# Online Chemical Analysis of Flowing n‑Hexane in a Pyrolysis Reactor by Optical Spectroscopy and Molecular Beam Mass Spectrometry

## TL;DR

The study uses optical and mass spectrometry techniques to analyze n-hexane pyrolysis in a reactor, revealing temperature-dependent chemical regimes and decomposition dynamics.

## Contribution

A novel experimental setup combining optical spectroscopy and online mass spectrometry to study hydrocarbon pyrolysis in real-time.

## Key findings

- Four chemical regimes of n-hexane pyrolysis are identified based on temperature.
- n-Hexane decomposition follows a first-order process with an activation energy of 217.7 ± 2.4 kJ·mol–1.
- A two-step model explains the temperature-dependent carbon deposition rate.

## Abstract

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 n-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 n-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. Ind.
Eng. Chem. Process Des. Dev.
1985, 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.