Interparticle Cohesive Forces Research Articles

Effects of Interparticle Forces on Fluidization Characteristics in Liquid-Containing and High-Temperature Fluidized Beds

Interparticle forces exert a critical effect on particle fluidization characteristics in gas–solid fluidized beds. Glass and steel fluidized beds are specially designed to study the effects of liquid and high temperature, two key parameters in the condensed-mode operation of industrial polymerization reactors, on particle fluidization characteristics mainly from two aspects: interparticle forces and minimum fluidization velocity. Variations in interparticle forces are reflected in the peak values and gas velocity ranges of the “hump” in the ΔP–ug curve. This is because an increase in liquid content or bed temperature enhances the compaction of fixed-bed particles and makes complete fluidization of the particles difficult to achieve. Moreover, the increase of the minimum fluidization velocity can be ascribed to the delay of particle fluidization caused by the growth of interparticle cohesive forces due to the increase in liquid content and temperature. Three types of methods, namely, pressure drop measurements, accelerometry, and visual observation, are compared, and accelerometry was found to be superior to the pressure drop method for measuring umf in a liquid-containing fluidized-bed reactor. In addition, the reversibility of agglomeration and its implications for industrial operation are discussed. This work aims at further understanding wet and high-temperature gas–solid fluidized beds, especially the condensed-mode operation of polyethylene fluidized-bed reactors.

Experimental Investigation on the Translational and Rotational Motion of Biomass Particle in a Spout-Fluid Bed

Abstract The translational and rotational motions of biomass particle in a spout-fluid bed have been investigated. Two kinds of cylindrical biomass particles with different densities were used as bed solids, which were obtained from the jujube tree. To describe the translational and rotational characteristics, virtual particle concentration and rotation angle were introduced. The former obtained by microwave heating-infrared thermal imaging technique was compared with the actual particle concentration determined by box-counting method. Based on the temperature distribution, the translational position and rotation angle of multiple tracers were distinguished. The results show that the heating characteristic of tracer depends on the tracer material. Compared to the spherical particles, non-spherical biomass particle is more likely to cause dead zone in the bed. In the flow regime of fluidizing, the flow characteristic of biomass particle relies on the jet gas velocity. The high resistance resulting from the interparticle cohesion forces hinders the rise of bubbles and weakens the radial dispersion of particle. In the fountain, homogeneous biomass particle rotates uniformly, while non-homogeneous biomass particle exhibits non-uniform rotation. In the annulus, the rotation characteristic of biomass particle depends on the structure of fountain and particle trajectory in the annulus.

Investigating agglomeration phenomena in an air-polyethylene fluidized bed using DEM–CFD approach

Soft-sphere discrete element method and Navier–Stokes equations were coupled with the equations of energy to simulate the agglomeration process at high temperature in an air-polyethylene fluidized bed. The inter-particle cohesive force was calculated based on solid bridging by the viscous flow. The agglomerates were tracked as a real object with irregular shapes in which their translational and rotational motions were calculated according to the multi-sphere method. Fluidization behavior of an agglomerating bed was successfully simulated in terms of increasing the size of agglomerates as well as decreasing the bed height. The influence of gas velocity on the size of agglomerates was investigated and it was shown that the formation of massive agglomerates is postponed as the gas velocity is increased. Microscopic mechanistic studies in terms of contact time, cohesive force and repulsive force, which are the key parameters in the formation of agglomerates, were also carried out.

Micromechanical modeling and analysis of different flow regimes in gas fluidization

The micromechanics of different particle–fluid flow regimes, such as fixed, expanded and fluidized beds, in gas fluidization is investigated for group A and B powders. To establish the connection between macroscopic and microscopic descriptions of complex particle–fluid flows, focus is given to the following two aspects: the formation of a stable expanded bed in relation to the interparticle cohesive, sliding and rolling frictional forces, and the correlation between coordination number (CN) and porosity (ε). The method employed is the combined approach of three-dimensional (3D) discrete element method (DEM) and two-dimensional (2D) computational fluid dynamics. The results show that compared to 2D DEM, 3D DEM is more reliable in investigating the micromechanics of granular media, although both can capture key features of different flow regimes. The roles of various forces between particles and between particles and fluid are examined, and the origin of different flow regimes is discussed. It is shown that the cohesive force is critical to the formation of a static expanded bed, while the sliding and rolling frictional forces also play a role here. The criterion for bed expansion is analyzed at bulk and particle scales, and the deficiency at a bulk scale is identified. CN, as a key measure of local structure, is analyzed. It is found that the CN–ε relationship for group A powders has a transitional point between the expanded and fluidized bed flow regimes at a bulk scale, unlike group B powders. A new phase diagram is established in terms of CN–ε relationship that has two branches representing expanded and fluidized (bed) states, which corresponds to the one in terms of interparticle forces.

Stochastic parameterization of dust emission and application to convective atmospheric conditions

Abstract. We develop a parameterization scheme of convective dust emission for regional and global atmospheric models. Convective dust emission occurs in the absence of saltation as large eddies intermittently produce strong shear stresses on the surface and entrain dust particles into the air. This dust emission mechanism has not been included in the traditional dust models. The scheme presented in this study is a new approach which takes account of the stochastic nature of convective dust emission. It consists of the statistical representations of soil particle size, inter-particle cohesion, and instantaneous surface shear stress. A method of determining the probability density function of the latter quantity is proposed. Dust emission is then estimated from the overlap of the probability density functions of the aerodynamic lifting and inter-particle cohesive forces. The new scheme is implemented into the WRF/Chem model and applied to dust modeling in the Taklimakan Desert. A comparison with lidar data shows that the model can reproduce the main features of the dust patterns and their diurnal variations. For the case studied, convective dust emission is typically several μg m−2 s−1 and at times up to 50 μg m−2 s−1.

Lowering of Clathrate Hydrate Cohesive Forces by Surface Active Carboxylic Acids

The present work uses a micromechanical force apparatus to directly measure hydrate particle–particle cohesion forces in hydrocarbon systems containing various carboxylic acids. Measured cohesive forces provide fundamental insight to the balance between surfactant adsorption kinetics and interfacial thermodynamics in hydrate systems. These results are essential to the accurate prediction of hydrate aggregation in multiphase flow, as encountered in oil/gas production. The present data support the existence of the water capillary bridge between hydrate particles as an essential mechanism for hydrate cohesion in oil continuous systems. The results indicate that, while all surface active compounds tested decreased the water–oil interfacial tension, only some chemicals were effective at reducing the interparticle cohesion force. Through systematic measurements, the data yield new insight into how some acids may alter hydrate surface wettability. Polynuclear aromatic carboxylic acids were found to be highly eff...

Does the dust explosion risk increase when moving from μm-particle powders to powders of nm-particles?

Based on experience with powders of particle sizes down to the 1–0.1 μm range one might expect that dust clouds from combustible nm-particle powders would exhibit extreme ignition sensitivities (very low MIEs) and extreme explosion rates (very high KSt-values). However, there are two basic physical reasons why this may not be the case. Firstly, complete transformation of bulk powders consisting of nm-particles into dust clouds consisting of well-dispersed primary particles is extremely difficult to accomplish, due to very strong inter-particle cohesion forces. Secondly, should perfect dispersion nevertheless be achieved, the extremely fast coagulation process in clouds of explosive mass concentrations would transform the primary nm-particles into much larger agglomerates within fractions of a second. Furthermore, for organic dusts and coal the basic mechanism of flame propagation in dust clouds suggests that increased cloud explosion rates would not be expected as the particle size decreases into the <1 μm range. An overall conclusion is that dust clouds consisting of nm primary particles are not expected to exhibit more severe KSt-values than clouds of μm primary particles, in agreement with recent experimental evidence. In the case of the ignition sensitivity recently published evidence indicates that MIEs of clouds in air of some metal powders are significantly lower for nm particles than for μm particles. A possible reason for this is indicated in the paper.

Are enhanced dust explosion hazards to be foreseen in production, processing and handling of powders consisting of nano-size particles?

Based on experience with powders having particle sizes down to the range 1 – 0.1 μm one might expect that dust clouds from combustible nm-particle powders would exhibit extreme ignition sensitivities (very low MIEs) and extreme explosion rates (very high KSt−values). However, there are two basic physical reasons why this may not necessarily be the case. Firstly, the formation of clouds of well-dispersed primary particles from bulk powders consisting of nm-particles is extremely difficult to achieve, due to the comparatively very strong inter-particle cohesion forces. Secondly, should such a dispersion process nevertheless be fully successful, extremely fast coagulation processes in clouds within the explosive mass concentration range, would transform the primary nm-particles into much larger agglomerates within fractions of a second. Furthermore, for organic dusts and coal, the basic mechanism of flame propagation in the dust cloud is a further reason for not expecting extreme ignition sensitivities and explosion rates dust clouds from nm-particles. The overall conclusion is that dust clouds consisting of nm primary particles would not necessarily be expected to exhibit more severe MIE and KSt−values than those of dust clouds of μm primary particles. Recent experimental evidence confirms that this is in fact the case for KSt, whereas for MIE the values for some metals are significantly lower for nm primary particles than for μm ones.

The role of scale resolution versus inter-particle cohesive forces in two-fluid modeling of bubbling fluidization of Geldart A particles

Coarse grid simulations of Geldart A particles in bubbling fluidized beds using standard two-fluid (TF) models, where the constitutive laws are based on the homogeneity of sub-grid scale structures, have been demonstrated to be unsuccessful due to the existence of significant sub-grid scale heterogeneous structures. However, a definite consensus on the fundamental origin of the failure is still lacking. Some claim that the existence of significant inter-particle cohesive forces results in the sub-grid scale heterogeneous structures which takes the form of agglomerates or clusters; others claim that the inter-particle cohesive forces are not the dominant factor, and that the poor performance of TF models is primarily due to the fact that the grid sizes and the time step used in previous studies are too coarse to fully resolve the bubbling structure. To this end, a short overview is firstly presented to discuss the role of scale resolution and inter-particle cohesive force in two-fluid modeling of Geldart A particles; We then qualitatively explain, using a state-of-the-art discrete particle method, why the methods based on the existence of aggregates or clusters work quite well, although the sub-grid scale structure is not properly identified, that is, the sub-grid scale structure takes the form of a bubble-emulsion phase.

Fluidization of nano and sub-micron powders using mechanical vibration

The fluidization behavior of nano and sub-micron powders belonging to group C of Geldart's classification was studied in a mechanically vibrated fluidized bed (vibro-fluidized bed) at room temperature. Pretreated air was used as the fluidizing gas whereas SiO 2, Al 2O 3, TiO 2, ZrSi, BaSO 4 were solid particles. Mechanical vibration amplitudes were 0.1, 0.25, 0.35, 0.45 mm, while the frequencies were 5, 20, 30, 40 Hz to investigate the effects of frequency and amplitude of mechanical vibration on minimum fluidization velocity, bed pressure drop, bed expansion, and the agglomerate size and size distribution. A novel technique was employed to determine the apparent minimum fluidization velocity from pressure drop signals. Richardson–Zaki equation was employed as nano-particles showed fluid like behavior when fluidized. The average size of agglomerates formed on top of the bed was smaller than those at the bottom. Size distribution of agglomerates on top was also more uniform compared to those near the distributor. Larger agglomerates at the bottom of the bed formed a small fraction of the bed particles. Average size of submicron agglomerates decreased with increasing the frequency of vibration, however nano particles were less sensitive to change in vibration frequency. Mechanical vibration enhanced the quality of fluidization by reducing channeling and rat-holing phenomena caused by interparticle cohesive forces.

Modelling and experiments on the effect of air humidity on the flow properties of glass powders

The effect of air humidity on flow properties of two different cuts of fine glass beads was experimentally studied by means of shear tests. The moisture content of powder samples was conditioned by humid air at relative humidities between 13% and 98% in a fluidization column. In spite of the very low moisture contents in the powder (<0.2%) obtained by this technique, a significant change in the powder cohesion was observed, while the effect on the angle of internal friction was limited. A model based on the Kelvin equation and the Laplace–Young equation was applied to describe the capillary condensation between touching asperities found on the surface of neighbouring particles and to estimate the relevant interparticle forces. This result was used to derive the powder tensile strength following the Rumpf approach and compare it with values derived from shear experiments. A single value of the unknown capillary bridge gap was chosen on the basis of the observed dependence of interparticle cohesive forces. This gap was able to provide quantitative agreement between model predictions and experimental results at relative humidity lower than 80%. Comparison between the total moisture content and the amount of water in liquid bridges indicated that water mainly condenses on rough surfaces of the particles and only a small portion of this condensed humidity contributes to change the powder flow properties by interparticle capillary forces.

Development and validation of calibration methods for discrete element modelling

Discrete element method (DEM) is proving to be a reliable and increasingly used tool to study and predict the behaviour of granular materials. Numerous particle-scale mechanisms influence the bulk behaviour and flow of bulk materials. It is important that the relevant measurable input parameters for discrete element models be measured by laboratory equipment or determined by physical calibration experiments for rational results. This paper describes some of the bench-scale experiments that have been developed to calibrate the DEM simulations to reflect actual dynamic behaviour. Relevant parameters such as static and rolling coefficients of friction, coefficient of restitution and inter-particle cohesion forces from the presence of liquid bridges have been investigated to model the bulk behaviour of dry and moist granular materials. To validate the DEM models, the results have been checked against experimental slump tests and hopper discharge experiments to quantitatively compare the poured and drained angles of repose and solids mass flow rate. The calibration techniques presented have the capability to be scaled to model and fine tune DEM parameters of granular materials of varying length scales to obtain equivalent static and dynamic behaviour.

Improving Powder Flow Properties of a Cohesive Lactose Monohydrate Powder by Intensive Mechanical Dry Coating

The objective of this study was to improve the cohesive lactose powder flowability. A cohesive lactose monohydrate powder was processed in either a tumbling blender or an intensive mechanical processor with either magnesium stearate or fumed silica. No substantial changes in particle size were detected by laser diffraction following either treatment. The untreated lactose sample exhibited very poor powder flow. Only limited improvements in powder flowability were indicated after the tumbling blending, intensive mechanical processing with the fumed silica or without additives. However, the intensive mechanical processing of the lactose sample with magnesium stearate demonstrated exceptionally large increases in both poured and tapped density as well as notable improvements in all powder flowability indicators examined. Our findings support the use of intensive mechanical processing technique as an effective method to coat cohesive pharmaceutical powders with selected additives, modify the surface nature of the particles, reduce the interparticle cohesive forces and hence improve powder flowability. The subtle differences in powder flow behaviour of lactose samples between the untreated and tumbling blended powders with magnesium stearate were only detected by the powder rheometer using its dynamic mode, indicating its potential advantages over traditional powder flow characterisation approaches. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:969–981, 2010

Effect of host particle size on the modification of powder flow behaviours for lactose monohydrate following dry coating

Our previous work demonstrated that the powder flowability of a cohesive lactose sample can be improved substantially using a dry coating technique. Our study reported here aims to investigate the influence of host particle size on the modification of powder flowability following dry coating process. Four commercial lactose monohydrate powders with different particle sizes were coated by an intensive mechanical process or mixed using a conventional tumbling process, both with magnesium stearate. All four untreated lactose samples showed a relatively poor powder flow. After dry coating, poured and tapped densities of all the lactose samples increased, while Carr indices and Hausner ratios decreased substantially. The angle of repose values were reduced to a notable extent only for particles with a median size larger than about 7 μm after dry coating. Both specific energy (SE) and cohesion values of lactose samples, measured by a powder rheometer system, decreased substantially after coating. In contrast, no apparent changes in powder flow were evident for conventionally mixed batches, except that in the dynamic powder rheometry measurement, a relatively small change in SE was observed. This study demonstrated that for the finer particles examined, cohesive forces were more influential in the powder bed after the surface treatment and resulted in a relatively poor flow. However, for the larger powders studied, the cohesive inter-particle forces could be overcome after this dry coating, whereby satisfactory flow could be obtained. This study indicated that the host particle size was a critical factor in influencing the modification of cohesive powder flowability.

Self-Organization of Nanoparticle Chains Using Oriented Poly(tetrafluoroethylene) Substrate

We found a novel self-organization phenomenon leading to formation of oriented one-dimensional assemblies of nanoparticles, namely nanoparticle chains. A friction-transferred poly(tetrafluoroethylene) (FT-PTFE) substrate was prepared by sliding a PTFE rod on a glass slide at elevated temperature. The nanoparticle chains were spontaneously formed in a drying drop of colloidal solution on the FT-PTFE substrate. The resulting gold nanoparticle chains showed dichroism on far-field optical observation, which was attributable to the anisotropic near-field optical interactions. Real-time polarizing light microscopy observations during the formation of the nanoparticle chains revealed that the chains were formed in the liquid phase. A specific flow pattern that was induced by convection of the colloidal solution between the basal glass slide and the FT-PTFE film was supposed to be responsible for the formation of the nanoparticle chains along with the interparticle cohesive force.

Effect of vibration on flow properties of fine glass beads

AbstractAn estimation of cohesion C and angle of internal friction φ of fine glass beads, which have been previously subjected to small intensity vertical vibrations of controlled frequency and amplitude for a fixed period of time, is presented. In the range of vibration intensities tested previbration causes powder compaction. Experimental measurements to obtain C and φ have been performed by means of a centrifuge powder tester in which the previbrated powder bed is rotated around its vertical axis. At a critical rotation velocity the shear stress induced by rotation is large enough to drive material avalanches. From a theoretical analysis of these avalanches, based on the Coulomb's method of wedges, C and φ are derived. We have found a significant increase of cohesion, and a slight decrease of the angle of internal friction as the effective consolidation stress due to previbration is increased. The estimated interparticle cohesion force ft from the experimental measurements, and using a Rumpf's averaging equation, is in agreement with the calculated interparticle attractive force due to van der Waals and capillary interaction for the unconsolidated beds. Moreover ft increases as the effective consolidation stress is increased in agreement with estimations from theoretical models based on contact plasticity. This result suggests that, at the microscopic level of asperities, compaction due to previbration is able to induce plastic deformation of the contacts. © 2008 American Institute of Chemical Engineers AIChE J, 2008

Defining the threshold wind velocity for moistened sediments

The moisture of surface sediments is one of the most significant factors governing the initiation of particle movement by the wind and hence the aeolian transport rate. This paper develops an equation for the threshold shear velocity of moistened sediments based more soundly on physics by means of the moment balance method, taking account of the interparticle cohesive forces produced by moisture. The equation relates threshold shear velocity directly to moisture content and contains three simple coefficients that need be determined from experiments. The threshold equation for moistened sediments basically follows Bagnold's threshold equation for loose, dry sediments but with a proportionality coefficient that accounts for the effect of moisture. Previously published data on the threshold shear velocities of moistened sediments from wind tunnel tests are used to determine the coefficients contained in the threshold equation, and it proves that the new equation describes the threshold velocities of moistened sediments reasonably well. Comparing the threshold shear velocity predicted using the new equation with the results of Belly's and Hotta et al.'s empirical equations reveals that the predicted results differed greatly. These differences can be attributed to differences in the definition of threshold velocity and moisture content in the physical and chemical properties of the sediment samples and in the experimental methods. Further theoretical and experimental investigation is required to fully understand the effects of sediment moisture content on threshold wind velocity. We also suggest that methodological improvement itself is also a topic of future research.

Natural Minerogenic Dust and Human Health

Natural Minerogenic Dust and Human Health

A novel experimental study of temperature enhanced cohesive interparticle forces

Interactions at elevated temperatures between solid particles occur in a wide range of industrial processes, for instance, in the filtering of hot gases, in the drying of pharmaceutical granules, in the curing of ceramics, in the combustion of solid fuels and regeneration of nuclear waste. Often these interactions can cause major problems in the operation of such processes. For example, it is well established that the fluidisation behaviour of certain powders is significantly affected by the presence of strong interparticle forces that, in turn, are the cause of both agglomerate formation and possible operative problems within the reactor. On the other hand, not much is known about the mechanisms of agglomeration, other than that it is mainly due to interfacial phenomena. High temperature adhesion forces arise from the formation of material bridges, usually due to the particle surfaces changing phase through either chemical reaction or simply melting. Moreover, thin liquid layers of sticky material, which may be present on particle surfaces, such as during reactive coating or drying processes, may also enhance strong interparticle bonds leading to solidification. It is obvious that for the reliable operation of high-temperature processes a good understanding of the fundamental mechanisms of adhesion and cohesion between particles at elevated temperatures is required. Here, adhesion is meant as the force that holds the particles together, after which they exhibit cohesive behaviour. Unfortunately, the level of understanding has been hampered by the lack of techniques available to observe and measure such interactions. In order to fulfil this technology gap, a novel device, termed a High Temperature Micro-Force Balance, has been designed and developed which combines force and direct observation measurements operated through an adapted micromanipulator technique. The flexibility of use for the HTMFB represents its strongest design advantage, permitting experimental investigations over different types of particle interactions at a small scale (e.g. crystallization of liquid binders, reactive coatings, sintering and composite material interactions). In this work a new approach is presented in order to determine the fundamental mechanisms behind the formation of agglomerates of uranium oxide in the thermal denitration process during nuclear fuel reprocessing. A new diagram shows results where the dependency of the rupture force for a material bridge (here Magnesium Nitrate Hexahydrate is used as a simulant of Uranyl Nitrate) is plotted when the initial liquid bridge volume and temperature are varied. The results provide an original contribution towards academic and industrial understanding of the micro-scale mechanisms of agglomeration and material properties at different operative conditions.

A DEM study of bubble formation in Group B fluidized beds with and without cohesive inter-particle forces

This paper presents a 2D soft-sphere discrete element method (DEM) simulation study of bubble formation in fluidized beds of Geldart Group B powders with and without externally imposed cohesive interparticle force. The effect of interparticle force on bubble formation and bubble characteristics is studied by inducing a single bubble at the centre of the distributor of a 2D bed that contains 36 000 mono-sized particles having a diameter of 500 μ m and density of 1000 kg m - 3 . It is observed that in the presence of externally imposed cohesive interparticle force, the bubble formation process not only requires a higher air velocity for its initiation, but it is also slower when compared to the case with no interparticle force.