(83e) Importance of Solubility Parameter Model for Prediction of Asphaltene Precipitation with Regular Solution Theory

Asphaltenes are the heaviest fraction of crude oil and when precipitated produce problems for oil refining. To remove these complex asphaltenes structures, the equilibria of current crude oils and the mechanism of asphaltene precipitation should be evaluated. The regular solution theory has been successfully implemented to predict asphaltene precipitation both for diluted heavy oils and depressurized conventional oils in the presence of a paraffinic solvent [1-3]. The regular solution theory is a thermodynamic approach to solving real solutions that requires molar volume, mole fraction and solubility parameter as input. In this study, we evaluate the effects of input parameters for asphaltene precipitation in n-alkane solvents. Of those, significant effects on the modelâ??s accuracy are only observed when varying the solvent solubility parameter and the beta distribution shape factor, while the effect of the number of pseudo-components is negligible beyond a minimum stability value.

Here, we discuss the effect of solubility parameter calculations on the regular solution model. We perform two different models to calculate the solubility parameter of a solvent for the regular solution model of asphaltene precipitation from bitumen mixed with n-alkane solvents. The two methods in question are the thermodynamic (Hildebrand) equation [4] and a linear model [2]. The Hildebrand equation for the solubility parameter produces results that disagree with the correlation between temperature and precipitation yield shown in experimental data. The latter method proposes a linear model for the solubility parameter, however, which follows the same temperature trend as the experimental data. The model is linear with respect to temperature and contains the solubility parameter for the solvent at 25 °C (δ₂₅) as a parameter utilized for the final fitting of the model to the experimental data. The tested solvents for the model are n-heptane and n-pentane, for which the determined δ₂₅ values are 7.33 (cal/cm³)^0.5 and 6.82 (cal/cm³)^0.5, respectively. These solubility parameters maintain validity for various oil samples and temperatures. The drastic effect of the solubility parameter model and its model parameters illustrate the complexity caused by the increased presence of asphaltenes in heavy oil and bitumen samples.

References

[1] H.W. Yarranton, and J.H. Masliyah, â??Molar mass distribution and solubility modeling of asphaltenesâ?, AIChE Journal 42 (1996) 3533-3543.

[2] H. Alboudwarej et al., â??Regular Solution model for asphaltene precipitation from bitumens and solvents,â? AIChE Journal 49 (2003) 2948-2956.

[3] K. Akbarzadeh et al., â??A generalized regular solution model for asphaltene precipitation from n-alkane diluted heavy oils and bitumens,â? Fluid Phase Equilibria, 232 (2005) 159-170.

 [4] A.F.M. Barton, CRC Handbook of Solubility Parameters and Other Cohesion Parameters, CRC Press, (1991) 24-34.

Acknowledgement: We gratefully appreciate the Schneider Electric internship program.