Unstable sinking of spheres at higher air velocity in a gas-solid fluidized bed

Float-sink of large objects (on order of cm) in a gas-solid fluidized bed of powder (on order of 100 s of microns) based on density difference has been utilized for dry density separation in industry. The air velocity u₀/u_mf is one of the important factors operating the fluidized bed, where u₀ and u_mf are the superficial air velocity and the minimum fluidization air velocity, respectively. It is empirically known that the sinking of heavy objects is “occasionally” unstable in the fluidized bed combustor, for which the higher air velocity u₀/u_mf > 4 is used. Unstable sinking means heavy objects that are expected to sink but sometimes do not. However, the precise conditions at which the unstable sinking occurs are not clear. In this study, we investigated the float-sink characteristics at a given air velocity u₀/u_mf = 2–7 using glass beads of size D_gb = 425–600 μm and 600–850 μm as the fluidized powder bed media. The float-sink experiments were carried out at the bed height h_gb = 150 mm and 75 mm using density adjusted spheres (diameter = 30 mm). We found that the spheres stably float or sink based on density difference at D_gb = 425–600 μm & h_gb = 150 mm and at D_gb = 600–850 μm & h_gb = 75 mm. However, the unstable sinking does occur at u₀/u_mf > 4 at D_gb = 600–850 μm & h_gb = 150 mm. These results indicate that the powder size and the bed height are key factors to induce the unstable sinking at the higher air velocity.     

Dry separation of particulate iron ore using density-segregation in a gas–solid fluidized bed

A gas–solid fluidized bed has been used to separate particulate iron ore (+250–500 μm in size) by segregating the particles by density. The ore particles were put into a cylindrical column of inner diameter of 100 mm and bed height of 50 mm, and were fluidized at a given air velocity u₀/u_mf = 1.2–3.2 for 10 min. u₀ and u_mf are the superficial air velocity and the minimum fluidization air velocity, respectively. The bulk density of the ore particles after fluidization was measured as a function of height through the bed in 5 mm increments (the 50 mm height was divided into 10 layers) to investigate the density-segregation. The size of the particles in each of the 10 layers was also measured to investigate size-segregation. It was found that both density-segregation and size-segregation occurred as a function of height through the bed after fluidization at u₀/u_mf = 2.0. However, the segregation did not occur near the bottom of the bed for lower u₀/u_mf and did not occur near the top of the bed for larger u₀/u_mf. The origin of the segregation-dependence on the air velocity was discussed considering the air bubbles size and the fluidizing intensity at upper and lower sections of the bed. The Fe content of the 10 layers at u₀/u_mf = 2.0 was measured to calculate the Fe-grade and Fe-recovery. The ore-recovery was also calculated using the weight of ore particles as a function of height through the bed. The feed Fe-grade (before separation) was 52.1 wt%. If the ore particles in the bottom half of the bed were regarded as the product, the Fe-grade was 59.0 wt%, and the Fe-recovery and the ore-recovery were 68.5 wt% and 60.5 wt%, respectively.     

Fluidization behavior of glass beads under different vibration modules

The expansion of free bubbling gas fluidized beds has been investigated experimentally in a two-dimensional perspex-walled bed. Glass beads were fluidized with dried air at varying gas velocities, while the bed was vibrated at different frequencies, amplitudes and directions to study their effects on the fluidization quality. The experimental results showed that the particle flow pattern depends on the vibration direction, especially at superficial gas velocities less than the minimum fluidization velocity U_mf. The effect of horizontal vibration on fluidization behavior of glass beads exists at superficial gas velocities less than U_mf, while the effect of vertical vibration on fluidization behavior still exists even at higher superficial gas velocities than U_mf.     

Experimental study on fluidization characteristics of vinegar residue in a vibrated fluidized bed

As a by-product in the vinegar brewing process, vinegar residue always has a high moisture content, which is detrimental to the storage and recycle process. The vibrated fluidized bed can be used to dry the vinegar residue. In present work, inert particles were added to a vibrated fluidized bed to improve the fluidization of vinegar residue. Experimental studies were carried out to investigate the fluidization behaviors of the binary mixtures. Flow pattern maps indicated that there was an upper limit to the vinegar residue mass concentration c_w at which stable fluidization could be achieved. The minimum fluidization velocity u_mf of the binary mixture increased as the vinegar residue mass concentration c_w increased and decreased with the increase of the vibration intensity Λ. As increasing vibration intensity Λ or decreasing vinegar residue mass concentration c_w, the drying rate of vinegar residue increased.     

Experimental and theoretical study on hydrodynamic characteristics of tapered fluidized beds

A novel fluidized bed ash cooler was developed for circulating fluidized bed boilers based on a proposed modified tapered fluidized bed. A cold model was built to study the hydrodynamic characteristics of the modified tapered fluidized bed, and its critical superficial gas velocity u_mf and critical velocity for full fluidization u_mff were particularly studied. The effects of taper angle α, static bed height H, air inlet section width δ and particle size d_p on the u_mf and u_mff were experimentally investigated. Furthermore, a theoretical model and an empirical correlation have been proposed to predict the u_mf and u_mff, respectively. The predicting capabilities of the model and correlation have been experimentally discussed. And the predicting capability of the model has also been compared with that of an existing representative model. It is found that both the u_mf and u_mff increase with the increase of taper angle α, static bed height H and particle size d_p, but decrease with the increase of air inlet section width δ, respectively. Additionally, the predicted values of u_mf and u_mff compare well with the experimental data, and the model has a better capability than the existing representative model in predicting the u_mf of the modified bed.     

Segregation of char and silica sand particles in a hot-fluidized-bed steam gasifier

Understanding the behavior of coal char particles in a hot fluidized bed is essential for the design and operation of low-temperature gasifiers. The segregation of char as flotsam is the main focal point of this work. Segregation is generally unfavorable; however, it is attractive if the char holdup can act advantageously as a strong promoter of in-bed decomposition of tar and its conversion to syngas. This study first demonstrates the segregation of char during its gasification in a hot fluidized bed at a relatively low U₀/U_mf and at a steady state under continuous feeding and discharge of lignite and char, respectively. The distribution of char and its conversion from the bottom to the top of the bed are obtained from the motion of the char particles and the distribution of the gas components.     

Fluidization of palm kernel shell,palm oil fronds,and empty fruit bunches in a swirling fluidized bed gasifier

The increased biomass utilization has triggered the use of palm oil waste as fuel for gasification in Malaysia. In this study, pioneering work was conducted on three types of palm oil wastes namely palm kernel shell (PKS), palm oil fronds (POF), and empty fruit bunches (EFB). Minimum air velocity (U_mf) required for fluidization of the tested biomass was determined experimentally in a swirling fluidized bed, by considering the effect of bed weight, density, particle size, fluidized bed height, pressure drop, and bed voidage. It was revealed that higher the particle size the smaller will be the voidage, which consequently affects the minimum fluidization velocity. U_mf was increased with a decrease in voidage size. However, a direct relationship was found between particle size and U_mf. Overall highest U_mf was determined for EFB followed by POF and PKS. Fluidized bed height was increased by decreasing the particle size regardless of the biomass type. Highest unsettled bed height was obtained with PKS on account of its low density among all the test fuels. It was concluded that optimization of the fluidized bed for each type of biomass, particle size, and density is explicitly required for a low-cost energy conversion process.     

Interaction effect of various factors and sulfur migration for pyrite recovery by vibrated fluidized bed

ABSTRACT

Gangue is a hazardous solid waste with high yield in the world. Due to higher proportion of pyrite in the gangue, pyrite recovery from gangue is significant for environmental protection. As effective recovery methods, dry separation methods have been received amount of attentions in the mineral processing field. In this study, vibrated fluidized bed was attempted to use for pyrite recovery. Vibration energy was introduced to strengthen the density segregation in the fluidized bed. The study also investigated the interaction effect of gas velocity, vibration intensity, and bed height in the vibrated fluidized bed. Moreover, sulfur migration has been studied by several advanced analytical techniques. The results showed that separation efficiency was directly related to the interaction effect among various factors. After the separation, the sulfur content of concentrates were increased to 37.31, 35.43, and 28.62% for ?6?+?3, ?3?+?1, and ?1?+?0.5?mm size fractions. The sulfur segregation’s standard deviation (S_sulfur) was beyond 0.70. In addition, elements of S and Fe accounted for higher proportion in the concentrates after the separation. The study indicated that pyrite could be effectively enriched by the vibrated fluidized bed.     

Characterization of temporal and spatial distribution of bed density in vibrated gas-solid fluidized bed

This study uses a Φ 200?mm?×?900?mm vibrated gas-solid fluidized bed (VGFB) with ?0.3?+?0.074?mm magnetite powder was utilized to characterize the temporal and spatial distribution of bed density in VGFB and the influence of bubble movement on fluctuations in bed density. The results indicate that the bed density decreases with an increase in gas velocity (U) and the frequency (f) and amplitude (A) of vibration and that the bed density spatial distribution is lower in the central region but higher in the border regions. The standard deviation of the density first increases then decreases and finally tends to stabilize with an increase in apparent gas velocity. Moreover, when A?=?2?mm, f?=?25?Hz and U?=?14?cm/s, the density distribution is 1.82–1.88?g/cm³ and the fluidization state is improved. The energy of the pressure signal increases with an increase in gas velocity and vibration amplitude. In particular, the low-frequency band of the pressure signal exhibits the highest amplitude and energy, which reveals that bubbles are the main cause of pressure fluctuation. Furthermore, the bed density decreases with an increase in bubble generation frequency, and the relationship between these follows the ExpDec 2 mathematical equation.     

Dry separation of fine particulate sand mixture based on density-segregation in a vibro-fluidized bed

Separation of fine particulate solid materials is one of most important unit operations in industry. Utilization of gas-solid fluidized beds has been considered where particulates are released from constraints due to contacts with surrounding particulates and segregation occurs according to density, size or combination of density and size. Addition of mechanical vibration to the gas-solid fluidized bed may improve dry solid separation. In this study, we investigated the dry separation characteristics of solid particulates using a vibro-fluidized bed especially focusing on the separation of fine particulate ores (≈100 μm) with small density differences. At first, we focused on the influence of fluidizing air velocity on the efficiency of segregation. Subsequently, the influence of vibration strength, vibration amplitude and frequency on segregation behavior was investigated. We found the density segregation does not occur with either gas-fluidization or vertical vibration alone. Only the combination of these effects produces density segregation. The fluidizing air velocity is an important factor to enhance the density-segregation of the particulates with small density difference.     

Influence of air velocity and powder bed height on local density and float–sink of spheres in a gas–solid fluidized bed

Depending on their density, large objects will either float or sink in a gas–solid fluidized bed due to the liquid–like properties and density of the fluidized bed. The float–sink technology has been applied to dry density separations in industry. It is important for optimized industrial application to understand how the air velocity and the powder bed height affect the float–sink as the key operating factors. In this study, we investigated the float–sink of spheres of various density by varying the air velocity and the powder bed height. Also, we obtained the local fluidized bed density and the buoyancy force working on the sphere at various heights. We used the weight of a stainless-steel sphere in the fluidized bed to estimate the local fluidized bed density and the buoyancy force based on Archimedes principle. We found that the spheres float–sink behavior changes dramatically with the air velocity and the powder bed height and that the spheres float–sink behavior is correlated to ΔF = F_b – F_g, where F_b is the buoyancy force and F_g is the gravity force acting on the sphere. We also found that the fluidized bed density is not constant as a function of height when the air velocity is relatively large; the local fluidized bed density is interestingly either minimal at approximately mid-height or surprisingly, gradually increases with height within the fluidized bed at higher air velocities. The possible reasons are discussed by considering the local variation of the motion of air bubbles and the fluidized medium which affect the fluid force acting on the sphere in the fluidized bed.     

Mixing and migration rule of binary medium in vibrated dense medium fluidized bed for fine coal separation

The vibrated dense medium fluidized bed is an efficient waterless dry coal separation technology. In order to reduce the interference of the macroscopic migration of the entire bed with the coal settlement process, the material composition and diffusion behavior of the binary medium are studied. The results show that the maximum mass fraction of fine coal particles is 18% in the uniformly mixed binary medium. Bubbles and vibration are the main factors influencing the mixing process. At low vibration intensity and high gas velocity (f = 15 Hz, U_v = 15 cm/s), the fine coal particles rapidly rise and back mixing under the entrainment of a large number of bubbles. If the vibration frequency is increased to 25 Hz, the excessive kinetic energy causes the entire bed to flow circularly in the vertical direction, interfering with the normal settlement of coal. After reducing the gas velocity (f = 25 Hz, U_v = 11 cm/s), the fine coal particles in the wall region rise rapidly, which is mainly driven by the vibration, whereas the fine coal particles in the central region migrate upward at a relatively low velocity, dragged by the bubbles. The bed impact forces in the opposite directions differ slightly, which promotes the directional settlement of coal.     

Estimating the segregation of a granular bed subjected to vibration in various modes

We present in this paper a DEM study on the segregation behavior of vibrated binary mixture, by focusing on the influence of vibration modes. The numerical simulation program includes a number of vibration tests on a binary mixture in a cylindrical bed, which are performed under different vibration velocity modes, frequencies and amplitudes. We utilize a novel segregation index, namely, the graphic segregation index (GPSI), to characterize the segregation behavior and trace the segregation evolution for binary mixtures. The numerical simulations reveal that the influence of velocity mode is coupled with that resulting from vibration frequency. The binary mixture tends to segregate at a low frequency, with little influence exerted by the velocity mode, whereas it does not fulfill complete segregation at a relatively high frequency, with the vibration frequency having some limited impact on the segregation behavior. Given an intermediate frequency the segregation behavior of the mixture is a little mixed for various velocity modes. It is also found that increasing vibration amplitude enhances the segregation degree of binary mixture. The operation of percolation mechanism in the broad sense is assumed to be primarily responsible for the occurrence of segregation in this study, and this alleged percolation mechanism, as an overall effect, is a result of three effects: the random gap effect, the size-and-mass dependent acceleration and the intrusion and expulsion effect.     

The effect of liquids on radial segregation of granular mixtures in rotating drums

The presence of liquids can significantly affect the dynamics of granular flow. This paper investigates the effect of liquids on radial segregation of granular mixture in a rotating drum using the discrete element method. The wet granular mixture, due to differences in particle size and density, segregates in a similar way to that of dry particles: lighter/larger particles move to the periphery of the bed while heavier/smaller particles stay in the centre. An index based on the variance of local concentration of one type of particles was proposed to measure the degree of segregation. While the liquid induced capillary force slows down the segregation process, its effect on the final state is more complicated: small cohesion shows no or even positive effect on segregation while high cohesion significantly reduces particle segregation. The effect can be explained by the change of flow regimes and the competing effects of mixing and segregation (un-mixing) in particle flow which are both reduced by the interparticle cohesion. A diagram is generated to describe the combined effect of particle size and density on segregation of wet particles. A theory is adopted to predict the segregation of particles under different density/size ratios.     

The role of interstitial gas in the Brazil Nut effect

We have studied the Brazil Nut effect – the rise of a large intruder particle within a vertically vibrated bed of smaller particles. In our study both intruder and bed particles were spherical and the vibration was such that bed convection was negligible. The rise of the intruder was found to be influenced by humidity of the air within the interstices of the particle bed and on the electrostatic charge developed on the bed particles during preparation and vibration. High relative humidity and high electrostatic charge each had the effect of slowing the rise of the intruder. Because increasing relative humidity of the interstitial air caused the electrostatic charge to diminish, the rise rate of the intruder achieved a maximum at a relative humidity of around 55%. Under controlled humidity and charge conditions, the time for the intruder to rise through the bed was found to decrease with increase in intruder diameter. As intruder density was varied under controlled humidity and charge conditions, the intruder rise time was found to exhibit a maximum when the intruder density of approximately one half of the bulk density of the bed of particles. This interesting trend was modelled by taking account of the pressure gradient that is generated across a bed of particles vertically vibrated within a gas. The tentative model suggests that the gas flows associated with this pressure gradient restrict the motion of the bed more than that of the intruder and that it is this difference that accounts for the rise of the intruder. Also incorporated in the model is the buoyancy force on the intruder that results from the pressure gradient across the bed. KeywordsBrazil nut effect, Vibration, Granular solids, Humidity, Electrostatics, Interstitial air Dr. Shintaro Takeuchi is a Research Fellow of the Japan Society for the Promotion of Science (JSPS). Ms. Muniandy received financial support from CSIRO grant CZ-25.     

Correlation for predicting minimum fluidization velocity with different size distributions and bed inventories at elevated temperature in gas-solid fluidized bed

This study presents extensive experimental details of the effects of the solid particle size distribution (PSD), bed inventory, and bed temperature on the minimum fluidization velocity (U_mf). Silica sand with three average solid particle diameters and five PSDs was used. The results showed that the U_mf values of the wide PSDs were lower than those of the narrow cut particles with the same average particle diameter. The standard deviation (SD) and skewness of the PSD also influenced U_mf variation. Furthermore, the U_mf decreased with increasing bed inventory and bed temperature. The influencing parameters were recast into a dimensionless group and included in a new correlation for predicting U_mf. The proposed correlation estimated an average absolute deviation of 14.6% between the experimental data and predicted values. Furthermore, the new correlation was evaluated with a dataset from the literature and gave an average absolute deviation of 15.6%.     

Dry dense medium separation of iron ore using a gas–solid fluidized bed

The dry dense medium separation of iron ore based on floating and sinking of ore particles in a gas–solid fluidized bed was investigated using zircon sand as the fluidized medium. The float-sink of ore particles with mean size D_ave = 23.6 mm was investigated as the fluidizing air velocity and the float-sink time were varied. It was found that gangue with density less than 2850 kg/m³ which float is able to be separated from valuable ore with density greater than 2850 km/m³ which sink. The set point (density where half the particles float and half the particles sink) decreases with increasing the air velocity, and that the float-sink separation is completed within 2 min. The influence of different sized ore particles in the float-sink experiments was also investigated. As a result, the iron ore with D_ave ? 17.6 mm are successfully separated. As D_ave decreases below 17.6 mm, the ore particles with density near the set point tend to scatter in the fluidized bed without floating or sinking, resulting in separation efficiency which decreases with decreasing D_ave. This indicates that the size of the ore particles is one of the major factors to achieve high separation efficiency.     

DEM study and machine learning model of particle percolation under vibration

This paper aims to understand model the effect of vibration on particle percolation. The percolation of small particles in a vibrated bed of big particles is studied by DEM. It is found the percolation velocity (V_p) decreases with increasing vibration amplitude (A) and frequency (f) when the size ratio of small to large particles (d/D) is smaller than the spontaneous percolation threshold of 0.154. Vibration can enable percolation when the size ratio is larger than 0.154, while V_p increases with increasing A and f first and then decreases. V_p can be correlated to the vibration velocity amplitude under a given size ratio. Previous radial dispersion model can still be applied while the dispersion coefficient is affected by vibration conditions and size ratio. Furthermore, a machine learning model is trained to predict V_p as a function of A, f and d/D, and is then used to obtain the percolation threshold size ratio as a function of vibration conditions.     

The impact of vertical internals array on the key hydrodynamic parameters in a gas-solid fluidized bed using an advance optical fiber probe

The effect of a circular configuration of intense vertical immersed tubes on the hydrodynamic parameters has been investigated in a gas-solid fluidized bed of 0.14?m inside diameter. The experiments were performed using glass beads solid particles of 365?μm average particle size, with a solid density of 2500?kg/m³ (Geldart B). An advanced optical fiber probe technique was used to study the behavior of six essential local hydrodynamic parameters (i.e., local solids holdup, particles velocity, bubble rise velocity, bubble frequency, and bubble mean chord length) in the presence of vertical immersed tubes. The experimental measurements were carried out at six radial positions and three axial heights, which represent the three key zones of the bed: near the distributor plate, the middle of the fluidizing bed, and near the freeboard of the column. Furthermore, four superficial gas velocities (u/u_mf?=?1.6, 1.76, 1.96, and 2.14) were employed to study the effect of operating conditions. The experimental results demonstrated that the vertical internals had a significant effect on all the studied local hydrodynamic characteristics such that when using internals, both the solids holdup and bubble mean chord length decreased, while the particles velocity, bubble rise velocity, and bubble frequency increased. The measured values of averaged bubble rise velocities and averaged bubble chord lengths at different axial heights and superficial gas velocities have been compared with most used correlations available in the literature. It was found that the measured values are in good agreement with values calculated using predicted correlation for the case without vertical internals. While, the absolute percentage relative error between the measured and calculated values of these two hydrodynamic parameters indicate large differences for the case of vertical internals.     

Upscaled DEM-CFD model for vibrated fluidized bed based on particle-scale similarities

Simulation based on discrete element method (DEM) coupled with computational fluid dynamics (CFD), coupled DEM-CFD, is a powerful tool for investigating the details of dense particle–fluid interaction problems such as in fluidized beds and pneumatic conveyers. The addition of a mechanical vibration to a system can drastically alter the particle and fluid flows; however, their detailed mechanisms are not well understood. In this study, a DEM-CFD model based on a non-inertial frame of reference is developed to achieve a better understanding of the influence of vibration in a vibrated fluidized bed. Because the high computational cost of DEM-CFD calculations is still a major problem, an upscaled coarse-graining model is also employed. To realize similar behaviors with enlarged model particles, non-dimensional parameters at the particle scale were deduced from the governing equations. The suitability and limitations of the proposed model were examined for a density segregation problem of a binary system. To reduce the computational costs, we show that the ratio between the bed width and model particle size can be reduced to a minimum value of 100; to obtain similar segregation behaviors, the ratio between the bed height and model particle size is considered unchanged.