Magnetic resonance imaging of gas–solid fluidization with liquid bridging

Magnetic resonance imaging is used to generate snapshots of particle concentration and velocity fields in gas–solid fluidized beds into which small amounts of liquid are injected. Three regimes of bed behavior (stationary, channeling, and bubbling) are mapped based on superficial velocity and liquid loading. Images are analyzed to determine quantitatively the number of bubbles, the bubble diameter, bed height, and the distribution of particle speeds under different wetting conditions. The cohesion and dissipation provided by liquid bridges cause an increase in the minimum fluidization velocity and a decrease in the number of bubbles and fast particles in the bed. Changes in liquid loading alter hydrodynamics to a greater extent than changes in surface tension or viscosity. Keeping U/U_mf at a constant value of 1.5 produced fairly similar hydrodynamics across different wetting conditions. The detailed results presented provide an important dataset for assessment of the validity of assumptions in computational models. © 2017 American Institute of Chemical Engineers AIChE J, 64: 2958–2971, 2018     

Fluidization of moist sawdust in binary particle systems in a gas–solid fluidized bed

The fluidized behavior of binary mixtures of moist sawdust and glass spheres has been investigated. The sawdust alone was observed to fluidize poorly, with extensive channelling occurring. The addition of 0.322 and 0.516 mm glass spheres to the fluidized bed of sawdust improved the fluidization characteristics. The mixtures of sawdust and 0.322 mm spheres were completely mixed when fluidized. Mixtures of sawdust and 0.516 mm spheres were either partially or completely mixed, depending upon gas velocity in the fluidized bed. As the moisture content of the sawdust was increased, the minimum fluidization velocity of the binary mixture also increased. There was an upper limit to the moisture content of the sawdust at which fluidization could be achieved. When the moisture content of the sawdust exceeded 33 and 54 wt% on a dry basis, agglomeration and channelling occurred in the mixtures of sawdust and glass spheres, with sizes 0.322 and 0.516 mm, respectively. The moisture likely contributes to interparticle liquid bridging forces. Binary mixtures of larger 0.777 and 1.042 mm glass spheres and up to 82% moisture sawdust did not readily agglomerate, but the two components completely segregated during fluidization.     

Experimental and computational study of the bed dynamics of semi‐cylindrical gas–solid fluidized bed

With computational fluid dynamics (CFD) it is possible to get a detailed view of the flow behaviour of the fluidized beds. A profound and fundamental understanding of bed dynamics such as bed pressure drop, bed expansion ratio, bed fluctuation ratio, and minimum fluidization velocity of homogeneous binary mixtures has been made in a semi‐cylindrical fluidized column for gas–solid systems, resulting in a predictive model for fluidized beds. In the present work attempt has been made to study the effect of different system parameters (viz., size and density of the bed materials and initial static bed height) on the bed dynamics. The correlations for the bed expansion and bed fluctuations have been developed on the basis of dimensional analysis using these system parameters. Computational study has also been carried out using a commercial CFD package Fluent (Fluent, Inc.). A multifluid Eulerian model incorporating the kinetic theory for solid particles was applied in order to simulate the gas–solid flow. CFD simulated bed pressure drop has been compared with the experimental bed pressure drops under different conditions for which the results show good agreements.     

Analysis of the effect of small amounts of liquid on gas–solid fluidization using CFD‐DEM simulations

Gas–solid fluidization involving small amounts of liquid is simulated using a CFD‐DEM model. The model tracks the amount of liquid on each particle and wall element and incorporates finite rates of liquid transfer between particles and pendular liquid bridges which form between two particles as well as between a particle and a wall element. Viscous and capillary forces due to these bridges are modeled. Fluidization–defluidization curves show that minimum fluidization velocity and defluidized bed height increase with Bond number (Bo), the ratio of surface tension to gravitational forces, due to cohesion and inhomogeneous flow structures. Under fluidized conditions, hydrodynamics and liquid bridging behavior change dramatically with increasing Bo, and to a lesser extent with capillary number, the ratio of viscous to surface tension forces. Bed fluidity is kept relatively constant across wetting conditions when one maintains a constant ratio of superficial velocity to minimum fluidization velocity under wet conditions. © 2017 American Institute of Chemical Engineers AIChE J, 63: 5290–5302, 2017     

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     

A novel approach for modeling bubbling gas–solid fluidized beds

A phenomenological discrete bubble model is proposed to help in the design and dynamic diagnosis of bubbling fluidized beds. An activation region mechanism is presented for bubble formation, making it possible to model large beds in a timely manner. The bubbles are modeled as spherical‐cap discrete elements that rise through the emulsion phase that is considered as a continuum. The model accounts for the simultaneous interaction of neighboring bubbles by including the trailing effects due to the wake acceleration force. The coalescence process is not irreversible and therefore, the coalescing bubble pair is free to interact with other rising bubbles originating the splitting phenomena. To validate the model, the simulated dynamics are compared with both experimental and literature data. Time, frequency, and state space analysis are complementarily used with a multiresolution approach based on the empirical method of decomposition to explore the different dynamic scales appearing in both the simulated time series and those obtained from experimental runs. It is concluded that the bubble dynamics interactions play the main role as the driver of the resulting bed dynamics, matching the main features of measured bubble dynamics. Exploding bubble phenomena have been identified by establishing a direct relation between the bubble generation, interaction and eruption, and the measured signals. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Realization and control of multiple temperature zones in liquid‐containing gas–solid fluidized bed reactor

Fluidized bed reactors (FBRs) have been developed to establish multiple temperature zones for various industrial processes. To overcome the common weakness, this work proposed to spray liquid into bottom and upper zones, respectively, to realize multiple temperature zones FBR (MTZFBR). Temperature, pressure, and acoustic emission techniques were applied to fully characterize liquid interaction and hydrodynamics. Compared with the bottom liquid‐spraying approach, the upper liquid‐spraying approach showed higher temperature difference (ΔT) and better fluidization stability, thus was selected for further control studies. Effects of liquid flow rate, static bed height, and inlet gas temperature on MTZFBR were studied systematically. The results showed that increasing liquid evaporation behavior or decreasing liquid bridge behavior enhance ΔT and fluidization stability and vice versa. G–L–S fluidization pattern depended mostly on the liquid behaviors and fluidization stability, and thus the stabilized MTZFBR could be regarded as a coexisted mode of two distinctive G–L–S fluidization patterns. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1454–1466, 2016     

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.     

Friction on a solid sphere exposed to gas–liquid and gas–liquid–solid flow in bubble column and fluidized bed reactors

Local velocity gradients on a solid spherical surface have been studied in a bubble column and in two- and three-phase fluidized beds, in order to clarify the influence of gas flow. The electrochemical method, measuring apparent local mass transfer coefficients, was verified and used to obtain the local velocity gradients, shear stresses and total frictional forces. The observed mass transfer rate was independent of liquid velocity, owing to a non-changing flow structure around the particles and not to averaging opposing effects. The identity in flow structure also held for three-phase fluidized beds up to a superficial gas velocity of 5 cm s^?1. The dramatic increase in velocity gradient on gas introduction was not a result of decreased homogenous density, but was caused by a change in the turbulent structure around a particle, leaving a larger portion of the total drag as frictional drag, thus improving the mass transfer characteristics of the bed. Use of velocity gradient measurements, including span of fluctuations and exposure time, to predict biomass growth and mechanical degradation in a reactor is also discussed.     

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.     

Radial profiles of solids loading and flux in a gas–solid circulating fluidized bed

An electrical-impedance tomography (EIT) system has been developed to non-invasively measure radial voidage profiles in the riser of a pilot-scale circulating fluidized bed (CFB), yielding quantitative information that is validated by comparison to a gamma-densitometry tomography (GDT) system. EIT and GDT were applied to the CFB riser (14-cm inner diameter, 5.77-m height) containing fluid catalytic cracking particles in air. For all cases, the average and near-wall voidages from EIT and GDT agreed to within 0.03 and 0.07, respectively. This good agreement suggests that, where feasible, EIT can be used in place of GDT, which is advantageous since EIT systems are often safer, less expensive, and faster than GDT systems. The results also compared well to two correlations for radial voidage profile from the literature. Finally, a procedure for determining radial solids flux profiles from radial voidage profiles using an additional correlation [M.J. Rhodes, X.S. Wang, H. Cheng, T. Hirama, B.M. Gibbs, Similar profiles of solids flux in circulating fluidized-bed risers, Chemical Engineering Science 47 (1992) 1635–1643] was investigated. It was found that the accuracy of this correlation strongly depends on the voidage and/or solids flux measurement at the riser center.     

Experimental study on solid circulation in a multiple jet fluidized bed

Particle image velocimetry was used to investigate the evolution of multiple inlet gas jets located at the distributor base of a two‐dimensional fluidized bed setup. Results were used to estimate the solid circulation rate of the fluidized bed as well as particle‐entrainment into the individual jets. The effects of fluidization velocity, orifice diameter, orifice pitch, particle diameter, and particle density were studied. It was determined from this study that the solid circulation rate linearly increased with an increase in the fluidization velocity until the multiple jet system transitioned from isolated to an interacting system. In the interacting system of jets, the solid circulation increased with fluidization velocity but at a much lower rate. For multiple jet systems, this phenomenon may indicate the presence of an optimum operating condition with high circulation rate and low air input in the bed. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3003–3015, 2012     

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%.     

Segregation in a stirred fluidized bed

A bed of particulate solids supported by an upward current of gas can be stirred at moderate power input per unit volume using thin horizontal rods mounted on a vertical shaft. The bed is fluidized, but bubbles are suppressed below a critical value of the fluidizing velocity. Stirring increases bed bulk density and reduces the minimum fluidizing velocity. Segregation in a stirred fluidized bed is enhanced with both flotsam and jetsam tracer particles, but the rate of segregation is reduced with flotsam tracers. The stirred fluidized bed may be useful as a device for dry separation of solids.     

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     

Investigation of nonuniformity in a liquid–solid fluidized bed with identical parallel channels

Previous work has demonstrated that multiphase flow through identical parallel channels and multiple cyclones can give rise to significant nonuniformity among the flow paths. This article presents results from a study where the distribution of voidage and flux through parallel channels in liquid–solid fluidized beds is investigated. Experiments and computational fluid dynamics simulations were performed with 1.2 mm glass beads fluidized by water where a cross baffle divided a 191 mm diameter column into four identical parallel channels. Voidages were measured by optical fiber probes. Simulations from a three‐dimensional unsteady‐state Eulerian–Eulerian model based on FLUENT software showed good agreement with the experimental results. Despite the symmetrical geometry of the system, the average voidage and particle velocities in one channel differed somewhat from those in the others. Increasing the superficial liquid velocity could increase voidage greatly and affect the degree of nonuniformity in the four channels. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Experimental investigation of electrostatic effect on bubble behaviors in gas‐solid fluidized bed

Electrostatics and hydrodynamics in the fluidized bed are mutually affected, and excess accumulation of electrostatic charges has a severe impact on hydrodynamics. However, there is a serious lack of experimental investigation of electrostatic effect on hydrodynamics. This work provides a first insight into the electrostatic effects on bubble behaviors experimentally by injecting a trace of liquid antistatic agents (LAA) into a fluidized bed. Different amounts of LAA (0–50 ppm) were injected to make the electrostatic charges vary in a wide range and the bubble behaviors were investigated simultaneously. Results showed that the charges on particles decreased with increasing amount of LAA, which resulted in larger bubble sizes, stronger fluctuations of dynamic bed height, and less wall sheeting, respectively. The maximum reduction ratio of bubble sizes due to electrostatic effect was 21%. When particles were charged, the bubble sizes were significantly smaller than those estimated from the classical correlation. This discrepancy was attributed to the neglect of electrostatic effect in classical correlation. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1160–1171, 2015     

Characterization on hydrodynamic behavior in liquid‐containing gas‐solid fluidized bed reactor

In many industrial processes involving gas–solid fluidized bed rectors, the addition of a liquid phase significantly alters the hydrodynamics. To fully characterize the hydrodynamics in the fluidized bed, pressure and acoustic measuring techniques were applied to study the behavior of gas bubbles and particles. A camera was used to take pictures to verify the pressure and acoustic results. During the liquid‐addition process, the pressure technique captured the bubble size variation and bubble motion while the acoustic technique reflected particle motion and particle size growth. Hurst and V‐statistics analyses of acoustic emission were used for the first time to detect periodic behavior during the injection process. The new break formation and change trend of V_max were used as the criteria to judge occurrence of abnormal fluidization states, such as agglomeration and gas channeling formation. These measurement techniques are beneficial in the elimination of adverse effects caused by the addition of liquid. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1056–1065, 2013