Molecular Dynamics Simulations of Asphaltene Aggregation in Supercritical Carbon Dioxide with and without Limonene

Abstract

We study the aggregation of asphaltene in supercritical carbon dioxide (sc-CO2) using molecular dynamics (MD) computer simulations. We use a well-defined asphaltene molecular structure (asphaltene “C” from Headen, T. F.; Boek, E. S.; Skipper, N. T. Energy Fuels 2009, 23, 1220−1229) obtained using a preliminary version of the quantitative molecular representation algorithm (Boek, E. S.; Yakovlev, D. S.; Headen, T. F. Energy Fuels 2009, 23, 1209−1219). We compare the aggregation of asphaltene in sc-CO2 with our previous MD aggregation studies in toluene and heptane. Also, we study asphaltene aggregation in the presence of limonene, a well-known aggregation inhibitor. First, we present simulations of sc-CO2 at a range of temperatures and pressures to test the ability of the force field used to correctly predict the system density. Simulations of asphaltenes were conducted in a similar fashion to those presented by Headen, T. F.; Boek, E. S.; Skipper, N. T. Energy Fuels 2009, 23, 1220−1229 for simulations in toluene and heptane, six asphaltene molecules in a bath of solvent at 7 wt %. Simulations were conducted at a range of different temperatures and pressures in both sc-CO2 and a 50 wt % mixture of sc-CO2 with limonene. Asphaltenes in sc-CO2 showed a strong propensity for aggregation, with all six molecules forming a strongly bound aggregate at all temperatures and pressures tested. Simulations in 50 wt % limonene/sc-CO2 show a considerable decrease in the aggregation compared to pure CO2. The temperature and pressure dependence of asphaltene aggregation in the mixture was complex, showing minimum aggregation (for the limited range of conditions studied) at 150 bar and 350 K.