EQUAL-AREA RULE AND ALGORITHM FOR DETERMINING PHASE COMPOSITIONS

The Maxwell equal area rule is a basic construct for determining satur ated phase volumes from equations of state. The fundamental idea is th at for a pure substance a horizontal line drawn through the trace of a n isotherm, which equalizes the van deu Waals loops area above and bel ow the line, intersects the isotherm at the saturated volumes. Another basic construct exists for determining the phase compositions in a bi nary mixture by establishing the tangent line to the total Gibbs energ y curve plotted against composition. We demonstrate that finding the t angent also reduces to an equal area construction for the derivative o f the total Gibbs energy plotted against composition. An efficient alg orithm suggested effects such equal area determinations. The trial-and -error procedure normally converges in one to three steps to the con e ct solution given any reasonable starting guess. Implementation. using modem calculators with graphics capabilities is simple and straight f orward.