Application of the modified molecular interaction volume model (M-MIVM) to vapor-liquid phase equilibrium of binary alloys in vacuum distillation

Thermodynamic models such as the Wilson model and molecular interaction volume model (MIVM) are known for their use in the calculation of activity coefficients of symmetric systems like Pb-Sn and Pb-Sb, but are deficient in handling asymmetric systems like Ag-Sn and Cu-Sn. Unfortunately, in metallurgical field, including vacuum metallurgy, the symmetric systems only account for a small part of liquid alloys, while the asymmetric systems take up a majority proportion. To overcome this application limitation of the Wilson model and MIVM in vacuum metallurgy, the modified molecular interaction volume model (M-MIVM) was applied. In contrast with Wilson model and MIVM, the M-MIVM exhibited remarkable superiority in data fitting of activity coefficient of binary asymmetric systems, which effectively increased the accuracy of predicting the activities of the multi-component systems containing corresponding binary asymmetric systems. Therefore, M-MIVM made possible a more accurate vapor-liquid phase equilibrium diagram of asymmetric systems. In addition, since M-MIVM is a modification of the MIVM, it retains MIVM's capabilities to predict separation coefficient and calculate vapor-liquid equilibrium (VLE) of symmetric systems under quasi-vacuum condition. These identified advantages made M-MIVM, when compared with existing models, to have better accuracy and wider scope of application in vacuum metallurgy.