Wall-to-bed heat transfer coefficient in gas—liquid—solid fluidized beds

Wall to bed heat transfer has been studied in three-phase fluidized beds with a cocurrent up-flow of water and air. Six sizes of glass beads, two sizes of activated carbon beads and one size of alumina beads, varying in average diameter from 0.61 to 6.9 mm and in density from 1330 to 3550 kg/m³, were fluidized in a 95.6 mm diameter brass column heated by a steam jacket. Complementary heat transfer experiments have been performed also for a gas–liquid cocurrent column and liquid–solid fluidized beds. The wall-to-bed coefficient for heat transfer in the gas–liquid–solid fluidized bed is evaluated on the basis of the axial dispersion model concept. The ratio of the wall-to-bed heat transfer coefficient in the gas–liquid–solid fluidized bed to that in the liquid–solid fluidized bed operated at the same liquid flow rate is correlated in terms of the ratio of the velocity of gas to that of liquid and the properties of solid particles. A correlation equation for estimating the wall-to-bed heat transfer coefficient in the liquid–solid fluidized bed is also developed.     

A method of evaluating limestone reactivity with SO2 under fluidized bed combustion conditions

The reactivity of limestone with sulphur dioxide has been evaluated during conditions similar to those existing in a fluidized bed combustor using a fixed-bed quartz reactor. The reactivity of up to 11 particle sizes of 2 limestones were evaluated, and an exponential decay function was found to best describe the rate behaviour versus conversion of CaO to CaSO₄. The two constants in the exponential decay approximation could both be expressed as functions of particle size. Subsequently, the limestone reactivity as a function of both size and conversion could be reasonably well described by a total of 4 to 6 constants. An analytical sulphur capture model for fluidized bed boilers (FBB) that incorporates this type of reactivity function is proposed.     

IMPEDIMENT TO INCIPIENT FLUIDIZATION IN WET BEDS OF POROUS NONSPHERICAL PARTICLES

Fluidized-bed units are efficiently and advantageously employed for drying various wet and sticky particulate materials provided that the bed of such materials can be kept under a fluidized condition. The effect was explored with the incremental addition of water to the bed of nonspherical, porous particles of ceramsite and lignite fluidized with wet air. The limiting fluidization-defluidization point was determined by experiments in a cold model fluidized bed contained in a transparent glass column using wet and dry particles. On the basis of the data amassed, an empirical correlation was developed. This relationship makes it possible to predict the dimensionless excess gas velocity that keeps the wet bed fluidized, as a function of the relative amount of moisture in the bed and the dimensionless particle size.     

IMPEDIMENT TO INCIPIENT FLUIDIZATION IN WET BEDS OF POROUS NONSPHERICAL PARTICLES

Fluidized-bed units are efficiently and advantageously employed for drying various wet and sticky particulate materials provided that the bed of such materials can be kept under a fluidized condition. The effect was explored with the incremental addition of water to the bed of nonspherical, porous particles of ceramsite and lignite fluidized with wet air. The limiting fluidization-defluidization point was determined by experiments in a cold model fluidized bed contained in a transparent glass column using wet and dry particles. On the basis of the data amassed, an empirical correlation was developed. This relationship makes it possible to predict the dimensionless excess gas velocity that keeps the wet bed fluidized, as a function of the relative amount of moisture in the bed and the dimensionless particle size.     

Modeling of Shallow Fluidized Bed Reactors: I. analytic solution of a simplified model

A fairly general dynamic model for shallow fluidized bed reactors is developed, and analytic solutions for the governing equations of the model are obtained after some simplification. The bubble size, the lateral dispersion coefficient of solids, the level of the excess fluid reactant and the structure of the bed are examined to determine their effects on the conversion of solids. It has been shown that the conversion of solids is influenced profoundly by the bubble size and that it is advantageous to employ a shallow fluidized bed reactor if a high conversion of solids is required.     

Phase holdup and liquid dispersion in gas-liquid-solid circulating fluidized beds

Axial and radial profiles of gas and solids holdups have been studied in agas-liquid-solid circulating fluidized bed at 140mm i.d..Experimental results indicate that the axialand radial profiles of gas and solids holdups are more uniform than those in a conventionalfluidized bed.Axial and radial liquid dispersion coefficients in the gas-liquid-solid circulating fluidizedbed are investigated for the first time.It is found that axial and radial liquid dispersioncoefficients increases with increaes in gas velocity and solids holdup.The liquid velocity has littleinfluence on the axial liquid dispersion coefficient,but would adversely affect the redial liquiddispersion coefficient.It can be concluded that the gas-liquid-solid circulating fluidized bed hasadvantages such as better interphase contact and lower liquid dispersion along the axial directionover the expanded bed.     

循环流化床密相区内颗粒的横向扩散的研究

Lateral solid mixing was investigated experimentally in the dense zone of a 900mm×100mm×5.2m rectangular circulating fluidized bed riser. Using heated tracer injection, the lateral solid dispersion was determined by measuring the temperature response at different lateral positions. Furthermore, a one-dimensional dispersion model, which describes the solid mixing in the dense zone, is presented. The experimental results were used to determine the lateral particle dispersion coefficient under various operating conditions. A correlation of dispersion coefficient with bed height, gas velocity, and particle size is also proposed.     

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.     

Horizontal gas mixing in rectangular fluidized bed: A novel method for gas dispersion coefficients in various conditions and distributor designs

In a rectangular fluidized bed combustor,the tracer gas is injected continuously into the bed from a point source at the center of the distributor plate.In this study,a general governing equation is formulated for tracer gas dispersion in the bed.An analytical solution is derived to estimate the dispersion coefficients,Dx and Dy,in a horizontal plane.The concentration profiles at different sampling heights with various gas velocities are plotted.Subsequently,to estimate the dispersion coefficients,surface fitting of the obtained analytical solution to the experimental data is performed.The dispersion coefficients obtained from this model are compared with those of a conventional model.Additionally,the effect of walls,bed height and gas injection rate on the dispersion coefficients in a horizontal plane is investigated,and the effect of distributor design on the dispersion coefficients in a horizontal plane is investigated with different tracer positions.It is found that Dx and Dy are nearly equivalent at a lower tracer gas ratio of the injected gas to the total gas flow rate.It is also demonstrated that the effect of bed height on Dx is minor.This model is also able to estimate the dispersion coefficients in the case of a multihorizontal nozzle distributor.     

串行流化床煤气化试验

针对串行流化床煤气化技术特点,以水蒸气为气化剂,在串行流化床试验装置上进行煤气化特性的试验研究,考察了气化反应器温度、蒸汽煤比对煤气组成、热值、冷煤气效率和碳转化率的影响。结果表明,燃烧反应器内燃烧烟气不会串混至气化反应器,该煤气化技术能够稳定连续地从气化反应器获得不含N₂的高品质合成气。随着气化反应器温度的升高、蒸汽煤比的增加,煤气热值和冷煤气效率均会提高,但对碳转化率影响有所不同。在试验阶段获得的最高煤气热值为6.9 MJ•m^-3,冷煤气效率为68％,碳转化率为92％。     

循环流化床气体返混及停留时间的分布

本文采用脉冲示踪技术,研究了直径140 mm循环流化床内气体的返混特性及其停留时间分布规律,并通过一维轴向扩散模型,得到了气体轴向扩散系数。实验结果表明,在循环流态化条件下,气体轴向流动明显偏离于平推流,并且随操作气速增大或颗粒循环速率的减小,气体轴向返混程度减小。     

快速流化床中气体轴向混合特性

在内径90mm、高8m的快速循环流化床试验装置上,用非理想脉冲示踪、双探头检测、计算机实时采样分析的方法,对湍流流态化、快速流态化和气力输送不同流动状态的气体混合程度分别进行了测量,结果表明均存在不同程度的轴向混合.轴向扩散系数主要与由气体速度和固体循环量决定的床层空隙度有关;用拟均相一维扩散模型对结果进行处理,时域拟合法求取模型参数,并将轴向扩散系数进行了关联,得到如下统一关联式D_e=0.1953ε~(-4.1197)     

Axial gas dispersion in a fluidized bed of polyethylene particles

Gas mixing behavior was investigated in a residence time distribution experiment in a bubbling fluidized bed of 0.07 m ID and 0.80 m high. Linear low density polyethylene (LLDPE) particles having a mean diameter of 772 Μm and a particle size range of 200-1,500 Μm were employed as the bed material. The stimulus-response technique with CO₂ as a tracer gas was performed for the RTD study. The effects of gas velocity, aspect ratio (H₀/D) and scale-up on the axial gas dispersion were determined from the unsteady-state dispersion model, and the residence time distributions of gas in the fluidized bed were compared with the ideal reactors. It was found that axial dispersion depends on the gas velocity and aspect ratio of the bed. The dimensionless dispersion coefficient was correlated with Reynolds number and aspect ratio.     

Effect of pseudoplastic behaviour of liquid in co-current three-phase fluidized beds on bed expansion

Gas hold-up and bed expansion measurements were carried out for a bed of glass beads fluidized in Newtonian liquids and non-Newtonian liquids with gas. The value of gas hold-up increased and decreased with increasing particle size and liquid velocity, respectively. The effect of rheological properties on gas hold-up was insignificant and therefore the gas hold-up data for both Newtonian and non-Newtonian fluids were reasonably fitted by the available correlation which had no liquid viscosity term. The bed voidage increased with increasing superficial liquid velocities and superficial gas velocities. The increase of the viscous non-Newtonian flow behaviours resulted in an increase of the bed voidage. The correlation for the bed voidage in three-phase fluidized beds was developed for gas-Newtonian or non-Newtonian liquid–solid three-phase systems by combining the generalized wake model and the correlation for liquid–solid two-phase systems proposed previously by the authors. The predictions for bed voidage were in reasonable agreement with the present experimental data for three-phase systems with Newtonian and non-Newtonian liquids in a wide range of Reynolds numbers.     

The effect of distributor rotation on gas mixing in a fluidized bed

The influence of distributor rotation on gas dispersion in a 15 cm diameter fluidized bed was investigated at four rotational speeds. Gas concentrations were determined at various bed depths and radial positions, according to a recently developed method. The concentration profiles were compared with those of a two-dimensional diffusion model. Rotation of the distributor plate caused an increase in the radial dispersion of the gas essentially at fluidization velocities within a range of u_mf.     

Investigating the eruptions of fine−grain material from a nonuniform fluidized bed

A model is proposed for the entrainment of particles from the surface of a fluidized bed. It is shown that the deep voids produced by coalescence of twThe gas flux across the elongated ellipsoidal voids at the bed surface was investigated when the superficial gas velocity was equal to w_mf andThe analytical solution for the gas flow through the ellipsoidal void when the Reynolds number is small was derived and an empirical correlation is givAn empirical correlation for prediction of the maximum entrainment height was obtained. This includes a dependence on diameter and height of the bed, f     

Solid-liquid mass transfer in a gas-liquid-solid fluidized bed

Solid-liquid mass transfer in co-current two- and three-phase fluidized beds of water, air and benzoic acid pellets is studied. An axial dispersion model is used to describe the liquid flow when evaluating the solid-liquid mass transfer. The axial concentration profile of benzoic acid in the liquid is compared to that obtained experimentally and is found to be accurate. Three-phase fluidized bed solid-liquid mass-transfer coefficients are higher than the corresponding two-phase bed coefficients. The mass-transfer coefficient increases with increasing gas rate and is independent of liquid rate over the entire range studied. The mass-transfer coefficient also appears to be dependent on particle size, but only at high gas rates. At low or zero gas rates, k is nearly independent of particle size. A generalized correlation is developed which accurately and conveniently predicts the mass transfer in both two- and three-phase fluidized beds. Comparison to the solid-liquid mass-transfer characteristics of slurry bubble columns is also performed.     

Axial dispersion of gas in a circulating fluidized bed

An axial dispersion of gas in a circulating fluidized bed was investigated in a fluidized bed of 4.0 cm I.D. and 279 cm in height. The axial dispersion coefficient of gas was determined by the stimulus-response method of trace gas of CO₂. The employed particles were 0.069 mm and 0.147 mm silica-sand. The results showed that axial dispersion coefficients were increased with gas velocity and solid circulation rates as well as suspension density. The experimentally determined axial dispersion coefficients in this study were in the range of 1.0-3.5 m²/s.     

EFFECTS OF PARTICLE CHARACTERISTICS ON THE PERFORMANCE OF A FAST FLUIDIZED BED REACTOR

A heterogeneous model for the fast fluidized bed reactor which carries out a gas-solid non catalytic reaction is presented. The hydrodynamics of the fast fluidized bed is characterized by the model of Kwauk et al. (1985) which assumes the existence of two phases; a dense phase and a dilute pneumatic transport phase. For a given solid flowrate, the length of the reactor occupied by each phase depends on gas velocity, particle diameter and density and average voidage within the reactor. The gas-solid reaction is assumed to follow the shrinking core model. The solids are assumed to be completely backmixed in the dense phase and move in plug How in the dilute pneumatic transport phase. The gas phase is assumed to be in plug flow in both phases

For given gas and solid flowrates, the transition from the dense phase flow to the fast fluidized bed (containing two regions) as functions of particle size and density is determined using the model of Kwauk et al. (1985). The numerical solution of the governing mass balance equations show that for given solid and gas flowrates, (and average voidage) the gas phase conversion shows an unusual behavior with respect to particle diameter and density. Such behavior is resulted from the effects of particle diameter and density on the reactor volume occupied by each phase and the effect of particle diameter on the apparent reaction rate. The numerical results show that a fast fluidized bed gives the best conversion at large particle density and for the particle diameter which results the fast fluidized bed to be operated near the pure dense phase flow.     

Autothermal CO2 reforming of methane over NiO–MgO solid solution catalysts under pressurized condition: Effect of fluidized bed reactor and its promoting mechanism

The effect of fluidized bed reactor in autothermal CO₂ reforming of methane over NiO–MgO solid solution catalysts was investigated by comparing with fixed bed reactor. Methane conversion to syngas was drastically enhanced by using a fluidized bed reactor. The catalyst was reduced and oxidized repeatedly in fluidized bed reactor during the reaction. The enhancement of methane conversion is related to the catalyst reducibility.