End effects on interphase heat and mass transfer in a cubically packed bed of spheres

Heat (and mass) transfer data were obtained for the constant rate evaporation of water from two layers of porous spheres in simple cubic array (? = 0.4764) to a flowing air stream, over a particle Reynolds number range of 200-900. It was found that the cumulative effect of adding inert layers of non-porous spheres at the entrance and exit was to lower (rather than raise, as in the case of randomly packed beds) the extent of heat and mass transfer between the active spheres and the fluid, the minimum number of inert layers required to eliminate end effects being three at the entrance and two at the exit. The specific effect of adding only one inert layer to the entrance, with none at the exit, was to raise somewhat the extent of heat and mass transfer. Interpretations are offered for these trends. The simple cubic assemblage showed substantially lower values of the heat (and mass) transfer factors than those of randomly packed beds.     

Numerical determination of fluid-to-particle mass and heat transfer coefficients in packed bed reactors

A method based on particle-resolved CFD is built and validated, to calculate the fluid-to-particle mass and heat transfer coefficients in packed beds of spheres with different tube-to-particle diameter ratios (N) and of various particle shapes with N = 5.23. This method is characterized by considering axial dispersion. The mass and heat transfer coefficients increase by 5%–57% and 9%–63% after considering axial dispersion, indicating axial dispersion should be included in the method. The mass and heat transfer coefficients are reduced as N decreases. The catalyst particles without inner holes show higher mass and heat transfer coefficients than the ones with inner holes, because of unfavorable fluid flow in inner holes. The bed of trilobes has the highest mass and heat transfer coefficients, being 85% and 95% higher than the one of spheres. This work provides a versatile method and some useful guidance for the design of packed bed reactors.     

Coupled heat and mass transfer in a packed bed with the lewis relation not satisfied

This paper forms part of the development of an analogy between heat transfer and coupled heat and mass transfer. It has been shown that for a particular case of coupled heat and mass transfer between a packed sorbent bed and the fluid flowing through, the linearised governing equations have the same form as those for heat transfer alone in the system. In that case transport of heat and sorbate between the fluid mixture and sorbent was determined by surface transfer coefficients, and the Lewis relation between these coefficients was satisfied.The Lewis relation is far from satisfied for many fluid mixtures, and when lumped transfer coefficients are employed to account for diffusion in the sorbent. The effect of such departures on system behaviour is explored by a numerical method. Also, the above analogy method is extended to deal with such systems. Comparison of results from these two methods shows that the extended analogy method gives a fairly close representation of the system behaviour resulting from a step change in the inlet fluid state.     

Heat transfer simulation in a raining packed bed exchanger

A model is presented that predicts the thermal performance of a raining packed bed heat exchanger (RPBE) from hydrodynamic data. The main feature of this gas-solids countercurrent heat exchanger is the presence of a packed zone which greatly enhances energy transfer between the two phases by slowing down the falling particles. The model based on the Ergun equation for evaluating an effective solids hold-up in the packed zone correctly predicts the fact that the efficiency passes by a maximum as the hot gas velocity increases. Experimental results obtained with sand particles of 205 μm mean dia. in a column filled with Pa11–15 rings agree reasonably well with the predicted ones.     

Dynamics of droplets and mass transfer in a rotating packed bed

To do further research on the mass‐transfer mechanism in rotating packed bed (RPB), dynamics of droplets in a RPB are studied by an analytical approach combined with a series of laboratory measurements. Based on the results of the fluid dynamics, mathematical models of mass‐transfer coefficient and mass‐transfer process in RPB are proposed, respectively. Mass‐transfer experiments in RPB are also carried out using ethanol–water solution. By comparison, the results of simulation agree well with that of the experiment, which demonstrate that both hydrodynamic model and mass‐transfer models can better describe the real conditions of RPB. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2705–2723, 2014     

超重力旋转床气相传质CFD仿真分析及实验验证

为了研究闭式循环柴油机中超重力旋转床的水吸收二氧化碳过程,开展了仿真与实验研究。建立旋转床三维物理模型,采用欧拉-拉格朗日两相流模型对气液流场进行流体力学计算。以希格比的溶质渗透理论为基础,设置气相源项,对液相吸收二氧化碳过程进行仿真。并通过实验验证不同操作参数对其吸收性能的影响。结果表明：吸收过程在靠近丝网与分布器位置较为强烈,吸收性能随着旋转床转速与吸收因数增加而加强,随着进气浓度增加而降低。仿真中设置的源项能模拟床内吸收的过程,仿真与实验得出的液相传质单元数有较高的一致性,平均相对误差在10%左右,最大误差为20.6%,在高进气浓度下仿真偏差较大。总的来说,仿真能较好地模拟实际二氧化碳在旋转床中的吸收过程。     

错流旋转填料床的质、热同传性能及传热机理研究

为了研究错流旋转填料床的质、热同传性能,采用热空气-氨水体系,考察了进气温度T、超重力因子β、液体喷淋密度q和气速u对错流旋转填料床传热性能的影响,在相同实验条件下对比了丝网填料和乱堆填料的传热性能。研究结果表明：气相体积传质系数k_ya_e、体积传热系数(Ua)_s随进气温度、超重力因子、气速、液体喷淋密度的增大而增大;传热效率ε、传热面积A随超重力因子、气速、液体喷淋密度的增大而增大;传热系数K随超重力因子、气速、液体喷淋密度的增大几乎不变,从而揭示了错流旋转填料床强化气液直接传热的机理是通过提高传热面积进而提高体积传热系数,而不是显著提高传热系数。在相同条件下,以丝网为填料时k_ya_e和(Ua)_s分别是乱堆填料的1.09~1.63倍和1.24~3.53倍。     

Experimental researches on mass and heat transfer in new typical cross-flow rotating packed bed

New typical cross-flow Rotating Packed Bed(RPB)called multi-pulverizing RPB was manufactured.There is enough void in multi-pulverizing RPB,where liquid easily flows and is repeatedly pulverized by light packing,which decreases the material consumed,lightens the weight,and compacts the structure.Mass and heat transfer property in the new type PRB were studied by two experimental models.In the mass transfer model,the axial fan pumping gas press is only 100 Pa,mass transfer coefficient and volumetric mass transfer coefficient are similar to countercurrent RPB,which are an order quantity lager than that in the conventional packed tower.In the heat transfer experiment,the axial fan pumping gas press is only 120 Pa;volumetric heatwhich especially suits the treatment of large gas flow and lower gas pressure drop.     

Modelling of non-catalytic gas—solid reactions—II. Transient simulation of a packed bed reactor

A Transient model for packed bed non-catalytic reactors, which avoids many of the simplifying assumptions of the earlier models, has been developed. The model includes the effects of inter- and intra-pellet transfer resistances and the additional effects of axial dispersion in the bulk fluid. The solution procedure to the system of equations is based on the orthogonal collocation method. The effects of various parameters on the temperature rise encountered in the bed and the possibility of reducing this rise by perturbing the inlet concentration and temperature of the gas are examined.     

Gas—particle heat transfer in a crossflow moving packed bed heat exchanger

A programme of work on a moving packed bed heat exchanger, whereby a gas is blown vertically upwards through a horizontally moving packed bed of particles, is described in this paper. Such a device can be used for the process heating or cooling of particulate solids. In the work rigorous and simplified analyses to describe the process of gas—particle heat transfer in the system were developed and the application of these analyses demonstrated by a series of experiments on a small-scale unit.     

A three-zone mass transfer model for a rotating packed bed

A rotating packed bed (RPB) is recognized for its merits in chemical process intensification. In most studies of RPB mass transfer modeling, however, the effects of the end and cavity zones have not been taken into consideration, since it was very difficult to distinguish the end and bulk zones by hydrodynamics and mass transfer process. In this work, the radial thickness of the end zone was obtained by developing a probability method and imaging experiments to separate the end and bulk zones. A three-zone model, including end, bulk, and cavity zones, of the overall gas-side volumetric mass transfer coefficient (K_Ga)t was first established. Experiments of dissolved MEA chemisorption of CO₂ were carried out to validate the proposed three-zone mass transfer model. The results of the MEA-CO₂ absorption experiments showed that the experimentally obtained values of CO₂ absorption efficiency were in agreement within ±20% with the model predictions.     

The heat transfer coefficient between a particle and a bed (packed or fluidised) of much larger particles

The heat transfer coefficient has been measured for a heated phosphor-bronze sphere (diam. 2.0, 3.0 or 5.56 mm) added to a bed of larger particles, through which air at room temperature was passed. The bronze heat transfer sphere was attached to a very thin, flexible thermocouple and was heated in a flame to before being immersed in the bed. The cooling of the bronze sphere enabled the heat transfer coefficient, h, to be measured for a variety of U/U_mf, as well as diameters of both the particles in the bed and the heat transfer sphere. It was found that before the onset of fluidisation, h rose with U, but h reached a constant value for U?U_mf. These measurements indicate that in this situation (of a relatively small particle in a bed of larger particles) all the heat transfer is between the hot bronze sphere and the gas flowing over it. Consequently, a Nusselt number, based on the thermal conductivity of the gas, is easy to define and for U?U_mf (i.e. a packed bed), Nu is given by

     

Efficient simulation of a separation column with axial diffusion and mass transfer resistance

The optimization of simulated moving bed systems is a complex task, and one of the difficulties is the lack of simulation methods that are sufficiently accurate and fast to be incorporated in the optimization algorithms. This paper presents a simulation of an adsorption column with finite differences based on a Lagrangian approach. The results obtained with this integration method were compared to values reported in the literature; the comparison shows that the accuracy of the integration method is not lower than that obtained with published methods and that this integration method requires a much lower cost in computation time. Various simulations were compared with experimental data for injections of caffeine and sodium 2-naphthalenesulfonate and with published results for the separation of isomers of omeprazole. The effects of axial diffusion and resistance to mass transfer on the elution curves were studied, and the simulation results were compared with the known theoretical analytical solution for a linear isotherm.     

Simultaneous heat and mass transfer in a packed binary distillation column

A simultaneous heat and mass transfer model was applied to a binary system in a 3-in. distillation column packed with $\text{1}\text{4}$in. Raschig Rings. MaThe experiments carried out with the toluene-trichloroethylene system show that the liquid phase is saturated, indicating no resistance to mass transfe     

固定床中丝状颗粒的传热传质特性

目前对于固定床中丝状填充颗粒传热传质特性的认识仍处于初始阶段。为了能够从颗粒尺度的微观层面揭示丝状颗粒与气体、颗粒与颗粒之间的热、质传递机理,建立了一种丝状颗粒传热传质数学模型,之后将离散单元法与计算流体力学相结合,对实验中较难获得的床层局部流动及传递信息进行了数值模拟,并着重分析比较了气流入口温度以及表观气速等关键因素对固定床中丝状颗粒温度和含水率变化的影响规律。通过模拟结果与实验数据的对比,验证了所建模型的可行性。研究结果表明:同一时刻,固定床中颗粒温度大体上是随着床层高度的增加而降低,含水率则是随着床层高度的增加而升高;气流入口温度对于固定床中丝状颗粒平均温度的提升起着主导作用,而颗粒的传质速度则受表观气速的影响更为明显。     

Heat,mass and momentum transfer in packed bed distillation columns

This contribution presents the basic equations for heat, mass and momentum transfer in multicomponent packed bed distillation processes. In some situations, the use of strongly simplified models is justified, but when approaching more difficult and, at the same time, economically more interesting regions of operation where non-linear effects are significant, these models are likely to fail. Consequently, a more rigorous vapour-liquid equilibrium model should be employed since the pressure drop in the column will not be negligible in those regions. Furthermore, neither constant parameter hold-ups nor heat and mass transfer coefficients are assumed. Simulations demonstrate some interesting process properties. The impact of the surroundings on the process is discussed and a three-dimensional model extension is outlined.     

新型旋转填料床强化气膜控制传质过程

转子结构为相互嵌套填料环的新型旋转填料床是基于强化气膜控制传质过程的新型高效传质设备,可适用于受气膜控制的吸收、精馏和低浓度工业气体的净化等过程。分别以化学吸收体系CO2-NaOH和物理吸收体系NH3-H2O测定了不同气量、液气比和超重力因子条件下的有效比表面积a和气相体积传质系数kya,并由此得到气相传质系数ky,对其传质性能进行研究。实验结果表明：a、kya和ky均随着气量、液气比和超重力因子的增大而增大。通过对比可知,新型旋转填料床的气相体积传质系数在相近操作条件下是文献逆流旋转填料床的2倍。并对实验数据进行了回归,拟合出了a、kya和ky分别与气相雷诺数ReG、液相韦伯数WeL和伽利略数Ga之间的关联式。     

Convective heat transfer and solid conversion of reacting particles in a copper(II) chloride fluidized bed

This paper develops a predictive model of convective heat transfer and conversion of cupric chloride particles in a fluidized bed reactor of a copper–chlorine (Cu–Cl) cycle of thermochemical hydrogen production. The hydrolysis reaction of particles in the fluidized bed is endothermic and it requires excess steam for complete conversion of cupric chloride solid. The excess steam supply may be used for partial heat supply to the endothermic reaction, and also to avoid defluidization in the bed. To avoid defluidization, the change of gas flow in the bed due to the reaction should be minimized at a given operating condition. The model predicts the maximum possible steam inlet temperature, steam conversion, amount of partial heat supply, and also gas flow rates through the bed to avoid defluidization. The new model presents significant new insight by analyzing the hydrodynamic and mass transport processes, considering the equilibrium limitation on the conversion of cupric chloride solid. The model results indicate that the chemical reaction requires a high mole ratio of steam for complete conversion of cupric chloride particles. The maximum steam conversion is limited by temperature, pressure, and the presence of hydrogen chloride gas. The maximum conversion of steam at 400 °C is 3.75% and it requires excess steam of 12.8 moles per unit mole of cupric chloride solid for complete conversion of solid. The heat supply by steam for the reaction, as well as raising the solid feed to the reaction temperature, varies with reaction temperature. The paper also adds significant new insight by analyzing the steam flow requirement in terms of temperature, conversion rate, and quality of fluidization. Additional new results are presented and applications discussed for the Cu–Cl cycle of nuclear hydrogen production.     

A study of mass transfer in a packed bed reactor by electrochemical technique

Electrochemical method was used to study the mass transfer between the solid particles and the flowing liquid in a packed bed. From the limiting current of a single active particle immersed in inactive glass particles of the same size and shape, the mass transfer coefficients can be derived.Various size and shape of packing particles were used. The experimental results indicate that smaller packing particles have higher mass transfer coefficient. In the meantime, spherical packing particles have higher mass transfer coefficients than cylindrical particles of the same equivalent diameter. However they approach each other when liquid flowing velocity is increased.The wall-effect of the reactor on mass transfer was also observed.     

Simulation of heat transfer and hydrodynamics for metal structured packed bed

Modeling and simulation based on computational hydrodynamics and heat transfer for metal structured packed bed are carried out to predict the flow field and temperature field, and to evaluate its performance in transport aspect. The comparison between the simulation results for the metal structured packed bed and the experimental heat transfer performance as well as pressure drop of the conventional pellet packed bed is made, which quantitatively validates that transport performance of the metal structured packed bed is much better. Furthermore, the effects of geometric parameters and the property of solid phase on heat transfer of the metal structured packed bed are discussed. It is found that at low Re, the specific surface area is a key factor to determine the heat transfer capability of the structured bed. However, when Re turns to be high, the property of solid phase and voidage of the structured packed bed will play an important role in the evaluation of its heat transfer. In light of above results, some feasible methods are available to enhance the heat transfer performance.