INTERPARTICLE FORCES IN HIGH TEMPERATURE FLUIDIZATION OF GELDART A PARTICLES

Previous reports and current studies show that fluidization of some Geldart A particles is enhanced by increasing bed temperature. Both the averaged local particle concentration and the particle concentration in the dense phase decrease with increasing bed temperature, at constant superficial gas velocities. However, conventional models fail to predict these changes, because the role of interparticle forces is usually neglected at different bed temperatures. Here, the interparticle forces are analyzed to explore the mechanism of gas-solid fluidization at high temperatures. Indeed, as the temperature increases, the interparticle attractive forces decrease while the interparticle repulsive forces increase. Consequently, fluidization behaviors of some Geldart A particles seem to increasingly shift from typical Geldart A towards B with increasing temperature.

Effect of packing method on packing formation and the correlation between packing density and interparticle force

The packing of cohesive particles is of paramount importance in many industries because the packing structure is closely related to process performance.A general relation between packing density and interparticle force was previously proposed based on packing structures formed without dynamic fluid flows.Its universality is examined here in two different packings,formed in settling and defluidization of static and dynamic fluids,respectively.First,it is shown that the packings of the same particles formed by two different methods have different structures because of different impact-induced pressures.Nevertheless,a one-to-one relationship between packing density and structural properties still holds regardless of the different packing methods,and the force distribution in those packings obeys similar rules.Finally,the packing densities obtained by the different methods are demonstrated to be universally correlated with the ratio of the interparticle force to the effective gravity.These findings indicate that different phenomena of particulate systems at a macro-or meso-scale may share similar microscopic origins,with the interparticle force playing a crucial role.

Alternating-current relaxation of a rotating metallic particle

Based on a first-principles approach,we establish an alternating-current（AC） relaxation theory for a rotating metallic particle with complex dielectric constant εα=εα-iσα/ω0.Here εα is the real part,σα the conductivity,ω0 the angular frequency of an AC electric field,and i=-11/2.Our theory yields an accurate interparticle force,which is in good agreement with the existing experiment.The agreement helps to show that the relaxations of two kinds of charges,namely,surface polarized charges（described by εα） and free charges（corresponding to σα）,contribute to the unusually large reduction in the attracting interparticle force.This theory can be adopted to determine the relaxation time of dynamic particles in various fields.

Temperature influence on minimum fluidization velocity:Complexity,mechanism,and solutions

Fluidized-bed reactors are widely employed in various high-temperature industrial processes.Thus,it is crucial to understand the temperature effect on various fluidization phenomena,specifically the minimum fluidization velocity(U_(mf))that governs various aspects of fluidized bed behavior.In this study,we comprehensively analyze U_(mf) data from the literature to unravel the complexity and underlying mechanisms of temperature influence on this critical velocity.The research examines experimental data encompassing a wide range of temperatures,pressures,and solid particles.The analysis reveals that the influence of temperature on U_(mf) is fundamentally determined by the relative importance of hydrodynamic forces and interparticle forces within fluidized beds and is realized by three distinctive temperature-induced changes:gas properties,bed voidage,and physiochemical characteristics of particles.On this basis,an equation is derived to enable predictions of temperature influences on the minimum fluidization velocity under broad temperature conditions.

Understanding the forces acting in self-assembly and the implications for constructing three-dimensional （3D） supercrystals

The assembly of nanocrystals into ordered structures called supercrystals or superstructures has become a pivotal frontier owing to numerous useful applications such as correlating the arrangements of atoms in macroscopic crystals and tuning the collective properties to meet the demands of various applications. In this article, recent progress in the preparation of three-dimensional superlattices of nanocrystals is outlined, with a particular emphasis on the driving forces and evolutionary routes beyond orderly assembly. First, the leading or repulsive forces that internally and externally govern the formation of three-dimensional supercrystals are systematically identified and discussed with respect to their origins and functions in three-dimensional self-organization. Then a synoptic introduction of commonly applied means of nanocrystal self-assembly based on growth scenarios such as droplet evaporation and a liquid/liquid interface is presented with specific cases and detailed analyses. Finally, the existing challenges and prospects for this field are briefly highlighted.

Dry powder platform for pulmonary drug delivery

The phenomenon of particle interaction involved in pulmonary drug delivery belongs to a wide variety of disciplines of particle technology, in particular, fluidization. This paper reviews the basic concepts of pulmonary drug delivery with references to fluidization research, in particular, studies on Geldart group C powders. Dry powder inhaler device-formulation combination has been shown to be an effective method for delivering drugs to the lung for treatment of asthma, chronic obstructive pulmonary disease and cystic fibrosis. Even with advanced designs, however, delivery efficiency is still poor mainly due to powder dispersion problems which cause poor lung deposition and high dose variability. Drug particles used in current inhalers must be 1–5 μm in diameter for effective deposition in small-diameter airways and alveoli. These powders are very cohesive, have poor flowability, and are difficult to disperse into aerosol due to cohesion arising from van der Waals attraction. These problems are well known in fluidization research, much of which is highly relevant to pulmonary drug delivery.

EFFECTS OF GAS TYPE AND TEMPERATURE ON FINE PARTICLE FLUIDIZATION

The influence of gas type （helium and argon） and bed temperature （77-473 K） on the fluidization behaviour of Geldart groups C and A particles was investigated. For both types of particles tested, i.e., Al2O3 （4.8μm） and glass beads （39 μm）, the fluidization quality in different gases shows the following priority sequence： Ar 〉 He. In the same gaseous atmosphere, the particles when fluidized at an elevated temperature usually show larger bed voidages, higher bed pressure drops, and a lower Umf for the group A powder, all indicating an enhancement in fluidization quality. Possible mechanisms governing the operations of gas type and temperature in influencing the fluidization behaviours of fine particles have been discussed with respect to the changes in both gas properties and interparticle forces （on the basis of the London-van der Waals theory）. Gas viscosity （varying significantly with gas-type and temperature） proves to be the key parameter that influences the bed pressure drops and Umf in fluidization of fine particles, while the interparticle forces （also varying with gas-type and temperature） may play an important role in fine-particle fluidization by affecting the expansion behaviour of the particle-bed.

Specific ion effects on soil aggregate stability and rainfall splash erosion

Soil interparticle forces can pose important effects on soil aggregate stability and rainfall splash erosion.Meanwhile,these interparticle forces are strongly influenced by specific ion effects.In this study,we applied three monovalent cations(Li^(+),Na^(+),and K^(+))with various concentrations to investigate the influence of specific ion effects on aggregate stability and splash erosion via pipette and rainfall simulation methods.The specific ion effects on soil interparticle forces were quantitatively evaluated by introducing cationic non-classical polarization.The results showed that aggregate stability and splash erosion had strong ion specificity.Aggregate breaking strength and splash erosion rate at the same salt concentration followed the sequence as Li^(+)>Na^(+)>K^(+).With decreasing salt concentration,the difference in aggregate breaking strength or splash erosion rate between different cation systems increased initially(1–10^(-2)mol L^(–1))and later was nearly invariable(10^(–2)–10^(–4)mol L^(–1)).The experimental results were well quantitatively explained by soil interparticle forces considering cationic non-classical polarization.Furthermore,both aggregate breaking strength and splash erosion rate of three cations revealed a strong positive linear relation with net force subjected to cationic non-classical polarization(R^(2)=0.81,R^(2)=0.81).These results demonstrated that different non-classical polarization of cations resulted in different soil interparticle forces,and thus led to differences in aggregate stability and splash erosion.Our study provides valuable information to deeply understand the mechanisms of rainfall splash erosion.

Bulk flow properties of fly ashes at ambient and high temperature

The bulk flow properties of four different fly ashes were assessed at ambient temperature and at 500 ~C, using a high temperature annular shear cell. These powders all resulted from industrial processes and had similar chemical compositions but different particle size distributions. Applying a high temperature was found to increase the powder cohesion, with this effect being more significant in the case of the sample with the highest proportion of fines. To better understand the effect of temperature on the bulk flow properties of these materials, a model previously proposed by some of the authors was used to correlate the powder isostatic tensile strength with the interparticle forces and microscale particle contact struc- ture. This model combines the continuum approach with description of particle-to-particle interactions. A comparison with experimental data indicated that the effects of consolidation and temperature on the tensile strength of the fly ashes were correctly described by the model. This theoretical approach also elucidates the mechanism by which the temperature affects the bulk flow properties of fly ashes through modifications of the microscale intemarticle contacts.