Powder Technology (v.184, #1)
Effect of injection-nozzle operating parameters on the interaction between a gas–liquid jet and a gas–solid fluidized bed
by Federica Portoghese; Lorenzo Ferrante; Franco Berruti; Cedric Briens; Edward Chan (pp. 1-10).
The objective of the present study was to provide insight into the effect of operating conditions on the performance of gas-assisted nozzles injecting liquid into gas–solid fluidized beds. Acquisition of such knowledge is relevant to many industrial applications where liquid injections into fluidized beds of solid particles are performed via spray nozzles. In the fluid coking process, for example, product yields and reactor operability strongly benefit from a rapid and uniform distribution of the liquid feed on fluidized solid particles, which, in turn, is greatly affected by the performance of the liquid-injection system used.A novel experimental technique was employed to investigate the effect of varying the air-to-liquid ratio (ALR), the liquid mass flow rate, and the nozzle size on the contact efficiency of injected liquid on fluidized bed particles. Increasing the ALR or the liquid flow rate increased the nozzle spraying efficiency. On the contrary, increasing the nozzle size while keeping the gas and the liquid flow rates constant, and hence decreasing the pressure at the nozzle tip, lowered the liquid–solid contact efficiency.The effect of increasing the ALR on the liquid–solid contact resulting from nozzle-injections into the fluid bed, was correlated to both the nozzle atomization performance, as determined by open-air tests using a laser-photocell equipment, and the solids entrainment into the gas–liquid jet, as predicted by a model.The effect of operating conditions on the performance of gas-assisted nozzles injecting liquid into gas–solid fluidized beds was studied. The liquid–solid contact efficiency was improved by increasing the air-to-liquid ratio, increasing the liquid flow rate or reducing the nozzle size. These results were correlated to nozzle atomization performance, as determined by open-air tests (see Figure), and solids entrainment into the jet.Display Omitted
Keywords: Gas–liquid jets; Gas–solid fluidized bed; Atomization; Nozzle; Spray
Preparation of aluminum/silicon carbide metal matrix composites using centrifugal atomization
by Morteza Eslamian; Joel Rak; Nasser Ashgriz (pp. 11-20).
This paper describes the development of a new technique to produce metal matrix composites (MMCs) by injecting silicon carbide particles into molten aluminum just prior to centrifugal atomization. A centrifugal atomization apparatus has been constructed for this study. Silicon carbide particles are injected during atomization of 6061 aluminum alloy to form metal matrix composite powder. The prepared aluminum/silicon carbide powder contains 18 vol.% of SiC particles and 1.2 vol.% of voids. The particle grain size is almost independent from the particle size.Aluminum metal matrix composites (MMC) reinforced with SiC particles have up to 20% improvement in yield strength, have a lower coefficient of thermal expansion and a higher modulus of elasicity and are more wear resistant than the corresponding non-reinforced matrix alloy systems. This paper describes a new technique to produce MMCs by injecting SiC particles into molten aluminum just prior to centrifugal atomizaion.Display Omitted
Keywords: Al/SiC metal matrix composites; Centrifugal atomization; Powder metallurgy
Preparation of silver/hydroxyapatite nanocomposite spheres
by Jin-Ku Liu; Xiao-Hong Yang; Xin-Gang Tian (pp. 21-24).
The silver nanoparticles of 7 nm in diameter were controlled to be synthesized under the cooperation of the ethylenediamine and the cetyl trimethyl ammonium bromide (CTAB). Then, under the electrostatic effect of the silver nanoparticles and with the strong adsorbability of the hydroxyapatite nanoribbon spherites, the two substances were combined to form special nanocomposite spheres.Silver nanoparticles of 7 nm in diameter were controlled to be synthesized under the cooperation of the ethylenediamine and the cetyl trimethyl ammonium bromide. Then, under the electrostatic effect of the silver nanoparticles and with the strong adsorbability of the hydroxyapatite nanoribbon spherites, the two substances were combined to form special nanocomposite spheres.Display Omitted
Keywords: Nanoparticle; Silver; Hydroxyapatite; Nanocomposite
Auto-ignition route to thermoelectric oxide Na xCo2O4 powder with high compactibility
by Weihua Wang; Yang Jiang; Man Niu; Lizhi Wang; Bailai Cao (pp. 25-30).
The ultra fine powder of thermoelectric oxide Na xCo2O4 has been synthesized via an auto-ignition route following with airflow shatter process, which has the flaky shape in different nominal x compositions. Single γ-phase Na xCo2O4 crystal structure was obtained in varying Na contents of 1.4–1.8 at the calcining temperature of 1153 K. The airflow shatter process was found to be beneficial for obtaining uniform particles with smaller the average size and larger BET surface area. The compaction module and the nonlinear exponent of powder compaction are about 4.037 MPa and 4.368 calculated form Huang Peiyun's compacting equation, respectively, which reveals that the Na xCo2O4 powder has high compactibility.The ultra fine powder of thermoelectric oxide Na xCo2O4 has been synthesized via an auto-ignition route following with airflow shatter process, which has the flaky shape in different nominal x compositions and the mean particle size of about 2 μm. The study of compaction module and the nonlinear exponent of powder illustrates the obtained powder has high compactibility.Display Omitted
Keywords: Thermoelectric oxide Na; x; Co; 2; O; 4; Auto-ignition; Crystal structure; Compactibility
Mixing in a vibrated granular bed: Diffusive and convective effects
by Li-Shin Lu; Shu-San Hsiau (pp. 31-43).
In this study, Discrete Elementary Method (DEM) is employed to simulate the motion and mixing behavior of granular materials in a three-dimensional vibrated bed, which is energized by vertical sinusoidal oscillations under different vibrating conditions. With frictional sidewalls, the convection flow is a very important phenomenon in the vibrated granular bed. The influence of vibrating conditions, including vibration acceleration and frequency, on the formation of symmetric convection flow is investigated in this study. In order to characterize the convective flow and the diffusive motion of granular materials, the dimensionless convection flow rate, Jconv, and the vertical self-diffusion coefficient, D yy, are defined, respectively. Péclet number, Pe, is employed to characterize the ratio of the convective flow to the diffusive motion in vertical direction. The role of Pe in the formation of symmetric convection flow is discussed in detail. Moreover, the top-bottom initial loading pattern of two groups of glass beads with different colors is employed to investigate mixing behavior of granular materials. The well-known Lacey index is employed as the mixing degree, M, to quantify the mixing quality. The mixing rate is calculated from a least-square fit using the time evolution of M. The simulation results demonstrate that the mixing rates increase with increasing Jconv and D yy in exponential relations.Péclet number characterizes the ratio of convective flow to diffusive motion in vertical direction. Pe increases first and then decreases with increasing vibration acceleration, Γ. The symmetric convection flow is formed at the inflection point ( Γ= Γinf). The mixing rate increases rapidly as Γ< Γinf due to the stronger convective effect and slowly as Γ> Γinf because of the stronger diffusion effect.Display Omitted
Keywords: Granular mixing; Vibrated bed; DEM; Convection; Diffusion; Péclet number
A new and rapid method for the evaluation of the liquid–solid contact resulting from liquid injection into a fluidized bed
by Aidan Leach; Federica Portoghese; Cedric Briens; Franco Berruti (pp. 44-51).
Liquid feeds are injected into fluidized bed reactors such as fluid cokers, fluid catalytic crackers and gas-phase polymerization reactors. In these industrial processes, it is of crucial importance to optimize the contact between the injected liquid and the bed solids to minimize agglomeration, to ensure good reactor operability, and to allow reactions to proceed under minimum heat and mass transfer limitations in order to maximize the yields of valuable products. It has been shown [P. House, M. Saberian, C. Briens, F. Berruti, E. Chan, Injection of a liquid spray into a fluidized bed: particle–liquid mixing and impact on fluid coker yields. Industrial & Engineering Chemistry Research 43 (2004) 5663–5669., S. Bruhns, J. Werther, An investigation of the mechanism of liquid injection into fluidized beds. AIChE Journal 51 (2005) 766–775] that the nozzle technology and the operating conditions have a significant effect on the quality of the liquid–solid interaction resulting from the injection of gas-atomized liquid feed. The goal of this study was to develop a rapid and reliable experimental technique to assess the liquid–solid contact efficiency resulting from the injection of a liquid feed into a fluidized bed. Air-fluidized silica sand particles were first charged by triboelectrification as a result of their random collisions with the inner walls of the fluidized bed. Immediately after the injection of water through an aerated nozzle, the fluidization air was stopped and the wetted bed solids were allowed to settle. While the bed was defluidized, the triboelectric charges accumulated on the particles migrated to a grounded electrode through the low-resistance paths offered by the conductive liquid. A stronger electric current flowing through the electrode indicated that the liquid was more evenly distributed on the solid particles. The intensity of the current flowing through the electrode was, therefore, used to define a spray nozzle performance index. This technique was used to examine the effect of increasing the nozzle aeration, and, specifically, the gas-to-liquid mass ratio (G/L) through the nozzle on the liquid–solid contact efficiency. The results showed that changing the nozzle geometry can change how the contact efficiency between atomized liquid and fluidized solids varies with (G/L), especially at relatively high G/L ratios. A model of the time-evolution of the electric current generated during defluidization of the bed solids is also presented.Display OmittedImmediately after water injection into a fluidized bed, the fluidization air was stopped. Triboelectric charges migrated from bed particles to a grounded electrode through the conductive liquid. A stronger discharge current indicated that the liquid was more evenly distributed on the particles. The figure shows that increasing the air-to-liquid mass ratio (ALR) through the nozzle improves liquid–solid contact.
Keywords: Fluidization; Liquid injection; Spray nozzle; Liquid–solid contact efficiency; Triboelectricity
Solids entrainment into gas, liquid, and gas–liquid spray jets in fluidized beds
by Cedric Briens; Franco Berruti; Vittorio Felli; Edward Chan (pp. 52-57).
The injection of liquid into a fluidized bed is a crucial step in many processes such as fluid coking, fluid catalytic cracking, or gas-phase polymerization, whose performance greatly depends on good and rapid contact between the injected liquid and the fluidized particles. The liquid spray, created by two-phase (gas–liquid) nozzles, forms a jet, i.e. a gas-rich cavity within the fluidized bed. Past studies have shown that good liquid–solid contact requires a large entrainment rate of particles into the jet, followed by intensive mixing of liquid droplets and entrained particles within the jet. The objective of this study is the experimental measurement of solids entrainment into spray jets. The specific application of interest is the enhancement of solids entrainment under conditions relevant to the fluid coking process.A novel and accurate experimental technique has been developed to measure the solids entrainment from a fluidized bed into two-phase gas–liquid jets, gas jets and liquid jets. The effects of operating conditions of the nozzle (sonic versus subsonic) and of the fluidized bed on the solids entrainment have been investigated. The differences between the mechanisms of solids entrainment for two-phase gas–liquid, gas and liquid jets have been analyzed.This experimental tool has been applied to the design and testing of a mixing chamber consisting of a cylindrical tube placed at a certain distance downstream of the nozzle tip, resulting in a confined, turbulent jet with enhanced liquid–solid mixing properties.When liquid is injected into a fluidized bed, good liquid–solid contact requires a large entrainment rate of particles into the jet. A novel and accurate experimental technique has been developed to measure solids entrainment from the pressure difference between the two halves of a partitioned bed. A cylindrical tube downstream of the nozzle tip confines the jet to enhance solid–liquid mixing.Display Omitted
Keywords: Fluid coking; Fluidized bed; Liquid injection; Jet; Entrainment
A study on the conversion of trona to sodium bicarbonate
by Kyungmin Jacob Cho; Timothy C. Keener; Soon-Jai Khang (pp. 58-63).
The deactivation model was used to explain kinetics underlying the conversion reaction of trona to NaHCO3 (sodium bicarbonate). The model showed good agreement with the experimental data obtained from the conversion reaction of trona to NaHCO3. It gave the value of 0.94 as an average correlation coefficient with the experimental data. However, at lower temperature, the model was in poor agreement with the data. This would be related to the structural variation of trona particles at the lower temperature. A trona particle is initially nonporous and then it begins to crack. This structural variation creates more surface area for the reaction with CO2 and water vapor. However, at the lower temperature, the fissures on the surface of the particles are not fully developed during the beginning of the reaction. As a consequence, the level of the conversion of trona at the lower temperature is low during the beginning of the reaction and the time to approach the complete conversion is shorter as temperature increases. However, since the deactivation model does not include the term articulating the degree of the structural variation during the reaction, it does not fit well to the experimental data at the lower temperature. The deactivation rate constant, kd is strongly temperature dependent and the change of the slope suggests the reaction mechanism changes as the reaction temperature increases.The deactivation model was used to explain kinetics underlying the conversion reaction of trona to NaHCO3. The model showed good agreement with the experimental data obtained from the conversion reaction of trona to NaHCO3. However, it does not fit well to the experimental data at the lower temperature. The deactivation rate constant, kd is strongly temperature dependent and the change of the slope suggests the reaction mechanism changes as the reaction temperature increases.Display Omitted
Keywords: Deactivation model; Trona; SO; 2; Desulfurization
Numerical simulation of liquid transfer between particles
by Deliang Shi; J.J. McCarthy (pp. 64-75).
Liquid transfer between particles plays a central role in the operation of a variety of particle processing equipment, including flotation, spray-coating, flocculation, granulation, and drying. In each of these applications, the local liquid concentration within the bed dramatically affects the flow behavior of the system and can strongly impact performance. In this work, we introduce a dynamic liquid transfer model for use in discrete element modeling (DEM) of heterogeneous particle systems. We explicitly track moisture levels on individual particles and utilize an experimentally validated rule-set for liquid transfer upon forming/breaking contacts. As a test of this new model we present results from the simulation of a rotary drum spray-coating system, but expect that this liquid transfer-modified DEM is general and would be applicable to wide range of processing operations.Here we describe and test a model for liquid transfer in heterogenous particle systems. Our model relies on two plausible assumptions: the “harvest area” for liquid contribution to a specific bridge is givenby the surface of a spherical cap near the contact spot; and the liquid bridge ruptures at its thinnest portion of the profile, thus redistributing the two “halves” of the bridge upon rupture.Display Omitted
Keywords: Granular materials; Discrete Element Method; Liquid bridge; Spray coating
Euler/Lagrange computations of pneumatic conveying in a horizontal channel with different wall roughness
by Santiago Laín; Martin Sommerfeld (pp. 76-88).
The present study is related to the particle behaviour and the pressure drop in a particle-laden six meter long horizontal channel with rectangular cross-section from both experimental and numerical perspectives. Experiments and calculations are carried out for different spherical glass beads with diameters between 60 and 625 μm and mass loadings up to 1.0 (kg particles/kg gas). Additionally, stainless steel walls with different wall roughness are considered. In all experiments the air volume flow rate is constant in order to maintain a fixed gas average velocity of 20 m/s. As a result, the pressure drop in the channel is strongly influenced by wall roughness. Higher wall roughness implies higher pressure drop because of the increase in wall collision frequency, whereby momentum is extracted from the fluid due to two-way coupling. The numerical computations were performed by the Euler/Lagrange approach accounting for two-way and four-way coupling. For the calculation of the particle motion all relevant forces (i.e. drag, transverse lift and gravity), inter-particle collisions and wall collisions with wall roughness were considered. The agreement of the computations with the experiments was found to be very good for the gas and particle velocities as well as the pressure drop.The Euler/Lagrange approach was applied to pneumatic conveying in a narrow horizontal channel. The results on velocities of both phases and pressure drop were compared with measurements. Variations of particle size, mass loading and degree of wall roughness were realised. The agreement of velocity profiles was found to be reasonable and the increase of pressure drop by the particles with increasing wall roughness was captured very well using a stochastic wall roughness model and accounting for inter-particle collisions (see Figure).Display Omitted
Keywords: Pneumatic conveying; Spherical particles; Wall roughness; Inter-particle collisions; Pressure loss; Numerical calculations; Euler/Lagrange approach; Modelling
Numerical simulation of horizontal jet penetration in a three-dimensional fluidized bed
by Tingwen Li; Konstantin Pougatch; Martha Salcudean; Dana Grecov (pp. 89-99).
The introduction of reactant gas as a jet into a fluidized bed chemical reactor is often encountered in various industrial applications. Understanding the hydrodynamics of the gas and solid flow resulting from the gas jet can have considerable significance in improving the reactor design and process optimization. In this work, a three-dimensional numerical simulation of a single horizontal gas jet into a cylindrical gas–solid fluidized bed of laboratory scale is conducted. A scaled drag model is proposed and implemented into the simulation of a fluidized bed of FCC particles. The gas and particles flow in the fluidized bed is investigated by analyzing the transient simulation results. The jet penetration lengths of different jet velocities have been obtained and compared with published experimental data as well as with predictions of empirical correlations. The predictions by several empirical correlations are discussed. A good agreement between the numerical simulation and experimental results has been achieved.A three-dimensional numerical simulation of a single horizontal gas jet into a cylindrical gas–solid fluidized bed of laboratory scale is conducted. A scaled drag model is proposed and implemented into the simulation of the fluidized bed of FCC particles. The jet penetration lengths have been compared with experimental data and a good agreement has been achieved.Display Omitted
Keywords: Jet penetration; Fluidized bed; Drag model; CFD
A new method to determine the true local strain in porous compacts during simple compaction
by A. Kasser; G.J. Richardson (pp. 100-104).
During hot working processes of metals, precise knowledge of parameters such as strain, strain rate and temperature, is of prime importance for a true prediction of subsequent structural changes in the processed materials. This paper outlines the effect of starting porosity on the true strain generated in 316 L stainless steel powder precompacts during hot deformation by axisymmetric compression tests. The study was carried out at constant temperature, using cylindrical samples with starting porosities of 13 and 37%.Results have shown that below a certain ideal porosity of 22%, the true uniform strain in the central region remote from the specimen ends is always higher than the nominal mean applied strain. However, if the starting porosity is above 22%, then the true uniform strain generated in the central region of the deformed specimen is always lower than the nominal mean applied strain. But if the starting porosity is about 22 %, then the true uniform strain is equal to the nominal applied mean strain. Empirical equations designed to quantify this true strain are proposed. These equations which are a function of the deformation variables can be very useful for computationnal purposes in controlled hot working operations of powder preforms.Pendant les processus de déformation à chaud des métaux, la valeur réelle de certains paramètres tels que: la déformation, la vitesse de déformation et la température sont d’une importance capitale pour une bonne prédiction des changements structuraux ultérieurs. Cette publication est destinée à montrer l'influence de la porosité initiale sur la déformation réelle locale, pendant le forgeage axisymétrique à chaud, pratiqué sur des comprimés de poudre d’acier inoxydables austénitiques de nuance 316 L. L’étude a été réalisée sous une température constante, en utilisant deux types d’échantillons de porosités 13 et 37%.Les résultats ont montré qu’en dessous d’une certaine porosité idéale de 22%, la déformation uniforme réelle locale est toujours supérieure à la déformation moyenne appliquée aux comprimés. Cependant, si la porosité initiale est supérieure à la porosité idéale, les résultats montrent alors que la déformation uniforme réelle locale est toujours inférieure à la déformation moyenne appliquée aux comprimés. Si par ailleurs la porosité initiale des comprimés est autour de 22%, dans ce cas la déformation uniforme réelle locale est égale à la déformation moyenne appliquée. Des équations pour la quantification de cette déformation uniforme réelle locale sont proposées. Ces équations qui sont une fonction des variables de déformation peuvent être très utiles lors de la simulation mathématique des opérations contrôlées de déformation à chaud des comprimés de poudre.During plastic deformation of metals precise knowledge of parameters such as strain and strain rate is very important for a true prediction of subsequent mechanical properties. In solid materials, the true strain is usually equal to the nominal applied strain if deformation occurs without friction. This strain can easily be determined if the dimensions of the specimen before and after deformation are known. However, in porous compacts, the true strain depends not only on the dimensions of the specimen but also on the volume fraction of porosity. This paper outlines the method to determine that strain through a set of equations which are a function of the nominal strain and the porosity prior to deformation.Display Omitted
Keywords: Powder; Compaction; Powder preforms; Porosity; Forging; Strain
Measurement and modeling of the settling velocity of isometric particles
by A. Hazzab; A. Terfous; A. Ghenaim (pp. 105-113).
The sedimentation of solid particles of simple form and complex form has been studied. The applied data in the case of isometric particles have been compared to those of spherical particles of same volume. This comparison allows highlighting an equivalent sedimentation diameter concept.Dimensionless factor depending on the extended equivalent diameter, the geometric aspect of the particle and the flow have been defined. The use of this factor allows finding an interrelationship with another dimensionless factor which is the Archimedes number. This last depends on the physical parameter of the particle and the carrying fluid. The numerical treatment of this interrelationship permitted to deduct an empirical model for the settling velocity of particles. This model is applied with success to the data of the considered measures and well compared to the results obtained by other models.The sedimentation of particles of simple form and complex form is studied. A dimensionless factor depending on the particle shape and flow characteristics is defined. It varies with Archimedes number. The relationship between these two parameters leads to a model calculating the settling velocity. The results obtained agreed well with those of other models (figure).Display OmittedComparison with the results of other models.
Keywords: Settling velocity; Solid particles; Isometric particles; Sedimentation; Model
Microencapsulation of Radix salvia miltiorrhiza nanoparticles by spray-drying
by Y.L. Su; Z.Y. Fu; J.Y. Zhang; W.M. Wang; H. Wang; Y.C. Wang; Q.J. Zhang (pp. 114-121).
About 133.5 nm Radix salvia miltiorrhiza nanoparticles were prepared by high speed centrifugal sheering pulverizer and the nanoparticles were characterized by TEM in this study. Microcapsules containing R. salvia miltiorrhiza nanoparticles were produced by spray-drying technique using different proportions of gelatin and sodium salt of carboxymethylcellulose (CMC-Na) as wall materials. The effects of inlet temperature, flow rate, spray-gas flow and the ratio of Mcore/ Mwall on encapsulation yield (EY) and encapsulation efficiency (EE) were investigated. The EE was determined by reverse high performance liquid chromatography (HPLC); the resulting microcapsules were characterized by FT-IR, SEM, and X-ray diffraction analysis. In addition, in vitro release characters of R. salvia miltiorrhiza raw powder, spray-dried powder and microcapsules were also studied. The results showed that spray-dried microcapsules had a regular spherical shape but the majority presented rough surfaces or invaginations with a diameter of 2–5 μm. R. salvia miltiorrhiza nanoparticles were embedded in the wall system consisting of gelatin and CMC-Na. Higher EE and EY were obtained under the inlet temperature of 80 °C and the ratio of Mcore/ Mwall of 1/4. In vitro release study showed that R. salvia miltiorrhiza microcapsules could regulate drug release. This study may be helpful to the pharmaceutical application of R. salvia miltiorrhiza. Radix salvia miltiorrhiza nanoparticles (showing as the following figure) were prepared by high speed centrifugal sheering pulverizer and 2–5 ìm microcapsules containing nanoparticles were produced by spray-drying using gelatin and CMC-Na as wall materials. A satisfactory encapsulation yield of 59.64% and encapsulation efficiency of 75.66–90.42% were obtained under the inlet temperature of 80 °C and the ratio of Mcore/ Mwall of 1/4.Display Omitted
Keywords: Radix salvia miltiorrhiza; Nanoparticles; Spray-drying; Gelatin; CMC-Na
Agglomeration of fine particles subjected to centripetal compaction
by R.Y. Yang; A.B. Yu; S.K. Choi; M.S. Coates; H.K. Chan (pp. 122-129).
Agglomeration is a common phenomenon in many processes. The mechanical properties of agglomerates strongly depend on their structures. This paper presents a numerical study of the agglomeration of fine particles down to 1 μm in size based on the discrete element method. The agglomerates were formed with particles initially generated randomly in a spherical space and then packed under an assumed centripetal force. Agglomerate structure, packing density, coordination number and tensile strength were analysed with particular reference to the effect of particle size associated with the van der Waals attraction. The results showed that both the packing density and coordination number of the agglomerates decay exponentially to their limits as agglomerate size increases. The tensile strength of the agglomerates was calculated from the simulations and shown to decrease with the increase of particle size. The strength was also estimated from the Rumpf model supported by the empirical equations formulated based on the present simulation results. The good agreement between the results from the simulations and the estimation indicates that the equations are useful to facilitate engineering applications.Agglomeration of fine particles was studied by DEM with focusing on the effect of particle size on the agglomerate structure and strength. The results showed that both packing density and coordination number decay exponentially to their planar limits as agglomerate size increases, and structural properties are dependent on the interparticle force. Equations were formulated to describe these relationships to facilitate engineering applications. Coupled with the Rumpt model, they can be used to calculate the tensile strength of agglomerates.Display Omitted
Keywords: Agglomerate; Particle packing; Discrete element method; Fine particles; Tensile strength