A discrete-sectional model for particle growth in aerosol reactor: Application to titania particles

A one-dimensional discrete-sectional model has been developed to simulate particle growth in aerosol reactors. Two sets of differential equations for volume and surface area, respectively, were solved simultaneously to determine the size distributions of agglomerates and primary particles. The surface area equations were derived in such a way that the coagulation integrals calculated for the volume equations could be used for the surface area equations as well, which is new in this model. The model was applied to a production of TiO₂ particles by oxidation of titanium tetrachloride. Model predictions were compared with experimental data and those of a two-dimensional sectional model. Good agreement was shown in calculated particle size distributions between the present model and the two-dimensional model, which is more rigorous but demands a large amount of computer time and memory. Compared to experimental data, the primary particle size calculated by the model was more sensitive to the variation of reactor temperature.     

Analysis of particle contamination in plasma reactor by 2-sized particle growth model

Rapid particle growth in the silane plasma reactor by coagulation between 2-sized particles was analyzed for various process conditions. The particle coagulation rate was calculated considering the effects of particle charge distribution based on the Gaussian distribution function. The large size particles are charged more negatively than the small size particles. Some fractions of small size particles are in neutral state or charged positively, depending on the plasma conditions. The small size particle concentration increases at first and decreases later and reaches the steady state by the balance of generation rate and coagulation rate. The large size particles grow with discharge time by coagulation with small size particles and their size reaches the steady state, while the large size particle concentration increases with discharge time by faster generation rate and reaches the steady state by the balance of generation and disappearance rates. As the diameter of small size particles decreases, the diameter of large size particles increases more quickly by the faster coagulation with small size particles of higher concentration. As the residence time increases, the concentration and size of large size particles increase more quickly and the average charges per small size and large size particle decrease.     

Analysis of non-spherical polydisperse particle growth in a two-dimensional tubular reactor

Polydisperse aggregate particle growth considering coalescence, coagulation, generation and spatial transport processes is studied in a two-dimensional reactor for the first time. Effects of two-dimensional spatial transport processes, such as convection, diffusion, deposition and thermophoresis as well as nucleation, coagulation and coalescence are of primary interests. An efficient particle dynamics model based on two sets of coupled sectional equations (J. Aerosol Sci. 32 (2001) 565) is used to facilitate the severe computation loads for analyzing the growth of non-spherical polydisperse particles in an axi-symmetric two-dimensional geometry. Fluid dynamics calculations indicate the existence of non-uniform distributions of temperature and flow fields in the radial direction as well as in the axial direction inside the reactor. Particle dynamics simulations also demonstrate the significant inhomogeneous spatial distributions of the characteristics of aggregate particles. The present two dimensional calculations for reactor temperatures and particle size distributions are in agreement with the previous experimental data. The validity of simplified one-dimensional analysis is also evaluated against the present two-dimensional analysis. While the one-dimensional analysis agreed well with the spatially two-dimensional one for the cases of low flow rates, it resulted in significant errors for high flow rates.     

Analysis of particle trajectories in a quick-contact cyclone reactor using a discrete phase model

The Eulerian–Lagrangian approach with a discrete phase model (DPM) is used to investigate the motion trajectories of the particles at the range of 1–50 μm in the quick-contact cyclone reactor, in which the cracking reactions and the separations of catalysts and products can occur respectively and simultaneously. The results show that the typical motion trajectories of the particles in the quick-contact cyclone reactor can be described as three types: trapping, escaping and dust ring. The first typical motion of particles corresponds to the particles successfully separated from the gas flow, while the other two types can lead to more coking and erosion in the reactor. Moreover, a pre-vortex flow is observed in the mixing-reaction chamber. Additionally, the grade separation efficiency of each particle size is also obtained by counting the numbers of escaping and capturing particles. The particles with diameter larger than 10 μm are separated completely from the gas. The reactor also has a strong capability to trap the particles of small diameters (5 μm <d_p<10 μm). Both results indicate that the separation efficiency of the reactor has met the requirement as a primary separator. Compared with the experimental results, the separation efficiency in the simulated method is higher than 98% with errors of no more than 1.31%. It is illustrated that separation efficiency of the reactor can be predicted by CFD simulation.     

Thresholds of secondary organic aerosol formation by ozonolysis of monoterpenes measured in a laminar flow aerosol reactor

The reactions of ozone with a series of monoterpenes (α-pinene, sabinene, limonene and myrcene) were investigated in a novel flow reactor dedicated to the investigation of secondary organic aerosol (SOA) formation. Rate constants for the gas phase reactions and nucleation thresholds were determined at T～296 K, P～764 Torr under dry conditions (dew point ≤?33 °C) and in absence of OH radicals scavenger and seed particles. Comparison with the literature as well as data from a simulation chamber showed good agreement. The experiments also show that the novel flow reactor improves the accuracy in evaluating the nucleation thresholds during the ozonolysis of monoterpenes and show that aerosol flow reactor is a useful tool to study the SOA nucleation step. Given as an upper limit, the nucleation thresholds obtained are (in molecule cm^?3/ppb): α-pinene, 3.9×10¹⁰/1.56; sabinene, 6.2×10⁹/0.26; limonene, 1.1×10¹⁰/0.43 and myrcene 2.1×10¹⁰/0.83.     

Gas distribution and particle mobility in a model fluid-bed polymer recycling reactor

The effect on fluidisation quality of a number of sieve-tray and tuyere distributor designs has been investigated in a cold scale model of a proposed pyrolytic fluid-bed polymer recycling reactor. Fluidisation quality in a scale model of the proposed reactor design was assessed visually, by comparing bed pressure drop to solids weight per unit area, by measuring activity of individual nozzles and by the use of probes measuring film heat transfer coefficients at a number of locations across the bed. It was found that all the distributors gave good-quality fluidisation at fluidising velocities greater than three times the minimum-fluidising value, except in a dead zone near the distributor. This appears to support the standard models of fluid-bed distribution, however a more surprising observation was that uniform activity of all orifices did not necessarily mean that fluidisation was uniform elsewhere in the bed. These dead areas could trap incompletely reacted polymer particles and hence favour the accumulation of carbonised plastic agglomerates in the full-scale reactor bed.The effect of the configuration of internals on the dispersion coefficient of polymer particles was also appraised, by injecting a representative polymer sample, defluidising the bed, dissecting it and recording the position of sample particles. Experiments using Positron Emission Particle Tracking (PEPT) were conducted, giving insight into circulation patterns of the particles in the bed. Both dispersion experiments and PEPT demonstrated the presence of recirculation vortices in the free space between successive stacks of internals and extending into the stacks. These results suggest injecting the polymer at several locations, in order to make use of these vortices for dispersion without overloading them during the initial melting stage.     

Aerodynamics and aerosol particle deaggregation phenomena in model oral-pharyngeal cavities

The results of numerical simulations of the aerodynamics and of solid aerosol deaggregation phenomena arising in the process of airflow through various model human oropharyngeal cavities are reported. Special attention is given to the relevance of these simulations to the inhalation of dry-powder therapeutic aerosols. Several two- and three-dimensional mouth and throat geometries (terminating just beyond the larynx) are considered. Cross-sectional area-averaged viscous stress values are numerically determined as a function of distance from the mouth opening. These values, ranging from approximately 10 to 500 dyn cm⁻², are compared with estimates of Van der Waals attractive forces per unit area of particle-particle contact so as to evaluate the ability of the flowing airstream to deaggregate aerosol particles that enter the mouth in an aggregated state (held together principally by Van der Waals attractive forces). Estimates of airstream viscous stress differ markedly depending on whether the geometry is two- or three-dimensional. Quantitative differences between flow in a 90°-bend model and an oropharyngeal geometry numerically reconstructed from a cast of a human mouth and throat are especially significant in regards to the ability of the airstream to break apart particle agglomerates. For all geometries it is observed that increasingly smaller particle agglomerates may potentially be separated as the airflow rate increases from 30 to 2001 min⁻¹. At the highest airflows, aggregated particles of diameter near to or even below 1 μm may potentially be separated by the airflow. If separation of particle agglomerates is to occur, it appears far more likely to take place in the throat than in the mouth. This is especially apparent for the more physiologically faithful oropharyngeal geometries considered.     

Analysis of ice crystallization in continuous crystallizers based on a particle size-dependent growth rate model

To predict the ice crystal size distribution in MSMPR type crystallizers easily, the population balance equation with respect to the crystal size was solved under steady state conditions with a size-dependent growth rate model, that is G = g₋₁/r + g₀. The kinetic parameters g₋₁ and g₀ obtained from the measurement of crystal size distribution in a dextran solution were consistent with those estimated by assuming that the growth process of ice crystals is governed by heat transfer process. We confirmed that the kinetic parameter β, determined from experiments in a batch crystallizer, can be adopted as the parameter for the nucleation rate per crystal in an MSMPR type crystallizer.     

Fine particle effects in a fluidized-bed reactor

Recent experimental work has shown that when the concentration of fine particles (45 μm) in a fluidized bed is increased the gas flow pattern within the bed is altered, more gas being caused to flow through the emulsion phase and less through the bubble phase. In the case of a catalytic reaction system this effect should result in an increase in conversion for a given throughput of reactant and the work described here was carried out to test this hypothesis. Experimental results are presented for the catalytic oxidative dehydrogenation of butene-1 in fluidized beds of catalyst containing 0, 16 and 27% fines and it is shown that conversion does in fact increase with increasing fines content. The system can be modelled on the basis of emulsion-phase gas flows that are in excess of the minimum fluidization flow, agreement between model predictions and experimental results being quite satisfactory.     

Controlled synthesis of alumina in a spray flame aerosol reactor

In this study, a spray flame aerosol reactor (S-FLAR) is used to synthesize alumina nanoparticles. The as-produced powders are then characterized by X-ray diffraction, N₂ physisorption, and transmission electron microscopy to determine the crystal phase, surface area, particle size distribution, and morphology. The effects of the precursor, dispersion oxygen, and sheath oxygen rates on the characteristics of synthesized alumina were investigated. On increasing the precursor rate, decreasing the dispersion oxygen rate or sheath oxygen rate; the alumina powder surface area decreased. With increasing precursor rates and decreasing dispersion oxygen rates, the proportion of theta alumina increased and that of eta alumina decreased. When using an S-FLAR to synthesize alumina, the dispersion oxygen rate offers the best control of the surface area, while the precursor rate controls the crystal phase proportions. This result is useful for the design and operation of spray flame aerosol reactors to produce alumina-based catalysts.     

A numerical investigation of aerosol dynamics in a wall-less reactor

This paper describes a numerical investigation of aerosol formation during silane decomposition in a wall-less reactor. The wall-less reactor is amenable to numerical investigation because the homogeneous chemical reactions leading to the formation of solid particles are isolated from heterogeneous effects, such as occur at the walls of a laminar flow aerosol reactor. The flow/heat transfer and gas-phase chemical kinetics are simulated utilizing separate one-way coupled models. The aerosol dynamics model is based on a simplified sectional model originally developed by Okuyama et al. This model is modified to allow for the simulation of particle growth via condensation. Simulations have been performed which indicate that particle growth via condensation may be an important process. Additionally, the effects of total reactor pressure, temperature and inlet silane concentration on the dynamics of the aerosol population have been investigated. Conditions which result in the formation of larger and more numerous particles have been identified.     

Evaporative cooling of micron-sized droplets in a low-pressure aerosol reactor

The results of experimental and computational investigation of evaporative cooling of micron-sized droplets in a low-pressure aerosol reactor (LPAR) are reported. The cooling rate of the aerosol was found to be about . A constant low pressure, the flow rates of the carrier gas and solution are major factors that affect droplet cooling. A higher total pressure accelerated the change in droplet radius. For some regimes it was predicted that an aerosol undergoes freezing and then melting. The characteristic time required for evaporative cooling is about 1 ms. The agreement between experimental results and calculated values, based on the free molecular approximation of heat and mass transfer processes, is reasonably good.     

Investigating particle emissions and aerosol dynamics from a consumer fused deposition modeling 3D printer with a lognormal moment aerosol model

ABSTRACT

Particle emissions from consumer-fused deposition modeling 3D printers have been reported previously; however, the complex processes leading to observed aerosols have not been investigated. We measured particle concentrations and size distributions between 7 nm and 25 μm emitted from a 3D printer under different conditions in an emission test chamber. The experimental data was combined with a moment lognormal aerosol dynamic model to better understand particle formation and subsequent evolution mechanisms. The model was based on particles being formed from nucleation of unknown semivolatile compounds emitted from the heated filament during printing, which evolve due to condensation of emitted vapors and coagulation, all within a small volume near the printer extruder nozzle. The model captured observed steady state particle number size distribution parameters (total number, geometric mean diameter and geometric standard deviation) with errors nominally within 20%. Model solutions provided a range of vapor generation rates, saturation vapor pressures and vapor condensation factors consistent with measured steady state particle concentrations and size distributions. Vapor generation rate was a crucial factor that was linked to printer extruder temperature and largely accounted for differences between filament material and brands. For the unknown condensing vapor species, saturation vapor pressures were in the range of 10^?3 to 10^?1 Pa. The model suggests particles could be removed by design of collection surfaces near the extruder tip.

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