Umf Values Research Articles

Correlation for predicting minimum fluidization velocity with different size distributions and bed inventories at elevated temperature in gas-solid fluidized bed

This study presents extensive experimental details of the effects of the solid particle size distribution (PSD), bed inventory, and bed temperature on the minimum fluidization velocity (Umf). Silica sand with three average solid particle diameters and five PSDs was used. The results showed that the Umf values of the wide PSDs were lower than those of the narrow cut particles with the same average particle diameter. The standard deviation (SD) and skewness of the PSD also influenced Umf variation. Furthermore, the Umf decreased with increasing bed inventory and bed temperature. The influencing parameters were recast into a dimensionless group and included in a new correlation for predicting Umf. The proposed correlation estimated an average absolute deviation of 14.6% between the experimental data and predicted values. Furthermore, the new correlation was evaluated with a dataset from the literature and gave an average absolute deviation of 15.6%.

Effect of Particle Size Distributions on Minimum Fluidization Velocity with Varying Gas Temperature

The particle size distribution (PSD) is an important property that can influence the hydrodynamics and chemical conversion in fluidized bed system. The objective of this study is to investigate the effect of PSDs of particle and gas temperature on minimum fluidization velocity (Umf). Here, the silica sand with three average diameters and five PSDs including narrow cut, Gaussian, Gaussian with high standard deviation, negative skewed distribution and positive skewed distribution were used. The considered gas temperature ranged from 30 to 120 °C. The results showed that the Umf values with wide PSDs were lower than the Umf values for narrow cut particle with the same average diameter. The reason can be explained by the addition of smaller particle will improve the fluidization characteristics. The standard deviation and skewness of PSD also influenced on the Umf. The Umf was observed to decrease with increasing gas temperature. In addition, the effect of average particle diameter could also be seen. The Umf increased with the increasing of average particle diameter.

Sensitivity analysis of the variational model for the particulate expansion of fluidized beds

Fluidization experiments were performed by using spherical and non-spherical particles to examine the sensitivity of the variational model to three input parameters: Umf, εmf and Ut. Several correlations for Umf, εmf and Ut were tested and the results were compared with the experimental values. The variational model’s ability to predict the bed expansion (in the form of U = f(ε)) and the interphase drag coefficient was investigated by varying the three input variables by ±20% compared with their experimental values. The variational model for the bed expansion showed the greatest sensitivity to changes in the εmf values. The overestimated values of εmf resulted in underestimating U(ε), and likewise, the underestimated values of εmf resulted in overestimating U(ε). The variational model’s sensitivity to changes in the Umf values is also important. Contrary to εmf, overestimating the values of Umf resulted in overestimating U(ε). The model’s least sensitivity was to the Ut value variation, which, if varied by ±20% compared with their experimental values, had a negligible effect on the prediction quality. This study aims to examine the sensitivity of the variational model to Umf, Ut and εmf values obtained from various correlations, in the absence of experimental values.

Empirical models for minimum fluidization velocity of particles with different size distribution in tapered fluidized beds

The particle size distribution (PSD) is known as an important parameter affecting the hydrodynamic behavior of fluidized beds. In this study, extensive experimental data for fluidization of particles with different PSDs in tapered fluidized beds was presented. In the experimentations, three Geldart B powders with four average diameters and various PSDs were used. The experimental results showed that the minimum fluidization velocity (Umf) for particles with flat and binary size distributions were roughly the same; moreover, they were up to 25% larger than the Umf values for narrow cut particle size distribution. The Umf values for Gaussian size distribution were lower than those of narrow cut PSD by about 15%. Using the experimental data and dimensional analysis, several empirical models for evaluating the minimum fluidization velocity for different PSD types in tapered fluidized beds were developed. The influence of operating conditions and geometric parameters of tapered fluidized beds recast into five dimensionless groups were included in the new empirical models. Comparing the predictions of the proposed models with the experimental data shows that the maximum absolute error for different PSDs was <18%. The average deviations of the proposed models from the experimental data are, respectively, 5.3%, 5.4%, 6.4% and 5.9% for narrow cut, Gaussian, flat and binary size distribution. The experimental data for different PSDs was also compared with the predictions of the available models for minimum fluidization velocity and the importance of particle size distribution was discussed. The effects of average particle size, initial bed height and tapered angle on Umf were also investigated and an increase of Umf with increasing these parameters was observed.

An examination of Wen and Yu’s formula for predicting the onset of fluidisation

Classical methods (measuring the pressure drop across a bed for different flowrates of air through the bed) were used to determine the superficial velocity for minimum fluidisation, Umf, of sieved particles of alumina. The particles were characterised optically using an instrument (Morphologi G3, Malvern Instruments), which, from pixelated, enlarged photographs, measured the particles’ mean diameter, dp, to be 0.48±0.04mm and their sphericity, ϕ, to be 0.77±0.09. Values of Umf were measured for the temperature in the electrically heated bed varying from 14 to 920°C. The results were analysed using Ergun’s equation; one outcome was that the voidage, εmf, in an incipiently fluidised bed was found to be related to the particles’ sphericity by:(1−εmf)/ϕ2εmf3=12.2±0.4.The measured Umf were significantly larger than the values predicted by Wen and Yu’s equation, if the mean diameter of the particles was taken, as recommended, to be the geometric mean of the upper and lower sieve sizes, used when preparing the particles. Alternatively, the measured values of Umf were over-predicted by Wen and Yu’s equation, when using the optically measured mean size of the particles. The best predictions of Umf were made by using the optically measured mean values of both dp and ϕ, together with Ergun’s equation and the above equation coupling εmf and ϕ. Such a procedure is proposed for estimating Umf in general. No evidence was found for εmf varying with temperature.

Prediction of bed expansion of polydisperse quartz sand mixtures fluidized with water

In this paper, the minimum fluidization velocity and bed expansion of polydisperse sand mixtures fluidized with water were investigated. The fluidization experiments were carried out using 19 different fractions of quartz filtration sand with sieve diameters in the interval dm=0.359 to 2.596mm. The intercept fluid velocity UE was determined from the plot of fluid superficial velocity vs. porosity. The results obtained indicated that UE was about 11% higher than particle terminal velocity Ut, for all of the sand fractions investigated.Based on the experimental data, the new model for bed expansion of quartz filtration sand was proposed. Using the proposed model, the mean absolute deviation in predicting the bed porosity ε, for a given superficial velocity U, was 2.66%, while the mean absolute deviation in predicting U at a given ε was 8.75%. For the prediction of the minimum fluidization velocity, Umf, several literature models were tested. The best results were obtained using the Ergun model, with mean absolute deviation in predicting Umf of 12.42%. A new simple correlation for the prediction of the minimum fluidization velocity Umf was proposed. The proposed correlation is the function only of the mean sieve diameter of the filtration sand fraction. The mean absolute deviation between the experimental and the predicted values of Umf was 7.65%.

Experimental and theoretical study on hydrodynamic characteristics of tapered fluidized beds

A novel fluidized bed ash cooler was developed for circulating fluidized bed boilers based on a proposed modified tapered fluidized bed. A cold model was built to study the hydrodynamic characteristics of the modified tapered fluidized bed, and its critical superficial gas velocity umf and critical velocity for full fluidization umff were particularly studied. The effects of taper angle α, static bed height H, air inlet section width δ and particle size dp on the umf and umff were experimentally investigated. Furthermore, a theoretical model and an empirical correlation have been proposed to predict the umf and umff, respectively. The predicting capabilities of the model and correlation have been experimentally discussed. And the predicting capability of the model has also been compared with that of an existing representative model. It is found that both the umf and umff increase with the increase of taper angle α, static bed height H and particle size dp, but decrease with the increase of air inlet section width δ, respectively. Additionally, the predicted values of umf and umff compare well with the experimental data, and the model has a better capability than the existing representative model in predicting the umf of the modified bed.

Pressure change and control of the solid circulation rate of Geldart A particles in a small diameter L-valve

In a circulating fluidized bed (CFB) with a riser 2.6 m in height and 0.009 m in ID, the solid circulation rate and pressure drop of Geldart A and B particles in an L-valve 0.016 m-ID × 0.22 m-horizontal length were investigated as a function of the change in aeration rate through the L-valve. The gas velocity in the riser was maintained at 2.1 m/s to circulate particles transported from the L-valve. The bubbling bed was maintained above the minimum fluidizing velocity, Umf. As a result, it was found that the solid circulation rate of KR40 (dp = 70 μm, ρp = 1730 kg/m3), located in the middle region of Geldart A, increased gradually with increasing aeration rate up to Umf in the L-valve and then increased sharply with increasing aeration rate. Therefore, the solid circulation rate of KR40 could not be controlled at velocities greater than Umf. On the other hand, in the case of FCC (dp = 85 μm, ρp = 1880 kg/m3), another set of Geldart A particles that are similar in size and density to KR40 is located at the boundary of Geldart A and B particles. These particles allowed the solid circulation rate to be controlled normally until the aeration rate in the L-valve reached the value of 3.5 Umf. The bubbling bed of KR40 exhibited a sharp increase in solid circulation rate at Umf and expanded by a factor of 1.05 as soon as the aeration rate reached the value of Umf. This phenomenon was not observed for the other particles in this study. At the moment of the sharp increase in the solid circulation rate of KR40, the pressure in the L-valve and standpipe drastically changed, indicating that the solid circulation rate was intimately related to the pressure drop in the L-valve. The correlation equation between the solid circulation rate and gas aeration in the L-valve was derived for Geldart A particles ranging from 70 ≤ dp ≤ 148 μm in particle size and 1,350 ≤ ρp ≤ 1880 kg/m3 in particle density.

Experimental flow behaviors of irregular particles with silica sand in solid waste fluidized bed

Co-fluidization characteristics of irregular particles with bed material have been investigated. Experiments were carried out in a solid waste fluidized bed with cross section of 0.2 m × 0.2 m and height of 2 m. Four particles differing in shapes, sizes and densities were used as simulative solid waste, and silica sand was employed as fluidization medium. The pressure drop, flow pattern and minimum fluidization velocity (Umf) under different operating conditions were investigated by recording pressure differential signals and fluidization images. A correlation of Umf was also developed. The results showed that during the fluidization of irregular particles with the bed material of silica sand, the pressure drop curve measured with increasing fluidization gas flow rate was visibly fluctuant and always underestimated the Umf value; while the pressure drop curve measured with decreasing fluidization gas flow rate was smooth and adequate to determine the value of Umf. The Umf was found to be increased with increasing volume proportion and effective particle density, while the initial static bed height has no significant effect on the minimum fluidization velocity. By comparing the Umf values predicted by the developed correlation with the present experimental data and those from literature, it was found that the correlation was quite satisfactory on predicting the Umf value for the fluidization of irregular particles with the bed material of silica sand. The correlation was also found to be applicable to the prediction of Umf values of a solid waste fluidized bed with a single kind or multi-kinds of irregular particles.

A New Method for Flow Regime Identification in a Fluidized Bed Based on Gamma-Ray Densitometry and Information Entropy

The accurate identification of the boundaries of the main flow regimes in fluidized beds is very important since the degrees of mixing and mass and heat transfer depend on the prevailing flow regime. Both the minimum fluidization velocity Umf (0.103 m·s−1) and the transition velocity Utrans (0.12 m·s−1) to bubbling fluidization regime have been successfully identified based on a newly developed maximum information entropy (IEmax) algorithm applied to gamma photon time series. The latter are recorded by means of gamma-ray densitometry scans performed in an air–polyethylene fluidized bed (0.438 m in ID) operated at ambient conditions. A comparison with the Kolmogorov entropy (KE) algorithm has found that the new approach yielded more accurate transition velocities. The value of Umf is also validated by means of the profiles of both bed pressure drop and bed height, respectively. It has been found, however, that these parameters are not capable of identifying the second transition velocity Utrans. It is demonstrated that the Umf value identified on the basis of IEmax is theoretically predictable. In the bubbling fluidization regime, a simple correlation between both the KE and IEmax values is developed.

Experimental and Theoretical Study on Fluidization of Stalk-Shaped Biomass Particle in a Fluidized Bed

Detailed knowledge of the fluidization mechanics of biomass itself, or of mixtures of biomass and inert particles is critical to successful design and operation of fluidized bed involving biomass. In this study, the fluidization behaviors of biomass alone and sand-biomass mixture were experimentally and theoretically studied. A segmented fluidized bed equipped with multiple pressure transducers was used as the experimental apparatus. The cotton stalk with a shape of stalk was employed as biomass material, and three different sizes of sand were utilized as inert particle. By mixing cotton stalk and sand, three binary systems were obtained, in which only the weight percentage of biomass particle in each mixture is different. The fluidized beds experienced the following states, depending on gas superficial velocity and initial mixture fraction: fixed, bubbling fluidization and turbulent fluidization. It is found that the additive of sand has positive influence on the fluidization of biomass and the initial arrangement of bed has influence on the development of pressure drop with the gas velocity. The developed equation for predicting the minimum fluidization velocity (Umf) of binary mixture quite satisfactorily predicts the Umf values of mixture with low biomass concentration. In addition, the correlation expressed in terms of Reynolds and Archimedes numbers to predict transition velocity (Uc) is proposed. Calculated Uc is in excellent agreement with the experimental data.

Sound-Assisted Fluidization of SiO2 Nanoparticles with Different Surface Properties

Sound-assisted fluidization of two kinds of SiO2 nanoparticles (having primary sizes of 5−10 nm) is investigated in this study. The two kinds of nanoparticles are SiO2 without any surface modification (NSM-SiO2) and SiO2 modified with an organic compound (SM-SiO2). The introduction of a 99.8−103.4 dB and 50 Hz acoustic field reduces the superficial minimum fluidization gas velocity, Umf,super, significantly for the two kinds of nanoparticles, and when the sound pressure level increases, the values of Umf,super decrease gradually. With the agitation of a 100 dB acoustic field, the SM-SiO2 nanoparticles could fluidize smoothly similar to Geldart-A particles over the frequency range of 40−60 Hz. NSM-SiO2 failed to fluidize so smoothly, but significant improvement was observed over such a frequency range. Different fluidization behavior, different bed expansion, and agglomerating behavior were also observed for the two kinds of nanoparticles, which indicate that the surface properties of nanoparticles have significant influences on their fluidization behaviors.

Particle Fluidization with Supercritical Carbon Dioxide: Experiments and Theory

Glass and Amberlite particles ranging in size from 0.18 to 1.0 mm were fluidized by supercritical CO2. For a given material in a narrow size range, the volume of the random packing increases exponentially with CO2 mass flow rate. In the supercritical region investigated (35−55 °C, 83−124 bar), the bed expansion has relatively weak temperature and pressure dependences due to the counteracting effects of the density and viscosity variations of supercritical CO2. Consistent with conventional particle fluidization correlations, the minimum fluidization velocity (umf) values obtained experimentally depend only on the packing material, size, and fluidization conditions (pressure and temperature). A plot (log u vs log e) of the Richardson−Zaki correlation is nearly linear with the value of the exponent (∼2.6) being in the range of values reported for spherical particles. The umf values and terminal velocity (ut) predicted with conventional correlations bracket those obtained from experiments, confirming that suc...

Air Gasification of Dried Sewage Sludge in a Fluidized Bed: Effect of the Operating Conditions and In-Bed Use of Alumina

Sewage sludge has recently become a particularly important problem all over the world because of its harmful impact on the environment. The consequent need to develop alternative processes for the use of dried sewage sludge for energy purposes, such as gasification, requires experimental tests in order to quantify the potential energy power of the sludge, as well as to evaluate the optimum conditions for its gasification. There is, however, little information available. In this study, the gasification with air of dried sewage sludge was experimentally investigated using a bubbling fluidized bed. Attention was focused on the influence of the temperature (750−850 °C), the equivalence ratio (25−35%), and the fluidizing velocity (5, 8, and 11 times the value of umf) on the product yields, gas composition, thermal efficiency, and tar content. The results obtained show the potential for using sewage sludge gasification with air as an option for energy recovery and waste treatment. However, the high tar yield obtained needs to be reduced. Consequently, experiments involving the in-bed use of alumina were carried out. This primary measure has proven effective for tar removal (the content decreases by nearly 40% with in-bed use of alumina at 5 wt%).

Fluidization Behavior of Vibrated and Aerated Beds of Starch Powders.

Batch experimental work on the fluidization hydrodynamic and entrainment phenomena of cohesive starch powders was studied. Results show that fluidization can be realized as vibration break particleparticle bond in the bed. The higher Umf values compared to predictions indicate the formation of agglomerates in the bed. This has increased the residence time of fines to reduce material loss by entrainment. It was found that using starch powders as feed materials, entrainment was only in the range of 0.5-1.7% of the total fines (should be entrained) in the bed. As has also been found by other workers, the entrainment rate constants of group C decrease as particle size decreases. By adapting the criterion proposed by Rietema (1984) as well as Ma and Kato (1998), it was found in this case that the critical cohesion number, Ncoh* is 19.