PvT and thermal-pressure coefficient measurements of diethyl ether (DEE) in the critical and supercritical regions

Abstract

The pressures (P) and its temperature derivatives or thermal-pressure coefficient, γV=(∂P/∂T)V, of DEE have been measured in the near- and supercritical regions as a function of temperature along the various liquid and vapour isochores. Measurements were made in the immediate vicinity of the liquid–gas phase transition and the critical points (single- and two-phase regions) using a high-temperature, high-pressure, nearly constant-volume adiabatic piezo-calorimeter. The constant-volume adiabatic calorimeter previously used for CV measurements was additionally supplied with high accurate strain gauge (calibrated piezoelectric transducer) to measure simultaneously the PvT, CV vT, and thermal-pressure coefficient γV. Measurements were made along 17 liquid and vapour isochores in the range from (212.6 to 534.6)kg·m−3 and at temperatures from (347 to 575)K and at pressures up to 18MPa. The quasi-static thermo- (reading of PRT, T–τ plot) and barograms (readings of the high accurate strain gauge, P–τ plot) techniques were used to accurate measure of the phase transition parameters (PS,ρS,TS) and γV at saturation curve. Temperatures at the liquid-gas phase transition curve, TS(ρ), for each measured density (isochore) and the critical parameters (TC and ρC) for DEE were obtained using the quasi-static thermograms technique. The expanded uncertainty of the pressure and its temperature derivative, (∂P/∂T)V, measurements at the 95% confidence level with a coverage factor of k=2 is estimated to be 0.05% and (0.12 to 1.5)% (depending on temperature and pressure), respectively. The measured pressures and temperature derivatives, (∂P/∂T)V, have been used to calculate the internal pressure (or energy–volume coefficient) as ∂U∂vT=T∂P∂TV-P. The effect of pressure and temperature on the internal pressure near the critical point was studied. The measured values of thermal-pressure coefficient, (∂P/∂T)V, were used to determine accurately the behaviour of second temperature derivative (∂2P/∂T2)V near the critical point and compared with our previous isochoric heat capacity measured results, ∂2P∂T2ρ=-ρ2T∂CV∂ρT. The measured and derived thermodynamic properties of DEE near the critical point were interpreted in terms of theory of critical phenomena.