Electric charge limits on settled powders

In flows of dry particulate systems, electric charge is generated on particle surfaces by their collision with walls and with other particles. Charge build-up on single particles can yield local charge values high enough to surpass the limiting electric field for corona discharge into the surrounding gas. Then, local charge is decreased to a lower value that becomes stabilized when flows stop and particles deposit in a container. In this paper, we have used a Faraday pail system to measure the residual particle charge after using two different devices—tribochargers—for particle charging. One of the tribochargers allowed us to directly measure the total charge that was transferred from the walls to the particles, and this was compared to the final values in the bulk powder once it was collected in the Faraday pail. The results show that the electric charge of particles dispersed in gas is limited by corona discharge and depends mainly on the particle size. In addition, we present a simple model of the discharge of the collected powder based on electrostatic considerations. If the powder effective conductivity and the electric charge of the settling particles are known, the model predicts the temporal evolution of the total charge of the collected powder and the spatial distribution of the electric charge and electric field.Ministerio español de Economía y Competitividad-FIS2014-54539-

Jamming Threshold of Dry Fine Powders

We report a novel experimental study on the jamming transition of dry fine powders with controlled attractive energy and particle size. Like in attractive colloids dry fine particles experience diffusionlimited clustering in the fluidlike regime. At the jamming threshold fractal clusters crowd in a metastable state at volume fractions depending on attractive energy and close to the volume fraction of hard nonattractive spheres at jamming. Near the phase transition the stress-(volume fraction) relationship can be fitted to a critical-like functional form for a small range of applied stresses J) as measured on foams, emulsions, and colloidal systems and predicted by numerical simulations on hard spheres

Physics of Compaction of Fine Cohesive Particles

Fluidized fractal clusters of fine particles display critical-like dynamics at the jamming transition, characterized by a power law relating consolidation stress with volume fraction increment. At a critical stress clusters are disrupted and there is a crossover to a logarithmic law resembling the phenomenology of soils. We measure _ _ _@__1=__=@ log_^ c / Bo0:2 g , where Bog is the ratio of interparticle attractive force (in the fluidlike regime) to particle weight. This law suggests that compaction is ruled by the internal packing structure of the jammed clusters at nearly zero consolidation

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 yg above the minimum fluidization velocity ym and below a critical gas velocity yc smaller than the minimum bubbling velocity yb 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 yg . yc 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

Fine cohesive powders in rotating drums: Transition from rigid-plastic flow to gas-fluidized regime

We investigate the dynamics of fine cohesive powders inside rotating drums. We show that these powders may be fluidized due to entrapment of ambient gas, and we determine the onset of fluidization. Experimental measurements on the bed expansion as a function of the rotation velocity have been performed. Drums of different diameters and fine powders of varying cohesiveness have been tested. We show that (i) fine powders transit directly from a rigid-plastic state to a gas-fluidized state in accordance with the flow regime boundaries predicted elsewhere [A. Castellanos et al., Phys. Rev. Lett. 82, 1156 ~1999], (ii) the onset of fluidization in the rotating drum is determined by the ratio of the powder kinetic energy per unit volume to its tensile strength, and ~iii! once the powder is completely fluidized the average interstitial gas velocity increases proportionally to the rotation velocity. The last two results imply that the required velocity to fluidize a powder, vR (v angular velocity, R radius of the drum), must increase as the square root of its tensile strength, and this has been confirmed by independent measurements and estimations

Correlation between bulk stresses and interparticle contact forces in fine powders

We present measurements of the tensile strength as a function of the consolidation stress for a set of fine cohesive powders ~xerographic toners! of 12.7 mm particle size and with a range of concentration of submicron fumed silica as flow control additive. This additive is well known for its ability to control interparticle adhesion force. Parallel measurements using an atomic force microscope have been carried out on the adhesion force between two individual grains as a function of a controlled previous load force. The effect of the additive on the tensile strength and adhesion force is analyzed. We have found a good correlation between bulk stresses and adhesion forces between individual particles. This correlation is compatible with the existence of a subnetwork of force chains

Granular avalanches: Deterministic, correlated and decorrelated dynamics

A statistical analysis of granular avalanches in a half-filled and slowly rotated drum is presented. For large-sized grains the classical coherent oscillation is reproduced, i.e. we observe a quasi-periodic succession of regularly sized avalanches. As particle size is decreased, we see a crossover to a new complex dynamics characterized by long-range time correlations of local avalanches without a typical size, although the size distribution is not scale invariant. In the limit of large values of the time lag avalanches turn gradually to be decorrelated. The trend observed in the system dynamics as particle size is decreased is ascribed to the increase of cohesiveness which promotes bulk disorder. We argue that our experimental findings can be qualitatively predicted by theoretical models with adjustable parameters such as unquenched disorder, random critical slope, fluidization length, inertia and dissipation.Ministerio de Ciencia y Tecnología BFM2003-0173

Aggregation and sedimentation in gas-fluidized beds of cohesive powders

We present measurements on the settling velocity of gas-fluidized beds of fine cohesive powders. In the solidlike regime ~solid volume fraction f.fc) particles are static, sustained by enduring contacts. The settling is hindered by interparticle contacts and is a very slow process. In the fluidlike regime (f,fc) permanent contacts no longer exist, and the bed displays a diffusive dynamics. The interparticle adhesive force leads to the formation of particle aggregates, and for this reason the sedimentation velocity exceeds the predicted value by empirical or theoretical laws on the settling of individual particles. We use an extension of the Richardson-Zaki empirical law for the settling of aggregates in the fluidlike regime to fit the experimental data. Aggregates are characterized by the number of aggregated particles N and by an effective radius R. The trend followed by these parameters with particle size is confirmed by direct visualization of the aggregates, and shows that cohesive effects become important when the adhesion force between particles is above particle weight. Results show that aggregates form open structures with a fractal dimension close to the predicted one in the diffusionlimited- aggregation model (D52.5)

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 ~10 mm). The powder is uniformly fluidized by an adjustable gas flow that enables us to control the average solid volume fraction f0. 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 f0. For f0.0.28 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 f0,0.28 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 A5Ac 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 Ac and as the vibration frequency is reduced. Moreover, as it should be expected from the predictions of hydrodynamic models, Ac is independent of cohesivity for particles of the same size and density

Enhancement of CO2 capture in limestone and dolomite granular beds by high intensity sound waves

The calcium looping (CaL) process, based on the calcination/carbonation of CaCO3 at high temperatures, has emerged in the last years as a potentially low cost technology for CO2 capture. In this work, we show that the application of high intensity sound waves to granular beds of limestone and dolomite in a CaL reactor enhances significantly their multicycle CO2 capture capacity. Sound waves are applied either during the calcination stage of each CaL cycle or in the carbonation stage. The effect of sound is to intensify the transfer of heat, mass and momentum and is more marked when sound is applied during calcination by promoting CaO regeneration. The application of sound would allow reducing the calcination temperature thereby mitigating the decay of capture capacity with the number of cycles and reducing the energy penalty of the technology.Ministerio de Economia y Competitividad CTQ2014-52763-C2-2-