Effect of elevated temperature and silica sand particle size on minimum fluidization velocity in an atmospheric bubbling fluidized bed

The impact of temperature and particle size on minimumfluidizing velocity was studied and analyzed in a small pilot scale of bubbling fluidized bed reactor.This study was devoted to providing some data about fluidization to the literature under high temperature conditions.The experiments were carried out to evaluate the minimum fluidizing velocity over a vast range of temperature levels from 20℃ to 850℃ using silica sand with a particle size of 300-425μm,425-500μm,500-600μm,and 600-710μm.Furthermore,the variation in the minimumfluidized voidage was determined experimentally at the same conditions.The experimental data revealed that the Umf directly varied with particle size and inversely with temperature,whileεmf increases slightly with temperature based on the measurements of height at incipient fluidization.However,for all particle sizes used in this test,temperatures above 700℃ has a marginal effect on Umf.The results were compared with many empirical equations,and it was found that the experimental result is still in an acceptable range of empirical equations used.In which,our findings are not well predicted by the widely accepted correlations reported in the literature.Therefore,a new predicted equation has been developed that also accounts for the affecting of mean particle size in addition to other parameters.A good mean relative deviation of 5.473% between the experimental data and the predicted values was estimated from the correlation of the effective dimensionless group.Furthermore,the experimental work revealed that the minimum fluidizing velocity was not affected by the height of the bed even at high temperature.

Temperature influence on minimum fluidization velocity:Complexity,mechanism,and solutions

Fluidized-bed reactors are widely employed in various high-temperature industrial processes.Thus,it is crucial to understand the temperature effect on various fluidization phenomena,specifically the minimum fluidization velocity(U_(mf))that governs various aspects of fluidized bed behavior.In this study,we comprehensively analyze U_(mf) data from the literature to unravel the complexity and underlying mechanisms of temperature influence on this critical velocity.The research examines experimental data encompassing a wide range of temperatures,pressures,and solid particles.The analysis reveals that the influence of temperature on U_(mf) is fundamentally determined by the relative importance of hydrodynamic forces and interparticle forces within fluidized beds and is realized by three distinctive temperature-induced changes:gas properties,bed voidage,and physiochemical characteristics of particles.On this basis,an equation is derived to enable predictions of temperature influences on the minimum fluidization velocity under broad temperature conditions.

Performance analysis of a 2D numerical model in estimating minimum fluidization velocity for fluidized beds

The minimum fluidization velocity of a fluid-solid particle fluidized bed is the primary focus of this paper.The computationally economic Eulerian Granular model has been used to analyze fluidization for both gas-solid particle and liquid-solid particle fluidized beds.The conventional approach of finding minimum fluidization velocity(umf)is either with a pressure drop across the particle bed or the change in bed height.However,these parameters are often unstable and cannot be used to generalize the degree of fluidization accurately.In this paper,the dominant factor of unstable pressure drop estimation in the 2D Two-Fluid Model(TFM)and a key non-dimensional Euler number has been investigated in deter-mining minimum fluidization velocity for different quasi-2D fluidized beds for different bed sizes,par-ticle sizes,and particle numbers.Averaging assumptions and limitations of these numerical models are discussed in detail for four different fluidized bed cases.A comparative study of the drag model shows little to no influence in unstable pressure drop estimation near fluidization velocity,and all drag models perform similarly.It is observed that particle-particle collision is not the dominant reason for unstable pressure drop near minimum fluidization.Instead,wall effects on the particle bed including frictional losses and wall-particle collision play a key role in unstable pressure drop calculation for the quasi-2D fluidized beds.Pressure drop characteristics alone do not suffice to obtain minimum fluidization ve-locity with 2D TFM using existing models.Thus,a different approach has been proposed to investigate minimum fluidization involving the Euler number,which has shown promising performance in deter-mining minimum fluidization velocity and characterizing fluidization with 2D TFM.Results show con-sistency in Euler number characteristics for all different fluidized bed cases considered in this paper.This can revitalize computationally economic 2D Eulerian simulations,increase the range of possible appli-cations,and provide guidance to the future development of computationally efficient and more accurate numerical models,and empirical correlations for minimum fluidization velocity.

Effect of Pressure on Minimum Fluidization Velocity

Minimum fluidization velocity of quartz sand and glass bead under different pressures of 0.5, 1.0, 1.5 and 2.0 MPa were investigated. The minimum fluidization velocity decreases with the increasing of pressure. The influence of pressure to the minimum fluidization velocities is stronger for larger particles than for smaller ones. Based on the test results and Ergun equation, an experience equation of minimum fluidization velocity is proposed and the calculation results are comparable to other researchers＇ results.

Effect of agitation on the characteristics of air dense medium fluidization

In order to study the effect of agitation on the characteristics of air dense medium fluidization, we designed and constructed an agitation device. Analyses were then conducted on the fluidization characteristics curves, the bed density stability and the average bubble rise velocity Uaunder different agitation conditions. The results indicated that a lower bed pressure drop(without considering lower gas velocity in a fixed bed stage) and higher minimum fluidized velocity are achieved with increasing agitation speed.The height d(distance between the lower blades and air distribution plate) at which the agitation paddle was located had a considerable effect on the stability of the bed density at 9.36 cm/s < U < 10.70 cm/s. The higher the value of d, the better the stability, and the standard deviation of the bed density fluctuation r dropped to 0.0364 g/cm^3 at the ideal condition of d = 40 mm. The agitation speed also had a significant influence on the fluidization performance, and r was only 0.0286 g/cm^3 at an agitation speed of N = 75 r/min. The average bubble rise velocity decreased significantly with increasing agitation speed under the operating condition of 1.50 cm/s < U–U_(mf)< 3.50 cm/s. This shows that appropriate agitation contributes to a significant improvement in the fluidization quality in a fluidized bed, and enhances the separation performance of a fluidized bed.

Effect of Gaussian size distribution width on minimum fluidization velocity in tapered gas–solid fluidized beds

This paper investigated the effect of Gaussian distribution width,average particle diameter,particle loading,and the tapered angle on minimum fluidization velocity(U_(mf))by conducting extensive experiments in tapered fluidized beds.Three powders with Gaussian size distribution and different distribution widths were used in the experiments.An increase in U_(mf)with increasing the average particle diameter,particle loading,and the tapered angle was observed.There was also a nonmonotonic behavior of Umf as the Gaussian distribution width increased.An empirical correlation including dimensionless groups for predicting Umf in tapered beds was developed in which the effect of distribution width was considered.The proposed correlation predictions were in good agreement with the experimental data,with a maximum deviation of 16.5%and average and standard deviations of,respectively,6.4%and 7.4%.The proposed correlation was also compared with three earlier models,and their accuracy was discussed.

Modified correlation for minimum fluidization velocity of low-density particles in inverse liquid-solid fluidized beds

The minimum fluidization velocity(U_(mf))is a key parameter for the scale-up of inverse liquid-solid flu-idized beds.Theoretical predictions using common correlations were compared against experimental minimum fluidization velocity measurements of low density(28-638 kg/m^(3)),0.80-1.13 mm Styrofoam particles in a fluidized bed with a height of 4.5 m and 0.2 m diameter.The average absolute relative deviation for the predicted minimum fluidization velocity for particles below 300 kg/m^(3) was above 40%using the studied common correlations.A modified Wen and Yu correlation was thus proposed based on novel and past measurements with low-density and small-diameter particles,expanding the range for predicting U_(mf).The new correlation predicted U_(mf) with deviations below 15%for ST028,ST122 and ST300.This modified correlation also improved U_(mf) predictions for comparable particles from a previous study,demonstrating its validity for a larger range of low-density particles.

Predicting minimum fluidization velocities of multi-component solid mixtures

Employing well-established mixing rules for mean properties, appropriate expressions are derived for predicting minimum fluidization velocities of multi-component solid mixtures in terms of mono- component values for the velocity and the bed voidage at incipient fluidization. Based on flow regime and the mixing level of constituent species, it is found that these relationships differ significantly from each other, whether related to size-different or density-different mixtures. For mixed beds of size-different mixtures, the effect of volume contraction is accounted for by the mean voidage term, which is absent for segregated beds. Incorporating the volume-change of mixing leads to values of the mixture minimum fluidization velocities even lower than corresponding values for segregated bed, thus conforming to the trend reported in the literature. Size-different mixtures exhibit flow regime dependence irrespective of whether the bed is mixed or segregated. On the other hand, the mixing of constituent species does not affect the minimum fiuidization velocity of density-different mixtures, as the difference in the expres- sions for a segregated and a mixed system is rather inconsequential. Comparison with experimental data available in the literature is made to test the efficacy of the minimum fluidization velocity expressions derived here.

Investigation into the hydrodynamics of liquid-solid inclined micro-fluidized beds

The inclined micro-fluidized bed(MFB)can enhance heat and mass transfer rates compared to the vertically aligned counterparts,but the increased significance of surface forces and wall effects may cause poor fluidization performance.In this paper,the effects of column inclination and different particle-to-bed ratios(d_(P)/d_(B))on the solid hydrodynamics are investigated in an inclined micro-fluidized bed.The results validated the suitability of using the Ergun equation to predict minimum fluidization velocities due to small deviations between 1.01 and 1.81 times the theoretical values,for a particle-to-bed ratio ranging from 0.025 to 0.165 at inclinations between 0°and 10°.Investigation into the effects on bed expansion behavior showed that the bed contracted with an increase in bed inclination.An unexpected observation during the bed expansion was the appearance of a secondary high voidage region and the appearance of strong circulation patterns with an increase in bed inclination.A detailed analysis of this phenomenon suggested the presence of a critical angle at 6°and 10°for the 85μm particles,4×4 mm bed cross-section and 165μm particles,1×1 mm bed cross-section,respectively.However,the liquid-solid back-mixing was observed due to the modified particle trajectories resulted in the disappearance of the high voidage region.This paper gives new insights into the micro-fluidization behavior in inclined beds thus contributing to the development of micro-fluidized beds and their future applications.

Investigation of drying characteristics of low rank coal of bubbling fluidization through experiment using lab scale

Lignite is a low rank coal which is evenly distributed throughout the world and accounts for 45% of the total coal reserves. As it has a higher moisture content, its moisture content must be reduced in order to utilize it in power plant. In the present work, experiments on lignite has been done using a lab scale fluidized-bed reactor. Drying lignite through fluidized-bed reactor has a higher drying rate because there is good contact between particles and gas in the fluidized-bed reactor. Fluidized-bed drying can use air of 1.5 times of the minimum fluidizing velocity performance at bubbling fluidized-bed. Experiments have been performed on coal particle sizes of 0.3-1 mm, 1-1.18 mm and 1.18-2.8 mm, with operating temperatures being 100℃, 125℃ and 150℃, respectively. It is found that fluidization has a higher drying rate due to the heat transfer rate through air velocity. Hence, moisture content in lignite can be dried to a desired value with a time interval of 10 rain. The experiment through fluidized-bed reactor is expected to be useful for saving money and time.

Hydrodynamic behavior of liquid-solid micro-fluidized beds determined from bed expansion

The bed-expansion characteristics of liquid-solid micro-fluidized beds were experimentally studied. Bed columns with inner diameters of 0.8, 1.45, and 2.3 mm were fabricated based on capillaries. Five parti- cle sizes in a range of 22-58 t^m were investigated. Bed-expansion curves were plotted using visually recorded bed-expansion heights. The bed expansion and initial fluidization behavior were compared with predictions for conventional-scale beds, Evident differences are reflected in lower expansion ratios and higher minimum fluidization velocities for micro-fluidized beds. These were attributed to the increase in the internal surface area of the particle beds and specific surface area of wall contact. The wall effect for micro-fluidized beds at higher particle/bed diameter ratios caused higher local voidage and an increase in expansion ratio. Correlations for the exponent and proportional coefficient in the Richardson-Zaki equation for micro-fluidized beds were proposed. The minimum fluidization velocities were correlated using a modification of the Ergun equation.

Fluidization of nano and sub-micron powders using mechanical vibration

The fluidization behavior of nano and sub-micron powders belonging to group C of Geldart＇s classification was studied in a mechanically vibrated fluidized bed （vibro-fluidized bed） at room temperature. Pretreated air was used as the fluidizing gas whereas SiO2. Al2O3, TiO2, ZrSi, BaSO4 were solid particles. Mechanical vibration amplitudes were 0.1, 0.25, 0.35, 0.45mm, while the frequencies were 5, 20, 30, 40 Hz to investigate the effects of frequency and amplitude of mechanical vibration on minimum fluidization velocity, bed pressure drop, bed expansion, and the agglomerate size and size distribution, A novel technique was employed to determine the apparent minimum fluidization velocity from pressure drop signals. Richardson-Zaki equation was employed as nano-particles showed fluid like behavior when fluidized. The average size of agglomerates formed on top of the bed was smaller than those at the bottom, Size distribution of agglomerates on top was also more uniform compared to those near the distributor. Larger agglomerates at the bottom of the bed formed a small fraction of the bed particles. Average size of submicron agglomerates decreased with increasing the frequency of vibration, however nano particles were less sensitive to change in vibration frequency. Mechanical vibration enhanced the quality of fluidization by reducing channeling and rat-holing phenomena caused by interparticle cohesive forces.

Statistical modeling and optimization of a multistage gas-solid fluidized bed for removing pollutants from flue gases

The present paper describes the statistical modeling and optimization of a multistage gas-solid fluidized bed reactor for the control of hazardous pollutants in flue gas. In this work, we study the hydrodynamics of the pressure drop and minimum fluidization velocity. The hydrodynamics of a three-stage fluidized bed are then compared with those for a single-stage unit. It is observed that the total pressure drop over all stages of the three-stage fluidized bed is less than that of an identical single-stage system. However, the minimum fluidization velocity is higher in the single-stage unit. Under identical conditions, the minimum fluidization velocity is highest in the top bed, and lowest in the bottom bed. This signifies that the behavior of solids changes from a well-mixed flow to a plug-flow, with intermediate behavior in the middle bed.

Effect of volume-contraction on incipient fluidization of binary-solid mixtures

Towards the development of a predictive model for computing the minimum fluidization velocity, the volume-contraction phenomenon arising from the mixing of unequal solid species is accounted for in the prediction of the bed void fraction of binary-solid mixtures at the incipient fluidization conditions. Com- parison with experimental data obtained from the literature clearly shows that significantly improved predictions are obtained except for cases where the stratification pattern whether arising from the slow defluidization or the difference in the densities of the two species affects the mixing of the constituent species.

Validated scale-up procedure to predict blockage condition for fluidized dense-phase pneumatic conveying systems

This paper presents results of an ongoing investigation into modelling fluidized dense-phase pneumatic conveying of powders. For the reliable design of dense-phase pneumatic conveying systems, an accurate estimation of the blockage boundary condition or the minimum transport velocity requirement is of sig- nificant importance. The existing empirical models for fine powder conveying in fluidized dense-phase mode are either based on only a particular pipeline and product or have not been tested for their accuracy under a wide range of scale-up conditions. In this paper, a validated test design procedure has been devel- oped to accurately scale-up the blockage boundary with the help of a modelling format that employs solids loading ratio and Froude number at pipe inlet conditions using conveying data of two different samples of fly ash, electro-static precipitation （ESP） dust and cement （particle densities： 2197-3637 kgJm3; loose poured bulk densities： 634-1070kg/m3; median size： 7-30 l^m）. The developed models （in power func- tion format） have been used to predict the blockage boundary for larger diameter and longer pipelines （e.g. models based on 69 mm I.D. ~ 168 m long pipe have been scaled up to 105 mm I.D. and 554 m length）. The predicted blockage boundaries for the scale-up conditions were found to provide better accuracy compared to the existing models.