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     

Minimum fluidization velocity and gas holdup in gas–liquid–solid fluidized bed reactors

Experiments were performed to study the hydrodynamics of a cocurrent three‐phase fluidized bed with liquid as continuous phase. Based on the 209 experimental data (with four liquid systems and five different particles) along with 115 literature data from six different sources on minimum fluidization velocity, a unique correlation for the estimation of minimum fluidization velocity in two‐phase (u_g = 0) as well as in three‐phase systems is developed. A data bank consisting of 1420 experimental measurements for the fractional gas phase holdup data with a wide range of variables is used for developing empirical correlations. Separate correlations are developed for two flow regimes observed in this present work. The proposed correlations are more accurate and simpler to use. © 2002 Society of Chemical Industry     

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.     

Statistical analysis of the phase holdup characteristics of a gas–liquid–solid fluidized bed

Experiments have been carried out to study the individual phase holdup characteristics in a cocurrent three‐phase fluidized bed. An antenna type modified air sparger has been used in the gas–liquid distributor section, for uniform mixing of the fluids with the gas moving as fine bubbles to the fluidizing section. This arrangement also reduces the pressure drop encountered through a conventional distributor used for the purpose. To overcome the non‐uniformity of flow through the column (i.e., the central region), a distributor plate with 20% open area has been fabricated with concentric circular punched holes of increased diameter from centre to the wall. Model equations have been developed by factorial design analysis for predicting various individual phase holdups.     

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.     

Towards monitoring electrostatics in gas–solid fluidized beds

The electrostatic charge distribution in a lab‐scale 2‐D fluidized bed of 900 µm glass beads was determined using arrays of induction probes, and the influence of relative humidity and superficial gas velocity was examined. The bubble presence, relative humidity, and superficial gas velocity were found to influence charge separation. Bipolar charging was observed; the net charge build‐up was found to be negligible. Moreover, the system was monitored by applying the attractor comparison method to the electrostatic charge signals from an induction probe. It was concluded that this approach can indeed be used to monitor changes in the electrostatic behaviour.     

Transient multi gas—solid reactions in a bubbling fluidized bed

A “multimodel” for gas‐solid reactions in a reacting particle has been applied to a bubbling fluidized bed reactor. The particle is tracked and bed and particle variables are determined continuously. The conservation equations of mass and heat with auxiliary relations are solved in an accelerating particle, which may rise or fall. The effects of bulk pressure, velocity and temperature, and particle diameter are studied. Heat and mass transfer coefficients may fluctuate up to 75% and 148% respectively. Doubling the pressure changes h_c by 75% and k_c by ?45%. Increase in pellet diameter reduces both h_c and k_c.     

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     

Mixing characteristics of irregular binaries in a promoted gas–solid fluidized bed: A mathematical model

The vertical as well as horizontal dispersion for the gas–solid fluidization has been analyzed in the present work. The fluidization and solids mixing characteristics of large irregular particles (Geldart‐BD type) for both homogeneous and heterogeneous materials (size‐variant and density‐variant respectively) have been investigated in a 15 × 100 cm² cylindrical column. A theoretical model for concentration of jetsam particles has been developed as a function of height of any layer of particles (measured from the grid) by considering the counterflow of solids and circulation model together with the dispersion model for the vertical and the horizontal displacement of the particles in the fluidized bed. The mixing index at any position in the bed has thereby been expressed as a function of the concentration of the jetsam. The values of mixing index calculated from the above model as well as the experimentally measured ones for both the homogeneous and the heterogeneous systems have been compared with each other. Attempt has also been made to develop correlations for the mixing index using various system parameters based on the dimensional analysis approach for the unpromoted and the promoted beds. The mixing index values thus obtained through the dimensional analysis approach have been compared with those obtained from the developed theoretical model. The values of the mixing index calculated for promoted beds have also been compared with the corresponding values for the unpromoted beds. Segregation effect is found to be stronger for the heterogeneous systems than the homogeneous ones. The density difference is the major factor affecting the segregation.     

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     

Phase distributions in a gas–liquid–solid circulating fluidized bed riser

The distributions of the three phases in gas–liquid–solid circulating fluidized beds (GLSCFB) were studied using a novel measurement technique that combines electrical resistance tomography (ERT) and optical fibre probe. The introduction of gas into a liquid–solid circulating fluidized bed (LSCFB), thus forming a GLSCFB, caused the increase of solids holdup due to the significantly decreased available buoyancy with the lower density of the gas, even with a somewhat increased liquid velocity due to the decreased liquid holdup giving space for the gas holdup. The gas passed through the riser in the form of bubbles, which tended to flow more through the central region of the riser, leading to more radial non‐uniformity in radial holdup of the phases. The gas velocity has the most significant effect on the gas phase holdup. While the gas velocity also has an obvious effect to the solids holdups, the liquid flow rate had a much more considerable effect on the phase holdups. The solids circulation rate also had a significant effect on the phase holdups, with increasing solids circulation rate causing much more increased solids holdup in the central region than close to the wall. A correlation was developed for the relative radial distributions of solids holdup in GLSCFB, as such radial profiles were found similar over a wide range of operating conditions, like those in a typical gas–solid circulating fluidized beds (GSCFB). Finally, the axial solids profiles in a GLSCFB was found to be much closer to those in an LSCFB which are very uniform, than those found in a GSCFB which are less uniform and sometime having a S shape. Water was used as the continuous and conductive phase, air was the gas phase and glass bead and lava rock particles were used as the solid and non‐conductive phase.     

Characterizing bubble behavior in a pseudo-2D fluidized bed of wet particles

By applying digital image analysis on the bubble characteristics in a two dimensional wet-particle fluidized-bed, we report two-stage evolution of bubble characteristics with increasing liquid content. In the first stage, bubble number and uniformity of bubble fraction increase, while bubble average diameter and aspect ratio decrease. In the second stage, these characteristics shift toward an opposite direction. This two-stage evolution of bubble characteristics are analogous to that of reducing particle size in dry-particle fluidization, and the fluidizing properties of particles shifts from Geldart Group B to Group A and then to Group C. Furthermore, liquid addition causes a continuous decrease of bubble fraction and bubble flow rate. This is different from dry-particle fluidization, in which reducing particle size causes an increase trend. An explanation for this difference is that liquid addition increases the equivalent size of agglomerates in wet-particle fluidization, which is opposite to the effect of reducing particle size.     

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     

基于递归分析的喷雾气固流化床团聚状态识别

液体会影响喷雾气固流化中的颗粒循环模式,形成不同程度的颗粒团聚物,从而直接降低反应器的传热、传质效果。实验中向流化床喷入不同黏度的乙二醇溶液,来人工制造团聚现象。研究结果表明,喷入不同黏度的液体后,流化床内会形成不同的团聚结构,按照团聚物大小可分为四类：微团聚、成核团聚、黏结团聚以及糊状团聚。通过对不同工况中的压差波动信号进行递归分析,发现不同团聚状态下的递归图纹理结构有着明显的差异。与此同时,对50种流动条件下的递归特征量分布情况进行分析,以此来识别不同的团聚结构,整体识别率高达96.39%。结果表明：通过对压差信号的递归分析可以快速识别流化床中的团聚状态。     

CFD simulation of gas–solid bubbling fluidized bed: A new method for adjusting drag law

In computational fluid dynamics modelling of gas–solid two phase flow, drag force is one of the dominant mechanisms for interphase momentum transfer. Despite the profusion of drag models, none of the available drag functions gives accurate results in their own original form. In this work the drag correlations of Syamlal and O'Brien (Syamlal and O'Brien, Int. J. Multiphase Flow. 1988; 14(4):473–481), Gidaspow (Gidaspow, Appl. Mech. Rev. 1986; 39:1–23), Wen and Yu (Wen and Yu, Chem. Eng. Prog. Symp. Ser. 1966; 62(2):100–111), Arastoopour et al. (Arastoopour et al., Powder Technol. 1990; 62(2): 163–170), Gibilaro et al. (Gibilaro et al., Chem. Eng. Sci. 1985; 40:1817–1823), Di Felice (Di Felice, Int. J. Multiphase Flow. 1994; 20(1):153–159), Zhang‐Reese (Zhang and Reese, Chem. Eng. Sci. 2003; 58(8):1641–1644) and Hill et al. (Hill et al., J. Fluid Mech. 2001; 448:243–278) are reviewed using a multi‐fluid model of FLUENT V6.3.26 (FLUENT, 2007. Fluent 6.3 User's Guide, 23.5 Eulerian Model, Fluent, Inc.) software with the resulting hydrodynamics parameters being compared with experimental data. The main contribution of this work is to propose an easy to implement and efficient method for adjustment of Di Felice drag law which is more efficient compared to the one proposed by Syamlal‐O'Brien. The new method adopted in this work showed a quantitative improvement compared to the adjusted drag model of Syamlal‐O'Brien. Prediction of bed expansion and pressure drop showed excellent agreement with results of experiments conducted in a Plexiglas fluidized bed. A mesh size sensitivity analysis with varied interval spacing showed that mesh interval spacing with 18 times the particle diameter and using higher order discretization methods produces acceptable results.     

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.     

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.