Bubbling and Bed Expansion Behavior Under Vibration in a Gas‐Solid Fluidized Bed

The effect of vibration on the flow patterns and fluidization characteristics including the minimum fluidization velocity (u_mf), the void fraction (ϵ_mf) at u_mf and the bed expansion ratio were examined. The powders used were spherical glass beads and their diameters were 6, 20, 30, 60 and 100μm. For group A powders, the manner in which the vibration affects the bubble formation was examined from the bed expansion ratio and the index of n/4.65. The area of the homogeneous fluidization region was also observed. The homogeneous fluidization region was broadened at a certain vibration strength, where the value of n/4.65 was a minimum. The bubble formation was observed even for 20μm powder (group C), at large vibration strengths and at high gas velocities. Under such conditions, the bed expansion ratio increased suddenly due to bubble formation. The bubbles broke the irregular bed structure, including various properties of agglomerates. Although the channel breakage was dominant flow pattern for group C powders, the bubbles also played an important role in the improvement of the fluidization.     

Influence of Effective Particle Porosity on Vacuum Fluidized Bed Drying

A series of vacuum fluidized bed drying experiments was carried out employing particles with distinct effective porosities. The experiments showed that decreasing the operating pressure as well as increasing the operating temperature produces higher drying rates in both drying periods. In all cases, the enhancement of the drying rates was found to be more significant in the case of particles with higher effective porosities. It is concluded that lower operating pressures as well as higher operating temperatures help to reduce the formation of air pockets in the pores and postpone the onset of the pendular state, enhancing the capillary action and the drying rates.     

Mass Transfer Coefficient for Drying of Moist Particulate in a Bubbling Fluidized Bed

Experiments on drying of moist particles by ambient air were carried out to measure the mass transfer coefficient in a bubbling fluidized bed. Fine glass beads of mean diameter 125 μm were used as the bed material. Throughout the drying process, the dynamic material distribution was recorded by electrical capacitance tomography (ECT) and the exit air condition was recorded by a temperature/humidity probe. The ECT data were used to obtain qualitative and quantitative information on the bubble characteristics. The exit air moisture content was used to determine the water content in the bed. The measured overall mass transfer coefficient was in the range of 0.0145–0.021 m/s. A simple model based on the available correlations for bubble‐cloud and cloud‐dense interchange (two‐region model) was used to predict the overall mass transfer coefficient. Comparison between the measured and predicted mass transfer coefficient have shown reasonable agreement. The results were also used to determine the relative importance of the two transfer regions.     

A Practical Method to Estimate the Bed Height of a Fluidized Bed of Fine Particles

Knowledge of both dense bed expansion and freeboard solids inventory are required for the determination of bed height in fluidized beds of fine particles, e.g., Fluidized Catalytic Cracking (FCC) catalysts. A more accurate estimation of the solids inventory in the freeboard is achieved based on a modified model for the freeboard particle concentration profile. Using the experimentally determined dense bed expansion and the modified freeboard model, a more practical method with improved accuracy is provided to determine the bed height both in laboratory and industrial fluidized beds of FCC particles. The bed height in a fluidized bed can exhibit different trends as the superficial gas velocity increases, depending on the different characteristics of the dense bed expansion and solids entrainment in the freeboard. The factors that influence the bed height are discussed, showing the complexity of bed height and demonstrating that it is not realistic to determine the bed height by a generalized model that can accurately predict the dense bed expansion and freeboard solids inventory simultaneously. Moreover, a method to determine the bed height, based on axial pressure fluctuation profiles, is proposed in this study for laboratory fluidized beds, which provides improved accuracy compared to observation alone or determining the turning points in the axial pressure profiles, especially in high‐velocity fluidized beds.     

Reduction of Devolatilization Rate of Fuel During Bubbling Fluidized Bed Combustion Using Porous Bed Material

Plastic waste combustion in bubbling fluidized bed combustors (BFBC) is characterized by the rapid devolatilization of the fuel. Noncombusted hydrocarbons are often formed, which have been reported to promote the formation of dioxins. In this work, porous bed material was employed instead of commonly used non‐porous sand to reduce the devolatilization rate. We measured (1) the heat transfer coefficient between an immersed object (brass sphere) and the bed and (2) the time required for the devolatilization of a plastic pellet after dropping it into the bed at 943 K. For porous particles we found a 30 % lower heat transfer coefficient, delayed onset of devolatilization and prolonged devolatilization time, compared with quartz sand. Therefore, porous particles were found to be effective in suppressing the rapid devolatilization of plastic waste.     

Granular Temperature in Bubbling Fluidized Beds

In the CFD simulation of industrial‐scale bubbling fluidized beds, coarse grids are necessary due to limited computation resources, which creates a problem about how to characterize the effect of meso‐scale bubbling structures on the particulate phase stresses and inter‐phase interaction force. In this paper, the two‐phase theory of bubbling fluidization is applied in the analysis of the fluctuation characteristics of solid volume fractions and then it was used to establish a simple model for granular temperature in bubbling fluidized beds, which is based on the local pseudo‐thermal energy balance. It was shown that the granular temperature obtained experimentally is well predicted by the present model.     

Reduction of the Volatile Matter Evolution Rate from a Plastic Pellet during Bubbling Fluidized Bed Pyrolysis by Using Porous Bed Material

Rapid volatile matter evolution from high‐volatile fuels such as wastes and biomass is one of problems associated with fluidized bed incinerators and gasifiers. When volatile matter evolves rapidly in the vicinity of the fuel feed point, the mixing of volatile matter with reactant gas is poor, and therefore, unreacted volatile matter is expected to be released from the reactor. In the present work, reduction of the volatile matter evolution rate was attempted by employing porous solids as bed materials instead of nonporous sand. The effect of bed material on the onset of devolatilization was measured by use of a bench‐scale bubbling fluidized bed reactor. Volatile matter capture by the porous solids (capacitance effect) and the heat transfer rate within the bed, both of which affect volatile matter evolution rate, were also measured. Four types of porous solids, both with and without capacitance effect, were employed as the bed material. By employing porous solids without capacitance effect, the contributions of reduced heat transfer rate and capacitance effect to the delay of volatile matter evolution can be evaluated separately. For porous bed materials with a moderate capacitance effect (volatile matter capture of up to 20 %), the delay of the onset of devolatilization, which was measured by detecting the flame combustion of the volatile matter, was explained by the lower heat transfer between the fuel and bed. However, for a porous particle with high capacitance effect (volatile matter capture of 30 %), the capacitance effect also affected the delay of the onset of the flame combustion.     

Fluidization Characteristics of SiO2 Nanoparticles in an Acoustic Fluidized

Under the action of an acoustic field, the fluidization behavior of 5–10 nm SiO₂ nanoparticles, with and without surface modification, was investigated. In a packed bed, the sound wave energy has a significant influence on the compact ratio of the bed. Experimental results indicated that the bed of nanoparticle agglomerates can be fluidized smoothly with the assistance of an acoustic field, and the minimum fluidization velocity is initially reduced dramatically with increasing sound frequency and then rises with increasing sound frequency. Under the same experimental conditions, the minimum fluidization velocity of 5–10 nm SiO₂ nanoparticles is greater than that of 5–10 nm SiO₂ nanoparticles with surface modification. The collapse of the bed demonstrates that SiO₂ nanoparticles, surface modified using organic compound, have longer minimum collapse times than SiO₂ nanoparticles.     

Measuring Average Particle Size for Fluidized Bed Reactors by Employing Acoustic Emission Signals and Neural Networks

Average particle size is one of the key parameters of fluidized bed reactors. A novel method to measure the average particle size in fluidized bed reactors by acoustic emission (AE) signal is proposed. The measurement of the average particle size by AE is superior to other methods since it is inherently safe to use and it is also a non‐invasive method. AE signals originating from different particle sizes were 8‐level decomposed by a Daubechies wavelet of order 3 after being denoised with a sym8 wavelet filter combined with the rigrsure threshold method. Principal component analysis (PCA) was applied to overcome the complex collinearity and reduce the number of input variables of the neural network. A feed‐forward back propagation neural network with two hidden layers was used to predict the average particle size according to the principal components. The results show that this soft‐measuring model is suitable for measuring the average particle size in the fluidized bed reactors by online AE. It achieved high accuracy when applied to a model‐scale fluidized bed.     

Expansion Behavior of a Binary‐Solid Liquid Fluidized Bed with Large Particle Size Difference

The overall expansion of two dissimilar solid particle species with over a tenfold difference in their size and substantial density difference is investigated here for different compositions of the fluidized bed. Contrary to the widely held notion that the total bed height would be the sum of the heights of the two segregated mono‐component beds, the actual bed heights were, in fact, found to be lower. This volume contraction is found to strongly depend upon the mixing behavior prevailing in the binary‐solid fluidized bed. At the complete mixing of the two solid species, the bed‐contraction versus liquid velocity profile shows a global maximum. As a result, the overall bulk density profiles are similarly affected. Moreover, it is found here that correlations meant for predicting the porosity of the packing of binary particle mixtures can be satisfactorily extended to binary‐solid fluidized beds where solid species differ significantly in size.     

声场流化床A类颗粒浓度分布研究

在内径140 mm,高1 600 mm的鼓泡流化床中,以流化催化裂化(FCC)颗粒为流化介质,采用光导纤维探针测定不同轴/径向位置的颗粒浓度分布.考察了操作气速和外加声场对密相区颗粒浓度的影响.结果表明,鼓泡床密相区颗粒浓度沿轴向逐渐减小,沿径向呈抛物线分布.声场的引入可以降低颗粒起始流化速度:声压级越大,起始流化速度越小:固定声压频率在150 Hz时颗粒起始流化速度最小.1随着声压强度的增大,床层中心区和上部密相区颗粒浓度增大.固定声压级,频率在100～400 Hz颗粒浓度较大,频率低于100 Hz或高于400 Hz时,声波的作用效果减弱.     

Fluid Dynamics of Spouted Bed Apparatus with Two Fluid Inlets: An Experimental Study

Spouted bed apparatuses are already used in some technical areas for a variety of chemical and metallurgical operations. One of the advantages of spouted beds compared to the fluidized beds is the possibility of achieving better conditions for an intense heat and mass transfer. Spouted beds are characterized by a relatively simple construction and an easy design, which allows the scale‐up of the spouted bed apparatuses and the creation of industrial equipments.     

Two-Phase and Three-Phase Modeling of an Industrial Fluidized Bed for Maximizing Iso-Paraffins

The maximizing iso-paraffins (MIP) process is a new fluid catalytic cracking route to produce cleaner gasoline. Its major innovation is the diameter-transformed fluidized bed reactor which can be flexibly regulated to multiple distinct reaction zones. This study aims to accurately reveal complex behaviors in MIP reactor by two-phase modeling and three-phase modeling to extend its application. Both simulations of an industrial MIP reactor are compared in terms of solids and liquid concentration, temperature, coke content, gas velocity, and product yield. It is found the two-phase case is enough for predicting the region far away from the oil feedstock injection, and the other is the better choice especially in the first zone if the heat transfer model can be reasonably built.     

Fluid Dynamics of Fluidized and Vibrofluidized Beds Operating with Geldart C Particles

The fluid dynamic behavior of a vibrofluidized bed operating with Geldart C particles was studied. The experiments were conducted in order to observe the influence of amplitude, frequency, and dimensionless vibration number on the minimum fluidization velocity, pressure drop, and standard deviation obtained. It was noted that the dimensionless vibration number should be used very carefully if it is to be the unique parameter to set the vibrational effect of the bed fluid dynamics. The results clearly indicate that the fluid dynamic behavior of the bed is very dependent on the different combinations of amplitude and frequency for the same dimensionless vibration number. Therefore, the use of the amplitude or frequency of vibration and the dimensionless vibration number is recommended for a better characterization of the vibrational effects on the fluid dynamic behavior of the particle bed.     

Design Parameter Effects on Expansion in Fluidized Beds with Inserts

The effect of some design parameters on the expansion of particles has been studied in a fluidized bed of 300 mm diameter. Four distributors were examined; three perforated plates, each perforated by holes of 0.8 mm in diameter but different hole densities at 6 mm, 9 mm and 12 mm pitch, and a porous plastic distributor 17 mm thick. Particles of different materials in the Archimedes number range from 100 to 10⁵ were fluidized. The inserts were held vertically as arrays. All experimental data for four distributors were correlated within experimental error by the equation: whereU, U_mf, U₀ are the gas velocity, velocity at minimum fluidization and real or apparent terminal velocity, while e is the bed porosity and e_mf is the porosity at the condition of minimum fluidization. P is the hole pitch of perforated plate distributor in millimeters.     

Modeling of Volatiles Combustion and Alkali Deposition in a Fluidized Bed Coal Combustor

A simplified kinetic model, coupled with the bed hydrodynamics and a volatile evolution region within the bed, was formulated to predict the extent of gas‐phase combustion in a laboratory‐scale fluidized bed coal combustor (FBC). A close examination has also been made to highlight the relevance of the reducing/oxidizing environment (computed with the present theoretical model) in relation to FBC materials exposed to fireside corrosion at high temperature, under various operating conditions. The model results revealed that, for high‐volatile coals with particle diameters (d_c) of 1–3 mm and sand particle size (d_s) of 0.674 mm, over one third of the original coal volatiles may burn in the freeboard region at bed temperature (T_b) ≤ 850 °C and excess air (XSA) ≤ 10 %. These values, together with the computed equilibrium conversion of alkali chlorides to sulfates, may suggest that sodium and potassium salts present in the vapor phase are likely to accelerate hot corrosion of heat exchange tubes above the bed when an FBC operates at T_b ≤ 840 °C, XSA ≤ 20 %, d_c < 5 mm, d_s < 1 mm, H_s ≤ 0.2 m and U_o < 1 m/s. Conversely, at T_b > 890 °C and XSA > 30 %, high oxidation rates may be present for the in‐bed tubes. At these higher T_b values and XSA < 10 %, a sulfidation mechanism presumably influences the extent of corrosion on the metallic components within the bed.     

Total Solution of the Gibilaro and Rowe Model for a Segregating Fluidized Bed

This study re‐examines the one‐dimensional equilibrium model of Gibilaro and Rowe (1974) for a segregating gas fluidized bed. The model was based on volumetric jetsam concentration and divided the bed contents into bulk and wake phases, taking account of bulk and wake flux, segregation, exchange between the bulk and wake phases, and axial mixing. Due to the complex nature of the model and its unstable solution, the lack of computing power at the time prevented the authors from doing little more than the analytical solutions to specific cases of this model. This paper provides a numerical total solution and allows the effect of the respective parameters to be compared for the first time. There is also a comparison with experimental results, which showed a reasonable agreement.     

A Novel Dry Process for Simultaneous Removal of SO2 and NO from Flue Gas in a Powder‐Particle Fluidized Bed

A novel sorbent, potassium carbonate impregnated on porous fine alumina, was produced, and its reactive and regenerative properties were evaluated for dry‐type simultaneous removal of SO₂ and NO from flue gas under stack temperatures, by using a powder‐particle fluidized bed (PPFB) with I.D. of 53 mm as the reactor. High removal efficiencies for SO₂ and NO were achieved simultaneously. An apparent beneficial effect of SO₂ on the enhancement of NO removal was found based on a large amount of data. The alumina carrier was successfully regenerated and used repeatedly for the production of fresh sorbent particles. With no ammonia, low temperature, high removal efficiency, and no second waste emission as main characteristics, this dry process can be a competitive technology for pollution control of flue gas from power plants in the future.