An Experimental Investigation on Solid Acceleration Length in the Riser of a Long Circulating Fluidized Bed

Systematic experimental work was conducted to investigate the solid acceleration length in a 16m long circulating fluidized bed riser with fluid cracking catalyst particles over a wide range of operating conditions. A more feasible method is proposed to determine the acceleration length from the measured axial profiles of pressure gradient (or apparent solid holdup). With this new method and large amount of experimental data, a clear picture on the variation of the acceleration length with both solid circulating rate and superficial gas velocity is obtained.It is found that the acceleration length increases generally with increasing solid flow rate and/or decreasing gas velocity. However, the trend in variation of the acceleration length with operating conditions are quite different in different operation ranges. Reasonable explanations are suggested for the observed variation patterns of acceleration length.

Investigation of the Acceleration Region in a Gas-Solid Co-current Down-flow Circulating Fluidized Bed

In this work, a mathematical model is established to describe the axial variation of the characteristic flow parameters (particle velocity, solid holdup and pressure gradient) in a downer. An empirical correlation is developed to estimate the particle velocity at the constant velocity section. Experimental investigations are made to validate the downer model. The model simulations have a good agreement with experimental data. Moreover, a formula is derived to predict the first acceleration section length and the whole acceleration section length.

Fluidization characteristics of different sizes of quartz particles in the fluidized bed

Fluidization characteristics of quartz particles with different sizes are experimentally investigated in a fluidized bed with an inner diameter of 300 mm and height of 8250 mm.Results show that the average solid holdup increases with the increase in superficial gas velocity and the decrease in initial solid holdup in the dense zone of the fluidized bed.The average cross-sectional solid holdup decreases with increasing bed height and superficial gas velocity.The bed expansion coefficient increases with the increase in superficial gas velocity and the decrease in solid holdup.Correlations of average solid holdup,average cross-sectional solid holdup and bed expansion coefficient are also established and discussed.These correlations can provide guidelines for better understanding of the fluidization characteristics.

A Sedimentation-Dispersion Model for both Non-attached and Attached Particles in Three-Phase Batchwise Fluidized Beds

The axial concentration distribution of both particles with betterwetting (forming non-attached system) and poorer wetting (formingattached system) was investigated in a vertical gas-liquid-solidfluidized bed of 4.2 cm in diameters and 130 cm in height with thesolids holdup less than 0.05. The one-dimensionalsedimentation-dispersion model could be used satisfactorily todescribe the axial distribution of solids holdup by modifying only amodel parameter, i.e. by means of the terminal settling velocityminus a certain value, which is a functions of gas velocity andconsiders the effect of an additional drag force resulted fromattached rising bubbles.

Experimental Investigation on the Hydrodynamic Characteristics of Fluidized Bed Particle Solar Receiver with Gas-Solid Countercurrent Flow Pattern

To design a particle solar receiver(PSR),a vital energy conversion system,is still a bottleneck for researchers.This study presents a novel PSR based on countercurrent fluidized bed(CCFB)technology,named CCFB receiver.In this design,downward-moving particles are subjected to the action of an up-flow gas to reduce the falling speed and enhance the radial disturbance,and hence increase the residence time of particles and improve the heat transfer.A cold-mold visual experimental setup is established.The influence factors are investigated experimentally,including the superficial gas velocity,solid flux,aeration gas,particle size and transport tube diameter.The results indicate that the maximum solid holdup can exceed 9%or so with fine particles of diameter d_(p)=113.5 μm and a tube diameter of 40 mm.It is proved that the CCFB can operate stably and adjust the solid flux rapidly.The results of this study provide a new structure for PSRs in the concentrated solar power field and could fill the research insufficiency in the gas-solid counterflow field.

GAS-SOLIDS FLOW BEHAVIOR WITH A GAS VELOCITY CLOSE TO ZERO

In a 9.3 m high and 0.10 m i.d. gas-solids downflow fluidized bed （downer）, the radial and axial distributions of the local solids holdups and particle velocities along the downer column were measured with the superficial gas velocity set to zero. A unique gas-solids flow structure was found in the downer system with zero gas velocity, which is completely different from that under conditions with higher gas velocities, in terms of its radial and axial flow structures as well as its micro flow structure. The gas-solids flow pattern under zero gas velocity conditions, together with that under low gas velocity conditions, can be considered as a special regime which differs from that under higher gas velocity conditions. According to the hydrodynamic properties of the two regimes, they can be named the ＂dense annulus＂ regime for the flow pattern under zero or low gas velocity conditions and the ＂dense core＂ regime for that under higher gas velocity conditions.

Continuous segregation of binary heterogeneous solids in a fast-fluidized bed

Continuous segregation of binary heterogeneous solids （different density mixtures） is carried out in a gas-solid fluidized bed to study the effects of gas velocity, solids feed rate, feed composition and density difference of solids on the separation factor （recovery of flotsam at top outlet） and the quality of the product （purity of flotsam at top outlet） in a continuous fast-fluidized bed. The holdup of the bed material is obtained in each experimental run. It is observed that the separation factor decreases with increase in solids feed rate or density difference of solids, and increases with gas velocity or proportion of flotsam in the feed. The quality of the product decreases with increase in gas velocity or solids flow rate, and increases with feed composition or density difference of solids. The experimental results show that the separation factor and the quality of the product are more sensitive to gas velocity than to other operating parameters, Empirical correlations for predicting the separation factor and quality of the product are proposed based on the Richards model for individual flotsam mass fraction in the feed, and the predictions agree satisfactorily with the present exlgerimental data,

Characteristics of Axial and Radial Development of Solids Holdup in a Countercurrent Fluidized Bed Particle Solar Receiver

A novel particle solar receiver(PSR)with gas-solids countercurrent fluidized bed(CCFB)was proposed.The cold-mold prototype was set up to investigate the gas-solids flow structure by using optical fiber probes.The local solids holdup distribution,its evolution with various operating conditions and the fluctuations of the local flow structures were investigated experimentally.The results show that the novel CCFB can achieve much higher solids holdup(~9%)compared to the traditional downer ones(~l%).The solid particles are mainly distributed in the near-wall region and the particles are more difficult to get a fully developed state in the near-wall region.The excellent gas-solids mixing and contacting demonstrated by the standard deviation and intermittency index means a better wall-to-bed heat transfer process.The distribution of the solid particles in the CCFB transport tube is revealed,which can provide a significant reference for the structure design of the hot-mold PSR.Moreover,the research can fill in the research gap in the gas-solids counterflow field.

Axial flow structure of solids holdup in an 18-m high-density CFB riser based on pressure measurements

The axial flow structure in a high-density CFB riser having a height of 18 m is investigated on the basis of pressure measurements.Solids circulation rates reach 1400 kg/(m2 s)at superficial gas velocities of 5-9 m/s and the apparent solids holdup exceeds 0.2,indicating high-density operations have been achieved.The apparent solids holdup increases with the solids circulation rate increasing and/or superficial gas velocity decreasing.Axial distributions of the apparent solids holdup have exponential shapes with denser regions at the bottom and more dilute regions in the upper part.The apparent slip velocity increases with the increasing solids holdup and reaches 14 m/s,showing that there are more opportunities of cluster formation in high-density operation.Furthermore,the apparent slip velocity has a power relation with the apparent solids holdup under a wide range of operating conditions.

Characterization of countercurrent liquid-upward and solid-downward fluidized system

Acting as an operating mode of fluidization,the flow characteristics of a countercurrent liquid–solid fluidized bed(CCLSFB)were experimentally investigated using a Plexiglas column of 1.5 m in height.Countercurrent liquid-upward and solid-downward fluidization was achieved under a limited solid flowrate before flooding occurred.The“flooding”phenomena and the flooding velocity were identified by measuring the variations in pressure drop in the axial direction of the column.Two different methods were used to quantify the flooding point that led to the instability of the system.Axial solids holdup profiles were also obtained from the pressure drop data along the column and the influences of device structure and operating conditions on the solids holdup were also studied.Seven types of particles with different diameters and densities were used.An agreement was found between the experimental results and the mathematic prediction derived from the Richardson–Zaki equation on the data of the solids holdup.

Local solids flow structure in a countercurrent liquid-upward and solids-downward fluidized bed(CCLSFB)system

The local solids holdup and local particle velocity in a Countercurrent Liquid-upward and Solids-downward Fluidized Bed(CCLSFB)were investigated in details using optical fiber probes with two different models in a Plexiglas column of 1.5 m in height and 7.0 cm in inner diameter.A new flow regime map including fluidized bed,transition,and flooding regimes was established.The axial solids holdup distribution is almost uniform at low liquid velocity and/or solids flowrate and becomes less uniform with higher solids holdup at the top of the column after the operating liquid velocity is reaching the flooding velocity.The radial solids holdup profile is also nearly flat with a slightly lower solids holdup in the near-wall region at low liquid velocity and solids flowrate but becomes nonuniform as the operating liquid velocity approaches the flooding velocity.Two equations were also proposed to correlate radial local solids holdups.The descending particle velocity in CCLSFB increases with the decrease of the liquid velocity and the increase of the solids flowrate.A generally uniform particle velocity distribution was found in the axial direction,as well as in the radial direction except for a small decrease near the wall.These results on the local solids flow structure would provide basic information and theoretical supports for the design and industrial application of CCLSFB.

Scale-up effect analysis and modeling of liquid-solid circulating fluidized bed risers using multigene genetic programming

Understanding scale-up effects on the hydrodynamics of a liquid-solid circulating fluidized bed(LSCFB)unit requires both experimental and theoretical analysis.We implement multigene genetic programming(MGGP)to investigate the solid holdup and distribution in three LSCFB systems with different heights.In addition to data obtained here,we also use a portion of data sets of LSCFB systems developed by Zheng(1999)and Liang et al.(1996).Model predictions are in good agreement with the experimental data in both radial and axial directions and at different normalized superficial liquid and solid velocities.The radial profiles of the solid holdup are approximately identical at a fixed average cross-sectional solid holdup for the three LSCFB systems studied.Statistical performance indicators including the mean absolute percentage error(6.19%)and correlation coefficient(0.985)are within an acceptable range.The results suggest that a MGGP modeling approach is suitable for predicting the solid holdup and distribution of a scaled-up LSCFB system.

Hydrodynamics in a new liquid–solid circulating conventional fluidized bed

A new type of liquid–solid fluidized bed,named circulating conventional fluidized bed(CCFB)which operates below particle terminal velocity was proposed and experimentally studied.The hydrodynamic behavior was systematically studied in a liquid–solid CCFB of 0.032 m I.D.and 4.5 m in height with five different types of particles.Liquid–solid fluidization with external particle circulation was experimentally realized below the particle terminal velocity.The axial distribution of local solids holdup was obtained and found to be fairly uniform in a wide range of liquid velocities and solids circulation rates.The average solids holdup is found to be significantly increased compared with conventional fluidization at similar conditions.The effect of particle properties and operating conditions on bed behavior was investigated as well.Results show that particles with higher terminal velocity have higher average solids holdup.

Effect of operating conditions on the performance of riser reactor for oxidative coupling of methane Author links open overlay panel

Oxidative coupling of methane(OCM)is characterized by high temperature and strong heat released.A riser reactor is designed for the OCM process to improve gas-solid contacting and heat transfer.This work illustrates the effect of hydrodynamic conditions and feed gas composition on the performance of an OCM riser reactor.The findings suggest that oxidative reactions predominantly occur within the acceleration zone and developed flow zone.Furthermore,at the low superficial gas velocity accompanied by a high solid circulating rate,the C_(2)yield initially increases in the acceleration zone,followed by a decrease in the developed flow zone and deceleration zone.The optimal operating conditions are a specific region corresponding to the solids holdup ranges from 0.015 to 0.02.Moreover,the C_(2)yield can be elevated using an appropriate feed mixture.

Characteristics of clusters in the jet influence zone of riser

By analyzing solid holdup signals in the jet influence zone of risers,the characteristics of clusters were studied.The solid holdups inside clusters and their distribution ranges were calculated under the cases of both upward and downward feed jets.Moreover,the effects of the jet velocity on the cluster characteristics were analyzed.The solid holdups inside clusters have higher values and wider distribution ranges in the upward feed injection zone than in the downward zone,implying that the number of individual particles in a cluster is unstable under the influence of upward jets.For the case of downward jets,cluster formations in the jet influence zone of risers can more easily maintain stability,offering a better mixing environment for jets and particles.As the jet velocity increases,the solid holdups inside clusters in the riser wall of the upward injection zone increase accordingly,while the distributions of solid holdups inside clusters have no significant changes if the feed jets are downward.This phenomenon confirms that improved operational flexibility can be obtained if downward jets are mixed with the prelift gas-solid flow in risers.

Hydrodynamic behavior of an internally circulating fluidized bed with tubular gas distributors

To better understand the hydrodynamic behavior of an internally circulating fluidized bed, solids holdup in the down-comer （Eso）, solids circulation rate （Gs） and gas bypassing fraction （from down-comer to riser y^R, and from riser to down-comer yRD） were experimentally studied. The effects of gas velocities in the riser and in the down-comer （UR and UD）, orifice diameter in the draft tube （dor）, and draft tube height （HR） were investigated. Experimental results showed that increase of gas velocities led to increase in Gs and yDR, and slight decrease in yeD. Larger orifice diameter on the draft tube led to higher 8sD, Gs and yDR, but had insignificant influence on YRD. with increasing draft tube height, both Gs and YDR first increased and then decreased, while yRD first decreased and then increased. Proposed correlations for predicting the hydrodynamic parameters agreed reasonably well with experimental values.

A modified wake model in bubble-induced three-phase inverse fluidized bed(BIFB)

A modified wake model was proposed for the newly developed bubble-induced three-phase inverse fluidized bed(BIFB),by combining the generalized wake model and the gas-perturbed liquid model.On the basis of the modified wake model,the solids and liquid holdups and the complete fluidization gas velocity in BIFB system have been successfully predicted with two established correlations.The predictions achieved very good agreements with the experimental data.