Minimum Bubbling Velocity Research Articles

Heat transfer efficiency in gas–solid fluidized beds with flat and corrugated walls

Abstract Gas–solid fluidized bed reactors exhibit improved heat and mass transfer performance as compared to packed beds. Corrugated walls installed in narrow gas–solid bubbling fluidized bed (CWBFB) enclosures have been observed to decrease minimum bubbling velocity, reduce bubble size, improve gas distribution, provide stable operation, and minimize particle carryover or loss. Thorough analyses of the wall-to-bed heat transfer coefficient in flat- (FWBFB) and corrugated- (CWBFB) wall bubbling fluidized beds have been performed for a variety of operating conditions and geometric parameters. Fast-response self-adhesive heat flux probes and thermocouples were used to simultaneously measure the wall-to-bed heat flux, surface and bed temperatures, and were used to determine the heat transfer coefficient (HTC) at various axial and lateral locations. For a given set of parameters, a significant increase in HTC was observed at lower gas flow rates in CWBFB as compared to FWBFB. It was shown that CWBFB inventory required lower U mb (gas flow rate) as compared to FWBFB. Full 3-D transient Euler–Euler CFD simulations using the kinetic theory of granular flow were also performed, which confirmed the experimental results.

Permeability of granular mixtures under shear

Fluidised granular flows are gas–particle mixtures that occur in a wide range of industrial applications and geophysical flows. One factor governing the fluidisation behaviour of a granular flow is its permeability — the ability of the gas to move through the particle column. Although extensive research has been carried out on the gas–particle permeability under static conditions, most of the industrial and geophysical processes are dynamic phenomena, where parameters such as the shear rate change with both time and space. Here, the effects of shear rate on the permeability of gas–particle mixtures were studied through novel experiments where a granular column comprising particles of diameter 63–250 μm was sheared at shear rates between 0 and 213s−1 while an increasing flux of compressed air was introduced into the base of the column. Regardless of the shear rate, the granular column starts to expand upon increasing the air velocity, however, columns sheared at higher rates require greater air velocities to exhibit bubbling (and column expansion). We also instrumented the column with a pressure sensor. For the static column, as the air velocity increases, the pressure gradient across the granular column increases linearly until it reaches a maximum value, slightly decreases, and then plateaus. Conversely, for high shear rates, the pressure gradient continuously increases with increased air velocity, never reaching a maximum value or a plateau. The pressure gradient data were analysed alongside the videography in terms of the minimum fluidisation velocity and the minimum bubbling velocity, and both were found to be greater for higher shear rates. Consequently, our results show that increased shear increases the permeability of granular columns and the experimental data further suggests that in order to accurately describe the evolution of the pressure gradient across a sheared granular column, processes in the vertical as well as radial plane should be taken into account.

Effects of temperature on gas-solid bubbling fluidization with Geldart A particles characterized by electrical capacitance tomography and pressure drop measurements

The fluidization with Geldart A particles could be usually operated at high temperature which can significantly influence the fluidization behaviors. This paper describes the impacts of temperature on the fluidization of Geldart A particles using electrical capacitance tomography (ECT) and pressure drop measurements. Alumina particles, classified as Geldart A particles, are studied in a fluidized bed across a temperature range of 20 °C to 600 °C. As the superficial gas velocity increases, the minimum fluidization velocity and the minimum bubbling velocity are observed based on the change of solid fraction and pressure drop at high temperatures. As temperature increases, both the minimum fluidization velocity and minimum bubbling velocity decrease, and the solid fraction decreases. The trend agrees well with empirical correlations which consider the change of gas density and viscosity with temperature. Specifically, the increase in temperature leads to a higher gas viscosity, which enhances the hydrodynamic forces and significantly influences the fluidization behaviors. The broad agreement between ECT and pressure drop measurements suggests that ECT can be applied for the characterization of fluidization behaviors at high temperatures.

Study of Fluidization Behavior Transition from Geldart B to A Induced by High Temperature Using Electrical Capacitance Tomography

The transition of the fluidization behavior of Geldart B particles to that of A particles induced by temperature change was investigated by a developed high-temperature electrical capacitance tomography (ECT) sensor. Silica particles with a Sauter mean diameter of 237 μm and density of 2650 kg/m3, typically Geldart B particles under ambient conditions, were fluidized in a column of 5.5 cm from 20 to 600 °C. With the increase in temperature, ECT measurements showed a decrease in minimum bubbling velocity (Umb), no bed expansion characteristic in the homogeneous fluidization regime, an absence of multiple-bubbles regime, and a larger bubble size. The pressure drop against superficial gas velocity curves at elevated temperatures confirmed homogeneous fluidization between the minimum fluidization velocity (Umf) and Umb. Our analysis demonstrates that cohesive interparticle forces, which increase linearly with temperature, are responsible for the fluidization behavior transition of the silica particles.

Segregation phenomena of binary solids in a pulsed fluidized bed

Segregation tests were conducted using a cold-bed system in a pulsed fluidized bed with binary solid species. The effects of superficial gas velocity (U g ), pulse frequency (f), and amplitude (A) on segregation efficiency were investigated. The range of U g was 0.423–1.41 cm/s, the range of f was 0–2 Hz, and the range of A was 0.052–0.331 cm/s. At non-pulsation conditions where U g was higher than the minimum bubbling velocity (U mb ), the jetsam was more prone to be located at the top of the bed than at the middle of the bed. For the pulsed gas flow, the effect of pulsation on segregation was observed when f exceeded 1 Hz. When U g is close to U mb , as the amplitude increases, the pulsation also affects the top of the bed, thus increasing the degree of segregation. When U g was 1.43 U mb , the degree of segregation did not increase, as the pulse effect did not appear at the top of the bed. At the optimal pulse condition, the degree of segregation was increased by 1.4 times compared to that in the non-pulsation condition. • The pulse effect was observed when the frequency was higher than 1 Hz. • Segregation increased as pulse-type B changed to A at U g = 0.564 cm/s. • At U g = 0.705 cm/s, the pulse effect did not appear at the top of the bed. • Segregation increased 1.4 times with pulsed flow.

Numerical simulation of homogeneous fluidization behaviour of Geldart Group A particles in gas tapered fluidized beds

AbstractA two‐dimensional gas–solid tapered fluidized bed at laboratory‐scale in a homogenous fluidization regime is simulated using the computational fluid dynamics (CFD) software COMSOL 5.4. The Eulerian–Eulerian (EE) hydrodynamic model is used to predict the minimum fluidization velocity, minimum bubbling velocity, bed height, and solid volume fraction of alumina particles (in Geldart Group A) in the bed with different tapered angles. The effects of static bed heights on the minimum fluidization velocity of tapered beds are investigated. The highest minimum fluidization velocity is 6 mm/s with a static height of 11 mm and a tapered angle of 12°. The bed height is increased with increasing superficial air velocity. At a given air velocity, the minimum bed height is obtained at a 12° tapered angle. A decrease in solid volume fraction is also observed upon an increase in the superficial air velocity. The CFD results show an almost uniform solid concentration over the bed height. The homogenous or non‐bubbling regime appears in a wider range of critical velocity at a 12° tapered angle. In the case of a 0° tapered angle, the results of this study and the discrete element method computational fluid dynamics (DEM‐CFD) simulation exhibit a good match. In the case of 6° and 12° tapered angles, the proposed model leads to good predictions based on the theory of homogenous fluidization of Geldart Group A. This research introduces a rapid and cost‐effective approach for the better design and operation of a tapered fluidized bed in large scales.

Flow Regimes in Gas‐Solid Fluidized Beds via Micro‐Foil Heat Flux Sensor

AbstractThe flow regime transitions in a gas‐solid fluidized bed were studied for the first time using a micro‐foil heat flux sensor. The time series of heat flux signals was analyzed by statistical and state space methods. The experimental measurements were performed in a lab‐scale gas‐solid fluidization column. Four different flow regimes, i.e., fixed‐bed flow regime, bubbling‐flow regime, slugging‐flow regime, and turbulent‐flow regime, and three intermediate transition velocities, i.e., minimum fluidizing velocity or minimum bubbling velocity, minimum slugging velocity, and minimum turbulent velocity, were successfully identified. The method based on the analysis of heat flux sensor measurements allowed to determine accurately the different flow regimes and the transition velocities.

Reduction in minimum fluidization velocity and minimum bubbling velocity in gas-solid fluidized beds due to vibration

Vibration of gas-solid fluidized beds is often used to enable fluidization of cohesive particles; however, it has also been shown to reduce the minimum fluidization velocity (Umf). Here, we show via experiments that vibration can reduce both Umf and the minimum bubbling velocity (Umb) in Geldart Group B and D particles. We observe that Umf is reduced more than Umb, thereby creating a densely packed, bubble-free fluidization state when the superficial gas velocity is in between Umf and Umb. Umf and Umb decrease with increasing vibration frequency and amplitude and the results for Umf and Umb across a variety of vibration conditions could be plotted along a single curve when plotted versus vibration strength, i.e. the ratio of peak vibration acceleration to gravitational acceleration. Changes in Umf and Umb due to vibration are not affected significantly by changing particle density, but in a non-monotonic way by changing particle diameter.

A Study of the Hydrodynamics Behavior of Cylindrical Gas-Solid Fluidized Beds for pharmaceutical material “Paracetamol “

The hydrodynamics behavior of gas - solid fluidized beds is complex and it should be analyzed and understood due to its importance in the design and operating of the units. The effect of column inside diameter and static bed height on the minimum fluidization velocity, minimum bubbling velocity, fluidization index, minimum slugging velocity and slug index have been studied experimentally and theoretically for three cylindrical columns of 0.0762, 0.15 and 0.18 m inside diameters and 0.05, 0.07 and 0.09 m static bed heights .The experimental results showed that the minimum fluidization and bubbling velocities had a direct relation with column diameter and static bed height .The minimum slugging velocity had an inverse relation with static bed height and a direct one with column diameter .There was no agreement between the experimental and calculated values of Umb for Di=0.0762m ,this was a result to the assumption used in the correlation development. The fluidization index values were around 1 in all cases and that proved that the material is of Geldart type B. There was not a significant dependence of fluidization index and slug index on static bed height and column diameter. 

Establishing fluidization parameters of different size of coal ash particles in bubbling fluidized bed

The Fluidization behavior of coal ash particles was studied to establish fluidization parameters in a cylindrical bubbling fluidized bed. A series of experiments were carried out with different sizes of irregular coal ash particles ranging from 1 mm to 3 mm at different bed heights at atmospheric pressure and room temperature (ca. 28 °C). Fluidization regimes of all selected coal ash particles were studied by varying flow rates of fluidization medium i.e., air. The change in bed height and pressure drop across the bed was carefully analyzed to see the effects of particle size and superficial gas velocity during fluidization. The minimum fluidization velocity (Umf), minimum bubbling velocity (Umb) and fluidization index for different sizes of ash particles in cold conditions were calculated. The validation of experimental values was supported by theoretical calculations. It was observed that the Umb is 1.5–1.8 times higher than the Umf at different bed heights (Hbed). No substantial deviation was found in Umf with the variations of Hbed at a particular mean particle diameter (dp). Finally, the fluidization index between 1.58 and 1.78 for coal ash particles of Geldart-D type (dp> 1000 µm) shows an ideal condition for fluidization.

Effect of Fines Content on Fluidity of FCC Catalysts for Stable Operation of Fluid Catalytic Cracking Unit

Effect of fines content (weight % of particles with diameter less than 45 μm) on bed fluidity was determined to get a base for good fluidization quality in the fluid catalytic cracking (FCC) unit. The fines content in equilibrium FCC catalysts (Ecat) from commercial units were controlled by adding or removing the fines to simulate commercial situation. To get the fluidity values (Umb/Umf) of seven different FCC catalysts (2 Ecats and 5 fresh catalysts) and their mixture, minimum fluidization velocity (Umf) and minimum bubbling velocity (Umb) were measured in a fluidized bed reactor (0.05 m ID). The fluidity decreased with loss of fines content and increased with increments of makeup of fresh catalysts or additive with the controlled fines content. The fluidities of catalysts increase with increases of normalized particle diameter variation by the fines addition. The obtained fluidities have been correlated with the fines contents and the catalyst and gas properties. The proposed correlation could guide to keep good catalyst fluidity in the FCC unit.

High-temperature electrical capacitance tomography for gas–solid fluidised beds

Electrical capacitance tomography (ECT) is a non-intrusive and non-invasive imaging technique for the visualisation of material distribution, e.g. in a gas–solid fluidised bed. So far, ECT has been successfully used in gas–solid fluidised beds at ambient temperature. However, ECT has been rarely used in high-temperature gas–solid fluidised beds, which is of practical importance in many applications. Considering that most fluidised bed reactors are operated at high temperature, it is necessary to investigate the application of ECT in high-temperature industrial processes. In this work, a high-temperature ECT sensor, which can withstand 1073 K, is designed and fabricated. It has been verified by three stationary objects. The results show that the ECT sensor can give satisfactory images at different high-temperature levels and the change in temperature has little effect on the signal-to-noise ratio. It has also been found that the minimum bubbling velocity estimated by ECT is the same as that obtained by pressure drop measurements, and two fluidisation regimes, i.e. bubbling and slugging can be identified by ECT.

Flow characteristics in pressurized oxy-fuel fluidized bed under hot condition

Pressurized oxy-fuel fluidized bed (POFB) combustion is regarded as a promising technology for carbon capture from coal-fired power plants. High pressure and temperature conditions have important impacts on the flow characteristic of fluidized bed, and understanding them will help to optimize the design and operation of the POFB boiler. In this work, experiments were carried out in two pressurized fluidized bed (PFB) devices (a hot PFB and a “visual PFB”) both operated under high temperature (20–800°C) and high pressure conditions (0.1–1.0 MPa). Four parameters including the minimum fluidization velocity (umf), the minimum bubbling velocity (umb), bubble diameter (Db) and bubble frequency (f) were examined in this study. Results showed that the umf decreases with rising pressure and temperature. Based on our results a formula was fitted for calculating the minimum fluidization velocity in PFB, with a relative error less than 15%. With the increase of fluidization number (w), the bubble size and tail vortex increased gradually, the bubbles tended to merge, and the shape of bubbles became more irregular. The Db decreases with the increase of temperature and pressure at the same w. The f increases with increased w, while it decreased with the increase of temperature and pressure.

Comparison of two-fluid and discrete particle modeling of gas-particle flows in micro fluidized beds

The formation of a more porous zone in the near wall region has been commonly suggested to be one of the main mechanisms of the delayed onset of fluidization in micro fluidized beds (MFBs). If such geometrical effect still plays a major role once the bed is fluidized, it will challenge the applications of Two-Fluid Model (TFM) to simulate micro fluidized beds. In this work this issue is assessed by taking the simulation results of the Computational Fluid Dynamics coupled with the Discrete Element Method (CFD-DEM) as the benchmark data. Both the delayed onset of fluidization and the advanced onset of turbulent fluidization predicted by CFD-DEM simulations are successfully captured by the TFM simulations. The radial profiles of solids volume fraction and solids axial velocity predicted by TFM and CFD-DEM simulations are also in well agreement. Such results demonstrate that TFM can be appropriately used to model micro fluidized beds systems. Parameter analyses show that TFM simulation results are closely related to the input parameter setting. The minimum bubbling velocity predicted by TFM simulations closely depends on the chosen value of frictional packing limit. And the specularity coefficient has a significant impact on the predicted solids flow behavior.

Fluidization of Geldart D type particles in a shallow vibrated gas-fluidized bed

The fluidization behavior of a binary mixture of Geldart D type particles in a shallow vibrated gas-fluidized bed (VFB) was systematically studied by using a high-speed dynamic camera. This study drew a comparison between the fluidization of a VFB and a conventional gas-fluidized bed (CFB). The results show that the characteristic air velocities in a VFB, such as the minimum fluidization velocity, the minimum bubbling velocity and the minimum slugging velocity, have considerable reductions in comparison with that in a CFB. It is indicated that the vibration significantly changes the gas-solid two-phase flow structure and improves the dispersion uniformity of air flow in the bed. The air pressure waves, as the main transmission mode of vibration energy, can improve the air-solid contact efficiency and organize the excess fluidizing air to form periodic slugs. In a VFB of Geldart D type particles, there are four distinct fluidization regimes: static bed, bubbling fluidization, slugging fluidization and turbulent fluidization. The results show that a good performance of density segregation in a VFB of Geldart D type particles happens in the stage of gentle slugging.

Fluidization behavior in high-pressure water at temperature from ambient to supercritical

The fluidization behaviors of Geldart-A and B particles in high pressure, sub-critical and supercritical water were investigated experimentally with the conditions of pressure up to 27.2MPa and temperature up to 482°C. Minimum fluidization velocity was determined by measuring pressure drop. Minimum bubbling velocity was obtained not only by analysis of the experimental differential pressure fluctuation signals, also by the relationship between bed voidage and superficial velocity. Correlations for the minimum fluidization velocity, homogeneous bed expansion rate and the minimum bubbling velocity were proposed. Transitions of fluidization regimes: fixed–homogeneous (F-H), fixed-homogeneous-bubbling (F-H-B) and fixed-bubbling (F-B) were uncovered at temperature from ambient to supercritical. The fluidization of F-H was found when discrimination number Dn is below 1.2×104, and the fluidization of F-H-B was found when discrimination number Dn is between 1.2×104 and 6×104, the fluidization of F-B was found when discrimination number Dn is above 6×104. These results are very useful for the design of the high pressure, sub-critical and supercritical water fluidized bed.

CFD–DEM modeling of gas fluidization of fine ellipsoidal particles

Particle characteristics are important factors affecting gas fluidization. In this work, the effects of both particle size and shape on fluidization in different flow regimes are studied using the combined computational fluid dynamic–discrete element method approach. The results are first analyzed in terms of flow patterns and fluidization parameters such as pressure drop, minimum fluidization, and bubbling velocities. The results show that with particle size decreasing, agglomerates can be formed for fine ellipsoidal particles. In particular, “chain phenomenon,” a special agglomerate phenomenon exists in expanded and fluidized beds for fine prolate particles, which is caused by the van der Waals force. The minimum fluidization velocity increases exponentially with the increase of particle size, and for a given size, it shows a “W” shape with aspect ratio. A correlation is established to describe the dependence of minimum fluidization velocity on particle size and shape. Ellipsoids have much higher minimum bubbling velocities and fluidization index than spheres. © 2015 American Institute of Chemical Engineers AIChE J, 62: 62–77, 2016

New methods for flow regime identification in bubble columns and fluidized beds

New methods for flow regime identification were developed and applied to photon count time series measured in a bubble column (0.162m in ID) and fluidized bed (0.438m in ID). The signals in the bubble column (operated with an air-therminol system) were measured by means of Computed Tomography (CT), whereas the data in the fluidized bed (operated with an air-polyethylene system) were recorded by means of Nuclear Gauge Densitometry (NGD). The hidden information in the time series was extracted by means of two new parameters: entropy (bit/s) and information entropy (bit). Both of them were calculated on the basis of multiple reconstructions of the time series. In the case of the bubble column, the well-pronounced local minima were used for identification of three transition velocities (0.04, 0.08 and 0.13m/s). They distinguished the boundaries of the bubbly flow, transition and churn-turbulent flow regimes. In the case of the fluidized bed, the minimum fluidization velocity (0.086m/s) and minimum bubbling velocity (0.12m/s) were also identified on the basis of the well-pronounced local minima in the profiles of the new parameters. They distinguished the boundaries of both the transition and bubbling fluidization regimes.

Flow structure and bubble dynamics in supercritical water fluidized bed and gas fluidized bed: A comparative study

Supercritical water (SCW) fluidized bed is a new reactor concept for hydrogen production from biomass or coal gasification. In this paper, a comparative study on flow structure and bubble dynamics in a supercritical water fluidized bed and a gas fluidized bed was carried out using the discrete element method (DEM). The results show that supercritical water condition reduces the incipient fluidization velocity, changes regime transitions, i.e. a homogeneous fluidization was observed when the superficial velocity is in the range of the minimum fluidization velocity and minimum bubbling velocity even the solids behave as Geldart B powders in the gas fluidized bed. Bubbling fluidization in the supercritical water fluidized bed was formed after superficial velocity exceeds the minimum bubbling velocity, as in the gas fluidized bed. Bubble is one of the most important features in fluidized bed, which is also the emphasis in this paper. Bubble growth was effectively suppressed in the supercritical water fluidized bed, which resulted in a more uniform flow structure. By analyzing a large number of bubbles, bubble dynamic characteristics such as diameter distribution, frequency, rising path and so on, were obtained. It is found that bubble dynamic characteristics in the supercritical water fluidized bed differ a lot from that in the gas fluidized bed, and there is a better fluidization quality induced by the bubble dynamics in the supercritical water fluidized bed.

Identification of Bubbling Onset Using Pressure Fluctuations

In this work, we outline an attempt to reduce operator bias in the determination of minimum bubbling velocity, Umb. We examine the utility of two simple statistical indicators, viz standard deviation, σ, and von Neumann ratio, T, of the pressure fluctuation data, for estimation of Umb. Samples of lactose, sand, refractory dust, and glass ballotini, were fluidized under ambient conditions in an 80mm column. Normalised σ and T–1 were plotted against normalised superficial velocity, U. Plots of normalised σ versus normalised U exhibit a step change in the vicinity of Umb,v, the visual estimate of Umb, while plots of normalised T-1 versus normalised U peak sharply, close to Umb,v.