High-temperature de fluidization

Operation of fluidized beds at high temperatures is limited by the tendency of the bed particles to agglomerate, causing defluidization. This operating limit is important in several processes, including coal conversion, iron ore reduction, and cement manufacture.At lower temperatures, the bed remains fluidized at velocities above the normal minimum fluidization velocity. Once the bed is operating above an ‘Initial Sintering Temperature’, the fluid velocity must be well above the normal minimum fluidization velocity in order to prevent defluidization. The defluidization velocity increases with increasing operating temperature. The ‘Initial Sintering Temperature’ can be estimated using a dilatometer to measure thermal expansion and contraction of a loosly packed sample of the bed particles.     

Comprehensive experimental investigation on biomass-glass beads binary fluidization: A data set for CFD model validation

Fluidization behavior of biomass and glass beads binary mixtures in a bubbling fluidized bed was experimentally investigated. Mixtures containing different mass fraction of Loblolly Pine white wood and glass beads were fluidized at different fluidization velocities. The particle properties were characterized in a QICPIC that uses a dynamic image processing method to measure both particle size and sphericity. The minimum fluidization velocity was determined using the pressure drop method. An image processing method was developed to capture the dynamic expanded bed height at a very high frequency. The effect of biomass mass fraction and inlet gas velocity on mixing and segregation behavior was studied and analyzed through pressure drop measurements. Pressure drop fluctuations and expanded bed height fluctuations via fast Fourier transform were analyzed and compared. The complete and accurate experimental data reported in this study could provide a benchmark data set for various computational fluid dynamics models validation, calibration, and identification.     

Dynamics of segregation and fluidization of binary mixtures in a cylindrical fluidized bed

Dynamics of segregation and fluidization of unary particles and binary mixtures in a cylindrical fluidized bed is investigated using temporally– and spatially–resolved measurements of solids volume fraction (α_s) performed using Electrical Capacitance Tomography (ECT). Through the comparison with high-speed imaging, we have shown that ECT can be used to measure the segregation behavior in cylindrical fluidized beds quantitatively. ECT measurements have been used further to quantify the effects of mixture composition, particle–diameter ratio, and superficial gas velocity on the bed segregation behavior. Dynamics of fluidization behavior is characterized using the time–evolution of local α_s fluctuations, corresponding frequency distribution, and bubble size distribution. Further, a relation between the measured variance of α_s fluctuations at different radial locations and corresponding flow structures under different fluidization conditions is established. The present work helps to understand dynamics of segregation and fluidization of binary mixtures and to provide a database for validation of Eulerian multifluid CFD models.     

流化床生物质惰性颗粒混合分离特性①

在Φ99×1000mm的圆柱流化床中,选用不同粒径的沙子／锯末、沙子／菜籽体系,研究了不同沉积组分含量,尤其研究了生物质、惰性颗粒在富沉积组分情形下床层表观气速、静床高度对流化床二组分混合物的混合与分离的影响。结果表明静床高度对分高度无影响,沉积组分适宜的粒度及高表观气速有利于流化床二组分混合物的混合,可使混合度达到87％。     

流化床生物质惰性颗粒混合分离特性

在Φ99×1000mm的圆柱流化床中,选用不同粒径的沙子／锯末、沙子／菜籽体系,研究了不同沉积组分含量,尤其研究了生物质、惰性颗粒在富沉积组分情形下床层表观气速、静床高度对流化床二组分混合物的混合与分离的影响。结果表明静床高度对分高度无影响,沉积组分适宜的粒度及高表观气速有利于流化床二组分混合物的混合,可使混合度达到87％。     

气-固微型流化床压降特性及最小流化速度的实验研究

在内径3～20 mm的4个气-固微型流化床中,分别考察了A类和B类两种类型颗粒的流化特性,同时研究了床几何结构、操作条件、物相性质等各因素对其最小流化速度的影响.结果 表明,气-固微型流化床中的床层压降特性与颗粒类型密切相关,不同的流动状态下两种类型颗粒的流动特性存在显著地差异.在固定床阶段,与B类颗粒相比,A类颗粒与...     

PVC类废塑料在冷态流化床中流化行为的研究

在Φ50 mm×800 mm圆柱体的冷态流化床反应器中,对PVC类废塑料、石英砂及其混合物的流化特性进行了研究。研究了PVC颗粒粒径与混合物料中PVC质量分数对混合物料的流化特性的影响规律,得到指导热态实验的关键参数。实验结果表明,PVC颗粒粒径与混合物料中PVC质量分数会影响混合物料的最小流化速度,也影响PVC颗粒与石英砂混合的均匀度。混合物料中PVC的质量分数越小,其最小流化速度就越小,混合物料也越容易实现充分混合;PVC颗粒为Geldart B类颗粒,但由于形状不规则,黏性力大,塌落特性明显,流化性能较差,显示出C类颗粒的流化特性,同时实际的最小流化速度要大于理论最小流化速度。PVC与石英砂混合物料冷态流化行为的研究结果为热态流化床降解PVC颗粒提供了基础数据和实践依据。     

Prediction of critical fluidization velocity and maximum bed pressure drop for binary mixture of regular particles in gas–solid tapered fluidized beds

The problems associated with conventional (cylindrical) fluidized beds, viz., fluidization of wider size range of particles, entrainment of particles and limitation of fluidization velocity could be overcome by using tapered fluidized beds. Limited work has been carried out to study the hydrodynamics of single materials with uniform size particles in tapered beds. In the present work, an attempt has been made to study the hydrodynamic characteristics of binary mixtures of homogeneous and heterogeneous regular particles (glass bead and sago) in tapered fluidized beds having different tapered angles. Correlations have been developed for critical fluidization velocity and maximum bed pressure drop for gas–solid tapered fluidized beds for binary mixtures of regular particles. Model predictions were compared with experimental data, which were in good agreement.     

Critical fluidization velocities and maximum bed pressure drops of homogeneous binary mixture of irregular particles in gas-solid tapered fluidized beds

Recently, tapered fluidized bed has become more attractive because of the problems associated with conventional (cylindrical) beds like fluidization of widely distributed particles, entrainment of particles and limitation of fluidization velocity. There have been some investigations on hydrodynamics of uniform single size particles but there have been no detailed studies of homogeneous binary mixture of particles of different sizes and different particles in tapered beds. In the present work, an attempt has been made to study the hydrodynamic characteristics of homogeneous binary mixture of irregular particles in tapered beds having different tapered angles. Correlations have been developed for important characteristics, especially critical fluidization velocities and maximum bed pressure drops of homogeneous binary mixture of irregular particles in gas-solid tapered fluidized beds. Experimental values of critical fluidization velocities and maximum bed pressure drops have been compared with the developed correlations.     

Composite fluidization in a circulating fluidized bed for flue gas desulfurization

Circulating fluidized beds (CFBs) for flue gas desulfurization (FGD) are widely used in small- and medium-scale power plants. However, even with multi-venturi tubes and guide plates, several problems, including the flow field deviation, axial velocity non-uniformity in the tower, and bad load adaptability to the boiler, are still encountered. To address these issues, the axial composite fluidization mode (ACFM) and tangential composite fluidization mode (TCFM) are developed. These modes form swirl flow using composite fluidization devices. The present study conducted laboratory-scale research to examine the regulations of venturi fluidization mode (VFM), ACFM, and TCFM. We found that the flow field for ACFM and TCFM is significantly improved. The turbulent kinetic energy and particle concentration for ACFM and TCFM are higher than those for VFM. A pilot research on Dual CFB-FGD with ACFM was conducted in a 145 MW unit power plant. The results show that CFB resistance is increased by 200 to 300 Pa compared with VFM, and its axial velocity and temperature profiles in the tower diameters are uniform. The CFB unit has good load adaptability to the boiler, high desulfurization at low Ca/S, and high approach to adiabatic saturation temperature. ACFM can weaken particle adhesion to the tower wall.     

Distribution of large biomass particles in a sand‐biomass fluidized bed: Experiments and modeling

The axial distribution of large biomass particles in bubbling fluidized beds comprised of sand and biomass is investigated in this study. The global and local pressure drop profiles are analyzed in mixtures fluidized at superficial gas velocities ranging from 0.2 to 1 m/s. In addition, the radioactive particle tracking technique is used to track the trajectory of a tracer mimicking the behavior of biomass particles in systems consisting of 2, 8, and 16% of biomass mass ratio. The effects of superficial gas velocity and the mixture composition on the mixing/segregation of the bed components are explored by analyzing the circulatory motion of the active tracer. Contrary to low fluidization velocity (U = 0.36 m/s), biomass circulation and distribution are enhanced at U = 0.64 m/s with increasing the load of biomass particles. The axial profile of volume fraction of biomass along the bed is modeled on the basis of the experimental findings. © 2014 American Institute of Chemical Engineers AIChE J, 60: 869–880, 2014     

3D simulation of effect of geometry on minimum fluidization velocity and flow regimes in a spout-fluidized bed

Spout–fluid beds are used for a variety of processes involving particulate solids, like coating, drying, granulation and etc. The spout–fluidized bed combines a number of favorable properties of both spouted and fluidized beds. In this study, the Granular Eulerian model is used in 3-D hydrodynamic simulation of spout fluidized bed for calculation of minimum fluidization velocity. The results of simulation were compared with experimental data and good agreement was obtained. Then the effect of geometry on minimum fluidization velocity was studied. Also a review of flow regimes in different spout fluidized bed geometries was studied.     

Classifying the fluidization and segregation behavior of binary mixtures using particle size and density ratios

Experimental investigations show that fluidized binary mixtures exhibit varied pressure drop profiles and segregation patterns, depending on the level of disparity due to size and/or density differences. In this study, different mixture types are mapped on a graph of density versus size ratios. It is found that the ratio of the minimum fluidization velocities of individual components can be used to categorize these mixtures. A simple correlation is developed to compute the ratio of the minimum fluidization velocities based on the density and size ratios. Categorizing the binary mixtures in this manner gives a qualitative understanding of how the different mixtures behave on fluidization. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Influence of drying conditions on the moisture diffusion and fluidization quality during multi-stage fluidized bed drying of bovine intestine for pet food

In order to establish the influence of the drying air characteristics on the drying performance and fluidization quality of bovine intestine for pet food, several drying tests have been carried out in a laboratory scale heat pump assisted fluid bed dryer. Bovine intestine samples were heat pump fluidized bed dried at atmospheric pressure and at temperatures below and above the materials freezing points, equipped with a continuous monitoring system. The investigation of the drying characteristics have been conducted in the temperature range ?10 to 25 °C and the airflow in the range 1.5–2.5 m/s. Some experiments were conducted as single temperature drying experiments and others as two stage drying experiments employing two temperatures. An Arrhenius-type equation was used to interpret the influence of the drying air temperature on the effective diffusivity, calculated with the method of slopes in terms of energy activation, and this was found to be sensitive to the temperature. The effective diffusion coefficient of moisture transfer was determined by the Fickian method using uni-dimensional moisture movement in both moisture, removal by evaporation and combined sublimation and evaporation. Correlations expressing the effective moisture diffusivity and drying temperature are reported.Bovine particles were characterized according to the Geldart classification and the minimum fluidization velocity was calculated using the Ergun Equation and generalized equation for all drying conditions at the beginning and end of the trials. Walli's model was used to categorize stability of the fluidization at the beginning and end of the drying for each trial. The determined Walli's values were positive at the beginning and end of all trials indicating stable fluidization at the beginning and end for each drying condition.     

生物质惰性颗粒混合物的混合分离特性

在D99mm×1000mm的圆柱流化床中,选用不同粒径的沙子／锯末、沙子／菜籽双组分混合物,研究了不同沉积组分含量,尤其是富沉积组分情形下床层表观气速、静床高度对流化床二组分混合物的混合与分离的影响．实验结果表明：静床高度对生物质惰性颗粒混合物分离指数无影响,沉积组分适宜的粒度及床层高和表观气速有利于生物质惰性颗粒混合物的混合．     

Coarse grained computational fluid dynamic simulation of sands and biomass fluidization with a hybrid drag

The bubbling fluidized bed reactor is widely used in fast pyrolysis of biomass. Discrete simulation of this reactor is challenging due to many sand particles and lack of accurate drag corrections accounting for the interaction of two different solid particles with different properties. In this research, the computational cost is reduced by using the coarse-grained computational fluid dynamic-discrete element method, where many sand particles are lumped into a larger numerical parcel. The Syamlal–O'Brien drag model is used for sand, while Ganser correction coupled with Gidaspow model is used for the nonspherical biomass particles. This hybrid approach shows superior behavior over other drag models using pressure drops as a benchmark. The predicted bed height and pressure fluctuating frequencies compare well with experiment. The mixing of biomass is close to perfect if the superficial velocity is larger than four times the minimum fluidization velocity.     

Prediction of minimum bubbling velocity, fluidization index and range of particulate fluidization for gas-solid fluidization in cylindrical and non-cylindrical beds

A uniform fluidization exists between minimum fluidization velocity and minimum bubbling velocity. Experimental investigations have been carried out for determination of minimum bubbling velocity and fluidization index for non-spherical particles in cylindrical and non-cylindrical beds. In the present paper equations have been developed for the prediction of minimum bubbling velocity for gas-solid fluidization in cylindrical and non-cylindrical (viz. semi-cylindrical, hexagonal and square) beds for non-spherical particles fluidized by air at ambient conditions. A fairly good agreement has been obtained between calculated and experimental values. Based on the experimental data it is concluded that under similar operating conditions minimum bubbling velocity and the fluidization index are maximum in case of either semi-cylindrical conduit or hexagonal conduit for most of the operating conditions and minimum in case of square one. It is further observed that the range of uniform (particulate) fluidization is maximum in case of semi-cylindrical bed for identical operating conditions.     

The fluidization process of binary mixtures of solids: Development of the approach based on the fluidization velocity interval

The main characteristic of the fluidization process of particulate mixtures is the interaction between bed suspension and segregation-remixing dynamics of their components.On addressing the fluidization properties of binary beds of solids differing either in particle density or diameter, the paper shows how interpretations based on definition of a minimum fluidization velocity of the mixture can lead to erroneous conclusions about the influence played by system variables on their behaviour. An alternative method of investigation is followed which takes into consideration the existence of a finite velocity interval, bounded by the “initial” and “final fluidization velocity” of the mixture, along which the whole process of fluidization has place. The results of a wide series of experiments demonstrate that this alternative approach allows recognizing the independent variables of binary fluidization; they also highlight the differences of behaviour between density- and size-segregating beds.     

Mixing–segregation phenomena of binary system in a fluidized bed

A study on mixing–segregation phenomena in a gas fluidized bed of binary density system was performed by analysis of the residence time distribution and mixing degree. The effect of particle mixing on the residence time distribution and solid mixing was studied in a binary particle system with different densities. Residence time distribution curve and mean residence time of each particle were measured according to the flotsam particle size, mixing ratio and gas velocity in a gas fluidized bed (0.109 m I.D., 1.8 m height). The characteristics of residence time distribution and the deviation of mean residence time of each particle are consistent with previous mixing index based on the axial concentration of jetsam. From this study, mixing index of binary particle system with different densities should be considered by not only axial concentration distribution of jetsam particle but also characteristics of residence time distribution. This result suggests that the solid movement by fluidization gas is more important than solid axial dispersion.     

Minimum fluidization velocities in beds of mixed solids

The fluidization behaviour of a bed of solids of mixed sizes and/or densities indicates that for a range of velocities between u_bf (the velocity of beginning of fluidization) and u_tf (the velocity of complete fluidization), such a bed is only partially fluidized. The phase diagrams of these systems show that the phenomenon is analogous to the fractional crystallization of a multicomponent solution and it can find use in selective classification of solids. In the present work, data on u_bf and u_tf for liquid fluidized beds of two, three, four and five component mixtures have been processed along with those reported earlier on gas-solid systems, to yield useful correlations for their prediction.