Molecular-thermodynamic framework for asphaltene-oil equilibria

Asphaltene precipitation is a perennial problem in producing and refining crude oils. To avoid precipitation, it is useful to know the solubility of asphaltenes in petroleum liquids as a function of temperature, pressure and liquid-phase composition. In the novel molecular-thermodynamic framework presented here, both asphaltenes and resins are represented by pseudo-pure components while all other components in the solution are represented by a continuous medium that affects interactions among asphaltene and resin particles. The effect of the medium on asphaltene-asphaltene, resin-asphaltene, resin-resin pair interactions is taken into account through its density and dispersion-force properties. To obtain expressions for the chemical potential of asphaltene and for the osmotic pressure of an asphaltene-containing solution, the SAFT model is used in the framework of McMillan-Mayer theory, which considers hard-sphere repulsive, association and dispersion-force interactions. By assuming that asphaltene precipitation is a liquid-liquid equilibrium process, a variety of experimental observations can be explained, including effects of temperature, pressure, and composition on the phase behavior of asphaltene-containing fluids. For practical quantitative applications, the model outlined here requires molecular parameters that must be estimated from a few experimental data.

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