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.     

Desorption of VOC from polymer adsorbent in multistage fluidized bed

The desorption process of volatile organic compounds(VOC) from a polymer adsorbent in counter-current multistage fluidized bed was studied. And a mathematical model considering the mass transfer dynamics was developed, which was validated from experiment data. The gaseous ethyl acetate mass transfer was discussed, and the limiting step is the intraparticle mass transfer of the desorption process. The value of intraparticle mass transfer coefficient is between 1.85 × 10~(-6) and 1.38 × 10~(-5) m·s~(-1) under temperature of 100–160 °C. Experiments under different operating conditions were carried out. The effects of operating conditions such as gas–solid flow ratio,gas inlet temperature and total stage number of multistage fluidized bed on outlet VOCs concentration and desorption efficiency were discussed. The maximum outlet VOCs concentration and corresponding desorption efficiency of the multistage fluidized bed desorber was calculated under different gas inlet temperatures and total stage numbers.     

Fundamental studies on the hydrodynamics and mixing of gas and solid in a downer reactor

The effect of flow direction on hydrodynamics and mixing in the upflow and downflowcirculating fluidized beds is discussed in details.Similar profiles of gas and solids velocities andsolids concentration are found in both risers and downers.When the flow is in the direction ofgravity(downer),the radial profiles of gas and particle velocity are more uniform than that inthe riser,the solids mixing is very small and the flow pattern approaches plug flow,while theflow is against gravity(riser),the solids backmixing significantly increase and the flow pattern isfar from plug flow.Among many of factors the flow direction has the largest influence onhydrodynamics and axial mixing of gas and solids.     

GAS-SOLIDS FLOW PATTERNS IN A CONCURRENT DOWNFLOW FAST FLUIDIZED BED(CDFFB)

Radial profiles of solid concentration and velocity for concurrent downward gas-solid suspension in a140mm inside diameter fast fluidized bed were investigated.The influence of gas velocity,solid circulating rateand axial position on radial profiles of solid concentration and particle velocity has been examined.It hasbeen found that an annular region of high solid concentration exists at r/R=0.94.At both the center and wallregion,the solid concentration and the particle velocities are relatively low.The shape of radial solid con-centration profile curves is mainly dependent on the cross-section averaged voidage,and the shape of radialparticle velocity profile is mainly affected by the gas velocity and cross-section averaged voidage.Based on the radial profiles of solid concentration and particle velocity,the solid mass flux profile and thenonuniformity of solids flow are discussed in this paper.It is shown that solids flow in CDFFB is much moreuniform than that in UFFB.     

流态化固体颗粒对载气蒸发传热的强化

Heat transfer characteristics are studied for gas carrying evaporation with fluidized solid particles in a vertical rectangular conduit. Experimental results show that heat transfer of gas carrying evaporation is enhanced and the superheat of liquid in contact with heating surface lowers remarkably by introducing solid particles. Nucleate boiling on the heating surface is suppressed to a considerable degree. The mechanism of heat transfer enhancement by fluidized solid particles is analyzed with the consideration of collisions of solid particles with the boiling vapor bubbles.     

Experimental research on steady-state operation characteristics of gas–solid flow in a 15.5 m dual circulating fluidized bed system

Gas-solid hydrodynamic steady-state operation is the operating basis in a chemical looping dual-reactor system. This study reported the experimental results on the steady-state operation characteristics of gas–solid flow in a 15.5 m high dual circulating fluidized bed(CFB) cold test system. The effects of superficial gas velocity, static bed material height and solid returning modes on the steady-state operation characteristics between the two CFBs were investigated. Results suggest that the solid distributions in the dual CFB test system was mainly determined by the superficial gas velocity and larger solid inventory may help to improve the solid distributions. Besides, cross-returning mode coupled with self-returning is good for steady-state running in the dual-reactor test system.     

Hydrodynamics of three-phase fluidization of homogeneous ternary mixture in a conical conduit——Experimental and statistical analysis

Hydrodynamics of conical fluidized bed differ from that of columnar beds by the fact that a velocity gradient exists along the axial direction of the bed.The gas–liquid–solid fluidized bed has emerged in recent years as one of the most promising devices for three-phase operations.Such a device is of considerable industrial importance as evident from its wide applications in chemical,refining,petrochemical,biochemical processing,pharmaceutical and food industries.To explore this,a series of experiments have been carried out for homogeneous well-mixed ternary mixtures of dolomite of varying compositions in a three-phase conical fluidized bed.The hydrodynamic characteristics determined included the bed pressure drop,bed fluctuation and bed expansion ratios.The single and combined effects of operating parameters such as superficial gas velocity,superficial liquid velocity,initial static bed height,average particle size and cone angle on the responses have been analyzed using response surface methodology(RSM).A 25 full factorial central composite experimental design has been employed.Analysis of variance(ANOVA) showed a high coefficient of determination value and satisfactory prediction second-order regression models have been derived.Experimental values of bed pressure drop,bed fluctuation and bed expansion ratios have been found to agree well with the developed correlations.     

Review on the effect of heat exchanger tubes on flow behavior and heat/mass transfer of the bubble/slurry reactors

Bubble/Slurry bubble column reactors(BCR/SBCR) are intensively used as multiphase reactors for a wide range of application in the chemical, biochemical and petrochemical industries. Most of these applications involve complicate gas–liquid/gas–liquid–solid flow behavior and exothermic process, thus it is necessary to equip the BCR/SBCR with heat exchanger tubes to remove the heat and govern the performance of the reactor. Amounts of experimental and numerical studies have been carried out to describe the phenomena taking place in BCR/SBCRs with heat exchanger tubes. Unfortunately, little effort has been put on reviewing the experiments and simulations for examining the effect of internals on the performance and hydrodynamics of BCR/SBCR. The objective of this work is to give a state-of-the-art review of the literature on the effects of heat exchanger tubes with different types and configurations on flow behavior and heat/mass transfer, then provide adequate information and scientific basis for the design and the development of heat exchanger tubes in BCR/SBCR, ultimately provide reasonable suggestions for better comprehend the performance of different heat exchanger tubes on hydrodynamics.     

流化床中旋风下料管的物理模型

In most industrial fluidization units, two- or three-stage cyclone systems are used to clean the product gases. To return the solids to the bed, these cyclones are fitted with diplegs. By pass of gas from the bed through the dipleg is partially overcome by the back pressure build-up in the dipleg and by adding a trickle valve at the bottom of the dipleg. Diplegs of primary cyclones, operating at a high solid loading behave differently from diplegs of secondary and tertiary cyclones which operate at low solid loading. Both types have been investigated by pressure drop measurements, visual observation and by measurements of the air flow rate flowing up the riser. The primary dipleg was also studied using electrical capacitance tomography. The results are reported hereafter and will give a first indication towards the right design of the dipleg and the selection of the trickle valve. The influence of gas flow in the dipleg on the conversion in a catalytic fluidized bed reactor is found to be negligible.     

加长循环流化床提升管中加速段长度的实验研究

Systematic experimental work was conducted to investigate the solid acceleration length in a 16 m 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 gasvelocity. 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.     

气-固循环流化床的流动特性

循环流化床是一种新型高效无气泡气-固反应器,应用广泛。文章首先介绍了气-固循环流化床的特点,总结了循环流化床上部区域的流动特性,并讨论了循环流化床底部区域的流动特性,最后展望了循环流化床流动特性研究的发展趋势。     

EFFECTS OF SECONDARY AIR INJECTION ON GAS-SOLID FLOW BEHAVIOR IN CIRCULATING FLUIDIZED BEDS

Effects of secondary air injection on the hydrodynamics such as solid holdup and gas-solid flow behavior were investigated in a circulating fluidized bed. The gas velocity in the riser, the ratio of secondary air velocity to that of primary air, and the solid circulating rate were chosen as operating variables. Fluid cracking catalyst(FCC) with a density of 1840 kg/m³ and a mean diameter of 74 um was employed as the solid phase. The secondary air was fed to the riser radially or tangentially at the wall of the column. Pressure drop fluctuations in the riser were measured and analyzed by adopting the stochastic method to characterize the effects of secondary air injection on the gas-solid flow behavior in the bed.

It has been found that the injection of secondary air into the riser can increase the solid holdup in the riser considerably, and that the tangential injection of secondary air is more effective for the increasing the solid holdup than the radial injection. However, the gas-solid flow behavior has been found to become less persistent with the injection of secondary air; the resultant flow behavior is more complex when the air is injected tangentially than radially. The solid holdups in the primary as well as secondary zones of the riser have been well correlated in terms of not only operating variables but also fractal dimension of the pressure fluctuations.     

EFFECTS OF SECONDARY AIR INJECTION ON GAS-SOLID FLOW BEHAVIOR IN CIRCULATING FLUIDIZED BEDS

Effects of secondary air injection on the hydrodynamics such as solid holdup and gas-solid flow behavior were investigated in a circulating fluidized bed. The gas velocity in the riser, the ratio of secondary air velocity to that of primary air, and the solid circulating rate were chosen as operating variables. Fluid cracking catalyst(FCC) with a density of 1840 kg/m³ and a mean diameter of 74 um was employed as the solid phase. The secondary air was fed to the riser radially or tangentially at the wall of the column. Pressure drop fluctuations in the riser were measured and analyzed by adopting the stochastic method to characterize the effects of secondary air injection on the gas-solid flow behavior in the bed.

It has been found that the injection of secondary air into the riser can increase the solid holdup in the riser considerably, and that the tangential injection of secondary air is more effective for the increasing the solid holdup than the radial injection. However, the gas-solid flow behavior has been found to become less persistent with the injection of secondary air; the resultant flow behavior is more complex when the air is injected tangentially than radially. The solid holdups in the primary as well as secondary zones of the riser have been well correlated in terms of not only operating variables but also fractal dimension of the pressure fluctuations.     

非均匀气固两相系统中多尺度传质模型

建立了适用于气固循环流化床的多尺度传质模型 .从过程与尺度的角度出发 ,将非均匀气固两相流中的传质过程分解为静态与动态的过程 ,并将前者分解为稀相内、密相内以及稀密相间 3个尺度下的传质 ;在用多尺度能量最小 (EMMS)模型求解已知物系性质和操作条件的非均匀气固两相流体动力学参数的基础上 ,借助于前人的研究结果 ,利用相对滑移速度、空隙率等参数求解传质系数 ,求得轴向的浓度场分布 ,并讨论非均匀两相流动结构对传质效率的影响     

MULTI-SCALE MASS TRANSFER MODEL FOR GAS-SOLID TWO-PHASE FLOW

This article is devoted to analyzing the mass transfer in heterogeneous gas-solid flow by means of structure and process decomposition. A multi-scale mass transfer model was developed on the basis of the hydrodynamics calculated from the so-called EMMS model. This resulted in the predictions of the steady-state two-dimensional concentration distributions of sublimated substance as well as total mass transfer coefficient for circular concurrent gas-solid contactors. The predictions were validated by experimentally measured (via an on-line HP GC-MS system) axial concentration distributions of sublimated naphthalene in air in a circulating fluidized bed riser 3.0 m in height and 72 mm in diameter. The experiment also obtained mass transfer coefficients comparable to theoretical predictions under conditions with various gas velocities, solid circulation rates, particle sizes, and active material fractions in the particles. Both the theoretical and experimental results demonstrated that the heterogeneous flow structure prevailing in the concurrent gas-solid flow greatly influenced the flow's mass transfer.     

MULTI-SCALE MASS TRANSFER MODEL FOR GAS-SOLID TWO-PHASE FLOW

This article is devoted to analyzing the mass transfer in heterogeneous gas-solid flow by means of structure and process decomposition. A multi-scale mass transfer model was developed on the basis of the hydrodynamics calculated from the so-called EMMS model. This resulted in the predictions of the steady-state two-dimensional concentration distributions of sublimated substance as well as total mass transfer coefficient for circular concurrent gas-solid contactors. The predictions were validated by experimentally measured (via an on-line HP GC-MS system) axial concentration distributions of sublimated naphthalene in air in a circulating fluidized bed riser 3.0 m in height and 72 mm in diameter. The experiment also obtained mass transfer coefficients comparable to theoretical predictions under conditions with various gas velocities, solid circulation rates, particle sizes, and active material fractions in the particles. Both the theoretical and experimental results demonstrated that the heterogeneous flow structure prevailing in the concurrent gas-solid flow greatly influenced the flow's mass transfer.     

Drying Kinetics in the Riser of Circulating Fluidized Bed with Internals

Solid particles were dried in the riser of circulating fluidized bed with internals to study the drying kinetics. Experiments were conducted in a circulating fluidized bed, having perforated plates as internals covering wide range in the operating parameters. The effects of various operating parameters, i.e., initial moisture content, temperature, and flow rate, of the heating medium and solid circulation rate on the rate of drying have been critically examined. It has been observed from the present investigation that the presence of internals enhances the solids holdup in the riser of circulating fluidized bed. The drying efficiency of a circulating fluidized bed with internals has been compared with the drying performance of a circulating fluidized bed without internals under the same operating conditions.     

Drying Kinetics in the Riser of Circulating Fluidized Bed with Internals

Solid particles were dried in the riser of circulating fluidized bed with internals to study the drying kinetics. Experiments were conducted in a circulating fluidized bed, having perforated plates as internals covering wide range in the operating parameters. The effects of various operating parameters, i.e., initial moisture content, temperature, and flow rate, of the heating medium and solid circulation rate on the rate of drying have been critically examined. It has been observed from the present investigation that the presence of internals enhances the solids holdup in the riser of circulating fluidized bed. The drying efficiency of a circulating fluidized bed with internals has been compared with the drying performance of a circulating fluidized bed without internals under the same operating conditions.     

The hydrodynamic behavior of the liquid-solid circulating fluidized bed ion exchange system for cesium removal

In this work, experimental studies on the hydrodynamics have been carried out in a cold model liquid-solid circulating fluidized bed (LSCFB) ion exchange system where the riser is operated in the circulating fluidization regime and the downcomer in the state of slow-moving packed bed, with the aim of removing cesium from high radioactive liquid waste. Two circulating fluidization zones, the initial and the fully developed, have been observed. The solids circulation rate can be controlled merely by the primary liquid flow if the auxiliary liquid flow is set to zero and the injection tap is located at the middle of the intersection of the riser and the solid-return pipe. Therefore, it is possible to minimize the number of the liquid feed assemblies (pumps, valves, flow meters, etc.), and this is crucial to the treatment of high radioactive waste. The liquid inevitably leaks through the solid-return pipe in most cases because of the “loosening effect” of the liquid feed flow, and the direction and magnitude of the leakage vary with the operating conditions. The system geometry, including the injection tap location and the riser outlet location, plays a very important role in the hydrodynamics of the LSCFB system.     

半干法循环流化床脱硫反应器内构件研究进展

常规脱硫反应器存在着床内气固流动不均匀、脱硫剂利用效率低等问题。安装内构件可以破碎气泡和颗粒团聚,改善流化质量,强化脱硫反应器内的气固传质和反应过程,提高脱硫效率。本文介绍了近年来半干法循环流化床脱硫反应器内构件的研究现状,主要介绍了惯性式内构件、钝体式内构件、孔板式内构件、复合型内构件;阐述了各种内构件的形式及特点,其中惯性式内构件能够强化气固分离,但不能优化流场;钝体式内构件能够增强反应器内气固湍动程度,但对轴向混合影响不大;孔板式内构件能够均布流场,但易堵塞;复合内构件能够改善流化性能,强化气固接触,但床层压降较高。通过对装有不同内构件的循环流化床脱硫反应器中气固两相流动特点的分析,指明了各类内构件对流化床内气固流动的作用原理及优缺点,综合床层压降及颗粒浓度分布,并根据现有流化床脱硫反应器内构件特点提出了促进气固高效接触、降低床层压降等新型内构件的开发方向。