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 sys tem (a stock tank oil, a residue, or an asphaltenic solution). Such measure ments are carried out by titrating the system with an asphaltene anti-solve nt (an n-paraffin) while measuring some physical property (e.g., light abso rbance, electrical conductivity, viscosity) that displays a distinctive dis continuity at the onset of phase separation. The quantity of antisolvent re quired to attain this condition is expressed in most cases as mass (or volu me) of anti-solvent per unit mass of oil. In the present work, the empirica l models that have been proposed to account for asphaltene behavior in the titration experiments are described and compared critically. It is shown th at they can all be reduced to a single general form that identifies a pecul iar behavior of these systems: the onset conditions for precipitation are i ndependent of the asphaltene concentration. This behavior cannot represente d by most of the theoretical models proposed in literature. It is argued th at the discordance between the observed behavior and the attempted mathemat ical descriptions derives, in large part, from an uncritical use of physica l models of the asphaltenes. A tentative, new physical interpretation of as phaltene phase behavior is offered together with a simple mathematical mode l for predicting the onset conditions based on the results of titration exp eriments.