The mass fraction and the properties of asphaltenes vary significantly with the n-alkane used to separate them from their parent oil and with the details of the separation procedure, such as washing steps. Measurement repeatability is challenging, with different error bounds reported within the American Society for Testing and Materials (ASTM) standards for a single operator using the same equipment and procedure vis-à-vis measurements performed by different operators in different laboratories. In this work, reversible interactions between Athabasca pentane asphaltenes and n-alkanes from pentane to hexadecane were observed using cross-polarized and visible light microscopy and were quantified using high-precision density measurements for mixtures ranging from 1000 to 8000 ppmw (from 0.8 to 6.5 g/L) and enthalpy of solution measurements for mixtures comprising from 1000 to 3500 ppmw (from 0.8 to 3 g/L) asphaltenes. The partial specific volumes of Athabasca pentane asphaltenes and the enthalpy of solution values, including a change of sign, were found to vary systematically with n-alkane carbon number. The microscopic observations revealed the formation of liquid crystals followed by isotropic liquid on the surface of the asphaltene particles. The interactions at low concentrations are consistent with n-alkane sorption by asphaltene particles, asphaltene particle swelling, and dissolution of a fraction of the asphaltenes in n-alkanes. The partial specific volume and enthalpy of solution results, simulated using a phenomenological model that includes these effects, explain the sensitivity of the repeatability of asphaltene mass fraction determinations to the details of the washing procedure applied during their preparation. Preparation techniques without washing appear to be preferred because the mass fractions of asphaltenes recovered are expected to be more repeatable and their properties are likely to be more consistent.
The similarity of the Hildebrand or Hansen solubility parameter is frequently used in petroleum science as a measure of the compatibility of constituents and for interpreting and correlating properties of asphaltene + diluent mixtures. A partial specific volume at near infinite dilution and enthalpies of solution are sensitive measures of solute−solvent interactions derived from high precision density and calorimetry measurements for dilute mixtures. In this contribution, the partial specific volumes and enthalpies of solution of pyrene and various Athabasca and Maya asphaltenes at near infinite dilution on a mole fraction basis, in decane, toluene, 1-methylnaphthalene, quinoline, anisole, 2,6-lutidine, pyridine, methylene chloride, and tetrahydrofuran are reported over the temperature range of 20−80°C. At 20°C, these diluents possess solubility parameters ranging from 15 MPa 0.5 (decane) to 22 MPa 0.5 (quinoline). The properties of pyrene + diluent mixtures are used to illustrate the application and misapplication of the regular solution theory to such mixtures. Thermodynamic measurements, partial specific volume, and enthalpy of solution are shown to be independent of both Hildebrand and Hansen solubility parameter values. These results do not support the use of the solubility parameter or other simple solution thermodynamic concepts to describe asphaltene + diluent mixture behavior. The need for a more detailed description of physiochemical phenomena arising upon mixing asphaltenes with diluents is discussed. ■ INTRODUCTIONRegular solution theory, as postulated by Hildebrand, is a modified version of van Laar's model for solutions. The theory assumes that the free-energy change upon mixing is correlated with the internal energy per unit volume of the species in the mixture and thereby makes predictions about the solubility behavior of materials. To apply regular solution theory, a mixture must meet the following criteria: 1 (1) components of the mixture should be subject to the same types of forces within the mixture as in pure liquids; (2) mutual energy (interactions) of two molecules should not depend upon their relative orientation or the presence of other molecules; (3) the distribution of molecules should be random; i.e., excess entropy of mixing is negligible; and (4) equilibrium relationships should be applicable to constituents collectively.Even though asphaltenes are defined as soluble in toluene, it is not clear whether asphaltene + diluent mixtures meet the above criteria because the operating definition for solubility of asphaltenes in toluene is based on macroscopic filtration measurements. 2 This definition is inconsistent with the thermodynamic definition of solubility of solids in liquids, which includes a phase change, from solid to liquid, for the solute followed by molecular-level dispersion in the solvent. Further, asphaltenes aggregate in organic media even at low concentrations. 3−8 There is a growing body of literature that asserts that asphaltenes can be separated from toluene by centri...
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