Modeling of vapor-liquid-liquid equilibria in binary mixtures

Vapor compression and Joule-Thomson (JT) cycles provide cooling power at the boiling temperatures of the refrigerants. Maintaining a fixed pressure in the evaporator allows for a stable cooling temperature at the boiling point of a pure refrigerant. In these coolers enhanced cooling power can be achieved by using mixed refrigerants. However, gas mixtures usually do not change their phase at a constant temperature, therefore, the cooling temperature has to be actively controlled. An exception to this rule holds for binary mixtures that can form a vapor-liquid-liquid equilibrium (VLLE). Phase equilibria in binary mixtures are usually modeled based on experimental results only. In the present study only the vapor pressures of the pure mixture components are required. The calculated results of nitrogen-ethane, nitrogen-ethylene, and nitrogen-propane mixtures are compared with experimental data presented in literature showing deviations of less than 1%.