A comparative hydrodynamic study of two types of spouted bed reactor designs

In order to properly design and scale up spouted beds, one needs to predict the minimum spouting velocity of specific systems having different bed dimensions, and properties of particle and spouting gas. Because of inherent complexity of predicting minimum spouting velocity, the prevailing approach has been to use empirical correlations, a number of which are available in the literature. Central jet distributors are commonly used in the experimental studies reported in the literature. Circular slit distributor is a new concept in which air is supplied to the bed of particles through a circular slit. This paper presents results of an experimental study on the hydrodynamics of central jet and circular slit distributors. In this paper a fully connected feed-forward neural network model was used to predict the minimum spouting velocity of central jet and circular slit spouted beds. A neural network model was also developed to predict minimum fluidization velocity. The actual experimental data obtained from published literature and from the experiments carried out in this study were used for training and validating the models. The minimum spouting and fluidization velocities predicted using the neural network models developed in this study show a better approximation to the actual experimental values than those obtained from correlations available in the open literature. The position of flow regime of circular slit spouted bed was also established relative to the flow regimes of central jet spouted bed and fluidized bed.     

PREDICTION OF MINIMUM SPOUTING VELOCITY IN TWO-DIMENSIONAL FLAT BOTTOM SPOUTED BEDS

Spouted beds have been used in industry for operations such as drying, catalytic reactions, and granulation. Conventional cylindrical spouted beds suffer from the disadvantage of scaleup. Two-dimensional beds have been proposed by other authors as a solution for this problem. Minimum spouting velocity has been studied for such two-dimensional beds. A force balance model has been developed to predict the minimum spouting velocity and the maximum pressure drop. Effect of porosity on minimum spouting velocity and maximum pressure drop has been studied using the model. The predictions are in good agreement with the experiments as well as with the experimental results of other investigators.     

A Discussion on Minimum Spout Velocity and Jet Penetration Length

The similarity and difference between the flat‐bottom and cone‐bottom cylindrical spouted beds, conical spouted beds and vertical upward jets in fluidized beds have been analyzed in this paper based on the effects of geometrical parameters on the minimum spouting velocity and operating stabilities of the spouted beds. The effect of angle on minimum spouting velocity was found to be only significant within the range of 30 to 60 degrees cone angles. Minimum spouting velocity in deep cylindrical spouted beds was proportional to the square root of the static bed height, but was proportional to the static bed height in conical spouted beds and large cylindrical spouted beds with small height‐to‐diameter ratio. The relationship between the minimum spouting velocity and the static bed height was consistent with that between jet velocity and the vertical jet penetration length in jetting fluidized beds.     

Minimum spouting velocity for binary mixture of particles in rectangular spouted beds

Minimum spouting velocity data are reported for seven mixtures of polystyrene, spouted with air in three rectangular spouted beds of square cross sectional areas, using nozzles of five sizes. Correlations for minimum spouting velocity in cylindrical columns were investigated for this geometry. A new dimensionless correlation for predicting the minimum spouting velocity is developed.     

Minimum spouting velocity of scaled-up beds

This note presents the results for minimum spouting velocity in beds up to 1.1 m diameter. New evidence is provided that for fixed aspect ratio the minimum spouting velocity first falls and then begins to rise as bed diameter is increased. A simple correction factor applied to the Mathur and Gishler minimum spouting velocity correlation is required for beds ≥ 0.4 m diameter.     

Pressure drop in slotted spouted beds of grains: Comparison of data with models

Data obtained in a rectangular slot spouted bed for two different grains (wheat and oats) have been analyzed to determine the minimum spouting velocity (V_ms) and the minimum spouting pressure drop (ΔP_ms) as a function of the bed height.Three different aerodynamic regimes occur as the bed height is increased.The results are compared with the literature models for the conventional (conical-cylindrical) spouted bed.For shallow beds, the experimental minimum spouting velocity and minimum spouting pressure drop can be well represented by the Littman et al.'s two-dimensional model.     

Minimum spouting velocity under vacuum and high temperature in conical spouted beds

A study has been carried out on spouting performance in conical spouted beds at high temperatures and under vacuum by using materials of different textures, densities and particle diameters. The applicability of existing correlations for determining the minimum spouting velocity has been checked and, based on a statistical analysis to assess the influence of the dimensionless modulus, a new one has been proposed for the prediction of the minimum spouting velocity under vacuum and at high temperatures. This correlation provides good predictions in the pyrolysis of tyres under vacuum.     

HYDRODYNAMIC STUDIES IN A HALF SLOT-RECTANGULAR SPOUTED BED COLUMN

Experiments were carried out in a half slot-rectangular spouted bed to investigate the effects of slot width and lower section basal angle on column hydrodynamics. Flow regimes, minimum spouting velocity, spouting and maximum pressure drops, and maximum spoutable bed height were determined for 4 mm diameter polyethylene particles. The results are compared with those for conventional cylindrical and rectangular spouted beds. Correlations for each hydrodynamic parameter are developed and compared with equations available in the literature.     

HYDRODYNAMIC STUDIES IN A HALF SLOT-RECTANGULAR SPOUTED BED COLUMN

Experiments were carried out in a half slot-rectangular spouted bed to investigate the effects of slot width and lower section basal angle on column hydrodynamics. Flow regimes, minimum spouting velocity, spouting and maximum pressure drops, and maximum spoutable bed height were determined for 4?mm diameter polyethylene particles. The results are compared with those for conventional cylindrical and rectangular spouted beds. Correlations for each hydrodynamic parameter are developed and compared with equations available in the literature.     

Spouting Enhancement by Addition of Small Quantities of Liquid to Gas‐Spouted Beds

The addition of liquid water to beds of polystyrene particles exceeding 3 mm in diameter enhanced spouting by causing a decrease in the minimum spouting velocity. While the degree of enhancement is quantitatively comparable to what we reported previously for fluidized beds of large light hydrophobic particles, the mechanism is quite different. Enhancement of spouting takes place because the liquid congregates at the spout‐annulus interface, forming an effective nearly‐impermeable draft tube which then increases the velocity of the gas up the spout.     

Pressure Fluctuations in Jet Spouted Beds

Jet spouted beds that consisted of a transparent Plexiglas cylindrical column of 1 m high and a conical base with cone angles of 30°, 36°, and 40° were used in this study. The particles used were spherical glass beads with an average diameter of 1.7, 2.1 and 3 mm, respectively, and particle size of 2.2 – 3.1 mm, non‐spherical rice particles. The effect of size and shape of particles, and static bed height on the minimum jet spouting velocity, and standard deviation of pressure fluctuations, was investigated. The results show that the minimum jet spouting velocity and pressure drop increased as the bed height and particle size increased. The minimum jet spouting velocity could be determined from the plot of standard deviation of pressure fluctuations vs. superficial gas velocity. The results obtained were in close agreement with the results of other methods in the literature.     

Minimum spouting velocity in multiple spouted beds

Studies have been carried out in multiple spouted beds having 2, 3 and 4 spout cells; different fluid inlet orifices and different solids have been used with air and water as spouting fluids. The minimum spouting velocities are measured for different bed depths. The experimental data for particle Reynolds number at minimum spouting have been correlated and the square root mean deviation between the calculated and experimental values is found to be 8.75 %.     

Effect of interparticle forces on the conical spouted bed behavior of wet particles with size distribution

This paper aims to analyze air-solid flow behavior in conical spouted beds composed of glass bead mixtures coated by glycerol. Four mixtures of glass beads are used as the solid phase. Although these mixtures have the same mean Sauter diameter, each one is characterized by a different size distribution function (mono-sized; flat, Gaussian or binary size distribution). When glycerol is added to the bed of these particles, which are spouted by air, the gas-solid flow characteristics are changed due to the growth of interparticle forces; however, the trends of these changes are affected by the glass bead mixture type as well as by the concentration of glycerol. For beds of mono-sized particles, the minimum spouting velocity is maintained almost unchanged as the glycerol concentration rises; while, for beds of inert particle mixtures, this velocity increases, becoming greater for flat and binary size distribution particles. Conversely, the minimum spouting pressure drop decreases as the glycerol concentration rises for all beds of particles used. Based on theoretical prediction of interparticle forces, it is shown that these changes in the minimum spouting conditions can be explained by the magnitude of these forces.     

Hydrodynamic characteristics of jet-spouted beds

Jet-spouted beds characterised by high velocity gas jets (above 1.7 U_msl), and shallow bed depths H₀ of around 2 D₁ were investigated on laboratory scale beds and industrial scale beds and the results obtained thereof are correlated and presented in this work. Compared with the classical spouted beds, important differences in bed structure, solid movements and basic hydrodynamic characteristics were observed. The minimum spouting velocity, bed voidage and pressure drop during stabilized spouting are described in terms of dimensionless equations. Bed expansion was used as the basis for the classification of different jet-spouting regimes (incoherent spouting, fast spouting, pneumatic conveying) and changes in the slope of the bed expansion curve are correlated with regime changes. This classification could be useful in the optimization of industrial scale jet-spouted beds. A typically applicable regime of fast spouting was identified.     

The characteristics of fluid flow in beds of small glass particles spouted with water

The characteristics of beds of small glass particles 0.28, 0.46 and 0.77mm in diameter spouted with water were studied in a half-cylindrical column 51mm in diameter with inlet tube diameter of 3.2mm. The minimum spouting velocity, bed pressure drop at minimum spouting and spout diameter were measured. Assuming Darcy flow, the fluid flow in the annulus is modeled and shown to represent the streamlines quite well. The residence time of the fluid in the annulus is calcuated from the model and compared with experimental data.     

Spouting of two-dimensional beds with draft plates

Two slotted two-dimensional spouted bed units with flexible bed dimensions were used with draft plates to study spouting pressure drop and minimum spouting superficial velocity. The data were collected while varying slant angle, spout width, separation distance, length of bed, height of bed, and size of bed (geometrical similarity) using shelled corn, soybean, and wheat. The variables which affect the spouting pressure drop and air flow through the beds are discussed. Empirical correlations are developed following the principles of dimensional analysis and similitude. The developed correlations agree closely with the collected data.     

粗颗粒在锥型床中的流化特性

通过Ｇｅｌｄａｒｔ－Ｄ类粗颗粒在三种锥形床中的实验表明：随着入口的表观气速的增加,在床中依次出现固定床流区,部分流化区和喷动流区。在固定床区内,压降和表观气速关系可以通过Ｅｒｇｕｎ公式求得：在部分流化床区内;实验和理论计算均表明了由于颗粒所受到的曳力档的轴向高度增加而减小,造成床层轴向上部未流化的颗粒对下部硫化颗粒膨和压制作用,在喷动流化区内,给出了预测锥形床的初始喷动气速和喷动压降的经验公式。     

双喷嘴矩形喷动床流动性能实验研究

在120 mm×240 mm的双喷嘴矩形不锈钢床内,对新型双喷嘴矩形导流管喷动床的最小喷动速度和喷动高度进行了研究,考察了喷动气速、粒径、静床层高度、导流管直径、导流管安装位置对最小喷动速度和喷动高度的影响。结果表明:最小喷动速度随颗粒直径、导流管直径、导喷距的增大而增大,随静床层高度的增大而减小;喷动高度随喷动气速的增大而增大,随导流管直径的增大而减小,受静床层高度和导喷距的影响不大,并得出了最小喷动速度的关联式。     

Determination of Minimum Spouting Velocities in Conical Spouted Beds

Minimum spouting velocities in conical spouted beds have been obtained from pressure drops versus the superficial gas velocity curves, based on both increasing and decreasing the superficial gas velocity. It has been shown that the minimum spouting velocity from decreasing the superficial gas velocity is lower than from increasing the superficial gas velocity in most cases. This phenomenon is similar to that in conventional spouted beds and different from the early works. The experimental results also showed that there isn't significant difference in the pressure drop and U_ms under identical operating conditions between semi‐circular and circular conical spouted beds, and the same U_ms can be obtained from absolute pressure drops at any position above the gas inlet. The U_ms is found to increase with increasing the cone angle and static bed height, as well as the gas inlet diameter to a less extent.     

An experimental study of spouting in large sector beds

In a sector spouted bed, fluid is introduced into the bottom at the junction of two vertical walls, the bed forming a sector in horizontal cross-section. The hydrodynamics of beds of 30° sector angle have been studied in equipment of radius up to 0.78 m. The spout radius is greater than would be predicted from standard correlations on small circular beds. The overall pressure drop can be less than 20% of that found for minimum fluidisation. The vertical pressure distribution is given approximately by a quarter cosine function. Minimum spouting velocity decreases with increasing size of bed, loss of spouting being caused by a decrease in fountain height followed by snuffing of the spout by annular solids.