Experimental study of the PVT and CVVT properties of n-butanol in the critical region

The PVT   and CVVTCVVT properties of n-butanol have been measured in the near- and supercritical regions. Measurements of PVT relation for n-butanol was made along 6 liquid and vapor near-critical isochores in the density range from (113.76 to 470.00) kg m−3 and at temperatures from (497 to 616) K and at pressures up to 10 MPa. The measurements were performed with a constant-volume piezometer technique. Caloric (CVVTCVVT) properties of n  -butanol were measured along the critical isochore and coexistence curve (liquid and vapor branches) in the one- and two-phase regions. The temperatures, densities, and pressures at the liquid-gas boundary curve (PS,TS,ρSPS,TS,ρS) and the critical parameters (TC=563.05±0.2 KTC=563.05±0.2 K, PC=4.405±0.01 MPaPC=4.405±0.01 MPa, ρC=270.0±2 kg m−3ρC=270.0±2 kg m−3) for n-butanol were obtained using the isochoric (P–T  ) break-point and quasi-static thermograms (CVCV jumps) techniques. The expanded uncertainty of the density, pressure, and temperature measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be 0.10% (at high densities) to 0.12% (at low densities), 0.0005–0.005 MPa, and 15 mK, respectively. The total experimental uncertainty of isochoric heat capacity (CVCV) measurements were estimated to be 2–3%. The measured CVCV, PVT  , saturated density (ρS,TSρS,TS) and vapor-pressure (PS,TSPS,TS) data near the critical point have been analyzed and interpreted in terms of extended scaling equations for the selected thermodynamic paths (critical isochore, critical isotherm, and coexistence curve) to accurately calculate the values of the asymptotic critical amplitudes (A0±, Γ0±, D0D0, B0B0) and to check their universal ratios. The measured saturated density data of n  -butanol near the critical point were also interpreted in terms of the “complete scaling” theory of critical phenomena. In particularly, the contributions of the “incomplete”, B2t1−αB2t1−α, and “complete scaling”, B4t2βB4t2β, terms on the coexistence-curve singular diameter were estimated. The values of the asymmetry parameters a3a3 and b2b2 of the coexistence curve singular diameter have been calculated. The strength of the Yang-Yang anomaly Rμ for n  -butanol was estimated using asymmetry parameters a3a3 and the contributions of the second temperature derivatives of vapor-pressure, (d2PS/dT2)(d2PS/dT2), and chemical potential, (d2μ/dT2)(d2μ/dT2), in the singularity of two-phase isochoric heat capacity, CV2CV2. The measured values of saturated one- (CV1′CV1′, CV1″CV1″) and two-phase (CV2′CV2′, CV2″CV2″) liquid and vapor isochoric heat capacities and saturated density data (ρS,TSρS,TS) together with measured vapor-pressure (PS,TSPS,TS) data were used to calculate other derived thermodynamic properties, such as KTKT, ΔHvapΔHvap, CPCP, CSCS, W  , (∂P/∂T)V′(∂P/∂T)V′, (∂V/∂T)P′(∂V/∂T)P′, (d2PS/dT2)(d2PS/dT2), and (d2μ/dT2)(d2μ/dT2) of n-butanol at saturation near the critical point. The results were compared with the direct measurements of these properties.

► PVT measurements of n-butanol near the critical point. ► CVVT measurements of n-butanol near the critical point. ► Yang-Yang anomaly strength and complete scaling behavior of singular diameter.