Effect of Volume Fraction of Material on Separation by Density Difference in a Liquid‐Fluidized Bed of Inert Particles

A liquid‐fluidized bed was used to separate a pure material from a mixture. A quantity of relatively large sized material was immersed in an inert‐particle fluidized bed and the behavior of materials was examined for different liquid velocities. In particular, the volume fraction of the material was varied and its effect on the separation characteristics was examined. The material floats on the inert‐particle fluidized bed when the density of the material is lower than the apparent density of the bed, regardless of the volume fraction of the material. The apparent density of the bed can be adjusted by changing the liquid velocity. The materials in the upper portion of the bed affect the properties of the bed below them, i.e., the void fraction decreases and the apparent density increases in the inert‐particle suspension when materials are present in the upper portion of the bed. Therefore, the materials float on the bed although the apparent density of the inert‐particle suspension obtained from the case without material is less than the density of the material at a relatively high volume fraction of material. This phenomenon occurs more easily for lighter and smaller materials. This means that small inert particles and low liquid velocities are the optimum operating conditions for the separation.     

Behavior of Pieces of Plastic Sheet in Solid‐Liquid Fluidized Bed with Stirring

A solid‐liquid fluidized bed of inert particles can be used to separate pure objects from a mixture. Pieces of plastic sheet were selected as the objects to be separated. To estimate the separation characteristics, the behavior of pieces of plastic sheet in the bed was examined experimentally. Slow stirring was used to improve the fluidization state of the bed. The object size and the volume ratio of objects to inert particles were varied. The use of stirring of the bed was effective in improving the fluidization state of the bed, and the objects, which sank in the bed without stirring, moved from the bottom to the upper portion of the bed at a certain liquid velocity. This liquid velocity increases with decreasing object size, and it also increases as the volume ratio of objects to inert particles in the bed increases. When the volume ratio of objects to inert particles is too high, the objects are distributed throughout the entire bed, regardless of the liquid velocity.     

Transition from Low Velocity to High Velocity in a Three Phase Fluidized Bed

The onset liquid velocity demarcating the conventional and the circulating fluidization regimes of three‐phase fluidized beds was determined by measuring the time required to empty all particles in a batch fluidized bed at various liquid and gas velocities. Experiments were performed in a gas‐liquid‐solid circulating fluidized bed of 2.7 m in height using glass beads of 0.508 mm in diameter as solid phase and air and tap water as the fluidizing gas and liquid, respectively. The results show that gas velocity is a strong factor on the onset liquid velocity. Higher gas velocity yields a lower onset liquid velocity. It is also demonstrated that the onset liquid velocity has the same value as particle terminal velocity in a gas‐liquid mixture. Within the gas‐liquid‐solid circulating fluidization regime, the solids circulation rate is increased with the total liquid velocity and the auxiliary liquid velocity.     

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.     

Intensification of mass transfer in liquid fluidized beds with inert particles

The influence of inert particles on liquid/solid mass transfer is studied in fluidized beds by using a binary-mixture of solids of differing size and density. The addition of inert particles of higher density and smaller diameter, e.g. glass beads, exerts remarkable effects on mass transfer coefficients in comparison to that of mono-component active particles at the same liquid velocity. The extent of the effect on liquid–solid mass transfer coefficients increases with an increasing fraction of the small inert particles in the mixture. The liquid–solid mass transfer coefficients for binary-mixtures are well correlated in terms of dimensionless groups and the voidage parameter.     

A simple correlation for solids holdup in a gas-liquid-solid fluidized bed

A simple empirical model was established which allows solids holdup in a gas-liquid-solid fluidized bed containing large and dense particles to be readily predicted based on the equation of Richardson and Zaki (1954, Trans. Inst. Chem. Engrs32, 35) for liquid-solid fluidized bed systems. The approach is applicable both to monocompnent particle systems and to binary mixtures of particles. For a monocomponent system, a correlation for model parameters was proposed which is expressed as a function of particle diameter, particle density, bed diameter and liquid density. For a binary mixture of particles, the averaging and serial approaches were shown to predict the solids holdup equally well within the range of the gas and liquid velocities considered. Experiments were also performed using eight solid particles for the monocomponent system and five binary mixtures of particles differing in diameter and/or density for the mixture system to substantiate the model.     

脱硅中液固循环流化床清洁传热

考察了铝土矿熟料溶出粗液脱硅加热过程中的结垢行为,采用有机-无机复合材质的惰性固体颗粒研究了循环流化床对脱硅加热过程中硅渣结垢的清除及防止性能.结果表明：硅渣结垢曲线为具有诱导期的渐近式曲线,硅渣结垢机理为结晶结垢和微颗粒沉积结垢混合机理;循环流化床不仅能有效防止硅渣结垢的形成,而且能完全清除已有的硅渣垢层,硅渣结垢的清除速率随操作流速及固体颗粒浓度的增大而增大;惰性固体颗粒的引入不影响硅渣结垢的机理.根据液固流化床的防垢机理建立了结垢模型,其预报值与实验值吻合较好.     

Measurement of Solid Circulation Patterns in Liquid—Solid and Gas—Liquid—Solid Fluidized Beds

Solid circulation patterns were determined by measuring collisions between tracer particles and cylindrical probes in liquid—solid and gas—liquid—solid fluidized beds. Special probes were used to obtain two‐ and three‐dimensional views of particle motion. Circulation patterns for 3 and 5 mm glass beads were studied. Mixing cells, which had been formed at low liquid velocities, grew in size and eventually merged as the liquid velocity increased. The flow patterns of smaller particles having the same density as bed particles and particles lighter than bed particles (graphite particles) with the same size were also measured.     

Simulation and experiment of segregating/mixing of rice husk–sand mixture in a bubbling fluidized bed

The fluidization behavior of rice husk–sand mixture in the gas bubbling fluidized bed is experimentally and theoretically studied. The relevancy of the pressure drop profile of rice husk–sand mixture to the definition of its minimum fluidization velocity is discussed, and the minimum fluidization velocity of rice husk–sand binary mixture is determined. The distributions of mass fraction of rice husk particles along the bed height are measured, and the profiles of the mean particle diameter of mixture are determined. A multi-fluid gas–solid flow model is presented where equations are derived from the kinetic theory of granular flow. Separate transport equations are constructed for each of the particle classes, allowing for the interaction between particle classes, as well as the momentum and energy are exchanged between the respective classes and the carrier gas. The distributions of the mass fraction of rice husk particles and the mean particle diameter of binary mixture are predicted. The numerical results are analyzed, and compared with experimental data.     

Clustering behaviour in gas–liquid–solid circulating fluidized beds with low solid holdups of resin particles

The flow in a gas–liquid–solid circulating fluidized bed is self‐organised and manifests itself with clustering of particles and bubbles. The clustering behaviour in the fluidized bed at low solid holdups of resin particles was experimentally investigated with a high‐speed image measurement and treatment technique of complementary metal oxide semiconductor to enhance the fundamental understanding on such a flow. Several new physical quantities were suggested to characterise such ordered flow structures. The main findings are as follows. The clusters of solid particles largely exist as doublets and triplets, the mixed groups of particles and bubbles mostly exist as one bubble carrying two to four particles. Increasing superficial liquid velocity, particle diameter or density weakens the aggregation degrees of both particle and mixed clusters in the riser and downer, except that the increase of superficial liquid velocity enhances the mixed clustering behaviour in the riser. The climbing of the auxiliary liquid velocity or liquid phase viscosity intensifies the aggregation behaviour, except that the increase of liquid phase viscosity reduces the mixed clustering degree in the riser. The influences of superficial gas velocity and surface tension of liquid phase on the clustering behaviour seem to be a little complex and the trends are not simply increasing or decreasing. The life cycle of solid particle clusters in the GLS riser is not sensitive to the operation conditions, being around 0.07 s. The mixed clusters' life cycle is more sensitive to the conditions and physical properties of phases, changing from 0.02 to 0.07 s.     

Bed Expansion and Particle Classification in Liquid Fluidized Beds with Structured Internals

The inclusion of a structured packing as internal in a liquid‐solid fluidized bed allows expansion of the liquid velocity operation range before elutriation, promoting the liquid solid contact and mixing. The bed expansion of liquid‐solid fluidized beds provided with structured packing as internals is examined, for solids denser than the liquid phase and within a wide range of operating conditions. A correlation to estimate the bed expansion in liquid‐solid fluidized beds using structured packing as internals is developed. In addition, the feasibility of employing structured packing as internals for favoring classification of different density particles is demonstrated by analyzing the mass elutriated from the column at different liquid velocities for single particles or binary mixtures.     

气-固环流反应器内瞬态流体力学特性的数值模拟

采用双流体模型和颗粒动力学理论,并考虑颗粒团聚现象对气固相间曳力的影响,对气-固环流反应器内的流体力学特性进行了数值模拟。模拟的时均固含率和颗粒速度与实验数据具有较好一致性,验证了模拟方法的可靠性。模拟结果表明：气-固环流反应器内瞬态固含率的分布具有典型聚式流态化的非均匀特征;压力脉动沿床层轴向的分布在一定程度上定性反映了气泡运动的信息;颗粒速度的时间序列和概率密度分布函数表明,床层各径向位置均存在颗粒的向上、向下运动,颗粒主体在床层内向上运动的同时还存在微观的内循环运动,模拟值为颗粒时均速度的径向非均一性宏观分布提供了合理的微观解释。     

Scale‐dependent nonequilibrium features in a bubbling fluidized bed

We investigate experimentally the nonequilibrium features in a pseudo 2‐D bubbling fluidized bed. Velocities of individual particles are measured by using a particle tracking velocimetry (PTV) method, and void fractions are obtained with the Voronoi tessellation. A bimodal shape of probability density function (PDF) for particle vertical velocity is found in not only time‐averaged but also time‐varying statistics, which is caused by the transition between the dense and dilute phases and breaks the local‐equilibrium assumption in continuum modeling of fluidized beds. The results of time‐varying radial distribution function and voidage distribution also confirm this finding. Moreover, the analysis of voidage, particle velocity, granular temperature and turbulent kinetic energy of particles shows that there is no scale‐independent plateau over the interface, and it seems hard to find a scale‐independent plateau to separate the micro‐ and meso‐scales of fluidized beds, which require sub‐grid meso‐scale modeling for continuum or coarse‐graining methods of gas‐fluidized systems. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2364–2378, 2018     

Dynamic characteristics of the volatile cloudy zone in a gas–solid fluidized bed with hydrocarbon liquid spray

In a gas–solid fluidized bed with continuous hydrocarbon liquid spray, a volatile “cloudy zone” could be formed, defined as a dynamically steady liquid-affected zone, including droplets, wet particles, and the gas which passes through the zone. A new flow pattern with the dynamic coexistence of cloudy zone and non-cloudy zone (gas–solid zone), is accordingly established. The temperature, particle concentration, and particle velocity fields are measured in real-time via infrared thermography and particle imaging velocimetry, respectively. Results show that the area and range of central position of the cloudy zone illustrate a heavier fluctuant trend with the increasing velocity of liquid spray, and the main frequency of area fluctuation is close to that of the bubble rising. Moreover, the particle concentration and particle velocity in the cloudy zone are lower than those in the non-cloudy zone, breaking the conventional symmetrical distributions of hydrodynamic parameters of particles in a gas–solid fluidized bed.     

气固环流燃烧器内颗粒流动行为

针对石油焦及气化余焦的燃烧特点和流态化特性,提出了一种采用气固密相环流烧焦与快速床管式烧焦技术相组合的新型燃烧器结构。在不同操作条件(导流筒区表观气速0.772～1.674m.s-1,环隙区表观气速0.223～0.519m.s-1,装置系统的颗粒外循环强度40.8～229.4kg.m-2.s-1)及两类颗粒体系下,采用光纤测量仪对组合燃烧器环流段内颗粒流动特性进行了系统的实验研究。结果表明,两类颗粒体系的固含率和颗粒轴向速度在导流筒区、底部区和颗粒分流区床层内沿径向的分布规律为中心区小、边壁区大的环-核型分布,体现了气固流化床典型聚式流态化的非均一性特征;在环隙区,受环流段结构的影响,两类颗粒体系的固含率和颗粒轴向速度参数沿床层径向的分布相对较均匀;混合颗粒体系的固含率、颗粒轴向速度较单一石英砂颗粒体系的要小,细颗粒的加入在一定程度上能改善气固混合的均匀程度;两类颗粒体系在底部区和颗粒分流区的径向流动具有剪切破碎气泡的作用,有利于环流段内气固的充分混合接触。     

INFLUENCE OF THE PARTICLE SIZE AND SUPERFICIAL GAS VELOCITY ON THE SUBLIMATION OF PURE SUBSTANCES IN FLUIDIZED BEDS OF DIFFERENT SIZES

In the present study the sublimation of large solid carbon dioxide particles inside fluidized beds of fine particles is investigated. A model which takes the surface area of the sublimable particles into account is used to describe the sublimation kinetics. Based on this model, the results of different experiments, namely single particle experiments using a precision scale, batch experiments in a laboratory-scale fluidized bed and continuous experiments in a larger circulating fluidized bed are compared. The main focus of the study is to evaluate the influences of the particle size, of the inert bed material, of the bed temperature and of the superficial gas velocity, respectively.     

Demarcation of a new circulating turbulent fluidization regime

Transient flow behaviors in a novel circulating‐turbulent fluidized bed (C‐TFB) were investigated by a multifunctional optical fiber probe, that is capable of simultaneously measuring instantaneous local solids‐volume concentration, velocity and flux in gas‐solid two‐phase suspensions. Microflow behavior distinctions between the gas‐solid suspensions in a turbulent fluidized bed (TFB), conventional circulating fluidized bed (CFB), the bottom region of high‐density circulating fluidized bed (HDCFB), and the newly designed C‐TFB were also intensively studied. The experimental results show that particle‐particle interactions (collisions) dominate the motion of particles in the C‐TFB and TFB, totally different from the interaction mechanism between the gas and solid phases in the conventional CFB and the HDCFB, where the movements of particles are mainly controlled by the gas‐particle interactions (drag forces). In addition, turbulence intensity and frequency in the C‐TFB are significantly greater than those in the TFB at the same superficial gas velocity. As a result, the circulating‐turbulent fluidization is identified as a new flow regime, independent of turbulent fluidization, fast fluidization and dense suspension upflow. The gas‐solid flow in the C‐TFB has its inherent hydrodynamic characteristics, different from those in TFB, CFB and HDCFB reactors. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Three-dimensional numerical modelling of interactions between a gas–liquid jet and a fluidized bed

This paper presents the development of a novel mathematical model that describes spray injection and spreading into a fluidized bed of solid particles. The model also includes the gas–liquid flow through the nozzle followed by the gas-assisted atomization. An Eulerian approach that is independent of the nature of the continuous phase is adopted for all phases, which are gas (or bubbles), liquid (or droplets), and solid particles that may be covered with a liquid layer. Variation in sizes of bubbles and droplets is represented by the particle number density approach that takes into account both break-up and coalescence. The atomization is considered as a catastrophic phase inversion triggered by a critical local volume fraction. New relationships were obtained for liquid spreading due to wet particle collisions and for heat conduction between a solid particle and a surrounding liquid layer. The model is applied to simulate liquid injection into the fluidized bed for conditions that were previously experimentally studied and published. The comparison reveals a reasonable agreement in prediction of the cumulative liquid distribution for two experimental cases. In addition, we evaluated a jet penetration distance with the model to compare it with the one measured in another set of experiments. This comparison also yields a good qualitative agreement. Finally, we evaluated the influence of the fluidization velocity on liquid distribution in the bed.     

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.     

倒锥型液固流化床减小液相返混型的研究

It is found analytically that the parabolic radial profile of liquid velocity in cylindrical liquid-solid fluidized bed (LSFB) causes particles to circulate around some radial position. This is the main reason for liquid-phase axial dispersions. The liquid-phase axial dispersion is depressed as the liquid velocity presents a flatter Bessel radial profile in a converging taper LSFB. The void fraction increases with axial distance in converging taper LSFB.The behavior produces less liquid-phase axial dispersion. Exverimental results show good coincidence.