Continuous Drying of Slurry in a Jet Spouted Bed

ABSTRACT

he performance of a laboratory scale jet spouted bed (JSB) for drying rice flour slurry was studied. The bed consisted of ceramic balls (5028 mm diameter) and the rice flour slurry was sprayed onto the moving particle surface near the inlet part. All the experiments were carried out at the jet spouting regime. This regime has high bed void fraction and violent movement and collision of bed particles. As a result, the dried product layer is attrited from particle surface as a fine powder and entrained from the bed by the spouting air. The experimental result were presented to show the effects of static bed height, inlet air flow rate and temperature, and feed concentration and flow rate on the outlet air temperature, thernal efficiency, and mean particle size and moisture content of the product. Asimple mathematical model, which is based on the conservation of mass and energy equations, was developed. Predicted results agreed well with those obtained from the experiment.     

A comparative hydrodynamic study of two types of spouted bed reactor designs

In order to properly design and scale up spouted beds, one needs to predict the minimum spouting velocity of specific systems having different bed dimensions, and properties of particle and spouting gas. Because of inherent complexity of predicting minimum spouting velocity, the prevailing approach has been to use empirical correlations, a number of which are available in the literature. Central jet distributors are commonly used in the experimental studies reported in the literature. Circular slit distributor is a new concept in which air is supplied to the bed of particles through a circular slit. This paper presents results of an experimental study on the hydrodynamics of central jet and circular slit distributors. In this paper a fully connected feed-forward neural network model was used to predict the minimum spouting velocity of central jet and circular slit spouted beds. A neural network model was also developed to predict minimum fluidization velocity. The actual experimental data obtained from published literature and from the experiments carried out in this study were used for training and validating the models. The minimum spouting and fluidization velocities predicted using the neural network models developed in this study show a better approximation to the actual experimental values than those obtained from correlations available in the open literature. The position of flow regime of circular slit spouted bed was also established relative to the flow regimes of central jet spouted bed and fluidized bed.     

Batch drying kinetics of corn in a novel rotating jet spouted bed

The batch drying kinetics of corn as a test material were investigated experimentally in a novel rotating jet spouted bed (RJSB) using both continuous and intermittent (on/off) spouting and heating schemes. The parameters investigated include inlet air temperature, bed height, superficial air velocity, nozzle diameter, distributor rotational speed and intermittency of spouting and heat input. The results indicate that the drying kinetics are comparable with conventional spouted and fluidized beds for slow drying materials and that intermittent drying can save up to 40% of the thermal energy as well as air consumption with better quality product.     

Hydrodynamics of spout-fluid beds

Experiments were carried out in a half-column incorporating auxiliary flow, introduced through up to five slots along a 60° conical base, in addition to a central flow of air. The column had a diameter of 0.15 m and inlet diameters of either 19 or 25 mm. Three different types of particle were investigated, all with mean particle sizes in the 2 – 4 mm range. Four different flow regimes — fixed bed, spouting with aeration, spout-fluidization and jet in a fluidized bed — were identified and mapped. The minimum total gas flow required for spouting with aeration and for spout-fluidization was always greater than that corresponding to minimum spouting. Gas percolation through the annulus increased by as much as 50% as the proportion of auxiliary flow was increased for a given total gas flow rate. Solids circulation was increased somewhat by addition of auxiliary flow for deep beds, but a decrease occurred for shallow beds. The overall bed pressure drop under minimum spouting with aeration conditions increased linearly with auxiliary flow. On the other hand, the fountain height decreased as the fraction of auxiliary flow was increased for a fixed total gas flow, and increased with auxiliary flow for a fixed central gas flow. The empirical correlation of McNab predicted the average spout diameter well if the sum of the central and auxiliary flows was used in the correlation.     

Aerodynamics of a novel rotating jet spouted bed

A novel rotating jet spouted bed (RJSB) is developed and tested. It consists of a rotating air distributor with two radially located spouting air nozzles. The effects of bed height, distributor rotational speed, nozzle diameter and particle properties on the flow characteristics were examined. Various flow regimes were mapped as functions of distributor rotational speed and superficial air velocity for different materials and column dimensions. Empirical correlations were developed for the minimum spouting velocity, peak pressure drop and steady spouting pressure drop.     

Spouting characteristics of small glass particles with water

Experimental data are reported for the minimum spouting velocity, bed pressure drop, maximum spoutable height and spout diameter which show that spouting of small particles with water represents a new spouting regime. The transition from large to small particle spouting occurs when the voidage at the inlet to the spout goes from unity to less than unity.     

ANALOGY BETWEEN HEAT AND MASS TRANSFER IN THREE SPOUTED BED ZONES DURING THE DRYING OF LIQUID MATERIALS

This work, presents an experimental study of simultaneous heat and mass transfer in spouted bed zones during the drying of liquid materials. Distilled water was used as the liquid material and glass beads as inert material. The effect of the inlet gas temperature (100 to 140 °C), the feed air flow rate relative to air flow at minimum spouting (1.1 to 1.3), and liquid feed flow rate to the bed were investigated. The similarity between heat and mass transfer was analyzed using the Lewis and Chitton-Colburn analogies. Results showed that the ratio between the heat and mass transfer Chilton-Colbum factors lay between 1,31 and 1.74.     

Jet penetration depth in a two-dimensional spout-fluid bed

The jet penetration depth was proposed to be an important parameter to describe the jet action during the chemical process of spout-fluid bed coal gasification. A two-dimensional cold model of a spout-fluid bed coal gasifier with its cross section of and height of 2000 mm was established to investigate the jet penetration depth. Four types of Geldart group D particles were used as bed materials. A multi-channel pressure sampling system and a high-resolution digital CCD camera were employed for experimental investigations. The effects of spouting gas velocity, spout nozzle diameter, static bed height, particle property and fluidizing gas flow rate on the jet penetration depth have been systematically studied by pressure signal analysis and image processing. Experimental results indicate that the jet penetration depth increases with increasing spouting gas velocity and spout nozzle diameter, while it decreases with increasing particle density, particle diameter, static bed height and fluidizing gas flow rate. Additional, a new correlation considered all of the above effects especially static bed height and fluidizing gas flow rate, was developed for predicting the jet penetration depth in spout-fluid beds. The correlation was compared with published experimental data or correlations, which was in well agreement with the present experimental results and some other references.     

Diffusion-Controlled Batch Drying of Particles in a Novel Rotating Jet Annular Spouted Bed

Results are presented for batch drying of rewetted wheat (used as model particles) in a newly developed annular spouted bed in which the drying air jet rotates slowly thus spouting bed intermittently. For particles drying in the falling rate period intermittent spouting is shown to result in significant energy savings with small increase in the drying time. A finite element simulation is employed to evaluate the effective moisture diffusivity of the test material which is nonspherical in geometry. Further, advantages of variable spouting and heating of the spouting air are demonstrated by a parametric study of the simulation model.     

Simultaneous Drying and Cleaning of Guava Seeds in a Spout‐Fluid Bed with Draft Tube

A half sectional spout‐fluid bed column with draft tube was used for the cleaning and drying of guava seeds in one single operation. The process took place on a batch basis with a constant spouting velocity of 1.5 U_ms and an additional annular air flow rate equivalent to 0.5 and 0.7 U_mf. A regression model based on the spouting air temperature T_s, the annular inlet air temperature T_a and the additional annular air flow rate xU_mf was obtained to calculate the drying time. An additional cleaning step was necessary to increase the separation efficiency of the dried pulp from about 70% obtained during the drying process to 84%.     

细颗粒喷动床的流体力学特性

在直径186 mm的喷动床中考察了细颗粒(d_p＝0.241～0.874 mm)体系的流体力学性质及夹带和磨损特性.采用不同的喷嘴直径(D_i＝6～14 mm)和锥底顶角(θ＝45°～80°)对其操作状态进行了研究,发现当D_i/d_p<19～21时可以形成稳定喷动.随着气体速度的增加,床层依次出现固定床、稳定喷动、不良喷动和腾涌等4个流动区域.实验测量的最小喷动速度与Mathur-Gishler关联式的误差在±15%之内.采用γ射线扫描仪测量了固体密度分布,结果表明喷动区的固体密度随高度的增加而增加,环流区的密度比松堆密度大3%～10%.颗粒的磨损和夹带随着气速的增大而增大,在稳定有序的喷动状态下变得最小.     

Diffusion-Controlled Batch Drying of Particles in a Novel Rotating Jet Annular Spouted Bed

Abstract

Results are presented for batch drying of rewetted wheat (used as model particles) in a newly developed annular spouted bed in which the drying air jet rotates slowly thus spouting bed intermittently. For particles drying in the falling rate period intermittent spouting is shown to result in significant energy savings with small increase in the drying time. A finite element simulation is employed to evaluate the effective moisture diffusivity of the test material which is nonspherical in geometry. Further, advantages of variable spouting and heating of the spouting air are demonstrated by a parametric study of the simulation model.     

喷动流化床流动形态变化的试验研究国家

在一个可视化的半圆柱喷动流化床试验台上研究了喷动流化床的流形变化规律。研究表明：随着喷动气量和流化气量的改变,床内会呈现不同的流动形态：固定床、带射流的流化床、喷动床、充气喷动床和喷动流化床。同时研究了量小喷动速度和最小喷动流化速度随喷动管内径、颗粒粒径、静止床高的变化规律,归纳了预测量小喷动速度和最小喷动流化速度的试验关联式,绘制了喷动流化床的流形划分相图。     

Hydrodynamics of a gas-driven gas-liquid-solid spouted bed with a draft tube

A cylindrical gas-liquid-solid spouted bed, driven exclusively by gas flow, has been developed with a high potential for use in biochemical processes, such as a biological wastewater treatment. A plexiglass column with a 152 mm inner diameter was used in combination with a 53 mm inner diameter plexiglass draft tube. Three particle types were studied with densities ranging from 1044 kg/m³-1485 kg/m³ and average particle sizes ranging from 0.7-2.5 mm. Four flow regimes were observed when increasing the gas velocity, including fixed bed, semispouted bed, full spouted bed, and internal circulating fluidized bed. The transition gas velocities between those regimes were experimentally measured and termed as minimum spouting velocity, full spouting velocity, and minimum circulating velocity, respectively. A measurement of the downward particle flux in the annulus was used to identify the minimum spouting velocity, while the particle velocity and dense phase retraction in the annulus were monitored for the full spouting and minimum circulating velocities. All regime transition velocities increased with more dense particles and longer draft tubes. The minimum spouting velocity and full spouting velocity were not affected when varying the nozzle-tube gap, while the minimum circulating velocity increased with longer nozzle-tube gaps. Experiments without a draft tube were carried, though the spouting stability was significantly reduced without the draft tube.     

Pressure Fluctuations in Jet Spouted Beds

Jet spouted beds that consisted of a transparent Plexiglas cylindrical column of 1 m high and a conical base with cone angles of 30°, 36°, and 40° were used in this study. The particles used were spherical glass beads with an average diameter of 1.7, 2.1 and 3 mm, respectively, and particle size of 2.2 – 3.1 mm, non‐spherical rice particles. The effect of size and shape of particles, and static bed height on the minimum jet spouting velocity, and standard deviation of pressure fluctuations, was investigated. The results show that the minimum jet spouting velocity and pressure drop increased as the bed height and particle size increased. The minimum jet spouting velocity could be determined from the plot of standard deviation of pressure fluctuations vs. superficial gas velocity. The results obtained were in close agreement with the results of other methods in the literature.     

ANALOGY BETWEEN HEAT AND MASS TRANSFER IN THREE SPOUTED BED ZONES DURING THE DRYING OF LIQUID MATERIALS

ABSTRACT

This work, presents an experimental study of simultaneous heat and mass transfer in spouted bed zones during the drying of liquid materials. Distilled water was used as the liquid material and glass beads as inert material. The effect of the inlet gas temperature (100 to 140 °C), the feed air flow rate relative to air flow at minimum spouting (1.1 to 1.3), and liquid feed flow rate to the bed were investigated. The similarity between heat and mass transfer was analyzed using the Lewis and Chitton-Colburn analogies. Results showed that the ratio between the heat and mass transfer Chilton-Colbum factors lay between 1,31 and 1.74.     

Separation and residence times of larger particles in a spout-fluid bed

Optimum conditions for the separation of added large particles from a spout-fluid bed of Geldart Type B powder have been determined. A shallow bed, 100 mm in diameter, with porous conical distributor was used. Mean and distributions of residence time of the large particles depend on the two gas flow rates, especially that to the distributor, and on spout inlet diameter and large particle size. Bed regimes studied were mainly spouting with aeration and spout-fluidization. In the latter regime an optimum range of fluidizing gas flow rates that minimised mean and spread of residence times was identified. RTD data were correlated by a model incorporating a minimum time supplemented by a distribution for an ideally mixed system.     

Hydrodynamic simulations of gas–solid spouted bed with a draft tube

Flow behavior of particles in a two-dimensional spouted bed with a draft tube is studied using a continuous kinetic-friction stresses model. The kinetic stress of particles is predicted from kinetic theory of granular flow, while the friction stress is computed from a model proposed by Johnson et al. (1990). A stitching function is used to smooth from the rapid shearing viscous regime to the slow shearing plastic regime. The distributions of concentration and velocities of particles are predicted in the spouted bed with a draft tube. Simulated results compare with the vertical velocity of particles (Zhao et al., 2008) measured and in the spout bed with draft plates and solid circulation rate (Ishikura et al., 2003) measured in the spouted bed with a draft tube. The impact of the friction stress of particles on the spout, annulus, fountain and entrancement regions is analyzed in gas–solid spouted bed with a draft tube. Numerical results show that the gas flow rate through the annulus increases with the increase of the entrainment zone. The solids circulation rate decreases with the decrease of inlet gas velocity and the height of the entrainment zone. The effect of spouting gas velocity on distributions of concentration, velocity and particle circulation is discussed.     

The spouting regime map,bubbling characteristics and bed expansion of deep beds of large hollow polyethylene spheres

The high velocity spouting regime of a deep bed containing 9.6 mm hollow polyethylene spheres of density 394 kg/m³ was investigated in a 152.4 mm flat bottom cylindrical half column with a 50.8 mm inlet. A bed height vs gas velocity regime diagram is presented showing the conditions under which type II spouting is observed. The transition velocity from bubbling to type II spouting is determined using the autocorrelation of the pressure fluctuations in the bed. The bubbling characteristics of the bed are described. Bed expansion in bubbling and type II spouted beds are reported.     

双喷嘴矩形喷动床流动性能实验研究

在120 mm×240 mm的双喷嘴矩形不锈钢床内,对新型双喷嘴矩形导流管喷动床的最小喷动速度和喷动高度进行了研究,考察了喷动气速、粒径、静床层高度、导流管直径、导流管安装位置对最小喷动速度和喷动高度的影响。结果表明:最小喷动速度随颗粒直径、导流管直径、导喷距的增大而增大,随静床层高度的增大而减小;喷动高度随喷动气速的增大而增大,随导流管直径的增大而减小,受静床层高度和导喷距的影响不大,并得出了最小喷动速度的关联式。