In Situ Chemical Imaging of Asphaltene Precipitation from Crude Oil

Article pubs.acs.org/EF

In Situ Chemical Imaging of Asphaltene Precipitation from Crude Oil Induced by n‑Heptane Anton A. Gabrienko, Chen H. Lai, and Sergei G. Kazarian* Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom ABSTRACT: Crude oil fouling in heat exchangers is one of the most challenging problems in a petroleum refinery. The fundamentals of the complex fouling process are not fully understood, which leads to inefficient prevention of deposition in heat exchangers. From an industrial point of view, it is difficult to predict the precipitation of some constituents from crude oil under particular conditions and to create precautionary measures. ATR-FTIR spectroscopic imaging is a powerful tool for providing visualization and chemical analysis when applied to study a range of dynamic systems and samples. Here, we report an ATRFTIR spectroscopic imaging approach for in situ visualization and chemical characterization of the formation of deposits from crude oil. To demonstrate ATR-FTIR spectroscopic imaging capabilities, experiments on n-heptane induced precipitation from Tatarstan crude oil have been performed. The dynamics of precipitation induced by n-heptane have been monitored starting from the formation of small particles on the measuring surface of the ATR crystal followed by their growth and aggregation. The deposits formed have been chemically characterized using extracted ATR-FTIR spectra. Asphaltenes have been identified as the main components of the observed deposits. This study has demonstrated the feasibility and potential applications of ATR-FTIR spectroscopic imaging to understand how the molecular and chemical structures of the deposits relate to the components of the crude oil. This information about asphaltene deposits will provide new insight and understanding about the fouling process.

1. INTRODUCTION Fouling of heat exchangers is one of the most serious problems in crude oil production, transportation, and refining nowadays. This undesirable process results in significant energy consumption, increased maintenance costs, and reduction of or loss of production. Therefore, the petroleum industry expects researchers to provide a clear understanding of the various fouling mechanisms and a proven recipe for mitigation. However, in spite of good progress in experimental studies,1−11 the mechanisms of crude oil fouling and possible ways of its prevention are still not fully understood. Asphaltene precipitation and deposition has been closely linked and related to crude oil fouling.1,11,12 Asphaltenes are defined as one of the heaviest fractions of crude oil, soluble in aromatic solvent, such as benzene and toluene, but insoluble in normal alkanes, for example, n-pentane or n-heptane.13 Asphaltenes mainly consist of alkyl substituted condensed or polycyclic aromatic structures joined by flexible aliphatic chains (rosary- or archipelago-type) or bridged by aliphatic rings (continental-type).14−17 The minor components of asphaltenes could be presented by heteroatomic compounds such as pyridine, thiophene, pyrrole, sulphoxide, carbonyl, ether, and ester.14,16,18,19 Thus, asphaltenes are the complex molecular structures, which tend to form aggregates that flocculate and can precipitate under certain physicochemical conditions. In general, temperature, pressure, and compositional changes are assumed to have an influence on the stability of crude oil, i.e. on asphaltene aggregation and precipitation.1,12 However, the effect these factors have on precipitation process is controversial. For example, high temperatures can result in either an increase1 or a decrease20 of asphaltene solubility in crude oils. High pressure CO2 can act as a flocculation inhibitor21 or asphaltene precipitant.22,23 Nevertheless, it could be stated that all these tendencies strongly depend on the © 2014 American Chemical Society

properties of crude oil, for example, ratio between light and heavy fractions, content of sulfur in asphaltenes, and others. Moreover, it is apparent that there is also a relationship between crude oil fouling and rheology, chemical reactions, surface properties, heat transfer, and other phenomena. A principal understanding of the aggregation and precipitation of asphaltenes in crude oils, as well as asphaltene chemical characterization at different stages, are key points for the development of preventive measures to avoid potential problems during oil production, transport, and refining. From this point of view, it is clear that a more fundamental approach to the problem is needed. To inquire into the mechanisms of fouling, one should have new data, especially for the initial stages of asphaltene precipitation. Fourier transform infrared (FTIR) spectroscopy is intensively used for the chemical characterization of crude oils and asphaltenes.24−30 The methodology to predict asphaltene content in crude oil, based on FTIR spectroscopic identification of particular functional groups, has been recently developed.31−34 Much attention has been paid to determining sulfur content and its chemical state, the two factors that influence the effectiveness of sulfur removal from crude oil.31,32,35,36 For the past decade, FTIR spectroscopic imaging, a technology which combines an analytical methodology for chemical characterization of various materials and the possibility to visualize dynamic processes, has been intensively developed.37−41 This approach has received wide acceptance as the result of its main advantages: relatively high sensitivity, fast acquisition times, the possibility to measure highly absorbing materials, little sample Received: November 15, 2013 Revised: December 31, 2013 Published: January 8, 2014 964

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range of 3900−500 cm−1 were registered with a Bruker Equinox-55 spectrometer equipped with a diamond ATR accessory (Specac, Ltd., U.K.) and with DTGS and MCT detectors. A spectral resolution of 4 cm−1 and 128 scans were used. For ATR-FTIR spectroscopic imaging experiments, an IFS 66/S step-scan FTIR spectrometer (Bruker Optics) with a macrochamber extension and a 64 × 64 focal plane array (FPA) detector was used. A diamond ATR accessory (Imaging Golden Gate, Specac, U.K.) was positioned in the macrochamber and carefully aligned prior to the macro-ATR-FTIR imaging measurements. The sample area measured by this macro-ATR spectroscopic imaging approach is 610 × 530 μm2. The number of scans accumulated for each spectrum was 64 for a better signal-to-noise ratio. ATR-FTIR spectra were registered from 1800 to 900 cm−1 with a resolution of 8 cm−1. For in situ imaging experiments, a custom designed cell was made from stainless steel (Figure 1). This cell allows crude oil and additives,

preparation, and the possibility to perform in situ experiments. FTIR spectroscopic imaging can be described as “chemical photography”42 of materials at a microscopic level because it obtains chemical information from different areas in the sample simultaneously using infrared array detectors. Each pixel of such the array measures an FTIR spectrum, thus the distribution of absorbance of characteristic spectral bands provides a chemical map showing the spatial distribution of components within the sample. Attenuated total reflection Fourier transform infrared (ATRFTIR) spectroscopic imaging has been successfully applied to various samples and systems, such as polymers,43−47 high pressure gases,46,47 drug release,42,48−50 biomaterials,39,47 paintings,50 latent fingermarks,51 and heat exchanger deposits.11 Tay and Kazarian have analyzed deposits extracted from crude oils and obtained from heat exchangers with macro- and microATR-FTIR spectroscopic imaging.11 Asphaltenes, carbonates, sulphates, sulphoxides, coke, and oxalates were detected in petroleum deposits. It was demonstrated that ATR-FTIR spectroscopic imaging could be effectively used for studying such materials as solid petroleum deposits. In that study, the ATR-FTIR imaging approach has not been applied for the in situ analysis of constituents precipitated from crude oil, which would be required for further understanding of the fouling processes. In the present paper we report, for the first time, chemical imaging results on the in situ precipitation of deposits from Tatarstan crude oil induced by n-heptane. The tendency of crude oil to be unstable in the presence of light alkanes, such as n-heptane, is a well-known cause for operational problems in the oil industry.52,53 Thus, n-heptane induced precipitation could be the way of studying crude oil fouling in laboratory conditions.1 We performed several ATR-FTIR spectroscopic imaging experiments of n-heptane induced precipitation. First of all, the mixture of n-heptane and benzene solution of deposits extracted from Tatarstan crude oil was analyzed with ATR-FTIR spectroscopic imaging. Then, experiments on induced precipitation from Tatarstan crude oil were carried out. In both cases, the precipitation process was visualized and the deposits formed were chemically analyzed using selected ATR-FTIR spectra. This work demonstrates the feasibility of in situ chemical imaging for studying such processes for a broader range of oil samples.

Figure 1. Schematic diagram of custom designed stainless steel cell and a diamond accessory for in situ ATR-FTIR imaging of the precipitation of deposits from crude oil induced by n-heptane. for example, n-heptane, to be isolated to prevent their evaporation or interaction with atmospheric gases. The PTFE O-rings were used for sealing between the diamond accessory and the cell and between the cell and the stopper. The experiments on the precipitation of crude oil deposits were carried out as follows: 30 μL of extracted deposits solution or Tatarstan crude oil was placed on the top of the measuring surface of diamond. ATR-FTIR spectra of the initial solution or crude oil were registered. Then 90 μL of n-heptane were added to either benzene solution or crude oil to induced the precipitation of deposits. The cell was covered by the stopper followed by fixing with the anvil by pressing from top. ATR-FTIR spectra of the mixtures were registered immediately and after given periods of time.

3. RESULTS AND DISCUSSION Prior to imaging experiments, the samples of Tatarstan crude oil and extracted deposits have been characterized with ATRFTIR spectroscopy to reveal characteristic spectral bands that can be further used for ATR-FTIR spectroscopic imaging of deposit detection and visualization during precipitation from benzene solution or crude oil. The ATR-FTIR spectra of Tatarstan crude oil and extracted deposits presented in Figure 2 show some similarities and differences between crude oil and extracted deposits. The following spectral bands have been observed for samples of crude oil and extracted deposits:30,34,56 major bands at 2952/ 2866 cm−1 and at 1444/1367 cm−1 that were assigned to asymmetric/symmetric stretching and bending of methyl groups, respectively; bands at 2921/2852 cm−1 and at 1457/ 1376 cm−1 that were assigned to asymmetric/symmetric stretching and bending of methylene groups, respectively; a minor broad band at around 3060 cm−1 and bands in the region of 865−745 cm−1 that can be assigned to aromatic C−H stretching and out-of-plane bending modes, respectively. The main difference between the spectra of crude oil and extracted

2. EXPERIMENTAL SECTION 2.1. Samples and Materials Characterization. The samples of crude oil originated from Tatarstan (Russia) and extracted deposits were kindly provided by Prof. O. N. Martyanov from Boreskov Institute of Catalysis. The deposits were extracted from crude oil by the described procedure using benzene instead of toluene as a solvent.54 The samples of crude oil and deposits were analyzed by chemical analysis, FTIR, and 1H NMR spectroscopy.55 It was observed that the sulfur content in crude oil is 4.1 wt %. According to FTIR and 1 H NMR data, extracted deposits mainly consist of asphaltenes (8.2 wt % of sulfur). A benzene solution of extracted deposits was prepared by the following method. 500 μL of benzene was added to 20 mg of extracted deposits. The mixture was intensively shaken for a few minutes and then left overnight for better dissolution. The freshly prepared solution was further used for ATR-FTIR imaging experiments. Benzene (99.9% purity) and n-heptane (≥99.0% purity) were purchased from VWR International Ltd. and were used without further purification. 2.2. ATR-FTIR Spectroscopy and Spectroscopic Imaging. First, FTIR spectra of Tatarstan crude oil and extracted deposits in the 965

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Figure 2. ATR-FTIR spectra of Tatarstan crude oil (a) and extracted deposits (b).

deposits is in the region of 1800−650 cm−1. The spectrum of extracted deposits is characterized by the higher absorbance of bands at 1600 (aromatic CC stretching mode), 1305 (aromatic CC stretching mode), and 1154 (aromatic C−H in-plane bending mode) cm−1 from polycyclic aromatic hydrocarbons (Figure 2b).57 The band at 1693 cm−1, due to the stretching mode of carbonyl CO groups, was detected in the spectrum of crude oil (Figure 2a). At the same time, the spectrum of extracted deposits shows the band at 1650 cm−1 from alkenes CC bond stretching. The overlapping bands in the 1070−1010 cm−1 region of both spectra can be assigned to sulphoxide groups. The presented spectrum of the extracted deposits is similar to the IR spectra of different asphaltenes registered previously.11,24−34 According to spectral data obtained, several characteristic chemical constituents have been detected for extracted deposits: polycyclic aromatic hydrocarbons, sulphoxides, and methyl and methylene groups. These compounds and functional groups are typical for asphaltenes.14,15,18,58 Thus, extracted deposits used in this study are mainly asphaltenes, as was stated by Kozhevnikov et al.55 The comparison of the spectra of crude oil and extracted deposits allowed us to choose the most appropriate spectral band for analysis of imaging data to be described further. So, for reliable observation and identification of the precipitation of deposits from crude oil, the absorbance of the band between 1650 and 1550 cm−1 was used, as it is the most intense characteristic band for the deposits. Previously, we have quantitatively demonstrated that the imaging approach using infrared array detectors provides an enhancement in the sensitivity for FTIR spectroscopy in a single measurement if the studied trace material is distributed in a localized area of the sample.59,60 Individual pixels of the infrared array detector can obtain an almost pure spectrum of a trace material from the localized area without contribution of other materials present in the studied sample. Therefore, this chemical imaging approach has a significant advantage for detecting trace materials present in crude oil deposits, as demonstrated here. The FPA detector (64 pixels × 64 pixels) used for imaging measurements registers a mid-IR spectrum at each pixel. Images are created from registered spectra by allocating a color to each pixel based on the integrated absorbance of the particular spectral band and then plotting its distribution for all pixels to create a 2D map. As was described

above, a band between 1650 and 1550 cm−1 has been used to create the images shown in Figures 3 and 5 by plotting the

Figure 3. In situ macro-ATR-FTIR spectroscopic images of the benzene solution of extracted deposits: the initial solution before the addition of n-heptane (a), the solution after the addition of n-heptane (b), the solution/n-heptane mixture after 10 min (c), 120 min (d), and 400 min (e). Images are based on the integration of the spectral band between 1650 and 1550 cm−1. The imaging area is ca. 610 μm × 530 μm.

distribution of the integrated absorbance of this band. This band characterizes the presence of deposits in the sampling area. The red color represents a high integrated absorbance, and blue color represents a low integrated absorbance, i.e. high or low concentrations, respectively, for a particular chemical component (for example, asphaltenes). First, the benzene solution of extracted deposits was used as a model system for in situ ATR-FTIR spectroscopic imaging of precipitation induced by n-heptane (Figure 3). The image of the initial benzene solution of extracted deposits shows no 966

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Figure 4. In situ macro-ATR-FTIR spectroscopic image of the benzene solution of extracted deposits and n-heptane, 120 min after mixing (a). The ATR-FTIR spectra were extracted from two locations: blue (b) and red (c). Black solid arrows point to the spectral bands of benzene. Black dashed arrows point to the absorbance bands of n-heptane. Blue dashed arrows point to the spectral bands that were assigned to polycyclic aromatic hydrocarbons. Red arrows point to the spectral bands that were assigned to thiophene. Green arrows point to the spectral bands that were assigned to pyridine and pyrrole. The purple arrow points to the spectral band that was assigned to sulphoxide. Orange arrows point to the spectral bands that were assigned to ethers.

Figure 5. In situ macro-ATR-FTIR spectroscopic images of the Tatarstan crude oil and n-heptane mixture: the initial crude oil before the addition of n-heptane (a), crude oil after the addition of n-heptane (b), crude oil/n-heptane mixture after 10 min (c), 60 min (d), 120 min (e), and 240 min (f). Images are based on the distribution of integrated absorbance of the band between 1650 and 1550 cm−1. The imaging area is ca. 610 μm × 530 μm.

particles (Figure 3a). So, extracted deposits were fully dissolved in benzene during solution preparation. However, after nheptane was added to the solution, the appearance of several particles was detected (Figure 3b). Thus, the precipitation of deposits from benzene solution was induced by n-heptane. Furthermore, the increase in the number and size of particles was observed after 10−400 min of n-heptane interaction with benzene solution (Figure 3c−e). So, the ATR-FTIR spectroscopic imaging approach can be used for in situ detection and observation of the precipitation of crude oil deposits. Moreover, it is possible to study the dynamics of the precipitation process using this approach. The ATR-FTIR spectroscopic imaging approach allows not only visualization of the precipitation of deposits but also

performance of chemical analysis of particles formed. Figure 4 presents the image of an n-heptane/solution mixture after 120 min of interaction (Figure 4a) and two selected ATR-FTIR spectra extracted from different locations of the image (Figure 4b,c). The spectrum extracted from the indicated location in blue area presents a spectrum of an n-heptane/benzene mixture (Figure 4b). The spectrum extracted from the indicated location in the red area reveals a spectrum of a particle precipitated from benzene solution upon interaction with nheptane (Figure 4c). The spectrum extracted from a location in the blue area of the image indeed contains only spectral bands of benzene and n-heptane (Figure 4b). Careful comparison of two selected spectra allows the identification of several spectral bands that belong to precipitated species. The spectral bands at 967

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1600, 1330, 1298, 1241, 1151, 949, 932, and 916 cm−1 were detected (Figure 4c). These spectral bands could be assigned to various vibrational modes of polycyclic aromatic hydrocarbons: aromatic CC stretching (1600, 1330, 1298, and 1241 cm−1), aromatic C−H in-plane bending (1151 cm−1), and aromatic C−H out-of-plane bending (949, 932, and 916 cm−1).57 The identification and assignment of other minor bands is difficult due to their low absorbance and overlap with bands of nheptane and benzene (Figure 4c). Nevertheless, bands at 1415, 1347, and 1238 cm−1 could be assigned to thiophene.61 The observation of bands at 1207, 1133, 1076, and 996 cm−1 might indicate the presence of pyridine or pyrrole as the components in deposits.61 Also bands at 1187 and 1109 cm−1 were detected. Such wavenumbers are typical for C−O−C stretching of ethers.34 Finally, bands at 1065 and 1030 cm−1 due to SO stretching modes of sulphoxides are usually observed in the spectra of deposits.30,34,56 The main components of precipitated deposits have been identified with analysis based on extracted ATR-FTIR spectra from the obtained imaging data sets. The compounds detected, namely condensed aromatic hydrocarbons, thiophene, pyridine, pyrrole, ethers, and sulphoxides, are common constituents of asphaltenes.14,15,18,58 These observations and assignments indicate that the main species precipitated from the solution are asphaltenes. Thus, the ATR-FTIR spectroscopic imaging approach was applied for the visualization of n-heptane induced asphaltene precipitation from a model solution. Next, we present results of similar experiments with Tatarstan oil rather than just the solution of deposits in benzene. Figure 5 presents the ATR-FTIR spectroscopic images of the Tatarstan crude oil and n-heptane mixture. The first image (Figure 5a) was measured for pure crude oil before the addition of n-heptane. It was described above (Figure 2a) that the FTIR spectrum of crude oil has a spectral band at 1600 cm−1 (aromatic CC bond stretching) because of the presence of complex aromatic compounds in its composition. Due to this reason, the image of the initial crude oil (Figure 5a) is colored in red and yellow according to the integrated absorbance of the band between 1650 and 1550 cm−1. The next image (Figure 5b) measured immediately after the addition of n-heptane to crude oil shows relatively high integral absorbance of the band between 1650 and 1550 cm−1 over the sample area. However, no changes in crude oil composition or no precipitation occurred, shown by the fact that the average ATR-FTIR spectra (Figure 6a,b) of the sample area extracted from the first two images (Figure 5a,b) are similar to each other. Unlike in the case of precipitation from the benzene solution described above, crude oil mixing with n-heptane results in light fraction dissolution in n-heptane, which is a slow process due to the high viscosity and density of Tatarstan crude oil. Due to this reason, after only 10 min of interaction between crude oil and n-heptane, we have observed some compositional changes (Figure 5c). Crude oil constituents were dissolved in nheptane, shown by the resulting sample area being mainly blue in color (zero integrated absorbance of the band between 1650 and 1550 cm−1 or zero concentration of the corresponding compounds). However, some particles (areas indicated by red color in the images) remained on the surface of diamond. It is reasonable to suppose that these particles are deposits that were precipitated by n-heptane from crude oil. Further, we observed increase and growth of particle numbers and sizes (Figure 5d− f), similar to the dynamics of asphaltene precipitation from benzene solution.

Figure 6. ATR-FTIR spectra of Tatarstan crude oil and n-heptane mixture: initial crude oil before the addition of n-heptane (a), crude oil after the addition of n-heptane (b), crude oil/n-heptane mixture after 10 min (c) and 240 min (d). The ATR-FTIR spectra were extracted from the overall sample area for corresponding images shown in Figure 5a−c and f.

However, one could assume that the observed changes in images (Figure 5c−f) might be due to crude oil deposition on the surface of diamond after mixing with n-heptane. To clarify this question, the average ATR-FTIR spectra, presented in Figure 6c,d, were extracted from whole sample area of corresponding images (Figure 5c,f). These two spectra are different from the spectra of initial crude oil (Figures 2a and 5a) and present the spectral bands of n-heptane and several other bands that could belong to the deposits formed. Thus, the changes we observed are indeed the precipitation of deposits from crude oil induced by n-heptane. It is not obvious from the imaging data (Figure 5) that namely asphaltenes were precipitated from crude oil after nheptane was added. To identify the deposits formed, we have performed analysis of observed particles by the method described previously. Two ATR-FTIR spectra have been extracted from selected areas of the image (Figure 7a) to analyze them. The first ATR-FTIR spectrum (Figure 7b) contains only the spectral bands of n-heptane. The second ATR-FTIR spectrum (Figure 7c) presents spectral bands of precipitated deposits. Comparison of these two ATR-FTIR spectra has revealed the particular spectral bands that could be definitely attributed to deposits. First of all, the spectral bands at 1600, 1333, 1309, and 1235 cm−1, due to the CC stretching modes of condensed aromatic hydrocarbons, and the bands at 1151 cm−1 and at 933, 921 cm−1 due to aromatic C−H in-plane and out-of-plane bending modes, respectively, were identified.57 The spectral bands at 1057 and 1025 cm−1 due to SO stretching modes point to the presence of sulphoxides. Moreover, some minor bands were also detected. However, their assignment is not well-defined. Deposits and crude oil are both complex mixtures of various chemical 968

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Figure 7. In situ macro-ATR-FTIR spectroscopic image of Tatarstan crude oil and n-heptane mixture after 120 min of interaction (a). ATR-FTIR spectra were extracted from two locations: blue (b) and red (c). Black solid arrows point to the spectral bands that were assigned to n-heptane soluble fractions of crude oil. Black arrows point to the spectral bands of n-heptane. Blue arrows point to the spectral bands that were assigned to polycyclic aromatic hydrocarbons. Red arrows point to the spectral bands that were assigned to thiophene. Green arrows point to the spectral bands that were assigned to pyridine and pyrrole. The purple arrow points to the spectral band that was assigned to sulphoxide. These assignments have been substantiated by the corresponding wavenumber for peaks of the observed spectral bands and demonstrated the feasibility of using ATR-FTIR spectroscopic imaging for studies of precipitation from crude oil in situ.

components. All these constituents have particular bands in the fingerprint region of the mid-IR spectrum, and some bands could overlap. The presence of pyridine or/and pyrrole and thiophene might be indicated by the presence of the following absorbance bands: 1205, 1130, 1078, 988 cm−1 and 1347, 1241 cm−1.61 Now it is clear that deposits formed after n-heptane addition to crude oil contain polycyclic aromatic hydrocarbons, sulphoxides, and, probably, thiophene, pyridine, and/or pyrrole. Therefore, in accordance with the obtained data by in situ ATRFTIR spectroscopic imaging, n-heptane induced precipitation of crude oil results in asphaltene formation. To prove our conclusion about chemical identification of deposits formed during in situ induced precipitation, the following experiment was also performed. The cell was opened, and all of the liquid phase was removed with a pipet. The solid deposits that remained were washed with n-heptane several times on the diamond accessory and dried at ambient temperature for 30 min. Then the ATR-FTIR spectrum of deposits was registered. Figure 8 presents the ATR-FTIR spectra of deposits that were formed during in situ precipitation (Figure 8a) and that were extracted from crude oil by n-heptane (Figure 8b; the same spectrum is presented in Figure 2b). These spectra are similar to each other and revealed spectral bands that belong to particular functional groups or constituents: polycyclic aromatic hydrocarbons (1600, 1305 and 1154, 1156 cm−1), alkenes (1680−1650 cm−1), and sulphoxides (1070−1010 cm−1). As was described above, the extracted deposits were identified as asphaltenes based on their ATR-FTIR spectrum (Figures 2b and 8b). The same conclusion can be drawn regarding deposits formed during the in situ experiment. Thus, chemical analysis of particles observed during in situ n-heptane induced precipitation from crude oil was correct. In summary, the experiments performed have shown that ATR-FTIR spectroscopic imaging can be used for studying nheptane induced precipitation of deposits from crude oil. This

Figure 8. ATR-FTIR spectra of deposits formed during in situ induced precipitation from Tatarstan crude oil (a) and extracted from Tatarstan crude oil (b).

approach allows in situ monitoring of the formation and growth of deposit particles during crude oil interaction with n-heptane. Also, the deposits formed can be chemically analyzed using selected ATR-FTIR spectra.

4. CONCLUSION The use of n-heptane to induce precipitation from crude oil has been chosen as a model to develop a new in situ ATR-FTIR spectroscopic imaging approach to study crude oil fouling. ATR-FTIR spectroscopic imaging has been successfully applied for in situ visualization and analysis of the precipitation of deposits from crude oil induced by n-heptane. It has been 969

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demonstrated that this approach can be used to detect the initial stages of precipitation, to monitor the dynamics of deposit particles formation and growth, and to chemically analyze the constituents of the deposits observed. Therefore, ATR-FTIR spectroscopic imaging can be reliably and beneficially applied to obtain information about the crude oil fouling process. For that, crude oil with different origins and fraction compositions could be analyzed in terms of deposit precipitation induced by n-heptane. In such cases, some similarities or differences in crude oil behavior could be found to make a correlation between crude oil properties (chemical composition) and its stability. Demonstration of this methodology also presents an opportunity for the ATR-FTIR spectroscopic imaging approach to be potentially applied for the in situ study crude oil heating or crude oil interaction with high pressure CO2 to inquire into the processes occurring under high temperatures and pressures that could cause fouling. Such chemical imaging data could lead to additional insight into the mechanisms of deposit formation from crude oils or provide information about the main features of fouling processes under various conditions, such as compositional, temperature, or pressure changes. Having such information, one could suggest reliable and effective measures to prevent negative processes from occurring during crude oil production, transportation, and refining. It is hoped that ATR-FTIR spectroscopic imaging will beneficially impact further development in this research to achieve such goals.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Funding

This research was performed under the UNIHEAT project. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was performed under the UNIHEAT project. The authors wish to acknowledge the Skolkovo Foundation and BP for financial support. The authors thank Prof. O. N. Martyanov from the Boreskov Institute of Catalysis (Novosibirsk, Russia) for providing samples of Tatarstan crude oil and extracted deposits and for his advice.

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