Minimum Fluidization Velocity Of Particles Research Articles

Minimum fluidization velocity of particles with different size distributions at elevated pressures and temperatures

This paper presents a detailed investigation into the effect of pressure, temperature and particle size distribution (PSD) on the minimum fluidization velocity (Umf). A series of measurement of Umf were carried out under the pressure from 0.1 MPa to 0.5 MPa and the temperature from 300 °C to 850 °C on a lab-scale pressurized fluidized bed reactor for three narrow PSD and three wide PSD (binary, uniform, and Gaussian-type) silica sand particles. The results showed that the Umf for narrow and wide PSDs both decreased with increasing temperature and decreased slightly with rising pressure. Besides, the Gaussian-type mixtures had almost the same Umf as the reference narrow PSD particle, while the Umf of binary and uniform PSD mixtures was larger than these two. Based on the experimental data, a new simple relationship for predicting Umf of the different PSD particles at elevated pressures and temperatures was developed.

Minimum Fluidization Velocity of Intermediate Sized Particles in Conventional and Packed Fluidized Bed

Abstract Typical Geldart B class particles are often used in a gas-solid fluidized bed for various industrial applications. The quality of fluidization of Geldart B particles depends upon the geometry of the bed containing vessel, properties of fluidization medium and the gas flow rate. The presence of large bubbles/slugs that can happen in beds with intermediate class particles is undesirable. The bubbling and slugging, which decrease the quality of fluidization can be prevented with the phenomenon called packed fluidization. Experiments were carried out on the conventional fluidized bed and packed fluidized bed to investigate some of the hydrodynamic characteristics as a function of bed temperature, small particle size, fluidization medium and fraction of voids of packing filled with small particles. The superficial gas velocity required for packed fluidization reduces significantly than conventional fluidization. The minimum fluidization velocity of particles increases with an increase in density of the fluidization gas and decreases with increase in temperature of the bed. A correlation established from experimental data fairly predicts the minimum fluidization velocity for the packed fluidized bed.

Determination of minimum fluidization velocity in fluidized bed at elevated pressures and temperatures using CFD simulations

The minimum fluidization velocity of particles is one of the most critical parameters for the design and operation of fluidized beds. It is difficult and expensive to experimentally measure this parameter at elevated pressures and temperatures. In this study, a three-dimensional model based on Eulerian−Lagrangian frames was used to predict the minimum fluidization velocity at elevated pressures and temperatures. The solid phase was simulated using the multi-phase particle-in-cell approach. The particle flow was described using the discrete particle approach, and a large eddy simulation was conducted to predict the turbulent gas motion. The simulation results were compared with the experimental data reported in previous literatures. The bed temperature ranged from 25 to 800 °C, and the operating pressure ranged from 0.1–4 MPa. In general, the minimum fluidization velocity decreased as the temperature and pressure increased. The variation trends were not significant at high temperatures and pressures (greater than 600 °C and 2 MP, respectively). With a decrease in the particle density, the minimum fluidization velocity decreased. With an increase in the particle size, the minimum fluidization velocity increased. Moreover, the influence of the particle size distribution on the minimum fluidization velocity was systematically evaluated. To precisely calculate the minimum fluidization velocity at elevated temperatures and pressures, a correlation was proposed based on the simulation results.

Empirical models for minimum fluidization velocity of particles with different size distribution in tapered fluidized beds

The particle size distribution (PSD) is known as an important parameter affecting the hydrodynamic behavior of fluidized beds. In this study, extensive experimental data for fluidization of particles with different PSDs in tapered fluidized beds was presented. In the experimentations, three Geldart B powders with four average diameters and various PSDs were used. The experimental results showed that the minimum fluidization velocity (Umf) for particles with flat and binary size distributions were roughly the same; moreover, they were up to 25% larger than the Umf values for narrow cut particle size distribution. The Umf values for Gaussian size distribution were lower than those of narrow cut PSD by about 15%. Using the experimental data and dimensional analysis, several empirical models for evaluating the minimum fluidization velocity for different PSD types in tapered fluidized beds were developed. The influence of operating conditions and geometric parameters of tapered fluidized beds recast into five dimensionless groups were included in the new empirical models. Comparing the predictions of the proposed models with the experimental data shows that the maximum absolute error for different PSDs was <18%. The average deviations of the proposed models from the experimental data are, respectively, 5.3%, 5.4%, 6.4% and 5.9% for narrow cut, Gaussian, flat and binary size distribution. The experimental data for different PSDs was also compared with the predictions of the available models for minimum fluidization velocity and the importance of particle size distribution was discussed. The effects of average particle size, initial bed height and tapered angle on Umf were also investigated and an increase of Umf with increasing these parameters was observed.

Prediction of the minimum fluidization velocity of particles of sugarcane bagasse

This study refers to an experimental analysis of the fluid-dynamics of particles of sugarcane bagasse in a fluidization column with an internal diameter of 190 mm, which determined the minimum fluidization velocity of the particles with different characteristic diameters (0 < dp<9.5 mm), using air as fluidization means. The results have shown that the minimum fluidization velocity has a tendency to increase as the diameter of the particle increases. However, in a certain range of diameter (0.88 mm < dp<9.5 mm), where the particles have a high aspect ratio (length/diameter), it has not been possible to fluidize them. High superficial air velocities have been used, mainly due to the strong trend to interlace and to develop high adhesion forces in this type of particles, as well as the high porosity that is displayed. Based on the experimental results, two new models have been developed in order to determine the minimum fluidization velocity and the complete fluidization velocity of the sugarcane bagasse with diameters that range from 0.075 to 0.445 mm. The comparisons have been made by using correlations from the literature for the determination of the minimum fluidization velocity, and the experimental results have shown that the new suggested correlations finely predict this parameter, with a maximum error range of 6%, respecting the experimental values.

Eksperimentalno istraživanje uticaja temperature vazduha i masenog odnosa zeolita i inertnog materijala na efikasnost procesa sušenja u fluidizovanom sloju

Fluidization has become a subject of wide interest in chemical engineering during the past few decades. This paper presents one part of the results of the kinetics research into the drying of fine grained material in a two-component fluidized bed. A review of theoretical and experimental investigations of aerodynamics of the fluidized bed and minimum fluidization velocity of particles are given, with a special insight into two-component fluidized beds (binary mixture), as well as the basics of heat and material transfer through a fluidized bed. The interaction between complex hydrodynamics and transfer mechanisms complicates the analysis, design and prediction of gas-fluidized-bed processes. Based on experimental investigations, where zeolits was used as a representative of fine grained material, and polyethylene as a representative of the inert material (another component), an analysis of the influence of working parameters on drying in a two-component fluidized bed has been performed. Before that, the minimum velocities for two-component fluidized bed with particles of different sizes and densities were studied experimentally. Also, the ability of elutriation of fine particles from the fluidized bed is described. The present paper deals with the experimental results on the influence of the considered parameters, such as the drying agent temperature and the mass ratio of zeolite and the inert material on the drying curve. The participation of the inert material can considerably increase the intensity of heat and material transfer in the flui dized bed.

CFD modeling to determine the minimum fluidization velocity of particles in gas-solid fluidized bed at different temperatures

It is often important to know minimum fluidization velocity of particles in gas-solid fluidized bed at different temperatures and with different fluidizing gases. Generally, carrying out such experiments are often time consuming as well as very expensive. Computational fluid dynamic (CFD) modeling is presently an important tool that can be extended to solve these problems. The present work demonstrates the use and advantages of CFD modeling to study the effect of temperature on fluidization velocity for lithium titanate (Li2TiO3) particles which fall under Geldart B type and air as fluidizing media. 2D simulation of fluidized bed was performed using Eulerian-Eulerian approach. The inlet superficial air velocity was varied and pressure drop across the bed was predicted at different temperatures from 30 to 600°C for spherical particles of diameter from 231 to 780μm. Minimum fluidization velocity was also determined using experiments for validation for different temperatures and diameter of particles. CFD predicts the minimum fluidization velocity with 95% accuracy when compared to experimental observations. The minimum fluidization velocity decreases with increase in temperature which is confirmed with validated model. The data from simulations was also compared with correlations reported in literature.

Hydrodynamics of particles in liquid–solid packed fluidized bed

Hydrodynamic properties of different particles in unary liquid–solid fluidized bed were well studied and are reported in literature. However, till date the same in liquid–solid binary packed fluidized bed has not been studied and no information is available in literature. Presently, it is well known that particles behave differently in a gas–solid packed fluidized bed; than in a unary gas–solid fluidized bed. A series of experiments were conducted to investigate the hydrodynamic characteristics of particles in liquid–solid packed fluidized bed as a function of particle size, fraction of interstitial void volume filled by particles and bed wall temperature. It was found that, like in gas–solid packed fluidized bed; the minimum fluidization velocity of particles in liquid–solid packed fluidized bed is lower than that of particles in conventional unary liquid–solid fluidized bed, but unlike gas–solid system, the minimum fluidization velocity increases with an increase in bed wall temperature. Experimental details and results are discussed in this paper.

Minimum fluidization velocity of particles with wide size distribution at high temperatures

The purpose of this paper is to explore the effects of bed temperature and particle size distribution (PSD) on minimum fluidization velocity (Umf). Quartz sand and bottom ash were fluidized in a bench-scale bubbling fluidized bed reactor at atmospheric pressure with four size distributions including narrow-cut, Gaussian-type, binary and uniformly distributed cases. Bed temperature ranged from 30 °C to 600 °C. The results show that the Umf of particles with wide PSD decreases with the increase in bed temperature and varies with the mass fraction of coarse particles due to different interparticle forces. In comparison with the narrow-cut particles with the same average diameter, a significant difference was found, which indicates that conventional equations predicting Umf of narrow-cut particles are not appropriate for wide size distributed particles. Due to apparent “filling” and “lubricating” effects, binary and uniformly distributed particles exhibit lower Umf than that of Gaussian-type particle distribution. The semi-fluidized region during fluidization of wide size distributed particles extends with increasing temperature and becomes stable at higher temperature region. To precisely calculate the Umf of wide-range sized particles, a more accurate and simpler correlation was proposed based on the experimental data.

Hydrodynamics of Reduced Pressure Fluidization Employing Particles with Variable Density

A series of experiments was carried out to study the hydrodynamics of a fluidized bed operating at reduced pressure and employing particles with variable moisture content; that is, of variable density. In these experiments, four different types of particles were used and fluidization characteristics very similar to the ones encountered in atmospheric pressure operations were observed. A novel method to measure the minimum fluidization velocity of particles with varying density was proposed. The experimental results demonstrated that the minimum fluidization velocity increased with decreased operating pressure, increased operating temperature, and increased particle moisture content. However, the bed voidage under minimum fluidization conditions showed very little sensitivity to variations in operating pressure. Two equations were developed to predict the minimum fluidization velocity and the results were compared with the experimental data as well as with the predictions of equations available in the tech...

Co-Fluidization of Fine Particles and Straw Pellets at Room and Elevated Temperatures

The results of the research on fluidization of multi-solid materials in cold and hot models have been introduced. It turned out that the change graphs of the statistical characteristic of pressure fluctuation can be used for evaluate values of fluidization velocity. Index Terms—Multi-solid bed, Pressure fluctuation, Straw pellets combustion, Turbulent fluidization. I. I NTRODUCTION The technology of dedicated biomass combustion and coal and biomass co-combustion in a fluidized bed is increasingly applied aiming to achieve a better control over a combustion process with a simultaneous decrease in greenhouse gases, ash and sulfur oxides emissions into the atmosphere. Straw is one of the most easily available biomass resources and its energy utilization is also constantly expanding requiring longer transport distances of the material. This stimulates the production of straw pellets, with a bulk density of 650-750 kg/m³ over 100-150 kg/m³ of baled straw. Fuel combustion can be carried out in a co-fluidized or multi-solid fluidized bed of fine coal ash particles or straw ash and straw char particles loaded with straw pellets. A multi - solid fluidized bed is a circulating fluidized bed in which an entrained bed consisting of fine particles is transiting through a fluidized dense bed of coarse particles, i.e. a bubbling bed in the bottom of the riser (1)-(4). The value of the minimum fluidization velocity of particles forming the bed is required for the design of fluidized bed furnaces. However, as has been proven (5)-(14) it is not possible to apply the known method of a minimum fluidization velocity evaluation from the pressure drop through a bed versus gas flow rate curve for a confined, multi solid or binary particle mixtures fluidized bed. The purpose of the present study is to define a method for the experimental evaluation of the minimum fluidization velocity of a bed compounded of fine and coarse particles mixtures at room and elevated temperatures.

The effect of column diameter and bed height on minimum fluidization velocity

AbstractExperiments show that the minimum fluidization velocity of particles increases as the diameter of the fluidization column is reduced, or if the height of the bed is increased. These trends are shown to be due to the influence of the wall. A new, semicorrelated model is proposed, which incorporates Janssen's wall effects in the calculation of the minimum fluidization velocity. The wall friction opposes not only the bed weight but also the drag force acting on the particles during fluidization. The enhanced wall friction leads to an increase in the minimum fluidization velocity. The model predictions compare favorably to existing correlations and experimental data. © 2010 American Institute of Chemical Engineers AIChE J, 2010