Critical Fluidization Velocity Research Articles

Experimental investigation of wet particle flow characteristics in a pseudo two-dimensional fluidized bed

The flow characteristics of wet particles in a fluidized bed are extremely complicated due to the presence of moisture. It is essential to understand the complex behavior and flow characteristics of wet particles in the drying, coating and granulation process for optimizing the process efficiency and product quality. Thus, the flow characteristics of wet particles, such as pressure fluctuation, bubble size and bubble fraction, in a pseudo two-dimensional fluidized bed were investigated through the multi-resolution time-frequency analysis of the pressure signal combined with the high-speed camera images. Results show that the critical fluidization velocity increases after the particles are wetted, and it shows a slight increase with higher moisture content levels. Besides, there are two main characteristic frequencies, which correspond to the gas-solid flow state of the gas channel and the large bubble in the bed, respectively. Notably, higher gas velocities promote the dominance of large bubbles. And increasing particle moisture inhibits bubble expansion and reduces bubble size, while dense particle areas expand vertically due to enhanced bonding. The bed height increases with the increase of moisture, and the fluctuation of bubble fraction decreases. In the drying experiment, two characteristic frequencies also appear and the pressure drop reduce slightly due to the reduction of water. During the drying process, both the average bubble fraction and fluctuation intensity increase.

Study on fluidized leaching of calcified vanadium slag clinker

The fluidized leaching process of calcified vanadium slag clinker was studied in a particulate fluidized bed reactor. The particle size distribution of the clinker was determined and seven size fractions were divided for the experiments. The critical velocity and entrainment velocity of different size fractions were firstly calculated. The effects of clinker particle size on the apparent fluidization velocity, residence time and vanadium leaching rate were investigated. The results show that the critical fluidization velocity and residence time distribution of different particle size fractions are quite different, and there exists significant short circuit or channeling which needs to be treated by stages. The best vanadium leaching efficiency of 94% can be obtained for particles lower than 39 μm, with the residual vanadium content of the tailing at 0.54%.

Intensification of the fluidization performances of polypropylene by using microwave absorbent

Fluidization is an effective method to better help the transfer and conversion of fuel particles. In this study, intensification of the fluidization performances of polypropylene was experimentally investigated by using microwave absorbent. The fluidization performances of polypropylene (PP) were initially presented and the effects of microwave absorbent (SiC) particle size, mass ratio of SiC to PP, and fluidization velocity on the co-fluidization performances were studied and presented. The results showed that the PP load significantly increased the initial bed height whereas slightly increased the critical fluidization velocity and bed expansion ratio. Adding microwave absorbent (SiC) can effectively improve the fluidization performance of PP particles. The smaller SiC particles (0.25−1 mm) had better fluidization performances than the larger ones (1–1.5 mm and 1.5−2 mm). The mass ratio of silicon carbide to polypropylene decreased the critical fluidization velocity. Fluidization velocity significantly affected the co-fluidization performance. For the co-fluidization of polypropylene and silicon carbide in the designed fluidized bed, the recommended fluidization parameters were: (a) the particle size of silicon carbide of 0.25–1 mm, (b) the mass ratio of silicon carbide to polypropylene of 3:2, and (c) the fluidization velocity of 0.66 m/s.

Numerical analysis of sand production during natural gas extraction from unconsolidated hydrate-bearing sediments

Sand production, which poses a great challenge for sustainable methane gas production, is commonly encountered during natural gas extraction from unconsolidated hydrate-bearing sediments. However, studies on the subject are still limited. This paper provides a numerical model, coupled with thermo-hydro-mechanical (THM) processes, which is capable to characterize the process of sand production in gas hydrate-bearing sediments. The numerical simulator is developed by introducing the newly developed particle migration module into the existing framework of the THM coupled simulator HydrateBiot. The dynamic of sand production was investigated numerically, and the influence of various parameters on sand, gas and water production was studied. The simulation results show that particle detachment and migration mainly occurs in the vicinity of wellbore, while lowering the depressurization rate can mitigate sand production to some extent without significantly inhibiting the gas production. The using of the sand control device can effectively inhibit sand production, but its blockage will cause a significant drop in gas productivity. Sensitivity analyses of the critical fluidization velocity and solid-liquid velocity show that the increase of solid particle concentration will significantly affect the fluidity of water, thus affecting the depressurization process and gas productivity. It is also found that the massive particle detachment and migration may cause the appearance of clogging area in the vicinity of the wellbore and severely restrict the sustainable gas production. The numerical model and analysis presented here could provide useful insight into sand detachment, migration and production processes induced by methane gas production using depressurization method in the hydrate-bearing marine sediments.

Experimental studies on combustion of composite biomass pellets in fluidized bed

This work presents studies on the combustion of Composite Biomass Pellets (CBPS) in fluidized bed using bauxite particles as the bed material. Prior to the combustion experiment, cold-flow characterization and thermogravimetric analysis are performed to investigate the effect of air velocity and combustion mechanism of CBPS. The cold-state test shows that CBPs and bauxite particles fluidize well in the fluidized bed. However, because of the presence of large CBPs, optimization of the fluidization velocity is rather challenging. CBPs can gather at the bottom of the fluidized bed at lower gas velocities. On the contrary, when the velocity is too high, they accumulate in the upper section of the fluidized bed. The suitable fluidization velocity for the system in this study was found to be between 1.5–2.0m/s. At the same time, it is found that the critical fluidization velocity and the pressure fluctuation of the two-component system increase with the increase of CBPs mass concentration. The thermogravimetric experiment verifies that the combustion of CBPs is a first-order reaction, and it is divided into three stages: (i) dehydration, (ii) release and combustion of the volatile and (iii) the coke combustion. The combustion of CBPs is mainly based on the stage of volatile combustion, and its activation energy is greater than that of char combustion. During the combustion test, CBPS are burned at a 10kg/h feed rate, while the excess air is varied from 25% to 100%. Temperatures of the bed and flue gas concentrations (O2, CO, SO2 and NO) are recorded. CBPs can be burnt stably, and the temperature of dense phase is maintained at 765–780°C. With the increase of the air velocity, the main combustion region has a tendency to move up. While the combustion is stable, O2 and CO2 concentrations are maintained at about 7%, and 12%, respectively. The concentration of SO2 in the flue gas after the initial stage of combustion is nearly zero. Furthermore, NO concentration is found to be closely related to O2: the NO reaches its peak value after initial stage and later decreases with the continued depletion of O2. Towards the end of combustion, NO increases with the increase of O2.

Granular compaction by fluidization

How to arrange a packing of spheres is a scientific question that aroused many fundamental works since a long time from Kepler’s conjecture to Edward’s theory (S. F. Edwards and R.B.S Oakeshott. Theory of powders . Physica A, 157 : 1080-1090, 1989), where the role traditionally played by the energy in statistical problems is replaced by the volume for athermal grains. We present experimental results on the compaction of a granular pile immersed in a viscous fluid when submited to a continuous or bursting upward flow. An initial fluidized bed leads to a well reproduced initial loose packing by the settling of grains when the high enough continuous upward flow is turned off. When the upward flow is then turned on again, we record the dynamical evolution of the bed packing. For a low enough continuous upward flow, below the critical velocity of fluidization, a slow compaction dynamics is observed. Strikingly, a slow compaction can be also observed in the case of “fluidization taps” with bursts of fluid velocity higher than the critical fluidization velocity. The different compaction dynamics is discussed when varying the different control parameters of these “fluidization taps”.

CFD Simulation Research on Flow Characteristics of Fluidized Beds

The Euler-Euler two-fluid model incorporating the kinetic theory of granular flow was applied to simulate the gas-solid flow in fluidized beds. The pressure drop, particle distribution and motion characteristics were studied in this paper. In order to investigate the effect of structure of the fluidized bed on flow characteristics, fluidized beds with different diameters and structures were applied. User defined functions (UDF) were applied to study the flow characteristics when the particle size and mass changed over time. The results showed that with the increase of particle size, higher minimum fluidization velocity was required, but lower pressure drop was obtained. For a certain fluidizing medium, the bed critical fluidization velocity depended only on the size and nature of the particles. The structure of a fluidized bed had an influence on the particle distribution and motion characteristics.

Hydrodynamic Characteristic of Large Particles in Vibrated Fluidization Bed

Hydrodynamic for 2D vibrated fluidized bed was studied with large particle glass beads of average diameters are less than 1.5mm.The effect of the vibration strength, the static bed height and the particle diameter on the critical fluidization velocity was analyzed. The results show that the critical fluidization velocity decreases with the increase of the vibration strength, and increases with increases of the static bed height and particle diameter. The effect of vibration on critical fluidization velocity was more significant than decrease the bed height and particle diameter. The empirical correlation equations to predict the critical fluidization velocity was established, and the results of the prediction were compared with the experimental data, the error is in range of ±10%. The results can provide important meaning on theory and instructing practice for developing the new drying technology with high drying efficiency and low dissipation of energy.

Hydrodynamics of gas–solid fluidization in tapered beds

AbstractGas–solid fluidization has a wide range of industrial applications like catalytic reactions, combustion, gasification, etc. In a number of these applications, there is particle size reduction during the operation leading to severe entrainment and limitation of operating velocity. The various problems associated with particles of different sizes or changing particles sizes could be overcome by adopting tapered beds in fluidization operation.In the present investigation, the fluidization phenomenon in tapered beds has been critically assessed through experimental investigations using particles of different sizes and materials and wide range of apex angles of the vessels. The effect of particle size and apex angle on the fluidization behaviour is clearly brought out which has not been reported so far in literature. The importance of compressive force existing in tapered beds is highlighted.In addition, correlations for all hydrodynamic characteristics, viz. critical fluidization velocity, minimum velocity for full fluidization, maximum velocity for defluidization, peak pressure drop, fluctuation ratio, compressive force, and hysteresis have been developed some of which are proposed for the first time.

Correlations for critical fluidization velocity and maximum bed pressure drop for heterogeneous binary mixture of irregular particles in gas–solid tapered fluidized beds

The tapered fluidized bed is a remedial measure for certain drawbacks of the gas–solid system, by the fact that a velocity gradient exists along the axial direction of the bed with increase in cross-sectional area. To study the dynamic characteristics of heterogeneous binary mixture of irregular particles, several experiments have been carried out with varying tapered angles and composition of the mixtures with various particles. The tapered angle of the bed has been found to affect the characteristics of the bed. Models based on dimensional analysis have been proposed to predict the critical fluidization velocity and maximum bed pressure drop for gas–solid tapered fluidized beds. Experimental values of critical fluidization velocity and maximum bed pressure drop compare well with that predicted by the proposed models and the average absolute errors are well within 15%.

Prediction of critical fluidization velocity and maximum bed pressure drop for binary mixture of regular particles in gas–solid tapered fluidized beds

The problems associated with conventional (cylindrical) fluidized beds, viz., fluidization of wider size range of particles, entrainment of particles and limitation of fluidization velocity could be overcome by using tapered fluidized beds. Limited work has been carried out to study the hydrodynamics of single materials with uniform size particles in tapered beds. In the present work, an attempt has been made to study the hydrodynamic characteristics of binary mixtures of homogeneous and heterogeneous regular particles (glass bead and sago) in tapered fluidized beds having different tapered angles. Correlations have been developed for critical fluidization velocity and maximum bed pressure drop for gas–solid tapered fluidized beds for binary mixtures of regular particles. Model predictions were compared with experimental data, which were in good agreement.

Phase hold-up and critical fluidization velocity in a three-phase inverse fluidized bed

We studied the hydrodynamic characteristics of a three-phase inverse fluidized bed made of a transpar- ent acrylic column of 0.115 m inner diameter and 2 m heights. Air, water and polyethylene particles were used as the gas, liquid and solid phase, respectively. We used both hydrophobic low density polyethylene (LDPE) and hydrophilic LDPE as solid phase, and distilled water as liquid phase, and filtered air as gas phase. The LDPE was chemically treated by chlorosulfonic acid to change the surface property from hydrophobic to hydrophilic. We tried to solely investigate the effect of the surface hydrophilicity of polymeric particles on the phase holdup and the critical fluidization velocity of three-phase inverse fluidization. Thus, we measured the static pressure and eventually observed critical fluidization velocity. Critical fluidization velocity became smaller in case of using MDPE hydrophobic particles than LDPE hy- drophilic particles. This was thought to be due to the retardation of rising bubbles near hydrophobic particles and, sub- sequently, the increase of gas hold-up.

Limits of stable fluidization regimes in vessel with inclined gas distributor grid

The uniformity of gas distribution, as one of the components of back-mixing, affects the driving force of mass transfer and reaction processes in commercial-size vessels. This paper examines the gas distribution in a rectangular-section vessel with an inclined grid; confirms that the angle of inclination of the grid, even a small angle, must be taken into account in calculating the critical fluidization velocity; emphasizes that for large vessels, the accuracy of fabrication and installation of the gas distributor grid is extremely important; and points out that different parts of the grid should be identical in resistance (e.g, the grid openings should be identical in diameter, and the edges should all be finished identically).

Some properties of powder-form nickel-chromium catalyst under fluidized conditions

1. Some physical characteristics of powder-form nickel-chrome catalyst have been determined. It has been shown that particles of various dimensions within the limits 0.16–0.5 mm, formed by grinding the standard pelletized catalyst, have practically identical shapes. 2. An empirical equation has been deduced, which allows calculation of the critical fluidization velocity of finely-dispersed nickel-chrome catalyst with an accuracy of ±5.5%. A method of conducting the experiment and of setting up the empirical equation has been described. 3. A simplified method of estimating the stability of fluidized catalysts for comparatively short terms of service has been recommended, according to which catalyst stability is characterized by a wear coefficient and a change of some parameter which unambiguously characterizes the hydrodynamic properties of the system. 4. The stability of nonpyrophoric nickel-chrome catalyst has been checked by the recommended method and it has been established that this catalyst, as one possessing sufficient stability under fluidizing conditions, can be proposed for use on an industrial scale.

Critical resistance and critical fluidization velocity of finely divided material in cylindro-conical apparatus

Critical resistance and critical fluidization velocity of finely divided material in cylindro-conical apparatus

Critical resistance and critical fluidization velocity of fine-grained material in conical equipment

The problem of the determination of the critical resistance and critical fluidization velocity of fine-grained material in conical equipment is investigated analytically. The relations obtained are compared with experiment.