HEAT TRANSFER IN TRICKLE-BED REACTORS

The mechanism of radial heat transfer in two-phase flow through packed beds is examined. A model with 2 parameters: an effective radial thermal conductivity in the bed, k_e, and a heat transfer coefficient, h_w, at the wall, give a satisfactory interpretation of the radial temperature profile.

k_e was expressed in terms of a stagnant contribution, due to the heat conduction through the solid and the fluid in the void space, and a radial mixing contribution of the gas and liquid phases, due to the radial component of the velocity of both fluids. The radial mixing contribution of the liquid ( k_e)_L was compared with radial mass dispersion data, and a satisfactory agreement was obtained.

Moreover, ( k_e)was much higher than the gas mixing and the stagnant contributions.

Correlations for hw and k_e)_L have been proposed in accordance with the hydrodynamic regimes of the two-phase flow.     

HEAT TRANSFER PHENOMENA IN TRICKLE-BED REACTORS

The interrelations between heat transfer phenomena and liquid hydrodynamics in trickle-bed reactors are investigated. The experiments are carried out in a large size flow model (0·6 m diameter; 1 m long) using a procedure which allows to determine simultaneously the effective thermal conductivity and the dynamic liquid holdup )continuous reactor weighing(. Both phenomena are related to a same hydrodynamic feature: the liquid flow maldistribution which may be characterized by the particle irrigation rate. We propose a model describing both phenomena at the particle and bed scales. The change of scale between these two observation levels is achieved by means of the percolation theory. Tentative correlations of the effective thermal conductivity and the dynamic liquid holdup are derived. They show a good agreement with the experimental data reported in this work.     

鼓泡浆态反应器的传热研究

在内径为98 mm 的鼓泡浆态反应器内,研究了浸没表面与浆液之间的传热特性。在实验的条件下,模拟了浆态床 F—T 合成的操作。三相体系为氮气-液体石腊-石英砂所组成。实验中考察了不同因素对传热模系数的影响。采用95组实验数据进行关联,建立了新的传热模系数关联式:h/u_GρC_P=0.179[(u_G)~3ρ/ug]~(-0.25)(C_Pu/k)~(-0.66)该方程适用于气—液两相体系及小粒子三相浆态体系。     

LIQUID FLOW MALDISTRIBUTIONS IN TRICKLE-BED REACTORS

The effect of zeolite crystal size distribution on sorption has been studied for a packed bed subjected to a pulse input. The swelling crystal approach proposed recently has been used lo incorporate the effect of size distribution. The effluent concentration pulse and its moments show significant dependence on size distribution. Use of conventional moment analysis for uniform particles is shown to introduce significant error in the estimates of sorption parameters. A simple technique is suggested to rectify these estimates for size distribution effects.     

HEAT TRANSFER IN PACKED BED REACTORS WITH ONE PHASE FLOW

An extensive array of literature data on the heat transfer from a reactor wall to a fluid flowing through a packed bed and those obtained from some experimental runs were interpreted with a model containing two parameters: k_e, (effective radial thermal conductivity within the bed) and h_w (heat transfer coefficient at the wall).

Both parameters were considered in terms of a stagnant contribution (due to the heat conduction through the solid particles and the fluid in the void space) and a radial mixing contribution (due to the heat convection by turbulent mechanism.

The stagnant contribution was interpreted with a model similar to that proposed by Kunii and Smith (1966) for heat transfer in a packed bed with motionless fluid.

General correlating equations for calculating the stagnant and the turbulent contributions of both k_e, and h_w are proposed.     

DISPERSION IN TRICKLE-BED REACTORS WITH PULSING GAS-LIQUID DOWN-FLOW

Pulsing flow in trickle-bed reactors occurs at high gas and liquid input flow rates. It refers to the periodic passage of liquid rich slug and gas rich pulse down the column. A model based on the combination of the method of characteristics and Monte Carlo simulation of liquid phase dispersion in the slug and the pulse is developed to predict liquid dispersion in a pulsing column packed with non-porous, spherical particles. The predicted Peclet number as a function of Reynolds number shows good agreement with experimental data. In addition, it is found that the pulsing phenomenon does not contribute significantly to dispersion.     

DISPERSION IN TRICKLE-BED REACTORS WITH PULSING GAS-LIQUID DOWN-FLOW

Pulsing flow in trickle-bed reactors occurs at high gas and liquid input flow rates. It refers to the periodic passage of liquid rich slug and gas rich pulse down the column. A model based on the combination of the method of characteristics and Monte Carlo simulation of liquid phase dispersion in the slug and the pulse is developed to predict liquid dispersion in a pulsing column packed with non-porous, spherical particles. The predicted Peclet number as a function of Reynolds number shows good agreement with experimental data. In addition, it is found that the pulsing phenomenon does not contribute significantly to dispersion.     

COMPUTATION OF THREE-DIMENSIONAL FLOW AND HEAT TRANSFER IN GAS AGITATED REACTORS

Equations have been written, and a computational scheme developed, for predicting flow fields and heat transfer phenomena associated with asymmetric gas-driven flows in systems of cylindrical geometry. Cases considered include axisymmetric gas injection wherein the gas-liquid plume is either of constant radius, or expands in a cone-like manner towards the surface. It is, thereby, shown that the model compares reliably with existing axisymmetric computations. Non-central gas flows are then considered and resulting 3-D liquid flow patterns computed. Finally, the temperature fields in an initially stagnant and thermally stratified liquid resulting from the local introduction of gas on the bottom surface is illustrated     

固定床反应器中传热参数的相关分析

传热参数在无反应下敏感性不高，改变模型参数的形式只是数学上的变换，并不能提高参数估计的准确程度，放热反应存在下每个参数都变得很重要，因此对参数的准确程度提高了很高要求。     

A MODEL FOR PULSING FLOW IN COCURRENT DOWN-FLOW TRICKLE-BED REACTORS

Based on macroscopic mass and momentum balances, a model is developed to characterize the hydrodynamic behavior of pulsing flow in cocurrent down-flow trickle-bed reactors. Predictions of the model regarding liquid saturation, length and velocity of both the liquid rich slug and gas rich pulse are reported. Also presented are experimental data of an air-water system for the effects of varying gas and liquid flow rates on pulse frequency, total pulse length and apparent slug velocity. To confirm the theory, predictions are compared with measured overall pressure gradient. Agreement is reasonably good.     

AXIAL AND RADIAL DISPERSION IN TRICKLE-BED REACTORS WITH TRICKLING GAS-LIQUID DOWN-FLOW

In cocurrent down-flow trickle-bed reactors in the trickling flow regime, the liquid flows down the column in the form of liquid films or rivulets on the surfaces of packings. Depending on system variables such as liquid flow rate, packing size, surface tension, etc., the liquid flow paths can be so well-connected that all packings are completely wetted or some of them remain dry. Dispersion of the liquid phase is confined within, and is controlled by the topology and flow field of, the flow paths. To provide a model for dispersion, flow paths are first developed on a computer-generated, two-dimensional packed column of uniform spheres. Then axial and radial dispersion coefficients are determined based on Monte Carlo simulation. Model predictions are in general agreement with experimental data in the literature.     

整装反应器起燃阶段传热的数学模拟

整装反应器已经广泛地应用于催化领域,其暂态传热特性对反应器的起燃有重要作用。本文研究了整装反应器在反应点火前起燃阶段的传热,通过对绝热反应器建立的简单一维传热数学模型的求解,分析了固相轴向导热、气流性质、催化剂性质及反应器设计参数对传热的影响,比较了用金属体和陶瓷体时基体热响应的不同。     

A STUDY OF TRICKLING-TO-PULSING FLOW TRANSITION IN TRICKLE-BED REACTORS (TBR)

In this work, visual observation as well as the probability density function and the standard deviation of the pressure drop fluctuation were used for determination of the boundary of trickling-to-pulsing flow transition. Extensive experimental work was carried out with air and three liquid media on four kinds of packings. Experimental results indicate the transition depends on the physical properties of liquid (viscosity and surface tension), gas and liquid flow rates as well as the size and shape of packing particles. The Baker coordinates (Baker, 1954) used in the flow map as proposed by Charpentier and Favier (1975) was examined. It is found that application of the flow pattern map in this form is of rather limited universality. Two new correlations were proposed with satisfactory prediction of the liquid flow rates for the transition of trickling-to-pulsing flow.     

HEAT TRANSFER IN ISOTROPIC TURBULENCE

The direct interaction approximation (Kraichnan 1959) was used to calculate thermal eddy diffusivities, intensities or temperature fluctuations, and various correlation functions for heat transfer in an isotropic turbulent flow with a uniform mean temperature gradient. The calculated results compare reasonably well with values from experiments in a heated wind tunnel and may be of value in the analysis of more complex heat transfer processes.     

HEAT TRANSFER IN MULTIPHASE CONTACTORS

A procedure for the prediction of wall-bed heat transfer coefficient for bubble columns and gas-solid fluidized beds is developed on the basis of hydrodynamic behavior of these contactors. A comparison between the predicted and experimental values of heat transfer coefficient over a wide range of design and operating variables is presented. An attempt is made to analyze the occurrence of maxima in heat transfer coefficient with respect to gas velocity in the case of fluidized beds. A procedure for the calculation of the optimum superficial gas velocity is outlined.     

HEAT TRANSFER IN MULTIPHASE CONTACTORS

A procedure for the prediction of wall-bed heat transfer coefficient for bubble columns and gas-solid fluidized beds is developed on the basis of hydrodynamic behavior of these contactors. A comparison between the predicted and experimental values of heat transfer coefficient over a wide range of design and operating variables is presented. An attempt is made to analyze the occurrence of maxima in heat transfer coefficient with respect to gas velocity in the case of fluidized beds. A procedure for the calculation of the optimum superficial gas velocity is outlined.     

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.     

PARTICLE-LIQUID MASS TRANSFER IN MULTI-IMPELLER AGITATED THREE PHASE REACTORS

The mass transfer coefficient between solid particle and liquid has been measured in high aspect ratio multiimpeller agitated three phase (gas-liquid-solid) reactors. Experiments were conducted in 0.15 and 0.30 m id acrylic columns of lm length each. Two types of impeller were used: disk turbine (DT) and pitched turbine downflow (PTD). Air used as the gas phase was introduced through a ring sparger of 0.8 D size located 0.1 T distance above the bottom. The interimpeller spacing was maintained at the tank diameter. The range of gas velocity used was 0-15 mm/s. A unique correlation between particle-liquid mass transfer coefficient and critical impeller speed for solid suspension has been presented.