ASPHALTENE PRECIPITATION AND LIQUID-LIQUID PHASE SEPARATION: A STATISTICAL-THERMODYNAMIC MODEL AND EXPERIMENT

P. Peczak, E. B. Sirota, and M. Sowlay

Corporate Strategic Research, ExxonMobil Research and Engineering Company,

Route 22 East, Annandale, New Jersey 08801, USA

We present a novel model of asphaltene precipitation based on a fundamental, physical approach. Our goal is to develop a predictive model for the solid-liquid and liquid-liquid phase behavior of asphaltene-containing streams based on the statistical picture of the interactions in mixtures of complex molecules.   The enthalpic interaction between molecules is calculated using the solubility theory. We have developed a novel approach to the entropy of mixing.  The petroleum molecules are neither rigid objects where a simple "ideal" form for the entropy of mixing is appropriate, nor are the asphaltenes long flexible polymers for which the Flory-Huggins form is appropriate.  We have therefore proposed a novel model where the molecules are represented by a system of rigid, super-structural units.  In addition, we recognize, based on scattering data, that the molecules are not randomly distributed, but tend to cluster ephemerally.  We therefore have modified the entropic and enthalpic terms of the free energy of mixing by using the effective local composition in the vicinity of a given species.

The novel thermodynamic model yielded first successes by being able to calculate and explain the crossover experimentally observed as the composition of the precipitate is varied, from phase separation where the minority phase is composed of solvent-rich light drops to one with asphaltene-rich dark aggregates.