New insights into the pyrolysis of n-tetradecane using ReaxFF molecular dynamics simulations: Implications for understanding the thermal cracking of subsurface crude oil

Abstract

The fragmentation of normal alkanes during the cracking of oil under geochemical conditions with high thermal stress is important for the preservation of crude oils in reservoirs and the formation of gaseous hydrocarbons. In this study, the kinetics behavior, cracking mechanisms, and products of n-tetradecane pyrolysis were investigated using reactive molecular dynamics (ReaxFF MD) simulations at high temperatures (2000K∼3000 K). The pyrolysis process itself, its main products and intermediates, and kinetic behavior were analyzed at an atomic/molecular level. Low molecular weight (C1–C5) alkanes and olefins, together with H2, were the predominant intermediates and products of the simulations. Three distinct stages—a stable stage, initial decomposition, and secondary pyrolysis—can be distinguished during the decomposition process. The reactant n-tetradecane was exhausted when the simulation temperature reaches ∼2250 K, accompanied by a rapid increase in hydrocarbons with molecular numbers C1–C5. The yield of C2–C5 peaked at ∼2500 K and then decreased. The number of CH4 molecules increased continuously throughout the entire simulation process because of the contribution of secondary pyrolysis. The Arrhenius parameters obtained from ReaxFF MD simulations, on the basis of first-order kinetic analysis of n-tetradecane, were reasonably consistent with experimental data and generally in agreement with results from pyrolysis experiments on crude oils in the laboratory. The pyrolysis process and reaction mechanism of n-tetradecane were also reasonably consistent with laboratory pyrolysis experiments on whole oils and their individual components. ReaxFF molecular dynamics simulation is therefore considered to be a valid approach for the study of thermal cracking of subsurface crude oils.