Prediction of minimum fluidization velocity for vibrated fluidized bed

The prediction of minimum fluidization velocity for vibrated fluidized bed was performed. The Geldart group A and C particles were used as the fluidizing particles. The method based on Ergun equation was used to predict the minimum fluidization velocity. The calculated results were compared with the experimental data.The calculated results of minimum fluidization velocity are in good agreement with experimental data for Geldart group A particles. For group C particles, the difference between the calculated results and experimental data is large because of the formation of agglomerates. In this case, the determination of agglomerate diameter is considered to be necessary to predict the minimum fluidization velocity.     

Experimental and numerical research for fluidization behaviors in a gas–solid acoustic fluidized bed

The effects of sound assistance on fluidization behaviors were systematically investigated in a gas–solid acoustic fluidized bed. A model modified from Syamlal–O'Brien drag model was established. The original solid momentum equation was developed and an acoustic model was also proposed. The radial particle volume fraction, axial root‐mean‐square of bed pressure drop, granular temperature, and particle velocity in gas–solid acoustic fluidized bed were simulated using computational fluid dynamics (CFD) code Fluent 6.2. The results showed that radial particle volume fraction increased using modified drag model compared with that using the original one. Radial particle volume fraction was revealed as a parabolic concentration profile. Axial particle volume fraction decreased with the increasing bed height. The granular temperature increased with increasing sound pressure level. It showed that simulation values using CFD code Fluent 6.2 were in agreement with the experimental data. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Determination of minimum fluidization velocity by pressure fluctuation measurement

Using the standard deviation of pressure fluctuations to find the minimum fluidization velocity, U_mf, avoids the need to de-fluidize the bed so U_mf, can be found for operational bubbling fluidized beds without disrupting the process provided only that the superficial velocity may be altered and that the bed remains in the bubbling fluidized state. This investigation has concentrated on two distinct aspects of the pressure fluctuation method for U_mf determination: (1) the minimum number of pressure measurements required to obtain reliable estimates of standard deviation has been identified as about 10000 and (2) pressure fluctuation measurements in the plenum below the gas distributor are suitable for U_mf determination so the problems of pressure probe clogging and erosion by bed particles may be avoided.     

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.     

Effect of agitation on the fluidization behavior of a gas–solid fluidized bed with a frame impeller

The effect of agitation on the fluidization performance of a gas–solid fluidized bed with a frame impeller is experimentally and numerically investigated. A 3‐D unsteady computational fluid dynamics method is used, combining a two‐fluid model and the kinetic theory of granular flow. The rotation of the impeller is implemented with a multiple reference frame method. The numerical model is validated using experimental data of the bed pressure drop and pressure fluctuation. Although the minimum fluidizing velocity and bed pressure drop are independent of the impeller agitation, a sufficiently high agitation speed yields higher fluidization performance with reduced bubble diameters and internal circulations of particles. The fluidized bed can be divided into three zones: inlet zone where the gas distribution plays a major role, agitated fluidization zone where the impeller agitation has a positive effect on fluidization, and free fluidization zone where the impeller agitation has no effect on fluidization. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1066–1074, 2013     

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

The tapered fluidized bed is a remedial measure for certain drawbacks of the gas–solid system, by the fact that a velocity gradient exists along the axial direction of the bed with increase in cross-sectional area. To study the dynamic characteristics of heterogeneous binary mixture of irregular particles, several experiments have been carried out with varying tapered angles and composition of the mixtures with various particles. The tapered angle of the bed has been found to affect the characteristics of the bed. Models based on dimensional analysis have been proposed to predict the critical fluidization velocity and maximum bed pressure drop for gas–solid tapered fluidized beds. Experimental values of critical fluidization velocity and maximum bed pressure drop compare well with that predicted by the proposed models and the average absolute errors are well within 15%.     

Regime classification of macroscopic gas—solid flow in a circulating fluidized bed riser

A conceptual flow regime diagram for a circulating fluidized bed riser is proposed, combining existing investigations with experimental data obtained under idealized conditions in which a fully independent control of gas velocity and solid circulation rate was conducted by use of a screw feeder for solid feed into the riser. The diagram classifies the flow state into five regimes by qualitative transition lines which describe the relationship between gas velocity and solid circulation rate. These regimes are particulate fluidization, bubbling fluidization, turbulent fluidization, dense-phase transport and dilute-phase transport. The diagram suggests that S-shaped bed-density distribution or dense/dilute region interface appears only at limited conditions in the bubbling and turbulent fluidization regimes. These experimental findings were generalized by further experiments in a conventional circulation system with a ball valve between the riser and the downcomer which permits changes in the solid circulation rate and the bed height in the downcomer. The experimental results showed that the bed height in the downcomer has no particular effect on the bed density distribution or the height of the dense/dilute region interface, but an appreciable effect on the lowest gas velocity to maintain steady solid circulation at a given rate. These results are consistent with the above diagram.     

Measuring and modelling lateral solid mixing in a three-dimensional batch gas—solid fluidized bed reactor

A 0.27 m diameter fluidized bed reactor has been designed to allow experimental measurement of the axial and radial mixing behaviour of the solids. A unique method has been developed which permits the continuous determination of solid tracer concentration with time at different radial and axial positions within the fluidized bed. Solids mixing has been described by a model in which vertical mixing is instantaneous and lateral mixing occurs by dispersion. The lateral solids dispersion coefficients have been evaluated at various operating conditions from the experimental results of tracer concentration versus time. Based on the results, a modification of an existing correlation is proposed.     

Particles mixing induced by bubbles in a gas‐solid fluidized bed

The effect of bubble injection characteristics on the mixing behavior of a gas‐solid fluidized bed is investigated using a discrete particle model. The effect of different parameters including gas injection time, velocity, and mode are studied. Simulation results show that injecting gas at a constant gas flow rate in the form of small bubbles results in a better overall particle mixing. It was also found that the injection velocities have limited effect on particle mixing behavior for the same total gas volume injected into the bed. Moreover, the mixing index (MI) of continuous gas jet bubbling regime is compared with the MI obtained in uniform gas injection regime and the results revealed that the MI of continuous jet bubbling regime has a larger value than that of uniform gas injection regime at the fixed total gas flow rate. In both regimes, z‐direction MI is larger than x‐direction index. The differences between two direction indices are more noticeable in continuous jet bubbling in comparison with the uniform gas injection regime. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1430–1438, 2016     

Minimum fluidization velocities for gas—liquid—solid three-phase systems

Cocurrent upward gas—liquid fluidization of coarse solids is actuated primarily by the motion of the liquid at relatively low gas velocities and by the momentum of the gas at zero or low liquid velocities. Our gas-perturbed liquid model, which has previously been shown to give good predictions of the minimum liquid fluidization velocity, U_lmf, at a fixed low gas velocity, is shown here also to give reasonable agreement with U_lmf measurements for inverse three-phase fluidization at a given upward gas velocity, using the coefficient in the gas hold-up equation of Yang et al. [X.L. Yang, G. Wild, J.P. Euzen, Int. Chem. Eng. 33 (1993) 72] as an adjustable parameter. It is further shown that a liquid-buoyed solids/liquid-perturbed gas model can predict with moderate success the minimum gas fluidization velocity, U_gmf, for three-phase cocurrent upward fluidization of coarse solids at zero or low liquid velocities.     

Prediction of minimum velocity and minimum bed pressure drop for gas-solid fluidization in conical conduits

Experimental investigations have been carried out for spherical and non-spherical particles using beds comprised of single-sized particles and mixtures in the size and particle density ranges of 439 to 1524 μm and 1303 to 4948 kg/m³, respectively. Five conical fluidizers with varying apex angles of 8.86, 14.77, 19.60, 32.0 and 43.2 degrees were used. Experimental values of minimum velocity and bed pressure drop with air as the fluidizing medium have been compared with their respective values obtained from different models available in the literature. Deviations for each chosen model have been presented.     

Minimum fluidization velocity in gas‐liquid‐solid minifluidized beds

The initial fluidization characteristics of gas‐liquid‐solid minifluidized beds (MFBs) were experimentally investigated based on the analyses of bed pressure drop and visual observations. The results show that U_Lmf in 3–5 mm MFBs can not be determined due to the extensive pressure drop fluctuations resulting from complex bubble behavior. For 8–10 mm MFBs, U_Lmf can be confirmed from both datum analyses of pressure drop and Hurst exponent at low superficial gas velocity. But at high superficial gas velocity, U_Lmf was not obtained because the turning point at which the flow regime changes from the packed bed to the fluidized bed disappeared, and the bed was in a half fluidization state. Complex bubble growth behavior resulting from the effect of properties of gas‐liquid mixture and bed walls plays an important role in the fluidization of solid particles and leads to the reduction of U_Lmf. An empirical correlation was suggested to predict U_Lmf in MFBs. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1940–1957, 2016     

Wall-to-bed heat transfer in liquid—solid and gas—liquid—solid fluidized beds part I: Liquid—solid fluidized beds

Experimental work was conducted to investigate the effect of particle size and particle density upon the wall-to-bed heat transfer characteristics in liquid—solid fluidized beds with a 95.6 mm column diameter over a wide range of operating conditions. The radial temperature profile was found to be parabolic, indicating the presence of a considerable bed resistance. The effective radial thermal conductivity and the apparent wall film coefficient were obtained on the basis of a series thermal resistance model. The modified Peclet number of the radial thermal conductivity decreases upon the onset of fluidization, has a minimum at a bed porosity of 0.6 to 0.7 and increases with further increase of bed porosity. The modified Peclet number decreases considerably with decreasing particle size or increasing particle density. The apparent wall heat transfer coefficient can be represented well by a Colburn j-factor correlation over a wide range of data as follows: j′_H = 0.137 Re′^?0.271 A close analogy is found to exist between the modified j-factor for wall heat transfer coefficient and that for wall mass transfer coefficient, in liquid—solid fluidized beds.     

Wall-to-bed heat transfer in liquid—solid and gas—liquid—solid fluidized beds part II: Gas—liquid—solid fluidized beds

Wall-to-bed heat transfer in gas—liquid—solid fluidized beds with a cocurrent upflow was analyzed on the basis of a series thermal resistance model. The effective radial thermal conductivity and the apparent wall heat transfer coefficient were determined over a wide range of experimental conditions. The behavior of the effective thermal conductivity strongly depends on the flow mode for the three-phase fluidized bed, directly indicating the trend of the radial liquid mixing. The modified Peclet number for the radial thermal diffusivity takes on a minimum with respect to the liquid velocity in a manner similar to that in a liquid—solid fluidized bed, but the value of the modified Peclet number decreases significantly with gas velocity. The apparent wall heat transfer coefficient can be correlated well with a Colburn type equation which at zero gas velocity reduces to the same equation as that proposed for liquid—solid fluidization, as follows: j′_H = 0.137 Re′_l.g^?0.271     

液固外循环流化床起始外循环液体流量的实验研究

为外循环流化床换热器的设计计算提供依据,以喷嘴为颗粒循环装置的液固外循环流化床换热器为研究对象,考察了颗粒直径、口径比、喷嘴安装位置、颗粒初始加入量及流体粘度对起始外循环液体流量的影响;得出了稳定操作情况下,液固外循环流化床换热器起始外循环液体流量与上述因素之间的经验关联式.结果表明:计算值与实验值吻合较好.     

Minimum fluidization velocity at elevated temperature in tapered fluidized bed

Very little data of minimum fluidization velocity at elevated temperatures of tapered bed are available in the literature. This study was undertaken to provide some data under elevated temperature conditions in tapered bed. Data on minimum fluidization velocity have been obtained experimentally for temperature up to 800 °C in case of 0.5 mm diameter of sand particles and up to 500 °C in case of 1 mm diameter of glass beads in tapered bed. An equation valid for the bed has been developed in terms of Archimedes number and Reynolds number. The experimental values for minimum fluidization velocity at elevated temperatures have been compared with the calculated values obtained from present equation and from earlier equations developed by other authors for ambient conditions in conventional (cylindrical) bed and tapered bed. Fairly good agreement was found to exist between the calculated (from present equation) and the experimental values.     

Experimental and computational study of gas–solid fluidized bed hydrodynamics

The hydrodynamics of a two-dimensional gas–solid fluidized bed reactor were studied experimentally and computationally. Computational fluid dynamics (CFD) simulation results from a commercial CFD software package, Fluent, were compared to those obtained by experiments conducted in a fluidized bed containing spherical glass beads of 250– in diameter. A multifluid Eulerian model incorporating the kinetic theory for solid particles was applied in order to simulate the gas–solid flow. Momentum exchange coefficients were calculated using the Syamlal–O’Brien, Gidaspow, and Wen–Yu drag functions. The solid-phase kinetic energy fluctuation was characterized by varying the restitution coefficient values from 0.9 to 0.99. The modeling predictions compared reasonably well with experimental bed expansion ratio measurements and qualitative gas–solid flow patterns. Pressure drops predicted by the simulations were in relatively close agreement with experimental measurements at superficial gas velocities higher than the minimum fluidization velocity, U_mf. Furthermore, the predicted instantaneous and time-average local voidage profiles showed similarities with the experimental results. Further experimental and modeling efforts are required in a comparable time and space resolutions for the validation of CFD models for fluidized bed reactors.     

Prediction of minimum fluidization velocity of gas-solid fluidized beds by pressure fluctuation measurements — Analysis of the standard deviation methodology

We analyze here an experimental method for the evaluation of U_mf in gas fluidized beds, based on pressure fluctuation measurements in which U_mf is determined by the relationship between the standard deviation of pressure measurements and fluid velocity [M. Puncochar, J. Drahos, J. Cermak, K. Selucky, Evaluation of minimum fluidizing velocity in gas fluidized bed from pressure fluctuations, Chem. Eng. Commun. 35 (1985) 81-87]. This was tested in four different particles: Sand, Microcrystalline Cellulose, FCC and Alumina using two circular plexiglass columns with internal diameters of 0.11 m and 0.14 m as the experimental apparatus. Validation of the method was made by comparing our U_mf results with those obtained by the fluid-dynamic curve. The experimental data revealed that: (1) the valid range of fluid velocity in the method employed by us is broader than those observed by other authors; (2) the method is suitable for both Geldart A and B solids. Influence of probe location and type of pressure measurement (either absolute or differential) were also analyzed and discussed.     

Phase hold-up and critical fluidization velocity in a three-phase inverse fluidized bed

We studied the hydrodynamic characteristics of a three-phase inverse fluidized bed made of a transparent acrylic column of 0.115 m inner diameter and 2 m heights. Air, water and polyethylene particles were used as the gas, liquid and solid phase, respectively. We used both hydrophobic low density polyethylene (LDPE) and hydrophilic LDPE as solid phase, and distilled water as liquid phase, and filtered air as gas phase. The LDPE was chemically treated by chlorosulfonic acid to change the surface property from hydrophobic to hydrophilic. We tried to solely investigate the effect of the surface hydrophilicity of polymeric particles on the phase holdup and the critical fluidization velocity of three-phase inverse fluidization. Thus, we measured the static pressure and eventually observed critical fluidization velocity. Critical fluidization velocity became smaller in case of using MDPE hydrophobic particles than LDPE hydrophilic particles. This was thought to be due to the retardation of rising bubbles near hydrophobic particles and, subsequently, the increase of gas hold-up.