Influence of conical nozzle attachments on horizontal spray dispersion in a fluidized bed

In this paper, we investigate numerically the influence of conical nozzle attachments on spray dispersion in a fluidized bed. We utilize an Eulerian approach to simulate flow through the nozzle and subsequent interactions between droplets and solid particles. We use process parameters and material properties relevant to the fluid coking process – bitumen upgrading by thermal cracking – by approximating available experimental data. After considering several attachments with varying expansion angles, we discovered that the best liquid distribution is obtained with the 40° angle attachment because of the jet instabilities facilitated by such a shape of an attachment.     

Effect of injection-nozzle operating parameters on the interaction between a gas-liquid jet and a gas-solid fluidized bed

The objective of the present study was to provide insight into the effect of operating conditions on the performance of gas-assisted nozzles injecting liquid into gas-solid fluidized beds. Acquisition of such knowledge is relevant to many industrial applications where liquid injections into fluidized beds of solid particles are performed via spray nozzles. In the fluid coking process, for example, product yields and reactor operability strongly benefit from a rapid and uniform distribution of the liquid feed on fluidized solid particles, which, in turn, is greatly affected by the performance of the liquid-injection system used.A novel experimental technique was employed to investigate the effect of varying the air-to-liquid ratio (ALR), the liquid mass flow rate, and the nozzle size on the contact efficiency of injected liquid on fluidized bed particles. Increasing the ALR or the liquid flow rate increased the nozzle spraying efficiency. On the contrary, increasing the nozzle size while keeping the gas and the liquid flow rates constant, and hence decreasing the pressure at the nozzle tip, lowered the liquid-solid contact efficiency.The effect of increasing the ALR on the liquid-solid contact resulting from nozzle-injections into the fluid bed, was correlated to both the nozzle atomization performance, as determined by open-air tests using a laser-photocell equipment, and the solids entrainment into the gas-liquid jet, as predicted by a model.     

Prediction of heat transfer to liquid-solid fluidized beds

Heat transfer coefficients to a liquid-solid fluidized bed in a cylindrical tube have been measured using water as liquid phase and three types of cylindrical steel particles, as well as glass, nickel, copper and lead spheres of different sizes as solid phase. The independent varaibles included heat flux, liquid velocity and particle physical properties. The experimental results as well as a data bank containing a large number of measured heat transfer coefficients for solid-liquid fluidization over a wide range of operational parameters have been compared with the predictions of most published correlations. A model for the prediction of heat transfer coefficients is proposed which predicts the present experimental data and the data of other investigators with good accuracy.     

喷嘴对液固外循环流化床内含固体积分数的影响

设计了实现液固外循环流化床中颗粒正常循环的关键部件——喷嘴,利用CCD图像采集分析系统,获得了其换热管束内固体颗粒的分布情况,考察了液体流量、口径比、喷嘴安装位置、颗粒直径、颗粒初始加入量以及液体黏度对液固外循环流化床换热器管束内含固体积分数的影响。结果表明:口径比及喷嘴安装位置对提高管束内的含固体积分数都存在一个最佳值,颗粒直径、颗粒初始加入量以及液体黏度对管束内含固体积分数也具有较大的影响,同时还通过对实验数据的综合分析,得到了喷嘴在最佳安装位置下液固外循环流化床内含固体积分数的经验关联式。     

Three-dimensional numerical study of heat and mass transfer in fluidized beds with spray nozzle

The numerical computations of temperature and concentration distributions inside a fluidized bed with spray injection in three-dimensions are presented. A continuum model, based on rigorous mass and energy balance equations developed from Nagaiah et al., is used for the three-dimensional simulations. The three-dimensional model equation for nozzle spray is reformulated in comparison to Heinrich. For solving the non-linear partial differential equations with boundary conditions a finite element method is used for space discretization and an implicit Euler method is used for time discretization.The time-dependent behavior of the air humidity, air temperature, degree of wetting, liquid film temperature and particle temperature is presented using two different sets of experimental data. The presented numerical results are validated with the experimental results. Finally, the parallel numerical results are presented using the domain decomposition methods.     

Solids entrainment into gas, liquid, and gas-liquid spray jets in fluidized beds

The injection of liquid into a fluidized bed is a crucial step in many processes such as fluid coking, fluid catalytic cracking, or gas-phase polymerization, whose performance greatly depends on good and rapid contact between the injected liquid and the fluidized particles. The liquid spray, created by two-phase (gas-liquid) nozzles, forms a jet, i.e. a gas-rich cavity within the fluidized bed. Past studies have shown that good liquid-solid contact requires a large entrainment rate of particles into the jet, followed by intensive mixing of liquid droplets and entrained particles within the jet. The objective of this study is the experimental measurement of solids entrainment into spray jets. The specific application of interest is the enhancement of solids entrainment under conditions relevant to the fluid coking process.A novel and accurate experimental technique has been developed to measure the solids entrainment from a fluidized bed into two-phase gas-liquid jets, gas jets and liquid jets. The effects of operating conditions of the nozzle (sonic versus subsonic) and of the fluidized bed on the solids entrainment have been investigated. The differences between the mechanisms of solids entrainment for two-phase gas-liquid, gas and liquid jets have been analyzed.This experimental tool has been applied to the design and testing of a mixing chamber consisting of a cylindrical tube placed at a certain distance downstream of the nozzle tip, resulting in a confined, turbulent jet with enhanced liquid-solid mixing properties.     

Using spray momentum flux measurements to understand the influence of diesel nozzle geometry on spray characteristics

Nowadays Diesel nozzle geometry is a major issue in order to fulfil new emission regulations due to the influence on internal flow, cavitation phenomenon, spray characteristics and therefore atomization behavior, which are very important for engines performance and pollutant formation.The aim of this article is to study the effect of cavitation on Diesel spray behavior. For this purpose, two bi-orifices nozzle geometries, a cylindrical nozzle and a convergent one, are characterized by means of two fundamental spray parameters: mass flux and momentum flux. Five injection pressure values and five discharge pressure levels have been measured in order to change the cavitation regime inside the nozzle flow. It is known from the literature that cavitation brings about a mass flux choke, but there are few studies that investigate its effects on momentum and outlet velocity in real geometries. The key point of this study is the measurement of spray momentum in order to explain the effects of nozzle geometry on spray behavior.     

流化床内颗粒混合研究

分析了流化床内颗粒混合机理，综合了床内水平方向和垂直方向上颗粒混合，将床内颗粒混合过程分成两部分：一是向上运动的尾迹相和向下运动的乳化相之间的对流交换，二是乳化相内横向扩散。建立了二维的对流扩散模型，数值结果和实验数据吻合。     

Effect of cross flow on the spray characteristics of an industrial feed nozzle

The spray characteristics of a scaled-down version of an industrial feed nozzle are studied in the presence of a cross flow. Aerated liquid nitrogen is injected through the nozzle to generate the spray. The aeration rate is low and held constant, while two different liquid flow rates are used to produce the spray. A nonuniform wind profile is chosen to represent the cross flow condition. The droplet diameter and velocity measurements are acquired using a phase-Doppler particle analyzer. The results of the present study indicate that the spray momentum flux determines the extent of the jet bending. The droplets are accelerated significantly in the initial jet region as a result of flashing. However, further downstream of the nozzle, the vaporization of the droplets is considered to be negligible. The size-velocity correlation changes significantly for the case where the spray is shifted due to the cross flow.     

Experimental investigation on atomizing characteristics of coal-water paste for pressurized fluidized bed

According to the properties of coal-water paste (CWP), a series of experimental nozzles were developed for a detailed study of the effect of nozzle design on CWPs behavior in a pressurized fluidized bed. The particle size and its distribution of atomized CWP using several types of nozzles are measured by PIV instrument and newly developed image processing software, and the effect of a number of factors on the atomizing properties are analyzed. The atomizing experimental results show that the atomization quality of CWP was found by correlating with the characteristics of CWP, gas to CWP mass ratio, atomization air motion, coal particle size distribution and the structure of nozzle etc. Mass median diameter (MMD) of atomized particles decreases with the increase of gas to CWP mass ratio. The proper type of nozzle type and proper parameters for engineering application are suggested.     

Modeling the mixing of a gas-liquid spray jet injected in a gas-solid fluidized bed: The effect of the draft tube

In several important industrial processes such as fluid coking, fluid catalytic cracking and gas-phase polymerization, a gas-liquid spray jet is injected into a fluidized bed. Recent findings [Chan et al., 2004. US Patent, US 2004065590] have shown that for the fluid coking operation, locating a cylindrical draft tube near the exit of the injector can enhance the contacting efficiency of the droplets and the particles.The main objective of this paper was to model the liquid-solids mixing in the draft tube. A two-dimensional model that characterizes the radial mixing by means of an eddy diffusivity coefficient is presented. The model predictions are validated by experimentally measuring the liquid-solids mixing by a technique based on temperature measurements in the spray region. The simulation results for a commercial fluid coking nozzle confirm the potential benefits of the mixing chamber. Moreover, the model allows ascertaining the effect of some important parameters such as nozzle size, gas and liquid properties, and the air-to-liquid mass ratio to the nozzle.     

Impact of a draft tube on industrial‐scale Fluid Coker™ Spray Jets in fluidized beds

In fluidized bed reactors such as Fluid Cokers?, liquid injections are used. Good contact between liquid and bed solids is required to maximize product yields and quality, and gas‐atomized nozzles are, therefore, used in all these processes. The spray nozzle technology is known to affect the liquid distribution. Therefore, the objective of this study is to assess the effect on liquid distribution of a draft tube located downstream of the spray nozzle, inside the fluidized bed. Experiments were conducted at a relevant scale, using a commercial‐scale nozzle with a liquid flow rate of about 100 L/min in a large‐scale pilot fluid bed containing about 7 tonnes of silica sand. Liquid injected into a fluidized bed either forms liquid‐solid agglomerates or free moisture, consisting of individual particles coated with a thin layer of liquid. Several electrodes were used to map the free moisture distribution throughout the bed. A draft tube greatly improves the contact efficiency throughout the bed. It also increases the penetration of the gas‐liquid jet formed by spray inside the fluidized bed.     

Proper-orthogonal decomposition of spatio-temporal patterns in fluidized beds

Numerical simulations of the hydrodynamics of a fluidized bed are carried out to investigate the complex interaction between the gas and the solid particles, and to explore the utility of a reduced-order model based on the proper orthogonal decomposition (POD). The behavior of a fluidized bed is modeled using a “two-fluid” theory, which involves conservation of mass, momentum, energy and species equations for the two interpenetrating continua. These equations are solved using a numerical algorithm that employs a conservative discretization scheme with mixed implicit and explicit formulations. We conducted simulations of gas-solid interaction in a narrow (two-dimensional) bed filled with sand particles which was uniformly fluidized at minimum fluidization but with additional air flow through a central nozzle. Aided by the proper orthogonal decomposition, spatial dominant features are identified and separated from the spatio-temporal dynamics of the simulations. The most dynamic region of the gas-solid interaction is confined to the central channel caused by the jet. The flow within this structure is successfully captured by a few POD eigenfunctions. Phase-space plots further indicate the existence of low-dimensional dynamics within the central channel. This conclusion supports the validity of a reduced-order model for fluidized beds, which can then be constructed by projecting the governing equations onto the POD modes, as it is commonly done in the Galerkin method.     

Mass transfer at the confining wall of helically coiled circular tubes with gas–liquid flow and fluidized beds

Experiments were conducted to investigate the effect of various dynamic and geometric parameters on mass transfer coefficients in two-phase helically coiled flow systems. Computation of mass transfer coefficients was facilitated by the measurement of limiting current at the electrodes fixed flush with the inner surface of the tube wall. Two flow systems were chosen: a two-phase liquid solid fluidized bed and a two-phase gas–liquid up flow. An equimolar potassium ferrocyanide and potassium ferricyanide solution in the presence of sodium hydroxide was used as the liquid phase. In the fluidized bed, glass spheres and sand of different sizes were employed as fluidizing solids. In two-phase flow system nitrogen was employed as inert gas. The pressure drop in the presence of fluidizing solids in helical coils was found to increase with increase in the pitch of the coil and was maximum for straight tube. The mass transfer coefficients were found to increase with increase in liquid velocity. The mass transfer coefficients in case of gas–liquid flow were found to be independent of liquid velocity and the pitch of the coil, and were largely influenced by gas velocity only. The data were correlated using j_D factor, Helical number, Froude number and Stanton number.     

Numerical investigation of gas mixing in gas‐solid fluidized beds

Gas mixing in a tall narrow fluidized bed operated in the slugging fluidization regime is simulated with the aid of computational fluid dynamics. In the first part, a parametric study is conducted to investigate the influence of various parameters on the gas mixing. Among the parameters studied, the specularity coefficient for the partial‐slip solid‐phase wall boundary condition had the most significant effect on gas mixing. It was found that the solid‐phase wall boundary condition needs to be specified with great care when gas mixing is modeled, with free slip, partial slip and no‐slip wall boundary conditions giving substantial differences in the extent of gas back mixing. Axial and radial tracer concentration profiles for different operating conditions are generally in good agreement with experimental data from the literature. Detailed analyses of tracer back mixing are carried out in the second part. Two parameters, the tracer backflow fraction and overall gas backflow fraction, in addition to axial profiles of cross‐sectional averaged tracer concentrations, are evaluated for different flow conditions. Qualitative trends are consistent with reported experimental findings. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

基于正交设计的旋流组合式喷嘴雾化性能实验

为寻求旋流组合式喷嘴雾化性能影响因素间的优化匹配方案,在分析和总结的基础上,选取旋流器的组合方式、喷头构型、进气流量和进气方式等作为主要影响因素,通过正交设计理论,安排试验方案,采用相位多普勒粒子分析仪（PDPA）对雾化性能的关键评判因素——喷雾场粒径进行了采集测量。实验结果的极差分析和方差分析表明,旋流器的组合方式对雾化粒径的影响最强,水流量次之,再次为喷头构型,而进气方式的影响最弱;此外,旋流器组合方式与喷头的交互作用产生的影响不应被忽略。     

Characterization of solids mixing patterns in bubbling fluidized beds

Behavior of the solid phase in fluidized beds was studied by a 2D CFD-DEM approach to obtain more information on the solid mixing and circulation. Hydrodynamic parameters, including solid diffusivity, and internal and gross circulations were considered in this study. To validate the simulation, time-position data obtained by the Radioactive Particle Tracking (RPT) technique were used. It was shown that the 2D model can satisfactorily predict the axial diffusivity, while the radial diffusivity calculated based on the model is an order of magnitude smaller than the experimental one in 3D. The influence of aspect ratio of the bed, type of distributor, and inlet gas velocity on solids mixing pattern were also studied. The solids flow pattern in the bed changed considerably by increasing the aspect ratio. Different solid circulations were captured by numerical model for the two types of distributors, namely porous and injection types. The results suggested that increasing the superficial gas velocity caused rigorous internal and gross circulations, which in return, improved solids mixing and decreased deviations from well mixed state.     

Experimental study of sawdust and coal particle mixing in sand or catalyst fluidized beds

This paper presents an experimental study in order to establish mixing and segregation conditions of wood sawdust and coal particles in a fluidized bed of sand or alumina particles. Experiments were performed at room temperature under atmospheric pressure. Simple equipment was used to divide the bed into layers and useful results could be obtained in a short period of time. The influence of fluidization velocity, time of an experiment, size and concentration of sawdust or coal particles was investigated. Uniformity of mixing was found to increase with an increase in the fluidization velocity. For u/u_mf less than 2.5, strong segregation occurred between sawdust and sand particles. Determination of good mixing conditions will be useful in gasification studies of sawdust and coal.     

循环流化床换热器中液固二相流的数值模拟

在循环流化床换热器的流体中加入固体粒子,可以对边界层有扰动作用,加快换热器中的传质与传热.对流化床换热器中的液固二相进行数值模拟,分别讨论了固体粒子对液体动量方程、k-ε方程及能量方程的影响,在单液相方程的基础上进行修正,建立了相应的液固二相流控制方程,确定了物理模型及边界条件.利用FLUENT进行数值计算,分别讨论了...