Computation of system turbulences and dispersion coefficients in circulating fluidized bed downer using CFD simulation

In this study, the Eulerian computational fluid dynamics model with the kinetic theory of granular flow model was effectively used to compute the system turbulences and dispersion coefficients in a circulating fluidized bed (CFB) downer. In addition, the obtained model was used to simulate all the system velocities.     

Experimental mass transfer coefficients in a pilot plant multistage column extractor

The volumetric overall mass transfer coefficients in a multistage column have been measured using axial dispersion model for toluene–acetone–water system. The effect of operating parameters on the volumetric overall mass transfer coefficients has been investigated for both mass transfer directions. The results show that the mass transfer performance is strongly dependent on rotor speed and mass transfer direction, although only slightly dependent on phase flow rates. In addition, empirical correlations to predict the overall mass transfer coefficients have been developed. The proposed correlations based on dimensionless numbers can be considered as a useful tool for the possible scale up of the multistage column extractor.     

Computation of the mass transfer coefficient of FCC particles in a thin bubbling fluidized bed using two- and three-dimensional CFD simulations

In this study, the standard kinetic theory based model with a modified drag correlation was successfully used to compute the mass transfer coefficients and the Sherwood numbers of FCC particles in a thin bubbling fluidized bed column using the additive diffusional and chemical reaction resistances concept. Also, the effects of the computational domain (two- or three-dimensional) and the reaction rate constant (low and high) are discussed.The computations show that the Sherwood numbers are in agreement with the measurement ranges for small particles in the fluidized bed system. The mass transfer coefficients and the Sherwood numbers are high near the inlet section, and decrease to a constant value with increasing height in the column. The two-dimensional computational domain simulations provide enough information to explain the phenomena inside a symmetrical system, but three-dimensional computational domain simulations are still needed for asymmetrical systems. Finally, the mass transfer coefficients and the Sherwood numbers increased with the larger reaction rate constant.     

气液固微型流化床的气液传质系数

气液固微型流化床兼具微流控系统和宏观流化床的优点,具有潜在的工业应用价值,但是,其应用基础研究十分缺乏。采用床径为1.6、2.0、2.4 mm的微型流化床,平均粒径为160、190、220 μm的玻璃珠,以NaOH水溶液吸收CO₂气体为气液传质研究物系,在三相流动研究的基础上,考察了表观气速、表观液速、床径、粒径等对三相微型流化床气液体积传质系数的影响。结果表明：给定其他条件,增加表观气速和表观液速,均使气液体积传质系数增大;表观气速主要改变气含率和气液相界面积,而表观液速主要改变液相传质系数;床径减小,气液相界面积和气液体积传质系数都有所增加;在气液两相微型鼓泡塔中加入固体颗粒,形成三相分散鼓泡流型,当其固含率在0.15~0.30范围内,可显著增强气液传质,其气液体积传质系数是气液微鼓泡塔的1.1~1.5倍;与宏观流化床相比,相同条件下微型床的相界面积为它的5~10倍,是微型流化床具有更大体积传质系数的主要影响因素。     

气液固微型流化床的气液传质系数

气液固微型流化床兼具微流控系统和宏观流化床的优点,具有潜在的工业应用价值,但是,其应用基础研究十分缺乏。采用床径为1.6、2.0、2.4 mm的微型流化床,平均粒径为160、190、220 μm的玻璃珠,以NaOH水溶液吸收CO₂气体为气液传质研究物系,在三相流动研究的基础上,考察了表观气速、表观液速、床径、粒径等对三相微型流化床气液体积传质系数的影响。结果表明：给定其他条件,增加表观气速和表观液速,均使气液体积传质系数增大;表观气速主要改变气含率和气液相界面积,而表观液速主要改变液相传质系数;床径减小,气液相界面积和气液体积传质系数都有所增加;在气液两相微型鼓泡塔中加入固体颗粒,形成三相分散鼓泡流型,当其固含率在0.15~0.30范围内,可显著增强气液传质,其气液体积传质系数是气液微鼓泡塔的1.1~1.5倍;与宏观流化床相比,相同条件下微型床的相界面积为它的5~10倍,是微型流化床具有更大体积传质系数的主要影响因素。     

A phenomenological model for heat transfer in freeboard of fluidized beds

A phenomenological model was developed to predict heat transfer to tubes located in the freeboard region of gas fluidized beds. The model is concerned with the conductive/convective mechanism of heat transfer. For high temperature applications, an additional contribution by thermal radiation would need to be incorporated. The model considers that the tube surface experiences alternating contact with a dense emulsion phase and a lean void phase. Contributions by dense and lean phases are represented by transient conduction and convection mechanisms, respectively. Particletube contact information was obtained experimentally for a wide range of operating conditions at room temperature and pressure. Predictions of the model were compared with measured heat transfer coefficients. Over a 20-fold range in magnitudes of heat transfer coefficients, the model successfully predicted the measured values with an average deviation of 44 percent.     

Heat transfer between very shallow fluidized beds and an immersed horizontal tube

A new correlation is proposed for the heat transfer coefficient between an immersed horizontal tube and very shallow fluidized beds (static bed heights of 10-40 mm). The correlation is based upon experimental data obtained in this work for a horizontal tube with an outside diameter of 13.1 mm, immersed in beds of spherical alumina particles with mean particle sizes of 335-1261 microns. The maximum bed pressure drop was 92.5 mm water. The effects of tube elevation, static bed height and distributor design were investigated. Nine different distributors were used, with maximum pressure drops ranging from 3 to 800 mm water and open areas from 2.2 to 36%. A comparison between the proposed correlation and data reported in the literature showed an agreement of approximately ±10%.     

Effect of annular fins on heat transfer of a horizontal immersed tube in bubbling fluidized beds

Experiments were conducted in a bubbling air-fluidized bed to investigate the effect of annular fins of constant thickness on heat transfer. Steady state time averaged local heat transfer coefficient measurements were made by the local thermal simulation technique in a cold bubbling fluidized bed (90 mm ID, 260 mm tall) with horizontally immersed tube initially with no fin and then with three fixed annular fins of constant thickness. Silica sand of mean particle diameter 307 μm and 200 μm were used as the bed materials. The superficial velocity of air was from minimum fluidization conditions, u_mf, to approximately 3 × u_mf. The results indicate that, although the heat transfer coefficient falls with the use of fins, the total heat transfer rises as a result of the greater surface area. Increasing the particle diameter reduces the heat transfer coefficient not only for unfinned horizontal tube but also for annular finned horizontal tube at the same conditions of fluidized bed. Based on the experimental data, correlations are proposed for predicting heat transfer coefficient from fluidized bed to horizontally immersed tubes with and without fins.     

Mass transfer in fluidized beds of inert particles. Part I: The role of collision currents in mass transfer to the electrode

The effect of particle-wall collision on the mass transfer rate mechanism in liquid fluidized bed electrochemical cells was studied. Collision frequencies and currents were measured at microelectrodes set in the bed wall. It is postulated that, at each particle–electrode collision, a specific microvolume of bulk concentration electrolyte is introduced into the near-electrode diffusion layer during particle movement towards the electrode causing an enhancement of the limiting diffusion current. Based on measurements made at microelectrodes calculations of the contribution of the particle collision mechanism to total mass transfer to a planar electrode are attempted and are in good agreement with experimental values.     

Kinetic theory based computation of PSRI riser: Part II—Computation of mass transfer coefficient with chemical reaction

The design of circulating fluidized bed systems requires the knowledge of mass transfer coefficients or Sherwood numbers. A literature review shows that these parameters in fluidized beds differ up to seven orders of magnitude.To understand the phenomena, a kinetic theory based computation was used to simulate the PSRI challenge problem I data for flow of FCC particles in a riser, with an addition of an ozone decomposition reaction. The mass transfer coefficients and the Sherwood numbers were computed using the concept of additive resistances. The Sherwood number is of the order of 4 × 10⁻³ and the mass transfer coefficient is of the order of 2 × 10⁻³ m/s, in agreement with the measured data for fluidization of small particles and the estimated values from the particle cluster diameter in part one of this paper. The Sherwood number is high near the inlet section, then decreases to a constant value with the height of the riser. The Sherwood number also varies slightly with the reaction rate constant. The conventionally computed Sherwood number measures the radial distribution of concentration caused by the fluidized bed hydrodynamics, not the diffusional resistance between the bulk and the particle surface concentration. Hence, the extremely low literature Sherwood numbers for fluidization of fine particles do not necessarily imply very poor mass transfer.     

Phase holdup and liquid dispersion in gas-liquid-solid circulating fluidized beds

Axial and radial profiles of gas and solids holdups have been studied in agas-liquid-solid circulating fluidized bed at 140mm i.d..Experimental results indicate that the axialand radial profiles of gas and solids holdups are more uniform than those in a conventionalfluidized bed.Axial and radial liquid dispersion coefficients in the gas-liquid-solid circulating fluidizedbed are investigated for the first time.It is found that axial and radial liquid dispersioncoefficients increases with increaes in gas velocity and solids holdup.The liquid velocity has littleinfluence on the axial liquid dispersion coefficient,but would adversely affect the redial liquiddispersion coefficient.It can be concluded that the gas-liquid-solid circulating fluidized bed hasadvantages such as better interphase contact and lower liquid dispersion along the axial directionover the expanded bed.     

New predictive correlation for mass transfer coefficient in structured packed extraction columns

Developments in the area of packed columns, particularly structured packed columns, are ongoing, specifically in the area of liquid–liquid extractions in different industries. In the present study, mass transfer coefficients have been obtained experimentally in a structured packed extraction column to develop a new correlation for prediction of continuous phase Sherwood number. The experiments were carried out for toluene/acetic acid/water and n-butyl acetate/acetic acid/water systems with counter current flow in different heights of column. A new dimensionless parameter, d₃₂/h, is introduced in proposed equation. This number considers the effect of column height (h) and mean drop diameter (d₃₂) jointly. The main advantage of this approach is that the principal effect of column height is considered in correlation without which the experimental data could not be fitted with a acceptable accuracy.     

Heat transfer between an isolated bubble and the dense phase in a fluidized bed

Heat transfer between the bubble and dense phases of a bubbling fluidized bed plays a very important role in the system performance, especially for applications involving solids drying and gas‐phase combustion. However, very few experimental data are available on this subject in the literature. An experimental and modelling investigation on the heat transfer behaviour of isolated bubbles injected into an incipiently fluidized bed is reported in this paper. A new single‐thermocouple technique was developed to measure the heat transfer coefficient. The effects of bed particle type and size, and bubble size on the heat transfer coefficient were examined. The heat transfer coefficient was found to exhibit a maximum as the bubble size increased in the bubble size range investigated. The bed particle size had a comparatively small effect on the heat transfer coefficient. A simple mathematical model was developed which provides good agreement with experimental data.     

Dynamics of gas-particle flow in circulating fluidized beds

The flow of gas—particle mixtures in a circulating fluidized bed has been studied, probing the flow behavior under both stable and unstable operating conditions. A novel feature of our work is the use of electrical capacitance tomography to image particle distribution over the cross-section at one elevation in the standpipe. In addition, we have also obtained data on pressure profile and aeration rate in the standpipe, particle circulation rate in the circulating fluidized bed and riser gas flow rate under various operating conditions. Here, we report experimental results obtained for two different particles, both belonging to Geldart type A. At low aeration rates, a stable dense phase flow is obtained in the standpipe. At high aeration rates, the flow became unstable, manifesting low frequency oscillations in the flow characteristics. Our results suggest that, under conditions explored in the present study, this instability originates in the standpipe and that any attempt to model it should consider the interaction between the various components of the circulating fluidized bed system.     

振动流化床与浸没水平管平均传热特性理论分析与实验研究

在二维流化床(240mm×80mm)中,以平均粒径dp为1.83mm的玻璃珠为物料,研究了振动流化床与浸没水平管间传热规律;考察了流化数、振动频率、床高、水平管管径等因素对平均传热系数的影响。采用自制探头对浸没加热管束和振动流化床层间平均传热系数进行实验测定,利用颗粒团模型,建立了振动流化床层与浸没水平管间平均传热模型,并对平均传热系数的理论预测值与实验测定值进行了比较。结果表明:计算值与实验值吻合较好,误差在±15%范围内。在较高流化数、低振动频率时,实验值处于理论值上方;随着振动频率、管径增大,平均传热系数实验值逐渐趋于理论预测值甚至低于理论预测值。结果可为带浸没水平管的振动流化床设计和研究提供参考。     

Effect of ring promoter on ionic mass transfer in a fluidized bed

Mass transfer data were obtained at the wall of a batch fluidized bed in the presence of a ring promoter used for the reduction of ferricyanide ion. The ring promoter was a copper rod on which rings of uniform size were fixed at equal distances. In these studies, the cross‐sectional diameter of the ring, the spacing between the rings, the particle size and the void fraction were varied and the limiting current data were measured. It was observed that the mass transfer coefficients were augmented with increased particle diameter, decreased spacing between the rings, and increased cross‐sectional diameter of the ring. Copyright © 2003 Society of Chemical Industry     

Wall-to-bed heat transfer in three-phase fluidized beds of low density particles

Bed-to-wall heat transfer was measured in three-phase fluidized beds under conditions typical of biochemical process applications. The thermal resistance of the fluidized bed, which was significant in the absence of gas, became negligible when gas was introduced. Decreasing the particle density at constant gas and liquid velocity increased the bed-to-wall heat transfer coefficient. Previously published heat transfer correlations were used and gave poor predictions of our data. A new correlation was developed which predicted very well all the heat transfer coefficient results in this paper.     

Heat transfer to the ceiling of the riser of a circulating fluidized bed

Very little information on the heat transfer to the ceiling of a circulating fluidized bed (CFB) boiler is available in the published literature though it constitutes a significant part of the furnace heat absorption. So, to explore this less-known heat transfer process a series of experiments were conducted at four different superficial gas velocities and three external solids circulation rates in a CFB pilot plant with a riser having a height of 5 m and a cross section of . The experimental results suggest that both solids circulation rates and superficial gas velocities had a significant influence on the local heat transfer to the ceiling close to the riser exit to the gas solids separator. However, on the ceiling, opposite of the exit, solids circulation rates and superficial gas velocities had only a minor influence on the local heat transfer coefficients.     

Numerical investigation of gas-solid heat transfer in rotating fluidized beds in a static geometry

Gas-solid heat transfer in rotating fluidized beds in a static geometry is theoretically and numerically investigated. Computational fluid dynamics (CFD) simulations of the particle bed temperature response to a step change in the fluidization gas temperature are presented to illustrate the gas-solid heat transfer characteristics. A comparison with conventional fluidized beds is made. Rotating fluidized beds in a static geometry can operate at centrifugal forces multiple times gravity, allowing increased gas-solid slip velocities and resulting gas-solid heat transfer coefficients. The high ratio of the cylindrically shaped particle bed “width” to “height” allows a further increase of the specific fluidization gas flow rates. The higher specific fluidization gas flow rates and increased gas-solid slip velocities drastically increase the rate of gas-solid heat transfer in rotating fluidized beds in a static geometry. Furthermore, both the centrifugal force and the counteracting radial gas-solid drag force being influenced by the fluidization gas flow rate in a similar way, rotating fluidized beds in a static geometry offer extreme flexibility with respect to the fluidization gas flow rate and the related cooling or heating. Finally, the uniformity of the particle bed temperature is improved by the tangential fluidization and resulting rotational motion of the particle bed.     

循环流化床中颗粒聚团特性的模拟

考虑到循环流化床中分散颗粒和颗粒聚团同时存在的多尺度结构,确定了密相和稀相加速度与计算网格局部参数之间的关系,建立了多尺度曳力消耗能量最小的稳定性条件,基于双变量极值理论,构建了考虑颗粒团聚效应的多尺度气固相间曳力模型。结合双流体模型,对循环流化床内气固流动特性以及颗粒聚团特性进行了模拟研究。通过与实验值比较,考虑颗粒聚团影响的计算模型可以更好地贴近实验结果,颗粒聚团直径随颗粒浓度增大呈现先增大后减小的分布趋势,气体和颗粒的加速度在模拟中与重力加速度同处一个数量级,求解过程中不能被忽略。