Bed Density Distribution Research Articles

Critical comparison of air dense medium fluidized bed (ADMFB) and pulsed fluidized bed (PFB) based on simulations and experiments

ABSTRACT In the present work, based on the traditional air dense medium fluidized bed (ADMFB), the constant gas velocity was substituted with pulsating velocity to propose pulsating fluidized bed (PFB) to investigate the distinct advantages in separating the fine coal particles in PFB over ADMFB. In this regard, the two-fluid model (TFM) was used to study the formation of bubbles, the distribution of density, and the gas-solid motion in PFB. The simulation results showed that lower bed height could decrease the energies of collision and mergers. The reason why the diameter of bubbles in PFB was much smaller than ADMFB was that the pulsating mechanism could effectively cut off the aggregation channels of gas below the lower pressure domain of bubbles. Moreover, the dense medium particles presented the upward movement in the middle of the bed and downward movement near the side walls. The distribution of bed density in PFB showed more stable condition from the spatial and temporal aspects. Finally, based on the orthogonal experiments, a correlation for estimating the fluctuations in bed density was established to illustrate the optimum values for bed height and pulsation frequency for airflow, which had values of approximately 100 mm and 2.0 hz, respectively.

Spatial distribution of bed density in a Gas-solid Fluidized Bed Coal Beneficiator (GFBCB) using Geldart A− particles

Spatial distribution of bed density in a Gas-solid Fluidized Bed Coal Beneficiator (GFBCB) using Geldart A− particles

Geldart A− particles: Hydrodynamics of Geldart A magnetite powder with modulation by ultrafine coal particles in Gas-solid Fluidized Bed Coal Beneficiator (GFBCB)

The Gas-solid Fluidized Bed Coal Beneficiator (GFBCB) is an important technology used in the dry coal beneficiation process, which requires lower bed density and very uniform density distribution. Geldart A− particles, Geldart A magnetite particles “modulated” by a small fraction of Geldart C ultrafine coal particles, is proposed. Fundamental studies are carried out to provide a comprehensive understanding of its hydrodynamics, including the minimum fluidization velocity, bed expansion, and spatial bed density distribution. By adding the ultrafine coal particles, the minimum fluidization velocity (Umf) is lowered, with a modified Umf correlation proposed, the bed expansion drastically increased (by up to 30%), with more gas being retained in the bed for improved flowability, and the corresponding bed density decreased to 1600–2000 kg/m3, with more uniform distribution, all contributing to a more desirable separation environment for coal beneficiation.

Pre-concentration of fine antimony oxide tailings using an agitated reflux classifier

Abstract Shaking table separation, the main method in dealing with antimony oxide tailings, has too many defects such as poor operational stability and low recovery. A new technical idea was proposed to solve these problems by using an agitated Reflux Classifier to pre-concentrate the tailings before they went to shaking tables. The principle of this separating system was based on hindered settling and agitation effect. This study focuses on the regulation of the bed density distribution in the agitated Reflux Classifier by mechanical agitation to influence particle separation. The experimental results showed that 73.13% of feed are discarded in the tailings by the agitated Reflux Classifier. At the same time, suitable mechanical agitation was able to improve particle properties (both in Sb grade and particle size), which is beneficial to the subsequent shaking table process. Hence this technology has clear potential for pretreating the antimony oxide tailings.

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

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

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

Abstract 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.

Characterization of temporal and spatial distribution of bed density in vibrated gas-solid fluidized bed

Abstract This study uses a Φ 200 mm × 900 mm vibrated gas-solid fluidized bed (VGFB) with −0.3 + 0.074 mm magnetite powder was utilized to characterize the temporal and spatial distribution of bed density in VGFB and the influence of bubble movement on fluctuations in bed density. The results indicate that the bed density decreases with an increase in gas velocity (U) and the frequency (f) and amplitude (A) of vibration and that the bed density spatial distribution is lower in the central region but higher in the border regions. The standard deviation of the density first increases then decreases and finally tends to stabilize with an increase in apparent gas velocity. Moreover, when A = 2 mm, f = 25 Hz and U = 14 cm/s, the density distribution is 1.82–1.88 g/cm3 and the fluidization state is improved. The energy of the pressure signal increases with an increase in gas velocity and vibration amplitude. In particular, the low-frequency band of the pressure signal exhibits the highest amplitude and energy, which reveals that bubbles are the main cause of pressure fluctuation. Furthermore, the bed density decreases with an increase in bubble generation frequency, and the relationship between these follows the ExpDec 2 mathematical equation.

Fluidization characteristics and fine coal dry beneficiation using a pronation-grille baffle dense phase medium fluidized bed

Abstract The fluidization of Geldart B particles commonly falls under the bubbling fluidization category, where the coalescence of bubbles is the dominant phenomenon. If the diameter of bubbles becomes too large, the fluidization quality will deteriorate and lead to significant maldistribution of bed density. In this study, a pronation-grille baffle was introduced into a gas-solid dense phase fluidized bed (GDPFB) in order to improve fluidization quality. This was termed as a pronation-grille baffle dense phase medium fluidized bed (PGBFB). The inhibitory effect of the pronation-grille baffle on the coalescence of bubbles and the large-scale particles back mixing, as well as the collaborative optimization of the baffle placing height and operating gas velocity were investigated. The density standard deviation ( S δ ) and density skewness ( SK δ ) were employed to evaluate the uniformity and stability of the bed density distribution. According to the experimental results, the S δ of a PGBFB was substantially reduced by 83.7%, compared to a GDPFB, and the lowest SK δ value was decreased to −0.0106, significantly improving the uniformity and stability of the bed density. Additionally, the beneficiation experiments were performed on a 1–6 mm raw coal in a PGBFB. The beneficiation results showed that the probable error, E, was 0.091 g/cm 3 and the ash content of clean coal was 16.63%, being reduced by 21.76%, compared to the ash content of the raw coal. This indicates that fine coal beneficiation using a PGBFB can provide a simple and efficient way for coal cleaning in arid and cold regions.

Numerical study on the effect of fine coal accumulation in a coal beneficiation fluidized bed

One important phenomenon in coal beneficiation fluidized bed (CBFB) is the accumulation of fine coal particles in beneficiation due to poor screening efficiency and attrition of coal samples. The effect of fine coal accumulation is numerically studied in this work using a TFM–DEM hybrid model. The gas phase and medium solid phase are modeled by a two-fluid model (TFM), while the fine coal particles are modeled by the discrete element method (DEM). Particles with a diameter of 0.9 mm are used as the fine coal sample in the simulation. The gas–solid flow pattern and particle dynamics are investigated with different concentrations of fine coal particles accumulated in the bed. For model validation purpose, the mean bed density distributions are compared with the experimental reports from He et al. (2013). The results show that a critical particle concentration exists in the fine coal accumulation process in CBFB. When the fine coal particles are less than 11 wt% in the bed, the flow pattern of medium phase is little affected and the coal particles are well mixed in the bed. However, when the particle concentration exceeds this threshold, the uniformity of bed density distribution is destroyed and particle stratification occurs along the bed height according to their density difference. Flow dynamics of the dense bed and main forces acting on the fine coal particles are analyzed to explain the underlying mechanism. With a large number of particles accumulated in the bed, the mixing effect of medium flow is suppressed. Motion of the fine coal particles is less dependent on the bed disturbance, instead, the particle gravity plays a decisive role in the particle distribution and results in the particle segregation in the fine coal accumulation process.

A model for predicting bubble rise velocity in a pulsed gas solid fluidized bed

Bed stability, and especially the bed density distribution, is affected by the behavior of bubbles in a gas solid fluidized bed. Bubble rise velocity in a pulsed gas–solid fluidized bed was studied using photographic and computational fluid dynamics methods. The variation in bubble rise velocity was investigated as a function of the periodic pulsed air flow. A predictive model of bubble rise velocity was derived: ub=ψ(Ut+Up-Umf)+kpgdb. The software of Origin was used to fit the empirical coefficients to give ψ = 0.4807 and kp = 0.1305. Experimental verification of the simulations shows that the regular change in bubble rise velocity is accurately described by the model. The correlation coefficient was 0.9905 for the simulations and 0.9706 for the experiments.

Bed density and circulation mass flowrate in a novel annulus-lifted gas–solid air loop reactor

Air loop reactors (ALR) have been widely used as promising and high-efficiency gas–liquid and gas–liquid–solid reactors. Extensive research on ALR has been conducted, but mostly limited to gas–liquid and gas–liquid–solid systems. Work associated with gas–solid systems has been rare and mainly focused on draft tube-lifted spouted bed treating coarse Geldart B, D particles. The present paper proposed a novel gas–solid air-loop reactor treating fine Geldart A particles and operating in a new annulus-lifted mode, with bubbling or turbulent bed upward flow in the annulus in parallel with bubbling bed downward flow in the draft tube. In view of these differences, distinct hydrodynamic behaviour can be anticipated for the gas–solid annulus-lifted air-loop reactor. The influence of operating conditions and geometric configuration on the distribution of bed density is discussed in a cold model annulus-lifted air loop reactor. A mechanistic model for the circulation mass flowrate is established based on an energy balance and resistance analysis. Nearly 50% and 30% of the energy dissipation rate occurs in the bottom and top regions, respectively. With increasing draft tube height, the energy dissipation rate increases in the annulus and draft tube regions, while it decreases in the top and bottom regions. The circulation mass flowrate decreases with increasing draft tube height. Analysis of the distribution of bed density and energy dissipation rate leads to suggestions regarding optimization of the design and axial location of the ring distributor and gap height.

Effect of Pressure Variation at Downcomer Bottom on Axial Bed Density Distribution in a Circulating Gas Fluidized Bed Riser.

The effect of static pressure variation at a downcomer bottom, Pdo, on axial bed density (ABD) distribution in a riser was examined in a circulating gas fluidized bed system consisting of a 0.1 m i.d. and 10.0 m high riser, a 0.2 m i.d. and 8.2 m high downcomer and a screw feeder between them. The ABD distribution and Pdo were determined from simultaneous measurements of static pressure distributions in the riser and the downcomer at a fixed superficial gas velocity of 1.3 m·s–1 as a function of the circulating solid flux, Gs, and the initial height of solid particles in the downcomer, Hsd, employing 0.054 mm FCC particles as a bed material. For most combinations of Gs and Hsd, no essential effect of the variation of Pdo was found on steady-state ABD distributions. The distributions depended appreciably on Pdo only when Gs takes a certain value where a dense region appears in the lower part of the riser. Under such conditions, a fluctuation of the height of the lower dense region was observed in correspondence to a transitional change in the pressure distribution in the downcomer. In addition, the steady-state dense region height was found to increase with the static pressure at the downcomer bottom which was varied through the change of Hsd and by aeration to the downcomer bottom.

Regime classification of macroscopic gas—solid flow in a circulating fluidized bed riser

Abstract A conceptual flow regime diagram for a circulating fluidized bed riser is proposed, combining existing investigations with experimental data obtained under idealized conditions in which a fully independent control of gas velocity and solid circulation rate was conducted by use of a screw feeder for solid feed into the riser. The diagram classifies the flow state into five regimes by qualitative transition lines which describe the relationship between gas velocity and solid circulation rate. These regimes are particulate fluidization, bubbling fluidization, turbulent fluidization, dense-phase transport and dilute-phase transport. The diagram suggests that S-shaped bed-density distribution or dense/dilute region interface appears only at limited conditions in the bubbling and turbulent fluidization regimes. These experimental findings were generalized by further experiments in a conventional circulation system with a ball valve between the riser and the downcomer which permits changes in the solid circulation rate and the bed height in the downcomer. The experimental results showed that the bed height in the downcomer has no particular effect on the bed density distribution or the height of the dense/dilute region interface, but an appreciable effect on the lowest gas velocity to maintain steady solid circulation at a given rate. These results are consistent with the above diagram.

CONCEPT OF SUCCESSIVE CONTACT MECHANISM FOR CATALYTIC REACTION IN FLUID BEDS

A concept of successive contact mechanism has been introduced to account for the influence of dilute phase catalyst on overall conversion of reaction in fluid beds. A theory has been given, as a simpler version of the proposed model, for an isothermal and irreversible first order reaction. To test the concept, axial distribution of bed density has been measured for small scale fluid beds. Rather freely suspended catalyst particles are observed in the dilute phase in enough amount to contribute to the progress of reaction. The theory has revealed for the progress of reaction that the catalyst particles located in the bubble phase are interchangeable with those suspended in the dilute phase and vice versa.

Cleaning tests of central Illinois coal

The fluidization of Geldart B particles commonly falls under the bubbling fluidization category, where the coalescence of bubbles is the dominant phenomenon. If the diameter of bubbles becomes too large, the fluidization quality will deteriorate and lead to significant maldistribution of bed density. In this study, a pronation-grille baffle was introduced into a gas-solid dense phase fluidized bed (GDPFB) in order to improve fluidization quality. This was termed as a pronation-grille baffle dense phase medium fluidized bed (PGBFB). The inhibitory effect of the pronation-grille baffle on the coalescence of bubbles and the large-scale particles back mixing, as well as the collaborative optimization of the baffle placing height and operating gas velocity were investigated. The density standard deviation (Sδ) and density skewness (SKδ) were employed to evaluate the uniformity and stability of the bed density distribution. According to the experimental results, the Sδ of a PGBFB was substantially reduced by 83.7%, compared to a GDPFB, and the lowest SKδ value was decreased to −0.0106, significantly improving the uniformity and stability of the bed density. Additionally, the beneficiation experiments were performed on a 1–6 mm raw coal in a PGBFB. The beneficiation results showed that the probable error, E, was 0.091 g/cm3 and the ash content of clean coal was 16.63%, being reduced by 21.76%, compared to the ash content of the raw coal. This indicates that fine coal beneficiation using a PGBFB can provide a simple and efficient way for coal cleaning in arid and cold regions.