Equal area rule and algorithm for determining phase compositions

Abstract

AbstractThe Maxwell equal area rule is a basic construct for determining saturated phase volumes from equations of state. The fundamental idea is that for a pure substance a horizontal line drawn through the trace of an isotherm, which equalizes the van der Waals loops area above and below the line, intersects the isotherm at the saturated volumes.Another basic construct exists for determining the phase compositions in a binary mixture by establishing the tangent line to the total Gibbs energy curve plotted against composition. We demonstrate that finding the tangent also reduces to an equal area construction for the derivative of the total Gibbs energy plotted against composition. An efficient algorithm suggested effects such equal area determinations. The trial‐and‐error procedure normally converges in one to three steps to the correct solution given any resonable starting guess. Implementation using modern calculators with graphics capabilities is simple and straightforward.