On the Applicability of the Regular Solution Theory to Asphaltene + Diluent Mixtures

Abstract

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...

Recent Advances in Nonaqueous Extraction of Bitumen from Mineable Oil Sands: A Review

“…The critical value of the solubility parameter difference between an asphaltene and solvent was dependent on the polarity and on the ability of a solvent to “self-associate” . Other interactions like polar hydrogen bonding, are important and should be taken into account in systems of bitumen and its fractions. − …”

“…If plotted as a function of the H/C ratio, solubility parameters of bitumen components are shown in Figure . ,, Interestingly, there appears to be a continuous increase of the solubility parameter with deceasing H/C from saturates, aromatics, and resins to asphaltenes. − ,− Depending on the sample composition, solubility parameters of saturates and crude oils are clustered near 17 MPa 1/2 and those of aromatics and resins near 19 MPa 1/2 . Maltenes have solubility parameters ranging from 18 to 20 MPa 1/2 .…”

“…In gravitational separations, the extraction of oil sand bitumen with an aliphatic solvent or analogue such as pentane, hexane, or heptane often produces relatively clean product with low solids content in bitumen. − ,− ,− ,− Depending on its carbon number and S/B, an aliphatic solvent has controlled ability to precipitate out partially or completely the heaviest fraction of bitumen, the asphaltenes. − Asphaltenes readily form floc networks, entrapping and capturing unwanted particulates and heavy metals, so asphaltenes and fines are separated from the extracted bitumen together . For example, Zahabi et al showed that the removal of solid particles in oil started effectively at the onset of minimal asphaltene precipitation .…”

Thermodynamic modelling of asphaltene precipitation and related phenomena

“…Although these are typically determined through correlation of experimental data, solubility parameters are also obtained using equations of state [93,109,123,[127][128][129][130][131][132][133]. Other authors have also highlighted the need for a more detailed description of physicochemical phenomena than that underlying regular solution theory, commenting on some of the limitations and misapplication of the theory to these mixtures [134].…”

Aggregation thermodynamics for asphaltene precipitation

“…This model was later generalized to cover asphaltene precipitation from heavy oils at various conditions . Although the regular solution model was capable of correlating asphaltene precipitation in reasonable agreement with experimental data, it was believed to be misapplied . As an evidence, experimental measurements showed that partial specific volumes and enthalpies of solution of asphaltene in various diluents have no correlation with solubility parameters, which is inconsistent with the thermodynamic concepts of regular solution theory.…”