Minimum Fluidization Velocity Research Articles

CFD simulations to study bed characteristics in gas–Solid fluidized beds with binary mixtures of Geldart-B particles: A qualitative analysis

The bed dynamics of unary and binary fluidized beds play a key role in understanding the pressure drop and hence provides an opportunity for performance improvement of the beds. In the present work, characteristics of fluidized beds with binary mixtures of Geldart-B particles were investigated using CFD simulations. The phenomena of segregation and mixing using simulations were studied, both qualitatively and quantitatively, at a range of superficial gas velocities (0.3–0.6 m/s) and two different bed heights. The study was divided into two parts. In Part I, the current study, a qualitative analysis of flow patterns for seven different binary mixtures, is presented. The quantitative analysis, including particle and gas velocity profiles, particle volume fraction profiles, and correlations for minimum fluidization velocity and pressure drop, will be presented in Part II of this work. A mathematical model consisting of an Eulerian-Eulerian model with RNG k-ε model and KTGF model to capture the bubble dynamics was used. The standardized values of coefficients and plastic stresses have been used for all simulations. The CFD model was validated using experimental data from the literature. Qualitative predictions of volume fraction profiles of small-sized particles showed that, for mixtures within a range of 40%–60% Geldart-B type large particles, the bubble and solid particle dynamics were different from those of single particles of the superficial gas velocities considered. In contrast to the single particles in the given superficial gas velocity range that were in bubbling regime, the binary particles showed a transition from bubbling to slugging to turbulent regime, as demonstrated by qualitative analysis. A homogeneous regime was observed for lower superficial gas velocities for mixtures consisting of 0%–20% large particles.

Minimum fluidization velocity and reduction behavior of combusted iron powder in a fluidized bed

The fluidization and reduction behavior of micron-sized iron oxide powder, produced by iron combustion, is studied in a lab-scale cylindrical fluidized bed. The minimum fluidization velocity umf is found to stabilize above normalized static bed heights of 0.5 H/D (static bed height divided by the bed diameter). umf is measured as a function of temperature between 280 and 860 K for both H2 and N2 as fluidizing gas. The experimental results start to deviate from the Ergun correlation at temperatures above 560 K, both for N2 and H2. A new correlation, taking the cohesive inter-particle solid bridge force into account, is proposed in this work to predict the minimum fluidization velocity at high temperature. Reduction experiments are carried out for a total time of 5 h at constant excess velocity with 50, 75 and 100 vol% of H2 and temperatures between 623 and 823 K. Gradual defluidization occurs when the operating temperature exceeds 800 K. A maximum reduction degree of 61% is obtained at 807 K and 100 vol% H2.

Predicting Behavior of Gas-Solids Systems for Non-Uniform Fluidization

Abstract A new method was developed to predict the pressure drop and minimum fluidization velocity as a function of inlet gas velocity for multicomponent fluidized beds. The method attributes the changing bed composition during the fluidization process by using a definition for the mass fractions of the bed not yet fluidized. First, a literature survey presents single and multicomponent system correlations that estimate the minimum fluidization velocity. Then, development of the “MASS method” was described and compared to published experimental data and numerical simulations for pressure drop from single-, binary-, and ternary-component fluidized bed systems, where good agreement was observed. Minimum fluidization velocities predicted using correlations in the literature were compared with the MASS method and corresponding experimental data and numerical simulations. The predicted minimum fluidization velocity from the MASS method provided very good results with a relative error of ±4% compared to experimental data. The MASS method was shown to accurately predict fluidization of complex multicomponent systems of granular material and can be used to predict other systems.

Minimum fluidization velocity for binary mixture of sand particles using pseudoplastic liquids

Minimum fluidization velocity for binary mixture of sand particles using pseudoplastic liquids

Thermo-economic optimization of a novel confined thermal energy storage system based on granular material

Concentrated solar power is a suitable technology for production of green electricity. However, to attain a uniform electricity production, concentrated solar power should be coupled with large Thermal Energy Storage (TES) systems. Among the different technologies of TES systems, storage of sensible heat in granular material is widely used due to its simple operation. These TES systems store energy as an increase of temperature of a large mass of small solid particles, through which a fluid circulates exchanging heat. TES systems are typically operated in a fixed bed regime, maximizing their exergy output, thus limiting the maximum allowable velocity of the fluid flow. In this work, a novel confined bed is proposed to mechanically prevent the motion of the solid particles conforming the TES system even for high fluid velocities, to guarantee that the exhaust temperature of the fluid is maximum during a discharge process. In this novel confined bed, a thermocline evolves from bottom to top of the system, separating the low and high temperature of the bed during the discharge process. An analytical model was applied to describe the evolution of the thermocline and the effect of the different operating parameters on the thermocline thickness.The effect of the thermocline thickness was combined with a thermo-economic analysis of a confined bed TES system proposed for a case of study. The new confined bed here proposed was optimized considering thermodynamics aspects, namely the fluid exergy increment in the bed, and economic factors, specifically the total investment cost of the TES system. The optimization resulted in low values of the fluid velocity, between 0.2 and 0.4 m/s, but still higher than the minimum fluidization velocity of sand particles of 750 μm, justifying the requirement of a confined bed, and low bed aspect ratios, between 0.25 and 0.9, to prevent excessively high fluid pressure drops. However, the bed aspect ratio increases significantly for higher granular material particle sizes, up to a ratio of bed height to diameter of 3 for a particle size of 10 mm and a TES demand time of 6 h.

Effect of particle shape on biomass pyrolysis in a bubbling fluidized bed

The effect of biomass particle shape on the conversion of beech wood during pyrolysis in a bubbling fluidized bed (BFB) was experimentally quantified. A lab-scale BFB installed on a high-precision scale was used to characterize the mass loss of the biomass particles immersed in the bed. The scale could monitor the mass loss of the beech wood particles while moving freely inside the bed, which was operated at 2.5 times the minimum fluidization velocity of the bed material employed. The tests were performed at 500 and 600 °C using beech wood particles of the same mass, but different in shape. All particles used were cylindrical in shape, with the same mass, and differing in their aspect ratio, analyzing particles from typical biomass chips to standard biomass pellets. The experimental results indicate that the velocity of pyrolysis for the different particles is proportional to the characteristic heat transfer length of the particles, with pyrolysis times ranging from 27 to 53 s for a bed temperature of 600 °C and from 43 to 85 s for a bed temperature of 500 °C. The minimum pyrolysis time was obtained for particles with a diameter of 20 mm and a length of 2 mm pyrolyzing in a bed at 600 °C, whereas the maximum pyrolysis time corresponds to particles of 10 mm in diameter and 8 mm in length converting in a bed at 500 °C. Estimations of the conversion time obtained from a Shrinking Unreacted Particle Model (SUPM), assuming a constant density and reducing volume of biomass during conversion, and a Uniform Conversion Model (UCM), considering uniform volume and decreasing density of biomass along the conversion process, were compared to experimental measurements of the conversion time. Qualitative agreement was found between the experimental values and the predictions of the conversion time from the simplified models, obtaining in all cases conversion times proportional to the characteristic length of heat transfer of each particle shape.

Fluidized Bed Membrane Reactor for the Direct Dehydrogenation of Propane: Proof of Concept.

In this work, the fluidized bed membrane reactor (FBMR) technology for the direct dehydrogenation of propane (PDH) was demonstrated at a laboratory scale. Double-skinned PdAg membranes were used to selectively remove H2 during dehydrogenation tests over PtSnK/Al2O3 catalyst under fluidization. The performance of the fluidized bed membrane reactor was experimentally investigated and compared with the conventional fluidized bed reactor (FBR) by varying the superficial gas velocity over the minimum fluidization velocity under fixed operating conditions (i.e., 500 °C, 2 bar and feed composition of 30vol% C3H8-70vol% N2). The results obtained in this work confirmed the potential for improving the PDH performance using the FBMR system. An increase in the initial propane conversion of c.a. 20% was observed, going from 19.5% in the FBR to almost 25% in the FBMR. The hydrogen recovery factor displayed a decrease from 70% to values below 50%, due to the membrane coking under alkene exposure. Despites this, the hydrogen extraction from the reaction environment shifted the thermodynamic equilibrium of the dehydrogenation reaction and achieved an average increase of 43% in propylene yields.

Analytical and numerical considerations of the minimum fluidization velocity of the molybdenite particles

Analytical and numerical considerations of the minimum fluidization velocity of the molybdenite particles

Enhanced fluidization of solid particles in an oscillating acoustic field

Compared with an ordinary fluidized bed, the fluidization quality of solid particles can be effectively improved by vibration induced by appropriate acoustic fields. The effects of sound on the hydrodynamic behavior of fluidized bed have been investigated under the application of acoustic fields of different intensities (110–130 dB) and frequencies (50–500 Hz). The obtained results show that the perturbation effect of the sound field on fixed-bed pressure drop becomes more significant with increasing sound pressure level, exerting a larger pressure than present under ordinary conditions, due to the change in particle arrangement induced by the acoustic field. Except for a particular frequency, the minimum fluidization velocity in the bed decreases gradually with the increase in the ratio of bed height to bed diameter. The rising velocity of the bubble and the average overflow velocity of residual gas in collapse tests are reduced by the acoustic field.

Tannery Sludge Gasification in a Fluidized Bed for Its Energetic Valorization

The present article deals with the valorization of the organic content of tannery sludges to produce energy vectors. In this scenario, gasification is a viable option to obtain a flexible gaseous stream (syngas) of interesting energetic value, under operating conditions that do not favor the oxidation of Cr(III) (typically found in tannery sludges) to the more harmful Cr(VI) state. To this end, an industrial tannery sludge was characterized through proximate/ultimate analyses and determination of the heating value, showing its capability to act as a solid fuel in a gasification process, and metal analyses, showing that its Cr(VI) content was below the detection limit (2 ppm). The material was subjected to gasification tests in a lab-scale fluidized bed (FB) reactor. The reactor, with a 41 mm inside diameter and a 1 m height, was electrically kept at an operating temperature of 850 °C. The fluidization velocity was 0.30 m/s at 850 °C, i.e., 7.5 times the value of the minimum fluidization velocity. The gasifying stream was composed by O2 (3% vol.) diluted in N2. The adopted oxidant equivalence ratio (ER) levels were 0.15 and 0.24, to ensure substoichiometric (i.e., reducing) conditions in the FB atmosphere. Under the most reducing operating conditions, it was possible to produce syngas with a lower heating value of 12.0 MJ/N m3 (dry and N2-free basis). It contained, under these conditions, about 42% H2, 36% CO, and 4% CH4, plus 16% CO2 and other components. The tar produced from the process, fully characterized by gas chromatography–mass spectrometry, showed a favorably low concentration of about 25 g/N m3. FB bottom and fly ashes were analyzed for their carbon and metal contents. In bottom ash, the total Cr concentration resulted in the range of 8–12 g/kg, with a Cr(VI) concentration between 8 and 10 ppm. In the elutriated stream, the total Cr concentration was about 55 g/kg, with a Cr(VI) concentration between 4 and 7 ppm. The Cr(VI) concentration was higher when higher values of the ER were used, but it resulted in 3–4 orders of magnitude lower than the total Cr concentration, showing the appropriateness of the process to produce syngas with very limited oxidation of chromium in the solid residues.

Gaseous fluidization of short fibers and powders, influence of temperature and pressure

The fluidization of short Hi Nicalon S fibers was investigated before considering CVD coatings with controlled composition and thicknesses. The fluidization of a short fiber batch alone being delicate and not reproducible, the study consisted in comparing the hydrodynamic behavior of a powder alone, to that of a short fiber/powder mixture. The fluidization behavior of the particle beds was studied by following their pressure drop as a function of pressure and temperature. The minimum fluidization velocity (Umf) was evaluated at room temperature (Tamb) at different reduced pressures, and at 200 mbar as a function of temperature. The results are in good agreement with the theoretical equations. At low pressures and Tamb, Umf increases rapidly. In contrast, Umf varies only slightly with temperature. While the pressure does not seem to affect the fluidization behavior of the powder alone, it displays a clear optimum at 200 mbar for the short fiber/powder mixture.

Computational Fluid Dynamics (CFD) modelling of the effect of distributor plate geometry in the bubbling fluidized bed reactor

Air distribution is the primary factor that influences the performance of fluidized beds. To understand the hydrodynamics and operation of a fluidized bed, an assessment of the airflow and its distribution is essential. The design and operation of the fluidized bed are complex therefore numerical models have been proposed to predict the behaviour of fluidized beds. Although fluidisation has been studied fundamentally, systematic investigation of air maldistribution due to fluidised bed reactor design and geometry has not been characterised. The effect of four distributor plates (Plate A, B, C and D with 8.67%, 6.12%,4,2% and 2.4% open area, respectively) and the two fluidised bed reactors (straight inlet and bend inlet reactors) on the fluidisation hydrodynamics have been investigated. OpenFOAM™ v1712 was used to develop a computational fluid dynamic (CFD11CFD- computational Fluid dynamics) model to predict the behaviour of a bubbling fluidised bed. The determination of these characteristics will allow for an improved CFD model and optimisation of an industrial fluidised bed granulator. The model was validated with experimental data. Plate pressure drops of 76 Pa,121 Pa,296 Pa and 886 Pa were achieved with plates A, B, C and D respectively. High bed pressure drops were observed for plate It was therefore concluded that the pitch and open area percentage of the distributor has an effect on both the plate and bed pressure drop. Particles with 1.3 mm diameter fluidised quicker with minimum fluidisation velocity range between 0.3 and 0.8 m/s. There was an agreement between the model and experiments for plate and bed pressure drop and bed expansion. Air maldistribution was observed for bend inlet reactor.

Bubble hydrodynamics of adding fine particles in 2D pulsed fluidized bed

The bubbling behavior of powders in the range of 30–2600 µm is well understood. However, the bubbling behavior in binary-medium pulsed fluidized beds is complex and has received little attention. Therefore, Geldart C and B magnetite powders were mixed to form a dense medium. The effects of C particles and frequency on the minimum fluidization gas velocity, bubble area, number, shape factor, and aspect ratio were studied. The results show that the introduction of pulsating energy can increase the bubble area but decrease the number of bubbles. Under an appropriate content of fine particles, the pulsating energy increases the bubble shape factor and reduces the aspect ratio. A bubble size growth model was developed. A pulsed fluidized bed mixed-medium minimum fluidization velocity model was developed with 10% error. A pulsed fluidized bed bubble size model was developed with a prediction error of 10% or less.

Investigation of wet particle drying process in a fluidized bed dryer by CFD simulation and experimental measurement

Wet particle drying process is complex due to the change of particle moisture, which has big effect on the gas–solid properties, flow dynamics and mass and heat transfer behavior. In this research, a two-fluid model (TFM) coupled with the mass and heat transfer was used to simulate the particle drying process in a fluidized bed dryer. Meanwhile, online electrical capacitance tomography (ECT) was used to measure the solids concentration and off-line moisture meter was employed to measure the particle moisture. Two periods, i.e., constant-rate period and falling-rate period, were captured in the experimental drying curve. The continuous drying curve was obtained by the 2D CFD simulation. The drying rate increases sharply in the pre-warming period, reaches high in the constant-rate period, and decreases markedly in the falling-rate period at the beginning and then decreases slowly. The particle temperature increases gradually until it reaches the air temperature and keeps constant. In order to capture detailed flow dynamics in the drying process, 3D CFD simulations at certain moisture contents were performed. It is found that the simulated cross-sectional particle volume fraction shows core-annular structure and is verified by the on-line ECT measurement. An internal circulation is formed along the reactor height. The mass and heat transfer mainly happens at the lower part of the dryer and at the contact region between the bubbles and the solids phase. The minimum fluidization velocity decreases, and the bubble rising velocity increases when the particle moisture level decreases.

Numerical Analysis and Experimental Verification on Flow Characteristics of a Pressurized Fluidized Bed System

Abstract The pressurized fluidized bed technology is recently shown to be a most promising technology for energy conversion and carbon capture. In the present work, two-phase flow characteristics of a laboratory-scale bubbling fluidized bed (BFB) system have been examined numerically under pressurized cold flow conditions. The BFB hydrodynamics is presented by axial pressure distributions and radial particle volume fractions. Moreover, the influence of drag models on minimum fluidization and bubble formation is studied using two different drag models, namely Wen-Yu/Ergun and EMMS–Yang, at both atmospheric and elevated pressure conditions. The numerical results are validated with a series of pressure drop and axial pressure measurements conducted at three different pressure conditions. The experimentally observed minimum fluidization velocity results are also compared with the theoretical correlations found in the literature. Good agreement has been found between experiments and numerical predictions.

Recent Development in Hydrodynamic and Heat Transfer Characteristics in the Three-phase Fluidized-bed System

Gas–liquid-solid fluidized beds are broadly utilized in the petrochemical, pharmaceutical, refining, food, biotechnology, and environmental industries. Due to complex phenomena, such as the particle-particle, liquid-particle, particle-bubble interactions, complex hydrodynamics, and heat transfer of three-phase (gas-liquid-solid) fluidized beds, they are incompletely understood. The ability to accurately predict the essential characteristics of the fluidized-bed system, such as hydrodynamics, individual phase mixing, and heat transfer parameters, is necessary for its successful design and operation. This paper investigates the pressure drop, minimum fluidization velocity, phase holdup, heat-transfer coefficient of a fluidized bed reactor, heat transfer studies, CFD simulation, and the effect of these parameters on the extent of fluidization. Many variables (fluid flow rate, particle density and size, fluid inlet, and bed height) affect the fluidizing quality and performance of the fluidization process. The hydrodynamics parameters, mixing of phases, and the behavior of heat transfer with various modes of fluidization were investigated to predict hydrodynamics parameters. Several publications have demonstrated the utility of (CFD) in explaining the hydrodynamics, heat, and mass transfer of fluidized beds. Principles of measurement, details of the experimental configurations, and the applied techniques by various researchers are also presented. Feng's model was statistically validated using experimental data that was both time-averaged and time-dependent. Furthermore, this model successfully predicted the instantaneous flow structures, which should provide strategies for the best design, scale-up, and operation in fluidized bed columns. The divergence between the simulated and observed values can be reduced by better understanding the fluidized bed's nature.

Numerical simulation study on mixing characteristics of binary Geldart-D particles in a pressurized fluidized bed

Understanding the mixing behaviors of different particles is significant for improving the mass and heat transfer performance of a pressurized fluidized bed. A numerical simulation study on the mixing behavior of binary Geldart-D particles in a pressurized bubbling fluidized bed is performed based on the CFD-DEM method. The mixing characteristics and bubble behavior of binary Geldart-D particles with different physical properties under elevated pressure are investigated in the current study. Increasing the fluidization gas velocity u g and operating pressure P could promote the mixing of the particles. Raising the operating pressure P would cause a smaller bubble size and large bubble quantity at the same fluidization number u g /u mg . Higher operating pressure P and fluidization number u g /u mg could create a stable and uniform distribution of jetsam in the fluidized bed. • Mixing behaviors in the pressurized fluidized bed are investigated by CFD-DEM. • Minimum fluidization velocity decrease as the increase of operating pressure. • Bubble size decreases as raising operating pressure of fluidized bed. • High operating pressure and gas velocity promote the mixing rate of particles.

Experimental study on heat transfer and hydrodynamics of a concentric double-pipe pulsed fluidized bed using microencapsulated phase change material and sand

This paper presents an experimental study on the heat transfer and hydrodynamic of a concentric double-pipe gas-solid pulsed fluidized bed filled in outer tube with microencapsulated phase change material and inner tube with sand. The effect of flow velocity, frequency and bed height are investigated on the heat transfer. Experiments are carried out at frequency in the range of 1 to 5Hz with the bed height of 15, 18.5and 22 cm and the ratio of air velocity to minimum fluidization velocity ( U / U mf ) of 1, 1.1, 1.2, 1.3, 1.4, 1.5. Results show that heat transfer enhances as the frequency, bed height and U / U mf increase. Besides, it is found that pulsed flow has higher heat transfer enhancement than continuous flow. For example, at U / U mf of 1.5, the heat transfer coefficient of the pulsed flow increases by 7.5% than that of the continuous flow at the bed height of 22 cm. • Hydrodynamics and heat transfer in a concentric double-pipe pulsed bed are studied. • Pulsed flow in bed has higher heat transfer coefficient than continuous flow. • Heat transfer coefficient increases as U / U mf , frequency, and bed height increase. • Heat transfer becomes higher as the fluidity increases.

Performance analysis of a 2D numerical model in estimating minimum fluidization velocity for fluidized beds

The minimum fluidization velocity of a fluid–solid particle fluidized bed is the primary focus of this paper. The computationally economic Eulerian Granular model has been used to analyze fluidization for both gas–solid particle and liquid-solid particle fluidized beds. The conventional approach of finding minimum fluidization velocity (umf) is either with a pressure drop across the particle bed or the change in bed height. However, these parameters are often unstable and cannot be used to generalize the degree of fluidization accurately. In this paper, the dominant factor of unstable pressure drop estimation in the 2D Two-Fluid Model (TFM) and a key non-dimensional Euler number has been investigated in determining minimum fluidization velocity for different quasi-2D fluidized beds for different bed sizes, particle sizes, and particle numbers. Averaging assumptions and limitations of these numerical models are discussed in detail for four different fluidized bed cases. A comparative study of the drag model shows little to no influence in unstable pressure drop estimation near fluidization velocity, and all drag models perform similarly. It is observed that particle-particle collision is not the dominant reason for unstable pressure drop near minimum fluidization. Instead, wall effects on the particle bed including frictional losses and wall-particle collision play a key role in unstable pressure drop calculation for the quasi-2D fluidized beds. Pressure drop characteristics alone do not suffice to obtain minimum fluidization velocity with 2D TFM using existing models. Thus, a different approach has been proposed to investigate minimum fluidization involving the Euler number, which has shown promising performance in determining minimum fluidization velocity and characterizing fluidization with 2D TFM. Results show consistency in Euler number characteristics for all different fluidized bed cases considered in this paper. This can revitalize computationally economic 2D Eulerian simulations, increase the range of possible applications, and provide guidance to the future development of computationally efficient and more accurate numerical models, and empirical correlations for minimum fluidization velocity.

Hydrodynamic Properties Investigation of Ebullated Bed Reactor Using Non-Newtonian Liquid

The importance of using EBR has been renewed recently due to the sharp increase in heavy feedstocks sent to refineries and the hydrocracking process. Most of these feedstocks have a non-Newtonian behavior. The performance of this type of reactor using non-Newtonian liquid is complicated and has not been covered well yet. Hence, the present work is devoted to elucidating the effect of the non-Newtonian behavior of fluid on the hydrodynamic properties of a three-phase (gas-liquid-solid) reactor under operating conditions of different values ​​of gas velocity (2, 4, 6) cm/sec, liquid velocity (0.9, 1.39, 1.8, 2.3) cm/sec, and recycle ratio (1.5, 2, 2.5). The study observed the effect of non-Newtonian behavior using polymethyl Cellulose (PMC) at different concentrations (0.1, 0.2, 0.3, and 0.4) wt%. The pressure gradient method was used to elucidate the minimum liquid fluidization velocity and to estimate hold up, while the imaging method was used to measure the bubble's size. The results showed that the higher the gas velocity, the lower the minimum liquid fluidization velocity. As the intensity of the non-Newtonian behavior increased, gas velocity showed the opposite effect. The results also showed that increasing the velocity of liquid and gas and the intensity of the non-Newtonian increase the gas hold-up. The bubbles characteristics, represented by bubble size results, show that small bubbles appear at low gas velocities, and these bubbles collapse as gas and liquid velocities increase as well as liquid viscosity.