Study of degradation of acid crude oil by high resolution analytical techniques

The extraction and refining process of unconventional oils, including heavy and extra heavy oils, has been a challenge to be overcome in the petrochemical industry. These oils typically comprise polar compounds responsible for some undesirable properties, such as naphthenic acids (NAs), which can be associated with the high corrosion rates in refineries. In this context, corrosion caused by NAs is among the most aggressive ones, and it can be accentuated when in presence of other compounds usually found in oil. Herein, a heavy oil sample was subjected to thermal degradation process at 350 degrees C aiming the understanding of its behavior at the molecular level and to evaluate its physicochemical properties as a function of degradation time. Additionally, the oil was characterized by classic and high resolution analytical techniques. The obtained results showed that the oil exposed to longer degradation times (t >= 24 h) presented characteristics of "lighter" oil due to mainly disaggregation of resins and asphaltenes. Total Acid number (TAN) analysis displayed a decrease of 68.3% (from 2.0 to 0.75 mg KOH g(-1)) of its initial value after 36 h of degradation at 350 degrees C. Other important changes observed included an increase in API gravity (of two decimal places from 17 to 19), and decreases in the pour point from -12 to -36 degrees C, in the kinematic viscosity in 86.5% and of total sulfur content in 5.2% (from 0.58 to 0.55 wt%). The HTGC and NMR results showed that the saturated fraction is not affected during the thermal stress process, whereas a subtle increase in the number of aromatic compounds was observed as a function of degradation time. This behavior can be attributed to the disaggregation of resins and asphaltenes as well as to the decarboxylation process. The last process was confirmed by ESI(-)-FT-ICR MS, where a reduction of average mass molar distribution from 466 to 450 Da was observed.