Chemical Speciation of Naphthenic Acids in Brazilian Crude Oils by Advanced Analytical Techniques

Abstract

The characterization of naphthenic acids (NAs) in petroleum samples has received particular attention and is still an analytical challenge as a result of the complexity of the matrix. This study aims to individually and comprehensively characterize NAs in real Brazilian crude oils by Orbitrap high-resolution mass spectrometry (Orbitrap− HRMS) and comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC × GC−TOFMS). Linear, branched, and cyclic carboxylic acids were identified and semi-quantified by GC × GC−TOFMS in four crude oils with a distinct total acid number (TAN) and American Petroleum Institute (API) gravity values. The P1 sample, a crude oil with an intermediate TAN value (1.25 mg KOH g −1 and 22.2°), presented a total acid quantification of 16.6 mg g oil −1 by GC × GC− TOFMS and a higher total concentration of identified linear acids. The P2 crude oil, which has a lower API gravity value and a higher TAN value (19.8°and 2.39 mg KOH g −1 , respectively), resulted in a higher total acid quantification of 35.2 mg g oil −1 and expresses the highest amount of identified cyclic acids, in particular, the tricyclic terpane acid homologous series. However, the P3 and P4 crude oils had similar API gravity and TAN values (29.9°and 29.8°and 0.22 and 0.24 mg KOH g −1 , respectively), and total acid quantification by GC × GC−TOFMS resulted in different values (2.185 and 9.636 mg g oil −1, respectively). These results indicate that the TAN value does not sufficiently express the actual quantification of acids when compared to the total acid quantification by GC × GC−TOFMS. The individual identification of the acidic species that comprise the oil can help to understand the real acidity of the samples. The data obtained via Orbitrap−HRMS show that a higher variability of molecular formulas with a large range of double bond equivalents (DBEs) was detected for the P1 and P2 oils, while the P3 and P4 oils showed a high percentage of linear carboxylic acids. Detailed information on the individual chemical structure of the NAs is very important for evaluating the physicochemical properties of the oil (e.g., TAN), using advanced analytical techniques and offering a new perspective for crude oil fingerprinting with greater analytical reliability.

A Kinetic Approach to the Pyrolysis of Naphthenic Acids

“…1,2 Crude oils that have a total acid number (TAN) over 0.5 are labeled as acidic. 3 TAN is measured by titrating 1 g of crude oil with KOH to assess the required milligrams of KOH. 4 From the refining point of view, high TAN crudes are cheaper to purchase, hence considered as opportunity crudes that generate high refining margins.…”

“…To provide an additional supply for the ever-growing fuel demand, high acid crude oils are considered as a good option, to be used as raw feedstocks in crude oil refineries. , Crude oils that have a total acid number (TAN) over 0.5 are labeled as acidic . TAN is measured by titrating 1 g of crude oil with KOH to assess the required milligrams of KOH .…”

Analysis of naphthenic acids in produced water samples by capillary electrophoresis‐mass spectrometry

“…Most studies have demonstrated the analysis of NAs utilizing gas chromatography (GC) [29][30][31][32][33][34], liquid chromatography [5,34,35], and direct MS analysis [36,37]. On the other hand, only a few studies involving the analysis of NAs by CE can be found in the literature.…”