Asphaltene aggregation problems are encountered both in the reservoir and during transport and process- ing of petroleum fluids. It is difficult to predict where and when in the production such problems might arise, but the consequences of unexpected asphaltene deposi- tion can be severe, both technically and environmen- tally. Due to the complex and highly polydisperse composi- tion of the asphaltene fraction it is defined operationally as the fraction of the crude oil precipitating in light alkanes such as n-pentane, hexane, or heptane. The precipitate is soluble in aromatic solvents such as toluene and benzene. Asphaltenes are the heaviest and most polar molecules in the crude oil, containing larger amounts of heteroatoms (N, S, O, metals) than the rest of the components in the crude oil. The molecular weight of asphaltenes is difficult to determine due to their strong tendency to self-aggregate, but a molecular weight of approximately 1000 g/mol is commonly men- tioned.1,2 It is generally accepted that the asphaltene molecules are dispersed in the oil by the polar molecules, aromat- ics, and resins, thus stating that the asphaltenes are colloidally dispersed in the crude oil.3,4 Resins adsorb
* Author to whom correspondence should be addressed. Fax: +47 73 58 46 28. E-mail: firstname.lastname@example.org. † Norwegian University of Science and Technology. ‡ Statoil R&D Centre. (1) Castillo, J.; Fernandez, A.; Ranaudo, M. A.; Acevedo, S. Pet. Sci. Eng. 2001, 19,75-106. (2) Sheu, E. Y.; Mullins, O. C. Asphaltenes: Fundamentals and Applications; Plenum Press: New York, 1995.
on to the asphaltene aggregates and keep them dis- persed in the crude oil medium. The molecules of the resin fraction are generally considered to be smaller than the asphaltene molecules. One common definition of resins is as the fraction soluble in light alkanes such as pentane and heptane, but insoluble in liquid pro- pane.2,5 The stability of this colloidal solution of as- phaltenes dispersed by resin molecules may be affected by temperature, pressure, and fluid composition, with the last two parameters having the greatest effect. The addition of light, paraffinic components to an asphaltene-containing solution will lower the solubility power with respect to the asphaltenes. Resin molecules will react to the addition by desorbing from the asphalt- enes in an attempt to reestablish thermodynamic equi- librium, thus increasing the probability of asphaltene self-aggregation.6 An increase in the temperature gen- erally affects the aggregation of asphaltenes by decreas- ing the solvating power of the crude oil,7 but some controversy regarding the temperature effect exists in the literature. Several authors state that the asphaltene aggregate size decreases with increasing temperature (aromatic solvents).8 Speight5 states that the precipita- tion of asphaltenes increases with temperature. A
(3) Pfeiffer, J. P.; Saal, R. N. Phys. Chem. 1940, 44, 139-149. (4) Swanson, J. J. Phys. Chem. 1942, 46, 141-150. (5) Speight, J. G. The Chemistry and Technology of Petroleum; Marcel Dekker: New York, 1999. (6) Hammami, A.; Phelps, C. H.; Monger-McClure, T.; Little, T. M. Energy Fuels 2000, 14,14-18. (7) Nielsen, B. B.; Svrcek, W. Y.; Mehrotra, A. K. Ind. Eng. Chem. Res. 1994, 33, 1324-1330. (8) Thiyagarajan, P.; Hunt, J. E.; Winans, R. E.; Anderson, K. B.; Miller, T. Energy Fuels 1995, 9, 829.
1288 Energy & Fuels, Vol. 16, No. 5, 2002