DSC enthalpy of vaporization measurements of high temperature two-phase working fluids

The enthalpies of vaporization, saturated pressure-temperature behavior, and melting points for high-temperature (300-400 degrees C) two-phase working fluids were measured using high-pressure DSC methodologies similar to those currently under development by ASTM E37. The materials, biphenyl, decafluorobiphenyl, o-terphenyl, naphthalene, quinoline, and perfluoro-1,3,5-triphenyl-benzene, were studied at seven pressures between 50 kPa and 1.4 MPa. The experimental measurements employed the use of a Mettler Pressure DSC and sealed aluminum sample/reference pans having a laser-drilled (0.10-0.13 mm orifice) lid. The vaporization pressure-temperature data were curve fitted to the Antoine equation using Marquardt-Levenberg non-linear regression, and the enthalpy of vaporization was then computed from the Clapeyron equation. The results were compared to literature values, where available, and water was also included in the study at each DSC-scan pressure to evaluate the accuracy of the method. Overall, the technique produced saturated pressures which were within 6% (2 standard deviations, 2 sigma) of literature values, and the standard deviation (sigma) associated with the Antoine curve fits was less than 3%. Enthalpy of vaporization data computed from the Clapeyron equation were within 4% of literature values for biphenyl, naphthalene, quinoline, and water, and within 12% for o-terphenyl.