填料塔液相混合的研究(Ⅱ)二维扩散模型

在对示踪剂采取连续点源稳态注入条件下,本文对同时考虑液相轴向和径向混合的二维扩散模型进行了求解.模型参数用非线性规划的最优化方法根据在1米直径填料塔中得到的二维停留时间分布的数据进行估计,得到了分别计算液相轴向和径向Peclet准数的无因次回归方程.     

脉冲萃取塔径向扩散系数的测定方法

引　言对于脉冲筛板萃取塔或脉冲填料萃取塔中的轴向混合 ,已有许多人用轴向扩散模型作了研究[1,2 ].但是 ,在脉冲萃取塔工业放大设计的过程中 ,径向混合程度是个不可忽略的重要因素 .然而 ,这方面的研究尚未见报道 .萃取塔中的混合情况会直接影响液液两相传质推动力的大小 .通常 ,希望塔内连续相出现尽可能小的轴向混合 ,使连续相的流形接近活塞流 ,以获得最大的传质推动力 .而对于连续相的径向混合 ,其混合程度越大越有利于径向浓度的均匀 ,有利于获得最大的传质推动力 .因此 ,径向扩散系数大小的确定 ,对于工业规模脉冲萃取塔的设计具有…     

RH真空精炼系统的混合行为

采用数值分析与水模型实验相结合的方法对RH装置的混合行为进行了研究.研究对象针对整个RH循环系统,并发展一种新的均相流方法建立流场模型.混合行为分析以真空室钢液表面示踪剂的混合速率为基准进行.研究结果表明,示踪剂的扩散速度在开始阶段非常迅速,只有上升管下部区域示踪剂的扩散较慢.数值计算的示踪剂浓度分布与实验观察的结果基本一致.     

固定床反应器中扩散反应问题的实验测定

为研究反应器尺度上的扩散反应问题，建立了一个可同时在轴向和径向进行温度和浓度测定的二维壁冷却式固定床反应器。选择３种操作条件进行了实验测定，利用所测实验数据对拟均相二维平推流模型进行了参数估计，发现按平推流模型得到的径向温度曲线在壁面附近同实验测定有很大偏差，本文认为是由于在颗粒大小不容忽视的条件下平推流假定过于简单之故。应当加以改进。     

脉冲筛板萃取柱中连续相的轴向混合

通过示踪剂实验方法对38mm脉冲筛板萃取柱中连续相的轴向混合进行了研究。分别采用亚甲基蓝溶液和氯化钾溶液为示踪剂。实验过程中,首先采用"扰动-响应"技术实测了示踪剂的停留时间分布(RTD)曲线,然后依照轴向扩散模型(ADM)应用最小二乘法拟合求出连续相的轴向混合系数Ec,并分析了连续相表观流速、分散相表观流速、脉冲强度对于Ec的影响。实验结果表明,示踪剂浓度、径向取样位置和轴向取样位置对轴向混合系数Ec值的影响可以忽略,轴向混合系数Ec随着脉冲强度和两相表观流速的增加而增大。最后在本实验参数范围内,拟合出了连续相的轴向混合系数随操作参数变化的经验关系式,与实验结果对比,相对偏差在±20%以内。     

气液固三相提升管中液相扩散特性

对气液固三相提升管内液相扩散行为进行了实验研究,考察了气速、液速以及颗粒循环量等操作因素对液相扩散系数的影响规律.实验研究结果表明,轴向、径向扩散系数随气速的增大均增大;轴向扩散系数随液速的变化基本保持不变,径向扩散系数随液速的增大而减小;轴向、径向扩散系数随颗粒循环量的增大均增大.与传统的气液固三相流化床相比,气液固三相提升管反应器更接近理想的平推流反应器.     

填料塔内流体轴向返混系数的确定

阐述了以示踪响应技术为基础确定填料塔内流体轴向返混系数的一种方法,首先对实验得到的示踪剂停留时间分布曲线采用等距法进行异常数据的剔除,再采用五点三次平滑公式进行滤波处理。然后用一维轴向扩散模型对曲线进行拟合,根据扩散模型在一定初始条件和边界条件下的解析解,采用时间域最小二乘法确定了轴向返混系数。结果表明,阐述的方法具有较高的精度。示踪停留时间曲线的实验测定值与其拟合值的均方差在0．034～0．096之间。     

旋转填料床与盘管组合反应器中的流动特性

以饱和的NaCl水溶液为示踪剂,采用脉冲法考察了液体流量、转子转速对旋转填料床与盘管组合反应器停留时间分布(RTD)曲线的影响。用轴向扩散模型对流动状况和返混程度进行了表征。结果表明,组合反应器内的流体流动型态与盘管相同,接近活塞流,且流量越大,平均停留时间越短。旋转填料床转子转速对组合反应器停留时间分布影响很小。     

气流在板波填料内的径向混合——(Ⅰ)试验与关联

本文在φ200和φ287mm两种塔内,对4.5型和6.3型板波填料进行了气流径向混合试验,试验采用CO_2作为示踪剂.试验结果表明,板波填料具有良好的径向混合性能.4.5型板波填料的Pe_r仅为1.5左右,而φ15mm拉西环的Pe_r约为3.5.影响气流在板波填料中径向混合的因素有:填料的盘高、盘径、波纹倾角以及气速等.在低Re下,板波填料的Pe_r较低,且随Re的增大而增加,当Re>800以后.Pe_r趋于定值.本文提出了Pe_r与诸参数间的经验关联式.     

气固并流下行床气体扩散行为的研究

采用氢气稳态示踪方法在内径140mm的气固并流下行循环流化床中对气体扩散行为进行了实验研究.实验结果表明:下行床中气体扩散行为可用二维拟均相模型进行描述,其气体的径向扩散系数与气速、固体循环量及颗粒密度的关系可用下列准数关联式表示Pe_r=4.35×10_(-3)Re~(0.95)ε~(-73.4) 1>ε>0.99而下行床中气体轴向扩散系数要比提升管中小1个数量级以上.     

Axial liquid dispersion in a liquid–solid circulating fluidized bed

Although axial liquid dispersion has been studied extensively in particulate fluidized beds, no data has been reported previously in a liquid–solid circulating fluidized bed (LSCFb). In this work, the axial liquid dispersions at various radial positions were measured in an LSCFB of 76 mm in diameter and 3.0 m in height using a dual conductivity probe. The results reveal that the axial liquid dispersion is affected not only by the operating conditions but by the radial positions as well. A local axial dispersion model is proposed to describe the axial liquid dispersion at various radial positions. The local axial liquid dispersion coefficients determined by the proposed model are greater at the axis than near the wall region of the riser. This nonuniformity of axial liquid dispersion is believed to be caused by the radial nonuniform distribution of liquid velocity, and bed voidage in the LSCFB can significantly affect the axial liquid dispersion.     

RADIAL DISPERSION AND BUBBLE CHARACTERISTICS IN THREE-PHASE FLUIDIZED BEDS

The effects of gas and liquid velocities, liquid viscosity and particle size on the radial dispersion coefficient of liquid phase (D_r) and the bubble properties in three-phase fluidized beds have been determined. A new flow regime map based on the drift flux theory in three-phase fluidized beds has been proposed.

In three-phase fluidized beds, D, increases with increasing gas velocity in the bubble coalescing and in the slug flow regimes, but it decreases in the bubble disintegrating regime. The coefficient exhibits a maximum value in the bed of small particles with increasing liquid velocity at lower gas velocities. However, it increases with increasing liquid velocity at higher gas velocities. In two and three-phase fluidized beds of larger particles (6,8 mm), D_r exhibits a maximum value with an increase in liquid viscosity at lower gas velocities, but it increases at higher gas velocities. The mean bubble chord length and its rising velocity increase with increasing gas velocity and liquid viscosity. However, the bubble chord length decreases with an increase in liquid velocity and it exhibits a maximum value with increasing particle size in the bed. The radial dispersion coefficients in the bubble coalescing and disintegrating regimes of three-phase fluidized beds in terms of the Peclet number in the present and previous studies have been well represented by the correlations based on the concept of isotropic turbulence theory.     

气液固三相循环流化床气液传质行为

<正>气液固三相流化床反应器在石油化工、湿法冶金、环境工程和煤的液化等工业领域得到了广泛应用,其基础研究也取得了很大进展.但是,传统三相床主要应用于低液速(U_L<     

Hydrodynamics and Mass Transfer in Gas‐Liquid‐Solid Circulating Fluidized Beds

Although extensive work has been performed on the hydrodynamics and gas‐liquid mass transfer in conventional three‐phase fluidized beds, relevant documented reports on gas‐liquid‐solid circulating fluidized beds (GLSCFBs) are scarce. In this work, the radial distribution of gas and solid holdups were investigated at two axial positions in a GLSCFB. The results show that gas bubbles and solid particles distribute uniformly in the axial direction but non‐uniformly in the radial direction. The radial non‐uniformity demonstrates a strong factor on the gas‐liquid mass transfer coefficients. A local mass transfer model is proposed to describe the gas‐liquid mass transfer at various radial positions. The local mass transfer coefficients appear to be symmetric about the central line of the riser with a lower value in the wall region. The effects of gas flow rates, particle circulating rates and liquid velocities on gas‐liquid mass transfer have also been investigated.     

以非晶态合金催化剂SRNA-4为固相的气液固磁稳定床的界面传质研究

1 INTRODUCTION Magnetically stabilized beds (MSB) exhibit an unique combination of packed-bed and fluidized-bed properties. Gas-liquid-solid (G-L-S) three-phase MSB has recently attracted more attention in the field of biotechnology processes (such as bioseparation or immobilized enzyme systems) and chemical engi- neering(such as the hydrogenation reaction system). The interphase mass transfer behavior plays an im- portant role in the optimal operation of practical MSB. However, many…     

Gas and solid behavior in cracking circulating fluidized beds

Gas and solid hydrodynamics have been studied in dilute circulating fluidized beds under conditions occurring in catalytic cracking risers. Gas radial velocity profiles and dispersions were established by a tracer technique in a cold set-up. The gas axial dispersion was determined in an industrial riser. The local concentrations of the solid phase were measured by a tomographic technique. This has allowed an assessment of the core—annulus structure of the bed and an estimate of the solid radial and axial dispersions. The axial solid concentration profiles were determined in pilot and industrial scale beds. These show an important accumulation upstream of the abrupt exit. The overall conclusion is that the gas flow can be considered to be plug flow with a radial velocity profile and a radial dispersion; the solid flow is slightly more dispersed due to the core—annulus structure and a high radial mixing.     

磁稳定床轴向液体分散特性

1 INTRODUCTION Magnetically stabilized beds (MSB) have at- tracted many research interests, owing to the unique feature of combination of characteristics for packed bed and conventional fluidized bed, especially in the field of biotechnology processes such as bioseparation or immobilized enzyme catalyzed systems. However, there are few reports about the effects of physical properties of fluids on the axial liquid dispersion coef- ficients in both L-S and G-L-S MSB. Siegell[1] , Goetz …     

Phase holdup and liquid dispersion in gas-liquid-solid circulating fluidized beds

Axial and radial profiles of gas and solids holdups have been studied in agas-liquid-solid circulating fluidized bed at 140mm i.d..Experimental results indicate that the axialand radial profiles of gas and solids holdups are more uniform than those in a conventionalfluidized bed.Axial and radial liquid dispersion coefficients in the gas-liquid-solid circulating fluidizedbed are investigated for the first time.It is found that axial and radial liquid dispersioncoefficients increases with increaes in gas velocity and solids holdup.The liquid velocity has littleinfluence on the axial liquid dispersion coefficient,but would adversely affect the redial liquiddispersion coefficient.It can be concluded that the gas-liquid-solid circulating fluidized bed hasadvantages such as better interphase contact and lower liquid dispersion along the axial directionover the expanded bed.     

Measuring and modelling lateral solid mixing in a three-dimensional batch gas—solid fluidized bed reactor

A 0.27 m diameter fluidized bed reactor has been designed to allow experimental measurement of the axial and radial mixing behaviour of the solids. A unique method has been developed which permits the continuous determination of solid tracer concentration with time at different radial and axial positions within the fluidized bed. Solids mixing has been described by a model in which vertical mixing is instantaneous and lateral mixing occurs by dispersion. The lateral solids dispersion coefficients have been evaluated at various operating conditions from the experimental results of tracer concentration versus time. Based on the results, a modification of an existing correlation is proposed.     

Axial dispersion characteristics in three phase fluidized beds

Axial dispersion coefficients in three-phase fluidized beds have been measured in a 0.152 m-ID x 1.8 m high column by the two points measuring technique with the axially dispersed plug flow model. The effects of liquid velocity (0.05–0.13 m/s), gas velocity (0.02–0.16 m/s) and particle size (3-8 mm) on the axial dispersion coefficient at the different axial positions (0.06–0.46 m) in the bed have been determined. The axial dispersion coefficient increases with increasing gas velocity but it decreases with an increase in particle size and exhibits a maximum value with an increase in the axial position from the distributor. The axial dispersion coefficients in terms of the Peclet number have been correlated in terms of the ratio of fluid velocities, the ratio of the panicle size to column diameter, and the dimensionless axial position in the bed based on the isotropic turbulence theory.