Determination of Asphaltene Particle Size: Influence of Flocculant

Article pubs.acs.org/EF

Determination of Asphaltene Particle Size: Influence of Flocculant, Additive, and Temperature Claudia R. E. Mansur,* Andressa R. de Melo, and Elizabete F. Lucas Federal University of Rio de Janeiro, Institute of Macromolecules, Laboratory of Macromolecules and Colloids for Petroleum Industry, Av. Horácio Macedo, 2030, Cidade Universitária, 21941598, Rio de Janeiro, RJ, Brazil ABSTRACT: Variations in temperature, pressure and composition during oil production, transport and refining can compromise the stability of asphaltenes. Flocculation occurs when the asphaltenes particles associate and form clusters that precipitate out of the solution. The objective of this work was to evaluate the variation in the size of the asphaltenes macromolecule particles as a function of the concentration of asphaltenes, the thermodynamic quality of the solvent medium, the addition of asphaltenes dispersants, temperature of the dispersion, and exposure time of the asphaltenes to the model solvent, using the photon correlation spectroscopy (PCS) technique with detection by backscattering. The average size of the asphaltenes particles in toluene at low concentrations ranged from 12 to 22 nm. With the successive addition of a flocculant (n-heptane), the particle size increased until their precipitation (>1000 nm). The presence of a dispersant additive in these systems caused a reduction in the size of the asphaltenes aggregates, inhibiting the formation of particles larger than 1000 nm. These results agree with those of the asphaltenes precipitation onset test. The increasing temperature of the asphaltenes dispersions caused a reduction in the sizes of the aggregates, characterizing UCST phase behavior (upper critical solution temperature) for the asphaltenes evaluated. On the other hand, increased contact time of the asphaltenes with the model solvent increased the average size of the asphaltenes particles.

1. INTRODUCTION Issues related to the crystallization and deposition of heavy organic fractions during the production, transport, and storage of crude oil can cause huge losses to the petroleum industry. The heavy organic fractions can contain waxes, resins, asphaltenes, and organometallic compounds, which exist in oil in different quantities, states, and forms. Problems associated with organic deposition can be encountered at any stage of the oil production process, from the well to the refinery.1 For instance, the stability of water in crude oil emulsions is attributed, at least in part, to the presence of asphaltenes at the emulsion interface.2−5 Resins are defined as the fraction of petroleum that is insoluble in ethyl acetate and soluble in hydrocarbons such as pentane, heptane, benzene, and toluene.6−8 Asphaltenes are defined as the fraction of petroleum insoluble in light hydrocarbons (pentane, hexane, heptane) but soluble in benzene and toluene. They have molar masses determined by different techiniques that generally vary from 5 × 102 to 103, but can reach the order of 104.9−14 If obtained by extraction with a solvent, the molar mass of the fraction can vary in function of the alkane chain used as a precipitating agent. In general, it is larger the longer this chain is.15−17 Due to their greater polarity, which causes them to be less hydrophobic than other petroleum fractions, asphaltenes have a tendency to agglomerate. Variations in the temperature, pressure, and composition of crude oil during the extraction, transport, and refining stages can compromise the stability of the asphaltenes in the oil.18−22 It is believed that asphaltenes are found in equilibrium, in which their molecules are partially dissolved and partially in the form of micelles, depending on the oil’s composition. The greater the presence of aromatics, the more free asphaltenes © 2012 American Chemical Society

give rise to micelle formation. The formation of micelles is a reversible process, in which the asphaltenes molecules (surfactants) associate, forming different geometric shapes, but remaining in suspension. This phenomenon only occurs when the surfactant concentration exceeds a given limit, known as the critical micelle concentration (CMC). It must be stressed that asphaltenes flakes and aggregates are caused in apolar (hydrophobic) media, such as paraffinic oils, and should not be confused with micelles and micelle aggregates, which are formed in more polar (aromatic) media, such as the case of oils with higher quantities of aromatic fractions.23 Hu and Guo24 studied the effect of temperature, size of the chain of the flocculant agent (n-alkane), and dilution ratio on the precipitation of the asphaltenes obtained from a Chinese oil sample. Their study was carried out by precipitating the asphaltenes from the oil at four temperatures, ranging from 293 to 338 K. Besides this, they used seven types of n-alkane as flocculant: n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, and n-dodecane. It was showed that the quantity of asphaltenes precipitated diminishes as the chain size of the flocculant increases. This result is expected25 because for flocculants with longer chains, only the larger asphaltenes molecules will be precipitated. Besides this, they found that increasing temperature causes a decrease in the final quantity of asphaltenes precipitated.24 Burya et al.26 showed that flocculation occurs through the association of asphaltenes particles and their subsequent growth until they form flakes that precipitate out. The solubilizing Received: March 1, 2012 Revised: July 19, 2012 Published: July 23, 2012 4988

dx.doi.org/10.1021/ef300365x | Energy Fuels 2012, 26, 4988−4994

Energy & Fuels

Article

controlled by a DECstation 320SX computer running IR data manager software from PerkinElmer. 2.2. Commercial Additives Evaluated as Asphaltenes Dispersants. The following commercial chemical additives were tested as asphaltenes stabilizers: 4-n-octyl-benzoic acid and linear alkyl benzene sulfonic acid (Sigma-Aldrich-USA). The alkyl chain present in the linear alkyl benzene sulfonic acid is composed of an average of 10− 13 carbon atoms. 2.3. Evaluation of the Additives in the Asphaltenes Dispersion Process. The efficiency of the additives as asphaltenes dispersants was evaluated by the asphaltenes precipitation onset test, in a Varian Cary 50 ultraviolet spectrometer, equipped with an external reader with optical paths of 2, 5, 10, and 40 mm. A wavelength of 850 nm was chose, because at shorter wavelengths (500, 600, and 700 nm), the absorption intensity values fell below the device’s scale or presented deviations from Beer−Lambert law. Successive dilutions from a solution of 0.1% w/v of asphaltenes in toluene were prepared with the addition of different volumes of nheptane. The solutions were left at rest for 24 h before the measurements in order to achieve or to get close to the thermodynamic equilibrium. After stirring each dispersion, we placed it in the device to read the absorption, from which the graphs of the absorption as a function of the ratio between the volumes of n-heptane and toluene in each of the systems were plotted. 2.4. Analysis of Particle Sizes. The size of the asphaltenes particles in toluene and in solutions containing heptane/toluene mixtures was analyzed by laser backscattering in a Malvern Nanosizer ZS, at 633 nm (Figure 1 shows the optical arrangement of the

power of the liquid phase in relation to the asphaltenes becomes insufficient to keep them in solution. The asphaltenes particles then pass from a state of agglomeration, which corresponds to an average diameter of roughly 0.02 μm, to form flakes of precipitate, whose diameter is significantly larger, at around 3 μm. In another study, Andreatta et al.27 showed that asphaltenes form nanoaggregates at low concentrations in a toluene solution. Clusters of nanoaggregates can form at much higher concentrations but are weakly bound and can dissociate at modest temperatures such as those found in oil reservoirs.28 Mullins et al.29 observed the formation of nanoaggregates in crude oil. Recently, he has reported that resins can associate with these asphaltenes aggregates; however they are not needed to stabilize asphaltenes nanoaggregates in toluene.9 Asphaltenes particle sizes have been studied by different techniques, such as dynamic light scattering,26,30,31 small angle neutron scattering,11,12,30,32 and small-angle X-ray scattering.30,32 However, due to the complexity of asphaltenes, further studies are required to understand their phase behavior under different conditions. The objective of this work was to evaluate the variation in size of asphaltenes macromolecule particles as a function of the asphaltenes concentration, thermodynamic quality of the model solvent medium, addition of a dispersant additive, dispersion temperature, and exposure time of the asphaltenes to the model solvent, using the photon correlation spectroscopy (PCS) technique with detection by backscattering. PCS studies the fluctuations of the intensity of light scattered by the particles of a diluted suspension and the relationship to their size. These fluctuations are a function of the random or Brownian motion of the particles resulting from the shock with the solvent molecules around them. The detection angle of the light scattered in this type of measurement is generally 90°, and it is not possible to observe a high absorption of the light radiated from the asphaltenes dispersions due to the presence of darkcolored polyaromatic aggregates.33 In this work, we evaluate the size of the asphaltenes particles using another type of equipment, which also operates by the dynamic light scattering technique, but where detection occurs at an angle of 173° and the incident ray does not pass through the entire sample. Therefore, the amount of scattered light collected by the detector is lower, making it possible to reduce the effect known as multiple scattering. This facilitates the analysis of high, opaque, or even dark concentrations.

Figure 1. Scheme of optical arrangement of Malvern ZS equipment. equipment). The tests varying the asphaltenes concentration in the solution were performed in the interval between 0.01% and 0.1% w/v and also varying the temperature of these systems, between 10 and 50 °C. In all cases, the flasks were sealed to avoid evaporation. The change in size of the asphaltenes particles in toluene was evaluated as a function of the concentration of flocculant solvent (nheptane), with or without the presence of a dispersant additive. All the tests were carried out after leaving the dispersion prepared at rest for 24 h. Besides this, we studied the speed of sedimentation of the asphaltenes aggregates in toluene/heptane mixtures, in this case not waiting 24 h. Thus, time zero corresponded to the recently prepared asphaltenes dispersion, and we made readings at various time intervals thereafter. Before analyzing the particle size, we measured the refraction indexes of all the solutions using a Bausch & Lomb Abbe refractometer and determined the viscosities of the solvents in an RS600 rheometer equipped with a DG41 coaxial cylinder sensor. These two parameters are important: the solvent viscosity (η) is required to calculate the particle size (RH) from the diffusion coefficient (Do), as established by Stokes−Einstein equation (eq 1);33the refractive index of the sample is

2. EXPERIMENTAL SECTION 2.1. Obtaining and Characterizing the Asphaltene Samples. Two asphaltene samples were used, extracted from a distillation residue and from a Brazilian petroleum sample, called in this work asphaltenes-DR and asphaltenes-BP, respectively. The asphaltene fractions were extracted by solubility difference, using a Soxhlet extractor, following the IP 143 standard procedure.34 Ten grams of material (residue or petroleum) were placed on filter paper and taken to the extractor. A first extraction was carried out using 500 mL of n-heptane for 24 h or until the solvent in the extractor was clear. The maltene fraction was separated in this step. The resulting material was then extracted with 500 mL of toluene for 48 h for complete solubilization of the asphaltenes fraction. The toluene was evaporated from the solution in a rotating evaporator and then the final asphaltenes sample was dried in a vacuum chamber. The asphaltenes samples were characterized by using a PerkinElmer Fourier transform infrared (FTIR) spectrometer (model 1720x), 4989


Energy & Fuels

Article

onset show that at the onset point, the first asphaltenes aggregates are already dispersed in the system. Figure 2 shows the precipitation onset test results for the pure asphaltenes-DR and in the presence of the additive 4-n-

needed to obtain the results as volume or number percent versus particle size. We can obtain graphs of intensity or volume or number against particle size. We have expressed our results as volume (%) versus particle size (nm).

RH = kT /6πηDo

(1)

where k = Boltzmann constant and T = absolute temperature.

3. RESULTS AND DISCUSSION 3.1. Obtaining and Characterizing the Asphaltenes Sample. The FTIR spectra were very similar for the asphaltenes samples extracted and matched the characteristic bands already observed in the literature:35,36 a doublet at 2923 and 2853 cm−1, related to the axial deformation of the CH2 and CH3; a peak at 1605 cm−1, corresponding to the conjugated bands of the CC and CO bonds; peaks at 1457 and 1376 cm−1, related, respectively, to the symmetric and asymmetric axial deformations of the CH3; a band at 1032 cm−1, characteristic of the sulphoxide grouping (C2SO); bands near 870 and 800 cm−1, corresponding to the deformation outside the plane of the CH ring; and a band at 750 cm−1, characteristic of the vibration of the four hydrogen atoms adjacent to the aromatic ring. 3.2. Evaluating the Additives in the Asphaltenes Dispersion Process. The chemical additives used to stabilize asphaltenes can be evaluated by determining the onset of asphaltenes precipitation. This onset refers to the concentration of the flocculant necessary to start the precipitation in a model system or oil sample. The onset of precipitation is characterized by the quantity of flocculant at which the first particles in the solution appear. In general, the flocculants used in this type of testing are light n-alkanes, such as n-pentane, n-heptane, or n-nonane, because asphaltenes are not soluble in these compounds.14 Various techniques can be employed to determine the onset of precipitation from petroleum, among them spectrometry using visible ultraviolet (UV−vis) and near-infrared (NIR). These techniques are based on the fact that the asphaltenes particles scatter light, which is detected by the equipment as an increase of the solution’s absorption. In general, a spectrometer compares the quantity of light that passes through the sample in relation to how much passes through the reference (blank) asphaltenes particles scatter part of the incident light, so that less light reaches the detector than do through the reference. Hence, the equipment interprets the phenomenon as if the scattered light were being absorbed by the solution. At the start of the test, the asphaltenes molecules are dispersed in the solution, which has a given absorbance. The addition of a relatively small quantity of flocculant causes a dilution in the solution, reducing its absorbance. As the quantity of flocculant added is increased, the first asphaltenes particles appear and the effects of the dilution and the light scattering compete, making the reduction in absorbance deviate from a linear relationship. When the quantity of aggregates present makes the effects of light scattering overcome that of the dilution, the absorbance begins to increase. This is the onset point of the system. The addition of more flocculant makes the quantity and size of the particles increase, and thus the absorbance continues to rise until a point at which the asphaltenes in the sample precipitates out. At this point, the addition of more flocculant once again results solely in dilution of the system, making the absorbance start to fall. This description of the effects involved and the determination of

Figure 2. Absorption intensity curves as a function of heptane/toluene ratio for the onset of precipitation test of asphaltene-DR, by UV− visible spectrometer: (a) asphaltene (initial concentration = 0.1wt/vol %) and (b) asphaltene + 4-n-octyl benzoic acid (initial concentrations = 0.1 and 0.5 wt/vol %, respectively).

octyl-benzoic acid with a 10-mm optical path. We also plotted the curves with optical paths of 2 and 5 mm. The results, although in agreement, showed less resolution than with the 10-mm path. It was not possible to plot a curve with the results obtained using a 40-mm path, because absorption values were very high (above 2) and very unstable. The onset of precipitation of the pure asphaltenes-DR was reached at a ratio of ∼0.9 of n-heptane/toluene, while with the presence of 4-n-octyl-benzoic acid in the system the onset shifted to 1.1 of heptane/toluene, evidencing the additive’s capacity to promote greater stability of the asphaltenes particles. The test conducted with linear alkyl benzene sulfonic acid is not shown because we did not observe any onset of precipitation from the asphaltenesDR. In a previous work,37 this additive proved to be an excellent dispersant, based on the observation of a discrete formation of precipitates from asphaltenes-DR only starting at an n-heptane/toluene ratio of 4.0. 3.3. Determining the Asphaltene Particle Sizes under Different Conditions. 3.3.1. Determining the Size of Asphaltene Particles As a Function of the Asphaltene Concentration. These tests were run in triplicate, with asphaltenes-DR in toluene, at concentrations of 0.01 to 0.1% w/v and a temperature of 25 °C. At the highest concentrations (0.05 and 0.1% w/v), it was not possible to measure using the current equipment, probably because the size distribution was too large and/or the solutions became very dark. Figure 3 shows the repeatability of measures obtained for concentration of 0.02% w/v. The reading of the particles size was obtained for the concentrations of 0.01, 0.017, 0.02 and 0.025% w/v of

Figure 3. Repeatability of measures obtained for asphaltenes-DR at a concentration of 0.02 wt/v %. 4990


Energy & Fuels

Article

Figure 5c compares the results of the asphaltenes-DR in toluene system (0.025% w/v) with the systems containing higher levels of n-heptane. With the increasing concentration of n-heptane up to 50% v/v, there is a decrease in the size of the asphaltenes aggregates, and we attribute that to the dilution of the solution. We can find in the literature another explanation that is the shrinkage of asphaltene cluster at the beginning of nheptane addition, observed by measuring intrinsic viscosity.11 However, it is already possible to observe at 40 and 50% v/v of n-heptane a small volume of asphaltenes particles of substantially larger size being formed (average size of 5000 nm). These results agree with those obtained in the precipitation onset test of this asphaltenes (Figure 2), where it was observed its precipitation starting at an n-heptane/ toluene ratio of 0.9. This result confirms that the onset test produces a precipitation value slightly above the real one, since at 40% v/v of n-heptane, that is, at a heptane/toluene ratio of 0.67, particles with average size of 5000 nm were already detected. Above 50% v/v, the average size of all the particles of this system increases sharply, reaching a wide distribution (1000− 8000 nm) at heptane concentrations of 75 and 80% v/v. The size of the particles of asphaltenes-DR with the addition of n-heptane in the presence of the additives linear alkyl benzene sulfonic acid and 4-n-octyl-benzoic acid was also evaluated. These additives were good dispersants for the asphaltenes-DR solutions, as shown previously. Figure 6 shows

asphaltenes (Figure 4), showing average particles sizes of 12, 14, 14, and 22 nm, respectively. A particle size of 20 nm has

Figure 4. Particle size of asphaltene-DR in toluene solution, at different concentrations. Temperature = 25 °C.

been cited in the literature for the agglomeration state of asphaltenes.26This behavior suggests a tendency for the asphaltenes aggregates in solution to increase in size as the concentration rises; however, the changes observed at lower concentrations have the same magnitude as the error. Because of these results, we limited the concentrations of asphaltenes in toluene to 0.01 and 0.025% w/v for subsequent tests. 3.3.2. Determining the Size of Asphaltenes Particles As a Function of the n-Heptane/Toluene Ratio. The particle sizes with the asphaltenes-DR dissolved in heptane/toluene mixtures at n-heptane concentrations of 15 and 30% v/v (Figure 5a and

Figure 6. Particle size of asphaltene-DR in toluene/heptane mixture in presence of (a) alkyl benzene sulfonic acid; (b) octyl benzoic acid. Initial asphaltene concentration =0.01% wt/v and initial additive concentration = 0.5% wt/v. Temperature = 25 °C. Figure 5. Particle size of asphaltene-DR in toluene/heptane mixture. Heptane concentration of (a) 15; (b) 30; (c) 0, 40, 50, 60, 75, and 80% v/v. Initial asphaltene concentration = 0.025% wt/v. Temperature = 25 °C.

that the presence of these additives in the asphaltenes dispersion, before the addition of the flocculant, causes a shift in the average particle size to a slightly higher figure (from 12 to 15 nm). Considering an error on the order of 10%, this variation can represent an interaction between the molecules of the asphaltenes and those of the additive, since the analyses of the pure additive in the solvent did not show particle sizes within the device’s detection range. An analysis of the results obtained for linear alkyl benzene sulfonic acid (Figure 6a) shows that the average particle sized declines with increasing addition of n-heptane up to 60% v/v, due to the effect of the medium’s dilution, that is, the additive keeps the asphaltenes stabilized in the medium, inhibiting the molecular aggregation process. At 80% v/v of n-heptane, the average particles size starts to increase, which agrees with the results of the

b, respectively) were also analyzed. The measurements were made at concentrations of 0.01 and 0.025% of asphaltenes-DR. We did not observe significant variations in the asphaltenes particle sizes in the dispersions (13 nm for 0.01% and 19 nm for 0.025%) in function of the variation in the n-heptane content, even when compared to the values obtained for the systems containing only toluene (12 nm for 0.01%v/v and 22 nm for 0.025% v/v), as shown in Figure 4. These results show that there is no variation in the size of the asphaltenes particles when the proportion of n-heptane in the mixture with toluene varied from 0 to 30%. 4991


Energy & Fuels

Article

precipitation test obtained by UV−vis reported in previous works.37 The results obtained with the addition of 4-n-octyl-benzoic acid (Figure 6b) show similar behavior, but at lower n-heptane concentrations (60% v/v) there is a significant increase in the size of the asphaltenes aggregates (>1000 nm). These results also agree with those obtained in the precipitation onset tests (Figure 2), where the presence of this additive retarded the precipitation of the asphaltenes-DR in toluene when a flocculant was added, but this additive was less efficient than linear alkyl benzene sulfonic acid, which is responsible for the formation of asphaltenes particles with smaller average size. 3.3.3. Determining the Size of Asphaltenes Particles as a Function of Time. The sizes of the asphaltenes particles as a function of time were also analyzed. These analyses sought to determine the speed of formation and sedimentation of the asphaltenes aggregates, or the kinetic of asphaltenes aggregates formation. The tests conducted with asphaltenes in toluene did not show variation in the particle size, within the time interval analyzed. Figure 7 presents the results obtained for the dispersion of asphaltenes-DR in toluene in the presence of 40% v/v of n-

Figure 8. Particle size of asphaltenes-PB (0.025 wt %/v), in toluene/nheptane (50/50 v/v) mixtures, measured at different resting times: (a) upper portion of dispersion and (b) lower portion of dispersion.

For both the asphaltenes samples, we observed a tendency for molecular aggregation as a function of time of contact of the particles with the solvent mixture. 3.3.4. Determining the Size of Asphaltenes Particles as a Function of Temperature. The size variation of the asphaltenes particles in function of temperature was studied with asphaltenes-PB. Figure 9a−c shows the results obtained for

Figure 7. Particle size of asphaltenes-DR (0.025 wt %/v) in toluene/nheptane (60/40 v/v) mixtures, measured at different resting times.

heptane. The solution was homogenized before the measurements. It can be observed that the average sizes of the aggregates of this asphaltenes increased with time, and only as of 18 h was the presence observed of a significant volume of particles larger than 1000 nm. However, since no measurements were made between 4 and 18 h, we cannot affirm that these larger particles would not have been observed in shorter intervals. Besides this, before the 4-h mark, the larger particles may have been too sparse for detection in the analyses. These results suggest that the formation of asphaltenes precipitates in oil, as a function of the variation of the oil’s composition or another condition unfavorable to the dispersal of asphaltenes, can occur well below the value determined experimentally in precipitation onset tests, if the system remains in an unfavorable condition for a determined period of time. Asphaltenes-PB were also analyzed. In this case, 50% v/v of heptane was added to the asphaltenes in toluene dispersion. The dispersions presented apparent separation of phases with rapid sedimentation of precipitates in the dispersion. Thus, in this case, we carried out two types of measurements: of the supernatant of the dispersion and of the lower part of the dispersion, where there would be a greater concentration of precipitated particles (Figure 8a and b, respectively). The results show that both in the supernatant, where the particles had smaller sizes (Figure 8a), and in the lower part of the solution, where the particles had sizes larger than 400 nm (Figure 8b), there was an increase in the size of these particles until 9 days.

Figure 9. Particle size of asphaltene-PB in toluene, at different temperatures, containing: (a) n-heptane (0% v/v); (b) n-heptane (25% v/v); and (c) n-heptane (90% v/v).

the asphaltenes-BP dispersions, respectively, in pure toluene (0% v/v of heptane) and at 25 and 90% v/v of heptane. In all the analyses, we observed a reduction in the average asphaltenes particle size with rising temperature, irrespective of the average size of the aggregate in the system, characterizing UCSTupper critical solution temperature behavior; that is the system tends to exhibit only one phase above critical temperature and two phases, below it. Similar results were obtained by Espinat et al.30,38 In the case of the systems containing flocculant, the size distribution of the asphaltenes particles was also narrower as temperature increased, that also evidence the increasing of asphaltenes solubility. 4992


Energy & Fuels

Article

(4) Maia Filho, D. C.; Ramalho, J. B. V. S.; Spinelli, L.; Lucas, E. F. Colloids Surf., A 2012, 396, 208−212. (5) Acevedo, S.; Escobar, G.; Gutierrez, L. B.; Rivas, H.; Gutierrez, X. Colloids Surf., A 1993, 71, 65−71. (6) Hirschberg, A.; de Jong, L. N. J.; Schipper, B. A.; Meijer, J. G. SPE J. 1984, 24, 283−293. (7) Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142, 497−502. (8) Andersen, S. I.; Speight, J. G. Pet. Sci. Technol. 2001, 19, 1−34. (9) Mullins., O. C. Annu. Rev. Anal. Chem. 2011, 4, 393−418. (10) Moschopedis, S. E.; Fryer, J. F.; Speight, J. G. Fuel 1976, 55, 227−232. (11) Fenistein, D.; Barré, L.; Broseta, D.; Espinat, D.; Livet, A.; Roux, J. N.; Scarsella, M. Langmuir 1998, 14, 1013−1020. (12) Spiecker, P. M.; Gawrys, K. L.; Kilpatrick, P. K. J. Colloid Interface Sci. 2003, 267, 178−193. (13) Rodgers, R. P.; Marshall, A. G. Petroleomics: Advanced characterization of petroleum derived materials by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). In Asphaltenes, heavy oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshal, A. G., Ed; Springer: New York, 2007; Chapter 3. (14) Honse, S. O.; Ferreira, S. R.; Mansur, C. R. E.; Gonzalez, G.; Lucas, E. F. Quim. Nova 2012. (15) González, G.; Middea, A. Colloids Surf. 1991, 52, 207−217. (16) Minssieux, L.; Bardon, C. Effects of asphaltenes deposition in production treatment and prevent tests. Proceedings of the International Symposium of Oil Production: Asphaltenes and Wax Deposition, Rio de Janeiro, Brazil, 1995, pp 129−134. (17) Middea, A.; Monte, M. B. M.; Lucas, E. F. Chem. Chem. Technol. 2008, 2 (2), 91−97. (18) Santos, R. G.; Mohamed, S. R.; Loh, W.; Bannwart, A. C. Influência de asfaltenos do petróleo sobre a reologia de emulsões O/A. Presented at the Rio Oil & Gas Expo and Conference 2004, Rio de Janeiro, Brazil, 2004; Paper IBP311-04. (19) Buckley, J. S.; Hirasaki, G. J.; Liu, Y.; Drasek, V.; Wang, J. X.; Gill, B. S. Pet. Sci. Technol. 1998, 16, 251−285. (20) Buckley, J. S. Energy Fuels 1999, 13 (2), 328−332. (21) Wang, J. X.; Buckley, J. S. Energy Fuels 2003, 17, 1445−1451. (22) Buckley, J. S.; Wang, J.; Creek, J. L. Solubility of the least-soluble asphaltenes. In Asphaltenes, heavy oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshal, A. G., Ed; Springer: New York, 2007; Chapter 16. (23) Branco, V. A. M.; Mansoori, G. A.; Xavier, L. C. A.; Park, S. J.; Manafi, H. J. Petrol. Sci. 2001, 32, 217−230. (24) Hu, Y. F.; Guo, T. M. Fluid Phase Equilib. 2001, 192, 13−25. (25) Speight, J. G. The chemistry and technology of petroleum, 3rd ed.; Marcel Dekker: New York, 1998. (26) Burya, Y. G.; Yudin, I. K.; Dechabo, V. A.; Kosov, V. I.; Anisimov., M. A. Appl. Opt. 2001, 40, 4028−4035. (27) Andreatta, G.; Bostrom, N.; Mullins, O. C. Langmuir 2005, 21, 2728−2736. (28) Anisimov, M. A.; Yudin, I. K.; Nikitin, V. V.; Nikolaenko, G. L.; Chernoutsan, A. I.; Toulhoat, H.; Frot, D.; Briolant, Y. J. Phys. Chem. 1995, 99, 9576−9580. (29) Mullins, O. C.; Betancourt, S. S.; Cribbs, M. E.; Dubost, F. X.; Creek, J. L.; Andrews, A. B.; Venkataramanan, L. Energy Fuels 2007, 21, 2785−2794. (30) Espinat, D.; Fenistein, D.; Barré, L.; Frot, D.; Briolant, Y. Energy Fuels 2004, 18, 1243−1249. (31) Yudin, I. K.; Anisimov, M. A. Dynamic light scattering monitoring of asphaltenes aggregation in crude oils and hydrocarbon solutions. In Asphaltenes, heavy oils and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshal, A. G., Ed; Springer: New York, 2007; Chapter 17. (32) Eyssautier, J.; Levitz, P.; Espinat, D.; Jestin, J.; Gummel, J.; Grillo, I.; Barré, L. J. Phys. Chem. B 2011, 115, 6827−6837.

A comparison of the results obtained for asphaltenes-BP (Figure 9) with those for asphaltenes-DR (Figures 4 and 5), at the same temperature and with the same solvent medium, show very different behaviors for the systems containing n-heptane in the mixture of solvents. This difference is probably due to the differences in the structures of the asphaltenes molecules, because differences only in the content of the various structures making up the fraction analyzed should have resulted in the presence of particles of the same size range, but in different volume percentages. This shows that although the FTIR technique is very useful to identify chemical groupings of molecules, it is not adequate to identify structural variations that cause significant behavior differences in solution/ dispersion. This difference of behavior is also evident in the study of the size variation of the particles with time.

4. CONCLUSIONS The results of the particle sizes show that asphaltenes have particles with average size on the order of 10 nm when dissolved in toluene at a concentration of 0.025% v/v and that, after precipitation, the particles size increases to a range of 1000−10 000 nm, depending on the thermodynamic quality of the solvent medium and time of contact of the asphaltenes with the solvent medium and temperature. It is evident that slightly below the precipitation onset of asphaltenes in a model solvent, determined by UV−vis, there is already a small volume of asphaltenes aggregated with average size over 1000 nm. Linear alkyl benzene sulfonic acid performs better as a stabilizer of asphaltenes particles than does 4-n-octyl-benzoic acid and consequently promotes asphaltenes particles with smaller average size. The particles size distribution results suggest there is an interaction between the asphaltenes macromolecules and the molecules of the dispersant additive. The size of the asphaltenes particles, in model systems, tends to decline with: (1) the reduction in the concentration of asphaltenes; (2) the increase in the thermodynamic quality of the solvent; (3) the addition of a dispersant additive; (4) the increase in the temperature of the analysis; and (5) the reduction in the contact time of the asphaltenes with the model solvent.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank the National Counsel of Technological and Scientific Development (CNPq), Coordination to Improve Higher Education Personnel (CAPES), and Petrobras.

■

REFERENCES

(1) Boukadi, A.; Philip, R. P.; Thanh, N. X. Appl. Geochem. 2005, 20, 1974−1983. (2) Ramalho, J. B. V. S.; Lechuga, F. C.; Lucas, E. F. Quim. Nova 2010, 33 (8), 1664−1670. (3) Lucas, E. F.; Mansur, C. R. E.; Spinelli, L.; Queirós, Y. G. C. Pure Appl. Chem. 2009, 81 (3), 476−494. 4993


Energy & Fuels

Article

(33) Lucas, E. F.; Soares, B. G.; Monteiro, E. Caracterização de ́ Polimeros: Determinação de peso molecular e análise térmica. E-papers: Rio de Janeiro, 2001; pp 103−106. (34) IP 143/84. Asphaltene Precipitation with Normal Heptane. Standard Methods for Analysis and Testing of Petroleum and Related Products; Institute of Petroleum: London, 1988; Vol. 1. (35) González, G.; Souza, M. A.; Lucas, E. F. Energy Fuels 2006, 20, 2544−2551. (36) Sousa, M. A.; Oliveira, G. E.; Lucas, E. F.; González, G. Prog. Colloid Polym. Sci. 2004, 128, 283−287. (37) Mansur, C. R. E.; Guimarães, A. R. S.; González, G.; Lucas, E. F. Anal. Lett. 2009, 42 (16), 2648−2664. (38) Espinat, D.; Rosenberg, E.; Scarsella, M.; Barré, L.; Fenistein, D.; Broseta, D. Colloidal structural evolution from stable to flocculated state of asphaltenes solutions and heavy crudes. In Structures and dynamics of asphaltenes; Mullins, O. C., Sheu, E. Y., Ed; Plenum Press: New York, 1998; Chapter 5.

4994


Determination of Asphaltene Particle Size: Influence of Flocculant

Recommend Documents