AREAS OF MAXWELL,J.C. FOR PURE-COMPONENT FLUID-PHASE EQUILIBRIA FROM EQUATIONS OF STATE

Cubic equations of state (EOS) cannot provide accurate multithermophys ical properties over wide ranges of temperature and pressure for pure components in the fluid state. More complex, noncubic EOS are required , but only recently has their use been made feasible by advances in co mputing. These equations often produce isotherms on a pressure-specifi c volume (P-V) plot with more than one van der Waals loop. This produc es multiple solutions to the Maxwell Equal Area Rule (MEAR) criterion for determining the saturated vapor pressure, P-sigma, at a fixed temp erature. A mechanical stability test is used to determine which one of these solutions gives stable vapor and liquid phases at equilibrium. We have developed a new corollary of MEAR, called the Maximum Positive Area (MPA) criterion, for determining the correct P-sigma of pure com ponents regardless of the complexity of the EOS used. In the MPA crite rion, the correct P-sigma occurs when the positive area, determined in the MEAR criterion, is maximized.