微小流化床流化特性分析

在内径4.3, 5.5, 10.5, 15.5, 20.5和25.5 mm的6个气固微小流化床中,考察了石英砂和不同粒径的催化裂化催化剂的流化特性. 研究了流化床尺寸、颗粒及流化介质物性对微小流化床床层压降及最小流化速度的影响. 结果表明,不同颗粒及流化介质的微小流化床床层压降实验值均小于计算值. 传统的压降关联式不能直接用于微小流化床. 其最小流化速度随床径减小呈指数增大,在高径比1:1~3:1范围内,最小流化速度随料高增大近似呈线性增大,其增大速度随床径增大而变缓. 基于实验数据得出了微小流化床最小流化速度的关联式.     

Transition from Low Velocity to High Velocity in a Three Phase Fluidized Bed

The onset liquid velocity demarcating the conventional and the circulating fluidization regimes of three‐phase fluidized beds was determined by measuring the time required to empty all particles in a batch fluidized bed at various liquid and gas velocities. Experiments were performed in a gas‐liquid‐solid circulating fluidized bed of 2.7 m in height using glass beads of 0.508 mm in diameter as solid phase and air and tap water as the fluidizing gas and liquid, respectively. The results show that gas velocity is a strong factor on the onset liquid velocity. Higher gas velocity yields a lower onset liquid velocity. It is also demonstrated that the onset liquid velocity has the same value as particle terminal velocity in a gas‐liquid mixture. Within the gas‐liquid‐solid circulating fluidization regime, the solids circulation rate is increased with the total liquid velocity and the auxiliary liquid velocity.     

Fluidization Characteristics in Micro‐Fluidized Beds of Various Inner Diameters

The fluidization characteristics of quartz sand and fluid catalytic crack (FCC) catalyst particles in six micro‐fluidized beds with inner diameters of 4.3, 5.5, 10.5, 15.5, 20.5, and 25.5 mm were investigated. The effects of bed diameter (D_t), static bed height (H_s), particles and gas properties on the pressure drop and minimum fluidization velocity (u_mf) were examined. The results show that the theoretical pressure drops of micro‐fluidized beds deviated from the experimental values under different particles and gas properties. The possible reason is due to an increase in bed voidage under smaller bed diameters. The equations for conventional fluidized beds did not fit for micro‐fluidized beds. u_mf increased with decreasing D_t. When the ratio of H_s to D_t ranged from 1:1 to 3:1, u_mf was characterized by a linear equation with H_s, while the slope of the equation u_mf versus H_s decreased with increasing D_t. In this paper, D_t/d_p and H_s/d_p were defined as dimensionless variables and a new equation was developed to predict u_mf in micro‐fluidized beds under the present experimental conditions.     

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.     

Bed Expansion and Particle Classification in Liquid Fluidized Beds with Structured Internals

The inclusion of a structured packing as internal in a liquid‐solid fluidized bed allows expansion of the liquid velocity operation range before elutriation, promoting the liquid solid contact and mixing. The bed expansion of liquid‐solid fluidized beds provided with structured packing as internals is examined, for solids denser than the liquid phase and within a wide range of operating conditions. A correlation to estimate the bed expansion in liquid‐solid fluidized beds using structured packing as internals is developed. In addition, the feasibility of employing structured packing as internals for favoring classification of different density particles is demonstrated by analyzing the mass elutriated from the column at different liquid velocities for single particles or binary mixtures.     

Design of an Efficient Gas Distribution System for a Fluidized Bed Dryer

Fluid bed dryers are commonly used to process granular solids. The design of the gas distributor has a significant impact on the performance of heat and mass transfer with or without chemical reactions in fluidized beds because it affects the quality of the fluidization obtained. In this work, an extensive study was carried out to design an optimal gas distribution system for a fluidized bed dryer. The air distribution chamber, also called a plenum chamber or gas chamber, was modified to obtain uniform air distribution across the bed cross section. Percentage maldistribution of the flow is considered as the basic evaluation parameter to quantify uniformity of the fluidizing gas distribution. Effects of various relevant design parameters such as the ratio of the orifice diameter to plate thickness (d_o/t), percentage free area, superficial gas velocity, etc., were examined experimentally and via modeling. The gas chamber was redesigned by inserting different types of packings in the chamber. In addition, the effect of height-to-diameter ratio (H/D) of the chamber on flow distribution was studied. Chambers of different H/D ratios and different air inlet nozzle diameters at various positions were examined to evaluate their effect on flow uniformity across the distributor. Three-dimensional computational fluid dynamic studies were carried out to evaluate the effects of pertinent parameters on flow uniformity, which has a direct bearing on the quality of fluidization and hence heat and mass transfer rates obtained.     

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.     

Entrainment Rate of Coarse Particles at Different Temperatures in Gas Fluidized Beds

Based on available experimental data, an empirical relationship is developed for the entrainment rate of coarse particles at the exit of gas‐fluidized beds, including the effects of fine particles in the bed, temperature, gas velocity, particle size and density, and column size. It predicts well the influence of fine particles on the entrainment rate of coarse particles, taking into account the momentum of the fine particles and the effect of superficial gas velocity. It also accounts well for the influence of temperature at different solid densities and gas velocities.     

高温鼓泡流化床流体动力学特性

流化床催化反应过程、煤的流化床燃烧与气化等多种流化床工业应用都涉及高温操作。文中结合研究结果 ,综述了温度对最小流化速度、最小流化空隙率、床层膨胀比和气泡直径的影响。在前人研究的基础上 ,对今后高温流化床的基础研究提出了建议     

Prediction of Layer‐Inversion Behavior of Down‐Flow Binary Solid‐Liquid Fluidized Beds

The layer‐inversion behavior of down‐flow binary solid‐liquid fluidized beds is predicted using the property‐averaging approach. The binary pair in this case consists of a larger solid species which is also heavier than its smaller counterpart, while both are lighter than the fluidizing medium. The model is based on using the generalized Richardson‐Zaki correlation for evaluation of the bed void fraction wherein mean values of particle properties are used. However, unlike the maximum bulk density condition for the conventional up‐flow binary solid fluidized bed, the model is based on a minimum bulk density condition for occurrence of layer inversion. This is due to the fact that the volume contraction phenomenon associated with the mixing of unequal solid species leads to a decrease in bulk density of the bed. Model predictions are also compared using the limited data available in the literature. Predictions are consistent with the observed mixing behavior.     

Experimental studies and empirical models for the prediction of bed expansion in gas–solid tapered fluidized beds

Studies in the expansion behaviour of tapered fluidized bed systems are important for specifying the height of the bed. Data have been obtained on the expanded heights of tapered fluidized beds and bed expansion ratios for spherical and non-spherical particles have been calculated. Based on dimensional analysis, models have been developed as a function of geometry of tapered bed, static bed height, particle diameter, density of solid and gas and superficial velocity of the fluidizing medium. The data used to derive the models cover a wide range of operating conditions, with varying fluidization velocities. Effects of static bed height, particle diameter, density, tapered angle and superficial gas velocity over minimum fluidization velocity on bed expansion ratios have been investigated experimentally. A comparison has been made between the calculated values of bed expansion ratios using proposed models and the experimental data. It has been seen that calculated values by models agree well with the experimental values. Models have also been compared with literature data of conventional bed and found its applicability at higher gas velocities with good accuracy.     

A Novel Model for Predicting the Dense Phase Behavior of 3D Gas‐Solid Fluidized Beds

A novel phenomenological discrete bubble model was developed and tested for prediction of the hydrodynamic behavior of the dense phase of a 3D gas‐solid cylindrical fluidized bed. The mirror image technique was applied to take into account the effects of the bed wall. The simulation results were validated against experimental data reported in the literature that were obtained by positron emission particle tracking. The time‐averaged velocity profiles of particles predicted by the developed model were found to agree well with experimental data. The initial bubble diameter had no significant influence on the time‐averaged circulating pattern of solids in the bed. The model predictions clearly indicate that the developed model can fairly predict the hydrodynamic behavior of the dense phase of 3D gas‐solid cylindrical fluidized beds.     

液–固脉冲流化床中浓度波传播与衰减

采用数值计算和实验验证相结合的方法研究液–固脉冲流化床中浓度波的传播和衰减. 当脉冲开半周期T2和闭半周期T1都远大于颗粒弛豫时间tp时，两相的惯性力之差在一个周期的绝大部分时间中相对于重力很小，可忽略，这时由双流体模型的动量方程可推导出推广的Richardson–Zaki公式，双流体模型简化为局部平衡模型. 采用五阶精度WENO格式求解浓度波传播方程，得到了脉冲流化过程中浓度波传播与衰减的规律，与实验结果符合良好.     

Studies on Liquid—Gas and Three‐Phase Fluidized Beds with Pulsating Air Flows

The effects of air‐flow pulsation and water and air flowrates on the hydrodynamics of liquid—gas and three‐phase fluidized beds containing 3‐mm glass beads have been studied in a 90‐mm i.d. column. Under steady‐flow conditions, both types of bed contained a relatively large number of small bubbles. With a pulsing air flow, however, a smaller number of much larger bubbles or slugs were formed. This was attributed to different mechanisms of bubble formation at the distributor. Variations in phase holdup were explained in terms of the effects of the operating parameters on the bubble characteristics.     

Tomographic diagnosis of gas maldistribution in gas-solid fluidized beds

Phase distribution is one of the key hydrodynamic parameters useful for the design and performance assessment of fluidized bed dryers (FBDs). It has direct influence on the drying rate, thermal efficiency, residence time distribution and degree of mixing. The quality of fluidization strongly depends on the uniformity of distribution of the fluidizing gas and the physical properties of the material to be fluidized. In the present work, gamma ray tomography (GRT) study was carried out in the form of chordal solid hold-up, which was found to be greatly influenced by the gas distributor design. The performance of a gas distributor due to the prevalent practice of operating at lower values of distributor-to-bed pressure drop ratio was characterized in a 0.15 m diameter fluidized bed dryer over a broad range of superficial gas velocity. The effects of various parameters such as solids loading, particle size and particle density were analyzed with the help of the reconstructed solid hold-up profiles. The fluidization was studied in terms of maldistribution factor (χ), a value of 5% or less can be obtained by properly designing distributor for a given bed loading, particularly for batch fluidized bed dryers. An industrial size fluidized bed dryer of 1 m diameter was also examined tomographically to obtain quantitative information on the solid hold-up distribution within the bed.     

Experimental Study on Flow Behavior in a Gas‐Solid Fluidized Bed for the Methanol‐to‐Olefins Process

A cold model experimental system is established to investigate the flow behavior in a gas‐solid fluidized bed for the methanol‐to‐olefins process catalyzed by SAPO‐34. The system comprises a gas distributor in a F 300 × 5000 mm acrylic column, double fiber optic probe system and a series of cyclones. The experiments are carried out under conditions of atmospheric pressure and room temperature with different superficial velocities (0.3930–0.7860 m s^–1) and different initial bed heights (600–1200 mm). The effects of radial distance, axial distance, superficial gas velocity, and initial bed height on the solid concentration and particle velocity in the bed are discussed. The time‐averaged solid concentration and rising particle velocity profiles under different conditions are obtained. The results show that an increase in the value of r/R or initial bed height results in an increase in the solid concentration but a decrease in the rising particle velocity in the dense phase area, while improvement of the superficial gas velocity has a negative influence on the solid concentration but results in an increase in the rising particle velocity.     

离心流化床初始流化状态的研究(Ⅰ)基本理论

通过简化求解离心流化床连续介质模型基本控制方程,获得了初始流化速度、压力、空隙率、空隙气速、床层膨胀和床层压降的计算方程式。理论预报的临界流化速度和床层压降与实验结果吻合得很好。揭示了离心流化床随流速增大由表面逐层初始流化;流化后各半径处流化程度不同。理论分析还表明气体压缩性的影响随着床体转速的增大而增大。     

Analysis and Interpretation of Multiple Radioactive Particle Tracking Data on Laboratory Scale Air/Polyethylene Fluidized Beds

Multiple radioactive particle tracking is a noninvasive technique used to study flow phenomena within gas‐solid fluidized beds. Five tagged polyethylene particles were added to a fluidized bed and tracked using two gamma cameras set perpendicular to one another. One camera collected x‐z particle locations and the other collected y‐z particle locations, while both cameras collected particle count rate values. The x‐z and y‐z particle movement information was combined to form three‐dimensional particle trajectories, and in turn used to calculate instantaneous flow information. The information from the five particles was then averaged and used to calculate three‐dimensional information regarding solid circulation parameters.     

Gas Filtration in Binary Fluidized Beds

Abstract

A systematic experimental study of aerosol filtration in a binary fluidized bed of dielectric material is carried out. Measurements of the collection efficiency when such parameters as gas velocity, bed height, collecting mixture, and column diameter are varied over a wide range have been made. Experimental evidence is given to show that charges generated naturally by triboelectrification of the bed dielectric particles can considerably increase the efficiency of such beds. Furthermore, it is demonstrated that a proper choice of the fluidized mixture can significantly improve the performance of such filters.     

流化床反应器过程强化技术

Fluidized beds enable good solids mixing, high rates of heat and mass transfer, and large throughputs, but there remain issues related to fluidization quality and scale-up. In this work I review modification techniques for fluidized beds from the perspective of the principles of process intensification (PI), that is, effective bubbling sup-pression and elutriation control. These techniques are further refined into (1) design factors, e.g. modifying the bed configuration, or the application of internal and external forces, and (2) operational factors, including altering the particle properties (e.g. size, density, surface area) and fluidizing gas properties (e.g. density, viscosity, or velocity). As far as two proposed PI principles are concerned, our review suggests that it ought to be possible to gain improve-ments of between 2 and 4 times over conventional fluidized bed designs by the application of these techniques.