Phase behavior of asphaltenes in organic media

Abstract

Unwanted phase separation of asphaltenes poses significant challenges in production and processing of heavy oils and bitumen. The analysis and prediction of asphaltene precipitation behaviors and asphaltene-diluent interactions are typically treated either from a solution thermodynamics or a colloid perspective. However, the polydispersity and multi-phase nature of asphaltenes allows for portions of these materials to exhibit both behaviors concurrently at fixed global composition. Partial specific volume at near infinite dilution and enthalpies of solution are sensitive thermodynamic measures of solute-solvent interactions derived from high precision density and calorimetry measurements for dilute mixtures. Solution calorimetry and density measurements are performed on mixtures of asphaltenes with a variety of diluents ranging from n-alkanes to bicyclic aromatics. The validity of the assumption of solubility of asphaltenes in various diluents is evaluated based on density and calorimetry data. The trends of changes in enthalpies of solution of asphaltenes and their fraction with variations in composition and temperature are used in identifying the interactions of asphaltenes with the diluents and determining the phase state of asphaltenes in organic media. The results indicate co-existence of a soluble and an insoluble fraction of asphaltenes in each organic diluent and at every fixed temperature and concentration. Endothermic phase transitions and exothermic sorption of diluent are detected in n-alkanes as well as 1-methylnaphthalene, tetrahydrofuran, toluene and quinoline. Asphaltenes are shown to have both intermolecular and interfacial interactions with the studied diluents. The results are in agreement with the hypothesis that only a fraction of asphaltenes undergo a solid-liquid phase change upon mixing with diluents and that fraction is a function of diluent properties, global composition and temperature. These results are inconsistent with the application of dissolution as classically defined to asphaltene + diluent mixtures. Quantitation of the asphaltene fractions potentially present as colloidal particles or as dissolved species in a diluent remains a subject for future study.