Fluidizing Gas Velocity Research Articles

Experimental study of fluidization characteristics of Geldart-D particles in pressurized bubbling fluidized bed

The pressurized bubbling fluidized bed shows great advantage in retreating municipal solid waste because it could effectively capture CO2 and enhance the reaction rate of the process of combustion and gasification. In the present work, fluidization characteristics of Geldart-D particles at elevated pressure were experimentally investigated, such as flow pattern, pressure drop, minimum fluidization gas velocity. At the same fluidization gas velocity, as elevating operating pressure, the fluidization of Geldart-D particles became more intense, the bubbles got larger, the standard deviation and the power density of dominant frequency of the pressure drop signal increased. While, under the same fluidization number, as increasing operating pressure, the fluidization of Geldart-D particles became smoother, the bubble size decreased, both the standard deviation and the power density of dominant frequency of the pressure drop signal decreased. It seems that, under elevated pressure, the fluidization behavior of Geldart-D particles would transition to that of Geldart-B particles. Finally, the minimum fluidization velocity of the Geldart-D particles was found decreased with the increase of the operating pressure. A new correlation for the prediction of the minimum fluidization velocity of Geldart-D particles at elevated pressure was also formulated based on the present experimental results.

Characteristics Analysis of the Blockage Zone in a Gas-Solid Fluidized Bed.

A gas–solid separation fluidized bed, which belongs to dry separation and can save a lot of water resources, is an important method for coal quality improvement. The effect of regional blockage of the air distributor on the fluidization effect was investigated by varying the blockage area and fluidization gas velocity, and the influence law of blockage of an air distributor on bed stability was revealed. In this paper, the micro-differential pressure sensor is used as a testing tool to analyze the regional characteristics of the fluidized bed blockage hole. The results show that when the air distributor is plugged, the standard deviations of the pressure fluctuation are largest at a height of 100 mm, all reaching more than 2.0, and that the pressure fluctuation at the bottom is greater than that at the top. When the plugged area fraction increases from 1/8 to 1/2, the density fluctuation range at the bottom of the plugged area decreases from 0.2 to 0.04 g/cm3, while the range of influence increases from 10 to 16 cm axially. With the increase of fluidization gas velocity, the mixing effect of particles is improved to an extent, and the range of the affected zone is gradually reduced.

Elutriation and agglomerate size distribution in a silica nanoparticle vibro-fluidized bed

Fluidization of nanoparticle agglomerates is a promising technique to process nanoparticles. However, possible elutriation of small agglomerates may cause significant loss of bed material. To obtain the elutriation behavior under stable operation, in this study the elutriation fraction of silica nanoparticle agglomerates is measured in a vibro-fluidized bed, which is operated for several hours. Among conditions with different fluidizing gas velocities, Ug, and vibration strength, Λ, the lowest elutriation fraction measured is around 5% after 7-hour fluidization. The elutriation fraction increases significantly with Ug, while varies slightly with Λ. To help elucidating the elutriation behavior, the agglomerates at three different locations (bed surface, splash zone, and bed outlet) are sampled and their size distributions are determined. The elutriation rate constant is found to be much smaller than the literature results for ordinary particles, and the reasons are discussed in detail. Finally, an empirical correlation considering size distribution is proposed to fit the elutriation rate constant for the conditions in this study.

Study on Fluidization Characteristics of Magnetically Fluidized Beds for Microfine Particles

The bed pressure drop, minimum fluidized gas velocity, bed density, and bed expansion rate are important parameters characterizing the fluidization characteristics of gas-solid fluidized beds. By analyzing these parameters, the advantages and disadvantages of the fluidization state can be known. In this study, experiments were conducted to study the fluidization characteristics of a gas-solid magnetically fluidized bed for microfine particles by changing the magnetic field strength, magnetic field addition sequence, and static bed height. The experimental results show that when the magnetic field strength increased from 0 KA/m to 5 KA/m, the minimum fluidized gas velocity of particles increased from 4.42 cm/s to 10.32 cm/s, while the bed pressure drop first increased and then decreased. When the magnetic field strength is less than 3.4 KA/m, the microfine particles in the bed are mainly acted on by the airflow; while when the magnetic field strength is greater than 3.4 KA/m, the microfine particles are mainly dominated by the magnetic field. The magnetic field addition sequence affects the fluidization quality of microfine particles. The fluidized bed with ‘adding magnetic field first’ shows a more stable fluidization state than ‘adding magnetic field later’. Increasing of the static bed height reduces the bed expansion rate. The bed expansion rate is up to 112.5% at a static bed height of h0 = 40 mm and H = 5 KA/m. This will broaden the range of density regulation of a single magnetic particle and lay the advantage of gas-solid magnetically fluidized bed for microfine particles in the field of separation of fine coal.

Experimental study on the fluidization discharging characteristics of Geldart-C kaolin powders in a blow tank with pulsed gas

Kaolin powders have been suggested to be able to adsorb heavy metal vapor from coal-fired flue gas. However, due to the influence of inter particle forces, such as liquid bridge force, it is difficult to realize stable pneumatic conveying. In the present work, the fluidization characteristics of kaolin powders were investigated. A series of unstable flow phenomena such as agglomeration, channeling, and slugging occurred during the fluidization process. Also, the fluidization discharging characteristics of kaolin powder in an optimized blow tank were experimentally studied. The results indicated that the introduction of pulsed gas can effectively destroy agglomeration and thus improving the stability of discharging. Visual experiments in pseudo-2D fluidized bed were also confirmed the destructive effect of pulsed gas on agglomeration. With an increase in either fluidization gas velocity Uf or pulsed gas velocity vpulsed, the mass flow rate of kaolin powder G first increased and then decreased. Finally, drying experiments demonstrated that there is free water on the surfaces of the kaolin powders. The analysis of forces indicated that the liquid bridge force Flb between particles is much larger than the particle gravity Fg. The liquid bridge force might be one of key reasons for kaolin powder agglomerating.

A modified wake model in bubble-induced three-phase inverse fluidized bed (BIFB)

A modified wake model was proposed for the newly developed bubble-induced three-phase inverse fluidized bed (BIFB), by combining the generalized wake model and the gas-perturbed liquid model. On the basis of the modified wake model, the solids and liquid holdups and the complete fluidization gas velocity in BIFB system have been successfully predicted with two established correlations. The predictions achieved very good agreements with the experimental data.

CFD-DEM Simulation of Fluidization of Polyhedral Particles in a Fluidized Bed

Fluidization of non-spherical particles is a common process in energy industries and chemical engineering. Understanding the fluidization of non-spherical particles is important to guide relevant processes. There already have been numerous studies which investigate the behaviors of different non-spherical particles during fluidization, but the investigations of the fluidization of polyhedral particles do not receive much attention. In this study, the investigation of the fluidization of polyhedral particles described by the polyhedron approach is conducted with a numerical CFD-DEM method. Experiments of the fluidization of three kinds of polyhedral particles are conducted under the same condition with corresponding simulations to validate the accuracy of our CFD-DEM model. The results indicate that our CFD-DEM model with the polyhedron approach can predict the behaviors of polyhedral particles with reasonable accuracy. Fluidization behaviors of different polyhedral particles are also investigated in this study. Compared to spherical particles, the motion of polyhedral particles is stronger, and mixing degree is higher under the same fluidization gas velocity.

Three-dimensional axial dispersion of gas-solid fluidized beds using 8-plane electrical capacitance volume tomography

Due to the complex flow characteristics and opaqueness of gas-solid fluidized beds, the axial behavior of circulating fluidized bed (CFB) risers is not entirely understood. A 32-electrode electrical capacitance volume tomography (ECVT) system with eight planes was developed to investigate the three-dimensional axial dispersion of gas-solid flow in a CFB riser. To maximize the imaging length in the axial direction, the 32-electrode ECVT sensor was designed with eight planes, with each plane containing four electrodes. The ratio of the axial imaging length to the sensor inner diameter was 8.3. We developed a 32-channel ECVT data acquisition system with an imaging speed of 120 frames per second to record the data of the ECVT sensor. Experimental results show that this ECVT system can effectively collect the flow details in the axial direction. The flow changed from slug flow to bubble flow when the fluidizing gas velocity Uf was adjusted from 2.34 m/s to 0.56 m/s. The distribution of the solid fraction, the velocity of the slug/bubble in the axial direction and the slug length were extracted from the 8-plane ECVT images to reveal the flow characteristics of the CFB riser.

A hydrodynamic model of U-type reduction chamber for iron ore suspension roaster

U-type reduction chamber plays a significant role in the suspension roaster which has been proven to be one of the most highly efficient and clean magnetization equipment to utilize complex refractory iron ores. In the present work, a hydrodynamic model has been developed for the operation that the materials in the fluidization chamber are fluidized while the particles in the supply chamber are in a moving packed bed. Through fluid dynamic analysis, a hydrodynamic model including four equations has been established, whose solution results in the solids concentration in the fluidization chamber and materials' height in the supply chamber. Meanwhile, the effects of operating conditions such as fluidizing gas velocity and delivery rate on solids concentration in the fluidization chamber and materials' height in the supply chamber were studied by a combination of model analysis and experiments. The experiments were carried out in a U-type reduction chamber cold test apparatus with alumina particles. The model predictions showed good agreement with the experimental data, giving average errors below 6.28% and 4.96% respectively.

Investigation of granulation calcined under the eutectic temperature in the fluidized bed

A new two-process cement calcination method using fluidized bed reactors to substitute rotary kilns was employed for cement manufacturing. Granulation of cement raw meal in the first calcination process under the eutectic temperature determined the quality of clinkers produced in the second process and the feasibility of the method. Therefore, the granulation process and the factors affecting granulation were investigated by comparing the morphology of different granules. X-ray diffraction and X-ray fluorescence tests verified CaO as a significant constitute for granulation. Morphological comparison between CaO and cement raw meal confirmed that sintering rather than solid-phase reactions was the main driving force of granulation. Thin layers of reaction zones contributed to the bonding of different clusters. Granulation was depicted as a two-step process involving agglomeration and sintering. Fluidized gas velocity, initial weight of cement raw meal and clinker amount all influenced granulation. An increase in the fluidized gas velocity suppressed granulation, while an increase in the initial weight of cement raw meal and clinker amount contributed to granulation. This study explains the granulation process of cement raw meal and the main driving force for granulation, and suggestions are also provided for granulation control.

Experimental study on weakly caking coal combustion preheated by circulating fluidized bed

In this study, we sought to achieve a stable high-efficiency and low-NOx combustion of weakly caking coal by preheating fuel in a pre-circulating fluidized bed (PCFB). When the operating temperature of the PCFB increased from 954 °C to 1048 °C and the fluidizing gas velocity increased from 2.02 m/s to 3.27 m/s, the stable operating time of the caking coal increased significantly from 290 min to 550 min. Moreover, the caking trend of the preheated char completely disappeared (RI = 0, GR.I = 0). The preheated char particles were obviously bonded together at 954 °C, while the char particles were still clearly separated at 1048 °C. These results indicate that the high preheating temperature and the high fluidizing gas velocity are beneficial to alleviate or even resolve the preheating instability of weakly caking coal. Based on the stable preheating operation, the temperature field and reaction field in the main combustion zone were relatively uniform. Gas-phase nitrogen-containing compounds in the post-combustion chamber existed mainly in the form of NO, and the NO concentration in the reducing zone was zero. With the gradual injection of the burnout air, the NO concentration first increased and then decreased to a stable level. By arranging the burnout air position separating the reducing zone and the oxidizing zone to broaden the reducing zone, NO emissions were further reduced without affecting the combustion effect. And the preheating combustion technology is verified to more flexibly arrange the number of air distribution layers in multilevel air distribution settings.

Fluidization and separation characteristics of a gas–solid separation fluidized bed in the presence of an acoustic field

The bed fluidization characteristics, such as pressure drop, minimum fluidization gas velocity, density, and voidage, play a vital role in understanding the hydrodynamic behavior of fluidization technologies. In this work, the variation in the bed pressure drop was analyzed in presence and absence of an acoustic field. Meanwhile, the minimum fluidization gas velocity was investigated for five different fine particle (−0.074 mm) contents as the acoustic source varied between 90 and 150 Hz, and the sound pressure level was within a range of 100–135 dB. The experimental results show that, upon an increase in the fine-grained particle (−0.074 mm) content, the sound frequency and sound pressure level have different effects on the minimum fluidization gas velocity. The introduction of an acoustic wave can effectively ensure the stability of the bed density, thus improving the separation performance of a gas–solid separation fluidized bed, with a lower probable error E.

The hydrodynamics of liquid-solid and gas-liquid-solid inverse fluidized beds with bioparticles

The hydrodynamic characteristics, such as minimum fluidization velocity (Ulmf for liquid-solid (LS) system and Ug,if for gas-liquid-solid (GLS) system) and bed expansion ratio (BER), of liquid-solid and gas-liquid-solid inverse fluidized beds (LSIFB and GLSIFB) with bare particles and particles with biofilm were investigated. In the LSIFB system, Ulmf and BER of the bare particles were independent of the solids loading. For bioparticles, the increase of the biofilm thickness reduced Ulmf and increased BER, suggesting that the fluidizability increases with the presence of the biofilm. In the GLSIFB system, the initial fluidization gas velocity (Ug,if) and the complete fluidization gas velocity (Ug,cf) both increased with increasing particle diameter and decreasing particle density under fixed superficial liquid velocities. Biofilm attachment led to a decrease of both Ug,if and Ug,cf, and an increase of bed expansion, again suggesting increased fluidizability with the presence of biofilm.

Experimental study of the solid circulation rate in a pressurized circulating fluidized bed

The solid circulation rate is essential for design of pressurized circulating fluidized beds (PCFBs). With increasing pressure from atmospheric pressure to a few bars, the gas density linearly increases with the pressure, which affects the gas–solid flow characteristics. In this work, experiments were performed at room temperature in a cold PCFB apparatus with a riser of 3.3m in height and 0.05m in diameter. The solid circulation rate was studied from 20 to 80kg/(m2·s) under various conditions with increasing pressure from 0.1 to 0.6MPa and fluidizing gas velocity from around 1.5 to 8.0m/s for different Geldart B group particles. Most of the conditions were in the flow regimes of core-annulus flow (CAF) only and CAF with a turbulent fluidized bed at the bottom. The trend of the apparent slip factor with the dimensionless slip velocity was similar at different pressures and for different average particle sizes, and it converged to an exponential function. An empirical equation was obtained by fitting the solid circulation rate with the operating parameters (particle transport velocity, particle volume fraction, Archimedes number, and Froude number), which is helpful for design and operation of PCFBs.

Study on dry separation technology of a continuous gas-solid separation fluidized bed with a moving scraper (И) – Separation performance

Separation performance is the most important parameter for a continuous gas-solid separation fluidized bed. In this work, the separation process and its influencing factors in fluidized bed were studied by using a combination of stratified sampling method and sink-float tests. The results showed that the axial density distribution of coal particles was mainly determined by the density difference and fluidizing gas velocity (or bubble), while the lateral migration process was mainly affected by the moving scraper. The use of suitable operational parameters (u < 14.74 cm/s or uf < 6.0 cm/s) can reduce the bed resistance to density stratification of coal particles, thus improving the overall separation efficiency. Furthermore, under the optimum operating parameters (uf = 4.4 cm/s, u = 13.26 cm/s), the lowest possible error E value of 0.06 g/cm3 was achieved, indicating that the fluidized bed has high separation efficiency under these operating conditions.

Study on the influence of different carrier gases on the fluidization properties of glass bead and FCC powders

The powder flow was influenced by carrier gas in the unit operation, such as pneumatic conveying and hopper discharge, in the pressurized entrained flow gasification technology. Carrier gases play a vital role in fluidization/ defluidization of powders. In this paper, we investigate the fluidization/ defluidization of absorbent Fluid Catalytic Cracking (FCC) and non-absorbent glass bead with two different carrier gases, N2 and CO2. The flowability of aerated powders with different carrier gases were obtained in the fixed bed region whose superficial gas velocity was lower than minimum fluidization gas velocity and fluidization region. The powder flowability was characterized by basic flow energy, flowability rate index and aeration energy with the help of FT4 powder Rheometer. The influence of different carrier gases on fluidization/ defluidization of powder bed was further discussed. Transition line of A/C powders with different carrier gases was revealed. Effect of gas type and interparticle force were calculated, compared with the flowability of aerated powder bed. In addition, combined with bed stress analysis, different bed fluidization mechanism with different gas velocities were revealed. Our findings might help people understand the influence of carrier gases on the fluidization/ defluidization of powders in the relative chemical unit operations.

On the characteristic heating and pyrolysis time of thermally small biomass particles in a bubbling fluidized bed reactor

Abstract Pyrolysis of crushed olive stone particles in a lab scale Bubbling Fluidized Bed (BFB) reactor was investigated. The time evolution of the pyrolysis conversion degree of the olive stone particles, while moving freely in the BFB, was determined from the evolution of the mass of olive stones remaining in the bed, measured by a precision scale holding the whole reactor installation. The experimental measurements of the pyrolysis conversion degree were employed to validate a simple model combining heat transfer and chemical kinetics, which is valid for thermally small particles. The model combines the Lumped Capacitance Method (LCM) and the simplified Distributed Activation Energy Model (DAEM) to account for heat transfer and pyrolysis chemical kinetics, respectively. The estimations of the combined LCM-DAEM model for the pyrolysis conversion degree were found to be in good agreement with the experimental measurements for the pyrolysis of olive kernels in a BFB operated at various bed temperatures, fluidizing gas velocities, and biomass particle size ranges. From the combined LCM-DAEM model, the characteristic heating time and the pyrolysis time of the olive stone particles were derived, obtaining a direct relation between these two parameters for constant values of the bed temperature.

Effect of elevated pressure on gas-solid flow characteristics in a circulating fluidized bed

Pressurized circulating fluidized beds are applied in a range of applications because of intensified heat and mass transfer and chemical reaction. The gas density increases linearly with the pressure, which influences the gas-solid flow characteristics. In this work, the effects of operating pressure (0.1 MPa–0.6 MPa) on the distribution of particle volume fraction and gas-solid slip velocity in the riser are studied under different conditions. The particle belongs to Geldart B group, the solids circulation rate ranges from 20 to 50 kg/(m2.s), and the fluidizing gas velocity ranges from around 2 to 8 m/s. With an increase in pressure from 0.1 to 0.6 MPa, in order to achieve the same solids circulation rate, the fluidizing gas velocity decreases approximately to a half. Under pressurized conditions, the profile of particle volume fraction along the axial direction shows a similar trend, while the particle volume fraction in the bed central zone at the upper riser increases slightly. Under different pressures, the plot of the nondimensional gas-solid slip velocity against the particle volume fraction converges on a power correlation. The present study is a first step towards the aim of correlating the solids circulation rate and gas-solid slip velocity with operating parameters under pressurized conditions.

Catalytic Oxychlorination Process Optimization for the Conversion of Ethylene to Ethylene Dichloride

The catalytic oxychlorination of ethylene is widely used for the production of ethylene dichloride (EDC) which is an essential step for the production of poly-vinyl chloride (PVC). However, catalyst entrainment from the fluidization reactor has formed a problem and caused the loss of expensive catalyst powder with the outlet gas, hence less conversion in achieved. In this research we studied the catalytic oxychlorination of ethylene over the surface of geldart A powder with average particle size of 80 microns. Simulation was developed using Aspen plus and Mat Lab. It was validated against the industrial data and they were matching. Various operating conditions and variables were studied. The effect of fluidized gas velocity, bubble diameter, temperature and pressure on solid catalyst entrainment rate. In addition, the copper content from the quencher bottom was analyzed. It was found that the entrainment rate increased with the increase in fluidized gas velocity, bubble diameter, reactor temperature and pressure. While on the other hand increasing orifice diameter decreased the amount of entrained catalyst within the fluidized bed. It was found that the optimum superficial gas velocity is 2570.4 m3/hr.and the optimum orifice diameter is 1 cm. At this condition the lowest rate of solids entrainment out of the bed will be reached. This will decrease the amount of lost catalyst and the process will be more economic with higher gas conversion.

Studies on Solids Flow in a Cold Model of a Dual Fluidized Bed Reactor for Chemical Looping Combustion of Solid Fuels

Abstract The results of investigations on solids flow in a cold model of the dual fluidized bed reactor designed for chemical looping combustion of solid fuels (DFB-CLC-SF) are presented in this paper. The constructed unit consists of two interconnected reactors. The first one, so-called fuel reactor (FR), is operated under bubbling fluidized bed (BFB) conditions, whereas the second one, so-called air reactor (AR), is structurally divided into two sections. The bottom part of AR works under BFB while the upper part, i.e., the riser, is operated in the fast fluidized bed (FFB) regime. In these studies, the air was used for fluidization process in all parts of the DFB-CLC-SF reactor. The glass beads with similar parameters to oxygen carriers (OCs) used in the CLC process were utilized as an inventory. The fluidization conditions are controlled by using the sets of pressure sensors installed around the circulation loop. The experimental data acquired in the tests are further employed to the analysis of solids behavior in a cold model of the DFB-CLC-SF reactor. The main goal of these studies was to establish the conditions for smooth fluidization, which concurrently provide the required residence time of solids in both reactors that is one of the most crucial factors in the CLC process. It was found that the fluidizing gas velocity in reactors has a significant impact on solids behavior and the investigated parameters. However, what is the most important, it was confirmed that the operation condition of the DFB-CLC-SF reactor can be adjusted to meet the requirements resulting from the properties of OCs.