N-heptane Pyrolysis Research Articles

High-Temperature Pyrolysis of N-Tetracosane Based on ReaxFF Molecular Dynamics Simulation.

In order to further understand the high-temperature reaction process and pyrolysis mechanism of hydrocarbon fuels, the high-temperature pyrolysis behavior of n-tetracosane (C24H50) was investigated in this paper via the reaction force field (ReaxFF) method-based molecular dynamics approach. There are two main types of initial reaction channels for n-heptane pyrolysis, C-C and C-H bond fission. At low temperatures, there is little difference in the percentage of the two reaction channels. With the temperature increase, C-C bond fission dominates, and a small amount of n-tetracosane is decomposed by reaction with intermediates. It is found that H radicals and CH3 radicals are widely present throughout the pyrolysis process, but the amount is little at the end of the pyrolysis. In addition, the distribution of the main products H2, CH4, and C2H4, and related reactions are investigated. The pyrolysis mechanism was constructed based on the generation of major products. The activation energy of C24H50 pyrolysis is 277.19 kJ/mol, obtained by kinetic analysis in the temperature range of 2400-3600 K.

Probing PAH Formation from Heptane Pyrolysis in a Single-Pulse Shock Tube

ABSTRACT To improve our capabilities to model surrogate fuels, particularly in respect to polycyclic aromatic hydrocarbons (PAHs) and soot formation, the pyrolysis of n-heptane is studied in a single-pulse shock across a wide temperature range (900–1700 K) at 20 bar nominal pressure and 4 ms residence time. Three different initial fuel mole fractions are considered, 103, 502, and 2000 ppm of n-heptane in argon. Fuel and intermediate species, including aromatics up to three-ring structures, are measured using gas chromatography and mass spectrometry diagnostics. An ongoing detailed chemical kinetic model for PAH chemistry has been updated to successfully capture the fuel decomposition, the formation of small hydrocarbons, and the concentration of the main PAH products. Major reaction pathways to PAHs are highlighted as well as the role of important intermediate species.

Soot formation from n-heptane counterflow diffusion flames: Two-dimensional and oxygen effects

Soot formation of n-heptane is experimentally and numerically studied in low-strain rate (K = 50 s−1) counterflow soot formation (SF) flames. Flame temperatures and soot volume fractions at different oxygen mole fractions on the oxidizer stream (xO2=0.3–0.45) are measured by using thermocouple-calibrated OH two colors laser-induced fluorescence (2C-PLIF) and laser-induced incandescence (LII) calibrated by light extinction methods, respectively. Mono (MAHs) and polycyclic aromatic hydrocarbons (PAHs) are also qualitatively measured by using the PAHs-LIF technique. Good agreement between measured and predicted maximum flame temperature (Tmax) and peak soot volume fraction (fv,peak) is obtained. However, discrepancies in the shape of the entire soot volume fraction profiles along the axial centerline are observed between 1D and 2D simulations. This result is found to be primarily related to the thermophoretic and radial effects in the low strain rate flames investigated, which hamper neglecting the underlying 2D effects. MAH/PAH peak mole fractions and fv,peak increase from xO2=0.3 to xO2=0.45 due to the related increase of flame temperature, which fosters n-heptane pyrolysis near the fuel nozzle leading to higher concentrations of C1C4 hydrocarbons. The latter are found to grow to MAHs/PAHs and finally to soot particles through analogous pathways independently of xO2. However, the higher flame temperature in the sooting region at xO2=0.45 leads to soot particles and aggregates characterized by larger sizes and more dehydrogenated than those produced at xO2=0.3.

Exploration of new olefin production process: Effects of the addition of CO2 and H2O on n-heptane pyrolysis

In order to study the new cracking process characteristics of hydrocarbon feedstock to light olefins by direct flue gas heating, the pyrolysis experiments of n-heptane in CO2 and H2O atmosphere were performed in pretreated STS316L flow reactor (ϕ10 mm × 2.5 mm × 700 mm) at a temperature range of 1073–1373 K, pressure range of 0.1–1.0 MPa. By changing the n-heptane flow and system pressure to adjust the residence time, results show that the latter is not conducive to olefin formation, but the residence time range about high olefin formation of the latter is wider than that of the former. Due to the promotion of CO and the high-temperature solubility of CO2, the types and contents of PAHs in tar under H2O&CO2 atmosphere are higher. Based on the analysis of n-heptane cracking products stream, the reactivity of H2O and CO2 gradually appears at high temperature and the reactivity of H2O is significantly higher than that of CO2, resulting in higher yields of H2 and CO. The mainstream of the process is the cracking of n-heptane, in which H2O is the supply source of •OH and •H, and CO2 acts as a coking agent. High olefin yields can be achieved in either H2O-rich or CO2-rich atmosphere, and the ratio of the two needs to be properly regulated.

Pyrolysis mechanism study of n-heptane as an endothermic hydrocarbon fuel: A reactive molecular dynamic simulation and density functional theory calculation study

The pyrolysis of endothermic hydrocarbon fuel with high heat sink can reduce the surface temperature of engines and aircraft fuselage. The pyrolysis mechanism of n-heptane as a model compound for endothermic hydrocarbon fuel was investigated by using ReaxFF reactive molecular dynamic simulations and density functional theory calculations. The results showed that the breakage of CC bonds of n-heptane were easier than that of CH bonds, in which the cleavage of C2C3 bond is the easiest to generate C2H5 and C5H11 radicals. Temperature has a significant effect on the pyrolysis process of n-heptane, and the increase of temperature will accelerate the pyrolysis of n-heptane and promote the formation of pyrolysis products. The pre-exponential factor and apparent activation energy of the n-heptane pyrolysis were 5.4456 × 1014 s−1 and 231.3 kJ·mol−1, respectively.

Empirical modeling of normal/cyclo-alkanes pyrolysis to produce light olefins

Due to the complexity of feedstock, it is challenging to build a general model for light olefins production. This work was intended to simulate the formation of ethylene, propene and 1,3-butadiene in alkanes pyrolysis by referring the effects of normal/cyclo-structures. First, the pyrolysis of n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, cyclohexane, methylcyclohexane, n-hexane and cyclohexane mixtures, and n-heptane and methylcyclohexane mixtures were carried out at 650–800 °C, and a particular attention was paid to the measurement of ethylene, propene and 1,3-butadiene. Then, pseudo-first order kinetics was taken to characterize the pyrolysis process, and the effects of feedstock composition were studied. It was found that chain length and cyclo-alkane content can be qualitatively and quantitively represented by carbon atom number and pseudo-cyclohexane content, which made a significant difference on light olefins formation. Furthermore, the inverse proportional/quadratic function, linear function and exponential function were proposed to simulate the effects of chain length, cycloalkane content and reaction temperature on light olefins formation, respectively. Although the obtained empirical model well reproduced feedstock conversion, ethylene yield and propene yield in normal/cyclo-alkanes pyrolysis, it exhibited limitations in simulating 1,3-butadiene formation. Finally, the accuracy and flexibility of the present model was validated by predicting light olefins formation in the pyrolysis of multiple hydrocarbon mixtures. The prediction data well agreed with the experiment data for feedstock conversion, ethylene yield and propene yield, and overall characterized the changing trend of 1,3-butadiene yield along with reaction temperature, indicating that the present model could basically reflect light olefins production in the pyrolysis process even for complex feedstock.

Experimental and kinetic study of hydrocarbon fuel pyrolysis in a shock tube

A pyrolysis study of hydrocarbon fuels (n-heptane and iso-octane) in a shock tube was conducted at temperatures range of 1200–2100 K and pressures range of 0.22–0.28 MPa. Several pyrolytic products were detected and quantified using gas chromatography. Different kinetic models were selected to simulate the fuel pyrolytic processes under the same experimental conditions. The results show that CH4, C2H2, C2H4, and C3H6 are the main products in the pyrolysis of n-heptane and iso-octane. The sensitivity and rate of production analysis indicate that olefins are the most important products in the pyrolysis, with the main decomposition pathways of n-heptane and iso-octane being hydrogen abstraction and C-C bonds scissions, respectively. In the pyrolysis of n-heptane, the main decomposition pathway is the hydrogen abstraction reaction by H/CH3 radicals, with ethylene as the main olefin product. In the pyrolysis of iso-octane, the main decomposition pathways are the direct cleavage reactions of the C-C bonds and isobutylene is the main olefin product. The difference in the pyrolysis characteristics between n-heptane and iso-octane is mainly caused by their different molecular structures.

Probing pyrolysis chemistry of 1-heptene pyrolysis with insight into fuel molecular structure effects

The pyrolysis of 1-heptene was studied in a flow reactor using synchrotron vacuum ultraviolet photoionization mass spectrometry at 0.04 and 1 atm and in a jet-stirred reactor using gas chromatography at 1 atm. Flow reactor pyrolysis products, including the allyl radical, cycloalkenes and aromatics, were identified and quantified. Alkenes are found to be the dominant product family, among which ethylene is the most abundant product. A detailed intermediate-to-high temperature model of 1-heptene was developed and validated against the new pyrolysis data in this work, as well as previous data of 1-heptene combustion in literature over a wide range of pressures, temperatures and equivalence ratios. Rate of production analysis and sensitivity analysis were performed to reveal the key pathways in fuel decomposition and product formation. The allylic CC bond dissociation reaction is concluded as the most important pathway in 1-heptene decomposition. Reactions of allyl, propargyl and cyclopentadienyl radicals play important roles in the formation of cycloalkenes and aromatics. Furthermore, comparative pyrolysis experiments of 1-hexene and n-heptane were also performed in the jet-stirred reactor at 1 atm using gas chromatography to explore fuel molecular structure effects on pyrolysis reactivity and product distributions among 1-alkene and n-alkane fuels. The comparison between 1-heptene and 1-hexene pyrolysis demonstrates that similar fuel molecular structure results in the similarities in primary fuel decomposition pathways and pyrolysis reactivity. Ethylene is the most abundant product in both 1-alkene pyrolysis, and the feature in 1-heptene molecular structure leads to enhanced formation of ethylene in its pyrolysis. The abundant formation of ethyl and methyl radicals leads to higher production of 1-pentene and 1-butene in 1-heptene and 1-hexene pyrolysis, respectively. The comparison between 1-heptene and n-heptane pyrolysis reveals that the existence of CC double bond enhances the pyrolysis reactivity of 1-heptene. Different from 1-heptene consumption, n-heptane consumption is dominantly controlled by H abstraction reactions instead of unimolecular decomposition reactions. Propene and 1-butene are prone to be produced in 1-heptene pyrolysis, while 1-hexene has higher mole fractions in n-heptane pyrolysis.

Comparison of Pyrolysis and Coking Performance of Methylcyclohexane and N-Heptane in Flow Reactor

Comparison of Pyrolysis and Coking Performance of Methylcyclohexane and N-Heptane in Flow Reactor

Plasma-assisted pyrolysis and ignition of pre-vaporized n-heptane, iso-octane and n-decane

We intensively investigated the kinetic enhancement of dielectric-barrier-discharge (DBD) plasma on the pyrolysis and ignition of n-heptane, iso-octane, and n-decane fuels in a counterflow diffusion burner. The DBD plasma generated upstream from the fuel nozzle can contribute to low-temperature pyrolysis and high-temperature oxidation of pre-vaporized liquid fuels. Stable and short-lived intermediate species generated by plasma were measured by ex situ gas chromatography (GC) and in situ planar laser induced fluorescence (PLIF), respectively. GC results indicate that with plasma assistance approximately 2–3% of large hydrocarbons decompose at temperatures as low as 370 K and within 1 s. High-temperature oxidation experiments demonstrate that DBD can lower the ignition temperature of all three fuels with various global flow stretch rates. Experiments were compared with numerical analysis performed using the OPPDIF code that is formulated for the counterflow geometry. Finally, the discrete chemical explosive mode analysis (CEMA) elucidates the promotion of plasma-induced species, such as H2, C2H4, and OH radicals.

Simulation of Microwave Discharge in Liquid n-Heptane in the Presence of Argon in the Discharge Region

A zero-dimensional model is presented for a microwave discharge in liquid n-heptane at atmospheric pressure with continuous introduction of argon into the plasma region. The model includes equations describing the formation of a solid phase from n-heptane degradation products. Along with the detailed kinetics of thermal pyrolysis of n-heptane, processes involving argon atoms and processes involving electrons, ions, and excited species are included. To determine the electron energy distribution function, the Boltzmann equation is used. For comparison with the spectral characteristics of the discharge, the kinetic scheme includes the processes of excitation of the radiative states of the hydrogen atom and the C2 molecule. Calculations have made it possible to explain the appearance of Hα lines at sufficiently high concentrations of introduced argon.

1D Modeling of the Microwave Discharge in Liquid n-Heptane Including Production of Carbonaceous Particles

Microwave plasma in the liquid is initiated inside a gas bubble formed at the end of the electrode-antenna, through which microwave energy is introduced into the liquid. A 1D model a set of gas phase kinetic reactions describes the evolution processes of ionization, heat transfer and formation of gas and solid products inside the plasma bubble. The code is based on joint solution of the Boltzmann equation for free electrons of the plasma, a simplified equation for the microwave field, the heat conduction equation, the balance equation for the electron density and the balance equations for the weight fraction for all gas and solid products of n-heptane pyrolysis. The Joule heat released in the plasma is expended on the evaporation of liquid n-heptane into the bubble and the decomposition of the n-heptane molecules. The model includes both the description of gas phase processes and formation of solid carbon-containing particles. The growth mechanism for generation of solid particles describes simultaneous processes of the initial nucleation, surface growth and coagulation of soot particles. The results of calculations are compared with known experimental results.

Modelling and numerical simulation of n-heptane pyrolysis coking characteristics in a millimetre-sized tube reactor

Based on the optimization of the recently proposed gas phase mechanism for n-heptane pyrolysis, mechanisms for polycyclic aromatic hydrocarbon (PAH) formation and a detailed wall surface reaction, a basic gaseous pyrolysis mechanism with 1127 steps and a simplified surface reaction mechanism with 34 steps were developed, respectively. After performing the detailed estimation and modelling procedures for the surface site density (SDEN) related to the particle diameter, particle density, C/H ratio and other parameters determined by the experimental tests and observations, numerical studies were integrated with the developed coke model to simulate the pyrolysis process and the coking properties in a millimetre-sized tube reactor on the Chemkin Pro software platform using a two-dimensional cylindrical shear flow (CSF) module. The prediction values of the pyrolysis product concentration and coking rate at different temperatures were compared with the experimental data, and agreement was obtained. Under pyrolysis conditions of an n-heptane inlet flow rate of 0.5 ml/min, an operating pressure of 1.0 MPa and a furnace temperature of 973–1073 K, the research results showed that the hydrogen abstraction reaction, as the first and key step of coke formation, is easily affected by gaseous diffusion. As the temperature and particle diameter increase, the inhibition of diffusion becomes stronger. Among all the considered radicals, the abstraction ability of the H radical is the strongest, and it is easily restricted by diffusion. The presence of abundant radicals such as CH3, C2H3, and C3H5 at low temperatures can compensate for this limitation. For the unsaturated species addition reactions, the calculation results showed that the addition of olefins, alkynes, diolefins and aromatic hydrocarbons occurs in sequence with an increase in residence time. With increasing temperature, the addition contributions of olefins such as C2H4 and C3H6 gradually weaken, while those of small molecule aromatic hydrocarbons such as benzene (A1) and styrene (A1C2H3) remarkably increase. Alkynes and diolefins, such as C2H2 and C3H4, have low concentrations under the experimental conditions, and their addition contributions are very low and can be ignored. Due to the addition of unsaturated species, the concentration of H2 in the gas phase is significantly improved. The modeling work can predict the coke formation characteristics and quantitatively evaluate the change in the coke formation rate, which is helpful for explaining the intrinsic thermal pyrolysis coking mechanism in a round tube, such as the regenerative cooling system of supersonic engines, among others.

A review of mechanistic and mathematical modeling of n-heptane and cyclohexane pyrolysis

An extensive literature review of the mechanistic modeling of n-heptane and cyclohexane pyrolysis was carried out. It was shown that Rice–Kossiakoff free radical theory does not adequately account for product distributions of n-heptane pyrolysis in the high conversion regime. Secondary reactions of alpha higher olefins and di-olefins accounted for the major products (ethene, propene and 1-butene) of n-heptane pyrolysis. Predicted product distributions (CH4, C2H4, C3H6, 1-C4H8 and 1,3-C4H6) of n-heptane pyrolysis showed very good agreement with experimental data. The product distributions of cyclohexane pyrolysis in the high conversion regime were rationalized and adequately accounted for using decomposition reactions of cyclohexyl bi-radicals followed by secondary reactions of major primary products such as C3H6 and 1,3-C4H6. The latter expanded mechanism can be used to model cyclohexane pyrolysis in the high conversion regime. Rate parameters (pre-exponential factors and activation energy) for each of the elementary reactions of n-heptane mechanistic model were either obtained from the literature or estimated using thermochemical parameters. The use of steady state approximation in mathematical modeling of n-heptane pyrolysis led to erroneous results.

Modeling and Experimental Study on Pyrolysis of Isooctane and n-Heptane behind Reflected Shock Waves

Species concentration profiles produced from pyrolysis of n-heptane and isooctane diluted in argon were measured behind reflected shock waves at temperatures of 1000–1500 K and pressures of 1.2–2.6 atm. Simulation was performed using a chemical kinetic model based on KUCRS. The model reproduced the concentration profiles of n-heptane and isooctane surprisingly well over experimental conditions, while it did not cover some product species. The model was updated including additional species and reactions to reproduce concentration profiles of products.

The effect of temperature and pressure on n-heptane thermal cracking in regenerative cooling channel

A thermal cracking experimental equipment of hydrocarbon fuels was built to study n-heptane pyrolysis and the effect of reaction conditions on this reaction process. The main species were measured and the change rules were analyzed on the range of temperature 873–1073 K and pressure 0.1–3.5 MPa. The total content of alkenes products was more than alkanes on this pyrolysis process. Compared to alkenes with same number carbons, the alkanes were more easy to decompose with temperature but more conducive to formation with pressure increasing. The content of ethylene is usually the most on above reaction conditions, but its descent is also the fastest with pressure increasing. A mechanism model of n-heptane pyrolysis (44 species and 166 reactions) was constructed and validated by experiments on different conditions. Compared with n-heptane oxidation detailed model of Version 3.1 from Lawrence Livermore National Laboratory (LLNL), the pyrolysis model present a better accordant with experiment results on a range of temperature and pressure. The kinetic reaction of n-heptane pyrolysis was analyzed with present pyrolysis model, and the pyrolysis reaction pathway for the main products was obtained. The formation of alkenes are mainly through CC bond dissociation reaction, especiallyβ-C dissociation, and small alkanes are formed mainly by radical metathetical or synthesis reaction, the former are endothermic reactions, but the latter are mostly exothermal reactions. The properties of some main reactions have a critical role for the change of product content with temperature and pressure, which is the main reason for the variety of products selectivity under different conditions. Pressure increased the pyrolysis residence time and mass density but it does not significantly affect the reaction energy, so its contribution to conversion rate of fuels thermal cracking is limited, although it changes the reaction pathway greatly. However, the temperature can increase obviously the reaction activation energy, even though the residence time and concentration is decreased, the conversion rate of n-heptane pyrolysis still increased.

A 0D kinetic model for the microwave discharge in liquid n-heptane including carbonaceous particles production

A 0D model of time dependent processes, occurring in a gas bubble created by a microwave discharge in liquid n-heptane, is presented. The model includes both the description of gas-phase processes and formation of solid carbon-containing particles at atmospheric pressure. Two mechanisms of generation of solid particles are considered. The first one relates to the ‘linear PAH growth model’ and the second to ‘planar PAH growth model’. Both of them describe simultaneous processes of the initial n-heptane pyrolysis and nucleation, surface growth and coagulation of soot particles. The results of calculations are compared with known experimental results. It is concluded that the second model describes the results of the experiments better.

Reaction kinetic analysis of the effect of pressure on ethylene selectivity of n-heptane pyrolysis

The n-heptane thermal cracking was studied by using the chemical reaction model extracted from the original mechanism developed by Lawrence Livermore National Laboratory (LLNL). The calculated results were compared to the available experiments data and the good agreement was achieved under different temperature and pressure. The decrease of ethylene selectivity with increasing pressure was found and analyzed by theoretical model, which shows that the radical scission reactions of nC3H7 (1-propyl), pC4H9 (1-butyl) and C2H5 (ethyl) play a critical role in this thermal cracking process. Ethylene formation was also studied with the methods of rate of production (ROP) analysis and sensitivity analysis (SA) under different conditions, respectively. The scission reactions of alkane radicals are the main source of ethylene formation, and the C2H5 (ethyl), nC3H7 (1-propyl), pC4H9 (1-butyl) and C5H11-1 (1-pentyl) radicals are the important intermediates. The reaction pathway is changed obviously with pressure increase. Compared with atmosphere pressure, the effect of C2H5 scission reaction becomes inconspicuous but C6H13-1 (1-hexyl) is remarkable for ethylene formation under high pressure.

Pyrolysis of n-pentane, n-hexane and n-heptane in a single pulse shock tube

The pyrolysis of n-pentane, n-hexane and n-heptane have been studied behind reflected shock waves at pressures of 1.0–2.5 atm and at temperatures of 1000–1500 K. A single-pulse shock tube (SPST) was used to measure reactant, intermediate, and product species profiles using GC samplings at different reaction times varying from 1.5 to 2.2 ms. Simulations have been performed using two chemical kinetic models for the three fuels, namely NUI Galway and JetSurf (Version2.0). Differences in simulated results between the models are described. Sensitivity and reaction path analyses were performed to determine the important reactions controlling fuel pyrolysis and their influence on the predicted concentrations of reactant and product species profiles compared to those measured in the experiments.

Low temperature oxidation and pyrolysis of n-heptane in nanosecond-pulsed plasma discharges

The present study seeks to explore the parametric effects of oxygen concentration, argon dilution and plasma discharge frequency on pyrolytic and oxidative reaction pathways of n-heptane assisted by non-equilibrium plasma discharge. Low temperature reaction pathways of n-heptane/O2/Ar mixtures with a nanosecond repetitively pulsed plasma discharge are experimentally investigated in both in situ time-dependent TDLAS and steady state gas sampling diagnostics. Fuel consumption is found to be more effective in higher argon than higher oxygen concentrations, indicating higher electron number densities with argon dilution is more effective than direct electron impact dissociation of oxygen. Steady state sampling results suggest a linear trend of n-heptane dissociation and product species formation with increasing plasma frequency, with different major product species for oxidation and pyrolysis. In the time-dependent measurements, the comparison between experiments and numerical modeling show that formation of a major intermediate species, formaldehyde, is significantly under-predicted while fuel and water production are over-predicted. This discrepancy suggests missing reactions in the current model, possibly involving excited alkyl peroxide radicals.