Fluidization behavior in a circulating slugging fluidized bed reactor. Part II: Plug characteristics

In the transporting square nosed slugging fluidization regime () a bed of polyethylene powder with a low density () and a large particle size distribution () was operated in two circulating fluidized bed systems (riser diameters 0.044 and 0.105 m). A relation was derived for the plug velocity as a function of the gas velocity, solids flux, riser diameter, particle size range and particle and powder properties. The influence of the plug length on the plug velocity, the raining rate of solids onto and from the plugs and the influence of the particle size range on the plug velocity is accounted for.     

Nanoparticle synthesis at high production rates by flame spray pyrolysis

Scaling-up of nanoparticle synthesis by the versatile flame spray pyrolysis process at production rates up to is investigated. Product silica powder is collected continuously in a baghouse filter unit which is cleaned periodically by air-pressure shocks. The effect of powder production rate, dispersion gas flow rate and precursor (hexamethyldisiloxane, HMDSO) concentration on product particle size, morphology and carbon content is investigated. Droplet size distributions of the cold spray are measured by laser diffraction, while N₂ adsorption (BET), transmission electron microscopy and thermogravimetric analysis coupled with a mass spectrometer are employed to characterize the product powder. The product primary particle size was precisely controlled from 10 to and compared to a well-established vapor-fed flame aerosol reactor.     

Numerical simulation of a dissolution process in a stirred tank reactor

A dissolution process of solid particles suspended in a turbulent flow of a Rushton turbine stirred tank is studied numerically by large eddy simulations including passive scalar transport and particle tracking. The lattice-Boltzmann flow solver and the Smagorinsky subgrid-scale model are adopted for solving the stirred tank flow. To the LES a finite volume scheme is coupled that solves the convection-diffusion equation for the solute. The solid particles are tracked in the Eulerian flow field through solving the dynamic equations of linear and rotational motion of the particles. Particle-particle and particle-wall collisions are included, and the particle transport code is two-way coupled. The simulation has been restricted to a lab-scale tank with a volume equal to . A set of 7×10⁶ spherical particles in diameter are released in the top part of the tank (10% of the tank volume), resulting in a local initial solids volume fraction of 10%. The particle properties are such that they resemble those of calcium chloride beads. The focus is on solids and scalar concentration distributions, particle size distributions, and the dissolution time. For the particular process considered, the dissolution time is found to be at most one order of magnitude larger than the time needed to fully disperse the solids throughout the tank.     

Demonstration of a minimum in the recovery of nanoparticles by flotation: Theory and experiment

The flotation of nano- and submicron particles does not follow the conventional collection theory based on the interception and collision mechanisms, which predicts extremely low collection efficiency for particles smaller than 10-μm. Brownian diffusion and colloidal forces strongly influence the collection of such particles by air bubbles in flotation. In this paper, a theoretical model is presented for predicting the collection efficiency of nanoparticles. The theory incorporates mass transfer by Brownian diffusion, microhydrodynamics of particles in the vicinity of a slip surface of rising air bubbles, and colloidal interactions that come into effect at small separation distances. The governing equation was solved numerically using the Crank-Nicolson method with variable step size. A finite difference scheme with mesh refinement in the vicinity of the air bubble surface was used to discretise the stiff partial differential equation for the particle concentration. The mesh refinement produced correct numerical solutions without oscillation in the particle concentration distribution, which otherwise occurred due to the stiffness of the differential equation and coarseness of the numerical mesh. Predictions from the model were compared with experimental results obtained with a small laboratory column cell, in which colloidal silica particles with diameters in the range were floated using fine bubbles of typical average diameter . The particle concentration in the pulp was about 1% by weight. Cetyltrimethyl ammonium bromide and Dowfroth 250 were used as the flotation collector and frother, respectively. Both the theory and experiment show significant effect of the electrical double-layer and non-DLVO hydrophobic attractive forces on the collection of nanoparticles by air bubbles. The theoretical and experimental results show the collection efficiency to have a minimum at a particle size in the order of . With larger particles, the interception and collision mechanisms predominate, while the diffusion and colloidal forces control the collection of particles with a size smaller than the transition size.     

Use of the attainable region analysis to optimize particle breakage in a ball mill

Particle size reduction is one of the most widely encountered, yet least energy efficient, processes. Therefore, potentially significant energy and cost savings exist with even the slightest increase in milling efficiency. Often one would like to mill particles to a certain size, and no smaller, while minimizing energy use and milling time. We use the attainable region (AR) analysis to optimize the comminution of silica sand particles in a bench top laboratory ball mill. When the mill is loaded with a large number of grinding media (J=volume of media/mill volume=10.7%), the breakage profiles are indistinguishable over all rotation rates investigated. However, operation at lower grinding media fill level (J=1.5%) reveals separation between the grinding profiles for different rotation rates, suggesting more efficient breakage occurs at a lower grinding media fill level for a given rotation rate. Our results show that operation at multiple speeds, fast at first and then slower (φ_c=0.03), takes advantage of the initially overlapping grinding profiles and produces a similar particle size distribution with a decreased amount of processing time—less than half the time required for the single rotation rate milling. A natural extension of this idea is continuous milling, where the first mill can operate at a higher energy input for a shorter amount of time and the second mill can operate at a lower energy input for a longer amount of time.     

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.     

Axial dispersion of particles in a slugging column—the role of the laminar wake of the bubbles

Axial solid dispersion promoted by Taylor bubbles in a batch liquid column was studied. A mechanistic model was developed to predict the axial solid dispersion. The model is based on the upward transport of particles inside closed wakes of non-interacting Taylor bubbles. The model predictions are compared with experimental data. The experimental data were obtained in a test tube of internal diameter. The particle volumetric distribution was measured by several differential pressure transducers placed along the column. Two classes of glass beads, mean diameter 180 and , were suspended in aqueous glycerol solutions, with glycerol percentage ranging from 40% (v/v) to 100% (v/v). The amount of particles in the column was such that the volumetric particle fractions were 0.1, 0.2 and 0.3, supposing homogeneous liquid-solid suspension. The air flow rate ranged from 90×10⁻⁶ to at PTN conditions. The obtained experimental data are in good agreement with the model predictions for laminar wakes, i.e., closed wakes with internal recirculation and without vortex shedding. The experimental data show a higher upward particle transport for wakes in the transition laminar-turbulent regime; closed wakes with internal recirculation and vortex shedding. The upward particle transport is higher for increasing air flow rate, decreasing particle diameter and increasing amount of particles in the column.     

Application of flow-focusing to the break-up of an emulsion jet for the production of matrix-structured microparticles

Powder behaviour and performances are intimately related to particle attributes defined by the size, shape and structure of the individual particles making up the powder. The heterogeneous nature of the vast majority of powders reflected in the broad distribution of these particle attributes gives rise to the well-known difficulties in the prediction of powder behaviours. To achieve a better predictability and ultimate control of the performances, significant research efforts have recently been made towards the production of particles with controlled attributes, i.e. structured particles with a very narrow size distribution. This paper presents a new method for the production of monodisperse matrix-structured microcapsules using a simple in-house designed and fabricated macroscopic flow cell. The method combines flow-focusing with laminar jet break-up of a water-in-oil emulsion jet in a bulk water phase that produces monodisperse water-in-oil-in-water dispersion templates. The templates are then converted into solid microcapsules with a matrix structure by solvent evaporation. Results presented in the paper confirm a transition of flow regime from jetting to dripping when the outer to inner flow rate ratio reaches a critical value, accompanied by an increase in the size of the resultant droplets from the break-up. Importantly, the transition imposes a limit to the smallest size of monodisperse droplets each flow cell can produce through flow-focusing. The smallest monodisperse microcapsules produced from the flow cell used in this study through flow-focusing are in diameter.     

Kinetic theory based computation of PSRI riser: Part I—Estimate of mass transfer coefficient

The PSRI benchmark challenge problem one is modeled using kinetic theory based CFD with the energy minimization multi-scale (EMMS) drag law. These computations give a better comparison than the previous models to measured solids mass flux, solids density and pressure drop.The computer model was also used to calculate axial and radial normal Reynolds stresses, energy spectra, power spectra, granular temperatures, the FCC viscosity and axial and radial dispersion coefficients. The computed cluster sizes agreed with the published empirical correlations. Then, the mass transfer coefficients and the Sherwood numbers are estimated based on particle cluster sizes. The conventional Sherwood number is scaled with the particle cluster diameter. The Sherwood number is the order of 10^-2 and the mass transfer coefficient is the order of . This Sherwood number is two orders of magnitude smaller than the diffusion controlled limit of two based on particle diameter, in agreement with the experimental data for fluidization of fine particles.     

Theoretical and experimental investigations on particle rotation speed in CFB riser

Particle rotation is a common phenomenon in gas-solid two-phase flows. The paper presents theoretical and experimental investigations on particle rotation speed in the gas-solid flow inside a cold CFB riser. The possible particle rotation speed, caused by non-homogeneous flow field and particle collision, and its variation with time were investigated. The average particle rotation speed was predicted with considerations of particle size, average particle collision velocity, particle collision rate and particle number density. It is found that particle collision is the most significant reason for particle rotation. The maximal and average rotation speed for particles with an average size of several hundred micrometers in the CFB riser under typical working condition may be several thousand revolutions per second and several hundred revolutions per second, respectively. The rotation speed of glass beads with an average size of in the upper dilute zone of a cold CFB riser was measured by using a high speed digital imaging system. The variation of particle rotation speed with time was observed, which is in accordance with the theoretical result. The average rotation speed for glass beads was statistically analyzed based on a large number of particle examples. With several factors taken into account, the experimental result is considered to agree with the theoretical one.     

Novel manufacturing process of hollow polymer microspheres

A novel manufacturing process of gas-filled, hollow, poly-butyl-2-cyanoacrylate (PBCA) microspheres in an aqueous phase is the subject of this paper. The two-step process enables the control of the particle-wall size, as well as the particle size. Particles of this type are useful in the medical and pharmaceutical industry for diagnostic purposes in connection with ultrasound and for life or food science applications, such as taste masking, release on demand, etc. Depending upon the process conditions, the particle size ranges from 1 to . The microparticles are formed by using surfactant-stabilized microbubbles, which act as a template. At the interfacial area of these templates, PBCA nanoparticles create deposits that form the particle wall, as the result of a partial filming process. The density of the particles has been calculated as ranging from 100 up to and the volume fraction of the entrapped gas can be 5% or more. The developed process can operate on a scale of several kilograms.     

Particle dispersion in viscous three-phase inverse fluidized beds

Dispersion characteristics of low density fluidized particles such as polyethylene and polypropylene were investigated by using the stochastic method in three-phase inverse fluidized beds with viscous liquid medium ( in height). To establish the relationship between the pressure drop variation and the particle dispersion in test section, the histogram of pressure drop fluctuations were also measured and analyzed. Effects of operating variables such as gas and liquid velocities, liquid viscosity and media particle kind (density) on the fluctuating frequency, dispersion coefficient and exiting rate of media particles from the test section were determined. The fluctuating frequency and dispersion coefficient of particles increased with increasing gas or liquid velocity, but decreased considerably with increasing liquid viscosity in three-phase inverse fluidized beds. The dispersion coefficient of media particles of relatively higher density exhibited a value higher than that of lower density particles. The dispersion coefficients of particles were well correlated with operating variables in terms of dimensionless groups.     

Effect of cohesive powders on the elutriation of particles from a fluid bed

The elutriation of fine particles (Group C or A particles in Geldart's classification) from a fluid bed of mixed fine and coarse particles is investigated in a steady state. Al(OH)₃ and alumina and TiO₂ powder of 0.5- were used as fines. FCC, alumina, Al(OH)₃, limestone, silica sand, SiC particles of 44- were used as coarse particles. The paper investigates the effect on the elutriation rate constant of both fine powders and coarse particles (i) of the weight fraction of Geldart C powders in the bed, (ii) of the superficial gas velocity, and (iii) of the size of C powder and size of coarse particles in the bed.The elutriation rate constant of group C or group A particles is not only affected by the properties of the elutriated powders or particles and gas velocity, but also by both the weight fraction and size of C powder in the bed. This finding differs from the elutriation result of A or B particles from a fluidized bed.     

Appraisal of three-dimensional numerical simulation for sub-micron particle deposition in a micro-porous ceramic filter

A computational, three-dimensional approach to investigate the behavior of diesel soot particles in the micro-channels of a wall-flow, porous-ceramic particulate filter is presented. Particle size examined is in the PM2.5 range. The flow field is simulated with a finite-volume Navier-Stokes solver and the Ergun equation is used to model the porous material. The permeability coefficients were obtained by fitting experimental data. Particle flow, dispersion, deposition and wall-particle interactions are investigated tracking large swarms of 2 and diameter particles in a Lagrangian frame of reference. Particle dynamics included rarefied gas hypotheses (the Knudsen number being larger than unity) and bounce/capture models based on impact kinetic energy loss. The influence of gas molecules-particle interaction on overall particle behavior is also examined by including Brownian motion and partial slip in particle equation of motion. Simulations help to highlight three-dimensional non-uniform particle deposition, mainly due to flow distribution in the micro-channel. All particles deposit onto the porous filter wall following the distribution of the through-wall velocity. The larger, , particles show a larger tendency to deposit at the end of the filter. Due to the flow contraction at the inlet, virtually no particle deposit in the inlet section of the filter. Reasons for the scarce influence on particle deposition due to particle-flow slip and Brownian motion are given.     

Nanoparticle formation by highly diffusion-controlled emulsion polymerisation

Emulsion polymerisations of several monomers with different water solubilities including styrene, butyl acrylate, methyl methacrylate, vinyl acetate, and methyl acrylate were carried out under highly diffusion-controlled conditions. The monomer was placed on top of an aqueous solution of an emulsifier and an initiator, while being gently stirred. Polymerisation occurred by diffusion of monomer from the monomer phase via the interface to the micellar solution. Nanoparticles as small as 25 nm were produced as a result of reduced particle growth and delayed depletion of emulsifier micelles. Nanolatexes with relatively high solids content (20%) but with a low surfactant/monomer ratio (1/50) were obtained. The monomers with the highest solubility, with the exception of methyl acrylate, produced the smallest particles. The rate of diffusion-controlled polymerisation was found to be almost proportional to the saturation monomer concentration in the water phase . The results were compared with those obtained with a high rate of agitation which allowed kinetics of polymerisation to become the rate determinant. While particles obtained by kinetics-controlled emulsion polymerisation of the monomers were large, as well as similar in size, particles obtained from diffusion-controlled runs were small, but different in size. On the other hand, particles made by kinetics-controlled emulsion polymerisation had increasing surface coverage (by emulsifier) with monomer solubility in water. Whereas, nanoparticles made by diffusion-controlled emulsion polymerisation reached almost a constant surfactant coverage independent of the monomer type (except for methyl acrylate), a surface coverage as low as 0.20 was found to be sufficient for stabilisation of nanoparticles.     

Solids flow pattern in gas-flowing solids-fixed bed contactors. Part I: Experimental

An experimental investigation of the solids flow pattern in gas-flowing solids-fixed bed contactors is presented. The apparatus and procedures for determining the dynamic and static solids holdups, solids residence time distribution and the extent and rate of the exchange between particles in the static and dynamic solids holdup are described in detail.Experiments were performed in a bench scale system, containing a column (diameter ) packed with glass beads of 16 mm in diameter packed up to the height of 0.8 m. Tracer experiments with a step input in flowing solids phase were used for determining the residence time distribution and exchange between particles. Fine solids (spheres with mean diameter of ) of two different colors (all other properties being the same) were used in the tracer experiments to determine the residence time distribution and the exchange between static and dynamic solids holdup. In both types of experiments, the response curves have been obtained via color analysis of digital photos. All experiments have been repeated at different operating conditions, with a broad variation of solids mass flux and gas velocity, and reproducibility at set conditions was checked.The obtained experimental results are discussed and the observed important characteristics of the solids flow pattern are outlined. The effects of the solids flux and gas velocity on the solids flow pattern are presented and analyzed.     

Modeling axial distributions of adsorbent particle size and local voidage in expanded bed

Axial distribution of adsorbent particle as well as local voidage variation is very important and fundamental for the predicting and understanding of adsorptive performance in expanded bed adsorption. Based on the analysis of bed expansion and fluid hydrodynamic behaviors, a model was developed to predict the mean particle size and local voidage variations with the axial bed height under various operation conditions in expanded bed. Experimental measurements of particle size and local voidage changes with bed height were conducted in a modified glass column of inner diameter with sampling ports. Experimental data obtained in this work and in the literature with Streamline SP, Streamline Phenyl and Streamline quartz base matrix particles in the columns of inner diameters 20-, were used to verify the model prediction. The results were in good agreement with the experimental data.