Progress in research of process intensification of spouted beds:A comprehensive review

The development of intensification technology for spouted beds has become a current research focus,and an effective way to improve the efficiency of spouted beds is to reform their structure.Although numerous studies have been conducted on conventional beds,there are few reviews on the comprehensive application of intensification technology for spouted beds.In this paper,we comprehensively review the role of intensification technology in spouted beds for use in hydrodynamics,drying,desulfurization,pyrolysis,coating,biomass and waste gasification,and biomass drying from the perspective of experiment and simulation.Finally,potential problems and challenges in current spouted-bed research are summarized.

Analysis of Gas-Solid Flow Characteristics in a Spouted Fluidized Bed Dryer by Means of Computational Particle Fluid Dynamics

In order to grasp the particle flow characteristics and energy consumption of industrial fluidized spouted beds,we conduct numerical simulations on the basis of a Computational Particle Fluid Dynamics(CPFD)approach.In particular,the traction model of Wen-Yu-Ergun is used and different inlet conditions are considered.Using a low-speed fluidizing gas,the flow state of the particles is better and the amount of particles accumulated at the bottom of the bed wall becomes smaller.For the same air intake,the energy loss of a circular nozzle is larger than that of a square nozzle.

Optimization of Draft Tube Position in a Spouted Bed Reactor using Response Surface Methodology

Optimization of draft tube position in a spouted bed reactor used for treatment of wastewater containing low concentration of heavy metals is investigated in this paper. Response surface methodology is used to optimize the draft tube height, the draft tube width and the gap between the bottom of the draft tube and the inlet nozzle. It is observed that the draft tube with a height of 60 millimeter, width of 12 millimeter and the gap of 13 millimeter between its bottom and inlet nozzle, results in optimum value of minimum spouting velocity, measured 45 cubic centimeter per second (2.7 Liter per minute) .

CFD-DEM simulation of high density particles fluidization behaviors in 3D conical spouted beds

The spouted bed has been used in the coating process of high-density nuclear fuel particle.The particle fluidization behaviors in pseudo-2D and 3D spouted beds were simulated and validated.The effects of four independent variables(cone angle,particle density,inlet gas velocity,and particle loading)on particle fluidization behaviors in the 3D spouted bed were investigated systematically.The cone angle effect on fluidization mechanism was studied quantitatively first time.A new fluidization quality index was proposed based on the particle entrainment principle,and an extreme value was obtained when the cone angle was 60°,considered to be the optimum value for the 3D conical spouted bed.It was indicated the gas–solid contact efficiency can be kept up if the gas velocity was proportional to ρ_(p)^(0.65) and N_(p)^(0.78) when the particle density or loading was increased.These results will be useful for geometry and operation parameters design of the 3D conical spouted bed and helpful for developing the fluidization mechanism of high-density particles.

A new method for identifying flow pattern of spouted fluidized bed by coupling Hilbert-Huang transform characteristics of differential pressure signals in different zones

On a cold spouted fluidized bed,this study compares the characteristic differences in intrinsic mode function(IMF)energy and Hilbert–Huang spectrum between the spout zone and annulus zone under different combinations of spouted gas and fluidized gas flow rates for five typical flow patterns.The energy distribution characteristics under different flow patterns are also analyzed.The Hilbert–Huang spectrum and IMF energy of pressure difference signals exhibit distinct variations in different zones as the flow pattern changes.Moreover,there exists a correlation between the energy in the middle-frequency range and the flow pattern.Leveraging the K-means algorithm,the middle-frequency range energy of IMFs in the spout zone and annulus zone is subjected to clustering analysis,leading to the identification of partition boundaries for each flow pattern.Based on this,a flow pattern identification method of spouted fluidized bed coupled with middle-frequency range energy in spout zone and annulus zone is proposed,which has very high identification accuracy.

Numerical simulation of uranium tetrafluoride fluorination in a multistage spouted bed using the improved CFD-DEM chemical reaction model

A CFD-DEM reaction coupling model was established to simulate UF_(4) fluorination process,in which heat and mass transfer,heterogeneous chemical reaction,and particle shrinkage model were considered.The gas behavior was described by the conservation laws of mass,momentum,and energy.The solid phase is modeled with the discrete element method,considering the gas-solid interphase force,contact force,heat transfer,and chemical reaction models based on the discretized surface.Each particle can be individually tracked and associated with specific physical properties.The proposed CFD-DEM reaction coupling model based on particle shrinking reaction model with discretized surface was validated by the experimental and literature results at first.Then a multistage conical spouted bed was proposed and the process of UF_(4) fluoridation reaction in it was investigated.The fluidization characteristics and the con-centration distribution of gaseous products in the spouted bed with an extended gas velocity range were obtained and analyzed.In addition,the effects of different parameters,such as superficial gas velocity,temperature,fluorine concentration,on fluoridation rate and the fluorine conversion rate were inves-tigated based on the proposed CFD-DEM reaction coupling model.The results obtained in this work are beneficial for method development of the chemical reaction simulation research in particle scale using the CFD-DEM model,and useful for operation and equipment parameters design of the uranium tetra-fluoride fluorinate industrial process in the future.

Axial gas mixing in conical spouted beds with high density particles

Conical spouted beds operating with high-density particles(ρp>2500 kg/m^(3))have recently gained attention because of their potential use as nuclear fuel coaters for next-generation nuclear reactors.In the literature,the number of axial gas mixing studies in conical and conical-cylindrical spouted beds is very limited and all axial mixing studies were carried out with relatively light particles(ρp≤2500 kg/m^(3)).Therefore,the objective of this study was to generate experimental data that can be used to explain the gas axial mixing behavior in conical spouted beds operating with both low-and high-density particles.Experiments were conducted in two(γ=30°,60°)conical spouted beds with three different types of particles:zirconia(ρp=6050 kg/m^(3)),zirconia toughened alumina(ρp=3700 kg/m^(3))and glass beads(ρp=2460 kg/m^(3)).In order to be able to compare experimental data obtained at different conditions,a 1-D convection-diffusion gas mixing model originally developed by San Joséet al.(1995)was implemented to determine the axial dispersion coefficients.The results show that the axial dispersion coefficients range between m^(2)/s and m^(2)/s,increase with superficial gas velocity and are higher than the corresponding dispersion coefficients of fixed beds,lower than the dispersion coefficients of fluidized beds and in the same range with the cylindrical spouted beds reported in the literature.The corresponding Peclet numbers were in the range of 0.6–7.8 for all operating conditions and slightly higher Peclet numbers were obtained with glass beads indicating the relative importance of gas convective transport over gas dispersion for light particles compared to heavy particles.

Hydrodynamic behaviour of Cellets^(TM)(Ph.Eur./USP)in a spouted bed using image processing method

The hydrodynamic behaviour of the spouted bed in the pharmaceutical industries has been found to be less addressed.The present paper has focused on the hydrodynamic characteristics of a spouted bed where the Cellets^(TM)(Ph.Eur./USP)is adopted as the bed material.Experiments are carried out with three different static bed heights(H_(0))of shallow depth(2D_(i)≤H_(0)<3D_(i))using two different particle sizes.The spouted bed employed with D_(i)/D_(0) of 5 has given the experimental information on external spouting(Ues)by mapping the pressure drop,and fountain height(H_(f))against the superficial gas velocity(U_(g))is rep-resented with the image contours,which show the intrinsic behaviour.All the 1000 μm and 700 μm particles have been found to exhibit symmetric and asymmetric spouting.With increasing U_(g),the fully suspended particles are limited to a certain height in the freeboard region due to the gas-solid cross-flow,which implies the clusters have identified with the image processing method.

Minimum spouting velocity of flat-base spouted fluid bed

Experiments were performed on spout characteristics of a cylindrical spout-fluidized bed （I.D. = 10 cm） with different static heights and two materials （A1203 and high density polyethylene）. Results of minimum spouting velocity obtained in this study were compared with reported correlations for both spouted and spout-fluidized beds. Considerable discrepancies were found between the values obtained using different model equations as well as with respect to experimental results. Based on the Mathur-Gishler correlation, a new correlation is proposed for calculating the minimum spouting velocity that introduces the ratio U/Umf. It was found that the minimum spouting velocity decreases with increasing fluidizing gas velocity （U/Umf）. The pressure drop at the point of minimum spouting velocity is also correlated using this dimensionless group and is presented in this work. This investigation demonstrates that the use of correlations reported in the literature that focus primarily on conical bottom spouted beds are not applicable to fiat-bottom spouted and spout-fluidized beds.

Flow regimes in wet conical spouted beds using glass bead mixtures

Using a high-viscosity Newtonian fluid, glycerol, an experimental investigation was carried out to evaluate the stable spouting regime in conical spouted beds using four particle mixtures： a reference （monoparticles）, a binary mixture, two ternary mixtures with flat and Gaussian distributions respectively. The mixtures were selected for particle diameters （dp） ranging from 1.09 to 4.98 mm and particle diameter ratios （dpL/dps） ranging from 1.98 to 4.0. Experimental data show that pressure fluctuation signals of the bed, as indicated by changes in their standard deviations, provide suitable information to identify the range of operational conditions for stable spouting. However, the analysis of skewness of curves of pressure fluctuation as a function of air velocity appears not sufficient to identify a particular flow regime. For glycerol in the spouting regime, the standard deviation is noted to increase with increasing glycerol concentration due to the growth of interparticle forces. The implications of these research findings on the drying of suspensions in conical spouted beds using glass bead mixtures are also discussed.

Spouting behaviour of binary mixtures in square-based spouted beds

From experime nts, the influe nee of the physical characteristics of different bin ary mixtures of solids on the spouting regime of a pyramidal square-based spouted bed reactor is assessed. The applied methodology permits a more precise evaluation of the effects of the tested variables (diameter, density, sphericity) on the response variables (minimum air flows at which spouting begins and at which to maintain spouting con ditions). The associated pressure drops along the bed of particles a nd the height of the formed fountai n are analysed in each case. During the initial stages of fluidisation, binary mixtures containing different density ratios show dead zones. Segregation becomes more evident at large-size and high-density ratios. The lack of sphericity was found to be the main reason leading to blocking, channelling, and start-up problems when system failures occur. Nevertheless, the extent of segregation in all cases decreases with increasing the spouting velocity. In addition, a computational fluid dynamic model based on the discrete element method, previously validated for a single solid bed, is proposed as a tool to predict and evaluate potential segregation phenomena in binary mixtures. This model reproduced with high accuracy the encountered segregation phenomena. Its use may help define the technical limts inherent in the pyramidal spouted bed reactor.

IMPROVED DEM-CFD MODEL AND VALIDATION:A CONICAL-BASE SPOUTED BED SIMULATION STUDY

An improved and efficient DEM-CFD approach is developed for spouted beds. A nonlinear Discrete Element Method （DEM）, with a concept of spring, dash-pot and friction slider, is used for tracing the movement of each individual particle. The gas flow is described by a set of reorganized governing equations. Two phases are coupled through contributions due to effects of porosity, viscosity and drag. All equations are solved with the commercial package Fluent with an implementation of User Defined Functions （UDF）. To validate the improved model, a two-dimensional conical-base spouted bed is chosen as a case study. An unstructured mesh system is adopted instead of regular grid system. The simulation also takes the Saffman force and Magnus effect into account. The calculation results show good agreement with the experimental observations which are taken from the literature.

Simulation of spray coating in a spouted bed using recurrence CFD

Although numerical models such as the computational fluid dynamics–discrete element method (CFD–DEM) have enabled the accurate simulation of laboratory-scale apparatuses, the application of these methods to large-scale apparatuses with many particles and time scales ranging from minutes to hours remains a challenge. The recently developed recurrence CFD (rCFD) method seeks to overcome these issues in pseudo-periodic processes by extrapolating globally recurring patterns in a physically meaningful way and describing the transport and interaction of passive scalars using Lagrangian tracers. Spouted beds represent an interesting target because of the associated variety of flow regimes. They can be effectively described by CFD–DEM on the time scale of tens of seconds, whereas industrially relevant processes typically take hours. In this contribution, we established the validity of applying the Lagrangian rCFD method to spouted beds by demonstrating the accurate reproduction of the particle residence time distribution in a fictitious spray zone. The deposition of spray droplets onto tracer particles was simulated for 1 h, and the particle surface coverage distribution was estimated using a statistical approach for both an unstabilized prismatic spouted bed and one stabilized by draft plates.

CFD–DEM simulation of fine particles in a spouted bed apparatus with a Wurster tube

In this work, a coupled computational fluid dynamics–discrete element method (CFD–DEM) approach was employed to evaluate the spouting behavior of fine, cohesive powders in a cylindrical spouted bed with a conical base and equipped with a Wurster tube. The particle and gas dynamics inside the apparatus were simulated with 1.7 million spherical ZrO2 particles with a particle size of 100 μm. For an accurate prediction of the interactions of cohesive particles in the spouted bed, the adhesion forces according to JKR theory were included in the Hertz–Tsuji contact model. The surface energy of the particles was varied over a wide range to determine the effect of the adhesion on the spouting (the fountain shape and maximum height as well as the distribution of the concentrations and velocities of particles in different zones of the apparatus). A detailed analysis of the collision dynamics was conducted. The spouting behavior of a spouted bed with the same dimensions, particles, and processing parameters was recorded with a high-speed camera. The CFD–DEM simulations showed good agreement with the experimentally captured spouting behavior.

Elutriation, attrition and segregation in a conical spouted bed with a fountain confiner

This study examined elutriation,attrition,and segregation in a conical spouted bed with a fountain confiner and incorporating an open sided draft tube.Fine silica sand with a wide particle size distribution was employed as a model material,operating in both the batch and continuous modes.The use of a fountain confiner is crucial when operating with fine particles,because otherwise the bed rapidly exhibits significant entrainment.The extent of attrition was quantified using a tracing technique based on differently-coloured sand fractions as well as monitoring size distributions by sieving.Particle breakage was found to be the primary attrition mechanism,and the fountain confiner was determined to limit the elutriation of fine particles resulting from breakage.Consequently,only a small fraction of the finest particles were entrained from the bed.The incorporation of a confiner increased operational stability while reducing segregation,especially in the upper half of the bed where the majority of segregation typically occurs.Thus,the bed was perfectly mixed apart from very minimal segregation close to the wall and at the bottom of the contactor.These results provide a basis for the design and operation of larger scale equipment for the continuous drying of materials.

Particle shape effect on hydrodynamics and heat transfer in spouted bed:A CFD-DEM study

Spouted bed has drawn much attention due to its good heat and mass transfer efficiency in many chemical units.Investigating the flow patterns and heat and mass transfer inside a spouted bed can help optimize the spouting process.Therefore,in this study,the effects of particle shape on the hydrodynamics and heat transfer in a spouted bed are investigated.This is done by using a validated computational fluid dynamics-discrete element method(CFD-DEM)model,considering volume-equivalent spheres and oblate and prolate spheroids.The results are analysed in detail in terms of the flow pattern,microstruc-ture,and heat transfer characteristics.The numerical results show that the prolate spheroids(Ar=2.4)form the largest bubble from the beginning of the spouting process and rise the highest because the fluid drag forces can overcome the interlocking and particle-particle frictional forces.Compared with spherical particles,ellipsoidal spheroids have better mobility because of the stronger rotational kinetic energy resulting from the rough surfaces and nonuniform torques.In addition,the oblate spheroid system exhibits better heat transfer performance benefiting from the larger surface area,while prolate spheroids have poor heat transfer efficiency because of their orientation distribution.These findings can serve as a reference for optimizing the design and operation of complex spouted beds.

NUMERICAL SIMULATIONS OF HYDRODYNAMIC BEHAVIORS IN CONICAL SPOUTED BEDS

The axial and radial distributions of static pressures and vertical particle velocities of conical spouted beds have been simulated and compared with experimental data. Simulation results show that, among all factors investigated, the Actual Pressure Gradient （the APG term） in conical spouted beds, introduced as the default gravity term plus an empirical axial solid phase source term, has the most significant influence on static pressure profiles, followed by the restitution coefficient and frictional viscosity, while other factors almost have no effect. Apart from the solid bulk viscosity, almost all other factors affect the radial distribution of the axial particle velocity, although the influence of the APG term is less significant. For complex systems such as conical spouted beds where a fluidized spout region and a defluidized annulus region co-exist, the new term introduced in this work can improve the CFD simulation. Furthermore, for other systems with the Actual Pressure Gradient different from either fluidized beds or packed beds, the new approach can also be applied.

Drying of suspensions and solutions on inert particle surface in mechanically spouted bed dryer

To eliminate some disadvantages of the conventional spouted bed dryers the mechanically spouted bed （MSB） system was developed. This dryer type is convenient to use inert particles providing an increased surface area for drying of materials of high-moisture content and heat sensitive materials. On three different drying tasks are demonstrated the experimental optimization of process parameters to obtain products of demanded quality. The main object was at drying of AIO（OH） suspension to preserve the particle size under 2.5μm and to obtain product with a moisture content of about 0.05 kg/kg （d.b.）. For this reason a very thin particle coating and intensive abrasion had to be assured. At drying of tomato concentrates the thermoplasticity makes the process very difficult. To jump over the deliquescent and sticky state developed at the critical temperature-moisture content values a very short drying time （8-10 s） must be provided. The third task was to form powder-like product from bovine serum albumin （BSA） solution having very low solid content （2-4%）. The selected process parameters given in this paper resulted in a mean particle size of less than 20 μm while the soluble oreserved orotein content was higher than 90%.

Mixing and segregation of solid particles in a conical spouted bed： Effect of particle size and density

In this work, the mixing and segregation of binary mixtures of particles with different sizes and densities in a pseudo-2D spouted bed were studied experimentally. A binary mixture of solid particles including sand, gypsum, and polyurethane was used. To determine the particles mass fraction, and their mixing and segregation in the bed, an image-processing technique was developed and used. Important hydrodynamic parameters, such as the axial and radial segregation profiles of the solid particles, were measured. The effects of air velocity, particle size, and particle mass fraction were also evaluated. The flow regime in the spouted bed and the time required for reaching the equilibrium state of the solid particles were discussed. The results showed that the segregation of solid particles and the time to equilibrium both decreased when the air velocity increased to much larger than the minimum spouting velocity. The axia! segregation increased with the diameter ratio of the particles. Upon completion of the test, coarse particles were concentrated mainly in the spout region, while fine particles were aggregated in the annulus region. Examination of the flow pattern in the spouted bed showed that the particles near the wall had longer flow paths, while those near the spout region had shorter flow paths.

Surface-to-bed heat transfer for high-density particles in conical spouted and spout–fluid beds

Bed-to-surface heat transfer experiments from a vertically submerged cylindrical surface were conducted in laboratory-scale (Dc = 25 cm) conical spouted and spout–fluid beds at two different conical angles (31° and 66°) in the high particle density range (2500 kg/m3 ≤ρp ≤ 6000 kg/m3). The effects of the bed design parameters (conical angle and inlet diameter of spouting gas entrance) and operating conditions (static bed height, particle size, density, and spouting and fluidization gas flow rates) on the heat transfer characteristics were investigated in detail. The heat transfer coefficients were shown to be dependent on the density and size of the particles. The minimum stable spouting velocities of the denser and larger particles were higher, which led to higher operational spouting velocities and thereby resulted in higher heat transfer coefficients. The positive effect of increasing the particle diameter on heat transfer was more pronounced in the spout and at the spout–annulus interface, whereas this effect was diminished in the annulus region. The heat transfer coefficient increased with increasing spouting gas velocity up to 1.0Ums–1.1Ums, beyond which no significant change was observed regardless of the particle type. The heat transfer coefficient in the annulus decreased with increasing conical angle because of reduced particle circulation. The spout–fluid operation increased the heat transfer coefficient by a maximum of 10% at the expense of a significant increase of the total gas flow rate. This result was attributed to the inability of the fluidizing gas to penetrate the annulus. An empirical correlation for the average heat transfer coefficient in the annulus was also proposed based on the data obtained in this work.