Application of the Wohl equation to ternary liquid-vapor equilibria



The use of the Wohl equation for activity coefficients is tested successfully for the most nonideal ternary systems for which experimental data could be found. The three-suffix form possessing seven constants is found to be successful in many cases. Owing to the high degree of nonideality resulting from one or more components being polar, however, the four-suffix form employing up to ten constants was required for ten of the twenty-five ternaries studied. Regardless of the form chosen, all the constants but one, C*, are derived from phase equilibrium data on the constituent binaries.

The ternary constant C* was found to be approximately zero. In only two instances did its value not fall in the region of −1.0 to +1.0. With the use of the binary constants and C*, predictions of vapor compositions were made for each experimental ternary liquid composition, temperature, and pressure. The results so obtained yielded average absolute errors in predicted vapor mole fractions, without respect to sign, as follows: for ten systems, less than 0.01; for seventeen systems, less than 0.02; and for twenty-four systems, less than 0.03. For an additional three sets the error was still less than 0.05. Results were analyzed statistically for twenty-two of the systems determining mean values of the vapor mole fraction deviations and the confidence limits of the means.

The mass testing of the Wohl equation reported here far exceeds the sum of all work previously published in the literature. It appears that this equation provides a very good way of expressing liquid nonideality for a wide range of components.