Isochoric Heat-Capacity Measurements for Pure Methanol in the Near-Critical and Supercritical Regions

Isochoric heat-capacity measurements for pure methanol are presented as a function of temperature at fixed densities between 136 and 750 kg·m⁻³. The measurements cover a range of temperatures from 300 to 556 K. The coverage includes the one- and two-phase regions, the coexistence curve, the near-critical, and the supercritical regions. A high-temperature, high-pressure, adiabatic, and nearly constant-volume calorimeter was used for the measurements. Uncertainties of the heat-capacity measurements are estimated to be 2–3% depending on the experimental density and temperature. Temperatures at saturation, T _S(ρ), for each measured density (isochore) were measured using a quasi-static thermogram technique. The uncertainty of the phase-transition temperature measurements is 0.02 K. The critical temperature and the critical density for pure methanol were extracted from the saturated data (T _S,ρ_S) near the critical point. For one near-critical isochore (398.92 kg·m⁻³), the measurements were performed in both cooling and heating regimes to estimate the effect of thermal decomposition (chemical reaction) on the heat capacity and phase-transition properties of methanol. The measured values of C _V and saturated densities (T _S,ρ_S) for methanol were compared with values calculated from various multiparametric equations of state (EOS) (IUPAC, Bender-type, polynomial-type, and nonanalytical-type), scaling-type (crossover) EOS, and various correlations. The measured C _V data have been analyzed and interpreted in terms of extended scaling equations for the selected thermodynamic paths (critical isochore and coexistence curve) to accurately calculate the values of the asymptotical critical amplitudes ( and B ₀).     

Thermodynamic properties of 1-chloro-1,2,2,2-tetrafluoroethane (R124)

The critical temperature and pressure, vapor pressure, and PVT relations for gaseous and liquid 1-chloro-1,2,2,2-tetrafluoroethane (R124) were determined experimentally. The vapor pressure was measured in the temperature range from 278.15 K to the critical temperature. The PVT measurements were carried out using two types of volumeters in the temperature range from 278.15 to 423.15 K, at pressure up to 100 MPa. The numerical PVT data of gaseous state are fitted as a function of density to a modified Benedict-Webb-Rubin equation. The pressure-volume relations of the liquid at each temperature are correlated satisfactorily as a function of pressure by the Tait equation. The critical density and saturated vapor and liquid densities are also determined and some of the thermodynamic properties are derived from the experimental results.     

Thermodynamic Properties of Methanol in the Critical and Supercritical Regions

The isochoric heat capacity of pure methanol in the temperature range from 482 to 533 K, at near-critical densities between 274.87 and 331.59 kg· m⁻³, has been measured by using a high-temperature and high-pressure nearly constant volume adiabatic calorimeter. The measurements were performed in the single- and two-phase regions including along the coexistence curve. Uncertainties of the isochoric heat capacity measurements are estimated to be within 2%. The single- and two-phase isochoric heat capacities, temperatures, and densities at saturation were extracted from experimental data for each measured isochore. The critical temperature (T_c = 512.78±0.02K) and the critical density (ρ_c = 277.49±2 kg · m⁻³) for pure methanol were derived from the isochoric heat-capacity measurements by using the well-established method of quasi-static thermograms. The results of the C_VVT measurements together with recent new experimental PVT data for pure methanol were used to develop a thermodynamically self-consistent Helmholtz free-energy parametric crossover model, CREOS97-04. The accuracy of the crossover model was confirmed by a comprehensive comparison with available experimental data for pure methanol and values calculated with various multiparameter equations of state and correlations. In the critical and supercritical regions at 0.98T_c≤ T ≤ 1.5T_c and in the density range 0.35ρ_c ≤ ρ leq 1.65 ρ_c, CREOS97-04 represents all available experimental thermodynamic data for pure methanol to within their experimental uncertainties.     

Volumetric (PVT) and Calorimetric (C V VT) Measurements for Pure Methanol in the Liquid Phase

Volumetric (PVT) and calorimetric (C _V VT) properties of pure methanol were measured in the liquid phase with a twin-cell adiabatic calorimeter. Temperatures were measured in a range from 314 to 411 K, densities between 699.3 and 775.6 kgm^–3, and pressures to 20 MPa. The calorimetric cell (70 cm³ capacity) was surrounded by adiabatic thermal shielding (high vacuum). The sample pressures were measured by means of a quartz crystal transducer to within an uncertainty of about ±7 kPa. The relative uncertainty of C _V was estimated to be 2%, with a coverage factor k = 2, by combining the various sources of experimental uncertainty using a root-sum-of-squares formula. The results for pure methanol were compared with other recent measurements performed with a second high-temperature, high-pressure adiabatic calorimeter. Deviations of less than 3% were found between the earlier C _V data and the present results for pure methanol. The uncertainty of the density measurements was estimated to be 0.2% (k = 2). The measured densities and isochoric heat capacities were compared with values calculated with an IUPAC equation of state. Agreement of density was within 0.088% and that for isochoric heat capacity was within 0.95%. Values of vapor pressure were determined by extrapolating experimental P–T data to the saturated temperature along a fixed isochore. In the temperature range of this study, decomposition of methanol was not observed.     

Vapor pressures and gas-phase PVT data for 1,1,1,2-tetrafluoroethane

We present new data for the vapor pressure and PVT surface of 1,1,1,2-tetrafluoroethane (Refrigerant 134a) in the temperature range 40° C (313 K) to 150° C (423 K). The PVT data are for the gas phase at densities up to one-half critical. Densities of the saturated vapor are derived at five temperatures from the intersections of the experimental isochores with the vapor pressure curve. The data are represented analytically in order to demonstrate experimental precision and to facilitate calculation of thermodynamic properties.Formerly National Bureau of Standards     

Isochoric Heat Capacity of CO2 + n-Decane Mixtures in the Critical Region

The isochoric heat capacity of two binary (CO₂+n-decane) mixtures (0.095 and 0.178 mole fraction of n-decane) have been measured with a high- temperature, high-pressure, nearly constant volume adiabatic calorimeter. Measurements were made at nineteen near-critical liquid and vapor densities between 87 and 658 kg·m⁻³ for the composition of 0.095 mole fraction n-decane and at nine densities between 83 and 458 kg·m⁻³ for the composition of 0.178 mole fraction n-decane. The range of temperatures was 295 to 568 K. These temperature and density ranges include near- and supercritical regions. The measurements were performed in both one- and two-phase regions including the vapor + liquid coexistence curve. The uncertainty of the heat- capacity measurements is estimated to be 2% (coverage factor k=2). The uncertainty in temperature is 15 mK, and that for density measurements is 0.06%. The liquid and vapor one- and two-phase isochoric heat capacities, temperatures (T _S), and densities (ρ_S) at saturation were measured by using the well-established method of quasi-static thermograms for each filling density. The critical temperatures (T _C), the critical densities (ρ_C), and the critical pressure (P _C) for the CO₂+n-decane mixtures were extracted from the isochoric heat-capacity measurements on the coexistence curve. The observed isochoric heat capacity along the critical isochore of the CO₂+n-decane mixture exhibits a renormalization of the critical behavior of C _{V X} typical for mixtures. The values of the characteristic parameters (K ₁, K ₂), temperatures (τ₁ ,τ₂), and the characteristic density differences were estimated for the CO₂+n-decane mixture by using the critical-curve data and the theory of critical phenomena in binary mixtures. The ranges of conditions were defined on the T-x plane for the critical isochore and the ρ-x plane for the critical isotherm, for which we observed renormalization of the critical behavior for the isochoric heat capacity.     

Gas-phasePVT properties of 1,1,1,2,3,3-Hexafluoropropane

We have measured the gas-phasePVT properties of 1,1,1,2,3,3,-hexafluoro-propane (R-236ea), which is considered to be a promising candidate for the replacement of 1,2-dichlorotetrafluoroethane (R-114). The measurements have been performed with a Burnett apparatus over a temperature range of 340 390 K and at pressures of 0.10–2.11 MPa. The experimental uncertainties of the measurements were estimated to be within ±0.5 kPa in pressure. ±8 mK in temperature, and ±0.15% in density. A truncated virial equation of state was developed to represent thePVT data and the second virial coefficients were also derived. The saturated vapor densities were also calculated by extrapolating the gas-phase isotherms to the vapor pressures. The critical density estimated from the rectilinear diameter was compared with the experimental value. The purity of the R-236ea sample used in the present measurements was 99.9 mol%. Paper presented at the Fourth Asian Thermophysical Properties Conference, September 5–8, 1995, Tokyo, Japan.     

PVTx Measurements for a H2O + Methanol Mixture in the Subcritical and Supercritical Regions

PVTx relationships for a H₂O + CH₃OH mixture (0.36 mole fraction of methanol) were measured in a range of temperatures from 373 to 673 K and pressures between 0.042 and 90.9 MPa. The density ranged from 37.76 to 559.03 kg · m^–3. Measurements were made with a constant-volume piezometer surrounded by a precision thermostat. The temperature inside the thermostat was maintained uniform within 5 mK. The volume of the piezometer (32.68 ± 0.01 cm³) was previously calibrated from well-established PVT values of pure water (IAPWS), and was corrected for both temperature and pressure expansions. Uncertainties of the density, temperature, and pressure measurements are estimated to be 0.16%, 30 mK, and 0.05%, respectively. The uncertainty in composition is 0.001 mole fraction. The method of isochoric and isothermal break points was used to extract the phase transition temperatures, pressures, and densities for each measured isochore and isotherm. The values of the critical temperature, pressure, and density of the mixture were also determined from PVTx measurements in the critical region.     

Experimental Study of the Isochoric Heat Capacity of Diethyl Ether (DEE) in the Critical and Supercritical Regions

Two- and one-phase liquid and vapor isochoric heat capacities (C _{V ρ} T relationship) of diethyl ether (DEE) in the critical and supercritical regions have been measured with a high-temperature and high-pressure nearly constant-volume adiabatic calorimeter. The measurements were carried out in the temperature range from 347 K to 575 K for 12 liquid and 5 vapor densities from 212.6 kg·m⁻³ to 534.6 kg·m⁻³. The expanded uncertainties (coverage factor k =  2, two-standard deviation estimate) for values of the heat capacity were 2% to 3% in the near-critical region, 1.0% to 1.5% for the liquid isochores, and 3% to 4% for the vapor isochores. The uncertainties of density (ρ) and temperature (T) measurements were 0.02% and 15 mK, respectively. The values of the internal energy, U(T, V), and second temperature derivative of pressure, (∂² P/∂T ²) _ρ , were derived using the measured C _V data near the critical point. The critical anomaly of the measured C _V and derived values of U(T, V) and (∂² P/∂T ²) _ρ in the critical and supercritical regions were interpreted in terms of the scaling theory of critical phenomena. The asymptotic critical amplitudes (A₀⁺ and A₀^- ){({A_0^+} {\rm and} {A_0^- )}} of the scaling power laws along the critical isochore for one- and two-phase C _V were calculated from the measured values of C _V . Experimentally derived values of the critical amplitude ratio for C_V (A₀⁺ /A₀^- = 0.521){C_{V} \left({A_0^+ /A_0^- = 0.521}\right)} are in good agreement with the values predicted by scaling theory. The measured C _V data for DEE were analyzed to study the behavior of loci of isothermal and isochoric C _V maxima and minima in the critical and supercritical regions.     

Vapor pressures and gas-phasePVT data for 1-Chloro-1,2,2,2-tetrafluoroethane (R124)

We present new data for the vapor pressure andPVT surface of 1-chloro-1,2,2,2-lelralluoroethane (designated R124 by the refrigeration industry) in the temperature range 278–423 K. ThePVT data are for the gas phase at densities up to 1.5 times the critical density. Correlating equations are given for the vapor pressures from 220 K to the critical temperature, 395.43 K, and for thePVT surface at densities up to 2 mol · L^–1 (approximately 0.5 times the critical density). Second and third virial coefficients have been derived from thePVT measurements.     

Isochoric Heat Capacity Measurements for 0.5 H2O+0.5 D2O Mixture in the Critical Region

The isochoric heat capacity C _V of an equimolar H₂O+D₂O mixture was measured in the temperature range from 391 to 655 K, at near-critical liquid and vapor densities between 274.05 and 385.36 kgm^–3. A high-temperature, high-pressure, nearly constant-volume adiabatic calorimeter was used. The measurements were performed in the one- and two-phase regions including the coexistence curve. The uncertainty of the heat-capacity measurement is estimated to be ±2%. The liquid and vapor one- and two-phase isochoric heat capacities, temperatures, and densities at saturation were extracted from the experimental data for each measured isochore. The critical temperature and the critical density for the equimolar H₂O+D₂O mixture were obtained from isochoric heat capacity measurements using the method of quasi-static thermograms. The measurements were compared with a crossover equation of state for H₂O+D₂O mixtures. The near-critical isochoric heat capacity behavior for the 0.5 H₂O+0.5 D₂O mixture was studied using the principle of isomorphism of critical phenomena. The experimental isochoric heat capacity data for the 0.5 H₂O+0.5 D₂O mixture exhibit a weak singularity, like that of both pure components. The reliability of the experimental method was confirmed with measurements on pure light water, for which the isochoric heat capacity was measured on the critical isochore (321.96 kgm^–3) in both the one- and two-phase regions. The result for the phase-transition temperature (the critical temperature, T _{C, this work}=647.104±0.003 K) agreed, within experimental uncertainty, with the critical temperature (T _{C, IAPWS}=647.096 K) adopted by IAPWS.     

Experimental Study of the Critical Behavior of the Isochoric Heat Capacity of Aqueous Ammonia Mixture

The isochoric heat capacity of a NH₃ + H₂O (0.2607 mole fraction of ammonia) mixture has been measured in the near- and supercritical regions. Measurements were made in the single- and two-phase regions including the coexistence curve using a high-temperature, high-pressure, nearly constant-volume adiabatic calorimeter. Measurements were made along 38 liquid and vapor isochores in the range from 120.03 kg · m⁻³ to 671.23 kg · m⁻³ and at temperatures from 478 K to 634 K and at pressures up to 28 MPa. Temperatures at the liquid–gas phase transition curve, T _S(ρ), for each measured density (isochore) and the critical parameters (T _C and ρ _C) for the 0.2607 NH₃ + 0.7393  H₂O mixture were obtained using the quasi-static thermograms technique. The expanded uncertainty of the heat-capacity measurements at the 95 % confidence level with a coverage factor of k = 2 is estimated to be 2 % to 3 % in the near-critical and supercritical regions, 1.0 % to 1.5 % in the liquid phase, and 3 % to 4 % in the vapor phase. Uncertainties of the density, temperature, and concentration measurements are estimated to be 0.06 %, 15mK, and 5×10⁻⁵ mole fraction, respectively. An unusual behavior of the isochoric heat capacity of the mixture was found near the maxcondetherm point (in the retrograde region). The value of the Krichevskii parameter was calculated using the critical properties data for the mixture and vapor-pressure data for the pure solvent (H₂O). The derived value of the Krichevskii parameter was used to analyze the critical behavior of the strong (C _P , K _T ) and weakly (C _V ) singular properties in terms of the principle of isomorphism of critical phenomena in binary mixtures. The values of the characteristic parameters (K ₁, K ₂), temperatures (τ ₁, τ ₂), and the characteristic density differences (Δρ ₁, Δρ ₂) were calculated for the NH₃ + H₂O mixture by using the critical-curve data.     

Densities and Excess,Apparent, and Partial Molar Volumes of Binary Mixtures of BMIMBF<Subscript>4</Subscript> + Ethanol as a Function of Temperature,Pressure, and Concentration

The densities of five BMIMBF₄ (1-butyl-3-methylimidazolium tetrafluoroborate) + ethanol binary mixtures with compositions of (0.0701, 0.3147, 0.5384, 0.7452, and 0.9152) mole fraction BMIMBF₄ and of pure BMIMBF₄ have been measured with a vibrating-tube densimeter. Measurements were performed at temperatures from 298 K to 398 K and at pressures up to 40 MPa. The total uncertainty of density, temperature, pressure, and concentration measurements were estimated to be less than 0.1 kg · m⁻³, 15 mK, 5 kPa, and 10⁻⁴, respectively. The uncertainties reported in this article are expanded uncertainties at the 95% confidence level with a coverage factor of k = 2. The measured densities were used to study derived volumetric properties such as excess, apparent, and partial molar volumes. It is shown that the values of excess molar volume for BMIMBF₄ + ethanol mixtures are negative at all measured temperatures and pressures over the whole concentration range. The effect of water content on the measured values of density is discussed. The volumetric (excess, apparent, and partial molar volumes) and structural (direct and total correlation integrals, cluster size) properties of dilute BMIMBF₄ + ethanol mixtures were studied in terms of the Krichevskii parameter. The measured densities were used to develop a Tait-type equation of state.     

PVT relationships in a carbon dioxide-rich mixture with ethane

Comprehensive isochoric PVT measurements have been obtained for the system (0.99 CO₂ + 0.01 C₂H₆). The range of state points studied includes those with densities from 2 to 24 mol·dm^–3, temperatures from 245 to 400 K, and pressures to 35 MPa. Extensive comparisons have been made with two predictive conformai solution models, one which uses the 32-term BWR-type equation of Stewart and Jacobsen as a reference and the other using the newer Schmidt-Wagner functional form. Results obtained with the Schmidt-Wagner equation are better in the near-critical region owing to the flatter critical isotherm associated with this functional form.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Measurements of the refractive index of alternative refrigerants

Measurements of coexistence curves of the refractive index of HCFC-22, HFC23, HFC-32, HFC-125, and HFC-152a have been carried out in the range from ambient to critical temperature. Near the critical temperature the refractive index has distribution in both the vapor and the liquid phases in the test cell. Thus the values at the boundary between vapor and liquid are selected at those of saturated vapor and liquid, respectively. The values of the critical temperature and critical refractive index for each substance are estimated. The refractive index is related to density by the Lorentz-Lorenz equation. In the case of HFC-32 the value of the LL function is assumed to be constant in the limited region near the critical point, and the values of density of saturated vapor and liquid are calculated and are compared with the experimental values of density obtained byPVT measurement.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

A thermodynamic property formulation for ethylene from the freezing line to 450 K at pressures to 260 MPa

A new thermodynamic property formulation based upon a fundamental equation explicit in Helmholtz energy of the form A=A(, T) for ethylene from the freezing line to 450 K at pressures to 260 MPa is presented. A vapor pressure equation, equations for the saturated liquid and vapor densities as functions of temperature, and an equation for the ideal-gas heat capacity are also included. The fundamental equation was selected from a comprehensive function of 100 terms on the basis of a statistical analysis of the quality of the fit. The coefficients of the fundamental equation were determined by a weighted least-squares fit to selected P--T data, saturated liquid and saturated vapor density data to define the phase equilibrium criteria for coexistence, C _v data, velocity of sound data, and second virial coefficients. The fundamental equation and the derivative functions for calculating internal energy, enthalpy, entropy, isochoric heat capacity (C _v), isobaric heat capacity (C _p), and velocity of sound are included. The fundamental equation reported here may be used to calculate pressures and densities with an uncertainty of ±0.1%, heat capacities within ±3 %, and velocity of sound values within ±1 %, except in the region near the critical point. The fundamental equation is not intended for use near the critical point. This formulation is proposed as part of a new international standard for thermodynamic properties of ethylene.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Measurements of PρT properties,vapor pressures,saturated densities,and critical parameters for R 1234ze(Z) and R 245fa

Pressure-density-temperature (PρT) properties, vapor pressures, and saturated liquid and vapor densities for refrigerants R 1234ze(Z) (cis-1,3,3,3-tetrafluoroprop-1-ene; CF₃CHCHF) and R 245fa (1,1,1,3,3-pentafluoropropane; C₃H₃F₅) were measured with two types of isochoric methods. Pressure was measured with a digital quartz pressure transducer. Temperature was measured with 25 Ω standard platinum resistance thermometer on the ITS-90 temperature scale. Density was calculated from the mass of sample and the inner volume of pressure vessel. By using the present vapor pressure data, new vapor pressure correlations for R 1234ze(Z) and R 245fa have been formulated. In addition, the critical temperature T_c, critical density ρ_c, and critical pressure P_c were directly determined on the basis of direct observation of the meniscus disappearance.     

Experimental study ofPVT properties of HFC-125 (CHF2CF3)

Pressure-volume-temperature (PVT) properties and vapor pressures of HFC125 (pentafuoroethane; CHF₂CF₃) have been experimentally obtained. Vapor pressures of HFC-125 have been measured in the range of temperatures from 223 to 338 K and pressures up to 3.54 M Pa with uncertainties of 5 mK and 2.5 kPa, respectively. The vapor pressure equation for this substance was correlated based on the present data. PVT properties of HFC-125 have been determined with a constant-volume apparatus in the range of temperatures from 280 to 473 K, pressures up to 17 M Pa, and densities up to 1145 kg · m^–3 with uncertainties of 5 mK, 2.5 kPa, and 0.01%, respectively. All of the available experimentalPVT property data were compared with the equation of state correlated by Wilson et al.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Molar Heat Capacity Cv, Vapor Pressure, and (p, ρ, T) Measurements from 92 to 350 K at Pressures to 35 MPa and a New Equation of State for Chlorotrifluoromethane (R13)

Measurements of the molar heat capacity at constant volume C _v for chlorotrifluoromethane (R13) were conducted using an adiabatic method. Temperatures ranged from 95 to 338 K, and pressures were as high as 35 MPa. Measurements of vapor pressure were made using a static technique from 250 to 302 K. Measurements of (p, , T) properties were conducted using an isochoric method; comprehensive measurements were conducted at 15 densities which varied from dilute vapor to highly compressed liquid, at temperatures from 92 to 350 K. The R13 samples were obtained from the same sample bottle whose mole fraction purity was measured at 0.9995. A test equation of state including ancillary equations was derived using the new vapor pressures and (p, , T) data in addition to similar published data. The equation of state is a modified Benedict–Webb–Rubin type with 32 adjustable coefficients. Acceptable agreement of C _v predictions with measurements was found. Published C _v(, T) data suitable for direct comparison with this study do not exist. The uncertainty of the C _v values is estimated to be less than 2.0% for vapor and 0.5% for liquid. The uncertainty of the vapor pressures is 1 kPa, and that of the density measurements is 0.1%.