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.     

Conversion air velocity at which reverse density segregation converts to normal density segregation in a vibrated fluidized bed of binary particulate mixtures

It is well known, when binary mixtures of different-density particles of the same size are vertically vibrated or fluidized by airflow through the bottom, the particles segregate by density. Reverse density segregation occurs in the vibrated bed; heavier particles move upward and lighter ones move downward, and normal density segregation occurs in the fluidized bed; lighter particles move upward and the heavier ones move downward. In this study, we investigated the particles’ behavior in a vertically vibrated fluidized bed at various air velocity using two types of particulate mixtures of glass beads (GB) and stainless steel powder (SP) or iron powder (IP) of same size. We found that reverse segregation converts to normal segregation at a certain air velocity; here we call it “conversion air velocity”. Then, we investigated the likely origin of the conversion air velocity considering the minimum fluidization air velocity u_mf determined for the three monocomponent particles (GB, SP and IP) with and without vibration. We found that the conversion air velocity is close to the u_mf of the lower density particles (GB) with vibration, indicating that the conversion from reverse segregation to normal segregation occurs around u_mf of lighter particles with vibration.     

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.     

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.     

Recovery of metal fractions from waste printed circuit boards via the vibrated gas-solid fluidized bed

The vibrated gas-solid fluidized bed based on fluidized separation technology was used to recycle the metallic fraction of waste printed circuit boards (WPCBs). The size fraction composition and element distribution of the crushed products were analyzed by sieving and X-ray fluorescence, respectively. The contents of Cu, Zn, Fe and Ti in various size fractions had significant differences, resulting in preliminary enrichment. The performance of vibration on the fluidization characteristics of WPCBs powder was described. With fluidization number, vibration frequency and vibration amplitude as variables, the separation performance of WPCBs powder under various operational conditions was studied. With the optimum operated conditions, the optimal recovery rates of metallic fraction of the three size fractions of 1–0.5 mm, 0.5–0.25 mm and 0.25–0.125 mm were 88.53%, 95.61% and 82.28%, respectively. The vibrated gas-solid fluidized bed can effectively enrich and recover the metallic fraction of WPCBs, providing convenience for subsequent separation.     

3D simulation on pressurized oxy-fuel combustion of coal in fluidized bed

Pressurized oxy-fuel combustion technology is considered as a perspective carbon capture technology in industrial process. A computational fluid dynamics (CFD) model based on Multi-Phase Particle-In-Cell (MP–PIC) method was developed to predict pressurized oxy-coal combustion process in fluidized bed. The heterogeneous and homogeneous combustion reactions of coal were considered in this model. The predicted results were validated the accuracy of this model with experimental data from a 15 kW_th pressurized fluidized bed combustor in terms of the gas component and temperature characteristics. The characteristics of gas–solid flow and combustion under different pressure (0.1–2 MPa) and oxygen atmosphere were studied in this work. The predicted results show that the intensity of particle motion and the expansion degree in the fluidized bed was gradually decreased with an increase in pressure. A correlation was proposed based on the simulation results to maintain suitable fluidization conditions in pressurized circulating fluidized bed at different pressures. The temperature of particle phase region gradually increased with combustion pressure and inlet O₂ concentration increased. In addition, the CO₂ concentration in outlet increased while the emission of CO and NO_x decreased as the combustion pressure increased.     

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.     

Fluidization behavior of binary mixtures of nanoparticles in vibro-fluidized bed

The fluidization behavior of different mixed SiO₂, TiO₂ and/or ZnO nanoparticles under the application of vibrated fields of constant vibrated frequencies (40 Hz) and amplitude (3.0 mm) is studied. The single nanoparticles experiments show that SiO₂ nanoparticles have a better fluidization quality than TiO₂ and ZnO nanoparticles. For binary mixtures of the nanoparticles, the amount of SiO₂ nanoparticles generally has a beneficial effect on the fluidization quality of the binary mixtures. Using the linear regression, the Richardson–Zaki exponents of three kinds single and their binary mixture of nanoparticles are calculated. The Richardson–Zaki analyses indicate that the particulate fluidization degree of mixed nanoparticles can be greatly improved in agglomerate particulate fluidization (APF) behavior.     

Characteristic gas velocity and fluidization quality evaluation of vibrated dense medium fluidized bed for fine coal separation

In order to remove incombustible impurity minerals, a vibrated dense medium fluidized bed (VDMFB) can be adopted for fine coal dry separation, while Geldart B magnetite powder can serve as the medium. The influence of vibration on characteristic gas velocity in flow pattern transition stage was experimentally investigated at a vibration amplitude range of 0.5–4?mm and frequency range of 5–35?Hz. The experimental results demonstrate that at a low frequency (f?<?10?Hz), the vibration effect results in a denser bed and a slightly smaller initial fluidization gas velocity. As the vibration frequency increases to a level similar to the bed’s natural frequency, the minimum fluidization gas velocity reduces sharply. The minimum fluidization gas velocity correlation in the VDMFB is obtained by means of theoretical deduction and experimental data fitting. Furthermore, a method is proposed for evaluating the effects of vibration on improving fluidization quality. Based on that, using the coal separation probable error $E_p?0.1$ as the evaluation index, a suitable effective operating gas velocity range for coal separation under different vibration parameters is determined. The ratio of the boundary operating gas velocity to $U_{\mathit{mfc}}$ is 0.67–2.28. Thus, a uniform and stable fluidization environment is provided for dry fine coal separation.     

Experimental study on the fluidization discharging characteristics of Geldart-C kaolin powders in a blow tank with pulsed gas

Kaolin powders have been suggested to be able to adsorb heavy metal vapor from coal-fired flue gas. However, due to the influence of inter particle forces, such as liquid bridge force, it is difficult to realize stable pneumatic conveying. In the present work, the fluidization characteristics of kaolin powders were investigated. A series of unstable flow phenomena such as agglomeration, channeling, and slugging occurred during the fluidization process. Also, the fluidization discharging characteristics of kaolin powder in an optimized blow tank were experimentally studied. The results indicated that the introduction of pulsed gas can effectively destroy agglomeration and thus improving the stability of discharging. Visual experiments in pseudo-2D fluidized bed were also confirmed the destructive effect of pulsed gas on agglomeration. With an increase in either fluidization gas velocity U_f or pulsed gas velocity v_pulsed, the mass flow rate of kaolin powder G first increased and then decreased. Finally, drying experiments demonstrated that there is free water on the surfaces of the kaolin powders. The analysis of forces indicated that the liquid bridge force F_lb between particles is much larger than the particle gravity F_g. The liquid bridge force might be one of key reasons for kaolin powder agglomerating.     

Partial Oxidation of Propane Over Mo1V0.3Te0.23Nb0.12Ox Catalyst in a Fluidized Bed Reactor

A fluidized bed reactor, for the first time, was employed to investigate the partial oxidation of propane to acrylic acid over Mo₁V_0.3Te_0.23Nb_0.12O_x catalyst. Effects of temperature, oxygen concentration, steam concentration, space velocity, and dilution with different sizes of SiC were studied and discussed. The reaction in the fluidized bed was compared with the reaction in the classical fixed bed. The results indicated that at constant temperature propane conversion obtained in the fixed bed reactor was higher. But the fluidized bed recompensed it by higher acrylic acid selectivity leading to acrylic acid yields and formation rates considerably over the fixed bed. In the fluidized bed reactor, an enhancement in the acrylic acid selectivity was observed over fixed bed at similar conversions. It is suggested that passing a large portion of gas phase oxygen through bubbles allows operation under lower oxygen partial pressure, which favors the desired reaction path. With the heat sink feature of the diluent, the catalytic performance is more influenced by dilution in the fixed bed. However, hot spots are less likely to occur in the fluidized bed and dilution has little effect on the catalytic performance. However, propane conversion increases slightly by increasing SiC particle diameter due to better fluidization quality.     

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.     

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.     

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.     

Effect of flow pulsation on fluidization degree of gas-solid fluidized beds by using coupled CFD-DEM

In this paper, the effect of inlet flow type on fluidization of a gas-solid fluidized bed was studied by using numerical simulations. Gas-solid fluidized beds are widely used in processes such as heating, cooling, drying, granulation, mixing, segregating and coating. To simulate the gas-particle flows, the unresolved surface CFD‐DEM was used considering Eulerian–Lagrangian approach. The fluid phase was modeled by computational fluid dynamics (CFD) while the solid phase was solved by discrete element method (DEM), and the coupling between gas and solid phases was considered to be four-way. The uniform and pulsed flows were injected through three nozzles located at the bottom of a rectangular bed. Three types of pulsed flow were considered: sinusoidal, rectangular and relocating. The fluidized bed behavior was discussed in terms of minimum fluidization velocity (MFV), pressure drop, bubble formation, bed expansion, particles velocity and, gas-solid interaction and particle contact forces. The results of different simulations indicated that the minimum fluidization velocity of the beds fluidized by pulsed flows was decreased by up to 33%. The influence of the pulsation amplitude on the minimum fluidization velocity was more significant than that of the pulsation frequency. The bed expansion and particles average velocity were increased by the pulsed flows, while the pressure drop and interaction force were decreased. As the pulsation frequency increased, the pressure drop and gas-solid interaction force increased, although size of the bubbles and bed expansion decreased. It was also observed that in large vibration frequencies, the bubbles became more regular. In the sinusoidal flow, the velocity and contact force between the particles were initially increased by frequency and in larger frequencies they were decreased.     

Bed expansion ratio of mono-sized and binary mixtures in fluidized,spouted, and spout-fluid beds

Studies on bed expansion ratio were carried out in fluidized, spouted, and spout-fluid beds. A single column has been used to compare the characteristics of fluidized, spouted, and spout-fluid beds. Experiments were carried out using air and glass beads under fluidized, spouted, and spout-fluid bed conditions separately to study the effect of gas velocity, bed mass, and particle size on bed expansion ratio. Glass beads of different sizes (0.75, 1.2, 1.7, and 3.075?mm) have been used as solid bed material. Bed expansion ratio was determined for mono-size particles and binary mixtures (different diameter ratios and composition). It was found that the bed expansion ratio decreases with increase in bed mass for only spouting condition and spout-fluidization conditions. The bed expansion ratio increases with increase in bed mass for only fluidization condition.     

Characteristics of vibro-fluidization for fine powder under reduced pressure

A cohesive powder (Geldart group C) was fluidized under reduced pressure (P = 0:5 - 10 kPa) with vibration. The fluidized powder was composed of glass beads 6 μm in diameter. The bed pressure drop was measured by decreasing gas velocity and the flow patterns in the bed were observed. A slanting particle flow, which was not observed at atmospheric pressure in a previous study, appeared at a lower pressure than about P = 20 kPa and with a larger vibration strength than the critical vibration strength, A_cr. Under the above conditions, the pressure drop curve changed abnormally due to the occurrence of this slanting particle flow. On the other hand, when the vibration strength was smaller than A_cr, a typical pressure drop curve was obtained. In light of these results, the interrelation between the slanting particle flow and the change in the pressure drop curve was examined.     

二元气-固流化系统临界流化速度的研究

在直径为4cm的有机玻璃流化床中,对由不同尺寸和质量分数的铁矿石和煤颗粒组成的二元气-固流态化系统进行了流化特征的实验研究,得到了该系统的流化特性曲线,给出了临界流化速度和二元系统混合颗粒平均直径之间的关系,用该关系式对临界流化速度进行了预测,并且将预测值和实验值进行了比较。结果表明,当煤颗粒的质量分数为10%时,临界流化速度umf与颗粒直径的平方dm2的关系为umf=0.13dm2+1.2;当煤颗粒的质量分数为20%时,两者的关系变为umf=0.12dm2+1.2,预测值与实验值误差在10%以内。     

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.     

Percolation of interacting particles deposited onto a planar substrate

A model for the diffusion of interacting particles deposited onto a planar substrate is analysed near its percolation threshold. In the Monte Carlo simulations on square lattices, the movement of particles depends on w/kT, where k is the Boltzmann constant, T the temperature and w is the lateral interaction energy of nearest neighbours; w>0 and w<0 correspond to the attractive and repulsive cases respectively. The extrapolated results for the infinite system show that the critical concentration ψ_c decreases continuously with w/kT, from ψ_c ≈ 0.661 (for w/kT = - 2.0) to ψ_c ≈ 0.512 (for w/kT = 2.0). Some details of the internal structure of clusters at ψ_c as well as the initial non-equilibrium regime are also studied as a function of w/kT.