Suppression of Phase Separation as a Hypothesis to Account for

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Suppression of Phase Separation as a Hypothesis to Account for Nuclei or Nanoaggregate Formation by Asphaltenes in Toluene. Socrates Acevedo, Jimmy Castillo, Vicmary Vargas, Alexandra Castro, Omaira Z Delgado, Camilo Andrés Franco Ariza, Farid Bernardo Cotés, and Brice Bouyssiere Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 09 May 2018 Downloaded from http://pubs.acs.org on May 9, 2018

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Suppression of Phase Separation as a Hypothesis to Account for Nuclei or Nanoaggregate Formation by Asphaltenes in Toluene. Sócrates Acevedo1*, Jimmy Castillo1, Vicmary Vargas1, Alexandra Castro1, Omaira Delgado1, Farid B. Cortes2, Camilo A. F.ranco Ariza2, Brice Bouyssiere3 1

Facultad de Ciencias, Escuela de Química, Universidad Central de Venezuela,

Caracas, 1041, Venezuela 2

Facultad de Minas, Grupo de Investigación en Yacimientos de Hidrocarburos,

Universidad Nacional de Colombia Sede Medellín, 5 Kra 80 No. 65-223, Medellín, Colombia 3

CNRS/ Univ Pau & Pays Adour, Institut des Sciences Analytiques et de Physico-

Chimie pour L'Environnement et les Materiaux, UMR5254, Lcabie, 64000, Pau, France

ACS Paragon Plus Environment

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ABSTRACT

Here, the concept of suppression of phase separation is proposed to account for the solubility behavior of asphaltenes at high dilution in toluene under ambient conditions. Nuclei formation at concentrations near 90 mg L-1 is the consequence of reaching A1 fraction solubility, and phase separation is suppressed by the intercalation of sufficient A2 in these nuclei or nanoaggregates. Presumably, such intercalation leads to media penetration of the nuclei periphery, hindering the growth and allowing for nuclei dispersion as a kinetic unit. Trapped compounds (TCs) or compounds trapped by asphaltene clusters were isolated, and their elemental analysis showed that they were neither resins nor asphaltenes. The information available regarding the A1 and A2 asphaltene subfractions is revised and complemented with new thermogravimetric analysis, simulation distillation (SimDis) curves, micro-carbon Conradson, softening points and nanoparticle results involving size-exclusion microchromatography. In general, physical results, such as solubility, SimDis, aggregation and the softening point, differ substantially, whereas structural results, such as elemental analysis, DBE and 13C nuclear magnetic resonance spectra, are similar. These results suggest that minor structural differences strongly affect the solubility, softening point and other physical characteristics.


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INTRODUCTION The sudden aggregation of asphaltenes in toluene at a very low concentration, C0, under ambient conditions1-3 combined with weak growth of these aggregates above C0 is the consequence of reaching the solubility concentration (near 90 mg L-1) of the very low soluble asphaltene subfraction, A1. Thus, at concentrations of approximately 90 mg L-1, solid-phase separation occurs with the formation of nuclei in which, although type-A1 molecules prevail, the nuclei contain sufficient intercalated A2-type molecules to remain in solution, thus suppressing nuclei growing and phase separation. Presumably, such intercalation leads to media penetration of the nuclei periphery, hindering growth and allowing for nuclei dispersion as a kinetic unit. This idea, henceforth called the suppression of phase separation (SPS) concept, is consistent with the one-step formation of aggregates containing n molecules instead of multimers, such as dimers or trimers. Polarity alone is not sufficient to induce the aforementioned start-stop mechanism. Indeed, generally, highly polar molecules in nonpolar solvents such as toluene form multimers after a substantial number of steps.4-6 Asphaltene models that bear high molecular polyaromatic rings and oxygen functionalities in one case7 and aromatic nitrogen in another case8 afforded dimers or trimers when measured in toluene or dichlorobenzene. In this interesting, important and laborious work, the authors combined polarity and high molecular weight to induce aggregation of molecular


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models. However, no effect that resembled phase separation was reported. Presumably, the absence of hydrogen bonding interactions combined with the use of archipelago-type structures led to a relatively weak aggregation tendency. For decades, the aforementioned phase separation was confused with “inverse micelle” formation because of its similarities with the behavior of surfactants in oily media. This confusion has made the term “asphaltene micelle” common in the past as well as in recent years.9-25 The references cited here are only a small sample of the many published articles that mention asphaltene micelles, which, according to Science Direct, is approximately 1200 papers over the course of 40 years and approximately 200 papers in the last two years. In the case of surfactants in an oily medium, however, aggregation may occur via the stepwise formation of multimers.26 Moreover, the start-stop aggregation observed in the asphaltene case is difficult to explain using molecular aggregation, micellar concepts or any other mechanism.27 In the present SPS concept, phase separation is promoted by A1, as previously described, and the growth and flocculation of nuclei is hindered by intercalation of soluble component A2 in the separated nuclei. Thus, the nuclei produced could be described as a solid A1-A2 solution with a solubility parameter sufficiently close to that of toluene to keep them in solution. Thus, the nuclei formed could be defined as a lipophilic colloid stabilized by A2.


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The expression “suppression of phase separation” can be found in the literature to have the same meaning and in contexts different from those for asphaltenes.28,29 Thus, we are here using a concept that is generally well established for phase behavior. A preliminary account of matters described here has been recently reported.30 The trapping of molecules within asphaltene clusters in addition to asphaltenes is very likely responsible for the presence of metallic porphyrins in asphaltenes.31,32 Thus, for this trapping, no specific interaction is required other than molecular fitting within the cluster (nuclei, nanoaggregate, flock). Similar arguments could be made in the case of so-called trapped compounds (TCs) or compounds soluble in n-heptane and that cannot be removed from asphaltene after thorough Soxhlet extraction with boiling n-heptane31,33 (see below). In recent decades, research has been devoted to the possible interaction between asphaltenes and resins. We speculate that this interaction is another case of molecular intercalation in which conditions other than fitting within the cluster are not required. As shown in a previous communication, a continuous curve was obtained when the softening (melting) point of asphaltene-resin mixtures were plotted against the percentage of resin in the mixture34; thus, a continuous depression of melting point (from 280 to 50°C) with an increase in resin content (10-70%) was observed (see below). This result is consistent with the formation of


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asphaltene-resin liquid solutions at different melting points, and it strongly suggests extensive intercalation of asphaltene clusters by the resins. The use of nanoparticles (NPs) to reduce the size of asphaltene flocks formed in n-C7-toluene mixtures has been reported.35 Large reductions in the average flock size of up to 50% were observed when NPs were added to an asphaltene-toluene solution before the addition of n-C7.35 We propose that removal of substantial amounts of subfraction A1 by NPs disrupts the flocks by increasing their solubility, resulting in large-sized reductions. Large reductions in viscosity of extra heavy, non-Newtonian oil were found after the addition of NPs.36 The authors proposed that NPs disrupted the 3-D web formed by asphaltic components (asphaltenes and resins), resulting in the observed change. Here again, preferential removal of A1 could account for the viscosity change. As reported several times,31,33,37-40 asphaltenes can be fractionated into the two main subfractions, A1 and A2, using the p-nitrophenol (PNP) method, where the solubility (in toluene, under laboratory conditions) of A1 is very low (approximately 90 mg L-1), whereas the solubility of A2 is similar to that observed for asphaltenes (5-12%, depending on the sample). This method has been tested numerous times and with different samples and always affords the two fractions with similar solubility differences. The solubility parameters of asphaltenes A1 and A2 were consistent, with A1 being less soluble than A2 in more than 50 different


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solvents.39 In the present work, we use both A1 and subfraction A1 interchangeably to designate this fraction; the same will be valid for A2. Compared with A2, the H/C values obtained from elemental analysis consistently show lower values for A131,33,37-40. However, these fractions share similarities, such as similar contents of heteroatoms, similar 13C-NMR spectra (see below) and similar LDI MS (laser desorption ionization mass spectrometry) spectra.31 These data, together with the results presented later, were used here to propose molecular models for asphaltene subfractions A1 and A2. When A1 and A2 solutions in chloroform, nitrobenzene and o-dichlorobenzene (ODB) were studied using VPO, ratio r between number averages of the molecular mass of these fractions was reported to be higher than one for all studied conditions40: r=

M n ( A1 ) M n ( A2 )

These results clearly show that, compared with A2, A1 has a higher aggregation capacity. Using diffusion-ordered spectroscopy 1H nuclear magnetic resonance (DOSY), Durand et al.41 observed asphaltene aggregates with radii near 1.56 nm in toluene at a very high dilution (approximately 100 mg L-1). Similar values for the smallest particles (approximately 1.25 nm radii) have been reported for asphaltenes


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dissolved in resins (3%) using the combined transmission electron microscopyfreeze fracture technique.42 The aforementioned nuclei have been called nanoaggregates by Mullins, who assigned this name without any consideration of phase separation.3 We propose that the word “nucleus” conveys the idea of phase separation, and hence, we chose to use it in this work. As reported earlier,1 thermal diffusivity, D , measurements in toluene under ambient conditions show a minimum near 100 mg L-1 when plotted versus the asphaltene concentration. This behavior is consistent with aggregate formation at concentrations near 100 mg L-1. By contrast, no significant change in D was observed in THF when the same sample concentrations were used. Asphaltene is known to form aggregates in THF under ambient conditions; for instance, numberaverage molecular weight measurements produced values near 4000 g mol-1,43 which is much higher than the now-accepted mean of approximately 600 g mol-1.3 Thus, these data are consistent with the SPS concept in toluene and with stepwise aggregation in THF because the A1 subfraction is soluble in THF; therefore, no phase separation is expected in these dilute solutions. On the basis of the previous discussion, SPS behavior will occur in solvents where asphaltenes are soluble and where A1 is not. Apart from toluene and other


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monoaromatics such as xylene and cumene, PSD behavior is expected for other solvents such as carbon tetrachloride and 1,2-dichloroethane.39 We conclude this section by highlighting the fact that nuclei or nanoaggregate formation is a consequence of phase separation promoted by A1 and suppression promoted by A2.

EXPERIMENTAL All samples used here were from Hamaca oil, API: 9°. Asphaltenes were obtained using the IP 143 standard norm (approximately 30 mL of n-heptane/g oil). The precipitated solid mixture of resins and asphaltenes was transferred to a thimble and Soxhlet extracted with boiling n-heptane until transparence of the liquid returned to the flask (approximately 3 days). When required, co-precipitated resins were obtained after evaporation of filtrated under vacuum. Asphaltene subfractions A1 and A2 and TCs were obtained from a cumene solution of asphaltenes after contact with p-nitrophenol (PNP), as described previously.31,33,37-40 Briefly, after the precipitate was filtrated, it was dissolved in chloroform and treated with aqueous sodium hydroxide to remove PNP to obtain subfraction A1. The filtrated cumene solution containing subfraction A2, PNP and the TC was distilled under reduced pressure to remove cumene, dissolved in chloroform and treated in the same manner to remove PNP. The solvent was


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removed to yield the solid TC-A2 mixture. This mixture was then dissolved in a small volume of toluene and treated with an excess of n-heptane, from which A2 was recovered after precipitation and filtration. The TC was obtained from the filtrate after vacuum distillation to dryness. Yields for A1, A2 and the TC, related to recovery (approximately 80%) were approximately 60 ± 5%, 30 ± 5% and 8 ± 2% as an average of two determinations. NMR spectra were obtained using a 400 MHz Bruker spectrometer provided with an Advance 400 console. Samples were analyzed using 10 mm diameter borosilicate tubes with a dual 1H/13C probe. Samples (80 mg) were dissolved in deuterated chloroform (2 mL) in the presence of dissolved (15 mg) relaxing agent. These conditions were similar to others previously reported.38,44 Elemental Analysis. The C, H, N, S and O contents were obtained by combustion using an organic elemental analyzer (Thermo Scientific model Flash EA 1112 (Milan, Italy)). Molecular models of subfractions A1 and A2 and a nuclei simulation were calculated using molecular mechanics software, such as the one found in HyperChem. All of the models were optimized in terms of both energy and geometry. Calculations were performed by assuming vacuum conditions. A TA Instruments model STD Q600, and a thermo balance, NETZSCH model STA 409 PC, were used for thermogravimetric analysis (TGA). For each analysis,


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sample weights between 5 and 12 mg were used with argon as a carrier gas flowing at 100 mL min-1; the ramp rate was 10° min-1, and the investigated temperature range was 50-800°C. Carbonaceous residues or MCC were determined using the ASTM D4530-6 standard test using a Tanaka oven (model ACR-M3) with nitrogen as the carrier gas. The weighted sample was placed in a glass vial and then heated to 500°C for a specific period of time. Simulated distillation (SimDis) was performed in a gas chromatograph from Agilent Technologies, model 6890 and injector 7683 B series, using a method according to the ASTM D7169 norm. Carbon disulfide, a good solvent for all of the samples (A1, A2 and asphaltenes), was used in the preparation of all of the solutions. Chromatography columns were calibrated with linear paraffins (C3-C100), and reference standard oil was used to check the calibration and the percentages of sample recollection. Uncertainties were approximately 2-3% in temperature and less than 2% in distillation. In this work, the softening point of the solid mixtures is defined as the temperature required for melting the solid at atmospheric pressure. Operationally, this definition is the same as the melting point for pure solids. Softening point measurements were recorded using a regular melting point apparatus such as the Techne™ Mel-Temp™ digital melting point apparatus from


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Fisher Scientific. Sample mixtures were prepared in chloroform solutions after dissolution of the corresponding quantities of resins plus asphaltenes or resins plus A1 or resins plus A2. The solutions were refluxed for approximately one hour and left standing overnight, and then the chloroform was evaporated under vacuum. The solid mixture was then transferred to a melting point capillary, flushed with nitrogen and sealed via glass blowing. The softening points (SP) were determined by preheating the capillary cell to a temperature close to but below the expected SP; the capillary with the mixture was then placed to measure the corresponding SP. Softening of the samples occurred in a small range (approximately 1-2°C), and the mean value was recorded. The average of two measurements was recorded. Size-exclusion microchromatography (µSEC) with high-resolution inductively coupled plasma mass spectrometric detection (HR ICP MS) method (µSEC, for short) was employed in this study using previously reported equipment and procedures.33 Chromatography profiles were obtained using 500 mg L-1 solutions. Silica NPs were prepared via hydrolysis and condensation of tetraethyl orthosilicate (TEOS, 98%) in ethanol and in the presence of ammonia using a previously reported method.45 The mean size (20 nm) and half-width distribution (10 nm) were determined using DLS.

RESULTS AND DISCUSSION


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The elemental analysis results of all samples isolated in this work are reported in Table 1. As was the case for previously reported data,37 the heteroatom contents (N and O) and the H/C ratios for subfractions A1 and A2 are similar and the H/C ratio is higher for A2 (see Table 2). DBE differences are approximately 2 per 100 C atoms. The results for the TCs show that they are neither asphaltenes nor resins (see Table 1). As previously mentioned (see Experimental), they are soluble in nheptane and contain a large DBE that is similar to that found for asphaltenes. Presumably, this large DBE reflex presence of polyaromatic sectors enables favorable fitting of these TCs within asphaltene aggregates (see previous discussion). A similar argument could account for trapping of both vanadyl and nickel porphyrins. As shown elsewhere, the µSEC profiles of these aggregates were consistent with trapping by asphaltenes and by both subfractions A1 and A2.33 Moreover, whereas nickel porphyrins were completely trapped, approximately 30% of the vanadyl one remained as “free” molecules outside the aggregates.33 This finding is consistent with the aforementioned molecular fitting because of the geometrical differences between these porphyrines in which the VO group is perpendicular to the pyrrolic ring46 and the nickel is almost coplanar with the pyrrolic ring.47 The trapping of compounds is very likely to be the consequence of the porosity of these 3-D asphaltene aggregate webs, which was previously used to represent


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asphaltene aggregates as fractals.48 When dissolved in the crude, the pores of these aggregates (host) are occupied by components (guest) other than asphaltenes in which resins and porphyrines are conspicuous components. For trapping, no structural requirement (such as hydrogen bonding, ligand formation or polarity) other than fitting in the pores is required. These ideas were used by our laboratories to analyze vanadium profiles using the µ-GPC HR ICP MS technique.33 The very low oxygen content measured for the TC is noteworthy (Table 1). Although more research is needed to confirm this finding, this result is consistent with the TC being shielded by asphaltenes in the aggregate, thus protecting them from reactions in the environment. A similar argument was previously employed when metallic potassium failed to reduce all free radicals in a sample of asphaltenes that were dissolved in tetrahydrofuran.49 These arguments are also consistent with the previously discussed trapping of metallic porphyrins. We share the view that TCs are confined during oil generation from kerogen.50,51 As such, they are shielded within asphaltene clusters and protected from the environment through geological time. As shown in Figure 1, 13C NMR shows no significant differences between asphaltenes and their subfractions A1 and A2, suggesting that such compositional differences are not a factor that affects either their solubility differences or their structure.


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Similarities between MM distributions31 and the 13C NMR combined with the small elemental analysis differences suggest that structural differences exist, such as the one shown in Figure 2 for molecular models M1 and M2, where the two aliphatic rings in M1 were opened to build molecular model M2. The calculated parameters for these models are shown in Table 3. These structures resemble others that have been recently proposed.3 We expect that, when the aforementioned nuclei are formed in toluene, the long and open aliphatic chains of M2 will hamper the approach of these nuclei to each other, resulting in its dissolution or dispersion. The presence of long aliphatic open chains in asphaltene derivatives could increase the solubility to the point of making them soluble in n-heptane, which is the case for asphaltenes alkylated with n-octyl alkyl groups.43 Of note, the 13C NMR technique cannot distinguish between aliphatic (open chains) and alyciclic (ring chains) carbons. Figure 3 shows a molecular mechanics model employed here to simulate phase nuclei or nanoaggregates. In this case, seven molecules were used. Using this approach, we determined that the distances (approximately 2.9 nm) were close to others experimentally measured (approximately 3 nm)41 (see Introduction). Notably, the projection of alkyl chains into the media will hamper aggregation of the nuclei, thus preventing phase separation.


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The TGA results in Figure 4 indicate that subfraction A2 lost more volatile products than subfraction A1. This difference is consistent with the loss of more open aliphatic chains in A2. For instance, breaking C-C bonds is very likely the case for open chains. In the case of alicyclic chains, conversion to aromatic rings is also possible, thus leading to a greater amount of carbon residue. These arguments are also supported by the MCC results in Table 4. According to the SimDis results in Figure 5, at any temperature, the distilled amount of A2 is higher than A1 and the difference steadily increases with increasing temperature; moreover, the difference is substantial and much larger than the expected uncertainties (see Experimental). According to the results and the previous discussion, these SimDis results are consistent with the trend of forming aggregates in the stationary phase. The bigger and strongest aggregate (as expected for A1) results in less distillated amount obtained. The SP results obtained here are plotted in Figure 6. The presented results are averages of two SP determinations with errors of approximately 2%. As shown, both the asphaltenes and A2 lead to similar continuous curves in the examined resin percentage range. Thus, relatively small amounts of resin (approximately 5 and 10% for the asphaltenes and A2, respectively) are required to begin the SP reduction. By contrast, the corresponding curve for A1 differs from the others, where approximately 50% of resin is required to begin the temperature decrease.


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According to basic thermodynamics, the melting point depression for any pure solid A will be expected for any pure substance B that is soluble in the melt of A. This behavior is attributed to an increase in the escape tendency of A from solid to liquid, leading to fusion at temperatures below the one corresponding to pure A. Indeed, at these high asphaltene concentrations, aggregate clusters will be abundant; hence, the softening we measured corresponds to breaking of these clusters into smaller aggregates via both resin mixing and temperature. In the case of A1, and considering the aforementioned structural characteristics, clustering is more difficult to break, requiring very large amounts of resin. The µGPC results combined with HR ICP MS spectra are shown in Figure 7. Here, profiles were obtained for the THF solutions of asphaltenes with and without added nanoparticles. Except for NPs, these profiles correspond to the same asphaltene solutions. For the purpose of this discussion, we divide the bands into high (1200 s to approximately 1500 s), medium (1500 s to approximately 1700 s) and low (1700 s and beyond) molecular weight regions. The drastic changes observed are due to asphaltene removal; however, in addition to the expected removal, the addition of NPs drastically changed the relation among the high, medium and low regions. As reported previously, both the high and medium molecular weight regions correspond to aggregates.33 Hence, we strongly suggest that removal of A1 disrupts


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the aggregates by increasing their solubility, leading to smaller aggregates or free molecules. CONCLUSIONS AND FINAL COMMENTS The SPS concept in which asphaltene subfractions A1 and A2 play different roles in the formation and maintenance in solution of a new nuclei phase has been described here. The very low solubility of A1 in toluene promotes nuclei formation and intercalation, and the soluble subfraction A2 promotes nuclei solubility and hinders nuclei growth. As a result of this process being similar to aggregation of surfactants in oily media, confusion has occurred, and terms such as “asphaltene micelles” have been and continue to be used in the literature. SPS is consistent with the start-stop aggregation of asphaltenes in toluene, which in turn is consistent with the literature (see previous discussion) that has reported sudden changes in the parameters measured. The process starts with the formation of A1 nuclei and stops when sufficient A2 has been intercalated, thus making these nuclei soluble in the media. A combination of high molecular weight, polarity and aromaticity is not sufficient to induce behavior similar to the SPS mechanism.7,8 Apparently, hydrogen bonding and island-type molecules should also be included. Results of analyses such as TGA, SimDis and the softening point measurements show clear differences between subfractions A1 and A2, whereas elemental analysis, 13C-NMR and LDI MS show similarities. These results are all consistent


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with subfraction A1 being responsible for nuclei formation and the formation of larger and stable aggregates. Presumably, these issues are responsible for many field problems. Molecular models that are consistent with solubility, nuclei size and nuclei stability were proposed here. The removal of A1 from asphaltene clusters (nuclei, aggregates or flocks) disrupts these arrays, leading to smaller bodies or lower-molecular weight entities. Trapping of compounds other than asphaltenes or TCs within the asphaltene cluster is likely to be the result of intercalation or fitting within the cluster; an interesting possibility is that these TCs became trapped during the genesis of the oil and remained there during geological time. The aforementioned soluble nuclei have been referred to as nanoaggregates3 to underscore the small size of the aggregates in the highly diluted concentration interval. The present SPS concept affords a solid platform for the origin and formation of these nuclei or nanoaggregates, and we hope that our findings contribute to abandoning the use of micelle concepts, which are erroneous and misleading when applied to asphaltene solutions.

AUTHOR INFORMATION Corresponding Author: Sócrates Acevedo Funding Sources. The financial support of the Conseil Regional d’Aquitaine (20071303002PFM) and FEDER (31486/08011464) is Acknowledged. ACS Paragon Plus Environment

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Notes: "The authors declare no competing financial interest."

ACKNOWLEDGMENTS: The authors acknowledge COLCIENCIAS, and Agencia Nacional de Hidrocarburos (Colombia) for their support provided through the 272-2017 agreement. They also acknowledge the Universidad Nacional de Colombia for logistical and financial support. We thank Fundacion Polar for partial funding of this work.


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REFERENCES (1) Acevedo, S.; Ranaudo, M. A.; Pereira, J. C.; Castillo, J.; Fernández, A.; Pérez, P.; Caetano, M. Thermo-Optical Studies of Asphaltene Solutions: Evidence for Solvent–Solute Aggregate Formation. Fuel 1999, 78 (9), 997-1003. (2) Andreatta, G.; Bostrom, N.; Mullins, O. C. High-Q Ultrasonic Determination of the Critical Nanoaggregate Concentration of Asphaltenes and the Critical Micelle Concentration of Standard Surfactants. Langmuir 2005, 21 (7), 2728-2736. (3) Mullins, O. C. Review of the Molecular Structure and Aggregation of Asphaltenes and Petroleomics. SPE Journal 2008, 13 (1), 48-57. (4) Dega-Szafran, Z.; Szafran, M.; Kreglewski, M. Aggregation of 4-Substituted Pyridine N-Oxide–Trifluoroacetates in Benzene. J. Chem. Soc., Perkin Trans. 2 1980, (10), 1516-1519. (5) Dega-Szafran, Z.; Szafran, M. Aggregation of Complexes of Some Phenols with Triethylamine in Benzene. J. Chem. Soc., Perkin Trans. 2 1987, (7), 897-899. (6) Pochapsky, S. S.; Mo, H.; Pochapsky, T. C. Closed-Shell Ion Pair Aggregation in Non-Polar Solvents Characterized by NMR Diffusion Measurements. J. Chem. Soc., Chem. Commun. 1995, (24), 2513.


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Energy & Fuels

(7) Akbarzadeh, K.; Bressler, D. C.; Wang, J.; Gawrys, K. L.; Gray, M. R.; Kilpatrick, P. K.; Yarranton, H. W. Association Behavior of Pyrene Compounds as Models for Asphaltenes. Energy Fuels 2005, 19 (4), 1268-1271. (8) Tan, X.; Fenniri, H.; Gray, M. R. Pyrene Derivatives of 2,2′-Bipyridine as Models for Asphaltenes: Synthesis, Characterization, and Supramolecular Organization. Energy Fuels 2008, 22 (2), 715-720. (9) Najafi, I.; Amani, M. Asphaltene Flocculation Inhibition with Ultrasonic Wave Radiation: A Detailed Experimental Study of the Governing Mechanisms. Adv. Pet. Explor. Dev. 2011, 2 (2), 32-36. (10) Meighani, H. M.; Ghotbi, C.; Behbahani, T. J.; Sharifi, K. Evaluation of PCSAFT Model and Support Vector Regression (SVR) Approach in Prediction of Asphaltene Precipitation Using the Titration Data. Fluid Phase Equilib. 2018, 456, 171-183. (11) Kargarpour, M. A.; Dandekar, A. Analysis of Asphaltene Deposition in Marrat Oil Well String: A New Approach. J. Pet. Explor. Prod. Technol. 2016, 6 (4), 845-856. (12) Andersen, S. I.; Christensen, S. D. The Critical Micelle Concentration of Asphaltenes as Measured by Calorimetry. Energy Fuels 2000, 14 (1), 38-42.


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(13) Paliukait, M.; Vaitkusa, A.; Zofkab, A. Evaluation of Bitumen Fractional Composition Depending on the Crude Oil Type and Production Technology. In 9th International Conference “Environmental Engineering”, Vilnius, Lithuania, May 22-23, 2014. (14) Rajagopal, K.; Silva, S. M. C. An Experimental Study of Asphaltene Particle Sizes in n-Heptane-Toluene Mixtures by Light Scattering. Braz. J. Chem. Eng. 2004, 21 (4), 601-609. (15) Priyanto, S.; Mansoori, G. A.; Suwono, A. Measurement of Property Relationships of Nano-Structure Micelles and Coacervates of Asphaltene in a Pure Solvent. Chem. Eng. Sci. 2001, 56 (24), 6933-6939. (16) Ferworn, K. A.; Svrcek, W. Y.; Mehrotra, A. K. Measurement of Asphaltene Particle Size Distributions in Crude Oils Diluted with n-Heptane. Ind. Eng. Chem. Res. 1993, 32 (5), 955-959. (17) Panuganti, S. R.; Tavakkoli, M.; Vargas, F. M.; Gonzalez, D. L.; Chapman, W. G. SAFT Model for Upstream Asphaltene Applications. Fluid Phase Equilib. 2013, 359, 2-16.; Sinnathamb, C. M.; Nor, N. M.; Ahmad, M. Z. Fouling Characteristic and Tendencies of Malaysian Crude Oils Processing. J. Appl. Sci. 2011, 11 (10), 1815-1820.


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Energy & Fuels

(18) Sheu, E. Y. Physics of Asphaltene Micelles and Microemulsions-Theory and Experiment. J. Phys.: Condens. Matter 1996, 8 (25A), A125-A141. (19) Ovalioglu, H.; Kirimli, H. E.; Akay, C. Intermolecular Magnetic Spin-Spin Interaction in Asphaltene Suspensions at 1.53 mT. Acta Phys. Pol., A 2016, 129 (4), 806-809. (20) Hussein, H. Q.; Mohammad, S. A.-W. Viscosity Reduction of Sharqi Baghdad Heavy Crude Oil Using Different Polar Hydrocarbons, Oxygenated Solvents. Iraqi Journal of Chemical and Petroleum Engineering 2014, 15, 39-48. (21) Hu, R.; Crawshaw, J. Measurement of the Rheology of Crude Oil in Equilibrium with CO(2) at Reservoir Conditions. J. Vis. Exp. 2017, (124), e55749. (22) Varanda, C.; Portugal, I.; Ribeiro, J.; Silva, A. M. S.; Silva, C. M. Influence of Polyphosphoric Acid on the Consistency and Composition of Formulated Bitumen: Standard Characterization and NMR Insights. J. Anal. Methods Chem. 2016, 2016, 1-16. (23) Sabbaghi, S.; Shariaty-Niassar, M.; Ayatollahi, S. H.; Jahanmiri, A. Characterization of Asphaltene Structure Using Atomic Force Microscopy. J. Microsc. 2008, 231 (3), 364-373.


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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(24) Eberhardsteiner, L.; Füssl, J.; Hofko, B.; Handle, F.; Hospodka, M.; Blab, R.; Grothe, H. Influence of Asphaltene Content on Mechanical Bitumen Behavior: Experimental Investigation and Micromechanical Modeling. Mater. Struct. 2014, 48 (10), 3099-3112. (25) Pacheco-Sánchez, J.; Mansoori, G. A. Tricritical Phenomena in Asphaltene/Aromatic Hydrocarbon Systems. Rev. Mex. Fis. 2013, 59 (6), 584-593. (26) Hiemenz, P. C.; Rajagopalan, R. Principles of Colloids and Surface Chemistry. Third Edition, Revised and Expanded; Marcel Dekker: New York, NY, 1997; Chapter 8, pp 386. (27) Dukhin, A.; Parlia, S. Ions, Ion Pairs and Inverse Micelles in Non-Polar Media. Curr. Opin. Colloid Interface Sci. 2013, 18 (2), 93-115. (28) Bai, P.; Cogswell, D. A.; Bazant, M. Z. Suppression of Phase Separation in LiFePO4 Nanoparticles During Battery Discharge. Nano Lett. 2011, 11 (11), 48904896. (29) Pande, J.; Berland, C.; Broide, M.; Ogun, O.; Melhuish, J.; Benedek, G. Suppression of Phase Separation in Solutions of Bovine Gamma IV-Crystallin by Polar Modification of the Sulfur-Containing Amino Acids. Proc. Natl. Acad. Sci. U. S. A. 1991, 88 (11), 4916-4920.


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(30) Acevedo, S. Phase Separation of Asphaltene Dissolved in Toluene Leads to Nuclei Formation Promoted by the Very Low Solubility of Fraction A1. Soluble Fraction A2 Hinders Nuclei Flocculation. Pet. Petro. Chem. Eng. J. 2017, 1 (3), 000115. (31) Acevedo, S. C.; Cordero T. J. M.; Carrier, H.; Bouyssiere, B.; Lobinski, R. Trapping of Paraffin and Other Compounds by Asphaltenes Detected by Laser Desorption Ionization−Time of Flight Mass Spectrometry (LDI−TOF MS): Role of A1 and A2 Asphaltene Fractions in this Trapping. Energy Fuels 2009, 23 (2), 842848. (32) Mujica, V.; Nieto, P.; Puerta, L.; Acevedo, S. Caging of Molecules by Asphaltenes. A Model for Free Radical Preservation in Crude Oils. Energy Fuels 2000, 14 (3), 632-639. (33) Acevedo, S. C.; Guzmán, K.; Labrador, H.; Carrier, H.; Bouyssiere, B.; Lobinski, R. Trapping of Metallic Porphyrins by Asphaltene Aggregates: A Size Exclusion Microchromatography with High-Resolution Inductively Coupled Plasma Mass Spectrometric Detection Study. Energy Fuels 2012, 26 (8), 49684977.


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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(34) Franco, C. A.; Lozano, M. M.; Acevedo, S.; Nassar, N. N.; Cortés, F. B. Effects of Resin I on Asphaltene Adsorption onto Nanoparticles: A Novel Method for Obtaining Asphaltenes/Resin Isotherms. Energy Fuels 2016, 30 (1), 264-272. (35) Nassar, N. N.; Betancur, S.; Acevedo, S.; Franco, C. A.; Cortés, F. B. Development of a Population Balance Model to Describe the Influence of Shear and Nanoparticles on the Aggregation and Fragmentation of Asphaltene Aggregates. Ind. Eng. Chem. Res. 2015, 54 (33), 8201-8211. (36) Taborda, E. A.; Franco, C. A.; Ruiz, M. A.; Alvarado, V.; Cortés, F. B. Experimental and Theoretical Study of Viscosity Reduction in Heavy Crude Oils by Addition of Nanoparticles. Energy Fuels 2017, 31 (2), 1329-1338. (37) Gutiérrez, L. B.; Ranaudo, M. A.; Méndez, B.; Acevedo, S. Fractionation of Asphaltene by Complex Formation with para-Nitrophenol. A Method for Structural Studies and Stability of Asphaltene Colloids. Energy Fuels 2001, 15 (3), 624-628. (38) Acevedo, S.; Escobar, O.; Echevarria, L.; Gutiérrez, L. B.; Méndez, B. Structural Analysis of Soluble and Insoluble Fractions of Asphaltenes Isolated Using the PNP Method. Relation between Asphaltene Structure and Solubility. Energy Fuels 2004, 18 (2), 305-311.


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(39) Acevedo, S. C.; Castro, A.; Vásquez, E.; Marcano, F.; Ranaudo, M. A. A. Investigation of Physical Chemistry Properties of Asphaltenes Using Solubility Parameters of Asphaltenes and Their Fractions A1 and A2. Energy Fuels 2010, 24 (11), 5921-5933. (40) Acevedo, S. C.; Guzman, K.; Ocanto, O. Determination of the Number Average Molecular Mass of Asphaltenes (Mn) Using Their Soluble A2 Fraction and the Vapor Pressure Osmometry (VPO) Technique. Energy Fuels 2010, 24 (3), 1809-1812. (41) Durand, E.; Clemancey, M.; Lancelin, J.-M.; Verstraete, J.; Espinat, D.; Quoineaud, A.-A. Aggregation States of Asphaltenes: Evidence of Two Chemical Behaviors by 1H Diffusion-Ordered Spectroscopy Nuclear Magnetic Resonance. J. Phys. Chem. C 2009, 113 (36), 16266-16276. (42) Acevedo, S.; García, L. A.; Rodríguez, P. Changes of Diameter Distribution with Temperature Measured for Asphaltenes and Their Fractions A1 and A2. Impact of These Measurements in Colloidal and Solubility Issues of Asphaltenes. Energy Fuels 2012, 26 (3), 1814-1819. (43) Acevedo, S.; Escobar, G.; Gutiérrez, L. B.; D'Aquino, J. Synthesis and Isolation of Octylated Asphaltene Standards for Calibration of G.P.C. Columns and Determination of Asphaltene Molecular Weights. Fuel 1992, 71 (9), 1077-1079.


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(44) Acevedo, S.; Gutierrez, L. B.; Negrin, G.; Pereira, J. C.; Mendez, B.; Delolme, F.; Dessalces, G.; Broseta, D. Molecular Weight of Petroleum Asphaltenes: A Comparison between Mass Spectrometry and Vapor Pressure Osmometry. Energy Fuels 2005, 19 (4), 1548-1560. (45) Vargas, V.; Castillo, J.; Ocampo-Torres, R.; Lienemann, C.-P.; Bouyssiere, B. Surface Modification of SiO2 Nanoparticles to Increase Asphaltene Adsorption. Pet. Sci. Technol. 2018, 36 (8), 618-624. (46) Drew, M. G. B.; Mitchell, P. C. H.; Scott, C. E. Crystal and Molecular Structure of Three Oxovanadium(IV) Porphyrins: Oxovanadium Tetraphenylporphyrin(I), Oxovanadium(IV) Etioporphyrin(II) and the 1:2 Adduct of (II) with 1,4-Dihydroxybenzene(III). Hydrogen Bonding involving the VO Group. Relevance to Catalytic Demetallisation. Inorg. Chim. Acta 1984, 82 (1), 6368. (47) Senge, M. O.; Davis, M. (5,15-Dianthracen-9-yl-10,20-dihexylporphyrinato) Nickel(II): A Planar Nickel(II) Porphyrin. Acta Crystallogr., Sect. E: Crystallogr. Commun. 2010, 66 (7), m790. (48) Cimino, R.; Correra, S.; Bianco, A. D.; Lockhart, T. P. Solubility and Phase Behavior of Asphaltenes in Hydrocarbon Media. In Asphaltenes: Fundamentals


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and Applications; Sheu, E. Y., Mullins, C., Eds.; Plenum Press: New York, NY, 1995; pp 97-130. (49) Acevedo, S.; Escobar, G.; Ranaudo, M. A.; Piñate, J.; Amorín, A.; Díaz, M.; Silva, P. Observations about the Structure and Dispersion of Petroleum Asphaltenes Aggregates Obtained from Dialysis Fractionation and Characterization. Energy Fuels 1997, 11 (4), 774-778. (50) Liao, Z.; Geng, A.; Graciaa, A.; Creux, P.; Chrostowska, A.; Zhang, Y. Saturated Hydrocarbons Occluded Inside Asphaltene Structures and their Geochemical Significance, as Exemplified by Two Venezuelan Oils. Org. Geochem. 2006, 37 (3), 291-303. (51) Liao, Z.; Geng, A.; Graciaa, A.; Creux, P.; Chrostowska, A.; Zhang, Y. Different Adsorption/Occlusion Properties of Asphaltenes Associated with Their Secondary Evolution Processes in Oil Reservoirs. Energy Fuels 2006, 20 (3), 1131-1136.


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Table captions Table 1. Elemental Analysis of Samplesa Performed in this Work Table 2. H/C Values a of Asphaltenes and Subfractions A1 and A2 Table 3. Atom Content, H/C, DBE and MM of Molecular Models M1 and M2 Table 4. Micro-Carbon Conradson (MCC) Content of Asphaltenes-H and the Subfractions


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Energy & Fuels

Figure captions Figure 1. 13C NMR spectra of asphaltenes (I) and subfractions A1 (II) and A2 (III) corresponding to Hamaca asphaltenes. As shown, the aromaticity values, Fa, are defined as the area of aromatics over the total area, and are approximately 0.5 in all cases. Spectra were measured in CD2Cl2 using conditions similar to those previously reported.38,44 Figure 2. Representation of two dimensional (2D) molecular models of A1 (M1, Top) and A2 (M2, bottom). Figure 3. Molecular model used to represent an asphaltene nucleus or nanoaggregate with four M1-type molecules and three M2-type molecules. The length of the red arrow is equal to 2.9 nm. Figure 4. Thermogram of the asphaltenes (black) and subfractions A1 (red) and A2 (blue). Compared to A1 (red curve), the weight loss of A2 is approximately 8% higher. Figure 5. Simulated distillation (SimDis) curves for asphaltenes and subfractions A1 and A2. Compared with A2 and the asphaltenes (Asph) and for equal distillation volumes, A1 exhibits a consistently higher distillation temperature. Figure 6. Softening temperatures as a function of percentage of resin for the asphaltene mixtures and asphaltene subfractions A1 and A2. ACS Paragon Plus Environment

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Figure 7. Comparison of the µSEC HR ICP MS profiles corresponding to asphaltenes and to asphaltenes in the presence of NPs.


34

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Energy & Fuels

Tables Table 1. Elemental Analysis of Samplesa Performed in this Work Sample

C

H

N

S

O

H/C

N/C

S/C

O/C

DBEb

Asp

83.41

7.62

1.51

4.29

2.12

1.093

0.016

0.019

0.019

47

A1

81.05

7.16

1.62

4.68

3.29

1.060

0.017

0.022

0.030

49

A2

80.62

7.47

1.32

4.51

4.43

1.112

0.014

0.021

0.041

46

TC

84.44

8.72

1.72

4.72

0.3

1.239

0.017

0.021

0.003

40

Resins 80.84 9.89 0.93 3.73 1.82 1.468 0.010 0.017 0.017 28 a Averages of two determinations. b Double-bond equivalents per 100 carbons = 1 + 50 (C-H/C + N/C).


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Table 2. H/C Values a of Asphaltenes and Subfractions A1 and A2 H/C Sample

As

A1

A2

%dif d

Boscan

1.192

1.105

1.17

5.5

Furrial

0.97

0.903

0.989

8.9

Cerro Negro

1.138

1.023

1.102

6.9

CNRb

1.115

1.084

1.185

9.1

Model c 1.11 1.024 1.11 7.7 a Experimental values from references 38 to 40. b Sample obtained from a Cerro Negro residue. c Calculated for the molecular models shown in Figure 2 (see below). In this case, the H/C value calculated for A2 was used for the asphaltenes. d

%dif

=

(

100 ( H / C ) A 2 − ( H / C ) A1

) ( H / C ) Asph

; the errors in the H/C values are typically better than

2%.


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Energy & Fuels

Table 3. Atom Content, H/C, DBE and MM of Molecular Models M1 and M2 M1

M2

C

42

42

H

43

47

N

1

1

S

1

1

H/C

1.024

1.119

DBEa

22

20

MM 593.877 597.909 fa a

Double-bond equivalent:

50

50

DBE = ( 2C + 2 + N − H ) / 2


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Page 38 of 46

Table 4. Micro-Carbon Conradson (MCC) Content of Asphaltenes-H and the Subfractions Sample

% Carbon Conradson

Asphaltenes-H

57.9 ± 2.0

Subfraction A1

60.2 ± 2.0

Subfraction A2

55.5 ± 1.9


38

Page 39 of 46

Figures Asfalteno Cerro Negro Fa= 0,51

I Cerro Negro As F = 0.51

Alifáticos

ppm (t1)

200

0.96

1.00

Aromaticos

150

100

50

0

A1 Hamaca Fa=0,51

II A1 subfraction F = 0.51

200 190 ppm (t1)

180

170

160

150

140

0.97

1.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

130

120

110

100

90

80

70

60

50

40

30

20

10

0

- 10


39

Energy & Fuels

A2 Hamaca

Fa=0,50

III A2 subfraction F = 0.5

ArC-H

C alifáticos

ArC-R

190 180 ppm (t1)

170

160

150

140

1.03

1.00

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 46

130

120

110

100

90

80

70

60

50

40

30

20

10

0

Figure 1


40

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Energy & Fuels

CH3

S

N

M1 H3C

CH3

CH3

S

N

M2

H3C

CH3

Figure 2


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Figure 3


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Energy & Fuels

Figure 4


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Figure 5


44

Page 45 of 46

Asphaltenes F Subfration A2 Subfraction A1 350

300

Temperature/°C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

250

200

150

100 0

10

20

30

40

50

60

70

Percentage Resins

Figure 6


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HMW

MMW

Page 46 of 46

LMW

Figure 7


46

Suppression of Phase Separation as a Hypothesis to Account for

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