Radial particle mixing in gas-solids fluidized beds

Difficulties involved in experimentally determining the mixing coefficients of particles in a gas-solids fluidized bed are well known. From the theoretical point of view, the mechanism of particle mixing in such a fluidized bed is influenced profoundly by complicated and stochastic phenomena, such as jetting and bubbling. It appears that a sufficiently effective deterministic approach is not available to analyse the mechanism.In the present work, the particle mixing in the radial or lateral direction of a gas-solids fluidized bed was studied by using heated particles as the tracer particles. The transient concentration distribution of these particles in the lateral direction was obtained by means of an impulse response technique; sensitive thermocouples were used for measurement. The mixing coefficients of particles have been estimated from the distribution by a stochastic approach, and the resultant coefficients are compared with those obtained by other investigators.     

Measuring and modelling lateral solid mixing in a three-dimensional batch gas—solid fluidized bed reactor

A 0.27 m diameter fluidized bed reactor has been designed to allow experimental measurement of the axial and radial mixing behaviour of the solids. A unique method has been developed which permits the continuous determination of solid tracer concentration with time at different radial and axial positions within the fluidized bed. Solids mixing has been described by a model in which vertical mixing is instantaneous and lateral mixing occurs by dispersion. The lateral solids dispersion coefficients have been evaluated at various operating conditions from the experimental results of tracer concentration versus time. Based on the results, a modification of an existing correlation is proposed.     

Quantifying lateral solids mixing in a fluidized bed by modeling the thermal tracing method

Thermal tracing is a simple method for studying solids mixing in fluidized beds. However, the measurement of temperatures is influenced by both mixing and heat transfer, which limits its usefulness for inferring mixing quantitatively. In this work, a semiempirical model is developed to quantify lateral solids mixing in fluidized beds. The model couples the tracer mass balance, the enthalpy balance of tracers and bed particles, and the response dynamic of thermometers. A series of tests is pezrformed in a lab‐scale fluidized bed, with particle sizes of 0.28–0.45, 0.45–0.6, 0.6–0.8, and 0.8–1.0 mm, and fluidizing velocity from 0.3 to 2.3 m/s. By evaluating the measured transient temperatures using the model, the lateral dispersion coefficient (D_sr) is determined to be between 0.0002 and 0.0024 m²/s. Its reliability is confirmed by bed collapse experiments. Finally, the values of D_sr is compared with a collection of data in the literature. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Cold flow model and computer simulation studies of a circulating fluidized bed reactor for the oxidative coupling of methane

A circulating fluidized bed configuration has been developed for application in the oxidative coupling process. The configuration comprises a bottom turbulent fluidized bed, wherein the oxidative coupling reaction is conducted, followed by a reduced-diameter top fast bed for catalyst entrainment and hydrocarbon cracking. The hydrodynamic characteristics of this configuration have been investigated in a pilot-plant cold flow unit. Detailed experimental results on the turbulent bed flow structure and the gas phase residence time distribution are presented and discussed. The performanceofthe proposed reactor is analyzed by computer simulation studies based on a published oxidative coupling kinetic model. It is shown that improved hydrocarbon yields can be obtained by optimizing the hydrodynamic structure and the mixing characteristics of the turbulent bed.     

Gas mixing in a turbulent fluidized bed reactor

A series of experiments has been conducted to study mixing and hydrodynamic behaviour of a downward facing sparger in a turbulent fluidized bed reactor. Using pressure measurement techniques, two flow discharge modes were identified around the sparger by injecting a gas tracer into the bed. These are bubbling and jetting conditions. Experimental results show that, under bubbling conditions, bubbles tend to keep their identity, while under jetting conditions a highly turbulent heterogeneous area is formed around the injection point. Due to attrition and erosion of internal heating or cooling surfaces in industrial reactors, the dominant discharge mode is the bubbling pattern. Therefore, in this investigation, the bubbling pattern is studied by measuring the radial and axial dispersion of gas tracer injected to a hot fluidized bed reactor of 20 cm diameter of FCC and sand particles. A three‐phase model is also proposed in order to predict the mixing length. In addition, the effect of sparger configuration on tracer gas mixing was examined for FCC particles.     

MODEL FOR PREDICTING THE LATERAL DISPERSION COEFFICIENT OF SOLIDS IN GAS-SOLIDS FLUIDIZED BEDS

The lateral mixing of solids in a gas-solids fluidized bed is very complicated.It can be caused by:(a)bubble movement through the bed,(b)bubble burst at the bed surface,and(c)gross particle circulation in thebed.However,experiments show that the major factors effected the lateral mixing of solids are the bubblemovement through the bed and the bubble burst at the bed surface.Thus a model with two mixing re-gions,i.e.mixing in bubble rising region and mixing in bubble breaking region,was proposed.Based on thismodel,an equation for predicting the lateral dispersion coefficient of solids in gas-solids fluidized beds wasderived without any adjustable parameter.The calculated values by this equation are well comparable withthe observed data including the present work and the other investigations.     

Experimental validation of CFD simulations of a lab‐scale fluidized‐bed reactor with and without side‐gas injection

Fluidized‐bed reactors are widely used in the biofuel industry for combustion, pyrolysis, and gasification processes. In this work, a lab‐scale fluidized‐bed reactor without and with side‐gas injection and filled with 500–600 μm glass beads is simulated using the computational fluid dynamics (CFD) code Fluent 6.3, and the results are compared to experimental data obtained using pressure measurements and 3D X‐ray computed tomography. An initial grid‐dependence CFD study is carried out using 2D simulations, and it is shown that a 4‐mm grid resolution is sufficient to capture the time‐ and spatial‐averaged local gas holdup in the lab‐scale reactor. Full 3D simulations are then compared with the experimental data on 2D vertical slices through the fluidized bed. Both the experiments and CFD simulations without side‐gas injection show that in the cross section of the fluidized bed there are two large off‐center symmetric regions in which the gas holdup is larger than in the center of the fluidized bed. The 3D simulations using the Syamlal‐O'Brien and Gidaspow drag models predict well the local gas holdup variation throughout the entire fluidized bed when compared to the experimental data. In comparison, simulations with the Wen‐Yu drag model generally over predict the local gas holdup. The agreement between experiments and simulations with side‐gas injection is generally good, where the side‐gas injection simulates the immediate volatilization of biomass. However, the effect of the side‐gas injection extends further into the fluidized bed in the experiments as compared to the simulations. Overall the simulations under predict the gas dispersion rate above the side‐gas injector. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Mixing dynamics in bubbling fluidized beds

Solids mixing affects thermal and concentration gradients in fluidized bed reactors and is, therefore, critical to their performance. Despite substantial effort over the past decades, understanding of solids mixing continues to be lacking because of technical limitations of diagnostics in large pilot and commercial‐scale reactors. This study is focused on investigating mixing dynamics and their dependence on operating conditions using computational fluid dynamics simulations. Toward this end, fine‐grid 3D simulations are conducted for the bubbling fluidization of three distinct Geldart B particles (1.15 mm LLDPE, 0.50 mm glass, and 0.29 mm alumina) at superficial gas velocities U/Umf = 2–4 in a pilot‐scale 50 cm diameter bed. The Two‐Fluid Model (TFM) is employed to describe the solids motion efficiently while bubbles are detected and tracked using MS3DATA. Detailed statistics of the flow‐field in and around bubbles are computed and used to describe bubble‐induced solids micromixing: solids upflow driven in the nose and wake regions while downflow along the bubble walls. Further, within these regions, the hydrodynamics are dependent only on particle and bubble characteristics, and relatively independent of the global operating conditions. Based on this finding, a predictive mechanistic, analytical model is developed which integrates bubble‐induced micromixing contributions over their size and spatial distributions to describe the gross solids circulation within the fluidized bed. Finally, it is shown that solids mixing is affected adversely in the presence of gas bypass, or throughflow, particularly in the fluidization of heavier particles. This is because of inefficient gas solids contacting as 30–50% of the superficial gas flow escapes with 2–3× shorter residence time through the bed. This is one of the first large‐scale studies where both the gas (bubble) and solids motion, and their interaction, are investigated in detail and the developed framework is useful for predicting solids mixing in large‐scale reactors as well as for analyzing mixing dynamics in complex reactive particulate systems. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4316–4328, 2017     

微型流化床内混合特性的数值模拟

微型流化床反应分析仪是中国科学院过程工程研究所研制的具有等温微分反应特性,且适合于气固反应分析的新仪器。细微样品与高温流化介质的瞬间混合是该仪器实现等温微分的必要条件。针对如何满足该要求,基于欧拉多流体模型对连接不同进样器的微型反应器本体进行了三维数值模拟,得到了不同喷口结构和位置下的流动图景及混合区浓度的相对标准偏差曲线,定量表征了各种进样器的混合质量。同时采用高速摄像手段获得了冷态实验中颗粒流动的快照,验证了模拟计算结果的可靠性。模拟结果对脉冲射流微量进样器结构的优化提出了如下建议：进样细管应避免采用弯角喷口,弯角结构会导致脉冲进样载流气喷出方向与流化气流相逆,使得细微颗粒试样堆积滞留,影响混合效果。     

布风方式对流化床混合特性的影响

通过将离散单元法同计算流体力学相结合,对流化床内物料混合过程进行了研究。给出了水平布风板均匀布风、倾斜布风板非均匀布风2种情况下的示踪颗粒场历变过程。模拟结果表明:瞬时颗粒场组图能够较为直观表征床内混合现象;其中,在均匀布风情况下,床内气泡横向运动受到限制,颗粒整体横向运动能力较弱,混合方式以扩散混合为主;而对于非均匀布风流化床,床内存在较大的横向颗粒浓度梯度,对流混和起主要作用,且混合速度较为迅速。     

An index to characterize gas-solid and solid-solid mixing from average volume fraction fields

A mixing index based on solid volume fraction fields is developed for gas-solid flows. Conventional mixing indices are based on particle realizations of granular mixing and are applicable to experimental data or discrete element method simulations. However, these indices cannot be used as-is for multifluid models, and an index for characterizing mixing in gas-solid flows from continuous fields is needed. The performance of the new mixing index is tested in two applications. The first is a 3D simulation of the mixing of biomass and sand in a fluidized bed reactor, and the second is a 2D simulation of binary particle segregation in a fluidized bed. The simulations are performed using OpenFOAM®. The mixing index is used to quantify gas-solid mixing using solid volume fractions and solid-solid mixing using solid fractions. The formulation of conventional mixing indices is extended to be used with solid volume fractions fields, and methods for performance improvement are presented.     

Numerical and experimental study on spout elevation in spout‐fluidized beds

The effect of elevating the spout on the dynamics of a spout‐fluidized bed, both numerically and experimentally is studied. The experiments were conducted in a pseudo‐two‐dimensional (2‐D) and a cylindrical three dimensional (3‐D) spout‐fluidized bed, where positron emission particle tracking (PEPT) and particle image velocimetry (PIV) were applied to the pseudo‐2‐D bed, and PEPT and electrical capacitance tomography (ECT) to the cylindrical 3‐D bed. A discrete particle model (DPM) was used to perform full 3‐D simulations of the bed dynamics. Several cases were studied, that is, beds with spout heights of 0, 2, and 4 cm. In the pseudo‐2‐D bed, the spout‐fluidization and jet‐in‐fluidized‐bed regime, were considered first, and it was shown that in the spout–fluidization regime, the expected dead zones appear in the annulus near the bottom of the bed as the spout is elevated. However, in the jet‐in‐fluidized‐bed regime, the circulation pattern of the particles is affected, without the development of stagnant zones. The jet‐in‐fluidized‐bed regime was further investigated, and additionally the experimental results obtained with PIV and PEPT were compared with the DPM simulation results. The experimental results obtained with PIV and PEPT agreed mutually very well, and in addition agreed well wtih the DPM results, although the velocities in the annulus region were slightly over predicted. The latter is probably due to the particle‐wall effects that are more dominant in pseudo‐2‐D systems compared with 3‐D systems. In the jet‐in‐fluidized‐bed regime, the background gas velocity is relatively high, producing bubbles in the annulus that interact with the spout channel. In the case of a non elevated spout, this interaction occurs near the bottom of the bed. As the spout is elevated, this interaction is shifted upwards in the bed, which allows the bubbles to remain undisturbed providing the motion of the particles in the annulus near the bottom of the bed. As a result, no dead zones are created and additionally, circulation patterns are vertically stretched. These findings were also obtained for the cylindrical 3‐D bed; although, the effects were less pronounced. In the cylindrical 3‐D bed the PEPT results show that the effect on the bed dynamics starts at h_spout =1 4 cm, which is confirmed by the ECT results. Additionally, ECT measurements were conducted for h_spout =1 6 cm to verify if indeed the effect happens at larger spout heights. The root mean square of the particle volume fraction slightly increased at h_spout =1 2 cm, whereas a larger increase is found at h_spout = 4 and 6 cm, showing that indeed more bubbles are formed. The presented results have not been reported so far and form valuable input information for improving industrial granulators. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2524–2535, 2012     

Numerical study on microscopic mixing characteristics in fluidized beds via DEM

In this paper, discrete element method (DEM), combined with computational fluid dynamics (CFD), is used to investigate the micro-mixing process in fluidized beds (FBs) of uniform particles. With the aid of snapshots and adoption of Lacey and Ashton indexes, mixing evolvement for two cases, fluidized bed using horizontal distributor with even gas supply and fluidized bed using inclined distributor with uneven gas supply, is discussed in detail. Results indicate that the Ashton index appears to be more effective in assessing the mixing dynamics in this work. Further analyses illustrate that in the case of horizontal distributor incorporated with even gas supply, diffusive mixing pattern is predominant, since bubbles lateral motion is reduced in such a bed; whereas, there is a faster convective mixing process in a fluidized bed using inclined distributor with uneven gas feed, followed by shear mixing. Generally, localized air supply induces the density gradient of particle distribution in the bed, which is the basic agent of convective particle stream. The analyses are confirmed by the comparison of solid flux during the simulations of the two cases. In addition, the mixing mechanism and the mixing time scale agree well with published experimental results.     

垂直内挡板对流态化流体动力学及谷物干燥特性之影响

The effect of vertical internal baffles on the particle mixing and grain drying characteristics in a batch fluidized bed column is investigated. Experimental work was carried out in a 3 m high rectangular fluidized bed dryer of cross sectional area of 0.15 mx0.61 m at different operating conditions using paddy, a group D particle, as the fluidizing material. The results of the study showed that the fluidized bed dryer system with vertical internal baffles gave better particle mixing effect in the bed of particles than that without vertical internal baffles. This is due to the fact that the vertical internal baffle act as gas bubble breakers by breaking up the large gas bubbles into smaller ones. The smaller bubbles cause a more vigorous mixing in the bed of particles before finally erupting at the bed surface. This improves the contacting efficiency and enhanced the heat and mass transfer of the fluidized bed system. Thus a higher drying rate was obtained in the falling rate period because the higher contactin efficiency increases the evaporation rate at the particle surface. However, the drying rate in the diffusion regiol shows little improvement because the moisture diffusivity does not depend on the contacting efficiency. The fluidized bed dryer with vertical internal baffles could therefore be used in the initial rapid drying stage in a two stage drying strategy for paddy. The insertion of vertical internal baffles into a fluidized bed system improves the processing of Group D particles in a fluidized bed system especially if the system is large in scale.     

Expansion Behavior of a Binary‐Solid Liquid Fluidized Bed with Large Particle Size Difference

The overall expansion of two dissimilar solid particle species with over a tenfold difference in their size and substantial density difference is investigated here for different compositions of the fluidized bed. Contrary to the widely held notion that the total bed height would be the sum of the heights of the two segregated mono‐component beds, the actual bed heights were, in fact, found to be lower. This volume contraction is found to strongly depend upon the mixing behavior prevailing in the binary‐solid fluidized bed. At the complete mixing of the two solid species, the bed‐contraction versus liquid velocity profile shows a global maximum. As a result, the overall bulk density profiles are similarly affected. Moreover, it is found here that correlations meant for predicting the porosity of the packing of binary particle mixtures can be satisfactorily extended to binary‐solid fluidized beds where solid species differ significantly in size.     

Heat transfer in a swirling fluidized bed with geldart type-D particles

A relatively new variant in fluidized bed technology, designated as the swirling fluidized bed (SFB), was investigated for its heat transfer characteristics when operating with Geldart type D particles. Unlike conventional fluidized beds, the SFB imparts secondary swirling motion to the bed to enhance lateral mixing. Despite its excellent hydrodynamics, its heat transfer characteristics have not been reported in the published literature. Hence, two different sizes of spherical PVC particles (2.61 mm and 3.65 mm) with the presence of a center body in the bed have been studied at different velocities of the fluidizing gas. The wall-to-bed heat transfer coefficients were measured by affixing a thin constantan foil heater on the bed wall. Thermocouples located at different heights on the foil show a decrease in the wall heat transfer coefficient with bed height. It was seen that only a discrete particle model which accounts for the conduction between the particle and the heat transfer surface and the gas-convective augmentation can adequately represent the mechanism of heat transfer in the swirling fluidized bed.     

Prediction of Layer‐Inversion Behavior of Down‐Flow Binary Solid‐Liquid Fluidized Beds

The layer‐inversion behavior of down‐flow binary solid‐liquid fluidized beds is predicted using the property‐averaging approach. The binary pair in this case consists of a larger solid species which is also heavier than its smaller counterpart, while both are lighter than the fluidizing medium. The model is based on using the generalized Richardson‐Zaki correlation for evaluation of the bed void fraction wherein mean values of particle properties are used. However, unlike the maximum bulk density condition for the conventional up‐flow binary solid fluidized bed, the model is based on a minimum bulk density condition for occurrence of layer inversion. This is due to the fact that the volume contraction phenomenon associated with the mixing of unequal solid species leads to a decrease in bulk density of the bed. Model predictions are also compared using the limited data available in the literature. Predictions are consistent with the observed mixing behavior.     

Verification of fluidized bed electrical capacitance tomography measurements with a fibre optic probe

Previous studies aimed at determining the spatial accuracy of electrical capacitance tomography (ECT) have employed phantoms placed within the ECT measurement space. No previous studies have compared ECT with a second independent measurement technique in an operating fluidized bed. In the present work, radial voidage profiles have been measured with ECT in the 0.14-m I.D. riser of a circulating fluidized bed (CFB) and in a bubbling fluidized bed with a 0.19-m I.D. The dynamic and time-averaged radial voidage profiles have been compared with measurements taken with a fibre optic probe in the same riser and in a slightly narrower (0.15-m I.D.) bubbling fluidized bed. In spite of the intrusiveness of the latter technique, the time-averaged radial profiles in the CFB riser fall within 10% of each other when the CFB is operated at high-flux conditions that lead to a very dense wall region. Iterative reconstruction of the ECT images is not needed in this case. Similar agreement is found between the two techniques in the bubbling fluidized bed, but off-line iterative image reconstruction is clearly necessary in this fluidization regime. These results suggest that ECT, which is often described as a tomographic imaging technique with low spatial resolution, can in fact provide semi-quantitative time-averaged images of the cross-section of fluidized beds of diameter comparable to or less than that used here.     

Particle injection and mixing experiments in a one-quarter scale model bubbling fluidized bed

One significant factor in the operation of a fluidized bed combustor is the manner in which coal particles disperse and mix with the bed material upon entering the bed. A thermal tracing technique was used to study the mixing characteristics in a 1/4 scale model of a pressurized bubbling fluidized bed combustor. Particles cooled by liquid nitrogen are injected into the bed in the same way that pulverized coal will be injected. An array of thermistors is mounted inside the bed. They are used to trace the path of the cooled particles as they enter the bed and mix with the other bed material. The approximate concentrations can also be determined since heat transfer from the cooled particles to the fluidizing gas is negligible during the course of the experiment. Time-resolved images of the particle concentration show that the lateral motion of the injected particles is much greater than the lateral motion of an injected gas jet. The extended lateral motion is due to the substantial momentum of the injected particles.     

Lateral mixing of solids in gas—solid fluidized beds with continuous flow of solids

Lateral mixing of solids in a gas—solid fluidized bed with continuous flow of solids can be adequately expressed by the dispersion model. An expression for estimating the lateral dispersion coefficient in such a bed is proposed.