IMPROVED PHASE-BOUNDARY FOR ONE-COMPONENT VAPOR-LIQUID-EQUILIBRIUM - INCORPORATING CRITICAL-BEHAVIOR AND CUBIC EQUATIONS OF STATE

Many current uses of fluid phase equilibria would benefit from new equ ations of state that are more accurate in fitting experimental data an d more certain of their predictions for unmeasured phase equilibria. W e present a first step towards a fitting procedure which incorporates the advantages of critical scaling theory in the vicinity of the criti cal point with the advantages of 'classical' cubic equations of state far from the critical point. For VLE, we propose a non-linear order pa rameter (density variable) that captures the familiar fluid asymmetry in the far from critical vapor and liquid regimes without affecting th e linear density-dependence in the near critical regime; this compleme nts earlier work for LLE. Use of this order parameter in describing th e liquid-vapor phase boundary effects a 'heuristic' crossover from acc urate critical point behavior to accurate far-from-critical behavior. The accuracy of this procedure has been verified by using it to fit da ta from the open literature for the vapor-liquid phase boundary for a variety of one component systems: with polar molecules (chlorotrifluor omethane), with small non-polar molecules (carbon dioxide, ethane), an d with the heavier normal alkanes (pentane, hexane, heptane, octane, a nd nonane). Within 100 k of the critical point, this method is substan tially more accurate than its competitors. At worst, in the far-from-c ritical (e.g. 200-300 K below the critical point) regime, this procedu re seems to consistently overestimate the vapor densities by a small a mount (less than 3%).