Kinetic Modeling of the Pyrolysis of Propylene Glycol

Abstract

A kinetic model was developed to predict product evolution during the thermal degradation of propylene glycol (PG) under N2 atmosphere. Main aldehydes that form from PG pyrolysis are formaldehyde (FA), acetaldehyde (AA), propanal (PA), acetone (Ace), glyoxal (GA) and methyglyoxal (MGA). Activation energies (Ea) of the proposed reaction pathways were determined experimentally under a range of temperatures via sequential parameter estimation and quasi-steady state approximation assumption. They were compared against Arrhenius parameters derived from ReaxFF-MD for aldehyde formation, with Ea errors between 3% for PA and up to 39% for Ace. ReaxFF-MD was also employed to assess reaction pathways and to compute Arrhenius parameters for PG cracking. PG decomposes at Ea=1.86E+05 J/mol and A0=6.84E+13 s-1. Model predictions were in agreement with experimental results and proved dehydration of PG to propylene oxide as the dominant pathway, which then undergoes ring opening to produce propen-1-ol and propen-2-ol. The two enols tautomerize to PA, Ace and FA - the major products from PG pyrolysis. Proposed elementary reactions were further studied on ReaxAMS to assess reaction pathways and transition states via the Nudged Elastic Band (NEB) method. This study's findings constitute a critical step in putting forward a comprehensive mathematical model for predicting toxicants in electronic cigarettes (ECIGS) emissions. © Engineered Science Publisher LLC 2022.