The Phase Behavior of Asphaltene + Polystyrene + Toluene Mixtures at 293 K

Polymers of various types are added to crude oils and oil products to prevent wax deposition, break water-in-oil emulsions, reduce drag in pipelines and to stabilize asphaltenes. In mixtures where a polymer does not adsorb on colloids, two stable liquid phases can arise due to depletion flocculation. Asphaltenes in heavy oils and toluene mixtures form sterically stabilized colloidal particles. In this work, the addition of a non-adsorbing polymer (polystyrene) to C5 Maya asphaltene + toluene mixtures was investigated experimentally and theoretically. As concentrated asphaltene + toluene mixtures are opaque to visible light, phase volumes and compositions were detected using ultrasound. The sensors comprised two commercial 64 element phased-array acoustic probes. The operation of the view cell, and kinetic and equilibrium data processing procedures were validated using mixtures of methanol + alkanes. Acoustic speed and attenuation profiles were found to provide independent measures of phase separation. At equilibrium, acoustic speed profiles are uniform in each phase with a step change at the interface. Acoustic wave attenuation profiles exhibit a sharp peak/spike at liquid-liquid interfaces. Mixtures of asphaltenes + polystyrene + toluene are shown to exhibit liquid-liquid phase behavior over broad ranges of composition. This is the first report of liquid-liquid phase behavior for such mixtures. One phase is asphaltene rich and the other phase is polystyrene rich. Liquid-liquid critical points were also identified along the liquid-liquid/liquid phase boundary for mixtures with two mean molar masses of polystyrene.;Compositions of co-existing phases were computed using phase volume variations along dilution lines [1], acoustic speed data and a mass balance model. A parameter was introduced to improve the agreement between calculated and experimental speeds of sound. The results of the model indicate that more than half of the asphaltenes, by volume, participate in the depletion flocculation process. Phase compositions were measured independently using UV-visible spectrophotometry. The nominal size of asphaltene colloidal particles participating in the phase separation mechanism was estimated by comparing calculated phase boundaries with the experimental phase diagram. The estimated size of asphaltene colloidal particles is in agreement with the expected size of asphaltenes in toluene mixtures obtained exogenously.