Pyrolysis Of N-dodecane Research Articles

Insight into microscopic mechanism of Pt/Pd-catalyzed pyrolysis of n-dodecane

Catalytic cracking by noble metals is an effective strategy to enhance the cooling capacity of endothermic hydrocarbon fuels (EHFs), but microscopic reaction mechanisms of noble catalysts assisted fuel cracking remain to be elucidated. The reactive molecular dynamics simulation (ReaxFF MD) was employed in this work to investigate the catalytic mechanisms of Pt and Pd in the cracking of a surrogate EHF, i.e. n-dodecane. The results demonstrate that both Pt and Pd catalysts can significantly reduce the apparent activation energies for overall pyrolysis, while Pd demonstrating 72 % superior reduction performance due to the higher absorption with reactants. Specially, excessively high temperatures may lead to catalyst deactivation. With metal catalyst included, the gas-phase product yield and the proportion of hydrogen dominate production increases, which can be ascribed to additional dehydrogenation reactions during the initial cracking of n-dodecane by Pt and Pd, resulting in more H2 and C2H4 formation.

Experimental and kinetic study of high pressure pyrolysis of n-dodecane

In this paper, a novel two-stage heating system of hydrocarbon fuel was used to simulate the fuel temperature, pressure and fuel residence time in cooling channels of an actively cooling scramjet, wherein the high-pressure pyrolysis of n-dodecane, a surrogate fuel, was experimentally studied. The fuel cracking experiment was carried out in the temperature range of 830–930 K and at a relatively constant pressure of 40 atm. When the pyrolysis temperature was below 890 K, the main pyrolysis products were macromolecular olefin. With the increase of pyrolysis temperature, the cracking degree of n-dodecane and the proportion of small olefin and alkane increased, and the cracking degree reached 53.5% at the pyrolysis temperature of 930 K. The proportion of each gas product is almost constant in the whole temperature range except the lowest temperature of 830 K, while the proportion of each liquid product to the total liquid products shows the same phenomenon. A preliminary detailed chemical kinetics model of 145 substances and 2024 reactions was established with RMG. The reaction mechanism generated by RMG and several published n-dodecane cracking reaction mechanisms were used in CHEMKIN simulation, respectively. By comparing the results of the experiment and the kinetic model, the obvious differences were observed and reasons were analyzed. On this basis, the RMG kinetic model was further optimized, and achieved in good accordance with the experimental results. The decomposition pathways of n-dodecane were further illustrated through the analysis of reaction flux. The primary decomposition of n-dodecane is mainly through the H-abstraction reaction with small-molecule free radicals, especially CH3 and C2H5. The H-abstraction reaction results into six isomers of n-dodecyl. The consumption of n-dodecyl is mainly through H-shift isomerization and β-scissions, which mainly influence the distributions of pyrolysis products.

A reactive molecular dynamics study of the effects of an electric field on n-dodecane combustion

A reactive Molecular Dynamics (MD) study of n-dodecane combustion at high temperatures under externally applied electrostatic fields is performed to investigate their effect on chemical kinetics. A local charge equilibration method is used to enable charge transfers up to the overlap of the atomic orbitals and introduce molecular polarization induced by an electric field. The atomic charges of an isolated n-dodecane molecule with and without external electrostatic fields are first compared with Density Functional Theory (DFT) computations, to assess the accuracy of the charge equilibration method and its ability to capture polarization. Then, the impact of external electrostatic fields on the reaction kinetics of fuel, oxidizer and products is studied for a range of ambient temperatures and densities. The activation energy and pre-exponential factor of Arrhenius-type reactions under various electrostatic fields are also investigated by performing Nudged Elastic Band (NEB) computations on selected reactions’ Minimum Energy Path (MEP) and by analysing the collision frequency, respectively. Results show that the atomic charge transfers due to close interactions and molecular polarisation are relatively weak in all investigated conditions, leading to the necessity of strong external electric fields to induce changes to chemical kinetics. The consumption rate of n-dodecane decreases for strong electrostatic fields, whereas for low values of the electrostatic field strength no clear trend is observed. In addition, at high temperature and density conditions, oxygen consumption increases under strong electrostatic fields, whereas the opposite trend is observed as the temperature and density decrease. NEB analysis shows alterations of the activation energy up to 2.3 kcal/mol for oxygen compound reactions with varying strength of the external electrostatic field. Furthermore, analysis of the translational, rotational and vibrational kinetic energy modes and collision frequency reveals the influence of translational motion and molecular stabilization on the reaction rates. The kinetics of oxygen molecules was found to be of primary importance to determine the reaction behaviour under external electrostatic fields, as oxygen molecules have a direct effect on the oxidation reactions and also indirectly affect n-dodecane pyrolysis when an electrostatic field is present. This study provides fundamental understanding of the interactions between heavy hydrocarbons and electrostatic fields for the development of future hybrid thermal-electrical technologies.

Investigating the Effects of Chemical Mechanism on Soot Formation Under High-Pressure Fuel Pyrolysis

We performed Computational Fluid Dynamics (CFD) simulations using a Reynolds-Averaged Navier-Stokes (RANS) turbulence model of high-pressure spray pyrolysis with a detailed chemical kinetic mechanism encompassing pyrolysis of n-dodecane and formation of polycyclic aromatic hydrocarbons. We compare the results using the detailed mechanism and those found using several different reduced chemical mechanisms to experiments carried out in an optically accessible, high-pressure, constant-volume combustion chamber. Three different soot models implemented in the CONVERGE CFD software are used: an empirical soot model, a method of moments, and a discrete sectional method. There is a large variation in the prediction of the soot between different combinations of chemical mechanisms and soot model. Furthermore, the amount of soot produced from all models is substantially less than experimental measurements. All of this indicates that there is still substantial work that needs to be done to arrive at simulations that can be relied on to accurately predict soot formation.

Kinetic study of plasma-assisted n-dodecane/O2/N2 pyrolysis and oxidation in a nanosecond-pulsed discharge

The present study investigates the kinetics of low-temperature pyrolysis and oxidation of n-dodecane/O2/N2 mixtures in a repetitively-pulsed nanosecond discharge experimentally and numerically. Time-resolved TDLAS measurements, steady-state gas chromatography (GC) sampling, and mid-IR dual-modulation Faraday rotation spectroscopy (DM-FRS) measurements are conducted to quantify temperature as well as species formation and evolution. A plasma-assisted n-dodecane pyrolysis and oxidation kinetic model incorporating the reactions involving electronically excited species and NOx chemistry is developed and validated. The results show that a nanosecond discharge can dramatically accelerate n-dodecane pyrolysis and oxidation at low temperatures. The numerical model has a good agreement with experimental data for the major intermediate species. From the pathway analysis, electronically excited N2* plays an important role in n-dodecane pyrolysis and oxidation. The results also show that with addition of n-dodecane, NO concentration is reduced considerably, which suggests that there is a strong NO kinetic effect on plasma-assisted low-temperature combustion via NO-RO2 and NO2-fuel radical reaction pathways. This work advances the understandings of the kinetics of plasma-assisted low-temperature fuel oxidation in N2/O2 mixtures.

Multiply accelerated ReaxFF molecular dynamics: coupling parallel replica dynamics with collective variable hyper dynamics

ABSTRACTTo tackle the time scales required to study complex chemical reactions, methods performing accelerated molecular dynamics are necessary even with the recent advancement in high-performance computing. A number of different acceleration techniques are available. Here we explore potential synergies between two popular acceleration methods – Parallel Replica Dynamics (PRD) and Collective Variable Hyperdynamics (CVHD), by analysing the speedup obtained for the pyrolysis of n-dodecane. We observe that PRD + CVHD provides additional speedup to CVHD by reaching the required time scales for the reaction at an earlier wall-clock time. Although some speedup is obtained with the additional replicas, we found that the effectiveness of the inclusion of PRD is depreciated for systems where there is a dramatic increase in reaction rates induced by CVHD. Similar observations were made in the simulation of ethylene-carbonate/Li system, which is inherently more reactive than pyrolysis, indicate that the speedup obtained via the combination of the two acceleration methods can be generalised to most practical chemical systems.

Investigation of n-dodecane pyrolysis at various pressures and the development of a comprehensive combustion model

n-Dodecane combustion was investigated experimentally and numerically in present study. Pyrolysis experiments of n-dodecane at pressures of 0.0066, 0.039, 0.197 and 1 atm, temperatures from 750 to 1430 K were studied in a flow reactor. Mole fractions of n-dodecane, argon and pyrolysis products (including active radicals) were evaluated. A kinetic model of n-dodecane was developed by validating both present and literature reported experiments. The rate of production analysis reveals H-abstraction and CC bond fission reactions are main consumption pathways of n-dodecane. The β-CC scission reactions of alkyls contribute to the formation of alkenes, which are mainly consumed via the allylic CC fission reactions. As a soot precursor, benzene is largely produced from the recombination of C3 species. Moreover, effects of carbon chain length on flow reactor pyrolysis were investigated for n-decane, n-dodecane and n-tetradecane. The decay of n-tetradecane is the fastest, followed by n-dodecane and n-decane, indicating that the pyrolysis reactivity of n-alkanes increases as the carbon chain length increases from C10 to C14n-alkanes. Ignition delay times and laminar burning velocities (LBVs) of n-alkanes under similar conditions were also compared, the result shows that effects of the carbon chain length on ignition delay times and LBVs are slight.

Measuring the soot onset temperature in high-pressure n-dodecane spray pyrolysis

Soot formation in pyrolyzing sprays of n-dodecane is visualized and quantified in a high-pressure, high-temperature, constant-volume spray chamber at 38 bar, 76 bar, and 114 bar. Sprays of n-dodecane are injected at 500 bar from a single-hole, 186-µm orifice diameter fuel injector. We quantify the temporal evolution of the soot optical thickness and the total soot mass formed in the pyrolyzing sprays using a high-speed extinction imaging diagnostic. The vessel ambient temperature and pressure are varied independently to identify the soot onset temperature for n-dodecane pyrolysis. Linear extrapolation of the maximum soot formation rates as a function of ambient temperature reveals a soot onset temperature near 1450 K. The onset temperature determined here for n-dodecane is within 50 K of those previously measured along the centerline of atmospheric pressure coflow diffusion flames for smaller alkane fuels.

Fuel pyrolysis in a microflow tube reactor–Measurement and modeling uncertainties of ethane, n-butane, and n-dodecane pyrolysis

A scaled down flow reactor consisting of 4 mm ID quartz tubing and rapid mixing of fuel with a preheated thermal carrier bath was developed to investigate the pyrolysis of both gaseous and pre-vaporized liquid fuels. Starting from a small mixing volume (less than 0.2 cm3), the temperature of the hot section (37 cm long) was controlled within ± 5 K. All species concentrations were measured at the exit plane of the reactor using a GC system while residence time variations were explored by varying the bulk flow velocity. For the atmospheric pressure cases reported here, the temperature and flow residence times explored were in the kinetically controlled regime and ranged from 1000 to 1100 K and 10–90 ms, respectively. The thermal pyrolysis of fuels investigated included ethane, n-butane, and n-dodecane, all diluted in a nitrogen carrier bath of 98% or higher (to minimize temperature departure from the target value). Because the ratio of the mixing volume compared to the kinetically controlled reactor volume is about 2.5%, the associated finite mixing time is shown to have a negligible effect on the temporal evolution of key C0–C4 species. As a consequence, no species profiles shifting (zero-time shifting) was required in comparisons with model predictions. Experimental and modeling uncertainty analysis are presented to determine whether the experimental data can be used in future efforts aimed at minimization of chemical kinetic model parameter uncertainties.

An experimental and kinetic modeling study of n-dodecane pyrolysis and oxidation

The current study investigates n-dodecane (n-C12H26) pyrolysis and oxidation kinetics in the temperature regime of 1000–1300 K in the Stanford Variable Pressure Flow Reactor facility. The reactor environment is vitiated and the experiments were conducted at atmospheric pressure. Species time history data were collected for n-dodecane and oxygen, as well as for 12 intermediate and product species over a span of 1–40 ms residence times using real time gas chromatography. The experimental data were compared against the predictions of four detailed kinetic models. The results showed that the fuel oxidation proceeds through an early pyrolytic stage where the fuel breaks down into smaller hydrocarbon fragments, including mostly C2–4 alkenes, and a late oxidation stage where the fragments oxidize to CO. The kinetic models were observed to diverge notably in their predictions from one another. Sensitivity and flux analysis identified the cause of the divergences to differences in the small hydrocarbon chemistry modeling. Finally, the flow reactor data were used to demonstrate how model uncertainty minimization can improve model predictions. It is shown that after uncertainty minimization against a selected set of n-dodecane combustion data, the predictions of the resulting optimized model are improved notably for all existing n-dodecane data sets tested, including those of the current flow reactor study that were not part of the optimization target list.

Fuel and Ethylene Measurements during n-dodecane, methylcyclohexane, and iso-cetane pyrolysis in shock tubes

Fuel and ethylene time histories, overall fuel decomposition rates, and ethylene yields were measured during the high-temperature (990–1520K) pyrolysis of n-dodecane, methylcyclohexane, and 2,2,4,4,6,8,8-heptamethylnonane in a heated high-pressure shock tube. Fuel measurements were made using IR HeNe laser absorption at 3.39μm and ethylene measurements were made using CO2 gas laser absorption near 10.5μm. Initial fuel mole fractions ranged from 0.1% to 0.4% fuel in argon and pressures from 17 to 23atm. Data provided here will enable the formulation of a fuel surrogate with pyrolytic kinetic properties (product species and overall fuel decomposition rates) similar to RP-fuels.

Experimental and modeling study on the pyrolysis and oxidation of n-decane and n-dodecane

High pressure n-decane and n-dodecane shock tube experiments were conducted to assist in the development of a Jet A surrogate kinetic model. Jet A is a kerosene based jet fuel composed of hundreds of hydrocarbons consisting of paraffins, olefins, aromatics and naphthenes. In the formulation of the surrogate mixture, n-decane or n-dodecane represent the normal paraffin class of hydrocarbons present in aviation fuels like Jet A. The experimental work on both n-alkanes was performed in a heated high pressure single pulse shock tube. The mole fractions of the stable species were determined using gas chromatography and mass spectroscopy. Experimental data on both n-decane and n-dodecane oxidation and pyrolysis were obtained for temperatures from 867 to 1739K, pressures from 19 to 74atm, reaction times from 1.15 to 3.47ms, and equivalence ratios from 0.46 to 2.05, and ∞. Both n-decane and n-dodecane oxidation showed that the fuel decays through thermally driven oxygen free decomposition at the conditions studied. This observation prompted an experimental and modeling study of n-decane and n-dodecane pyrolysis using a recently submitted revised n-decane/iso-octane/toluene surrogate model. The surrogate model was extended to n-dodecane in order to facilitate the study of the species and the 1-olefin species quantified during the pyrolysis of n-dodecane and n-decane were revised with additional reactions and reaction rate constants modified with rate constants taken from literature. When compared against a recently published generalized n-alkane model and the original and revised surrogate models, the revised (based on our experimental work) and extended surrogate model showed improvements in predicting 1-olefin species profiles from pyrolytic and oxidative n-decane and n-dodecane experiments. The revised and extended model when compared to the published generalized n-alkane and surrogate models also showed improvements in predicting species profiles from flow reactor n-decane oxidation experiments, but similarly predicted n-decane and n-dodecane ignition delay times.

Fuel pyrolysis through porous media: Coke formation and coupled effect on permeability

The development of hypersonic vehicles (up to Mach 10) leads to an important heating of the whole structure. The fuel is thus used as a coolant. It presents an endothermic decomposition with possible coke formation. Its additional permeation through the porous structure involves internal convection. This implies very complex phenomena (heat and mass transfers with chemistry). In this paper, the n-dodecane pyrolysis is studied through stainless steel porous medium up to 820K and 35bar (supercritical state). The longitudinal profiles of chemical compositions inside the porous medium are given thanks to a specific sampling technique with off-line Gas Chromatograph and Mass Spectrometer analysis. By comparison with previous experiments under plug flow reactor, the conversion of dodecane is higher for the present experimental configuration. The pyrolysis produces preferentially light gaseous species, which results in a higher gasification rate for a similar pyrolysis rate. The effects of the residence time and of the contact surface area are demonstrated. The transient changes of Darcy's permeability are related to the coke formation thanks to previous experimental relationship with methane production. A time shift is observed between coke chemistry and permeability change. This work is quite unique to the author's knowledge because of the complex chemistry of heavy hydrocarbon fuels pyrolysis, particularly in porous medium.

Supercritical thermal decompositions of normal- and iso-dodecane in tubular reactor

Thermal decompositions of a serials of normal- and iso-dodecane mixtures with different iso/ normal ratios were performed in a tubular reactor under supercritical conditions (550–680 °C, 4.0 MPa). It was found that iso-dodecane has an accelerating effect on the pyrolysis of n-dodecane depending upon iso/ normal ratio of the mixture, and that accelerating factor reaches up to almost 4 at iso/ normal ratio of 3/1. Decomposition conversions of the mixtures show a strong dependence on the iso/ normal ratio and the temperature owing to their possible effect on the radical generation mechanism and the decomposition kinetics. In addition, iso/ normal ratio of the mixture also affects the formation rates and morphologies of cokes due to the variation of decomposition conversion.

Experimental and modeling study of the thermal decomposition of methyl decanoate

The experimental study of the thermal decomposition of methyl decanoate was performed in a jet-stirred reactor at temperatures ranging from 773 to 1123 K, at residence times between 1 and 4 s, at a pressure of 800 Torr (106.6 kPa) and at high dilution in helium (fuel inlet mole fraction of 0.0218). Species leaving the reactor were analyzed by gas chromatography. Main reaction products were hydrogen, carbon oxides, small hydrocarbons from C 1 to C 3, large 1-olefins from 1-butene to 1-nonene, and unsaturated esters with one double bond at the end of the alkyl chain from methyl-2-propenoate to methyl-8-nonenoate. At the highest temperatures, the formation of polyunsaturated species was observed: 1,3-butadiene, 1,3-cyclopentadiene, benzene, toluene, indene, and naphthalene. These results were compared with previous ones about the pyrolysis of n-dodecane, an n-alkane of similar size. The reactivity of both molecules was found to be very close. The alkane produces more olefins while the ester yields unsaturated oxygenated compounds. A detailed kinetic model for the thermal decomposition of methyl decanoate has been generated using the version of software EXGAS which was updated to take into account the specific chemistry involved in the oxidation of methyl esters. This model contains 324 species and 3231 reactions. It provided a very good prediction of the experimental data obtained in jet-stirred reactor. The formation of the major products was analyzed. The kinetic analysis showed that the retro-ene reactions of intermediate unsaturated methyl esters are of importance in low reactivity systems.

Reactive molecular dynamics simulation and chemical kinetic modeling of pyrolysis and combustion of n-dodecane

The initiation mechanisms and kinetics of pyrolysis and combustion of n-dodecane are investigated by using the reactive molecular dynamics (ReaxFF MD) simulation and chemical kinetic modeling. From ReaxFF MD simulations, we find the initiation mechanisms of pyrolysis of n-dodecane are mainly through two pathways, (1) the cleavage of C–C bond to form smaller hydrocarbon radicals, and (2) the dehydrogenation reaction to form an H radical and the corresponding n-C 12H 25 radical. Another pathway is the H-abstraction reactions by small radicals including H, CH 3, and C 2H 5, which are the products after the initiation reaction of n-dodecane pyrolysis. ReaxFF MD simulations lead to reasonable Arrhenius parameters compared with experimental results based on first-order kinetic analysis of n-dodecane pyrolysis. The density/pressure effects on the pyrolysis of n-dodecane are also analyzed. By appropriate mapping of the length and time from macroscopic kinetic modeling to ReaxFF MD, a simple comparison of the conversion of n-dodecane from ReaxFF MD simulations and that from kinetic modeling is performed. In addition, the oxidation of n-dodecane is studied by ReaxFF MD simulations. We find that formaldehyde molecule is an important intermediate in the oxidation of n-dodecane, which has been confirmed by kinetic modeling, and ReaxFF leads to reasonable reaction pathways for the oxidation of n-dodecane. These results indicate that ReaxFF MD simulations can give an atomistic description of the initiation mechanism and product distributions of pyrolysis and combustion for hydrocarbon fuels, and can be further used to provide molecular based robust kinetic reaction mechanism for chemical kinetic modeling of hydrocarbon fuels.

Experimental and modelling investigation of the thermal decomposition of n-dodecane

An important challenge in developing the Scramjet engine is cooling of the walls. One possibility is circulating the fuel itself through the walls of the engine, with cooling achieved through the strongly endothermic decomposition of the fuel. This paper presents an experimental and modelling study of the pyrolysis of n-dodecane, a component of some jet fuels. The experiments have been performed in a stainless steel isothermal plug flow reactor at temperatures of 950, 1000 and 1050 K and atmospheric pressure, with GC analysis of the end products, mainly methane, ethane and alkenes from C 2 to C 10. A detailed kinetic model containing 1175 reactions has been produced using EXGAS software. This model is able to represent with reasonable accuracy the experimental results presented here, both for the conversion of n-dodecane and for the formation of products, as well as literature data obtained from 623 to 823 K.