Cyclohexane Pyrolysis Research Articles

Formal kinetics of catalytic oxidation of diesel paraffin-naphthenic fraction in the presence of carbon nanotubes containing Co, Mn and Fe

This study identified the formal kinetics of aerobic oxidation of the paraffin-naphthenic fraction of diesel fuel in the presence of multi-walled carbon nanotubes (MWCNTs) containing three transition metals—Fe, Co, and Mn. The synthesis of metal-containing multi-walled carbon nanotubes (M@MWCNTs) was carried out by pyrolysis of cyclohexane in the presence of ferrocene (CVD synthesis), followed by sequential doping of the resulting iron-containing carbon nanotubes Fe@MWCNTs with Co and Mn clusters. The resulting Fe–Co–Mn@MWCNT samples were identified using electron microscopy (SEM, TEM) and elemental analysis. The resulting carbon nanotubes were found to have a diameter of 30–60 nm and length of 500–800 μm. Iron clusters were observed to be present within the channels, while Co and Mn particles were observed to be present in the outer space of the nanotubes. The formal kinetic regularities of the dearomatized diesel fraction aerobic oxidation promoted by M@MWCNT additives were studied. It has been demonstrated that metal-nanocarbon additives effectively catalyze the oxidation process in the absence of hydroperoxides, resulting in thermal decomposition. The introduction of Co and Mn clusters into the nanotube structure has been observed to enhance catalytic activity. A catalytic oxidation scheme is currently under consideration, with corresponding effective oxidation rates being determined. It is postulated that these findings may prove useful in the estimation of effective nano-catalytic systems for the oxidative conversion of multicomponent hydrocarbon feedstocks.

Understanding the impact of molecular structure on the formation of stable intermediates during the pyrolysis of monoalkylated cyclohexanes in a shock tube

Alkylcyclohexanes are common constituents of conventional jet fuels and are expected to be a vital molecular subclass in next-generation, sustainable aviation fuels (SAFs). In an effort to advance the current understanding of the combustion chemistry of these species at engine-relevant conditions, we measured the time-resolved evolution of key stable intermediates during the pyrolysis of cyclohexane (CH) and four monoalkylated cyclohexanes: methylcyclohexane (MCH), ethylcyclohexane (ECH), propylcyclohexane (PCH), and butylcyclohexane (BCH), using laser absorption spectroscopy (LAS) in a shock tube. These experiments were conducted using 2 % Fuel/Argon test mixtures at a nominal pressure of 2 atm over the temperature range 1150–1530 K. Simultaneous tracking of the mole fraction time histories of methane, ethylene, and larger alkenes (>C2) provided new, valuable insight into the high-temperature reactivity of the five fuels studied. Studying all five fuels under similar test conditions enabled us to observe clear trends in the yields of different pyrolysis products with varying molecular structure, specifically increasing the number of carbon atoms in the alkyl chain. The fuel structure effects investigated in this work are instrumental in characterizing the pyrolysis product distribution, and thereby the overall combustion behavior of cyclohexane derivatives with longer (>C4) alkyl chains. We believe the multi-wavelength speciation data presented in this work can significantly contribute towards the development of robust, simplified, and fuel-specific kinetic models for monoalkylated cyclohexanes.

Pyrolysis of Cyclohexane and 1-Hexene at High Temperatures and Pressures—A Photoionization Mass Spectrometry Study

Pyrolysis of Cyclohexane and 1-Hexene at High Temperatures and Pressures—A Photoionization Mass Spectrometry Study

Experimental investigation on the low-pressure pyrolysis of cyclohexane initiated by 1-nitropropane in a flow tube reactor

An experimental study of 5% 1-nitropropane (1-NP)-initiated cyclohexane pyrolysis was carried out in a flow reactor at 30 Torr (4.0 kPa) over a temperature range of 673–1273 K. The pyrolysis species were detected with synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) technique. Pyrolysis species from m/z 15–106, including C1-C8 hydrocarbons, CO, NO, NO2, CH2O, CH3OH and C3H5NO, were identified with near-threshold photoionization efficiency (PIE) spectra and quantified at different temperatures. The experimental results indicated that the addition of 1-NP obviously reduced the initial pyrolysis temperature of cyclohexane from 1048 to 1098–723 K, and caused the decreases of the initial formation temperatures for the species from cyclohexane pyrolysis. Almost all the high-temperature pyrolysis species in the mechanisms proposed in the literature were identified in the experiments. Based on the experimental results in this work and rate coefficients proposed in the literature, the primary pyrolysis mechanisms of cyclohexane in the presence of 1-NP were discussed. The 1-NP pyrolysis occurs at a lower temperature than the cyclohexane does, and produces HONO, NO2, C3H7, etc. It was demonstrated that cyclohexane was mainly consumed via the H-atom abstractions by H and OH attacks in this work and the cyclohexyl radical formed via H-atom abstractions mainly isomerized into 1-C6H11-6 radical, which further combines with H to form 1-C6H12.

Pyrolysis and coking behavior of CxHy with different structures in microchannel continuous flow reactor

Coking with carbon deposition in hydrocarbon fuel (CxHy) pyrolysis at high temperature is a serious problem to be considered. In this work, single component CxHy with different structures was used as the probe of the carbon deposition. Thirteen simple and representative of alkanes, cycloalkanes, aromatics and O-containing CxHy were selected as the model compounds to decompose and coking in microchannel continuous flow reactor under the same conditions. The distributions of gas, liquid and solid phase products formed by CxHy pyrolysis were characterized by GC, GC-MS, SEM and TPO-IR respectively. The results demonstrate that the molecular structure of CxHy has a great influence on pyrolysis and coking behavior. For all alkanes, CH4, C2H4, C3H6 and C4H8 are the most abundant in gaseous products, and their total content exceeds 70 %. For cycloalkanes, monocyclic cyclohexane pyrolysis tends to produce C2H4, C4H8 and 1,3-C4H6, while bicyclic decahydronaphthalene pyrolysis tends to produce CH4, C2H4 and C3H6. For aromatics, CH4 and C2H4 are the main products in the pyrolysis gas phase of benzene, toluene and ethylbenzene. Interestingly, in the liquid phase of the pyrolysis of O-containing CxHy (ethanol, acetone and tetrahydrofuran), most of the products are oxygenated compounds and no aromatics. SEM and TPO-IR revealed that at least three carbon species were formed by cracking of CxHy molecules with different structures, including amorphous, filamentous and granular carbon. However, due to the great and complex influence of CxHy molecular structure on the micro morphology of carbon deposition, it is difficult to classify all carbon species at present, which needs further experiments and researches.

A comparative single-pulse shock tube experiment and kinetic modeling study on pyrolysis of cyclohexane, methylcyclohexane and ethylcyclohexane

The pyrolysis of cyclohexane, methylcyclohexane, and ethylcyclohexane have been studied behind reflected shock waves at pressures of 5 and10 bar and at temperatures of 930–1550 K for 0.05% fuel diluted by Argon. A single-pulse shock tube (SPST) is used to perform the pyrolysis experiments at reaction times varying from 1.65 to 1.74 ms. Major products are obtained and quantified using gas chromatography analysis. A flame ionization detector and a thermal conductivity detector are used for species identification and quantification. Kinetic modeling has been performed using several detailed and lumped chemical kinetic mechanisms. Differences in modeling results among the kinetic models are described. Reaction path analysis and sensitivity analysis are performed to determine the important reactions controlling fuel pyrolysis and their influence on the predicted concentrations of reactant and product species profiles. The present work provides new fundamental knowledge in understating pyrolysis characteristics of cyclohexane compounds and additional data set for detailed kinetic mechanism development.

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.

Role of normal/cyclo-alkane in hydrocarbons pyrolysis process and product distribution

In order to reveal the effects of normal/cyclo-alkane on the pyrolysis process, n-hexane, n-heptane, cyclohexane, methylcyclohexane and n-hexane/cyclohexane mixtures were employed as model reactants, and a particular attention was paid to the measurement of the product distribution at 650−830 °C under atmosphere. Compared with the increase of chain length of normal alkane and the presence of methyl substituent on cycloalkane, the distinction between normal and cyclic alkane led to a significant difference on the pyrolysis process. Due to the normal structure, n-hexane directly generated alkyl radicals via the homolysis of CC bond; due to the cyclic structure, cyclohexane generated alkenyl and alkyl radicals via the successive reactions of ring-opening, isomerization and decomposition. As a result, cyclohexane pyrolysis obtained a lower conversion and selectivity to light alkanes and alkenes, while a higher selectivity to 1,3-butadiene, cyclohexene and aromatics compared to n-hexane pyrolysis. Furthermore, a synergistic effect between n-hexane and cyclohexane was observed in the mixture pyrolysis. The synergistic effect mainly depended on the hydrogen abstraction and was modulated as a function of reaction temperature and feedstock composition. In the mixture pyrolysis: (1) n-Hexane preferentially generated radicals and triggered the hydrogen abstraction, which promoted cyclohexane decomposition while was weakened by increasing cyclohexane content. (2) Cyclohexane competed for radicals with n-hexane in the hydrogen abstraction, which inhibited n-hexane decomposition and was enhanced by increasing cyclohexane content. (3) An increase of reaction temperature weakened both the inhibition effect of cyclohexane on n-hexane decomposition and the acceleration effect of n-hexane on cyclohexane decomposition, and it was attributed to the reduction of hydrogen abstraction at high temperature.

Carbon nanotubes catalysis in liquid-phase aerobic oxidation of hydrocarbons: Influence of nanotube impurities

Pure carbon nanotubes (CNTs) have a high electron affinity and as such are able to actively absorb free radicals. This functional feature of CNTs leads to a linear chain breakage with the formation of inert spin adducts and effective inhibition of the oxidation process. However, there is a clear contradiction in this issue in the literature with the analysis of research results indicating that CNTs exhibit antioxidant activity mainly in polymeric materials, under conditions of diffusional restrictions for oxygen access. While, in the liquid-phase oxidation of hydrocarbons by CNTs (CNTs synthesised by the thermal catalytic pyrolysis of carbon-containing raw materials (CVD-process)), the CNTs enhance catalytic processes. In this study the aerobic liquid phase oxidation of cumene and initiated (by azobisisobutyronitrile) at low-temperature (333 K) in the presence of multi-walled carbon nanotubes (MWCNTs) obtained by thermal catalytic pyrolysis of cyclohexane (catalyst-ferrocene) has been undertaken. Kinetic analysis establishes that the catalysis of the oxidation process is associated with the presence of metal compounds in the structure of the MWCNTs. These metals are residues of metal catalysts remaining in the synthesis and in the process of pyrolysis. The metals are converted, as a rule, into metal carbides and are not easily removable by treatment with mineral acids. Thus, in the presence of metals in the composition of MWCNTs, interfering parallel reactions are observed with two processes running in parallel — MWCNTs + R• (RO2 •) → •MWCNTs-R (RO2) and ROOH + M @ MWCNTs → RO• radicals (RO2•). The branching of the chain processes involving hydroperoxides suppresses the route of attachment of alkyl and peroxide radicals to the carbon cage structure of the nanotubes and the reaction proceeds in an autocatalytic mode. Contradictory conclusions regarding the effect of CNTs on the chain processes in the oxidation of organic substances (hydrocarbons, polymers) that exist in the literature are attributed to the lack of control over the presence and nature of the metal containing impurities in the CNTs.

Variable high‐pressure and concentration study of cyclohexane pyrolysis at high temperatures

AbstractA high‐pressure and temperature cyclohexane pyrolysis shock tube study was completed with the goal of extending the experimental cyclohexane pyrolysis data to pressures relevant to current and future combustors and to investigate whether ring contraction products observed in high‐pressure, supercritical phase cyclohexane and cycloalkane pyrolysis experiments can form at matching, and higher, pressures in the gas phase. The experiments in the current work were completed over a range of 950–1650 K and at nominal pressures of 40, 100, and 200 bar. No alkylcyclopentanes, possible ring contraction products, were observed to form under the conditions of the current study. The production of methylenecyclopentane and 1,3‐cyclopentadiene, and the other three cyclic species quantified: cyclohexene, benzene, and toluene, increased significantly with a substantial increase in the initial fuel concentration. Two sets of experimental data obtained at 200 bar were compared with a literature and laboratory‐generated model. Both models had difficulty capturing the propadiene and propyne profiles, and the literature model significantly overestimated the benzene observed in the set of experiments completed with the more dilute fuel mixture. The literature model was able to better predict propadiene, propyne, and benzene product profiles in the 200 bar set of experiments, which used a higher concentration of fuel in the test gas. These results suggest that despite both cyclohexane and benzene being well‐studied and important species in combustion chemistry, their reaction pathways and reaction rates would benefit from further refinement.

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.

Experimental and Kinetic Modeling Study of Cyclohexane Pyrolysis

The pyrolysis of undiluted cyclohexane has been studied in a continuous flow tubular reactor at temperatures from 913 to 1073 K and inlet feed flow rates in the range 288–304 g·h–1 at 0.17 MPa reactor pressure with average reactor residence time of 0.5 s calculated based on the pressure in the reactor, the temperature profile along the reactor, and the molar flow rate along the reactor estimated by the logarithmic average of the inlet and outlet molar flows. The reactions lead to conversions between 2% and 95%. Forty-nine products were identified and quantified using two-dimensional gas chromatography equipped with thermal conductivity and flame ionization detectors. The products with molecular weights between those of hydrogen and naphthalene constitute more than 99 mass % of the total products. A kinetic mechanism composed exclusively of elementary step reactions with high pressure limit rate coefficients has been generated with the automatic network generation tool “Genesys”. The kinetic parameters for the reactions originate either directly from high level ab initio calculations or from reported group additive values which were derived from ab initio calculations. The Genesys model performs well when compared to five models available in the literature, and its predictions agree well with the experimental data for 15 products without any adjustments of the kinetic parameters. Reaction path analysis shows that cyclohexane consumption is initiated by the unimolecular isomerization to 1-hexene but is overall dominated by hydrogen abstraction reactions by hydrogen atoms and methyl radicals. Dominant pathways to major products predicted with the new model are discussed and compared to other well performing models in the literature.

An experimental and simulated investigation on pyrolysis of blended cyclohexane and benzene under supercritical pressure

The pyrolysis characteristics of blended cyclohexane and benzene mixture are investigated under supercritical pressure. The experimental results show that cyclohexane pyrolysis is greatly easier than thermal cracking of benzene, and the addition of cyclohexane can significantly promote the benzene pyrolysis, while the thermal cracking of cyclohexane itself is weakened by the addition of benzene. A simulation of the blended fuel is carried out by using Chemkin program, indicating that the chemical heat sink of cyclohexane pyrolysis can account for nearly 50% of its total heat absorption capacity, while benzene is inactive in thermal cracking process.

Synthesis of novel hollow graphitic vesicle-supported Pt nanoparticles for oxygen reduction reaction

A new type of interconnected hollow graphitic vesicle (HGV-T) with high purity was successfully synthesized by pyrolysis of cyclohexane over Co3O4 nanoparticles. Heat treatment was proven to be an effective way to make the shell permeable, thus allowing to easily remove metal core by facile acid leaching at room temperature. The overall transition process was investigated in detail on the basis of the structures of formed carbons and the crystal changes of cobalt species at different reaction stages. It was found that the wall thickness of HGV-T could be tailored by varying the deposition time, and the graphitization degree of HGV-T was highly dependent on the annealing temperature. Upon support of Pt, HGV-T shows much higher electrocatalytic activity and durability than commercial Pt/C catalyst in the oxygen reduction reaction as results of its open hollow network, high dispersion of Pt, excellent electrical conductivity and high corrosion resistance. The electrocatalytic activity of Pt/HGV-700 is 2.7 times as high as that of commercial Pt/C under the same testing conditions, which endows it with a potential for serving as an alternative support of fuel cell.

Characterization of MOCVD TiO2 coating and its anti-coking application in cyclohexane pyrolysis

Abstract In order to inhibit the surface coking, TiO 2 coatings were prepared on the 310S stainless steel surface by the metal-organic chemical vapor deposition method in a horizontal hot-wall tubular reactor under atmospheric pressure condition. According to the coating deposition kinetics, the apparent activation energy of 29.30 kJ/mol was obtained in the low temperature range from 250 to 400 °C due to the domination of the surface reaction. However, the coating deposition rate declined at temperatures above 400 °C because of the simultaneous generation of TiO 2 powder in the gas phase. The morphology, elemental and phase composition of TiO 2 coatings were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction, respectively. The results show that the surface TiO 2 particle size increased with the rising temperature, and anatase phase was achieved above 350 °C with the preferential growth of crystal structure from (101) face at 350 °C to (220) face at 450 °C. Correspondingly, the coating thickness increased with the deposition time, and the coating with the thickness of about 2 μm covered the substrate surface completely, with the elemental composition in atomic percentage of Cr, Fe and Ni being close to 0, respectively. Based on the parallel experiments of cyclohexane pyrolysis for each TiO 2 coating specimen, it is indicated that the best performance was achieved on TiO 2 coating deposited at 250 °C for 80 min with an anti-coking ratio of 64.0%. The ratio was 48.0% for that deposited at 450 °C for 30 min. And the coating deposited at 450 °C for 30 min has better effect on inhibiting the formation of filamentous carbon.

Characterization of CVD TiN coating at different deposition temperatures and its application in hydrocarbon pyrolysis

In this study, titanium nitride (TiN) coatings at different deposition temperatures have been designed as anti-coking coatings during cyclohexane steam pyrolysis. The TiN coatings were deposited on 310S foils (10mm×10mm×0.9mm) between 750 and 950°C by a chemical vapor deposition (CVD) method. To evaluate the anti-coking property of TiN coatings, cyclohexane was employed for cracking in a tubular quartz reactor. The as-deposited and coked TiN coatings were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, Raman spectroscopy and X-ray diffraction. The results show that the deposition temperature has a great influence on the morphology and microstructure of CVD TiN coatings. The structures of TiN coatings with apparent differences tend to lie on (220) preferred orientation as the deposition temperature increases. When the deposition temperature is below 850°C, the TiN coatings cannot cover the substrate surface completely and some filamentous coke was observed. However, the anti-coking ratios of TiN coatings at different deposition temperatures are all above 90% during cyclohexane pyrolysis at 770°C for 1.5h. In summary, the TiN coatings at high deposition temperatures can completely cover the cracks and gaps on the rough surface of blank substrate, and thus provide a very good protective layer to prevent the substrate from metal dusting and severe coking. Moreover, the Raman spectra show that the dust-like coke on the surface of TiN coatings has a higher degree of graphitization than the filamentous coke on 310S substrate, which also proves the good anti-coking performance of TiN coatings.

Experimental and kinetic modeling study of 2,5-dimethylfuran pyrolysis at various pressures

The pyrolysis of 2,5-dimethylfuran (DMF) in a flow reactor was investigated at various pressures (30, 150 and 760Torr) by synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Dozens of pyrolysis products, especially a series of radicals and aromatics, were identified from the measurement of photoionization efficiency spectra; and their mole fraction profiles were measured at 780–1470K. Phenol, 1,3-cyclopentadiene, 2-methylfuran, vinylacetylene and 1,3-butadiene were observed with high concentrations in the decomposition of DMF. The pressure-dependent rate constants of the major unimolecular decomposition reactions of DMF were theoretically calculated, and was adopted in the pyrolysis model of DMF with 285 species and 1173 reactions developed in the present work. The model was validated against the species profiles measured in both the present work and the previous pyrolysis studies of DMF. Based on the rate of production and sensitivity analyses, main pathways in the decomposition of DMF and the growth of aromatics were determined. The unimolecular decomposition to produce CH3CHCCH and acetyl radicals, H-atom abstraction to produce 5-methyl-2-furanylmethyl radical, ipso substitution by H-atom to produce 2-methylfuran and H-atom attack to produce 1,3-butadiene and acetyl radical were concluded to dominate the primary decomposition of DMF. Further decomposition of 5-methyl-2-furanylmethyl radical leads to great production of phenol and 1,3-cyclopentadiene which can be readily converted to precursors of large aromatics such as cyclopentadienyl radical, phenyl radical and benzene. As a result, the formation of aromatics in the pyrolysis of DMF is promoted compared with the pyrolysis of cyclohexane and methylcyclohexane under very close conditions. This observation implies the potentially high sooting tendency of DMF, and emphasizes the necessity to investigate the sooting behavior and soot formation mechanism in DMF combustion for the potential application of DMF as an alternative engine fuel.

Influence of TiN coating on products distribution for hydrocarbon fuel cracking under high temperature and pressure

To understand the cracking behavior of hydrocarbon fuels when a passivation coating covers the metal substrate, the pyrolysis of n-hexane, cyclohexane and benzene in uncoated and titanium nitride (TiN) coated stainless steel 304 (SS304) continuous tubular reactors (3mm diameter, 700mm long) under high temperature and pressure were studied. The TiN coating was prepared on the inner surface of SS304 tubes by atmospheric pressure chemical vapor deposition (APCVD) using TiCl4-H2-N2 system. The solid, liquid and gaseous products distribution of the pyrolysis of n-hexane, cyclohexane and benzene for bare and TiN-coated tubes were analyzed and discussed. The results show that the influence of TiN coating on hydrocarbon fuels cracking is not a simple effect of promotion or inhibition. As a whole, the TiN coating has little effects on the distribution of the gas products for n-hexane cracking, but has great influence on that of cyclohexane and benzene cracking. For the liquid products distribution, the mass fractions of alkane for bare tubes reduce compared with those for TiN-coated tubes, while the alkene contents are higher than those for TiN-coated tubes. Moreover, the morphologies of carbon deposits are mainly filamentous and spherical on the inner surface of SS304 tubes, but no morphologies of carbon deposits are observed on TiN-coated tubes surface after thermal cracking of hydrocarbon fuels. Meanwhile, the amount and carbon deposition rate of cokes deposited in the system for TiN-coated tubes are higher than those of bare tubes, regardless of the hydrocarbon fuels are n-hexane, cyclohexane and benzene.

An experimental and kinetic modeling study of cyclohexane pyrolysis at low pressure

The pyrolysis of cyclohexane at low pressure (40mbar) was studied in a plug flow reactor from 950 to 1520K by synchrotron VUV photoionization mass spectrometry. More than 30 species were identified by measurement of photoionization efficiency (PIE) spectra, including some radicals like methyl, propargyl, allyl and cyclopentadienyl radicals, and stable products (e.g., 1-hexene, benzene and some aromatics). Among all the products, 1-hexene is formed at the lowest temperature, indicating that the isomerization of cyclohexane to 1-hexene is the dominant initial decomposition channel under the condition of our experiment. We built a kinetic model including 148 species and 557 reactions to simulate the experimental results. The model satisfactorily reproduced the mole fraction profiles of most pyrolysis products. The rate of production (ROP) analysis at 1360 and 1520K shows that cyclohexane is consumed mainly through two reaction sequences: cyclohexane→1-hexene→allyl radical+n-propyl radical, and cyclohexane→cyclohexyl radical→hex-5-en-1-yl radical that further decomposes to 1,3-butadiene via hex-1-en-3-yl and but-3-en-1-yl radicals. Besides the stepwise dehydrogenation of cyclohexane, C3+C3 channels, i.e. C3H3+C3H3 and C3H3+aC3H5 also have important contribution to benzene formation. The simulation reveals that C3H3+C3H3=phenyl+H reaction is the key step for other aromatics formation, i.e. toluene, phenylacetylene, styrene, ethylbenzene and indene in this work.

Kinetics and Selectivity of Cyclohexane Pyrolysis

Pyrolysis of cyclohexane was conducted with a plug-flow tube reactor at 873 K. The data of feed conversion fit first-order kinetics adequately, giving the apparent rate constant of 0.0092 s-1 . A chain mechanism of free radical reactions is proposed to interpret consumption of cyclohexane by four processes: homolysis of C-C bond (Path I) and homolysis of C-H bond (Path II ) in reaction chain initiation, H-abstraction of various radicals from feed molecule in reaction chain propagation (Path III ), and the process associated with coke formation (Path IV). The reaction path probability ratio of X I：X II：X III ：X IV was 0.5420: 0.0045 : 0.3897 : 0.0638.