Naphthenic
acids (NAs), present in a typical Brazilian acid crude
oil and its thermal degradation products, were investigated using
two separation methodologies: solid-phase extraction (SPE) and liquid–liquid
extraction (LLE). Fractions produced were characterized by proton
nuclear magnetic resonance spectroscopy (1H NMR) and negative-ion-mode
electrospray ionization Fourier transform ion cyclotron resonance
mass spectrometry (ESI(−)-FT-ICR MS). Of the NAs extraction
methods studied, SPE was more efficient than LLE. Further, ESI(−)-FT-ICR
MS results showed that the SPE method with eluent phase variation
allowed for the detection of a larger amplitude of NAs compounds (m/z 200–1200), reducing the occurrence
of ion suppression on the NAs of higher average molecular weight (M
w) distribution. It was noted that the aromaticity
or double bond equivalent (DBE) of these produced collective fractions
as well as their M
w values increased as
a function of the polarity of the extraction system (DCM →
DCM:MeOH:FA). Also, 1H NMR analysis revealed the alkyl
predominance evidenced by the presence of high Hβ content in fractions, suggesting that the NAs compounds have long
and unbranched chains. The behavior of NAs species during the thermal
degradation process was also evaluated, and the results showed their
presence in only five SPE extracts out of six, containing different M
w values (M
w = 366,
417, 531, 662, and 836 Da). This suggests that, in the last SPE fraction
(named SF6: m/z 700–1150,
carbon number of C52–C72, and DBE = 0–15;
detected only in virgin crude oil), the NAs were selectively cracked
during the thermal degradation process.
Microscopic techniques were combined to study the influence of corrosion rate on the morphologic behavior of AISI 1020 steel specimens submitted to thermal degradation of a typical acid crude oil (total acid number (TAN) = 2.1390 mg KOH g -1 and total sulfur (S) = 0.7778 wt.%). The techniques used were light microscopy (LM), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDX), atomic force microscopy (AFM) as well as Raman spectroscopy. Assays were performed in six different degradation time (t = 6, 12, 24, 36, 48 and 72 h) at 320 °C. After the exposure of the specimens to petroleum, a reduction above 37% in the TAN after t = 72 h was observed, with a maximum corrosion rate during the first periods of degradation (t = 6 and 12 h). Correlating the TAN and corrosion rate data with the microscopic data, the images of LM, AFM, and SEM/EDX showed that after 6 h of exposure to petroleum, a passivation film was formed on the surface of the steel. This film consisted of two layers, an external one, formed of FeS, and an internal one, composed of iron oxides and oxyhydroxides. However, after 48 h of thermal degradation, this morphology was altered to a single layer of FeS coating the steel surface.
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