Comparative analysis of numerically derived drag models for development of bed expansion ratio correlation in a bubbling fluidized bed

For designing and operating fluidized bed reactors, bed expansion ratio is one of the most important parameters. In this research, a bubbling fluidized bed is simulated using three-dimensional Eulerian-Eulerian method that is incorporated with kinetic theory of granular flow (KTGF) to calculate the pressure drop, gas volume fraction (GVF) and bed expansion ratio. Grid optimization is firstly conducted to achieve suitable solution for further simulations. Subsequently, different numerically derived drag models are employed to investigate the effect of these models on gas-solid flow dynamics. Afterwards, the fluidized bed is simulated at different gas superficial velocities employing two different drag models respectively. Simulation results have been comprehensively validated against experimental data. Finally, an expression for bed expansion ratio has been formulated and compared with the empirical correlation. The proposed correlation holds reasonably well with various experimental values. This work provides a scalable way to aid in designing and operating process reactors.     

Artificial Neural Network Approach to Segregation Characteristic of Binary Heterogeneous Mixtures in Promoted Gas-Solid Fluidized Beds

Binary mixtures of particles of the same size but of different densities are fluidized in a 15 cm diameter column with a perforated plate distributor and two coaxial promoters. In the present work an attempt has been made to study the fluidization and the segregation characteristic of density-variant solids of the same size in terms of segregation distance. The dimensionless segregation distance has been correlated with other dimensionless groups relating to various system parameters: ratio of the density of jetsam particles to that of flotsam, initial static bed height, height of layer of particles above the bottom grid, superficial gas velocity, and average density of the mixture on the basis of the dimensional analysis approach for both un-promoted and promoted beds. Correlations have also been developed with the above system parameters by using an artificial neural network approach for different types of fluidized beds, and the findings with respect to both approaches have been compared with each other. The values of segregation distance for promoted beds have also been compared with those for the un-promoted bed in this work.     

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.     

Computational and experimental studies on bed dynamics of a gas–solid fluidized bed using Geldart-A particle: A comparison

The bed dynamics of a two-dimensional gas–solid fluidized bed is studied experimentally and computationally using Geldart-A particles. Commercial software ANSYS FLUENT 13 is used for computational studies. Unsteady behavior of gas–solid fluidized bed is simulated by using the Eulerian–Eulerian model coupled with the kinetic theory of granular flow. The two-equation standard k?? model is used to describe the turbulent quantities. The simulation predictions are compared with experimentally observed data on volume fraction, bed pressure drop and bed expansion ratio. The results of simulations are found to be in close agreement with the experimental observations, implying that computational fluid dynamics (CFD) can be used for the design of an efficient bench-scale catalytic fluidized bed reactor.     

Fluctuations of the number of solid-phase particles in a fluidized bed

An analytic expression is obtained for calculating the relative fluctuation of the number of particles in a fluidized bed. The fluctuation of the number of particles in a bed consisting of glass pellets fluidized with water has been experimentally investigated. The relative fluctuations obtained in these experiments are compared with the theoretical values for a fluidized bed and an ideal gas.     

On Startup of a Boiler with a Low-Power Fluidized Bed

Journal of Engineering Physics and Thermophysics - An investigation has been made into the process of heating up the granular bed in two regimes: heating up a roomtemperature fluidized bed and...     

Modeling of superheated-steam drying of biofuel in a fluidized bed

The drying of a dispersed material in a fluidized bed has been modeled mathematically. The dependence of the bed’s mass, which ensures a prescribed final humidity, on the regime parameters of the fluidized system has been established. The superheated-steam drying of wood granules in a fluidized bed has been investigated experimentally.     

Study the particle loss from vortex chamber fluidized bed with different conditions

Vortex chamber fluidized bed (VCFB) is a novel type of fluidized bed which has many advantages compare to conventional fluidized beds due to its high-G operation. Hence, the VCFB can replace the conventional fluidized bed for process intensification. However, particle loss at the initial stage of particle injection from VCFB is an important issue which may prevent the particle bed formation inside the VCFB. Therefore, the quality of the fluidized bed will largely depends upon it. The present work aims to study the particle loss from a VCFB at the beginning of particle injection by varying different conditions such as particle feeding location, number of feeding points and particle diameter. A three-dimensional computational fluid dynamics (CFD) model of the VCFB is developed and the particle loss is studied numerically. It is observed that particle loss is minimal when particles are injected from the rear wall of the vortex chamber. Particle loss increases with increasing the number of particle injection points and particle diameter.     

CONTINUOUS COOLING OF WET FOUNDRY SAND USING A FLUIDIZED BED

In order to establish the optimum cooling system for hot returned sand in metal casting processes, a mathematical model including simultaneous heat and mass transfer has been constructed. The present study is especially directed towards clarifying the dynamical behavior of sand cooling in a one-pass fluidized bed. A series of continuous cooling experiments has been made for various conditions. Characteristics of both transient and steady-state bed temperature are explained reasonably by the present model. Furthermore, a possible proposal has been given on the optimal operation, which raises heat exchange efficiency in fluidization cooling of hot molding sand.     

Two-dimensional CFD simulation of chemical reactions in tapered-in and tapered-out fluidized bed reactors

This article presents a simulation study of tapered-in and tapered-out fluidized bed reactors to investigate the influences of apex angle on the fractional conversion and the pressure drop of the fluidized beds in the presence of two types of chemical reaction with gas volume increase and reduction. The 2D behavior of tapered-in and -out fluidized beds was also compared with a columnar one from fractional conversion and bed pressure drop point of views. To validate the simulation results, the numerical predictions for the expansion ratio and the pressure drop of a tapered fluidized bed were compared with experimental data and good agreement was observed. The obtained simulation results clearly indicate that an apt apex angle exists in tapered-in reactors in which the fractional conversion reaches a maximum value; while the variations of the apex angle slightly affect the fractional conversion in tapered-out fluidized beds. Increasing the residence time of the gas phase in the upper section of tapered-in beds has positive influences on the fractional conversion, while a further decrease in the gas phase velocity in the tapered-in reactors has a negative effect on the fractional conversion. Moreover, higher bed pressure drop was observed in tapered-in reactors than that in the columnar and tapered-out ones.     

Phase Diagram of a Disperse System with Ascending Gas Flow

The phase diagram of an infiltrated disperse medium which includes a circulating fluidized bed in addition to fixed and fluidized beds and vertical pneumatic transport has been constructed. The expression to calculate the most important characteristic of a flow system, i.e., the transport velocity, has been obtained. The definition of a circulating fluidized bed has been given.     

Viscosity and separation performance of a gas-solid separation fluidized beds

A theoretical model of viscosity in gas-solid separation fluidized beds is established according to the two-phase flow theory of fluidized beds. After comparing theoretical and measured values, the correlation coefficient between the two is as high as 0.99, showing that the model has good predictability for the viscosity of fluidized beds. Meanwhile, the viscosity and its influencing factors were studied using a Brookfield viscometer. The study shows that smaller medium particles (0.074–0.15?mm) can reduce the viscosity of fluidized beds, but they will aggravate the viscosity fluctuation at more than 5?wt% addition, which is unfavorable to the stability of fluidized beds. In addition, in the actual separation process, the external factors (such as moisture and coal powder content) also affect the viscosity of the fluidized beds. Increasing the moisture increases the viscosity of the fluidized bed, whereas coal dust has the opposite effect. In order to ensure the stability of the fluidized bed, the bed moisture content should be controlled below 1?wt%, while the content of coal powder should be limited below 5?wt%. Based on separation tests, reducing the viscosity will improve the separation performance of a fluidized bed at the proper fluidized gas velocity, with the lowest possible error E_p of 0.085.     

CONTINUOUS COOLING OF WET FOUNDRY SAND USING A FLUIDIZED BED

Abstract

In order to establish the optimum cooling system for hot returned sand in metal casting processes, a mathematical model including simultaneous heat and mass transfer has been constructed. The present study is especially directed towards clarifying the dynamical behavior of sand cooling in a one-pass fluidized bed. A series of continuous cooling experiments has been made for various conditions. Characteristics of both transient and steady-state bed temperature are explained reasonably by the present model. Furthermore, a possible proposal has been given on the optimal operation, which raises heat exchange efficiency in fluidization cooling of hot molding sand.     

Variation of heat-transfer intensity around the perimeter of a horizontal tube in a fluidized bed

An experimental investigation has been made of the variation of local heat-transfer coefficient in the cross section of a horizontal tube in a fluidized bed by the method of measuring local temperature difference and local heat flux power.     

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.     

Features of the Motion of a Circulating Bed in a Large‐Volume Spherical Packing

An experimental study has been made of the motion of a circulating fluidized bed in a column filled with a largevolume spherical packing. It has been established that, once the feed of the loose material ceases, the motion of the bed continues up to the total removal of the disperse medium from the column with a velocity acquired in constant circulation. A procedure based on the processing of experimental results has been proposed for calculation of the operating conditions of apparatuses in which the indicated motion can be employed.     

Hydrodynamics of a dual fluidized bed gasifier

The hydrodynamics of a circulating fluidized bed gasification system with dual fluidized bed concept has been studied through a cold model investigation. The article focuses on the axial voidage, the pressure drops across various components and the solid circulation under different operating conditions. The control of solids circulation between the dual fluidized beds has been done through an L-valve system. A mathematical model of hydrodynamic behavior of the system has been presented. It is observed from the investigated operating parameters that the aeration flow, the secondary air flow and particle diameter have the strongest influence on system pressure drop and solid circulation.     

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.     

Simulation of 3D gas–solid fluidized bed reactor hydrodynamics

In this article, an attempt is made to develop a 3D gas–solid fluidized bed reactor (FBR). Basically, it deals with simulation of a FBR in computational fluid dynamics (CFD) using the software, Ansys Fluent v14. The simulation of gas–solid flow is carried out using Eulerian multifluid model which is integrated with the solid particle kinetic theory. The coefficients of exchange momentum are estimated using the Syamlal & O'Brien, Gidaspow, Wen-Yu, and Huilin–Gidaspow drag functions. The results of the simulation have been validated with the experimental data available in literature and had proven that the model is capable to predict the hydrodynamics of FBR. The variation in kinetic energy of the solid phase is calculated by varying the restitution coefficient (RC) from 0.90 to 0.99. The predictions of pressure drop compare excellently with the experimental data. Finally, the effect of particle diameter on the expanded bed height has been studied for FBR.     

Solid Flow Transition in Liquid and Three-Phase Fluidized Beds

Flow transition of solids in liquid and three phase fluidized beds of Newtonian and non-Newtonian fluids have been studied in a 15.2 cm-ID pyrex glass column. The relation between the fluid flow rate and the bed porosity in three phase fluidized beds have been determined in terms of effective volumetric flux of fluid phases from the modification of the Richardson and Zaki's equation. The modified particle Reynolds Number exhibited its maximum value with the variation of bed porosity in liquid and three phase fluidized beds. The drag coefficient changed its slope apparently at the bed porosity where the maximum value of the modified particle Reynolds number could be attained. At the flow transition condition, the continuity wave velocity, energy dissipation rate, and the continuity shock wave velocity found to have their maximum values. Also, the immersed heater-to-bed and wall-to-bed heat transfer coefficients, wall-to-bed mass transfer coefficient, liquid radial mixing coefficient and solid particle diffusivity in the literature data were found to have maximum values at the transition condition of liquid and three phase fluidized beds.