CFD Model of Dense Gas-solid Systems in Jetting Fluidized Beds

A CFD code has been developed based on the conservation principles describing gas and solid flow in fluidized beds. This code is employed to simulate not only the spatiotemporal gas and solid phase velocities and voidage profiles in a two dimensional bed but also fluid dynamics in the jet region. The computational results show that gas flow direction is upward in the entire bed accompanied with random local circulations, whilst solid flow direction is upward at the center and downward near the wall. The radical reason of strong back mixing of solid particles and good transfer behavior between two phases is that the jet entrains solid particles. Numerical calculation indicates that gas velocity, solid velocity and pressure profile have a significant change when the voidage is 0 8. The simulated time averaged voidage profiles agree with the experimental results and simulated data reported by Gidaspow and Ettehadieh(1983). Therefore, CFD model can be regarded as a useful tool to study the jet characteristics in dense gas solid fluidized beds.

CFD-DEM simulation of fluorination reaction in fluidized beds with local grid and time refinement method

The gas-solid reaction process with wide particle size distribution is extensively used in the chemical engineering field,especially the particle reacts with the gas gradually,such as fluorination reactions in fluidized beds.When the computational fluid dynamics-discrete element method(CFD-DEM)is used for the coupling simulation of multiphase and polydisperse particle reaction system,the grid size directly affects the accuracy of flow field information and simulation of chemical reaction.Furthermore,particle calculation time step will directly affect the efficiency of coupling calculation.In this work,a local grid and time step refinement method is proposed to simulate multiphase and polydisperse particle fluid-ization reaction system.In this method,the refined DEM grids are automatically generated in the computational domain around the fine particles,and the detailed fluid phase information is obtained with the interpolation algorithm.In the two-phase coupling process,particles are divided into different groups based on physical properties,each group has its own independent time step.The multistage conical-cylindrical spouted bed is proposed for the fluorination reaction process;the operating gas ve-locity range of the polydisperse particle system is extended by the new design while the particle size distribution changes with the gas-solid reaction process.It is demonstrated that the local grid and time step refinement method can improve the accuracy and efficiency of the traditional CFD-DEM method in the reaction process simulation,which describes a polydisperse particle system with wide particle size distribution.Aimed at improving the simulation accuracy and efficiency,this paper will be helpful for simulating the particle reaction process in the gas-solid fluidized bed and beneficial for the development of the CFD-DEM method.

Hydrodynamic analysis of carbon nanotube clusters in distributor-less conical fluidized beds with step-by-step scaling

As a high-performance material with great application potential,the application of carbon nanotubes has been limited by their production volume.A distributor-less conical fluidized bed is the main equipment used in the industrial production of carbon nanotubes.To improve the production volume and product quality of carbon nanotubes,the study of fluidized-bed-diameter scaling is important.Three different diameters of distributor-less conical fluidized beds were established,and then the particle behavior and bubble characteristics of carbon nanotube clusters at these bed diameters were investigated.Time-series and wavelet analysis methods were used to analyze the pressure-fluctuation signals inside the fluidized beds.Results showed that the distributor-less design caused the airflow to break through the middle of the bed,which did not change with the change in bed diameter.The powder-bridging phenomenon of carbon nanotube clusters in a 100-mm-diameter fluidized bed was related to the special microstructure of carbon nanotube clusters.The frequency of pressure fluctuations in the bed decreased nonlinearly with increasing bed diameter.This study can guide the design and scale-up of distributor-less conical fluidized beds,especially for the scale-up of carbon nanotube production equipment,which can contribute to the improvement of carbon nanotubes’capacity and quality in industrial production.

Multi-scale numerical simulation of fluidized beds: Model applicability assessment

In the past few decades,multi-scale numerical methods have been developed to model dense gas-solidflow in fluidized beds with different resolutions,accuracies,and efficiencies.However,ambiguity needsto be clarified in the multi-scale numerical simulation of fluidized beds:(i)the selection of the submodels,parameters,and numerical resolution;(ii)the multivariate coupling of operating conditions,bed configurations,polydispersity,and additional forces.Accordingly,a state-of-the-art review is performed to assess the applicability of multi-scale numerical methods in predicting dense gas-solid flow influidized beds at specific fluidization regimes(e.g.,bubbling fluidization region,fast fluidization regime),with a focus on the inter-particle collision models,inter-phase interaction models,collision parameters,and polydispersity effect.A mutual restriction exists between resolution and efficiency.Higherresolution methods need more computational resources and thus are suitable for smaller-scale simulations to provide a database for closure development.Lower-resolution methods require fewercomputational resources and thus underpin large-scale simulations to explore macro-scale phenomena.Model validations need to be further conducted under multiple flow conditions and comprehensivemetrics(e.g.,velocity profiles at different heights,bubbles,or cluster characteristics)for furtherimprovement of the applicability of each numerical method.

CFD simulation for reduction of pyrolusite in fluidized beds

In this study,a CFD model coupled with heterogeneous flow structure,mass transfer equations,and chemical reaction kinetics is established to forecast the pyrolusite reduction reaction behavior.Compared with the previous studies which ignore the volume change of solids phase,the influence of volume shrinkage on reaction and flow behavior is explored in this research.Volume shrinkage of pyrolusite is proved to be non-negligible in predicting the conversion rate.The negligence of volume shrinkage leads to the overestimation of conversion rate for its inaccurate estimation of surface area for reaction.Besides,the influence of volume shrinkage on the reaction is found smaller in the scaled-up reactor.

Numerical simulations of high pressure carbon dioxide fluid fluidized beds

Hydrodynamics of carbon dioxide fluid-particle mixtures are predicted using a low density ratio-based kinetic theory of granular flow in high pressure carbon dioxide fluid fluidized beds.A coexistence of particle waves and particle aggregates exists along bed height.The threshold to identify the occurrence of particle aggregates is suggested based on standard deviation of solid volume fractions in aggregative fluidization.The existence time fractions and frequencies of particle aggregates are predicted along axial direction.The effect of carbon dioxide fluid temperature and pressure on volume fraction and velocity distributions are analyzed at different inlet carbon dioxide velocities and particle densities in high pressure carbon dioxide fluidized beds.Simulated results indicate that the carbon dioxide-particles fluidization transits from particulate to aggregative states with the increase of inlet carbon dioxide ve-locities.The computed fluid volume fractions and heterogeneity indexes are close to the measurements in a high pressure carbon dioxide fluidized bed.

Injection of gas-liquid jets into gas-solid fluidized beds:A review

Injection of gas-liquid sprays into gas-solid fluidized beds finds application in many industries.Effective mixing and distribution of liquid feed and solid bed material is paramount to ensure an efficient and profitable process.Despite its long-term use,the mechanism of liquid injection into gas-solid fluidized beds continues to raise questions and is only partially understood.This paper provides a thorough and up-to-date review of experimental and numerical investigations of gas-liquid sprays into gas-solid fluidized beds conducted over the past decades.Based on the surveyed literature,a phenomenological description of the prevalent mechanisms of gas-liquid injection under different operating conditions is presented.This review identifies suitable computational fluid dynamic models for simulating the mechanisms involved in gas-liquid-solid interactions along with recommendations for future numer-ical and experimental work.

Performance analysis of a 2D numerical model in estimating minimum fluidization velocity for fluidized beds

The minimum fluidization velocity of a fluid-solid particle fluidized bed is the primary focus of this paper.The computationally economic Eulerian Granular model has been used to analyze fluidization for both gas-solid particle and liquid-solid particle fluidized beds.The conventional approach of finding minimum fluidization velocity(umf)is either with a pressure drop across the particle bed or the change in bed height.However,these parameters are often unstable and cannot be used to generalize the degree of fluidization accurately.In this paper,the dominant factor of unstable pressure drop estimation in the 2D Two-Fluid Model(TFM)and a key non-dimensional Euler number has been investigated in deter-mining minimum fluidization velocity for different quasi-2D fluidized beds for different bed sizes,par-ticle sizes,and particle numbers.Averaging assumptions and limitations of these numerical models are discussed in detail for four different fluidized bed cases.A comparative study of the drag model shows little to no influence in unstable pressure drop estimation near fluidization velocity,and all drag models perform similarly.It is observed that particle-particle collision is not the dominant reason for unstable pressure drop near minimum fluidization.Instead,wall effects on the particle bed including frictional losses and wall-particle collision play a key role in unstable pressure drop calculation for the quasi-2D fluidized beds.Pressure drop characteristics alone do not suffice to obtain minimum fluidization ve-locity with 2D TFM using existing models.Thus,a different approach has been proposed to investigate minimum fluidization involving the Euler number,which has shown promising performance in deter-mining minimum fluidization velocity and characterizing fluidization with 2D TFM.Results show con-sistency in Euler number characteristics for all different fluidized bed cases considered in this paper.This can revitalize computationally economic 2D Eulerian simulations,increase the range of possible appli-cations,and provide guidance to the future development of computationally efficient and more accurate numerical models,and empirical correlations for minimum fluidization velocity.

Heat and Mass Transfer for a Nanofluid Flow in Fluidized Bed Dryer in Presence of Induced Magnetic Field

This research entails the study of heat and mass transfer of nanofluid flow in a fluidized bed dryer used in tea drying processes in presence of induced magnetic field. A mathematical model describing the fluid flow in a Fluidized bed dryer was developed using the nonlinear partial differential equations. Due to their non-linearity, the equations were solved numerically by use of the finite difference method. The effects of physical flow parameters on velocity, temperature, concentration and magnetic induction profiles were studied and results were presented graphically. From the mathematical analysis, it was deduced that addition of silver nanoparticles into the fluid flow enhanced velocity and temperature profiles. This led to improved heat transfer in the fluidized bed dryer, hence amplifying the tea drying process. Furthermore, it was noted that induced magnetic field tends to decrease the fluid velocity, which results in uniform distribution of heat leading to efficient heat transfer between the tea particles and the fluid, thus improving the drying process. The research findings provide information to industries on ways to optimize thermal performance of fluidized bed dryers.

Development of a hydrodynamic model and the corresponding virtual software for dual-loop circulating fluidized beds

Dual-loop circulating fluidized bed(CFB)reactors have been widely applied in industry because of their good heat and mass transfer characteristics and continuous handling ability.However,the design of such reactors is notoriously difficult owing to the poor understanding of the underlying mechanisms,meaning it has been heavily based on empiricism and stepwise experiments.Modeling the gas-solid CFB system requires a quantitative description of the multiscale heterogeneity in the sub-reactors and the strong coupling between them.This article proposed a general method for modeling multiloop CFB systems by utilizing the energy minimization multiscale(EMMS)principle.A full-loop modeling scheme was implemented by using the EMMS model and/or its extension models to compute the hydrodynamic parameters of the sub-reactors,to achieve the mass conservation and pressure balance in each circulation loop.Based on the modularization strategy,corresponding interactive simulation software was further developed to facilitate the flexible creation and fast modeling of a customized multi-loop CFB reactor.This research can be expected to provide quantitative references for the design and scale-up of gas-solid CFB reactors and lay a solid foundation for the realization of virtual process engineering.

Comparative study of two fluid model and dense discrete phase model for simulations of gas-solid hydrodynamics in circulating fluidized beds

Computational fluid dynamics(CFD)has become a valuable tool to study the complex gas-solid hydrodynamics in the circulating fluidized bed(CFB).Based on the two fluid model(TFM)under the Eulerian-Eulerian framework and the dense discrete phase model(DDPM)under the Eulerian-Lagrangian framework,this work conducts the comparative study of the gas-solid hydrodynamics in a CFB riser by these two different models.Results show that DDPM could be used to predict gas-solid hydrodynamics in the circulating fluidized bed,and there are differences between TFM and DDPM,especially in the radial distribution profiles of solid phase.Sensitivity analysis results show that the gas-solid drag model exhibits significant effects on the results for both the two models.The specularity coefficient and the restitution coefficient in the TFM,as well as the reflection coefficient and the parcel number in the DDPM,exhibit less impact on the simulated results.

Effect of bed size on the gas-solid flow characterized by pressure fluctuations in bubbling fluidized beds

Pressure fluctuations in four bubbling fluidized beds having different bed sizes (three square cross-sections of 5, 10, and 15 cm in side length, and one rectangular cross-section of 2 × 10 cm2) were measured at four axial positions (P1, P2, P3, and P4). Several characteristic indicators of the flow specifically of the pressure were calculated. In terms of these characteristic indicators, the effect of bed size on flow behavior was investigated. The results show that in the fully fluidized state, the pressure drop is slightly higher in smaller beds, but the pressure drops in the 10- and 15-cm beds are close. The 15-cm bed has the lowest pressure-fluctuation amplitude. The amplitudes at P1 and P2 in the lower part of the bed are very close for bed sizes below 10 cm, but the amplitude at P3 near the bed surface increases with decreasing bed size. No general trend was observed regarding the effect of bed size on skewness and kurtosis of the pressure for all four axial heights. For the average, standard deviation, skewness, and kurtosis of the pressure at P4, the values are close for the two small beds (2 × 10 and 5 × 5 cm2) and the two large beds (10 × 10 and 15 × 15 cm2), and hence the effect of bed size separates the beds into two groups. In the fully fluidized state, for P1, P2, and P3, the Kolmogorov entropy and the dominant frequency both increase with increasing bed size, but in the pseudo-2D bed both are between the values for the 5- and 10-cm beds.

Effect of structure parameters on forces acting on baffles used in gas-solids fluidized beds

Effects of some important structural parameters,i.e.slat pitch,and layout position,on dynamic forces acting on the baffles were examined in the fluidized bed of FCC particles operating under different superficial gas velocities.The experimental baffles were made of multiple inclined slats.We found that the forces acting on the baffles decreased significantly with reducing pitch between the slats.For the baffles with a small slat pitch,the forces acting on the baffles increased slightly and then decreased with increasing superficial gas velocity,which is very different from the measured results of a single slat or tube immersed in fluidized beds.The different results are greatly related to the appearance of the“gas cushion”beneath the baffles,whose height increases with increasing superficial gas velocity.On the other hand,a region with stronger particle circulation induced by the inclined slat array was observed in the experiments.The slat near the wall and located below the region of downward-flowing particles was found to be subjected to the severest forces.Therefore,the slats located in similar locations of industrial baffles are suggested to be reinforced to increase their structural strength.

Characteristics of gas-solid micro fluidized beds for thermochemical reaction analysis

Fuel conversion and clean energy reaction systems involve a variety of catalytic and non-catalytic gas-solid thermochemical reactions.A good understanding of the correct reaction mechanism and kinetics,as well as the profiles of reaction products,is of great significance to the development,design,and operation of such reaction systems.The micro fluidized bed reaction analysis provides an efficient and reliable method to acquire this essential information with low capital and operating costs,low energy consumption and enhanced safety.This paper provides an overview of the system and its characteristics for the micro fluidized bed reaction analyzer that has been well proven to be a reliable new approach as well as an instrument for characterizing various gas-solid thermochemical reactions.

MP-PIC simulation of circulating fluidized beds using an EMMS based drag model for Geldart B particles

In this study the multiphase particle-in-cell(MP-PIC)method is used for the simulation of two pilot-scale circulating fluidized beds(CFBs)with quartz sand belonging to Geldart’s group B as bed material.The simulations were performed using a homogeneous drag model as well as a structure dependent drag model based on the energy minimization multi-scale method(EMMS).The results are compared with experimental data from literature as well as experiments.The simulations with the EMMS based drag model show a good agreement of the time-averaged axial solids concentration,circulation rate and riser pressure drop.Furthermore,a lower grid sensitivity is observed compared to the homogeneous drag model.In contrast to the conventional drag model a dense bottom zone is predicted by the EMMS based drag model.An overprediction of the solid concentration in the dense bottom zone is presumably due to an overprediction of the cluster diameter that is calculated using an empirical cluster diameter correlation.This shows the necessity for a new meso-scale cluster correlation for the simulation of Geldart B particles.Furthermore,the results of the time-averaged radial solids concentration differ from the expectations of a core-annulus flow indicating that a mesh refinement at the walls is necessary.Finally,the importance of using a realistic particle size distribution is identified.

Hydrodynamics in commercial-scale internally circulating fluidized beds with different central downcomer outlets

Standpipes,or downcomers,are commonly used in fluidized beds to transport particles.The outlet structure of the downcomer greatly affects the performance of flow from it and even overall reactor performance.In this study,the hydrodynamics in commercial-scale internally circulating fluidized beds(ICFBs)with central downcomers having different outlet structures was investigated using computational fluid dynamics simulations with an energy minimization multi-scale drag model.The predicted results closely agreed with experimental data.Results showed that in an ICFB with a downcomer outlet directly open to the bed(model A),nearly 12.7%to 5.4%of the gas in the draft tube bypasses into the downcomer.In the ICFB models B and C with a conic baffle below the downcomer,the gas bypass is significantly weakened or even eliminated when the diameter of the conic baffle is 1.1 times that of the downcomer(model C).In addition,the solids circulation mass flux in ICFBs increased by about 62.5%,from 126.8 kg/(m2 s)in model A to 206 kg/(m2 s)in model C.

Gas-liquid mass transfer in the gas-liquid-solid mini fluidized beds

The gas-liquid-solid mini fluidized bed(GLSMFB)combines the advantages of fluidized bed and micro-reactor,and meets the requirements for safety and efficiency of green development of process industry.However,there are few studies on its flow performance and no studies on its mass and heat transfer performance.In this paper,the characteristics of gas-liquid mass transfer in a GLSMFB were studied in order to provide basic guidance for the study of GLSMFB reaction performance and application.Using CO_(2)absorption by NaOH as the model process,the gas-liquid mass transfer performance of GLSMFB was investigated.The results show that the liquid volumetric mass transfer coefficient and the gas-liquid interfacial area both increase with the increase of the superficial gas velocity within the experimental parameter range under the same given superficial liquid velocity.At the same ratio of superficial gas to liquid velocity,the liquid volumetric mass transfer coefficient increases with the increase of the super-ficial liquid velocity.Fluidized solid particles strengthen the liquid mass transfer process,and the liquid volumetric mass transfer coefficient is about 13%higher than that of gas-liquid mini bubble column.

Experimental study on reactor performance of gas-solids low-velocity fluidized beds

Reactor performance of bubbling fluidized bed(BFB)and turbulent fluidized bed(TFB)was carefully examined and systematically compared using catalytic ozone decomposition as a model reaction,based on a complete mapping of local flow structures and spatial distributions of ozone conversion and solids holdup.TFB clearly has a higher conversion and shows better reactor performance than BFB as a result of the vigorously turbulent flow and the relatively homogeneous gas–solids mixing in TEB.Besides,the intensive interaction between gas and solids in TFB leads to greater gas–solids contact efficiency of TFB over that of BFB.Due to gas bypassing and backmixing caused by bubbling behaviours and two-phase structure,BFB deviates significantly from a plug flow reactor and sometimes from a continuously stirred tank reactor.The flow structures essentially dictate the reactor performance in the low-velocity fluidized beds.

Effects of riser geometry on gas-solid flow characteristics in circulating fluidized beds

The performance of a circulating fluidized bed strongly depends on its parameter settings,including that of riser geometry.In this study,a laboratory-scale circulating fluidized bed with three different riser geometries(circular,square,and rectangular)that had the same cross-sectional area and height was operated under two static bed heights(20,and 35 cm).Electrical capacitance tomography was combined with differential pressure transducers and an optical-fiber probe to measure the solids'volume fraction,differential pressure fluctuations,and radial particle concentration variations.Computational particle fluid dynamics simulations were also performed.The results showed that single bubbles appeared in the bottom region of the circular and square risers and double bubbles in the bottom region of the rectangular riser.The autocorrelation of capacitance signals was periodic for the circular and square risers and non-periodic for the rectangular riser.The radial particle concentration profiles showed a single-core annulus structure in the circular and square risers,but a double-core annulus structure along the long side and single-core annulus structure along the short side in the rectangular riser.Shannon entropy analysis showed that fluidization was less disordered and most predictable for the rectangular riser.

Effect of Gaussian size distribution width on minimum fluidization velocity in tapered gas–solid fluidized beds

This paper investigated the effect of Gaussian distribution width,average particle diameter,particle loading,and the tapered angle on minimum fluidization velocity(U_(mf))by conducting extensive experiments in tapered fluidized beds.Three powders with Gaussian size distribution and different distribution widths were used in the experiments.An increase in U_(mf)with increasing the average particle diameter,particle loading,and the tapered angle was observed.There was also a nonmonotonic behavior of Umf as the Gaussian distribution width increased.An empirical correlation including dimensionless groups for predicting Umf in tapered beds was developed in which the effect of distribution width was considered.The proposed correlation predictions were in good agreement with the experimental data,with a maximum deviation of 16.5%and average and standard deviations of,respectively,6.4%and 7.4%.The proposed correlation was also compared with three earlier models,and their accuracy was discussed.