Minimum Fluidization Velocity Research Articles

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.

Studying the effect of direction and strength of magnetic field on fluidization of nanoparticles by recurrence analysis

Effect of the intensity and direction of the magnetic field was studied on fluidization of titanium oxide nanoparticles (anatase phase) mixed with ferromagnetic Iron (III) oxide nanoparticles using the recurrence analysis. The bed expansion and visual observation revealed that these particles operate in the ABF regime in all cases. Minimum fluidization velocities were obtained through the standard deviation of pressure fluctuations. Agglomerates samples were taken to study the effect of the magnetic field on the size of agglomerates. It was observed that at low field strength, the vibration has a major effect on fluidization than the magnetic force. At field strengths less than 400 Gauss, the vibration of solenoid improved the quality of fluidization by decreasing the size of agglomerate and minimum fluidization velocity, regardless of the field direction. At field strengths greater than 400 Gauss, the upward direction increases the interaction of agglomerate which results in a stochastic pattern of fluidization and lower ABF characteristics. At high magnetic field strengths, the effect of field direction becomes more noticeable. At high field strengths, the downward field results in less movement of the bed material and resistance against fluidization as well as the formation of larger bubbles and their deterministic effect on the bed.

Analysis of the Usability of Iron Ore Ultra-Fines for Hydrogen-Based Fluidized Bed Direct Reduction—A Review

This review focuses on the usability of iron ore ultra-fines for hydrogen-based direct reduction. Such technology is driven by the need to lower CO2 emissions and energy consumption for the iron and steel industry. In addition, low operational and capital expenditures and a high oxide yield because of the direct use of ultra-fines can be highlighted. The classification of powders for a fluidized bed are reviewed. Fluid dynamics, such as minimum fluidization velocity, entrainment velocity and fluidized state diagrams are summarized and discussed regarding the processing of iron ore ultra-fines in a fluidized bed. The influence of the reduction process, especially the agglomeration phenomenon sticking, is evaluated. Thus, the sticking determining factors and the solutions to avoid sticking are reviewed and discussed. The essential theoretical considerations and process-relevant issues are provided for the usability of iron ore ultra-fines for hydrogen-based fluidized bed direct reduction.

Minimum Velocity of Impingement Fluidization for Parachute-Shaped Vegetables

Accurate calculation of the minimum fluidization velocity makes it possible to reduce raw material losses due to the use of excessively high or excessively low air velocities. This is particularly true for impingement fluidization, which is little studied, especially when treating parachute-shaped raw material. This paper focused on determining the drag coefficient for cauliflower florets, mushrooms, and broccoli. Analysis of the critical particle lift velocity showed that the lowest value of the drag coefficient was found for mushrooms (0.9). The parachute-shaped vegetables analyzed had a large scatter of drag coefficient values associated with their specific shape (standard deviation: mushrooms 0.10 broccoli 0.14, and for cauliflower 0.15). The measured mean values of the minimum fluidization velocity of the tested vegetables in the impingement fluidization method ranged from 6.9 m∙s−1 to 10.97 m∙s−1. Application of the procedure recommended by Shilton and Narajan for calculating the minimum fluidization velocity on the basis of the shape coefficient ε resulted in large discrepancies between the calculated and experimental values (from 2.4 m∙s−1 to 3.8 m∙s−1).

Coarse-grained CFD-DEM simulations of fluidization with large particles

Detailed validation coarse-grained CFD-DEM model in the open source code MFIX is performed. Metrics used include fluidization behavior, minimum fluidization velocity, averaged pressure drop, mean and rms particle velocity. The choice of the spring constant alters fluidization dynamics. Of drag correlations tested, the Tang et al. (2015) formed poorly overall at low superficial velocities of 2 Umf and 3 Umf and exceptionally well at 4 Umf suggesting it is suitable at higher velocities. A deterioration in the predicted minimum fluidization velocity is noticed using the coarse-grained model. A trend of reducing error of average pressure drop with increasing statistical weight is observed at low gas velocities. The mean particle velocities profiles for the coarse grain and conventional CFD-DEM models do not show appreciable differences unlike rms profiles that exhibit a reducing trend with increasing statistical weight. Some gain on the wall clock time required to complete a case were recorded.

Computational modeling of high pressure bubbling fluidized bed hydrodynamics using ilmenite particles

The multiphase particle-in-cell approach was used to model the hydrodynamics of five different ilmenite particle size distributions in a high pressure cylindrical fluidized bed. The model was created using the commercial software Barracuda Virtual Reactor® and validated against previously obtained experimental data. Simulations for minimum fluidization velocity were dependent on the model input close pack factor, with a value of 0.58 resulting in the best fit with an average absolute deviation (AARD) of 3.9%. Afterwards, trends in bed expansion with fluidization ratio and pressure were well represented by the model, but consistently under predicting by an average of 4.8%. The model also predicted bubble size distribution with change in relation to fluidization ratio, pressure, and elevation. The calculated mean bubble diameter deviated by 18.7% from the experimental results, and the Mori & Wen correlation for bubble growth was adapted to the simulated data resulting in an AARD of 22%.

Experimental study of convective heat transfer coefficient for pulsed fluidized bed using microcapsule phase change material

AbstractThe convective heat transfer coefficient in a pulsating cylindrical fluidized bed using microcapsule particle‐phase change material (MPCM) was examined experimentally. A solenoid on and off valve was used to provide the pulsation and was varied from 1 to 10 Hz. The test section in the bed was loaded with granular microparticle‐phase change material (MPCM) with an average diameter of 200 µm. The pulsation effect on the thermal field, thermal storage, and heat transfer coefficient was investigated for 1.5, 2, and 2.5 of minimum fluidization velocity. Results indicated that the amplitude of the oscillation decreases with increasing in pulsation frequency. An increase in heat transfer rate was shown to be related to the superficial velocity. The maximum performance was obtained for the frequency of 7 Hz at the velocity ratio of 2.5. The convective heat transfer increased by 17% for frequency pulsation of 7 Hz. Furthermore, the Duty cycle (η) is defined as the ratio of turn‐on duration to the total turn‐on and off duration for entering airflow to the solenoid valve. It was shown that the η > 0.4 led to better mixing and higher heat transfer. However, the pulsation effect was shown to disappear for the duty cycle of higher than 0.8. Comparison with the available experimental data of others for continuous flow was in good agreement.

Effect of internal friction among unfluidized particles on measuring minimum fluidization velocity and segregation index

The internal friction among unfluidized particles makes them polymorphic, thus confusing measurements of minimum fluidization velocity (U mf) and segregation index (S). Such a polymorphic characteristic of the packed bed caused the ΔP b–U g↑ curve measured subsequently to be poorly reproducible. The poor reproducibility of determining U mf from this curve resulted eventually from the ambiguity in the traditional definition of U mf. It did not clearly indicate which of the many states the particles should be initially packed into before measuring the ΔP b–U g↑ curve. The internal friction caused it difficult to accurately measure S for low gas velocities. The corresponding stable fluidization states could not be obtained through reducing the gas flow slowly from complete fluidization to these velocities. Some particles lost the capability to move before reaching their stable equilibrium positions. Only some steady states which could not represent the given U g were obtained. Many states of the unfluidized particles were demonstrated to be metastable and belong to the amorphous/glass state. They could not recover/restore themselves after relatively large perturbations.

Separation performance of 0.5–1 mm fine coal in a gas–solid fluidized bed without dense medium

The polluting elements in coal are mainly concentrated in gangue, and it is difficult for existing dry coal preparation technologies to separate fine gangue below 1 mm effectively. Because 0.5–1 mm raw coal particles belonging to the Geldart B group can form a stable bubbling fluidized bed, the fluidization and segregation of simulated coal particles composed of 0.5–1 mm quartz sand and clean coal were investigated in this study. The results showed that the optimum operation gas velocity was slightly higher than the minimum fluidization gas velocity when the content of quartz sand was less than 60%; when it was higher than 60%, the optimum gas velocity was somewhat less than or equal to the minimum fluidization velocity. When the content of quartz sand was 60%, the separation effect was the most obvious, and the segregation standard deviation was 0.37. The initial bed height was in the range of 150–210 mm, and the separation effect was good. In addition, the separation tests of three raw coal samples showed that the research results of binary-simulated coal particles composed of quartz sand and clean coal were valid. Under the optimal conditions, the ash content of clean coal products of three raw coal samples can be reduced by 75.65%, 79.03%, and 83.03%, respectively.

Correlation for predicting minimum fluidization velocity with different size distributions and bed inventories at elevated temperature in gas-solid fluidized bed

This study presents extensive experimental details of the effects of the solid particle size distribution (PSD), bed inventory, and bed temperature on the minimum fluidization velocity (Umf). Silica sand with three average solid particle diameters and five PSDs was used. The results showed that the Umf values of the wide PSDs were lower than those of the narrow cut particles with the same average particle diameter. The standard deviation (SD) and skewness of the PSD also influenced Umf variation. Furthermore, the Umf decreased with increasing bed inventory and bed temperature. The influencing parameters were recast into a dimensionless group and included in a new correlation for predicting Umf. The proposed correlation estimated an average absolute deviation of 14.6% between the experimental data and predicted values. Furthermore, the new correlation was evaluated with a dataset from the literature and gave an average absolute deviation of 15.6%.

Modified correlation for minimum fluidization velocity of low-density particles in inverse liquid–solid fluidized beds

The minimum fluidization velocity (Umf) is a key parameter for the scale-up of inverse liquid–solid fluidized beds. Theoretical predictions using common correlations were compared against experimental minimum fluidization velocity measurements of low density (28–638 kg/m3), 0.80–1.13 mm Styrofoam particles in a fluidized bed with a height of 4.5 m and 0.2 m diameter. The average absolute relative deviation for the predicted minimum fluidization velocity for particles below 300 kg/m3 was above 40% using the studied common correlations. A modified Wen and Yu correlation was thus proposed based on novel and past measurements with low-density and small-diameter particles, expanding the range for predicting Umf. The new correlation predicted Umf with deviations below 15% for ST028, ST122 and ST300. This modified correlation also improved Umf predictions for comparable particles from a previous study, demonstrating its validity for a larger range of low-density particles.

Experimental study of fluidization characteristics of Geldart-D particles in pressurized bubbling fluidized bed

The pressurized bubbling fluidized bed shows great advantage in retreating municipal solid waste because it could effectively capture CO2 and enhance the reaction rate of the process of combustion and gasification. In the present work, fluidization characteristics of Geldart-D particles at elevated pressure were experimentally investigated, such as flow pattern, pressure drop, minimum fluidization gas velocity. At the same fluidization gas velocity, as elevating operating pressure, the fluidization of Geldart-D particles became more intense, the bubbles got larger, the standard deviation and the power density of dominant frequency of the pressure drop signal increased. While, under the same fluidization number, as increasing operating pressure, the fluidization of Geldart-D particles became smoother, the bubble size decreased, both the standard deviation and the power density of dominant frequency of the pressure drop signal decreased. It seems that, under elevated pressure, the fluidization behavior of Geldart-D particles would transition to that of Geldart-B particles. Finally, the minimum fluidization velocity of the Geldart-D particles was found decreased with the increase of the operating pressure. A new correlation for the prediction of the minimum fluidization velocity of Geldart-D particles at elevated pressure was also formulated based on the present experimental results.

Towards a general correlation for minimum fluidization velocity in gas-fluidized beds: Based on a database mining from the literature

As one of the most significant parameters in fluidized beds design and operation, the minimum fluidization velocity (Umf) has received utmost attention since 1940s. As an effective reference for evaluating the fluidization characteristic, Umf is conventionally predicted using the empirical correlation developed based on experiments for the specified gas-solid system. Despite more than 100 correlations proposed in the literature, these correlations show great diversity in either the applicable regimes or the mathematical formulae. Thus a general correlation for accurately determining Umf is highly desired. In this work, with a recently established database of Umf mined from published papers, the empirical correlations with four different formulae were first assessed for Geldart Groups A, B, and D particles, respectively. In view of the application limitations, modifications to different formulae of empirical correlations have been made to achieve satisfactory performances in predicting Umf for either Geldart Groups A, B, or D particles. Note that Umf is essentially determined by the balance between the gravitational force and drag force for a gas-solid system, a new formula based on a fluidized bed drag correlation derived from direct numerical simulations (DNS) was proposed. By using this formula, we developed a general correlation for Umf in gas-fluidized beds, which exhibits the best performance for all Geldart Groups A, B, and D particles when compared with empirical correlations reported in the literature.

Removal of Amoxicillin From Aqueous Solution in Batch and Circulated Fluidized Bed System Using Zinc Oxide Nanoparticles: Hydrodynamic and Mass Transfer Studies

In this study pistachio shell waste coated with zinc oxide nanoparticles was investigated for its capacity to adsorb the antibiotic amoxicillin (AMO) from aqueous solutions. The pistachio shells, ZnO nanoparticles, and the prepared zinc oxide pistachio shell nanocomposite (CPS) were characterised using various techniques including Brunauer, Emmett and Teller, X-Ray Fluorescence, Fourier Transform Infrared Spectroscopy, Scanning Electron Microscope, and Energy Dispersive Spectroscopy. Batch and continuous flow system studies were carried out to study the adsorption behavior of CPS towards antibiotics. In the batch system, the factors affecting the sorption capacity of CPS including pH, contact time, initial AMO concentration, adsorbent dose, shaking speed, CPS particle size and temperature were investigated. The higher adsorption efficiency was reported at pH 5 for AMO at 70 mg/l initial AMO concentration with 150 rpm. The best adsorbent dose for above conditions was 0.08g/100 mL for AMO removal. The efficiency for removing AMO is affected by temperature and decreased with increasing temperature and is exothermic in nature. With increasing temp, negative value of ΔG° increase indicating that the adsorption process of AMO on CPS becomes unfavorable at higher temperatures. By the Langmuir model, the higher adsorption ability was 163 mg/g for AMO. The Pseudo-second order kinetic model very well simulated the kinetic experimental data and the adsorption rate parameters values indicating the removal process of AMO onto CPS was chemisorption and rate limiting contribute to the diffusion of the boundary layer, not intra-particular diffusion. A circulated fluidized bed reactor was adopted for the continuous removal of AMO, the influences of operating factors were studied including bed depth, air and water flow rate, and initial AMO concentration. The minimum fluidization velocity of bed was affected by Ug, dp, Hs and particle density. Different operating conditions were used, and the removal efficiency decreased when the liquid flow rate increased. Increasing air flow rate increase the time required for CPS to reach saturated. The obtained results show that CPS is an excellent medium for the adsorption of AMO from aqueous solution.

Investigation into the hydrodynamics of liquid-solid inclined micro-fluidized beds

The inclined micro-fluidized bed (MFB) can enhance heat and mass transfer rates compared to the vertically aligned counterparts, but the increased significance of surface forces and wall effects may cause poor fluidization performance. In this paper, the effects of column inclination and different particle-to-bed ratios (dP/dB) on the solid hydrodynamics are investigated in an inclined micro-fluidized bed. The results validated the suitability of using the Ergun equation to predict minimum fluidization velocities due to small deviations between 1.01 and 1.81 times the theoretical values, for a particle-to-bed ratio ranging from 0.025 to 0.165 at inclinations between 0° and 10°. Investigation into the effects on bed expansion behavior showed that the bed contracted with an increase in bed inclination. An unexpected observation during the bed expansion was the appearance of a secondary high voidage region and the appearance of strong circulation patterns with an increase in bed inclination. A detailed analysis of this phenomenon suggested the presence of a critical angle at 6° and 10° for the 85 µm particles, 4 × 4 mm bed cross-section and 165 µm particles, 1 × 1 mm bed cross-section, respectively. However, the liquid-solid back-mixing was observed due to the modified particle trajectories resulted in the disappearance of the high voidage region. This paper gives new insights into the micro-fluidization behavior in inclined beds thus contributing to the development of micro-fluidized beds and their future applications.

Investigation on dynamic movement of cylindrical biomass particles in a fast fluidized bed

The processing of biomass particles is complex in a fluidized bed due to their heterogeneous characteristics. To further understand biomass particles, the dynamic movement of cylindrical biomass particles were investigated in a fast fluidized bed. Cylindrical particles were studied via the impulse momentum theorem and the Discrete Element Method (DEM). Meanwhile, the contact action, the drag force and other conventional forces were considered. To validate the present method, the predicted orientation and the minimum fluidization velocities of cylindrical particles were compared with the relative results, and validate the present approach. Then the characteristics of biomass particle flow dynamics were analyzed in terms of the particle concentration, the orientation distribution and the residence time distribution (RTD). It was found that most particles present as the horizontal or nearly horizontal states (0°) during the fluidization, and the percentage can reach and exceed 16%. High concentration occurs near the wall due to the back flow. The lower fluidization velocity corresponds to a wider RTD. The particles with the same size leave the riser with various residence times. The applied method and the obtained results provide helpful consults to study the cylindrical and other non-spherical biomass particles in an extensive way.

Study on Fluidization Characteristics of Magnetically Fluidized Beds for Microfine Particles

The bed pressure drop, minimum fluidized gas velocity, bed density, and bed expansion rate are important parameters characterizing the fluidization characteristics of gas-solid fluidized beds. By analyzing these parameters, the advantages and disadvantages of the fluidization state can be known. In this study, experiments were conducted to study the fluidization characteristics of a gas-solid magnetically fluidized bed for microfine particles by changing the magnetic field strength, magnetic field addition sequence, and static bed height. The experimental results show that when the magnetic field strength increased from 0 KA/m to 5 KA/m, the minimum fluidized gas velocity of particles increased from 4.42 cm/s to 10.32 cm/s, while the bed pressure drop first increased and then decreased. When the magnetic field strength is less than 3.4 KA/m, the microfine particles in the bed are mainly acted on by the airflow; while when the magnetic field strength is greater than 3.4 KA/m, the microfine particles are mainly dominated by the magnetic field. The magnetic field addition sequence affects the fluidization quality of microfine particles. The fluidized bed with ‘adding magnetic field first’ shows a more stable fluidization state than ‘adding magnetic field later’. Increasing of the static bed height reduces the bed expansion rate. The bed expansion rate is up to 112.5% at a static bed height of h0 = 40 mm and H = 5 KA/m. This will broaden the range of density regulation of a single magnetic particle and lay the advantage of gas-solid magnetically fluidized bed for microfine particles in the field of separation of fine coal.

Minimum Fluidization Velocity and Reduction Behavior of Combusted Iron Powder in a Fluidized Bed

Minimum Fluidization Velocity and Reduction Behavior of Combusted Iron Powder in a Fluidized Bed

A comparative investigation of flow structures in three-dimensional supercritical water and gas-solid fluidized bed via two-fluid model simulations

A comparative study of flow characteristics in three-dimensional supercritical water fluidized beds (SCWFB, 440–560 ℃, 22–26 MPa), cold gas-solid fluidized beds (CGSFB, 20 ℃, 0.1 MPa), and high-pressure and high-temperature gas-solid fluidized beds (HGSFB, 440–560 ℃, 22–26 MPa) was conducted via two-fluid model simulations across different fluid velocities, un-fluidized bed heights, pressures and temperatures. Bubble (or slug) and particle dynamics, fluid velocities, and bed expansions are investigated. The results show that, the drag forces of particulate phase are around 0.4 times particulate phase weights in SCWFB. Compared with CGSFB, SCWFB has lower minimum fluidization velocities, smaller bubble and slug sizes, more bubble and slug numbers, higher bed expansions and slug onset heights, and more uniform gas-solid interactions. Flow structures in SCWFB are similar to those in HGSFB. Pressure and temperature have less effects on the flow structures than fluid velocity and un-fluidized bed height in both SCWFB and HGSFB.

Batch scale study on magnetizing roasting of low-grade iron ore tailings using fluidized bed roaster

The present steel production of India is 111.24 MTPA in January 2019 as per the Ministry of Steel, Govt. of India. The demand for steel production is increasing day by day. Depleting the high-grade iron ore resources compels steel industries to utilize the low/lean grade iron ore to meet future demand. However, there is no technology available that can directly use these low-grade iron ore. Further, these iron ores are dumped in different mine heads and slime ponds as waste creating environmental and societal problems. Therefore, it is need of the hour to develop suitable scientific methods where these low and lean grade iron ore can be effectively utilized. Magnetizing roasting of this low and lean-grade iron-ore followed by low-intensity magnetic separation can produce high-grade magnetite. The present study focused on the reduction roasting of a tailing iron ore sample having 47.47 %Fe in a batch-type fluidized bed roaster. The roasting of the sample was conducted at a temperature of 1000 °C and varying residence time from 2 to 10 min. Non-coking coal was employed as a reductant for the reduction reaction. The fluidization velocity was maintained at 1.5–1.7 times the minimum fluidization velocity. Then, the roasted sample was subjected to LIMS to obtain a high-grade magnetite concentrate. The study reveals that 61.1-grade iron ore can be obtained at 2 min residence time with 48.4% and 62.5% yield and recovery.