Hydrodynamics of a spouted bed with an impermeable draft tube for binary particle systems

A spouted bed with an impermeable draft tube was employed to obtain fundamental data of binary mixtures of glass beads for both the operating conditions and the design factors. These data were compared with those for the coarser particle system only. From this view point, minimum spouting velocity, pressure drop, hold-up of solid particles within a draft tube, upward gas flow rate within the annulus and solids circulation rate were determined by changing the total gas flow rate and mass fraction of finer particles as operating parameters and by changing distance of entrainment zone and draft tube diameter as geometric parameters.     

Grain Drying in a Paraboloid-Based Spouted Bed with and without Draft Tube

In this study, grain drying in a spherical-based spouted bed (SBSB), a cone-based spouted bed (CBSB), and a paraboloid-based spouted bed (PBSB) with and without draft tube was investigated. Spouted-bed bases with the same volume in different shapes—spherical, cone, and paraboloid—were used for the drying experiments to investigate the effect of the spouted-bed base shape on drying. The drying experiments were carried out with perforated and solid draft tubes. The effects of the distance between the gas inlet nozzle and the bottom of the draft tube (entrainment zone height) and the draft tube diameter as geometric parameters on drying were also investigated. It was seen that the geometrical shape of the contactor base influenced the drying time. The highest drying rate was achieved for drying in a paraboloid-based spouted bed. The results also showed that using a draft tube caused a significant increase in drying time. Because the perforated draft tube allows a higher gas flow rate through the annulus, it decreases the drying time when compared with the solid draft tube. Drying time decreased slightly with the decreasing height of the entrainment zone but draft tube diameter did not have a considerable effect on drying.     

基于颗粒流运动论的喷动流化床气固流动行为三维模拟

1 INTRODUCTION Spout-fluid beds have been of increasing interest in the petrochemical, chemical and metallurgic indus-tries since spout-fluid beds can reduce some of the limitations of both spouting and fluidization by su-perimposing the two type of systems[1―4]. In recent years, spout-fluid beds have become an alternative for gas/solid contactors in coal gasification. Spout-fluid bed coal gasifiers have been adopted for APFBC-CC (advanced pressurized fluidized bed combus-tion-combined…     

用正交配置技术估计固定床内的传热参数

用正交配置技术与非线性最小二乘估计固定床内的传热参数.用多通道巡回检测装置同时测定沿床层径向分布的40个正交配置点上的温度.系统地考察了床高、Re_p、d_t/d_p、气体入口温度、壁温对K_(er)和h_w的影响,并提出了以下的关联式k_(er)=0.199+0.015d_t/d_p+0.0020/1+14.15(d_p/d_t)~2Re_ph_w=33.4+4.23d_t/d_p+0.331/1+8.45(d_p/d_t)~2Re_p(5.5相似文献   

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.     

Effects of Draft Tubes on Particle Velocity Profiles in Spouted Beds

The vertical particle velocity profiles in a full‐column cylindrical conical spouted bed, with or without a draft tube, are measured using a fibre optic probe system. The profiles have different characteristics for a draft tube spouted bed (DTSB) than for a conventional spouted bed (CSB). The spout of a CSB consists of a central flow where particle velocities fit exponential distributions, and a boundary layer where particle velocities are nearly uniform. The spout of a DTSB has no boundary layer and its radial particle velocity profiles are approximately linear. The particle velocities in the spout of a DTSB increase when superficial gas velocity increases, draft tube diameter decreases, or when entrainment height decreases. A kinematic model has been used to simulate the granular flow in the annulus of a CSB and DTSB, and they are compared with the experiments. The particle velocities in the annulus of a DTSB are much lower than that of a CSB. Their radial profiles are also different with a CSB. The dependence of particle velocities in the annulus of a DTSB on superficial gas velocity, draft tube diameter, and entrainment height are also discussed. One concludes that the draft tube diameter and entrainment height are two key factors for the solid circulation rate of a DTSB.     

Modelling of gas interstitial velocity radial distribution over cross-section of a tube packed with granular catalyst bed; effects of granule shape and of lateral gas mixing

The previously presented [Zió?kowska, I., Zió?kowski, D., 1993. Modelling of gas interstitial velocity radial distribution over a cross-section of a tube packed with granular catalyst bed. Chemical Engineering Science 48, 3283-3292] mathematical model of gas flow field within a tube packed with a bed of spherical elements has been modernised. The modernisation consists in more rigorous treating of the radial gas dispersion within the bed voids in the fluid dynamic equations and in involving the formulae correlating the flow resistance in beds packed with various non-spherical elements (Raschig rings, cylinders) with their characteristics. The model solution relates the gas interstitial and superficial radial distributions with an empirical parameter—the local effective viscosity or corresponding Reynolds number, dependent on the geometric, aerodynamic and physical properties of the system which are usually known. The effective viscosity is associated with the kinetic energy dissipation due to the interface friction, the shear stresses in molecular and turbulent motion and the radial dispersion in the gas stream. Its knowledge makes possible the evaluation of the radial profiles of the gas interstitial velocity, as well as the dispersion coefficient, or corresponding Péclet number and the drag coefficient for individual element within the bed. The effective viscosity has been determined experimentally for beds of Raschig rings and cylinders by the method presented previously [Zió?kowska, I., Zió?kowski, D., 2001. Experimental analysis of isothermal gas flow field in tubes packed with spheres. Chemical Engineering and Processing 40, 221-233] and the results have been correlated with the system characteristics. Then the correlations have been used, according to the model, in evaluation of the radial distributions of the gas interstitial velocity, the radial dispersion coefficient and the drag coefficient for individual element within the bed.     

超细粉在导向管喷动床中的固体循环速率

Ultra-fine powders are difficult to be fluidized due to the strong particle to particle cohesiveness.However, the authors‘ experiments showed that the ultra-fine powder CaCO3 could be stably fluidized in a spouted bed with a draft tube. The effects of geometric and operating parameters on solid circulation rate of ultra-fine powder CaCO3 were investigated in a 120 mm diameter transparent semicircular spouted bed with a draft tube. Three draft tubes with different sizes were used in this study. It was found that the solids circulation rate was mainly dependent on the drawing rate of the gas jet from the nozzle, then on the gas transport capacity in the draft tube. With increasing gas feed rate, distance between the nozzle and the draft tube inlet and draft tube diameter, the solids circulation rate could be increased. Based on the jet theory, a quantitative correlation was proposed for predicting the solid circulation rate of ultra-fine powders in a spouted bed with a draft tube by taking into account the gas transport capacity in the draft tube.     

Solid circulation and gas bypassing in an internally circulating fluidized bed with an orifice-type draft Tube

The effects of orifice diameter in the draft tube, particle size, gas velocities and bed height on the circulation rate of solids and gas bypassing between the draft tube and annulus have been determined in an internally circulating fluidized bed (i.d., 0.3 m ; height, 2.5 m) with an orifice-type draft tube. A conical shape gas separator has been employed above the draft tube to facilitate the separation of gases from the two beds. The circulation rate of solids and the quantity of gas bypass from the annulus to draft tube show their minimums when the static bed height is around the bottom of the separator. The circulation rate of solids increases with an increase in orifice diameter in the draft tube. At fixed aeration to the annulus, gas bypassing from the draft tube to annulus sections decreases, whereas reverse gas bypassing from the annulus to the draft tube increases with increasing the inlet gas velocity to the draft tube. The obtained solids circulation rate has been correlated by a relationship developed for the cocurrent flow of gas and solid through the orifice.     

IMPELLER CLEARANCE EFFECT ON OFF BOTTOM PARTICLE SUSPENSION IN AGITATED VESSELS

The effect of impeller off bottom clearance on the power input requirement for off bottom solid suspension was examined for 45° pitched blade impellers in flat and round bottom fully baffled agitated vessels. Results showed a similar dependence as obtained for radial flow impellers when similar flow patterns were observed. The dependence appears to be independent of the impeller diameter to tank diameter ratio and vessel shape.     

SUPERCRITICAL FLUID DISTRIBUTION IN A PACKED COLUMN

Supercritical fluid distribution in a packed column was investigated by measuring the weight loss of a solid in coaxial cylinders where the fluid passed through. Two criteria to make the equilibrium distribution be uniform were suggested: (1) the ratio of the column diameter to the particle diameter is at least 10, and (2) the particle layers are larger than 40 when a point source is used. The radial diffusion model was found to be applicable to describe the flow in a packed column under supercritical operations. The spreading coefficients in this model were evaluated and were observed to be dependent on particle size and flow rate. A correlation is proposed for predicting the spreading coefficients which gives a deviation smaller than 3.0%.     

Numerical Analysis of the Impact of Particle Friction on Bed Voidage in Fixed‐Beds

The impact of the static friction coefficient, the initial particle orientation, and the particle height‐to‐diameter ratio on the bed voidage of random packings with a low tube‐to‐particle diameter ratio is investigated for spherical and cylindrical particles using the discrete element method. Based on the numerical results a correlation is proposed to predict the bed voidage as a function of the static friction coefficient and the tube‐to‐particle diameter ratio for spheres, equilateral and non‐equilateral cylinders. This correlation is extended for the use of hollow cylinders.     

节能型立式热虹吸再沸器的模拟

为了提高再沸器的用能效率，特将固体颗粒引入立式热虹吸再沸器的管程形成气－液－固多相流系统，以强化传热，并提出了该节能型立式热虹吸再沸器的数学模型。模拟计算表明，流态化技术的引入能够强化再沸器的传热，减小换热面积或系统的有效能损失。颗粒的密度对传热的影响较大，其增加可使管内传热系数增大；随着固含率的增加，管内传热系数增大；随着粒径的增加，管内传热 数先增加后减小，有一极大值，颗粒密度的增加可使该极值增大并出现得更早。     

Effects of geometric and operating parameters and feed characters on the motion of solid particles in hydrocyclones

The effects of geometric and operating parameters and feed characters on the motion of solid particles in hydrocyclones were experimentally investigated by using a new type of laser surveying instrument named particle dynamics analyzer. The absolute radial velocity of solid particles decreases with increasing the positional radius, and the axial distribution curves of the particle radial velocity are parabolic. The particle radial velocity increases with increasing the inlet pressure or with increasing the diameter of the underflow pipe. When the particle density or the particle size increases, the absolute radial velocity of the solid particles decreases. The particle radial velocity also decreases with increasing the feed particle concentration. The axial distribution curves of the particle axial velocity are also parabolic. The axial velocity in the inner helical flow increases with the increase of the flow rate of overflow; while that in the lower positions in the outer helical flow increases with increasing the flow rate of underflow.     

导向管直径对喷动床流动特性影响的计算颗粒流体力学数值模拟

导向管喷动床是较为常见的一种喷动床改进床型,通过阻断喷动区与环隙区气固接触来提高颗粒循环的规律性与稳定性。本文采用计算颗粒流体力学（CPFD）方法对于直径150mm的柱锥式导向管喷动床进行了数值模拟研究,考察了导向管直径对于喷动床内颗粒流动特性的影响,从环隙区死区分布、颗粒速度分布、固体循环量等方面分析了具有不同直径导向管喷动床的运行状态。结果表明,加入导向管在减少床内死区的同时也降低了运行时的固体循环量,对于本次采用的喷动床结构尺寸与运行参数,只有在导向管直径为40～60mm时才能保证床内具有良好喷动状态,综合考虑各因素,选用直径50～55mm的导向管最为合适。对于具有类似结构与运行条件的柱锥喷动床,导向管直径可考虑选为无导向管运行时喷动区直径的1.2～1.375倍。     

用于细颗粒分离的水力旋流器的压力特性研究

对用于细颗粒分离的水力旋流器的压力特性 (压力降及压降比 )与流量、分流比、旋数、溢流口和底流口直径及气液比等主要参数之间的关系进行了深入的研究与分析。研究发现 ,水力旋流器内部压力降分别随流量、分流比、旋数及气液比的提高而加大 ,压降比则分别随流量、分流比、旋数的提高而降低。随着溢流口直径的加大 ,水力旋流器的溢流压力降减小 ,而压降比也随之降低 ;随着底流口直径的加大 ,底流压力降减小 ,压降比随之升高。分析可知 ,减少旋流器能耗的有效方法是降低旋数 ,或者减少混合介质中的气液比     

填充床层热之传导——床层之温度分布

作者用低导热系数(包括玻璃、磁)的球体、圆柱体、环柱体与高导热系数(包括铜、铁的球体,圆柱体为填充物,以空气为传热介质,使其在管径为81毫米之填充床层内冷却,改变流体流量,床层高度及填充物大小,通过试验测出在不同的条件下床层的径向温度分布,并应用积分法、直流电模拟计算法及图解法求得床层的有效导热系数及管壁薄膜传热系数.在试验范围:低导热系数填充物D_P/D_t自0.074—0.254;高导热系数填充物D_p/D_t自0.12—0.2,L/D_t自5—15,Re汇数自130—1400,即直线速度自0.5—1.6公尺/分,若以床层进出口平均温度之数学平均值为定性温度,则床层之有效导热系数及管壁薄膜传热系数可分别归纳于下式:低导热系数填充物:K_e=0.182(D_t/D_p)~(0.45)Re~(0.75),h_w=65e~(-4)(D_p/D_t)(K/D_t)((D_t/L))~(0.2)Re~(0.4)高导热系数填充物:K_e=0.3k(D_t/D_p)~(0.6)Re~(0.72),h_w=5.1(K/D_t)(D_t/D_p)~(0.8)(D_t/L)~(0.1)Re~(0.46)填充物形状对K_e及h_w的影响,仅需将D_p用 D’_p代替,同时把K_e式中之常数0182及03各改为0.22及0.38即可.直流电模拟计算法系利用电压表示温度,电阻表示传热阻力,电流表示热的流动,是简单的模拟计算机的一种,它在近代工程上的应用日渐广泛,有了传热数据应用它来求床层的温度分布异常方便.     

Cold flow studies of a slurry transport reactor-hydrocyclon system

The fluid dynamics of a new reaction system composed of a slurry transport reactor-hydrocyclon were studied using two column diameters at different gas and liquid linear velocities. The cold models used a gas collector in the top that allowed measurement of the gas disengaged by radial zones and a conductimetric probe that measured the frequency of the bubbles exiting from the top of the reactor. Gas and liquid hold-ups were determined. Liquid and solid tracers were also employed to determine the resident time distribution (RTD), global residence time, and the recycle of slurry near the wall. The results show the effect of sparger and disengaging design, as well as the effect of gas and liquid flow rate on the radial and axial gas hold-up profiles and on the recycle of slurry by the wall. This recycle is similar to those observed with a draft tube. No significant effect of column diameter was observed. A smooth circulation of slurry and solid was achieved through a mechanical optimization of the inlet and outlet of the reactor. It was demonstrated that the RTD of the system can be simulated using a set of continuous stirred tank reactors and plug flow reactor in a recycle (three parameters). Empirical equations are proposed for predicting the hold-up and the three parameters needed by the model. The similarity to a spouted bed reactor is discussed.     

Modeling Oxidation of Soot Particles Within a Laminar Aerosol Flow Reactor Using Computational Fluid Dynamics

This work examines the measurement of surface specific soot oxidation rates with the High Temperature Oxidation-Tandem Differential Mobility Analyzer (HTO-TDMA) method. The Computational Fluid Dynamics package CFD-ACE⁺ is used to understand particle flow, oxidation and size dependent particle losses in the laminar aerosol flow reactor using an Eulerian-Lagrangian framework. Decrease of DMA selected mono-disperse particle size distribution due to oxidation within the aerosol tube is modeled using fitted kinetic soot oxidation parameters. The effects of Brownian diffusion and thermophoresis on particle flow and loss to the reactor walls are evaluated. The position of peak particle diameter, which is used as an indicator to determine oxidation rate, is found to be independent of diffusion, thermophoresis and secondary flow effects, thus validating its use in deriving kinetic soot oxidation parameters. Diffusion does not affect the evolution of particle size distribution within the reactor. However, thermophoresis is found to be the dominant mechanism influencing both shape of particle size distribution and particle loss to the walls of the aerosol reactor. Simulations show reduced effects of secondary recirculating flows on the particle flow trajectories in a vertical furnace as compared to horizontal furnace orientation. This work highlights the importance of making accurate measurements of temperature within the modeling domain. Since gas temperature within the flow tube could not be measured with high radial resolution using radiation shielded thermocouple, the derived soot oxidation rate may be uncertain by a factor of 2. Importantly, CFD simulations suggest that a distribution of temperature and size-dependent particle reactivities may be present in the reactor, requiring further theoretical and experimental investigation.     

Three‐phase airlift internal loop reactor: correlations for predicting the main fluid dynamic parameters

Three‐phase airlift loop reactors have many industrial applications. Examples include applications in the chemical, metallurgical, biochemical and minerals‐processing industries; fluid dynamics in such systems is a critical factor affecting efficiency. The experimental work was carried out at a pilot‐plant scale in a tank with a cylindrical shape and conical bottom (height 1.25 m; diameter 0.42 m), water and air were used as liquid and gas phases, and for different solid phases, different loads of glass spheres (diameters: 0.25 and 1 mm; density 2.6 g cm^?3), and polystyrene cylinders (diameter and length 3 mm; density 1.0 g cm^?3) were introduced. Air was injected through the bottom of the tank by means of 12 nozzles (diameter 1 mm each). An internal draft tube riser was tested on different configurations as its diameter was varied (44, 82, 125 and 240 mm) as was its height (1050 and 630 mm). Corresponding liquid velocities in the adjacent annular downcomer were determined by a thermal tracer technique, solid holdups were determined by conductivity methods, riser overall gas holdups were deduced from the liquid level, and riser gas holdup from manometer readings. Several adjusted correlations were considered in a method to predict the main fluid dynamic parameters (solids and gas holdups, and superficial liquid velocities). Copyright © 2005 Society of Chemical Industry