Excess Gas Velocity Research Articles

Bubble dynamics in a binary Gas-Solid fluidization system of Geldart B and Geldart D particles

In order to gain a more fundamental understanding of binary particle systems, a two-dimensional (2D) fluidized bed was used to study the bubble dynamics and dense phase composition. Digital image analysis (DIA) was employed to measure bubble diameter, bubble rise velocity, bed expansion, and the distribution of the two types of particles in the bed. Magnetite and sand particles as the single components and binary mixtures were used as the bed materials. The bubble diameter and bubble rise velocity both increased with the distance above the gas distributor and the excess gas velocity. The bubble rise velocity also increased while bubbles ascending in the bed due to an increase in bubble size and buoyancy. Both mono particle systems had higher average bubble sizes than that of the system with nearly identical particle composition, which had the smallest bubble size and the lowest bubble rise velocity. The bubble coalescence and splitting processes were also imaged and the total bubble volume was found to increase after coalescence and decrease after splitting due to some uncaptured smaller bubbles. In addition, a preliminary experiment was carried out to trace dense the phase composition and the overall bed density was estimated based on the dense phase composition and bed expansion.

Bubble dynamics in 2‐D gas–solid fluidized bed with Geldart A or Geldart B particles by image processing method

AbstractGas bubbles in a two‐dimensional gas–solid fluidized bed were identified using an image processing method to characterize the bubble dynamics in a binary fluidization system. The magnetite, sand (two types), and fluid catalytic cracking (FCC) catalyst particles belonging to Geldart groups A and B were used as the particulate materials. Image processing and MATLAB were applied for bubble size and velocity measurements. Bubble properties and bed expansion in fluidized beds of those four types of particles were studied. Bubble size and bubble rise velocity were found to increase with the distance above the gas distributor and excess gas velocity. Geldart A particles have a higher bed expansion than Geldart B particles due to the large number of small bubbles in the bed. A new empirical correlation for estimating bubble diameter along the bed height was proposed, giving good agreement with the experimental data.

Modeling the motion of fuel particles in a fluidized bed

A semiempirical model for the mixing of fuel particles in a fluidized bed is presented and validated against experimental data from the literature regarding lateral fuel mixing. The model of fuel particle mixing categorizes the fluidized bed into three mixing zones: a rising bubble wake solid zone, an emulsion zone with sinking bulk solids, and a splash zone located above the dense bed. In the emulsion zone, the axial motion of the fuel particle is described by a force balance, applying a viscoplastic stress model, i.e., with a dominant yield stress and only a minor contribution of the shear stress, using an empirical expression from the literature. In the lateral direction, the model is divided into so-called ‘recirculation cells’, which are crucial for the lateral mixing.Comparisons of the modeled and measured lateral dispersion coefficients of different fuel types measured in three different large-scale fluidized bed units under both hot and cold conditions (covering a broad range of coefficients: 10−4–10−1 m2/s) reveal satisfactory agreement. The validated model was used to investigate how the lateral mixing of fuel particles depends on the excess gas velocity, the bed height, and the lateral distribution of bubbles over the bed cross-section (which is typically uneven in industrial FB furnaces), as well as the size and density of the fuel particles.

Mixing and segregation behavior in an air dense medium fluidized bed with binary mixtures for dry coal beneficiation

Particle mixing and segregation behavior in an Air Dense Medium Fluidized Bed with binary mixtures of solid particles were investigated for dry coal beneficiation. Magnetite mixed with coal/gangue/sand particles belonging to Geldart B/D group were tested individually for the bed density adjustment. The effects of operating parameters including particle density ratio, particle size ratio, mixture composition, superficial gas velocity, and fluidized bed height on the mixing and segregation pattern were examined. The results demonstrated that the segregation becomes more severe with increasing density difference of binary mixtures. An increase in particle size ratio may also lead to partial segregation. Mixing and segregation of binary systems are almost independent of lower excess gas velocity and initial bed height when it is over 15 cm. Moreover, a mixing index was employed to evaluate the mixing and segregation performance, and the criteria for good mixing to achieve the bed density adjustment were identified.

Quantification of Solid Mixing in Bubbling Fluidized Beds via Two-Fluid Model Simulations

The influence of particle density and fluidization velocity on the mixing behavior of Geldart B particles was investigated in bubbling fluidized beds for various operating conditions using a two-fluid model approach. The simulation results proved that the predicted solid flow pattern for a wide range of Geldart B particles differs from particles of Geldart B/D classification at identical fluidization numbers. Quantitative analysis of the predicted solid velocity variances revealed that the solid axial dispersion rate is higher than the lateral one. In detail, a Peclet number of 1.54 and 3.92 was predicted for axial and lateral mixings, respectively. Also, the solid dispersion and diffusion coefficients were linearly correlated with the excess gas velocity for a range of particle properties. Finally, the characteristic global particle mixing times were computed. Thereby, the characteristic mixing time can be correlated with the particle Froude number, expanded bed height, and solid dispersion coefficient with high accuracy.

Investigation of fluidized bed behaviour using electrical capacitance tomography

AbstractThe temporal and cross‐sectional distributions of particles in a 127 mm diameter fluidized bed have been obtained using a new generation, high‐speed electrical capacitance tomography. Two planes of eight electrodes were used and mounted at 160 and 660 mm from the gas distributor which was a 3 mm thick porous plastic plate (maximum pore size of 50‐70 μm). 3 mm diameter, nearly‐spherical polyethylene granules made up the bed. Experiments at sampling frequencies of 200‐2000 cross‐sections per second and gas superficial velocities from just below the minimum fluidization to 83% above minimum fluidization velocities were used. The time series of the cross‐sectional average void fractions have been examined both directly and in amplitude and frequency space. The last two used probability density functions and power spectral densities. The information gathered shows that the fluidized bed was operating in the slugging mode, which is not surprising given the size of the particles. It has been found that an increase in the excess gas velocity above the minimum fluidization velocity resulted in an increase in the mean void fraction, an increase in the length and velocity of the slug bubbles as well as the bed height, and a slight decrease in the slug frequency. The results are presented in a level of detail suitable for comparison with later numerical simulation.

Experimental evaluation of the convection heat transfer coefficient of large particles moving freely in a fluidized bed reactor

Heat transfer is a key aspect in the performance of fluidized bed reactors. In particular, the dense phase heat transfer coefficient, which determines the heat exchanged between the bed and fuel particles, is of special relevance. This work presents a study on the heat transfer that occur when introducing pelletized particles of dry ice in a fluidized bed. The convection heat transfer coefficient is experimentally evaluated by means of the sublimation of dry ice particles using a novel methodology, which consists in a measurement system based on a macro-TGA fluidized bed and the corresponding evaluation of the acquired data. The system is capable of measuring the real time mass of the fluidized bed, which is a direct measure of the mass loss of the dry ice particles. The evaluation of such measurements allows to determine the convection heat transfer coefficient between the dry ice particles and the bed. Fixed and fluidized bed regimes, varying the excess gas velocity from -0.03 m/s up to 0.35 m/s, were tested for two different bed materials, namely Ballotini glass beads and alumina. The results of the sublimation and heat transfer coefficients are in good agreement with values reported in the literature, confirming the reliability of the experimental and postprocessing methodologies. In general, higher gas velocities enhance the dry ice particles mixing in the bed up to a limit in which the sublimation and convection heat transfer coefficients do not further increase. Additionally, the influence of the buoyancy behaviour of the dry ice particles in the bed on the convection heat transfer coefficient is discussed. The convection heat transfer coefficient decreases substantially if the dry ice particles present a flotsam behaviour and the gas velocity is not sufficiently high to ensure a proper circulation of the particles in the whole bed.

Heat transfer in a pressurized fluidized bed with continuous addition of fines

The goal of this work was to investigate the impact of pressure and the presence of fine particles (i.e., a surrogate for pulverized fuel) on the overall surface-to-bed heat transfer coefficient in relation to an oxygen-fired pressurized fluidized bed combustor. Experiments were conducted in a pilot-scale fluidized bed with an inner diameter of 0.15 m under cold flow conditions. A tube bundle consisting of five horizontal staggered rows was completely submerged in the bed. One of the tubes was replaced by a heating cartridge housed in a hollowed copper rod. Five thermocouples distributed at 45° intervals along the copper rod circumference measured the surface temperature and ensured that local effects were included. The bed material was glass beads of 1.0 mm in diameter while the fines were glass beads of 60 μm in diameter and thus susceptible to entrainment. The fine particles were continuously fed to the fluidized bed and then captured downstream by a filter system. Fluidization was conducted at 101, 600 and 1200 kPa (abs) with excess gas velocities (Ug-- Umf) of 0.21, 0.29 and 0.51 m/s. Fine particle feed rates were 0, 9.5 and 14.4 kg/h. Two heating rod positions (2nd row and 4th row) were studied. Overall, the heat transfer coefficient approximately doubled when pressure was increased from 101 to 1200 kPa. At atmospheric conditions, where the slug flow regime occurred, the maximum heat transfer coefficient was at the bottom of the rod, while it moved to the side of the rod at high pressures where the bubbling regime occurred. When the heating rod was moved from the 2nd row to the 4th row, the averaged heat transfer coefficient increased by 18%, 9% and 6% at 101, 600 and 1200 kPa. The addition of fine particles decreased the average heat transfer coefficient by 10 to 20 W/m2 K, where the averaged coefficients were approximately 220 and 450 W/m2 at 101 and 1200 kPa respectively, but there was no effect on the angular heat transfer profile around the tube surface. The results showed that average heat transfer coefficients matched the correlation developed by Molerus et al. (1995) within a 5% difference across all conditions when fines were not present.

The distribution of bed density in an air dense medium fluidized bed with single and binary mixtures of Geldart B and/or D particles

The distribution of bed density in an Air Dense Medium Fluidized Bed (ADMFB) with single and binary mixtures of solid particles was investigated for dry coal beneficiation. The effects of particle properties, superficial gas velocity, and mixture composition on the axial distribution of fluidized bed density were examined. A lower density region at the bottom of the fluidized bed with single particles has been observed, whereas the bed density at the upper part remains almost consistent. The increasing excess gas velocity does not change this trend although decreasing the overall bed density, however the binary mixtures of solid particles can be used to balance this non-uniform density distribution. A new equation has been derived to predict the bed density distribution, considering particle properties and fluidization characteristics. This correlation successfully accounts for the estimation of density distribution in ADMFB involving both single and binary mixtures of Geldart B and/or D particles.

On the two-phase theory of fluidization for Geldart B and D particles

Knowledge of the division of gas flow between the bubble and dense phases is important for modelling and operation of bubbling fluidized beds. The two-phase theory of fluidization, which suggested that the bubble flow rate being equal to the excess gas flow above the incipient fluidization, has been proved to be an overestimation in most cases. While the two-phase theory has been modified by introducing a correction factor (Y), most previous studies were conducted for Geldart Group A powders. In the present work, the contribution to predict the parameter Y for Geldart Group B and D particles has been formulated based on almost all the available experimental data. The experimental results demonstrated that the Y value increases with decreasing particle size or density and increasing excess gas velocity. A new correlation has been developed to estimate the Y value for Geldart Group B and D particlesY=1.72Ar−0.133Ug−Umf0.024with an overall standard deviation of 19%. It only requires the knowledge of Archimedes number and excess gas velocity. This correlation is in reasonable agreement with almost all the available data in literature and the present work.

Fluidization of electrically charged particles

Fluidization hydrodynamics of uncharged, bed-charged and pre-charged particles was studied experimentally. Charge accumulation in the bed-charged case exhibits a linear trend against the excess gas velocity, regardless of the particle sizes. By charge accumulation on particles, minimum fluidization velocity increases while transition velocity to turbulent fluidization decreases. In the case of pre-charged particles, minimum fluidization velocity is higher than that of bed-charged particles and their transition velocity is higher. These trends can be explained with electrostatic forces which affects the bed voidage and bubble formations. The developed model confirms the effect of initial charge of particles on the bed voidage.

Dry coal beneficiation by the semi-industrial Air Dense Medium Fluidized Bed with binary mixtures of magnetite and fine coal particles

Air Dense Medium Fluidized Bed (ADMFB) is deemed to be one of the most efficient methods for dry coal beneficiation. In the present work, a semi-industrial ADMFB system in continuous operation was utilized to study the effects of operating gas velocity, feed coal size, and mixture composition of medium particles on the coal beneficiation in industrial practice. Binary mixtures of magnetite and fine coal particles were used as the medium material, and four different feed coal samples with the size ranges of −50 + 25, −25 + 13, −13 − 6, and −6 + 2 mm were tested individually. The experimental results showed that the influence of excess gas velocity on the dry coal separation is relatively small in the lower flow rates. The separation density and probable error increase with the decreasing of feed coal size, regardless of the type of feed coal. The separation density can be continually reduced by further increasing the fraction of fine coal in the medium material, with the compromise of the increased probable error. Moreover, the ash content and calorific value of −50 + 6 mm coarse coal can be effectively upgraded, but the beneficiation of −6 + 2 mm fine coal was less efficient.

Pyrolysis of Cynara cardunculus L. samples – Effect of operating conditions and bed stage on the evolution of the conversion

The effect of different parameters on the pyrolysis of Cynara cardunculus L. was studied through an innovative technique based on a precision scale, capable of measuring the time evolution of the biomass samples mass during their thermochemical conversion process while moving freely inside a fluidized bed. A silica sand bed reactor, operated under different values of excess gas velocity and reactor temperature, was employed to hold the pyrolysis reaction of cardoon particles of three different size ranges. The pyrolysis was accelerated for higher excess gas velocities, obtaining pyrolysis times as short as 17.3 s for experiments conducted under bubbling fluidized bed regimes, compared to 185.9 s required to complete the pyrolysis of the same sample in a fixed bed configuration. Similarly, the effect of increasing the reactor temperature promoted faster heating rates across the fuel samples, especially under fixed bed configurations, for which the pyrolysis time is reduced from 321.7 s to 132.0 s when increasing the bed temperature from 450 to 650 °C. Regarding the biomass particle size, small sizes are preferred to minimize the conduction thermal resistance inside the fuel particles and, thus, reduce pyrolysis times and increase volatile yields for the pyrolysis in a bubbling fluidized bed reactor. The opposite result was found when the pyrolysis took place in non-bubbling beds, where the use of larger particles is beneficial to accelerate the biomass pyrolysis reaction.

Borescopy in pressurized gas‐solid fluidized beds

A borescopic technique was used for finding the effect of pressure on the hydrodynamics of gas‐solid fluidized beds. The results showed that solids radial distribution may become more or less uniform with increasing pressure depending on the superficial gas velocity. Moreover, it is found that the solids volume fraction of the emulsion phase may decrease at relatively high pressures, only in the central region of the bed. Additionally, it is observed that with increasing pressure the bubble size generally decreased in the central regions and increased near the wall regions. This trend was more complicated at low excess gas velocities. The number of bubbles increased for the central regions and near the walls for all the performed experiments. However, this parameter showed a different trend at other radial positions. © 2018 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 64: 3303–3311, 2018

Some general aspects of a gas-solid fluidized bed using digital image analysis

Digital image analysis (DIA) was used to achieve some information related to aspect ratio of the bubbles, bed expansion fluctuations and the emulsion area at different gas velocities and different bed heights. All experiments were done in a pseudo-2D gas-solid fluidized bed. Variations of the bubble fraction at different gas velocities and different bed heights were investigated. It was found that when the excess gas velocity increases, the dimensionless bed height increases linearly with slope of about 0.4 for LLDPE particles. To validate the analysis, the bubble diameter achieved by DIA results in this work was compared with the bubble diameter correlation presented by Shen et al. Bubble aspect ratio in different heights of the bed was extracted through image analysis, and it was observed that the bubble aspect ratio first increases with the bed height and near the freeboard becomes flattened. Results of the experiments show that the emulsion phase becomes more expanded at higher excess gas velocities. As demonstrated by the analysis results, by increasing the bed height, bubble fraction parameter is decreased.

The heat transfer between an immersed surface of moving lignite and small particles in a fluidized bed equipped with an inclined slotted distributor

In the process of fluidized bed combustion, the particle-convective component (hpc) can evidently affect the overall heat transfer coefficient (ho). Therefore, a vigorous mixing of fuel that enhances heat transfer is required. The use of an inclined slotted distributor (IS distributor) is a promising technique to improve the gas-particles contact and particle dispersion. In the present study, experiments were carried out in a fluidized bed equipped with an IS distributor, and the surface-renewal theory was used to investigate the heat transfer between an immersed surface of moving lignite and small particles. The effects of excess gas velocity (Ue), immersed surface diameter (dI), and small particle size (dp) on ho were discussed, and a comparison with the conventional fluidized bed was made. The test results confirm that the use of a bed equipped with an IS distributor is effective to enhance the heat transfer performance when compared to a conventional bed. Based on the test results obtained at Umf and different Ue, the parameters of the surface-renewal theory were determined, and the correlations were established for predicting ho. A good agreement between the predictions and test results could be achieved when the radiative component (hr) is negligible.

Influence of particle size distribution on minimum fluidization velocity and bed expansion at elevated pressure

This paper presents an experimental study on the effects of elevated pressure and particle size distribution (PSD) on the minimum fluidization velocity (Umf) and the bed expansion. It was developed with Geldart B and D-type polystyrene particles. Four wide PSDs and five narrow PSDs were investigated. The four wide PSDs studied included a Gaussian distribution, a flat distribution, a ternary mixture, and a binary mixture, all with the same mean diameters. Operation pressure ranged from 0.1MPa (absolute pressure) to 2.7MPa. The results revealed that the Umfs of the wide PSDs were lower than that of the reference narrow PSD with the same diameter at pressures exceeding 0.3MPa. The segregation tendency of wide PSDs decreases with the increase in the operating pressure. The four wide PSDs had obviously different Umfs at pressures below 0.9MPa; however, this difference in the Umfs was gradually reducing with a further increase in the pressure (>0.9MPa). The bed expansion increased with increasing the pressure at same excess fluidized gas velocity. Correlations basing on experimental data were proposed in order to predict the Umf and bed expansion for the narrow PSD at elevated pressures, and three correlations were investigated for predicting the Umf of wide PSD.

On the internal solids circulation rates in freely-bubbling gas-solid fluidized beds

The solids mass flux distribution and internal solids circulation rates in freely-bubbling gas-solid fluidized beds has been studied in detail in a pseudo-2D column. A non-invasive Particle Image Velocimetry (PIV) combined with Digital Image Analysis (DIA) technique has been further extended to investigate and quantify the gas and solids phase properties simultaneously for different particle types and sizes (all Geldart B type) at different fluidization velocities. It is found that the solids fluxes increase strongly, practically linearly, as a function of the vertical position and depend on the excess gas velocity but not on the particle size, while the most often used phenomenological two-phase fluidized bed models assume the vertical solids fluxes to be constant. To further investigate this important discrepancy, the underlying assumptions of the phenomenological models have been validated, especially concerning the average solids fraction inside the bubbles, the laterally and time-averaged axial bubble fraction profile (or visual bubble flow rate) and the wake parameter (the amount of solids carried along a bubble relative to the bubble volume). To this end, the PIV/DIA technique was further extended and a new method for the determination of the wake parameter is proposed. From the experimental results, it was concluded that i) the average solids fraction inside the bubbles is about 2.5–3% for glass beads and alumina particles and is practically independent of the excess gas velocity and particle size; ii) the measured laterally and time-averaged bubble fractions are considerably lower compared to often used correlations from literature, which would lead to a significant over-prediction of the visual bubble flow rate and iii) the wake parameter depends strongly on the bubble size and with the developed correlation the axial solids mass fluxes as a function of the vertical position can be well described. Finally, the influence of these findings was evaluated by performing a sensitivity analysis with an existing phenomenological model for fluidized beds with the new values and closures considering the case of the heterogeneously catalyzed steam methane reforming. With the developed findings and correlations the predictions with the two-phase phenomenological models can be further improved, especially concerning the hydrodynamics of the solids phase.

Bubbling and Slugging of Geldart Group A Powders in Small Diameter Columns

Bubbling and slugging were investigated in a 0.05 m internal diameter bed of Geldart group A powders. The slugging characteristics were assessed in terms of the operating parameters. Neither bed height nor particle properties affect the excess gas velocities at the onset of slugging, as predicted by the Stewart criterion. Slugging characteristics (length and frequency) and fluidized bed properties were determined. The bottom zone of the bed is freely bubbling, and the bubble sizes can be predicted by dB = 0.047H0.37, where dB is the frontal diameter of the bubble and H is the fluidized bed height. Mostly, wall slugs occur higher in the bed when dB exceeds about 50% of the column diameter. The slug frequency varies with the height in the bed but reaches an equilibrium value of 1 Hz for bed heights exceeding ∼1 m. Three distinct zones appear in a deep bed of group A powders. The design and scale-up should combine the bubbling and slugging modes within their corresponding regions in the bed.

Monitoring of particle motions in gas-solid fluidized beds by electrostatic sensors

Gas-solid fluidized beds are widely applied in numerous industrial processes. Particle motions significantly affect the performance of fluidized bed reactors and the characterization of particle movements is therefore important for fluidization quality monitoring and scale-up of reactors. Electrostatic charge signals in the fluidized bed contain much dynamic information on particle motions, which are poorly understood and explored. In this work, correlation velocities of Geldart B and D particles were measured, analyzed and compared by induced electrostatic sensors combined with cross-correlation method in the fluidized bed. The results indicated that the average correlation velocity of particle clouds increased and the normalized probability density distributions of correlation velocities broadened when the superficial gas velocity increased in the dense-phase region. Both upward and downward correlation velocities could be acquired in the dynamic bed level region. Under the same excess gas velocity, the average correlation velocity of Geldart D particles was significantly smaller than that of Geldart B particles, which was caused by the smaller bubble sizes caused by the dominant bubble split over coalescence and less volume of gas forming bubbles for Geldart D particles. The experimental results verified the reliability and repeatability of particle correlation velocity measurement by induced electrostatic sensors in the gas-solid fluidized bed, which provides definite potential in monitoring of particle motions.