Homogeneous Fluidization Research Articles

The Influence of Gas Pulsed Flow on Hydrodynamic Behaviors of Refined Standard Sugar

The influence of gas-pulsed flow when supplied in the perpendicular direction to the refined standard (RS) sugar layer has been studied in this article. Some hydrodynamic parameters of the RS sugar in the gas-pulsed fluidized bed have also been determined. The research results have been applied in the design of the RS sugar dryer using the modern pulsed fluidized bed drying method. The bed porosity in the static particle layer (e0) was 0.44 while the bed porosity in the minimum fluidized bed (emg) was 0.484 and the minimum fluidized bed velocity (Umg) was 0.65 m/s. The bed porosity in the homogeneous fluidization bed (ehg) was 0.67, and the homogeneous fluidization velocity (Uhg) of 1.63 m/s has been calculated. The critical velocity (Ucg) of 2.7 m/s and the bed porosity (ecg) of 0.8 in the circulating particle bed were determined. The pressure drop through the layer of RS sugar with a thickness of 300 mm was 3808 N/m2.

Bubbles and bed expansion in low pressure fluidization

Fluidization at low pressure has significant benefits over traditional fluidized beds for the fine chemical industry. Yet there is a lack of knowledge of bubbling and bed behaviour under the slip flow regime (low pressure), and this presents a hindrance to the optimization of this process. In this work, the fundamental aspects of gas bubbles and bed expansion are investigated for a wider range of operating pressure, flow rates and bed aspect ratio. Typically, rising individual bubbles undergo a series of breakups and coalesce as they rise in the bed. The lowering of pressure below 14,884 Pa was found to inhibit this behaviour indicating a qualitative indication of the impact of pressure on the bubble dynamics. The bubbles formed at low pressure were larger in average size, greater in number at the inlet and have a higher aspect ratio. All these factors were directly related to the expansion of gas due to the lowering of operating pressure. Under a controlled environment with single rising bubbles, the gas expansion was predictable using the ideal gas equation. However, where the pressure is low enough (≤ 24,231 Pa) the inhomogeneous fluidization impacted the predictability and trend of the bubbling zone and bed expansion. Despite this, the overall consumption of fluidizing mass was magnitudes lower to generate similar sizes of bubbles at atmospheric pressure. This has a direct impact on the usage of fluidizing gas while maintaining similar gas-solid mixing behaviour. Overall, the benefits of low pressure can be obtained at moderate pressure between 49,130 pa and 24,231 Pa at AR 3 with homogeneous fluidization.

Extending homogeneous fluidization flow regime of Geldart-A particles by exerting axial uniform and steady magnetic field

The homogeneous/particulate fluidization flow regime is particularly suitable for handling the various gas–solid contact processes encountered in the chemical and energy industry. This work aimed to extend such a regime of Geldart-A particles by exerting the axial uniform and steady magnetic field. Under the action of the magnetic field, the overall homogeneous fluidization regime of Geldart-A magnetizable particles became composed of two parts: inherent homogeneous fluidization and newly-created magnetic stabilization. Since the former remained almost unchanged whereas the latter became broader as the magnetic field intensity increased, the overall homogeneous fluidization regime could be extended remarkably. As for Geldart-A nonmagnetizable particles, certain amount of magnetizable particles had to be premixed to transmit the magnetic stabilization. Among others, the mere addition of magnetizable particles could broaden the homogeneous fluidization regime. The added content of magnetizable particles had an optimal value with smaller/lighter ones working better. The added magnetizable particles might raise the ratio between the interparticle force and the particle gravity. After the magnetic field was exerted, the homogeneous fluidization regime was further expanded due to the formation of magnetic stabilization flow regime. The more the added magnetizable particles, the better the magnetic performance and the broader the overall homogeneous fluidization regime. Smaller/lighter magnetizable particles were preferred to maximize the magnetic performance and extend the overall homogeneous fluidization regime. This phenomenon could be ascribed to that the added magnetizable particles themselves became more Geldart-A than -B type as their density or size decreased.

Development of novel waste tea-derived activated carbon promoted with SiO2 nanoparticles as highly robust and easily fluidizable sorbent for low-temperature CO2 capture

Waste-derived biomass-based sorbents being employed in many applications, e.g., post-combustion CO2 capture process, are in rising interest. This work investigates the CO2 capture technique by activated carbon (AC) derived from waste tea (WT) biomass. The synthesis protocol consists of carbonization followed by activation with KOH and NaOH, known as chemical activators, utilizing the wet impregnation method. With the aim of developing the AC sorbent fluidizable in fluidized-bed systems, an essential factor in the industrialization of the CO2 capture process, these sorbents are mixed with the easily fluidizable SiO2 nanoparticles (NPs). Prepared samples are characterized via SEM, EDS, BET, FTIR, and TG analysis. Based on the TG analysis assessed in three consecutive adsorption/desorption cycles, 457 %, 141 %, and 69 % higher average CO2 capture capacity is recorded for KOH-promoted AC (2.316 mmol/g) compared to raw waste tea (0.416 mmol/g), NaOH-promoted AC (0.961 mmol/g) and commercial AC (1.370 mmol/g), respectively. Fluidization experiments show that the WT-derived AC mixed with 2.5 wt% SiO2 NPs presents a 45 % higher average bed expansion ratio linked with a homogeneous and bubbleless fluidization regime than its silica-free counterpart, which presents the bubble- and channel-containing bed with a non-uniform regime. This modified sorbent shows a 9% more CO2 capture capacity (2.53 mmol/g) than the WT-derived AC and superior multicyclic stability during 25 multiple adsorption/desorption cycles.

Study of Fluidization Behavior Transition from Geldart B to A Induced by High Temperature Using Electrical Capacitance Tomography

The transition of the fluidization behavior of Geldart B particles to that of A particles induced by temperature change was investigated by a developed high-temperature electrical capacitance tomography (ECT) sensor. Silica particles with a Sauter mean diameter of 237 μm and density of 2650 kg/m3, typically Geldart B particles under ambient conditions, were fluidized in a column of 5.5 cm from 20 to 600 °C. With the increase in temperature, ECT measurements showed a decrease in minimum bubbling velocity (Umb), no bed expansion characteristic in the homogeneous fluidization regime, an absence of multiple-bubbles regime, and a larger bubble size. The pressure drop against superficial gas velocity curves at elevated temperatures confirmed homogeneous fluidization between the minimum fluidization velocity (Umf) and Umb. Our analysis demonstrates that cohesive interparticle forces, which increase linearly with temperature, are responsible for the fluidization behavior transition of the silica particles.

Fluidization expansion of novel generation dense medium and flow regime transition in gas-solid separation fluidized bed

Gas-solid separation fluidized beds are important for coal cleaning through the removal of ash-forming impurities. Homogeneous fluidization is considered as an ideal separation method because it results in fewer pressure fluctuations and smaller bubbles. And Geldart C ultrafine powder could further intensify the fluidization stability of Geldart A particles. Thus, the present work provides a breakthrough in the density adjustment method in the gas-solid separation field, namely, combining Geldart A magnetite particles and Geldart C fine coal particles as a novel dense medium. The results showed that the addition of ultrafine coal effectively increased the overall expansion of the dense phase by the adhesion of the coal particles on the surfaces of the magnetite particles. To comprehensively understand the difference in the dense phase expansion ratio between the Geldart B/D and Geldart A particles, the flow regime was investigated to determine the transition point of homogeneous expansion using various dense media. The propagation velocities of the shock and continuity waves were analyzed using the theory of elastic systems. A quantitative criterion is proposed to identify the transition point. Based on the error analysis, the available data in the literature and the present work gave an overall in 5 × 10−5 error range compared to the prediction data. Overall, this research provides a comprehensive understanding of homogeneous fluidization characteristics using a novel dense medium and a reliable quantitative transition criterion of the flow regime for Geldart B/D and Geldart A particles in a gas-solid separation fluidized bed.

Multifunctional Core–Shell Particle Electrodes for Application in Fluidized Bed Reactors

Fluidized particle electrodes are well-suited for a wide range of electrochemical reactions due to their large specific surface area and their tolerance toward suspended solid contaminants and gas bubbles. For effective electrode performance, particles require (i) high conductivity, (ii) efficient interparticle contact, and (iii) chemical inertness toward a broad range of reagents and solvents. In particular, sufficient interparticle contact remains a critical challenge. Magnetic stabilization of the fluidized bed provides a potential solution but requires the use of magnetic electrode materials. However, the synthesis of electrode particles with high magnetic susceptibility remains an ongoing challenge. Herein, electrojetting of magnetite/poly(methyl methacrylate) suspensions into a graphite powder bed is demonstrated to be a facile and effective route toward magnetically stabilizable electrode particles. Energy-dispersive X-ray spectroscopy and Raman spectroscopy confirmed the composition and core–shell structure of the particles. Characterization of their magnetic and electric properties showed an exceptionally high saturation magnetization of 11.1 A·m2/kg and a conductivity of 28 S/m. Further, scanning electron microscopy revealed an average diameter of approximately 200 μm, large surface areas, and spherical shapes, three prerequisites for homogeneous fluidization. Compared to previously reported fluidized bed electrodes, electrodes comprised of these novel core/shell particles showed 2-fold increases in current densities and conversion rates. These results underline the potential of the electrohydrodynamic jetting process for the design and synthesis of novel, tailor-made core–shell particles as key components of fluidized bed electrodes for electrochemical reactions.

Improvement in cyclic CO2 capture performance and fluidization behavior of eggshell-derived CaCO3 particles modified with acetic acid used in calcium looping process

Although calcium-based materials are the most promising adsorbents used in calcium looping process for carbon dioxide removing, their CO2 capture capacity decaying besides poor fluidization, still are the important challenges. In the present investigation, eggshell as a cheap, easily available and unpolluted source of calcium carbonate was used for CO2 capturing in calcium looping process. Eggshell particles were treated with various volume concentrations of acetic acid to improve its sorption capacity. According to the TGA results after 20 carbonation/calcination cycles, the effective carbonation conversion of modified eggshell with 5%, 20%, 30% and 40%. v/v acetic acid was 21.33%, 24.26%, 25.97% and 28.97%, respectively, which is considerable compared to 20.54% for untreated eggshell. The effect of initial eggshell particle size on the adsorption behavior of final adsorbent was also investigated by using two different sizes including dp< 45 µm and dp> 320 µm. The results showed that the effective conversion of the adsorbent containing 40%. v/v acetic acid derived from small particle size eggshells was 9.32% higher than that from larger particle size eggshells. In terms of fluidization behavior, surprisingly the addition of acetic acid to the eggshell particles also increased the bed expansion ratio as 8% and 36.2% at gas velocities of 0.27 and 6.67 cm/s, respectively. Further improvement in the fluidity of eggshell modified with 40% acid was performed by manually mixing of SiO2 nanoparticles at different weight percentages. According to the results, adding 7.5 wt% SiO2 leaded to the homogeneous and agglomerate particulate fluidization.

Numerical investigation for the suitable choice of bubble diameter correlation for EMMS/bubbling drag model

Mesoscale bubbles exist inherently in bubbling fluidized beds and hence should be considered in the constitutive modeling of the drag force. The energy minimization multiscale bubbling (EMMS/bubbling) drag model takes the effects of mesoscale structures ( i.e ., bubbles) into the modeling of drag coefficient and thus improves the coarse-grid simulation of bubbling and turbulent fluidized beds. However, its dependence on the bubble diameter correlation has not been thoroughly investigated. The hydrodynamic disparity between homogeneous and heterogeneous fluidization is accounted for by the heterogeneity index, H d , which can be affected by choice of bubble diameter correlation. How this choice of bubble diameter correlation influences the model prediction calls for further fundamental research. This article incorporated seven different bubble diameter correlations into EMMS/bubbling drag model and studied their effects on H d . The performance of these correlations has been compared with the correlation used previously by EMMS/bubbling drag model. We found that some of the correlations predicted lower H d by order of a magnitude than the correlation used by the original EMMS/bubbling drag. Based on such analysis, we proposed a modification in the EMMS drag model for bubbling and turbulent fluidized beds. A computational fluid dynamics (CFD) simulation using two-fluid model with the modified EMMS/bubbling drag model was performed for two bubbling and one turbulent fluidized beds. Voidage distribution, time averaged solid concentration and axial solid concentration profiles were studied and compared with the previous version of the EMMS/bubbling drag model and experimental data. We found that the right choice of bubble diameter correlations can significantly improve the results for CFD simulations.

Influence of catalyst characteristics on the formation of MWCNT agglomerates during the synthesis in a fluidized bed reactor

The problem of the loss of homogeneous fluidization and the formation of large clumps of MWCNTs during their catalytic synthesis in a fluidized bed reactor has been studied experimentally and theoretically in terms of the impact of catalyst properties and synthesis process parameters. It has been shown that two factors play the most important role in the formation of large clumps of MWCNTs, namely, the ability of high-performance catalyst to produce agglomerates of MWCNTs with high cohesion, and a high concentration of catalyst particles in the fluidized bed volume. A relationship has been established between chemical components of the catalyst, its structure and cohesive properties of the MWCNT agglomerates formed on it. An approximate semi-analytical scheme for calculating the formation and growth of large fractal agglomerates (clumps) of MWCNTs during their catalytic synthesis in a fluidized bed has been developed. This approach is based on the model of sequential coagulation of MWCNT agglomerates starting from the primary ones, and made it possible to formulate the requirements for characteristics of the catalyst and parameters of the synthesis process, which ensure homogeneous fluidization in the reactor.

Theoretical and experimental study on the fluidity performance of hard-to-fluidize carbon nanotubes-based CO2 capture sorbents

Carbon nanotubes-based materials have been identified as promising sorbents for efficient CO2 capture in fluidized beds, suffering from insufficient contact with CO2 for the high-level CO2 capture capacity. This study focuses on promoting the fluidizability of hard-to-fluidize pure and synthesized silica-coated amine-functionalized carbon nanotubes. The novel synthesized sorbent presents a superior sorption capacity of about 25 times higher than pure carbon nanotubes during 5 consecutive adsorption/regeneration cycles. The low-cost fluidizable-SiO2 nanoparticles are used as assistant material to improve the fluidity of carbon nanotubes-based sorbents. Results reveal that a minimum amount of 7.5 and 5 wt% SiO2 nanoparticles are required to achieve an agglomerate particulate fluidization behavior for pure and synthesized carbon nanotubes, respectively. Pure carbon nanotubes + 7.5 wt% SiO2 and synthesized carbon nanotubes + 5 wt% SiO2 indicates an agglomerate particulate fluidization characteristic, including the high-level bed expansion ratio, low minimum fluidization velocity (1.5 and 1.6 cms−1), high Richardson—Zaki n index (5.2 and 5.3 > 5), and low Π value (83.2 and 84.8 < 100, respectively). Chemical modification of carbon nanotubes causes not only enhanced CO2 uptake capacity but also decreases the required amount of silica additive to reach a homogeneous fluidization behavior for synthesized carbon nanotubes sorbent.

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.

Computational simulations of hydrodynamics of supercritical methanol fluid fluidized beds using a low density ratio-based kinetic theory of granular flow

Hydrodynamics of fluid and particles were simulated using a low density ratio-based kinetic theory of granular flow (KTGF) in supercritical methanol (ScM) fluidized beds (FB). Results indicated that the fluidization state of the methanol fluid and particle mixtures change progressively from particulate to aggregative fluidization. A transition exists with wavy-like flows near the bottom and churn-like flows at the upper part along bed height, unlike the homogeneous fluidization in atmospheric methanol fluid FB and turbulent fluidization regime with particle strands in ScM FB. The threshold to identify the occurrence of strands was proposed in terms of mean value and standard deviation of solid volume fractions. The frequencies of particle strands increased with increasing methanol fluid temperatures and pressures. The computed fluid volume fractions agreed with experimental data in a supercritical carbon dioxide fluid fluidized bed.

On the concentration models in electrical capacitance tomography for gas-fluidized bed measurements

Electrical capacitance tomography (ECT) has been widely applied in gas-fluidized bed measurements, which however is constrained by the sensor designs, image reconstruction algorithms and concentration models owing to the “soft-field” nature. Among the three factors, the concentration models that map the permittivity to solid concentration distribution are less understood. In this paper we present an experimental investigation on the effects of concentration models for the ECT measurements of gas-fluidized beds containing Geldart’s group A particles. The results show that the bubble velocity, bubbling frequency and fluidization regime transition velocity are intrinsic whatever the concentration model employed. However, the time-averaged solid concentrations measured by ECT with the conventional concentration models are usually higher than the corresponding results obtained by the bed expansion experiments in homogeneous fluidization. It has been further found that the concentration models can significantly influence the threshold selection in identifying the bubbles and surrounding emulsion phase in gas-fluidized beds. Thus, a new concentration model given in the formula of a power function (ϕ=Gα) has been proposed, with the optimal exponent α obtained via the least-square fitting of solid concentration in bed expansion during homogeneous fluidization. The model is robust and the sensitivity analysis shows that the variation of time-averaged solid concentration is negligible (≤ 3%) suppose that α changes by ± 20%.

Computational simulations using a low density ratio-based kinetic theory of granular flow in subcritical water fluidized beds

Hydrodynamics of fluid and particles were simulated using a low density ratio-based kinetic theory of granular flow (KTGF) in subcritical water (SbW) fluidized beds (FB). Results indicated that the fluidization state of the SbW-particles mixture changes progressively from particulate to aggregative, indicating that a transitional state exists between aggregative and particulate fluidization. Wavy-like and churn-like flows were found along bed height in SbW FBs, unlike the homogeneous fluidization found in atmospheric water (AW) FBs and wispy-annular-like flow found in supercritical water (SCW) FBs. The predicted axial velocities and RMS fluctuating velocities of particles agreed approximately with measurements reported in the literature. The effect of fluid pressure and temperature, inlet fluid velocity and particle density on distributions of solid volume fractions and velocities were analyzed in SbW FBs.

Granular Stack Density’s Influence on Homogeneous Fluidization Regime: Numerical Study Based on EDEM-CFD Coupling

FLUENT and EDEM were applied to simulate liquid–solid coupling in a 3D homogenous fluidization. The dynamics of destabilization of the granular material immersed by homogeneous fluidization were observed. The effect of initial packing density of granular stack and fluidization rate on the fluidization’s transient regime, the configuration of particles in the fluidized bed and the variation of bed height were analyzed and discussed. According to the results, there was an original observation of a strong impact of the initial density of an initially static granular stack on the transient fluidization regime. Depending on the material initial volume fraction, there was a difference in grain dynamics. For an initially loose stack, a homogeneous turbulent fluidization was observed, whereas for an initially dense stack, there was a mass takeoff of the stack. The propagation of wave porosity instability, from the bottom to the top of the stack with fast kinetics that decompacted the medium, followed this mass takeoff.

A CFD‐DEM study of the solid‐like and fluid‐like states in the homogeneous fluidization regime of Geldart A particles

AbstractThe mechanisms underlying homogeneous fluidization of Geldart A particles have long been debated. Recent experiments shed fresh insights that both a solid‐like and a fluid‐like state exist. Herein, 3D computational fluid dynamics‐discrete element method simulations with the incorporation of interparticle van der Waals forces were performed for five typical Geldart A particles to investigate the bed structure in the homogeneous fluidization regime. Simulation results reproduce the existence of the solid‐like and fluid‐like states, showing that in the solid‐like state, particles have enduring and strong interparticle van der Waals forces that stabilize the bed to keep the bed stationary; while in the fluid‐like state, the interparticle van der Waals forces are dominated by the drag force, and particles show global circulation. Both interparticle van der Waals forces and friction are found to be necessary for the existence of the solid‐like state, and increasing the former has an effect to widen the solid‐like state.

CFD Modeling and Simulation of the Hydrodynamics Characteristics of Coarse Coal Particles in a 3D Liquid-Solid Fluidized Bed

In this study, a Eulerian-Eulerian liquid-solid two-phase flow model combined with kinetic theory of granular flow was established to study the hydrodynamic characteristics and fluidization behaviors of coarse coal particles in a 3D liquid-solid fluidized bed. First, grid independence analysis was conducted to select the appropriate grid model parameters. Then, the developed computational fluid dynamics (CFD) model was validated by comparing the experimental data and simulation results in terms of the expansion degree of low-density fine particles and high-density coarse particles at different superficial liquid velocities. The simulation results agreed well with the experimental data, thus validating the proposed CFD mathematical model. The effects of particle size and particle density on the homogeneous or heterogeneous fluidization behaviors were investigated. The simulation results indicate that low-density fine particles are easily fluidized, exhibiting a certain range of homogeneous expansion behaviors. For the large and heavy particles, inhomogeneity may occur throughout the bed, including water voids and velocity fluctuations.

Influence of restitution and friction coefficients on the velocity field of polydisperse TiO2 agglomerates in a conical fluidized bed by the adhesive CFD-DEM simulation

The hydrodynamic behavior of cohesive TiO2 nanoparticle agglomerates was studied in a conical fluidized bed. The effects of size, particle polydispersity, and airflow velocity were analyzed in the transition regime from heterogeneous to homogeneous and turbulent fluidization. The simple-agglomerates size was ~25–75 μm with fractal dimensions of ~1.95–2.50. Adhesive CFD-DEM simulations were performed to study the interaction between the simple-agglomerates by combining Mindlin-Dersiewicz and Johnson-Kendall-Roberts (JKR) contact models. The effects of restitution and friction coefficients, probability density function, and coordination number of agglomerates were studied on the particle velocity profiles in the spout and annular zones. The error analysis results showed that bed dynamics is more sensitive to a change in the friction coefficient than in the restitution coefficient. A good agreement between the results was found for friction and restitution coefficients close to one, which is evidence of nearly elastic collisions between TiO2 agglomerates.

Agglomeration of nickel oxide particle during hydrogen reduction at high temperature in a fluidized bed reactor

This study examined the effect of temperature and time on the reduction reaction, agglomeration, and fluidization of NiO particles in a fluidized bed reactor. NiO particles, which are classified as Geldart group B, showed homogeneous fluidization at 600°C. Above 700°C, heterogeneous fluidization and a low reduction reaction were observed, and the agglomeration rate of the particles increased. From 600 to 900°C, the reduction rate increased up to 40min of reaction time and decreased with increasing agglomeration rate at a reaction time over 40min. Above 700°C and over 40min resulted in a low extent of gas-solid reaction owing to agglomeration of the particles. The structure and distribution of nickel and oxygen in the product were verified at various reaction temperatures, based on SEM, TEM, and EDS analyses.