Quimica

Energy & Fuels 2004, 18, 1243-1249

1243

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 Francais du Petrole, 1&4 Ave de Bois-Preau, 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 asphaltenemacrostructure 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 pressurehas 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 greatimportance 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 veryimportant 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:didier.espinat@ifp.fr. (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 fromStable 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 ofAsphaltene 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 catalyticcracking 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...