Evolution of Naphthenic Acids during the Corrosion Process

Dec 2, 2013 - This acidity, related to the presence of naphthenic acids (NAs), is considered ... an index of potential corrosion, although any clear c...
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Evolution of Naphthenic Acids during the Corrosion Process Cristina Flego,*,† Luigi Galasso,† Luciano Montanari,† and Maria Elena Gennaro‡ †

SDM CHIF, Refining and Marketing Division, eni S.p.A., Via Maritano 26, I-20097 San Donato Milanese, Italy Venezia Tecnologie S.p.A., Via delle Industrie 39, I-30175 Porto Marghera, Italy

‡

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.

1. INTRODUCTION The strong interest in the demand of energy earned by the global economic growth is nowadays dealing with the decline of conventional oil reserves. The shortage of known petroleum reserves makes more attractive less attended energy resources, especially considering that their availability ratio to conventional crude oils is 2.5:1.1 Processing unconventional or opportunity crudes, i.e., heavy sour crudes, oil sands/bitumen, extra-heavy oil, high total acid number (TAN) crudes, and oil shale, are considered a huge potential resource to fulfill energy requirements and the most viable option in a time of exploitation of oil prices and high demand for light sweet crudes. Among the unconventional crudes, acidic crudes, i.e., crude oils with TAN higher than 0.5 mg of KOH/g, may result in significant economic benefits for refiners.2 However, the associated risk for enhanced corrosion may negatively impact reliability of operations and increase health, safety, and environmental (HSE) risks. A deep knowledge (amount and distribution) of the acid components and the mechanism of corrosion results in the implementation of adequate treatment strategies. Naphthenic acids (NAs) are minor components of crudes, although they are considered the main components responsible for the acidity and the problems caused in both up- and downstream processing. In fact, they can lead to several detrimental effects, such as plugging of gas hydrate in crude oils,3 stabilizing crude oil−water emulsions,4 increasing corrosivity in refinery units, and foaming in the desalter and catalytic plants.5 NAs comprise a large part of the carboxylic acid fraction in crude oils.6 Generally, they are described as a complex mixture of alkyl-substituted acyclic and cycloaliphatic carboxylic monoacids, with molecular weight (MW) in the range of 200−700, centered around 300 Da,7 and having the general formula CnH2n + zO2, where n stands for the number of carbon atoms (varying from 10 to 50) and z is a negative even © 2013 American Chemical Society

integer that specifies the hydrogen deficiency, i.e., the number of naphthenic rings that can be obtained from z/2 (usually from 0 to 6 fused saturated rings). The acid content in crude oils (conventionally expressed as TAN) was used in the past as an index of potential corrosion, although any clear correlation was established.2,8 Recently, the determination of ring type and carbon number distributions of NAs is considered more important because the degree of corrosivity of NAs is dependent upon their size and structure.9 Different kinds of tests have been described in the literature to determine NA aggressiveness. Laboratory tests have been mainly conducted in confined environments (glass flask10 or autoclave11,12) in static conditions or with rotating cages and with fluid replenishment for long durations.13 A system built by Venezia Tecnologie to carry out corrosivity testing14 was applied in this work, able to simulate plant operating conditions, avoiding oil degradation by gas evolution and thermal cracking. Several methods have been proposed to extract the NAs from crude oil and organic derivatives, to concentrate them, and to allow for a “cleaner” analysis. Extraction by ion-exchange resins has also been proposed with a long and complex procedure of interaction between acids and resins, removal of non-acid components, and extraction of the remaining acids with a proper acid mixture.15,16 Solid-phase extraction applies cyano17 or quaternary amino18 columns or amino-propylmodified silica19 to separate the organic acids by polarity in a very time-consuming procedure. Another approach to the Special Issue: 14th International Conference on Petroleum Phase Behavior and Fouling Received: September 30, 2013 Revised: November 25, 2013 Published: December 2, 2013 1701

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separation of NAs requires bubbling gaseous ammonia into the oil to transform the acids into salts that are precipitated by refrigeration and then separated by filtration and dissolved into methanol or toluene.15 Alternatively to gaseous ammonia, liquid−liquid separation of acids through the corresponding salts was proposed using ammonia-saturated ethylene glycol20 or a solution of 0.5% ammonia in methanol versus a solution of crude oil in the acetonitrile/methanol mixture.21 The reaction of acids by NaOH in water/ethanol solution to form salts from crude oil diluted in hexane has been applied for separation of the NA species, eventually recovered by the addition of concentrated HCl.3 An efficiency higher than 90% in NA extraction has been obtained with alkaline solution at pH 14.22 The use of ionic liquids (e.g., 20% 2-methylimidazole in ethanol) has also been proposed for liquid−liquid extraction of NA from crude oil, with yield efficiency up to 67%.5 Separation of NA mixtures after derivatization steps, as methylation or reaction with 2-nitrophenylhydrazine, was proposed using gas chromatography−mass spectrometry (GC−MS)23,24 and high-pressure liquid chromatography (HPLC)25,26 with a variety of apolar and polar solvents and columns. The application of chromatographic separation without derivatization of NA was obtained by HPLC/ quadrupole time-of-flight−MS (HPLC/qTOF−MS), using a reverse-phase capillary column and water and acidified methanol as solvents, from tailing water samples.27 The coupling of this kind of separation with high-resolution MS has shown unequivocal results,28,29 although the use of this sophisticated apparatus limits its application to few laboratories. The general trends observed for the NA distribution for a given batch of samples are similar using either low- or ultrahighresolution mass spectrometry.6 Among the mass spectrometric techniques, negative-ion atmospheric pressure chemical ionization (APCI) with acetonitrile as a solvent and mobile phase was proposed to yield the cleanest spectra from solid-extracted NA mixtures, without the discrimination of heavier ions and the formation of fragments and cluster ions.9 Reversely, negative-ion ESI has been considered a more suited soft technique for the analysis of NA mixtures, especially when dissociated to facilitate the ion formation.30,31 In this paper, a new protocol for determining NA distribution is applied to identification and quantification of NAs in crude oils. This procedure combines an extractive procedure with a liquid chromatographic separation coupled with identification by mass spectrometry. The combination of NA maps and corrosivity data may explain the evolution of NAs when involved in corrosion processes.

Table 1. List of Samples and Some of Their Properties name A B C D gas oil + DNA A + DNA A + B + DNA

composition (wt %) crude oil crude oil crude oil crude oil mixture (99.07/0.93) mixture (99.76/0.24) mixture (58.52/41/0.48)

TAN (mg of KOH/g)

S (wt %)

0.06 0.18 0.75 3.58 2.0 0.5 1.0

0.12 2.13 0.36 3.60 0.0 0.12 1.0

with 1 cm−1 resolution. An IR cell with an optical pathway of 0.5 mm and KBr windows was filled with crude oil as received. The intensity of the IR carbonyl signal with a maximum at 1705−1710 cm−1 was transformed into the NAN value (expressed as mg of KOH/g), using the extinction coefficient determined by calibration with standard solutions of known concentration (expressed as μg/mL or mg of KOH/g) of NAs. 2.3. NA Distribution. NAs were extracted from crude oil by the liquid−liquid method. The crude oil was contacted (1:1, vol) with a solution of water/ethanol (1:1, vol) at pH 11 by the addition of drops of concentrated NH4OH. A two-phase system was formed, and the aqueous solution was analyzed by the liquid chromatography−mass spectrometer detector/diode array detector (LC−MSD/DAD) method without further treatments. In the case of high-density crude oil, the sample was diluted in CH2Cl2 (1:10, vol) before extraction to reduce the viscosity of the system and allow for a proper liquid−liquid interaction with the extractive mixture. The LC−MSD/DAD analysis was performed with an Agilent HPLC 1100 Series system equipped with ultraviolet (UV) and MS ionic trap detectors. Negative ionization in the m/z 50−2000 range was obtained with an electrospray ionization (ESI) source at 350 °C, 30 psi, and 10 L/min drying gas. The UV chromatograms were obtained at wavelengths of 260, 290, and 350 nm, to emphasize and distinguish the response of the carboxylic groups and aromatic rings. The chromatographic separation was performed with an Agilent Zorbax Eclipse XDB C8 column (4.6 × 150 mm, 5 μm particle size) as the stationary phase and an isocratic mixture of 50:50 acetonitrile/ methanol (+1% acetic acid) as the mobile phase (0.5 mL/min). A total of 1 μL of sample solution was injected per analysis. 2.5. Corrosivity Tests. Corrosion tests were performed in a labscale plant specifically designed by Venezia Tecnologie for the same operating conditions as refinery crude distillation units. The plant includes a feeding jacketed tank, from which the testing fluid is pumped through the system, a two-step heating area, which consists of a counter-current exchanger and a heating section, a testing section, from which the fluid flows into the exchanger for cooling and returns to the vessel, and a number of pressure and temperature sensors to monitor conditions. In Figure 1, a schematic representation of the plant is shown. The sample continuously flows from the hot section, where the carbon steel coupons were located, to the reservoir at ca. 70 °C. High fluid velocity on the surface of specimens can be reached with shear stress up to 300−500 Pa. The short residence time in the hot section avoids degradation of the testing fluid by gas evolution and thermal cracking, allowing for several days of testing. At the beginning of the test, the vessel can be heated by a heating tape (in the case of heavy oils), whereas during the experiment, further cooling inside the jacket may be provided. Several pressure and temperature sensors allow for controlling experimental conditions and the safety of the operating personnel. The plant was built according to European Pressure Equipment and ATEX Directives and was CE-marked.14 The corrosivity tests were performed at 350 °C (or 330 °C in the presence of the gas oil sample), with a flow rate of 5 mL/min and at a pressure of 7 bar.

2. EXPERIMENTAL SECTION 2.1. Samples. A commercial reference NA sample (DNA, batch VKO/2011/977 from Umicore Specialty Materials Brugge) was applied for optimization of the analytical method and to reach higher selected values of naphthenic acidity of some crude samples. This reference sample was characterized by a distillation fraction of 200− 210 °C and a TAN value of 215 mg of KOH/g. Crude oils and mixtures with a wide range of naphthenic acidity and S content were studied before and after corrosivity tests. The main properties of the samples are listed in Table 1. 2.2. TAN and Naphthenic Acid Number (NAN) Determination. TAN was measured according ASTM method D664-11. NAN was determined by Fourier transform infrared spectroscopy (FTIR) at ambient temperature and atmospheric pressure with a FTIR spectrometer model 2000 Perkin-Elmer in the 4000−400 cm−1 range 1702

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Figure 1. Scheme of the plant for corrosivity tests.

3. RESULTS AND DISCUSSION 3.1. NA Distribution. Corrosion effects of NAs depend upon a combination of the amount, molecular weight, and structure of the NAs (and, consequently, acidity, volatility, viscosity, etc.) in the crude. The presence of other species (e.g., S and additives) contributes with synergic or protective actions toward the native corrosion of the sole NAs.9,32 The concentration of NAs in the samples was determined by TAN (Table 1) and NAN (Table 2) values, with the former

all of their negative molecular ions gave rise to an unresolved hump in the total ion current (TIC) profile (Figure 2a). The chromatographic area of this hump increases with the increasing naphthenic acidity of the sample (Figure 2a), and a direct relation is found between the area of the TIC humps and NAN values (Figure 2b). Therefore, by application of the response factor, it is possible to transform the absolute composition (i.e., number of counts per negative molecular ion) into the concentration expressed as mg of KOH/g (i.e., NAN) of each NA. This assumption is important to evaluate different contents and distributions of NAs in crudes and to detect differences between samples before and after corrosivity tests. At the same time, the sum of the intensity of all negative molecular ions in the elution range of NAs gives rise to the comprehensive mass spectrum representing the fingerprint of the naphthenic acidity of the sample (Figure 2c). The main NAs present in the DNA sample are those with negative molecular ions with m/z 211 and 225 (Figure 2c) that correspond to a homologous series with one naphthenic ring (and 13−14 C atoms, respectively) and with m/z 195, 237, 251, 265, and 279 of the homologous series with two naphthenic rings (and 12 and 15−18 C atoms, respectively). This distribution is aligned to the distillation properties of the sample and confirms that no fragmentation6,32 and only negligible dimerization30 occur under the experimental conditions applied for the measurement. The same procedure applied for the reference DNA sample was followed for describing real crude oils. In Figure 3, the NA distribution for three crudes and one mixture (crude A plus the reference DNA sample) is represented. The amount of NAs expressed as the NAN value (i.e., mg of KOH/g; y axis) is reported as a function of the number of carbon atoms (x axis) and naphthenic rings present in their structure (series from 0 to 6 NRs). Because crude A is not acidic, the addition of the reference DNA mixture was imposed to prepare one crude oil with known naphthenic acidity. The same series with 1 and 2 naphthenic rings and 13−20 C atoms evidenced in the mass spectrum of the reference DNA sample (Figure 2b) seems to dominate the NA distribution of the mixture (top left panel of Figure 3).

Table 2. NAN Variation before and after Corrosivity Tests NAN (mg of KOH/g) name

before

A B C D gas oil + DNA A + DNA A + B + DNA

0.00 0.05 0.41 2.23 1.01 0.36 0.60

after 0.04 0.33 0.97 0.37 0.58

measuring all of the species with acid properties (including also inorganic acids, phenols, sulfides, and esters) and the latter specifically measuring the acidity of carboxylic acids only. For this reason, NAN values are always lower that TAN values. The NA distribution was evaluated from the negative molecular ion profile obtained by ESI(−)−MS and represented with a series of NAs corresponding to the general formula of CnH2n + zO2 and with hydrogen deficiency (z = −2H) defining the number of naphthenic rings in the structure.6,9,33−35 The m/z values of the negative ions are used to assign the structure to each NA, while the percentage of each NA is determined by the relative abundance of each NA molecular ion, considering all homologous series having equal ionization sensitivity. The map build up from the number of carbon atoms (n°C) and naphthenic rings (NRs, i.e., z or two hydrogen deficiency per naphthenic ring) described the NA distribution for each sample. The NAs eluted according to the molecular weight, and the sum of the absolute intensity (i.e., number of ionic counts) of 1703

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Figure 2. (a) TIC profiles of the reference DNA sample at the concentrations of (a) 24.00, (b) 12.00, (c) 6.00, (d) 2.40, (e) 0.24, and (f) 0.024 μg/ mL in gas oil. (b) Correlation between the area of the TIC profiles (concentrations a−f) and the acidity (NAN, mg of KOH/g) of the reference DNA sample. (c) Mass spectrum of the reference DNA sample in the elution range of 3−9 min with the absolute intensity (ionic counts) on the y axis and m/z values on the x axis.

consequently, a minor acidity concentration per single refinery unit, especially those working in the range of 200−400 °C.12,36 3.2. Evolution of NAs after Corrosivity Tests. The influence on the corrosion rate of the operating parameters (e.g., temperature and fluid velocity) of the refinery units and the physicochemical properties (e.g., TAN and S content) of crude oils was described in several papers.8,36−38 The complexity of the factors affecting corrosion processes limits the absolute value of the laboratory measurements and the comprehension of the mechanism, to which NAs are subjected. The approach described and applied to the creation of NA maps in crude oils was also applied to highlight differences in NA distribution before and after corrosivity tests. In Table 2, the NAN values are reported for the series of crude oils and mixtures with different TANs and S contents, before and after corrosivity tests. The evolution of the NAs after corrosivity tests is expressed as a variation of the concentration (NAN, mg of KOH/g) of the species grouped by number of C atoms and naphthenic rings. This difference was determined at different times (in days) of the corrosivity tests with respect to the native crude (i.e., time 0) to follow the changes in NA composition for the mixture of gas oil and reference DNA sample and B and C crude oils (Figure 4).

In contrast with this mixture and according to Figure 3, the most pronounced NAs for crude B are those associated with 0 and 1 naphthenic rings and 12−20 C atoms in the structure. The NA distribution in crude C seems more similar to the DNA sample than to the other crudes: the main NAs have 2 and 3 naphthenic rings in their structure, and their masses are located in the range of 14−22 C atoms, while NAs with different types of structure, i.e., linear or with a larger number of naphthenic rings, prevail in the 24−28 C atom range. The different NA distributions of these two crudes in both the number of C atoms and naphthenic rings agrees with the general properties of these crudes, as commonly reported in the crude assays published on the Web. In crude D, the NAs with 0 and 2 naphthenic rings mainly contribute to the organic acidity, although other structures (acyclic and with 4 or 5 naphthenic rings) are also represented in Figure 3. The possibility to draw NA maps of crude oils is not only for academic interest, because it appears substantial for predicting which sections of refinery plants may be subjected to corrosion during crude processing. In fact, in the refinery, a low TAN crude feed does not always mean that corrosion will not occur because of the presence of NAs and very low amount of sulfur in defined petroleum fractions.1 Vice versa, crude oils with high TAN values may show limited corrosion effects when NAs show a wide distribution along the boiling point cuts and, 1704

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Figure 3. NA distribution for (a) mixture A + DNA and (b) B, (c) C, and (d) D crude oils. The amount (expressed as NAN, mg of KOH/g, in the y axis) of NAs is depicted versus the number of C atoms (x axis) as a function of the series corresponding to the number of naphthenic rings (NRs), labeled by colors and markers: (blue diamond) 0 NR, (crimson square) 1 NR, (red triangle) 2 NRs, (green circle) 3 NRs, (violet diamond) 4 NRs, (brown circle) 5 NRs, and (black square) 6 NRs.

In the temperature range of NA corrosion (200−400 °C, with a maximum at 370 °C), NAs can decompose producing CO2, causing a change of TAN and corrosiveness with time.39 The experimental device applied in this work for corrosivity measures avoids the continuous heating of the sample during the test (up to 10 days) and, therefore, suppresses the thermal decomposition of NAs (as confirmed by TAN and NAN measures), while the coupons remain constantly at the fixed reaction conditions. In this way, changes in NA distribution may only be ascribed to their interaction with iron to form an initial layer of corrosion products and later oil-soluble iron naphthenates.36,39 The NA corrosion process is controlled and can be divided into four steps: (i) transportation of the NAs toward the metal surface, (ii) adsorption of NA at the surface active sites, (iii) reactions at the active surface sites, and (iv) dissolution of corrosion products from the metal surface.40 The first step is mainly a function of the flow rate of the crude oil in the device that is experimentally controlled. The adsorption of the acid on the metal surface depends upon the physicochemical properties of the single NA, increasing with molecular weight up to 9 C atoms and then decreasing because of steric hindrance limitations.8 The crude oils studied in this work do not contain any light NAs (with less than 9 C atoms); therefore, accelerated adsorption because of this kind of NA is negligible. As far as reactivity of acyclic NAs with iron atoms is concerned, when the molecular weight increases, the corresponding acidic constant diminishes (i.e., it becomes less aggressive toward iron) and the solubility in oil of the corresponding iron naphthenates increases.41 The presence of naphthenic rings in the structure seems to favor the dissolution of iron salts with respect to acyclic species.41 The experimental

The NA evolution in the mixture of gas oil and reference DNA (Figure 4a) shows a progressive decrease of the species with two naphthenic rings (2 NRs) in the structure, while NAs acyclic and with 1 and 6 NRs in the structure increase. As far as the amount of NAs according to their number of C atoms is concerned, the species with 16 and 18 C atoms increase to the detriment of other species. NAs with 2 NRs and 11−17 C atoms seem to be the most reactive in corrosivity tests and undergo structural transformation. It is interesting to note that the NAs showing the greatest variations in amount are not necessarily those with the highest concentration in the native sample; in fact, in the reference DNA sample, NAs with one naphthenic ring in the structure are present to the highest extent. The evolution of NAs in crude B (Figure 4b) also involves the 2 NR species, although those with one and three naphthenic rings show a stronger decrease, in favor of species either acyclic or with a higher number of NRs. The NAs with 15−19 C atoms decrease in amount, while lighter species develop. Also, in this crude oil, the NAs showing the highest evolution are not those more represented in the native distribution. The analysis of crude C (Figure 4c) confirms the decrease of 1−3 NR species and the formation of acyclic species, while the evolution of NAs as a function of the C atoms is not welldefined. As found with the other samples, the NAs that are more involved in the transformation after corrosivity tests are not necessarily those more concentrated in the crude oil, i.e., series with one and two naphthenic rings and a high number of C atoms. The possibility to rationalize this evidence is challenging. We have to start from a series of statements from the literature. 1705

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Figure 4. NA evolution during corrosivity tests. The variation of the amount (NAN, mg of KOH/g) of NAs after different times (days) of corrosivity tests with respect to the native sample (time 0) is depicted as a function of the number of naphthenic rings (NRs, left) and C atoms (n°C, right) for (a) one mixture of gas oil and the reference DNA sample and (b) B and (c) C crude oils. The variation is determined at short (3−5 days, blue bar) and long (8−9 days, brown bar) corrosivity times.

evidence of Figure 4 agrees with this statement, because the species more involved in the transformation are those with 1−3 naphthenic rings, i.e., the more easily dissolved naphthenates. In fact, the rate-determining step of corrosion is proposed to be the dissolution of iron naphthenates in oil, with the solubility of a given naphthenate mainly contributing rather than the acidity of the parent acid.41 Iron naphthenates are proposed to undergo thermal decomposition at temperatures higher than 300 °C (or 200 °C in the case of long linear acids),41 although the stability of the bonds of −COO− groups and FeII cations in complexes between organic acids and metal ions is particularly high.42 In agreement with literature data, we never observed the presence of iron naphthenate as a corrosion product on the surface of coupons. On the basis of experimental evidence and literature statements, the transformation of NAs is proposed, with the NA−Fe complexes formed during the corrosion process undergoing thermal transformation of the aliphatic moiety. In fact, because the carboxylic group acquires high stability in the COO−Fe bond with respect to the aliphatic chain,43 the aliphatic portion of the NA may be subject to thermal transformation, changing the distribution of naphthenic rings, with a progressive decrease of the species with 1−3 naphthenic rings in the favor of branched aliphatic chains and species with a higher number of naphthenic rings. The release of NAs (the only species detected by IR and LC−MSD analyses) from iron naphthenates may be obtained

during corrosivity tests by reaction with H2S, according to the conventional mechanism of NA and S corrosion.32 Fe 0 + 2RCOOH → Fe(RCOO)2 + H 2 Fe(RCOO)2 + H 2S → FeS + 2RCOOH

In fact, when S is present in the crude oil, a more complex picture has to be considered, because NA and S participate to corrosion with different corrosion mechanisms and morphologies of the attack.12 Sulfur corrosion causes wastage of the exposed surface with the formation of an iron sulfide corrosion layer, insoluble in oil, while NA corrosion causes a more localized attack with oil-soluble corrosion products that progressively expose fresh metal. The mechanism of NA corrosion was related to the loss of the protective sulfide film on the metal surface, because of either chemical dissolution or mechanical action of the flowing fluids.12 As far as our evidence is concerned, the content of S does not interfere with the nature of the NA transformation. In fact, the reference DNA sample (without S), crude C (crude oil with a low S content), and crude B (crude oil with a high content of S) show similar transformations of NAs: species with 1−3 naphthenic rings and 14−17 C atoms preferentially undergo dissolution and are eventually transformed to iron naphthenates, before being released as transformed NAs. It is also possible that, during the extraction procedure of NAs from crude oils, NAs are released from naphthenate 1706

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complexes by reaction with NH4OH in an aqueous environment.

4. CONCLUSION One protocol combining IR and LC−MSD techniques was applied for identification and quantification of NAs 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−MSD technique by ESI(−). The identification of the NAs was obtained from the corresponding molecular ions, according to the number of C atoms and naphthenic rings. The distribution of the single NAs was calculated from the intensity of the MS peaks in the range of elution of NAs, linearly correlated with the concentration of NAs as measured by IR spectroscopy. NA maps of crude oils were obtained with indication of the main characteristics (number of C atoms and naphthenic rings) of the most abundant NAs and the possible prediction of the processes and sections of refinery more involved in NA corrosion. The same kind of NA maps were drawn for crude oils coming from corrosivity tests. The comparison between the two sets of NA maps (before and after corrosivity tests) allows for evaluation of which NAs are mainly involved and transformed during the process. The following reaction mechanism is proposed. NAs with 1−3 naphthenic rings and 14−17 C atoms preferentially undergo dissolution and eventually transformation as iron naphthenates, before being released as NAs. Information about the evolution of the NAs after corrosivity tests could help prevent corrosion and fouling in refinery by focusing the attention on the specific root causes.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +390252046678. Fax: +390252036347. E-mail: cristina.fl[email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Energy & Fuels

Article

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