Kinetic Modeling of the Pyrolysis of Propylene Glycol 

Christina AlGemayel1

Edward Honein1

Ahmad El Hellani2,4

Rola Salman2,4

Najat A. Saliba2,4

Alan Shihadeh3,4

Joseph Zeaiter1,Email

1Baha and Walid Bassatne Department of Chemical and Petroleum Engineering, Maroun Semaan Faculty of Engineering and Architecture, American University of Beirut, Beirut 1107 2020, Lebanon
2Department of Chemistry, Faculty of Arts and Science, American University of Beirut, Beirut 1107 2020, Lebanon
3Department of Mechanical Engineering, Maroun Semaan Faculty of Engineering and Architecture, American University of Beirut, Beirut 1107 2020, Lebanon
4Center for the Study of Tobacco Products, Virginia Commonwealth University, Richmond, VA 23284, USA

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+05J/mol and A0=6.84E+13s-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 ECIG emissions.

Kinetic Modeling of the Pyrolysis of Propylene Glycol