Molecular Signatures of Asphaltene Precipitation in a Depressurization Process

Abstract

The effect of pressure on asphaltene aggregation is studied in a mixture of toluene–methane (90:10 wt/wt %, 61:39 mol/mol %) using coarse-grained (CG) molecular simulations to mimic a depressurization (flash liberation) process. The created CG model allows us to sample systems containing over 10000 molecules for 250 ns. At each of the pressures considered, we quantify asphaltene aggregation by computing the mass-averaged aggregation number using three metrics that differentiate among the aggregation stages proposed in the Yen–Mullins model, as well as the radius of gyration of the aggregates. The total potential energy of asphaltenes allows us to identify the pressure at which aggregation begins. The potential energy curve exhibits local maxima at high pressures and a global maximum near the bubble point pressure. A breakdown of the total potential energy of asphaltenes shows that asphaltene–solvent interactions dominate asphaltene behavior. Our identification of the asphaltene onset pressure provides molecular-level interpretation of experimental asphaltene aggregation results.