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.     

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.     

Rate of CO2 adsorbent attrition induced by gas jets on perforated plate distributors in bubbling fluidized beds

Particle attrition is a major challenge when handling bulk solid materials with fluidized beds due to its ability to cause particle loss. Herein, the particle attrition induced by the gas jets on a perforated plate distributor in a bubbling fluidized bed was investigated for CO₂ adsorbent particles. An attrition tube, which used air as the fluidizing gas, was used as the fluidized bed. At a constant fluidizing velocity, the initial static bed height and orifice gas velocity were considered as variables. It was confirmed that abrasion dominated the particle attrition. The trend indicating the change in the maximum size of the particles (d_pm,a) formed by attrition followed that of the attrition rate (i.e., the formation rate of fine particles via attrition). A new stirring factor that combined the model developed by Werther and Xi with the original stirring factor adequately explained the effect of the static bed height on both the attrition rate and d_pm,a when the initial static bed height was greater than the length of the orifice gas jet that penetrated the bed. The attrition rate increased linearly with the new stirring factor. However, d_pm,a increased exponentially with the new stirring factor. Relationships were successfully proposed to enable the estimation of the attrition rate and d_pm,a for the CO₂ adsorbent particles. This study provided the evidence indicating the significance of the effect of bed height on particle attrition induced by the gas jet on the distributor. Moreover, proper models for correlating the attrition rate and the maximum size of the fine particles formed by attrition in the bubbling fluidized bed were provided.     

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.     

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.     

Attrition rate of CO2 adsorbent in bubbling fluidized beds

Particle attrition induced by bubbles in a bubbling fluidized bed was investigated with CO₂ adsorbent particles (0.128 mm in diameter, 1770 kg/m³ in apparent density). The theoretical relationship between the rate of attrition by gas jets on the perforated plate distributor (R_a,j) and the rate of attrition by bubbles (R_a,b) in the bed was revealed that the rate constant of attrition by bubbles (K_a,b) was the product of the rate constant of attrition by gas jets (C_a) and dimensionless particle diameter (d_pbc). An attrition tube (0.035 m-i.d.) using the perforated-plate distributor designed for reducing the attrition by gas jets was employed as the fluidized bed, and the air as the fluidizing gas. The mode of attrition by bubbles was identified as abrasion. The rate of attrition by bubbles (R_a,b) was linearly proportional to the power given to the bed solids by bubbles. The top size of the fine particles formed by attrition (d_pm,ab) increased exponentially with an increase of bed mass and gas velocity. The effects of temperature, pressure, and area of internal surface contacting particle bed on the R_a,b and d_pm,ab were negligible under the tested condition. Empirical relationships on R_a,b and d_pm,ab were proposed based on the experimental data. When both jet and bubble attrition were significant, there existed the static bed heights that gave respectively the minimum attrition rate and the minimum of the top size of fine particles formed by attrition. Each optimal static bed heights increased with an increase of the orifice jet velocity of the perforated plate distributor.     

Bed pressure drop of a bubbling fluidized-bed with overflow solid discharge

The pressure drop of a bubbling fluidized-bed that employed an in-bed inlet and an overflow outlet for continuous flow of solid particles was investigated with variation in the particle size and density, the solid flow rate and the gas velocity. The bed pressure drop decreased with increasing the gas velocity, but increased with the solid flow rate. The characteristics in lifting the solid particles vertically to the level of the overflow outlet by bubbles appeared different from the ones of particle entrainment and bed expansion. Regardless of size and density of particles, bed height in minimum fluidizing condition (pressure head by solid bed weight, H_mf,f) decreased with increasing the volume flow rate of bubble but increased with the mass flow rate of solid particles. The nominal vertical height from H_mf,f to the level of the overflow outlet that the particles should overcome in the course of discharging out of the fluidized-bed with the aid of bubbles increased as either the volume flow rate of bubble increased or the mass flow rate of solid particles decreased. The power consumed while bubbles lifted particles to be discharged appeared to be same at the fixed volume flow rate of bubble. A correlation was proposed successful even for predicting the bed pressure drop of the recycle chamber of the loop seal and the external solid circulation rate in the circulating fluidized-bed system.     

Minimum spouting velocity of binary mixture with non-spherical particles

The minimum spouting velocity, U_ms, defined for stable external spouting, is found also crucial to ensure good mixing when multi-component particles are involved in spouted beds. In this study, experiments were performed in a cylindrical-conical spouted bed to study the influences of diverse factors on the U_ms of a binary system with the cylindrical particles and the spherical bed material. The results showed that the changes in U_ms with the particle properties (particle shape, size, and density) and operating conditions were closely related to the blending ratio of the mixture. When the volume fraction of the non-spherical particles was relatively small (less than 40% to 50%), U_ms mainly depended on the properties of the bed material. It was considered acceptable to estimate U_ms by assuming that the system only consisted of the spherical bed material. Otherwise, the cylindrical particle shape has a significant influence on the flow dynamics and U_ms. For such spouting systems, an equivalent diameter of the bed material was proposed to reflect the shape effects of non-spherical particles, whereby U_ms would be independent of the blending ratio. Consequently, a novel empirical correlation is proposed to quantitatively predict the U_ms of binary mixtures.     

The hydrodynamics of liquid-solid and gas-liquid-solid inverse fluidized beds with bioparticles

The hydrodynamic characteristics, such as minimum fluidization velocity (U_lmf for liquid-solid (LS) system and U_g,if for gas-liquid-solid (GLS) system) and bed expansion ratio (BER), of liquid-solid and gas-liquid-solid inverse fluidized beds (LSIFB and GLSIFB) with bare particles and particles with biofilm were investigated. In the LSIFB system, U_lmf and BER of the bare particles were independent of the solids loading. For bioparticles, the increase of the biofilm thickness reduced U_lmf and increased BER, suggesting that the fluidizability increases with the presence of the biofilm. In the GLSIFB system, the initial fluidization gas velocity (U_g,if) and the complete fluidization gas velocity (U_g,cf) both increased with increasing particle diameter and decreasing particle density under fixed superficial liquid velocities. Biofilm attachment led to a decrease of both U_g,if and U_g,cf, and an increase of bed expansion, again suggesting increased fluidizability with the presence of biofilm.     

Experimental study of fluidization characteristics of Geldart-D particles in pressurized bubbling fluidized bed

The pressurized bubbling fluidized bed shows great advantage in retreating municipal solid waste because it could effectively capture CO₂ and enhance the reaction rate of the process of combustion and gasification. In the present work, fluidization characteristics of Geldart-D particles at elevated pressure were experimentally investigated, such as flow pattern, pressure drop, minimum fluidization gas velocity. At the same fluidization gas velocity, as elevating operating pressure, the fluidization of Geldart-D particles became more intense, the bubbles got larger, the standard deviation and the power density of dominant frequency of the pressure drop signal increased. While, under the same fluidization number, as increasing operating pressure, the fluidization of Geldart-D particles became smoother, the bubble size decreased, both the standard deviation and the power density of dominant frequency of the pressure drop signal decreased. It seems that, under elevated pressure, the fluidization behavior of Geldart-D particles would transition to that of Geldart-B particles. Finally, the minimum fluidization velocity of the Geldart-D particles was found decreased with the increase of the operating pressure. A new correlation for the prediction of the minimum fluidization velocity of Geldart-D particles at elevated pressure was also formulated based on the present experimental results.     

Prediction ability of a new minimum bubbling criterion

The recently developed minimum bubbling criterion of Brandani and Zhang [19] for a prediction of the minimum bubbling point was validated using an experimental determination of the minimum bubbling points of spherical rigid non-porous powders with various particle size distributions. These powders include a narrow size cut powder, a “natural” size distribution powder and a “bimodal” size distribution powder. The minimum bubbling points were correctly identified using the ε_d and U_d characteristic curves, obtained from a correct interpretation of 1-valve and 2-valve bed collapse curves using the bed collapse model, developed by Cherntongchai and Brandani in [21]. In order to enhance the prediction ability of the stability criterion, an appropriate drag force correlation was introduced into the criterion. Then, it was pointed out that the characteristic parameter of the criterion has a strong dependence on the voidage term as an exponential function. As a result, a simple empirical correlation is proposed. The new stability criterion was, then, tested against a detailed comparison of 700 minimum bubbling points taken from literature. The criterion can very well predict the minimum bubbling voidage for various operating conditions of rigid non-porous materials and predict fairly well the minimum bubbling velocity.     

Effect of level of overflow solid outlet on pressure drop of a bubbling fluidized-bed

The effect of level of the overflow outlet for continuous flow of solid particles on the pressure drop of a bubbling fluidized-bed that employed an in-bed inlet for solid feed was investigated with changing solid properties, solid feed rate, gas velocity, and level of the overflow outlet. The pressure drop of fluidized-bed (Δp_bed,f) decreased with increasing gas velocity, but increased with either solid feed rate or level of the overflow solid outlet (L). The Δp_bed,f/L increased with L. Irrespective of particle size and density, bed height converted for minimum fluidization condition (pressure head by bed weight, H_mf,f) decreased with increasing the volume flow rate of bubble but increased with either the solid feed rate or the level of the overflow solid outlet. The nominal vertical height, height between the H_mf,f and the level of the overflow outlet, that bubbles transported particles while drawing the solid particles out of the fluidized-bed increased as either the volume flow rate of bubble or level of the overflow outlet increased. However, it decreased as the solid feed rate increased. It appeared that the power of bubble for lifting solid to be discharged through the overflow outlet was same at the fixed volume flow rate of bubble, solid feed rate, and level of the overflow solid exit. The power of bubble increased with the level of the overflow outlet but not linearly. The correlation proposed for the pressure drop across the bubbling fluidized-bed was useful to predict the pressure drop across the recycle chamber of the loop seal and the external solid circulation rate in the circulating fluidized-bed system.     

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%.     

The yield stress of jammed magnetofluidized beds

By means of a magnetic field externally imposed, fluidized beds of magnetizable particles may experience a transition from a fluidlike unstable to a solidlike stable state. In our work, measurements have been taken of the gas velocity and particle volume fraction at the jamming transition as well as of the tensile yield stress of the stabilized bed subjected to a small consolidation. The influence of diverse physical parameters such as initialization mode, magnetic field orientation, average particle size and size polydispersion, are analyzed. Noninvasive visualization of the bed structure has revealed that magnetic stabilization is determined by the formation of particle chains. Due to the enhancement of the interparticle attractive force with field strength and particle size, the transition to stability takes place at higher gas velocities as the magnitude of these parameters is increased. The magnetic yield stress of magnetofluidized beds of naturally aggregated particles because of a large presence of fines is significantly larger than that corresponding to naturally nonaggregated particles. Moreover, the jamming transition occurs at larger gas velocities (or smaller field strengths) in the former case since the agglomerates behave magnetically as large effective particles. The effect of the magnetic field on the yield stress ia only relevant when it is applied during initialization of the bed in the bubbling regime and particles are free to move and restructure in chains. Measurements of the yield stress are presented when the applied magnetic field is oriented either in the vertical or horizontal direction (B co-flow and B cross-flow modes, respectively). The variation of the magnetic yield stress with particle size was found to be dependent on the field direction. This can be justified by the dependence of the interparticle magnetic force on the chain average angle with the field, which is affected by particle size as the stabilized bed is subjected to small consolidations.     

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.     

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.     

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.     

Hydrodynamic studies on fluidization of Red mud: CFD simulation

Hydrodynamic studies are carried out for the fluidization process using fine i.e. Geldart-A particles. Effects of superficial velocity on bed pressure drop and bed expansion is studied in the present work. Commercial CFD software package, Fluent 13.0 is used for simulations. Red mud obtained as waste material from Aluminum industry having average particle size of 77 microns is used as the bed material. Eulerian–Eulerian model coupled with kinetic theory of granular flow is used for simulating unsteady gas–solid fluidization process. Momentum exchange coefficients are calculated using the Gidaspow drag functions. Standard k–ε model has been used to describe the turbulent pattern. Bed pressure drop and bed expansion studies are simulated by CFD which are explained with the help of contour and vector plots. CFD simulation results are compared with the experimental findings. The comparison shows that CFD modeling is capable of predicting the hydrodynamic behaviors of gas–solid fluidized bed for fine particles with reasonable accuracy.     

Using particle trajectory for determining the fluidization regime in gas–solid fluidized beds

Transition from bubbling to turbulent in a conventional gas–solid fluidized bed was evaluated from trajectory of particles in fluidized bed. A series of experiments were carried out in a lab-scale fluidization bed using radioactive particle tracking (RPT) technique for recording the position of a tracer in the bed. Statistical parameters, such as standard deviation and skewness of the time–position data, were utilized to determine the transition velocity from bubbling to turbulent regime. The results showed that the data obtained by the RPT technique can predict transition velocity. It was shown that the standard deviation of position fluctuations reach a maximum with increasing superficial gas velocity corresponding to regime transition. It was shown that transition from bubbling to turbulent can be determined using skewness and kurtosis of time–position data. The velocities obtained in this work are in good agreement with the available correlations.     

A novel technique for residence time distribution (RTD) measurements in solids unit operations

This work presents a novel technique with fast response for Residence Time Distribution (RTD) measurements in gas-solid unit operations (e.g., fluidized bed reactors). This technique is based on an optical method which eliminates the requirement of knowing the velocity and concentration profiles at the exit section of the system. Experiments were carried out with SiC particles and a phosphorescent pigment used as a tracer. A concentration measurement system was developed to measure the tracer concentration in SiC/pigment mixtures. The corresponding pigment concentrations were evaluated at the bottom of this system using a photomultiplier. The pigment concentration was derived from the integral of the signal intensity received by the photomultiplier. Then, a calibration curve was established which provided the empirical relationship between the integral and pigment concentration. In order to validate this RTD measurement technique, a series of experiments was performed in a bubbling fluidized bed and the effect of the bed height was studied. It was shown that the experimental RTD curves were in good agreement with the theoretical RTD of bubbling fluidized beds. This solids RTD measurement technique can be used to provide a better understanding of the hydrodynamics of complex solids unit operations.