Reversible Elementary Reactions Research Articles

A new skeletal mechanism for ethanol using a modified implementation methodology based on directed relation graph (DRG) technique

A different methodology for implementing the standard mechanism reduction technique directed relation graph (DRG) is presented and applied to develop a new skeletal mechanism for ethanol. Two combustion processes, ignition delay time and flame speed, that are mandatory for a mechanism to reproduce are used to calculate the species coupling through DRG index. Based on a detailed mechanism of 57 species among 383 reversible elementary reactions, a skeletal mechanism of 37 species and 184 reactions is obtained, which represents a decrease of 35% in the number of species and almost 52% in the number of reactions. The new mechanism is validated for ignition delay time and flame speed measurements, and also for one-dimensional burner stabilized flat and counterflow flames simulations, which were not considered in the development of the skeletal mechanism. Comparisons with experimental data and with the behaviour of other mechanisms in the literature are also displayed. The methodology presented can be useful and contribute to generate skeletal mechanisms with less effort that can reproduce a more demanding simulation.

Simple Program for Step-by-Step Time Integration in Chemical Kinetics, Applied to Simple Model for Hydrogen Combustion

A simple algorithm is proposed for step-by-step time integration of stiff ODEs in Chemical Kinetics. No predictor-corrector technique is used within each step of the algorithm. It is assumed that species concentrations less than 10-6 mol·L-1 do not activate any chemical reaction. So, within each step, the time steplength Δt of the algorithm is determined from the fastest reaction rate maxR by the formula Δt = 10-6mol·L-1/max R. All the reversible elementary reactions occur simultaneously; however, by a simple book-keeping technique, the updating of species concentrations, within each step of the algorithm, is performed within each elementary reaction separately. The above proposed simple algorithm for Chemical Kinetics is applied to a simple model for hydrogen combustion with only five reversible elementary reactions (Initiation, Propagation, First and Second Branching, Termination by wall destruction) with six species (H2, O2, H, O, HO, H2O). These five reversible reactions are recommended in the literature as the most significant elementary reactions of hydrogen combustion [1] [2]. Based on the proposed here simple algorithm for Chemical Kinetics, applied to the global mechanism of proposed five reversible elementary reactions for hydrogen combustion, a simple and short computer program has been developed with only about 120 Fortran instructions. By this proposed program, the following are obtained: 1) The total species concentration of hydrogen combustion, starting from the sum of initial reactants concentrations [H2] + [O2], gradually diminishes, due to termination reaction by wall destruction, and tends to the final concentration of the product [H2O], that is to the 2/3 of its initial value, in accordance to the established overall stoichiometric reaction of hydrogen combustion 2H2 + O2 → 2H2O. 2) Time-histories for concentrations of main species H2, O2, H, H2O of hydrogen combustion, in explosion and equilibrium regions, obtained by the proposed program, are compared to corresponding ones obtained by accurate computational studies of [3]. 3) In the first step of the algorithm, the only nonzero species concentrations are those of reactants [H2], [O2]. So, the maximum reaction rate is that of the forward initiation reaction max R = Rif = kif[H2] [O2], where the rate constant kif is very slow. Thus, the first time steplength Δt1 = 10-6mol·L-1/max R results long in sec. After the first step, the sequences of all the following Δt’s are very short, in μsec. So, the first time steplength Δt1 can be considered as ignition delay time. 4) It is assumed that explosion corresponds to ignition delay time Δt1 < 10 sec. So, the curve on T-P plane, obtained by proposed program for Δt1 = 10 sec., can be considered as explosion limit curve. This curve is compared to the corresponding one obtained by the accurate computational studies of [2].

Kinetic study and kinetic parameters of lipase-catalyzed glycerolysis of sardine oil in a homogeneous medium

The production of polyunsaturated fatty acids (PUFAs) concentrates by enzymatic catalysis has gained interest due to their stereospecificity and the milder conditions employed compared to the use of inorganic catalysts. The enzymatic glycerolysis of sardine oil by Lipozyme® 435 to get PUFA concentrates in the forms of di- and monoacylglycerols (DAGs, MAGs) in an optimized amount of tert-butanol as the organic solvent was studied. First, mass transfer limitation of the reaction system was analyzed. The effects of different operating variables such as lipase loading, temperature and feed composition were investigated. A semi-empirical kinetic model based on the reversible elementary reactions of glycerolysis and hydrolysis of the glycerides was employed to correlate the experimental kinetic data. A molar ratio glycerol:oil of 3:1 was the optimum, which produced more than 84 wt% of MAG at 323 K. A comparison with other glycerolysis systems was performed using MAG yield, reaction rate and significance of kinetic parameters.

Experimental and modelling study of speciation and benzene formation pathways in premixed 1-hexene flames

An existing detailed and broadly validated kinetic scheme is augmented to capture the flame chemistry of 1-hexene under stoichiometric and fuel rich conditions including benzene formation pathways. In addition, the speciation in a premixed stoichiometric 1-hexene flame (flat-flame McKenna-type burner) has been studied under a reduced pressure of 20–30 mbar applying flame-sampling molecular-beam time-of-flight mass spectrometry and photoionization by tunable vacuum-ultraviolet synchrotron radiation. Mole fraction profiles of 40 different species have been measured and validated against the new detailed chemical reaction model consisting of 275 species and 3047 reversible elementary reactions. A good agreement of modelling results with the experimentally observed mole fraction profiles has been found under both stoichiometric and fuel rich conditions providing a sound basis for analyzing benzene formation pathways during 1-hexene combustion. The analysis clearly shows that benzene formation via the fulvene intermediate is a very important pathway for 1-hexene.

Combustion and pyrolysis of iso-butanol: Experimental and chemical kinetic modeling study

Abstract The first reaction mechanism for iso-butanol (372 species and 8723 reversible elementary reactions) pyrolysis and combustion that includes pressure dependent kinetics and proposes reaction pathways to soot precursors has been automatically generated using the open-source software package RMG. High-pressure reaction rate coefficients for important hydrogen abstraction reactions from iso-butanol by hydrogen, methyl and HO2 were calculated using quantum chemistry at the CBS-QB3 level. The mechanism was validated with recently published iso-butanol combustion experiments as well as new pyrolysis speciation data under diluted and undiluted conditions from 900 to 1100 K at 1.72 atm representative of fuel rich combustion conditions. Sensitivity and rate of production analysis revealed that the overall good agreement for the pyrolysis species, and in particular for the soot precursors like benzene, toluene and 1,3-cyclopentadiene, depends strongly on pressure dependent reactions involving the resonantly stabilized iso-butenyl radical. Laminar flame speed, opposed flow diffusion flame speciation profiles, and autoignition are also well-captured by the model. The agreement with speciation profiles for the jet-stirred reactor could be improved, in particular for temperatures lower than 850 K. Flux and sensitivity analysis for iso-butanol consumption revealed that this is primarily caused by uncertainty in iso-butanol + OH, iso-butanol + HO2 and the low temperature peroxy chemistry rates. Further theoretical and quantum chemical studies are needed in understanding these rates to completely predict the combustion behavior of iso-butanol using detailed chemistry.

Bioethanol combustion based on a reduced kinetic mechanism

Bioethanol is a fuel additive or a fuel substitute that has the benefit of being cleaner and price competitive with gasoline. Therefore, we develop a reduced kinetic mechanism capable of modeling the ethanol combustion and the generation of the combustion products \(\text{H}_{2}\text{O},~\text{CO}_{2},~\text{CO},~ \text{H}_{2},~ \text{C}_{2}\text{H}_{4}\) and OH. Based on a mechanism composed by 372 reversible elementary reactions among 56 reactive species, we propose a reduction strategy to obtain an eight-step mechanism for the ethanol. The reduction strategy consists in estimating the order of magnitude of the reaction rate coefficients, defining the main chain, applying the steady-state and partial equilibrium hypotheses, and justifying the assumptions through an asymptotic analysis. The main advantage of the obtained reduced mechanism is the decrease of the work needed to solve the system of chemical equations proportionally to the number of elementary reactions present in the complete mechanism. Numerical tests are carried out for a jet diffusion flame of ethanol and the results compare well with available data in the literature.

Exploring Chemical Equilibrium with Poker Chips: A General Chemistry Laboratory Exercise

A hands-on laboratory exercise at the general chemistry level introduces students to chemical equilibrium through a simulation that uses poker chips and rate equations. More specifically, the exercise allows students to explore reaction tables, dynamic chemical equilibrium, equilibrium constant expressions, and the equilibrium constant based on rate constants. Poker chips are used to simulate the reactants and products of two reversible elementary reactions.

Evaluation of the reaction rate constants for the gas-phase Al-CH4–air combustion chemistry

The most likely reaction pathways and reaction products in the Al-CH4-O2-N2 system are investigated using density functional theory and ab initio calculations. The B3LYP functional with extended 6–311+G(3df,2p) basis set as well as the CBS-QB3 composite method are mainly utilised. Theoretical analysis of corresponding reaction rate constants is also performed with the use of simple theoretical models. A critical overview of current knowledge on combustion-relevant reactions with aluminium compounds is given. On the basis of critical comparison of available experimental kinetic data with theoretical calculations, the approximations for rate constants for 44 reversible elementary reactions involving Al-containing species are recommended for use in combustion issues.

Development of reduced reaction mechanisms for hydrogen and methanol diffusion flames

Biofuels, such as biohydrogen, biomethanol, bioethanol and biodiesel may be the future of energy for transportation. The methanol can be used directly for powering Otto engines or fuel cells; it is commonly used in biodiesel production for its reactivity. Biohydrogen is a replacement for fossil and biorenewable liquid fuels. Based on a mechanism composed by 20 reversible elementary reactions among 8 reactive species, we propose a reduction process to obtain the mechanism of hydrogen, and from 129 reversible elementary reactions among 23 reactive species, we obtain a reduced mechanism for the methanol. Therefore, the aim of this work is the development of an algorithm consisting of 7 steps, which eliminates the fastest reactions of the reaction mechanism since the slowest reactions are rate-determining. The main advantage of the strategy is the decrease of the work needed to solve the system of chemical equations.

Construction of an ab initio kinetic model for industrial ethane pyrolysis

AbstractThe industrial steam cracking of ethane was simulated using an ab initio kinetic model. The reaction network consists of 20 species and 150 reversible elementary reactions. The thermodynamic and kinetic parameters were obtained from ab initio CBS‐QB3 and W1U calculations and agree well with available experimental data. Predicted C2H6, C2H4, and H2 yields are within 5% of experimental data for the three sets of conditions tested. Though CH4 yields and outlet temperatures are particularly sensitive to the accuracy of the kinetic parameters, they are simulated with an accuracy of better than 10%. Larger deviations for the C3H6 and C2H2 yields are attributed to the limited size of the reaction network. The effect of total pressure on the rate coefficients was evaluated using Quantum Rice‐Ramsberger‐Kassel theory with the Modified Strong‐Collision approximation, and was found to be relatively minor for the reaction conditions tested. This study hence demonstrates the feasibility of simulating complex radical reactions using a predictive kinetic model derived from state‐of‐the‐art quantum chemical calculations. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Microkinetic Modeling and Reduced Rate Expressions of Ethylene Hydrogenation and Ethane Hydrogenolysis on Platinum

Using a hierarchical multiscale methodology, a thermodynamically consistent surface reaction mechanism has been developed to capture the C−C and C−H chemistries of ethylene hydrogenation and ethane hydrogenolysis on platinum. This mechanism, consisting of 32 reversible elementary reactions, describes kinetically limited ethane hydrogenolysis and ethylene hydrogenation experimental data under diverse temperature and composition conditions. Under hydrogenolysis conditions, sensitivity analysis shows that C−C bond cleaving reactions are rate-determining and that most C−C bond cleaving reactions occur through the ethyl and ethylidene surface intermediates. Under hydrogenation conditions, sensitivity analysis shows that both hydrogenation reactions (π-bonded ethylene to ethyl and associative desorption of ethyl to ethane) are rate-controlling. Finally, to enable reactor design and systems simulations, the full mechanism is reduced to two separate, reduced microkinetic models and one-step rate expressions that ...

Complete thermodynamically consistent kinetic model of particle nucleation and growth: Numerical study of the applicability of the classical theory of homogeneous nucleation

A complete thermodynamically consistent elementary reaction kinetic model of particle nucleation and growth from supersaturated vapor was developed and numerically evaluated to determine the conditions for the steady-state regime. The model treats all processes recognized in the aerosol science (such as nucleation, condensation, evaporation, agglomerationcoagulation, etc.) as reversible elementary reactions. It includes all possible forward reactions (i.e., of monomers, dimers, trimers, etc.) together with the thermodynamically consistent reverse processes. The model is built based on the Kelvin approximation, and has two dimensionless parameters: S0-the initial supersaturation and Theta-the dimensionless surface tension. The time evolution of the size distribution function was obtained over the ranges of parameters S0 and Theta. At low initial supersaturations, S0, the steady state is established after a delay, and the steady-state distribution function corresponds to the predictions of the classical nucleation theory. At high initial supersaturations, the depletion of monomers due to condensation on large clusters starts before the establishing of the steady state. The steady state is never reached, and the classical nucleation theory is not applicable. The boundary that separates these two regimes in the two dimensionless parameter space, S0 and Theta, was determined. The model was applied to several experiments on water nucleation in an expansion chamber [J. Wolk and R. Strey, J. Phys. Chem. B 105, 11683 (2001)] and in Laval nozzle [Y. J. Kim et al., J. Phys. Chem. A 108, 4365 (2004)]. The conditions of the experiments performed using Laval nozzle (S0=40-120) were found to be close to the boundary of the non-steady-state regime. Additional calculations have shown that in the non-steady-state regime the nucleation rate is sensitive to the rate constants of the initial steps of the nucleation process, such as the monomer-monomer, monomer-dimer, etc., reactions. This conclusion is particularly important for nucleation from supersaturated water vapor, since these processes for water molecules at and below the atmospheric pressure are in the low pressure limit, and the rate constants can be several orders of magnitude lower than the gas kinetic. In addition, the impact of the thermodynamic inconsistency of the previously developed partially reversible kinetic numerical models was assessed. At typical experimental conditions for water nucleation, S0=10 and Theta=10 (T=250 K), the error in the particle nucleation rate introduced by the thermodynamic inconsistency exceeds one order of magnitude.

Numerical simulation of the combustion of hydrogen–air mixture in micro-scaled chambers. Part I: Fundamental study

Abstract Understanding of micro-scale combustion mechanism is very essential to the development of combustion-based micro-power devices, which may supply much higher energy density than the batteries used nowadays. In part I of this paper, Computational Fluid Dynamics (CFD)-based numerical simulations have been performed to study the combustion of premixed hydrogen–air mixture in a series of chambers with same shape aspect ratio but various dimensions from millimetre to micron level. The transition of the combustion phenomena in the chambers from relatively large scale to micro-scale has been studied numerically to investigate the micro-combustion mechanism. The combustion model of premixed hydrogen–air mixture represents the detailed reaction mechanism with 19 reversible elementary reactions and nine species. The effect of various heat transfer conditions at chamber wall, e.g. adiabatic wall, with heat loss and heat conduction within the wall, on the combustion is analysed. These thermal conditions have strong effects on the combustion especially when the chamber dimension goes smaller and the ratio of surface area to volume becomes larger. Both factors, such as larger heat loss through the chamber wall and smaller chamber dimension size, may lead to the thermal quenching of micro-scale combustion. The simulation results also indicate that the stable combustion in a micro-scaled chamber may be sustained through increasing the ratio of flow residence time in chamber to chemical reaction time, and maintaining proper thermal condition. In part II of this paper, the numerical modelling method developed here is applied to analyse the micro-combustion characteristics in a three-dimensional micro-combustor based on the prototype developed by the MIT group for a micro gas turbine engine.

Rate constants for the homogeneous gas-phase Al/HCl combustion chemistry

Experimental reaction rate data for the Al/HCl system are very scarce. Such data are needed for the comprehension and for the numerical simulation of the combustion of aluminum particles as encountered in solid segmented motors. Toward this end, we have examined the homogeneous chemistry of this system, computed rate parameters for important reactions using conventional Transition State Theory (TST) and RRKM/master equation simulations, and estimated rate parameters for reactions where rigorous computations are not presently feasible. The reaction mechanism presented in this study consists of 15 species participating in 39 reversible elementary reactions for which rate parameters have been estimated or computed.

Kinetics of Sol Particle Formation as a Function of pH Studied by Viscosity Measurements in Silica Solutions

A model for the dissolution of oxides has been developed recently. It has been tested with silica. The model describes the overall rate constant of dissolution as composed of several reversible elementary reactions. By this way the overall rate constant of dissolution can be expressed in dependence on pH and ionic strength. It is also possible to derive an expression for the rate constant of polycondensation in dependence on pH and ionic strength. The exact evaluation of these expressions is difficult and only possible by numerical methods. Approximate analytical expressions are therefore deduced, which are valid for special pH regions. The pH dependence of the overall rate constant of polycondensation predicted by the model is confirmed by both synthesis of silica sols from alkali silicate solutions and synthesis from tetraethylorthosilicate.

Ethylene Ignition and Detonation Chemistry, Part 1: Detailed Modeling and Experimental Comparison

A numerical investigation is reported on the chemical kinetics of C 2 H 4 ignition and detonation in oxygen-diluent mixtures. Conditions addressed cover initial (postshock) temperatures between 1000 and 2500 K, pressures between 0.5 and 100 bar and equivalence ratios between 0.5 and 2. Attention is paid to histories of species concentrations and temperature, to autoignition times, and to subsequent heat release that is relevant to detonation structure. A detailed mechanism is proposed consisting of 148 reversible elementary reactions among 34 chemical species. This mechanism is shown to provide good agreement with most ignition times measured in shock tubes and with burning-velocity measurements available in the literature. The results provide a basis for developing simplified descriptions that can be used in multidimensional detonation studies for applications to propulsion devices.

Simulation of the Effects of CCl4 on the Ethylene Dichloride Pyrolysis Process

Modeling of the ethylene dichloride (EDC) pyrolysis reaction was performed with respect to 108 reversible elementary reactions with 47 molecular and radical species. Kinetic schemes and reaction pathways have been developed based on thermochemical kinetic theories, such as thermal decomposition of molecules and radical-chain reactions, especially the abstraction of H by a Cl radical. In particular, mass, energy, and momentum conservation equations in a gas-phase plug reactor were solved simultaneously with the established reaction mechanisms using a numerical scheme for stiff ordinary differential equations. Because of the characteristics of Cl-catalyzed reaction mechanisms, Cl suppliers play key roles in promoting the reaction conversion. This study reports on the calculation and analysis of the effects of CCl4, as a promotor, on the process conversion to vinyl chloride monomer, concentration of the coking precursor, changes in the heat required, and the pressure drop. The study addresses quantitatively ...

Extinction and autoignition of n-heptane in counterflow configuration

A study is performed to elucidate the mechanisms of extinction and autoignition of n-heptane in strained laminar flows under nonpremixed conditions. A previously developed detailed mechanism made UP of 2540 reversible elementary reactions among 557 species is the starting point for the study. The detailed mechanism was previously used to calculate ignition delay times in homogeneous reactors, and concentration histories of a number of species in plug-flow and jet-stirred reactors. An intermediate mechanism made up of 1282 reversible elementary reactions among 282 species and a short mechanism made up of 770 reversible elementary reactions among 160 species are assembled from this detailed mechanism. Ignition delay times in an isochoric homogeneous reactor calculated using the intermediate and the short mechanism are found to agree well with those calculated using the detailed mechanism. The intermediate and the short mechanism are used to calculate extinction and autoignition of n-heptane in strained laminar flows. Steady laminar flow of two counter flowing Streams toward a stagnation plane is considered. One stream made up of prevaporized n-heptane and nitrogen is injected from the fuel boundary and the other stream made up of air and nitrogen is injected from the oxidizer boundary. Critical conditions of extinction and autoignition given by the strain rate, temperature and concentrations of the reactants at the boundaries, are calculated. The results are found to agree well with experiments. Sensitivity analysis is carried out to evaluate the influence of various elementary reactions on autoignition. At all values of the strain rate investigated here, high temperature chemical processes are found to control autoignition. In general, the influence of low temperature chemistry is found to increase with decreasing strain. A key finding of the present study is that strain has more influence on low temperature chemistry than the temperature of the reactants.

Chemometric analysis of a detailed chemical reaction mechanism for methane oxidation

A widely used detailed reaction mechanism for methane oxidation (Gas Research Institute (GRI) mechanism 2.11) has been analysed, in order to evaluate if reactions were to be added to the mechanism. This mechanism consists of 279 reversible elementary reactions between 48 different species, each with a temperature dependence described by a modified Arrhenius expression. The mechanism was transformed to 558 irreversible reactions, and the rate constants were analysed at a fixed temperature, to reduce the complexity of the analysis. A partial least squares (PLS) model was generated, which estimated reaction rate constants as a function of a reaction descriptor vector. This vector characterized the different chemical bonds in the reactants and products of a chemical reaction. The model was validated through full cross validation. The original mechanism was unable to correctly predict oxidation of methane in a natural gas engine exhaust manifold: Oxidation of 2300 ppm methane in the presence of 300 ppm NO, 9% oxygen and 2% water. Therefore, these conditions were used for evaluation of the reaction mechanism. The potential reactions for expanding the mechanism were selected among reactions with one or two reactants and one or two products. A stepwise analysis combining rate of production (ROP) analysis with sensitivity analysis was used to reduce 2138 potential elementary reactions to nine important reactions, which were added to the mechanism. The analysis was based on PLS estimates of the reaction rate constants, but in the final model, literature values were included where available. This modification of the mechanism improved model predictions.

Evaluation of Ignition Improvers for Methane Autoignition

The objective of this study was to estimate the efficiency of methane autoignition promotion by testing different ignition improvers including hydrogen, H2, hydrogen peroxide, H2O2, ozone,O3 and dimethyl ether, DME, (CH3)2O. This was accomplished by computing ignition delays for CH4/O2/Ar or N2 mixtures of various compositions, concentrations of the promoters, pressures and temperatures. Ignition delay times for additive-free mixtures were used for tuning a methane oxidation mechanism consisting of 185 reversible elementary reactions between 32 species. A selection of the reaction rate parameters available in the standard databases was made to optimize the agreement between simulation and experimental results for one particular set of test conditions (reference mixture) by refining the rate parameters of the most sensitive stages revealed by sensitivity analysis. The agreement achieved between model predictions and shock tube experimental data is very good. To investigate the effect of dimethyl ether on methane autoignition, the mechanism was extended up to 301 reactions and 53 species to predict the autoignition delay at high pressures. Methane autoignition promotion is proven to be much stronger for H2O2, O3 and (CH3)2O additives than for H2. The mechanism of ignition acceleration is attributed mainly to the dominant role of the O and OH-radicals generated by the rapid decomposition of hydrogen peroxide and ozone, and to the peroxide intermediates and low temperature branching, in the case of dimethyl ether.