The effect of pressure on the flow of gas in fluidized beds of fine particles

The division of gas flow between the bubble and dense phases of fluidized beds of six different types of Group A powders has been studied at pressures of up to 20 bar using surface collapse and X-ray absorption measurements. It was found that with these fine powders as pressure increases at constant volumetric gas flowrate so the size and hold-up of bubbles decrease while their frequency increases. Contrary to previous measurements the average bubb velocity appears to decrease with increasing pressure. The dominant mode of bubble break-up in all the powders was found to be division from the rear, contrast to that observed with Group B powders at atmospheric pressure. Interstitial phase voidage was found to increase with increasing pressure.The results are interpreted in terms of a model which assumes a difference between the voidages, and hence the gas flow, of powder in the wakes behind     

Improvement of the Fluidizability of Cohesive Powders through Mixing with Small Proportions of Group A Particles

The gas fluidization behaviour of fine cohesive powders, classified as Geldart group C, is known to be characterized by cracks and channels leading to severe non‐homogeneities in the bed. Geldart group A particles, on the other hand, are known to show more homogeneous and regular fluidization behaviour. This paper studies the effects of the addition of small proportions of group A on the fluidization behaviour of a group C powder. Differential pressure fluctuations data at a sampling frequency of 200 Hz were recorded for two cases. In the first case, the bed contained only group C powder while in the second case small amounts of group A particles were added to the existing group C powder. Visual/image observations coupled with time series analysis showed that the addition of small proportions of group A particles substantially improved the fluidization behaviour of the bed even at low superficial gas velocities, leading to a more uniform fluidization. Evaluation of mean and standard deviations has shown the advantage of mixing the two powders as it allowed larger pressure fluctuations and smaller standard deviations. Power spectra, on the other hand, showed that unlike group C, for which fluctuations were small in magnitude and broadband in structure, the mixture showed stronger periodic behaviour as result of the attenuation of the small and rapid fluctuations caused by the flow of gas in the cracks and channels. Advanced methods such as the principal component analysis of the embedded trajectories allowed a quantitative comparison between the fluidization behaviour of the two systems.     

Oscillations in gas-fluidized beds: Ultra-fast magnetic resonance imaging and pressure sensor measurements

Ultra-fast Magnetic Resonance Imaging (MRI) and pressure sensor measurements have been applied to study: (i) pressure fluctuations, (ii) the eruption of bubbles at the top of a bed and (iii) the formation of bubbles in a gas-fluidized bed. Ultra-fast MRI has been applied for the first time to study the formation and eruption of bubbles; the technique is non-intrusive and provides measurements with good temporal and spatial resolutions. The MRI measurements revealed that bubbles are formed periodically, rather than randomly at a distributor, which in this case was a perforated plate. The frequency at which bubbles erupted from the top of the bed matched the frequency of the pressure fluctuations measured just above the distributor, where the measured pressure is predicted very well for the case of slug flow by Kehoe and Davidson's [P.W.K. Kehoe, J.F. Davidson, Pressure fluctuations in slugging fluidized beds, AIChE Symp. Ser. 128 (69) (1973) 34-40] correlation, originally developed for locations high up a bed. Both findings lead to the conclusion that the passage and eruption of bubbles at the top of a bed are the dominant cause of the pressure fluctuations, which are subsequently propagated downwards through the bed. Two new correlations are proposed for predicting the frequency of pressure fluctuations in a bubbling bed; both correlations agree well with experimental measurements. A modification of Baeyens and Geldart's [J. Baeyens, D. Geldart, An investigation into slugging fluidized beds, Chem. Eng. Sci. 29 (1974) 255-265] correlation predicts the frequency of pressure fluctuations when slugs are formed, but are not fully developed. The frequency of bubble formation, as measured by MRI, is equal to or higher than both the frequency of bubble eruption at the top of the bed and the frequency of pressure fluctuations, depending on the depth of the bed. The frequency of bubble formation is significantly lower than predicted by Davidson and Schüler's [J.F. Davidson, B.O.G. Schüler, Bubble formation at an orifice in an inviscid liquid, Trans. Inst. Chem. Eng. 38 (1960) 335-342] equation, originally developed for gas-liquid systems.     

Bubbling and Bed Expansion Behavior Under Vibration in a Gas‐Solid Fluidized Bed

The effect of vibration on the flow patterns and fluidization characteristics including the minimum fluidization velocity (u_mf), the void fraction (ϵ_mf) at u_mf and the bed expansion ratio were examined. The powders used were spherical glass beads and their diameters were 6, 20, 30, 60 and 100μm. For group A powders, the manner in which the vibration affects the bubble formation was examined from the bed expansion ratio and the index of n/4.65. The area of the homogeneous fluidization region was also observed. The homogeneous fluidization region was broadened at a certain vibration strength, where the value of n/4.65 was a minimum. The bubble formation was observed even for 20μm powder (group C), at large vibration strengths and at high gas velocities. Under such conditions, the bed expansion ratio increased suddenly due to bubble formation. The bubbles broke the irregular bed structure, including various properties of agglomerates. Although the channel breakage was dominant flow pattern for group C powders, the bubbles also played an important role in the improvement of the fluidization.     

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.     

Analysis of pressure fluctuations during water evaporation in spouted bed

The aim of this work was to study the behaviour of conventional spouted beds during water evaporation and to analyze the pressure fluctuations at the maximum water evaporative capacity for different bed heights and air flow rates. The results showed that spout pressure drop could not indicate the proximity of maximum evaporative capacity; however this condition is denoted by a minimum in fountain height. The standard deviation and amplitude of the pressure fluctuations also showed a minimum point at the maximum water evaporation capacity. The frequency domain analysis of pressure fluctuations revealed that the dry bed has a dominant frequency varying from 6 to 8.2 Hz and that the peak of dominant frequency tends to disappear with the increase in water feed rate.     

气固脉冲流化床的流体力学特性

在直径７０ｍｍ的流化床中,采用ＦＣＣ、空心玻璃珠、细沙等Ａ类和Ｂ类颗粒,在０￣１０Ｈｚ的频率范围内,测定了气固脉冲流化床的流体力学特性。采用时间继电器改变脉冲气流的频率和脉宽周期比,利用微压传感器记录床层压力变化,研究了瞬时床层压力、平均床层压力,最大床层压降、起始流化速度、床层高度等随操作条件的变化规律,并对脉冲流化床中的气泡现象进行了初步的观察和研究,发现在脉冲流化床中气泡的形成和发展受到了有     

Characterization of spouted bed regimes using pressure fluctuation signals

This work compares time, frequency and phase space analyses of pressure measurements in different spouted beds. The experiments were carried out in different constructions of spouted bed apparatuses, operated under ambient conditions and under different spouting regimes. Spouted beds are used when the conventional fluidized beds fail to achieve a homogeneous and stable flow regime as, for example, in the case of non-spherical particles and in poly dispersed and finely dispersed systems. Different fluidization regimes in spouted beds have been characterized by the analysis of pressure fluctuation signals. Several flow regimes are found to exist as: fixed bed, channel formation, bubbling formation, stable spouting and slugging bed regimes. Analyses of standard deviation and chaotic time series on pressure fluctuation signals are conducted to determine the transition gas velocities. A treatment technique using the Fast Fourier Transformation of measured pressure fluctuations was developed to create plots describing the bed behaviour evolution from fixed to slugging bed. At the beginning of stable spouting the amplitude of pressure fluctuations is uniform and small.     

Fluidization and slugging in large-particle systems

Square-nose slugging that occurs with large particles in relatively small-diameter fluidized beds shows certain similarities with the fluidization behaviour in a fluidized bed coal combustion system with closely packed heat-exchanger tubes. In the present investigation, square-nose slugging is studied in fluidized beds of 0.1 and 0.15 m I.D. with coarse sand and alumina particles, at ambient conditions. Recording of pressure fluctuations was used to analyse the fluidization behaviour. A remarkable change in the pressure fluctuation pattern occurs at the transition from normal fluidization to slugging: a more regular signal with a narrowed frequency spectrum is found.In the square-nose slugging regime, the pressure fluctuations seem to be caused by the disintegration of a rising solids slug, followed by the raining down of the particles. Experimental evidence for this mechanism was found in the behaviour of the magnitude of the pressure fluctuations as a function of operating variables.The frequency of square-nose slugging increases with approximately the square root of the bed diameter and appears to be independent of the type of particles used. The slug frequency decreases slightly with the gas velocity between about 0.8 and 1.8 m.s⁻¹, and is inversely proportional to the stationary bed height between 0.15 and 0.4 m.     

Control of particle cohesion with a polymer coating and temperature adjustment

A new approach is presented that introduces interparticle forces induced by a change of temperature at which polymer coated particles are exposed. The particle cohesive flow behavior is shown for two different applications. The first one considers a dense granular flow that is typically observed for wet powder granulation using a modified spheronizer. The other application shows the possibility of mimicking the particle flow behavior observed in high‐temperature fluidized beds with the advantage of being operated at ambient conditions. For the first application, as the temperature increases, significant changes in the particle bed surface morphology were observed and an important reduction of the dynamic density was noticed. For the gas–solid fluidized‐bed application, pressure drop measurements revealed that the behavior of the particles transited from Geldart group B to Geldart group A and even Geldart group C as the temperature increased. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

A critical review of the complex pressure fluctuation phenomenon in gas-solids fluidized beds

The complex pressure fluctuation phenomenon in gas-solid fluidized beds is systematically examined in this paper based on a comprehensive review of the literature data. The local pressure fluctuations are composed of multiple sources, including local bubble induced fluctuations, global bed oscillations and propagating pressure waves originating in other locations (e.g. bed surface, distributor and windbox). The interaction and coupling among bubble motion, under-damped oscillations of fluidized particles and bed surface, propagating compressible pressure waves and flow pulsation in gas-solid fluidized beds creates the complexity of local pressure fluctuations, and is likely responsible for the formation of complex but unique flow patterns. A few attempts have been reported in the literature on examining the interaction between bed oscillations, plenum chamber air pulsation and propagating pressure waves in fluidized beds, showing some promises on predicting the local pressure fluctuations. Future work should be focused on predicting local and global pressure fluctuations and the formation of unique surface flow patterns by coupling different contributing mechanisms.     

Characterization of the void size distribution in fluidized beds using statistics of pressure fluctuations

Experiments carried out in fluidized beds of Geldart A and B powders revealed that in-bed pressure fluctuations typically have power-law statistics, which are well described by the Student's distribution. We show that the pressure statistics encode information about the bubble size distribution, and propose a novel method that allows an easy determination of the approximate shape of the bubble size distribution from the statistics of pressure fluctuations.     

Numerical studies of the effects of fines on fluidization

Euler‐Lagrange simulations of fluidized beds of Geldart Group A particles containing different levels of fines are performed in periodic domains with various domain‐averaged solid volume fractions. Bubble‐like voids readily form when no fines are added. Introducing fines does not reduce bubble sizes if van der Waals force between particles is not accounted for. In contrast, the addition of van der Waals force produces significant changes. With no fines, bubbles are found to be suppressed at sufficiently high solid volume fractions, corresponding to the expanded bed regime for Group A particles. With the addition of fines, bubbles can be suppressed at lower solid volume fractions. With more fines added, bubbles can be suppressed at even lower solid volume fractions. When bubbles are suppressed, the system is found to be in a stable solid‐like regime. In this regime, forces on each particle are balanced, and the particle velocity fluctuations are dampened. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2271–2281, 2016     

Modification and re-interpretation of Geldart's classification of powders

In developing his powder classification, Geldart [D. Geldart, Powder Technol. 7 (1973) 285.] employed fluidization data obtained only at ambient temperature and pressure and from beds fluidized only with air. Unfortunately, industrial applications of fluidized bed technology invariably are at elevated pressure and temperature and with fluidizing gas other than air. Geldart classification of powders does not apply at elevated pressure and temperature. There are ample evidences reported in the literature indicating that normally Geldart Group B powders at ambient conditions, such as polymer particles, can behave like a Group A powder under polymerization conditions at elevated pressure and moderate temperature with substantial emulsion-phase expansion, relatively small bubbles, smooth fluidization, and reduced gas bypassing [J.R. Grace, Can. J. Chem. Eng. 64 (1986) 353; I.D. Burdett, R.S. Elsinger, P. Cai, K.H. Lee, Gas-phase fluidization technology for production of polyolefins, in Fluidization X, Eds. M. Kwauk, J. Li, W.C. Yang, 2001, pp. 39-52; P.N. Rowe, P.U. Foscolo, A.C. Hoffmann, J.G. Yates, X-ray observation of gas fluidized beds under pressure, in Fluidization IV, Eds. D. Kunii, R. Toei, 1983, pp. 53-60]. Similar findings were also reported for Geldart Group B powders fluidized by supercritical carbon dioxide at elevated pressures [C. Vogt, R. Schreiber, J. Werther, G. Brunner, Fluidization at supercritical fluid conditions, in Fluidization X, Eds. M. Kwauk, J. Li, W.C. Yang, 2001, pp. 117-124; C. Vogt, R. Schreiber, G. Brunner, J. Werther, Powder Technol. 158 (2005) 102; D. Liu, M. Kwauk, H. Li, Chem. Eng. Sci. 51 (1996) 4045; M. Poletto, P. Salatino, L. Massimilla, Chem. Eng. Sci. 48 (1993) 617; A. Marzocchella, P. Salatino, AIChE J. 46 (2000) 901].The original Geldart's classification is modified and re-interpreted in this paper by plotting a dimensionless density against the Archimedes number. The new parameters allow powders with different properties fluidized at different pressures and temperatures with gases of different properties to be plotted in the same graph. The proposed modification successfully transforms the normally Geldart Group B particles at ambient conditions to Group A classification when fluidized at elevated pressure and temperature. The selection of these two parameters, the dimensionless density and the Archimedes number, for plotting is not arbitrary, however. The experimental and theoretical development is discussed.     

Simultaneous measurement of gas and solid holdups in multiphase systems using ultrasonic technique

Ultrasonic technique has been recently developed to measure dispersed phase holdups in multiphase flows. The experimental results obtained in this work have shown that the fluctuations of the sound speed and attenuation of ultrasound are well-defined functions with solid and gas holdups in liquid-solid and gas-liquid two-phase flows. When both solid particles and gas bubbles are present in a liquid flow, gas bubbles appear to be a dominant factor for the instability of ultrasound signals. These findings lead to a new approach for simultaneous detection of gas and solid holdups in a gas-liquid-solid three-phase flow. In this work, canonical analysis and optimization methods are successfully applied to differentiate the contributions of gas bubbles and solid particles to the ultrasonic signals recorded from three-phase systems. The gas and solid holdups determined by the proposed approach are in a good agreement with the experimental results obtained using differential pressure transducers and conductivity probes.     

The clustered dense phase model for group A fluidization: I. Dense phase hydrodynamics

A new hydrodynamic model is proposed to represent the gas flow in the dense phase of fluidized Group A powders. The model views that the particles form clusters under the influence of inter-particle forces, giving rise to the formation of a heterogeneous void structure consisting of clusters of particles and interstitial cavities. The model contains two parameters, one representing the intrinsic void structure of the clusters and the other representing their size.The model predictions have been tested against reported experimental dense phase hydrodynamic data of Group A powders, both in bubble-free beds and in freely bubbling beds. The results show 2-3 particle clusters in bubble-free beds, but considerably larger clusters, containing 100 particles or so, in the dense phase of freely bubbling beds. The model also provides predictions for bubble through-flow, bubble splitting from below, dense-phase solids circulation, and interstitial gas bypassing in freely bubbling beds.     

Fluidization of fine particles in a sound field and identification of group C/A particles using acoustic waves

Effects of sound field on the fluidization of fine particles have been comprehensively examined by using fine powders (4.8-65 μm average in size) including Al₂O₃, TiO₂, glass beads and FCC catalyst. It is found that the fluidization quality of fine particles can be enhanced with the assistance of a sound field, resulting in higher pressure drops and a lower u_mf. The effect of sound on the fluidization of fine particles is strongly dependent on the particle properties (Geldart type and particle size) as well as the parameters of the sound field such as sound pressure level (or intensity) and frequency. Given a fixed sound frequency, the effect becomes more significant at a higher sound pressure level. For the present sound-aided fluidized bed system, there is a resonant frequency at about 100-110 Hz, at which the effectiveness of the sound wave in improving fluidization of fine particles is most remarkable. In addition, based on the different attenuation features of sonic waves in the gas-solid suspension of group C and A particles, a novel acoustic method is explored to distinguish group C from group A particles.     

从浆态床压力脉动信号中辨识气含率的多分辨分析方法

采用多分辨分析方法研究了浆态床中压力脉动时序信号的定量特征,借以从这类信号中提取气泡运动参数——气含率.具体做法是：确定压力脉动信号功率谱图上的主频,同时通过小波变换将压力脉动信号分解成不同分辨率下的低频和高频信号,据此确定对应主频的小波主尺度,并对该尺度上的信号的间歇行为以局部间歇性值（local intermittency measure,LIM）定量表征;选取合适的阈值后,对LIM二值化可得床层中的局部气含率.与压降法测得的平均气含率值比较表明,根据LIM分析方法计算得到的气含率与压降法实测气含率相一致,这也表明反应器内气泡的发生是导致浆态床压力脉动的主要因素.本文提出的方法提供了一种适用于高温、高压场合下测取床层气含率的简便、实用方法.     

Periodic pressure fluctuations in fluidized beds

In shallow gas fluidized beds harmonic oscillations of the pressure around its equilibrium value can be observed. Three aspects of these vibrations have been analysed: the frequency, the critical bed height and the damping. The frequency decreases with the inverse of the square root of the bed height for values below the critical height.For bed heights larger than the critical height the fluctuations cease to be harmonic, the bed breaks up and voids are formed leading to the formation of bubbles or slugs. The critical bed height can be calculated from the frequency and the wave velocity. The maximum value of the critical bed height is a few hundred particle diameters, thus most beds will fluidize heterogeneously. Damping of the oscillations is governed by the ratio of the fluid- to solids-density; the lower this values the higher the damping. The damping is liquid fluidized beds is such that oscillations are prevented.     

Investigation of Hydrodynamics of High‐Temperature Fluidized Beds by Pressure Fluctuations

Hydrodynamics of a gas‐solid fluidized bed at elevated temperatures was investigated by analyzing pressure fluctuations in time and frequency domains. Sand particles were fluidized with air at various bed temperatures. At a constant gas velocity, the standard deviation, power spectrum density function, and wide‐band energy of pressure fluctuations reach a maximum at 300 °C. Increasing the temperature to this value causes larger bubble sizes and after the bubbles reach their maximum size, they break into smaller bubbles. The Archimedes number decreases with higher temperature and the type of fluidization becomes closer to that of Geldart A boundary at this maximum temperature. Based on estimation of the drag force acting on the emulsion phase, it was concluded that 300 °C was a transition temperature at which the drag force reaches a minimum due to a significant change of interparticle and hydrodynamic forces.