Uniformity Of Fluidization Research Articles

A comprehensive understanding of the non-uniform characteristics and regulation mechanism of six external loops in a 600 MW supercritical CFB boiler

As the carrier of material circulation, the uniformity of multi-parallel external loops directly affects the operation of large-scale circulating fluidized bed (CFB) boilers. Some non-uniform phenomena have been found, but the non-uniform characteristics and the absorptive capacity in full process of external loops under changeable operating parameters were seldom considered. To solve this, comprehensive cold/thermal field tests were conducted on the external loops of the first 600 MW supercritical CFB boiler. Operating characteristics of the loops and the fluidization uniformity of fluidized bed heat exchangers (FBHEs) under various cases were fully demonstrated. The influence of air-coal distribution on the operating non-uniformity of the external loops were analyzed, and the absorptive capacity to thermal deviations were quantified. The regulation mechanisms of cyclones and FBHEs were clarified, and the key reason of thermal deviations in FBHEs was also figured out. Eventually, a new coal feeding method to maintain heat balance in dense phase zone was proposed. The results of this paper fully reflected the operating characteristic of the full process in external loops and the response characteristics to air-coal adjustments, which could provide the most direct guidance for academic research as well as optimal design and operation of large-scale CFB boilers.

Experimental and Numerical Investigation of Backwash Flow in Nutshell Filter

Summary The granular nutshell filter is commonly used as deep-bed media filter to remove insoluble hydrocarbons and suspended solids from water. The important advantage of using such a filter is the ability to backwash and regenerate the media. The process of backwash includes fluidization of the media by applying two fluid flows: backwash flow that is upward vs. a jet flow that is downward. A computational fluid dynamics (CFD) simulation of such fluidization was performed for a pilot-scale nutshell filter. The effect of the backwash flow, jet flow, and medium depth in the filter on the fluidization quality during the backwash mode of the filter were investigated. The media fluidization uniformity was studied as the media depth and flow patterns varied. A fair consistency was observed between the experimental values and the results obtained by CFD analysis. It was observed that the longer the media depth, the more uniform the media fluidization will occur. Increasing the media depth resulted in longer fluidization times. Shorter media-depth filters are more sensitive to jet flows. For shorter filters, when the jet flow was reduced, a more uniform fluidization was obtained. The fluidization time (the time for the media to reach the top of the vessel) is strongly related to the jet flow for both shorter- and longer-depth media filters. For long-depth media filters, the effect of increasing the backwash flow and reducing the jet flow on the uniformity of media fluidization vs. the fluidization time was studied. The velocity pattern for the fluid showed vortices close to the floor screen in lower mid-depths of the filter. The CFD analysis for the commercial-scale nutshell filter with a diameter of 12 ft (3.65 m) and height of 8 ft (2.4 m) showed a similar vortex pattern with uniform media fluidization.

A modified model for predicting bed density of air dense medium gas-solid fluidized bed (ADMGFB) using binary dense media

The binary mixture of magnetite powder and fine coal was used to investigate the adjustment of bed density and fluidization stability of air dense medium gas-solid fluidized bed (ADMGFB) through theoretical and experimental approaches. An optimum calculated model was selected to predict incipient fluidization velocities with different weight proportions of fine coal (xc). A modified model of the bed density of ADMGFB using binary dense media was derived based on the two-phase fluidization theory with experimental verification. Bed density stability and fluidization uniformity were achieved based on the investigation distribution of bed density and fine coal of each bed layer. The separation experiments of raw coal using ADMGFB were carried out with N of 1.3. When xc was less than 16%, the probable errors were no more than 0.07 g/cm3. Results indicated that the satisfactory separation performance of raw coal was achieved using ADMGFB based on the binary dense media.

Modeling multiple gas jet interactions during fluidization in a pseudo-2D bed

The hydrodynamics of fluidized beds involving gas and particle interactions are very complex, and must be carefully considered when modeling such a system using computational fluid dynamics (CFD). One of the issues is the interaction of multiple jets developing above the distributor plate, which affects the uniformity of fluidization. To model the distributor plate, one approach is to use a uniform velocity at the inlet and the other is to model the distributor holes and specify the jet velocity. To predict and examine the hydrodynamics of interacting jets, a multi-fluid Eulerian-Eulerian CFD model was employed. In the present work, simulations of a pseudo-two-dimensional (2D) fluidized bed were compared to a corresponding experiment designed to examine multiple jet interactions for two distributor plate configurations with 9 and 5 holes. Two-dimensional and three-dimensional simulations of the pseudo-2D bed were used to investigate fluidization characteristics, volume fractions and solids velocity distributions. The deadzones formed due to multiple jet configurations of the distributor were quantified and their distributions over the plate were analyzed. It was found that three-dimensional simulations captured the effect of multiple jets and provided more accurate predictions of pressure, particle velocity and particle distribution. It was shown that increasing the number of holes along the gas inlet plane changed the fluidization characteristics, whereby fewer holes restricted the amount of particles that fluidized, irrespective of inlet velocity. When the number of holes increased, more material fluidized with increasing velocity. It was also found that although the reactor geometry is effectively two-dimensional, using two-dimensional simulations did not always capture all aspects of the flow, especially, the particle velocity.

Acoustic streaming in gas-fluidized beds of small particles

An increasing number of applications rely on the use of gas-fluidized beds of small particles to enhance processes based on the mass/heat transfer between a gas and a solid. On the other hand, experimental studies have shown that application of high-intensity acoustic fields serves to further intensify fluidized bed processes. It is well known that the fluid streaming developed around the surface of a solid body subjected to a sound wave leads to a notable increase of the mass/heat transfer. However, several additional phenomena are observed in sonoprocessed fluidized beds requiring a close examination. In a sonoprocessed fluidized bed, acoustic streaming is superposed on the fluidizing gas flow, which exhibits large fluctuations within the bed at a local level due to long-range hydrodynamical interactions. Attenuation due to viscous friction and thermal conduction on the surface of the small particles is expected to play a relevant role. Fine particles may be entrained by the acoustic flow field whereas acoustic streaming on the surface of not entrained particles gives rise to an increased drag. Entrainment of fine particles in the sound wave may serve to improve fluidization uniformity in dense beds or to induce particle agglomeration in diluted beds. Acoustic streaming leads also to a large-scale fluid recirculation characterized by velocities sufficiently high as to affect the fluidized bed dynamics. A substantial modification of the dynamics of fluidized beds may thus occur depending on intervening physical parameters such as particle size and density, sound wave intensity and frequency, fluid properties, fluidization regime and system size. In this manuscript, the main features of acoustic streaming and the associated phenomena due to the application of high-intensity acoustic fields on fluidized beds are analyzed from a review on the fundamentals of acoustic streaming and observations on sonoprocessed fluidized beds. Results inferred from this analysis are expected to help assess the optimum conditions under which sound assisted process intensification would be maximized in each particular application.

Effect of gas type on the fluidization and discharge characteristics of the pulverized coal

The influence of gas type (air, He, H2 and CO2) on the fluidization and discharge characteristics of the pulverized coal was investigated in this paper. It was concluded that the gas viscosity was dominant over the gas density in influencing the fluidization quality. The increase of gas viscosity improved the fluidization uniformity, delayed the visible bubbling and promoted the discharge stability. Compared with air, a higher proportion of H2 rises through the coal bed while a higher proportion of CO2 escapes through the hopper outlet during the discharge process. Experimental results show that the discharge rate increases with the addition of H2 in the whole experimental range and CO2 always corresponds to the minimum discharge rate. In spite of the very different effects produced by the various gases, the relationship between the discharge rate and the pressure difference does not change a lot. Nedderman's mode was used to predict the discharge rate, which agreed well with the experimental data with the error kept below 20% except individual points at low aeration rate.

Effect of inclination on gas-fluidized beds of fine cohesive powders

Results are presented of an experimental investigation on how bed inclination affects the fluidization and sedimentation behavior of fine cohesive particles. In contrast with the expected Geldart C behavior, and due to self-agglomeration, these fine particles are uniformly fluidized by gas in a vertically oriented bed, displaying a fluid-like regime and expanding smoothly as the gas velocity is increased. When the gas flow supply to the bed is suddenly stopped, the initial sedimentation velocity of the vertically oriented bed is similar to the fluidizing gas velocity as corresponds to uniform fluidization. The main effect of inclination is to induce fluidization heterogeneity. The local gas velocity increases in the adjacent region to the upper wall at the expense of the region adjacent to the lower wall. This situation anticipates the onset of local bubbling in the region adjacent to the upper wall. Meanwhile the region adjacent to the lower wall remains in a solid-like state and does not reach the fluid-like state until values of the gas flow are applied much higher than those needed in a vertical fluidized bed. As a consequence, the expansion and fluidization uniformity of the tilted bed are hindered. If the gas supply to the inclined bed is suddenly stopped, and because of induced heterogeneity, sedimentation takes place at a decreased rate as compared with sedimentation velocity in the uniformly fluidized vertical bed.

High viscosity gas fluidization of fine particles: An extended window of quasihomogeneous flow

We explore the role of gas viscosity in the behavior of gas-fluidized beds of fine powders by means of experimental measurements using nitrogen and neon as fluidizing gases, and theoretical considerations. The existence of a nonbubbling fluidlike regime has been recently observed in beds of fine powders fluidized with nitrogen. Our experiments with neon reveal a discontinuous transition from heterogeneous fluidization to a highly expanded homogeneous fluidization state. We point out that increasing gas viscosity enhances the coherence of agglomerate swarms, which promotes a local void-splitting mechanism, thus improving the uniformity of fluidization. Our theoretical analysis predicts that further increase of gas viscosity would produce a full suppression of the bubbling regime, i.e., the uniformly fluidized bed would undergo a direct transition to a turbulent regime as seen in beds of nanoparticles fluidized by nitrogen and in liquid-fluidized beds of moderate-density beads.

Relationship between pressure fluctuations and generation of organic pollutants with different particle size distributions in a fluidized bed incinerator

The hydrodynamic behaviors of fluidization perhaps significantly influence the uniformity of fluidization in fluidized bed incinerator. Good uniformity of fluidization expressed the air across uniformly through the bed and the particles being distributed well in the fluid stream. The aggregates, flocs and channels of particles do not happen during fluidization. The Good uniformity will maintain high heat and mass distribution to improve reaction efficiency. These parameters include the height of static bed, gas velocity, mixing and distribution of bed particle, which have rarely been studied in previous investigations. Consequently, this study examines how the hydrodynamic parameters affect the generation of organic pollutants (BTEXs and PAHs) during incineration. The statistical and power spectral analysis of the measured pressure fluctuation during incineration are used to elucidate the relationship between behaviors of fluidization and generation of pollutants during incineration. Experimental results show the organic concentration does not increase with uniformity of fluidization decreasing. The reason may be the explosion of the gas and the consequent thermal shock destroy the coalescent bubbles to form small bubbles again and enhance the efficiency of transfer of oxygen to increase combustion efficiency. Additionally, the mean amplitude and fluidized index of pressure fluctuation similarly vary with the concentration of organic pollutants. These two indices can be used to assess the efficiency of combustion. The four particle size distributions could be divided into two groups by statistical analysis. The Gaussian and narrow distributions belong to one group and the binary and flat the other. The organic concentration of the Gaussian and narrow distributions are lower than that of the other distributions. Consequently, the bed materials should maintain narrow or Gaussian distributions to maintain a good combustion efficiency during incineration.

On the influence of aspect ratio and distributor in gas fluidized beds

The influence of distributor on the uniformity of fluidization and the selection of the aspect ratio at a critical fluidization quality which corresponds to the maximum of the experimental to theoretical bed pressure drop ratio have been studied. Two types of multi-orifice distributors and three bed materials having their particle size in the range 70–161 μm have been investigated in an air fluidized bed at ambient conditions. The distributor to bed pressure drop ratio and the operating velocity for achieving uniform fluidization have been analyzed. The aspect ratio is shown to have a significant role in selecting a distributor and fixing the operating velocity particularly when the bed is shallow. A model equation derived from the first principles is proposed to predict the distributor to bed pressure drop ratio in terms of the critical aspect ratio and compared with the data reported in the literature.

Uniformity of fluidization on a multi‐orifice gas distributor

AbstractAs the gas flow rate is increased through a bed of particles lying on a multi‐orifice gas distributor, several gas pressure maxima arise in the plenum chamber which correspond to the occurrence of fluidization in different sections of the bed. The gas pressure at the distributor orifices may considerably exceed the quantiy ρsg(1 − ϵ0)H. A formula has been derived for the complete‐fluidization index at which all caps come into play.

Effect of distributor to bed resistance ratio on uniformity of fluidization

On propose un nouveau modele expliquant la formation de cheminements preferentiels dans le lit fluidise, et on etablit un critere de fluidisation uniforme

Establishment of a nonuniform mode of fluidization with a uniform distribution of the flow of the cooling medium

Using the principle of the mechanical stability of a system, a criterion for the disturbance of the uniformity of fluidization is obtained and the initial properties of the nonuniform fluidization state of a layer of fine particles close to a distributed grating (the porosity of the dense phase, and the concentration and dimensions of the drops which appear) are considered.

Effect of uniformity of fluidization on catalytic cracking of cumene

AbstractThe effect of uniformity of fluidization on the catalytic cracking of cumene, with a silicaalumina catalyst, was studied in a 3‐in.–diameter reactor operated at 800°F. Superficial gas velocities were varied from 0.06 to 0.24 ft./sec. for bed heights of 1.5, 3, 5, and 8 in. The percentage conversion in a fluidized bed was found to be lower than in a fixed bed and to be dependent on the type of catalyst support used. A porous plate support was up to 30% more efficient than a screen support, which is attributed to its better gas‐distribution characteristics and to its ability to introduce the discontinuous phase as smaller gas bubbles. Indications are that the first few inches of bed are very effective in bringing in contact the continuous and discontinuous phase, and therefore most of the conversion due to the interaction of the two phases takes place here. This can be explained in terms of the rapid growth of gas bubbles as they proceed through the bed.

Factors affecting density transients in a fluidized bed

AbstractThe effects of five operating variables on the uniformity of fluidization in a 4‐in.‐diameter column were determined by a statistically designed experiment. Density fluctuations inside the bed were measured by a capacitance method. Certain parts of the experiment were repeated in a 24‐in.‐diameter column for comparison with the 4‐in.‐column data.In the 4‐in. column gas velocity had by far the greatest effect on uniformity, with uniformity generally decreasing with an increase in gas velocity. This is consistent with the theory that most gas introduced in excess of the rate for incipient fluidization passes through the bed in the form of various‐sized bubbles. Better uniformity at the lower bed level indicates that bubbles grow in size as they proceed up the column. An entirely different gas‐flow pattern was indicated in the 24‐in. column.

Characteristics of gas pockets in fluidized beds

AbstractSome characteristics of gas pockets rising through beds of fluidized solids have been measured directly with a light probe technique. The vertical thickness, numerical frequency, and rate of rise of the bubbles were obtained from oscillographs of dual probe signals. Room‐temperature air at 1‐atm. pressure was used in 4‐ and 6‐in. columns. Glass beads, a crushed rock, commerical cracking catalysts, coal, and hollow resin spheres were studied in particle sizes from 12‐µ fluid cracking catalyst to 450‐µ coal.Vertical bubble thickness was found to increase with particle size, distance above the bed support, and gas velocity. Size growth upon rising was mainly the result of coalescence of bubbles. The rise velocity ranged from 1 to 2 ft./sec., relatively unaffected by operating conditions. Bubble frequency decreased with height above the bed support owing to coalescence. Total bed‐depth variation from 1.0 to 2.5 ft. did not significantly influence the results, which should be of interest in studying gas by‐passing in fluidized beds and predicting the slugging behavior and fluidization uniformity.