Precipitation onset and physical models of asphaltene solution behavior

The onset of asphaltene precipitation provides important information about the thermodynamic state of the asphaltene solute in a given hydrocarbon system (a stock tank oil, a residue, or an asphaltenic solution). Such measurements are carried out by titrating the system with an asphaltene anti-solvent (an n-paraffin) while measuring some physical property (e.g., light absorbance, electrical conductivity, viscosity) that displays a distinctive discontinuity at the onset of phase separation. The quantity of antisolvent required to attain this condition is expressed in most cases as mass (or volume) of anti-solvent per unit mass of oil. In the present work, the empirical models that have been proposed to account for asphaltene behavior in the titration experiments are described and compared critically. It is shown that they can all be reduced to a single general form that identifies a peculiar behavior of these systems: the onset conditions for precipitation are independent of the asphaltene concentration. This behavior cannot represented by most of the theoretical models proposed in literature. It is argued that the discordance between the observed behavior and the attempted mathematical descriptions derives, in large part, from an uncritical use of physical models of the asphaltenes. A tentative, new physical interpretation of asphaltene phase behavior is offered together with a simple mathematical model for predicting the onset conditions based on the results of titration experiments.