Segregation of cohesive powders in a vibrated granular bed

The effects of small amounts of added liquid on the segregation behavior of a granular system under vertical vibration by DEM simulation are investigated in this study. The cohesive forces of grains are incorporated into DEM simulations via a simplified dynamic liquid bridge force model. The simulation results show that capillary forces in addition to viscous forces have an important effect on the segregation phenomenon. The segregation rate of larger intruder rises to the top of the bed is found to depend on the liquid content. The segregation rate is sharply increased when a small amount of liquid is added to granular system. A transition to the reduction of segregation rate occurs at a critical liquid content. It has shown that this transition can be interpreted as the increase of attractive force between grains due to viscous force. The viscous forces make the particles stick more tightly to each other and retard the movement of particles, thus reducing the segregation rate. The segregation rate is also related to the convection motion of the granular system. The presence of convection enhances the segregation rate of wet granular materials.     

Fluid Dynamics of Fluidized and Vibrofluidized Beds Operating with Geldart C Particles

The fluid dynamic behavior of a vibrofluidized bed operating with Geldart C particles was studied. The experiments were conducted in order to observe the influence of amplitude, frequency, and dimensionless vibration number on the minimum fluidization velocity, pressure drop, and standard deviation obtained. It was noted that the dimensionless vibration number should be used very carefully if it is to be the unique parameter to set the vibrational effect of the bed fluid dynamics. The results clearly indicate that the fluid dynamic behavior of the bed is very dependent on the different combinations of amplitude and frequency for the same dimensionless vibration number. Therefore, the use of the amplitude or frequency of vibration and the dimensionless vibration number is recommended for a better characterization of the vibrational effects on the fluid dynamic behavior of the particle bed.     

Effect of cohesive powders on the elutriation of particles from a fluid bed

The elutriation of fine particles (Group C or A particles in Geldart's classification) from a fluid bed of mixed fine and coarse particles is investigated in a steady state. Al(OH)₃ and alumina and TiO₂ powder of 0.5- were used as fines. FCC, alumina, Al(OH)₃, limestone, silica sand, SiC particles of 44- were used as coarse particles. The paper investigates the effect on the elutriation rate constant of both fine powders and coarse particles (i) of the weight fraction of Geldart C powders in the bed, (ii) of the superficial gas velocity, and (iii) of the size of C powder and size of coarse particles in the bed.The elutriation rate constant of group C or group A particles is not only affected by the properties of the elutriated powders or particles and gas velocity, but also by both the weight fraction and size of C powder in the bed. This finding differs from the elutriation result of A or B particles from a fluidized bed.     

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 cohesive powders on pressure fluctuation characteristics of a binary gas-solid fluidized bed

The effect of cohesive particles on the pressure fluctuations was experimentally investigated in a binary gas-solid fluidized bed. The pressure fluctuation signals were measured by differential pressure sensors under conditions of various weight percentages of cohesive particles. The cohesive particles increased the fixed bed pressure drop per unit height and decreased the minimum fluidization velocity. The Wen & Yu equation well predicts the minimum fluidization velocity of the binary system. The addition of cohesive particles slightly decreased the bubble size in bubbling flow regime when the cohesive particles and the coarse particles mixed well, while the bubble size greatly decreased when the cohesive particles agglomerated on the bed surface. The time series of pressure fluctuations was analyzed by using the methods of time domain, frequency domain and wavelet transformation. The normalized standard deviation of pressure fluctuations decreased with increasing weight percentages of cohesive particles. A wide bandwidth frequency of 0 to 1Hz got narrower with a single peak around 0.6Hz with an increase in proportion of the cohesive particles. The meso-energy and micro-energy of pressure fluctuations were decreasing with increasing cohesive particles proportions, which indicated that adding cohesive particles could reduce the energy dissipation of bubble and particle fluctuations.     

Avalanching flow of cohesive powders

The flow dynamics of cohesive powders is investigated in rotating cylinders with an L : R ratio of 3 : 1 using experiments and DEM simulations. Flow onset and steady-state behavior are compared for free-flowing (cohesionless) dry glass beads, wet glass beads, and “dry” cohesive powders (lactose, microcrystalline cellulose). The avalanching dynamics of powders is substantially different from those observed for free-flowing or wet-cohesive glass beads. Dry cohesive powders exhibit history-dependent flow dynamics, significant dilation, aperiodic avalanche frequencies, and variable avalanche size. These behaviors also provide a route for effective characterization of cohesive forces under dilated conditions characteristic of unconfined flows.     

Entrainment behaviour of high-density Geldart A powders with different shapes

Atomised and milled Ferrosilicon with average particle diameters of 38 and 50 µm respectively were fluidised with air at ambient conditions. The entrainment rate of the more spherical atomised particles was on average six times that of the irregularly shaped milled particles over the range of superficial velocities investigated. In an attempt to decouple the effect of particle size from shape, the bed was divided into theoretically isolated bins based on the distributions of particle sizes. This indicated that the atomised particles had a higher entrainment rate for particles smaller than approximately 25 µm whereas the opposite was true for particles greater than this size. None of the entrainment correlations investigated was able to predict the switch in entrainment behaviour as a function of particle sphericity and diameter. Furthermore, the traditional critical particle diameter associated with cohesive Geldart A particles was not observed for either of the two particle shapes. It is therefore concluded that neither the hydrodynamic nor Van der Waals forces acting on the particles can adequately explain the entrainment rate behaviour of differently shaped high-density Geldart A particles.     

Fluidization and coating of very dense powders by Fluidized Bed Chemical Vapour Deposition

The hydrodynamic behaviour of a very dense tungsten powder, 75 μm in median diameter and 19,300 kg/m³ in grain density, has been studied in a fluidized bed at room temperature using nitrogen and argon as carrier gas. Even if fluidization was achieved, the small bed expansion indicated that it was imperfect. Then, the fluidization was studied at 400 °C in order to investigate the feasibility of coating this powder by Fluidized Bed Chemical Vapour Deposition (FBCVD). In particular, the influence of the H₀/D ratio (initial fixed bed height to reactor diameter) on the bed thermal behaviour was analyzed. It appeared that at least 1.5 kg of powder (corresponding to a H₀/D ratio of 1.8) was necessary to obtain an isothermal bed at 400 °C. Finally, first results about alumina coatings on the tungsten powder by FBCVD from aluminium acetylacetonate are detailed. They show that for the quite low temperatures tested, the coatings are uniform on all bed particles and are formed of amorphous carbon containing alumina. This study demonstrates the efficiency to combine fluidization (instead of spouted bed) and CVD to coat such very dense powders.     

Theory of yield of cohesive powders

In many problems in process technology, the influence of the adhesion forces on the mechanics of fine powders plays a considerable role. The existing knowledge is characterized by intensive experimental and theoretical investigations into the mechanism of adhesion for single particle contacts without consolidation by external forces. On the other hand, the mechanical behaviour of cohesive powders has been investigated mainly on the basis of continuum mechanics.For science and technology, it is useful to combine the particles approach and the continuum approach in order to provide a better understanding of the mechanical properties of cohesive powders. A theoretical model has therefore been developed 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. Contrary to the commonly used phenomenological view, this theory yields a physically based understanding of the effective yield locus for steady-state flow which is reduced to material properties of the solid particles and is therefore shown to be an invariant of a respective powder.The opinion found in the literature that a single yield locus uniquely belongs to one consolidation locus is shown to be not valid. The microstructural view of a cohesive powder is able to show that, contrary to the continuum mechanical view, in the general case a cohesive powder is mechanically anisotropic due to its loading history.     

Fluidized bed spray granulation: Investigation of the coating process on a single sphere

The coating and granulation of solid particles in a fluidized bed is a process which converts pumpable and atomizable liquids (solutions, slurries, melts) into granular solids in one step by means of drying. The solution to be processed is sprayed onto a fluidized bed. Particle growth can take place either via surface layering or agglomeration. In the case of surface layering the atomized droplets deposit a thin layer of liquid onto the seed particles. The solvent is then evaporated by the hot fluidizing, leaving behind the dissolved material on the surface. Although fluidized bed spray granulation and film coating have been applied in industry for several years, there is still a lack of understanding of the physical fundamentals and the mechanisms by which spherical granules are formed. Hence a new method was developed which allows the direct observation of the subsequent particle-forming mechanisms such as droplet deposition, spreading, wetting and drying. The authors present a laboratory scale apparatus in which a single freely suspended particle can be coated under well defined and constant coating and drying conditions. With this device, particle-growth-rate and the development of particle morphology were measured and investigated under various experimental conditions.     

Fluidized bed gasification of coal

Gasification of high ash India coal has been studied in a laboratory-scale, atmospheric fluidized bed gasifier using steam and air as fluidizing media. A one-dimensional analysis of the gasification process has been presented incorporating a two-phase theory of fluidization, char gasification, volatile release and an overall system energy balance. Results are presented on the variation of product gas composiiton, bed temperature, calorific value and carbon conversion with oxygen and steam feed. Comparison between predicted and experimental data has been presented, and the predictions show similar trends as in the experiments.     

Hydrodynamics of Geldart group A particles in gas-solid fluidized beds

Geldart group A particles were fluidized in a 10 cm i.d.×1.8 m high Plexiglas-made bed with ambient air to determine the hydrodynamic properties in a gas-solid fluidized bed. The effects of static bed heights, position of pressure measuring points, differential and absolute pressure fluctuations on the hydrodynamic behavior of a Geldart group A particles in a gas-solid fluidized bed were investigated. The particles used in this study were 80 micrometer FCC powders and 60 micrometer glass beads. The variance of pressure fluctuations was used to find the minimum bubbling velocity. The obtained minimum bubbling velocity was compared with the other methods available in the literature. This method was found to be much easier and had better data reproducibility than the classical visual method or sedimentation method. The variance of pressure fluctuations increased due to the increase of superficial gas velocity and static bed height. The obtained minimum bubbling velocity and pressure fluctuations were found to depend on the measuring position along the axial direction. The effect of measuring position was discussed. Cross-correlation of two pressure signals was used to find the delay time, then the bubble rising velocity.     

Fluidized bed combustion of coal

To conserve oil an alternative start-up procedure for the fluidized-bed combustor has been established which uses charcoal. The elutriated carbon loss has been measured for different grades of coal, and the variation of carbon loss with fluidization velocity is also reported. The results could be useful in the design of fluidized-bed combustion systems.     

Hydrodynamic modeling of the entrainment of Geldart A group particles in gas-solid fluidized bed: The effect of column diameter

A multi-fluid Eulerian computational fluid dynamics (CFD) model is used to simulate the entrainment of fluid catalytic cracking (FCC) particles in gas-solid fluidized beds. Entrainment of Geldart A group particles was studied because of their wide range of industrial use. The model was based on the kinetic theory of granular flow. The CFD model was used to investigate the effect of column diameter on the entrainment flux of particles in a binary mixture. Two different sizes of particles were used because many engineering applications deal with binary mixture of particles in fluidized beds. Various column diameters, including 38 mm, 76 mm, 114 mm, 152 mm, and 190 mm, were investigated. The entrainment flux of particles was increased with decreasing column diameter. The effect of column diameter was not significant for column diameters larger than 114 mm. Furthermore, increasing the superficial gas velocity increased the entrainment flux of particles. Model predictions were also compared with experimental findings.     

声场流化床A类颗粒浓度分布研究

在内径140 mm,高1 600 mm的鼓泡流化床中,以流化催化裂化(FCC)颗粒为流化介质,采用光导纤维探针测定不同轴/径向位置的颗粒浓度分布.考察了操作气速和外加声场对密相区颗粒浓度的影响.结果表明,鼓泡床密相区颗粒浓度沿轴向逐渐减小,沿径向呈抛物线分布.声场的引入可以降低颗粒起始流化速度:声压级越大,起始流化速度越小:固定声压频率在150 Hz时颗粒起始流化速度最小.1随着声压强度的增大,床层中心区和上部密相区颗粒浓度增大.固定声压级,频率在100～400 Hz颗粒浓度较大,频率低于100 Hz或高于400 Hz时,声波的作用效果减弱.     

The tensile properties of mixtures of cohesive powders

The theory of tensile stress proposed by Cheng has been extended to deal with binary and three-component mixtures of powders by means of an equation which relates the tensile strength to the particle size parameters and the composition of the mixtures and also to the interparticle force. The interparticle force is expressed in terms of a “strength” parameter ε₀ and a range parameter to by applying the law of corresponding states.By means of ε₀, the tensile strength of the mixtures can be compared relative to the strength of an arbitrarily chosen (single) material.The modified equation has been used in combination with the law of corresponding states to predict the tensile strengths of binary and three-component mixtures with an accuracy of about ± 20%.     

Uniaxial compaction behaviour and elasticity of cohesive powders

The compression and compaction behaviour of bentonite, limestone and microcrystalline cellulose (MCC) — three cohesive powders widely used in industry were studied. Uniaxial compression was performed in a cylindrical die, 40 mm in diameter and 70 mm high, for three selected cohesive powder samples. The initial density, instantaneous density and tablet density were determined. The influence of maximum pressure and deformation rate was examined. The secant modulus of elasticity E_sec was calculated as a function of deformation rate v, maximum pressure p and powder sample. After compaction experiments in hydraulic press at three pressures - p = 30, 45 and 60 MPa - and two different deformation rates, the strength of the produced tablets was examined in a material strength testing machine.From uniaxial compression tests performed on the universal testing machine for loading and unloading, the modulus of elasticity E was calculated on the basis of the first linear phase of unloading. The total elastic recovery of tablets was also obtained.     

Hopper discharge of cohesive powders using pulsated airflow

An aerated hopper with pulsated aeration was designed to study the effect of square-wave pulsated airflow on the discharge characteristics of cohesive and non-cohesive powders. Experiments were carried out with pulsation frequencies between 0 (continuous) and 50 Hz. Two flow patterns were observed at different aeration rates and pulsation frequencies: intermittent and smooth. For non-cohesive powders, pulsated aeration has a worsening effect on discharge flow. For cohesive powders, results revealed that pulsated aeration can reduce the minimum aeration necessary to allow continuous discharge. And the maximum discharge rate that can be attained applying pulsated aeration is higher than that of continuous aeration and it is significantly more uniform. The aggregates size of the powder was estimated and used to understand the stabilizing mechanism of pulsated aeration on the aggregative discharge. The model value of the minimum aeration rate and the discharge rate were obtained and compared with the experimental values.     

Fluidized bed feeding of pulverized coal

Some general characteristics of fluidized bed feeders are discussed. Experimental results for the feeding of pulverized coal in the range from 0 to 25 g/min are presented and discussed. Operating procedures that allow virtually trouble-free feeding and a quick method of obtaining solid flow rates are noted. A calculational scheme is also given for predicting the effects of design and operating procedure changes on feed rate. Although we view the computations as being only qualitative, they do give the required bed pressure needed to initiate feeding. Further, the approximately linear relationship between bed pressure and solid flow rate obtained experimentally is predicted also.     

Fluidized bed coating and granulation: influence of process-related variables and physicochemical properties on the growth kinetics

This study, which deals with the coating and granulation of solid particles by aqueous solutions of polymers or inorganic salts, aims to understand the effect of:

process-related variables such as the excess gas velocity, atomizer location, liquid flow rate and concentration, and atomizing air flow rate,

physicochemical-related variables such as the viscosity of solutions, wettability of the granulating liquid on solid particle surfaces, initial particle mean size, and porosity of the particles on the agglomeration kinetics of solid particles in a fluidized bed.

The results showed that for a given particle size, the fluidizing air velocity was the most important factor affecting the growth kinetics and the stability of the operation. An increase of the relative humidity, depending on the liquid flow rate as well as the air flow rate, favor agglomeration mechanism especially for values greater than 0.4. An increase in the particle initial size leads to an enhancement of the layering mechanism, especially for values greater than 300 μm.

The effect of the interfacial tension is investigated by adding different concentrations of a non-ionic surfactant to the binding solution. The effect of the contact angle is then studied using non-hydrophobic, partly hydrophobic, or totally hydrophobic particles. The growth of agglomerates appears to be favoured when the interfacial tension increases and the contact angle decreases. The viscosity of the solution has less effect than the interfacial parameters. The results show that the dominant forces in the granulation process are the capillary forces.