Guide to Asphaltene Solubility

Abstract

A guide to the solubility of asphaltenes in a range of solvents is constructed through the use of an association model to account for asphaltene nanoaggregation and its effect on phase behavior and Hildebrand solubility parameters to model interactions between aggregates and solvent. Solvents are classified according to their polarity and ability to self-associate (e.g., through hydrogen bonds). In addition, estimates of the contribution of free volume terms to interaction parameters indicate that a further distinction must be made between solvents with flexible molecules (such as the n-paraffins) and those that are relatively inflexible (such as toluene). A "bare" interaction parameter (χ0) is calculated, and it is this parameter that is related to Hildebrand solubility parameters. For nonpolar and weakly polar solvents, a critical value of the solubility parameter difference (Δδc) between an asphaltene or asphaltene component and solvent is calculated to be ±3.5 MPa0.5 at 25 °C for a nonpolar or weakly polar solvents with largely inflexible molecules and a molar volume of 100 cm3/mol. For flexible solvents such as the n-paraffins, free volume effects are larger and Δδc is about ±2.8 MPa0.5. For strongly polar solvents that have limited flexibility, the equivalent critical value of the solubility parameter difference is also calculated to be ±2.8 MPa0.5. Hydrogen bonded solvents like methanol are calculated to be immiscible with asphaltenes, with miscibility being defined as forming a single phase across the composition range (at 25 °C). Miscibility maps are constructed in terms of the calculated phase boundary at the critical point, δA ± Δδc, where δA is the asphaltene component solubility parameter, plotted as a function of solvent molar volume. The solubility of asphaltenes and asphaltene components in various solvents is discussed. The solvent that defines asphaltene identity, toluene, is predicted to dissolve only a limited range of asphaltene components. This is consistent with various reported experimental observations. However, solubility is often defined in terms of an absence of a visible precipitate, and on the basis of recent work in the literature, toluene solutions may contain some microphase-separated material stabilized against further aggregation by steric and kinetic factors.