Measurement of the Specific Enthalpy of Vaporization on 1,1,1,2-Tetrafluoroethane in the Temperature Range from 180 to 240 K

An apparatus is described which is capable of measuring the enthalpy of vaporization in the temperature range from 100 to 250 K. The sample (R134a; purity, at least 99.999%) is located in the measuring cell at the saturated vapor pressure, p = p _s. A control circuit allows p to be kept constant by opening a motor-operated valve to a weighing cylinder after having switched on the electrical measuring cell heater. During the experiment, the temperature is kept constant within a 10mK. In the range 180 to 230 K, the data for R134a are compared with calculated values from the fundamental equation given by Tillner-Roth and Baehr, which is recommended by Annex 18 of the International Energy Agency (IEA) as an international standard. Good agreement within a standard uncertainty of 1.6×10^–3 is obtained. At temperatures of only 10 K above the triple-point temperature, the enthalpy of vaporization calculated from the Clausius–Clapeyron equation shows considerable uncertainty due to the determination of the small vapor pressure. It is chiefly in this range that it is advantageous to have the new apparatus.     

Compression factor and vapor pressure of bromotrifluoromethane in the temperature range 245–393 K at pressures up to 12 MPa

The compression factors and vapor pressures have been measured on bromotrifluoromethane using a Burnett apparatus. The results on the compression factor cover the range of temperatures 263 to 393 K and of pressures 0.14 to 11.6 MPa, corresponding to a density variation from 7 to 1367 kg· m^–3. The experimental uncertainty of these 176 measurements of compression factor was estimated to be 0.2%. Thirty measurements of vapor pressure were made for temperatures 245 to 339 K, with an experimental uncertainty of 0.1%. Based on these results, the second virial coefficients were determined for temperatures 293 to 393 K.     

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.     

Thermal conductivity andPVT measurements of pentafluoroethane (refrigerant HFC-125)

By means of the transient and steady-state coaxial cylinder methods, the thermal conductivity of pentafluoroethane was investigated at temperatures from 187 to 419 K and pressures from atmospheric to 6.0 MPa. The estimated uncertainty of the measured results is ±(2–3)%. The operation of the experimental apparatus was validated by measuring the thermal conductivity of R22 and R12. Determinations of the vapor pressure andPVT properties were carried out by a constant-volume apparatus for the temperature range 263 to 443 K, pressures up to 6 MPa, and densities from 36 to 516 kg m^–3. The uncertainties in temperature, pressure, and density are less than ±10 mK, ±0.08%, and ±0.1%, respectively.Paper presented at the Twelfth Symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

A Small-Volume Apparatus for the Measurement of Phase Equilibria

An apparatus has been designed and constructed for the measurement of vapor-liquid equilibrium properties. The main components of the apparatus consist of an equilibrium cell and a vapor circulation pump. The cell and all of the system valves are housed inside a temperature controlled, insulated aluminum block. The temperature range of the apparatus is 260 K to 380 K to pressures of 6 MPa. The uncertainty of the temperature measurement is 0.03 K, and the uncertainty in the pressure measurement is 9.8 × 10⁻⁴ MPa. An automated data acquisition system is used to measure temperature and pressure at equilibrium. The apparatus has been performance tested by measuring the vapor pressures of propane, butane, and a standard mixture of propane + butane.     

PVT measurements of water-ammonia refrigerant mixture in the critical and supercritical regions

The PVTx properties of H₂O + NH₃ mixture (0.2607 mole fraction of ammonia) have been measured in the near- and supercritical regions. Measurements were made along 40 liquid and vapor isochores in the range from 120.03 to 727.75 kg m⁻³ and at temperatures from 301 to 634 K and at pressures up to 28 MPa. Temperatures and densities at the liquid–gas phase transition curve, dew- and bubble-pressure points, and the critical parameters for the 0.7393 H₂O + 0.2607 NH₃ mixture were obtained using the quasi-static thermograms and isochoric (P–T) break-point techniques. The expanded uncertainty of the density, pressure, temperature measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be 0.06%, 0.02–0.09%, and 15 mK, respectively.     

Ultrasonic speeds and densities of poly(dimethylsiloxane) (viscosity grades 30 and 50×10−4 m · s−1) at 298.15, 303.15, and 308.15 K under high pressures

The ultrasonic speeds and densities of poly(dimethylsiloxane), viscosity grades 30 and 50×10^–4 m · s^–1 at 298.15 K, were measured at 298.15, 303.15, and 308.15 K. The measurements were carried out using new apparatuses, one for measurement of the speed under pressures up to 200 MPa and another for measurement of the density under pressures up to 100 MPa. The former is constructed with a sing-around technique of the fixed-path type operated at a frequency of 2 MHz, and the latter is a dynamic bellows piezometer. The probable uncertainty in the present results is within ±0.23% for speed and ±0.19% for density for all the experimental conditions. The ultrasonic speed in these fluids at first increases rapidly with pressure and then indicates a mild rise in the highpressure region. Similar pressure effects are observed for the density. The relationship between the speed and the density satisfied a first-order function well. The isentropic compressibility, derived from the speed and density, also showed a large pressure effect. The values and its pressure effects seemed almost independent of the viscosity of poly(dimethylsiloxane).     

Thermal conductivity of methane-ethane mixtures at temperatures between 140 and 330 K and at pressures up to 70 MPa

The paper presents new measurements on the thermal conductivity of three methane-ethane mixtures with methane mole fractions of 0.69, 0.50, and 0.35. The thermal conductivity surface for each mixture is defined by up to 13 isotherms at temperatures between 140 and 330 K with pressures up to 70 MPa and densities up to 25 mol · L^–1. The measurements were made with a transient hot-wire apparatus. They cover a wide range of physical states including the dilute gas, the single-phase fluid at temperatures above the maxcondentherm, the compressed liquid states, and the vapor at temperatures below the maxcondentherm. The results show an enhancement in the thermal conductivity in the single-phase fluid down to the maxcondentherm temperature, as well as in the vapor and in the compressed liquid. A curve fit of the thermal conductivity surface is developed separately for each mixture.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Thermodynamic properties of HFC-32 (difluoromethane)

An 18-coefficient modified Benedict–Webb–Rubin equation of state of HFC-32 (difluoromethane) has been developed, based on the updated available PVT measurements, heat capacity measurements and speed of sound measurements. Correlations of vapor pressure and saturated liquid density are also presented. The correlations have been developed based on the reported experimental saturation properties data. This equation of state is effective both in the superheated gaseous phase and compressed liquid phase at pressures up to 70 MPa, densities to 1450 kg/m³, and temperatures from 150 to 475 K, respectively.     

Thermodynamic Properties for R-404A

An 18-coefficient modified Benedict–Webb–Rubin equation of state has been developed for R-404A, a ternary mixture of 44% by mass of pentafluoroethane (R-125), 52% by mass of 1,1,1-trifluoroethane (R-143a), and 4% by mass of 1,1,1,2-tetrafluoroethane (R-134a). Correlations of bubble point pressures, dew point pressures, saturated liquid densities, and saturated vapor densities are also presented. This equation of state has been developed based on the reported experimental data of PVT properties, saturation properties, and isochoric heat capacities by using least-squares fitting. These correlations are valid in the temperature range from 250 K to the critical temperature. This equation of state is valid at pressures up to 19 MPa, densities to 1300 kg·m^–3, and temperatures from 250 to 400 K. The thermodynamic properties except for the saturation pressures are calculated from this equation of state.     

Dew Point, Liquid Volume, and Dielectric Constant Measurements in a Vapor Mixture of Methane + Propane Using a Microwave Apparatus

An apparatus based on a microwave resonant cavity has been used to measure dew points and liquid volume fractions in a zC₃H₈+(1–z)CH₄ mixture with z=0.250±0.001 mole fraction. The microwave cavity is optimized for the measurement of small liquid volume fractions in lean natural gases. Argon and carbon dioxide were used to calibrate the resonator for dielectric constant and liquid volume measurements in mixtures. Estimated uncertainties are 1×10^–4 for dielectric constants and (0.05 K, 0.05 MPa) for dew points. The novel use of multiple cavity modes, each sensitive to different liquid volume regimes, substantially improves the reliability of liquid volume measurements. Liquid volume fractions can be resolved to better than 0.01%. Densities inferred from (P,T,) measurements agree within 0.6% of equation of state (EOS) densities with an estimated uncertainty of 0.1%. Liquid volume fractions measured with the microwave apparatus compare well with values determined using a conventional PVT cell. Fourteen dew points were measured at ten different temperatures. From these data, the mixture cricondentherm is estimated to be (293.45±0.05) K, which is 0.15 K higher than the value predicted using the Peng–Robinson equation of state.     

Vapor pressures of new fluorocarbons

The vapor pressures of four fluorocarbons have been measured at the following temperature ranges: R123 (2,2-dichloro-l,l,l-trifluoroethane), 273–457 K; R123a (1,2-dichloro-1,1,2-trifluoroethane), 303–458 K; R134a (1,1,1,2-tetrafluoroethane), 253–373 K; and R132b (l,2-dichloro-l,l-difluoroethane), 273–398 K. Determinations of the vapor pressure were carried out by a constant-volume apparatus with an uncertainty of less than 1.0%. The vapor pressures of R123 and R123a are very similar to those of R11 over the whole experimental temperature range, but the vapor pressures of R134a and R132b differ somewhat from those of R12 and R113, respectively, as the temperature increases. The numerical vapor pressure data can be fitted by an empirical equation using the Chebyshev polynomial with a mean deviation of less than 0.3 %.     

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.     

Thermodynamic Properties of Air from 60 to 2000 K at Pressures up to 2000 MPa

A thermodynamic property formulation for standard dry air based upon experimental P––T, heat capacity, and speed of sound data and predicted values, which extends the range of prior formulations to higher pressures and temperatures, is presented. This formulation is valid for temperatures from the solidification temperature at the bubble point curve (59.75 K) to 2000 K at pressures up to 2000 MPa. In the absence of experimental air data above 873 K and 70 MPa, air properties were predicted from nitrogen data. These values were included in the fit to extend the range of the fundamental equation. Experimental shock tube measurements ensure reasonable extrapolated properties up to temperatures and pressures of 5000 K and 28 GPa. In the range from the solidification point to 873 K at pressures to 70 MPa, the estimated uncertainty of density values calculated with the fundamental equation for the vapor is ±0.1%. The uncertainty in calculated liquid densities is ±0.2%. The estimated uncertainty of calculated heat capacities is ±1% and that for calculated speed of sound values is ±0.2%. At temperatures above 873 K and 70 MPa, the estimated uncertainty of calculated density values is ±0.5%, increasing to ±1% at 2000 K and 2000 MPa.     

Subatmospheric vapor pressures evaluated from internal-energy measurements

Vapor pressures were evaluated from measured internal-energy changes in the vapor+liquid two-phase region, ΔU ⁽²⁾. The method employed a thermodynamic relationship between the derivative quantity (ϖU ⁽²⁾/ϖV) _T and the vapor pressure (p _σ) and its temperature derivative (ϖp/ϖT)_σ. This method was applied at temperatures between the triple point and the normal boiling point of three substances: 1,1,1,2-tetrafluoroethane (R134a), pentafluoroethane (R125), and difluoromethane (R32). Agreement with experimentally measured vapor pressures near the normal boiling point (101.325 kPa) was within the experimental uncertainty of approximately ±0.04 kPa (±0.04%). The method was applied to R134a to test the thermodynamic consistency of a publishedp-p-T equation of state with an equation forp _σ for this substance. It was also applied to evaluate publishedp _σ data which are in disagreement by more than their claimed uncertainty.     

A thermodynamic property formulation for nitrogen from the freezing line to 2000 K at pressures to 1000 MPa

A new fundamental equation explicit in Helmholtz energy for thermodynamic properties of nitrogen from the freezing line to 2000 K at pressures to 1000 MPa is presented. A new vapor pressure equation and equations for the saturated liquid and vapor densities as functions of temperature are also included. The techniques used for development of the fundamental equation are those reported in a companion paper for ethylene. 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 also included in that paper. The property formulation using the fundamental equation reported here may generally be used to calculate pressures and densities with an uncertainty of ±0.1%, heat capacities within ± 2%, and velocity of sound values within ±2%. The fundamental equation is not intended for use near the critical point.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Isochoric p–ρ–T and Heat Capacity Cv Measurements for Ternary Refrigerant Mixtures Containing Difluoromethane (R32), Pentafluoroethane (R125), and 1,1,1,2-Tetrafluoroethane (R134a) from 200 to 400 K at Pressures to 35 MPa

The p––T relationships and constant volume heat capacity C _v were measured for ternary refrigerant mixtures by isochoric methods with gravimetric determinations of the amount of substance. Temperatures ranged from 200 to 400 K for p––T and from 203 to 345 K for C _v, while for both data types pressures extended to 35 MPa. Measurements of p––T were carried out on compressed gas and liquid samples with the following mole fraction compositions: 0.3337 R32+0.3333 R125+0.3330 R134a and 0.3808 R32+0.1798 R125+0.4394 R134a. Measurements of C _v were carried out on liquid samples for the same two compositions. Published p––T data are in good agreement with this study. For the p––T apparatus, the uncertainty is 0.03 K for temperature and is 0.01% for pressure at p>3 MPa and 0.05% at p<3 MPa. The principal source of uncertainty is the cell volume (28.5 cm³), with a standard uncertainty of 0.003 cm³. When all components of experimental uncertainty are considered, the expanded relative uncertainty (with a coverage factor k=2 and, thus, a two-standard deviation estimate) of the density measurements is estimated to be 0.05%. For the C _v calorimeter, the uncertainty of the temperature rise is 0.002 K and for the change-of-volume work it is 0.2%; the latter is the principal source of uncertainty. When all components of experimental uncertainty are considered, the expanded relative uncertainty of the heat capacity measurements is estimated to be 0.7%.     

Isochoric p–ρ–T Measurements for Binary Refrigerant Mixtures Containing Difluoromethane (R32), Pentafluoroethane (R125), 1,1,1,2-Tetrafluoroethane (R134a), and 1,1,1-Trifluoroethane (R143a) from 200 to 400 K at Pressures to 35 MPa

The p––T relationships were measured for binary refrigerant mixtures by an isochoric method with gravimetric determinations of the amount of substance. Temperatures ranged from 200 to 400 K, while pressures extended up to 35 MPa. Measurements were conducted on compressed gas and liquid samples with the following mole fraction compositions: 0.4997 R32+0.5003 R134a, 0.3288 R32+0.6712 R134a, 0.4996 R32+0.5004 R125, 0.5001 R125+0.4999 R134a, and 0.5000 R125+0.5000 R143a. Most published p––T data are in good agreement with this study. The uncertainty is 0.03 K for temperature and is 0.01% for pressure at p>3 MPa and 0.05% at p<3 MPa. The principal source of uncertainty is the cell volume (28.5 cm³), with a standard uncertainty of 0.003 cm³. When all components of experimental uncertainty are considered, the expanded relative uncertainty (with a coverage factor k=2 and, thus, a two-standard deviation estimate) of the density measurements is estimated to be 0.05%.     

Water and Gallium at Absolute Negative Pressures. Loci of Maximum Density and of Melting

Several physical properties of liquids as well as those of the coexistence between liquid and solid can be determined at absolute negative pressures. Examples for this include thermal pressure coefficients, loci of temperature of maximum density, melting lines, speed of propagation of low-intensity sound waves, and (p, T, x) conditions of occurrence of liquid/liquid phase separation. Three model temperature-pressure cycles, which allow for the measurement of temperature-pressure conditions of the occurrence of maxima of liquid density, negatively sloped fusion lines, and the upper critical solution temperature (UCST) of liquid solutions in these metastable regimes are described. A new apparatus for measuring negative pressures was developed. The temperature and pressure are determined within an uncertainty of ±0.05°C and ±5 bar, respectively. Water and heavy water have been used as testing systems with respect to the location of their temperatures of maximum density (TMD) loci. Empirical equations of state whose parameters have been fitted to experimental data located in the normal positive pressure region have proven to extrapolate well to the negative pressure regime. Furthermore, an attempt was made to use SAFT in order to provide a more theoretically founded framework. Preliminary results for gallium have shown that a TMD exists 45 K inside the supercooled regime, and that the continuation of its melting line down to –80 bar evolves with a slope of –515±25 bar·K^–1.     

The P–V–T–xProperties and Liquid–Liquid and Liquid–Vapor Phase Equilibria of n-Hexane–Water Binary System

The P–V–T–xproperties of an n-hexane–water stratifying binary system are investigated in the ranges of temperatures from 372.75 to 620.55 K, densities from 66.87 to 834.30 kg/m³, and pressures from 0.40 to 65.34 MPa for seven values of concentration of water (in molar fractions), namely, 0.166, 0.257, 0.347, 0.615, 0.827, 0.935, and 0.964. The measurements cover a wide range of the parameters of state, including the regions of liquid–liquid and liquid–vapor phase equilibria. The Soave–Redlich–Quong and scaling equations are used to describe the properties of the n-hexane–water system.