Gas-solid Fluidized Bed Reactor Research Articles

A CFD-DEM study of hydrodynamics with heat transfer in a gas-solid fluidized bed reactor for solar thermal applications

The particles flow and heat transfer characteristics of a high temperature solar thermochemical fluidized bed reactor have been studied for solar beam-down concentrating systems. A numerical model has been developed by the combined approach of computational fluid dynamics (CFD) and discrete element method (DEM) collisional model since it is an effective approach for studying the gas-solid flow. The discrete ordinate model has been used to solve the radiation heat transfer. An experimental visualization of particles circulation pattern and mixing of two-tower fluidized bed system has been presented. A good agreement has been found between the experimental measurements and numerical predictions. The effect of gas superficial velocity, bed mass and inlet gas temperature on the flow pattern and temperature characteristics of the bed have been investigated. The results showed that the maximum and average temperature of the bed, depends on the top layer position and focal point of the concentrated radiation, decreased when increasing the total mass of the bed.

Particle agglomeration and control of gas-solid fluidized bed reactor with liquid bridge and solid bridge coupling actions

Fluidized beds reactors with liquid bridge and solid bridge coupling actions have been widely used in industry and thus studies of particle agglomerations under such fluidization conditions are of great significance. This work proposed an experimental apparatus combining electro-magnetic induction heating system with fluidized bed to simulate the real conditions for particle reaction heat release and heat transfer in industrial polymerization fluidized bed reactors. The effects of liquid content on agglomeration behavior of wax/graphite composite particles under different fluidization gas temperatures, gas velocities and heating powers have been studied. Through a force balance analysis of particles with liquid bridge and solid bridge coupling actions, relative solid bridge force was taken as a key parameter to demonstrate the effects of solid bridge force and other forces on agglomerations. Results showed that as the relative solid bridge force varied in different ranges, the agglomeration mass presented three different variation trends with liquid increases, namely monotonic increasing, non-monotonic changing and monotonic decreasing, due to different dominating agglomerating mechanisms (liquid evaporation and liquid bridge) during fluidization. According to the results of this study, the proposed criterion based on relative solid bridge force can be used to guide the regulation and control of particle agglomerations in fluidization with liquid bridge and solid bridge coupling actions.

Thermal-fluid characteristics on near wall of gas-solid fluidized bed reactor

Gas-solid circulating fluidized beds (CFBs) are for carbon capture process because of their remarkable heat and mass transfer characteristics in the target reactor. Bed-to-wall heat transfer is an important issue in designing a reactor to satisfy the required conditions. Bed-to-wall heat transfer is characterized by near-wall particle behavior. Thus, in this study, local bed-to-wall heat transfer and near-wall particle dynamic characteristics were measured in a lab-scale CFB riser. The clustered particle behavior near the wall was investigated primarily using a non-intrusive method, particle-imaging velocimetry (PIV), within the CFB riser. To determine the effects of bed instability on the clustered particle behavior at the wall, bed instability was controlled. Bed instability was estimated by the degree of pressure fluctuation in the riser, and the fluctuation effect was compared, depending on the fluidization regime. Ultimately, the clustered particle behavior on the adjacent wall was compared depending on the pressure fluctuations using the PIV system. We estimated the clustered particle behavior on the adjacent wall of the CFB riser considering fluidization instability. Detailed analysis of the relationship between the bed-to-wall heat transfer and the particle behavior was conducted.

Magnetic Resonance Imaging and Velocity Mapping in Chemical Engineering Applications.

This review aims to illustrate the diversity of measurements that can be made using magnetic resonance techniques, which have the potential to provide insights into chemical engineering systems that cannot readily be achieved using any other method. Perhaps the most notable advantage in using magnetic resonance methods is that both chemistry and transport can be followed in three dimensions, in optically opaque systems, and without the need for tracers to be introduced into the system. Here we focus on hydrodynamics and, in particular, applications to rheology, pipe flow, and fixed-bed and gas-solid fluidized bed reactors. With increasing development of industrially relevant sample environments and undersampling data acquisition strategies that can reduce acquisition times to <1 s, magnetic resonance is finding increasing application in chemical engineering research.

ChemInform Abstract: The Calcium‐Looping Technology for CO2 Capture: On the Important Roles of Energy Integration and Sorbent Behavior

AbstractReview: 162 refs.

Multi-fluid model with variable particle density and diameter based on mass conservation at the particle scale

The multi-fluid model (MFM) has been widely used in the simulation of gas–solid fluidized bed reactor, in which the density and diameter of particles normally change due to the heterogeneous reactions. However, the particle diameter always keeps constant although the particle density varies along with reaction in the previous MFM, which may disobey the law of mass conservation at the particle scale. Therefore, in order to overcome the limitation of constant diameter in the previous MFM, the biomass pyrolysis reaction was taken as an example to develop the variable particle density and diameter model based on the mass conservation at the particle scale, which was successfully incorporated into MFM to simulate the biomass pyrolysis process in a fluidized bed reactor. The results indicate that the segregation/mixing and entrainment behavior are significantly affected by the variation model of particle density and diameter and the species density in the solid phase.

Realization and control of multiple temperature zones in liquid‐containing gas–solid fluidized bed reactor

Fluidized bed reactors (FBRs) have been developed to establish multiple temperature zones for various industrial processes. To overcome the common weakness, this work proposed to spray liquid into bottom and upper zones, respectively, to realize multiple temperature zones FBR (MTZFBR). Temperature, pressure, and acoustic emission techniques were applied to fully characterize liquid interaction and hydrodynamics. Compared with the bottom liquid‐spraying approach, the upper liquid‐spraying approach showed higher temperature difference (ΔT) and better fluidization stability, thus was selected for further control studies. Effects of liquid flow rate, static bed height, and inlet gas temperature on MTZFBR were studied systematically. The results showed that increasing liquid evaporation behavior or decreasing liquid bridge behavior enhance ΔT and fluidization stability and vice versa. G–L–S fluidization pattern depended mostly on the liquid behaviors and fluidization stability, and thus the stabilized MTZFBR could be regarded as a coexisted mode of two distinctive G–L–S fluidization patterns. © 2016 American Institute of Chemical Engineers AIChE J, 62: 1454–1466, 2016

Development of an endoscopic-laser PIV/DIA technique for high-temperature gas–solid fluidized beds

To enable the investigation of the hydrodynamics of gas–solid fluidized bed reactors at high-temperatures and reactive conditions, a new non-invasive experimental technique has been developed, based on the extension of Particle Image Velocimetry (PIV) coupled with Digital Image Analysis (DIA) using dedicated high-temperature endoscopes for image recording and laser illumination. The new endoscopic-laser PIV/DIA technique (ePIV/DIA) allows to determine simultaneously information from the emulsion and bubble phase with high spatial and temporal resolution in a pseudo-2D columns.The ePIV/DIA technique with single-source laser illumination has first been thoroughly validated at cold-flow conditions by comparison with a standard PIV/DIA technique with LED-illumination, showing very good agreement of the results in terms of particle velocity and solids mass flux profiles. The technique has subsequently been applied to fluidized beds in the bubbling fluidization regime at elevated temperatures to demonstrate its unique possibilities to measure detailed bed hydrodynamics at elevated temperatures.

Simulation of 3D gas–solid fluidized bed reactor hydrodynamics

ABSTRACTIn this article, an attempt is made to develop a 3D gas–solid fluidized bed reactor (FBR). Basically, it deals with simulation of a FBR in computational fluid dynamics (CFD) using the software, Ansys Fluent v14. The simulation of gas–solid flow is carried out using Eulerian multifluid model which is integrated with the solid particle kinetic theory. The coefficients of exchange momentum are estimated using the Syamlal & O'Brien, Gidaspow, Wen-Yu, and Huilin–Gidaspow drag functions. The results of the simulation have been validated with the experimental data available in literature and had proven that the model is capable to predict the hydrodynamics of FBR. The variation in kinetic energy of the solid phase is calculated by varying the restitution coefficient (RC) from 0.90 to 0.99. The predictions of pressure drop compare excellently with the experimental data. Finally, the effect of particle diameter on the expanded bed height has been studied for FBR.

Effects of liquid action mechanisms on hydrodynamics in liquid-containing gas–solid fluidized bed reactor

This study mainly investigated the effects of liquid evaporation and liquid bridge actions on hydrodynamics in liquid-containing gas–solid fluidized bed reactor (LGSFBR). A liquid flow-rate increasing process and a gas velocity decreasing process were studied by using temperature, acoustic emission (AE) and pressure fluctuation (PF) measurement techniques. When the liquid was added with lower flow-rates, particle circulation pattern was dominated by gas flow/liquid evaporation action, and thus particles display ascending at the center and descending near the wall (ACDW) with particle motion enhanced, meanwhile bubble sizes increased at the beginning and then decreased to steady values. With increasing liquid flow-rate or decreasing gas velocity, liquid bridge action relative to gas flow action intensity (LRGI) increased, and thus the particle circulation pattern gradually became dominated by the liquid bridge action. Specifically, the downward motion of dynamic particle agglomerations was enhanced while particle motion was weakened, and the gas bubble size was repressed gradually. Several unstable fluidization states would be caused by continuous increases in the liquid bridge action.

Performance of a Catalytic Gas–Solid Fluidized Bed Reactor in the Presence of Interparticle Forces

Abstract The influence of interparticle forces (IPFs) on the hydrodynamics of a gas–solid fluidized bed was experimentally investigated with the help of a polymer coating approach. The results showed that the presence of IPFs in the bed can considerably change the hydrodynamic parameters. The tendency of the fluidizing gas passing through the bed in the emulsion phase increased with IPFs in the bubbling regime. The performance of a fluidized bed reactor was then studied through simulation of a reactive catalytic system using three different hydrodynamic models: (a) a simple two-phase flow model, (b) a dynamic two-phase flow model, and (c) a dynamic two-phase flow model, integrating the effects of superficial gas velocity and IPFs. The simple two-phase flow model was found to underestimate the reactor performance for catalytic reaction most likely due to the oversimplified assumptions involved in this model. Also, the simulation results showed that modification of the bed hydrodynamics due to IPFs resulted in a better performance for a bubbling fluidized bed reactor. This suggests that the hydrodynamic models should take into account the effects of superficial gas velocity and variation in the ratio of the magnitude of IPFs/hydrodynamic forces, due to any operational reason, to yield a more reliable evaluation of the performance of the fluidized bed reactor.

Characterization of pressure fluctuations from a gas–solid fluidized bed by structure density function analysis

Hydrodynamics in a gas–solid fluidized bed reactor were studied based on differential pressure signals measured over several height intervals at different temperatures. Structure density function analysis was utilized to study the dynamics of multi-scale structures. An amplitude division method based on the Gaussian distribution and the Kolmogorov–Smirnov test was developed to divide the structure density distribution of pressure signals into three characteristic regions linked to: I. fine-scale structures, II. small-scale structures, and III. large-scale structures. The frequency of large voids and the uniformity of multi-scale flow structures were quantitatively characterized by parameters SDFb,II and KSDF, respectively. KSDF reached a maximum when large bubbles or slugs dominated the flow, then decreased until it remained almost constant in the turbulent flow regime. Structure density function analysis showed that increasing the operating temperature enhanced the transition of flow regimes, while the transition direction was determined by the superficial gas velocity. Increased gas velocity and decreased minimum fluidization velocity at higher temperature were major factors promoting earlier flow regime transitions. The frequency of large-scale structures decreased at higher temperature for the same flow regime.

Effects of external electric field on bubble and charged particle hydrodynamics in a gas–solid fluidized bed

In the gas–solid fluidized bed reactors, the interactions between particles and the wall, the gas, or the other particles always generate electrostatic charges, which affect the flow characteristics significantly. Imposing an external electric field to regulate the particle charging and reactor hydrodynamics is a potential way of process intensification. This work focuses on the incipient gas–solid fluidization under external electric fields of direct current (DC) or alternating current (AC), with each in the form of cross-flow and co-flow respectively. The two-dimensional multi-fluid CFD model coupled with electrostatic model is established to simulate the electric effects on the fluidized bed hydrodynamics. As the particle charge magnitude increases, the bubble size and bubble detachment time are reduced, while the electrostatic force on particle is enhanced obviously. By imposing the DC electric field, the bubble is split in cross-flow field and stretched to be sharper in co-flow field. Meanwhile, the particle self-generating electric field and particle velocity are strengthened around the bubble, distributor and wall. For the application of AC electric field, the bubble deforms in the lower frequency field, and the particle self-generating electric field declines near the wall, which will reduce the particle segregation and agglomeration towards the wall.

Statistical modeling and optimization of a multistage gas–solid fluidized bed for removing pollutants from flue gases

The present paper describes the statistical modeling and optimization of a multistage gas–solid fluidized bed reactor for the control of hazardous pollutants in flue gas. In this work, we study the hydrodynamics of the pressure drop and minimum fluidization velocity. The hydrodynamics of a three-stage fluidized bed are then compared with those for a single-stage unit. It is observed that the total pressure drop over all stages of the three-stage fluidized bed is less than that of an identical single-stage system. However, the minimum fluidization velocity is higher in the single-stage unit. Under identical conditions, the minimum fluidization velocity is highest in the top bed, and lowest in the bottom bed. This signifies that the behavior of solids changes from a well-mixed flow to a plug-flow, with intermediate behavior in the middle bed.

Residence time distribution study in a pilot-scale gas–solid fluidized bed reactor using radiotracer technique

This paper describes a radiotracer study carried out to measure residence time distribution (RTD) of coal particles in a pilot-scale gas–solid fluidized bed reactor (FBR). Gold-198 labelled on coal particles was used as radiotracer. RTD measurements were conducted for selected operating conditions and mean residence times (MRTs) of the coal particles were determined. Gamma function model was used to simulate the measured RTD data and mixing of coal particles in the reactor was investigated. Based on the results, the performance of the air distributor used in the reactor was evaluated.

Gas bubble behaviors in a gas–solid fluidized bed with an arch agitator

The performance of gas–solid fluidized bed reactors (FBRs) strongly depends on the behavior of gas bubbles. Mechanical agitation is widely used in industrial FBRs to enhance gas–solid mixing; however, its influence on bubble behaviors has been poorly studied, especially in experimental study. Pressure fluctuations in a 300mm inner diameter plexiglas column fluidized bed of Geldart D particles with an arch agitator were measured under various gas velocities and agitation speeds. The bubble sizes were evaluated by incoherent analysis of pressure fluctuations. The effects of mechanical agitation on axial bubble growth, bubble sizes, regime transition velocity and fines elutriation were comprehensively investigated. Results show that mechanical agitation promotes bubble growth at 1.7 Umf but reduces the bubble sizes with increasing gas velocities. The axial maximum bubble size with increasing gas velocity occurs at 1.2m/s without agitation and 1.0m/s with agitation speeds of 14 and 21rpm. Interestingly, mechanical agitation leads to a small increase in minimum fluidizing velocity and a slight decrease in critical turbulent velocity. Furthermore, the entrainment rate is reduced with mechanical agitation, due to the decreasing bed height and small fines ejection at the bed surface. The arch agitator helps to reduce the channel flow, enhance the gas–solid contact efficiency and improve the fluidization quality.

A Coarse-Grained Two-Fluid Model for Gas-Solid Fluidized Beds

Due to increasing computer power the numerical simulation of fluidized and moving beds has become feasible. However, while kinetic theory based CFD (Computational Fluid Dynamics) has become a valuable design tool for modeling pilot plant scale gassolid fluidized bed reactors, a fully resolved simulation of industrial scale reactor is still nearly unfeasible. It is, therefore, common to use sub-grid models to account for the effect of the small unresolved structures on large resolved scales when using coarse grids. It is generally agreed that the influence of these small scales on the drag force is a key parameter in the prediction of the hydrodynamics of fluidized beds. We present a sub-grid drag modification dealing with the influence of heterogeneous structures on the drag force. It is assumed that these structures appear as distinct clusters of particles within an interstitial dilute particle phase. The clusters and the dilute phase itself consist of homogeneously distributed particles enabling the application of a homogenous drag correlation to these structures. In contrast to the established sub-grid drag modification EMMS (Energy-Minimization Multi-Scale Method), the presented model distinguishes between resolved and unresolved clusters by computing the expectation value of the diameter of the unresolved clusters. This reveals a grid and slip velocity dependent drag modification, which recovers the homogenous drag law as the solids volume fraction approaches the maximum packing of frictional spheres. The presented model is validated on the one hand, in case of industrial scale bubbling and turbulent fluidized beds. On the other hand, the model is applied to the coarse grid simulation of a riser flow. The numerical results obtained on a coarse gird demonstrate that our model reveals fairly good agreement with experimental data of bed expansion and solids volume fraction distributions. Thus, the results proof that the presented drag modification is applicable to a wide range of particle diameters (Geldart A, B and D group particles) and different fluidization regimes.

Experimental and computational fluid dynamic study of the change of volumetric flow in fluidized-bed reactors

Gas–solid fluidized-bed reactors are complex systems that involve multiple interrelated phenomena. One key phenomenon that affects reactor performance is the change in gas volumetric flow. The volumetric flow in a fluidized-bed reactor can change with height due to reaction, feed distribution, change of phase, selective removal/addition of species and changes in temperature and pressure. This paper presents results from an experimental program and gives a qualitative comparison with modeling results obtained with computational fluid dynamics (CFD). A high-pressure two-dimensional column was used to investigate the effects of sudden increase/decrease in pressure on fluidization behavior by recording multiple bubble properties using high-speed video recording. A CFD model was developed to investigate the same variations in process conditions. Changing the volumetric flow can radically alter the bed hydrodynamics. The CFD code is able to predict the experimental trends correctly.

Numerical analysis of the mixing process in a gas–solid fluidized bed reactor

Abstract Hydrodynamics of a two dimensional gas–solid fluidized bed reactor was investigated by applying computational fluid dynamics (CFD). Unsteady behavior of this multiphase flow was simulated by using the Eulerian–Eulerian model coupled with kinetic theory of granular flow. Momentum exchange coefficients are calculated by using the Gidaspow drag functions. Simulation results were compared with the experimental data in order to validate the CFD model. Good agreement between the numerical results and experimental data was observed under different operating conditions. The effect of mixture composition, inlet gas velocity, restitution coefficient of particles and bed deviation on gas–solid flow behavior were investigated. A mixture of particles with diameter of 1 and 2 mm and density of 2400 and 2500 kg/m3 were fluidized under inlet gas velocity of 0.5–2.5 m/s. An increase in mass fraction of small particles from 45% to 59%, led to an increase in the bed height and a decrease in average diameter of particles in the bed. Restitution coefficient differences can be the cause of segregation in a bed including particles with the same size and density. Time averaged radial profiles were smoother in higher gas velocities. Bed deviation did not affect the mixing rate, but it could be beneficial to simplify the particle derivation process on the bed. Appropriate ranges of the Courant number were used to attain reasonable results and decrease the computational cost.

A comparative study of different CFD-codes for numerical simulation of gas–solid fluidized bed hydrodynamics

The hydrodynamics of a gas–solid fluidized bed reactor were studied numerically. Computational two-dimensional results from open source software packages MFIX and OpenFOAM, and those obtained from the commercial software package Fluent were discussed and compared with numerical and experimental data existing in the literature. The gas–solid flow was simulated applying the multifluid Eulerian–Eulerian model, where the solid phase is treated as a continuum. The solid-phase properties were calculated by using the kinetic theory of granular flow. Momentum exchange coefficients were calculated using the Gidaspow and Syamlal–O’Brien drag functions. Pressure drop and bed expansion ratio predicted by the simulations were in relatively close agreement with benchmark numerical and experimental data sets in the bubbling regime. Contrary to the OpenFOAM predictions, computations with MFIX and Fluent predicted instantaneous and time-average local voidage and velocity profiles which are comparable with results from the literature.