Beds Of Fine Powders Research Articles

Elucidation of Particle Behavior under External Vibration and Development of Devices to Improve Flowability—Effective Use of Bubbles and Convection Rolls Formed in a Fine-Powder Bed—

Elucidation of Particle Behavior under External Vibration and Development of Devices to Improve Flowability—Effective Use of Bubbles and Convection Rolls Formed in a Fine-Powder Bed—

Convection induced by vibrating rod in fine-powder bed

In this study, vibration-induced convection was studied experimentally using a fine powder with a mass median particle diameter of 8μm. A cylindrical rod arranged vertically in a powder container was vibrated horizontally with simple harmonic motion at a frequency of 300Hz using a piezoelectric vibrator. For a vibration amplitude of 10μm, particles around the cylindrical rod were consolidated to a certain extent due to gravity; however, for a vibration amplitude of 70μm or more, a pair of convection rolls formed on both sides of the vibrating rod. The strength of the convection was quantified from the particle velocity distribution in the convection rolls, and the relationship between the convection strength and vibration amplitude was elucidated. In addition, the air-pressure distribution in the powder bed was measured showing that the convection strength correlates with the characteristic positive pressure, i.e., the average value of positive pressure measurements. Elliptical motion and circular motion as well as simple harmonic motion were applied to the cylindrical rod by adding two harmonic motions in directions orthogonally crossing each other with a phase difference of π/2rad. The convection of the particles varied according to the Lissajous trajectory of the cylindrical rod. Even for simple harmonic motion, heaps of a pair of convection cells overlapped each other. In the case of elliptical motion, the overlapping portion of the heaps became larger. In the case of circular motion, the two heaps were integrated into one circular heap, and there were no effects of the circumferential angle on the particle velocity and the characteristic positive pressure.

Gas distribution of a downward micro-nozzle assisted fluidized bed of fine powder

A downward micro-nozzle system is investigated for the fluidization of fine particles. To study the impact of the gas flow from the nozzle on the fluidization, the gas distribution below the nozzle is measured in a bed filled with 76μm Puralox particles. A high speed X-ray tomography system, consisting of 3 X-ray sources and 2 layers of 32 detectors for each source, is employed to visualize the gas distribution with a temporal resolution of 2500fps. Both the time-averaged and time resolved results are analyzed by combining the tomographic images from different measurement heights. A bubbling area, diluted area, and compacted area are found from the time averaged results. From time-series analysis, a stable bubbling flow is detected from both the cross-correlation of the raw data and image reconstruction. The penetration depth is also estimated. The results are validated with correlations from literature.

Fluid to solid transition in magnetofluidized beds of fine powders

Experimental observations on the fluid to solid transition in beds of magnetized fine particles fluidized by gas are reported for different particle sizes (dp). Contrarily to stability analysis prediction, the fluidized bed is stabilized by a sufficiently strong magnetic field in the cross-flow configuration. As the strength H of the horizontally applied magnetic field is increased, particle chaining in the bubbling bed becomes apparent due to the induced attractive magnetostatic forces between the particles. In close analogy with magnetorheological fluids chain stability is determined by the balance between gas flow shear and the interparticle magnetostatic force. The jamming transition occurs at a gas velocity scaling proportionally to dp2H2 when the length of the stable chains reaches a critical size which is independent of particle size.

Two-fluid modeling of Geldart A particles in gas-fluidized beds

We have investigated the effect of cohesion and drag models on the bed hydrodynamics of Geldart A particles based on the two-fluid (TF) model. For a high gas velocity U 0 = 0 .03 m/s, we found a transition from the homogeneous fluidization to bubbling fluidization with an increase of the coefficient C 1, which is used to account for the contribution of cohesion to the excess compressibility. Thus cohesion can play a role in the bed expansion of Geldart A particles. Apart from cohesion, we have also investigated the influence of the drag models. When using the Wen and Yu drag correlation with an exponent n = 4 .65, we find an under-prediction of the bed expansion at low gas velocities ( U 0 = 0 .009 m/s). When using a larger exponent ( n = 9 .6), as reported in experimental studies of gas-fluidization, a much better agreement with the experimental bed expansion is obtained. These findings suggest that at low gas velocity, a scale-down of the commonly used drag model is required. On the other hand, a scale-up of the commonly used drag model is necessary at high gas velocity ( U 0 = 0 .2 and 0.06 m/s). We therefore conclude that scaling the drag force represent only an ad hoc way of repairing the deficiencies of the TF model, and that a far more detailed study is required into the origin of the failure of the TF model for simulating fluidized beds of fine powders.

Development of a tribolelectric procedure for the measurement of mixing and drying in a vibrated fluidized bed

Fluidization of fine, pharmaceutical powders makes them easier to dry, coat and mix. Fine powders, however, are difficult to fluidize well with gas flow only. Vibration can often help achieve smooth fluidization at a lower gas flow. The objective of the present study was, thus, to develop reliable and quick experimental methods to characterize mixing and drying in vibrated fluidized beds of fine powders. Effective mixing is critical in many industrial applications and, in gas–solid fluidized beds, requires gas velocities greater than the minimum bubbling velocity ( U mb). There are a number of techniques available for determining U mb. However, they often are impossible or impractical to use in an industrial application. A new measurement technique involving the use of triboelectric probes was developed. Signal characteristics obtained from sophisticated signal analysis were used to identify the minimum bubbling velocities. These predictions corresponded well with the values obtained from more traditional laboratory methods such as the bed pressure gradient. In a fluidized bed, particles hitting a metal probe will generate a small triboelectric current. Triboelectric probes are able to detect rapid changes in particle surface properties. Surface properties of the particles were modified by wetting the particles in a low shear mixer. This change was detected by triboelectric probes at various locations inserted throughout the bed. The water adsorbed on the particles will begin to evaporate when exposed to the gas stream and the surface properties of the particles will gradually return to their original dry state. The triboelectric probes were able to monitor this drying process. The effects of vibration amplitude on the mixing and drying rate of the bed were also determined.

High viscosity gas fluidization of fine particles: An extended window of quasihomogeneous flow

We explore the role of gas viscosity in the behavior of gas-fluidized beds of fine powders by means of experimental measurements using nitrogen and neon as fluidizing gases, and theoretical considerations. The existence of a nonbubbling fluidlike regime has been recently observed in beds of fine powders fluidized with nitrogen. Our experiments with neon reveal a discontinuous transition from heterogeneous fluidization to a highly expanded homogeneous fluidization state. We point out that increasing gas viscosity enhances the coherence of agglomerate swarms, which promotes a local void-splitting mechanism, thus improving the uniformity of fluidization. Our theoretical analysis predicts that further increase of gas viscosity would produce a full suppression of the bubbling regime, i.e., the uniformly fluidized bed would undergo a direct transition to a turbulent regime as seen in beds of nanoparticles fluidized by nitrogen and in liquid-fluidized beds of moderate-density beads.

The effect of van der Waals and charge induced forces on bed modulus of elasticity in ac/dc electrofluidized beds of fine powders—a unified theory

Perturbation theory is applied to an ac electrofluidzed bed of fine powder (glass and FCC) using electric field bubble control to infer the relation between interparticle forces (microscale) and the bulk bed modulus of elasticity (macroscale). Electrostatically induced and permanent van der Waals forces are modeled in a unified theory with bulk fluidized bed behavior. The extrapolation of the electric field to zero strength gives the permanent bed force and bulk bed modulus of elasticity as limiting cases. The resulting equations involve atomic as well as macros-scale parameters. The charge induced forces are identified through the bed modulus of elasticity as a function of the applied electric field strength. The semi-empirical approach is based on the principle that the conservation equations for the perturbed fluidized bed become unstable at the onset of bubbling giving characteristic eigenvalues for the bed modulus, a condition that is readily identifiable experimentally. Eigenvalues from the one-dimensional linearized conservation equations for the fluidized state are examined for growth, neutrality, or decay from the perturbation, which together with bed data are evaluated at bubbling conditions to give the bed modulus of elasticity. Both Richardson–Zaki and Carman–Kozeny bed expansion models of fluidization are examined. The former approach is found to give self-consistent results in which the bed modulus varies linearly with the electric field strength. The results are extended to dc beds as a limiting case of zero field frequency. The modulus of elasticity (a macroscopic bed property) is finally related to particle charge separation at the particle level through an interparticle force model applicable to ac–dc electric fields.

Development of a split-type tensile-strength tester and analysis of mechanism of increase of adhesion behavior of inorganic fine powder bed at high-temperature conditions

To analyze the phenomenon of the increase in cohesive force between fine particles under elevated temperature conditions, a new split-type tensile-strength tester of powder beds for high temperature was developed. Fused silica glass with low thermal expansion coefficient was used as the material for the suspended cell of a commercial split-type tensile-strength measuring device at room temperature. The powder samples used were pure fine silica and two kinds of fly ash powders collected in a pulverized coal (PC) combustion and a pressurized fluidized coal combustion systems. Under conditions of relatively low temperatures below 1000 K, the adhesive force of all powders increased gradually in proportion to the temperature. Rapid increases of tensile strength of both fly ash powder beds were observed in the high-temperature range (above 1100 K). Based on the results of thermo-mechanical analysis (TMA), it was estimated that a viscous flow sintering mechanism controlled this rapid increase phenomenon of adhesive behavior of ash powders at high-temperature conditions above 1100 K. On the other hand, using FT-IR analysis, it was demonstrated that the increase of the van der Waals force with the change of surface molecular structure controlled the stickiness of pure silica and both ash powders at relatively low-temperature conditions below 1000 K.

Effect of vibration on the stability of a gas-fluidized bed of fine powder

We have investigated the effect of vibrations on the stability of gas-fluidized beds of fine powders (particle size approximately 10 microm). The powder is uniformly fluidized by an adjustable gas flow that enables us to control the average solid volume fraction straight phi(0). The fluidized bed is then subjected to a vertical oscillatory motion of controlled amplitude and frequency. The response of the fluidized bed depends essentially on the value of straight phi(0). For straight phi(0)>0.28 the fluidized bed is in a weak solidlike regime, it has a mechanical strength, and particles are static. In this regime vibration causes compaction of the loosely packed bed. For straight phi(0)<0.28 the mechanical strength vanishes and stresses are carried by interstitial gas and collisions. In this fluidlike regime the fluidized bed displays a diffusive dynamics and particles aggregate due to the strong interparticle van der Waals forces. When vibration is applied the powder expands due to the partial disruption of aggregates. However at a critical value of the vibration amplitude A=A(c) either surface (sloshing) or flow (bubbling) instabilities develop. The nucleation of gas bubbles has been correlated to the saturation in particle diffusivity measured elsewhere. The size of the bubbles increases as A is further increased above A(c) and as the vibration frequency is reduced. Moreover, as it should be expected from the predictions of hydrodynamic models, A(c) is independent of cohesivity for particles of the same size and density.

Self-diffusion in a gas-fluidized bed of fine powder.

We have investigated the self-diffusion in a stable gas-fluidized bed of fine powder. Two regimes have been observed: for gas velocities v(g) above the minimum fluidization velocity v(m) and below a critical gas velocity v(c) smaller than the minimum bubbling velocity v(b) the powder does not mix. Experimental measurements show the existence of yield stresses in this regime which are responsible for the static behavior of the bed. For v(g)>v(c) the yield stress vanishes; the bed behaves like a fluid and displays a diffusive dynamics. In this region we have found that the diffusion coefficient D increases with gas velocity until the bed expansion approaches its maximum value.

Settling regimes for initially fluidized beds of find powders

Two empirical flux laws, one continuous and one discontinuous, are used to construct a kinematic wave description for the settling and compaction of an initially fluidized bed of fine powder. Both flux laws are simple generalizations of the Richardson—Zaki equation. The continuous flux law is found to produce two regimes during settling and requires an infinite time for complete compaction. The discontinuous flux law produces up to three regimes during settling and requires a finite time for complete compaction. The discontinuous flux law predicts that the depth of the fully compacted bed increases with a constant speed either due to a contact discontinuity or a simple shock; that initially the head of the bed falls with constant speech due to a simple shock. The predictions of the continuous flux law are similar except that the bed does not fully compact anywhere. An elastic bulk modulus for the solid phase is introduced, resulting in a non-linear convective—diffusion equation, which describes approximately the voidage changes in the settling bed. It is conjectured that the initial settling of the bed is described by a hyperbolic equation, but finally, a diffusive equation may determine voidage evolution.

A new bed-collapsing technique for measuring the dense phase properties of gas-fluidized beds

A new bed-collapsing technique, the “optimum dual-drainage method”, is proposed to determine the dense phase properties in gas-fluidized beds of fine powders. The effects of gas flow resistance of the distributor and the windbox volume on the bed collapse of UO 2, ammonium uranyl carbonate (AUC) and alumina powders have been determined in a 0.07-m I.D.×2.5-m high Plexiglas column. In the single-drainage method, the measured voidage and gas velocity in the dense phase increase with increasing windbox volume, gas flow resistance of distributor and gas velocity whereas in the optimum dual-drainage method, the dense phase properties are found to be independent of distributor resistance and windbox volume. It has been found that the period of bed expansion after the bubble escape stage on the bed-collapse curve varied with the type of powders, and the bed expansion was most pronounced in the bed of UO 2 powders in the single-drainage method. Momentum transfers from the windbox to the bed have been correlated with the pertinent experimental variables ( U o, V w, K d).

The significance of bed collapse experiments in the characterization of fluidized beds of fine powders

Bed collapse experiments are interpreted by a three-phase model of a gas-fluidized bed. Application of the model leads to a relationship between bed voidage in the sedimentation zone of the collapse and the properties of the emulsion and bubble phases of the bed prior to the collapse. Experimental results show that for beds of small particles fluidized under pressure the flow of gas through the emulsion phase is higher than that expected on the basis of existing theories, but it is shown that average rise velocities of bubbles may be obtained from results of bed collapse measurements.

微粉体層の時間依存型力学特性に関する研究

微粉体層の時間依存型力学特性に関する研究

On dense phase voidage and bubble size in high pressure fluidized beds of fine powders

AbstractDense phase voidage, εD, dense phase superficial gas velocity, uDo, and absolute bubble rise velocity, uB, were measured at pressures up to 8300 kPa in a pilot‐scale, fluidized bed of Group A and boundary Group A/B powders. The mean equivalent bubble diameter, db, near the top of the bed was inferred from uB, and known bed operating conditions. Increased pressure at fixed superficial gas velocity, uo, increased εD and uDo and decreased db for Group A powders. The marked decrease in inferred maximum bubble size, d with increased pressure could not be explained by a decrease in gas contributing to bubble flow, uBo, but rather appeared to be the result of a bubble instability phenomenon limiting bubble growth.

Hydrodynamic and heat-transfer studies in fine particle gas-fluidized beds

A 0.152 m internal diameter fluidized-bed pilot-plant facility designed to operate up to a maximum temperature of 1500 K at ambient pressures has been fabricated, installed and tested. High temperatures are obtained by the combustion of propane-air mixtures in the bed. A specially designed bubble cap distributor plate ensures good gas dispersion and solids mixing in the bed. Preliminary measurements dealing with hydrodynamic and heat-transfer characteristics in the moderate temperature range 330–500 K at ambient pressures in beds of fine powders of sand, glass beads and alumina particles are reported here. These include the bed pressure drop and bed expansion as a function of fluidizing velocity and bed temperature. The characteristics of the bubble cap air distributor plate are also investigated in view of its role in controlling the quality of fluidization. The heat transfer between the bed and an immersed water-cooled vertical tube is analyzed to compute the heat-transfer coefficient and its dependence on the superficial air-fluidizing velocity and on bed temperature. These data are critically examined in the light of mechanistic processes taking place in the bed.

Bubble Velocities in Fluidized Beds of Fine, Dense Powders

A method is presented for measuring bubble velocities in gas-fluidized beds of fine powders at high bubble concentrations. The technique effectively quantifies the transmission time of a packet of tracer-rich gas between two points in the bed, and is shown to yield results consistent with those obtained by other methods. Bubble and slug velocities in 0.64 and 0.05 m diameter beds of a fine iron oxide powder are presented. In the small unit, slugs are shown to rise at a velocity independent of bed height but somewhat higher than might be expected from literature predictions. The slug flow results are well described by ▪ It is suggested that the high slug rise coefficient (0.63) is the result of a low dense phase viscosity associated with a high dense phase voidage. Bubble velocities in the large unit display a strong dependence on axial position, and are higher than might have been expected. Beyond about 2 m above the grid, bubble velocities in excess of 2 m/s are observed. These data also suggest the presence of a low retarding force associated with a low dense phase viscosity or, alternatively, the existence of bubble chains and preferred bubble tracks.

The measurement of emulsion phase voidage in gas fluidized beds of fine powders

The results of two different experimental methods, X-ray absorption and bed collapse, for the determination of the emulsion-phase voidage of gas fluidized beds are compared. It is shown that good agreement exists between them over the range of gas velocities studied, but that possible differences in voidage in an axial direction are only revealed by the X-ray technique. The bed collapse method applied to a Geldart Group A powder confirms that there is little difference between the surface settling rates of a bubbling bed and of a uniformly expanded bed of the same voidage.

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, ϵ D , in terms of the physical properties of the gas and powder, and our predictive equations fit literature data. ϵ 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 ϵ 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 ϵ 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.