Steam Reforming of Methane Over Nickel: Development of a Multi-Step Surface Reaction Mechanism

A detailed multi-step reaction mechanism is developed for modeling steam reforming of methane over nickel-based catalysts. The mechanism also includes partial and total oxidation reactions, water?Cgas shift reactions, formation of carbon monolayers, and methanation reactions. A method is presented for ensuring thermodynamic consistency in the development of surface reaction mechanisms. The applicability of the mechanism is tested by simulating experimental investigations of SR of methane on a Ni-coated monolithic cordierite catalyst as well as experimental studies from literature. The reactive flow in the channels of the experimentally used monolithic structures is modeled by a two-dimensional flow field analysis of a single monolith channel coupled with the reaction mechanism developed. The gas composition and surface coverage with adsorbed species are calculated as function of the position in the channel. The model developed is able to properly describe steam reforming of methane over the nickel catalysts for wide ranges of temperature and steam/methane ratio.     

Enhancing mass and energy integration by external recycle in reactive distillation columns

The synthesis and design of reactive distillation columns separating reacting mixtures with the most unfavorable relative volatilities (i.e., the reactants are the heaviest and lightest components with the products being the intermediate ones) are described. The unfavorable thermodynamics poses great difficulties in combining the reaction operation and the separation operation involved and limits severely the potential of reactive distillation columns in the reduction of capital investment (CI) and operating cost. To remove the limitation, we propose two strategies for facilitating the synthesis and design of this kind of reactive distillation columns in this article. One is to arrange prudentially the reactive section so as to strengthen internal energy integration between the reaction operation and the separation operation involved; that is, while the reactive section should be placed at the bottom of the reactive distillation columns separating exothermic reactions, it should be at the top of the reactive distillation columns separating endothermic reactions. The other is to introduce an external recycle flow between the two ends of the reactive distillation columns to reinforce internal mass integration and internal energy integration between the reaction operation and the separation operation involved; that is, whereas the external recycle flow should be directed from the top to bottom of the reactive distillation columns separating exothermic reactions, it should be from the bottom to top of the reactive distillation columns separating endothermic reactions. Separation of four hypothetical ideal (i.e., two quaternary and two ternary systems, respectively) and two real nonideal (i.e., two quaternary systems) reacting mixtures is chosen to evaluate the proposed strategies. The results show that they can considerably lower energy requirement besides a further reduction in CI. © 2012 American Institute of Chemical Engineers AIChE J, 59: 2015–2032, 2013     

Effects of transport and phase equilibrium on fast, nearly irreversible reactive extraction

Integration of reaction and separation can be exploited to drive reversible reactions in the direction of the desired product using multiphase flow contacting. In the case of nearly irreversible, fast reactions, however, the dynamics of the product have little influence on the reactor efficiency in say liquid-liquid reactive extraction. A similar intensification in reaction efficiency to reactive separation can be achieved by exploiting phase equilibrium or asymmetry in mass transfer rates of the reactants. Here, a model for two-layer biphasic flow and homogeneous reaction is proposed for co-current reactive extraction, demonstrating that localization and intensification of reaction occurs in the region between the entrance and crossover. Crossover occurs if the reactant in stoichiometric deficit preferentially populates the reacting phase due to sufficient imbalance in either mass transfer coefficients or phase equilibrium. We develop an infinite Peclet number (convection dominates over bulk diffusion) model that indicates that crossover occurs when

     

Micromixing effects on series–parallel and autocatalytic reactions in a turbulent single-phase gas flow

A hybrid solution algorithm is implemented to simulate turbulent reactive single-phase gas flow in an isothermal tubular reactor. This algorithm is a combination of a Finite Volume (FV) and a Probability Density Function (PDF) method. The FV method is used to solve the mass and momentum conservation equations combined with a standardized k–ε model for single-phase gas flow. The PDF method is applied to solve the species continuity equations. The advantage of using the PDF method is the fact that there is no need of any closures for chemical reaction source terms in a turbulent flow. The mesomixing and the micromixing contributions in the PDF equation are closed using the gradient-diffusion model and the Interaction-by-Exchange-with-the-Mean (IEM) model, respectively. This hybrid solution algorithm is applied to simulate a series–parallel and an autocatalytic reactive single-phase gas flow. The mechanical-to-scalar time-scale ratio, i.e. the IEM model parameter, is found to have an influence on the simulation results that cannot be neglected. As expected, in series–parallel reactions, the desired product selectivity increases when the reaction rate coefficient, corresponding to its formation, increases. Moreover multiple solutions are observed in the autocatalytic reaction for a given feed ratio, Damköhler (Da^I) and Péclet (Pe) numbers. To validate the hybrid FV–PDF solution algorithm, the calculated results are compared with the results obtained when using the Reynolds-averaged species continuity equation model. A good agreement is observed when infinite-rate mixing is applied.     

Application of a thermochemical method for studying a low-temperature polycondensation process

A thermochemical method has been used for investigating the course of a polycondensation process. Measurements were performed in a Calvet-type calorimeter. The reactions of triethylamine complex formation with bisphenols and terephthaloyl chloride were investigated. Both hydroxyl groups in the bisphenols are found to be equally reactive with triethylamine. Stability constants, rate constants, and the ΔH⁰, ΔG⁰ and ΔS⁰ values for the reaction of triethylamine with hydroxyl groups were determined. The acid chloride groups in the terephthaloyl chloride are found to exhibit different reactivities with triethylamine. Thermal effects on the reaction of these groups were determined. Results of the measurements are also presented in the form of thermokinetic curves. Such curves obtained for the polycondensation process were compared with those determined for reactions involving the formation of the complex of triethylamine with each of the monomers separately. A mechanism of the polycondensation process is proposed.     

醋酸酐与水在化学反应量热方法确认中的应用

热流法化学反应量热测试获取的目标反应热（ΔH）、反应体系比热容（c_p）和最大热累积度（X_ac）等热安全参数是精细化工反应安全风险评估中不可或缺的关键参数,其测试结果的准确度（即正确度与精密度）直接影响化学反应的工艺危险度分级。目前关于测得的化学反应热安全参数准确度和有效性的研究未见报道。为解决这一问题,提出了以醋酸酐和水作为参考物质,采用“变异系数（CV值）法” 和“道格拉斯（Grubbs）检验法”分别验证热流法反应量热测试醋酸酐水解反应的ΔH、C_p 、X_ac等参数的精密度（即重复性和再现性偏差）,采用“t验证法”评估热流法反应量热测试水的比热容的正确度。研究结果表明：热流法反应量热测试醋酸酐水解反应的ΔH、C_p 、X_ac等参数在同一实验室内的重复性标准偏差和与其他4家实验室比对的再现性标准偏差均满足要求;采用热流法量热测试得到25℃水的比热容数据与标准比热容值t验证结果不存在实质性偏倚。因此,测试结果的精密度和正确度均满足要求。     

A reactive distillation column with double reactive sections for the separations of two-stage consecutive reversible reactions

In this paper, a novel reactive distillation column with double reactive sections (RDC-DRS) is proposed for the separations of two-stage consecutive reversible reactions. The arrangement of two reactive sections not only allows the careful coordination of the two reaction operations involved, but also provides additional degrees of freedom for the reinforcement of internal mass integration and/or internal energy integration between the reaction operations and the separation operation involved, which could facilitate the RDC-DRS to be more advantageous than the conventional reactive distillation column with a single reactive section (RDC-SRS) in operating cost and capital investment. A representative hypothetical and real two-stage consecutive reversible reactions are chosen to evaluate the steady-state performance of the RDC-DRS. With the constraints of the same total number of stages and the same total number of reactive stages, the RDC-DRS is demonstrated to require less utility consumption than the RDC-SRS and this outcome indicates that the former could be a competitive alternative to the latter in the separations of the two-stage consecutive reversible reactions. The number of reactive sections should therefore be viewed as an important decision variable for the synthesis and design of reactive distillation columns especially in the separations of complicated reacting mixtures involving multiple reversible reactions.     

旋转填充床内微观混合的数值模拟

旋转填充床作为新型的高效反应传质设备,广泛应用于快速反应过程,如制备纳米粉体材料.对旋转填充床内微观混合进行研究,有助于进一步认识旋转填充床内高度分散液体微元在填料丝网中的流动行为和分散混合机制,为旋转填充床内液液反应混合制备纳米材料提供理论基础.基于公开文献报道的实验观测结果,通过合理假设,建立了旋转填充床内微元流动的物理模型.在该物理模型的基础上,结合此前提出的湍流混合与反应模型,模拟计算了液体微元经过实验条件下50层丝网填料最终流出填料空间的浓度分布.由浓度分布得到的微观混合特征指数与实验值进行了对比,吻合良好.     

Feasibility study of a thermally coupled reactive distillation process for biodiesel production

Biodiesel fuel represents an interesting alternative as a clean and renewable substitute of fossil fuels. A typical biodiesel production process involves the use of a catalyst, which implies high energy consumptions for the separation of the catalyst and the by-products of the reaction, including those of undesirable side reactions (such as the saponification reaction). A recently proposed process involves the use of short-chain alcohols at supercritical conditions, avoiding the use of a catalyst and the occurrence of the saponification reaction. This process requires fewer pieces of equipment than the conventional one, but its high energy requirements and the need of special materials that support the reaction conditions makes the main product, biodiesel fuel, more expensive than petroleum diesel. In this work, a modification of the supercritical process for the production of biodiesel fuel is proposed. Two alternatives are proposed. The process involves the use of either reactive distillation or thermally coupled reactive distillation. Simulations have been carried out by using the Aspen One™ process simulator to demonstrate the feasibility of such alternatives to produce biodiesel with methanol at high pressure conditions. A design method for the thermally coupled system is also proposed. Both systems have been tested and the results indicate favorable energy performance when compared to the original scheme. Furthermore, the thermally coupled system shows lower energy consumptions than the reactive distillation column.     

SIMULATION OF STEADY DISTILLATION PROCESS ACCOMPANIED WITH MULTIPLE CHEMICAL EQUILIBRIUM REACTIONS BASING ON TRANSFORMED VARIABLES

The equations describing the steady state operation of distillation column with simultaneous equilibrium chemical reactions have been transformed into a new set or MESH equations by introducing transrormed composition, flow rate and enthalpy variables. The new set of equations are similar to equations describing traditional non-reactive distillation, and it is general when one or several reactions occur and inert components are present.

An algorithm combining relaxation method and modified Newton-Raphson method is proposed to solve the new equations, the relaxation method is used to estimate initial values, while the modified Newton-Raphson method is used to set solution. The reactive distillation processes of MTBE synthesis with n-butane and the separation of meta- and para-xylene are simulated as numerical examples     

Efficient structure synthesis of reactive distillation processes using the IDEAS approach

A new synthesis method for reactive distillation processes is proposed. At each stage of a column, vapor–liquid equilibrium (VLE) is assumed and kinetically controlled reaction in liquid phase is considered. First, the liquid composition space is divided into small subspaces. Then, for each subspace a representative liquid composition is decided and assigned to a module corresponding to a stage of a distillation column. Then, after the calculation of the VLE and the reaction rate, the distribution network (superstructure) connecting all modules by vapor and liquid flow paths is constructed. The feature of the proposed model is that all constraints are linear to the optimization variables: the liquid and vapor flow rate and the liquid hold-up. The developed method was applied to the metathesis reaction of 2-pentene, and a completely new process structure was obtained. The effectiveness of the implied structure was confirmed through a comparison with conventional structures.     

Rheological changes during the polymerization of polyurethane based interpenetrating polymer network(IPN)

Experimental results on reactions forming polyurethanes and polyurethane-polyester Interpenetrating polymer network (IPN) are discussed with particular interest in rheological and kinetic changes before the gel point. In the IPN formation, viscosity rise is affected by the amount of styrene and polyester in the reaction system. Plots of reduced viscosity vs. conversion do not fall into a single curve. The results may be explained by the concept of having intra-molecular reaction or ring formation. The existence of hydroxyl group at the end of polyester molecule causes the graft reaction between the polyurethane phase and the polyester phase, which may speed up the viscosity rise of the reactive system. A simple method which based on rheological measurements only is proposed. This method of plotting reduced viscosity vs. reduced reaction time provides similar results as in the plots of reduced viscosity vs. conversion.     

Kinetics of the liquid phase reaction between carbon dioxide and diethanolamine

The kinetics of the reaction between carbon dioxide in aqueous solution and aqueous diethanolamine have been studied using the thermal flow method. The data indicate that the kinetics of the reaction is complex. The particular mechanism proposed consists of two reactions which initially compete for the available carbon dioxide. One of these reactions is reversible, and regenerates carbon dioxide as the overall reaction proceeds. The species participating in the proposed reversible reaction were not identifiable from the data obtained.     

Kinetics of the liquid phase reaction between carbon dioxide and diethanolamine

The kinetics of the reaction between carbon dioxide in aqueous solution and aqueous diethanolamine have been studied using the thermal flow method. The data indicate that the kinetics of the reaction is complex. The particular mechanism proposed consists of two reactions which initially compete for the available carbon dioxide. One of these reactions is reversible, and regenerates carbon dioxide as the overall reaction proceeds. The species participating in the proposed reversible reaction were not identifiable from the data obtained.     

THE LAMINAR FLOW TUBULAR REACTOR WITH HOMOGENEOUS AND HETEROGENEOUS REACTIONS II. Application to Reactor Modeling Involving Free Radical Reactions

The simulation of the complex polychlorination of methane in a non-isothermal photoreactor with laminar flow regime shows the usefulness of the integral equations deduced in Part I of this contribution. Improved iterative schemes for numerical solution have been implemented by using the particular equations proposed for species having slow, very fast and instantaneous reaction regimes.

From such integral equations, in a mathematically consistent form, deviations in the fields of predicted concentrations with and without the application of the local steady-state approximation for homogeneous reactions involving atomic and free radical species have been obtained. Similarly the fields of predicted concentrations applying the usual assumption of negligible wall reactions for the same highly reactive intermediate species has been compared with the reaction mechanisms that includes the wall reactions. A comprehensive analysis of the validity of these simplifying assumptions is presented.     

On-the-fly reduction of kinetic mechanisms using element flux analysis

A systematic approach for on-the-fly mechanism reduction is proposed in this paper. The approach consists of element flux analysis and dynamic generation of reduced mechanisms during reactive flow computation. At each time step of the computation, a reduced mechanism is generated based on local conditions and no previous information or analysis is necessary. The approach is demonstrated in a plug-flow reactor (PFR) model and a pairwise mixing stirred reactor (PMSR) model. A detailed n-pentane oxidation mechanism with 385 species and 1895 elementary reactions and a primary reference fuel (PRF) mechanism with 1034 species and 4236 reactions are used to illustrate the proposed approach. Auto-ignition delay, temperature, and species composition profile predicted by the on-the-fly reduction scheme agree with the detailed simulation and experimental results over a wide range of temperatures and equivalence ratios.     

一种新的氯化钠热化学分解循环

提出了一种新的热化学分解循环.研究了循环中各主要化学反应的适宜条件,并从热力学计算出了循环中的能量流动过程;用TG法研究了循环中多相反应的动力学问题,实验和理论证明了该循环具有很大的节能优势.     

A reactive molecular dynamics model of thermal decomposition in polymers. II. Polyisobutylene

An improved version of the reactive molecular dynamics method is presented. The method, which extends conventional (force-field-based) molecular dynamics to modeling chemical reactions, is used to simulate the thermal decomposition of polyisobutylene. The results of the simulations are generally consistent with experimental observations. A quantitative analysis of the results shows that the rate constant of the key initiation reaction, backbone scission, depends on the size of the molecular model of the polymer. This implies that the kinetics of some elementary reactions that take place in a polymer melt are affected by the macromolecular nature of the environment.     

Application of the channel flow cell to the investigation of dyeing kinetics and mechanism: new perspectives on dyeing processes

The ability of the channel flow cell to probe reactions at a solution/non-conducting solid interface makes it a powerful method for investigating dyeing reactions. The technique gives direct information about the surface concentrations of dye at the fibre surface and thus allows the elucidation of dyeing mechanisms and the determination of kinetic parameters. The experimental realisation of the channel flow cell in this context and its basic theory are outlined and the strength of the methodology illustrated with investigations of various reactive dyes and model acid dyes. These studies ailow a generalised model of dyeing phenomena to be proposed.     

Limitations of the Augis and Bennett Method for Kinetic Analysis of the Crystallization of Glasses and Conditions for Correct Use

The equation proposed by Augis and Bennett for determining the kinetic exponent of the Johnson–Mehl–Avrami (JMA) model is thoroughly analyzed; a new expression, calculated accurately with no assumptions introduced, is proposed. This new method of calculation has been extended to the different kinetic models more commonly used in the literature for describing solid-state reactions. However, determining the JMA exponent from the Augis and Bennett method can lead to an incorrect interpretation of the reaction mechanism unless an additional, independent test is used. A testing method for verifying the applicability of the Augis and Bennett method is proposed. The kinetic analysis of the crystallization of Ge_0.3Sb_1.4S_2.7 has been used for checking this method.