(87i) Crossover Hr-Saft Equation of State for Fluid Mixtures: Application to Binary Mixtures of Carbon Dioxide, Water, and Methanol

In this work, we extend the pure crossover statistical associating fluid theory (HRX-SAFT) equation of state (EOS) fluids [1], to fluid mixtures of polar and associating components. HRX-SAFT incorporates non-analytic scaling laws in the critical region and is transformed into the analytical, classical HR-SAFT EOS [2] far away from the critical point. Here, similar to our previous work for n-alkanols [1], CO₂, H₂O, and CH₃OH are modeled as associating chain molecules with two association sites, i.e. model 2B [2]. For all three substances, the HRX-SAFT EOS reproduces the vapor pressure data from the triple point to the critical temperature with an average absolute deviation (AAD) of about 1%, the saturated liquid and vapor densities with an AAD of about 1-3%, and the single phase pressures in the one-phase region with an AAD of about 2-3%. Using the classical mixing rules in terms of composition, we have also applied the HRX-SAFT EOS to binary mixtures. For the crossover function in binary mixtures, we used the simplified representation as employed earlier by Kiselev and co-workers in the crossover cubic EOS [3,4]. For the non-association terms in the classical HR-SAFT, we used the vdW1 mixing rules, as described in [5], with one constant binary interaction parameter (k_ij). For the association term in the classical HR-SAFT, we assumed that in mixtures there is cross association between the oxygen in carbon dioxide molecule and the hydrogen in the methanol and water molecules, and between the carbon in carbon dioxide molecule and the oxygen in the methanol and water molecules as well. We used a generalized procedure developed recently by Tan et al. [6] to estimate the fractions of non-bonded molecules in the association term. The HRX-SAFT mixture model was tested against extensive experimental data for VLE, PVTx, and excess properties in carbon dioxide + water, carbon dioxide + methanol, and water + methanol mixtures. We show that the HRX-SAFT EOS not only yields a better description of the PVTx and VLE properties of binary mixtures in the critical region, but also improves the representation of the entire thermodynamic surface. Unlike the original HR-SAFT EOS for binary mixtures [6], the HRX-SAFT EOS is capable of simultaneous representation of the pressure-composition, pressure-density, and density-composition isotherms in associating binary mixtures with high accuracy. Shortcomings of the current HRX-SAFT EOS formulation in the representation of the asymptotic critical region of binary mixtures are also discussed.

[1] S.B. Kiselev, J.F. Ely, H. Adidharma, M. Radosz, Fluid Phase Equilib., 183-184, 53 (1998). [2] S.H. Huang and M. Radosz, Ind. Eng. Chem. Res. 29, 2284 (1990). [3] S.B. Kiselev and D.G. Friend, Fluid Phase Equilib., 155, 33 (1999). [4] S.B. Kiselev and J.F. Ely, J. Chem. Phys., 119, 8645 (2003). [5] S.H. Huang and M. Radosz, Ind. Eng. Chem. Res. 30, 1994 (1991). [6] S.P. Tan, H. Adidharma, M. Radosz, Ind. Chem. Eng. Res. 43, 203 (2004).