Non‐intrusive characterization of particle size changes in fluidized beds using recurrence plots

An on‐line method is developed for monitoring of mean particle size in fluidized beds using pressure fluctuations (PFs) and acoustic emissions (AE) signal by recurrence plot (RP) and recurrence quantification analysis (RQA). PFs and AE signals of a lab‐scale fluidized bed were measured simultaneously at various superficial gas velocities and mean particle sizes. Although the AE signals are often very complicated due to many different acoustic sources in the bed, applying RP analyses showed that small changes in mean particle size can be detected by visual comparison of AE‐RP structures, while this cannot be distinguished by graphical RP analysis of PFs. Moreover, the hydrodynamics of the bed was inspected through RQA analysis of both signals. For this purpose, recurrence rate, determinism, laminarity, average length of diagonal and vertical lines were extracted from RPs showing the effect of an increase in the mean particle size. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3547–3561, 2016     

基于声发射信号递归分析的气固流化床流型转变

利用声发射技术采集不同流化气速下流化床内颗粒与壁面碰撞的声信号,结合声能量及递归分析法研究不同流型下颗粒运动特征,得到鼓泡流态化到湍动流态化的临界转变速度及流型转变规律。特别是针对声能量分析无法准确区分不同床层高度处流型转变的不足,利用递归分析可有效预测系统周期性的特点,将声信号进行递归分析,研究了流化床不同位置的流型转变性质。结果表明,鼓泡流态化下颗粒运动的周期性较湍动流态化强,并能够清晰地检测到由鼓泡流态化向湍动流态化的流型转变速度,而且床层较低处的流型转变速度比床层较高处大。由此获得了一种便捷灵敏、安全环保的非侵入式流化床流型转变速度的测量技术,可用于对整个流化床内不同位置流型转变过程的实时在线监控。     

隔板式内循环流化床压力脉动信号递归分析

隔板式内循环流化床中流化态及颗粒循环特性对压力脉动信号特征具有重要影响,其作用机制尚未完全清楚。测量了隔板式内循环流化床在不同气速比条件下的压力脉动信号,通过时域及递归分析,获得了压力脉动信号的标准差、递归率、确定性及香农熵等特征参数。结果表明,随着表观气速比的增加,内循环中颗粒循环状态存在未循环、鼓泡循环、过渡循环和湍动循环四个阶段;通过压力脉动信号的标准差、递归图黑白结构占比和递归特征参数可识别这四种循环状态,递归特征参数在不同循环区域内显示出良好的线性关系,可用于识别隔板式内循环流化床系统的循环状态。     

流化床内生物质石英砂双组分混合流动混沌递归分析

采用递归分析方法对流化床内的压力脉动信号进行分析,研究了不同工况下石英砂与柱形生物质颗粒混合双组分颗粒的流动特性。柱形生物质颗粒尺寸为10 mm×10 mm（直径×长）,石英砂粒径为0.8 mm。在鼓泡床阶段,随着生物质含量的增加,系统的周期性呈现先减弱再增强的变化趋势;该阶段确定性与层流率均表现为随气速的增加而增大,其值在生物质含量低时会明显减小,系统熵则随着气速的增大而减小。在腾涌阶段,生物质添加前后系统信号的递归图呈现相似的纹理特征,确定性与层流率变化较小,此时熵随着气速的增加而减小,与单组分流动呈现相反的趋势。     

Measurements and numerical predictions of gas vortices formed by single bubble eruptions in the freeboard of a fluidised bed

Gas vortices generated in the freeboard of a bubbling fluidised bed have become the centre of increasingly more research due to the advances in experimental technology. The behaviour of gas flow in the freeboard of a bubbling fluidised bed is of interest for applications such as the gasification of coal where reactions of gas mixtures, as well as gas–particle heat and mass transfer take place. Knowledge of the hydrodynamics of the gas within the freeboard can be hard to characterise, especially the detailed behaviour of gases escaping from bubbles that erupt at the bed surface. In the present study, experiments were conducted on a rectangular three-dimensional gas–solid fluidised bed. The experiments used a particle imaging velocimetry (PIV) measurement technique to visualise and measure the gas flow within the freeboard after a single bubble eruption. A computational study was carried out using Eulerian–Eulerian, kinetic theory of granular flow approach with a quasi-static flow model and with LES used to account for gas turbulence. Results from a three dimensional simulation of the experimental fluidised bed were compared with experimental velocity profiles of gas flow in the freeboard of the gas–solid fluidised bed after a bubble eruption. The CFD simulations showed a qualitative agreement with the formation of the gas vortices as the bubble erupted. Consistent with experimental findings the CFD simulations showed the generation of a pair of vortices. However, the simulations were unable to demonstrate downward flow at the centre of the freeboard due to particles in free fall after a bubble eruption event was observed in the experiments. Velocity profiles from the CFD data are in reasonably good agreement with the characteristic trends observed in the experiments, whereas the CFD model was able to predict the gas vortices phenomena and the velocity magnitudes were over-predicted.     

Measurement and simulation of bubbling fluidised beds

The effective control of systems requires the formulation of suitably robust models of their behaviour. The work described in this paper describes the simulation and modelling of the behaviour of a bubbling fluidised bed. A simple system is investigated consisting of a vertical planar bed. The performance of the bed is characterised by measuring the proportion of the bed occupied by the voids associated with bubbles. From these measurements it is possible to evaluate the response of the bed to changes in the gas flow rate into it in the time domain and through transformation into the frequency domain. These techniques allow a simulation of the bed based on the work of Clift and Grace [R. Clift, J. Grace, Coalescence of bubbles in fluidised beds, A.I.Ch.E. Symp. Ser. 67 (116) (1970) 23–33.] to be validated. The simulation can then be used to evaluate a simple but effective physical model of a bubbling fluidised bed which treats it as being primarily a temporary store of gas. The model represents the dynamics of the bed well and in the form of a transfer function which can be used successfully as a basis for controlling the bed.     

Control of the state of a bubbling fluidised bed

Bubbling fluidised beds are used for a wide variety of industrial processes. As such it is desirable to be able to specify the conditions within the bed. These conditions can be subject to unwanted change in areas such as feedstock or fluctuations in process settings such as gas flow. This paper shows how pressure measurements can be used to characterise changing conditions within a three-dimensional bubbling fluidised bed. These characterisation techniques are then used to show that a linear controller can control the conditions within a bubbling bed. This control is demonstrated to be robust, being insensitive to the gas flow conditions, controller gains and location at which the characterisation measurement is made.     

Prediction of bubble behaviour in fluidised beds based on solid motion and flow structure

It has been demonstrated that the non-intrusive positron emission particle tracking (PEPT) could be a potential technique for observing bubble flow pattern, measuring bubble size and rise velocity in bubbling fluidised beds according to the solid motion in bubble and its wake. The results indicate that the behaviour of air bubbles varies greatly with the bed materials and superficial gas velocity. Three types of bubbling patterns (namely A, B and C) have been reported in this study, in which the pattern C is observed when the polyethylene fluidised bed is operated at the superficial gas velocity (U − U_mf) of 0.25–0.5 m/s and the ratio of bed height to bed diameter is unity. After the comparison of the results measured by the PEPT technique with the values calculated by using a number of empirical correlations, two modified correlations are recommended to calculate the bubble size based on the PEPT data.     

CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors. Part B: Heat, momentum and mass transport in bubbling fluidised beds

The fluid-particle interaction inside a 150 g/h fluidised bed reactor is modelled. The biomass particle is injected into the fluidised bed and the heat, momentum and mass transport from the fluidising gas and fluidised sand is modelled. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. Heat transfer from the bubbling bed to the discrete biomass particle, as well as biomass reaction kinetics are modelled according to the literature. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase. FLUENT 6.2 has been used as the modelling framework of the simulations with the whole pyrolysis model incorporated in the form of user-defined function (UDF). The study completes the fast pyrolysis modelling in bubbling fluidised bed reactors.     

Using Electrical Capacitance Tomography to Investigate Gas Solid Contacting

An ECT system has been shown to be useful in observing differences in particle behaviour in a bubbling fluidised bed. An image analysis technique is further described that utilises the construction of solids concentration profiles at three key values, x = 0.2, x = 0.5 and x = 0.8, which have been identified as important solids concentrations in a new Bubble Structure Model. Significant differences in the bubble structure are shown to result when the concentration of fine particles is slightly increased in a bubbling fluidised bed. Changes of this type would seriously alter the gas solid contact efficiency in the fluidised bed, which would significantly influence selectivity of in‐bed catalytic reactions and gas residence time.     

3D simulation of the effects of sphericity on char entrainment in fluidised beds

The paper presents a 3-dimensional simulation of the effect of particle shape on char entrainment in a bubbling fluidised bed reactor. Three char particles of 350 μm side length but of different shapes (cube, sphere, and tetrahedron) are injected into the fluidised bed and the momentum transport from the fluidising gas and fluidised sand is modelled. Due to the fluidising conditions, reactor design and particle shape the char particles will either be entrained from the reactor or remain inside the bubbling bed. The sphericity of the particles is the factor that differentiates the particle motion inside the reactor and their efficient entrainment out of it. The simulation has been performed with a completely revised momentum transport model for bubble three-phase flow, taking into account the sphericity factors, and has been applied as an extension to the commercial finite volume code FLUENT 6.3.     

水平气液两相流流型图像信息递归特征分析

采用高速摄像机在气液两相流实验台上拍摄各种典型流型的流动时频,将单帧图像提取信息熵组成时间序列。对信息熵时间序列进行WVD分析,对气液两相流几种典型流型以及过渡流型的演化轨迹进行解析。同时对序列进行了近年来较少用到的非线性混沌递归图分析（RP）,将其分析结果同WVD的分析结果进行对比,在对流型演化轨迹提出新的见解外也对WVD的分析结果进行验证。在分析后将100组不同流型的信息熵序列进行平均值计算,列出随气体体积流量增大的变化趋势图,进一步对流型信息熵与气流量的关系进行说明。最后讨论了气液两相流容积含气率与信息熵的关系。结果表明：流动图像的信息熵序列结合WVD与RP可以对气液两相流型演化轨迹进行很好表征。同时流型图像的信息熵能够对气液两相流各种流型的容积含气率进行表征,是一种有效的流型图像特征参数。     

Reduction of hematite with hydrogen in fixed and fluidised beds

Reduction of hematite with hydrogen has been carried out in fixed and fluidised beds at identical gas flow rates. Influence of gas velocity and particle size has been ascertained. The performances of the two types of bed for the reduction of hematite with hydrogen have been compared. The observed difference in performance is considered to be primarily due to the solids movement characteristics of systems studied. This is also influenced by the bubbling phenomena occurring in fluidised systems.     

A new version of CSFB,comprehensive simulator for fluidised bed equipment

A comprehensive simulation program for fluidised bed equipment (CSFB) has been improved and is now able to predict operational conditions of bubbling and circulating fluidised bed equipment such as boilers, gasifiers, dryers, shale-retorting reactors, and pyrolysers with various designs and consuming a wide range of feedstock. This paper concentrates on the improvements on the original bubbling model. Comparisons between simulation results and real equipment operational conditions under bubbling fluidised bed techniques are presented. Since those comparisons showed relatively low deviations, it is believed that CSFB is useful for improving operational conditions of existing industrial units and as auxiliary tool for designing of new equipment.     

Turbulence in the freeboard of a gas-fluidised bed: The significance of ghost bubbles

The turbulence in the upward gas flow above a gas-fluidised bed was measured by hot wire anemometry. In this region above the bed—defined as the free is the fluidising velocity. The irregularity of the motion is caused by bursting bubbles at the surface of the bed. It appears that the gas within a bu are termed “ghost bubbles”.Ghost bubble mechanics were studied by releasing single bubbles from: (i) stagnant liquid; and (ii) an incipiently fluidised bed. Tle velocity field wa The velocity of a ghost bubble diminishes as it rises, due to fluid entrainment caused by turbulent circulatory flow within the bubble.From the freeboard turbulence measurements for a continuously bubbling fluidised bed, it is clear that the turbulence is due to ghost bubbles, but the ghost bubble.     

The combustion of polymer pellets in a bubbling fluidised bed

Wastes burned in incinerators usually contain polymers, whose combustion can be associated with noxious emissions, unless the conditions are properly selected. This paper investigates how polymers burn in a fluidised bed; in fact, the combustion of a number of polymers, including several types of polyethylene, polystyrene and a polyamide, was studied in a laboratory-size, bubbling, fluidised bed, filled with quartz sand, with no external heating. Pellets of a polymer were mostly thrown into such a bed of sand, fluidised and maintained hot by a fuel-lean mixture of propane, methane or hydrogen in air, which burned soon after entering the bed. In addition, polymers were also used as the only fuel, i.e., added to a hot bed fluidised by only air. Visual observations of burning polymer pellets up to 240 mg were made, as well as video records obtained and the flue gas composition monitored, when the combustor was run at 800–1000 °C with 1.1–2.0 times more O₂ than required for complete combustion. It is clear that a polymer burns as if its volatile content were 100%. The polymer pellet first melts at a rate controlled by heat transfer. However, the melt and the gaseous products of thermal decomposition are dispersed, albeit sometimes slowly, in a fluidised bed. Although the high U/U_mf of above 10 caused some back-mixing of the gas leaving the bed and the combustion efficiency was high (assessed from O₂ consumption and CO₂ production), long streaks or plumes of fuel-rich gases (from each polymer pellet) did reach the freeboard, i.e., these plumes burned as transient diffusion flames at a rate controlled by mixing. By increasing the temperature and the residence times of gas in the bed and freeboard, the observed emissions of CO and hydrocarbons could be considerably reduced. The concentrations of NO were low, except when the polymer contained chemically-combined nitrogen, as in a polyamide. It is concluded that bubbling fluidised beds can be good for incinerating polymers, possibly together with other wastes.     

Thermal inefficiency model for determination of the bed-to-gas heat transfer coefficients with effectiveness factor in a fluidised bed

Process operations often involve the physical interaction of a gas and a solid phase. Fluidised bed heat transfer can be characterised by limited space–time (τ) on the basis of particle volume in the bed. As aimed in this study, a thermal inefficiency model (TIM) was developed using a pseudo-steady-state heat balance, i.e., equating the electrical power input to the rate of heat transfers from the bed to the gas. A bench-scale fluidised bed (105 × 200 mm) was operated for obtaining the gas temperature profiles. Temperature data were used for extracting the bed-to-gas heat transfer coefficients (h_BG) with effectiveness factors (η) from the TIM. Fluidised bed experiments at low temperature range (290–473 K) were conducted avoiding excessive instrumentation and time. Compressed dry air entered the bed through a distributor of a 200-mesh brass sieve and fluidised the single charge of alumina particles (1.3 kg) with a mean diameter approximately 250 μm. The superficial gas velocity was changed from 0.085 to 0.469 m s^− 1. The bed-to-gas heat transfer coefficients (h_BG0×η₀) at initial bed hight and thermal inefficiency constants (k_I) were calculated from the intercept and slope of the linear form of the TIM, respectively. The agreement between the experimental and predicted values of gas temperatures confirmed by the TIM. The latter may be successfully used to design fluidised beds for, e.g., drying or combustion.     

The division of gas between bubble and interstitial phases in fluidised beds of fine powders

The division of gas flow between the two phases of a fluidised bed of commercial catalyst powder has been determined from measurements of interstitial phase voidage and bed height over a wide range of gas velocities. Voidage measurements were made by comparing the X-ray absorption of the interstitial phase of the freely bubbling catalyst with that of a calibration wedge containing the same material. X-Ray photography was also used in the measurement of bed height. Three batches of catalyst powder containing different amounts of ‘fines’ were examined.The results clearly demonstrate that increasing the fines content of a fluidised powder leads to an increase in the relative proportion of gas flowing interstitially at all fluid velocities. Furthermore the interstitial gas flow is shown to be up to 25 times greater than the minimum fluidisation value for the powder containing the highest proportion of fines. The implications of this for the two-phase theory of fluidisation and for the design and operation of fluidised bed reactors is discussed.     

CFD modelling of the fast pyrolysis of biomass in fluidised bed reactors, Part A: Eulerian computation of momentum transport in bubbling fluidised beds

The fluid-particle interaction inside a 150 g/h fluidised bed reactor is modelled. The biomass particle is injected into the fluidised bed and the momentum transport from the fluidising gas and fluidised sand is modelled. The Eulerian approach is used to model the bubbling behaviour of the sand, which is treated as a continuum. The particle motion inside the reactor is computed using drag laws, dependent on the local volume fraction of each phase, according to the literature. FLUENT 6.2 has been used as the modelling framework of the simulations with a completely revised drag model, in the form of user defined function (UDF), to calculate the forces exerted on the particle as well as its velocity components. 2-D and 3-D simulations are tested and compared. The study is the first part of a complete pyrolysis model in fluidised bed reactors.