GRAPHICAL SOLUTION OF MULTISTAGE FLUIDIZED BED HEAT EXCHANGER

The design of multistage fluidized beds for heat exchange necessitates the solution of the mass and energy balance equations combined with the equilibrium relations for each stage. This paper presents a novel way of applying well known methodology to a different technology, one where it has not been widely applied.

In the present work a McCabe-Thiele type of graphical approach is presented for both counter-current and cross-current contacting multiple fluidized beds. The necessary equations for a multistage calciner are developed and the application of the proposed method is demonstrated. Generalized fluidized bed efficiency for counter-current and cross-current multiple fluidized bed is presented.     

Desorption of VOC from polymer adsorbent in multistage fluidized bed

The desorption process of volatile organic compounds(VOC) from a polymer adsorbent in counter-current multistage fluidized bed was studied. And a mathematical model considering the mass transfer dynamics was developed, which was validated from experiment data. The gaseous ethyl acetate mass transfer was discussed, and the limiting step is the intraparticle mass transfer of the desorption process. The value of intraparticle mass transfer coefficient is between 1.85 × 10~(-6) and 1.38 × 10~(-5) m·s~(-1) under temperature of 100–160 °C. Experiments under different operating conditions were carried out. The effects of operating conditions such as gas–solid flow ratio,gas inlet temperature and total stage number of multistage fluidized bed on outlet VOCs concentration and desorption efficiency were discussed. The maximum outlet VOCs concentration and corresponding desorption efficiency of the multistage fluidized bed desorber was calculated under different gas inlet temperatures and total stage numbers.     

Investigation on gas–solid flow regimes in a novel multistage fluidized bed

Gas–solid flow regime in a novel multistage circulating fluidized bed is investigated in this study. Pressure fluctuations are first sampled from gas–solid flow systems and then are analyzed through frequency and time–frequency domain methods including power spectrum and Hilbert–Huang transform. According to the flow characteristics obtained from pressure fluctuations, it is found that the gas–solid motions in the multistage circulating fluidized bed exhibit two dominant motion peaks in low and high frequencies. Moreover, gas-cluster motions become intensive for the multistage circulating fluidized bed in comparison with the fast bed. Unlike the traditional methods, the fuzzy C-means clustering method is introduced to objectively identify flow regime in the multistage circulating fluidized bed on the basis of the flow characteristics extracted from bubbling, turbulent, fast, and multistage fluidized beds. The identification accuracy of fuzzy C-means clustering method is first verified. The identification results show that the flow regime in the multistage circulating fluidized bed is in the scope of fast flow regime under examined conditions. Moreover, the results indicate that the consistency of flow regime between two enlarged sections exists. In addition, the transition onset of fast flow regime in the multistage circulating fluidized bed is higher than that in the fast bed.     

Performance of a three-phase fluidized bed as a reactive distillation device

A new reactive distillation device, the multistage gas/liquid/solid three-phase fluidized bed, has been developed. The flow regimes of the multistage three-phase fluidized bed have been studied and the regimes can be divided into the liquid leakage regime, the dispersed bubble regime, and the coalesced bubble regime. Liquid velocity has a much smaller effect on phase holdups in this device than in conventional three-phase fluidized beds. The three phase fluidized bed is used as a reactive distillation device for the hydrolysis of methyl acetate. Much higher reaction conversion than the equilibrium value and high catalyst-contacting efficiency are obtained. Different methods of feeding the water into the reactive distillation section are studied.     

The uses of magnetic fields in the processing of solids

This paper shows how properly applied magnetic fields can advantageously be used to control the behavior of streams of solids which contain some magnetizable material. In particular, we describe the development of a new magnetic flow meter for solids, a quick acting on-off valve with no mechanical action, a sliding solids reactor which may be used for particle filtration and related operations, a positively controlled multistage moving bed contactor, fluidized beds with no gross bubbling or gas bypassing, and especially, a reliably operating distributor-downcomer for fluidized beds.The prospects are considered that these devices may open the way to new types of gas/solids contactors including non-plugging trouble-free multistage fluidized beds.     

Flow patterns and mass transfer rates in fluidized bed ion exchange equipment

The axial mixing of ion exchange resin and liquid in fluidized beds is modelled by a series of well-mixed cells for the resin and an equal or larger number of mixed cells for the liquid. Concentration breakthrough curves are calculated using this mixing model, assuming finite mass transfer kinetics and irreversible equilibrium. The model is used to describe mixing patterns and kinetics in static fluidized beds (no resin flow); the information so obtained can be used to design continuous and periodic flow counter-current fluidized bed ion exchange systems.     

多层筛板流化床料层高度均匀性研究

通过测定流化床料层压差,研究了多层流化床顶层进料均匀性、进气及排气方式对多层流化床流化料层高度均匀性的影响。结果表明,布料越均匀,上中二料层高度越接近,且在低气速下易于形成较良好的流化层,但随着气速增大,其影响逐渐减弱;在低气速下进气方式对中下二料层均匀性有较大影响,但随气速增大影响也减弱,底部进气方式更易达到较好的料层均匀性;流化床的排气方式对流化料层基本无影响。因此均匀的布料和较均匀的进气预分布有助于均化各料层高度,并拓宽多层流化床的操作弹性。     

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.     

DYNAMIC ANALYSIS OF CONTINUOUS WELL-MIXED FLUIDIZED/SPOUTED BED DRYERS

A diffusion-based mathematical model is presented for the prediction of the dynamics of drying in continuous well-mixed fluidized/spouted beds. Numerical techniques are used to solve the model equations. The outlet solids moisture content, the outlet air humidity and solids temperature are predicted as a function of time for the falling rate drying period. The model is helpful in describing the drying process during the startup periods and in studying open loop behavior of drying process. The model is also useful in designing control system for fluidized/spouted bed dryers.     

Wärme- und Stoffübertragung in der Wirbelschicht

Heat and mass transfer in fluidized beds . Methods of calculation are presented which enable reasonably accurate prediction of heat and mass transfer coefficients in fluidized beds. Some well established equations already given in the literature could be used for estimating the range of existence of the fluidized bed and for calculating maximum fluidization velocities, bed expansion, and particle-to-fluid heat and mass transfer. However, a new model had to be developed for the wall-to-bed heat transfer from solid surfaces immersed in fluidized beds. This model makes use of some basic ideas adopted from the kinetic theory of gases in order to describe the mechanism of energy transfer through the moving particles. Predictions with this new model are in good agreement with most of the experimental observations, particularly regarding the effects of particle diameter, temperature, pressure, physical properties of gas and particles, and gas velocity.     

Power-law fluids flow through multiparticle system

A study on the flow of power-law fluids through a multi-particle system including both fixed bed and fluidized bed is presented. Equations for the pressure drop, the minimum fluidization velocity, and the bed expansion are obtained by extending the Blake-Kozeny's equation for the pressure drop through packed beds to power-law fluids. Bed expansion equations are also obtained by extending the Richard-son-Zaki's theory for the drag force in a multiparticle suspension to power-law fluids. These equations are compared with experimental data.     

Identification of stable operating ranges of a counter-current multistage fluidized bed reactor with downcomer

This paper describes the stable operating range for a multistage gas–solid fluidized bed reactor and the influence of downcomer and aspect ratio on stable and uniform gas–solid fluidization. A three-stage counter-current fluidized bed reactor was designed, fabricated and operated and range of stable operation for lime and sand particles has been determined. In addition to that a comprehensive discussion on dynamics of downcomer has been made to ensure smooth transfer of solids from one stage to another. A theoretical model has been developed to predict the solid height in downcomer and tested for experimental conditions. The influence of aspect ratio on distributor to bed ratio (P_p/P_b) was investigated for the reactor for uniform fluidization.     

Numerical and experimental study on multiple-spout fluidized beds

In this paper we study the effect of multiple spouts on the bed dynamics in a pseudo-2D triple-spout fluidized bed, employing the discrete particle model (DPM) and non-intrusive measurement techniques such as particle image velocimetry (PIV) and positron emission particle tracking (PEPT). A flow regime map was constructed, revealing new regimes that were not reported so far. The multiple-interacting-spouts regime (C) has been studied in detail for a double- and triple-spout fluidized bed, where the corresponding fluidization regime for a single-spout fluidized bed has been studied as a reference case. The experimental results obtained with PIV and PEPT agree very well for all the three cases, showing the good performance of these techniques. The DPM simulation results slightly deviate from the experiments which is attributed to particle–wall effects that are more dominant in pseudo-2D beds than in 3D systems. The investigated multiple-interacting-spouts regime is a fully new flow regime that does not appear in single-spout fluidized beds. Two flow patterns have been observed, viz. particle circulation in between the spouts near the bottom of the bed, and an apparent single-spout fluidization motion at a higher location upwards in the bed. These findings show that the presence of multiple spouts in a spout fluidized bed highly affect the flow behaviour, which cannot be distinguished by solely investigating single-spout fluidized beds.     

The Magnetic Distributor—Downcomer (MDD) for fluidized beds

This paper evaluates the performance of the magnetic valve for solids when used as a combined distributor—downcomer (MDD) for fluidized beds of magnetic solids. Two kinds of MDD, the grate and the collar designs, are selected for study, and regimes of behavior are mapped out in the form of phase diagrams. The difference in behavior of these designs is contrasted, and is found to be great.Also studied in detail are the bed pressure drop, power requirements, and the mass flux through the MDD when the current is intermittently applied, and hence when the solids are allowed to drop from the bed in a controlled way.We conclude that either design of MDD can be made to operate satisfactorily, and this opens the door to the possibility of developing reliable trouble-free multistage fluidized beds for solids which contain magnetic materials.     

Devolatilization of large coal particles in fluidized beds

Analysis of devolatilization of predried large coal particles in fluidized beds requires consideration of both the chemical kinetics of coal decomposition and transport processes. Models available either assume the devolatilization particle to be isothermal (whereas it may be shown that, in general, large temperature gradients may exist within the particle) or require extensive numerical integration procedures. This Paper describes a model which permits formulation of analytical and easy-to-use equations for the estimation of the devolatilization history of a large predried coal particle in a fluidized bed. The model predictions are compared with experimental data collected for Mississippi lignite. A correlation is proposed for the estimation of the total devolatilization time. The analytical solutions presented may be used with ease in coupling the devolatilization process to the other phenomena, such as drying and/or combustion of volatiles and residual char, occuring during fluidized bed combustion of coal.     

Heat transfer between gas fluidized beds of solid particles and the surfaces of immersed heat exchanger elements,part I

After some general remarks about fluidization, and a section on the hydrodynamic behaviour of fluidized beds, the mechanisms of heat transfer between the surfaces of heat exchanger elements and gas—solid fluidized beds are discussed in detail. A theoretical model, presented some years ago, is slightly modified and further developed to improve its applicability within a wide range of variables. The model makes use of some of the basic concepts of molecular kinetic theory as applied to solid particles in a fluidized bed. A complete derivation as well as all the parameters required to apply the model equations are given.     

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。     

ATTRITION IN THE BUBBLING ZONE OF A STEADY-STATE FLUIDIZED BED

This paper describes the kinetics of attrition in the bubbling zone of a fluidized bed and focuses on the development of an equation for attrition in continuous fluidized beds operating at steady state.

Laboratory data describing batch attrition of a limestone sorbent are applied to the integrated equations to describe overall attrition rate in a full-scale continuous system.