Effect of pressure on thermo-oxidative reactions of saturates, aromatics, and resins (S-Ar-R) from extra-heavy crude oil

This work investigates the thermo-oxidative behaviour of saturates, aromatics, and resins, at different pressures of 0.084, 3.0, and 6.0 MPa. Saturates (S), aromatics (Ar), and resins (R) were characterized by elemental analysis, vapor pressure osmometry, 1H- and 13C-nuclear magnetic resonance, and X-ray photoelectron spectroscopy. The atmospheric evaporation of S, Ar, and R was evaluated by simulated distillation, whereas high-pressure distillation curves were created by calculation using the Clapeyron equation with the Redlich Kwong equation of state. The mass change and rate of mass change profiles were assessed in a high-pressure thermogravimetric analyzer (HP-TGA) by injecting nitrogen and air to obtain the pyrolysis and oxidation responses. Kinetic analysis was performed using a first-order kinetic model discretizing thermal oxidation profiles in four different regions, namely, oxygen chemisorption (OC), decomposition of chemisorbed oxygen-based compounds (DCO), first combustion (FC), and second combustion (SC). As main results, it was obtained that the saturates were just described by FC and SC regions at all pressures, while aromatics and resins by the four stages (OC, DCO, FC, and SC) for pressures higher than 3.0 MPa. At 6.0 MPa, aromatics exhibit a rapid consumption in the FC region, while resins present a more controlled consumption, distributed in the three decomposition regions. This implies that as pressure increases, aromatics have a greater influence on oxidation reactions at high temperatures. From pyrolysis curves, it was noted that the amount of coke generated follows the increasing order saturates < aromatics < resins for all pressures used. The kinetic parameters corroborated the results obtained. For the higher chemisorption systems, the effective activation energy values ​​were higher, and therefore the DCO values. At the same time, the pre-exponential factor showed opposite results given slower reaction kinetics in these stages.