HYDRODYNAMICS AND STABILITYOF SLOT-RECTANGULAR SPOUTED BEDS. PART I: THIN BED

Spouted beds of rectangular cross-section with gas entry through bottom slots have been proposed as a means of overcoming scale-up difficulties of conventional spouted beds. A study was undertaken of bed hydrodynamics in a thin slot-rectangular column of width 150 mm and slot width 2 to 20 mm for four types of particles. Flow regimes and bed hydrodynamics are qualitatively similar to those in cylindrical spouted beds, but there are significant quantitative differences caused by the different geometry. Slot width exerts a strong influence on such features as flow regimes, maximum spoutable bed height, minimum spouting velocity, pressure drop and fountain height.     

HYDRODYNAMIC STUDIES IN A HALF SLOT-RECTANGULAR SPOUTED BED COLUMN

Experiments were carried out in a half slot-rectangular spouted bed to investigate the effects of slot width and lower section basal angle on column hydrodynamics. Flow regimes, minimum spouting velocity, spouting and maximum pressure drops, and maximum spoutable bed height were determined for 4?mm diameter polyethylene particles. The results are compared with those for conventional cylindrical and rectangular spouted beds. Correlations for each hydrodynamic parameter are developed and compared with equations available in the literature.     

HYDRODYNAMIC STUDIES IN A HALF SLOT-RECTANGULAR SPOUTED BED COLUMN

Experiments were carried out in a half slot-rectangular spouted bed to investigate the effects of slot width and lower section basal angle on column hydrodynamics. Flow regimes, minimum spouting velocity, spouting and maximum pressure drops, and maximum spoutable bed height were determined for 4 mm diameter polyethylene particles. The results are compared with those for conventional cylindrical and rectangular spouted beds. Correlations for each hydrodynamic parameter are developed and compared with equations available in the literature.     

Hydrodynamics of slot‐rectangular spouted beds: Effect of slot configuration on the local flow structure

The local flow structure in a slot‐rectangular spouted bed column of 300 mm × 100 mm cross‐section was investigated with slots of equal area but different length‐to‐width ratios. Dead‐zones, spout shapes, and distributions of pressure, particle velocity, and voidage were explored. The local flow structure of the slot‐rectangular spouted bed became similar to conventional spouted beds as the height increased, with spouts from slots of different length‐to‐width ratios approaching similarity in the upper part of the bed. Static bed height was found to have little influence on the flow in the spout at a given level.     

Characterization of Gas Spouted Beds Using the Rescaled Range Analysis

Differential pressure fluctuation measurements were conducted in a gas spouted bed of 120 mm in diameter at different axial and radial positions. Hurst's rescaled range analysis of the differential pressure fluctuation signals was successfully employed to recognize different flow regimes, i.e. packed bed, stable spouting and unstable spouting, and characterize their transitions. Obvious two‐phase behaviour, as suggested by two Hurst exponents, was observed in the stable spouting regime. It had also been found that the spouting behaviour in deeper spouted beds was significantly different from that in shallow spouted beds. The influences of measurement location and bed height on the Hurst exponents were discussed.     

Gas holdup above the bed surface and grid gas jet hydrodynamics for three phase fluidized beds

A three dimensional column was used to study the hydrodynamics of a three phase system: air, water and 3 mm glass beads. Various effects of the grid jets on bed hydrodynamics were investigated for both increasing and decreasing liquid superficial velocities. Three regimes were observed: spouted bed, spouted fluidized bed and three phase fluidized bed. The hydrodynamics of the two phase region above the bed was also studied. The gas holdup increased when the gas superficial velocity was increased but decreased when the liquid superficial velocity was increased. A correlation for the estimation of the gas holdup as a function of gas and liquid superficial velocities was established.     

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.     

A Discussion on Minimum Spout Velocity and Jet Penetration Length

The similarity and difference between the flat‐bottom and cone‐bottom cylindrical spouted beds, conical spouted beds and vertical upward jets in fluidized beds have been analyzed in this paper based on the effects of geometrical parameters on the minimum spouting velocity and operating stabilities of the spouted beds. The effect of angle on minimum spouting velocity was found to be only significant within the range of 30 to 60 degrees cone angles. Minimum spouting velocity in deep cylindrical spouted beds was proportional to the square root of the static bed height, but was proportional to the static bed height in conical spouted beds and large cylindrical spouted beds with small height‐to‐diameter ratio. The relationship between the minimum spouting velocity and the static bed height was consistent with that between jet velocity and the vertical jet penetration length in jetting fluidized beds.     

Stability of slot‐rectangular spouted beds: Effect of slot configuration

Experiments were carried out to determine the flow stability of slot‐rectangular spouted beds of 300 × 100 mm cross‐section with slots of different widths, lengths, and depths. The effects of slot expansion angle and diverging base were also investigated. Dependent variables included the minimum spouting velocity, bed pressure drop and standard deviation of pressure fluctuations. Based on the flow regimes and spout termination mechanisms, instability was found to be mainly due to the interaction of multiple spouts. Criteria were identified relating stable spouting to slot dimensions and particle size. Slots of limited cross‐sectional area can provide sustainable and symmetric spouting with little fluctuation, as long as the slot length‐to‐width ratio and depth are within certain limits, related to those for conventional spouted beds.     

串行流化床内气固流动控制

针对化学链燃烧分离CO₂技术特点,在一串行流化床（循环床+喷动床）冷态实验装置上,以CaSO₄载氧体为实验原料（d_p= 0.6 mm）,研究串行流化床气固流动特性。基于床内压力分布特征,提出将循环床（空气反应器）沿床高方向划分为鼓泡段和快速流化段2个流型区域,将喷动床（燃料反应器）沿床高方向划分为喷动段、鼓泡段和悬浮段3个流型区域,得出串行流化床内气固流动控制机理。研究并考察了循环床流化风速度、喷动床喷动风速度对串行流化床内反应器间（空气反应器和燃料反应器）气体串混、颗粒循环速率以及床层压降的影响。研究结果表明,流化风是床内颗粒循环的驱动力,流化风速度应控制在 3.77～4.05 m·s^-1;喷动风速度对床内颗粒循环以及系统稳定运行起着关键作用,建议将喷动风速度控制在0.42～0.56 m·s^-1。     

Mass transfer in liquid-fluidized and spouted beds of ion exchange resin at low Reynolds number

Rates of mass transfer from the liquid phase to small ion exchange resin particles (0.78 mm in mean diameter) in fluidized and spouted beds were studied experimentally. Dilute aqueous solution of sodium hydroxide was fed into the beds of strong cation exchange resin and the exit concentration of the solution was determined by conductivity measurement. In spouted beds, the initial conversion and K_l increased with bed height, but decreased with fluid flowrate. The model, applying material balance of the reactant and axisymmetric flow of fluid in the annulus of a spouted bed, predictions of the initial conversion in spouted beds are satisfactory. In fluidized beds, the obtained mass transfer coefficients were correlated and compared with other works.     

Comparison of numerical simulations and experiments in conical gas-solid spouted bed

Flowbehavior of gas and particles in conical spouted beds is experimentally studied and simulated using the twofluid gas-solid model with the kinetic theory of granular flow. The bed pressure drop and fountain height are measured in a conical spouted bed of 100mmI.D. at different gas velocities. The simulation results are compared with measurements of bed pressure drop and fountain height. The comparison shows that the drag coefficient model used in cylindrical beds under-predicted bed pressure drop and fountain height in conical spouted beds due to the partial weight of particles supported by the inclined side walls. It is found that the numerical results using the drag coefficient model proposed based on the conical spouted bed in this study are in good agreement with experimental data. The present study provides a useful basis for further works on the CFD simulation of conical spouted bed.     

Pressure drop in slotted spouted beds of grains: Comparison of data with models

Data obtained in a rectangular slot spouted bed for two different grains (wheat and oats) have been analyzed to determine the minimum spouting velocity (V_ms) and the minimum spouting pressure drop (ΔP_ms) as a function of the bed height.Three different aerodynamic regimes occur as the bed height is increased.The results are compared with the literature models for the conventional (conical-cylindrical) spouted bed.For shallow beds, the experimental minimum spouting velocity and minimum spouting pressure drop can be well represented by the Littman et al.'s two-dimensional model.     

A comparative hydrodynamic study of two types of spouted bed reactor designs

In order to properly design and scale up spouted beds, one needs to predict the minimum spouting velocity of specific systems having different bed dimensions, and properties of particle and spouting gas. Because of inherent complexity of predicting minimum spouting velocity, the prevailing approach has been to use empirical correlations, a number of which are available in the literature. Central jet distributors are commonly used in the experimental studies reported in the literature. Circular slit distributor is a new concept in which air is supplied to the bed of particles through a circular slit. This paper presents results of an experimental study on the hydrodynamics of central jet and circular slit distributors. In this paper a fully connected feed-forward neural network model was used to predict the minimum spouting velocity of central jet and circular slit spouted beds. A neural network model was also developed to predict minimum fluidization velocity. The actual experimental data obtained from published literature and from the experiments carried out in this study were used for training and validating the models. The minimum spouting and fluidization velocities predicted using the neural network models developed in this study show a better approximation to the actual experimental values than those obtained from correlations available in the open literature. The position of flow regime of circular slit spouted bed was also established relative to the flow regimes of central jet spouted bed and fluidized bed.     

Scale-up relationships of spouted beds by solid stress analyses

This article describes a new set of scale-up parameters of spouted beds derived from solid stress analyses. Compared with the first set of scale-up parameters of spouted beds proposed by He et al. [He Y. L., Lim C. J., Grace J. R., Scale-up studies of spouted beds, Chemical Engineering Science, 52 (2), 329-339, 1997], this set introduced a new parameter, the coefficient of restitution of particles, that accounts for the effects of particle-particle collisions in the spout region of a spouted bed. To verify the present set of scale-up parameters, a series of experiments were designed and conducted in two spouted beds of 80 mm and 120 mm in diameter, respectively, operated in the different testing cases consisting of different combinations of the involved scale-up parameters. The results showed that the more closely the scale-up parameters of the spouted beds tested were matched, the higher hydrodynamic similarity could be achieved. The comparisons of the hydrodynamic properties such as fountain height, spout diameter and bed voidage measured in the different testing cases revealed that the coefficient of restitution of particles could significantly impact the particle-particle interactions and thus its effects should be taken into account in scaling-up spouted beds.     

Voidage Profiles in a Slot‐Rectangular Spouted Bed

Local voidages were determined using optical fibre probes for 1.4 and 2.4 mm glass beads in slot‐rectangular spouted beds of width 150 mm. The effects of air inlet flow rate, slot width and bed thickness were investigated. Spout widths were determined from standard deviations of local voidage fluctuations. Both spout and annulus voidages increased with increasing air flow. Annulus voidages were usually higher than corresponding loose‐packed voidages and decreased with increasing height. Annulus and spout voidages were very sensitive to the air entry slot width and particle diameter. With increasing height, spouts became increasingly circular (three‐dimensional), despite the initial rectangular geometry.     

Hydrodynamics of a Novel Biomass Autothermal Fast Pyrolysis Reactor: Flow Pattern and Pressure Drop

A novel biomass, autothermal, fast pyrolysis reactor with a draft tube and an internal dipleg dividing the reactor into two interconnected beds is proposed. This internally interconnected fluidized beds (IIFB) reactor is designed to produce high‐quality bio‐oil using catalysts. Meanwhile, the pyrolysis by‐products, i.e., char, coke and non‐condensable gases, are expected to burn in the combustion bed to provide the heat for the pyrolysis. On the other hand, the catalysts can be regenerated simultaneously. In this study, experiments on the hydrodynamics of a cold model IIFB reactor are reported. Geldart group B and D sand particles were used as the bed materials. The effects of spouting and fluidizing gas velocities, particle size, static bed height and the total pressure loss coefficient of the pyrolysis bed exit, on the flow patterns and pressure drops of the two interconnected beds are studied. Six distinct flow patterns, i.e., fixed bed (F), periodic spouted/bubbling bed (PS/B), spouted bed with aeration (SA), spout‐fluidized bed (SF), spout‐fluidized bed with slugging (SFS) and spouted bed with backward jet (SBJ) are identified. The investigations on the pressure drops of the two beds show that both of them are seen to increase at first (mainly in the F flow pattern), then to decrease (mainly in the PS/B and SA flow patterns) and finally to increase again (mainly in the SA and SF flow patterns), with the increase of the spouting gas velocity. It is observed that a larger particle size and lower static bed height lead to lower pressure drops of the two beds.     

Pressure distribution in spout-fluid bed reactors

The occurrence of several flow regimes in spout-fluid beds was investigated. Four different flow regimes, viz. a packed bed flow regime, a bubbling and a fluctuating spouted bed flow regime and a stable spouted bed flow regime, were found to exist. Pressure distributions in a spout-fluid bed were measured in several of these flow regimes by means of a moveable pressure probe. A theoretical model that describes the flow pattern in spout-fluid beds was developed from fundamental relationships that govern the flow of gases through porous media. Pressure distributions calculated from this model agree fairly well with measured values.     

狭缝式矩形喷动床中多粒度颗粒体系的最大喷动压降

在 15 0 mm× 5 0 mm× 110 0 mm的矩形喷动床中 ,研究了单一粒径体系和二组分及三组分混合粒径颗粒体系的最大喷动压降受颗粒粒径及粒度组成、静止床高和气体入口狭缝宽度的影响情况。实验采用宽度为2、4、6 mm三种宽度的狭缝式气体分布板 ,实验物料为单一粒径分别为 1、1.5、2 mm的玻璃珠。实验表明矩形喷动床的最大喷动压降与上述三种影响因素都有关系。本文还给出了最大喷动压降随这三种因素变化的实验关联式     

Spouted bed and spout-fluid bed behaviour in a column of diameter 0.91 m

Hydrodynamic measurements were obtained in a half-column of diameter 0.91 m equipped with a conical base using particles of diameter 3.3 to 6.7mm with operation both as pure spouted beds and in the spout-fluid bed mode. Comparison of the experimental results for minimum spouting velocity with equations in the literature generally gave unsatisfactory agreement. On the other hand, the correlations of McNab (1972) and Had?isdmajlovi? et al. (1983) gave reasonable predictions of spout diameters in spouted and spout-fluid beds respectively. Hydrodynamic regimes with auxiliary air present were broadly similar to those determined in smaller columns. However, there were substantial dead regimes at the bottom of the column. A finite difference model based on the vector form of the Ergun equation gave good predictions of air flow distribution and longitudinal pressure profiles.