Minimum Fluidization Research Articles

Numerical and experimental studies of microcapsules phase change material in a pulsating fluidized bed as an energy storage medium

Phase change materials (PCMs) and particularly microencapsulated phase change materials (MPCM) have introduced new ways to enhance the energy storage capacity of a system due to the high latent heat and high storage density of these materials. This experimental and numerical research aims to investigate the process in a pulsating fluidized bed with MPCMs operating as an energy storage device. Experiments were carried out with MPCM (PX52) in a pulsating fluidized bed with the 0.2 m ID and 1 m height. Numerical simulation was performed using the Eulerian-Lagrangian approach in a symmetric two-dimensional model. Pulsating gas flow at a temperature of 60 °C with a frequency range of 1–10Hz for 200 μm mean diameter was used in the experiment and simulation. The effect of pulsating on the efficiency of the energy storage was examined at 1.5, 2, and 2.5 of the minimum fluidization velocity (Umf). Also, simulation and experiment were performed in the continuous mode for comparison with the published research of others. Results of simulation and experimental data of this study and others were noticed in close agreement. The simulation and experimental results revealed an optimal pulsating frequency, at which the energy storage in the fluidized bed is highest for 2 Umf in 7Hz, which is 92% for pulsating and 64% for continuous flow. Besides, using 7Hz and 10Hz frequencies enhanced the charging time with higher efficiency and reached the stabilized temperature in a shorter time.

Experimental study on fluidization characteristics of vinegar residue in a vibrated fluidized bed

• Vibro-fluidizing behaviors of vinegar residue are experimentally studied. • Inert Geldart-D plastic particles are used to improve the fluidization process. • Minimum fluidization velocities of binary particles are investigated. • Drying rate of vinegar residue increases due to the improving of fluidization. As a by-product in the vinegar brewing process, vinegar residue always has a high moisture content, which is detrimental to the storage and recycle process. The vibrated fluidized bed can be used to dry the vinegar residue. In present work, inert particles were added to a vibrated fluidized bed to improve the fluidization of vinegar residue. Experimental studies were carried out to investigate the fluidization behaviors of the binary mixtures. Flow pattern maps indicated that there was an upper limit to the vinegar residue mass concentration c w at which stable fluidization could be achieved. The minimum fluidization velocity u mf of the binary mixture increased as the vinegar residue mass concentration c w increased and decreased with the increase of the vibration intensity Λ . As increasing vibration intensity Λ or decreasing vinegar residue mass concentration c w , the drying rate of vinegar residue increased.

Geldart A− particles: Hydrodynamics of Geldart A magnetite powder with modulation by ultrafine coal particles in Gas-solid Fluidized Bed Coal Beneficiator (GFBCB)

The Gas-solid Fluidized Bed Coal Beneficiator (GFBCB) is an important technology used in the dry coal beneficiation process, which requires lower bed density and very uniform density distribution. Geldart A− particles, Geldart A magnetite particles “modulated” by a small fraction of Geldart C ultrafine coal particles, is proposed. Fundamental studies are carried out to provide a comprehensive understanding of its hydrodynamics, including the minimum fluidization velocity, bed expansion, and spatial bed density distribution. By adding the ultrafine coal particles, the minimum fluidization velocity (Umf) is lowered, with a modified Umf correlation proposed, the bed expansion drastically increased (by up to 30%), with more gas being retained in the bed for improved flowability, and the corresponding bed density decreased to 1600–2000 kg/m3, with more uniform distribution, all contributing to a more desirable separation environment for coal beneficiation.

Aeration and cohesive effects on flowability in a vibrating powder conveyor

A CFD-DEM model for aerated vibratory convection was developed to explore the coupled gas-solid interactions governing bulk powder bed dynamics. Each simulation was prepared by carefully characterizing the rough, porous conveyor baseplate and four candidate particle sizes representative of typical powder beds. Trends in the vibratory convection of particles between 20 and 250 μm in diameter could be explained by considering each powder's minimum fluidization velocity and the magnitude of van der Waals cohesive forces. Simulations of fine powders under high cohesive forces exhibited competing effects from drag and cohesion; drag promotes powder-frit liftoff while cohesion suppresses contact separation. Experimental convection velocities were observed to be in good agreement with the simulated mean powder velocity for throw numbers between 0.25 and 0.50.

Effective adsorption of methylene blue from aqueous solution by coal gangue-based zeolite granules in a fluidized bed: Fluidization characteristics and continuous adsorption

Methylene blue (MB) pollution in wastewater has become an environmental problem. Batch and continuous adsorption experiments were performed to study the adsorption mechanism of MB by coal gangue-based zeolite granules in a fluidized absorber. The results showed that MB adsorption of coal gangue-based zeolite granules was mainly chemical adsorption in a fluidized bed, which was mainly affected by its size fraction ( d ), fluid velocity ( v ), bed weight ( m ), and initial concentration ( c o ). Under suitable operating conditions ( d ≤ 0.5 mm, v = 1.3 v mf (i.e., minimum fluidization velocity), m = 100 g, and c o = 100 mg/L), the fluidized bed formed by zeolite granules had the best adsorption performance for MB, with an adsorption saturation of 0.444 g/g and an equilibrium uptake of 108.041 mg/g, which is higher than that of the traditional packed bed. The results provide a theoretical basis for the industrial application of coal gangue-based zeolite granules. • NaX-type zeolite granules were synthesized from coal gangue. • Batch and continuous experiments were conducted to test the efficiency. • Coal gangue-based zeolite can adsorb methylene blue from industrial discharge. • Adsorption process of methylene blue was dominated by chemical adsorption. • Fluidized bed formed by zeolite granules had a good adsorption performance.

Prediction of minimum and complete fluidization velocity and transport disengaging height of the segregated coal in a cold flow fluidized bed

The low-grade coal has been taken in the 40 mm ID acrylic fluidized bed column to examine the minimum fluidization velocity (Umf), complete fluidization velocity (Ucf), and transport disengaging height (TDH) of different size groups of fine coal. The results reveal that the Umf and Ucf increase as the particle diameter increases, but the TDH has the opposite trend. Ucf is found to be two to three times of Umf for all particle sizes. New correlations for the prediction of Umf and TDH in terms of Reynolds number and Archimedes number for coal fines have been established based on the experimental study. The developed correlations have shown strong agreement with experimental values, with variances for all samples falling within a ± 10% range.

Modelling the minimally fluidized state under reduced pressure

Several aspects of numerically modelling a minimally fluidized gas–solid system have been investigated in this work. The numerical results show that voidage and the resulting pressure drop are not a function of the fluidizing cycle. More interestingly, the pressure drop was not impacted by introducing the lateral axis of gas and solid flow in the 3D models. Under a reduced pressure environment, none of the well-known drag models could capture the effect of the slip flow. A relatively new but not well-known slip flow drag model showed the ability to capture the impact of the slip flow regime. However, improvements in its overall accuracy are desirable. To this extent, the Ergun pressure drop equation was modified to introduce the effect of the slip flow regime. The losses in the slip flow regime were captured by deriving a new correlation using experimental work that predicted a linear relationship between the laminar coefficient and the Knudsen number. The modified Ergun equation showed notable improvement in its pressure drop accuracy. Furthermore, the modified Ergun equation was implemented as a modified Gidaspow drag model. It showed better accuracy in predicting pressure drop and minimum fluidization velocity at reduced pressure for various alumina particle sizes.

Performance improvement methods for quartz tube solid particle fluidized bed solar receiver

The conditions to improve performance of quartz tube silicon carbide (SiC) solid particle fluidized bed solar receiver was investigated with computational fluid dynamics (CFD) simulations. The difficulty of experimenting all possible operating conditions was overcome by preparing CFD base input with appropriate models and parameters. The amount of SiC in the bed, the size of particles, and the air inlet velocity were considered as variables. After model verification, in order to evaluate the effect of particle addition, bed without solid particles were simulated first. Outlet temperature of single-phase receiver was calculated as 421 K. Outlet temperatures of 913 K, 895 K, and 881 K were obtained for 400 μm diameter particles in 0.3 m bed height for air inlet velocities of 0.25, 0.3, and 0.35 m/s. Air outlet temperature decreases as air inlet velocity increases. On the other hand, too much reduction at inlet velocity retards the system performance since it affects fluidization. For 400 μm particle diameter and bed height of 0.2 m, outlet temperatures of 994 K, 974 K, and 955 K were found for the same air inlet velocities above. As bed height decreases, air outlet temperature increases. For particle diameters of 300 and 500 μm for bed height of 0.3 m, outlet temperatures of 980 K and 878 K were calculated for appropriate minimum fluidization velocities. Outlet temperature increased with decreasing particle size.

Comparison of hydrodynamic characteristics of Tulsion and Siran immobilised beads in a fluidised bed bioreactor

The fluidised bed reactors are successfully utilised in various bioprocesses to produce high value products using immobilised cells and enzymes. The present work aims at evaluating hydrodynamic characteristics of a three phase fluidised bed bioreactor using Tulsion® A-1X MP, Tulsion® ADS-400 and Tulsion® ADS-800, polystyrene and polyacrylic based three types of free and immobilised cells solid carriers viz. and comparing their performance with Siran® SIKUG41 glass beads. The hydrodynamic parameters such as particle density (cells and beads), drag coefficient, minimum and maximum fluidisation velocity, particle settling velocity were estimated and compared in water and biological media (E2 and MSM) having varying rheological properties. The relationships between drag coefficient (CD)–Reynolds numbers (NRe), superficial liquid velocity (Uf)–void fraction(ε), expansion index(n)–NRe were obtained.The scanning electron microscopy studies revealed that the pore sizes of Tulsion beads (20–300 nm) were significantly smaller compared to Siran carriers (7–120 μm) leading to cells growth only on the surface of the beads. The biomass grown was estimated to be 0.75–1.2 mgCDW/g for Tulsion beads and 1.27–1.3 mgCDW/g for Siran beads with immobilised cells of P. putida and R. erythropolis, respectively. As per the results obtained, the reactor could be operated as a fluidised bed reactor with Tulsion and Siran beads at ε>0.4 and ε>0.6. The cell immobilised Tulsion and Siran beads have shown higher drag co-efficient in E2 and MSM media compared to water. The empirical correlations describing beads properties in falling (NRe) and fluidisation conditions in the form of Archimedes Number (NAr) were developed for ε in the range of 0.3–0.8 at 0.3<NRe<100 and 10<NAr<1000. The experimental data was statistically analysed using percentage mean relative error ± standard deviation and values were found to be at 95% confidence level.The evaluation of hydrodynamic characteristics of Tulsion and Siran carriers could be used to set up an operating window for three phase fluidised bed reactor for a bioprocess.

Improve reproducibility of determining minimum fluidization velocity via endowing it with a precise definition

When the fluidizing agent is given, the minimum fluidization velocity (U mf) of particles is generally reckoned to be the sole function of their physical properties. However, U mf determined as per its original definition was found to depend also on how the particles were packed before the experimental measurement, causing the well-known poor reproducibility and difficulty in comparing the U mf reported by different scholars. The original definition of U mf neglected the polymorphism of packed particles and did not clearly indicate which of the many states the particles should be packed into before the experimental measurement. To solve this problem, this work proposed a precise definition for U mf by specifying the initial packing of particles: U mf characterizes the state transition of particles from “free” packed to fluidized. Such a definition could endow U mf with a clear physical meaning and a definite value. However, as for mixed particles, no precise and realizable definition for apparent U mf could be proposed yet. The difficulty lies in specifying a reproducible packed state as the starting point for the experimental measurement, which, first and foremost, should have good mixing. Under such circumstances, a compromise method was proposed to improve the reproducibility of measuring U mf as per its preliminary definition.

Assessment of turbulent contact absorber hydrodynamics with application in carbon capture

Chemical gas–liquid absorption systems are one of the most employed technologies for CO2 removal from post-combustion processes, due to the high capture efficiency. This work presents a complex hydrodynamics model used to describe a novel CO2 capture technique that relies on the use of a three phase, gas–solid-liquid absorption column, in which the gas phase acts as the continuous phase, the liquid as the dispersed phase and the low-density solid particles are inert.To determine and analyze the hydrodynamic parameters of the process: minimum fluidization velocity, fluidized bed height, pressure drop, fluid retention, an air–water system was used. The developed hydrodynamics model was validated against experimental data from a pilot column (correlation coefficients above 0.97).For the study of the chemisorption process, the liquid phase consists of NaOH solution and the gas phase is a mixture of CO2 and air. The implementation of the hydrodynamic model along with the mass transfer model contributes to increasing the correlation coefficient for the outlet CO2 concentration to a value above 0.99. In this case, the increased value of the mass transfer area leads to a value up to 10 times higher of the transferred flow of carbon dioxide between the two phases, compared to regular packed bed columns.The model proves useful in the analysis of the influence of liquid phase and solid phase characteristics on the system’s performance. The results show that the capture rate is highly dependent on the fluidized bed height. Increasing the size or density of solid particles leads to a decrease in the height of the fluidized bed resulting in a 20 – 40% decrease in the carbon capture rate.

Development of a novel carbon capture and utilization approach for syngas production based on a chemical looping cycle

The present work assesses the potential of reducing CO2 emissions associated with steel production through the introduction of a decarbonization process downstream of a steel mill eventually producing an alternative fuel/syngas. The analysed system is composed of a calcium looping process for CO2 separation followed by a chemical looping section for syngas production from CO2 and H2Os. The main units in the chemical looping cycle are: the oxidizer, where a flux of CO2 and H2Os reacts with an oxygen carrier to produce CO and H2; the air reactor, where the oxidation of the oxygen carrier is completed by the interaction with air; the reducer, where the reduced oxygen carrier is regenerated to the initial state (Fe2O3 or NiFe2O4 in the present case) through an endothermic reaction occurring at high temperatures. A MATLAB model was created to determine the molar flow rate of the components flowing through the thermochemical cycle and the thermal power associated with each unit at the operating conditions. The analysis is carried out focusing on the treatment of 1t/h of CO2, resulting in 7.1t/h of NiFe2O4 or 12.1t/h of Fe2O3. The syngas at the outlet from the oxidizer reactor is composed of equimolar H2 and CO with a mass flow rate of 0.05t/h and 0.64t/h, respectively. A separate MATLAB model was developed to identify the experimental conditions necessary to reach fluidization of FeO particles in a lab-scale oxidizer reactor (umf=0.162m/s). Companion CFD simulations were carried out to evaluate the hydrodynamics of the lab-scale oxidizer reactor and the associated reaction kinetics (Langmuir-Hishelwood) above minimum fluidization conditions with the aim of assessing the assumptions performed in the MATLAB in terms of conversion rates. For the imposed inlet velocity conditions of the gas mixture (2.6 times above the minimum fluidization velocity) large bubbles with low frequency are observed, while full consumption of the reactant gases is achieved during the first 15 s of simulation, due to the significant reaction rate (2.6kmol/sm3). The results of the CFD simulation and the comparison with existing literature allow to validate the assumptions on the oxidizer conversion and the overall accuracy of the model.

Analysis of collision characteristics in a 3D gas-solid tapered fluidized bed

This study concerns the dynamic of particle-particle collision characteristics in a three-dimensional gas-solid tapered fluidized bed. To this aim, a two-way coupled discrete element method and computational fluid dynamics is used to obtain the position and velocity of particles in the bed. By post-processing of data, the collision characteristics such as the collision frequency, relative collision velocity, and contact time of colliding particles are calculated. Next, the effect of inlet air velocity (vinlet) on the particulate flow dynamics and collision characteristics is studied statistically. Predictive equations for collision parameters are obtained in terms of the dimensionless vinlet using the regression analysis. Furthermore, it is shown that the relative collision velocity has a Log-normal distribution and its parameters are computed in time. Additionally, the collision characteristics are calculated and analyzed in the different zones of the bed. The results indicate that no collision occurs at heights upper than 350mm for vinlet up to 6.8 times the minimum fluidization velocity. Finally, the particle-particle collision frequencies are compared with the Martin and Gidaspow models where, on average, the minimum differences at vinlet=2.2m/s as 29.7 and 22.6s−1 and the maximum ones at vinlet=0.7m/s as 48.1 and 29s−1, respectively.

Hydrodynamics of gas-solid fluidization at ultra-high temperatures

The interest in processing granular materials in fluidized beds at ultra-high temperatures has grown recently to reduce energy consumption and CO2 emissions in relation to numerous high-temperature solid-state reactions. The present research focuses on understanding the hydrodynamics of gas-solid fluidization at temperatures up to 1650 °C to aid the development of ultra-high temperature fluidized bed reactors. Experiments are conducted in a fluidized bed of 30 mm diameter at temperatures up to 1650 °C with corundum and magnesite oxide particles. The fluidization stability is examined based on bed pressure drop measurements using a non-isothermal experimental method. The results demonstrate that stable fluidization of these particles at ultra-high temperatures can be attained by using particles of large and broad size distribution and operating at sufficiently high gas velocities. This article also discusses the bed pressure drop fluctuation, minimum fluidization velocity, and pressure drop offset at ultra-high temperatures.

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.

Comparative hydrodynamics study of fluidized bed gasifier incorporating static and rotating air distributor plates: A CFD approach

This paper studies hydrodynamics of bubbling fluidized bed gasifier using static and rotating perforated air distributor plates. Pressure drop across the bed is calculated numerically using a static distributor plate to validate the model results with available experimental data. CFD results are good enough within the acceptable range of 10% difference. Dynamic mesh methodology coupled with the siding mesh technique of ANSYS FLUENT is incorporated to rotate distributor plates for specified rotational velocity using a user-defined function (UDF) hooked to the solver. Hydrodynamics study of fluidized bed with varying operational parameters using rotating plate distributor is carried out extensively. It is revealed that the impact of tangential and radial velocities on the fluidized bed due to rotation of plate is highest for lower superficial velocities, shallow initial depth, and the maximum rotational velocity of the distributor plate. The tangential and radial velocities variations with column height up to 3Umf showed a similar trend that changed significantly for higher superficial velocities (4Umf, 5Umf), resulting in more chaotic solids. The comparative evaluation of static and rotating distributor plates results shows an appreciable difference. The pressure drop across the bed is increased about 6–7% and minimum fluidization velocity is decreased upto 10% when compared to static plate distributor. Moreover, bed height rise is on a higher side for static plate distributors due to the dominant axial component of air velocity compared to the rotating plate distributor. It is also revealed that the pressure fluctuations within the fluidized bed are significantly reduced (88%) when using a rotating distributor plate compared with static plate distributors.

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.

Conversion air velocity at which reverse density segregation converts to normal density segregation in a vibrated fluidized bed of binary particulate mixtures

It is well known, when binary mixtures of different-density particles of the same size are vertically vibrated or fluidized by airflow through the bottom, the particles segregate by density. Reverse density segregation occurs in the vibrated bed; heavier particles move upward and lighter ones move downward, and normal density segregation occurs in the fluidized bed; lighter particles move upward and the heavier ones move downward. In this study, we investigated the particles’ behavior in a vertically vibrated fluidized bed at various air velocity using two types of particulate mixtures of glass beads (GB) and stainless steel powder (SP) or iron powder (IP) of same size. We found that reverse segregation converts to normal segregation at a certain air velocity; here we call it “conversion air velocity”. Then, we investigated the likely origin of the conversion air velocity considering the minimum fluidization air velocity umf determined for the three monocomponent particles (GB, SP and IP) with and without vibration. We found that the conversion air velocity is close to the umf of the lower density particles (GB) with vibration, indicating that the conversion from reverse segregation to normal segregation occurs around umf of lighter particles with vibration.

Numerical prediction on minimum fluidization velocity of a supercritical water fluidized bed reactor: Effect of particle shape

Supercritical water fluidized bed reactor (SCWFBR) is a novel gasification reactor for hydrogen production. Compared with traditional hydrogen production technique, the SCW-based gasification has outstanding advantages such as high hydrogen production efficiency, no emission of gaseous pollutants, low water consumption and strong material adaptability. In this paper, based on Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) simulations, numerical investigation is conducted on the effect of particle aspect ratio (α) on the minimum fluidization velocity (Umf) of a SCWFBR containing cylindrical biomass particles. The coupled CFD-DEM model is firstly validated against previously published experimental data. Then, the Umf for six types of particle shapes are calculated under different working conditions of temperature ranging from 633 to 693 K, pressure ranging from 25 to 27 MPa and α ranging from 1 to 6. Numerical results show that Umf increases with the temperature, but decreases with the pressure. Moreover, Umf is increasing with α. The underlying mechanisms are revealed by investigating the micro-structure of different shaped particles. It is shown that the increase of Umf with α is caused by the weakened fluid-solid interaction force and enhanced inter-particle locking. And the weakened fluid-solid interaction force is mainly due to the increase of the voidage. Finally, a predictive correlation of Umf for the SCWFBR is proposed considering α which demonstrates strong predictive capability for cylindrical particles.

Detailed biomass fast pyrolysis kinetics integrated to computational fluid dynamic (CFD) and discrete element modeling framework: Predicting product yields at the bench-scale

Fast pyrolysis is an intricate process due to the variability and anisotropy of lignocellulosic biomass and the complicated chemistry and physics during conversion in a bubbling fluidized bed reactor (BFBR). The complexity of biomass fast pyrolysis lends itself well to computational fluid dynamics (CFD) and discrete element (DEM) analysis, which promises to reduce experimental time and its associated cost. This study investigated switchgrass fast pyrolysis simulated by computational fluid dynamics coupled with a discrete element method to track individual reacting biomass particles throughout a bench-scale BFBR reactor. We accounted for the fast pyrolysis chemistry through a comprehensive reaction scheme with secondary cracking reactions. We performed a three-step reduction for secondary cracking reactions to convert the full cracking scheme into a reduced scheme easily incorporated into our model. We assessed the impact of operational conditions on the steady-state yields of liquid bio-oil, non-condensable gases (NCG), at 550 °C over a range of fluidization numbers (2 – 6 Umf), reported as a ratio to the minimum fluidization velocity (Umf). At steady-state, the volatile bio-oil yield had a range of 49.3–50.4 wt%. Levoglucosan was the primary volatile component present with 21 wt% of the bio-oil while water was the second largest with 20 wt%. The reduction of the secondary reaction schemes did not appreciably affect the overall yields of switchgrass pyrolysis compared to the full secondary scheme.