The asphaltenes precipitation onset: influence of the addition of a

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The asphaltenes precipitation onset: influence of the addition of a second crude oil or its asphaltenes fractions (C3I and C5I) Fabio Rosa Barreira, Leidiane Guimarães Reis, Rita de Cassia Pessanha Nunes, Sofia Dornellas Filipakis, and Elizabete F. Lucas Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01749 • Publication Date (Web): 11 Sep 2018 Downloaded from http://pubs.acs.org on September 11, 2018

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The asphaltenes precipitation onset: influence of the addition of a second crude oil or its asphaltenes fractions (C3I and C5I) Fabio R. Barreira1, Leidiane G. Reis1, Rita de Cassia P. Nunes2, Sofia D. Filipakis2, Elizabete F. Lucas*1,2 1

Universidade Federal do Rio de Janeiro, Instituto de Macromoléculas, Laboratório de Macromoléculas e Colóides na Indústria de Petróleo – Av. Horácio Macedo, 2030, bloco J, Cidade Universitária, 21941598, RJ, Brazil - Phone# 552139387033 2

Universidade Federal do Rio de Janeiro, COPPE, Programa de Engenharia

Metalúrgica e de Materiais, Av. Horácio Macedo, 2030, bloco F, Cidade Universitária, 21941972, RJ, Brazil * Corresponding author: [email protected]

ABSTRACT. Predicting the asphaltene stability in crude oils from different production streams is very useful in the petroleum industry because it allows avoiding serious problems caused by formation of solid deposits during oil flow. That prediction can be carried out by applying the solubility parameter (δ) of each oil, as calculated by the asphaltene precipitation onset value, obtained by titration with n-heptane. However, many crude oils do not have a well-defined precipitation onset point, which can be overcome by adding a crude oil assumed as the standard. This article analyzes the influence of a crude oil (called APS) on the precipitation onset of two other petroleum samples (called APA and APB). For this purpose, the asphaltene fractions C3I and C5I were extracted from APS, and the influence of the addition of this crude oil as well as its asphaltenes fractions in samples of oils APA and APB was evaluated by tests involving titration of n-heptane with detection by near-infrared spectroscopy (NIR). The calculation of the solubility parameters of the oils without well-defined precipitation onset, by adding the oil with well-defined precipitation onset, led to varied errors in function of the type of oil in question. The smallest errors were obtained when using, as the solubility parameter of the mixture (δM), the solubility parameter of the solvent system at the precipitation onset of the asphaltene C3I fraction (extracted from the crude oil assumed as the standard) in toluene, determined by titration with n-heptane. Keywords: Solubility parameter; asphaltenes precipitation onset; near infrared; crude oils miscibility

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INTRODUCTION Many analytical techniques has been used to characterize crude oil samples.1-2 Asphaltenes has been characterized as chemical structures containing condensed aromatic rings, aliphatic rings, aliphatic hydrocarbon chains, polar groups, heteroatoms and metals. The asphaltenes fraction is the heaviest one of petroleum and is defined as that insoluble in n-alkanes, such as n-heptane and n-pentane, and soluble in benzene or toluene.3-5 Asphaltenes present amphiphilic character and it can become destabilized, forming solid deposits, in function of variations in the oil composition and pressure of the system. These deposits can occur at different points, from the reservoir rock to refinery lines, including production lines, valves and storage tanks, impairing the production, transport and refining of crude oil. The stability of asphaltenes can be evaluated experimentally, at room temperature and normal atmospheric pressure, by adding n-heptane into the oil, to detect the formation of precipitates by means of optical microscopy, interfacial tension, ultraviolet, near-infrared spectroscopy or other technique. The oil is considered to be more stable the greater the quantity of n-heptane that is necessary to cause precipitation of the asphaltenes.3,6-12 The stability of the asphaltene fractions in petroleum has been the subject of many studies.13-19 The solubility parameter of asphaltenes is another very important aspect that is attracting growing research interest, because it can enable prediction of the factors that will cause precipitation and deposition of asphaltenes.20 Infrared (IR), near-infrared (NIR) and microcalorimetry techniques have been used to determine the solubility parameters of the various components of crude oil.21-23 Besides this, compatibility models have been developed.24-31


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Prediction of the asphaltene stability in crude oils from different production streams is very useful to the petroleum industry, because this knowledge can avoid serious problems from formation of solid deposits during oil flow in lines. That prediction can be achieved by determining the solubility parameter (δ) of each oil, by using the following correlations:32 = √ =

∆

Equation 1

Where c is the cohesive energy, ∆Hv is the enthalpy of vaporization, R is the universal gas constant, T is the temperature, and Vm is the molar volume. However, in the case of asphaltenes, which degrades before vaporizing, it is not possible to determine ∆Hv directly from experimental analyses, like calorimetry. Nikooyek and Shaw33 have demonstrated a procedure to determine ∆H, by measuring asphaltenes-diluent interaction heat. Wiehe and Kennedy25, studying oil compatibility, have determined the critical solubility parameter at the onset of asphaltenes flocculation for Souedie Crude oil at around 16.35 MPa1/2. Even though this value can change for different crude oils and Nikooyek and Shaw33 study has concluded that the use of the solubility parameter is not suitable to describe asphaltenes + diluent mixture behavior, Equation 2 has been used to determine the solubility parameter of the crude oil, assuming δΜ = 16.35 MPa1/2.31,34-36 = ∑

Equation 2

Where δi is the solubility parameter of component i and ϕi is the fraction by volume of component i in the mixture. In this way, the solubility parameter of petroleum (δp) can be easily and quickly estimated based on experimental values of the precipitation onset of each oil sample,


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using titration with n-heptane and monitoring by NIR spectroscopy. The value of δp can be calculated by Equation 3. = +

Equation 3

Where δh is the solubility parameter of n-heptane, ϕh is the volumetric fraction of nheptane at the asphaltenes precipitation onset, δp is the solubility parameter of the crude oil (targeted for calculation), and ϕp is the volumetric fraction of that oil in relation to the volume of n-heptane added. Knowing that δh = 15.2 MPa1/2

37

and determining the volumetric fractions via

titration until reaching the flocculation point allows obtaining δp. However, this method is not suitable when the determination of the oil’s precipitation onset, by the experimental procedure described above, is not well defined. Therefore, in one alternative to overcome this limitation, a crude oil that has a well-defined precipitation onset (standard oil) can be mixed with the target oil to enable reliable measurement of the precipitation onset. When using this procedure, the solubility parameter of the target petroleum (δp) based on the precipitation onset can be obtained by Equation 4. = + +

Equation 4

Where δpp is the solubility parameter of the standard oil and ϕpp is the volumetric fraction of the standard oil in the final mixture, at the flocculation point, with the target oil and n-heptane added. A Brazilian crude oil, called APS, which has a very well defined curve (absorbance against n-heptane volume) for determining precipitation onset, has been used by the industry as the standard oil for mixture with the target oil whose curve for measurement of precipitation onset is not well defined. However, the results obtained by this procedure have not been evaluated in a systematic way.


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In this work, the crude oil APS was added to two other crude oils (APA and APB), in order to evaluate its influence on the asphaltenes precipitation onset of such crude oils. In addition, two fractions of crude oil APS (C3I and C5I) were separated and evaluated the influence of adding them on the asphaltenes precipitation onset of the same crude oil samples (APA and APB). Finally, the solubility parameters were studied to compare the prediction and experimental behavior of the asphaltenes in these oils, evidencing the uncertainties of this simple methodology used by the industry and suggesting changes to improve its reliability.

EXPERIMENTAL Materials Ethyl alcohol 95% P.A., n-heptane 99.5% P.A., n-pentane 99.0% P.A. and commercial toluene (further distilled and dried on alumina) were supplied by Vetec Quimica Fina Ltda, Duque de Caxias, Brazil; 1,4 anhydrous dioxane 99.8% was supplied by Sigma- Aldrich, São Paulo, Brazil. Three crude oil samples from Brazilian fields (called APS, APA and APB) were donated by Petrobras. Some characteristics of these samples are listed in Table 1. Petrobras also donated samples of APS fractions deasphalted with propane (liquid extract and solid extract) obtained from the deasphaltation procedure, using a highpressure fractionation unit.38 The amounts extracted in relation to the total amount of crude oil were 31.5% solids and 60.1% liquids. Table 1. Some characteristics of crude oils named APS, APA and APB CENPES/Petrobras39


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Preparation of the model systems

The model systems were prepared with 15 and 30 wt% of C3I in toluene, using 1.05 and 2.10 g of C3I in toluene, respectively.

Extraction of asphaltene C5I from APS solid extract A sample of 10 g of C3I asphaltene was placed in an extraction cartridge of a Soxhlet extractor for separation and solubilization of the resin residues still adsorbed in the asphaltene. A total of 300 mL of n-pentane was added to the round-bottom beaker. The extraction was considered complete when the paraffinic solvent (n-pentane) appeared clear in the extractor. The resins were evaporated with an IKA RV 05 Basic rotary evaporator connected to a vacuum pump, at a temperature of 50°C, followed by drying in an exhaust hood at room temperature for 3 days. A resin mass of 3.5 g was obtained. The asphaltenes that remained precipitated in the cartridge were dissolved in an aromatic solvent, using 300 mL of dry toluene added to the flask. This extraction was considered complete when the dry toluene appeared clean in the extractor. The dissolved asphaltenes were recovered after evaporation of the solvent in the rotary evaporator at 80°C, followed by drying in the exhaust hood for 3 days. The mass of C5I obtained was 5.05 g, corresponding to a yield of approximately 50 wt%. Determination of asphaltene precipitation onset using NIR spectrometry The precipitation onset of the asphaltene fractions was determined by near-infrared spectrometry (NIR)7,23,40-41 in a Bruker MATRIX-F system, operating with the Opus 6.5 software and a Jasco PU 2087 Plus displacement pump. Initially, 7 g of sample (crude oil or asphaltene solution prepared previously) was poured into the device’s cup and kept under constant magnetic stirring. Then a 5 mm optical probe was introduced in the


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cup and the device was turned on, at the same time as starting titration of the flocculant, at a flow of 0.1 mL/min. The analyses were carried out at room temperature from 12000 cm-1 to 4000 cm-1 by 0.5 scan/min with a resolution of 16 cm-1 in adsorption mode. Each analysis lasted 8 hours and used 48 mL of flocculant. The data obtained were used to plot a graph of absorbance at 1600 nm versus volume of n-heptane, to ascertain the amount of n-heptane corresponding to the lowest absorbance point, indicating precipitation of asphaltenes. The precipitation onset was obtained by dividing this value for total n-heptane volume by 7, since the onset is expressed in terms of mL of nheptane/g of oil phase. The tests were performed in duplicate and the values presented are the average of two measurements, with an error of < 3%. Calculation of the Hildebrand solubility parameter of the solvent mixtures The solubility parameter of the solvent mixtures (δΜ) at the precipitation onset was calculated using the weighted average of the solubility parameter values of the solvent and the asphaltene flocculant, considering the respective volumetric fractions in the mixture, according to Equation 2, where δi denotes the solubility parameters of the solvent and flocculant and φi denotes their respective volumetric fractions at the onset point. Since in some cases we performed tests with two types of flocculants, one with δ value lower than that of the solvent and the other with value higher than that of the solvent, it was possible to determine the solubility parameter range of the solute in question. For this purpose, we applied the procedure described previously, based on the onset values obtained for the two flocculants.

RESULTS AND DISCUSSION


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Evaluation of asphaltene precipitation onset by near infrared spectroscopy (NIR) We first conducted asphaltene precipitation onset tests for the oil samples. Figure 1 presents the curves of absorption intensity (at 1600 nm) versus volume of n-heptane titrated with the oil samples APS (a), APA (b) and APB (c). It can be seen that the curve of APS, as expected, clearly shows the onset value, at the point of minimum absorption, which was reached after adding 23.25 mL of n-heptane. Considering that 7 grams of APS was used, the precipitation onset corresponds to 3.32 mL of n-heptane/g of oil. The onset curve of petroleum APA is also well defined, showing 3.86 mL of n-heptane/g of oil. On the other hand, the curve of oil APB does not have this quality, hampering determination of its precipitation onset. At the minimum absorption intensity, the nheptane volume corresponds to onset of 3.35 mL of n-heptane/g of oil. In practice, cases that are more critical occur, where it is not possible to observe the minimum absorption intensity. In this study, we only used this type of behavior to represent the problem addressed.

Figure 1. Absorption intensity (at 1600 nm) versus volume of n-heptane for the crude oils: (a) APS, (b) APA and (c) APB.

As mentioned before, crude oil APS, because of its well-defined curve to determine the precipitation onset (Figure 1a), is being used in Brazil as a standard oil, meaning it is mixed with oil samples whose curves are not well defined. Therefore, this oil was mixed pairwise with oils APA and APB and these mixtures were submitted to tests to determine the asphaltenes precipitation onset using NIR. As expected, the onset values of oils APA and APB were altered by adding the APS. The precipitation onset of oil APA was shifted from 3.86 to 3.30 mL of n-heptane/g of oil (Figure 2a), a value much


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nearer to that of APS, even though only 20 wt% of this oil was added. In turn, the onset of APB was shifted from 3.35 to 2.46 (Figure 2c) and 1.96 (Figure 2b) mL of nheptane/g of oil, respectively, after adding 50 and 20 wt% of APS.

Figure 2. Absorption intensity (at 1600 nm) versus volume of n-heptane for the crude oils mixtures: (a) APA:APS (80:20 m/m), (b) APB:APS (80:20 m/m) and (c) APB:APS (50:50 m/m).

The crude oil APB, which has a very low concentration of asphaltenes (< 0.5 wt%), was influenced much more by adding the standard oil (APS) than the crude oil APA, which has 3.6 wt% of asphaltenes. These different results were to a certain extent expected. However, the mixtures of oils APS and APB increased the instability of the asphaltenes of this system. The results can be interpreted as indicating that the nonasphaltenic phase of oil APB destabilizes the asphaltenes of oil APS, and the larger the quantity of APB in the mixture, the greater the instability caused. This behavior can be explained by the solubility parameter theory of Hansen37 the solubility parameter of a mixture of two solvents depends on the specific interactions of the two systems, and gives rise to a new system with its own solubility parameter, having specific contributions from the interactions involving hydrogen bonds and van der Waals and dipole-dipole forces. In the case in question here, the non-asphaltenic fraction of oil APB appears not to be a good solvent of the asphaltenes of oil APS, inducing precipitation with a lower volume of n-heptane. In fact, the non-asphaltenic fractions of these oils differ in terms of solubility parameter, as can be observed from the results of asphaltenes precipitation onset obtained for each one by adding only 20 wt% of toluene. The precipitation onset values of samples APS, APA and APB were reduced,


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respectively, from 3.32 to 3.09, from 3.86 to 3.82 and from 3.35 to 3.08 mL of nheptane per gram of oil/mixture. These results show that the three samples are influenced differently by adding toluene, indicating that their non-asphaltenic fractions have different solubility parameters. Since sample APB has a very low asphaltene concentration (~0.5%), it is nearly all non-asphaltenic, and because it has a solubility parameter different from that of the non-asphaltenic fraction of APS, it exerts a stronger influence on the result of the mixture of these two oils. The curve in Figure 2 show that oil APA (Figure 2a), which had a well-defined onset for the pure sample (Figure 1b), had a curve with similar precipitation onset definition in the mixture. In turn, the curve of mixture with sample APB was much better defined than that of the pure sample (Figure 1c), both with addition of 50 wt% of APS (Figure 2c) and only 20 wt% of APS (Figure 2b). Since the objective of this study was to understand the influence of adding asphaltenes fractions on the detection of the precipitation onset point in crude oil samples (utilizing titration with n-heptane and detection by NIR), such as what happens in the petroleum industry when adding a standard oil, we also investigated the behavior of the fractions of oil APS (liquid and solid extracts from deasphaltation with propane) in terms of precipitation onset when added to oils APA and APB, to better understand the influence of these fractions separately. We first evaluated the precipitation onset of the mixture of these two extracts in the proportion of their respective yields, as indicated in the experimental section, to characterize the recombination of these two fractions of sample APS. The curve obtained had exactly the same profile as that of APS, and the precipitation onset of the liquid extract:solid extract mixture (67:33 m/m) was 3.57 mL of n-heptane/g of recombined APS. Since the sum of the masses of the liquid and solid extracts was not


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equal to that of the APS used in the deasphaltation process with propane, the difference between the onset values of the original APS (3.32) and the recombined APS (3.57) might be related to the loss of light compounds that occurred during the deasphaltation process, making the mixture more stable. It also should be considered that due to the loss of this material, the proportion used in the recombination (67:33 m/m) might have differed slightly from the original ratio of these fractions in the crude oil APS. To study the characteristics of the molecules that compose the solid extract (C3I) of the standard oil, we dissolved this fraction in toluene and assessed its precipitation by induction, with n-heptane and ethanol. The concentration of the solid extract in toluene was set at 30 wt%, to reproduce the content in the APS sample. The objective of these tests was to determine the lower and upper solubility parameter values of this asphaltene fraction. Since the curve of absorption intensity in function of the volume of titrant was not well defined when ethanol was used, we also performed the same type of test with dioxane. Table 2 summarizes the asphaltenes precipitation onset values as well as the solubility parameter values of the solvent/non-solvent mixture at the precipitation onset. The results show that although the curve was not well defined when applying titration with ethanol, the solubility parameter value obtained was 19.5 MPa1/2, very close to that obtained by titration with dioxane (19.6 MPa1/2). This indicates that the solubility parameter range in which the asphaltenes of the solid extract (C3I) are soluble is between 15.9 and 19.6 MPa1/2. These results are in accordance with those obtained in a previous study19 for fractions C5I (insoluble in pentane) and C7I (insoluble in heptane), for which the lower limit of δ was 2.045 MPa1/2 higher than the value of δ of the solvent used to extract the referred fractions from the crude oil. In this study, the solvent employed to separate the solid extract from oil APS was propane (δ = 13.4 MPa1/2) and the lower limit found was 15.9 MPa1/2, approximately 2.045 MPa1/2 higher than the δ


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value of the extraction solvent (propane). Regarding the upper solubility parameter limit of the asphaltenes from the solid extract (C3I), although the separation procedure applied (high-pressure fractionation) did not involve solubilization in toluene, like in the usual procedures for separation of fractions C5I and C7I,7,16,41 the value obtained of 19.6 MPa1/2 is the same as that obtained previously for fractions C5I and C7I.22

Table 2. Precipitation onset of C3I asphaltenes fraction (at 30 wt% in toluene), obtained when using n-heptane, ethanol and dioxane as titrant, and solubility parameters (δ) of the solvent/non-solvent mixture at the precipitation onset

Since asphaltenes are soluble in petroleum (solvent medium), toluene was chosen to dissolve the solid extract of APS, and the solution was added to APB to compare the results with those already obtained with addition of APS. The solutions of C3I in toluene were prepared at concentrations of 15 and 30 wt%. To ascertain which fraction would behave closest to that of APS, solutions of C5I in toluene were also prepared at these two concentrations and added to APB. The results are summarized in Table 3.

Table 3. Precipitation onset of crude oil APB after adding C3I in toluene or C5I in toluene. Data obtained for the APB added of APS is also described in this table for comparison

The precipitation onset values obtained for sample APB containing the solution of C3I in toluene were relatively close to those obtained for the same crude oil with addition of the same quantity of APS, both for the 50:50 proportion (respectively 2.46 and 2.33 mL of heptane/mL of oil phase) and the 80:20 proportion (respectively 1.96


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and 2.04 mL of heptane/mL of oil phase). This did not occur when using the solution of C5I in toluene. This result is in accordance with the results obtained by Garetto et al. (2013),8 who reported that model systems of asphaltenes in solvents are better represented when using asphaltene fractions separated from crude oil with alkanes having a lower number of carbon atoms. Besides this, the obtainment of the lowest precipitation onset values with the addition of the C5I fraction solution was coherent with the fact that C5I is more susceptible for precipitation induced by n-heptane than C3I, because although both fractions have the same more polar molecules, C3I has other less polar molecules that are not present in C5I, and these help stabilize the more polar molecules. The results of this study with model systems also confirmed that 20 wt% of the solution of asphaltenes added to APB leads to a lower precipitation onset value than the addition of 50 wt%, for both asphaltene solutions evaluated. This can again be explained by the Hansen solubility parameter theory for solvent mixtures, which takes in account that the total energy of vaporization of a liquid arises from dispersion forces, permanent dipole-permanent dipole forces, and hydrogen bonding.37 The concentration of asphaltenes in the model system (15 or 30 wt%) did not affect asphaltenes precipitation onset values obtained, because the differences were within the range of experimental error. This was verified for the C3I and C5I systems in the APB:model system proportions of 50:50 and 80:20. This result demonstrates that the inducement of precipitation depends only on the molecules present in the model system, not their concentration, although the quantity of the model system added to the crude oil significantly affected the precipitation onset. In other words, the quantity of the solvent medium has a stronger influence on the stability than the quantity of asphaltene molecules added.


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Besides this, the profile of the curve of APB with addition of the model system at 15 wt% of C5I asphaltenes was better defined than that with addition of the model system at 30 wt% of C5I asphaltenes (comparative figure not shown). However, in both cases the onset values were easily identified. To confirm that the best way to add the asphaltenes fractions (C3I or C5I) to the crude oil was previous dissolution in a solvent, having selected toluene, we also added the solid extract C3I directly in APB, in the proportion (m/m) 85:15. After homogenizing the mixture, we measured the precipitation onset. The value obtained was 2.17 mL of n-heptane/mL of oil. Although the concentration of asphaltenes added to APB was the same as in the test using the C3I solution at 30 wt% in toluene (added in the APB:asphaltene solution proportion of 50:50 m/m), the result obtained with direct addition was slightly lower, probably related to the fact that the solubility parameter of the medium was not significantly affected by the presence of toluene. With respect to the curve’s profile, although the curve showed a slightly better defined onset point (Figure 3a) than did the curve of pure APB (Figure 1c), we decided to continue using this solvent, since it provided a much better defined onset (Figure 3b). This is extremely important, mainly in more critical cases of oils for which it is not possible to observe any indication of asphaltenes precipitation onset.

Figure 3. Absorption intensity (at 1600 nm) versus volume of n-heptane for the crude oils mixtures: (a) APB:C3I (85:15 m/m), (b) APB:C3I 30 m/m/% in toluene (80:20 m/m).

Since the objective of using a standard petroleum sample is to enable calculating the solubility parameter of an oil whose asphaltenes precipitation onset is not easy to detect,


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using Equation 4, the results of this study allow verification of how the addition of the standard oil (APS) influenced the Hildebrand solubility parameter values calculated for oils APA and APB, by comparison of the parameter values calculated for these pure oils (Equation 3) and their respective mixtures (Equation 4), as indicated in Table 4. By applying the approximation of Wiehe, we observed that the Hildebrand solubility parameters did not present good correlations between the values obtained for the pure samples (APA and APB) that those obtained for their respective mixtures with the standard oil (80:20 proportion), causing shifts of 4.1 and 11.5% in the solubility parameters. Therefore, we propose that the Hansen solubility parameter is better for systems with different oils than the Hildebrand solubility parameter, since the overall parameters can be the result of distinct contributions of the three components described by Hansen, so that the absolute value is the same, but is shifted in distinct forms in a tridimensional diagram.37

Table 4. Calculation of global solubility parameters (Hildebrand) using Wiehe approximation and using lower, mean and upper parameters calculated for the asphaltenes C3I (extracted from crude oil APS)

To confirm whether these variations observed between the parameters calculated for the pure oils and their mixtures with the standard oil were due to the approximation proposed by Wiehe, we recalculated the Hildebrand solubility parameters, but substituted the value of 16.35 MPa1/2 in Equations 3 and 4 with the lower (15.9 MPa1/2), mean (17.7 MPa1/2) and upper (19.5 MPa1/2) parameters calculated for the C3I asphaltenes fraction of APS (Table 2). The results of these calculations showed that the variations previously observed in the parameters calculated using the approximation of


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Wiehe are also present in the new results, as indicated in Table 4. However, these variations were smaller, for both oil samples, when using in Equations 3 and 4 the lower solubility parameter that was calculated for the C3I asphaltenes fraction of APS, as shown in Table 5. Also, the tendency for variation in function of the solubility parameter value used in Equations 3 and 4 was the same for both oil types, that is, always higher for APB.

Table 5. Percentage difference between the solubility parameter calculated from the precipitation onset for the crude oil using Equation 3 and the solubility parameter calculated from the precipitation onset for the crude oil mixed to the APS using Equation 4.

CONCLUSIONS The solubility parameter range of the C3I fraction of crude oil APS was found to be 15.9 to 19.6 MPa1/2, confirming that the lower solubility parameter limit of this oil is nearly 2.4 MPa1/2 higher than the solubility parameter of the solvent used in the extraction (δ of propane = 13.4 MPa1/2). The upper limit was similar to the values observed in a previous study for the C5I and C7I fractions.23 With respect to the behavior of the asphaltenes precipitation onset curve of a determined crude oil sample, the model systems of C3I asphaltene in toluene behaved more closely to that of the oil from which the fraction was extracted than the C5I fraction in toluene. The similarity of the influence of adding a standard oil and a model system containing C3I in toluene was confirmed for different proportions of the mixtures.


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Within the range from 15 to 30 wt%, the concentration of asphaltenes in toluene did not significantly affect the precipitation behavior of any of the crude oils. The main influence on this behavior was the quantity of the model system added to the oil. In other words, the quantity of the solvent medium has a stronger influence on the crude oil stability than the quantity of asphaltenes added. The asphaltenes precipitation onset of crude oil samples can be detected more reliably with the addition of an oil sample for which the absorption intensity versus flocculant volume curve is better defined. However, mixture of oils can result in greater instability of the asphaltenes in relation to the stability of the pure oils. Asphaltenes phase behavior in oil blends can be explained better by the Hansen solubility parameter theory than that of Hildebrand. The calculation of the solubility parameter of oils with poorly defined precipitation onset by the addition of an oil sample with well-defined precipitation onset can lead to various errors, depending on the type of oil under study. In this work, the smallest errors were obtained when using, as the solubility parameter of the mixture (δM), the solubility parameter of the solvent system at the precipitation onset of the asphaltene C3I fraction (from the crude oil assumed as the standard) in toluene determined by titration with nheptane (δ lower).

ACKNOWLEDGEMENTS We thank CNPq (307193/2016-0), CAPES, FAPERJ (E-26/201.233/2014), ANP and Petrobras for financial support (0050.0086965.13.9). We also thank Laboratório de Escoamento de Petróleo e Emulsões / Gerência de Tecnologia de Elevação Artificial e


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Garantia de Escoamento / CENPES / Petrobras for the donation of crude oil APS fractions deasphalted with propane (liquid extract and solid extract).

REFERENCES 1. Correia, R. M.; Domingos, E.; Cáo, V. M.; Araujo, B. R. F.; Romão, W. Portable near infrared spectroscopy applied to fuel quality control. Talanta 2018, 176, 26-33. 2. Catelani, T. A.; Santos, J. R.; Páscoa, R. N .M. J.; Pezza, L.; Lopes, J. A. Real-time monitoring of a coffee roasting process with near infrared spectroscopy using multivariate statistical analysis: A feasibility study. Talanta 2018,179, 292-299. 3. Mullins, O. C.; Sheu, E. Y.; Hammami, A.; Marshal, A. G. Asphaltenes, Heavy Oils, and Petroleomics. Springer, New York, 2007. 4. Gray, M. R. Upgrading petroleum residues and heavy oils. Marcel Dekker, New York, 1994. 5. Powers, D. P.; Sadeghi, H.; Yarranton, H. W.; van den Berg, F. G. A. Regular solution based approach to modeling asphaltene precipitation from native and reacted oils: Part 1, molecular weight, density, and solubility parameter distributions of asphaltenes. Fuel 2016,178, 218-233. 6. Mousavi-Dehghani, S. A.; Riazi, M. R.; Vafaie-Sefti, M.; Mansoori, G. A. An analysis of methods for determination of onsets. J. Petrol. Sci. Eng. 2004, 42, 145-156. of asphaltene phase separations 7. Garreto, M. S. E.; Gonzalez, G.; Ramos, A. C.; Lucas, E. F. Looking for a model solvent to disperse asphaltenes. Chem.Chem.Technol. 2010, 4, 317-323. 8. Garreto, M. S. E.; Mansur, C. R. E.; Lucas, E. F. A model system to assess the phase behavior of asphaltenes in crude oil. Fuel 2013, 113, 318-322.


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9. Palermo, L. C. M.; Souza, N. F.; Louzada, H. F.; Bezerra, M. C. M.; Spinelli, L. F.; Lucas, E. F. Development of multifunctional formulation for inhibition of waxes and asphaltenes deposition. Braz. J. Petrol. Gas 2014, 7, 181-192. 10. Ferreira, S. R.; Louzada, H. F.; Moyano, R. M.; Gonzalez, G.; Lucas, E. F. Influence of the architecture of additives on the stabilization of asphaltenes and waterin-oil emulsions separation. Energy Fuel. 2015, 29, 7213-7220. 11. Palermo, L. C. M.; Lucas, E. F. Asphaltene aggregation: influence of composition of copolymers based on styrene-stearyl methacrylate and styrene-stearyl cinnamate containing sulfate groups. Energy Fuel. 2016, 30, 3941-3946. 12. Brostow, W.; Lobland, H. E. H. Materials Introduction and Applications. John Wiley & Sons, New Jersey, 2017. 13. Sousa, M. A.; Oliveira, G. E.; Lucas, E. F.; González, G. The onset of precipitation of asphaltenes in solvents of different solubility parameters. Prog. Colloid Polym. Sci. 2004, 128, 283-287. 14. Lucas, E. F.; Mansur, C. R. E.; Spinelli, L.; Queirós, Y. G. C. Polymer science applied to petroleum production. Pure Appl. Chem. 2009, 81, 476-494. 15. Lima, A. F.; Mansur, C. R. E.; Lucas, E. F.; González, G. Polycardanol or sulfonated polystyrene as flocculants for asphaltene dispersions. Energy Fuel. 2010, 24, 2369-2375. 16. Mansur, C. R. E.; Melo, A. R.; Lucas, E. F. Determination of asphaltenes particles size: Influence of flocculant, additive and temperature. Energy Fuel. 2012, 26, 49884994. 17. Aguiar, J. I. S.; Mansur, C. R. E. Study of the interaction between asphaltene and resins by microcalorimetry and ultravioleta-visible spectroscopy. Fuel. 2015, 140, 462469.


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18. Lucas, E .F.; Ferreira, L. S.; Khalil, C. N. Polymers Applications in Petroleum Production. In Mark, H. F. (ed), Encyclopedia of Polymer Science and Technology, John Wiley & Sons, Michigan, 2015. 19. Hartmann, D.; Lopes, H. E.; Teixeira, C. L. S.; Oliveira, M. C. K.; Gonzalez, G.; Lucas, E. F.; Spinelli, L. S. Alkanes induced asphaltenes precipitation studies at high pressure and temperature. Energy Fuel. 2016, 30, 3693-3706. 20. Likhatsky, V. V.; Syunyaev, R. Z. New Colloidal Stability Index for Crude Oils Based on Polarity of Crude Oil Components. Energy Fuel. 2010, 24, 6483-6488. 21. Fossen, M.; Hemmingsen, P. V.; Hannisdal, A.; Sjoblom, J.; Kallevik, H. Solubility Parameters Based on IR and NIR Spectra: I. Correlation to Polar Solutes and Binary Systems. J. Dispersion Sci. Technol. 2005, 26, 227-241. 22. Carvalho, S. P.; Gonzalez, G.; Lucas, E. F.; Spinelli, L. S. Determining Hildebrand solubility parameter by ultraviolet spectroscopy and microcalorimetry. J. Braz. Chem. Soc. 2013, 24(12), 1998-2007. 23. Aguiar, J. I. S.; Garreto, M. S. E.; González, G.; Lucas, E. F.; Mansur, C. R. E. Microcalorimetry as a new technique for experimental study of solubility parameter of crude oil and asphaltenes. Energy Fuel. 2014, 28, 409-416. 24. Wiehe, I. A.; Kennedy, R. J. Application of the oil compatibility model to refinery streams. Energy Fuel. 2000, 14, 60-63. 25. Wiehe, I. A.; Kennedy, R. J. The oil compatibility model and crude oil incompatibility. Energy Fuel. 2000, 14, 56-59. 26. Asomaning, S. Test method for determining asphaltenes stability in crude oils. Pet. Sci. Technol. 2003, 21, 581-590. 27. Schermer, W. E. M.; Melein, P. M. J.; Van Den Berg, F. G. A. Simple techniques for evaluation of crude oil compatibility. Pet. Sci. Technol. 2004, 22, 1045-1054.


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28. Redelius, P. Bitumen solubility model using Hansen solubility parameter. Energy Fuel. 2004, 18, 1087-1092. 29. Moura, L. G. M.; Rolemberg, M. P.; Ramos, A. C. S.; Santos, M. F. P.; Zílio, E. L. Avaliação das incertezas associadas à determinação do parâmetro de solubilidade de Hildebrand de petróleos. Quím. Nova. 2011, 34, 226-231. 30. Ramos, A. C. S.; Rolemberg, M. P.; Moura, L. G. M.; Zilio; E. L.; Santos, M. F. P.; González G. Determination of solubility parameters of oils and prediction of oil compatibility. J. Pet. Sci. Eng. 2013, 102, 36-40. 31. Santos, D. C.; Filipakis, S. D.; Rolemberg, M. P.; Lima, E. R. A.; Paredes, M. L. L. Asphaltene flocculation parameter in Brazilian crude oils and syntehtic polar and nonpolar mixtures: Experimental and modeling. Fuel 2017,199, 606-615. 32. Huggins, M. L. Some properties of solutions of long-chain compounds. J. Phys. Chem. 46 (1942) 151-158. 33. Nikooyeh, K.; Shaw, J. On the applicability of the regular solution theory to asphaltene + diluent mixtures. Energy Fuel. 2012, 26, 576-585. 34. Mutelet, F.; Ekulu, G.; Solimando, R.; Rogalski, M. Solubility Parameters of Crude Oils and Asphaltenes. Energy Fuel. 2004, 18, 667-673. 35. Zilio, E. L.; Santos, M. F. P.; Ramos, A. C. S.; Rolemberg, M. P. Comparação entre parâmetros de estabilidade de petróleos. In: Rio Oil & Gas Expo & Conference, Rio de Janeiro, 2006. 36. Henriques, C. B.; Winter, A.; Koroishi, E. T.; Maciel Filho, R.; Bueno, M. I. M. S. Estudo da influência dos particulados no fenômeno de agregação dos asfaltenos por espectrometria de varredura óptica. Quim. Nova 2011, 34(3), 424-429. 37. Hansen, C. M. Hansen solubility parameters: a user's handbook, 2nd ed., CRC Press, New York, 2007.


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38. Altoé, R.; Lopes, H. E.; Teixeira, C.; Cirilo, L. C. M.; Lucas, E. F.; Gonzalez, G. Solution behavior of asphaltic residues and desasphalted oil prepared by extraction of heavy oil. Colloids Surf. A. 2014, 445, 59-66. 39. CENPES/Petrobras. Internal Communication, 2016. 40. Ferreira, S. R.; Barreira, F. R.; Spinelli, L.; Seidl, P.; Leal, K. Z.; Lucas, E.F. Comparison between asphaltenes (sub)fractions extracted from two different asphaltic residues: chemical characterization and phase behavior. Quim. Nova 2016, 39, 26-31. 41. Figueira, J. N.; Simão, R. A.; Soares, B. G.; Lucas, E. F. The influence of chemicals on asphaltenes precipitation: a comparison between atomic force microscopy and near infrared techniques. Fuentes Reventón Energ. 2017, 15, 7-17. 42. Ethel, B. Toxicity and Metabolism of Industrial Solvents, Elsevier, London, 1965.


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Energy & Fuels

Table 1. Some characteristics of crude oils named APS, APA and APB CENPES/Petrobras39 Crude oil samples

APS

APA

APB

Density (g/cm3)

0.976

0.935

0.880

º API

13.0

14.8

31.0

Asphaltenes (m/m%)

6.20

3.6

< 0.5

Water content (Karl Fisher) (m/m%)

0.05

0.09

0.03


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Table 2. Precipitation onset of C3I asphaltenes fraction (at 30 wt% in toluene), obtained when using n-heptane, ethanol and dioxane as titrant, and solubility parameters (δ) of the solvent/non-solvent mixture at the precipitation onset Asphaltenes precipitation onset

δ of titrant30

δ at the onset

(mL of titrant / g oil phase)

(MPa)1/2

(MPa)1/2

n-Heptane

3.48

15.3

15.9

Ethanol

0.18

26.5

19.5

Dioxane

1.70

20.5

19.6

Titrant

Solubility parameter (δ) of toluene = 18.2 (MPa)1/2,37 Density of toluene = 0.867 g/mL42


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Table 3. Precipitation onset of crude oil APB after adding C3I in toluene or C5I in toluene. Data obtained for the APB added of APS is also described in this table for comparison Model system or APS crude oil

Crude

Asphaltenes

Asphaltenes

Concentration

Proportion

precipitation onset

fraction

of asphaltenes in

APB:oil

(mL of n-heptane/g of

toluene

phase

oil phase)

(m/m%)

(m/m)

(± 0.1)

-

50:50

2.46

-

80:20

1.96

15

50:50

2.33

30

50:50

2.32

30

80:20

2.04

15

50:50

2.17

30

50:50

2.14

15

80:20

1.50

30

80:20

1.47

oil

APS

C3I APB

C5I


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Table 4. Calculation of global solubility parameters (Hildebrand) using Wiehe approximation and using lower, mean and upper parameters calculated for the asphaltenes C3I (extracted from crude oil APS) 1/2

Global solubility parameter (MPa ) Crude oil and Wiehe approximation

δ lower

δ mean

δ upper

(δ δ=16.35)

(δ δ=15.9)

(δ δ=17.7)

(δ δ=19.5)

APS

19.68

17.80

25.30

32.80

APA

20.07

18.03

26.21

34.39

APB

19.50

17.70

24.90

32.10

APA:APS 80:20

19.25

17.56

24.34

31.13

APB:APS 80:20

17.26

16.20

20.46

24.71

APB:APS 50:50

18.19

16.97

21.97

26.97

system

The solubility parameters of the pure crude oils and their mixtures were calculated using, respectively, Equations 3 and 4.


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Table 5. Percentage difference between the solubility parameter calculated from the precipitation onset for the crude oil using Equation 3 and the solubility parameter calculated from the precipitation onset for the crude oil mixed to the APS using Equation 4 Difference between the solubility parameter calculated (%) Crude oil Wiehe approximation

δ lower

δ mean

δ upper

(δ δ=16.35)

(δ δ=15.9)

(δ δ=17.7)

(δ δ=19.5)

APA

4.1

2.6

7.1

9.5

APB

11.5

8.5

17.8

23


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Figure Captions Figure 1. Absorption intensity (at 1600 nm) versus volume of n-heptane for the crude oils: (a) APS, (b) APA and (c) APB.

Figure 2. Absorption intensity (at 1600 nm) versus volume of n-heptane for the crude oils mixtures: (a) APA:APS (80:20 m/m), (b) APB:APS (80:20 m/m) and (c) APB:APS (50:50 m/m).

Figure 3. Absorption intensity (at 1600 nm) versus volume of n-heptane for the crude oils mixtures: (a) APB:C3I (85:15 m/m), (b) APB:C3I 30 m/m/% in toluene (80:20 m/m).


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Figure 1


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Figure 2


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Figure 3


The asphaltenes precipitation onset: influence of the addition of a

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