Liquid-solid Fluidized Bed Research Articles

The effect of column tilt on flow homogeneity and particle agitation in a liquid fluidized bed

The motion of particles in a solid-liquid fluidized bed was experimentally studied by video tracking of marked particles in a matched refractive index medium. In this study, two fluidized states are compared, one carefully aligned in the vertical direction ensuring a homogeneous fluidisation and another one with a non-homogeneous fluidisation regime that results from a slight tilt of the fluidisation column of 0.3° with respect to the vertical. As a result of the misalignment, large recirculation loops develop within the bed in a well-defined spatial region. It is found that in that range of solid fraction (between 0.3 and 0.4), the inhomogeneous motion of the particles leads to significant differences in velocity fluctuations as well as in self-diffusion coefficient of the particles in the vertical direction, whereas the fluidisation height remains unaffected. At lower (less than 0.2) or higher (higher than 0.5) concentration, particle agitation characteristics are almost unchanged in the vertical direction.

Impact of catalyst density distribution on the fluid dynamics of an ebullated bed operating at high gas holdup conditions

Experiments were conducted to investigate the impact of particle density distribution on ebullated bed phase holdups and local fluidization behaviour when operating under high gas holdup conditions. Fresh and spent heavy oil hydroprocessing catalyst having relatively narrower and wider density distributions were compared. A 0.5wt.% aqueous ethanol solution was used to obtain relatively high gas holdups as observed in many industrial reactors containing liquid mixtures with surface-active compounds. Axial pressure profiles were used to assess the degree of segregation on liquid-solid and gas-liquid solid fluidized beds. While marginal axial holdup variation occurred when operating the liquid-solid fluidized bed, introduction of gas significantly impacted the fluidized bed dynamics by rendering the bed-freeboard interface diffuse at low superficial liquid velocity as relatively large bubbles were formed. This was observed visually and experimentally based on the pressure profile curvature. At elevated liquid flow rates, the bed interface became more stable due to smaller bubbles being formed because of the greater shear stress at the gas-liquid distributor. Solid holdup was the most affected by the particle density distribution, where bed expansion/contraction was dependent of the liquid flow rate due to varying particle-bubble dynamics. Such information provided guidance on potential factors that can lead to the loss of bed-freeboard interface in the operation of heavy oil hydroprocessors such as the LC-FinerSM.

Settling/rising of a foreign particle in solid-liquid fluidized beds: Application of dynamic mesh technique

The modeling of moving objects has been the focus of many studies and has succeeded to attract sufficient attention by researchers. However, commonly used modeling approaches such as discrete element modeling (DEM), direct numerical simulations (DNS) lack simplicity and have been computationally intensive. In the present work, simple method of dynamic mesh in the framework of computational fluid dynamics has been employed. Eulerian-Eulerian simulations of a monodisperse solid-liquid fluidized bed (SLFB) have been carried out. A foreign particle (settling particle or rising bubble) was inserted in the system to study the effect of turbulence in SLFB on the motion of settling particle. The operating and geometrical parameters have been chosen based on the experiments performed by Ghatage et al. (2013). The results showed that the model can satisfactorily predict the settling velocity for low voidage fluidization in 2D as well as 3D simulations. Computational fluid dynamics (CFD) simulations at higher values of superficial liquid velocity showed liquid bubbles confirming the transition to heterogeneous regime. These liquid bubbles directed the settling particle to move zig-zag resulting in lower settling velocity. The size and number of the bubbles increase with an increase in the liquids velocity indicating increased heterogeneity. However, CFD predicted larger and higher number of bubbles than experimentally noted. This resulted in an increase in the deviation of predicted settling velocities from experimentally observed with an increase in superficial liquid velocity. In case of bubbles, it was observed that the dynamic mesh method is greatly dependent on the regime of operation in the column and works only in the range of low voidage when the fluidized bed is homogeneous and does not contain liquid bubbles.

Study of particle hydrodynamics and misplacement in liquid–solid fluidized bed separator

Liquid–solid fluidized bed separators are one of the important types of equipment for the beneficiation of low-grade iron ore fines. Particles separate based on the difference in their terminal velocities in these types of separators. Misplacement of particles is a common phenomenon in such type of separators. Qualitative and quantitative studies of the misplacement of particles are essential for the complete understanding of the particle separation. The performance of fluidized bed separators depends on the particle separation. The present study looks into the qualitative and quantitative evaluation of the misplacement of particles inside a fluidized bed separator. The misplacement of particles depends on the particle flow or hydrodynamics. The misplacement of particles, studied using iron ore, quartz and their binary mixtures. The present study uses a new index called “misplacement index” to understand the particle misplacement quantitatively. Effects of different parameters on the misplacement of particles, evaluated experimentally. It is found that mean particle size ratio (MPSR), particle size, static bed height, particle component concentration in the feed, overflow height and superficial velocity have a significant effect on the misplacement of the particles.

Particle resolved direct numerical simulation of a liquid–solid fluidized bed: Comparison with experimental data

Particle-resolved direct numerical simulations of a 3-D liquid–solid fluidized bed experimentally investigated by Aguilar-Corona (2008) have been performed at different fluidization velocities (corresponding to a range of bed solid volume fraction between 0.1 and 0.4), using Implicit Tensorial Penalty Method. Particle Reynolds number and Stokes number are O(100) and O(10), respectively. In this paper, we compare the statistical quantities computed from numerical results with the experimental data obtained with 3-D trajectography and High Frequency PIV. Fluidization law predicted by the numerical simulations is in very good agreement with the experimental curve and the main features of trajectories and Lagrangian velocity signal of the particles are well reproduced by the simulations. The evolution of particle and flow velocity variances as a function of bed solid volume fraction is also well captured by the simulations. In particular, the numerical simulations predict the right level of anisotropy of the dispersed phase fluctuations and its independence of bed solid volume fraction. They also confirm the high value of the ratio between the fluid and the particle phase fluctuating kinetic energy. A quick analysis suggests that the fluid velocity fluctuations are mainly driven by fluid–particle wake interactions (pseudo-turbulence) whereas the particle velocity fluctuations derive essentially from the large scale flow motion (recirculation). Lagrangian autocorrelation function of particle fluctuating velocity exhibits large-scale oscillations, which are not observed in the corresponding experimental curves, a difference probably due to a statistical averaging effect. Evolution as a function of the bed solid volume fraction and the collision frequency based upon transverse component of particle kinetic energy correctly matches the experimental trend and is well fitted by a theoretical expression derived from Kinetic Theory of Granular Flows.

Modeling the hydrodynamic forces between fluid–granular medium by coupling DEM–CFD

The hydrodynamics flow of solid–liquid fluidized beds has been studied through the simulation by means of coupling Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD). The coupling has been done through the drag force. The interaction forces between a fluid (liquid) and a granular medium (particles) is translated through a drag force. The effect of local particles concentration on the drag force is modeled by a porosity function f(ε)m=ε−m.n. The value of exponent of porosity (m.n) shows substantial dependence on the size of the REV (Representative Volume Element). A validation of the method by comparison with experimental data is presented. It is proposed also as an illustration of three different geometries of fluidizing columns (column simple, column with interior plate and column with two lateral inlets). For each of them, simulation results are analyzed, through a fluidization indicator. These examples illustrate the ability of the coupling DEM–CFD method presented in this work to optimize complex granular flow.

CFD-DEM simulation of liquid-solid fluidized bed with dynamic restitution coefficient

In liquid-solid fluidized beds, the existence of interstitial liquid among colliding particles generates more energy dissipation than that in the gas-solid fluidized bed. In this paper, a numerical modelling method to account for the energy dissipation by means of dynamic restitution coefficient for wet particles in liquid-solid fluidized beds is evaluated. The dynamic restitution coefficient model is based on a semi-empirical correlation generated from freely falling particles bouncing on a wet plate. We employed the Discrete Element Method (DEM) to numerically investigate the hydrodynamics of wet particles under the effect of interstitial fluid by incorporating dynamic restitution coefficient into damping coefficient. Simulation of the liquid-solid two-phase flow used a coupled DEM-Navier Stokes solver. Simulated distribution of granular pressure as well as particle velocities in the liquid fluidized beds were in good agreement with experimental data, thus the effects of dynamic restitution coefficient on the hydrodynamics of wet particle fluidization should be considered in numerical simulations. Two granular pressure models were also compared at different solid fraction for different particle sizes, the results indicated that Gidaspow's granular pressure model shows better predicted results when applied for small particles, while Batchelor's model provided more reasonable results for particles with larger diameter.

Simulations of flow behavior of particles in a liquid-solid fluidized bed using a second-order moments model

Flow behavior of liquid and particles is simulated using an Eulerian-Eulerian two-fluid model based on the theory of granular flow in a liquid-solid fluidized bed. The kinetic interaction of particle collisions is modeled using a second-order moments method. Comparing to simulated results from the kinetic theory of granular flow, the simulated results from the second-order moments method are in agreement with the experimental data of Razzak et al. [25] in a riser. Simulated results indicate that the component of second-order moments in the axial direction is higher than that in the radial direction. The distribution of velocity and volume fraction is predicted at the different inlet liquid velocities. Simulations indicate that axial velocities of particles increase with increasing inlet liquid velocity. The granular temperature is computed from simulations as a function of solid volume fraction. Roughly, the granular temperature increases, reach a maximum, and then decreases with the increase of solid volume fraction. To verify the numerical model further, the liquid-solid flow in the downcomer is simulated, and the simulated results are agreement with the experimental results from Lan et al. [33]. As well as, the liquid-solid circulation fluidized bed loop is investigated, and comparisons with Roy et al. simulations [12] are conducted.

Comparison of Separation Performance of Liquid-Solid Fluidized Bed Separator and Dense Medium Cyclone for Cleaning Coal

ABSTRACTBoth a liquid-solid fluidized bed separator (LSFBS) and a dense medium cyclone (DMC) are used for cleaning coal of -1+0.25 mm size. However, a comparison of LSFBS and DMC’s performance in separation of difficult-to-separate coal has seldom been reported. Difficult-to-separate coarse coal from the Tangkou plant was processed using LSFBS and DMC. The products with the same ash content were analyzed by the float-sink tests to compare the separation efficiencies. According to the results, the concentrate yield and Ep value obtained by the DMC were 48.85% and 0.045, respectively, while the values obtained by LSFBS were 32.86% and 0.12, respectively. The results showed that DMC was more efficient than LSFBS for separating the Tangkou difficult-to-separate coal with a higher yield and a lower Ep value.

Characterization of gas–liquid–solid fluidized beds by S statistics

The hydrodynamics of a gas–liquid–solid fluidized bed was investigated by applying the S statistics method to pressure fluctuations measured under various operating conditions in a laboratory-scale bed. S statistics tests reveal the existence of three transition velocities, especially at low gas velocities. Four distinct fluidization regimes, namely, the compacted bed, agitated bed and coalesced and discrete bubble regimes were detected. A comparison of reconstructed attractors of pressure fluctuations measured at different axial positions along the riser and with various solid loadings showed significant differences in the signals compared before fluidization, especially at minimum liquid agitation velocity. Close to the minimum liquid fluidization velocity and high liquid velocities, the variation in particle size has an insignificant effect on the bed hydrodynamics. Therefore, S statistics is a reliable method to demarcate different fluidization regimes and to characterize the influence of various operating conditions on the hydrodynamics of gas–liquid–solid fluidized beds. The method is applicable in large-scale industrial installations to detect dynamic changes within a bed, such as regime transitions or agglomeration.

Multi-scale vibration behavior of a graphite tube with an internal vapor–liquid–solid boiling flow

In this study, a fluidized bed evaporator mainly made of a brittle graphite heat transfer tube was developed and the vibration acceleration signals of the tube were measured by means of accelerometer sensors and datum acquisition systems at varied steam pressure, solid holdup, particle diameter and axial position to investigate the vibration characteristics of the tube induced by an internal vapor–liquid–solid boiling flow. Multi-scale characteristics of the vibration acceleration signals are identified by the power spectral density and wavelet transformation analyses. The high (3000–9000Hz), intermediate (500–3000Hz) and low (0–500Hz) frequency components of the signals are motivated by the micro-scale motion of solid particles, meso-scale motion of vapor bubbles and macro-scale motion of the circulating liquid flow, respectively. The axial distributions of solid particles and vapor bubbles in the opaque graphite tube can be well reflected by the power spectral density analysis. The tube vibration energy enhances with the increase of solid holdup. Especially, the energy of the micro-scale vibration sub-signal gradually rises and then dominates the system with solid holdup. However, the energy of the micro-scale sub-signal shows a slight increase with heating steam pressure due to the uniform axial distribution of solid particles. The influence of particle diameter is mainly reflected in the enhancement of micro-scale motion. In order to balance the possible destruction caused by tube vibration and heat transfer enhancement due to the addition and fluidization of solid particles, operating conditions of lower solid holdup and higher steam pressure are recommended for the industrial application of vapor–liquid–solid fluidized bed evaporators.

Hydrodynamic characteristics at the layer inversion point in three-phase fluidized beds with binary solids

Layer inversion in gas–liquid–solid fluidized beds with binary solids was examined in a semi-acrylic column of 1.8m height and 0.21m inner diameter. Binary solid combinations were composed of polymer beads (PB, dp=3.3mm, ρs=1,280kg/m3) and glass beads (GB, dp=0.385mm, ρs=2500kg/m3). Five volumetric ratios (PB:GB=0.67:0.33, 0.6:0.4, 0.5:0.5, 0.4:0.6, 0.33:0.67) were used with superficial liquid velocities of 21.6–34.2mm/s, and superficial gas velocities of 0 to 16.6mm/s. Bubble size, bed height, and superficial liquid velocity at the layer inversion point vary with the binary composition. Upon gas introduction into a liquid–solid fluidization column, the superficial liquid layer-inversion velocity increased if it occurred when there was initial bed contraction. On the other hand, the liquid layer inversion velocity decreased when there was initial bed expansion. For all binary solid combinations showing initial bed expansion, liquid layer-inversion velocity decreased with increasing superficial gas velocity in three-phase fluidization, consistent with the gas-perturbed liquid model.

Numerical prediction of flow hydrodynamics of wet molecular sieve particles in a liquid-fluidized bed

The Eulerian–Eulerian framework was used in the numerical simulation of liquid hydrodynamics and particle motion in liquid-fluidized beds. The kinetic theory of granular flow, which accounts for the viscous drag influence on the interstitial liquid phase, was used in combination with two-fluid models to simulate unsteady liquid–solid two-phase flows. We focus on local unsteady features predicted by the numerical models. The solid fraction power spectrum was analyzed. A typical flow pattern, such as core annular flow and particle back-mixing near the wall region of liquid–solid fluidized beds is obtained from this calculation. Effects of the restitution coefficient of particle–particle collisions on the distribution of granular pressure and temperature are discussed. Good agreement was achieved between the simulated results and experimental findings.

Design and experiments using a spiral-liquid-solid fluidized bed system

Liquid-solid fluidized bed (LSFB) has been widely known and used for separation of coarse coal particles (normally larger than 0.25 mm). The process of separation by LSFB needs fluidization water from the bottom to the top of LSFB. The fluidization water is formed by the water addition at the bottom of the LSFB. Normally the quantity of water addition is very large, which increases the burden of water treatment in coal preparation processes. In this investigation, a spiral unit was introduced into the conventional LSFB and the new separation equipment was named S-LSFB. The spiral unit could provide an upward force for the upward movement of coarse low density coals into the concentrate, and hence the quantity of water addition for fluidization water may be reduced. Samples of 0.5-0.25 mm size fraction coal were used to investigate the difference in separation performance between S-LSFB and LSFB. It was found that the separation performance of S-LSFB was nearly equal to that of LSFB. S-LSFB may be beneficial to coarse coal separation in coal preparation plant since the burden of water treatment can be reduced by the application of S-LSFB.

Onset velocity of circulating fluidization and particle residence time distribution: A CFD–DEM study

Until now, the onset velocity of circulating fluidization in liquid–solid fluidized beds has been defined by the turning point of the time required to empty a bed of particles as a function of the superficial liquid velocity, and is reported to be only dependent on the liquid and particle properties. This study presents a new approach to calculate the onset velocity using CFD–DEM simulation of the particle residence time distribution (RTD). The onset velocity is identified from the intersection of the fitted lines of the particle mean residence time as a function of superficial liquid velocity. Our results are in reasonable agreement with experimental data. The simulation indicates that the onset velocity is influenced by the density and size of particles and weakly affected by riser height and diameter. A power-law function is proposed to correlate the mean particle residence time with the superficial liquid velocity. The collisional parameters have a minor effect on the mean residence time of particles and the onset velocity, but influence the particle RTD, showing some humps and trailing. The particle RTD is found to be related to the particle trajectories, which may indicate the complex flow structure and underlying mechanisms of the particle RTD.

Dominant flow structures in gas–liquid–solid fluidized beds

Effective time series analysis techniques in time and frequency domains were applied to characterize three‐phase fluidization under a wide range of gas and liquid velocities. The experiments were carried out in a laboratory scale fluidized bed, operated under ambient conditions. Standard deviation of pressure fluctuations successfully detected four distinct regimes, namely compacted bed, agitated bed, coalesced and discrete bubble regimes (or discrete and dispersed bubble regimes at extremely low gas velocities). A minimum in skewness and average cycle frequency and a maximum in flatness indicated a minimum deviation from larger structures of the bed. A transition between macro and finer structures occurred during agitated bed regime and accordingly, minimum liquid fluidization velocity was perceived by skewness, flatness, and average cycle frequency analyses. The results showed that at very low liquid velocities in the compacted bed regime, the power spectrum at lower frequencies slightly increased with increasing liquid velocity. This results in the consecutive appearance of single bubbles of low frequencies. With further increase in the liquid velocity at agitated bed regime the power spectrum at lower frequencies gradually decreased. At higher liquid velocities near ULmf, the peak dominant frequency is transmitted from low frequency ranges of 2 Hz to the higher frequency of about 10 Hz.

Ebullated bed fluid dynamics relevant to industrial hydroprocessing

This study investigates the overall fluid dynamics of an ebullated bed operating at high gas holdup conditions to provide relevant observations for industrial residue hydroprocessors. Scaling approaches for three-phase fluidized beds were compared specifically for the scale-down of the industrially observed high gas holdup conditions. Five dimensionless groups, a binary approach for bubble coalescence behaviour in multi-component liquids, and geometric considerations are proposed to achieve dynamic similitude. Experiments were carried out in a 101.6mm diameter co-current gas–liquid–solid fluidized bed operating at 0.1 and 6.5MPa with liquids that do (e.g., 0.5wt% aqueous ethanol) and do not (e.g., tap water) significantly inhibit bubble coalescence. A comparison of the overall phase holdups for two sizes of cylindrical particles (dSV of 1.6 and 3.9mm) at matching dimensionless groups provided a preliminary verification of the proposed scaling approach when bubble coalescence was sufficiently and consistently inhibited. The impacts of increased liquid viscosity (e.g., greater vacuum distillation tower residue feed fraction), varying superficial gas velocity (e.g., inlet gas flow rate and gas entrainment in the liquid recycle line), and varying superficial liquid velocity (e.g., liquid recycle pump speed) were experimentally studied due to their relevance for industrial ebullated bed hydroprocessors. For the studied operating conditions, gas holdups in the ebullated bed were much greater when bubble coalescence was sufficiently inhibited (up to approx. 45vol%) compared to a coalescing system at equivalent conditions (up to approx. 25 and 35vol% for 0.1 and 6.5MPa, respectively). When increasing the liquid viscosity of the 0.5wt% aqueous ethanol, a fraction of the gas was entrained in the liquid recirculation, increasing gas holdups and exhibiting operational similarities to industrial hydroprocessors. After adjusting the gas and liquid flow rates based on the gas recirculation, the increased liquid viscosity mainly reduced the solid holdups while gas holdups were approximately unchanged. Enhanced bubble break-up or bubble coalescence due to particles resulted in an overestimation or underestimation, respectively, of the freeboard gas holdups from a solids-free estimate based on the bed region phase holdups, where this model could not capture the complex bubble–particle interactions. Experimental results at high gas holdups conditions were used to correlate the bed and freeboard phase holdups based on the proposed dimensionless groups.

Experimental and CFD Investigation the Effect of Solid Particle Height in Water-Solid Flow in Fluidized Bed Column

The liquid solid fluidized beds have been usually used in catalytic cracking, ion exchange, crystallization, and particle classification. Fluidized bed column has investigated experimentally and numerically. A numerical investigation has been done by using computational fluid dynamic software ANSYS Fluent 15.0. In this study, a spherical stainless steel has used as solid particle fluidized continuously by water. Three different values of water flow rate (10, 15, 20 l/min) have used with three different values of static head of the solid particles (2, 2.5, 3 cm). The pressure of the fluidized bed has been measured at four different location of the test section 20 cm apart and the flow behavior at the test section has monitored by using AOS high speed camera. The experimental data has compared with numerical solution and a good agreement has been found. It can be observed that the expansion of the solid particles and the pressure inside the bed increased as the water flow rate and static bed height increased.

Analogy between momentum and heat transfer in liquid–solid fluidized beds

Wall-to-bed heat transfer in particulate fluidized beds of spherical particles was studied. Experiments were performed using spherical glass particles of 0.80–2.98mm in diameter with water in a 25.4mm I.D. copper tube equipped with a steam jacket.Heat transfer data related to the fluid–particle interphase drag coefficient were obtained and compared with previous results for wall-to-bed mass transfer in fluidized beds [Bošković et al., Powder Technol., 79 (1994) 217].All the data for momentum, heat and mass transfer in particulate fluidized beds of spherical particles, showed the existence of an analogy among these three phenomena.

Stability analysis in solid–liquid fluidized beds: Experimental and computational

Abstract In this study the transition from homogeneous to heterogeneous flow in a solid–liquid fluidized bed (SLFB) is examined both experimentally and numerically. The experimental apparatus comprised a refractive index-matched SLFB, comprising 5 mm diameter borosilicate glass and sodium iodine solution, which allowed for both instantaneous particle image velocimetry of the liquid flow field and solids hold-up measurements to be undertaken for superficial liquid velocities in the range of 0.06–0.22 m/s. The motion of individual, spherical steel balls (with diameters 6, 7, 8, 9 mm) was then tracked as it settled through the fluidized bed for differing superficial liquid velocities. It was observed that, for all the steel balls covered in this work, there was a change in slope in their respective classification velocity curves at a superficial liquid velocity of 0.08 m/s. This value was very close to the critical velocity of 0.085 m/s predicted from 1-D linear stability analysis; and therefore deemed to be the critical condition that marked the transition from homogeneous to non-homogenous flow. It is proposed that the change in slope of the classification velocity curve is due to the encounter of the settling foreign particles with liquid bubbles whose presence marks the onset of heterogeneous flow. Additional computational analysis, involving both Eulerian–Eulerian (E–E) and Eulerian–Lagrangian (E–L) approaches, is used to confirm the presence of liquid bubbles at a critical liquid hold-up of 0.54, which corresponds to that predicted from 1-D linear stability analysis. In summary, the study has highlighted that experimentally the transition condition for a SLFB can be obtained simply by observing the behavior of the classification velocity of a single foreign particle at different superficial liquid velocities. This transition condition was found to agree with the 1D linear stability criterion, Eulerian–Eulerian CFD (3D) and Eulerian–Lagrangian DEM (3D) approaches.