Fluidizing Gas Velocity Research Articles

Characteristic gas velocity and fluidization quality evaluation of vibrated dense medium fluidized bed for fine coal separation

In order to remove incombustible impurity minerals, a vibrated dense medium fluidized bed (VDMFB) can be adopted for fine coal dry separation, while Geldart B magnetite powder can serve as the medium. The influence of vibration on characteristic gas velocity in flow pattern transition stage was experimentally investigated at a vibration amplitude range of 0.5–4 mm and frequency range of 5–35 Hz. The experimental results demonstrate that at a low frequency (f < 10 Hz), the vibration effect results in a denser bed and a slightly smaller initial fluidization gas velocity. As the vibration frequency increases to a level similar to the bed’s natural frequency, the minimum fluidization gas velocity reduces sharply. The minimum fluidization gas velocity correlation in the VDMFB is obtained by means of theoretical deduction and experimental data fitting. Furthermore, a method is proposed for evaluating the effects of vibration on improving fluidization quality. Based on that, using the coal separation probable error Ep⩽0.1 as the evaluation index, a suitable effective operating gas velocity range for coal separation under different vibration parameters is determined. The ratio of the boundary operating gas velocity to Umfc is 0.67–2.28. Thus, a uniform and stable fluidization environment is provided for dry fine coal separation.

Control of pressure balance and solids circulation characteristics in DCFB reactors

In the paper, a novel dual circulating fluidized bed (DCFB) reactor is introduced, which can be widely applied in catalytic cracking, chemical looping combustion, coal/biomass combustion and gasification, and gas adsorption, etc. The study mainly investigates the effects of specific operating parameters on pressure balance and solid circulation rate, including total solids inventory, riser gas velocity, bubbling fluidized bed gas velocity and pot-seal gas velocity. It is found that changes of those parameters lead to variations in pressure balance and riser axial voidage in DCFB system, and further affect solid concentration and solid circulation rate in the system. To be more specific, total solids inventory, riser gas velocity and bubbling fluidized bed gas velocity have a major impact on solid concentration and solid circulation rate, while pot-seal gas velocity only exerts a minor impact. Besides, the changing of overall pressure drop across the bubbling fluidized bed is consistent with variations in pressure drops in the other parts of the system, with the former equaling △Pr+△Pc+△Pp-t+△Pud1+△Pud2. In this way pressure balance in the system is maintained. Eventually the paper reaches the conclusion that relevant operating parameters can be adjusted to control pressure distribution and solid circulation rate in different parts of the system and further to ensure the safe and stable operation of DCFB reactors.

Fluidization Characteristics of TiO2 Ultrafine Particles in an Acoustic Spout-fluid Bed with a Draft Tube

Abstract An experimental study was performed to investigate the effects of spouting gas velocity, fluidizing gas velocity, sound wave frequency and sound wave pressure level on fluidization behaviors of ultrafine particles in an acoustic spout-fluid bed with a draft tube. A half-cylinder with a diameter of 120 mm and height of 1200 mm was used as the fluidization column, TiO2 ultrafine particles with an average diameter of 290 nm were employed as the bed materials while the high-speed atmosphere jet was utilized as the spouting gas. Results showed that high-speed jet as spouting gas could effectively disrupt the agglomerates of TiO2 ultrafine particles and reduce the agglomerate size significantly. The introduced fluidized gas in annulus was conducive to eliminate dead-zone in the bottom of bed. Sound wave could break bubbles, restrain channeling making the axial fluidization state in annular region to be more uniform and homogeneous, thus significantly improved the fluidization quality of agglomerates in the annulus, reduced the minimum spouting velocity of particles, and promoted the stable circulation of inner particles in spout-fluid bed with a draft tube.

Agglomerate behavior in a recirculating fluidized bed with sheds: Effect of bed properties

The Radioactive Particle Tracking (RPT) technique was used to study agglomerates behavior inside a cold flow recirculating fluidized bed with internals (known as sheds), mimicking the stripper baffles of a Fluid Coker™. A higher fluidization gas velocity increases the time that agglomerates spend above the sheds and reduces the time spent in the shed zone and below the sheds, which is highly desirable. The residence time of the agglomerate in the stripper zone quadruples when the solid recirculation rate is cut by half. The release of vapors from agglomerates can be estimated by combining the RPT results with a coking reaction model. As the concentration of agglomerates inside the fluidized increases from 0 to 10wt%, this study predicts wet agglomerates entering the stripper section with 30wt% liquid would release 17% more hydrocarbon vapors below the top stripper shed.

Study of material agglomeration during nozzle injection of multiviscosity liquid into a fluidized bed reactor by the electric conductance method

Fluidized bed agglomeration is an important and challenging problem for thermal cracking in fluid cokers. A low coker temperature can be problematic because the bitumen is injected into the fluidized bed with a different viscosity, resulting in formation of agglomerates of varying sizes, which slows the cracking reactions. In the present study, the bed material agglomeration process during nozzle injection of multiviscosity liquid was investigated in a fluidized bed operated at different mass ratios of the atomization gas to the liquid jets (GLR=1%–3.5%) and gas velocities (3.9Umf and 5.9Umf) based on a conductance method using a water–sand system to simulate the hot bitumen–coke system at room temperature. During the tests of liquid-jet dispersion throughout the bed, different agglomeration stages are observed at both gas velocities. The critical amount of tert-butanol in the liquid jets that could lead to severe agglomeration of the bed materials (poor fluidization) at GLR=1% is about 10wt% at the low fluidizing gas velocity (3.9Umf) and 18wt% at the high gas velocity (5.9Umf). This study provides a new approach for on-line monitoring of bed agglomeration during liquid injection to guarantee perfect contact between the atomized liquid and the bed particles.

A comparison of fluidized bed pyrolysis of oil sand from Utah, USA, and Alberta, Canada

Characterization and thermal pyrolysis of oil sand was conducted. The experiment was performed on Circle Cliffs, Utah, U.S.A. and the results were compared with the data from Alberta, Canada. The reaction character identified by TGA was dual mode of vaporization of light hydrocarbon and thermal cracking of high molecular hydrocarbon. The pyrolysis experiment was performed in a 2 kg/h capacity fluidized bed externally heated by electricity. The process variables investigated were a temperature range of 723-923 K, fluidization gas velocity of 1.5-2 times of the minimum fluidization velocity, solid retention time of 15-30 minutes, and average particle size of 435 microns. The results of TGA and elemental analysis of bitumen provided necessary information regarding maximum liquid yield from the pyrolysis prior to pyrolysis experiment. The oil yield was maximum at 823 K. The yield of liquid was not exceeding the weight percent of maltenes in original bitumen. The optimum reaction condition should be fast vaporization of light hydrocarbon and minimizing thermal cracking of high molecular hydrocarbon. To maximize the liquid yield, fast heating and vaporization of oil sand bitumen and then the rapid removal of the vaporized product from the heating zone is recommended, i.e., operation in a fluidized bed reactor.

Collaborative optimization of vibration and gas flow on fluidization quality and fine coal segregation in a vibrated dense medium fluidized bed

In a vibrated dense medium fluidized bed (VDMFB), the uniform and stable gas-solid fluidized bed where medium particles belong to the Geldart B, was formed and considered suitable for 1–6mm fine coal separation under the combined effects of both vibration energy and air flow. Driven by the excitation force, the particles in the VDMFB had a higher kinetic activity and lower minimum fluidization gas velocity. Prior to the bed fluidizing initiation (fluidization number, N<1), the solid particles absorbed energy during inelastic collisions with the gas distributor. Subsequently to the bed uniform fluidization, the vibration energy was continuously transferred to the quasi-elastic bed in the form of waves. The vibration amplitude and frequency played a different role in the gas-solid contact. When the operating gas velocity, Ui, was 6.26cm/s and the vibration acceleration level, K, was lower than 7.1, it could have been quite effective for the vibration energy transfer in the bed to be enhanced by the vibration frequency increase. As the frequency increased, the transmission efficiency of vibration energy decreased. When K>7.1, the vibration amplitude effect on the vibration energy transmission in the bed was quite significant. In the study of the vibration energy effective range, it was discovered that the bed could be utilized for the fine coal effective separation in the area below 60mm for the VDMFB with 80mm of static bed height, where the vibration energy was continuously and steadily transmitted and attenuated along the bed height. Regarding the raw coal with a washability variety, the separation density could be flexibly regulated in the VDMFB. The separation results demonstrated that the ash content of three clean coal products was decreased exceeding 50%, compared to the ash content of the raw coal when the separation density was set at 1.49g/cm3, 1.67g/cm3 and 1.78g/cm3 respectively, indicating that the VDMFB could effectively separate fine coal by a dry method.

Particle agglomeration and control of gas-solid fluidized bed reactor with liquid bridge and solid bridge coupling actions

Fluidized beds reactors with liquid bridge and solid bridge coupling actions have been widely used in industry and thus studies of particle agglomerations under such fluidization conditions are of great significance. This work proposed an experimental apparatus combining electro-magnetic induction heating system with fluidized bed to simulate the real conditions for particle reaction heat release and heat transfer in industrial polymerization fluidized bed reactors. The effects of liquid content on agglomeration behavior of wax/graphite composite particles under different fluidization gas temperatures, gas velocities and heating powers have been studied. Through a force balance analysis of particles with liquid bridge and solid bridge coupling actions, relative solid bridge force was taken as a key parameter to demonstrate the effects of solid bridge force and other forces on agglomerations. Results showed that as the relative solid bridge force varied in different ranges, the agglomeration mass presented three different variation trends with liquid increases, namely monotonic increasing, non-monotonic changing and monotonic decreasing, due to different dominating agglomerating mechanisms (liquid evaporation and liquid bridge) during fluidization. According to the results of this study, the proposed criterion based on relative solid bridge force can be used to guide the regulation and control of particle agglomerations in fluidization with liquid bridge and solid bridge coupling actions.

Hydrodynamic behavior of silicon particles with a wide size distribution in a draft tube spout-fluid bed

An experimental study on the hydrodynamic behavior of silicon particles with a wide size distribution in a draft tube spout-fluid bed was performed in a 182 diameter cylindrical column with a flat gas distributor. Effects of spouting and fluidizing gas velocity, static bed height, entrainment zone height and draft tube diameter on the maximum spouting pressure drop, minimum stable spouting velocity and solids circulation rate were investigated. The results show that the maximum spouting pressure drop increases with static bed height, entrainment zone height, draft tube diameter and fluidizing gas velocity. The minimum spouting velocity increases with entrainment zone height, while it decreases with static bed height, draft tube diameter and fluidizing gas velocity. Two correlations on the maximum pressure drop and minimum spouting velocity are proposed respectively based on the experimental data, which are in good agreement with the present experiments. In addition, the solids circulation rate increases with static bed height, draft tube diameter, fluidizing and spouting gas velocity. It also increases with entrainment zone height at a lower fluidizing gas velocity, however it decreases with entrainment zone height at a higher fluidizing gas velocity.

Study on the flow behavior of irregular plastic particles in a spout-fluid bed with a draft tube

ABSTRACTThis work presents an experimental investigation on the hydrodynamic performance of a draft tube spout-fluid bed with irregular particles. Nonmetal particles from waste printed circuit boards (NPCBs) were used as a spouting solid, and polypropylene (PP) particles were selected as an assistant to fluidization particles. The flow pattern, minimum spouting velocity (Ums), and minimum spout-fluidization velocity (Umsf) were investigated under different operating conditions. The irregular cohesive particles from NPCBs showed poor flowability and channeling, which restrained stable spouting in the spout-fluid bed. The quality of fluidization and spouting improved when greater than 40 wt.% PP particles were added into the NPCB/PP mixtures. The mechanism was that the PP particles accelerated the movement of NPCB particles. Meanwhile, lower density differences between NPCB and PP particles decreased the segregation of the mixtures. The minimum spouting velocity decreased with an increase in fluidization gas velocity and the ratio of NPCB particle in the NPCB/PP mixtures. Two flow patterns, unstable spouting and unstable spouting fluidization, were observed over a large range of gas velocity. The ranges of gas velocity in these two flow patterns enlarged with the increase in mass fraction of NPCB particles within the NPCB/PP mixtures.

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.

Comparison of electrostatic charge generation in gas-solid fluidized beds in turbulent versus pre-turbulent flow regime

In this work, the effect of gas velocity on the electrostatic charge generation in gas-solid fluidized beds was studied with a specific focus on the transition to turbulent flow regime. Experiments were conducted at a pressure of 2600kPa (abs) with fluidizing gas velocities of 1.5, 3, and 5 times of Umf (pre-turbulent regimes) and 7.5 times of Umf (turbulent regime). Increasing the gas velocity and transitioning to the turbulent flow regime improved particle-wall contacts; and thus, augmented the extent of wall fouling, which indicates the increase in bed electrostatic charge generation. The amount of fouling was approximately five times larger in turbulent flow regime (7.5Umf) in comparison to that for the lowest gas velocity examined in bubbling flow regime (1.5Umf). The particles coating on the column wall consisted of a thick bottom layer which extended to a height of approximately 1m above the distributor plate, and a thin top layer which extended to the top of the column near the outlet. The particles net specific charge in the top and bottom layers did not vary with the increase in gas velocity. However, the net charge of these particles increased. The fine particles entrained from the bed had a net negative charge resulting in a net positive charge to be left behind in the bed contributing to the increase in the magnitude of wall fouling at higher gas velocities, especially in turbulent flow regime.

Conveying Characteristics of Dual Pneumatic Feeder Used for Biomass Pyrolysis

A novel dual pneumatic feeder was developed to achieve constant and steady biomass conveying for pyrolysis. To facilitate the feedstock replenishment, an injection pipe was installed inside a pressure chamber to convey the feedstock. Another stream of gas entered the pressure chamber from the bottom to fluidize the particles. Experiments were performed to test the performance of the new feeder, and three injection pipes and gas distributors were used. Results showed that the feeding rate depended on both the injection and fluidizing gas velocities. The feeding rate decreased with the inner diameter (ID) of an injection pipe, due to its impact on gas velocity, while the effective injection distance had a negative effect within a certain range. The opening ratio of the gas distributors had a positive effect on the feeding rate. Then, a model was developed, based on the Ergun equation, to describe relationships between the feeding rate and the gas velocities. The classical equation was further reformed to establish the correlation between the solid mass flowrate and the construction parameters of the feeder. The developed model deviated from the measured values within ± 15%, which was considered capable to predict the feeder performances.

Experimental investigation of bubble and particle motion behaviors in a gas-solid fluidized bed with side wall liquid spray

Bubble and particle motion behaviors are investigated experimentally in a gas solid fluidized bed with liquid spray on the side wall. The particles used in the experiment are classified as Geldart B particles. The results reveal that when the fluid drag force is less than the liquid bridge force between particles, liquid distribute all over the bed. Bubble size increases as the increase of inter-particle force, then decreases owing to the increase of particle weight with increasing liquid flow rate. When the fluid drag force is greater than the liquid bridge force, liquid mainly distribute in the upper part of the bed. And it is difficult for the wet particles to form agglomerates. Bubble size decreases with increasing liquid flow rate due to the increasing of minimum fluidization velocity. Besides, the acoustic emission (AE) measurements illustrate that the liquid adhesion and evaporation on particles could enhance the particles motion intensity. Consequently, the bubble and particle behaviors change due to the variation in fluidized gas velocity and liquid flow rate should be seriously considered when attempting to successfully design and operate the side wall liquid spray gas solid fluidized bed.

Minimum spouting velocity of flat-base spouted fluid bed

Experiments were performed on spout characteristics of a cylindrical spout-fluidized bed (I.D.=10cm) with different static heights and two materials (Al2O3 and high density polyethylene). Results of minimum spouting velocity obtained in this study were compared with reported correlations for both spouted and spout-fluidized beds. Considerable discrepancies were found between the values obtained using different model equations as well as with respect to experimental results. Based on the Mathur–Gishler correlation, a new correlation is proposed for calculating the minimum spouting velocity that introduces the ratio U/Umf. It was found that the minimum spouting velocity decreases with increasing fluidizing gas velocity (U/Umf). The pressure drop at the point of minimum spouting velocity is also correlated using this dimensionless group and is presented in this work. This investigation demonstrates that the use of correlations reported in the literature that focus primarily on conical bottom spouted beds are not applicable to flat-bottom spouted and spout-fluidized beds.

Properties of an inclined standpipe for feeding solids into a bubbling fluidized-bed

Flow properties of solids in an inclined standpipe were investigated while the solid particle was fed to the bottom of a bubbling fluidized bed (diameter 0.1m, 1.34m tall) via the standpipe and discharged out of the bed through the overflow exit. A group of K-based CO2 adsorbent particles was used as solids. The pressure drop of the fluidized bed, the height of solids bed in the standpipe, and flow rate of gas bypassing the fluidized bed through the standpipe were measured at room temperature and pressure as varying the fluidizing gas velocity, solids flow rate, and height of the overflow exit of fluidized solids. The pressure drop of the fluidized bed, the height of the solids bed in the standpipe, and the ratio of gas bypassing through the standpipe to the total gas in flow rate decreased with increasing the fluidizing gas velocity. Both the pressure drop of the fluidized bed and the height of the solids bed in the standpipe increased a little with the solids feed rate. The effect of the solids feed rate on the flow rate of gas bypassing through the standpipe could be ignored. The pressure drop of the fluidized bed and the height of the solids bed in the standpipe increased as the height of the overflow exit for solids increased in the fluidized bed. However, the effect of the height of the overflow exit on the flow rate of gas bypassing through the standpipe was negligible. Correlations on the pressure drop of the fluidized bed, the height of the solids bed in the standpipe, and the flow rate of gas bypassing through the standpipe were proposed successfully.

A hydrodynamic model of loop seal with a fluidized standpipe for a circulating fluidized bed

Loop seals are among the most common non-mechanical valves used in circulating fluidized bed systems. In the present work, a fundamental study is conducted of a fluidized loop seal, which consists of a fluidized standpipe, a fluidized supply chamber, and a fluidized recycling chamber. Based on the principles of momentum, mass, and energy conservation, a hydrodynamic model for a loop seal is established, consisting of 13 equations. The effects of operating conditions such as the bottom aeration rate, total solid inventory, and fluidizing gas velocity in the riser on the solids flow rate and the performance of the loop seal are studied by a combination of model analysis and experiments. The experiments are carried out in a circulating fluidized bed with silica gel particles (Geldart group A). The model predictions show good agreement with the experimental data.

Analysis of maximum pressure drop for a flat-base spouted fluid bed

For design purposes of the blower power requirements during startup of a spouting bed process, knowledge of the maximum pressure drop across a spouting bed at the onset of spouting is required. A survey of the literature for this information lead to only a few correlations which required information from a spouting bed test to be applicable. Therefore, an experimental investigation of this pressure drop and its associated superficial gas velocity in a flat based spouted bed operating with and without fluidization has been conducted. Measurements were performed using a 10cm cylindrical bed with different static bed heights and two Geldard group B particles (Al2O3 and HDPE). The results show that the taller static bed height creates a larger maximum pressure drop across the bed, as expected. The maximum pressure drop also increases with increasing fluidized gas velocity. The evolution of maximum pressure drop and its velocity were evaluated and a correlation for the maximum pressure drop and its corresponding velocity for spouted and spouted fluid beds were obtained.

CFD simulation of solids residence time distribution in a multi-compartment fluidized bed

Abstract The present work focuses on a numerical investigation of the solids residence time distribution (RTD) and the fluidized structure of a multi-compartment fluidized bed, in which the flow pattern is proved to be close to plug flow by using computational fluid dynamics (CFD) simulations. With the fluidizing gas velocity or the bed outlet height rising, the solids flow out of bed more quickly with a wider spread of residence time and a larger RTD variance ( σ 2 ). It is just the heterogeneous fluidized structure that being more prominent with the bed height increasing induces the widely non-uniform RTD. The division of the individual internal circulation into double ones improves the flow pattern to be close to plug flow.

Development and study of measurement methods for bogging in a fluidized bed

In the Fluid Coking™ process, heavy oil is contacted with hot fluidized coke particles. If the local concentration of liquid is too high, fluidization is poor, a condition commonly known as bogging. The objective of this study is to identify practical methods for early bogging detection, simulating heavy oil at coker temperatures with a lighter oil at room temperature and determining the impact of the oil concentration on bubble properties, on fluctuations of the bed pressure gradient, and on the transmission of sound through the bed. Bogging, as determined from changes in bubble properties, occurred when the oil mass fraction was increased from 0.25 to 0.275wt%.A Kolmogorov-Smirnov test of the wavelet coefficients of pressure fluctuations, optimised with a genetic algorithm, can detect early bogging more effectively than other methods using pressure fluctuations. A major advantage of this method is that its results are not affected by moderate variations in fluidization gas velocity. The success of this new bogging detection method is explained by studying the transmission of sound of different frequencies through dry and wet fluidized beds, which could also be used to detect bogging: bogging sharply increases the speed of sound from below 30m/s to well above 40m/s. A theoretical model confirmed that the changes in bubble properties caused by bogging affect the transmission of sound through the fluidized bed. The geometry of the gas bubbles and their distance from the wall were measured with capacitance sensors to understand how bubble properties affect sound transmission. The propagation of sound with bubbles of various geometries was simulated with Comsol.