen production has been studied in this work. A Fe/Al2O3 catalyst previously developed by our research group has been tested in a fluidized bed reactor (FBR). A parametric study of the effects of some process variables,including reaction temperature and space velocity,is undertaken. The operating conditions strongly affect the catalyst performance. Methane conversion was increased by increasing the temperature and lowering the space velocity. Using temperatures between 700 and 900℃ and space velocities between 3 and 6LN/(gcat·h),a methane conversion in the range of 25%-40% for the gas exiting the reactor could be obtained during a 6 hrun. In addition,carbon was deposited in the form of nanofilaments (chain like nanofibers and multiwall nanotubes) with similar properties to those obtained in a fixed bed reactor.     

Two-temperature modeling of an arc plasma reactor

In this paper a two-temperature plasma model is established and applied to the injection of cold gases into an atmospheric-pressure, high-intensity argon arc. The required nonequilibrium plasma composition and the non-equilibrium transport properties are also calculated. The results show that the arc becomes constricted at the location of gas injection due to thermal and fluid dynamic effects of the injected cold flow. Enhanced Joule heating in the constricted arc path raises the electron as well as the heavy-particle temperatures. This temperature increase resists, via secondary effects, the penetration of the cold gas into the hot arc core which behaves more or less as a solid body as far as the injected flow is concerned. The temperature discrepancy between electrons and heavy particles is most severe at the location of cold flow injection, a finding which may have important consequences on chemical reactions in an arc plasma reactor.     

Study of a transferred-arc plasma reactor with a converging wall and flow through a hollow cathode

In this paper the behavior of an arc in a transferred-arc plasma reactor with a converging wall geornetrr and flow through a hollow cathode is investigated numerically with emphasis on the fluid dynamics. The general conservation equations and auxiliary relations for the calculation domain are established based on reasonable assumptions Then, the coupled nonlinear differential equations are solved with suitable boundary conditions and temperature-dependent argon plasma properties at atmospheric pressure, by employing an efficient finite-difference method. The results, for a hollow cathode geornetrr with low injection flow rates, clear/y demonstrate the existence of the Maecker elect which is responsible Joy the formation of two recirculation zones. As the plasma gas flow rate is increased, the downstream recirculation zoner is swept away leaving only an upstream recirculation zone.     

Volumetric emission of argon plasmas in the presence of vapors of Fe,Si, and Al

The net volumetric emission was calculated for argon plasmas at atmospheric pressure in the presence of metal vapors for different elements, over the temperature range from 3000 to 30,000 K. The computations are based on the escape factor model, using a semi-empirical method for the determination of line profiles and line broadening effects. Results for iron, .silicon, and aluminum show an important influence of the presence of even the smallest concentrations of the metal vapors on the net emission coefficient of the plasma. The effect is strongest for iron, followed by aluminum and .silicon. Special attention is given to self-absorption effects which are most important in the first millimeter o% the optical path of the emitted radiation. The effect is incorporated into the calculation procedure of the net emission coefficient and can be used as a volumetric energy sink as long as the absorption length is shorter than the radius of the control volume used in the computation scheme.     

p-Iodinations in hydrocarbon media: continuous flow reactor application

Regiospecific iodination of aryl amines, that is, aryl compounds possessing strong electron donating groups (EDG’s) in the p-position, is described. This procedure features not only the unique use of hydrocarbon media for such substitutions but also the absence of any oxidants aside from iodine itself. Further potential of this hydrocarbon media based electrophilic aromatic substitution is demonstrated by the coupling of the iodination with an in situ halogen/lithium exchange and product forming nucleophilic addition in a batch process. The protocol was ultimately scaled to a continuous flow reactor using an isolated p-iodoarylamine. Constituted as described, these procedures possess enhanced atom-economical, green and safety aspects compared to existing literature protocols.     

Formation of octacalcium phosphate by heterogeneous nucleation on a titania surface

Biocompatibility of the surfaces of titanium dental implants can be improved by covering them with calcium phosphate crystals, which makes the surface bioreactive. Possibly the most effective bioreactive foreign material that improves osteointegration and adsorption/binding of extracellular proteins and structural proteins is crystalline octacalcium phosphate {2×[Ca₄H(PO₄)₃·2.5H₂O] or Ca₈(HPO₄)₂(PO₄)₄·5H₂O, OCP}. In this work the building up of OCP crystals on the surface of TiO₂ anatase is examined in the process of heterogeneous nucleation from constant-composition solutions of CaCl₂ and KH₂PO₄ at constant pH (pH 6.8) and ionic strength (I=0.05 M), in dense titania suspensions. Constant relative supersaturation with regard to the calcium phosphate formation was maintained by the controlled addition of the reagent solutions, according to the desired speed of crystallization. The surface saturation value of calcium ion adsorption was measured by detecting the pH decrease during CaCl₂ addition in a separate experiment. The OCP crystallization was also conducted on the surface of an evaporated titanium layer, and on titanium metal disks. The surface of the disks was modified by the laser ablation method in order to increase the oxide layer thickness. Calcium phosphate crystals formed on the surface of the modified titanium disks, but not in an appreciable amount on the surface of the evaporated titanium layer.     

Flow field in a shrinking-bed reactor for pretreatment of cellulosic biomass

A shrinking-bed reactor was designed by the National Renewable Energy Laboratory to maintain a constant bulk packing density of cellulosic biomass. The high solid-to-liquid ratio in the pretreatment process allows a high sugar yield and avoids the need to flush large volumes of solution through the reactor. The shrinking-bed reactor is a promising pretreatment reactor with the potential for scale-up for commercial applications. To scale up the shrinking-bed reactor, it is necessary to understand the flow pattern in the reactor. In this study, flow field is simulated with computational fluid dynamics using a porous medium model. Different discrete “snapshots” and multiple steady states are utilized. The bulk flow pattern, velocity distribution, and pressure drop are determined from the simulation and can be used to guide reactor design and scale-up.     

Homogeneous nucleation of droplets from supersaturated vapor in a closed system

Kinetic equations describing homogeneous nucleation kinetics within standard model are solved numerically under the condition of a constant number of molecules in the considered system. It has consequences to decrease the supersaturation of the supersaturated vapor during the process of the formation of small droplets of a new phase. The decrease of supersaturation occurs in a short time and reaches some value which remains unchanged for a relatively long time (quasistationary regime), especially at lower initial supersaturations. This time interval decreases with increasing value of the initial supersaturation. In the quasistationary regime the nucleation rate reaches its stationary value. At higher initial supersaturation, the rate of formation of nuclei goes to some maximum value corresponding to the stationary nucleation rate and then decreases with time due to the decrease of supersaturation.     

Computational study of high-speed plasma flow impinging on an enthalpy probe

Enthalpy probe measurements in supersonic plasma flows are subject to various sources of error which are difficult to quantify experimentally v. The relative importance of several such errors has been assessed by means of detailed two-dimensional numerical simulations of high-speed plasma flow impinging on an enthalpy probe. The simulations show that moderate uncertainties in upstream pressure and composition (i.e., degree of ionization) can lead to significant errors in the velocity and temperature inferred from the measurements. These errors tend to be larger in velocity than temperature A second potential source of error is that enthalpy probe data are generally interepreted by means of simplified analytical relations which neglect the effects of finite-rate ionization, internal electronic excitation, thermal radiation, probe cooling, and probe sampling. The importance of these effects was also assessed, and the resulting errors were not ,significant under the conditions examined. We conclude that enthalpy probe measurements in supersonic plasma flows are use f d in situations where the upstream pressure and degree of ionization are known to reasonable accucary.     

Kinetic of hydrate formation of propane and its mixture with methane in a circulating flow reactor

Kinetics of hydrate formation for propane and its mixture with 73% methane have been studied experimentally and theoretically at pressure up to 2 MPa and temperature up to 277.65 K in a 10 m circulating flow reactor. A mathematical model has been developed for the process of hydrate formation based on crystallization, mass transfer and thermodynamics concepts. The amounts of gas consumptions due to hydrate formation are measured experimentally and predicted by the model. The agreement between the experimental measured gas consumptions and predicted values by the mathematical model are very good and the average deviation errors in the prediction of gas consumption are less than 10%.     

Fractal approach of nucleation in liquid and gas phases

Summary The authors present a generalised fractal treatment of the nucleation from the liquid solution or gas-phase reactant.     

Use of computational fluid dynamics simulations for design of a pretreatment screw conveyor reactor

Computational fluid dynamics simulations were employed to compare performance of various designs of a pretreatment screw conveyor reactor. The reactor consisted of a vertical screw used to create cross flow between the upward conveying solids and the downward flow of acid. Simulations were performed with the original screw design and a modified design in which the upper flights of the screw were removed. Results of the simulations show visually that the modified design provided favorable plug flow behavior within the reactor. Pressure drop across the length of the reactor without the upper screws in place was predicted by the simulations to be 5 vs 40 kPa for the original design.     

ESR spectra of radical anions — Products of the reaction of fullerene with alkali metals

The ESR spectra of the products of the reaction of fullerene C₆₀ with lithium, sodium, and potassium, including doping in time, were described. The formation of a radical anion with homogeneous reduction of fullerene by samarium iodide in solution was demonstrated.A. N. Nesmeyanov Institute of Organoelemental Compounds, Russian Academy of Sciences, 117813 Moscow. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 12, pp. 2809–2812, December, 1992.     

Interpretation of the nonisothermal crystallization kinetics of polypropylene using a power law nucleation rate function

A nucleation rate function is proposed for use in analyzing the overall crystallization kinetics of polymers. This function allows for the possibility that the nucleation rate varies substantially during the crystallization. This feature is particularly useful in analyzing nonisothermal crystallization, but it can be used to analyze isothermal crystallization as well. The nucleation rate function was used in the derivation of a modified transformation kinetics equation of the Avrami type. The modified Avrami equation was found to be suitable for kinetics analysis for the data obtained from nonisothermal crystallization at rapid cooling rates. Kinetics parameters used to describe nonisothermal crystallization under rapid cooling rates are presented and discussed. These include crystallization induction time, plateau (crystallization) temperature, crystallization half-time, crystallization rate constant, Avrami index, and newly defined quantities called nucleation index, geometric index, and nucleation rate constant. The procedure used to obtain the nucleation rate constant and nucleation index for the nucleation rate function is described and illustrated by application to the analysis of the crystallization kinetics of polypropylene. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1077–1093, 1997     

Modelling particle formation and growth in a plasma synthesis reactor

A model for particle nucleation and growth in a thermal plasma reactor is discussed. A nondimensional form of the aerosol general dynamic equation is derived under a set of simplifying assumptions which are appropriate to plasma powder synthesis, and the resulting set of equations is solved numerically. The results are converted to dimensional form for the case of iron powder, for which experimental data are available, and for silicon carbide. Calculated particle sizes increase significantly with increasing reactant concentrations and with decreasing cooling rate, although the influence of cooling rate is mainly a residence time effect.     

Cathode erosion phenomena in a transferred-arc plasma reactor

Phenomena occurring on file surface of a thoriated tungsten cathode operating in a transferred-arc reactor were investigated. The effects of cathode geometry (pointed-tip vs. flat-tip) and plasma gas composition (argon vs. helium) on the rate and mechanisms of cathode erosion were studied experimentally by examining the morphology of the surface before and after runs of prespecified duration, up to one hour in length. For flat-tip cathodes in argon, the major characteristic was the migration of thoria and its concentration at segregated sites. Both geometries in helium operated at much higher temperatures, around the boiling point of tungsten, giving rise to extensive vaporization of cathode material, followed by apparent redeposition of the ionized species carried by file ionic current, in characteristic ringlike sites on the surface. Erosion rates were low and similar in magnitude, except for pointed-tip cathodes operated in argon, where the formation of a large molten sphere of tungsten and its subsequent release gave rise to a higher rate of erosion.     

Design of a fluidized bed reactor for microencapsulated urease

A fluidized bed reactor was designed, constructed, and tested for handling microencapsulated urease. The working volume of the reactor was 10 mL, with a minimum fluidization velocity of 7.7×10⁻⁵ m/s. An even suspension of the microcapsules was obtained at fluid velocities between 1.5×10⁻⁴ and 6.0×10⁻⁴ m/s without breakage of the shear-sensitive microcapsules. The mixing behavior in the reactor was evaluated using pulse input tracer experiments and the hydrolysis rates of urea in continuous flow experiments were evaluated under various operating conditions.