Beds Of Large Particles Research Articles

Effect of Horizontal Tube Position on Wall-to-Bed Heat Transfer in Air-Solid Fluidized Bed of Large Particles

Fluidized beds are extensively used in a variety of industrial applications, such as heat recovery systems, coal combustion, and solid particle drying. The effect of the position of the heat transfer tube on heat transfer from a horizontal bare tube in a 150 mm square cross-section fluidized bed containing large particles, ragi, and mustard is investigated. The influence of varying fluidizing velocities on heat transport is also examined for a fixed bed height of the large particles. When the position of the tube from the distributor plate was increased in 0.028m increments from 0.52 m to 0.080 m and then to 0.108 m, the heat transfer coefficient was reduced. Due to enhanced fluidization, all bed heights yielded almost comparable heat transfer coefficient values at velocities exceeding 1 m/s, 170 and 160 W/m2K for ragi and mustard, respectively. The dependency of the heat transfer coefficient on fluidizing velocity is qualitatively like that of small particles. An error deviation of around 8.8% was found when comparing experimental heat transfer coefficient values to those predicted from empirical correlations in the published literature.

Influence of Open Area Ratio in Distributor on Wall to Bed Heat Transfer Coefficient from a Single Horizontal Tube to Fluidized Bed of Large Particles

Influence of Open Area Ratio in Distributor on Wall to Bed Heat Transfer Coefficient from a Single Horizontal Tube to Fluidized Bed of Large Particles

Gasification of powdered coal in filtration regime with a super adiabatic heating

A new method is proposed for gasification of a powdered coal in filtration regime with superadiabatic heating. The fuel is supplied suspended with the flow of oxidant gas, whereas the gas flow filters through a porous bed of large particles making a heat recovery medium. Owing to the heat exchange between countercurrent solid and gas flows part of the heat is recuperated and transferred from the products to the reaction zone. The heat accumulation in a relatively narrow combustion zone can result in the temperature within the combustion front substantially higher than the adiabatic temperature calculated for the initial temperature of the reactants being that ambient and the heat of the reaction is evenly distributed over the products.

Study of particle residence time in a pressurized fluidized bed with in-bed heat exchanger tubes

Anthropogenic climate change is amongst the greatest of challenges to human civilization. A key area that will play a large role in mitigating its effects are clean fossil fuel applications. Clean coal combustion can be achieved with an oxygen-fired pressurized fluidized bed combustor incorporating carbon capture and storage. In relation to pressurized fluidization processes, understanding the influence of pressure on fluidized bed hydrodynamics and, in turn, their effect on parameters including fuel residence time is essential. For the combustor under consideration here, a fraction of the heat exchanger boiler tubes are submerged in the fluidized bed such that the effect of the horizontal tube bundle on the fuel residence time is of great importance. The main focus of the present work was to evaluate the impact of gas velocity, pressure, presence or absence of a horizontal tube bundle and fuel feed rate on the average fuel residence time in a dense gas-solid fluidized bed. Experiments were conducted under cold flow conditions in a pressurized fluidized bed with an inner diameter of 0.15 m. The fluidization material was large glass beads (1.0 mm in diameter) while fuel particles were simulated with smaller glass beads (64 and 83 μm in Sauter mean diameter) that were susceptible to entrainment. Operating pressures and superficial gas velocities were maintained between 101.3 and 1200 kPa and 1.5 and 3.2 Umf, respectively. To simulate continuous fuel injection, experiments were conducted with the fuel surrogate particles being continuously fed to the fluidized bed of large particles over a desired period of time. Downstream, entrained particles were captured to determine the average entrainment rate and average mass of fuel particles inside the fluidized bed at steady state, which yielded the average fuel residence time. The combination of elevated pressure with the tube bundle present was found to have the most influential impact when compared to base conditions of atmospheric pressure and with no tube bundle present. It was found to enhance gas bubble break up and reduce the average gas bubble size substantially. In turn, this increased the average residence time of 83 μm particles by nearly three-fold in comparison to the case of atmospheric pressure with no tube bundle present. The effect of gas velocity on particle residence time was not found to be statistically significant under the range tested. Similarly, the effect of increasing fuel feed rate by 50% had no statistically significant impact.

Magnetic resonance imaging of single bubbles injected into incipiently fluidized beds

Rapid magnetic resonance imaging is used to investigate the volume, shape and rise velocity of single isolated bubbles injected into incipiently fluidized beds as well as the particle velocity field surrounding these bubbles. The volume of gas injected and particle size are varied to investigate their effects on bubble behavior. Image processing is used to quantify data into plots which can be compared directly with predictions from analytical and numerical models. Results show that more injected gas leaks into the interstitial flow in beds of large particles than beds of small particles because of the higher permeability to gas flow. Bubbles develop from a taller shape with a large wake angle into a wider shape with a smaller wake angle as they develop from a spherical shape to a spherical cap shape as they rise. Bubble rise velocities for fully formed bubbles are consistent with correlations in the literature.

DEM study of blockage behaviour of cohesive fine particles in a packed structure of large particles

Blockage of cohesive fine particles in a packed structure of large particles may happen in many industrial processes, of particular interests to ironmaking COREX technology. In this work, blockage behaviour of cohesive fine particles in a simplified packing structure is studied by means of discrete element method. The typical phenomena about effects of sticking force on descending velocity and blockage frequency is described. Two kinds of blockage mechanisms of fine particles in a packed bed of large particles are explored: mechanical interactions between fine particles will create arches on the pore throat and stop the flow; fine particles with high interparticle stickiness will adhere to the surface near the pore throat of the orifice, thus the sectional area of the flow channel will decrease. With the increase of sticking force, the descending velocity will decrease, while the frequency of the blockage increases. In addition, the effects of size ratio of fine particle to packing particle, mass flow rate and magnitude of the upward force (e.g. from a gas stream) on blockage behaviour are also discussed. The results show that increasing the size ratio, mass flow rate and external force will increase the frequency of the blockage. The study provides a fundamental understanding on blockage behaviour of cohesive fine particles in a packed bed, and is useful for industry processes' understanding and optimization.

Effect of material type and particle size distribution on pressure drop in packed beds of large particles: Extending the Ergun equation

Abstract The dependence of packed bed pressure drop on variables such as particle size distribution (PSD) and material type are of vital importance in the design of industrial equipment including fixed- and fluidized-bed reactors, blast furnaces and fixed-bed gasifiers. The pressure drop across a packed bed is commonly calculated using the Ergun equation that was developed using experimental data from laboratory-scale beds comprised of small, mono-sized, smooth, non-porous, spherical or nearly spherical particles. In the industrial applications much larger poly-dispersed particles are used, raising the need for a correlation based on more relevant measurements. This study deals with an extension of the Ergun equation to packed beds of large coal, char and ash particles with different average particle diameters and different PSD widths. The research presented here has shown the influence of material type and PSD on both particle properties (sphericity) and packed bed properties (voidage and Sauter mean diameter). In turn these have a significant impact on the subsequent bed pressure drop. It was determined that a bed of ash particles has the highest voidage, followed by the char bed and then coal bed with the lowest voidage. The difference may be attributed to differences in particle sphericity as well as the surface roughness of the particles. In all cases the particle diameter had a lesser effect on bed voidage compared to PSD width, as wider PSD was associated with a lower bed voidage due to smaller particles filling the spaces between the larger particles. New values of the Ergun equation constants were obtained via regression analysis from pressure drop data generated for coal, ash and char particles. The values applicable to coal (77.4 and 2.8), char (160.4 and 2.8) and ash (229.7 and 2.3) particles were found to better approximate bed pressure drop compared to those used in the original form of the Ergun equation (150 and 1.75). The modified Ergun equation can successfully be used to predict pressure drop in a composite packed bed of coal, char and ash particles mimicking the bed structure in an industrial packed-bed gasifier.

Artificial Neural Network Based Prediction of Heat Transfer From Horizontal Tube Bundles Immersed in Gas–Solid Fluidized Bed of Large Particles

Artificial neural network (ANN) modeling of heat transfer from horizontal tube bundles immersed in gas fluidized bed of large particles (mustard, raagi and bajara) was investigated. The effect of fluidizing gas velocity on the heat transfer coefficient in the immersed tube bundles in in-line and staggered arrangement is discussed. The parameters particle diameter, temperature difference between bed and immersed surface were used in the neural network (NN) modeling along with fluidizing velocity. The feed-forward network with back propagation structure implemented using Levenberg–Marquardt's learning rule in the NN approach. The predictions of the ANN were found to be in good agreement with the experiment's values, as well as the results achieved by the developed correlations.

Neural network optimization by comparing the performances of the training functions -Prediction of heat transfer from horizontal tube immersed in gas–solid fluidized bed

This paper describes the selection of training function of an artificial neural network (ANN) for modeling the heat transfer prediction of horizontal tube immersed in gas–solid fluidized bed of large particles. The ANN modeling was developed to study the effect of fluidizing gas velocity on the average heat transfer coefficient between fluidizing bed and horizontal tube surface. The feed-forward network with back propagation structure implemented using Levenberg–Marquardt’s learning rule in the neural network approach. The objective of this work is to compare performances of five training functions (TRAINSCG, TRAINBFG, TRAINOSS, TRAINLM and TRAINBR) implemented in training neural network for predicting the heat transfer coefficient. The comparison is shown on the basis of percentage relative error, coefficient of determination, root mean square error and sum of the square error. The predictions by training function TRAINBR found to be in good agreement with the experiment’s values.

Experimental investigation of horizontal tube immersed in gas–solid fluidized bed of large particles using artificial neural network

The average heat transfer coefficient is determined between the fluidizing bed and horizontal tube surface immersed in the bed of large particles. The mustard (dp=1.8mm), raagi (dp=1.4mm) and bajara (dp=2.0mm) were used as particles in the bed. The effect of fluidizing gas velocity on the heat transfer coefficient in the immersed horizontal tube is discussed. The results obtained by experiment are compared with correlations and artificial neural network modeling. The parameters particle size, temperature difference between bed and immersed surface were used in the neural network modeling along with fluidizing velocity. The feed-forward network with back propagation structure implemented using Levenberg–Marquardt’s learning rule in the neural network approach. The network’s performance tested with regression analysis. The predictions of the artificial neural network were found to be in good agreement with the experiment’s values, as well as the results achieved by the developed correlations.

Axial transport within bidisperse granular media in horizontal rotating cylinders

The discrete element method has been used to examine axial dispersion within rotating cylinders containing two sizes of particle. Two bed configurations are considered: initially segregated, which consists of a pulse (narrow axial band) of small particles within a bed of large particles, and initially mixed, in which the cylinder is loaded with a homogeneous mixture of the two particle sizes. The dispersion of the small particles within initially segregated beds is found to depend strongly on the initial length of the pulse of small particles. Initially mixed beds are found to undergo a transient period in which the small particles disperse rapidly. Following this transient, axial dispersion of both particles sizes is found to follow Fick's second law, in that the mean squared deviation of the axial position of the particles is proportional to time. Axial dispersion coefficients have been calculated for initially mixed beds that have reached steady state; the axial dispersion coefficients of both particle sizes decrease as the volume fraction of small particles is increased.

スフェアパック燃料ピン製造時における小粒子浸透過程の観察

In order to optimize sphere-pack fuel pin fabrication process, internal structure of fuel pin under small particle infiltration process was observed by micro focus X-ray CT system. During the settling of small particles through the bed of large particles, some small particles are localized in the space between large particles. When small particles reach the bottom of the fuel column, they fill the space between large particles forming a complete packing layer. Sphere-pack process proceeds with the growth of the layer from the bottom to the top of the column. A unique layer is formed near the top of fuel column. In the layer, small particles completely intrude between large particles and pin wall. The structure did not change after the completion of small particle infiltration process. The formation of the unique layer was controlled by the strength of a constraint of the layer surface.

SORPTION DRYING OF SOYBEAN SEEDS WITH SILICA GEL. I. HYDRODYNAMICS OF A FLUIDIZED BED DRYER

ABSTRACT Fluidization technology has been employed to mix soybean seeds and silica gel particles in aim of sorption drying of particulate agricultural products. The characteristic fluidization velocities, mixing mechanisms and fluidization quality have been studied in a 180 mm I.D. fluidized bed. Two sizes of silica gel particles (SG2 and SG3) were selected, with the mass fractions in the range of 0.33–0.75 to form a static bed with height from 100 to 280 mm (H/D = 0.56–1.56). The results show that the dispersion rate of soybean seeds is increased with addition of either SG2 or SG3, and that the frequency and the span of pressure fluctuations within the bed are increased. It is inferred that the gas–solids contacting is improved with addition of small particles in the bed of large particles. In addition, for practical design and operation of such a fluidized bed of binary mixtures, empirical correlations for characteristic fluidization velocities were developed with a wide application.

Unsaturated liquid percolation flow through nonwetted packed beds

AbstractThe unsaturated flow of liquid through packed beds of large particles was studied using six different liquids, all with contact angles greater than 90° on the bed packing (wax spheres of 9, 15 and 19.4 mm diameter). The liquidflow was discrete in nature, as drops for low flow rates and rivulets for high flow rates. For unsaturated liquid flows, the actual percolation velocity, not superficial velocity, should be used to characterize the flow. The percolation velocity did not vary with packed‐bed depth, but was a strong function of liquid flow rate, liquid and particle properties. Effects of liquid and particle properties (but not flow rate) are well captured by a simple correlation between the liquid‐particle fiction factor and Reynolds number based on actual percolation velocities. Liquid dispersion, characterized by the maximum dispersion angle, varies significantly with liquid and particle properties. The tentative correlation suggested here needs further validation for a wider range of conditions.

Hydrodynamics of large particle fluidization in reduced pressure operations: an experimental study

Experiments were carried out to study effect of vacuum conditions over hydrodynamics of fluidization in large particle beds. Fluidization is one of the most attractive industrial processes due to its very high rates of heat and mass transfer provided by a good contact between particles and fluid. A series of experiments was carried out to analyze the hydrodynamics of a vacuum fluidized bed. A fluidization column, made of glass, with an i.d. of 100 mm was employed and the vacuum conditions were established through a vacuum pump. The tests were run with 10 different types of large particles (2250–7730 μm) under various temperatures and vacuum pressures. Experimental results show that the minimum fluidization velocity increases, decreasing the operation pressure. This tendency fully confirms the results obtained by Llop et al. [Chem. Eng. Sci. 51 (1996) 5149] under vacuum conditions. Using the experimental results two equations, one for sharp particles the other one for round particles, were developed to predict the minimum fluidization velocity and the results were compared with the other equations proposed in the technical literature. On the other hand, the experimentally obtained values of bed voidage under minimum fluidization conditions were found to be insensitive to operation pressure as also observed by Chitester et al. [Chem. Eng. Sci. 39 (1984) 253].

Effects of Packed Structure and Liquid Physical Properties on Liquid Flow Behaviors in Trickle Bed with Non-uniformly Packed Structure.

The packed structure of the lower part of a blast furnace is modeled with packing two different sizes of hydrophobic particles in a rectangular container. The interface between the small and large particle beds is inclined at a certain angle. Glycerin aqueous solutions with different concentrations are used to vary the liquid viscosity from 1 to 100 mPa·s. Isoamyl alcohol aqueous solution is used as the liquid with low surface tension. Two-dimensional distribution of liquid flow rate in the trickle bed with non-uniformly packed structure is measured. Both the packed angle and liquid physical properties have significant effcts on the distribution of liquid flow rate in the trickle bed with non-uniformly packed structure. The liquid flow rate in the small particle bed decreases with increasing packed angle. The liquid flow rate along the interface between different particle beds increases with increasing liquid viscosity. When the liquid surface tension is lower, the liquid flow rate at the interface between different particle beds becomes lower and the distribution of liquid flow rate becomes more uniform. The experimental equation proposed in previous paper is modified by using the data for small particle and low surface tension. A percolation model combined with the above experimental equation is developed to predict the liquid flow distribution in non-uniformly packed trickle bed. The calculated results show the same tendencies of the experimental data.

99/00790 Effect of in-bed recirculation of particle on the design of heat transfer surface of dense phase zone in circulating fluidized bed

The circulating fluidized bed combustion technology has been extensively developed in recent years all over the world. The size of fed coal ranges from 0 to 10 mm and the mean diameter of coal ranges from 2 mm to 3.5 mm leading to a the fluidized bed which consists of a dense phase zone (large particle bed) and a dilute phase zone (small particle bed). This means that the coal combustion fraction in the dense phase zone is large, so one must pay more attention to the heat balance calculation and amount of heat exchange surface area in the dense phase zone. If the heat exchange surface area is not sized correctly, the circulating fluidized bed boiler can not be operated in a steady manner. This paper analyses the heat balance and heat exchange surface design in the dense phase zone for a circulating fluidized bed boiler of 75 t/h steam capacity which is widely used in China. The effect of in-bed particle recirculation on the amount of heat exchange surface area in the dense phase zone is also described.

Confirmation of Faraday's explanation of bunkering in vibrated granular beds

Faraday, like Chladni earlier, saw a shallow layer of a fine powder collect in a circular heap (a ‘minibunker’) at an antinode of a vibrating plate. He saw ‘Faraday circulation’: powder motion in a shallow surface layer of the heap, downward from its center to its edge; inward motion centerward at the edge; by inference, motion upward toward the middle of the heap's interior. What he saw led him to postulate formation of a partial vacuum beneath the powder heap, creating (1) an external wind blowing the powder toward an antinode and (2) a flow of air inward from the heap's edge, driving powder toward its center. Although some experimentalists have recently advanced alternative explanations of vibrated-bed heaping (‘bunkering’), we are able to confirm the essentials of Faraday's thought. At suitable amplitude and frequency, vertical sinusoidal vibration of a fine-powder bed causes it, in each vibration cycle, to experience a free-flight interval during which pressure gradients in its interior drive powder centerward. A center-high bunker forms, displaying Faraday circulation. When bed-floor collision terminates flight, pressure gradients reverse direction; but passage of a compaction front has locked particles against further movement. Before a next flight interval, an increase in porosity will reverse the compaction that accompanied heap-floor collision. In particles 177-μm and larger, we see the compaction front cinematographically; in 88-μm particles, we infer it from floor-pressure data. Although, given sufficient time, a bed of large particles (e.g., 707 μm) will form a bunker, it displays the wall-friction-driven circulation elucidated by Muchowski, not the Faraday pattern.

EVALUATION OF OVERALL HEAT TRANSFER RATES BETWEEN BUBBLING FLUIDIZED BEDS AND IMMERSED SURFACES

A mechanistic model to evaluate heat transfer rates between the dense phase of gas fluidized beds and immersed surfaces has been recently presented by the authors. This model, denoted Generalized Heterogeneous Model (GHM), is formulated in terms of effective thermal properties for particles and interstitial gas. It has been conceived with the purpose of achieving a generalized formulation accounting simultaneously for conductive, gas convective and radiant effects. The model was previously tested as regards its capability to predict radiative heat transfer rates in beds at high temperature and gas convective contribution in beds of large particles and high operating pressures. It is the principal object of this contribution to evaluate the performance of the GHM for a wide range of particle sizes, covering from The purely conductive regime to the gas convection dominant regime. Also, the main assumptions incorporated in the model are revised and some modifications are introduced, mainly on the basis of the results obtained by Mazza et al. (1997b).

Radiation Heat Transfer in Fluidized Beds: A Comparison of Exact and Simplified Approaches

Radiation heat transfer at heat exchanger walls in fluidized beds has never been examined through a complete formulation of the problem. In this paper a wall-to-bed heat transfer model is proposed to account for particle convection, gas convection, and radiation exchange in a variable porosity medium. Momentum, energy, and intensity equations are solved in order to determine the velocity, temperature, radiative heat flux profiles and heat transfer coefficients. The discrete-ordinates method is used to compute the radiative intensity equation and the radiative flux divergence in the energy equation. Both the gray and the non-gray assumptions are considered, as well as dependent and independent scattering. The exact solution obtained is compared with several simplified approaches. Large differences are shown for small particles at high temperature but the simplified solutions are valid for large particle beds. The dependency of radiative contribution on controlling parameters is discussed.