Minimum Fluidization Velocity Research Articles

Dependence of the Fluidizing Condition on Operating Parameters for Sorption-Enhanced Methanol Synthesis Catalyst and Adsorbent

The fluidization of two different solids was investigated by varying the temperature and pressure conditions and the fluidizing gas. The solids are a novel catalyst and a water sorbent that could be used to perform sorption-enhanced methanol synthesis; the operating conditions were selected accordingly to this process. The aim of this investigation was to find an expression for predicting the minimum fluidization conditions of a methanol synthesis catalyst and an adsorbent in the presence of their process stream and operating conditions. The findings of this study highlighted how umf (STP) decreases with a rise in temperature and increases with a rise in pressure, according to other works in the literature with different solids. Furthermore, the type of gas was found to influence the minimum fluidization velocity significantly. The experimental results agreed well with a theoretical expression of the minimum fluidization velocity adjusted for temperature, pressure, and viscosity. The choice of the expression for viscosity calculation in the case of gas mixtures was found to be of key importance. These results will be useful for researchers aiming to calculate the minimum fluidization velocity of a catalyst or other solids under reaction conditions using results obtained at ambient conditions with air or inert gas.

Permeability of granular mixtures under shear

Fluidised granular flows are gas–particle mixtures that occur in a wide range of industrial applications and geophysical flows. One factor governing the fluidisation behaviour of a granular flow is its permeability — the ability of the gas to move through the particle column. Although extensive research has been carried out on the gas–particle permeability under static conditions, most of the industrial and geophysical processes are dynamic phenomena, where parameters such as the shear rate change with both time and space. Here, the effects of shear rate on the permeability of gas–particle mixtures were studied through novel experiments where a granular column comprising particles of diameter 63–250 μm was sheared at shear rates between 0 and 213s−1 while an increasing flux of compressed air was introduced into the base of the column. Regardless of the shear rate, the granular column starts to expand upon increasing the air velocity, however, columns sheared at higher rates require greater air velocities to exhibit bubbling (and column expansion). We also instrumented the column with a pressure sensor. For the static column, as the air velocity increases, the pressure gradient across the granular column increases linearly until it reaches a maximum value, slightly decreases, and then plateaus. Conversely, for high shear rates, the pressure gradient continuously increases with increased air velocity, never reaching a maximum value or a plateau. The pressure gradient data were analysed alongside the videography in terms of the minimum fluidisation velocity and the minimum bubbling velocity, and both were found to be greater for higher shear rates. Consequently, our results show that increased shear increases the permeability of granular columns and the experimental data further suggests that in order to accurately describe the evolution of the pressure gradient across a sheared granular column, processes in the vertical as well as radial plane should be taken into account.

Developing and Assessing Performance of a Laboratory-Scale Fluidized Bed Dryer

A laboratory-scale batch fluidized bed dryer with 75 mm bed diameter was designed, fabricated and evaluated to study the hydrodynamics of river sand as well as the drying of cassava mash and bitter kola particulates. The hydrodynamics properties such as minimum fluidization velocity, effect of bed height and pressure drop across the bed, effect of particle size and density on minimum fluidization velocity and stability of the bed column of river sand were studied. Drying of cassava mash to edible garri was carried out at in fluidized bed dryer at controlled temperatures. Drying characteristics of the laboratory fluidized bed dryer was compared with the laboratory WiseVen oven (model number: WOF - 105) using bitter kola particulate material sample. The experimental results from laboratory-scale fluidized bed dryer showed that the minimum fluidization velocity increases as material density increases and the value of the minimum fluidization velocity obtained from the fluidized bed gave good agreement with other empirical correlation such as Kozeny-Carman Equation. The fluidized bed dryer showed high rates of moisture removal over the conventional oven with the ratio of 1:29 under the same operating conditions. Drying of cassava mash to edible garri was achieved at lower drying temperatures of 83 oC \(\pm\) 3 oC at 55 minutes when compared to conventional frying temperatures of cassava mash to edible at 180 – 200 oC thereby saving energy cost. Hence, this fluidized bed dryer is recommended for use in demonstrating hydrodynamics and drying of particulate materials in the laboratory.

Characterization of different darken sand particles and behavior under fluidized and irradiated conditions

The use of solid particles in a fluidized bed with concentrated solar irradiation from the top is a promising technology for the next generation of concentrated solar power (CSP) plants. Sand is an inexpensive and abundant material easy to fluidize, but it has a low absorptivity, which is around 0.5 according to different previous works. This optical property is a key parameter for CSP applications with solid particles. This work presents a novel methodology to induce darkening of the sand surface by inducing solid state diffusion of Mn in SiO2, rendering a stable material resistant to abrasion upon the fluidization process. For this study, two different samples considering different MnCO3:SiO2 weight ratios (1:30 and 1:50) were analyzed. The objective is to compare the two samples and optimize the concentration of MnCO3 to get the desired darkening of the sand and high absorptivity. First, the main properties of the particles were analyzed (particle size, morphology, color and absorptivity). Second, the samples were tested in a lab-scale fluidized bed directly irradiated from the top with a beam-down 4kW Xe lamp. Both samples were tested under three different fluidized velocities: 1.5, 2.0, and 2.5 times the minimum fluidization velocity (Umf). In both cases, there is a significant increase in the maximum temperature reached during the process, with temperatures exceeding 260 °C. This is clearly higher than the case of raw sand, which reaches 230 °C under the same conditions. Furthermore, these values exceed the highest temperature reached by SiC in the same facility, which is 250 °C, and it is considered as one of best tested materials for CSP applications.

Calibrating the frictional-pressure model from two-fluid simulation of fluidised beds in the defluidisation regime

The two-fluid simulation of a fluidised bed of non-adhesive particles, in which particles are represented as a continuous medium, requires a model for the pressure associated with particle contacts. While closures may be derived in the framework of the kinetic theory of granular flow for short-duration contacts, the frictional pressure associated with enduring particle contacts is generally modelled with an empirical closure involving several free parameters. This article proposes to use the experimental transition between the fluidised-bed and fixed-bed regimes in the determination of these parameters. The prediction of the minimum fluidisation velocity is shown to be governed by the frictional parameters, with either a discrete-element method or a two-fluid model. The shape of the particles, proxied by a sphericity coefficient, also influences the minimum fluidisation velocity. Such dependences of the model prediction can be used to select relevant frictional parameters, by comparison with experimental data or CFD-DEM simulation results.

A modified CFD-DEM method for accurate prediction of the minimum fluidization velocity

Monodisperse fluidized bed experiments have been carried out on 13 kinds of particles with different particle sizes in a cylindrical fluidized bed. The accuracy of nine commonly used monodisperse drag models, including the Gidsapow, BVK, and Zhou–Fan models is verified. It is found that the Zhou–Fan model is the most accurate at low and moderate values of the particle Reynolds number. It is found that with the traditional computational fluid dynamics–discrete element method (CFD-DEM) method it is difficult to obtain an accurate minimum fluidization velocity owing to difficulty in obtaining an accurate value of the minimum fluidization solid volume fraction. A modified method is proposed to increase the accuracy of prediction of the minimum fluidized solid volume fraction by using a virtual particle size in the calculation of the particle collision process. The effectiveness of the modified method is verified by comparing the CFD-DEM simulation results with experimental results for Geldart-D particles. The relative error of the minimum fluidization velocity is found to be reduced from 24.6% to 2%.

Experimental modeling to design a heat exchanger control strategy for a Fischer–Tropsch fluidized bed

Fluidized beds are ideal contactors for high temperature Fischer–Tropsch (FT) synthesis because of their superior heat transfer and low pressure drop compared to fixed bed reactors. However, as the reaction is extremely exothermic, even in these systems, runaway reactions sinter catalyst when the heat removal is insufficient or the control strategy is deficient. Although FT technology has been well established for decades, new catalysts, process intensification, and modular designs require a cooling strategy and control system a priori to ensure the process operates safely through the development stages. Here, we designed a safe mode experiment for a pilot fluidized bed reactor (200mm in diameter) charged with 36kg of alumina powder with a vertical cooling coil bundle. We measured the heat transfer rate between hot water in the coils and the fluidized bed operating up to 300°C, 0.5-1MPa, and superficial gas velocities from 5 × to 10 × the minimum fluidization velocity (umf=2.4mms−1). The dynamics of the cooling was 35% faster at higher gas velocities and the preliminary estimates concluded two tube bundles with 4 vertical runs each with 3/4 in. Schedule 10 pipe was sufficient to remove the heat generated. The calculated convective heat transfer coefficients were comparable to several published models but they were lower than expectation. We compared heat transfer rates of a single bundle and a set of two bundles and designed a PI and a PID controller plus filter (PIDf) separately using the internal mode control approach. Based on the results, the performance of the PIDf controller is better than the PI, as it minimizes oscillations and responds faster.

Exergy analysis of packed and fluidized bed thermal energy storage systems

This work presents a detailed exergy analysis that compares two different thermal energy storage (TES) systems that use particles as energy storage media and air as the heat transfer fluid: packed and fluidized beds. The study permits the optimization of both TES systems in terms of exergy storage capacity and quantifies the irreversibilities that appear in both cases. In addition, the influence of air flow rate and particle size has been quantified.The results indicate that when focusing solely on the storage process, there is an optimal charging time that minimizes the exergy destruction in both cases: topt∼mscp,s/ṁacp,a. Although the exergy destruction of the fluidized bed during the beginning of the charging process is higher than that in the packed bed, at the end of the process, the total exergy destruction in both beds is similar. However, in the full storage–recovery process the total exergy destruction is doubled in the fluidized case, with an entropy generation number of approximately 0.6 in comparison with a value of 0.3 for the packed bed. Besides, a sensitivity analysis varying the particle size and the air flow rate indicates that the particle size has a much higher influence on the fluidized bed case due to the change in the minimum fluidization velocity and its influence on the heat transfer rate. Finally, a case study is presented using the storage bed for building heating, and an exergy analysis concludes that the fluidized bed generates 534W/K of entropy, more than three times the entropy generated in the packed bed (161W/K), due to the higher pressure drop needed to fluidize the particles.

Multi-scale simulation of particle density effects on hydrodynamics in dense gas-solid fluidized beds

Gas-solid fluidization has gained prominence in chemical engineering, particularly for processes involving high-density particles. However, a notable dearth of research on the fundamental fluidization of high-density particles exists, distinct from the well-studied low-density particles. This knowledge gap presents significant challenges in designing and operating fluidized beds for such processes. In this study, we employ the CPFD scheme to comprehensively investigate how particle density affects hydrodynamics. Simulation results consistently align with experiments for pressure drop and minimum fluidization velocity, underscoring the model reliability. Importantly, typical empirical correlations for predicting minimum fluidization velocity display substantial inaccuracies, particularly for high-density particles. Additionally, we identify marked disparities in fundamental fluidization behavior between high and low-density particles, with increasing density leading to deteriorating fluidization performance, characterized by larger bubbles, defluidized regions, gas streaming, and reduced bed expansion ratios. Investigating high-density particle gas-solid fluidization systems promises valuable theoretical insights, bridging the gap between theory and practice.

Mixing/segregation characteristics and bubble behaviors of density-segregated binary particles in a pressurized fluidized bed

To enhance comprehension and achieve more targeted optimization of pressurized fluidized beds, experiments were carried out in a “cold” pseudo 2D pressurized fluidized bed with binary mixtures differing in densities via high-speed digital image analysis. The impacts of operating pressure and gas velocity on fluidization pattern, mixing/segregation characteristics, and bubble behaviors were comprehensively analyzed. Results show that the fluidization of binary mixtures is a gradual process which occurs within the range between the minimum fluidization velocity of flotsam and jetsam components. Particles with lighter density tend to produce larger size of bubbles, larger bubble fraction and increased fluctuation compared to the heavier ones when particle sizes are similar. Increasing the pressure results in an overall decreased bubble diameter and bubble fraction, which prohibits the bubble-particle interaction and ultimately deteriorates the mixing quality. The carried-out whole-field, non-intrusive measurements in this work would provide the much-needed database for validation of CFD models.

The impact of particle size in fluidized bed on heat transfer behavior: A review

This review paper explores the significance of fluidized bed heat exchangers in various industrial applications. By delving into the operation of fluidized beds as multiphase flow systems, the aim is to enhance their capabilities and efficiency. Key parameters such as minimum fluidization velocity and local gas holdup are crucial for characterizing the hydrodynamic behavior of materials within fluidized beds. Fluidization, achieved by passing atmospheric air through particulate solids, imparts fluid-like properties to the bed. Fluidized beds serve as reactors where this phenomenon takes place, offering several advantages in industrial processes, including high rates of heat and mass transfer, low pressure drops, and uniform temperature distribution. In future work, a focus on understanding and optimizing the fluidization process will contribute to further advancements in the performance of fluidized bed heat exchangers.

A numerical investigation on segregation in gas-fluidized beds of short, flexible fibers

Numerical simulations of gas-fluidized beds of short, flexible fibers are performed using a coupled approach of discrete element method (DEM) and computational fluid dynamics (CFD), and segregation of bidisperse fibers is investigated. A gas critical velocity exists for achieving the maximum degree of segregation, characterized by a segregation index SI. and it is about 1.5 times minimum fluidization velocity for present particulate systems. The segregation is determined by two mechanisms: air entrainment mechanism, attributed to the difference in air effects on fiber motion, and sifting mechanism, due to the difference in fiber sizes. Smaller and lighter fibers tend to move to upper region of a fiber bed due to the air entrainment mechanism, while the smaller fibers more likely migrate to lower region due to the sifting mechanism. The interlinked effects of the two mechanisms are analyzed, and a phase diagram to determine SI is consequently created based on an air-effect ratio and a fiber diameter ratio. When the air-effect ratio is higher than 1.3, significant segregation occurs due to the enhancement from both the mechanisms; while when the air-effect ratio is below 0.8, weaker segregation is observed as the air entrainment and sifting effects counterbalance each other.

Dimensionless Number Group Analysis of Surface-Treated Powders

Modeling powder properties remains a complex and difficult area of study because particulate materials can behave differently under variable conditions based on their bulk and surface-level properties. The research presented in this manuscript was designed to support the fundamental understanding of powder systems by joining experimental and theoretical calculations of dimensionless numbers groups for design purposes. In order to do so, this work focused on two critical variables to better understand fluidization design: physical and chemical surface properties. To better resolve the influence of surface properties, surface-treated powders were used. Five different powder samples of varying particle size distribution were characterized using physical property measurements, including pressure drop profiles to obtain the minimum fluidization velocity, density measurements, and particle sizing. Using theoretical equations, the minimum fluidization velocity was also calculated to compare with those obtained experimentally and determine typical dimensionless number groups used in bulk handling system design. The results showed that the theoretically determined values were lower than those calculated using the experimentally umf. In the case of the Reynolds number, the experimental values were 3–20% higher than the theoretical values, which is an important distinction for designing conveying systems and pipeline flow. Similar results were observed for the theoretical and experimental Froude numbers, indicating an important dependence on the cohesive properties of the particle interactions. Additional dimensionless number groups were considered, including the granular bond number and flow factors. To investigate the influence of surface forces, Hamaker constants were utilized for alumina and polydimethylsiloxane in the calculation of the granular bond number. A lower granular bond was observed with a decrease in the Hamaker constant for PDMS, suggesting that the surface forces would be lower for our surface-treated powders.

Investigating the Influence of Distributor Type, Particle Size and Rice Husk Percentage on Fluidized Beds through Cold Fluidization Experiments

This work studies a fluidization system through cold experiments by using a mixture of rice husk and sand to investigate three parameters: type of bed distributor (perforated plate and plate with Tuyere-type injectors), sand granulometry (mean diameters of 324 µm and 647 µm) and rice husk mass ratio (from 1% to 10% of rice husk). The results reveal that the perforated distributor plate achieved a lower minimum fluidization velocity. However, the plate with Tuyere injectors generated better mixing, thus reducing possible stagnation points. An increase in the mean diameter of the sand raises the minimum fluidization velocity but also facilitates the formation of preferential channels. As for the rice husk mass ratio, values of over 5% cause stagnation points and preferential channels. It was also found that the relation between minimum fluidization velocity and rice husk ratio follows an exponential behavior, and an equation was developed to better describe their relation.

Experimental investigation of the behavior of non-spherical particles in a small-scale gas-solid fluidized bed

The present work examined the effect of particle shape on the fluidization behavior at different inlet superficial gas velocities. The experiments are performed in a laboratory-scale 3D circular fluidized bed column of 1 m height and 0.067 m inner diameter with Geldart D particles of four different shapes. The effect of particle shape on pressure drop, bed expansion, and solids holdup are analyzed. Non-spherical particles have lower minimum fluidization velocities and higher bed expansion than spherical particles. The shape of particles significantly impacts the solids holdup, and it has been observed that spherical particles exhibit a higher solids holdup at the same superficial velocity. The time series analysis of the pressure signals is investigated by the frequency domain method using the Fast Fourier Transform (FFT). The FFT is implemented in Matlab R2021b to obtain the Power Spectral Density (PSD). The analysis of the PSD pattern shows the flow regime transition with the change in the particle shape. PSD pattern has observed a broad range of frequencies between 1 and 5 Hz. The dominant frequency of around 3 Hz corresponds to the bubbles or slugs that pass the bed.

Combustion of polyethylene and polypropylene in the fluidized bed with a variable vertical density profile

A new type of fluidized bed with a variable vertical density profile was developed. A mixture of sand (Ønominal = 100–140 μm, 2.6 g/cm3) and cenospheres (Ønominal = 140–160 μm, 0.85 g/cm3) was used to form a binary fluidized bed. The minimum fluidization velocity of the binary fluidized bed was determined experimentally. Two points of inflection on the fluidization curve were observed. Additionally, a measurement of the pressure as a function of height was used to determine the profile of fluidized bed density. It was proven that the fluidized bed made out of cenospheres and sand can be characterized by a variable vertical density profile. The binary fluidized bed was used for the combustion of polyolefins. The results were compared to combustion in a conventional fluidized bed made out of sand. The surface of the fluidized bed was video recorded and the changes in the image brightness were used to identify diffusion combustion in the freeboard. A Fourier Transform Infrared (FTIR) analyzer with high spectral resolution (1 cm−1) was used to record the IR spectra of flue gases. In-house software was used to deconvolve complex IR spectra. In the process performed in the fluidized bed made out of sand, regardless of temperature, around 30 % of the polymeric carbon left the reactor in the form of soot. The lack of dependence on the temperature results from the fact that in such an organized process combustion did not take place inside the fluidized bed but on its surface. The binary fluidized bed eliminated the problem of the presence of diffusion flames and soot formation. High fuel conversion to CO2 (above 90 %) was observed at temperatures above over 800 °C during polyolefin combustion in the binary fluidized bed.

Temperature influence on minimum fluidization velocity: Complexity, mechanism, and solutions

Fluidized-bed reactors are widely employed in various high-temperature industrial processes. Thus, it is crucial to understand the temperature effect on various fluidization phenomena, specifically the minimum fluidization velocity (Umf) that governs various aspects of fluidized bed behavior. In this study, we comprehensively analyze Umf data from the literature to unravel the complexity and underlying mechanisms of temperature influence on this critical velocity. The research examines experimental data encompassing a wide range of temperatures, pressures, and solid particles. The analysis reveals that the influence of temperature on Umf is fundamentally determined by the relative importance of hydrodynamic forces and interparticle forces within fluidized beds and is realized by three distinctive temperature-induced changes: gas properties, bed voidage, and physiochemical characteristics of particles. On this basis, an equation is derived to enable predictions of temperature influences on the minimum fluidization velocity under broad temperature conditions.

Gas fluidization of natural graphite flakes

Demand for high purity graphite is expected to greatly increase in the coming decades. Mined graphite is purified to the final purity specification via thermal and/or chemical fluid-solid processes. As fluidized bed electro-thermal purification is a promising technology, the physical characterization and cold-flow fluidization of Canadian natural flake graphite was undertaken to better understand the gas-solid dynamics. Three commercial graphite particle batches were characterized. Mean Sauter diameters of the three particle populations measured by microscopy were 89, 63 and 51 μm. The modified sieve analysis and permeametry yielded similar mean diameters. Particle sphericities were determined to be 0.34, 0.36 and 0.40 respectively. Channelling near the surface occurred near minimum fluidization (vmf) but eventually disappeared with increasing gas velocity. This led to a partial fluidization state before achieving complete fluidization with the delay, which was observed to be up to around 3vmf, being greater for the smaller particles. Minimum fluidization velocity was determined to be 0.063, 0.028 and 0.011 m/s respectively. Complete fluidization was observed at velocities of approximately 0.070, 0.046 and 0.035 m/s respectively. Finally, wall effects due to the presence of a central electrode did not significantly impact overall fluidization characteristics.

Minimum Fluidization Velocity and Expansion Behavior of Spherical Particle Beds Fluidized with Inelastic and Viscoelastic Fluids

Minimum Fluidization Velocity and Expansion Behavior of Spherical Particle Beds Fluidized with Inelastic and Viscoelastic Fluids

Influence of distributor plate design on mixing characteristics of rice husk biomass in a bubbling fluidized bed gasifier: An experimental and CFD study

This study investigates the mixing characteristics of rice husk biomass in a bubbling fluidized bed gasifier (BFBG) using different distributor plates: perforated, 45° slotted, and hybrid plates. The study explores the fluidization characteristics of a biomass/sand mixture using three distributor plates, observing the maximum difference in initial minimum fluidization velocity (Umf,i) and final minimum fluidization velocity (Umf,f) for perforated plate, indicating axial mixing and segregation, while the 45° slotted plate represents uniform bed expansion and fluidization, and the hybrid plate exhibits an intermediate behavior. Axial and lateral biomass distribution is also studied, and it is found that with the 45° slotted plate the highest axial distribution of rice husk is observed at the lower portion due to lateral and swirling flow whereas perforated plate causes the highest deviation at the lower portion but approaches ideal mixing at the mid and upper sections due to axial dominant flow. The hybrid plate exhibits a blend of axial and lateral flows, resulting in intermediate axial distribution at lower and upper section of gasifier. Radially, the perforated plate has the highest concentration of biomass at maximum height whereas the 45° slotted plate has concentration near the walls, and the hybrid plate has optimal mixing throughout. The hybrid plate also achieves the highest mixing index at all air velocities. Overall, the study provides insights into biomass mixing in BFBGs and highlights the superior performance of the hybrid distributor plate.The CFD study also carried out to complement the experimental results in terms of pressure drop and mixing behavior by employing different distributor plats. Multiphase two fluid model (TFM) with kinetic theory of granular flow has been employed for CFD simulations with the engagement of k-epsilon turbulence model. The simulation results are in good agreement with experimental results in terms of bed pressure drop quantitatively and mixing characteristics of rice husk biomass qualitatively.