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

Abstract

A 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 lumped kinetic model for high-temperature pyrolysis and combustion of 50 surrogate fuel components and their mixtures

“…Therefore, the speciation data of C4H6, C6H6 and C6H5CH3 were incorporated to validate the present model. Liszka et al measured the speciation data in the pyrolysis of cyclohexane [74] and methylcyclohexane [78] methylcyclohexane [75], (b) ethylcyclohexane [80], (c) n-propylcyclohexane [81] and (d) n-butylcyclohexane [75] in shock tubes at 1-3 atm and  = 0.5-2.0. n-butylcyclohexane at Pu = 1 atm.…”

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

“…The only high pressure and temperature study of cyclohexane pyrolysis is one by Liszka and Brezinsky [10] who used a single pulse shock tube (950-1650 K, ~2.2 ms) to obtain stable species concentrations at reaction pressures up to 200 bar. An important aspect of this work was to examine the effect of pressure and initial fuel concentration on the product distributions.…”

“…The importance of reaction ( 7), H-abstraction as a source of cyclohexyl radicals, was initially addressed by Tsang [4] and later supported by Wang et al [8]. Three paths were proposed for the loss of the cyclohexyl radical: ring opening to an unsaturated radical (reaction (8)); dehydrogenation to cyclohexene (reaction (9)); isomerization to a cyclopentylmethyl radical (reaction (10)).…”

“…Recent theoretical works by Knepp et al [16] and Gong et al [13] support the ring opening and dehydrogenation channels for pyrolysis of cyclohexyl. However, they also suggest that isomerization of 1-C 6 H 11 -6 generates the cyclopentylmethyl radical rather than it being formed directly by reaction (10).…”

“…The model was later updated and embedded as a sub-mechanism in a detailed model for methylcyclohexane (MCH) combustion [17]. Liszka and Brezinsky [10] later extended the MCH mechanism to 200 bar and through a Monte Carlo-based optimization approach identified and modified three reactions that improved predictions of benzene formation at very high pressures.…”