Spout-fluid Bed Research Articles

Mixing characteristics of binary mixtures in a spout-fluid bed

ABSTRACTMixing characteristics of binary mixtures in a flat-bottom cylindrical spout-fluid bed using glass beads and air are reported in this work. Experiments were carried out to investigate the mixing characteristics for binary mixtures at three flow conditions, i.e., only spouting, only fluidization and spout-fluidization. The experiments were performed at different gas velocities, diameter ratios of binary mixtures and three different bed arrangements. Mixing index was determined for fluidized bed and static bed conditions. It was found that, in all cases, lowest-diameter ratio mixture gave good mixing index values. For all flow conditions, mixing index for large–small bed arrangement was increasing with time, whereas for small–large bed arrangement, the mixing index deteriorated with time. However, in both cases, the mixing index reached almost a constant value. For well-mixed bed arrangement and spout-fluidization flow condition, segregation and re-mixing were observed.

Coupling of discrete element model (DEM) with computational fluid mechanics (CFD): A validation study

A numerical model is developed in the framework of OpenFOAM; an open source computational fluid dynamics (CFD) simulation code to simulate particle laden dense flows. The model uses discrete element method (DEM) for the discrete/particle phase and computational fluid dynamics approach (CFD) for the fluid/continuum phase. In current study, validation of the model is done in two steps. In first step the drag model is validated by comparing the results of settling velocity of spherical particle. In second step, spout fluidization test cases with different operating conditions are simulated and results of numerical simulation of spout-fluidized bed are compared against experimental and simulation results reported in literature. The isosurface plots of solid volume fraction show a good qualitative prediction of different flow regimes. The particle velocity profiles in the vertical direction for different test cases corresponding to different flow regimes are plotted and compared with the literature data. The predictions of the model are in good agreement with the experimental and numerical results reported in the literature.

Influence of operating parameters and flow regime on solid dispersion behavior in a gas–solid spout-fluid bed

The dispersion behavior of the solid phase in a three-dimensional spout-fluid bed was numerically investigated via the approach of coupled computational fluid dynamics and discrete element method (CFD–DEM), in which the fluid and solid phases were solved using the Eulerian and Lagrangian framework, respectively. Detailed trends of both the local and overall dispersion behaviors of the solid phase in the system were quantitatively studied, and the impact of various operating parameters (namely, spouting gas velocity (Usp), background velocity (Ubg), particle diameter (dp), the bed depth (L) and bed height) and flow regime on lateral and vertical solid dispersion was investigated. The results show that varying Usp, Ubg and dp affects the local solid dispersion in the spout and fountain regions more than the annulus. To enhance the overall solid dispersion, either Usp or Ubg or L can be increased, or dp and bed height can be decreased. Meanwhile, the total gas flow rate plays the dominated role for overall dispersion of solid phase. Finally, solid dispersion is the most effective in the Jet-in-fluidization regime, followed by the Spouting-with-aeration then the Intermediate/spout-fluidization regimes.

DEM numerical simulation of wet cohesive particles in a spout fluid bed

The dynamic behavior of wet cohesive particles is greatly different from that of dry particles in a spout–fluid bed. A modified discrete element model (DEM) for wet particles is established by adding an additional module to consider the presence of the liquid bridge. DEM simulations are conducted for the dry and wet granular systems and the geometry of the bed is the same as the experimental one of Link et al. The influences of drag models and liquid contents are investigated. It is found that the Koch–Hill drag model predicts a dominant peak, the same as the measured results of Link et al., in the frequency domain of the dynamic pressure drop under Case A. However, the Gidaspow drag model fails to catch this phenomenon. With an increase of the liquid content, the fluctuation range of the pressure drop increases firstly and then decreases rapidly under Case A, but it monotonically increases under Case B. In the spout–fluid bed, dead zones, where the particles are de-fluidized, appear at the corners. The inclined angle and the area of the dead zone expand with the increase of the liquid content, which makes the value of the final Lacey mixing index decrease.

Combined phase distribution and particle velocity measurement in spout fluidized beds by ultrafast X-ray computed tomography

The spout fluidized bed is a very successful combination of spouted and fluidized bed, which is most interesting for processing particles with larger or varying diameter. However, the more complex hydrodynamics of the combined configuration is yet not fully understood. Here, we propose the application of ultrafast X-ray computed tomography (CT), which has proven to be a valuable measurement technique for the analysis of highly dynamic processes, for example multiphase flows. It is able to resolve material distributions with up to 8000 frames per second and a spatial resolution down to 1mm. Especially for opaque systems, such as fluidized beds, ultrafast X-ray CT can reveal details, which are not recoverable by e.g. optical measurement techniques. Besides the recovering of the dynamic phase distribution, determining local particle velocities is essential to understand the complex flow in fluidized beds. As we will demonstrate here for the first time, this is possible via a dual-plane ultrafast X-ray CT by using suitable marker particles. In this article, the methodology for determining particle velocities under highly dynamic conditions within a cylindrical spout fluidized bed is presented. Complementary analysis techniques for different flow conditions have been combined to increase confidence in the velocity data.

The influence of different chemical compositions in biomass on gasification tar formation

To elucidate the relationship between biomass composition and tar formation, forest residue sawdust, rich in lignin, and agriculture waste cornstalks, rich in cellulose, were gasified in a spout-fluidized bed reactor from 700 °C to 900 °C. Gel permeation chromatography (GPC) coupled with a photodiode array detector (PDA) and gas chromatography – mass spectrometry (GC–MS) were used to analyze the tar character. The GPC results showed that the molecular mass distribution of the gasified tars were unchanged, only the amount of each component changed when the temperature increased during gasification. The amount of heaviest molecular mass components decreased, while the lighter components increased with temperature. Sawdust tar and cornstalks tar both showed aromatic character, while cornstalks tar contained more aliphatic compounds than sawdust tar. The tar formation mechanism has been proposed from the experimental data analysis.

Hydrodynamic investigation of gas-solid flow in rectangular spout-fluid bed using CFD-DEM modeling

The hydrodynamics of rectangular spout-fluid beds are studied using Computational Fluid Dynamics-Discrete Element Method (CFD-DEM), a powerful approach in simulating gas-solid flows. Modified Navier-Stokes equations for the gas phase mass and momentum balance, and Newton's second law for the solid phase motion are coupled with Newton's third law of motion. Contact forces of particles and particle-wall collisions are based on a spring-damper system. The results show good agreement with 24,500 spherical particles for a column of 15mm thickness and 150mm width. The effect of column thickness and 3D effects are investigated by studying thicknesses of 30, 60, and 90mm with 49,000, 98,000, and 147,000 particles. The effects of 2D and 3D CFD modeling on the pressure drop, air velocity, particle flux, and voidage profiles in different directions are compared in a column of thickness 30mm. In addition to demonstrating the capability of CFD-DEM to model rectangular spout-fluid beds, it is shown that bed thickness and CFD modeling dimensions affect the flow and should be considered in numerical simulations.

New Type of Draft Tube Spout-Fluid Bed. Part 2: Modeling and Design of the Acceleration Section of the Riser for the Pneumatic Transport of 1 mm Glass Spheres

A new type of draft tube spout-fluid bed (DTSFB) generically similar to a circulating fluidized bed (CFB) possesses a unique combination of capabilities as a fluid-particle contacting system or multiphase reactor. This paper focuses on the modeling of the acceleration section of the draft tube (a riser) of a DTSFB for the dilute phase pneumatic transport of 1 mm glass spheres in a turbulent air stream. An analytical expression for a dimensionless dynamic pressure gradient distribution in the acceleration section (y(ζ)) is derived from the solution of an isoperimetric problem of the calculus of variations. The gradient is shown to be a function of a single dimensionless parameter, which is the ratio of the dynamic pressure drop in the acceleration section and the dynamic pressure gradient in the nonacceleration section. It is shown that ∫01 y(ζ) dζ is maximized in the acceleration section of the riser. As a result, the pressure drop required to overcome the resisting forces in the acceleration section is minimized in any run. The pressure distribution obtained by integrating the pressure gradient is shown to be an excellent fit to experimental data. The modeling of the acceleration section is based on the one-dimensional, two-fluid continuity and momentum equations plus the variational pressure gradient distribution making it possible to calculate the solids fraction, gas and particle phase velocities, and effective drag coefficient distributions in the riser from five independent equations. The particle–wall friction factor in the model is calculated from a theory developed by the authors in a previous paper.

The Effects of Ca(OH)<sub>2</sub> on the Steam Gasification Process of Acid-Washed and Raw Lignite

The effect of Ca (OH)2 prepared by the wet-mixing method on acid-washed and raw lignite steam gasification process at 800°C were analyzed in a spout-fluid bed reactor. The addition of Ca (OH)2 on acid-washed lignite obviously increased the coal conversion, gas yield and reduced the tar yield, and the Ca (OH)2 addition on raw lignite significantly reduced the tar yield but just slightly increased the coal conversion and gas yield. As a catalyst, Ca (OH)2 can promote the tar steam reforming in the gasification process. Based on GC-MS data, it can be deduced that Ca (OH)2 has different catalytic activity on the steam reforming of tar with different molecular structures. Aliphatic hydrocarbons, Class 2 and Class 5 tars were clearly catalytic reformed, while the concentration of Class 4 tars increased. The CO concentration decreased and the CO2 and H2 concentration increased with the addition of Ca (OH)2 on both acid-washed and raw lignites, which was mainly caused by the catalytic effect of Ca (OH)2 on WGS reaction. The effect of Ca (OH)2 addition on acid-washed lignite was more significant than on raw lignite, and no adsorption of CO2 was detected in the process by using active carbon as a model compound.

Influence of fluid-mechanical parameters on volumetric mass transfer coefficient in a spout–fluid bed with a draft tube

In order to apply a spout–fluid bed reactor with a draft tube for the nitrification process of wastewater treatment, the influence of fluid-mechanical parameters on volumetric mass transfer coefficient was analyzed. Experiments were carried out in a 1000mm high 2D semi-column with a draft tube. The draft tube had a cross section of 50×50mm2 and the annular region of the reactor had a cross section of 50×140mm2. The work described in this paper reports the influence of several fluid-mechanical parameters including liquid and gas flow rates, particle diameters and particle circulation on volumetric mass transfer coefficient, kLa. The liquid flow ranged from 3.5–4.5m3/h and gas flow ranged from 300 to 800L/h. The particles used were glass spheres with diameters of 3, 4, 5 and 6mm. Tap water and air were used as the liquid and gas phase, respectively. The experimental results have shown that particle circulation through the draft tube contributes to kLa increase as well as the increase in the particle diameter, the gas and liquid flow rates.

Numerical investigation of the time-related properties of solid phase in a 3-D spout-fluid bed

Numerical simulation of the two-phase flow in a 3-D spout-fluid bed is carried out to investigate the time-related properties of solid phase based on the In-house code by CFD–DEM coupling approach. The mixing time, distribution properties of the cycle time, global residence time and local residence time of solid phase are explored. Moreover, the influences of spouting gas velocity, background velocity, particle diameter and bed thickness on these properties are discussed. The results illustrate that the mixing time is lowered with the increase of spouting velocity or the decrease of background velocity. Cycle time shows a log-normal distribution behavior and grows with the increase of the particle diameter but decreases with the increase of spouting velocity or background velocity. Moreover, nearly 60% of the total particle time elapses in the annulus and 34% of that in the fountain while only 6% of that is paid in the spout. The solid residence time (SRT) in the spout or annulus diminishes with increasing spouting velocity or background velocity and also the decrease of particle diameter. However, the SRT in the fountain region shows the opposite response behavior. The scale-up of the bed thickness enlarges the cycle time and SRTs in three regions of the bed. Besides, small local SRT appears in the spout while large value exists in the annulus especially the corners of the bed bottom. The geometrical configuration of the bottom leads to the opposite distribution of local SRT near walls in the annulus and fountain. Moreover, the local SRTs in different regions of the system show distinct distribution properties.

Effects of Operating Parameters on Hydrodynamic Behavior of Spout-fluid Beds without and with a Draft Tube

The spout-fluid bed (SFB) is one of many modifications to conventional spouted bed, which is achieved by introducing an auxiliary gas flow into the annular region of a spouted bed through a distributor. Therefore, the SFB has better gas-solid contact and mixing, and reduces the risks of particle agglomeration and dead zone. In addition, the gas-solid contact and the solids circulation rate can be controlled more easily and flexibly by installing a draft tube in the bed (DSFB). In this study, the effects of operating parameters on flow characteristics of the SFB and the DSFB were investigated and they were compared, using the same semi-cylindrical column and the same particles. As the results, the flow regime maps were recognized based on pressure fluctuation data and the DSFB had wide range of operating conditions for stable spouting, comparing with the SFB. The minimum spouting gas velocity for stable spouting state (spouting with aeration) decreased with increasing the auxiliary gas velocity. Also, longitudinal and radial distributions of particle velocity in the spout and the annulus were obtained in detail for the stable spouting state by means of the particle image velocimetry (PIV). Particle velocities in the spout and the annulus were increased with increasing the auxiliary gas velocity. Moreover, the distributions of particle velocities in the DSFB showed that particles in the draft tube were accelerated remarkably and particles in the annulus descended uniformly with small velocity, comparing with the SFB.

Numerical Simulations of Particle-laden jet/spout Flows Using Eulerian-Lagrangian Approach

Discrete Element Method (DEM) had been successfully coupled with Computational Fluid Dynamics (CFD) in the framework of OpenFOAM; an open source CFD simulation code. It is used to simulate particle laden flows using Eulerian approach for continuum phase and Lagrangian approach for solid phase. This numerical model is validated with simple test cases of spherical particle comparing the results with the analytical solution and is reported in earlier work. In current study, results of numerical simulation of spout-fluid bed are compared against experimental and simulation results reported in literature. The pressure drop fluctuations and isosurface plots of solid volume fraction show a good qualitative prediction of different flow regimes. The particle velocity profiles in the vertical direction for different test cases are plotted and compared with the literature data. The comparison of results shows a good quantitative behavior of the model predictions.

Numerical Investigation on Effect of Food Particle Mass on Spout Elevation of a Gas–Particle Spout Fluidized Bed in a Microwave–Vacuum Dryer

The effect of food particle mass on the instantaneous fluid dynamic characteristics of a gas–particle spout fluidized bed in a pulsed spouted microwave–vacuum drying (PSMVD) system was investigated by a numerical method. The spout fluidization process in a pseudo-2D spout fluidized bed was simulated by computational fluid dynamics (CFD) using the inviscid two-fluid theory method (TFM) based on the kinetic theory of granular flow. The effect of particle mass on the particle flow pattern, jet penetration depth, and pressure drop was evaluated. Under a specific particle mass, the spout velocity was an important factor controlling particle status in the spout fluidized bed and a critical velocity was identified for effective transition of the flow pattern. With an increase in particle mass, the spout velocity tended to increase to obtain the expected gas–particle flow pattern and the correlation between the particle mass and particle flow pattern was revealed. This research on the fluid dynamic characteristics of a gas–particle spout fluidized bed could be used to improve the uniformity of particle mixing and microwave heating.

Encapsulation of Orange Essential Oil in a Spout-Fluid Bed Dryer with a Draft Tube on a Bed of Inert Solids

Encapsulation of orange essential oil in modified starch (N-Lok) was performed in a spout-fluid bed dryer with a draft tube (SFB) on a bed of inert solids under different operating conditions. The essential oil retention and encapsulation efficiencies were determined and compared with those obtained in a spray-drying encapsulation process. The results showed that, in general, both SFB efficiencies were lower than those obtained in the spray dryer; however, it was observed that most of the essential oil remaining in the SFB was encapsulated (94% compared with 70% for the spray dryer). The formation of particles with thicker encapsulation walls in the SFB provided better protection of the orange essential oil than that achieved in spray drying.

The synergistic effect of Ca(OH)2 on the process of lignite steam gasification to produce hydrogen-rich gas

The synergistic effect of Ca(OH)2 prepared by the wet-mixing method on lignite steam gasification process at different temperatures (700–900 °C) was analyzed in a spout-fluid bed reactor. Firstly, to avoid disturbance of volatile and tar, active carbon was used as a model compound. On the one hand, Ca(OH)2 effectively catalyzed the water-gas shift (WGS) reaction to improve H2 concentration, but the performance was weaker at higher temperature due to the enhancement of boudouard reaction and the weakening of WGS reaction. On the other hand, it was found that the (CO+2CO2)/H2 ratio of syngas produced at 700 °C in the presence of Ca(OH)2 was 0.82, which was much lower than that of the other cases, owning to the absorption of CO2. The synergistic effect was observed at this temperature, for the adsorption of CO2 altered equilibrium of the WGS reaction and further improved H2 concentration. Then two kinds of Chinese lignite (HLH and XM) were selected to further study the performance of Ca(OH)2 on optimizing the lignite steam gasification process. In the presence of Ca(OH)2, tar and char yields greatly reduced at the same reaction temperature, whereas the gas yields significantly increased. As a catalyst, Ca(OH)2 can not only promote solid–gas reaction to decrease char yield, but also accelerate tar decomposition to reduce its yield in syngas. Based on GC–MS data, it can be deduced that Ca(OH)2 has different catalytic activity on the steam reforming of tar with different molecular structures. Contrast to Class 4, tars of aliphatic hydrocarbons, Class 2 and Class 5 were clearly catalytic reformed. Hydrogen-rich gas can be produced at 800 °C and 900 °C owning to the catalytic effect of Ca(OH)2, but the highest H2 concentration was found at 700 °C due to the additional effect of CO2 absorption, which was supported by the results of thermogravity experiments.

Pressure Drop and Gas Holdup Studies in a Spout-Fluid Bed

Spout-fluid beds are used for a variety of processes involving particulate solids. They are employed where the particle agglomeration, dead zones, and sticking of particles to the vessel are the common problems in conventional spouted beds. Applications involved are granulation, coating, drying, combustion, and gasification. In this study, experimental studies have been carried out in a cylindrical Perspex column (0.094 m internal diameter and 1.217 m height) using glass beads and air. The effects of initial bed loading, spout velocity, and background (fluidization) velocity on pressure drop and gas holdup have been investigated. It is found that the minimum spout-fluidizing velocity increases with increase in initial bed loading. The pressure drop and gas holdup increase with increasing bed loading. In spout-fluid bed condition, at a constant spout velocity, as the background gas velocity increases, the gas holdup increases, and it is found to be high for smaller bed loading and is low for larger bed loading at higher velocities. The fountain height increases as spouting velocity increases and it decreases with initial bed loading. The total velocity required to fluidize the particles in spout fluidization is lower in comparison to spouted beds and fluidized beds.

Influences of operating parameters on the hydrodynamics of a 3-D spout–fluid bed based on DEM modeling approach

Abstract The influences of operating parameters on the gas–solid hydrodynamics of a 3-D spout–fluid bed operated in the flow regime of Jet-In-Fluidization are numerically investigated at the particle-scale level. The gas motion and solid motion are solved under the frameworks of computational fluid dynamics and discrete element method, respectively. The dynamical property of solid phase obtained numerically is compared with the experimental data in literature. Then, the influences of the spouting velocity, the background velocity and the particle diameter on the bed hydrodynamics are discussed. The results show that vigorously lateral motion of solid phase exists in the spout–annulus interface and the vertical transportation intensity is larger than the lateral one. Increasing the spouting velocity enlarges the solid velocity but decreases its concentration in the spout and enhances the solid circulating flux of the system. Meanwhile, the enlargement of background velocity gives rise to an obvious change of voidage in the annulus. However, enlarging the particle diameter lowers the solid motion in three regions of the bed. Spout and annulus interact with each other through the spout–annulus interface. The spout boundary diverges initially, and then keeps nearly constant, finally diverges again until the bed surface. Moreover, the shape of the spouting boundary is strongly influenced by the inlet geometry. Along the bed height, this influence diminishes initially but is enhanced in the vicinity of bed surface. The enlargement of spouting velocity or background velocity leads to the expansion of the spout boundary along the bed height. In contrast, the increase of particle diameter reduces the spout boundary.

Hydrodynamic Performance of a Spout–Fluid Bed with Draft Tube at Different Temperatures

Temperature is one of the key factors exerting a strong effect on the hydrodynamic performance of spout–fluid beds. In this study, experiments were conducted in a 160-mm-diameter spout–fluid bed with a nonporous draft tube at four temperatures: 293, 373, 440, and 500 K. The effect of temperature on the hydrodynamic performance indexes of pressure drop, gas bypassing, minimum spouting velocity, and flow pattern was investigated. The pressure drop in the annular region was higher at 500 K than at 293 K. The minimum spouting velocity (Ums) increased with temperature at a low superficial fluidizing gas velocity (Ums < 0.010 m/s). However, Ums decreased with temperature at a high superficial fluidizing gas velocity (Ums > 0.020 m/s) because the amount of gas flow rate transferred from the fluidizing gas into the draft tube increases with temperature. Six flow patterns identified by pressure drop fluctuations and digital images show that the gas velocity ranges in the two flow regimes of spouting with aeration and spout–fluidization are narrow or limited at 500 K compared with those at 293 K. The effect of temperature on these hydrodynamic performance indexes should be considered in the use and design of spout–fluid beds.

Simulation of Gas-Solid Flow Characteristics in Three-Dimensional Rotational Spouted-Fluidized Bed

In order to study the gas-solid flow characteristics in a rotational spouted-fluidized bed dryer, the eulerian multi-phase model was applied in three-dimensional numerical simulation of a rotational spouted-fluidized bed to analyze the effect of different velocity ratios between bottom and tangential wind on gas and particle velocity distribution characteristics, and the change rule of gas-solid flow state with the time at the velocity ratio of 30 m·s-1/30 m·s-1 was derived. The results show that the increase of tangential wind velocity is propitious to enhance the gas flow rate in the region near the wall and make the gas-solid phase mix sufficiently as well as augment of the contact area of gas and particle phase, and decrease of the gas flow dead zones and the adhesion of viscous materials to cylinder wall. However, the negative pressure formed by the entrainment effect of tangential wind goes against the development of gas flow along the axial direction reducing the penetration effect of axial wind to the granular layer.