Fuel, Vol.108, 647-655, 2013
Monitoring the liquid/liquid extraction of naphthenic acids in brazilian crude oil using electrospray ionization FT-ICR mass spectrometry (ESI FT-ICR MS)
Although the term "naphthenic acids" was originally used to describe acids that contain naphthenic rings, today this term is used in a more general sense and refers to all cyclic, acyclic, and aromatic acids in crude oil. In crude oil, naphthenic acids exist as a complex mixture of compounds with broad polydispersity with respect to both molecular weight and structure. Recently, there has been increasing interest in acidic fractions in crude oil because of the corrosion problems that these compounds cause during oil refinery. This corrosion is associated with the total acid number (TAN). However, it has been argued that there is no clear correlation between the TAN and the level of corrosion. Herein, naphthenic acids were extracted from two crude oil samples (TAN = 4.95 and 3.19 mg KOH g(-1)) using liquid/liquid extraction with alkaline solutions at three different pHs (pH 7, 10 and 14), thus evaluating the efficiency and selectivity among different acidic extraction methods to prevent future corrosion processes in petroleum industry such as regions of the refineries working. The original oil samples and their naphthenic acidic fractions and residues (washed oil from the acidic extraction) were analyzed using negative-ion electrospray ionization (ESI) Fourier transform ion cyclotron mass spectrometry (FT-ICR MS) and Fourier transform infrared spectroscopy (FTIR). The TAN and sulfur content were also determined. We observe a decreasing TAN in the original oil with increasing pH (7 -> 14), reaching a value of almost zero at pH 14. Consequently, an intense band at approximately 1700 cm(-1) was observed for the naphthenic acidic fraction. Fractions produced ESI(-)-FT-ICR MS spectra with average molecular weight distribution, M-w, and m/z values ranging from m/z 200-350 and M-w = 270 (for pH 7) to m/z 200-650 and M-w = 390 (for pH 14). Therefore, the acidic extraction method at pH 14 was more efficient and the ESI FT-ICR MS analysis reveals that the most abundant naphthenic acids are those with short alkyl chain lengths (