Rectangular Fluidized Bed Research Articles

CFD-DEM simulation study on the bed dynamics of a binary mixture with super-quadric particles in a fluidized bed

In biomass thermal conversion processes such as pyrolysis and gasification, biomass often takes on a cylindrical shape. However, most previous studies have modeled biomass as spherical, leaving the fluidization and mixing behaviors of non-spherical biomass particles with approximately spherical bed materials insufficiently explored. To bridge this gap, a super-quadratic particle model coupled with CFD-DEM was used to explore the fluidization characteristics and mixing dynamics of non-spherical binary mixtures. The numerical model successfully validated the pressure drop and spatial distribution within the binary mixture. The results indicate that particle mixing is primarily driven by the rise and movement of bubbles. The large aspect ratio of the cylindrical particles, combined with the narrow thickness of the 2D rectangular fluidized bed, leads to a significant interlocking effect during fluidization. This interlocking hinders both fluidization and mixing, resulting in poor overall performance for non-spherical particles. At a low superficial velocity (Ug), only the cylindrical particles in the lower part are fluidized as a result of the rising central bubble. The two particle types achieve effective mixing as the Ug rises to a high value. The mixing index increases from 0.78 to 0.94 as the Ug increases from 1.8 to 2.1 m/s. However, the stacking pattern of cylindrical particles creates a dead zone at the bed bottom, occupied solely by spherical particles, which limits local bed mixing and fluidization. As the Ug increases from 1.8 to 2.1 m/s and the dimensionless inventory height increases from 0.6 to 1.0, the axial dispersion coefficient of the cylinders increases from 1.17×10−3 to 4.35×10−3 m2/s and from 1.91×10−3 to 9.22×10−3 m2/s, respectively. This study provides new insights into the behavior of non-spherical particles, offering potential avenues for optimizing chemical engineering processes.

Jet injection and spout formation in a fluidized bed

A single jet was studied in a rectangular fluidized bed by analyzing the pressure fluctuations data to characterize the jet hydrodynamics. The bed contained with glass beads of different mean sizes (450, 650, and 920 µm) as well as pharmaceutical pellets (920 µm). The analysis of the pressure fluctuation data was performed using the power spectral density function (PSDF) and discrete wavelet transforms technique. This study investigated how the flow regime changed from a jet in a fluidized bed to a spouted regime as the gas injection velocity was increased. Increasing the mean particle size led to an increase in the minimum spouting velocity. The minimum spouting velocity for 450, 650, and 920 μm glass bead particles and for 920 μm pharmaceutical pellets were determined to be 32.47 m s-1, 38.66 m s-1, 44.78 m s-1, and 44.47 m s-1, respectively. The study also examined how the dominant frequency of the various particles and the energy percentage of scales changed with increasing injection velocities. The ratio of energy percentages of meso-scale changes as the injection velocity increases and these changes were used to estimate the minimum spouting velocity. Wavelet sub-signals analysis revealed that the Daubechies 2 (DB2) wavelet was the most effective at capturing the characteristics of the jet. Based on the Shannon entropy of the approximate coefficients, the wavelet analysis showed that 11 levels of decomposition were required. By combining the wavelet analysis and PSDF, a more detailed analysis of the meso-scale was achieved. This study provided valuable insights into the behavior of jets and spouts in fluidized beds, which can greatly contribute to the optimization of a jet in fluidized beds and spout-fluidized beds by enhancing our understanding of jet behavior in such environments.

Bubble and particle dynamics in a thin rectangular bubbling fluidized bed under emulated marine instabilities

Floating fluidized beds are affected by swell/sea waves, limiting their effectiveness in seagoing applications as an alternative to fixed beds for the removal of exhaust pollutants. To address this, experiments were conducted using a rectangular fluidized bed mounted on a hexapod to emulate marine operation. The bed was subjected to dynamic perturbations, and Digital Image Analysis and Particle Image Velocimetry were used to analyze the effects of motion on fluidization, bubble growth/shape, void/velocity fields, and gas/solids circulations. Our investigation showed that non-vertical orientations resulted in a decrease in minimum fluidization velocity, but the most significant effect was seen in the trajectory and shape of the bubbles near both static and dynamic walls. This resulted in a slug regime with a high degree of gas bypass. These outcomes have important implications for the design/operation of marinized fluidized beds, and for process intensification solutions to address the constraints associated with ship emissions.

Quantitative Measurement of Solids Holdup for Group A and B Particles Using Images and Its Application in Fluidized Bed Reactors

Solids holdup as one of the main parameters in characterizing the performance of fluidized bed reactors is widely concerned. With its development and improvement, visualization technology has been applied in fluidization because of its little disturbance to the flow. In this study, four types of particles with different properties are tested in a narrow rectangular fluidized bed equipped with a high-speed video camera. Calibration curves of these different types of particles are achieved by correlating the grayscale of the digital images with the corresponding solids holdup. These calibration curves are further applied to obtain the average solids holdup across the sectional area and local solids holdup from the center towards the wall in both a gas-solids turbulent fluidized bed and a circulating fluidized bed to verify the results. The calibration method works well for solids holdup of different types of particles in both dense and dilute fluidization systems. This method is important to characterize the fluidization quality and reactor performance with a wide operating condition.

Gas flow distribution and solid dynamics in a thin rectangular pressurized fluidized bed using CFD-DEM simulation

Operating a fluidized bed at elevated pressure is typically used to intensify processes. The effects of pressure on bubble behavior have been extensively studied; however, the gas flow distribution and particle dynamics are much less studied due to difficulties in measurement. In this work, CFD-DEM simulations are conducted to study hydrodynamics in a thin rectangular fluidized bed at pressures increased from 1 bar to 10 bar. The important findings can be summarized as follows. With an increase of pressure, the bubble size and pressure fluctuations decrease. With increasing pressure, the percentage of visible flow of the rising bubbles increases, while the percentage of the gas throughflow through bubbles decreases. At elevated pressures, the solid flow flux and mixing rate are increased. Finally, the reasons of the influence of pressure on fluidization characteristics are explored in view of the increased inertia force on particles and the increased differential pressure on bubbles.

Role of bubbling behaviour in segregation and mixing of binary gas-solids flow of particles with different density

Dynamics of segregation and mixing phenomena of binary gas-solids flow in fluidized beds is predominantly influenced by the bubbling behaviour. In the present work, we have investigated experimentally the effect of the local bubbling behaviour on the segregation and mixing phenomena of binary gas-solids flow with particles differing in density (of the same size) in a pseudo–2D rectangular fluidized bed. The bubbling characteristics were controlled using two different modes of air injection: uniform distributor and two-jet distributor under the same superficial gas velocity (UG). The local individual phase area fraction fluctuations of all the three phases were measured using the high-speed imaging. These measurements were further used to quantify the effects of bed composition, mode of air injection, and UG on the bubbling characteristics and eventually on the segregation/mixing behaviour. The present study shows that the use of the two-jet distributor led to an increase in the amplitude of the local gas-phase area fraction fluctuations in comparison to that of the uniform distributor and thus enhanced the mixing of the particles. Also, with an increase in the flotsam component in the mixture, due to increase in the bubble size and frequency, the bubble-induced mixing of the particles was enhanced further. The results reported here provide the understanding of the effect of local bubbling characteristics on the dynamics of segregation and mixing phenomena of binary mixtures with particles differing in density and also provide the much-needed database for validation of CFD-DEM and Eulerian multi-fluid CFD models.

Investigating the bubble dynamics in fluidized bed by CFD-DEM

Fluidized beds are of great importance in various industries and presence of bubbles significantly influence their efficiency. In order to gain a more accurate vision on the dynamics of a single bubble in fluidized beds, formation, rising, and eruption of a single bubble in a rectangular fluidized bed were studied by CFD-DEM and validated by experimental data. Single bubble in a 2-D rectangular fluidized bed at different stages, from forming to erupting, was modeled by the CFD-DEM technique. Investigating the pressure fluctuations and bubble dynamics were the main focus of this study. Pressure fluctuations were measured using a pressure transducer and a high-speed digital camera was used to record the whole bubbling process, from bubble formation on the distributor to its eruption at the surface. Validation was performed by comparing experimental and simulated bubble diameter and pressure fluctuations. CFD-DEM results such as particles and gas velocity vectors, bed voidage and pressure contours revealed new information on characteristics of pressure fluctuations which was not possible to gain by experimental studies. In the formation stage, due to the compression wave from injection, particles accelerate and bed reaches to a packed state. This causes a pressure peak in the pressure. At the beginning of bubble formation on the distributor, pressure decreases and when the bubble detaches from the distributor, pressure reaches a minimum. While bubble is rising through the bed, pressure increases again, however, it remains below zero because of the lower voidage behind the bubble in comparison to pre-injection state and the greater velocity of the gas above and below the bubble. By bubble eruption, another small compression wave is seen in the pressure.

Rice husk combustion characteristics in a rectangular fluidized-bed combustor with triple pairs of chevron-shaped discrete ribbed walls

An experimental combustion work using the rice husk as a fuel was performed in a rectangular fluidized bed combustor with triple pairs of chevron-shaped discrete ribbed walls. The effect of percent excess air (EA = 40%–70%) on temperature distribution and gas emissions in a rectangular fluidized-bed combustor on combustion behaviors (temperature distributions inside the bed, exhaust gas emissions and combustion efficiency) is reported. The free-board had a rectangular shape which modified by increasing the chamber size from 300 × 100 mm2 to 600 × 200 mm2 or two times of the bottom chamber and 2400 mm in height. Triple pairs of rib configurations, namely, 30° chevron-shaped discrete ribbed walls were introduced and placed in the bottom part of the combustion chamber to generate counter-rotating vortices in the chamber. The results show that the fluidized bed combustor with 30° chevron-shaped discrete ribbed walls at 40% percent excess air provides the higher temperature than other of 861 °C. Among the studied excess air percentages, the highest combustion efficiency of 99.2% is obtained at EA = 60%. In addition, combustion at EA = 60% shows the lowest emissions CO, CO2, O2 and NOx of 236.8%Vol, 2.55 ppm, 13.53 %Vol, 110.2 ppm, respectively.

Modeling Average Pressure and Volume Fraction of a Fluidized Bed Using Data-Driven Smart Proxy

Simulations can reduce the time and cost to develop and deploy advanced technologies and enable their rapid scale-up for fossil fuel-based energy systems. However, to ensure their usefulness in practice, the credibility of the simulations needs to be established with uncertainty quantification (UQ) methods. The National Energy Technology Laboratory (NETL) has been applying non-intrusive UQ methodologies to categorize and quantify uncertainties in computational fluid dynamics (CFD) simulations of gas-solid multiphase flows. To reduce the computational cost associated with gas-solid flow simulations required for UQ analysis, techniques commonly used in the area of artificial intelligence (AI) and data mining are used to construct smart proxy models, which can reduce the computational cost of conducting large numbers of multiphase CFD simulations. The feasibility of using AI and machine learning to construct a smart proxy for a gas-solid multiphase flow has been investigated by looking at the flow and particle behavior in a non-reacting rectangular fluidized bed. The NETL’s in house multiphase solver, Multiphase Flow with Interphase eXchanges (MFiX), was used to generate simulation data for the rectangular fluidized bed. The artificial neural network (ANN) was used to construct a CFD smart proxy, which is able to reproduce the CFD results with reasonable error (about 10%). Several blind cases were used to validate this technology. The results show a good agreement with CFD runs while the approach is less computationally expensive. The developed model can be used to generate the time averaged results of any given fluidized bed with the same geometry with different inlet velocity in couple of minutes.

Mixing Characteristics of Binary Mixture with Biomass in a Gas-Solid Rectangular Fluidized Bed

Aiming to better understand the biomass pyrolysis and gasification processes, a detailed experimental study of the mixing characteristics is conducted in a fluidized bed with binary mixtures. Rapeseed is used as biomass, and silica sand or resin as inert material. The effect of mixture composition, initial packing manner, and superficial gas velocity on the concentration distribution is investigated in a rectangular fluidized bed by means of photography and sampling methods. The results show that the mixture composition plays an important role in the axial solids profile of binary mixtures. The mixing behavior of binary mixture is dominated by the bubble movement. The axial distribution of binary mixtures becomes uniform with increasing superficial gas velocity, whilst no obvious effect of initial packing manner is observed in this study.

COMPARATIVE ANALYSIS OF COAL BENEFICIATION PERFORMANCE IN GAS-SOLID FLUIDIZED SEPARATION BEDS WITH DIFFERENT BED STRUCTURES

Fluidization characteristics and separation properties of fluidized beds with different bed structures were studied. The stability of fluidized beds with different structures decreases with the increase of bed height. The uniformity and stability of density of rectangular fluidized bed is better than that of square and circular fluidized bed at the same bed height. The influence of fine coal content on the density distribution of fluidized bed has little relation with the bed structure. Separation results showed that ash segregation degrees of the three fluidized beds were generally high and the separation effects were good when the static bed height was 130 mm, the fluidization number was 1.5, and the fine coal content was 10%. The rectangular gas-solid fluidized bed has the best separation effect: the lowest ash content of cleaned coal is 20.11% and the probable error Ep is 0.11g/cm3. Its Ep value is significantly better than that of square fluidized bed (Ep=0.155 g/cm3) or circular fluidized bed (Ep=0.16 g/cm3).

On the limitations of 2D CFD for thin-rectangular fluidized bed simulations

Thin rectangular fluidized beds enable detailed optical diagnostics providing high quality data for validating numerical simulations. Because of their lower computational costs, 2D CFD continues to be employed despite the high wall surface-area to bed-volume ratio characterizing this geometric setup. 2D simulations do not resolve the gas and solids flow in the third (spanwise) direction nor the true boundary condition along the front and back walls, both of which are critical because the hydrodynamics are significantly affected by the presence these walls whose surface area is often much larger than the walls modeled in 2D analyses. Through highly-resolved simulations of three independent experimental setups, we show that 2D CFD may not capture, even qualitatively, the fluidization hydrodynamics because (a) bubble rise and coalescence mechanisms along the spanwise direction are not resolved, and (b) solids momentum and energy dissipation are under-predicted, and bubble rise velocities are over-predicted, because effects of the front and back walls are not modeled. 3D simulations with suitable wall boundary conditions predict bubbling dynamics and solids mixing in excellent agreement with experimental observations without further tuning of model parameters. Overall, we recommended that 3D numerical simulations be employed to model thin lab-scale setups for model development and validation purposes.

Horizontal gas mixing in rectangular fluidized bed: A novel method for gas dispersion coefficients in various conditions and distributor designs

Abstract In a rectangular fluidized bed combustor, the tracer gas is injected continuously into the bed from a point source at the center of the distributor plate. In this study, a general governing equation is formulated for tracer gas dispersion in the bed. An analytical solution is derived to estimate the dispersion coefficients, D x and D y , in a horizontal plane. The concentration profiles at different sampling heights with various gas velocities are plotted. Subsequently, to estimate the dispersion coefficients, surface fitting of the obtained analytical solution to the experimental data is performed. The dispersion coefficients obtained from this model are compared with those of a conventional model. Additionally, the effect of walls, bed height and gas injection rate on the dispersion coefficients in a horizontal plane is investigated, and the effect of distributor design on the dispersion coefficients in a horizontal plane is investigated with different tracer positions. It is found that D x and D y are nearly equivalent at a lower tracer gas ratio of the injected gas to the total gas flow rate. It is also demonstrated that the effect of bed height on D x is minor. This model is also able to estimate the dispersion coefficients in the case of a multi-horizontal nozzle distributor.

Three-dimensional multi-phase simulation of the mixing and segregation of binary particle mixtures in a two-jet spout fluidized bed

This study presents a three-dimensional numerical study of the mixing and segregation of binary particle mixtures in a two-jet spout fluidized bed based on an Eulerian–Eulerian three-fluid model. Initially, the particle mixtures were premixed and packed in a rectangular fluidized bed. As the calculation began, the gas stream was injected into the bed from the distributor and jet nozzles. The model was validated by comparing the simulated jet penetration depths with corresponding experimental data. The main features of the complex gas–solid flow behaviors and the mechanism of mixing and segregation of the binary mixtures were analyzed. Moreover, further simulations were carried out to evaluate the effects of operating conditions on the mixing and segregation of binary particle mixtures. The results illustrate that mixing can be enhanced by increasing the jet velocity or enlarging the difference of initial proportions of binary particle mixtures.

3D Eulerian modeling of thin rectangular gas–solid fluidized beds: Estimation of the specularity coefficient and its effects on bubbling dynamics and circulation times

This study aims at investigating the influence of the wall boundary conditions and specifically the specularity coefficient on the fluidization behavior of a thin rectangular fluidized bed by means of 3D numerical simulation employing an Eulerian description of the gas and the solid phases. Thin rectangular fluidized beds have been extensively used in the research literature since it is assumed that the flow behaves like a simpler two-dimensional flow and hence they offer validation data for 2D simulations. However, the effects of the front and the back walls are significant, influencing the sensitivity of the fluidization hydrodynamics to the third dimension whose consideration is thus necessary. In order to investigate the influence of the specularity coefficient, ϕ (a parameter controlling the momentum transfer from the particles to the wall), on the fluidization hydrodynamics, a parametric analysis is conducted and the response of the bubble dynamics, reflecting the gasmotion, and the circulation fluxes, displaying the solids motion, are examined in detail. The computational results are compared with available experimental data in order to determine the values of ϕ that lead to the accurate description of the fluidization hydrodynamics via a two-fold validation strategy which involves the calculation of the circulation time and the solids concentration maps. It is observed that the appropriate value of the specularity coefficient depends rather strongly on the superficial gas velocity of the bed.

CFD–DEM study of effect of bed thickness for bubbling fluidized beds

The effect of bed thickness in rectangular fluidized beds is investigated through the CFD–DEM simulations of small-scale systems. Numerical results are compared for bubbling fluidized beds of various bed thicknesses with respect to particle packing, bed expansion, bubble behavior, solids velocities, and particle kinetic energy. Good two-dimensional (2D) flow behavior is observed in the bed having a thickness of up to 20 particle diameters. However, a strong three-dimensional (3D) flow behavior is observed in beds with a thickness of 40 particle diameters, indicating the transition from 2D flow to 3D flow within the range of 20–40 particle diameters. Comparison of velocity profiles near the walls and at the center of the bed shows significant impact of the front and back walls on the flow hydrodynamics of pseudo-2D fluidized beds. Hence, for quantitative comparison with experiments in pseudo-2D columns, the effect of walls has to be accounted for in numerical simulations.

Research of the gas-solid flow character based on the DEM method

Numerical simulation of gas-solid flow behaviors in a rectangular fluidized bed is carried out three dimensionally by the discrete element method (DEM). Euler method and Lagrange method are employed to deal with the gas phase and solid phase respectively. The collided force among particles, striking force between particle and wall, drag force, gravity, Magnus lift force and Saffman lift force are considered when establishing the mathematic models. Soft-sphere model is used to describe the collision of particles. In addition, the Euler method is also used for modeling the solid phase to compare with the results of DEM. The flow patterns, particle mean velocities, particles’ diffusion and pressure drop of the bed under typical operating conditions are obtained. The results show that the DEM method can describe the detailed information among particles, while the Euler-Euler method cannot capture the micro-scale character. No matter which method is used, the diffusion of particles increases with the increase of gas velocity. But the gathering and crushing of particles cannot be simulated, so the energy loss of particles’ collision cannot be calculated and the diffusion by using the Euler-Euler method is larger. In addition, it is shown by DEM method, with strengthening of the carrying capacity, more and more particles can be schlepped upward and the dense suspension upflow pattern can be formed. However, the results given by the Euler-Euler method are not consistent with the real situation.

Influence of multiple gas inlet jets on fluidized bed hydrodynamics using Particle Image Velocimetry and Digital Image Analysis

A rectangular fluidized bed setup was developed to study the evolution of inlet gas jets located at the distributor. Experiments were conducted with varying distributor types and bed media to understand the motion of particles and jets in the grid-zone region of a fluidized bed. Particle Image Velocimetry and Digital Image Analysis were used to quantify the parameters that characterize these jets. A grid-zone phenomenological model was developed to compare these parameters with those available in the literature. It was determined from this study that jet penetration length behavior is consistently different for fluidization velocities below and above the minimum fluidization. For velocities above minimum fluidization, jet lengths were found to increase more rapidly with increase in orifice velocity than for operating conditions below minimum fluidization.

Drying characteristics of peppercorns in a rectangular fluidized-bed with triangular wavy walls

An experimental study on drying kinetics of peppercorns has been conducted in two different drying fluidized-bed configurations: rectangular fluidized-bed ( RFB) and rectangular fluidized-bed with wavy walls ( RFBW). In the RFBW, two opposite triangular wavy walls with three blockage ratios ( e/ H) are formed to produce vortex/swirl flows leading to stronger turbulence and longer residence time of the flow in the bed. For each bed, three inlet hot airs ( T in) at 60 °C, 80 °C and 100 °C and two superficial air velocity, U* of 1.2 and 2.0 ( U* = U/ U mf) are introduced. The experimental results reveal that the air temperature and air velocity show significant effects on the drying rate of both beds, especially at T in = 100 °C and U* = 2.0. The RFBW performs much better than the RFB due to shorter drying time. The average drying time of the RFBW with e/ H = 0.3125, 0.3750 and 0.4375 is, respectively, around 29%, 36% and 43% less than that of the RFB. In addition, three mathematical drying models are offered for both the beds and the effect of the air temperature and velocity on the drying model constants was determined by fitting the experimental data using regression analysis techniques. The three models satisfactorily described the drying characteristics of peppercorns especially for the Henderson and Pabis model. The RFBW with e/ H = 0.4375 is preferable in the study.

Adjustment of drag coefficient correlations in three dimensional CFD simulation of gas–solid bubbling fluidized bed

Fluidized beds have been widely used in power generation and in chemical, biochemical, and petroleum industries. 3D simulation of commercial scale fluidized beds has been computationally impractical due to the required memory and processor speeds. In this study, 3D Computational Fluid Dynamics simulation of a gas–solid bubbling fluidized bed is performed to investigate the effect of using different inter-phase drag models. The drag correlations of Richardon and Zaki, Wen–Yu, Gibilaro, Gidaspow, Syamlal–O’Brien, Arastoopour, RUC, Di Felice, Hill Koch Ladd, Zhang and Reese, and adjusted Syamlal are reviewed using a multiphase Eulerian–Eulerian model to simulate the momentum transfer between phases. Furthermore, a method has been proposed to adjust the Di Felice drag model in a three dimensional domain based on the experimental value of minimum fluidization velocity as a calibration point. Comparisons are made with both a 2D Cartesian simulation and experimental data. The experiments are performed on a Plexiglas rectangular fluidized bed consisting of spherical glass beads and ambient air as the gas phase. Comparisons were made based on solid volume fractions, expansion height, and pressure drop inside the fluidized bed at different superficial gas velocities. The results of the proposed drag model were found to agree well with experimental data. The effect of restitution coefficient on three dimensional prediction of bed height is also investigated and an optimum value of restitution coefficient for modeling fluidized beds in a bubbling regime has been proposed. Finally sensitivity analysis is performed on the grid interval size to obtain an optimum mesh size with the objective of accuracy and time efficiency.