APPLICATIONS OF STATISTICAL THERMODYNAMICS TO THE CALCULATION OF PHYSICAL PROPERTIES AND FLUID-PHASE EQUILIBRIA (HARD-SPHERE, METAL, LATTICE)

Three related topics in applied statistical thermodynamics are addressed with the intent of developing rigorous models for correlating and predicting phase-equilibria in engineering practice.^ In the first part, the simultaneous correlation of activity coefficients and excess energies for liquid metal mixtures are carried out by the modified UNIQUAC model. Here, the modifications are achieved by making the structural parameters and coordination number in the UNIQUAC model as functions of temperature by introducing the concept of shrinking, hard-sphere. The correlated results by this modifications are remarkably successful. For both positive and for negative deviations from ideal behaivor, the proposed equation gives a good representation for binary alloy systems even at high temperatures. The approach is also applicable to multicomponent, multiphase metal alloys and allow reasonable extrapolation of limited data to elevated temperatures.^ In the second part, a new engineering-oriented equation of state is proposed; this equation is on the basis of virial treatment to the reference force and on the thermodynamic perturbation theory to the attractive forces of the separable hard-core pair potential function. For efficient parameter estimation, a constrained nonlinear programming technique is applied to the equation of state. With this technique, an optimum set of paraemeters is obtained. At the same time the parameters fit exactly the constraints imposed by the gas-liquid critical points. The proposed equation shows a wider range of applicability than existing equations of state for a molecularly simple pure system.^ In the final part, the concepts of the decorated lattice theory are extended to represent non-classical critical points as well as gas-liquid coexistence curves of real systems. Also presented is efficient programming technique for successive numerical fitting of experimental data using this theory. The predicted results consistently show improvement in equilibrium density calculations. These results overcome the deficiencies found in the classical equations of state.