ARTICLE pubs.acs.org/EF
LiquidLiquid Phase Equilibria in Asphaltene + Polystyrene + Toluene Mixtures at 293 K M. Khammar and John M. Shaw* Chemical and Materials Engineering, University of Alberta, Edmonton, T6G2 V4, Canada ABSTRACT: The phase behavior of hydrocarbon mixtures where one of the constituents self-aggregates is a subject of significant industrial and academic interest. Here, a nonintrusive acoustic phased-array technique operated in pulse echo mode is used to investigate the phase behavior of asphaltenes, a well-known self-aggregating species, in mixtures with polystyrene and toluene at 293 K and atmospheric pressure. This mixture exhibits liquidliquid phase behavior where both liquids are opaque to visible light, are of uniform composition, and are stable over broad ranges of composition. One phase is asphaltene rich and the other phase is polystyrene rich. Varying the polystyrene mean molar mass had little impact on the liquid to liquidliquid phase boundaries. Liquidliquid critical points were identified and phase compositions were confirmed for a fixed global composition using the UVvisible spectrophotometry and mass balance equations. This is the first report of liquidliquid phase behavior for such mixtures. Depletion flocculation is hypothesized to be the mechanism causing phase separation in this ternary mixture.
1. INTRODUCTION Asphaltenes are defined as the fraction of crude oil insoluble in alkanes and soluble in benzene and toluene on the basis of filtration experiments (ASTM D4055). However, in toluene, asphaltenes aggregate to form sterically stabilized colloidal particles.15 The size of these particles has been the subject of several investigations. For example, Barre et al.6 measured the radius of gyration of asphaltenes colloidal particles in a 3 vol % toluene solution using small-angle X-ray scattering (SAXS). They found that the asphaltene aggregates fall in the size range 6.3 to 16 nm. In heavy oils (15 to 20 wt % asphaltenes7) nanofiltration experiments showed that asphaltene nanoaggregates form large aggregates up to 50100 nm in both Athabasca bitumen and Maya crude oil.8 Espinat et al.9 measured the size of asphaltenes in toluene using small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and dynamic light scattering to investigate the effect of temperature and pressure on the size of asphaltene aggregates in toluene. They found that the size of asphaltene aggregates decreased with temperature, while pressure did not appear to have a significant effect on size. The interaction forces between asphaltene colloidal particles in toluene are dominated by repulsion. For example, Wang et al.10 measured steric repulsive forces between asphaltene-coated surfaces in toluene. If asphaltene-containing mixtures are treated as colloidal solutions, conventional separation methods include ultracentrifugation, variation of pressure, temperature, solvent evaporation, and addition of antisolvent.11,12 Alternatives include polymer addition.12 In a recent work, Lima et al.13 focused on the impact of adsorbing polymers, polycardanol and sulfonated polystyrene, on asphaltene solutions. Polycardanol polymers were added to asphaltene in toluene solutions (60 mg/L) and sulfonated polystyrene was added to asphaltene in toluene and acetone solution. Asphaltene + polymer solutions were left for 24 h and then centrifuged at 3000 rpm for 30 min. The effect of polymer addition was estimated by measuring the concentration of r 2011 American Chemical Society
asphaltenes remaining dispersed using UVvisible spectrophotometry. At low polymer concentrations (
