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Effect of Temperature and Composition on the Stability of Crude Oil Blends Studied with Chemical Imaging In Situ Anton A. Gabrienko, Oleg Nikolaevich Martyanov, and Sergei G. Kazarian Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.5b01880 • Publication Date (Web): 20 Oct 2015 Downloaded from http://pubs.acs.org on October 21, 2015
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Effect of Temperature and Composition on the Stability of Crude Oil Blends Studied with Chemical Imaging In Situ Anton A. Gabrienko,a Oleg N. Martyanov,a,b Sergei G. Kazarian*,c a
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Prospekt
Akademika Lavrentieva 5, Novosibirsk 630090, Russia b
Department of Natural Sciences, Novosibirsk State University, Pirogova street 2, Novosibirsk
630090, Russia c
Department of Chemical Engineering, Imperial College London, South Kensington Campus,
London, UK SW7 2AZ Corresponding Author *E-mail:
[email protected]. KEYWORDS: chemical imaging, ATR-FTIR spectroscopy, crude oil blend, stability, asphaltene aggregation, precipitation. Abstract: Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopic imaging approach has been applied in situ to investigate the behavior of crude oil blends at different conditions. The ratio between mixed crude oils and temperature has been varied to
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study how these factors influence the stability of the blends. Obtained spectroscopic results have been used to correlate the composition of the blends with respect to the incompatibility of crude oils. The chemical compositions of precipitates formed from the blends at 60 and 200 °C have been examined by FTIR spectroscopy. The change of asphaltene solubility in crude oil blends as well as the precipitation of inorganic salts, sulfates, have been observed at higher temperature. Based on available spectroscopic data, the formation of hydrogen bonds between sulfoxide groups of precipitated inorganic sulfates and hydroxy groups of asphaltenes or similar condensed aromatic species has been proposed. 1. INTRODUCTION Blending of crude oils, particularly lower and higher grade fluids, is used in petroleum industry to improve the properties of feedstock for more efficient refining. However, some crude oils can be incompatible and cause significant fouling due to asphaltene precipitation after mixing.1-5 Knowledge of chemical and physical properties of petroleum fluids, in principal, leads to more efficient prediction of their behavior as well as mitigation of undesirable processes such as fouling. This opens new ways to intensify crude oil production, transportation and processing. For the last several decades, there have been numerous studies that provided better understanding of the composition, structure, properties and thus behavior of crude oils.1,
2, 6-14
However, the
issues, such as mechanism of fouling, interrelation between composition and stability of petroleum fluids, influence of temperature or pressure, have yet to be addressed. Several types of crude oil fouling are possible: crystallization, particulate formation, corrosion, biological and chemical fouling.15 Clearly, the main reason for that is the complexity of the chemical composition of crude oils and interactions between different components. The changes
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of temperature, pressure or chemical composition can lead to the precipitation or deposition of various constituents of crude oil due to due their destabilization.3, 4, 6, 8, 11, 16-18 Crystallization fouling occurs when inorganic salts, such as carbonates, sulphates, chlorides, precipitate during crude oil processing. The salts, if present, should be mostly dissolved in water, emulsified in hydrocarbon media of crude oil, thus forming dispersed brine. However, water exhibits an appreciable solubility in crude oil at elevated temperatures.19 Hence, as crude oil is heated to operating temperature, water from the dispersed brine is solubilized into the oil phase while leaving behind precipitated salts. This leads to the formation of crystalline salts. Hydrocarbon species that have impact on chemical fouling are waxes, asphaltenes, diamondoids and gas hydrates. Different organic constituents of crude oil can be involved in precipitation and deposition or auto-oxidation, polymerization, thermal decomposition and cracking reactions.2,
5, 13, 15, 20
Among others, asphaltenes attract much attention as their
precipitation and deposition cause significant problems for petroleum industry.6,
9, 14, 21
Asphaltenes are usually defined as a fraction of crude oil that contains heaviest hydrocarbons of the stock soluble in aromatic fluids and insoluble in medium-chain paraffins.22 The molecular structure and molecular weight of asphaltenes and the way of their stabilization in crude oil are still debatable,2, 23, 24 but it is generally accepted that the influence of asphaltenes on fouling is very important and sometimes it can overcome the contribution of other crude oil components.25 Asphaltenes can be forced to flocculate, form agglomerates and thus precipitate by different factors, such as flow velocity, bulk and surface temperatures, particulate concentrations, pressure and composition. At particular conditions, asphaltenes can become incompatible with other components of crude oil resulting in the loss of aggregation stability.
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There is no doubt that the mechanism of crude oil fouling is complex and such processes as crystallization of inorganic salts, deposition of particulates, asphaltenes precipitation, and coke formation can contribute to it. This assumption is supported by the fact that deposits obtained at conditions of crude oil refinery, for instance, in the heat exchangers, show complex chemical composition. Such deposits consist of inorganic salts and particulates, complex organic compounds and coke.26 Therefore, the fouling is a complex process involving destabilization of different crude oil components and thus occurring via different mechanisms. Thus, the understanding of how the conditions, used for crude oil processing, can affect the stability of different constituents is crucial for further improvement of existing models used for describing crude oil behavior. The attenuated total reflection infrared (ATR-FTIR) spectroscopic imaging or chemical imaging approach27 has already demonstrated its advantages and feasibility to study various dynamic systems and samples corresponding to different fields of science: drying of latex films,28 biotherapeutic antibodies,29 tablet dissolution and drug release,30-34 crude oils and deposits,26,
35-37
proteins,38,
39
polymers,40-44 biomaterials.45-47 This approach allows one to
investigate heterogeneous samples with high spatial resolution (10–15 µm for macro ATR-FTIR spectroscopic imaging mode) and obtain chemical information about the components of the sample and processes occurring in the dynamic systems. The important feature of this method is the ability to perform measurements as a function of time using in situ regime for various samples at high temperatures and pressures. Hence, ATR-FTIR spectroscopic imaging can potentially provide valuable information about crude oil behavior at different conditions in situ. In this work, the blends of crude oils have been studied using ATR-FTIR spectroscopic imaging in situ to obtain information about possible mechanisms of fouling, which is the first
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study of its type to the best of our knowledge. The behavior of the blends having different proportions of crude oils have been monitored at 60 and 200 °C to investigate the effect of temperature and composition on the stability of the blend and mechanism of fouling. Additionally, similar experiments have been performed with crude oil samples after desalting procedure to validate the proposed interpretation of obtained spectroscopic data on possible role of inorganic salts in the process of deposit formation. 2. EXPERIMENTAL SECTION Samples and materials characterisation. The properties of crude oil samples used in this work are listed in Table 1. One light (C) and two heavy crude oils (A, B) were studied. The analysis of saturates, aromatics, resins, asphaltenes content in the crude oil samples was performed based on the SAR-AD method from Western Research Institute.48, 49 The method used for elemental analysis (metals) is ASTM D-5708. The determination of nickel, vanadium, silicon, aluminum and iron, as well as calcium and sodium, in crude oils was performed with Inductively Coupled Plasma (ICP) Atomic Emission Spectroscopy. The experiments on crude oils behavior at elevated temperatures were performed with both pure crude oils A, B, C and blends consisting of these crude oils. Two pairs of light and heavy crude oils (A+C and B+C) were used to prepare blends having different proportions of initial crude oils. For the blend of crude oils, the partial volume of crude oil C was varied from ca. 5 to 95 vol. %. For the measurements with ATR-FTIR spectroscopic imaging approach, the blends of required ratio were prepared by the mixing of certain volumes of light and heavy crude oils followed by stirring during 10 min. The overall volume of the blend used for each measurement was 140 µL. Only freshly prepared blends were studied.
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Table 1. Chemical composition of studied crude oil samples.
Crude oil sample
Composition (wt%) A
B
C
Saturates
24.0
18.4
44.3
Aromatics
61.9
60.6
53.3
Resins
8.7
12.1
1.7
Asphaltenes
5.3
8.9
0.9
Sulfur
2.2
3.8
0.4
API gravity
19.6
21.5
39.5
Metal content (ppm)
A
B
C
Ca
8.9
1
9.5
Na
