On the Prandtl or Schmidt number dependence of the turbulent heat or mass transfer coefficient

Numerical experiments using a direct numerical simulation (DNS) of turbulent flow between two parallel plates in conjunction with Lagrangian scalar tracking (LST) of trajectories of thermal markers in the flow field are conducted for Prandtl or Schmidt numbers between 0.01 and 50,000. The LST methodology is used to generate mean temperature profiles as a function of the entry distance in the case of a step change in heat or mass flux at the walls of the channel. The heat transfer coefficient and the Nusselt number ratio, Nu(x)/Nu(x→∞), downstream from the step change in the wall flux are determined for the range of Pr or Sc fluids examined. Relations between the heat or mass transfer coefficient at the fully developed part of the channel and Pr or Sc are proposed for low and high Pr or Sc cases. Finally, unified correlations, which provide the heat or mass transfer coefficient for all Pr or Sc, in the Reynolds number range examined, are proposed. Also, the exponent of the asymptotic dependence of the eddy diffusivity close to the wall is obtained.     

合成MgO-CaO砂的水化反应动力学研究

研究了合成MgO CaO砂的水化反应过程 ,建立了水化反应物理模型 ,推导出水化反应动力学公式。结果表明 :合成MgO CaO砂水化反应机理呈现化学反应速度控制和扩散速度控制交替进行的特性     

On the second-order moment turbulence model for simulating a bubble column

Two versions of the second-order moment two-phase turbulence model are proposed in this study for simulating bubble-liquid two-phase turbulent velocity fluctuations and their interactions in bubble-liquid flows under the dispersed bubble regime. One of them is a full transport equation model; the other is an algebraic stresses model. The proposed model is used to simulate liquid and gas mean velocities, gas volume fraction, liquid and gas Reynolds stresses and turbulent kinetic energy in a 2-D bubble column. Furthermore, the bubble and liquid velocities, Reynolds stresses and gas volume fraction are measured using the PIV. The simulation results are in good agreement with the PIV results and experimental data in the literature. The studies reveal the liquid recirculation and bubble up-rising flow patterns, and anisotropic liquid and bubble normal Reynolds stresses. Bubble fluctuation is observed to be stronger than liquid fluctuation. Moreover, both the liquid velocity gradient and bubble-liquid interaction are important for the generation of liquid turbulence.     

Mass transport effects during measurements of gas-solid reaction kinetics in a fluidised bed

Design, modelling and scaling of many gas-solid reaction processes in a fluidised bed (FB) require reliable intrinsic kinetic data obtained under conditions similar to those in industrial-scale units. The determination of such data in a laboratory-scale FB seems to be the best route. Despite this, owing to practical difficulties, few experimental reactivity data are available from FB. One of the drawbacks of FB reactors is the plausible interference of physical processes on kinetic measurements. In this work a simple methodology is developed for the assessment of fluid-dynamic and mass transport effects during kinetic experiments in FB (FBKE). A modelling approach is proposed, which combines a kinetic particle model with a simple two-phase flow model. The parameters resulting from the model are expressed in terms of three observable quantities, making it possible to evaluate the transport effects in a straightforward way from gas concentration measurements. Charts for direct evaluation of transport effects in FBKE are included. The analysis presented aims at facilitating the selection of optimum operating conditions for FB tests to determine gas-solid kinetics. Moreover, it may support dimensioning of new rigs designed for this purpose. This study primarily aims at the assessment of transport effects in batch-operated laboratory FBKE, where isothermal conditions are assumed and only one reaction occurs: the reaction between CO₂ and char is taken as an example. The methodology developed can be applied to other isothermal systems to estimate the influence of diffusional effects on the observed reaction rate, i.e., char oxidation and other catalytic or non-catalytic systems.     

多种复杂化学反应动力学统一的数值模拟

研究各种经典复杂化学反应动力学方程如简单级次反应、平行反应、连续反应和对峙反应等的求解方法时,可以发现它们之间存在共同点,并得到统一的求解形式.文中给出了将多种复杂化学反应动力学的求解统一起来的方法,并用C++程序语言,编制了可以求解物理化学教学中各种经典复杂化学反应动力学方程的数值模拟程序.使用该程序,只要根据具体情...     

Effects of different molecular weights of polystyrene on the acylation reaction and on the reaction kinetics

In this study, chemical modification reactions of polystyrenes with different molecular weights with maleic and acetic anhydrides in the presence of a catalyst [BF₃ · O(C₂H₅)₂] were examined. The number of polyfunctional groups bound to the polymer structure changed according to the molecular weight of the polymer, and more functional groups were bound to lower molecular weight polystyrenes. The bound functional groups and their amounts were determined with spectral and chemical analysis methods. The effects of the molecular weights of the polystyrenes on the chemical modification kinetics with maleic and acetic anhydrides in the presence of BF₃ · O(C₂H₅)₂ were examined. The kinetic parameters of the reaction and its activation energy were calculated. The higher the molecular weight of polystyrene was, the lower its rate of modification was and the greater its activation energy was; the lower the molecular weight of polystyrene was, the higher its rate of modification was and the lower its activation energy was. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 253–259, 2005     

Analysis of the kinetics of agglomerate erosion in simple shear flows

A model for the erosion kinetics of particle agglomerates in simple shear flows has been developed. The erosion rate is taken to be proportional to the difference between hydrodynamic and cohesive forces and to the rotation velocity of the dispersing agglomerate. For dispersion under identical hydrodynamic conditions, the model predicts faster erosion for larger agglomerates. Moreover, at equivalent hydrodynamic stress, erosion is enhanced at higher shear rates. Dispersion experiments using silica agglomerates of various densities and liquid low molecular weight polymers (e.g., poly(dimethyl siloxane)) of different viscosities were conducted in an oscillatory shear flow device. The experimental results validate the erosion model; the general shape of the erosion kinetic curve, and the effect of viscosity, shear rate and agglomerate size on dispersion kinetics are well predicted. The model can also predict erosion for agglomerates with fractal structure.     

Reaction kinetics determination based on microfluidic technology

Microfluidic technology has been successfully applied to determine the reaction kinetics relying on its great characteristics including narrow residence time distribution,fast mixing,high mass and heat trans-fer rates and very low consumption of materials.In this review,the recent progresses about the reaction kinetics measured in microreactors are comprehensively organized,and the kinetic modeling thoughts,determination methods and essential kinetic regularities contained in these studies are summarized according to the reaction types involving nitration,oxidation,hydrogenation,photochemical reaction,polymerization and other reactions.Besides,the significant advances in the innovation of micro-platform are also covered.The novel reactor configuration methods were established mainly to achieve rapid and efficient data collection and analysis.Finally,the advantages of microfluidic technology for the kinetic measurement are summarized,and a perspective for the future development is provided.     

Flow field and residence time distribution simulation of a cross-flow gas-liquid wastewater treatment reactor using CFD

A three-dimensional Eulerian-Eulerian two-phase approach has been used for the simulation of a cross-flow gas-liquid wastewater treatment reactor. Two different turbulence models have been tested: the k-ε and Reynolds Stress Model (RSM) models. Bubble induced turbulence source terms have been added to these models. Numerical results have been validated using Laser Doppler Velocimetry (LDV) measurements. Simulations with both turbulence models successfully predicted the hydrodynamics of the reactor. Then particle tracking with a stochastic approach has been used to calculate residence time distributions (RTD) with the flow previously simulated. It has been shown that dispersion in the reactor is primarily due to turbulence. Results have been compared with experimental RTD for various liquid and gas flowrates both on a bench scale and full scale plant. The RSM model accurately predicted the dispersion whereas the standard k-ε model slightly underestimated the dispersion.     

A theoretical study of solvent effects on Kolbe-Schmitt reaction kinetics

A theoretical DFT study was employed to confirm the Kolbe-Schmitt reaction mechanism and investigate solvent effects on this reaction. The use of a solvent in the Kolbe-Schmitt reaction is desirable to facilitate a homogeneous reaction mixture and potentially improve the reaction rate. The candidate solvents were designed using computer aided molecular design (CAMD) and tested using DFT solvation calculations. The results from the quantum mechanical calculations were then used to determine the rate constants for each elementary step, the overall reaction yields and the corresponding residence time. The methodology was tested on the reaction without solvent, with solvents reported in the literature, and with the designed solvents. The study revealed that in the presence of solvents with high dielectric constant the reaction becomes reversible, leading to low product yields.     

A distribution kinetics model for chromatography of block-like molecules

We develop the theory for chromatographic separation of isomers such as polycyclic aromatic hydrocarbons, polychlorinated biphenyls, dioxins, steroids, and carotenoids based on shape-selective mechanisms for either exclusion or adsorption. Block-like solute transport and retention on oriented stationary phases are modeled by a dispersed-flow chromatographic equation combined with simultaneous exclusion partitioning and shaped-based adsorption. Molecules to be separated are considered distributed continuously with respect to the shape and sorption parameters. The population balance equations of distribution kinetics provide the governing differential equations that are solved for the temporal moments of the concentration in a chromatographic column. By including dispersive mass transport effects, the model allows estimation of peak variance and HETP and is thus an improvement over theories that focus solely on retention time. The model predictions are compared with experimental data from the literature for gas chromatography.     

镁钙砂细粉碳酸化反应的热分析动力学研究

采用单一扫描速率法对镁钙砂细粉的碳酸化反应进行了热分析动力学研究,为镁钙砂颗粒表面碳酸化改性动力学研究提供相关信息。结果表明:镁钙砂中的MgO仅发生轻微碳酸化,因其碳酸化温度较低、反应量较小,且反应产物MgCO3在400℃后即开始分解,因此不会对镁钙砂中CaO的碳酸化反应产生太大影响;镁钙砂中CaO碳酸化反应的最大转化速率出现在700℃左右,碳酸化反应前期(相对转化率约小于38%)机理可用n=2的Avrami-Erofeev成核生长模式(机理)函数描述,后期(相对转化率约大于38%)则可用Ginstling-Brounshtein三维扩散模式(机理)函数描述。     

Light-off curve of catalytic reaction and kinetics

The light-off curve is the conversion-temperature plot of a catalytic reaction; it is usually used in catalyst development. The possibility to deduce kinetic information such as the apparent reaction order or mass transfer limitation from the shape of the curve is examined here. The light-off plots obtained by numerical simulation with several different rate equations are compared between themselves and with experimental curves. This is illustrated with the oxidation of carbon monoxide and methane on noble metal catalysts with different degree of mass transfer resistance. The comparison is complemented with the Arrhenius plots, i.e. the logarithm of the apparent rate constant versus the reverse of temperature.From the shape of the curves, the three following situations can be discriminated: first-order kinetics, negative first-order kinetics in the absence or in the presence of external mass transfer limitation. The range of diffusional limitation is well defined for negative-order reaction. However, the mass transfer resistance is difficult to evidence for first-order reaction. It is observed that the Langmuir-Hinshelwood reaction kinetic model is not convenient to represent the effect of strong external transport limitation on a reaction in the regime of reactant inhibition (negative apparent reaction order).     

碳酸钠脱除湿法磷酸中氟的反应动力学研究

采用碳酸钠为脱氟剂,研究了湿法磷酸的脱氟反应动力学,研究结果显示碳酸钠脱氟反应为二级反应,反应速率常数与温度的关系为k＝0.2487exp(-3455.8/T),反应活化能为28.73kJ/mol.动力学方程可靠性较好,25℃及30℃下动力学方程计算值与实验值的相对误差小于2％,40℃下动力学方程计算值与实验值的相对误差小于7％.     

碳酸钙分解动力学TGA实验数据的新诠释

对热学分析(TGA)实验测得的CaCO3分解反应动力学数据的解释曾一直采用了气固反应缩核模型的假设，这一简化处理方法导致了难以对实验中反应过程和输运现象间的关系作出定量描述，所获的动力学数据均具表观性质。作者基于气固反应结构模型观点提出了描述CaCO3分解反应等TGA实验的数学模型及相关数值模拟程序，并对文献记载的一组CaCO3分解动力学TGA实验数据作了新诠释。     

Hydrogenation of acetophenone using a 10% Ni supported on zeolite Y catalyst: kinetics and reaction mechanism

The kinetics of hydrogenation of acetophenone was studied using a 10% Ni supported on zeolite Y catalyst in a temperature range 353–393 K. The effect of H₂ pressure, initial concentration of acetophenone and catalyst loading on the concentration-time profiles was studied. Water formed during the course of hydrogenation showed a strong inhibiting influence on the rate of reaction. A rate equation has been proposed based on L–H type rate mechanism assuming that the reaction between the non-competitively adsorbed hydrogen and the adsorbed organic substrate as rate limiting step. A semi-batch reactor model was developed to predict the concentration-time, H₂ consumption-time profiles at different sets of initial conditions. The model predictions were found to agree with experimental data very well at all temperatures. This model incorporates the inhibition of hydrogenation rate due to water. The inhibiting effect of water is also explained based on quantum chemical calculations.     

基于简单碰撞理论化学反应动力学数值模拟

寻找合适和有效的数值模拟方法,对于越来越复杂的化学反应动力学研究有着重要的意义.以简单碰撞理论(SCT)模型为出发点,可以得到一种准确有效的化学动力学方程的数值求解方法.基于SCT的化学反应动力学数值模拟方法编程方便简单,具有较高的精度,与化学动力学过程机理相似,能体现化学动力学的过程.使用该方法对n级简单反应、连续反...     

Photocatalytic intrinsic reaction kinetics. II: Effects of oxygen concentration on the kinetics of the photocatalytic degradation of dichloroacetic acid

The effect of oxygen on the reaction kinetics of the photocatalytic oxidation of dichloroacetic acid at pH below the titanium dioxide isoelectric point (pH from 5 to 4) was studied. A kinetic scheme based on the direct hole attack to the dichloracetate ion in conjunction with the classical role of oxygen acting as an electron acceptor is proposed. Oxygen also intervenes in a direct addition reaction to one of the intermediate reaction products. Experiments were conducted in a well-mixed, small, flat plate reactor employing Aldrich titanium dioxide as the photocatalyst in the suspension mode and using polychromatic irradiation with energy in the range from 275 to 385 nm in wavelength. A complete mathematical model, including the effect of the absorbed radiation intensities and catalyst concentration was developed. Experimental data agree well with theoretical predictions employing just two kinetic parameters derived from the proposed reaction mechanism.     

Study of auto-accelerated reactions in heterogeneous liquid/liquid mixture: Reaction products effects on mass transfer kinetics

The alkaline hydrolysis of aromatic esters exhibits auto-accelerated kinetics when performed under two phase conditions. This auto-acceleration behavior is linked to reaction products effects (surfactant and co-solvent) on mass transfer kinetics. The co-solvent increases the equilibrium concentration of organic compounds in aqueous solution, while the surfactant modifies the mass transfer coefficient and interfacial area. In this research work, the selected reaction is the alkaline hydrolysis of methyl benzoate carried out in a calorimetric reactor Mettler RC1 in isothermal semi-batch mode. The parameters estimation of mass transfer and chemical kinetics has been conducted using thermal power profile. Two functions—giving the evolution during reaction of the mass transfer coefficient and the equilibrium concentration—are proposed accordingly to surfactant and co-solvent concentrations.     

化学反应动力学应用于约束爆炸的研究进展

约束爆炸会产生高温高压环境,该环境下,爆炸产物和空气中的氧气等成分极易发生化学反应.研究约束爆炸中涉及到的化学反应动力学过程,将动力学过程的具体参数耦合到约束爆炸中,可以获得约束爆炸后密封容器或者密闭爆室内更精确的总压力、静态压力和爆炸释放的总热量值,进而更准确地描述约束爆炸中的热应变、力学效应等重要物理现象,对于完善...