Modeling the Phase Behavior of Asphaltene plus Toluene plus Polystyrene Mixtures-A Depletion Flocculation Approach

Mixtures of polystyrene + Maya and Athabasca pentane asphaltene + toluene split into two stable phases, one toluene + polymer rich and one toluene + asphaltene rich. This phase behavior was attributed to depletion flocculation, previously, and a Fleer-Tuinier based model was used to simulate the phase diagrams including one-phase to two-phase boundaries, tie lines, critical points, and relative phase volumes in the two-phase region. The distribution of asphaltenes between molecular and aggregated species and the variation of the mean size and size distribution of aggregated asphaltene species with global composition are not known a priori. This knowledge gap presents a key conceptual challenge. Consequently, the variation of the fraction of asphaltenes participating in the depletion flocculation mechanism, gamma(eta), and variation of asphaltene mean size R-s(eta) with asphaltene volume fraction, eta, are fitted parameters introduced in the phase behavior model. In this work, the phase behavior modeling approach is presented and illustrated, and the joint conformance of R-s(eta) and gamma(eta) with trends anticipated by the well-known Ostwald-Freundlich equation for solute solubility with solute particle size is demonstrated for both Maya and Athabasca pentane asphaltenes. With this conformance, the behavior of asphaltenes in toluene is shown to be consistent with the behavior of a temperature invariant distribution of solid, spherical, colloids where the mean size varies with asphaltene volume fraction, eta. Furthermore, the correlations gamma(eta) and R-s(eta) are validated with measured differential enthalpies of solution for asphaltenes in toluene. These outcomes provide additional theoretical underpinnings for a phase behavior modeling approach that may lead to the development of predictive models for asphaltene + polystyrene + toluene and analogous mixtures as additional phase behavior data become available.