Investigation of the Precipitation Behavior of Asphaltenes in the Presence of Naphthenic Acids Using Light Scattering and Molecular Modeling Techniques
A delay in the onset of flocculation is observed for asphaltenes in the presence of several naphthenic acids: methyl abietate, hydrogenated methyl abietate, 5 beta-cholanic acid, and 5 beta-cholanic acid-3-one. This flocculation behavior is monitored as a function of the added precipitant (n-heptane) to solutions of suspended asphaltenes and naphthenic acids in model solutions of toluene/n-heptane, using a combination of dynamic light scattering (DLS) and near-infrared (NIR) spectroscopic techniques. DLS and NIR show very good correlation in indentifying the onsets of flocculation, which varied among the series of naphthenic acids. Specific interaction energies and equilibrium intermolecular distances of asphaltenes and naphthenic acids are calculated using molecular mechanics. The results from molecular mechanics calculations support the experimental results of the titrations, and structure property relationships are defined. Structure property relationships are established for naphthenic acids, defining the relative contributions and importance of various functional groups: C=C, C=O, COOR, and COOH. The additive effects of naphthenic acids, defined by an increase in the precipitation onset, increase in the order of 5 beta-cholanic acid-3-one < hydrogenated methyl abietate < methyl abietate < 5 beta-cholanic acid, with experiments containing 5 beta-cholanic acid-3-one containing unexpected and interesting results.