A study of new mixture combining rules for prediction of vapor-liquid equilibria

Abstract

Wong and Sandler published two important articles in 1992 that established new mixture combining rules (MCR) for use with cubic equations of state (EOS) in the prediction and correlation of vapor-liquid equilibrium (VLE) over wide ranges of pressure and temperature for binary and ternary systems including those containing polar components. These new MCR appeared to contain more basic physics than that did the familiar van der Waals (vdW), one fluid model rules that generally could not predict VLE without at least one tuning factor kl2, the binary interaction constant. The MCR proposed by Wong and Sandler consist of a gas-side equation and a liquid-side equation. The former is based upon the familiar quadratic MCR of the second virial coefficient, Bm(yi,T). They re-introduced kl2 to get the cross second virial coefficient B 12. The first major contribution of this thesis is to use experimental values of B12(l) and thus eliminate kl2. Thus we predict VLE results a priori rather than correlate kl2 to match those results. When experimental values of B12 are not available, they can be estimated independently by the well-known method of Tsonopoulos. We have also shown that the WS/MCR can be better expressed in terms of 312 = 2 B 12 -B 11 -B22. The second major contribution of this thesis concerns the influence of temperature upon A- = G- in the liquid-side MCR. We have shown that for cubic EOS with (1) vdW repulsion and (2) the attraction constant independent of temperature, S@- is equal to zero. Thus, A! is independent of temperature, a regular solution assumption. However, Wong and Sandler chose the atherrnal solution assumption of H'=O or (A!/RT) is independent of temperature. Thus, calculations by Wong and Sandler over wide ranges of temperature are inconsistent with their liquid-side MCR. We have corrected this problem in this thesis to improve somewhat the agreement with VLE data. In summary, this thesis has made important changes in how the new W-S/A4CR are used but has not changed the MCR themselves.