Elutriation Rate Constant Research Articles

Solids inventory and external solids circulation rate in risers of circulating fluidized beds

• Solids flow rate in riser exit F s depended on riser solids amount M s . • F s depended on elutriation rate constant. • F s for circulating fluidized bed is similar with that for batch riser. • F s decreased with increasing riser diameter D. • M s increased with D. This study was conducted to provide improved relationships on the external solids circulation rate and solids inventory in the riser of the circulating fluidized bed (CFB) through increasing the basic understanding on how the solids flow rate in the riser exit establishes. The relation between the solids inventory in the riser and the solids flow rate in the riser exit was investigated in batch elutriation and CFBs, using air as fluidizing gas at atmospheric pressure and temperature. A batchwise riser (0.1 m-i.d., 2.75 m-height) and the riser of a CFB system (0.1 m-i.d., 2.5 m-height) were used to measure the relation for different gas velocities (1.5–2.4 m/s) and groups of solids (0.064–0.201 mm in diameter, 2045–4080 kg/m 3 in apparent density). The batch elutriation indicated that the amount of solids inventory in the riser determined the solids flow rate in the riser exit, increasing with the amount of the solids inventory in the riser. The solids flow rate in the exit of the riser strongly depended on the elutriation rate constant combined with the amount of the solids inventory in the riser. When the riser was same in size, the relationship on the solids flow rate in the riser exit, derived in the batch elutriation, was valid in the CFB. Relationships on the external solids circulation rate and solids inventory in the riser of the CFB were proposed based on the data from this study and the literature. It was found that the external solids circulation rate per cross-sectional area of the riser decreased with increasing the riser diameter, however, the static bed height of solids in the riser (h st,c ) increased with the riser diameter. The dependency of h st,c on riser diameter was greater in the fast bed than in the bed of pneumatic conveying.

Elutriation and agglomerate size distribution in a silica nanoparticle vibro-fluidized bed

Fluidization of nanoparticle agglomerates is a promising technique to process nanoparticles. However, possible elutriation of small agglomerates may cause significant loss of bed material. To obtain the elutriation behavior under stable operation, in this study the elutriation fraction of silica nanoparticle agglomerates is measured in a vibro-fluidized bed, which is operated for several hours. Among conditions with different fluidizing gas velocities, Ug, and vibration strength, Λ, the lowest elutriation fraction measured is around 5% after 7-hour fluidization. The elutriation fraction increases significantly with Ug, while varies slightly with Λ. To help elucidating the elutriation behavior, the agglomerates at three different locations (bed surface, splash zone, and bed outlet) are sampled and their size distributions are determined. The elutriation rate constant is found to be much smaller than the literature results for ordinary particles, and the reasons are discussed in detail. Finally, an empirical correlation considering size distribution is proposed to fit the elutriation rate constant for the conditions in this study.

Elutriation characteristics of multi-component mixtures in a fluidized bed pyrolysis process

Multi-component mixtures were prepared to simulate a bed of walnut shells undergoing pyrolysis in acold fluidized bed. Four mixtures with different ratios of walnut shell, semi-chars, and clean char were used. Batch elutriation experiments using these mixtures were performed in a transparent fluidized bed to allow direct observation of the flow patterns and particle composition. The results indicated that the bubble wake rather than bubble nose was primarily responsible for particle entrainment at higher gas velocities, and that coarser particles would be “transformed” into elutriable particles. Dimensionless gas velocities ranged from 0.2 to 1.0 and an exponential decrease in particle concentration with respect to bed height was observed. The transport disengagement height (TDH) gradually increased with the gas velocity until the entire bed layer enters a pneumatic transport state. Notably, larger TDHs were required when the bed contained a larger fraction of light components. Two characteristic parameters were used to evaluate particle elutriation: the elutriation rate constant (K), and the residual volatile content of the elutriated particles (v′). These parameters were used to optimize operating gas velocity for the fluidized bed.

Elutriation of fines from binary particle mixtures in bubbling fluidized bed cold model

The elutriation of fine particles from a binary mixture of particles with different densities and diameters has been investigated in bubbling fluidized bed (BFB). A series of experiments were conducted in a 10cm diameter, 170cm tall cylindrical bubbling fluidized bed under various operating conditions. Bed materials with different particle sizes, ranging from 93μm to 1000μm powder, and particle densities ranging from 7.9 to 2.45g/cm3 were used in these experiments. Various combinations of these solids were mixed and fluidized at several superficial gas velocities. Solid elutriation was measured by collecting the carryover solids exiting the column with a filter. Experimental data on the effects of particle density, particle size, and gas velocity on the elutriation of particles from bubbling fluidized bed of binary mixture are examined. Influence of weight fraction of fines in the binary mixture on coarse particles was also investigated and discussed. The results indicated that the elutriation rate constant increases with increasing superficial gas velocities and weight fraction of fines in the bed. A generalized correlation for the elutriation rate constant is proposed using the ratio of Ug/Ut, weight fraction of fines, and fines to coarse particle density ratio.

The sequential elutriation behavior of wide particle size distributions

Batch elutriation of a metallurgical-grade silicon powder with a wide particle size distribution in a laboratory scale fluidized bed was studied, highlighting the influence of carryover polydispersity. The smallest elutriable fines, namely superfines (<10μm), whose terminal velocity Ut is far lower than the superficial gas velocity Ug are entrained first, while the largest elutriable particles (Ut≈Ug) begin to be entrained with a delay that is as long as the time required for the superfines to leave the bed, thus inducing sequential elutriation. When no superfines were present, the entrainment was not delayed. This peculiar phenomenon was observed at all of the tested gas velocities (0.05–0.2ms−1) and for different wide particle size distributions belonging overall to the Geldart group A. The superfines thus seem to strongly limit the elutriation of the larger elutriable particles. In addition, the elutriation rate constants were found to increase with increasing superficial gas velocity and with decreasing particle size. When superfines were present, the elutriation rate constant leveled off under a critical size. Increasing the superfine particle content appears to reduce the elutriation rate constant of all of the elutriable particles. These phenomena are related to interparticle interactions within the bed and/or the freeboard and confirm the importance of polydispersity in the elutriation behavior.

Hot syngas filtration in the freeboard of a fluidized bed gasifier: Development of a CFD model

An Eulerian–Lagrangian CFD model was described and validated against experimental findings, able to simulate the fluid dynamic behavior of gas and solid particles inside a fluidized bed freeboard where filtration candles are inserted, with the aim to integrate the steam gasification of biomass and the hot gas cleaning system into one reactor vessel. Fluidization tests with a cold model consisting of a bed of sand particles containing also a fraction of Geldart group A/C fine powder were carried out batchwise in a 10-cm ID column, at different operating conditions, to quantify solid ejection into the freeboard zone. A Particle Elutriation Model (PEM) was proposed to set the boundary condition at the bed surface (freeboard inlet) for fine particles. This model accounted for elutriation and attrition based on the assumption that the generation of fines by attrition is a nonlinear function of time and depends on the percentage of agglomerated fines. By fitting the experimental data with PEM equations, the elutriation rate constants and attrition rates were evaluated at various particle diameters, and it could be observed that the entrainment rate at low air velocities was affected by interparticle adhesion forces. The PEM was then interfaced with the CFD open source software MFIX in order to simulate transport of particles in the freeboard, the deposition of fine particles on the filter candle and thus the pressure drop profile as a function of time due to cake formation on the candle surface. A k-ε model was used for turbulence. A Discrete Random Walk (DRW) model was implemented to consider the particles turbulent dispersion. 2D simulations of the freeboard were carried out at different static bed height (10 and 20cm), superficial gas velocity (11 and 16cm/s) and mass of fines initially charged inside the bed. The numerical predictions were compared with experimental results obtained when a filtration candle is inserted in the freeboard of a bubbling fluidized bed system with similar dimensions and operated at the same conditions as those simulated by the model. In order to match experimental and numerical results, it was necessary to assume that clusters of very fine particles (with a diameter smaller than 20μm) were transported in the freeboard. The results of the numerical model were in fair agreement with the experimental results.

Effect of fines hold-up in the freeboard on elutriation from a fluidized bed

Elutriation of fines within a bubbling fluidized bed of a two-particle system was investigated. The coarse particles were glass beads with a mean size of 545 μm. The fine particles with mean sizes of 81, 97, 115, 137 and 163 μm were used, respectively. Fines elutriated from the fluidized-bed column were weighted continually by an electronic balance (OHAUS TP4KD). Weight of fines hold-up in the freeboard was measured by the method of the pressure drop. The first-order equation was used to determine the elutriation rate of fines. Weight of fines hold-up in the freeboard was removed from the bed weight. The results show that the elutriation rate constant increases with superficial gas velocity. Also it was found that the elutriation rate constant decreases with increasing fine particle size and the freeboard height. In this study, the elutriation rate constant is lower than that from the first-order equation which does not consider fines hold-up in the freeboard indicating that the previous researchers overestimate the elutriation rate constant. Moreover, a generalized correlation was proposed correlating with superficial gas velocity, fine particle size and the freeboard height.

Entrainment Rates and Elutriation Rate Constants of Bed Particles and Group C Fine Powder in a Fluidized Bed and in a Circulating Fluidized Bed

The effects of particle size and weight fraction of fine powder (group C) upon the entrainment rate of bed particles (groups B + C or groups A + C) and fine powder were investigated in a fluidized bed and in a circulating fluidized bed. The entrainment rates and elutriation rate constants of the bed particles and fine powder were strongly affected by the particle size and the weight fraction of fine powder in the bed. A clear difference was observed in the dependence on the weight fraction of fine powder in both the entrainment rate of bed particles and the ratio of the elutriation rate constant of fine powder to that of coarse particles between the B–C and A–C binary particle systems. When fine powder of 6 μm or smaller was added, the elutriation rate constant of fine powder decreased as the mean diameter of the bed particles decreased from 40 μm. On the other hand, addition of group C powder of larger than 10 μm to group A particles showed very good fluidization with the large entrainment rate of the bed particles.

Model of Fluidized Bed Elutriation: Effect of the Initial Mass of Fine Particles in the Bed

A model is proposed to account for elutriation and attrition based on the assumption that the generation of fine particles by means of attrition depends on the percentage of the mass of fine particles in the bed. Elutriation rate constants and attrition rates were evaluated for various types of particles (TiO2, CaCO3). A decrease in the elutriation rate constant and in the attrition rate of the model was observed when the fines loaded in the bed increased. The decrease in the elutriation and attrition rates was significant when the powder cohesiveness diminished. The effect of the initial mass of fine particles on fluidized bed elutriation is also discussed.

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 TiO 2 powder of 0.5– 32 μm were used as fines. FCC, alumina, Al(OH) 3, limestone, silica sand, SiC particles of 44– 97 μm 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.

Empirical correlation for prediction of the elutriation rate constant

In vessels containing fluidized solids, the gas leaving carries some suspended particles. This flux of solids is called entrainment, E or carryover and the bulk density of solids on this leaving gas stream is called the holdup. For design we need to know the rate of this entrainment and the size distribution of these entrained particles Rim in relation to the size distribution in the bed, Rib, as well as the variation of both these quantities with gas and solids properties, gas flow rate, bed geometry and location of the leaving gas stream. Steady-state elutriation experiments have been done in a fluidized bed 0,2 m diameter by 2,94 m high freeboard with superficial gas velocities up to 1 m/s using solids ranging in mean size from 0,15 to 0,58 mm and with particle density 2660 kg/m3. When the fine and coarse particles were mixed, the total entrainment flux above the freeboard was increased. None of the published correlations for estimating the elutriation rate constant were useful. A new simple equation, which is developed on the base of experimental results and theory of dimensional analyses, is presented.

A correlation of the elutriation rate constant for adhesion particles (group C particles)

Abstract The elutriation of fine particles (groups C or A particles) from a fluidized bed of fine–coarse particle mixtures was investigated at steady state. FCC and alumina particles were used as fines, Alumina, FCC, PVC, SiC and silica sand particles (groups A or B particles) were used as coarse particles, respectively. The elutriation rate constant of group C particles (adhesion particles) decreased with increases in the mean diameter of bed particles and with decreases in the size of elutriated particles under the condition of a constant superficial gas velocity. This finding is quite different from that of elutriation of groups A or B particles. The elutriation rate constant of group C particles was hardly affected by the density of coarse particles in the bed. To correlate the elutriation rate constant for adhesion particles, the particle size coefficient (Cps) is introduced. Empirical equations of elutriation rate constant for cohesive particles were obtained. The proposed correlation well predicts the values of elutriation rate constant of cohesive particles.

Effect of electrostatic and capillary forces on the elutriation of fine particles from a fluidized bed

Abstract The elutriation of fine particles (group C or A particles) from a fluidized bed of fine-coarse particle mixtures was investigated in a steady state. Alumina particles were used as fine; PVC, alumina, SiC and silica sand particles (group A or B particles) were used as coarse particles. The effect of the relative humidity on the elutriation rate constant was investigated for different elutriated particle sizes and bed particles. There were three different zones between the elutriation rate constant and the relative humidity. At the lower relative humidity, the elutriation rate constant increased with increasing relative humidity. When the relative humidity reached a certain value, the elutriation rate constant was not affected by the relative humidity. We defined this relative humidity as the electrostatic relative humidity (RHe). In the middle relative humidity region, the elutriation rate constant was not affected by the relative humidity. When the relative humidity increased and reached a certain higher value, the elutriation rate constant decreased with an increase of relative humidity. We defined this relative humidity as the capillary relative humidity (RHc). When the relative humidity is smaller than RHe, the elutriation rate constant is affected by the electrostatic forces of the bed particles and the elutriation rate constant increased with an increase of relative humidity. When the relative humidity is higher than RHc, the elutriation rate constant is affected by the capillary forces of the bed particles and the elutriation rate constant decreased with an increase of relative humidity.

Estimation of the Critical Particle Size of Elutriation of Very Small Particles from Fluidized Bed.

The elutriation of fine particles (group C or group A particles) from a fluidized bed of fine-coarse particle mixtures was investigated at a steady state. FCC, AL(OH)3, limestone, alumina, SiC, PVC and silica sand particles in the range of 22.44 to 650 μm were used as the fluidized particles. The elutriation rate constant of particles increased with increase of particle size in the range of group C particles at the constant superficial gas velocity and decreased with increase of the particle size in the rage of group A and B particles. The particle size corresponding to the maximum elutriation rate constant is defined as the critical particle size of the elutriation. The critical particle size decreases with increasing elutriated particle density and with increasing gas velocity. On the contrary, the critical particle size increases with increasing the mean diameter of bed particles. But, the coarse particle density has no effect on the critical particle size. An empirical equation is obtained to estimate the critical particle size.

Fluidization and elutriation of iron oxide particles. A study of attrition and agglomeration processes in fluidized beds

Elutriation is a way of separating particles by means of fluidodynamic forces as well as a very common method to generate solid aerosols to be used for many different purposes. In this work, a study of the elutriation of different size iron oxide particles from a fluidized bed has been carried out in order to determine the elutriation rate constants and the magnitude of the attrition and agglomeration processes in a fluidized bed of this sort of powder, which can be considered as C-type in Geldart's particle classification. The fluidized bed was formed by a mixture of sand and iron oxide particles. Sand particles were much bigger than iron oxide particles and the weight of the latter particles, which were elutriated was very small compared with the initial weight in the bed so that the weight of the bed could be considered as a constant. Theory about elutriation and its application to study the iron oxide behaviour and to determine the elutriation rate constants are presented. Experimental results showed the importance of attrition and agglomeration in iron oxide powders. The smallest iron oxide particles were not those most easily elutriated from fluidized beds of mixed size particles. These particles may be agglomerated between them or with larger sand particles. Elutriation results were compared with previous literature and it was found that a correlation similar in form to Yagi and Aochi's correlation fits the experimental results quite well.

Elutriation of Very Fine Particles from Fluidized Bed-Effect of Mean Diameter of Bed Particles.

The elutriation of fine powders (group C or group A powders in Geldart’s classification) from a fluidized bed of fine-coarse particle mixtures in a steady state is investigated. FCC and alumina particles of 12 μm to 91 μm in diameter (group C and group A powders) were used as fines. Alumina particles of 69 μm to 650 μm in diameter (group A and group B particles) were used as coarse particles. The effects of the weight fraction of powder in the bed, superficial gas velocity, size of fine powder, size of coarse particle, and mean diameter of bed particles on the elutriation rate constant were investigated.The following results are obtained. The elutriation rate constant of group A particles is affected by the properties of the elutriated particles and gas and by operating conditions such as gas velocity. However, the elutriation rate constant of group C powders is not only affected by the above conditions, but also by the weight fraction of the elutriated powder in the bed and sizes of both coarse particles and elutriated powder in the bed, that is, the mean diameter of the bed particles. The elutriation rate constant of group C powders decreases with increases in the mean diameter of bed particles and with decreases in the size of group C powders under the condition of a constant superficial gas velocity. This finding is quite different from that of elutriation of group A or group B particles.

Modelling fluidized bed elutriation of fine particles

Experimental work with Geldart groups A and C powders was carried out in a 6.4-cm i.d. column fluidized batchwise at superficial velocities between 0.24 and 0.6 m s −1. A new model was proposed to account for elutriation and attrition based on the assumption that the generation of fines by attrition is a nonlinear function of time and depends on the percentage of agglomerated fines. Elutriation rate constants and attrition rates were evaluated at various particle mixing ratios and it could be observed that the entrainment rate at low air velocities was affected by interparticles adhesion forces.

Biomass Gasification with Air in a Fluidized Bed: Effect of the In-Bed Use of Dolomite under Different Operation Conditions

The performance of a biomass gasifier, fluidized-bed type, is improved by in-bed use of calcined dolomite. Tar contents in the raw flue gas below 1 g/mn3 are obtained by using a bed with a percentage between 15 and 30 wt % of dolomite (the rest being silica sand). The work is carried out at small pilot-plant scale (10 kg of biomass/h) with equivalence ratios (ER) between 0.20 and 0.35 and temperatures of 800−840 °C in the gasifier bed. To replace the eroded and elutriated dolomite (from the gasifier bed), an amount of dolomite (0.40−0.63 mm) is continuously fed, mixed with the biomass at 3 wt %. When the results obtained with in-bed dolomite are compared to the ones gained in a gasifier bed without dolomite, change of the following variables is reported: gas composition and its corresponding heating value, gas and char yields, apparent thermal efficiency, and tar contents. Once the usefulness of the in-bed use of dolomite is established, three main operation variables (ER and temperatures of the gasifier bed and freeboard) are studied in the improved gasifier. Carryover of solids from the gasifier also increases when calcined dolomite is used because of its softness. Elutriation rate constants are calculated for several operational parameters.

The effect of swirling flow on elutriation in a vortexing fluidized bed

The elutriation of fine particles in a vortexing fluidized bed (VFB) was studied by a batch and binary system. The diameter of the coarse particles was 545 Μm, and the diameter of the fine particles for the elutriation test was 81, 97, 115, 137, 163, and 193 Μm, respectively. It was found that the swirling flow caused by the secondary air injection is the dominating factor to influence the elutriation rates. The effect of primary air velocity, swirling flow, injection angle of secondary air nozzle, and diameter of fine particles on the elutriation rate constant was also studied. The Taguchi experimental method and Regular analysis are used to identify the effects of various operating variables. A correlation was developed to estimate the specific elutriation rate constant (K∞*) in the vortexing fluidized bed. The specific elutriation rate constant (K∞*) was found to be a function of the primary air velocity, the diameter of fine particles, the secondary air velocity, and the height of secondary air injection.

Carry-over from shallow fluidized beds

Shallow fluidized beds are widely used for physical unit operations, i.e. powder drying, cooling, agglomeration, etc. The paper reports experimental results on entrainment from such fluidized beds. Batch and continuously operated beds of four commercially relevant powders are compared. Results are also compared with predictions by the empirical correlation of Geldart et al. Predicted and experimental entrainment rates differ widely, especially for the inner particle sizes. The empirical correlation overestimates the influence of the particle size on the elutriation rate constant.