Prediction of the maximum spoutable bed height in spout-fluid beds

A model for predicting the ratio of the maximum spoutable bed height in a spout-fluid bed to the maximum spoutable bed height in a spouted bed is presented and experimentally verified. The model is a further extension of the basic Mamuro and Hattori force balance model. The ratio H_mSF/H_m is found to be a function of the external annular fluid flowrate and a parameter C, which is the ratio of fluid velocity at the top of the annulus to that at minimum fluidization. C = 0.935 gives a best fit of our data.     

A new spouting regime in beds of coarse particles deeper than the maximum spoutable height

A new spouting regime for coarse particles in beds deeper than the maximum spoutable height has been identified and its location on a phase diagram is presented for an air spouted bed of 5 mm glass spheres. The periodic formation and breakup of single bubbles originating at the top of the spout at lower gas velocities than required to observe the new regime has been studied using high speed photographs. An analogy is drawn between these observations and the breakup of bubbles in a turbulently fluidized bed.     

A theory for predicting the maximum spoutable height in a spouted bed

A theory is presented for predicting the maximum spoutable height, in a spouted bed where fluidization of the annular solids limits the penetration of the fluid jet entering the bed. The quantity (H_md_s/(D – d) is found to be a function of d_s/D_c only for spherical particles and \[ \frac{{H_m d_S }} {{(D_c^2 - d_S^2 )}} = 0.345\left( {\frac{{d_S }} {{D_c }}} \right)^{ - 0.384} \] where the spout diameter is calculated using McNab's correlation⁽⁷⁾. Insertion of McNab's correlation into Equation (1) shows that H_m is proportional to D and the effect of d_p on H_m varies from d to d as the particle Reynolds number increases assuming 1 ≥ (d_s/D). Using experimental value of d_s, the calculated values of H_m differ from the experimental ones by 8.5% on average. When small particles are spouted with air, the penetration of the jet is limited by the formation of a slug in he spout and the theoretical value provides an upper limit for H_m.     

Hydrodynamics of spout-fluid beds

Experiments were carried out in a half-column incorporating auxiliary flow, introduced through up to five slots along a 60° conical base, in addition to a central flow of air. The column had a diameter of 0.15 m and inlet diameters of either 19 or 25 mm. Three different types of particle were investigated, all with mean particle sizes in the 2 – 4 mm range. Four different flow regimes — fixed bed, spouting with aeration, spout-fluidization and jet in a fluidized bed — were identified and mapped. The minimum total gas flow required for spouting with aeration and for spout-fluidization was always greater than that corresponding to minimum spouting. Gas percolation through the annulus increased by as much as 50% as the proportion of auxiliary flow was increased for a given total gas flow rate. Solids circulation was increased somewhat by addition of auxiliary flow for deep beds, but a decrease occurred for shallow beds. The overall bed pressure drop under minimum spouting with aeration conditions increased linearly with auxiliary flow. On the other hand, the fountain height decreased as the fraction of auxiliary flow was increased for a fixed total gas flow, and increased with auxiliary flow for a fixed central gas flow. The empirical correlation of McNab predicted the average spout diameter well if the sum of the central and auxiliary flows was used in the correlation.     

The mechanics of spout-fluid beds at the minimum spout-fluid flowrate

Equations for predicting the minimum fluid flowrate in spout-fluid beds of coarse spherical particles are developed and experimentally verified. Correlations for the spout diameter and maximum spoutable height are also given. The annular pressure drop equations developed in previous work are experimentally verified.     

Maximum spoutable bed depths of mixed particle-size beds

Maximum spoutable bed depths for mixed particle-size beds of alundum, glass and polystyrene have been found experimentally to reach a peak at a critical particle Reynolds number of about 70. Using Becker's criterion that V_{m f} = V_mf and the correlations of Smith-Reddy for V, and of Wen-Yu for V_mf, the maximum spoutable bed depths H_max were theoretically predicted to reach a maximum at a critical N_Rem = 68. The experimental maximum spoutable bed depts in a 6-in. I. D. column fitted with a 60° cone were in good agreement with theoretical predictions.     

Wall-to-bed heat transfer characteristics of spout-fluid beds

Wall-to-bed heat transfer characteristics have been investigated in a rectangular spout-fluid (S–F) bed segment column (20 cm length, 5 cm width and 50 cm height) utilizing glass beads (D_p = 0.254, 0.388 and 0.461 mm) and air as fluid. Results indicate that h values in the S–F bed increase with increasing air mass velocity and particle diameter, and decrease with increasing bed height. Under identical flow conditions h values in the S–F bed were about 30% more than for the corresponding fluidized bed.     

The maximum spoutable bed heights of fine particles spouted with air

The maximum spoutable bed heights of systems of fine glass spheres spouted with air are studied in flat based semi circular columns of 80 and 152.4 mm diameters using particles with average diameters ranging from 0.3 to 1.3 mm. New correlations are proposed to predict the maximum spoutable bed heights of air spouted fine particle systems. It is concluded that the influence of the column diameter on the maximum spoutable bed height is significantly decreased compared to coarse particle systems and that it further varies with particle size within the fine particle spouting regime. Data reported in the literature agree with the proposed correlations.     

The maximum spoutable bed heights of water-spouted fine particle systems

The maximum spoutable bed height of systems of fine glass spheres spouted with water is studied in a flat based, semi-circular column of 80 mm diameter using glass spheres of diameters 0.300, 0.377, and 0.450 mm. A new correlation is proposed to predict the maximum spoutable bed height in water spouted fine particle systems. The same correlation is suggested for use with air spouted fine particles larger than 0.5 mm. It is concluded that in fine particle systems the jet penetration is significantly lower than that in coarse particle systems. The influence of the column diameter on the maximum spoutable bed height is also decreased significantly compared to coarse particle systems. Data reported in the literature for other water spouted fine particle systems agree with the correlation proposed in this work.     

Flow regimes and combustion behaviour in coal-burning spouted and spout-fluid beds

A study of coal combustion was carried out in a 0.15-m diameter half-column spout-fluid bed combustor, in which a sub-bituminous Alberta coal was burned in inert beds of sand. High temperatures promoted spout instability leading to earlier appearance of pulsatory spouting, the jet-in-fluidized-bed regime, and slugging than for room temperature operation. While radial temperature profiles below the bed surface were quite uniform, axial profiles showed a significant temperature jump in the fountain region above the bed surface. Axial profiles of oxygen concentration fell abruptly at the bed surface due to rapid burning there. The size of inert particles and the amount of auxiliary air were found to influence the hydrodynamics and combustion behaviour.     

喷动流化床最大喷动压降的多因素影响与关联

基于截面200 mm×20 mm,高1600 mm,锥角60°的矩形喷动流化床,以二组分混合颗粒、单一组分球形颗粒及非球形颗粒为物料进行最大喷动压降的实验研究.结果表明,最大喷动压降随静止床高、颗粒密度、颗粒球形度及二组分混合颗粒体系中沉积组分分率增加而增大,随流化气速增大而减小;增大颗粒粒径或喷口宽度,呈现先减小后增...     

Prediction of attrition in agitated particle beds

The majority of pharmaceutical powders produced through crystallisation are dried in agitated dryers. The rotation of the impeller causes shear deformation of the bed, which enhances the drying rate, but also leads to particle breakage. A method of predicting the extent of breakage occurring due to agitation is described and applied for Paracetamol in a small-scale dryer. The distributions of stresses and strains in the bed are estimated using the Distinct Element Method (DEM). The information obtained here is then coupled with the measured attrition of Paracetamol in an annular shear cell in order to predict the attrition in the agitated bed. The experiments are carried out on dry material so as to establish purely the effect of stresses and strains on attrition, whilst keeping moisture content and temperature constant.The shear cell provides uniform condition for stresses and strains so that the breakage taking place under relatively well-defined conditions is quantified. In contrast, the prevailing shear stresses and strains in the agitated bed have wide distributions, as little shearing takes place near the impeller shaft, whilst there are considerable shearing stresses near the impeller tip. Therefore, the bed is divided into a number of segments for which the extent of attrition can be evaluated for each segment, based on the shear cell data. A good quantitative agreement is found between the predictions and experimental results obtained for the attrition of Paracetamol in the small scale dryer. The resulting prediction also suggests that, for a given number of impeller rotations, the extent of breakage is independent of impeller speed in the range tested (20–78 rpm). This is expected as the prevailing strain rates are too low for the inertial effects to be dominating and the shear stresses are independent of shear rates within the range investigated. The attrition prediction suggest that over half of the attrition occurs in the bottom third of the bed, with increased attrition at greater radial distances. The attrition is also predicted to occur predominantly within the region extending from 30° in front of to 30° behind the impeller.     

Pressure and flow characteristics of a gas phase spout-fluid bed and the minimum spout-fluid condition

Spoutfluidization is a new technique for solid-fluid contact which aims at incorporating the advantages of spouted bed and fluidized bed technique. The characteristics of physical state of the bed with the variation of the variables which include flow of fluid, particle diameter, orifice diameter, bed height, are studied in this investigation. Experimental study of minimum spoutfluidizing velocity using glass beads with mean particle diameters from 0.254 to 0.600 mm has been carried out in a 90 mm glass column with three spouting inlet orifice sizes at different bed heights. Phase diagrams indicate that the minimum spout-fluid flow rate in a gas-solid system may be a point property for a given bed. A correlation is presented in which the standard deviation of experimental from calculated minimum spoutfluidizing velocity is within 5%.     

Prediction of liquid phase hold-up in random packed beds

Experimental data on liquid phase hold-up in packed beds consisting of Raschig rings, Lessing rings and spheres are presented. A generalized correlation has been proposed for predicting the liquid phase hold-up in terms of packing density equivalent spherical diameter, liquid viscosity, density and flow rate.     

Pressure drop and flowrate characteristics of a liquid phase spout-fluid bed at the minimum spout-fluid flowrate

Data on the minimum spout-fluid flowrates and the pressure drop in the annulus at that flowrate are presented as a function of bed height and nozzle diameter for a bed spout-fluidized with water. The column was 62.6 mm. in diameter and contained 3.09 mm. glass particles. The particle Reynolds number at the minimum fluidizing velocity was 91.6. A plot of the annular vs. the nozzle flowrate shows that the minimum spout-fluid flowrates fall on a straight line between the minimum fluidizing and minimum spouting flowrates. The annular pressure drop is explained using an extension of the theory of Mamuro and Hattori.     

Pressure distribution in spout-fluid bed reactors

The occurrence of several flow regimes in spout-fluid beds was investigated. Four different flow regimes, viz. a packed bed flow regime, a bubbling and a fluctuating spouted bed flow regime and a stable spouted bed flow regime, were found to exist. Pressure distributions in a spout-fluid bed were measured in several of these flow regimes by means of a moveable pressure probe. A theoretical model that describes the flow pattern in spout-fluid beds was developed from fundamental relationships that govern the flow of gases through porous media. Pressure distributions calculated from this model agree fairly well with measured values.     

Prediction of heat transfer to liquid-solid fluidized beds

Heat transfer coefficients to a liquid-solid fluidized bed in a cylindrical tube have been measured using water as liquid phase and three types of cylindrical steel particles, as well as glass, nickel, copper and lead spheres of different sizes as solid phase. The independent varaibles included heat flux, liquid velocity and particle physical properties. The experimental results as well as a data bank containing a large number of measured heat transfer coefficients for solid-liquid fluidization over a wide range of operational parameters have been compared with the predictions of most published correlations. A model for the prediction of heat transfer coefficients is proposed which predicts the present experimental data and the data of other investigators with good accuracy.     

Prediction of critical fluidization velocity and maximum bed pressure drop for binary mixture of regular particles in gas–solid tapered fluidized beds

The problems associated with conventional (cylindrical) fluidized beds, viz., fluidization of wider size range of particles, entrainment of particles and limitation of fluidization velocity could be overcome by using tapered fluidized beds. Limited work has been carried out to study the hydrodynamics of single materials with uniform size particles in tapered beds. In the present work, an attempt has been made to study the hydrodynamic characteristics of binary mixtures of homogeneous and heterogeneous regular particles (glass bead and sago) in tapered fluidized beds having different tapered angles. Correlations have been developed for critical fluidization velocity and maximum bed pressure drop for gas–solid tapered fluidized beds for binary mixtures of regular particles. Model predictions were compared with experimental data, which were in good agreement.