Expansion Behavior of a Binary‐Solid Liquid Fluidized Bed with Large Particle Size Difference

The overall expansion of two dissimilar solid particle species with over a tenfold difference in their size and substantial density difference is investigated here for different compositions of the fluidized bed. Contrary to the widely held notion that the total bed height would be the sum of the heights of the two segregated mono‐component beds, the actual bed heights were, in fact, found to be lower. This volume contraction is found to strongly depend upon the mixing behavior prevailing in the binary‐solid fluidized bed. At the complete mixing of the two solid species, the bed‐contraction versus liquid velocity profile shows a global maximum. As a result, the overall bulk density profiles are similarly affected. Moreover, it is found here that correlations meant for predicting the porosity of the packing of binary particle mixtures can be satisfactorily extended to binary‐solid fluidized beds where solid species differ significantly in size.     

Numerical simulation of local and global mixing/segregation characteristics in a gas–solid fluidized bed

Researches on solids mixing and segregation are of great significance for the operation and design of fluidized bed reactors. In this paper, the local and global mixing and segregation characteristics of binary mixtures were investigated in a gas–solid fluidized bed by computational fluid dynamics-discrete element method (CFD-DEM) coupled approach. A methodology based on solids mixing entropy was developed to quantitatively calculate the mixing degree and time of the bed. The mixing curves of global mixing entropy were acquired, and the distribution maps of local mixing entropy and mixing time were also obtained. By comparing different operating conditions, the effects of superficial gas velocity, particle density ratio and size ratio on mixing/segregation behavior were discussed. Results showed that for the partial mixing state, the fluidized bed can be divided into three parts along the bed height: complete segregation area, transition area and stable mixing area. These areas showed different mixing/segregation processes. Increasing gas velocity promoted the local and global mixing of binary mixtures. The increase in particle density ratio and size ratio enlarged the complete segregation area, reduced the mixing degree and increased the mixing time in the stable mixing area.     

Characterization of Mixing and Size Segregation in a Rotating Drum by a Particle Tracking Method

The mechanisms of segregation in solids mixing, even in simple rotating drums, are not clearly understood. Although most past studies have focused on binary mixtures, this work investigates the effect of polydispersity on granular flow, mixing, and segregation in a rotating drum operated in rolling regime through particle trajectories obtained from the radioactive particle tracking technique. Velocity profiles, radial segregation, and axial dispersion coefficients for monodisperse and polydisperse systems of glass beads are analyzed with respect to rotational speed and particle size. A model is introduced to predict the residence times along streamlines and evaluate the rate at which the material renews at the free surface and within the inner layers of the bed. Our results reveal similar velocity profiles and residence times for monodisperse and polydisperse systems. They also indicate that the particles distribute along the radial direction of the drum, although not necessarily in a core/shell configuration. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1894–1905, 2013     

Cohesive effects in powder mixing in a tumbling blender

Rotary drums are used as mixers, dryers, kilns and granulators. In all of these systems, powder cohesion deeply affects mixing and segregation, and it is critical in process scale up. In this paper, we focus on the effect of cohesion in mixing and size segregation of binary mixtures of uniform and non-uniform sizes in a partially filled rotating drum. The cohesive force between particle is simulated using a square-well potential and the numerical model is used to characterize flow and mixing properties. The model is validated by comparison to experimental images. Results show a time-dependent spatial distribution of cohesive powder that depends on the magnitude of cohesion and friction. In uniform binary systems, as cohesion increases, the rate of mixing first increases and then decreases, however for the case of non-uniform binary systems, we observe different mixing patterns depending on the relative magnitude of forces acting between particles of same/different sizes. Unlike free flowing material, for cohesive mixtures, a higher rotation speed is found to enhance mixing performance.     

Discrete particle simulation of solids motion in a gas–solid fluidized bed

The solids motion in a gas–solid fluidized bed was investigated via discrete particle simulation. The motion of individual particles in a uniform particle system and a binary particle system was monitored by the solution of the Newton's second law of motion. The force acting on each particle consists of the contact force between particles and the force exerted by the surrounding fluid. The contact force is modeled by using the analogy of spring, dash-pot and friction slider. The flow field of gas was predicted by the Navier–Stokes equation. The solids distribution is non-uniform in the bed, which is very diluted near the center but high near the wall. It was also found that there is a single solids circulation cell in the fluidized bed with ascending at the center and descending near the wall. This finding agrees with the experimental results obtained by Moslemian. The effects of the operating conditions, such as superficial gas velocity, particle size, and column size on the solids movement, were investigated. In the fluidized bed containing uniform particles better solids mixing was found in the larger bed containing smaller size particles and operated at higher superficial gas velocity. In the system containing binary particles, it was shown that under suitable conditions the particles in a fluidized bed could be made mixable or non-mixable depending on the ratios of particle sizes and densities. Better mixing of binary particles was found in the system containing particles with less different densities and closer sizes. These results were found to follow the mixing and segregation criteria obtained experimentally by Tanaka et al.     

铁基载氧体与干化市政污泥二元混合物流化特性

化学链燃烧技术处置污泥可有效抑制有害气体排放，但干化污泥与铁基载氧体的物理性质相差较大，在流化床内会产生混合分离等问题。在内径(φ)为100 mm的有机玻璃冷态流化床装置上，进行了干化污泥与载氧体二元颗粒流化实验，讨论了颗粒粒径大小、干化污泥与载氧体质量比和操作风速对二元颗粒流化特性的影响。结果表明，污泥与平均粒径为0.66 mm的载氧体能实现混合流化，最小流化速度 和最小混合操作风速Um相等；污泥与平均粒径为1.46 mm的载氧体流化时，随操作风速增大，逐渐由分离流化状态过渡到混合流化状态，Um? ；污泥与平均粒径为2.43 mm的载氧体流化时，始终保持分离流化状态。基于提出的表征混合/分离流化状态的无量纲数Gd，当00.8时处于分离流化。     

A particle‐scale index in the quantification of mixing of particles

Discrete element method (DEM) is a useful tool for obtaining details of mixing processes at a particle scale. It has been shown to satisfactorily describe the flow structure developed in bladed mixers. Here, the advantage is taken of the microstructure gained from DEM to evaluate how best to quantify the microstructure created by mixing. A particle‐scale mixing index (PSMI) is defined based on coordination numbers to represent the structure of a particle mixture. The mixture quality is then analyzed qualitatively and quantitatively in three different ways: a macroscopic mixing index based on the conventional approach, coordination number, and PSMI. Their effectiveness is examined based on DEM data generated for different particle loading arrangements and binary mixtures of particles with various volume fractions, size ratios, and density ratios. Unlike the two other methods, PSMI reveals in a straightforward manner whether a binary mixture of different particles is mixing or segregating over time, while being able to detect particle‐scale structural changes accompanying the mixing or segregation processes in all the mixtures investigated. Moreover, PSMI is promising in that it is not influenced by the size and number of samples, which afflict conventional mixing indexes. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Effect of particle shape on liquid-fluidized beds of binary (and ternary) solids mixtures: segregation vs. mixing

Experiments were carried out in water-fluidized binary (and ternary) mixtures of teflon spheres, discs and rods. All particles had the same volume, while the discs and rods had nearly the same sphericity. It is shown that segregation can occur by shape, with similar segregated and mixed zones as when binary mixtures of different size or density are fluidized. The model of Pruden and Epstein (1964; Stratification by size in particulate fluidisation and in hindered settling. Chemical Engineering Science 19, 696), in which the degree of segregation depends on the bulk density difference(s) of the corresponding monocomponent beds at the same liquid velocity, is vindicated qualitatively for each system, but sphericity is not sufficient as a single shape factor to yield a single quantitative correlation of the transitions between segregation patterns for the different systems. Segregation by shape of non-isometric particles appears to require higher reduced density differences than sizing of spheres, probably because of the greater bed instabilities generated by the non-isometric particles. Overall bed voidage is predicted well by the serial model of Epstein et al. (1981; Liquid fluidisation of binary particle mixture- I.Overall bed expansion. Chemical Engineering Science 36, 1803).     

Modelling transverse mixing in a rolling drum

In this paper, a new model of transverse mixing in a rotating drum is derived from dynamic data collected from an experimental rig. Since the active layer has often been declared as the zone that is responsible for the mixing of solids in a rolling bed, the active layer was characterized so that its properties could be predicted for a wide range of experimental conditions. The mixing model consisted of two correlations, one to predict the mixing rate in the drum and the other to predict the final contact between the two materials. These correlations were linked back to the operational variables of the drum, such as the drumfs loading, size and rotational velocity. By combining these two correlations the time required to fully mix the material in the drum could be predicted. The mixing model was tested against independent data and good agreement was observed between the experimentally derived results and those predicted by the mixing model. Furthermore, this mixing model was designed such that it would be easily applicable to different sized drums. The extrapolation ability was tested on different smaller‐sized drums and found to agree considerably with experimental results.     

Comprehensive experimental investigation on biomass-glass beads binary fluidization: A data set for CFD model validation

Fluidization behavior of biomass and glass beads binary mixtures in a bubbling fluidized bed was experimentally investigated. Mixtures containing different mass fraction of Loblolly Pine white wood and glass beads were fluidized at different fluidization velocities. The particle properties were characterized in a QICPIC that uses a dynamic image processing method to measure both particle size and sphericity. The minimum fluidization velocity was determined using the pressure drop method. An image processing method was developed to capture the dynamic expanded bed height at a very high frequency. The effect of biomass mass fraction and inlet gas velocity on mixing and segregation behavior was studied and analyzed through pressure drop measurements. Pressure drop fluctuations and expanded bed height fluctuations via fast Fourier transform were analyzed and compared. The complete and accurate experimental data reported in this study could provide a benchmark data set for various computational fluid dynamics models validation, calibration, and identification.     

蔗糖三相流态化结晶过程中离集与混合特性的研究

在一总高６５０ｍｍ，直径６０ｍｍ的三相流化床实验装置上，分别采用压降法和取样法测定了不同操作条件下二元混合物蔗糖晶体颗粒浓度沿轴向的分布，并建立了表征颗粒混合与分级程度的数学模型，得到不同条件下的混合系数。结果表明，混合系数即混合程度随气速增加而增大，但随液速增加略有减小；随大颗粒比率的增加，床中的离集程度有所增大。     

Prediction of Layer‐Inversion Behavior of Down‐Flow Binary Solid‐Liquid Fluidized Beds

The layer‐inversion behavior of down‐flow binary solid‐liquid fluidized beds is predicted using the property‐averaging approach. The binary pair in this case consists of a larger solid species which is also heavier than its smaller counterpart, while both are lighter than the fluidizing medium. The model is based on using the generalized Richardson‐Zaki correlation for evaluation of the bed void fraction wherein mean values of particle properties are used. However, unlike the maximum bulk density condition for the conventional up‐flow binary solid fluidized bed, the model is based on a minimum bulk density condition for occurrence of layer inversion. This is due to the fact that the volume contraction phenomenon associated with the mixing of unequal solid species leads to a decrease in bulk density of the bed. Model predictions are also compared using the limited data available in the literature. Predictions are consistent with the observed mixing behavior.     

Particle segregation in a spouted bed of binary mixtures of particles

Minimum spouting velocity and segregation behaviour of binary mixtures of particles differing in size have been studied. The experiments were carried out in a bed of 20 cm diameter at superficial gas velocities up to 1.3 U_ms by use of silica sand of four different particle sizes from 0.655 to 2.23 mm. An empirical equation was proposed for the minimum spouting velocity of binary mixtures. The effects of the particle size difference and the superficial gas velocity on segregation were investigated. Results showed that considerable radial segregation as well as axial segregation occurred even for high gas velocity under the condition of large particle size difference.     

Agglomerate Sizes of Binary Nanoparticle Mixtures in a Vibro‐Fluidized Bed

Agglomerate size is one of the key factors influencing the fluidization behavior of nanoparticles. The effect of fluidization time, superficial gas velocity, and vibration frequency on agglomerate sizes for different binary mixtures of nanoparticles at the top of the bed was investigated in a vibrated fluidized bed (VFB). The agglomerate sizes decreased and fluidization quality was significantly enhanced owing to introduction of vibration energy. The Richardson‐Zaki equation combined with Stokes' law permitted the prediction of mean agglomerate sizes. Experimental and estimated results indicated that vibration led to a smaller agglomerate size. The mean predicted agglomerate sizes were in agreement with those determined experimentally in the VFB.     

Mixing–segregation phenomena of binary system in a fluidized bed

A study on mixing–segregation phenomena in a gas fluidized bed of binary density system was performed by analysis of the residence time distribution and mixing degree. The effect of particle mixing on the residence time distribution and solid mixing was studied in a binary particle system with different densities. Residence time distribution curve and mean residence time of each particle were measured according to the flotsam particle size, mixing ratio and gas velocity in a gas fluidized bed (0.109 m I.D., 1.8 m height). The characteristics of residence time distribution and the deviation of mean residence time of each particle are consistent with previous mixing index based on the axial concentration of jetsam. From this study, mixing index of binary particle system with different densities should be considered by not only axial concentration distribution of jetsam particle but also characteristics of residence time distribution. This result suggests that the solid movement by fluidization gas is more important than solid axial dispersion.     

Prediction of bed pressure drop and top packed bed formation in gas-liquid-solid semi-fluidized bed with irregular homogeneous binary mixtures

We studied the hydrodynamics of a gas-liquid-solid semi-fluidized bed relating to packed bed formation and bed pressure drop with irregular homogeneous binary mixtures in a 0.05 m internal diameter Perspex column, with water and air (secondary) as fluidizing medium at constant static bed height of 0.08 m. A homogeneous binary mixture has been taken for easy formation of a semi-fluidized bed. Air is supplied centrally below the bottom grid in radial direction with a special design air sparger after the bed is first fluidized by the liquid. Experimental parameters studied included superficial gas and liquid velocities, average particle size and density and the bed expansion ratio. Empirical and semi-empirical models were developed. The calculated values from predicted models were compared with the experimental values and fairly good agreement was obtained.     

Segregation behavior of binary mixtures of cylindrical particles with different length ratios in the rotating drum

Particle shape impacts the flow behavior of granular material but this effect is still far from being fully understood. Using discrete element method, the current work explores the segregation phenomena of the binary mixtures of cylindrical particles (differing in length but with the same diameter) in the three-dimensional rotating drum operating in the rolling regime, with each cylindrical particle fully represented by the superquadric equation. The important characteristics and the effect of length ratio on the flow dynamics of the binary mixtures are discussed. Some trends are in sync with those of binary mixtures of spherical particles. Unique to nonspherical particles is the orientation of particles, with results indicating that the cylindrical particles align their major axes perpendicular to the drum axis and this behavior becomes more significant for large particles when the length ratio increases. The length-induced radial segregation causes the orientation of large cylindrical particles to be less uniform.     

Investigation of axial segregation in a rotating drum

A binary mixture of particles is used to investigate axial segregation in a rotating drum. The effects of the rotational speed, the fill level, the particle size ratio and the resilience of particles on the segregated band widths are evaluated. When rigid glass particles 0.35 mm or 0.70 mm as the key components are mixed particles with a size ratio of 2, 3, or 4, the dimensionless band width is less affected by the rotational speed, except at fill levels 10% and 20% with 0.35 mm particles as the key components. The dimensionless band width decreases with the increase of the level of fill. Experimental results show that there exists a size ratio where the dimensionless band width has a minimum value using rigid particles. The value is approximately 3. The existence of this value was attributed to the diffusion and inertia mechanisms of the motion of a single particle.A comparison was made on the influence of the fill level and the speed of rotation on the dimensionless band width using rigid glass beads and non-rigid rubber particles. The dimensionless band width increases with the increase of the level of fill using 1 mm rubber particles as the key components with a size ratio 2 at all rotational speeds studied. Whereas the dimensionless band width decreases with increasing level of fill when 1 mm glass particles as the key components with a size ratio 2 are mixed at all rotational speeds studied. When particles of different elastic properties are mixed, segregated bands are formed but are less pure compared to the bands formed due to size segregation. The interfaces between the rubber-rich bands and the glass-rich bands are curved. A remarkable convective motion at the surface of the particle bed is observed.     

A comprehensive interpretation of solid layer inversion in liquid fluidised beds

Solid mixing and segregation in liquid fluidised beds containing binary mixtures of spherical particles of different density and size has been studied for a range of liquid velocities, bulk bed compositions and particle properties. It was shown that a bed of denser particles expands with liquid velocity independently of the presence of the lighter particles. When the bulk volume fraction of the lighter particles is high and the liquid velocity is relatively low, the bed forms two layers, i.e. the upper layer consisting almost entirely of the lighter and the lower mixed layer consisting of both components in which the volume of the lighter increases with liquid velocity. A completely mixed bed is obtained at a certain velocity and then a further increase of the velocity causes “layer inversion”. The liquid velocity at which complete mixing occurs depends on the bulk bed composition, and at that velocity the volume fraction of the lighter in the lower mixed layer is constant regardless of the bulk bed composition. It is shown that layer inversion occurs for a given particle mixture when the liquid velocity passes through a value at which the volume fraction of the lighter in the lower layer becomes equal to the bulk bed composition; or for a given velocity, when the bulk bed composition becomes equal to the fraction of the lighter component which exists in the lower layer. The dependency of the fraction on the liquid velocity and the particle properties is examined to some extent.     

All the Brazil nuts are not on top: Vibration induced granular size segregation of binary,ternary and multi-sized mixtures

Vertically vibrated systems of granular materials have been used to gain insight on granular segregation for decades. However, the majority of studies have focused on the rise of a single large “intruder” particle in an otherwise monodisperse bed, or binary mixtures. As most industrially relevant granular materials are characterized by some degree of polydispersity, a study of granular mixtures with additional particle sizes is warranted to determine the role of polydispersity in granular mixing and segregation. In this work, the segregation of binary, ternary and polydisperse mixtures of nuts and spheres in a vertically vibrated cylinder is studied experimentally and computationally. We find that the presence of the other species – besides the smallest and largest size – is responsible for dramatic reductions in the final degree of segregation compared to a binary mixture. In addition, we quantify orders of magnitude reduction in the segregation rate of a mixture of polydisperse spheres compared to a binary mixture. This reduction in segregation coincides with an increase in diffusive and convective mixing and correlates with a lower average system density. Voidage distributions demonstrate the bed packing structure plays an important role in enabling multi-sized systems to remain in a more mixed state as compared to binary systems. Our observations show that this prototypical segregation experiment provides as much insight into why materials do not segregate as why they do.