Modeling phase equilibria for acid gas mixtures using the CPA equation of state. Part IV. Applications to mixtures of CO₂ with alkanes

The thermodynamic properties of pure gaseous, liquid or supercritical CO₂ and CO₂ mixtures with hydrocarbons and other compounds such as water, alcohols, and glycols are very important in many processes in the oil and gas industry. Design of such processes requires use of accurate thermodynamic models, capable of predicting the complex phase behavior of multicomponent mixtures as well as their volumetric properties. In this direction, over the last several years, the cubic-plus-association (CPA) thermodynamic model has been successfully used for describing volumetric properties and phase behavior of several mixtures of interest in oil and gas industry.The purpose of this work is to evaluate the performance of CPA for CO₂-alkane mixtures. CPA calculations were performed using different association schemes for CO₂. Firstly, CO₂ was treated as non-associating fluid and, at a second level, CO₂ was treated as self associating fluid, using the 2B, 3B and 4C association schemes. A systematic investigation of the CPA performance in correlating the phase behavior of CO₂-alkane mixtures has been performed. Mixtures with alkanes up to n-hexatriacontane (n-C₃₆) were investigated and the corresponding binary interaction parameters were estimated with all modeling approaches for CO₂. CPA parameters were previously available for alkanes up to n-eicosane (n-C₂₀) and new parameters have been estimated for heavier alkanes based on DIPPR correlations. CPA correlations were compared with literature experimental data for three heavy alkanes and a satisfactory agreement was observed. The CPA performance is discussed including trends of the interaction parameters with chain length and comparison to literature studies.