Solvent Swelling of Petroleum Asphaltenes - Energy & Fuels (ACS

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Energy & Fuels 2002, 16, 1348-1358

Solvent Swelling of Petroleum Asphaltenes Lante Carbognani*,† and Estrella Rogel‡,§ Departments of Analytical Chemistry and Integrated Oil Production Management, PDVSA-Intevep, P.O. Box 76343, Caracas-1070A, Venezuela Received December 20, 2001

Volumetric changes of crude oils and derived fractions can cause phase separation. Crude oil asphaltene swelling has been recently reported in the open literature. Solvent swelling processes have been formerly studied in greater detail for coal samples. Important structural and chemical information on coals have been gathered through the use of swelling as a characterization technique. In this study, crude oil isolated asphaltenes were successfully characterized by solvent swelling, employing polar as well as nonpolar solvents. Operationally stable and unstable crude oils, virgin and hydrocracked vacuum residua, and solid deposits from oil production tubings were the selected matrices for asphaltene isolation and characterization. The main factor that governs swelling appears to be dispersive forces, since swelling maxima were observed for alkane type solvents. Consistently, compositional asphaltene parameters like elemental hydrogen and aliphatic chain content significantly influenced the solvent swelling process. However, solvent donor properties and asphaltene acceptor properties also played an important role in the swelling phenomenom. Asphaltene swelling in nonpolar solvents was successfully fitted using the FloryRehner equation. This procedure allows to estimate the solubility parameters for the samples, which turned out relatively low. The former results allowed us to present a structural model for solid asphaltenes. This model proposes the solids to be composed of inner aromatic cores that are impermeable to solvents, surrounded by aliphatic moieties capable of incorporating solvents and swell. Kinetic dissolution experiments carried out with solvent swollen asphaltenes show consistency with the proposed model.

Introduction Solid precipitation during oil production is a serious problem that reduces the revenue of Venezuela’s petroleum industry. Asphaltene type hydrocarbons are important components found within these precipitated solids.1-5 On the basis of the selective elution order of alkane probes from columns that were packed with solid asphaltenes, Carbognani and Orea proposed a porous structure for these materials.6 On the other hand, the colloidal nature of crude oils has been recognized for quite a long time.7,8 Recently, Sirota described the capability of colloidal asphaltenes to further organize into a swollen regime.9 This author demonstrated that * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Analytical Chemistry. ‡ Department of Integrated Oil Production Management. § Present address: 15315 SW 78 Court, Miami, FL 33157. (1) Rivas, O. Visio´ n Tecnolo´ gica 1995, 2, 4-17. (2) Carbognani, L.; Espidel, J. Visio´ n Tecnolo´ gica 1995, 3, 35-42. (3) Carbognani, L.; Fonseca, M.; Izquierdo, A.; Leon, O. In Proceedings of the 9th International Oilfield Chemical Symposium, Geilo, Norway, March 22-25, 1998; Paper 12. (4) Carbognani, L.; Orea, M.; Fonseca, M. Energy Fuels, 1999, 13, 351-358. (5) Carbognani, L.; Espidel, J.; Izquierdo, A. In Asphaltenes and Asphalts. 2. Developments in Petroleum Science 40 B, Yen, T. F, Chilingarian, G. V., Eds.; Elsevier: Amsterdam, 2000; Chapter 13, pp 335-362. (6) Carbognani, L.; Orea, M. Pet. Sci. Technol. 1999, 17, 165-187. (7) Nellenstein, F. J. In The Science of Petroleum, 4th ed.; Dunston, A. E., Ed.; Oxford Press: New York, 1938; pp 2760-2764. (8) Pfeiffer, J. P., Saal, R. N. J. J. Phys. Chem, 1940, 49, 139-149. (9) Sirota, E. B. Pet. Sci. Technol. 1998, 16, 415-431.

the asphaltene concentration in the oil medium controlled their existence within one or the other structural regime. Volumetric changes of crude oils and derived fractions can cause phase separation. Temperature reduction induces paraffin crystallization, which results into an up to 30% volume shrinkage.10 This phenomenon causes the development of cracks in paraffin containing materials.11 It is believed that crude oil expansion along production pipelines decreases the average density of the fluid, thus promoting settling of dense fractions, such as minerals and flocculated asphaltenes.12-14 Volumetric expansion (swelling) of coal, also a fossil fuel, has been studied in greater detail for three decades. This technique is useful to characterize structural features of the material, like the nature of networks responsible for its elastic behavior and the distribution of functional groups on the sample surface. Two types of approaches have been described for swelling measurements. The first technique, initially described by (10) Srivastava; S. P.; Handoo, J.; Agrawal, K. M.; Joshi, G. C. J. Phys. Chem. Solids 1993, 54, 639-670. (11) Youtcheff, J. S.; Jones, D. R. Guideline for Asphalt Refiners and Suppliers. Report SHRP-A-686; Strategic Highway Research Program, National Research Council: Washington, DC, 1994. (12) Angulo, R.; Borges, A.; Fonseca, M.; Gil, C. In Proceedings ISCOP’95, 1st International Symposium on Colloid Chemistry in Oil Production, Rio de Janeiro, Brazil, 1995; pp 109-111. (13) Turta, A.; Najman, J.; Fisher, D.; Singhal, A. In Proceedings of the 48th Annual CIM Petroleum Society Technology Meeting, Calgary, ON, Canada, June 8-11, 1999; Vol. 2, Paper CIM 97-81. (14) Sheu, E. Y.; Acevedo, S. Energy Fuels 2001, 15, 702-707.

10.1021/ef010299m CCC: $22.00 © 2002 American Chemical Society Published on Web 10/29/2002

Swelling of Petroleum Asphaltenes

Sanada and Honda, relies on the gravimetric determinations of the amount of solvent vapors absorbed by the solid.15 The second approach is based on volumetric determinations, and was first described by Liotta et al.16 In this case, a known volume of the solid is soaked in excess solvent. After reaching the swelling equilibrium, the final volume is measured. The second technique is by far the simpler one and has been widely applied by many authors after its initial publication. Swelling of organic materials often suggests a crosslinked polymeric structure.17 Many authors considered the existence of cross-linked domains within coals,18-32 following van Krevelen’s work.33In addition to covalent bonds, other binding forces have been reported to contribute to the network type structures within coals. These are21-23,26-31,34-36 (1) dispersive or van der Waals, (2) π-π bonds, (3) bond polarity, (4) hydrogen bonds, (5) charge transfer, and (6) aliphatic chain entangling. Polymer swelling experiments are based on the fact that the liquid contacting the material exerts an osmotic pressure that promotes solvent absorption by the solid until this phenomenon is equilibrated by the elastic restoring forces of the network.37,38 In principle, the theory of solvent swelling for cross-linked polymers developed by Flory and Rehner can be used to describe this phenomenum.22 According to this theory, the largest swelling of coals should occur with solvents whose solubility parameters closely match those of the samples.24 Indeed, open literature reports many coals that behave consistently with this model.22,24,31,39 High rank coals rich in elemental carbon display swelling maxima with nonpolar solvents. Low rank coals, which are rich in oxygen containing functional groups, show their maxima with polar solvents. However, there are also evidences of many coal samples whose swelling behavior does not follow a regular solution behavior,40 where chemical functionalities play significant roles in the swelling process. (15) Sanada, Y.; Honda, H. Fuel 1966, 45, 295-300. (16) Liotta, R.; Brown, G.; Isaacs, J. Fuel 1983, 62, 781-791. (17) van Krevelen, D. W. Properties of Polymers; Elsevier: Amsterdam, 1972. (18) Green, T. K.; Kovac, J.; Larsen, J. W. Fuel 1984, 63, 935-938. (19) Larsen, J. W.; Shawer, S. Energy Fuels 1990, 4, 74-77. (20) Larsen, J. W.; Cheng, J. C.; Pan, C.-H. Energy Fuels 1991, 5, 57-59. (21) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T., Milosavljevic, I. Energy Fuels 1994, 8, 1247-1262. (22) Larsen, J. W.; Li, S. Energy Fuels 1994, 8, 932-936. (23) Larsen, J. W.; Gurevich, I. Energy Fuels 1996, 10, 1269-1272. (24) Larsen, J. W.; Li, S. Org. Geochem. 1997, 26, 305-309. (25) Otake, Y.; Suuberg, E. M. Energy Fuels 1997, 1, 1155-1164. (26) Yun, Y.; Suuberg, E, M. Energy Fuels 1998, 12, 798-800. (27) Wang, N.; Sasaki, M.; Yoshida, T.; Kotanigawa, T. Energy Fuels 1998, 12, 531-535. (28) Chen, C.; Gao, J.; Yan, Y. Energy Fuels 1998, 12, 1328-1334. (29) Norinaga, K.; Iino, M.; Cody, G. D.; Thiyagarajan, P. Energy Fuels 2000, 14, 1245-1251. (30) Iino, M. Fuel Process. Technol. 2000, 62, 89-101. (31) Parikh, H. M.; Larsen, J. W. Prepr. Pap.-Am. Chem. Soc., Div. Pet. Chem. 2000, 45, 556-558. (32) Krzesinska, M. Energy Fuels 2001, 5, 324-330. (33) van Krevelen, D. W. Coal; Elsevier: Amsterdam, 1961. (34) Szeliga, J.; Marzec, A. Fuel 1983, 62, 1229-1231. (35) Jones, J. C.; Hewitt, R. G.; Innes, R, A. Fuel 1997, 76, 575577. (36) Miura, K.; Mae, K.; Li, W.; Kusawaka, T.; Morozumi, F.; Kumano, A. Energy Fuels 2001, 15, 599-610. (37) Hildebrand, J. H.; Scott, R. L. The Solubility of Nonelectrolytes; Dover Publications: New York, 1964. (38) Hildebrand, J. H.; Prausnitz, J. L.; Scott R. L. Regular and Related Solutions; van Nostrand Reinhold: New York, 1970. (39) Painter, P. C.; Graf, J.; Coleman, M. M. Energy Fuels 1990, 4, 379-384.

Energy & Fuels, Vol. 16, No. 6, 2002 1349

Studies of chemical functionalities showed that basic/ electron donor solvents influence coal swelling the most.20-21,25-26,34,41 According to this fact, coals are acidic in nature. Experiments showed that the number of acceptor sites in each sample is limited.21,23,25 Additionally, the acid-base solid/solvent interaction depends on parameters such as temperature and steric hindrance of the basic solvent. The latter is caused by the molecular size as well as by the topology.25 Many authors have discussed the important role played by these specific reagents for the release of coal-coal interactions and the formation of new coal-solvent bonds.20-21,23-28,30,32,34,36,41-42 Moreover, coals show enhanced swelling in mixed polar/nonpolar solvents.30,43 This fact led to propose a dual mechanism to explain the swelling process. According to this model, covalent bonds are not as important as initially believed. Instead, the six intermolecular forces cited above are those responsible for the three-dimensional structure of coal. The first step in the swelling phenomenom is coal-coal interactions releasing, followed then by the uptake of the apolar solvent.23,26,28,30 The fact that swelling experiments reportedly were nonreproducible further supports this model because it allows for possible network rearrangements that can occur during the first swelling experiments.20,22,31-32 Along with the fundamental knowledge of coal structure unveiled by the above-mentioned swelling studies, theses studies also allowed the development of optimized coal upgrading operations for swollen coals.44 Keeping these facts in mind, a swelling characterization study of petroleum asphaltenes was carried out. To the best of our knowledge, this is the first successful attempt reported on the open literature. The present study was carried out with petroleum asphaltenes isolated from operationally stable and unstable crude oils, virgin and hydrocracked vacuum residua, and solid deposits from oil production tubings. The swelling experiments were carried out with both nonpolar and polar solvents. The aim was to develop fundamental insights into asphaltene macromolecular structure and their capability to adsorb solvents as a function of their chemical properties. According to the results found within the study we believe that the fundamental knowledge developed can be further applied for understanding the properties and behavior of mixtures of asphaltenes plus high molecular waxes, which is a topic recently investigated in our laboratory.45,46 Composite solid deposits formed by these hydrocarbon group types are often observed in storage facilites.4 (40) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (41) Takahashi, K.; Norinaga, K.; Masui, Y.; Iino, M. Energy Fuels 2001, 15(1), 141-146. (42) Painter, P. C.; Park, Y.; Sobkowiak, M.; Coleman, M. Energy Fuels 1990, 4, 384-393. (43) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100-106. (44) Hu, H.; Sha, G.; Chen, G. Fuel Process. Technol. 2000, 68, 3343. (45) Garcia, M. C.; Carbognani, L. Energy Fuels 2001, 15, 10211027. (46) Carbognani, L.; Rogel, E. Accepted for publication in the special issue of Pet. Sci. Technol. devoted to papers presented at the AIChE Spring National Meeting, New Orleans, LA, March 10-14, 2002.

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Energy & Fuels, Vol. 16, No. 6, 2002

Experimental Section Samples and Solvents. Asphaltenes were precipitated with n-Heptane, following a standard procedure.47 One of the samples was precipitated in a pilot plant, with a commercial C6 raffinate. Some properties of the samples are presented in the results and discussion section. Solvents selected for the study were liquid chromatography grade (HPLC). They were used as received, without any further purification step. The manufacturers were Burdick & Jackson (n-heptane and acetonitrile), E. M. Science (n-hexane and methanol), J. T. Baker (i-octane, ethyl acetate, acetone, and n-pentane), and Aldrich (n-hexadecane). Sample Preparation. Isolated asphaltenes were dried in a vacuum oven at 80 °C and 130 mmHg. The oven was flushed with nitrogen prior to sample drying, to avoid oxidation. Dried samples were ground using an aghata mortar, and stored in a desiccator. Particle size distributions of ground asphaltenes were determined with ASTM standard sieves. Swelling Experiments. Nearly 300 mg (weighted to the nearest 0.1 mg) of dried and ground asphaltenes were poured into a centrifuge tube. The solid was compacted with the aid of a metal piston. The height of the compacted dry solid (h0) was measured with the help of a transparent ruler with millimeter division marks. Approximately 1.5 mL of solvent was poured over the compacted asphaltene. Air was released with the aid of a microspatula. About 0.5 mL of solvent were added to fill the tube just beneath its upper rim. The tube was capped to avoid solvent evaporation and the sample was allowed to swell for 20-24 h. The mixture was shaken at least twice during the swelling period to aid bubble release. Swollen asphaltenes were compacted by centrifugation. A Dupont model EasySpin equipment was used. Centrifugation was repeated during consecutive 15 min periods at 4000 rpm, until a constant solid height was reached. This height (h1) was recorded as well. In some cases, the sample amount that has been solubilized was quantified. For this, after completion of the swelling experiment, the remaining solvent from the tube was transferred with the help of a pipet into a tared vial. The solvent was distilled off and the residue brought to constant weight. The centrifuge tubes were custom-built 6 mm diameter and 9.5 cm height Pyrex glass tubes. The bottom was manually closed, achieving nearly round shapes. Corrections for variations in the bottom thickness (1-2 mm) were considered in order to measure neat sample heights (h0 and h1). To fit into the centrifugue, the small diameter tubes were inserted into custom-built Teflon sleeves. Asphaltene Properties. Carbon and Hydrogen elemental contents were determined with a Leco-932 analyzer. Oxygen content was determined with the same instrument, provided with a VTF-900 oven. Sulfur content was determined with a Leco IR-432 equipment. Nitrogen contents of asphaltene solutions in toluene, were measured using an Antek-9000 chemiluminiscence system. Asphaltene aromaticity was determined by C13 NMR, following procedures previously published.48,49 Asphaltene flocculation onsets during n-Heptane titration of toluene solutions were determined following published procedures.49-53 Asphalt(47) Determination of Asphaltenes (Heptane Insolubles); Institute of Petroleum: London, 1993; IP 143/90 (BS 2000. Part 143.). (48) Carbognani, L.; DeLima, L.; Orea, M.; Ehrmann, U. Pet. Sci. Technol. 2000, 18, 607-634. (49) Rogel, E.; Leon, O.; Contreras, E.; Carbognani, L.; Torres, G.; Espidel, J. Energy Fuels 2001. Submitted for publication. (50) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels, 2000, 14, 6-10. (51) Leon, O.; Contreras, E.; Rogel, E. Colloids Surf. A 2001, 189, 123-130. (52) Carbognani, L.; Contreras, E.; Guimerans, R.; Leon, O.; Flores, E.; Moya, S. In Proceedings of the SPE International Symposium on Oilfield Chemistry, Houston, TX, Feb 13-16, 2001 (Paper SPE 64993).

Carbognani and Rogel ene densities were determined by pycnomety following a method described earlier.54,55 Apparent density of compacted dry solid asphaltenes was estimated as the ratio of the known mass to their apparent volume. Apparent volumes were estimated comparing the heights of known solvent volumes poured into the centrifuge tubes with the observed heights of the compacted solids. Solvent volumes were measured with gastight syringes calibrated to the nearest 10 µL. Dissolution Kinetics of Swollen Asphaltenes. A technique for the determination of asphaltene dissolution kinetics has been already described.45,54 By adding a sample preparation step, it was adapted for the determination of the dissolution kinetics of swollen asphaltenes. n-Heptane was poured over the asphaltene aliquot weighted inside the HPLC column used as dissolution cell. The cell was tightly closed for 24 h, with occasional shaking to aid the swelling process. After this period of time, the dissolution kinetic was performed following the published procedure.

Results and Discussion Sample Properties and Analytical Aspects. Some recent evidence indicates that crude oil asphaltenes swell,9 in a way that resemble the swelling of coal in organic solvents. Asphaltenes were isolated from crude oil samples that exhibit a wide variety of properties. Operationally stable or unstable oils prone to solid precipitation, their vacuum residua and hydrocracked products, as well as solid deposits from production tubings, were the selected matrices for asphaltene isolation and characterization. Table 1 summarizes the samples used and some of their main properties. Volumetric swelling of hydrocarbon samples has been proposed as a simple technique well suited for probing physical and chemical properties of the material.16,18 It only requires a couple of heights: initially, of the compacted dry solid (h0) and, then, of the compacted solvent swollen material (h1). The volumetric swelling (Qv) is defined as the ratio of the final to the initial volume. Since container cross sections in these experiments were kept constant, Qv is equivalent to the height ratio:

Qv ) h1/h0

(1)

When dealing with asphaltenes isolated from crude oils, one of the common constraints is the reduced sample amount available. Therefore, recommended sieving procedures that reportedly improve precision during coal swelling analyses,25 could not be followed. Nevertheless, particle size distribution of two samples was determined; Figure 1 shows the results. The particle size distribution spans over a wide range (30+ to 325- mesh); the highest abundance lies in the 3080 and 100-230 mesh ranges. It is important to point out that while there are some large particles prone to fast settling (30+ mesh) there are at the same time small particles that are less likely to settle (325- mesh). This wide distribution apparently caused some initial (53) Leon, O.; Contreras, E.; Rogel, E.; Dambakli, G.; Acevedo, S.; Carbognani, L.; Espidel, J. Langmuir 2002, 18, 5105-5112. (54) Carbognani, L. Energy Fuels 2001, 15, 1004-1012. (55) Rogel, E.; Carbognani, L. In Proceedings International Symposium on Asphaltene and Wax Deposition, Cancun, Mexico, July 2327, 2001.



Table 1. Studied Asphaltenes and Some of Their Properties elemental content (wt %) asphaltene CN550+ CN550+ ox 48h CN crude Boscan nC7 Boscan C6 S1-0 feedGU HCKGU D T9

origin

δa

fac

Floc. Onset. (mL n-C7)

C

H

S

N

O

H/C

1.21

0.70

0.57

11.1

85.04

8.54

5.55

3.22

1.71

1.21

1.17

0.77

0.52

12.4

86.62

8.54

5.41

3.38

2.75

1.24

1.17 1.17

0.75 0.75

0.54 0.44

13.7 15.5

83.90 81.34

7.79 8.09

5.18 6.72

1.92 1.86

1.77 1.46

1.11 1.19

heavy oil

1.16

0.71

0.43

9.0

81.99

8.96

6.01

2.62

1.99

1.31

unstable crudee vacuum residuef hydrocracked product from the former samplef solid deposit from production tubingg

1.28 1.20 1.22

0.72 0.67 0.72

0.71 0.55 0.75

15.9 10.5 6.6

86.56 85.87 89.34

7.45 7.90 6.56

2.44 1.29 0.62

1.61 1.66 3.94

2.17 1.88 1.38

1.03 1.10 0.88

1.25

0.77

0.59

0

83.90

7.00

4.74

2.72

2.11

1.00

vacuum residue from CN cruded former, oxidized for 48 hoursd extraheavy oil heavy oil

(25 °C)

appb

δ

a Density at 25 °C (g/mL). b Apparent density of compacted solid (g/mL). c C13 NMR aromaticity. more information.52 f For more information.69-70 g For more information.45,54

Figure 1. Particle size distribution for a couple of dried and ground asphaltenes. Grinding was performed with an aghata mortar. Standard ASTM sieves were employed for providing the distributions.

drawbacks in the measurement of h0, as will be discussed next. Some of the initially tested samples appeared to shrink instead of swell, when contacted with excess polar solvents such as methanol and acetonitrile. Those samples, when dry, were compacted by centrifugation, as the original method indicated. Apparently, the nonuniformity of the sample particle sizes precluded efficient compaction of the dry sample; consequently, measured h0 values were larger than the actual values. Compaction was then performed with help of a metal piston. This method was more efficient, as the results of apparent density values presented in Figure 2 show. Solid samples compacted in this way did no longer exhibit shrinking when exposed to excess solvent. All experiments conducted afterward were performed using piston compaction. From the average apparent packed density (0.7 g/mL, see Figure 2) and the average asphaltene density (1.20 g/mL, see Table 1) it is possible to estimate that the porous or void volume of dried asphaltene packings is about 40% vol. Considering that ideal packings for rigid spheres range from 26 to 48% void volume,56 there is ample room for packing improvements of solid asphaltenes. The optimization can be achieved by a careful selection of small monodisperse particles, which has been shown to be efficient for coal (56) Berg, R. R. Trans.-Gulf Coast Assoc. Geol. Soc. 1970, 20, 303317.

d

For more information.68-69

e

For

Figure 2. Effect of compaction technique on the apparent density of solid asphaltenes. Boscan C6 was the sample selected for the described experiments.

samples.25 It is to be said that the experiments described in the present study are deemed a reasonable tradeoff that successfully allowed to get consistent results when sample availability was scarce. Some of the studied crude oils have e1wt % heptane asphaltenes, which made the preparative separation of more than 3 g of isolated solids pretty difficult. Repeatability of the Qv determinations was evaluated with some samples. The typical standard deviations (SD) obtained were e0.05 and agree with those reported elsewhere for coal samples.20 The typical Qv values ranged between 1.10 and 1.70. Qv values of different samples showed statistically significant differences; that is, the determinations are meaningful. The common practice during the present work was to delete replicas differing by more than 0.10 Qv. General Trends on Solvent Swelling for Crude Oil Asphaltenes. A recent work by Sirota9 described swelling experiments conducted with mixed solvents which partially solubilize asphaltenes. Reports of work done with coal show that solvents able to partially solubilize the sample promote less swelling since some of the restoring network disappears in the solution.20,26 Consequently, we searched for solvents that do not solubilize significantly the asphaltenes. At the same time, the boiling points of the solvents had to be low enough to allow recovery of the highly valuable asphaltene fractions for future work. The tests presented in this work were carried out with a set of commonly found

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Carbognani and Rogel Table 2. Asphaltene Calculated Solubility Parametersa

a

asphaltene

δ (Mpa1/2)

CN550+ CN550+ ox CN crude Boscan n-C7 Boscan C6 S1-0 feedGU HCKGU D T9

20.15 20.98 21.53 21.53 20.15 20.63 21.28 22.58 22.90

According to the eq 2, proposed by Painter et al.

39

Figure 4. Asphaltene flocculation onset as a function of solubility parameters. Flocculation onsets were determined by titration with n-C7, as described in the literature.49-53

by Painter et al.39 allows the estimatation of the solubility parameters δ for the studied asphaltenes. Table 2 presents the results. Specifically, the equation applied (eq 2) states:

δ ) (7.0 + 63.5fa + 63.5(H/C) + 106(O/C) + 51.8((N+S)/C))/(-10.9 + 12fa + 13.9(H/C) + 5.5(O/C) - 2.8((N+S)/C)) (2)

Figure 3. Volumetric solvent swelling (Qv) of studied asphaltenes. Qv was determined according to eq 1.

laboratory solvents that fulfill both constraints. The solvent set comprises normal alkanes, i-octane, acetone, ethyl acetate, acetonitrile, and methanol. Commonly, sample swelling is reported as a function of the solvent solubility parameters.19,22,24,28,31,32,34,35,57 Figure 3 presents a plot of the swelling results obtained with seven different solvents for the asphaltenes studied in this work, as a function of the solvent solubility parameters. With one exception, swelling maxima were observed with n-C6 or n-C7. The sample differing is the asphaltene isolated from the severely hydrocracked vacuum residue (HCKGU), which displayed the maximum swelling with acetone. These results are unexpected if the swelling of asphaltenes in a first approximation would follow regular solution theory. According to the theory, maxima should appear with solvents that show solubility parameters similar to those determined or estimated for the asphaltenes.22,24,31,40 Reportedly, asphaltenes solubility parameters range between 18 and 22 (MPa1/2).58-62 Substitution of asphaltene characterization parameters (see Table 1) into the equation proposed (57) Barton, A. F. M Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press, Inc.: Boca Rato´n, FL, 1983.

where fa is aromaticity, and H/C, O/C, and (N+S)/C are, respectively, the atomic ratios of hydrogen, oxygen, and (nitrogen+sulfur) divided by carbon, as calculated from the respective mass contents. Indeed, the calculated values obtained for the asphaltene solubility parameters span the range previously cited for other asphaltene samples (18-22 MPa1/2). Among the solvents employed to carry out the swelling experiments, only acetone and ethyl acetate match this range. However, neither of both induced the maximum swelling of the asphaltene fractions. Instead, experimental results show that most of the studied asphaltenes displayed swelling maxima at around 15 MPa1/2 (Figure 3), which is a typical value for alkane type compounds. On the other hand, asphaltene calculated solubility parameters appear to correlate inversely with their intrinsic stability as measured by their flocculation onset,49-52 as shown in Figure 4. In fact, it has been reported that unstable asphaltenes typically show high solubility parameters.62 The preceding facts can appear confusing at this point. Initially we aimed at using the swelling data for asphaltene solubility parameters (58) Wiehe, I. A. Fuel Sci. Technol. Int. 1996, 14, 289-312. (59) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117, 201210. (60) DeBoer, R. B.; Leerloyer, K.; Eigner, M. R. P.; VanBergen, A. R. D. SPE 24987. Presented at the European Petroleum Conference, Cannes, Nov 16-18, 1992. (61) Hirschberg, A.; deJong, L. N. J.; Schipper, B. A.; Meyers, J. G. SPE 11202. Presented at the SPE 57th Annual Fall Technical Conference and Exhibition, New Orleans, Sept 26-29, 1982. (62) Rogel, E. Energy Fuels 1997, 11, 920-925.


Figure 5. Swelling of crude oil asphaltenes in alkane solvents. Volumetric swelling (Qv) was determined according to Equation 1.

prediction and apply to the Flory-Huggins theory as a convenient way to estimate the phase behavior of crude oil. However, the low solubility parameter values found compelled us to consider all these aspects in greater detail, and to find coherent explanations that will be covered along the ensuing sections of the discussion. The graphics plotted in Figure 3 shows another interesting result. Most of the asphaltenes tested show smaller swelling ratios with i-octane than with the two n-alkanes included in the experiments. According to the literature, the topology of solvent affects coal swelling.25 Suspecting a steric effect exerted by the three methyl groups of the i-octane, a sequence of swelling experiments was performed with different alkane type solvents. Figure 5 displays the results. For the four samples tested, swelling maxima appeared around 15 MPa1/2 (n-C6 and n-C7). The existence of a maximum value along the normal alkane series suggests that, in addition to solvent steric hindrance, other factors simultaneously affect the swelling phenomenon. These could be higher diffusivity of the smaller molecules or difference in solubility parameters. The combination of these factors is believed to generate the maxima in Figure 5. Published studies of solute solubilization into micelles dispersed in aqueous media showed similar simultaneous effects.63 Finally, Figure 3 reveals that the Boscan C6 asphaltene exhibits lower swelling ratios than the Boscan C7 asphaltene. The liquid phase in equilibrium with the Boscan C6 asphaltene was particularly colored; this led us to suspect that partial solubility influences the swelling behavior of this sample. Indeed, the Boscan C6 asphaltene is 3-4 times more soluble in the set of solvents selected for swelling experiments (Figure 6) than the other tested asphaltenes. As reported for coals,20,26 this demonstrates on a first approximation that solubilization decreases the amount of structured networks ready for solvent uptaking, decreasing asphaltene swelling as a direct consequence. Asphaltene Elemental Composition and Swelling. In many instances, fundamental properties of oil fractions can explain their behavior. For example, stability, solubility, and crackability relate directly with the hydrogen content (H/C atomic ratio) and inversely with the aromaticity (fa).4,5,54,58,59 This study explores simple correlation between the elemental composition (63) Nagarajan, R.; Ruckenstein, E. Langmuir 1991, 7, 2934-2969.


Figure 6. Asphaltene solubility in the organic solvents at equilibium at the end of the swelling experiments.

Figure 7. Correlation between swelling maxima and hydrogen content for studied asphaltenes. Hydrogen content is reported as the atomic ratio H/C. The anomalous, very soluble Boscan C6 outlyer is identified in the plot.

of the studied asphaltenes and their solvent swelling ratios. The existence of such correlation might suggest preliminary evidences for the molecular interactions that govern the swelling with a particular solvent. Although most of the explored relationships showed no significant correlation, some were identified and are presented in Figures 7 and 8. Figure 7 reveals an exponential increase of the swelling maxima with the relative increase of the hydrogen content (H/C ratio). High H/C ratios for asphaltenes indicate high proportions of alkyl substituents, since the aromatic fragments are more hydrogen deficient. Therefore, the exponential correlation between Qv and the H/C ratio further supports the hypothesis that alkane moieties play a crucial role in the swelling process. A first indication in this sense was provided by the swelling maxima observed with alkane solvents for most of the studied samples (Figure 3). The asphaltene swelling in medium polarity solvents (acetone and ethyl acetate) showed gross linear correlation with the oxygen and nitrogen contents of the asphaltenes, as seen in Figure 8. These findings suggest that not only alkane fractions govern the swelling process; other phenomena that involve polar functional groups are key players too. This is not surprising, since asphaltenes are known to be complex mixtures of molecules comprising large molecular masses plus highpolarity compounds.64 Heteroatomic groups such as (64) Speight, J. G. In The Chemistry and Technology of Petroleum, 2nd ed.; Marcel Dekker: New York, 1991; Chapter 11, pp 401-471.

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Figure 8. Correlation between swelling in medium polarity solvents (ethyl-acetate and acetone) with N,O elemental content for studied asphaltenes. The severely oxidized CN550+/ ox outlayer is identified in the graphics.

oxygen and nitrogen containing functional groups represent the high-polarity compounds. However, at this point, there is no evidence available that may shed light on the particular groups that contribute to the observed effects on asphaltene swelling. To help unveil the effects posed by the chemical functionalities, influences from polar groups will be examined in greater detail in the following sections. Asphaltene Swelling and the Donor-Acceptor Nature of the Solvent. A systematic approach which allows to correlate solute behavior (solubility, redox potential, and ionization degree) with solvent basicity, is the donor number DN. A relative scale has been proposed to estimate solvent DN, based on its reaction enthalpy with a reference acid. The most common referred acid is SbCl5, proposed by Gutmann.65 Similarly, the acceptor number AN correlates to the interaction between the solvent and a basic solute that serves as reference. Commonly, Et3PO is considered as reference base; AN correlates with the 31P NMR chemical shift that occurs when the base is dissolved in the solvent of interest. AN numbers are arbitrarily scaled to 100, where 100 is the equivalent value assigned to the shift of the 1:1 adduct of Et3PO:SbCl5 in 1,2dichloroethane.65 The DN-AN scale has proven useful for correlation of coal swelling with solvent properties34 and for prediction of coal preasphaltene solubilization.66 A commonly used predictor is the DN/AN ratio of the solvent.66 Figure 9 shows the correlation of this predictor with the swelling of the asphaltene samples in polar solvents (65) Jensen, W. B. Chem. Rev. 1978, 78, 1-22. (66) Jones, M. B.; Argasinski, J. K. Fuel 1985, 64, 1547-1551.

Carbognani and Rogel

Figure 9. Asphaltene swelling and donor-acceptor properties of solvents. Donor numbers DN and acceptor numbers AN were taken from the published literature.34,65,66

(ethyl acetate, acetone, acetonitrile, and methanol). In general, swelling increases with the value of the DN/ AN ratio, although some results are shown appreciable scattering. This correlation demonstrates that basic solvents interact significantly with acidic sites on the asphaltenes, promoting a relaxation of the asphaltene network and thus, facilitating their swelling. These results also indicate that asphaltenes resemble coals when interacting with polar solvents; that is, both behave like acidic compounds. 20-21,25-26,34,41 Extrapolating from previously reported studies related to coals,21,23,25 the number of asphaltene polar functional groups should be finite, as is the case in coals. Consequently, a correlation between the maximum number of polar solvent molecules at equilibrium and the number of polar heteroatoms in asphaltenes should exist. Indeed, Figure 10 reveals such a linear correlation. The values on the y-axis correspond to the number of solvent moles adsorbed per volume unit of dry asphaltene. These were calculated from the volume of adsorbed solvent (difference between the final-swollen and initial asphaltene volumes), the solvent density and its molecular weight, neglecting any possible solvent density change due to the adsorption. The x-axis corresponds to the sum of (nitrogen + oxygen) moles per dry asphaltene volume unit (calculated from the asphaltene characterization parameters reported in Table 1.) As Figure 10 shows, in most cases, the calculated number of adsorbed solvent moles is slightly higher than expected on the basis of heteroatoms content and the assumption of a 1:1 interaction. With the exception of



Figure 10. Moles of solvent adsorbed per moles of (nitrogen+ oxygen) on the asphaltenes. Results are plotted based on the initial dry asphaltene volume. Asphaltenes from CN550+ and Boscan C6, falling on the diagonal line, are identified.

two samples, the response is linear, giving the following equation:

NS ) 1.077(NO) + 0.001

(r2 ) 0.982)

(3)

where NS represents the number of adsorbed solvent moles and NO the number of nitrogen plus oxygen moles within the asphaltenes. It is important to note that, in comparison to the full scale of NS, the intercept of 0.001 is significantly different from zero. Figure 10 as well as eq 3 show that the line found for the NS vs NO relationship is parallel to the diagonal that would describe a biunivocal correlation between the number of moles of adsorbed solvent and the number of moles of available N+O heteroatoms. This fact suggests that the accessibility of the solvent molecules to the polar sites of most of the studied asphaltenes is similar, i.e., the functional groups on the different asphaltenes seem to be distributed in similar arrays. Otherwise, results would have been scattered and/or not parallel to the diagonal line. However, the precise placement of chemical functionalities is far from understood. Similarly, other aspects remain unknown as well: for example, the reason most asphaltenes show similar polar site accessibility given their remarkably diverse origin and properties (Table 1). Asphaltene Donor-Acceptor Properties. Role of Oxygen Functionalities. Results discussed in the former section suggested that donor-acceptor (DN-AN) interactions between the solvent and the sample influence asphaltene swelling in polar solvents. If the swelling depends directly and exclusively from these interactions, the phenomenon could be described by eq 4 as follows:

Q ) KaDN + KdAN

(4)

where Q is the asphaltene volumetric swelling, and Ka and Kd are parameters that describe the acceptor and donor capacity of asphaltenes. A plot of Q/AN as a function of DN/AN should be a straight line of slope Ka and intercept Kd. Equation 4 resembles an equation used to characterize acid/basic properties of surfaces through the determination of adsorption enthalpies by inverse gas chromatography.67 Figure 11 illustrates some of the results found with the asphaltenes studied in this work. For (67) Tshabalala, M. A. J. Appl. Polym. Sci. 1997, 65, 1013-1020.

Figure 11. Asphaltene swelling as a function of their acceptor-donor properties. Table 3. Asphaltene Acceptor Ka and Donor Kd Parametersa asphaltene

Ka

Kd

r2

CN550+ CN550+ Ox. CN crude Boscan n-C7 Boscan C6 S1-0 feedGU HCKGU D T9

0.0862 0.1028 0.0864 0.0850 0.0774 0.0906 0.0858 0.0756 0.0860

-0.0112 -0.0170 -0.0127 -0.0097 -0.0054 -0.0117 -0.0123 -0.0004 -0.0072

0.9958 0.9955 0.9833 0.9936 0.9960 0.9911 0.9742 0.9756 0.9971

a Determined with Equation 4. Linear correlation coefficient is included (r2).

Figure 12. Acceptor parameters for asphaltenes Ka as a function of their oxygen content. Ka values (reported in Table 3) were determined with eq 4 and the plotts illustrated in Figure 11. The anomalous highly soluble Boscan C6 sample is identified.

all the other asphaltene samples, linear responses were observed as well. Table 3 summarizes the numerical values calculated using eq 4; correlation coefficients >0.97 in all cases further support linear dependence of the variables. The acceptor parameters for the studied asphaltenes (Ka, Table 3) were plotted as a function of the oxygen content of the samples (Figure 12). A linear response is observed for most of the samples. Again, the sample that deviates most from the straight line is the one that corresponds to the anomalous, C6 soluble Boscan asphaltene. The linear relationship between Ka and oxygen content suggests that the acidic functionalities of asphaltenes, responsible for their interaction with basic solvents, can be attributed with high certainty to oxygen functionalities. CN550+ Ox‚asphaltene (Table

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3) yielded the largest Ka value, which is consistent with the severe oxidation process that gave origin to this sample.68 On the other hand, the severely hydrocracked HCKGU asphaltene yielded the lowest absolute Kd value, which is not surprising because these type of products usually loose their basic nitrogen components during the upgrading process.69-71 Asphaltene Swelling in Apolar Solvents. Proposed Structural Model for Solid Asphaltenes. The former discussions demonstrate that two main factors govern crude oil asphaltene swelling. The first one is related to the presence of hydrogen rich alkyl moieties within these hydrocarbon types. The second factor is the presence of acidic sites in asphaltenes; that is, functionalities that are prone to interact with basic solvents. As Figure 3 evidences, swelling maxima achieved with alkane solvents suggest that the first of the two factors mentioned is the more important one in controlling asphaltene swelling. The present section discusses this topic in greater detail and proposes an asphaltene structural model on the bases of all evidences found within this work. When asphaltene swelling is carried out with apolar solvents, dispersive interactions are the most important forces that govern the process. Consequently, the FloryRehner model developed for cross-linked polymers, and the corresponding Flory-Rehner equation (eq 5) can be applied.72

Mc ) -V1F (φ1/3 -φ)/(ln(1 - φ) + φ + χ12φ 2)

(5)

In this equation, V1 is the solvent molar volume, F is the asphaltene density, and φ is the asphaltene volumetric fraction. The expression is commonly used to estimate the average molecular weight of the crosslinking moieties Mc.21-22,65 If φ is determined from swelling experiments and the Flory interaction parameter χ12 is known, Mc can be calculated. However, for asphaltenes, both Mc and χ12 are unknown. As a first approximation, χ12 can be calculated from the known expression:57

χ12 ) (V1/kT)(δ1 - δ2)2

(6)

where δ1 and δ2 are the solvent and the asphaltene solubility parameters. Substitution of calculated asphaltene solubility parameters (Table 2), and published solvent solubility parameters 57 into eqs 5 and 6, yielded meaningless results for Mc (Mc < 0). A possible reason could be that calculated asphaltene solubility parameters are not correct. Because of these shortcomings, an alternative method was developed to simultaneously determine Mc, χ12, and δ2. The method assumes that each asphaltene sample has a unique pair of Mc and δ2 values, regardless of the solvent employed. The search (68) Carbognani, L.; Espidel, J.; Carbognani, N.; Albujas, L.; Rosquete, M.; Parra, L.; Mota, J.; Espidel, A.; Querales, N. Pet. Sci. Technol. 2000, 18, 671-699. (69) Izquierdo, A.; Carbognani, L.; Leo´n, V.; Parisi, A. Fuel Sci. Technol. Int. 1989, 7, 561-570. (70) Carbognani, L.; Izquierdo, A. Fuel Sci. Technol. Int. 1990, 8, 1-15. (71) Solari, B. R. In Asphaltenes and Asphalts. 2. Developments in Petroleum Science, 40B; Yen, T. F, Chilingarian, G. V., Eds.; Elsevier: Amsterdam, 2000; Chapter 7, pp 149-171. (72) Munk, P. Introduction to Macromolecular Science; John Wiley & Sons: New York, 1989.

Table 4. Asphaltene Solubility Parameters δ and Average Molecular Weight Mc of Crosslinks, Calculated with the Flory-Rehner Equation 521,22,65,72 asphaltene

δ (Mpa1/2)

Mc (g/mol)

CN550+ CN550+ ox CN crude Boscan n-C7 Boscan C6 S1-0 feedGU HCKGU D T9

16.3 15.8 16.2 17.9 17.2 14.6 15.2 14.4 15.7

252 303 252 461 195 252 151 35 160

for Mc and δ2 values is based in the minimization of the average deviation on Mc and was carried out using a standard iterative scheme. Table 4 shows the calculated results. Solubility parameters for asphaltenes span the 14.4-17.9 MPa0.5 range. These values are noticeably lower than the corresponding ones calculated from compositional parameters (Table 2). Moreover, they are also noticeably lower than the ones determined22,24,31,40,58-61 or estimated62 in previous publications. These low values resemble those of alkane type compounds and do not agree with aromatic type compounds.57 To explain these unexpected results, a structural model was envisioned. The model proposes that asphaltene particles are composed of inner aromatic cores surrounded by alkyl shells. The alkane solvents employed during the swelling experiments were not able to penetrate the aromatic asphaltene cores, and interacted only with alkane moieties on the particle periphery. Accordingly, the calculated δ2 and Mc values would reflect exclusively the nature of the aliphatic-naphtenic peripheral surface. This would explain the apparent correlation observed between the parameters δ2 and Mc (Table 4). For example, the lowest solubility parameter corresponds to the severely hydrocracked sample (HCKGU), whose alkane cross-links are the shortest determined, as expected from the nature of the process that originated the sample. To validate the proposed model, that is, the hypothesis that peripheral naphteno-aliphatic regions of the solid asphaltene particles are the only ones that swell, an independent calculation of their volume was performed as follows. An average asphaltene molecule with 100 atoms was assumed and a previously published equation (eq 7) was used for the calculation: 73

Vol (aliphatic-naphtenic) ) 32.8 + 16.3Nt(1 - fa) (7) where Nt is the number of carbon atoms and fa is the aromaticity of the average molecule. The aliphaticnaphtenic volume fractions calculated from eq 7 were plotted against the solubility parameters calculated from the Flory-Rehner equation (Table 4, Figure 13). Figure 13 shows that solubility parameters increase if the volume of the aliphatic-naphtenic region increases, which supports the proposed hypothesis. Assumption of an average asphaltene molecular mass of 1000 daltons and combination of asphaltene volumes obtained from density data with the calculated aliphatic-naphtenic volumes allowed to estimate that the volume of the (73) Satou, M.; Nakamura, T.; Hattori, H.; Chiba, T. Fuel, 2000, 79, 1057-1066.



Figure 13. Asphaltene solubility parameters δ as a function of the volume of the aliphatic-naphtenic region. δ values were calculated with the Flory-Rehner eq 5.21,22,65,72 Aliphaticnaphtenic volumes were determined with eq 7.73

alkane fraction spans from a minimum of 40% for the hydrocracked HCKGU asphaltene to a maximum of 80% for Boscan and CN (nonoxidized) asphaltenes. It is worth noting that, for the preceding calculations, naphtenic structures were not differentiated from the lineal moieties. However, solubility parameters for cycloalkanes are reportedly higher than those corresponding to lineal alkanes.57 Consequently, if naphtenic contributions to the peripheral alkane regions could be estimated, the correlation plotted in Figure 13 should improve. Combined evidence discussed in this work allows to propose of an structural model for solid asphaltenes isolated from crude oils. These hydrocarbon types in their isolated-solid form possess inner aromatic cores, surrounded by an outer alkane shell. Permeability of the inner zone toward nonpolar and very polar solvents is limited. Swelling takes place in the external alkane shell. Figure 14 shows a schematic representation of this model, illustrating the original solid state as well as the swollen state after alkane type solvent uptake. A similar model has recently been proposed for coals, based on neutron scattering and proton diffusion.29 The authors described the existence of two domains within solid coals. Accordingly, these materials comprise inner aromatic regions, surrounded by aliphatic zones which are the ones prone to solvent swelling. One of the authors of the present work recently published a similar asphaltene model that allows to explain much of the already known asphaltene behavior in addition to some controversial experimental results, as for instance, the existence or absence of critical aggregate concentration within diverse crude oils.74 To complete the present study, the proposed model was subjected to an experimental test. With this aim, dissolution kinetics of two dried and alkane swollen asphaltenes were performed with toluene at ambient temperature. Recently, asphaltene dissolution kinetics have been studied and reported in greater detail.54 The hypothesis supposes that swollen asphaltenes should dissolve at a faster rate because the swollen alkane shell should facilitate particles incorporation into the solvent stream during the dissolution process. Interparticle core interaction decreases in the swollen regime, increasing (74) Rogel, E. Langmuir 2001, 18, 1928-1937.

Figure 14. Proposed structural model for asphaltenes. The initialy dried solid and the swollen sample are shown, allowing for the visualization of how the alkane shell is swelled by a paraffinic solvent.

Figure 15. Dissolution kinetics for D T9 asphaltene in the solid and swollen state as well. Dissolution was carried out with toluene at ambient temperature, following procedures already described.45,54 Asphaltene swelling was achieved with n-C7.

the dissolution kinetic. Indeed, for both of the samples tested, n-C7 preswollen asphaltenes dissolved at a faster rate during the initial stages of the experiment than original dry asphaltenes, as shown in Figure 15 for one of the samples. However, after a certain point, the dissolution proceeded at a slower rate for the preswollen asphaltene than for the original dry asphaltene. No clear explanation has been found for this behavior. Possibly, incorporation of n-C7 in the external asphaltene framework facilitated the early dissolution of more soluble fractions, leaving more refractory portions behind. These more soluble fractions could act as cosolu-

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bilizers for the refractory portions in case of the dry asphaltenes. Recently, similar behavior was observed during kinetic dissolution studies of composite materials formed from wax mixtures and asphaltenes.45 The trend reported for these composite materials is similar to the trend observed for alkane solvent swollen asphaltenes, which leads to the belief that the proposed model could explain composite dissolution behavior as well. However, a deeper understanding and further studies are necessary to enhance the comprehension of the dissolution behavior found in this work. Conclusions Crude oil asphaltene swelling was demonstrated to be a useful technique to unravel some of their structural properties. Experimental results evidenced the existence of structural networks within isolated crude oil asphaltenes. Dispersive forces proved to be the main factor governing asphaltene swelling. However, peripheral acidic functions of the asphaltenes, in conjunction with basic solvent functionalities were assessed to significantly influence the phenomenon as well. The largest swelling of virgin crude oil asphaltene was observed with alkane solvents. The solubility parameters of the asphaltenes match the typical alkane range. Swelling of these samples depends on their hydrogen


content and the volume of their alkyl moieties within the asphaltene. On the other hand, the only hydrocracked sample studied did not swell significantly in alkane solvents, but exhibited the largest swelling with medium polarity solvents instead. The evidence gathered permitted to propose an asphaltene structural model to account for all the results. The model considers the existence of inner aromatic regions, surrounded by external aliphatic zones. The latter are permeable toward solvents; therefore, the swelling takes place exclusively within these peripheral regions. Dissolution kinetics of dry and solvent swollen asphaltenes support the proposed model and are in agreement with recently published articles on coal and crude oil asphaltenes as well. Acknowledgment. The authors thank PDVSAIntevep for funding and permission to publish this work. Colleagues from PDVSA-Intevep are acknowledged for sample characterization: titration data were provided by E. Contreras, NMR spectra were obtained by Y. Espidel, and elemental analysis was carried out by M. Matos and N. Badillo. The detailed review of Dr. U. Ehrmann is greatly appreciated for improvement of the original manuscript. EF010299M

Solvent Swelling of Petroleum Asphaltenes - Energy & Fuels (ACS

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