(650d) Phase Behavior of Asphaltenes + Organic Diluents

Asphaltenes, defined operationally on the basis of ASTM standards that include mixing and macroscopic filtration steps, are typically asserted to be soluble in toluene [1]. A similar definition is used to classify organic diluents as solvents and non-solvent for asphaltenes. In analysis of phase separation mechanisms of asphaltenes from hydrocarbon media, the assumption of solubility in the original medium is a key one. From a solution thermodynamics standpoint, dissolution of a solid into a liquid includes the enthalpy of transition to liquid for the solute plus the enthalpy associated with homogeneous mixing at a molecular level [2]. The present work challenges the application of the concept of solubility to mixtures of asphaltenes with organic diluents and attempts to explain the behavior of asphaltenes in organic liquids based on thermodynamic measurements.

Solution calorimetry, differential scanning calorimetry (DSC), and density measurements were performed on mixture of asphaltenes with a variety of diluents ranging from n-alkanes to bicyclic aromatics. Athabasca n-pentane asphaltenes were mixed with diluents at compositions ranging from 0.1 to 10 wt%.  In each case 25 mg of asphaltenes were added to 25 ml of a mixture with an underlying composition. Solution calorimetry measurements performed using a TA Instruments TAM Precision solution calorimeter, in the range 15 to 80 °C provided information on phase change with composition at fixed temperature. Differential scanning calorimetry measurements with a TG-DSC 111 (Setaram, France) calorimeter, provided information on phase change with temperature at fixed composition and on the reversibility of such phase behavior. Asphaltene specific partial volume data and mixture density data obtained using an Anton Paar DMA5000 densimeter, complemented the calorimetric measurements.

Solution calorimetry measurements reflect the net enthalpy change resulting from addition of asphaltenes to diluents. The value of enthalpy of solution is a weak function of temperature and composition if the dissolution process is complete, i.e. the solute undergoes a solid-liquid phase change. This behavior was demonstrated for the benchmark case pyrene (solute) + toluene (diluent). The enthalpies of dispersion of asphaltenes in all diluents used in this study, however, change significantly with changes in temperature and composition. The enthalpies trend to higher values with increasing temperature and to lower values at increasing asphaltene concentration at fixed temperature. This is 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. Differential scanning calorimetry and density measurements for asphaltenes + toluene and 1-methylnaphthalene, show a hysteresis in the temperature interval 120 to 200 °C, indicating that the impact of mixing on phase behavior is irreversible.

These results are inconsistent with the application of dissolution as classically defined to asphaltene + diluent mixtures. The ranges of values, their trends with temperature and composition are consistent with overlapping exothermic and endothermic processes occurring simultaneously.

References

1.         ASTM-D6560, Standard Test Method for Determination of Asphaltenes (Heptane Insolubles) in Crude Petroleum and Petroleum Products. 2000, ASTM International: West Conshohocken, PA.

2.         Zhang, Y., et al., Estimating the Interaction Energy of Asphaltene Aggregates with Aromatic Solvents. Energy Fuels, 2005. 19(3): p. 1023-1028.