Study of Asphaltene Precipitation in Crude Oils at Desalter Conditions

Article pubs.acs.org/EF

Study of Asphaltene Precipitation in Crude Oils at Desalter Conditions by Near-Infrared Spectroscopy Denisson Santos,† Elvio B. M. Filho,† Raul S. Dourado,† Monique Amaral,† Sofia Filipakis,‡ Lize M. S. L. Oliveira,‡ Regina C. L. Guimaraẽ s,‡ Alexandre F. Santos,§ Gustavo R. Borges,† Elton Franceschi,† and Cláudio Dariva*,† †

Center for Study on Colloidal Systems, Núcleo de Estudos em Sistema Coloidais (NUESC)/Instituto de Tecnologia e Pesquisa (ITP), Pós-Graduaçaõ em Engenharia de Processos (PEP)/Universidade Tiradentes (UNIT), Avenida Murilo Dantas 300, Aracaju, Sergipe 49031-490, Brazil ‡ PETROBRAS/CENPES/PDISO/AP, Avenida Horacio de Macedo 950, Cidade Universitária, Rio de Janeiro, Rio de Janeiro 21941-915, Brazil § Department of Chemical Engineering, Paraná Federal University (UFPR), Polytechnic Center (DTQ/ST/UFPR), Jardim das Américas, Curitiba, Paraná 82530-990, Brazil S Supporting Information *

ABSTRACT: The asphaltene precipitation and deposition cause severe operational, economic, and environmental problems to the petroleum industry in all of its streams, such as pipe clogging, emulsion stabilization, and incrustation in separators. It is induced by temperature, pressure, and composition changes at crude oil production and refinement. The crude oil incompatibility is the main reason for asphaltene precipitation at refinery plants. Although it has been the subject of studies for the last 50 years, there is still a lack of robust and reliable tools that can detect and monitor the asphaltene precipitation online. In this scenario, the present work aims to use near-infrared spectroscopy to analyze the oil compatibility regarding asphaltene stability as well as to study the influences of pressure, temperature, and composition variation at desalter conditions. For this, a near-infrared spectrometer equipped with a transflectance probe connected to a variable volume high-pressure cell was used. The results showed that the temperature, pressure, and rate of flocculant agent addition on the crude oil presented a small influence on the asphaltene precipitation onset at the experimental range studied. The interaction among the compounds in oil blends was the only factor that changed the asphaltene stability level.

1. INTRODUCTION It is commonly accepted that the asphaltenes are not a welldefined group of chemical compounds but a class of solubility. Therefore, it is considered as the molecules originated from the less volatile portion of the crude oil that is insoluble in nalkanes and soluble in aromatics.1−3 This classification as a solubility class is imprecise but convenient because it provides enough chemical restrictions for both the operational and scientific senses.4 It is also known that the molecular structure of asphaltenes contains polycondensed aromatic rings with side chains and a large variety of heteroatoms, such as nitrogen, sulfur, oxygen, and metals.5 It confers high molecular weight and polarity to the asphaltenes. These characteristics can promote the asphaltene aggregation.6−9 The aggregation, precipitation, and flocculation of asphaltene have been studied during the last 50 years. However, the understanding of this phenomenon is still a controversial matter, even though it has a huge economic importance to the petroleum industry. Such an understanding will facilitate the development of more efficient mitigation techniques. The main issue is related to the asphaltene nature into the crude oil.10−13 Some studies state that those molecules are completely dissolved in the oil.14 On the other hand, other authors defend that asphaltenes are found as suspended colloidal particles, in which the resins rule the colloidal stabilization.15 © 2017 American Chemical Society

The precipitation is generally divided into four stages. The first level is the nucleation, in which asphaltene clusters are formed. As the clusters grow, they will generate larger aggregates that will flocculate into fractal structures. The flocs destabilize, precipitate, and then form deposits that will generate severe problems, such as well and pipe clogging, the formation of stable emulsions, wax precipitation, and incrustation in separators.16−18 Variations in the temperature, pressure, and oil composition during oil production are the key factors that dictate the asphaltene stability level.7,19−21 For instance, the chemical composition alteration promoted by blending of incompatible dead oils at refineries tends to destabilize the asphaltenes. In short, incompatible crude oils are those that produce precipitated asphaltenes when mixed, whereas those oils that keep the asphaltenes stable after mixing are called compatible oils.22 The asphaltene precipitation caused by oil incompatibility will affect the oil characteristics as well as the quality of some refined products, such as fuel oil and marine diesel. Macroscopic properties (e.g., density and viscosity) of both the crude oil and the products are affected by the dispersion degree Received: February 27, 2017 Revised: April 18, 2017 Published: April 19, 2017 5031

DOI: 10.1021/acs.energyfuels.7b00602 Energy Fuels 2017, 31, 5031−5036

Article

Energy & Fuels of the asphaltene particles.23−26 The oil compatibility issue regarding asphaltene stability has been the subject of recent studies. However, there is still a lack of robust tools that allow for the observation of asphaltene behavior in crude oil medium instead of model systems, without sampling step or asphaltene extraction, because oil is the ambient of great interest for this kind of study.27 One tool that arises as a promising alternative is near-infrared (NIR) spectroscopy. NIR is an analytic technique based on absorption/transmission of electromagnetic waves with a wavelength range between 700 and 2500 nm (14 285−4000 cm−1). It allows for the analysis of multicomponent solutions in a fast and nondestructive way, without demanding complex pretreatment of the samples.28,29 There are three well-stablished ways to detect and monitor the asphaltene precipitation phenomenon using NIR. The first way consists of reading all of the spectra peaks. The absorbance reading at the specific wavelength of 1600 nm (6250 cm−1) is the second way. The third way, in turn, is the optical density variation along all of the spectra.30 The optical density is the sum of light absorption and light scattering, wherein the minimum of optical density is taken as the asphaltene precipitation onset. Therefore, the spectral data can provide the same conclusion, even if submitted to different treatments.31 In this scenario, this study aims to apply the NIR technique to detect the asphaltene precipitation in petroleum at desalter conditions of pressure and temperature. Furthermore, the oil blend compatibility regarding asphaltene stability will be analyzed through the same methodology.

consolidated methodology. The crude oil pre-homogenization was performed by shaking the samples manually for 5 min. In turn, for the blends, there was a further step, which was ultrasonication by 30 min at 60 °C. 2.2.1. Optical Microscopy. The crude oil samples (around 2 g) were weighted in a vial. Then, known volumes of the chosen flocculant agent, n-heptane [Sigma-Aldrich, high-performance liquid chromatography (HPLC) grade, 99%], were added and vigorously mixed. The solutions were left to rest at room pressure and temperature for 20 min and then analyzed. A Carl Zeiss inverted optical microscope (model Axiovert 40 MAT) was employed for this analysis. It was equipped with a charge-coupled device (CCD) video camera, which allowed for the acquisition and processing of images taken from the solutions. Through the image analysis, it was possible to detect the asphaltene aggregate appearance and, hence, the precipitation onset. A microcomputer equipped with the Axio Vision software (version 4.7.2) was used to capture and process the images. 2.2.2. NIR Spectroscopy. First, a methodological strategy was set to check the analysis obtained through microscopy. Thus, the onset measurements were carried out at room temperature and pressure. For this first step, a three-neck round-bottom flask (Pyrex) was used as the flocculation vessel. Around of 100 g of pre-homogenized crude oil sample or pre-prepared crude oil blend was added to the vessel. It was kept under constant homogenization by magnetic stirring. Then, the flocculant was gradually added (2 mL min−1) by a positive displacement pump (PU-2080 Plus, Jasco). Preliminary titrations were carried out using titration rates of 0.05, 1, 2, and 4 mL min−1, and no modification on the asphaltene precipitation onset was observed in this experimental range for the initial 100 g of oil. The asphaltene precipitation was continuously monitored online by a NIR probe. The second strategy was set up to reproduce the pressure and temperature conditions verified in desalter plants at the refinery. According to Fahim et al.,24 the inlet temperature of the oil is usually kept between 50 and 55 °C in the desalting process, as a way to reduce oil viscosity and, consequently, increase the settling rate, and this temperature is normally enhancing along the process. At the same time, the operational pressure is maintained between 3 and 18 bar to keep the oil as a liquid phase.32 On the basis of this information, the experimental variables (conditions) were selected and are shown in Table 2. For this purpose, a variable-volume cell with an internal volume of 250 cm3 was designed. It was equipped with a frontal sapphire window and a lateral opening where the NIR probe was inserted. An internal piston was used to increase and keep a constant pressure with the aid of a syringe pump (Teledyne ISCO 500D). Temperature and pressure transducers were used to monitor these variables. The experimental apparatus diagram is presented in Figure 1. The Fourier transform (FT)-NIR (model FTLA-2000-160D, ABB Bomem) spectrometer was operated in the absorbance mode. It was equipped with a transflectance probe that had 2 mm of optical path length. This equipment operates by the continuous scanning of Michelson’s interferometer. The probe was connected to the spectrometer by an optical fiber cable. The collected spectrum was an average of 16 spectra with a time lapse of 25 s. Software was used to compile the spectra (GRAMS, Thermo Scientific). The spectrometer was operated with the scan spectral region ranging from 1000 to 2500 nm (10 000−4000 cm−1). However, only the spectral region from 8500 to 4500 cm−1 was employed in the analysis to reduce the spectral noise. The asphaltene onset precipitation was detected through reading of spectra variation. A clear baseline change is caused by the variation of the heptane concentration in solution. In general, there is a dilution in the analyzed sample promoted by the addition of heptane. It results in an absorbance reduction and, consequently, a baseline decrease. As the heptane addition continues, the first asphaltene particles start to precipitate from crude oil and, thus, the light extinction starts to increase. It is a result of the light scattering promoted by the precipitated asphaltene particles. At this moment, there will be a baseline turn point. This turn point is taken as the precipitation onset as related by Fossen et al.33

2. EXPERIMENTAL SECTION 2.1. Crude Oil Characterization. Three Brazilian crude oils were selected for this study. They were supplied by Petroleo Brasileiro S/A (Petrobras). The oils were characterized with regard to density and saturate, aromatic, resin, and asphaltene (SARA) composition. Their physicochemical characteristics are summarized in Table 1.

Table 1. Crude Oil Characteristics oil property API gravity (deg) saturate aromatic resin asphaltene

R

A

25 23 SARA composition (wt %)

B

22

57 27 15 1

32 34 28 6

27 29 33 11

2.1.1. Density. The density measurements [American Petroleum Institute (API) gravity] were carried out using a digital density analyzer, model DMA 4500M (Anton Paar), which operates based on the principle of “U”-tube oscillation. The equipment provided a precision of ±0.000 01 g/cm3 and accuracy of ±0.000 05 g/cm3. The methodology can be seen at the ASTM D5002-16 standard test method. All of the analyses were performed at 20 °C. 2.1.2. SARA Characterization. SARA contents were determined through a modified standard test method (ASTM D2007-11). This modification was carried out in the asphaltene washing step. Instead of washing it through Soxhlet reflux, asphaltenes were washed by percolation of heptane during 5 days or until the percolation became colorless. 2.2. Asphaltene Onset Measurements. The onset measurements were performed through two distinct methodologies. The first methodology was the commonly used optical microscopy. The second methodology, in turn, was NIR spectroscopy. Thus, it was possible to validate the methodology proposed by this work through a 5032

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Energy & Fuels Table 2. Experimental Parameters test

technique

flow rate (mL min−1)

T (°C)

P (bar)

volume fraction of heptane

B1 B2 B3 B4 B5 B6 B7 R1 R2 R3 R4 R5 R6 R7 R8 R9 A1 A2 BR1 BR2 RA1 RA2 BA1 BA2 BRA1 BRA2

NIR NIR NIR NIR NIR NIR microscopy microscopy NIR NIR NIR NIR NIR NIR NIR NIR NIR microscopy NIR microscopy NIR microscopy NIR microscopy NIR microscopy

2 2 2 2 2 2

25 25 25 25 25 60 25 25 25 25 25 25 40 40 60 60 60 25 60 25 60 25 60 25 60 25

10 1 5 10 20 10

0.70 0.68 0.68 0.69 0.69 0.71 0.75 0.41 0.37 0.45 0.38 0.42 0.35 0.35 0.32 0.32 0.75 0.80 0.72 0.72 0.74 0.76 0.70 0.75 0.71 0.75

2 2 2 2 2 2 2 2 2 2 2 2 2

1 5 10 20 5 20 5 20 10 10 10 10 10

solubility parameter (MPa1/2) 18.8 18.7 18.7 18.8 18.8 18.9 19.5 17.1 16.9 17.2 17.0 17.1 16.9 16.9 16.8 16.8 19.5 20.3 19.1 19.1 19.3 19.4 18.8 19.3 18.9 19.3

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.1 0.5 0.3 0.2 0.1 0.1 0.5 0.5 0.1 0.1 0.2 0.2 0.1 0.2 0.1 0.1 0.1 0.5 0.1 0.5 0.1 0.5 0.2 0.5 0.1 0.5

In this scenario, a new way to read the spectra is proposed by this study. The reading strategy consists in summing the absorbance values at the entire wavelength range, which is between 2200 nm (4500 cm−1) and 1170 nm (8500 cm−1). The sum is then plotted against the volume fraction of heptane. It results in a clear detection of the turning point, hence facilitating the detection of the asphaltene precipitation onset. The solubility parameter of asphaltene is then calculated through eq 1, where δmix is the solubility parameter of the mixture, δoil and δheptane are the solubility parameters of oil and heptane, respectively, and Φoil and Φheptane are the volume fractions of oil and heptane, respectively. The n-heptane solubility parameter is taken as 15.3 MPa1/2. At flocculation conditions, the solubility parameter δmix is equal to δfloc. Santos et al.34 suggested that the exact value used for δfloc is still an open question. According to Wiehe and Kennedy,35 however, such δfloc shall be between the solubility parameters of methylcyclohexane and that of cyclohexane, which are 15.9 and 16.8 MPa1/2, respectively. Hence, this work considered the average of such values for the calculations, i.e., 16.35 MPa1/2. It is also assumed that δfloc is invariant for all experimental conditions, as suggested by Andersen.36

Figure 1. Schematic diagram of the experimental apparatus designed to monitor the asphaltene precipitation phenomenon by NIR spectroscopy at desalter conditions: (1) reservoir of pneumatic fluid, (2, 4, and 7) needle valves, (3) syringe pump, (5) stainless tube, (6) pressure transducer, (8) reservoir of the flocculant agent (n-heptane), (9) positive displacement pump, (10) backpressure valve, (11) temperature indicator, (12) variable-volume pressurizable cell, (13) piston, (14) sapphire window, (15) optical path of the transflectance probe, (16) NIR spectrometer, and (17) desktop computer.

δmix = δfloc = Φoil δoil + Φ heptaneδ heptane

(1)

3. RESULTS AND DISCUSSION 3.1. Validation of the Experimental Setup. The experimental setup was validated by a comparison of the results obtained through microscopy and NIR spectroscopy. Figure 2 shows the asphaltene precipitation onset measured by the microscopy technique, which is the typical methodology used for this kind of measurement. It was verified that oil R has a solubility parameter (SP) of 17.1 MPa1/2. Figure 3, in turn, shows the SP measured by NIR in the open vessel and in the pressurizable cell for the same oil R. As seen, the measured solubility parameter was also 17.1 MPa1/2, which is the same observed through microscopy. The uncertainties of the NIR quantities reported in Table 2 were obtained through

Sometimes, the exact baseline turn point is not easily detected, thus demanding the use of chemometric tools. In addition, choosing a specific wavelength can generate a misreading of the precipitation onset, because there is also a relationship between the particle size growth and the increase of light extinction at a distinct wavelength. According to Oh et al.,22 the baseline increase will be at the spectral region of the short wavelength during the growth of small particles. This is called wavelength-dependent particle growth. On the other hand, a baseline elevation in the entire NIR spectral region is verified when bigger asphaltene particles are formed. 5033

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Energy & Fuels

solubility parameter for both oils B and R, whether the standard deviation is considered. Commonly, the pressure variation causes asphaltene destabilization as a result of alteration in the oil composition. It happens because of the light fractions that volatilize/solubilize after pressure changes. Because the oils B and R had no gas in solution, the pressure variation at desalter plants does not promote significant composition alteration; hence, it did not result in solubility parameter changes. 3.3. Influence of the Temperature. The effect of the temperature increasing upon the asphaltene stability is complex. There is a competitiveness among the temperature, pressure, and composition variation for the live oils. On the other hand, one expects that there will be an asphaltene stability enhancement as long as the temperature increases for dead oils, such as the oils analyzed in this study.7,20 However, as seen in Figure 4b, no significant changes in the solubility parameter of the oil R at temperatures of 25, 40, and 60 °C were verified. Furthermore, the pressure increasing at each temperature produced the same result. 3.4. Influence of the Oil Blending. The compatibility of oil blends regarding asphaltene stability was verified by comparing the solubility parameter of the oils and their proportional mixtures. As shown in Table 2, the temperature and pressure conditions of 60 °C and 10 bar were used for this analysis to guarantee the homogeneity of the blends. First, Figure 5a presents the solubility parameters for the oils. It is noticed that oil A is the most stable oil. Although it has a higher asphaltene content, it shows at the same time a higher resin content and a lower saturate content. Thus, the peptizing power of the resins overcomes the flocculation power of the saturates. On the other hand, oil R, which has only a 4.4% higher saturate content than oil A, showed a substantially lower solubility parameter when compared to oil A. Oil B, in turn, has almost the same solubility parameter as oil A, despite the

Figure 2. Asphaltene onset precipitation detected by optical microscopy for the crude oil blend composed of oils B, R, and A: (a) BRA pure crude oil and (b−d) BRA with a volume fraction of nheptane of (b) 0.5, (c) 0.7, and (d) 0.75, with detection of asphaltene particles.

duplicated experiments. For the microscopy tests, it is estimated that the standard deviation of the measurements was ±0.5 MPa1/2. Thus, even though applying the new proposed methodology of reading the spectra, one can obtain the same SP, which validates it. It allowed for the analysis of variable influences upon the SP. The influence of the pressure, temperature, and composition simulating desalter conditions was analyzed as follows. 3.2. Influence of the Pressure. The influence of the pressure on the flocculation index of asphaltenes was analyzed at 1, 5, 10, and 20 bar. Figure 4a and the data in Table 2 show that such pressure variation has little influence upon the

Figure 3. Asphaltene onset precipitation detected by NIR spectroscopy: (a) diagram of the sum of absorbances for the open vessel, (b) diagram of the sum of absorbances for the pressurizable cell at 1 bar, (c) NIR spectra for the open vessel, and (d) NIR spectra for the pressurizable cell at 1 bar. 5034

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Figure 4. Solubility parameter of asphaltene in crude oils detected by NIR spectroscopy at different conditions of (a) pressure and (b) temperature.

Figure 5. Solubility parameter of oils: (a) B, R, and A and (b) their blends.

■

distinctions on the saturate and resin contents between these two oils. The analysis by SARA composition is a simplistic way to look at oil stability; however, this is the most popular method applied at oil fields and refinery plants.37−39 The results presented by this study highlight the inconsistency eventually related to this method. Figure 5b shows the solubility parameter for the oil blends. There were binary blends with a volume proportion of 50:50 (%) and a ternary blend of 33.3%. As one can see, the solubility parameter of the binary blend of oils B and A is almost the same as that of oil B. Also, the asphaltenes in the blend of oils R and A are as stable as they are in oil A. It denotes an interaction between the oil components, which results in the stabilization of the asphaltenes. Such interactions are more prominent for the blend composed by oils B and R. For this case, the instability extent verified with oil R seems to be overcome by the oil B composition. The ternary blend (BRA) followed the same tendency.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b00602. Micrographs of the crude oil blends + heptane, showing the asphaltene particles (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Elton Franceschi: 0000-0002-2675-7250 Cláudio Dariva: 0000-0002-5239-9039 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors thank Petrobras (Petróleo Brasileiro S.A., Brazil) and CNPq for supporting the present work.

4. CONCLUSION NIR spectroscopy was successfully validated and applied for the detection of the asphaltene precipitation in crude oils. Furthermore, the proposed methodology for reading the spectra was effective and arises as a new and promising alternative. At the same time, the results showed that none of the analyzed variables (temperature and pressure) has an influence on the asphaltene precipitation at the studied desalter conditions. Finally, the incongruences of the simplistic way to predict the oil compatibility based on SARA characterization were highlighted. Equally important, it was shown that the chemical interaction among the oil components can stabilize the asphaltenes from a relatively unstable oil when it is blended with a stable oil.

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