Hydrodynamic study of fine metallic powders in an original spouted bed contactor in view of chemical vapor deposition treatments

An original gas-solid contactor was developed so as to treat by chemical vapor deposition, fine (mean diameter 23 μm) and dense (bulk density 7700 kg/m³) NiCoCrAlYTa powders with large size distribution. In order to avoid the presence of a distributor in the reactive zone, a spouted bed configuration was selected, consisting in a glass cylindrical column associated through a 60° cone to an inlet tube, connected at its bottom to a grid so as to support the powders at rest. A hydrodynamic study was conducted at ambient temperature and pressure, combining pressure drop measurements and visual observations as a function of gas velocity and of the ratio H/D of the height of the bed at rest over the bed diameter. Using conventional alumina particles belonging to Geldart's group B, it was shown that this equipment is able to ensure conventional spouted bed behavior, especially for H/D ratio equal to 1. From numerous experiments conducted with the fine metallic powders of interest, it was shown that (i) conventional pressure drop curves for spouted beds are obtained for H/D ratios between 1 and 1.8, (ii) due to the large grain size distribution of particles, minimum spouted bed velocities occur in a range rather than at precise values. Visual observations reveal the presence of the spout and fountain at the minimum spouted bed velocity and for H/D equal to 1.     

Behaviour of gas-fluidized beds of fine powders part I. Homogeneous expansion

The minimum bubbling velocity has been correlated from the results of experiments on 48 gas/solid systems and is a function of the density and viscosity of the fluidizing gas, the mean sieve size of the powder and the fraction of fines less than 45 μm. An improved equation is presented to predict U_mb/U_mf; this parameter also correlates well with the maximum non-bubbling bed expansion ratio H_mb/H_mf which is used to calculate the maximum dense phase voidage ?_mb. An equation based on the Carman-Kozeny theory can be used to predict bed voidages between incipient fluidization and bubbling. An accurate value of particle density is essential in these equations and a simple comparative method has been used to determine particle densities of fine porous powders.     

Effect of competition between particle-particle and gas-particle interactions on flow patterns in dense gas-fluidized beds

Particle-particle (P-P) and particle-fluid (P-F) interactions are two fundamental phenomena in dense particulate flows which interact mutually to compose a variety of flow structures. Using discrete particle simulation and energy budget analysis, we quantitatively explore the effect of competition between these interactions on flow patterns in dense gas-fluidized beds. A diagram, picturing flow pattern formation and its evolution, along with the altering roles of P-P collision and P-F interaction in their competition, is presented and detailed energy analysis is used to explore how P-P collision and P-F interaction drives flow pattern formation and transition.It is shown that the flow structures in various flow regimes, ranging from the fixed to the turbulent regime, can be reproduced from these simulations. Systems with strong collisional dissipation but weak gas-particle interaction display a distinct emulsion-bubble two-phase flow structure. On the contrary, systems with strong gas-particle interaction but less pronounced collisional dissipation produce uniform structures, as often observed in the uniform regime near the incipient fluidization point. If these two interactions are equally important, the system features complex flow patterns resembling those displayed in the turbulent fluidization regime.Energy analysis demonstrates that the competition between P-P collision and P-F interaction determines flow structure formation and its evolution. The flow regime transition is actually the macro-scale expression of the altering of degree of dominance of P-P and P-F interactions.     

Behaviour of gas-fluidized beds of fine powders part II. Voidage of the dense phase in bubbling beds

The collapse rate technique has been used to evaluate the average dense phase properties in vigorously bubbling beds of fine powders. The results of experiments on 13 air/solid systems are used to correlate the average dense phase voidage, $\text{?}$_D, in terms of the physical properties of the gas and powder, and our predictive equations fit literature data. $\text{?}$_D increases as the particle density and mean particle size decrease, and as the fraction of fine <45 μm, gas viscosity and gas density increase.The gas velocity, U_D, through the dense phase of a bubbling fluidized bed has been calculated from $\text{?}$_D assuming Darcy's law and can now be predicted. Since it is less than the minimum bubbling velocity, the improved performance of fluidized bed reactors when the gas/solid properties are changed so as to increase $\text{?}$_D can be attributed more to smaller bubbles splitting and coalescing frequently rather than to the small amount of extra gas passing through the dense phase.     

Surface-to-bed heat transfer in fluidised beds of fine particles

This work reports experimental results on the heat transfer between a fluidised bed of fine particles and a submerged surface. Experiments have been carried out using different bed materials (polymers, ballotini, corundum, carborundum and quartz sand) with Archimedes number between 2 and 50. Dry air at ambient pressure and temperature has been used as fluidising gas. Three different exchange surfaces, namely a sphere and two cylinders with different base diameter and same height, have been used.Experimental results show that the heat transfer coefficient increases with particle Archimedes number and is almost independent from particle thermal conductivity for K_p/K_g > 30. Finally, the comparison of heat transfer coefficient for the different surfaces shows that the effect of the surface geometry may account for a 30% variation in the heat transfer coefficient, with higher differences occurring for coarser particles.     

Effects of fine reactive alumina powders on properties of alumina-magnesia castables with TiO2 additions

The design of alumina-magnesia castables was optimized based on the action mechanism of raw materials. In this study, the effects of fine reactive alumina powders on the properties of alumina-magnesia castables with TiO₂ additions were investigated. The phase composition and microstructure of castables with different contents of TiO₂ and two fine reactive alumina powders were characterized by X-ray diffraction and scanning electron microscopy. The results of these studies showed that the Na₂O/SiO₂ ratio of reactive alumina affected on the phase evolution and properties of castables. The changes in the spinel and CA₆ content and their solid solubility could account for the comprehensive effect of TiO₂ and reactive alumina on the castables. The difference in the expansion of castables with 1 wt% TiO₂ addition after calcination at 1350 °C was significant. The permanent linear changes and the apparent porosity of castables first increased and then decreased with increasing the calcination temperature. Castables containing reactive alumina with lower Na₂O/SiO₂ ratio had higher cold moduli of rupture.     

Effect of cohesive forces on fluidized and spouted beds of wet particles

Fluid beds are now being used for processing pasty materials including production of fine powders through drying suspensions in beds of inert particles; coating of tablets or pellets; granulation, etc. In such processes, the fluid bed operation becomes more complex due to the development of cohesive forces resulting from liquid bridges between particles. Such forces can affect gas and solids flow leading to uncontrollable particle agglomeration and to poor gas–solid contact. This work is aimed at analyzing and quantifying the differences of flow behavior in fluidized and spouted beds of wet and dry particles. Experimentally, surface stickiness is induced by application of metered amounts of glycerol. Based on pressure drop vs. fluid flow rate curves, solids circulation rates and bed porosity variations, two types of particle–particle interaction forces are identified and their effect on air–solid flow is quantified as a function of glycerol concentration. Implications of these results in coating, granulation and drying of suspensions in these beds are also discussed.     

Hydrodynamic modeling of particle rotation for segregation in bubbling gas-fluidized beds

A multi-fluid Eulerian model has been improved by incorporating particle rotation using kinetic theory for rapid granular flow of slightly frictional spheres. A simplified model was implemented without changing the current kinetic theory framework by introducing an effective coefficient of restitution to account for additional energy dissipation due to frictional collisions. Simulations without and with particle rotation were performed to study the bubble dynamics and bed expansion in a monodispersed bubbling gas-fluidized bed and the segregation phenomena in a bidispersed bubbling gas-fluidized bed. Results were compared between simulations without and with particle rotation and with corresponding experimental results. It was found that the multi-fluid model with particle rotation better captures the bubble dynamics and time-averaged bed behavior. The model predictions of segregation percentages agreed with experimental data in the fluidization regime where kinetic theory is valid to describe segregation and mixing.     

The influence of inclined plates on expansion behaviour of solid suspensions in a liquid fluidised bed — A computational fluid dynamics study

A significant increase in the particle sedimentation rate can be achieved by introducing inclined plates into conventional fluidised beds. In turn, high suspension densities are possible at fluidisation velocities in excess of the particle terminal velocity. The installation of the inclined plates, however, alters the dynamic characteristics of the fluidised bed, in particular, impacting upon the expansion behaviour of the suspension. In the present work a Computational Fluid Dynamics (CFD) approach was employed to investigate the influence of inclined plates on the expansion behaviour of solids suspensions in liquid fluidised beds. The model is based on the solution of the Eulerian multiphase equations for up to two different particle sizes with a continuous phase of water. The momentum equations treat hindered settling behaviour via the inclusion of a volume fraction dependent drag law. The computational model was validated against our experimental data and compared with the predictions of a kinematic model developed in one of our earlier works. In general the predictions made by both the CFD and the kinematic models were found to be in good agreement with the experimental results.     

Experimental study on fluidization of fine powders in rotating drums with various wall friction and baffled rotating drums

Fine powders were found to be fluidized in a rotating drum by internal cycling gas by the drum rotation. It is essentially a fluidized bed without requiring any external fluidizing gas. Such a rotating drum can be regarded as a new gasless fluidized bed for fine powders in contrast to a traditional fluidized bed, possibly leading to a considerable amount of energy savings. In addition, the fluidization quality of fine powders was found to be further improved with the assistance of drum rotation because of the shearing movement among particles that eliminates channeling and cracks and possibly also breaks agglomerates. Five regimes were identified in the rotating drum including slipping, avalanching-sliding, aerated, fluidization and re-compacted regimes. It was also found that drum wall friction plays an important role to fluidize fine powders because the friction carries particles to the freeboard, leading to gas cycling that fluidizes the powders. As well, three types of specially designed baffles were utilized to promote powder fluidization in rotating drums. These baffles effectively bring an early onset of all the regimes in rotating drums by reducing powder-wall slipping, carrying particles and bringing additional gas to the powders.     

Behaviour of gas-fluidized beds of fine powders part III. Effective thermal conductivity of a homogeneously expanded bed

The effective thermal conductivity of the dense phase in expanded non-bubbling fluidized beds has been studied between minimum fluidization and minimum bubbling for 9 gas—solid systems. Except close to minimum bubbling, the effective thermal conductivity is not a function of bed voidage and is most sensitive to the gas phase thermal conductivity. The experimental results have been compared with 11 packed bed correlations and with suitable modification three of these can be used to accurately predict the effective thermal conductivity of a non-bubbling fluidized bed.     

Simulations of vibrated fine powders

During the past few decades, several studies have been conducted to understand the behaviour of powders in vibrated beds. This paper introduces a technique of incorporating the agglomeration and deagglomeration phenomena in the simulation of vibrated fine powders. Two-dimensional direct simulations are performed using 300 spheres 2.99 mm in diameter in a trapezoidal container vibrated vertically at an amplitude of 2.5 mm and 20 Hz frequency as preliminary conditions. Under non-cohesive conditions, the results are in agreement with those found in the literature. As a preliminary effort to predict the behaviour of cohesive fine powders under vibrated conditions, agglomeration and deagglomeration processes are modelled as the formation and destruction, respectively, of interparticle bonds during particle collisions. Two parameters used to model agglomeration and deagglomeration are the ease of cohesion and cohesivity of the powder. Dependencies of these parameters on certain physical properties of cohesive powders have been suggested. Simulation results reveal two aggregate populations, one with uniform size aggregates and another population with multi-sized aggregates. The former aggregates were more prevalent in weakly cohesive powders while the latter in highly cohesive powders. Interesting macroscopic bed behaviour such as alternating cycles of agglomeration and deagglomeration were also observed. Further work is needed in which the aerodynamic forces are taken into account and cohesion mechanisms at the particle surface are modelled.     

Evaluation of the turnover times of the bed particles and of the fine powders in a circulating powder-particle fluidized bed (CPPFB)

Circulating fluidizing system of binary Geldart C powders and Geldart A particles was formed, and was called a circulating powder-particle fluidized bed (CPPFB). Solid residence in the CPPFB was concerned in two aspects, namely, bed turnover time and average turnover time of fine powders. The former represented the average time needed for all the bed particles to be circulated, while the latter represented the time needed for all the fine powders in the bed to be discharged out of the bed. Both parameters were investigated under different operating conditions as to the superficial gas velocity, size and hold-up of fine powders. FCC particles of 66 μm were used as coarse particles and 1-5 wt.% Al(OH)₃ powders of different sizes ranging from 0.5 to 15 μm were used as fine powders.The bed turnover time decreased with increasing the size of fine powders to a certain level and then became almost constant with further increase of the size of fine powders. When the size of fine powders was larger than this critical size, neither the size nor the hold-up of fine powders affected the bed turnover time. The bed turnover time drastically increased with increasing the hold-up of fine powders for the cases of using very fine powders of 1.0 μm or smaller. On the other hand, the average turnover time of fine powders decreased with increasing the size of fine powders to a minimum at around 3.5 μm and then increased with increasing the size of fine powders. It also decreased with increasing the gas velocity and decreasing the hold-up of fine powders in the bed. The average turnover time of fine powders was several times larger than the bed turnover time at the same operating conditions.     

A study of the mixing of solids in gas-fluidized beds, using ultra-fast MRI

Magnetic resonance imaging has been used to measure the rate of axial mixing in a vertical direction of a small batch of poppy seeds suddenly added to the upper surface of a bed of sugar crystals fluidized by air.     

The bubbling behavior of cohesive particles in the 2D fluidized beds

The present work focuses on a fully statistical analysis of bubbling behavior in the two-dimensional (2D) fluidized beds with cohesive particles. Various significant bubble properties such as bubble size, rising velocity, aspect ratio, bed expansion and bubble hold-up, etc., were investigated. An equation for bubble diameter is developed, , and the observed bubbles are generally smaller than the ones generated in the beds with A or B type powders. Both the average bubble size and rising velocity initially increase with the elevation above the distributor and keep constant beyond certain heights. The bubbles exhibit oblong with the most density aspect ratio (β) equal to 0.7. In addition, the bubble rising velocity coefficient ranges from 0.8 to 1.5. Two core-annular flows form in the large diameter, shallow fluidized bed used in this experiment.     

重力沉降原理在微粉和超细粉粒度分析中的应用

探讨了利用重力沉降原理对粉体颗粒粒度分布进行测试时 ,颗粒的布朗运动对测定精度的影响。并推导出常见的耐火材料粉体最小的测试颗粒粒径范围应为 0 .6～ 0 .95 μm。同时介绍了采用国产KCT - 1型沉降天平进行微粉与超细粉的粒度分布测定时的操作要点与技巧。并对测试过程中出现的异常现象和解决的方法进行了阐述     

Arch-Free flow in aerated silo discharge of cohesive powders

Arching can occur during silo discharge of cohesive powders. In general this happens when the outlet size is not wide enough. Flow aid devices, such as aeration pads, are commonly used in the industry to achieve proper flow of cohesive materials. However, no design criteria are presently available for such kind of devices and, in particular, for the intensity of aeration to be used to avoid arching. Aim of this paper is the evaluation of the limiting aeration condition to produce the collapse of established arches and the minimum aeration rate necessary for no arching discharge flow. Experimental tests are carried out in an aerated flat bottom silo. The measured quantities are the aeration rate at arch collapse and the arch size. Powder permeability is characterized by fluidization experiments. A simplified model is proposed to assess on the prevailing physical phenomena and predictively evaluate the minimum aeration rate to determine no arching discharge flow.     

Favorable vibrated fluidization conditions for cohesive fine particles

The present study was performed to clarify the operational range for vibro-fluidization of fine cohesive particles (glass beads, d_p = 6 μm). Decreasing and increasing gas velocity methods were examined to clarify the favorable vibro-fluidization region. The upper limit of the gas velocity for intermittent channel breakage was higher in the case of the increasing gas velocity method than the decreasing gas velocity method. This was because the changes in the bed flow pattern from a favorable (intermittent channel breakage) to an unfavorable fluidization state (stable channels) were moderate in the case of the increasing gas velocity method. In the increasing gas velocity method, two kinds of cross-points were obtained from the relationship between the gas velocity and the bed pressure drop. At one of the gas velocities at these cross-points, the bed void fraction reached its maximum. In the present study, the above-mentioned gas velocity was defined as the upper limit of gas velocity for favorable vibro-fluidization, u_chu. A favorable vibro-fluidization region was determined by combining u_chu with u_chl, which is the lower limit of gas velocity for intermittent channel breakage obtained in a previous study. The value of u_chu was found to have a maximum corresponding to a certain vibration strength.     

Oscillations in gas-fluidized beds: Ultra-fast magnetic resonance imaging and pressure sensor measurements

Ultra-fast Magnetic Resonance Imaging (MRI) and pressure sensor measurements have been applied to study: (i) pressure fluctuations, (ii) the eruption of bubbles at the top of a bed and (iii) the formation of bubbles in a gas-fluidized bed. Ultra-fast MRI has been applied for the first time to study the formation and eruption of bubbles; the technique is non-intrusive and provides measurements with good temporal and spatial resolutions. The MRI measurements revealed that bubbles are formed periodically, rather than randomly at a distributor, which in this case was a perforated plate. The frequency at which bubbles erupted from the top of the bed matched the frequency of the pressure fluctuations measured just above the distributor, where the measured pressure is predicted very well for the case of slug flow by Kehoe and Davidson's [P.W.K. Kehoe, J.F. Davidson, Pressure fluctuations in slugging fluidized beds, AIChE Symp. Ser. 128 (69) (1973) 34-40] correlation, originally developed for locations high up a bed. Both findings lead to the conclusion that the passage and eruption of bubbles at the top of a bed are the dominant cause of the pressure fluctuations, which are subsequently propagated downwards through the bed. Two new correlations are proposed for predicting the frequency of pressure fluctuations in a bubbling bed; both correlations agree well with experimental measurements. A modification of Baeyens and Geldart's [J. Baeyens, D. Geldart, An investigation into slugging fluidized beds, Chem. Eng. Sci. 29 (1974) 255-265] correlation predicts the frequency of pressure fluctuations when slugs are formed, but are not fully developed. The frequency of bubble formation, as measured by MRI, is equal to or higher than both the frequency of bubble eruption at the top of the bed and the frequency of pressure fluctuations, depending on the depth of the bed. The frequency of bubble formation is significantly lower than predicted by Davidson and Schüler's [J.F. Davidson, B.O.G. Schüler, Bubble formation at an orifice in an inviscid liquid, Trans. Inst. Chem. Eng. 38 (1960) 335-342] equation, originally developed for gas-liquid systems.     

How to elutriate particles according to their density

It is well known that particles settle at a velocity dependent on both their size and density. This dependence makes it difficult to efficiently concentrate valuable minerals of a given density, unless the density range between the valuable and gangue particles is sufficient to overcome the segregation arising from the particle size range. Here we reveal a method for suppressing the effects of particle size so that particles can be routinely elutriated on the basis of their density. The particles are suspended by fluidizing with water, and then conveyed into channels inclined at 70° to the horizontal, closely spaced with a gap of 1.77 mm. Through the application of these conditions, a fortuitous coincidence arises. Firstly, a high shear rate is produced, sufficient to achieve inertial lift and hence promote the transport of the particles along the inclined surfaces, free of significant lubrication and mechanical friction forces. Secondly, the flow through the channels is laminar, with an almost linear variation in the local velocity with distance from the inclined surface. Hence a given particle resident near an inclined surface experiences a local elutriation velocity proportional to its diameter. These conditions suppress particle size segregation thus allowing the imposed hydraulic velocity to convey particles on the basis of their density.