Voidage profiles in magnetically fluidized beds

Voidage profiles in a fluidized bed of iron particles (230 μm) were investigated under the influence of an external uniform axial magnetic field. Passing a direct current through five solenoids generated uniform magnetic field. The five solenoids were arranged elaborately to get larger uniform magnetic space than that generated by Helmholtz electromagnet coils. A sensitive optical measuring system, based on detection of light reflected by particles, was used to measure local voidage in both dense and dilute phases.

Local voidage was measured as a function of superficial fluidizing air velocity, magnetic field intensity and the position in the bed. At a given magnetic field intensity and at the same position in the bed, the voidage was constant for a low air velocity range (in a fixed bed). The local voidage changed irregularly with increasing air velocity for an intermediate air velocity range (in a magnetically stabilized fluidized bed, MSFB). The local voidage changed linearly with increasing air velocity for a slightly high air velocity range (in a magnetized bubbling fluidized bed, MBFB). A general correlation was developed to predict the local solids fraction at the arbitrary position in the bed: (1−)=(1−)_c+[(1−)_w−(1−)_c](r/R)^B where (1−), (1−)_c and (1−)_w represent the local solids fraction at arbitrary position in the bed, at the bed center and on the bed wall; and B, (1−)_c and (1−)_w are the function of air velocity, distance from the distributor and magnetic field intensity.     

Experimental study of sawdust and coal particle mixing in sand or catalyst fluidized beds

This paper presents an experimental study in order to establish mixing and segregation conditions of wood sawdust and coal particles in a fluidized bed of sand or alumina particles. Experiments were performed at room temperature under atmospheric pressure. Simple equipment was used to divide the bed into layers and useful results could be obtained in a short period of time. The influence of fluidization velocity, time of an experiment, size and concentration of sawdust or coal particles was investigated. Uniformity of mixing was found to increase with an increase in the fluidization velocity. For u/u_mf less than 2.5, strong segregation occurred between sawdust and sand particles. Determination of good mixing conditions will be useful in gasification studies of sawdust and coal.     

Devolatilization of large coal particles in fluidized beds

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

Attrition and abrasion models for oil shale process modelling

As oil shale is processed, fine particles, much smaller than the original shale, are created. This process is called attrition or, more accurately, abrasion. In this paper, models of abrasion are presented for oil shale processed in several unit operations. Two of these unit operations, a fluidized bed and a lift pipe, are used in the Lawrence Livermore National Laboratory hot recycle solid (HRS) process being developed for the above-ground processing of oil shale. Abrasion occurs so commonly in the handling and processing of paniculate materials that numerous studies have been conducted to characterize the phenomenon and to attempt to minimize it. In the review of the literature, materials which have been studied for attrition potential are examined, as are the specific unit operations for which either experimental or modelling studies have been conducted. Several papers are discussed in which attrition in fluidized beds or lift pipes is addressed. In this paper, empirical models are derived for the processes occurring in the HRS process from the experimental studies conducted on oil shale. The derived models are presented, as are comparisons with experimental results.     

The behaviour of segregating fluidized beds and its relevance to ash management

Visual observation of the movement of particles across a transparent, drilled plate distributor allowed their directional and RMS velocities to be determined. Particles too big and/or dense to be lifted by bubble wakes are moved towards collection zones by the circulating bed material. The minimum fluidizing velocity has to be exceeded by an amount which increases with increasing density and/or number of particles before there is significant transport. Direct and RMS velocities increase linearly with the gas velocity. Circulation is controllable by producing imbalance in the air supplies to different zones of the distributor; tilting the distributor (flow is then up the slope); or use of a spout as a pump. Spouts can remix large/dense particles back into the bulk of the bed. Particles can be selectively withdrawn through a pneumatic off-take tube.     

Prediction of heat transfer to liquid-solid fluidized beds

Heat transfer coefficients to a liquid-solid fluidized bed in a cylindrical tube have been measured using water as liquid phase and three types of cylindrical steel particles, as well as glass, nickel, copper and lead spheres of different sizes as solid phase. The independent varaibles included heat flux, liquid velocity and particle physical properties. The experimental results as well as a data bank containing a large number of measured heat transfer coefficients for solid-liquid fluidization over a wide range of operational parameters have been compared with the predictions of most published correlations. A model for the prediction of heat transfer coefficients is proposed which predicts the present experimental data and the data of other investigators with good accuracy.     

Solids motion and holdup profiles in liquid fluidized beds

Non-invasive gamma rays-based techniques, computer tomography and computer-aided radioactive particle tracking, were used to measure solid holdup and solid velocity profiles in liquid-solid fluidized beds. The time averaged velocity measurements indicated that multiple circulation cells exist in the column. The solid motion was upward in the center and downward near the walls in the fully developed part of the column. The flow pattern is reversed in the entry region of the column. The solid holdup values increase slightly with the increase in radial position in the fully developed region. The average values of holdup in the column were in agreement with other measurements and with the modified Richardson-Zaki equation. The solids mean velocities and eddy diffusivities increase with increase in liquid superficial velocity, column size, particle size and density. Distributor-type affects the mean velocity and turbulence parameters while the column height has a relatively minor effect. The solids motion and turbulence parameters presented here are useful for validation of CFD models.     

鼓泡流化床离散模拟中的一个局部空隙率模型

气固流化床离散颗粒模拟中通常采用面积加权平均法计算局部空隙率,不能较好地反映流化床中显著的非均匀结构特性.为了考虑非均匀结构对局部空隙率的影响,文中提出一个适用鼓泡流化床的局部空隙率模型.将流场划分为稀区(气泡区)和密区(乳相区);非均匀结构对密区局部空隙率的影响通过引入局部空隙率下限和非均匀影响系数描述;将提升管流动...     

Particle scale study of heat transfer in packed and bubbling fluidized beds

The approach of combined discrete particle simulation (DPS) and computational fluid dynamics (CFD), which has been increasingly applied to the modeling of particle‐fluid flow, is extended to study particle‐particle and particle‐fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale. The development of this model is described first, involving three heat transfer mechanisms: fluid‐particle convection, particle‐particle conduction and particle radiation. The model is then validated by comparing the predicted results with those measured in the literature in terms of bed effective thermal conductivity and individual particle heat transfer characteristics. The contribution of each of the three heat transfer mechanisms is quantified and analyzed. The results confirm that under certain conditions, individual particle heat transfer coefficient (HTC) can be constant in a fluidized bed, independent of gas superficial velocities. However, the relationship between HTC and gas superficial velocity varies with flow conditions and material properties such as thermal conductivities. The effectiveness and possible limitation of the hot sphere approach recently used in the experimental studies of heat transfer in fluidized beds are discussed. The results show that the proposed model offers an effective method to elucidate the mechanisms governing the heat transfer in packed and bubbling fluidized beds at a particle scale. The need for further development in this area is also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

温度对循环流化床双床气化中热解炉产物的影响

在循环流化床双床煤高温热解气化试验台上,以神木烟煤为燃料在不同温度下进行了热解气化试验,该试验台利用上下返料器将热解炉和气化炉耦合在一起,其中热解炉为N2气氛,气化炉通入空气作为气化剂,试验主要研究热解炉底部温度对热解煤气及热解炉底渣的影响。试验结果表明:随热解炉底部温度升高,热解煤气中H2体积分数升高,CH4,CO2体积分数降低,CO体积分数先降低后升高,热解煤气主要组分气体的收率增加。试验所取热解炉底渣样品的孔比表面积分布和孔比体积分布主要集中于中小孔(0—50 nm),其总比表面积和总孔体积大小顺序为在817℃最大,844℃次之,766℃最小,在N2气氛、1 200℃条件下,CO2反应的活性大小顺序为817℃最大,766℃次之,844℃最小。     

Wall-to-bed heat transfer in three-phase fluidized beds of low density particles

Bed-to-wall heat transfer was measured in three-phase fluidized beds under conditions typical of biochemical process applications. The thermal resistance of the fluidized bed, which was significant in the absence of gas, became negligible when gas was introduced. Decreasing the particle density at constant gas and liquid velocity increased the bed-to-wall heat transfer coefficient. Previously published heat transfer correlations were used and gave poor predictions of our data. A new correlation was developed which predicted very well all the heat transfer coefficient results in this paper.     

基于气固流态化原理的油页岩干燥动力学

为了考察气固流化床干燥器能否使油页岩含水质量分数达到要求,以柳树河油页岩颗粒为原料,研究进口气体温度和颗粒直径对油页岩干燥性能的影响,采用薄层干燥模型,对油页岩干燥实验数据进行模拟,确定油页岩干燥方程和干燥速率方程,建立油页岩干燥速率特征常数和有效扩散系数之间的关联式。研究结果表明:薄层干燥模型中修正Page模型Ⅰ适合描述油页岩的干燥过程;油页岩在流化床内干燥过程主要发生在降速干燥阶段,进口气体温度越高,油页岩颗粒直径越小,所需干燥时间越短,进口气体温度为350℃时,使2.4 mm油页岩含水质量分数低于5%,所需干燥时间为2.5 min。     

Circulating fluidized bed combustion of pitches derived from heavy oil upgrading

Combustion trials were conducted with two pitch materials in the University of British Columbia pilot scale (152 mm × 152 mm × 7.3 m) circulating fluidized bed combustion unit. The pitches, which were bottom end by-products of heavy oil upgrading processes containing high concentrations of sulphur, heavy metals and asphaltenes, were fed to the combustor in liquid form and atomized in the combustion chamber. Limestone was fed also to capture sulphur in situ. The combustion efficiency was always above 99%. NO_x emissions increased as the Ca:S ratio increased. The heavy metal elements present in the pitch were found to remain essentially in the solid residue.     

Modelling the kinetics of fast catalytic cracking reactions

Instantaneous kinetic constants and gasoline selectivities have been determined for catalytic cracking of n-hexadecane. The pulse technique was used in order to model the sequential build-up of coke which occurs on cracking catalyst within a riser transport-line reactor. The total amount of hydrocarbon injected per unit weight of catalyst was between 0 and 10. The mathematical model used to analyze the data was based on the unsteady state mass balance of the microcatalytic reactor with the assumption of plug flow. Results suggest a fast deactivation process during the run with fresh catalyst, while regenerated catalyst showed a slower deactivation. The catalyst regenerated three times evidenced a low apparent activation energy when temperature was increased from 500°C to 550°C.     

Optimal placement of probes for dynamic pressure measurements in large-scale fluidized beds

Pressure data sampled at sufficiently high frequency (typically 20 Hz or higher) can yield much information about the hydrodynamic state of a fluidized bed. Since part of the pressure waves travelling through large (industrial) fluidized beds is only detectable in a limited area of the bed, pressure measurements need to be performed at several positions to cover the whole bed. We examine these local pressure waves (caused by, e.g., passing bubbles or coalescing bubbles) in a 0.80 m i.d. bubbling fluidized bed of Geldart B particles. Experiments and simulations are performed to determine the intensity decrease as local pressure waves propagate from their origin. A new spectral method is applied to determine the degree of coherence for pressure signals measured at two different positions in a fluidized bed. For a superficial gas velocity of 5u_mf, local pressure waves can be detected up to a radial distance of about 0.5 m from their origin; this distance is somewhat lower for lower gas velocities. This means that the radial spacing of pressure probes should not exceed 1 m. For large diameter beds with a bed height below 1.5 m, a set of probes at a single level and at several radial positions is sufficient to observe or monitor the dynamic state of the complete bed; the probes should preferably be placed at a height of 30% to 40% of the total bed height.     

Multi-scale analysis on particle-phase stresses of coarse particles in bubbling fluidized beds

Despite the restless efforts devoted to the hydrodynamics of bubbling fluidized beds, the particle-phase stress, which is one of the critical issues in modeling bubbling fluidized beds by the Eulerian approach, is not well understood. To address this problem, a mathematical model is developed to describe the kinetic properties of coarse particles, such as granular temperature and granular pressure, based on a multi-scale analysis of particle velocity fluctuation. Contributions from both high-frequency particle-scale fluctuations and low-frequency bubble-induced fluctuations are considered. It is proven to agree well with experimental data available up to date from bubbling fluidized beds for Geldart B and D particles.     

Density segregation of dry and wet granular mixtures in gas fluidized beds

The Discrete Element Method combined with Computational Fluid Dynamics was coupled to a capillary liquid bridge force model for computational studies of mixing and segregation behaviors in gas fluidized beds containing dry or wet mixtures of granular materials with different densities. The tendency for density segregation decreased with increasing fluidizing velocity, coefficient of restitution, and amount of liquid present. Due to the presence of strong capillary forces between wet particles, there was a high tendency for particles to form agglomerates during the fluidization process, resulting in lower segregation efficiency in comparison with fluidization of dry particles. Particle‐particle collision forces were on average stronger than both fluid drag forces and capillary forces. The magnitudes of drag forces and particle‐particle collision forces increased with increasing fluidizing velocity and this led to higher mixing or segregation efficiencies observed in dry particles as well as in wet particles at higher fluidizing velocities. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4069–4086, 2015     

Numerical and experimental study on multiple-spout fluidized beds

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

Pyrolysis behaviour of Australian oil shales in a fluidized bed reactor and in a material balance modified Fischer assay retort

The pyrolysis behaviour of several Australian oil shales was determined using the material balance modified Fischer assay and a bench-scale fluidized bed pyrolysis reactor, with nitrogen or steam as the sweep gas. The assay oil yield, which ranged from 5.3 to 15.7 wt% of the dry shale, did not correlate well with the organic carbon contents of the shales. However, under both assay and fluidized bed pyrolysis conditions, a shale of high kerogen H/C had high organic carbon conversions to oil. Compared with the Fischer assay, nitrogen pyrolysis gave 7 ± 4 wt% more oil for the shales studied, and steam pyrolysis gave 15 ± 4 wt% more oil for all shales except one, which showed a 35 wt% increase in oil yield. However, the oils from both nitrogen and steam pyrolysis had lower H/Cs, higher sulphur and nitrogen contents, and more high boiling point fractions than those from the Fischer assay. Nitrogen pyrolysis oils were of higher quality than those produced by steam pyrolysis. With steam as the sweep gas, much more hydrogen and hydrogen sulphide were produced for all shales; in most cases, there was also more carbon monoxide and less hydrocarbon gases.