Experimental study of PVT and phase-transition properties of binary water+n-hexane mixture near the upper critical endpoint

Abstract

The PρT and phase transition (LLV and LV) properties (PS,TS,ρS) of binary water+n-hexane mixture with fixed concentration of 0.5 mol fraction of n-hexane (equimolar mixture) were measured. Measurements were focused in the immediate vicinity of the upper critical endpoint (UCEP) and supercritical regions in order to closely study the features of the mixture critical behavior near the UCEP. Measurements were made along 15 liquid and vapor isochores between (63.16 and 554.41) kg·m−3 and over a temperature range from (373.15 to 673.15) K at pressures up to 53.3 MPa. Phase transition phenomena on isochoric heating of the binary mixture was detailed study. For each isochore most measurements were made in the immediate vicinity of the LLV and LV phase transition temperatures (in the single-, two-, and three-phase regions) where the break of the P-T isochore's slopes are observing. Temperatures and pressures (PS,TS) at the LLV and LV phase transition points for each fixed density (isochore, ρ) were measured using the isochoric P-T break-point technique in the immediate vicinity of the UCEP of the mixture. The UCEP property data TUCEP = 469.15 K, PUCEP = 5.229 MPa, and ρUCEP = 263.95 kg·m−3 for the binary H2O + n-hexane mixture (x = 0.5 mol fraction of n-hexane) were extracted from the detailed PρT and three-phase equilibrium data (PLLV, TLLV, ρLLV) measurements near the UCEP. The combined expanded uncertainty of the density, ρ, pressure, P, temperature, T, and concentration, x, measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be 0.2% (density in the liquid phase), 0.3% (density in the gas phase), 0.5% (density in the critical and supercritical regions); 0.05% (pressure); 15 mK (temperature); and 0.01 mol% (concentration). The excess molar volumes (VmE) of the binary mixture were calculated using the measured molar volumes Vm of the mixture and the reference molar volumes of the pure components (water and n-hexane) for various experimental temperatures and pressures in the supercritical region. Also, the measured PρT data at low densities (vapor phase) and high temperatures were used to calculated the second, third, and cross-term virial coefficients of the mixture.