褐煤脉冲式气流干燥传递过程研究

褐煤干燥是褐煤固体热载体新法干馏工艺的重要组成部分。本文对脉冲式气流干燥器内基于气-固两相流的褐煤干燥动量、热量、质量传递过程进行了研究。依托高效成熟的气流干燥技术,以颗粒在气流中的受力情况为基础,建立了气流干燥过程中颗粒加速运动的热量传递模型和颗粒减速运动的动量及热量传递模型,并提出了新的脉冲式气流干燥器高度的设计及优化方法。通过将脉冲式气流干燥器的干燥模拟数据与相同高度的等径式气流干燥器的数据进行对比,验证了脉冲式气流干燥的高效性,并根据烟气温度、颗粒湿含量、体积传热系数等模拟参数沿干燥器高度的分布情况对其高效性的原因进行了详细分析,为脉冲式气流干燥装置工艺设计提供准确的数据依据和理论参考。     

脉冲式气流干燥器的设计

一、概述气流干燥器因有其干燥强度大,干燥时间短,可适用于热敏性物料,热效率较高,设备简单、占地面积小等优点而能越来越广泛地应用于各行各业。但是由于普通气流干燥器在等速运动段的气固相对速度较小(等于固体颗粒的沉降速度),气固间的给热系数也就较小,     

气流干燥管数值计算方法

在工程设计、运行的实践基础上,本文从气—固两相流动、传热的基本方程出发,建立了等直径管气流干燥过程的数学模型并进行解算,所得结果与实践积累的经验比较一致,说明该数学程模型反映了气流干燥过程的基本规律。它的主要特点是在气、固两相流动与传热计算中,以沿流程连续变化的气流参数作为依据,避免了以前沿用气流平均参数作为计算依据的缺点,从而得到了沿程气,固两相流动与传热各参数的变化规律,为改善干燥效果及合理利用热能挺供一定依据。     

气流干燥含湿氧化铝颗粒传热传质数值模拟

基于二相流理论,提出了一个描述含湿氧化铝颗粒气流干燥过程的一维数学模型。模型考虑了干燥管内气固二相间的传热传质和动量传递、含水质量分数和气固二相温度的变化。根据B ird所提出的努赛尔数经验公式,利用标准四阶龙格-库塔法对由非线性常微分方程组成的气流干燥耦合模型进行计算,得到了含湿氧化铝颗粒在不同气流干燥条件的干燥曲线。整个干燥过程中数值计算结果与试验实测数据吻合得很好,可以用来预测含湿氧化铝颗粒的干燥含水质量分数。     

旋流干燥器流场模拟研究

旋流干燥器(SMTD)作为一种新型的干燥装置,其结构简单、干燥效率高、处理量大,成为气固二相流干燥研究的新方向.文中用流体力学软件Fluent对旋流干燥器内部的流场进行了数值模拟研究,采用雷诺应力模型(RSM)模拟气相流动,模拟结果表明:旋流干燥器各干燥事内气流主要做旋转运动,穿过旋流板时,主要做返混运动;切向速度分布...     

脉冲气流干燥管内固体颗粒运动轨迹分析

本文利用气-固两相流动的相关理论,在夏诚意法的基础上对脉冲管内固体颗粒的加速运动轨迹和减速运动轨迹进行了全面的分析。通过对脉冲气流干燥管进行分段、分区间的逐次计算,建立了颗粒在加速运动段和减速运动段的高度及时间方程。颗粒运动轨迹的分析为气固两相流跨域操作模型的建立和开发新型干燥过程及装置提供了一定的理论支撑。     

有关脉冲式气流干燥器若干问题的探讨

<正> 气流干燥器因其结构简单、操作方便、造价低廉、干燥时间短而受到欢迎,是化工、医药、食品等工业中广泛应用的一种干燥器。 在一般的直管气流干燥器中,颗粒进入气流管后由热气流加速,在这加速过程中,颗粒与气流间相对速度U_r较大,因此具有较高的传热传质速率。但到了一定高度(一般为1～2米)以后,颗粒即处于等速运动状态,颗粒与气流间的相对速度达到恒定,数值上等于颗粒沉降速度U_t,一直维持到颗粒     

正压旋风气流干燥装置在制药工业中的应用

正压旋风气流干燥是利用流态化技术结合传热过程原理,使气体夹带物料颗粒以切线方向进入旋风干燥室,沿壁产生旋转运动而达到干燥物料之目的。由于物料颗粒处于悬浮、搅拌、旋转运动状态,因此,即使在雷诺数较低的情况下,颗粒周围的气体边界层处亦能呈高度湍流状态。由于切线运动,气固相的相对速度大大增加,又因气流带着固体颗粒在内管与外管间作旋转运动而粉碎、加速,颗粒表面不断更新,气固相接触面     

强化气固两相并流干燥传递过程的模型与计算

对气固两相并流流动的干燥过程的强化方式进行了研究，对脉冲流动中气固两相的运动与传递方程提出了模型并进行了数值计算，同时对相同条件下直管型干燥器的干燥过程进行了计算与比较。实际应用结果表明，该计算模型能较好地反应气固两相并流流动干燥过程的规律。     

稀相输送床中气固两相运动及换热特征

本文针对输送床颗粒加速段气固两相对流换热系数和温度都连续变化的实际,建立了毕渥准数小于0.1时的群颗粒非稳态对流换热数学模型,可求解气固有效换热时间,结合颗粒运动方程,能确定出有效换热管的高度。数学模型的理论计算、热模试验及现场实测均表明,水泥生料粉和高温气流的换热主要发生在加速段的起始区,其距离不超过20厘米,时间不大于0.03秒。强调降低各级预热器输送管的长度,增加预热器的级数,是提高热效率的重要途径。     

Three-dimensional modelling of pneumatic drying process

Steady-state three-dimensional calculations of heat and mass transfer in vertical pneumatic dryer were performed. The theoretical model of the drying process is based on two-phase Eulerian-Lagrangian approach for gas-particles flow and incorporates advanced drying kinetics for wet particles. The model was utilized for simulation of the drying process of wet PVC and silica particles in a large-scale vertical pneumatic dryer. The influence of wall thermal boundary conditions was investigated by assuming either known value of the wall temperature or adiabatic flow in the dryer. Analyzing the predicted particle drying kinetics, an uneven product quality was predicted due to non-uniform drying conditions in the central and peripheral zones of the pneumatic dryer. Moreover, for the case of non-insulated chamber walls such quality unevenness was estimated to be substantially greater than for the case with thermally insulated drying chamber. The examination of the predicted temperature profiles within the silica and PVC wet particles showed that the latter is subjected to higher temperature gradients potentially resulting in the greater rate of thermally-degraded final product.     

TWO-FLUID MODEL FOR PNEUMATIC DRYING OF PARTICULATE MATERIALS

The Two-Fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved for a one-dimensional steady-state condition and was applied to the drying process of wet PVC particles in a large-scale pneumatic dryer and to the drying process of wet sand in a laboratory-scale pneumatic dryer. A two-stage drying process was implemented. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The drying process is assumed to stop when the moisture content of a particle falls to a predefined value or when the particle riches the exit of the pneumatic dryer. Our developed model was solved numerically and two operating conditions, adiabatic and given pneumatic dryer wall temperature, were simulated. Comparison between the prediction of the numerical models of Rocha and DryPak, (Pakowski, 1996), which were presented by Silva and Correa (1998), with the prediction of our numerical simulation reviled better agreements with DryPak then with the models of Rocha. The results of the developed model were also compared with experimental results of Baeyens et al. (1995) and Rocha.     

TWO-FLUID MODEL FOR PNEUMATIC DRYING OF PARTICULATE MATERIALS

The Two-Fluid model has been used for modeling the flow of particulate materials through pneumatic dryer. The model was solved for a one-dimensional steady-state condition and was applied to the drying process of wet PVC particles in a large-scale pneumatic dryer and to the drying process of wet sand in a laboratory-scale pneumatic dryer. A two-stage drying process was implemented. In the first drying stage, heat transfer controls evaporation from the saturated outer surface of the particle to the surrounding gas. At the second stage, the particles were assumed to have a wet core and a dry outer crust; the evaporation process of the liquid from a particle assumed to be governed by diffusion through the particle crust and by convection into the gas medium. As evaporation proceeds, the wet core shrinks while the particle dries. The drying process is assumed to stop when the moisture content of a particle falls to a predefined value or when the particle riches the exit of the pneumatic dryer. Our developed model was solved numerically and two operating conditions, adiabatic and given pneumatic dryer wall temperature, were simulated. Comparison between the prediction of the numerical models of Rocha and DryPak, (Pakowski, 1996), which were presented by Silva and Correa (1998), with the prediction of our numerical simulation reviled better agreements with DryPak then with the models of Rocha. The results of the developed model were also compared with experimental results of Baeyens et al. (1995) and Rocha.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.     

STEAM DRYING OF WOOD CHIPS IN PNEUMATIC CONVEYING DRYERS

ABSTRACT

A model for a pneumatic conveying dryer is presented. Although the main emphasis is put on superheated steam drying of wood chips, it can be used for other porous materials as well

The model includes a comprehensive two-dimensional model for the drying of single wood chips which accounts for the main physical mechanisms occurring in wood during drying. The external drying conditions in a pneumatic conveying dryer were calculated by applying the mass, heat and momentum equations for each incremental step in dryer length. A plug flow assumption was made for the dryer model and the single particle and dryer models were solved in an iterative manner. The non-spherical nature of wood chips were accounted for by measuring the drag and heat transfer coefficients

Model calculations illustrate the complex interactions between steam, particles and walls which occur in a flash dryer. The drying rate varies in a very complex manner through the dryer. The internal resistance to mass transfer becomes very important in The drying of less permeable wood species such as spruce. Two effects were observed as the particle size was increased: firstly the heat transfer rate decreased, and secondly the residence time increased. To some extent, these effects compensate for each other, however, the net result is that larger chips have a higher final moisture content.     

磷酸氢钙的大型气流干燥装置设计

介绍了一种大型气流干燥磷酸氢钙的工业生产流程，对干燥管内颗粒和空气的干燥特性进行了模拟和计算机辅助设计，模拟设计结果与实际生产情况较吻合。     

组合干燥技术在间苯二甲腈生产中的应用

介绍了脉冲气流干燥和旋风干燥器的干燥原理及特点，并把这两种干燥方式组合起来，成功地应用于间苯二甲腈的干燥中。     

Simulation Study for Pneumatic Conveying Drying of Sawdust for Pellet Production

Pneumatic conveying drying (PCD) is a combination of heat and mass transfer and pneumatic handling technology. This technology has been extensively used in chemical, pharmaceutical, and food industries, as well as many others. The PCD technique is beneficial for agricultural products, because it can achieve high-quality drying with reduced heat damage in a very short time. In this study, one-dimensional and three-dimensional mathematical models for the drying of sawdust particles in a pneumatic dryer were developed and verified with experiments. The three-dimensional modeling was done with a computational fluid dynamics (CFD) package (ANSYS FLUENT, Ver. 13.0, Ansys, Inc.), in which the gas phase is modeled as a continuum using the Euler approach, and the droplet/particle phase is modeled by a discrete phase model with a Lagrange approach. One-dimensional analysis was performed in MATLAB (Ver. 7.0). The experiments were carried out to validate the model in a pneumatic dryer with a horizontal length of 1 m, vertical height of 1.1 m, and diameter of 0.14 m. Sawdust, a raw material used for producing pellets, was prepared from well-seasoned pinewood timber. The initial moisture content of the sawdust was 22% (wb). The hot air inlet temperature in the dryer was fixed at 100°C. The variations in air pressure, air velocity, air temperature, and particle moisture content were investigated along the length of the dryer. The final moisture contents of sawdust and air temperature were reduced by 2% (wb) and 5°C, respectively. The simulated values were in good agreement with the experimental values. The developed model was then employed for the design of a pilot-scale pneumatic dryer (length 7 m and diameter 0.14 m). The final moisture content of the sawdust particles was reduced to 14% (wb) when the dryer length was increased from 1 to 7 m. In addition, the modeling was performed using buffers in the pilot-scale dryers. The use of a buffer noticeably increased the drying efficiency.     

Eulerian–Lagrangian Simulation and Experimental Validation of Pneumatic Conveying Dryer

This paper explores numerical and experimental studies on the performance of a pneumatic conveying dryer. The four-way coupling Eulerian–Lagrangian approach is utilized in the numerical study and the experimental study is carried out in a pilot-scale vertical pneumatic conveying dryer of diameter 8.1 cm and 4.5 m length. The effects of Reynolds number, particle size, solid mass flow rate, and inlet gas temperature on the dryer performance are investigated. It is found that the present model predictions agree well with the experimental data. Generally, it is concluded that the drying rate increases as the Reynolds number increases, while increasing the particle size or the solid mass flow rate decreases the drying rate.