Online Chemical Analysis of Flowing <i>n</i> -Hexane in a Pyrolysis Reactor by Optical Spectroscopy and Molecular Beam Mass Spectrometry

Hydrocarbon pyrolysis at high pressures and temperatures is relevant to the decomposition of aviation fuel in advanced thermal management applications. To unravel the dynamics of hydrocarbon cracking and surface deposition, we have developed a novel experimental technique to characterize a neat, supercritical hydrocarbon fluid undergoing pyrolysis in a glass tube reactor (GTR). Using optical absorption spectroscopy, we sensitively measure the onset and rate of amorphous carbon deposition. Simultaneously, we unravel the chemical speciation of the fluid by online quadrupole mass spectrometry (MS). For <i>n</i>-hexane, we reveal four chemical regimes with increasing temperature: (1) no chemistry, (2) cracking with little-or-no deposition, (3) cracking with deposition, and (4) rapid, severe deposition. By modeling the GTR using computational fluid dynamics, we validate its representation as a simple plug flow reactor. The fluid phase decomposition of <i>n</i>-hexane, evident by MS, is consistent with an overall first-order process with an activation energy of 217.7 ± 2.4 kJ·mol^-1. The temperature-dependent deposition rate is analyzed by a crude two-step model, and we compare our findings to those previously reported [e.g., Pramanik, M., et al. <i>Ind. Eng. Chem. Process Des. Dev.</i> <b>1985</b>, 24 (4), 1275-1281]. We anticipate that our experimental methods will provide a powerful means to quickly evaluate purported decomposition mechanisms of hydrocarbon fuel surrogates.