Hydrodynamics Of Gas-solid Fluidized Bed Research Articles

Effect of particle shape on the hydrodynamics of gas-solid fluidized bed

Non-spherical particles are common in industrial applications such as coal gasification and biomass combustion. The present work employs a coupled Computational Fluid Dynamics – Discrete Element Method (CFD-DEM) approach to numerically investigate the particle shape effect on the hydrodynamics of a rectangular gas-solid fluidized bed. The CFD-DEM model capability is tested by comparing the mean horizontal and vertical velocities and pressure drop against the National Energy Technology Laboratory (NETL) experimental data. The effect of different numerical parameters (time step, grid size, discretization technique) on the predicted data is also studied. A grid size of three particle diameters and a 2nd Superbee discretization scheme are selected to run the simulations based on the numerical accuracy and simulation time. Seven different drag models are utilized to model the particle-fluid interaction. Further, simulations are conducted using the Di Felice-Ganser drag model for five different sphericities. A strong relationship between sphericity and fluidization behavior is noted. At the constant superficial gas velocity of 2.19 m/s, the fluidization behavior is changed from bubbling to a turbulent regime when the sphericity decreases from 1 to 0.42.

Experimental and numerical investigations on the hydrodynamics of gas-solid fluidized bed with an inclined agitator

The effect of inclined agitation on the fluidization quality of a gas-solid fluidized bed is experimentally and numerically investigated. The pressure characteristics are characterized by the standard deviation of pressure drop and power spectra. The particle velocity, solid volume fraction and granular temperature distributions are investigated by numerical modeling. It is found that inclined agitation enhances the gross circulation of solid particles and enlarges the affect region of the agitator, improving the gas-solid fluidization quality. The increase in the agitation speed leads to higher frequency of collisions and more activity of particles, resulting in decreases of bubble sizes and a more homogeneous solid volume fraction distribution. Wide-blade agitator can also improve the fluidization performance.

Effect of electrostatic charge of particles on hydrodynamics of gas-solid fluidized beds

The aim of this work was to investigate effect of electrostatic charge of particles on the fluidization hydrodynamics. Behavior of bubbles in beds of polyethylene particles was studied through analysis of pressure fluctuations in the frequency domain. Fluidized beds of uncharged, pre-charged and bed-charged particles were used in the experiments. Results revealed that in the bed of pre-charged particles, compared to uncharged experiments, particle-particle repulsive force increases the bed voidage and reduces equilibrium bubble size while the transition velocity to turbulent fluidization is decreased. In the case of bed-charged particles, at low gas velocities bubble fraction is greater compare to the other cases due to faster bubble coalescence in the presence of particle-wall attractive electrostatic force. Electrostatic charge of bulk increases by increasing the gas velocity. At high gas velocities, the repulsion force between highly charged particles overcomes the particle-wall effect on bubble formation and reduces the bubble size to less than in uncharged experiments. Accumulation of particles near the wall in the bed od bed-charged particles affects the hydrodynamics in two ways: first it accelerates bubble growth via bubble coalescence at low gas velocities, second it limits the bubble growth and reduces the transition velocity to turbulent regime to a value less than for pre-charged particles.

CFD-DEM Investigation on van der Waals Force in Gas-Solid Bubbling Fluidized Beds

Effect of interparticle force on the hydrodynamics of gas-solid fluidized beds was investigated using the combined method of computational fluid dynamics and discrete element method (CFD-DEM). The cohesive force between particles was considered to follow the van der Waals equation form. The model was validated by experimental results from literature in terms of bed voidage distribution and Eulerian solid velocity field. The results revealed that the incorporated model can satisfactorily predict the hydrodynamics of the fluidized bed in the presence of interparticle forces. Effect of interparticle force on the bubble rise characteristics, such as bubble stability, bubble diameter and bubble velocity, was investigated. It was shown that the emulsion voidage increases with increasing the interparticle force in the bed and it can hold more gas inside its structure. In addition, by increasing the interparticle force, size of bubbles and rise velocity of bubbles increase while the average velocity of particles decreases.

Investigation of hydrodynamics of gas-solid fluidized beds using cross recurrence quantification analysis

Hydrodynamics of gas-solid fluidized bed was investigated by analyzing its pressure fluctuations using cross recurrence quantification analysis (CRQA). Pressure fluctuations were measured in a lab scale fluidized bed of various particle sizes at different gas velocities. First, the CRQA was applied to a number of well-known dynamic systems and the results demonstrated that it is a powerful method to detect similarities between nonlinear signals. Then, it was shown that graphical structures within the cross recurrence plot of pressure fluctuations of a fluidized bed vary with both superficial gas velocity and particle size. It was found that determinism and cross recurrence rate of non-normalized data initially decrease and then increase with increasing the gas velocity. When the signal is initially normalized, determinism and entropy do not change with the superficial gas velocity while cross recurrence rate is sensible to changes in the superficial gas velocity. It was concluded that entropy can be used for detecting changes of particle size and if a proper reference state is chosen, entropy can be a powerful index for detecting changes in the size of particles in a fluidized bed.

Investigating effect of pulsed flow on hydrodynamics of gas-solid fluidized bed using two-fluid model simulation and experiment

Gas-solid flow behavior in a pulsed fluidized bed was studied experimentally and numerically. Eulerian-Eulerian two-fluid model approach in conjunction with the kinetic theory of granular flow was used for simulation of dense gas-solid flow. The effect of pulsating flow with frequency range of 1 to 10Hz for particle sizes of Geldart B and A/B group with variation of particle density was investigated. Model simulation results were compared reasonably well with researcher's findings and the experimental data. Time-averaged local pressure drop and solid volume fraction were obtained and the effect of various pulsation frequencies on the bed expansion ratio, solid volume fraction, voidage and solid axial velocity was discussed. By increasing the pulsation frequency up to 10Hz, results show reduction in the bubble size, bed expansion ratio and pressure fluctuation. Time-averaged axial velocity and voidage have a flatter profile with increasing pulsation frequency.

Online Detection of Hydrodynamic Changes in Fluidized Bed using Cross Average Diagonal Line

Online detection of hydrodynamics of gas-solid fluidized bed was characterized using pressure fluctuations by cross recurrence plot (CRP) and cross recurrence quantification analysis (CRQA). Experiments were conducted in a lab scale fluidized bed of various particle sizes 150 μm, 280 μm and 490 μm at different gas velocities. Firstly, pattern changes of cross recurrence plot were discussed and then reference states was selected. Afterwards, cross average diagonal line (CLave) of other states corresponding to reference states were obtained. It was found that cross average diagonal line of non-normalized data initially decreases and then increases with increasing the gas velocity. When the signal is initially normalized, cross average diagonal line does not change with the superficial gas velocity. It was concluded that cross average diagonal line could be used for detecting small changes of particle size and if a proper reference state is chosen, it can be perceived as a powerful index for detecting changes in the size of particles in a fluidized bed.

A CFD-DEM study of single bubble formation in gas fluidization of spherical and non-spherical particles

Bubble dynamics significantly affect the hydrodynamics of gas-solid fluidized bed since they influence the gas-solid mixing. In this study, simulations using CFD-DEM were carried out to characterize the bubble size and shape for a bubble formed at a single orifice in gas-solid fluidized bed. Impact of parameters such as jet velocity, orifice size and particle shape on bubble equivalent diameter and bubble aspect ratio were analysed and discussed. Bubble equivalent diameter was found to increase with increasing jet velocity, decreasing bed width to orifice width ratio, and particle shape deviating from spherical. The bubble shape illustrated by aspect ratio, was found to elongate more as it rise through the bed and then commence to expand horizontally after it was detached from the orifice. Aspect ratio was found to be closer to a circle for the bubble at higher jet velocity, lower orifice width to bed ratio and for non-spherical particles.

Effect of changes in particle size on the hydrodynamics of gas-solid fluidized beds through wall vibration

Vibration signatures of a wall of a bubbling fluidized bed were measured in order to investigate the effect of change in particle size on fluidized bed hydrodynamics. Vibration signatures were analyzed using wavelet transform and decomposed into three scales: micro-, meso- and macro-scales. Principal component analysis (PCA) was employed to gain a deeper insight into the dynamic behaviour of each sub scale. Bubble and emulsion phases were distinguished by the PCA as first and second components.

Hydrodynamics Study in a fluidized bed reactor Using Gamma Ray Computed Tomography (CT) Technique

The main goal of this study is to further understand the fluidized bed hydrodynamics by determining the effects of superficial gas velocity and bed density on the hydrodynamics of gas-solid fluidized beds by using a 14-cm-diameter Plexiglas cylindrical fluidized bed column, air at room temperature was used as the fluidizing gas, two different Geld art type-B particles were used glass bead and copper particle with material densities of 2500, 5300 Kg/m3 respectively with the same size particle 210 µm. Three different superficial gas velocities were investigated Ug=25, 30, and 35 cm/s. Non-invasive flow measurement technique gamma ray computed tomography (CT) has been used to measure the time-averaged cross-sectional and radial profiles of solids and gas holdups distribution. The results show that local time-averaged gas holdup is a function of bed material density, that as material density decreases, gas holdup increases, gas holdup increases with increasing superficial gas velocities

Hydrodynamic studies in fluidized bed with internals and modeling using ANN and ANFIS

The hydrodynamics of gas-solid fluidized beds were studied covering a range of hydrodynamic parameters such as inlet air flow rate, bed height, spacing of internal and column diameter with a particular percentage open area of internal. Cross type internals with constant percentage open area were used. The influence of the above mentioned parameters on pressure drop, expansion ratio (R), fluctuation ratio (r), minimum fluidization velocity and Fluidization index (FI) were studied. The pressure drop was found to increase with increase in bed height regardless of presence or absence of internals. But in presence of internals (more numbers or less spacing) pressure drop initially decreases but beyond certain level the resistance offered by internals is more and it caused slight increase in pressure drop. The expansion ratio reduces significantly with decrease in the spacing of internals. The effect of internals on quality of fluidization has been studied. By the introduction of internals, the quality of fluidization in terms of FI had improved. Fluctuation ratio was found to reduce with the use of internals and also decrease with decrease in internal spacing. Correlations for pressure drop and expansion ratio have been developed using significant dimensionless groups. Correlated values of pressure drop and expansion ratio have been compared with their respective experimental values and were found to be in good agreement. The collected experimental data was used in the development of a model using Expert systems such as Artificial Neural Networks (ANN) and Adaptive Neuro-Fuzzy Inference Systems (ANFIS) and a comparison was made. Error analysis made for both ANFIS and ANN shows that ANFIS has better ability to model under non-linearity conditions over ANN.

Fluid Dynamic Study of the Fluidized Bed Wit Jambu Seeds (<i>Sphilantes oleracea</i> L.)

This work has as objective the study experimental and computational of the hydrodynamics of gas-solid fluidized bed. The gas-solid fluidized beds present important applications in chemical industry; among typical examples of fluidized beds some applications could be outlined: the syntheses and catalytic reactions, the catalytic regeneration, the combustion and gasification of coal, coating, drying, etc. The simulated data were obtained through the commercial CFD (Computational fluid dynamics), and the results were compared to those obtained by experiments conducted in a fluidized bed, containing jambu seeds (Sphilantes oleracea L.) of 772.21  2.98 μm in diameter, sphericity de 0.45  0.0066, bulk density 1130.8 ± 0.22 Kg/m3 and with porosity 0.2303  0.0099. In this work the porosity profile and characteristic curve of fluidized bed were calculated by using an Eulerian multiphase model. The comparisons showed a very good agreement between experimental and simulation results.

Effect of Bed Diameter on the Hydrodynamics of Gas-Solid Fluidized Beds

Effect of scale on the hydrodynamics of gas-solid fluidized beds was investigated in two fluidized beds of 152 mm and 78 mm in diameter. Air at room temperature was used as the fluidizing gas in the bed of sand particles. The Radioactive Particle Tracking (RPT) technique was employed to obtain the instantaneous positions of the particles at every 20 ms of the experiments. These data were used to calculate hydrodynamic parameters, such as mean velocity of upward and downward-moving particles, jump frequency, cycle frequency and axial and radial diffusivities, which are representative of solid mixing and diffusion of particles in the bed. These hydrodynamic parameters were compared in both scales in order to determine the scale effect on the hydrodynamics of the gas-solid fluidized bed. In all cases, it was shown that solid mixing and diffusivity of particles increase by increasing column diameter. The results of this study would help to understand solid mixing which might be critical in industrial fluidized bed reactors.

Analogous description of the hydrodynamics of gas-solid fluidized beds and bubble columns

In this paper we stress analogies in the hydrodynamic behaviour of gassolid fluidized beds and bubble columns. Using published experimental data, it is demonstrated that the analogous hydrodynamic-behaviour is not only qualitative but also quantitative in nature. Specifically, we show the following. (1) The gas holdup in the homogeneous regimes of bubble columns and fluidized beds can be modelled in a unified way using V slip = υ ∞(1 − ϵ d) n−1 , where V slip refers to the slip velocity between the dispersed (bubbles or particles) and continuous phases and ϵ d the dispersed phase holdup. The Richardson-Zaki exponent n decreases with increasing gas density. (2) The transition from homogeneous to heterogeneous flow regimes in gasliquid bubble columns and gassolid fluid beds is delayed by increasing system pressure. Extrapolation of the influence of increased gas density allows us to consider liquidliquid dispersions and liquidsolid fluid beds as limiting cases. (3) In the heterogeneous flow regime of operation the classic two-phase theory of fluidized beds can be applied with profit to also describe the hydrodynamics of gasliquid bubble columns provided that the “dilute” phase is identified with the fast-rising large bubbles and the “dense” phase is identified with the liquid phase containing entrained “small” bubbles. Tentative analogies can also be drawn for the interphase mass transfer processes. (4) The “dense” phase backmixing can be modelled in a unified manner. (5) The two-phase theory can be extended to describe slurry reactors. It is argued that, because of cross-fertilization of concepts and information, appreciation of analogies can be invaluable tool in scaling up.

Review of experimental methods for studying the hydrodynamics of gas-solid fluidized beds

A state-of-the-art review of experimental methods for studying the hydrodynamics of gas-solids fluidization is presented. Techniques applied by various investigators to obtain statistically representative properties of bubble behavior and particle mixing in the dense region and particle dynamics in the region above the bed surface are described. Measurement principles, descriptions of the experimental setups, and accuracy of the measurements are presented. Guidelines for measurement system selection are given.