2 nd Mercosur Congress on Chemical Engineering 4 th Mercosur Congress on Process Systems Engineering A TRANSITION FROM MONOMERIC TO ASSOCIATED ASPHALTENES IN CRUDE OIL SOLUTIONS Igor N. Evdokimov * Department of Physics – Gubkin Russian State University of Oil and Gas Abstract. It is widely recognized that physicochemical properties of crude oil asphaltenes in processes of refining and processing (i.e. transfer rates in catalyst’s porous networks) depend on their association state in a hydrocarbonated environment. For various industrial applications it is necessary to have a detailed knowledge of physico-chemical and structural properties of complex asphaltene colloids, in order to be able to master their behavior. We have studied optical and dielectric properties (molar absorptivity, refractive index, dielectric constant, dielectric loss) in dilute toluene solutions of Russian crude oils and of solid asphaltenes. Sharp inflections in measured concentration dependencies are indicative of changes in association states of asphaltenes. Apparently, asphaltenes may exist in a monomeric state only at concentrations below 1-2 mg/l, while at 60-90 mg/l there is a demixing transition to a colloidal system of asphaltene aggregates. By analysis of correlations between measured properties we expect to obtain quantitative information on dipole moments of asphaltene monomers. Keywords: Crude Oil, Asphaltenes, Associated State. 1. Introduction Properties of crude oil asphaltenes in refining and processing depend on their association state in a hydrocarbon environment. Increased association may result in lower transfer rates in catalyst’s porous networks, in higher levels of entrapment of light fractions within asphaltene aggregates. For various industrial applications it is necessary to have a detailed knowledge of complex asphaltene colloids in order to be able to master their behavior. Refractive index (RI) represents various important properties of multicomponent crude oils as well of the respective components. Values of RI can be measured very accurately and are used to correlate density, parachor, and other properties of hydrocarbons with high reliability) (Touba et al., 1997). Information obtained from RI measurements can be applied for various reservoir engineering calculations. The RI of light crude oils is directly measured in conventional refractometers (Yang et al., 2002; Buckley and Wang, 2002; Buckley, 1999). However, direct measurements of the RI of heavy crudes are unattainable since these liquids are too opaque. In these cases RI is determined for a series of oil/solvent mixtures and the data are extrapolated (in an assumption of a certain mixing rule) to determine the RI of the crude oil (Yang et al., 2002; Goual and Firoozabadi, 2002; Taylor et al., 2001). It is usually assumed that a solution of a crude oil behaves as an ideal binary mixture of the components (Taylor et al., 2001; Buckley, 1999; Buckley et al., 1998). It should be noted that there are no universally accepted mixing rules for interpreting the refractive indices of such presumably ideal mixtures. Most frequent are mixing rules based on the Lorentz-Lorenz additivity rule for specific polarizabilities/refractions of the components (Shoemaker et al., 1996). In notations of Buckley and Wang, 2002; Buckley et al., 1998 and Wattana et al., 2005 : * Address: Department of Physics, Gubkin Russian State University of Oil and Gas, Leninsky Prospekt, 65, Moscow B- 296, GSP-1, 119991 – Russia. E-mail: physexp@gubkin.ru , URL: http://eee.gubkin.ru 1