MOLECULAR THEORY OF PHASE-EQUILIBRIA IN MODEL AND REAL ASSOCIATED MIXTURES .3. BINARY-SOLUTIONS OF INERT-GASES AND N-ALKANES IN AMMONIA ANDMETHANOL

Using new molecular models of ammonia and methanol and thermodynamic p erturbation theory, the global phase diagrams of model mixtures of the se compounds with a van der Waals fluid, representing a simple nonpola r fluid, have been calculated, The global phase diagram of these mixtu res is much richer than that of corresponding aqueous mixtures. More t ypes of critical line behavior are found, including the presence of va n Laar points and a small region when the mixtures exhibit a closed li quid-liquid immiscibility loop (Type VI phase behavior). The individua l mixture components are characterized by two molecular parameters, wh ich can be adjusted to their critical temperature and critical volume; the mixture model itself contains no adjustable parameters. It is sho wn that the theory gives qualitatively correct predictions of mixtures with n-alkanes. This includes the prediction of Type III critical lin e behavior for small and large values of the ratio of the critical tem peratures of the components, and Type II over a large range of conditi ons, including the presence or absence of absolute or limited azeotrop y, and temperature and pressure extrema of critical lines and their de pendence on the number of carbon atoms. (C) 1997 Elsevier Science B.V.