An updated experimental and kinetic modeling study of n-pentane pyrolysis at atmospheric pressure in a flow reactor
Introduction
From the generation of petrochemical industry chemicals to the application of novel technologies, fuel pyrolysis plays an important role in almost every fuel process, and it has a significant function in all combustion devices [1], [2], [3]. The study of hydrocarbon fuel pyrolysis offers concrete verification for the pyrolysis reaction in combustion kinetic models and further deepens the understanding of the chemistry of key polycyclic aromatic hydrocarbons (PAHs) in soot formation mechanisms under pyrolysis conditions [4], [5], [6]. Therefore, insight into the pyrolysis reaction pathway of hydrocarbon fuel to molecular weight growth is conducive for reducing particulate emissions of engines. n-Pentane, as a representative fuel with gasoline components, is the smallest linear alkane consisting of a liquid phase under atmospheric pressure and ambient temperature [7]. In the past few years, with many kinetic simulations and theoretical and experimental investigations, there has been a novel interest in the understanding of n-pentane pyrolysis [1,4,[8], [9], [10], [11]. These investigations not only emphasize the concrete aspects that support our entire understanding of pyrolysis but also use a multifaceted approach, combining the distinct factors, linking individual efforts, and offering more insight into complex underlying processes. Nevertheless, data on n-pentane pyrolysis are relatively limited.
A large number of prior kinetic models and experimental investigations in the literature focused on the combustion and oxidation of dual fuel and/or n-pentane mixtures. Experimental investigations have centered on the distinct performances of fuel oxidation under various conditions, such as the delay time of ignition in rapid compression machines [12], species profiles to temperature and/or the time detected in jet-stirred reactors (JSRs) [13], [14], [15] and flow reactors [16], as well as laminar flame velocities [17,18]. In addition, the ignition time and reduced oxidation kinetic model of n-pentane and its mixtures with isooctane were performed by Harstad et al. [19] and Aworinde et al. [20]. However, the existing literature mainly focuses on the investigation of the combustion and oxidation of n-pentane. The experimental data for n-pentane pyrolysis at atmospheric pressure are limited, particularly considering the pathway of the pressure-dependent reaction and the intermediate product temperature evolution. In addition, although previous kinetic models can well predict the consumption of n-pentane, there is still large uncertainty in predicting the type of intermediate products.
The reaction mechanism of hydrocarbon fuel for combustion applications consists of hundreds of species and thousands of reactions. A dependable database of the experimental determinations over pressure and temperature ranges is the key to verifying these mechanisms. Compared to oxidation experiments of hydrocarbon fuel, pyrolysis experiments of hydrocarbon fuel give rise to relatively small temperature and pressure variations, and thus, the experimental data can be fully modeled with the reactor. Furthermore, pyrolysis experiments are extensively utilized to understand the mechanism of molecular growth and thermal decomposition in models of combustion because isolation from oxidants offers a relatively simple and specific reaction system [21]. In addition, in the process of fuel pyrolysis, the problem of spectral interference from other species is less than that in fuel oxidation [9]. For n-pentane pyrolysis, most of the previous experimental studies on product species concentrations, such as methane, acetylene, ethylene, and propylene, were performed using gas chromatography (GC) [1,4,22,23], Raman spectroscopy [8,24], and shock tube/laser absorption techniques [9,25]. Nevertheless, the GC approach utilized in these works could offer information only on stable products, while no intermediate species profiles were reported to validate and improve the kinetic model of n-pentane pyrolysis. As a result, the concentration profiles of intermediate species in n-pentane pyrolysis are very limited, and the pyrolysis chemistry of n-pentane is insufficiently understood.
Synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) has been confirmed to be an important method to study combustion chemical kinetics, such as pyrolysis in flow reactors [26], [27], [28], [29], oxidation in JSRs [7,30], and laminar premixed flames [31,32]. In addition to the traditional GC method, the new diagnostic technique using SVUV-PIMS should be applied to distinguish unstable species (for instance, radicals) in new reaction pathways, which are utilized in the kinetic model to better understand the process of n-pentane pyrolysis. Nevertheless, information about the detailed pyrolysis reaction and the mole fractions of intermediate products in the n-pentane pyrolysis process is lacking. An updated database of the n-pentane pyrolysis experiment needs to cover a wider range of conditions than the existing database to validate the kinetic model based on published research [1,4,9,11].
Motivated by the above discussion, the present work aims to close this gap and improve the detailed understanding of the pyrolysis reaction behavior of n-pentane by extending the applicable range to low and intermediate temperatures (450–750 °C) at atmospheric pressure (760 Torr). n-Pentane pyrolysis in a flow reactor was explored, and the pyrolysis species were detected. The related species concentration profiles were also evaluated using SVUV-PIMS. Starting from a previously validated mechanism [4], a reduced kinetic model of n-pentane pyrolysis was developed and validated by models in the literature for ignition delay times and species profiles measured in JSRs in this study. Moreover, the simulation results of the reactant and product species profiles from the current model and the literature model were contrasted and compared to the experimental data. The rate of production (ROP) and sensitivity analyses were also conducted to understand n-pentane decomposition and the formation of major species. In addition, the pyrolysis processes for hydrogen atom abstraction and hydrogen atom addition were described, offering guidance for the reasonable design and in-depth latent application of n-pentane thermal conversion systems.
Section snippets
Experimental method
n-Pentane pyrolysis experiments in a flow reactor were carried out in the Hefei National Synchrotron Radiation Laboratory, China. A detailed description of the vacuum ultraviolet beamline of the synchrotron [26,33] and the apparatus of the flow reactor [5,27,34] has been previously reported in detail. Fig. S1 in Supplemental Material (SM) shows the schematic diagram of the flow reactor pyrolysis apparatus. Briefly, the present pyrolysis apparatus consists of a pyrolysis chamber with an
Kinetic modeling
The n-pentane pyrolysis model described in recent literature was used as the starting point to simulate the new experimental data in a flow reactor [4]. In the CHEMKIN-Pro-software, the detected temperature distribution was utilized as the input parameter for the plug flow reactor module [40]. The plug flow model assumes that the axial diffusion is negligible and that there is no radial gradient [1]. The thermochemical data for most of the species were obtained from the thermodynamic database
Results and discussion
In this work, new experimental data and kinetic modeling results of n-pentane pyrolysis in a flow reactor are discussed. Dozens of the pyrolysis species, including pentane and stable products such as alkane (CH4, C2H6), alkene (C2H4, C3H6, etc.), alkyne (C2H2, C3H4), and some free radicals, including methyl (CH3), propyl (C3H7), butyl (C4H9) and pentyl (C5H11) radicals, were identified and quantified in this study. A detailed description of the simulation methods was reported in the literature
Conclusions
In this work, to understand the chemistry of n-pentane pyrolysis, SVUV-PIMS was used to study n-pentane pyrolysis in a flow reactor at 760 Torr. A kinetic model of n-pentane pyrolysis was developed and validated against the new experimental data. In addition, the decomposition reaction of n-pentane and the influence of related radical reactions on the concentration profiles of the measured alkene products were also investigated. The major conclusions of this present work are summarized as
Declaration of Competing Interest
The authors declare no conflict of interest.
Acknowledgments
This work is sponsored by “open fund of National Synchrotron Radiation Laboratory (NSRL) in Hefei (2018-HLS-PT-001746,2018-HLS-PT-001757)”, “State Key Laboratory of Engines at Tianjin University (K2018-09)”, and “the Key Laboratory of Marine Power Engineering & Technology, Ministry of Transport (2020Ⅲ021GX)”. The authors also appreciate the technical assistance from Dr. Jiuzhong Yang, Dr. Chuangchuang Cao, Dr. Jiabiao Zou, Mr. Qiang Xu and Mr. Huaijiang Su at the National Synchrotron Radiation
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