Initial Fluidization Research Articles

A Numerical Simulation and Experimental Study of Fluidization Characteristics of a Bubbling Fluidized Bed in Biomass Gasification

Traditional fossil energy sources still dominate the world energy structure. And fully utilizing biomass is a viable approach for energy transition. A bubbling fluidized bed has better heat and mass transfer, while particle agglomeration limits the development of its industrial application. In this paper, two-phase flow characteristics of a bubbling fluidized bed are investigated by combining numerical simulations and fluidized bed gasification experiments. Numerical simulations found that the bed fluidization height reached twice the initial fluidization height at the 0.054 m initial fluidization height with uniform particle distribution. Fluidized bed gasification experiments found that syngas yield increased with increasing temperature. The carbon conversion efficiency reached 79.3% and the effective gas production was 0.64 m3/kg at 850 °C. In addition, when the water vapor concentration reached 15%, the carbon conversion efficiency and effective gas production reached the maximum values of 86.01% and 0.81 m3/kg, respectively.

Comparative analysis of particle density effects on initial fluidization in gas-solid fluidized beds

Gas-solids fluidization technology is commonly used in chemical engineering processes involving high-density particles, often encountering poor defluidization phenomena. However, there is limited research examining the applicability of flow characteristics and empirical correlations established for low-density, easily fluidizable particles in guiding the flow behavior of high-density particles. This knowledge gap poses significant challenges for designing and operating fluidized beds in relevant processes involving high-density particles. This study comprehensively evaluates the effect of particle density on hydrodynamics during the initial fluidization stage using experimental, empirical correlations and simulation methods. Results of preliminary experiments indicate that empirical correlations for predicting Umf may exhibit significant errors of high-density particles. However, the experiments face challenges in eliminating the influence of different particle size distribution. In contrast, the simulation results present a high consistency with the experiments after sensitivity analysis. Further simulation results illustrate that empirical correlations for Umf may only yield relatively accurate predictions within the narrow density range. As the particle density increases, the predicted errors tend to increase. During the initial fluidization stage, three distinct fluidization stages can be observed, as revealed by both experiments and simulations. These stages can be attributed to the practical particle size distribution. Furthermore, it is noted that as the particle density increases, the fluidization performance tends to deteriorate. This study endeavors to advance the understanding of the effect of particle density on hydrodynamics during the initial fluidization stage. Nonetheless, more studies are still needed to enhance and deepen the current knowledge of the fluidization of high-density particles.

Numerical Study of Dry Reforming of Methane in Packed and Fluidized Beds: Effects of Key Operating Parameters

Replacing the conventionally used steam reforming of methane (SRM) with a process that has a smaller carbon footprint, such as dry reforming of methane (DRM), has been found to greatly improve the industry’s utilization of greenhouse gases (GHGs). In this study, we numerically modeled a DRM process in lab-scale packed and fluidized beds using the Eulerian–Lagrangian approach. The simulation results agree well with the available experimental data. Based on these validated models, we investigated the effects of temperature, inlet composition, and contact spatial time on DRM in packed beds. The impacts of the side effects on the DRM process were also examined, particularly the role the methane decomposition reaction plays in coke formation at high temperatures. It was found that the coking amount reached thermodynamic equilibrium after 900 K. Additionally, the conversion rate in the fluidized bed was found to be slightly greater than that in the packed bed under the initial fluidization regime, and less coking was observed in the fluidized bed. The simulation results show that the adopted CFD approach was reliable for modeling complex flow and reaction phenomena at different scales and regimes.

Hydrodynamic characteristics of a rectangular gas-driven inverse liquid-solid fluidized bed

The hydrodynamic characteristics of a rectangular gas-driven inverse liquid-solid fluidized bed (GDFB) using particles of different diameters and densities were investigated in detail. Rising gas bubbles cause a liquid upflow in the riser portion, enabling a liquid downflow that causes an inverse fluidization in the downer portion. Four flow regimes (fixed bed regime, initial fluidization regime, complete fluidization regime, and circulating fluidization regime) and three transition gas velocities (initial fluidization gas velocity, minimum fluidization gas velocity, and circulating fluidization gas velocity) were identified via visual observation and by monitoring the variations in the pressure drop. The axial local bed voidage (ε) of the downer first decreases and then increases with the increase of the gas velocity. Both the liquid circulation velocity and the average particle velocity inside the downer increase with the increase of the gas velocity in the riser, but decrease with the particle loading. An empirical formula was proposed to successfully predict the Richardson-Zaki index “n”, and the predicted ε obtained from this formula has a ±5% relative error when compared with the experimental ε.

Hydrodynamic characteristics of a pilot-scale gas-driven inverse liquid-solid fluidized bed with a central draft tube

A new pilot-scale gas-driven inverse liquid-solid fluidized bed with a central draft tube (GDFB-II) was studied. With gas bubbles rising through the draft tube and enabling an upflow liquid, the associated liquid downflow fluidizes light particles downwards in the main column, forming a GDFB. Hydrodynamic characteristics of the GDFB-II were carefully studied using particles of different diameters and densities. Three flow regimes including packed bed, fluidized bed, and circulating bed regimes were identified and these regimes were demarcated by the initial fluidization gas velocity (Ug,if) and the minimum circulating gas velocity (Ug,mc). Ug,if and Ug,mc increase with the increase of particle diameter or the decrease of particle density. The effects of the draft tube diameter and the gas injection tube diameter on Ug,if and Ug,mc were also investigated. As the superficial gas velocity increases, the particle bed height increases first and then plateaus, while the average solids holdup decreases rapidly first and then remains constant.

Hydrodynamics of a bubble-induced inverse fluidized bed reactor with a nanobubble tray

This paper reports on the hydrodynamics of a bubble-induced inverse fluidized bed reactor, using a nanobubble tray gas distributor, where solid particles are fluidized only by an upward gas flow. Increasing the gas velocity, the fixed layer of particles initially packed at the top of the liquid starts to move downwards, due to the rise of bubbles in this system, and then gradually expands downwards until fully suspended. The axial local pressure drops and standard deviation were examined to delineate the flow regime comprehensively under different superficial gas velocities. Four flow regimes (fixed bed regime, initial fluidization regime, expanded regime, and post-homogeneous regime) were observed and three transitional gas velocities (the initial fluidization velocity, minimum fluidization velocity, and homogeneous fluidization velocity) were identified to demarcate the flow regime. Three correlations were developed for the three transitional velocities. As the fine bubbles generated from the nanobubble tray gas distributor are well distributed in the entire column, the bed expansion process of the particles is relatively steady.

Experimental investigation of instability of fluid mud layer under surface wave motion

Motivated by environmental impacts of surface-wave induced mixing of fluid mud with clear water in nearshore areas, this paper presents quantitative measurements of excitation of interfacial waves over a bed mud layer by a surface wave in a wave flume. After an initial fluidization process, a quasi-standing interfacial wave comprised of four interfacial waves was observed at the interface as a result of a resonant wave interaction with the surface wave. The interfacial waves were subharmonic to the surface wave and traveled at the maximum possible angle from it. The growth rate and kinematic properties of the interfacial waves were measured, and good agreement with theoretical predictions of a two-layer interaction model was obtained. It was found that excitation of these waves was highly dependent on the surface wave height and fluid mud viscosity. Furthermore, thickening of the lutocline layer as a result of resuspension of fluid mud highly influenced the wavelengths as well as the damping rate of the interfacial waves.

Experimental research of apparatuses for peanuts and cocoa beans roasting with rotating-pulsed fluidized bed

The physical parameters of two products widely used in the confectionery industry – peanuts and cocoa beans – have been studied. The hydrodynamic characteristics of these products, influencing the fluidized bed roasting regime, have been determined. The hydrodynamics of a rotating-pulsed fluidization of the cited materials has been examined. Dependences are derived for calculating the bed pressure drops and the initial fluidization velocity of the particles during rotating-pulsed creation of fluidization.

The hydrodynamics of liquid-solid and gas-liquid-solid inverse fluidized beds with bioparticles

The hydrodynamic characteristics, such as minimum fluidization velocity (Ulmf for liquid-solid (LS) system and Ug,if for gas-liquid-solid (GLS) system) and bed expansion ratio (BER), of liquid-solid and gas-liquid-solid inverse fluidized beds (LSIFB and GLSIFB) with bare particles and particles with biofilm were investigated. In the LSIFB system, Ulmf and BER of the bare particles were independent of the solids loading. For bioparticles, the increase of the biofilm thickness reduced Ulmf and increased BER, suggesting that the fluidizability increases with the presence of the biofilm. In the GLSIFB system, the initial fluidization gas velocity (Ug,if) and the complete fluidization gas velocity (Ug,cf) both increased with increasing particle diameter and decreasing particle density under fixed superficial liquid velocities. Biofilm attachment led to a decrease of both Ug,if and Ug,cf, and an increase of bed expansion, again suggesting increased fluidizability with the presence of biofilm.

Experimental study on defluidization behaviours and its influence factors during gas switching fluidization

In this work, an investigation on the fluidization behaviour of solid particles was carried out while the fluidizing agent was switched from one gas to another with an equal volumetric flow. Four different gases, namely helium, nitrogen, argon and carbon dioxide (He, N2, Ar and CO2), were adopted as fluidizing agents, and fluid catalytic cracking (FCC) was used as the experimental material. When changing the gas supply a sharp decrease of the bed pressure drop and significant compression of the emulsion phase are observed, as well as the disappearing of the bubbles and the formation of channels inside the bed. However, these disturbances are temporary, and the initial fluidization conditions can be restored after a certain amount of time. Such behaviour is quantitatively described by measuring the maximum relative decrease of bed pressure drop, the time duration and the intensity of the defluidization phase. The effects of the initial powder bed height, the switching gas velocity and the gas properties are analyzed as well. Finally, a semi-empirical model was developed to predict the defluidization time based on the understanding of this physical process and analyzing the experimental data.

Time-series analysis of the characteristic pressure fluctuations in a conical fluidized bed with negative pressure

The negative pressure conical fluidized bed is widely used in the pharmaceutical industry. In this study, experiments based on the negative pressure conical fluidized bed are carried out by changing the material mass and particle size. The pressure fluctuation signals are analyzed by the time and the frequency domain methods. A method for absolutely characterizing the degree of the energy concentration at the main frequency is proposed, where the calculation is to divide the original power spectrum by the average signal power. A phenomenon where the gas velocity curve temporarily stops growing is observed when the material mass is light, and the particle size is small. The standard deviation and kurtosis both rapidly change at the minimum fluidization velocity and thus can be used to determine the flow regime, and the variation rule of the kurtosis is independent of both the material mass and particle size. In the initial fluidization stage, the dominant pressure signal comes from the material movement; with the increase in the gas velocity, the power of a 2.5 Hz signal continues to increase. A method of dividing the main frequency by the average cycle frequency can conveniently determine the fluidized state, and a novel concept called stable fluidized zone proposed in this paper can be obtained. Controlling the gas velocity within the stable fluidized zone ensures that the fluidized bed consistently remains in a stable fluidized state.

Hydrodynamic characteristics of bubble-induced three-phase inverse fluidized bed (BIFB)

A new bubble-induced three-phase inverse fluidized bed (BIFB) is proposed, where particles lighter than water are fluidized downwards by upward flowing gas without net liquid flow due to the reduced net gas-liquid mixture density and the turbulence caused by bubble flow. Key advantages include uniform fluidization of particles, zero or low liquid flowrate, low energy consumption and easy re-fluidization, showing great potential in biochemical, bioprocess, food, environmental and petrochemical industry. In a 0.33 m diameter pilot-scale BIFB column, four flow regimes; initial fluidization gas velocity and complete fluidization gas velocity were identified, using particles of different diameters and densities. A new concept, the fluidization ratio, is devised to best describe the fluidization quality in BIFB.

The Effect of Very Cohesive Ultra-Fine Particles in Mixtures on Compression, Consolidation, and Fluidization

This paper focuses on the effect of ultra-fine ( d < 10 µm) powders in mixtures with fine ( d < 100 µm) bulk material on compression processes and also evaluates the re-fluidization behavior of the compressed bed (history effect). Achieving this goal, different mixtures of fine and ultra-fine Ground-Carbonate-Calcium were compressed at three pressure levels. The results show that by increasing the applied pressure, the compressibility decreases due to change in compaction regime. Subsequently, for the higher pressure, the slope of packing density versus applied stress curves is noticeably different. However, this slope does not depend on the size distribution of mixtures, but on the type of material. Comparing fluidization and re-fluidization curves (bed pressure drop vs. gas velocity) shows an increase in the maximum bed pressure drop ( Δ P p e a k ) for re-fluidization. By increasing the portion of ultra-fine particles in the binary mixture, Δ P p e a k increases in a non-linear manner. Furthermore, the incipient fluidization point moves to a higher gas velocity. After compression, the peak of the bed pressure drop in the re-fluidization test happens at a lower gas velocity than in the initial fluidization test. Thus, the slope of the loading curve is much larger for re-fluidization. The opposite is observed for the unloading curves.

Hydrodynamics of a bubble-driven liquid-solid fluidized bed

A new fluidization system, the bubble-driven liquid-solid fluidized bed, adopting particles with a slightly higher density than the fluid, has been proposed. During the experiment process, air was introduced into the bottom of the column as driving force, and superficial gas velocity ranged from 0 to 28 mm/s and liquid was continuous phase but maintained at the stationary state. The variation of pressure drops in the bottom section near the gas distributor was measured to identify the demarcation of flow regimes and determine the loose velocity (Uloose) and initial fluidization velocity (Uin). It was found that Uloose and Uin both increase with the initial hydrostatic height but are independent of the initial liquid height. Moreover, solids holdup was analyzed using a photographic technique while gas holdup was calculated from the pressure drop variation and solids holdup. A non-uniform axial solids distribution exists for all gas velocities. Additionally, two fluidization mechanisms were proposed based on the experimental observations.

From elastic deformation to flow in tempered chocolate

We characterize the yield behavior and flow curve of a commercially available, molten, tempered milk chocolate with a fat content of 30%, from rheological measurements performed with a rotational rheometer fitted with a four-bladed vane spindle in a large-gap setup. We show that the yield behavior can be captured by the viscoelastic White–Metzner model, which indicates that elastic deformation precedes flow as the shear stress is increased. Following an initial fluidization cycle, the shear modulus of the tempered chocolate increases by a factor of 60 to G=4.4×104 Pa. We find that the Bingham model provides the best fit to our extensive flow curve data, where the yield stress is τyield=36.1±0.6 Pa and the Newtonian plateau viscosity is μB=8.7±0.02 Pa s.

Characteristic gas velocity and fluidization quality evaluation of vibrated dense medium fluidized bed for fine coal separation

In order to remove incombustible impurity minerals, a vibrated dense medium fluidized bed (VDMFB) can be adopted for fine coal dry separation, while Geldart B magnetite powder can serve as the medium. The influence of vibration on characteristic gas velocity in flow pattern transition stage was experimentally investigated at a vibration amplitude range of 0.5–4 mm and frequency range of 5–35 Hz. The experimental results demonstrate that at a low frequency (f < 10 Hz), the vibration effect results in a denser bed and a slightly smaller initial fluidization gas velocity. As the vibration frequency increases to a level similar to the bed’s natural frequency, the minimum fluidization gas velocity reduces sharply. The minimum fluidization gas velocity correlation in the VDMFB is obtained by means of theoretical deduction and experimental data fitting. Furthermore, a method is proposed for evaluating the effects of vibration on improving fluidization quality. Based on that, using the coal separation probable error Ep⩽0.1 as the evaluation index, a suitable effective operating gas velocity range for coal separation under different vibration parameters is determined. The ratio of the boundary operating gas velocity to Umfc is 0.67–2.28. Thus, a uniform and stable fluidization environment is provided for dry fine coal separation.

Hydrodynamic behavior of liquid–solid micro-fluidized beds determined from bed expansion

The bed-expansion characteristics of liquid–solid micro-fluidized beds were experimentally studied. Bed columns with inner diameters of 0.8, 1.45, and 2.3mm were fabricated based on capillaries. Five particle sizes in a range of 22–58μm were investigated. Bed-expansion curves were plotted using visually recorded bed-expansion heights. The bed expansion and initial fluidization behavior were compared with predictions for conventional-scale beds. Evident differences are reflected in lower expansion ratios and higher minimum fluidization velocities for micro-fluidized beds. These were attributed to the increase in the internal surface area of the particle beds and specific surface area of wall contact. The wall effect for micro-fluidized beds at higher particle/bed diameter ratios caused higher local voidage and an increase in expansion ratio. Correlations for the exponent and proportional coefficient in the Richardson–Zaki equation for micro-fluidized beds were proposed. The minimum fluidization velocities were correlated using a modification of the Ergun equation.

Operational Factor Optimization of Fine Lignite Cleaning Through a Dry Vibrated Fluidized Bed Separation

ABSTRACTEfficient cleaning of fine lignite is difficult through conventional coal-separation technology. This study focused on the dry cleaning of fine lignite using vibrated fluidized bed separation. Various operational factors that influence the segregation efficiency of fine lignite include the initial bed height, fluidization number, vibrational amplitude, and frequency. The factors were optimized using response surface methodology, which was based on the Box–Behnken design. Results indicated that the satisfied segregation efficiency could be achieved under suitable operational conditions. The optimum segregation index was obtained when the initial bed height, fluidization number, vibrational amplitude, and frequency were optimally set to 80 mm, 1.5, 2.0 mm, and 16 Hz, respectively. In this case, the favorable recovery efficiency of clean coal was achieved as well. A mathematical quadratic model was established to predict the relationship between the segregation index and operational factors. Predicted results of the segregation index obtained under the optimized conditions by the model were consistent with the experimental results.

Minimum fluidization velocity in gas‐liquid‐solid minifluidized beds

The initial fluidization characteristics of gas‐liquid‐solid minifluidized beds (MFBs) were experimentally investigated based on the analyses of bed pressure drop and visual observations. The results show that ULmf in 3–5 mm MFBs can not be determined due to the extensive pressure drop fluctuations resulting from complex bubble behavior. For 8–10 mm MFBs, ULmf can be confirmed from both datum analyses of pressure drop and Hurst exponent at low superficial gas velocity. But at high superficial gas velocity, ULmf was not obtained because the turning point at which the flow regime changes from the packed bed to the fluidized bed disappeared, and the bed was in a half fluidization state. Complex bubble growth behavior resulting from the effect of properties of gas‐liquid mixture and bed walls plays an important role in the fluidization of solid particles and leads to the reduction of ULmf. An empirical correlation was suggested to predict ULmf in MFBs. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1940–1957, 2016

Enhanced run-out of dam-break granular flows caused by initial fluidization and initial material expansion

We report results of the run-out of experimental dam-break flows in a horizontal channel generated from the collapse of columns of fine (75 \(\upmu \)m) particles fluidized at various degrees. We find that the flow run-out (x) made dimensionless by the initial column length (\(x_{o})\) is a power function of the initial column height-to-length ratio (r), as shown in previous works with non-fluidized flows. The run-out of flows initially fluidized at different degrees is accounted by \(x/x_{o }=\alpha r^{n}\). For initially non-fluidized flows, our values of \(\alpha \) are significantly higher than those reported earlier for flows of coarser granular material (\(>\)0.15 mm), showing that finely grained flows have longer run-outs compared to their coarser counterparts. The coefficient \(\alpha \) is a function of the initial degree of fluidization, with a higher growth above 93 % of fluidization, which coincides with the onset of bed expansion, and it accounts for a flow run-out increase being up to more than twice that of non-fluidized flows. The parameter \(\alpha \) is well correlated with the amount of initial bed expansion, which undergoes a sharp transition at high degrees of fluidization that has shown to be an important mechanism for reducing flow friction. Our results are consistent with earlier findings that showed that bed expansion significantly increases pore pressure diffusion timescales in static columns, suggesting that the long run-out of initially expanded finely grained flows is due to their ability to diffuse pore pressure slowly.