Double azeotropy in binary mixtures of NH₃ and CHF₂CF₃

Vapor-liquid equilibrium pressures have been measured as a function of overall composition for binary mixtures of NH₃ and CHF₂CF₃ at constant temperatures of - 19.00, 3.23, 35.00, and 49.90°C. The coexistence curves for vapor-liquid equilibrium at each temperature were regressed from these measurements using a modified form of the Peng-Robinson equation of state to calculate molar volumes and the fugacities of each component in the two phases. The coexistence curves at 35.00 and 49.90°C exhibit both a maximum and a minimum in pressure, establishing the existence of double azeotropes at these two temperatures. The double azeotropes are unusual in that they exist at temperatures for which the vapor pressures of the two components are notably different. Double azeotropy is attributed to the weak association of NH₃ in the liquid phase, leading to the maximum-pressure azeotrope at high NH₃ concentrations, and to similar vapor pressures for unassociated NH₃ and CHF₂CF₃ in nearly ideal (Raoult's law) dilute NH₃ solutions, which leads to the minimum-pressure azeotrope.