Das Verhalten kohäsiver Schüttgüter

In this paper dealing with the behaviour of cohesive solids it is first discussed why and under what conditions the property ‘cohesion’ is of particular interest. The results of measurements relating the dependence of cohesive forces on previous compression are then reported. Theoretical considerations based on these measurements show which aspects of the behaviour of cohesive solids are of particular interest in experiment as well as in theory. Experiments with a bunker centrifuge show the relevance of the previous loading history for practical problems. A theory of flow of cohesive solids interprets experimentally measurable data in terms of physically based material properties.     

Measurements with multi-point microprobes: Effect of suspended solids on the hydrodynamics of bubble columns for application in chemical and biotechnological processes

Many experimental studies reveal that suspended solids considerably alter the coalescence behaviour and hydrodynamic functions of wo-phase flow. But no systematic efforts have yet been undertaken to separate the effects of different particle properties on local gas hold-up, bubble size and interfacial area gas/liquid. The aim of this paper is to present the local values of these parameters in three-phase fluidized beds of different solids, using miniaturized optical fiber and conductivity needle probes. It is shown that particle concentration, size and, in particular, density are decisive for the change in coalescence behaviour. Ranges of normal (?_S > ?_L) and inverse fluidization (?_S < ?_L) must be distinguished and the flow regime also exerts a strong influence on the interactions between the dispersed phases, the transition point itself being a function of particle properties can be observed, This effect is evaluated for different column diameters, between 0.1 and 0.3 m.     

Relationship between particle scale capillary forces and bulk unconfined yield strength

One of the main problems in industrial applications is the handling of fine cohesive materials. This cohesion can cause hang-ups in process equipment and stop or slow down processing. Conversely, just a small quantity of cohesion in a bulk powder can prevent particle size segregation. It is obvious that understanding cohesion from a fundamental point of view is useful in describing process flow behavior. Capillary force between particles is one source of cohesion between particles. This paper presents a theory that can be used to model the cohesive behavior of bulk materials based on capillary forces between particles.     

INVITED REVIEW: FLOW BEHAVIOUR OF COHESIVE MATERIALS

Many problems associated with solids handling originate in the influence of the cohesive forces on the flow behaviour of fine powders. Therefore first it is discussed why and under what conditions the property "cohesion" is of particular interest. The results of measurements relating the dependence of cohesive forces on previous compression are then reported. A theoretical model is derived which takes into account the forces acting in interparticle contacts. The theory especially considers the increase in the adhesion forces transmitted in interparticle contacts with increasing consolidation. Permitted simplification of the theory results in a graphic representation allowing immediate visualization of the basic theoretical parameters. Determination of the latter is possible directly, by means of shear tests on cohesive powders. The usefulness of the theory is demonstrated through the representation of unconfined yield stress characteristics of various powders.     

INVITED REVIEW: FLOW BEHAVIOUR OF COHESIVE MATERIALS

Many problems associated with solids handling originate in the influence of the cohesive forces on the flow behaviour of fine powders. Therefore first it is discussed why and under what conditions the property "cohesion" is of particular interest. The results of measurements relating the dependence of cohesive forces on previous compression are then reported. A theoretical model is derived which takes into account the forces acting in interparticle contacts. The theory especially considers the increase in the adhesion forces transmitted in interparticle contacts with increasing consolidation. Permitted simplification of the theory results in a graphic representation allowing immediate visualization of the basic theoretical parameters. Determination of the latter is possible directly, by means of shear tests on cohesive powders. The usefulness of the theory is demonstrated through the representation of unconfined yield stress characteristics of various powders.     

Wet granule breakage in a breakage only high-hear mixer: Effect of formulation properties on breakage behaviour

Wet granule breakage can occur in the granulation process, particularly in granulators with high agitation forces, such as high-shear mixers. In this paper, the granule breakage is studied in a breakage only high-shear mixer. Granule pellets made from different formulations with precisely controlled porosity and binder saturation were placed in a high-shear mixer in which the bulk medium is a non-granulating cohesive sand mixture. After subjecting the pellets to different mixing time in the granulator, the numbers of whole pellets without breakage are counted and taken as a measure of granule breakage. The experimental results showed that binder saturation, binder viscosity and surface tension as well as the primary powder size have significant influence on granule breakage behaviour. It is postulated that granule breakage is closely related to the granule yield strength, which can be calculated from a simple equation which includes both the capillary and viscous force of the liquid bridges in the granule. The Stokes deformation number calculated from the impact velocity and the granule dynamic strength gives a good prediction of whether the granule of certain formulation will break or not. The model is completely based on the physical properties of the formulations such as binder viscosity, surface tension, binder saturation, granule porosity and particle size as well as particle shape.     

Solids separation from the products of the solvent extraction of coal

In the first state of the NCB's coal liquefaction process, coal is extracted with a high-boiling process solvent. The removal of residual solids following extraction is an essential part of the process, which is achieved by filtration. In this study, the characteristics of the coal digest in relation to its filtration properties have been investigated. Filtration rates are discussed in relation to particle composition, fluid viscosity and the existing filtration theory as applied to coal digest. The filtration properties of the solids are principally dependent upon the size and nature of the particles and in particular the composition of the undissolved coal component, which is related to the maceral composition of the coal. A novel technique for characterising coal digest, which embodies the influence of particle size and nature upon filtration, is discussed.     

Particle clusters in and above fluidized beds

High-speed video imaging of particle clusters in and above a fluidized bed suggests that clustering is significant for FCC catalyst and polyethylene powders. Based on fluidized bed experiments at varying fines concentration, bed heights and bed internals location, the dominant mechanism for clusters in the freeboard appears to be cluster formation in the bed. Some of these clusters are then subsequently ejected into the freeboard region. Hydrodynamics does not appear to be solely responsible for cluster formation. Cohesive forces such as electrostatics, capillary and van der Waals forces, appear to play a significant role in particle cluster formation. The proposed mechanism suggests that particle shear produces collisional cooling that allows the granular temperature to decay to where these cohesive forces can dominate. The decrease in the granular temperature appears to be dependent on the particle properties and surface morphology. Collisions that only redirect the particle or increase particle rotation, limits this reduction in the granular temperature such that cohesive forces are less of an impact. In the case of risers, where large shear streams are prevalent, these clusters and the corresponding drag forces may result in the formation of larger clusters or streamers.     

Fluidization of cohesive powders

Relatively small changes in particle size and other parameters which affect interparticle forces can transform a fine free-flowing powder into one which is cohesive. The influence of these parameters on the fluidization behaviour of fine powders has been investigated and can be readily assessed by measuring the ratio of tap to aerated bulk density.     

Effect of particle size on the rheology of Athabasca clay suspensions

The success of the separation process conventionally used in Alberta for oil sands extraction is highly influenced by the rheology of the oil sands slurry. In the gravity separation vessel, high slurry viscosities can hinder the rise of aerated bitumen and reduce the efficiency of the recovery process. In this study, the effect of particle size on the viscosity of oil sands slurries is investigated. Solids from mature fine tails (MFT) obtained from tailings pond were fractionated into three fractions of different particle size distributions and their rheological properties were studied. The solids in each fraction were characterized by XRD analysis which showed the presence of different types of clays in each fraction. For the rheological measurements, dispersions of the three fractions were prepared in the supernatant water decanted from centrifuged MFT to maintain the solution chemistry of the solids. Suspensions of each fraction showed a non‐Newtonian shear thinning behaviour as well as yield stress that is characteristic of structure formation within the suspensions. For all solids fractions, increasing solids concentration led to higher viscosities and higher yield stress values. Viscoelastic properties of the suspensions showed stronger solid‐like behaviour at higher particle concentrations. Among the three fractions numbered from 1 to 3, solids in fraction 3 were coated with organic matters, exhibiting the highest suspension viscosities. Also for fraction 3, higher gelling potency was observed at much lower weight fractions of solids as compared to the other fractions.     

Capillary Curves for Ex‐situ Washing of Oil‐Coated Particles

The oil removal efficiency for the ex situ extraction of bitumen from oil sands, or ex situ washing of oil‐contaminated sand and related processes is determined by the balance of forces at the oil/water and solid/fluid interfaces. The objective of this work is to estimate the balance of forces at the interface using dimensionless numbers, and their use in evaluating and engineering ex situ soil washing processes. To this end, bitumen was removed from bitumen‐coated sand particles using a two‐step process. In the first step, the particles were mixed with a suitable solvent (toluene) used, primarily, to reduce the viscosity of bitumen. The particles were then mixed with water or an aqueous surfactant solution capable of producing low interfacial tensions with the solvent‐bitumen mixture. The fraction of oil retained after washing was evaluated as a function of interfacial tension, solvent/bitumen ratio, mixing time, mixing velocity, and particle size. These ex situ washing conditions were normalized using dimensionless film and particle‐based Weber and Capillary numbers. The fraction of oil retained by the particles was plotted against these dimensionless numbers to generate capillary curves similar to those used in enhanced oil recovery. These curves reveal the existence of a critical film‐based Weber number and a particle‐based Capillary number that can be used in the design or evaluation of soil washing processes. The film‐based Weber number also explained literature data that associates interfacial tension with the removal of oil from oil‐based drill cuttings, as well as field observations on the role that particle size plays on the removal of oil in soil washing operations.     

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.     

Effect of powder properties on discharge characteristics of cohesive carbonaceous fuel powders from a top discharge blow tank at high pressure

Abstract

Discharge experiments of two cohesive carbonaceous fuel powders (anthracite coal and petroleum coke) were carried out in a top discharge blow tank experimental system with design pressure up to 6.4?MPa. The effects of powder properties (particle size and powder category) on the discharge characteristics (including solid discharge rate, solids loading ratio, and discharge stability) from the top discharge blow tank at high pressure were investigated. Meanwhile, shear tests with a ShearTrac II shear tester were performed to investigate the flowability of these two cohesive powders. The results indicate that there exists a near-linear relationship between major consolidation stress and flow function (FF). As particle size decreases, the values of FF decrease and powder flowability becomes worse. However, as particle size decreases, the particle suspension velocity decreases correspondingly, and the gas permeability becomes worse and gas maintain ability becomes better, which contribute to higher solid discharge rate and better discharge stability, whereas solids loading ratio may have opposite variation tendency. Compared to petroleum coke with similar particle size, anthracite coal has better flowability and can obtain higher solid discharge rate and solids loading ratio, as well as better discharge stability. During the discharge processes of petroleum coke, it was found that as the number of discharge experiment increases, the discharge stability decreases correspondingly. After petroleum coke being discharged continuously for 4–6 times, the discharge process could not start again, adhesion phenomena of petroleum coke to the wall of conveying pipelines could be observed.     

A two-dimensional study of the rupture of funicular liquid bridges

An analysis of the rupture behaviour of liquid bridges in simple granular systems is reported. Wet granules present a complex structure in which primary particles are held together at a microscopic scale by means of capillary forces mediated by a liquid binder. These capillary interactions control the mechanical properties of the particle assemblies as well as the kinetics and pathways of granule growth. The evaluation of the interaction between contacting particles by means of the interstitial liquid binder in its various possible configurations has recently been reported for two-dimensional systems of contacting particles (J. Colloid Interface Sci. 220 (1999) 42). In the current work particle separation forces and bridge rupture have been examined for the symmetric separation pathways in all possible wetted states of triplets of particles. The separation force and the energy of the system due to the liquid capillary action have been calculated during the process of separation, which is assumed to occur slowly through a sequence of equilibrium configurations. A wide range of behaviour is found for the rupture processes of the different liquid configurations.     

Effects of particle properties on flow structure in a 2-D circulating fluidized bed: Solids concentration distribution and flow development

The effects of particle properties (particle density, size and sphericity) on solids concentration in a 2-D riser were comprehensively investigated by measuring the axial and lateral solids concentration with an optical fiber probe. In this study, solids concentration of different types of particles shows that heavier particles have higher solids concentration laterally and axially than lighter particles; larger particles result in more compact solids distribution and such size effect is more evident at the riser bottom; more spherical particles lead to higher solids concentration. Various forces acting on particles and the change in the slip velocity between gas and particles were used to explain the effects of particle properties on solids distribution.     

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.     

Numerical simulations of the normal impact of adhesive microparticles with a rigid substrate

The rebound behavior of elastic and elastoplastic microspheres impacting normally with a rigid wall is studied using the finite element method. The interfacial adhesion forces are introduced by adding piecewise-linear spring elements with a particular constitutive relation characterizing the adhesion property. The effect of adhesion hysteresis is taken into account by assuming that the adhesion work during the incident stage is smaller than that during rebounding. The influences of the interfacial adhesion parameters, the constitutive relations, size, and incident velocity of the particle on the coefficient of restitution (COR) are all examined. We also analyze the changing tendency of the kinetic energy, elastic strain energy, adhesion work, and their interchange during impact. It is found that besides interfacial adhesion and plastic dissipation, the residual stress field caused by incompatible plastic deformation has a considerable influence on the impact behavior of the sphere as well. For smaller impact velocities, interfacial adhesion plays a dominant role in the impact process, while for higher incident velocities, the COR depends mainly on plastic deformation. In addition, the COR shows a distinct dependence on the particle size. Finally, our numerical results are compared with the relevant experimental results.