Energy & Fuels 2004, 18, 1243-1249
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Effects of Temperature and Pressure on Asphaltenes Agglomeration in Toluene. A Light, X-ray, and Neutron Scattering Investigation D. Espinat,* D. Fenistein, L. Barre´, D. Frot, and Y. Briolant Institut Franc¸ ais du Pe´ trole, 1&4 Ave de Bois-Pre´ au, 92852 Rueil-Malmaison, France Received December 22, 2003
Small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and dynamic light scattering (DLS) are used to study the temperature dependence of asphaltene aggregate sizes in toluene solutions. A large range of aggregate sizes is covered by combining the three scattering methods. The effect of temperature on aggregate size is also investigated over a considerable temperature range. A huge modification of asphaltene macrostructure is observed. At high temperatures, reversible aggregation of asphaltene leads to stable small entities. When decreasing the temperature, irreversible aggregation of asphaltene occurs, corresponding to a large increase of the aggregate size. We have also investigated the effect of pressure on asphaltene solution in toluene as a function of temperature. Our results indicate that pressure has a minor effect, which is much less important than that of temperature, on the weight of the asphaltene aggregates.
Introduction It is well-known that asphaltenes exhibit a colloidal structure in crude oils or in heavy petroleum fractions. Many investigations have been undertaken in earlier studies to secure a better description of this colloidal macrostructure.1-4 This approach is of great importance to gain a better understanding of the industrial problems found during oil recovery, transport, or refining. Asphaltenes are often involved in these practical difficulties. Asphaltenes and resins may flocculate in the reservoir during its exploitation as a result of pressure, temperature, or changes in the composition of the oil. It was observed that asphaltene flocculation is very important near the bubble point.5,6 Asphaltene flocculation may lead to formation damage, because of pore plugging or permeability reduction.7,8 Asphaltene deposition can induce fouling of the production and surface handling facilities. To meet current environmental polution stan* Author to whom correspondence should be addressed. Telephone: +33-0478022943. Fax: +33-0478022745. E-mail address:
[email protected]. (1) Yen, T. F.; Erdman, J. G.; Pollack, S. S. Anal. Chem. 1961, 33 (11), 1587. (2) Sheu, E. Y.; Mullins, O. C. Asphaltenes: Fundamentals and Applications; Plenum Press: New York and London, 1995. (3) Pfeiffer, J. P.; Saal, R. N. J. J. Phys. Chem. 1940, 44, 139. (4) Espinat, D.; Rosenberg, E.; Scarsella, M.; Barre, L.; Fenistein, D.; Broseta, D. “Colloidal Structural Evolution from Stable to Flocculated State of Asphaltene Solutions and Heavy Crudes ”. In Structures and Dynamics of Asphaltenes ; Mullins, O. C., Sheu, E. Y., Eds.; Plenum Press: New York, 1998; pp 145-201. (5) Briant, J. Rev. Inst. Fr. Pet. 1963, 1. (6) Szewczyk, V.; Thomas, M.; Behar, E. Rev. Inst. Fr. Pet. 1998, 53 (1), 51. (7) Islam, M. R. Role of Asphaltenes on Oil Recovery and Mathematical Modeling of Asphaltene Properties. Dev. Pet. Sci. 1994, 40A, 249. (8) Minssieux, L. Presented at the SPE International Symposium on Oilfield Chemistry, Houston, TX, 1997, SPE Paper 37250, p 703.
dards, it is necessary to upgrade heavier crude oils or petroleum fractions that contain high concentrations of asphaltenes, sulfur, and nitrogen.9-11 Various processes often used are based on thermal or catalytic cracking under a high pressure of hydrogen. Asphaltenes are the heaviest and most polar among the molecules contained in oils and residues. They are defined as the fraction that is insoluble in light hydrocarbons such as n-pentane or n-heptane. Important investigations have been conducted to have a good description of asphaltene molecules. The major portion of these studies has been dedicated to the chemical characterization.12-14 We can simply represent the asphaltene molecule by two different regions: (i) the first one consists of aromatic or naphtenic rings more or less extended as a function of the aromaticity of the molecule, and (ii) a second one that is composed of aliphatic chains. More recently, a huge effort has been made to gain an insight in the colloidal state of asphaltenes in good solvents or in their natural medium. Many techniques have been applied; they have shown, for similar types of asphaltenes, a wide range of molecular (9) Billon, A.; Morel, F.; Morrison, M. E.; Peries, J. P. Rev. Inst. Fr. Pet. 1994, 49 (5), 495. (10) Morel, F.; Kressmann, S.; Harle´, V.; Kasztelan, S. Hydrotreatment and Hydrocracking of Oil Fractions; Froment, G. F., Delmon, B., Grange, P., Eds.; Elsevier Science: Amsterdam, 1997; p 1. (11) Speight, J. G. Asphaltene Characterization and Use in Understanding Processes. In Symposium on the Role of Asphaltenes in Petroleum Exploration, Production and Refining, 207th National Meeting of the American Chemical Society (ACS), San Diego, CA, 1994; p 200. (12) Bunger, J. W.; Li, N. C. Chemistry of Asphaltenes; Advances in Chemistry Series 195; American Chemical Society: Washington, DC, 1981. (13) Speight, J. G. The Chemistry and Technology of Petroleum, 2nd ed.; Marcel Dekker: New York, 1991. (14) Altgelt, K. H.; Boduszynski, M. M. Composition and Analysis of Heavy Petroleum Fractions; Marcel Dekker: New York, 1994.
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weights.15 These experiments have demonstrated that the molecular weight of asphaltene is not unique and is strongly dependent on the asphaltene concentration, the nature of the solvent, and the temperature.15-20 One of the major conclusions of these investigations was that asphaltenes have a strong tendency toward self-association in solution, resulting in, more or less, extended aggregates, particularly at room temperature. An asphaltene micellization mechanism has been proposed in the literature;21-24 for very low asphaltene concentrations, elementary asphaltene molecules can be observed. However, it is very difficult to measure the asphaltene molecular weight under such conditions, because all scattering techniques are not sensitive enough for solutions of low asphaltene concentration. The addition of resins does result in the formation of a smaller micellar-like structure. Various fractionation techniques of asphaltene solutions have shown their large polydispersity, in regard to size and chemical composition.21,24-29 Fractal-like behavior can be observed in these asphaltene solutions.30-35 The purpose of this study is to investigate the asphaltene macrostructure as a function of temperature and pressure. Small-angle X-ray or neutron scattering techniques16 have shown their capabilities for investigating the colloidal state of asphaltene molecules, either in solution, in more or less good solvents, or in real petroleum products. These tools allow the measurement of the molecular weight and the radius of gyration. Photon correlation spectroscopy (PCS) has been applied for the investigation of asphaltene aggregation.31 As we have previously mentioned, the effect of temperature on asphaltene agglomeration in solution has been (15) Speight, J. G.; Wernick, D. L.; Gould, K. A.; Overfield, R. E.; Rao, B. M. L.; Savage, D. W. Rev. Inst. Fr. Pet. 1985, 40 (1), 51. (16) Espinat, D. Application des Techniques de Diffusion de la Lumie` re, des Rayons X et des Neutrons a` l’E Ä tude des Syste` mes Colloı¨daux; Editions Technip, Paris, 1990; p 111. (17) Ravey, J. C.; Ducouret, G.; Espinat, D. Fuel 1988, 67, 1560. (18) Thiyagarajan, P.; Hunt, J. E.; Winans, R. E.; Anderson, K. B.; Miller, J. T. Energy Fuels 1995, 9, 829-833. (19) Sheu, E. Y.; Liang, K. S.; Sinha, S. K.; Overfield, R. E. J. Colloid Interface Sci. 1992, 153 (2), 399. (20) Tanaka, R.; Hunt, J. E.; Winans, R. E.; Thiyagarajan, P.; Sato, S.; Takanohashi, T. Energy Fuels 2003, 17, 127. (21) Andersen, S. I.; Speight, J. G. Fuel 1993, 72 (9), 1343. (22) Andersen, S. I.; Birdi, K. S. J. Colloid Interface Sci. 1991, 142 (2), 497. (23) Storm, D. A.; Barresi, R. J.; Sheu, E. Y. Evidence for the Micellization of Asphaltenic Molecules in Vacuum Residue. In Symposium on Petroleum Chemistry and Processing, The Division of Petroleum Chemistry, Inc. 210th National Meeting (Chicago, IL); American Chemical Society: Washington, DC, 1995; p 776. (24) Bardon, Ch.; Barre, L.; Espinat, D.; Guille, V.; Li, M. H.; Lambard, J.; Ravey, J.; Rosenberg, C.; Th Zemb, E. Fuel Sci. Technol. Intl. 1996, 14 (1-2), 203. (25) Szewczyk, V.; Behar, F.; Behar, E.; Scarsella, M. Rev. Inst. Fr. Pet. 1996, 51 (4), 575. (26) Andersen, S. I.; Stenby, E. H. Fuel Sci. Technol. Int. 1994, 14, 261. (27) Mitchell, D. L.; Speight, J. G. Fuel 1973, 52, 149. (28) Ali, M. F.; Saleem, M. Fuel Sci. Technol. Int. 1988, 6, 541. (29) Ali, L. H.; Al Ghannam, K. A. Fuel 1981, 60, 1043. (30) Liu, Y. C.; Sheu, E. Y.; Chen, S. H.; Storm, D. A. Fuel 1995, 74, 1352. (31) Anisimov, M. A.; Yudin, I. K.; Nikitin, V.; Nikolaenko, G.; Chernoustan, A.; Toulhoat, H.; Frot, D.; Briolant, Y. J. Phys. Chem. 1995, 99, 9576. (32) Raghunathan, P. Chem. Phys. Lett. 1991, 182, 331. (33) Fenistein, D.; Barre, L.; Broseta, D.; Espinat, D.; Livet, A.; Roux, J. N.; Scarsella, M. Langmuir 1998, 14, 101. (34) El Mohamed, S.; Hardouin, F.; Gasparoux, H. J. Chim. Phys. 1998, 85, 135. (35) Janardhan, A. S.; Mansoori, G. A. J. Pet. Sci. Eng. 1993, 9, 17.
Espinat et al. Table 1. Elemental Analysis of Safaniya Vacuum Residue (VR) Asphaltenes element
content (%, w/w)
carbon hydrogen oxygen nitrogen sulfur
82.5 7.5 1.4 1.0 7.5
investigated; the range of temperature examined was confined to temperatures of 300 K, and a huge variation of the hydrodynamic radius is highlighted for lower temperatures. A decrease of temperature is followed by a strong aggregation, which first generates a fractal-like structure and then an acceleration of the aggregation process, resulting in flocculation. Differences between high- and low-temperature behavior may be explained by the sizes of the particles involved in the aggregation or disaggregation processes. During cooling of the asphaltene solution, very large aggregates can be detected by the naked eye or when sedimentation occurs. This observation indicates the formation of dense aggregates. This behavior can be compared with the n-heptane flocculation process.33 Temperature decrease induces simultaneously (i) a desolvation or densification of the aggregates, due to solvent expulsion, and (ii) an aggregation between different asphaltene entities. A certain nonreversibility of asphaltenes at low temperatures was observed, probably because of strong interactions between molecules or difficult solvation, which is necessary for aggregate dispersion. These results are in agreement with the model of multiple orders of asphaltene aggregation.44 First, a micelle is formed, including several asphaltene molecules. Interaction forces involved in the micellization can be of different types and, thus, different bonding energies. These are strong interactions compared to (44) Yen, T. F. In Encyclopedia of Polymer Science and Engineering, 2nd Ed.; Grayson, M., Krochwitz, J. I., Eds.; Wiley: New York, 1988; p 1.
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those involved in further steps of aggregation. The size of the micelle is probably close to the asphaltene size observed at room temperature. Further micelles aggregation occurs, resulting in supermicelles and then giant supermicelles and flocs. Conclusion We have studied asphaltene aggregation as a function of temperature. A new apparatus was constructed to investigate high-temperature asphaltene evolution using small-angle neutron scattering (SANS). Low-temperature effects were monitored using a small-angle X-ray scattering (SAXS) technique and photon correlation spectroscopy (PCS). From room temperature to very high temperatures (563 K), a continuous disaggregation process is observed. Small asphaltene entities are present at 563 K; however, we are not sure that they are asphaltene basic units, which should be observed below the critical micelle concentration. A further increase of temperature is not straightforward, because thermal cracking can occur. We have suggested two different aggregation processes: (i) when the temperature increases, the asphaltene molecule dissociates, following a micellar-type equilibrium; (ii) when the temperature decreases, a fractal-like association is observed, resulting in the formation of flocs. The micellar association is a reversible process. The size (diameter) of the micelle is approximately some tens of angstroms. At low temperatures, the aggregation seems quite irreversible, leading to dramatic changes in the sample (flocculation, sedimentation). We have suggested that low-temperature behavior is similar to the flocculation process induced by n-heptane addition. Note that temperature variation can be, experimentally, an easier solution than n-heptane flocculation, to modify the aggregation state of asphaltenes in solution or in pure crude oils. Acknowledgment. The authors are grateful to “Laboratoire Leon Brillouin”, CEN-Saclay (France)s particularly, J. P. Cotton and J. Teixeirasfor making the SANS spectrometer available to us. We would like to thank J. N. Roux and D. Broseta for interesting discussions. We have also benefited from very judicious technical advice from J. Brandely, M. Garcia, and A. Stanek. EF030190+
