Thermal rate constants of the pyrolysis of n-Heptane

doi:10.1016/j.combustflame.2011.04.015

Combustion and Flame

Volume 158, Issue 12, December 2011, Pages 2314-2324

https://doi.org/10.1016/j.combustflame.2011.04.015 Get rights and content

Abstract

Rate coefficients for straight chain alkane and free radical decomposition are important in combustion process. This work reports a theoretical study of the pyrolysis of n-Heptane. The barrier heights of the C−C fission reaction, β-scission reaction and H-atom abstraction reaction, as well as geometrical parameters of the reactants, products, and transition states involved in the decomposition of n-Heptane have been calculated at the CCSD(T)/6–311G(d,p)//B3LYP/6–311G(d,p) level. The temperature-dependent rate constants for individual reaction have been obtained in the temperature range of 200–3000 K using variational transition state theory and Rice–Ramsperger–Kassel–Marcus theory. The pressure dependence rate constants have been treated by one-dimensional master equation calculations at different pressure as well as high-pressure limit. In order to facilitate the use of the reaction rate constants for chemical kinetics modeling, all of the individual rate constants were fitted to a modified three-parameter Arrhenius expression: k(T) = ATⁿ exp(−E_b/RT) at various pressures. Some of the predicted rate constants are in reasonable agreement with the available experimental and previous theoretical results. The pyrolysis mechanism and RRKM-based rate constants presented in this paper may be used in high accuracy combustion modeling.

Introduction

A basic understanding of combustion kinetics of fossil fuels is critical to a large range of practical applications; including combustor designs and pollution reduction in our living environment [1], [2], [3]. While straight chain hydrocarbons is one of the important elements of fossil fuel, so the thermal decomposition of straight chain hydrocarbons and free radicals have been the subject of interest since the beginning of petroleum, not only due to the demand for ethylene and propylene, but also it is an important class of elementary reactions in high-temperature pyrolysis and combustion processes [1], [2], [3]. Analysis revealed that for a wide range of combustion conditions, the kinetics of fuel cracking to form smaller molecular fragments is essential and coupled with the oxidation kinetics of the fragments. The ultimate products formed by combustion processes, rates of burning, and temperature dependencies of flame properties are sensitive to the relative importance of the competing reactions of decomposition and other elementary reactions [1], [2]. In any quantitative combustion modeling study, the proper inclusion of the kinetics of these reactions, including the pressure and temperature dependencies of their rate constants is essential [4], [5]. Moreover, in term of supersonic flight, the fuel is subjected to high pressures, sometimes exceeding its critical value [6], however, the kinetics rate constants at high pressure are not widely available in the literature. Besides, laboratory studies of unimolecular decompositions are conducted far from the conditions (T and P) encountered in high-temperature and high-pressure processes [7]. So, there is a continuing need to obtain pyrolysis rate constants under much different conditions.

Some studies on high pressure pyrolysis of long chain paraffins are available. Voge and Good [8] studied atmospheric as well as high pressure pyrolysis of n-hexadecane at 773 K. Fabuss et al. [9] investigated the thermal decomposition of the pure n-hexadecane in the temperature range of 866–977 K and pressure range of 1.48–6.99 MPa. Song et al. [10] investigated the condensed-phase pyrolysis of n-tetradecane at elevated pressures. Yu and Eser [11] studied the near-critical and super-critical phase thermal decomposition of C10–C14 normal alkanes at temperatures from 673 to 723 K and pressures from 1 to 10 MPa (initial pressure). Recently, Yuan et al. [12] do an experimental and theoretical study of n-Heptane pyrolysis, they detected the pyrolysis products by using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Beside these, several studies on the pyrolysis of n-Heptane at atmospheric pressure have been published [6], [13], [14]. However, detailed theoretical studies about the pyrolysis of n-Heptane are not widely available in the literature.

It is well-established that pyrolysis of hydrocarbons follows a free radical chain mechanism. A free radical mechanism was first proposed by Rice [15] and later modified by Kossiakoff and Rice [16], who considered resonance stabilization and radical isomerization in order to better predict the available experimental product distribution of some n-paraffins. A free radical mechanism includes: C−C fission; alkyl radical’s β-scission to produce olefins and lower alkyl radicals; and the subsequent H-atom abstraction from C atoms in parent hydrocarbons by H and small alkyl radical. These three kinds of reaction are the critical reaction during the pyrolysis of hydrocarbons.

Reaction rate parameters in a kinetic model are usually derived from three sources: experimental studies, estimations, and theoretical studies [2]. Even with the advances made in the experimental techniques to obtain accurate kinetic data, there is still disagreement among different people regarding the elementary radical reactions to be included for a particular n-alkane, as well as on the value of kinetic constants for the same elementary reaction [17], [18], [19]. Most of the rate constants of unimolecular reaction were derived from the recombination direction where the recombination reaction may be measured more directly. Dissociation, on the other hand, is difficult to measure it in isolation. Therefore, a theoretical study of rate parameters may be necessary when they are not well known. For instance, quantum mechanical electronic structure calculations are useful tools for exploring elementary reaction pathways. Statistical mechanics and reaction rate theories can be applied to determine rate parameters of elementary reactions relevant to fuel combustion.

The objective of this work is to provide a comprehensive treatment of the kinetics and mechanisms for the n-Heptane decomposition reaction at a high level of computation, from a detailed mapping of the potential energy surface (PES) of the system to the rate constant prediction for all reaction channels involved. n-Heptane was chosen because n-Heptane is one of the important elements of jet fuel, the kinetics and rate constants of n-Heptane pyrolysis at a high level of theoretical study has not been reported. We plan to use a combined high-level ab initio method and Rice–Ramsperger–Kassel–Marcus (RRKM) theory to investigate the effect of temperature and pressure on the rate constants and kinetics during high pressure pyrolysis of n-Heptane, in order to provide a reliable estimation of these rate constants for practical combustion applications.

Section snippets

Potential energy surface

The geometries for all the reactants, transition states, and products considered here were optimized using the hybrid density functional B3LYP method, i.e., Becke’s three parameter nonlocal exchange functional [20], [21], [22] with the nonlocal correlation functional of Lee, Yang, and Parr [23] with 6–311G (d,p) basis set [24]. Vibrational frequencies of all species, calculated at the B3LYP/6–311G (d,p) level and scaled by standard factor of 0.9986 [25] were used for the calculation of

Results and discussion

As alluded to in Section 1, pyrolysis of hydrocarbons follows a free radical chain mechanism, which includes several kinds of reaction: (a) C−C fission, (b) H-atom abstraction from C atoms in parent n-alkanes by H and small alkyl radical, (c) their β-scission to produce olefins and lower alkyl radicals. In this paper, we just consider those reactions that relevant to alkanes and small alkyl radicals (C1−C4). In the case of n-Heptane, we divide the pyrolysis reactions into three categories: the

Conclusions

Using statistical–theoretical methodology, we have studied the pyrolysis mechanism of n-Heptane and the temperature and pressure dependence rate constants in some detail. The pyrolysis mechanism includes the important P-reaction, β-scission reaction and H-atom abstraction reaction, consisting of 22 species, 28 reactions and 25 transition states. The energy barrier and geometrical parameters of transition states have been calculated at a high level of CCSD (T)/6–311G (d,p)//B3LYP/6–311G (d,p)

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Thermal rate constants of the pyrolysis of n-Heptane

Abstract

Introduction

Section snippets

Potential energy surface

Results and discussion

Conclusions

Prog. Energy. Combust. Sci.

J. Anal. Appl. Pyrolysis.

J. Anal. Appl. Pyrolysis.

Chem. Eng. Sci.

Appl. Catal.

Chem. Phys. Lett.

Chem. Phys. Lett.

Chem. Phys. Lett.

Chemistry of Hydrocarbon Combustion

J. Phys. Chem. Ref. Data

J. Phys. Chem. Ref. Data

J. Phys. Chem.

J. Am. Chem. Soc.

Ind. Eng. Chem. Process Des. Dev.

Ind. Eng. Chem. Res.

Ind. Eng. Chem. Res.

J. Phys. Chem. A

J. Am. Chem. Soc.

J. Am. Chem. Soc.

J. Am. Chem. Soc.

Int. J. Chem. Kinet.

J. Chem. Phys.