Rate Coefficients Of Elementary Reactions Research Articles

Design of combustion experiments using differential entropy

The aim of several combustion experiments is the determination of the rate coefficients of important elementary reactions. The experimental conditions are usually selected on the basis of local sensitivity analysis. Shock tube and tubular flow reactor experiments are often designed in such a way that only one reaction step is important at the investigated conditions. Sheen and Manion (J. Phys. Chem. A, 118 (2014) 4929–4941) suggested a method for the design of shock tube experiments based on differential entropy. Their method was modified and extended in this work. In the extended method, both the experimental and residual errors of the measurements are considered at the calculation of the posterior uncertainty of the determined rate parameters, the differential entropy matrix is calculated in an analytical way and the net information flux value is calculated for each suggested experimental point. In an iterative procedure, all investigated experimental points with negative net information flux values are discarded and the remaining experimental conditions are recommended for the measurements. The most valuable candidate experimental points can be determined based on the net information flux values. The method was used for the selection of experimental conditions for the determination of the rate coefficient of reaction NO2 + H = NO + OH at conditions similar to the tubular flow reactor experiments of Alzueta et al. (Energy Fuels, 15 (2001) 724–729). In these experiments, the oxidation of methanol was investigated with and without NO addition. Our method suggested a range of temperature, equivalence ratio and initial NO concentration, where the experimental data carry the most information on the rate coefficient of this elementary reaction.

Growth network of PAH with 5-membered ring: Case study with acenaphthylene molecule

Numerous experimental and theoretical evidence indicates that polycyclic aromatic hydrocarbon (PAH) is the precursor of soot. However, the formation mechanism of PAH at flame conditions is not well understood. In this study, the growth of PAHs with a 5-membered ring via the hydrogen-abstraction-acetylene-addition route was systematically investigated using acenaphthylene (A2R5) as a model molecule. Potential energy surfaces were searched using DFT/B3LYP/6-311+G(d,p) and ROCCSD(T)/cc-pVDZ methods. The reaction rate coefficients of elementary reactions were evaluated using the transition state theory in the temperature range of 500–2500 K. The results showed that H abstractions by H, OH, and CH3 radicals, and the subsequent C2H2 addition reactions, were highly sensitive to the 5-membered ring. H abstraction reaction from the C atom on a 5-membered ring is not favorable due to the relatively higher energy barrier among all competing reactions. In contrast, the reaction rate coefficient of subsequent C2H2 addition reaction on a 5-membered ring is the highest. The planar PAH with no substitution, planar PAH with C2H substitution, curved PAH with two adjacent 5-membered rings, and PAH with a CCH2C functional group, can be formed in the A2R5H-C2H2 system investigated. The reactions investigated were merged into a detailed PAH mechanism to check the product distribution in a premixed C2H4 sooting flame. Owing to the existence of the 5-membered ring, the formation of planar PAHs with a C2H substitution was much more favorable than the formation of new 5-membered ring and 6-membered rings. The predicted mole fraction of all C14H8 species with one C2H substitution was as high as 2.8 × 10−5, higher than that of phenanthrene with same C atom number (1.2 × 10−5). A similar phenomenon was observed for C16H8 species with two C2H substitutions. In the future, more attention should be given to PAHs with C2H substitution, due to their high concentrations and enhanced reactivity, as compared to the benzenoid PAH.

Chemical kinetic model reduction through species-targeted global sensitivity analysis (STGSA)

The uncertainty of the rate coefficients of elementary reactions in detailed chemical kinetic mechanisms can affect the predicted species concentrations and global properties. The size of reduced mechanisms generated by reduction methods, such as directed relation graph (DRG), can also be greatly affected by the uncertainties in the rate coefficients, because these methods eliminate species by comparing contributions of the production rates of important species from other species. In order to obtain the optimum reduced mechanism in the uncertainty domain of reaction rate parameters, the species-targeted global sensitivity analysis (STGSA) method is proposed to measure the importance of species in kinetic mechanisms and to simplify them by considering the input uncertainties. The demonstrations are performed for the ethylene oxidation mechanism using the USC-Mech II with 111 species and the n-heptane oxidation mechanism using the JetSurf (version 1.0) with 194 species. It is found that STGSA can accurately identify the redundant global species and generate more compact reduced mechanisms compared with the DRG-based methods, resulting in a 33-species reduced mechanism for ethylene oxidation and a 37-species reduced mechanism for n-heptane oxidation. These reduced mechanisms show accurate predictions for the ignition delay times and laminar flame speeds over a wide range of operating conditions.

Impact of thermochemistry on optimized kinetic model predictions: Auto-ignition of diethyl ether

Diethyl ether (DEE) is considered a potential diesel substitute because of its advantageous properties, such as high cetane number and liquid state at ambient conditions. In the present study, an optimized kinetic model for auto-ignition of DEE is developed based on a mechanism from the literature (Sakai et al., 2017). By means of sensitivity and uncertainty analyses, the impact of kinetic and thermochemical parameters on model predictions is compared first. It is found that enthalpies of formation of the most sensitive species have an even larger impact on ignition delay times than the most sensitive reactions. This motivates the joint optimization of both thermochemical parameters and reaction rate constants within their respective uncertainty limits against the experimental targets. In addition to conventional model modification, which often only considers the rate coefficients of elementary reactions as uncertain, this study thus also takes the enthalpies of formation, standard entropies, and heat capacities of species into account, and calibrates the rate rules instead of elementary reactions, when needed. For comparison, a second model is developed by modifying rate coefficients alone. Both optimized models show good agreement with the experimental ignition delay times at low and high temperatures, with slightly improved predictions by the model with additionally optimized thermochemistry. However, modified model parameters and the resulting reaction fluxes at intermediate temperature exhibit notable differences between the two models, which highlights the effect of the shifted chemical equilibria of important reactions when the thermochemistry is optimized as well.

Using sensitivity entropy in experimental design for uncertainty minimization of combustion kinetic models

Due to the inherent uncertainties in combustion kinetic model parameters, especially the rate coefficients of elementary reactions, the uncertainties of model predictions can be quite large. Uncertainty minimization using experimental measurements can reduce the uncertainty space of the rate coefficients of elementary reactions, and further reduce the uncertainties of model predictions. Many mathematical methods have been developed for this purpose, while little research has been done to guide us in designing experiments which are relatively efficient for uncertainty minimization. In this work, “sensitivity entropy” is proposed as a measure of the degree of dispersion of uncertainty sources of a model output. The smaller the sensitivity entropy is, the lower degree of dispersion of uncertainty sources will be. The experimental measurement of a target which has smaller sensitivity entropy will be more efficient for the uncertainty minimization. To illustrate the practicability of using sensitivity entropy to guide the selection of relatively efficient experimental systems for specific targets for model uncertainty minimization, the methanol/O2/Ar laminar premixed flame system is investigated. Based on the analysis of sensitivity entropy, two experiments in which many targets have small sensitivity entropies are designed. Results show that these well designed experiments can provide strong constraints on the uncertainty space of some rate coefficients, which are expected to be useful in further kinetic model development.

Rule-based ab initio kinetic model for alkyl sulfide pyrolysis

The use of a rule-based automated kinetic model builder Genesys is illustrated for the thermal decomposition of diethyl sulfide and ethyl methyl sulfide. Rule-based automatic kinetic model generation builds-upon the users’ expert knowledge to define constraints per reaction family to limit the model size and exclude species and reactions that are considered irrelevant. In the case of alkyl sulfide pyrolysis intermolecular hydrogen abstractions, intermolecular additions/β-scissions and intermolecular homolytic substitutions are used to iteratively expand the model not considering species with more than 5 heavy atoms. Furthermore, the formation of biradical species and cyclic structures was avoided. Rate coefficients of elementary reactions and thermochemical properties of molecules were estimated through group additive methods, with parameters solely derived from high level ab initio calculations. 39 reactions were added to the model after the automatic generation out of which 37 got rate constants assigned from ab initio calculations. The generated model, consisting of 444 reactions between 28 molecules and 38 radical species, was validated using experimental data for the thermal decomposition of diethyl sulfide and ethyl methyl sulfide and showed that measured and predicted species mole fractions are in good agreement. However, there is a clear need for more accurate and more detailed experimental data for the pyrolysis of sulfur containing compounds.

Modeling Kinetics and Structural Properties in High-Pressure Fluid-Phase Polymerization

The present contribution provides an overview of actual applications in modeling free-radical polymerizations. Topics of interest are the simulation of pulsed laser polymerization experiments with subsequent analysis of the formed product by size exclusion chromatography (PLP-SEC), single pulse laser experiments, experimental techniques for determining rate coefficients of elementary reactions that control polymer properties, and technical applications. Aspects being investigated are model validation and testing predictive potential in polymerization models using well-defined experiments as well as developing and testing experimental strategies for deriving rate coefficients of elementary reactions that exist (especially when dealing with copolymerizations) within a network of complex coupled reactions. In any of these fields remarkable success in modeling can be achieved. This demonstrates the great potential that can grow from combining modern mathematical methods, computational power and detailed kinetic insights into the mechanism of polymerization. It is the wide scope of applications, e. g. ranging from modeling kinetics to the investigation of termination processes being dependent on the chain-length of the macroradical (as an example of pure fundamental research) to modeling of technical reactors, that provides attractiveness and defines challenges. Especially, the success in transforming results directly from laboratory experiments into technical applications justifies laborious efforts in determining highly precise rate coefficients and proves the concept breaking down a complex process into elementary subparts. A necessary boundary condition for this is keeping in mind the demands along the whole scope of applications and avoiding simplifications that are only applicable for part of them. Although at a first glance this may appear to hinder fast progress in one discipline, it is the essential requirement for final success.