Study of Asphaltenes Aggregation in Toluene/n-Heptane/CO2 Mixtures under High-Pressure Conditions

In this work, we present a novel experimental method to obtain an asphaltenes precipitation envelope (APE) of complex systems composed of a mixture of toluene and n-heptane in the presence of CO2 at high pressure and different temperatures. The method has the feasibility to detect phase changes of asphaltenes in a pressure range from 1 MPa to 100 MPa and temperatures from 283 K to 473 K. For the first time, a complete APE using experimental data was obtained. The solubility behavior of the asphaltenes with the pressure of the system is in agreement with previous studies. However, the temperature aggregation behavior at high pressure showed a minimum near 353 K that contrasts with reported data. This complex tendency can be explained by changes in viscosity, density, and aggregate composition with temperature. Also, the coexistence of two subfractions (denoted A1 and A2) in the asphaltenes and their differences in solubility modifies the solubility, and the entropy variation of the system can help to understand the asphaltene aggregation behavior at higher temperatures.