LIQUID-LIQUID VAPOR-PHASE EQUILIBRIA IN THE SYSTEM ETHANE+PROPANE+EICOSANE - RETROGRADE BEHAVIOR OF THE HEAVY LIQUID-PHASE

The fluid phase behaviour of the ternary system ethane + propane + eicosane was studied experimentally with special emphasis on liquid-liquid-vapour equilibria and retrograde condensation of the second, heavier liquid in this region. P,T-sections for mixtures of constant composition were determined according to the synthetic method for six constant molar ratios of ethane and eicosane and different mole fractions of propane. The temperature range investigated was 309-322 K and the pressure range 5.0-5.6 MPa. The liquid-liquid-vapour regions of the ternary mixtures are found at higher temperatures and pressures than the three-phase curve of the binary system. Addition of propane to a binary ethane + eicosane mixture of constant composition extends the three-phase line to a region in accordance with the phase rule. This effect also gives rise to the phenomenon of retrograde condensation of the heavier liquid phase. A description of the behaviour of ternary mixtures and a comprehensive explanation of the retrograde phenomena are given. The experimental results also allowed estimation of the temperature and pressure of the tricritical point.