Evolution of Naphthenic Acids during the Corrosion Process

Abstract

Acidic crudes, i.e., oils with total acid number (TAN) higher than 0.5 mg of KOH/g, represent an important option for the refiner. This acidity, related to the presence of naphthenic acids (NAs), is considered responsible for a number of drawbacks in refinery, such as corrosion of pipelines and devices. The understanding of the corrosion activity of NAs is complex because of their wide distribution of physical properties and the simultaneous presence in the oil of other species contributing to corrosion (e.g., H2S). In this work, the distribution of NAs was determined by a combination of infrared (IR) and liquid chromatography–mass spectrometry (LC–MS) techniques in crude oils before and after corrosivity tests. An analytical procedure was developed to efficiently extract NAs from crude oils and to monitor them by the LC–MS technique, via electrospray ionization in negative mode [ESI(−)]. The identification of the NAs was possible from the corresponding molecular ions, according to the number of C atoms and naphthenic rings. The sum of the intensities of these molecular ions was linearly correlated with the total amount of NAs, as measured by IR spectroscopy, and allowed for the calculation of the distribution of the NAs. The evaluation of NAs in crude oils allowed for drawing maps of distribution, following their evolution after corrosivity tests. A mechanism of reaction of NAs was also proposed after the interaction with corrosivity coupons.