THE STABILITY OF WET SPOUTED BEDS

ABSTRACT

In granulation, coating or drying of slurries on inert particles a liquid phase is added to a gas-spouted bed. The effect of liquid injection on spouting velocity, fountain height, bed pressure drop and stability of the spouting regime is examined in a 0.15 m diameter bed with four types of inert particles 2 to 5 mm in size, 1140 to 2880 kg/m³ in true density, and 1.3 and 1400 mPa·s in liquid viscosity. Key factors are the liquid content and the presence of cohesive forces due to liquid bonds between particles. In a spoutability chart the maximum spoutable liquid content is related to the ratio of inertial force of a particle to the viscous or cohesive force exerted onto the particle by the liquid film. The spoutability chart serves to define regions of stable spouting.     

HYDRODYNAMICS AND STABILITY OF SLOT-RECTANGULAR SPOUTED BEDS PART II: INCREASING BED THICKNESS

Tests were carried out with rectangular spouted bed columns of different thickness, i.e., front-to-back dimension, while holding the column width and air-entry slot width constant, to investigate the effects on spoul stability and bed hydrodynamics. For the three sizes of glass beads and one size of polyethylene beads examined, increasing the column thickness led to three-dimensional effects, such as formation of multiple spouts, and affected such hydrodynamics variables as the minimum spouting velocity, maximum spoutable bed depth and maximum pressure drop.     

HYDRODYNAMICS AND STABILITY OF SLOT-RECTANGULAR SPOUTED BEDS PART II: INCREASING BED THICKNESS

Tests were carried out with rectangular spouted bed columns of different thickness, i.e., front-to-back dimension, while holding the column width and air-entry slot width constant, to investigate the effects on spoul stability and bed hydrodynamics. For the three sizes of glass beads and one size of polyethylene beads examined, increasing the column thickness led to three-dimensional effects, such as formation of multiple spouts, and affected such hydrodynamics variables as the minimum spouting velocity, maximum spoutable bed depth and maximum pressure drop.     

HYDRODYNAMICS OF SPOUTED BEDS AT ELEVATED TEMPERATURES

A review of the literature reveals little information on the hydrodynamics of spouted beds at high temperature. Most existing correlations are based upon experiments done at ambient conditions; they have not usually been tested with data at higher temperatures. The present study focussed on obtaining data over a temperature range of 20-420°C. Spouting of three sizes of Ottawa sand with preheated air was couducted in a 156 mm diameter stainless steel semi-cylindrical column with a 60° included angle half-conical base. A transparent wire-glass panel on the flat face allowed measurement of parameters which are otherwise difficult to obtain in a full stainless steel column. In addition to air, helium and methane at room conditions were used as spouting gases. With these two additional gases, it became possible to investigate the effect of changing gas density at constant viscosity and the effect of changing gas viscosity at constant gas density. In general, it was found that the range of stable spouting decreased with decreasing gas density and increasing gas viscosity, hence with increasing air temperature. Existing equations for various spouted bed parameters were tested and, where necessary, empirically modified to fit the new data obtained. @KEYWORDS Spouted bed hydrodynamics, High temperature gas spouting, Temperature effect on spoutability     

喷动床内气固两相流体动力行为的数值模拟

引　言喷动床被广泛应用于不同工业领域中 ,如石油裂解反应 -再生器、煤和农业废弃物气化和燃烧 ,喷动床还被应用于粮食和药品的干燥等[1] .因此 ,喷动床设计应满足不同应用的要求 .喷动床内气相反应物的反应时间和停留时间依赖于床体几何结构和运行参数 .尽管已有许多的实验对喷动床内气固两相流动进行了研究 ,得到了喷射区、环形区和喷泉区内的气固两相流动流体动力特性 ,然而由于喷动床内气固两相流动的复杂性 ,人们对床体几何结构和运行参数对喷动床动力学的影响至今并不清楚 .因此 ,床体几何结构和运行参数等对喷动床动力学的影响成为…     

HEAT TRANSFER IN SPOUTED BEDS

Experiments were carried out in order to analyse the wall-to-bed and fluid-to-particle heat transfer coefficients in spouted Beds. wall-to-bed heat transfer coefficients were determined in cylindrical-conical and conical spouted beds for various gas flow rates, particle sizes and bed heights for spouted beds with and without draft tubes.

A new definition for wall-to-bed transfer coefficient was proposed baaed on experimental observations.

The heat tranefer area was also studied to ensure that a physically significant fluid-to-particle heat transfer coefficient was achieved.     

喷动床发展与现状

介绍了流化床的一个分支——喷动床的发展与现状。首先简述了喷动床的发展史，然后着重讲解了有关喷动床的几个基本概念，并对喷动床的流动特性及其与传统流化床的异同点作了描述。给出了一些喷动床的床型变化及应用实例，并提供了一份有关喷动床研究的较完整的文献清单     

SPOUTED AND SPOUT-FLUIDIZED BEDS FOR GRAM DRYING

The objectives of this work are to analyze the drying performance of conical-cylindrical spouted bed (CSB) dryers for three different grains (rice, corn and wheat), and to compare the drying efficiency of CSB dryers with that of spout-fluid bed (SFB) dryers. A PC-program was developed for: (I) -optimization of the CSB dryer dimensions; (2) -simulation of drying grains in the optimized CSB dryer (including start-up period); and, (3) -analysis of the drying performance in a similar SFB dryer.

The liquid diffusion model is used to describe the falling rate drying period. Semi-empirical correlations available in the literature as well as information obtained in the authors' laboratory for spouted and spout-fluidized beds of grains are used to describe the aerodynamic parameters.

The results are presented in terms of the size of the dryer, energy consumption, air handling requirement, drying characteristics etc for different drying conditions. The drying effeciency in a CSB is compared with that in a similar SFB for different grain feed rates and drying temperatures.     

PREDICTION OF MINIMUM SPOUTING VELOCITY IN TWO-DIMENSIONAL FLAT BOTTOM SPOUTED BEDS

Spouted beds have been used in industry for operations such as drying, catalytic reactions, and granulation. Conventional cylindrical spouted beds suffer from the disadvantage of scaleup. Two-dimensional beds have been proposed by other authors as a solution for this problem. Minimum spouting velocity has been studied for such two-dimensional beds. A force balance model has been developed to predict the minimum spouting velocity and the maximum pressure drop. Effect of porosity on minimum spouting velocity and maximum pressure drop has been studied using the model. The predictions are in good agreement with the experiments as well as with the experimental results of other investigators.     

SPOUTED AND SPOUT-FLUIDIZED BEDS FOR GRAM DRYING

The objectives of this work are to analyze the drying performance of conical-cylindrical spouted bed (CSB) dryers for three different grains (rice, corn and wheat), and to compare the drying efficiency of CSB dryers with that of spout-fluid bed (SFB) dryers. A PC-program was developed for: (I) -optimization of the CSB dryer dimensions; (2) -simulation of drying grains in the optimized CSB dryer (including start-up period); and, (3) -analysis of the drying performance in a similar SFB dryer.

The liquid diffusion model is used to describe the falling rate drying period. Semi-empirical correlations available in the literature as well as information obtained in the authors' laboratory for spouted and spout-fluidized beds of grains are used to describe the aerodynamic parameters.

The results are presented in terms of the size of the dryer, energy consumption, air handling requirement, drying characteristics etc for different drying conditions. The drying effeciency in a CSB is compared with that in a similar SFB for different grain feed rates and drying temperatures.     

CONVECTIVE HEAT TRANSFER DURING COATING OF PARTICLES IN TWO-DIMENSIONAL SPOUTED BEDS

ABSTRACT

This work deals with the evaluation of gas-to-particle convective heat transfer in a process of coating of particles in two-dimensional spouted beds. A detailed calculus routine is presented and two correlations of heat transfer coefficient, for the spout and annular regions are derived. The study is performed with two, two-dimensional spouted beds with different sizes and with two different particles, placebo and soybean.     

BATCH DRYING OF SHELLED CORN IN TWO-DIMENSIONAL SPOUTED BEDS WITH DRAFT PLATES

Geometrically similar spouted beds with draft plates were used to obtain the drying characteristics of freshly harvested shelled corn with 0.28 to 0.31 kg/kg initial moisture content at different air inlet temperatures and bed heights. Thermal equilibrium between air and grains was achieved at minimum spouting conditions. The drying kinetics of shelled corn in a drafted two-dimensional spouted bed was found to be of the "thin layer" type. Expressions for the model parameters in Page's equation accounting for the bed geometry, grain moisture content, and drying conditions were developed.     

BATCH DRYING OF SHELLED CORN IN TWO-DIMENSIONAL SPOUTED BEDS WITH DRAFT PLATES

ABSTRACT

Geometrically similar spouted beds with draft plates were used to obtain the drying characteristics of freshly harvested shelled corn with 0.28 to 0.31 kg/kg initial moisture content at different air inlet temperatures and bed heights. Thermal equilibrium between air and grains was achieved at minimum spouting conditions. The drying kinetics of shelled corn in a drafted two-dimensional spouted bed was found to be of the "thin layer" type. Expressions for the model parameters in Page's equation accounting for the bed geometry, grain moisture content, and drying conditions were developed.     

THE MINIMUM OF TOTAL AVAILABLE ENERGY (ENERGY) DISSIPATION IN THE MULTISTAGE FLUIDIZED WET BEDS

This article represents a second half of the work in optimization of the fluidized drying and moistening processes by the method of dynamic programming. The problem considered in Part I concerns the case of fluidized drying and moistening in the continuous processes for the variable inlet gas temperature. In this work, the generalized aspect of the continuous and multistage adiabatic processes for the case when the decision variables on the stage are gas enthalpy, gas humidity and dry gas flow rate, is considered. The continuous processes are considered here only as a limiting case of the multistage ones.

The two types of the thermodynamic performance indexes based on the idea of energy as a thermodynamic measure of the substance value are considered. The first type is related to the economic costs of production and the second to the overall economic costs. The equivalency of the optimization results for either of the two types of energy costs is discussed- Also, the nature of the optimal trajectories and decisions is considered.     

THE MINIMUM OF TOTAL AVAILABLE ENERGY (ENERGY) DISSIPATION IN THE MULTISTAGE FLUIDIZED WET BEDS

This article represents a second half of the work in optimization of the fluidized drying and moistening processes by the method of dynamic programming. The problem considered in Part I concerns the case of fluidized drying and moistening in the continuous processes for the variable inlet gas temperature. In this work, the generalized aspect of the continuous and multistage adiabatic processes for the case when the decision variables on the stage are gas enthalpy, gas humidity and dry gas flow rate, is considered. The continuous processes are considered here only as a limiting case of the multistage ones.

The two types of the thermodynamic performance indexes based on the idea of energy as a thermodynamic measure of the substance value are considered. The first type is related to the economic costs of production and the second to the overall economic costs. The equivalency of the optimization results for either of the two types of energy costs is discussed- Also, the nature of the optimal trajectories and decisions is considered.     

SOLIDS CIRCULATION IN SPOUTED AND SPOUT-FLUID BEDS WITH DRAFT-TUBES

A 20 cm semi-cylindrical spout-fluid bed with a draft-tube has been used to investigate the effect of various physical parameters on the solids circulation rate, A new method has been developed by which these data can be easily and accurately collected. Equations are presented which can be used to predict the maximum attainable solids circulation rate and the spouting velocity required to achieve this maximum for the unit operating in spouting mode. The circulation trends are shown to be a direct result of the phenomena of entrainment and jetting occurring in the entrainment zone investigated visually with a video camera. Preliminary gas bypassing experiments were also performed which show the ability to almost eliminate spout gas bypassing to the annulus with appropriate auxiliary gas addition.     

振动流化床床层均匀流化行为

在φ148mm的振动圆柱型流化床中,采用光导纤维浓度测量仪测量了床层局部空隙率的波动．对空隙率波动偏差的分析表明,振动参数、颗粒物性和操作参数均对床层均匀流化行为有不同程度的影响,且振幅和振动频率越大越能明显提高以小米或玻璃珠为物料的床层的均匀稳定性,两颗粒物性和操作参数直接影响振动能量在床层中的传播．根据实验结果,提出并定义了一个振动均匀操作判别系数,这对选择实际振动流化床的操作范围具有一定的指导意义．     

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.