By using a spline function for curved surface used in simulating the bending of a plate to describe the ternary excess free enthalpy function Q

a new indirect method for calculating ternary vapor composition y from T

p and liquid composition x was established. Broyden-Nowton-Raphson iteration procedure was used for solving the nonlinear equations. The calculation of 30 ternary systems from nearly ideal to highly nonideal phase behavior showed that this new method was very stable and reliable. The calculated results were in good agreement with the experimental data. More-over

this method had a variety of distinguishing features such as good convergence

no special requirement for initial values and little influence caused by neglecting the excess properties during calculation.