Experiments and modeling of octanoic acid pyrolysis in a plug flow reactor

Abstract

Pyrolysis of carboxylic acids plays a key role in various industrial applications (production of bio-energy vectors from waste cooking oil reforming or of biosourced chemicals from lignocellulosic biomass pyrolysis, or even combustion of ethyl biodiesel). In this work, pyrolysis of octanoic acid selected as departure model compound was carried out in a plug flow reactor. Profiles of the reactant, stable intermediates, and final products, identified by gas chromatography/mass spectrometry, were measured by non-dispersive infrared absorption and gas chromatography equipped with thermal conductivity detector or flame ionization detector. Within the operating conditions of the plug flow reactor (mean pressure/kPa: 112; oven set point temperature/K: 923–1073, mean residence times/s: 0.42-0.83; reactant mole fraction in nitrogen selected as inert diluent: 3 and 5%), the major products formed are 1-olefins and carboxylic acids (saturated and monounsaturated) together with small species like hydrogen, carbon monoxide, carbon dioxide, methane, and ethylene. These experimental data were used to further develop a detailed chemical kinetic mechanism for octanoic acid pyrolysis, mainly generated from EXGAS software with kinetic and thermodynamic properties of key reactions specific to carboxylic acids revisited. Satisfactory repeatability and material balances were obtained for the generated kinetic data, which also showed a satisfactory agreement with the proposed mechanism. Rate of production and sensitivity analyses were carried out for selected experimental conditions (high temperature and various residence times), highlighting the significant contribution of H-abstractions (particularly the α-hydrogens) and the sensitive key role of the unimolecular decomposition in octanoic acid consumption.