Elutriation Rate Constant Research Articles

Entrainment of FCC from fluidized beds — A new correlation for the elutriation rate constants Ki∞ *

Batch and continuous experiments of fluidization have been carried out in columns of diameter 76 and 152 mm. Gas velocities of 0.1–0.8 m/s were used with mixtures of coarse and fine FCC as the feed materials. The total entrainment flux E Z and the elutriation rate constants K i∞ * are found to depend on the gas velocity in the form of E Z ∝ U n and K i∞ * ∝ U m , respectively. The values of n and m depend on the bed compositions and particle size; they become smaller at higher gas velocities. For FCC, n has an average value of 4.0 in the velocity range 0.2–0.4 m/s and tends to a value of 2 at higher gas velocities. The effect of column diameter depends on the gas velocity profiles and is reduced for column diameters above 0.1 m. Empirical correlations to predict K i∞ * and E Z have been developed from the results.

Effect of interparticle adhesion forces on elutriation of fine powders from a fluidized bed of a binary particle mixture

Abstract The elutriation of fine powders from a fluidized bed of a fine-coarse particle mixture was investigated in a transparent column with a diameter of 7.1 cm and a height of 102 cm at superficial gas velocities from 0.3 to 0.9 m/s. In the experiments, a silic sand (group B particle) was used as the coarse particles, whereas several kinds of limestone powder varying in Geldart's classification from A to C were used as fines. The elutriation rate constant of the powders was obtained by measuring the elutriation rate of the powders for a concentration of the powders in the bed at a steady state. Experimental results showed that the elutriation rate constant of powders reaches a maximum at the critical powder diameter d p ∗ corresponding approximately to the boundary between group A and C powders, giving an indication of the different elutriation mechanisms between group A and C powders. This finding further confirms the discovery in recent work, contrary to the assumptions implicit in existing correlations for the prediction of elutriation rates of group A and B particles, that changes in the elutriation rate may be attributed to the interparticle adhesion forces. A new empirical correlation for the elutriation rate constant of cohesive powders smaller than the critical powder diameter in a fluidized bed of a binary particle mixture is therefore proposed based on the experimental results by taking into account the powder cohesiveness. Moreover, a critical cohesion number N coh ∗ corresponding to the critical powder diameter was also obtained by which the critical powder diameter can be predicted.

ENTRAINMENT OF FINES FROM FLUIDIZED BEDS WITH AN EXPANDED FREEBOARD

Apart from accomodating cyclones, expanded freeboards are very effective at reducing the particle carryover from fluidized beds by reducing the superficial gas velocity. However, few experimental studies have been done on this aspect of entrainment. Batch experiments on entrainment have been carried out at velocities of 0.2m/s to 0.8 m/s using Group A FCC powders in 76 mm and 152 mm diameter columns and also in a column constructed from a 76 mm diameter section 1.1m high at the bottom having a 152 mm diameter section with a height of 1.3 m above it. This is referred to as the expanded column. Results show that by doubling the diameter of the top section the total entrainment fluxes, Ez , are reduced to about 1% of the original values. The elutriation rate constants, K* i∞ , of fines entrained from fluidized beds with an expanded freeboard are best calculated using as the equilibrium concentrations those in the dense bed, Xbi , but the gas velocities and the cross sectional area used are those based on the larger expanded section of the column.

Elutriation of Fine Particles from a Vortexing Fluidized Bed.

A systematic investigation of elutriation of particles is reported in the cold model of bubbling fluidized bed (BFB) and vortexing fluidized bed (VFB) of 0.19 m diameter and 4.0 m height. Glass beads were used as the fluidized material. The diameter of the coarse particles was 545 μm, the diameter of the fine particles used for the elutriation test ranged from 49 to 194 μm. The results indicate that the specific elutriation rate constant (K∞*) is a function of the initial concentration of elutriated fine particles. The minimum value of K∞* is obtained when the initial concentration of elutriated fine particles is reached 16 wt% in a BFB system. Based on the experimental data obtained from the BFB, a correlation is presented to estimate the K∞*. The K∞* is found to be a function of the particle terminal velocity, gas density, and modified Froude number (Fr*). For a wide particle size distribution (PSD) system, a new parameter, called particle size distribution factor, γ, is introduced to describe the specific elutriation rate constant of wide PSD elutriated fine particles. A method to estimate the specific elutriation rate constant of a multi-component system by using the specific elutriation rate constant of each component obtained from a binary particle mixture system is also presented in this study.

Elutriation of fine powders from a fluidized bed of a binary particle-mixture.

The elutriation of fine powders (Group C or A powders in Geldart’s classification) from a fluidized bed of fines-coarse particle mixtures had been investigated in a transparent column with diameter of 7.1 cm and height of 102 cm. In the experiments, a silica sand (Group B in Geldart’s classification) was used as the coarse particles, whereas several kinds of limestone powder varying in Geldart’s classification from A to C were used as fines. The elutriation rate constant of fine powders was obtained by measuring the elutriation rate of powders, and the concentration of powders in the bed at steady state. The effects of superficial gas velocity, concentration of powders in the bed and diameter of powders on the elutriation rate constant were investigated. Experimental results showed that the elutriation rate constant of powders reaches a maximum at a certain powder size corresponding approximately to the boundary between Group A and C powders, giving an indication of the different elutriation mechanisms between Group A (or B) particles and C powders. This finding is contrary to the assumption made by existing models for the prediction of elutriation rate of Group A and B particles and may be attributed to the powder cohesivity.

95/06307 Effect of secondary superficial air velocity on the elutriation rate of fine particles in a cold-model two-stage swirl-flow fluidized-bed combustor

The elutriation rate of fine particles were measured in a cold model Two-Stage Swirl-Flow Fluidized Bed Combustor(TSSF-FBC). The range of secondary air flow rate was varied from 0.1 m/s to 0.35 m/s while the primary air flow rate was kept to a constant value of 1 m/s. It was found that the elutriation rate of fine particles slightly increased with the secondary superficial air velocity. When the secondary superficial air velocity exceeded over 0.3 m/s, however, the elutriation rate rapidly increased. The elutriation rate of fine particles was strongly dependent upon the swirl intensity. It was exponentially decreased as the swirl intensity increased. The elutriation rate constant increased with the secondary superficial air velocity, and the critical particle diameters were in range of . Under the experimental conditions, the elutriation rate constant of fine particles can be correlated with secondary excess air velocity() and as follows;

Fluidization and entrainment of difficult-to-fluidize fine powdermixed with easy-to-fluidize large particles

Entrainment of silicon fines mixed with large silicon nitride particles and mixed with large silicon particles wasmeasured. A model is proposed to account for attrition and elutriation based on the assumption that the fine powder is in one of three forms: (a) elutriable freely moving fines, (b) agglomerates of fines, (c) fines attached to large particles. Elutriation rate constants and attrition rates were then evaluated at various particle mixing ratios and gas velocities.

予備還元流動層における粉鉄鉱石の流動化および粉化挙動

The integrated test plant, consisting of a prereduction fluidized bed and in-bath smelting furnace characterized by high post combustion, was installed at NKK's Fukuyama Works, and the operation was started in 1988. The prereduction fluidized bed uses iron ore fines as feed, and the fluidizing condition and pressure fluctuation characteristics to keep the stable fluidizing were clarified by the operation of the integrated test plant. The classification behavior of fines was analyzed by the measurement of the elutriation rate constant. Then, the degradation phenomena of iron ore fines were observed during the preheating and prereduction of iron ore fines in the fluidized bed, and it was quantitatively analyzed by the application of Rittinger's law for size reduction. The model including classification and degradation behavior was constructed so as to estimate the operating condition. These results were considered to be availabe in the scale-up of the prereduction fluidized bed.

Fluidization and Degradation Characteristics of Iron Ore Fines in Prereduction Fluidized Bed.

The integrated test plant, consisting of a prereduction fluidized bed and in-bath smelting reduction furnace characterized by high post combustion, was installed at NKK Fukuyama Works, and the operation was started in 1988. The prereduction fluidized bed used iron ore fines as feed, and the fluidizing conditions and pressure fluctuation characteristics to keep the stable fluidization were clarified by the operation of the integrated test plant. The classification behavior of fines was analyzed by the measurement of elutriation rate constant. Then, the degradation phenomena of iron ore fines were observed during the preheating and prereduction of iron ore fines in the fluidized bed, and it was quantitatively analyzed by the application of Rittinger's law for size reduction. The model including classification and degradation behavior was constructed so as to estimate the operating condition. These results were considered to be useful in the scale-up of the prereduction fluidized bed.

Application of the obstruction model to elutriation of more than one species of particles from multicomponent fluidized bed.

In the previous paper, the authors proposed a simple model which describes elutriations of one particle species from bubbling fluidized beds made up of one- or two-component particles, by consideration of the obstructive effect due to suspended particles in the freeboard. In this report the proposed model was applied to the elutriations of more than one species of particles from multicomponent fluidized beds consisting of glass beads, FCC particles and iron powders. The applicability of the model to such systems and the obtained best-fit values of the model parameters were examined thoroughly. It was proved that the model also describes well the elutriation of more than one species of particles. The virtual elutriation rate constant in the proposed model, which is one of the model parameters and means the imaginary elutriation rate constant without the obstructive effect, agreed well with the conventional elutriation rate constant predicted by the correlation proposed by Wen and Hashinger.The other parameter kp was in proportion to the reciprocal of the slip velocity.

On the entrainment of solid particles from a fluidized bed

This paper presents a generalized approach to the phenomenon of entrainment of solids from a fluidized bed. Starting with the discussion of the transport disengaging height (TDH) and of the elutriation of particles above the TDH, one arrives finally at the relationship between the elutriation rate constant, saturation carrying capacity and choking parameters of pneumatic transport.

Effect of particle size on elutriation rate constant for a fluidized bed.

The elutriation rate constant for the gas-solid fluidized bed with and without vertical multi-tube internals was determined by a continuous steady-state operation. Silica sand, FCC, and glass beads of a wide size distribution were used. For fine particles belonging to group A of Geldart''s classification, the elutriation rate was not affected by bed internals. For coarse particles, however, the rate was affected by the internals. The elutriation rate constant for the fluidized bed with multitube internals was affected by the gas velocity, terminal velocity of particles, hydraulic diameter of bed and minimum fluidization velocity. Empirical equations for the elutriation rate constant are presented.

Elutriation of fine particles from batch gas—solid fluidized bed — at sufficiently high freeboard height —

An experimental and semi-theoretical work on the elutriation of fine particles from a fluidized bed was carried out. The fluidizing equipment, consisting of an iron pipe of 3 in i.d., was designed in such a way that its freeboard height can be adjusted properly. At the different freeboard heights and air flow rates, the accumulated weights of fine particles with time were measured. The same procedure was repeated for different initial contents of the fine particles in the bed. In order to interpret the experimental results, a simple model was constructed by assuming that the fine particles were transported only by convection of gas. The model included two parameters; one was a critical height above which the transport of fine particles was controlled by convection of gas, and the other was the elutriation rate constant. The critical height was estimated experimentally. The elutriation rate constant was determined by a method proposed in this work. It was found that the elutriation rate constant was independent of the freeboard height and that its value was consistent with the calculated value based on the correlation of Wen and Hashinger [3]. Further, a physical meaning of the elutriation rate constant was eluciated from this model.

Elutriation and attrition of char form a large fluidized bed

Previous correlations for elutriation of particle fines did not fit the present data obtained in a large fluidized bed using char/sand mixture as the bed material. A new form of correlation (after Zenz and Weil [1]) for the elutriation rate constants, E i, has been developed which gives a good agreement with the experimental data: E/ ϱ g U g = 9.43 × 10 −4 ( U 2 g/ gd p) 1.65. The attrition rate of production of fines of char particles in char/sand fluidized bed has been found to increase with U g— U mf as an exponential function: k a = ae b( U g — U mf ) . The dependence of attrition rate constant on d p appears small, if it is present at all. Attrition would appear to be occuring only in the bubble wakes. A small char layer (without sand) was found on the char/sand bed surface when it was fluidized. The elutriation rate was found not to be significantly influenced by this thin char layer. This is a unique confirmation of the theory of entrainment that the majority of ejected particles originate from the wakes of bubbles which have grown as they rise through the bed.

THE ELUTRIATION RATE FROM A PACKED FLUIDIZED BED

The particle elutriation rate from a packed fluidized bed consisting of particles of almost uniform size is measured for various reduced bed heights, Lmf/LT various reduced gas velocities U/UT, and various packing sizes. An experimental equation for that rate is obtained. The elutriation rate of fine particles from a packed fluidized bed comprising particles of two sizes is also measured under various experimental conditions. That rate is proportional to the concentration of fine particles in the bed. An experimental equation for the elutriation rate constant is obtained Finally, the mechanism of elutriation from the packed fluidized bed is discussed.

Elutriation of solid particles from a dense‐phase fluidized bed

AbstractThe elutriation of fines from fluidized beds of two‐ and multiparticle systems is investigated. Materials used are glass beads and coal powder with air and helium as the elutriating gases.A specific elutriation rate constant which takes into consideration the particle stratification and thus is nearly independent of the bed dimensions is employed in the correlation of the elutriation of fines.The proposed generalized correlation applicable to a system of two particle sizes and to more complex ones is presented. Recorrelation of pertinent literature data by the dimensionless equation indicates good agreement in spite of a diversity of bed geometries. The effect of fluidized‐bed parameters on the rate of elutriation is examined, and applications and limitations of the correlation are shown.