Assessment of the degree of agglomeration is suspensions

A survey is given of the methods for determining the size of agglomerates in suspensions. Counting methods, photometric methods, and sedimentation methods are most frequently used. With the latter two methods, the agglomerate size can only be determined if the porosity of the agglomerates is known. The mean size and the mean porosity of agglomerates in suspensions can be determined by the simultaneous measurement of the specific extinction cross-section area by means of a photometer, and the mean settling velocity by means of a sedimentation balance. As shown for suspension of quartz in water with NaCl or a cationic polyelectrolyte as agglomerating agent, the porosity of the agglomerates increases with increasing agglomerate size.     

Structural Property Effect of Nanoparticle Agglomerates on Particle Penetration through Fibrous Filter

Most filtration studies have been conducted with spherical particles; however, many aerosol particles are agglomerates of small primary spheres. Filtration efficiency tests were conducted with silver NP agglomerates, with the agglomerate structure controlled by altering the temperature of a sintering furnace. The mobility diameter and mass of the silver NP agglomerates were measured using a differential mobility analyzer together with an aerosol particle mass analyzer. From these measurements, it was found that the fractal-like dimension, D _fm, varied from 2.07 to 2.95 as the sintering temperatures was increased from ambient to 600°C. The agglomerates were essentially fully coalesced at 600°C allowing direct comparison of the filtration behavior of the agglomerate to that of a sphere with the same mobility diameter. Other agglomerate properties measured include the primary diameter, the agglomerate length and aspect ratio, and the dynamic shape factor.

Agglomerate filtration modeling with no adjustable parameters has been investigated in terms of diffusion, impaction, and interception. The model results agree qualitatively with the experimental results in the particle size range of 50 to 300 nm. The results indicated that the larger interception length of agglomerates is responsible for the smaller penetration through a fibrous filter in comparison to spherical particles with the same mobility diameters.     

The setting of a dense bed of fine powders in air Part II. A simple model of the settling process

In the sedimentation of fine particles it is widely recognized that there is flocculation and that the settling rate is governed by that. By using a simple model it is possible to deduce an order-of-magnitude estimate of the size of the agglomerates from the sedimentation rate at the surface of a settling bed of powder in air. For some of the materials used, namely fine clay particles, the results seem plausible. On the other hand, the results for glass beads indicate that in this case the assumptions were open to doubt; the one most suspect was that the agglomerate could be assumed to be impermeable.All beds were very dense with a sharp upper interface, hence the model dealt with dense beds of fine powders which were assumed to form spherical agglomerates of relatively large diameter, settling in close proximity. The variables used had to take account, at least in an empirical fashion, of the effect of flow of air around each agglomerate due to (a) their proximity, (b) air displaced upwards by solids settling in a closed vessel. Thus the quantities of significance were (a) the diameter of the agglomerate, (b) the separation between agglomerates; these were combined into variables used in the equations, namely the hydraulic diameter and the density of the suspension.     

Agglomeration behavior of anhydrous L-ornithine-L-aspatate crystals during semi-batch drowning-out crystallization

Crystallization of L-ornithine-L-aspartate (LOLA) by drowning out was carried out to produce the anhydrous form of agglomerates. The primary crystal size in the agglomerate remained unchanged after completion of the crystallization. The LOLA aqueous solution introduced into the system was immediately dispersed and cluster coagulated on the surface of the crystals. On the surface of the crystals, a cluster reached critical nuclei size, nucleated and intergrowth to form agglomerates. It was proposed that a spherical agglomeration occurred during secondary nucleation by coagulation model and intergrowth. The agglomerates size and size distribution were varied with the process parameters. The agglomerate sizes of LOLA crystals appeared to be ruled not only by secondary nucleation rate but also by the mass of suspended agglomerates. Moreover, the agglomeration rates of fine particles were higher than the agglomeration rates of large agglomerates. Using these properties, the uniform agglomerates size distribution could be obtained.     

Effect of size ratio on the behaviour of agglomerates embedded in a bed of particles subjected to shearing: DEM analysis

This paper presents the results of analysis of the deformation and breakage of spherical agglomerates embedded in a bed of particles and subjected to shearing, a situation commonly encountered in powder granulation. The study is based on three dimensional distinct element method (DEM), in which the inter-particle interactions are governed by theories of contact mechanics. An agglomerate was first generated in a bed of particles having the same size as the primary particles forming the agglomerate. Different size ratios (i.e., the ratio of the diameter of agglomerate to the diameter of surrounding particles) in the range 3-10 were then simulated by varying the size and number of surrounding particles. The agglomerates were subjected to shearing (shear rate and strain of about and 0.3, respectively) and their breakage characteristics were analysed. The agglomerate with the size ratio 10 does not break but undergoes some structural deformation by re-arrangements of contacts. However, the agglomerates with ratio about 7 or smaller suffer breakage. For the size ratio equal or smaller than 5, the agglomerate breaks significantly leading to full disintegration. The results of stress analysis of the agglomerates suggest that the resistance to breakage for the agglomerate with size ratio of 10 is due to the nature of stresses exerted on the agglomerate. For large size ratios the stress on the agglomerate is predominantly hydrostatic. The ratio of deviatoric stress over hydrostatic pressure increases as the size ratio of the agglomerate is reduced. The nature of stresses experienced by agglomerates with smaller size ratios is predominantly deviatoric, thus causing shear deformation and breakage. The results are compared with physical experiments and a satisfactory agreement is obtained.     

Influence of interface energy of primary particles on the deformation and breakage behaviour of agglomerates sheared in a powder bed

In this paper, the parameters that affect the deformation and breakage of agglomerates embedded in a bed of particles subjected to rapid shearing are identified and analysed. The influences of interface energy between the primary particles and the size ratio (between agglomerates and particles of the bed) on the deformation characteristics of the agglomerate are addressed. The study is based on computer simulations using the distinct element method (DEM). It has recently been shown that for agglomerates having a size ratio greater than about 7, the nature of stresses experienced by the agglomerates when sheared inside a particulate bed is predominantly hydrostatic, hence it is difficult to break them (Hassanpour et al., 2007). However, the role of the interface energy between primary particles coupled with the effect of size ratio on the breakage and deformation characteristics of agglomerates during shearing has not been analysed. This feature is of great interest in the agglomeration process and is hence addressed in the present study. It is found that despite the predominantly hydrostatic nature of stresses responsible for retarding the breakage, agglomerates with size ratio greater than about 7 could undergo macroscopic deformation when the surface energy between the primary particles is decreased below a critical value. Furthermore, a failure map of agglomerates is presented in terms of their size ratio and the value of interface energy of the primary particles.     

Characterization of Morphological Changes in Agglomerates Subject to Condensation and Evaporation Using Multiple Fractal Dimensions

Multiple fractal dimensions are used to characterize morphological changes that occur when an aerosol composed of irregularly shaped agglomerates is subject to condensation followed by evaporation. The agglomerates change from a branched, chainlike structure to a more regular, near-spherical or clumplike structure reflected in a decrease in the structural fractal dimension. The textural fractal dimension remains constant because the primary particles, of which the agglomerates are composed, do not change in shape. The degree of supersaturation and the number of condensation-evaporation cycles that the aerosol undergoes are major factors that influence morphological change. Even at low supersaturations, increasing the number of condensation-evaporation cycles makes the agglomerates more regular and thus decreases the structural fractal dimension. The transition point in the Richardson plot is a good indicator of the size of the primary particles in the agglomerate.     

Morphology of Highly Dispersing Precipitated Silica: Impact of Drying and Sonication

Light and X-ray scattering are used to examine the structure of two commercial precipitated silicas (Zeosil 1165 and Ultrasil 7005) and one developmental precipitated silica, Dimosil 288. All three products have a four-level hierarchical structure consisting of primary particles, aggregates, hard agglomerates and soft agglomerates, with Dimosil 288 showing the clearest evidence of the four structural levels. The impact of sonication and drying protocol was explored by light scattering for Dimosil 288. With the exception of the large-scale soft agglomerates, all the structural levels are robust under sonication of aqueous suspensions. Sonication breaks down soft agglomerates leaving hard structures approximately 11 μm in radius-of-gyration. Drying plays a critical role in hardening the soft agglomerates. If the product is never dried, sonication reduces the agglomerate size to 3.5 μm, which is identified as the size of the hard agglomerate. Although agglomerates larger than 3.5 μm are present in the never-dried product, they are friable. Large-scale agglomerates are marginally more robust when dried at 150 °C compared to room temperature drying. Given the similarity of mechanical properties of rubbers filled with these silicas, it appears that the four-level structure with friable large-scale soft agglomerates is a characteristic of highly dispersing silica products.     

Flow induced phase inversion agglomeration: Fundamentals and batch processing

A novel agglomeration technique, based on flow induced phase inversion (FIPI) is described and applied to the batch preparation of polyethylene-bound abrasive calcite agglomerates. Water soluble polymers are used to agglomerate the needle-like crystals of tetraacetylethylene diamine and also sodium chloride crystals. In a typical isothermal FIPI agglomeration process primary particles are dispersed in the molten binder, which is subsequently inverted by the addition of sufficient amount of primary particles, which also defines the critical filler concentration at phase inversion, C_c. Agglomerate particle size is primarily a function of C_p – C_c where C_p is the mean concentration of filler. C_c decreases with increasing binder molecular weight and primary particle surface area. Agglomerate size distribution is affected by processing, mainly by the mixing time after phase inversion. For the non-isothermal FIPI agglomeration process, phase inversion is induced locally, by the addition of fine particles in the molten binder. Phase inversion is then propagated by cooling the dispersion during mixing. Agglomerate characteristics such as particle size, particle size distribution, binder concentration distribution in each agglomerate size range, agglomerate topology, binder morphology in the agglomerates, agglomerate strength, and agglomerate dissolution rate in water were evaluated. These agglomerate characteristics are related to the binder and filler properties as well as to the processing conditions.     

Deposition of silica agglomerates in a cast of human lung airways: Enhancement relative to spheres of equal mobility and aerodynamic diameter

This paper reports on an experimental study of the deposition of well-characterized silica agglomerates in a cast of a section of a human lung. Deposition of the agglomerates is compared with the deposition of oleic acid spheres and sodium chloride particles for a range of mobility sizes, agglomerate properties (primary particle size and mass–mobility exponent) and inspiratory flow rates. In most cases, agglomerate deposition was significantly greater than that of the oleic acid and sodium chloride particles. Deposition of agglomerates with a more open structure was greater than that of relatively more compact (but still non-spherical) agglomerates. Deposition also increased with the flow rate. Because of the large physical size of the agglomerates, as well as the crenulated flow path through the model and the flow rate dependence, it is likely that interception is responsible for the enhanced deposition of the agglomerates.     

Agglomeration of magnesium particles in magnesium-sodium nitrate combustion

Agglomeration phenomenon of magnesium particles during combustion of Mg NaNO₃ propellant has been studied. High speed photographs of combustion zones and the burning surface temperature data indicate that the metal particles form agglomerates on the burning surface in varying degree depending on the mass fraction of NaNO₃. It is found that the increase of oxidizer content increases the metal agglomeration and the agglomerate size depends on the initial particle size of the ingredients. An attempt has been made to predict the size of the agglomerates based on the consideration that the agglomerate size depends on the thickness of the molten oxidizer layer enveloping the metal particles in the condensed phase and surface heat flux providing local temperature environment to agglomerate the metal particles and to eject from the burning surface for the vapour phase combustion. The results were compared with the experimental data. The prediction describes fairly well the observed effects of the concentration and particle size.     

A One-Dimensional Model for Coagulation,Sintering, and Surface Growth of Aerosol Agglomerates

The size of the primary particles in aerosol agglomerates is determined in part by the interplay of surface growth, coagulation, and sintering. These processes are modelled by a one-dimensional (1-D) discrete-sectional model, DISGLOM2, which predicts the evolution of agglomerate and primary particle size distributions. DISGLOM2 is an extended version of DISGLOM (Rogak 1997), in which particles smaller than the "melting diameter" were assumed to sinter instantly while bigger particles did not sinter at all. Gradual sintering, "condensational obliteration" (whereby primary particles are lost during heavy surface growth), and diffusional wall deposition have been incorporated into DISGLOM2. Results from DISGLOM2 were comparable with those from 2-D sectional models, but DISGLOM2 was much faster. In addition, DISGLOM2 includes the effects of "condensation" of small spherical particles on large agglomerates, which were not modelled previously. The effect of condensation was shown to be significant at low temperature. DISGLOM2 was used to predict the primary particle diameter of titania particles generated by precursor reaction. By adding gradual sintering, the growth rate of agglomerate particles by coagulation was slightly decreased and the primary particle size considerably increased compared with the results given by DISGLOM. Although DISGLOM2 is an efficient model of the relevant physical processes, the predictions are sensitive to the kinetics of precursor reactions and particle sintering, which can be difficult to characterize in real experimental systems.     

Experimental and theoretical study on the agglomeration arising from fluidization of cohesive particles—effects of mechanical vibration

A novel technique that can prevent the disruption of agglomerates when sampling the agglomerates from a fluidized bed has been developed and has been applied to the investigation of the agglomeration behaviour of cohesive particles during fluidization with and without mechanical vibration. A new model for the prediction of agglomerate size has also been established on the basis of the energy balance between the agglomerate collision energy, the energy due to cohesive forces and the energy generated by vibration. The accuracy of the model is tested by comparing the theoretical results with the experimental data obtained both in the present work and in the literature. Effects of gas velocity and mechanical vibration on agglomeration for two cohesive (Geldart group C) powders in fluidization are examined experimentally and theoretically. The experimental results prove that mechanical vibration can significantly reduce both the average size and the degree of the size-segregation of the agglomerates throughout the whole bed. However, the experiments also reveal that the mean agglomerate size decreases initially with the vibration intensity, but increases gradually as the vibration intensity exceeds a critical value. This suggests that the vibration cannot only facilitate breaking the agglomerates due to the increased agglomerate collision energy but can also favour the growth of the agglomerates due to the enhanced contacting probability between particles and/or agglomerates. Both the experimental and theoretical results show that a higher gas velocity leads to a smaller agglomerate size.     

Growth and breakup of a wet agglomerate in a dry gas–solid fluidized bed

Using CFD‐DEM simulations, a wet agglomerate of particles was placed in a void region of a dry vigorously fluidized bed to understand how wet agglomerates grow or breakup and how liquid spreads when agglomerates interact with dry fluidized particles. In the CFD‐DEM model, cohesive and viscous forces arising from liquid bridges between particles were modeled, as well as a finite rate of liquid bridge filling. The liquid properties were varied between different simulations to vary Bond number (surface tension forces/gravitational forces) and Capillary number (viscous forces/surface tension forces) in the system. Resulting agglomerate behavior was divided into regimes of (i) the agglomerate breaking up, (ii) the agglomerate retaining its initial form, but not growing, and (iii) the agglomerate retaining its initial form and growing. Regimes were mapped based on Bo and Ca. Implications of agglomerate behavior on spreading of liquid to initially dry particles were investigated. This article identifies a new way to map agglomerate growth and breakup behavior based on Bo and Ca. In modeling both liquid forces and a finite rate of liquid transfer, it identifies the complex influence viscosity has on agglomeration by strengthening liquid bridges while slowing their formation. Viewing Ca as the ratio of bridge formation time to particle collision and separation time capture why agglomerates with high Ca struggle to grow. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2520–2527, 2017     

Application of discrete element method simulation for studying fluidization of nanoparticle agglomerates

The usefulness of discrete element method simulation for studying fluidization of nanoparticle agglomerates is explored. Nanoparticle agglomerates were simulated by using solid particles of equivalent sizes and densities. Validity of the present simulation was assessed through comparisons of simulation results and experimental observations of bed expansion, characteristic fluidization behaviour, and dense‐bed settling. The simulation was then used to investigate initial bed expansion and bed uniformity under particulate fluidization conditions. The role of inter‐agglomerate interparticle force in fluidization of nanoparticle agglomerates was examined. A stability analysis originally developed for addressing the transition from particulate to bubbling fluidization for conventional particles was used for predicting the start of bubbling in fluidized beds of nanoparticle agglomerates.     

超细粉在声场导向管喷流床中的聚团尺寸预测模型

超细粉的流化性能与聚团尺寸密切相关。通过分析超细粉聚团在声场导向管喷流床中的形成过程,提出了高速射流的剪切作用和聚团间的碰撞作用是决定聚团尺寸的主要原因。在此基础上,结合聚团在射流剪切过程和聚团间碰撞过程中的力平衡分析,建立了声场导向管喷流床中聚团尺寸分布的预测模型;并运用这一模型成功预测了不同射流气速下,超细TiO₂颗粒在声场导向管喷流床中的聚团平均直径和聚团尺寸分布。     

Mathematical modelling of the MCFC cathode

Different models for the NiO cathode have been compared with respect to their abilities to predict polarization curves and the influence of the amount of electrolyte on the electrode performance. It has been shown that the agglomerate model for the MCFC cathode gives more reasonable results when the exterior agglomerate surface area is specifically taken into account. In the cathode only the outermost layer of nickel oxide particles in the agglomerate is utilized for the electrochemical reaction. The pseudohomogeneous approach is questionable for these agglomerates since the individual particles constituting the agglomerate are of the same size as the reaction zone thickness. A thin-film model with a roughness factor for the electrode surface appears to be as good a model as the agglomerate model. A model based on a chain of spherical agglomerates and the partially drowned agglomerate model are physically more realistic models than the homogeneous agglomerate model for the prediction of the influence of electrolyte fill on the electrochemical performance.     

Effect of glidants in binary powder mixtures

The intention of this study is to investigate on a particulate level the flow properties of dry powder mixtures consisting of cornstarch and a second nanoscaled material. Special attention is paid to the question on the working mechanism of glidants. In 1974, Rumpf showed that a roughness on the surface of a smooth particle leads to a reduction of its forces of interaction with another particle. The interaction forces are reduced as the surface roughness increases the distance between the centers of gravity of the two interacting particles. Agglomerates as well as the primary particles of materials used as glidants are characterized by diameters in the lower nanometer range. In consequence they are strongly adsorbed at the surface of larger particles and act as a surface roughness. If the effect of a glidant would be due to its ability to act as a surface roughness then all nanoparticles being small enough to reduce the net interaction forces could be used as glidants almost irrespective of their chemical nature. Indeed we have been able to demonstrate that nanoparticles of titanium dioxide, aluminum oxide, silicon dioxide or of carbon black act as glidants. Mixing time directly influences the efficiency of a nanomaterial to act as a glidant. Due to increasing ratio of adhesive force to particle weight with decreasing particle radius, nanomaterials tend to aggregate and agglomerate. With increasing mixing time the size of agglomerates decreases. At the same time the number of primary particles available for adsorption on the surfaces of the cornstarch particles increases. An optimum in flow properties is achieved at a characteristic mixing time. At a further increase in mixing time, the size of agglomerates decreases and the coverage of the cornstarch particles by nanoparticles increases. Eventually cornstarch particles are obtained being completely coated with nanoparticles. The surfaces of these coated particles are smooth. Accordingly they show a poor flow behavior. The property of the nanomaterial to act as a glidant is lost.     

Aluminum Agglomeration on Burning Surface of NEPE Propellants at 3–5 MPa

Aluminum agglomeration and agglomerate sizes of NEPE propellants were studied by cinephotomicrography at pressures of 3 and 5 MPa. Accumulation, aggregation, and agglomeration of aluminum particles similar to that at pressures below 1 MPa were observed. Coalescence of two agglomerates on the burning surface is obtained for the first time. A decrease in the burning rate from 8 to 5 mm s⁻¹ leads to about 20 % increase in the agglomerate diameter. The pressure is found to have no direct influence on the agglomerate diameter when the burning rate is kept constant. The evolution of the agglomerate diameter according to the increase of the virgin aluminum size from 16 to 36 μm is convex in shape and reached its maximum at a particle diameter of 29 μm. Increasing the amount of RDX crystals added in the propellants causes a larger agglomerate diameter. The experimental mass average agglomerate diameters were compared with various agglomeration models. It is found that Hermsen and Salita empirical models have a higher accuracy for NEPE propellants rather than Becksted, Liu, or pocket models.     

Agglomeration of fine particles subjected to centripetal compaction

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.