Extending the Modified Regular Solution Model To Predict Component Partitioning to the Asphaltene-Rich Phase

The modified regular solution (MRS) model is an activity coefficient approach previously developed to predict the onset and amount of asphaltene precipitation from n-paraffin-diluted crude oils. The activity coefficients in this model are determined from an enthalpic contribution based on the regular solution theory and a Flory-Huggins (configurational) entropic contribution. The inputs are the molar volume and solubility parameter of the crude oil pseudo-components, which are defined on the basis of the saturate, aromatic, resin, and asphaltene (SARA) fractions; however, only asphaltenes and resins are free to partition between the two phases. Hence, the model cannot predict the amount and composition of the heavy asphaltene-rich phase. In this study, the MRS model was updated to account for the partitioning of all components based on phase equilibrium data from a Western Canada bitumen mixed with n-pentane and n-heptane at ambient conditions. To match the measured phase compositions, the entropic contribution in the solvent-rich phase was set to the Flory-Huggins term and the contribution from the asphaltene-rich phase was set to zero. An approach to predict the solubility parameters and density of the pseudo-components at other temperatures and pressures was also proposed. The model was tested on a data set containing phase mass and phase compositions of a Western Canada bitumen mixed with n-heptane, n-pentane, n-butane, and propane at temperatures from 20 to 250 degrees C and pressures up to 13 MPa. The average absolute deviations in the phase mass and compositions were 7 and 12 wt %, respectively. The maximum deviations were found for the asphaltene-rich phase compositions. The model was also tested on a data set of asphaltene yields from oils from different disparate geographical locations at temperatures from 0 to 50 degrees C at 0.1 MPa. The average absolute deviation was 1 wt %.