Fluidized Bed Of Fine Particles Research Articles

DPM Simulations of A-Type FCC Particles’ Fast Fluidization by Use of Structure-Dependent Nonlinear Drag Force

Nonlinear drag force has been a research frontier in complex gas-solid systems. The literature has reported that the commonly-used drag correlations often overestimate drag force and, thus, cause unrealistic homogeneous flow structures in gas-solid fluidized beds of fine particles. For solving this problem, the structure-dependent drag model, derived from energy-minimization multi-scale approach, is used in discrete simulations of fluid catalytic cracking particles in a small riser. The gas phase is dealt with by computational fluid dynamics. Particles are considered as a discrete phase and described by Newton’s second law of motion. Gas-particle phases are coupled according to Newton’s third law of motion. Simulations show that use of structure-dependent drag model results in drag reduction, the effect of which is not so apparent as that in simulations of the two fluid model. The particle clustering tendency, however, is more distinct and leads to more heterogeneous flow structures in riser flow with a much greater amplitude of outlet solid flux fluctuations. Moreover, the behaviors of particle and gas back-mixing can be captured in the present simulations, which was supported by past simulations and experimental data. The simulation time resolution is discussed. The spring constant can be artificially brought down for safe setting of larger time step when modelling the collision process between fine particles with a higher calculation load. To appropriately mimic the continuous decay of van der Waals force may, however, need a much smaller time step. There is also an obvious effect of space resolution on simulations. When using a grid size smaller than 3 times the particle diameter, the simulated clusters turn extraordinarily large, and the effect of gas-solid back-mixing turns insignificant.

Heat transfer from an immersed fixed silver sphere to a gas fluidised bed of very small particles

Results of unique heat transfer measurement in beds of fine, cracking catalyst particles, fluidized by air or helium gas, are compared with predictions from a theoretical model presented in the literature, and also with an earlier established empirical correlation. Moreover, the results have been related to dense phase flow conditions around a silver heat transfer probe by a simple turbulence model. A maximum heat transfer coefficient of h = 2300 W/m2K has been measured in a bed of 14?m (average diameter) particles, fluidized by helium gas. The data collected, and the model developed, can be used for the design of heat transfer tubes in fluidized beds of fine particles as for instance in fluid catalytic cracking (FCC) of crude oil heavy residues. The FCC is one of the most important conversion processes in the petroleum refineries.

CPFD simulation of bed-to-wall heat transfer in a gas-solids bubbling fluidized bed with an immersed vertical tube

Based on previous experimental study, CPFD method was used to simulate the bed-to-wall heat transfer between an immersed vertical heat tube and bed material in a gas-solids fluidized bed of fine particles. Radial and axial profiles of heat transfer coefficients at different superficial gas velocities, as well as their circumferential profiles around heat tube surface were obtained. By comparing with experimental results, it is found that trends of the predicted radial and axial profiles of heat transfer coefficients at different superficial gas velocities agree well with experimental results. However, all the simulated values of heat transfer coefficients are smaller than experimental values due to the default heat transfer coefficient correlation for dense particle phase in the built-in heat transfer module of Barracuda. There exists strong heterogeneity on the distribution of heat transfer coefficient around the circumference of the heat tube, especially near the column wall. The circumferential uniformity of heat transfer coefficient becomes worse as heat tube moves to the column wall and at increasing superficial gas velocities. Both averaged and instantaneous results show a good linear correlation between heat transfer coefficient and solids renewal flux, demonstrating the dominant role of solids renewal on the efficiency of fluidized heat exchangers.

Dynamic forces on a horizontal slat immersed in a fluidized bed of fine particles

Forces on a single horizontal slat immersed in a fluidized bed of Geldart A particles were investigated for the purpose of providing useful design guidelines for long-period reliability of horizontal baffles in industrial reactors. The characteristics of slat forces in the fluidized and de-fluidized states of a reactor were measured using adhered strain gauges. The main parameters influencing these characteristics were superficial gas velocity, installation height, inclination angle, and method of slat installation. The experimental results show that the measured force on the slat in the fluidized state is highly fluctuating, but its root-mean-square is usually significantly lower than for the de-fluidized state, especially at the bottom of the bed. The effects of various operating and structural parameters were determined. Critical locations and operating conditions under high load are indicated and warrant more attention in structural design of horizontal baffles.

Characterization of the upward motion of an object immersed in a bubbling fluidized bed of fine particles

The axial motion of HDPE, Acetal, and PTFE spheres immersed in a 3-D bed of the conventional fluidization materials was studied using the non-invasive radioactive particle tracking (RPT) technique. Coarse and fine sand as well as FCC were chosen as the fluidization material. The experiments were carried out at two excess gas velocities, i.e. Ue=0.25m/s and Ue=0.50m/s.Rise of an object immersed in a bubbling fluidized bed can be descried by two characteristic velocities, i.e. rise and upward velocity. When circulation pattern is fully established, the mean upward and rise velocity to the mean bubble velocity ratios become comparable. It is shown that the object rise takes place principally in the drift of bubbles. The mean object rise velocity is primarily influenced by the excess gas velocity. Comparatively, the ratio between the physical properties of the fluidization medium and the object such as size and density has trivial impact on the rise velocity of the object. The mean object upward velocity can satisfactorily be predicted with a theoretical model with no adjustable parameter.The frequency of the object rise along the bed is independent of the object properties but influenced by the fluidization material. Comparing to coarse sand and FCC, the objects immersed in fine sand rise less frequently. This is attributed to the relatively longer residence time of the object in the emulsion phase of fine sand particles. The intermittent circulation of the medium and large size Acetal spheres in beds of FCC and fine sand is featured by the low upward and rise object velocities and the very limited height travelled by the object.

Long-range interactions in randomly driven granular fluids

We study the long-range spatial correlations in the nonequilibrium steady state of a randomly driven granular fluid with the emphasis on obtaining the explicit form of the static structure factors. The presence of immobile particles immersed in such a fluidized bed of fine particles leads to the confinement of the fluctuation spectrum of the hydrodynamic fields, which results in effective long-range interactions between the intruders. The analytical predictions are in agreement with the results of discrete element method simulations. By changing the shape and orientation of the intruders, we address how the effective force is affected by small changes in the boundary conditions.

Heat transfer characteristics in a pressurized fluidized bed of fine particles with immersed horizontal tube bundle

The effect of pressure on the average and local heat transfer coefficients between a submerged horizontal tube (25.4mm-O.D.) and a fluidized bed has been determined in a fluidized-bed-heat-exchanger (FBHE; 0.20×0.26×0.58m-high) of silica sand particles. The heat transfer coefficients were measured around the tube circumference by thermocouples. The average heat transfer coefficient (havg) exhibits a maximum value with variation of gas velocity (Ug) irrespective of pressure. The havg increases with increasing pressure at a given fluidizing number (Ug/Umf) due to increase of gas density and improvement of fluidizing quality with indication of low standard deviation of solid holdup fluctuation. Variation of instantaneous local heat transfer coefficient (hi) is a function of bubble behavior around the tube surface. The hi exhibits higher values at the bottom than top location of the tube, and the highest value at the side of the tube (0°) at the given bed pressure. Instantanous hi variations at the top regions (+45°, +90°) becomes much uniform with high peak frequency at higher pressures. The obtained maximum heat transfer coefficients (hmax) in terms of the maximum Nusselt numbers (Numax) have been correlated with Archimedes, Prandtl and Froude numbers.

Bubbling behavior of a fluidized bed of fine particles caused by vibration-induced air inflow

We demonstrate that a vibration-induced air inflow can cause vigorous bubbling in a bed of fine particles and report the mechanism by which this phenomenon occurs. When convective flow occurs in a powder bed as a result of vibrations, the upper powder layer with a high void ratio moves downward and is compressed. This process forces the air in the powder layer out, which leads to the formation of bubbles that rise and eventually burst at the top surface of the powder bed. A negative pressure is created below the rising bubbles. A narrow opening at the bottom allows the outside air to flow into the powder bed, which produces a vigorously bubbling fluidized bed that does not require the use of an external air supply system.

Evaluating solids dispersion in fluidized beds of fine particles by gas backmixing experiments

In this study, solids dispersion coefficients in fluidized beds of fine Group A particles were derived by modeling the data obtained in steady-state gas tracer experiments with a two-phase gas mixing model proposed by May [May, W.G., 1959, Fluidized-bed reactor studies. Chem Eng Prog 55 (12): 49–56]. Both baffle-free and baffled fluidized beds of FCC particles were tested in a two-dimensional cold model column with broad operating conditions covering both bubbling and turbulent flow regimes. Modeled results showed the axial solids dispersion coefficient of baffle-free fluidized bed ranges from 0.01 to 0.13 m 2/s, increasing monotonously with increasing superficial gas velocity in both bubbling and turbulent flow regimes, agreeing well with previous solids-tracer studies with similar particles and operating conditions. The modeled results in baffled fluidized beds showed that the axial solids dispersion coefficient is significantly reduced when multilayer louver baffles are inserted in the bed, demonstrating louver baffle's strong suppression on solids backmixing. The suppression on solids backmixing by louver baffles augments with increasing superficial gas velocity. However, for the multilayer mesh-grid baffles, effective suppression of solids backmixing only functions at u 0 ≥ 0.6 m/s. As superficial gas velocity increases, the axial solids dispersion coefficients change in a similar trend as the axial gas dispersion coefficients in both baffle-free and baffled fluidized beds. The effects of different baffles on solids mixing are also similar to those on gas mixing. This demonstrates the close relationship between gas mixing and solids mixing in fluidized bed of fine particles.

A Practical Method to Estimate the Bed Height of a Fluidized Bed of Fine Particles

AbstractKnowledge of both dense bed expansion and freeboard solids inventory are required for the determination of bed height in fluidized beds of fine particles, e.g., Fluidized Catalytic Cracking (FCC) catalysts. A more accurate estimation of the solids inventory in the freeboard is achieved based on a modified model for the freeboard particle concentration profile. Using the experimentally determined dense bed expansion and the modified freeboard model, a more practical method with improved accuracy is provided to determine the bed height both in laboratory and industrial fluidized beds of FCC particles. The bed height in a fluidized bed can exhibit different trends as the superficial gas velocity increases, depending on the different characteristics of the dense bed expansion and solids entrainment in the freeboard. The factors that influence the bed height are discussed, showing the complexity of bed height and demonstrating that it is not realistic to determine the bed height by a generalized model that can accurately predict the dense bed expansion and freeboard solids inventory simultaneously. Moreover, a method to determine the bed height, based on axial pressure fluctuation profiles, is proposed in this study for laboratory fluidized beds, which provides improved accuracy compared to observation alone or determining the turning points in the axial pressure profiles, especially in high‐velocity fluidized beds.

Simulation of bubbling fluidized bed of fine particles using CFD

Computational fluid dynamics (CFD) simulation for bubbling fluidized bed of fine particles was carried out. The reliability and accuracy of CFD simulation was investigated by comparison with experimental data. The experimental facility of the fluidized bed was 6 cm in diameter and 70 cm in height and an agitator of pitched-blade turbine type was installed to prevent severe agglomeration of fine particles. Phosphor particles were employed as the bed material. Particle size was 22 μm and particle density was 3,938 kg/m3. CFD simulation was carried by two-fluid module which was composed of viscosity input model and fan model. CFD simulation and experiment were carried out by changing the fluidizing gas velocity and agitation velocity. The results showed that CFD simulation results in this study showed good agreement with experimental data. From results of CFD simulation, it was observed that the agitation prevents agglomeration of fine particles in a fluidized bed.

Modeling the Injection of Gas‐Liquid Jets into Fluidized Beds of Fine Particles

AbstractContacting of gas‐liquid jets with particles/ catalyst is essential in many fluidization processes. This paper discusses a momentum conservation model to determine the entrainment rate of solids and gas into a gas‐liquid jet, injected in a fluidized bed. A novel experimental technique developed by Felli (2002) is used to verify the model results. Correction factors to the initial momentum calculated from the homogenous model are addressed. The model predictions are in good agreement with the experimental data. The present model can be used to characterize the behaviour of gas‐liquid jets (fine sprays) injected into fluidized beds of fine particles.

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.

Technique to Produce Particles from Melts, Suspensions or Solutions by Solidification of Drops in a Fluidized Bed

The stability of large drops consisting of melts and suspensions is examined when these drops are solidified in a fluidized bed of fine particles. The results show that solidified drops at boundary conditions remain spheres at We′ < 20. The We′ – number represents the ratio of deceleration force to capillary force.

96/00706 Lateral mixing of coarse particles in fluidized beds of fine particles

96/00706 Lateral mixing of coarse particles in fluidized beds of fine particles

Expansion behaviour of confined fluidized beds of fine particles

AbstractThe expansion behaviour of fluidized beds of fine particles confined within packings of coarse spheres has been studied experimentally. An expansion law of the Richardson‐Zaki type has been demonstrated to be valid in a wide range of voidages for fine particles belonging to A as well as to B group of Geldart's classification. Parameters of the proposed correlation have been evaluated and their dependence on fines properties discussed.

Simultaneous heat and mass transfer between a fluidized bed of fine particles and immersed coarse porous particles

L'etude experimentale fait apparaitre deux regimes hydrodynamiques differents selon la valeur de la vitesse du gaz qui aboutissent pour chacun d'entre eux a caracteristiques differentes pour le transfert de chaleur et le transfert de matiere. Le systeme etudie est constitue de spheres d'alumine poreuses dans un lit fluidise contenant des particules de sable

Emulsion phase expansion and sedimentation velocity in fluidized beds of fine particles.

Mesure pour des lits de fines particules de : la vitesse minimale de fluidisation (Vmf), la vitesse de sedimentation (Vs) et la desaturation de la phase emulsion (E). La relation entre la vitesse et la desaturation est etablie experimentalement a partir de l'equation de Richardson et Zaki (pour Vmf et Vs). En ce qui concerne E, il est correle a un parametre adimensionnel qui prend en compte l'effet de la densite de gaz. Les resultats obtenus a l'aide des correlations sont en bon accord avec les resultats experimentaux

The effect of pressure on the flow of gas in fluidized beds of fine particles

The division of gas flow between the bubble and dense phases of fluidized beds of six different types of Group A powders has been studied at pressures of up to 20 bar using surface collapse and X-ray absorption measurements. It was found that with these fine powders as pressure increases at constant volumetric gas flowrate so the size and hold-up of bubbles decrease while their frequency increases. Contrary to previous measurements the average bubb velocity appears to decrease with increasing pressure. The dominant mode of bubble break-up in all the powders was found to be division from the rear, contrast to that observed with Group B powders at atmospheric pressure. Interstitial phase voidage was found to increase with increasing pressure. The results are interpreted in terms of a model which assumes a difference between the voidages, and hence the gas flow, of powder in the wakes behind