Verification of Onsager's reciprocal relations for evaporation and condensation using non-equilibrium molecular dynamics

Non-equilibrium molecular dynamic (NEMD) simulations have been used to study heat and mass transfer across a vapor-liquid interface for a one-component system using a Lennard-Jones spline potential. It was confirmed that the relation between the surface tension and the surface temperature in the non-equilibrium system was the same as in equilibrium (local equilibrium). Interfacial transfer coefficients were evaluated for the surface, which expressed the heat and mass fluxes in temperature and chemical potential differences across the interfacial region (film). In this analysis it was assumed that the Onsager reciprocal relations were valid. In this paper we extend the number of simulations such that we can calculate all four interface film transfer coefficients along the whole liquid-vapor coexistence curve. We do this analysis both for the case where we use the measurable heat flux on the vapor side and for the case where we use the measurable heat flux on the liquid side. The most important result we found is that the coupling coefficients within the accuracy of the calculation are equal. This is the first verification of the validity of the Onsager relations for transport through a surface using molecular dynamics. The interfacial film transfer coefficients are found to be a function of the surface temperature alone. New expressions are given for the kinetic theory values of these coefficients which only depend on the surface temperature. The NEMD values were found to be in good agreement with these expressions.     

在非平衡态热力学的基础上探索建立催化理的新途径

平衡态热力学一直被认作多相催化理论的基石之一。但是,它并不能概括工作中的催化剂的状态和行为,这主要是这里还发生一些非平衡过程。催化体系常常处于非平衡状态之下,而非平衡态条件下体系状态和行为,同时取决于体系的动力学和热力学。联系动力学和热力学最一般的关系式并非原来的De Donder不等式：Ar≥0,而是新的De Donde方程ln r^-/r^-=A/RT。同时发生的总反应之间的热力学耦合对总反应的作用只是形式上的,远不及催化反应链中各基元步骤之间在动力学上的耦合那么重要。通过在动力学方程中引入反应亲和力（热力学位）得到的动力学-热力学结合近似分析,可以用来分析非平衡态的催化反应和催化剂状态。非平衡态热力学在建立多相催化理论中,较之原来的平衡态热力学将能提供更能采纳的和更有意义的物理化学背景。     

Modeling the oxidative coupling of methane： Heterogeneous chemistry coupled with 3D flow field simulation

The oxidative coupling of methane (OCM) over titanate perovskite catalyst has been developed by three-dimensional numerical simulations of flow field coupled with heat transfer as well as heterogeneous kinetic model. The reaction was assumed to take place both in the gas phase and on the catalytic surface. Kinetic rate constants were experimentally obtained using a ten step kinetic model. The simulation results agree quite well with the data of OCM experiments, which were used to investigate the effect of temperature on the selectivity and conversion obtained in the methane oxidative coupling process. The conversion of methane linearly increased with temperature and the selectivity of C2 was practically constant in the temperature range of 973–1073 K. The study shows that CFD tools make it possible to implement the heterogeneous kinetic model even for high exothermic reaction such as OCM.     

Indications for a surface temperature excess in heterogeneous catalysis

We explored the surface temperature excess DeltaT = T(r) - T(m) (real reaction temperature T(r), measured catalyst temperature T(m)) on the basis of experimental data, a gradually curved Arrhenius plot for CO oxidation reactions over Pd/gamma-Al(2)O(3) catalysts. Such a plot could be an indication of the surface temperature excess in the 2-dimensional reaction surfaces of catalysts. The positive or negative surface temperature excess could be developed to be a general explanation for a gradually curved Arrhenius plot of a gas-solid catalytic system. This is a new insight into solving the puzzle on such common phenomena in heterogeneous catalysis. By using the reciprocal of the real reaction temperature T(r) in the hypothetical 2-D reaction surface, instead of the experimentally determined catalyst temperature T(m) or the gas temperature T(g), the gradually curved Arrhenius plot becomes linear. We investigated the implications of such a difference among T(r), T(m), and T(g). The surface temperature excess could be the effect of coupling between the fluxes of a chemical reaction and heat transport in the 2-D reaction surface. Its order of magnitude is 10 K for the present model system.(1) The surface temperature excess increases exponentially with the reaction temperature.     

Heats of transfer in the diffusion layer before the surface and the surface temperature for a catalytic hydrogen oxidation (H2 + (1/2)O2 --> H2O) reaction

The surface temperature and surface mole fractions are calculated for a catalytic hydrogen oxidation reaction over a Pt/Al2O3 catalyst pellet. The thermodynamics of irreversible processes was used in order to ensure the correct introduction of coupled heat and mass transfer. Two pathways, one using the 4 x 4 resistivity matrix and the other using a simplified effective conductivity matrix, were proven to yield equivalent results. By using expressions for the thermal diffusion coefficients, heats of transfer, and the Maxwell-Stefan diffusion coefficients given in the literature, available experimental data could be reproduced. The Dufour effect was found to be negligible for the prediction of the surface temperature. Neglecting the Soret effect would increase the predicted value of the surface temperature significantly-more than 30 K out of an average of about 400 K. It is found that the reaction rate can be used to predict the surface temperature.     

Phenomenological study on heat and mass coupling mechanism of waxy crude oil pipeline transport process

Based on the phase equilibrium model of the paraffin wax precipitation in the process of oil pipeline transportation, theory and method of non-equilibrium thermodynamics were applied to obtain the linear phenomenological equations for the cross-interaction of heat and mass transfer during pipeline transport, which were derived from the irreversible entropy production rate equation. Then, the analysis of the irreversible heat flow and the mass flow were carried out, and the mathematical expressions of the phenomenological coefficient of liquid phase, the phenomenological coefficient of solid phase flow, and the heat flow phenomenological coefficient were obtained. Taking a waxy crude oil transportation pipeline in Daqing Oilfield as an example, based on the analysis of liquid–solid phase equilibrium, the irreversible linear phenomenological mechanism of heat and mass coupling in waxy crude oil pipeline transportation was analyzed in detail from three levels: phenomenological coefficients which reflect characteristic of the effect of force on flow in heat and mass transfer; thermodynamic forces which trigger heat and mass transfer; transmitted heat and mass flow density, providing a theoretical basis for the further study of the wax deposition in the process of pipeline transportation.     

添加助熔剂后朱集西煤灰熔融特性规律研究

以高灰熔点朱集西洗煤为对象,研究了助熔剂CaCO₃、Fe₂O₃、CaCO₃/Fe₂O₃复合助熔剂以及CaMg(CO₃)₂对其煤灰熔融特性的影响。结果表明,各助熔剂均可降低煤灰熔融温度,但助熔效果与助熔剂种类和添加量有关,采用CaCO₃/Fe₂O₃复合助熔剂以及CaMg(CO₃)₂在添加量较小时,助熔效果明显;利用FactSage热力学软件,分析了添加助熔剂对煤灰中矿物高温熔融行为的影响,为进一步掌握助熔剂的助熔机理提供理论帮助。     

Changing the Mechanism for CO2 Hydrogenation Using Solvent‐Dependent Thermodynamics

A critical scientific challenge for utilization of CO₂ is the development of catalyst systems that function in water and use inexpensive and environmentally friendly reagents. We have used thermodynamic insights to predict and demonstrate that the HCo^I(dmpe)₂ catalyst system, previously described for use in organic solvents, can hydrogenate CO₂ to formate in water with bicarbonate as the only added reagent. Replacing tetrahydrofuran as the solvent with water changes the mechanism for catalysis by altering the thermodynamics for hydride transfer to CO₂ from a key dihydride intermediate. The need for a strong organic base was eliminated by performing catalysis in water owing to the change in mechanism. These studies demonstrate that the solvent plays a pivotal role in determining the reaction thermodynamics and thereby catalytic mechanism and activity.     

Adsorption Calculations Using the Film Model Approximation for Intraparticle Mass Transfer

An approximate rate equation based on a film-model representation of diffusional mass transfer has been developed to describe the kinetics of multicomponent adsorption. The model describes mass transfer as a pseudo-steady state diffusion process through a flat film of thickness equal to one fifth of the particle radius. The flux relationships are integrated across the film yielding analytical expressions for the rate of mass transfer in a multicomponent adsorption system. The usefulness of the film model approximation is tested by carrying out calculations for three different practical adsorption systems: the adsorption of n-pentane and n-heptane mixtures on NaCaA zeolite discussed by Marutovsky and Bülow (1987); the adsorption of air in molecular sieve RS-10 discussed by Farooq et al. (1993); and the separation of air in a kinetically-controlled nitrogen PSA process discussed by Farooq and Ruthven (1990) and Sundaram and Yang (1998). In each case, the film model approximation predicts the expected trends accounting for the coupling of diffusion fluxes in the adsorbed phase.     

Influence of protein–protein interactions on bulk mass transport during ultrafiltration

Bulk mass transfer limitations can have a significant effect on the flux and selectivity during membrane ultrafiltration. Most previous studies of these phenomena have employed the simple stagnant film analysis, but this model is unable to account for the effects of solute–solute interactions on mass transport. We have developed a generalized framework for multicomponent mass transfer that includes both thermodynamic and hydrodynamic (frictional) interactions. Thermodynamic (virial) coefficients were evaluated from osmotic pressure data for albumin (BSA) and immunoglobulins (IgG), while hydrodynamic interaction parameters were determined from filtrate flux data obtained in a stirred cell using fully retentive membranes. The protein concentration profiles in the bulk solution were evaluated by numerical solution of the governing continuity equations incorporating the multicomponent diffusive flux. This model was used to analyze flux and protein transmission data obtained for the filtration of BSA and IgG mixtures through partially permeable membranes. The model accurately predicted the large reduction in flux and BSA transmission upon addition of IgG. These effects were due to the coupling between BSA and IgG mass transfer caused by protein–protein interactions.     

丙烷甲醇制烯烃共反应的热力学和动力学分析

通过计算和实验研究相结合的方法研究丙烷甲醇共进料制烯烃反应热力学及动力学过程.热力学过程采用Gibbs最小自由能法模拟丙烷甲醇制烯烃反应体系的平衡组成,同时结合响应面分析法建立了温度、压力、丙烷甲醇进料摩尔比对产物中丙烯的摩尔分数的函数关系,通过回归方程分析最佳工艺范围.热力学分析了反应条件对平衡产物的影响,随着反应温度升高,平衡产物丙烯的质量分数先增高后降低;平衡产物中丙烯的质量分数随着丙烷甲醇进料中丙烷摩尔比增高而增高,但是实际的反应状态和催化剂也是相关的,因此研究了存在催化剂情况下,丙烷脱氢和丙烷甲醇共进料反应的活化能.反应活化能动力学实验表明,通过添加少量甲醇可以降低耦合过程中丙烷脱氢表观活化能.     

Nitrogen-rich porous covalent imine network (CIN) material as an efficient catalytic support for C-C coupling reactions

In an effort to expand the realm of possibilities of nitrogen-rich porous materials that could be used in catalysis, herein we report the synthesis of a new highly nitrogen rich (ca. 45%) porous covalent imine network (CIN-1) material employing simple Schiff base chemistry and further grafting its surface with palladium. Pd-loaded CIN-1 support acts as a truly heterogeneous catalyst towards Suzuki C-C coupling reaction between aryl halides with arylboronic acids. High surface area and excellent accessibility of the catalytic sites make it very efficient for heterogeneous catalysis. The stability of the catalyst due to intimate contact between nitrogen-rich organic support and metal allows several reuses with only a minor loss in catalytic activity.     

A short perspective of modeling electrode materials in lithium-ion batteries by the ab initio atomistic thermodynamics approach

Atomic-scale insights into the performance of electrode materials in lithium-ion batteries require thermodynamic considerations as first step in order to determine potential surface structures that are relevant for subsequent kinetic studies. Within the last 20 years, research in heterogeneous catalysis as well as in electrocatalysis has been spurred by the ab initio atomistic thermodynamics approach, whose application for electrode materials in lithium-ion batteries is eyed and discussed in this perspective article.     

Heat transfer and thermographic analysis of catalyst surface during multiphase phenomena under spray-pulsed conditions for dehydrogenation of cyclohexane over Pt catalysts

Dehydrogenation of cyclohexane over Pt/alumite and Pt/activated carbon catalysts has been carried out for hydrogen storage and supply to fuel cell applications. An unsteady state has been created using spray pulsed injection of cyclohexane over the catalyst surface to facilitate the endothermic reaction to occur efficiently. Higher temperature of the catalyst surface is more favorable for the reaction, thus the heat transfer phenomena and temperature profile under alternate wet and dry conditions created using spray pulsed injection becomes important. IR thermography has been used for monitoring of temperature profile of the catalyst surface simultaneously with product analysis. The heat flux from the plate-type heater to the catalyst has been estimated using a rapid temperature recording and thermocouple arrangement. The estimated heat flux under transient conditions was in the range of 10-15 kW/m(2), which equates the requirement for endothermic reactions to the injection frequency of 0.5 Hz, as used in this study. The analysis of temperature profiles, reaction products over two different supports namely activated carbon cloth and alumite, reveals that the more conductive support such as alumite is more suitable for dehydrogenation of cyclohexane.     

Thermo-chemical characteristics of R134a flow boiling in helically coiled tubes at low mass flux and low pressure

The characteristics of R134a heat transfer coefficients and wall temperature distribution were investigated under low mass flux and low pressure conditions in a helically coiled tube with heated length of 7070 mm, outer diameter of 10 mm, inner diameter of 7.6 mm, coil diameter of 300 mm and helical pitch of 40 mm. System pressures, mass fluxes and inlet qualities range from 0.20 to 0.75 MPa, 50 to 260 kg/m² s and ?0.18 to 0.40, respectively. It was found that the wall temperatures in descending segments of coiled tube were higher than those of climbing ones, while the heat transfer coefficients varied inversely. Around the section circumference, the outside temperature was lower than the inside one; this is more apparent at very low mass flux and pressure conditions. The heat transfer coefficient increases with increasing mass flux, vapor quality and heat flux. However, the pressure has an indeterminate effect. New heat transfer coefficient correlations for current conditions were developed comparing with existing correlations.