Precise parameter estimation for chemical batch reactions in heterogeneous medium

A sequential experimental strategy for precise parameter estimation has been used in the case of liquid-liquid dispersions in batch-stirred tank reactors where slow chemical reactions take place. The mathematical model for a batch reaction in a stirred tank reactor is formulated as a system of non-linear differential equations standing for the mass balance of each component. Physical kinetic parameters and chemical kinetic parameters which arise from this model are estimated simultaneously. The estimation problem is posed as a weighted least squares problem and solved by using a standard Levenberg-Marquardt algorithm. In this work, we intend to show how it is possible to develop efficient experimental design strategies that lead to an accurate estimation of the parameters involved in phenomenological models and most particularly in kinetic models. Three design criteria for designing the experiments have been employed in order to increase the precision on the parameter estimates of the model. A standard non-linear sequential quadratic programming method ensures the determination of the operating conditions which define the experimental design. The well-known alkaline hydrolysis of esters in aqueous phase has been treated as a numerical application example.     

A new continuous-flow recycle microwave reactor for homogeneous and heterogeneous chemical reactions

A new continuous-flow recycle microwave reactor suitable for organic synthesis has been developed to handle 0.51 quantities of reagents. The apparatus, which is designed for laboratory use, operates at atmospheric pressure in open- or closed-loop mode. It is fitted with a temperature control system. We describe this novel reactor and illustrate its efficiency with examples of organic syntheses carried out using both homogeneous and heterogeneous reactions.     

Optimal temperature profiles for heterogeneous catalytic parallel reactions

The problem of determination of the optimal temperature profiles in a fixed-bed catalytic reactor for the catalytic parallel reactions including transport phenomena (external or internal diffusion) has been worked out.Special attention was given to predict the shapes of the optimal temperature profiles. The general considerations are illustrated by some numerical examples. It has been found that for every studied case the optimal temperature profile need not to end by the isotherm T = T_max.     

Useful experimental technique for the study of heterogeneous reactions

The heterogeneous CaO/SO₂ reaction has been thoroughly investigated by developing a series of new experimental techniques including the TGA reactor, the volulmetric reactor and the entrained flow reactor. The heterogeneous system is designed in such a way that most of the gas film and pore diffusion resistances are reduced. The modelling of each step related to the reaction is discussed while the chemical reaction and product layer diffusion are emphasized as the main influences on the SO₂removal. The unchanging size shrinking core model is used to describe the reaction progress with a two stage assumption which has been confirmed in the TGA reactor: first, a very fast surface reaction, followed by a product layer diffusion controlled reaction. It was found from the experiments that the SO₂-partial pressure aat the very beginning is very important for a high removal efficiency during the initial reaction period.     

Thermal chemical vapor deposition grown graphene heat spreader for thermal management of hot spots

Graphene of different layer numbers was fabricated using thermal chemical vapor deposition (TCVD), and it was demonstrated as a heat spreader in electronic packaging. Platinum thermal evaluation chips were used to evaluate the thermal performance of the graphene heat spreaders. The temperature of a hot spot driven at a heat flux of up to 430 W cm⁻² was decreased from 121 °C to 108 °C (ΔT ≈ 13 °C) with the insertion of the monolayer graphene heat spreader, compared with the multilayer (n = 6–10) ones’ temperature drop of ∼8 °C. Various parameters affecting the thermal performance of graphene heat spreaders were discussed, e.g. layer numbers of graphene, phonon scattering, thermal boundary resistance. We demonstrate the potentials of using a complementary metal oxide semiconductor compatible TCVD process to utilize graphene as a heat spreader for heat dissipation purposes.     

Criteria for thermal stability of parabolically oxidable heterogeneous systems

A system of equations that interaction of an isolated particle or a dense ensemble of particles with an oxidizing agent in the case of parabolic growth of the product layer is analytically treated in the one-temperature approximation. It is shown that a jumplike transition from the lower reacting mode (regime with a temperature maximum) to the upper reacting mode (regime with a monotonically increasing temperature) occurs with a continuously changing ignition parameter. The critical ignition parameter obtained differs from its numerically predicted value by 7%. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 53–56, May–June, 2006.     

动态现场原位(operando)表征技术在多相催化反应中的应用与进展

动态现场原位（operando）表征是在接近过程工业反应条件下,揭示催化反应机理及工业催化剂结构演变的新兴动态结构解析技术。本文综述了operando表征技术在多相催化反应中的应用及发展趋势,从operando红外、operando拉曼、operando X射线衍射、operando穆斯堡尔谱、operando X射线吸收谱及operando X射线光电子能谱6个方面概述了operando技术的最新进展。此外,还介绍了正在兴起的operando联用技术,该技术综合多种operando技术为一体,能够在反应过程中对催化剂的结构全貌进行深度表征,实现工业催化剂的理性设计,将成为未来多相催化研究的重要手段。然而,目前operando技术的时间分辨率和空间分辨率仍需进一步提升,其巨大潜力依然有待开发。     

The effect of mass transfer on selectivity for heterogeneous reactions

A simple model based on existing penetration theory solutions is proposed to predict the selectivity for consecutive reactions in gas-liquid systems. The model is limited to cases of moderate to high selectivity, where the first reaction is much faster than the second. When the reaction greatly increases the mass transfer rate, the model gives about the same selectivity as the film theory, but predicts higher selectivities than the film theory when the enhancement factor ø is small and the reaction is rapid. When ø is small, an appreciable fraction of the gas that is absorbed reacts after surface elements are mixed with the bulk, and this contribution is neglected in the film theory. The model can also be applied to estimate the effect of diffusion on parallel reactions.     

Mathematical modelling of heterogeneous—homogeneous reactions

Mathematical models have been developed for a reaction network involving concurrent and consecutive heterogenous and homogeneous reactions. Comparisons have been drawn between models of decreasing complexity involving two dimensions—two phases, two dimensions—one phase and one dimension—one phase. The catalytic oxidation of methanol to formaldehyde coupled to the gas phase oxidation of formaldehyde to carbon dioxide is used as an example of a consecutive reaction: the decomposition of nitrous oxide in the gas phase or over gold catalysts is used as an example of a concurrent reaction. Differences between the prediction of the three models are discussed, as is the effect on the catalytic reaction of the gas phase reaction.     

Shrinking-core model for systems with facile heterogeneous and homogeneous reactions

The shrinking-core equation for pore diffusion control has been extended to the case of a facile heterogeneous reaction coupled to a facile homogeneous reaction occurring within the pores of the product layer and in the bulk solution. The model considered is very general in that the simultaneous transport of all reacting species is included. The resulting equation is identical to the standard one for diffusional control by a single species except that the parabolic rate constant k_d is considerably more complex and contains useful information concerning the system. Analysis of its dependence on the system parameters yields important criteria that determine the identity of the rate-controlling species and the direction of the heterogeneous reaction. Depending upon the relative values of the equilibrium constants of the two reactions, the dependence of k_d on the bulk reactant concentration can vary significantly from the linearity expected from the standard model.     

Some strategies for the kinetic study of complex heterogeneous catalytic reactions

This paper presents a method of transforming the non-liner regression problem in the kinetic study of complex heterogeneous catalytic reactions into a linear regression problem. Application of this method reduced the number of parameters to be estimated by n-1, where n is the number of independent reactions. In addition, a stepwise model discrimination strategy in introduced to reduce the number of equation sets ad equations in the set undergoing parameter estimation. These two new approaches are very advantageous in reducing the computation effort, especially when the number of independent reactions is large. The linear regression method and the stepwise model discrimination strategy are successfully applied in the kinetic study of the methanol synthesis system in which the formation rates of methanol, methane, ethanol and ethane are considered.     

A model for primary and heterogeneous secondary reactions of wood pyrolysis

A chain growth model for heterogeneous secondary reactions is developed for the pyrolysis of large wood particles and the parameters determined by nonlinear optimization. The model takes both the volatile retention time and cracking and repolymerization reactions of the vapours with the decomposing solid as well as sutocatalysis into consideration. The extent of the secondary reactions is strongly influenced by the time and the ratio of the autocatalytic (propagation) reaction rate to noncatalytic (initiation) reaction rate. The wood which has a higher value of the autocatalytic/noncatalytic ratio also has a higher exothermic heat of reaction and yields a higher amount of final char residue. This fact confirms the heterogeneous secondary reactions lead to carbon enrichment of the final residue and are accompanied with an exothermic heat of reaction. The lower activation energies of the initiation and propagation reactions as compared to primary reactions (competitive reaction model consisting of weight loss and char forming reactions) confirm autocatalysis in large particles. The sealed reactor studies of small quantities of fine wood samples show that heterogeneous secondary reactions and not lower heating rates in large particles are the main source of char formed during the thermal decomposition of large wood particles. The model predictions are in agreement with the weight loss and temperature versus time curves over a wide range of particle size and furnace temperatures.     

Carbon foams for thermal management

A unique process for the fabrication of high-thermal-conductivity carbon foam was developed at Oak Ridge National Laboratory (ORNL). This process does not require the traditional blowing and stabilization steps and therefore is less costly. The resulting foam can have density values of between 0.2 and 0.6 g/cc and can develop a bulk thermal conductivity of between 40 and 180 W/m K. Because of its low density, its high thermal conductivity, its relatively high surface area, and its open-celled structure, the ORNL carbon foam is an ideal material for thermal management applications. Initial studies have shown the overall heat transfer coefficients of carbon foam-based heat sinks to be up to two orders of magnitude greater than those of conventional heat sinks.     

Laser-assisted high-pressure chemical reactions

The growing need for production methods with a reduced environmental impact drives the search for new synthetic approaches in chemistry. Here we report the state of the art of a recently developed branch of ultra-high-pressure chemistry, employing high pressure to create the necessary reaction conditions to transform simple hydrocarbons and monochromatic light to trigger and direct, according to selective paths, the chemical reaction. For the available systems studied up to now, we will provide an overview of the different effects that can be obtained by the combined use of pressure and laser light.     

Thermodynamics of heterogeneous systems with chemical interaction

The principles of analysis of phase transitions and shifts in equilibrium in systems with chemical reactions using elements of linear algebra and conditions of thermodynamic equilibrium and stability are presented.     

Decoding complexity of chemical reactions

Three approaches for grasping chemical complexity using data of kinetic measurements are presented, i.e. ‘gray-box’ approach, non-steady-state kinetic monitoring (“chemical calculus”) and pattern analysis. All approaches are illustrated by original results.