Minimum spouting velocity of dense particles in shallow spouted beds

Quantitative method is used to experimentally measure the minimum spouting velocity in shallow conical spouted bed. And a new minimum spouting correlation for shallow conical spouted beds is developed. It is based on spherical ZrO₂ particles whose density is as high as 5890 kg/m³ while the other U_ms correlations published so far are mainly based on relatively deep conical beds composed of lower density particles with density around or lower than 3000 kg/m³. The new U_ms correlation can predict U_ms of heavy particles well within the range of the experimental matrix. © 2011 Canadian Society for Chemical Engineering     

Evaluation of flow regimes in a semi‐cylindrical spouted bed through statistical,mutual information,spectral and Hurst's analysis

Hydrodynamic studies were conducted in a semi‐cylindrical spouted bed column of diameter 150 mm, height 1000 mm, conical base included angle of 60° and inlet orifice diameter 25 mm. Pressure transducers at several axial positions were used to obtain pressure fluctuation time series with 1.2 and 2.4 mm glass beads at U/U_ms from 0.3 to 1.6, and static bed depths from 150 to 600 mm. The conditions covered several flow regimes (fixed bed, incipient spouting, stable spouting, pulsating spouting, slugging, bubble spouting and fluidization). Images of the system dynamics were also acquired through the transparent walls with a digital camera. The data were analyzed via statistical, mutual information theory, spectral and Hurst's Rescaled Range methods to assess the potential of these methods to characterize the spouting quality. The results indicate that these methods have potential for monitoring spouted bed operation.     

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.     

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.     

Flow Characteristics in Slot‐Rectangular Spouted Beds with Draft Plates

An experimental investigation was conducted of slot‐rectangular spouted beds with air entry slots spanning the full thickness of the column and vertical draft plates intended to help control the solids circulation rate. With increasing superficial gas velocity, the flow between the draft plates changed from bubbling to slugging and then to spouting with dilute pneumatic between the plates and moving‐bed downward motion on both sides. However, there was difficulty maintaining stability and symmetrical flow on the two sides. Once spouting is established, pressure drops and local voidages vary with gas velocity, particle size and gas entry size in broadly similar manners as for conventional spouted beds     

Spouted bed hydrodynamics in a 0.91 m diameter vessel

Hydrodynamic measurements were obtained in a flat-based half-cylindrical column of diameter 0.91 m and inlet orifice diameters of 76 to 114 mm. Beds of 3.5 to 6.7 mm diameter particles with static depths of 0.53 to 1.83 m were spouted with air. In agreement with measurements by earlier workers in smaller columns, it was found necessary to operate with inlet orifice diameters less than about 30 times the mean particle diameter in order to be able to achieve stable spouting. Correlations for minimum spouting velocity developed on small vessels generally gave poor predictions for the large diameter vessel employed in this work and failed to predict the observed dependence of U_ms on the static bed height. Substantial dead regions where particles were stagnant were observed in the lower outer portion of the vessel. Other aspects of behaviour studied, including spout diameters and shapes, fountain heights, pressure profiles and gas velocities in the annulus, were qualitatively similar to those in smaller columns, although equations developed for the smaller vessels did not always provide accurate predictions.     

Minimum Spouting Velocity of Draft Tube Conical Spouted Beds Using the Neural Network Approach

A multilayer perceptron with back‐propagation learning algorithm is developed to predict the minimum spouting velocity (u_ms) in draft tube conical spouted beds. Six dimensionless variables involving ten essential geometric and operating parameters of the beds were taken as model inputs. To compare the model results with both experimental data and those predicted by the limited existing empirical equations, the root mean square error and the mean relative error are utilized. Although there is a complex relationship between the input variables and u_ms, and despite the huge number of data available, the steps of training and testing show good agreement with the corresponding experimental values. This demonstrates that an artificial neural network is a useful approach to predict u_ms, especially when the relationship between the geometric and operating parameters and u_ms is complex and difficult to define.     

Particle segregation in a spouted bed of binary mixtures of particles

Minimum spouting velocity and segregation behaviour of binary mixtures of particles differing in size have been studied. The experiments were carried out in a bed of 20 cm diameter at superficial gas velocities up to 1.3 U_ms by use of silica sand of four different particle sizes from 0.655 to 2.23 mm. An empirical equation was proposed for the minimum spouting velocity of binary mixtures. The effects of the particle size difference and the superficial gas velocity on segregation were investigated. Results showed that considerable radial segregation as well as axial segregation occurred even for high gas velocity under the condition of large particle size difference.     

Prediction of pressure drop and minimum spouting velocity in draft tube conical spouted beds using genetic programming approach

The smart method of genetic programming (GP) is used to predict the operating pressure drop (ΔP_s) and the minimum spouting velocity u_ms for conical spouted beds (CSBs) equipped with nonporous draft tubes. Accordingly, six dimensionless variables have been taken as model inputs, including crucial parameters associated with the bed and tube geometric and operating conditions. Two general correlations comprising almost all constitutive and operating variables have been derived for the first time by the GP approach. Both ΔP_s and u_ms values predicted by the GP technique are in a fair agreement with the values corresponding to the experiments, with average absolute relative errors (AARE) of 18.9 and 19.9 %, respectively. The results of the proposed correlations show that the GP method is a powerful tool to make reasonable estimates.     

Pressure gradients,voidage and gas flow in the annulus of spouted beds

Parallel measurements of pressure gradients with a differential pressure probe and voidage profiles with a fibre optic system have been carried out to study gas flow distributions in the annulus of spouted beds. The observation of Grbavcic et al. (1976) that for a given fluid‐solid combination and column geometry the annulus pressure gradient at any bed level is independent of bed depth was corroborated again. Calibration curves of pressure drops versus superficial gas velocities for beds of voidage higher than the loose‐packed voidage were obtained by applying the Ergun (1952) equation, making it possible to estimate superficial gas velocities in the annulus using the static pressure gradient method. The local superficial gas velocity in the annulus was found to be higher in a deep bed than in a shallow bed of the same material, contrary to the conclusion (Grbavcic et al., 1976) that, for a given fluid‐solid combination and column geometry, the annulus fluid velocity at any level is independent of bed depth. Theoretical models and equations which do not account for the conical geometry near the bottom were found to underpredict superficial gas velocities in the annulus. Increasing the spouting gas flow was found to increase the net gas flow through the annulus.     

HYDRODYNAMICS AND MASS TRANSFER IN A SPOUTED BED DRYER

ABSTRACT

The disposal of sludge generated by water treatment plants poses major financial and environmental problems. Drying of the sludge is an essential pan of any disposal process which may include incinerating, landfilling or upgrading. In the present study, experiments were carried out to investigate the drying of sludge in a spouted bed and to characterize the hydrodynamics and mass transfer mechanisms. The effect of bed moisture content on the minimum spouting velocity (U_ms) was examined for sludge granules. U_ms was found to increase with increasing the moisture content of the sludge particles. The gas phase mass transfer coefficient in the dryer was determined using porous calcined alumina particles and the results were compared with predictions by correlations available in the literature. The existing correlations gave poor predictions for the mass transfer coefficient. A new correlation for the mass transfer coefficient in a spouted bed dryer was developed based on the experimental results.     

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.     

The spouting regime map,bubbling characteristics and bed expansion of deep beds of large hollow polyethylene spheres

The high velocity spouting regime of a deep bed containing 9.6 mm hollow polyethylene spheres of density 394 kg/m³ was investigated in a 152.4 mm flat bottom cylindrical half column with a 50.8 mm inlet. A bed height vs gas velocity regime diagram is presented showing the conditions under which type II spouting is observed. The transition velocity from bubbling to type II spouting is determined using the autocorrelation of the pressure fluctuations in the bed. The bubbling characteristics of the bed are described. Bed expansion in bubbling and type II spouted beds are reported.     

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.     

Simultaneous Drying and Cleaning of Guava Seeds in a Spout‐Fluid Bed with Draft Tube

A half sectional spout‐fluid bed column with draft tube was used for the cleaning and drying of guava seeds in one single operation. The process took place on a batch basis with a constant spouting velocity of 1.5 U_ms and an additional annular air flow rate equivalent to 0.5 and 0.7 U_mf. A regression model based on the spouting air temperature T_s, the annular inlet air temperature T_a and the additional annular air flow rate xU_mf was obtained to calculate the drying time. An additional cleaning step was necessary to increase the separation efficiency of the dried pulp from about 70% obtained during the drying process to 84%.     

喷动床内气固两相流体动力行为的数值模拟

引　言喷动床被广泛应用于不同工业领域中 ,如石油裂解反应 -再生器、煤和农业废弃物气化和燃烧 ,喷动床还被应用于粮食和药品的干燥等[1] .因此 ,喷动床设计应满足不同应用的要求 .喷动床内气相反应物的反应时间和停留时间依赖于床体几何结构和运行参数 .尽管已有许多的实验对喷动床内气固两相流动进行了研究 ,得到了喷射区、环形区和喷泉区内的气固两相流动流体动力特性 ,然而由于喷动床内气固两相流动的复杂性 ,人们对床体几何结构和运行参数对喷动床动力学的影响至今并不清楚 .因此 ,床体几何结构和运行参数等对喷动床动力学的影响成为…     

Pressure and voidage fluctuations in slugging fluidized beds

In fully developed slugging fluidized beds, the maximum amplitude of absolute pressure fluctuations is reached with increasing superficial gas velocity when the slug length reaches a maximum, and the separation distance between successive slugs starts to decrease. U_c, the superficial gas velocity at which absolute pressure fluctuations reach a maximum, thus indicates the early stage of transition from slugging to core-annular flow. U_c, identified based on standard deviations of differential pressure fluctuations or local voidage fluctuations, can be predicted by slug flow models and does not signify a transition to turbulent fluidization.     

Liquid spouting of pulp fibers in a conical vessel

Pulp fibers can be spouted in water in a conical vessel. The entities which are spouted are fiber flocs rather than individual fibers. Synthetic fibers, which do not flocculate, cannot be spouted. For comparison, rigid spherical particles were spouted with water in the same conical vessel. Liquid spouting of rigid particles was similar to gaseous spouting. For pulp spouting, the minimum spouting velocity is proportional to the mass of fibers in the bed and inversely proportional to the diameter of the inlet. For rigid particles, the minimum spouting velocity is proportional to the height of the bed and inversely proportional to the square of the diameter of the inlet. A model for the minimum spouting velocity was developed for pulp spouting.     

EXPERIMENTAL ANALYSIS OF A MECHANICALLY STIRRED SPOUTED BED DRYER

ABSTRACT

In order to increase the movement of the particles in the annular region of a conical spouted bed, a mechanical stirrer was incorporated. This led to a study of the fluid dynamics of the conical spouted bed of inert particles under different operating conditions: mass of inert particles, ratio of inert particles to suspension volumes, type of stirrer (2 designs) and stirrer speed.

The action of the stirrers resulted in increased movement of the particles, especially at low rpm values (≤ 90 rpm), with the loss of the annular region, and thus approaching the beds behavior to that of a fluidized bed. To characterize this novel type of spouted beds, the “minimum spouting flow” (Q_ms) parameter which is used usually was replaced by the “minimum pseudo-fluidizing flow” (Qmpr). The value of Q_mpf decreases when the agitation in the bed increases. A correlation for Q_mpf with the different operating conditions was developed.

Drying suspensions in the stirred spouted bed shows a considerable increase of drying capacity, over units without agitation.     

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.