High-temperature and high-pressure density and liquid–vapor phase transition properties of methyl octanoate as main biofuel component

Abstract

Thermodynamic properties of compressed fuels as density at liquids phase (PρT data) and two liquids-gas phases as saturated pressure, density, and temperature (PS,ρS,TS) are extremely important for engine performance as well as the transportation and storage of fuels. In this work single – and two–phase PρT data and liquid + gas phase equilibrium properties (PS,TS,ρS) of methyl octanoate (caprylate) as one of the key components of biofuels have been studied. The measurements were performed along 15 liquid and vapor isochores between (44.7 and 832.0) kg·m−3 over a temperature range from (298 to 438) K at pressures up to 17.33 MPa using a constant-volume piezometer technique. The study was concentrated in the two-phase region to accurately determine vapor-pressures and in the immediate vicinity of the phase-transition temperatures to precisely determine the phase boundary properties (PS,ρS,TS) on the liquid + gas equilibrium curve. In the low temperature range (far from the critical point) where the values of vapor-pressure are negligible small (<1 kPa, approximately at temperatures below 350 K), the saturated vapor phase of methyl octanoate was considered as ideal gas with volume of VG=RT/PS and ΔVLG≈VG=RT/PS, where the saturated liquid volume can be neglected, VG>>VL. This approach was used to simplify the calculation of the thermodynamic properties of methyl octanoate at saturation.